EX-10.1





 Exhibit 10.1 

FORM OF SUPPORT AGREEMENT 

This Support Agreement (this “Agreement”) is made and entered into as of November [_], 2018, by and among X4
Pharmaceuticals, Inc., a Delaware corporation (“Merger Partner”), Arsanis, Inc., a Delaware corporation (“Public Company”), and the undersigned stockholder (the “Stockholder”)
of Public Company. 
 RECITALS 

A. Concurrently with the execution and delivery hereof, Public Company, Merger Partner and Artemis AC Corp., a Delaware corporation and a
wholly owned subsidiary of Public Company (the “Merger Sub”), have entered into an agreement and plan of merger (as such agreement may be amended or supplemented from time to time pursuant to the terms thereof, the
“Merger Agreement”), pursuant to which Merger Sub will merge with and into Merger Partner, with Merger Partner as the surviving corporation and a wholly owned subsidiary of Public Company (the
“Merger”). 
 B. The Stockholder is the beneficial owner (as defined in Rule
13d-3 under the Exchange Act) of such number of shares of Public Company Common Stock as indicated in Appendix A to this Agreement. 

C. As an inducement to the willingness of Merger Partner to enter into the Merger Agreement, Merger Partner has required that certain
stockholders of Public Company (including the Stockholder) enter into this Agreement. 
 NOW, THEREFORE, intending to be legally bound, the
parties hereby agree as follows: 
 1. Certain Definitions. Capitalized terms used but not otherwise defined herein shall have the
meanings ascribed thereto in the Merger Agreement. For all purposes of this Agreement, the following terms shall have the following respective meanings: 

“Constructive Sale” means, with respect to any security, a short sale with respect to such security,
entering into or acquiring a derivative contract with respect to such security, entering into or acquiring a futures or forward contract to deliver such security or entering into any other hedging or other derivative transaction that has the effect
of either directly or indirectly materially changing the economic benefits or risks of ownership of such security. 

“Recommendation Change Requirement” means 70% of the Shares held by the Stockholder. 

“Shares” means (i) all shares of Public Company Common Stock owned, beneficially or of record, by
the Stockholder as of the date hereof, and (ii) all additional shares of Public Company Common Stock acquired by the Stockholder, beneficially or of record, during the period commencing with the execution and delivery of this Agreement and
expiring on the Closing Date. 
 “Transfer” means, with respect to any security, the direct or
indirect assignment, sale, transfer, tender, exchange, pledge or hypothecation, or the grant, creation or suffrage of a lien, security interest or encumbrance in or upon, or the gift, grant or placement in trust, or the Constructive Sale or other
disposition of such security (including transfers by testamentary or intestate succession, by domestic relations order or other court order, or otherwise by operation of law) or any right, title or interest therein (including any right or power to
vote to which the holder 












 
thereof may be entitled, whether such right or power is granted by proxy or otherwise), or the record or beneficial ownership thereof, the offer to make such a sale, transfer, Constructive Sale
or other disposition, and each agreement, arrangement or understanding, whether or not in writing, to effect any of the foregoing. 
 2.
Transfer and Voting Restrictions. The Stockholder covenants to Merger Partner as follows: 
 (a) During the period commencing with the
execution and delivery of this Agreement and expiring on the Lock-Up Termination Date (as defined below), such Stockholder shall not Transfer any of such Stockholder’s Shares, or publicly announce its
intention to Transfer any of its Shares. 
 (b) Except as otherwise permitted by this Agreement or by order of a court of competent
jurisdiction, the Stockholder will not commit any act that would restrict the Stockholder’s legal power, authority and right to vote all of the Shares held by the Stockholder or otherwise prevent or disable the Stockholder from performing any
of his, her or its obligations under this Agreement. Without limiting the generality of the foregoing, except for this Agreement and as otherwise permitted by this Agreement, the Stockholder shall not enter into any voting agreement with any person
or entity with respect to any of the Stockholder’s Shares, grant any person or entity any proxy (revocable or irrevocable) or power of attorney with respect to any of the Stockholder’s Shares, deposit any of the Stockholder’s Shares
in a voting trust or otherwise enter into any agreement or arrangement with any person or entity limiting or affecting the Stockholder’s legal power, authority or right to vote the Stockholder’s Shares in favor of the approval of the
Merger. 
 (c) Notwithstanding anything else herein to the contrary, the Stockholder may, at any time, Transfer its Shares (i) by will
or other testamentary document or by intestacy, (ii) to any investment fund or other entity controlled or managed by the Stockholder, (iii) to any member of the Stockholder’s immediate family or (iv) to any trust for the direct
or indirect benefit of the Stockholder or the immediate family of the Stockholder or otherwise for estate planning purposes; provided, that the applicable transferee shall have executed and delivered a support agreement substantially
identical to this Agreement. 
 3. Agreement to Vote Shares. The Stockholder covenants to Merger Partner as follows: 

