title: Quinine-dependent antibodies bind a restricted set of epitopes on the glycoprotein Ib-IX complex: characterization of the epitopes.
abstract: 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-specific quinine-dependent antibodies. Antibody binding to Chinese hamster ovary cells or mouse L cells stably transfected with various combinations of the three genes (Ibalpha, Ibbeta, 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 GPIbalpha 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 GPIbalpha. Thus, the quinine-dependent antibodies fall into two distinct populations that bind to GPIbalpha and GPIX independently. Using proteases which cleave GPIbalpha at specific sites, we have shown that the GPIbalpha-specific antibody binds to an 11-amino acid (283 to 293) region. Peptide inhibition studies provide confirmatory evidence that this region contains the epitope for the GPIbalpha-specific quinine-dependent antibody.
authors.affiliation: University of New South Wales, Centre for Thrombosis and Vascular Research, Department of Haematology, Prince of Wales Hospital, Sydney, Australia.
authors.forename: J K
authors.initials: JK
authors.lastname: Burgess
authors.forename: J A
authors.initials: JA
authors.lastname: Lopez
authors.forename: M C
authors.initials: MC
authors.lastname: Berndt
authors.forename: I
authors.initials: I
authors.lastname: Dawes
authors.forename: C N
authors.initials: CN
authors.lastname: Chesterman
authors.forename: B H
authors.initials: BH
authors.lastname: Chong
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000918:Antibody Specificity; D001323:Autoantibodies / Q000276:immunology*; D001327:Autoimmune Diseases / Q000139:chemically induced* / Q000276:immunology; D001665:Binding Sites; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D000939:Epitopes / Q000737:chemistry / Q000187:drug effects / Q000276:immunology*; D006801:Humans; D007163:Immunosorbent Techniques; D007739:L Cells; D008666:Metalloendopeptidases / Q000494:pharmacology; D051379:Mice; D008969:Molecular Sequence Data; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000737:chemistry / Q000235:genetics / Q000276:immunology*; D011803:Quinine / Q000009:adverse effects*; D011993:Recombinant Fusion Proteins / Q000276:immunology; D013921:Thrombocytopenia / Q000139:chemically induced* / Q000276:immunology; D014162:Transfection; D014357:Trypsin / Q000494:pharmacology
chemical_list: D001323:Autoantibodies; D000939:Epitopes; D019038:Platelet Glycoprotein GPIb-IX Complex; D011993:Recombinant Fusion Proteins; D011803:Quinine; D014357:Trypsin; D008666:Metalloendopeptidases; C099092:mocarhagin