The interferon (IFN) system is an extremely powerful antiviral response that is capable of controlling most, if not all, virus infections in the absence of adaptive immunity. However, viruses can still replicate and cause disease in vivo, because they have some strategy for at least partially circumventing the IFN response. We reviewed this topic in 2000 [Goodbourn, S., Didcock, L. & Randall, R. E. (2000). J Gen Virol 81, 2341-2364] but, since then, a great deal has been discovered about the molecular mechanisms of the IFN response and how different viruses circumvent it. This information is of fundamental interest, but may also have practical application in the design and manufacture of attenuated virus vaccines and the development of novel antiviral drugs. In the first part of this review, we describe how viruses activate the IFN system, how IFNs induce transcription of their target genes and the mechanism of action of IFN-induced proteins with antiviral action. In the second part, we describe how viruses circumvent the IFN response. Here, we reflect upon possible consequences for both the virus and host of the different strategies that viruses have evolved and discuss whether certain viruses have exploited the IFN response to modulate their life cycle (e.g. to establish and maintain persistent/latent infections), whether perturbation of the IFN response by persistent infections can lead to chronic disease, and the importance of the IFN system as a species barrier to virus infections. Lastly, we briefly describe applied aspects that arise from an increase in our knowledge in this area, including vaccine design and manufacture, the development of novel antiviral drugs and the use of IFN-sensitive oncolytic viruses in the treatment of cancer. Biology of the interferon systemThe interferons (IFNs) are a group of secreted cytokines that elicit distinct antiviral effects. They are grouped into three classes called type I, II and III IFNs, according to their amino acid sequence. Type I IFNs (discovered in 1957;Isaacs & Lindenmann, 1957) comprise a large group of molecules; mammals have multiple distinct IFN-a genes (13 in man), one to three IFN-b genes (one in man) and other genes, such as IFN-v, -e, -t, -d and -k. The IFN-a and -b genes are induced directly in response to viral infection, whereas IFN-v, -e, -d and -k play less well-defined roles, such as regulators of maternal recognition in pregnancy. Thus, rather than use the term 'type I IFN', we will use IFN-a/b when referring to the virally induced cytokines. Although the multigenic nature of IFN-a has been known for over 20 years, the significance of this is still debatedi.e. whether these genes are expressed differentially in distinct cell types, whether they are inducible by different types of viruses or whether they are functionally specialized (Brideau-Andersen et al., 2007). For the rest of this review, we will not distinguish between IFN-a subtypes. Type III IFNs have been described more recently and comprise IFNl1, -l2 and -l3, also referred to as IL-2...

Members of the Paramyxovirinae subfamily include viruses such as measles, mumps, parainfluenza viruses (PIV) of humans, Newcastle Disease virus of birds, Sendai virus (SeV) of rodents, and simian virus 5 (SV5), which has been isolated from monkeys, dogs, pigs, and humans. Paramyxoviruses also have zoonotic potential, as has been observed with the newly emergent Hendra (HeV) and Nipah viruses, which naturally infect fruit bats but can cause serious, often fatal infections when transmitted to farm and domestic animals and to humans (reviewed in ref.2). Like all viruses, upon infection of cells, paramyxoviruses are subjected to a variety of intracellular antiviral responses, including the IFN response (reviewed in refs. 3-5). Over the last few years, it has become clear that protein products of the P͞V͞C gene of viruses within the Paramyxovirinae subfamily (for review of the molecular biology of paramyxoviruses, see ref. 1) specifically reduce the effectiveness of the IFN response. For example, the V protein of SV5 targets signal transducer and activator of transcription 1 (STAT1) for degradation, thereby blocking both IFN-␣͞␤ and IFN-␥ signaling within infected cells (6), whereas the C proteins of SeV block IFN signaling by interfering with STAT phosphorylation or stability (reviewed in refs. 7-9). As well as blocking IFN signaling, these viruses also specifically limit the production of IFN by virus-infected cells (10-12). The block on IFN-␤ production is at the level of transcription, because very little IFN-␤ mRNA is induced in cells infected with SV5. In contrast, large amounts of IFN-␤ mRNA (and thus IFN-␤) are produced by cells infected with a recombinant of SV5 (SV5V⌬C) that produces a truncated V protein lacking the cysteine-rich C terminus (which is dispensable for virus replication), suggesting that the V protein is responsible for the block on IFN production. This conclusion is supported by the observation that in gene reporter assays, the V proteins of SV5, PIV2, and SeV inhibit the activation of the IFN-␤ promoter in response to intracellular dsRNA (11).Initial transcription from the IFN-␤ promoter requires the activation of a number of cellular transcription factors, including IFN regulatory factor (IRF)-3 and NF-B, leading to the formation of an enhanceosome complex that associates with the basal transcriptional machinery to recruit RNA polymerase II to the IFN-␤ promoter (reviewed in refs. 3 and 13). The molecular details of how the V proteins of paramyxoviruses block IFN production are not known, but the block affects the signal transduction pathway that activates both NF-B and IRF-3 in response to dsRNA. Thus, these transcription factors are not activated in cells infected with wild-type SV5 but are activated in cells infected with SV5V⌬C. Furthermore, ectopic expression of SV5 V inhibits the activation of IRF-3 and NF-B by both dsRNA and infection with SV5V⌬C (10, 11). Unlike the targeted degradation of signal transducer and activator of transcription 1 (STAT1), which requires both the N-and C-t...

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Molecular genetics approaches have been used to identify and characterize cis-acting DNA sequences required for eukaryotic gene regulation. These sequences are modular in nature, consisting of arrays of short (10- to 12-base pair) recognition elements that interact with specific transcription factors. Some transcription factors have been extensively purified and the corresponding genes have been cloned, but the mechanisms by which they promote transcription are not yet understood. Positive and negative regulatory elements that function only in specific cell types or in response to extracellular inducers have been identified. A number of cases of inducible and tissue-specific gene expression involve the activation of preexisting transcription factors, rather than the synthesis of new proteins. This activation may involve covalent modification of the protein or an allosteric change in its structure. The modification of regulatory proteins may play a central role in the mechanisms of eukaryotic gene regulation.

The induction of IFN-beta by the paramyxovirus PIV5 (formerly known as SV5) is limited by the action of the viral V protein that targets the cellular RNA helicase mda-5. Here we show that 12 other paramyxoviruses also target mda-5 by a direct interaction between the conserved cysteine-rich C-terminus of their V proteins and the helicase domain of mda-5. The inhibition of IFN-beta induction is not species-restricted, being observed in a range of mammalian cells as well as in avian cells, and we show that the inhibition of mda-5 function is also not restricted to mammalian cells. In contrast, the V proteins do not bind to the related RNA helicase RIG-I and do not inhibit its activity. The relative contributions of mda-5 and RIG-I to IFN-beta induction are discussed.

Viruses must enter host cells to replicate, assemble and propagate. Because of the restricted size of their genomes, viruses have had to evolve efficient ways of exploiting host cell processes to promote their own life cycles and also to escape host immune defence mechanisms. Many viral open reading frames (viORFs) with immune-modulating functions essential for productive viral growth have been identified across a range of viral classes. However, there has been no comprehensive study to identify the host factors with which these viORFs interact for a global perspective of viral perturbation strategies. Here we show that different viral perturbation patterns of the host molecular defence network can be deduced from a mass-spectrometry-based host-factor survey in a defined human cellular system by using 70 innate immune-modulating viORFs from 30 viral species. The 579 host proteins targeted by the viORFs mapped to an unexpectedly large number of signalling pathways and cellular processes, suggesting yet unknown mechanisms of antiviral immunity. We further experimentally verified the targets heterogeneous nuclear ribonucleoprotein U, phosphatidylinositol-3-OH kinase, the WNK (with-no-lysine) kinase family and USP19 (ubiquitin-specific peptidase 19) as vulnerable nodes in the host cellular defence system. Evaluation of the impact of viral immune modulators on the host molecular network revealed perturbation strategies used by individual viruses and by viral classes. Our data are also valuable for the design of broad and specific antiviral therapies.

Bovine viral diarrhea virus (BVDV) is a pestivirus that can establish a persistent infection in the developingfetus and has the ability to disable the production of type I interferon. In this report, we extend our previous observations that BVDV encodes a protein able to specifically block the activity of interferon regulatory factor 3 (IRF-3), a transcription factor essential for interferon promoter activation, by demonstrating that this is a property of the N-terminal protease fragment (NPro) of the BVDV polyprotein. Although BVDV infections cause relocalization of cellular IRF-3 from the cytoplasm to the nucleus early in infection, NPro blocks IRF-3 from binding to DNA. NPro has the additional property of targeting IRF-3 for polyubiquitination and subsequent destruction by cellular multicatalytic proteasomes. The autoprotease activity of NPro is not required for the inhibition of type I interferon induction or the targeting of IRF-3 for degradation.Bovine viral diarrhea virus (BVDV) has a global spread and is a major reproductive pathogen of cattle (12). BVDV, along with classical Swine fever virus (CSFV), Border disease virus of sheep, and a small number of isolates from undomesticated species, make up the genus Pestivirus in the family Flaviviridae. The viruses are positive-strand RNA viruses with a genome on the order of 12.5 kb in length. The RNA is translated into a single viral polyprotein that is processed by both viral and host proteases to either 11 or 12 polypeptides, depending on the virus biotype. BVDV is generally noncytopathogenic (ncp), although the virus associated with the development of mucosal disease in persistently infected animals is a cytopathogenic (cp) biotype. The result of infection of a susceptible host with BVDV is unusual; BVDV can cause an acute infection like other viruses, but infection of a pregnant animal can result in transmission of virus from the dam to the developing fetus, where the virus can replicate. Infection of the fetus prior to immune competence results in a failure of the fetus to control virus infection and can result in the birth of persistently infected offspring that fail to mount an acquired immune response to BVDV. These offspring then serve as a reservoir for further acute virus infection and are held to be critical for BVDV transmission in areas of endemicity.Although infection during fetal development takes place in the absence of a functional acquired immune response, viruses still have to evade innate immunity in order to establish a persistent infection; the ability of ncpBVDV to avoid the induction of interferon (IFN) seems critical in this regard (5). We and others have shown that ncpBVDV encodes an active block to type I IFN induction (3, 46), and there is evidence that ncpBVDV employs more than one mechanism to bring about this block. BVDV-infected cells are refractory to the addition of double-stranded RNA (dsRNA) to the medium. At least in part, the failure of this extracellular dsRNA to induce type I IFN can be explained by the action of the s...

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