ABSTRACT
During viral infection, host cells sense the invasion by detecting viral components. Toll-like receptors (TLRs) comprise one of the systems that recognize viral nucleotides. TLR-mediated recognition of viruses leads to the production of type I interferons and pro-inflammatory cytokines. The TLR system plays an important role in plasmacytoid dendritic cells. In addition, the RIG-I and MDA5 RNA helicases located in the cytoplasm function as viral detectors by recognizing viral doublestranded (ds) RNA. Gene targeting has shown that RIG-I and MDA5 recognize different types of RNA viruses, as well as synthetic double-stranded RNAs. Moreover, 5′ triphosphate single-stranded (ss) RNA has also been shown to be a RIG-I ligand. The way these helicases sense distinct dsRNA structures, and the reason only RIG-I recognizes ssRNA structures, remain to be determined. Structural studies of the interactions of ligands with RIG-I and MDA5 are critical to the further understanding of antiviral immunity.
