Construction and immunogenicity of DNA vaccines encoding fusion protein of murine complement C3d-p28 and GP5 gene of porcine reproductive and respiratory syndrome virus.

Porcine reproductive and respiratory syndrome virus (PRRSV) has recently caused catastrophic losses in swine industry worldwide. The commercial vaccines only provide a limited protection against PRRSV infection. At present, DNA vaccine is the focus on the new vaccines. The gene fragment (p28) coding for the molecular adjuvants complement protein C3d (mC3d) from BALB/c mouse was cloned and expressed as a fusion protein for its application in the vaccine study of mice. Three potential vaccines construct units were engineered to contain two, four and six copies of mC3d-p28 coding gene linked to the GP5 gene of PRRSV and one vaccine expressing GP5 alone (pcDNA3.1-GP5) was constructed. Subsequently, the vaccines' abilities to elicit the humoral and cellular immune responses were investigated in mice. These results showed that significantly enhanced GP5-specific ELISA antibody, GP5-specific neutralizing antibody, IFN-γ level, and IL-4 level, could be induced in mice immunized with DNA construct units encoding the pcDNA3.1-C3d-p28.n-GP5 than those received DNA vaccine expressing GP5 alone (pcDNA3.1-GP5). Analysis of the immunogenicity of different repeats of mC3d-p28 revealed that mC3d-p28 had an enhancing effect on the immunogenicity of antigens, and that six or more repeats of mC3d-p28 may be necessary for efficient enhancement of antigen specific immune responses. This approach may provide a new strategy for the development of efficient vaccines against the PRRSV for pigs in the future.