Sec Form 4 Filing - Binder Gwendolyn @ Cabaletta Bio, Inc. - 2025-05-19

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Binder Gwendolyn 2. Issuer Name and Ticker or Trading Symbol Cabaletta Bio, Inc. [ CABA] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) President, Science & Tech. (Last) (First) (Middle) C/O CABALETTA BIO, INC., 2929 ARCH STREET, SUITE 600 3. Date of Earliest Transaction (MM/DD/YY) 05/19/2025 (Street) PHILADELPHIA, PA19104 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Stock Option (Right to Buy) $ 9.54 05/19/2025 D( 1 ) 66,667 ( 2 ) 08/21/2029 Common Stock 66,667 $ 0 0 D Stock Option (Right to Buy) $ 1.92 ( 3 ) 05/19/2025 A( 1 ) 66,667 ( 2 ) 08/21/2029 Common Stock 66,667 $ 0 66,667 D Stock Option (Right to Buy) $ 11 05/19/2025 D( 1 ) 29,908 ( 2 ) 10/23/2029 Common Stock 29,908 $ 0 0 D Stock Option (Right to Buy) $ 1.92 ( 3 ) 05/19/2025 A( 1 ) 29,908 ( 2 ) 10/23/2029 Common Stock 29,908 $ 0 29,908 D Stock Option (Right to Buy) $ 11.47 05/19/2025 D( 1 ) 125,000 ( 2 ) 02/28/2031 Common Stock 125,000 $ 0 0 D Stock Option (Right to Buy) $ 1.92 ( 3 ) 05/19/2025 A( 1 ) 125,000 ( 2 ) 02/28/2031 Common Stock 125,000 $ 0 125,000 D Stock Option (Right to Buy) $ 3.21 05/19/2025 D( 1 ) 160,000 ( 2 ) 01/17/2032 Common Stock 160,000 $ 0 0 D Stock Option (Right to Buy) $ 1.92 ( 3 ) 05/19/2025 A( 1 ) 160,000 ( 2 ) 01/17/2032 Common Stock 160,000 $ 0 160,000 D Stock Option (Right to Buy) $ 11.09 05/19/2025 D( 1 ) 176,000 ( 2 ) 01/17/2033 Common Stock 176,000 $ 0 0 D Stock Option (Right to Buy) $ 1.92 ( 3 ) 05/19/2025 A( 1 ) 176,000 ( 2 ) 01/17/2033 Common Stock 176,000 $ 0 176,000 D Stock Option (Right to Buy) $ 23.97 05/19/2025 D( 1 ) 130,000 ( 2 ) 02/28/2034 Common Stock 130,000 $ 0 0 D Stock Option (Right to Buy) $ 1.92 ( 3 ) 05/19/2025 A( 1 ) 130,000 ( 2 ) 02/28/2034 Common Stock 130,000 $ 0 130,000 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Binder Gwendolyn C/O CABALETTA BIO, INC. 2929 ARCH STREET, SUITE 600 PHILADELPHIA, PA19104 President, Science & Tech. Signatures By: /s/ Michael Gerard, as Attorney-in-Fact 05/19/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On May 15, 2025, the Issuer's board of directors approved an option repricing (the "Repricing"), to be effective May 19, 2025 (the "Repricing Date"). All of the other terms of the options remain unchanged. Such transactions were exempt pursuant to Rule 16b-6(d) and Rule 16b-3 of the Exchange Act, as applicable. ( 2 )This stock option award was issued pursuant to the Cabaletta Bio, Inc. 2018 Stock Option and Incentive Plan (the "2018 Plan") or the Cabaletta Bio, Inc. 2019 Stock Option and Incentive Plan (the "2019 Plan"), as applicable, and becomes exercisable in accordance with the vesting schedule specified in the award agreement and as previously reported on applicable Form 4, subject to the Reporting Person's continued service with the Issuer as of the applicable vesting date. ( 3 )The exercise price of the option is $1.92 per share, representing the fair market value per share of the Issuer's Common Stock on the Repricing Date. Under the terms of the stock option award repricing, a repriced stock option award will revert to its original exercise price if exercised during the Retention Period. The "Retention Period", through which the Reporting Person must remain in service to the Issuer, commenced on the Repricing Date and ends upon the earliest of (i) the one-year anniversary of the Repricing Date, (ii) a Sale Event (as defined in the 2018 Plan and 2019 Plan, as applicable) and (iii) the termination of the Reporting Person's Service Relationship (as defined in the 2018 Plan and 2019 Plan, as applicable) due to the Reporting Person's death or disability.

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