Objective: To explore a strategy to enhance the specific immune response induced by secreted form of Coxsackie virus B3 (CVB3) capsid protein 1 (sVP1) gene vaccination against Coxsackie virus infection.
Methods: Eukaryotic expression plasmid pcDNA3/sVP1-C3d3 was constructed by conjugating the 3 copies of C3d (C3d3) with the secreted form of sVP1 of CVB3. 42 BALB/c mice were randomly divided into 3 equal groups to be inoculated with pcDNA3/sVP1-C3d3, pcDNA3/sVP1 and pcDNA3 plasmids respectively once every 4 weeks for 3 times. 14 days after every inoculation venous blood was collected to measure the titer of neutralizing antibody. Three weeks after the last inoculation their spleens were taken out to detect the specific CTL cytotoxic activity. Three mice from each group underwent intraperitoneal injection of 3 LD(50) CVB3 and were killed 7 days later, and then the titer of blood viruses was measured. The remaining 8 mice underwent intraperitoneal injection of 5 LD(50) CVB3 and the survival status was observed for 21 days.
Results: The titer of neutralizing antibody in the mouse blood increased along with the time after inoculation. The antibody titer and specific CTL cytotoxic activity 3 days after inoculation of the pcDNA3/sVP1-C3d3 group were 33.6 +/- 1.7 and 66.1% +/- 2.9% respectively, both significantly stronger than those of the pcDNA3/sVP1 group [(28.3 +/- 1.7) and (52.8% +/- 3.3%) respectively, both P < 0.05]. After lethal CVB3 challenge, the blood virus titer of the pcDNA3/sVP1-C3d3 group was significantly lower than that of the pcDNA3 group (P < 0.05), and the survival rate of pcDNA3/sVP1-C3d3 group was 50%, significantly higher than that of the pcDNA3 group (0, P < 0.05), however, not significantly different from that of the pcDNA3/sVP1 group (25%, P > 0.05).
Conclusion: C3d strongly enhances the specific immune response induced by SVP1 gene vaccination.