September 17, 2019

Steven Nichtberger
Chief Executive Officer and President
Cabaletta Bio, Inc.
2929 Arch Street, Suite 600
Philadelphia, PA 19104

       Re: Cabaletta Bio, Inc.
           Amendment No. 1 to
           Draft Registration Statement on Form S-1
           Submitted August 2, 2019
           CIK No. 0001759138

Dear Mr. Nichtberger:

      We have reviewed your amended draft registration statement and have the
following
comments. In some of our comments, we may ask you to provide us with
information so we
may better understand your disclosure.

       Please respond to this letter by providing the requested information and
either submitting
an amended draft registration statement or publicly filing your registration
statement on
EDGAR. If you do not believe our comments apply to your facts and circumstances
or do not
believe an amendment is appropriate, please tell us why in your response.

      After reviewing the information you provide in response to these comments
and your
amended draft registration statement or filed registration statement, we may
have additional
comments.

Amended Draft Registration Statement on Form S-1

Financial Statements
Statements of Operations, page F-4

1.     With regard to your response to comment 13, please:
         tell us why you believe it is appropriate to weight the period of
April 3, 2017 to
          August 22, 2017 at 51% when there were no shares outstanding to share
in the loss,
         reconcile the issuance of 4,183,250 common shares on August 22, 2017
shown in your
          response letter dated September 6, 2019 to the 5,000,000 common
shares shown as
          issued in 2017 on page F-5 of the draft registration statement and
explain why the
          computation is not based on the 5,000,000 shares,
 Steven Nichtberger
Cabaletta Bio, Inc.
September 17, 2019
Page 2

             provide us your computation of the 204,187 shares and 3,156 shares
that vest in some
             quarters as shown in your response letter dated September 6, 2019,
and
             tell us where the vesting of the 3,130,874 shares is shown in your
calculation in your
             response.
Notes to the Financial Statements
6. Commitments and Contingencies
The Regents of the University of California, page F-15

2.       Refer to your response to comment 15. Please disclose the amount of
obligation you are
         committed to fund or disclose that such amount is not material as we
believe this is a
         significant term of the agreement that should be disclosed.
       You may contact Keira Nakada at 202-551-3267 or Lisa Vanjoske at
202-551-3614 if
you have questions regarding comments on the financial statements and related
matters. Please
contact Julie Griffith at 202-551-3267 or Dietrich King at 202-551-8071 with
any other
questions.



FirstName LastNameSteven Nichtberger                          Sincerely,
Comapany NameCabaletta Bio, Inc.
                                                              Division of
Corporation Finance
September 17, 2019 Page 2                                     Office of
Healthcare & Insurance
FirstName LastName