Virus entry and innate immune activation

A temperature-dependent virus-binding assay reveals the presence of neutralizing antibodies in human cytomegalovirus gB vaccine recipients' sera

Human cytomegalovirus (HCMV) remains an important cause of mortality in immune-compromised transplant patients and following congenital infection. Such is the burden, an effective vaccine strategy is considered to be of the highest priority. The most successful vaccines to date have focused on generating immune responses against glycoprotein B (gB) - a protein essential for HCMV fusion and entry. We have previously reported that an important component of the humoral immune response elicited by gB/MF59 vaccination of patients awaiting transplant is the induction of non-neutralizing antibodies that target cell-associated virus with little evidence of concomitant classical neutralizing antibodies. Here we report that a modified neutralization assay that promotes prolonged binding of HCMV to the cell surface reveals the presence of neutralizing antibodies in sera taken from gB-vaccinated patients that cannot be detected using standard assays. We go on to show that this is not a general feature of gB-neutralizing antibodies, suggesting that specific antibody responses induced by vaccination could be important. Although we can find no evidence that these neutralizing antibody responses are a correlate of protection in vivo in transplant recipients their identification demonstrates the utility of the approach in identifying these responses. We hypothesize that further characterization has the potential to aid the identification of functions within gB that are important during the entry process and could potentially improve future vaccine strategies directed against gB if they prove to be effective against HCMV at higher concentrations.

How Immunosenescence and Inflammaging May Contribute to Hyperinflammatory Syndrome in COVID-19

Aging is characterized by the dynamic remodeling of the immune system designated “immunosenescence,” and is associated with altered hematopoiesis, thymic involution, and lifelong immune stimulation by multitudinous chronic stressors, including the cytomegalovirus (CMV). Such alterations may contribute to a lowered proportion of naïve T-cells and to reduced diversity of the T-cell repertoire. In the peripheral circulation, a shift occurs towards accumulations of T and B-cell populations with memory phenotypes, and to accumulation of putatively senescent and exhausted immune cells. The aging-related accumulations of functionally exhausted memory T lymphocytes, commonly secreting pro-inflammatory cytokines, together with mediators and factors of the innate immune system, are considered to contribute to the low-grade inflammation (inflammaging) often observed in elderly people. These senescent immune cells not only secrete inflammatory mediators, but are also able to negatively modulate their environments. In this review, we give a short summary of the ways that immunosenescence, inflammaging, and CMV infection may cause insufficient immune responses, contribute to the establishment of the hyperinflammatory syndrome and impact the severity of the coronavirus disease 2019 (COVID-19) in elderly people.

Interferon-Independent Upregulation of Interferon-Stimulated Genes during Human Cytomegalovirus Infection is Dependent on IRF3 Expression

The antiviral activity of type I interferons (IFNs) is primarily mediated by interferon-stimulated genes (ISGs). Induction of ISG transcription is achieved when type I IFNs bind to their cognate receptor and activate the Janus Kinase/Signal Transducer and Activator of Transcription (JAK/STAT) signaling pathways. Recently it has become clear that a number of viruses are capable of directly upregulating a subset of ISGs in the absence of type I IFN production. Using cells engineered to block either the response to, or production of type I IFN, the regulation of IFN-independent ISGs was examined in the context of human cytomegalovirus (HCMV) infection. Several ISGs, including IFIT1, IFIT2, IFIT3, Mx1, Mx2, CXCL10 and ISG15 were found to be upregulated transcriptionally following HCMV infection independently of type I IFN-initiated JAK-STAT signaling, but dependent on intact IRF3 signaling. ISG15 protein regulation mirrored that of its transcript with IFNβ neutralization failing to completely inhibit ISG15 expression post HCMV infection. In addition, no detectable ISG15 protein expression was observed following HCMV infection in IRF3 knockdown CRISPR/Cas-9 clones indicating that IFN-independent control of ISG expression during HCMV infection of human fibroblasts is absolutely dependent on IRF3 expression.

Protection from cytomegalovirus viremia following glycoprotein B vaccination is not dependent on neutralizing antibodies

Human cytomegalovirus (HCMV) is an important pathogen in transplant patients and in congenital infection. Previously, we demonstrated that vaccination with a recombinant viral glycoprotein B (gB)/MF59 adjuvant formulation before solid organ transplant reduced viral load parameters post transplant. Reduced posttransplant viremia was directly correlated with antibody titers against gB consistent with a humoral response against gB being important. Here we show that sera from the vaccinated seronegative patients displayed little evidence of a neutralizing antibody response against cell-free HCMV in vitro. Additionally, sera from seronegative vaccine recipients had minimal effect on the replication of a strain of HCMV engineered to be cell-associated in a viral spread assay. Furthermore, although natural infection can induce antibody-dependent cellular cytotoxicity (ADCC) responses, serological analysis of seronegative vaccinees again presented no evidence of a substantial ADCC-promoting antibody response being generated de novo. Finally, analyses for responses against major antigenic domains of gB following vaccination were variable, and their pattern was distinct compared with natural infection. Taken together, these data argue that the protective effect elicited by the gB vaccine is via a mechanism of action in seronegative vaccinees that cannot be explained by neutralization or the induction of ADCC. More generally, these data, which are derived from a human challenge model that demonstrated that the gB vaccine is protective, highlight the need for more sophisticated analyses of new HCMV vaccines over and above the quantification of an ability to induce potent neutralizing antibody responses in vitro.

Virus infection facilitates the development of severe pneumonia in transplant patients with hematologic malignancies

Allogeneic hematopoietic stem cell transplantation (HSCT) is an effective therapy for patients with hematologic malignancies. Severe pneumonia is associated with high mortality rate in HSCT recipients. Viral co-infection indicates a poor prognosis of HSCT recipients. In this study, a total of 68 allogeneic HSCT recipients were included. Cytomegalovirus (CMV) and Respiratory syncytial virus (RSV) infection was assessed by testing peripheral blood and oropharynx swabs, respectively, collected in the first 180 days after transplantation. We analysed the correlation of CMV and RSV co-infection with severe pneumonia and mortality. The incidence of CMV and RSV co-infection was 26.5% (18/68). Severe pneumonia was diagnosed in 61% (11/18) cases with co-infection compared to only 10% (5/50) cases with mono-infection or no infection. The analysis of potential risk factors for severe pneumonia showed that CMV and RSV co-infection was significantly associated with severe pneumonia (p < 0.001). The 5 patients who died of severe pneumonia were all co-infected with CMV and RSV. In conclusion, CMV and RSV co-infection appears to be an important factor and facilitates the development of severe pneumonia in allogeneic HSCT patients with hematologic malignancies.

