from Ann B. Hill writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

In recent years it has been suggested that CMV may be involved in the pathogenesis of a variety of conditions in which there may not be clear evidence of viral replication. These "non-traditional" disease associations include glioblastoma and various other cancers, atherosclerotic cardiovascular disease, Alzheimer's disease, and immunosenescence, amongst others. The pathologies fall into two broad groups: tumors, and inflammatory diseases of aging. In the case of tumors, some groups have used ultrasensitive detection techniques and report finding CMV in the majority of tumor cells. In the case of inflammatory diseases of aging, the evidence mostly comes from epidemiological studies that have associated CMV serology or CMV-driven alterations in T cell populations with various outcomes. While CMV's biology provides ready explanations for these putative disease associations, the actual evidence for its being involved remains controversial. This review will review the evidence for several putative disease associations.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Michael Mach, Anna-Katharina Wiegers, Nadja Spindler and Thomas Winkler writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

The generation of antibodies represents a powerful tool of the adaptive immune system in the battle against viral infections. Targets for antibodies with potential antiviral activity are glycoproteins in the viral envelope and/or on the surface of infected cells. In recent years, considerable progress has been made in our understanding of the protective antibody response against cytomegaloviruses. Animal studies have unambiguously demonstrated the protective capacity of antibodies both in prophylaxis as well as in therapy of existing primary infection or reactivation. A number of human monoclonal antibodies have been isolated which show potent virus-neutralizing capacity and new antibody targets have been identified. However, we still need to expand our knowledge on the mechanisms of virus neutralization by antibodies and the mode of action of protective antibodies in vivo. Increasing this knowledge will help us to rationally design strategies to limit the consequences of infections in populations at risk for CMV-disease.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Robert F. Kalejta writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

As virions disassemble during viral entry, they must expertly navigate and manage the complex and unwelcoming environments they encounter in order to successfully infect host cells. Herpesviruses incorporate proteins into their virions in a layer between the capsid and envelope termed the tegument to assist in this hostile takeover. When delivered to infected cells subsequent to membrane fusion, tegument proteins begin to facilitate viral infection after entry but before immediate early (IE) gene expression (referred to as the pre-IE stage of infection). Tegument-delivered proteins mediate capsid migration through the cytoplasm to nuclear pore complexes and the transmission of the genome into the nucleus. Furthermore, they modulate viral transcription, and help infected cells avoid all three classes of immune function (intrinsic, innate and adaptive). While they are most often studied during lytic infections, a new appreciation for the role that the proper regulation of tegument-delivered protein function may play during viral latency is emerging. Here the pre-IE functions of tegument proteins during both lytic and latent infections are reviewed and analyzed.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Klaus Früh, Daniel Malouli, Kristie L. Oxford and Peter A. Barry writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

The last few years have witnessed significant expansion of the Non-Human-Primate (NHP) models of CMV persistence and pathogenesis. Progress in the utilization of the NHP CMV models has been highlighted by a better understanding of natural history, comparative genomic sequence analyses, and in vivo studies addressing mechanisms of tissue tropism, immune modulation, vaccine development, and optimization of the use of CMV as a vaccine vector for ectopic expression of heterologous antigens. The earliest observations of CMV infection in NHP during the first part of the twentieth century were remarkable for their prescient descriptions of CMV-host relationships based entirely on microscopic characterization of the protozoan-like (cytomegalic) cells that had been noted in congenitally infected human infants (Ribbert, 1904; Goodpasture and Talbot, 1921) and guinea pigs (Jackson, 1920). In particular, it was noted in the 1920's and 1930's that NHP CMV (1) is a ubiquitous infectious agent, (2) infects multiple cell types, (3) is characterized by low virulence, and (4) modifies host inflammatory responses. In addition, the first use of the term "latency" to describe the ability of CMV to reactivate may have been used for NHP CMV. In 1935, Cowdry and Scott recognized that treatment of CMV-infected monkeys with irradiated ergosterol stimulated reactivation of CMV in multiple tissues, and they noted that the treatment "may have activated or intensified a process already latent in the kidneys" (Cowdry and Scott, 1935). The recent progress in the NHP models follows these earliest insights into the hallmarks of CMV infections, and now enables the unique positioning of NHP models to provide a better understanding, treatment, and prevention of HCMV infection and disease in humans. This review summarizes the current status of our understanding of NHP CMVs with particular emphasis on viral gene function and viral disease models.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Gavin W. G. Wilkinson, Rebecca J. Aicheler and Eddie C. Y. Wang writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

