Document:

EX-10.3

			
	

	  	Exhibit 10.3

 2018 Amended and Restated Employment Agreement for Michael Gray 

November 26, 2018 
 Dear Mr. Gray, 

This letter (the “Employment Agreement”) amends and restates the terms and conditions of your employment with Arsanis, Inc.
(“Arsanis” or “Company”), as set forth in the Amended and Restated Letter Agreement dated October 10, 2017 (the “Existing Agreement”), and will take effect as of November 27, 2018 (the
“Effective Date”), provided that you remain employed by the Company as of the Effective Date. Until the Effective Date, the Existing Agreement will remain in force and effect and continue to govern your employment with the Company.
This Employment Agreement contains the following terms: 
 1. Employment: You will be employed to serve as the Company’s
President and Chief Executive Officer, effective as of the Effective Date. You will report directly to the Company’s Board of Directors (the “Board”) and will have those duties as are customarily performed by a president and
chief executive officer, and such other duties as may be assigned by the Board. 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 Board. As an exempt employee, you are not eligible for overtime pay. 

3. Base Salary: Your base salary will be at the rate of $18,750 per bi-monthly pay
period (annualized rate of $ 450,000) (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 Board. 
 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 55% 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 Board 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

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
  

 
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”). For the avoidance of doubt, the bonus (if
any) for calendar year 2018 would be calculated based on (i) your base salary and target bonus percentage under the Existing Agreement for the portion of 2018 prior to the Effective Date (the “Prior Period”), and (ii) your Base
Salary and target bonus percentage under this Employment Agreement for the remainder of 2018 (the “Current Period”) (i.e., the bonus (if any) would be calculated at the rate of up to 40% of an annualized rate of $400,000 for the
Prior Period, and at the rate of up to 55% of an annualized rate of $450,000 for the Current Period). 
 5. Restricted Stock. The
Company shall, subject to the approval of the Board (or the Compensation Committee of the Board), grant you a restricted stock award with respect to 250,000 shares of the Company’s common stock (the “Restricted Stock Award”),
which Restricted Stock Award shall (i) vest with respect to 25% of the shares subject to the award on the one-year anniversary of the Effective Date, and, with respect to 2.0833% of the shares subject to
the award, monthly thereafter until the fourth anniversary of the Effective Date, (ii) vest in full on a Change of Control of the Company (as defined below), and (iii) be granted under the Company’s 2017 Equity Incentive Plan (the
“Plan”) and be subject to the terms and conditions set forth the Plan and a restricted stock agreement.  

6. 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.

 7. 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. 
 8. 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). 

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
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 9. 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. 
 10. 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 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; 
 (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 Employment 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.

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
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 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 Employment Agreement. 
 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 Employment 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. 

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
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 11. 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 11(b) and 11(f) below and
Section 409A, in the event that your employment is terminated by the Company without Cause pursuant to Section 10(b) of this Employment Agreement or by you for Good Reason pursuant to Section 10(c) of this Employment 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 11(f) hereof (the “Severance Period”); 

(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 , 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 11(f) and Section 409A, in the event that your employment is terminated by the Company without Cause pursuant to Section 10(b) of this Employment Agreement or by you for Good Reason pursuant to 10(c) of this Employment
Agreement, in either case within eighteen (18) 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 11(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) one and one-half times your annualized Base Salary, at the rate then in effect, and (B) one and one-half times 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 eighteen (18) months commencing at the
time set forth in Section 10(f) hereof (the “Change of Control Severance Period”); 

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
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 (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 , on a monthly and taxable basis, an amount equal to the full monthly
premium cost of such participation until the conclusion of the twelve-month period commencing at the time set forth in Section 11(f) hereof (the “Change of Control COBRA 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 other than those granted to you in June of 2017 (the “June Grant”), those options shall remain
exercisable for the period of time set forth in the applicable grant agreement. The Company will extend until the date that is the earlier of (i) two years following the date of your termination from employment and (ii) the final exercise
date, the period during which you may exercise the June Grant. You understand that the stock options subject to this extended exercise period shall cease to be treated for tax purposes as incentive stock options. 

c. Termination by the Company Due to Your Disability or Death: Subject to Section 11(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 10(e) of this Employment 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 11(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 Section10(b) of this Employment Agreement, by you for Good Reason pursuant to Section 10(c) of this Employment Agreement, or by the Company due to your disability or death pursuant to Section 10(e) of this
Employment Agreement, the Company shall pay you the Accrued Compensation. For purposes of this Employment 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. 

