Effects of β2-Glycoprotein I and Monoclonal Anticardiolipin Antibodies on Extrinsic Fibrinolysis

ABSTRACT Antiphospholipid antibodies (aPLs) are associated with an increased incidence of thrombosis, but the mechanisms responsible for thrombosis are unclear. The present study investigated the effect of both β2-glycoprotein I (β2-GPI) and aPLs on the activity of extrinsic fibrinolysis. The remaining tissue-plasminogen activator (t-PA) of the sample consisting of β2-GPI, two-chain recombinant t-PA, plasminogen activator inhibitor (PAI) -1 was measured by a chromogenic assay using synthetic substrate S-2251, Glu-plasminogen, and soluble fibrin monomer. Without PAI-1, β2-GPI did not affect t-PA activity. When 14.3 ng/ml PAI-1 was added to 3.6 U/ml t-PA, the remaining t-PA activity was increased from 48.9% to 60.4% by the addition of β2-GPI (190 μg/ml). The effect of β2-GPI did not require phospholipids. The β2-GPI seems to protect t-PA activity from the inhibition by PAI-1. When monoclonal anticardiolipin antibodies (aCLs), EY1C8, and EY2C9, which were established from a patient with antiphospholipid syndrome, were further added to the mixture with a diluted phospholipid (Platelin®) to investigate the influence of aPL, the remaining t-PA activity decreased to 50.1 and 80.7%. Monoclonal aCLs appeared to inhibit the effect of β2-GPI, that is, these monoclonals inhibited the fibrinolytic activity by an elevation in PAI-1 activity. These results suggest the possibility that the impairment of fibrinolytic activity by aCLs is one of reasons for the increased incidence in thrombosis in patients with aCLs.