Immunomodulatory roles of myeloid cells in gliomas

Abstract Myeloid cells represent the most abundant immune cell-type infiltrating glioblastoma (GBM). These myeloid cells are a heterogeneous population with primarily tumor-promoting functions. Macrophage activation status has been shown to span a broad spectrum of phenotypes that are highly dependent on the stimuli present in a specific tissue microenvironment. The paradigm defining macrophages as either classically activated antitumor M1 macrophages or immunosuppressive M2 macrophages provides points of reference; however, recent data indicate the differentiation of GBM-infiltrating macrophages is a continuous process, characterized by the large numbers of relatively immature myeloid cells that can express both canonical M1 and M2 markers at the same time. These cells are likely to contribute to the outcomes of GBM patients treated with immune therapies given their high abundance and immunosuppressive functions.