The highly conserved HA2 protein of the influenza A virus induces a cross protective immune response.

Existing influenza vaccines protect mostly homologous subtypes and acted most effectively only when well matched to the circulating strain. Immunization with an updated vaccine is therefore necessary to maintain long-term protection and the development of a broadly protective influenza vaccine against the threat of pandemic outbreak. The highly conserved HA2 glyco-polypeptide (HA2 gp) is a promising new candidate for such an influenza vaccine. Helical domain and the fusion peptide (residues 15-137) of surface antigen from influenza A subtype A/EM/Korea/W149/06 (H5N1) was used to assess the potentiality of HA2 vaccination against multiple subtypes of the influenza viruses. The construct, named H5HA2 was expressed in Escherichia coli and allowed to refold from inclusion bodies. Purified proteins were used to investigate the immunogenicity of H5HA2 and its potential for cross protection. The immunization of mice with H5HA2 induced HA2 antibodies, HA2 specific T-cell responses, and protection against homologous A/EM/Korea/W149/06 (H5N1) influenza. Immunized mice were also protected from two distinct heterosubtypes of influenza: A/Puerto Rico/1/34(H1N1) and bird/Korea/w81/2005(H5N2). Results suggest that recombinant proteins based on the highly conserved residues within HA2 are candidates for the development of vaccines against pandemic outbreaks of emergent influenza variants.