CAR T cell trogocytosis and cooperative killing regulate tumour antigen escape

Chimeric antigen receptors (CARs) are synthetic receptors for antigen that reprogram T cell specificity, function and persistence. Patient-derived CAR T cells have demonstrated remarkable efficacy against a range of B-cell malignancies, and early trial results suggest activity in multiple myeloma. Despite high complete response rates, relapses occur in a large fraction of patients, some of which are antigen-negative and others antigen-low. Unlike mechanisms resulting in complete and permanent antigen loss, those leading to antigen-low tumour escape remain obscure. In murine leukaemia models, we show that CARs provoke reversible antigen loss through trogocytosis, an active process whereby target antigen is transferred to T cells, thereby decreasing target density on tumour cells and abating T cell activity by promoting fratricide killing and exhaustion. These mechanisms affect both CD28 and 4-1BB-based CARs, albeit differentially, depending on antigen density. They can be offset by cooperative killing and combinatorial targeting. Examination of mRNA CD19 expression in NALM6 retrieved from relapsed mice treated with 19-BBζ CAR T cells (D0), retrieved NALM6 after ex-vivo culture (D6), and NALM6 culture in vitro (ctrl).