SEC FORM 3 SEC Form 3
FORM 3 UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940
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1. Name and Address of Reporting Person*
SVLSF V, LLC

(Last) (First) (Middle)
ONE BOSTON PLACE, SUITE 3900
201 WASHINGTON STREET

(Street)
BOSTON MA 02108

(City) (State) (Zip)
2. Date of Event Requiring Statement (Month/Day/Year)
11/15/2017
3. Issuer Name and Ticker or Trading Symbol
Arsanis, Inc. [ ASNS ]
4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
Director X 10% Owner
Officer (give title below) Other (specify below)
5. If Amendment, Date of Original Filed (Month/Day/Year)
6. Individual or Joint/Group Filing (Check Applicable Line)
Form filed by One Reporting Person
X Form filed by More than One Reporting Person
Table I - Non-Derivative Securities Beneficially Owned
1. Title of Security (Instr. 4) 2. Amount of Securities Beneficially Owned (Instr. 4) 3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5) 4. Nature of Indirect Beneficial Ownership (Instr. 5)
Table II - Derivative Securities Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 4) 2. Date Exercisable and Expiration Date (Month/Day/Year) 3. Title and Amount of Securities Underlying Derivative Security (Instr. 4) 4. Conversion or Exercise Price of Derivative Security 5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5) 6. Nature of Indirect Beneficial Ownership (Instr. 5)
Date Exercisable Expiration Date Title Amount or Number of Shares
Series A-1 Convertible Preferred Stock (1) (1) Common Stock 19,128 (1) D(6)
Series A-1 Convertible Preferred Stock (1) (1) Common Stock 404 (1) D(7)
Series A-2 Convertible Preferred Stock (2) (2) Common Stock 247,010 (2) D(6)
Series A-2 Convertible Preferred Stock (2) (2) Common Stock 5,219 (2) D(7)
Series B Convertible Preferred Stock (3) (3) Common Stock 322,102 (3) D(6)
Series B Convertible Preferred Stock (3) (3) Common Stock 6,806 (3) D(7)
Series C Convertible Preferred Stock (4) (4) Common Stock 111,955 (4) D(6)
Series C Convertible Preferred Stock (4) (4) Common Stock 2,365 (4) D(7)
Series D Convertible Preferred Stock (5) (5) Common Stock 202,915 (5) D(6)
Series D Convertible Preferred Stock (5) (5) Common Stock 4,288 (5) D(7)
Series D Convertible Preferred Stock (5) (5) Common Stock 431,980 (5) D(8)
Series D Convertible Preferred Stock (5) (5) Common Stock 14,789 (5) D(9)
1. Name and Address of Reporting Person*
SVLSF V, LLC

(Last) (First) (Middle)
ONE BOSTON PLACE, SUITE 3900
201 WASHINGTON STREET

(Street)
BOSTON MA 02108

(City) (State) (Zip)
1. Name and Address of Reporting Person*
SVLSF VI, LLC

(Last) (First) (Middle)
ONE BOSTON PLACE, SUITE 3900
201 WASHINGTON STREET

(Street)
BOSTON MA 02108

(City) (State) (Zip)
1. Name and Address of Reporting Person*
SV Life Sciences Fund VI Strategic Partners, L.P.

(Last) (First) (Middle)
ONE BOSTON PLACE, SUITE 3900,
201 WASHINGTON STREET

(Street)
BOSTON MA 02108

(City) (State) (Zip)
1. Name and Address of Reporting Person*
SV Life Sciences Fund VI, L.P.

(Last) (First) (Middle)
ONE BOSTON PLACE, SUITE 3900,
201 WASHINGTON STREET

(Street)
BOSTON MA 02108

(City) (State) (Zip)
1. Name and Address of Reporting Person*
SV Life Sciences Fund V Strategic Partners, L.P.

(Last) (First) (Middle)
ONE BOSTON PLACE, SUITE 3900,
201 WASHINGTON STREET

(Street)
BOSTON MA 02108

(City) (State) (Zip)
1. Name and Address of Reporting Person*
SV LIFE SCIENCES FUND V LP

(Last) (First) (Middle)
ONE BOSTON PLACE, SUITE 3900,
201 WASHINGTON STREET

(Street)
BOSTON MA 02108

(City) (State) (Zip)
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. 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.
3. 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.
4. 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.
5. 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.
6. These shares are owned directly by SV Life Sciences Fund V, L.P. ("SVLS V LP"). SV Life Sciences Fund V (GP), LP ("SVLS V GP") is the general partner of SVLS V LP. The general partner of SVLS V GP is SVLSF V, LLC. Kate Bingham, James Garvey, Eugene D. Hill, III and Michael Ross are members of the investment committee of SVLSF V, LLC. SVLS V GP, SVLSF V, LLC and each of the individuals comprising the SVLSF V, LLC investment committee may be deemed to share voting, dispositive and investment power over the shares held of record by SVLS V LP. Each of SVLS V GP, SVLSF V, LLC and the individual members of the SVLSF V, LLC investment committee disclaims beneficial ownership of these securities and this report shall not be deemed to be an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or any other purpose, except to the extent of their pecuniary interest therein.
7. These shares are owned directly by SV Life Sciences Fund V Strategic Partners, L.P. ("SVLS V SPP"). SVLS V GP is the general partner of SVLS V SPP. The general partner of SVLS V GP is SVLSF V, LLC. Kate Bingham, James Garvey, Eugene D. Hill, III and Michael Ross are members of the investment committee of SVLSF V, LLC. SVLS V GP, SVLSF V, LLC and each of the individuals comprising the SVLSF V, LLC investment committee may be deemed to share voting, dispositive and investment power over the shares held of record by SVLS V SPP. Each of SVLS V GP, SVLSF V, LLC and the individual members of the SVLSF V, LLC investment committee disclaims beneficial ownership of these securities and this report shall not be deemed to be an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or any other purpose, except to the extent of their pecuniary interest therein.
8. These shares are owned directly by SV Life Sciences Fund VI, L.P. ("SVLS VI LP"). SV Life Sciences Fund VI (GP), LP ("SVLS VI GP") is the general partner of SVLS VI LP. The general partner of SVLS VI GP is SVLSF VI, LLC. Kate Bingham, James Garvey, Eugene D. Hill, III, Paul LaViolette, Thomas Flynn and Michael Ross are members of the investment committee of SVLSF VI, LLC. SVLS VI GP, SVLSF VI, LLC and each of the individuals comprising the SVLSF VI, LLC investment committee may be deemed to share voting, dispositive and investment power over the shares held of record by SVLS VI LP. Each of SVLS VI GP, SVLSF VI, LLC and the individual members of the SVLSF VI, LLC investment committee disclaims beneficial ownership of these securities and this report shall not be deemed to be an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or any other purpose, except to the extent of their pecuniary interest therein.
9. These shares are owned directly by SV Life Sciences Fund VI Strategic Partners, L.P. ("SVLS VI SPP"). SVLS VI GP is the general partner of SVLS VI SPP. The general partner of SVLS VI GP is SVLSF VI, LLC. Kate Bingham, James Garvey, Eugene D. Hill, III, Paul LaViolette, Thomas Flynn and Michael Ross are members of the investment committee of SVLSF VI, LLC. SVLS VI GP, SVLSF VI, LLC and each of the individuals comprising the SVLSF VI, LLC investment committee may be deemed to share voting, dispositive and investment power over the shares held of record by SVLS VI SPP. Each of SVLS VI GP, SVLSF VI, LLC and the individual members of the SVLSF VI, LLC investment committee disclaims beneficial ownership of these securities and this report shall not be deemed to be an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or any other purpose, except to the extent of their pecuniary interest therein.
Remarks:
Exhibit Index: 24.1 Power of Attorney 24.2 Power of Attorney 24.3 Power of Attorney 24.4 Power of Attorney 24.5 Power of Attorney 24.6 Power of Attorney
/s/ Mike Gray, as attorney-in-fact on behalf of SVLSF V, LLC 11/15/2017
/s/ Mike Gray, as attorney-in-fact on behalf of SVLSF VI, LLC 11/15/2017
/s/ Mike Gray, as attorney-in-fact on behalf of SV Life Sciences Fund V, L.P. 11/15/2017
/s/ Mike Gray, as attorney-in-fact on behalf of SV Life Sciences Fund V Strategic Partners, L.P. 11/15/2017
/s/ Mike Gray, as attorney-in-fact on behalf of SV Life Sciences Fund VI, L.P. 11/15/2017
/s/ Mike Gray, as attorney-in-fact on behalf of SV Life Sciences Fund VI Strategic Partners, L.P. 11/15/2017
** Signature of Reporting Person 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 5 (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).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.
                                                                    EXHIBIT 24.1

