Design and evaluation of an Edwardsiella tarda DNA vaccine co-encoding antigenic and adjuvant peptide

Edwardsiella tarda is associated with edwardsiellosis in cultured fish, resulting in heavy losses in aquaculture. So far, different types of vaccine have been attempted against E. tarda. In this study, an optimized eukaryotic expression plasmid was developed and an optimized DNA vaccine co-encoding antigenic and adjuvant peptide using a bicistronic expression system was designed. As a result, a modified plasmid harbored cytomegalovirus (CMV) promoter attached with R region of long terminal repeat from human T-cell leukemia virus type 1 (CMV/R) and woodchuck hepatitis virus post-transcriptional response element (WPRE) component showed an increased antigenic gene expression compared with unmodified plasmid. Moreover, the designed system based on bicistronic system exhibited a stronger ability to express antigenic gene and the RPS achieved 87.3% compared with plasmid encoding antigentic gene. Finally, immunological analysis showed that the DNA vaccine induced both innate and adaptive immune responses. These results suggest that co-encoding antigenic and adjuvant proteins might be an efficient strategy to develop DNA vaccines in aquaculture in the future.