Dynamic changes in immune checkpoint and cell populations following lymphodepletion and CAR-T therapy in hematologic malignancies.

7542 Background: Chimeric antigen receptor T-cell (CAR-T) therapy has significantly improved outcomes in relapsed and refractory hematologic malignancies; however, durable remissions remain limited in patients with high-risk features, including early progression to ≥2 extranodal sites, high tumor burden, and elevated serum lactate dehydrogenase. Proposed mechanisms of resistance include CAR-T cell exhaustion, impaired expansion and cytotoxicity, antigen loss or downregulation, and inadequate tumor infiltration. In addition, immunosuppressive interactions among myeloid-derived suppressor cells (MDSCs), regulatory T cells, tumor-associated macrophages, and inhibitory immune checkpoints such as PD-1, PD-L1, LAG-3, TIM-3, and VISTA may further limit CAR-T efficacy. Lymphodepleting chemotherapy alters immune homeostasis by affecting T-, B-, and NK-cell populations, modulating co-stimulatory signaling, reducing regulatory populations, and reshaping cytokine networks, influencing both response and toxicity. Methods: We conducted a pilot study (n = 10) to characterize peripheral immune alterations in patients receiving lymphodepleting chemotherapy followed by CAR-T therapy. Serial peripheral blood samples were collected at predefined time points before and after lymphodepletion and CAR-T infusion (D -5, D0 and D28+/-2). Flow cytometry was used to quantify immune cell subsets and evaluate immune checkpoint expression, including PD-1, PD-L1, LAG-3, and VISTA. Results: Lymphodepleting chemotherapy resulted in a marked reduction in T cells, B cells, and MDSCs. By D28, most immune populations recovered to pre-lymphodepletion levels; however, granulocytic MDSCs remained relatively suppressed. Preliminary molecular analyses demonstrated trends in both VISTA and LAG-3 expressions on T cells in multiple myeloma patients following lymphodepletion, without significant changes in overall circulating immune cell frequencies. Among patients who experienced disease recurrence (n = 3), a trend toward decreased CD45⁺CD3⁺ and CD45⁺CD4⁺ T-cell frequencies on D28 compared with D0 was observed relative to patients achieving complete remission (n = 7). Conclusions: These preliminary data suggest that lymphodepleting chemotherapy and CAR-T therapy induce dynamic peripheral immune remodeling, including sustained suppression of select myeloid populations and differential immune checkpoint expression. Interpretation is limited by the small sample size and pilot nature of the study. Nonetheless, observed trends in post-infusion T-cell dynamics associated with disease recurrence support further investigation. Larger, longitudinal studies are needed to validate these findings, elucidate mechanisms of resistance, and identify immune biomarkers predictive of response, toxicity, and durable remission following CAR-T therapy.