TDO2 and neoplasm: Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are expressed in tumor cells and promote immunosuppression via the recruitment of immunosuppressive cell subsets, such as T regulatory cells (Tregs) and myeloid derived suppressor cells (MDSC) and trough secretion of inhibitory cytokines and growth factors (IL-6, IL-10, and TGF-β) [44].