Sequential immunization with consensus influenza hemagglutinins raises cross-reactive neutralizing antibodies against various heterologous HA strains

Seasonal and emerging epidemics caused by influenza virus remain as a public health concern and an economic burden. The weak immunogenicity of conserved epitopes on hemagglutinin that induces broad protective immune responses is the main obstacle to the development of universal vaccines. In the present report, we designed the cross-subtypic sequential vaccination strategy and evaluated its neutralizing antibody (nAb) activity by pseudovirus-based neutralization assays. The results clearly indicated that compared with traditional vaccines strategy, the cross-subtypic sequential immunization could significantly induce a broad serum cross-reactive nAb response in mice as well as against homologous strains, and provide protection from heterologous virus PR8 (H1N1) challenge. Furthermore, we isolated two monoclonal antibodies from sequentially immunized mice, which had potent broadly neutralizing activity against multiple influenza strains. These data suggest the feasibility of sequential immunization in universal flu vaccine development.