Document:

EX-10.20

 Exhibit 10.20 
  

 
  
 Amended and Restated Letter Agreement
for David Mantus 
 October 10, 2017 
 Dear
Mr. Mantus, 
 This letter (the “Letter Agreement”) amends and restates the terms and conditions of your employment
with Arsanis, Inc. (“Arsanis” or “Company”), as initially set forth in the offer letter dated September 24, 2015 (the “Original Offer Letter”), and will take effect upon the closing of the
Company’s initial public offering (the “Effective Date”), provided that you remain employed by the Company as of the Effective Date. Until the Effective Date, the Original Offer Letter will remain in force and effect and
continue to govern your employment with the Company. This Letter Agreement contains the following terms: 

1.    Employment: You will continue to be employed to serve as the Company’s Chief Development Officer,
effective as of the Effective Date. You will report directly to the Chief Executive Officer and will have those duties as are customarily performed by a chief development officer, and such other duties as may be assigned by the Company. You agree to
devote your full business time, best efforts, skill, knowledge, attention and energies to the advancement of the Company’s business and interests and to the performance of your duties and responsibilities as an employee of the Company. You
agree to abide by the rules, regulations, personnel practices and policies of the Company and any changes therein that may be adopted from time to time by the Company. 

2.    Time Commitment: The time commitment for this position will continue to be a full-time professional
commitment. You will be expected to work a minimum of 40 hours per week. You may be required to work more than 40 hours per week as needed. Arsanis, however, believes in a flexible schedule policy that encourages employees to work hard but allows
flexibility around when work is done, consistent with the Company’s business needs and the approval of the Company. As an exempt employee, you are not eligible for overtime pay. 

3.    Base Salary: Your base salary will be at the rate of $14,166.66 per
bi-monthly pay period (annualized rate of $340,000) (the “Base Salary”), subject to deductions for taxes and other withholdings as required by law or the standard and lawful policies of the
Company. Such Base Salary may be adjusted from time to time in accordance with normal business practice and in the sole discretion of the Company. 

4.    Bonus Potential: At the sole discretion of the Company, you will be eligible to receive an annual
retention and performance bonus of up to 30% of your Base Salary, which will be dependent upon your individual performance and the Company’s performance during the applicable year, all as determined by the Company in its sole discretion. Any
bonus that may be awarded to you hereunder will be paid no later than March 15th of the year following the calendar year to which the bonus relates, subject to your continuous employment through
the end of the calendar year to which such bonus relates. The foregoing shall be construed and applied so that any bonus payable to you hereunder qualifies as a “short-term deferral” under Section 409A of the Internal Revenue Code of
1986, as amended (“Section 409A”). 
  
 

 

 

 
  

5.    Vacation and Other Paid Time Off: You will receive paid holiday, vacation, and sick time, on an
annualized basis, as follows: 
  

	 	•	 	 up to eleven (11) paid holidays per calendar year, such days to be allocated to those official U.S. federal
and/or state holidays observed in Massachusetts, unless otherwise mutually agreed upon between you and Arsanis; 

  

	 	•	 	 up to twenty-five (25) days of paid vacation per calendar year, (accruing at a rate of 2.08 days per month),
subject to the Company’s vacation policy and to be taken at such times as may be approved by the Company; and 

  

	 	•	 	 paid sick leave as required, subject to the Company’s sick leave policy and applicable law.

 6.    Location: You will be primarily located in the Company’s Boston area
offices (currently located at 890 Winter Street, Suite 230, Waltham, MA 02451), but, consistent with the Company’s business needs, you will also be required to travel as directed by the Company to other locations, including, without limitation,
to the Company’s offices in Vienna, Austria. 
 7.    Benefits: You may participate in any and all
benefit programs that the Company establishes and makes available to its employees from time to time, provided you are eligible under (and subject to all provisions of) the plan documents governing those programs. The benefit programs made available
by the Company, and the rules, terms and conditions for participation in such benefit programs, may be changed by the Company at any time without advance notice (other than as required by such programs or under law). 

8.    Expense Reimbursement: Arsanis will reimburse you for all reasonable and necessary out-of-pocket expenses incurred in the course of your employment, subject in certain circumstances to approval, and in accordance with all Company policies and procedures
regarding documentation and accounting for such expenses. 
 9.    Termination of Employment: You or the
Company may terminate your employment at any time for any reason, with or without cause, subject to the following provisions: 

a.    Termination for Cause: The Company may terminate your employment for Cause, as defined below, upon written
notice to you setting forth in reasonable detail the nature of the Cause. The following, as determined by the Company’s Board of Directors (the “Board”) in its reasonable judgment, shall constitute “Cause” for
termination: 
 (i)    the commission of, or indictment or conviction for, any felony, or any other
crime involving dishonesty; 
 (ii)    participation in any fraud, deliberate and substantial
misconduct, breach of duty of loyalty or breach of fiduciary duty against the Company; 
  
 

 

  
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(iii)    intentional and substantial damage to any property of the Company; 

