Fractionated radiation therapy stimulates anti-tumor immunity mediated by both resident and infiltrating polyclonal T-cell populations when combined with PD1 blockade.

Abstract

Purpose: Radiotherapy (RT) is a highly effective anti-cancer treatment forming part of the standard of care for the majority of patients, but local and distal disease recurrence remains a major cause of mortality. RT is known to enhance tumor immunogenicity; however, the contribution and mechanisms of RT induced immune responses are unknown. Experimental Design: The impact of low-dose fractionated RT (5 x 2 Gy) alone and in combination with αPD-1 mAb on the tumor microenvironment was evaluated by flow cytometry and next-generation sequencing (NGS) of the T-cell receptor (TCR)-repertoire. A dual-tumor model was used, with fractionated RT delivered to a single tumor site to enable evaluation of the local and systemic response to treatment and ability to induce abscopal responses outside the radiation field. Results: We show that fractionated RT leads to T-cell infiltration at the irradiated site; however, the TCR landscape remains dominated by polyclonal expansion of pre-existing T-cell clones. Adaptive resistance via the PD-1/PD-L1 pathway restricts the generation of systemic anti-cancer immunity following RT which can be overcome through combination with αPD-1 mAb leading to improved local and distal tumor control. Moreover, we show that effective clearance of tumor following combination therapy is dependent on both T-cells resident in the tumor at the time of RT and infiltrating T-cells. Conclusions: These data provide evidence that RT can enhance T-cell trafficking to locally-treated tumor sites and augment pre-existing anti-cancer T-cell responses with the capacity to mediate regression of out-of-field tumor lesions when delivered in combination with αPD-1 mAb therapy.