Document:

EX-10.37

 Exhibit 10.37 

Execution Version 

ARSANIS, INC. 
 SECOND
AMENDED AND RESTATED 
 STOCKHOLDERS’ AGREEMENT 

Dated as of April 12, 2016 

 TABLE OF CONTENTS 

 

					
	 	  	Page No.	 
	 1. VOTING AGREEMENT.
	  	 	2	 
	 1.1 VOTING PROVISIONS REGARDING THE
BOARD OF DIRECTORS
	  	 	2	 
	 1.2 VOTING SHARES
	  	 	3	 
	 1.3 TRANSFERS OF VOTING
RIGHTS
	  	 	3	 
	 1.4 SUCCESSOR DIRECTORS
	  	 	3	 
	 1.5 NO LIABILITY FOR ELECTION
OF RECOMMENDED DIRECTORS
	  	 	4	 
	 1.6 VOTE TO INCREASE AUTHORIZED
COMMON STOCK
	  	 	4	 
		
	 2. DRAG-ALONG RIGHTS.
	  	 	4	 
	 2.1 DEFINITION
	  	 	4	 
	 2.2 ACTIONS TO BE TAKEN
	  	 	4	 
	 2.3 EXCEPTIONS
	  	 	5	 
	 2.4 RESTRICTIONS ON SALES OF
CONTROL OF THE COMPANY
	  	 	7	 
		
	 3. TRANSFER OF SHARES.
	  	 	7	 
	 3.1 RESTRICTIONS ON TRANSFER BY
RESTRICTED STOCKHOLDERS
	  	 	7	 
	 3.2 TRANSFERS BY RESTRICTED
STOCKHOLDERS NOT SUBJECT TO RESTRICTIONS
	  	 	7	 
	 3.3 OFFER OF TRANSFER; NOTICE
OF PROPOSED TRANSFER
	  	 	7	 
	 3.4 COMPANY’S OPTION TO
PURCHASE
	  	 	7	 
	 3.5 INVESTORS’ OPTION TO
PURCHASE
	  	 	8	 
	 3.6 FAILURE TO FULLY EXERCISE
OPTIONS; CO-SALE RIGHTS WITH RESPECT TO SELLING RESTRICTED
STOCKHOLDER
	  	 	9	 
	 3.7 EXCEPTION FOR DRAG-ALONG
RIGHTS
	  	 	10	 
		
	 4. RESTRICTIVE LEGENDS.
	  	 	10	 
	 4.1 VOTING SHARES
	  	 	10	 
	 4.2 TRANSFER RESTRICTION
	  	 	11	 
		
	 5. TERMINATION OF AGREEMENT.
	  	 	11	 
	 5.1 TERMINATION
	  	 	11	 
	 5.2 EXCEPTION
	  	 	11	 
		
	 6. TRANSFER OF RIGHTS
	  	 	11	 
		
	 7. IRREVOCABLE PROXY AND POWER OF ATTORNEY
	  	 	12	 
		
	 8. STAND-OFF AGREEMENT
	  	 	12	 
		
	 9. GENERAL.
	  	 	13	 
	 9.1 ADDITIONAL PARTIES
	  	 	13	 
	 9.2 SEVERABILITY
	  	 	13	 
	 9.3 COVENANTS OF THE
COMPANY
	  	 	13	 
	 9.4 SPECIFIC PERFORMANCE
	  	 	13	 
	 9.5 REMEDIES CUMULATIVE
	  	 	14	 

  
 i 

  

			
	 9.6 GOVERNING LAW
	  	14
	 9.7 NOTICES
	  	14
	 9.8 COMPLETE AGREEMENT; AMENDMENTS
	  	15
	 9.9 CONSTRUCTION
	  	16
	 9.10 COUNTERPARTS; FACSIMILE SIGNATURES
	  	16
	 9.11 AGGREGATION OF SHARES
	  	16

 EXHIBITS 
 Exhibit
A     Schedule of Purchasers 

  
 ii 

 ARSANIS, INC. 

SECOND AMENDED AND RESTATED STOCKHOLDERS’ AGREEMENT 

This Second Amended and Restated Stockholders’ Agreement (the “Agreement”) is entered into as of April 12, 2016, by
and among Arsanis, Inc., a Delaware corporation (the “Company”), the persons and entities listed on Exhibit A attached hereto (collectively with any subsequent investors or transferees who become parties
hereto as Investors pursuant to Section 9.1(a), the “Investors”), and Eszter Nagy, Tillman U. Gerngross and Errik B. Anderson (each, individually, a “Founder” and collectively, the “Founders,”
and collectively with any parties who become parties hereto as Restricted Stockholders pursuant to Section 9.1(b), the “Restricted Stockholders”). The Investors and Restricted Stockholders are referred to herein collectively as
the “Stockholders”). As used in this Agreement, the term “Shares” shall include all shares of capital stock of the Company held by the Stockholders, whether now owned or hereafter acquired. For purposes of
calculating an Investor’s pro rata ownership of Shares, all shares of Series A-1 Convertible Preferred Stock, par value $0.001 per share, of the Company (the “Series A-1 Preferred Stock”), Series A-2 Convertible Preferred Stock, par value $0.001 per share, of the Company (the “Series
A-2 Preferred Stock”), Series B Convertible Preferred Stock, par value $0.001 per share, of the Company (the “Series B Preferred Stock”) and Series C Convertible Preferred Stock, par
value $0.001 per share, of the Company (the “Series C Preferred Stock” and together with the Series A-1 Preferred Stock, Series A-2 Preferred Stock and
Series B Preferred Stock, the “Preferred Stock”) shall be deemed to have been converted into shares of Common Stock, par value $0.001 per share of the Company (the “Common Stock”). 

Recitals 
 WHEREAS, the
Founders own certain outstanding shares of the Company’s Common Stock; 
 WHEREAS, concurrently with the execution of this Agreement,
the Company and certain of the Investors are entering into a Series C Securities Purchase Agreement (the “Purchase Agreement”) providing for, among other things, the sale of shares of the Company’s Series C Preferred Stock.
Certain of the Investors (the “Existing Investors”) and the Restricted Stockholders are parties to that certain Amended and Restated Stockholders’ Agreement, dated as of July 30, 2013, by and among the Company and the
parties thereto (such agreement, as amended as of February 9, 2015 and further amended as of May 22, 2015, the “Prior Agreement”). 

WHEREAS, the parties to the Prior Agreement desire to amend and restate the Prior Agreement in its entirety to provide those Investors
purchasing shares of Series C Preferred Stock with the right, among other rights, to elect certain members of the board of directors of the Company (the “Board”) in accordance with the terms of this Agreement; 

WHEREAS, the Company, the Founders and the Investors wish to (i) protect the management and control of the Company from influence by any
person not acceptable to the Company and the Investors and (ii) assist the Investors in selling their Shares under certain circumstances. 

 NOW, THEREFORE, the parties agree that the provisions of the Prior Agreement are hereby amended
and restated in their entirety, and hereby further agree, as follows: 
 1. Voting Agreement. 

1.1 Voting Provisions Regarding the Board of Directors. In any and all elections of directors of the Company (whether at a meeting or by written
consent in lieu of a meeting), each Stockholder shall vote or cause to be voted all Voting Shares (as defined in Section 1.2 below) owned by him, her or it or over which he, she or it has voting control, and otherwise use his, her or its best
efforts, so as to: 
 (a) fix the number of directors at ten (10); and 

(b) elect as directors: 

(i) Five individuals elected by the holders of a majority of the Preferred Stock voting on an
as-converted to Common Stock basis held by the Investors; provided, that each holder of Preferred Stock (including for purposes of this section, their transferees) agrees to vote all of its
shares of the capital stock of the Company having voting power (and any other shares over which he, she or it exercises voting control), to the extent it holds such voting stock at the relevant time, so that at all times (A) one individual then
designated by Polaris Venture Partners V, L.P. (“Polaris”) so long as Polaris is the holder of any shares of Preferred Stock, the first such designee being Terrance McGuire, (B) one individual then designated by OrbiMed Private
Investments IV, LP (“OrbiMed”) so long as OrbiMed is the holder of any shares of Preferred Stock, the first such designee being Carl Gordon, (C) one individual then designated by SV Life Sciences Fund V, L.P.
(“SVLS”) so long as SVLS is the holder of any shares of Preferred Stock, the first such designee being Michael Ross, (D) one individual then designated by NeoMed Innovation V L.P. (“NeoMed”) so long as NeoMed
is the holder of any shares of Preferred Stock, the first such designee being Claudio Nessi, and (E) one individual then designated by EMBL Technology Fund II GmbH & Co. KG (“EMBL”) so long as EMBL is the holder of any
shares of Preferred Stock, the first such designee being Jan Adams, are members of the Board of Directors of the Company, provided, however, that should any of Polaris, OrbiMed, SVLS, NeoMed or EMBL, or their respective affiliates,
cease to hold any shares of Preferred Stock, then such party shall immediately lose its right to designate a director and from that point forward, such director shall be nominated and elected by the holders of a majority of the shares of Preferred
Stock held by the Investors, voting on an as-converted to Common Stock basis; 
 (ii)
One individual designated by Tillman Gerngross so long as he is the holder of any shares of Common Stock, such designee to initially be Tillman Gerngross; 

(iii) One individual designated by the Founders holding a majority of the aggregate Shares held by the Founders; 

  
 - 2 - 

 (iv) The Company’s most senior executive officer who is an employee of the
Company, as determined by the Board of Directors, currently Eszter Nagy; and 
 (v) Two individuals designated by a majority
of the members of the Board, such designees to initially be Dan Burgess and Amy Schulman. 
 1.2 Voting Shares. “Voting Shares”
shall mean the Shares and any and all other shares of capital stock of the Company, by whatever name called, which carry voting rights (including voting rights which arise by reason of default) which are now owned or subsequently acquired by a
Stockholder, however acquired, including, without limitation, through stock splits, stock dividends, reclassifications, recapitalizations and other similar events. 

