Document:

EX-10.16

 Exhibit 10.16 

EMPLOYMENT AGREEMENT 

This Employment Agreement (“Agreement”) is made between Cabaletta Bio, Inc., a Delaware corporation (the “Company”), and
Steven Nichtberger, M.D. (the “Executive”) and is effective as of the closing of the Company’s first underwritten public offering of its equity securities pursuant to an effective registration statement under the Securities Act of
1933, as amended (the “Effective Date”). Subject to Section 11, except with respect to the Restrictive Covenant Agreement and the Equity Documents (each as defined below), this Agreement supersedes in all respects all prior agreements
between the Executive and the Company regarding the subject matter herein, including without limitation (i) the letter agreement between the Executive and the Company dated October 10, 2018, as amended on January 2, 2019 (the
“Prior Agreement” except as specifically preserved herein), and (ii) any offer letter, employment agreement or severance agreement. 

WHEREAS, the Company desires to continue to employ the Executive and the Executive desires to continue to be employed by the Company on the
new terms and conditions contained herein. 
 NOW, THEREFORE, in consideration of the mutual covenants and agreements herein
contained and other good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged, the parties agree as follows: 

1. Employment. 
 (a)
Term. The Company shall employ the Executive and the Executive shall be employed by the Company pursuant to this Agreement commencing as of the Effective Date and continuing until such employment is terminated in accordance with the
provisions hereof (the “Term”). The Executive’s employment with the Company will continue to be “at will,” meaning that the Executive’s employment may be terminated by the Company or the Executive at any time and for
any reason subject to the terms of this Agreement. 
 (b) Position and Duties. The Executive shall serve as the Chief Executive
Officer of the Company (the “CEO”) and President, and shall have such powers and duties as may from time to time be prescribed by the Board of Directors (the “Board”). In addition, the Company shall cause the Executive to be
nominated for election to the Board and to be recommended to the stockholders for election to the Board as long as the Executive remains the CEO, provided the Executive shall resign from the Board and from any related positions upon ceasing to serve
as CEO for any reason. The Executive shall devote substantially all of his working time and efforts to the business and affairs of the Company. Notwithstanding the foregoing, the Executive may continue to serve in the outside roles/positions
referenced in Section 1 of the Prior Agreement, subject to Section 12 of the Prior Agreement, and may serve on other boards of directors, with the approval of the Board, or engage in religious, charitable or other community activities as
long as such services and activities are disclosed to the Board and do not interfere with the Executive’s performance of the Executive’s duties to the Company. To the extent applicable, the Executive shall be deemed to have resigned
from all officer and board member positions that the Executive holds with the Company or any of its respective subsidiaries and affiliates upon the termination of the Executive’s employment for any reason. The Executive shall execute any
documents in reasonable form as may be requested to confirm or effectuate any such resignations. 

 2. Compensation and Related Matters. 

(a) Base Salary. The Executive’s initial base salary shall be paid at the rate of $497,000 per year. The Executive’s base
salary will increase to $520,200 effective March 1, 2020, subject to the Executive’s continued employment. The Executive’s base salary shall be subject to periodic review by the Board or the Compensation Committee of the Board
(the “Compensation Committee”). The base salary in effect at any given time is referred to herein as “Base Salary.” The Base Salary shall be payable in a manner that is consistent with the Company’s usual payroll practices
for executive officers. 
 (b) Incentive Compensation. The Executive shall be eligible to receive cash incentive compensation as
determined by the Board or the Compensation Committee from time to time. The Executive’s initial target annual incentive compensation shall be 50 percent of the Executive’s Base Salary; provided that any annual incentive compensation
awarded to the Executive with respect to performance in calendar year 2019 will be pro-rated based on (i) the target bonus in effect prior to the Effective Date for the period of time the Executive was
employed in calendar year 2019 prior to the Effective Date and (ii) the target bonus set forth in this Section 2(b) for the period of time the Executive is employed between the Effective Date and December 31, 2019. The target bonus in
effect at any given time is referred to herein as “Target Bonus.” The actual amount of the Executive’s annual incentive compensation, if any, shall be determined in the sole discretion of the Board or the Compensation Committee,
subject to the terms of any applicable incentive compensation plan that may be in effect from time to time. Except as otherwise provided herein, to earn incentive compensation, the Executive must be employed by the Company on the day such incentive
compensation is paid. 
 (c) Expenses. The Executive shall be entitled to receive prompt reimbursement for all reasonable expenses
incurred by the Executive during the Term in performing services hereunder, in accordance with the policies and procedures then in effect and established by the Company for its executive officers. 

(d) Other Benefits. The Executive shall be eligible to participate in or receive benefits under the Company’s employee benefit
plans in effect from time to time, subject to the terms of such plans. Consistent with the Prior Agreement (which, for the avoidance of doubt, is fully superseded), in lieu of the Executive receiving any Company-provided benefit for life insurance,
the Company will reimburse the Executive his personal existing life policy annually upon his prompt submission of receipts, in accordance with the Company’s expense reimbursement policy, as in effect from time to time; provided that if the
Executive and the Company agree that the Company will cease reimbursements of the Executive’s existing life policy, then the Executive shall be entitled to any Company-provided benefit for life insurance, subject to any eligibility requirements
that may apply. 

 (e) Paid Time Off. The Executive shall be eligible to accrue up to five
(5) weeks of paid vacation per calendar year in accordance with the Company’s vacation policy, as in effect from time to time. The Executive shall also be eligible for Company designated holidays. 

(f) Equity. The equity awards held by the Executive shall continue to be governed by the terms and conditions of the Company’s
applicable equity incentive plan(s) and the applicable award agreement(s) governing the terms of such equity awards held by the Executive (collectively, the “Equity Documents”); provided, however, and notwithstanding anything to the
contrary in the Equity Documents, Section 6(a)(ii) of this Agreement shall apply in the event of a termination by the Company without Cause or by the Executive for Good Reason in either event within the Change in Control Period (as such terms
are defined below). For the avoidance of doubt, nothing in this Agreement or otherwise shall be construed to affect or limit the Executive’s acceleration rights pursuant to Section 2(f) of the Amendment No 1 to the Founder Stock
Restriction Agreement dated May 4, 2018. 
 3. Termination. The Executive’s employment hereunder may be terminated without
any breach of this Agreement under the following circumstances: 
 (a) Death. The Executive’s employment hereunder shall
terminate upon death. 
 (b) Disability. The Company may terminate the Executive’s employment if the Executive is disabled and
unable to perform the essential functions of the Executive’s then existing position or positions under this Agreement with or without reasonable accommodation for a period of 180 days (which need not be consecutive) in any 12-month period. If any question shall arise as to whether during any period the Executive is disabled so as to be unable to perform the essential functions of the Executive’s then existing position or
positions with or without reasonable accommodation, the Executive may, and at the request of the Company shall, submit to the Company a certification in reasonable detail by a physician selected by the Company to whom the Executive or the
Executive’s guardian has no reasonable objection as to whether the Executive is so disabled or how long such disability is expected to continue, and such certification shall for the purposes of this Agreement be conclusive of the issue. The
Executive shall cooperate with any reasonable request of the physician in connection with such certification. If such question shall arise and the Executive shall fail to submit such certification, the Company’s determination of such issue
shall be binding on the Executive. Nothing in this Section 3(b) shall be construed to waive the Executive’s rights, if any, under existing law including, without limitation, the Family and Medical Leave Act of 1993, 29 U.S.C. §2601
et seq. and the Americans with Disabilities Act, 42 U.S.C. §12101 et seq. 

 (c) Termination by Company for Cause. The Company may terminate the Executive’s
employment hereunder for Cause. For purposes of this Agreement, “Cause” shall mean any of the following: 
 (i) conduct by
the Executive in connection with the Executive’s service to the Company that is fraudulent or which is grossly negligent; 
 (ii) the
commission by the Executive of acts satisfying the elements of (A) any felony or (B) a misdemeanor involving deceit, dishonesty or fraud related to the Executive’s employment; 

(iii) the Executive’s willful failure or refusal to comply with lawful directives of the Board which, if curable, is not cured by the
Executive within 30 days following the Executive’s receipt of written notice from the Company; 
 (iv) the Executive’s willful
material breach of a written Company policy or the Executive’s representations, warranties, covenants and/or obligations under this Agreement or the Restrictive Covenant Agreement; 

(v) materially unsatisfactory performance by the Executive which, if curable, is not cured by the Executive within 30 days following the
Executive’s receipt of written notice from the Company; 
 (vi) material misconduct by the Executive which discredits or damages the
Company or any of its affiliates; and/or 
 (vii) the Executive’s failure to cooperate with a bona fide internal investigation or an
investigation by regulatory or law enforcement authorities, after being instructed by the Company to cooperate, or the willful destruction or failure to preserve documents or other materials known to be relevant to such investigation or the
inducement of others to fail to cooperate or to produce documents or other materials in connection with such investigation. 
 (d)
Termination by the Company without Cause. The Company may terminate the Executive’s employment hereunder at any time without Cause. Any termination by the Company of the Executive’s employment under this Agreement which does not
constitute a termination for Cause under Section 3(c) and does not result from the death or disability of the Executive under Section 3(a) or (b) shall be deemed a termination without Cause. 

