Effects of the Glycosylation of the HA Protein of H9N2 Subtype Avian Influenza Virus on the Pathogenicity in Mice and Antigenicity

As the H9N2 subtype avian influenza virus (H9N2 AIV) evolves naturally, mutations in the hemagglutinin (HA) protein still occur, which involves some sites with glycosylations. It is widely established that glycosylation of the H9N2 AIV HA protein has a major impact on the antigenicity and pathogenicity of the virus. However, the biological implications of a particular glycosylation modification site (GMS) have not been well investigated. In this study, we generated viruses with different GMSs based on wild-type (WT) viruses. Antigenicity studies revealed that the presence of viruses with a 200G+/295G− mutation (with glycosylation at position 200 and deletion of glycosylation at position 295 in the HA protein) combined with a single GMS, such as 87G+, 127G+, 148G+, 178G+, or 265G+, could significantly affect the antigenicity of the virus. Pathogenicity assays revealed that the addition of GMS, such as 127G+, 188G+, 148G+, 178G+, or 54G+, decreased the virulence of the virus in mice, except for 87G+. The removal of GMS, such as 280G− or 295G−, increased the pathogenicity of the virus in mice. Further studies on pathogenicity revealed that 87G+/295G− could also enhance the pathogenicity of the virus. Finally, we selected the WT, WT-87G+, WT-295G−, and WT-87G+/295G− strains as our further research targets to investigate the detailed biological properties of the viruses. GMS, which can enhance viral pathogenicity, did not significantly affect replication or viral stability in vitro but significantly promoted the expression of proinflammatory factors to enhance inflammatory responses in mouse lungs. These findings further deepen our understanding of the influence of the glycosylation of the HA protein of H9N2 AIV on the pathogenicity and antigenicity of the virus in mice.