Relapse after allogeneic hematopoietic stem cell transplantation (alloHSCT) is associated with poor survival in advanced B-cell malignancies. Donor-derived CD19-CAR T cells offer the potential for durable remissions but are limited by poor engraftment, expansion, and persistence. We report results from a sequential Phase 1 study (NCT01087294) in which patients with B-cell malignancies relapsed after alloHSCT received either donor-derived standard or stem cell memory T (TSCM)-enriched CD19-CAR T cells without lymphodepletion preconditioning. CAR TSCM cells exhibited delayed but superior expansion and persistence, leading to higher complete remission rates (45% vs. 10%) for doses < 3×108 CAR T cells. Despite their potency, CAR TSCM cells induced milder cytokine-mediated toxicities, driven by IFN-γ rather than IL-6, as observed in standard CAR T cell recipients. While both T cell products differentiated into effectors, only CAR TSCM cells robustly reconstituted the stem-like compartment. Longitudinal clonal-tracking revealed CAR TSCM cells were maintained by clonal succession, whereas standard CAR T cells persisted through maintenance or attrition of early-expanded clones. Unlike standard CAR T cells, CAR TSCM treatment failure was primarily due to tumor- and host-related factors rather than intrinsic T cell defects. These findings establish TSCM cells as a safe and highly effective CAR T cell platform.