(e) Termination by the Executive. The Executive may terminate employment hereunder at any time for any reason, including but not limited
to, Good Reason. For purposes of this Agreement, “Good Reason” shall mean, consistent with the Prior Agreement (which, for the avoidance of doubt, is fully superseded): 

(i) that the Executive has completed all steps of the Good Reason Process (hereinafter defined) following the Executive’s first awareness
of the first occurrence of any of the following events without the Executive’s consent (each, a “Good Reason Condition”): 

(A) a material diminution in the Executive’s responsibilities, authority or duties; 

(B) a material diminution in the Executive’s Base Salary; 

 (C) a relocation of the Executive’s principal place of employment by
more than 50 miles from the current location; or 
 (D) a material breach of this Agreement by the Company; or 

(ii) within six (6) months following the appointment of a Chairperson of the Board pursuant to Section 1.7 of the
Amended and Restated Voting Agreement, dated as of January 2, 2019 by and among the Company and the other parties thereto (a “Chairperson Designation”), the Executive has provided written notice to the Board of the Executive’s
reasonable dissatisfaction with such Chairperson, and the Board fails to designate a new Chairperson of the Board within 30 days of receipt of such written notice. 

The “Good Reason Process” consists of the following steps: 

(i) the Executive reasonably determines in good faith that a Good Reason Condition has occurred; 

(ii) the Executive notifies the Company in writing of the first occurrence of the Good Reason Condition within 90 days of the
Executive’s first awareness of such condition; 
 (iii) the Executive cooperates in good faith with the Company’s
efforts, for a period of not less than 30 days following such notice (the “Cure Period”), to remedy the Good Reason Condition; 

(iv) notwithstanding such efforts, the Good Reason Condition continues to exist; and 

(v) the Executive terminates employment within 30 days after the end of the Cure Period. 

If the Company cures the Good Reason Condition during the Cure Period, Good Reason shall be deemed not to have occurred. 

If the Executive’s employment with the Company is terminated for any reason, the Company shall pay or provide to the Executive (or to the
Executive’s authorized representative or estate) (i) any Base Salary plus any accrued but unused vacation, earned through the Date of Termination; (ii) unpaid expense reimbursements (subject to, and in accordance with,
Section 2(c) of this Agreement); and (iii) any vested benefits the Executive may have under any employee benefit plan of the Company through the Date of Termination, which vested benefits shall be paid and/or provided in accordance with
the terms of such employee benefit plans (collectively, the “Accrued Obligations”). 
 4. Notice and Date of Termination.

 (a) Notice of Termination. Except for termination as specified in Section 3(a), any termination of the Executive’s
employment by the Company or any such termination by the Executive shall be communicated by written Notice of Termination to the other party hereto. For purposes of this Agreement, a “Notice of Termination” shall mean a notice which shall
indicate the specific termination provision in this Agreement relied upon. 

 (b) Date of Termination. “Date of Termination” shall mean: (i) if the
Executive’s employment is terminated by death, the date of death; (ii) if the Executive’s employment is terminated on account of disability under Section 3(b) or by the Company for Cause under Section 3(c), the date on which
Notice of Termination is given; (iii) if the Executive’s employment is terminated by the Company without Cause under Section 3(d), the date on which a Notice of Termination is given or the date otherwise specified by the Company in
the Notice of Termination; (iv) if the Executive’s employment is terminated by the Executive under Section 3(e) other than for Good Reason, 30 days after the date on which a Notice of Termination is given, and (v) if the
Executive’s employment is terminated by the Executive under Section 3(e) for Good Reason, the date on which a Notice of Termination is given after the end of the Cure Period. Notwithstanding the foregoing, in the event that the Executive
gives a Notice of Termination to the Company, the Company may unilaterally accelerate the Date of Termination and such acceleration shall not result in a termination by the Company for purposes of this Agreement. 

5. Severance Pay and Benefits Upon Termination by the Company without Cause or by the Executive for Good Reason Outside the Change in
Control Period. If the Executive’s employment is terminated by the Company without Cause as provided in Section 3(d), or the Executive terminates employment for Good Reason as provided in Section 3(e), each outside of the Change
in Control Period (as defined below), then, in addition to the Accrued Obligations, and subject to (i) the Executive signing a separation agreement and release in a form and manner satisfactory to the Company, which shall include,
without limitation, a general release of claims against the Company and all related persons and entities, a reaffirmation of all of the Executive’s Continuing Obligations (as defined below), and shall provide that if the Executive breaches any
of the Continuing Obligations, all payments of the Severance Amount shall immediately cease (the “Separation Agreement and Release”), and (ii) the Separation Agreement and Release becoming irrevocable, all within 60 days after the
Date of Termination (or such shorter period as set forth in the Separation Agreement and Release), which shall include a seven (7) day revocation period: 

(a) the Company shall pay the Executive an amount equal to 12 months of the Executive’s Base Salary (the “Severance
Amount”); and 
 (b) subject to the Executive’s copayment of premium amounts at the applicable active employees’ rate and the
Executive’s proper election to receive benefits under the Consolidated Omnibus Budget Reconciliation Act of 1985, as amended (“COBRA”), the Company shall pay to the group health plan provider, the COBRA provider or the
Executive a monthly payment equal to the monthly employer contribution that the Company would have made to provide health insurance to the Executive if the Executive had remained employed by the Company until the earliest of (A) the 12 month
anniversary of the Date of Termination; (B) the Executive’s eligibility for group medical plan benefits under any other employer’s group medical plan; or (C) the cessation of the Executive’s continuation rights under COBRA;
provided, however, if the Company determines that it cannot pay such amounts to the group 

 
health plan provider or the COBRA provider (if applicable) without potentially violating applicable law (including, without limitation, Section 2716 of the Public Health Service Act), then
the Company shall convert such payments to payroll payments directly to the Executive for the time period specified above. Such payments shall be subject to tax-related deductions and withholdings and paid on
the Company’s regular payroll dates. 
 The amounts payable under Section 5, to the extent taxable, shall be paid out in substantially equal
installments in accordance with the Company’s payroll practice over 12 months commencing within 60 days after the Date of Termination; provided, however, that if the 60-day period begins in one
calendar year and ends in a second calendar year, the Severance Amount, to the extent it qualifies as “non-qualified deferred compensation” within the meaning of Section 409A of the Internal
Revenue Code of 1986, as amended (the “Code”), shall begin to be paid in the second calendar year by the last day of such 60-day period; provided, further, that the initial payment shall include a catch-up payment to cover amounts retroactive to the day immediately following the Date of Termination. Each payment pursuant to this Agreement is intended to constitute a separate payment for purposes of Treasury
Regulation Section 1.409A-2(b)(2). 
 Notwithstanding anything to the contrary herein, in the event that the
Executive resigns his employment for Good Reason within six (6) months following a Chairperson Designation and such Date of Termination is outside of the Change in Control Period, and provided that the Executive is eligible for the benefits set
forth in this Section 5 and the Separation Agreement and Release has become fully effective within the timeframe set forth therein and in no event more than 60 days after the Date of Termination, then in addition to the severance benefits
described in this Section 5, all stock options and other stock-based awards held by the Executive, including without limitation all equity awards subject to milestone vesting (the “Equity Awards”) shall immediately accelerate and
become fully exercisable or nonforfeitable as of the later of (i) the Date of Termination or (ii) the effective date of the Separation Agreement and Release (the “Accelerated Vesting Date”); provided that any termination
or forfeiture of the unvested portion of such Equity Awards that would otherwise occur on the Date of Termination in the absence of this Agreement will be delayed until the effective date of the Separation Agreement and Release and will only occur
if the vesting pursuant to this subsection does not occur due to the absence of the Separation Agreement and Release becoming fully effective within the time period set forth therein. Notwithstanding the foregoing, no additional vesting of any
time-based Equity Awards shall occur during the period between the Executive’s Date of Termination and the Accelerated Vesting Date. 

6. Severance Pay and Benefits Upon Termination by the Company without Cause or by the Executive for Good Reason within the Change in Control
Period. The provisions of this Section 6 shall apply in lieu of, and expressly supersede, the provisions of Section 5 if (i) the Executive’s employment is terminated either (a) by the Company without Cause as provided in
Section 3(d), or (b) by the Executive for Good Reason as provided in Section 3(e), and (ii) the Date of Termination is within the 60 days before or 12 months after the occurrence of the first event constituting a Change in
Control (such period, the “Change in Control Period”). These provisions shall terminate and be of no further force or effect after a Change in Control Period. 

