X0306 4 2021-08-30 0 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 2021-08-30 4 P 0 44042 17.8661 A 4309385 I See Footnotes Common Stock, $0.001 par value 2021-08-31 4 P 0 48645 18.9261 A 4358030 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 only 3,378,881 of the 4,309,385 shares deemed to be beneficially owned by Aquilo Capital Management, LLC. Aquilo Capital, LP did not purchase any of the 44,042 shares purchased in this transaction on 08/30/2021. Aquilo Capital, LP owns only 3,378,881 of the 4,358,030 shares deemed to be beneficially owned by Aquilo Capital Management, LLC. Aquilo Capital, LP did not purchase any of the 48,645 shares purchased in this transaction on 08/31/2021. Aquilo Capital Management, LLC By: its Managing Member, /s/ Marc Schneidman 2021-09-01 Aquilo Capital, L.P., By: Aquilo Capital Management, LLC, its General Partner By: Marc Schneidman, its Managing Member /s/ Marc Schneidman 2021-09-01