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
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 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 Employment 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 Employment Agreement or otherwise are
required to be reduced as described in Section 11(e)(i) of this Employment Agreement, the adjustment will be made, first, by reducing the amount of base salary payable pursuant to Section 11(a)(i) or 11(b)(i), as applicable; second, if
additional reductions are necessary, by reducing the payment of the amounts due to you pursuant to Section 11(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 Employment 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

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
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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 Employment 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, 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 Employment Agreement
shall be cancelled with no consideration due to you, and the Company will not have any further obligations to you under this Employment 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 COBRA 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 Employment Agreement
shall survive any termination if so provided in this Employment Agreement or if necessary or desirable to accomplish the purposes of other surviving provisions of the Employment 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

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
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performance during any applicable severance periods, of your material obligations under this Employment 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 11(h). 

12. Definitions: For purposes of this Employment Agreement, the following definitions apply: 

a. “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. 

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. 

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
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 13. Section 409A. 

a. You and the Company agree that this Employment 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 Employment 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 Employment 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 Employment 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 13(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 Employment Agreement shall be paid or provided in accordance with the normal payment dates specified for
them herein. 
 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 Employment
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 Employment Agreement to comply with, or be exempt from, the requirements of Section 409A. 

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
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 14. 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 Employment Agreement. This Employment 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 Board. 

15. 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 as its President and Chief Executive Officer, the increases in
your current compensation and bonus, and the Restricted Stock Award hereunder, are conditioned on your signing and returning this Employment Agreement and your continued compliance with the Invention,
Non-Competition, Non-Solicitation and Non-Disclosure Agreement that you previously executed in connection with your employment
(the “Non-Disclosure Agreement”). 
 16. 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 Employment 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. 

17. Miscellaneous: Your rights and obligations under this Employment 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 Employment 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 Employment 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 Employment
Agreement supersedes all prior understandings, whether written or oral, relating to the terms of your employment, including without limitation the Existing Agreement. 

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
 - 11 - 

 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 Employment Agreement by the end of the day on November 26, 2018. 
 [Remainder of
page intentionally blank] 

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
 - 12 - 

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

 

	
	Sincerely,
	
	/s/ Tillman U. Gerngross
	Tillman U. Gerngross
	Chairman of the Board of Directors

  

	
	AGREED TO:
	
	/s/ Michael Gray
	Michael Gray

  

	
	November 26, 2018
	Date

  
 Arsanis, Inc. | 890
Winter Street Suite 230 • Waltham, MA • 02451 | Phone: (781) 819-5153 | www.arsanis.com 
 - 13 -EX-10.4

			
	

	  	Exhibit 10.4

 VIA HAND DELIVERY 

November 26, 2018 
 Rene Russo 

Dear Rene: 
 As we have discussed, in connection
with your separation from employment with, and all offices in, Arsanis, Inc. (the “Company”) (and, as may be applicable, any and all of its parents, affiliates and subsidiaries (together, the “Affiliates”)), effective
November 26, 2018 (the “Separation Date”), you will be eligible to receive the severance benefits described in paragraph 1 below if you sign and return this letter agreement to me no later than December 18, 2018, but no
earlier than close of business on the Separation Date, and do not revoke your agreement (as described below). 
 By signing and
returning this letter agreement and not revoking your acceptance, you will be entering into a binding agreement with the Company and will be agreeing to the terms and conditions set forth in the numbered paragraphs below, including the release of
claims set forth in paragraph 2. Therefore, you are advised to consult with an attorney before signing this letter agreement and you have been given at least twenty-one (21) days to do so. If you sign
this letter agreement, you may change your mind and revoke your agreement during the seven (7) day period after you have signed it (the “Revocation Period”) by notifying me in writing. If you do not so revoke, this letter
agreement will become a binding agreement between you and the Company upon the expiration of the Revocation Period. 
 Although your receipt
of the severance benefits is expressly conditioned on your entering into this letter agreement, the following will apply regardless of whether or not you do so: 
  

	 	•	 	 As of the Separation Date, all salary payments from the Company will cease and any benefits you had as of the
Separation Date under Company-provided benefit plans, programs, or practices will terminate, except as required by federal or state law. 