1.3 Transfers of Voting Rights. Any transferee to whom Voting Shares are transferred by a Stockholder, whether voluntarily or by operation of
law, shall be bound by the obligations imposed upon the transferor under this Agreement, to the same extent as if such transferee were a Stockholder hereunder, and, as a condition precedent to the Company’s recognizing such transfer, each
transferee or assignee shall agree in writing to be subject to each of the terms of this Agreement by executing and delivering an Adoption Agreement substantially in the form attached hereto as Exhibit B. Upon the execution and delivery of an
Adoption Agreement by any transferee, such transferee shall be deemed to be a party hereto as if such transferee were the transferor and such transferee’s signature appeared on the signature pages of this Agreement and shall be deemed to be an
Investor and Stockholder, or Restricted Stockholder and Stockholder, as applicable. The Company shall not permit the transfer of the Voting Shares subject to this Agreement on its books or issue a new certificate representing any such Voting Shares
unless and until such transferee shall have complied with the terms of this Section 1.3. Each certificate representing Voting Shares subject to this Agreement if issued on or after the date of this Agreement shall be endorsed by the Company
with the legends set forth in Section 4. 
 1.4 Successor Directors. If a person who has been elected as a director pursuant to a designation
under Section 1.1(b)(i), 1.1(b)(ii) or 1.1(b)(iii) above shall cease to serve as a director for any reason, the persons or entities who had the right to designate such person as a director under Section 1.1(b)(i), 1.1(b)(ii) or 1.1(b)(iii)
above (the “Nominating Party”), shall have the right to designate a successor nominee and each of the other parties hereto who or which are entitled under the Company’s Certificate of Incorporation to vote or act by written
consent with respect to the election of such successor nominee as a director shall vote or cause to be voted, or execute or cause to be executed a written consent with respect to, all Voting Shares in favor of such successor nominee. If a person who
has been elected as a director under Section 1.1(b)(iv) above shall cease to serve as a director for any reason, the Board of Directors shall promptly designate the Company’s most senior executive officer who is an employee of the Company
as a successor nominee and each of the other parties hereto who or which are entitled under the Company’s Certificate of Incorporation to vote or act by written consent with respect to the election of such successor nominee as a director shall
vote or cause to be voted, or execute or cause to be executed a written consent with respect to, all Voting Shares in favor of such successor nominee. If a person who has been elected as a director under Section 1.1(b)(v) above shall cease to
serve as a director for any reason, the Board of Directors shall promptly designate a successor nominee and each of the other parties hereto who or which are entitled under the Company’s Certificate of 

  
 - 3 - 

 Incorporation to vote or act by written consent with respect to the election of such successor nominee as a
director shall vote or cause to be voted, or execute or cause to be executed a written consent with respect to, all Voting Shares in favor of such successor nominee. Except as provided herein, a director designated under Section 1.1(b)(i),
1.1(b)(ii), 1.1(b)(iii) or 1.1(b)(v) above may be removed during his or her term of office, with or without cause, by and only by the affirmative vote or written consent of those Nominating Parties that had the right to designate the director (or
the Board of Directors in the case of a director nominated under Section 1.1(b)(v)). A director designated under Section 1.1(b)(iv) shall not be removed so long as such director remains (a) the Company’s most senior executive
officer and (b) an employee of the Company. If a Nominating Party (or the Board of Directors in the case of a director nominated under Section 1.1(b)(v)) notifies the other parties hereto that it desires to remove its nominee as a
director, each of the other parties hereto who or which are entitled under the Certificate of Incorporation to vote or act by written consent with respect to the removal of such nominee as a director, shall vote or cause to be voted, or execute or
cause to be executed a written consent with respect to, all Voting Shares in favor of such removal. If a Nominating Party (or the Board of Directors in the case of a director nominated under Section 1.1(b)(v)) notifies the Company that it
desires to remove its nominee as a director and/or designate a successor nominee by means of stockholder vote, the Company shall, at the request of such Nominating Party or the Board of Directors, as applicable, use its reasonable best efforts to
ensure that a meeting of stockholders of the Company is promptly called for such purpose. 
 1.5 No Liability for Election of Recommended Directors.
No Stockholder, nor any affiliate of any Stockholder, shall have any liability as a result of designating a person for election as a director for any act or omission by such designated person in his or her capacity as a director of the Company, nor
shall any Stockholder have any liability as a result of voting for any such designee in accordance with the provisions of this Agreement. 
 1.6 Vote to
Increase Authorized Common Stock. Each Stockholder agrees to vote or cause to be voted all Voting Shares owned by such Stockholder, or over which such Stockholder has voting control, from time to time and at all times, in whatever manner as
shall be necessary to increase the number of authorized shares of Common Stock from time to time to ensure that there will be sufficient shares of Common Stock available for conversion of all of the shares of Preferred Stock outstanding at any given
time. 
 2. Drag-Along Rights. 
 2.1
Definition . A “Sale of the Company” shall mean a transaction that qualifies as an “Event” pursuant to the Company’s Certificate of Incorporation. 

2.2 Actions to be Taken. In the event that the holders of at least a majority of the shares of Common Stock then issuable upon conversion of the shares
of the then-outstanding Preferred Stock (the “Selling Investors”) approve a Sale of the Company in writing, specifying that this Section 2 shall apply to such transaction, then each Stockholder hereby agrees: 

(a) if such transaction requires stockholder approval, with respect to all Voting Shares and/or any other Company securities
that such Stockholder owns or over which such Stockholder otherwise exercises voting power, (i) to vote (in person, by 

  
 - 4 - 

 
proxy or by action by written consent, as applicable) all Voting Shares in favor of, and adopt, such Sale of the Company and to vote in opposition to any and all other proposals that could
reasonably be expected to delay or impair the ability of the Company to consummate such Sale of the Company, and (ii) to be present, in person or by proxy, as a holder of Voting Shares or other securities of the Company, at any related
stockholder meetings that have been duly noticed and held, and be counted for the purposes of determining the presence of a quorum at such meetings; 

(b) if such transaction is a sale of stock by Selling Investors, to sell the same proportion of shares of capital stock of the
Company beneficially held by such Stockholder as is being sold by the Selling Investors to the person to whom the Selling Investors propose to sell their Shares, and on the same terms and conditions as the Selling Investors; 

(c) to execute and deliver all related documentation and take such other action in support of the Sale of the Company as shall
reasonably be requested by the Company or the Selling Investors in order to carry out the terms and provision of this Section 2, including without limitation executing and delivering instruments of conveyance and transfer, and any purchase
agreement, merger agreement, indemnity agreement, escrow agreement, consent, waiver, governmental filing, share certificates duly endorsed for transfer (free and clear of impermissible liens, claims and encumbrances), and any similar or related
documents; 
 (d) not to deposit, and to cause their affiliates not to deposit, except as provided in this Agreement, any
voting securities of the Company owned by such party or affiliate in a voting trust or subject any such voting securities to any arrangement or agreement with respect to the voting of such shares of capital stock, unless specifically requested to do
so by the acquirer, the Company or the Selling Investors in connection with the Sale of the Company; and 
 (e) to refrain
from exercising any dissenters’ rights or rights of appraisal under applicable law at any time with respect to such Sale of the Company. 
 2.3
Exceptions. Notwithstanding the forgoing, a Stockholder will not be required to comply with Section 2.2 above in connection with any specific Sale of the Company (the “Proposed Sale”) unless: 

(a) any representations and warranties to be made by such Stockholder in connection with the Proposed Sale are limited to
representations and warranties related to authority, ownership of the Shares held by such Stockholder and the ability to convey title to such Shares, including but not limited to representations and warranties that (i) the Stockholder holds all
right, title and interest in and to the Company’s securities such Stockholder purports to hold, free and clear of all liens and encumbrances, (ii) the obligations of the Stockholder in connection with the transaction have been duly
authorized, if applicable, (iii) the documents to be entered into by the Stockholder have been duly executed and delivered to the acquirer and are enforceable against the Stockholder in accordance with their respective terms and
(iv) neither the execution and 

  
 - 5 - 

 
delivery of documents to be entered into in connection with the transaction, nor the performance of the Stockholder’s obligations thereunder, will cause a breach or violation of the terms of
any agreement, law or judgment, order or decree of any court or governmental agency; 
 (b) the liability for
indemnification, if any, of such Stockholder in the Proposed Sale, and with respect to any representation or warranty made by the Company in connection with such Proposed Sale, is several and not joint with any other person, and is pro rata in
accordance with such Stockholder’s relative stock ownership of the Company; 
 (c) such liability shall be limited to
the amount of consideration actually paid to such Stockholder in connection with such Proposed Sale, except with respect to (i) representations and warranties of such Stockholder related to authority, ownership of the Shares held by such
Stockholder and the ability to convey title to such Shares, (ii) any covenants made by such Stockholder with respect to confidentiality or voting related to the Proposed Sale or (iii) claims related to fraud or willful breach by such
Stockholder, the liability for which need not be limited; 
 (d) upon the consummation of the Proposed Sale, each holder of
each series of the Company’s Preferred Stock and each holder of Common Stock will receive the same form and amount of consideration per share of such series of Preferred Stock or Common Stock, respectively, and the holders of each series of
Preferred Stock and Common Stock shall, together, receive the same form and amount of consideration per share of such stock, taking into account the relative liquidation preference to which the holders of each respective series of Preferred Stock
and the holders of Common Stock are entitled in a liquidation (if applicable) in accordance with the Company’s Certificate of Incorporation in effect immediately prior to the Proposed Sale, provided, however that if the consideration to be paid
in exchange for the Shares pursuant to the Proposed Sale includes any securities and due receipt thereof by any Stockholder would require under applicable law (x) the registration or qualification of such securities or of any person as a broker
or dealer or agent with respect to such securities or (y) the provision to any Stockholder of any information other than such information as a prudent issuer would generally furnish in an offering made solely to “accredited investors”
as defined in Regulation D promulgated under the Securities Act of 1933, as amended, the Company may cause to be paid to any such Stockholder in lieu thereof, against surrender of the Shares which would have otherwise been sold by such Stockholder,
an amount in cash equal to the fair value (as determined in good faith by the Company) of the securities which such Stockholder would otherwise receive as of the date of the issuance of such securities in exchange for the Shares; and 

(e) if any holders of any series of Preferred Stock or of Common Stock are given an option as to the form and amount of
consideration to be received as a result of the Proposed Sale, all holders of such respective series of Preferred Stock or of Common Stock will be given the same option. 

  
 - 6 - 

 2.4 Restrictions on Sales of Control of the Company. No Stockholder shall be a party to any sale or
transfer, or series of related sales or transfers, of Voting Shares representing fifty percent (50%) or more of the then-outstanding voting power of the Company to any person or entity, or group of related persons or entities (a “Stock
Sale”), unless all holders of Preferred Stock are allowed to participate in such transaction and the consideration received pursuant to such transaction is allocated among the parties thereto in the manner specified in the Company’s
Certificate of Incorporation in effect immediately prior to the Stock Sale (as if such transaction were an Event pursuant to the Company’s Certificate of Incorporation), unless the holders of at least a majority of the then-outstanding shares
of Preferred Stock voting as a single class on an as-converted to Common Stock basis elect otherwise by written notice given to the Company at least ten (10) days prior to the effective date of any such
transaction or series of related transactions. 
 3. Transfer of Shares. 

3.1 Restrictions on Transfer By Restricted Stockholders. Any sale, assignment, transfer or other disposition, whether voluntarily or by operation of
law (collectively, “Transfer”), of any Shares by a Restricted Stockholder other than according to the terms of this Agreement, shall be void and shall transfer no right, title, or interest in or to any of such Shares to the
purported transferee. 
 3.2 Transfers By Restricted Stockholders Not Subject to Restrictions. A Restricted Stockholder may Transfer without
compliance with the provisions of Sections 3.3 through 3.6 hereof (i) any or all of his Shares to his spouse, lineal ancestors or descendants (including adopted children and grandchildren) (collectively, “Family Members”)
or to a trust established for the benefit of any of his Family Members, (ii) any or all of his Shares under his will or otherwise pursuant to the laws of descent and distribution, (iii) any or all of his Shares to a partnership, limited
liability company or other entity controlled by such Restricted Stockholder, the partners or members of which consist solely of such Restricted Stockholder, his spouse and/or any of his other Family Members, or (iv) to a charitable organization
or educational institution qualifying for exemption from Federal income taxes pursuant to Section 501(c)(3) of the Internal Revenue Code, provided in the case of each such Transfer that the transferee delivers a written instrument to the other
parties hereto agreeing to be bound by the terms hereof as if he, she or it were a Restricted Stockholder and, in the case of Transfer from a Founder, a Founder. 

