Innate cytokine responses in porcine macrophage populations: evidence for differential recognition of double-stranded RNA.

Pulmonary airways are vulnerable to infection because of exposure to Ag during respiration. The innate, antiviral response must be activated rapidly after pathogen recognition, and alveolar macrophages (AMphi) play a role in this response. TLR3 and protein kinase R (PKR) recognize dsRNA, a replication intermediate of RNA viruses, and initiate transcription of IFN-alphabeta. In this study, synthetic dsRNA poly(I:C) was used to investigate innate responses of porcine AMphi compared with responses of peritoneal macrophages (PMphi). Poly(I:C) triggered IFN-alphabeta in AMphi and PMphi, but levels in AMphi were higher. In contrast, mRNA levels of IFN-stimulated genes, Mx and PKR, were greater in PMphi than AMphi. Low levels of Mx and PKR transcription in AMphi were not due to deficient type I IFN receptor signaling, as exogenous IFN-alpha induced nuclear translocation of phosphorylated STAT1. To investigate the differential mechanism by which IFN-alphabeta transcription is activated in AMphi and PMphi, 2-aminopurine (2-AP) was used to block dsRNA-mediated activation of PKR. IFN-alphabeta, Mx, and PKR mRNA levels in AMphi after poly(I:C) treatment were unaffected by 2-AP; conversely, transcription of IFN-alphabeta, Mx, or PKR remained at baseline levels in PMphi. Phosphorylated PKR was detected in PMphi, but not AMphi, after poly(I:C) treatment. In addition to IFN-alphabeta gene induction, mRNA levels of TNF-alpha and RANTES were higher in AMphi than PMphi after poly(I:C) stimulation. In summary, differential dsRNA-induced cytokine expression patterns between AMphi and PMphi provide evidence that dsRNA recognition and subsequent signaling is likely mediated via TLR3 in AMphi and PKR in PMphi.