Role of murine cytomegalovirus US22 gene family members in replication in macrophages

No Time to Die: How Cytomegaloviruses Suppress Apoptosis, Necroptosis, and Pyroptosis

Viruses are obligate intracellular pathogens as their replication depends on the metabolism of the host cell. The induction of cellular suicide, known as programmed cell death (PCD), has the potential to hinder viral replication and act as a first line of defense against viral pathogens. Apoptosis, necroptosis, and pyroptosis are three important PCD modalities. Different signaling pathways are involved in their execution, and they also differ in their ability to cause inflammation. Cytomegaloviruses (CMV), beta-herpesviruses with large double-stranded DNA genomes, encode a great variety of immune evasion genes, including several cell death suppressors. While CMV inhibitors of apoptosis and necroptosis have been known and studied for years, the first pyroptosis inhibitor has been identified and characterized only recently. Here, we describe how human and murine CMV interfere with apoptosis, necroptosis, and pyroptosis signaling pathways. We also discuss the importance of the different PCD forms and their viral inhibitors for the containment of viral replication and spread in vivo.

Cytomegalovirus inhibitors of programmed cell death restrict antigen cross-presentation in the priming of antiviral CD8 T cells

CD8 T cells are the predominant effector cells of adaptive immunity in preventing cytomegalovirus (CMV) multiple-organ disease caused by cytopathogenic tissue infection. The mechanism by which CMV-specific, naïve CD8 T cells become primed and clonally expand is of fundamental importance for our understanding of CMV immune control. For CD8 T-cell priming, two pathways have been identified: direct antigen presentation by infected professional antigen-presenting cells (pAPCs) and antigen cross-presentation by uninfected pAPCs that take up antigenic material derived from infected tissue cells. Studies in mouse models using murine CMV (mCMV) and precluding either pathway genetically or experimentally have shown that, in principle, both pathways can congruently generate the mouse MHC/H-2 class-I-determined epitope-specificity repertoire of the CD8 T-cell response. Recent studies, however, have shown that direct antigen presentation is the canonical pathway when both are accessible. This raised the question of why antigen cross-presentation is ineffective even under conditions of high virus replication thought to provide high amounts of antigenic material for feeding cross-presenting pAPCs. As delivery of antigenic material for cross-presentation is associated with programmed cell death, and as CMVs encode inhibitors of different cell death pathways, we pursued the idea that these inhibitors restrict antigen delivery and thus CD8 T-cell priming by cross-presentation. To test this hypothesis, we compared the CD8 T-cell responses to recombinant mCMVs lacking expression of the apoptosis-inhibiting protein M36 or the necroptosis-inhibiting protein M45 with responses to wild-type mCMV and revertant viruses expressing the respective cell death inhibitors. The data reveal that increased programmed cell death improves CD8 T-cell priming in mice capable of antigen cross-presentation but not in a mutant mouse strain unable to cross-present. These findings strongly support the conclusion that CMV cell death inhibitors restrict the priming of CD8 T cells by antigen cross-presentation.

Construction and Characterization of a High-Capacity Replication-Competent Murine Cytomegalovirus Vector for Gene Delivery

We investigated the basic characteristics of a new murine cytomegalovirus (MCMV) vector platform. Using BAC technology, we engineered replication-competent recombinant MCMVs with deletions of up to 26% of the wild-type genome. To this end, we targeted five gene blocks (m01-m17, m106-m109, m129-m141, m144-m158, and m159-m170). BACs featuring deletions from 18% to 26% of the wild-type genome exhibited delayed virus reconstitution, while smaller deletions (up to 16%) demonstrated reconstitution kinetics similar to those of the wild type. Utilizing an innovative methodology, we introduced large genomic DNA segments, up to 35 kbp, along with reporter genes into a newly designed vector with a potential cloning capacity of 46 kbp (Q4). Surprisingly, the insertion of diverse foreign DNAs alleviated the delayed plaque formation phenotype of Q4, and these large inserts remained stable through serial in vitro passages. With reporter-gene-expressing recombinant MCMVs, we successfully transduced not only mouse cell lines but also non-rodent mammalian cells, including those of human, monkey, bovine, and bat origin. Remarkably, even non-mammalian cell lines derived from chickens exhibited successful transduction.

Human cytomegalovirus UL36 inhibits IRF3-dependent immune signaling to counterbalance its immunoenhancement as apoptotic inhibitor

Apoptotic inhibition and immune evasion have particular importance to efficient viral infection, while a dilemma often faced by viruses is that inhibiting apoptosis can up-regulate antiviral immune signaling. Herein, we uncovered that in addition to inhibiting caspase-8/extrinsic apoptosis, human cytomegalovirus (HCMV)-encoded UL36 suppresses interferon regulatory factor 3 (IRF3)-dependent immune signaling by directly targeting IRF3 to abrogate IRF3 interaction with stimulator of interferon genes or TANK-binding kinase 1 and inhibit IRF3 phosphorylation/activation. Although UL36-mediated caspase-8/extrinsic apoptosis inhibition enhances immune signaling, the immunosuppressing activity of UL36 counterbalances this immunoenhancing "side effect" undesirable for virus. Furthermore, we used mutational analyses to show that only the wild-type, but not the UL36 mutant losing either inhibitory activity, is sufficient to support effective HCMV replication in cells, showing the functional importance of the dual inhibition by UL36 for the HCMV life cycle. Together, our findings demonstrate a sophisticated mechanism by which HCMV tightly controls innate immune signaling and extrinsic apoptosis for efficient infection.

Human cytomegalovirus in cancer: the mechanism of HCMV-induced carcinogenesis and its therapeutic potential

Cancer is one of the leading causes of death worldwide. Human cytomegalovirus (HCMV), a well-studied herpesvirus, has been implicated in malignancies derived from breast, colorectal muscle, brain, and other cancers. Intricate host-virus interactions are responsible for the cascade of events that have the potential to result in the transformed phenotype of normal cells. The HCMV genome contains oncogenes that may initiate these types of cancers, and although the primary HCMV infection is usually asymptomatic, the virus remains in the body in a latent or persistent form. Viral reactivation causes severe health issues in immune-compromised individuals, including cancer patients, organ transplants, and AIDS patients. This review focuses on the immunologic mechanisms and molecular mechanisms of HCMV-induced carcinogenesis, methods of HCMV treatment, and other studies. Studies show that HCMV DNA and virus-specific antibodies are present in many types of cancers, implicating HCMV as an important player in cancer progression. Importantly, many clinical trials have been initiated to exploit HCMV as a therapeutic target for the treatment of cancer, particularly in immunotherapy strategies in the treatment of breast cancer and glioblastoma patients. Taken together, these findings support a link between HCMV infections and cellular growth that develops into cancer. More importantly, HCMV is the leading cause of birth defects in newborns, and infection with HCMV is responsible for abortions in pregnant women.

Programmed Necrosis in Host Defense

Host control over infectious disease relies on the ability of cells in multicellular organisms to detect and defend against pathogens to prevent disease. Evolution affords mammals with a wide variety of independent immune mechanisms to control or eliminate invading infectious agents. Many pathogens acquire functions to deflect these immune mechanisms and promote infection. Following successful invasion of a host, cell autonomous signaling pathways drive the production of inflammatory cytokines, deployment of restriction factors and induction of cell death. Combined, these innate immune mechanisms attract dendritic cells, neutrophils and macrophages as well as innate lymphoid cells such as natural killer cells that all help control infection. Eventually, the development of adaptive pathogen-specific immunity clears infection and provides immune memory of the encounter. For obligate intracellular pathogens such as viruses, diverse cell death pathways make a pivotal contribution to early control by eliminating host cells before progeny are produced. Pro-apoptotic caspase-8 activity (along with caspase-10 in humans) executes extrinsic apoptosis, a nonlytic form of cell death triggered by TNF family death receptors (DRs). Over the past two decades, alternate extrinsic apoptosis and necroptosis outcomes have been described. Programmed necrosis, or necroptosis, occurs when receptor interacting protein kinase 3 (RIPK3) activates mixed lineage kinase-like (MLKL), causing cell leakage. Thus, activation of DRs, toll-like receptors (TLRs) or pathogen sensor Z-nucleic acid binding protein 1 (ZBP1) initiates apoptosis as well as necroptosis if not blocked by virus-encoded inhibitors. Mammalian cell death pathways are blocked by herpesvirus- and poxvirus-encoded cell death suppressors. Growing evidence has revealed the importance of Z-nucleic acid sensor, ZBP1, in the cell autonomous recognition of both DNA and RNA virus infection. This volume will explore the detente between viruses and cells to manage death machinery and avoid elimination to support dissemination within the host animal.

