Variability of antigenic epitopes of the fusion protein of Newcastle disease virus.

Using a panel of 10 monoclonal antibodies (Mab) against fusion (F) protein of Newcastle disease virus (NDV), strain Australia-Victoria, three non-overlapping antigenic sites (F1, F2 and F3) and one site partially overlapping with the sites F1 and F2 (F1.2) have been identified. The sites F2 and F3 are clusters that each include four antigenic epitopes. The antigenic stability of the above epitopes was estimated by comparison of the binding capacity of the corresponding Mabs towards 63 NDV strains isolated in different years and places from various avian species. The results demonstrated high variability of the site F1.2 and of all the four epitopes of the site F2. At the same time, the only epitope of the site F1 can be defined as highly conservative: the corresponding Mab gave positive binding with 60 from 63 NDV strains, one from the four epitopes pertaining to site F3 was the most conservative--the corresponding Mab reacted with all the 63 strains used in the studies, while the other three Mabs showed rather low stability--the corresponding Mabs reacted with 34-39 NDV strains. Thus, as opposed to the published data asserting antigenic stability of the F protein contrary to the high variability of the haemagglutinin-neuraminidase (HN) protein, our results have revealed a number of variable epitopes on the F protein. This demonstrates an evolutionary changeability of the F protein, which is of importance from the theoretical (viral antigenic evolution) as well as practical point of view.