IDO1 and neoplasm: They are an important group that maintains the tumor immunosuppressive microenvironment by expressing Arg1, iNOS, ROS, TGF-β, indoleamine-2,3-dioxygenase 1 (IDO), and PD-L1 to inhibit T cell proliferation.37,123,127 PMN-MDSCs are immature neutrophils, and it is difficult to distinguish PMN-MDSCs from the protumor phenotype neutrophils (N2s) because both are identified as CD11b+LY6G+LY6Clow cells in mice.