Cabaletta Bio, Inc. — Major Shareholding Notification 2021

Feb 11, 2021

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).

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CUSIP No. 12674W109 13G/A Page 2 of 11 Pages

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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

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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

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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

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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

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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

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

Item 1(b).
The Company's principal executive offices are located at 2929 Arch Street, Suite 600, Philadelphia, Pennsylvania 19104.

Item 2(a).
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).
The address of the business office of each of the Reporting Persons is 200 Clarendon Street, 52nd Floor, Boston, Massachusetts 02116.

Item 2(c).
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).
Voting Common Stock , $0.00001 par value per share (the " Voting Common Stock ").

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Item 2(e).
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.

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ITEM 4. OWNERSHIP

A. — (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. — (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.

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Item 5.
Not applicable.

Item 6.
Not applicable.

Item 7.
Not applicable.

Item 8.
Not applicable.

Item 9.
Not applicable.

Item 10.
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.

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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