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

Mpox Virus and its Ocular Surface Manifestations

The Mpox virus (MPXV) is the causative agent of human Mpox disease - a debilitating rash illness similar to smallpox. Although Clade I MPXV has remained endemic to West and Central Africa, Clade II MPXV has been responsible for many outbreaks worldwide. The most recent outbreak in 2022 resulted from the rapid spread of a new clade of MPXV, classified into Clade IIb - a distinct lineage from the previously circulating viral strains. The rapid spread and increased severity of Mpox disease by the Clade IIb strain have raised the serious public health imperative of better understanding the host and viral determinants during MPXV infection. In addition to typical skin rashes, including in the periorbital area, MPXV causes moderate to severe ophthalmic manifestations - most commonly, ocular surface complications (e.g., keratitis, conjunctivitis, blepharitis). While ocular manifestations of Clade I Mpox within the Congo basin have been well-reported, global incidence trends of ocular Mpox cases by Clade IIb are still emerging. Given the demonstrated ability of all MPXV strains to auto-inoculate ocular tissue alongside the enhanced transmissibility of the Clade IIb virus, there is an urgent need to elucidate the mechanisms by which MPXV causes ocular anomalies. In this review, we discuss the viral and genomic structures of MPXV, the epidemiology, and pathology of systemic and ocular Mpox, as well as potential prophylactic and therapeutic interventions.

Disruption of the cGAS/STING axis does not impair sensing of MVA in BHK21 cells

Modified vaccinia Ankara (MVA) is an attenuated strain of vaccinia virus (VACV), a dsDNA virus that replicates its genome in the cytoplasm and as a result is canonically sensed by the cyclic GMP-AMP synthase (cGAS) and its downstream stimulator of interferon genes (STING). MVA has a highly restricted host range due to major deletions in its genome including inactivation of immunomodulatory genes, only being able to grow in avian cells and the hamster cell line BHK21. Here we studied the interplay between MVA and the cGAS/STING DNA in this permissive cell line and determined whether manipulation of this axis could impact MVA replication and cell responses. We demonstrate that BHK21 cells retain a functional cGAS/STING axis that responds to canonical DNA sensing agonists, upregulating interferon stimulated genes (ISGs). BHK21 cells also respond to MVA, but with a distinct ISG profile. This profile remains unaltered after CRISPR/Cas9 knock-out editing of STING and ablation of cytosolic DNA responses, indicating that MVA responses are independent of the cGAS/STING axis. Furthermore, infection by MVA diminishes the ability of BHK21 cells to respond to exogenous DNA suggesting that MVA still encodes uncharacterised inhibitors of DNA sensing. This suggests that using attenuated strains in permissive cell lines may assist in identification of novel host-virus interactions that may be of relevance to disease or the therapeutic applications of poxviruses.

Single Immunization with Recombinant ACAM2000 Vaccinia Viruses Expressing the Spike and the Nucleocapsid Proteins Protects Hamsters against SARS-CoV-2-Caused Clinical Disease

Increasing cases of SARS-CoV-2 breakthrough infections from immunization with current spike protein-based COVID-19 vaccines highlight the need to develop alternative vaccines using different platforms and/or antigens. In this study, we expressed SARS-CoV-2 spike and nucleocapsid proteins based on a novel vaccinia virus (VACV) ACAM2000 platform (rACAM2000). In this platform, the vaccinia virus host range and immunoregulatory gene E3L was deleted to make the virus attenuated and to enhance innate immune responses, and another host range gene, K3L, was replaced with a poxvirus ortholog gene, taterapox virus 037 (TATV037), to make virus replication competent in both hamster and human cells. Following a single intramuscular immunization, the rACAM2000 coexpressing the spike and nucleocapsid proteins induced significantly improved protection against SARS-CoV-2 challenge in comparison to rACAM2000 expressing the individual proteins in a hamster model, as shown by reduced weight loss and shorter recovery time. The protection was associated with reduced viral loads, increased neutralizing antibody titer, and reduced neutrophil-to-lymphocyte ratio. Thus, our study demonstrates that rACAM2000 expressing a combination of the spike and nucleocapsid antigens is a promising COVID-19 vaccine candidate, and further studies will investigate if the rACAM2000 vaccine candidate can induce a long-lasting immunity against infection by SARS-CoV-2 variants of concern. IMPORTANCE Continuous emergence of SARS-CoV-2 variants which cause breakthrough infection from the immunity induced by current spike protein-based COVID-19 vaccines highlights the need for new generations of vaccines that will induce long-lasting immunity against a wide range of the variants. To this end, we investigated the protective efficacy of the recombinant COVID-19 vaccine candidates based on a novel VACV ACAM2000 platform, in which an immunoregulatory gene, E3L, was deleted and both the SARS-CoV-2 spike (S) and nucleocapsid (N) antigens were expressed. Thus, it is expected that the vaccine candidate we constructed should be more immunogenic and safer. In the initial study described in this work, we demonstrated that the vaccine candidate expressing both the S and N proteins is superior to the constructs expressing an individual protein (S or N) in protecting hamsters against SARS-CoV-2 challenge after a single-dose immunization, and further investigation against different SARS-CoV-2 variants will warrant future clinical evaluations.

Translational control during poxvirus infection

Poxviruses are an unusual family of large double-stranded (ds) DNA viruses that exhibit an incredible degree of self-sufficiency and complexity in their replication and immune evasion strategies. Indeed, amongst their approximately 200 open reading frames (ORFs), poxviruses encode approximately 100 immunomodulatory proteins to counter host responses along with complete DNA synthesis, transcription, mRNA processing and cytoplasmic redox systems that enable them to replicate exclusively in the cytoplasm of infected cells. However, like all other viruses poxviruses do not encode ribosomes and therefore remain completely dependent on gaining access to the host translational machinery in order to synthesize viral proteins. Early studies of these intriguing viruses helped discover the mRNA cap and polyadenylated (polyA) tail that we now know to be present on most eukaryotic messages and which play fundamental roles in mRNA translation, while more recent studies have begun to reveal the remarkable lengths poxviruses go to in order to control both host and viral protein synthesis. Here, we discuss some of the central strategies used by poxviruses and the broader battle that ensues with the host cell to control the translation system, the outcome of which ultimately dictates the fate of infection. This article is categorized under: Translation > Translation Regulation.

