IDO1 and glioma: The immunosuppressive subgroup characterized by the high expression of indoleamine 2,3-dioxygenase 1 (IDO1; [15]) and low expression of MHC class II molecule (HLA-DR) was identified in rGBMs as one sort of mononuclear phagocytes, glioma-associated microglia, and macrophages (GAMs) [14].