3.3 Offer of Transfer; Notice of Proposed Transfer. If a Restricted Stockholder desires to Transfer any of his Shares, or any interest in such Shares,
in any transaction other than pursuant to Section 3.2 of this Agreement, such Restricted Stockholder (a “Selling Restricted Stockholder”) shall first deliver written notice of his desire to do so (the
“Seller’s Notice”) to the Company and each Investor in the manner prescribed in Section 9.7 of this Agreement. The Seller’s Notice must specify (i) the name and address of the party to which
the Selling Restricted Stockholder proposes to Transfer the Shares or an interest in the Shares (the “Offeror”), (ii) the number of Shares the Selling Restricted Stockholder proposes to Transfer (the “Offered
Shares”), (iii) the consideration per Offered Share to be delivered to the Selling Restricted Stockholder (if applicable) for the proposed Transfer, and (iv) all other material terms and conditions of the proposed transaction.

 3.4 Company’s Option to Purchase. 

  
 - 7 - 

 (a) The Company shall have the first option to purchase all or any part of the
Offered Shares of a Selling Restricted Stockholder for the consideration per share and on the terms and conditions no less favorable than those specified in the Seller’s Notice. The Company must exercise such option no later than 15 days after
such Seller’s Notice is deemed under Section 9.7 hereof to have been delivered to it by giving written notice to the Selling Restricted Stockholder. 

(b) In the event the Company does not exercise its option within such 15-day period
with respect to all or part of the Offered Shares of a Selling Restricted Stockholder, the Secretary of the Company shall, by the last day of such period, give written notice of that fact to each Investor (the “Investor Notice”).
The Investor Notice shall specify the number of Offered Shares of a Selling Restricted Stockholder not purchased by the Company (the “Remaining Shares”). 

(c) In the event the Company duly exercises its option to purchase all or part of the Offered Shares of a Selling Restricted
Stockholder, the closing of such purchase shall take place at the offices of the Company on the later of (i) the date that is five days after the expiration of such 15-day period or (ii) the date
that the Investors consummate their purchases of Offered Shares under Section 3.5 hereof. 
 (d) To the extent that the
consideration proposed to be paid by the Offeror for the Offered Shares consists of property other than cash or a promissory note, the consideration required to be paid by the Company and/or the Investors when exercising their options under
Sections 3.4 and 3.5 hereof may consist of cash equal to the value of such property, as determined in good faith by agreement of the Selling Restricted Stockholder, the Company and/or the Investors acquiring such Offered Shares. 

(e) Notwithstanding anything to the contrary in this Agreement, neither the Company nor the Investors shall have any right to
purchase any of the Offered Shares hereunder unless the Company and/or the Investors exercise their option or options to purchase all of the Offered Shares. 

3.5 Investors’ Option to Purchase. 

(a) Each Investor shall have an option, exercisable for a period of 15 days from the date of delivery of the Investor Notice,
to purchase its pro rata share of the Remaining Shares for the consideration per share and on terms and conditions no less favorable than those set forth in the Seller’s Notice. Such option shall be exercised by delivery of written notice to
the Secretary of the Company. Alternatively, each Investor may within the same 15-day period notify the Secretary of the Company and the Selling Restricted Stockholder in writing of its desire to participate
in the sale of the Shares pursuant to Section 3.6 on the terms set forth in the Seller’s Notice and the number of Shares it proposes to sell. 

(b) In the event options to purchase have been exercised by the Investors with respect to some but not all of the Remaining
Shares, those Investors who have exercised their options within the 15-day period specified in Section 3.5(a) shall have an additional 

  
 - 8 - 

 
option, for a period of five (5) days next succeeding the expiration of such 15-day period, to purchase all or any part of the balance of such
Remaining Shares on terms and conditions no less favorable than those set forth in the Seller’s Notice, which option shall be exercised by the delivery of written notice to the Secretary of the Company. In the event there are two or more such
Investors that choose to exercise the last-mentioned option for a total number of Shares in excess of the number available, the Shares available for each such Investor’s option shall be allocated to such Investor pro rata based on the number of
shares of Preferred Stock calculated on an as-converted to Common Stock basis owned by the Investors so electing. 

(c) If the options to purchase the Remaining Shares are exercised in full by the Investors, the Secretary of the Company shall
promptly notify all of the exercising Investors of that fact. The closing of the purchase of the Remaining Shares shall take place at the offices of the Company no later than five days after the date of such notice to the Investors. 

(d) Any Shares sold by a Selling Restricted Stockholder in compliance with the provisions of Sections 3.3 through 3.6 of
this Agreement shall be transferred free and clear of the covenants, agreements and restrictions set forth in Section 3 of this Agreement, provided, that, the transferee and any subsequent transferee of the Shares shall hold the
Shares so acquired subject to all other covenants, agreements and restrictions imposed by this Agreement and shall be deemed a party to this Agreement for all other purposes. As a condition to any such Transfer, (i) the transferor shall notify
the Company and the other Stockholders thereof, which notice shall include a brief description of such Transfer and (ii) each transferee shall execute and deliver a written instrument agreeing to be bound by the provisions of this Agreement
(other than the provisions of this Section 3). 
 3.6 Failure to Fully Exercise Options; Co-Sale Rights with
Respect to Selling Restricted Stockholder. 
 (a) If the Company and the Investors do not exercise their options to
purchase all of the Offered Shares within the periods described in this Agreement (the “Option Period”), then all options of the Company and the Investors to purchase the Offered Shares, whether exercised or not, shall terminate;
but each Investor who has, pursuant to Section 3.5(a), expressed a desire to sell Shares in the transaction (“Participating Investors”) shall be entitled to do so pursuant to this Section. The Secretary of the Company shall
promptly, upon expiration of the Option Period, notify the Selling Restricted Stockholder of the aggregate number of Shares the Participating Investors wish to sell. The Selling Restricted Stockholder shall use his best efforts to interest the
Offeror in purchasing, in addition to the Offered Shares, the Shares the Participating Investors wish to sell. If the Offeror does not wish to purchase all of the Shares made available by the Selling Restricted Stockholder and the Participating
Investors, then each Participating Investor and the Selling Restricted Stockholder shall be entitled to sell, at the price and on terms and conditions no less favorable than those set forth in the Seller’s Notice, a portion of the Shares being
sold to the Offeror, in the same proportion as their respective ownership of Shares calculated on an as-converted to Common Stock 

  
 - 9 - 

 
basis bears to the aggregate number of Shares calculated on an as-converted to Common Stock basis owned by the Selling Restricted Stockholder and the
Participating Investors. The transaction contemplated by the Seller’s Notice shall be consummated no later than 60 days after the expiration of the Option Period. 

(b) The Selling Restricted Stockholder shall be entitled to sell to the Offeror, according to the terms set forth in the
Seller’s Notice, that number of Offered Shares which equals the difference between the number of Offered Shares desired to be purchased by the Offeror and (if applicable) the number of Shares the Participating Investors are entitled to sell in
accordance with the provisions of this Section 3.6. If the Selling Restricted Stockholder wishes to Transfer any such Offered Shares at a price per Offered Share which differs from that set forth in the Seller’s Notice, upon terms that
differ in any material respect from those previously offered to the Company and the Investors, or more than 60 days after the expiration of the Option Period, then, as a condition precedent to such transaction, such Offered Shares must first be
offered to the Company and the Investors (if applicable) on terms and conditions no less favorable than those given the Offeror, and in accordance with the procedures and time periods set forth above. 

(c) The proceeds of any sale made by a Selling Restricted Stockholder without compliance with the provisions of this
Section 3.6 shall be deemed to be held in constructive trust in such amount as would have been due the Participating Investors if the Selling Restricted Stockholder had complied with this Agreement. The contents of any such trust shall be
delivered to the Participating Investors upon surrender of the applicable Shares to the Selling Restricted Stockholder. 
 3.7 Exception for Drag-Along
Rights. This Section 3 shall not apply to any sale by a Restricted Stockholder pursuant to a Sale of the Company that the Selling Investors (i) have approved and (ii) with respect to which the Selling Investors have specified in
writing that Section 2 of this Agreement shall apply. 
 4. Restrictive Legends. 

4.1 Voting Shares. All certificates representing Voting Shares owned or hereafter acquired by the Stockholders or any transferee bound by this
Agreement shall have affixed thereto a legend substantially in the following form: 
 The shares of stock represented by this certificate are
subject to certain agreements as set forth in a Stockholders’ Agreement, as may be amended from time to time, by and among the registered owner of this certificate, the Company and certain other stockholders of the Company, a copy of which is
available for inspection by a prospective investor without charge at the office of the Secretary of the Company. 
 Each Restricted Stockholder agrees to
present the certificates representing Voting Shares owned or hereafter acquired by him to the Secretary of the Company and cause the Secretary to stamp on the certificate in a prominent manner such a legend. 

  
 - 10 - 

 4.2 Transfer Restriction. Each Restricted Stockholder agrees to present the certificates representing the
Shares presently owned or hereafter acquired by him to the Secretary of the Company and cause the Secretary to stamp on the certificate in a prominent manner the following legend: 

The sale or other disposition of any of the shares represented by this certificate is restricted by a Stockholders’ Agreement, as may be
amended from time to time, among certain of the shareholders of this Company and this Company, a copy of which is available for inspection by a prospective purchaser without charge at the office of the Secretary of the Company. 

5. Termination of Agreement. 
 5.1
Termination. This Agreement shall terminate upon the earlier to occur of the following events: 
 (a) the consummation
of a Sale of the Company, including the distribution of the net proceeds of such Sale of the Company in accordance with the Company’s Certificate of Incorporation, provided that the provisions of Section 2 hereof will
continue after the closing of any Sale of the Company to the extent necessary to enforce the provisions of Section 2 with respect to such Sale of the Company; 

(b) the closing of the sale of shares of Common Stock in a firm commitment underwritten public offering pursuant to an
effective registration statement under the Securities Act of 1933, as amended, in which (i) the price to the public per share is at least $28.95 (subject to equitable adjustment for any stock dividend, stock split, stock split-up, combination of shares or the like) and (ii) the aggregate offering price is at least $50,000,000 (based on the market price or fair value at the time of such offering); 

(c) the redemption or conversion of all shares of Preferred Stock; or 

(d) upon vote of the parties hereto pursuant to Section 9.8(b) hereof. 

