The immunobiology and obstetrical consequences of antiphospholipid antibodies

Two classes of antiphospholipid antibodies (APA) are associated with adverse pregnancy outcomes. Those APA identified by immunoassays using phospholipid-coated surfaces (e.g., anticardi-olipin antibodies) seem to bind to the 57 kD anticoagulant protein, β₂-glycoprotein-l, when complexed with anionic phospholipid bilayers. Such APA may or may not prolong phospholipid-dependent clotting assays. A second class of APA are identified by their interference with phospholipid-dependent clotting assays (i.e., lupus anticoagulants). The latter bind to phospholipids present in a unique hexagonal phase either alone or complexed with prothrombin or β₂-glycoprotein-l. There is evidence that both classes of APA are directly responsible for adverse pregnancy outcomes including spontaneous abortions, stillbirths, fetal growth retardation, thrombosis, thrombocytopenia, and preeclampsia. Putative APA-mediated pathogenic mechanisms include intervillous thrombosis, intravillous infarctions and decidual vasculopathy. The thrombogenicity of APA may result from their interference with endothelial phospholipids required for antithrombin III and protein C and S anticoagulant activity and prostacyclin synthesis and/or increased endothelial expression of the procoagulants: tissue factor, von Willebrand factor, platelet-activating factor, and plasminogen activator inhibitor type-1. Other prothrombotic properties seem to include: increased platelet aggregation, and reduced β₂-glycoprotein-l and annexin V anticoagulant activity. Rigorous diagnostic criteria must be applied to the detection of both classes of APA because the prevention of adverse pregnancy outcomes requires potentially hazardous anticoagulant therapy.