Radiation Therapy Increases Circulating Early-Stage Natural Killer Cells With Enhanced Cytokine Responsiveness in Breast Cancer.

Abstract

Although radiation therapy (RT) can modulate diverse immune cells, its effects on natural killer (NK) cells are poorly understood. Here, we aimed to investigate alterations in NK cells following RT. Peripheral blood samples from patients with breast cancer undergoing stereotactic body radiation therapy (SBRT) for bone metastasis, obtained before and after treatment, as well as spleens and tumors of the 4T1 breast cancer mouse model, were analyzed. Phenotypes and functional capacity of NK cells were assessed using flow cytometry, bulk RNA sequencing, and ex vivo functional assays. Although the proportion of peripheral blood NK cells was not significantly changed following SBRT in patients with breast cancer, transcriptomic analysis showed upregulation of early-stage NK cell gene signatures at 1 week (W1) post-SBRT compared with baseline. The increase in CD16^-CD57^- NK cells at W1 was verified by flow cytometry. In addition, the increase in Ki-67 expression at W1 compared with baseline was significant among CD16^-CD57^- NK cells. Importantly, interferon (IFN)-γ production by NK cells following interleukin -12/18 or interleukin-15 stimulation was significantly increased at W1 compared with baseline. In mouse models, splenic CD27⁺CD11b^- early-stage NK cells were more abundant in irradiated mice than in controls. Notably, CD27^-CD11b⁺ terminally differentiated NK cells and IFN-γ-secreting NK cells were increased among intratumoral NK cells in irradiated tumors after RT. RT increased cytokine-responsive early-stage NK cells in circulation, which was associated with increased terminally differentiated cells within tumors with high IFN-γ production capability. This finding reveals how NK cells are modulated following RT in breast cancer.