Blood, 15 September 2001, Vol. 98, No. 6, pp. 1819-1827
HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY
Platelet-associated anti-GPIIb-IIIa autoantibodies in
chronic immune thrombocytopenic purpura recognizing epitopes close to
the ligand-binding site of glycoprotein (GP) IIb
Satoru Kosugi,
Yoshiaki Tomiyama,
Shigenori Honda,
Hisashi Kato,
Teruo Kiyoi,
Hirokazu Kashiwagi,
Yoshiyuki Kurata, and
Yuji Matsuzawa
From the Department of Internal Medicine and Molecular
Science, Graduate School of Medicine, Osaka University; and the
Department of Blood Transfusion, Osaka University Hospital, Suita,
Osaka, Japan.
Localization of epitopes for platelet-associated
(PA) anti-GPIIb-IIIa (
IIb
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.
