Poxvirus MC160 protein utilizes multiple mechanisms to inhibit NF-kappaB activation mediated via components of the tumor necrosis factor receptor 1 signal transduction pathway

Innate Immune Sensing of Parapoxvirus Orf Virus and Viral Immune Evasion

Orf virus (ORFV) is the type species of Parapoxvirus of the Poxviridae family that induces cutaneous pustular skin lesions in sheep and goats, and causes zoonotic infections in humans. Pattern recognition receptors (PRRs) sense pathogen-associated molecular patterns (PAMPs), leading to the triggering of the innate immune response through multiple signalling pathways involving type I interferons (IFNs). The major PAMPs generated during viral infection are nucleic acids, which are the most important molecules that are recognized by the host. The induction of type l IFNs leads to activation of the Janus kinase (JAK)-signal transducer activator of transcription (STAT) pathway, which results in the induction of hundreds of interferon-stimulated genes (ISGs), many of which encode proteins that have antiviral roles in eliminating virus infection and create an antiviral state. Genetic and functional analyses have revealed that ORFV, as found for other poxviruses, has evolved multiple immunomodulatory genes and strategies that manipulate the innate immune sensing response.

Molluscum Contagiosum Virus: Biology and Immune Response

Molluscum contagiosum virus is a poxvirus belonging to the Poxviridae family, which includes Orthopoxvirus, Parapoxvirus, Yantapoxvirus, Molluscipoxvirus, Smallpox virus, Cowpox virus and Monkeypox virus. MCV belongs to the genus Molluscipoxvirus and has a tropism for skin tissue. MCV infects keratinocytes and, after an incubation period of 2 weeks to 6 weeks, causes a breakdown of the skin barrier with the development of papules of variable size depending on the proper functioning of the immune response (both adaptive and acquired). MCV only infects humans and does not cause viraemia. MCV encodes for several inhibitory proteins responsible to circumvent the immune response through different signalling pathways. Individuals who can be infected with MCV are children, immunocompromised individuals such as organ transplant recipients and Human Immunodeficiency Virus (HIV)-infected individuals. Current treatments to manage MCV-induced lesions are different and include the use of immunomodulators, which, however, do not provide an effective response.

The MC160 protein of the molluscum contagiosum virus dampens cGAS/STING-induced interferon-β activation

Molluscum contagiosum virus (MCV) is a poxvirus that causes benign, persistent skin lesions. MCV encodes a variety of immune evasion molecules to dampen host immune responses. Two of these proteins are the MC159 and MC160 proteins. Both MC159 and MC160 contain two tandem death effector domains and share homology to the cellular FLIPs, FADD, and procaspase-8. MC159 and MC160 dampen several innate immune responses such as NF-κB activation and mitochondrial antiviral signaling (MAVS)-mediated induction of type 1 interferon (IFN). The type 1 IFN response is also activated by the cytosolic DNA sensors cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING). Both cGAS and STING play a vital role in sensing a poxvirus infection. In this study, we demonstrate that there are nuanced differences between both MC160 and MC159 in terms of how the viral proteins modulate the cGAS/STING and MAVS pathways. Specifically, MC160 expression, but not MC159 expression, dampens cGAS/STING-mediated induction of IFN in HEK 293 T cells. Further, MC160 expression prevented the K63-ubiquitination of both STING and TBK1, a kinase downstream of cGAS/STING. Ectopic expression of the MC160 protein, but not the MC159 protein, resulted in a measurable decrease in the TBK1 protein levels as detected via immunoblotting. Finally, using a panel of MC160 truncation mutants, we report that the MC160 protein requires both DEDs to inhibit cGAS/STING-induced activation of IFN-β. Our model indicates MC160 likely alters the TBK1 signaling complex to decrease IFN-β activation at the molecular intersection of the cGAS/STING and MAVS signaling pathways.

Molluscum Contagiosum Virus Protein MC008 Targets NF-κB Activation by Inhibiting Ubiquitination of NEMO

