Indoleamine 2, 3 dioxygenase (IDO1) differentiallyModulates Microbial Aromatic Amino Acid Metabolites and Colonic Inflammation in an Age-Dependent Manner

Background and Objective: Indoleamine 2,3-dioxygenase (IDO1) metabolizes tryptophan (Trp) into kynurenine (Kyn), an immunosenescence pathway linked to immune dysfunction in aging, though its mechanism remains unclear. Kynurenine activates aryl hydrocarbon receptor (AHR) which further regulates immunity and gut homeostasis by shaping the microbiota. H350A transgenic mice that express IDO1 but are incapable of IDO1 Trp metabolism show distinct immune responses compared to constitutional IDO1 knockout (IDOKO) mice, confirming that IDO1 possesses both enzyme- and non-enzyme-dependent functions. Gut microbes also metabolize aromatic amino acids including Trp, tyrosine (Tyr), and phenylalanine (Phe) into bioactive aryl metabolites that influence IDO1-driven immune responses. We examined how age and IDO1 activity interact to influence the gut microbiome, microbrial-derived aryl metabolites, and colonic inflammation across the lifespan. Hypothesis: Microbial aryl metabolites are lower in WT mice as compared to IDOKO and H350A (IDO1 enzyme null) mice, and in-turn, intestinal inflammation is increased in WT as compared to IDOKO and H350A mice during advanced age. Methods: Serum, ileal content, and colonic content of young 12 –22-week-old WT, H350A, and IDOKO mice, (n=6-8/group) and old 114-138 week old (n=10/group) wwere analyzed for microbial aryl metabolites by LC/MS/MS. Microbe compositionof ileum and colon contents were assessed by 16S rRNA sequencing. Inflammatory markers were assessedin ileum and colon tissues by multiplex qPCR. Results: Tryptophan-derived microbial aryl metabolites, indole-lactate and indole-acetatein aged WT mice as compared to IDOKO or H350A mice (p<0.05).Immune-modulating Tyr-derived 4-hydroxyphenyllactic acid (4-HPLA) and Phe-derived phenyllactate (PLA), were also increased in the serum of aged IDOKO and H350A mice (p<0.05) as compared to aged WT mice. A known microbial derived anti-inflammatory aryl metabolite producer, Bifidobacterium sp, was increased in the colon of young and aged IDOKO mice and also in the ileum of H350A mice compared to WT mice. In contrast, the pro-inflammatory pathobiont, Helicobacter hepaticus, was elevated in the colon of both IDOKO and H350A aged mice compared to aged WT mice, correlating to increased colonic inflammatory markers (Lbp, Tnfrsf1b, S100a8, S100a9, and Ccr2). Conclusion: Enzymatic and non-enzymatic activity of IDO1 influences microbial-derived aryl production, microbiota composition, and colonic inflammation, with effects more pronounced in aging. IDO1 deficiency (genetic/metabolic) alters gut microbial metabolism, with protective consequences depending on microbiota composition.Ongoing studies are examining the role of AHRin mediating IDO1-dependent and independent regulation of gut microbiota and inflammation. This work was supported in part by National Institutes of Health (NIH) grants R01NS097851 (D.A.W.), K02AG068617 (D.A.W.), R01NS129835 (D.A.W.), RO1DK131133 (J.M.A) This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.