Natural killer (NK) cells are an important line of host innate immune defense against cancer. Capable of directly killing tumor cells, they also possess regulatory functions that influence the quality and strength of adaptive immune defenses. We have examined the differential effects of radiation dose-fractionation on the ability of external beam radiation therapy to augment NK cell responses to tumors, with the goal of identify ways of better leveraging the therapeutic potential of NK cells with cancer immunotherapy.
For this study, established AT-3-OVA mammary tumors growing orthotopically in wildtype C57BL/6 mice were locally irradiated with different radiation dose-fractionations to dissect the effects of radiation dose per fraction (DPF) from total radiation dose (biological effective dose, BED). Flow cytometry and RNA sequencing were used to examine the differential effects of radiation dose-fractionation on the tumor immune microenvironment. Measurement of tumor growth post-irradiation in mice treated with NK or CD8 T cell depleting antibodies reveal the biological implications caused by radiation induced changes to the NK and T cell compartments of AT-3-OVA tumors.
We have demonstrated that radiation dose-fractionations are not equivalent in their capacity to evoke local effector CD8+ T and NK cell responses. Treatment with the 3x4Gy, 9x4Gy and 3x8Gy regimens elicited an effective CD8+ T cell responses that supported the anti-cancer actions of anti-PD-1 therapy. The high DPF regimen of 1x20Gy, despite having a similar BED to 9x4Gy was ineffective at priming host CD8+ T cell responses. In contrast, only the higher BED regimens (36-45Gy) could evoke an NK cell response that significantly slowed AT-3-OVA tumor growth. While all radiation regimens tested promoted an early increase in tumor-associated NK cell numbers, only the higher BED regimen supported a sustained elevation in tumor-associated NK cell numbers with a corresponding enrichment of the IFN-γ production gene signature. These early and late...