 (a) If the Executive’s employment is terminated by the Company without Cause as
provided in Section 3(d) or the Executive terminates employment for Good Reason as provided in Section 3(e) and in each case the Date of Termination occurs during the Change in Control Period, then, in addition to the Accrued Obligations,
and subject to the signing of the Separation Agreement and Release by the Executive and the Separation Agreement and Release becoming fully effective, all within the time frame set forth in the Separation Agreement and Release but in no event more
than 60 days after the Date of Termination: 
 (i) the Company shall pay the Executive in an amount equal to 1.5 times
the sum of (A) the Executive’s then current Base Salary (or the Executive’s Base Salary in effect immediately prior to the Change in Control, if higher) plus (B) the Executive’s Target Bonus for the then-current year; and

 (ii) notwithstanding anything to the contrary in any applicable option agreement or other stock-based award agreement,
Equity Awards shall immediately accelerate and become fully exercisable or nonforfeitable as of the later of (i) the Date of Termination or (ii) the Accelerated Vesting Date; provided that any termination or forfeiture of the
unvested portion of such Equity Awards that would otherwise occur on the Date of Termination in the absence of this Agreement will be delayed until the effective date of the Separation Agreement and Release and will only occur if the vesting
pursuant to this subsection does not occur due to the absence of the Separation Agreement and Release becoming fully effective within the time period set forth therein. Notwithstanding the foregoing, no additional vesting of any time-based Equity
Awards shall occur during the period between the Executive’s Date of Termination and the Accelerated Vesting Date; and 

(iii) subject to the Executive’s copayment of premium amounts at the applicable active employees’ rate and the
Executive’s proper election to receive benefits under COBRA, the Company shall pay to the group health plan provider, the COBRA provider or the Executive a monthly payment equal to the monthly employer contribution that the Company would have
made to provide health insurance to the Executive if the Executive had remained employed by the Company until the earliest of (A) the 18 month anniversary of the Date of Termination; (B) the Executive’s eligibility for group medical
plan benefits under any other employer’s group medical plan; or (C) the cessation of the Executive’s continuation rights under COBRA; provided, however, if the Company determines that it cannot pay such amounts to the group health
plan provider or the COBRA provider (if applicable) without potentially violating applicable law (including, without limitation, Section 2716 of the Public Health Service Act), then the Company shall convert such payments to payroll payments
directly to the Executive for the time period specified above. Such payments shall be subject to tax-related deductions and withholdings and paid on the Company’s regular payroll dates. 

The amounts payable under Section 5, to the extent taxable, shall be paid out in substantially equal installments in accordance with the Company’s
payroll practice over 18 months commencing within 60 days after the Date of Termination, however, that if the 60-day period begins in one calendar year and ends in a second calendar year, such payments
to the extent they qualify as “non-qualified deferred compensation” within the meaning of Section 409A of the Code, shall commence to be paid in the second calendar year by the last day of such 60-day period. For the avoidance of doubt, in the event that the Executive’s Date of Termination is 

 
within the 60 days before a Change in Control and the Executive has signed the Separation Agreement and Release that has become irrevocable and is entitled to the benefits under Section 5 of
this Agreement, then the Executive will receive the benefits set forth in this Section 6 following the occurrence of a Change in Control; provided that the amount to be paid to the Executive pursuant to Section 6(a)(i) will be decreased by
any benefits previously paid to the Executive pursuant to Section 5, and the Executive will receive no further benefits pursuant to Section 5. In no event may there be duplication of benefits under Section 5 and Section 6. 

(b) Additional Limitation. 

(i) Anything in this Agreement to the contrary notwithstanding, in the event that the amount of any compensation, payment or
distribution by the Company to or for the benefit of the Executive, whether paid or payable or distributed or distributable pursuant to the terms of this Agreement or otherwise, calculated in a manner consistent with Section 280G of the Code,
and the applicable regulations thereunder (the “Aggregate Payments”), would be subject to the excise tax imposed by Section 4999 of the Code, then the Aggregate Payments shall be reduced (but not below zero) so that the sum of all of
the Aggregate Payments shall be $1.00 less than the amount at which the Executive becomes subject to the excise tax imposed by Section 4999 of the Code; provided that such reduction shall only occur if it would result in the Executive receiving
a higher After Tax Amount (as defined below) than the Executive would receive if the Aggregate Payments were not subject to such reduction. In such event, the Aggregate Payments shall be reduced in the following order, in each case, in reverse
chronological order beginning with the Aggregate Payments that are to be paid the furthest in time from consummation of the transaction that is subject to Section 280G of the Code: (1) cash payments not subject to Section 409A of the
Code; (2) cash payments subject to Section 409A of the Code; (3) equity-based payments and acceleration; and (4) non-cash forms of benefits; provided that in the case of all the foregoing
Aggregate Payments all amounts or payments that are not subject to calculation under Treas. Reg. §1.280G-1, Q&A-24(b) or (c) shall be reduced before any
amounts that are subject to calculation under Treas. Reg. §1.280G-1, Q&A-24(b) or (c). 

(ii) For purposes of this Section 6(b), the “After Tax Amount” means the amount of the Aggregate Payments less
all federal, state, and local income, excise and employment taxes imposed on the Executive as a result of the Executive’s receipt of the Aggregate Payments. For purposes of determining the After Tax Amount, the Executive shall be deemed to pay
federal income taxes at the highest marginal rate of federal income taxation applicable to individuals for the calendar year in which the determination is to be made, and state and local income taxes at the highest marginal rates of individual
taxation in each applicable state and locality, net of the maximum reduction in federal income taxes which could be obtained from deduction of such state and local taxes. 

(iii) The determination as to whether a reduction in the Aggregate Payments shall be made pursuant to Section 6(b)(i)
shall be made by a nationally recognized accounting firm selected by the Company (the “Accounting Firm”), which shall provide detailed supporting calculations both to the Company and the Executive within 15 business days of the Date of
Termination, if applicable, or at such earlier time as is reasonably requested by the Company or the Executive. Any determination by the Accounting Firm shall be binding upon the Company and the Executive. 

 (c) Definitions. For purposes of this Section 6, a “Change in Control”
shall mean a “Sale Event” as defined in the Cabaletta Bio, Inc. 2019 Stock Option and Incentive Plan, as may be amended from time to time, but only to the extent such Sale Event is also a “change in control event” within the
meaning of Section 409A of the Code and the regulations promulgated thereunder. 
 7. Section 409A. 

(a) Anything in this Agreement to the contrary notwithstanding, if at the time of the Executive’s separation from service within the
meaning of Section 409A of the Code, the Company determines that the Executive is a “specified employee” within the meaning of Section 409A(a)(2)(B)(i) of the Code, then to the extent any payment or benefit that the Executive
becomes entitled to under this Agreement or otherwise on account of the Executive’s separation from service would be considered deferred compensation otherwise subject to the 20 percent additional tax imposed pursuant to
Section 409A(a) of the Code as a result of the application of Section 409A(a)(2)(B)(i) of the Code, such payment shall not be payable and such benefit shall not be provided until the date that is the earlier of (A) six months and one
day after the Executive’s separation from service, or (B) the Executive’s death. If any such delayed cash payment is otherwise payable on an installment basis, the first payment shall include a
catch-up payment covering amounts that would otherwise have been paid during the six-month period but for the application of this provision, and the balance of the
installments shall be payable in accordance with their original schedule. 
 (b) All in-kind benefits
provided and expenses eligible for reimbursement under this Agreement shall be provided by the Company or incurred by the Executive during the time periods set forth in this Agreement. All reimbursements shall be paid as soon as administratively
practicable, but in no event shall any reimbursement be paid after the last day of the taxable year following the taxable year in which the expense was incurred. The amount of in-kind benefits provided or
reimbursable expenses incurred in one taxable year shall not affect the in-kind benefits to be provided or the expenses eligible for reimbursement in any other taxable year (except for any lifetime or other
aggregate limitation applicable to medical expenses). Such right to reimbursement or in-kind benefits is not subject to liquidation or exchange for another benefit. 

(c) To the extent that any payment or benefit described in this Agreement constitutes
“non-qualified deferred compensation” under Section 409A of the Code, and to the extent that such payment or benefit is payable upon the Executive’s termination of employment, then such
payments or benefits shall be payable only upon the Executive’s “separation from service.” The determination of whether and when a separation from service has occurred shall be made in accordance with the presumptions set forth in
Treasury Regulation Section 1.409A-1(h). 

 (d) The parties intend that this Agreement will be administered in accordance with
Section 409A of the Code. To the extent that any provision of this Agreement is ambiguous as to its compliance with Section 409A of the Code, the provision shall be read in such a manner so that all payments hereunder comply with
Section 409A of the Code. Each payment pursuant to this Agreement or the Restrictive Covenant Agreement is intended to constitute a separate payment for purposes of Treasury Regulation
Section 1.409A-2(b)(2). The parties agree that this Agreement may be amended, as reasonably requested by either party, and as may be necessary to fully comply with Section 409A of the Code and all
related rules and regulations in order to preserve the payments and benefits provided hereunder without additional cost to either party. 

(e) The Company makes no representation or warranty and shall have no liability to the Executive or any other person if any provisions of this
Agreement are determined to constitute deferred compensation subject to Section 409A of the Code but do not satisfy an exemption from, or the conditions of, such Section. 

