IDO2 and neoplasm: TAMs are well-known facilitators of tumor progression due to numerous growth factors and cytokines that promote tumor cell proliferation, invasiveness and angiogenesis [40], and immunosuppressive factors such as TGF-β, arginase 1, indoleamine 2,3-dioxygenase (IDO), IL-10 and programmed death ligand 1 (PD-L1).