Role of the vaccinia virus E3L and K3L gene products in rescue of VSV and EMCV from the effects of IFN-alpha

Subversion of Programed Cell Death by Poxviruses

Poxviruses have been long regarded as potent inhibitors of apoptotic cell death. More recently, they have been shown to inhibit necroptotic cell death through two distinct strategies. These strategies involve either blocking virus sensing by the host pattern recognition receptor, ZBP1 (also called DAI) or by influencing receptor interacting protein kinase (RIPK)3 signal transduction by inhibition of activation of the executioner of necroptosis, mixed lineage kinase-like protein (MLKL). Vaccinia virus E3 specifically blocks ZBP1 → RIPK3 → MLKL necroptosis, leaving virus-infected cells susceptible to the TNF death-receptor signaling (e.g., TNFR1 → FADD → RIPK1 → RIPK3 → MLKL), and, potentially, TLR3 → TRIF → RIPK3 → MLKL necroptosis. While E3 restriction of necroptosis appears to be common to many poxviruses that infect vertebrate hosts, another modulatory strategy not observed in vaccinia or variola virus manifests through subversion of MLKL activation. Recently described viral mimics of MLKL in other chordopoxviruses inhibit all three modes of necroptotic cell death. As with inhibition of apoptosis, the evolution of potentially redundant viral mechanisms to inhibit programmed necroptotic cell death emphasizes the importance of this pathway in the arms race between pathogens and their hosts.

Antagonism of Protein Kinase R by Large DNA Viruses

Decades of research on vaccinia virus (VACV) have provided a wealth of insights and tools that have proven to be invaluable in a broad range of studies of molecular virology and pathogenesis. Among the challenges that viruses face are intrinsic host cellular defenses, such as the protein kinase R pathway, which shuts off protein synthesis in response to the dsRNA that accumulates during replication of many viruses. Activation of PKR results in phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α), inhibition of protein synthesis, and limited viral replication. VACV encodes two well-characterized antagonists, E3L and K3L, that can block the PKR pathway and thus enable the virus to replicate efficiently. The use of VACV with a deletion of the dominant factor, E3L, enabled the initial identification of PKR antagonists encoded by human cytomegalovirus (HCMV), a prevalent and medically important virus. Understanding the molecular mechanisms of E3L and K3L function facilitated the dissection of the domains, species-specificity, and evolutionary potential of PKR antagonists encoded by human and nonhuman CMVs. While remaining cognizant of the substantial differences in the molecular virology and replication strategies of VACV and CMVs, this review illustrates how VACV can provide a valuable guide for the study of other experimentally less tractable viruses.

Small Hero with Great Powers: Vaccinia Virus E3 Protein and Evasion of the Type I IFN Response

Poxviridae have developed a plethora of strategies to evade innate and adaptive immunity. In this review, we focused on the vaccinia virus E3 protein, encoded by the E3L gene. E3 is present within the Chordopoxvirinae subfamily (with the exception of the avipoxviruses and molluscum contagiosum virus) and displays pleiotropic effects on the innate immune system. Initial studies identified E3 as a double-stranded RNA (dsRNA)-binding protein (through its C terminus), able to inhibit the activation of protein kinase dependent on RNA (PKR) and the 2′5′-oligoadenylate synthetase (OAS)/RNase L pathway, rendering E3 a protein counteracting the type I interferon (IFN) system. In recent years, N-terminal mutants of E3 unable to bind to Z-form nucleic acids have been shown to induce the cellular death pathway necroptosis. This pathway was dependent on host IFN-inducible Z-DNA-binding protein 1 (ZBP1); full-length E3 is able to inhibit ZBP1-mediated necroptosis. Binding to what was identified as Z-RNA has emerged as a novel mechanism of counteracting the type I IFN system and has broadened our understanding of innate immunity against viral infections. This article gives an overview of the studies leading to our understanding of the vaccinia virus E3 protein function and its involvement in viral pathogenesis. Furthermore, a short summary of other viral systems is provided.

Collateral Lethal Effects of Complementary Oncolytic Viruses

Virus-infected cells release type 1 interferons, which induce an antiviral state in neighboring cells. Naturally occurring viruses are therefore equipped with stealth replication strategies to limit virus sensing and/or with combat strategies to prevent or reverse the antiviral state. Here we show that oncolytic viruses with simple RNA genomes whose spread was suppressed in tumor cells pretreated with interferon were able to replicate efficiently when the cells were coinfected with a poxvirus known to encode a diversity of innate immune combat proteins. In vivo the poxvirus was shown to reverse the intratumoral antiviral state, rescuing RNA virus replication in an otherwise restrictive syngeneic mouse tumor model leading to antitumor efficacy. Pairing of complementary oncolytic viruses is a promising strategy to enhance the antitumor activity of this novel class of anticancer drugs.

Characterization of a PKR inhibitor from the pathogenic ranavirus, Ambystoma tigrinum virus, using a heterologous vaccinia virus system

Ambystoma tigrinum virus (ATV) (family Iridoviridae, genus Ranavirus) was isolated from diseased tiger salamanders (Ambystoma tigrinum stebbinsi) from the San Rafael Valley in southern Arizona, USA in 1996. Genomic sequencing of ATV, as well as other members of the genus, identified an open reading frame that has homology to the eukaryotic translation initiation factor, eIF2α (ATV eIF2α homologue, vIF2αH). Therefore, we asked if the ATV vIF2αH could also inhibit PKR. To test this hypothesis, the ATV vIF2αH was cloned into vaccinia virus (VACV) in place of the well-characterized VACV PKR inhibitor, E3L. Recombinant VACV expressing ATV vIF2αH partially rescued deletion of the VACV E3L gene. Rescue coincided with rapid degradation of PKR in infected cells. These data suggest that the salamander virus, ATV, contains a novel gene that may counteract host defenses, and this gene product may be involved in the presentation of disease caused by this environmentally important pathogen.

Vaccinia virus and Cowpox virus are not susceptible to the interferon-induced antiviral protein MxA

MxA protein is expressed in response to type I and type III Interferon and constitute an important antiviral factor with broad antiviral activity to diverse RNA viruses. In addition, some studies expand the range of MxA antiviral activity to include particular DNA viruses like Monkeypox virus (MPXV) and African Swine Fever virus (ASFV). However, a broad profile of activity of MxA to large DNA viruses has not been established to date. Here, we investigated if some well characterized DNA viruses belonging to the Poxviridae family are sensitive to human MxA. A cell line inducibly expressing MxA to inhibitory levels showed no anti-Vaccinia virus (VACV) virus activity, indicating either lack of susceptibility of the virus, or the existence of viral factors capable of counteracting MxA inhibition. To determine if VACV resistance to MxA was due to a virus-encoded anti-MxA activity, we performed coinfections of VACV and the MxA-sensitive Vesicular Stomatitis virus (VSV), and show that VACV does not protect VSV from MxA inhibition in trans. Those results were extended to several VACV strains and two CPXV strains, thus confirming that those Orthopoxviruses do not block MxA action. Overall, these results point to a lack of susceptibility of the Poxviridae to MxA antiviral activity.

