| H9N2 subtype avian influenza viruses (AIV) have widely spreaded in the world. It has made not only huge economic losses for poultry industries but damage for human being's health.Most viruses exhibit a relatively restricted host range, with efficient viral replication occurring in the natural host and complete or partial restriction of viral replication occurring in other host species. Many avian influenza A viruses exhibit a host range phenotype characterized by restricted replication only in the respiratory tract of domestic poultry. However, some H9N2 avian influenza viruses, which isolated from domestic poultry in China mainland, could cross the avian-mammalian host species barrier and caused the virus replication in the lung organ of mouse model. We characterized two H9N2 subtype avian influenza viruses that were isolated from chicken, A/Chicken/Shandong/6/96 (CK/SD/6/96) and A/Chicken/Guangdong/5/97 (CK/GD/5/97). These two viruses are similar to mild pathogenicity for chicken, but differ pathogenicity in mice model. We use reverse genetics to create a series of single-gene segments from CK/SD/6/96 and the remaining seven gene segment from CK/GD/5/97. We found that the PB2 gene of CK/SD/6/96 could attenuate CK/GD/5/97 virus to some extent. And an Asn-to-Asp substitution at position 448 of PB2 gene plays a key role in this function. Conversely, of the recombinant viruses in the CK/SD/6/96 background, only the one that contains the PB2 gene of CK/GD/5/97 was able to replicate in mice. A single amino acid substitution (Asp to Asn) at position 448 of PB2 enabled CK/SD/6/96 to infect in mice.The results demonstrate that amino acid Asn 448 is one of the important determinants for avian influenza virus to cross the host species barrier and infect mice, though the replication of H9N2 influenza viruses involve multiple genes and result from a constellation of gene. Our findings may help us to explain of the host range of replication H9N2 influenza viruses to mouse model.
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