SC 13G/A 1 p21-0458sc13ga.htm CABALETTA BIO, INC.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G/A

Under the Securities Exchange Act of 1934
(Amendment No. 1)*

Cabaletta Bio, Inc.
(Name of Issuer)

Voting Common Stock, $0.00001 par value per share
(Title of Class of Securities)

12674W109
(CUSIP Number)

December 31, 2020
(Date of Event Which Requires Filing of This Statement)


Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

¨	Rule 13d-1(b)
x	Rule 13d-1(c)
¨	Rule 13d-1(d)

(Page 1 of 11 Pages)

*The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 12674W109

13G/A

Page 2 of 11 Pages

1	NAME OF REPORTING PERSON Adage Capital Partners, L.P.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) ¨ (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of shares of Non-Voting Common Stock)
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of shares of Non-Voting Common Stock)
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of shares of Non-Voting Common Stock)
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.99%
12	TYPE OF REPORTING PERSON PN

CUSIP No. 12674W109

13G/A

Page 3 of 11 Pages

1	NAME OF REPORTING PERSON Adage Capital Partners GP, L.L.C.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) ¨ (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of shares of Non-Voting Common Stock)
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of shares of Non-Voting Common Stock)
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of shares of Non-Voting Common Stock)
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.99%
12	TYPE OF REPORTING PERSON OO

CUSIP No. 12674W109

13G/A

Page 4 of 11 Pages

1	NAME OF REPORTING PERSON Adage Capital Advisors, L.L.C.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) ¨ (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of shares of Non-Voting Common Stock)
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of shares of Non-Voting Common Stock)
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of shares of Non-Voting Common Stock)
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.99%
12	TYPE OF REPORTING PERSON OO

CUSIP No. 12674W109

13G/A

Page 5 of 11 Pages

1	NAME OF REPORTING PERSON Robert Atchinson
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) ¨ (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION United States
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.99%
12	TYPE OF REPORTING PERSON IN

CUSIP No. 12674W109

13G/A

Page 6 of 11 Pages

1	NAME OF REPORTING PERSON Phillip Gross
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) ¨ (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION United States
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 9.99%
12	TYPE OF REPORTING PERSON IN

CUSIP No. 12674W109

13G/A

Page 7 of 11 Pages

Item 1(a).	NAME OF ISSUER
	The name of the issuer is Cabaletta Bio, Inc. (the "Company").

Item 1(b).	ADDRESS OF ISSUER'S PRINCIPAL EXECUTIVE OFFICES
	The Company's principal executive offices are located at 2929 Arch Street, Suite 600, Philadelphia, Pennsylvania 19104.

Item 2(a).	NAME OF PERSON FILING
	This statement is filed by:

	(i)	Adage Capital Partners, L.P., a Delaware limited partnership ("ACP") with respect to the shares of Voting Common Stock directly owned by it;

	(ii)	Adage Capital Partners GP, L.L.C., a limited liability company organized under the laws of the State of Delaware ("ACPGP"), as general partner of ACP with respect to the shares of Voting Common Stock directly owned by ACP;

	(iii)	Adage Capital Advisors, L.L.C., a limited liability company organized under the laws of the State of Delaware ("ACA"), as managing member of ACPGP, general partner of ACP, with respect to the shares of Voting Common Stock directly owned by ACP;

	(iv)	Robert Atchinson ("Mr. Atchinson"), as managing member of ACA, managing member of ACPGP, general partner of ACP with respect to the shares of Voting Common Stock directly owned by ACP; and

	(v)	Phillip Gross ("Mr. Gross"), as managing member of ACA, managing member of ACPGP, general partner of ACP with respect to the shares of Voting Common Stock directly owned by ACP.

	The foregoing persons are hereinafter sometimes collectively referred to as the "Reporting Persons." Any disclosures herein with respect to persons other than the Reporting Persons are made on information and belief after making inquiry to the appropriate party.

	The filing of this statement should not be construed in and of itself as an admission by any Reporting Person as to beneficial ownership of the securities reported herein.

Item 2(b).	ADDRESS OF PRINCIPAL BUSINESS OFFICE OR, IF NONE, RESIDENCE
	The address of the business office of each of the Reporting Persons is 200 Clarendon Street, 52nd Floor, Boston, Massachusetts 02116.

Item 2(c).	CITIZENSHIP
	ACP is a limited partnership organized under the laws of the State of Delaware. ACPGP and ACA are limited liability companies organized under the laws of the State of Delaware. Messrs. Gross and Atchinson are citizens of the United States.

Item 2(d).	TITLE OF CLASS OF SECURITIES
	Voting Common Stock, $0.00001 par value per share (the "Voting Common Stock").

CUSIP No. 12674W109

13G/A

Page 8 of 11 Pages

Item 2(e).	CUSIP NUMBER
	12674W109

Item 3.

IF THIS STATEMENT IS FILED PURSUANT TO Rules 13d-1(b), OR 13d-2(b) OR (c), CHECK WHETHER THE PERSON FILING IS A:

(a)	¨	Broker or dealer registered under Section 15 of the Act;
(b)	¨	Bank as defined in Section 3(a)(6) of the Act;
(c)	¨	Insurance company as defined in Section 3(a)(19) of the Act;
(d)	¨	Investment company registered under Section 8 of the Investment Company Act of 1940;
(e)	¨	An investment adviser in accordance with Rule 13d-1(b)(1)(ii)(E);
(f)	¨	An employee benefit plan or endowment fund in accordance with Rule 13d-1(b)(1)(ii)(F);
(g)	¨	A parent holding company or control person in accordance with Rule 13d-1(b)(1)(ii)(G);
(h)	¨	A savings association as defined in Section 3(b) of the Federal Deposit Insurance Act;
(i)	¨	A church plan that is excluded from the definition of an investment company under Section 3(c)(14) of the Investment Company Act;
(j)	¨	A non-U.S. institution in accordance with Rule 13d-1(b)(1)(ii)(J);
(k)	¨	Group, in accordance with Rule 13d-1(b)(1)(ii)(K).