Human Cytomegalovirus UL24 and UL43 Cooperate to Modulate the Expression of Immunoregulatory UL16 Binding Protein 1

The human cytomegalovirus (HCMV) UL24 and UL43 are tegument proteins that have recently been shown to interact with each other in a yeast two-hybrid system. By their overexpression in MRC5 cells, we demonstrate that these viral proteins interact with several important host proteins, especially Dicer and trans-activation response RNA binding protein. As these hots proteins are involved in regulating the production of cellular micro-RNAs, the cytomegalovirus (CMV) proteins could interfere with their actions to favor viral replication directly or through an immune escape mechanism. Double knockout of UL24 and UL43 does not show a remarkable effect on CMV entry or replication, but it significantly downregulates the expression of CMV-encoded miR-UL59, which is thought to regulate the expression of a downstream target UL16 binding protein 1 (ULBP1). Interestingly, the double knockout increases the expression of the ULBP1 recognized by the NKG2D activating receptor of natural killer cells. This study investigates the potential role of several proteins encoded by HCMV in regulating the host cellular environment to favor escape from immunity, and it also provides some basis for the future development of RNA-targeted small molecules to control HCMV infection.

Antagonism of Protein Kinase R by Large DNA Viruses

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

Characterization of M116.1p, a murine cytomegalovirus protein required for efficient infection of mononuclear phagocytes

Broad tissue tropism of cytomegaloviruses (CMVs) is facilitated by different glycoprotein entry complexes, which are conserved between human CMV (HCMV) and murine CMV (MCMV). Among the wide array of cell types susceptible to the infection, mononuclear phagocytes (MNPs) play a unique role in the pathogenesis of the infection as they contribute both to the virus spread and immune control. CMVs have dedicated numerous genes for the efficient infection and evasion of macrophages and dendritic cells. In this study, we have characterized the properties and function of M116, a previously poorly described but highly transcribed MCMV gene region which encodes M116.1p, a novel protein necessary for the efficient infection of MNPs and viral spread in vivo. Our study further revealed that M116.1p shares similarities with its positional homologs in HCMV and RCMV, UL116 and R116, respectively, such as late kinetics of expression, N-glycosylation, localization to the virion assembly compartment, and interaction with gH - a member of the CMVs fusion complex. This study, therefore, expands our knowledge about virally encoded glycoproteins that play important roles in viral infectivity and tropism. Importance Human cytomegalovirus (HCMV) is a species-specific herpesvirus that causes severe disease in immunocompromised individuals and immunologically immature neonates. Murine cytomegalovirus (MCMV) is biologically similar to HCMV, and it serves as a widely used model for studying the infection, pathogenesis, and immune responses to HCMV. In our previous work, we have identified the M116 ORF as one of the most extensively transcribed regions of the MCMV genome without an assigned function. This study shows that the M116 locus codes for a novel protein, M116.1p, which shares similarities with UL116 and R116 in HCMV and RCMV, respectively, and is required for the efficient infection of mononuclear phagocytes and virus spread in vivo. Furthermore, this study establishes the α-M116 monoclonal antibody and MCMV mutants lacking M116, generated in this work, as valuable tools for studying the role of macrophages and dendritic cells in limiting CMV infection following different MCMV administration routes.

Species-Specific Inhibition of Necroptosis by HCMV UL36

Viral infection activates cellular antiviral defenses including programmed cell death (PCD). Many viruses, particularly those of the Herpesviridae family, encode cell death inhibitors that antagonize different forms of PCD. While some viral inhibitors are broadly active in cells of different species, others have species-specific functions, probably reflecting the co-evolution of the herpesviruses with their respective hosts. Human cytomegalovirus (HCMV) protein UL36 is a dual cell death pathway inhibitor. It blocks death receptor-dependent apoptosis by inhibiting caspase-8 activation, and necroptosis by binding to the mixed lineage kinase domain-like (MLKL) protein and inducing its degradation. While UL36 has been shown to inhibit apoptosis in human and murine cells, the specificity of its necroptosis-inhibiting function has not been investigated. Here we show that UL36 interacts with both human and murine MLKL, but has a higher affinity for human MLKL. When expressed by a recombinant mouse cytomegalovirus (MCMV), UL36 caused a modest reduction of murine MLKL levels but did not inhibit necroptosis in murine cells. These data suggest that UL36 inhibits necroptosis, but not apoptosis, in a species-specific manner, similar to ICP6 of herpes simplex virus type 1 and MC159 of molluscum contagiosum virus. Species-specific necroptosis inhibition might contribute to the narrow host range of these viruses.

Multiple Autonomous Cell Death Suppression Strategies Ensure Cytomegalovirus Fitness

Programmed cell death pathways eliminate infected cells and regulate infection-associated inflammation during pathogen invasion. Cytomegaloviruses encode several distinct suppressors that block intrinsic apoptosis, extrinsic apoptosis, and necroptosis, pathways that impact pathogenesis of this ubiquitous herpesvirus. Here, we expanded the understanding of three cell autonomous suppression mechanisms on which murine cytomegalovirus relies: (i) M38.5-encoded viral mitochondrial inhibitor of apoptosis (vMIA), a BAX suppressor that functions in concert with M41.1-encoded viral inhibitor of BAK oligomerization (vIBO), (ii) M36-encoded viral inhibitor of caspase-8 activation (vICA), and (iii) M45-encoded viral inhibitor of RIP/RHIM activation (vIRA). Following infection of bone marrow-derived macrophages, the virus initially deflected receptor-interacting protein kinase (RIPK)3-dependent necroptosis, the most potent of the three cell death pathways. This process remained independent of caspase-8, although suppression of this apoptotic protease enhances necroptosis in most cell types. Second, the virus deflected TNF-mediated extrinsic apoptosis, a pathway dependent on autocrine TNF production by macrophages that proceeds independently of mitochondrial death machinery or RIPK3. Third, cytomegalovirus deflected BCL-2 family protein-dependent mitochondrial cell death through combined TNF-dependent and -independent signaling even in the absence of RIPK1, RIPK3, and caspase-8. Furthermore, each of these cell death pathways dictated a distinct pattern of cytokine and chemokine activation. Therefore, cytomegalovirus employs sequential, non-redundant suppression strategies to specifically modulate the timing and execution of necroptosis, extrinsic apoptosis, and intrinsic apoptosis within infected cells to orchestrate virus control and infection-dependent inflammation. Virus-encoded death suppressors together hold control over an intricate network that upends host defense and supports pathogenesis in the intact mammalian host.

Rapid adaptation to human protein kinase R by a unique genomic rearrangement in rhesus cytomegalovirus

Cytomegaloviruses (CMVs) are generally unable to cross species barriers, in part because prolonged coevolution with one host species limits their ability to evade restriction factors in other species. However, the limitation in host range is incomplete. For example, rhesus CMV (RhCMV) can replicate in human cells, albeit much less efficiently than in rhesus cells. Previously we reported that the protein kinase R (PKR) antagonist encoded by RhCMV, rTRS1, has limited activity against human PKR but is nonetheless necessary and sufficient to enable RhCMV replication in human fibroblasts (HF). We now show that knockout of PKR in human cells or treatment with the eIF2B agonist ISRIB, which overcomes the translational inhibition resulting from PKR activation, augments RhCMV replication in HF, indicating that human PKR contributes to the inefficiency of RhCMV replication in HF. Serial passage of RhCMV in HF reproducibly selected for viruses with improved ability to replicate in human cells. The evolved viruses contain an inverted duplication of the terminal 6.8 kb of the genome, including rTRS1. The duplication replaces ~11.8 kb just downstream of an internal sequence element, pac1-like, which is very similar to the pac1 cleavage and packaging signal found near the terminus of the genome. Plaque-purified evolved viruses produced at least twice as much rTRS1 as the parental RhCMV and blocked the PKR pathway more effectively in HF. Southern blots revealed that unlike the parental RhCMV, viruses with the inverted duplication isomerize in a manner similar to HCMV and other herpesviruses that have internal repeat sequences. The apparent ease with which this duplication event occurs raises the possibility that the pac1-like site, which is conserved in Old World monkey CMV genomes, may serve a function in facilitating rapid adaptation to evolutionary obstacles.

