Studies on the interaction of placental anticoagulant protein I, beta 2 glycoprotein 1, and antiphospholipid antibodies in the prothrombinase reaction and in the solid phase anticardiolipin assays.

Recently it has been suggested that antiphospholipid antibodies may not be specific for phospholipids but directed to beta2glycoprotein 1 (beta2GP1), phospholipid-beta2GP1 complexes, prothrombin, or prothrombin-phospholipid complexes. To explore this issue further, we examined the influence of two phospholipid binding proteins, annexin V (placental anticoagulant protein I (PAP I)) and beta2GP1, on the activity of immunoglobulin G (IgG) fractions from patients with antiphospholipid syndrome (APS), both in the prothrombin-thrombin conversion assay and in the anticardiolipin enzyme-linked immunosorbent assay (ELISA). Results showed that each of eight IgG-APS; fractions, as well as PAP I and beta2GP1, individually inhibited the prothrombinase reaction. When IgG-APS samples were combined with PAP I or beta2GP1, or both PAP I and beta2GP1, inhibition of the prothrombinase reaction was additive. In the anticardiolipin ELISA, PAP I inhibited IgG-APS binding to cardiolipin, but beta2GP1 enhanced IgG-APS binding to cardiolipin. The "enhancing" effect of beta2GP1 in the ELISA system was neutralized by PAP I, an effect partially overcome by increasing the concentration of beta2GP1. Similar results were observed when affinity-purified anticardiolipin antibodies were used in place of whole IgG-APS preparations. These data indicate that IgG preparations obtained from the 8 patients with APS recognize similar epitopes; on anionic phospholipids in the anticardiolipin test and in the prothrombin-thrombin conversion assay. These data do not exclude the possibility that the IgG preparations may bind prothrombin-phospholipid or beta2GP1-phospholipid complexes.