(iv)    failure of performance of your duties hereunder (not attributable to sickness, disability or
death) after reasonable written notice no later than thirty (30) days following the occurrence of the failure and a 30-day opportunity to cure, provided, however, that such opportunity to cure shall only
apply to any failure that the Board, in its reasonable discretion, deems susceptible to cure; or 

(v)    your breach of any material provision of this Letter Agreement, your Invention, Non-Competition, Non-Solicitation and Non-Disclosure Agreement (as defined below, the
“Non-Disclosure Agreement”), or any other agreement to which you and the Company are both parties, after reasonable written notice no later than thirty (30) days following the occurrence
of the breach and a 30-day opportunity to cure, provided, however, that such opportunity to cure shall only apply to any breach that the Board, in its reasonable discretion, deems susceptible to cure, and that
any breach by you of your obligations of confidentiality or non-competition under the Non-Disclosure Agreement shall be deemed not susceptible to cure. 

Termination of your employment by the Company for Cause will result in no severance pay or benefits. 

b.    Termination without Cause: The Company may terminate your employment at any time other than for Cause upon
written notice to you. 
 c.    Termination for Good Reason: You may terminate your employment hereunder for Good
Reason, as defined below, by providing written notice to the Company of the condition giving rise to the Good Reason, specifying in reasonable detail the basis for such claim of Good Reason, no later than thirty (30) days following the
occurrence of the condition, by giving the Company thirty (30) days to remedy the condition and by terminating employment for Good Reason within thirty (30) days thereafter if the Company fails to remedy the condition. The following, if
occurring without your consent, shall constitute “Good Reason” for termination by you: 

(i)    a material and adverse diminution of your duties and responsibilities with the Company, provided
that such change is not in connection with a termination of your employment relationship with the Company; 

(ii)    a material diminution of your then Base Salary, provided that such change is not in connection
with a termination of your employment relationship with the Company; 
 (iii)    relocation of your
principal place of employment outside a thirty (30) mile radius from Boston, Massachusetts, if such relocation increases your daily commuting distance; or 

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

 
 

 

  
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d.    Termination without Good Reason: You may terminate your employment with the Company other than for Good Reason
at any time subject to your provision of thirty (30) days’ advance written notice to the Company (the “Applicable Notice Period”), provided, however, that the Company may, in its sole discretion, in lieu of all or part of
the Applicable Notice Period, pay you an amount equal to the Base Salary that would otherwise have been payable to you had you remained employed for the duration of the Applicable Notice Period. In such instance, your termination will become
effective on the date set forth in a written notice of termination to be provided by the Company (the “Early Termination Date”), and you will be paid an amount equal to the Base Salary you would have received had you remained
employed by the Company between the Early Termination Date and the end of the Applicable Notice Period (the “Early Termination Payment”), with the Early Termination Payment to be made no later than the 30th day following the end of the Applicable Notice Period. For the avoidance of doubt, except for the Early Termination Payment, you will not be entitled to receive any severance pay or benefits. 

e.    Termination Due to Death or Disability: Your employment shall automatically terminate in the event of your
death during employment. The Company may terminate your employment, upon notice to you, in the event you become disabled during employment through any illness, accident, injury or condition of either a physical or psychological nature and, as a
result, are unable to continue to perform substantially all of your duties and responsibilities (notwithstanding the provision of any reasonable accommodation) for 180 days (whether or not consecutive) during any period of 365 consecutive calendar
days. If any question shall arise as to whether you are disabled to the extent that you are unable to perform substantially all of your duties and responsibilities for the Company, you shall, at the Company’s request and expense, submit to a
medical examination by a mutually acceptable physician in the Boston area who is board-certified in the area of practice involved in the disability and such determination shall, for the purposes of this Letter Agreement, be conclusive of the issue.
If such a question arises and you fail to submit to the requested medical examination, the Company’s determination of the issue shall be binding on you. 

10.    Severance and other Matters Related to Termination; Change of Control: 

a.    Termination by the Company without Cause or by You for Good Reason: Subject to Sections 10(b) and 10(f) below
and Section 409A, in the event that your employment is terminated by the Company without Cause pursuant to Section 9(b) of this Letter Agreement or by you for Good Reason pursuant to Section 9(c) of this Letter Agreement, in addition
to the Accrued Compensation (as defined below), the Company shall provide you with the severance payments and benefits specified below: 

(i)    the Company shall pay you an amount equal to your annualized Base Salary, at the rate then in
effect and payable in equal installments in accordance with the Company’s standard payroll policy as then in effect, for a period of twelve (12) months commencing at the time set forth in Section 10(f) hereof (the “Severance
Period”); 
  
 

 

  
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(ii)    the Company shall pay you in one lump sum, on the date that annual bonuses are paid to active and
similarly situated employees, a pro-rata annual bonus for the year in which your termination occurs in such amount, if any, as you would have received based on your actual performance during such calendar
year, had you remained employed through the end of such calendar year, calculated by multiplying the full bonus amount (based on actual performance) for such year by a fraction, the numerator of which is the number of days you were employed during
such year and the denominator of which is 365 (the “Lump Sum Pro-Rata Performance Bonus”); and 

(iii)    subject to your timely election to continue participation in the Company’s group health and
dental plans under COBRA or Massachusetts law, and only for so long as you are eligible for such coverage through COBRA or Massachusetts law, the Company shall pay you, on a monthly and taxable basis, an amount equal to the full monthly premium cost
of such participation until the conclusion of the Severance Period, or, if earlier, until the date you become eligible to enroll in such plans of any new employer. 

b.    Termination by the Company without Cause or by You for Good Reason in Connection with a Change of Control:
Subject to Section 10(f) and Section 409A, in the event that your employment is terminated by the Company without Cause pursuant to Section 9(b) of this Letter Agreement or by you for Good Reason pursuant to 9(c) of this Letter
Agreement, in either case within twelve (12) months following a Change of Control (as defined below), in addition to the Accrued Compensation (as defined below), in lieu of any payments and benefits provided in Section 10(a) above, the
Company shall provide you with the severance payments and benefits specified below: 
 (i)    the
Company shall pay you an amount equal to the sum of (A) your annualized Base Salary, at the rate then in effect, and (B) your target annual bonus for the year in which your termination of employment occurs, payable in equal installments
and in accordance with the Company’s standard payroll policy as then in effect, for a period of twelve (12) months commencing at the time set forth in Section 10(f) hereof) (the “Change of Control Severance Period”);

 (ii)    subject to your timely election to continue participation in the Company’s group health
and dental plans under COBRA, and only for so long as you are eligible for such coverage through COBRA (or Massachusetts laws), the Company shall pay you, on a monthly and taxable basis, an amount equal to the full monthly premium cost of such
participation until the conclusion of the Change of Control Severance Period, or, if earlier, until the date you become eligible to enroll in such plans of any new employer; and 

(iii)    all outstanding and unvested stock options and other equity awards then held by you shall become
fully vested and exercisable and, with respect to any stock options then held by you, those options shall remain exercisable for the period of time set forth in the applicable grant agreement. 

 
 

 

  
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c.    Termination by the Company Due to Your Disability or Death: Subject to Section 10(f) and
Section 409A, in the event your employment with the Company is terminated by the Company due to your disability or is terminated due to your death pursuant to Section 9(e) of this Letter Agreement, in addition to the Accrued Compensation
(as defined below), the Company shall pay you in a lump sum a pro-rata amount of your target annual bonus for the year in which your death or disability occurs, calculated by multiplying your target annual
bonus for such year by a fraction, the numerator of which is the number of days you were employed during such year and the denominator of which is 365 (the “Pro-Rata Bonus”) payable at the
time set forth in Section 10(f) hereof. 
 d.    Any Other Termination: In the event your employment with
the Company terminates for any reason other than by the Company without Cause pursuant to Section 9(b) of this Letter Agreement, by you for Good Reason pursuant to Section 9(c) of this Letter Agreement, or by the Company due to your
disability or death pursuant to Section 9(e) of this Letter Agreement, the Company shall pay you the Accrued Compensation. For purposes of this Letter Agreement, “Accrued Compensation” means any base salary earned but not paid
through the date of the termination of employment and an amount equal to the value of any vacation time accrued but unused as of such date. 

e.    Parachute Payments: 

(i)    In the event of the consummation of a change in ownership or control within the meaning of
Section 280G (a “280G Change in Control”) of the Company following the time that the Company has stock readily tradeable on an established securities market (within the meaning of Section 280G and the regulations
thereunder), if all or a portion of the payments and benefits under this Letter Agreement, together with any other payments and benefits provided to you by the Company or its Affiliates (including, without limitation, any accelerated vesting of
stock options and other equity awards) (the “Total Payments”), would constitute an “excess parachute payment” within the meaning of Section 280G (the aggregate of such payments (or portions thereof) being hereinafter
referred to as the “Excess Parachute Payments”), you will be entitled to receive (A) an amount limited so that no portion thereof shall fail to be tax deductible under Section 280G (the “Limited Amount”),
or (B) if the amount otherwise payable hereunder or otherwise (without regard to clause (A)) reduced by all taxes applicable thereto (including, for the avoidance of doubt, the excise tax levied under Section 4999 of the Code (the
“Excise Tax”)) would be greater than the Limited Amount reduced by all taxes applicable thereto, the amount otherwise payable hereunder or otherwise. 

(ii)    The determination as to whether the Total Payments include Excess Parachute Payments and, if so,
the amount of such Excess Parachute Payments, the amount of any Excise Tax with respect thereto, and the amount of any reduction in Total Payments shall be made at the Company’s expense by the independent public accounting firm most recently
serving as the Company’s outside auditors or such other accounting, law or benefits consulting group or firm as the Company may designate (the “Accountants”). In the event that any payments under this Letter Agreement or
otherwise are required to be 
  
 

 

  
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 reduced as described in
Section 10(e)(i) of this Letter Agreement, the adjustment will be made, first, by reducing the amount of base salary payable pursuant to Section 10(a)(i) or 10(b)(i), as applicable; second, if additional reductions are necessary, by
reducing the payment of the amounts due to you pursuant to Section 10(a)(ii) as applicable; and third, if additional reductions are still necessary, by eliminating the accelerated vesting of stock option awards and other equity awards, if any,
starting with those awards for which the amount required to be taken into account under Section 280G is the greatest. 

(iii)    In the event that there has been an underpayment or overpayment under this Letter Agreement or
otherwise as determined by the Accountants, the amount of such underpayment or overpayment shall forthwith be paid to you or refunded to the Company, as the case may be, with interest at the applicable federal rate provided for in
Section 7872(f)(2) of the Code. 
 f.    Release: Any obligation of the Company to provide you severance
payments or other benefits (including accelerated vesting of stock options and other equity awards) or any Pro-Rata Bonus or Lump Sum Pro-Rata Performance Bonus (for the
avoidance of doubt, other than Accrued Compensation), is conditioned on your (or your legal representative, if applicable, in the case of a termination due to your disability) signing within a period of time not to exceed forty-five (45) days
following the date of such termination of employment (or such shorter period as may be directed by the Company) a separation and release of claims agreement in the form provided by the Company (the “Release”) following the
termination of your employment, and on your (or your legal representative, if applicable) not revoking the Release within any revocation period provided therein following your (or your legal representative’s, if applicable) execution of the
Release, which Release shall not require you to release (i) claims for indemnification in your capacity as an officer or director of the Company under the Company’s Certificate of Incorporation, Bylaws, insurance or other written
agreements, if any, providing for director or officer indemnification, (ii) rights to receive insurance payments under any policy maintained by the Company, (iii) vested rights as an equity holder or option holder, (iv) rights to
receive retirement and other benefits that are accrued and fully vested at the time of your termination, and (v) any other claims that cannot be released as a matter of law. Except as otherwise provided in Section 409A, any payments to be
made in either in a lump sum or in the form of salary continuation pursuant to the terms of this Letter Agreement shall be payable in accordance with the normal payroll practices of the Company, with such payment or, as may be applicable, the first
such payment (which shall be retroactive to the day immediately following the date of your termination of employment) due and payable as soon as administratively practicable following the date the Release becomes effective, but not later than the
date that is sixty (60) days following the date your employment terminates. Notwithstanding the foregoing, if the date your employment terminates occurs in one taxable year and the date that is sixty (60) days following such termination
date occurs in a second taxable year, to the extent required by Section 409A, such payment or, as may be applicable, first payment shall not be made prior to the first day of the second taxable year. For the avoidance of doubt, if you (or your
legal representative, if applicable) do not execute a Release within the period specified in this Section 10(f), or if you (or your legal representative, if applicable) revoke the executed Release within the time period permitted by law, 

 
 

 

  
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 you will not be entitled to any payments or benefits
(including the accelerated vesting of stock options or other equity awards) or any Pro-Rata Bonus or Lump Sum Pro-Rata Performance Bonus set forth herein (other than the
Accrued Compensation), any stock options and other equity awards that vested on account of such termination as provided for in this Letter Agreement shall be cancelled with no consideration due to you, and the Company will not have any further
obligations to you under this Letter Agreement or otherwise. You agree that, should you become eligible to participate in the health and, if applicable, dental, plan of any subsequent employer prior to the conclusion of the Severance Period or
Change of Control Severance Period, as may be applicable, you will provide the Company with written notice thereof within five (5) business days of such eligibility. You further agree to repay any overpayment of health and, if applicable,
dental, benefit premiums made by the Company hereunder. Notwithstanding anything to the contrary herein, in the event that the Company’s payment of the amounts described in Section 10(a)(iii) or (b)(ii), as applicable, would subject the
Company to any tax or penalty under the Patient Protection and Affordable Care Act (as amended from time to time, the “ACA”) or Section 105(h) of the Internal Revenue Code of 1986, as amended
(“Section 105(h)”), or applicable regulations or guidance issued under the ACA or Section 105(h), you and the Company agree to work together in good faith to restructure such benefit. 

g.    Survival, Conditions to Severance: Provisions of this Letter Agreement shall survive any termination if so
provided in this Letter Agreement or if necessary or desirable to accomplish the purposes of other surviving provisions of the Letter Agreement and the Non-Disclosure Agreement. The obligation of the Company
to make severance payments to you or on your behalf is expressly conditioned upon (i) your full performance, and continued performance during any applicable severance periods, of your material obligations under this Letter Agreement, the Non-Disclosure Agreement, and any subsequent agreement between you and the Company relating to, without limitation, confidentiality, non-competition, proprietary information
or the like, and (ii) your (or your legal representative’s, if applicable, in the case of a termination due to your disability) execution and non-revocation of the Release as set forth above. 

h.    Change of Control: In the event of a Change of Control, the vesting of all equity awards granted to you
before the closing date of the Company’s first initial public offering (“IPO”) of its common stock shall accelerate in full and with respect to any stock options then held by you, those options shall remain exercisable
for the period of time set forth in the applicable grant agreement. For the avoidance of doubt, any equity awards granted to you on or after the closing of the IPO shall not be entitled to the accelerated vesting benefits set forth in this
Section 10(h). 
 11.    Definitions: For purposes of this Letter Agreement, the following
definitions apply: 
 a.    “Change of Control” means the first to occur of any of the following:
(i) a merger or consolidation in which (A) the Company is a constituent party, or (B) a subsidiary of the Company is a constituent party and the Company issues shares of its capital stock pursuant to such merger or consolidation,
except in the case of either clause (A) or (B) any such merger or consolidation involving the Company or a subsidiary of the Company in which the beneficial owners of the shares of capital stock of the Company outstanding immediately prior to
such merger 
  
 

 

  
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 or consolidation continue beneficially to own,
immediately following such merger or consolidation, at least a majority by voting power of the capital stock of (x) the surviving or resulting corporation or (y) if the surviving or resulting corporation is a wholly owned subsidiary of
another corporation immediately following such merger or consolidation, the parent corporation of such surviving or resulting corporation; (ii) the sale, lease, transfer, exclusive license or other disposition, in a single transaction or series
of related transactions, by the Company or a Company subsidiary of all or substantially all the assets of the Company and the Company subsidiaries taken as a whole (except in connection with a merger or consolidation not constituting a Change of
Control under clause (i) or where such sale, lease, transfer, exclusive license or other disposition is to a wholly owned Company subsidiary); or (iii) the sale or transfer, in a single transaction or series of related transactions, by the
stockholders of the Company of more than 50% by voting power of the then-outstanding capital stock of the Company to any Person or entity or group of affiliated Persons or entities. 

b.    “Code” means the Internal Revenue Code of 1986, as amended. 

c.    “Person” means an individual, a corporation, an association, a partnership, an estate, a trust and
any other entity or organization, other than the Company. 
 d.    “Section 280G”
means Section 280G of the Code, together with the regulations thereunder. 
 12.    Section 409A.

 a.    You and the Company agree that this Letter Agreement shall be interpreted to comply with or be exempt from
Section 409A, and the regulations and guidance promulgated thereunder to the extent applicable, and all provisions of this Letter Agreement shall be construed in a manner consistent with the requirements for avoiding taxes or penalties under
Section 409A. 
 b.    A termination of employment shall not be deemed to have occurred for purposes of any
provision of this Letter Agreement providing for the payment of any amounts or benefits considered “nonqualified deferred compensation” under Section 409A upon or following a termination of employment unless such termination is also a
“separation from service” within the meaning of Section 409A (after giving effect to the presumptions contained therein) and, for purposes of any such provision of this Letter Agreement, references to a “termination,”
“termination of employment” or like terms shall mean “separation from service.” If you are deemed on the date of termination to be a “specified employee” within the meaning of that term under Section 409A(a)(2)(B),
then with regard to any payment or the provision of any benefit that is considered nonqualified deferred compensation under Section 409A payable on account of a “separation from service,” such payment or benefit shall be made or
provided at the date which is the earlier of (a) the expiration of the six-month period measured from the date of such “separation from service”, and (b) the date of your death (the
“Delay Period”). Upon the expiration of the Delay Period, all payments and benefits delayed pursuant to this Section 12(b) (whether they would have otherwise been payable in a single sum or in installments in the absence of
such delay) shall be paid or reimbursed on the first business day following the expiration of the Delay Period to you in a lump sum, and any remaining payments and benefits due under this Letter Agreement shall be paid or provided in accordance with
the normal payment dates specified for them herein. 
  
 

 

  
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 c.    With regard
to any provision herein that provides for payment or reimbursement of costs and expenses or in-kind benefits, except as permitted by Section 409A, (a) the right to payment, reimbursement or in-kind benefits shall not be subject to liquidation or exchange for another benefit; (b) the amount of expenses eligible for payment or reimbursement, or in-kind
benefits, provided during any taxable year shall not affect the expenses eligible for payment or reimbursement, or in-kind benefits, to be provided in any other taxable year; and (c) such payments shall
be made on or before the last day of your taxable year following the taxable year in which the expense occurred. 

d.    For purposes of Section 409A, your right to receive any installment payments pursuant to this Letter Agreement
shall be treated as a right to receive a series of separate and distinct payments. 
 e.    In no event shall the
Company have any liability relating to the failure or alleged failure of any payment or benefit under this Letter Agreement to comply with, or be exempt from, the requirements of Section 409A. 

13.    At-Will Status: As is true for all Company employees, your
employment with Arsanis will be “at-will.” This means that your employment is for no specified period of time, and may be terminated at any time by either you or the Company, with or without
cause, subject only to the provisions of this Letter Agreement. This Letter Agreement is not meant to be a contract of employment for any specific duration. You agree that although your title, duties, compensation or benefits may change from time to
time, such changes will not change the at-will nature of your employment, which may only be changed by an express written agreement that is signed by you and the Chief Executive Officer of the Company or other
officer duly authorized by the Board. 
 14.    Conditions/Required Documentation: The Company’s
premises, including all workspaces, furniture, documents, and other tangible materials, and all information technology resources of the Company (including computers, data and other electronic files, and all internet and email) are subject to
oversight and inspection by the Company at any time, and employees should have no expectation of privacy with regard to any Arsanis premises, materials, resources, or information. Further, you hereby acknowledge that your continued employment with
the Company is conditioned on your signing and returning, together with this Letter Agreement, the Invention, Non-Competition, Non-Solicitation and Non-Disclosure Agreement that is being provided to you contemporaneously herewith (the “Non-Disclosure Agreement”). 

15.    No Inconsistent Obligations: You represent and warrant to the Company that you are under no
obligations or commitments, whether contractual or otherwise, that are inconsistent with your obligations set forth in this Letter Agreement or that would be violated by your employment by the Company. You agree that you will not take any action on
behalf of the Company or cause the Company to take any action that will violate any agreement that you have with a prior employer or any other third party. 
  

 

  
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16.    Miscellaneous: Your rights and obligations under this Letter Agreement shall be neither assignable nor
delegable by you, except to the extent that any rights to compensation hereunder may be assigned to your estate or legal representative in the event of your death or disability. This Letter Agreement shall be binding upon and inure to the benefit of
you and the Company and your and its respective permitted successors and assigns. The terms of this Letter Agreement and the resolution of any disputes as to the meaning, effect, performance or validity of this letter or arising out of, related to,
or in any way connected with this letter, your employment with the Company or any other relationship between you and the Company (the “Disputes”) will be governed by Massachusetts law, without regard to conflict of laws provisions.
You and the Company submit to the exclusive personal jurisdiction of the federal and state courts located in the Commonwealth of Massachusetts in connection with any Dispute or any claim related to any Dispute. As of the Effective Date, this Letter
Agreement supersedes all prior understandings, whether written or oral, relating to the terms of your employment, including without limitation the Original Offer Letter. 

If you would like to accept this offer of continued employment on the terms set forth herein as of the Effective Date, please sign and return
this Letter Agreement, together with the Non-Disclosure Agreement, by the end of the day on October 13, 2017. 

We look forward to your continuing contributions to the Arsanis team. 

 

	
	Sincerely,
	
	/s/ René Russo
	René Russo, President and CEO

  

	
	AGREED TO:
	
	/s/ David Mantus
	David Mantus

  

	
	13 Oct 2017
	Date

  
 

 

  
 - 11 -EX-10.21

 Exhibit 10.21 
  

 
 Confidential 

November 26, 2018 
 David Mantus 

Retention Bonus Agreement 
 Dear Dave, 

On behalf of Arsanis, Inc. (the “Company”), I would like to thank you for your continued service to the Company and
emphasize your importance to our organization and its success. I am pleased to inform you that, as an incentive for your continued service to the Company, the Company is making you eligible for a special retention bonus, subject to the satisfaction
of the terms and conditions described in this letter agreement. 
  

	1.	 Retention Bonus – You are eligible to receive a retention bonus in an amount equal to
$481,000, less all applicable taxes and withholdings, (the “Retention Bonus”) upon the earliest to occur of the following (the date on which the earliest of the following to occur, the “Payment Eligibility
Date”), provided you remain employed by the Company through the Payment Eligibility Date and that you otherwise satisfy the terms and conditions of this letter agreement: 

 

	 	(i)	 March 31, 2019; 

 

	 	(ii)	 the closing of a Change of Control (as defined in Exhibit A to this letter agreement) (the
“Closing”); 

  

	 	(iii)	 the termination of your employment by the Company without “Cause” (as defined in
Exhibit A to this letter agreement); or 

  

	 	(iv)	 your death. 

  

	 	2.	 Interaction with Terms of Employment Letter Agreement – You agree and acknowledge that, in
order to receive the Retention Bonus, you hereby relinquish your rights to receive any and all severance pay and bonus amounts you would otherwise be entitled to receive upon a termination of your employment (together, the “Severance
Payments”) pursuant to Sections 10(a), 10(b) or 10(c) of the Amended and Restated Letter Agreement between you and the Company dated October 10, 2017 (the “Employment Letter”), whether or not the Retention
Bonus is paid to you as a result of the termination of your employment by the Company without Cause. For the avoidance of doubt, you shall continue to be eligible to receive payment from the Company for benefits continuation and any equity
acceleration described in the Employment Letter, in accordance with the terms and conditions of the Employment Letter. The terms of your Employment Letter shall otherwise remain in full force and effect. By signing this agreement, you agree and
acknowledge that you shall have no further rights with respect to the Severance Payments. 

	3.	 Conditions to Payment – Release, Compliance, and Cooperation. In order to receive payment of
the Retention Bonus, you (or, in the event of your death, the Trust or other representatives, as set forth in Section 7 below) must sign a release and, if applicable, separation agreement in a form with customary terms to be supplied by the
Company at or promptly following the Payment Eligibility Date that includes, among other provisions, (i) a release of claims in favor of the Company and its agents, (ii) confirmation of continued compliance with the Invention, Non-Competition, Non-Solicitation and Non-Disclosure Agreement you previously signed in connection with your employment with the
Company (the “NDA”), and, (iii) if the payment of the Retention Bonus is as a result of a termination without Cause, an agreement to provide post-employment cooperation to the Company, and which release (the
“Release”) becomes enforceable within 60 days (or such shorter period as the Company specifies) following the Payment Eligibility Date. The “Release Effective Date” is the date the Release becomes
enforceable, provided that if the 60-day period for providing an enforceable Release extends into a calendar year subsequent to the year containing the Payment Eligibility Date, the Release Effective Date will
be treated, solely for payment timing purposes, as occurring no earlier than the first business day of such subsequent year unless payment may be made earlier consistent with Section 409A (as defined below). 

You must continue to comply with all of the provisions of your NDA in order to be eligible for this program; provided, however, that the
Company agrees that, in consideration of the Release, it will waive the post-employment non-competition restriction set forth in Section 4(a) of the NDA. You must also, while employed, perform the
material duties assigned to you that are consistent with your position (or those of a position to which you have been transferred by the Company or that are reasonably required of you in connection with a Closing transition, if applicable) and must
assist the Company to the best of your ability in any efforts to effect a Change of Control or to transition services thereafter, if applicable. 
  

	4.	 Payment Timing – Assuming you satisfy the conditions to payment, you will receive the
Retention Bonus in a single lump sum, less all applicable taxes and withholdings, on the first regular payroll date occurring after the Release Effective Date but no later than March 15 of the year following the year in which the Payment
Eligibility Date occurs. 

  

	5.	 Amendment – This letter agreement shall be binding upon the parties and may not be modified
in any manner, except by an instrument in writing of concurrent or subsequent date signed by duly authorized representatives of the parties hereto. 

  

	6.	 Withholding; Section 409A. All payments and benefits hereunder
will be subject to reduction for applicable tax withholdings. Any payments made over time are to be treated as a series of separate payments for purposes of Section 409A of the Internal Revenue Code of 1986, as amended
(“Section 409A” of the “Code”). This letter agreement is intended to provide for payments that are exempt from or comply with the

  
 2 

	 	
provisions of Section 409A and this letter agreement must, to the extent practicable, be construed in accordance therewith. Terms defined in this letter agreement will have the meanings
given such terms under Section 409A if and to the extent required to comply with Section 409A. In any event, the Company makes no representations or warranty and will have no liability to you or any other person if any provisions of or
payments under this letter agreement are determined to constitute deferred compensation subject to Section 409A but not to satisfy the conditions of that section. 

 

	7.	 Payment to Heirs. If you should die after signing this letter agreement, any unpaid amounts will
be paid in accordance with the terms of this letter agreement (i) to the extent permitted by law, to the Rupich Mantus Family Trust (the “Trust”), as your beneficiary, or (ii) to the extent payment to the Trust is not permitted
by law, to the executors, personal representatives, or administrators of your estate. All references to you in this letter agreement shall, where applicable, refer to the Trust or your other representatives as described in this Section 7 in the
event the Payment Eligibility Date occurs by reason of your death. 

  

	8.	 Source of Payment. Nothing herein may be construed as establishing a trust or as requiring the
Company to set aside funds to meet its obligations hereunder. 

  

	9.	 Notices. Any notice, request, demand, and other communication provided for by this letter
agreement must be in writing and will be effective when delivered in person or three business days after it is deposited in the United States mail, postage prepaid, registered or certified, or one business day after it is provided to a national
overnight carrier, and addressed to you at your last known address on the books of the Company or, in the case of the Company, at its principal place of business, attention of the President and Chief Executive Officer, or to such other address as
either party may specify by notice in the manner provided in this Section 9. 

  

	10.	 Severability. Each provision of this letter agreement must be considered severable such
that if any one provision or clause conflicts with existing or future applicable law, or may not be given full effect because of such law, this will not affect any other provision of the letter agreement, which, consistent with such law, will remain
in full force and effect. All surviving clauses must be construed so as to effectuate the purpose and intent of the parties. 

  

	11.	 Interpretation. The parties agree that this letter agreement will be construed without regard to
any presumption or rule requiring construction or interpretation against the drafting party. References in this letter agreement to “include” or “including” should be read as though they said “without limitation” or
equivalent forms. 

  

	12.	 Counterparts. This letter agreement may be executed in two or more counterparts, each of which
will be an original and all of which together will constitute one and the same instrument. 

  
 3 

	13.	 Binding Effect; Assignment and Assumption. This letter agreement will be binding upon and inure
to the benefit of the parties, any successors or assigns of the Company, and the Trust or your other representatives as described in Section 7 above. The Company may assign this letter agreement in connection with a Change of Control to an
acquirer in a Change of Control (the “Acquirer”). Any obligation of the Company to pay any compensation under this letter agreement will be deemed to have been appropriately satisfied if the Acquirer assumes the obligations
under this letter agreement. You may not assign this letter agreement to others, provided, however, that nothing precludes you from changing your designated beneficiaries to receive compensation or benefits, if any, payable under this letter
agreement upon your death. 