TNF Signaling Dictates Myeloid and Non-Myeloid Cell Crosstalk to Execute MCMV-Induced Extrinsic Apoptosis

Cytomegaloviruses all encode the viral inhibitor of caspase-8-induced apoptosis (vICA). After binding to this initiator caspase, vICA blocks caspase-8 proteolytic activity and ability to activate caspase-3 and/or caspase-7. In this manner, vICA has long been known to prevent apoptosis triggered via tumor necrosis factor (TNF) family death receptor-dependent extrinsic signaling. Here, we employ fully wild-type murine cytomegalovirus (MCMV) and vICA-deficient MCMV (∆M36) to investigate the contribution of TNF signaling to apoptosis during infection of different cell types. ∆M36 shows the expected ability to kill mouse splenic hematopoietic cells, bone marrow-derived macrophages (BMDM), and dendritic cells (BMDC). Antibody blockade or genetic elimination of TNF protects myeloid cells from death, and caspase-8 activation accompanies cell death. Interferons, necroptosis, and pyroptotic gasdermin D (GSDMD) do not contribute to myeloid cell death. Human and murine fibroblasts or murine endothelial cells (SVEC4-10) normally insensitive to TNF become sensitized to ∆M36-induced apoptosis when treated with TNF or TNF-containing BMDM-conditioned medium. We demonstrate that myeloid cells are the natural source of TNF that triggers apoptosis in either myeloid (autocrine) or non-myeloid cells (paracrine) during ∆M36 infection of mice. Caspase-8 suppression by vICA emerges as key to subverting innate immune elimination of a wide variety of infected cell types.

Murine cytomegaloviruses m139 targets DDX3 to curtail interferon production and promote viral replication

Cytomegaloviruses (CMV) infect many different cell types and tissues in their respective hosts. Monocytes and macrophages play an important role in CMV dissemination from the site of infection to target organs. Moreover, macrophages are specialized in pathogen sensing and respond to infection by secreting cytokines and interferons. In murine cytomegalovirus (MCMV), a model for human cytomegalovirus, several genes required for efficient replication in macrophages have been identified, but their specific functions remain poorly understood. Here we show that MCMV m139, a gene of the conserved US22 gene family, encodes a protein that interacts with the DEAD box helicase DDX3, a protein involved in pathogen sensing and interferon (IFN) induction, and the E3 ubiquitin ligase UBR5. DDX3 and UBR5 also participate in the transcription, processing, and translation of a subset of cellular mRNAs. We show that m139 inhibits DDX3-mediated IFN-α and IFN-β induction and is necessary for efficient viral replication in bone-marrow derived macrophages. In vivo, m139 is crucial for viral dissemination to local lymph nodes and to the salivary glands. An m139-deficient MCMV also replicated to lower titers in SVEC4-10 endothelial cells. This replication defect was not accompanied by increased IFN-β transcription, but was rescued by knockout of either DDX3 or UBR5. Moreover, m139 co-localized with DDX3 and UBR5 in viral replication compartments in the cell nucleus. These results suggest that m139 inhibits DDX3-mediated IFN production in macrophages and antagonizes DDX3 and UBR5-dependent functions related to RNA metabolism in endothelial cells.

Human cytomegalovirus is an opportunistic pathogen that causes severe infections in immunocompromised individuals. The virus infects certain cell types, such as macrophages and endothelial cells, to ensure its dissemination within the body. Little is known about the viral factors that promote a productive infection of these cell types. The identification of critical viral factors and the molecular pathways they target can lead to the development of novel antiviral treatment strategies. Using the mouse cytomegalovirus as a model, we studied the viral m139 gene, which is important for virus replication in macrophages and endothelial cells and for dissemination in the mouse. This gene encodes a protein that interacts with the host proteins DDX3 and UBR5. Both proteins are involved in gene expression, and the RNA helicase DDX3 also participates in mounting an innate antiviral response. By interacting with DDX3 and UBR5, m139 ensures efficient viral replication in endothelial cells. Importantly, we identify m139 as a new viral DDX3 inhibitor, which curtails the production of interferon by macrophages.

Identification and functional analyses of a cell-death inhibitor encoded by guinea pig cytomegalovirus gp38.1 in cell culture and in animals

Cytomegaloviruses (CMVs) employ an array of strategies designed to interfere with host defence responses against pathogens. Studies on such evasion mechanisms are important for understanding the pathogenesis of CMV diseases. Although guinea pig CMV (GPCMV) provides a useful animal model for congenital CMV infection, its evasion strategies are not fully elucidated. Here, we analysed a genome locus that may encode gene products for the GPCMV evasion mechanisms and found the following. (1) RACE analyses identified five transcripts in the GP38-gp38.4 locus, one of which was a spliced product encoding gp38.1. Similarities in the splicing pattern and gene position of gp38.1 to human CMV UL37 and its exon 1 encoding vMIA (viral mitochondria-localized inhibitor of apoptosis) suggest that the gp38.1 gene encodes an apoptosis inhibitor. (2) In a transient transfection assay, gp38.1 localized in the mitochondria and relocated BAX from the cytoplasm to the mitochondria, although its co-localization with BAK was not evident. Further, the expression of gp38.1 partially reduced staurosporine-induced apoptosis. (3) GPCMV defective in the gp38.1 ORF (Δ38.1) and the virus that rescues the defect (r38.1) were generated. Guinea pig fibroblast cells infected with Δ38.1 died earlier than r38.1-infected cells, which resulted in the lower yields of Δ38.1. (4) In animals, viral loads in the spleens of r38.1-infected guinea pigs were higher than those in the spleens of Δ38.1-infected animals. In conclusion, although GPCMV gp38.1 exerts a vMIA-like function, its inhibitory effect was not robust, suggesting the presence of additional inhibitory molecule(s), such as a BAK-specific inhibitor.

Varicella zoster virus encodes a viral decoy RHIM to inhibit cell death

Herpesviruses are known to encode a number of inhibitors of host cell death, including RIP Homotypic Interaction Motif (RHIM)-containing proteins. Varicella zoster virus (VZV) is a member of the alphaherpesvirus subfamily and is responsible for causing chickenpox and shingles. We have identified a novel viral RHIM in the VZV capsid triplex protein, open reading frame (ORF) 20, that acts as a host cell death inhibitor. Like the human cellular RHIMs in RIPK1 and RIPK3 that stabilise the necrosome in TNF-induced necroptosis, and the viral RHIM in M45 from murine cytomegalovirus that inhibits cell death, the ORF20 RHIM is capable of forming fibrillar functional amyloid complexes. Notably, the ORF20 RHIM forms hybrid amyloid complexes with human ZBP1, a cytoplasmic sensor of viral nucleic acid. Although VZV can inhibit TNF-induced necroptosis, the ORF20 RHIM does not appear to be responsible for this inhibition. In contrast, the ZBP1 pathway is identified as important for VZV infection. Mutation of the ORF20 RHIM renders the virus incapable of efficient spread in ZBP1-expressing HT-29 cells, an effect which can be reversed by the inhibition of caspases. Therefore we conclude that the VZV ORF20 RHIM is important for preventing ZBP1-driven apoptosis during VZV infection, and propose that it mediates this effect by sequestering ZBP1 into decoy amyloid assemblies.

RIP homotypic interaction motifs (RHIMs) are found in host proteins that can signal for programmed cell death and in viral proteins that can prevent it. Complexes stabilized by intermolecular interactions involving RHIMs have a fibrillar amyloid structure. We have identified a novel RHIM within the ORF20 protein expressed by Varicella zoster virus (VZV) that forms amyloid-based complexes with human cellular RHIMs. Whereas other herpesvirus RHIMs inhibit necroptosis, this new VZV RHIM targets the host RHIM-containing protein ZBP1 to inhibit apoptosis during infection. This is the first study to demonstrate the importance of the ZBP1 pathway in VZV infection and to identify the role of a viral RHIM in apoptosis inhibition. It broadens our understanding of host defense pathways and demonstrates how a decoy amyloid strategy is employed by pathogens to circumvent the host response.

