Attenuation Of Porcine Reproductive And Respiratory Syndrome Virus By Molecular Breeding Of The Virus Envelope Genes From Genetically Divergent Strains.

Molecular breeding via DNA shuffling can direct the evolution of viruses with desired traits. By using a positive-strand RNA virus, porcine reproductive and respiratory syndrome virus (PRRSV), as a model, rapid attenuation of the virus is achieved in this study by DNA shuffling of the viral envelope genes from multiple strains. The GP5 envelope genes of 7 genetically divergent PRRSV strains and the GP5-M genes of 6 different PRRSV strains were molecularly bred by DNA shuffling and iteration of the process, and the shuffled genes were cloned into the backbone of a DNA-launched PRRSV infectious clone. Two representative chimeric viruses, DS722 with shuffled GP5 genes and DS5M3 with shuffled GP5-M genes, were rescued and shown to replicate at a lower level and formed smaller plaques in vitro when compared to its parental virus. An in vivo pathogenicity study revealed that pigs infected with the two chimeric viruses have significant reductions in viral RNA loads in sera and lungs, and in gross and microscopic lung lesions, indicating attenuation of the chimeric viruses. Furthermore, pigs vaccinated with the chimeric virus DS722, but not with DS5M3, still induced protection against PRRSV challenge at a level similar to that of its parental virus. Therefore, this study reveals a unique approach through DNA shuffling of viral envelope genes to attenuate a positive-strand RNA virus. The results have important implications for future vaccine development and will generate broad general interest in the scientific community for rapidly attenuating other important human and veterinary viruses.