  

	 	•	 	 You will receive payment for your final wages and any unused vacation time accrued through the Separation Date.

  

	 	•	 	 You may, if eligible and at your own cost, elect to continue receiving group medical insurance pursuant to the
“COBRA” law. Please consult the COBRA materials to be provided under separate cover for details regarding these benefits. 

	 	•	 	 You are obligated to keep confidential and not to use or disclose any and all
non-public information concerning the Company that you acquired during the course of your employment with the Company, including any non-public information concerning
the Company’s business affairs, business prospects, and financial condition, except as otherwise permitted by paragraph 8 below. Further, you remain subject to your continuing obligations to the Company as set forth in the Invention, Non-Competition, Non-Solicitation and Non-Disclosure Agreement that you previously executed for the benefit of the Company (the “Non-Disclosure Agreement”), which remain in full force and effect, except as and to the extent set forth in paragraph 3 below. 

 

	 	•	 	 You must immediately return to the Company no later than the Separation Date all Company property.

 If you elect to timely sign and return this letter agreement and do not revoke your acceptance within the Revocation
Period, the following terms and conditions will also apply: 
 Severance Benefits – The Company will provide you with the
following severance benefits (the “severance benefits”): 
  

	 	a.	 Severance Pay. The Company shall pay you one million forty-six
thousand two hundred fifty dollars ($1,046,250), less applicable taxes and withholdings, as severance pay (the “Severance Pay”). The Severance Pay shall be payable to you in one lump sum, as soon as administratively practicable following
the date that this letter agreement becomes effective and irrevocable. 

  

	 	b.	 COBRA Benefits. Provided that you are eligible for and timely elect to continue participation in the
Company’s group health and dental plans under applicable COBRA law, the Company will, subject to and in accordance with the terms set forth in Section 10(b)(ii) of the Amended and Restated Letter Agreement dated as of October 10, 2017
between you and the Company (the “Employment Agreement”), pay, on a monthly basis, the full monthly premium cost of such participation. 

  

	 	c.	 Option Acceleration. Effective immediately after this letter agreement becomes effective and
irrevocable, all outstanding and unvested options to purchase shares of the Company’s common stock which you hold as of the Separation Date shall become fully vested and exercisable, and shall remain exercisable for the period of time set forth
in paragraph 1(d) below. 

  

	 	d.	 Extension of Exercise Period. Effective immediately after this letter agreement becomes effective and
irrevocable, the Company will extend until the date that is the earlier of (i) two years following the Separation Date and (ii) the final exercise date, the period during which you may exercise those stock options granted to you in June of
2017 and in which you have vested as of the Separation Date (including those in which you have vested pursuant to paragraph 2(c) above), subject to the terms of the applicable agreement(s) and plan(s) governing such options. You understand that the
stock options subject to this extended exercise period shall cease to be treated for tax purposes as incentive stock options. 

