SynNotch CAR circuits enhance solid tumor recognition and promote persistent antitumor activity in mouse models

The first clinically approved engineered chimeric antigen receptor (CAR) T cell therapies are remarkably effective in a subset of hematological malignancies with few therapeutic options. Although these clinical successes have been exciting, CAR T cells have hit roadblocks in solid tumors that include the lack of highly tumor-specific antigens to target, opening up the possibility of life-threatening "on-target/off-tumor" toxicities, and problems with T cell entry into solid tumor and persistent activity in suppressive tumor microenvironments. Here, we improve the specificity and persistent antitumor activity of therapeutic T cells with synthetic Notch (synNotch) CAR circuits. We identify alkaline phosphatase placental-like 2 (ALPPL2) as a tumor-specific antigen expressed in a spectrum of solid tumors, including mesothelioma and ovarian cancer. ALPPL2 can act as a sole target for CAR therapy or be combined with tumor-associated antigens such as melanoma cell adhesion molecule (MCAM), mesothelin, or human epidermal growth factor receptor 2 (HER2) in synNotch CAR combinatorial antigen circuits. SynNotch CAR T cells display superior control of tumor burden when compared to T cells constitutively expressing a CAR targeting the same antigens in mouse models of human mesothelioma and ovarian cancer. This was achieved by preventing CAR-mediated tonic signaling through synNotch-controlled expression, allowing T cells to maintain a longlived memory and non-exhausted phenotype. Collectively, we establish ALPPL2 as a clinically viable cell therapy target for multiple solid tumors and demonstrate the multifaceted therapeutic benefits of synNotch CAR T cells.