5.2 Exception. The provisions of Sections 3.3 through 3.6, inclusive, of this Agreement shall not apply to sales of Shares pursuant to a
transaction referred to in Section 5.1 above. 
 6. Transfer of Rights. This Agreement, and the rights and obligations of each
Investor hereunder, may be assigned by such Investor (a) to any person or entity to which such Investor transfers a number of shares of Preferred Stock equal to not less than five percent (5%) of the total number of shares of Preferred Stock
held by such Investor (subject to adjustment for any stock dividend, stock split, stock split-up, combination or shares or the like) immediately following the Closing (as defined in the Purchase Agreement),
(b) if the Investor is an individual, to any family member or trust or partnership established for such family member, or (c) if the Investor is a corporation, partnership, limited liability company or other entity, to any current or
former partner (including general partner and limited partner), shareholder, member or other 

  
 - 11 - 

 
affiliate of the Investor. Such transferee shall be deemed an “Investor” for purposes of this Agreement, provided that the transferee provides written notice of such assignment to the
Company and agrees in writing to be bound by the terms and conditions set forth herein as if he, she or it were the original Investor. 
 7.
Irrevocable Proxy and Power of Attorney. Each party to this Agreement hereby constitutes and appoints as the proxies of the party and hereby grants a power of attorney to the President of the Company, and a designee of the Investors, and
each of them, with full power of substitution, with respect to the matters set forth herein, including without limitation, election of persons as members of the Board in accordance with Sections 1.1 and 1.4 hereto, votes to increase authorized
shares pursuant to Section 1.6 hereof and votes regarding any Sale of the Company pursuant to Section 2 hereof, and hereby authorizes each of them to represent and to vote, if and only if the party (i) fails to vote or
(ii) attempts to vote (whether by proxy, in person or by written consent), in a manner which is inconsistent with the terms of this Agreement, all of such party’s Shares in favor of the election of persons as members of the Board
determined pursuant to and in accordance with the terms and provisions of this Agreement or the increase of authorized shares or approval of any Sale of the Company pursuant to and in accordance with the terms and provisions of Sections 1.1, 1.4,
1.6 and 2, respectively, of this Agreement or to take any action necessary to effect Sections 1.1, 1.4, 1.6 and 2, respectively, of this Agreement. Each of the proxy and power of attorney granted pursuant to the immediately preceding sentence is
given in consideration of the agreements and covenants of the Company and the parties in connection with the transactions contemplated by this Agreement and, as such, each is coupled with an interest and shall be irrevocable unless and until this
Agreement terminates or expires pursuant to Section 5 hereof. Each party hereto hereby revokes any and all previous proxies or powers of attorney with respect to the Shares and shall not hereafter, unless and until this Agreement terminates or
expires pursuant to Section 5 hereof, purport to grant any other proxy or power of attorney with respect to any of the Shares, deposit any of the Shares into a voting trust or enter into any agreement (other than this Agreement), arrangement or
understanding with any person, directly or indirectly, to vote, grant any proxy or give instructions with respect to the voting of any of the Shares, in each case, with respect to any of the matters set forth herein. 

8. Stand-Off Agreement. Each Stockholder, if requested by the Company and the managing
underwriter of an offering by the Company of Common Stock or other securities of the Company pursuant to a registration statement, shall agree not to sell publicly or otherwise transfer or dispose of any Shares held by such Stockholder for a
specified period of time (not to exceed 180 days, which period may be extended upon the request of the managing underwriter for a period of up to fifteen (15) days if the Company issues or proposes to issue an earnings or other public release
within fifteen (15) days of the expiration of the 180-day lockup period) following the effective date of such Registration Statement; provided, that: 

(a) any discretionary modification, waiver or termination of the restrictions of such agreements (including this Agreement) by
the Company or the managing underwriter shall apply to all persons subject to such agreements on a pro rata basis, based upon the number of shares held by each subject to such agreements; 

  
 - 12 - 

 (b) such agreement shall only apply to the Company’s first registration
statement covering Shares to be sold by or on behalf of the Company to the public in an underwritten offering; and 
 (c)
such agreement shall not apply to securities acquired in an open market transaction after such registration statement is declared effective. 

9. General. 
 9.1 Additional
Parties. 
 (a) Notwithstanding anything to the contrary contained herein, if the Company issues additional shares of
Preferred Stock after the date hereof, as a condition to the issuance of such shares the Company shall require that any purchaser become a party to this Agreement by executing and delivering (i) the Adoption Agreement attached to this Agreement
as Exhibit B, or (ii) a counterpart signature page hereto agreeing to be bound by and subject to the terms of this Agreement as an Investor and Stockholder hereunder. In either event, each such person shall thereafter shall be deemed an
Investor and Stockholder for all purposes under this Agreement. 
 (b) In the event that after the date of this Agreement,
the Company enters into an agreement with any person or entity to issue shares of capital stock to such person or entity (other than to a purchaser of Preferred Stock described in Section 9.1(a) above), then, the Company shall cause such person
or entity, as a condition precedent to entering into such agreement, to become a party to this Agreement by executing an Adoption Agreement in the form attached hereto as Exhibit A, agreeing to be bound by and subject to the terms of this Agreement
as a Restricted Stockholder and a Stockholder and thereafter such person shall be deemed a Restricted Stockholder and a Stockholder for all purposes under this Agreement. Further, following the date of this Agreement, the Company will use
commercially reasonable efforts to cause each existing holder of shares of the capital stock of the Company that are not otherwise a party to this Agreement to become a party to this Agreement by executing an Adoption Agreement in the form attached
hereto as Exhibit A, agreeing to be bound by and subject to the terms of this Agreement as a Restricted Stockholder and a Stockholder. 
 9.2
Severability. The provisions of this Agreement are severable, so that the invalidity or unenforceability of any provision of this Agreement shall not affect the validity or enforceability of any other term or provision of this Agreement,
which shall remain in full force and effect. 
 9.3 Covenants of the Company. The Company agrees to use its commercially reasonable efforts, within
the requirements of applicable law, to ensure that the rights granted under this Agreement are effective and that the parties enjoy the benefits of this Agreement. Such actions include, without limitation, the use of the Company’s commercially
reasonable efforts to cause the nomination and election of the directors as provided in this Agreement. 
 9.4 Specific Performance. In addition to
any and all other remedies that may be available at law in the event of any breach of this Agreement, each Investor shall be entitled to specific performance of the agreements and obligations of the Company, the Restricted Stockholders and the other
Investors hereunder and to such other injunctive or other equitable relief as may be granted by a court of competent jurisdiction. 

  
 - 13 - 

 9.5 Remedies Cumulative. All remedies, either under this Agreement or by law or otherwise afforded to any
party, shall be cumulative and not alternative. 
 9.6 Governing Law. 

(a) This Agreement shall be governed by and construed in accordance with the laws of the State of Delaware (without reference
to the conflicts of law provisions thereof). Subject to Subsection 9.6(b), the parties (i) hereby irrevocably and unconditionally submit to the jurisdiction of the state courts of Massachusetts and to the jurisdiction of the United States
District Court for the District of Massachusetts for the purpose of any suit, action or other proceeding arising out of or based upon this Agreement, (ii) agree not to commence any suit, action or other proceeding arising out of or based upon
this Agreement except in the state courts of Massachusetts or the United States District Court for the District of Massachusetts, and (iii) hereby waive, and agree not to assert, by way of motion, as a defense, or otherwise, in any such suit,
action or proceeding, any claim that it is not subject personally to the jurisdiction of the above-named courts, that its property is exempt or immune from attachment or execution, that the suit, action or proceeding is brought in an inconvenient
forum, that the venue of the suit, action or proceeding is improper or that this Agreement or the subject matter hereof may not be enforced in or by such court. 

(b) Notwithstanding the foregoing Subsection 9.6(a), in the event there is a suit, action or other proceeding of the type
described in Article Eleventh of the Company’s Amended and Restated Certificate of Incorporation (i) pending in the Court of Chancery in the State of Delaware or (ii) to be filed simultaneously with the Court of Chancery in the State
of Delaware, in either case with respect to facts related to any suit, action or proceeding under this Agreement, then any suit, action or other proceeding under this Agreement must be brought exclusively in the Court of Chancery in the State of
Delaware and the parties (x) hereby irrevocably and unconditionally submit to the jurisdiction of the Court of Chancery in the State of Delaware and (y) hereby waive, and agree not to assert, by way of motion, as a defense, or otherwise,
in any such suit, action or proceeding, any claim that it is not subject personally to the jurisdiction of the above-named courts, that its property is exempt or immune from attachment or execution, that the suit, action or proceeding is brought in
an inconvenient forum, that the venue of the suit, action or proceeding is improper or that this Agreement or the subject matter hereof may not be enforced in or by such court. 

9.7 Notices. All notices and other communications given or made pursuant to this Agreement shall be in writing and shall be deemed effectively given
upon the earlier of actual receipt or (i) personal delivery to the party to be notified, (ii) when sent, if sent by electronic mail or facsimile during normal business hours of the recipient, and if not sent during normal business hours,
then on the recipient’s next business day, (iii) five (5) days after having been sent by registered or certified mail, return receipt requested, postage prepaid, or (iv) one (1) business day after deposit with a nationally recognized
overnight courier, freight prepaid, specifying next business day delivery, with written verification of receipt: 

  
 - 14 - 

 (a) If to the Company, or a Restricted Stockholder, at 890 Winter Street, Suite
230, Waltham, Massachusetts 02451, Attn: Michael Gray, or at such other address or addresses as may have been furnished in writing by the Company to the Investors, with a copy to Foley Hoag LLP, 155 Seaport Boulevard, Boston, Massachusetts 02210,
Attention: Robert L. Birnbaum, Esq. 
 (b) If to an Investor, at the address set forth on Exhibit A
attached hereto. 
 9.8 Complete Agreement; Amendments. 

(a) This Agreement constitutes the full and complete agreement of the parties hereto with respect to the subject matter hereof
and amends and restates in its entirety the Prior Agreement. 
 (b) No amendment, modification or termination of any
provision of this Agreement shall be valid unless in writing and signed by (i) the Company and (ii) the Stockholders holding at least a majority of the Shares held by all Stockholders voting on an
as-converted to Common Stock basis. Notwithstanding the foregoing, (i) Section 1.1(b)(i)(A) of this Agreement regarding Polaris’ right to designate a director may not be amended, terminated or
waived without the written consent of Polaris so long as Polaris has the right to designate a director, (ii) Section 1.1(b)(i)(B) of this Agreement regarding OrbiMed’s right to designate a director may not be amended, terminated or
waived without the written consent of OrbiMed so long as OrbiMed has the right to designate a director, (iii) Section 1.1(b)(i)(C) of this Agreement regarding SVLS’ right to designate a director may not be amended, terminated or
waived without the written consent of SVLS so long as SVLS has the right to designate a director, (iv) Section 1.1(b)(i)(D) of this Agreement regarding NeoMed’s right to designate a director may not be amended, terminated or waived
without the written consent of NeoMed so long as NeoMed has the right to designate a director, (v) Section 1.1(b)(i)(E) of this Agreement regarding EMBL’s right to designate a director may not be amended, terminated or waived without
the written consent of EMBL so long as EMBL has the right to designate a director, (vi) Section 1.1(b)(ii) many not be amended, terminated or waived without the written consent of Tillman Gerngross so long as Tillman Gerngross has the
right to designate a director and (vii) Section 1.1(b)(iii) may not be amended, terminated or waived without the written consent of the Founders holding a majority of the aggregate Shares held by the Founders so long as such Founders have
the right to designate a director. The applicability of any provisions of this Agreement in a particular instance may be waived by the party entitled to the benefit of such provision(s) as follows: in the case of the Company, by written instrument
signed on behalf of the Company by a duly authorized officer; in the case of the Founders, by written instrument signed on behalf of the Founders holding at least a majority of the shares of Common Stock held by all Founders; in the case of the
Restricted Stockholders, by written instrument signed on behalf of the Restricted Stockholders holding at least a majority of the shares of Common Stock held by the Restricted Stockholders; and in the case of the Investors, by a written instrument
signed by the Investors holding at least a majority of the shares of 

  
 - 15 - 

 
Preferred Stock voting on an as-converted to Common Stock basis. No waivers of or exceptions to any term, condition or provision of this Agreement, in any
one or more instances, shall be deemed to be, or construed as, a further or continuing waiver of any such term, condition or provision. Any such amendment or waiver effected in accordance with this Section 9.8(b) shall be binding on all parties
hereto, even if they did not consent to such amendment or waiver. 
 9.9 Construction. A reference to a Section or Exhibit shall mean a Section in,
or Exhibit to, this Agreement unless otherwise expressly stated. The titles and headings herein are for reference purposes only and shall not in any manner limit the construction of this Agreement which shall be considered as a whole. The words
“include,” “includes” and “including” when used herein shall be deemed in each case to be followed by the words “without limitation.” Whenever the context may require, any pronouns used herein shall include
the corresponding masculine, feminine or neuter forms, and the singular form of names and pronouns shall include the plural and vice-versa. 
 9.10
Counterparts; Facsimile Signatures. This Agreement may be executed in any number of counterparts, each of which shall be deemed to be an original, and all of which together shall constitute one Agreement binding on all the parties hereto.
This Agreement may be executed by facsimile signatures. 
 9.11 Aggregation of Shares. All shares of capital stock of the Company held or acquired by
affiliated entities or persons shall be aggregated together for the purpose of determining the availability of any rights under this Agreement. 

