Sec Form 4 Filing - Flagship Ventures Fund IV General Partner LLC @ Axcella Health Inc. - 2020-05-18

FORM 4 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 Flagship Ventures Fund IV General Partner LLC 2. Issuer Name and Ticker or Trading Symbol Axcella Health Inc. [ AXLA] 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) C/O FLAGSHIP PIONEERING INC., 55 CAMBRIDGE PARKWAY, SUITE 800E 3. Date of Earliest Transaction (MM/DD/YY) 05/18/2020 (Street) CAMBRIDGE, MA02142 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 Common Stock 05/18/2020 P 1,520,000 A $ 4.75 4,808,780 I See Footnote ( 1 ) Common Stock 05/18/2020 P 380,000 A $ 4.75 1,056,752 I See Footnote ( 2 ) Common Stock 05/18/2020 P 1,900,000 A $ 4.75 2,886,023 I See Footnote ( 3 ) Common Stock 2,035,830 I See Footnote ( 4 ) Common Stock 1,761,029 I See Footnote ( 5 )

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

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Flagship Ventures Fund IV General Partner LLC C/O FLAGSHIP PIONEERING INC. 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE, MA02142 X Flagship Ventures Fund IV, L.P. 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE, MA02142 X Flagship Ventures Fund IV-Rx, L.P. 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE, MA02142 X Flagship Ventures Opportunities Fund I General Partner LLC 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE, MA02142 X Flagship Ventures Opportunities Fund I, L.P. 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE, MA02142 X AFEYAN NOUBAR 55 CAMBRIDGE PARKWAY, SUITE 800E CAMBRIDGE, MA02142 X Signatures Flagship Ventures Fund IV General Partner LLC, By: /s/ Noubar B. Afeyan, Ph.D., Name: Noubar B. Afeyan, Ph.D., Title: Manager 05/20/2020 Signature of Reporting Person Date Flagship Ventures Fund IV, L.P., By: Flagship Ventures Fund IV General Partner LLC, its general partner, By: /s/ Noubar B. Afeyan, Ph.D., Name: Noubar B. Afeyan, Ph.D., Title: Manager 05/20/2020 Signature of Reporting Person Date Flagship Ventures Fund IV-Rx, L.P., By: Flagship Ventures Fund IV General Partner LLC, its general partner, By: /s/ Noubar B. Afeyan, Ph.D., Name: Noubar B. Afeyan, Ph.D., Title: Manager 05/20/2020 Signature of Reporting Person Date Flagship Ventures Opportunities Fund I General Partner LLC, By: /s/ Noubar B. Afeyan, Ph.D., Name: Noubar B. Afeyan, Ph.D., Title: Manager 05/20/2020 Signature of Reporting Person Date Flagship Ventures Opportunities Fund I, L.P. By: Flagship Ventures Opportunities Fund I General Partner LLC, its general partner, By: /s/ Noubar B. Afeyan, Ph.D., Name: Noubar B. Afeyan, Ph.D., Title: Manager 05/20/2020 Signature of Reporting Person Date Noubar B. Afeyan, Ph.D. By: /s/ Noubar B. Afeyan, Ph.D. 05/20/2020 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Shares held by Flagship Ventures Fund IV, L.P. ("Flagship Fund IV"). Flagship Ventures Fund IV General Partner LLC ("Flagship Fund IV GP") is the general partner of Flagship Fund IV. Noubar B. Afeyan, Ph.D. is the sole manager of Flagship Fund IV GP. Each of the reporting persons except for Flagship Fund IV disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. Edwin M. Kania, Jr. is retired both from Flagship Pioneering, Inc. and as a manager of Flagship Fund IV GP. ( 2 )Shares held by Flagship Ventures Fund IV-Rx, L.P. ("Flagship Fund IV-Rx"). Flagship Fund IV GP is the general partner of Flagship Fund IV-Rx. Noubar B. Afeyan, Ph.D. is the sole manager of Flagship Fund IV GP. Each of the reporting persons except for Flagship Fund IV-Rx disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. ( 3 )Shares held by Flagship Ventures Opportunities Fund I, L.P. ("Flagship Opportunities I"). Flagship Ventures Opportunities Fund I General Partner LLC ("Flagship Opportunities GP") is the general partner of Flagship Opportunities I. Noubar B. Afeyan, Ph.D. is the sole manager of Flagship Opportunities. Each of the reporting persons except for Flagship Opportunities I disclaims beneficial ownership of the shares except to the extent of his or its pecuniary interest therein. ( 4 )Shares held by Flagship VentureLabs IV, LLC ("VentureLabs IV"). Flagship Fund IV is a member of VentureLabs IV and also serves as its manager. Flagship Fund IV GP is the general partner of Flagship Fund IV. Noubar B. Afeyan, Ph.D. is the sole manager of Flagship Fund IV GP. Each of the reporting persons except for VentureLabs IV disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. ( 5 )Shares held by Flagship Ventures Fund 2007, L.P. ("Flagship Fund 2007"). Flagship Ventures 2007 General Partner LLC ("Fund 2007 GP") is the general partner of Flagship Fund 2007. Noubar B. Afeyan, Ph.D. is the sole manager of Fund 2007 GP. Each of the reporting persons except for Flagship Fund 2007 disclaims beneficial ownership of such shares except to the extent of his or its pecuniary interest therein. Edwin M. Kania, Jr. is retired both from Flagship Pioneering, Inc. and as a manager of Fund 2007 GP.

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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