Are immunoglobulins with lupus anticoagulant activity specific for phospholipids?

Recent studies have suggested that the lupus anticoagulant (LA) may be specific for prothrombin, prothrombin-phospholipid complexes, or beta 2 glycoprotein 1 (beta 2GP1) rather than phospholipids. We performed a series of experiments to determine whether LA is indeed phospholipid specific. IgG was purified from sera of six patients with the antiphospholipid syndrome (APS) and 10 healthy controls. The six IgG-APS preparations had both LA and anticardiolipin (aCL) activity. Incubation of the six IgG-APS preparations with cardiolipin (CL), phosphatidylserine (PS), phosphatidylcholine (PC), or PS/PC (20:80) liposomes in Tris-buffered saline, resulted in loss of LA activity from the supernatant. We postulated that loss of activity might have resulted from absorption of IgG LA antibodies by phospholipids, a dilutional effect, or the presence of phospholipids in the supernatant causing 'by-pass' of IgG LA inhibitory activity. To distinguish between these possibilities, we re-isolated IgG from the supernatants and re-tested them for LA activity. IgG re-isolated from the PS. CL and PS/PC supernatants had no LA activity, but LA activity remained in the PC supernatant. This suggested that IgG with LA activity was absorbed by negatively charged but not by zwitterionic phospholipids. In like manner, PS, CL and PS/PC, but not PC liposomes, absorbed IgG with aCL activity. Mixtures of the phospholipid liposomes with beta 2GP1 did not modify the absorption of IgG with LA or aCL activity. Finally, we demonstrated that IgG eluted from immunoglobulin-cardiolipin liposome complexes had LA activity. Based on these findings, we conclude that at least one population of antibodies with LA activity is phospholipid specific.