Positively Selected Sites at HCMV gB Furin Processing Region and Their Effects in Cleavage Efficiency

Human cytomegalovirus is a ubiquitous infectious agent that affects mainly immunosuppressed, fetuses, and newborns. The virus has several polymorphic regions, in particular in the envelope glycoproteins. The UL55 gene encodes the glycoprotein B that has a variable region, containing a furin cleavage site and according to the variability different genotypes are characterized. Here we investigated variability and existence of selective pressure on the UL55 variable region containing the furin cleavage site in 213 clinical sequences from patients worldwide. We showed the occurrence of positive selective pressure on gB codons 461 and 462, near the furin cleavage site. Cleavage analysis of synthesized peptides demonstrated that most mutations confer better cleavage by furin, suggesting that evolution is acting in order to increase the efficiency cleavage and supporting the hypothesis that gB processing is important in the host. We also demonstrated that peptides containing sequences, that characterize genotypes gB2 and 3, are differentially cleaved by furin. Our data demonstrate for the first time that variability in the cleavage site is related to degree of gB processing by furin.

Cytomegalovirus Vaccines: Current Status and Future Prospects

Congenital human cytomegalovirus (HCMV) infection can result in severe and permanent neurological injury in newborns, and vaccine development is accordingly a major public health priority. HCMV can also cause disease in solid organ transplant (SOT) and hematopoietic stem-cell transplant (HSCT) recipients, and a vaccine would be valuable in prevention of viremia and end-organ disease in these populations. Currently there is no licensed HCMV vaccine, but progress toward this goal has been made in recent clinical trials. A recombinant HCMV glycoprotein B (gB) vaccine has been shown to have some efficacy in prevention of infection in young women and adolescents, and has provided benefit to HCMV-seronegative SOT recipients. Similarly, DNA vaccines based on gB and the immunodominant T-cell target, pp65 (ppUL83), have been shown to reduce viremia in HSCT patients. This review provides an overview of HCMV vaccine candidates in various stages of development, as well as an update on the current status of ongoing clinical trials. Protective correlates of vaccine-induced immunity may be different for pregnant woman and transplant patients. As more knowledge emerges about correlates of protection, the ultimate licensure of HCMV vaccines may reflect the uniqueness of the target populations being immunized.

The Microtubule Inhibitor Podofilox Inhibits an Early Entry Step of Human Cytomegalovirus

Human cytomegalovirus is a ubiquitous β-herpesvirus that infects many different cell types through an initial binding to cell surface receptors followed by a fusion event at the cell membrane or endocytic vesicle. A recent high-throughput screen to identify compounds that block a step prior to viral gene expression identified podofilox as a potent and nontoxic inhibitor. Time-of-addition studies in combination with quantitative-PCR analysis demonstrated that podofilox limits an early step of virus entry at the cell surface. Podofilox was also able to drastically reduce infection by herpes simplex 1, an α-herpesvirus with a very similar entry process to CMV. Podofilox caused a reduced maximal plateau inhibition of infection by viruses with single step binding processes prior to fusion-like Newcastle disease virus, Sendai virus, and influenza A virus or viruses that enter via endocytosis like vesicular stomatitis virus and a clinical-like strain of CMV. These results indicate that microtubules appear to be participating in the post-binding step of virus entry including the pre- and post-penetration events. Modulation of the plasma membrane is required to promote virus entry for herpesviruses, and that podofilox, unlike colchicine or nocodazole, is able to preferentially target microtubule networks at the plasma membrane.

Human cytomegalovirus vaccination: progress and perspectives of recombinant gB

A vaccine for Human cytomegalovirus (HCMV) remains a high priority as complications following infection are observed in immunocompromised individuals and in congenitally infected neonates. Numerous preclinical and clinical studies have investigated vaccine strategies ranging from live attenuated preparations, nucleic acid-based approaches and recombinant delivery systems to subunit vaccines. These have defined the importance of both cell-mediated and humoral immunity to viral gB in the control of HCMV infection. This review will cover clinical trials investigating vaccine approaches that have incorporated gB and discuss the future perspectives of the recombinant gB subunit vaccine for HCMV.

The Human Cytomegalovirus UL116 Gene Encodes an Envelope Glycoprotein Forming a Complex with gH Independently from gL

Human cytomegalovirus (HCMV) is a major cause of morbidity and mortality in transplant patients and is the leading viral cause of birth defects after congenital infection. HCMV infection relies on the recognition of cell-specific receptors by one of the viral envelope glycoprotein complexes. Either the gH/gL/gO or the gH/gL/UL128/UL130/UL131A (Pentamer) complex has been found to fulfill this role, accounting for HCMV entry into almost all cell types. We have studied the UL116 gene product, a putative open reading frame identified by in silico analysis and predicted to code for a secreted protein. Virus infection experiments in mammalian cells demonstrated that UL116 is expressed late in the HCMV replication cycle and is a heavily glycosylated protein that first localizes to the cellular site of virus assembly and then inserts into the virion envelope. Transient-transfection studies revealed that UL116 is efficiently transported to the plasma membrane when coexpressed with gH and that gL competes with UL116 for gH binding. Further evidence for gH/UL116 complex formation was obtained by coimmunoprecipitation experiments on both transfected and infected cells and biochemical characterization of the purified complex. In summary, our results show that the product of the UL116 gene is an HCMV envelope glycoprotein that forms a novel gH-based complex alternative to gH/gL. Remarkably, the gH/UL116 complex is the first herpesvirus gH-based gL-less complex.

IMPORTANCE

HCMV infection can cause severe disease in immunocompromised adults and infants infected in utero The dissection of the HCMV entry machinery is important to understand the mechanism of viral infection and to identify new vaccine antigens. The gH/gL/gO and gH/gL/UL128/UL130/UL131 (Pentamer) complexes play a key role in HCMV cell entry and tropism. Both complexes are formed by an invariant gH/gL scaffold on which the other subunits assemble. Here, we show that the UL116 gene product is expressed in infected cells and forms a heterodimer with gH. The gH/UL116 complex is carried on the infectious virions, although in smaller amounts than gH/gL complexes. No gH/UL116/gL ternary complex formed in transfected cells, suggesting that the gH/UL116 complex is independent from gL. This new gH-based gL-free complex represents a potential target for a protective HCMV vaccine and opens new perspectives on the comprehension of the HCMV cell entry mechanism and tropism.

