Summary
We produced a murine monoclonal antibody, 7H2, and localized its epitope to one or
more small regions on platelet glycoprotein (GP) Ilia. 7H2-IgG and 7H2-F(ab’)2 completely inhibit platelet aggregation and fibrinogen binding at low agonist concentrations,
but only partially inhibit aggregation and fibrinogen binding at high agonist concentrations;
7H2-Fab has no effect on aggregation or fibrinogen binding at any agonist concentration.
7H2-IgG binds to the entire platelet population as judged by flow cytometry. At near
saturating concentrations, ∼40,000 7H2-IgG antibody molecules bind per platelet. In
contrast, ∼80,000 7H2 Fab molecules bind per platelet, suggesting that 7H2-IgG binding
is bivalent. 7H2 was unable to inhibit fibrinogen binding to purified, immobilized
GPIIb/IIIa. These data indicate that the bivalent binding of 7H2 to GPIIIa is required
for its partial inhibition of fibrinogen binding to platelets, perhaps through dimerization
of GPIIb/IIIa surface receptors (or more complex GPIIb/IIIa redistribution triggered
by 7H2 binding) resulting in limited accessibility of fibrinogen to its binding site(s).
