Immune-defense molecules of molluscum contagiosum virus, a human poxvirus

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.

MC159 of Molluscum Contagiosum Virus Suppresses Autophagy by Recruiting Cellular SH3BP4 via an SH3 Domain-Mediated Interaction

MC159 is a viral FLIP (FLICE inhibitory protein) encoded by the molluscum contagiosum virus (MCV) enabling MCV to evade antiviral immunity and to establish persistent infections in humans. Here, we show that MC159 contains a functional SH3 binding motif, which mediates avid and selective binding to SH3BP4, a signaling protein known to regulate endocytic trafficking and suppress cellular autophagy. The capacity to bind SH3BP4 was dispensable for regulation of NF-κB-mediated transcription and suppression of proapoptotic caspase activation but contributed to inhibition of amino acid starvation-induced autophagy by MC159. These results provide new insights into the cellular functions of MC159 and reveal SH3BP4 as a novel host cell factor targeted by a viral immune evasion protein.IMPORTANCE After the eradication of smallpox, molluscum contagiosum virus (MCV) is the only poxvirus restricted to infecting humans. MCV infection is common and causes benign skin lesions that usually resolve spontaneously but may persist for years and grow large, especially in immunocompromised individuals. While not life threatening, MCV infections pose a significant global health burden. No vaccine or specific anti-MCV therapy is available. MCV encodes several proteins that enable it to evade antiviral immunity, a notable example of which is the MC159 protein. In this study, we describe a novel mechanism of action for MC159 involving hijacking of a host cell protein called SH3BP4 to suppress autophagy, a cellular recycling mechanism important for antiviral immunity. This study contributes to our understanding of the host cell interactions of MCV and the molecular function of MC159.

Evolution of the Large Nucleocytoplasmic DNA Viruses of Eukaryotes and Convergent Origins of Viral Gigantism

The Nucleocytoplasmic Large DNA Viruses (NCLDV) of eukaryotes (proposed order "Megavirales") comprise an expansive group of eukaryotic viruses that consists of the families Poxviridae, Asfarviridae, Iridoviridae, Ascoviridae, Phycodnaviridae, Marseilleviridae, Pithoviridae, and Mimiviridae, as well as Pandoraviruses, Molliviruses, and Faustoviruses that so far remain unaccounted by the official virus taxonomy. All these viruses have double-stranded DNA genomes that range in size from about 100 kilobases (kb) to more than 2.5 megabases. The viruses with genomes larger than 500kb are informally considered "giant," and the largest giant viruses surpass numerous bacteria and archaea in both particle and genome size. The discovery of giant viruses has been highly unexpected and has changed the perception of viral size and complexity, and even, arguably, the entire concept of a virus. Given that giant viruses encode multiple proteins that are universal among cellular life forms and are components of the translation system, the quintessential cellular molecular machinery, attempts have been made to incorporate these viruses in the evolutionary tree of cellular life. Moreover, evolutionary scenarios of the origin of giant viruses from a fourth, supposedly extinct domain of cellular life have been proposed. However, despite all the differences in the genome size and gene repertoire, the NCLDV can be confidently defined as monophyletic group, on the strength of the presence of about 40 genes that can be traced back to their last common ancestor. Using several most strongly conserved genes from this ancestral set, a well-resolved phylogenetic tree of the NCLDV was built and employed as the scaffold to reconstruct the history of gene gain and loss throughout the course of the evolution of this group of viruses. This reconstruction reveals extremely dynamic evolution that involved extensive gene gain and loss in many groups of viruses and indicates that giant viruses emerged independently in several clades of the NCLDV. Thus, these giants of the virus world evolved repeatedly from smaller and simpler viruses, rather than from a fourth domain of cellular life, and captured numerous genes, including those for translation system components, from eukaryotes, along with some bacterial genes. Even deeper evolutionary reconstructions reveal apparent links between the NCLDV and smaller viruses of eukaryotes, such as adenoviruses, and ultimately, derive all these viruses from tailless bacteriophages.

