H9 avian influenza reassortant with engineered polybasic cleavage site displays high pathogenic phenotype in chicken

In the field, highly pathogenic avian influenza viruses (HPAIV) originate from low pathogenic strains of the hemagglutinin (HA) serotypes H5 and H7 that have acquired a polybasic HA cleavage site. This observation suggests the presence of a cryptic virulence potential of H5 and H7 low pathogenic avian influenza viruses (LPAIV). Among all other LPAIV, the H9N2 strains are of particular relevance as they became wide-spread across many countries in several avian species and have transmitted to humans. To assess the potential of those strains to transform into an HPAIV, we introduced a polybasic cleavage site into the HA of a contemporary H9N2 isolate. Whereas the engineered polybasic HA cleavage site mutant remained low pathogenic like its parent virus, a reassortant expressing the modified H9 HA with engineered polybasic cleavage site and all the other genes from an H5N1 HPAIV became highly pathogenic in chicken with an IVPI of 1.23. These results suggest that an HPAIV with other subtype than H5 or H7 would only emerge on the conditions that the HA gene were able to acquire a polybasic cleavage site and the other viral genes carry additional virulence determinants.