IDO1 and neoplasm: TAM2 and MDSCs are well known for their ability to suppress T cell-mediated tumor killing (Figure 3) since both can exert direct immunosuppressive effects using cell-cell contact (e.g., CD80 and PD-L1), secreted factors (e.g., IL-10 and TGF-β) and expression of enzymes (e.g., ARG-1, iNOS, IDO) (69–75).