FLT3L release by natural killer cells enhances response to radioimmunotherapy in preclinical models of HNSCC

Purpose: Natural killer (NK) cells are type I innate lymphoid cells that are known for their role in killing virally infected cells or cancer cells through direct cytotoxicity. In addition to direct tumor cell killing, NK cells are known to play fundamental roles in the tumor microenvironment through secretion of key cytokines, such as FMS-like tyrosine kinase 3 ligand (FLT3L). Although radiotherapy is the mainstay treatment in most cancers, the role of radiotherapy on NK cells is not well characterized. Experimental Design: This study combines radiation, immunotherapies, geneticmousemodels, and antibody depletion experiments to identify the role of NK cells in overcoming resistance to radiotherapy in orthotopicmodels of head and neck squamous cell carcinoma. Results: We have found that NK cells are a crucial component in the development of an antitumor response, as depleting them removes efficacy of the previously successful combination treatment of radiotherapy, anti-CD25, and anti-CD137. However, in the absence of NK cells, the effect can be rescued through treatment with FLT3L. But neither radiotherapy with FLT3L therapy alone nor radiotherapy with anti-NKG2A yields any meaningful tumor growth delay. We also identify a role for IL2 in activating NK cells to secrete FLT3L. This activity, we show, is mediated through CD122, the intermediate affinity IL2 receptor, and can be targeted with anti-CD25 therapy. Conclusions: These findings highlight the complexity of using radio-immunotherapies to activate NK cells within the tumor microenvironment, and the importance of NK cells in activating dendritic cells for increased tumor surveillance.