asns-ex1037_392.htm

This SHARE PURCHASE AGREEMENT (this “Agreement”) is entered into as of November 15, 2017 (the “Effective Date”), by and between New Enterprise Associates 16, L.P., a Delaware limited partnership (the “Investor”), and Arsanis, Inc., a Delaware corporation (the “Company”).

WHEREAS, the Company is proposing to issue and sell to the Investor (the “Offering”) $20,000,000 of the Company’s common stock, $0.001 par value per share (the “Common Stock”), in connection with the Company’s initial public offering of Common Stock (“IPO”), pursuant to the terms and subject to the conditions set forth in this Agreement;

WHEREAS, the parties hereto have executed this Agreement on the Effective Date, which is prior to the effectiveness of the registration statement on Form S-1 filed by the Company with the Securities and Exchange Commission (the “SEC”) for the Company’s IPO;

WHEREAS, the closing of the Offering shall take place concurrently with the closing of the IPO and at a price per share equal to the initial public offering price per share that the Common Stock is sold to the public in the IPO, provided, however, that such initial public offering price per share shall not exceed $10.00 per share (the “IPO Price” and such time, the “IPO Closing Time”), as set forth on the cover of the final prospectus filed with the SEC;

WHEREAS, the Shares are being offered to the Investor pursuant to a private placement exemption from registration under the Securities Act of 1933, as amended (the “Securities Act”); and

WHEREAS, in order to effect the IPO, the Company shall enter into an Underwriting Agreement (the “Underwriting Agreement”) with Citigroup Global Markets Inc., Cowen and Company, LLC and Piper Jaffray & Co., as representative of the several underwriters named therein (the “Underwriters”), substantially in the form attached hereto as Exhibit A;

NOW, THEREFORE, in consideration of the foregoing and the mutual promises and covenants set forth below, the parties hereto hereby agree as follows:

(a) Purchase and Sale. Subject to the terms and conditions set forth in this Agreement, the Company hereby agrees to sell to the Investor, and the Investor hereby agrees to purchase from the Company, $20,000,000 of the Company’s Common Stock (the “Investment Amount”) at the IPO Price. The number of shares of Common Stock to be sold by the Company and purchased by the Investor hereunder (the “Shares”) shall equal the number of shares determined by dividing the Investment Amount by the IPO Price (rounded down to the nearest whole share). The total purchase price to be paid by the Investor for the Shares is equal to (i) the number of Shares multiplied by (ii) the IPO Price (the “Purchase Price”).

(b) Closing. The closing of the purchase and sale of the Shares (the “Closing”) shall take place at the offices of Davis Polk & Wardwell LLP, 450 Lexington Avenue, New York, New York 10017, or at such other place as shall be agreed upon by the parties hereto, after the satisfaction or waiver of each of the conditions set forth in Section 4 (other than those conditions that by their nature are to be satisfied at the Closing, but subject to the fulfillment or waiver of those conditions) concurrently with the IPO Closing Time. At the Closing, (i) the Company shall cause the Company’s transfer agent to deliver the Shares to the Investor registered in the name of the Investor, and (ii) the Purchase Price for the Shares shall be delivered by or on behalf of the Investor to the Company.

(c) Payment of Purchase Price. Payment by the Investor of the Purchase Price to the Company shall be made at the Closing by wire transfer of immediately available funds equal to the aggregate purchase price for the Shares to an account specified in writing by the Company.

Payment of the Purchase Price for the Shares shall be made against delivery to the Investor of the Shares, which Shares shall be uncertificated and shall be registered in the name of the Investor on the books of the Company by the Company’s transfer agent.

2.1 Representations and Warranties of the Company. The Company represents and warrants to the Investor as follows:

(a) The representations and warranties of the Company set forth in the Underwriting Agreement are true and correct on and as of the date hereof, with the same effect as if made herein on the date hereof (except to the extent any such representations and warranties expressly relate to a particular date, in which case such representations and warranties are true and correct as of such particular date) (after giving effect to any materiality or other qualifiers contained therein);

(b) The Company has the full right, power, authority and capacity to enter into this Agreement and to consummate the transactions contemplated hereby and has taken all necessary action to authorize the execution, delivery and performance of this Agreement;

(c) This Agreement constitutes the valid and legally binding obligation of the Company, enforceable in accordance with its terms, subject to bankruptcy, reorganization, insolvency and other similar laws affecting the enforcement of creditors’ rights generally and to general principles of equity. Upon issuance in accordance with, and payment pursuant to, the terms of this Agreement, the Shares will be duly authorized, validly issued and fully-paid and nonassessable;

(d) The execution and delivery by the Company of, and the performance by the Company of its obligations under this Agreement, the issuance, sale and delivery of the Shares, the consummation of the transactions contemplated herein and compliance by the Company with its obligations hereunder does not and will not contravene any provision of applicable law, or the certificate of incorporation or by laws or other organizational documents of the Company, or any

agreement or other instrument binding upon the Company or the Shares or any judgment, order or decree of any governmental body, agency or court having jurisdiction over the Company, and no consent, approval, authorization or order of, or qualification with, any governmental body or agency is required for the performance by the Company of its obligations under this Agreement, except, in each case, where any such contravention or where the failure to obtain any such consent, approval, authorization or order would not reasonably be expected to have a material adverse effect on the consummation of the transactions contemplated hereby;

(e) To the knowledge of the Company, the Company has not taken any action which could reasonably be expected to cause the sale of the Shares to be sold by the Company to the Investor to fail to qualify as exempt from the registration requirements of the Securities Act; and

(f) None of the Company, any predecessor of the Company, any affiliated issuer, any director, executive officer, other officer of the Company participating in the Offering, general partner or managing member of the Company, any beneficial owner of 20% or more of the Company’s outstanding voting equity securities, any promoter connected with the Company at the time of any sale of the Shares or any investment manager of the Company (if the Company is a pooled investment fund), as applicable, is subject to the “bad actor” disqualifications set forth in Rule 506(d)(1) of the Securities Act.

(a) (i) It is an institutional “accredited investor” as defined in Rule 501(a) promulgated under the Securities Act; (ii) it has sufficient knowledge and experience in investing in companies similar to the Company so as to be able to evaluate the risks and merits of its investment in the Company and it is able financially to bear the risks thereof; (iii) it has had an opportunity to discuss the Company’s business, management and financial affairs with the Company’s management; (iv) all documents, records, and information pertaining to its investment in the Common Stock and the Company that have been requested by it, if any, have been made available or delivered to it prior to the date hereof; (v) its financial condition is such that it is able to bear the risk of holding the Shares for an indefinite period of time and can bear the loss of the entire investment in such Shares; and (vi) it is not purchasing the Shares as the result of any form of general solicitation or general advertising within the meaning of Rule 502(c) under the Securities Act or as a result of the Investor's review of public filings by the Company;

(b) The Investor understands that this Agreement is made in reliance upon the Investor’s express representations, which it hereby represents and warrants to the Company, that (i) the Shares being purchased by the Investor are being acquired for the Investor’s own account (and not on behalf of any other person or entity) for the purpose of investment and not with a view to, or for sale in connection with, the distribution thereof, nor with any present intention of distributing or selling the Shares or any portion thereof, (ii) the Investor was not organized for the specific purpose of acquiring the Shares and (iii) the Shares will not be sold by the Investor without registration under the Securities Act or applicable state securities laws, or an exemption therefrom;

(c) The Investor further understands that the Shares being purchased by the Investor hereunder have not been registered under the Securities Act or any state securities laws and are instead being offered and sold in reliance on an exemption from such registration requirements. The Investor represents and warrants to the Company that, to the Investor’s knowledge, the Investor has not taken any action which could reasonably be expected to cause the sale of the Shares to be sold by the Company to the Investor to fail to qualify as exempt from the registration requirements of the Securities Act. The Investor further understands that until such time as the Shares shall have been registered under the Securities Act and applicable state securities laws or shall have been transferred in accordance with an opinion of counsel reasonably satisfactory to the Company that such registration is not required, stop transfer instructions shall be issued to the Company’s transfer agent and any certificate or certificates representing such securities shall bear a restrictive legend stating that such securities have not been registered under the Securities Act and applicable state securities laws and referring to restrictions on the transferability and sale thereof.

(d) The Investor further understands that its representations and warranties hereunder will not preclude disposition of the Shares without registration thereof, in compliance with Rule 144 promulgated under the Securities Act (“Rule 144”). The Investor understands and acknowledges, however, that there may not be available when the Investor wishes to sell the Shares, or any portion thereof, the adequate current public information with respect to the Company which would permit offers or sales of such securities pursuant to Rule 144, and, therefore, compliance with the Securities Act or some other exemption from the registration and prospectus delivery requirements of the Securities Act may be required for any such offer or sale; and

(e) (i) The Investor is validly existing as a limited partnership in good standing under the laws of the State of Delaware; (ii) the Investor has all requisite power and authority to execute and deliver this Agreement; and (iii) this Agreement constitutes the valid and legally binding obligation of the Investor, enforceable against the Investor in accordance with its terms, subject to bankruptcy, reorganization, insolvency and other similar laws affecting the enforcement of creditors’ rights generally and to general principles of equity.

4. Conditions to the Closing. The obligations of the Company and the Investor hereunder are subject to the satisfaction of the conditions set forth below on or before the Closing. If for any reason any of the conditions set forth in this Section ‎4 are not satisfied or waived by each party entitled to the benefit of such conditions at or prior to the Closing, then each party by written notice given to the other parties hereto shall have the right to elect to terminate this Agreement and each party shall be released from their obligations hereunder and shall have no further liability hereunder, provided, however, that nothing contained in this Section ‎4 shall relieve either party from liabilities or damages arising out of any fraud or willful breach by such party of this Agreement prior to such termination.

(a) Conditions to the Investor’s Obligations. The Investor’s obligation to purchase the Shares at the Closing is subject to the satisfaction of the following conditions:

(i) delivery of an opinion of counsel to the Company, dated the date of the Closing, substantially in the form attached hereto as Exhibit B;

(ii) the representations and warranties of the Company contained in Section 2.1 shall be true and accurate on and as of the Closing with the same force and effect as if they had been made at the Closing (except to the extent any such representations and warranties expressly relate to a particular date, in which case such representations and warranties are true and correct as of such particular date) (after giving effect to any materiality or other qualifiers contained therein); and

(b) Conditions to the Company’s Obligations. The Company's obligation to sell the Shares at the Closing is subject to the satisfaction of the following conditions:

(i) the Investor shall have paid the Purchase Price to the Company by wire transfer of immediately available funds as specified in Section 1(c) of this Agreement;

(ii) the representations and warranties of the Investor contained in Section 2.2 shall be true and accurate on and as of the Closing with the same force and effect as if they had been made at the Closing; and

(iii) the Underwriters shall have purchased, concurrent with the purchase of the Shares by the Investor hereunder, the Firm Shares (as defined in the Underwriting Agreement) at the IPO Price (less any underwriting discounts or commissions).

(a) Defined Terms. As used in this Section 5, the following terms shall have the following meanings:

(i) “Prospectus” means (x) the prospectus included in any Registration Statement contemplated by this Section 5, as amended or supplemented by any prospectus

supplement, with respect to the terms of the offering of any portion of the Registrable Securities covered by such Registration Statement and by all other amendments and supplements to the prospectus, including post-effective amendments and all material incorporated by reference in such prospectus, and (y) any “free writing prospectus” as defined in Rule 405 promulgated under the Securities Act.

(ii) “Registrable Securities” means (x) the Shares, and (y) any shares of Common Stock issued or issuable with respect to the Shares by way of a stock dividend or stock split or in connection with a combination of shares, recapitalization, merger, consolidation or other reorganization. As to any particular Registrable Securities, such securities shall cease to be Registrable Securities when (i) a Registration Statement covering such securities has been declared effective by the SEC and such securities have been disposed of pursuant to such effective Registration Statement, (ii) such securities are sold under circumstances in which all of the applicable conditions of Rule 144 (or any similar provisions then in force) are met, (iii) such securities are otherwise transferred and such securities may be resold without subsequent registration under the Securities Act, or (iv) such securities shall have ceased to be outstanding.

(iii) “Registration Statement” means any registration statement of the Company which covers any of the Registrable Securities pursuant to the provisions of this Section 5, including the Prospectus, amendments and supplements to such Registration Statement, including post-effective amendments, all exhibits and all materials incorporated by reference in such Registration Statement.

