Polymorphisms in the F pocket of HLA-B27 subtypes strongly impact on assembly, chaperone interactions and heavy chain misfolding

Objective - HLA-B27 is associated with the inflammatory spondyloarthropathies (SpAs). Of significance, subtypes HLA-B*27:06 and HLA-B*27:09 are not associated with the SpAs. These subtypes primarily differ from the HLA-B*27:05 disease associated allele at residues 114 and 116 of the heavy chain, part of the F pocket of the antigen-binding groove. Dimerisation of HLA-B27 during assembly has been implicated in disease onset. This study investigated the factors influencing differences in dimerisation between disease associated and non-associated HLA-B27 alleles.

Methods – HLA-B*27:05 and mutants resembling the HLA-B*27:06 and 09 subtypes were expressed in the rat C58 T cell line, the human CEM T cell line and its calnexin deficient variant CEM.NKR. Immunoprecipitation, pulse chase, flow cytometry and immunoblotting were performed to study the assembly kinetics, heavy chain dimerisation and chaperone associations.

Results - By expressing HLA-B*27:05, 06-like and 09 alleles on a restrictive rat TAP peptide transporter background, we demonstrate that a tyrosine expressed at p116 or together with an aspartic acid residue at p114 inhibited HLA-B27 dimerisation and increased the assembly rate. F pocket residues alter the associations with chaperones of the early MHC class I folding pathway. Calnexin was demonstrated to participate in endoplasmic reticulum (ER) stress mediated degradation of dimers, whereas the oxidoreductase ERp57 does not appear to influence dimerization.

Conclusion - Residues within the F pocket of the peptide-binding groove differing between disease-associated and non-disease-associated HLA-B27 subtypes can influence the assembly process and heavy chain dimerisation, events which have been linked to the initiation of disease pathogenesis.

© 2016 American College of Rheumatology. This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as an ‘Accepted Article’, doi: 10.1002/art.39948