In vivo characterization of target cells for acute elephant endotheliotropic herpesvirus (EEHV) infection in Asian elephants (Elephas maximus)

Elephant endotheliotropic herpesvirus-hemorrhagic disease (EEHV-HD) is a dangerous viral infectious disease in young Asian elephants. Despite hypotheses underlying pathogenesis of the disease, it is unclear which cell types the virus targets during acute or persistent infections. This study investigated the tissues and target cells permissive for EEHV infection and replication in vivo. Rabbit polyclonal antibodies against the non-structural proteins of EEHV, DNA polymerase (EEHV DNAPol), were generated and validated. These were used to examine EEHV infection and replication in various tissues of acute EEHV-HD cases and compared to an EEHV-negative control. The results indicated that viral antigens were distributed throughout the epithelia of the alimentary tract and salivary glands, endothelia and smooth muscle cells, and monocytic lineage cells of the EEHV-infected elephants. Moreover, EEHV DNAPol proteins were also found in the bone marrow cells of the EEHV1A-HD and EEHV1A/4-HD cases. This study demonstrated for the first time the target cells that favor in vivo EEHV replication during acute infection, providing a promising foundation for investigating EEHV propagation in vitro.

Cytomegalovirus inhibition of extrinsic apoptosis determines fitness and resistance to cytotoxic CD8 T cells

Viral immune evasion is currently understood to focus on deflecting CD8 T cell recognition of infected cells by disrupting antigen presentation pathways. We evaluated viral interference with the ultimate step in cytotoxic T cell function, the death of infected cells. The viral inhibitor of caspase-8 activation (vICA) conserved in human cytomegalovirus (HCMV) and murine CMV (MCMV) prevents the activation of caspase-8 and proapoptotic signaling. We demonstrate the key role of vICA from either virus, in deflecting antigen-specific CD8 T cell-killing of infected cells. vICA-deficient mutants, lacking either UL36 or M36, exhibit greater susceptibility to CD8 T cell control than mutants lacking the set of immunoevasins known to disrupt antigen presentation via MHC class I. This difference is evident during infection in the natural mouse host infected with MCMV, in settings where virus-specific CD8 T cells are adoptively transferred. Finally, we identify the molecular mechanism through which vICA acts, demonstrating the central contribution of caspase-8 signaling at a point of convergence of death receptor-induced apoptosis and perforin/granzyme-dependent cytotoxicity.

Cytomegaloviruses and Macrophages-Friends and Foes From Early on?

Starting at birth, newborn infants are exposed to numerous microorganisms. Adaptation of the innate immune system to them is a delicate process, with potentially advantageous and harmful implications for health development. Cytomegaloviruses (CMVs) are highly adapted to their specific mammalian hosts, with which they share millions of years of co-evolution. Throughout the history of mankind, human CMV has infected most infants in the first months of life without overt implications for health. Thus, CMV infections are intertwined with normal immune development. Nonetheless, CMV has retained substantial pathogenicity following infection in utero or in situations of immunosuppression, leading to pathology in virtually any organ and particularly the central nervous system (CNS). CMVs enter the host through mucosal interfaces of the gastrointestinal and respiratory tract, where macrophages (MACs) are the most abundant immune cell type. Tissue MACs and their potential progenitors, monocytes, are established target cells of CMVs. Recently, several discoveries have revolutionized our understanding on the pre- and postnatal development and site-specific adaptation of tissue MACs. In this review, we explore experimental evidences and concepts on how CMV infections may impact on MAC development and activation as part of host-virus co-adaptation.

User-Friendly Reverse Genetics System for Modification of the Right End of Fowl Adenovirus 4 Genome

A novel fowl adenovirus 4 (FAdV-4) has caused significant economic losses to the poultry industry in China since 2015. We established an easy-to-use reverse genetics system for modification of the whole right and partial left ends of the novel FAdV-4 genome, which worked through cell-free reactions of restriction digestion and Gibson assembly. Three recombinant viruses were constructed to test the assumption that species-specific viral genes of ORF4 and ORF19A might be responsible for the enhanced virulence: viral genes of ORF1, ORF1b and ORF2 were replaced with GFP to generate FAdV4-GFP, ORF4 was replaced with mCherry in FAdV4-GFP to generate FAdV4-GX4C, and ORF19A was deleted in FAdV4-GFP to generate FAdV4-CX19A. Deletion of ORF4 made FAdV4-GX4C form smaller plaques while ORF19A deletion made FAdV4-CX19A form larger ones on chicken LMH cells. Coding sequence (CDS) replacement with reporter mCherry demonstrated that ORF4 had a weak promoter. Survival analysis showed that FAdV4-CX19A-infected chicken embryos survived one more day than FAdV4-GFP- or FAdV4-GX4C-infected ones. The results illustrated that ORF4 and ORF19A were non-essential genes for FAdV-4 replication although deletion of either gene influenced virus growth. This work would help function study of genes on the right end of FAdV-4 genome and facilitate development of attenuated vaccines.

HCMV Infection and Apoptosis: How Do Monocytes Survive HCMV Infection?

Human cytomegalovirus (HCMV) infection of peripheral blood monocytes plays a key role in the hematogenous dissemination of the virus to multiple organ systems following primary infection or reactivation of latent virus in the bone marrow. Monocytes have a short life span of 1⁻3 days in circulation; thus, HCMV must alter their survival and differentiation to utilize these cells and their differentiated counterparts-macrophages-for dissemination and long term viral persistence. Because monocytes are not initially permissive for viral gene expression and replication, HCMV must control host-derived factors early during infection to prevent apoptosis or programmed cell death prior to viral induced differentiation into naturally long-lived macrophages. This review provides a short overview of HCMV infection of monocytes and describes how HCMV has evolved to utilize host cell anti-apoptotic pathways to allow infected monocytes to bridge the 48⁻72 h viability gate so that differentiation into a long term stable mature cell can occur. Because viral gene expression is delayed in monocytes following initial infection and only occurs (begins around two to three weeks post infection in our model) following what appears to be complete differentiation into mature macrophages or dendritic cells, or both; virally-encoded anti-apoptotic gene products cannot initially control long term infected cell survival. Anti-apoptotic viral genes are discussed in the second section of this review and we argue they would play an important role in long term macrophage or dendritic cell survival following infection-induced differentiation.

Remarkably Robust Antiviral Immune Response despite Combined Deficiency in Caspase-8 and RIPK3

Caspase-8 (Casp8)-mediated signaling triggers extrinsic apoptosis while suppressing receptor-interacting protein kinase (RIPK) 3-dependent necroptosis. Although Casp8 is dispensable for the development of innate and adaptive immune compartments in mice, the importance of this proapoptotic protease in the orchestration of immune response to pathogens remains to be fully explored. In this study, Casp8^-/-Ripk3^-/- C57BL/6 mice show robust innate and adaptive immune responses to the natural mouse pathogen, murine CMV. When young, these mice lack lpr-like lymphoid hyperplasia and accumulation of either B220 ⁺ CD3⁺ or B220^-CD3⁺CD4⁺ and CD8⁺ T cells with increased numbers of immature myeloid cells that are evident in older mice. Dendritic cell activation and cytokine production drive both NK and T cell responses to control viral infection in these mice, suggesting that Casp8 is dispensable to the generation of antiviral host defense. Curiously, NK and T cell expansion is amplified, with greater numbers observed by 7 d postinfection compared with either Casp8^+/-Ripk3^-/- or wild type (Casp8^+/+Ripk3^+/+ ) littermate controls. Casp8 and RIPK3 are natural targets of virus-encoded cell death suppressors that prevent infected cell apoptosis and necroptosis, respectively. It is clear from the current studies that the initiation of innate immunity and the execution of cytotoxic lymphocyte functions are all preserved despite the absence of Casp8 in responding cells. Thus, Casp8 and RIPK3 signaling is completely dispensable to the generation of immunity against this natural herpesvirus infection, although the pathways driven by these initiators serve as a crucial first line for host defense within virus-infected cells.

Adaptation by copy number variation in monopartite viruses

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Viral gene amplification allows rapid adaptation, especially for large DNA viruses.

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Amplifications often incur fitness costs for viral replication.

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Adaptive mutations can arise in the expanded locus or elsewhere enabling collapse.

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Genome amplifications provide genetic substrate for the evolution of modified or new functions.

Viruses evolve rapidly in response to host defenses and to exploit new niches. Gene amplification, a common adaptive mechanism in prokaryotes, archaea, and eukaryotes, has also contributed to viral evolution, especially of large DNA viruses. In experimental systems, gene amplification is one mechanism for rapidly overcoming selective pressures. Because the amplification generally incurs a fitness cost, emergence of adaptive point mutations within the amplified locus or elsewhere in the genome can enable collapse of the locus back to a single copy. Evidence of gene amplification followed by subfunctionalization or neofunctionalization of the copies is apparent by the presence of families of paralogous genes in many DNA viruses. These observations suggest that copy number variation has contributed broadly to virus evolution.

