The ATPase RIG-I senses viral RNAs that contain 5'-triphosphates in the cytoplasm. It initiates a signaling cascade that activates innate immune response by interferon and cytokine production, providing essential antiviral protection for the host. The mode of RNA 5'-triphosphate sensing by RIG-I remains elusive. We show that the C-terminal regulatory domain RD of RIG-I binds viral RNA in a 5'-triphosphate-dependent manner and activates the RIG-I ATPase by RNA-dependent dimerization. The crystal structure of RD reveals a zinc-binding domain that is structurally related to GDP/GTP exchange factors of Rab-like GTPases. The zinc coordination site is essential for RIG-I signaling and is also conserved in MDA5 and LGP2, suggesting related RD domains in all three enzymes. Structure-guided mutagenesis identifies a positively charged groove as likely 5'-triphosphate-binding site of RIG-I. This groove is distinct in MDA5 and LGP2, raising the possibility that RD confers ligand specificity.

The mammalian target of rapamycin (mTOR) can be viewed as cellular master complex scoring cellular vitality and stress. Whether mTOR controls also innate immune-defenses is currently unknown. Here we demonstrate that TLR activate mTOR via phosphoinositide 3-kinase/Akt. mTOR physically associates with the MyD88 scaffold protein to allow activation of interferon regulatory factor-5 and interferon regulatory factor-7, known as master transcription factors for pro-inflammatory cytokine-and type I IFN-genes. Unexpectedly, inactivation of mTOR did not prevent but increased lethality of endotoxinmediated shock, which correlated with increased levels of IL-1b. Mechanistically, mTOR suppresses caspase-1 activation, thus inhibits release of bioactive IL-1b. We have identified mTOR as indispensable component of PRR signal pathways, which orchestrates the defense program of innate immune cells.Key words: Caspase-1 . IRF . mTOR . TLR Supporting Information available online IntroductionThe phosphoinositide 3-kinase (PI3K) represents a signaling gateway for the activation of various cellular effector functions including cell growth, proliferation, survival and vesicular transport [1,2]. Activated PI3K catalyzes the phosphorylation of membrane-anchored phosphoinositides (PI) and binding of PI-3,4,5-tri-phosphate to both Akt and PI-dependent protein kinase 1, which then drives PI-dependent protein kinase 1 to activate Akt via Thr308 phosphorylation [1]. It is known that inhibition of PI3K interferes with functions of innate immune cells [3,4], yet the molecular basis for this is still unclear.Upon inhibition of Akt, TLR-activated macrophages and DC mimic the phenotype of TLR-stimulated PI3K deficient cells [5]. Therefore, we reasoned that PI3K executes its regulatory function along the Akt pathway. One of the major targets of Akt is the mammalian target of rapamycin (mTOR), known to influence multiple cellular functions including cell cycle control, cellular growth, apoptosis, transcription and translational efficacy [6,7]. Whether and how mTOR signaling becomes integrated into TLR signaling pathways is unknown. Eur. J. Immunol. 2008. 38: 2981-2992 DOI 10.1002 HIGHLIGHTS 2981 FrontlineHere we describe that mTOR signaling is indispensable for the signal pathways of various PRR. First we show that membrane bound TLR directly activate mTOR via the PI3K/Akt axis. Activated mTOR subsequently transcriptionally controls in innate immune cells cytokine and type I IFN production. Essential steps in this transcriptional process include recruitment of activated mTOR to the MyD88 scaffold protein, the site at which interferon regulatory factor (IRF)-5 and IRF-7 become activated in an mTOR-dependent fashion. In addition, mTOR negatively regulates bioactive IL-1b production by inhibiting caspase-1 activation. These data characterize mTOR as transcriptional regulator and controller of acute innate immune reactions. Results TLR activate mTORTo analyze whether TLR signaling drives mTOR activation, we asked whether TLR-mediated activation of bon...

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Pattern recognition receptors (PRRs) sensing commensal microorganisms in the intestine induce tightly controlled tonic signaling in the intestinal mucosa, which is required to maintain intestinal barrier integrity and immune homeostasis. At the same time, PRR signaling pathways rapidly trigger the innate immune defense against invasive pathogens in the intestine. Intestinal epithelial cells and mononuclear phagocytes in the intestine and the gut-associated lymphoid tissues are critically involved in sensing components of the microbiome and regulating immune responses in the intestine to sustain immune tolerance against harmless antigens and to prevent inflammation. These processes have been mostly investigated in the context of the bacterial components of the microbiome so far. The impact of viruses residing in the intestine and the virus sensors, which are activated by these enteric viruses, on intestinal homeostasis and inflammation is just beginning to be unraveled. In this review, we will summarize recent findings indicating an important role of the enteric virome for intestinal homeostasis as well as pathology when the immune system fails to control the enteric virome. We will provide an overview of the virus sensors and signaling pathways, operative in the intestine and the mononuclear phagocyte subsets, which can sense viruses and shape the intestinal immune response. We will discuss how these might interact with resident enteric viruses directly or in context with the bacterial microbiome to affect intestinal homeostasis.

IFN regulatory factor 7 (IRF7) has been described as the master regulator of type I IFN responses and has been shown to be critical for innate antiviral immunity in vivo. In addition to type I IFN, NK cell responses are involved in the control of viral replication during acute viral infection. To investigate the role of IRF7 in the context of a viral infection that induces a strong NK cell response, the murine cytomegalovirus (MCMV) infection model was used. WT, IRF7-deficient and IRF3/IRF7-double deficient mice were infected with MCMV. The systemic IFN-a response to MCMV was entirely dependent on IRF7, but independent of IRF3. However, peak IFN-b production during MCMV infection was not affected by the lack of IRF7 or both IRF7 and IRF3. Despite the complete lack of IFN-a production IRF7-and IRF3/IRF7-deficient mice were surprisingly efficient in controlling MCMV replication and were only modestly more susceptible to MCMV infection than WT mice. NK cell cytotoxicity was unimpaired and NK cell IFN-c production was enhanced in IRF7-deficient mice correlating with increased levels of bioactive IL-12. Owing to these compensatory mechanisms IRF7-dependent antiviral immune responses were not essential for resistance against acute MCMV infection in vivo.

Dendritic cells which are located at the interface of innate and adaptive immunity are targets for infection by many different DNA and RNA viruses. Dendritic cell subpopulations express specific nucleic acid recognition receptors belonging to the Toll-like receptor family (TLR3, 7, 8, 9) and the cytosolic RNA helicase family (RIG-I, MDA5, LGP2). Activation of dendritic cells by viral DNA and RNA via these receptors is essential for triggering the innate antiviral immune response and shaping the ensuing adaptive antiviral immunity. This review will summarize our current knowledge of viral nucleic acid recognition and signaling by Toll-like receptors and RNA helicases focusing on recent evidence for their specific functions in antiviral defense in vivo.

Key Points Systemic virus infection leads to rapid disruption of the Peyer’s patches but not of peripheral lymph nodes. Virus-associated innate immune activation and type I IFN release blocks trafficking of B cells to Peyer’s patches.