         LIMITED POWER OF ATTORNEY FOR SECTION 16 REPORTING OBLIGATIONS

    Know all by these presents, that the undersigned hereby makes, constitutes
and appoints each of Rene Russo and Mike Gray, signing singly and each acting
individually, as the undersigned's true and lawful attorney-in-fact with full
power and authority as hereinafter described to:

    (1)  execute for and on behalf of the undersigned, Forms 3, 4, and 5
(including any amendments thereto) in accordance with Section 16(a) of the
Securities Exchange Act of 1934 and the rules thereunder (the "Exchange Act");

    (2)  do and perform any and all acts for and on behalf of the undersigned
which may be necessary or desirable to prepare, complete and execute any such
Form 3, 4, or 5, prepare, complete and execute any amendment or amendments
thereto, and timely deliver and file such form with the United States Securities
and Exchange Commission (the "SEC") and any stock exchange or similar authority,
including without limitation the filing of a Form ID or any other application
materials to enable the undersigned to gain or maintain access to the Electronic
Data Gathering, Analysis and Retrieval system of the SEC;

    (3)  seek or obtain, as the undersigned's representative and
 on the
undersigned's behalf, information regarding transactions in the securities of
Arsanis, Inc. (the "Company") from any third party, including brokers, employee
benefit plan administrators and trustees, and the undersigned hereby authorizes
any such person to release any such information to such attorney-in-fact and
approves and ratifies any such release of information; and

    (4)  take any other action of any type whatsoever in connection with the
foregoing which, in the opinion of such attorney-in-fact, may be of benefit to,
in the best interest of, or legally required by, the undersigned, it being
understood that the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Power of Attorney shall be in such form and shall
contain such terms and conditions as such attorney-in-fact may approve in such
attorney-in-fact's discretion.

    The undersigned hereby grants to each such attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever requisite,
necessary, or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in-fact's substitute or substitutes, shall lawfully do or cause to be
done by virtue of this power of attorney and the rights and powers herein
granted.  The undersigned acknowledges that the foregoing attorneys-in-fact, in
serving in such capacity at the request of the undersigned, are not assuming nor
relieving, nor is the Company assuming nor relieving, any of the undersigned's
responsibilities to comply with Section 16 of the Exchange Act.  The undersigned
acknowledges that neither the Company nor the foregoing attorneys-in-fact assume
(i) any liability for the undersigned's responsibility to comply with the
requirement of the Exchange Act, (ii) any liability of the undersigned for any
failure to comply with such requirements, or (iii) any obligation or liability
of the undersigned for profit disgorgement under Section 16(b) of the Exchange
Act.

    This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4, and 5 with respect to the
undersigned's holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to the
foregoing attorneys-in-fact.

    IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of this 13th day of November, 2017.

                                        SVLSF V, LLC.


                                        By: /s/ Denise W. Marks
                                            ------------------------------
                                        Name: Denise W. Marks
                                        Title: SVLSF V, LLC, Member
 
                                                                    EXHIBIT 24.2

           LIMITED POWER OF ATTORNEY FOR SECTION 16 REPORTING OBLIGATIONS

    Know all by these presents, that the undersigned hereby makes, constitutes
and appoints each of Rene Russo and Mike Gray, signing singly and each acting
individually, as the undersigned's true and lawful attorney-in-fact with full
power and authority as hereinafter described to:

    (1)  execute for and on behalf of the undersigned, Forms 3, 4, and 5
(including any amendments thereto) in accordance with Section 16(a) of the
Securities Exchange Act of 1934 and the rules thereunder (the "Exchange Act");

    (2)  do and perform any and all acts for and on behalf of the undersigned
which may be necessary or desirable to prepare, complete and execute any such
Form 3, 4, or 5, prepare, complete and execute any amendment or amendments
thereto, and timely deliver and file such form with the United States Securities
and Exchange Commission (the "SEC") and any stock exchange or similar authority,
including without limitation the filing of a Form ID or any other application
materials to enable the undersigned to gain or maintain access to the Electronic
Data Gathering, Analysis and Retrieval system of the SEC;

    (3)  seek or obtain, as the undersigned's representative and
 on the
undersigned's behalf, information regarding transactions in the securities of
Arsanis, Inc. (the "Company") from any third party, including brokers, employee
benefit plan administrators and trustees, and the undersigned hereby authorizes
any such person to release any such information to such attorney-in-fact and
approves and ratifies any such release of information; and

    (4)  take any other action of any type whatsoever in connection with the
foregoing which, in the opinion of such attorney-in-fact, may be of benefit to,
in the best interest of, or legally required by, the undersigned, it being
understood that the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Power of Attorney shall be in such form and shall
contain such terms and conditions as such attorney-in-fact may approve in such
attorney-in-fact's discretion.

