ERAP1 deficiency mitigates HLA-B27 misfolding and ER stress, and protects HLA-B27 transgenic rats from arthritis and orchitis

Abstract

Abstract Endoplasmic reticulum aminopeptidase 1 (ERAP1) is associated with ankylosing spondylitis (AS) in HLA-B27-positive individuals. ERAP1 optimizes peptide length for binding to MHC class I molecules. However, the role of HLA-B27-bound peptides in AS pathogenesis remains unclear. We investigated epistasis between ERAP1 and HLA-B27 in a rat model of spondyloarthritis (SpA). ERAP1-deficient rats generated by TALEN-mediated genome editing were crossed with disease prone HLA-B27 transgenic (B27 Tg) rats (33-3 transgene). ERAP1 deficiency reduced the prevalence of arthritis (4/34 in ERAP1−/− vs. 12/37 in ERAP1+/+; p<0.05) and testicular inflammation (6/34 vs. 15/37, p<0.05) in male B27 Tg rats. Wild type and HLA-B7 transgenic (B7 Tg) rats with and without ERAP1 remained healthy. Bone marrow-derived macrophages (BMDMs) from B27 Tg ERAP1−/− rats exhibited a 12-fold (p<0.05) increase in MARB4-reactive cell surface B27 complexes containing long peptides, a 30% reduction of total cellular unfolded B27 heavy chains (HC) (p<0.01), and a 32% increase in the ratio of folded/unfolded B27 HC (p=0.01). ERAP1 deficiency also reduced by 50% the formation of disulfide-linked dimers and B27-BiP complexes (p<0.01). ERAP1-deficiency attenuated BiP and CHOP induction and XBP1 splicing (50%, 27%, and 25%, respectively; p<0.01) caused by IFNg-induced B27 upregulation. These results indicate that ERAP1-deficiency is protective for arthritis development in 33-3 B27 Tg rats, and raise the possibility that alleviation of ER stress may contribute to protection, although further investigation is needed to define the mechanism.