Oral DNA vaccination with the polyprotein gene of infectious bursal disease virus (IBDV) delivered by attenuated Salmonella elicits protective immune responses in chickens

Our previous study showed that vaccination with plasmid DNA containing infectious bursal disease virus (IBDV) gene which encodes complete polyprotein (VP2/4/3) induced protective immune responses. In this study, we examined the efficacy of an oral DNA vaccine carrying the IBDV polyprotein antigen delivered by attenuated Salmonella enterica sv. Typhimurium (S. typhimurium). The recombinant plasmid pCI-VP2/4/3 was transformed by electroporation into an attenuated S. typhimurium strain (Dam̄ PhoP̄) (designated hereafter as SV/pCI-VP2/4/3). The IBDV polyprotein gene was expressed in chicken embryo fibroblast (CEF) cells infected with strain SV/pCI-VP2/4/3, as shown by gene-specific RT-PCR and Western blot. Oral immunization of 7-day-old specific-pathogen-free (SPF) chickens with SV/pCI-VP2/4/3 elicited specific humoral responses as measured by ELISA. Vaccination with the strain SV/pCI-VP2/4/3 at 109 CFU per chicken offered 11/15 (73.3%) protection of the chickens against virulent IBDV challenge. Our results have implications in the development of DNA vaccines against avian viral diseases by bacteria-vectored oral delivery system.