Antigenic mapping and functional analysis of the F protein of Newcastle disease virus using monoclonal antibodies

Twelve monoclonal antibodies to the F protein of a velogenic strain of Newcastle disease virus (NDV) were established. Each of these antibodies inhibited virus-induced plaque formation in BHK-21 cells. Seven antibodies neutralized viral infectivity in eggs; thus, antigenic variants could be selected with these antibodies and used for antigenic mapping. Based on the reactivity of the antigenic variants with the antibodies used in selection, 4 distinct antigenic sites (I-IV) were defined on the F protein molecule. In competitive binding assay, sites II and III were found to be spatially close to each other. Each antibody to sites I, II and III inhibited both virus-induced hemolysis of chicken erythrocytes and syncytium formation of BHK-21 cells. On the other hand, some of the antibodies to site IV selectively inhibited either hemolysis or cell fusion. This finding may indicate that the fusion of the viral envelope with erythrocytes and host cell membrane is modulated through different ways. Comparative analysis of different NDV strains using monoclonal antibodies to each of the different antigenic sites showed that the antigenicity of the F protein is highly conserved.