The efficient downregulation of HLA-I by HCMV has the clear potential to render infected cells extremely vulnerable to NK cells. Moreover, the major IE genes activate cell responses that stimulate efficient transcription of multiple ligands for NK cell activating receptors. The capacity of HCMV to persist in vivo can clearly be ascribed to its ability to modulate NK cell responses. To date 7 functions encoded by HCMV have been formally demonstrated to suppress NK cell activation. UL18 is an MHC-I homologue that binds the inhibitory receptor LIR-1, while UL40 rescues expression of HLA-E, a ligand for the inhibitory receptor CD94-NKG2A. UL16, UL142 and miR-UL112 target multiple ligands for the ubiquitous NK activating receptor NKG2D, while UL141 targets ligands for the ubiquitous activating receptors DNAM-1 and TACTILE. The UL83-encoded major tegument protein (pp65) is unique in that it binds directly to inhibit the activating receptor NKp30. It is becoming evident that a substantial proportion of the remarkable coding capacity of this virus is directed at systematically addressing the NK cell response. Outwith the immediate goal of understanding HCMV pathogenesis, research on these immunodulatory functions are providing remarkable insights into the mechanisms that regulate human NK cell responses. Recent studies demonstrate that during its lifelong persistent/latent infection, HCMV induces dramatic changes in the NK cell repertoire leading specifically to expansions of NK cell subsets expressing CD94-NKG2C, LIR-1 and CD57. There is growing interest in these changes in the NK cell response as they potentially contribute to an emerging paradox: an 'adaptive' response by a supposedly innate arm of the immune system. This amplification of specific NK cell subsets to the virus may be instrumental in controlling infections, and may also be disrupted by immunosuppression. NK cells undoubtedly play a crucial role in controlling HCMV infections. There is a compelling need to understand the mechanisms by which HCMV evades, modulates, and ultimately is recognised by 'innate' defence systems.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Lisa P. Daley-Bauer and Edward S. Mocarski writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Cytomegalovirus pathogenesis, dissemination and immunity are tied to the behaviour of myelomonocytic cells. Investigations of the roles of monocyte subsets in the dissemination of cytomegalovirus revealed that the MCMV-encoded chemokine, MCK2, controls recruitment patterns of the two major monocyte subsets (inflammatory and patrolling) to sites of infection. Monocytes give rise to both macrophages and dendritic cells that populate tissues. Mice deficient in the chemokine axis (CCR2 and CCL2 /MCP-1) do not support inflammatory monocyte emigration from bone marrow. Inflammatory monocyte-derived lineages, which are nonpermissive for MCMV, are dispensable for pathogenesis and dissemination as well as for the establishment of latency set-points. Nevertheless, recruitment of inflammatory monocytes is enhanced by elaboration of MCK2 and impairs the CTL response to delay viral clearance. In contrast, patrolling monocytes support MCMV replication and contribute to dissemination patterns in the host. Studies in CX₃CR1-deficient infected mice show reduced viral dissemination to salivary glands, consistent with the reduced survival of patrolling monocytes in these animals. HCMV studies suggest myelomonocytic progenitor cells, including inflammatory and patrolling monocyte subsets, are associated with acute as well as latent infections. MCMV latency in mice occurs in myeloid as well as epithelial and endothelial cell lineages. In this paper, we review the current understanding of myelomonocytic lineage cells in the establishment of cytomegalovirus infections based on human and murine studies.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from William J. Britt, Djurdjica Cekinović and Stipan Jonjić writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Congenital HCMV infection of developing CNS is a major cause of long-term neuronal morbidity. Prevention of neurological damage due to CNS infection is considered as the primary goal of vaccine strategies and passively administered biologics. Due to species specificity of CMVs animal models have been developed in order to paradigmatically analyze the course of CMV infections. While different animal models provide good tools for studies of HCMV infections in the immunocompromised host, no single animal model completely recapitulates the pathogenesis of congenital HCMV infection. The murine model of perinatal MCMV infection in newborn mice has been proven as powerful tool to analyze the pathogenesis of congenital HCMV infection and mechanisms of the immune response which control CNS infection in the developing brain. This review describes a model of MCMV infection in neonatal mice that represents various aspects of HCMV infection in neonates, and thus, could be highly predictive for possible antiviral interventions in humans.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Rafaela Holtappels, Stefan Ebert, Jürgen Podlech, Annette Fink, Verena Böhm, Niels A.W. Lemmermann, Kirsten Freitag, Angélique Renzaho, Doris Thomas and Matthias J. Reddehase writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Cytomegalovirus (CMV) disease is a clinically relevant complication in hematopoietic (stem) cell transplantation (HCT). The murine model of CMV infection in the phase of hematopoietic reconstitution after experimental HCT has been pivotal in defining efficient endogenous reconstitution of donor (D)-derived antiviral CD8⁺ T cells as the decisive immune parameter for the control of lytic CMV replication and the prevention of acute manifestations of end-organ disease in HCT recipients (R). Endogenous reconstitution of protective immunity occurs with some delay, since hematopoietic stem cells and T-cell lineage lymphopoietic progenitors need to engraft in bone marrow (BM) stroma, migrate to the thymus for thymic self/non self T-cell receptor specificity selection, and emigrate as mature but naïve CD44^low T cells to the periphery where they encounter viral antigen to become 'primed', expand clonally, and differentiate into antiviral effector and memory cell subsets. These processes take time, and depending on when CMV reactivates in HCT recipients, endogenous reconstitution may come too late. In addition, the murine model has shown that CMV directly interferes with endogenous reconstitution by infecting BM stromal cells and inhibiting the expression of stromal cell-derived hemopoietins, specifically