Identification of novel E347Q and G362K amino acid substitutions in HN neutralization epitope and major antigenic difference in novel sub-genotype VIIj isolates.

Hemagglutinin-neuraminidase (HN) protein of Newcastle disease virus (NDV) plays a crucial role in induction of immune response against Newcastle disease infection. Mutation in residues 347 and 362 of HN linear antigenic site has been identified to be responsible for antigenic variations. Hence we studied antigenic difference between sub-genotype VIIj isolates and vaccine strains by the use of polyclonal serum against LaSota strain in hemagglutination inhibition test. Furthermore, epitope patterns of the isolates under study were analyzed and compared to HN sequences in GenBank. The results demonstrated that new Newcastle disease isolates (Mazandaran and Behshahr) had hemagglutination inhibition (HI) titer three and five, respectively while LaSota strain titer was eight. In addition, observation of sequences and epitope patterns revealed three unique amino acid substitutions (D144N, E347Q and G362K) in HN protein. E347Q and G362K mutations were located in neutralization antigenic site. Thus, we suggest that, these two novel amino acid substitutions in major linear epitope might be responsible for antigenic variation and decrease of HI activity.