Impaired expression of erythrocyte glycosyl-phosphatidylinositol-anchored membrane CD59 in patients with psoriatic arthritis. Relation to terminal complement pathway activation.

Objective. Complement-mediated injury is regulated by many factors; among these CD59 has been identified as a widely distributed glycoprotein that inhibits membrane C5b-9 (terminal complement component) formation. The aim of the study was to assess erythrocyte CD59 expression in patients with psoriatic arthritis in order to understand the role of CD59 in the pathogenesis. Methods. Washed erythrocytes from 50 patients with psoriatic arthritis, 8 with cutaneous psoriasis and 24 healthy subjects were incubated with monoclonal anti-CD59 antibody followed by a second FITC conjugated antibody and fluorescence intensity analysed by FAC-Scan flow cytometer to assess their CD59 membrane expression. SC5b-9 levels were measured in the plasma by ELISA and results compared with CD59 values. Immune complexes, complement C3 and C4 and rheumatoid factor were also determined. Results. Impaired expression of erythrocyte membrane-anchored CD59 was found in patients with psoriatic arthritis; the lowest levels were seen in active patients (p<0.01). Increased SC5b-9 was seen in the plasma of patients with active disease. An inverse correlation was also found between plasma C5b-9 and the CD59 expression levels (r = -0.81, p < 0.001). Conclusion. The low CD59 expression on erythrocytes from patients with psoriatic arthritis may be an index of a low tissue CD59 expression. This impairment could facilitate the activation of complement pathway and increase the risk for arthritis. Membrane attack complex formation in deficient membrane bound CD59 may also exacerbate synovial cell injury and inflammation.