Molluscum contagiosum virus (MCV) is a human-adapted poxvirus that causes a common and persistent yet mild infection characterized by distinct, contagious, papular skin lesions. These lesions are notable for having little or no inflammation associated with them and can persist for long periods without an effective clearance response from the host. Like all poxviruses, MCV encodes potent immunosuppressive proteins that perturb innate immune pathways involved in virus sensing, the interferon response, and inflammation, which collectively orchestrate antiviral immunity and clearance, with several of these pathways converging at common signaling nodes. One such node is the regulator of canonical nuclear factor kappa B (NF-κB) activation, NF-κB essential modulator (NEMO). Here, we report that the MCV protein MC008 specifically inhibits NF-κB through its interaction with NEMO, disrupting its early ubiquitin-mediated activation and subsequent downstream signaling. MC008 is the third NEMO-targeting inhibitor to be described in MCV to date, with each inhibiting NEMO activation in distinct ways, highlighting strong selective pressure to evolve multiple ways of disabling this key signaling protein. IMPORTANCE Inflammation lies at the heart of most human diseases. Understanding the pathways that drive this response is the key to new anti-inflammatory therapies. Viruses evolve to target inflammation; thus, understanding how they do this reveals how inflammation is controlled and, potentially, how to disable it when it drives disease. Molluscum contagiosum virus (MCV) has specifically evolved to infect humans and displays an unprecedented ability to suppress inflammation in our tissue. We have identified a novel inhibitor of human innate signaling from MCV, MC008, which targets NEMO, a core regulator of proinflammatory signaling. Furthermore, MC008 appears to inhibit early ubiquitination, thus interrupting later events in NEMO activation, thereby validating current models of IκB kinase (IKK) complex regulation.

How to Inhibit Nuclear Factor-Kappa B Signaling: Lessons from Poxviruses

The Nuclear Factor-kappa B (NF-κB) family of transcription factors regulates key host inflammatory and antiviral gene expression programs, and thus, is often activated during viral infection through the action of pattern-recognition receptors and cytokine–receptor interactions. In turn, many viral pathogens encode strategies to manipulate and/or inhibit NF-κB signaling. This is particularly exemplified by vaccinia virus (VV), the prototypic poxvirus, which encodes at least 18 different inhibitors of NF-κB signaling. While many of these poxviral NF-κB inhibitors are not required for VV replication in cell culture, they virtually all modulate VV virulence in animal models, underscoring the important influence of poxvirus–NF-κB pathway interactions on viral pathogenesis. Here, we review the diversity of mechanisms through which VV-encoded antagonists inhibit initial NF-κB pathway activation and NF-κB signaling intermediates, as well as the activation and function of NF-κB transcription factor complexes.

Poxviral Strategies to Overcome Host Cell Apoptosis

Apoptosis is a form of cellular suicide initiated either via extracellular (extrinsic apoptosis) or intracellular (intrinsic apoptosis) cues. This form of programmed cell death plays a crucial role in development and tissue homeostasis in multicellular organisms and its dysregulation is an underlying cause for many diseases. Intrinsic apoptosis is regulated by members of the evolutionarily conserved B-cell lymphoma-2 (Bcl-2) family, a family that consists of pro- and anti-apoptotic members. Bcl-2 genes have also been assimilated by numerous viruses including pox viruses, in particular the sub-family of chordopoxviridae, a group of viruses known to infect almost all vertebrates. The viral Bcl-2 proteins are virulence factors and aid the evasion of host immune defenses by mimicking the activity of their cellular counterparts. Viral Bcl-2 genes have proved essential for the survival of virus infected cells and structural studies have shown that though they often share very little sequence identity with their cellular counterparts, they have near-identical 3D structures. However, their mechanisms of action are varied. In this review, we examine the structural biology, molecular interactions, and detailed mechanism of action of poxvirus encoded apoptosis inhibitors and how they impact on host–virus interactions to ultimately enable successful infection and propagation of viral infections.

Molecular and genomic characterization of a novel equine molluscum contagiosum-like virus

Cases of pox-like lesions in horses and donkeys have been associated with poxviruses belonging to different genera of the family Poxviridae. These include the orthopoxviruses vaccinia virus (VACV), horsepoxvirus (HPXV) and cowpoxvirus (CPXV), as well as a potentially novel parapoxvirus and molluscum contagiosum virus (MOCV). However, with the exception of VACV, HPXV and CPXV, the genomic characterization of the causative agents remains largely elusive with only single short genome fragments available. Here we present the first full-length genome sequence of an equine molluscum contagiosum-like virus (EMCLV) directly determined from skin biopsies of a horse with generalized papular dermatitis. Histopathological analysis of the lesions revealed severe epidermal hyperplasia with numerous eosinophilic inclusion bodies within keratinocytes. Virions were detected in the lesions in embedded tissue by transmission electron microscopy. The genome sequence determined by next- and third-generation sequencing comprises 166 843 nt with inverted terminal repeats (ITRs) of 3473 nt. Overall, 20 of the predicted 159 ORFs have no equivalents in other poxviruses. Intriguingly, two of these ORFs were identified to encode homologues of mammalian proteins involved in immune signalling pathways, namely secreted and transmembrane protein 1 (SECTM1) and insulin growth factor-like family receptor 1 (IGFLR1), that were not described in any virus family so far. Phylogenetic analysis with all relevant representatives of the Poxviridae suggests that EMCLV should be nominated as a new species within the genus Molluscipoxvirus.