Is Pulmonary non-Tuberculous Mycobacterial Disease Linked with a High Burden of Latent Cytomegalovirus?

Cytomegalovirus (CMV) establishes lifelong infections with episodes of active replication. We hypothesized that recurrent CMV replication in older individuals may suppress protective immune responses to non-tuberculous mycobacteria (NTM) and so potentiate pulmonary disease. Accordingly, levels of antibodies to three CMV antigen preparations were higher in NTM patients than in age-matched controls. This did not reflect broad-spectrum B cell activation as total immunoglobulin levels were not equivalently increased.

The life cycle and pathogenesis of human cytomegalovirus infection: lessons from proteomics

Viruses have coevolved with their hosts, acquiring strategies to subvert host cellular pathways for effective viral replication and spread. Human cytomegalovirus (HCMV), a widely-spread β-herpesvirus, is a major cause of birth defects and opportunistic infections in HIV-1/AIDS patients. HCMV displays an intricate system-wide modulation of the human cell proteome. An impressive array of virus-host protein interactions occurs throughout the infection. To investigate the virus life cycle, proteomics has recently become a significant component of virology studies. Here, we review the mass spectrometry-based proteomics approaches used in HCMV studies, as well as their contribution to understanding the HCMV life cycle and the virus-induced changes to host cells. The importance of the biological insights gained from these studies clearly demonstrate the impact that proteomics has had and can continue to have on understanding HCMV biology and identifying new therapeutic targets.

A peptide inhibitor of cytomegalovirus infection from human hemofiltrate

Naturally occurring substances with antimicrobial activity can serve as a starting point for the rational design of new drugs to treat infectious diseases. Here, we screened a library of peptides derived from human hemofiltrate for inhibitory effects on human cytomegalovirus (CMV) infection. We isolated a previously unknown derivative of the neutrophil-activating peptide 2, which we termed CYVIP, for CMV-inhibiting peptide. The peptide blocked infection with human and mouse CMV as well as with herpes simplex virus type 1 in different cell types. We found that CYVIP interferes with virus attachment to the cell surface, and structure-activity relationship studies revealed that positively charged lysine and arginine residues of CYVIP are essential for its inhibitory activity. The N-terminal 29 amino acids of the peptide were sufficient for inhibition, and substitution with an acidic residue further improved its activity. The target structure of CYVIP on the cell surface seems to be the sulfate residues of heparan sulfate proteoglycans, which are known to serve as herpesvirus attachment receptors. Our data suggest that O-sulfation of heparan sulfate is required for binding of CYVIP, and furthermore, that the initial interaction of CMV particles with cells takes place preferentially via 6-O-linked sulfate groups. These findings about CYVIP's mode of action lay the basis for further development of antivirals interfering with attachment of CMV to cells, a crucial step of the infection cycle.

The tiers and dimensions of evasion of the type I interferon response by human cytomegalovirus

Human cytomegalovirus (HCMV) is a member of the β-herpesvirus family that invariably occupies hosts for life despite a consistent multi-pronged antiviral immune response that targets the infection. This persistence is enabled by the large viral genome that encodes factors conferring a wide assortment of sophisticated, often redundant phenotypes that disable or otherwise manipulate impactful immune effector processes. The type I interferon system represents a first line of host defense against infecting viruses. The physiological reactions induced by secreted interferon act to effectively block replication of a broad spectrum of virus types, including HCMV. As such, the virus must exhibit counteractive mechanisms to these responses that involve their inhibition, tolerance, or re-purposing. The goal of this review is to describe the impact of the type I interferon system on HCMV replication and to showcase the number and diversity of strategies employed by the virus that allow infection of hosts in the presence of interferon-dependent activity.

Broad-spectrum antiviral activity of chebulagic acid and punicalagin against viruses that use glycosaminoglycans for entry

BACKGROUND

We previously identified two hydrolyzable tannins, chebulagic acid (CHLA) and punicalagin (PUG) that blocked herpes simplex virus type 1 (HSV-1) entry and spread. These compounds inhibited viral glycoprotein interactions with cell surface glycosaminoglycans (GAGs). Based on this property, we evaluated their antiviral efficacy against several different viruses known to employ GAGs for host cell entry.

RESULTS

Extensive analysis of the tannins' mechanism of action was performed on a panel of viruses during the attachment and entry steps of infection. Virus-specific binding assays and the analysis of viral spread during treatment with these compounds were also conducted. CHLA and PUG were effective in abrogating infection by human cytomegalovirus (HCMV), hepatitis C virus (HCV), dengue virus (DENV), measles virus (MV), and respiratory syncytial virus (RSV), at μM concentrations and in dose-dependent manners without significant cytotoxicity. Moreover, the natural compounds inhibited viral attachment, penetration, and spread, to different degrees for each virus. Specifically, the tannins blocked all these steps of infection for HCMV, HCV, and MV, but had little effect on the post-fusion spread of DENV and RSV, which could suggest intriguing differences in the roles of GAG-interactions for these viruses.

CONCLUSIONS

CHLA and PUG may be of value as broad-spectrum antivirals for limiting emerging/recurring viruses known to engage host cell GAGs for entry. Further studies testing the efficacy of these tannins in vivo against certain viruses are justified.

Discovery of cellular proteins required for the early steps of HCV infection using integrative genomics

Successful viral infection requires intimate communication between virus and host cell, a process that absolutely requires various host proteins. However, current efforts to discover novel host proteins as therapeutic targets for viral infection are difficult. Here, we developed an integrative-genomics approach to predict human genes involved in the early steps of hepatitis C virus (HCV) infection. By integrating HCV and human protein associations, co-expression data, and tight junction-tetraspanin web specific networks, we identified host proteins required for the early steps in HCV infection. Moreover, we validated the roles of newly identified proteins in HCV infection by knocking down their expression using small interfering RNAs. Specifically, a novel host factor CD63 was shown to directly interact with HCV E2 protein. We further demonstrated that an antibody against CD63 blocked HCV infection, indicating that CD63 may serve as a new therapeutic target for HCV-related diseases. The candidate gene list provides a source for identification of new therapeutic targets.