Human Cytomegalovirus Protein pUL38 Prevents Premature Cell Death by Binding to Ubiquitin-Specific Protease 24 and Regulating Iron Metabolism

Human cytomegalovirus (HCMV) protein pUL38 has been shown to prevent premature cell death by antagonizing cellular stress responses; however, the underlying mechanism remains unknown. In this study, we identified the host protein ubiquitin-specific protease 24 (USP24) as an interaction partner of pUL38. Mutagenesis analysis of pUL38 revealed that amino acids TFV at positions 227 to 230 were critical for its interaction with USP24. Mutant pUL38 TFV/AAA protein did not bind to USP24 and failed to prevent cell death induced by pUL38-deficient HCMV infection. Knockdown of USP24 suppressed the cell death during pUL38-deficient HCMV infection, suggesting that pUL38 achieved its function by antagonizing the function of USP24. We investigated the cellular pathways regulated by USP24 that might be involved in the cell death phenotype by testing several small-molecule compounds known to have a protective effect during stress-induced cell death. The iron chelators ciclopirox olamine and Tiron specifically protected cells from pUL38-deficient HCMV infection-induced cell death, thus identifying deregulated iron homeostasis as a potential mechanism. Protein levels of nuclear receptor coactivator 4 (NCOA4) and lysosomal ferritin degradation, a process called ferritinophagy, were also regulated by pUL38 and USP24 during HCMV infection. Knockdown of USP24 decreased NCOA4 protein stability and ferritin heavy chain degradation in lysosomes. Blockage of ferritinophagy by genetic inhibition of NCOA4 or Atg5/Atg7 prevented pUL38-deficient HCMV infection-induced cell death. Overall, these results support the hypothesis that pUL38 binds to USP24 to reduce ferritinophagy, which may then protect cells from lysosome dysfunction-induced cell death.IMPORTANCE Premature cell death is considered a first line of defense against various pathogens. Human cytomegalovirus (HCMV) is a slow-replicating virus that encodes several cell death inhibitors, such as pUL36 and pUL37x1, which allow it to overcome both extrinsic and intrinsic mitochondrion-mediated apoptosis. We previously identified HCMV protein pUL38 as another virus-encoded cell death inhibitor. In this study, we demonstrated that pUL38 achieved its activity by interacting with and antagonizing the function of the host protein ubiquitin-specific protease 24 (USP24). pUL38 blocked USP24-mediated ferritin degradation in lysosomes, which could otherwise be detrimental to the lysosome and initiate cell death. These novel findings suggest that iron metabolism is finely tuned during HCMV infection to avoid cellular toxicity. The results also provide a solid basis for further investigations of the role of USP24 in regulating iron metabolism during infection and other diseases.

RHIM-based protein:protein interactions in microbial defence against programmed cell death by necroptosis

The Receptor-interacting protein kinase Homotypic Interaction Motif (RHIM) is an amino acid sequence that mediates multiple protein:protein interactions in the mammalian programmed cell death pathway known as necroptosis. At least one key RHIM-based complex has been shown to have a functional amyloid fibril structure, which provides a stable hetero-oligomeric platform for downstream signaling. RHIMs and related motifs are present in immunity-related proteins across nature, from viruses to fungi to metazoans. Necroptosis is a hallmark feature of cellular clearance of infection. For this reason, numerous pathogens, including viruses and bacteria, have developed varied methods to modulate necroptosis, focusing on inhibiting RHIM:RHIM interactions, and thus their downstream cell death effects. This review will discuss current understanding of RHIM:RHIM interactions in normal cellular activation of necroptosis, from a structural and cell biology perspective. It will compare the mechanisms by which pathogens subvert these interactions in order to maintain their replicative and infective cycles and consider the similarities between RHIMs and other functional amyloid-forming proteins associated with cell death and innate immunity. It will discuss the implications of the heteromeric nature and structure of RHIM-based amyloid complexes in the context of other functional amyloids.

RIPK3 in cell death and inflammation: the good, the bad, and the ugly

Necroptosis is a form of cell death that can be observed downstream of death receptor or pattern recognition receptor signaling under certain cellular contexts, or in response to some viral and bacterial infections. The receptor interacting protein kinases-1 (RIPK1) and RIPK3 are at the core of necroptotic signaling, among other proteins. Because this pathway is normally halted by the pro-apoptotic protease caspase-8 and the IAP ubiquitin ligases, how and when necroptosis is triggered in physiological settings are ongoing questions. Interestingly, accumulating evidence suggests that RIPK3 has functions beyond the induction of necroptotic cell death, especially in the areas of tissue injury and sterile inflammation. Here, we will discuss the role of RIPK3 in a variety of physiological conditions, including necroptotic and non-necroptotic cell death, in the context of viral and bacterial infections, tissue damage, and inflammation.

The M25 gene products are critical for the cytopathic effect of mouse cytomegalovirus

Cell rounding is a hallmark of the cytopathic effect induced by cytomegaloviruses. By screening a panel of deletion mutants of mouse cytomegalovirus (MCMV) a mutant was identified that did not elicit cell rounding and lacked the ability to form typical plaques. Altered cell morphology was assigned to the viral M25 gene. We detected an early 2.8 kb M25 mRNA directing the synthesis of a 105 kDa M25 protein, and confirmed that a late 3.1 kb mRNA encodes a 130 kDa M25 tegument protein. Virions lacking the M25 tegument protein were of smaller size because the tegument layer between capsid and viral envelope was reduced. The ΔM25 mutant did not provoke the rearrangement of the actin cytoskeleton observed after wild-type MCMV infection, and isolated expression of the M25 proteins led to cell size reduction, confirming that they contribute to the morphological changes. Yields of progeny virus and cell-to-cell spread of the ΔM25 mutant in vitro were diminished and replication in vivo was impaired. The identification of an MCMV gene involved in cell rounding provides the basis for investigating the role of this cytopathic effect in CMV pathogenesis.

Die Another Day: Inhibition of Cell Death Pathways by Cytomegalovirus

Multicellular organisms have evolved multiple genetically programmed cell death pathways that are essential for homeostasis. The finding that many viruses encode cell death inhibitors suggested that cellular suicide also functions as a first line of defence against invading pathogens. This theory was confirmed by studying viral mutants that lack certain cell death inhibitors. Cytomegaloviruses, a family of species-specific viruses, have proved particularly useful in this respect. Cytomegaloviruses are known to encode multiple death inhibitors that are required for efficient viral replication. Here, we outline the mechanisms used by the host cell to detect cytomegalovirus infection and discuss the methods employed by the cytomegalovirus family to prevent death of the host cell. In addition to enhancing our understanding of cytomegalovirus pathogenesis we detail how this research has provided significant insights into the cross-talk that exists between the various cell death pathways.

Immune Protection by a Cytomegalovirus Vaccine Vector Expressing a Single Low-Avidity Epitope

Experimental CMV-based vaccine vectors expressing a single MHC class I-restricted high-avidity epitope provided strong, T cell-dependent protection against viruses or tumors. In this study we tested the low-avidity epitope KCSRNRQYL, and show that a mouse CMV (MCMV) vector provides complete immune control of recombinant vaccinia virus expressing the same epitope if KCSRNRQYL is expressed within the immediate-early MCMV gene ie2 The same epitope expressed within the early M45 gene provided no protection, although MCMV vectors expressing the high-avidity epitope SSIEFARL induced protective immunity irrespective of gene expression context. Immune protection was matched by Ag-induced, long-term expansion of effector memory CD8 T cells, regardless of epitope avidity. We explained this pattern by observing regularities in Ag competition, where responses to high-avidity epitopes outcompeted weaker ones expressed later in the replicative cycle of the virus. Conversely, robust and early expression of a low-avidity epitope compensated its weak intrinsic antigenicity, resulting in strong and sustained immunity and immune protection.

