Anti-CD21 Chimeric Antigen Receptor T cells for the Treatment of T Cell Acute Lymphoblastic Leukemia

Abstract

Abstract Relapsed/refractory (R/r) T cell acute lymphoblastic leukemia (T-ALL) has a dismal prognosis, with an unmet need for effective novel therapies. The successes seen in chimeric antigen receptor (CAR)-T cell therapy for B-ALL have yet to be fully translated to T-ALL. Most strategies have targeted pan-T antigens (CD7, CD5) but these may be limited by T cell aplasia and fratricide, requiring elimination of CAR-T antigen expression during manufacture and salvage hematopoietic stem cell transplantation (HSCT). Here, we describe CD21 as a novel immunotherapeutic strategy for the treatment of T-ALL. CD21 is largely confined to malignant T cells with expression in 57% of diagnostic T-ALL but only on a minor fraction of mature T cells (10%). While anti-CD21 CAR-T targeting membrane distal epitopes were ineffective, CAR-T cells utilising a novel Fab-CAR architecture and binding to membrane proximal epitopes showed no fratricide and were potent against low antigen density cell line and patient-derived xenograft models of T-ALL in vitro and in vivo. Further, we showed that CD21 expression in T-ALL can be upregulated by inhibition of the PI3K/ axis. CD21 is a novel target for CAR-T cell therapy in T-ALL, avoiding the fratricide and T cell aplasia seen with many T-ALL CAR-T strategies.