Scientists Map Two New Linear Epitopes on H9N2 Bird Flu Hemagglutinin

Researchers have identified and precisely mapped two previously uncharacterized linear B-cell epitopes on the hemagglutinin protein of H9N2 avian influenza virus, offering new molecular targets for subtype-specific diagnostic assays. The study, published as an open-access Original Article in the journal Animal Diseases on 22 July 2026, describes the generation of two monoclonal antibodies against the viral HA protein, the fine mapping of the short continuous peptide segments they recognize, and a detailed analysis of the residues and structural contexts that govern antibody binding. Although neither antibody showed neutralizing activity, the authors argue that the epitopes themselves are valuable precisely because they are conserved, surface-exposed, and largely specific to the H9N2 subtype.

H9N2 is a low-pathogenicity avian influenza virus that was first isolated from turkeys in Wisconsin, USA, in 1966. Since then it has become endemic in poultry across Asia, the Middle East, Africa, and parts of Europe, and in China it emerged in the 1990s and now ranks among the most prevalent subtypes in chickens and other domestic birds. While infections are typically associated with mild respiratory signs, the virus causes substantial economic losses by reducing egg production, impairing immune function, and increasing susceptibility to secondary bacterial and viral infections. H9N2 also infects a range of avian hosts and occasionally spills over into mammals, including pigs, dogs, and humans, underscoring its zoonotic potential. In addition, the virus frequently donates internal gene segments to other avian influenza subtypes such as H5N1, H7N9, and H10N8, facilitating the emergence of novel reassortant strains with pandemic potential.

The hemagglutinin protein sits at the center of both immunity and diagnostics for influenza A viruses. HA functions as a homotrimer on the viral surface, with each monomer consisting of two disulfide-linked subunits: HA1, the globular head that binds sialic acid receptors, and HA2, the stem that mediates membrane fusion. Because HA is strongly immunogenic, it is the principal target of neutralizing antibodies and the backbone of most vaccine and diagnostic antigen design. However, frequent antigenic drift, particularly within the HA1 domain, can alter antibody recognition and erode both vaccine efficacy and the reliability of diagnostic tests. Linear epitopes, which consist of continuous amino acid sequences that remain recognizable even under denaturing conditions, are considered especially useful building blocks for peptide-based diagnostic assays.

In the new work, the team, led by Dong Chen and Hongyun Wei with corresponding author Rui Luo, expressed the extracellular domain of HA from the H9N2 strain A/chicken/Hubei/222/2022 as a recombinant fusion protein carrying a human IgG Fc tag. The HA-hFc protein was produced in HEK293 cells and purified by protein A affinity chromatography. SDS-polyacrylamide gel electrophoresis revealed a single band above the predicted molecular weight of roughly 90 kilodaltons, a shift consistent with glycosylation, and Western blotting with H9N2-specific mouse antiserum and an anti-human IgG reagent confirmed successful expression and purification of the immunogen.

Immunization of BALB/c mice with this recombinant protein yielded two hybridoma clones, designated 9C12 and 9F4. Both monoclonal antibodies specifically recognized viral HA in H9N2-infected MDCK cells, as demonstrated by Western blotting and an indirect immunofluorescence assay, and both bound the recombinant antigen in ELISA. Functional testing, however, showed that neither antibody inhibited hemagglutination in a hemagglutination inhibition assay, and neutralization assays returned IC50 values greater than 50 micrograms per milliliter, indicating that no neutralizing activity was detectable in vitro. The antibodies therefore recognize antigenically accessible but non-neutralizing regions of the HA molecule, a profile the authors consider well suited to epitope mapping and diagnostic development rather than therapeutic or vaccine applications.

To locate the exact binding sites, the researchers used a panel of N-terminal EGFP-tagged HA truncation constructs expressed in HEK293T cells and screened by Western blot. For 9C12, sequential subdivision narrowed the reactive region from the full HA1 domain, amino acids 19 to 338, down through fragments covering residues 96 to 195, 96 to 145, and 116 to 135, before overlapping short peptides defined the minimal linear epitope as the eight-residue sequence FSSSRSYQ at positions 123 to 130. The same strategy applied to 9F4 traced its binding site through HA1 subfragments of residues 176 to 279, 176 to 225, and 196 to 215, ultimately defining the minimal epitope as the nine-residue sequence NLYTRTDTT at positions 201 to 209.

