Figure 2.

Modulation of kynurenine pathway in extrahepatic immune system. The activity of IDO can be stimulated by inflammatory cytokines and blocked by anti-inflammatory cytokines. The metabolites of KP catabolized by IDO play the suppressive role in the immune responses. They reduce activities of CD8+T cells and innate cells such as NKT cells, DCs and macrophages. They also inhibit Th1 cell proliferation. However, They promote Th2-phenotype polarization and Treg differentiation, as well as TGF-βproduction.