(a) Until the Expiration Date (as defined below), at any meeting of the stockholders of Public Company, however called, and at every
adjournment or postponement thereof, and on every action or approval by written consent of the stockholders of Public Company: (i) if no Public Company Board Recommendation Change has occurred, the Stockholder shall be present (in person or by
proxy) and vote, or exercise its right to consent with respect to, all Shares held by the Stockholder (A) in favor of the issuance of shares of common stock of Public Company in connection with the Merger, as well as the adoption of the Merger
Agreement and (B) against any Acquisition Proposal; and (ii) if a Public Company Board Recommendation Change has occurred, the Stockholder shall be present (in person or by proxy) and vote, or exercise its right to consent with respect to,
at least the Recommendation Change Requirement (rounded up to the nearest whole number of Shares) of Shares held by the Stockholder (A) in favor of the issuance of shares of common stock of Public Company in connection with the Merger, as well
as the adoption of the Merger Agreement and (B) against any Acquisition Proposal.
 (b) If the Stockholder is the beneficial owner, but
not the record holder, of Shares, the Stockholder agrees to take all actions necessary to cause the record holder and any nominees to be present (in person or by proxy) and vote all of the Stockholder’s Shares in accordance with this
Section 3. 

  
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 (c) In the event of a stock split, stock dividend or distribution, or any change in the
capital stock of Public Company by reason of any split-up, reverse stock split, recapitalization, combination, reclassification, reincorporation, exchange of shares or the like, the term “Shares”
shall be deemed to refer to and include such shares as well as all such stock dividends and distributions and any securities into which or for which any or all of such shares may be changed or exchanged or which are received in such transaction.

 (d) The Stockholder hereby waives and agrees not to exercise any rights of appraisal or any dissenters’ rights (including under
Section 262 of the DGCL) that the Stockholder may have (whether under applicable law or otherwise) or could potentially have or acquire in connection with the Merger. 

4. Action in Stockholder Capacity Only. The Stockholder is entering into this Agreement solely in the Stockholder’s capacity as a
record holder and beneficial owner, as applicable, of its Shares and not in the Stockholder’s capacity as a director or officer of Public Company. Nothing herein shall limit or affect any Stockholder’s ability to act as an officer or
director of Public Company. 
 5. Irrevocable Proxy. The Stockholder hereby revokes (or agrees to cause to be revoked) any proxies
that the Stockholder has heretofore granted with respect to its Shares. In the event and to the extent that the Stockholder fails to vote its Shares in accordance with Section 3 at any applicable meeting of the stockholders
of Public Company or pursuant to any applicable written consent of the stockholders of Public Company, the Stockholder shall be deemed to have irrevocably granted to, and appointed, Merger Partner, and any individual designated in writing by it, and
each of them individually, as his, her or its proxy and attorney-in-fact (with full power of substitution), for and in its name, place and stead, to vote his, her or its
Shares in any action by written consent of Public Company stockholders or at any meeting of the Public Company stockholders called with respect to any of the matters specified in, and in accordance and consistent with,
Section 3 of this Agreement; provided, that, for the avoidance of doubt, if a Public Company Board Recommendation Change has occurred the Stockholder shall only be deemed to have granted proxies to the extent of the
Recommendation Change Requirement. Merger Partner agrees not to exercise the proxy granted herein for any purpose other than the purposes described in this Agreement. Except as otherwise provided for herein, the Stockholder hereby affirms that the
irrevocable proxy is coupled with an interest and may under no circumstances be revoked and that such irrevocable proxy is executed and intended to be irrevocable. Notwithstanding any other provisions of this Agreement, the irrevocable proxy granted
hereunder shall automatically terminate upon the termination of this Agreement. 
 6. No Solicitation. Subject to
Section 4, the Stockholder agrees not to, directly or indirectly, including through any of its officers, directors or agents, (a) solicit, seek or initiate or knowingly take any action to facilitate or encourage, any
offers, inquiries or the making of any proposal or offer that constitutes, or could reasonably be expected to lead to, any Acquisition Proposal or (b) enter into, continue or otherwise participate or engage in any discussions or negotiations
regarding any Acquisition Proposal, or furnish to any person any non-public information or afford any person, other than Public Company or Merger Partner, as applicable, access to such party’s property,
books or records (except pursuant to a request by a Governmental Entity) in connection with, any Acquisition Proposal; provided, however, that nothing in this Section 6 shall prevent the Stockholder from
referring a person to this Section 6 or to the Merger Agreement. 
 7. Representations and Warranties of the
Stockholder. The Stockholder hereby represents and warrants to Merger Partner as follows: 
 (a) (i) The Stockholder is the beneficial
or record owner of the shares of Public Company Common Stock indicated in Appendix A to this Agreement (each of which shall be deemed to be “held” by the Stockholder for purposes of Section 3 unless
otherwise expressly stated with respect to any shares in Appendix A), free and clear of any and all Liens; and (ii) the Stockholder does not beneficially own any securities of Public Company other than the shares of Public Company Common
Stock and rights to purchase shares of Public Company Common Stock set forth in Appendix A to this Agreement. 

