The hemagglutinin-neuraminidase (HN) protein of Newcastle disease virus (NDV) is an important multifunctional envelope protein, which plays key roles in virus attachment, neuraminidase and fusion promotion activities. Using the established reverse genetics technique for the recovery of infectious NDV from cloned cDNA, the role of the HN protein in NDV pathogenesis was explored. By exchanging the HN gene between an avirulent NDV strain LaSota and a virulent NDV strain Beaudette C (BC), two chimeric viruses were generated. In vitro and in vivo studies done with the chimeras indicated an important role of the HN protein in modulating NDV virulence in hosts.

The HN gene of NDV has six glycosylation sites, two of which are not used for addition of carbohydrates. The exact role of the four functional glycosylation sites in NDV pathogenesis is unknown. To understand the importance of these sites in influencing NDV virulence, each site was eliminated individually and in combination on a cDNA clone of NDV strain BC. Four single mutant viruses with each of the sites (1-4) and one double mutant virus with sites 1 and 2 eliminated, were recovered. Results from this study established the key role of glycosylation site 4 and glycosylation sites 1 and 2 in combination, in influencing NDV pathogenesis.

A recent crystal structure of the HN protein of NDV gives valuable information about the location of amino acids involved in receptor-binding, neuraminidase and fusion promotion activities of the protein. To study the effect of mutagenesis of such key amino acids on NDV pathogenesis in vivo, five recombinant viruses with mutations in residues at the receptor-binding domains of the HN protein and in residues differing between the LaSota and BC strains of NDV, were generated. In vitro and in vivo studies with these viruses indicated the important role of certain residues in viral infectivity.

In summary, the importance of the HN protein in NDV pathogenesis was established. Further studies indicated the role of glycosylation sites in modulating NDV virulence. The relevance of certain key amino acid residues on the HN protein in NDV pathogenesis was also investigated.