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.

Functional Comparison of Molluscum Contagiosum Virus vFLIP MC159 with Murine Cytomegalovirus M36/vICA and M45/vIRA Proteins

Molluscum contagiosum virus (MCV) gene MC159 encodes a viral FLICE inhibitory protein (vFLIP) that inhibits caspase-8-mediated apoptosis. The MC159 protein was also reported to inhibit programmed necrosis (necroptosis) and modulate NF-κB activation by interacting with RIP1 and NEMO. The importance of MC159 during MCV infection has remained unknown, as there is no system for propagation and genetic manipulation of this virus. Here we investigated the functions of MC159 during viral infection using murine cytomegalovirus (MCMV) as a surrogate virus. MC159 was inserted into the MCMV genome, replacing M36 or M45, two MCMV genes with functions similar to those reported for MC159. M36 encodes a viral inhibitor of caspase-8-induced apoptosis (vICA) and M45 a viral inhibitor of RIP activation (vIRA), which inhibits RIP1/RIP3-mediated necroptosis. The M45 protein also blocks NF-κB activation by interacting with NEMO. When expressed by MCMV, MC159 blocked tumor necrosis factor alpha (TNF-α)-induced apoptosis of infected cells and partially restored MCMV replication in macrophages. However, MC159 did not fully replace M45, as it did not inhibit necroptosis in murine cells, but it reduced TNF-α-induced necroptosis in MCMV-infected human HT-29 cells. MC159 also differed from M45 in its effect on NF-κB. While MCMV-encoded M45 blocked NF-κB activation by TNF-α and interleukin-1β (IL-1β), MC159 inhibited TNF-α- but not IL-1β-induced NF-κB activation in infected mouse fibroblasts. These results indicate that the spectrum of MC159's functions differs depending on cell type and expression system and that a cell culture system for the propagation of MCV is needed to determine the biological relevance of presumed viral gene functions.

IMPORTANCE

MCV is a human-pathogenic poxvirus that cannot be propagated in cell culture or laboratory animals. Therefore, MCV gene products have been studied predominantly in cells expressing individual viral genes. In this study, we analyzed the function of the MCV gene MC159 by expressing it from a different virus and comparing its functions to those of two well-characterized MCMV genes. In this system, MC159 displayed some but not all of the previously described functions, suggesting that the functions of a viral gene depend on the conditions under which it is expressed. Until a cell culture system for the analysis of MCV becomes available, it might be necessary to analyze MCV genes in several different systems to extrapolate their biological importance.

Chordopoxvirus protein F12 implicated in enveloped virion morphogenesis is an inactivated DNA polymerase

Through the course of their evolution, viruses with large genomes have acquired numerous host genes, most of which perform function in virus reproduction in a manner that is related to their original activities in the cells, but some are exapted for new roles. Here we report the unexpected finding that protein F12, which is conserved among the chordopoxviruses and is implicated in the morphogenesis of enveloped intracellular virions, is a derived DNA polymerase, possibly of bacteriophage origin, in which the polymerase domain and probably the exonuclease domain have been inactivated. Thus, F12 appears to present a rare example of a drastic, exaptive functional change in virus evolution.

Reviewers: This article was reviewed by Frank Eisenhaber and Juergen Brosius. For complete reviews, go the Reviewers’ Reports section.

