Abstract
Avian influenza A(H9N2) viruses are an increasing threat to global poultry production and, through zoonotic infection, to human health where they are considered viruses with pandemic potential. Vaccination of poultry is a key element of disease control in endemic countries, but vaccine effectiveness is persistently challenged by the emergence of antigenic variants. Here we employed a combination of techniques to investigate the genetic basis of H9N2 antigenic variability and evaluate the role of different molecular mechanisms of immune escape. We systematically tested the influence of published H9N2 monoclonal antibody escape mutants on chicken antisera binding, determining that many have no significant effect. Substitutions introducing additional glycosylation sites were a notable exception, though these are relatively rare among circulating viruses. To identify substitutions responsible for antigenic variation in circulating viruses, we performed an integrated meta-analysis of all published H9 haemagglutinin sequences and antigenic data. We validated this statistical analysis experimentally and allocated several new residues to H9N2 antigenic sites, providing molecular markers that will help explain vaccine breakdown in the field and inform vaccine selection decisions. We find evidence for the importance of alternative mechanisms of immune escape, beyond simple modulation of epitope structure, with substitutions increasing glycosylation or receptor-binding avidity, exhibiting the largest impacts on chicken antisera binding. Of these, meta-analysis indicates avidity regulation to be more relevant to the evolution of circulating viruses, suggesting that a specific focus on avidity regulation is required to fully understand the molecular basis of immune escape by influenza, and potentially other viruses.
Highlights
In recent years, novel avian influenza virus (AIV) strains have emerged as a major threat to animal and human health
At 10 of the 30 published escape sites, no evidence of an antigenic impact in haemagglutination inhibition (HI) assays was detected, even when taking alternative substitutions at the site into account. These results suggest that antibodies in chicken polyclonal antisera do not consistently recognise the same epitopes as mouse monoclonal antibody (mAb) or that there are differences in immunodominance, and that further sites likely contribute to antigenic variation of H9N2 viruses in nature
We assessed the contribution of residues reported by the literature to be antigenic and their relation to virus antigenic diversity and showed that previous studies based on murine mAb escape mutants are only moderately reliable at providing relevant antigenic information; several false positives found to have no impact on inhibition of chicken antisera were identified, as were residues that are very highly conserved in the field
Summary
Novel avian influenza virus (AIV) strains have emerged as a major threat to animal and human health. H9N2 AIVs are endemic across much of Asia, the Middle East and North Africa, where they cause severe economic losses to the poultry industry through moderate-to-high morbidity and mortality[1,2,3]. Owing to the threat to poultry and human health posed by endemic and emerging H9N2 AIVs, many countries vaccinate poultry as a major method of viral control, with conventional inactivated vaccines being used most commonly[12,13,14]. As with human influenza virus, poor vaccine matching owing to antigenic drift often results in vaccine failure[3,12,14,15]. Immunity, following infection or vaccination, is primarily achieved by the generation of neutralising antibodies that interact with haemagglutinin (HA), the major influenza antigen and receptor-binding protein, sterically blocking attachment to target cells[16]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
