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ABSTRACT H6 influenza A viruses are enzootic and genetically diverse in both domestic poultry and wild birds, occasionally causing spillovers to mammals, including humans, posing threat to public health. However, the antigenic sites of the hemagglutinin (HA) of H6 are poorly understood. In this study, a panel of murine monoclonal antibodies (MAbs) with different reactivity spectra against HA of H6 was generated and nine key residues of antigenic epitopes in H6 were identified through escape mutants of these MAbs. Of these, residues 139 and 140 belonged to antigenic Sa, residue 89 belonged to antigenic Cb, residue 149 belonged to Ca2, while residue 221 belonged to Ca1 regarding the H1 antigenic sites. However, the other four residues 69, 120, 124, and 246 were not located in the identified antigenic regions of H1. Although most HA variants of the escape mutants had the preference for avian-like receptor, whereas escape mutants m3B9 and m4C2 increased the binding affinity for both avian and human-like receptors, and m1A5 and m6D11 decreased the binding affinity for avian-like receptors. Moreover, a single dose of MAbs 4C2 or 6E3 could fully protect mice against lethal challenge of mouse-adapted A/Eurasian teal/Jiangxi/2018WB0417(H6N2) (MA E-Teal/417) in the prophylactic treatment and also could provide partial protection in the therapeutic treatment. Overall, the generated MAbs and the identified key residues of antigenic epitopes not only provide novel insights into the key markers for monitoring the antigenic variation of H6 but also demonstrate the potential for developing MAb-based passitive treatments for H6. IMPORTANCE Since the escape immunity of influenza A viruses (IAVs) is mainly caused by the continuous antigenic variations in HA, the identification of key antigenic epitopes is crucial for better understanding of the escape immunity and vaccine development for IAVs. The antigenic sites of several HA subtypes, including H1, H3, H5, and H9, have been well characterized, whereas those of H6 subtype are poorly understood. Here, we mapped nine key residues of antigenic epitopes in H6 through escape mutants using a panel of MAbs. Moreover, MAbs 4C2 and 6E3, targeting 140 and 89 residues, respectively, could protect mice against lethal challenge of MA E-Teal/417. These key residues of antigenic epitopes identified here provide the molecular targets for further elucidating the antigenic evolution of H6 and better preparing the vaccine against H6 IAV.

H6 avian influenza viruses (AIVs) not only continue to circulate in both domestic poultry and wild waterfowl, but also have occasionally caused spillovers infections in pigs and humans, posing a potential threat to public health. However, the molecular mechanism of H6 AIV adaptation to mammals remains largely unknown. In this study, two mouse-adapted (MA) H6 AIV strains, named as MA E-Teal/417 and MA GWF-Goose/740, were generated through blind passages in BALB/c mice. The two MA H6 strains replicated more efficiently and showed higher virulence than the corresponding wild type (WT) H6 strains in mice. Genome sequencing revealed that MA E-Teal/417 and MA GWF-Goose/740 carried six amino acid mutations (PB2-T224A/E627K, HA-G124R, NA-F167L/Y356H and M1-M92R), and four amino acid mutations (PB1-K577E, PA-T97I/D514E and HA-T276K), respectively, when compared to the corresponding WT virus. Receptor binding assay showed MA E-Teal/417 had stronger binding activity to α-2,3 SA than WT E-Teal/417. Moreover, the polymerase activity analysis found the RNP polymerase activity of both MA H6 viruses was significantly higher than that of the corresponding WT virus in 293T cells. All these demonstrate that H6 AIV can acquire limit amino acid substitutions to adapt to mammals and increase virulence, highlighting the significance of monitoring such mutations of H6 AIV in the field for alarming the potential of its cross-transmission and pathogenesis in mammals.

Background Avian leukosis virus subgroup J (ALV-J) causes tumours and immunosuppression in chickens. The host-ALV interactions at the transcriptional level are unknown. In this study, gene expression profiling was performed to analyse the bursa response induced by ALV-J strain JS09GY3 in chickens. Results A total of 594 gene transcripts displaying differential expression during ALV-J infection were identified. These differentially expressed genes are involved in binding, biological regulation, metabolic processes (MYF6 and FABP3), response to stimulus (F13A1 and CNGA3) and immune system processes (LY86, CATHL2, CCL4, and OASL), and several differentially expressed genes (e.g., ETV7, MMP9, and NOV) are involved in tumourigenesis. Eight differentially expressed genes were confirmed by quantitative reverse transcription-PCR (qRT-PCR). Based on pathway analysis, the notable signalling pathways mainly included cytokine-cytokine receptor interaction, the JAK-STAT signalling pathway and the RIG-1-like receptor signalling pathway. Conclusions The gene expression profile obtained in this study may aid a better understanding of the molecular pathogenesis of ALV-J infection in chickens. [ABSTRACT FROM AUTHOR]