A serologic marker attenuated live vaccine protects piglets against highly pathogenic porcine reproductive and respiratory syndrome virus infection

Currently, there are no commercial serologic marker or differentiation of infected and vaccinated animal (DIVA) vaccines for the eradication of porcine reproductive and respiratory syndrome virus (PRRSV) infection from pig farms. In a previous study, a nanobody-based competitive ELISA (cELISA) was specifically developed to detect anti-genotype 2 PRRSV (PRRSV-2) antibodies. On the basis of the epitope recognized by the nanobody and the prevalence of PRRSV-2 infection in China, a DIVA vaccine candidate strain was designed and evaluated in the present study. First, an infectious cDNA clone based on the genomic sequence of the highly pathogenic PRRSV-2 (HP-PRRSV) isolate SX-HD was constructed and named rSX-HD. Using the infectious clone as the backbone, a chimeric infectious cDNA clone in which the gene encoding the nucleocapsid (N) protein was replaced with the gene encoding the genotype 1 PRRSV N protein was generated and named rSX-HD^2M1. The chimeric PRRSV rSX-HD^2M1 was subsequently rescued successfully in Marc-145 cells, which were then passaged for 120 generations for attenuation. A safety study indicated that rSX-HD^2M1-F120 is not pathogenic to piglets. In vivo inoculation and challenge experiments suggested that rSX-HD^2M1-F120 vaccination significantly reduced serum viral loads and lung tissue lesions and that vaccinated piglets did not show any clinical symptoms or histopathological changes. Furthermore, this recombinant marker virus, in conjunction with the previously developed nanobody-based cELISA, enables serological differentiation between marker virus-infected animals and those infected with wild-type PRRSV-2. These results suggest that rSX-HD^2M1-F120 is a good candidate for providing a live attenuated DIVA vaccine against PRRSV-2 infection in piglets.