Neutrophils, historically known for their involvement in acute inflammation, are also targets for infection by many different DNAand RNA viruses. However, the mechanisms by which they recognize and respond to viral components are poorly understood.Polyinosinic:polycytidylic acid (poly(I:C)) is a synthetic mimetic of viral dsRNA that is known to interact either with endosomalTLR3 (not expressed by human neutrophils) or with cytoplasmic RNA helicases such as melanoma differentiation-associated gene5 (MDA5) and retinoic acid-inducible gene I (RIG-I). In this study, we report that intracellularly administered poly(I:C) stimulateshuman neutrophils to specifically express elevated mRNA levels encoding type I IFNs, immunoregulatory cytokines, and chemokines,such as TNF-, IL-12p40, CXCL10, CXCL8, CCL4, and CCL20, as well as classical IFN-responsive genes (IRG), includingIFIT1 (IFN-induced protein with tetratricopeptide repeats 1)/IFN-stimulated gene (ISG)56, G1P2/ISG15, PKR (dsRNA-dependentprotein kinase), and IFN-regulatory factor (IRF)7. Investigations into the mechanisms whereby transfected poly(I:C) promotesgene expression in neutrophils uncovered a crucial involvement of the MAPK-, PKR-, NF-B-, and TANK (TNF receptorassociatedNF-B kinase)-binding kinase (TBK1)/IRF3-signaling transduction pathways, as illustrated by the use of specificpharmacological inhibitors. Consistent with the requirement of the cytoplasmic dsRNA pathway for antiviral signaling, humanneutrophils were found to constitutively express significant levels of both MDA5 and RIG-I, but not TLR3. Accordingly, neutrophilsisolated from MDA5-deficient mice had a partial impairment in the production of IFN- and TNF- upon infection withencephalomyocarditis virus. Taken together, our data demonstrate that neutrophils are able to activate antiviral responses viahelicase recognition, thus acting at the frontline of immunity against viruses.
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