Our knowledge regarding the contribution of the innate immune system in recognizing and subsequently initiating a host response to an invasion of RNA virus has been rapidly growing over the last decade. Descriptions of the receptors involved and the molecular mechanisms they employ to sense viral pathogen-associated molecular patterns have emerged in great detail. This review presents an overview of our current knowledge regarding the receptors used to detect RNA virus invasion, the molecular structures these receptors sense, and the involved downstream signaling pathways.

Type I interferons (IFNs) (alpha/beta interferon [IFN-␣/␤]) are expressed as a first line of defense against viruses and are known to play a critical role in the antiviral response (38). Type I IFNs combat viruses both directly by inhibiting virus replication in the cells and indirectly by stimulating the innate and adaptive immune responses (38). The direct antiviral activity of type I IFNs is exerted by a number of different mechanisms, e.g., blockage of viral entry into the cell, control of viral transcription, cleavage of RNA, and preventing translation (16,31,37). In addition to the direct effects, type I IFNs play immunoregulatory roles and thereby shape the innate and adaptive immune responses. For instance, IFN-␣/␤ induce natural killer cell cytotoxicity and up-regulate expression of major histocompatibility complex class I on most cells and costimulatory molecules on antigen-presenting cells (9, 37). Furthermore, type I IFNs enhance cross-presentation of exogenous antigen in major histocompatibility complex class I and promote T-cell expansion (14,19,36 (11,15,20,23,32), although they exert their action through a receptor complex distinct from the type I IFNs (20,32). Most of the reports demonstrating antiviral activity of IFN-have addressed the issue in an in vitro experimental setup, but one report has shown that a recombinant IFN--expressing vaccinia virus is attenuated in vivo (4), whereas recombinant IFN-had no antiviral effect in vivo in the transgenic hepatitis B virus mouse model (30). Thus, we still do not have a clear picture of the antiviral potential of IFN-in vivo or of the mechanisms of action.The IFN-s have been demonstrated to be induced after stimulation with several single-stranded RNA (ssRNA) viruses, whereas the information on viruses with other genomes (DNA and double-stranded RNA [dsRNA]) is sparse (11). Virtually all cell types are capable of producing type I IFNs in response to viral infections, with the amount of IFN being virus and cell type dependent (7) and with plasmacytoid dendritic cells (pDCs) being the most potent producers of type I IFNs (3). IFN-s can be produced by a number of cell types, although the pattern of expression has not been elucidated. One report has demonstrated that IFN-s are produced by pDCs to a greater extent than by monocyte-derived DCs after influenza A virus (IAV) infection, suggesting that pDCs are the primary IFN--producing cells (13). However, this needs to be confirmed for other virus infections.Here, we have investigated the expression of type I and III IFNs after infection with DNA and RNA viruses in lymphoid, myeloid, and epithelial cell lines, and we have also examined the ability of type I and III IFNs to cross-induce one another. Subsequently, we investigated the antiviral activity of IFN-in

Type III IFNs (IFN-λ/IL-28/29) are cytokines with type I IFN-like antiviral activities, which remain poorly characterized. We herein show that most cell types expressed both types I and III IFNs after TLR stimulation or virus infection, whereas the ability of cells to respond to IFN-λ was restricted to a narrow subset of cells, including plasmacytoid dendritic cells and epithelial cells. To examine the role of type III IFN in antiviral defense, we generated IL-28Rα-deficient mice. These mice were indistinguishable from wild-type mice with respect to clearance of a panel of different viruses, whereas mice lacking the type I IFN receptor (IFNAR−/−) were significantly impaired. However, the strong antiviral activity evoked by treatment of mice with TLR3 or TLR9 agonists was significantly reduced in both IL-28RA−/− and IFNAR−/− mice. The type I IFN receptor system has been shown to mediate positive feedback on IFN-αβ expression, and we found that the type I IFN receptor system also mediates positive feedback on IFN-λ expression, whereas IL-28Rα signaling does not provide feedback on either type I or type III IFN expression in vivo. Finally, using bone-marrow chimeric mice we showed that TLR-activated antiviral defense requires expression of IL-28Rα only on nonhemopoietic cells. In this compartment, epithelial cells responded to IFN-λ and directly restricted virus replication. Our data suggest type III IFN to target a specific subset of cells and to contribute to the antiviral response evoked by TLRs.

Viruses are recognized by the innate immune system through pattern recognition receptors (PRRs). For instance, HSV virions and genomic DNA are recognized by TLR2 and TLR9, respectively. Although several viruses and viral components have been shown to stimulate cells through TLRs, only very few studies have defined essential roles for single TLRs in innate immune defense in vivo. This could suggest that PRRs act in concert to mount the first line of defense against virus infections. To test this hypothesis we have examined the host response of C57BL/6, TLR2−/−, TLR9−/−, and TLR2/9−/− mice toward HSV-2 infection. After a systemic infection, the cytokine serum response was markedly reduced in the double knockout mice, but only partly affected in either strain of the single knockout mice. This was supported by in vitro data showing that HSV-induced cytokine expression relayed on TLR2 and TLR9 in a cytokine- and cell type-dependent manner. With respect to the cellular response to infection, we found that recruitment but not activation of NK cells was impaired in TLR2/9−/− mice. Importantly, the viral load in the brain, but not liver, was significantly higher in the brain of TLR2/9−/− mice whereas the viral loads in organs of single knockout mice were statistically indistinguishable from C57BL/6 mice. In the brain we found that TNF-α and the IFN-stimulated gene CXCL9 were expressed during infection and were dependent on either TLR2 or TLR9. Thus, TLR2 and TLR9 synergistically stimulate innate antiviral activities, thereby protecting against HSV infection in the brain.

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