IDO1 and neoplasm: The immunosuppression of tumour-infiltrating DCs can be facilitated by various soluble factors secreted in the TME such as IL-6, IL-10, IDO, M-CSF, transforming growth factor-β1 (TGF-β1), PGE2, VEGF (Figure 1) (84–91); although promisingly some of these defects in DC development or function have been proven to be reversible in pre-clinical models and clinical trials (27, 91–94).