Lupus anticoagulant (LA) positive SLE patients seem to have a thrombotic tendency. We have examined that plasma levels of antigen and protein C (PC) activity were lower in LA positive than in negative SLE patients. To elucidate the mechanism of thrombosis in LA positive SLE patients, we investigate the effects of IgG fraction obtained from LA and antiphospholipid antibody (APA) positive SLE patients on the activation of PC in the reaction of human umbilical vein endothelial cells (HUVEC) and thrombin. The results were as follows:
1. The activation rate of PC was inhibited by the addition of IgG obtained from LA and APA positive SLE patients (inhibition rate: 17.8%, p<0.05).
2. The activation rate of PC was reduced much more by the addition of IgG fraction obtained from the patients who showed high level of APA.
3. Among five antiphospholipid antibodies, anticardiolipin antibody strongly inhibited the activation of PC.
4. IgG fraction obtained from LA and APA positive SLE patients did not injure HUVEC.
In addition to the previous reports which showed the reduced plasma level of PC in LA positive SLE patients, the activation rate of PC on HUVEC was inhibited by their IgG. From these results, we concluded that LA positive SLE patients show higher possibility of thrombosis than LA negative patients.
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