Neutrophils, historically known for their involvement in acute
inflammation, are also targets for
infection by many different
DNA and 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 endosomal TLR3 (not expressed by human neutrophils) or with cytoplasmic
RNA helicases such as
melanoma differentiation-associated gene 5 (MDA5) and
retinoic acid-inducible gene I (RIG-I). In this study, we report that intracellularly administered
poly(I:C) stimulates human neutrophils to specifically express elevated
mRNA levels encoding type I IFNs, immunoregulatory
cytokines, and
chemokines, such as
TNF-alpha,
IL-12p40, CXCL10, CXCL8, CCL4, and CCL20, as well as classical IFN-responsive genes (IRG), including IFIT1 (IFN-induced
protein with tetratricopeptide repeats 1)/IFN-stimulated gene (ISG)56, G1P2/ISG15, PKR (dsRNA-dependent
protein kinase), and IFN-regulatory factor (IRF)7. Investigations into the mechanisms whereby transfected
poly(I:C) promotes gene expression in neutrophils uncovered a crucial involvement of the MAPK-, PKR-,
NF-kappaB-, and TANK (
TNF receptor-associated
NF-kappaB kinase)-binding
kinase (TBK1)/IRF3-signaling transduction pathways, as illustrated by the use of specific pharmacological inhibitors. Consistent with the requirement of the cytoplasmic dsRNA pathway for
antiviral signaling, human neutrophils were found to constitutively express significant levels of both MDA5 and RIG-I, but not TLR3. Accordingly, neutrophils isolated from MDA5-deficient mice had a partial impairment in the production of IFN-beta and
TNF-alpha upon
infection with encephalomyocarditis virus. Taken together, our data demonstrate that neutrophils are able to activate
antiviral responses via helicase recognition, thus acting at the frontline of immunity against viruses.