Poxviruses and the evolution of host range and virulence

Poxviruses as a group can infect a large number of animals. However, at the level of individual viruses, even closely related poxviruses display highly diverse host ranges and virulence. For example, variola virus, the causative agent of smallpox, is human-specific and highly virulent only to humans, whereas related cowpox viruses naturally infect a broad spectrum of animals and only cause relatively mild disease in humans. The successful replication of poxviruses depends on their effective manipulation of the host antiviral responses, at the cellular-, tissue- and species-specific levels, which constitutes a molecular basis for differences in poxvirus host range and virulence. A number of poxvirus genes have been identified that possess host range function in experimental settings, and many of these host range genes target specific antiviral host pathways. Herein, we review the biology of poxviruses with a focus on host range, zoonotic infections, virulence, genomics and host range genes as well as the current knowledge about the function of poxvirus host range factors and how their interaction with the host innate immune system contributes to poxvirus host range and virulence. We further discuss the evolution of host range and virulence in poxviruses as well as host switches and potential poxvirus threats for human and animal health.

Molluscum contagiosum virus infection

Molluscum contagiosum virus is an important human skin pathogen: it can cause disfigurement and suffering in children, in adults it is less common and often sexually transmitted. Extensive and persistent skin infection with the virus can indicate underlying immunodeficiency. Traditional ablative therapies have not been compared directly with newer immune-modulating and specific antiviral therapies. Advances in research raise the prospect of new approaches to treatment informed by the biology of the virus; in human skin, the infection is localised in the epidermal layers, where it induces a typical, complex hyperproliferative lesion with an abundance of virus particles but a conspicuous absence of immune effectors. Functional studies of the viral genome have revealed effects on cellular pathways involved in the cell cycle, innate immunity, inflammation, and cell death. Extensive lesions caused by molluscum contagiosum can occur in patients with DOCK8 deficiency-a genetic disorder affecting migration of dendritic and specialised T cells in skin. Sudden disappearance of lesions is the consequence of a vigorous immune response in healthy people. Further study of the unique features of infection with molluscum contagiosum virus could give fundamental insight into the nature of skin immunity.

Horizontal transfer and the evolution of host-pathogen interactions

Horizontal gene transfer has been long known in viruses and prokaryotes, but its importance in eukaryotes has been only acknowledged recently. Close contact between organisms, as it occurs between pathogens and their hosts, facilitates the occurrence of DNA transfer events. Once inserted in a foreign genome, DNA sequences have sometimes been coopted by pathogens to improve their survival or infectivity, or by hosts to protect themselves against the harm of pathogens. Hence, horizontal transfer constitutes a source of novel sequences that can be adopted to change the host-pathogen interactions. Therefore, horizontal transfer can have an important impact on the coevolution of pathogens and their hosts.

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.

The evolution of infectious agents in relation to sex in animals and humans: brief discussions of some individual organisms

The following series of concise summaries addresses the evolution of infectious agents in relation to sex in animals and humans from the perspective of three specific questions: (1) what have we learned about the likely origin and phylogeny, up to the establishment of the infectious agent in the genital econiche, including the relative frequency of its sexual transmission; (2) what further research is needed to provide additional knowledge on some of these evolutionary aspects; and (3) what evolutionary considerations might aid in providing novel approaches to the more practical clinical and public health issues facing us currently and in the future?

Evaluating the orthopoxvirus type I interferon-binding molecule as a vaccine target in the vaccinia virus intranasal murine challenge model

The biological threat imposed by orthopoxviruses warrants the development of safe and effective vaccines. We developed a candidate orthopoxvirus DNA-based vaccine, termed 4pox, which targets four viral structural components, A33, B5, A27, and L1. While this vaccine protects mice and nonhuman primates from lethal infections, we are interested in further enhancing its potency. One approach to enhance potency is to include additional orthopoxvirus immunogens. Here, we investigated whether vaccination with the vaccinia virus (VACV) interferon (IFN)-binding molecule (IBM) could protect BALB/c mice against lethal VACV challenge. We found that vaccination with this molecule failed to significantly protect mice from VACV when delivered alone. IBM modestly augmented protection when delivered together with the 4pox vaccine. All animals receiving the 4pox vaccine plus IBM lived, whereas only 70% of those receiving a single dose of 4pox vaccine survived. Mapping studies using truncated mutants revealed that vaccine-generated antibodies spanned the immunoglobulin superfamily domains 1 and 2 and, to a lesser extent, 3 of the IBM. These antibodies inhibited IBM cell binding and IFN neutralization activity, indicating that they were functionally active. This study shows that DNA vaccination with the VACV IBM results in a robust immune response but that this response does not significantly enhance protection in a high-dose challenge model.

