PCV Cap proteins fused with calreticulin expressed into polymers in Escherichia coli with high immunogenicity in mice

Background： Porcine circovirus type 2 (PCV2) is considered one of the critical viral pathogens of porcine circovirus diseases (PCVDs), which results in economic losses in the pig breeding industry. Capsid protein (Cap) of PCV2 acts as the most protective antigen in the course of immune response. Therefore, developing a novel and safe subunit vaccine against PCV2 infection is needed.Results: In this study, the Cap gene bound to the truncated calreticulin (CRT) (120-250 aa/120-308 aa) at the N/C terminal, and then the CRT-Cap fusion genes were expressed in Escherichia coli (E.coli) . The size-exclusion chromatography and dynamic light scattering (DLS) data showed that the purified recombinant CRT-Cap fusion protein (rP5F) existed in the form of polymers. The results of ELISA and NA indicated that humoral immune responses induced by rP5F in mice were almost identical to those by the commercial subunit and inactivated vaccine. The lymphocyte proliferation and cytokine secretion were also detected in rP5F immunized mice. According to the results of the PCV2-challenge experiment, virus loads significantly decreased in mice immunized with rP5F. The data obtained in the current study revealed that rP5F might have the potential to be a subunit vaccine candidate against PCV2 in the future. Conclusions: We have successfully expressed Cap-CRT fusion proteins in E.coli and optimized rP5F could form into immunogenic polymers. Mice immunized with rP5F efficiently induced humoral and part of cellular immune responses and decreased the virus content against PCV2-challenge, which suggested that rF5P could be a potential subunit vaccine candidate.