Expression of the Vaccinia Virus Antiapoptotic F1 Protein Is Blocked by Protein Kinase R in the Absence of the Viral E3 Protein

Poxviruses encode many proteins with the ability to regulate cellular signaling pathways. One such protein is the vaccinia virus innate immunity modulator E3. Multiple functions have been ascribed to E3, including modulating the cellular response to double-stranded RNA, inhibiting the NF-κB and IRF3 pathways, and dampening apoptosis. Apoptosis serves as a powerful defense against damaged and unwanted cells and is an effective defense against viral infection; many viruses therefore encode proteins that prevent or delay apoptosis. Here, we present data indicating that E3 does not directly inhibit the intrinsic apoptotic pathway; instead, it suppresses apoptosis indirectly by stimulating expression of the viral F1 apoptotic inhibitor. Our data demonstrate that E3 promotes F1 expression by blocking activation of the double-stranded RNA-activated protein kinase R (PKR). F1 mRNA is present in cells infected with E3-null virus, but the protein product does not detectably accumulate, suggesting a block at the translational level. We also show that two 3' coterminal transcripts span the F1 open reading frame (ORF), a situation previously described for the vaccinia virus mRNAs encoding the J3 and J4 proteins. One of these is a conventional monocistronic transcript of the F1L gene, while the other arises by read-through transcription from the upstream F2L gene and does not give rise to appreciable levels of F1 protein.IMPORTANCE Previous studies have shown that E3-deficient vaccinia virus triggers apoptosis of infected cells. Our study demonstrates that this proapoptotic phenotype stems, at least in part, from the failure of the mutant virus to produce adequate quantities of the viral F1 protein, which acts at the mitochondria to directly block apoptosis. Our data establish a regulatory link between the vaccinia virus proteins that suppress the innate response to double-stranded RNA and those that block the intrinsic apoptotic pathway.

Ectromelia virus lacking the E3L ortholog is replication-defective and nonpathogenic but does induce protective immunity in a mouse strain susceptible to lethal mousepox

All known orthopoxviruses, including ectromelia virus (ECTV), contain a gene in the E3L family. The protein product of this gene, E3, is a double-stranded RNA-binding protein. It can impact host range and is used by orthopoxviruses to combat cellular defense pathways, such as PKR and RNase L. In this work, we constructed an ECTV mutant with a targeted disruption of the E3L open reading frame (ECTVΔE3L). Infection with this virus resulted in an abortive replication cycle in all cell lines tested. We detected limited transcription of late genes but no significant translation of these mRNAs. Notably, the replication defects of ECTVΔE3L were rescued in human and mouse cells lacking PKR. ECTVΔE3L was nonpathogenic in BALB/c mice, a strain susceptible to lethal mousepox disease. However, infection with ECTVΔE3L induced protective immunity upon subsequent challenge with wild-type virus. In summary, E3L is an essential gene for ECTV.

Modulating Vaccinia Virus Immunomodulators to Improve Immunological Memory

The increasing frequency of monkeypox virus infections, new outbreaks of other zoonotic orthopoxviruses and concern about the re-emergence of smallpox have prompted research into developing antiviral drugs and better vaccines against these viruses. This article considers the genetic engineering of vaccinia virus (VACV) to enhance vaccine immunogenicity and safety. The virulence, immunogenicity and protective efficacy of VACV strains engineered to lack specific immunomodulatory or host range proteins are described. The ultimate goal is to develop safer and more immunogenic VACV vaccines that induce long-lasting immunological memory.

E3L and F1L Gene Functions Modulate the Protective Capacity of Modified Vaccinia Virus Ankara Immunization in Murine Model of Human Smallpox

The highly attenuated Modified Vaccinia virus Ankara (MVA) lacks most of the known vaccinia virus (VACV) virulence and immune evasion genes. Today MVA can serve as a safety-tested next-generation smallpox vaccine. Yet, we still need to learn about regulatory gene functions preserved in the MVA genome, such as the apoptosis inhibitor genes F1L and E3L. Here, we tested MVA vaccine preparations on the basis of the deletion mutant viruses MVA-ΔF1L and MVA-ΔE3L for efficacy against ectromelia virus (ECTV) challenge infections in mice. In non-permissive human tissue culture the MVA deletion mutant viruses produced reduced levels of the VACV envelope antigen B5. Upon mousepox challenge at three weeks after vaccination, MVA-ΔF1L and MVA-ΔE3L exhibited reduced protective capacity in comparison to wildtype MVA. Surprisingly, however, all vaccines proved equally protective against a lethal ECTV infection at two days after vaccination. Accordingly, the deletion mutant MVA vaccines induced high levels of virus-specific CD8+ T cells previously shown to be essential for rapidly protective MVA vaccination. These results suggest that inactivation of the anti-apoptotic genes F1L or E3L modulates the protective capacity of MVA vaccination most likely through the induction of distinct orthopoxvirus specific immunity in the absence of these viral regulatory proteins.

Ectromelia virus accumulates less double-stranded RNA compared to vaccinia virus in BS-C-1 cells

Most orthopoxviruses, including vaccinia virus (VACV), contain genes in the E3L and K3L families. The protein products of these genes have been shown to combat PKR, a host defense pathway. Interestingly, ectromelia virus (ECTV) contains an E3L ortholog but does not possess an intact K3L gene. Here, we gained insight into how ECTV can still efficiently evade PKR despite lacking K3L. Relative to VACV, we found that ECTV-infected BS-C-1 cells accumulated considerably less double-stranded (ds) RNA, which was due to lower mRNA levels and less transcriptional read-through of some genes by ECTV. The abundance of dsRNA in VACV-infected cells, detected using a monoclonal antibody, was able to activate the RNase L pathway at late time points post-infection. Historically, the study of transcription by orthopoxviruses has largely focused on VACV as a model. Our data suggest that there could be more to learn by studying other members of this genus.

