Effects of IgG Fraction from Antiphospholipid Syndrome Patients on the Activation of Coagulation Factor X

Antiphospholipid syndrome patients often suffer from thrombotic episodes, but the mechanism of thrombosis remains unclear. β2-Glycoprotein I (β2-GPI) is known to play an important role in the reaction between antiphospholipid antibodies (APA) and cardiolipin, but the true antigen for APA is not known. Although there are many laboratory tests to detect APA, none of them shows good correlation with the clinical severity of thrombosis. Moreover, cases of antiphospholipid syndrome vary in their APA types. To elucidate the interaction among APA, β2-GPI and phospholipids, we investigated the effects of the IgG fraction of APA on the activation of factor X by the using the chromogenic substrate S-2222.The IgG fraction from 2 of 4 patients dose-dependently inhibited the activation of factor X. β2-GPI also dose-dependently inhibited phospholipids and decreased the generation of Xa. To investigate the interaction between β2-GPI and IgG fractions, they were incubated together before adding phospholipids. The inhibitory effects in Xa generation were increased, and the effects did not seem to neutralize each other.