of stem cell factor (SCF). Depending on the precise conditions, BM pathogenesis of CMV, often referred to as 'myelosuppression', can range from reduced engraftment to a complete graft failure resulting in BM aplasia. Substituting HCT with already primed, 'ready-to-go' CMV-specific CD8⁺ effector and/or memory T cells, also known as 'adoptive T-cell transfer' or 'preemptive cytoimmunotherapy', can provide immediate protection. Key parameters of protection by CD8⁺ T cells were revealed by the murine model: (i) memory cell subsets isolated ex vivo from immune donors are ~ 100-fold more efficient than cell culture-propagated short-term cytolytic effector cell lines of identical specificity, (ii) the magnitude of the primary immune response to a viral epitope, its 'immunodominance', does not correlate with protection mediated by cognate T cells; (iii) viral immune evasion proteins determine if an epitope is 'protective', and (iv) genetic deletion of dominant protective epitopes in either the donor's or the recipient's virus has little impact on protection by a broadly-specific T-cell population, predicting 'robustness' of T-cell immunotherapy towards antigenic variation in virus strains or clinical isolates.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Marco Thomas, Nina Reuter and Thomas Stamminger writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Research of the last two decades revealed that human cytomegalovirus (HCMV) developed a multitude of sophisticated mechanisms to usurp and manipulate the cellular gene expression machinery in order to achieve efficient viral protein synthesis. Furthermore, there is increasing evidence that the virus has to antagonize cellular restriction factors in order to avoid a silencing of viral transcription. This review summarizes our present knowledge on how viral regulatory proteins modulate chromatin structure, promoter activities, transcriptional elongation, RNA processing and mRNA export. Interestingly, as exemplified by the pleiotropic effector pUL69 of HCMV, specific viral regulatory proteins appear to be able to affect different cellular machineries, thus providing evidence for an extensive interconnection between transcriptional and post-transcriptional regulatory processes.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Wade Gibson and Elke Bogner writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Formation and maturation of the cytomegalovirus capsid is reviewed. Recent information about protein-protein interactions involved in capsid assembly, molecular interactions relating to the mechanism of DNA packaging, and the sequence of events in primary envelopment is considered as it establishes similarities and reveals differences between CMV and other herpesviruses.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Alec J. Redwood, Geoffrey R. Shellam and Lee M. Smith writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Cytomegaloviruses have co-evolved with their hosts since the mammalian radiation. The MCMV genome appears to be highly conserved and unlike the HCMV genome contains no large-scale deletions and rearrangements following serial in vitro passage. The genome of MCMV is both highly conserved and highly variable. The central regions of the genome, containing the betaherpesvirus and herpesvirus conserved genes, are highly conserved. However, significant variation occurs in species-specific genes at genomic termini, where the known and putative immune evasion genes reside. Variation in the MCMV genome consists of presence/absence polymorphisms in individual genes, grouping of genes into specific genotypes and random nucleotide diversity across the genome. However, much of the genotypic variation is under strong purifying selection indicating that these genotypes are fixed and conserved at the population level. The individual genotypes are either known to, or are likely to, target variant host gene products. Consequently, replication of MCMV is likely to be viral strain dependent and reflect the particular repertoire of genes encoded by the infecting strain. A total of 22 MCMV genes are genotypic, indicating considerable potential for variation in the MCMV population. This variation likely reflects genetic heterogeneity in the target population and suggests exquisite adaptation of the virus to its host.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Wolfram Brune writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Cytomegaloviruses (CMVs) are highly species-specific as they replicate almost exclusively in cells of their natural host species. However, the molecular basis of species specificity remains poorly understood. In cells of a foreign host a post-penetration block to viral gene expression and genome replication appears to restrict viral replication and spread. In some cases, infected cells of a foreign host undergo programmed cell death, indicating that apoptosis acts as a cellular antiviral defense mechanism to prevent viral replication. A few recent studies suggested that mediator and effector molecules of the interferon system and antiviral defenses operating at PML nuclear bodies (PML-NBs) might also be involved in restricting the host range of CMVs. Moreover, a recently isolated spontaneous mutant of murine CMV, which is capable of replicating to high titers in human cells, provided a new opportunity to study the mechanisms of CMV host species specificity. In this spontaneously adapted virus, mutations in the region encoding the viral Early1 (E1) proteins were found to be responsible for the extended host range phenotype. Further investigations of the CMV host species specificity should lead to a better understanding of the viral replication machinery, interfering host cell factors, and viral countermeasures.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Zsolt Ruzsics, Eva M. Borst, Jens B. Bosse, Wolfram Brune and Martin Messerle writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Cloning of a CMV genome as a BAC was reported for the first time more than a decade ago, and since then this approach has virtually opened the avenue for unrestricted mutagenesis of CMVs. This review gives an overview of recent developments in BAC-based mutagenesis techniques and their application to specific questions of CMV biology. One focus is on mutagenesis of essential CMV genes and the design of complementation strategies, as well as on conditional CMVs that allow the analysis of this class of viral genes in vitro and in vivo.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Jeffery L. Meier and Mark F. Stinski writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

