IDO1 and neoplasm: In addition to their direct tumor-promoting functions, both tumor-associated macrophages (TAMs) and tumor-associated neutrophils (TANs) may also suppress antitumor adaptive immune responses through the production of IL-10 and TGF-beta as well as the enzymes arginase 1 and IDO, which are detrimental for T cell-mediated immunity (13, 31).