    The undersigned hereby grants to each such attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever requisite,
necessary, or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in-fact's substitute or substitutes, shall lawfully do or cause to be
done by virtue of this power of attorney and the rights and powers herein
granted.  The undersigned acknowledges that the foregoing attorneys-in-fact, in
serving in such capacity at the request of the undersigned, are not assuming nor
relieving, nor is the Company assuming nor relieving, any of the undersigned's
responsibilities to comply with Section 16 of the Exchange Act.  The undersigned
acknowledges that neither the Company nor the foregoing attorneys-in-fact assume
(i) any liability for the undersigned's responsibility to comply with the
requirement of the Exchange Act, (ii) any liability of the undersigned for any
failure to comply with such requirements, or (iii) any obligation or liability
of the undersigned for profit disgorgement under Section 16(b) of the Exchange
Act.

    This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4, and 5 with respect to the
undersigned's holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to the
foregoing attorneys-in-fact.

    IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of this 13th day of November, 2017.

                                        SVLSF VI, LLC

                                        By: /s/ Denise W. Marks
                                            ------------------------------------
                                        Name: Denise W. Marks
                                        Title: SVLSF VI, LLC, Member
 
                                                                    Exhibit 24.3

           LIMITED POWER OF ATTORNEY FOR SECTION 16 REPORTING OBLIGATIONS

    Know all by these presents, that the undersigned hereby makes, constitutes
and appoints each of Rene Russo and Mike Gray, signing singly and each acting
individually, as the undersigned's true and lawful attorney-in-fact with full
power and authority as hereinafter described to:

    (1)  execute for and on behalf of the undersigned, Forms 3, 4, and 5
(including any amendments thereto) in accordance with Section 16(a) of the
Securities Exchange Act of 1934 and the rules thereunder (the "Exchange Act");

    (2)  do and perform any and all acts for and on behalf of the undersigned
which may be necessary or desirable to prepare, complete and execute any such
Form 3, 4, or 5, prepare, complete and execute any amendment or amendments
thereto, and timely deliver and file such form with the United States Securities
and Exchange Commission (the "SEC") and any stock exchange or similar authority,
including without limitation the filing of a Form ID or any other application
materials to enable the undersigned to gain or maintain access to the Electronic
Data Gathering, Analysis and Retrieval system of the SEC;

    (3)  seek or obtain, as the undersigned's representative and
 on the
undersigned's behalf, information regarding transactions in the securities of
Arsanis, Inc. (the "Company") from any third party, including brokers, employee
benefit plan administrators and trustees, and the undersigned hereby authorizes
any such person to release any such information to such attorney-in-fact and
approves and ratifies any such release of information; and

    (4)  take any other action of any type whatsoever in connection with the
foregoing which, in the opinion of such attorney-in-fact, may be of benefit to,
in the best interest of, or legally required by, the undersigned, it being
understood that the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Power of Attorney shall be in such form and shall
contain such terms and conditions as such attorney-in-fact may approve in such
attorney-in-fact's discretion.

    The undersigned hereby grants to each such attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever requisite,
necessary, or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in-fact's substitute or substitutes, shall lawfully do or cause to be
done by virtue of this power of attorney and the rights and powers herein
granted.  The undersigned acknowledges that the foregoing attorneys-in-fact, in
serving in such capacity at the request of the undersigned, are not assuming nor
relieving, nor is the Company assuming nor relieving, any of the undersigned's
responsibilities to comply with Section 16 of the Exchange Act.  The undersigned
acknowledges that neither the Company nor the foregoing attorneys-in-fact assume
(i) any liability for the undersigned's responsibility to comply with the
requirement of the Exchange Act, (ii) any liability of the undersigned for any
failure to comply with such requirements, or (iii) any obligation or liability
of the undersigned for profit disgorgement under Section 16(b) of the Exchange
Act.

