Company: ATRA
Filing Date: 2025-04-28
Form Type: DEFA14A
Source: 0000950170-25-059333
Chunk: 0

Company: Atara Biotherapeutics, Inc.
Filing Date: 2025-04-28
Form: DEFA14A
Chunk 0
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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No.__)

| Filed by the Registrant                    |     | ☒ |
| Filed by a Party other than the Registrant |     | ☐ |

Check the appropriate box:

| ☐ |     | Preliminary Proxy Statement                                                     |
| ☐ |     | Confidential, for Use of the Commission only (as permitted by Rule 14a-6(e)(2)) |
| ☐ |     | Definitive Proxy Statement                                                      |
| ☒ |     | Definitive Additional Materials                                                 |
| ☐ |     | Soliciting Material Pursuant to § 240.14a-12                                    |

Atara Biotherapeutics, Inc. (Name of Registrant as Specified In Its Charter) (Name of Person(s) Filing Proxy Statement if Other Than the Registrant) Payment of Filing Fee (Check the appropriate box):

| ☒ | No fee required.                                                                                     |
| ☐ | Fee paid previously with preliminary materials.                                                      |
| ☐ | Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11. |

Your Vote Counts! ATARA BIOTHERAPEUTICS, INC. 2025 Annual Meeting Vote by June 9, 2025 11:59 PM ET ATARA BIOTHERAPEUTICS, INC. 1280 RANCHO CONEJO BOULEVARD THOUSAND OAKS, CA 91320 V68538-P31357 You invested in ATARA BIOTHERAPEUTICS, INC. and it’s time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy materials for the stockholder meeting to be held on June 10, 2025. Get informed before you vote View the Notice and Proxy Statement and 2024 Annual Report on Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 27, 2025. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-