HLA–B27 Subtype Oligomerization and Intracellular Accumulation Patterns Correlate With Predisposition to Spondyloarthritis

Abstract

Objective. Mechanisms underlying the striking association of spondyloarthritis (SpA) with the class I major histocompatibility complex molecule HLA-B27 remain poorly understood. SpA-like disease develops spontaneously in B*2705-transgenic rats, in conjunction with high HLA-B27 expression levels. This study was undertaken to examine the effects of increased expression of HLA-B27 alleles that are differentially associated with SpA on oligomerization and intracellular redistribution.Methods. HeLa cells were transfected with complementary DNA encoding for HLA-B proteins fused to yellow fluorescent protein and/or Renilla luciferase and harvested at an early phase and a later phase of expression. We monitored HLA-B intracellular trafficking and localization by means of microscopy and livecell imaging. Bioluminescence resonance energy transfer (BRET) and Western blotting were used to monitor HLA-B oligomerization.Results. At low expression levels, BRET signals were similarly elevated for all SpA-associated HLA-B27 alleles tested, but were lower for the nonassociated B*2706. Of note, at higher expression levels, HLA-B27 signals remained steady while signal for HLA-B7 decreased sharply, reaching the level observed for B*2706. This was due at least in part to a decreased oligomer proportion without unfolded protein response outbreak. Such differential behavior was not abrogated by proteasome inhibition. With increased expression, all HLA-B proteins accumulated to a high density in cytoplasmic vesicles with labile form and size. The extent of this phenomenon was closely correlated with the level of association with predisposition to SpA.Conclusion. To our knowledge, this is the first report of a correlation between the level of predisposition to SpA conferred by HLA-B27 alleles and their biochemical behavior. These findings open new perspectives for understanding the pathogenicity of HLA-B27.The class I major histocompatibility complex (MHC) molecule HLA-B27 is strongly associated with ankylosing spondylitis (AS) and other forms of spondyloarthritis (SpA), a group of related inflammatory rheumatic diseases characterized by axial and peripheral joint inflammation, sometimes with concomitant extraarticular manifestations including uveitis, psoriasis, and inflammatory bowel disease. Although this association was demonstrated 40 years ago (1,2) and has been extensively studied since then, its molecular basis remains largely unknown.

Ankylosing Spondylitis and Axial Spondyloarthritis

“…These substitutions affect the repertoire of bound peptides, biochemical and intracellular behaviors, and the conformational flexibility of the HLA-B27 heavy chain, and these features correlate with susceptibility to disease. [47][48][49] A recent study of single-nucleotide polymorphisms (SNPs) in the HLA region identified a small number of other statistically significant but weakly associated HLA class I and class II alleles (odds ratio for the alleles, 1.06 to 2.35). 50 The basis for the association between HLA-B27 and axial spondyloarthritis and ankylosing spondylitis remains unexplained.…”

The interplay between HLA-B27 and ERAP1/ERAP2 aminopeptidases: from anti-viral protection to spondyloarthritis

“…Moreover, the two structurally close but differently AS-predisposing B*2705 and B*2709 alleles have been shown to have similar intracellular trafficking and propensity to form oligomers [33]. However, more recently, it has been described a greater tendency of AS-associated B*2702, B*2705, B*2707 alleles to accumulate in 'dynamic' intracellular vesicles as misfolded/ aggregate proteins in comparison with the non-associated B*2706 allele [37]. This behaviour has been correlated with the B27 protein level.…”

Editorial: HLA–B27: The Story Continues to Unfold

“…Furthermore, recent studies by Jeanty et al showed increased oligomerization and intracellular accumulation of B*27:05 compared to B*27:06 when these heavy chains were transiently overexpressed in cells , which is precisely what one might predict if these 2 subtypes differ in their folding and/or unfolding properties. Of course, conformational flexibility could also be hypothesized to lead to autoreactivity.…”