Abstract FDA approval of chimeric antigen receptor (CAR) T cell therapy for multiple myeloma (MM) has reshaped the therapeutic landscape for this incurable disease. In pivotal clinical trials CAR T cells outperformed standard-of-care chemotherapy, yet most patients experienced MM relapse within two years, underscoring the need to improve CAR T cell therapy for MM. In the current study, we set out to determine if inhibition of MM survival signaling through CD28 could increase sensitivity to CAR T cell therapy. Contrary to expectations, we found that blocking CD28 interaction with its ligands CD86/CD80 using abatacept (CTLA4-Ig) accelerated MM relapse following CAR T therapy in preclinical models. Knockout studies determined that endogenous CD28 expressed on 4-1BB co-stimulated (BBζ) CAR T cells sustained in vivo anti-MM activity. Mechanistically, endogenous CD28 reprogrammed BBζ CAR T cell mitochondrial metabolism to maintain redox balance, stimulated proliferation, and increased inflammatory cytokine production in the MM bone marrow microenvironment (BME). In agreement, higher grade cytokine-mediated toxicities were associated with CD86 expression on CD138+ cells from CAR T cell treated MM patients and short-term abatacept exposure decreased inflammatory cytokine levels in the MM BME of CAR T cell treated mice without affecting their long-term survival. Overall, data directly demonstrate that CD28 signaling sustains in vivo function of BBζ CAR T cells and indicate that transient CD28 inhibition could reduce cytokine release and associated toxicities in MM patients. Citation Format: Mackenzie M. Lieberman, Jason H. Tong, Nkechi U. Odukwe, Colin A. Chavel, Kimberly M. Crasti, Terrence J. Purdon, Craig M. Brackett, Spencer R. Rosario, A.J. Robert McGray, Jonathan L. Bramson, Renier J. Brentjens, Ehsan Malek, Kelvin P. Lee, Scott H. Olejniczak. CD28 drives CAR T cell responses in multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr LB037.