Spontaneous regression of highly immunogenic Molluscum contagiosum virus (MCV)-induced skin lesions is associated with plasmacytoid dendritic cells and IFN-DC infiltration

Molluscum contagiosum virus (MCV) infection induces self-limiting cutaneous lesions in an immunocompetent host that can undergo spontaneous regression preceded by local inflammation. On histology, a large majority of MCV-induced lesions are characterized by islands of hyperplastic epithelium containing infected keratinocytes and surrounded by scarce inflammatory infiltrate. However, spontaneous regression has been associated with the occurrence of a dense inflammatory reaction. By histology and immunohistochemistry, we identified MCV-induced lesions showing a dense inflammatory infiltrate associated with cell death in keratinocytes (inflammatory Molluscum contagiosum (I-MC)). In I-MC, hyperplastic keratinocytes were highly immunogenic as demonstrated by the expression of major histocompatibility complex class I and II molecules. Immune cell infiltration consisted of numerous cytotoxic T cells admixed with natural killer cells and plasmacytoid dendritic cells (PDCs). Accordingly, a type I IFN signature associated with PDC infiltration was demonstrated in both keratinocytes and inflammatory cells. Among the latter, a cell population resembling IFN-DC (CD123(+)CD11c(+)CD16(+)CD14(+)MxA(+)) was identified in proximity to islands of apoptotic keratinocytes. In vitro-generated IFN-DCs expressed a strong cytotoxic signature, as demonstrated by high levels of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and Fas ligand (FasL). This study establishes a previously unreported model to underpin the role of innate immune cells in viral immune surveillance.

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

Poxviruses express proteins that limit host immune responses to infection. For example, the molluscum contagiosum virus MC160 protein inhibits tumor necrosis factor alpha (TNF-alpha)-induced NF-kappaB activation. This event correlates with MC160-induced IKK1 protein degradation, suggesting a mechanism for the above-mentioned phenotype. IKK1 is stabilized when it associates with the cellular heat shock protein 90 (Hsp90). Here, Hsp90 overexpression restored IKK1 levels in MC160-expressing cells, suggesting that MC160 competitively interacted with Hsp90. In support of this, further investigation showed that a mutant MC160 protein comprising only the C-terminal region (C protein) immunoprecipitated with Hsp90. In contrast, Hsp90 IP with a mutant MC160 protein consisting of only the N-terminal tandem death effector domains (DEDs) (N protein) was dramatically decreased. Since cells expressing either the N or C mutant MC160 protein remained similarly resistant to TNF-alpha-induced NF-kappaB activation, the N mutant protein probably utilized a different mechanism for inhibiting NF-kappaB. One likely mechanism for the N protein lies in its association with the DED-containing procaspase-8 protein, a cellular apoptosis precursor protein that regulates NF-kappaB activation. Here, IPs revealed that this association relied on the presence of the DED-containing N terminus of the MC160 protein but not the C-terminal portion. These interactions appear to have relevance with NF-kappaB activation, since the expression of the viral DEDs strongly inhibited procaspase-8-mediated NF-kappaB activation, an event not substantially altered by the C protein. Thus, the MC160 protein utilizes at least two distinct mechanisms for impeding NF-kappaB activation, association with Hsp90 to result in IKK1 protein degradation or interaction with procaspase-8.

Identification of multiple independent horizontal gene transfers into poxviruses using a comparative genomics approach

Background

Poxviruses are important pathogens of humans, livestock and wild animals. These large dsDNA viruses have a set of core orthologs whose gene order is extremely well conserved throughout poxvirus genera. They also contain many genes with sequence and functional similarity to host genes which were probably acquired by horizontal gene transfer.

