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 (AM phi) 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-alpha 3. In this study, synthetic dsRNA poly(I:C) was used to investigate innate responses of porcine AM phi compared with responses of peritoneal macrophages (PM phi). Poly(I:C) triggered IFN-alpha beta; in AM phi and PM phi, but levels in AM phi were higher. In contrast, mRNA levels of IFN-stimulated genes, Mx and PKR, were greater in PM phi than AM phi. Low levels of Mx and PKR transcription in AM phi 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-alpha beta; transcription is activated in AM phi and PM phi, 2-aminopurine (2-AP) was used to block dsRNA-mediated activation of PKR. IFN-alpha beta, Mx, and PKR mRNA levels in AM phi after poly(I:C) treatment were unaffected by 2-AP; conversely, transcription of IFN-alpha beta, Mx, or PKR remained at baseline levels in PM phi. Phosphorylated PKR was detected in PM phi, but not AM phi, after poly(I:C) treatment. In addition to IFN-alpha beta gene induction, mRNA levels of TNF-alpha and RANTES were higher in AM phi than PM phi after poly(I:C) stimulation. In summary, differential dsRNA-induced cytokine expression patterns between AM phi and PM phi provide evidence that dsRNA recognition and subsequent signaling is likely mediated via TLR3 in AM phi and PKR in PM phi.