The optimized capsid gene of porcine circovirus type 2 expressed in yeast forms virus-like particles and elicits antibody responses in mice fed with recombinant yeast extracts.

Porcine circovirus type 2 (PCV2)-associated diseases are considered to be the biggest problem for the worldwide swine industry. The PCV2 capsid protein (Cap) is an important antigen for development of vaccines. At present, most anti-PCV2 vaccines are produced as injectable formulations. Although effective, these vaccines have certain drawbacks, including stress with concomitant immunosuppresion, and involve laborious and time-consuming procedures. In this study, Saccharomyces cerevisiae was used as a vehicle to deliver PCV2 antigen in a preliminary attempt to develop an oral vaccine, and its immunogenic potential in mice was tested after oral gavage-mediated delivery. The cap gene with a yeast-optimized codon usage sequence (opt-cap) was chemically synthesized and cloned into Escherichia coli/Saccharomyces cerevisiae shuttle vector, pYES2, under the control of the Gal1 promoter. Intracellular expression of the Cap protein was confirmed by Western blot analysis and its antigenic properties were compared with those of baculovirus/insect cell-produced Cap protein derived from the native PCV2 cap gene. It was further demonstrated by electron micrography that the yeast-derived PCV2 Cap protein self-assembles into virus-like particles (VLPs) that are morphologically and antigenically similar to insect cell-derived VLPs. Feeding raw yeast extract containing Cap protein to mice elicited both serum- and fecal-specific antibodies against the antigen. These results show that it is feasible to use S. cerevisiae as a safe and simple system to produce PCV2 virus-like particles, and that oral yeast-mediated antigen delivery is an alternative strategy to efficiently induce anti-PCV2 antibodies in a mouse model, which is worthy of further investigation in swine.