Influenza A virus lacking the NS1 gene replicates in interferon-deficient systems

Nipah virus V and W proteins have a common STAT1-binding domain yet inhibit STAT1 activation from the cytoplasmic and nuclear compartments, respectively

In previous reports it was demonstrated that the Nipah virus V and W proteins have interferon (IFN) antagonist activity due to their ability to block signaling from the IFN-alpha/beta receptor (J. J. Rodriguez, J. P. Parisien, and C. M. Horvath, J. Virol. 76:11476-11483, 2002; M. S. Park et al., J. Virol. 77:1501-1511, 2003). The V, W, and P proteins are all encoded by the same viral gene and share an identical 407-amino-acid N-terminal region but have distinct C-terminal sequences. We now show that the P protein also has anti-IFN function, confirming that the common N-terminal domain is responsible for the antagonist activity. Truncation of this N-terminal domain revealed that amino acids 50 to 150 retain the ability to block IFN and to bind STAT1, a key component of the IFN signaling pathway. Subcellular localization studies demonstrate that the V and P proteins are predominantly cytoplasmic whereas the W protein is localized to the nucleus. In all cases, STAT1 colocalizes with the corresponding Nipah virus protein. These interactions are sufficient to inhibit STAT1 activation, as demonstrated by the lack of STAT1 phosphorylation on tyrosine 701 in IFN-stimulated cells expressing P, V, or W. Therefore, despite their common STAT1-binding domain, the Nipah virus V and P proteins act by retaining STAT1 in the cytoplasm while the W protein sequesters STAT1 in the nucleus, creating both a cytoplasmic and a nuclear block for STAT1. We also show that the IFN antagonist activity of the P protein is not as strong as that of V or W, perhaps explaining why Nipah virus has evolved to express these two edited products.

NF-kappaB-dependent induction of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and Fas/FasL is crucial for efficient influenza virus propagation

Activation of the transcription factor NF-kappaB is a hallmark of infections by viral pathogens including influenza viruses. Because gene expression of many proinflammatory and antiviral cytokines is controlled by this factor, the concept emerged that NF-kappaB and its upstream regulator IkappaB kinase are essential components of the innate antiviral immune response to infectious pathogens. In contrast to this common view we report here that NF-kappaB activity promotes efficient influenza virus production. On a molecular level this is due to NF-kappaB-dependent viral induction of the proapoptotic factors tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and FasL, which enhance virus propagation in an autocrine and paracrine fashion. Thus, NF-kappaB acts both proapoptotically and provirally in the context of an influenza virus infection.

Defective RNA replication and late gene expression in temperature-sensitive influenza viruses expressing deleted forms of the NS1 protein

Influenza A virus mutants expressing C-terminally deleted forms of the NS1 protein (NS1-81 and NS1-110) were generated by plasmid rescue. These viruses were temperature sensitive and showed a small plaque size at the permissive temperature. The accumulation of virion RNA in mutant virus-infected cells was reduced at the restrictive temperature, while the accumulation of cRNA or mRNA was not affected, indicating that the NS1 protein is involved in the control of transcription versus replication processes in the infection. The synthesis and accumulation of late virus proteins were reduced in NS1-81 mutant-infected cells at the permissive temperature and were essentially abolished for both viruses at the restrictive temperature, while synthesis and accumulation of nucleoprotein (NP) were unaffected. Probably as a consequence, the nucleocytoplasmic export of virus NP was strongly inhibited at the restrictive temperature. These results indicate that the NS1 protein is essential for nuclear and cytoplasmic steps during the virus cycle.

Essential role of the NV protein of Novirhabdovirus for pathogenicity in rainbow trout

Novirhabdovirus, infectious hematopoietic necrosis virus (IHNV), and viral hemorrhagic septicemia virus (VHSV) are fish rhabdoviruses that, in comparison to the other rhabdoviruses, contain an additional gene coding for a small nonvirion (NV) protein of unassigned function. A recombinant IHNV with the NV gene deleted but expressing the green fluorescent protein (rIHNV-Delta NV) has previously been shown to be efficiently recovered by reverse genetics (S. Biacchesi et al., J. Virol. 74:11247-11253, 2000). However, preliminary experiments suggested that the growth in cell culture of rIHNV-Delta NV was affected by the NV deletion. In the present study, we show that the growth in cell culture of rIHNV-Delta NV is indeed severely impaired but that a normal growth of rIHNV-Delta NV can be restored when NV is provided in trans by using fish cell clones constitutively expressing the NV protein. These results indicate that NV is a protein that has a crucial biological role for optimal replication of IHNV in cell culture. Although IHNV and VHSV NV proteins do not share any significant identity, we show here that both NV proteins play a similar role since a recombinant IHNV virus, rIHNV-NV(VHSV), in which the IHNV NV open reading frame has been replaced by that of VHSV, was shown to replicate as well as the wild-type (wt) IHNV into fish cells. Finally, data provided by experimental fish infections with the various recombinant viruses strongly suggest an essential role of the NV protein for the pathogenicity of IHNV. Furthermore, we show that juvenile trout immunized with NV-knockout IHNV were protected against challenge with wt IHNV. That opens a new perspective for the development of IHNV attenuated live vaccines.

The influenza A virus NS1 protein binds small interfering RNAs and suppresses RNA silencing in plants

RNA silencing comprises a set of sequence-specific RNA degradation pathways that occur in a wide range of eukaryotes, including animals, fungi and plants. A hallmark of RNA silencing is the presence of small interfering RNA molecules (siRNAs). The siRNAs are generated by cleavage of larger double-stranded RNAs (dsRNAs) and provide the sequence specificity for degradation of cognate RNA molecules. In plants, RNA silencing plays a key role in developmental processes and in control of virus replication. It has been shown that many plant viruses encode proteins, denoted RNA silencing suppressors, that interfere with this antiviral response. Although RNA silencing has been shown to occur in vertebrates, no relationship with inhibition of virus replication has been demonstrated to date. Here we show that the NS1 protein of human influenza A virus has an RNA silencing suppression activity in plants, similar to established RNA silencing suppressor proteins of plant viruses. In addition, NS1 was shown to be capable of binding siRNAs. The data presented here fit with a potential role for NS1 in counteracting innate antiviral responses in vertebrates by sequestering siRNAs.

Human influenza virus NS1 protein enhances viral pathogenicity and acts as an RNA silencing suppressor in plants

RNA silencing has a well-established function as an antiviral defence mechanism in plants and insects. Using an Agrobacterium-mediated transient assay, we report here that NS1 protein from human influenza A virus suppresses RNA silencing in plants in a manner similar to P1/HC-Pro protein of Tobacco etch potyvirus, a well-characterized plant virus silencing suppressor. Moreover, we have shown that NS1 protein expression strongly enhances the symptoms of Potato virus X in three different plant hosts, suggesting that NS1 protein could be inhibiting defence mechanisms activated in the plant on infection. These data provide further evidence that an RNA silencing pathway could also be activated as a defence response in mammals.

Attenuation of classical swine fever virus by deletion of the viral N(pro) gene

We have reported earlier that replacement of the N(pro) gene of classical swine fever virus (CSFV) by the murine ubiquitin gene only slightly affects the characteristics of virus replication in the porcine kidney cell line SK-6 [J. Virol. 72 (1998) 7681]. Here, for the moderately virulent CSFV strain Alfort/187 as well as for the highly virulent strain Eystrup we show that the respective N(pro)-deleted viruses are attenuated. Vaccination of pigs with either of the two deletion mutants resulted in the induction of a strong antibody response. Animals were protected against challenge with a lethal dose of highly virulent CSFV indicating that N(pro) deletion mutants are excellent candidates for a modified live virus vaccine. A chimeric virus obtained by replacement of the N(pro) gene in the Eystrup virus by the corresponding sequence of the avirulent CSFV vaccine strain Riems resulted in a virus that was highly virulent. This indicates that the virulence of CSFV correlates with the presence of N(pro) and also suggests that N(pro) is not responsible for the varying virulence observed between individual strains of CSFV.

Generation of high-yielding influenza A viruses in African green monkey kidney (Vero) cells by reverse genetics

Influenza A viruses are the cause of annual epidemics of human disease with occasional outbreaks of pandemic proportions. The zoonotic nature of the disease and the vast viral reservoirs in the aquatic birds of the world mean that influenza will not easily be eradicated and that vaccines will continue to be needed. Recent technological advances in reverse genetics methods and limitations of the conventional production of vaccines by using eggs have led to a push to develop cell-based strategies to produce influenza vaccine. Although cell-based systems are being developed, barriers remain that need to be overcome if the potential of these systems is to be fully realized. These barriers include, but are not limited to, potentially poor reproducibility of viral rescue with reverse genetics systems and poor growth kinetics and yields. In this study we present a modified A/Puerto Rico/8/34 (PR8) influenza virus master strain that has improved viral rescue and growth properties in the African green monkey kidney cell line, Vero. The improved properties were mediated by the substitution of the PR8 NS gene for that of a Vero-adapted reassortant virus. The Vero growth kinetics of viruses with H1N1, H3N2, H6N1, and H9N2 hemagglutinin and neuraminidase combinations rescued on the new master strain were significantly enhanced in comparison to those of viruses with the same combinations rescued on the standard PR8 master strain. These improvements pave the way for the reproducible generation of high-yielding human and animal influenza vaccines by reverse genetics methods. Such a means of production has particular relevance to epidemic and pandemic use.

The influenza B virus nonstructural NS1 protein is essential for efficient viral growth and antagonizes beta interferon induction

We analyzed the functions of the influenza B virus nonstructural NS1-B protein, both by utilizing a constructed mutant virus (Delta NS1-B) lacking the NS1 gene and by testing the activities of the protein when expressed in cells. The mutant virus replicated to intermediate levels in 6-day-old embryonated chicken eggs that contain an immature interferon (IFN) system, whereas older eggs did not support viral propagation to a significant extent. The Delta NS1-B virus was a substantially stronger inducer of beta IFN (IFN-beta) transcripts in human lung epithelial cells than the wild type, and furthermore, transiently expressed NS1-B protein efficiently inhibited virus-dependent activation of the IFN-beta promoter. Interestingly, replication of the Delta NS1-B knockout virus was attenuated by more than 4 orders of magnitude in tissue culture cells containing or lacking functional IFN-alpha/beta genes. These findings show that the NS1-B protein functions as a viral IFN antagonist and indicate a further requirement of this protein for efficient viral replication that is unrelated to blocking IFN effects.

Interferon resistance promotes oncolysis by influenza virus NS1-deletion mutants

NS1 protein of influenza virus is a virulence factor that counteracts Type I interferon (IFN)-mediated antiviral response by the host. A recombinant influenza A virus that lacks the NS1 protein only replicates efficiently in systems that contain defective IFN pathways. We demonstrate that the conditional replication properties of NS1-modified influenza A virus mutants can be exploited for the virus-mediated oncolysis of IFN-resistant tumor cells. IFN resistance in analyzed tumor cell lines correlated with a reduced expression of STAT1. Addition of exogenous IFNalpha or supernatant of virus-infected endothelial cells inhibited viral oncolysis in IFN-sensitive but not in IFN-resistant cell lines. The oncolytic potential of NS1-modified influenza A virus mutants could be exploited in vivo in a SCID mouse model of a subcutaneously-implanted human IFN-resistant melanoma. The data indicate that IFN-resistant tumors are a suitable target for oncolysis induced by NS1-modified influenza virus mutants. STAT1 might serve as a marker to identify these IFN-resistant tumors.

Interferon antagonist proteins of influenza and vaccinia viruses are suppressors of RNA silencing

Homology-dependent RNA silencing occurs in many eukaryotic cells. We reported recently that nodaviral infection triggers an RNA silencing-based antiviral response (RSAR) in Drosophila, which is capable of a rapid virus clearance in the absence of expression of a virus-encoded suppressor. Here, we present further evidence to show that the Drosophila RSAR is mediated by the RNA interference (RNAi) pathway, as the viral suppressor of RSAR inhibits experimental RNAi initiated by exogenous double-stranded RNA and RSAR requires the RNAi machinery. We demonstrate that RNAi also functions as a natural antiviral immunity in mosquito cells. We further show that vaccinia virus and human influenza A, B, and C viruses each encode an essential protein that suppresses RSAR in Drosophila. The vaccinia and influenza viral suppressors, E3L and NS1, are distinct double-stranded RNA-binding proteins and essential for pathogenesis by inhibiting the mammalian IFN-regulated innate antiviral response. We found that the double-stranded RNA-binding domain of NS1, implicated in innate immunity suppression, is both essential and sufficient for RSAR suppression. These findings provide evidence that mammalian virus proteins can inhibit RNA silencing, implicating this mechanism as a nucleic acid-based antiviral immunity in mammalian cells.

A recombinant influenza A virus expressing an RNA-binding-defective NS1 protein induces high levels of beta interferon and is attenuated in mice

Previously we found that the amino-terminal region of the NS1 protein of influenza A virus plays a key role in preventing the induction of beta interferon (IFN-beta) in virus-infected cells. This region is characterized by its ability to bind to different RNA species, including double-stranded RNA (dsRNA), a known potent inducer of IFNs. In order to investigate whether the NS1 RNA-binding activity is required for its IFN antagonist properties, we have generated a recombinant influenza A virus which expresses a mutant NS1 protein defective in dsRNA binding. For this purpose, we substituted alanines for two basic amino acids within NS1 (R38 and K41) that were previously found to be required for RNA binding. Cells infected with the resulting recombinant virus showed increased IFN-beta production, demonstrating that these two amino acids play a critical role in the inhibition of IFN production by the NS1 protein during viral infection. In addition, this virus grew to lower titers than wild-type virus in MDCK cells, and it was attenuated in mice. Interestingly, passaging in MDCK cells resulted in the selection of a mutant virus containing a third mutation at amino acid residue 42 of the NS1 protein (S42G). This mutation did not result in a gain in dsRNA-binding activity by the NS1 protein, as measured by an in vitro assay. Nevertheless, the NS1 R38AK41AS42G mutant virus was able to replicate in MDCK cells to titers close to those of wild-type virus. This mutant virus had intermediate virulence in mice, between those of the wild-type and parental NS1 R38AK41A viruses. These results suggest not only that the IFN antagonist properties of the NS1 protein depend on its ability to bind dsRNA but also that they can be modulated by amino acid residues not involved in RNA binding.

Identification and characterization of viral antagonists of type I interferon in negative-strand RNA viruses

Interferons are cytokines secreted in response to viral infections with potent antiviral activity, and they represent a critical component of the innate immune response against viruses. It has now become apparent that many viruses have evolved different mechanisms to counteract the interferon response, allowing their efficient replication and propagation in their hosts. This review discusses how the development of reverse genetics techniques and the increase in our knowledge of the interferon response have led to the discovery of interferon-antagonistic functions of different genes of viruses belonging to the negative-strand RNA virus group. In many cases, these viral genes encode accessory pro- teins that are not required for viral infectivity but are critical for optimal replication and for virulence in the host.

Persistent infection and suppression of host response by alphaviruses

Alphaviruses cause chronic noncytopathic infection in mosquito cells and develop a highly cytopathic infection in a wide variety of cells of vertebrate origin. Upon infection, alphaviruses modify cellular processes to meet the virus needs for propagation. Downregulation of translation and transcription caused by viral infection appears to reduce interferon (IFN) and cytokine gene expression and allows more efficient dissemination of infection. Alphaviruses with mutations in nonstructural protein nsP2 can become less cytopathic and capable of persisting in some vertebrate cell lines for a number of passages. nsP2 likely functions as an important regulator of virus-host cell interactions and plays a significant role in suppressing the antiviral response. Mammalian cells having no defects in type I IFN system react to replication of the nsP2 viral mutants by more efficient activation of IFN and IFN-dependent genes and are capable of eliminating established alphavirus infection. Blocking of IFN-alpha/beta signaling makes mouse fibroblasts unable to stop replication of Sindbis virus (SINV) with mutated nsP2 and leads to persistent infection. Downregulation of transcription and translation during alphavirus infection are quite independent events, and both probably are involved in inhibition of the antiviral response.

Influenza

Although most influenza infections are self-limited, few other diseases exert such a huge toll of suffering and economic loss. Despite the importance of influenza, there had been, until recently, little advance in its control since amantadine was licensed almost 40 years ago. During the past decade, evidence has accrued on the protection afforded by inactivated vaccines and the safety and efficacy in children of live influenza-virus vaccines. There have been many new developments in vaccine technology. Moreover, work on viral neuraminidase has led to the licensing of potent selective antiviral drugs, and economic decision modelling provides further justification for annual vaccination and a framework for the use of neuraminidase inhibitors. Progress has also been made on developing near-patient testing for influenza that may assist individual diagnosis or the recognition of widespread virus circulation, and so optimise clinical management. Despite these advances, the occurrence of avian H5N1, H9N2, and H7N7 influenza in human beings and the rapid global spread of severe acute respiratory syndrome are reminders of our vulnerability to an emerging pandemic. The contrast between recent cases of H5N1 infection, associated with high mortality, and the typically mild, self-limiting nature of human infections with avian H7N7 and H9N2 influenza shows the gaps in our understanding of molecular correlates of pathogenicity and underlines the need for continuing international research into pandemic influenza. Improvements in animal and human surveillance, new approaches to vaccination, and increasing use of vaccines and antiviral drugs to combat annual influenza outbreaks are essential to reduce the global toll of pandemic and interpandemic influenza.

Innate immune activation as a broad-spectrum biodefense strategy: prospects and research challenges

Biodefense strategies require protection against a broad and largely unforeseen spectrum of pathogens—the forte of innate immune system defenses—that have evolved over millennia to function within moments of encountering either ancient or newly emerging pathogens. Although constitutive, the innate immune system is activated by the presence of microbes or their products, providing a rationale for a potential biodefense strategy. Both prophylactic and postexposure strategies involving innate immune stimulation have been shown to be plausible to prevent or ameliorate infections in animal models. Innate immune-activating compounds based on conserved microbial components recognized by toll-like molecules and other receptors could be synthesized and delivered like drugs by using an entirely different strategy from conventional vaccination. However, important theoretic and practical questions emerge about developing and deploying innate immune protective strategies for biodefense. This rostrum discusses prospects and problems in the overall approach itself. Important topics include microbe-specific issues about innate immune system effectiveness against highly virulent pathogens and general questions, such as whether innate immune responses will be safe and effective if used in a diverse human population of different age groups and with different genetic makeups.

Influenza virus induction of apoptosis by intrinsic and extrinsic mechanisms

It is now firmly established that apoptosis is an important mechanism of influenza virus-induced cell death both in vivo and in vitro. Data are predominantly from experiments with influenza A virus and in vitro experimental systems. Multiple influenza virus factors have been identified that can activate intrinsic or extrinsic apoptotic induction pathways. Currently there is no evidence for influenza virus directly accessing the apoptosis execution factors. The best-studied influenza virus inducers of apoptosis are dsRNA, NS1, NA, and a newly described gene product PB1-F2. PB1-F2 is the only influenza virus factor to date identified to act intrinsically by localization and interaction with the mitochondrial-dependent apoptotic pathway. Both dsRNA and NA have been shown to act via an extrinsic mechanism involving proapoptotic host-defense molecules: PKR by induction of Fas-Fas ligand and NA by activation of TGF-beta. PKR is capable of controlling several important cell-signaling pathways and therefore may have multiple effects; a predominant one is increased interferon (IFN) production and activity. NS1 has been shown to be both proapoptotic and antiapoptotic. Use of influenza virus NS1 deletion mutants has provided evidence for NS1 interference with apoptosis, IFN induction, and related cell-signaling pathways. Influenza virus also has important exocrine paracrine effects, which are likely mediated via TNF family ligands and oxygen, free radicals capable of inducing apoptosis. Little is known about activation of inhibitors of apoptosis such as inhibitory apoptotic proteins. Whether all these factors always have a role in influenza virus-induced apoptosis is unknown. The kinetics of synthesis of influenza virus factors affecting apoptosis during the replication cycle may be an important aspect of apoptosis induction.

Newcastle disease virus V protein is a determinant of host range restriction

It has been demonstrated that the V protein of Newcastle disease virus (NDV) functions as an alpha/beta interferon (IFN-alpha/beta) antagonist (M. S. Park, M. L. Shaw, J. Muñoz-Jordan, J. F. Cros, T. Nakaya, N. Bouvier, P. Palese, A. García-Sastre, and C. F. Basler, J. Virol. 77:1501-1511, 2003). We now show that the NDV V protein plays an important role in host range restriction. In order to study V functions in vivo, recombinant NDV (rNDV) mutants, defective in the expression of the V protein, were generated. These rNDV mutants grow poorly in both embryonated chicken eggs and chicken embryo fibroblasts (CEFs) compared to the wild-type (wt) rNDV. However, insertion of the NS1 gene of influenza virus A/PR8/34 into the NDV V(-) genome [rNDV V(-)/NS1] restores impaired growth to wt levels in embryonated chicken eggs and CEFs. These data indicate that for viruses infecting avian cells, the NDV V protein and the influenza NS1 protein are functionally interchangeable, even though there are no sequence similarities between the two proteins. Interestingly, in human cells, the titer of wt rNDV is 10 times lower than that of rNDV V(-)/NS1. Correspondingly, the level of IFN secreted by human cells infected with wt rNDV is much higher than that secreted by cells infected with the NS1-expressing rNDV. This suggests that the IFN antagonist activity of the NDV V protein is species specific. Finally, the NDV V protein plays an important role in preventing apoptosis in a species-specific manner. The rNDV defective in V induces apoptotic cell death more rapidly in CEFs than does wt rNDV. Taken together, these data suggest that the host range of NDV is limited by the ability of its V protein to efficiently prevent innate host defenses, such as the IFN response and apoptosis.

Newcastle disease virus V protein is associated with viral pathogenesis and functions as an alpha interferon antagonist

Newcastle disease virus (NDV) edits its P gene by inserting one or two G residues at the conserved editing site (UUUUUCCC, genome sense) and transcribes the P mRNA (unedited), the V mRNA (with a +1 frameshift), and the W mRNA (with a +2 frameshift). All three proteins are amino coterminal but vary at their carboxyl terminus in length and amino acid composition. Little is known about the role of the V and W proteins in NDV replication and pathogenesis. We have constructed and recovered two recombinant viruses in which the expression of the V or both the V and W proteins has been abolished. Compared to the parental virus, the mutant viruses showed impaired growth in cell cultures, except in Vero cells. However, transient expression of the carboxyl-terminal portion of the V protein enhanced the growth of the mutant viruses. In embryonated chicken eggs, the parental virus grew to high titers in embryos of different gestational ages, whereas the mutant viruses showed an age-dependent phenomenon, growing to lower titer in more-developed embryos. An interferon (IFN) sensitivity assay showed that the parental virus was more resistant to the antiviral effect of IFN than the mutant viruses. Moreover, infection with the parental virus resulted in STAT1 protein degradation, but not with the mutant viruses. These findings indicate that the V protein of NDV possesses the ability to inhibit alpha IFN and that the IFN inhibitory function lies in the carboxyl-terminal domain. Pathogenicity studies showed that the V protein of NDV significantly contributes to the virus virulence.

Viral infection switches non-plasmacytoid dendritic cells into high interferon producers

Type I interferons (IFN-I) are important cytokines linking innate and adaptive immunity. Plasmacytoid dendritic cells make high levels of IFN-I in response to viral infection and are thought to be the major source of the cytokines in vivo. Here, we show that conventional non-plasmacytoid dendritic cells taken from mice infected with a dendritic-cell-tropic strain of lymphocytic choriomeningitis virus make similarly high levels of IFN-I on subsequent culture. Similarly, non-plasmacytoid dendritic cells secrete high levels of IFN-I in response to double-stranded RNA (dsRNA), a major viral signature, when the latter is introduced into the cytoplasm to mimic direct viral infection. This response is partially dependent on the cytosolic dsRNA-binding enzyme protein kinase R and does not require signalling through toll-like receptor (TLR) 3, a surface receptor for dsRNA. Furthermore, we show that sequestration of dsRNA by viral NS1 (refs 6, 7) explains the inability of conventional dendritic cells to produce IFN-I on infection with influenza. Our results suggest that multiple dendritic cell types, not just plasmacytoid cells, can act as specialized interferon-producing cells in certain viral infections, and reveal the existence of a TLR-independent pathway for dendritic cell activation that can be the target of viral interference.

The Ebola virus VP35 protein inhibits activation of interferon regulatory factor 3

The Ebola virus VP35 protein was previously found to act as an interferon (IFN) antagonist which could complement growth of influenza delNS1 virus, a mutant influenza virus lacking the influenza virus IFN antagonist protein, NS1. The Ebola virus VP35 could also prevent the virus- or double-stranded RNA-mediated transcriptional activation of both the beta IFN (IFN-beta) promoter and the IFN-stimulated ISG54 promoter (C. Basler et al., Proc. Natl. Acad. Sci. USA 97:12289-12294, 2000). We now show that VP35 inhibits virus infection-induced transcriptional activation of IFN regulatory factor 3 (IRF-3)-responsive mammalian promoters and that VP35 does not block signaling from the IFN-alpha/beta receptor. The ability of VP35 to inhibit this virus-induced transcription correlates with its ability to block activation of IRF-3, a cellular transcription factor of central importance in initiating the host cell IFN response. We demonstrate that VP35 blocks the Sendai virus-induced activation of two promoters which can be directly activated by IRF-3, namely, the ISG54 promoter and the ISG56 promoter. Further, expression of VP35 prevents the IRF-3-dependent activation of the IFN-alpha4 promoter in response to viral infection. The inhibition of IRF-3 appears to occur through an inhibition of IRF-3 phosphorylation. VP35 blocks virus-induced IRF-3 phosphorylation and subsequent IRF-3 dimerization and nuclear translocation. Consistent with these observations, Ebola virus infection of Vero cells activated neither transcription from the ISG54 promoter nor nuclear accumulation of IRF-3. These data suggest that in Ebola virus-infected cells, VP35 inhibits the induction of antiviral genes, including the IFN-beta gene, by blocking IRF-3 activation.

Classical swine fever virus interferes with cellular antiviral defense: evidence for a novel function of N(pro)

Classical swine fever virus (CSFV) replicates efficiently in cell lines and monocytic cells, including macrophages (MPhi), without causing a cytopathic effect or inducing interferon (IFN) secretion. In the present study, the capacity of CSFV to interfere with cellular antiviral activity was investigated. When the porcine kidney cell line SK-6 was infected with CSFV, there was a 100-fold increased capacity to resist to apoptosis induced by polyinosinic-polycytidylic acid [poly(IC)], a synthetic double-stranded RNA. In MPhi, the virus infection inhibited poly(IC)-induced alpha/beta IFN (type I IFN) synthesis. This interference with cellular antiviral defense correlated with the presence of the viral N(pro) gene. Mutants lacking the N(pro) gene (DeltaN(pro) CSFV) did not protect SK-6 cells from poly(IC)-induced apoptosis, despite growth properties and protein expression levels similar to those of the wild-type virus. Furthermore, DeltaN(pro) CSFV did not prevent poly(IC)-induced type I IFN production in MPhi but rather induced type I IFN in the absence of poly(IC) in both MPhi and the porcine kidney cell line PK-15, but not in SK-6 cells. With MPhi and PK-15, an impaired replication of the DeltaN(pro) CSFV compared with wild-type virus was noted. In addition, DeltaN(pro) CSFV, but not wild-type CSFV, could interfere with vesicular stomatitis virus replication in PK-15 cells. Taken together, these results provide evidence for a novel function associated with CSFV N(pro) with respect to the inhibition of the cellular innate immune system.

Activation of tumor antigen-specific cytotoxic T lymphocytes (CTLs) by human dendritic cells infected with an attenuated influenza A virus expressing a CTL epitope derived from the HER-2/neu proto-oncogene

The development of cancer vaccines requires approaches to induce expansion and functional differentiation of tumor antigen-specific cytotoxic T lymphocyte (CTL) effectors which posses cytolytic capability and produce cytokines. Efficient induction of such cells is hindered by the poor immunogenicity of tumor antigens and by the poor transduction efficiency of dendritic cells (DCs) with current nonreplicating vectors. We have investigated the use of influenza A virus, a potent viral inducer of CTLs, as a vector expressing the immunodominant HER-2 CTL epitope KIF (E75). For this purpose, an attenuated influenza A/PR8/34 virus with a truncated nonstructural (NS1) gene was generated containing the E75 epitope in its neuraminidase protein (KIF-NS virus). Stimulation of peripheral blood mononuclear cells from healthy donors and of tumor-associated lymphocytes from ovarian and breast cancer patients with DCs infected with KIF-NS virus (KIF-NS DC) induced CTLs that specifically recognized the peptide KIF and HER-2-expressing tumors in cytotoxicity assays and secreted gamma interferon (IFN-gamma) and interleukin-2 at recall with peptide. Priming with KIF-NS DCs increased the number of E75(+) CD45RO(+) cells by more than 10-fold compared to nonstimulated cells. In addition, KIF-NS virus induced high levels of IFN-alpha in DCs. This is the first report demonstrating induction of human epitope-specific CTLs against a tumor-associated antigen with a live attenuated recombinant influenza virus vector. Such vectors may provide a novel approach for tumor antigen delivery, lymphocyte activation, and differentiation in human cancer vaccine development.

PKR is not required for interferon-gamma inhibition of VSV replication in neurons

In this report, the contribution of PKR to the IFN-gamma mediated inhibition of VSV replication in neurons was examined. IFN-gamma treatment of NB41A3 murine neuroblastoma cells resulted in the reduced expression of VSV protein during infection. PKR was found to be modestly upregulated in NB41A3 cells following IFN-gamma treatment. The phosphorylation state of PKR and its downstream target, eIF2alpha, were unaffected by either IFN-gamma or VSV infection. Inhibition of PKR through the use of 2-aminopurine or the expression of the Influenza A NS1 gene had no effect on the ability of IFN-gamma to inhibit the replication of VSV in vitro. These data indicate that endogenously expressed PKR is not required for the IFN-gamma mediated inhibition of VSV replication in NB41A3 neuroblastoma cells.

Localization of influenza virus proteins to nuclear dot 10 structures in influenza virus-infected cells

We studied influenza virus M1 protein by generating HeLa and MDCK cell lines that express M1 genetically fused to green fluorescent protein (GFP). GFP-M1 was incorporated into virions produced by influenza virus infected MDCK cells expressing the fusion protein indicating that the fusion protein is at least partially functional. Following infection of either HeLa or MDCK cells with influenza A virus (but not influenza B virus), GFP-M1 redistributes from its cytosolic/nuclear location and accumulates in nuclear dots. Immunofluorescence revealed that the nuclear dots represent nuclear dot 10 (ND10) structures. The colocalization of authentic M1, as well as NS1 and NS2 protein, with ND10 was confirmed by immunofluorescence following in situ isolation of ND10. These findings demonstrate a previously unappreciated involvement of influenza virus with ND10, a structure involved in cellular responses to immune cytokines as well as the replication of a rapidly increasing list of viruses.

Impaired innate host defense causes susceptibility to respiratory virus infections in cystic fibrosis

Viral infection is the primary cause of respiratory morbidity in cystic fibrosis (CF) infants. Here, we identify that host factors allow increased virus replication and cytokine production, providing a mechanism for understanding the severity of virus disease in CF. Increased virus is due to lack of nitric oxide synthase 2 (NOS2) and 2', 5' oligoadenylate synthetase (OAS) 1 induction in response to virus or IFNgamma. This can be attributed to impairment of activation of signal transducer and activator of transcription (STAT)1, a fundamental component to antiviral defense. NO donor or NOS2 overexpression provides protection from virus infection in CF, suggesting that NO is sufficient for antiviral host defense in the human airway and is one strategy for antiviral therapy in CF children.

Viral evasion of the interferon system: old viruses, new tricks

Interferon-alpha/beta (IFN-alpha/beta) plays an important role in antiviral defense, but many viruses have evolved mechanisms to evade this barrier. The emerging understanding of how viral proteins intercept the IFN response was the focus of a workshop held at the University of Freiburg, Germany, on September 27-28, 2002.

IFN-alpha antagonistic activity of HCV core protein involves induction of suppressor of cytokine signaling-3

Eighty percent of patients newly infected with the hepatitis C virus (HCV) develop chronic infection, suggesting that HCV can develop effective strategies to escape the unspecific and specific immune response of the host. Because SOCS molecules have been recognized to be powerful inhibitors of cytokine signaling via the Jak/STAT pathway, virus-induced expression of these molecules should be an efficient instrument to counteract the cellular response toward interferons (IFNs), an essential part of first line antiviral immune response. This study shows that overexpression of HCV core protein inhibits IFN-alpha-induced tyrosine phosphorylation and activation of STAT1 in hepatic cells. With the use of a STAT1-YFP fusion protein, further evidence is given that HCV core is capable to inhibit nuclear translocation of STAT1. Inhibition of STAT1-tyrosine phosphorylation was accompanied by the induction of SOCS3-mRNA expression, suggesting that the HCV core protein impairs IFN-alpha-induced signal transduction via induction of SOCS3 expression. HCV core protein was competent to partially rescue growth of a genetically engineered influenza A virus lacking its own IFN antagonist. These IFN-antagonistic properties of the HCV core protein may be part of the molecular basis of IFN-alpha unresponsiveness in about one-half of chronically infected HCV-patients.

Alternative base pairs attenuate influenza A virus when introduced into the duplex region of the conserved viral RNA promoter of either the NS or the PA gene

The development of plasmid-based rescue systems for influenza virus has allowed previous studies of the neuraminidase (NA) virion RNA (vRNA) promoter to be extended, in order to test the hypothesis that alternative base pairs in the conserved influenza virus vRNA promoter cause attenuation when introduced into other gene segments. Influenza A/WSN/33 viruses with alternative base pairs in the duplex region of the vRNA promoter of either the polymerase acidic (PA) or the NS (non-structural 1, NS1, and nuclear export, NEP, -encoding) gene have been rescued. Virus growth in MDBK cells demonstrated that one of the mutations, the D2 mutation (U-A replacing G-C at nucleotide positions 12'-11), caused significant virus attenuation when introduced into either the PA or the NS gene. The D2 mutation resulted in the reduction of PA- or NS-specific vRNA and mRNA levels in PA- or NS-recombinant viruses, respectively. Since the D2 mutation attenuates influenza virus when introduced into either the PA or the NS gene segments, or the NA gene segment, as demonstrated previously, this suggests that this mutation will lead to virus attenuation when introduced into any of the eight gene segments. Such a mutation may be useful in the production of live-attenuated viruses.

Virus replication in engineered human cells that do not respond to interferons

The V protein of the paramyxovirus simian virus 5 blocks interferon (IFN) signaling by targeting STAT1 for proteasome-mediated degradation. Here we report on the isolation of human cell lines that express the V protein and can no longer respond to IFN. A variety of viruses, particularly slow-growing wild-type viruses and vaccine candidate viruses (which are attenuated due to mutations that affect virus replication, virus spread, or ability to circumvent the IFN response), form bigger plaques and grow to titers that are increased as much as 10- to 4,000-fold in these IFN-nonresponsive cells. We discuss the practical applications of using such cells in vaccine development and manufacture, virus diagnostics and isolation of newly emerging viruses, and studies on host cell tropism and pathogenesis.

Novel gene product of Thogoto virus segment 6 codes for an interferon antagonist

Thogoto virus (THOV) is a tick-transmitted orthomyxovirus with a genome of six negative-stranded RNA segments. The sixth segment encodes two different transcripts: a spliced transcript that is translated into the matrix protein (M) and an unspliced transcript. Here, we report that the unspliced transcript encodes an elongated form of M named ML. A THOV isolate deficient in ML expression was an efficient interferon inducer, whereas ML-expressing wild-type strains were poor interferon inducers. These results were confirmed with recombinant THOVs rescued from cDNAs. Expression of ML efficiently suppressed activation of the beta interferon promoter by double-stranded RNA. These results indicate that ML is an accessory protein that functions as a potent interferon antagonist by blocking transcriptional activation of alpha/beta interferons.

Influenza-virus-induced signaling cascades: targets for antiviral therapy?

Influenza viruses continue to pose a severe threat worldwide, causing thousands of deaths and an enormous economic loss every year. The major problem in fighting influenza is the high genetic variability of the virus, resulting in the rapid formation of variants that escape the acquired immunity against previous virus strains, or have resistance to antiviral agents. Every virus depends on its host cell and, hence, cellular functions that are essential for viral replication might be suitable targets for antiviral therapy. As a result, intracellular signaling cascades induced by the virus, in particular mitogen-activated protein kinase pathways, have recently come into focus.

Newcastle disease virus (NDV)-based assay demonstrates interferon-antagonist activity for the NDV V protein and the Nipah virus V, W, and C proteins

We have generated a recombinant Newcastle disease virus (NDV) that expresses the green fluorescence protein (GFP) in infected chicken embryo fibroblasts (CEFs). This virus is interferon (IFN) sensitive, and pretreatment of cells with chicken alpha/beta IFN (IFN-alpha/beta) completely blocks viral GFP expression. Prior transfection of plasmid DNA induces an IFN response in CEFs and blocks NDV-GFP replication. However, transfection of known inhibitors of the IFN-alpha/beta system, including the influenza A virus NS1 protein and the Ebola virus VP35 protein, restores NDV-GFP replication. We therefore conclude that the NDV-GFP virus could be used to screen proteins expressed from plasmids for the ability to counteract the host cell IFN response. Using this system, we show that expression of the NDV V protein or the Nipah virus V, W, or C proteins rescues NDV-GFP replication in the face of the transfection-induced IFN response. The V and W proteins of Nipah virus, a highly lethal pathogen in humans, also block activation of an IFN-inducible promoter in primate cells. Interestingly, the amino-terminal region of the Nipah virus V protein, which is identical to the amino terminus of Nipah virus W, is sufficient to exert the IFN-antagonist activity. In contrast, the anti-IFN activity of the NDV V protein appears to be located in the carboxy-terminal region of the protein, a region implicated in the IFN-antagonist activity exhibited by the V proteins of mumps virus and human parainfluenza virus type 2.

Induction of proinflammatory cytokines in human macrophages by influenza A (H5N1) viruses: a mechanism for the unusual severity of human disease?

BACKGROUND

In 1997, the first documented instance of human respiratory disease and death associated with a purely avian H5N1 influenza virus resulted in an overall case-fatality rate of 33%. The biological basis for the severity of human H5N1 disease has remained unclear. We tested the hypothesis that virus-induced cytokine dysregulation has a role.

METHODS

We used cDNA arrays and quantitative RT-PCR to compare the profile of cytokine gene expression induced by viruses A/HK/486/97 and A/HK/483/97 (both H5N1/97) with that of human H3N2 and H1N1 viruses in human primary monocyte-derived macrophages in vitro. Secretion of tumour necrosis factor alpha (TNF alpha) from macrophages infected with the viruses was compared by ELISA. By use of naturally occurring viral reassortants and recombinant viruses generated by reverse genetic techniques, we investigated the viral genes associated with the TNF-alpha response.

FINDINGS

The H5N1/97 viruses induced much higher gene transcription of proinflammatory cytokines than did H3N2 or H1N1 viruses, particularly TNF alpha and interferon beta. The concentration of TNF-alpha protein in culture supernatants of macrophages infected with these viruses was similar to that induced by stimulation with Escherichia coli lipopolysaccharide. The non-structural (NS) gene-segment of H5N1/97 viruses contributed to the increase in TNF alpha induced by the virus.

INTERPRETATION

The H5N1/97 viruses are potent inducers of proinflammatory cytokines in macrophages, the most notable being TNF alpha. This characteristic may contribute to the unusual severity of human H5N1 disease.

Unlocking the mysteries of virus-host interactions: does functional genomics hold the key?

The interactions between viruses and the cells they infect are complex and multifaceted. While viruses strive to usurp cellular functions to their advantage, the cell strives to thwart these efforts by mounting a variety of defensive responses. These responses may include the induction of interferon, stress response, or apoptotic pathways, all of which are accompanied by changes in gene expression. Some viruses consistently win this tug of war, whereas others succumb to cellular defense mechanisms. The viral and cellular factors that determine the outcome are for the most part still unknown. With the advent of functional genomics, potent new technologies are now available to probe the complexities of virus-host interactions in ever increasing depth and detail. We describe here our efforts to use microarrays, proteomics, and bioinformatics to focus in on the changes in gene expression and protein production that occur in a virus-infected cell and to use these technologies to unlock the mysteries of virus-host interactions.

Functional replacement of the carboxy-terminal two-thirds of the influenza A virus NS1 protein with short heterologous dimerization domains

The NS1 protein of influenza A/WSN/33 virus is a 230-amino-acid-long protein which functions as an interferon alpha/beta (IFN-alpha/beta) antagonist by preventing the synthesis of IFN during viral infection. In tissue culture, the IFN inhibitory function of the NS1 protein has been mapped to the RNA binding domain, the first 73 amino acids. Nevertheless, influenza viruses expressing carboxy-terminally truncated NS1 proteins are attenuated in mice. Dimerization of the NS1 protein has previously been shown to be essential for its RNA binding activity. We have explored the ability of heterologous dimerization domains to functionally substitute in vivo for the carboxy-terminal domains of the NS1 protein. Recombinant influenza viruses were generated that expressed truncated NS1 proteins of 126 amino acids, fused to 28 or 24 amino acids derived from the dimerization domains of either the Saccharomyces cerevisiae PUT3 or the Drosophila melanogaster Ncd (DmNcd) proteins. These viruses regained virulence and lethality in mice. Moreover, a recombinant influenza virus expressing only the first 73 amino acids of the NS1 protein was able to replicate in mice lacking three IFN-regulated antiviral enzymes, PKR, RNaseL, and Mx, but not in wild-type (Mx-deficient) mice, suggesting that the attenuation was mainly due to an inability to inhibit the IFN system. Remarkably, a virus with an NS1 truncated at amino acid 73 but fused to the dimerization domain of DmNcd replicated and was also highly pathogenic in wild-type mice. These results suggest that the main biological function of the carboxy-terminal region of the NS1 protein of influenza A virus is the enhancement of its IFN antagonist properties by stabilizing the NS1 dimeric structure.

The influenza A virus NS1 protein inhibits activation of Jun N-terminal kinase and AP-1 transcription factors

The influenza A virus nonstructural NS1 protein is known to modulate host cell gene expression and to inhibit double-stranded RNA (dsRNA)-mediated antiviral responses. Here we identify NS1 as the first viral protein that antagonizes virus- and dsRNA-induced activation of the stress response-signaling pathway mediated through Jun N-terminal kinase.

A decade after the generation of a negative-sense RNA virus from cloned cDNA - what have we learned?

Since the first generation of a negative-sense RNA virus entirely from cloned cDNA in 1994, similar reverse genetics systems have been established for members of most genera of the Rhabdo- and Paramyxoviridae families, as well as for Ebola virus (Filoviridae). The generation of segmented negative-sense RNA viruses was technically more challenging and has lagged behind the recovery of nonsegmented viruses, primarily because of the difficulty of providing more than one genomic RNA segment. A member of the Bunyaviridae family (whose genome is composed of three RNA segments) was first generated from cloned cDNA in 1996, followed in 1999 by the production of influenza virus, which contains eight RNA segments. Thus, reverse genetics, or the de novo synthesis of negative-sense RNA viruses from cloned cDNA, has become a reliable laboratory method that can be used to study this large group of medically and economically important viruses. It provides a powerful tool for dissecting the virus life cycle, virus assembly, the role of viral proteins in pathogenicity and the interplay of viral proteins with components of the host cell immune response. Finally, reverse genetics has opened the way to develop live attenuated virus vaccines and vaccine vectors.

Roles of nonstructural protein nsP2 and Alpha/Beta interferons in determining the outcome of Sindbis virus infection

Alphaviruses productively infect a variety of vertebrate and insect cell lines. In vertebrate cells, Sindbis virus redirects cellular processes to meet the needs of virus propagation. At the same time, cells respond to virus replication by downregulating virus growth and preventing dissemination of the infection. The balance between these two mechanisms determines the outcome of infection at the cellular and organismal levels. In this report, we demonstrate that a viral nonstructural protein, nsP2, is a significant regulator of Sindbis virus-host cell interactions. This protein not only is a component of the replicative enzyme complex required for replication and transcription of viral RNAs but also plays a role in suppressing the antiviral response in Sindbis virus-infected cells. nsP2 most likely acts by decreasing interferon (IFN) production and minimizing virus visibility. Infection of murine cells with Sindbis virus expressing a mutant nsP2 leads to higher levels of IFN secretion and the activation of 170 cellular genes that are induced by IFN and/or virus replication. Secreted IFN protects naive cells against Sindbis virus infection and also stops viral replication in productively infected cells. Mutations in nsP2 can also attenuate Sindbis virus cytopathogenicity. Such mutants can persist in mammalian cells with defects in the alpha/beta IFN (IFN-alpha/beta) system or when IFN activity is neutralized by anti-IFN-alpha/beta antibodies. These findings provide new insight into the alphavirus-host cell interaction and have implications for the development of improved alphavirus expression systems with better antigen-presenting potential.

Suppression of lipopolysaccharide-induced antiviral transcription factor (STAT-1 and NF-kappa B) complexes by antibody-dependent enhancement of macrophage infection by Ross River virus

Subneutralizing concentrations of antibody may enhance virus infection by bringing the virus-antibody complex into contact with the cell surface Fc receptors; this interaction facilitates entry of virus into the cell and is referred to as antibody-dependent enhancement (ADE) of infection. Northern analysis of macrophage RNA demonstrated that ADE infection by the indigenous Australian alphavirus Ross River (RRV-ADE) ablated or diminished message for tumor necrosis factor alpha (TNF-alpha), nitric-oxide synthase 2 (NOS2), and IFN regulatory factor 1 (IRF-1), as well as for IFN-inducible protein 10 (IP-10) and IFN-beta; the transcription of a control gene was unaffected. Additionally, electrophoretic mobility-shift assay (EMSA) studies showed that transcription factor IFN-alpha-activated factor (AAF), IFN-stimulated gene factor 3 (ISGF3), and nuclear factor-kappaB (NF-kappaB) complex formation in macrophage nuclear extracts were specifically suppressed post-RRV-ADE infection, emphasizing the capacity for ADE infections to compromise antiviral responses at the transcriptional level. The suppression of antiviral transcription factor complexes was shown to depend on replicating virus and was not simply a result of general antibody-Fc-receptor interaction. Although only a minority of cells ( approximately 15%) were shown to be positive for RRV by immunostaining techniques post ADE, molecular (RT-PCR) analysis showed that unstained cells carried RRV-RNA, indicating a higher level of viral infectivity than previously suspected. Electron microscopy studies confirmed this observation. Furthermore, levels of cellular IL-10 protein were dramatically elevated in RRV-ADE cultures. This evidence demonstrates that RRV can potently disrupt the activation of specific antiviral pathways via ADE infection pathways, and may suggest a significant mechanism in the infection and pathogenesis of other ADE viruses.

Effect of double-stranded RNA and interferon on the antiviral activity of Atlantic salmon cells against infectious salmon anemia virus and infectious pancreatic necrosis virus

Type I interferons (IFN) establish an antiviral state in vertebrate cells by inducing expression of Mx and other antiviral proteins. We have studied the effect of Atlantic salmon interferon-like activity (AS-IFN) and poly I:C on the Mx protein expression and antiviral activity against infectious salmon anaemia virus (ISAV) and infectious pancreatic necrosis virus (IPNV) in the Atlantic salmon cell lines SHK-1 and TO. The double-stranded RNA poly I:C is an inducer of type I IFN in vertebrates. A cell cytotoxicity assay and measurements of virus yield were used to measure protection of cells against virus infection. Maximal induction of Mx protein in TO and SHK-1 cells occurred 48 h after poly I:C stimulation and 24 h after AS-IFN stimulation. TO cells pretreated with AS-IFN or poly I:C were protected from infection with IPNV 24 to 96 h after stimulation. Poly I:C or AS-IFN induced a minor protection against ISAV infection in SHK-1 cells, but no protection was induced against ISAV in TO cells. Western blot analysis showed that ISAV induced expression of Mx protein in TO and SHK-1 cells whereas IPNV did not induce Mx protein expression. These results suggest that ISAV and IPNV have very different sensitivities to IFN-induced antiviral activity and have developed different strategies to avoid the IFN-system of Atlantic salmon. Moreover, Atlantic salmon Mx protein appears not to inhibit replication of ISAV.

Lethal H5N1 influenza viruses escape host anti-viral cytokine responses

The H5N1 influenza viruses transmitted to humans in 1997 were highly virulent, but the mechanism of their virulence in humans is largely unknown. Here we show that lethal H5N1 influenza viruses, unlike other human, avian and swine influenza viruses, are resistant to the antiviral effects of interferons and tumor necrosis factor alpha. The nonstructural (NS) gene of H5N1 viruses is associated with this resistance. Pigs infected with recombinant human H1N1 influenza virus that carried the H5N1 NS gene experienced significantly greater and more prolonged viremia, fever and weight loss than did pigs infected with wild-type human H1N1 influenza virus. These effects required the presence of glutamic acid at position 92 of the NS1 molecule. These findings may explain the mechanism of the high virulence of H5N1 influenza viruses in humans.

Viruses and interferon: a fight for supremacy

The action of interferons (IFNs) on virus-infected cells and surrounding tissues elicits an antiviral state that is characterized by the expression and antiviral activity of IFN-stimulated genes. In turn, viruses encode mechanisms to counteract the host response and support efficient viral replication, thereby minimizing the therapeutic antiviral power of IFNs. In this review, we discuss the interplay between the IFN system and four medically important and challenging viruses -- influenza, hepatitis C, herpes simplex and vaccinia -- to highlight the diversity of viral strategies. Understanding the complex network of cellular antiviral processes and virus-host interactions should aid in identifying new and common targets for the therapeutic intervention of virus infection. This effort must take advantage of the recent developments in functional genomics, bioinformatics and other emerging technologies.

Cellular transcriptional profiling in influenza A virus-infected lung epithelial cells: the role of the nonstructural NS1 protein in the evasion of the host innate defense and its potential contribution to pandemic influenza

The NS1 protein of influenza A virus contributes to viral pathogenesis, primarily by enabling the virus to disarm the host cell type IFN defense system. We examined the downstream effects of NS1 protein expression during influenza A virus infection on global cellular mRNA levels by measuring expression of over 13,000 cellular genes in response to infection with wild-type and mutant viruses in human lung epithelial cells. Influenza A/PR/8/34 virus infection resulted in a significant induction of genes involved in the IFN pathway. Deletion of the viral NS1 gene increased the number and magnitude of expression of cellular genes implicated in the IFN, NF-kappaB, and other antiviral pathways. Interestingly, different IFN-induced genes showed different sensitivities to NS1-mediated inhibition of their expression. A recombinant virus with a C-terminal deletion in its NS1 gene induced an intermediate cellular mRNA expression pattern between wild-type and NS1 knockout viruses. Most significantly, a virus containing the 1918 pandemic NS1 gene was more efficient at blocking the expression of IFN-regulated genes than its parental influenza A/WSN/33 virus. Taken together, our results suggest that the cellular response to influenza A virus infection in human lung cells is significantly influenced by the sequence of the NS1 gene, demonstrating the importance of the NS1 protein in regulating the host cell response triggered by virus infection.

Bunyamwera bunyavirus nonstructural protein NSs counteracts the induction of alpha/beta interferon

Production of alpha/beta interferons (IFN-alpha/beta) in response to viral infection is one of the main defense mechanisms of the innate immune system. Many viruses therefore encode factors that subvert the IFN system to enhance their virulence. Bunyamwera virus (BUN) is the prototype of the Bunyaviridae family. By using reverse genetics, we previously produced a recombinant virus lacking the nonstructural protein NSs (BUNdelNSs) and showed that NSs is a nonessential gene product that contributes to viral pathogenesis. Here we demonstrate that BUNdelNSs is a strong inducer of IFN-alpha/beta, whereas in cells infected with the wild-type counterpart expressing NSs (wild-type BUN), neither IFN nor IFN mRNA could be detected. IFN induction by BUNdelNSs correlated with activation of NF-kappaB and was dependent on virally produced double-stranded RNA and on the IFN transcription factor IRF-3. Furthermore, both in cultured cells and in mice lacking a functional IFN-alpha/beta system, BUNdelNSs replicated to wild-type BUN levels, whereas in IFN-competent systems, wild-type BUN grew more efficiently. These results suggest that BUN NSs is an IFN induction antagonist that blocks the transcriptional activation of IFN-alpha/beta in order to increase the virulence of Bunyamwera virus.