  

	14.	 Governing Law; Jury Trial Waiver. This letter agreement will be governed by the laws of the
Commonwealth of Massachusetts without regard to its conflicts of laws principles. Any action, suit or other legal proceeding arising under or relating to any provision of this letter agreement must be commenced only in a court of the Commonwealth of
Massachusetts (or, if appropriate, a federal court located within the Commonwealth of Massachusetts, and the Company and you each consents to the jurisdiction of such a court. IF AND TO THE EXTENT PERMITTED BY APPLICABLE LAW, EACH OF THE COMPANY AND
YOU HEREBY IRREVOCABLY WAIVE ANY RIGHT TO A TRIAL BY JURY IN ANY ACTION, SUIT OR OTHER LEGAL PROCEEDING ARISING UNDER OR RELATING TO ANY PROVISION OF THIS LETTER AGREEMENT OR THE RELEASE IT CONTEMPLATES. 

 

	15.	 Effect of this Letter Agreement – Nothing in this letter agreement imposes any requirement
on the Company or the Company’s shareholders to complete any Change of Control with respect to the Company. Nothing in this letter agreement restricts either your or the Company’s rights to terminate your employment at any time, with or
without Cause or notice, and for any or no reason, nor does it change your status as an at-will employee. This letter agreement supersedes any written or oral communications between the Company and you with
respect to retention bonuses. For the avoidance of doubt, notwithstanding the terms of the Employment Letter, you will not be eligible for, nor shall you have a right to receive, the Severance Payments. 

If you have any questions about the matters covered in this letter agreement, please call contact Sheila Daly at sheila.daly@arsanis.com or (781)819-4645. 
  

			
	Very truly yours,
		
	By:	 	/s/ Michael Gray
		 	Michael Gray
		 	Chief Financial Officer and Chief Operating Officer
		 	Arsanis, Inc.

  
 4 

 Intending to be legally bound, I have signed this letter agreement as of the date set forth below. 

 

									
					
		 	/s/ David Mantus	 		 	Date:	 	26 Nov 2018
		 	David Mantus	 		 		 	

  
 5 

 Exhibit A 

“Cause” means, for purposes of this letter agreement, the following, as determined by the Board in its reasonable judgment: 

 

	(i)	 the commission of, or indictment or conviction for, any felony, or any other crime involving dishonesty;

  

	(ii)	 participation in any fraud, deliberate and substantial misconduct, breach of duty of loyalty or breach of
fiduciary duty against the Company; 

  

	(iii)	 intentional and substantial damage to any property of the Company; 

 

	(iv)	 failure of performance of your duties (not attributable to sickness, disability or death) after reasonable
written notice no later than thirty (30) days following the occurrence of the failure and a 30-day opportunity to cure, provided, however, that such opportunity to cure shall only apply to any failure
that the Board, in its reasonable discretion, deems susceptible to cure; or 

  

	(v)	 your breach of any material provision of your Employment Letter, the NDA, or any other agreement to which you
and the Company are both parties, after reasonable written notice no later than thirty (30) days following the occurrence of the breach and a 30-day opportunity to cure, provided, however, that such
opportunity to cure shall only apply to any breach that the Board, in its reasonable discretion, deems susceptible to cure, and that any breach by you of your obligations of confidentiality or non-competition
under the NDA shall be deemed not susceptible to cure. 

 “Change of Control” means the first to occur of any of
the following: (i) a merger or consolidation, business combination, acquisition or similar transaction (a “Transaction”) in which (A) the Company is a constituent party, or (B) a subsidiary of the Company is a constituent
party and the Company issues shares of its capital stock pursuant to such Transaction, except in the case of either clause (A) or (B) any such Transaction involving the Company or a subsidiary of the Company in which the beneficial owners of
the shares of capital stock of the Company outstanding immediately prior to such Transaction continue beneficially to own, immediately following such Transaction, at least a majority by voting power of the capital stock of (x) the surviving or
resulting corporation or (y) if the surviving or resulting corporation is a wholly owned subsidiary of another corporation immediately following such Transaction, the parent corporation of such surviving or resulting corporation; (ii) the
sale, lease, transfer, exclusive license or other disposition, in a single transaction or series of related transactions, by the Company or a Company subsidiary of all or substantially all the assets of the Company and the Company subsidiaries taken
as a whole (except in connection with a Transaction not constituting a Change of Control under clause (i) or where such sale, lease, transfer, exclusive license or other disposition is to a wholly owned Company subsidiary); or (iii) the
sale or transfer, in a single transaction or series of related transactions, by the stockholders of the Company of more than 50% by voting power of the then-outstanding capital stock of the Company to any Person or entity or group of
affiliated Persons or entities. For purposes of this definition, “Person” means an individual, a corporation, an association, a partnership, an estate, a trust and any other entity or organization, other than the Company. 

  
 6

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