Increased level of HLA-B27 expression in ankylosing spondylitis patients compared with healthy HLA-B27 positive subjects: a possible further susceptibility factor for the development of disease

Objective. In B27 transgenic rats, susceptibility to the development of a spondyloarthropathy‐like disease has been shown to correlate with the level of B27 transgene expression on lymphoid cells. The aim of this work was to study HLA‐B27 molecule expression in peripheral blood mononuclear cells (PBMCs) from patients with ankylosing spondylitis (AS) and from normal controls (NC). Methods. Twenty B27+ AS patients and 16 B27+ NC were studied. HLA‐B27 whole molecules and free heavy chains (HCs) and total HLA class I molecules were evaluated at the surface of PBMCs by immunofluorescence and flow cytometry. B27 subtypes were defined with the PCR‐SSP (polymerase chain reaction–sequence‐specific primer) technique. Cellular activation was evaluated by the expression of CD69, CD25 molecules and interferon γ (IFN‐γ) production. Results. B27 expression was 55 536.3±18 961.0 MESF (molecules of equivalent soluble fluorochromes) units in AS and 25 936.0±12 117.5 MESF in NC (P=0.00009), total HLA class I expression was 448 840.2±136 293.8 MESF in AS and 533 494.4±232 931.1 MESF in NC (not significant), HC expression was 10 593.4±6396.1 MESF in AS and 14 843.0±7544.2 MESF in NC (not significant). The higher B27 expression in the SA group was not due to higher cell activation as it was not correlated with CD69 and CD25 expression in PBMCs or with the level of IFN‐γ. HLA‐B27 expression did not correlate with indexes of disease status [Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Bath Ankylosing Spondylitis Functional Index (BASFI) and Bath Ankylosing Spondylitis Metrology Index (BASMI)]. Conclusions. We found greater expression of HLA‐B27 molecules in patients with AS than in healthy subjects. This phenomenon was not accompanied by general up‐regulation of HLA class I molecules or by greater expression of classical T‐cell activation markers. On this basis we propose that the higher expression of the HLA‐B27 molecules is a further predisposing factor for the development of AS.