Sensing of viral and endogenous RNA by ZBP1/DAI induces necroptosis

Nucleic acids are potent triggers for innate immunity. Double‐stranded DNA and RNA adopt different helical conformations, including the unusual Z‐conformation. Z‐DNA/RNA is recognised by Z‐binding domains (ZBDs), which are present in proteins implicated in antiviral immunity. These include ZBP1 (also known as DAI or DLM‐1), which induces necroptosis, an inflammatory form of cell death. Using reconstitution and knock‐in models, we report that mutation of key amino acids involved in Z‐DNA/RNA binding in ZBP1's ZBDs prevented necroptosis upon infection with mouse cytomegalovirus. Induction of cell death was cell autonomous and required RNA synthesis but not viral DNA replication. Accordingly, ZBP1 directly bound to RNA via its ZBDs. Intact ZBP1‐ZBDs were also required for necroptosis triggered by ectopic expression of ZBP1 and caspase blockade, and ZBP1 cross‐linked to endogenous RNA. These observations show that Z‐RNA may constitute a molecular pattern that induces inflammatory cell death upon sensing by ZBP1.

UL36 Rescues Apoptosis Inhibition and In vivo Replication of a Chimeric MCMV Lacking the M36 Gene

Apoptosis is an important defense mechanism mounted by the immune system to control virus replication. Hence, cytomegaloviruses (CMV) evolved and acquired numerous anti-apoptotic genes. The product of the human CMV (HCMV) UL36 gene, pUL36 (also known as vICA), binds to pro-caspase-8, thus inhibiting death-receptor apoptosis and enabling viral replication in differentiated THP-1 cells. In vivo studies of the function of HCMV genes are severely limited due to the strict host specificity of cytomegaloviruses, but CMV orthologues that co-evolved with other species allow the experimental study of CMV biology in vivo. The mouse CMV (MCMV) homolog of the UL36 gene is called M36, and its protein product (pM36) is a functional homolog of vICA that binds to murine caspase-8 and inhibits its activation. M36-deficient MCMV is severely growth impaired in macrophages and in vivo. Here we show that pUL36 binds to the murine pro-caspase-8, and that UL36 expression inhibits death-receptor apoptosis in murine cells and can replace M36 to allow MCMV growth in vitro and in vivo. We generated a chimeric MCMV expressing the UL36 ORF sequence instead of the M36 one. The newly generated MCMV^UL36 inhibited apoptosis in macrophage lines RAW 264.7, J774A.1, and IC-21 and its growth was rescued to wild type levels. Similarly, growth was rescued in vivo in the liver and spleen, but only partially in the salivary glands of BALB/c and C57BL/6 mice. In conclusion, we determined that an immune-evasive HCMV gene is conserved enough to functionally replace its MCMV counterpart and thus allow its study in an in vivo setting. As UL36 and M36 proteins engage the same molecular host target, our newly developed model can facilitate studies of anti-viral compounds targeting pUL36 in vivo.

Mouse cytomegalovirus M36 and M45 death suppressors cooperate to prevent inflammation resulting from antiviral programmed cell death pathways

The complex interplay between caspase-8 and receptor-interacting protein (RIP) kinase RIP 3 (RIPK3) driving extrinsic apoptosis and necroptosis is not fully understood. Murine cytomegalovirus triggers both apoptosis and necroptosis in infected cells; however, encoded inhibitors of caspase-8 activity (M36) and RIP3 signaling (M45) suppress these antiviral responses. Here, we report that this virus activates caspase-8 in macrophages to trigger apoptosis that gives rise to secondary necroptosis. Infection with double-mutant ΔM36/M45mutRHIM virus reveals a signaling pattern in which caspase-8 activates caspase-3 to drive apoptosis with subsequent RIP3-dependent activation of mixed lineage kinase domain-like (MLKL) leading to necroptosis. This combined cell death signaling is highly inflammatory, greater than either apoptosis induced by ΔM36 or necroptosis induced by M45mutRHIM virus. IL-6 production by macrophages is dramatically increased during double-mutant virus infection and correlates with faster antiviral responses in the host. Collaboratively, M36 and M45 target caspase-8 and RIP3 pathways together to suppress this proinflammatory cell death. This study reveals the effect of antiviral programmed cell death pathways on inflammation, shows that caspase-8 activation may go hand-in-hand with necroptosis in macrophages, and revises current understanding of independent and collaborative functions of M36 and M45 in blocking apoptotic and necroptotic cell death responses.

Anti-apoptotic Bcl-XL but not Mcl-1 contributes to protection against virus-induced apoptosis

Infection of mammalian cells with viruses often induces apoptosis. How the recognition of viruses leads to apoptosis of the infected cell and which host cell factors regulate this cell death is incompletely understood. In this study, we focussed on two major anti-apoptotic proteins of the host cell, whose abundance and activity are important for cell survival, the Bcl-2-like proteins Mcl-1 and Bcl-X_L. During infection of epithelial cells and fibroblasts with modified vaccinia virus Ankara (MVA), Mcl-1 protein levels dropped but the MVA Bcl-2-like protein F1L could replace Mcl-1 functionally; a similar activity was found in vaccinia virus (VACV)-infected cells. During infection with murine cytomegalovirus (MCMV), Mcl-1-levels were not reduced but a viral Mcl-1-like activity was also generated. Infection of mouse macrophages with any of these viruses, on the other hand, induced apoptosis. Virus-induced macrophage apoptosis was unaltered in the absence of Mcl-1. However, apoptosis was substantially increased in infected Bcl-X_L-deficient macrophages or macrophages treated with the Bcl-2/Bcl-X_L-inhibitor ABT-737. Genetic loss of Bcl-X_L or treatment of macrophages with ABT-737 reduced the generation of infectious VACV. These data show that Mcl-1 is dispensable for the regulation of apoptosis during infection with different large DNA viruses, either because the viruses replace its function (in fibroblasts and epithelial cells) or because the pro-apoptotic activity generated by the infection appears not to be blocked by it (in macrophages). Bcl-X_L, on the other hand, can be important to maintain survival of virus-infected cells, and its activity can determine outcome of the infection.

Targeted Mutagenesis of Guinea Pig Cytomegalovirus Using CRISPR/Cas9-Mediated Gene Editing

The cytomegaloviruses (CMVs) are among the most genetically complex mammalian viruses, with viral genomes that often exceed 230 kbp. Manipulation of cytomegalovirus genomes is largely performed using infectious bacterial artificial chromosomes (BACs), which necessitates the maintenance of the viral genome in Escherichia coli and successful reconstitution of virus from permissive cells after transfection of the BAC. Here we describe an alternative strategy for the mutagenesis of guinea pig cytomegalovirus that utilizes clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated genome editing to introduce targeted mutations to the viral genome. Transient transfection and drug selection were used to restrict lytic replication of guinea pig cytomegalovirus to cells that express Cas9 and virus-specific guide RNA. The result was highly efficient editing of the viral genome that introduced targeted insertion or deletion mutations to nonessential viral genes. Cotransfection of multiple virus-specific guide RNAs or a homology repair template was used for targeted, markerless deletions of viral sequence or to introduce exogenous sequence by homology-driven repair. As CRISPR/Cas9 mutagenesis occurs directly in infected cells, this methodology avoids selective pressures that may occur during propagation of the viral genome in bacteria and may facilitate genetic manipulation of low-passage or clinical CMV isolates.

IMPORTANCE

The cytomegalovirus genome is complex, and viral adaptations to cell culture have complicated the study of infection in vivo Recombineering of viral bacterial artificial chromosomes enabled the study of recombinant cytomegaloviruses. Here we report the development of an alternative approach using CRISPR/Cas9-based mutagenesis in guinea pig cytomegalovirus, a small-animal model of congenital cytomegalovirus disease. CRISPR/Cas9 mutagenesis can introduce the same types of mutations to the viral genome as bacterial artificial chromosome recombineering but does so directly in virus-infected cells. CRISPR/Cas9 mutagenesis is not dependent on a bacterial intermediate, and defined viral mutants can be recovered after a limited number of viral genome replications, minimizing the risk of spontaneous mutation.

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.

Rationally designed chemokine-based toxin targeting the viral G protein-coupled receptor US28 potently inhibits cytomegalovirus infection in vivo

The use of receptor-ligand interactions to direct toxins to kill diseased cells selectively has shown considerable promise for treatment of a number of cancers and, more recently, autoimmune disease. Here we move the fusion toxin protein (FTP) technology beyond cancer/autoimmune therapeutics to target the human viral pathogen, human cytomegalovirus (HCMV), on the basis of its expression of the 7TM G protein-coupled chemokine receptor US28. The virus origin of US28 provides an exceptional chemokine-binding profile with high selectivity and improved binding for the CX3C chemokine, CX3CL1. Moreover, US28 is constitutively internalizing by nature, providing highly effective FTP delivery. We designed a synthetic CX3CL1 variant engineered to have ultra-high affinity for US28 and greater specificity for US28 than the natural sole receptor for CX3CL1, CX3CR1, and we fused the synthetic variant with the cytotoxic domain of Pseudomonas Exotoxin A. This novel strategy of a rationally designed FTP provided unparalleled anti-HCMV efficacy and potency in vitro and in vivo.