  
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 (b) Except as otherwise provided in this Agreement, the Stockholder has full power and
authority to (i) make, enter into and carry out the terms of this Agreement and (ii) vote all of its Shares in the manner set forth in this Agreement without the consent or approval of, or any other action on the part of, any other person
or entity (including any Governmental Entity). Without limiting the generality of the foregoing, the Stockholder has not entered into any voting agreement (other than this Agreement) with any person with respect to any of the Stockholder’s
Shares, granted any person any proxy (revocable or irrevocable) or power of attorney with respect to any of the Stockholder’s Shares, deposited any of the Stockholder’s Shares in a voting trust or entered into any arrangement or agreement
with any person limiting or affecting the Stockholder’s legal power, authority or right to vote the Stockholder’s Shares on any matter. 

(c) This Agreement has been duly and validly executed and delivered by the Stockholder and (assuming the due authorization, execution and
delivery by the other parties hereto) constitutes a valid and binding agreement of the Stockholder enforceable against the Stockholder in accordance with its terms, subject to the Bankruptcy and Equity Exception. The execution and delivery of this
Agreement by the Stockholder and the performance by the Stockholder of the agreements and obligations hereunder will not result in any breach or violation of or be in conflict with or constitute a default under any term of any Contract or if
applicable any provision of an organizational document (including a certificate of incorporation) to or by which the Stockholder is a party or bound, or any applicable law to which the Stockholder (or any of the Stockholder’s assets) is subject
or bound, except for any such breach, violation, conflict or default which, individually or in the aggregate, would not reasonably be expected to materially impair or adversely affect the Stockholder’s ability to perform its obligations under
this Agreement. 
 (d) The Stockholder has had the opportunity to review the Merger Agreement and this Agreement with the Stockholder’s
legal counsel. The Stockholder understands and acknowledges that Merger Partner is entering into the Merger Agreement in reliance upon the Stockholder’s execution, delivery and performance of this Agreement. 

(e) The execution, delivery and performance of this Agreement by the Stockholder do not and will not require any consent, approval,
authorization or permit of, action by, filing with or notification to, any Governmental Entity, except for any such consent, approval, authorization, permit, action, filing or notification the failure of which to make or obtain, individually or in
the aggregate, has not and would not materially impair the Stockholder’s ability to perform its obligations under this Agreement. 

