Construction of a replication-defective recombinant virus and cell-based vaccine for H9N2 avian influenza virus.

The H9N2 subtype of avian influenza is highly contagious, and although it is classified as a low-pathogenic avian influenza virus, its tendency to recombine with other subtypes of avian influenza viruses has made it a potential problem for the poultry industry. Vaccines currently used to prevent this disease are all inactivated, making it difficult to stimulate long-lasting immunity, and have a very weak ability to trigger cellular immunity, thus failing to address the problem of virus shedding. Live-attenuated vaccines are capable of stimulating cellular immunity but carry the risk of recombination with wild-type strains. In this study, we successfully rescued a replication-deficient H9N2 strain (H9-SD18GD12HA) using reverse genetic techniques, which was obtained by replacing the neuraminidase (NA) gene with the open reading frame of the hemagglutinin (HA) gene with the PR8 strain as the backbone. Dynamic growth results showed that H9-SD18GD12HA can proliferate only under NA-containing conditions and therefore cannot grow in normal animals or cells. After immunization of chickens with H9-SD18GD12HA using eye and nose drops, both humoral and cellular immunity were stimulated, and some degree of reduction in virus shedding was observed. These results indicate that H9-SD18GD12HA has good immunogenicity, does not proliferate in vivo, and has the potential to be developed into a novel live-attenuated vaccine for the H9N2 subtype of avian influenza.