A systemic lupus erythematosus-derived human hybridoma autoantibody reactive with antigens expressed on ADP-activated platelets.

Hematological complications seen in systemic lupus erythematosus (SLE) patients may be caused by the binding of specific autoantibodies to platelets, but the epitopes on platelets responsible for antibody binding and the mechanisms by which autoantibodies induce hemostatic abnormalities in SLE patients remain unknown. We have previously demonstrated that polyspecific platelet-binding antibodies can be derived from SLE patients. In the present study, we have characterized an SLE-derived polyspecific hybridoma antibody, 9604, which was previously shown to be strongly cytotoxic to platelets in vitro and to have weak lupus anticoagulant activity. We demonstrated that this antibody does not bind to fixed intact platelets in an enzyme-linked immunoassay (ELISA), but does react with lysed, washed or ADP-activated platelets. By Western blotting analysis, antibody 9604 was unique among other platelet-binding autoantibodies in that it reacted mainly with polypeptides of approximately 200,000 and 32,000 molecular weight (MW) in platelets. In blots of endothelial cell proteins, 9604 reacted with a band of approximately 200,000 MW, but no 32,000 MW reactive band was observed. Based on these findings, we postulate that antibody 9604 may bind to a protein or proteins of 32,000 MW exposed on the platelet surface during activation. To our knowledge, this is the first demonstration of a human hybridoma monoclonal antibody derived from an SLE patient which distinguishes between activated and resting platelets. Further characterization of the proteins recognized by this autoantibody may provide insight into the mechanisms responsible for the production and pathogenesis of anti-platelet autoantibodies in patients with SLE.