The RIG-I-like receptors (RLRs) RIG-I, MDA5, and LGP2 play a major role in pathogen sensing of RNA virus infection to initiate and modulate antiviral immunity. The RLRs detect viral RNA ligands or processed self RNA in the cytoplasm to triggers innate immunity and inflammation and to impart gene expression that serves to control infection. Importantly, RLRs cooperate in signaling crosstalk networks with Toll-like receptors and other factors to impart innate immunity and to modulate the adaptive immune response. RLR regulation occurs at a variety of levels ranging from autoregulation to ligand and co-factor interactions and post-translational modifications. Abberant RLR signaling or dysregulation of RLR expression is now implicated in the development of autoimmune diseases. Understanding the processes of RLR signaling and response will provide insights to guide RLR-targeted therapeutics for antiviral and immune modifying applications.

The cellular protein retinoic acid-inducible gene I (RIG-I) senses intracellular viral infection and triggers a signal for innate antiviral responses including the production of type I IFN. RIG-I contains a domain that belongs to a DExD/H-box helicase family and exhibits an N-terminal caspase recruitment domain (CARD) homology. There are three genes encoding RIG-I-related proteins in human and mouse genomes. Melanoma differentiation associated gene 5 (MDA5), which consists of CARD and a helicase domain, functions as a positive regulator, similarly to RIG-I. Both proteins sense viral RNA with a helicase domain and transmit a signal downstream by CARD; thus, these proteins share overlapping functions. Another protein, LGP2, lacks the CARD homology and functions as a negative regulator by interfering with the recognition of viral RNA by RIG-I and MDA5. The nonstructural protein 3/4A protein of hepatitis C virus blocks the signaling by RIG-I and MDA5; however, the V protein of the Sendai virus selectively abrogates the MDA5 function. These results highlight ingenious mechanisms for initiating antiviral innate immune responses and the action of virus-encoded inhibitors.

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Alpha/beta interferon immune defenses are essential for resistance to viruses and can be triggered through the actions of the cytoplasmic helicases retinoic acid-inducible gene I (RIG-I) and melanoma differentiationassociated gene 5 (MDA5). Signaling by each is initiated by the recognition of viral products such as RNA and occurs through downstream interaction with the IPS-1 adaptor protein. We directly compared the innate immune signaling requirements of representative viruses of the Flaviviridae, Orthomyxoviridae, Paramyxoviridae, and Reoviridae for RIG-I, MDA5, and interferon promoter-stimulating factor 1 (IPS-1). In cultured fibroblasts, IPS-1 was essential for innate immune signaling of downstream interferon regulatory factor 3 activation and interferon-stimulated gene expression, but the requirements for RIG-I and MDA5 were variable. Each was individually dispensable for signaling triggered by reovirus and dengue virus, whereas RIG-I was essential for signaling by influenza A virus, influenza B virus, and human respiratory syncytial virus. Functional genomics analyses identified cellular genes triggered during influenza A virus infection whose expression was strictly dependent on RIG-I and which are involved in processes of innate or adaptive immunity, apoptosis, cytokine signaling, and inflammation associated with the host response to contemporary and pandemic strains of influenza virus. These results define IPS-1-dependent signaling as an essential feature of host immunity to RNA virus infection. Our observations further demonstrate differential and redundant roles for RIG-I and MDA5 in pathogen recognition and innate immune signaling that may reflect unique and shared biologic properties of RNA viruses whose differential triggering and control of gene expression may impact pathogenesis and infection.

Virus-responsive signaling pathways that induce alpha/beta interferon production and engage intracellular immune defenses influence the outcome of many viral infections. The processes that trigger these defenses and their effect upon host permissiveness for specific viral pathogens are not well understood. We show that structured hepatitis C virus (HCV) genomic RNA activates interferon regulatory factor 3 (IRF3), thereby inducing interferon in cultured cells. This response is absent in cells selected for permissiveness for HCV RNA replication. Studies including genetic complementation revealed that permissiveness is due to mutational inactivation of RIG-I, an interferon-inducible cellular DExD/H box RNA helicase. Its helicase domain binds HCV RNA and transduces the activation signal for IRF3 by its caspase recruiting domain homolog. RIG-I is thus a pathogen receptor that regulates cellular permissiveness to HCV replication and, as an interferonresponsive gene, may play a key role in interferon-based therapies for the treatment of HCV infection.Hepatitis C virus (HCV) is a major public health problem, infecting nearly 200 million people worldwide and causing hepatic fibrosis, end-stage cirrhosis, and hepatocellular carcinoma (16). A member of the Flaviviridae, HCV's positivesense RNA genome contains highly structured 5Ј and 3Ј nontranslated regions (NTRs) flanking a large open reading frame (ORF) encoding a polyprotein that is processed into both structural (core-E2) and nonstructural (NS) proteins (Fig. 1A). The NS3-NS5B proteins support viral genome replication, which is also dependent upon conserved RNA sequences within the 5ЈNTR and 3ЈNTR that are highly structured and required for both protein translation and RNA replication (15,20). HCV infection is treated with alpha interferon (IFN-␣)-based therapy, but treatment is effective at best in only 50% of patients (10). The nearly unique ability of HCV to establish persistent infections in humans has been attributed, in part, to a variety of strategies to evade host immune and IFN-induced defenses (12). Epidemiological studies suggest that 25 to 50% of all persons resolve acute HCV infection without treatment (16), however, indicating that innate and/or adaptive immune responses are indeed capable of controlling the outcome of HCV infection. Processes that regulate innate intracellular antiviral responses may therefore serve as pivotal points of control, potentially limiting host permissiveness for HCV replication and favorably modulating subsequent adaptive immune responses.Virus-induced production of IFN-␣ and IFN-␤ and the subsequent expression of IFN-stimulated genes (ISGs) are central to these antiviral defenses (22). This host response is initiated by cellular recognition of a pathogen-associated molecular pattern (PAMP) presented by the infection, in which a host protein receptor is engaged by the PAMP ligand and signals downstream components to activate intracellular immune defenses. In mammalian cells, replicating viral RNAs present features of nucleic...