(f) With respect to the Stockholder, as of the date hereof, there is no action, suit, investigation or proceeding pending against, or, to the
knowledge of the Stockholder, threatened against, the Stockholder or any of the Stockholder’s properties or assets (including the Shares) that would reasonably be expected to prevent or materially delay or impair the ability of the Stockholder
to perform its obligations hereunder or to consummate the transactions contemplated hereby. 
 8. Termination. This Agreement shall
terminate and shall cease to be of any further force or effect as of the earlier of (a) such date and time as the Merger Agreement shall have been terminated pursuant to the terms thereof or (b) the date on which the Public Company Voting
Proposal shall have been approved by the requisite holders of Public Company Common Stock (the “Expiration Date”); provided, however, that (i) Section 9 shall survive
the termination of this Agreement, (ii) Section 2(a) (and, to the extent relating to thereto, Section 2(c)) shall terminate and shall cease to be of any further force or effect as of the
earlier of (A) such date and time as the Merger Agreement shall have been terminated pursuant to the terms thereof or (B) the date which is one hundred and eighty (180) calendar days following the Closing Date (the “Lock-Up Termination Date”) and (iii) the termination of this Agreement shall not relieve any party hereto from any liability for any material and willful breach of this Agreement prior to the
Expiration Date. 

  
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 9. Miscellaneous Provisions. 

(a) Amendments. No amendment of this Agreement shall be effective against any party unless it shall be in writing and signed by each of
the parties hereto. 
 (b) Entire Agreement. This Agreement constitutes the entire agreement between the parties to this Agreement
and supersedes all other prior agreements, arrangements and understandings, both written and oral, among the parties with respect to the subject matter hereof. 

(c) Governing Law. All matters arising out of or relating to this Agreement and the transactions contemplated hereby (including its
interpretation, construction, performance and enforcement) shall be governed by and construed in accordance with the internal laws of the State of Delaware without giving effect to any choice or conflict of law provision or rule (whether of the
State of Delaware or any other jurisdiction) that would cause the application of laws of any jurisdictions other than those of the State of Delaware. 

(d) Jurisdiction. Each of the parties to this Agreement (i) consents to submit itself to the exclusive personal jurisdiction of
the Court of Chancery of the State of Delaware, New Castle County, or, if that court does not have jurisdiction, a federal court sitting in Wilmington, Delaware in any action or proceeding arising out of or relating to this Agreement or any of the
transactions contemplated by this Agreement, (ii) agrees that all claims in respect of such action or proceeding shall be heard and determined in any such court, (iii) agrees that it shall not attempt to deny or defeat such personal
jurisdiction by motion or other request for leave from any such court and (iv) agrees not to bring any action or proceeding arising out of or relating to this Agreement or any of the transactions contemplated by this Agreement in any other
court. Each of the parties hereto waives any defense of inconvenient forum to the maintenance of any action or proceeding so brought and waives any bond, surety or other security that might be required of any other party with respect thereto. Any
party may make service on another party by sending or delivering a copy of the process to the party to be served at the address and in the manner provided for the giving of notices in Section 9(j). Nothing in this
Section 9(d), however, shall affect the right of any party to serve legal process in any other manner permitted by law. 

(e) WAIVER OF JURY TRIAL. EACH OF THE PARTIES TO THIS AGREEMENT HEREBY IRREVOCABLY WAIVES ALL RIGHT TO TRIAL BY JURY IN ANY ACTION,
PROCEEDING OR COUNTERCLAIM (WHETHER BASED ON CONTRACT, TORT OR OTHERWISE) ARISING OUT OF OR RELATING TO THIS AGREEMENT OR THE TRANSACTIONS CONTEMPLATED HEREBY OR THE ACTIONS OF ANY PARTY TO THIS AGREEMENT IN THE NEGOTIATION, ADMINISTRATION,
PERFORMANCE AND ENFORCEMENT OF THIS AGREEMENT. 
 (f) Assignment. Except as otherwise provided in
Section 2(c) hereof, no party may assign any of its rights or delegate any of its performance obligations under this Agreement, in whole or in part, by operation of law or otherwise, without the prior written consent of the
other parties hereto, and any such assignment without such prior written consent shall be null and void. Subject to the preceding sentence, this Agreement shall be binding upon, inure to the benefit of, and be enforceable by, the parties hereto and
their respective successors and permitted assigns. Any purported assignment of rights or delegation of performance obligations in violation of this Section 9(f) is void. 

