A virulent strain of porcine reproductive and respiratory syndrome virus does not up-regulate interleukin-10 levels in vitro or in vivo

Porcine reproductive and respiratory syndrome virus (PRRSV) causes reproductive failure and respiratory illness in infected pigs. It has been postulated that the ability of PRRSV to induce the anti-inflammatory cytokine interleukin-10 (IL-10) in macrophages of infected pigs would be important for PRRSV immunopathogenesis, although this property would be variable and might be dependent on the strain. Several strains were reported to induce IL-10 in infected macrophages while others would not. In this study, we analyzed the IL-10 expression during in vitro and in vivo infections by a well-characterized virulent strain of PRRSV, vFL12, which is derived from an infectious clone. Our results showed that the vFL12 strain did not up-regulate IL-10 at mRNA or protein levels in either infected macrophages or dendritic cells in vitro. Furthermore, immunofluorescence staining for IL-10 on tonsil sections of PRRSV-infected pigs did not produce any evidence of IL-10 induction in PRRSV-infected cells or in bystander cells of the lymphoid tissues. Hence, based on these results obtained with a well-characterized highly pathogenic PRRSV strain it may be concluded that the induction of IL-10 release is not a part of the PRRSV virulence mechanisms.