Radiotherapy (RT) has traditionally been seen as a means to induce targeted tumor cell death, and more than 50% of all cancer patients receive RT. RT is also known to induce immune cell activation, and the advent of immunotherapeutic treatments such as checkpoint inhibition has sparked increased interest in using RT to enhance an anti-tumor immune response. However, to take advantage of the immunological effects of RT, a deeper understanding of the effects of RT on tumor-infiltrating leukocytes (TILs) is essential. Therefore, we systematically analyzed the effects of RT on tumor growth and the tumor-infiltrating immune cells in five syngeneic tumor models with diverse immune cell-infiltration profiles. These tumor types show strong differences in the overall immune cell infiltration, as well as in the composition of the infiltrating immune cell populations. After RT, tumors that were characterized as hot (e.g. CT26) showed increased sensitivity and dose-dependent tumor growth inhibition compared to cold tumor models (e.g. B16F10). Immune cell profiling indicated that RT led to strong changes in TILs of some tumor types, such as MC38 and CT26. These changes included reduced fractions of macrophages and increases in NK cells and also CD8 T cells. Single cell RNA sequencing also revealed an increase in CD8 T cells expressing proliferation-related genes. Furthermore, macrophage clusters expressing markers of proliferation were specifically eradicated by RT, while monocytes and neutrophils were less affected. The monocytes and neutrophils in the models that showed little changes in TILs after RT expressed marker genes of type-I interferon response. These findings predict that tumors that are highly infiltrated by neutrophils and monocytes, with little intra-tumoral proliferation and type-I interferon response signature, are likely resistant to an RT-mediated anti-tumor immune response. The present work has laid a strong foundation to develop next-generation combinatorial treatments using RT and immunotherapy.

Citation Format: Tristan Wirtz, Catherine Lee, Tao Xie, Lisa Manzuk, Manfred Kraus, Christopher Dillon, Timothy Affolter, Anand Giddabasappa. Effects of targeted radiotherapy on tumor immune landscape in diverse murine tumor models [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2021 Oct 5-6. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(1 Suppl):Abstract nr P065.