Background Chimeric Antigen Receptor (CAR) T cell therapy has shown strong antitumor potency in the context of hematological malignancies. However, CAR T cells face multiple barriers in treating solid tumors, such as trafficking to the tumor and a failure to expand and persist in the periphery. While ideally solid tumor target expression is restricted to the tumor to prevent on-target off-tumor toxicity, this scenario provides no opportunity for antigen-driven CAR T cell expansion in the periphery which can be correlated with response. To address this issue and increase the expansion and persistence of solid tumor-directed CAR T cells, we sought to design a dual-targeted CAR T cell against CD19 and the solid tumor antigen, mesothelin (MSLN), in order to maintain peripheral MSLN CAR T cell fitness and increase the number of CAR T cells which traffic to the tumor.

Methods Leveraging the paradigm of CD19 CAR T cells, which target a dispensable cell population and have exhibited success and safety in the clinic, we developed and optimized a CD19 and MSLN directed CAR T cell. We first assessed our technology in immunocompetent C57BL/6 mice where we established two syngeneic flank tumor models of pancreatic ductal adenocarcinoma (PDA) from KPC (Kras^LSL.G12D/+p53^R172H/+) mice, ranging in immunogenicity. In addition to assessing their antitumor efficacy, we also assessed their trafficking patterns compared to control CAR T cells. To determine the antitumor efficacy of human CAR T cells in NOD scid gamma (NSG) mice, we have developed a novel model to mimic CD19⁺ B cells in the periphery.

Results We have demonstrated specific cognate expansion of CD45.1⁺ dual murine CAR T cells and the induction of B cell aplasia in CD45.2⁺ C57BL/6 mice. Ultimately, we find enhanced B cell driven expansion in the blood, increased antitumor efficacy, and higher levels of early activation without increasing downstream exhaustion or toxicity compared to single CAR T cells. We have also identified alternative mechanisms of trafficking, likely owing to boosting in the spleen. Further, we have established a novel humanized model for CD19+ B cells in NSG mice and demonstrate that human dual CAR T cells expand in the periphery and demonstrate anti-tumor activity.

Conclusions Dual-targeted CAR T cells are a successful strategy to enhance solid tumor-directed expansion in the periphery and tumor infiltration resulting in increased antitumor efficacy in syngeneic and humanized models of PDA.