CMV major immediate-early (MIE) gene expression activates the viral replicative cycle in both acute and reactivation infections and is greatly restricted in latent infection. Specific signaling cascades, transcriptional regulatory hierarchies, and cis-regulatory codes govern the initiation efficacy, magnitude, and sustainability of MIE gene transcription. The MIE enhancer/promoter, a major determinant in viral fitness, is at the heart of this control. It is equipped with complex regulatory circuitry that integrates diverse viral, cellular, and environmental cues. The MIE genes via differential RNA splicing produce a set of multifunctional proteins that function directly in advancing the viral life cycle. CMV-induced disease genesis is driven by the regulatory mechanisms underlying both the expression of the MIE genes and the actions of the MIE gene products. A better understanding of the MIE enhancer and its gene products could potentially spawn novel strategies for preventing CMV-related disease.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Anamaris M. Colberg-Poley and Chad D. Williamson writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

As with most DNA viruses, which require nuclear and cytoplasmic phases of virion maturation, proper and coordinated trafficking of viral proteins is crucial for the CMV lifecycle. Trafficking of CMV proteins enables jumpstarting its infection, partly determines whether lytic or latent infection is established, promotes nuclear and cytoplasmic assembly of virions, and enhances their stability and egress. To allow complex processes including viral DNA replication, packaging, nuclear and cytoplasmic egress, trafficking of CMV proteins is temporally and spatially regulated by modifications, particularly phosphorylation, and by interactions between viral or cellular proteins. Thus, orchestrated recruitment and colocalization of necessary components to enable functions are assured. In addition to conventional nuclear and cytoplasmic trafficking of viral proteins, HCMV encodes an antiapoptotic UL37 exon 1 protein or viral mitochondria-localized inhibitor of apoptosis, which circuitously traffics from the ER to mitochondria and through ER subdomains known as mitochondria-associated membranes. By this unconventional trafficking, HCMV is able to commandeer ER-mitochondrial cross-talk as well as mitochondrial functions, metabolism, antiviral responses, and apoptosis. The importance of proper intracellular trafficking of some key HCMV proteins such as those required for its DNA replication and assembly is supported by the deleterious effects of their inhibition on HCMV permissive growth.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Silvia Vidal, Astrid Krmpotić, Michal Pyzik and Stipan Jonjić writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Cells of the innate immune system, including macrophages, DCs and NK cells play an important role in the control of viral infection before the induction of a specific immune response, of which generation they are also crucial. The infection of mice with MCMV as a model of HCMV infection has been particularly informative in elucidating the role of innate and adaptive immune response mechanisms during infection. NK cells are considered the most important effector cells in early CMV surveillance. An evolutionary struggle between NK cells and CMVs can be inferred from the existence of a broad range of viral mechanisms designed to compromise NK-cell function. This review describes major innate immune response mechanisms involved in control of MCMV, with an emphasis on NK-cell mediated viral detection as well as virally-encoded immune evasion mechanisms.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Marc Dalod and Christine A. Biron writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

In addition to mediating early defense during primary infection, innate immunity delivers immunoregulatory functions to shape innate and adaptive immunity. Basic knowledge on the mechanisms inducing innate cytokine responses and the consequences for orchestrating downstream immunity is being advanced at a dramatic rate. Studies of murine cytomegalovirus (MCMV) infections in mice have contributed many of the breakthrough discoveries in these areas. Although intrinsic differences in viruses, hosts, and infection sites result in unique host-microbe relationships, there are overlapping effects mediated by host cell subsets and molecules. Thus, the information resulting from the leading-edge characterization of responses to MCMV has set the framework for understanding responses to a variety of infections in humans and mice and provided insights on approaches for enhancing resistance to virus-induced diseases. Key areas of progress include sensing of infection, co-ordination of cellular and innate cytokine cascades, consequences for delivery of innate and adaptive immunity to mediate defense, and regulation to protect from immune-mediated damage. These are reviewed here with a focus on the early cytokine/chemokine networks as infection spreads through a host. The surprisingly elegant picture emerging is one of profound flexibility for orchestrating optimal subset innate responses to protect against diverse and complex infectious organisms.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Christof K. Seckert, Marion Grieβl, Julia K. Büttner, Kirsten Freitag and Niels A.W. Lemmermann and Mary A. Hummel, Xue-Feng Liu and Michael I. Abecassis and Ana Angulo and Martin Messerle and Charles H. Cook and Matthias J. Reddehase writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Cytomegalovirus (CMV) disease with cytopathogenic viral replication and multiple organ involvement is typically confined to the immunocompromised or immunologically immature host. In the immunocompetent host, productive primary CMV infection is efficiently controlled, and is eventually resolved at all tissue sites, by well-orchestrated mechanisms of the innate and adaptive branches of the immune system in due time to prevent overt disease manifestations. At the earliest stages of an acute infection, NK cells rapidly followed by virus epitope-specific CD8⁺ T cells play major antiviral roles, and recent findings indicate that these two effector systems are cross-talking for keeping the virus in check despite the fact that during co-speciation with their specific host species all CMVs have evolved strategies to reduce the infected cells' susceptibility to both NK cell-mediated and CD8⁺ T cell-mediated antiviral immune functions. The outcome of this virus-host struggle for survival is a ceasefire in which the viral genome is not cleared but is maintained for the lifetime of the individual host in the presence of a fully developed, protective antiviral 'immune memory' without producing infectious viral progeny but retaining the functional capacity to complete the productive replication cycle under conditions of waning immune surveillance and transcription factor-mediated viral gene desilencing as a result of inflammatory cytokine signaling. These phenomena, known as 'latency' and 'reactivation' are biological hallmarks that CMVs share with all other members of the herpesvirus family. Notably, while immune surveillance appears to play a central role in maintaining latency, that is in preventing the virus from completing the productive replication cycle and, if it nevertheless should happen locally, preventing recurrent virus from further rounds of infection and spreading, increasing evidence suggests that the establishment of latency on the molecular level may not be immune-driven. Rather, molecular latency results from the cells' intrinsic antiviral defense by epigenetic silencing of viral gene expression associated with rapid circularization and chromatinization of incoming linear viral genomes within repressive nuclear domains. In this view, 'latency' is the default state, whereas productive infection, from the hosts' perspective, is the accident when viral genomes evade epigenetic silencing, with the chance for this being dependent on cell type, cell differentiation stage, cell cycle stage, and an overall nuclear environment that favors open chromatin structures, collectively defining what we describe as 'permissively' for productive infection. It is proposed that during latency stochastic episodes of promiscuous desilencing of single or combinatorial sets of viral genes can lead to the expression of transcripts (transcript expressed in latency, TEL), which, when translated into proteins, can result in the presentation of antigenic peptides sensed by tissue-patrolling effector-memory T cells. Importantly, promiscuous gene desilencing, unlike reactivation, does not usually initiate the productive viral replication cycle and can affect any viral gene regardless of its temporal expression in the kinetic classes immediate-early (IE), early (E) and late (L), and regardless of the function it takes during lytic infection. It is our current understanding that these limited desilencing episodes are the molecular motor that drives the CMV-typic expansion of T cells, of CD8⁺ T cells in particular, a phenomenon commonly known under the catchphrase 'memory inflation'. The 'classical era' of research in diverse murine models of CMV latency and reactivation has been reviewed by Jordan (1983) and authors of this review have provided updates (Hummel and Abecassis, 2002; Reddehase et al., 2002; 2008). Here, independent research groups have joined to review their more recent results and current views on CMV latency and reactivation based on murine CMV models with focus on neonatal infection, hematopoietic (stem) cell transplantation (HCT), solid organ transplantation (SOT), and sepsis.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from M. Shane Smith, Daniel N. Streblow, Patrizia Caposio, and Jay A. Nelson writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

