Proxy Results, Amendments to Bylaws (Form 8-K)

Item 5.03 Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year.

On June 9, 2026, Cabaletta Bio, Inc. (the "Company") held its Annual Meeting of Stockholders (the "Annual Meeting"). As further described in Item 5.07 to this Current Report on Form 8-K, at the Annual Meeting, the Company's stockholders approved an amendment to the Company's Third Amended and Restated Certificate of Incorporation to increase the number of authorized shares of common stock from 300,000,000 to 600,000,000. On June 9, 2026, the Company filed a Certificate of Amendment to the Company's Third Amended and Restated Certificate of Incorporation (the "Certificate of Amendment") with the Secretary of State of the State of Delaware and the Certificate of Amendment became effective on filing. The foregoing description does not purport to be complete and is qualified in its entirety by reference to the full text of the Certificate of Amendment, a copy of which is filed as Exhibit 3.1 to this Current Report on Form 8-K and incorporated herein by reference.

Item 5.07 Submission of Matters to a Vote of Security Holders.

On June 9, 2026, the Company held its Annual Meeting. As of April 20, 2026, the record date for the Annual Meeting, there were 111,324,796 outstanding shares of the Company's voting common stock, par value $0.00001 per share (the "Common Stock"). As set forth in the Supplement to the Proxy Statement, dated June 1, 2026 (the "Supplement"), on May 31, 2026, the Board of Directors withdrew Proposal 3 from stockholder consideration at the Annual Meeting. The Company's stockholders voted on the following matters, which are described in detail in the Company's definitive proxy statement on Schedule 14A filed on April 28, 2026 (the "Proxy Statement"), as supplemented by the Supplement: (i) to elect two directors, Scott Brun, M.D. and Shawn Tomasello, MBA, as Class I directors of the Company, each to serve for a three-year term expiring at the Company's 2029 annual meeting of stockholders and until their successor has been duly elected and qualified, subject to their earlier death, resignation or removal ("Proposal 1"), (ii) to ratify the appointment of Ernst & Young LLP as the Company's independent registered public accounting firm for the fiscal year ending December 31, 2026 ("Proposal 2"), (iii) to approve an amendment to the Company's Third Amended and Restated Certificate of Incorporation to increase the number of authorized shares of common stock from 300,000,000 to 600,000,000 (the "Charter Amendment Proposal") ("Proposal 4"), (iv) to approve, on a non-binding, advisory basis, the compensation of the Company's named executive officers ("Proposal 5"), and (v) to approve a proposal to adjourn the Annual Meeting to a later date or dates, if necessary or appropriate, to permit further solicitation and vote of proxies in the event that there are insufficient votes for, or otherwise in connection with, the approval of the Charter Amendment Proposal ("Proposal 6").

The Company's stockholders approved the Class I director nominees, Scott Brun, M.D. and Shawn Tomasello, MBA, recommended for election in Proposal 1 at the Annual Meeting. The votes cast at the Annual Meeting were as follows:

The Company's stockholders ratified the appointment of Ernst & Young LLP as the Company's independent registered public accounting firm for the fiscal year ending December 31, 2026, recommended for ratification in Proposal 2 at the Annual Meeting. The votes cast at the Annual Meeting were as follows:

As set forth in the Supplement, the Board of Directors withdrew Proposal 3, the Plan Amendment Proposal, from stockholder consideration at the Annual Meeting. Accordingly, proxy cards or voting instructions received with direction on Proposal 3 were not voted on Proposal 3.

The Company's stockholders approved the amendment to the Company's Third Amended and Restated Certificate of Incorporation to increase the number of authorized shares of common stock from 300,000,000 to 600,000,000, recommended for approval as Proposal 4 at the Annual Meeting. The votes cast at the Annual Meeting were as follows:

The Company's stockholders approved, by non-binding advisory vote, the compensation of the Company's named executive officers in Proposal 5. The votes cast at the Annual Meeting were as follows:

Although Proposal 6 was deemed not necessary because there was a quorum present and there were sufficient proxies at the time of the Annual Meeting to approve the Charter Amendment Proposal, it was approved by the requisite vote as follows:

No other matters were submitted to or voted on by the Company's stockholders at the Annual Meeting.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits