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Chimeric antigen receptor (CAR) T cell therapies targeting B cell maturation antigen (BCMA) are transforming treatment for relapsed or refractory multiple myeloma (RRMM). We analyze 61 RRMM patients receiving idecabtagene vicleucel (Ide-cel; n = 34) or ciltacabtagene autoleucel (Cilta-cel; n = 27) and find that Cilta-cel achieves higher complete response (CR) rates (78% vs. 38%) and longer progression-free survival. Using a longitudinal single-cell multi-omics atlas of 135 blood samples, we show that Cilta-cel induces expansion of CD4 cytotoxic T cells associated with CR and immune-related toxicities, whereas non-CR CD8 T cells display impaired effector programs. Among non-B cells, plasmacytoid dendritic cells (pDCs) show the highest BCMA expression and BCMA-targeted agents eradicate a blastic plasmacytoid dendritic cell neoplasm line, suggesting a novel therapeutic avenue for this disease. Greater reductions in soluble BCMA correlate with enhanced CAR T expansion and systemic inflammation. These findings reveal cellular mechanisms driving differential efficacy and toxicity of BCMA-directed immunotherapy.