Figure 1.

IDO- and cell-mediated immunosuppression. Tryptophan is an essential amino acid that is catalyzed by the IDO enzyme into kynurenine. The kynurenine pathway is immunosuppressive in nature because the catabolites have inhibitory effects on lymphocytes, and the depletion of tryptophan leads to T cell cycle arrest and reduced proliferation. PD-1 and CTLA-4 are immune checkpoint molecules expressed on T cells, which also negatively regulate T cell immunity upon recognition of their cognate ligands on antigen-presenting cells. D-1-MT is an inhibitor of the IDO enzyme.