Generation of highly productive and mammalian nonpathogenic recombinant H9N2 avian influenza viruses by optimization of 3’end promoter and NS genome

•Recombinant H9N2 virus with weak PB2 activity can be rescued by promoter optimization.•Strong 3’end promoter of PB2 and PB1 genomes can compensate weak PB2 activity.•The 3’end promoter optimization improves the productivity of recombinant H9N2 viruses.•NS1 gene plays common role in mammalian and chicken embryonic pathogenicity.•Non-pathogenic NS1 gene improves recombinant H9N2 virus productivity in chicken eggs. We developed A/PR/8/34 (PR8) virus-based reverse genetics system in which six internal genes of PR8 and attenuated hemagglutinin and intact neuraminidase genes of field avian influenza viruses (AIVs) have been used for the generation of highly productive recombinant vaccine strains. The 6 + 2 recombinant vaccine strains can induce protective humoral immunity against intended field AIVs; however, the epitopes of B and T cells encoded by internal genes may be important for heterosubtypic protection. Therefore, it is advantageous to use homologous internal genes of field AIVs for recombinant vaccine strains. However, the rescue of recombinant viruses having whole internal genes of field AIVs by the PR8-based reverse genetics system was unsuccessful in some cases. Although partial replacement of an internal gene has been successful for generation of highly productive and mammalian nonpathogenic recombinant viruses, complete replacement of internal genes may be more favorable. In this study, we successfully generated complete recombinant H9N2 AIVs possessing 8 genomes of H9N2 AIVs by optimal combinations of 3’ end promoter sequences of polymerase genomes, and a NS genome. All the generated recombinant viruses showed highly productive and mammalian nonpathogenic traits but some of them showed much higher virus titers in embryonated chicken eggs. Additionally, we found the same mutations of NS1 gene determined pathogenicity of AIVs in chicken embryos as well as mammals. Thus, the 3’ end promoter optimization, and highly productive and mammalian nonpathogenic internal genes may be useful to develop vaccines against AIVs.