The molluscum contagiosum virus protein MC163 inhibits TNF-α-induced NF-κB activation

Aim: The molluscum contagiosum virus (MCV) expresses several immune evasion molecules that inhibit activation of NF-κB. Presumably, inhibition of inflammatory responses mediated by NF-κB allows MCV to cause persistent infections. Materials & methods: MC163-IKK-α interactions were detected by immunoprecipitations. Results: Here, we identify a novel MCV inhibitor of NF-κB. Ectopic expression of the MC163 protein resulted in a significant decrease in TNF-α-induced NF-κB activation. However, MC163 had no detectable effect on mitochondrial antiviral-signaling protein-induced activation of the IFN-β-promoter. MC163 dampened NF-κB activation induced via the overexpression of either IKK-α or IKK-β suggesting MC163 targets the IKK complex. Conclusion: Our data highlight a previously unknown function for the MC163 protein and may represent an additional strategy used by MCV to subvert host immune responses.

Molluscum contagiosum virus MC80 sabotages MHC-I antigen presentation by targeting tapasin for ER-associated degradation

The human specific poxvirus molluscum contagiosum virus (MCV) produces skin lesions that can persist with minimal inflammation, suggesting that the virus has developed robust immune evasion strategies. However, investigations into the underlying mechanisms of MCV pathogenesis have been hindered by the lack of a model system to propagate the virus. Herein we demonstrate that MCV-encoded MC80 can disrupt MHC-I antigen presentation in human and mouse cells. MC80 shares moderate sequence-similarity with MHC-I and we find that it associates with components of the peptide-loading complex. Expression of MC80 results in ER-retention of host MHC-I and thereby reduced cell surface presentation. MC80 accomplishes this by engaging tapasin via its luminal domain, targeting it for ubiquitination and ER-associated degradation in a process dependent on the MC80 transmembrane region and cytoplasmic tail. Tapasin degradation is accompanied by a loss of TAP, which limits MHC-I access to cytosolic peptides. Our findings reveal a unique mechanism by which MCV undermines adaptive immune surveillance.

A comparison of the effect of molluscum contagiosum virus MC159 and MC160 proteins on vaccinia virus virulence in intranasal and intradermal infection routes

Molluscum contagiosum virus (MCV) causes persistent, benign skin neoplasm in children and adults. MCV is refractive to growth in standard tissue culture and there is no relevant animal model of infection. Here we investigated whether another poxvirus (vaccinia virus; VACV) could be used to examine MCV immunoevasion protein properties in vivo. The MCV MC159L or MC160L genes, which encode NF-κB antagonists, were inserted into an attenuated VACV lacking an NF-κB antagonist (vΔA49), creating vMC159 and vMC160. vMC160 slightly increased vΔA49 virulence in the intranasal and intradermal routes of inoculation. vMC159 infection was less virulent than vΔA49 in both inoculation routes. vMC159-infected ear pinnae did not form lesions, but virus replication still occurred. Thus, the lack of lesions was not due to abortive virus replication. This system provides a new approach to examine MCV immunoevasion proteins within the context of a complete and complex immune system.

Poxviruses Utilize Multiple Strategies to Inhibit Apoptosis

Cells have multiple means to induce apoptosis in response to viral infection. Poxviruses must prevent activation of cellular apoptosis to ensure successful replication. These viruses devote a substantial portion of their genome to immune evasion. Many of these immune evasion products expressed during infection antagonize cellular apoptotic pathways. Poxvirus products target multiple points in both the extrinsic and intrinsic apoptotic pathways, thereby mitigating apoptosis during infection. Interestingly, recent evidence indicates that poxviruses also hijack cellular means of eliminating apoptotic bodies as a means to spread cell to cell through a process called apoptotic mimicry. Poxviruses are the causative agent of many human and veterinary diseases. Further, there is substantial interest in developing these viruses as vectors for a variety of uses including vaccine delivery and as oncolytic viruses to treat certain human cancers. Therefore, an understanding of the molecular mechanisms through which poxviruses regulate the cellular apoptotic pathways remains a top research priority. In this review, we consider anti-apoptotic strategies of poxviruses focusing on three relevant poxvirus genera: Orthopoxvirus, Molluscipoxvirus, and Leporipoxvirus. All three genera express multiple products to inhibit both extrinsic and intrinsic apoptotic pathways with many of these products required for virulence.