[Remainder of page intentionally left blank.] 

  
 - 16 - 

 IN WITNESS WHEREOF, this Second Amended and Restated Stockholders’ Agreement has been
executed under seal as of the date first above written. 
  

			
	COMPANY:
	
	ARSANIS, INC.
		
	By:	 	 /s/ Tillman U. Gerngross

		 	Name: Tillman U. Gerngross
		 	Title:   President

  
 Signature Page to
Second Amended and Restated Stockholders Agreement 

 
	
	 FOUNDERS:
  

/s/ Eszter Nagy

	 Eszter Nagy
  

/s/ Tillman U. Gerngross

	 Tillman U. Gerngross
  

/s/ Errik B. Anderson

	Errik B. Anderson

  
 Signature Page to
Second Amended and Restated Stockholders Agreement 

 
	
	 RESTRICTED STOCKHOLDERS:
  

/s/ Gabor Nagy                April 9, 2016

	 Gabor Nagy
  

/s/ Zoltan Magyarics                 2016-APR-09

	Zoltan Magyarics 

  
 Signature Page to
Second Amended and Restated Stockholders Agreement 

 
							
	INVESTORS:
	
	ORBIMED PRIVATE INVESTMENTS IV LP
		
	By:	 	OrbiMed Capital GP IV LLC
		 	Its General Partner
		
	By:	 	OrbiMed Advisors LLC,
		 	Its Managing Member

 
					
			
		 	By:	 	 /s/ Carl L. Gordon

		 		 	Name: Carl L. Gordon
		 		 	Title: Member

  
 Signature Page to
Second Amended and Restated Stockholders Agreement 

 
							
	INVESTORS:
	
	POLARIS VENTURE PARTNERS V, L.P.
		
	By:	 	Polaris Venture Management Co. V, L.L.C.,
		 	Its General Partner

 
					
			
		 	By:	 	 /s/ William E. Bilodeau

		 		 	Name: William E. Bilodeau
		 		 	Title: Attorney-in-fact

  

							
	 POLARIS VENTURE PARTNERS

ENTREPRENEURS’ FUND V, L.P.

		
	By:	 	Polaris Venture Management Co. V, L.L.C.,
		 	Its General Partner
			
		 	By:	 	 /s/ William E. Bilodeau

		 		 	Name: William E. Bilodeau
		 		 	Title: Attorney-in-fact

  

							
	 POLARIS VENTURE PARTNERS FOUNDERS’

FUND, V, L.P.

		
	By:	 	Polaris Venture Management Co. V, L.L.C.,
		 	Its General Partner
			
		 	By:	 	 /s/ William E. Bilodeau

		 		 	Name: William E. Bilodeau
		 		 	Title: Attorney-in-fact

  

							
	 POLARIS VENTURE PARTNERS SPECIAL

FOUNDERS’ FUND V, L.P.

		
	By:	 	Polaris Venture Management Co. V, L.L.C.,
		 	Its General Partner
			
		 	By:	 	 /s/ William E. Bilodeau

		 		 	Name: William E. Bilodeau
		 		 	Title: Attorney-in-fact

  
 Signature Page to
Second Amended and Restated Stockholders Agreement 

 
					
	INVESTORS:
	
	SV LIFE SCIENCES FUND V, L.P.
		
	By:	 	SV Life Sciences Fund V (GP), L.P.,
		 	Its sole General Partner
		
	By:	 	SVLSF V, LLC,
		 	Its sole General Partner
			
		 	By:	 	 /s/ Denise W. Marks

		 		 	Name: Denise W. Marks
		 		 	Title: SVLSF V, LLC, Member

  

					
	 SV LIFE SCIENCES FUND V STRATEGIC

PARTNERS, L.P.

		
	By:	 	SV Life Sciences Fund V (GP), L.P.,
		 	Its sole General Partner
		
	By:	 	SVLSF V, LLC,
		 	Its sole General Partner
			
		 	By:	 	 /s/ Denise W. Marks

		 		 	Name: Denise W. Marks
		 		 	Title: SVLSF V, LLC, Member

  
 Signature Page to
Second Amended and Restated Stockholders Agreement 

 
					
	INVESTORS:
	
	NEOMED INNOVATION V L.P.
			
	By:	 	/s/ PETER CANHAM	 	/s/ TAMARA WILLIAMS
		 	Name: PETER CANHAM	 	TAMARA WILLIAMS
		 	Title: ALTERNATE DIRECTOR        	 	DIRECTOR

  
 Signature Page to
Second Amended and Restated Stockholders Agreement 

			
	INVESTORS:
	
	EMBL TECHNOLOGY FUND II GMBH & CO. KG
		
	By:	 	EMBL VENTURES VERWALTUNGS GMBH,
		 	its General Partner

  

			
	By:	 	/s/ Jan Adams
		 	  
 Name: Jan Adams

		 	Title:   Executive Director

  

			
	By:	 	/s/ Stefan Herr
		 	  
 Name: Stefan Herr

		 	Title:   Executive Director

  
 Signature Page to
Second Amended and Restated Stockholders Agreement 

			
	INVESTORS:
	
	Anna-Maria and Stephen Kellen Foundation, Inc.
		
	By:	 	/s/ Michael M. Kellen
	Name:	 	  
 Michael M. Kellen

	Title:	 	President

  
 Signature Page to
Second Amended and Restated Stockholders Agreement 

 JOINDER AGREEMENT TO SECOND AMENDED AND RESTATED 

STOCKHOLDERS’ AGREEMENT 

This Joinder Agreement is executed on April 24, 2017, by the undersigned (the “Investor”) pursuant to the terms of
that certain Second Amended and Restated Stockholders’ Agreement by and among Arsanis, Inc., a Delaware corporation (the “Company”) and certain of its Stockholders, dated as of April 12, 2016, and as amended by that
certain First Amendment to the Second Amended and Restated Stockholders’ Agreement dated as of April 24, 2017 (as so amended and as the same may be amended or amended and restated hereafter, the “Agreement”).
Capitalized terms used but not defined in this Joinder Agreement shall have the respective meanings ascribed to such terms in the Agreement. By the execution of this Joinder Agreement, the Investor agrees as follows. 

1.1    Acknowledgement.    Investor acknowledges that Investor is acquiring certain shares of
the capital stock of the Company (the “Stock”) as a new Investor in accordance with Section 9.1(a) of the Agreement and will be an “Investor” and a “Stockholder” for all purposes of the Agreement. 

1.2    Agreement.    Investor hereby (a) agrees that the Stock, and any other shares of
capital stock or securities required by the Agreement to be bound thereby, shall be bound by and subject to the terms of the Agreement, and (b) adopts the Agreement with the same force and effect as if the Investor were originally a party
thereto. 
 1.3    Notice.    Any notice required or permitted by the Agreement shall be
given to Investor at the address or facsimile number listed below the Investor’s signature hereto. 
  

			
	 ALEXANDRIA VENTURE INVESTMENTS, LLC,

a Delaware limited liability company

		
	By:    	 	ALEXANDRIA REAL ESTATE EQUITIES, INC.,
		 	a Maryland corporation, managing member
		
	By:    	 	 /s/ Aaron Jacobson

		 	 Name: Aaron Jacobson
 Title: VP - Corporation
Counsel

		
	Address:	 	     385 E. Colorado Blvd., Suite 299

    Pasadena, CA 91101

  

			
	Accepted and Agreed:
	
	ARSANIS, INC.
		
	By:	 	 /s/ Rene Russo

	Name:	 	Rene Russo
	Title:	 	President and Chief Executive Officer

 —Joinder Agreement to Second Amended and Restated Stockholders’ Agreement— 

 JOINDER AGREEMENT TO SECOND AMENDED AND RESTATED 

STOCKHOLDERS’ AGREEMENT 

This Joinder Agreement is executed on April 24, 2017, by the undersigned (the “Investor”) pursuant to the terms of
that certain Second Amended and Restated Stockholders’ Agreement by and among Arsanis, Inc., a Delaware corporation (the “Company”) and certain of its Stockholders, dated as of April 12, 2016, and as amended by that
certain First Amendment to the Second Amended and Restated Stockholders’ Agreement dated as of April 24, 2017 (as so amended and as the same may be amended or amended and restated hereafter, the “Agreement”).
Capitalized terms used but not defined in this Joinder Agreement shall have the respective meanings ascribed to such terms in the Agreement. By the execution of this Joinder Agreement, the Investor agrees as follows. 

1.1    Acknowledgement.    Investor acknowledges that Investor is acquiring certain shares of
the capital stock of the Company (the “Stock”) as a new Investor in accordance with Section 9.1(a) of the Agreement and will be an “Investor” and a “Stockholder” for all purposes of the Agreement. 

1.2    Agreement.    Investor hereby (a) agrees that the Stock, and any other shares of
capital stock or securities required by the Agreement to be bound thereby, shall be bound by and subject to the terms of the Agreement, and (b) adopts the Agreement with the same force and effect as if the Investor were originally a party
thereto. 
 1.3    Notice.    Any notice required or permitted by the Agreement shall be
given to Investor at the address or facsimile number listed below the Investor’s signature hereto. 
  

			
	GV 2016, L.P.
	By:	 	GV 2016 GP, L.P., its General Partner
	By:	 	GV 2016 GP, L.L.C., its General Partner
		
	By:	 	 /s/ Jennifer L. Kercher

	Name:	 	Jennifer L. Kercher
	Title:	 	Authorized Signatory

  

			
	Address:	 	 Attn: Jennifer L. Kercher
 c/o GV

1600 Amphitheatre Parkway
 Mountain View, CA 94043

  

			
	Accepted and Agreed:
	
	ARSANIS, INC.
		
