Background/Purpose: HLA-B27 and human b₂m expression in rats induces a spontaneous inflammatory disease resembling human spondyloarthritis (SpA). While aspects of rat SpA have been studied in detail, understanding of gut inflammation remains limited. We recently showed that HLA-B27 affects the gut microbiota in rats. Here, we aimed to determine the relationship between intestinal microbiota and the gut immune response in B27 rats developing SpA-like disease.

Methods: HLA-B27/human b₂m transgenic (B27) rats (33-3 transgene locus) on 3 genetic backgrounds (Dark Agouti, DA; Lewis, LEW; and Fischer, F344), and HLA-B7/human b₂m transgenic (B7) rats on the LEW background, were examined at 2, 3, and 6 months of age. In a total of 194 rats, microbiota in the ileum, cecum and colon was determined by 16S rRNA gene sequencing. Cecum and colon histology was scored for inflammation, and RNA from whole tissue was analyzed by RNA-Seq to assess gene expression. Differences in microbiota and host immune response were identified as a function of genotype, genetic background, and disease severity.

Results: DA rats are resistant to B27-induced gut inflammation, while it progresses with age in LEW B27, and occurs early and is more severe in F344 rats. LEW B7 rats remain unaffected. Disease severity correlates with increased relative abundance of Proteobacteria and Verrucomicrobia at the expense of Firmicutes. While disease-associated microbes are similar in B27 LEW and F344, their relative frequencies are background specific. For example, Clostridium and Akkermansia are abundant in F344, while Prevotella and Sutterella exhibit greater increases in LEW. B27 DA and B7 LEW animals exhibit different microbial profiles compared to their WT controls, yet neither develops disease. Metagenome predictions revealed perturbed glutathione and steroid hormone biosynthesis B27 LEW and B27 F344. A striking finding was that LEW and F344 rats harbor segmented filamentous bacteria (SFB) in the ileum, whereas SFB are absent from the DA background and B7 LEW rats. Transcriptome analysis revealed robust activation of the IL-23/IL-17 axis, with IFN-g as well as TNF-a pathways upregulated with concomitant downregulation of metabolic pathways in B27 LEW and F344 rats. Interestingly, DA B27 showed a transient upregulation of these pathways including IL-17, IL-22, and TNF-a, but not IFN-g. LEW B7 rats had a remarkably similar transcriptome to LEW WT rats despite dramatic differences in microbial communities.

Conclusion: Integrated analysis of the gut microbiota and host transcriptome on different genetic backgrounds provides an unprecedented picture of the relationship between HLA-B27-induced gut inflammation and microbial communities. Microbial dysbiosis varies with genetic background, but contributes to a common disruption of gut metabolic pathways with activation of the IL-23/IL-17 axis, and IFN/TNF signaling leading to gut inflammation. The lack of SFB in DA rats may protect B27 animals given the well-known requirement for SFB for Th17 development in the lamina propria. Understanding SpA-associated microbial communities may lead to a better understanding of HLA-B27-associated disease pathogenesis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/spondyloarthritis-pathogenesis-involves-interplay-between-gut-microbiota-and-genetic-background/