IDO1 and cancer: Next, we analyzed the expression of the major enzymes (COX2, IDO1, NOS2, ARG1) and immune checkpoints (CD-47/SIRPα [39], PD-L1/PD1 [40] and HLA class I/LILRB1 [41]), respectively, driving the immunosuppressive activity of tumor-infiltrating myeloid cells and hampering the ability of macrophages to eliminate cancer cells by phagocytosis (Figure 2C,D).