DISCUSSION
We generated recombinant NDV expressing either the wild-type or mutant HA gene of an H5N1 avian influenza virus using reverse genetics and evaluated its potential use as a bivalent vaccine against pathogenic influenza infection as well as against pathogenic NDV infection. After a single immunization dose, the recombinant viruses expressing wild-type or mutant HA induced a strong HI antibody response to NDV and H5 AIV in chickens and protected chickens from disease signs and deaths from highly pathogenic NDV. Most importantly, the vaccinated chickens were completely protected from homologous and heterologous H5N1 virus challenges and displayed no virus shedding, signs of disease, or deaths. We also determined that the recombinant virus expressing the wild-type HA could induce complete protection in mice against lethal doses of homologous and heterologous H5N1 virus challenges. Our results demonstrated that the recombinant viruses could work as live bivalent vaccines to provide protection against infection from highly pathogenic NDV and H5 influenza virus strains.
The vaccine efficacy of recombinant NDVs may be affected by several factors. The replication ability of the vector virus is an important determinant. Currently, two attenuated NDV strains, B1 and LaSota, have been used as vectors for recombinant vaccine construction. Swayne et al. (35) constructed a recombinant virus to express the HA gene of H7 avian influenza virus using the highly attenuated NDV B1 strain, and this recombinant virus only provided 40% protection against highly pathogenic H7 AIV or NDV challenge. The LaSota virus used in our experiments is less attenuated than the B1 strain, and our study as well as those of others indicate that the LaSota virus-based recombinant vaccine provides much better protection against both NDV and the targeted pathogen challenge than does the B1 strain (14, 38).
The insertion site of the foreign gene also affects the immunogenicity and vaccine efficacy of recombinant NDV. The genome of NDV contains six genes in the order of 3′-NP-P-M-F-HN-L-5′ (9). Expression levels of the proteins are attenuated in a sequential manner from the 3′ end to the 5′ end of the viral genome (17, 27). Huang et al. (14) inserted the VP2 gene of infectious bursal disease virus into the far 3′ end of the genome of NDV LaSota, and after a single dose the resultant virus only induced 90% protection against highly pathogenic Newcastle disease virus challenge, while the wild-type NDV virus induced 100% protection. We chose to insert the foreign HA gene within the NDV genome at a more 5′ position, where it would be assumed that expression of the foreign HA gene would be more attenuated than if the HA gene would have been inserted at the 3′ end of the genome, as chosen by Huang et al. These results may suggest that the overexpression of a foreign gene may impair the replication of the recombinant virus in vivo and diminish its efficacy as a vaccine.