Local Radiation Enhances Systemic CAR T Cell Efficacy by Augmenting Antigen Cross-Presentation and T-cell Infiltration

• Two fractions of low-dose RT enhance antitumor responses in both irradiated and non-irradiated sites when combined with CART-19 therapy.

• Enhanced responses from two fractions of RT are linked to activation of the STING pathway, TAA cross-priming, and epitope spreading.

Chimeric antigen receptor (CAR) T cell therapy targeting CD19 (CART-19) represents a significant advance in the treatment of patients with relapsed or refractory CD19-positive B-cell lymphomas. However, a significant portion of patients either relapse or fail to respond. Moreover, many patients have symptomatic disease, requiring bridging radiation therapy (RT) during the period of CAR-T cells manufacturing. To investigate the impact of 1-2 fractions of low-dose RT on CART-19 treatment response, we developed a mouse model using A20 lymphoma cells for CART-19 therapy. We found that low dose fractionated RT had a positive effect on generating abscopal systemic antitumor responses beyond the irradiated site. The combination of RT with CART-19 therapy resulted in additive effects on tumor growth in irradiated masses. Notably, a significant additional increase in antitumor effect was observed in non-irradiated tumors. Mechanistically, our results validate activation of the cGAS/STING pathway, tumor-associated antigen (TAA) cross-priming, and elicitation of epitope spreading. Collectively, our findings suggest that RT may serve as an optimal priming and bridging modality for CAR-T cell therapy overcoming treatment resistance and improving clinical outcomes in patients with CD19-positive hematologic malignancies.