CD274 and glioma: Immune suppression is mediated primarily by glioma cells, myeloid-derived suppressor cells (MDSCs), Tregs, and tumor-associated macrophages and microglia (TAMMs) via the expression of immune checkpoint receptor ligands, such as PD-L1, and the secretion of immunosuppressive cytokines, such as TGF-β and IL-10 (44).