(i) At any time after the date that is 180 days after the Closing, the Investor may request registration under the Securities Act of all of its Registrable Securities then held on a Form S-1 or Form S-3 registration statement (or any successor to each such form) (or, if Form S-1 or Form S-3 is not then available, on such form of registration statement as is then available to effect a registration of the Registrable Securities pursuant to this subsection (b)(i)) (each a “Demand Registration”). Each request for a Demand Registration shall specify the approximate number of Registrable Securities required to be registered. Upon receipt of a Demand Registration request, the Company shall cause a Registration Statement to be filed within forty-five (45) days after the date on which such request was received by the Company. The Company shall not be required to effect a Demand Registration (A) more than two (2) times for the Investor; provided, however, that a Registration Statement shall not count as a Demand Registration requested under this subsection (b)(i)(A) unless and until it has become effective, or (B) if the Company furnishes to the Investor a certificate signed by an authorized officer of the Company stating that (a) within sixty (60) days of receipt of the Demand Registration request under this subsection (b)(i), the Company expects to file a registration statement for the public offering of securities for the account of the Company (other than a registration of securities (x) issuable pursuant to an employee stock option, stock purchase or similar plan, (y) issuable pursuant to a merger, exchange offer or a transaction of the type

specified in Rule 145(a) under the Securities Act or (z) in which the only securities being registered are securities issuable upon conversion of debt securities which are also being registered), provided, that the Company is actively employing good faith efforts to cause such registration statement to become effective, or (b) the Company is engaged in a material transaction or has an undisclosed material corporate development, in either case, which would be required to be disclosed in the Registration Statement, and in the good faith judgment of the Company’s Board of Directors, such disclosure would be materially detrimental to the Company and its stockholders at such time (in which case, the Company shall disclose the matter as promptly as reasonably practicable and thereafter file the Registration Statement, and the Investor agrees not to disclose any information about such material transaction to third parties until such disclosure has occurred or such information has entered the public domain other than through breach of this provision by such Investor), provided, however, that the Company shall have the right to defer the filing of the Registration Statement pursuant to this subsection twice in any twelve (12) month period and any such deferral may not exceed a period of more than sixty (60) days in the aggregate during such twelve-month period.

(ii) If the Investor requests a Demand Registration and elects to distribute the Registrable Securities covered by its request in an underwritten offering, the Investor shall so advise the Company as a part of its request made pursuant to subsection (b)(i). The Company shall select the investment banking firm or firms to act as the managing underwriter or underwriters in connection with such offering; provided, however, that such selection shall be subject to the consent of the Investor, which consent shall not be unreasonably withheld, delayed or conditioned.

(i) In connection with the filing by the Company of any Registration Statement, the Company shall furnish to the Investor (x) a copy of the Prospectus, including a preliminary prospectus, in conformity with the requirements of the Securities Act, and (y) such other documents as the Investor may reasonably request, in order to facilitate the public sale or other disposition of the Registrable Securities.

(ii) The Company shall use its reasonable best efforts to cause each Registration Statement contemplated by this Section 5 to be declared effective or become effective as soon as practicable following the filing thereof with the SEC. The Company shall notify the Investor in writing after any Registration Statement is declared effective.

(iii) In the event of any stock split, stock dividend or transaction with respect to the Registrable Securities that increases the number of Registrable Securities, if a then-effective Registration Statement does not cover the resale of such additional number of Registrable Securities, the Company shall amend or supplement any Registration Statement to cover such additional number of Registrable Securities.

(iv) The Company shall use its best efforts to register or qualify the Registrable Securities covered by any Registration Statement under the securities laws of each state of the United States; provided, however, that the Company shall not be required in connection with this subsection (c)(iv) to qualify as a foreign corporation or execute a general consent to service of process in any jurisdiction.

(v) If the Company has delivered preliminary or final Prospectuses to the Investor and, after having done so, the Prospectus is amended or supplemented to comply with the requirements of the Securities Act, the Company shall promptly notify the Investor and, if requested by the Company, the Investor shall immediately cease making offers or sales of shares under the applicable Registration Statement and return all Prospectuses to the Company. The Company shall promptly provide the Investor with revised or supplemented Prospectuses and, following receipt of the revised or supplemented Prospectuses, the Investor shall be free to resume making offers and sales under the applicable Registration Statement.

(vi) The Company shall advise the Investor promptly after it shall receive notice or obtain knowledge of the issuance of any stop order by the SEC delaying or suspending the effectiveness of any Registration Statement or of the initiation or threat of any proceeding for that purpose, and it will promptly use commercially reasonable efforts to prevent the issuance of any stop order or to obtain its withdrawal at the earliest possible moment if such stop order should be issued.

(d) Requirements of the Investor. The Company shall not be required to include any Registrable Securities in any Registration Statement contemplated by this Section 5 unless the Investor furnishes to the Company, in writing, such information regarding the Investor and the proposed sale of the Registrable Securities by the Investor as the Company may reasonably request in writing in connection with such Registration Statement or as shall be required in connection therewith by the SEC or any state securities law authorities.

(e) Suspension. The Company may suspend the use of any Registration Statement or Prospectus (a “Suspension”) by the Investor if the Company determines in good faith that such Suspension is necessary to (x) delay the disclosure of material non-public information concerning the Company, the disclosure of which at the time, in the good faith opinion of the Company’s Board of Directors, would be materially detrimental to the Company or its stockholders for a registration to be effected at such time; (y) amend or supplement the affected Registration Statement or the related Prospectus so that such Registration Statement or Prospectus shall not include an untrue statement of a material fact or omit to state a material fact required to be stated therein; or (z) amend or supplement the affected Registration Statement or Prospectus in order to make the statements therein, in light of the circumstances under which they were made, not misleading; provided, however, in each case of clauses (x) through (z), that the Company shall (a) promptly notify the Investor in writing of such Suspension and the reasons therefor, but shall not disclose to the Investor any material non-public information giving rise to a Suspension under clause (x); (b) advise the Investor in writing to cease all sales under the Registration Statement or Prospectus until the end of the Suspension; and (c) use its reasonable

best efforts to terminate such Suspension as promptly as practicable. The Company may not exercise its rights pursuant to this subsection (e) for more than ninety (90) days in the aggregate in any twelve (12) month period.

(f) Expenses. Except as set forth below, the Company will pay all of the expenses incurred in connection with complying with this Section 5 (whether or not any Registration Statement or Prospectus becomes final or effective), including, without limitation: all registration, filing and printing fees, the Company’s counsel and accounting fees and expenses, costs and expenses associated with clearing the Registrable Securities for sale under applicable state securities laws (including, without limitation, fees, charges and disbursements of counsel in connection with such clearance), all listing fees, expenses incurred by the Company (but not the Investor) in connection with any “road show,” and reasonable fees, charges and disbursements of counsel to the Investor. The Company shall not be required to pay or reimburse the Investor for any underwriting discounts or commissions and fees of underwriters, selling brokers, dealer managers or similar securities industry professionals with respect to the Registrable Securities being sold.

(i) The Company agrees to indemnify and hold harmless the Investor, each underwriter, and each other person, if any, who controls the Investor or underwriter within the meaning of the Securities Act or the Securities Exchange Act of 1934, as amended (the “Exchange Act”), from and against any losses, claims, damages or liabilities to which the Investor, such underwriter or controlling person may become subject (under the Securities Act, the Exchange Act, state securities or blue sky laws or otherwise) insofar as such losses, claims, damages or liabilities (or actions or proceedings in respect thereof) arise out of, or are based upon, any untrue statement of a material fact contained in any registration statement covering the Shares or in any preliminary prospectus or final prospectus contained in such registration statement, or any amendment or supplement to such registration statement, or the omission or alleged omission to state a material fact required to be stated therein or necessary to make the statements therein not misleading, and the Company will reimburse the Investor for any reasonable legal or other expenses reasonably incurred in investigating, defending or preparing to defend any such action, proceeding or claim, or preparing to defend any such action, proceeding or claim; provided, however, that the Company shall not be liable in any such case to the extent that such loss, claim, damage or liability arises out of, or is based upon, an untrue statement made in such registration statement, preliminary prospectus or prospectus, or any amendment or supplement in reliance upon and in conformity with written information relating to the Investor furnished to the Company by or on behalf of the Investor specifically for use in the preparation thereof or any statement or omission in any prospectus that is corrected in any subsequent prospectus that was delivered to the Investor prior to the pertinent sale or sales by the Investor.

(ii) The Investor agrees to indemnify and hold harmless the Company, each underwriter and each person, if any, who controls the Company within the meaning of

Section 15 of the Securities Act, each officer of the Company who signs the registration statement and each director of the Company, from and against any losses, claims, damages or liabilities to which the Company or any such underwriter, officer, director or controlling person may become subject under the Securities Act, the Exchange Act, state securities or blue sky laws or otherwise, insofar as such losses, claims, damages or liabilities (or actions or proceedings in respect thereof) arise out of, or are based upon, any untrue statement of a material fact contained in any registration statement covering the Shares or in any preliminary prospectus, final prospectus contained in such registration statement, or any amendment or supplement to such registration statement or the omission or alleged omission to state a material fact required to be stated therein or necessary to make the statements therein not misleading, if such untrue statement or omission was made in reliance upon and in conformity with written information relating to the Investor furnished by or on behalf of the Investor specifically for use in preparation of the registration statement, prospectus, amendment or supplement and the Investor will reimburse the Company, or such underwriter, officer, director or controlling person, as the case may be, for any legal or other expenses reasonably incurred in investigating, defending or preparing to defend any such action, proceeding or claim; provided, however, that the Investor’s obligation to indemnify the Company shall be limited to the net amount received by the Investor from the sale of the Shares.

(iii) Promptly after receipt by any indemnified person of a notice of a claim or the beginning of any action in respect of which indemnity is to be sought against an indemnifying person pursuant to this Section 5(g), such indemnified person shall notify the indemnifying person in writing of such claim or of the commencement of such action, but the omission to so notify the indemnifying party will not relieve it from any liability which it may have to any indemnified party under this Section 5(g) (except to the extent that such omission materially and adversely affects the indemnifying party’s ability to defend such action). Subject to the provisions hereinafter stated, in case any such action shall be brought against an indemnified person, the indemnifying person shall be entitled to participate therein, and, to the extent that it shall elect by written notice delivered to the indemnified party promptly after receiving the aforesaid notice from such indemnified party, shall be entitled to assume the defense thereof, with counsel reasonably satisfactory to such indemnified person. After notice from the indemnifying person to such indemnified person of its election to assume the defense thereof, such indemnifying person shall not be liable to such indemnified person for any legal expenses subsequently incurred by such indemnified person in connection with the defense thereof; provided, however, that if there exists or shall exist a conflict of interest that would make it inappropriate, in the opinion of counsel to the indemnified person, for the same counsel to represent both the indemnified person and such indemnifying person or any affiliate or associate thereof, the indemnified person shall be entitled to retain its own counsel at the expense of such indemnifying person; provided, however, that no indemnifying person shall be responsible for the fees and expenses of more than one separate counsel (together with appropriate local counsel) for all indemnified parties. In no event shall any indemnifying person be liable in respect of any amounts paid in settlement of any action unless the indemnifying person shall have approved the terms of such settlement;

provided, however, that such consent shall not be unreasonably withheld. No indemnifying person shall, without the prior written consent of the indemnified person, effect any settlement of any pending or threatened proceeding in respect of which any indemnified person is or could have been a party and indemnification could have been sought hereunder by such indemnified person, unless such settlement includes an unconditional release of such indemnified person from all liability on claims that are the subject matter of such proceeding.

(iv) If the indemnification provided for in this Section 5(g) is unavailable to or insufficient to hold harmless an indemnified party under subsections (g)(i) or (ii) above in respect of any losses, claims, damages or liabilities (or actions or proceedings in respect thereof) referred to therein, then each indemnifying party shall contribute to the amount paid or payable by such indemnified party as a result of such losses, claims, damages or liabilities (or actions in respect thereof) in such proportion as is appropriate to reflect the relative fault of the Company on the one hand and the Investor, as well as any other selling stockholders under such registration statement on the other in connection with the statements or omissions or other matters which resulted in such losses, claims, damages or liabilities (or actions in respect thereof), as well as any other relevant equitable considerations. The relative fault shall be determined by reference to, among other things, in the case of an untrue statement, whether the untrue statement relates to information supplied by the Company on the one hand or the Investor or other selling stockholder on the other and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such untrue statement. The Company and the Investor agree that it would not be just and equitable if contribution pursuant to this subsection (g)(iv) were determined by pro rata allocation (even if the Investor and other selling stockholders were treated as one entity for such purpose) or by any other method of allocation which does not take into account the equitable considerations referred to above in this subsection (g)(iv). The amount paid or payable by an indemnified party as a result of the losses, claims, damages or liabilities (or actions in respect thereof) referred to above in this subsection (g)(iv) shall be deemed to include any legal or other expenses reasonably incurred by such indemnified party in connection with investigating or defending any such action or claim. Notwithstanding the provisions of this subsection (g)(iv), the Investor shall not be required to contribute any amount in excess of the amount by which the net amount received by the Investor from the sale of the Shares to which such loss relates exceeds the amount of any damages which the Investor has otherwise been required to pay by reason of such untrue statement. No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Securities Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation.

(h) Survival. The rights and obligation of the Company and the Investor under this Section 5 shall survive the termination of this Agreement

(i) Termination. All of the Company’s obligations to register the Shares under this Agreement shall terminate on the earlier of (a) the three year anniversary of the date of this Agreement or (b) the date on which all of the Shares have been sold by the Investor.