Diverse immune evasion strategies by human cytomegalovirus

Members of the Herpesviridae family have the capacity to undergo both lytic and latent infection to establish a lifelong relationship with their host. Following primary infection, human cytomegalovirus (HCMV) can persist as a subclinical, recurrent infection for the lifetime of an individual. This quiescent portion of its life cycle is termed latency and is associated with periodic bouts of reactivation during times of immunosuppression, inflammation, or stress. In order to exist indefinitely and establish infection, HCMV encodes a multitude of immune modulatory mechanisms devoted to escaping the host antiviral response. HCMV has become a paradigm for studies of viral immune evasion of antigen presentation by both major histocompatibility complex (MHC) class I and II molecules. By restricting the presentation of viral antigens during both productive and latent infection, HCMV limits elimination by the human immune system. This review will focus on understanding how the virus manipulates the pathways of antigen presentation in order to modulate the host response to infection.

An endothelial cell-specific requirement for the UL133-UL138 locus of human cytomegalovirus for efficient virus maturation

Human cytomegalovirus (HCMV) infects a variety of cell types in humans, resulting in a varied pathogenesis in the immunocompromised host. Endothelial cells (ECs) are considered an important target of HCMV infection that may contribute to viral pathogenesis. Although the viral determinants important for entry into ECs are well defined, the molecular determinants regulating postentry tropism in ECs are not known. We previously identified the UL133-UL138 locus encoded within the clinical strain-specific ULb' region of the HCMV genome as important for the latent infection in CD34(+) hematopoietic progenitor cells (HPCs). Interestingly, this locus, while dispensable for replication in fibroblasts, was required for efficient replication in ECs infected with the TB40E or fusion-inducing factor X (FIX) HCMV strains. ECs infected with a virus lacking the entire locus (UL133-UL138(NULL) virus) complete the immediate-early and early phases of infection but are defective for infectious progeny virus production. ECs infected with UL133-UL138(NULL) virus exhibited striking differences in the organization of intracellular membranes and in the assembly of mature virions relative to ECs infected with wild-type (WT) virus. In UL133-UL138(NULL) virus-infected ECs, Golgi stacks were disrupted, and the viral assembly compartment characteristic of HCMV infection failed to form. Further, progeny virions in UL133-UL138(NULL) virus-infected ECs inefficiently acquired the virion tegument and secondary envelope. These defects were specific to infection in ECs and not observed in fibroblasts infected with UL133-UL138(NULL) virus, suggesting an EC-specific requirement for the UL133-UL138 locus for late stages of replication. To our knowledge, the UL133-UL138 locus represents the first cell-type-dependent, postentry tropism determinant required for viral maturation.

Nanoemulsion mucosal adjuvant uniquely activates cytokine production by nasal ciliated epithelium and induces dendritic cell trafficking

While the nasal mucosa is a potentially useful site for human immunization, toxin-based nasal adjuvants are generally unsafe and less effective in humans. Safe mucosal adjuvants that activate protective immunity via mucosal administration are highly dependent on barrier antigen sampling by epithelial and DCs. Here, we demonstrate that protein antigens formulated in unique oil-in-water nanoemulsions (NEs) result in distinctive transcellular antigen uptake in ciliated nasal epithelial cells, leading to delivery into nasal associated lymphoid tissue. NE formulation also enhances MHC class II expression in epithelial cells and DC activation/trafficking to regional lymphoid tissues in mice. These materials appear to induce local epithelial cell apoptosis and heterogeneous cytokine production by mucosal epithelial cells and mixed nasal tissues, including G-CSF, GM-CSF, IL-1a, IL-1b, IL-5, IL-6, IL-12, IP-10, KC, MIP-1a, TGF-β, and TSLP. This is the first observation of a nasal adjuvant that activates calreticulin-associated apoptosis of ciliated nasal epithelial cells to generate broad cytokine/chemokine responses in mucosal tissue.

Interplay between human cytomegalovirus and intrinsic/innate host responses: a complex bidirectional relationship

The interaction between human cytomegalovirus (HCMV) and its host is a complex process that begins with viral attachment and entry into host cells, culminating in the development of a specific adaptive response that clears the acute infection but fails to eradicate HCMV. We review the viral and cellular partners that mediate early host responses to HCMV with regard to the interaction between structural components of virions (viral glycoproteins) and cellular receptors (attachment/entry receptors, toll-like receptors, and other nucleic acid sensors) or intrinsic factors (PML, hDaxx, Sp100, viperin, interferon inducible protein 16), the reactions of innate immune cells (antigen presenting cells and natural killer cells), the numerous mechanisms of viral immunoevasion, and the potential exploitation of events that are associated with early phases of virus-host interplay as a therapeutic strategy.

Antibodies against the gH/gL/UL128/UL130/UL131 complex comprise the majority of the anti-cytomegalovirus (anti-CMV) neutralizing antibody response in CMV hyperimmune globulin

Anti-cytomegalovirus (anti-CMV) hyperimmune globulin (HIG) has demonstrated efficacy in preventing CMV disease in solid-organ transplant patients as well as congenital disease when administered to pregnant women. To identify the neutralizing component of cytomegalovirus hyperimmune globulin (CMV-HIG), we performed serial depletions of CMV-HIG on cell-surface-expressed CMV antigens as well as purified antigens. Using this approach, we demonstrate that the major neutralizing antibody response is directed at the gH/gL/UL128/UL130/UL131 complex, suggesting little role for anti-gB antibodies in CMV-HIG neutralization.

The US16 gene of human cytomegalovirus is required for efficient viral infection of endothelial and epithelial cells

The human cytomegalovirus (HCMV) US12 gene family comprises a set of 10 contiguous genes (US12 to US21), each encoding a predicted seven-transmembrane protein and whose specific functions have yet to be ascertained. While inactivation of individual US12 family members in laboratory strains of HCMV has not been found to affect viral replication in fibroblasts, inactivation of US16 was reported to increase replication in microvascular endothelial cells. Here, we investigate the properties of US16 further by ascertaining the expression pattern of its product. A recombinant HCMV encoding a tagged version of the US16 protein expressed a 33-kDa polypeptide that accumulated with late kinetics in the cytoplasmic virion assembly compartment. To elucidate the function(s) of pUS16, we generated US16-deficient mutants in the TR clinical strain of HCMV. According to previous studies, inactivation of US16 had no effect on viral replication in fibroblasts. In contrast, the US16-deficient viruses exhibited a major growth defect in both microvascular endothelial cells and retinal pigment epithelial cells. The expression of representative IE, E, and L viral proteins was impaired in endothelial cells infected with a US16 mutant virus, suggesting a defect in the replication cycle that occurs prior to IE gene expression. This defect must be due to an inefficient entry and/or postentry event, since pp65 and viral DNA did not move to the nucleus in US16 mutant-infected cells. Taken together, these data indicate that the US16 gene encodes a novel virus tropism factor that regulates, in a cell-specific manner, a pre-immediate-early phase of the HCMV replication cycle.

