Broadly protective anti-hemagglutinin stalk antibodies induced by live attenuated influenza vaccine expressing chimeric hemagglutinin

Highlights: • A panel of LAIV candidates bearing chimeric hemagglutinins was generated and compared to classical LAIVs in vitro and in vivo; • LAIVs expressing chimeric HA molecules have similar growth characteristics to classical LAIVs in eggs; • cHA-based LAIVs replicate in mouse respiratory tract to a lesser extent, compared to classical LAIVs; • cHA-based LAIVs demonstrate enhanced cross-protection compared to the LAIVs expressing natural HAs in a mouse model; • The enhanced protection of cHA-based LAIVs is driven by stalk-HA-reactive IgG antibodies. The development of influenza vaccines that can provide broad protection against all drifted seasonal virus variants, zoonotic infections and emerging pandemic strains, has been a priority for two decades. Here we propose a strategy of inducing broadly-reactive anti-stalk antibody by sequential immunizations with live attenuated influenza vaccines (LAIVs) expressing chimeric HAs (cHAs). These vaccines are designed to contain identical hemagglutinin stalk domains from H1N1 virus but antigenically unrelated globular head domains from avian influenza virus subtypes H5, H8 and H9. Mouse experiments demonstrated enhanced cross-protection of cHA-containing LAIVs compared to the relevant vaccine viruses expressing natural HAs, and this enhanced protection was driven by stalk-HA-reactive IgG antibodies. The establishment of fully functional cross-protective immunity after two doses of cHA LAIV vaccination in naïve animals suggests that a similar effect might be expected after a single cHA LAIV dose in primed individuals, or after two to three doses in naïve children.