Generation and immunogenicity of porcine circovirus type 2 chimeric virus-like particles displaying porcine reproductive and respiratory syndrome virus GP5 epitope B

Virus-like particles (VLPs) can be used as transfer vehicles carrying foreign proteins or antigen epitopes to produce chimeric VLPs for bivalent or multivalent vaccines. Based on the crystal structure of porcine circovirus type 2 (PCV2) capsid protein (Cap), in addition to alignment of the Cap sequences collected from various isolates of PCV2 and PCV1, we predicted that Loop CD of the PCV2 Cap should tolerate insertion of foreign epitopes, and furthermore that such an insertion could be presented on the surface of PCV2 VLPs. To validate this, the GP5 epitope B of porcine reproductive and respiratory syndrome virus (PRRSV) was inserted into Loop CD of the PCV2 Cap. The 3D structure of the recombinant PCV2 Cap (rCap) was simulated by homology modeling; it appeared that the GP5 epitope B was folded as a relatively independent unit, separated from the PCV2 Cap backbone. Furthermore, based on transmission electron microscopy, the purified PCV2 rCap self-assembled into chimeric VLPs which entered PK-15 cells. In addition, PCV2 chimeric VLPs induced strong humoral (neutralizing antibodies against PCV2 and PRRSV) and cellular immune responses in mice. We concluded that the identified insertion site in the PCV2 Cap had great potential to develop PCV2 VLPs-based bivalent or multivalent vaccines; furthermore, it would also facilitate development of a nano-device to present a functional peptide on the surface of the VLPs that could be used for therapeutic purposes.