6. Expenses. The Company will pay the fees, charges and disbursements of counsel to the Investor in connection with this Offering, provided, however, that such fees, charges and disbursements shall not exceed $35,000.

7. Successors and Assigns. This Agreement shall be binding upon and inure to the benefit of the parties hereto and their respective successors and permitted assigns.

8. Notices. Notices given hereunder shall be deemed to have been duly given, only if given in writing, and on (i) the date of personal delivery, (ii) on the date one day after being delivered to a reputable overnight courier with proper delivery instructions, or (iii) if delivered by facsimile, upon electronic confirmation of receipt to the party being notified as addressed as follows: (a) if to the Investor, to: New Enterprise Associates, 1954 Greenspring Drive, Suite 600, Timonium, Maryland 21093, Attention: Louis Citron, with a copy to: Davis Polk & Wardwell LLP, 450 Lexington Avenue, New York, New York 10017, Attention: Richard D. Truesdell, Jr., (b) if to the Company, to: Arsanis, Inc., 890 Winter Street, Suite 230, Waltham, Massachusetts 02451, Attention: René Russo, with a copy to: Wilmer Cutler Pickering Hale and Dorr LLP, 60 State Street, Boston, Massachusetts 02109, Attention: Cynthia T. Mazareas.

9. Entire Agreement and Amendments. This Agreement constitutes the entire agreement of the parties with respect to the subject matter hereof. This Agreement may be amended or waived if, and only if, such amendment or waiver is in writing and signed, in the case of an amendment, by each of the parties hereto or, in the case of a waiver, by the party waiving compliance. No waiver shall be deemed a waiver of any subsequent breach or default.

10. Governing Law. This Agreement shall be governed by and interpreted in accordance with the laws of the State of New York without regard to the conflict of laws principles thereof.

11. Severability. The invalidity or unenforceability of any particular provision of this Agreement shall not affect the other provisions hereof.

12. Assignment. This Agreement may not be assigned by the Company or the Investor without the prior written consent of the other parties hereto.

13. Captions. Captions are for convenience only and are not deemed to be part of this Agreement. All references herein to numbered Sections are to Sections of this Agreement unless otherwise indicated.

14. Survival. The representations and warranties contained herein shall survive the Closing.

15. Counterparts. This Agreement may be executed by pdf and in counterparts, each of which shall be deemed an original, but all of which together shall constitute one and the same instrument.

16. Further Assurances. The parties agree to execute such further instruments and to take such further action as may reasonably be necessary to carry out the intent of this Agreement.

17. Termination. This Agreement shall automatically terminate upon the earliest to occur, if any, of: (a) either the Company, on the one hand, or the Underwriters, on the other hand, advising the other in writing, prior to the execution of the Underwriting Agreement, that they have determined not to proceed with the IPO, (b) termination of the Underwriting Agreement (other than the provisions thereof which survive termination) prior to the sale of any of the Common Stock to the Underwriters, (c) the initial public offering price per share that the Common Stock is sold to the public in the IPO exceeds $10.00 per share, (d) the registration statement filed with the SEC with respect to the IPO is withdrawn, (e) the Underwriting Agreement has not become effective by February 16, 2018, or (f) the written consent of each of the Company and the Investor.

IN WITNESS WHEREOF, the parties hereto have executed this Agreement to be effective as of the date first set forth above.

ARSANIS, INC.

By:		/s/ René Russo
		Name:René Russo
		Title:President and Chief Executive Officer

NEW ENTERPRISE ASSOCIATES 16, L.P.

By:		NEA Partners 16, L.P.
By:		NEA 16 GP, LLC

By:		/s/ Louis S. Citron
		Name: Louis S. Citron
		Title:Chief Legal Officer

Arsanis, Inc., a corporation organized under the laws of Delaware (the “Issuer”), proposes to sell to the several underwriters named in Schedule I hereto (the “Underwriters”), for whom Citigroup Global Markets Inc., Cowen and Company, LLC and Piper Jaffray & Co. (the “Representatives”) are acting as representatives, 4,000,000 shares of common stock, $0.001 par value per share (“Common Stock”) of the Issuer (said shares to be issued and sold by the Issuer being hereinafter called the “Underwritten Securities”). The Issuer also proposes to grant to the Underwriters an option to purchase up to 600,000 additional shares of Common Stock to cover over-allotments, if any (the “Option Securities;” the Option Securities, together with the Underwritten Securities, hereinafter called the “Securities”). To the extent there are no additional Underwriters listed on Schedule I other than you, the term Representatives as used herein shall mean you, as Underwriters, and the terms Representatives and Underwriters shall mean either the singular or plural as the context requires. The offering and sale of the Securities contemplated by this Agreement is referred to herein as the “Offering.”

1. Representations and Warranties. The Issuer represents and warrants to, and agrees with, each Underwriter as set forth below in this Section 1:

(a) The Issuer has prepared and filed with the Securities and Exchange Commission (the “SEC”) a registration statement (file number 333-221050) on Form S-1 including exhibits and financial statements and any prospectus supplement relating to the Securities that is filed with the SEC pursuant to Rule 424(b) under the Securities Act and deemed part of such registration statement pursuant to Rule 430A under the Securities Act, as amended at the Execution Time (as defined herein) and, in the event any post-effective amendment thereto or any registration statement and any amendments thereto filed pursuant to Rule 462(b) under the Securities Act (as defined herein) relating to the Offering (the “Rule 462(b) Registration Statement”) becomes effective prior to the Closing Date (as defined herein), shall also mean such registration statement as so amended or such Rule 462(b) Registration Statement, as the case may be (the “Registration Statement”), including the Preliminary Prospectus (as defined herein), for registration under the Securities Act of 1933, as amended, and the rules and regulations of the SEC promulgated thereunder (the “Securities Act”) of the Offering. Such Registration Statement, including any amendments thereto filed prior to the date and time that this agreement (the “Underwriting Agreement”) is executed and delivered by the parties hereto (the “Execution Time”), has become effective under the Securities Act. The Issuer may have filed one or more amendments thereto, including a related preliminary prospectus relating to the Securities which is used prior to the filing of the Prospectus (the “Preliminary Prospectus”), each of which has previously been furnished to you. The Issuer will file with the SEC a final prospectus relating to the Securities in accordance with Rule 424(b) after the Execution Time (the “Prospectus”). As filed, such Prospectus shall contain all information required by the Securities Act and the rules thereunder and, except to the extent the Representatives shall agree in writing to a modification, shall be in all substantive respects in the form furnished to you prior to the Execution Time or, to the extent not completed at the Execution Time, shall contain only such specific additional information and other changes (beyond that contained in the latest Preliminary Prospectus) as the Issuer has advised you, prior to the Execution Time, will be included or made therein;

(b) On each date and time that the Registration Statement, any post-effective amendment or amendments thereto and any Rule 462(b) Registration Statement became or becomes effective (the “Effective Date”), the Registration Statement did, and when the Prospectus is first filed in accordance with Rule 424(b) under the Securities Act and on the Closing Date and on any date on which Option Securities are purchased, if such date is not the Closing Date (a “settlement date”), the Prospectus (and any supplement thereto) will, comply in all material respects with the applicable requirements of the Securities Act and the rules thereunder; on the Effective Date, at the Execution Time and on the Closing Date, the Registration Statement did not and will not contain any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary in order to make the statements therein not misleading; and on the date of any filing pursuant to Rule 424(b) and on the Closing Date and any settlement date, the

Prospectus (together with any supplement thereto) will not include any untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; provided, however, that the Issuer makes no representations or warranties as to the information contained in or omitted from the Registration Statement, or the Prospectus (or any supplement thereto) in reliance upon and in conformity with information furnished in writing to the Issuer by or on behalf of any Underwriter through the Representatives specifically for inclusion in the Registration Statement or the Prospectus (or any supplement thereto), it being understood and agreed that the only such information furnished by any Underwriter consists of the information described as such in Section 8 hereof;

(c) The “Disclosure Package” shall mean (i) the Preliminary Prospectus that is generally distributed to investors and used to offer the Securities, (ii) any issuer free writing prospectus, as defined in Rule 433 under the Securities Act (the “Issuer Free Writing Prospectuses”), if any, identified in Schedule II hereto and (iii) any other free writing prospectus, as defined in Rule 405 under the Securities Act (a “Free Writing Prospectus”) that the parties hereto shall hereafter expressly agree in writing to treat as part of the Disclosure Package. The (i) Disclosure Package, (ii) each electronic road show, when taken together as a whole with the Disclosure Package and (iii) any individual Written Testing-the-Waters Communication (as defined herein), when taken together as a whole with the Disclosure Package and the price to the public, the number of Underwritten Securities and the number of Option Securities to be included on the cover page of the Prospectus, does not contain any untrue statement of a material fact or omit to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading. The preceding sentence does not apply to statements in or omissions from the Disclosure Package based upon and in conformity with written information furnished to the Issuer by or on behalf of any Underwriter through the Representatives specifically for use therein, it being understood and agreed that the only such information furnished by or on behalf of any Underwriter consists of the information described as such in Section 8 hereof;

(d) (i) At the time of filing the Registration Statement and (ii) as of the Execution Time (with such date being used as the determination date for purposes of this clause (ii)), the Issuer was not and is not an ineligible issuer, as defined in Rule 405 under the Securities Act (an “Ineligible Issuer”), without taking account of any determination by the SEC pursuant to Rule 405 that it is not necessary that the Issuer be considered an Ineligible Issuer;

(e) From the time of initial confidential submission of the Registration Statement to the SEC (or, if earlier, the first date on which the Issuer engaged directly or through any person authorized to act on its behalf in any Testing-the-Waters Communication) through the Execution Time, the Issuer has been and is an “emerging growth company,” as defined in Section 2(a) of the Securities Act (an “Emerging Growth Company”).

“Testing-the-Waters Communication” means any oral or written communication with potential investors undertaken in reliance on Section 5(d) of the Securities Act;

(f) The Issuer (i) has not alone engaged in any Testing-the-Waters Communication other than Testing-the-Waters Communications with the consent of the Representatives with entities that are qualified institutional buyers within the meaning of Rule 144A under the Securities Act or institutions that are accredited investors within the meaning of Rule 501 under the Securities Act and (ii) has not authorized anyone other than the Representatives to engage in Testing-the-Waters Communications. The Issuer reconfirms that the Representatives have been authorized to act on its behalf in undertaking Testing-the-Waters Communications. The Issuer has not distributed any Written Testing-the-Waters Communications other than those listed on Schedule III hereto. “Written Testing-the-Waters Communication” means any Testing-the-Waters Communication that is a written communication within the meaning of Rule 405 under the Securities Act;

(g) Each Issuer Free Writing Prospectus does not include any information that conflicts with the information contained in the Registration Statement, including any document incorporated by reference therein that has not been superseded or modified. The foregoing sentence does not apply to statements in or omissions from any Issuer Free Writing Prospectus based upon and in conformity with written information furnished to the Issuer by or on behalf of any Underwriter through the Representatives specifically for use therein, it being understood and agreed that the only such information furnished by or on behalf of any Underwriter consists of the information described as such in Section 8 hereof;

(h) The interactive data in the eXtensible Business Reporting Language included as an exhibit to the Registration Statement fairly presents the information called for in all material respects and has been prepared in accordance with the SEC’s rules and guidelines applicable thereto. Each of the Issuer and its subsidiaries has been duly incorporated and is validly existing as a corporation in good standing under the laws of the jurisdiction in which it is chartered or organized with full corporate power and authority to own or lease, as the case may be, and to operate its properties and conduct its business as described in the Disclosure Package and the Prospectus, and is duly qualified to do business as a foreign corporation and is in good standing under the laws of each jurisdiction which requires such qualification, except where the failure to be so qualified or in good standing would not reasonably be expected to have a material adverse effect (i) on the Issuer’s performance of this Underwriting Agreement, (ii) the Issuer’s consummation of the Offering, or (iii) individually or in the aggregate, on the condition (financial or otherwise), prospects, earnings, business or properties of the Issuer and its subsidiaries, taken as a whole, whether or not arising from transactions in the ordinary course of business (clauses (i), (ii) or (iii), a “Material Adverse Effect”);

(i) All the outstanding shares of capital stock of each subsidiary have been duly and validly authorized and issued and are fully paid and non-assessable, and, except as

otherwise set forth in the Disclosure Package and the Prospectus, all outstanding shares of capital stock of the subsidiaries are owned by the Issuer either directly or through wholly owned subsidiaries free and clear of any perfected security interest or any other security interests, claims, liens or encumbrances;

