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 live-cell 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.
Copyright © 2014 by the American College of Rheumatology.