Necroptosis: The Trojan horse in cell autonomous antiviral host defense

Herpesviruses suppress cell death to assure sustained infection in their natural hosts. Murine cytomegalovirus (MCMV) encodes suppressors of apoptosis as well as M45-encoded viral inhibitor of RIP activation (vIRA) to block RIP homotypic interaction motif (RHIM)-signaling and recruitment of RIP3 (also called RIPK3), to prevent necroptosis. MCMV and human cytomegalovirus encode a viral inhibitor of caspase (Casp)8 activation to block apoptosis, an activity that unleashes necroptosis. Herpes simplex virus (HSV)1 and HSV2 incorporate both RHIM and Casp8 suppression strategies within UL39-encoded ICP6 and ICP10, respectively, which are herpesvirus-conserved homologs of MCMV M45. Both HSV proteins sensitize human cells to necroptosis by blocking Casp8 activity while preventing RHIM-dependent RIP3 activation and death. In mouse cells, HSV1 ICP6 interacts with RIP3 and, surprisingly, drives necroptosis. Thus, herpesviruses have illuminated the contribution of necoptosis to host defense in the natural host as well as its potential to restrict cross-species infections in nonnatural hosts.

Cytomegalovirus immune evasion of myeloid lineage cells

Cytomegalovirus (CMV) evades the immune system in many different ways, allowing the virus to grow and its progeny to spread in the face of an adverse environment. Mounting evidence about the antiviral role of myeloid immune cells has prompted the research of CMV immune evasion mechanisms targeting these cells. Several cells of the myeloid lineage, such as monocytes, dendritic cells and macrophages, play a role in viral control, but are also permissive for CMV and are naturally infected by it. Therefore, CMV evasion of myeloid cells involves mechanisms that qualitatively differ from the evasion of non-CMV-permissive immune cells of the lymphoid lineage. The evasion of myeloid cells includes effects in cis, where the virus modulates the immune signaling pathways within the infected myeloid cell, and those in trans, where the virus affects somatic cells targeted by cytokines released from myeloid cells. This review presents an overview of CMV strategies to modulate and evade the antiviral activity of myeloid cells in cis and in trans.

Suppression of RIP3-dependent necroptosis by human cytomegalovirus

Necroptosis is an alternate programmed cell death pathway that is unleashed by caspase-8 compromise and mediated by receptor-interacting protein kinase 3 (RIP3). Murine cytomegalovirus (CMV) and herpes simplex virus (HSV) encode caspase-8 inhibitors that prevent apoptosis together with competitors of RIP homotypic interaction motif (RHIM)-dependent signal transduction to interrupt the necroptosis. Here, we show that pro-necrotic murine CMV M45 mutant virus drives virus-induced necroptosis during nonproductive infection of RIP3-expressing human fibroblasts, whereas WT virus does not. Thus, M45-encoded RHIM competitor, viral inhibitor of RIP activation, sustains viability of human cells like it is known to function in infected mouse cells. Importantly, human CMV is shown to block necroptosis induced by either TNF or M45 mutant murine CMV in RIP3-expressing human cells. Human CMV blocks TNF-induced necroptosis after RIP3 activation and phosphorylation of the mixed lineage kinase domain-like (MLKL) pseudokinase. An early, IE1-regulated viral gene product acts on a necroptosis step that follows MLKL phosphorylation prior to membrane leakage. This suppression strategy is distinct from RHIM signaling competition by murine CMV or HSV and interrupts an execution process that has not yet been fully elaborated.

Staying alive: cell death in antiviral immunity

Programmed cell death is an integral part of host defense against invading intracellular pathogens. Apoptosis, programmed necrosis, and pyroptosis each serve to limit pathogen replication in infected cells, while simultaneously promoting the inflammatory and innate responses that shape effective long-term host immunity. The importance of carefully regulated cell death is evident in the spectrum of inflammatory and autoimmune disorders caused by defects in these pathways. Moreover, many viruses encode inhibitors of programmed cell death to subvert these host responses during infection, thereby facilitating their own replication and persistence. Thus, as both virus and cell vie for control of these pathways, the battle for survival has shaped a complex host-pathogen interaction. This review will discuss the multifaceted role that programmed cell death plays in maintaining the immune system and its critical function in host defense, with a special emphasis on viral infections.

Resolving the titer of murine cytomegalovirus by plaque assay using the M2-10B4 cell line and a low viscosity overlay

BACKGROUND

Murine cytomegalovirus (MCMV) is increasingly used as an infectious model to investigate host-pathogen interactions in mice. Detailed methods have been published for using primary murine embryonic fibroblasts (MEFs) for preparing stocks and determining viral titers of MCMV. For determining the titer of MCMV by plaque assay, these methods rely on a high viscosity media that restricts viral spreading through the supernatant of the culture, but is also usually too viscous to pipet. Moreover, MEFs must be repeatedly generated and can vary widely from batch-to-batch in purity, proliferation rates, and the development of senescence. In contrast, the M2-10B4 bone marrow stromal cell line (ATCC # CRL-1972), which is also permissive for MCMV, has been reported to produce high-titer stocks of MCMV and has the considerable advantages of growing rapidly and consistently. However, detailed methods using these cells have not been published.

METHODS

We modified existing protocols to use M2-10B4 cells for measuring MCMV titers by plaque assay.

RESULTS

We found that MCMV plaques could be easily resolved on monolayers of M2-10B4 cells. Moreover, plaques formed normally even when cultures of M2-10B4 cells were less than 50% confluent on the day of infection, as long as we also used a reduced viscosity overlay.

CONCLUSIONS

Overall, our protocol enabled us to use a consistent cell line to assess viral titers, rather than repeatedly producing primary MEFs. It also allowed us to start the assay with 4-fold fewer cells than would be required to generate a confluent monolayer, reducing the lead-time prior to the start of the assay. Finally, the reduced viscosity CMC could be handled by pipet and did not need to be pre-mixed with media, thus increasing its shelf-life and ease-of-use. We describe our results here, along with detailed protocols for the use of the M2-10B4 cell lines to determine the titer and grow stocks of MCMV.

A modified screening system for loss-of-function and dominant negative alleles of essential MCMV genes

Inactivation of gene products by dominant negative mutants is a valuable tool to assign functions to yet uncharacterized proteins, to map protein-protein interactions or to dissect physiological pathways. Detailed functional and structural knowledge about the target protein would allow the construction of inhibitory mutants by targeted mutagenesis. Yet, such data are limited for the majority of viral proteins, so that the target gene needs to be subjected to random mutagenesis to identify suitable mutants. However, for cytomegaloviruses this requires a two-step screening approach, which is time-consuming and labor-intensive. Here, we report the establishment of a high-throughput suitable screening system for the identification of inhibitory alleles of essential genes of the murine cytomegalovirus (MCMV). In this screen, the site-specific recombination of a specifically modified MCMV genome was transferred from the bacterial background to permissive host cells, thereby combining the genetic engineering and the rescue test in one step. Using a reference set of characterized pM53 mutants it was shown that the novel system is applicable to identify non-complementing as well as inhibitory mutants in a high-throughput suitable setup. The new cis-complementation assay was also applied to a basic genetic characterization of pM99, which was identified as essential for MCMV growth. We believe that the here described novel genetic screening approach can be adapted for the genetic characterization of essential genes of any large DNA viruses.

Natural killer cells are required for extramedullary hematopoiesis following murine cytomegalovirus infection

The immune response against a variety of pathogens can lead to activation of blood formation at ectopic sites, a process termed extramedullary hematopoiesis (EMH). The underlying mechanisms of EMH have been enigmatic. Investigating splenic EMH in mice infected with murine cytomegalovirus (MCMV), we find that, while cells of the adaptive immune system were dispensable for EMH, natural killer (NK) cells were essential. EMH required recognition of infected cells via activating NK cell receptors Ly49H or NKG2D, and correspondingly, viral interference with NK cell recognition abolished EMH. Surprisingly, development of EMH was not induced by NK cell-derived cytokines but was dependent on perforin-mediated cytotoxicity in order to control virus spread. Spreading virus reduced the numbers of F4/80(+) macrophages that were crucial for inflammatory EMH. Hence, whereas MCMV suppresses inflammation-induced EMH, NK cells confine virus spread, thereby protecting extramedullary hematopoietic niches and facilitating EMH.