(j) There is no franchise, contract or other document of a character required to be described in the Registration Statement or Prospectus, or to be filed as an exhibit thereto, which is not described or filed as required (and the Preliminary Prospectus contains in all material respects the same description of the foregoing matters contained in the Prospectus); and the statements in the Preliminary Prospectus and the Prospectus under the headings “Material U.S. Federal Income and Estate Tax Considerations for Non-U.S. Holders of Common Stock,” “Risk Factors—Risks Related to Our Intellectual Property,” “Risk Factors—Risks Related to Regulatory Approval and Other Legal Compliance Matters,” “Business—Intellectual Property” and “Business—Government Regulation and Product Licensure,” insofar as such statements summarize legal matters, agreements, documents or proceedings discussed therein, are accurate and fair summaries of such legal matters, agreements, documents or proceedings;

(k) This Underwriting Agreement has been duly authorized, executed and delivered by the Issuer;

(l) The Issuer is not, and immediately after giving effect to the Offering and the application of the proceeds thereof as described in the Disclosure Package and the Prospectus will not be, an “investment company” as defined in the Investment Company Act of 1940, as amended;

(m) No consent, approval, authorization, filing with or order of any court or governmental agency or body is required in connection with the transactions contemplated herein, except such as have been obtained under the Securities Act and such as may be required under the blue sky laws of any jurisdiction in connection with the purchase and distribution of the Securities by the Underwriters in the manner contemplated herein and in the Disclosure Package and the Prospectus;

(n) Neither the issue and sale of the Securities nor the consummation of any other of the transactions herein contemplated nor the fulfillment of the terms hereof will conflict with, result in a breach or violation of, or imposition of any lien, charge or encumbrance upon any property or assets of the Issuer or any of its subsidiaries pursuant to, (i) the charter or by-laws of the Issuer or any of its subsidiaries, (ii) the terms of any indenture, contract, lease, mortgage, deed of trust, note agreement, loan agreement or other agreement, obligation, condition, covenant or instrument to which the Issuer or any of its subsidiaries is a party or bound or to which its or their property is subject, or (iii) any statute, law, rule, regulation, judgment, order or decree applicable to the Issuer or any of its subsidiaries of any court, regulatory body, administrative agency, governmental body, arbitrator or other authority having jurisdiction over the Issuer or any

of its subsidiaries or any of its or their properties, except in the case of clauses (ii) and (iii) as would not have a Material Adverse Effect;

(o) No holders of securities of the Issuer have rights to the registration of such securities under the Registration Statement except for such registration rights as have been effectively waived in connection with this Offering;

(p) The consolidated historical financial statements and schedules of the Issuer and its consolidated subsidiary included in the Preliminary Prospectus, the Prospectus and the Registration Statement present fairly in all material respects the financial condition, results of operations and cash flows of the Issuer as of the dates and for the periods indicated, comply as to form in all material respects with the applicable accounting requirements of the Securities Act and have been prepared in conformity with generally accepted accounting principles in the United States applied on a consistent basis throughout the periods involved (except as otherwise noted therein); The selected financial data set forth under the caption “Selected Financial Information” in the Preliminary Prospectus, the Prospectus and Registration Statement fairly present in all material respects, on the basis stated in the Preliminary Prospectus, the Prospectus and the Registration Statement, the information included therein;

(q) No action, suit or proceeding by or before any court or governmental agency, authority or body or any arbitrator involving the Issuer or any of its subsidiaries or its or their property is pending or, to the best knowledge of the Issuer, threatened that could reasonably be expected to have a Material Adverse Effect, except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any supplement thereto);

(r) Each of the Issuer and each of its subsidiaries owns or leases all such properties as are necessary to the conduct of its operations as presently conducted, except for intellectual property which is separately addressed in subsection (oo) below, and except as would not be reasonably be expected to have a Material Adverse Effect;

(s) Neither the Issuer nor any subsidiary is in violation or default of (i) any provision of its charter or bylaws, (ii) the terms of any indenture, contract, lease, mortgage, deed of trust, note agreement, loan agreement or other agreement, obligation, condition, covenant or instrument to which it is a party or bound or to which its property is subject, or (iii) any statute, law, rule, regulation, judgment, order or decree of any court, regulatory body, administrative agency, governmental body, arbitrator or other authority having jurisdiction over the Issuer or such subsidiary or any of its properties, as applicable except in the case of clauses (ii) and (iii), for such breach, violation or default as would not, individually or in the aggregate, reasonably be expected to have a Material Adverse Effect;

(t) PricewaterhouseCoopers LLP, who have certified certain financial statements of the Issuer and its consolidated subsidiary and delivered their report with respect to the

audited consolidated financial statements and schedules included in the Disclosure Package and the Prospectus, are independent public accountants with respect to the Issuer within the meaning of the Securities Act and the applicable published rules and regulations thereunder;

(u) There are no transfer taxes or other similar fees or charges under Federal law or the laws of any state, or any political subdivision thereof, required to be paid in connection with the execution and delivery of this Underwriting Agreement or the issuance by the Issuer or sale by the Issuer of the Securities;

(v) The Issuer has filed all tax returns that are required to be filed or has requested extensions thereof (except in any case in which the failure so to file would not reasonably be expected to have a Material Adverse Effect) and has paid all taxes required to be paid by it and any other assessment, fine or penalty levied against it, to the extent that any of the foregoing is due and payable, except for any such assessment, fine or penalty that is currently being contested in good faith or as would not reasonably be expected to have a Material Adverse Effect;

(w) No labor problem or dispute with the employees of the Issuer or any of its subsidiaries exists or is threatened or, to the Issuer’s knowledge, imminent, and the Issuer is not aware of any existing or imminent labor disturbance by the employees of any of its or its subsidiaries’ principal suppliers, contractors or customers, that could have a Material Adverse Effect;

(x) The Issuer and each of its subsidiaries are insured by insurers of recognized financial responsibility against such losses and risks and in such amounts as the Issuer reasonably believes are prudent and customary in the businesses in which the Issuer and its subsidiaries are engaged; all policies of insurance insuring the Issuer or any of its subsidiaries or their respective businesses, assets, employees, officers and directors are in full force and effect; the Issuer and its subsidiaries are in compliance with the terms of such policies and instruments in all material respects; and there are no claims by the Issuer or any of its subsidiaries under any such policy or instrument as to which any insurance company is denying liability or defending under a reservation of rights clause; neither the Issuer nor any such subsidiary has been refused any insurance coverage sought or applied for; and neither the Issuer nor any such subsidiary has any reason to believe that it will not be able to renew its existing insurance coverage as and when such coverage expires or to obtain similar coverage from similar insurers as may be necessary to continue its business at a cost that would not reasonably be expected to have a Material Adverse Effect;

(y) The Issuer and its subsidiaries possess all licenses, certificates, permits and other authorizations required to be issued by all applicable authorities necessary to conduct their respective businesses, except where the failure to possess such license, certificate, permit and other authorization would not, individually or in the aggregate, reasonably be expected to have a Material Adverse Effect, and neither the Issuer nor any

such subsidiary has received any notice of proceedings relating to the revocation or modification of any such certificate, authorization or permit which, singly or in the aggregate, if the subject of an unfavorable decision, ruling or finding, would individually or in the aggregate, reasonably be expected to have a Material Adverse Effect;

(z) The Issuer maintains a system of internal accounting controls designed to provide reasonable assurance that (i) transactions are executed in accordance with management’s general or specific authorizations; (ii) transactions are recorded as necessary to permit preparation of financial statements in conformity with generally accepted accounting principles in the United States and to maintain asset accountability; (iii) access to assets is permitted only in accordance with management’s general or specific authorization; (iv) the recorded accountability for assets is compared with the existing assets at reasonable intervals and appropriate action is taken with respect to any differences; and (v) the interactive data in eXtensible Business Reporting Language included or incorporated by reference in the Registration Statement, the Preliminary Prospectus and the Prospectus is in compliance with the SEC’s published rules, regulations and guidelines applicable thereto. The Issuer’s internal controls over financial reporting are effective and the Issuer is not aware of any material weakness in its internal controls over financial reporting;

(aa) The Issuer maintains “disclosure controls and procedures” (as such term is defined in Rule 13a-15(e) under the Securities and Exchange Act 1934, as amended and the rules and regulations of the SEC promulgated thereunder (the “Exchange Act”); such disclosure controls and procedures are effective at the reasonable assurance level;

(bb) The Issuer has not taken, directly or indirectly, without giving effect to activities by the Underwriters, any action designed to or that would constitute or that would reasonably be expected to cause or result in, under the Exchange Act or otherwise, stabilization or manipulation of the price of any security of the Issuer to facilitate the sale or resale of the Securities;

(cc) The Issuer and its subsidiaries are (i) in compliance with any and all applicable foreign, federal, state and local laws and regulations relating to the protection of human health and safety, the environment or hazardous or toxic substances or wastes, pollutants or contaminants (“Environmental Laws”), (ii) have received and are in compliance with all permits, licenses or other approvals required of them under applicable Environmental Laws to conduct their respective businesses and (iii) have not received notice of any actual or potential liability under any environmental law, except where such non-compliance with Environmental Laws, failure to receive required permits, licenses or other approvals, or liability would not, individually or in the aggregate, reasonably be expected to have a Material Adverse Effect. Except as set forth in the Disclosure Package and the Prospectus, neither the Issuer nor any of the subsidiaries has been named as a “potentially responsible party” under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980, as amended;

(dd) None of the following events has occurred or exists: (i) a failure to fulfill the obligations, if any, under the minimum funding standards of Section 302 of the United States Employee Retirement Income Security Act of 1974, as amended (“ERISA”), and the regulations and published interpretations thereunder with respect to a Plan (as defined herein), determined without regard to any waiver of such obligations or extension of any amortization period; (ii) an audit or investigation by the Internal Revenue Service, the U.S. Department of Labor, the Pension Benefit Guaranty Corporation or any other federal or state governmental agency or any foreign regulatory agency with respect to the employment or compensation of employees by any of the Issuer or any of its subsidiaries that would reasonably be expected to have a Material Adverse Effect; (iii) any breach of any contractual obligation, or any violation of law or applicable qualification standards, with respect to the employment or compensation of employees by the Issuer or any of its subsidiaries that would reasonably be expected to have a Material Adverse Effect. None of the following events has occurred or is reasonably likely to occur: (i) a material increase in the aggregate amount of contributions required to be made to all Plans in the current fiscal year of the Issuer and its subsidiaries compared to the amount of such contributions made in the most recently completed fiscal year of the Issuer and its subsidiaries; (ii) a material increase in the “accumulated post-retirement benefit obligations” (within the meaning of Statement of Financial Accounting Standards 106) of the Issuer and its subsidiaries compared to the amount of such obligations in the most recently completed fiscal year of the Issuer and its subsidiaries; (iii) any event or condition giving rise to a liability under Title IV of ERISA that would reasonably be expected to have a Material Adverse Effect; or (iv) the filing of a claim by one or more employees or former employees of the Issuer or any of its subsidiaries related to their employment that would reasonably be expected to have a Material Adverse Effect. For purposes of this paragraph, the term “Plan” means a plan (within the meaning of Section 3(3) of ERISA) subject to Title IV of ERISA with respect to which the Issuer or any of its subsidiaries may have any liability;

(ee) There is and has been no failure on the part of the Issuer and any of the Issuer’s directors or officers, in their capacities as such, to comply with any provision of the Sarbanes-Oxley Act of 2002 and the rules and regulations promulgated in connection therewith (the “Sarbanes-Oxley Act”), that are in effect and with which the Issuer is required to comply as of the Effective Date including Section 402 relating to loans;

(ff) Neither the Issuer nor any of its subsidiaries nor, to the knowledge of the Issuer, any director, officer, agent, employee, affiliate or other person acting on behalf of the Issuer or any of its subsidiaries is aware of or has taken any action, directly or indirectly, that would result in a violation or a sanction for violation by such persons of the Foreign Corrupt Practices Act of 1977 or the U.K. Bribery Act 2010, each as may be amended, or similar law of any other relevant jurisdiction, or the rules or regulations thereunder; and the Issuer and its subsidiaries have instituted and maintain policies and procedures to ensure compliance therewith. No part of the proceeds of the Offering will be used, directly or indirectly, in violation of the Foreign Corrupt Practices Act of 1977

or the U.K. Bribery Act 2010, each as may be amended, or similar law of any other relevant jurisdiction, or the rules or regulations thereunder;

(gg) The operations of the Issuer and its subsidiaries are and have been conducted at all times in compliance with applicable financial recordkeeping and reporting requirements and the money laundering statutes and the rules and regulations thereunder and any related or similar rules, regulations or guidelines, issued, administered or enforced by any governmental agency (collectively, the “Money Laundering Laws”) and no action, suit or proceeding by or before any court or governmental agency, authority or body or any arbitrator involving the Issuer or any of its subsidiaries with respect to the Money Laundering Laws is pending or, to the knowledge of the Issuer, threatened;

(hh) Neither the Issuer nor any of its subsidiaries nor, to the knowledge of the Issuer, any director, officer, agent, employee or affiliate of the Issuer or any of its subsidiaries (i) is, or is controlled or 50% or more owned in the aggregate by or is acting on behalf of, one or more individuals or entities that are currently the subject of any sanctions administered or enforced by the United States (including any administered or enforced by the Office of Foreign Assets Control of the U.S. Department of the Treasury, the U.S. Department of State or the Bureau of Industry and Security of the U.S. Department of Commerce), the United Nations Security Council, the European Union, a member state of the European Union (including sanctions administered or enforced by Her Majesty’s Treasury of the United Kingdom) or other relevant sanctions authority (collectively, “Sanctions” and such persons, “Sanctioned Persons” and each such person, a “Sanctioned Person”), (ii) is located, organized or resident in a country or territory that is, or whose government is, the subject of Sanctions that broadly prohibit dealings with that country or territory (collectively, “Sanctioned Countries” and each, a “Sanctioned Country”) or (iii) will, directly or indirectly, use the proceeds of this Offering, or lend, contribute or otherwise make available such proceeds to any subsidiary, joint venture partner or other individual or entity in any manner that would result in a violation of any Sanctions by, or could result in the imposition of Sanctions against, any individual or entity (including any individual or entity participating in the Offering, whether as underwriter, advisor, investor or otherwise);

(ii) Neither the Issuer nor any of its subsidiaries has engaged in any dealings or transactions with or for the benefit of a Sanctioned Person, or with or in a Sanctioned Country, in the preceding 3 years, nor does the Issuer or any of its subsidiaries have any plans to engage in dealings or transactions with or for the benefit of a Sanctioned Person, or with or in a Sanctioned Country;

(jj) The Issuer’s only subsidiary at the Effective Date is Arsanis Biosciences GmbH.

