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Atara Biotherapeutics reports a reported 8.56% ownership position held by Adage Capital Management and affiliated individuals.
Adage Capital Management, L.P., together with Robert Atchinson and Phillip Gross, states shared dispositive and voting power over 700,000 shares of Common Stock. The filing calculates the percentage using 8,178,114 shares outstanding as of March 10, 2026. The statement is a joint Schedule 13G disclosure of beneficial ownership and identifies the Reporting Persons and their business addresses.
Insights
Adage and two principals report a sizable passive stake in Atara.
The filing shows 700,000 shares held with shared voting and dispositive power, equal to 8.56% of the 8,178,114 shares outstanding as of March 10, 2026. The statement is a Schedule 13G joint filing and identifies the reporting structure and addresses.
Voting and disposition are disclosed as shared rather than sole; cash‑flow treatment and any plans to buy or sell are not stated in the excerpt. Subsequent filings would provide any updates to position or intentions.
Filing clarifies attribution and managerial roles behind the reported stake.
The filing attributes holdings to ACM as investment manager and to Messrs. Atchinson and Gross in their managerial roles for related entities. It includes the business office address and cites the Company’s Form 10‑K for the share count used in the percentage calculation.
Disclosure preserves that filing is not an admission of beneficial ownership by any Reporting Person; any rights to dividends or proceeds on behalf of others are not expanded upon in the excerpt.
Key Figures
Reported shares held:700,000 sharesPercent of class:8.56%Shares outstanding used:8,178,114 shares+1 more
4 metrics
Reported shares held700,000 sharesshared voting and dispositive power reported on Schedule 13G
Percent of class8.56%calculated using shares outstanding as of <date>March 10, 2026</date>
Shares outstanding used8,178,114 sharesoutstanding as of <date>March 10, 2026</date> per Company Form 10‑K
CUSIP046513206Atara Biotherapeutics Common Stock CUSIP
"This statement is filed by: Adage Capital Management, L.P."
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.
shared dispositive powerfinancial
"Shared Dispositive Power 700,000.00"
beneficially ownedregulatory
"Amount beneficially owned: The information required by Item 4(a)"
Beneficially owned describes securities or assets where a person has the economic rights and control—such as the right to receive dividends and to direct voting—even if legal title is held in another name. Think of it like having the keys and using a car that’s registered to someone else: you get the benefits and make decisions. Investors care because beneficial ownership reveals who truly controls value and voting power, affecting corporate decisions and takeover dynamics.
FAQ
AI-generated questions and answers. How Rhea-AI works. Not financial advice.
What stake does Adage report in Atara Biotherapeutics (ATRA)?
The filing reports a 700,000‑share position, representing 8.56% of common stock. The percentage uses 8,178,114 shares outstanding as of March 10, 2026, per Atara’s Form 10‑K.
Who are the reporting persons in the Schedule 13G for ATRA?
The statement is filed by Adage Capital Management, L.P. and two individuals, Robert Atchinson and Phillip Gross, in their managerial roles for related Adage entities.
Does the Schedule 13G state whether the stake is passive or active?
The form type (Schedule 13G) and disclosed shared powers indicate a passive beneficial‑ownership filing. The excerpt does not state any plans to acquire or dispose of shares.
How was the ownership percentage calculated in the filing for ATRA?
The percentage is calculated using 8,178,114 shares outstanding reported in Atara’s Form 10‑K for the year ended December 31, 2025, as of March 10, 2026.
What voting and dispositive powers are reported by the filers?
The cover rows show 0 sole voting/dispositive power and 700,000 shared voting and dispositive power, reflecting shared authority over the reported shares.
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13G
UNDER THE SECURITIES EXCHANGE ACT OF 1934
Atara Biotherapeutics, Inc.
(Name of Issuer)
Common Stock, par value $0.0001 per share
(Title of Class of Securities)
046513206
(CUSIP Number)
03/31/2026
(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):
046513206
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
700,000.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
700,000.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
700,000.00
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)
8.56 %
12
Type of Reporting Person (See Instructions)
IA, PN
SCHEDULE 13G
CUSIP Number(s):
046513206
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
700,000.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
700,000.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
700,000.00
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)
8.56 %
12
Type of Reporting Person (See Instructions)
HC, IN
SCHEDULE 13G
CUSIP Number(s):
046513206
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
700,000.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
700,000.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
700,000.00
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)
8.56 %
12
Type of Reporting Person (See Instructions)
HC, IN
SCHEDULE 13G
Item 1.
(a)
Name of issuer:
Atara Biotherapeutics, Inc.
(b)
Address of issuer's principal executive offices:
1280 Rancho Conejo Boulevard, Thousand Oaks, California 91320
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 $0.0001 per share ("Common Stock") of Atara Biotherapeutics, Inc., a Delaware corporation (the "Company") 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 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 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 $0.0001 per share
(e)
CUSIP Number(s):
046513206
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 8,178,114 shares of Common Stock outstanding as of March 10, 2026, as reported in the Company's Annual Report on Form 10-K for the year ended December 31, 2025, filed with the Securities and Exchange Commission on March 16, 2026.
(b)
Percent of class:
8.56%
(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, Managing Member
