Characterization of field isolates of Newcastle disease virus using antipeptide antibodies

A recent Australian field isolate of Newcastle disease virus (NDV) was analyzed with antipeptide antibodies capable of differentiating between the sequences at the cleavage activation sites of fusion proteins of different NDV pathotypes. The isolate was found to have the same fusion protein cleavage activation signal as the V4 isolate of the Queensland strain of NDV. However, the isolate failed to react with an antibody specific for the carboxyl-terminal extension on the hemagglutinin/neuraminidase (HN)-protein precursor (HNo-protein) of the V4 isolate and other similar strains (e.g., Ulster and D26). Identity of the fusion protein cleavage activation motif of the isolate was confirmed by chemical analysis of purified fusion protein subunits of the isolate. The combination of a V4-like fusion protein cleavage activation motif but lack of an HNo-protein has not been described before, and the findings indicate that the isolate is a distinct strain of NDV. Analysis of a range of isolates from the state of Victoria, Australia, indicated that similar strains have been present in Australia since at least 1976. Other isolates examined appeared to have fusion protein cleavage activation motifs that could not be defined with the fusion-protein-targeted antibodies. These isolates also appeared to lack the HNo-protein characteristic of the Queensland strain.