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.
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.
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.
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.