A longitudinal single-cell atlas to predict outcome and toxicity after BCMA-directed CAR T cell therapy in multiple myeloma

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.