BackgroundChimeric antigen receptor (CAR) T cell therapy has proven its clinical utility in hematological malignancies. Optimization is still required for its application in solid tumors. Here, the lack of cancer-specific structures along with tumor heterogeneity represent a critical barrier to safety and efficacy. Modular CAR T cells indirectly binding the tumor antigen through CAR-adaptor molecules have the potential to reduce adverse events and to overcome antigen heterogeneity. We hypothesized that a platform utilizing unique traits of clinical grade antibodies for selective CAR targeting would come with significant advantages. Thus, we developed a P329G-directed CAR targeting the P329G mutation in the Fc part of tumor-targeting human antibodies containing P329G L234A/L235A (LALA) mutations for Fc silencing.MethodsA single chain variable fragment-based second generation P329G-targeting CAR was retrovirally transduced into primary human T cells. These CAR T cells were combined with IgG1 antibodies carrying P329G LALA mutations in their Fc part targeting epidermal growth factor receptor (EGFR), mesothelin (MSLN) or HER2/neu. Mesothelioma, pancreatic and breast cancer cell lines expressing the respective antigens were used as target cell lines. Efficacy was evaluated in vitro and in vivo in xenograft mouse models.ResultsUnlike CD16-CAR T cells, which bind human IgG in a non-selective manner, P329G-targeting CAR T cells revealed specific effector functions only when combined with antibodies carrying P329G LALA mutations in their Fc part. P329G-targeting CAR T cells cannot be activated by an excess of human IgG. P329G-directed CAR T cells combined with a MSLN-targeting P329G-mutated antibody mediated pronounced in vitro and in vivo antitumor efficacy in mesothelioma and pancreatic cancer models. Combined with a HER2-targeting antibody, P329G-targeting CAR T cells showed substantial in vitro activation, proliferation, cytokine production and cytotoxicity against HER2-expressing breast cancer cell lines and induced complete tumor eradication in a breast cancer xenograft mouse model. The ability of the platform to target multiple antigens sequentially was shown in vitro and in vivo.ConclusionsP329G-targeting CAR T cells combined with antigen-binding human IgG1 antibodies containing the P329G Fc mutation mediate pronounced in vitro and in vivo effector functions in different solid tumor models, warranting further clinical translation of this concept.