Cabaletta Bio Inc美国股票发行说明书（2026年5月4日版）

51,725,000 Shares of Common Stock We are selling 51,725,000 shares of our common stock, par value $0.00001 per share, or the common stock, in this offering pursuant to thisprospectus supplement and the accompanying prospectus. Our common stock is listed on the Nasdaq Global Select Market under the symbol “CABA.” On May 1, 2026, the last reported sale price of ourcommon stock on the Nasdaq Global Select Market was $2.94 per share. Investing in our common stock involves a high degree of risk. See the information contained under “Risk Factors”beginning on page S-10 of this prospectus supplement and the documents incorporated by reference herein. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities ordetermined if this prospectus supplement is truthful or complete. Any representation to the contrary is a criminal offense. Offering priceUnderwriting discounts and commissions(1)Proceeds, before expenses, to us (1)See the section entitled “Underwriting” for additional disclosure regarding underwriter compensation and estimated offering expenses. Delivery of the shares of common stock is expected to be made on or about May5, 2026. Cantor TD Cowen H.C. Wainwright& Co. ge1236 Table of Contents ABOUT THIS PROSPECTUS SUPPLEMENT This prospectus supplement is part of a shelf registration statement on Form S-3 that we filed with the Securities and Exchange Commission, orthe SEC. Under the shelf registration process, we may offer from time to time various securities, of which this offering of shares of our common stock is If the information contained in this prospectus supplement differs or varies from the information contained in any document incorporated byreference herein that was filed with the SEC before the date of this prospectus supplement, you should rely on the information set forth in thisprospectus supplement. If any statement in one of these documents is inconsistent with a statement in another document having a later date (for You should rely only on the information contained or incorporated by reference in this prospectus supplement. We have not, and the underwritershave not, authorized anyone to provide you with information that is in addition to or different from that contained or incorporated by reference in thisprospectus supplement or contained in any permitted free writing prospectuses we have authorized for use in connection with this offering. We and the The information contained in this prospectus supplement and the documents incorporated by reference herein is accurate only as of theirrespective dates, regardless of the time of delivery of any such document or the time of any sale of our common stock. Our business, financial condition,results of operations and prospects may have changed since those dates. It is important for you to read and consider all information contained orincorporated by reference in this prospectus supplement in making your investment decision. You should read this prospectus supplement, as well as the We further note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that isincorporated by reference in this prospectus supplement were made solely for the benefit of the parties to such agreement, including, in some cases, forthe purpose of allocating risk among the parties to such agreements, and should not be deemed to be a representation, warranty or covenant to you. We use various trademarks, trade names and service marks in our business, including without limitation our corporate name and logo. All othertrademarks, trade names or service marks referred to in this prospectus supplement are the property of their respective owners. Solely for convenience,the trademarks, trade names and service marks in this prospectus supplement may be referred to without the®,™and SM symbols, but such referencesshould not be construed as any indicator that their respective owners will not assert, to the fullest extent under applicable law, their rights thereto. Thisprospectus supplement and the documents incorporated by reference herein also contain estimates, projections and other information concerning our Table of Contents circumstances may differ materially from events and circumstances reflected in this information. Unless otherwise expressly stated, we obtained thisindustry, business, market and other data from reports, research surveys, studies and similar data prepared by market research firms and other third We are offering to sell, and seeking offers to buy, shares of our common stock only in jurisdictions where such offers and sales are permitted. Thedistribution of this prospectus supplement and the offering of our common stock in certain jurisdictions may be restricted by law. Persons outside theUnited States who come into possession of this prospectus supplement must inform themselves about, and observe any r