INCREASED LEVELS OF β2 GLYCOPROTEIN-I ANTIGEN AND β2 GLYCOPROTEIN-I BINDING ANTIBODIES ARE ASSOCIATED WITH A HISTORY OF THROMBOEMBOLIC COMPLICATIONS IN PATIENTS WITH SLE AND PRIMARY ANTIPHOSPHOLIPID SYNDROME
β2 Glycoprotein-I (β2GPI), a plasma component with in vitro anticoagulant properties, has been identified as a cofactor for the binding of some antiphospholipid antibodies (aPAs). In order to determine whether β2GPI changes were associated with the thromboembolic complications of aPAs, we measured β2GPI antigen (β2GPI: Ag), β2GPI aPA cofactor activity (β2GPI: Cof) and antibodies to β2GPI (αβ2GPI) in 44 systemic lupus erythematosus (SLE) patients, of whom 19 had evidence of aPAs (SLE-aPA +) and 17 patients with primary antiphospholipid syndrome (PaPS). β2GPI: Ag levels were significantly increased in SLE-aPA+ patients and PaPS patients compared with SLE-aPA – patients and normal healthy controls. The ratio of β2GPI: Cof/Ag was significantly reduced in SLE-aPA + patients compared with SLE-aPA – patients, indicating functional modification of β2GPI in SLE-aPA + patients. Eighty per cent of patients with anticardiolipin (aCL) IgG also had αβ2GPI, and 13% patients with no detectable aCL IgG had αβ2GPI. Increased β2GPI: Ag and αβ2GPI were associated with a clinical history of thrombosis or recurrent fetal loss. The results of these investigations suggest that β2GPI may play a role in the pathogenic mechanism of thrombosis associated with aPAs.