The generation of mice engrafted with human hematopoietic stem cells (HSC) has allowed, for the first time, the study of human specific viruses in an in vivo setting. These humanized mouse models have been developed and improved over the past 30 years. It is now possible to achieve high levels of human cell engraftment producing human myeloid and lymphoid lineage cells. Humanized mouse models have been increasingly utilized in the study of human cytomegalovirus, a human-specific beta-herpesvirus that infects myeloprogenitor cells and establishes a life-long latency in the infected host. Upon mobilization and differentiation of infected bone marrow progenitor cells the latent virus reactivates and disseminates to other tissues. In this paper, we review the current status of the HSC-engrafted mouse models used to study HCMV latency and reactivation. We will first highlight the role myeloid lineage cells plays in HCMV biology and then describe the types of humanized mouse models that have been used in HIV, EBV, KSHV and HCMV anti-viral therapy studies. We will then describe recent studies utilizing the latest generation of humanized mice for the study of HCMV latency and reactivation and outline the future role that these models may play in the study of human-specific viruses.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Joshua D. Rabinowitz and Thomas Shenk writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Metabolic changes at the whole organism and cellular levels have been described for many diseases, and the alterations often underlie disease progression. Known metabolites can be quantified by using liquid chromatography to fractionate a complex mixture of compounds with analysis of the output by mass spectrometry. This approach has been applied to quantify steady state levels of metabolites as well as to monitor the flux of isotopically labeled metabolites through pathways in human cytomegalovirus-infected fibroblasts. Cytomegalovirus hijacks cellular metabolism, markedly inducing flux through much of central carbon metabolism, including glycolysis, nucleotide metabolism, the tricarboxylic acid cycle and the fatty acid metabolic enzyme acetyl-CoA carboxylase. This review details the metabolic changes that accompany infection, discusses the current understanding of mechanisms underlying the changes, and considers the physiological roles of the changes in human cytomegalovirus replication and spread.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Chris A. Benedict, Karine Crozat, Mariapia Degli-Esposti and Marc Dalod writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Mouse cytomegalovirus (MCMV) was first isolated more than half a century ago (Smith, 1954). Subsequent studies of MCMV in its natural host have yielded enormous information regarding the cellular and molecular immune mechanisms that regulate the various phases of this lifelong β-herpesvirus infection. As the techniques and tools for studying mechanisms of immune defense in mice have advanced, so has our understanding of the specific host pathways that operate to control this complex host-pathogen relationship. In this paper we will review how various mouse genetic models have defined an initial blueprint for how immune control of MCMV is achieved, both at the level of innate and adaptive immunity, and where we foresee the advancements in this model of CMV infection will come from in the future.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Deborah H. Spector writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Successful replication of HCMV requires the deployment of multiple approaches to commandeer the host cell machinery and create a cellular milieu that is optimal for viral gene expression, DNA replication, and formation of infectious progeny. The complex regulatory network that drives cell cycle progression provides a rich source of factors that can be co-opted and combined in different ways to tailor the host cell's environment to meet the needs of the virus for productive infection. To this end, HCMV dramatically alters cell cycle regulatory pathways, leading to cell cycle arrest. These alterations begin as soon as the viral particle enters the cell and continue throughout the entire replicative cycle. The molecular mechanisms underlying the viral-mediated effects operate at multiple levels, including altered RNA transcription, inhibition of cell DNA synthesis, changes in the levels and activity of cyclin dependent kinases as well as other cellular kinases involved in cell cycle control, modulation of protein stability through targeted effects on the ubiquitin-proteasome degradation pathway, and movement of proteins to different cellular locations. This review will focus on the interplay between the viral and cellular factors and the mechanisms utilized to effect these changes as they relate to the cell cycle.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Matthew Reeves and John Sinclair writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

