Associations of microbial and indoleamine-2,3-dioxygenase-derived tryptophan metabolites with immune activation in healthy adults

Authors: RIAZATI, NIKNAZ - University Of California, Davis; Kable, Mary; Newman, John; Adkins, Yuriko; Freytag, Tammy; Jiang, Xiaowen; Stephensen, Charles

Submitted to: Frontiers in Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2022
Publication Date: 9/29/2022
Citation: Riazati, N., Kable, M.E., Newman, J.W., Adkins, Y.C., Freytag, T.L., Jiang, X., Stephensen, C.B. 2022. Associations of microbial and indoleamine-2,3-dioxygenase-derived tryptophan metabolites with immune activation in healthy adults. Frontiers in Immunology. 13. Article 917966. https://doi.org/10.3389/fimmu.2022.917966.
DOI: https://doi.org/10.3389/fimmu.2022.917966

Interpretive Summary: The amino acid tryptophan (Trp) is an essential nutrient. One of the roles of Trp in maintaining human health involves its conversion to active metabolites. One metabolite is kynurenine (Kyn), produced in the immune system via the indoleamine-2,3-dioxygenase (IDO) pathway. Other metabolites are produced by intestinal bacteria and include indole, indole acetic acid (IAA) and indole propionic acid (IPA). These metabolites have biological activity, including regulation of gene expression in immune cells. In addition, the ratio of Kyn to Trp (K:T ratio) in plasma is an index of IDO activity, which occurs largely in cells of the immune system. In the present study we examined the association of plasma concentrations of indole, IAA, IPA and the K:T ratio from 362 healthy adults to markers of immune activation. In addition, we measured the association of indole, IAA and IPA to the relative abundance of bacterial taxa in the intestine with the goal of identifying bacteria that may produce or consume these metabolites. Our goal was to determine if these bacteria themselves, rather than just the Trp metabolites in question, were associated with markers of immune activation. In brief we found that indole and IAA were associated with natural killer (NK) T-cells in peripheral blood. NK T-cells are also prominent in intestinal immune tissue in close contact with intestinal bacteria and we speculate that indole and IAA from these bacteria may enhance NK T-cell numbers, which could provide protection against potentially damaging bacteria found in the intestine. Several bacterial taxa that were negatively associated with plasma indole, IAA or IPA concentrations and these taxa were found to be associated, in general, with higher levels of immune activation in peripheral blood suggesting that these bacteria, perhaps by decreasing the level of Trp metabolite availability to immune tissue, may enhance inflammation in the intestine. Finally, the K:T ratio was associated with markers of systemic inflammation and the acute phase response, including C-reactive protein, consistent with the role of the IDO pathway in regulating systemic immune activation. This work suggests that intestinal bacteria, perhaps by regulating available of indole, IAA and IPA to host immune cells, may play a novel role in regulating immunity, in addition to the known role of the IDO pathway in regulating systemic inflammation.

Technical Abstract: Background Tryptophan (Trp) metabolites from intestinal bacteria, including indole, indole acetic acid (IAA) and indole propionic acid (IPA), and from the mammalian indoleamine 2,3-dioxygenase (IDO) pathway, including kynurenine (Kyn), can regulate immune activation. IDO activity in vivo is reflected in the plasma Kyn/Trp ratio. In this study we hypothesized that (1) the plasma Kyn/Trp ratio, (2) plasma indole, IPA, and IAA concentrations, and (3) intestinal bacteria associated with indole metabolism, could all be associated with inflammation and immune activation markers in a study population of healthy adults. Methods A broad group of 88 immune markers was assessed using plasma, flow cytometric analysis of whole blood and of peripheral blood mononuclear cells (PBMC), and ex vivo culture of PBMC, in 362 healthy, fasting adults. Plasma Trp metabolites were also measured. Bacterial taxa from stool were identified by 16S rRNA gene analysis and those associated with plasma Trp metabolites were identified. Multiple linear regression analysis was used to identify statistically significant associations (adjusted for multiple comparisons) of these metabolites and taxa with immune markers. Results Several robust positive associations were found between Kyn/Trp and markers of inflammation including neopterin and IP-10, markers of type 1 immunity, TNF-a and C-reactive protein (CRP) indicating activation of the acute phase response, and the regulatory cytokine IL-10. Only one significant positive association was found between bacterial Trp metabolites and immune markers, a positive association between the sum of indole and IAA and natural killer (NK) T-cells, involved in type 1 immunity. However, three taxa that were negatively associated with bacterial Trp metabolites were associated with elevated markers of immune activation: the family Lachnospiraceae with higher lymphocyte counts, the genus Dorea with higher production of the type 1 cytokine IFN-' by T-cells in PBMC culture, and the genus Ruminococcus with higher production of the inflammatory cytokine IL-6 in PBMC cultures stimulated with bacterial lipopolysaccharide (LPS). Conclusions IDO activity, bacterial Trp metabolites and indole-associated commensal bacteria were all associated with markers of systemic immune activation, particularly involving inflammatory cytokines, the acute phase response and type 1 innate and adaptive immunity. These findings suggest sub-clinical levels of systemic and/or intestinal immune activity that might be precursors to adverse health outcomes related to chronic immune activation.