Generation and immunogenicity assessment of ELPylated virus-like particles of porcine circovirus type 2

BACKGROUND: Porcine circovirus type 2 (PCV2) is an economically important pathogen affecting swine industry worldwide. The production of current PCV2 vaccines is time-consuming and expensive. Elastin-like polypeptides (ELP) undergo temperature-dependent inverse phase transition and ELPylated proteins can be purified simply by inverse transition cycling (ITC). METHODS: The Cap protein of PCV2b, together with the virus neutralizing (VN) epitopes of PCV2a, PCV2d and PCV2e, was expressed in E. coli as an ELPylated protein, and purified by ITC in the presence of mild detergents. For the control purpose, the Cap protein was also expressed as a His-tagged protein and purified by nickel affinity chromatography. The formation of ELPylated VLP (ELP-VLP) and His-tagged VLP (VLP) was revealed by transmission electron microscopy. Mice were immunized two times with the two forms of VLP and the antigen-specific IgG antibody, VN antibody, cytokine responses and immunoprotection against PCV2 challenge were compared. RESULTS: ELPylated Cap protein was expressed as a soluble protein and purified to 94.3% purity by ITC in the presence of 1% Triton X-100 and 0.5 M urea. His-tagged Cap fusion protein was expressed as insoluble inclusion bodies and purified to 90% purity under denatured conditions. The two purified fusion proteins assembled into VLP with similar morphology. Compared to immunization with VLP, immunization with ELP-VLP induced significantly (p < 0.01) stronger VN antibody response and slightly (p < 0.05) stronger Cap-specific IgG antibody response, cytokine production and immunoprotection against PCV2 challenge. CONCLUSION: A novel ELPylation platform for easy preparation of PCV2 VLP was established and the prepared ELP-VLP was more immunogenic than VLP. The ELPylation technology could be used for other VLP preparation and the prepared ELP-VLP could be developed as a novel PCV2 subunit vaccine.