Although phylogenetic trees can indicate the occurrence of horizontal gene transfer and even uncover multiple events, their use may be hampered by uncertainties in both the topology and the rooting of the tree. We propose to use synteny conservation around the horizontally transferred gene (HTgene) to distinguish between single and multiple events.

Results

Here we devise a method that incorporates comparative genomic information into the investigation of horizontal gene transfer, and we apply this method to poxvirus genomes. We examined the synteny conservation around twenty four pox genes that we identified, or which were reported in the literature, as candidate HTgenes. We found support for multiple independent transfers into poxviruses for five HTgenes. Three of these genes are known to be important for the survival of the virus in or out of the host cell and one of them increases susceptibility to some antiviral drugs.

Conclusion

In related genomes conserved synteny information can provide convincing evidence for multiple independent horizontal gene transfer events even in the absence of a robust phylogenetic tree for the HTgene.

Targeting leukocyte trafficking to inflamed skin - still an attractive therapeutic approach?

Research into leukocyte trafficking and its therapeutic exploitation appears to be a multistep process, just like the trafficking cascade itself. The initial euphoria evoked by an early understanding of the trafficking steps was followed by considerable disappointment following the clinical failure of the first selectin antagonist Cylexin (CY-1503), a sialyl Lewis(X) mimetic. The research area recovered and identified additional attractive pharmacological targets such as chemokine receptors and integrins. However, after lack of efficacy in anti-chemokine trials and the fatalities associated with anti VLA-4 therapy (Tysabri), the question arose again whether targeting leukocyte trafficking is really promising or whether such a complex, multistep process with many redundant and/or functionally overlapping molecules is simply too challenging to deal with. In this article, we delineate some pros and cons of this approach followed by a brief update on where we stand in the field and where we might move in the future.

Genome of crocodilepox virus

Here, we present the genome sequence, with analysis, of a poxvirus infecting Nile crocodiles (Crocodylus niloticus) (crocodilepox virus; CRV). The genome is 190,054 bp (62% G+C) and predicted to contain 173 genes encoding proteins of 53 to 1,941 amino acids. The central genomic region contains genes conserved and generally colinear with those of other chordopoxviruses (ChPVs). CRV is distinct, as the terminal 33-kbp (left) and 13-kbp (right) genomic regions are largely CRV specific, containing 48 unique genes which lack similarity to other poxvirus genes. Notably, CRV also contains 14 unique genes which disrupt ChPV gene colinearity within the central genomic region, including 7 genes encoding GyrB-like ATPase domains similar to those in cellular type IIA DNA topoisomerases, suggestive of novel ATP-dependent functions. The presence of 10 CRV proteins with similarity to components of cellular multisubunit E3 ubiquitin-protein ligase complexes, including 9 proteins containing F-box motifs and F-box-associated regions and a homologue of cellular anaphase-promoting complex subunit 11 (Apc11), suggests that modification of host ubiquitination pathways may be significant for CRV-host cell interaction. CRV encodes a novel complement of proteins potentially involved in DNA replication, including a NAD(+)-dependent DNA ligase and a protein with similarity to both vaccinia virus F16L and prokaryotic serine site-specific resolvase-invertases. CRV lacks genes encoding proteins for nucleotide metabolism. CRV shares notable genomic similarities with molluscum contagiosum virus, including genes found only in these two viruses. Phylogenetic analysis indicates that CRV is quite distinct from other ChPVs, representing a new genus within the subfamily Chordopoxvirinae, and it lacks recognizable homologues of most ChPV genes involved in virulence and host range, including those involving interferon response, intracellular signaling, and host immune response modulation. These data reveal the unique nature of CRV and suggest mechanisms of virus-reptile host interaction.

