Role of β2-glycoprotein I and anti-phospholipid antibodies in activation of protein C in vitro

Abstract

Aims-To investigate the effect of β2- glycoprotein I (β2GPI) on the thrombin/ thrombomodulin dependent activation of protein C; and to determine whether β2 GPI dependent anticardiolipin antibodies have any effect. Methods-Protein C was activated by thrombin in the presence of thrombomodulin and phospholipid vesicles in an in vitro system. The effect of adding purified β2 GPI to this system was observed. Affinity purified anticardiolipin antibodies and total IgG from patients with anticardiolipin antibodies and the lupus anticoagulant were studied for their effects on protein C activation in the presence and absence of β2GPI. Results-β2-Glycoprotein I had no effect on the activity of preformed activated protein C. When the phospholipid vesicles were incubated with β2 GPI before the addition of protein C, the activation of protein C was inhibited in a dose dependent manner. With phosphatidylserine: phosphatidylcholine vesicles at a concentration of 1 4μM:2 μM, β2 GPI began to inhibit the reaction at a concentration of 15 nM, and at 4μM (the normal plasma concentration) the activation of protein C was reduced to 40%. Anticardiolipin antibodies had no demonstrable effect. Conclusions-β2-Glycoprotein I inhibits protein C activation in an in vitro system. Its physiological role is unknown but it has potential procoagulant as well as anticoagulant properties. An effect of antiphospholipid antibodies on protein C activation, which might explain their association with thrombosis, could not be shown.