Radiotherapy and cGAS/STING signaling: Impact on MDSCs in the tumor microenvironment

•Radiotherapy affects immune populations in the TME, including MDSCs.•Conventional fractionated radiation schedules are associated with MDSC recruitment.•Radiotherapy, inflammation and hypoxia play a role in MDSC function and recruitment.•Both MDSC-intrinsic and -extrinsic STING signaling are able to alter MDSC behavior.•Future studies should focus on elucidating a potential role of cGAS in MDSC function. Myeloid derived suppressor cells (MDSCs) are a highly heterogeneous population of immature immune cells with immunosuppressive functions that are recruited to the tumor microenvironment (TME). MDSCs promote tumor growth and progression by inhibiting immune effector cell proliferation and function. MDSCs are affected by both novel anti-cancer therapies targeting the immune system to promote anti-tumor immunity, as well as by conventional treatments such as radiotherapy. Following radiotherapy, cytoplasmic double stranded DNA stimulates the cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) pathway, resulting in type I interferon production. Effectiveness of radiotherapy and cGAS/STING signaling are closely intertwined: activation of cGAS and STING is key to generate systemic anti-tumor immunity after irradiation. This review focuses on how radiotherapy and cGAS/STING signaling in MDSCs and/or tumor cells impact MDSC recruitment, expansion and function. The influence of conventional and ablative radiotherapy treatment schedules, inflammatory response following radiotherapy, and hypoxia are discussed as MDSC modulators.