The myeloid lineage is now accepted to be an important site in vivo for the carriage of latent HCMV genomes, but the mechanisms underlying how the latent state is maintained and how latent virus reactivates are still far from clear. In this review, we discuss how analyses of promoter binding proteins and post-translational modifications of histones on viral promoters during virus infection have led to an understanding that the higher-order chromatin structure around the viral major immediate-early promoter region has profound effects on the control of viral latency and reactivation. We further discuss the role of chromatin during lytic infection and how this may also give insights into the cellular mechanisms important for the establishment and control of latent infection.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Mirko Trilling and Hartmut Hengel writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Interferons (IFNs) comprise a family of three different subtypes (I, II and III) of related cytokines which share their potent immuno-stimulatory and antiviral function. IFN secretion is initiated by synchronous activation of distinct classes of transcription factors (ATF/cJun, IRFs, NF-κB) upon recognition of conserved pathogen-associated molecular patterns (PAMPs) by germ-line-encoded pathogen recognition receptors (PRRs). Binding of the transcription factors to the ifn-b promoter/enhancer assembles the IFN enhanceosome, leading to IFN transcription. Secreted IFNs signal in an autocrine and paracrine manner via Jak-STAT signal transduction pathways stimulating a far-reaching transcriptional program of >300 differentially expressed genes to orchestrate intrinsic, innate and adaptive immunity. The intimate co-adaptation of cytomegaloviruses with their respective host species led to the evolution of multiple viral countermeasures which mitigate the antiviral effect of IFNs. The number of identified HCMV- and MCMV-encoded gene products interfering with IFN induction, IFN receptor signalling or IFN effector functions, is steadily growing. This review aims to provide a snapshot of our current understanding of the balance of power between pro- and antiviral measures positioned between CMV and the host IFN system. Given the immense selective pressure elicited by IFNs, it is tempting to speculate that IFNs have driven CMV to evolve a high number of antagonistic genes ensuring the complex counterbalance with IFNs and promoting CMV replication in an IFN containing environment. Counterintuitively, CMV appears also to exploit IFN induced transcription to enhance its gene expression under appropriate conditions.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Meaghan H. Hancock, Igor Landais, Lauren M. Hook, Finn Grey, Rebecca Tirabassi and Jay A. Nelson writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

MicroRNAs (miRNAs) represent an important class of small regulatory RNAs regulate cellular processes including development and malignancies. Since the discovery that viruses encode miRNAs over 240 viral miRNAs have been identified primarily in the herpesvirus family. The cytomegalovirus (CMV) family encodes multiple miRNAs encoded throughout the viral genome. Recent work has shown that the CMV miRNAs regulate expression of both viral and cellular genes including the CMV immediate early gene 1 (IE1) and cellular genes involved in immune recognition and cell cycle regulation. In this paper we will review our current knowledge of the targets and function of CMV encoded miRNAs.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Stanley A. Plotkin and Bodo Plachter writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Prenatal transmission of CMV is a frequent cause of mental retardation and hearing loss in children. Furthermore, infection with this virus is a severe threat to immunocompromised patients. Consequently, development of a vaccine to prevent CMV disease has been identified as a first rank medical priority. Goals and target populations for such a vaccine have been identified. Antigens to be targeted by vaccine-induced immune responses have been defined indicating that only a subset of the more than 150 viral proteins may be sufficient to induce protective immunity. Using this information, strategies for the development of live virus vaccines as well as subunit vaccines have been developed. At this point at least seven candidate vaccines have been tested clinically and many other approaches are being explored.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Lenore Pereira, Takako Tabata, Matthew Petitt and June Fang-Hoover writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

