| Summary: | Developing new vaccine candidates is considered the best strategy for protecting poultry against artificial haemagglutinin (A/H5N1) strains. The transient expression system in plants has been a very efficient method for rapidly producing haemagglutinin-based recombinant vaccines. In this study, two novel artificial trimeric haemagglutinin constructs representing A/H5N1 strains that were detected in poultry from 2005 to 2015 in Vietnam, H5.c1 (representing all of the subclades 1.1, 1.1.1, and 1.1.2) and H5.c2 (representing all of the subclades 2.3.2.1, 2.3.2.1a, 2.3.2.1b, and 2.3.2.1c), were designed for transient expression in <i>Nicotiana benthamiana</i> via agroinfiltration. However, only the H5.c1 protein, which showed the best expression and biofunction via the haemagglutination test, was selected for purification by immobilized metal ion affinity chromatography (IMAC). The trimeric structure of the IMAC-purified H5.c1 protein was well characterized by cross-linking reaction and size exclusion chromatography. An indirect ELISA and Western blot analysis of vaccinated mouse sera demonstrated that the H5.c1 protein strongly induced HA-specific Immunoglobulin G (IgG) immune responses. Notably, the H5.c1 protein induced strongly neutralizing antibodies against homologous H5.c1 protein and that of three heterologous native strains of clade, 1, 1.1, and 2.3.2.1c, in haemagglutination inhibition assays. Therefore, the plant-based artificial H5.c1 protein can be a promising vaccine candidate for conferring poultry resistance against A/H5N1 viruses in Vietnam.
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