Adage Capital Discloses 9,002,580 CABA Shares, Includes Warrants

Rhea-AI Filing Summary

Adage Capital Management, L.P. and affiliated reporting persons disclose beneficial ownership of 9,002,580 shares of Cabaletta Bio common stock, representing 9.99% of the outstanding class. The reported total includes 172,822 shares issuable upon exercise of warrants held by Adage's affiliate and is calculated using 89,943,101 shares reported outstanding in the company's prospectus.

The filing identifies Robert Atchinson and Phillip Gross as reporting persons and lists Adage as investment manager for Adage Capital Partners with a business address at 200 Clarendon Street, Boston. The cover rows show 0 shares of sole voting or dispositive power and 9,002,580 shares of shared voting and dispositive power. The reporting persons certify the securities are held in the ordinary course of business and not for the purpose of changing or influencing control.

Positive

• Material stake disclosed: Adage reports ownership of 9,002,580 shares, representing 9.99% of common stock.
• Includes warrants: The position explicitly includes 172,822 shares issuable upon exercise of warrants.
• Passive intent stated: Reporting persons certify holdings are in the ordinary course of business and not held to change or influence control.

Negative

• None.

Insights

Financial Analyst

TL;DR: Adage discloses a near-10% passive stake in Cabaletta Bio, including warrants, signaling a material but non-control position.

The Schedule 13G/A shows Adage Capital and principals beneficially own 9,002,580 shares, or 9.99% of Cabaletta Bio, calculated on 89,943,101 shares outstanding per the prospectus. The position includes 172,822 shares issuable on warrants. The filing reports shared voting and dispositive power for the entire stake and no sole voting or dispositive power. Because the statement is on Schedule 13G/A and includes the ordinary-course certification, this is presented as a passive investment rather than an attempt to influence control. For investors, this represents a sizable strategic holding by a sophisticated asset manager without an overt activist intent.

Corporate Governance Expert

TL;DR: The disclosure is material for governance monitoring but indicates no declared move to seek control or board influence.

The reporting identifies Adage Capital Management, L.P., and individuals Robert Atchinson and Phillip Gross as reporting persons, with holdings held on behalf of Adage Capital Partners. The position is reported as 9.99%, just below common 10% thresholds that can trigger additional investor scrutiny. The filing explicitly states the securities were acquired and are held in the ordinary course of business and not to influence control. Governance teams should note the concentration and the inclusion of warrant-derived shares, but the statement contains no language indicating intent to nominate directors or pursue corporate actions.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 6)

Cabaletta Bio, Inc.

(Name of Issuer)

Common Stock, par value $0.00001 per share

(Title of Class of Securities)

12674W109

(CUSIP Number)

06/30/2025

(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)

Rule 13d-1(c)

Rule 13d-1(d)

SCHEDULE 13G

CUSIP No. 12674W109

1

Names of Reporting Persons

Adage Capital Management, L.P.

2

Check the appropriate box if a member of a Group (see instructions)

(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 0.00
6	Shared Voting Power 9,002,580.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 9,002,580.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

9,002,580.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

9.99 %

12

Type of Reporting Person (See Instructions)

IA, PN

Comment for Type of Reporting Person: Includes 172,822 shares of Common Stock (as defined in Item 2(a)) issuable upon exercise of warrants.

SCHEDULE 13G

CUSIP No. 12674W109

1

Names of Reporting Persons

Robert Atchinson

2

Check the appropriate box if a member of a Group (see instructions)

(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 0.00
6	Shared Voting Power 9,002,580.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 9,002,580.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

9,002,580.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

9.99 %

12

Type of Reporting Person (See Instructions)

HC, IN

Comment for Type of Reporting Person: Includes 172,822 shares of Common Stock issuable upon exercise of warrants.

SCHEDULE 13G

CUSIP No. 12674W109

1

Names of Reporting Persons

Phillip Gross

2

Check the appropriate box if a member of a Group (see instructions)

(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 0.00
6	Shared Voting Power 9,002,580.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 9,002,580.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

9,002,580.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

9.99 %

12

Type of Reporting Person (See Instructions)

HC, IN

Comment for Type of Reporting Person: Includes 172,822 shares of Common Stock issuable upon exercise of warrants.

