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. 2019 Apr 15;31(8):477–488. doi: 10.1093/intimm/dxz034

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© The Japanese Society for Immunology. 2019. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com

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Fig. 3. — Proposed model of dietary tryptophan-mediated regulation of anti-viral innate responses and immune homeostasis. An endogenous AHR ligand, kynurenine, is synthesized via oxidation of tryptophan, which is catalyzed by TDO and IDO1/2 in human liver and other organs. Constitutive AHR signaling activated by endogenous ligand(s), such as kynurenine, negatively modulates the type I interferon anti-viral response to prevent excessive responses. On the other hand, in the gut, some tryptophan metabolites such as kynurenine, IAld and I3S are generated by the bacterial catabolism of tryptophan, and act as AHR ligands to induce regulatory responses for the maintenance and function of ILC3 cells and the control of Treg and T_h17 differentiation.