PL2‐49, a monoclonal antibody against glycoprotein IIb which is a platelet activator

Abstract

PL2-49 is a murine monoclonal IgG₁ antibody obtained after immunization of Balb/c mice with EDTA washed platelets. Binding could be detected on Zw^a(+) as well as Zw^a(—) platelets, but not on type I Glanzmann's thrombasthenia platelets using an ELISA screening test. Immunoprecipitation studies showed that PL2-49 bound to glycoprotein IIb when the glycoprotein IIb/IIIa complex dissociation was performed after the monoclonal antibody binding. Experiments with a human alloantibody against Zwa antigen were run in parallel to control the complex dissociation. Ascitic fluid, as well as the purified antibody, induced activation and aggregation of washed platelets and ATP release. PL2-49-induced aggregation did not require exogenous fibrinogen and was inhibited, partially, in the presence of aspirin, apyrase, isosorbide dinitrate. Raising intra-platelet cyclic AMP with a stable PGI₂ analogue, iloprost, and/or a phosphodiesterase inhibitor, RA 233, suppressed the responses to PL2-49. F(ab')₂ fragments did not induce aggregation of normal platelets but inhibited the response to the whole immunoglobulin. Finally PL2-49 was shown to induce aequorin-detected elevations in intraplatelet Ca⁺⁺ levels. Thus PL2-49 seems to differ from monoclonal antibodies so far described, since it binds to glycoprotein IIb in a complex-dependent manner at least under our experimental conditions for immunoprecipitation studies, and it induces platelet Ca⁺⁺ mobilization and platelet aggregation after a lag-time. These reactions depend both on Fab and Fc domains of the antibody and require neither complement nor exogenous fibrinogen.