Evolutionary lineage and host origin influence virulence and mammalian adaptation of H7N9 avian influenza viruses.

The H7N9 avian influenza virus (AIV) has posed a major global public health concern since its first detection in China in 2013. Transmitted among wild birds and poultry, this virus has crossed the species barrier to infect humans, causing severe respiratory disease and high mortality. Although the widespread use of H7 vaccines has markedly reduced human infections, the ongoing circulation and adaptive evolution of the virus in poultry remain a serious threat. In this study, we analyzed three highly pathogenic H7N9 isolates collected in China in 2022, representing two hemagglutinin (HA) gene evolutionary lineages: Group.y.2.3 (isolate 229-4, chicken origin; isolate 782-2, quail origin) and Group.y.2.4 (isolate 621, quail origin). Pathogenicity was compared through phylogenetic analysis, molecular characterization, and infection experiments in both avian and mammalian models. Group.y.2.3 isolates displayed stronger replication and pathogenicity in chickens and mice, with isolate 782-2 being the most virulent. The chicken-origin isolate 229-4 caused more severe weight loss and higher viral loads in the lungs of mice, indicating that host origin influences cross-species transmission potential. Molecular analyses revealed that all isolates possessed multiple basic cleavage sites and mutations linked to mammalian adaptation, including HA 186 V. Some isolates also harbored newly acquired glycosylation sites associated with immune evasion. Overall, our findings demonstrate that both genetic lineage and host origin shape the biological characteristics of H7N9 isolates. Group.y.2.3 isolates warrant priority in surveillance, providing critical insights for vaccine updates and risk assessment.