	By:	 	 /s/ Rene Russo

	Name:	 	Rene Russo
	Title:	 	President and Chief Executive Officer

 —Joinder Agreement to Second Amended and Restated Stockholders’ Agreement— 

 JOINDER AGREEMENT TO SECOND AMENDED AND RESTATED 

STOCKHOLDERS’ AGREEMENT 

This Joinder Agreement is executed on April 24, 2017, by the undersigned (the “Investor”) pursuant to the terms of
that certain Second Amended and Restated Stockholders’ Agreement by and among Arsanis, Inc., a Delaware corporation (the “Company”) and certain of its Stockholders, dated as of April 12, 2016, and as amended by that
certain First Amendment to the Second Amended and Restated Stockholders’ Agreement dated as of April 24, 2017 (as so amended and as the same may be amended or amended and restated hereafter, the “Agreement”).
Capitalized terms used but not defined in this Joinder Agreement shall have the respective meanings ascribed to such terms in the Agreement. By the execution of this Joinder Agreement, the Investor agrees as follows. 

1.1    Acknowledgement.    Investor acknowledges that Investor is acquiring certain shares of
the capital stock of the Company (the “Stock”) as a new Investor in accordance with Section 9.1(a) of the Agreement and will be an “Investor” and a “Stockholder” for all purposes of the Agreement. 

1.2    Agreement.    Investor hereby (a) agrees that the Stock, and any other shares of
capital stock or securities required by the Agreement to be bound thereby, shall be bound by and subject to the terms of the Agreement, and (b) adopts the Agreement with the same force and effect as if the Investor were originally a party
thereto. 
 1.3    Notice.    Any notice required or permitted by the Agreement shall be
given to Investor at the address or facsimile number listed below the Investor’s signature hereto. 
  

			
	N5 Investments AS

 
			
		
	By:	 	 /s/ Pål Jensen

			
	Pål Jensen, an authorized person, for and on
its behalf

  

			
	Address:	 	 Parkveien 55
 0256 Oslo

Norway

  

			
	Accepted and Agreed:
	
	ARSANIS, INC.
		
	By:	 	 /s/ Rene Russo

	Name:	 	Rene Russo
	Title:	 	President and Chief Executive Officer

 —Joinder Agreement to Second Amended and Restated Stockholders’ Agreement— 

 JOINDER AGREEMENT TO SECOND AMENDED AND RESTATED 

STOCKHOLDERS’ AGREEMENT 

This Joinder Agreement is executed on April 24, 2017, by the undersigned (the “Investor”) pursuant to the terms of
that certain Second Amended and Restated Stockholders’ Agreement by and among Arsanis, Inc., a Delaware corporation (the “Company”) and certain of its Stockholders, dated as of April 12, 2016, and as amended by that
certain First Amendment to the Second Amended and Restated Stockholders’ Agreement dated as of April 24, 2017 (as so amended and as the same may be amended or amended and restated hereafter, the “Agreement”).
Capitalized terms used but not defined in this Joinder Agreement shall have the respective meanings ascribed to such terms in the Agreement. By the execution of this Joinder Agreement, the Investor agrees as follows. 

1.1    Acknowledgement.    Investor acknowledges that Investor is acquiring certain shares of
the capital stock of the Company (the “Stock”) as a new Investor in accordance with Section 9.1(a) of the Agreement and will be an “Investor” and a “Stockholder” for all purposes of the Agreement. 

1.2    Agreement.    Investor hereby (a) agrees that the Stock, and any other shares of
capital stock or securities required by the Agreement to be bound thereby, shall be bound by and subject to the terms of the Agreement, and (b) adopts the Agreement with the same force and effect as if the Investor were originally a party
thereto. 
 1.3    Notice.    Any notice required or permitted by the Agreement shall be
given to Investor at the address or facsimile number listed below the Investor’s signature hereto. 
  

			
	Bill & Melinda Gates Foundation

 
			
		
	By:	 	 /s/ Jim Bromley

	Name:	 	Jim Bromley
	Title:	 	Chief Financial Officer

 
			
		
	Address:	 	1432 Elliott Ave W.

		 	Seattle, WA 98119

		 	Attn: General Counsel

  

			
	Accepted and Agreed:
	
	ARSANIS, INC.
		
	By:	 	 /s/ Rene Russo

	Name:	 	Rene Russo
	Title:	 	President and Chief Executive Officer

 —Joinder Agreement to Second Amended and Restated Stockholders’ Agreement— 

 JOINDER AGREEMENT TO SECOND AMENDED AND RESTATED 

STOCKHOLDERS’ AGREEMENT 

This Joinder Agreement is executed on April 24, 2017, by the undersigned (the “Investor”) pursuant to the terms of
that certain Second Amended and Restated Stockholders’ Agreement by and among Arsanis, Inc., a Delaware corporation (the “Company”) and certain of its Stockholders, dated as of April 12, 2016, and as amended by that
certain First Amendment to the Second Amended and Restated Stockholders’ Agreement dated as of April 24, 2017 (as so amended and as the same may be amended or amended and restated hereafter, the “Agreement”).
Capitalized terms used but not defined in this Joinder Agreement shall have the respective meanings ascribed to such terms in the Agreement. By the execution of this Joinder Agreement, the Investor agrees as follows. 

1.1    Acknowledegment.    Investor acknowledges that Investor is acquiring certain shares of
the capital stock of the Company (the “Stock”) as a new Investor in accordance with Section 9.1(a) of the Agreement and will be an “Investor” and a “Stockholder” for all purposes of the Agreement. 

1.2    Agreement.    Investor hereby (a) agrees that the Stock, and any other shares of
capital stock or securities required by the Agreement to be bound thereby, shall be bound by and subject to the terms of the Agreement, and (b) adopts the Agreement with the same force and effect as if the Investor were originally a party
thereto. 
 1.3    Notice.    Any notice required or permitted by the Agreement shall be
given to Investor at the address or facsimile number listed below the Investor’s signature hereto. 
  

			
	SV LIFE SCIENCES FUND VI, L.P.

 
			
		
	By:	 	SV Life Sciences Fund VI (GP), L.P.,
		 	Its sole General Partner
		
	By:	 	SVLSF VI, LLC,
		 	Its sole General Partner

 
					
			
		 	By:	 	 /s/ Denise W. Marks

		 		 	 Name: Denise W. Marks
 Title: SVLSF VI, LLC,
Member

 
			
		
	Address:	 	 One Boston Place
201 Washington Street, Suite 3900
Boston, MA 02108

Attn: Denise Marks

  

			
	Accepted and Agreed:
	
	ARSANIS, INC.
		
	By:	 	 /s/ Rene Russo

	Name:	 	Rene Russo
	Title:	 	President and Chief Executive Officer

 —Joinder Agreement to Second Amended and Restated Stockholders’ Agreement— 

 JOINDER AGREEMENT TO SECOND AMENDED AND RESTATED 

STOCKHOLDERS’ AGREEMENT 

This Joinder Agreement is executed on April 24, 2017, by the undersigned (the “Investor”) pursuant to the terms of
that certain Second Amended and Restated Stockholders’ Agreement by and among Arsanis, Inc., a Delaware corporation (the “Company”) and certain of its Stockholders, dated as of April 12, 2016, and as amended by that
certain First Amendment to the Second Amended and Restated Stockholders’ Agreement dated as of April 24, 2017 (as so amended and as the same may be amended or amended and restated hereafter, the “Agreement”).
Capitalized terms used but not defined in this Joinder Agreement shall have the respective meanings ascribed to such terms in the Agreement. By the execution of this Joinder Agreement, the Investor agrees as follows. 

1.1    Acknowledgement.    Investor acknowledges that Investor is acquiring certain shares of
the capital stock of the Company (the “Stock”) as a new Investor in accordance with Section 9.1(a) of the Agreement and will be an “Investor” and a “Stockholder” for all purposes of the Agreement. 

1.2    Agreement.    Investor hereby (a) agrees that the Stock, and any other shares of
capital stock or securities required by the Agreement to be bound thereby, shall be bound by and subject to the terms of the Agreement, and (b) adopts the Agreement with the same force and effect as if the Investor were originally a party
thereto. 
 1.3    Notice.    Any notice required or permitted by the Agreement shall be
given to Investor at the address or facsimile number listed below the Investor’s signature hereto. 
  

			
	 SV LIFE SCIENCES FUND VI STRATEGIC

PARTNERS, L.P.

 
			
		
	By:    	 	SV Life Sciences Fund VI (GP), L.P.,
		 	Its sole General Partner
		
	By:    	 	SVLSF VI, LLC,
		 	Its sole General Partner

 
					
			
		 	By:	 	 /s/ Denise W. Marks

		 		 	 Name: Denise W. Marks
 Title: SVLSF VI, LLC,
Member

 
			
		
	Address:    	 	 One Boston Place
201 Washington Street, Suite 3900
Boston, MA 02108

Attn: Denise Marks

  

			
	Accepted and Agreed:
	
	ARSANIS, INC.
		
	By:	 	 /s/ Rene Russo

	Name:	 	Rene Russo
	Title:	 	President and Chief Executive Officer

 —Joinder Agreement to Second Amended and Restated Stockholders’ Agreement— 

 JOINDER AGREEMENT TO SECOND AMENDED AND RESTATED 

STOCKHOLDERS’ AGREEMENT 

This Joinder Agreement is executed on April 24, 2017, by the undersigned (the “Investor”) pursuant to the
terms of that certain Second Amended and Restated Stockholders’ Agreement by and among Arsanis, Inc., a Delaware corporation (the “Company”) and certain of its Stockholders, dated as of April 12, 2016, and as amended by
that certain First Amendment to the Second Amended and Restated Stockholders’ Agreement dated as of April 24, 2017 (as so amended and as the same may be amended or amended and restated hereafter, the “Agreement”).
Capitalized terms used but not defined in this Joinder Agreement shall have the respective meanings ascribed to such terms in the Agreement. By the execution of this Joinder Agreement, the Investor agrees as follows. 

1.1    Acknowledgement.    Investor acknowledges that Investor is acquiring certain shares of
the capital stock of the Company (the “Stock”) as a new Investor in accordance with Section 9. 1(a) of the Agreement and will be an “Investor” and a “Stockholder” for all purposes of the Agreement. 

1.2    Agreement.    Investor hereby (a) agrees that the Stock, and any other shares of
capital stock or securities required by the Agreement to be bound thereby, shall be bound by and subject to the terms of the Agreement, and (b) adopts the Agreement with the same force and effect as if the Investor were originally a party
thereto. 
 1.3    Notice.    Any notice required or permitted by the Agreement shall be
given to Investor at the address or facsimile number listed below the Investor’s signature hereto. 
  

													
		 	Date:                     	 		 	 /s/ Tillman U. Gerngorss
	 	
		 		 		 	Tillman U. Gerngorss	 	
							
		 		 		 	Address:	 		 	  
	 	
		 		 		 		 		 	  
	 	
		 		 		 		 		 	  
	 	

  

			
	Accepted and Agreed:
	
	ARSANIS, INC.

			
		
	By:	 	/s/ Rene Russo

			
	Name: Rene Russo
	Title: President and Chief Executive Officer

 —Joinder Agreement to Second Amended and Restated Stockholders’ Agreement— 

 JOINDER AGREEMENT TO SECOND AMENDED AND RESTATED 

STOCKHOLDERS’ AGREEMENT 

This Joinder Agreement is executed on April 24, 2017, by the undersigned (the “Investor”) pursuant to the terms
of that certain Second Amended and Restated Stockholders’ Agreement by and among Arsanis, Inc., a Delaware corporation (the “Company”) and certain of its Stockholders, dated as of April 12, 2016, and as amended by that
certain First Amendment to the Second Amended and Restated Stockholders’ Agreement dated as of April 24, 2017 (as so amended and as the same may be amended or amended and restated hereafter, the “Agreement”). Capitalized
terms used but not defined in this Joinder Agreement shall have the respective meanings ascribed to such terms in the Agreement. By the execution of this Joinder Agreement, the Investor agrees as follows. 

