Human hybridoma lupus anticoagulants distinguish between lamellar and hexagonal phase lipid systems

Antibodies to phospholipids may have important physiological and biological functions. Lupus anticoagulants represent a subclass of anti-phospholipid antibodies which are characterized by their ability to prolong the clotting time in in vitro coagulation assays measuring partial thromboplastin time (PTT) (Thiagarajan, P., Shapiro, S. S., and DeMarco, L. (1980) J. Clin. Invest. 66, 397-405). In the present study, we produced hybridomas by fusing lymphocytes from 13 systemic lupus erythematosus patients with the GM 4672 lymphoblastoid line. Of the resulting 67 hybridoma autoantibodies, 14 (21%) were found to prolong a modified PTT assay, and 11 of these antibodies were analyzed further. Competition experiments, using a modified PTT assay, demonstrated that hexagonal phase phospholipids, including natural and synthetic forms of phosphatidylethanolamine, were able to neutralize the lupus anticoagulant activity of all 11 hybridoma antibodies. In contrast, lamellar phospholipids, such as phosphatidylcholine and synthetic lamellar forms of phosphatidylethanolamine, had no effect on the anticoagulant activity. Thus, these antibodies are capable of recognizing phospholipids on purely structural criteria. The demonstration that anti-phospholipid antibodies are able to distinguish between different structural arrangements of phospholipid may have important implications regarding the immunoregulation of autoimmunity.