  
 - 2 - 

 You will not be eligible for, nor shall you have a right to receive, any
payments or benefits from the Company following the Separation Date other than as set forth in this paragraph. 
 Release of Claims – In
consideration of the severance benefits, which you acknowledge you would not otherwise be entitled to receive, you hereby fully, forever, irrevocably and unconditionally release, remise and discharge the Company, its Affiliates, predecessors, and
successors, and all of their respective past and present officers, directors, stockholders, partners, members, employees, agents, representatives, plan administrators, attorneys, insurers and fiduciaries (each in their individual and corporate
capacities) (collectively, the “Released Parties”) from any and all claims, charges, complaints, demands, actions, causes of action, suits, rights, debts, sums of money, costs, accounts, reckonings, covenants, contracts, agreements,
promises, doings, omissions, damages, executions, obligations, liabilities, and expenses (including attorneys’ fees and costs), of every kind and nature that you ever had or now have against any or all of the Released Parties, whether known or
unknown, including, but not limited to, any and all claims arising out of or relating to your employment with and/or separation from the Company, including, but not limited to, all claims under Title VII of the Civil Rights Act, the Americans With
Disabilities Act, the Age Discrimination in Employment Act, the Genetic Information Nondiscrimination Act, the Family and Medical Leave Act, the Worker Adjustment and Retraining Notification Act, the Rehabilitation Act, Executive Order 11246,
Executive Order 11141, the Fair Credit Reporting Act, and the Employee Retirement Income Security Act, all as amended; all claims arising out of the Massachusetts Fair Employment Practices Act, Mass. Gen. Laws ch. 151B, § 1 et
seq., the Massachusetts Wage Act, Mass. Gen. Laws ch. 149, § 148 et seq. (Massachusetts law regarding payment of wages and overtime), the Massachusetts Civil Rights Act, Mass. Gen. Laws ch. 12, §§ 11H and 11I, the
Massachusetts Equal Rights Act, Mass. Gen. Laws. ch. 93, § 102 and Mass. Gen. Laws ch. 214, § 1C, the Massachusetts Labor and Industries Act, Mass. Gen. Laws ch. 149, § 1 et seq., Mass. Gen. Laws ch. 214, § 1B
(Massachusetts right of privacy law), the Massachusetts Maternity Leave Act, Mass. Gen. Laws ch. 149, § 105D, and the Massachusetts Small Necessities Leave Act, Mass. Gen. Laws ch. 149, § 52D, all as amended; all common law claims
including, but not limited to, actions in defamation, intentional infliction of emotional distress, misrepresentation, fraud, wrongful discharge, and breach of contract (including, without limitation, any claims arising out of or related to the
Employment Agreement); all claims to any non-vested ownership interest in the Company, contractual or otherwise; all state and federal whistleblower claims to the maximum extent permitted by law; and any claim
or damage arising out of your employment with and/or separation from the Company (including a claim for retaliation) under any common law theory or any federal, state or local statute or ordinance not expressly referenced above; provided, however,
that this release of claims does not prevent you from filing a charge with, cooperating with, or participating in any investigation or proceeding before, the Equal Employment Opportunity Commission or a state fair employment practices agency (except
that you acknowledge that you may not recover any monetary benefits in connection with any such charge, investigation, or proceeding, and you further waive any rights or claims to any payment, benefit, attorneys’ fees or other remedial relief
in connection with any such charge, investigation or proceeding). Further, nothing herein shall release (i) any claims you may have for indemnification under the Company’s certificate of 

  
 - 3 - 

 
incorporation, by-laws, insurance and/or any written agreement between you and the Company for director or officer indemnification (recognizing that any
such indemnification is not guaranteed by this letter agreement and shall be governed by the instrument, if any, providing for such indemnification), (ii) any rights you may have to receive insurance payments under any policy maintained by the
Company; (iii) any vested rights you may have as an equity holder or option holder, (iv) any rights you may have to receive retirement and other benefits that are accrued and fully vested as of the Separation Date, or (v) any other
claims that cannot be released as a matter of law. 
 Continuing Obligations – You acknowledge and reaffirm your confidentiality
and non-disclosure obligations discussed above in this letter agreement, as well as the obligations set forth in the Non-Disclosure Agreement, which survive your
separation from employment with the Company; provided, however, that in consideration of your entering into this letter agreement, the Company will waive the post-employment non-competition restriction set
forth in Section 4(a) thereof. 
 Non-Disparagement – You understand and agree that, to the
extent permitted by law and except as otherwise permitted by paragraph 8 below, you will not, in public or private, make any false, disparaging, derogatory or defamatory statements, online (including, without limitation, on any social media,
networking, or employer review site) or otherwise, to any person or entity, including, but not limited to, any media outlet, industry group, financial institution or current or former employee, board member, consultant, client or customer of the
Company, regarding the Company or any of the other Released Parties, or regarding the Company’s business affairs, business prospects, or financial condition. In turn, the Company will instruct those with knowledge of this letter agreement not
to make, in public or private, any false, disparaging, derogatory or defamatory statements about you. 
 Return of Company Property
– You confirm that you have returned to the Company all keys, files, records (and copies thereof), equipment (including, but not limited to, computer hardware, software, printers, flash drives and other storage devices, wireless handheld
devices, cellular phones, tablets, etc.), Company identification, and any other Company owned property in your possession or control, and that you have left intact all, and have otherwise not destroyed, deleted, or made inaccessible to the Company,
any electronic Company documents, including, but not limited to, those that you developed or helped to develop during your employment, and that you have not (a) retained any copies in any form or media; (b) maintained access to any copies
in any form, media, or location; (c) stored any copies in any physical or electronic locations that are not readily accessible or not known to the Company or that remain accessible to you; or (d) sent, given, or made accessible any copies
to any persons or entities that the Company has not authorized to receive such electronic or hard copies. Notwithstanding the foregoing, you may retain possession and assume ownership of the iPhone, MacBook, and iPad you have been using in
connection with your employment; provided, however, that you hereby represent that you have deleted from such devices all documents, e-mails, files, information, and data related to the Company. You further
confirm that you have cancelled all accounts for your benefit, if any, in the Company names, including but not limited to, credit cards, telephone charge cards, cellular phone accounts, and computer accounts. 

