PT - JOURNAL ARTICLE AU - Chen, Liye AU - Shi, Hui AU - Yuan, Jack AU - Bowness, Paul TI - Position 97 of HLA-B, a residue implicated in pathogenesis of ankylosing spondylitis, plays a key role in cell surface free heavy chain expression AID - 10.1136/annrheumdis-2016-209512 DP - 2017 Mar 01 TA - Annals of the Rheumatic Diseases PG - 593--601 VI - 76 IP - 3 4099 - http://ard.bmj.com/content/76/3/593.short 4100 - http://ard.bmj.com/content/76/3/593.full SO - Ann Rheum Dis2017 Mar 01; 76 AB - Objective Association of position 97 (P97) residue polymorphisms in human leucocyte antigen (HLA)-B, including HLA-B*27, with ankylosing spondylitis (AS) has recently been reported. We studied the effect of P97 variations on cell surface expression of the AS-associated HLA-B*27 and HLA-B*51, and the AS-protective HLA-B*7.Methods Flow cytometry was used to measure surface expression of HLA-B*27 in C1R/HeLa cells expressing HLA-B*27 (N97) and six mutants at P97 (N97T, N97S, N97V, N97R, N97W and N97D). Transporter associated with antigen processing-deficient T2, tapasin-deficient 220, β2m-deficient HCT15 and endoplasmic reticulum aminopeptidase 1 or β2m-clustered regularly interspaced short palindromic repeats/Cas9-knockout HeLa cells were used to provide evidence for specific protein interactions. Surface expression of HLA-B*7/HLA-B*51 P97 mutants was also studied.Results Mutation of HLA-B*27 P97 to the AS risk residue threonine increased cell surface free heavy chain (FHC) expression. Protective residues (serine or valine) and non-AS-associated residues (arginine or tryptophan) did not alter FHC expression. The N97D mutation reduced expression of conventional and FHC forms of HLA-B*27. Differences in FHC expression levels between HLA-B*27, HLA-B*27-N97T and HLA-B*27-N97D were dependent on the presence of functional β2m. HLA-B*7, which has an AS-protective serine at P97, expressed lower levels of FHC than HLA-B*27 or HLA-B*51. Introduction of asparagine at P97 of both HLA-B*7 and HLA-B*51 increased FHC expression.Conclusions The nature of P97 residue affects surface expression of HLA-B*27, B*7 and B*51, with AS-associated residues giving rise to higher FHC expression levels. The association of P97 amino acid polymorphisms with AS could be, at least in part, explained by its effect on HLA-B*27 FHC cell surface expression.