[Circulating lupus-type anticoagulant, a risk factor for thrombosis by inhibition of protein C activation].

The lupus anticoagulant is a risk factor of thrombosis. The non thrombogenic endothelial surface could be a target for the lupus anticoagulant. We have investigated the effect of purified immunoglobulins G of five patients with LA on the thrombomodulin activity of cultured human endothelial cells from umbilical cord vein. The rate of activation of purified protein C (PC) (30 nM) by the endothelial cells in the presence of thrombin (0.1 U/dish) has been measured by hydrolysis of substrate S 2366. Activated PC has been 7.37 +/- 0.78 pmoles X ml-1 X h-1 in the presence of buffer and 7.2 +/- 0.78 pmoles X ml-1 X h-1 in the presence of control IgG (2 mg/dish). Heat aggregated IgG did not induce any significant change. Patient's IgG lowered significantly the rate of PC activation (4.86 +/- 1.04 pmoles X ml-1 X h-1, p less than 0.001). Fab fragment from two of these patient's IgG displayed the same inhibition. Moreover neutralization of this effect was obtained by addition of phospholipids (70% phosphatidylcholine, 30% phosphatidylserine) in excess to patient's IgG. Activation of PC has been also performed using purified rabbit thrombomodulin and a similar inhibition by patient's IgG was found. These results seem to indicate that antibodies present in the IgG fractions containing LA could be directed against phospholipids associated to thrombomodulin activity. Reduction of PC activation if present in the patients with LA could play a role in the occurrence of thrombosis.