Dissecting the role of the HA1-226 leucine residue in the fitness and airborne transmission of an A(H9N2) avian influenza virus
A better understanding of viral factors that contribute to influenza A virus (IAV) airborne transmission is crucial for pandemic preparedness. A limited capacity for airborne transmission was recently observed in a human A(H9N2) virus isolate (A/Anhui-Lujiang/39/2018, AL/39) that possesses a leucine (L) residue at position HA1-226 (H3 numbering), indicative of human like receptor binding potential. To evaluate the roles of the residue at this position in virus fitness and airborne transmission, a wild-type AL/39 (AL/39-wt) and a mutant virus (AL/39-HA1-L226Q) with a single substitution at the position HA1-226 from leucine to glutamine (Q), a consensus residue in avian influenza viruses, were rescued and assessed in the ferret model. The AL/39-HA1-L226Q virus lost the ability to transmit by air, although the virus had a comparable capacity for replication, induced similar levels of host innate immune responses, and was detected at comparable levels in the air surrounding the inoculated ferrets relative to AL/39-wt virus. However, ferrets showed a lower susceptibility to AL/39-HA1-L226Q virus infection compared to the AL/39-wt virus. Furthermore, the AL/39-wt and AL/39-HA1-L226Q viruses each gained dominance in different anatomic sites in the respiratory tract in a co-infection competition model in ferrets. Taken together, our findings demonstrate that the increasing dominance of HA1-L226 residue in an avian A(H9N2) virus plays multifaceted roles in virus infection and transmission in the ferret model, including improved virus fitness and infectivity.