Sec Form 4 Filing - Aquilo Capital Management, LLC @ Spero Therapeutics, Inc. - 2021-08-20

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 Aquilo Capital Management, LLC 2. Issuer Name and Ticker or Trading Symbol Spero Therapeutics, Inc. [ SPRO] 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 LETTERMAN DRIVE, SUITE D4900, BUILDING D 3. Date of Earliest Transaction (MM/DD/YY) 08/20/2021 (Street) SAN FRANCISCO, CA94129 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, $0.001 par value ( 1 ) 08/20/2021 P 221,190 A $ 14.8601 4,050,399 ( 2 ) I See Footnotes ( 1 ) Common Stock, $0.001 par value ( 1 ) 08/23/2021 P 137,835 A $ 16.3336 4,188,234 ( 3 ) I See Footnotes ( 1 ) Common Stock, $0.001 par value ( 1 ) 08/24/2021 P 40,170 A $ 16.2757 4,228,404 ( 4 ) I See Footnotes ( 1 )

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 Aquilo Capital Management, LLC ONE LETTERMAN DRIVE SUITE D4900, BUILDING D SAN FRANCISCO, CA94129 X Aquilo Capital, L.P. ONE LETTERMAN DRIVE, SUITE D4900 BUILDING D, THE PRESIDIO SAN FRANCISCO, CA94129 X Signatures Aquilo Capital Management, LLC By: its Managing Member, /s/ Marc Schneidman 08/24/2021 Signature of Reporting Person Date Aquilo Capital, L.P., By: Aquilo Capital Management, LLC, its General Partner By: Marc Schneidman, its Managing Member /s/ Marc Schneidman 08/24/2021 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The reporting persons are Aquilo Capital, L.P. (the "Fund"), Aquilo Capital Management, LLC ("Aquilo") and Marc R. Schneidman. Aquilo is the general partner and investment adviser of the Fund and other funds managed by Aquilo that hold securities directly for the benefit of their investors. Aquilo may be deemed to beneficially own such securities indirectly as the investment adviser and general partner of the Fund and affiliated funds, and Mr. Schneidman may be deemed to beneficially own them indirectly as the control person of Aquilo. Each of, Aquilo, the Fund and Mr. Schneidman disclaims beneficial ownership of such securities except to the extent of his or its pecuniary interest therein. ( 2 )Aquilo Capital, LP owns only 3,378,881 of the 4,050,399 shares deemed to be beneficially owned by Aquilo Capital Management, LLC. Aquilo Capital, LP did not purchase any of the 221,190 shares purchased in this transaction on 08/20/2021. ( 3 )Aquilo Capital, LP owns only 3,378,881 of the 4,188,234 shares deemed to be beneficially owned by Aquilo Capital Management, LLC. Aquilo Capital, LP did not purchase any of the 137,835 shares purchased in this transaction on 08/23/2021. ( 4 )Aquilo Capital, LP owns only 3,378,881 of the 4,228,404 shares deemed to be beneficially owned by Aquilo Capital Management, LLC. Aquilo Capital, LP did not purchase any of the 40,170 shares purchased in this transaction on 08/24/2021.

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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