8. Continuing Obligations. 

(a) Restrictive Covenant Agreement. The terms of the Employee Restrictive Covenant Agreement, dated October 12, 2018 (the
“Restrictive Covenant Agreement”), between the Company and the Executive, attached hereto as Exhibit A, continue to be in full force and effect and are incorporated by reference as material terms of this Agreement provided
Section 8(b) of the Restrictive Covenant Agreement shall be modified as follows: “the Company will consider in good faith any request I may make to work with a Company vendor during the Restricted Period and will not unreasonably deny such
a request” shall be deleted and replaced with “this Section 8(b)(ii) shall only apply in the event my work with any such vendor during the Restricted Period is reasonably expected to have an adverse effect on the Company”. For
purposes of this Agreement, the obligations in this Section 8 and those that arise in the Restrictive Covenant Agreement, Sections 7 through 12, 14 and 15 of the Founders Agreement dated August 22, 2017, as amended by Amendment No. 1
to Founders Agreement dated May 4, 2018 (the “Preserved Provisions”) and any other agreement relating to confidentiality, assignment of inventions, or other restrictive covenants shall collectively be referred to as the
“Continuing Obligations.” 
 (b) Third-Party Agreements and Rights. The Executive hereby confirms that the Executive is not
bound by the terms of any agreement with any previous employer or other party which restricts in any way the Executive’s use or disclosure of information, other than confidentiality restrictions (if any), or the Executive’s engagement in
any business. The Executive represents to the Company that the Executive’s execution of this Agreement, the Executive’s employment with the Company and the performance of the Executive’s proposed duties for the Company will not
violate any obligations the Executive may have to any such previous employer or other party. In the Executive’s work for the Company, the Executive will not disclose or make use of any information in violation of any agreements with or rights
of any such previous employer or other party, and the Executive will not bring to the premises of the Company any copies or other tangible embodiments of non-public information belonging to or obtained from
any such previous employment or other party. 

 (c) Litigation and Regulatory Cooperation. During and after the Executive’s
employment, the Executive shall cooperate fully with the Company in (i) the defense or prosecution of any claims or actions now in existence or which may be brought in the future against or on behalf of the Company which relate to events or
occurrences that transpired while the Executive was employed by the Company, and (ii) the investigation, whether internal or external, of any matters about which the Company believes the Executive may have knowledge or information. The
Executive’s full cooperation in connection with such claims, actions or investigations shall include, but not be limited to, being available to meet with counsel to answer questions or to prepare for discovery or trial and to act as a witness
on behalf of the Company at mutually convenient times. During and after the Executive’s employment, the Executive also shall cooperate fully with the Company in connection with any investigation or review of any federal, state or local
regulatory authority as any such investigation or review relates to events or occurrences that transpired while the Executive was employed by the Company. The Company shall reimburse the Executive for any reasonable out-of-pocket expenses incurred in connection with the Executive’s performance of obligations pursuant to this Section 8(c). For the avoidance of doubt, this Section 8(c) shall not apply in the
event the Executive is pursuing claims or considering potential claims against the Company. 
 (d) Relief. The Executive agrees that
it would be difficult to measure any damages caused to the Company which might result from any breach by the Executive of the Continuing Obligations, and that in any event money damages would be an inadequate remedy for any such breach.
Accordingly, the Executive agrees that if the Executive breaches, or proposes to breach, any portion of the Continuing Obligations, the Company shall be entitled, in addition to all other remedies that it may have, to an injunction or other
appropriate equitable relief to restrain any such breach without showing or proving any actual damage to the Company. 
 (e) Protected
Disclosures and Other Protected Action. Nothing in this Agreement shall be interpreted or applied to prohibit the Executive from making any good faith report to any governmental agency or other governmental entity (a “Government
Agency”) concerning any act or omission that the Executive reasonably believes constitutes a possible violation of federal or state law or making other disclosures that are protected under the anti-retaliation or whistleblower provisions of
applicable federal or state law or regulation. In addition, nothing contained in this Agreement limits the Executive’s ability to communicate with any Government Agency or otherwise participate in any investigation or proceeding that may be
conducted by any Government Agency, including the Executive’s ability to provide documents or other information, without notice to the Company. In addition, for the avoidance of doubt, pursuant to the federal Defend Trade Secrets Act of 2016,
the Executive shall not be held criminally or civilly liable under any federal or state trade secret law or under this Agreement or the Restrictive Covenant Agreement for the disclosure of a trade secret that (a) is made (i) in confidence
to a federal, state, or local government official, either directly or indirectly, or to an attorney; and (ii) solely for the purpose of reporting or investigating a suspected violation of law; or (b) is made in a complaint or other
document filed in a lawsuit or other proceeding, if such filing is made under seal. 
 9. Conflicts. The Company acknowledges that as
of the Effective Date, the Executive is a part time faculty member of the University of Pennsylvania (the “University”). The Company further acknowledges that as of the Effective Date, the Executive’s activities may be subject to the
Staff Policies and regulations of the University, including but not limited to the University’s Conflict-of-Interest and Consulting Policies (the “Applicable
Policies”). In the event 

 
that the performance of the Executive’s duties as Chief Executive Officer of the Company will result in a knowing violation of the Applicable Policies, the Executive will promptly notify the
Company of such potential conflict and may (i) terminate this Agreement immediately (which shall constitute a resignation other than for Good Reason), or (ii) together with the Company, take such actions as are reasonably necessary to
resolve such potential conflict prior to performance of duties, and if thereafter the Executive reasonably believes that a conflict with the Applicable Policies would result from performance of such duties, the Executive may terminate this Agreement
immediately (which shall constitute a resignation other than for Good Reason). 
 10. Consent to Jurisdiction. The parties hereby
consent to the jurisdiction of the state and federal courts of the Commonwealth of Pennsylvania. Accordingly, with respect to any such court action, the Executive (a) submits to the personal jurisdiction of such courts; (b) consents to
service of process; and (c) waives any other requirement (whether imposed by statute, rule of court, or otherwise) with respect to personal jurisdiction or service of process. 

11. Integration. This Agreement constitutes the entire agreement between the parties with respect to the subject matter hereof and
supersedes all prior agreements between the parties concerning such subject matter, including the Prior Agreement, provided that the Restrictive Covenant Agreement, the Preserved Provisions and the Equity Documents (including, without limitation,
the Founder Stock Restriction Agreement dated August 22, 2017, as amended by Amendment No. 1 to Founder Stock Restriction Agreement dated May 4, 2018 (the “Founder Stock Restriction Agreement”)) remain in full force and
effect. To the extent that there is any conflict between this Agreement and the Founder Stock Restriction Agreement as amended, the Founder Stock Restriction Agreement shall govern. 

12. Withholding; Tax Effect. All payments made by the Company to the Executive under this Agreement shall be net of any tax or other
amounts required to be withheld by the Company under applicable law. Nothing in this Agreement shall be construed to require the Company to make any payments to compensate the Executive for any adverse tax effect associated with any payments or
benefits or for any deduction or withholding from any payment or benefit. 
 13. Assignment. Neither the Executive nor the Company may
make any assignment of this Agreement or any interest in it, by operation of law or otherwise, without the prior written consent of the other; provided, however, that the Company may assign its rights and obligations under this Agreement (including
the Restrictive Covenant Agreement) without the Executive’s consent to any affiliate or to any person or entity with whom the Company shall hereafter effect a reorganization, consolidate with, or merge into or to whom it transfers all or
substantially all of its properties or assets; provided further that if the Executive remains employed or becomes employed by the Company, the purchaser or any of their affiliates in connection with any such transaction, then the Executive shall not
be entitled to any payments, benefits or vesting pursuant to Section 5 or pursuant to Section 6 of this Agreement. This Agreement shall inure to the benefit of and be binding upon the Executive and the Company, and each of the
Executive’s and the Company’s respective successors, executors, administrators, heirs and permitted assigns. 

 14. Enforceability. If any portion or provision of this Agreement (including, without
limitation, any portion or provision of any section of this Agreement) shall to any extent be declared illegal or unenforceable by a court of competent jurisdiction, then the remainder of this Agreement, or the application of such portion or
provision in circumstances other than those as to which it is so declared illegal or unenforceable, shall not be affected thereby, and each portion and provision of this Agreement shall be valid and enforceable to the fullest extent permitted by
law. 
 15. Survival. The provisions of this Agreement shall survive the termination of this Agreement and/or the termination of the
Executive’s employment to the extent necessary to effectuate the terms contained herein. 
 16. Waiver. No waiver of any
provision hereof shall be effective unless made in writing and signed by the waiving party. The failure of any party to require the performance of any term or obligation of this Agreement, or the waiver by any party of any breach of this Agreement,
shall not prevent any subsequent enforcement of such term or obligation or be deemed a waiver of any subsequent breach. 
 17.
Notices. Any notices, requests, demands and other communications provided for by this Agreement shall be sufficient if in writing and delivered in person or sent by a nationally recognized overnight courier service or by registered or
certified mail, postage prepaid, return receipt requested, to the Executive at the last address the Executive has filed in writing with the Company or, in the case of the Company, at its main offices, attention of the Board. 