Evolutionary genomics of nucleo-cytoplasmic large DNA viruses

A previous comparative-genomic study of large nuclear and cytoplasmic DNA viruses (NCLDVs) of eukaryotes revealed the monophyletic origin of four viral families: poxviruses, asfarviruses, iridoviruses, and phycodnaviruses [Iyer, L.M., Aravind, L., Koonin, E.V., 2001. Common origin of four diverse families of large eukaryotic DNA viruses. J. Virol. 75 (23), 11720-11734]. Here we update this analysis by including the recently sequenced giant genome of the mimiviruses and several additional genomes of iridoviruses, phycodnaviruses, and poxviruses. The parsimonious reconstruction of the gene complement of the ancestral NCLDV shows that it was a complex virus with at least 41 genes that encoded the replication machinery, up to four RNA polymerase subunits, at least three transcription factors, capping and polyadenylation enzymes, the DNA packaging apparatus, and structural components of an icosahedral capsid and the viral membrane. The phylogeny of the NCLDVs is reconstructed by cladistic analysis of the viral gene complements, and it is shown that the two principal lineages of NCLDVs are comprised of poxviruses grouped with asfarviruses and iridoviruses grouped with phycodnaviruses-mimiviruses. The phycodna-mimivirus grouping was strongly supported by several derived shared characters, which seemed to rule out the previously suggested basal position of the mimivirus [Raoult, D., Audic, S., Robert, C., Abergel, C., Renesto, P., Ogata, H., La Scola, B., Suzan, M., Claverie, J.M. 2004. The 1.2-megabase genome sequence of Mimivirus. Science 306 (5700), 1344-1350]. These results indicate that the divergence of the major NCLDV families occurred at an early stage of evolution, prior to the divergence of the major eukaryotic lineages. It is shown that subsequent evolution of the NCLDV genomes involved lineage-specific expansion of paralogous gene families and acquisition of numerous genes via horizontal gene transfer from the eukaryotic hosts, other viruses, and bacteria (primarily, endosymbionts and parasites). Amongst the expansions, there are multiple families of predicted virus-specific signaling and regulatory domains. Most NCLDVs have also acquired large arrays of genes related to ubiquitin signaling, and the animal viruses in particular have independently evolved several defenses against apoptosis and immune response, including growth factors and potential inhibitors of cytokine signaling. The mimivirus displays an enormous array of genes of bacterial provenance, including a representative of a new class of predicted papain-like peptidases. It is further demonstrated that a significant number of genes found in NCLDVs also have homologs in bacteriophages, although a vertical relationship between the NCLDVs and a particular bacteriophage group could not be established. On the basis of these observations, two alternative scenarios for the origin of the NCLDVs and other groups of large DNA viruses of eukaryotes are considered. One of these scenarios posits an early assembly of an already large DNA virus precursor from which various large DNA viruses diverged through an ongoing process of displacement of the original genes by xenologous or non-orthologous genes from various sources. The second scenario posits convergent emergence, on multiple occasions, of large DNA viruses from small plasmid-like precursors through independent accretion of similar sets of genes due to strong selective pressures imposed by their life cycles and hosts.

The MC160 protein expressed by the dermatotropic poxvirus molluscum contagiosum virus prevents tumor necrosis factor alpha-induced NF-kappaB activation via inhibition of I kappa kinase complex formation

