Protein C Activity in Patients with Antiphospholipid Syndrome

The interaction between antiphospholipid antibodies and the protein C system may explain at least a part of the mechanisms underlying thrombosis in the antiphospholipid syndrome (APS). We evaluated the protein C activity, factor V Leiden mutation and the presence of several types of antiphospholipid antibodies in 60 patients with antiphospholipid syndrome. Nineteen patients (31.6%) and 5 controls (8.3%) had decreased protein C activity (95% CI, 10%-37%). 14 patients with high levels of antiphospholipid antibodies and normal factor V molecule (37.8%) and none of the patients with antiphospholipid antibodies in normal ranges and normal factor V molecule had decreased protein C activity (95% CI, 24%-52%; (x =10.4; p = 0.001). Leiden mutation was found in 5 controls (8.3%) and in 4 patients (8.3%; 95% CI, 8–24%).We found a strong association between decreased protein C activity and presence of anticardiolipin antibodies and antibodies against β2-glycoprotein I. The decreased protein C activity depended on the antibody titre. Although abnormalities of the natural anticoagulant proteins like protein C and protein S seem to be involved in the pathogenesis of the APS, activation of protein C also occurs during the process of clotting. Studies such as we describe may help us identify subsets of patients whose clotting is dependent on protein C-antiphospholipid interactions that may lead to future novel therapies.