The immune response to human CMV

This review will summarize and interpret recent literature regarding the human CMV immune response, which is among the strongest measured and is the focus of attention for numerous research groups. CMV is a highly prevalent, globally occurring infection that rarely elicits disease in healthy immunocompetent hosts. The human immune system is unable to clear CMV infection and latency, but mounts a spirited immune-defense targeting multiple immune-evasion genes encoded by this dsDNA β-herpes virus. Additionally, the magnitude of cellular immune response devoted to CMV may cause premature immune senescence, and the high frequencies of cytolytic T cells may aggravate vascular pathologies. However, uncontrolled CMV viremia and life-threatening symptoms, which occur readily after immunosuppression and in the immature host, clearly indicate the essential role of immunity in maintaining asymptomatic co-existence with CMV. Approaches for harnessing the host immune response to CMV are needed to reduce the burden of CMV complications in immunocompromised individuals.

Human cytomegalovirus activation of ERK and myeloid cell leukemia-1 protein correlates with survival of latently infected cells

The ability of human CMV (HCMV) to enter and establish a latent infection in myeloid cells is crucial for survival and transmission in the human population. Initial pathogen binding and entry triggers a number of antiviral responses, including the activation of proapoptotic cell death pathways, which must be countered during latency establishment. However, mechanisms responsible for a prosurvival state in myeloid cells upon latent HCMV infection remain completely undefined. We hypothesized that the cellular antiapoptotic machinery must be initially activated by HCMV to promote early survival events upon entry. Here we show that HCMV transiently protects nonpermissive myeloid cells from chemical and virus entry induced cell death by up-regulating a key myeloid cell survival gene, myeloid cell leukemia (MCL)-1 protein. The induction of MCL-1 expression was independent of viral gene expression but dependent on activation of the ERK-MAPK pathway by viral glycoprotein B. Inhibition of ERK-MAPK signaling, inhibition of HCMV fusion, antibody-mediated neutralization of glycoprotein B signaling or expression of a shRNA against MCL-1 all correlated with increased cell death in response to virus infection or chemical stimulation. Finally we show that activation of ERK-MAPK signaling impacts on long-term latency and reactivation in hematopoietic cells. Thus, HCMV primes myeloid cells for from the initial virus-cell encounter. Given the importance of ERK and MCL-1 for myeloid cell survival, the successful establishment of HCMV latency in myeloid progenitors begins at the point of virus entry.

Expression of the RAE-1 family of stimulatory NK-cell ligands requires activation of the PI3K pathway during viral infection and transformation

Natural killer (NK) cells are lymphocytes that play a major role in the elimination of virally-infected cells and tumor cells. NK cells recognize and target abnormal cells through activation of stimulatory receptors such as NKG2D. NKG2D ligands are self-proteins, which are absent or expressed at low levels on healthy cells but are induced upon cellular stress, transformation, or viral infection. The exact molecular mechanisms driving expression of these ligands remain poorly understood. Here we show that murine cytomegalovirus (MCMV) infection activates the phosphatidylinositol-3-kinase (PI3K) pathway and that this activation is required for the induction of the RAE-1 family of mouse NKG2D ligands. Among the multiple PI3K catalytic subunits, inhibition of the p110α catalytic subunit blocks this induction. Similarly, inhibition of p110α PI3K reduces cell surface expression of RAE-1 on transformed cells. Many viruses manipulate the PI3K pathway, and tumors frequently mutate the p110α oncogene. Thus, our findings suggest that dysregulation of the PI3K pathway is an important signal to induce expression of RAE-1, and this may represent a commonality among various types of cellular stresses that result in the induction of NKG2D ligands.

Human and mouse cytomegaloviruses (HCMV and MCMV) are members of the Herpesvirus family. Both viruses cause disease in individuals with a compromised immune system, such as transplant patients and AIDS patients. Natural killer (NK) cells are essential players in the immune response against these viruses. NK cells recognize self-proteins, such as NKG2D ligands, that are poorly expressed on healthy cells but are upregulated on cells that are undergoing stress, such as infection and tumor development. The biological processes associated with NKG2D ligand expression in infected cells are unknown. The PI3K pathway, which controls many cellular processes, is activated by a variety of viruses to prime cells for efficient viral replication. We observed that MCMV activates the PI3K pathway and that this activation is required for NKG2D ligand expression. We also found that the expression of NKG2D ligands on cancer cell lines is dependent on this pathway. Our data suggest that NKG2D ligand expression, and thus recognition of infected and cancer cells by NK cells, is associated with a dysregulation in the PI3K pathway.

Inhibition of inflammatory interleukin-6 activity via extracellular signal-regulated kinase-mitogen-activated protein kinase signaling antagonizes human cytomegalovirus reactivation from dendritic cells

Human cytomegalovirus (HCMV) remains a major cause of viral disease in immunosuppressed transplant patients. The ability of HCMV to establish lifelong infection in humans and reactivate with devastating clinical consequences underscores the importance of understanding the triggers of HCMV reactivation in mature myeloid cells. Dendritic cell (DC) differentiation is concomitant with the activation of cellular signaling pathways and inflammatory gene expression and also HCMV reactivation. Here, we show a major role for interleukin-6 (IL-6) through extracellular signal-regulated kinase-mitogen-activated protein kinase (ERK-MAPK) signaling upon DC differentiation to promote HCMV reactivation. IL-6 drives reactivation by transcriptional upregulation of the major immediate-early (IE) genes, resulting in efficient progression of the virus life cycle and, ultimately, higher titers of infectious virus. Furthermore, the interception of IL-6 signaling with biological inhibitors significantly abrogated HCMV reactivation from experimental latency. Crucially, using cells derived from healthy seropositive donors, we observed a key role for IL-6 during reactivation from natural latency ex vivo in interstitial DCs. Clinically, HCMV reactivation occurs in highly inflammatory environments (i.e., transplantation); thus, the implications of this study could potentially provide novel approaches for therapeutic intervention.