(kk) Except as described in the Registration Statement, the Disclosure Package and the Prospectus, as applicable, the Issuer (i) is and at all times has been in compliance in all material respects with all statutes, rules and regulations applicable to the ownership,

testing, development, manufacture, packaging, processing, use, distribution, marketing, advertising, labeling, promotion, sale, offer for sale, storage, import, export or disposal of any product manufactured or distributed by the Issuer including, without limitation the Federal Food, Drug and Cosmetic Act (21 U.S.C. §301 et seq.), the federal Anti-Kickback Statute (42 U.S.C. §1320a-7b(b)), the Health Insurance Portability and Accountability Act of 1996, as amended by the Health Information Technology for Economic and Clinical Health Act of 2009, and the Patient Protection and Affordable Care Act of 2010, as amended by the Health Care and Education Affordability Reconciliation Act of 2010, the regulations promulgated pursuant to such laws, and any successor government programs and comparable state laws, regulations relating to Good Clinical Practices and Good Laboratory Practices and all other local, state, federal, national, supranational and foreign laws, manual provisions, policies and administrative guidance relating to the regulation of the Company (collectively, the “Applicable Laws”); (ii) has not received any written notice from any court or arbitrator or governmental or regulatory authority or third party alleging or asserting material noncompliance with any Applicable Laws or any licenses, exemptions, certificates, approvals, clearances, authorizations, permits, registrations and supplements or amendments thereto required by any such Applicable Laws (“Authorizations”); (iii) possesses all Authorizations and such Authorizations are valid and in full force and effect and are not in material violation of any term of any such Authorizations; (iv) has not received written notice of any claim, action, suit, proceeding, hearing, enforcement, investigation, arbitration or other action from any court or arbitrator or governmental or regulatory authority or third party alleging that any product operation or activity is in violation of any Applicable Laws or Authorizations nor, to Issuer’s knowledge, is any such claim, action, suit, proceeding, hearing, enforcement, investigation, arbitration or other action threatened; (v) has received any written notice that any court or arbitrator or governmental or regulatory authority has taken, is taking or intends to take, action to limit, suspend, materially modify or revoke any Authorizations nor, to Issuer’s knowledge is any such limitation, suspension, modification or revocation threatened; (vi) has filed, obtained, maintained or submitted all material reports, documents, forms, notices, applications, records, claims, submissions and supplements or amendments as required by any Applicable Laws or Authorizations and that all such reports, documents, forms, notices, applications, records, claims, submissions and supplements or amendments were complete and accurate in all material respects on the date filed (or were corrected or supplemented by a subsequent submission); and (vii) is not a party to any corporate integrity agreements, monitoring agreements, consent decrees, settlement orders, or similar agreements with or imposed by any governmental or regulatory authority.

(ll) The clinical and pre-clinical trials conducted by or on behalf of or sponsored by the Issuer, or in which the Issuer has participated, that are described in the Registration Statement, the Disclosure Package and the Prospectus or the results of which are referred to in the Registration Statement, the Disclosure Package and the Prospectus, as applicable, and are intended to be submitted to Regulatory Authorities as a basis for product approval, were and, if still pending, are being conducted in all material respects in accordance with standard medical and scientific research procedures and all applicable

statutes, rules and regulations of the United States Food and Drug Administration and comparable drug regulatory agencies outside of the United States to which they are subject (collectively, the “Regulatory Authorities”), including, without limitation, 21 C.F.R. Parts 50, 54, 56, 58, and 312, and current Good Clinical Practices and Good Laboratory Practices; the descriptions in the Registration Statement, the Disclosure Package or the Prospectus of the results of such studies and trials are accurate and complete in all material respects and fairly present the data derived from such trials; the Issuer has no knowledge of any other trials the results of which are inconsistent with or otherwise call into question the results described or referred to in the Registration Statement, Disclosure Package and the Prospectus; the Company has not received any written notices, correspondence or other communication from the Regulatory Authorities or any governmental authority which could lead to the termination or suspension of any clinical or pre-clinical trials that are described in the Registration Statement, the Disclosure Package and the Prospectus or the results of which are referred to in the Registration Statement, Disclosure Package or the Prospectus, and to Issuer’s knowledge there are no reasonable grounds for same.

(mm) Except as set forth in the Disclosure Package and the Prospectus, the Issuer and its subsidiaries own, possess, license or have other rights to use, on reasonable terms, all patents, patent applications, trade and service marks, trade and service mark registrations, trade names, copyrights, licenses, inventions, trade secrets, technology, know-how and other intellectual property necessary for the conduct of the Issuer’s business as now conducted or as proposed in the Disclosure Package and Prospectus to be conducted (collectively, the “Intellectual Property”). Except as set forth in the Disclosure Package and the Prospectus under the caption “Business—Intellectual Property,” (a) there are no rights owned by third parties in any such Intellectual Property; (b) to the Issuer’s knowledge, there is no material infringement by third parties of any such Intellectual Property; (c) there is no pending or, to the Issuer’s knowledge, threatened action, suit, proceeding or claim by others challenging the Issuer’s rights in or to any such Intellectual Property, and, the Issuer is unaware of any facts which would form a reasonable basis for any such claim; (d) there is no pending or threatened action, suit, proceeding or claim by others challenging the validity or scope of any such Intellectual Property, and the Issuer is unaware of any facts which would form a reasonable basis for any such claim; (e) there is no pending or, to the Issuer’s knowledge, threatened action, suit, proceeding or claim by others that the Issuer infringes or otherwise violates any patent, trademark, copyright, trade secret or other proprietary rights of others, and, the Issuer is unaware of any other fact which would form a reasonable basis for any such claim; and (f) to the Issuer’s knowledge, there is no prior art of which the Issuer is aware that may render any U.S. patent held by the Issuer invalid or any U.S. patent application held by the Issuer un-patentable which has not been disclosed to the U.S. Patent and Trademark Office;

(nn) Except as disclosed in the Registration Statement, the Disclosure Package and the Prospectus, the Issuer (i) does not have any material lending or other relationship with any bank or lending affiliate of Citigroup Global Markets Holdings Inc. and (ii) does

not intend to use any of the proceeds from the sale of the Securities hereunder to repay any outstanding debt owed to any affiliate of Citigroup Global Markets Holdings Inc.; and

(oo) Any certificate signed by any officer of the Issuer and delivered to the Representatives or counsel for the Underwriters in connection with the Offering of the Securities shall be deemed a representation and warranty by the Issuer, as to matters covered thereby, to each Underwriter.

(a) Subject to the terms and conditions and in reliance upon the representations and warranties herein set forth, the Issuer agrees to sell to each Underwriter, and each Underwriter agrees, severally and not jointly, to purchase from the Issuer, at a purchase price of $9.30 per share, the amount of the Underwritten Securities set forth opposite such Underwriter’s name in Schedule I hereto; and

(b) Subject to the terms and conditions and in reliance upon the representations and warranties herein set forth, the Issuer hereby grants an option to the several Underwriters to purchase, severally and not jointly, up to 600,000 Option Securities at the same purchase price per share as the Underwriters shall pay for the Underwritten Securities, less an amount per share equal to any dividends or distributions declared by the Issuer and payable on the Underwritten Securities but not payable on the Option Securities. Said option may be exercised only to cover over-allotments in the sale of the Underwritten Securities by the Underwriters. Said option may be exercised in whole or in part at any time on or before the 30th day after the date of the Prospectus upon written or telegraphic notice by the Representatives to the Issuer setting forth the number of shares of the Option Securities as to which the several Underwriters are exercising the option and the settlement date. The number of Option Securities to be purchased by each Underwriter shall be the same percentage of the total number of shares of the Option Securities to be purchased by the several Underwriters as such Underwriter is purchasing of the Underwritten Securities, subject to such adjustments as you in your absolute discretion shall make to eliminate any fractional shares.

3. Delivery and Payment. Delivery of and payment for the Underwritten Securities and the Option Securities (if the option provided for in Section 2(b) hereof shall have been exercised on or before the third Business Day (as defined herein) immediately preceding the Closing Date) shall be made at 10:00 AM, Eastern Standard Time, on November 20, 2017, or at such time on such later date not more than three Business Days after the foregoing date as the Representatives shall designate, which date and time may be postponed by agreement between the Representatives and the Issuer or as provided in Section 9 hereof (such date and time of delivery and payment for the Securities being herein called the “Closing Date”). For purposes herein, “Business Day” shall mean any day other than a Saturday, a Sunday or a legal holiday or a day on which banking institutions or trust companies are authorized or obligated by law to close in New York, New York. Delivery of the Securities shall be made to the Representatives

for the respective accounts of the several Underwriters against payment by the several Underwriters through the Representatives of the purchase price thereof to or upon the order of the Issuer by wire transfer payable in same-day funds to an account specified by the Issuer. Delivery of the Underwritten Securities and the Option Securities shall be made through the facilities of The Depository Trust Company unless the Representatives shall otherwise instruct.

If the option provided for in Section 2(b) hereof is exercised after the third Business Day immediately preceding the Closing Date, the Issuer will deliver the Option Securities (at the expense of the Issuer) to the Representatives, at 620 Eighth Avenue, New York, New York, 10018 on the date specified by the Representatives (which shall be within three Business Days after exercise of said option) for the respective accounts of the several Underwriters, against payment by the several Underwriters through the Representatives of the purchase price thereof to or upon the order of the Issuer by wire transfer payable in same-day funds to an account specified by the Issuer. If settlement for the Option Securities occurs after the Closing Date, the Issuer will deliver to the Representatives on the settlement date for the Option Securities, and the obligation of the Underwriters to purchase the Option Securities shall be conditioned upon receipt of, supplemental opinions, certificates and letters confirming as of such date the opinions, certificates and letters delivered on the Closing Date pursuant to Section 6 hereof.

4. Offering by Underwriters. It is understood that the several Underwriters propose to offer the Securities for sale to the public as set forth in the Prospectus.

