411. CD5 CAR for the Treatment of T-Cell Malignancies

Abstract

Chimeric antigen receptors (CARs) have emerged as a powerful therapeutic tool by redirecting patients’ own T cells to tackle hematologic malignancies – in particular, those of B cell origin. However, T cell malignancies remain a more challenging task for CAR T cells owing to the shared expression of targetable tumor-associated antigens between normal and cancerous T cells, potentially invoking fratricide of CAR T cells and/or profound immunodeficiency.We designed a novel CAR targeting CD5, a common marker expressed in ~80% of T-cell leukemia/lymphoma blasts as well as in normal T cells. Upon transduction with CD5 CAR, more than 95% of T cells expressed the CAR while producing limited and transient fratricide. Fratricide of CD5 CAR T cells was mainly directed against naive phenotype and central memory cells, eliminating more than 50% of cells in these subsets and resulting in preferential expansion of effector memory and effector T cells. CD5 CAR T cells acquired resistance to self-killing by 3 days post-transduction and could then expand normally, up to 100,000-fold. Expansion of CD5 CAR T cells coincided with downregulation of CD5 protein from the cell surface while preserving CAR expression.CD5 CAR T cells mounted an effective anti-tumor response, selectively killing five CD5-positive T-ALL and T lymphoma cell lines in 4-hour chromium release assays while displaying no activity against CD5-negative cell lines. Upon longer-term coculture, CD5 CAR T cells eliminated >95% of leukemia cells from 3 T-ALL lines within 48h and 100% by day 7. We also observed the ability of CD5 CAR T cells to eliminate leukemia cells in sequential killing assays where we recurrently replenished fresh target cells for at least 4 iterations. Lack of functional exhaustion in sequential killing assays supports the fitness of CD5 CAR T cells for eradicating large numbers of tumor cells in vivo. CD5 CAR T cells dramatically suppressed systemic in vivo disease progression in 3 different xenograft mouse models, doubling median survival. Importantly, CD5 CAR T cells demonstrated significant cytokine production and cytotoxicity against primary T-ALL blasts (n=6), highlighting the therapeutic potential of CD5 CAR for patients with T cell malignancies. Overall, we demonstrated for the first time that CD5 CAR redirects T cells to eliminate CD5-positive malignant T cells in vitro and in vivo while producing only limited fratricide of the normal T cell population.