General blockade of human cytomegalovirus immediate-early mRNA expression in the S/G2 phase by a nuclear, Daxx- and PML-independent mechanism

The onset of human cytomegalovirus (HCMV) lytic replication is strictly controlled by the host cell division cycle. Although viral entry of S/G2-phase cells is unperturbed expression of major immediate-early (MIE) genes IE1 and IE2 is tightly blocked in these cells. Besides the finding that cyclin-dependent kinase (CDK) activity is required for IE1/IE2 repression little is known about the nature of this cell cycle-dependent block. Here, we show that the block occurs after nuclear entry of viral DNA and prevents the accumulation of IE1/IE2 mRNAs, suggesting an inhibition of transcription. Remarkably, the presence of cis-regulatory regions of the MIE locus is neither sufficient nor necessary for IE1/IE2 repression in the S/G2 phase. Furthermore, the block of viral mRNA expression also affects other immediate-early transcribed regions, i.e. the US3 and UL36-38 gene loci. This suggests a mechanism of repression that acts in a general and not a gene-specific fashion. Such a nuclear, genome-wide repression of HCMV is typically mediated by the intrinsic immune defence at nuclear domain 10 (ND10) structures. However, we found that neither Daxx nor PML, the main players of ND10-based immunity, are required for the block to viral gene expression in the S/G2 phase. In addition, the viral tegument protein pp71 (pUL82), a major antagonist of the intrinsic immunity at pre-immediate-early times of infection, proved to be functional in S-phase cells. This suggests the existence of a yet undiscovered, CDK-dependent mechanism exerting higher-level control over immediate-early mRNA expression in HCMV-infected cells.

Herpesviruses and intermediate filaments: close encounters with the third type

Intermediate filaments (IF) are essential to maintain cellular and nuclear integrity and shape, to manage organelle distribution and motility, to control the trafficking and pH of intracellular vesicles, to prevent stress-induced cell death, and to support the correct distribution of specific proteins. Because of this, IF are likely to be targeted by a variety of pathogens, and may act in favor or against infection progress. As many IF functions remain to be identified, however, little is currently known about these interactions. Herpesviruses can infect a wide variety of cell types, and are thus bound to encounter the different types of IF expressed in each tissue. The analysis of these interrelationships can yield precious insights into how IF proteins work, and into how viruses have evolved to exploit these functions. These interactions, either known or potential, will be the focus of this review.

A systemic network triggered by human cytomegalovirus entry

Virus entry is a multistep process that triggers various cellular pathways that interconnect into a complex network; yet the molecular complexity of this network remains largely elusive. Here, by employing systems biology approaches, we reveal a systemic virus-entry network initiated by human cytomegalovirus (HCMV), a widespread opportunistic pathogen. This network contains ten functional modules (i.e., groups of proteins) that coordinately respond to HCMV entry. Functional modules activated (up- and downregulated) in this network dramatically decline shortly within 25 minutes post infection. While modules annotated as receptor system, ion transport, and immune response are continuously activated during the entire process of HCMV entry, those annotated for cell adhesion and skeletal movement are specifically activated during viral early attachment. The up-regulated network contains various functional modules, such as cell surface receptors, skeletal development, endocytosis, ion transport, and chromatin remodeling. Interestingly, macromolecule metabolism and chromatin remodeling module predominates this over-expressed system, suggesting that the fundamental nuclear process modulation is one of the most important events in HCMV entry. The entire up-regulated network is primarily controlled by multiple elements like SLC10A1. Thus, virus entry triggers multiple cellular processes especially nuclear processes to facilitate its entry.

Cytomegalovirus: pathogen, paradigm, and puzzle

Human cytomegalovirus (CMV), one of the eight herpesviruses that commonly infect humans, is best known for its propensity to cause disease in immunocompromised patients, especially transplant recipients, patients with advanced AIDS, and congenitally infected newborns. Advances in molecular virology coupled with improvements in diagnostic methods and treatment options have vastly improved our understanding of and ability to manage CMV, but many uncertainties remain, including the mechanisms of persistence and pathogenesis and its hypothesized roles in a variety of human illnesses. Here we review recent advances that are reshaping our view and approach to this fascinating virus.

Human cytomegalovirus immunity and immune evasion

Human cytomegalovirus (HCMV) infection induces both innate immune responses including Natural Killer cells as well as adaptive humoral and cell mediated (CD4+ helper, CD8+ cytotoxic and γδ T cell) responses which lead to the resolution of acute primary infection. Despite such a robust primary immune response, HCMV is still able to establish latency. Long term memory T cell responses are maintained at high frequency and are thought to prevent clinical disease following periodic reactivation of the virus. As such, a balance is established between the immune response and viral reactivation. Loss of this balance in the immunocompromised host can lead to unchecked viral replication following reactivation of latent virus, with consequent disease and mortality. HCMV encodes multiple immune evasion mechanisms that target both the innate and acquired immune system. This article describes the current understanding of Natural killer cell, antibody and T cell mediated immune responses and the mechanisms that the virus utilizes to subvert these responses.

The essential role of clathrin-mediated endocytosis in the infectious entry of human enterovirus 71

Little is currently known about the infectious entry process of human enterovirus 71 (HEV71) into host cells, which may represent potential anti-viral targeting sites. In this study a targeted small-interfering RNA (siRNA) screening platform assay was established and validated to identify and profile key cellular genes involved in processes of endocytosis, cytoskeletal dynamics, and endosomal trafficking essential for HEV71 infection. Screen evaluation was conducted via the expression of well characterized dominant-negative mutants, bioimaging studies (double-labeled immunofluorescence assays, transmission electron microscopy analysis), secondary siRNA-based dosage dependence studies, and drug inhibition assays. The infectious entry of HEV71 into rhabdomyosarcoma cells was shown to be significantly inhibited by siRNAs targeting genes associated with clathrin-mediated endocytosis (CME) that include AP2A1, ARRB1, CLTC, CLTCL1, SYNJ1, ARPC5, PAK1, ROCK1, and WASF1. The functional role of CME was verified by the observation of strong co-localization between HEV71 particles and clathrin as well as dose-dependent inhibition of HEV71 infection upon siRNA knockdown of CME-associated genes. HEV71 entry by CME was further confirmed via inhibition by dominant-negative EPS15 mutants and treatment of CME drug inhibitors, with more than 80% inhibition observed at 20 μm chlorpromazine. Furthermore, HEV71 infection was shown to be sensitive to the disruption of human genes in regulating early to late endosomal trafficking as well as endosomal acidic pH. The identification of clathrin-mediated endocytosis as the entry pathway for HEV71 infection of susceptible host cells contributes to a better understanding of HEV71 pathogenesis and enables future development of anti-viral strategies against HEV71 infection.

