The Role of Myeloid-derived Suppressor Cells in High-dose Irradiated TRAMP-C1 tumors: A Therapeutic Target and an Index for Assessing Tumor Microenvironment

Abstract

To investigate the chronological and spatial responses of myeloid-derived suppressor cells (MDSCs) to high-dose irradiation in TRAMP-C1 tumor and explore their potential role in tumor re-growth after irradiation. Murine prostate TRAMP-C1 tumors were irradiated with a single dose of 25Gy (SD-RT). Using immunohistochemical staining (IHC) and flow cytometry to verify the spatial distribution and dynamic mobilization of MDSCs. Blood and tumor lysate were collected for multiplex immunoassay. Furthermore, mice were injected Gr-1 antibody to verify the role of MDCSs in tumor re-growth after irradiation. Recruitment of Gr-1+ cells into irradiated tumors rapidly occurred in 4 hours and persisted up to 2 weeks. Flow cytometry showed that the main subpopulations of recruited MDSCs within the tumor are CD11b+Ly6G+Ly6C+ polymorphonuclear-MDSCs (PMN-MDSCs) and CD11b+Ly6G-Ly6Chi monocytic-MDSCs (M-MDSCs), not CD11b+Ly6G-Ly6C- tumor-associated macrophages (TAMs). The percentage of PMN-MDSCs and M-MDSCs was also increased in peripheral blood after radiation therapy. At two weeks after irradiation, levels of multiple cytokines, including GM-CSF, G-CSF, CCL-3, CCL-5, CXCL-5, IL-6, IL-17α, and VEGF-a were increased in the irradiated tumors; and levels of G-CSF, IL-6 and TNF-α were increased in the blood. The PMN-MDSCs chronologically aggregated at the central necrotic region of chronic hypoxia. The infiltrated Ly6G + PMN-MDSCs dominantly possessed pro-angiogenic factor, matrix metalloproteinase-9 (MMP-9), and highly expressed arginase-1, an immunosuppressive molecule, at the necrotic area. The depletion of MDSCs by the Gr-1 antibody had no effects on sham-irradiated tumors, but significantly delayed the regrowth of irradiated tumor. PMN-MDSCs are actively involved in tumor responses to high-dose irradiation, and their spatial distribution and phenotype changes are associated with the evolution of chronic hypoxia in the irradiated tumors. PMN-MDSCs could become a therapeutic target. Checking the MDSCs component and cytokine levels in peripheral blood could be used as an index for assessing responses in the tumor microenvironment.