18. Amendment. This Agreement may be amended or modified only by a written instrument signed by the Executive and by a duly authorized
representative of the Company. 
 19. Effect on Other Plans and Agreements. An election by the Executive to resign for Good Reason
under the provisions of this Agreement shall not be deemed a voluntary termination of employment by the Executive for the purpose of interpreting the provisions of any of the Company’s benefit plans, programs or policies. Nothing in this
Agreement shall be construed to limit the rights of the Executive under the Company’s benefit plans, programs or policies except as otherwise provided in Section 8 hereof, and except that the Executive shall have no rights to any severance
benefits under any Company severance pay plan, offer letter or otherwise. In the event that the Executive is party to an agreement with the Company providing for payments or benefits under such plan or agreement and under this Agreement, the terms
of this Agreement shall govern and the Executive may receive payment under this Agreement only and not both. Further, Section 5 and Section 6 of this Agreement are mutually exclusive and in no event shall the Executive be entitled to
payments or benefits pursuant to both Section 5 and Section 6 of this Agreement. 
 20. Governing Law. This is a
Pennsylvania contract and shall be construed under and be governed in all respects by the laws of the Commonwealth of Pennsylvania without giving effect to the conflict of laws principles thereof. With respect to any disputes concerning federal law,
such disputes shall be determined in accordance with the law as it would be interpreted and applied by the United States Court of Appeals for the Third Circuit. 

 21. Counterparts. This Agreement may be executed in any number of counterparts, each
of which when so executed and delivered shall be taken to be an original; but such counterparts shall together constitute one and the same document. 

IN WITNESS WHEREOF, the parties have executed this Agreement effective on the Effective Date. 

 

			
	CABALETTA BIO, INC.
		
	By:	 	 /s/ Brian Daniels

	Its:	 	 Director

	
	EXECUTIVE
	
	 /s/ Steven Nichtberger

	Steven Nichtberger, M.D.

 Exhibit A 

Employee Restrictive Covenant AgreementEX-10.17

 Exhibit 10.17 

EMPLOYMENT AGREEMENT 

This Employment Agreement (“Agreement”) is made between Cabaletta Bio, Inc., a Delaware corporation (the “Company”), and
Anup Marda (the “Executive”) and is effective as of the closing of the Company’s first underwritten public offering of its equity securities pursuant to an effective registration statement under the Securities Act of 1933, as amended
(the “Effective Date”). Except with respect to the Restrictive Covenant Agreement and the Equity Documents (each as defined below), this Agreement supersedes in all respects all prior agreements between the Executive and the Company
regarding the subject matter herein, including without limitation (i) the letter agreement between the Executive and the Company dated January 4, 2019 (the “Prior Agreement”), and (ii) any offer letter, employment agreement
or severance agreement. 
 WHEREAS, the Company desires to continue to employ the Executive and the Executive desires to continue to be
employed by the Company on the new terms and conditions contained herein. 
 NOW, THEREFORE, in consideration of the mutual covenants
and agreements herein contained and other good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged, the parties agree as follows: 

1. Employment. 
 (a)
Term. The Company shall employ the Executive and the Executive shall be employed by the Company pursuant to this Agreement commencing as of the Effective Date and continuing until such employment is terminated in accordance with the
provisions hereof (the “Term”). The Executive’s employment with the Company will continue to be “at will,” meaning that the Executive’s employment may be terminated by the Company or the Executive at any time and for
any reason subject to the terms of this Agreement. 
 (b) Position and Duties. The Executive shall serve as the Chief Financial
Officer of the Company and shall have such powers and duties as may from time to time be prescribed by the Chief Executive Officer (the “CEO”) or other duly authorized executive. The Executive shall devote the Executive’s full working
time and efforts to the business and affairs of the Company. Notwithstanding the foregoing, the Executive may serve on other boards of directors, with the approval of the Board of Directors of the Company (the “Board”), or engage in
religious, charitable or other community activities as long as such services and activities are disclosed to the Board and do not interfere with the Executive’s performance of the Executive’s duties to the Company. To the extent
applicable, the Executive shall be deemed to have resigned from all officer and board member positions that the Executive holds with the Company or any of its respective subsidiaries and affiliates upon the termination of the Executive’s
employment for any reason. The Executive shall execute any documents in reasonable form as may be requested to confirm or effectuate any such resignations. 

2. Compensation and Related Matters. 

(a) Base Salary. The Executive’s initial base salary shall be paid at the rate of $366,900 per year. The Executive’s base
salary will increase to $381,900 effective March 1, 2020, subject to the Executive’s continued employment. The Executive’s base salary shall be subject to periodic review by the Board or the Compensation Committee of the Board
(the “Compensation Committee”). The base salary in effect at any given time is referred to herein as “Base Salary.” The Base Salary shall be payable in a manner that is consistent with the Company’s usual payroll practices
for executive officers. 

 (b) Incentive Compensation. The Executive shall be eligible to receive cash incentive
compensation as determined by the Board or the Compensation Committee from time to time. The Executive’s initial target annual incentive compensation shall be 40 percent of the Executive’s Base Salary; provided that any annual
incentive compensation awarded to the Executive with respect to performance in calendar year 2019 will be pro-rated based on (i) the target bonus in effect prior to the Effective Date for the period of
time the Executive was employed in calendar year 2019 prior to the Effective Date and (ii) the target bonus set forth in this Section 2(b) for the period of time the Executive is employed between the Effective Date and December 31,
2019. The target bonus in effect at any given time is referred to herein as “Target Bonus.” The actual amount of the Executive’s annual incentive compensation, if any, shall be determined in the sole discretion of the Board or the
Compensation Committee, subject to the terms of any applicable incentive compensation plan that may be in effect from time to time. Except as otherwise provided herein, to earn incentive compensation, the Executive must be employed by the Company on
the day such incentive compensation is paid. In addition, if the Executive is employed by the Company on January 22, 2020, then the Company will pay the Executive on its next regular payroll date a
one-time bonus in the amount of $60,000 (the “One-Year Anniversary Bonus”), less applicable deductions and withholdings; provided that if the Executive
terminates his employment other than for Good Reason (as defined below) within the three (3) month period following January 22, 2020, the Executive shall repay the One-Year Anniversary Bonus within
10 days of the Date of Termination (as defined below). 
 (c) Expenses. The Executive shall be entitled to receive prompt
reimbursement for all reasonable expenses incurred by the Executive during the Term in performing services hereunder, in accordance with the policies and procedures then in effect and established by the Company for its executive officers. 

(d) Other Benefits. The Executive shall be eligible to participate in or receive benefits under the Company’s employee benefit
plans in effect from time to time, subject to the terms of such plans. 
 (e) Paid Time Off. The Executive shall be eligible to accrue
up to four (4) weeks of paid vacation per calendar year in accordance with the Company’s vacation policy, as in effect from time to time. The Executive shall also be eligible for Company designated holidays. 

(f) Equity. The equity awards held by the Executive shall continue to be governed by the terms and conditions of the Company’s
applicable equity incentive plan(s) and the applicable award agreement(s) governing the terms of such equity awards held by the Executive (collectively, the “Equity Documents”); provided, however, and notwithstanding anything to the
contrary in the Equity Documents, Section 6(a)(ii) of this Agreement shall apply in the event of a termination by the Company without Cause or by the Executive for Good Reason in either event within the Change in Control Period (as such terms
are defined below). 

 3. Termination. The Executive’s employment hereunder may be terminated without
any breach of this Agreement under the following circumstances: 
 (a) Death. The Executive’s employment hereunder shall
terminate upon death. 
 (b) Disability. The Company may terminate the Executive’s employment if the Executive is disabled and
unable to perform the essential functions of the Executive’s then existing position or positions under this Agreement with or without reasonable accommodation for a period of 180 days (which need not be consecutive) in any 12-month period. If any question shall arise as to whether during any period the Executive is disabled so as to be unable to perform the essential functions of the Executive’s then existing position or
positions with or without reasonable accommodation, the Executive may, and at the request of the Company shall, submit to the Company a certification in reasonable detail by a physician selected by the Company to whom the Executive or the
Executive’s guardian has no reasonable objection as to whether the Executive is so disabled or how long such disability is expected to continue, and such certification shall for the purposes of this Agreement be conclusive of the issue. The
Executive shall cooperate with any reasonable request of the physician in connection with such certification. If such question shall arise and the Executive shall fail to submit such certification, the Company’s determination of such issue
shall be binding on the Executive. Nothing in this Section 3(b) shall be construed to waive the Executive’s rights, if any, under existing law including, without limitation, the Family and Medical Leave Act of 1993, 29 U.S.C. §2601
et seq. and the Americans with Disabilities Act, 42 U.S.C. §12101 et seq. 
 (c) Termination by Company for Cause.
The Company may terminate the Executive’s employment hereunder for Cause. For purposes of this Agreement, “Cause” shall mean any of the following: 

