Inhibition of lupus anticoagulant activity by hexagonal phase phosphatidylethanolamine in the presence of prothrombin

We have previously demonstrated that lupus anticoagulant antibodies from pa tients with systemic lupus erythematosus (SLE) specifically recognize hexag onal (II) phase phosphatidylethanolamine (PE), but not bilayer PE (Thromb H aemost 1989; 62: 892). In those studies, the involvement of proteins in thi s recognition was not evaluated. To address this issue, we have isolated Ig G lupus anticoagulant antibodies from the plasma of SLE patients and evalua ted the inhibition of lupus anticoagulant activity by hexagonal (II) phase PE in the presence and absence of purified plasma proteins. All six of the IgG lupus anticoagulant antibodies tested were inhibited by hexagonal (II) phase PE in the presence, but not the absence, of human prothrombin. In con trast, little or no inhibition was observed with prothrombin alone or with PE in combination with either beta(2)-glycoprotein I or annexin V. These da ta indicate that, for certain lupus anticoagulant antibodies, inhibition by hexagonal (II) phase PE is dependent on prothrombin, suggesting that these antibodies recognize a complex of PE and prothrombin.