Background/Purpose: Common variants in endoplasmic reticulum (ER)-associated aminopeptidase-1 (ERAP1) affect susceptibility to spondyloarthritis (SpA) in HLA-B27-positive human subjects, with loss-of-function/expression conferring protection. Using HLA-B27/human b2-microglobulin transgenic (HLA-B27 TG) rats, an experimental model of SpA, we have preliminary results that ERAP1 deficiency partially protects from the development of arthritis and orchitis, but not gut inflammation. The mechanism(s) underlying the protective effect of ERAP1 loss-of-function remain unknown. Here, we examined the gut microbiota of HLA-B27-TG rats with and without arthritis and with different ERAP1 genotypes, with the goal of identifying arthritis-associated gut microbes.

Methods: Cohorts of HLA-B27-TG rats with 3 ERAP1 genotypes (+/+, +/−, and −/−) were generated by crossing HLA-B27-TG Lewis (LEW) rats with Sprague-Dawley (SD) rats carrying 0, 1, or 2 ERAP1 null alleles. The mixed LEW-SD background (SDM) accelerates the onset of arthritis and orchitis, while gut inflammation remains unchanged. Animals were examined 2-3 times per week for the presence of arthritis and orchitis for up to 6 months and then euthanized. Gut inflammation in the cecum and colon was assessed by histological scoring. Microbial profiles were determined using DNA isolated from the cecum luminal contents, with 16S rRNA gene sequencing performed using Illumina MiSeq. Data were quality-filtered using Quantitative Insights Into Microbial Ecology (QIIME 2). Microbe relative frequency was determined at the species level (maximum >0.1%; p<0.05, q<0.1).

Results: HLA-B27 TG SDM rats with arthritis have a distinct gut microbial signature compared to HLA-B27 TG non-arthritic SDM rats as measured by principal component analysis, despite similar gut histology scores. Moreover, arthritis-associated microbes differ from HLA-B27-assocated dysbiotic microbes. Arthritis is associated with a significant loss of microbial alpha diversity, as well as differences in microbial community structure. HLA-B27 TG SDM rats with arthritis have increased Bacteroidesand Parabacteroidesat the expense of Firmicutes. At the species level, we see an increased relative frequency of certain microbes such as Akkermansia muciniphilaand Blautia, that are often associated with inflammation, whereas other inflammation-associated microbes ([Prevotella], Lachnospiraceae, [Ruminococcus] gnavus) were decreased in comparison with non-arthritic HLA-B27 TG rats. Further analysis based upon different ERAP1 genotypes also revealed ERAP1-associated differences on the microbial diversity and community structure.

Conclusion: In-depth analysis of HLA-B27 TG rats with and without arthritis has revealed the presence of arthritis-associated microbiota, which may explain why only some animals develop arthritis. These distinct arthritis-associated microbial communities will be further tested by microbiota transfer experiments in other genetic backgrounds. These results may provide valuable insights into the relationship between HLA-B27 and ERAP1, and their effects on arthritis and gut inflammation in experimental spondyloarthritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/epistasis-between-hla-b27-and-erap1-affects-gut-microbial-dysbiosis-and-arthritis-in-experimental-spondyloarthritis/