Immunogenicity and protective efficacy of DHBV DNA vaccines expressing envelope and capsid fusion proteins in ducks delivered by attenuated Salmonella typhimurium

Duck hepatitis B virus (DHBV) shares many basic characteristics with hepatitis B virus (HBV) and is an attractive model for vaccine development. In this study, DHBV DNA vaccines were designed to express envelope and capsid fusion proteins to enhance the breadth of immune response in ducks. Attenuated Salmonella typhimurium (SL7207) was used as a carrier and adjuvant to boost the magnitude of immune response. Based on this strategy, novel DNA vaccines (SL7207-pVAX1-LC and SL7207-pVAX1-SC) were generated. Growth kinetics, genetic stabilities and relative transcription levels of the L, S and C genes introduced by these vaccine strains were measured before inoculation to guarantee safety and efficacy. The relative transcript levels of the CD4 and CD8 T genes and the antibody levels (IgY) in ducks receiving the vaccines were higher than those in single gene delivered groups. Additionally, the copy number of covalently closed circular DNA in hepatocytes after DHBV challenge also provided evidence that our fusion vaccines could enhance the protective efficiency against DHBV infection in ducks.