Abstract C036: Radiotherapy increases circulating early-stage natural killer cells with enhanced cytokine responsiveness in breast cancer

Abstract Purpose: Although radiotherapy (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. Methods: Peripheral blood samples from breast cancer patients undergoing stereotactic body radiotherapy (SBRT) for bone metastasis, obtained before and after treatment, as well as spleens and tumors of 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. Results: Although the proportion of peripheral blood NK cells was not significantly changed following SBRT in breast cancer patients, 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. Additionally, 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 (IL)-12/18 or IL-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. Conclusion: 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. Work supported by grant from the National Research Foundation of Korea (NRF-2023R1A2C3003782) Citation Format: In Ah Kim, Seong Hyck Jeon, Nawon Park, Yumin Kim. Radiotherapy increases circulating early-stage natural killer cells with enhanced cytokine responsiveness in breast cancer [abstract]. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr C036.