P28 derived from C3d enhances the immune responses against HBV-preS2/S induced by gene immunization
Objective To investigate the possibility of P28 derived from C3d to enhance the immune response to HBV-preS2/S that is induced by direct injection of naked plasmids containing different repeats of P28 and HBV-preS2/S in fusion form. Methods Copies of C3d-P28 coding gene, amplified by PCR and modified by restriction endonucleases digestion, were subcloned into a eukaryotic expression vector pVAON33 to construct pVAON33-P28.[1-4], respectively. HBV-preS2/S coding sequence was then introduced into pVAON33, pVAON33-P28.[1-4] and then identified by both PCR and DNA sequencing. BALB/c mice were primed by im gene immunization with different recombinant plasmids on day 0 and were boosted by sc inoculation with HBsAg protein 12 weeks post-priming. The levels of specific IgG in sera collected at the indicated times from each group were determined by ELISA. Results HBsAg specific antibody response was elicited in groups primed with plasmids pVAON33-S2/S-P28.[1-4] and pVAON33-S2/S. However, the response against HBsAg in the groups primed with pVAON33-S2/S-P28.[1-4] was significant higher than that in pVAON33-S2/S group, the highest level of the specific antibody response was observed in the group primed with pVAON33-S2/S-P28.4 (P0.01). The booster immunization with HBsAg protein accelerated and increased specific antibody in all groups except pVAON33 control, a stronger response was still found in the group vaccinated with pVAON33-S2/S-P28.[1-4] than that in pVAON33-S2/S group (P0.05). Conclusion Different repeats of C3d-P28 enhanced both primary and secondary response induced by gene immunization, in which 4 copies of C3d-P28 sequence showed the strongest effect of enhancement.