Interferon-β induction through Toll-like receptor 3 depends on double-stranded RNA structure

Type I interferons (IFN-alpha/beta) play an essential role in both innate and adaptive antiviral immune responses. IFN-beta is produced by fibroblasts and myeloid dendritic cells (DCs) upon viral infection or in response to double-stranded RNA ( dsRNA). Several intracellular molecules having a dsRNA-binding motif such as dsRNA-dependent protein kinase recognize dsRNA in a sequence-independent manner and induce antiviral innate responses. Toll-like receptor (TLR) 3, a member of TLR family proteins, recognizes extracellular dsRNA and activates NF-kappa B and the IFN-beta promoter leading to the induction of IFN-beta production. Here we analyzed the dsRNA structure capable of inducing TLR3-mediated IFN-beta production using various synthetic RNA duplexes. In contrast to the recognition of dsRNA by intracellular molecules, TLR3 preferentially recognizes polyriboinocinic: polyribocytidylic acid ( poly( I: C)) rather than synthetic virus-derived dsRNAs. 2'-O-methyl or 2'-fluoro modification of cytidylic acid abolished the IFN-beta-inducing ability of the poly( I: C) duplex, and these modified dsRNAs inhibited poly( I: C)-induced TLR3-mediated IFN-beta production by fibroblasts and DCs. In addition, poly(dI: dC), a non-IFN inducer, also blocked poly( I:C)-induced IFN-beta induction. Since TLR3 is localized in the intracellular compartment of DCs where signaling occurs, modified dsRNAs may compete with poly( I: C) for binding to the cell-surface receptor that transfers dsRNA into TLR3-enriched vesicles. Thus, TLR3 recognizes a unique dsRNA structure that largely differs from those recognized by other dsRNA-binding proteins.