1.1    Acknowledgement.    Investor acknowledges that Investor is acquiring certain shares of
the capital stock of the Company (the “Stock”) as a new Investor in accordance with Section 9.1(a) of the Agreement and will be an “Investor” and a “Stockholder” for all purposes of the Agreement. 

1.2    Agreement.    Investor hereby (a) agrees that the Stock, and any other shares of
capital stock or securities required by the Agreement to be bound thereby, shall be bound by and subject to the terms of the Agreement, and (b) adopts the Agreement with the same force and effect as if the Investor were originally a party
thereto. 
 1.3    Notice.    Any notice required or permitted by the Agreement shall be
given to Investor at the address or facsimile number listed below the Investor’s signature hereto. 
  

					
	 /s/ Michael W. Bonney

	Michael W. Bonney
			
	Address:	 		 	536 Commercial Street
		 		 	Boston, MA 02109

  

			
	Accepted and Agreed:
	
	ARSANIS, INC.

			
		
	By:	 	 /s/ Rene Russo

			
	Name: Rene Russo
	Title: President and Chief Executive Officer

 —Joinder Agreement to Second Amended and Restated Stockholders’ Agreement— 

 JOINDER AGREEMENT TO SECOND AMENDED AND RESTATED 

STOCKHOLDERS’ AGREEMENT 

This Joinder Agreement is executed on September 1, 2017, by the undersigned (the “Investor”) pursuant to the
terms of that certain Second Amended and Restated Stockholders’ Agreement by and among Arsanis, Inc., a Delaware corporation (the “Company”) and certain of its Stockholders, dated as of April 12, 2016, and as amended by
that certain First Amendment to the Second Amended and Restated Stockholders’ Agreement dated as of April 24, 2017 (as so amended and as the same may be amended or amended and restated hereafter, the “Agreement”).
Capitalized terms used but not defined in this Joinder Agreement shall have the respective meanings ascribed to such terms in the Agreement. By the execution of this Joinder Agreement, the Investor agrees as follows: 

1.1    Acknowledgement.    Investor acknowledges that Investor is acquiring certain shares of
the capital stock of the Company (the “Stock”) as a new Investor in accordance with Section 9.1(a) of the Agreement and will be an “Investor” and a “Stockholder” for all purposes of the Agreement. 

1.2    Agreement.    Investor hereby (a) agrees that the Stock, and any other shares of
capital stock or securities required by the Agreement to be bound thereby, shall be bound by and subject to the terms of the Agreement, and (b) adopts the Agreement with the same force and effect as if the Investor were originally a party
thereto. 
 1.3    Notice.    Any notice required or permitted by the Agreement shall be
given to Investor at the address or facsimile number listed below the Investor’s signature hereto. 
  

			
	SECTION 32 FUND 1, LP
		
	By:	 	Section 32 GP 1, LLC
		 	its General Partner
		
	By:	 	 /s/ Jennifer L. Kercher

		 	Jennifer L. Kercher
		 	Chief Operating Officer

  

			
	ACCEPTED AND AGREED:
	
	ARANIS, INC.

			
		
	By:	 	 /s/ René Russo

			
	Name: René Russo
	Its: President and Chief Executive Officer

 —Joinder Agreement to Second Amended and Restated Stockholders’ Agreement— 

 EXHIBIT A 

SCHEDULE OF INVESTORS 
 OrbiMed Private
Investments IV, LP 
 601 Lexington Avenue (at 53rd Street) 

54th Floor 
 New York, NY 10022-4629 

Polaris Venture Partners V, L.P. 
 One Marina Park Drive, 10th
Floor 
 Boston, MA 02210 
 Polaris Venture Partners
Entrepreneurs’ Fund V, L.P. 
 [Same address as above] 

Polaris Venture Partners Founders’ Fund V, L.P. 
 [Same
address as above] 
 Polaris Venture Partners Special Founders’ Fund V, L.P. 

[Same address as above] 
 SV Life Sciences Fund V, L.P. 

One Boston Place 
 201 Washington Street, Suite 3900 

Boston, MA 02108 
 Attn: Denise Marks 

SV Life Sciences Fund V Strategic Partners, L.P. 
 [Same address
as above] 
 SV Life Sciences Fund VI, L.P. 
 [Same address as
above] 
 SV Life Sciences Fund VI Strategic Partners, L.P. 

[Same address as above] 
 NeoMed Innovation V L.P. 

13, Castle Street 
 Jersey, JE4 5UT 

cc to: claudio@neomed.net 
 EMBL Technology Fund II
GmbH & Co. KG 
 Boxbergring 107 
 D-69126 Heidelberg 
 Germany 

Attn: Jan Adams 

  
 1 

 Anna-Maria and Stephen Kellen Foundation, Inc. 

1345 Avenue of the Americas, 48th Floor 

New York, NY 10105-0048 
 Attn. Michael M. Kellen 

Bill & Melinda Gates Foundation 
 Address: 

For UPS, FedEx, DHL: 

Bill & Melinda Gates Foundation 

1432 Elliott Ave West 
 Seattle,
WA 98119 
 For United States Postal Service 

Bill & Melinda Gates Foundation 

PO Box 23350 
 Seattle, WA 98102

 For Messengers & Courier Service 

Bill & Melinda Gates Foundation 

ATTN: Loading Dock 
 500 Fifth Ave
N 
 Seattle, WA 98109-4636 

Fax No. 206.497.7100 
 Attn:
Jim Bromley, Chief Financial Officer 
 With a copy (which shall not constitute notice): 

Andrew Farnum, Director Program-Related Investments 

With a copy (which shall not constitute notice): 

Claire White 
 K&L Gates LLP

 925 4th Ave, Suite 2900 

Seattle WA, 98104 
 GV 2016, L.P. 

Email: notice@gv.com 
 Attn: Jennifer L. Kercher 

c/o GV 
 1600 Amphitheatre Parkway 

Mountain View, CA 94043 
 Alexandria Venture Investments, LLC

 385 E. Colorado Blvd., Suite 299 
 Pasadena, CA 91101 

  
 2 

 Michael W. Bonney 

536 Commercial Street 
 Boston, MA 02109 

N5 Investments AS 
 Parkveien 55 

0256 Oslo 
 Norway 

Tillman U. Gerngross 
 Section 32 Fund 1, LP 

2033 San Elijo Avenue #565 
 Cardiff by the Sea, CA 92007 

  
 3 

 

ARSANIS, INC. 
 FIRST AMENDMENT 

TO THE 
 SECOND AMENDED
AND RESTATED 
 STOCKHOLDERS’ AGREEMENT 

This FIRST AMENDMENT TO THE SECOND AMENDED AND RESTATED STOCKHOLDERS’ AGREEMENT (this “Amendment”) is entered into as of
April 24, 2017, by and among Arsanis, Inc. (the “Company”), the existing investors listed on the signature pages hereto under the heading “Investors” (the “Existing Investors”), the individuals listed
on the signature pages hereto under the heading “Founders” (the “Founders”), and the individuals listed on the signature pages hereto under the heading “Restricted Stockholders” (the “Restricted
Stockholders”). Capitalized terms used herein but not defined herein shall have the meanings ascribed to them in the Stockholders Agreement (each as defined below). 

RECITALS 

WHEREAS, the Existing Investors are holders of the Company’s Series A Preferred Stock, Series B Preferred Stock and/or Series C Preferred
Stock; 
 WHEREAS, the Company, the Existing Investors, the Founders and the Restricted Stockholders are parties to a Second Amended
and Restated Stockholders’ Agreement dated as of April 12, 2016 (the “Stockholders Agreement”); 
 WHEREAS, the
Company proposes to issue and sell shares of its Series D Convertible Preferred Stock, $0.001 par value per share (the “Series D Preferred Stock”), pursuant to the Series D Preferred Stock Purchase Agreement (the “Purchase
Agreement”) of even date herewith, to be entered into by the Company, the Existing Investors and certain new investors; 
 WHEREAS,
the Company, the Existing Investors, the Founders and the Restricted Stockholders wish to amend the Stockholders Agreement to, among other things, clarify that the Series D Preferred Stock shall be “Preferred Stock” as defined in the
preamble to the Stockholders Agreement; 
 WHEREAS, pursuant to Section 9.8 of the Stockholders Agreement, the Stockholders Agreement
may be amended with the written consent of (i) the Company and (ii) the Stockholders holding at least a majority of the Shares held by all Stockholders voting on as-converted to Common Stock basis
(the “Required Holders”); 
 WHEREAS, the undersigned Existing Investors, Founders and Restricted Stockholders constitute
the Required Holders; and 
 WHEREAS, the parties intend that these recitals be a part of the Stockholders Agreement, as amended by this
Amendment. 

 NOW, THEREFORE, in consideration of the mutual agreements, covenants and considerations contained
herein, the receipt and sufficiency of which are hereby acknowledged, the parties hereto agree as follows: 

1.    Amendments to the Stockholders Agreement. 

1.1    Definitions. The Stockholders Agreement is hereby amended so that the defined term “Preferred
Stock” set forth therein (or herein) shall include the Series D Preferred Stock for all purposes thereunder (or hereunder), all terms defined in this Amendment shall apply as if set forth in full in the Stockholders Agreement, and in the event
of any conflict between a term defined in the Stockholders Agreement and this Amendment, the meaning set forth in this Amendment shall control. 

1.2    Exceptions to Drag-Along Rights. The Stockholders Agreement is hereby amended so that the following
provisions are added to Section 2.3 at the end thereof as follows: 
 “(f)    any covenants that any Investor
shall be required to make (other than in relation to such Investor’s indemnification obligations) in connection with a Proposed Sale shall be limited to reasonable covenants regarding confidentiality, publicity, and similar matters; 

(g)    the Bill & Melinda Gates Foundation shall not be required to amend, extend or terminate any contractual or
other relationship with the Company, the acquirer, or their respective affiliates (including without limitation, that certain Letter Agreement, dated on or about April , 2017, between the Company and the Bill & Melinda Gates Foundation, and
any grant agreement from the Bill & Melinda Gates Foundation then currently in effect); except the Stockholders’ Agreement or that certain Second Amended and Restated Investors’ Rights Agreement, as amended, to the extent such
agreements (or any specific provisions therein) terminate by their terms in connection with a Proposed Sale; 
 (h)
    no Investor shall be required to agree to any covenant not to compete or covenant not to solicit customers, employees, or suppliers of any party unless such Investor is an employee, consultant, or similar service provider to
the Company at the time of the closing of the Proposed Sale.” 
 1.3    Termination. The Stockholders
Agreement is hereby amended such that the dollar amount “$28.95” as it appears in Section 5.1(b) thereof is hereby deleted and replaced with the dollar amount “$9.83.” 

