Notably, upregulation of IDO1 or TDO2 enzymes by tumor cells, stromal cells and/or mononuclear phagocytes in the TME results in activation of Trp catabolism, depriving T cells of the essential amino acid Trp, and, at the same time, generating Trp metabolites that are toxic to T cell responses [51] or are able to induce Treg-cell differentiation or immunosuppressive function of immature myeloid cells [52]. Here, TDO2 is linked to neoplasm.