Improve immunogenicity of DNA vaccine against Foot-and-Mouth disease virus: Role of Intron and probably Viral 3C Protease as biological adjuvants

Foot-and-mouth disease (FMD) is one of the most important diseases with heavy economic losses. The causative agent of the disease is a virus, named as FMD virus, belonging to the picornavirus family. There is no treatment for the disease and vaccination is the main control strategy. Several vaccination methods have been introduced against FMD including DNA vaccines. In this study, two genetic constructs, which were defined by absence and presence of an intron, were tested for their ability to induce the anti-FMD virus responses in mouse. Both constructs encoded a fusion protein consisting of viral (P12A and 3C) and EGFP proteins under the control of CMV promoter. The protein expression was studied in the COS-7 cells transfected with the plasmids by detecting EGFP protein. Cell death was induced in the cells expressing the P12A3C-EGFP, but not the EGFP, protein. This might be explained by the protease activity of the 3C protein which cleaved critical proteins of the host cells. Mice injected with the intron-containing plasmid induced 16-fold higher antibody level than the intronless plasmid. In addition, serum neutralization antibodies were only induced in the mice injected with intron-containing plasmid. In conclusion, the use of intron might be a useful strategy for enhancing antibody responses by DNA vaccines. Moreover, cell death inducing activity of the 3C protein might suggest applying it along with DNA vaccines to improve immunogenicity.