IDO1 and neoplasm: Complex multi-factors derived from the tumor microenvironment, including VEGF, non-classical HLA class I, death ligands (FasL and TRAIL), pro-and-anti-inflammatory cytokines (TNF-α, IL-10, IL-1β, TGFβ, IL-8) and metabolites (e.g., IDO, ROS, RNS, NO), are directly responsible for DCs functional transformation [156,157,158,159,160].