Function of Beta2-GPI in Inhibiting the Oxidized Phospholipids Dependent APC Activity by Antiphospholipid IgG from APS Patients.

Beta2-GPI (ß2GPI), a plasma glycoprotein with phospholipid-binding properties, is the actual target antigen for autoimmune type antiphospholipid antibodies. Certain groups of these antibodies exert lupus anticoagulant (LA) activity by directly inhibiting the Protein C anticoagulant pathway and associated with an increased risk of thrombotic disease. Our previous studies have shown that the activated protein C (APC) activity and the ability to be inhibited by antiphospholipid antibodies associated with thrombosis are strongly augmented by the presence of Phosphatidyl-ethanolamine (PE) and phospholipid oxidation. In this study, we investigated the relationship between ß2GPI and anti-APC activity by IgG derived from APS patients.

IgG from patients with APS syndrome were tested using a modified oxidized PE containing phospholipids dependent dRVVT test, in the presence of APC at concentration of 0.2ug/ml, the clotting times was prolonged to 3 fold of the baseline level, when the anti-phospholipids IgG antibodies added into the system, its showed a significant impact on the APC prolongation clotting times in the presence of ß2GPI. However, no affect was observed on the APC prolongation clotting times in the absence of ß2GPI. To eliminate the possible confounding effect from other plasma component, we investigated the effect on factor Va inactivation by APC in a purified system. As expected, the inhibition was observed only when the B2GPI was present. We also found that the presence of beta2GPI-IgG complex had its strongest effect when Protein S was included.

We conclude that ß2GPI is necessary to inhibit the oxidized phospholipids dependent APC activity by IgG antibodies from APS patients. On oxidized lipids surface, ß2GPI -APS IgG complex had its strongest effect on APC activity. This is a potentially important result indicating the functional assay may be more sensitive for pathological antibodies.