Immune microenvironment of experimental rat C6 gliomas resembles human glioblastomas

Glioblastoma (GBM) is the most aggressive primary brain tumor, with ineffective anti-tumor responses and a poor prognosis despite aggressive treatments. GBM immune microenvironment is heterogenous  and activation of specific immune populations in GBM is not fully characterized. Reliable animal models are critical for defining mechanisms of anti-tumor immunity. First we analyzed the immune subpopulations present in rat C6 gliomas. Using flow cytometry we determined kinetics of infiltration of myeloid cells and T lymphocytes into glioma-bearing brains. We found significant increases of the amoeboid, pro-tumorigenic microglia/macrophages, T helper (Th) and T regulatory (Treg) cells in tumor-bearing brains, and rare infiltrating T cytotoxic (Tc) cells. Transcriptomic analyses of glioma-bearing hemispheres revealed overexpression of invasion and immunosuppression-related genes, reflecting the immunosuppressive microenvironment. Microglia, sorted as CD11b⁺CD45^low cells from gliomas, displayed the pro-invasive and immunosuppressive type of activation. Accumulation of Th and Treg cells combined with the reduced presence of Tc lymphocytes in rat gliomas may result in the lack of effective anti-tumor responses. Transcriptional profiles of CD11b⁺ cells and composition of immune infiltrates in C6 gliomas indicate that rat C6 gliomas employ similar immune system evasion strategies as human GBMs.