Quinine-dependent antibodies bind a restricted set of epitopes on the glycoprotein Ib-IX complex: Characterization of the epitopes

Severe immune thrombocytopenia is an idiosyncratic complication of quinine therapy. Although in most cases the responsible antibody is directed against platelet membrane glycoprotein (GP) Ib-IX, specificity for GPIIb-IIIa or both epitopes has also been reported. The objective of this study was to characterize the binding site of GPIb-IX-speciiic quinine-dependent antibodies. Antibody binding to Chinese hamster ovary cells or mouse L cells stably transfected with various combinations of the three genes (Ib alpha, Ib beta, or IX) that encode this complex was detected using flow cytometry, monoclonal antibody-specific immobilization of platelet antigens assay, and differential adsorption studies. IgG in sera from 15 patients with quinine-induced thrombocytopenia binding to the cells, in the presence of quinine, showed three distinct patterns. Group 1 sera contained at least two antibody populations, one which binds to GPIb alpha and another which recognizes GPIX, Group 2 sera contained an antibody which binds drug dependently to GPIX, and Group 3 sera contained an antibody which recognizes a quinine-dependent epitope on GPIb alpha. Thus, the quinine-dependent antibodies fall into two distinct populations that bind to GPIb alpha and GPIX independently. Using proteases which cleave GPIb alpha at specific sites, we have shown that the GPIb alpha-specific antibody binds to an II-amino acid (283 to 293) region. Peptide inhibition studies provide confirmatory evidence that this region contains the epitope for the GPIb alpha-specific quinine-dependent antibody. (C) 1998 by The American Society of Hematology.