  
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 (g) No Third Party Rights. This Agreement is not intended to, and shall not, confer
upon any other person any rights or remedies hereunder other than the parties hereto to the extent expressly set forth herein. 
 (h)
Severability. Any term or provision of this Agreement that is invalid or unenforceable in any situation in any jurisdiction shall not affect the validity or enforceability of the remaining terms and provisions hereof or the validity or
enforceability of the offending term or provision in any other situation or in any other jurisdiction. If the final judgment of a court of competent jurisdiction declares that any term or provision hereof is invalid or unenforceable, the parties
hereto agree that the court making such determination shall have the power to limit the term or provision, to delete specific words or phrases, or to replace any invalid or unenforceable term or provision with a term or provision that is valid and
enforceable and that comes closest to expressing the intention of the invalid or unenforceable term or provision, and this Agreement shall be enforceable as so modified. In the event such court does not exercise the power granted to it in the prior
sentence, the parties hereto agree to replace such invalid or unenforceable term or provision with a valid and enforceable term or provision that will achieve, to the extent possible, the economic, business and other purposes of such invalid or
unenforceable term. 
 (i) Specific Performance. The parties hereto agree that irreparable damage would occur in the event that any
of the provisions of this Agreement were not performed in accordance with their specific terms or were otherwise breached. It is accordingly agreed that the parties shall be entitled to an injunction or injunctions to prevent breaches of this
Agreement and to enforce specifically the terms and provisions of this Agreement, this being in addition to any other remedy to which they are entitled at law or in equity. 

(j) Notices. All notices and other communications hereunder shall be in writing and shall be deemed duly delivered (i) three
Business Days after being sent by registered or certified mail, return receipt requested, postage prepaid, or (ii) one Business Day after being sent for next Business Day delivery, fees prepaid, via a reputable overnight courier service, in
each case to the intended recipient as follows: (A) if to Merger Partner or Public Company, to the address, electronic mail address or facsimile provided in the Merger Agreement, including to the persons designated therein to receive copies;
and/or (B) if to the Stockholder, to the Stockholder’s address, electronic mail address or facsimile shown below the Stockholder’s signature to this Agreement. 

(k) Counterparts. This Agreement may be executed in two or more counterparts (including by facsimile, by an electronic scan delivered
by electronic mail or any electronic signature), each of which shall be deemed an original but all of which together shall be considered one and the same agreement and shall become effective when counterparts have been signed by each of the parties
hereto and delivered to the other parties, it being understood that all parties need not sign the same counterpart. This Agreement may be executed and delivered by facsimile, by an electronic scan delivered by electronic mail or by delivery of any
electronic signature. 
 (l) Interpretation. When reference is made in this Agreement to a Section, such reference shall be to a
Section of this Agreement, unless otherwise indicated. The headings contained in this Agreement are for convenience of reference only and shall not affect in any way the meaning or interpretation of this Agreement. The language used in this
Agreement shall be deemed to be the language chosen by the parties hereto to express their mutual intent, and no rule of strict construction shall be applied against any party. Whenever the context may require, any pronouns used in this Agreement
shall include the corresponding masculine, feminine or neuter forms, and the singular form of nouns and pronouns shall include the plural, and vice versa. Any reference to any federal, state, local or foreign statute or law shall be deemed also to
refer to all rules and regulations promulgated thereunder, unless the context requires 

  
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otherwise. Whenever the words “include,” “includes” or “including” are used in this Agreement, they shall be deemed to be followed by the words “without
limitation.” 
 [Remainder of Page Left Intentionally Blank] 

  
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 IN WITNESS WHEREOF, the undersigned have caused this Agreement to be duly executed as of the
date first above written. 
 MERGER PARTNER: 
 X4
PHARMACEUTICALS, INC. 
  

	
	
	   

	By:
	Title:













 IN WITNESS WHEREOF, the undersigned have caused this Agreement to be duly executed as of the
date first above written. 
 PUBLIC COMPANY: 
 ARSANIS, INC.

  

	
	
	   

	By:
	Title:













 IN WITNESS WHEREOF, the undersigned have caused this Agreement to be duly executed as of the
date first above written. 
 [STOCKHOLDER], 
 in its capacity
as the Stockholder: 
  

	
	
	   

	
	By:
	Title:

  

	
	Address:

 __________________________________ 

__________________________________ 

__________________________________ 












 Appendix A 
  

					
	 Name of Beneficial Owner
	  	 Number of Shares Beneficially Owned
	  	 Percentage of Shares Beneficially Owned*

		  		  	

  

	*	 Based on 14,315,410 shares of common stock, $0.001 par value per share, outstanding as of October 31,
2018.