Additional human β₂‐microglobulin curbs HLA–B27 misfolding and promotes arthritis and spondylitis without colitis in male HLA–B27–transgenic rats

Abstract

Objective. Ankylosing spondylitis and related spondylarthritides are associated with HLA-B27, and also with intestinal inflammation, by unknown mechanisms. The folded HLA-B27 molecule is a trimer of heavy chain, ␤ 2 -microglobulin (␤ 2 m), and short peptide. However, B27 heavy chain has an unusual propensity to misfold and trigger the unfolded protein response (UPR). This study was initiated to test the hypothesis that B27 misfolding plays a role in the pathogenesis of spondylarthritis.Methods. Rats with high transgene copy numbers of HLA-B27 heavy chain together with human ␤ 2 m (Hu␤ 2 m) spontaneously develop colitis, peripheral arthritis, and occasional spondylitis, and those with lower transgene copy numbers remain healthy. We crossed disease-prone and healthy HLA-B27/Hu␤ 2 mtransgenic rat lines with a healthy line, 283-2, carrying only the Hu␤ 2 m transgene. HLA-B27 assembly was assessed by pulse-chase analysis of B27 molecules, and UPR triggering was assessed by measuring BiP/Grp78 messenger RNA (mRNA) in splenic concanavalin A blasts. Surface expression of B27 and Hu␤ 2 m was determined by flow cytometry. Disease manifestations were identified by clinical observation, histology, and measurement of cytokine mRNA.Results. The extra Hu␤ 2 m from the 283-2 line significantly reduced B27 misfolding and UPR triggering. Unexpectedly, however, F 1 male offspring of the healthy 21-3 line crossed with the 283-2 line showed a high prevalence, severity, and duration of arthritis and spondylitis, in the absence of colitis. The arthropathy showed many features characteristic of human spondylarthritis.Conclusion. These results suggest that B27 misfolding is associated with intestinal inflammation, but that neither B27 misfolding nor intestinal inflammation is critical to the development of B27-associated arthropathy.The spondylarthritides are a group of inflammatory rheumatic diseases characterized by axial and peripheral arthropathy and a variety of extraarticular lesions (1). In the prototype of these disorders, ankylosing spondylitis (AS), there is inflammation in the spinal and sacroiliac joints and in ligamentous attachments that, in severe cases, leads to bony fusion. The pathogenesis of spondylarthritis is poorly understood. Two factors strongly associated with these conditions are intestinal inflammation and the major histocompatibility complex (MHC) class I gene HLA-B27.With regard to the association with intestinal inflammation, the prevalence of inflammatory bowel disease (IBD) (both Crohn's disease and ulcerative colitis) is increased in individuals with AS. An even higher prevalence of IBD is found in individuals with other forms of spondylarthritis (2), and endoscopic studies with biopsy have shown a prevalence of microscopic inflammatory bowel lesions in 60-70% of patients

HLA–B27 up‐regulation causes accumulation of misfolded heavy chains and correlates with the magnitude of the unfolded protein response in transgenic rats: Implications for the pathogenesis of spondylarthritis‐like disease

“…This conclusion was based on the observation that overexpression of additional Hu␤ 2 m increased the incidence and severity of arthritis, while it reduced the tendency of HLA-B27 to misfold (15). Additional Hu␤ 2 m reduced the proportion of newly synthesized HLA-B27 heavy chains that formed disulfide-linked dimers by approximately half in Con A-treated splenocytes, and this was associated with a 30-50% reduction in levels of BiP mRNA (15).…”

“…This conclusion was based on the observation that overexpression of additional Hu␤ 2 m increased the incidence and severity of arthritis, while it reduced the tendency of HLA-B27 to misfold (15). Additional Hu␤ 2 m reduced the proportion of newly synthesized HLA-B27 heavy chains that formed disulfide-linked dimers by approximately half in Con A-treated splenocytes, and this was associated with a 30-50% reduction in levels of BiP mRNA (15). It seems unlikely, based on the results presented here, that these relatively modest effects of Hu␤ 2 m overexpression will be sufficient to prevent HLA-B27 misfolding and UPR activation in macrophages and possibly other cell types when HLA-B27 is up-regulated.…”

“…We observed little difference in BiP (and CHOP) expression in whole spleen and thymus from B27-transgenic rats (10), although we occasionally see small differences in older rats with active disease (Turner MJ, et al: unpublished observations). Tran et al reported that Con A-treated B27-transgenic splenocytes express ϳ1.5-fold HLA-B27 MISFOLDING AND THE UNFOLDED PROTEIN RESPONSEmore BiP mRNA compared with Con A-treated wildtype cells (15). Whether these differences represent sudden activation of the UPR in response to HLA-B27 up-regulation as observed in BMMs or, perhaps, baseline differences (i.e., long-term) in expression is not clear.…”

“…Recently, it was reported that overexpression of additional Hu␤ 2 m in rats with high transgene copy numbers of HLA-B27 heavy chain increased the frequency and severity of arthritis without influencing colitis (15). In rats with lower transgene copy numbers, which would otherwise remain healthy, additional Hu␤ 2 m caused arthritis without inducing colitis.…”

HLA-B27 Misfolding and Spondyloarthropathies

“…Although axial inflammation can occur, it does not recapitulate the spinal inflammation and ankylosis seen in humans. 18 However, recently Tran et al have reported that overexpressing additional hβ 2 m can alter the phenotype in transgenic rats that already overexpress HLA-B27 and hβ 2 m. 19 High copy number B27/hβ 2 m transgenics with additional hβ 2 m develop more severe arthritis and significant axial disease with no apparentchange in colitis. Interestingly, rats with low copy number B27/hβ 2 m that normally do not develop spontaneous disease, develop axial and peripheral arthritis without colitis, when additional hβ 2 m is overexpressed.…”