Exhibit 10.3

CERTAIN CONFIDENTIAL PORTIONS OF THIS EXHIBIT HAVE BEEN OMITTED AND REPLACED WITH “[***]”. SUCH IDENTIFIED INFORMATION HAS BEEN EXCLUDED FROM THIS EXHIBIT BECAUSE IT IS (I) NOT MATERIAL AND (II) THE TYPE OF INFORMATION THAT THE REGISTRANT TREATS AS PRIVATE OR CONFIDENTIAL.

AMENDMENTTOSERVICEAGREEMENT,July1,2026-November15,2027

CICandClientherebyagreetosupplementtheServiceAgreementasfollows:

Section1.License

NotwithstandingthefinalsentenceofSection1,thepartiesagreethatClientwillhavealicensetouseandoccupy the Space as provided in Section 2 below from July 1, 2026 - November 15, 2027 [the Term].

ThisSpace-SpecificAmendmentreplacestheamendmentsexecutedbyClientprevioustothesigningofthis document, which the parties agree is now null and void.

As this is a fixed term agreement, you will be responsible for payment of your fees for your term, even if you choosetovacateyourspacebeforetheendofyourterm.Exceptionstothismayonlybemadebymutualagreement in writing signed by both parties.

If Client occupies any other part of any CIC Premises beyond the Space listed in Section 2 during the Term or the 12monthextensionterms,suchoccupancywillnotbesubjecttotheamendedterminationrightsassetforthherein.

Section2.SpaceandServices

ThebelowSpacesareincludedinthisTerm:

SERVICEAGREEMENT

Amendment

CICServiceAgreement

SERVICEAGREEMENT

Amendment

CICServiceAgreement

Section3.FeesandPayments

The parties agree that the monthly fees associated with the Spaces listed in Section 2 shall be a [***]reduction from the rate listed in the Section 2 table.If the client terminates the agreement before the Term expires, they are obligated to pay [***]of the remaining value for the uncompleted period. Any additional spaces added during the Termwillalsoincludethe[***]pricereductionfrommarketrate,butthosespaceswillremainonCIC’sstandard [***]daynoticeforofficesand[***] daysnoticeforlabs.[***].

SERVICEAGREEMENT

Amendment

CICServiceAgreement

DuringtheTerm,thisamendmentsupplementsthethirdparagraphofSection6("FeesandPayments")ofthe Service Agreement by providing Client with an opportunity for deferral and possible forgiveness of price adjustments otherwise permitted under Section 6.

AllothertermsoftheServiceAgreement,includingwithoutlimitationSection1("License"),remainunchanged.

Name of Licensee legal entity: Cabaletta Bio CIC Innovation Communities, LLC (“CIC”)

Licensee Federal Tax ID#: 82-1685768

(If left blank, agreement becomes personal obligation of signer)

Signature: /s/ Anup Marda Signature: /s/ Sarajane Blair

Name of Authorized Signer: Anup Marda Officer's Name: Sarajane Blair

5/19/26

Title: CFO Title: General Manager

DATE: 18 May 2026 | 1:44:09 PM PDF DATE: