A novel eight amino acid insertion contributes to the hemagglutinin cleavability and the virulence of a highly pathogenic avian influenza A (H7N3) virus in mice

Generation of Stable Influenza Virus Hemagglutinin through Structure-Guided Recombination

Hemagglutinin (HA) is the major surface antigen of influenza virus and the most promising influenza vaccine immunogen. In 2013, the devastating H7N9 influenza virus was identified in China, which induced high mortality. The HA of this virus (H7) is relatively unstable, making it challenging to produce an effective vaccine. To improve the stability of HA protein from H7N9 influenza virus for better vaccine antigens without impairing immunogenicity, we recombined the HA from H7N9 (H7) with a more stable HA from H3N2 (H3) by structure-guided recombination, resulting in six chimeric HAs, FrA-FrF. Two of these chimeric HAs, FrB and FrC, exhibited proper hemagglutination activity and presented improved thermal stability compared to the original H7. Mice immunized with FrB and FrC elicited H7-specific antibodies comparable to those induced by parental H7, and the antisera collected from these immunized mice successfully inhibited H7N9 infection in a microneutralization assay. These results suggest that our structural-recombination approach can create stabilizing chimeric antigens while maintaining proper immunogenicity, which may not only benefit the construction of more stable HA vaccines to fight against H7N9 infection, but also facilitate effective vaccine improvements for other influenza viruses or infectious pathogens. In addition, this study also demonstrates the potential for better engineering of multimeric protein complexes like HA to achieve improved function, which are often immunologically or pharmaceutically important but difficult to modify.

Identification of key hemagglutinin residues responsible for cleavage, acid stability, and virulence of fifth-wave highly pathogenic avian influenza A(H7N9) viruses

We previously demonstrated that despite no airborne transmissibility increase compared to low pathogenic avian influenza viruses, select human isolates of highly pathogenic avian influenza A(H7N9) virus exhibit greater virulence in animal models and a lower threshold pH for fusion. In the current study, we utilized both in vitro and in vivo approaches to identify key residues responsible for hemagglutinin (HA) intracellular cleavage, acid stability, and virulence in mice. We found that the four amino acid insertion (-KRTA-) at the HA cleavage site of A/Taiwan/1/2017 virus is essential for HA intracellular cleavage and contributes to disease in mice. Furthermore, a lysine to glutamic acid mutation at position HA2-64 increased the threshold pH for HA activation, reduced virus stability, and replication in mice. Identification of a key residue responsible for enhanced acid stability of A(H7N9) viruses is of great significance for future surveillance activities and improvements in vaccine stability.

Role of H7 hemagglutinin in murine infectivity of influenza viruses following ocular inoculation

H7 subtype influenza viruses have demonstrated an ocular tropism in humans, causing conjunctivitis and not respiratory symptoms in many infected individuals. However, the molecular determinants which confer ocular tropism are still poorly understood. Here, we used a murine model of ocular inoculation to demonstrate that H7 influenza viruses are more likely to cause infection following ocular exposure than are non-H7 subtype viruses. We included investigation regarding the potential role of several properties of influenza viruses with murine infectivity following ocular inoculation, including virus lineage, pathogenicity, and HA cleavage site composition. Furthermore, we examined the potential contribution of internal proteins to murine ocular infectivity. These studies establish a link between H7 subtype viruses and the risk of heightened infectivity in a mammalian species following ocular exposure, and support the development of non-traditional inoculation methods and models to best understand the human risk posed by influenza viruses of all subtypes.