Similar cell surface expression of β2‐microglobulin–free heavy chains by HLA–B27 subtypes differentially associated with ankylosing spondylitis

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Abstract
Objective To determine whether the cell surface features of HLA–B27 subtypes reported to be differentially associated with ankylosing spondylitis (AS) differ in a way that correlates with disease susceptibility. Methods Human cell transfectants expressing or lacking the transporter associated with antigen processing were used to determine the cell surface expression of B27 subtypes by flow cytometry with antibodies recognizing the B27 heterodimer or β2-microglobulin (β2m)–free heavy chains. Results In lymphoid cells with an intact peptide-loading complex, all B27 subtypes, irrespective of their association with disease, showed similar ratios of free heavy chain to heterodimer, suggesting similar surface stability. A substantial decrease in dissociated heavy chains, which never reached 100%, was observed upon addition of a B27 ligand, with no significant differences among subtypes. This is compatible with similar surface expression of irreversible β2m-free heavy chain forms among subtypes differentially associated with disease. In cells lacking the transporter associated with antigen processing, both disease-associated and non–disease-associated subtypes expressed a population of heterodimers at 26°C that was less stable than the population expressed at 37°C. In the presence of exogenous peptide, the expression of heterodimers increased, without a concomitant decrease in β2m-free heavy chains. This suggests that in these cells, and for all subtypes tested, most of the dissociated heavy chains at the cell surface are in irreversible forms. At 37°C, the expression of β2m-free B27 heavy chains was very low on T2 transfectant cells. Conclusion HLA–B27 subtypes showing differential associations with AS are similar in their extent of β2m dissociation and surface expression of free heavy chains.