Higher organisms including humans are equipped to counteract infecting viruses using two kinds of immune responses: innate and adaptive immunity. Unlike adaptive immunity, which is characterized by its specificity and memory, innate immunity is provoked early in infection and is critical for an initial antiviral response. The type I interferon (IFN) system plays a major role in antiviral innate immunity (Samuel 2001; Stetson and Medzhitov 2006). Upon viral infection, type I IFN is secreted in body fluid and expands IFN response signals, resulting in the activation of various enzymes that prevent viral replication. In addition to antiviral activity, type I IFN has been known to exert various biological effects such as cell cycle regulation, differentiation, and immune modulation. Furthermore, innate immune responses lead to the activation of specific cells with antigen-presenting functions to facilitate the initiation of adaptive immunity.

The triggering of the IFN system is the activation of IFN genes. Since the initial discovery of type I IFN, the activation mechanism of the type I IFN genes has been a major focus of many researchers. Although several double-stranded (ds) RNA-binding proteins such as protein kinase-activated by RNA (PKR) have been attributed to the detection of replicating viral RNA, gene knockout studies do not support its role (Yang et al. 1995). Recent functional analyses revealed that TLRs function as pathogen receptors including those of viral origin (Takeda and Akira 2005). TLR3 has been identified as a receptor for exogenous dsRNA (Alexopoulou et al. 2001); however, TLR3-deficient cells can still activate type I IFN genes (Diebold et al. 2003; Yoneyama et al.

2004), suggesting the existence of other receptor(s). Screening of an expression cDNA library identified RIG-I as an essential receptor for virus-derived dsRNA (Yoneyama et al. 2004). In this article, we describe the recently identified function of the RIG-I family of RNA helicases in innate immune reactions to infecting viruses.

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