    This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4, and 5 with respect to the
undersigned's holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to the
foregoing attorneys-in-fact.

    IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of this 31st day of October, 2017.

                                        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
 
                                                                    EXHIBIT 24.4

           LIMITED POWER OF ATTORNEY FOR SECTION 16 REPORTING OBLIGATIONS

    Know all by these presents, that the undersigned hereby makes, constitutes
and appoints each of Rene Russo and Mike Gray, signing singly and each acting
individually, as the undersigned's true and lawful attorney-in-fact with full
power and authority as hereinafter described to:

    (1)  execute for and on behalf of the undersigned, Forms 3, 4, and 5
(including any amendments thereto) in accordance with Section 16(a) of the
Securities Exchange Act of 1934 and the rules thereunder (the "Exchange Act");

    (2)  do and perform any and all acts for and on behalf of the undersigned
which may be necessary or desirable to prepare, complete and execute any such
Form 3, 4, or 5, prepare, complete and execute any amendment or amendments
thereto, and timely deliver and file such form with the United States Securities
and Exchange Commission (the "SEC") and any stock exchange or similar authority,
including without limitation the filing of a Form ID or any other application
materials to enable the undersigned to gain or maintain access to the Electronic
Data Gathering, Analysis and Retrieval system of the SEC;

    (3)  seek or obtain, as the undersigned's representative and
 on the
undersigned's behalf, information regarding transactions in the securities of
Arsanis, Inc. (the "Company") from any third party, including brokers, employee
benefit plan administrators and trustees, and the undersigned hereby authorizes
any such person to release any such information to such attorney-in-fact and
approves and ratifies any such release of information; and

    (4)  take any other action of any type whatsoever in connection with the
foregoing which, in the opinion of such attorney-in-fact, may be of benefit to,
in the best interest of, or legally required by, the undersigned, it being
understood that the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Power of Attorney shall be in such form and shall
contain such terms and conditions as such attorney-in-fact may approve in such
attorney-in-fact's discretion.

    The undersigned hereby grants to each such attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever requisite,
necessary, or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in-fact's substitute or substitutes, shall lawfully do or cause to be
done by virtue of this power of attorney and the rights and powers herein
granted.  The undersigned acknowledges that the foregoing attorneys-in-fact, in
serving in such capacity at the request of the undersigned, are not assuming nor
relieving, nor is the Company assuming nor relieving, any of the undersigned's
responsibilities to comply with Section 16 of the Exchange Act.  The undersigned
acknowledges that neither the Company nor the foregoing attorneys-in-fact assume
(i) any liability for the undersigned's responsibility to comply with the
requirement of the Exchange Act, (ii) any liability of the undersigned for any
failure to comply with such requirements, or (iii) any obligation or liability
of the undersigned for profit disgorgement under Section 16(b) of the Exchange
Act.

    This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4, and 5 with respect to the
undersigned's holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to the
foregoing attorneys-in-fact.

    IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of this 31st day of October, 2017.

                              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
 
                                                                    EXHIBIT 24.5

           LIMITED POWER OF ATTORNEY FOR SECTION 16 REPORTING OBLIGATIONS

    Know all by these presents, that the undersigned hereby makes, constitutes
and appoints each of Rene Russo and Mike Gray, signing singly and each acting
individually, as the undersigned's true and lawful attorney-in-fact with full
power and authority as hereinafter described to:

    (1)  execute for and on behalf of the undersigned, Forms 3, 4, and 5
(including any amendments thereto) in accordance with Section 16(a) of the
Securities Exchange Act of 1934 and the rules thereunder (the "Exchange Act");

    (2)  do and perform any and all acts for and on behalf of the undersigned
which may be necessary or desirable to prepare, complete and execute any such
Form 3, 4, or 5, prepare, complete and execute any amendment or amendments
thereto, and timely deliver and file such form with the United States Securities
and Exchange Commission (the "SEC") and any stock exchange or similar authority,
including without limitation the filing of a Form ID or any other application
materials to enable the undersigned to gain or maintain access to the Electronic
Data Gathering, Analysis and Retrieval system of the SEC;

    (3)  seek or obtain, as the undersigned's representative and
 on the
undersigned's behalf, information regarding transactions in the securities of
Arsanis, Inc. (the "Company") from any third party, including brokers, employee
benefit plan administrators and trustees, and the undersigned hereby authorizes
any such person to release any such information to such attorney-in-fact and
approves and ratifies any such release of information; and

    (4)  take any other action of any type whatsoever in connection with the
foregoing which, in the opinion of such attorney-in-fact, may be of benefit to,
in the best interest of, or legally required by, the undersigned, it being
understood that the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Power of Attorney shall be in such form and shall
contain such terms and conditions as such attorney-in-fact may approve in such
attorney-in-fact's discretion.

