In vivo growth of porcine reproductive and respiratory syndrome virus engineered nsp2 deletion mutants

Prior studies on PRRSV strain VR-2332 non-structural protein 2 (nsp2) had shown that as much as 403 amino acids could be removed from the hypervariable region without losing virus viability in vitro. We utilized selected nsp2 deletion mutants to examine in vivo growth. Young swine (4 pigs/group; 5 control swine) were inoculated intramuscularly with one of 4 nsp2 deletion mutants (rΔ727–813, rΔ543–726, rΔ324–523, rΔ324–726) or full-length recombinant virus (rVR-2332). Serum samples were collected on various days post-inoculation and analyzed by HerdChek* ELISA, PRRSV real time RT-PCR, gamma interferon (IFN-γ) ELISA, and nucleotide sequence analysis of the entire nsp2 coding region. Tracheobronchial lymph node weight compared to body weight was recorded for each animal and used as a clinical measurement of viral pathogenesis. Results showed that all deletion mutants grew less robustly than full-length recombinant virus, yet all but the large deletion virus (rΔ324–726) recovered to parental viral RNA levels by study end. Swine receiving the rΔ727–813 mutants had a significant decrease in lymph node enlargement compared to rVR-2332. While swine infection with rVR-2332 caused a rapid rise in serum IFN-γ levels, the IFN-γ protein produced by infection with 3 of the 4 deletion mutant viruses was significantly reduced, perhaps due to differences in viral growth kinetics. The rΔ543–726 nsp2 mutant virus, although growth impaired, mimicked rVR-2332 in inducing a host serum IFN-γ response but exhibited a 2-week delay. Targeted sequencing showed that all deletions were stable in the region coding for nsp2 after one swine passage. The data suggested that the selected nsp2 deletion mutants were growth attenuated in swine, altered the induction of serum IFN-γ, an innate cytokine of unknown function in PRRSV clearance, and pointed to a domain that may influence tracheobronchial lymph node size.