<SEC-DOCUMENT>0001591986-21-000012.txt : 20210901
<SEC-HEADER>0001591986-21-000012.hdr.sgml : 20210901
<ACCEPTANCE-DATETIME>20210901174257
ACCESSION NUMBER:		0001591986-21-000012
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20210830
FILED AS OF DATE:		20210901
DATE AS OF CHANGE:		20210901

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Aquilo Capital, L.P.
		CENTRAL INDEX KEY:			0001491371
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-38266
		FILM NUMBER:		211231173

	BUSINESS ADDRESS:	
		STREET 1:		ONE LETTERMAN DRIVE, SUITE D4900
		STREET 2:		BUILDING D, THE PRESIDIO
		CITY:			SAN FRANCISCO
		STATE:			CA
		ZIP:			94129
		BUSINESS PHONE:		(415) 635-0141

	MAIL ADDRESS:	
		STREET 1:		ONE LETTERMAN DRIVE, SUITE D4900
		STREET 2:		BUILDING D, THE PRESIDIO
		CITY:			SAN FRANCISCO
		STATE:			CA
		ZIP:			94129

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Aquilo Capital Management, LLC
		CENTRAL INDEX KEY:			0001591986
		STATE OF INCORPORATION:			CA
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-38266
		FILM NUMBER:		211231174

	BUSINESS ADDRESS:	
		STREET 1:		ONE LETTERMAN DRIVE
		STREET 2:		SUITE D4900, BUILDING D
		CITY:			SAN FRANCISCO
		STATE:			CA
		ZIP:			94129
		BUSINESS PHONE:		415-635-0140

	MAIL ADDRESS:	
		STREET 1:		ONE LETTERMAN DRIVE
		STREET 2:		SUITE D4900, BUILDING D
		CITY:			SAN FRANCISCO
		STATE:			CA
		ZIP:			94129

	FORMER NAME:	
		FORMER CONFORMED NAME:	Aquilo Capital Managment, LLC
		DATE OF NAME CHANGE:	20131114

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			Spero Therapeutics, Inc.
		CENTRAL INDEX KEY:			0001701108
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				000000000
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		675 MASSACHUSETTS AVENUE
		STREET 2:		14TH FLOOR
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02139
		BUSINESS PHONE:		857-242-1600

	MAIL ADDRESS:	
		STREET 1:		675 MASSACHUSETTS AVENUE
		STREET 2:		14TH FLOOR
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02139

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	Spero Therapeutics, LLC
		DATE OF NAME CHANGE:	20170316
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
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<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-08-30</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001701108</issuerCik>
        <issuerName>Spero Therapeutics, Inc.</issuerName>
        <issuerTradingSymbol>SPRO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001591986</rptOwnerCik>
            <rptOwnerName>Aquilo Capital Management, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>ONE LETTERMAN DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE D4900, BUILDING D</rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94129</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001491371</rptOwnerCik>
            <rptOwnerName>Aquilo Capital, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>ONE LETTERMAN DRIVE, SUITE D4900</rptOwnerStreet1>
            <rptOwnerStreet2>BUILDING D, THE PRESIDIO</rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94129</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, $0.001 par value</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2021-08-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>44042</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>17.8661</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4309385</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnotes</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, $0.001 par value</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2021-08-31</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>48645</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>18.9261</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4358030</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnotes.</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The reporting persons are Aquilo Capital, L.P. (the &quot;Fund&quot;), Aquilo Capital Management, LLC (&quot;Aquilo&quot;) 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.</footnote>
        <footnote id="F2">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.</footnote>
        <footnote id="F3">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.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Aquilo Capital Management, LLC By: its Managing Member, /s/ Marc Schneidman</signatureName>
        <signatureDate>2021-09-01</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Aquilo Capital, L.P., By: Aquilo Capital Management, LLC, its General Partner By: Marc Schneidman, its Managing Member /s/ Marc Schneidman</signatureName>
        <signatureDate>2021-09-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
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</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
