Effect of anticardiolipin/beta2-glycoprotein I complexes on production of thromboxane A2 by platelets from patients with the antiphospholipid syndrome.

OBJECTIVE: Antiphospholipid antibodies (aPL) reactive with anionic phospholipids and beta2-glycoprotein I (beta2-GPI) are found in the sera of patients with autoimmune diseases. Clinically, aPL/beta2-GPI complexes are associated with arterial and venous thrombosis, fetal loss, and thrombocytopenia, i.e., the antiphospholipid syndrome (APS). The mechanism of thrombosis is not known. We hypothesized that aPL/beta2-GPI complexes could perturb the platelet membrane and increase production of thromboxane A2 (TXA2, a proaggregatory prostanoid). METHODS: We isolated an IgG fraction containing anticardiolipin antibody (aCL) and the plasma cofactor, beta2-GPI, from a patient with a high titer of aCL and thrombotic cerebrovascular disease. We then examined the effect of aCL, beta2-GPI, and the aCL/beta2-GPI complex on platelet TXB2 (a stable metabolite of TXA2) biosynthesis in vitro from 7 healthy controls. We also measured in vitro platelet TXB2 biosynthesis in 7 patients with APS and in 8 controls. RESULTS: We found: (1) significantly increased in vitro TXB2 production by platelets from controls after incubation with aCL/beta2-GPI complexes; (2) moderately increased TXB2 production by aCL alone; (3) no increase in TXB2 production by beta2-GPI alone; and (4) significantly increased 11-dehydro-TXB2, a metabolite of TXB2 production in vivo, in the urine of patients with APS compared with controls. CONCLUSION: These data suggest that aCL/beta2-GPI complexes play a role in activating platelets to produce TXA2, which could contribute to the prothrombotic state found in patients with APS.