Adage Capital Management, L.P. and related parties report a significant ownership position in Cabaletta Bio, Inc. (CABA). The reporting persons collectively beneficially own 14,174,758 shares of Common Stock, including 5,000,000 shares issuable upon exercise of warrants held by Adage Capital Partners, L.P.
This position represents 8.43% of Cabaletta Bio’s Common Stock, based on 163,054,796 shares outstanding as of May 7, 2026, and assuming exercise of the reported warrants. The reporting persons have shared voting and dispositive power over all 14,174,758 shares and no sole voting or dispositive power. Adage Capital Management, L.P., and individuals Robert Atchinson and Phillip Gross file jointly as reporting persons, with the securities directly held by Adage Capital Partners, L.P.
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Key Figures
Beneficially owned shares:14,174,758 sharesOwnership percentage:8.43%Shares outstanding:163,054,796 shares+3 more
6 metrics
Beneficially owned shares14,174,758 sharesTotal Cabaletta Bio Common Stock beneficially owned by the reporting persons
Ownership percentage8.43%Portion of Cabaletta Bio Common Stock class beneficially owned by reporting persons
Shares outstanding163,054,796 sharesCabaletta Bio Common Stock outstanding as of May 7, 2026
Warrant shares included5,000,000 sharesCommon Stock issuable upon exercise of warrants held by Adage Capital Partners, L.P.
Shared voting power14,174,758 sharesShares over which reporting persons have shared power to vote
Sole voting power0 sharesShares over which reporting persons have sole power to vote
"The filing of this statement should not be construed in and of itself as an admission by any Reporting Person as to beneficial ownership"
Beneficial ownership means the person or entity that actually enjoys the benefits of owning shares or other assets — such as receiving dividends, voting rights, or price gains — even if the legal title is held in another name. For investors it matters because knowing who truly controls and profits from a company reveals who can influence decisions, exposes potential conflicts of interest or hidden concentration of power, and affects transparency and risk in the stock.
shared voting powerfinancial
"Shared Voting Power 14,174,758.00"
Shared voting power occurs when two or more parties jointly have the right to vote or decide how a block of company shares is cast, like co-owners who must agree before moving a piece of furniture. Investors care because who controls voting rights affects board elections, major corporate decisions and takeover outcomes, and shared control can alter regulatory disclosures and the practical influence any holder has over a company’s direction and value.
shared dispositive powerfinancial
"Shared Dispositive Power 14,174,758.00"
warrantsfinancial
"Includes 5,000,000 shares of Common Stock issuable upon exercise of warrants"
Warrants are special documents that give you the right to buy a company's stock at a set price before a certain date. They are often used as a way for companies to attract investors or raise money, and their value can increase if the company's stock price goes up.
Schedule 13Gregulatory
"The percentage set forth in this is calculated based upon an aggregate of 163,054,796 Common Stock outstanding"
A Schedule 13G is a formal document that investors file with the government when they acquire a large ownership stake in a company, usually for investment purposes rather than control. It helps keep the public informed about who owns significant parts of a company's shares, which can influence how the company is managed and how investors make decisions. Filing this schedule is important for transparency and understanding the ownership landscape of publicly traded companies.
What percentage of Cabaletta Bio (CABA) does Adage Capital report owning?
Adage Capital and related reporting persons report beneficial ownership of 8.43% of Cabaletta Bio’s Common Stock. This is based on 163,054,796 shares outstanding as of May 7, 2026, and assumes exercise of certain warrants.
How many Cabaletta Bio (CABA) shares does Adage Capital beneficially own?
The reporting persons beneficially own 14,174,758 shares of Cabaletta Bio Common Stock. This total includes 5,000,000 shares issuable upon exercise of warrants held by Adage Capital Partners, L.P.
How many Cabaletta Bio (CABA) shares underlie Adage’s warrants?
Adage Capital’s position includes 5,000,000 shares of Cabaletta Bio Common Stock issuable upon exercise of warrants. These warrant shares are counted in the 14,174,758 total beneficially owned and in the 8.43% ownership calculation.
What is the share count used to calculate Adage’s 8.43% stake in CABA?
The reported 8.43% ownership is calculated using 163,054,796 shares of Cabaletta Bio Common Stock outstanding as of May 7, 2026, as stated in the company’s Quarterly Report for the quarter ended March 31, 2026.
Do Adage Capital and its principals have sole or shared voting power over CABA shares?
The reporting persons have 0 shares with sole voting power and 14,174,758 shares with shared voting power. They also have 0 shares with sole dispositive power and 14,174,758 shares with shared dispositive power.
Who are the reporting persons on the Cabaletta Bio (CABA) Schedule 13G/A?
The reporting persons are Adage Capital Management, L.P., and individuals Robert Atchinson and Phillip Gross. The securities are directly held by Adage Capital Partners, L.P., for which Adage Capital Management acts as investment manager.
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13G
UNDER THE SECURITIES EXCHANGE ACT OF 1934
(Amendment No. 7)
Cabaletta Bio, Inc.
(Name of Issuer)
Common Stock, $0.00001 par value per share
(Title of Class of Securities)
12674W109
(CUSIP Number)
06/30/2026
(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)
schemaVersion:
SCHEDULE 13G
CUSIP Number(s):
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
14,174,758.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
14,174,758.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
14,174,758.00
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)
8.43 %
12
Type of Reporting Person (See Instructions)
IA, PN
Comment for Type of Reporting Person: Includes 5,000,000 shares of Common Stock (as defined in Item 2(a)) issuable upon exercise of warrants.
SCHEDULE 13G
CUSIP Number(s):
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
14,174,758.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
14,174,758.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
14,174,758.00
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)
8.43 %
12
Type of Reporting Person (See Instructions)
HC, IN
Comment for Type of Reporting Person: Includes 5,000,000 shares of Common Stock issuable upon exercise of warrants.
SCHEDULE 13G
CUSIP Number(s):
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
14,174,758.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
14,174,758.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
14,174,758.00
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)
8.43 %
12
Type of Reporting Person (See Instructions)
HC, IN
Comment for Type of Reporting Person: Includes 5,000,000 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, Pennsylvania 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 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, L.L.C., a Delaware limited liability company ("ACPLLC"), general partner of ACM, with respect to the 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 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, $0.00001 par value 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 163,054,796 Common Stock outstanding as of May 7, 2026, as reported in the Company's Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2026 filed with the Securities and Exchange Commission on May 14, 2026, and assumes the exercise of the warrants held by ACP.
(b)
Percent of class:
8.43%
(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.
Not Applicable
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, L.L.C., its General Partner, By: Robert Atchinson, its Managing Member
