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 no t be specific for phospholipids but directed to beta(2)glycoprotein 1 (beta(2)GP1), phospholipid-beta(2)GP1 complexes, prothrombin, or proth rombin-phospholipid complexes. To explore this issue further, we exami ned the influence of two phospholipid binding proteins, annexin V (pla cental anticoagulant protein I ((PAPI)) and beta(2)GP1, on the activit y of immunoglobulin G (IgG) fractions from patients with antiphospholi pid 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 PAPI a nd beta(2)GP1, individually inhibited the prothrombinase reaction. Whe n IgG-APS samples were combined with PAPI or beta(2)GP1, or both PAPI and beta(2)GP1, inhibition of the prothrombinase reaction was additive . in the anticardiolipin ELISA, PAPI inhibited IgG-APS binding to card iolipin, but beta(2)GP1 enhanced IgG-APS binding to cardiolipin. The ' 'enhancing'' effect of beta(2)GP1 in the ELISA system was neutralized by PAPI, an effect partially overcome by increasing the concentration of beta(2)GP1. Similar results were observed when affinity-purified an ticardiolipin antibodies were used in place of whole IgG-APS preparati ons. These data indicate that IgG preparations obtained from the 8 pat ients with APS recognize similar epitopes on anionic phospholipids in the anticardiolipin test and in the prothrombin-thrombin conversion as say. These data do not exclude the possibility that the lgG preparatio ns may bind prothrombin-phospholipid or beta(2)GP1-phospholipid comple xes.