Experimental infection and comparative genomic analysis of a highly pathogenic PRRSV-HBR strain at different passage levels

A highly pathogenic strain of porcine reproductive and respiratory syndrome virus (PRRSV-HBR) was passaged on Marc-145 cells for 125 passages. In order to elucidate the change in virulence of PRRSV-HBR strain during the process of passage in vitro, swine infection experiment was performed with the viruses of low (F5 and F10) and high passage (F125). In addition, to identify the mutations related to the change in virulence of PRRSV-HBR strain, we compared and analyzed the genomic sequences of the F5, F10 and F125 of the strain. The virulence of F125 was significantly lower than that of F5 in the virus-infected pigs. In comparison with F5 and F125, there were 45 amino acids (aa) mutations and a deletion of 2 continuous aa by means of the virus genome sequence analysis. For these mutations, 33 aa (73.3%) occurred in the viral nonstructural proteins and the other 12 aa (26.7%) were contained in the viral structural proteins. Of the mutations, only 15 aa (33.3%) appeared in F10 and 30 aa (66.7%) occurred during passage from F10 to F125. The data showed that the latter 30 aa mutations were probably associated with attenuation of PRRSV-HBR strain, and that the change in virulence of the virus was determined by multiple alterations both in the structural and nonstructural genes. The virulence of PRRSV-HBR strain was remarkably attenuated after serial passages, and it can be used as vaccine candidate for control of the PRRS.