Encapsidated-CpG ODN enhances immunogenicity of porcine circovirus type 2 virus-like particles

A DNA fragment containing CpG motifs (CpG ODN) is one of the potent immunopotentiators used to improve vaccine efficacy. It can enhance a protective immunity by stimulating both innate and adaptive immune responses. In this study, we designed and constructed a recombinant plasmid carrying the combined CpG ODN to generate an immunopotentiator for boosting the immunogenicity of porcine circovirus type 2 (PCV2) virus-like particles (VLPs). The capsid protein of PCV2b was expressed in insect cells and purified by affinity chromatography. The purified capsid protein was incubated with the CpG ODN in the reaction that allowed VLPs formation and encapsidation of the CpG ODN to occur simultaneously. Morphology of the reassembled VLPs was similar to the PCV2 virions as observed using an electron microscope. When the CpG ODN-encapcidated VLPs was treated with DNase I, the VLPs could protect the packaged CpG ODN from the enzyme digestion. Moreover, we immunized mice subcutaneously with VLPs, CpG ODN-loaded VLPs, or phosphate buffer saline for three times at two-week intervals. The results showed that the CpG ODN-loaded VLPs could elicit significantly higher levels of PCV2-specific neutralizing antibodies and interferon gamma (IFN-γ) expression in the immunized mice compared to those conferred by the VLPs alone. Conclusively, we have proved that the CpG ODN incorporated in VLPs can serve as a potent immunopotentiator for PCV2 vaccine development.