Ionizing radiation enhances CAR T-cell infiltration and efficacy in solid tumors

Chimeric antigen receptor (CAR) T cells represent an exciting therapeutic strategy with improved survival outcomes for patients with hematological malignancies. However, the efficacy of CAR T-cell therapy in the treatment of solid tumors remains suboptimal due to therapeutic barriers associated with the solid tumor microenvironment. We investigated whether ionizing radiation could improve vascular perfusion and CAR T-cell delivery in an EGFRvIII-expressing B16F10 melanoma model. Tumors received radiation doses of 2-12 Gy, and perfusion was evaluated at multiple time points using immunofluorescence detection of intravenously administered fluorescent dyes. We found that a single 8-Gy dose of ionizing radiation produced the most significant increase in B16F10 tumor perfusion 4 h after irradiation. Consistently, the irradiation of tumors 4 h prior to a systemic administration of EGFRvIII-targeting CAR T cells led to higher intratumoral CAR T-cell accumulation than in non-irradiated tumors. This approach also resulted in a significantly delayed tumor growth and improved survival relative to radiation or CAR T cells alone. Interestingly, the CD28ζ EGFRvIII-CAR T-cell levels substantially increased in irradiated tumors over time relative to 4-1BBζ EGFRvIII-CAR T cells and produced greater tumor growth delays and survival improvements in comparison to 4-1BBζ EGFRvIII-CAR T cells administered at a 10-fold higher concentration. Taken together, these data highlight the importance of co-stimulatory domains in CAR T-cell function in vivo and demonstrate that irradiating tumors prior to systemic CAR T-cell infusion can increase CAR T-cell infiltration and efficacy in solid tumors.