The ORF5-encoded major envelope glycoprotein (GP5) of porcine reproductive and respiratory syndrome virus (PRRSV) is one of the three major structural proteins of this virus. While some porcine convalescent sera and monoclonal antibodies directed against GP4 and GP5 have the capacity to neutralize the virus in vitro, the protein specificity of porcine neutralizing sera has not yet been established. DNA immunization with a plasmid encoding GP5 of PRRSV, under the control of a human cytomegalovirus promoter, induced anti-GP5-specific neutralizing antibodies in pigs and BALB/c mice. The GP5 protein specificity of neutralizing sera was confirmed by immunoblotting and ELISA. Peripheral blood mononuclear cells obtained from DNA-vaccinated pigs underwent blastogenic transformation in the presence of E. coli-expressed recombinant ORF5-encoded protein, indicating the specificity of the cellular immune response to GP5. Following a massive intratracheal challenge with the virulent IAF-Klop strain of PRRSV, DNA-vaccinated pigs were protected from generalized viraemia and the development of typical macroscopic lung lesions that were observed in unvaccinated, virus-challenged controls, as well as in pigs that were immunized with E. coli-expressed GST-ORF5 recombinant fusion protein. Interstitial pneumonitis and broncho-alveolitis were remarkably milder in DNA-vaccinated animals. These results suggest that the GP5 of PRRSV is a good candidate for a subunit recombinant-type vaccine.


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