1.4    Transfer of Rights. The Stockholders Agreement is hereby amended so that the first sentence of
Section 6 is replaced in its entirety to read as follows: 
 “This Agreement, and the rights and obligations of each Investor
hereunder, may be assigned by such Investor (a) to any person or entity to which such Investor transfers a number of shares of Preferred Stock equal to not less than five percent (5%) of the total number of shares of Preferred Stock held by
such Investor 

 
(subject to adjustment for any stock dividend, stock split, stock split-up, combination or shares or the like) immediately following the Closing (as
defined in the Purchase Agreement), (b) if the Investor is an individual, to any family member or trust or partnership established for such family member, (c) if the Investor is a corporation, partnership, limited liability company or other
entity, to any current or former partner (including general partner and limited partner), shareholder, member, or other affiliate of the Investor or (d) in the case of the Bill & Melinda Gates Foundation, to a charitable trust or other
entity a majority of the trustees of which are also trustees of the Bill & Melinda Gates Foundation.” 

1.5    Stand-Off Agreement. Section 8 of the Stockholders Agreement is hereby amended by adding the following
paragraph after Section 8(c): 
 “Notwithstanding the foregoing, with respect to the Investors, to the extent that the provisions
of this Section 8 are in conflict with the “stand-off” agreement provisions in the Second Amended and Restated Investors’ Rights Agreement, dated as of April 12, 2016, and as amended
from time to time (the “Rights Agreement”), the provisions in the Rights Agreement shall control.” 

1.6    Complete Agreement; Amendments. The Stockholders Agreement is hereby amended such that a new sentence is
added to Section 9.8(b) at the end thereof as follows: 
 “Notwithstanding the foregoing, Subsections 2.3(f) through 2.3(h) may not
be amended or waived without the written consent of the Bill & Melinda Gates Foundation.” 

1.7    Exhibit A. Each individual and entity listed on Exhibit A attached hereto is hereby added to Exhibit A of
the Stockholders Agreement, subject to the execution by such individual or entity of a counterpart signature page to the Stockholders Agreement, as amended by this Amendment, agreeing to be bound by and subject to the terms of the Stockholders
Agreement as so amended as an Investor and a Stockholder thereunder, and the term “Investors” thereunder and hereunder shall include such individuals and entities. 

2.    No Further Amendment. The Stockholders Agreement, as amended by this Amendment, is hereby ratified and
confirmed in all respects, shall continue in full force and effect and shall, together with this Amendment, be read and construed as a single agreement. 

3.    Successors and Assigns. This Amendment shall be binding upon and inure to the benefit of all of the parties
to the Stockholders Agreement, their successors and assigns, heirs, devisees, legates and personal representatives. 

4.    Counterparts. This Amendment may be executed in multiple counterparts, each of which shall be deemed an
original for all purposes and all of which shall be deemed collectively to be one agreement. 

 5.    Governing Law. This Amendment shall be governed by and construed
under the laws of the State of Delaware as applied to agreements among Delaware residents entered into and to be performed entirely within Delaware. 

[Remainder of page intentionally left blank.] 

 IN WITNESS WHEREOF, the
parties hereto have executed this First Amendment to the Second Amended and Restated Stockholders’ Agreement as of the date set forth in the first paragraph hereof. 

 

			
	COMPANY:
	
	ARSANIS, INC.
		
	By:	 	 /s/ Rene Russo

		 	Name: Rene Russo
		 	Title:   President

 Signature Page to First Amendment to Second Amended and Restated Stockholders’ Agreement 

  

	
	FOUNDERS:
	
	 /s/ Eszter Nagy

	Eszter Nagy
	  
 /s/ Tillman U. Gerngross

	Tillman U. Gerngross
	  
 /s/ Errik B. Anderson

	Errik B. Anderson

 Signature Page to First Amendment to Second Amended and Restated Stockholders’ Agreement 

 
	
	RESTRICTED STOCKHOLDERS:
	
	 /s/ Gabor
Nagy                Apr 20, 2017

	Gabor Nagy
	  
 /s/ Zoltan Magyarics

	Zoltan Magyarics

 Signature Page to First Amendment to Second Amended and Restated Stockholders’ Agreement 

 
			
	INVESTORS:
	
	ORBIMED PRIVATE INVESTMENTS IV LP
		
	By:	 	OrbiMed Capital GP IV LLC
		 	Its General Partner
		
	By:	 	OrbiMed Advisors LLC,
		 	Its Managing Member

  

							
		 		 	By:	 	 /s/ Carl L. Gordon

		 		 		 	Name: Carl L. Gordon
		 		 		 	Title: Member

 Signature Page to First Amendment to Second Amended and Restated Stockholders’ Agreement 

 
			
	INVESTORS:
	
	POLARIS VENTURE PARTNERS V, L.P.
		
	By:	 	Polaris Venture Management Co. V, L.L.C.,
		 	Its General Partner

  

							
		 		 	By:	 	 /s/ Max Eisenberg

		 		 		 	Name: Max Eisenberg
		 		 		 	Title: Attorney-in-fact

  

			
	POLARIS VENTURE PARTNERS ENTREPRENEURS’ FUND V, L.P.
		
	By:	 	Polaris Venture Management Co. V, L.L.C.,
		 	Its General Partner

  

							
		 		 	By:	 	 /s/ Max Eisenberg

		 		 		 	Name: Max Eisenberg
		 		 		 	Title: Attorney-in-fact

  

			
	 POLARIS VENTURE PARTNERS FOUNDERS’

FUND, V, L.P.

		
	By:	 	Polaris Venture Management Co. V, L.L.C.,
		 	Its General Partner

  

							
		 		 	By:	 	 /s/ Max Eisenberg

		 		 		 	Name: Max Eisenberg
		 		 		 	Title: Attorney-in-fact

  

			
	 POLARIS VENTURE PARTNERS SPECIAL

FOUNDERS’ FUND V, L.P.

		
	By:	 	Polaris Venture Management Co. V, L.L.C.,
		 	Its General Partner

  

							
		 		 	By:	 	 /s/ Max Eisenberg

		 		 		 	Name: Max Eisenberg
		 		 		 	Title: Attorney-in-fact

 Signature Page to First Amendment to Second Amended and Restated Stockholders’ Agreement 

 
			
	INVESTORS:
	
	SV LIFE SCIENCES FUND V, L.P.
		
	By:	 	SV Life Sciences Fund V (GP), L.P.,
		 	Its sole General Partner
		
	By:	 	SVLSF V, LLC,
		 	Its sole General Partner

  

							
		 		 	By:	 	 /s/ Denise W. Marks

		 		 		 	Name: Denis W. Marks
		 		 		 	Title: SVLSF V, LLC, Member

  

			
	SV LIFE SCIENCES FUND V STRATEGIC PARTNERS, L.P.
		
	By:	 	SV Life Sciences Fund V (GP), L.P.,
		 	Its sole General Partner
		
	By:	 	SVLSF V, LLC,
		 	Its sole General Partner

  

							
		 		 	By:	 	 /s/ Denise W. Marks

		 		 		 	Name: Denis W. Marks
		 		 		 	Title: SVLSF V, LLC, Member

 Signature Page to First Amendment to Second Amended and Restated Stockholders’ Agreement 

 
					
	INVESTORS:
	
	NEOMED INNOVATION V L.P.
			
	By:	 	/s/ TAMARA WILLIAMS        	 	/s/ JAMES KEATING
		 	Name: TAMARA WILLIAMS        	 	JAMES KEATING
		 	Title: DIRECTOR	 	DIRECTOR
	  
 ACTING BY ITS GENERAL PARTNER

NEOMEO INNOVATION V LIMITED

 Signature Page to First Amendment to Second Amended and Restated Stockholders’ Agreement 

 
			
	INVESTORS:
	
	EMBL TECHNOLOGY FUND II GMBH & CO. KG
		
	By:	 	EMBL VENTURES VERWALTUNGS GMBH,
		 	 its General Partner

  

			
	By:	 	 /s/ Jan Adams

		 	Name: Jan Adams
		 	Title:   Executive Director

  

			
	By:	 	 /s/ Dr. Stefan Herr

		 	Name: Stefan Herr
		 	Title:   Executive Director

 Signature Page to First Amendment to Second Amended and Restated Stockholders’ Agreement 

 
			
	INVESTORS:
	
	Anna-Maria and Stephen Kellen Foundation, Inc.

 
			
		
	By:	 	/s/ Michael M. Kellen

 
			
	Name: Michael M. Kellen
	Title: President

 Signature Page to First Amendment to Second Amended and Restated Stockholders’ Agreement 

 EXHIBIT A 

Bill & Melinda Gates Foundation 
 Address: 

For UPS, FedEx, DHL: 

Bill & Melinda Gates Foundation 

1432 Elliott Ave West 
 Seattle,
WA 98119 
 For United States Postal Service 

Bill & Melinda Gates Foundation 

PO Box 23350 
 Seattle, WA 98102

 For Messengers & Courier Service 

Bill & Melinda Gates Foundation 

ATTN: Loading Dock 
 500 Fifth Ave
N 
 Seattle, WA 98109-4636 

Fax No. 206.497.7100 
 Attn:
Jim Bromley, Chief Financial Officer 
 With a copy (which shall not constitute notice): 

Andrew Farnum, Director Program-Related Investments 

With a copy (which shall not constitute notice): 

Claire White 
 K&L Gates LLP

 925 4th Ave, Suite 2900 

Seattle WA, 98104 
 GV 2016, L.P. 

Email: notice@gv.com 
 Attn: Jennifer L. Kercher 

c/o GV 
 1600 Amphitheatre Parkway 

Mountain View, CA 94043 
 Alexandria Venture Investments, LLC

 385 E. Colorado Blvd., Suite 299 
 Pasadena, CA 91101 

Michael W. Bonney 
 536 Commercial Street 

Boston, MA 02109 

 N5 Investments AS 

Parkveien 55 
 0256 Oslo 

Norway 
 SV Life Sciences Fund VI, L.P. 

One Boston Place 
 201 Washington Street, Suite 3900 

Boston, MA 02108 
 Attn: Denise Marks 

SV Life Sciences Fund VI Strategic Partners, L.P. 
 One Boston
Place 
 201 Washington Street, Suite 3900 
 Boston, MA 02108

 Attn: Denise Marks 
 Tillman U. GerngrossEX-10.38

 EXHIBIT 10.38 
  

 
 Amended and Restated Letter Agreement for Rene Russo 

October 10, 2017 
 Dear Ms. Russo, 

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 July 12, 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 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 Board, 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 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 Board 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 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; 

  
 

 
 - 2 - 

 

 
  

 (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. 

  
 

 
 - 3 - 

 

 
  

 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”); 

  
 

 
 - 4 - 

 

 
  

 (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 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 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) 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”); 
 (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. 

  
 

 
 - 5 - 

 

 
  

 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 

  
 

 
 - 6 - 

 

 
  

 
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, 

  
 

 
 - 7 - 

 

 
  

 
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 

  
 

 
 - 8 - 

 

 
  

 
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. 

  
 

 
 - 9 - 

 

 
  

 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. 

  
 

 
 - 10 - 

 

 
  

 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/ Michael Gray
	
	Michael Gray
	
	AGREED TO:
	
	
	/s/ Rene Russo
	Rene Russo
	
	October 20, 2017
	Date

  
 

 
 - 11 -

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00275-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00275-of-00352.parquet"}]]