Background/Purpose: HLA-B27 (B27) confers a strong predisposition to spondyloarthritis (SpA), but accounts for < 25% of overall heritability of ankylosing spondylitis. Rats transgenic for HLA-B27 and human beta-2-microglobulin (B27-TG) develop SpA-like disease with gut inflammation and arthritis. Common variants in ERAP1, which trims peptide cargo for HLA class I molecules in the ER, have been associated with SpA in B27(+) and some B27(-) individuals, although the influence on heritability is much smaller than for B27. Protective variants have been associated with lower enzymatic activity and lower expression of ERAP1, although analysis of entire allotypes suggests that many AS patients express very active variants. The objective of this study is to determine how ERAP1 loss-of-function affects SpA-like disease in rats.

Methods: The first exon of the rat ERAP1 gene was edited using TALEN technology (Transposagen) causing a deletion of 29-nucleotides. The resulting frame-shift results in 3 in-frame stop codons in the first exon. ERAP1 protein expression was assessed by Western blotting, which revealed complete lack of detectable protein in ERAP1-/-, and about 50% expression in ERAP1+/- rats. ERAP1-/- or ERAP1+/- Sprague-Dawley (SD) rats (disease permissive background) were crossed with B27-TG Lewis rats to generate cohorts of B27-TG with ERAP1+/+, ERAP1+/-, and ERAP1-/- genotypes. B7-TG rats and WT rats with ERAP1+/+, +/- or -/- were used as controls. Rats were observed for arthritis, fecal scores and other disease manifestations for up to 6 months of age. Gut inflammation was accessed by histology score of gut collected from 2, 4 and 6 months old rats.

Results: B27+ males with either ERAP1+/+ or ERAP1+/- genotype developed arthritis with a frequency of 29% (5/17), and 36% (8/22), respectively, with comparable arthritis severity scores (2.0) and age of onset (4.1 months), whereas only 6.7% (1/15) of rats with an ERAP1-/- genotype developed arthritis (P=0.02). Only 13.4% of B27+ERAP1+/- females developed arthritis with arthritis score of 1.25, while ERAP1+/+ and ERAP1-/- females remained healthy. Furthermore, B7-TG and WT rats were healthy regardless of the ERAP1 genotype, except one WT ERAP1+/- male that developed arthritis. All B27+ rats developed gut inflammation as assessed by histology, with increased fecal scores, in animals that were ERAP1+/- or ERAP1-/- with an increase in frequency (i.e. 3/16 (19%) for ERAP1+/+, 6/16 (38%) for ERAP1+/-, and 3/9 (33%) for ERAP1-/-), and histology scores were slightly higher in the complete absence of ERAP1 .

Conclusion: Complete absence of ERAP1 protected B27-TG rats from arthritis, but led to an increase in gut inflammation, suggesting different effects on pathogenic mechanisms involved in arthritis and colitis. B27-TG rats heterologous for ERAP1 exhibited higher incidence of arthritis concomitant with lower inflammation in the gut as compared to ERAP1-/- .This novel model provides an opportunity to better understand the mechanism(s) by which HLA-B27 contributes to SpA pathogenesis

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/erap1-deficiency-protects-hla-b27-transgenic-rats-from-arthritis/