A parapoxviral virion protein inhibits NF-κB signaling early in infection

Poxviruses have evolved unique proteins and mechanisms to counteract the nuclear factor κB (NF-κB) signaling pathway, which is an essential regulatory pathway of host innate immune responses. Here, we describe a NF-κB inhibitory virion protein of orf virus (ORFV), ORFV073, which functions very early in infected cells. Infection with ORFV073 gene deletion virus (OV-IA82Δ073) led to increased accumulation of NF-κB essential modulator (NEMO), marked phosphorylation of IκB kinase (IKK) subunits IKKα and IKKβ, IκBα and NF-κB subunit p65 (NF-κB-p65), and to early nuclear translocation of NF-κB-p65 in virus-infected cells (≤ 30 min post infection). Expression of ORFV073 alone was sufficient to inhibit TNFα induced activation of the NF-κB signaling in uninfected cells. Consistent with observed inhibition of IKK complex activation, ORFV073 interacted with the regulatory subunit of the IKK complex NEMO. Infection of sheep with OV-IA82Δ073 led to virus attenuation, indicating that ORFV073 is a virulence determinant in the natural host. Notably, ORFV073 represents the first poxviral virion-associated NF-κB inhibitor described, highlighting the significance of viral inhibition of NF-κB signaling very early in infection.

Molluscum Contagiosum Virus MC159 Abrogates cIAP1-NEMO Interactions and Inhibits NEMO Polyubiquitination

Molluscum contagiosum virus (MCV) is a dermatotropic poxvirus that causes benign skin lesions. MCV lesions persist because of virally encoded immune evasion molecules that inhibit antiviral responses. The MCV MC159 protein suppresses NF-κB activation, a powerful antiviral response, via interactions with the NF-κB essential modulator (NEMO) subunit of the IκB kinase (IKK) complex. Binding of MC159 to NEMO does not disrupt the IKK complex, implying that MC159 prevents IKK activation via an as-yet-unidentified strategy. Here, we demonstrated that MC159 inhibited NEMO polyubiquitination, a posttranslational modification required for IKK and downstream NF-κB activation. Because MCV cannot be propagated in cell culture, MC159 was expressed independent of infection or during a surrogate vaccinia virus infection to identify how MC159 prevented polyubiquitination. Cellular inhibitor of apoptosis protein 1 (cIAP1) is a cellular E3 ligase that ubiquitinates NEMO. Mutational analyses revealed that MC159 and cIAP1 each bind to the same NEMO region, suggesting that MC159 may competitively inhibit cIAP1-NEMO interactions. Indeed, MC159 prevented cIAP1-NEMO interactions. MC159 also diminished cIAP1-mediated NEMO polyubiquitination and cIAP1-induced NF-κB activation. These data suggest that MC159 competitively binds to NEMO to prevent cIAP1-induced NEMO polyubiquitination. To our knowledge, this is the first report of a viral protein disrupting NEMO-cIAP1 interactions to strategically suppress IKK activation. All viruses must antagonize antiviral signaling events for survival. We hypothesize that MC159 inhibits NEMO polyubiquitination as a clever strategy to manipulate the host cell environment to the benefit of the virus.IMPORTANCE Molluscum contagiosum virus (MCV) is a human-specific poxvirus that causes persistent skin neoplasms. The persistence of MCV has been attributed to viral downregulation of host cell immune responses such as NF-κB activation. We show here that the MCV MC159 protein interacts with the NEMO subunit of the IKK complex to prevent NEMO interactions with the cIAP1 E3 ubiquitin ligase. This interaction correlates with a dampening of cIAP1 to polyubiquitinate NEMO and to activate NF-κB. This inhibition of cIAP1-NEMO interactions is a new viral strategy to minimize IKK activation and to control NEMO polyubiquitination. This research provides new insights into mechanisms that persistent viruses may use to cause long-term infection of host cells.

The molluscum contagiosum virus death effector domain containing protein MC160 RxDL motifs are not required for its known viral immune evasion functions

The molluscum contagiosum virus (MCV) uses a variety of immune evasion strategies to antagonize host immune responses. Two MCV proteins, MC159 and MC160, contain tandem death effector domains (DEDs). They are reported to inhibit innate immune signaling events such as NF-κB and IRF3 activation, and apoptosis. The RxDL motif of MC159 is required for inhibition of both apoptosis and NF-κB activation. However, the role of the conserved RxDL motif in the MC160 DEDs remained unknown. To answer this question, we performed alanine mutations to neutralize the arginine and aspartate residues present in the MC160 RxDL in both DED1 and DED2. These mutations were further modeled against the structure of the MC159 protein. Surprisingly, the RxDL motif was not required for MC160's ability to inhibit MAVS-induced IFNβ activation. Further, unlike previous results with the MC159 protein, mutations within the RxDL motif of MC160 had no effect on the ability of MC160 to dampen TNF-α-induced NF-κB activation. Molecular modeling predictions revealed no overall changes to the structure in the MC160 protein when the amino acids of both RxDL motifs were mutated to alanine (DED1 = R67A D69A; DED2 = R160A D162A). Taken together, our results demonstrate that the RxDL motifs present in the MC160 DEDs are not required for known functions of the viral protein.