    The undersigned hereby grants to each such attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever requisite,
necessary, or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in-fact's substitute or substitutes, shall lawfully do or cause to be
done by virtue of this power of attorney and the rights and powers herein
granted.  The undersigned acknowledges that the foregoing attorneys-in-fact, in
serving in such capacity at the request of the undersigned, are not assuming nor
relieving, nor is the Company assuming nor relieving, any of the undersigned's
responsibilities to comply with Section 16 of the Exchange Act.  The undersigned
acknowledges that neither the Company nor the foregoing attorneys-in-fact assume
(i) any liability for the undersigned's responsibility to comply with the
requirement of the Exchange Act, (ii) any liability of the undersigned for any
failure to comply with such requirements, or (iii) any obligation or liability
of the undersigned for profit disgorgement under Section 16(b) of the Exchange
Act.

    This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4, and 5 with respect to the
undersigned's holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to the
foregoing attorneys-in-fact.

    IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of this 31st day of October, 2017.

                              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
 
                                                                    EXHIBIT 24.6

           LIMITED POWER OF ATTORNEY FOR SECTION 16 REPORTING OBLIGATIONS

    Know all by these presents, that the undersigned hereby makes, constitutes
and appoints each of Rene Russo and Mike Gray, signing singly and each acting
individually, as the undersigned's true and lawful attorney-in-fact with full
power and authority as hereinafter described to:

    (1) execute for and on behalf of the undersigned, Forms 3, 4, and 5
(including any amendments thereto) in accordance with Section 16(a) of the
Securities Exchange Act of 1934 and the rules thereunder (the "Exchange Act");

    (2)  do and perform any and all acts for and on behalf of the undersigned
which may be necessary or desirable to prepare, complete and execute any such
Form 3, 4, or 5, prepare, complete and execute any amendment or amendments
thereto, and timely deliver and file such form with the United States Securities
and Exchange Commission (the "SEC") and any stock exchange or similar authority,
including without limitation the filing of a Form ID or any other application
materials to enable the undersigned to gain or maintain access to the Electronic
Data Gathering, Analysis and Retrieval system of the SEC;

    (3)  seek or obtain, as the undersigned's representative and
 on the
undersigned's behalf, information regarding transactions in the securities of
Arsanis, Inc. (the "Company") from any third party, including brokers, employee
benefit plan administrators and trustees, and the undersigned hereby authorizes
any such person to release any such information to such attorney-in-fact and
approves and ratifies any such release of information; and

    (4)  take any other action of any type whatsoever in connection with the
foregoing which, in the opinion of such attorney-in-fact, may be of benefit to,
in the best interest of, or legally required by, the undersigned, it being
understood that the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Power of Attorney shall be in such form and shall
contain such terms and conditions as such attorney-in-fact may approve in such
attorney-in-fact's discretion.

    The undersigned hereby grants to each such attorney-in-fact full power and
authority to do and perform any and every act and thing whatsoever requisite,
necessary, or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in-fact's substitute or substitutes, shall lawfully do or cause to be
done by virtue of this power of attorney and the rights and powers herein
granted.  The undersigned acknowledges that the foregoing attorneys-in-fact, in
serving in such capacity at the request of the undersigned, are not assuming nor
relieving, nor is the Company assuming nor relieving, any of the undersigned's
responsibilities to comply with Section 16 of the Exchange Act.  The undersigned
acknowledges that neither the Company nor the foregoing attorneys-in-fact assume
(i) any liability for the undersigned's responsibility to comply with the
requirement of the Exchange Act, (ii) any liability of the undersigned for any
failure to comply with such requirements, or (iii) any obligation or liability
of the undersigned for profit disgorgement under Section 16(b) of the Exchange
Act.

    This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4, and 5 with respect to the
undersigned's holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to the
foregoing attorneys-in-fact.

    IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of this 31st day of October, 2017.

                              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