An oral double-targeted DNA vaccine induces systemic and intestinal mucosal immune responses and confers high protection against Vibrio mimicus in grass carps

Abstract

Vibrio mimicus (V. mimicus) is an enteric pathogen causing serious vibriosis in aquatic animals, and intestinal mucosal immunity elicited by oral DNA vaccines plays a vital role in the resistance to the pathogen. Yet, the immunogenicity of oral naked DNA vaccine is poor due to the degradation in the digestive tract. Therefore, it is necessary to use targeted DNA delivery strategy for improving the potency of vaccine. In our previous studies, the epitopes of OmpU and VMH proteins from V. mimicus have been identified. Here, a double-targeted DNA vaccine based on the multi-epitope peptide (named OVepis) of V. mimicus was developed by using E. coli DH5α bacterial ghosts (BGs) and invariant chain-like protein (Iclp) of grass carps as exogenous and endogenous targeting delivery carriers, respectively, and then the efficacy was evaluated in grass carps via oral gavage administration. The results showed that the loading efficiency and capacity of DH5α-BGs to pcDNA3.1-Iclp-OVepis were 89.91% and 16.6 μg per milligram BGs, respectively. DH5α-BGs loaded with pcDNA3.1-Iclp-OVepis targeted efficiently RAW264.7 cells and mediated effective gene expression. Higher transcription levels of the Iclp-OVepis gene were detectable in the different tissues of fish in the double-targeted DNA vaccine (DH5α-BGs/pcDNA3.1-Iclp-OVepis) group than that in the endogenous-targeted DNA vaccine (pcDNA3.1-Iclp-OVepis) group at 21 dpv. The double-targeted DNA vaccine significantly enhanced SOD activity, LZM activity and C3 contents in the serum and intestinal mucus of immunized fish. Specific antibody levels in the serum and intestinal mucus were markedly increased in the double-targeted DNA vaccine group compared to the other DNA vaccine groups. at each examined time point. Likewise, the proliferative activities of peripheral blood and intestinal intraepithelial lymphocytes of fish in the double-targeted DNA vaccine group were also significantly higher at 28 dpv. Importantly, the vaccine conferred high protection against V. mimicus challenge with 81.11% RPS. This study suggested that the oral double-targeted DNA vaccine might be used as a promising vaccine candidate to control V. mimicus infection.