Molluscum Contagiosum Virus Transcriptome in Abortively Infected Cultured Cells and a Human Skin Lesion

Molluscum contagiosum virus (MOCV), the only circulating human-specific poxvirus, has a worldwide distribution and causes benign skin lesions that may persist for months in young children and severe infections in immunosuppressed adults. Studies of MOCV are restricted by the lack of an efficient animal model or a cell culture replication system. We used next-generation sequencing to analyze and compare polyadenylated RNAs from abortive MOCV infections of several cell lines and a human skin lesion. Viral RNAs were detected for 14 days after MOCV infection of cultured cells; however, there was little change in the RNA species during this time and a similar pattern occurred in the presence of an inhibitor of protein synthesis, indicating a block preventing postreplicative gene expression. Moreover, a considerable number of MOCV RNAs mapped to homologs of orthopoxvirus early genes, but few did so to homologs of intermediate or late genes. The RNAs made during in vitro infections represent a subset of RNAs detected in human skin lesions which mapped to homologs of numerous postreplicative as well as early orthopoxvirus genes. Transfection experiments using fluorescent protein and luciferase reporters demonstrated that vaccinia virus recognized MOCV intermediate and late promoters, indicating similar gene regulation. The specific recognition of the intermediate promoter in MOCV-infected cells provided evidence for the synthesis of intermediate transcription factors, which are products of early genes, but not for late transcription factors. Transcriptome sequencing (RNA-seq) and reporter gene assays may be useful for testing engineered cell lines and conditions that ultimately could provide an in vitro replication system.

IMPORTANCE

The inability to propagate molluscum contagiosum virus, which causes benign skin lesions in young children and more extensive infections in immunosuppressed adults, has constrained our understanding of the biology of this human-specific virus. In the present study, we characterized the RNAs synthesized in abortively infected cultured cells and a human skin lesion by next-generation sequencing. These studies provided an initial transcription map of the MOCV genome, suggested temporal regulation of gene expression, and indicated that the in vitro replication block occurs prior to intermediate and late gene expression. RNA-seq and reporter assays, as described here, may help to further evaluate MOCV gene expression and define conditions that could enable MOCV replication in vitro.

Immune evasion strategies of molluscum contagiosum virus

Molluscum contagiosum virus (MCV) is the causative agent of molluscum contagiosum (MC), the third most common viral skin infection in children, and one of the five most prevalent skin diseases worldwide. No FDA-approved treatments, vaccines, or commercially available rapid diagnostics for MCV are available. This review discusses several aspects of this medically important virus including: physical properties of MCV, MCV pathogenesis, MCV replication, and immune responses to MCV infection. Sequencing of the MCV genome revealed novel immune evasion molecules which are highlighted here. Special attention is given to the MCV MC159 and MC160 proteins. These proteins are FLIPs with homologs in gamma herpesviruses and in the cell. They are of great interest because each protein regulates apoptosis, NF-κB, and IRF3. However, the mechanism that each protein uses to impart its effects is different. It is important to elucidate how MCV inhibits immune responses; this knowledge contributes to our understanding of viral pathogenesis and also provides new insights into how the immune system neutralizes virus infections.

The role of the IKK complex in viral infections

The NF‐κB signal transduction pathway is a critical regulator of multiple cellular functions that ultimately shift the balance between cell survival and death. The cascade is activated by many intrinsic and extrinsic stimuli, which is transduced via adaptor proteins to phosphorylate the IκB kinase (IKK) complex, which in turn phosphorylates the inhibitory IκBα protein to undergo proteasomal degradation and sets in motion nuclear events in response to the initial stimulus. Viruses are important modulators of the NF‐κB cascade and have evolved multiple mechanisms to activate or inhibit this pathway in a manner conducive to viral multiplication and establishment of a productive infectious cycle. This is a subject of extensive research by multiple laboratories whereby unraveling the interactions between specific viral components and members of the NF‐κB signal transduction cascade can shed unique perspectives on infection associated pathogenesis and novel therapeutic targets. In this review, we highlight the interactions between components of the IKK complex and multiple RNA and DNA viruses with the emphasis on mechanisms by which the interaction feeds the infection. Understanding these interactions will shed light on the exploitative capabilities of viruses to maintain an environment favorable for a productive infection.

