beta(2),-glycoprotein I is necessary to inhibit protein C activity by monoclonal anticardiolipin antibodies

Objective, To clarify mechanisms of the thrombosis associated with anticard iolipin antibodies (aCL), we examined the effects on activated protein C (A PC) of monoclonal aCL and beta(2)-glycoprotein I (beta(2)GPI), which is req uired for formation of the epitopes of aCL, Methods. We developed the chromogenic assay, in which the degradation of co agulation factor Va by APC is reflected in the reduced generation of thromb in from prothrombin, using soybean trypsin inhibitor to inhibit APC, APC ac tivities were measured in the presence and absence of 3.4 mu M beta(2)GPI a nd/or 2.5 mu g/ml of IgM monoclonal aCL (EY2C9 and EY1C8) established from peripheral blood lymphocytes obtained from a patient with aCL. Results, Without APC, the formed thrombin activity decreased by the additio n of 3.4 mu M beta(2)GPI, When 12.8 nM APC was added, beta(2)GPI partially reversed the APC-induced inhibition of thrombin generation in a concentrati on-dependent manner. With 3.4 mu M beta(2)GPI, the thrombin generation in m onoclonal aCL (2.5 mu g/ml) decreased to 77.1-80.2% by the addition of 12.8 nM APC, but the values were above that in the control IgM (72.7%). Without beta(2)GPI, the APC activity was unaffected by the addition of monoclonal aCL. Conclusion, Beta(2)-glycoprotein I exhibits procoagulant activity by inhibi ting APC activity and anticoagulant activity by inhibiting thrombin generat ion. Any further inhibition of APC activity was caused by monoclonal aCL an d only in the presence of beta(2)GPI.