Exhibit 99.3

Filed by Sesen Bio, Inc.

Pursuant to Rule 425 under the Securities Act of 1933

and deemed filed pursuant to Rule 14a-12

under the Securities Exchange Act of 1934

Subject Company: Sesen Bio, Inc.

Commission File No.: 001-36296

On September 21, 2022, CARISMA Therapeutics Inc. published the following post on Twitter:

We have entered into a definitive merger agreement w/ Sesen Bio $SESN. We expect this transaction to put us in a strong financial position to continue developing our lead candidate CT-0508 & potentially unlock the value of CAR-M’s. Impt Info here: [Link to Press Release on Carisma’s Website] 

On September 21, 2022, CARISMA Therapeutics Inc. published the following post on LinkedIn:

We are thrilled to announce that Carisma and Sesen Bio have entered into a definitive merger agreement and, upon closing, expect the combined company to operate under the Carisma name and trade on the Nasdaq Capital Market. We anticipate this transaction will put us in a strong financial position to continue developing our lead candidate CT-0508 and unlock the potential value of CAR-Macrophages for patients.

It’s been nearly a decade since our Chief Scientific Officer, Michael Klichinsky, discovered a new method of genetically modifying patients’ own immune cells – called macrophages – to destroy solid tumor cancer cells while generally sparing normal tissue. This discovery led to the formation of Carisma Therapeutics back in 2016. Since then, we’ve continued to evaluate and develop the potential for using engineered macrophages to treat cancer and other serious disorders. We believe this transaction will further fuel our efforts, and we’re looking forward to the bright future ahead.

Important information here: [Link to Press Release on Carisma’s Website] 

On September 21, 2022, Steven Kelly, President and Chief Executive Officer ofCARISMA Therapeutics Inc., published the following post on LinkedIn:

Today is an exciting day at Carisma Therapeutics. As a combined company with Sesen Bio, we intend to continue to focus on advancing Carisma’s proprietary cell therapy platform that utilizes engineered macrophages to potentially transform the treatment of cancer and other serious disorders. We believe this transaction will further our ability to advance the clinical development of our potentially ground-breaking treatment, and we can’t wait for what is next. Important Info here: [Link to Press Release on Carisma’s Website] 

On September 21, 2022, Richard Morris, Chief Financial Officer of CARISMA Therapeutics Inc., published the following post on LinkedIn:

Good news for Carisma Therapeutics today. Looking forward to a future where we can potentially help more patients through our CAR-M technology. Important Info here: [Link to Press Release on Carisma’s Website] 

Cautionary Note on Forward-Looking Statements

Any statements in this communication about future expectations, plans and prospects for Sesen Bio, Carisma or the combined company, Sesen Bio’s, Carisma’s or the combined company’s strategy or future operations, and other statements containing the words “anticipate,” “believe,” “contemplate,” “expect,” “intend,” “may,” “plan,” “predict,” “target,” “potential,” “possible,” “will,” “would,” “could,” “should,” “continue,” and similar expressions, constitute forward-looking statements within the meaning of The Private Securities Litigation Reform Act of 1995. For example, statements concerning the proposed transaction, the concurrent financing, the contingent value rights and other matters, including without limitation: statements relating to the satisfaction of the conditions to and consummation of the proposed transaction, the expected timing of the consummation of the proposed transaction and the expected ownership percentages of the combined company, Sesen Bio’s and Carisma’s respective businesses, the strategy of the combined company, future operations, advancement of the combined company’s product candidates and product pipeline, clinical development of the combined company’s product candidates, including expectations regarding timing of initiation and results of clinical trials of the combined company, the ability of Sesen Bio to remain listed on the Nasdaq Stock Market, the completion of the concurrent financing and the receipt of any payments under the contingent value rights are forward-looking statements. Actual results may differ materially from those indicated by such forward-looking statements as a result of various important factors, including without limitation: (i) the risk that the conditions to the closing of the proposed transaction are not satisfied, including the failure to obtain stockholder approval of matters related to the proposed transaction in a timely manner or at all; (ii) uncertainties as to the timing of the consummation of the proposed transaction and the ability of each of Sesen Bio and Carisma to consummate the proposed transaction, including completing the concurrent financing; (iii) risks related to Sesen Bio’s ability to correctly estimate its expected net cash at closing and Sesen Bio’s and Carisma’s ability to correctly estimate and manage their respective operating expenses and expenses associated with the proposed transaction; (iv) risks related to Sesen Bio’s continued listing on the Nasdaq Stock Market until closing of the proposed transaction; (v) the risk that as a

