In vitro effect of anti-β2 glycoprotein I antibodies on P-selectin expression: a marker of platelet activation

Objective. Antiphospholipid antibodies (aPL) associated with thrombembolic events and/or pregnancy morbidity characterize the so-called antiphospholipid syndrome (APS). Beta2glycoprotein I (beta(2) GPI) represents the major target antigen for aPL, but the pathogenic role of anti-beta(2) GPI antibodies (a beta(2) GPI) is still unclear. Some authors assume they play a role in activating platelets. The effects of a beta(2) GPI antibodies on platelet P-selectin expression were evaluated in this study. Methods. A beta(2) GPI antibodies in the plasma of a pregnant APS patient were isolated by affinity chromatography during two different stages (catastrophic and quiescent) of the disease. Gel filtered platelets (100,000/mu l) from healthy volunteers were incubated with beta(2)-GPI (20 mu g/ml) and with different concentrations (5, 25 e 50 mu g/ml) of a beta(2) GPI antibodies. P-selectin surface expression on platelets was assessed by flow cytometry using a specific fluorescent antibody directed against P-selectin. Results. A beta(2) GPI antibodies induced platelet activation only in the presence of thrombin receptor activator for peptide 6 (TRAP-6), a platelet agonist, at a subthreshold concentration. A beta(2) GPI antibody enhancement on platelet surface P-selectin expression was stronger in the catastrophic than in the quiescent phase of the disease (47% versus 15%). Conclusions. TRAP-6 dependent platelet activation by a beta(2) GPI antibodies is consistent with the two hit pathogenetic hypothesis for thrombosis. A beta(2) GPI antibodies induce higher platelet P-selectin expression during the active rather than in the acute phases.