HLA-B27 subtypes predisposing to ankylosing spondylitis accumulate in endoplasmic reticulum-derived compartment apart from the peptide-loading complex.

Objective It was previously shown thatHLA-B27 subtypes predisposing to spondyloarthritis (SpA), i.e., B*27:02, B*27:05, and B*27:07, displayed an increased propensity to form intracellular oligomers and to accumulate at a high density in cytoplasmic vesicles, as compared to the non-SpA-associatedHLA-B*07:02 andHLA-B*27:06. This study was undertaken to characterize the nature and content ofHLA-B-containing vesicles and to further examine their relevance to SpA predisposition. Methods Vesicles containingHLA-B proteins were detected in transfected HeLa cells and in cells from SpA patients orHLA-B27/human beta(2)-microglobulin (h beta(2)m)-transgenic rats, by microscopy. The nature and content ofHLA-B-containing vesicles were characterized in colocalization experiments with appropriate markers. Results The SpA-associatedHLA-B*27:04 subtype accumulated at higher levels (P< 10(-5)) in cytoplasmic vesicles compared toHLA-B*27:06, from which it differs only by 2 substitutions, reinforcing the correlation between vesicle formation and SpA predisposition. Colocalization studies showed that those vesicles contained misfoldedHLA-B heavy chain along with beta(2)m and endoplasmic reticulum (ER) chaperones (calnexin, calreticulin, BiP, glucose-regulated protein 94-kd) and belonged to theERbut were distinct from the peptide-loading complex (PLC). Similar vesicles were observed in immune cells fromHLA-B27+ SpA patients, in greater abundance than in healthy controls (P< 0.01), and in dendritic cells fromHLA-B27/h beta(2)m transgenic rats, correlating with SpA susceptibility. Conclusion Accumulation of misfoldedHLA-B heavy chain along with beta(2)m andERchaperones intoER-derived vesicles distinct from thePLCis a characteristic feature ofHLA-B27 subtypes predisposing to SpA. This phenomenon could contribute toHLA-B27 pathogenicity, via a noncanonical mechanism.