Partial inhibition of platelet aggregation and fibrinogen binding by a murine monoclonal antibody to GPIIIa: requirement for antibody bivalency.
We produced a murine monoclonal antibody, 7H2, and localized its epitope to one or more small regions on platelet glycoprotein (GP) IIIa. 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, approximately 40,000 7H2-IgG antibody molecules bind per platelet. In contrast, approximately 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).
Kutok
J L
JL
Division of Hematology, State University of New York at Stony Brook.
Coller
B S
BS
D000818:Animals; D000911:Antibodies, Monoclonal; D000937:Antigen-Antibody Reactions; D001666:Binding Sites, Antibody; D000939:Epitopes / Q000097:blood; D005340:Fibrinogen / Q000276:immunology*; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000276:immunology; D007074:Immunoglobulin G / Q000276:immunology*; D007457:Iodine Radioisotopes; D051379:Mice; D010975:Platelet Aggregation Inhibitors / Q000276:immunology*; D010980:Platelet Membrane Glycoproteins / Q000276:immunology*; D011485:Protein Binding
D000911:Antibodies, Monoclonal; D000939:Epitopes; D007140:Immunoglobulin Fab Fragments; D007074:Immunoglobulin G; D007457:Iodine Radioisotopes; D010975:Platelet Aggregation Inhibitors; D010980:Platelet Membrane Glycoproteins; D005340:Fibrinogen