Anti-cardiolipin antibody from a patient with antiphospholipid syndrome (APS) recognizes only an epitope expressed by cardio- lipin/ 2-glycoprotein-I ( 2GPI) complex and induces APS

Objective As the antiphospholipid syndrome (APS) is characterized by antibodies which bind negatively charged phospholipids either directly or mainly through different target epitopes located on the beta-2-glycoprotein-I ( 2GPI) molecule, the aim of this study is to describe an additional target epitope for anti-cardiolipin binding. Methods The binding characteristics of affinity purified anti-cardiolipin antibodies from a patient with monoclonal gammopathy associated with clinically overt APS were studied; inhibition studies were also carried out. These antibodies were used for the active induction of experimental APS. Results The affinity purified anti-cardiolipin antibodies were found to bind a target epitope created by the complex of cardiolipin/ 2GPI, while not reacting with a complex composed by another phospholipid (phosphatidylserine/ 2GPI), as confirmed by direct binding and competition assays. Immunization of naive mice with this unique affinity purified anticardiolipin antibody resulted in the induction of experimental APS (thrombocytopenia, prolonged coagulation timed and fetal resorptions). The anti-cardiolipin/ 2GPI injected mice developed high titers of mouse anti-cardiolipin/ 2GPI antibodies with the same binding characteristics as the human antibody which was used for disease induction. Conclusion APS is a unique syndrome that is characterized by a diversity of pathogenic anti-phospholipid antibodies which may explain the diversity of clinical manifestations reported in patients.