RBIO-02. DEVELOPMENT OF FLASH-CAR RADIOIMMUNOTHERAPY FOR PEDIATRIC BRAIN TUMORS

Abstract Pediatric brain tumors including medulloblastoma and high-grade glioma are generally refractory to T cell-based immunotherapy, largely due to an immune-hostile microenvironment infiltrated extensively with immunosuppressive macrophages. Ultra-high dose rate (FLASH) radiotherapy holds promise for treating solid tumors, given the potential lower toxicity in normal tissues and possible favorable impact on tumor immunity. Using a genetically engineered mouse model of medulloblastoma and glioma, we show that FLASH radiation stimulates pro-inflammatory polarization in tumor macrophages. Single-cell transcriptome analysis shows that FLASH proton beam radiation skews macrophages towards proinflammatory phenotypes and increases T cell infiltration in the medulloblastoma model. Further bulk transcriptome analyses reveal that FLASH radiation reduces peroxisome proliferator-activated receptor (PPAR)-γ and arginase-1 expression and inhibits immunosuppressive macrophage polarization under stimulus-inducible conditions. Mechanistically, FLASH radiation abrogates lipid oxidase expression and oxidized low-density lipid (oxLDL) generation to reduce PPARγ activity, while standard radiation induces reactive oxygen species (ROS)-dependent PPARγ activation in macrophages. Notably, FLASH radiotherapy improves infiltration and activation of chimeric antigen receptor (CAR) T cells and sensitizes tumors to GD2 CAR T immunotherapy in the autochthonous medulloblastoma and syngeneic glioma models. These findings suggest that FLASH radiotherapy may reprogram macrophage lipid metabolism to reverse tumor immunosuppression and that combination FLASH-CAR radioimmunotherapy may offer exciting opportunities for treatment of pediatric brain tumor.