(i) conduct by the Executive in connection with the Executive’s service to the Company that is fraudulent or which is grossly negligent;

 (ii) the commission by the Executive of acts satisfying the elements of (A) any felony or (B) a misdemeanor involving deceit,
dishonesty or fraud related to the Executive’s employment; 
 (iii) the Executive’s willful failure or refusal to comply with
lawful directives of the CEO or the Board which, if curable, is not cured by the Executive within 30 days following the Executive’s receipt of written notice from the Company; 

(iv) the Executive’s willful material breach of a written Company policy or the Executive’s representations, warranties, covenants
and/or obligations under this Agreement or the Restrictive Covenant Agreement; 
 (v) materially unsatisfactory performance by the Executive
which, if curable, is not cured by the Executive within 30 days following the Executive’s receipt of written notice from the Company; 

 (vi) material misconduct by the Executive which discredits or damages the Company or any of
its affiliates; and/or 
 (vii) the Executive’s failure to cooperate with a bona fide internal investigation or an investigation by
regulatory or law enforcement authorities, after being instructed by the Company to cooperate, or the willful destruction or failure to preserve documents or other materials known to be relevant to such investigation or the inducement of others to
fail to cooperate or to produce documents or other materials in connection with such investigation. 
 (d) Termination by the Company
without Cause. The Company may terminate the Executive’s employment hereunder at any time without Cause. Any termination by the Company of the Executive’s employment under this Agreement which does not constitute a termination for
Cause under Section 3(c) and does not result from the death or disability of the Executive under Section 3(a) or (b) shall be deemed a termination without Cause. 

(e) Termination by the Executive. The Executive may terminate employment hereunder at any time for any reason, including but not limited
to, Good Reason. For purposes of this Agreement, “Good Reason” shall mean that the Executive has completed all steps of the Good Reason Process (hereinafter defined) following the Executive’s first occurrence of any of the
following events without the Executive’s consent (each, a “Good Reason Condition”): 
 (i) a material
diminution in the Executive’s responsibilities, authority or duties, in each case within the 12 months following a Change in Control; 

(ii) a material diminution in the Executive’s Base Salary; 

(iii) a relocation of the Executive’s principal place of employment by more than 50 miles from the current location; or

 (iv) a material breach of this Agreement by the Company. 

The “Good Reason Process” consists of the following steps: 

(i) the Executive reasonably determines in good faith that a Good Reason Condition has occurred; 

(ii) the Executive notifies the Company in writing of the first occurrence of the Good Reason Condition within 90 days of such
condition; 
 (iii) the Executive cooperates in good faith with the Company’s efforts, for a period of not less than 30
days following such notice (the “Cure Period”), to remedy the Good Reason Condition; 
 (iv) notwithstanding
such efforts, the Good Reason Condition continues to exist; and 

 (v) the Executive terminates employment within 30 days after the end of the
Cure Period. 
 If the Company cures the Good Reason Condition during the Cure Period, Good Reason shall be deemed not to have occurred. 

If the Executive’s employment with the Company is terminated for any reason, the Company shall pay or provide to the Executive (or to the
Executive’s authorized representative or estate) (i) any Base Salary plus any accrued but unused vacation, earned through the Date of Termination; (ii) unpaid expense reimbursements (subject to, and in accordance with,
Section 2(c) of this Agreement); and (iii) any vested benefits the Executive may have under any employee benefit plan of the Company through the Date of Termination, which vested benefits shall be paid and/or provided in accordance with
the terms of such employee benefit plans (collectively, the “Accrued Obligations”). 
 4. Notice and Date of Termination.

 (a) Notice of Termination. Except for termination as specified in Section 3(a), any termination of the Executive’s
employment by the Company or any such termination by the Executive shall be communicated by written Notice of Termination to the other party hereto. For purposes of this Agreement, a “Notice of Termination” shall mean a notice which shall
indicate the specific termination provision in this Agreement relied upon. 
 (b) Date of Termination. “Date of Termination”
shall mean: (i) if the Executive’s employment is terminated by death, the date of death; (ii) if the Executive’s employment is terminated on account of disability under Section 3(b) or by the Company for Cause under
Section 3(c), the date on which Notice of Termination is given; (iii) if the Executive’s employment is terminated by the Company without Cause under Section 3(d), the date on which a Notice of Termination is given or the date
otherwise specified by the Company in the Notice of Termination; (iv) if the Executive’s employment is terminated by the Executive under Section 3(e) other than for Good Reason, 30 days after the date on which a Notice of Termination
is given, and (v) if the Executive’s employment is terminated by the Executive under Section 3(e) for Good Reason, the date on which a Notice of Termination is given after the end of the Cure Period. Notwithstanding the foregoing, in
the event that the Executive gives a Notice of Termination to the Company, the Company may unilaterally accelerate the Date of Termination and such acceleration shall not result in a termination by the Company for purposes of this Agreement. 

5. Severance Pay and Benefits Upon Termination by the Company without Cause or by the Executive for Good Reason Outside the Change in
Control Period. If the Executive’s employment is terminated by the Company without Cause as provided in Section 3(d), or the Executive terminates employment for Good Reason as provided in Section 3(e), each outside of the Change
in Control Period (as defined below), then, in addition to the Accrued Obligations, and subject to (i) the Executive signing a separation agreement and release in a form and manner satisfactory to the Company, which shall include,
without limitation, a general release of claims against the Company and all related persons and entities, a reaffirmation of all of the Executive’s Continuing Obligations (as defined below), and shall provide that if the Executive breaches any

 
of the Continuing Obligations, all payments of the Severance Amount shall immediately cease (the “Separation Agreement and Release”), and (ii) the Separation Agreement and Release
becoming irrevocable, all within 60 days after the Date of Termination (or such shorter period as set forth in the Separation Agreement and Release), which shall include a seven (7) day revocation period: 

(a) the Company shall pay the Executive an amount equal to 9 (nine) months of the Executive’s Base Salary (the “Severance
Amount”); and 
 (b) subject to the Executive’s copayment of premium amounts at the applicable active employees’ rate and the
Executive’s proper election to receive benefits under the Consolidated Omnibus Budget Reconciliation Act of 1985, as amended (“COBRA”), the Company shall pay to the group health plan provider, the COBRA provider or the
Executive a monthly payment equal to the monthly employer contribution that the Company would have made to provide health insurance to the Executive if the Executive had remained employed by the Company until the earliest of (A) the 9 (nine)
month anniversary of the Date of Termination; (B) the Executive’s eligibility for group medical plan benefits under any other employer’s group medical plan; or (C) the cessation of the Executive’s continuation rights under
COBRA; provided, however, if the Company determines that it cannot pay such amounts to the group health plan provider or the COBRA provider (if applicable) without potentially violating applicable law (including, without limitation,
Section 2716 of the Public Health Service Act), then the Company shall convert such payments to payroll payments directly to the Executive for the time period specified above. Such payments shall be subject to
tax-related deductions and withholdings and paid on the Company’s regular payroll dates. 
 The amounts payable
under Section 5, to the extent taxable, shall be paid out in substantially equal installments in accordance with the Company’s payroll practice over nine (9) months commencing within 60 days after the Date of Termination;
provided, however, that if the 60-day period begins in one calendar year and ends in a second calendar year, the Severance Amount, to the extent it qualifies as
“non-qualified deferred compensation” within the meaning of Section 409A of the Internal Revenue Code of 1986, as amended (the “Code”), shall begin to be paid in the second calendar
year by the last day of such 60-day period; provided, further, that the initial payment shall include a catch-up payment to cover amounts retroactive to the day
immediately following the Date of Termination. Each payment pursuant to this Agreement is intended to constitute a separate payment for purposes of Treasury Regulation Section 1.409A-2(b)(2). 

6. Severance Pay and Benefits Upon Termination by the Company without Cause or by the Executive for Good Reason within the Change in Control
Period. The provisions of this Section 6 shall apply in lieu of, and expressly supersede, the provisions of Section 5 if (i) the Executive’s employment is terminated either (a) by the Company without Cause as provided in
Section 3(d), or (b) by the Executive for Good Reason as provided in Section 3(e), and (ii) the Date of Termination is within the 60 days before or 12 months after the occurrence of the first event constituting a Change in
Control (such period, the “Change in Control Period”). These provisions shall terminate and be of no further force or effect after a Change in Control Period. 