The pluripotent cytokine tumor necrosis factor alpha (TNF-alpha) binds to its cognate TNF receptor I (TNF-RI) to stimulate inflammation via activation of the NF-kappaB transcription factor. To prevent the detrimental effects of TNF-alpha in keratinocytes infected with the molluscum contagiosum virus (MCV), this poxvirus is expected to produce proteins that block at least one step of the TNF-RI signal transduction pathway. One such product, the MC160 protein, is predicted to interfere with this cellular response because of its homology to other proteins that regulate TNF-RI-mediated signaling. We report here that expression of MC160 molecules did significantly reduce TNF-alpha-mediated NF-kappaB activation in 293T cells, as measured by gene reporter and gel mobility shift assays. Since we observed that MC160 decreased other NF-kappaB activation pathways, namely those activated by receptor-interacting protein, TNF receptor-associated factor 2, NF-kappaB-inducing kinase, or MyD88, we hypothesized that the MC160 product interfered with I kappa kinase (IKK) activation, an event common to multiple signal transduction pathways. Indeed, MC160 protein expression was associated with a reduction in in vitro IKK kinase activity and IKK subunit phosphorylation. Further, IKK1-IKK2 interactions were not detected in MC160-expressing cells, under conditions demonstrated to induce IKK complex formation, but interactions between the MC160 protein and the major IKK subunits were undetectable. Surprisingly, MC160 expression correlated with a decrease in IKK1, but not IKK2 levels, suggesting a mechanism for MC160 disruption of IKK1-IKK2 interactions. MCV has probably retained its MC160 gene to inhibit NF-kappaB activation by interfering with signaling via multiple biological mediators. In the context of an MCV infection in vivo, MC160 protein expression may dampen the cellular production of proinflammatory molecules and enhance persistent infections in host keratinocytes.

The TRAF3-binding site of human molluscipox virus FLIP molecule MC159 is critical for its capacity to inhibit Fas-induced apoptosis

Members of the viral Flice/caspase-8 inhibitory protein (v-FLIP) family prevent induction of apoptosis by death receptors through inhibition of the processing and activation of procaspase-8 and -10 at the level of the receptor-associated death-inducing signaling complex (DISC). Here, we have addressed the molecular function of the v-FLIP member MC159 of the human molluscum contagiosum virus. MC159 FLIP powerfully inhibited both caspase-dependent and caspase-independent cell death induced by Fas. The C-terminal region of MC159 bound TNF receptor-associated factor (TRAF)3, was necessary for optimal TRAF2 binding, and mediated the recruitment of both TRAFs into the Fas DISC. TRAF-binding-deficient mutants of MC159 showed impaired inhibition of FasL-induced caspase-8 processing and Fas internalization, and had reduced antiapoptotic activity. Our findings provide evidence that a MC159/TRAF2/TRAF3 complex regulates a new aspect of Fas signaling, and identify MC159 FLIP as a molecule that targets multiple features of Fas-induced cell death.

Molluscum contagiosum prevents progression of staphylococcal scalded skin syndrome

Prevention of progression of staphylococcal scalded skin syndrome by molluscum contagiosum indicated a possible interference by viral anti-cytokine molecule such as interleukin-18 binding protein.

Viral mimicry of cytokines, chemokines and their receptors

Viruses have evolved elegant mechanisms to evade detection and destruction by the host immune system. One of the evasion strategies that have been adopted by large DNA viruses is to encode homologues of cytokines, chemokines and their receptors--molecules that have a crucial role in control of the immune response. Viruses have captured host genes or evolved genes to target specific immune pathways, and so viral genomes can be regarded as repositories of important information about immune processes, offering us a viral view of the host immune system. The study of viral immunomodulatory proteins might help us to uncover new human genes that control immunity, and their characterization will increase our understanding of not only viral pathogenesis, but also normal immune mechanisms. Moreover, viral proteins indicate strategies of immune modulation that might have therapeutic potential.

Poxvirus immune modulators: functional insights from animal models

Poxviruses express several different classes of immune modulators that suppress the host response to infection, including soluble cytokine binding proteins, serpins, chemokine binding proteins, a complement control protein, and members of the semaphorin and Toll/IL-1 receptor families. Biochemical activity of these proteins has been demonstrated by many in vitro studies. Conservation in evolution of poxvirus immune modulators implies that these genes are functional in vivo, but the results of infecting animals with knockout viruses have not always been clear cut. Studies involving different animal models are reviewed, and the criteria for suitable models are discussed. Challenges include finding an appropriate animal host, and using an inoculation route that resembles the process of natural infection. The fact that multiple immune modulators can target the same pathway at different steps may explain why single knockout mutants are not always attenuated in animals.