Opposing Roles of Double-Stranded RNA Effector Pathways and Viral Defense Proteins Revealed with CRISPR-Cas9 Knockout Cell Lines and Vaccinia Virus Mutants

Vaccinia virus (VACV) decapping enzymes and cellular exoribonuclease Xrn1 catalyze successive steps in mRNA degradation and prevent double-stranded RNA (dsRNA) accumulation, whereas the viral E3 protein can bind dsRNA. We showed that dsRNA and E3 colocalized within cytoplasmic viral factories in cells infected with a decapping enzyme mutant as well as with wild-type VACV and that they coprecipitated with antibody. An E3 deletion mutant induced protein kinase R (PKR) and eukaryotic translation initiation factor alpha (eIF2α) phosphorylation earlier and more strongly than a decapping enzyme mutant even though less dsRNA was made, leading to more profound effects on viral gene expression. Human HAP1 and A549 cells were genetically modified by clustered regularly interspaced short palindromic repeat-Cas9 (CRISPR-Cas9) to determine whether the same pathways restrict E3 and decapping mutants. The E3 mutant replicated in PKR knockout (KO) HAP1 cells in which RNase L is intrinsically inactive but only with a double knockout (DKO) of PKR and RNase L in A549 cells, indicating that both pathways decreased replication equivalently and that no additional dsRNA pathway was crucial. In contrast, replication of the decapping enzyme mutant increased significantly (though less than that of wild-type virus) in DKO A549 cells but not in DKO HAP1 cells where a smaller increase in viral protein synthesis occurred. Xrn1 KO A549 cells were viable but nonpermissive for VACV; however, wild-type and mutant viruses replicated in triple-KO cells in which RNase L and PKR were also inactivated. Since KO of PKR and RNase L was sufficient to enable VACV replication in the absence of E3 or Xrn1, the poor replication of the decapping mutant, particularly in HAP1 DKO, cells indicated additional translational defects.

IMPORTANCE

Viruses have evolved ways of preventing or counteracting the cascade of antiviral responses that double-stranded RNA (dsRNA) triggers in host cells. We showed that the dsRNA produced in excess in cells infected with a vaccinia virus (VACV) decapping enzyme mutant and by wild-type virus colocalized with the viral E3 protein in cytoplasmic viral factories. Novel human cell lines defective in either or both protein kinase R and RNase L dsRNA effector pathways and/or the cellular 5' exonuclease Xrn1 were prepared by CRISPR-Cas9 gene editing. Inactivation of both pathways was necessary and sufficient to allow full replication of the E3 mutant and reverse the defect cause by inactivation of Xrn1, whereas the decapping enzyme mutant still exhibited defects in gene expression. The study provided new insights into functions of the VACV proteins, and the well-characterized panel of CRISPR-Cas9-modified human cell lines should have broad applicability for studying innate dsRNA pathways.

Modified Vaccinia Virus Ankara: History, Value in Basic Research, and Current Perspectives for Vaccine Development

Safety tested Modified Vaccinia virus Ankara (MVA) is licensed as third-generation vaccine against smallpox and serves as a potent vector system for development of new candidate vaccines against infectious diseases and cancer. Historically, MVA was developed by serial tissue culture passage in primary chicken cells of vaccinia virus strain Ankara, and clinically used to avoid the undesirable side effects of conventional smallpox vaccination. Adapted to growth in avian cells MVA lost the ability to replicate in mammalian hosts and lacks many of the genes orthopoxviruses use to conquer their host (cell) environment. As a biologically well-characterized mutant virus, MVA facilitates fundamental research to elucidate the functions of poxvirus host-interaction factors. As extremely safe viral vectors MVA vaccines have been found immunogenic and protective in various preclinical infection models. Multiple recombinant MVA currently undergo clinical testing for vaccination against human immunodeficiency viruses, Mycobacterium tuberculosis or Plasmodium falciparum. The versatility of the MVA vector vaccine platform is readily demonstrated by the swift development of experimental vaccines for immunization against emerging infections such as the Middle East Respiratory Syndrome. Recent advances include promising results from the clinical testing of recombinant MVA-producing antigens of highly pathogenic avian influenza virus H5N1 or Ebola virus. This review summarizes our current knowledge about MVA as a unique strain of vaccinia virus, and discusses the prospects of exploiting this virus as research tool in poxvirus biology or as safe viral vector vaccine to challenge existing and future bottlenecks in vaccinology.

Mutational analysis of vaccinia virus E3 protein: the biological functions do not correlate with its biochemical capacity to bind double-stranded RNA

Vaccinia E3 protein has the biochemical capacity of binding to double-stranded RNA (dsRNA). The best characterized biological functions of the E3 protein include its host range function, suppression of cytokine expression, and inhibition of interferon (IFN)-induced antiviral activity. Currently, the role of the dsRNA binding capacity in the biological functions of the E3 protein is not clear. To further understand the mechanism of the E3 protein biological functions, we performed alanine scanning of the entire dsRNA binding domain of the E3 protein to examine the link between its biochemical capacity of dsRNA binding and biological functions. Of the 115 mutants examined, 20 were defective in dsRNA binding. Although the majority of the mutants defective in dsRNA binding also showed defective replication in HeLa cells, nine mutants (I105A, Y125A, E138A, F148A, F159A, K171A, L182A, L183A, and I187/188A) retained the host range function to various degrees. Further examination of a set of representative E3L mutants showed that residues essential for dsRNA binding are not essential for the biological functions of E3 protein, such as inhibition of protein kinase R (PKR) activation, suppression of cytokine expression, and apoptosis. Thus, data described in this communication strongly indicate the E3 protein performs its biological functions via a novel mechanism which does not correlate with its dsRNA binding activity.

