Effect of antiphospholipid antibodies on procoagulant activity of activated platelets and platelet-derived microvesicles.

We have recently described the in vitro mechanism of action of anticardiolipin (aCL) and lupus anticoagulant (LA) antibodies in patients with the antiphospholipid syndrome. LA antibodies inhibit coagulation reactions in plasma because they appear to recognize the complex of lipid-bound (human) prothrombin, whereas aCL antibodies require beta 2-glycoprotein I (beta 2-GPI) for binding to anionic phospholipids. aCL antibodies can be divided into two subgroups, according to their behaviour in lipid-dependent coagulation reactions: aCL-type A enhances the anti-coagulant effect of beta 2-GPI, whereas aCL-type B does not. In the present study we investigated the effect of purified aCL-type A and B and of LA antibodies on the procoagulant activity of both Ca-ionophore activated platelets and platelet-derived microvesicles, using an assay system with highly purified bovine coagulation factors Xa, Va, and prothrombin from human and bovine origin. In the absence of beta 2-GPI neither type of aCL was able to inhibit the prothrombinase activity of platelets or microvesicles. However, a strong and dose-dependent inhibition of the prothrombinase activity of both platelets and platelet-derived microvesicles was observed within a few minutes, when aCL-type A antibodies were added in combination with beta 2-GPI. This inhibitory effect was dependent also on the concentration of beta 2-GPI. Conversely, no inhibitory effect of aCL-type B antibodies on platelet- (or microvesicle) prothrombinase activity in the presence of beta 2-GPI could be observed. LA antibodies were able to inhibit in a dose-dependent way the procoagulant activity of activated platelets and platelet-derived microvesicles. With two LA preparations this inhibition was only apparent when human prothrombin was used as substrate, while a third preparation exhibited its inhibitory effect both in the presence of human and bovine prothrombin. The data indicate that, in the presence of their respective cofactors beta 2-GPI and prothrombin, aCL and LA antibodies interact with the membrane of activated platelets and platelet-derived microvesicles in a very similar way as previously observed for their interaction with anionic phospholipid surfaces.