(a) Prior to the termination of the Offering, the Issuer will not file any amendment of the Registration Statement or supplement to the Prospectus or any Rule 462(b) Registration Statement unless the Issuer has furnished you a copy for your review prior to filing and will not file any such proposed amendment or supplement to which you reasonably object. The Issuer will cause the Prospectus and any supplement thereto to be filed in a form approved by the Representatives with the SEC pursuant to the applicable paragraph of Rule 424(b) under the Securities Act within the time period prescribed and will provide evidence satisfactory to the Representatives of such timely filing. The Issuer will promptly advise the Representatives (i) when the Prospectus, and any supplement thereto, shall have been filed (if required) with the SEC pursuant to Rule 424(b) or when any Rule 462(b) Registration Statement shall have been filed with the SEC, (ii) when, prior to termination of the Offering, any amendment to the Registration Statement shall have been filed or become effective, (iii) of any request by the SEC or its staff for any amendment of the Registration Statement, or any Rule 462(b) Registration Statement, or for any supplement to the Prospectus or for any additional information, (iv) of the issuance by the SEC of any stop order suspending the effectiveness of the Registration Statement or of any notice objecting to its use or the institution or threatening of any proceeding for that purpose and (v) of the receipt by the Issuer of any notification with respect to the suspension of the qualification of the Securities for sale in any jurisdiction or the institution or threatening of any proceeding for such purpose. The Issuer will use

its reasonable best efforts to prevent the issuance of any such stop order or the occurrence of any such suspension or objection to the use of the Registration Statement and, upon such issuance, occurrence or notice of objection, to obtain as soon as possible the withdrawal of such stop order or relief from such occurrence or objection, including, if necessary, by filing an amendment to the Registration Statement or a new registration statement and using its reasonable best efforts to have such amendment or new registration statement declared effective as soon as practicable;

(b) If, at any time prior to the filing of the Prospectus pursuant to Rule 424(b) under the Securities Act, any event occurs as a result of which the Disclosure Package would include any untrue statement of a material fact or omit to state any material fact necessary in order to make the statements therein in the light of the circumstances under which they were made at such time not misleading, the Issuer will (i) notify promptly the Representatives so that any use of the Disclosure Package may cease until it is amended or supplemented; (ii) amend or supplement the Disclosure Package to correct such statement or omission; and (iii) supply any amendment or supplement to you in such quantities as you may reasonably request;

(c) If, at any time when a prospectus relating to the Securities is required to be delivered under the Securities Act (including in circumstances where such requirement may be satisfied pursuant to Rule 172), any event occurs as a result of which the Prospectus as then supplemented would include any untrue statement of a material fact or omit to state any material fact necessary in order to make the statements therein in the light of the circumstances under which they were made or the circumstances then prevailing not misleading, or if it shall be necessary to amend the Registration Statement or supplement the Prospectus to comply with the Securities Act or the rules thereunder, the Issuer promptly will (i) notify the Representatives of any such event; (ii) prepare and file with the SEC, subject to the second sentence of paragraph (a) of this Section 5, an amendment or supplement which will correct such statement or omission or effect such compliance;and (iii) supply any supplemented Prospectus to you in such quantities as you may reasonably request;

(d) As soon as practicable, the Issuer will make generally available to its security holders and to the Representatives an earnings statement or statements of the Issuer which will satisfy the provisions of Section 11(a) of the Securities Act and of Rule 158 under the Securities Act;

(e) The Issuer will furnish to the Representatives and counsel for the Underwriters, without charge, signed copies of the Registration Statement (including exhibits thereto) and to each other Underwriter a copy of the Registration Statement (without exhibits thereto) and, so long as delivery of a prospectus by an Underwriter or dealer may be required by the Securities Act (including in circumstances where such requirement may be satisfied pursuant to Rule 172), as many copies of each Preliminary Prospectus, the Prospectus and each Issuer Free Writing Prospectus and any supplement

thereto as the Representatives may reasonably request. The Issuer will pay the expenses of printing or other production of all documents relating to the Offering;

(f) The Issuer will arrange, if necessary, for the qualification of the Securities for sale under the laws of such jurisdictions as the Representatives may reasonably designate and will use its reasonable best efforts to maintain such qualifications in effect so long as required for the distribution of the Securities; provided that in no event shall the Issuer be (i) obligated to qualify to do business in any jurisdiction where it is not now so qualified, (ii) obligated to take any action that would subject it to service of process in suits, other than those arising out of the Offering, in any jurisdiction where it is not now so subject or (iii) subject itself to taxation in any jurisdiction in which it is not otherwise subject to taxation on the date hereof.

(g) The Issuer will not, without the prior written consent of the Citigroup Global Markets Inc. and Cowen and Company, LLC, offer, sell, contract to sell, pledge, or otherwise dispose of, (or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition (whether by actual disposition or effective economic disposition due to cash settlement or otherwise) by the Issuer or any affiliate of the Issuer or any person in privity with the Issuer or any affiliate of the Issuer) directly or indirectly, including the filing (or participation in the filing) of a registration statement (other than a registration statement on Form S-8) with the SEC in respect of, or establish or increase a put equivalent position or liquidate or decrease a call equivalent position within the meaning of Section 16 of the Exchange Act, any other shares of Common Stock or any securities convertible into, or exercisable, or exchangeable for, shares of Common Stock; or publicly announce an intention to effect any such transaction, for a period of 180 days after the date of the Underwriting Agreement, provided, however, that (i) the Issuer may issue and sell Common Stock or any securities convertible into, or exercisable, or exchangeable for, shares of Common Stock, pursuant to any employee stock option plan, incentive plan, employee stock purchase plan, stock ownership plan or dividend reinvestment plan of the Issuer in effect at the Execution Time or disclosed in the Prospectus, (ii) and the Issuer may issue Common Stock issuable upon the conversion of securities or the exercise of warrants outstanding at the Execution Time, (iii) offer, issue and sell shares of Common Stock, or any securities convertible into or exercisable or exchangeable for Common Stock, in connection with any acquisition or strategic investment (including any joint venture, strategic alliance or partnership, (iv) issue, offer or sell any shares of Common Stock or securities convertible into or exercisable or exchangeable for shares of Common Stock, on an arm’s-length basis, to unaffiliated financial institutions or lessors pursuant to a commercial agreement, equipment financing transaction or commercial property lease transaction and (v) issue, offer or sell any shares of Common Stock or securities convertible into or exercisable or exchangeable for shares of Common Stock, on an arm’s-length basis, to unaffiliated collaborators, vendors, manufacturers, lessors, distributors, customers or other similar parties pursuant to a collaboration, licensing agreement, strategic alliance, lease, manufacturing or distribution arrangement or similar transaction; provided, further, that (x) the aggregate number of shares of Common Stock issued or issuable in accordance

with clauses (iii), (iv) or (v) of this paragraph do not exceed 10% of the number of shares of Common Stock outstanding immediately after the issuance and sale of the Securities and (y) each recipient of any such shares or other securities agrees to restrictions on the resale of securities that are consistent with the provisions set forth in the lock-up letter described in Section 6(j) for the remainder of the 180-day restricted period;

(h) If Citigroup Global Markets Inc. and Cowen and Company, LLC, in their sole discretion, agree to release or waive the restrictions set forth in a lock-up letter described in Section 6(j) hereof for an officer or director of the Issuer and provides the Issuer with notice of the impending release or waiver at least three Business Days before the effective date of the release or waiver, the Issuer agrees to announce the impending release or waiver by a press release substantially in the form of Exhibit D hereto through a major news service at least two Business Days before the effective date of the release or waiver;

(i) The Issuer will not take, directly or indirectly, without giving effect to the activities by the Underwriters, any action designed to or that would constitute or that would reasonably be expected to cause or result in, under the Exchange Act or otherwise, stabilization or manipulation of the price of any security of the Issuer to facilitate the sale or resale of the Securities;

(j) The Issuer agrees to pay the costs and expenses relating to the following matters: (i) the preparation, printing or reproduction and filing with the SEC of the Registration Statement (including financial statements and exhibits thereto), each Preliminary Prospectus, the Prospectus and each Issuer Free Writing Prospectus, and each amendment or supplement to any of them; (ii) the printing (or reproduction) and delivery (including postage, air freight charges and charges for counting and packaging) of such copies of the Registration Statement, each Preliminary Prospectus, the Prospectus and each Issuer Free Writing Prospectus, and all amendments or supplements to any of them, as may, in each case, be reasonably requested for use in connection with the Offering; (iii) the preparation, printing, authentication, issuance and delivery of certificates for the Securities, including any stamp or transfer taxes in connection with the original issuance and sale of the Securities; (iv) the printing (or reproduction) and delivery of this Underwriting Agreement, any blue sky memorandum and all other agreements or documents printed (or reproduced) and delivered in connection with the Offering; (v) the registration of the Securities under the Exchange Act and the listing of the Securities on the NASDAQ Global Market; (vi) any registration or qualification of the Securities for offer and sale under the securities or blue sky laws of the several states (including filing fees and the reasonable fees and expenses of counsel for the Underwriters relating to such registration and qualification); (vii) any filings required to be made with the Financial Industry Regulatory Authority, Inc. (“FINRA”) (including filing fees and the reasonable fees and expenses of counsel for the Underwriters relating to such filings), with such fees and expenses of counsel contained in clauses (vi) and (vii) not to exceed $30,000 in the aggregate); (viii) the transportation and other expenses incurred by or on behalf of Issuer representatives in connection with presentations to

prospective purchasers of the Securities; provided that in relation to such presentations, if the Issuer and the Underwriters agree that a private or chartered aircraft should be used for such presentations, the Issuer will only be responsible for 50% of the costs and expense associated with such aircraft and the Underwriters shall be responsible for the balance, (ix) the fees and expenses of the Issuer’s accountants and the fees and expenses of counsel (including local and special counsel) for the Issuer; and (x) all other costs and expenses incident to the performance by the Issuer of its obligations hereunder;

(k) The Issuer agrees that, unless it has or shall have obtained the prior written consent of the Representatives, and each Underwriter, severally and not jointly, agrees with the Issuer that, unless it has or shall have obtained, as the case may be, the prior written consent of the Issuer, it has not made and will not make any offer relating to the Securities that would constitute an Issuer Free Writing Prospectus or that would otherwise constitute a Free Writing Prospectus required to be filed by the Issuer with the SEC or retained by the Issuer under Rule 433 under the Securities Act; provided that the prior written consent of the parties hereto shall be deemed to have been given in respect of the Free Writing Prospectuses included in Schedule II hereto and any electronic road show. Any such free writing prospectus consented to by the Representatives or the Issuer is hereinafter referred to as a “Permitted Free Writing Prospectus.” The Issuer agrees that (x) it has treated and will treat, as the case may be, each Permitted Free Writing Prospectus as an Issuer Free Writing Prospectus and (y) it has complied and will comply, as the case may be, with the requirements of Rules 164 and 433 applicable to any Permitted Free Writing Prospectus, including in respect of timely filing with the SEC, legending and record keeping;

(l) The Issuer will notify promptly the Representatives if the Issuer ceases to be an Emerging Growth Company at any time prior to the later of (a) completion of the distribution of the Securities within the meaning of the Securities Act and (b) completion of the 180-day restricted period referred to in Section 5(g) hereof; and

(m) If at any time following the distribution of any Written Testing-the-Waters Communication, any event occurs as a result of which such Written Testing-the-Waters Communication would include any untrue statement of a material fact or omit to state any material fact necessary in order to make the statements therein in the light of the circumstances under which they were made at such time not misleading, the Issuer will (i) notify promptly the Representatives so that use of the Written Testing-the-Waters Communication may cease until it is amended or supplemented; (ii) amend or supplement the Written Testing-the-Waters Communication to correct such statement or omission; and (iii) supply any amendment or supplement to the Representatives in such quantities as may be reasonably requested.

6. Conditions to the Obligations of the Underwriters. The obligations of the Underwriters to purchase the Underwritten Securities and the Option Securities, as the case may be, shall be subject to the accuracy of the representations and warranties on the part of the Issuer contained herein as of the Execution Time, the Closing Date and any settlement date pursuant to

Section 3 hereof, to the accuracy of the statements of the Issuer made in any certificates pursuant to the provisions hereof, to the performance by the Issuer of its obligations hereunder and to the following additional conditions:

(a) The Prospectus, and any supplement thereto, have been filed in the manner and within the time period required by Rule 424(b) under the Securities Act; any material required to be filed by the Issuer pursuant to Rule 433(d) under the Securities Act shall have been filed with the SEC within the applicable time periods prescribed for such filings by Rule 433; and no stop order suspending the effectiveness of the Registration Statement or any notice objecting to its use shall have been issued and no proceedings for that purpose shall have been instituted or threatened.

(b) The Issuer shall have requested and caused Wilmer Cutler Pickering Hale and Dorr LLP, counsel for the Issuer, to have furnished to the Representatives their opinion, dated the Closing Date and addressed to the Representatives, substantially in the form attached hereto as Exhibit A.

(c) The Issuer shall have requested and caused Redl Life Science Patent Attorneys, intellectual property counsel for the Issuer, to have furnished to the Representatives their opinion, dated the Closing Date and addressed to the Representatives, substantially in the form attached hereto as Exhibit B.

(d) The Representatives shall have received from Goodwin Procter LLP, counsel for the Underwriters, such opinion or opinions, dated the Closing Date and addressed to the Representatives, with respect to the issuance and sale of the Securities, the Registration Statement, the Disclosure Package, the Prospectus (together with any supplement thereto) and other related matters as the Representatives may reasonably require, and the Issuer shall have furnished to such counsel such documents as they may reasonably request for the purpose of enabling them to pass upon such matters;

(e) The Issuer shall have furnished to the Representatives a certificate of the Issuer, signed by the Chief Executive Officer and the Chief Operating Officer and Chief Financial Officer of the Issuer, dated the Closing Date, to the effect that the signers of such certificate have carefully examined the Registration Statement, the Disclosure Package, the Prospectus and any amendment or supplement thereto, as well as each electronic road show used in connection with the Offering, and this Underwriting Agreement and that:

(i) the representations and warranties of the Issuer in this Underwriting Agreement are true and correct on and as of the Closing Date with the same effect as if made on the Closing Date and the Issuer has complied with all the agreements and satisfied all the conditions on its part to be performed or satisfied at or prior to the Closing Date;

(ii) no stop order suspending the effectiveness of the Registration Statement or any notice objecting to its use has been issued and no proceedings for that purpose have been instituted or, to the Issuer’s knowledge, threatened; and

(iii) since the date of the most recent financial statements included in the Disclosure Package and the Prospectus (exclusive of any amendment or supplement thereto), there has been no Material Adverse Effect, except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any amendment or supplement thereto).

