Stalking influenza by vaccination with pre-fusion headless HA mini-stem for broadly reactive antibodies

Abstract

Background: Influenza vaccine has been available for over 70 years, yet influenza still causes epidemics or pandemic with substantial morbidity and mortality. The protective responses induced by current human influenza vaccines still primarily depend on vaccine-induced neutralizing antibodies (nAbs) against the HA head. However, the continually evolving influenza virus evades herd immunity induced through natural infection and vaccination by means of antigenic drift and shift. These antigenic drift and shift events render vaccine stockpiling unviable in case of an outbreak or pandemic. In addition, a major shortcoming of current influenza vaccines is its long production time because of existing egg-based or cell-based vaccine manufactory pipelines. Thus, these factors combined necessitate the development of novel influenza vaccine with increased breadth of protection and potential for rapid production and deployment. Method: Recent studies indicated the conserved stem domain contains a greater proportion of vulnerable sites targeted by broadly neutralizing antibodies. Importantly, anti-stem broadly neutralizing antibodies are detectable in some individuals at a low level, suggesting they can be induced naturally by infection and optimized by vaccination. Here, we reported the design of a bacterially expressed polypeptide that mimics a H5 HA stem (i.e. group 1 HA) in the pre-fusion conformation by protein minimization. Results: The absent of HA head domain of this protein could focus host’s antibody responses toward the HA stem. The HA mini-stem folded as a trimer and it was resistant to thermal/chemical stress. It bound to various broadly neutralizing HA stem-specific antibodies with high affinity. Mice vaccinated with the group 1 HA mini-stems were protected from morbidity and mortality against lethal challenge by group 1 and group 2 influenza viruses, the first report of cross-group protection. Vaccine-induced antibodies showed broad HA reactivity and no antibody-dependent enhancement activity. Protection from lethal infection was attributed to a broadly reactive antibody response that was able to provide passive protection. Conclusion: The HA mini-stem vaccination can elicit cross-reactive antibody responses that confer robust protection against lethal heterologous influenza A virus challenge from both group 1 and 2 viruses. The recombinant protein is highly stable at room temperature and it can be readily produced in large scale. Our study provides a promising foundation for developing a HA stem-based ‘universal’ influenza vaccine.