Anti-β2-glycoprotein I and anti-prothrombin antibodies in patients with the ‘antiphospholipid’ syndrome: Immunological specificity and clotting profiles

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Abstract
Lupus anticoagulant (LA) antibodies have been shown to be directed to protein-phospholipid complexes. In this study, we report on LA antibodies from patients with the ‘antiphospholipid’ syndrome (APS), that are directed to prothrombin and b2-glycoprotein I, but not to the complexes of these plasma proteins to anionic phospholipids. The anti-prothrombin antibodies studied had different reactivities in two clotting assays: the dilute Russell's viper venom time (dRVVT) and the dilute kaolin clotting time (dKCT). Anti-prothrombin and anti-b2-glycoprotein I (anti-b2GPI) antibodies, affinity-purified from one patient with APS were not cross-reactive and had different effects in the dRVVT and dKCT clotting tests. Polyclonal anti-prothrombin antibodies, affinity-purified on a prothrombin column, from two patients with prothrombin reactivity in their plasma, have affinity constants to prothrombin of 104 and 192 nM. The patient with affinity-purified antibodies to prothrombin and b2GPI, had affinity constants to prothrombin and b2GPI, respectively, of 192 nM and 3030 nM, respectively. LA antibodies are a heterogenous population of antibodies that have different immunological specificities and clotting test reactivities in different patients.