Plasmacytoid dendritic cells, tumor-associated macrophagesand myeloid-derived suppressor cells secreting anti-inflammatory cytokines andexpressing immunosuppressive metabolic enzymes (such as inducible nitric oxidesynthase (iNOS), indoleamine 2,3-dioxygenase (IDO), tryptophan 2,3-dioxygenase(TDO), and arginase) play an important role in the development of theimmunosuppressive tumor microenvironment [108, 109]. This evidence concerns the gene IDO1 and neoplasm.