Targeting the membrane-proximal C2-set domain of CD33 for improved CAR T cell therapy.

Current CD33-targeted immunotherapies typically recognize the membrane-distal V-set domain of CD33. Here, we show that decreasing the distance between T cell and leukemia cell membrane increases the efficacy of CD33 chimeric antigen receptor (CAR) T cells. We therefore generated and optimized second-generation CAR constructs containing single-chain variable fragments from antibodies raised against the membrane-proximal C2-set domain, which bind CD33 regardless of whether the V-set domain is present (CD33 antibodies). CD33 CAR T cells resulted in efficient tumor clearance and improved survival of immunodeficient mice bearing human AML cell xenografts and, in an AML model with limited CD33 expression, forced escape of CD33 leukemia. Compared to CD33 CAR T cells, CD33 CAR T cells showed greater and efficacy against several human AML cell lines with differing levels of CD33 without increased expression of exhaustion markers. CD33 moieties were detected at a higher frequency on human leukemic stem cells, and CD33 CAR T cells had greater efficacy against primary human AML cells. Together, our studies demonstrate improved efficacy with CAR T cells binding CD33 close to the cell membrane, providing the rationale to investigate CD33 CAR T cells further toward possible clinical application.