Since 2003, H5N1 highly pathogenic
avian influenza viruses (HPAIV) have not only caused outbreaks in poultry but were also transmitted to humans with high mortality rates. Vaccination is an efficient and economical means of increasing immunity against
infections to decrease the shedding of infectious agents in immunized animals and to reduce the probability of further
infections.
Subunit vaccines from plants are the focus of modern
vaccine developments. In this study, plant-made
hemagglutinin (H5) trimers were purified from transiently transformed N. benthamiana plants. All chickens immunized with purified H5 trimers were fully protected against the severe HPAIV H5N1 challenge. We further developed a proof-of-principle approach by using
disulfide bonds, homoantiparallel
peptides or homodimer
proteins to combine H5 trimers leading to production of H5 oligomers. Mice vaccinated with crude leaf extracts containing H5 oligomers induced
neutralizing antibodies better than those induced by crude leaf extracts containing trimers. As a major result, eleven out of twelve chickens (92%) immunized with adjuvanted H5 oligomer
crude extracts were protected from lethal disease while nine out of twelve chickens (75%) vaccinated with adjuvanted H5 trimer
crude extracts survived. The solid protective immune response achieved by immunization with
crude extracts and the stability of the oligomers form the basis for the development of inexpensive protective veterinary
vaccines.