Recognition of viruses by innate immunity.

The innate immune system plays critical roles in recognizing viral infections and evoking initial anti-viral responses. Nucleotides from RNA viruses are recognized by retinoic acid-inducible gene I (RIG-I)-like helicases (RLHs) and Toll-like receptors (TLRs), and the recognition triggers signaling cascades that induce anti-viral mediators such as type I interferons (IFNs) and pro-inflammatory cytokines. The RLH signaling pathways play essential roles in the recognition of RNA viruses in various cells, with the exception of plasmacytoid dendritic cells (pDCs). However, TLRs are important for the production of type I IFNs in pDCs but not in other cell types. The contributions of RLHs and TLRs to the production of type I IFNs in response to RNA viruses vary depending on the route of infection. Specifically, local infections induce IFNs through RLHs but not TLRs, whereas systemic infections strongly stimulate TLRs in pDCs. In this review, we discuss recent advances toward clarifying the signaling pathways activated by RLHs and TLRs.