During human pregnancy, HCMV spreads from the infected mother to the fetus, navigating the complex architecture of the human placenta, which anchors the fetus to the uterus. Primary sites of virus replication in the placenta include cytotrophoblast progenitor cells in chorionic villi and differentiating/invading cytotrophoblasts that breach uterine blood vessels and form a hybrid vasculature, increasing blood flow to the surface of the placenta. Focal virus replication and induction and release of paracrine factors result in pathology and a hypoxic intrauterine environment that stimulates compensatory development of vascularized floating villi. This review summarizes recent insights into the molecular changes that occur during virus replication, strategies to rescue development of the human placenta and the utility of placental villous explants and xenografts in SCID mice for quantifying infection in vitro and pathogenesis in vivo.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Patrizia Caposio, Daniel N. Streblow and Jay A. Nelson writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Proteomics is the large-scale study of proteins, particularly their structure and interaction. In this review we will discuss the results obtained using a proteomic approach to analyse what is secreted from cytomegalovirus infected cells: the composition of the viral and subviral particles as well as the cellular factors that are involved in the viral pathogenesis. In the previous edition we described the viral and cellular proteins that compose the infectious HCMV virion, the entry competent, non-replicating viral particles such as Dense Bodies (DBs) and Non-Infectious Enveloped Particles (NIEPs). Using a gel-free 2-D capillary liquid chromatography (LC)-MS/MS and a Fourier transform ion cyclotron resonance (FTICR) mass spectrometry we were able to identify the relative abundance of viral and cellular proteins in purified HCMV particles. The first part of this review will be an up-date of the literature that has been published in these last few years on the structure and composition of the viral particles. The second part of the review will be dedicated to the analysis of the cellular factors secreted from infected cells that act in a paracrine fashion to enhance wound healing (WH) and angiogenesis (AG) associated with the development of long term diseases like atherosclerosis, transplant vascular sclerosis (TVS), chronic allograft rejection (CR) and glioblastoma.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Barbara Adler and Christian Sinzger writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Cytomegaloviruses (CMVs) are host species-specific pathogens that cause life-long persistent infections. Under conditions of reduced immune responses CMVs can cause acute systemic infections with replication in virtually any organ. The broad organ tropism is based on an equally broad range of target cell types. Epithelial cells, fibroblasts, endothelial cells and smooth muscle cells are the major target cells that support highly productive HCMV infection. Hepatocytes, trophoblasts, neurons, macrophages and dendritic cells are also susceptible to the full replication cycle of HCMV but are apparently less productive. Granulocytes and monocytes are non-productively infected by HCMV but are assumed to contribute to hematogenous dissemination as passive vehicles. Glycoprotein complexes containing gH-gL were identified as major determinants of the cell tropism of HCMV. They are assumed to recognize entry receptors and to trigger fusion of viral and cellular membranes during entry either directly at the plasma membrane or within endosomes. Virus strains that only incorporate gH-gL-gO in their envelope have a restricted target cell range excluding endothelial cells, epithelial cells and leukocytes whereas strains that also incorporate gH-gL-pUL128-pUL130-pUL131A have an extended target cell range including these cell types. HCMV progeny of the latter strains consists of distinct populations containing either high levels or low levels of the gH-gL-pUL128-pUL130-pUL131A complex thus allowing cells to navigate virus progeny by selectively releasing or retaining virion populations that differ in their tropism.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Andrew J. Davison, Mary Holton, Aidan Dolan, Derrick J. Dargan, Derek Gatherer and Gary S. Hayward writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

The subfamily Betaherpesvirinae contains four genera and three species not yet assigned to genera. CMVs belong to this subfamily, and are usually reckoned to consist of primate viruses (such as HCMV) in the genus Cytomegalovirus, rodent viruses (such as MCMV) in the genus Muromegalovirus, and two of the unassigned viruses (GPCMV and tupaiid herpesvirus 1). In this review, we focus primarily on members of the genus Cytomegalovirus, which infect apes (including humans) and Old and New World monkeys. The genomes of nine of these viruses, representing six species, have been sequenced fully. They are distinguished by being among the largest and most complex in the family Herpesviridae, by exhibiting an extraordinary degree of interspecies and interstrain variation, and by all having suffered functional losses via mutation. These characteristics continue to contribute to the fascination and challenge of CMV genomics.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Robin K. Avery writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Heart and lung transplantation can be lifesaving therapies for end-stage organ disease in some patients. Despite advances in antiviral prevention, cytomegalovirus infection is still an important issue in post-transplant management and may contribute to survival-limiting dysfunction of the transplanted organ. This paper reviews the clinical presentations and risk factors for CMV infection in thoracic transplant recipients, as well as the direct and indirect effects of CMV. Recent studies on CMV-specific immunity and allograft dysfunction have shed further light on the differential benefits of prophylaxis and pre-emptive strategies for CMV prevention, as well as the potential benefits of an extended duration of prophylaxis. Finally, the development of newer anti-CMV agents holds promise for therapeutic management in the future.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Vincent C. Emery, Richard S.B. Milne and Paul D. Griffiths writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

CMV infection significantly impacts on the success of transplantation of abdominal organs. There are a range of both direct and indirect effects attributable to CMV. In this review we survey our current understanding of CMV pathogenesis, the immune control of CMV replication in these transplanted patients, the antiviral chemotherapeutic options available for managing infection/disease and consider the newer options available for drug therapy and for vaccination.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Sachiko Seo and Michael Boeckh writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

HCMV infection remains an important complication after hematopoietic cell transplantation (HCT), although significant progress in the management of HCMV infection and disease has been made in the last two decades. A major achievement has been the optimization of preemptive therapy strategies based on surveillance by the pp65 antigenemia assay or HCMV DNA or RNA detection. However, current strategies are limited by the toxicity of antiviral agents and breakthrough disease in highly immunosuppressed individuals. Antiviral resistance occurs infrequently after HCT, but can be a considerable management challenge. This review outlines the current topics in host immunity, diagnosis, prevention, treatment of HCMV disease in HCT recipients, and future directions of HCMV management.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Stuart P. Adler and Giovanni Nigro writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