	If filing as a non-U.S. institution in accordance with Rule 13d-1(b)(1)(ii)(J), please
	specify the type of institution: Not applicable.

CUSIP No. 12674W109

13G/A

Page 9 of 11 Pages

Item 4.	OWNERSHIP

A.	Adage Capital Partners, L.P., Adage Capital Partners GP, L.L.C. and Adage Capital Advisors, L.L.C.
	(a)	Amount beneficially owned: 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares).
	(b)	Percent of class: 9.99%. The percentage set forth in this Schedule 13G/A is calculated based upon a total of 19,597,463 shares of Voting Common Stock, which is the sum of (i) 19,379,852 shares of Voting Common Stock outstanding as of September 30, 2020, as reported in the Company's Quarterly Report on Form 10-Q for the quarterly period ended September 30, 2020, filed with the Securities and Exchange Commission on November 10, 2020 and (ii) 217,611 shares of Voting Common Stock issued to the Reporting Persons on December 28, 2020 upon conversion of shares of the Company's Non-Voting Common Stock, and assumes the conversion of such shares of Non-Voting Common Stock.
	(c)	(i)	Sole power to vote or direct the vote: 0
		(ii)	Shared power to vote or direct the vote: 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)
		(iii)	Sole power to dispose or direct the disposition: 0
		(iv)	Shared power to dispose or direct the disposition of: 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)

ACP has the power to dispose of and the power to vote the shares of Voting Common Stock beneficially owned by it, which power may be exercised by its general partner, ACPGP. ACA, as managing member of ACPGP, directs ACPGP's operations. Neither ACPGP nor ACA directly own any shares of Voting Common Stock. By reason of the provisions of Rule 13d-3 of the Act, ACPGP and ACA may be deemed to beneficially own the shares owned by ACP.

B.	Robert Atchinson and Phillip Gross
	(a)	Amount beneficially owned: 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)
	(b)	Percent of class: 9.99%
	(c)	(i)	Sole power to vote or direct the vote: 0
		(ii)	Shared power to vote or direct the vote: 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)
		(iii)	Sole power to dispose or direct the disposition: 0
		(iv)	Shared power to dispose or direct the disposition: 1,985,182 (including 275,233 shares of Voting Common Stock issuable upon conversion of non-voting shares)

Messrs. Atchinson and Gross, as managing members of ACA, have shared power to vote the shares of Voting Common Stock beneficially owned by ACP. Neither Mr. Atchinson nor Mr. Gross directly own any shares of Voting Common Stock. By reason of the provisions of Rule 13d-3 of the Act, each may be deemed to beneficially own the shares beneficially owned by ACP.

On the date of the event which requires the filing of this Schedule 13G/A, each of the Reporting Persons may have been deemed to be the beneficial owner of 1,911,707 shares of Voting Common Stock (including 1,511,707 shares of Voting Common Stock issuable upon conversion of non-voting shares), representing 9.99% of the outstanding shares of Voting Common Stock as of such time assuming the conversion of such shares of Non-Voting Common Stock.

CUSIP No. 12674W109

13G/A

Page 10 of 11 Pages

Item 5.	OWNERSHIP OF FIVE PERCENT OR LESS OF A CLASS
	Not applicable.

Item 6.	OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON
	Not applicable.

Item 7.	IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH ACQUIRED THE SECURITY BEING REPORTED ON BY THE PARENT HOLDING COMPANY OR CONTROL PERSON
	Not applicable.

Item 8.	IDENTIFICATION AND CLASSIFICATION OF MEMBERS OF THE GROUP
	Not applicable.

Item 9.	NOTICE OF DISSOLUTION OF GROUP
	Not applicable.

Item 10.	CERTIFICATION

	Each of the Reporting Persons hereby makes the following certification:

	By signing below each Reporting Person certifies that, to the best of his or its knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

CUSIP No. 12674W109

13G/A

Page 11 of 11 Pages

SIGNATURES

After reasonable inquiry and to the best of his or its knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

DATE: February 11, 2021

ADAGE CAPITAL PARTNERS, L.P.
By: Adage Capital Partners GP, L.L.C.,
its general partner

By: Adage Capital Advisors, L.L.C.,
its managing member

/s/ Robert Atchinson
Name: Robert Atchinson
Title: Managing Member

ADAGE CAPITAL PARTNERS GP, L.L.C.
By: Adage Capital Advisors, L.L.C.,
its managing member

/s/ Robert Atchinson
Name: Robert Atchinson
Title: Managing Member

ADAGE CAPITAL ADVISORS, L.L.C.

/s/ Robert Atchinson
Name: Robert Atchinson
Title: Managing Member

ROBERT ATCHINSON

/s/ Robert Atchinson
ROBERT ATCHINSON, individually

PHILLIP GROSS

/s/ Phillip Gross
PHILLIP GROSS, individually

SEC Form SC 13G/A filed