Bacterial metabolites controlling Th17 and Treg cells

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Abstract Maintaining an equilibrium between inflammatory Th17 cells and anti-inflammatory Treg cells is critical to support intestinal barrier function and tissue homeostasis. Nuclear hormone receptors (NhRs) have been shown to play crucial roles in the development and function of key immune cells, including Th17 and Treg cells. Based on our prior work, we hypothesize that host-produced, bacterially modified steroids bind to host NhRs and modulate T cell differentiation and function. Specifically, we posit that secondary bile acids (microbial metabolites of host-produced primary bile acids) bind to host NhRs and modulate T cell differentiation and function. We previously demonstrated that two bile acid metabolites modulate T cell differentiation: 3-oxo- lithocholic acid (3oxoLCA) inhibits the differentiation of naïve T cells into inflammatory Th17 cells, and isoallo- lithocholic acid (isoalloLCA) enhances the differentiation of naïve T cells into anti-inflammatory Treg cells. Although we determined that 3oxoLCA inhibited Th17 cells by acting as a ligand for RORγt (retinoic acid receptor-related orphan nuclear receptor γ t), it is unknown whether isoalloLCA exerts its Treg cell-modulating activity by acting through NhR(s). In addition, it is likely that there are additional bile acids that modulate T cell responses. In preliminary work, we have identified an abundant bile acid metabolite, iso-lithocholic acid (isoLCA) that inhibits Th17 cell differentiation and function, as well as human gut bacteria that produce isoLCA and isoalloLCA. We have found that the levels of these metabolites are significantly decreased in the feces of human patients with Crohn’s disease compared to healthy controls. We propose to (1) identify human gut bacteria and bacterial genes responsible for the production of isoLCA and isoalloLCA, (2) determine the molecular mechanisms by which these compounds influence Th17 and Treg differentiation and function, and (3) investigate whether gut bacteria producing isoLCA and isoalloLCA affect host immune responses in vivo. Elucidating the pathways that produce immunomodulatory bile acids and their mechanisms of action will open up exciting avenues to study unique regulatory interactions between gut-residing microorganisms and host immune cells. This research will lay the groundwork for the development of new therapies to treat autoimmune diseases, including inflammatory bowel disease.