T160A mutation-induced deglycosylation at site 158 in hemagglutinin is a critical determinant of the dual receptor binding properties of clade 2.3.4.4 H5NX subtype avian influenza viruses

Most clade 2.3.4.4 H5NX subtype avian influenza viruses possess a T160A amino acid substitution in the hemagglutinin (HA) protein that has been shown to affect the receptor binding properties of a clade 2.3.4 H5N1 virus. However, the effect of this single site mutation on the HA backbone of clade 2.3.4.4 H5NX viruses remains unclear. In this study, two H5N6 field isolates possessing HA-160A with dual alpha-2,3 and alpha-2,6 receptor binding properties (Y6 virus) and HA-160T with alpha-2,3 receptor binding affinity (HX virus), respectively, were selected to generate HA mutants containing all of the internal genes from A/PR8/H1N1 virus for comparative investigation. We found that the Y6-P-160A and RHX-P-160A viruses each with 160A in the HA resulting in loss of glycosylation at site 158 exhibited binding to the two receptor types, whereas the RY6-P-160T and HX-P-160T viruses each with 160T in the HA displayed selective binding to alpha-2,3 receptors only. In addition, differences were noted in the replication of these four 1-15N6 recombinants in avian and mammalian cells, as well as in their pathogenicity in mice. The contribution of deglycosylation at site 158 to the acquisition of human-like receptors was further verified in H5N2, H5N5 and H5N8 reassortants. Therefore, we conclude that the lack of glycosylation at site 158 induced by the T160A mutation in HA is a critical determinant for the dual receptor binding properties of Glade 2.3.4.4 H5NX viruses. This new insight may be helpful in assessing the pandemic potential of novel H5 isolates.