Dendritic cells accelerate CAR T cells in irradiated tumors through chimeric synapses

The persistence of adoptively transferred T cells is vital for anti-tumor efficacy. Chimeric antigen receptor (CAR) T cells can persist indefinitely when delivered to patients with B cell cancers and can confer long-term remission. For patients with solid tumors, however, sustaining CAR T cell activity remains a major challenge. This has been attributed in part to the immune microenvironment within solid tumors, though the contribution of specific immune subsets to resistance to CAR T cells is not clear. Here we resolve how the immunology of irradiated tumors dramatically enhances persistence and efficacy of CAR T cells targeted to advanced lung metastases in a syngeneic mouse model. Remarkably, CAR T cell persistence depended critically on dendritic cells (DC) that underwent trogocytic "antigen-dressing" of tumor target antigens and stimulated CAR T cells through the chimeric receptor. Furthermore, tumor irradiation increased antigen-dressing onto DCs. In the absence of functional DCs, CAR T cell activity in irradiated tumor was short-lived and tumors relapsed. These findings establish a critical mechanism through which DCs maintain the CAR T cell pool in irradiated tumors, thus supporting translation of this approach to advance CAR T cell therapy for solid tumors.