(f) The Issuer shall have requested and caused PricewaterhouseCoopers LLP to have furnished to the Representatives, at the Execution Time and at the Closing Date, letters, dated respectively as of the Execution Time and as of the Closing Date, in form and substance reasonably satisfactory to the Representatives, containing statements and information of the type customarily included in accountant’s “comfort letters” to underwriters, delivered according to Statement of Auditing Standards No. 72 (or any successor bulletin), with respect to the audited and unaudited financial statements and certain financial information contained in the Registration Statement, the Disclosure Package, and each free writing prospectus, if any.

(g) Subsequent to the Execution Time or, if earlier, the dates as of which information is given in the Registration Statement (exclusive of any amendment thereof) and the Prospectus (exclusive of any amendment or supplement thereto), there shall not have been (i) any change or decrease specified in the letter or letters referred to in paragraph (e) of this Section 6 or (ii) any change, or any development involving a prospective change, in or affecting the condition (financial or otherwise), earnings, business or properties of the Issuer and its subsidiaries taken as a whole, whether or not arising from transactions in the ordinary course of business, except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any supplement thereto) the effect of which, in any case referred to in clause (i) or (ii) above, is, in the sole judgment of the Representatives, so material and adverse as to make it impractical or inadvisable to proceed with the Offering or delivery of the Securities as contemplated by the Registration Statement (exclusive of any amendment thereof), the Disclosure Package and the Prospectus (exclusive of any amendment or supplement thereto).

(h) Prior to the Closing Date, the Issuer shall have furnished to the Representatives such further customary information, certificates and documents as the Representatives may reasonably request.

(i) The Securities shall have been listed and admitted and authorized for trading on the NASDAQ Global Market, and satisfactory evidence of such actions shall have been provided to the Representatives.

(j) At the Execution Time, the Issuer shall have furnished to the Representatives a letter substantially in the form of Exhibit C hereto from each officer and director of the Issuer and specified stockholders and option holders of the Issuer, addressed to the Citigroup Global Markets Inc. and Cowen and Company, LLC.

If any of the conditions specified in this Section 6 shall not have been fulfilled when and as provided in this Underwriting Agreement, or if any of the opinions and certificates mentioned above or elsewhere in this Underwriting Agreement shall not be reasonably satisfactory in form and substance to the Representatives and counsel for the Underwriters, this Underwriting Agreement and all obligations of the Underwriters hereunder may be canceled at, or at any time prior to, the Closing Date by the Representatives. Notice of such cancellation shall be given to the Issuer in writing or by telephone or facsimile confirmed in writing.

The documents required to be delivered by this Section 6 shall be delivered at the office of Goodwin Procter LLP, counsel for the Underwriters, at 620 Eighth Avenue, New York, New York 10018, on the Closing Date.

7. Reimbursement of Underwriters’ Expenses. If the sale of the Securities provided for herein is not consummated because any condition to the obligations of the Underwriters set forth in Section 6 hereof is not satisfied, because of any termination pursuant to Section 10 hereof or because of any refusal, inability or failure on the part of the Issuer to perform any agreement herein or comply with any provision hereof other than by reason of a default by any of the Underwriters, the Issuer will reimburse the Underwriters severally through Citigroup Global Markets Inc. on demand for all reasonable and documented out of pocket expenses (including reasonable fees and documented disbursements of counsel) that shall have been incurred by them in connection with the proposed purchase and sale of the Securities.

(a) The Issuer agrees to indemnify and hold harmless each Underwriter, the directors, officers, employees, affiliates (within the meaning of Rule 405 under the Securities Act, each an “Affiliate”) and agents of each Underwriter and each person who controls any Underwriter within the meaning of either the Securities Act or the Exchange Act against any and all losses, claims, damages or liabilities, joint or several, to which they or any of them may become subject under the Securities Act, the Exchange Act or other Federal or state statutory law or regulation, at common law or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based upon any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement for the registration of the Securities as originally filed or in any amendment thereof, or in any Preliminary Prospectus, or the Prospectus, or any Issuer Free Writing Prospectus, or any Written Testing-the-Waters Communication or in any amendment thereof or supplement thereto or arise out of or are based upon the omission or alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, and agrees to reimburse each such indemnified party, as incurred, for any legal or other out of pocket expenses

reasonably incurred by them in connection with investigating or defending any such loss, claim, damage, liability or action; provided, however, that the Issuer will not be liable in any such case to the extent that any such loss, claim, damage or liability arises out of or is based upon any such untrue statement or alleged untrue statement or omission or alleged omission made therein in reliance upon and in conformity with information furnished in writing to the Issuer by or on behalf of any Underwriter through the Representatives specifically for inclusion therein. This indemnity agreement will be in addition to any liability which the Issuer may otherwise have.

(b) Each Underwriter severally and not jointly agrees to indemnify and hold harmless the Issuer, each of its directors, each of its officers who signs the Registration Statement, and each person who controls the Issuer within the meaning of either the Securities Act or the Exchange Act, to the same extent as the foregoing indemnity from the Issuer to each Underwriter, but only with reference to written information relating to such Underwriter furnished to the Issuer by or on behalf of such Underwriter through the Representatives specifically for inclusion in the documents referred to in the foregoing indemnity. This indemnity agreement will be in addition to any liability which any Underwriter may otherwise have. The Issuer acknowledges that the statements set forth (i) in the last paragraph of the cover page regarding delivery of the Securities and, under the heading “Underwriting,” (ii) the list of Underwriters and their respective participation in the sale of the Securities, (iii) the paragraph related to concessions and reallowances and (iv) the paragraphs related to stabilization in the Preliminary Prospectus and the Prospectus constitute the only information furnished in writing by or on behalf of the several Underwriters for inclusion in the Preliminary Prospectus, the Prospectus or any Issuer Free Writing Prospectus.

(c) Promptly after receipt by an indemnified party under this Section 8 of notice of the commencement of any action, such indemnified party will, if a claim in respect thereof is to be made against the indemnifying party under this Section 8, notify the indemnifying party in writing of the commencement thereof; but the failure so to notify the indemnifying party (i) will not relieve it from liability under paragraph (a) or (b) above unless and to the extent it did not otherwise learn of such action and such failure results in the forfeiture by the indemnifying party of substantial rights and defenses and (ii) will not, in any event, relieve the indemnifying party from any obligations to any indemnified party other than the indemnification obligation provided in paragraph (a) or (b) above. The indemnifying party shall be entitled to appoint counsel of the indemnifying party’s choice at the indemnifying party’s expense to represent the indemnified party in any action for which indemnification is sought (in which case the indemnifying party shall not thereafter be responsible for the fees and expenses of any separate counsel retained by the indemnified party or parties except as set forth below); provided, however, that such counsel shall be reasonably satisfactory to the indemnified party. Notwithstanding the indemnifying party’s election to appoint counsel to represent the indemnified party in an action, the indemnified party shall have the right to employ separate counsel (including local counsel), and the indemnifying party shall bear the reasonable and documented fees, costs and expenses of such separate counsel (which, if

the Issuer is the indemnifying party, shall be limited to one such separate counsel for any Underwriter together with all persons who control such Underwriter within the meaning of either the Act or the Exchange Act) if (i) the use of counsel chosen by the indemnifying party to represent the indemnified party would present such counsel with a conflict of interest, (ii) the actual or potential defendants in, or targets of, any such action include both the indemnified party and the indemnifying party and the indemnified party shall have reasonably concluded that there may be legal defenses available to it and/or other indemnified parties which are different from or additional to those available to the indemnifying party, (iii) the indemnifying party shall not have employed counsel reasonably satisfactory to the indemnified party to represent the indemnified party within a reasonable time after notice of the institution of such action or (iv) the indemnifying party shall authorize the indemnified party to employ separate counsel at the expense of the indemnifying party. An indemnifying party will not, without the prior written consent of the indemnified parties, settle or compromise or consent to the entry of any judgment with respect to any pending or threatened claim, action, suit or proceeding in respect of which indemnification or contribution may be sought hereunder (whether or not the indemnified parties are actual or potential parties to such claim or action) unless such settlement, compromise or consent (i) includes an unconditional release of each indemnified party from all liability arising out of such claim, action, suit or proceeding and (ii) does not include a statement as to or an admission of fault, culpability or a failure to act, by or on behalf of any indemnified party.

(d) In the event that the indemnity provided in paragraph (a), (b) or (c) of this Section 8 is unavailable to or insufficient to hold harmless an indemnified party for any reason, the Issuer and the Underwriters severally agree to contribute to the aggregate losses, claims, damages and liabilities (including legal or other expenses reasonably incurred in connection with investigating or defending the same) (collectively “Losses”) to which the Issuer and one or more of the Underwriters may be subject in such proportion as is appropriate to reflect the relative benefits received by the Issuer on the one hand and by the Underwriters on the other from the Offering. If the allocation provided by the immediately preceding sentence is unavailable for any reason, the Issuer and the Underwriters severally shall contribute in such proportion as is appropriate to reflect not only such relative benefits but also the relative fault of the Issuer on the one hand and of the Underwriters on the other in connection with the statements or omissions which resulted in such Losses as well as any other relevant equitable considerations. Benefits received by the Issuer shall be deemed to be equal to the total net proceeds from the Offering (before deducting expenses) received by it, and benefits received by the Underwriters shall be deemed to be equal to the total underwriting discounts and commissions, in each case as set forth on the cover page of the Prospectus. Relative fault shall be determined by reference to, among other things, whether any untrue or any alleged untrue statement of a material fact or the omission or alleged omission to state a material fact relates to information provided by the Issuer on the one hand or the Underwriters on the other, the intent of the parties and their relative knowledge, access to information and opportunity to correct or prevent such untrue statement or omission. The Issuer and the Underwriters agree that it would not be just and equitable if contribution

were determined by pro rata allocation or any other method of allocation which does not take account of the equitable considerations referred to above. Notwithstanding the provisions of this paragraph (d), (i) in no event shall an Underwriter be required to contribute any amount in excess of the amount by which the total underwriting discounts and commissions received by such Underwriter with respect to the Offering exceeds the amount of any damages that such Underwriter has otherwise been required to pay by reason of such untrue or alleged untrue statement or omission or alleged omission and (ii) no person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Securities Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation. For purposes of this Section 8, each person who controls an Underwriter within the meaning of either the Securities Act or the Exchange Act and each director, officer, employee, Affiliate and agent of an Underwriter shall have the same rights to contribution as such Underwriter, and each person who controls the Issuer within the meaning of either the Securities Act or the Exchange Act, each officer of the Issuer who shall have signed the Registration Statement and each director of the Issuer shall have the same rights to contribution as the Issuer, subject in each case to the applicable terms and conditions of this paragraph (d).

9. Default by an Underwriter. If any one or more Underwriters shall fail to purchase and pay for any of the Securities agreed to be purchased by such Underwriter or Underwriters hereunder and such failure to purchase shall constitute a default in the performance of its or their obligations under this Underwriting Agreement, the remaining Underwriters shall be obligated severally to take up and pay for (in the respective proportions which the amount of Securities set forth opposite their names in Schedule I hereto bears to the aggregate amount of Securities set forth opposite the names of all the remaining Underwriters) the Securities which the defaulting Underwriter or Underwriters agreed but failed to purchase; provided, however, that in the event that the aggregate amount of Securities which the defaulting Underwriter or Underwriters agreed but failed to purchase shall exceed 10% of the aggregate amount of Securities set forth in Schedule I hereto, the remaining Underwriters shall have the right to purchase all, but shall not be under any obligation to purchase any, of the Securities, and if such non-defaulting Underwriters do not purchase all the Securities, this Underwriting Agreement will terminate without liability to any non-defaulting Underwriter or the Issuer. In the event of a default by any Underwriter as set forth in this Section 9, the Closing Date shall be postponed for such period, not exceeding five Business Days, as the Representatives shall determine in order that the required changes in the Registration Statement and the Prospectus or in any other documents or arrangements may be effected. Nothing contained in this Underwriting Agreement shall relieve any defaulting Underwriter of its liability, if any, to the Issuer and any non-defaulting Underwriter for damages occasioned by its default hereunder.

10. Termination. This Underwriting Agreement shall be subject to termination in the absolute discretion of the Representatives, by notice given to the Issuer prior to delivery of and payment for the Securities, if at any time prior to such delivery and payment (i) trading in the Issuer’s Common Stock shall have been suspended by the SEC, the NASDAQ Global Market or trading in securities generally on the New York Stock Exchange shall have been suspended or limited or minimum prices shall have been established on either of such exchanges, (ii) a

banking moratorium shall have been declared either by Federal or New York State authorities, (iii) there shall have occurred a material disruption in commercial banking or securities settlement or clearance services or (iv) there shall have occurred any outbreak or escalation of hostilities, declaration by the United States of a national emergency or war, or other calamity or crisis the effect of which on financial markets is such as to make it, in the sole judgment of the Representatives, impractical or inadvisable to proceed with the Offering or delivery of the Securities as contemplated by the Preliminary Prospectus or the Prospectus (exclusive of any supplement thereto).

