Dimers of beta(2)-glycoprotein I mimic the in vitro effects of beta(2)-glycoprotein I-anti-beta(2)-glycoprotein I antibody complexes

Anti-beta (2)-glycoprotein I antibodies are thought to cause lupus anticoag ulant activity by forming bivalent complexes with beta (2)-glycoprotein I ( beta (2)GPI). To test this hypothesis, chimeric fusion proteins were constr ucted of the dimerization domain (apple 4) of factor XI and beta (2)GPI. Bo th a covalent (apple 4-beta (2)GPI) and a noncovalent (apple 4-C321S-beta ( 2)GPI) chimer were constructed. As controls, apple 2-beta (2)GPI and apple 4-C321S-beta (2)GPI-W316S, in which beta (2)GPI-W316S is not able to bind t o phospholipids, were made. In a phospholipid binding assay, apple 4-beta ( 2)GPI and apple 4-C321S-beta (2)GPI were able to bind to phospholipids with an affinity 35 times higher than that of plasma-derived beta (2)GPI and ap ple 2-beta (2)GPI. Apple 4-C321S-beta (2)GPI-W316S did not bind at all. Onl y apple 4-beta (2)GPI and apple 4-C321S-beta (2)GPI were able to bind to ad hered platelets as shown by immunofluorescence. Using the prothrombin time, which was the most responsive coagulation assay, the clotting time was app roximately doubled when 200 mug/ml apple 4-beta (2)GPI or apple 4-C321S-bet a (2)GPI was added. Addition of 200 mug/ml plasma-derived beta (2)GPI, appl e 2-beta (2)GPI, or apple 4-C321S-beta (2)GPI-W316S did not affect clotting time. Clotting time could be corrected with the addition of extra phosphol ipids, which is indicative for lupus anticoagulant activity. An additional increase in clotting times for apple 4-beta (2)GPI or apple 4-C321S-beta (2 )GPI was achieved by the addition of monoclonal antibodies against beta (2) GPI. In conclusion, dimerization of beta (2)GPI explains the in vitro obser ved effects of beta (2)GPI-anti-beta (2)GPI antibody complexes.