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. 2019 Mar 29;10:607. doi: 10.3389/fimmu.2019.00607

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Copyright © 2019 Cheng, Ning, Chen and Ma.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Gut microbiota enhance the expression of IL-22 against invading pathogens. IL-22 is important in maintaining the integrity of mucosal barriers and can be produced by many different types innate immune cells, which induce the expression of various antimicrobial proteins, including lipocalin-2, calprotectin, C-type lectins, S100A8, S100A9, and so on to clear pathogens. Increasing evidence identified that gut microbiota enhances the expression of IL-22 to protect the host against pathogens. The results show that gut microbiota utilize tryptophan as an energy source and produce a metabolite, indole-3-aldehyde (IAld), which in turns activated Ahr on innate immune cells. Once activated, innate immune cells will secrete IL-22, which protects the host against opportunistic pathogens by reducing their colonization.