Alanine-scanning mutagenesis then resolved which individual residues within each epitope are essential for antibody recognition. Within the 9C12 epitope, replacing S124, S125, R127, S128, Y129, or Q130 with alanine abolished antibody binding, whereas substitutions at F123 and S126 had no detectable effect. For the 9F4 epitope, the indispensable residues were N201, Y203, T204, T206, and D207, while L202, R205, T208, and T209 could be replaced without disrupting recognition. These single-residue definitions, the authors note, provide a rational molecular basis for assay design, since the critical positions are predominantly hydrophilic or charged, consistent with their role as anchor points stabilizing the antibody-epitope interaction.

Structural and bioinformatic analyses placed both epitopes in functional context. Immunological profiling with the PROTEAN module of DNAStar showed that both peptides display strong hydrophilicity, high antigenic indices, and elevated surface probability. A three-dimensional homology model of the HA trimer, generated with SWISS-MODEL and based on the A/chicken/Hubei/222/2022 sequence, confirmed that both epitopes are exposed on the surface of the trimer and solvent accessible. Domain-level mapping revealed that the 9F4 epitope NLYTRTDTT lies within the receptor-binding subdomain, whereas the 9C12 epitope FSSSRSYQ resides in the vestigial esterase subdomain. Notably, even though the 9F4 epitope sits within the receptor-binding region, its recognition by 9F4 was insufficient to confer detectable neutralizing activity.

Conservation analysis strengthened the diagnostic case for both motifs. Multiple sequence alignment of HA proteins from 30 representative H9N2 isolates retrieved from the NCBI database showed that the 9C12 epitope is largely conserved, with key residues F123, S124, S125, S128, Y129, and Q130 strongly stable and only minor substitutions observed at S126 and R127. The 9F4 epitope, by contrast, displayed near-complete invariance across all strains analyzed, which the authors describe as exceptional stability within the H9N2 subtype. When the same regions were compared with homologous sequences in other influenza A virus subtypes, including H1 through H13 and H16, both epitopes were poorly conserved in non-H9 viruses, suggesting subtype-associated conservation that could serve as a molecular marker for H9N2-associated detection and surveillance.

The authors position these findings against the limitations of current serological tools. Traditional hemagglutination inhibition tests remain widely used but are often limited by cross-reactivity and insufficient resolution when distinguishing closely related subtypes such as H9, H5, and H7. Monoclonal antibodies directed against short, well-defined linear epitopes such as NLYTRTDTT may help improve the specificity of H9N2 serological detection, a capability relevant to field surveillance, vaccine strain selection, outbreak containment, and monitoring of antigenic evolution. The authors emphasize that because both epitopes are nonneutralizing, they are unsuitable for direct use in protective vaccine design, and they caution that future work must validate diagnostic performance across large-scale poultry samples and multiple assay platforms, including peptide-based ELISAs and lateral-flow immunoassays. Within those limits, the study delivers experimentally validated peptide motifs and paired monoclonal antibodies that expand the antigenic map of H9N2 hemagglutinin and provide concrete raw material for subtype-specific diagnostic reagents.

Subject of Research: Mapping of two novel linear B-cell epitopes on the hemagglutinin protein of H9N2 avian influenza virus using monoclonal antibodies

Article Title: Identification of two novel B-cell epitopes targeting the hemagglutinin protein of H9N2 avian influenza virus

Article References: Chen, D., Wei, H., Zhang, Y., Liu, D., Kong, D., Zeinalzadeh, Z., Li, K., Zhou, P., Jin, H., Zhou, H., & Luo, R. (2026). Identification of two novel B-cell epitopes targeting the hemagglutinin protein of H9N2 avian influenza virus. Animal Diseases, 6(1), Article 47. https://doi.org/10.1186/s44149-026-00261-7

Image Credits: AI Generated

DOI: 10.1186/s44149-026-00261-7

Keywords: H9N2 avian influenza virus, hemagglutinin, B-cell epitopes, monoclonal antibodies, epitope mapping, alanine scanning, diagnostic assays, antigenic drift, poultry health, zoonotic potential, conservation analysis, vestigial esterase domain