RASCAL is a new human cytomegalovirus-encoded protein that localizes to the nuclear lamina and in cytoplasmic vesicles at late times postinfection

The products of numerous open reading frames (ORFs) present in the genome of human cytomegalovirus (CMV) have not been characterized. Here, we describe the identification of a new CMV protein localizing to the nuclear envelope and in cytoplasmic vesicles at late times postinfection. Based on this distinctive localization pattern, we called this new protein nuclear rim-associated cytomegaloviral protein, or RASCAL. Two RASCAL isoforms exist, a short version of 97 amino acids encoded by the majority of CMV strains and a longer version of 176 amino acids encoded by the Towne, Toledo, HAN20, and HAN38 strains. Both isoforms colocalize with lamin B in deep intranuclear invaginations of the inner nuclear membrane (INM) and in novel cytoplasmic vesicular structures possibly derived from the nuclear envelope. INM infoldings have been previously described as sites of nucleocapsid egress, which is mediated by the localized disruption of the nuclear lamina, promoted by the activities of viral and cellular kinases recruited by the lamina-associated proteins UL50 and UL53. RASCAL accumulation at the nuclear membrane required the presence of UL50 but not of UL53. RASCAL and UL50 also appeared to specifically interact, suggesting that RASCAL is a new component of the nuclear egress complex (NEC) and possibly involved in mediating nucleocapsid egress from the nucleus. Finally, the presence of RASCAL within cytoplasmic vesicles raises the intriguing possibility that this protein might participate in additional steps of virion maturation occurring after capsid release from the nucleus.

Multifaceted evasion of the interferon response by cytomegalovirus

Human cytomegalovirus (HCMV), which infects the majority of the population worldwide, causes few, if any, symptoms in otherwise healthy people but is responsible for considerable morbidity and mortality in immunocompromised patients and in congenitally infected newborns. The evolutionary success of HCMV depends in part on its ability to evade host defense systems. Here we review recent progress in elucidating the remarkable assortment of mechanisms employed by HCMV and the related beta-herpesviruses, murine cytomegaloviruses (MCMV) and rhesus cytomegaloviruses (RhCMV), for counteracting the host interferon (IFN) response. Very early after infection, cellular membrane sensors such as the lymphotoxin beta receptor initiate the production of antiviral cytokines including type I IFNs. However, virion factors, such as pp65 (ppUL83) and viral proteins made soon after infection including the immediate early gene 2 protein (pUL122), repress this response by interfering with steps in the activation of IFN regulatory factor 3 and NF-kappaB. CMVs then exert a multi-pronged attack on downstream IFN signaling. HCMV infection results in decreased accumulation and phosphorylation of the IFN signaling kinases Jak1 and Stat2, and the MCMV protein pM27 mediates Stat2 down-regulation, blocking both type I and type II IFN signaling. The HCMV immediate early gene 1 protein (pUL123) interacts with Stat2 and inhibits transcriptional activation of IFN-regulated genes. Infection also causes reduction in the abundance of p48/IRF9, a component of the ISGF3 transcription factor complex. Furthermore, CMVs have multiple genes involved in blocking the function of IFN-induced effectors. For example, viral double-stranded RNA-binding proteins are required to prevent the shutoff of protein synthesis by protein kinase R, further demonstrating the vital importance of evading the IFN response at multiple levels during infection.

Small interference RNA profiling reveals the essential role of human membrane trafficking genes in mediating the infectious entry of dengue virus

Background

Dengue virus (DENV) is the causative agent of Dengue fever and the life-threatening Dengue Haemorrhagic fever or Dengue shock syndrome. In the absence of anti-viral agents or vaccine, there is an urgent need to develop an effective anti-viral strategy against this medically important viral pathogen. The initial interplay between DENV and the host cells may represent one of the potential anti-viral targeting sites. Currently the involvements of human membrane trafficking host genes or factors that mediate the infectious cellular entry of dengue virus are not well defined.

Results

In this study, we have used a targeted small interfering RNA (siRNA) library to identify and profile key cellular genes involved in processes of endocytosis, cytoskeletal dynamics and endosome trafficking that are important and essential for DENV infection. The infectious entry of DENV into Huh7 cells was shown to be potently inhibited by siRNAs targeting genes associated with clathrin-mediated endocytosis. The important role of clathrin-mediated endocytosis was confirmed by the expression of well-characterized dominant-negative mutants of genes in this pathway and by using the clathrin endocytosis inhibitor chlorpromazine. Furthermore, DENV infection was shown to be sensitive to the disruption of human genes in regulating the early to late endosomal trafficking as well as the endosomal acidic pH. The importance and involvement of both actin and microtubule dynamics in mediating the infectious entry of DENV was also revealed in this study.

Conclusions

Together, the findings from this study have provided a detail profiling of the human membrane trafficking cellular genes and the mechanistic insight into the interplay of these host genes with DENV to initiate an infection, hence broadening our understanding on the entry pathway of this medically important viral pathogen. These data may also provide a new potential avenue for development of anti-viral strategies and treatment of DENV infection.

Biphasic recruitment of transcriptional repressors to the murine cytomegalovirus major immediate-early promoter during the course of infection in vivo

Our previous studies showed that establishment of murine cytomegalovirus (MCMV) latency in vivo is associated with repression of immediate-early gene expression, deacetylation of histones bound to the major immediate-early promoter (MIEP), changes in patterns of methylation of histones, and recruitment of cellular repressors of transcription to the MIEP. Here, we have quantitatively analyzed the kinetics of changes in viral RNA expression, DNA copy number, and recruitment of repressors and activators of transcription to viral promoters during the course of infection. Our results show that changes in viral gene expression correlate with changes in recruitment of RNA polymerase and acetylated histones to viral promoters. Binding of the transcriptional repressors histone deacetylase type 2 (HDAC2), HDAC3, YY1, CBF-1/RBP-Jk, Daxx, and CIR to the MIEP and HDACs to other promoters showed a biphasic pattern: some binding was detectable prior to activation of viral gene expression, then decreased with the onset of transcription and increased again as repression of viral gene expression occurred. Potential binding sites for CBF-1/RBP-Jk and YY1 in the MIEP and for YY1 in the M100 promoter (M100P) were identified by in silico analysis. While recruitment of HDACs was not promoter specific, binding of CBF-1/RBP-Jk and YY1 was restricted to promoters with their cognate sites. Our results suggest that sequences within viral promoters may contribute to establishment of latency through recruitment of transcriptional repressors to these genes. The observation that repressors are bound to the MIEP and other promoters immediately upon infection suggests that latency may be established in some cells very early in infection.

