Background/Purpose: Common endoplasmic reticulum (ER) aminopeptidase 1 (ERAP1) haplotypes influence the risk of developing axial spondyloarthritis (SpA) in HLA-B27-positive individuals, with loss-of-function and reduced expression conferring protection. ERAP1 increases the pool of short peptides, which are thought to be optimal for binding to most HLA class I molecules. The role of B27 in this model is not clearly defined, although misfolding and expression of cell surface dimers have both been implicated, while CD8+ T cells appear to be dispensible. Thus, the role of peptides in axial SpA remains unclear. We investigated the interaction between ERAP1 and HLA-B27 in the rat model of SpA.

Methods: ERAP1-deficient rats generated by genome editing were crossed with disease prone HLA-B27 transgenic (B27 Tg) rats (33-3 Tg locus), and B7 Tg rats as controls. The phenotype was monitored for up to 6 months. Colitis was assessed by stool and histology scores. Bone marrow-derived macrophages (BMM) were used to examine HLA class I by FACS and immunoprecipitation with conformation-specific antibodies. ER stress was assessed by quantitative PCR.

Results: ERAP1 deficiency reduced the prevalence of arthritis and orchitis by 64% and 58%, respectively (p<0.05), in male B27 Tg rats. Clinical colitis was unaffected, while histology scores were slightly increased in rats lacking ERAP1 (p<0.05). Wild type and B7 Tg rats with and without ERAP1 remained healthy. BMM from B27 Tg ERAP1-/- rats exhibited a 12-fold increase in cell surface B27 displaying long peptides (MARB4), a 40% and 50% increase in free heavy chains (FHC) and FHC dimers of B27, respectively (p<0.05), while folded complexes (ME1) were increased 25% (NS). Immunoprecipitation of total cellular folded and FHC revealed that ERAP1 deficiency reduced the pool of B27 FHC by 30%, including a 50% reduction in misfolded disulfide-linked and BiP-bound dimers, and increased the ratio of folded/free HC by 56% (p<0.05 for each). In contrast, for B7, total cellular FHC were increased 3-fold (p<0.05) by ERAP1 deficiency, and the ratio of folded/free HC was reduced by 60% (p<0.05). B27 upregulation results in the accumulation of misfolded, disulfide-linked dimers. ERAP1-deficiency resulted in a significant reduction in the accumulation of misfolded and BiP-bound B27 during upregulation. This was associated with attenuated activation of the unfolded protein response, with upregulation of Hsp5a and Ddit3 reduced by up to 50%, and attenuated Xbp1 splicing (p<0.05).

Conclusion: ERAP1-deficiency protects B27 Tg line 33-3 rats from arthritis and orchitis, without reducing the severity of gut inflammation. We demonstrate that ERAP1 deficiency promotes B27 folding and reduces misfolding and its consequences, while increasing the expression of cell surface dimers and aberrant long peptide complexes. Interestingly, ERAP1 deficiency impairs the formation of folded B7 complexes, consistent with the importance of short peptides for most other HLA alleles. Together, these results suggest that the preservation of long peptides in the absence of ERAP1 may rescue B27 from misfolding, and identify ERAP1 as a potential target for therapeutic intervention in SpA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/erap1-deficiency-implicates-long-peptides-in-protection-from-hla-b27-induced-experimental-spondyloarthritis/