Complexes of anti-prothrombin antibodies and prothrombin cause lupus anticoagulant activity by competing with the binding of clotting factors for catalytic phospholipid surfaces.

We investigated the mechanism by which anti-prothrombin antibodies cause lupus anticoagulant (LAC) activity. Addition of affinity-purified anti-prothrombin antibodies from LAC-positive plasma samples (alpha-FII-LAC+) to normal plasma induced LAC activity. Upon increasing the phospholipid concentration, LAC activity was neutralized. Addition of purified alpha-FII-LAC+ to normal plasma strongly inhibited factor Xa formation. No inhibition was measured when alpha-FII-LAC+ were added to prothrombin-deficient plasma or when purified anti-prothrombin antibodies from LAC-negative plasma samples (alpha-FII-LAC-) were added. When a combination of prothrombin and alpha-FII-LAC+ was added to the purified clotting complex, a strong inhibition of factor Xa and IIa formation was seen. The alpha-FII-LAC+ alone or a combination of prothrombin and alpha-FII-LAC- did not show inhibition. Ellipsometry studies showed that, in the presence of alpha-FII-LAC+, the affinity of prothrombin for a phospholipid surface increased dramatically, whereas a much lower increase was observed with alpha-FII-LAC-. Our results show that complexes of prothrombin and anti-prothrombin antibodies with LAC activity inhibit both prothrombinase and tenase. The antibodies increase the affinity of prothrombin for the phospholipid surface, thereby competing with clotting factors for the available catalytic phospholipid surface, a mechanism similar to that of anti-beta2-glycoprotein I antibodies.