Evasion of the Innate Immune Type I Interferon System by Monkeypox Virus

The vaccinia virus (VACV) E3 protein has been shown to be important for blocking activation of the cellular innate immune system and allowing viral replication to occur unhindered. Mutation or deletion of E3L severely affects viral host range and pathogenesis. While the monkeypox virus (MPXV) genome encodes a homologue of the VACV E3 protein, encoded by the F3L gene, the MPXV gene is predicted to encode a protein with a truncation of 37 N-terminal amino acids. VACV with a genome encoding a similarly truncated E3L protein (VACV-E3LΔ37N) has been shown to be attenuated in mouse models, and infection with VACV-E3LΔ37N has been shown to lead to activation of the host antiviral protein kinase R pathway. In this report, we present data demonstrating that, despite containing a truncated E3 homologue, MPXV phenotypically resembles a wild-type (wt) VACV rather than VACV-E3LΔ37N. Thus, MPXV appears to contain a gene or genes that can suppress the phenotypes associated with an N-terminal truncation in E3. The suppression maps to sequences outside F3L, suggesting that the suppression is extragenic in nature. Thus, MPXV appears to have evolved mechanisms to minimize the effects of partial inactivation of its E3 homologue.

IMPORTANCE

Poxviruses have evolved to have many mechanisms to evade host antiviral innate immunity; these mechanisms may allow these viruses to cause disease. Within the family of poxviruses, variola virus (which causes smallpox) is the most pathogenic, while monkeypox virus is intermediate in pathogenicity between vaccinia virus and variola virus. Understanding the mechanisms of monkeypox virus innate immune evasion will help us to understand the evolution of poxvirus innate immune evasion capabilities, providing a better understanding of how poxviruses cause disease.

A single vertebrate DNA virus protein disarms invertebrate immunity to RNA virus infection

Virus-host interactions drive a remarkable diversity of immune responses and countermeasures. We found that two RNA viruses with broad host ranges, vesicular stomatitis virus (VSV) and Sindbis virus (SINV), are completely restricted in their replication after entry into Lepidopteran cells. This restriction is overcome when cells are co-infected with vaccinia virus (VACV), a vertebrate DNA virus. Using RNAi screening, we show that Lepidopteran RNAi, Nuclear Factor-κB, and ubiquitin-proteasome pathways restrict RNA virus infection. Surprisingly, a highly conserved, uncharacterized VACV protein, A51R, can partially overcome this virus restriction. We show that A51R is also critical for VACV replication in vertebrate cells and for pathogenesis in mice. Interestingly, A51R colocalizes with, and stabilizes, host microtubules and also associates with ubiquitin. We show that A51R promotes viral protein stability, possibly by preventing ubiquitin-dependent targeting of viral proteins for destruction. Importantly, our studies reveal exciting new opportunities to study virus-host interactions in experimentally-tractable Lepidopteran systems.

Tumor Restrictions to Oncolytic Virus

Oncolytic virotherapy has advanced since the days of its conception but therapeutic efficacy in the clinics does not seem to reach the same level as in animal models. One reason is premature oncolytic virus clearance in humans, which is a reasonable assumption considering the immune-stimulating nature of the oncolytic agents. However, several studies are beginning to reveal layers of restriction to oncolytic virotherapy that are present before an adaptive neutralizing immune response. Some of these barriers are present constitutively halting infection before it even begins, whereas others are raised by minute cues triggered by virus infection. Indeed, we and others have noticed that delivering viruses to tumors may not be the biggest obstacle to successful therapy, but instead the physical make-up of the tumor and its capacity to mount antiviral defenses seem to be the most important efficacy determinants. In this review, we summarize the constitutive and innate barriers to oncolytic virotherapy and discuss strategies to overcome them.

A survey of host range genes in poxvirus genomes

Poxviruses are widespread pathogens, which display extremely different host ranges. Whereas some poxviruses, including variola virus, display narrow host ranges, others such as cowpox viruses naturally infect a wide range of mammals. The molecular basis for differences in host range are poorly understood but apparently depend on the successful manipulation of the host antiviral response. Some poxvirus genes have been shown to confer host tropism in experimental settings and are thus called host range factors. Identified host range genes include vaccinia virus K1L, K3L, E3L, B5R, C7L and SPI-1, cowpox virus CP77/CHOhr, ectromelia virus p28 and 022, and myxoma virus T2, T4, T5, 11L, 13L, 062R and 063R. These genes encode for ankyrin repeat-containing proteins, tumor necrosis factor receptor II homologs, apoptosis inhibitor T4-related proteins, Bcl-2-related proteins, pyrin domain-containing proteins, cellular serine protease inhibitors (serpins), short complement-like repeats containing proteins, KilA-N/RING domain-containing proteins, as well as inhibitors of the double-stranded RNA-activated protein kinase PKR. We conducted a systematic survey for the presence of known host range genes and closely related family members in poxvirus genomes, classified them into subgroups based on their phylogenetic relationship and correlated their presence with the poxvirus phylogeny. Common themes in the evolution of poxvirus host range genes are lineage-specific duplications and multiple independent inactivation events. Our analyses yield new insights into the evolution of poxvirus host range genes. Implications of our findings for poxvirus host range and virulence are discussed.

Host-range restriction of vaccinia virus E3L deletion mutant can be overcome in vitro, but not in vivo, by expression of the influenza virus NS1 protein

During the last decades, research focused on vaccinia virus (VACV) pathogenesis has been intensified prompted by its potential beneficial application as a vector for vaccine development and anti-cancer therapies, but also due to the fear of its potential use as a bio-terrorism threat. Recombinant viruses lacking a type I interferon (IFN) antagonist are attenuated and hence good vaccine candidates. However, vaccine virus growth requires production in IFN-deficient systems, and thus viral IFN antagonists that are active in vitro, yet not in vivo, are of great value. The VACV E3 and influenza virus NS1 proteins are distinct double-stranded RNA-binding proteins that play an important role in pathogenesis by inhibiting the mammalian IFN-regulated innate antiviral response. Based on the functional similarities between E3 and NS1, we investigated the ability of NS1 to replace the biological functions of E3 of VACV in both in vitro and in vivo systems. For this, we generated a VACV recombinant virus lacking the E3L gene, yet expressing NS1 (VVΔE3L/NS1). Our study revealed that NS1 can functionally replace E3 in cultured cells, rescuing the protein synthesis blockade, and preventing apoptosis and RNA breakdown. In contrast, in vivo the VVΔE3L/NS1 virus was highly attenuated after intranasal inoculation, as it was unable to spread to the lungs and other organs. These results indicate that there are commonalities but also functional differences in the roles of NS1 and E3 as inhibitors of the innate antiviral response, which could potentially be utilized for vaccine production purposes in the future.

Innate immune evasion mediated by the Ambystoma tigrinum virus eukaryotic translation initiation factor 2alpha homologue

Ranaviruses (family Iridoviridae, genus Ranavirus) are large, double-stranded DNA (dsDNA) viruses whose replication is restricted to ectothermic vertebrates. Many highly pathogenic members of the genus Ranavirus encode a homologue of the eukaryotic translation initiation factor 2α (eIF2α). Data in a heterologous vaccinia virus system suggest that the Ambystoma tigrinum virus (ATV) eIF2α homologue (vIF2αH; open reading frame [ORF] 57R) is involved in evading the host innate immune response by degrading the interferon-inducible, dsRNA-activated protein kinase, PKR. To test this hypothesis directly, the ATV vIF2αH gene (ORF 57R) was deleted by homologous recombination, and a selectable marker was inserted in its place. The ATVΔ57R virus has a small plaque phenotype and is 8-fold more sensitive to interferon than wild-type ATV (wtATV). Infection of fish cells with the ATVΔ57R virus leads to eIF2α phosphorylation, in contrast to infection with wtATV, which actively inhibits eIF2α phosphorylation. The inability of ATVΔ57R to prevent phosphorylation of eIF2α correlates with degradation of fish PKZ, an interferon-inducible enzyme that is closely related to mammalian PKR. In addition, salamanders infected with ATVΔ57R displayed an increased time to death compared to that of wtATV-infected salamanders. Therefore, in a biologically relevant system, the ATV vIF2αH gene acts as an innate immune evasion factor, thereby enhancing virus pathogenesis.

Inhibition of type III interferon activity by orthopoxvirus immunomodulatory proteins

The type III interferon (IFN) family elicits an antiviral response that is nearly identical to that evoked by IFN-alpha/beta. However, these cytokines (known as IFN-lambda1, 2, and 3) signal through a distinct receptor, and thus may be resistant to the evasion strategies used by some viruses to avoid the IFN-alpha/beta response. Orthopoxviruses are highly resistant to IFN-alpha/beta because they encode well-characterized immunomodulatory proteins that inhibit IFN activity. These include a secreted receptor (B18R) that neutralizes IFN-alpha/beta, and a cytoplasmic protein (E3L) that blocks IFN-alpha/beta effector functions in infected cells. We therefore determined the ability of these immunomodulators to abrogate the IFN-lambda-induced antiviral response. We found that (i) vaccinia virus (VACV) replication is resistant to IFN-lambda antiviral activity; (ii) neither VACV B18R nor the variola virus homolog B20R neutralizes IFN-lambda; (iii) VACV E3L inhibits the IFN-lambda-mediated antiviral response through a PKR-dependent pathway; (iv) VACV infection inhibits IFN-lambdaR-mediated signal transduction and gene expression. These results demonstrate differential sensitivity of IFN-lambda to multiple distinct evasion mechanisms employed by a single virus.

Binding and relocalization of protein kinase R by murine cytomegalovirus

Many viruses have evolved mechanisms to evade the repression of translation mediated by protein kinase R (PKR). In the case of murine cytomegalovirus (MCMV), the protein products of two essential genes, m142 and m143, bind to double-stranded RNA (dsRNA) and block phosphorylation of PKR and eukaryotic initiation factor 2alpha. A distinctive feature of MCMV is that two proteins are required to block PKR activation whereas other viral dsRNA-binding proteins that prevent PKR activation contain all the necessary functions in a single protein. In order to better understand the mechanism by which MCMV evades the PKR response, we investigated the associations of pm142 and pm143 with each other and with PKR. Both pm142 and pm143 interact with PKR in infected and transfected cells. However, the approximately 200-kDa pm142-pm143 complex that forms in these cells does not contain substantial amounts of PKR, suggesting that the interactions between pm142-pm143 and PKR are unstable or transient. The stable, soluble pm142-pm143 complex appears to be a heterotetramer consisting of two molecules of pm142 associated with each other, and each one binds to and stabilizes a monomer of pm143. MCMV infection also causes relocalization of PKR into the nucleus and to an insoluble cytoplasmic compartment. These results suggest a model in which the pm142-pm143 multimer interacts with PKR and causes its sequestration in cellular compartments where it is unable to shut off translation and repress viral replication.

Poxvirus host range genes

As a family of viruses, poxviruses collectively exhibit a broad host range and most of the individual members are capable of replicating in a wide array of cell types from various host species, at least in vitro. At the cellular level, poxvirus tropism is dependent not upon specific cell surface receptors, but rather upon: (1) the ability of the cell to provide intracellular complementing factors needed for productive virus replication, and (2) the ability of the specific virus to successfully manipulate intracellular signaling networks that regulate cellular antiviral processes downstream of virus entry. The large genomic coding capacity of poxviruses enables the virus to express a unique collection of viral proteins that function as host range factors, which specifically target and manipulate host signaling pathways to establish optimal cellular conditions for viral replication. Functionally, the known host range factors from poxviruses have been associated with manipulation of a diverse array of cellular targets, which includes cellular kinases and phosphatases, apoptosis, and various antiviral pathways. To date, only a small number of poxvirus host range genes have been identified and studied, and only a handful of these have been functionally characterized. For this reason, poxvirus host range factors represent a potential gold mine for the discovery of novel pathogen-host protein interactions. This review summarizes our current understanding of the mechanisms by which the known poxvirus host range genes, and their encoded factors, expand tropism through the manipulation of host cell intracellular signaling pathways.

Vaccinia virus E3 protein prevents the antiviral action of ISG15

The ubiquitin-like modifier ISG15 is one of the most predominant proteins induced by type I interferons (IFN). In this study, murine embryo fibroblast (MEFs) and mice lacking the gene were used to demonstrate a novel role of ISG15 as a host defense molecule against vaccinia virus (VACV) infection. In MEFs, the growth of replication competent Western Reserve (WR) VACV strain was affected by the absence of ISG15, but in addition, virus lacking E3 protein (VVDeltaE3L) that is unable to grow in ISG15+/+ cells replicated in ISG15-deficient cells. Inhibiting ISG15 with siRNA or promoting its expression in ISG15-/- cells with a lentivirus vector showed that VACV replication was controlled by ISG15. Immunoprecipitation analysis revealed that E3 binds ISG15 through its C-terminal domain. The VACV antiviral action of ISG15 and its interaction with E3 are events independent of PKR (double-stranded RNA-dependent protein kinase). In mice lacking ISG15, infection with VVDeltaE3L caused significant disease and mortality, an effect not observed in VVDeltaE3L-infected ISG15+/+ mice. Pathogenesis in ISG15-deficient mice infected with VVDeltaE3L or with an E3L deletion mutant virus lacking the C-terminal domain triggered an enhanced inflammatory response in the lungs compared with ISG15+/+-infected mice. These findings showed an anti-VACV function of ISG15, with the virus E3 protein suppressing the action of the ISG15 antiviral factor.

Vaccinia viruses with mutations in the E3L gene as potential replication-competent, attenuated vaccines: scarification vaccination

In this study, we evaluated the efficacy of vaccinia virus (VACV) containing mutations in the E3L virulence gene to protect mice against a lethal poxvirus challenge after vaccination by scarification. VACV strains with mutations in the E3L gene had significantly decreased pathogenicity, even in immune deficient mice, yet retained the ability to produce a potent Th1-dominated immune response in mice after vaccination by scarification, while protecting against challenge with wild type, pathogenic VACV. Initial experiments were done using the mouse-adapted, neurovirulent Western Reserve (WR) strain of vaccinia virus. Testing of the full E3L deletion mutation in the Copenhagen and NYCBH strains of VACV, which are more appropriate for use in humans, produced similar results. These results suggest that highly attenuated strains of VACV containing mutations in E3L have the potential for use as scarification administered vaccines.

Antagonizing activity of vaccinia virus E3L against human interferons in Huh7 cells

The E3L protein of vaccinia virus (VV) is well known for its capacity to evade cellular innate antiviral immunity related to interferon (IFN), for example PKR and RNaseL mediated antiviral activities. However, due to the limited range of cells that support VV E3L deletion mutant replication, the full capacity of E3L inhibiting the innate immune response induced by IFNs remains to be examined. In this report, the inhibition activity of VV E3L against a wide spectrum of human IFNs, including type I IFNs (12 IFN-alpha subtypes, IFN-beta, and IFN-omega), and type II IFN (gamma), was comparatively examined using the Copenhagen strain E3L deletion mutant and its revertant control virus in a human hepatoma cell line, Huh7. Deletion of the E3L open reading frame rendered the mutant VV sensitive to all types of IFNs, while the revertant VV was strongly resistant to these treatments. Furthermore, we show that the inhibition of VV E3L deletion mutant by IFN occurs at the stage of intermediate gene translation, while the expression of early genes and transcription of intermediate genes are largely unaffected. Using specific siRNAs to suppress the classical IFN-induced antiviral pathways, we found that PKR is the key factor modulated by E3L, while the RNaseL and MxA pathways play limited roles in this Huh7 cell system. Thus, our data demonstrates that VV E3L can mediate strong inhibition activity against all human type I and type II IFNs, mainly through modulation of the PKR pathway in Huh7 cells.

Loss of protein kinase PKR expression in human HeLa cells complements the vaccinia virus E3L deletion mutant phenotype by restoration of viral protein synthesis

The E3L proteins encoded by vaccinia virus bind double-stranded RNA and mediate interferon resistance, promote virus growth, and impair virus-mediated apoptosis. Among the cellular proteins implicated as targets of E3L is the protein kinase regulated by RNA (PKR). To test in human cells the role of PKR in conferring the E3L mutant phenotype, HeLa cells stably deficient in PKR generated by an RNA interference-silencing strategy were compared to parental and control knockdown cells following infection with either an E3L deletion mutant (DeltaE3L) or wild-type (WT) virus. The growth yields of WT virus were comparable in PKR-sufficient and -deficient cells. By contrast, the single-cycle yield of DeltaE3L virus was increased by nearly 2 log(10) in PKR-deficient cells over the impaired growth in PKR-sufficient cells. Furthermore, virus-induced apoptosis characteristic of the DeltaE3L mutant in PKR-sufficient cells was effectively abolished in PKR-deficient HeLa cells. The viral protein synthesis pattern was altered in DeltaE3L-infected PKR-sufficient cells, characterized by an inhibition of late viral protein expression, whereas in PKR-deficient cells, late protein accumulation was restored. Phosphorylation of both PKR and the alpha subunit of protein synthesis initiation factor 2 (eIF-2alpha) was elevated severalfold in DeltaE3L-infected PKR-sufficient, but not PKR-deficient, cells. WT virus did not significantly increase PKR or eIF-2alpha phosphorylation in either PKR-sufficient or -deficient cells, both of which supported efficient WT viral protein production. Finally, apoptosis induced by infection of PKR-sufficient HeLa cells with DeltaE3L virus was blocked by a caspase antagonist, but mutant virus growth was not rescued, suggesting that translation inhibition rather than apoptosis activation is a principal factor limiting virus growth.

The identification and characterization of a monoclonal antibody to the vaccinia virus E3 protein

Monoclonal antibodies with reactivity to vaccinia virus specific proteins are useful reagents to study the proteins as well as to help understand aspects of the poxvirus life cycle. Using a vaccinia virus proteomics microarray, we found a hybridoma (MAb 3015B2) from a vaccinia virus vaccinated mouse that reacted with the product of the E3L gene. The specificity to the E3 protein was confirmed by Western blotting and immunofluorescence of cells infected with either wild-type vaccinia virus or a mutant virus with the E3L gene deleted. Antibody reactivity with E3 was also seen in cells transfected with a plasmid expressing the E3 protein. A panel of mutated vaccinia viruses with truncations in the E3L gene revealed that while MAb 3015B2 reacted with E3 lacking the C-terminal 7 amino acids, it lost reactivity with a mutant E3 lacking the C-terminal 26 amino acids. This indicates that the antigenic site recognized by 3015B2 is on the C-terminus, somewhere between amino acids 164 through 183. The antibody also recognizes the E3 protein encoded by other orthopoxviruses. This antibody will be useful for further investigations of the E3 protein as well as a useful reagent to indicate vaccinia virus early protein expression.

Overproduction of double-stranded RNA in vesicular stomatitis virus-infected cells activates a constitutive cell-type-specific antiviral response

In a companion paper (D. Ostertag, T. M. Hoblitzell-Ostertag, and J. Perrault, J. Virol. 81:492-502, 2007), we provided indirect evidence that cell-type-specific growth restriction of the vesicular stomatitis virus (VSV) polR mutants may be due to enhanced production of double-stranded RNA (dsRNA). We show here that polR growth in mouse L-929 cells was rescued by vaccinia virus coinfection and that sole expression of the vaccinia virus dsRNA-binding E3L protein, via coinfection with an engineered VSV minigenome, also restored polR growth. Expression of dsRNA-binding protein NS1A or NS1B from influenza virus, but not C protein from Sendai virus, which does not bind dsRNA, likewise effected polR rescue. The N-terminal dsRNA-binding domain of NS1A, only 73 amino acids in length, but not a full-size mutant NS1A lacking dsRNA-binding activity, restored polR growth. Both key aspects of polR growth restriction, namely inhibition of genome replication and release of low-infectivity virus particles, were countered by expression of the dsRNA-binding proteins. We tested the effects of overproducing dsRNA in wild-type VSV infections by coinfecting cells with a VSV recombinant expressing the sense strand of the enhanced green fluorescent protein gene (VSV-GFP) and one expressing the antisense strand (VSV-PFG). These coinfections mimicked all aspects of polR restriction, including host range, lack of effect on transcription, reduced virus particle infectivity, and insensitivity to inhibition of host gene transcription or dsRNA-activated protein kinase activity. We conclude that, for some cell types, overproduction of dsRNA during VSV infection triggers an immediate and constitutive host cell antiviral effector response independent of interferon induction or signaling.

Double-stranded RNA is produced by positive-strand RNA viruses and DNA viruses but not in detectable amounts by negative-strand RNA viruses

Double-stranded RNA (dsRNA) longer than 30 bp is a key activator of the innate immune response against viral infections. It is widely assumed that the generation of dsRNA during genome replication is a trait shared by all viruses. However, to our knowledge, no study exists in which the production of dsRNA by different viruses is systematically investigated. Here, we investigated the presence and localization of dsRNA in cells infected with a range of viruses, employing a dsRNA-specific antibody for immunofluorescence analysis. Our data revealed that, as predicted, significant amounts of dsRNA can be detected for viruses with a genome consisting of positive-strand RNA, dsRNA, or DNA. Surprisingly, however, no dsRNA signals were detected for negative-strand RNA viruses. Thus, dsRNA is indeed a general feature of most virus groups, but negative-strand RNA viruses appear to be an exception to that rule.

Double-stranded RNA-binding protein E3 controls translation of viral intermediate RNA, marking an essential step in the life cycle of modified vaccinia virus Ankara

Infection of human cells with modified vaccinia virus Ankara (MVA) activates the typical cascade-like pattern of viral early-, intermediate- and late-gene expression. In contrast, infection of human HeLa cells with MVA deleted of the E3L gene (MVA-DeltaE3L) results in high-level synthesis of intermediate RNA, but lacks viral late transcription. The viral E3 protein is thought to bind double-stranded RNA (dsRNA) and to act as an inhibitor of dsRNA-activated 2'-5'-oligoadenylate synthetase (2'-5'OA synthetase)/RNase L and protein kinase (PKR). Here, it is demonstrated that viral intermediate RNA can form RNase A/T1-resistant dsRNA, suggestive of activating both the 2'-5'OA synthetase/RNase L pathway and PKR in various human cell lines. Western blot analysis revealed that failure of late transcription in the absence of E3L function resulted from the deficiency to produce essential viral intermediate proteins, as demonstrated for vaccinia late transcription factor 2 (VLTF 2). Substantial host cell-specific differences were found in the level of activation of either RNase L or PKR. However, both rRNA degradation and phosphorylation of eukaryotic translation initiation factor-2alpha (eIF2alpha) inhibited the synthesis of VLTF 2 in human cells. Moreover, intermediate VLTF 2 and late-protein production were restored in MVA-DeltaE3L-infected mouse embryonic fibroblasts from Pkr(0/0) mice. Thus, both host-response pathways may be involved, but activity of PKR is sufficient to block the MVA molecular life cycle. These data imply that an essential function of vaccinia virus E3L is to secure translation of intermediate RNA and, thereby, expression of other viral genes.

Role of viral factor E3L in modified vaccinia virus ankara infection of human HeLa Cells: regulation of the virus life cycle and identification of differentially expressed host genes

Modified vaccinia virus Ankara (MVA) is a highly attenuated virus strain being developed as a vaccine for delivery of viral and recombinant antigens. The MVA genome lacks functional copies of numerous genes interfering with host response to infection. The interferon resistance gene E3L encodes one important viral immune defense factor still made by MVA. Here we demonstrate an essential role of E3L to allow for completion of the MVA molecular life cycle upon infection of human HeLa cells. A deletion mutant virus, MVA-DeltaE3L, was found defective in late protein synthesis, viral late transcription, and viral DNA replication in infected HeLa cells. Moreover, we detected viral early and continuing intermediate transcription associated with degradation of rRNA, indicating rapid activation of 2'-5'-oligoadenylate synthetase/RNase L in the absence of E3L. Further molecular monitoring of E3L function by microarray analysis of host cell transcription in MVA- or MVA-DeltaE3L-infected HeLa cells revealed an overall significant down regulation of more than 50% of cellular transcripts expressed under mock conditions already at 5 h after infection, with a more prominent shutoff following MVA-DeltaE3L infection. Interestingly, a cluster of genes up regulated exclusively in MVA-DeltaE3L-infected cells could be identified, including transcripts for interleukin 6, growth arrest and DNA damage-inducible protein beta, and dual-specificity protein phosphatases. Our data indicate that lack of E3L inhibits MVA antigen production in human HeLa cells at the level of viral late gene expression and suggest that E3L can prevent activation of additional host factors possibly affecting the MVA molecular life cycle.

Inhibition of PKR by vaccinia virus: role of the N- and C-terminal domains of E3L

The process of eukaryotic translation initiation can be regulated by a highly conserved mechanism involving the phosphorylation of the translation initiation factor eIF2 on the alpha subunit. This mechanism is recognized as an efficient step in the host antiviral response. Vaccinia virus (VV), like many other viruses, encodes proteins to overcome this inhibitory process. The C-terminus of the vaccinia virus E3L is known to bind to double-stranded RNA (dsRNA) thereby sequestering the activator of this antiviral response. In this report, the N-terminus of E3L was found to be required for the additional regulation of eIF2alpha phosphorylation. This phosphorylation event did not lead to a global shutdown in protein synthesis. Because the N-terminus of E3L is required for full viral pathogenesis in mice, these results suggest an alternative role of eIF2alpha phosphorylation in regulating viral replication.

Characterization of three paralogous members of the Mammalian vaccinia related kinase family

Members of the novel vaccinia related kinase (VRK) protein family are characterized by notable sequence homology to the vaccinia virus-encoded B1 kinase (vvB1). vvB1 plays an essential role in viral DNA replication, and Boyle and Traktman have demonstrated that VRK1 enzymes complement the replication defect of a temperature-sensitive viral mutant defective in vvB1 (Boyle, K., and Traktman, P. (2004) J. Virol. 78, 1992-2005). This mammalian kinase family comprises three members, VRK1, VRK2, and VRK3. We have annotated the gene structure for the members of this family and have characterized the enzyme activity and subcellular localization for the human and mouse proteins. VRK1 enzymes show robust autophosphorylation activity and will phosphorylate casein; VRK2 enzymes show modest autophosphorylation activity and will also phosphorylate casein. The VRK3 proteins have key amino acid substitutions that disrupt invariant motifs required for catalytic activity, rendering them enzymatically inert. The VRK1 and VRK2 proteins contain COOH-terminal extracatalytic sequences that mediate intracellular localization. VRK1 proteins possess a basic nuclear localization signal and are indeed nuclear; the extreme C termini of the VRK2 proteins are highly hydrophobic, and the proteins are membrane-associated and colocalize with markers of the endoplasmic reticulum. The NH(2)-terminal region of the VRK3s contains a bipartite nuclear localization signal, which directs these proteins to the nucleus. Our findings provide the basis for further studies of the structure and function of this newly discovered family of protein kinases.

The Orf virus E3L homologue is able to complement deletion of the vaccinia virus E3L gene in vitro but not in vivo

Orf virus (OV), the prototypic parapoxvirus, is resistant to the effects of interferon (IFN) and this function of OV has been mapped to the OV20.0L gene. The protein product of this gene shares 31% amino acid identity to the E3L-encoded protein of vaccinia virus (VV) that is required for the broad host range and IFN-resistant phenotype of VV in cells in culture and for virulence of the virus in vivo. In this study we investigated whether the distantly related OV E3L homologue could complement the deletion of E3L in VV. The recombinant VV (VV/ORF-E3L) expressing the OV E3L homologue in place of VV E3L was indistinguishable from wt VV in its cell-culture phenotype. But VV/ORF-E3L was over a 1000-fold less pathogenic than wt VV (LD(50) > 5 x 10(6) PFU, compared to LD(50) of wtVV = 4 x 10(3) PFU) following intranasal infection of mice. While wt VV spread to the lungs and brain and replicated to high titers in the brain of infected mice, VV/ORF-E3L could not be detected in the lungs or brain following intranasal infection. VV/ORF-E3L was at least 100,000-fold less pathogenic than wt VV on intracranial injection. Domain swap experiments demonstrate that the difference in pathogenesis maps to the C-terminal domain of these proteins. This domain has been shown to be required for the dsRNA binding function of the VV E3L.

Genomic analysis of the host response to hepatitis C virus infection

We have examined the progression of hepatitis C virus (HCV) infections by gene expression analysis of liver biopsies in acutely infected chimpanzees that developed persistent infection, transient viral clearance, or sustained clearance. Both common responses and outcome-specific changes in expression were observed. All chimpanzees showed gene expression patterns consistent with an IFN-alpha response that correlated with the magnitude and duration of infection. Transient and sustained viral clearance were uniquely associated with induction of IFN-gamma-induced genes and other genes involved in antigen processing and presentation and the adaptive immune response. During the early stages of infection, host genes involved in lipid metabolism were also differentially regulated. We also show that drugs that affect these biosynthetic pathways can regulate HCV replication in HCV replicon systems. Our results reveal genome-wide transcriptional changes that reflect the establishment, spread, and control of infection, and they reveal potentially unique antiviral programs associated with clearance of HCV infection.

The quest for an efficacious antiviral for respiratory syncytial virus

Respiratory syncytial virus (RSV) continues as an emerging infectious disease not only among infants and children, but also for the immune-suppressed, hospitalized and the elderly. To date, ribavirin (Virazole) remains the only therapeutic agent approved for the treatment of RSV. The prophylactic administration of palivizumab is problematic and costly. The quest for an efficacious RSV antiviral has produced a greater understanding of the viral fusion process, a new hypothesis for the mechanism of action of ribavirin, and a promising antisense strategy combining the 2'-5' oligoadenylate antisense (2-5A-antisense) approach and RSV genomics.

The role of the PKR-inhibitory genes, E3L and K3L, in determining vaccinia virus host range

Vaccinia virus encodes two regulators of the cellular antiviral response. The E3L gene is thought to act primarily by sequestering double-stranded RNA, whereas the K3L gene is thought to act as a competitive inhibitor of the double-stranded RNA-dependent protein kinase, PKR. The broad host range associated with vaccinia virus replication appears to be related to the presence of these genes. The E3L gene is required for replication in HeLa cells, but is not required for replication in BHK cells. On the contrary, the K3L gene is required for replication in BHK cells, but is dispensable for replication in HeLa cells. Our results suggest that these cell lines varied in the expression of endogenous activatable PKR and that replication of vaccinia virus in different cell lines led to altered levels of double-stranded RNA synthesis from the virus. Vaccinia virus was able to overcome these cellular variations by regulating PKR activity through the synthesis of either E3L or K3L. The results suggest that vaccinia virus has evolved a broad host range by maintaining both the E3L and the K3L genes.

Complementation of vaccinia virus lacking the double-stranded RNA-binding protein gene E3L by human cytomegalovirus

The cellular response to viral infection often includes activation of pathways that shut off protein synthesis and thereby inhibit viral replication. In order to enable efficient replication, many viruses carry genes such as the E3L gene of vaccinia virus that counteract these host antiviral pathways. Vaccinia virus from which the E3L gene has been deleted (VVDeltaE3L) is highly sensitive to interferon and exhibits a restricted host range, replicating very inefficiently in many cell types, including human fibroblast and U373MG cells. To determine whether human cytomegalovirus (CMV) has a mechanism for preventing translational shutoff, we evaluated the ability of CMV to complement the deficiencies in replication and protein synthesis associated with VVDeltaE3L. CMV, but not UV-inactivated CMV, rescued VVDeltaE3L late gene expression and replication. Thus, complementation of the VVDeltaE3L defect appears to depend on de novo CMV gene expression and is not likely a result of CMV binding to the cell receptor or of a virion structural protein. CMV rescued VVDeltaE3L late gene expression even in the presence of ganciclovir, indicating that CMV late gene expression is not required for complementation of VVDeltaE3L. The striking decrease in overall translation after infection with VVDeltaE3L was prevented by prior infection with CMV. Finally, CMV blocked both the induction of eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation and activation of RNase L by VVDeltaE3L. These results suggest that CMV has one or more immediate-early or early genes that ensure maintenance of a high protein synthetic capacity during infection by preventing activation of the PKR/eIF2alpha phosphorylation and 2-5A oligoadenylate synthetase/RNase L pathways.

Comparison of the eIF-2alpha homologous proteins of seven ranaviruses (Iridoviridae)

The alpha-subunit of the eukaryotic initiation factor 2 (eIF-2alpha) is a key component of the translation machinery of the cell. In response to cellular stress such as viral infections, eIF-2alpha is phosphorylated by double-stranded RNA-dependent protein kinase (PKR) leading to the inhibition of cellular protein synthesis. The importance of eIF-2alpha as a regulatory mechanism for protein synthesis is illustrated by the wide variety of strategies employed by viruses to down-regulate PKR. Thus, Vaccinia virus encodes K3L protein, which resembles eIF-2alpha and acts as a pseudo-substrate inhibitor of PKR. Nucleotide sequencing of the genome of epizootic haematopoietic necrosis virus (EHNV), a member of the genus ranavirus of Iridoviridae, has revealed an eIF-2alpha equivalent gene. We have cloned and sequenced eIF-2alpha genes of several iridoviruses of fishes and frogs. The eIF-2alpha open reading frames and deduced proteins of the iridoviruses investigated exhibit a high degree of homology of both nucleotide and amino acid sequences. At the N-terminus, the iridoviral eIF-2alpha shows significant homology to the N-termini of cellular initiation factor 2-alpha of various species, to full-length poxviral eIF-2alpha proteins, and to the S1 domain of ribosomal proteins. Comparison of amino acid sequences of corresponding iridoviral proteins with eIF-2alpha homologous proteins of poxviruses and eukaryotes has revealed a high conservation of motifs. A phylogenetic analysis of eukaryotic eIF-2alpha and poxvirus and iridovirus eIF-2alpha sequences has demonstrated the relationship of these iridoviruses. In order to investigate the role of the eIF-2alpha equivalent, respective genes have been expressed in prokaryotic and eukaryotic (insect, fish and chicken cell) systems. The iridoviral eIF-2alpha protein has a molecular weight of 31 kDa and is cytoplasmic. The cellular and viral protein synthesis of iridoviruses is probably regulated by a mechanism similar to that of Vaccinia virus. Frog-virus 3, the type species of the genus ranavirus of Iridoviridae, has a unique translational efficiency and, moreover, down-regulates the cellular protein synthesis of infected cells.

Vaccinia virus E3L interferon resistance protein inhibits the interferon-induced adenosine deaminase A-to-I editing activity

The RNA-specific adenosine deaminase (ADAR1) is an interferon-inducible editing enzyme that converts adenosine to inosine. ADAR1 contains three distinct domains: a N-terminal Z-DNA binding domain that includes two Z-DNA binding motifs; a central double-stranded RNA binding domain that includes three dsRNA binding motifs (dsRBM); and a C-terminal catalytic domain responsible for A-to-I enzymatic activity. The E3L protein of vaccinia virus mediates interferon resistance. E3L, similar to ADAR1, also contains Z-DNA binding and dsRNA binding motifs. To assess the possible role of E3L in modulating RNA editing by ADAR1, we examined the effect of E3L on ADAR1 deaminase activity. Wild-type E3L protein was a potent inhibitor of ADAR1 deaminase enzymatic activity. Analysis of mutant E3L proteins indicated that the carboxy-proximal dsRBM of E3L was essential for antagonism of ADAR1. Surprisingly, disruption of the Z-DNA binding domain of E3L by double substitutions of two highly conserved residues also abolished its antagonistic activity, whereas deletion of the entire Z domain had little effect on the inhibition. With natural neurotransmitter pre-mRNA substrates, E3L weakly inhibited the site-selective editing activity by ADAR1 at the R/G site of the glutamate receptor B subunit (GluR-B) pre-mRNA and the A site of serotonin 2C receptor (5-HT2CR) pre-mRNA; editing of the intronic hotspot (+)60 site of GluR-B was not affected by E3L. These results demonstrate that the A-to-I RNA editing activity of the IFN-inducible adenosine deaminase is impaired by the product of the vaccinia virus E3L interferon resistance gene.

Antiviral actions of interferons

Tremendous progress has been made in understanding the molecular basis of the antiviral actions of interferons (IFNs), as well as strategies evolved by viruses to antagonize the actions of IFNs. Furthermore, advances made while elucidating the IFN system have contributed significantly to our understanding in multiple areas of virology and molecular cell biology, ranging from pathways of signal transduction to the biochemical mechanisms of transcriptional and translational control to the molecular basis of viral pathogenesis. IFNs are approved therapeutics and have moved from the basic research laboratory to the clinic. Among the IFN-induced proteins important in the antiviral actions of IFNs are the RNA-dependent protein kinase (PKR), the 2',5'-oligoadenylate synthetase (OAS) and RNase L, and the Mx protein GTPases. Double-stranded RNA plays a central role in modulating protein phosphorylation and RNA degradation catalyzed by the IFN-inducible PKR kinase and the 2'-5'-oligoadenylate-dependent RNase L, respectively, and also in RNA editing by the IFN-inducible RNA-specific adenosine deaminase (ADAR1). IFN also induces a form of inducible nitric oxide synthase (iNOS2) and the major histocompatibility complex class I and II proteins, all of which play important roles in immune response to infections. Several additional genes whose expression profiles are altered in response to IFN treatment and virus infection have been identified by microarray analyses. The availability of cDNA and genomic clones for many of the components of the IFN system, including IFN-alpha, IFN-beta, and IFN-gamma, their receptors, Jak and Stat and IRF signal transduction components, and proteins such as PKR, 2',5'-OAS, Mx, and ADAR, whose expression is regulated by IFNs, has permitted the generation of mutant proteins, cells that overexpress different forms of the proteins, and animals in which their expression has been disrupted by targeted gene disruption. The use of these IFN system reagents, both in cell culture and in whole animals, continues to provide important contributions to our understanding of the virus-host interaction and cellular antiviral response.

IRF3 and IRF7 phosphorylation in virus-infected cells does not require double-stranded RNA-dependent protein kinase R or Ikappa B kinase but is blocked by Vaccinia virus E3L protein

Induction of interferon-alpha (IFNalpha) gene expression in virus-infected cells requires phosphorylation-induced activation of the transcription factors IRF3 and IRF7. However, the kinase(s) that targets these proteins has not been identified. Using a combined pharmacological and genetic approach, we found that none of the kinases tested was responsible for IRF phosphorylation in cells infected with Newcastle disease virus (NDV). Although the broad-spectrum kinase inhibitor staurosporine potently blocked IRF3 and -7 phosphorylation, inhibitors for protein kinase C, protein kinase A, MEK, SAPK, IKK, and protein kinase R (PKR) were without effect. Both IkappaB kinase and PKR have been implicated in IFN induction, but cells genetically deficient in IkappaB kinase, PKR, or the PKR-related genes PERK, IRE1, or GCN2 retained the ability to phosphorylate IRF7 and induce IFNalpha. Interestingly, PKR mutant cells were defective for response to double-stranded (ds) RNA but not to virus infection, suggesting that dsRNA is not the only activating viral component. Consistent with this notion, protein synthesis was required for IRF7 phosphorylation in virus-infected cells, and the kinetics of phosphorylation and viral protein production were similar. Despite evidence for a lack of involvement of dsRNA and PKR, vaccinia virus E3L protein, a dsRNA-binding protein capable of inhibiting PKR, was an effective IRF3 and -7 phosphorylation inhibitor. These results suggest that a novel cellular protein that is activated by viral products in addition to dsRNA and is sensitive to E3L inhibition is responsible for IRF activation and reveal a novel mechanism for the anti-IFN effect of E3L distinct from its inhibition of PKR.

Latently expressed human herpesvirus 8-encoded interferon regulatory factor 2 inhibits double-stranded RNA-activated protein kinase

Human herpesvirus 8 (HHV-8; Kaposi's sarcoma herpesvirus) encodes four open reading frames with homology to cellular proteins of interferon regulatory factor (IRF) family. Three of them, viral IRF-1 (vIRF-1), vIRF-2, and vIRF-3, have been cloned and found, when overexpressed, to down-regulate the transcriptional activity of interferon type I gene promoters in infected cells by interfering with the transactivating activity of cellular IRFs. In this study, we have further characterized vIRF-2 and shown that it is a nuclear protein which is constitutively expressed in HHV-8-positive pleural effusion lymphoma cell lines. Nuclear localization of vIRF-2 was confirmed by in situ detection of ectopically expressed enhanced green fluorescent protein/vIRF-2 fusion protein. We found that the expression of vIRF-2 in HEK293 cells inhibited the antiviral effect of interferon and rescued translation of vesicular stomatitis virus mRNA from interferon-induced translational block. To provide insight into the mechanism of this effect we have demonstrated that vIRF-2 physically interacts with PKR consequently inhibiting autophosphorylation of double-stranded RNA-activated protein kinase (PKR) and blocking phosphorylation of PKR substrates histone 2A and eukaryotic translation initiation factor 2alpha. These results suggest that the latently expressed vIRF-2 has a role in viral mimicry which targets the activity of interferon-induced PKR kinase. By inhibiting the kinase activity of PKR and consequent down-modulation of protein synthesis, HHV-8 has evolved a mechanism by which it can overcome the interferon-mediated antiviral effect. Thus, the anti-interferon functions of vIRF-2 may contribute to the establishment of a chronic or latent infection.

Both carboxy- and amino-terminal domains of the vaccinia virus interferon resistance gene, E3L, are required for pathogenesis in a mouse model

The vaccinia virus (VV) E3L gene is responsible for providing interferon (IFN) resistance and a broad host range to VV in cell culture. The E3L gene product contains two distinct domains. A conserved carboxy-terminal domain, which is required for the IFN resistance and broad host range of the virus, has been shown to bind double-stranded RNA (dsRNA) and inhibit the antiviral dsRNA-dependent protein kinase, PKR. The amino-terminal domain, while conserved among orthopoxviruses, is dispensable in cell culture. To study the role of E3L in whole-animal infections, WR strain VV recombinants either lacking E3L (VVDeltaE3L) or expressing an amino-terminal (VVE3LDelta83N) or carboxy-terminal (VVE3LDelta26C) truncation of E3L were constructed. Whereas wild-type VV had a 50% lethal dose of approximately 10(4) PFU after intranasal infection, and elicited severe weight loss and morbidity, VVDeltaE3L was apathogenic, leading to no death, weight loss, or morbidity. VVDeltaE3L was also apathogenic after intracranial injection. Although the amino-terminal domain of E3L is dispensable for infection of cells in culture, both the amino- and carboxy-terminal domains of E3L were required for full pathogenesis in intranasal infections. These results demonstrate that the entire E3L gene is required for pathogenesis in the mouse model.

Translational control of viral gene expression in eukaryotes

As obligate intracellular parasites, viruses rely exclusively on the translational machinery of the host cell for the synthesis of viral proteins. This relationship has imposed numerous challenges on both the infecting virus and the host cell. Importantly, viruses must compete with the endogenous transcripts of the host cell for the translation of viral mRNA. Eukaryotic viruses have thus evolved diverse mechanisms to ensure translational efficiency of viral mRNA above and beyond that of cellular mRNA. Mechanisms that facilitate the efficient and selective translation of viral mRNA may be inherent in the structure of the viral nucleic acid itself and can involve the recruitment and/or modification of specific host factors. These processes serve to redirect the translation apparatus to favor viral transcripts, and they often come at the expense of the host cell. Accordingly, eukaryotic cells have developed antiviral countermeasures to target the translational machinery and disrupt protein synthesis during the course of virus infection. Not to be outdone, many viruses have answered these countermeasures with their own mechanisms to disrupt cellular antiviral pathways, thereby ensuring the uncompromised translation of virion proteins. Here we review the varied and complex translational programs employed by eukaryotic viruses. We discuss how these translational strategies have been incorporated into the virus life cycle and examine how such programming contributes to the pathogenesis of the host cell.

Antiapoptotic and oncogenic potentials of hepatitis C virus are linked to interferon resistance by viral repression of the PKR protein kinase

Hepatitis C virus (HCV) is prevalent worldwide and has become a major cause of liver dysfunction and hepatocellular carcinoma. The high prevalence of HCV reflects the persistent nature of infection and the large frequency of cases that resist the current interferon (IFN)-based anti-HCV therapeutic regimens. HCV resistance to IFN has been attributed, in part, to the function of the viral nonstructural 5A (NS5A) protein. NS5A from IFN-resistant strains of HCV can repress the PKR protein kinase, a mediator of the IFN-induced antiviral and apoptotic responses of the host cell and a tumor suppressor. Here we examined the relationship between HCV persistence and resistance to IFN therapy. When expressed in mammalian cells, NS5A from IFN-resistant HCV conferred IFN resistance to vesicular stomatitis virus (VSV), which normally is sensitive to the antiviral actions of IFN. NS5A blocked viral double-stranded RNA (dsRNA)-induced PKR activation and phosphorylation of eIF-2alpha in IFN-treated cells, resulting in high levels of VSV mRNA translation. Mutations within the PKR-binding domain of NS5A restored PKR function and the IFN-induced block to viral mRNA translation. The effects due to NS5A inhibition of PKR were not limited to the rescue of viral mRNA translation but also included a block in PKR-dependent host signaling pathways. Cells expressing NS5A exhibited defective PKR signaling and were refractory to apoptosis induced by exogenous dsRNA. Resistance to apoptosis was attributed to an NS5A-mediated block in eIF-2alpha phosphorylation. Moreover, cells expressing NS5A exhibited a transformed phenotype and formed solid tumors in vivo. Disruption of apoptosis and tumorogenesis required the PKR-binding function of NS5A, demonstrating that these properties may be linked to the IFN-resistant phenotype of HCV.