title: Platelet-associated anti-GPIIb-IIIa autoantibodies in chronic immune thrombocytopenic purpura recognizing epitopes close to the ligand-binding site of glycoprotein (GP) IIb.
abstract: Localization of epitopes for platelet-associated (PA) anti-GPIIb-IIIa (alpha(IIb)beta(3)) autoantibodies in chronic immune thrombocytopenic purpura remains elusive. Previous studies suggest that PA antibodies recognize the tertiary structure of intact glycoprotein (GP) IIb-IIIa. To localize their epitopes using antigen-capture enzyme-linked immunosorbent assay (ELISA), the reactivity of 34 PA anti-GPIIb-IIIa antibodies was examined with recombinant GPIIb-IIIa having a defect in ligand-binding sites in either GPIIb or GPIIIa, and no major conformational change was induced: KO variant GPIIb-IIIa was attributed to a 2-amino acid insertion between residues 160 and 161 in the W3 4-1 loop in GPIIb, and CAM variant GPIIb-IIIa was attributed to D119Y in GPIIIa. In one third (11 of 34) of the patients, PA antibodies showed a marked decrease (less than 50%) in reactivity with KO compared with wild-type GPIIb-IIIa. Their reactivity was also impaired against GPIIbD163A-IIIa. In sharp contrast, they reacted normally with CAM GPIIb-IIIa. OP-G2, a ligand-mimetic monoclonal antibody, markedly inhibited their binding to GPIIb-IIIa in patients with impaired binding to KO GPIIb-IIIa, but small GPIIb-IIIa antagonists did not. In addition, a newly developed sensitive ELISA indicated that autoantibodies showing impaired binding to KO are more potent inhibitors for fibrinogen binding. The present data suggest that certain PA anti-GPIIb-IIIa autoantibodies recognize epitopes close to the ligand-binding site in GPIIb, but not in GPIIIa.
authors.affiliation: Department of Internal Medicine and Molecular Science, Graduate School of Medicine, Osaka University, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Kosugi
authors.forename: Y
authors.initials: Y
authors.lastname: Tomiyama
authors.forename: S
authors.initials: S
authors.lastname: Honda
authors.forename: H
authors.initials: H
authors.lastname: Kato
authors.forename: T
authors.initials: T
authors.lastname: Kiyoi
authors.forename: H
authors.initials: H
authors.lastname: Kashiwagi
authors.forename: Y
authors.initials: Y
authors.lastname: Kurata
authors.forename: Y
authors.initials: Y
authors.lastname: Matsuzawa
mesh_terms: D000328:Adult; D000368:Aged; D000911:Antibodies, Monoclonal / Q000276:immunology; D001323:Autoantibodies / Q000276:immunology*; D001324:Autoantigens / Q000235:genetics / Q000276:immunology; D001665:Binding Sites; D001792:Blood Platelets / Q000276:immunology; D002460:Cell Line; D002908:Chronic Disease; D004797:Enzyme-Linked Immunosorbent Assay; D000939:Epitopes / Q000276:immunology; D005260:Female; D005340:Fibrinogen / Q000378:metabolism; D006801:Humans; D008297:Male; D008875:Middle Aged; D009154:Mutation; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D016553:Purpura, Thrombocytopenic, Idiopathic / Q000276:immunology*
chemical_list: D000911:Antibodies, Monoclonal; D001323:Autoantibodies; D001324:Autoantigens; D000939:Epitopes; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D005340:Fibrinogen