Background/Purpose: The HLA-B27/β2m transgenic rat is a leading model of Anklylosing Spondylitis (AS) and other B27-associated spondyloarthopathies. The development of both bowel and joint inflammation is dependent on the presence of intestinal bacteria and we have shown previously that adult HLA-B27/β2m rats have an altered intestinal microbiota.  In this study we sought to better define age-dependent changes to immune function, host gene expression and the development of dysbiosis in this system.

Methods: Ileal, cecal and colonic contents were collected from WT and HLA-B27/β2m+ rats during the post-weaning period (3 and 6 weeks), at disease onset (10 wks) and after the establishment of disease (16wks). Microbial community structure was determined by 16s sequencing and qRT-PCR. Mucosal and systemic Th1, Th17 and Treg responses were analyzed by flow cytometry. Flow cytometry was also used to determine the frequency of IgA-coated bacteria in intestinal contents.  Host expression of inflammatory cytokines and antimicrobial peptides (AMPs) was determined in intestinal tissue by qRT-PCR. Histopathological evaluation was performed on H&E stained intestinal and joint sections.

Results: Both HLA-B27/β2m rats and controls exhibited a highly age-dependent intestinal microbiota. The microbiota of these rats diverged from that of controls by 10wks of age. Akkermansia muciniphila was greatly expanded in the intestinal mucosa of transgenic animals. Notably, an inflammatory cytokine signature and elevated AMP expression during the post-weaning period preceded the development of clinical bowel inflammation and dysbiosis. An early and sustained expansion of the Th17 pool was specifically observed in cecal and colonic mucosa of HLA-B27/β2m rats, whereas FoxP3 +ve Treg and Th1 cells were either unchanged or decreased at these tissue sites. By contrast, CD4+FoxP3+ T cells were significantly expanded in both mesenteric lymph node and spleen of HLA-B27/ β2m rats. Strikingly, a vast expansion in the frequency of IgA-coated intestinal bacteria (30-fold over controls) was observed at a late stage (16 weeks) concurrent with the age of arthritis onset in these animals.

Conclusion: HLA-B27/β2m expression renders the host hyper-responsive to microbial antigens from infancy. Early activation of innate immunity and expansion of a mucosal Th17 signature is soon followed by dysbiosis in HLA-B27/β2m+ve animals. The study of mucosal immune response and dysbiotic changes to the intestinal microbiota strongly merit further study in both pre-diseased and diseased SpA patient populations. IgA-coating has been reported to identify colitogenic bacteria in CD patients. Efforts are ongoing to identify sIgA+ve bacteria observed in the presence of HLA-B27 expression and their arthritogenic potential.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/age-dependent-effects-of-hla-b272m-expression-on-host-immunity-and-the-intestinal-microbiota/