Sec Form 4 Filing - FORMELA JEAN FRANCOIS @ Spero Therapeutics, Inc. - 2017-11-06

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 FORMELA JEAN FRANCOIS 2. Issuer Name and Ticker or Trading Symbol Spero Therapeutics, Inc. [ SPRO] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O SPERO THERAPEUTICS, INC., 675 MASSACHUSETTS AVENUE, 14TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 11/06/2017 (Street) CAMBRIDGE, MA02139 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ 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 11/06/2017 C 245,605 A 288,386 I See Footnote ( 3 ) Common Stock 11/06/2017 C 246,272 A 534,658 I See Footnote ( 3 ) Common Stock 11/06/2017 C 328,992 A 863,650 I See Footnote ( 3 ) Common Stock 11/06/2017 C 513,318 A 1,376,968 I See Footnote ( 3 ) Common Stock 11/06/2017 C 602,589 A 1,979,557 I See Footnote ( 4 ) Common Stock 11/06/2017 P 428,571 ( 1 ) A $ 14 2,408,128 I See Footnote ( 4 )

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 Junior Preferred Stock ( 2 ) 11/06/2017 C 1,492,645 ( 2 ) ( 2 ) Common Stock 245,605 $ 0 0 I See Footnote ( 3 ) Series A Preferred Stock ( 2 ) 11/06/2017 C 982,906 ( 2 ) ( 2 ) Common Stock 246,272 $ 0 0 I See Footnote ( 3 ) Series B Preferred Stock ( 2 ) 11/06/2017 C 1,250,000 ( 2 ) ( 2 ) Common Stock 328,992 $ 0 0 I See Footnote ( 3 ) Series C Preferred Stock ( 2 ) 11/06/2017 C 3,119,633 ( 2 ) ( 2 ) Common Stock 513,318 $ 0 0 I See Footnote ( 3 ) Series C Preferred Stock ( 2 ) 11/06/2017 C 3,662,178 ( 2 ) ( 2 ) Common Stock 602,589 $ 0 0 I See Footnote ( 4 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other FORMELA JEAN FRANCOIS C/O SPERO THERAPEUTICS, INC. 675 MASSACHUSETTS AVENUE, 14TH FLOOR CAMBRIDGE, MA02139 X Signatures /s/ Frank Castellucci, Attorney-in-Fact for Jean-Francois Formela 11/07/2017 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Reflects shares of the Issuer's common stock that were purchased in connection with the Issuer's initial public offering. ( 2 )Amount of underlying securities reflects the application of the applicable conversion ratio as set forth in the Issuer's Amended and Restated Certificate of Incorporation in effect immediately prior to the closing of the initial public offering (as adjusted by the 1-for-6.0774 reverse stock split of the Issuer's common stock effected on October 20, 2017). Each series of preferred stock was automatically converted upon the closing of the Issuer's initial public offering without payment of further consideration and had no expiration date. ( 3 )Shares held by Atlas Venture Fund IX, L.P. ("Atlas Venture Fund IX"). The general partner of Atlas Venture Fund IX is Atlas Venture Associates IX, L.P. ("AVA IX LP"). Atlas Venture Associates IX, LLC ("AVA IX LLC") is the general partner of AVA IX LP. Dr. Formela is a member of AVA IX LLC and disclaims beneficial ownership of such shares, except to the extent of his pecuniary interest therein, if any. ( 4 )Shares held by Atlas Venture Fund X, L.P. ("Atlas Venture Fund X"). The general partner of Atlas Venture Fund X is Atlas Venture Associates X, L.P. ("AVA X LP"). Atlas Venture Associates X, LLC ("AVA X LLC") is the general partner of AVA X LP. Dr. Formela is a member of AVA X LLC and disclaims beneficial ownership of such shares, except to the extent of his pecuniary interest therein, if any.

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).

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