IMPORTANCE

dsRNAs produced during virus replication are important pathogen-associated molecular patterns (PAMPs) for inducing antiviral immune responses. One of the strategies used by many viruses to counteract such antiviral immune responses is achieved by producing dsRNA binding proteins, such as poxvirus E3 family proteins, influenza virus NS1, and Ebola virus V35 proteins. The most widely accepted model for the biological functions of this class of viral dsRNA binding proteins is that they bind to and sequester viral dsRNA PAMPs; thus, they suppress the related antiviral immune responses. However, no direct experimental data confirm such a model. In this study of vaccinia E3 protein, we found that the biological functions of the E3 protein are not necessarily linked to its biochemical capacity of dsRNA binding. Thus, our data strongly point to a new concept of virus modulation of cellular antiviral responses triggered by dsRNA PAMPs.

Recombinant modified vaccinia virus Ankara generating excess early double-stranded RNA transiently activates protein kinase R and triggers enhanced innate immune responses

Double-stranded RNA (dsRNA) is an important molecular pattern associated with viral infection and is detected by various extra- and intracellular recognition molecules. Poxviruses have evolved to avoid producing dsRNA early in infection but generate significant amounts of dsRNA late in infection due to convergent transcription of late genes. Protein kinase R (PKR) is activated by dsRNA and triggers major cellular defenses against viral infection, including protein synthesis shutdown, apoptosis, and type I interferon (IFN-I) production. The poxviral E3 protein binds and sequesters viral dsRNA and is a major antagonist of the PKR pathway. We found that the highly replication-restricted modified vaccinia virus Ankara (MVA) engineered to produce excess amounts of dsRNA early in infection showed enhanced induction of IFN-β in murine and human cells in the presence of an intact E3L gene. IFN-β induction required a minimum overlap length of 300 bp between early complementary transcripts and was strongly PKR dependent. Excess early dsRNA produced by MVA activated PKR early but transiently in murine cells and induced enhanced systemic levels of IFN-α, IFN-γ, and other cytokines and chemokines in mice in a largely PKR-dependent manner. Replication-competent chorioallantois vaccinia virus Ankara (CVA) generating excess early dsRNA also enhanced IFN-I production and was apathogenic in mice even at very high doses but showed no in vitro host range defect. Thus, genetically adjuvanting MVA and CVA to generate excess early dsRNA is an effective method to enhance innate immune stimulation by orthopoxvirus vectors and to attenuate replicating vaccinia virus in vivo.

IMPORTANCE

Efficient cellular sensing of pathogen-specific components, including double-stranded RNA (dsRNA), is an important prerequisite of an effective antiviral immune response. The prototype poxvirus vaccinia virus (VACV) and its derivative modified vaccinia virus Ankara (MVA) produce dsRNA as a by-product of viral transcription. We found that inhibition of cellular dsRNA recognition established by the virus-encoded proteins E3 and K3 can be overcome by directing viral overexpression of dsRNA early in infection without compromising replication of MVA in permissive cells. Early dsRNA induced transient activation of the cellular dsRNA sensor protein kinase R (PKR), resulting in enhanced production of interferons and cytokines in cells and mice. Enhancing the capacity of MVA to activate the innate immune system is an important approach to further improve the immunogenicity of this promising vaccine vector.

Modified vaccinia virus Ankara triggers type I IFN production in murine conventional dendritic cells via a cGAS/STING-mediated cytosolic DNA-sensing pathway

Modified vaccinia virus Ankara (MVA) is an attenuated poxvirus that has been engineered as a vaccine against infectious agents and cancers. Our goal is to understand how MVA modulates innate immunity in dendritic cells (DCs), which can provide insights to vaccine design. In this study, using murine bone marrow-derived dendritic cells, we assessed type I interferon (IFN) gene induction and protein secretion in response to MVA infection. We report that MVA infection elicits the production of type I IFN in murine conventional dendritic cells (cDCs), but not in plasmacytoid dendritic cells (pDCs). Transcription factors IRF3 (IFN regulatory factor 3) and IRF7, and the positive feedback loop mediated by IFNAR1 (IFN alpha/beta receptor 1), are required for the induction. MVA induction of type I IFN is fully dependent on STING (stimulator of IFN genes) and the newly discovered cytosolic DNA sensor cGAS (cyclic guanosine monophosphate-adenosine monophosphate synthase). MVA infection of cDCs triggers phosphorylation of TBK1 (Tank-binding kinase 1) and IRF3, which is abolished in the absence of cGAS and STING. Furthermore, intravenous delivery of MVA induces type I IFN in wild-type mice, but not in mice lacking STING or IRF3. Treatment of cDCs with inhibitors of endosomal and lysosomal acidification or the lysosomal enzyme Cathepsin B attenuated MVA-induced type I IFN production, indicating that lysosomal enzymatic processing of virions is important for MVA sensing. Taken together, our results demonstrate a critical role of the cGAS/STING-mediated cytosolic DNA-sensing pathway for type I IFN induction in cDCs by MVA. We present evidence that vaccinia virulence factors E3 and N1 inhibit the activation of IRF3 and the induction of IFNB gene in MVA-infected cDCs.

Attenuated NYCBH vaccinia virus deleted for the E3L gene confers partial protection against lethal monkeypox virus disease in cynomolgus macaques

The New York City Board of Health (NYCBH) vaccinia virus is the currently licensed vaccine for use in the US against smallpox. The vaccine under investigation in this study has been attenuated by deletion of the innate immune evasion gene, E3L, and shown to be protective in homologous virus mouse challenge and heterologous virus mouse and rabbit challenge models. In this study we compared NYCBH deleted for the E3L gene (NYCBHΔE3L) to NYCBH for the ability to induce phosphorylation of proinflammatory signaling proteins and the ability to protect cynomolgus macaques from heterologous challenge with monkeypox virus (MPXV). NYCBHΔE3L induced phosphorylation of PKR and eIF2α as well as p38, SAPK/JNK, and IRF3 which can lead to induction of proinflammatory gene transcription. Vaccination of macaques with two doses of NYCBHΔE3L resulted in negligible pock formation at the site of scarification in comparison to vaccination using a single dose of NYCBH, but still elicited neutralizing antibodies and protected 75% of the animals from mortality after challenge with MPXV. However, NYCBHΔE3L-vaccinated animals developed a high number of secondary skin lesions and blood viral load similar to that seen in unvaccinated controls. The NYCBHΔE3L-vaccinated animals that survived MPXV challenge were able to show resolution of blood viral load, a decrease in number of skin lesions, and an improved clinical score by three weeks post challenge. These results suggest that although the highly attenuated NYCBHΔE3L allows proinflammatory signal transduction to occur, it does not provide full protection against monkeypox challenge.

Comparative analysis of poxvirus orthologues of the vaccinia virus E3 protein: modulation of protein kinase R activity, cytokine responses, and virus pathogenicity

Poxviruses are important human and animal pathogens that have evolved elaborate strategies for antagonizing host innate and adaptive immunity. The E3 protein of vaccinia virus, the prototypic member of the orthopoxviruses, functions as an inhibitor of innate immune signaling and is essential for vaccinia virus replication in vivo and in many human cell culture systems. However, the function of orthologues of E3 expressed by poxviruses of other genera with different host specificity remains largely unknown. In the present study, we characterized the E3 orthologues from sheeppox virus, yaba monkey tumor virus, swinepox virus, and myxoma virus for their ability to modulate protein kinase R (PKR) function, cytokine responses and virus pathogenicity. We found that the E3 orthologues of myxoma virus and swinepox virus could suppress PKR activation and interferon (IFN)-induced antiviral activities and restore the host range function of E3 in HeLa cells. In contrast, the E3 orthologues from sheeppox virus and yaba monkey tumor virus were unable to inhibit PKR activation. While the sheeppox orthologue was unable to restore the host range function of E3, the yaba monkey tumor virus orthologue partially restored E3-deficient vaccinia virus replication in HeLa cells, correlated with its ability to suppress IFN-induced antiviral activities. Moreover, poxvirus E3 orthologues show varying ability to inhibit the induction of antiviral and proinflammatory cytokines. Despite these in vitro results, none of the E3 orthologues tested was capable of restoring pathogenicity to E3-deficient vaccinia virus in vivo.

Induction of Noxa-mediated apoptosis by modified vaccinia virus Ankara depends on viral recognition by cytosolic helicases, leading to IRF-3/IFN-β-dependent induction of pro-apoptotic Noxa

Viral infection is a stimulus for apoptosis, and in order to sustain viral replication many viruses are known to carry genes encoding apoptosis inhibitors. F1L, encoded by the orthopoxvirus modified vaccinia virus Ankara (MVA) has a Bcl-2-like structure. An MVA mutant lacking F1L (MVAΔF1L) induces apoptosis, indicating that MVA infection activates and F1L functions to inhibit the apoptotic pathway. In this study we investigated the events leading to apoptosis upon infection by MVAΔF1L. Apoptosis largely proceeded through the pro-apoptotic Bcl-2 family protein Bak with some contribution from Bax. Of the family of pro-apoptotic BH3-only proteins, only the loss of Noxa provided substantial protection, while the loss of Bim had a minor effect. In mice, MVA preferentially infected macrophages and DCs in vivo. In both cell types wt MVA induced apoptosis albeit more weakly than MVAΔF1L. The loss of Noxa had a significant protective effect in macrophages, DC and primary lymphocytes, and the combined loss of Bim and Noxa provided strong protection. Noxa protein was induced during infection, and the induction of Noxa protein and apoptosis induction required transcription factor IRF3 and type I interferon signalling. We further observed that helicases RIG-I and MDA5 and their signalling adapter MAVS contribute to Noxa induction and apoptosis in response to MVA infection. RNA isolated from MVA-infected cells induced Noxa expression and apoptosis when transfected in the absence of viral infection. We thus here describe a pathway leading from the detection of viral RNA during MVA infection by the cytosolic helicase-pathway, to the up-regulation of Noxa and apoptosis via IRF3 and type I IFN signalling.

Viruses have come up with a diverse set of mechanisms to stop infected cells from committing suicide and hence secure their own propagation. In this study we use the DNA virus Modified Vaccinia virus Ankara, a highly attenuated version Vaccinia Virus, to study how cells detect viral infection and induce apoptosis. Modified Vaccinia virus Ankara is currently in clinical trials for its use in various vaccination protocols. By using a broad array of immortalized and primary cell types we observed that viral infection induced programmed cell death was controlled by proteins predominantly involved in detection of viral RNA, in particular proteins involved in the type 1 interferon response. The novelty of our findings lies on the observation that not only can RNA from DNA viruses be detected and activate the type 1 interferon response to infection, but that these responses can also directly modulate the levels of proteins regulating programmed cell death. Future treatments of infections by viral pathogens could exploit the synergistic ability of the type 1 interferon responses and programmed cell death in order to inhibit viral propagation.

RNA species generated in vaccinia virus infected cells activate cell type-specific MDA5 or RIG-I dependent interferon gene transcription and PKR dependent apoptosis

RNA species produced during virus replication are pathogen-associated molecular patterns (PAMPs) triggering cellular innate immune responses including induction of type I interferon expression and apoptosis. Pattern recognition receptors (PRRs) for these RNAs include the retinoic acid-inducible gene I (RIG-I) like receptors (RLRs) RIG-I and melanoma differentiation associated gene 5 (MDA5) and the dsRNA dependent protein kinase (PKR). Currently, poxvirus PAMPs and their associated PRRs are not well characterized. We report that RNA species generated in vaccinia infected cells can activate MDA5 or RIG-I dependent interferon-β (IFN-β) gene transcription in a cell type-specific manner. These RNA species also induce the activation of apoptosis in a PKR dependent, but MDA5 and RIG-I independent, manner. Collectively our results demonstrate that RNA species generated during vaccinia virus replication are major PAMPs activating apoptosis and IFN-β gene transcription. Moreover, our results delineate the signaling pathways involved in the recognition of RNA-based poxvirus PAMPs.

Viral double-stranded RNAs from vaccinia virus early or intermediate gene transcripts possess PKR activating function, resulting in NF-kappaB activation, when the K1 protein is absent or mutated

PKR is a potent antiviral molecule that can terminate infection by inhibiting protein synthesis and stimulating NF-κB activation and apoptosis. Originally, it was thought that only intermediate and late gene transcription produced double-stranded (ds) RNA to activate PKR during vaccinia virus (VACV) infection. The VACV E3 or K3 proteins squelch this effect by binding to either dsRNA or PKR. However, in the absence of the K1 protein, VACV infection activates PKR at very early times post-infection and despite the presence of E3 and K3. These data suggest that VACV infection induces PKR activation by a currently unknown mechanism. To determine this mechanism, cells were infected with K1L-containing or -deficient VACVs. By using conditions that limited the progression of the poxvirus replication cycle, we observed that early gene transcripts activated PKR in RK13 cells, identifying a new PKR-activating mechanism of poxvirus infection. Using a similar approach for HeLa cells, intermediate gene transcription was sufficient to activate PKR. RNA isolated from infected RK13 or HeLa cells maintained PKR-activating properties only when dsRNA was present. Moreover, viral dsRNA was directly detected in infected cells either by RT-PCR or immunofluorescent microscopy. Interestingly, dsRNA levels were higher in infected cells in which the K1 protein was nonfunctional. Only K1 proteins with PKR inhibitory function prevented downstream NF-κB activation. These results reveal a new PKR activation pathway during VACV infection, in which the K1 protein reduces dsRNA levels early in VACV infection to directly inhibit PKR and several of its downstream antiviral effects, thereby enhancing virus survival.

Formation of antiviral cytoplasmic granules during orthopoxvirus infection

Vaccinia virus (VV) mutants lacking the double-stranded RNA (dsRNA)-binding E3L protein (ΔE3L mutant VV) show restricted replication in most cell types, as dsRNA produced by VV activates protein kinase R (PKR), leading to eIF2α phosphorylation and impaired translation initiation. Here we show that cells infected with ΔE3L mutant VV assemble cytoplasmic granular structures which surround the VV replication factories at an early stage of the nonproductive infection. These structures contain the stress granule-associated proteins G3BP, TIA-1, and USP10, as well as poly(A)-containing RNA. These structures lack large ribosomal subunit proteins, suggesting that they are translationally inactive. Formation of these punctate structures correlates with restricted replication, as they occur in >80% of cells infected with ΔE3L mutant VV but in only 10% of cells infected with wild-type VV. We therefore refer to these structures as antiviral granules (AVGs). Formation of AVGs requires PKR and phosphorylated eIF2α, as mouse embryonic fibroblasts (MEFs) lacking PKR displayed reduced granule formation and MEFs lacking phosphorylatable eIF2α showed no granule formation. In both cases, these decreased levels of AVG formation correlated with increased ΔE3L mutant VV replication. Surprisingly, MEFs lacking the AVG component protein TIA-1 supported increased replication of ΔE3L mutant VV, despite increased eIF2α phosphorylation and the assembly of AVGs that lacked TIA-1. These data indicate that the effective PKR-mediated restriction of ΔE3L mutant VV replication requires AVG formation subsequent to eIF2α phosphorylation. This is a novel finding that supports the hypothesis that the formation of subcellular protein aggregates is an important component of the successful cellular antiviral response.

Viral host-range factor C7 or K1 is essential for modified vaccinia virus Ankara late gene expression in human and murine cells, irrespective of their capacity to inhibit protein kinase R-mediated phosphorylation of eukaryotic translation initiation factor 2alpha

Vaccinia virus (VACV) infection induces phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha), which inhibits cellular and viral protein synthesis. In turn, VACV has evolved the capacity to antagonize this antiviral response by expressing the viral host-range proteins K3 and E3. This study revealed that the host-range genes K1L and C7L also prevent eIF2alpha phosphorylation in modified VACV Ankara (MVA) infection of several human and murine cell lines. Moreover, C7L-deleted MVA (MVA-DeltaC7L) lacked late gene expression, which could be rescued by the function of host-range factor K1 or C7. It was demonstrated that viral gene expression was blocked after viral DNA replication and that it was independent of apoptosis induction. Furthermore, it was found that eIF2alpha phosphorylation in MVA-DeltaC7L-infected cells is mediated by protein kinase R (PKR) as shown in murine embryonic fibroblasts lacking PKR function, and it was shown that this was not due to reduced E3L gene expression. The block of eIF2alpha phosphorylation by C7 could be complemented by K1 in cells infected with MVA-DeltaC7L encoding a reinserted K1L gene (MVA-DeltaC7L-K1L). Importantly, these data illustrated that eIF2alpha phosphorylation by PKR is not responsible for the block of late viral gene expression. This suggests that other mechanisms targeted by C7 and K1 are essential for completing the MVA gene expression cycle and probably also for VACV replication in a diverse set of cell types.

Modified vaccinia virus Ankara can activate NF-kappaB transcription factors through a double-stranded RNA-activated protein kinase (PKR)-dependent pathway during the early phase of virus replication

Modified vaccinia virus Ankara (MVA), which is a promising replication-defective vaccine vector, is unusual among the orthopoxviruses in activating NF-kappaB transcription factors in cells of several types. In human embryonic kidney (HEK 293T) cells, the MVA-induced depletion of IkappaBalpha required to activate NF-kappaB is inhibited by UV-inactivation of the virus, and begins before viral DNA replication. In HEK 293T, CHO, or RK13 cells, expression of the cowpox virus CP77 early gene, or the vaccinia virus K1L early gene suppresses MVA-induced IkappaBalpha depletion. In mouse embryonic fibroblasts (MEFs), MVA induction of IkappaBalpha depletion is dependent on the expression of mouse or human double-stranded RNA-activated protein kinase (PKR). These results demonstrate that events during the early phase of MVA replication can induce PKR-mediated processes contributing both to the activation of NF-kappaB signaling, and to processes that may restrict viral replication. This property may contribute to the efficacy of this vaccine virus.

Early viral protein synthesis is necessary for NF-kappaB activation in modified vaccinia Ankara (MVA)-infected 293 T fibroblast cells

Modified vaccinia Ankara (MVA) is an attenuated vaccinia virus, and is a promising vaccine vector for variola and monkeypox viruses, as well as for other pathogens. The MVA determinants important for vaccine efficacy and immunogenicity are poorly defined. MVA infection of fibroblast cells activates NF-kappaB, a characteristic not ascribed to wild-type vaccinia viruses. Thus, NF-kappaB activation, and the subsequent upregulation of host immune molecules, could be one of the determinants for MVA's immunogenicity. We report that ERK2 phosphorylation, an event preceding and required for NF-kappaB activation, occurred rapidly after virus infection. ERK2 and NF-kappaB remained inert when virus endocytosis was prevented, suggesting that virus-host cell interactions were insufficient for activating NF-kappaB. Inhibition of viral protein synthesis decreased NF-kappaB activation, and elimination of intermediate and late gene expression did not alter MVA-induced NF-kappaB activation. Thus, early gene expression activates NF-kappaB.

Vaccinia virus E3 suppresses expression of diverse cytokines through inhibition of the PKR, NF-kappaB, and IRF3 pathways

The vaccinia virus double-stranded RNA binding protein E3 has been demonstrated to inhibit the expression of cytokines, including beta interferon (IFN-beta) and tumor necrosis factor alpha (TNF-alpha). However, few details regarding the molecular mechanisms of this inhibition have been described. Using real-time PCR arrays, we found that E3 suppressed the induction of a diverse array of cytokines representing members of the IFN, interleukin (IL), TNF, and transforming growth factor cytokine families. We discovered that the factor(s) responsible for the induction of IL-6, TNF-alpha, and inhibin beta A (INHBA) was associated with the early and late phases of virus infection. In contrast, the factor(s) which regulates IFN-beta induction was associated with the late phase of replication. We have found that expression of these cytokines can be induced by transfection of cells with RNA isolated from vaccinia virus-infected cells. Moreover, we provide evidence that E3 antagonizes both PKR-dependent and PKR-independent pathways to regulate cytokine expression. PKR-dependent activation of p38 and NF-kappaB was required for vaccinia virus-induced INHBA expression, whereas induction of TNF-alpha required only PKR-dependent NF-kappaB activation. In contrast, induction of IL-6 and IFN-beta was largely PKR independent. IL-6 induction is regulated by NF-kappaB, while IFN-beta induction is mediated by IFN-beta promoter stimulator 1 and IFN regulatory factor 3/NF-kappaB. Collectively, these results indicate that E3 suppresses distinct but interlinked host signaling pathways to inhibit the expression of a diverse array of cytokines.

The orthopoxvirus 68-kilodalton ankyrin-like protein is essential for DNA replication and complete gene expression of modified vaccinia virus Ankara in nonpermissive human and murine cells

Modified vaccinia virus Ankara (MVA) is a highly attenuated and replication-deficient vaccinia virus (VACV) that is being evaluated as replacement smallpox vaccine and candidate viral vector. MVA lacks many genes associated with virulence and/or regulation of virus tropism. The 68-kDa ankyrin-like protein (68k-ank) is the only ankyrin repeat-containing protein that is encoded by the MVA genome and is highly conserved throughout the Orthopoxvirus genus. We showed previously that 68k-ank is composed of ankyrin repeats and an F-box-like domain and forms an SCF ubiquitin ligase complex together with the cellular proteins Skp1a and Cullin-1. We now report that 68k-ank (MVA open reading frame 186R) is an essential factor for completion of the MVA intracellular life cycle in nonpermissive human and murine cells. Infection of mouse NIH 3T3 and human HaCaT cells with MVA with a deletion of the 68k-ank gene (MVA-Delta68k-ank) was characterized by an extensive reduction of viral intermediate RNA and protein, as well as late transcripts and drastically impaired late protein synthesis. Furthermore, infections with MVA-Delta68k-ank failed to induce the host protein shutoff that is characteristic of VACV infections. Although we demonstrated that proteasome function in general is essential for the completion of the MVA molecular life cycle, we found that a mutant 68k-ank protein with a deletion of the F-box-like domain was able to fully complement the deficiency of MVA-Delta68k-ank to express all classes of viral genes. Thus, our data demonstrate that the 68k-ank protein contains another critical domain that may function independently of SCF ubiquitin ligase complex formation, suggesting multiple activities of this interesting regulatory protein.

Gamma interferon-induced interferon regulatory factor 1-dependent antiviral response inhibits vaccinia virus replication in mouse but not human fibroblasts

Vaccinia virus (VACV) replicates in mouse and human fibroblasts with comparable kinetics and efficiency, yielding similar titers of infectious progeny. Here we demonstrate that gamma interferon (IFN-gamma) but not IFN-alpha or IFN-beta pretreatment of mouse fibroblasts prior to VACV infection induces a long-lasting antiviral state blocking VACV replication. In contrast, high doses of IFN-gamma failed to establish an antiviral state in human fibroblasts. In mouse fibroblasts, IFN-gamma impeded the viral replication cycle at the level of late gene transcription and blocked the multiplication of VACV genomes. The IFN-gamma-induced antiviral state invariably prevented the growth of different VACV strains but was not effective against the replication of ectromelia virus. The IFN-gamma effect required intact IFN-gamma receptor signaling prior to VACV infection through Janus kinase 2 (Jak2) and signal transducer and activator of transcription 1 (STAT1). The permissive state of IFN-gamma-treated human cells was unrelated to the VACV-encoded IFN decoy receptors B8 and B18 and associated with a complete disruption of STAT1 homodimer formation and DNA binding. Unlike human fibroblasts, mouse cells responded with long-lasting STAT1 activation which was preserved after VACV infection. The deletion of the IFN regulatory factor 1 (IRF-1) gene from mouse cells rescued efficient VACV replication, demonstrating that IRF-1 target genes have a critical role in VACV control. These data have implications for the understanding of VACV pathogenesis and identify an incongruent IFN-gamma response between the human host and the mouse model.

Vaccinia virus subverts a mitochondrial antiviral signaling protein-dependent innate immune response in keratinocytes through its double-stranded RNA binding protein, E3

Skin keratinocytes provide a first line of defense against invading microorganisms in two ways: (i) by acting as a physical barrier to pathogen entry and (ii) by initiating a vigorous innate immune response upon sensing danger signals. How keratinocytes detect virus infections and generate antiviral immune responses is not well understood. Orthopoxviruses are dermatotropic DNA viruses that cause lethal disease in humans. Virulence in animal models depends on the virus-encoded bifunctional Z-DNA/double-stranded RNA (dsRNA)-binding protein E3. Here, we report that infection of mouse primary keratinocytes with a vaccinia DeltaE3L mutant virus triggers the production of beta interferon (IFN-beta), interleukin-6 (IL-6), CCL4, and CCL5. None of these immune mediators is produced by keratinocytes infected with wild-type vaccinia virus. The dsRNA-binding domain of E3 suffices to prevent activation of the innate immune response. DeltaE3L induction of IFN-beta, IL-6, CCL4, and CCL5 secretion requires mitochondrial antiviral signaling protein (MAVS; an adaptor for the cytoplasmic viral RNA sensors RIG-I and MDA5) and the transcription factor IRF3. IRF3 phosphorylation is induced in keratinocytes infected with DeltaE3L, an event that depends on MAVS. The response of keratinocytes to DeltaE3L is unaffected by genetic ablation of Toll-like receptor 3 (TLR3), TRIF, TLR9, and MyD88.

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.

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.

The highly conserved orthopoxvirus 68k ankyrin-like protein is part of a cellular SCF ubiquitin ligase complex

The 68k ankyrin-like protein (68k-ank) of unknown function is highly conserved among orthopoxviruses and contains ankyrin repeats and an F-box-like domain. We performed a yeast-two-hybrid screen with 68k-ank to find interacting proteins. From a human and a murine cDNA library, 99% of the interaction partners were S-phase kinase-associated protein 1a (Skp1a), a part of the SCF ubiquitin ligase complex. 68k-ank co-immunoprecipitated with components of the endogenous, mammalian SCF ubiquitin ligase. This interaction was F-box domain dependent and could also be observed in infected cells, indicating that SCF complex formation might be important for the viral life cycle.

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.

Vaccinia virus double-stranded RNA-binding protein E3 does not interfere with siRNA-mediated gene silencing in mammalian cells

Vaccinia virus (VACV) evolved several strategies to evade antiviral cellular defence. The vaccinia virus E3 protein for example binds and sequesters double stranded RNA (dsRNA) and counteracts interferon action. We were interested to find out whether and to what extend E3 interferes with RNA silencing mediated by short interfering RNA (siRNA) in mammalian cells. We could show that the expression of a VACV-encoded marker gene can be efficiently inhibited by siRNA independently of the presence of the E3 protein. In addition, expression of E3 had no impact on RNA polymerase III promoter-derived shRNA-induced silencing of a cellular gene in human cells. Both VACV early and late gene expression could be inhibited by siRNA. Furthermore, downregulation of the expression of the E3L gene itself by siRNA in VACV infected cells produced the previously described phenotype of a knock-out virus, which illustrates the power of siRNA for vaccinia virus gene function analysis.

Suppression of proinflammatory signal transduction and gene expression by the dual nucleic acid binding domains of the vaccinia virus E3L proteins

Cells have evolved elaborate mechanisms to counteract the onslaught of viral infections. To activate these defenses, the viral threat must be recognized. Danger signals, or pathogen-associated molecular patterns, that are induced by pathogens include double-stranded RNA (dsRNA), viral single-stranded RNA, glycolipids, and CpG DNA. Understanding the signal transduction pathways activated and host gene expression induced by these danger signals is vital to understanding virus-host interactions. The vaccinia virus E3L protein is involved in blocking the host antiviral response and increasing pathogenesis, functions that map to separate C-terminal dsRNA- and N-terminal Z-DNA-binding domains. Viruses containing mutations in these domains allow modeling of the role of dsRNA and Z-form nucleic acid in the host response to virus infection. Deletions in the Z-DNA- or dsRNA-binding domains led to activation of signal transduction cascades and up-regulation of host gene expression, with many genes involved in the inflammatory response. These data suggest that poxviruses actively inhibit cellular recognition of viral danger signals and the subsequent cellular response to the viral threat.

A poxvirus host range protein, CP77, binds to a cellular protein, HMG20A, and regulates its dissociation from the vaccinia virus genome in CHO-K1 cells

Vaccinia virus does not grow in Chinese hamster ovary (CHO-K1) cells in the absence of a viral host range factor, cowpox protein CP77. In this study, CP77 was fused to the C terminus of green fluorescence protein (GFP-CP77) and a series of nested deletion mutants of GFP-CP77 was constructed for insertion into a vaccinia virus host range mutant, VV-hr, and expressed from a viral early promoter. Deletion mapping analyses demonstrated that the N-terminal 352 amino acids of CP77 were sufficient to support vaccinia virus growth in CHO-K1 cells, whereas the C-terminal residues 353 to 668 were dispensable. In yeast two-hybrid analyses, CP77 bound to a cellular protein, HMG20A, and GST pulldown analyses showed that residues 1 to 234 of CP77 were sufficient for this interaction. After VV-hr virus infection of CHO-K1 cells, HMG20A was translocated from the nucleus to viral factories and bound to the viral genome via the HMG box region. In control VV-hr-infected CHO-K1 cells, binding of HMG20A to the viral genome persisted from 2 to 8 h postinfection (h p.i.); in contrast, when CP77 was expressed, the association of HMG20A with viral genome was transient, with little HMG20A remaining bound at 8 h p.i. This indicates that dissociation of HMG20A from viral factories correlates well with CP77 host range activity in CHO-K1 cells. Finally, in cells expressing a CP77 deletion protein (amino acids 277 to 668) or a DeltaANK5 mutant that did not support vaccinia virus growth and did not contain the HMG20A binding site, HMG20A remained bound to viral DNA, demonstrating that the binding of CP77 to HMG20A is essential for its host range function. In summary, our data revealed that a novel cellular protein, HMG20A, the dissociation of which from viral DNA is regulated by CP77, providing the first cellular target regulated by viral host range CP77 protein.