SecProbe.io

August 28, 2019
Steven Nichtberger
Chief Executive Officer and President
Cabaletta Bio, Inc.
2929 Arch Street, Suite 600
Philadelphia, PA 19104
Re:Cabaletta Bio, Inc.
Draft Registration Statement on Form S-1
Submitted August 2, 2019
CIK No. 0001759138
Dear Mr. Nichtberger:
            We have reviewed your 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 amending your registration statement and providing the
requested information.  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 any amendment to your registration statement and the information you
provide in response to these comments, we may have additional comments.
Draft Registration Statement on Form S-1
Prospectus Summary, page 1
1.Please revise the disclosure in the first paragraph of the Overview to clarify what you
mean by "IND-ready."
Pipeline, page 2
2.We note your disclosure regarding "in vivo  evidence of efficacy and safety in an animal
model..." We also note other examples of statements regarding safety and efficacy on
pages 3, 117, 119, and 124.  Since none of your product candidates has received FDA
approval, please revise your disclosure regarding safety and efficacy in the summary and
throughout your prospectus to clarify this point.

 FirstName LastNameSteven Nichtberger
 Comapany NameCabaletta Bio, Inc.
 August 28, 2019 Page 2
 FirstName LastNameSteven Nichtberger
Cabaletta Bio, Inc.
August 28, 2019
Page 2
3.We note your statement on page 13 that you are early in your development efforts and
have not initiated clinical trials for any product candidates.  Please expand the table on
page 2 to include more information about the progress of the company's pre-clinical trials,
 and to clarify in the table that no products are past the phase of IND enabling studies. In
addition, please include columns for Phase 1, 2 and 3 testing that are equally prominent
with the pre-clinical columns, and clarify that these columns refer to clinical testing. In
this regard, our concern is that potential investors see a balanced graphical presentation of
where you are in the drug development process, and that the table adequately depict, if
true, that you have not yet filed an IND application for any product.  Please make
conforming changes in the Business section as well.
Implications of Being an Emerging Growth Company, page 7
4.Please supplementally provide us with copies of all written communications, as defined in
Rule 405 under the Securities Act, that you, or anyone authorized to do so on your behalf,
present to potential investors in reliance on Section 5(d) of the Securities Act, whether or
not they retain copies of the communications.
Risk Factors, page 12
5.We note that there are references to foreign regulators and foreign markets throughout the
Risk Factors and other sections of your prospectus. Please revise to explain what non-U.S.
markets, if any, you plan to enter, and what steps you have taken to attain the necessary
regulatory approvals.
6.Please provide us with a basis for your statement on page 21 that although clinical trials
for cell-based therapies have historically included "a lymphodepleting chemotherapeutic
regime to condition the patient prior to infusion [with CAR T-cell based therapies]," and
that such infusions have resulted in neurotoxicity and other serious side effects,  you
believe that  the use of CAR T cell therapies without preconditioning will avoid
neurotoxicity or other side effects.  We note your statement that "Based on evidence from
other CAR T cell clinical trials demonstrating clinical activity without prior conditioning
and the levels of certain cytokines that promote T cell expansion in the patients we are
treating relative to cancer patients, as well as data from engineered T cell therapy in the
setting of HIV without conditioning, we believe that CAAR T cell therapy may be
functional in our autoimmune target patient populations without preconditioning
regimens. Based on preclinical studies where DSG3-CAART is combined with
stimulatory DSG3 antibodies, we observed these antibodies generate a modest level of
cytokine activity that is an order of magnitude less than what was observed when DSG3-
CAART engaged with target B cells. We believe this data indicates the presence of
soluble DSG3 antibodies could stimulate DSG3-CAART expansion and potentially
facilitate engraftment. This information coupled with the risks associated with certain
lymphodepleting regimens used for preconditioning, we believe exposing autoimmune
patients to these regimens without data to support the benefit is difficult to justify."  Please