The context of gene expression defines the immunodominance hierarchy of cytomegalovirus antigens

Natural immunity to CMV dominates the CD4 and CD8 memory compartments of the CMV-seropositive host. This property has been recently exploited for experimental CMV-based vaccine vector strategies, and it has shown promise in animal models of AIDS and Ebola disease. Although it is generally agreed that CMV-based vaccine vectors may induce highly protective and persistent memory T cells, the influence of the gene expression context on Ag-specific T cell memory responses and immune protection induced by CMV vectors is not known. Using murine CMV (MCMV) recombinants expressing a single CD8 T cell epitope from HSV-1 fused to different MCMV genes, we show that magnitude and kinetics of T cell responses induced by CMV are dependent on the gene expression of CMV Ags. Interestingly, the kinetics of the immune response to the HSV-1 epitope was paralleled by a reciprocal depression of immune responses to endogenous MCMV Ags. Infection with a recombinant MCMV inducing a vigorous initial immune response to the recombinant peptide resulted in a depressed early response to endogenous MCMV Ag. Another recombinant virus, which induced a slowly developing "inflationary" T cell response to the HSV-1 peptide, induced weaker long-term responses to endogenous CMV Ags. Importantly, both mutants were able to protect mice from a challenge with HSV-1, mediating strong sterilizing immunity. Our data suggest that the context of gene expression markedly influences the T cell immunodominance hierarchy of CMV Ags, but the immune protection against HSV-1 does not require inflationary CD8 responses against the recombinant CMV-expressed epitope.

Novel gene therapy viral vector using non-oncogenic lymphotropic herpesvirus

Despite the use of retroviral vectors, efficiently introducing target genes into immunocytes such as T cells is difficult. In addition, retroviral vectors carry risks associated with the oncogenicity of the native virus and the potential for introducing malignancy in recipients due to genetic carryover from immortalized cells used during vector production. To address these issues, we have established a new virus vector that is based on human herpesvirus 6 (HHV-6), a non-oncogenic lymphotropic herpesvirus that infects CD4⁺ T cells, macrophages, and dendritic cells. In the present study, we have altered the cell specificity of the resulting recombinant HHV-6 by knocking out the U2–U8 genes. The resulting virus proliferated only in activated cord blood cells and not in peripheral blood cells. Umbilical cord blood cells produced replication-defective recombinant virus in sufficiently high titer to omit the use of immortalized cells during vector production. HHV-6 vectors led to high rates (>90%) of gene transduction in both CD4⁺ and CD8⁺ T cells. These viruses showed low-level replication of viral DNA that supported greater expression of the induced genes than that of other methods but that was insufficient to support the production of replication-competent virus. Furthermore, HHV-6 vectors containing short hairpin RNAs against CD4 and HIV Gag remarkably inhibited the production of these proteins and HIV particles. Here we demonstrate the utility of HHV-6 as a new non-carcinogenic viral vector for immunologic diseases and immunotherapy.

Viral inhibition of BAK promotes murine cytomegalovirus dissemination to salivary glands

Apoptosis induction is an important host defense mechanism to control viral infection, which is antagonized by viral proteins. Murine cytomegalovirus m41.1 encodes a viral inhibitor of BAK oligomerization (vIBO) that blocks the mitochondrial apoptosis mediator BAK. However, its importance for viral fitness in vivo has not been investigated. Here, we show that an m41.1-deficient virus attains reduced titers in salivary glands of wild-type but not Bak1(-/-) mice, indicating a requirement of BAK inhibition for optimal dissemination in vivo.

Mouse cytomegalovirus egress protein pM50 interacts with cellular endophilin-A2

The herpesvirus replication cycle comprises maturation processes in the nucleus and cytoplasm of the infected cells. After their nuclear assembly viral capsids translocate via primary envelopment towards the cytoplasm. This event is mediated by the nuclear envelopment complex, which is composed by two conserved viral proteins belonging to the UL34 and UL31 protein families. Here, we generated recombinant viruses, which express affinity-tagged pM50 and/or pM53, the pUL34 and pUL31 homologues of the murine cytomegalovirus. We extracted pM50- and pM53-associated protein complexes from infected cells and analysed their composition after affinity purification by mass spectrometry. We observed reported interaction partners and identified new putative protein-protein interactions for both proteins. Endophilin-A2 was observed as the most prominent cellular partner of pM50. We found that endophilin-A2 binds to pM50 directly, and this interaction seems to be conserved in the pUL34 family.

Block of death-receptor apoptosis protects mouse cytomegalovirus from macrophages and is a determinant of virulence in immunodeficient hosts

The inhibition of death-receptor apoptosis is a conserved viral function. The murine cytomegalovirus (MCMV) gene M36 is a sequence and functional homologue of the human cytomegalovirus gene UL36, and it encodes an inhibitor of apoptosis that binds to caspase-8, blocks downstream signaling and thus contributes to viral fitness in macrophages and in vivo. Here we show a direct link between the inability of mutants lacking the M36 gene (ΔM36) to inhibit apoptosis, poor viral growth in macrophage cell cultures and viral in vivo fitness and virulence. ΔM36 grew poorly in RAG1 knockout mice and in RAG/IL-2-receptor common gamma chain double knockout mice (RAGγC^−/−), but the depletion of macrophages in either mouse strain rescued the growth of ΔM36 to almost wild-type levels. This was consistent with the observation that activated macrophages were sufficient to impair ΔM36 growth in vitro. Namely, spiking fibroblast cell cultures with activated macrophages had a suppressive effect on ΔM36 growth, which could be reverted by z-VAD-fmk, a chemical apoptosis inhibitor. TNFα from activated macrophages synergized with IFNγ in target cells to inhibit ΔM36 growth. Hence, our data show that poor ΔM36 growth in macrophages does not reflect a defect in tropism, but rather a defect in the suppression of antiviral mediators secreted by macrophages. To the best of our knowledge, this shows for the first time an immune evasion mechanism that protects MCMV selectively from the antiviral activity of macrophages, and thus critically contributes to viral pathogenicity in the immunocompromised host devoid of the adaptive immune system.

The majority of adult people are infected with human cytomegalovirus (CMV), but in hosts with a healthy immune system it is kept in check and does not cause disease. On the other hand, in patients suffering from innate or acquired immune deficiencies, CMV can cause severe disease or death. Infection of mice with the mouse CMV (MCMV) is an experimental model to study the biology of CMV infection, and mice that lack all of their lymphocytes are very susceptible to MCMV and die typically within three weeks of infection. In this article we show that MCMV causes disease and death in mice lacking lymphocytes because its gene M36 blocks programmed cell death, or apoptosis. MCMV lacking the M36 gene grew thousand folds less well in these mice, which significantly improved survival. This was because M36 deletion made MCMV susceptible to the action of macrophages, cells that secrete soluble factors that induce apoptosis. Importantly, viral growth and virulence of the M36-deficient MCMV could be restored by blocking apoptosis by other means, showing that the block of apoptosis was critical for viral replication. Therefore, our data imply that viral inhibition of apoptosis may be a key molecular target for antiviral strategies in immunodeficient hosts.

Multiplicity-dependent activation of a serine protease-dependent cytomegalovirus-associated programmed cell death pathway

At a low MOI (≤0.01), cytomegalovirus-associated programmed cell death terminates productive infection via a pathway triggered by the mitochondrial serine protease HtrA2/Omi. This infected cell death is associated with late phase replication events naturally suppressed by the viral mitochondrial inhibitor of apoptosis (vMIA). Here, higher MOI (ranging from 0.1-3.0) triggers cell death earlier during infection independent of viral DNA synthesis. Thus, MOI-dependent activating signals early, at high MOI, or late, at low MOI, during replication promote serine protease-dependent death that is suppressed by vMIA. Treatment with an antioxidant targeting reactive oxygen species (ROS) or the serine protease inhibitor N-alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK) delays cell death, and the combination has an additive impact. These studies identify serine proteases and ROS as important factors triggering programmed cell death induced by vMIA-deficient virus, and show that this death pathway occurs earlier and reduces viral yields as the MOI is increased.