Viral and cellular FLICE-inhibitory proteins: a comparison of their roles in regulating intrinsic immune responses

FLICE-inhibitory proteins (FLIPs) are a family of viral (poxvirus and herpesvirus) and cellular proteins. The hallmark of this family is the presence of tandem death-effector domains (DEDs). Despite this shared motif, each protein possesses different abilities to modulate apoptosis, NF-κB, and interferon regulatory factor 3 (IRF3). These similarities and differences are discussed and highlighted here. The comparative study of FLIPs provides a unique basis to understand virus-host interactions, viral pathogenesis, and cellular regulation of immune system signal transduction pathways.

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.

Inhibition of interferon gene activation by death-effector domain-containing proteins from the molluscum contagiosum virus

Apoptosis, NF-κB activation, and IRF3 activation are a triad of intrinsic immune responses that play crucial roles in the pathogenesis of infectious diseases, cancer, and autoimmunity. FLIPs are a family of viral and cellular proteins initially found to inhibit apoptosis and more recently to either up- or down-regulate NF-κB. As such, a broad role for FLIPs in disease regulation is postulated, but exactly how a FLIP performs such multifunctional roles remains to be established. Here we examine FLIPs (MC159 and MC160) encoded by the molluscum contagiosum virus, a dermatotropic poxvirus causing skin infections common in children and immunocompromised individuals, to better understand their roles in viral pathogenesis. While studying their molecular mechanisms responsible for NF-κB inhibition, we discovered that each protein inhibited IRF3-controlled luciferase activity, identifying a unique function for FLIPs. MC159 and MC160 each inhibited TBK1 phosphorylation, confirming this unique function. Surprisingly, MC159 coimmunoprecipitated with TBK1 and IKKε but MC160 did not, suggesting that these homologs use distinct molecular mechanisms to inhibit IRF3 activation. Equally surprising was the finding that the FLIP regions necessary for TBK1 inhibition were distinct from those MC159 or MC160 regions previously defined to inhibit NF-κB or apoptosis. These data reveal previously unappreciated complexities of FLIPs, and that subtle differences within the conserved regions of FLIPs possess distinct molecular and structural fingerprints that define crucial differences in biological activities. A future comparison of mechanistic differences between viral FLIP proteins can provide new means of precisely manipulating distinct aspects of intrinsic immune responses.

Molluscum contagiosum virus: persistence pays off

The molluscum contagiosum (MC) virus (MCV) is a dermatotropic poxvirus, and the causative agent of MC. Unlike smallpox and human monkeypox diseases, MC is nonlethal, common and worldwide. Additionally, little inflammation is associated with MC papules, and MC can persist for months to years. Such a prolonged infection implies that MCV successfully manipulates the host environment. This review highlights recent findings that reveal how MCV infections manipulate localized host immune responses and which immune response are key for the eventual resolution of MC. Also highlighted here are the MCV proteins that inhibit apoptosis, inflammation and immune cell recruitment or that induce cellular proliferation, with discussion as to how these proteins dampen localized antiviral immune responses. Lastly, this review discusses how the immune evasion tactics of MCV have led to insights about specific functions of the human innate and adaptive immune responses.

The MC159 protein from the molluscum contagiosum poxvirus inhibits NF-κB activation by interacting with the IκB kinase complex

Molluscum contagiosum virus (MCV) causes persistent neoplasms in healthy and immunocompromised people. Its ability to persist likely is due to its arsenal of viral immunoevasion proteins. For example, the MCV MC159 protein inhibits TNF-R1-induced NF-κB activation and apoptosis. The MC159 protein is a viral FLIP and, as such, possesses two tandem death effector domains (DEDs). We show in this article that, in human embryonic kidney 293 T cells, the expression of wild-type MC159 or a mutant MC159 protein containing the first DED (MC159 A) inhibited TNF-induced NF-κB, or NF-κB activated by PMA or MyD88 overexpression, whereas a mutant protein lacking the first DED (MC159 B) did not. We hypothesized that the MC159 protein targeted the IκB kinase (IKK) complex to inhibit these diverse signaling events. Indeed, the MC159 protein, but not MC159 B, coimmunoprecipitated with IKKγ. MC159 coimmunoprecipitated with IKKγ when using mouse embryonic fibroblasts that lack either IKKα or IKKβ, suggesting that the MC159 protein interacted directly with IKKγ. MC159-IKKγ coimmunoprecipitations were detected during infection of cells with either MCV isolated from human lesions or with a recombinant MC159-expressing vaccinia virus. MC159 also interacts with TRAF2, a signaling molecule involved in NF-κB activation. However, mutational analysis of MC159 failed to reveal a correlation between MC159-TRAF2 interactions and MC159's inhibitory function. We propose that MC159-IKK interactions, but not MC159-TRAF2 interactions, are responsible for inhibiting NF-κB activation.

