Immunodominant epitopes in nsp2 of porcine reproductive and respiratory syndrome virus are dispensable for replication, but play an important role in modulation of the host immune response

Non-structural protein 2 (nsp2) of porcine reproductive and respiratory syndrome virus (PRRSV) is the largest protein of this virus. In addition to its crucial role in virus replication, recent studies have indicated its involvement in modulating host immunity. In this study, each of the six identified immunodominant nsp2 B-cell epitopes (ES2-ES7) was deleted using a type I PRRSV cDNA infectious clone. Deletion of ES3, ES4 or ES7 allowed the generation of viable virus. In comparison with the parental virus, the Delta ES3 mutant showed increased cytolytic activity and more vigorous growth kinetics, whilst the Delta ES4 and Delta ES7 mutants displayed decreased cytolytic activity and slower growth kinetics in MARC-145 cells. These nsp2 mutants were characterized further in a nursery pig disease model. The results showed that the Delta ES4 and Delta ES7 mutants exhibited attenuated phenotypes, whereas the Delta ES3 mutant produced a higher peak viral load in pigs. The antibody response reached similar levels, as measured by IDEXX ELISA at 21 days post-infection, and slightly higher levels of mean virus neutralizing titres were observed from pigs infected by the Delta ES4 and Delta ES7 mutants. The expression of innate and T-helper 1 cytokines was measured in peripheral blood mononuclear cells or virus-infected macrophages. The results consistently showed that interleukin-1 beta and tumour necrosis factor alpha expression levels were downregulated in cells that were stimulated (or infected) with the Delta ES3 mutant compared with parental virus and the other nsp2 deletion mutants. These results suggest that certain regions in nsp2 are non-essential for PRRSV replication but may play an important role in modulation of host immunity in vivo.