Innate Cytokine Responses in Porcine Macrophage Populations: Evidence for Differential Recognition of Double-Stranded RNA

Abstract

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 (AMφ) 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-αβ. In this study, synthetic dsRNA poly(I:C) was used to investigate innate responses of porcine AMφ compared with responses of peritoneal macrophages (PMφ). Poly(I:C) triggered IFN-αβ in AMφ and PMφ, but levels in AMφ were higher. In contrast, mRNA levels of IFN-stimulated genes, Mx and PKR, were greater in PMφ than AMφ. Low levels of Mx and PKR transcription in AMφ were not due to deficient type I IFN receptor signaling, as exogenous IFN-α induced nuclear translocation of phosphorylated STAT1. To investigate the differential mechanism by which IFN-αβ transcription is activated in AMφ and PMφ, 2-aminopurine (2-AP) was used to block dsRNA-mediated activation of PKR. IFN-αβ, Mx, and PKR mRNA levels in AMφ after poly(I:C) treatment were unaffected by 2-AP; conversely, transcription of IFN-αβ, Mx, or PKR remained at baseline levels in PMφ. Phosphorylated PKR was detected in PMφ, but not AMφ, after poly(I:C) treatment. In addition to IFN-αβ gene induction, mRNA levels of TNF-α and RANTES were higher in AMφ than PMφ after poly(I:C) stimulation. In summary, differential dsRNA-induced cytokine expression patterns between AMφ and PMφ provide evidence that dsRNA recognition and subsequent signaling is likely mediated via TLR3 in AMφ and PKR in PMφ.