Sec Form 3 Filing - Polaris Venture Management Co. V, L.L.C. @ Arsanis, Inc. - 2017-11-15

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Polaris Venture Management Co. V, L.L.C. 2. Issuer Name and Ticker or Trading Symbol Arsanis, Inc. [ ASNS] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) ONE MARINA PARK DRIVE, 10TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 11/15/2017 (Street) BOSTON, MA02210 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Series A-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 18,848 I See Footnotes ( 2 ) ( 3 ) Series A-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 367 I See Footnote ( 4 ) Series A-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 129 I See Footnote ( 5 ) Series A-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 188 I See Footnote ( 6 ) Series A-2 Convertible Preferred Stock ( 7 ) ( 7 ) ( 7 ) Common Stock 243,385 I See Footnotes ( 2 ) ( 3 ) Series A-2 Convertible Preferred Stock ( 7 ) ( 7 ) ( 7 ) Common Stock 4,743 I See Footnote ( 4 ) Series A-2 Convertible Preferred Stock ( 7 ) ( 7 ) ( 7 ) Common Stock 1,667 I See Footnote ( 5 ) Series A-2 Convertible Preferred Stock ( 7 ) ( 7 ) ( 7 ) Common Stock 2,433 I See Footnote ( 6 ) Series B Convertible Preferred Stock ( 8 ) ( 8 ) ( 8 ) Common Stock 317,376 I See Footnotes ( 2 ) ( 3 ) Series B Convertible Preferred Stock ( 8 ) ( 8 ) ( 8 ) Common Stock 6,185 I See Footnote ( 4 ) Series B Convertible Preferred Stock ( 8 ) ( 8 ) ( 8 ) Common Stock 2,173 I See Footnote ( 5 ) Series B Convertible Preferred Stock ( 8 ) ( 8 ) ( 8 ) Common Stock 3,173 I See Footnote ( 6 ) Series C Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 110,312 I See Footnotes ( 2 ) ( 3 ) Series C Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 2,150 I See Footnote ( 4 ) Series C Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 755 I See Footnote ( 5 ) Series C Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 1,102 I See Footnote ( 6 ) Series D Convertible Preferred Stock ( 10 ) ( 10 ) ( 10 ) Common Stock 631,041 I See Footnotes ( 2 ) ( 3 ) Series D Convertible Preferred Stock ( 10 ) ( 10 ) ( 10 ) Common Stock 12,298 I See Footnote ( 4 ) Series D Convertible Preferred Stock ( 10 ) ( 10 ) ( 10 ) Common Stock 4,322 I See Footnote ( 5 ) Series D Convertible Preferred Stock ( 10 ) ( 10 ) ( 10 ) Common Stock 6,310 I See Footnote ( 6 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Polaris Venture Management Co. V, L.L.C. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON, MA02210 X Polaris Venture Partners V, L.P. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON, MA02210 X Polaris Venture Partners Entrepreneurs' Fund V, L.P. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON, MA02210 X Polaris Venture Partners Founders' Fund V, L.P. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON, MA02210 X Polaris Venture Partners Special Founders' Fund V, L.P. ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON, MA02210 X Flint Jonathan A ONE MARINA PARK DRIVE, 10TH FLOOR BOSTON, MA02210 X Signatures /s/ Max Eisenberg, Attorney-in-Fact for Polaris Ventures Management Co. V., L.L.C. 11/15/2017 Signature of Reporting Person Date /s/ Max Eisenberg, Attorney-in-Fact for Polaris Venture Partners V, L.P. 11/15/2017 Signature of Reporting Person Date /s/ Max Eisenberg, Attorney-in-Fact for Polaris Venture Partners Entrepreneurs' Fund V, L.P. 11/15/2017 Signature of Reporting Person Date /s/ Max Eisenberg, Attorney-in-Fact for Polaris Venture Partners Founders' Fund V 11/15/2017 Signature of Reporting Person Date /s/ Max Eisenberg, Attorney-in-Fact for Polaris Venture Partners Special Founders' Fund V, L.P. 11/15/2017 Signature of Reporting Person Date /s/ Max Eisenberg, Attorney-in-Fact for Jonathan Flint 11/15/2017 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The Series A-1 Convertible Preferred Stock is convertible into Common Stock on a 0.29300-for-one basis into the number of shares of Common Stock shown in column 3 at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date. ( 2 )These shares are owned directly by Polaris Venture Partners V, L.P. ("Polaris V"), whose general partner is Polaris Venture Management Co. V, L.L.C. ("Polaris Management"). Each of Jonathan A. Flint ("Flint") and Terrance G. McGuire ("McGuire"), who is a member of the Issuer's Board of Directors and whose beneficial ownership is reported on a separate Form 3, are the managing members of Polaris Management and may each be deemed to share voting and dispositive power with respect to the shares held by each of Polaris V, Polaris Venture Partners Entrepreneurs' Fund V, L.P. ("Polaris Entrepreneurs' V"), Polaris Venture Partners Founders' Fund V, L.P. ("Polaris Founders' V") and Polaris Venture Partners Special Founders' Fund V, L.P. ("Polaris Special Founders' V", and together with Polaris V, Polaris Entrepreneurs' V and Polaris Founders' V, the "Polaris V Funds"). Polaris Management may be deemed to have sole power to vote and dispose of the shares held by the Polaris V Funds. ( 3 )(Continued from footnote 2) Each of Flint, McGuire and Polaris Management disclaims beneficial ownership of these securities and this report shall not be deemed to be an admission that any of them is the beneficial owner of such securities for purposes of Section 16 or any other purpose, except to the extent of their respective pecuniary interests therein. ( 4 )These shares are owned directly by Polaris Entrepreneurs' V, whose general partner is Polaris Management. Each of Flint and McGuire are the managing members of Polaris Management and may each be deemed to share voting and dispositive power over the shares held by each of the Polaris V Funds. Polaris Management may be deemed to have sole power to vote and dispose of the shares held by the Polaris V Funds. Each of Flint, McGuire and Polaris Management disclaims beneficial ownership of these securities and this report shall not be deemed to be an admission that any of them is the beneficial owner of such securities for purposes of Section 16 or any other purpose, except to the extent of their respective pecuniary interests therein. ( 5 )These shares are owned directly by Polaris Founders' V, whose general partner is Polaris Management. Each of Flint and McGuire are the managing members of Polaris Management and may each be deemed to share voting and dispositive power over the shares held by each of the Polaris V Funds. Polaris Management may be deemed to have sole power to vote and dispose of the shares held by the Polaris V Funds. Each of Flint, McGuire and Polaris Management disclaims beneficial ownership of these securities and this report shall not be deemed to be an admission that any of them is the beneficial owner of such securities for purposes of Section 16 or any other purpose, except to the extent of their respective pecuniary interests therein. ( 6 )These shares are owned directly by Polaris Special Founders' V, whose general partner is Polaris Management. Each of Flint and McGuire are the managing members of Polaris Management and, may each be deemed to share voting and dispositive power over the shares held by each of the Polaris V Funds. Polaris Management may be deemed to have sole power to vote and dispose of the shares held by the Polaris V Funds. Each of Flint, McGuire and Polaris Management disclaims beneficial ownership of these securities and this report shall not be deemed to be an admission that any of them is the beneficial owner of such securities for purposes of Section 16 or any other purpose, except to the extent of their respective pecuniary interests therein. ( 7 )The Series A-2 Convertible Preferred Stock is convertible into Common Stock on a 0.35785-for-one basis into the number of shares of Common Stock shown in column 3 at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date. ( 8 )The Series B Convertible Preferred Stock is convertible into Common Stock on a 0.44650-for-one basis into the number of shares of Common Stock shown in column 3 at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date. ( 9 )The Series C Convertible Preferred Stock is convertible into Common Stock on a 0.49832-for-one basis into the number of shares of Common Stock shown in column 3 at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date. ( 10 )The Series D Convertible Preferred Stock is convertible into Common Stock on a 0.29300-for-one basis into the number of shares of Common Stock shown in column 3 at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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