Development of a differentiable virus via a spontaneous deletion in the nsp2 region associated with cell adaptation of porcine reproductive and respiratory syndrome virus.

Porcine reproductive and respiratory syndrome virus (PRRSV) is renowned for its genetic, antigenic, and pathogenic heterogeneity. As a consequence, highly pathogenic PRRSV (HP-PRRSV) has emerged and caused tremendous economic losses in the swine industry. In this study, a Chinese HP-PRRSV JX143 isolate was serially passaged in MARC-145 cells up to 100 times. We found that phenotypic changes involved with the cell adaptation process of PRRSV JX143 were characterized by higher titers, faster growth kinetics, and larger plaque sizes as the passage number increased compared with the parental virus. We found that the virulence of the JX143 strain in piglets was decreased greatly at passage 100 (JXM100). The attenuated strain JXM100 could protect piglets from lethal challenge by HP-PRRSV JX143. Genome-wide analysis showed that JXM100 contained a continuous 264 nucleotide (88 amino acids; 88aa) deletion in the nsp2 region and 75 random nucleotide mutations throughout the genome. The nucleotide changes that arose during MARC-145 passaging of HP-PRRSV JX143 provide a potential molecular basis for the observed cell-adapted phenotype in MARC-145 cells and the attenuated phenotype in vivo. We also showed that pigs inoculated with JXM100 with an 88aa deletion (del88) in nsp2 elicited a strong antibody response against the N protein but they did not develop antibody against the del88, whereas strong reactivity was observed in sera obtained from piglets infected with JX143 using the same del88-based ELISA. This suggests that del88 can be used as a genetic marker for serologically differentiating JXM100 from JX143.