  
 - 4 - 

 Business Expenses and Final Compensation – You acknowledge that you have been reimbursed
by the Company for all business expenses incurred in conjunction with the performance of your employment and that no other reimbursements are owed to you. You further acknowledge that you have received payment in full for all services rendered in
conjunction with your employment by the Company, including payment for all wages, bonuses, and accrued, unused vacation time, and that no other compensation is owed to you except as provided herein. 

Confidentiality – You understand and agree that, to the extent permitted by law and except as otherwise permitted by paragraph 8 below, the
contents of the negotiations and discussions resulting in this letter agreement, shall be maintained as confidential by you and your agents and representatives and shall not be disclosed except as otherwise agreed to in writing by the Company. 

Scope of Disclosure Restrictions – Nothing in this letter agreement prohibits you from communicating with government agencies about
possible violations of federal, state, or local laws or otherwise providing information to government agencies, filing a complaint with government agencies, or participating in government agency investigations or proceedings. You are not required to
notify the Company of any such communications; provided, however, that nothing herein authorizes the disclosure of information you obtained through a communication that was subject to the attorney-client privilege. Further, notwithstanding your
confidentiality and nondisclosure obligations, you are hereby advised as follows pursuant to the Defend Trade Secrets Act: “An individual shall not be held criminally or civilly liable under any Federal or State trade secret law 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. An individual who files a lawsuit for retaliation by an employer for reporting a suspected
violation of law may disclose the trade secret to the attorney of the individual and use the trade secret information in the court proceeding, if the individual (A) files any document containing the trade secret under seal; and (B) does
not disclose the trade secret, except pursuant to court order.” 
 Cooperation – You agree that, to the extent
permitted by law, you shall cooperate fully with the Company in the investigation, defense or prosecution of any claims or actions which already have been brought, are currently pending, or which may be brought in the future against the Company by a
third party or by or on behalf of the Company against any third party, whether before a state or federal court, any state or federal government agency, or a mediator or arbitrator. Your full cooperation in connection with such claims or actions
shall include, but not be limited to, being available to meet with the Company’s counsel, at reasonable times and locations designated by the Company, to investigate or prepare the Company’s claims or defenses, to prepare for trial or
discovery or an administrative hearing, mediation, arbitration or other proceeding and to act as a witness when requested by the Company. You further agree that, to the extent permitted by law, you will notify the Company promptly in the
event that you are served with a subpoena (other than a subpoena issued by a government agency), or in the event that you are asked to provide a third party (other than a government agency) with information concerning any actual or potential
complaint or claim against the Company. All 

  
 - 5 - 

 
reasonable out-of-pocket expenses incurred by you in providing such reasonable cooperation under this paragraph 9
shall be reimbursed by the Company, and shall be paid promptly upon request no later than 30 days after submission of documentation thereof. In addition, the Company shall continue to indemnify you as and to the extent set forth in any
indemnification provision that may exist in the Company’s certificate of incorporation, by-laws, insurance and/or any written agreement between you and the Company for director or officer indemnification.

 Amendment and Waiver – 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. This letter agreement is binding upon and shall inure to the benefit of the parties and their respective agents, assigns,
heirs, executors, successors and administrators. No delay or omission by the Company in exercising any right under this letter agreement shall operate as a waiver of that or any other right. A waiver or consent given by the Company on any one
occasion shall be effective only in that instance and shall not be construed as a bar to or waiver of any right on any other occasion. 

Validity – Should any provision of this letter agreement be declared or be determined by any court of competent jurisdiction to be illegal
or invalid, the validity of the remaining parts, terms or provisions shall not be affected thereby and said illegal or invalid part, term or provision shall be deemed not to be a part of this letter agreement. 

