Therapeutic Targeting of Mesothelin with Chimeric Antigen Receptor T Cells in Acute Myeloid Leukemia

Purpose: We previously identified mesothelin (MSLN) as highly expressed in a significant fraction of acute myeloid leukemia (AML) but entirely silent in normal hematopoiesis, providing a promising antigen for immunotherapeutic targeting that avoids hematopoietic toxicity. Given that T cells genetically modified to express chimeric antigen receptors (CAR) are effective at eradicating relapsed/refractory acute lymphocytic leukemia, we developed MSLN-directed CAR T cells for preclinical evaluation in AML. Experimental Design: The variable light (VL) and heavy (VH) sequences from the MSLN-targeting SS1P immunotoxin were used to construct the single-chain variable fragment of the standard CAR containing 41-BB costimulatory and CD3Zeta stimulatory domains. The preclinical efficacy of MSLN CAR T cells was evaluated against AML cell lines and patient samples expressing various levels of MSLN in vitro and in vivo. Results: We demonstrate that MSLN is expressed on the cell surface of AML blasts and leukemic stem cell–enriched CD34+CD38− subset, but not on normal hematopoietic stem and progenitor cells (HSPC). We further establish that MSLN CAR T cells are highly effective in eliminating MSLN-positive AML cells in cell line– and patient-derived xenograft models. Importantly, MSLN CAR T cells can target and eradicate CD34+CD38− cells without impacting the viability of normal HSPCs. Finally, we show that CAR T-cell functionality can be improved by inhibition of the ADAM17 metalloprotease that promotes shedding of MSLN. Conclusions: These findings demonstrate that MSLN is a viable target for CAR T-cell therapy in AML and that inhibiting MSLN shedding is a promising approach to improve CAR T-cell efficacy.