Cellular and molecular mechanisms determining allogeneic CAR T cell rejection and expansion.

Allogeneic CAR T cells could overcome limitations of autologous therapies but are limited by immune rejection. We evaluated 11 patients with large B‑cell lymphoma treated with a single lot of cemacabtagene ansegedleucel (cema-cel), an allogeneic anti‑CD19 CAR T product. Despite receiving identical infusion products, patients exhibited heterogeneous cema-cel expansion and clinical outcomes. Our integrated analyses using longitudinal TCRβ sequencing, single‑cell molecular profiling, and mixed lymphocyte reaction assays revealed that high frequencies of pre‑existing, recipient-derived alloreactive CD8+ T cells mediated rapid CAR T rejection in non-expanders. Furthermore, effector-like features, rather than stem/central memory programs, drove robust clonal CAR T expansion consistently across expanders. We confirmed similar expansion patterns in two independent cohorts treated with a separate lot of cema-cel or an allogeneic anti-BCMA CAR T product. These findings highlight distinct cell‑extrinsic and cell‑intrinsic mechanisms that influence allogeneic CAR T performance and provide insights to optimize donor selection, manufacturing, and product design.