P08.20 Heterogeneous long-term T cell fitness following CAR-T Therapy in DLBCL and multiple myeloma

Background Chimeric antigen receptor (CAR) T cell therapy is highly effective in relapsed or refractory B cell malignancies, yet long-term outcomes remain heterogeneous, with frequent late relapses. Early post-infusion parameters are strongly influenced by lymphodepletion and antigen burden and therefore provide limited insight into durable immune control. In contrast, evaluation of T cell fitness one year after CAR-T therapy reflects long-term immune adaptation following sustained proliferative and antigenic stress and may reveal mechanisms underlying relapse and durable remission.Methods Serum and peripheral blood mononuclear cells (PBMCs) were obtained from patients with diffuse large B cell lymphoma (DLBCL) and multiple myeloma (MM) one year after CAR-T cell therapy, as well as from healthy donors (HD). T cell counts, phenotype, and activation were assessed after overnight thawing. T cell activation and cytokine release were evaluated using GFP + JeKo-1 cells in the presence of Glofitamab. Long-term T cell function was assessed in a co-culture system with repeated tumor rechallenge. T cell phenotype, proliferation, exhaustion, and residual tumor cells were analyzed by flow cytometry.Results Serum cytokine profiles were largely comparable between DLBCL and MM patients but differed markedly from those of healthy donors (HD), indicating a shared inflammatory state in both patient cohorts. Compared with MM and HD, DLBCL patients exhibited a pronounced reduction in naïve T cells in both CD4 + and CD8+ compartments (HD: 47.257% ± 14.803, DLBCL: 19.814% ± 19.106, MM: 24.672% ± 21.127), accompanied by an increased frequency of terminally differentiated effector memory (TEMRA) T cells (HD: 16.021% ±4.753, DLBCL: 40.315% ± 25.129, MM: 36.692% ± 13.207), and elevated CD69 expression on CD8+ T cells (HD: 18.423% ±12.484, DLBCL: 84.32% ± 12.28, MM: 69.942% ± 18.512). Although T cells from DLBCL patients displayed increased interferon-α (IFN-α) expression in a 12-hour cytotoxicity assay (HD:39.171% ± 10.662, DLBCL: 58.044% ± 18.821, MM: 56.667% ± 13.576), their functional capacity was inferior to that of T cells from HD and MM. Both DLBCL and MM T cells exhibited limited proliferative capacity during co-culture, with markedly reduced peak expansion (T cell peak number/µl: HD:6068.971 ± 1513.484, DLBCL: 514.567 ± 415.767, MM: 1042.596 ± 1396.375), consistent with decreased IL-2 expression. Patient cohorts showed increased exhausted T cell subsets and reduced naïve T cell frequencies compared with HD. Notably, T cells from DLBCL patients demonstrated the poorest long-term cytotoxicity, followed by MM, as evidenced by fewer effective rechallenge cycles and increased residual tumor burden.Conclusion Despite achieving clinical remission, patients with DLBCL and MM exhibit persistent T cell dysfunction up to one year after CAR-T cell therapy, marked by reduced immune fitness, sustained inflammation, and depletion of naïve T cells, suggesting durable immune remodeling beyond tumor clearance that may contribute to heterogeneous long-term remission and late complications.