High-dose capsaicin exposure is associated with more severe inflammation, intestinal dysregulation, and IL-17 elevation in experimental autoimmune uveitis

Purpose To investigate whether capsaicin exposure increases the severity of experimental autoimmune uveitis (EAU) and to further characterize the associated alterations in intestinal homeostasis and immune regulation. Methods Mice were orally administered different doses of capsaicin (CAP) from the day of EAU induction through the peak of disease on day 21. EAU severity was evaluated by fundus imaging, clinical scoring, and retinal histopathological scoring. Intestinal morphological changes were assessed by hematoxylin and eosin staining, while intestinal barrier integrity was evaluated by detecting colonic ZO-1 and MUC2 expression using immunofluorescence staining and enzyme-linked immunosorbent assays. IL-17A levels in the colon and spleen were quantified by ELISA. Fecal 16S rRNA gene sequencing, untargeted metabolomics, and colonic RNA sequencing were performed to characterize alterations in gut microbiota, metabolic profiles, and inflammatory pathways. Based on the identification of reduced kynurenic acid (KYNA) levels, KYNA co-treatment was subsequently performed to evaluate its effects on ocular inflammation, IL-17A levels, and intestinal barrier-related alterations during high-dose CAP exposure. Results In the 80 mg/kg CAP group, retinal inflammation and tissue injury were more pronounced than in the other groups, accompanied by higher clinical and histopathological scores and increased IL-17A levels in the colon and spleen. High-dose CAP was also associated with ileal architectural abnormalities, reduced colonic mucosal thickness, and decreased colonic ZO-1 and MUC2 expression. 16S rRNA gene sequencing revealed alterations in gut microbial diversity and community structure. CAP-associated differential metabolites were enriched in the tryptophan metabolism pathway, and fecal KYNA levels were significantly reduced. KEGG over-representation analysis identified significant enrichment of the IL-17 signaling pathway among CAP-upregulated genes, whereas GSEA was not significant after multiple-testing correction, upregulation of Il17a , Il17f , and Rorc , and altered expression of genes involved in tryptophan metabolism. KYNA co-treatment reduced clinical and histopathologic scores, decreased IL-17A levels in the colon and spleen, and partially restored colonic ZO-1 and MUC2 expression. Conclusion High-dose CAP exposure during EAU induction was associated with increased disease severity, accompanied by disrupted intestinal homeostasis, reduced fecal KYNA levels, and enhanced IL-17-related inflammation. KYNA co-treatment partially attenuated CAP-associated ocular and intestinal alterations.