Nature of Agreement – You understand and agree that this letter agreement is a severance agreement and does not constitute an
admission of liability or wrongdoing on the part of the Company. 
 Acknowledgments – You acknowledge that you
have been given at least twenty-one (21) days to consider this letter agreement, and that the Company is hereby advising you to consult with an attorney of your own choosing prior to signing this letter
agreement. You understand that you may revoke this letter agreement for a period of seven (7) days after you sign this letter agreement by notifying me in writing, and the letter agreement shall not be effective or enforceable until the
expiration of this seven (7) day revocation period. You understand and agree that by entering into this letter agreement, you are waiving any and all rights or claims you might have under the Age Discrimination in Employment Act, as amended by
the Older Workers Benefit Protection Act, and that you have received consideration beyond that to which you were previously entitled. 

Voluntary Assent – You affirm that no other promises or agreements of any kind have been made to or with you by any
person or entity whatsoever to cause you to sign this letter agreement, and that you fully understand the meaning and intent of this letter agreement. You further state and represent that you have carefully read this letter agreement, understand the
contents herein, freely and voluntarily assent to all of the terms and conditions hereof, and sign your name of your own free act. 
 Applicable
Law – This letter agreement shall be interpreted and construed by the laws of the Commonwealth of Massachusetts, without regard to conflict of laws provisions. You hereby irrevocably submit to and acknowledge and recognize the
jurisdiction of the courts of the Commonwealth of Massachusetts, or if appropriate, a federal court located in the Commonwealth of Massachusetts (which courts, for purposes of this letter agreement, are the only courts of competent jurisdiction),
over any suit, action or other proceeding arising out of, under or in connection with this letter agreement or the subject matter hereof. 

  
 - 6 - 

 Entire Agreement – This letter agreement contains and constitutes the entire
understanding and agreement between the parties hereto with respect to your severance benefits and the settlement of claims against the Company and cancels all previous oral and written negotiations, agreements, and commitments in connection
therewith.  
 Tax Acknowledgement – In connection with the severance benefits provided to you pursuant to this letter agreement,
the Company shall withhold and remit to the tax authorities the amounts required under applicable law, and you shall be responsible for all applicable taxes with respect to such severance benefits under applicable law. You acknowledge that you are
not relying upon the advice or representation of the Company with respect to the tax treatment of any of the severance benefits set forth in paragraph 1 of this letter agreement. 

Section 409A – This letter agreement, and all payments hereunder, are intended to be exempt from, or if
not so exempt, to comply with the requirements of, Section 409A of the Internal Revenue Code of 1986, as amended, and the guidance issued thereunder (“Section 409A”), and this letter agreement shall be interpreted and
administered accordingly. Notwithstanding anything to the contrary in this letter agreement, if at the time of your termination of employment or otherwise when any severance benefit is payable hereunder, you are a “specified employee” as
defined under Section 409A, any and all amounts payable hereunder on account of such termination of employment that would (but for this provision) be payable within six (6) months following the Separation Date, shall instead be paid on the
next business day following the expiration of such six (6) month period or, if earlier, upon your death; except to the extent of amounts that do not constitute a deferral of compensation within the meaning of Treasury regulation
Section 1.409A – 1(b) or other amounts or benefits that are exempt from or otherwise not subject to the requirements of Section 409A. For purposes of this letter agreement, whether or not a termination of employment has occurred shall
be determined consistently with Section 409A. In addition, each payment made pursuant to the letter agreement shall be treated as a separate payment and the right to a series of installment payments hereunder is to be treated as a right to a
series of separate payments. Neither the Company, nor any of its Affiliates, nor any of its or their respective agents shall have any liability to you should the benefits and payments hereunder that are intended to be exempt from or compliant with
Section 409A, not be so exempt or compliant. 
 [Remainder of page is intentionally left blank] 

  
 - 7 - 

 If you have any questions about the matters covered in this letter agreement, please call me. 

 

			
	Very truly yours,
		
	By:	 	/s/ Tillman U. Gerngross
		 	Tillman U. Gerngross
		 	Chairman of the Board of Directors

 I hereby agree to the terms and conditions set forth above. I have been given at least
twenty-one (21) days to consider this letter agreement, and I have chosen to execute this on the date below. I intend that this letter agreement will become a binding agreement between me and the Company
if I do not revoke my acceptance in seven (7) days. 
  

					
	/s/ René Russo	 		 	11-26-18
	René Russo	 		 	Date

 To be returned in a timely manner as set forth on the first page of this letter agreement. 

  
 - 8 -

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