EFFECT OF ANTIPHOSPHOLIPID ANTIBODIES ON BETA(2)GLYCOPROTEIN I-PHOSPHOLIPID INTERACTION

PROBLEM: beta(2) glycoprotein I (beta(2)GPI) physiologically binds to negatively charged phospholipids (PLs) and is a natural regulator of t he coagulation cascade. Thrombotic clinical complications and recurren t fetal loss associated with autoimmune antiphospholipid (aPL) antibod ies are thought to be related to their binding to beta(2)GPI-PL comple x and interference with the physiological function of beta(2)GPI. METH OD OF STUDY: To investigate the effect of aPL on beta(2)GPI-PL interac tion, we studied the binding of biotinylated beta(2)GPI to cardiolipin (CL) by enzyme-linked immunosorbent assay (ELISA) in the presence and absence of purified aPL immunoglobulin G (IgG) antibodies. RESULTS: A dding five different aPL IgG antibodies with different levels of aPL a ctivity isolated from the sera of five patients with aPL-associated re current fetal death greatly increased the binding of biotinylated beta (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 t o 0.425 in the absence of aPL Igc. In contrast, normal human IgG had n o enhancing effect. The OD was 0.480 and 0.425, respectively. The enha ncement of beta(2)GPI binding to CL by aPL IgG correlated with the tit ers of aPL antibodies. The use of phosphate-buffered saline with incre asing salt concentrations as a washing buffer for the ELISA resulted i n more stable binding of beta(2)GPI to PL in the presence of aPL IgG. CONCLUSIONS: These findings suggest that the binding of autoimmune aPL antibodies to beta(2)GPI-PL complex results in abnormally tighter int eraction between beta(2)GPI and PLs. which may lead to physiological d ysfunction of beta(2)GPI as a regulator of coagulation.