Abstract 5571: Disruption of cell-intrinsic checkpoint regulator CTLA-4 in CD19 directed CAR T cells provides clinical efficacy in CLL patients

The field of cancer immunotherapy has led to successful treatments such as monoclonal antibodies (MAb), inhibitory receptor (IR) blockade, and adoptive cell transfer (ACT). ACT modified to express chimeric antigen receptors (CARs) can redirect T cells to tumor antigens and has shown remarkable clinical efficacy in patients with relapse and refractory leukemia and lymphoma. 90% of pediatric patients with acute lymphoblastic leukemia (ALL) respond to CD19 CAR T cell therapy (CAR T19), although only 26-35% of patients with chronic lymphocytic leukemia (CLL) show complete responses (CR). It is unclear why responses are less frequent in CLL compared to ALL. The heavy pre-treatment received by CLL patients likely contributes to lower response rates with CAR T cell therapy, and disease progression is worsened due to profound T cell defects characterized by elevated expression of IRs such as PD-1 and CTLA-4. Fraietta et. al. assessed CLL patient apheresis samples and CART19 products to show that CR was associated with elevated levels of CD27+PD1-CD8+ T memory cells whereas the non-responders (NR) showed an exhausted phenotype with high levels of multiple IRs. We studied 14 patients with advanced, heavily pretreated CLL who received at least one dose of CART19. Patients with CRs exhibited high in vivo expansion and persistence of infused CAR T cells, as opposed to NR’s. Importantly, at peak levels of in vivo CAR expansion, NR’s had elevated levels of CTLA-4 expression which correlated with poor CLL patient responses to CART19 therapy. In summary, these data suggest that eliminating CTLA-4 mediated T cell inhibition can be clinically beneficial. Thus, we tested the hypothesis that disruption of CTLA4 would improve CAR T cell efficacy in CLL based on numerous observations. Our data demonstrate that knockout (KO) of CTLA-4 in T cells from healthy donors using CRISPR Cas9 technology leads to maintenance of surface CAR expression and higher tumor clearance in a chronic re-stimulation model using CART19 cells against NALM6 tumor cells. In xenograft models of ALL, KO of CTLA-4 increases the anti-tumor efficacy of CART19 cells. We then performed CTLA4 disruption in T cells banked from CLL patients that did not respond to CAR T cell therapy to determine whether dysfunctional CAR T infusion products can be invigorated by CTLA-4 KO. In both chronic re-stimulation model and xenograft models of NALM6, CTLA-4 KO CD19 CAR T cells from CLL patients maintained surface CAR expressio