Viral interference with innate immunity by preventing NF-κB activity

Viruses are the most abundant and diverse pathogens challenging the host immune system, and as such are a severe threat to human health. To this end, viruses have evolved multiple strategies to evade and subvert the host immune response. Host-pathogen interactions are usually initiated via recognition of pathogen-associated molecular patterns (PAMPs) by host sensors known as pattern recognition receptors (PRRs), which include, Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), NOD-like receptors (NLRs) and DNA receptors. Effective sensing of PAMPs rapidly triggers host immune responses, via activation of complex signalling pathways that culminates in the induction of inflammatory responses and the eradication of pathogens. Activation of the nuclear factor-κB (NF-κB) transcription pathway is crucial for the immediate early step of immune activation. This review discusses the recent evidence describing a variety of viral effectors that have been shown to prevent NF-κB signalling. Most of these viral effectors can be broadly classified into three categories based on the site of inhibition within the NF-κB pathway, that is, at the (i) TLRs, (ii) IKK complex or (iii) the transcriptional level.

Modulation of NF-κB signalling by microbial pathogens

The nuclear factor-κB (NF-κB) family of transcription factors plays a central part in the host response to infection by microbial pathogens, by orchestrating the innate and acquired host immune responses. The NF-κB proteins are activated by diverse signalling pathways that originate from many different cellular receptors and sensors. Many successful pathogens have acquired sophisticated mechanisms to regulate the NF-κB signalling pathways by deploying subversive proteins or hijacking the host signalling molecules. Here, we describe the mechanisms by which viruses and bacteria micromanage the host NF-κB signalling circuitry to favour the continued survival of the pathogen.

Orf virus ORFV121 encodes a novel inhibitor of NF-kappaB that contributes to virus virulence

Orf virus (ORFV), the type member of the genus Parapoxvirus of the Poxviridae, has evolved novel strategies (proteins and/or mechanisms of action) to modulate host cell responses regulated by the nuclear factor-κB (NF-κB) signaling pathway. Here, we present data indicating that ORFV ORFV121, a gene unique to parapoxviruses, encodes a novel viral NF-κB inhibitor that binds to and inhibits the phosphorylation and nuclear translocation of NF-κB-p65. The infection of cells with an ORFV121 deletion mutant virus (OV-IA82Δ121) resulted in increased NF-κB-mediated gene transcription, and the expression of ORFV121 in cell cultures significantly suppressed NF-κB-regulated reporter gene expression. ORFV ORFV121 physically interacts with NF-κB-p65 in the cell cytoplasm, thus providing a mechanism for the inhibition of NF-κB-p65 phosphorylation and nuclear translocation. Notably, the deletion of ORFV121 from the viral genome markedly decreased ORFV virulence and disease pathogenesis in sheep, indicating that ORFV121 is a virulence determinant for ORFV in the natural host.

A nuclear inhibitor of NF-kappaB encoded by a poxvirus

Poxviruses have evolved various strategies to inhibit cytoplasmic events leading to activation of the nuclear factor κB (NF-κB) signaling pathway, with individual viruses often encoding multiple NF-κB inhibitors. Here, the novel orf virus (ORFV)-encoded protein ORFV002 was shown to inhibit nuclear events regulating NF-κB transcriptional activity. ORFV002 expression in cell cultures significantly decreased wild-type-virus-, tumor necrosis factor alpha (TNF-α)-, and lipopolysaccharide (LPS)-induced NF-κB-mediated gene expression. Expression of ORFV002 in cells, while not affecting phosphorylation or nuclear translocation of NF-κB-p65, markedly decreased TNF-α- and wild-type-virus-induced acetylation of NF-κB-p65, a p300-mediated nuclear modification of NF-κB-p65 that regulates its transactivating activity. ORFV002 was shown to colocalize and interact with NF-κB-p65, and expression of ORFV002 in cell cultures resulted in a reduced interaction of NF-κB-p65 with p300, suggesting that ORFV002 interferes with NF-κB-p65/p300 association. Deletion of ORFV002 from the OV-IA82 genome had no significant effect on ORFV pathogenesis in sheep, indicating that ORFV002 is nonessential for virus virulence in the natural host. This represents the first description of a nuclear inhibitor of NF-κB encoded by a poxvirus.

