Abstract
The polymerase of avian influenza A virus (AIV) is a heterotrimer composed of PB2, PB1, and PA. PB2 plays a role in overcoming the host barrier; however, the genetic prerequisites for avian PB2 to acquire mammalian pathogenic mutations have not been well elucidated. Previously, we identified a prototypic avian PB2 that conferred non-replicative and non-pathogenic traits to a PR8-derived recombinant virus when it was used to infect mice. Here, we demonstrated that key amino acid mutations (I66M, I109V, and I133V, collectively referred to as MVV) of this prototypic avian PB2 increase the replication efficiency of recombinant PR8 virus carrying the mutated PB2 in both avian and mammalian hosts. The MVV mutations caused no weight loss in mice, but they did allow replication in infected lungs, and the viruses acquired fatal mammalian pathogenic mutations such as Q591R/K, E627K, or D701N in the infected lungs. The MVV mutations are located at the interfaces of the trimer and are predicted to increase the strength of this structure. Thus, gaining MVV mutations might be the first step for AIV to acquire mammalian pathogenicity. These results provide new insights into the evolution of AIV in birds and mammals.
Highlights
Aquatic birds are natural reservoirs for influenza A virus (IAV), and 16 hemagglutinin (HA) and 9 neuraminidase (NA) subtypes for IAV have been identified[1, 2]
The PB2 gene of the H9N2 low-pathogenic AIV (LPAIV) strain A/Korea/KBNP0028/2000 (H9N2) (0028) was replicative but non-pathogenic in BALB/c mice, and we identified candidate amino acids related to the replication of 0028 PB2 in mouse lungs by comparing the amino acid sequences of PB2 proteins[22]
Recombinant viruses with the I133V, L373I, or E627K mutations produced higher titres than the parent strain rPB2(01310) (P < 0.05) (Fig. 1b)
Summary
Aquatic birds are natural reservoirs for influenza A virus (IAV), and 16 hemagglutinin (HA) and 9 neuraminidase (NA) subtypes for IAV have been identified[1, 2]. PB1 functions as an RNA-dependent RNA polymerase, and PA cleaves the cap containing 10-13 nucleotides from host pre-mRNA, which is captured by PB28, 9 These three subunits determine the host range, tissue tropism and mammalian pathogenicity of AIV6, 10–13. We identified key mutations (I66M, I109V, and I133V, collectively referred to as “MVV”) that increase replication efficiency in both avian and mammalian hosts and are predicted to increase the structural integrity of the trimeric polymerase. These MVV mutations are essential prerequisites for the subsequent acquisition of fatal mammalian pathogenic mutations. These results provide important insights into the first evolutionary step taken by AIV to acquire pathogenicity in mammals
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