X0306 4 2022-09-22 1 0001701108 Spero Therapeutics, Inc. SPRO 0001591986 Aquilo Capital Management, LLC ONE LETTERMAN DRIVE SUITE D4900, BUILDING D SAN FRANCISCO CA 94129 0 0 1 0 0001491371 Aquilo Capital, L.P. ONE LETTERMAN DRIVE, SUITE D4900 BUILDING D, THE PRESIDIO SAN FRANCISCO CA 94129 0 0 1 0 Common Stock, $0.001 par value 2022-09-22 4 S 0 1335512 2.2503 D 3985719 I See Footnotes Common Stock, $0.001 par value 2022-09-22 4 S 0 566284 2.2503 D 3419435 I See Footnotes 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. Aquilo Capital, LP owns 2,176,470 of the 3,985,719 shares deemed to be beneficially owned by Aquilo Capital Management, LLC. Aquilo Capital, LP sold 1,335,512 shares in this transaction on 09/22/2022. Aquilo Capital, LP owns only 2,176,470 of the 3,419,435 shares deemed to be beneficially owned by Aquilo Capital Management, LLC. Aquilo Capital, LP did not sell any of the 566,284 shares sold in this transaction on 09/22/2022. Aquilo Capital Management, LLC By: Marc Schneidman, its Managing Member, /s/ Marc Schneidman 2022-09-26 Aquilo Capital, L.P., By: Aquilo Capital Management, LLC, its General Partner By: Marc Schneidman, its Managing Member /s/ Marc Schneidman 2022-09-26