Peptide-derivatized dendrimers inhibit human cytomegalovirus infection by blocking virus binding to cell surface heparan sulfate

Dendrimers are hyperbranched synthetic well-defined molecules with a number of potential applications, especially in relation to the need for new antiviral agents. One subclass of dendrimers are peptide-derivatized dendrimers which consist of a peptidyl branching core and covalently attached surface peptide functional units. Few studies have addressed the potential uses of peptide dendrimers as direct-acting antiviral agents. Here, we report on the ability of two peptide dendrimers, SB105 and SB105_A10, to directly and almost completely inhibit human cytomegalovirus (HCMV) replication in both primary fibroblasts and endothelial cells; the agents were also found to inhibit murine CMV replication, whereas they were not able to inhibit adenovirus or vesicular stomatitis virus. The peptide dendrimers prevented adsorption of the HCMV to cells at 4 degrees C, whereas SB104, a dendrimer with a different amino acid sequence within the functional group and minimal anticytomegaloviral activity, was ineffective in blocking HCMV attachment. In effect, SB105_A10 bound to human cells through an interaction with cell surface heparan sulfate and thereby blocked virion attachment to target cells. These results indicate that the SB105 and SB105_A10 dendrimers could provide a useful starting point for the development of novel molecules to block HCMV infection.

Isolation of human monoclonal antibodies that potently neutralize human cytomegalovirus infection by targeting different epitopes on the gH/gL/UL128-131A complex

Human cytomegalovirus (HCMV) is a widely circulating pathogen that causes severe disease in immunocompromised patients and infected fetuses. By immortalizing memory B cells from HCMV-immune donors, we isolated a panel of human monoclonal antibodies that neutralized at extremely low concentrations (90% inhibitory concentration [IC(90)] values ranging from 5 to 200 pM) HCMV infection of endothelial, epithelial, and myeloid cells. With the single exception of an antibody that bound to a conserved epitope in the UL128 gene product, all other antibodies bound to conformational epitopes that required expression of two or more proteins of the gH/gL/UL128-131A complex. Antibodies against gB, gH, or gM/gN were also isolated and, albeit less potent, were able to neutralize infection of both endothelial-epithelial cells and fibroblasts. This study describes unusually potent neutralizing antibodies against HCMV that might be used for passive immunotherapy and identifies, through the use of such antibodies, novel antigenic targets in HCMV for the design of immunogens capable of eliciting previously unknown neutralizing antibody responses.

Age-dependent association between low frequency of CD27/CD28 expression on pp65 CD8+ T cells and cytomegalovirus replication after transplantation

In this cross-sectional study of 42 solid organ transplant recipients, the association of human cytomegalovirus (HCMV) replication and age with the phenotype of the HCMV-specific CD8(+) T cells was analyzed by using the CMV pp65 HLA-A*0201 pentamer. A correlation between the proportion of CD28(-) HCMV-specific CD8(+) T cells and age was observed in patients without HCMV replication (r = 0.50; P = 0.02) but not in patients with HCMV replication (r = -0.05; P = 0.83), a finding which differs from that observed for total CD8(+) T cells. Within the group of patients younger than 50 years of age, patients with HCVM replication after transplantation had higher percentages of CD28(-) HCMV-specific CD8(+) T cells (85.6 compared with 58.7% for patients without HCMV replication; P = 0.004) and CD27(-) HCMV-specific CD8(+) T cells (90.7 compared with 68.8% for patients without HCMV replication; P = 0.03). However, in patients older than age 50 years, a high frequency of these two subpopulations was observed in patients both with and without previous HCMV replication (for CD28(-) HCMV-specific CD8(+) T cells, 84.4 and 80.9%, respectively [P = 0.39]; for CD27(-) HCMV-specific CD8(+) T cells 86.6 and 81.5%, respectively [P = 0.16]). In conclusion, the present study shows that in the group of recipients younger than age 50 years, HCMV replication after transplantation is associated with a high percentage of CD27(-) and CD28(-) HCMV-specific CD8(+) T cells. These results suggest that the increased percentage of CD27(-) or CD28(-) HCMV-specific subsets can be considered a biomarker of HCMV replication in solid organ transplant recipients younger than age 50 years but not in older patients. Further studies are necessary to define the significance of these changes in HCMV-associated clinical complications posttransplantation.

Onset of human cytomegalovirus replication in fibroblasts requires the presence of an intact vimentin cytoskeleton

Like all viruses, herpesviruses extensively interact with the host cytoskeleton during entry. While microtubules and microfilaments appear to facilitate viral capsid transport toward the nucleus, evidence for a role of intermediate filaments in herpesvirus entry is lacking. Here, we examined the function of vimentin intermediate filaments in fibroblasts during the initial phase of infection of two genotypically distinct strains of human cytomegalovirus (CMV), one with narrow (AD169) and one with broad (TB40/E) cell tropism. Chemical disruption of the vimentin network with acrylamide, intermediate filament bundling in cells from a patient with giant axonal neuropathy, and absence of vimentin in fibroblasts from vimentin(-/-) mice severely reduced entry of either strain. In vimentin null cells, viral particles remained in the cytoplasm longer than in vimentin(+/+) cells. TB40/E infection was consistently slower than that of AD169 and was more negatively affected by the disruption or absence of vimentin. These findings demonstrate that an intact vimentin network is required for CMV infection onset, that intermediate filaments may function during viral entry to facilitate capsid trafficking and/or docking to the nuclear envelope, and that maintenance of a broader cell tropism is associated with a higher degree of dependence on the vimentin cytoskeleton.

Tegument proteins of human cytomegalovirus

SUMMARY

Human cytomegalovirus (HCMV) is a common, medically relevant human herpesvirus. The tegument layer of herpesvirus virions lies between the genome-containing capsids and the viral envelope. Proteins within the tegument layer of herpesviruses are released into the cell upon entry when the viral envelope fuses with the cell membrane. These proteins are fully formed and active and control viral entry, gene expression, and immune evasion. Most tegument proteins accumulate to high levels during later stages of infection, when they direct the assembly and egress of progeny virions. Thus, viral tegument proteins play critical roles at the very earliest and very last steps of the HCMV lytic replication cycle. This review summarizes HCMV tegument composition and structure as well as the known and speculated functions of viral tegument proteins. Important directions for future investigation and the challenges that lie ahead are identified and discussed.