Effect of antiphospholipid antibodies on β2Glycoprotein I-phospholipid interaction

PROBLEM: β 2 glycoprotein I (β 2 GPI) physiologically binds to negatively charged phospholipids (PLs) and is a natural regulator of the coagulation cascade. Thrombotic clinical complications and recurrent fetal loss associated with autoimmune antiphospholipid (aPL) antibodies are thought to be related to their binding to β 2 GPI-PL complex and interference with the physiological function of β 2 GPI. METHOD OF STUDY: To investigate the effect of aPL on β 2 GPI-PL interaction, we studied the binding of biotinylated β 2 GPI to cardiolipin (CL) by enzyme-linked immunosorbent assay (ELISA) in the presence and absence of purified aPL immunoglobulin G (IgG) antibodies. RESULTS: Adding five different aPL IgG antibodies with different levels of aPL activity isolated from the sera of five patients with aPL-associated recurrent fetal death greatly increased the binding of biotinylated β 2 GPI to CL-coated plates. The optical densities (ODs) were 0.635, 0.890, and 1.265 in the presence of three aPL IgG antibodies, compared to 0.425 in the absence of aPL IgG. In contrast, normal human IgG had no enhancing effect. The OD was 0.480 and 0.425, respectively. The enhancement of β 2 GPI binding to CL by aPL IgG correlated with the titers of aPL antibodies. The use of phosphate-buffered saline with increasing salt concentrations as a washing buffer for the ELISA resulted in more stable binding of β 2 GPI to PL in the presence of aPL IgG. CONCLUSIONS: These findings suggest that the binding of autoimmune aPL antibodies to β 2 GPI-PL complex results in abnormally tighter interaction between β 2 GPI and PLs, which may lead to physiological dysfunction of β 2 GPI as a regulator of coagulation.