 (a) If the Executive’s employment is terminated by the Company without Cause as
provided in Section 3(d) or the Executive terminates employment for Good Reason as provided in Section 3(e) and in each case the Date of Termination occurs during the Change in Control Period, then, in addition to the Accrued Obligations,
and subject to the signing of the Separation Agreement and Release by the Executive and the Separation Agreement and Release becoming fully effective, all within the time frame set forth in the Separation Agreement and Release but in no event more
than 60 days after the Date of Termination: 
 (i) the Company shall pay the Executive a lump sum in cash in an amount equal
to 1.0 times the sum of (A) the Executive’s then current Base Salary (or the Executive’s Base Salary in effect immediately prior to the Change in Control, if higher) plus (B) the Executive’s Target Bonus for the
then-current year; and 
 (ii) notwithstanding anything to the contrary in any applicable option agreement or other
stock-based award agreement, all time-based stock options and other stock-based awards subject to time-based vesting held by the Executive (the “Time-Based Equity Awards”) shall immediately accelerate and become fully exercisable or
nonforfeitable as of the later of (i) the Date of Termination or (ii) the effective date of the Separation Agreement and Release (the “Accelerated Vesting Date”); provided that any termination or forfeiture of the unvested
portion of such Time-Based Equity Awards that would otherwise occur on the Date of Termination in the absence of this Agreement will be delayed until the effective date of the Separation Agreement and Release and will only occur if the vesting
pursuant to this subsection does not occur due to the absence of the Separation Agreement and Release becoming fully effective within the time period set forth therein. Notwithstanding the foregoing, no additional vesting of the Time-Based Equity
Awards shall occur during the period between the Executive’s Date of Termination and the Accelerated Vesting Date; and 

(iii) subject to the Executive’s copayment of premium amounts at the applicable active employees’ rate and the
Executive’s proper election to receive benefits under COBRA, the Company shall pay to the group health plan provider, the COBRA provider or the Executive a monthly payment equal to the monthly employer contribution that the Company would have
made to provide health insurance to the Executive if the Executive had remained employed by the Company until the earliest of (A) the 12 month anniversary of the Date of Termination; (B) the Executive’s eligibility for group medical
plan benefits under any other employer’s group medical plan; or (C) the cessation of the Executive’s continuation rights under COBRA; provided, however, if the Company determines that it cannot pay such amounts to the group health
plan provider or the COBRA provider (if applicable) without potentially violating applicable law (including, without limitation, Section 2716 of the Public Health Service Act), then the Company shall convert such payments to payroll payments
directly to the Executive for the time period specified above. Such payments shall be subject to tax-related deductions and withholdings and paid on the Company’s regular payroll dates. 

The amounts payable under this Section 6(a), to the extent taxable, shall be paid or commence to be paid within 60 days after the Date of Termination;
provided, however, that if the 60-day period begins in one calendar year and ends in a second calendar year, such payments to the extent they qualify as
“non-qualified deferred compensation” within the meaning of Section 409A of the Code, shall be paid or commence to be paid in the second calendar year by the last

 
day of such 60-day period. For the avoidance of doubt, in the event that the Executive’s Date of Termination is within the 60 days before a Change in
Control and the Executive has signed the Separation Agreement and Release that has become irrevocable and is entitled to the benefits under Section 5 of this Agreement, then the Executive will receive the benefits set forth in this
Section 6 following the occurrence of a Change in Control; provided that the lump sum amount to be paid to the Executive pursuant to Section 6(a)(i) will be decreased by any benefits previously paid to the Executive pursuant to
Section 5, and the Executive will receive no further benefits pursuant to Section 5. In no event may there be duplication of benefits under Section 5 and Section 6. 

(b) Additional Limitation. 

(i) Anything in this Agreement to the contrary notwithstanding, in the event that the amount of any compensation, payment or
distribution by the Company to or for the benefit of the Executive, whether paid or payable or distributed or distributable pursuant to the terms of this Agreement or otherwise, calculated in a manner consistent with Section 280G of the Code,
and the applicable regulations thereunder (the “Aggregate Payments”), would be subject to the excise tax imposed by Section 4999 of the Code, then the Aggregate Payments shall be reduced (but not below zero) so that the sum of all of
the Aggregate Payments shall be $1.00 less than the amount at which the Executive becomes subject to the excise tax imposed by Section 4999 of the Code; provided that such reduction shall only occur if it would result in the Executive receiving
a higher After Tax Amount (as defined below) than the Executive would receive if the Aggregate Payments were not subject to such reduction. In such event, the Aggregate Payments shall be reduced in the following order, in each case, in reverse
chronological order beginning with the Aggregate Payments that are to be paid the furthest in time from consummation of the transaction that is subject to Section 280G of the Code: (1) cash payments not subject to Section 409A of the
Code; (2) cash payments subject to Section 409A of the Code; (3) equity-based payments and acceleration; and (4) non-cash forms of benefits; provided that in the case of all the foregoing
Aggregate Payments all amounts or payments that are not subject to calculation under Treas. Reg. §1.280G-1, Q&A-24(b) or (c) shall be reduced before any
amounts that are subject to calculation under Treas. Reg. §1.280G-1, Q&A-24(b) or (c). 

(ii) For purposes of this Section 6(b), the “After Tax Amount” means the amount of the Aggregate Payments less
all federal, state, and local income, excise and employment taxes imposed on the Executive as a result of the Executive’s receipt of the Aggregate Payments. For purposes of determining the After Tax Amount, the Executive shall be deemed to pay
federal income taxes at the highest marginal rate of federal income taxation applicable to individuals for the calendar year in which the determination is to be made, and state and local income taxes at the highest marginal rates of individual
taxation in each applicable state and locality, net of the maximum reduction in federal income taxes which could be obtained from deduction of such state and local taxes. 

 (iii) The determination as to whether a reduction in the Aggregate Payments
shall be made pursuant to Section 6(b)(i) shall be made by a nationally recognized accounting firm selected by the Company (the “Accounting Firm”), which shall provide detailed supporting calculations both to the Company and the
Executive within 15 business days of the Date of Termination, if applicable, or at such earlier time as is reasonably requested by the Company or the Executive. Any determination by the Accounting Firm shall be binding upon the Company and the
Executive. 
 (c) Definitions. For purposes of this Section 6, a “Change in Control” shall mean a “Sale
Event” as defined in the Cabaletta Bio, Inc. 2019 Stock Option and Incentive Plan, as may be amended from time to time, but only to the extent such Sale Event is also a “change in control event” within the meaning of Section 409A
of the Code and the regulations promulgated thereunder. 
 7. Section 409A. 

(a) Anything in this Agreement to the contrary notwithstanding, if at the time of the Executive’s separation from service within the
meaning of Section 409A of the Code, the Company determines that the Executive is a “specified employee” within the meaning of Section 409A(a)(2)(B)(i) of the Code, then to the extent any payment or benefit that the Executive
becomes entitled to under this Agreement or otherwise on account of the Executive’s separation from service would be considered deferred compensation otherwise subject to the 20 percent additional tax imposed pursuant to
Section 409A(a) of the Code as a result of the application of Section 409A(a)(2)(B)(i) of the Code, such payment shall not be payable and such benefit shall not be provided until the date that is the earlier of (A) six months and one
day after the Executive’s separation from service, or (B) the Executive’s death. If any such delayed cash payment is otherwise payable on an installment basis, the first payment shall include a
catch-up payment covering amounts that would otherwise have been paid during the six-month period but for the application of this provision, and the balance of the
installments shall be payable in accordance with their original schedule. 
 (b) All in-kind benefits
provided and expenses eligible for reimbursement under this Agreement shall be provided by the Company or incurred by the Executive during the time periods set forth in this Agreement. All reimbursements shall be paid as soon as administratively
practicable, but in no event shall any reimbursement be paid after the last day of the taxable year following the taxable year in which the expense was incurred. The amount of in-kind benefits provided or
reimbursable expenses incurred in one taxable year shall not affect the in-kind benefits to be provided or the expenses eligible for reimbursement in any other taxable year (except for any lifetime or other
aggregate limitation applicable to medical expenses). Such right to reimbursement or in-kind benefits is not subject to liquidation or exchange for another benefit. 

(c) To the extent that any payment or benefit described in this Agreement constitutes
“non-qualified deferred compensation” under Section 409A of the Code, and to the extent that such payment or benefit is payable upon the Executive’s termination of employment, then such
payments or benefits shall be payable only upon the Executive’s “separation from service.” The determination of whether and when a separation from service has occurred shall be made in accordance with the presumptions set forth in
Treasury Regulation Section 1.409A-1(h). 

 (d) The parties intend that this Agreement will be administered in accordance with
Section 409A of the Code. To the extent that any provision of this Agreement is ambiguous as to its compliance with Section 409A of the Code, the provision shall be read in such a manner so that all payments hereunder comply with
Section 409A of the Code. Each payment pursuant to this Agreement or the Restrictive Covenant Agreement is intended to constitute a separate payment for purposes of Treasury Regulation
Section 1.409A-2(b)(2). The parties agree that this Agreement may be amended, as reasonably requested by either party, and as may be necessary to fully comply with Section 409A of the Code and all
related rules and regulations in order to preserve the payments and benefits provided hereunder without additional cost to either party. 

(e) The Company makes no representation or warranty and shall have no liability to the Executive or any other person if any provisions of this
Agreement are determined to constitute deferred compensation subject to Section 409A of the Code but do not satisfy an exemption from, or the conditions of, such Section. 