 FirstName LastNameSteven Nichtberger
 Comapany NameCabaletta Bio, Inc.
 August 28, 2019 Page 3
 FirstName LastNameSteven Nichtberger
Cabaletta Bio, Inc.
August 28, 2019
Page 3
expand your disclosure to include references to the specific studies on which you base
your beliefs, and clarify the reasons for your belief that preconditioning is not necessary
for your product candidate.
7.We note your disclosure on page 80 and on page 184 that your exclusive forum provision
does not apply to actions arising under the Securities Act or the Exchange Act. Please also
ensure that the exclusive forum provision in the bylaws (as effective on the closing of the
offering) states this clearly, or tell us how you will inform investors in future filings that
the provision does not apply to any actions arising under the Securities Act or Exchange
Act.
8.Please refer to the Risk Factor on page 36 which states that you expect to grow the size of
your organization by expanding your employee base.  Please explain, here and in the Use
of Proceeds section on page 84, what portion of the proceeds of this offering you plan to
spend on this endeavor, how long you expect that application of proceeds to last, or what
additional source of funds you expect to use in growing your employee base.
9.Please revise your risk factor disclosure to address more specifically the fact that 88.7% of
the company will be held by the current stockholders following the offering, and detail
more clearly the dilutive impact on investors in this offering that the conversion of
outstanding convertible notes, convertible preferred stock, and the exercise of outstanding
stock options would have, or tell us why you believe this is not a material risk.
Use of Proceeds, page 84
10.Please revise to clarify whether you believe the net proceeds will be sufficient to complete
the Phase 1 clinical trials for your four product candidates, and if not, how far into those
trials you expect the proceeds to last.
Management’s Discussion and Analysis of Financial Condition and Results of Operations
Components of Operating Results
Research and Development, page 95
11.Please quantify the research and development expenses by types of costs incurred for each
of the periods presented.
Critical Accounting Policies and Significant Judgements and Estimates
Stock-Based Compensation, page 103
12.Once you have an estimated offering price or range, please explain to us the reasons for
any differences between the recent valuations of your common stock leading up to the
initial public offering and the estimated offering price. This information will help facilitate
our review of your accounting for equity issuances including stock compensation and
beneficial conversion features.
Financial Statements

 FirstName LastNameSteven Nichtberger
 Comapany NameCabaletta Bio, Inc.
 August 28, 2019 Page 4
 FirstName LastNameSteven Nichtberger
Cabaletta Bio, Inc.
August 28, 2019
Page 4
Statements of Operations, page F-4
13.Please provide us your computation of weighted-average number of shares used in
computing net loss per share for all periods presented.  Also, tell us your consideration of
disclosing in the financial statements how you determined weighted-average number of
shares used in computing net loss per share.
Notes to the Financial Statements
6. Commitments and Contingencies
Operating Lease Agreement, page F-14
14.Please tell us how you recognize rental expense for leases in which the rent varies from
year to year.
The Regents of the University of California, page F-15
15.Please quantify the obligation you are committed to fund.  If no such amount is defined in
the contract, disclose the estimate of such obligations and whether there is a limit to such
obligations.
General
16.Please provide us mockups of any pages that include any additional pictures or graphics to
be presented, including any accompanying captions. Please keep in mind, in scheduling
your printing and distribution of the preliminary prospectus, that we may have comments
after our review of these materials.
            We remind you that the company and its management are responsible for the accuracy
and adequacy of their disclosures, notwithstanding any review, comments, action or absence of
action by the staff.
            Refer to Rules 460 and 461 regarding requests for acceleration.  Please allow adequate
time for us to review any amendment prior to the requested effective date of the registration
statement.
            You may contact Keira Nakada at 202-551-3659 or Lisa Vanjoske at 202-551-3614 if
you have questions regarding comments on the financial statements and related matters.  Please
contact Julia Griffith at 202-551-3267 or Dietrich King at 202-551-8071 with any other
questions.
Sincerely,
Division of Corporation Finance

 FirstName LastNameSteven Nichtberger
 Comapany NameCabaletta Bio, Inc.
 August 28, 2019 Page 5
 FirstName LastName
Steven Nichtberger
Cabaletta Bio, Inc.
August 28, 2019
Page 5
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