HA198 Mutations in H9N2 Avian Influenza: Molecular Dynamics Insights into Receptor Binding

The H9N2 avian influenza virus is widely disseminated in poultry and poses a zoonotic threat, despite vaccination efforts. Mutations at residue 198 of hemagglutinin (HA) are critical for antigenic variation and receptor-binding specificity, but the underlying molecular mechanisms remain unclear. This study explores the molecular mechanisms by which mutations at the HA 198 site affect the antigenicity, receptor specificity, and binding affinity of the H9N2 virus. Using the sequence of the A/Chicken/Jiangsu/WJ57/2012 strain, we constructed recombinant H9N2 viruses, including rWJ57, rWJ57/HA _198A , and rWJ57/HA _198T , using reverse genetics. These variants were analyzed through hemagglutination inhibition (HI) assays, solid-phase receptor binding assays, and enzyme-linked immunosorbent assays (ELISA). Additionally, molecular dynamics (MD) simulations were performed to further dissect the atomic-level interactions between HA and sialic acids (SA). The results showed that HA _198V and HA _198T mutations enhanced viral binding to human α2,6-linked SA, while HA _198A exhibited a preference for avian α2,3 SA. These mutations altered interactions with non-neutralizing antibodies but did not affect neutralizing antibodies, potentially contributing to immune evasion. MD simulations revealed that HA _198V/T formed stable complexes with the α2,6 SA, mediated by specific residues and water bridges, whereas HA _198A formed stable complexes with the α2,3 SA. Interestingly, residue 198 interacted with the α2,6 SA via water bridges but had minimal direct interaction with the α2,3 SA. This study provides new insights into the molecular basis of receptor specificity, binding affinity, and antigenic drift in H9N2 viruses, highlighting the critical role of HA 198 mutations in regulating host adaptation. These findings are of great significance for H9N2 virus surveillance, vaccine development, and zoonotic transmission risk assessment.