Exploring Mechanisms of Tumor Resistance to CAR T Cell Therapy in Immunocompetent Mouse Models

Abstract Chimeric antigen receptor (CAR) T cell therapies have shown striking clinical efficacy in the treatment of hematological malignancies, but limited success so far against solid tumors, suggesting that solid tumors may harbor mechanisms of resistance. To investigate mechanisms of CAR T cell therapy resistance in solid tumors, we developed immunocompetent mouse models to explore immune cell interactions and the effect of an immunosuppressive tumor microenvironment on CAR T cell behavior. We generated murine CAR T cells against a model antigen that contained a CD3z intracellular signaling domain linked to either a CD28 or a 4-1BB signaling domain. We found that murine CAR T cells exhibited a predominantly central memory phenotype together with potent cytotoxic activity and antigen-dependent cytokine release in vitro. Further, we found that murine CAR T cells exhibited strong anti-tumor efficacy against both disseminated (EL4) and subcutaneous (MC38) murine tumors in vivo, whereas non-binding control CAR T cells had no anti-tumor effect. However, in both the disseminated and subcutaneous models, tumors were suppressed but not completely eradicated. Although murine CAR T cells possessing either CD28 or 41BB co-stimulatory domains demonstrated similar in vitro activities, they exhibited strikingly different efficacies and resistance mechanisms in vivo. Collectively, these results highlight the contributions of specific co-stimulatory signaling pathways to CAR T cell therapy in immunocompetent murine models and provide a platform to explore resistance mechanisms to CAR T therapy against solid tumors.