8. Continuing Obligations. 

(a) Restrictive Covenant Agreement. The terms of the Employee Restrictive Covenant Agreement, dated January 5, 2019 (the
“Restrictive Covenant Agreement”), between the Company and the Executive, attached hereto as Exhibit A, continue to be in full force and effect and are incorporated by reference as material terms of this Agreement. For purposes of
this Agreement, the obligations in this Section 8 and those that arise in the Restrictive Covenant Agreement and any other agreement relating to confidentiality, assignment of inventions, or other restrictive covenants shall collectively be
referred to as the “Continuing Obligations.” 
 (b) Third-Party Agreements and Rights. The Executive hereby confirms that
the Executive is not bound by the terms of any agreement with any previous employer or other party which restricts in any way the Executive’s use or disclosure of information, other than confidentiality restrictions (if any), or the
Executive’s engagement in any business. The Executive represents to the Company that the Executive’s execution of this Agreement, the Executive’s employment with the Company and the performance of the Executive’s proposed duties
for the Company will not violate any obligations the Executive may have to any such previous employer or other party. In the Executive’s work for the Company, the Executive will not disclose or make use of any information in violation of any
agreements with or rights of any such previous employer or other party, and the Executive will not bring to the premises of the Company any copies or other tangible embodiments of non-public information
belonging to or obtained from any such previous employment or other party. 
 (c) Litigation and Regulatory Cooperation. During and
after the Executive’s employment, the Executive shall cooperate fully with the Company in (i) the defense or prosecution of any claims or actions now in existence or which may be brought in the future against or on behalf of the Company
which relate to events or occurrences that transpired while the Executive was employed by the Company, and (ii) the investigation, whether internal or external, of any matters about which the Company believes the Executive may have knowledge or
information. The Executive’s full cooperation in connection with such claims, actions or investigations shall include, but not be limited to, being available to meet with counsel to answer questions or to prepare for discovery or trial and to
act as a witness on behalf of the Company at 

 
mutually convenient times. During and after the Executive’s employment, the Executive also shall cooperate fully with the Company in connection with any investigation or review of any
federal, state or local regulatory authority as any such investigation or review relates to events or occurrences that transpired while the Executive was employed by the Company. The Company shall reimburse the Executive for any reasonable out-of-pocket expenses incurred in connection with the Executive’s performance of obligations pursuant to this Section 8(c). 

(d) Relief. The Executive agrees that it would be difficult to measure any damages caused to the Company which might result from any
breach by the Executive of the Continuing Obligations, and that in any event money damages would be an inadequate remedy for any such breach. Accordingly, the Executive agrees that if the Executive breaches, or proposes to breach, any portion
of the Continuing Obligations, the Company shall be entitled, in addition to all other remedies that it may have, to an injunction or other appropriate equitable relief to restrain any such breach without showing or proving any actual damage to the
Company. 
 (e) Protected Disclosures and Other Protected Action. Nothing in this Agreement shall be interpreted or applied to
prohibit the Executive from making any good faith report to any governmental agency or other governmental entity (a “Government Agency”) concerning any act or omission that the Executive reasonably believes constitutes a possible violation
of federal or state law or making other disclosures that are protected under the anti-retaliation or whistleblower provisions of applicable federal or state law or regulation. In addition, nothing contained in this Agreement limits the
Executive’s ability to communicate with any Government Agency or otherwise participate in any investigation or proceeding that may be conducted by any Government Agency, including the Executive’s ability to provide documents or other
information, without notice to the Company. In addition, for the avoidance of doubt, pursuant to the federal Defend Trade Secrets Act of 2016, the Executive shall not be held criminally or civilly liable under any federal or state trade secret law
or under this Agreement or the Restrictive Covenant Agreement for the disclosure of a trade secret that (a) is made (i) in confidence to a federal, state, or local government official, either directly or indirectly, or to an attorney; and
(ii) solely for the purpose of reporting or investigating a suspected violation of law; or (b) is made in a complaint or other document filed in a lawsuit or other proceeding, if such filing is made under seal. 

9. Consent to Jurisdiction. The parties hereby consent to the jurisdiction of the state and federal courts of the Commonwealth of
Pennsylvania. Accordingly, with respect to any such court action, the Executive (a) submits to the personal jurisdiction of such courts; (b) consents to service of process; and (c) waives any other requirement (whether imposed by
statute, rule of court, or otherwise) with respect to personal jurisdiction or service of process. 
 10. Integration. This Agreement
constitutes the entire agreement between the parties with respect to the subject matter hereof and supersedes all prior agreements between the parties concerning such subject matter, including the Prior Agreement, provided that the Restrictive
Covenant Agreement and the Equity Documents remain in full force and effect. 

 11. Withholding; Tax Effect. All payments made by the Company to the Executive under
this Agreement shall be net of any tax or other amounts required to be withheld by the Company under applicable law. Nothing in this Agreement shall be construed to require the Company to make any payments to compensate the Executive for any adverse
tax effect associated with any payments or benefits or for any deduction or withholding from any payment or benefit. 
 12.
Assignment. Neither the Executive nor the Company may make any assignment of this Agreement or any interest in it, by operation of law or otherwise, without the prior written consent of the other; provided, however, that the Company may
assign its rights and obligations under this Agreement (including the Restrictive Covenant Agreement) without the Executive’s consent to any affiliate or to any person or entity with whom the Company shall hereafter effect a reorganization,
consolidate with, or merge into or to whom it transfers all or substantially all of its properties or assets; provided further that if the Executive remains employed or becomes employed by the Company, the purchaser or any of their affiliates in
connection with any such transaction, then the Executive shall not be entitled to any payments, benefits or vesting pursuant to Section 5 or pursuant to Section 6 of this Agreement. This Agreement shall inure to the benefit of and be
binding upon the Executive and the Company, and each of the Executive’s and the Company’s respective successors, executors, administrators, heirs and permitted assigns. 

13. Enforceability. If any portion or provision of this Agreement (including, without limitation, any portion or provision of any
section of this Agreement) shall to any extent be declared illegal or unenforceable by a court of competent jurisdiction, then the remainder of this Agreement, or the application of such portion or provision in circumstances other than those as to
which it is so declared illegal or unenforceable, shall not be affected thereby, and each portion and provision of this Agreement shall be valid and enforceable to the fullest extent permitted by law. 

14. Survival. The provisions of this Agreement shall survive the termination of this Agreement and/or the termination of the
Executive’s employment to the extent necessary to effectuate the terms contained herein. 
 15. Waiver. No waiver of any
provision hereof shall be effective unless made in writing and signed by the waiving party. The failure of any party to require the performance of any term or obligation of this Agreement, or the waiver by any party of any breach of this Agreement,
shall not prevent any subsequent enforcement of such term or obligation or be deemed a waiver of any subsequent breach. 
 16.
Notices. Any notices, requests, demands and other communications provided for by this Agreement shall be sufficient if in writing and delivered in person or sent by a nationally recognized overnight courier service or by registered or
certified mail, postage prepaid, return receipt requested, to the Executive at the last address the Executive has filed in writing with the Company or, in the case of the Company, at its main offices, attention of the Board. 

17. Amendment. This Agreement may be amended or modified only by a written instrument signed by the Executive and by a duly authorized
representative of the Company. 

 18. Effect on Other Plans and Agreements. An election by the Executive to resign for
Good Reason under the provisions of this Agreement shall not be deemed a voluntary termination of employment by the Executive for the purpose of interpreting the provisions of any of the Company’s benefit plans, programs or policies. Nothing in
this Agreement shall be construed to limit the rights of the Executive under the Company’s benefit plans, programs or policies except as otherwise provided in Section 8 hereof, and except that the Executive shall have no rights to any
severance benefits under any Company severance pay plan, offer letter or otherwise. In the event that the Executive is party to an agreement with the Company providing for payments or benefits under such plan or agreement and under this Agreement,
the terms of this Agreement shall govern and the Executive may receive payment under this Agreement only and not both. Further, Section 5 and Section 6 of this Agreement are mutually exclusive and in no event shall the Executive be
entitled to payments or benefits pursuant to both Section 5 and Section 6 of this Agreement. 
 19. Governing Law. This is a
Pennsylvania contract and shall be construed under and be governed in all respects by the laws of the Commonwealth of Pennsylvania without giving effect to the conflict of laws principles thereof. With respect to any disputes concerning federal law,
such disputes shall be determined in accordance with the law as it would be interpreted and applied by the United States Court of Appeals for the Third Circuit. 

20. Counterparts. This Agreement may be executed in any number of counterparts, each of which when so executed and delivered shall be
taken to be an original; but such counterparts shall together constitute one and the same document. 
 IN WITNESS WHEREOF, the parties have
executed this Agreement effective on the Effective Date. 
  

			
	CABALETTA BIO, INC.
		
	By:	 	 /s/ Steven Nichtberger

	Its:	 	 President and Chief Executive Officer

	
	EXECUTIVE
	
	 /s/ Anup Marda

	Anup Marda

 Exhibit A 

Employee Restrictive Covenant Agreement

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