Sequential DNA immunization of chickens with bivalent heterologous vaccines induce highly reactive and cross-specific antibodies against influenza hemagglutinin

Vaccines against avian influenza are mostly based on hemagglutinin (HA), which is the main antigen of this virus and a target for neutralizing antibodies. Traditional vaccines are known to be poorly efficient against newly emerging strains, which is an increasing worldwide problem for human health and for the poultry industry. As demonstrated by research and clinical data, sequential exposure to divergent influenza HAs can boost induction of universal antibodies which recognize conserved epitopes. In this work, we have performed sequential immunization of laying hens using monovalent or bivalent compositions of DNA vaccines encoding HAs from distant groups 1 and 2 (H5, H1, and H3 subtypes, respectively). This strategy gave promising results, as it led to induction of polyclonal antibodies against HAs from both groups. These polyclonal antibodies showed cross-reactivity between different HA strains in ELISA, especially when bivalent formulations were used for immunization of birds. However, cross-reactivity of antibodies induced against H3 and H5 HA subtypes was rather limited against each other after homologous immunization. Using a cocktail of HA sequences and/or sequential DNA vaccination with different strains presents a good strategy to overcome the limited effectiveness of vaccines and induce broader immunity against avian influenza. Such a strategy could be adapted for vaccinating laying hens or parental flocks of different groups of poultry.