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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 (β2GPI) represents the major target antigen for aPL, but the pathogenic role of anti-β2GPI antibodies (aβ2GPI) is still unclear. Some authors assume they play a role in activating platelets. The effects of aβ2GPI antibodies on platelet P-selectin expression were evaluated in this study.


METHODS
Aβ2GPI 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/μl) from healthy volunteers were incubated with β2-GPI (20 μg/ml) and with different concentrations (5, 25 e 50 μg/ml) of aβ2GPI antibodies. P-selectin surface expression on platelets was assessed by flow cytometry using a specific fluorescent antibody directed against P-selectin.


RESULTS
Aβ2GPI antibodies induced platelet activation only in the presence of thrombin receptor activator for peptide 6 (TRAP-6), a platelet agonist, at a subthreshold concentration. Aβ2GPI 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 aβ2GPI antibodies is consistent with the “two hit” pathogenetic hypothesis for thrombosis. Aβ2GPI antibodies induce higher platelet P-selectin expression during the active rather than in the acute phases.