SCHEDULE 13G

Item 1.
(a)	Name of issuer: Cabaletta Bio, Inc.
(b)	Address of issuer's principal executive offices: 2929 Arch Street, Suite 600, Philadelphia, PA 19104
Item 2.
(a)	Name of person filing: This statement is filed by: (i) Adage Capital Management, L.P., a Delaware limited partnership ("ACM"), as the investment manager of Adage Capital Partners, L.P., a Delaware limited partnership ("ACP"), with respect to the shares of common stock, par value $0.00001 per share ("Common Stock") of Cabaletta Bio, Inc., a Delaware corporation (the "Company") and shares of Common Stock issuable upon exercise of warrants directly held by ACP; (ii) Robert Atchinson ("Mr. Atchinson"), as (1) managing member of Adage Capital Advisors, L.L.C., a limited liability company organized under the laws of the State of Delaware ("ACA"), managing member of Adage Capital Partners GP, L.L.C., a limited liability company organized under the laws of the State of Delaware ("ACPGP"), general partner of ACP and (2) managing member of Adage Capital Partners LLC, a Delaware limited liability company ("ACPLLC"), general partner of ACM, with respect to the shares of Common Stock and shares of Common Stock issuable upon exercise of warrants directly held by ACP; and (iii) Phillip Gross ("Mr. Gross"), as (1) managing member of ACA, managing member of ACPGP and (2) managing member of ACPLLC, general partner of ACM, with respect to the shares of Common Stock and shares of Common Stock issuable upon exercise of warrants directly held 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.
(b)	Address or 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.
(c)	Citizenship: ACM is a limited partnership organized under the laws of the State of Delaware. Messrs. Gross and Atchinson are citizens of the United States.
(d)	Title of class of securities: Common Stock, par value $0.00001 per share
(e)	CUSIP No.: 12674W109
Item 3.	If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)	Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)	Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)	Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)	Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)	An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)	An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)	A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)	A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)	A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:
(k)	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.	Ownership
(a)	Amount beneficially owned: The information required by Item 4(a) is set forth in Row 9 of the cover page for each of the Reporting Persons and is incorporated herein by reference. The percentage set forth in this Schedule 13G is calculated based upon an aggregate of 89,943,101 shares of Common Stock, reported to be outstanding in the Company's Prospectus filed pursuant to Rule 424(b)(5) with the Securities and Exchange Commission on June 12, 2025 and the Company's Current Report on Form 8-K filed with the Securities and Exchange Commission on June 12, 2025, after giving effect to the completion of the offering, as described therein, and assumes the reported warrants held by ACP.
(b)	Percent of class: 9.99%
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: The information required by Item 4(c)(i) is set forth in Row 5 of the cover page for each of the Reporting Persons and is incorporated herein by reference.
	(ii) Shared power to vote or to direct the vote: The information required by Item 4(c)(ii) is set forth in Row 6 of the cover page for each of the Reporting Persons and is incorporated herein by reference.
	(iii) Sole power to dispose or to direct the disposition of: The information required by Item 4(c)(iii) is set forth in Row 7 of the cover page for each of the Reporting Persons and is incorporated herein by reference.
	(iv) Shared power to dispose or to direct the disposition of: The information required by Item 4(c)(iv) is set forth in Row 8 of the cover page for each of the Reporting Persons and is incorporated herein by reference.
Item 5.	Ownership of 5 Percent or Less of a Class.
Item 6.	Ownership of more than 5 Percent on Behalf of Another Person.
	If any other person is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, such securities, a statement to that effect should be included in response to this item and, if such interest relates to more than 5 percent of the class, such person should be identified. A listing of the shareholders of an investment company registered under the Investment Company Act of 1940 or the beneficiaries of employee benefit plan, pension fund or endowment fund is not required. See Item 2(a).
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.

Certifications:

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and 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, other than activities solely in connection with a nomination under ?? 240.14a-11.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Adage Capital Management, L.P.

Signature:	/s/ Robert Atchinson
Name/Title:	By: Adage Capital Partners LLC, its General Partner, By: Robert Atchinson, its Managing Member
Date:	08/12/2025

Robert Atchinson

Signature:	/s/ Robert Atchinson
Name/Title:	Robert Atchinson, individually
Date:	08/12/2025

Phillip Gross

Signature:	/s/ Phillip Gross
Name/Title:	Phillip Gross, individually
Date:	08/12/2025

FAQ

Who filed the Schedule 13G/A for Cabaletta Bio (CABA)?

The filing was made by Adage Capital Management, L.P. and reporting persons Robert Atchinson and Phillip Gross.

How many Cabaletta Bio (CABA) shares does Adage beneficially own?

Adage and the reporting persons beneficially own 9,002,580 shares, representing 9.99% of the class.

Does the disclosed stake include warrant-related shares?

Yes. The reported ownership includes 172,822 shares issuable upon exercise of warrants.

Does the filer claim intent to influence control of Cabaletta Bio?

No. The filing certifies the securities are held in the ordinary course of business and were not acquired to change or influence control.

What voting and dispositive powers are reported?

The cover information shows 0 sole voting/dispositive power and 9,002,580 shared voting and dispositive power.

What address is listed for the reporting persons?

The business address for the reporting persons is 200 Clarendon Street, 52nd Floor, Boston, MA 02116.