Improved Immunogenicity of DNA Constructs Co-expressing the GP5 and M Proteins of Porcine Reproductive and Respiratory Syndrome Virus by Glycine-proline-glycine-proline (GPGP) Linker in Mice

The objective of the study was to evaluate whether co-expressing porcine reproductive and respiratory syndrome virus (PRRSV) envelop glycoprotein 5 (GP5) and matrix (M) protein linked by glycine-proline-glycine-proline (GPGP) could enhance its immunogenicity in mice. Three DNA constructs expressing GP5/M without GPGP linker (pcDNA-56), GP5/M conjugated by GPGP linker (pcDNA-5L6), and M/GP5 conjugated by GPGP linker (pcDNA-6L5) were included. These constructs were then inserted into an eukaryotic expression vector, pcDNA3.1/V5-His TOPO, as DNA vaccines to inject mice intramuscularly for four times at a 2-week interval. Serum samples were collected at various designated time points for the measurement of PRRSV-specific antibodies and splenocytes were isolated at time of sacrifice for lymphocyte blastogenesis assay. The results showed that pcDNA-5L6- and pcDNA-6L5-inoculated mice displayed higher PRRSV-specific neutralizing antibody (NA) titers, serum IgG responses, and lymphocyte proliferative responses than did the pcDNA-56-inoculated mice. The data indicated that co-expressing GP5/M with GPGP can indeed improve the immunogenicity of the heterodimeric complex.