11. Representations and Indemnities to Survive. The respective agreements, representations, warranties, indemnities and other statements of the Issuer or its officers and of the Underwriters set forth in or made pursuant to this Underwriting Agreement will remain in full force and effect, regardless of any investigation made by or on behalf of any Underwriter or the Issuer or any of the officers, directors, employees, agents, Affiliates or controlling persons referred to in Section 8 hereof, and will survive delivery of and payment for the Securities. The provisions of Sections 7 and 8 hereof shall survive the termination or cancellation of this Underwriting Agreement.

12. Notices. All communications hereunder will be in writing and effective only on receipt, and, if sent to the Representatives, will be mailed, delivered or telefaxed to Citigroup Global Markets Inc. General Counsel (fax no.: 1(646) 291-1469) and confirmed to the General Counsel, Citigroup Global Markets Inc., at 388 Greenwich Street, New York, New York, 10013, Attention: General Counsel; and to Cowen and Company, LLC (fax no. 1(646) 562-1124) and to be confirmed to the Head of Equity Capital Markets, Cowen and Company, LLC at 599 Lexington Avenue, New York, New York 10022, Attention: Head of Equity Capital Markets; or, if sent to Arsanis, Inc., will be mailed, delivered or telefaxed to 890 Winter Street, Suite 230, Waltham, MA 02451, Attention: Chief Executive Officer.

13. Successors. This Underwriting Agreement will inure to the benefit of and be binding upon the parties hereto and their respective successors and the officers, directors, employees, agents and controlling persons referred to in Section 8 hereof, and no other person will have any right or obligation hereunder.

14. Jurisdiction. The Issuer agrees that any suit, action or proceeding against the Issuer brought by any Underwriter, the directors, officers, employees, Affiliates and agents of any Underwriter, or by any person who controls any Underwriter, arising out of or based upon this Underwriting Agreement or the transactions contemplated hereby may be instituted in any State or U.S. federal court in The City of New York and County of New York, and waives any objection which it may now or hereafter have to the laying of venue of any such proceeding, and irrevocably submits to the non-exclusive jurisdiction of such courts in any suit, action or proceeding. The Issuer hereby appoints René Russo, Pharm.D., BCPS, Arsanis, Inc., 890 Winter Street, Suite 230, Waltham, Massachusetts 02451, as its authorized agent (the “Authorized Agent”) upon whom process may be served in any suit, action or proceeding arising out of or based upon this Underwriting Agreement or the transactions contemplated herein that may be instituted in any State or U.S. federal court in The City of New York and County of New York,

by any Underwriter, the directors, officers, employees, Affiliates and agents of any Underwriter, or by any person who controls any Underwriter, and expressly accepts the non-exclusive jurisdiction of any such court in respect of any such suit, action or proceeding. The Issuer hereby represents and warrants that the Authorized Agent has accepted such appointment and has agreed to act as said agent for service of process, and the Issuer agrees to take any and all action, including the filing of any and all documents that may be necessary to continue such appointment in full force and effect as aforesaid. Service of process upon the Authorized Agent shall be deemed, in every respect, effective service of process upon the Issuer. Notwithstanding the foregoing, any action arising out of or based upon this Underwriting Agreement may be instituted by any Underwriter, the directors, officers, employees, Affiliates and agents of any Underwriter, or by any person who controls any Underwriter, in any court of competent jurisdiction in Massachusetts.

15. No Fiduciary Duty. The Issuer hereby acknowledges that (a) the purchase and sale of the Securities pursuant to this Underwriting Agreement is an arm’s-length commercial transaction between the Issuer, on the one hand, and the Underwriters and any affiliate through which it may be acting, on the other, (b) the Underwriters are acting as principal and not as an agent or fiduciary of the Issuer and (c) the Issuer’s engagement of the Underwriters in connection with the Offering and the process leading up to the Offering is as independent contractors and not in any other capacity. Furthermore, the Issuer agrees that it is solely responsible for making its own judgments in connection with the Offering (irrespective of whether any of the Underwriters has advised or is currently advising the Issuer on related or other matters). The Issuer agrees that it will not claim that the Underwriters have rendered advisory services of any nature or respect, or owe an agency, fiduciary or similar duty to the Issuer, in connection with such transaction or the process leading thereto.

16. Integration. This Underwriting Agreement supersedes all prior agreements and understandings (whether written or oral) between the Issuer and the Underwriters, or any of them, with respect to the subject matter hereof.

17. Applicable Law. This Underwriting Agreement will be governed by and construed in accordance with the laws of the State of New York applicable to contracts made and to be performed within the State of New York.

18. Waiver of Jury Trial. The Issuer and the Underwriters hereby irrevocably waive, to the fullest extent permitted by applicable law, any and all right to trial by jury in any legal proceeding arising out of or relating to this Underwriting Agreement or the Offering.

19. Counterparts. This Underwriting Agreement may be signed in one or more counterparts, each of which shall constitute an original and all of which together shall constitute one and the same agreement.

20. Headings. The section headings used herein are for convenience only and shall not affect the construction hereof.

If the foregoing is in accordance with your understanding of our agreement, please sign and return to us the enclosed duplicate hereof, whereupon this letter and your acceptance shall represent a binding agreement among the Issuer and the several Underwriters.

Very truly yours,

Arsanis, Inc.

By:
Name:
Title:

Citigroup Global Markets Inc.

By:
		Name:
		Title:

Cowen and Company, LLC

By:
		Name:
		Title:

Piper Jaffray & Co.

By:
		Name:
		Title:

Underwriters		Number of Underwritten Securities to be Purchased

Citigroup Global Markets Inc.		1,600,000

Cowen and Company, LLC		1,300,000

Piper Jaffray & Co.		1,100,000
		__________

Total		4,000,000

This letter is being delivered to you in connection with the proposed underwriting agreement (the “Underwriting Agreement”), between Arsanis, Inc., a Delaware corporation (the “Issuer”), and each of you as representatives of a group of underwriters named therein (the “Underwriters”), relating to an underwritten public offering of common stock, $0.001 par value per share (the “Common Stock”), of the Issuer (the “Offering”).

In order to induce you and the other Underwriters to enter into the Underwriting Agreement, the undersigned will not, without the prior written consent of Citigroup Global Markets Inc. and Cowen and Company, LLC (collectively, the “Representatives”), offer, sell, contract to sell, pledge or otherwise dispose of, (or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition (whether by actual disposition or effective economic disposition due to cash settlement or otherwise) by the undersigned or any affiliate of the undersigned or any person in privity with the undersigned or any affiliate of the undersigned), directly or indirectly, including the filing, whether publicly or confidentially, (or participation in the filing) of a registration statement (other than a registration statement on Form S-8) with the Securities and Exchange Commission (the “SEC”) in respect of, or establish or increase a put equivalent position or liquidate or decrease a call equivalent position within the meaning of Section 16 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and the rules and regulations of the SEC promulgated thereunder with respect to, any shares of capital stock of the Issuer or any securities convertible into, or exercisable or exchangeable for such capital stock, or publicly announce an intention to effect any such transaction, for a period from the date hereof through 180 days after the date of the Underwriting Agreement (the “Lock-Up Period”). If the undersigned is an officer or director of the Issuer, the

undersigned further agrees that the foregoing restrictions shall be equally applicable to any issuer-directed shares of Common Stock the undersigned may purchase in the Offering.

The provisions of the immediately preceding paragraph shall not apply to or prohibit any of the following:

(iv)

transfers or dispositions of shares of capital stock of the Issuer or any securities convertible into, or exercisable or exchangeable for such capital stock to any trust for the direct or indirect benefit of the undersigned or the immediate family of the undersigned in a transaction not involving a disposition for value;

(v)

transfers or dispositions of shares of capital stock of the Issuer or any securities convertible into, or exercisable or exchangeable for such capital stock to any corporation, partnership, limited liability company or other entity all of the beneficial ownership interests of which are held by the undersigned or the immediate family of the undersigned in a transaction not involving a disposition for value;

(vi)

transfers or dispositions of shares of capital stock of the Issuer or any securities convertible into, or exercisable or exchangeable for such capital stock by will, other testamentary document or intestate succession to the legal representative, heir, beneficiary or a member of the immediate family of the undersigned; or

(vii)

distributions of shares of capital stock of the Issuer or any securities convertible into, or exercisable or exchangeable for such capital stock to partners, members or stockholders of the undersigned;

provided, that, in the case of clauses (i) and (ii) (a) no filing under Section 13 or Section 16(a) of the Exchange Act reporting a reduction in beneficial ownership of shares of Common Stock shall be required during the Lock-Up Period, other than Forms 5 and Schedule 13F, and (b) no filing under Section 13 or Section 16(a) of the Exchange Act or other public announcement shall be voluntarily made by the undersigned during the Lock-Up Period, other than Forms 5 and Schedule 13F; provided further that in the case of any transfer or distribution pursuant to clauses (iii), (iv), (v), (vi) and (vii) (a) the recipient agrees to be bound in writing by the same restrictions set forth herein for the duration of the Lock-Up Period, (b) no filing under Section 13 or Section

16(a) of the Exchange Act reporting a reduction in beneficial ownership of shares of Common Stock shall be required during the Lock-Up Period, other than Forms 5 and Schedule 13F and (c) no filing under Section 13 of Section 16(a) of the Exchange Act or other public announcement shall be voluntarily made by the undersigned or the transferee during the Lock-Up Period, other than Forms 5 and Schedule 13F and (d) any such transfer shall not involve a disposition for value.

Notwithstanding the restrictions imposed by this letter, the undersigned may, without the prior written consent of the Representatives, (a) exercise, in cash or by means of a net exercise, an option to purchase shares of Common Stock granted under any stock incentive plan or stock purchase plan of the Issuer or exercise outstanding warrants to purchase shares of the Issuer’s capital stock, provided that the underlying shares issuable upon exercise thereof shall continue to be subject to the restrictions on transfer set forth in this letter, (b) establish a trading plan pursuant to Rule 10b5-1 under the Exchange Act for the transfer of shares of Common Stock, provided that such plan does not provide for any transfers of Common Stock, and no filing with the SEC or other public announcement shall be required or voluntarily made by the undersigned or any other person in connection therewith, in each case during the 180-day period referred to in the immediately preceding paragraph and (c) transfer shares of Common Stock to the Issuer in connection with the termination of the undersigned’s employment with the Issuer.

If the undersigned is an officer or director of the Issuer, (i) the Representatives agree that, at least three business days before the effective date of any release or waiver of the foregoing restrictions in connection with a transfer of shares of Common Stock, the Representatives will notify the Issuer of the impending release or waiver and (ii) the Issuer has agreed in the Underwriting Agreement to announce the impending release or waiver by press release through a major news service at least two business days before the effective date of the release or waiver. Any release or waiver granted by the Representatives hereunder to any such officer or director shall only be effective two business days after the publication date of such press release. The provisions of this paragraph will not apply if (a) the release or waiver is effected solely to permit a transfer not for consideration and (b) the transferee has agreed in writing to be bound by the same terms described in this letter to the extent and for the duration that such terms remain in effect at the time of the transfer.

This letter shall automatically terminate and the undersigned shall be released from all obligations under this letter upon the earliest to occur, if any, of (i) either the Issuer, on the one hand, or the Representatives, on the other hand, advising the other in writing, prior to the execution of the Underwriting Agreement, that they have determined not to proceed with the Offering, (ii) the Underwriting Agreement being terminated prior to the Closing Date (as defined in the Underwriting Agreement), (iii) the registration statement filed with the SEC with respect to the Offering being withdrawn and (iv) February 16, 2018, in the event that the Underwriting Agreement has not been executed by such date.

Yours very truly,

Exact Name of Stockholder

Authorized Signature

Title (if signing on behalf of an entity)

This letter is being delivered to you in connection with the offering by Arsanis, Inc. (the “Issuer”) of [•] shares of common stock, $0.001 par value per share (the “Common Stock”), of the Issuer and the lock-up letter dated [●], 2017 (the “Lock-up Letter”), executed by you in connection with such offering, and your request for a [waiver] [release] dated [insert date], 20[•], with respect to [•] shares of Common Stock (the “Shares”).

Citigroup Global Markets Inc. and Cowen and Company, LLC hereby agree to [waive] [release] the transfer restrictions set forth in the Lock-up Letter, but only with respect to the Shares, effective [insert date], 20[•]; provided, however, that such [waiver] [release] is conditioned on the Issuer announcing the impending [waiver] [release] by press release through a major news service at least two business days before effectiveness of such [waiver] [release]. This letter will serve as notice to the Issuer of the impending [waiver] [release].

Except as expressly [waived] [released] hereby, the Lock-up Letter shall remain in full force and effect.

Yours very truly,

Citigroup Global Markets Inc.

By:
		Name:
		Title:

Cowen and Company, LLC

By:
		Name:
		Title: