Inhibition of apoptosis and NF-κB activation by vaccinia protein N1 occur via distinct binding surfaces and make different contributions to virulence

Attenuation of monkeypox virus by deletion of genomic regions

Monkeypox virus (MPXV) is an emerging pathogen from Africa that causes disease similar to smallpox. Two clades with different geographic distributions and virulence have been described. Here, we utilized bioinformatic tools to identify genomic regions in MPXV containing multiple virulence genes and explored their roles in pathogenicity; two selected regions were then deleted singularly or in combination. In vitro and in vivo studies indicated that these regions play a significant role in MPXV replication, tissue spread, and mortality in mice. Interestingly, while deletion of either region led to decreased virulence in mice, one region had no effect on in vitro replication. Deletion of both regions simultaneously also reduced cell culture replication and significantly increased the attenuation in vivo over either single deletion. Attenuated MPXV with genomic deletions present a safe and efficacious tool in the study of MPX pathogenesis and in the identification of genetic factors associated with virulence.

Phosphatidylserine receptors: enhancers of enveloped virus entry and infection

A variety of both RNA and DNA viruses envelop their capsids in a lipid bilayer. One of the more recently appreciated benefits this envelope is incorporation of phosphatidylserine (PtdSer). Surface exposure of PtdSer disguises viruses as apoptotic bodies; tricking cells into engulfing virions. This mechanism is termed apoptotic mimicry. Several PtdSer receptors have been identified to enhance virus entry and we have termed this group of proteins PtdSer-mediated virus entry enhancing receptors or PVEERs. These receptors enhance entry of a range of enveloped viruses. Internalization of virions by PVEERs provides a broad mechanism of entry with little investment by the virus itself. PVEERs may allow some viruses to attach to cells, thereby making viral glycoprotein/cellular receptor interactions more probable. Alternatively, other viruses may rely entirely on PVEERs for internalization into endosomes. This review provides an overview of PtdSer receptors that serve as PVEERs and the biology behind virion/PVEER interaction.

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Phosphatidylserine (PtdSer) receptors can mediate entry of enveloped viruses.

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PtdSer is present on the outer leaflet of the virion envelope.

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PtdSer receptors are expressed on a variety of primary cells and cell lines.

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Characteristics of PtdSer receptors that mediate virus entry are defined.

Analysis of the anti-apoptotic activity of four vaccinia virus proteins demonstrates that B13 is the most potent inhibitor in isolation and during viral infection

Vaccinia virus (VACV) is a large dsDNA virus encoding ~200 proteins, several of which inhibit apoptosis. Here, a comparative study of anti-apoptotic proteins N1, F1, B13 and Golgi anti-apoptotic protein (GAAP) in isolation and during viral infection is presented. VACVs strains engineered to lack each gene separately still blocked apoptosis to some degree because of functional redundancy provided by the other anti-apoptotic proteins. To overcome this redundancy, we inserted each gene separately into a VACV strain (vv811) that lacked all these anti-apoptotic proteins and that induced apoptosis efficiently during infection. Each protein was also expressed in cells using lentivirus vectors. In isolation, each VACV protein showed anti-apoptotic activity in response to specific stimuli, as measured by immunoblotting for cleaved poly(ADP ribose) polymerase-1 and caspase-3 activation. Of the proteins tested, B13 was the most potent inhibitor, blocking both intrinsic and extrinsic stimuli, whilst the activity of the other proteins was largely restricted to inhibition of intrinsic stimuli. In addition, B13 and F1 were effective blockers of apoptosis induced by vv811 infection. Finally, whilst differences in induction of apoptosis were barely detectable during infection with VACV strain Western Reserve compared with derivative viruses lacking individual anti-apoptotic genes, several of these proteins reduced activation of caspase-3 during infection by vv811 strains expressing these proteins. These results illustrated that vv811 was a useful tool to determine the role of VACV proteins during infection and that whilst all of these proteins have some anti-apoptotic activity, B13 was the most potent.

Vaccinia virus virulence factor N1 can be ubiquitylated on multiple lysine residues

Ubiquitylation is a covalent post-translational modification that regulates protein stability and is involved in many biological functions. Proteins may be modified with mono-ubiquitin or ubiquitin chains. Viruses have evolved multiple mechanisms to perturb the cell ubiquitin system and manipulate it to their own benefit. Here, we report ubiquitylation of vaccinia virus (VACV) protein N1. N1 is an inhibitor of the nuclear factor NF-κB and apoptosis that contributes to virulence, has a Bcl-2-like fold, and is highly conserved amongst orthopoxviruses. The interaction between N1 and ubiquitin occurs at endogenous protein levels during VACV infection and following ectopic expression of N1. Biochemical analysis demonstrated that N1 is covalently ubiquitylated, and heterodimers of ubiquitylated and non-ubiquitylated N1 monomers were identified, suggesting that ubiquitylation does not inhibit N1 dimerization. Studies with other VACV Bcl-2 proteins, such as C6 or B14, revealed that although these proteins also interact with ubiquitin, these interactions are non-covalent. Finally, mutagenesis of N1 showed that ubiquitylation occurs in a conventional lysine-dependent manner at multiple acceptor sites because only an N1 allele devoid of lysine residues remained unmodified. Taken together, we described a previously uncharacterized modification of the VACV protein N1 that provided a new layer of complexity to the biology of this virulence factor, and provided another example of the intricate interplay between poxviruses and the host ubiquitin system.

Ectromelia virus encodes a BTB/kelch protein, EVM150, that inhibits NF-κB signaling

The NF-κB signaling pathway plays a critical role in inflammation and innate immunity. Consequently, many viruses have evolved strategies to inhibit NF-κB in order to facilitate replication and evasion of the host immune response. Recently, we determined that ectromelia virus, the causative agent of mousepox, contains a family of four BTB/kelch proteins that interact with cullin-3-based ubiquitin ligases. We demonstrate here that expression of EVM150, one of the four BTB/kelch proteins, inhibited NF-κB activation induced by tumor necrosis factor alpha (TNF-α) and interleukin-1β (IL-1β). Although EVM150 inhibited NF-κB p65 nuclear translocation, IκBα degradation was observed, indicating that EVM150 functioned downstream of IκBα degradation. Significantly, expression of the BTB-only domain of EVM150 blocked NF-κB activation, demonstrating that EVM150 functioned independently of the kelch domain and its role as an adapter for cullin-3-based ubiquitin ligases. Furthermore, cullin-3 knockdown by small interfering RNA demonstrated that cullin-3-based ubiquitin ligases are dispensable for TNF-α-induced NF-κB activation. Interestingly, nuclear translocation of IRF3 and STAT1 still occurred in the presence of EVM150, indicating that EVM150 prevented NF-κB nuclear translocation specifically. In addition to identifying EVM150 as an inhibitor of the NF-κB pathway, this study provides new insights into the role of BTB/kelch proteins during virus infection.

IMPORTANCE

With the exception of virulence studies, little work has been done to determine the role of poxviral BTB/kelch proteins during infection. This study, for the first time, has identified a mechanism for the ectromelia virus BTB/kelch protein EVM150. Here, we show that EVM150 is a novel inhibitor of the cellular NF-κB pathway, an important component of the antiviral response. This study adds EVM150 to the growing list of NF-κB inhibitors in poxviruses and provides new insights into the role of BTB/kelch proteins during virus infection.

Deletion of the vaccinia virus N2L gene encoding an inhibitor of IRF3 improves the immunogenicity of modified vaccinia virus Ankara expressing HIV-1 antigens

A modified vaccinia virus Ankara poxvirus vector expressing the HIV-1 Env, Gag, Pol, and Nef antigens from clade B (MVA-B) is currently being tested in clinical trials. To improve its immunogenicity, we have generated and characterized the immune profile of MVA-B containing a deletion of the vaccinia viral gene N2L, which codes for an inhibitor of IRF3 (MVA-B ΔN2L). Deletion of N2L had no effect on virus growth kinetics or on the expression of HIV-1 antigens; hence, the N2 protein is not essential for MVA replication. The innate immune responses triggered by MVA-B ΔN2L revealed an increase in beta interferon, proinflammatory cytokines, and chemokines. Mouse prime-boost protocols showed that MVA-B ΔN2L improves the magnitude and polyfunctionality of HIV-1-specific CD4(+) and CD8(+) T cell adaptive and memory immune responses, with most of the HIV-1 responses mediated by CD8(+) T cells. In the memory phase, HIV-1-specific CD8(+) T cells with an effector phenotype were predominant and in a higher percentage with MVA-B ΔN2L than with MVA-B. In both immunization groups, CD4(+) and CD8(+) T cell responses were directed mainly against Env. Furthermore, MVA-B ΔN2L in the memory phase enhanced levels of antibody against Env. For the vector immune responses, MVA-B ΔN2L induced a greater magnitude and polyfunctionality of VACV-specific CD8(+) T memory cells than MVA-B, with an effector phenotype. These results revealed the immunomodulatory role of N2L, whose deletion enhanced the innate immunity and improved the magnitude and quality of HIV-1-specific T cell adaptive and memory immune responses. These findings are relevant for the optimization of poxvirus vectors as vaccines.

IMPORTANCE

On the basis of the limited efficacy of the RV144 phase III clinical trial, new optimized poxvirus vectors as vaccines against HIV/AIDS are needed. Here we have generated and characterized a new HIV/AIDS vaccine candidate on the basis of the poxvirus MVA vector expressing HIV-1 Env, Gag, Pol, and Nef antigens (MVA-B) and containing a deletion in the vaccinia virus N2L gene. Our findings revealed the immunomodulatory role of N2L and proved that its deletion from the MVA-B vector triggered an enhanced innate immune response in human macrophages and monocyte-derived dendritic cells. Furthermore, in immunized mice, MVA-B ΔN2L induced improvements in the magnitude and quality of adaptive and memory HIV-1-specific CD4(+) and CD8(+) T cell immune responses, together with an increase in the memory phase of levels of antibody against Env. Thus, the selective deletion of the N2L viral immunomodulatory gene is important for the optimization of MVA vectors as HIV-1 vaccines.

Vaccinia virus inhibits NF-κB-dependent gene expression downstream of p65 translocation

The transcription factor nuclear factor kappa light-chain enhancer of activated B cells (NF-κB) plays a critical role in host defense against viral infection by inducing the production of proinflammatory mediators and type I interferon. Consequently, viruses have evolved many mechanisms to block its activation. The poxvirus vaccinia virus (VACV) encodes numerous inhibitors of NF-κB activation that target multiple points in the signaling pathway. A derivative of VACV strain Copenhagen, called vv811, lacking 55 open reading frames in the left and right terminal regions of the genome was reported to still inhibit NF-κB activation downstream of tumor necrosis factor alpha (TNF-α) and interleukin-1β (IL-1β), suggesting the presence of one or more additional inhibitors. In this study, we constructed a recombinant vv811 lacking the recently described NF-κB inhibitor A49 (vv811ΔA49), yielding a virus that lacked all currently described inhibitors downstream of TNF-α and IL-1β. Unlike vv811, vv811ΔA49 no longer inhibited degradation of the phosphorylated inhibitor of κBα and p65 translocated into the nucleus. However, despite this translocation, vv811ΔA49 still inhibited TNF-α- and IL-1β-induced NF-κB-dependent reporter gene expression and the transcription and production of cytokines induced by these agonists. This inhibition did not require late viral gene expression. These findings indicate the presence of another inhibitor of NF-κB that is expressed early during infection and acts by a novel mechanism downstream of p65 translocation into the nucleus.

Structural biology of the Bcl-2 family and its mimicry by viral proteins

Intrinsic apoptosis in mammals is regulated by protein–protein interactions among the B-cell lymphoma-2 (Bcl-2) family. The sequences, structures and binding specificity between pro-survival Bcl-2 proteins and their pro-apoptotic Bcl-2 homology 3 motif only (BH3-only) protein antagonists are now well understood. In contrast, our understanding of the mode of action of Bax and Bak, the two necessary proteins for apoptosis is incomplete. Bax and Bak are isostructural with pro-survival Bcl-2 proteins and also interact with BH3-only proteins, albeit weakly. Two sites have been identified; the in-groove interaction analogous to the pro-survival BH3-only interaction and a site on the opposite molecular face. Interaction of Bax or Bak with activator BH3-only proteins and mitochondrial membranes triggers a series of ill-defined conformational changes initiating their oligomerization and mitochondrial outer membrane permeabilization. Many actions of the mammalian pro-survival Bcl-2 family are mimicked by viruses. By expressing proteins mimicking mammalian pro-survival Bcl-2 family proteins, viruses neutralize death-inducing members of the Bcl-2 family and evade host cell apoptosis during replication. Remarkably, structural elements are preserved in viral Bcl-2 proteins even though there is in many cases little discernible sequence conservation with their mammalian counterparts. Some viral Bcl-2 proteins are dimeric, but they have distinct structures to those observed for mammalian Bcl-2 proteins. Furthermore, viral Bcl-2 proteins modulate innate immune responses regulated by NF-κB through an interface separate from the canonical BH3-binding groove. Our increasing structural understanding of the viral Bcl-2 proteins is leading to new insights in the cellular Bcl-2 network by exploring potential alternate functional modes in the cellular context. We compare the cellular and viral Bcl-2 proteins and discuss how alterations in their structure, sequence and binding specificity lead to differences in behavior, and together with the intrinsic structural plasticity in the Bcl-2 fold enable exquisite control over critical cellular signaling pathways.

Poxviral protein A52 stimulates p38 mitogen-activated protein kinase (MAPK) activation by causing tumor necrosis factor receptor-associated factor 6 (TRAF6) self-association leading to transforming growth factor β-activated kinase 1 (TAK1) recruitment

Vaccinia virus encodes a number of proteins that inhibit and manipulate innate immune signaling pathways that also have a role in virulence. These include A52, a protein shown to inhibit IL-1- and Toll-like receptor-stimulated NFκB activation, via interaction with interleukin-1 receptor-associated kinase 2 (IRAK2). Interestingly, A52 was also found to activate p38 MAPK and thus enhance Toll-like receptor-dependent IL-10 induction, which was TRAF6-dependent, but the manner in which A52 manipulates TRAF6 to stimulate p38 activation was unclear. Here, we show that A52 has a non-canonical TRAF6-binding motif that is essential for TRAF6 binding and p38 activation but dispensable for NFκB inhibition and IRAK2 interaction. Wild-type A52, but not a mutant defective in p38 activation and TRAF6 binding (F154A), caused TRAF6 oligomerization and subsequent TRAF6-TAK1 association. The crystal structure of A52 shows that it adopts a Bcl2-like fold and exists as a dimer in solution. Residue Met-65 was identified as being located in the A52 dimer interface, and consistent with that, A52-M65E was impaired in its ability to dimerize. A52-M65E although capable of interacting with TRAF6, was unable to cause either TRAF6 self-association, induce the TRAF6-TAK1 association, or activate p38 MAPK. The results suggest that an A52 dimer causes TRAF6 self-association, leading to TAK1 recruitment and p38 activation. This reveals a molecular mechanism whereby poxviruses manipulate TRAF6 to activate MAPKs (which can be proviral) without stimulating antiviral NFκB activation.

Vaccinia virus immune evasion: mechanisms, virulence and immunogenicity

Virus infection of mammalian cells is sensed by pattern recognition receptors and leads to an innate immune response that restricts virus replication and induces adaptive immunity. In response, viruses have evolved many countermeasures that enable them to replicate and be transmitted to new hosts, despite the host innate immune response. Poxviruses, such as vaccinia virus (VACV), have large DNA genomes and encode many proteins that are dedicated to host immune evasion. Some of these proteins are secreted from the infected cell, where they bind and neutralize complement factors, interferons, cytokines and chemokines. Other VACV proteins function inside cells to inhibit apoptosis or signalling pathways that lead to the production of interferons and pro-inflammatory cytokines and chemokines. In this review, these VACV immunomodulatory proteins are described and the potential to create more immunogenic VACV strains by manipulation of the gene encoding these proteins is discussed.

Vaccinia virus protein N2 is a nuclear IRF3 inhibitor that promotes virulence

Vaccinia virus (VACV) expresses many proteins that are non-essential for virus replication but promote virulence by inhibiting components of the host immune response to infection. These immunomodulators include a family of proteins that have, or are predicted to have, a structure related to the B-cell lymphoma (Bcl)-2 protein. Five members of the VACV Bcl-2 family (N1, B14, A52, F1 and K7) have had their crystal structure solved, others have been characterized and a function assigned (C6, A46), and others are predicted to be Bcl-2 proteins but are uncharacterized hitherto (N2, B22, C1). Data presented here show that N2 is a nuclear protein that is expressed early during infection and inhibits the activation of interferon regulatory factor (IRF)3. Consistent with its nuclear localization, N2 inhibits IRF3 downstream of the TANK-binding kinase (TBK)-1 and after IRF3 translocation into the nucleus. A mutant VACV strain Western Reserve lacking the N2L gene (vΔN2) showed normal replication and spread in cultured cells compared to wild-type parental (vN2) and revertant (vN2-rev) viruses, but was attenuated in two murine models of infection. After intranasal infection, the vΔN2 mutant induced lower weight loss and signs of illness, and virus was cleared more rapidly from the infected tissue. In the intradermal model of infection, vΔN2 induced smaller lesions that were resolved more rapidly. In summary, the N2 protein is an intracellular virulence factor that inhibits IRF3 activity in the nucleus.

Vaccinia virus protein K7 is a virulence factor that alters the acute immune response to infection

Vaccinia virus (VACV) encodes many proteins that antagonize the innate immune system including a family of intracellular proteins with a B-cell lymphoma (Bcl)-2-like structure. One of these Bcl-2 proteins called K7 binds Toll-like receptor-adaptor proteins and the DEAD-box RNA helicase DDX3 and thereby inhibits the activation of NF-κB and interferon regulatory factor 3. However, the contribution of K7 to virus virulence is not known. Here a VACV lacking the K7R gene (vΔK7) was constructed and compared with control viruses that included a plaque purified wt (vK7), a revertant with the K7R gene reinserted (vK7-rev) and a frame-shifted virus in which the translational initiation codon was mutated to prevent K7 protein expression (vK7-fs). Data presented show that loss of K7 does not affect virus replication in cell culture or in vivo; however, viruses lacking the K7 protein were less virulent than controls in murine intradermal (i.d.) and intranasal (i.n.) infection models and there was an altered acute immune response to infection. In the i.d. model, vΔK7 induced smaller lesions than controls, and after i.n. infection vΔK7 induced a reduced weight loss and signs of illness, and more rapid clearance of virus from infected tissue. Concomitantly, the intrapulmonary innate immune response to infection with vΔK7 showed increased infiltration of NK cells and CD8⁺ T-cells, enhanced MHC class II expression by macrophages, and enhanced cytolysis of target cells by NK cells and VACV-specific CD8⁺ T-cells. Thus protein K7 is a virulence factor that affects the acute immune response to infection.

Poxvirus targeting of E3 ligase β-TrCP by molecular mimicry: a mechanism to inhibit NF-κB activation and promote immune evasion and virulence

The transcription factor NF-κB is essential for immune responses against pathogens and its activation requires the phosphorylation, ubiquitination and proteasomal degradation of IκBα. Here we describe an inhibitor of NF-κB from vaccinia virus that has a closely related counterpart in variola virus, the cause of smallpox, and mechanistic similarity with the HIV protein Vpu. Protein A49 blocks NF-κB activation by molecular mimicry and contains a motif conserved in IκBα which, in IκBα, is phosphorylated by IKKβ causing ubiquitination and degradation. Like IκBα, A49 binds the E3 ligase β-TrCP, thereby preventing ubiquitination and degradation of IκBα. Consequently, A49 stabilised phosphorylated IκBα (p-IκBα) and its interaction with p65, so preventing p65 nuclear translocation. Serine-to-alanine mutagenesis within the IκBα-like motif of A49 abolished β-TrCP binding, stabilisation of p-IκBα and inhibition of NF-κB activation. Remarkably, despite encoding nine other inhibitors of NF-κB, a VACV lacking A49 showed reduced virulence in vivo.

Sheeppox virus SPPV14 encodes a Bcl-2-like cell death inhibitor that counters a distinct set of mammalian proapoptotic proteins

Many viruses express inhibitors of programmed cell death (apoptosis), thereby countering host defenses that would otherwise rapidly clear infected cells. To counter this, viruses such as adenoviruses and herpesviruses express recognizable homologs of the mammalian prosurvival protein Bcl-2. In contrast, the majority of poxviruses lack viral Bcl-2 (vBcl-2) homologs that are readily identified by sequence similarities. One such virus, myxoma virus, which is the causative agent of myxomatosis, expresses a virulence factor that is a potent inhibitor of apoptosis. In spite of the scant sequence similarity to Bcl-2, myxoma virus M11L adopts an almost identical 3-dimensional fold. We used M11L as bait in a sequence similarity search for other Bcl-2-like proteins and identified six putative vBcl-2 proteins from poxviruses. Some are potent inhibitors of apoptosis, in particular sheeppox virus SPPV14, which inhibited cell death induced by multiple agents. Importantly, SPPV14 compensated for the loss of antiapoptotic F1L in vaccinia virus and acts to directly counter the cell death mediators Bax and Bak. SPPV14 also engages a unique subset of the death-promoting BH3-only ligands, including Bim, Puma, Bmf, and Hrk. This suggests that SPPV14 may have been selected for specific biological roles as a virulence factor for sheeppox virus.

Vaccinia virus protein C4 inhibits NF-κB activation and promotes virus virulence

Vaccinia virus (VACV) strain Western Reserve protein C4 has been characterized and its function and contribution to virus virulence assessed. Bioinformatic analysis showed that C4 is conserved in six orthopoxvirus species and shares 43 % amino acid identity with VACV protein C16, a known virulence factor. A recombinant VACV expressing a C-terminally tagged version of C4 showed that, like C16, this 37 kDa protein is expressed early during infection and localizes to both the cytoplasm and the nucleus. Functional assays using a firefly luciferase reporter plasmid under the control of a nuclear factor kappa B (NF-κB)-dependent promoter demonstrated that C4 inhibits NF-κB activation at, or downstream of, the inhibitor of kappa kinase (IKK) complex. Consistent with this, C4 inhibited interleukin-1β-induced translocation of p65 into the nucleus. A VACV lacking the C4L gene (vΔC4) showed no significant differences from wild-type virus in growth kinetics or spread in cell culture, but had reduced virulence in a murine intranasal model of infection. vΔC4-infected mice exhibited fewer symptoms, lost less weight and recovered 7 days earlier than animals infected with control viruses expressing C4. Furthermore, bronchoalveolar lavage fluid from vΔC4-infected mice had increased cell numbers at day 5 post-infection, which correlated with reduced lung virus titres from this time onward. C4 represents the ninth VACV protein to inhibit NF-κB activation and remarkably, in every case examined, loss of each protein individually caused an alteration in virus virulence, despite the presence of other NF-κB inhibitors.

Orthopoxvirus genes that mediate disease virulence and host tropism

In the course of evolution, viruses have developed various molecular mechanisms to evade the defense reactions of the host organism. When understanding the mechanisms used by viruses to overcome manifold defense systems of the animal organism, represented by molecular factors and cells of the immune system, we would not only comprehend better but also discover new patterns of organization and function of these most important reactions directed against infectious agents. Here, study of the orthopoxviruses pathogenic for humans, such as variola (smallpox), monkeypox, cowpox, and vaccinia viruses, may be most important. Analysis of the experimental data, presented in this paper, allows to infer that variola virus and other orthopoxviruses possess an unexampled set of genes whose protein products efficiently modulate the manifold defense mechanisms of the host organisms compared with the viruses from other families.

Innate immune response of human plasmacytoid dendritic cells to poxvirus infection is subverted by vaccinia E3 via its Z-DNA/RNA binding domain

Plasmacytoid dendritic cells (pDCs) play important roles in antiviral innate immunity by producing type I interferon (IFN). In this study, we assess the immune responses of primary human pDCs to two poxviruses, vaccinia and myxoma virus. Vaccinia, an orthopoxvirus, was used for immunization against smallpox, a contagious human disease with high mortality. Myxoma virus, a Leporipoxvirus, causes lethal disease in rabbits, but is non-pathogenic in humans. We report that myxoma virus infection of human pDCs induces IFN-α and TNF production, whereas vaccinia infection does not. Co-infection of pDCs with myxoma virus plus vaccinia blocks myxoma induction effects. We find that heat-inactivated vaccinia (Heat-VAC; by incubating the virus at 55°C for 1 h) gains the ability to induce IFN-α and TNF in primary human pDCs. Induction of IFN-α in pDCs by myxoma virus or Heat-VAC is blocked by chloroquine, which inhibits endosomal acidification required for TLR7/9 signaling, and by inhibitors of cellular kinases PI3K and Akt. Using purified pDCs from genetic knockout mice, we demonstrate that Heat-VAC-induced type I IFN production in pDCs requires the endosomal RNA sensor TLR7 and its adaptor MyD88, transcription factor IRF7 and the type I IFN feedback loop mediated by IFNAR1. These results indicate that (i) vaccinia virus, but not myxoma virus, expresses inhibitor(s) of the poxvirus sensing pathway(s) in pDCs; and (ii) Heat-VAC infection fails to produce inhibitor(s) but rather produces novel activator(s), likely viral RNA transcripts that are sensed by the TLR7/MyD88 pathway. Using vaccinia gene deletion mutants, we show that the Z-DNA/RNA binding domain at the N-terminus of the vaccinia immunomodulatory E3 protein is an antagonist of the innate immune response of human pDCs to poxvirus infection and TLR agonists. The myxoma virus ortholog of vaccinia E3 (M029) lacks the N-terminal Z-DNA/RNA binding domain, which might contribute to the immunostimulating properties of myxoma virus.