Two ´alleles´ of segment 8 (NS) circulate in non-chiropteran influenza A viruses. The A-allele is found in avian and mammalian viruses, but the B-allele is viewed as almost exclusively avian. This might reflect that one or both of its encoded proteins (NS1 and NEP) are maladapted for replication in mammalian hosts. To test this, a number of clade A and B avian NS segments were introduced into human H1N1 and H3N2 viruses. In no case was peak virus titre substantially reduced following infection of various mammalian cell types. Exemplar reassortant viruses also replicated to similar titres in mice, although the avian segment 8s reduced weight-loss compared to the PR8 parent. In vitro, the viruses coped similarly with type I interferons. Temporal proteomics analysis of cellular responses to infection showed that the avian NS segments provoked lower expression of IFN-stimulated genes in cells than the WT. Thus, neither A- nor B-alleles of avian virus-derived NS segments necessarily attenuate virus replication in a mammalian host although they can attenuate disease. Phylogenetic analyses identified 32 independent incursions of an avian-derived A-allele into mammals compared to 6 introductions of a B-allele. However, A-allele isolates from birds outnumber B-allele samples and the relative rates of Aves to Mammalia transmission are not significantly different. We conclude that while the introduction of an avian virus segment 8 into mammals is a relatively rare event, the dogma of the B-allele being especially restricted is misleading - with implications in the assessment of pandemic potential of avian influenza viruses.

IMPORTANCE:

Influenza A virus (IAV) can adapt to poultry and mammalian species, inflicting a great socioeconomic burden on farming and healthcare sectors. Host adaptation likely involves multiple viral factors. Here, we investigated the role of IAV segment 8. Segment 8 has evolved into two distinct clades - the ´A´ and ´B´ alleles. The B-allele genes have previously been suggested to be avian-restricted. We introduced a selection of avian A- and B-allele segment 8s into human H1N1 and H3N2 backgrounds, and found that these reassortant viruses were fully competent in mammalian host systems. We also analysed the currently available public data on segment 8 gene distribution, finding surprisingly little evidence for specific avian-host restriction of the B clade segment. We conclude that B-allele segment 8 genes are in fact capable of supporting infection in mammals and that they should be considered during the assessment of pandemic risk of zoonotic influenza A viruses