SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No.     )*

Axcella Health Inc.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

05454B105

(CUSIP Number)

December 31, 2019

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

☐ Rule 13d-1(b)

☐ Rule 13d-1(c)

☒ Rule 13d-1(d)

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 05454B105

CUSIP No. 05454B105

CUSIP No. 05454B105

CUSIP No. 05454B105

CUSIP No. 05454B105

Item 1.

Axcella Health Inc. (the “Issuer”)

840 Memorial Drive

Cambridge, MA 02139

(857) 320-2200

Item 2.

HarbourVest Partners, LLC

HarbourVest Partners L.P.

HVST-TOPE LLC

SMRS-TOPE LLC

SMRS-TOPE LLC, HVST-TOPE LLC, HarbourVest Partners L.P. and HarbourVest Partners, LLC have entered into a Joint Filing Agreement, dated February 5, 2020, a copy of which is filed with this Schedule 13G as Exhibit 99.1, pursuant to which such reporting persons have agreed to file this statement jointly in accordance with the provisions of 13d-1(k)(1) under the Securities Exchange Act of 1934.

The principal business office of each reporting person is One Financial Center, Boston, MA 02111.

Reporting entities are all organized and exist under the laws of the State of Delaware.

Common stock, par value $0.001 per share

05454B105

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

Consists of 1,229,034 shares common stock owned directly by SMRS-TOPE LLC. HarbourVest Partners, LLC (“HarbourVest”) is the General Partner of HarbourVest Partners L.P., which is the Manager of HVST-TOPE LLC, which is the Managing Member of SMRS-TOPE LLC. Each of HarbourVest, HarbourVest Partners L.P. and HVST-TOPE LLC may be deemed to have a beneficial interest in the shares held by SMRS-TOPE LLC.

Voting and investment power over the securities owned directly by SMRS-TOPE LLC is exercised by the Investment Committee of HarbourVest. Each of HarbourVest, HarbourVest Partners L.P. and HVST-TOPE LLC and the members of the HarbourVest Investment Committee disclaim beneficial ownership of the shares held directly by SMRS-TOPE LLC.

5.3% (based on the Issuer’s aggregate of 23,113,715 shares of common stock outstanding as of November 8, 2019, as reported in the Issuer’s Form 10-Q, filed with the SEC on November 12, 2019).

(i) Sole power to vote or to direct the vote:

SMRS-TOPE LLC has the sole power to vote or to direct the vote of 1,229,034 shares of common stock.

(ii) Shared power to vote or to direct the vote:

HarbourVest, HarbourVest Partners L.P. and HVST-TOPE LLC may be deemed to have shared power to vote or direct the vote of 1,229,034 shares of common stock. Each of HarbourVest, HarbourVest Partners L.P. and HVST-TOPE LLC disclaim such beneficial ownership of such shares.

(iii) Sole power to dispose or to direct the disposition:

SMRS-TOPE LLC has the sole power to dispose or to direct the disposition of 1,229,034 shares of common stock.

(iv) Shared power to dispose or to direct the disposition:

HarbourVest, HarbourVest Partners L.P. and HVST-TOPE LLC may be deemed to have shared power to dispose or direct the disposition of 1,229,034 shares of common stock. Each of HarbourVest, HarbourVest Partners L.P. and HVST-TOPE LLC disclaim such beneficial ownership of such shares.

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following [ ]:

See item 4 above.

Not Applicable.

Not Applicable.

Not Applicable.

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under §240.14a–11.

Signatures

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Date: February 5, 2020

Exhibit 99.1

AGREEMENT REGARDING THE JOINT FILING OF SCHEDULE 13G

Pursuant to Rule 13d-1(k)(1) promulgated under the Securities Exchange Act of 1934, as amended, the undersigned agree that the Statement on Schedule 13G to which this Exhibit is attached is filed on behalf of each of them. Each of them is individually eligible to use the Schedule 13G to which this Exhibit is attached. Each of them is responsible for the timely filing of such Schedule 13G and any amendments thereto, and for the completeness and accuracy of the information concerning such person contained therein; but none of them is responsible for the completeness or accuracy of the information concerning the other persons making the filing, unless such person knows or has reason to believe that such information is inaccurate.

IN WITNESS WHEROF, the undersigned hereby execute this Joint Filing Agreement as of February 5, 2020.