Background CAR (Chimeric antigen receptor)-T cell therapy achieved unprecedented success in treating blood cancers but showed limited effects in solid tumors so far. Major challenges include T cells’ poor infiltration, T cell exhaustion and low persistence under an immune-suppressive tumor microenvironment. Given the critical need to develop effective cell therapies for solid tumors, mast cells present a promising new carrier for CAR because 1) mast cell are long-lived and tissue resident, and can undergo repetitive activation; 2) they secret cytotoxic factors that can directly kill target cells; 3) they release a plethora of chemicals, chemokines, and cytokines that can increase tissue permeability, recruit immune cells and modify tissue environment; 4) they do not express conventional checkpoint receptors, therefore might be resistant to the inhibitory signals in tumors. We hypothesize that CAR-mast cells eliminate solid tumors by directly killing cancer cells, enhancing T cell and NK cell infiltration, and remodeling the immune-suppressive tumor microenvironment.
Methods We established GD2 and B7-H3 CAR-mast cells via introducing CARs into mouse bone marrow and spleen derived mast cells via lentiviral transduction. Flow cytometry, ELISA and cytokine profiling were used to examine CAR-dependent mast cell activation in vitro, and utilized B78D14 and MC38 xenograft mouse model to examine the anti-tumor efficacy of CAR-mast cell in vivo. We also measured rectal temperature to assess potential anaphylactic side effects.
Results Both GD2 and B7-H3 CAR-mast cells were activated in an antigen-dependent manner in vitro. Specific killing of cancer cells were observed in vitro. Multiple cytokines were released, including CCL2, CCL3, CCL4 and CCL5, which recruit T cells, dendritic cells and NK cells. Slowed tumor growth and improved mice survival were observed after CAR-mast cell treatment in B78D14 and B7-H3 MC38 mouse models. Neglectable acute anaphylactic responses were observed after CAR-mast cell treatment.
Conclusions Here we developed the first CAR-mast cell that displayed potent anti-tumor effects in mouse xenograft models. Two mechanisms were demonstrated to explain the anti-tumor effects of CAR-mast cells: direct killing of cancer cells and release of chemokines that promote tumor infiltration of T cells, dendritic cells, and NK cells. Importantly, no drastic acute anaphylactic responses were observed. These findings expanded the repertoire of cell therapy to mast cells and highlight the potential of mobilizing broad immune cell response to combat solid tumors.
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