Tryptophan Metabolism, Regulatory T Cells, and Inflammatory Bowel Disease: A Mini Review

Figure 2

The schematic representation of Trp metabolism and its influence on Tregs. Trp metabolism produces AhR ligands through KP and microbial-mediated degradation, which affects the generation of Tregs. The relationship between IDO and Tregs is bidirectional, because they can regulate each other via DCs. CTLA-4, GITR, IL-10, IL-35, TGF-β, and IFN-γ are main components of the regulatory responses. Trp: tryptophan; IDO: indoleamine 2, 3-dioxygenase; KP: kynurenine pathway; Kyn: kynurenine; Treg: regulatory T cell; AhR: aryl hydrocarbon receptor; ARNT: aryl hydrocarbon receptor nuclear translocator; FoxP3: forkhead box P3; IL-35: interleukin-35; IL-10: interleukin-10; TGF-β: transforming growth factor beta; GITR: glucocorticoid-induced TNF receptor; CTLA-4: cytotoxic T-lymphocyte antigen-4; DC: dendritic cell; 1-MT: 1-Methyl-tryptophan.