Mechanism and effects of the binding of lupus anticoagulant IgG and prothrombin to surface phospholipid

We report here experiments on how lupus anticoagulant antibodies (LA IgG) that react with prothrombin bind to surface phospholipid and affect prothrombin's affinity for surface phospholipid and activation to thrombin. LA IgG was purified by protein A chromatography from the plasma of 16 patients of whom four had associated hypoprothrombinemia and 10 had experienced thrombosis. Many LA IgG bound, in the absence of phospholipid and calcium, not only to immobilized prothrombin but to both prothrombin 1 and fragment 1, which established at least an oligoclonal origin of LA IgG. No LA IgG bound to thrombin. Although prothrombin and Ca2+ were required to support binding of LA IgG to immobilized phosphatidylserine (PS), prothrombin at higher concentrations inhibited binding, presumably by competing with prothrombin/LA IgG complexes for PS binding sites. Prothrombin 1, which cannot bind to PS, also inhibited binding of many LA IgG to PS, presumably by forming competing soluble prothrombin 1/LA IgG complexes. Despite their ability to react with prothrombin independent of phospholipid, LA IgG enhanced binding of prothrombin to immobilized phospholipid and to cultured human umbilical vein endothelial cells. Prothrombin bound with LA IgG to the surface of endothelial cell monolayers could be activated to thrombin after supernatant prothrombin and LA IgG were washed away. The relation is discussed of these observations to a hypothesis that LA IgG mediated concentration of prothrombin on cell surface phospholipid represents a mechanism by which LA IgG could increase thrombotic risk.