Enhanced Co-Stimulatory Signaling Improves CAR T cell Effector Responses in CLL

Abstract CD19-redirected chimeric antigen receptor (CAR) T cells have shown remarkable activity against B-cell cancers. While second-generation CARs induce complete remission in >80% of acute lymphoblastic leukemia patients, similar mono-therapy induces long-term remissions in only 26% of chronic lymphocytic leukemia (CLL) patients. This disparity is attributed to cell-intrinsic effector defects in autologous CLL-derived T cells. However, the mechanisms by which leukemic cells impact CAR T cell potency are poorly understood. Herein we describe an in vitro assay that recapitulates endogenous CLL-mediated T cell defects in healthy donor CAR T cells. Contact with CLL cells insufficiently activates, but does not irreversibly impair, CAR T cell function. This state is rescuable by strong antigenic stimulation or IL-2, and is not driven by immune-suppression. Rather, this activation defect is attributable to low levels of co-stimulatory molecules on CLL cells, and exogenous co-stimulation enhanced CAR T cell activation. We next assessed the stimulatory phenotype of CLL cells derived from different niches within the same patient. Lymph node-derived (LN) CLL cells had a strong co-stimulatory phenotype and promoted better CAR T cell degranulation and cytokine production than matched peripheral blood (PB) CLL cells. Finally, in vitro CD40L-activated CLL cells acquired a co-stimulatory phenotype similar to the LN-derived tumor and stimulated improved CAR T cell proliferation, cytokine production, and cytotoxicity. Together these data identify insufficient activation as a driver of poor CAR T cell responses in CLL. The co-stimulatory phenotype of CLL cells drives differential CAR T cell responses, and can be augmented by improving co-stimulatory signaling.