result of adjustments to the exchange ratio, Sesen Bio stockholders or Carisma stockholders could own less of the combined company than is currently anticipated; (vi) the risk that the conditions to payment under the contingent value rights will not be met and that the contingent value rights may otherwise never deliver any value to Sesen Bio stockholders; (vii) risks associated with the possible failure to realize certain anticipated benefits of the proposed transaction, including with respect to future financial and operating results; (viii) uncertainties regarding the impact any delay in the closing would have on the anticipated cash resources of the combined company upon closing and other events and unanticipated spending and costs that could reduce the combined company’s cash resources; (ix) the occurrence of any event, change or other circumstance or condition that could give rise to the termination of the merger agreement; (x) the effect of the announcement, pendency or completion of the merger on Sesen Bio’s or Carisma’s business relationships, operating results and business generally; (xi) costs related to the merger; (xii) the outcome of any legal proceedings that may be instituted against Sesen Bio, Carisma or any of their respective directors or officers related to the merger agreement or the transactions contemplated thereby; (xiii) the ability of Sesen Bio or Carisma to protect their respective intellectual property rights; (xiv) competitive responses to the proposed transaction and changes in expected or existing competition; (xv) the success and timing of regulatory submissions and pre-clinical and clinical trials; (xvi) regulatory requirements or developments; (xvii) changes to clinical trial designs and regulatory pathways; (xviii) changes in capital resource requirements; (xix) risks related to the inability of the combined company to obtain sufficient additional capital to continue to advance its product candidates and its preclinical programs; (xx) legislative, regulatory, political and economic developments; and (xxi) other factors discussed in the “Risk Factors” section of Sesen Bio’s Annual Report on Form 10-K, Quarterly Reports on Form 10-Q and other reports filed with the Securities and Exchange Commission (“SEC”). In addition, the forward-looking statements included in this communication represent Sesen Bio’s and Carisma’s views as of the date hereof. Sesen Bio and Carisma anticipate that subsequent events and developments will cause the respective company’s views to change. However, while Sesen Bio may elect to update these forward-looking statements at some point in the future, Sesen Bio specifically disclaims any obligation to do so, except as required under applicable law. These forward-looking statements should not be relied upon as representing Sesen Bio’s views as of any date subsequent to the date hereof.

Important Additional Information

In connection with the proposed transaction, Sesen Bio will file materials with the SEC, including a registration statement on Form S-4 (Form S-4), which will include a document that serves as a proxy statement/prospectus of Sesen Bio and an information statement of Carisma, and other documents regarding the proposed transaction. INVESTORS AND SECURITY HOLDERS ARE URGED TO READ THESE MATERIALS, INCLUDING THE FORM S-4 AND THE PROXY STATEMENT/PROSPECTUS, WHEN THEY BECOME AVAILABLE, BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION AND THE PARTIES TO THE PROPOSED TRANSACTION. Investors and security holders will be able to obtain the Form S-4, the proxy statement/prospectus and other materials filed by Sesen Bio with the SEC free of charge from the SEC’s website at www.sec.gov or from Sesen Bio at the SEC Filings section of www.sesenbio.com.

No Offer or Solicitation

This communication shall not constitute an offer to sell or the solicitation of an offer to buy any securities, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offering of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended. Subject to certain exceptions to be approved by the relevant regulators or certain facts to be ascertained, a public offer will not be made directly or indirectly, in or into any jurisdiction where to do so would constitute a violation of the laws of such jurisdiction, or by use of the mails or by any means or instrumentality (including without limitation, facsimile transmission, telephone or internet) of interstate or foreign commerce, or any facility of a national securities exchange, of any such jurisdiction.

Participants in the Solicitation

Sesen Bio and Carisma and their respective directors, executive officers and other members of management may be deemed to be participants in the solicitation of proxies in respect of the proposed transaction. Information about Sesen Bio’s directors and executive officers is available in Sesen Bio’s Annual Report on Form 10-K for the fiscal year ended December 31, 2021, its definitive proxy statement dated April 28, 2022 for its 2022 Annual Meeting of Stockholders and its Current Report on Form 8-K filed with the SEC on August 31, 2022. Other information regarding the participants in the proxy solicitation and a description of their interests in the transaction, by security holdings or otherwise, will be included in the proxy statement/prospectus and other relevant materials to be filed with the SEC regarding the proposed transaction when they become available. Investors should read the proxy statement/prospectus carefully when it becomes available before making any voting or investment decisions. You may obtain free copies of these documents from Sesen Bio or the SEC’s website as indicated above.