Porcine reproductive and respiratory syndrome virus non-structural protein 1 suppresses tumor necrosis factor-alpha promoter activation by inhibiting NF-κB and Sp1

The objective of this study was to identify porcine reproductive and respiratory syndrome virus (PRRSV)-encoded proteins that are responsible for the inhibition of TNF-α expression and the mechanism(s) involved in this phenomenon. Using a TNF-α promoter reporter system, the non-structural protein 1 (Nsp1) was found to strongly suppress the TNF-α promoter activity. Such inhibition takes place especially at the promoter's proximal region. Both Nsp1α and Nsp1β, the two proteolytic fragments of Nsp1, were shown to be involved in TNF-α promoter suppression. Furthermore, using reporter plasmids specific for transcription factors (TFs) that bind to TNF-α promoter, Nsp1α and Nsp1β were demonstrated to inhibit the activity of the TFs that bind CRE-κB(3) and Sp1 elements respectively. Subsequent analyses showed that Nsp1α moderately inhibits NF-κB activation and that Nsp1β completely abrogates the Sp1 transactivation. These findings reveal one of the important mechanisms underlying the innate immune evasion by PRRSV during infection.

A novel inhibitor of the NF-{kappa}B signaling pathway encoded by the parapoxvirus orf virus

The parapoxvirus orf virus (ORFV) is a pathogen of sheep and goats that has been used as a preventive and therapeutic immunomodulatory agent in several animal species. However, the functions (genes, proteins, and mechanisms of action) evolved by ORFV to modulate and manipulate immune responses are poorly understood. Here, the novel ORFV protein ORFV024 was shown to inhibit activation of the NF-kappaB signaling pathway, an important modulator of early immune responses against viral infections. Infection of primary ovine cells with an ORFV024 deletion mutant virus resulted in a marked increase in expression of NF-kappaB-regulated chemokines and other proinflammatory host genes. Expression of ORFV024 in cell cultures significantly decreased lipopolysaccharide (LPS)- and tumor necrosis factor alpha (TNF-alpha)-induced NF-kappaB-responsive reporter gene expression. Further, ORFV024 expression decreased TNF-alpha-induced phosphorylation and nuclear translocation of NF-kappaB-p65, phosphorylation, and degradation of IkappaBalpha, and phosphorylation of IkappaB kinase (IKK) subunits IKKalpha and IKKbeta, indicating that ORFV024 functions by inhibiting activation of IKKs, the bottleneck for most NF-kappaB activating stimuli. Although ORFV024 interferes with activation of the NF-kappaB signaling pathway, its deletion from the OV-IA82 genome had no significant effect on disease severity, progression, and time to resolution in sheep, indicating that ORFV024 is not essential for virus virulence in the natural host. This represents the first description of a NF-kappaB inhibitor encoded by a parapoxvirus.

Co-regulation of NF-kappaB and inflammasome-mediated inflammatory responses by myxoma virus pyrin domain-containing protein M013

NF-kappaB and inflammasomes both play central roles in orchestrating anti-pathogen responses by rapidly inducing a variety of early-response cytokines and chemokines following infection. Myxoma virus (MYXV), a pathogenic poxvirus of rabbits, encodes a member of the cellular pyrin domain (PYD) superfamily, called M013. The viral M013 protein was previously shown to bind host ASC-1 protein and inhibit the cellular inflammasome complex that regulates the activation and secretion of caspase 1-regulated cytokines such as IL-1beta and IL-18. Here, we report that human THP-1 monocytic cells infected with a MYXV construct deleted for the M013L gene (vMyxM013-KO), in stark contrast to the parental MYXV, rapidly induce high levels of secreted pro-inflammatory cytokines like TNF, IL-6, and MCP-1, all of which are regulated by NF-kappaB. The induction of these NF-kappaB regulated cytokines following infection with vMyxM013-KO was also confirmed in vivo using THP-1 derived xenografts in NOD-SCID mice. vMyxM013-KO virus infection specifically induced the rapid phosphorylation of IKK and degradation of IkappaBalpha, which was followed by nuclear translocation of NF-kappaB/p65. Even in the absence of virus infection, transiently expressed M013 protein alone inhibited cellular NF-kappaB-mediated reporter gene expression and nuclear translocation of NF-kappaB/p65. Using protein/protein interaction analysis, we show that M013 protein also binds directly with cellular NF-kappaB1, suggesting a direct physical and functional linkage between NF-kappaB1 and ASC-1. We further demonstrate that inhibition of the inflammasome with a caspase-1 inhibitor did not prevent the induction of NF-kappaB regulated cytokines following infection with vMyxM013-KO virus, but did block the activation of IL-1beta. Thus, the poxviral M013 inhibitor exerts a dual immuno-subversive role in the simultaneous co-regulation of both the cellular inflammasome complex and NF-kappaB-mediated pro-inflammatory responses.