Co-expression of IL15 promotes effector differentiation and sustained proliferative capacity in ALPPL2-specific human CAR T cells

Chimeric antigen receptor (CAR) T cells have robust antitumor activity against hematologic malignancies and have the potential to benefit patients with solid tumors. Immune recognition of murine proteins expressed in adoptively transferred T cells and lack of homeostatic cytokines in the tumor microenvironment can limit the expansion and persistence of CAR T cells. CARs generated only from human sequences could reduce the risk of immune-mediated rejection and interleukin-15 (IL15), which promotes T-cell survival and fitness, may improve the expansion and persistence of CAR T cells. In this study, we report a CAR construct (ABBz) assembled from human sequences including a single-chain variable fragment (scFv) specific to ALPPL2. This binder was selected through an unbiased, high-throughput screen of a human antibody-derived, phage-displayed scFv library based on binding specificity, stringency, and low dissociation constant. We demonstrated specificity to the antigen, effective cytolytic function, and cytokine production in ABBz T cells. We showed NK-like effector differentiation with sustained proliferative capacity specific to secreted IL15 coexpression in ABBz T cells. Lastly, we demonstrated that ABBz CAR T cells had robust antitumor activity, which was further enhanced through IL15 co-expression, resulting in NK-like effector differentiation with increased cytotoxicity and superior expansion capacity due to reduced apoptosis of CAR T cells. These results demonstrate that IL15 co-expression can promote effector differentiation while maintaining the proliferative capacity of huALPPL2-CAR T cells and provide a foundation for further clinical development of IL15 co-expressing huALPPL2-CAR T cells in patients.