A time to kill: viral manipulation of the cell death program

Many viruses have as part of their arsenal the ability to modulate the apoptotic pathways of the host. It is counter-intuitive that such simple organisms would be efficient at regulating this the most crucial pathway within the host, given the relative complexity of the host cells. Yet, viruses have the potential to initiate or stay the onset of programmed cell death through the manipulation of a variety of key apoptotic proteins. It is the intention of this review to provide an overview of viral gene products that are able to promote or inhibit apoptotic death of the host cell and to discuss their mechanisms of action. It is not until recently that the depth at which viruses exploit the apoptotic pathways of their host has been seen. This understanding may provide a great opportunity for future therapeutic ventures.

MC159L protein from the poxvirus molluscum contagiosum virus inhibits NF-kappaB activation and apoptosis induced by PKR

Molluscum contagiosum virus (MCV) is a human poxvirus that causes abnormal proliferation of epithelial cells. MCV encodes specific molecules to control host defences, such as MC159L, which as previously shown prevents apoptosis induced by death receptors. However, unlike most poxviruses, MCV lacks a homologue to the E3L and K3L proteins of vaccinia virus, which are involved in the control of the key antiviral and pro-apoptotic dsRNA-dependent protein kinase, PKR. In this study, we analysed the relationship of MC159L to PKR. We found that MC159L is not a direct inhibitor of PKR since it does not associate with PKR and cannot block PKR-induced phosphorylation of eIF-2alpha. However, expression of MC159L inhibits apoptosis triggered by PKR through death receptor-mediated pathways. In addition, MC159L inhibits NF-kappaB activation induced in response to PKR. Expression of MC159L cannot counteract the PKR-mediated antiviral action in the context of a poxvirus infection, despite its ability to affect these signalling events. These findings show that MC159L is able to interfere with downstream events triggered by PKR in the absence of a direct physical interaction, and assign a role to MC159L in the control of some PKR-mediated biological effects.

Correspondence of the functional epitopes of poxvirus and human interleukin-18-binding proteins

Molluscum contagiosum virus, a human poxvirus that causes persistent small benign skin tumors, encodes a variety of putative immune defense proteins. Three such proteins, MC51L, MC53L, and MC54L, have 20 to 35% amino acid sequence identities with human interleukin-18 (hIL-18)-binding protein (hIL-18BP), a naturally occurring antagonist of the proinflammatory cytokine IL-18. We previously demonstrated that seven amino acids within the immunoglobulin-like domain of hIL-18BP were important for high-affinity binding to hIL-18. Model building indicated that MC54L, which has been shown to bind hIL-18, contains five of the seven amino acids at corresponding positions in its immunoglobulin-like domain, the exceptions being the conservative substitution of isoleucine for a leucine and the nonconservative substitution of valine for a phenylalanine. We found that individual alanine substitutions for these six identical or highly conserved amino acids of MC54L caused changes in affinity and binding free energy for hIL-18 that were quantitatively similar to those produced by mutagenesis of hIL-18BP. Furthermore, when the nonconserved valine of MC54L was mutated to phenylalanine, making it more like hIL-18BP, its affinity for hIL-18 increased more than 10-fold. In addition, the carboxyl-terminal half of MC54L, which has no similarity with hIL-18BP, was dispensable for hIL-18 binding. Thus, despite their relatively low overall sequence identity, MC54L and hIL-18BP have similar hIL-18 binding sites and functional epitopes. On the other hand, MC51L and MC53L have nonconservative substitutions of three to six of the seven critical amino acids of hIL-18BP and neither protein bound hIL-18, suggesting that they may interact with unidentified ligands.