The epidemiology and pathogenesis of CMV infections among pregnant women have been intensely studied over the last three decades. This review focuses on recent research developments related to CMV infections during pregnancy. The developments include an understanding of the pathogenesis of CMV infections, knowledge of high risk women, and potentially effective interventions to prevent maternal infections during pregnancy.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from A. Louise McCormick and Edward S. Mocarski writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Cytomegalovirus (CMV) deploys multiple strategies to overcome host intrinsic, innate, and adaptive responses that limit infection by triggering cell death. Multiple cell death suppressors are encoded by cytomegaloviruses infecting humans, monkeys and rodents. The viral inhibitor of caspase activation (vICA) and even the viral mitochondrial-localized inhibitor of apoptosis (vMIA) represent evolutionarily conserved strategies, whereas viral inhibitor of receptor-interacting protein kinase (RIP) activation (vIRA), the mitochondrial complex I-associated b2.7 RNA and other viral gene products whose mechanisms are not fully understood, appear to have evolved independently in primate and rodent CMVs. Initiators, effectors and interactions between CMV-induced cell death pathways have begun to emerge, through studies in cell culture and intact animals. It has become very clear that many cell death pathways are targeted by CMV-encoded cell death suppressors. Indeed, this subfamily of viruses has provided fundamental insights into pathogen-triggered regulated cell death pathways in mammals. Whereas both intrinsic and extrinsic caspase-dependent apoptosis was well-established, studies in CMV brought serine protease-dependent programmed cell death and RIP3 programmed necrotic death pathways to the fore. Virus-encoded cell death suppressors contribute resistance to cell stress as well as resistance to disruption of critical metabolic and respiratory activities. Challenges for investigators interested in this area continue to be integration of findings using diverse viral strains that impact metabolic and stress pathways with seemingly subtle differences.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Detlef Michel, Meike Chevillotte and Thomas Mertens writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Three drugs are currently used for treatment of human cytomegalovirus (HCMV) disease or infection: ganciclovir (GCV)/valganciclovir (valGCV), cidofovir (CDV), and foscarnet (FOS). They all target the viral DNA polymerase pUL54, thereby inhibiting viral DNA replication. In contrast to antiviral treatment of overt HCMV disease, antiviral prophylaxis and pre-emptive therapy aim at prevention of HCMV disease. All current anti-HCMV compounds cause drug-specific and severe side effects and have been reported to select for clinically relevant drug resistant virus variants. Active systemic HCMV infection can be first asymptomatic or symptoms can be non-specific. Therefore, a reliable and fast diagnosis of active systemic viral infection is needed, based on virological markers. This includes early and quantitative detection of drug resistant HCMV variants. Genotyping will become the method of choice for identification of viral drug resistance, but absolutely requires previous quantitative characterization of the effect of individual and combined mutations on the resistance phenotype. A database has been made available in the www containing all published mutations and according information on the quantitative resistance phenotypes. In view of the many published resistance mutations, the database is a helpful tool to correlate diagnosed mutations in a viral genome with resistance. New compounds to treat HCMV infection and disease are urgently needed. They should ideally combine few adverse effects, good oral bioavailability, the option to treat children in utero, as well as novel mechanisms of action, to possibly reduce selection of resistant virus variants by combination therapy. Some substances should also target early steps of viral infection, thereby inhibiting IE- and E-gene expression in infected cells. Modified and new nucleoside analogues, small-molecule compounds, benzimidazoles, indolocarbazole protein kinase inhibitors as well as drugs with originally other indications (leflunomide, artesunate) will soon augment the HCMV antiviral portfolio. They will expand treatment options, but will also increase the number of resistance-conferring mutations, thereby rendering resistance analyses even more complex.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Mark R. Wills, Gavin M. Mason and J. G. Patrick Sissons writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

Primary HCMV infection induces robust CD8⁺ cytotoxic, and CD4⁺ helper, T cell mediated immune responses, which are associated with the resolution of acute primary infection: these responses are maintained at high frequency in long-term memory as the virus establishes persistent infection, with latency and periodic reactivation. Many of these T cells are specific for epitopes in the pp65 and IE1 HCMV proteins, but it is apparent that many other viral proteins can also be T cell targets, and in some individuals pp65 and IE1 responses are not immunodominant. During long-term carriage of the virus a balance is established between the T cell mediated immune response and viral reactivation: the T cell response controls viral spread following reactivation, but the virus encodes multiple genes that interfere with MHC-I antigen processing (US 2, 3, 6 and 11), and with MHC-II processing and NK cell killing, allowing limited viral evasion of the response. Loss of this balance is most evident in the immunocompromised host in whom reactivation of latent virus or primary infection can lead to unchecked viral replication, with consequent disease and mortality. This review describes current understanding of CD8⁺ and CD4⁺ T cell responses to HCMV, and how these responses reconstitute after bone marrow transplantation and might be used as therapies to protect against HCMV disease in immunocompromised subjects. The phenomenon of "memory T cell inflation" associated with HCMV and its relationship to immunosenesence is also discussed.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention

from Peter Ghazal, Alexander Mazein, Steven Watterson, Ana Angulo and Kai A. Kropp writing in Cytomegaloviruses: From Molecular Pathogenesis to Intervention:

This review discusses the use of systems biology towards understanding the combinatorially complex set of molecular interactions that underpin the infection process by CMV. A hallmark of systems biology is the elucidation of pathways rather than single gene or protein activities. This generally involves the use of bioinformatics and computational modelling to analyse unbiased high throughput data such as those derived from whole genome sequencing, genome-wide transcriptomics, proteomics and metabolomics. The emerging studies in the area of CMV systems biology have to date underscored the requirement for host-dependencies on transcription factor networks, cell signalling, metabolism and cellular trafficking. Here we consider at the systems pathway level the importance of host-dependency and host-protection pathways in regulating the CMV transcription-replication cycle.

Further reading: Cytomegaloviruses: From Molecular Pathogenesis to Intervention