A derivative of platelet-derived growth factor receptor alpha binds to the trimer of human cytomegalovirus and inhibits entry into fibroblasts and endothelial cells

The N Terminus of Human Cytomegalovirus Glycoprotein O Is Important for Binding to the Cellular Receptor PDGFRα

The human cytomegalovirus (HCMV) glycoprotein complex gH/gL/gO is required for the infection of cells by cell-free virions. It was recently shown that entry into fibroblasts depends on the interaction of gO with the platelet-derived growth factor receptor alpha (PDGFRα). This interaction can be blocked with soluble PDGFRα-Fc, which binds to HCMV virions and inhibits entry. The aim of this study was to identify parts of gO that contribute to PDGFRα binding. In a systematic mutational approach, we targeted potential interaction sites by exchanging conserved clusters of charged amino acids of gO with alanines. To screen for impaired interaction with PDGFRα, virus mutants were tested for sensitivity to inhibition by soluble PDGFRα-Fc. Two mutants with mutations within the N terminus of gO (amino acids 56 to 61 and 117 to 121) were partially resistant to neutralization. To validate whether these mutations impair interaction with PDGFRα-Fc, we compared binding of PDGFRα-Fc to mutant and wild-type virions via quantitative immunofluorescence analysis. PDGFRα-Fc staining intensities were reduced by 30% to 60% with mutant virus particles compared to wild-type particles. In concordance with the reduced binding to the soluble receptor, virus penetration into fibroblasts, which relies on binding to the cellular PDGFRα, was also reduced. In contrast, PDGFRα-independent penetration into endothelial cells was unaltered, demonstrating that the phenotypes of the gO mutant viruses were specific for the interaction with PDGFRα. In conclusion, the mutational screening of gO revealed that the N terminus of gO contributes to efficient spread in fibroblasts by promoting the interaction of virions with its cellular receptor.IMPORTANCE The human cytomegalovirus is a highly prevalent pathogen that can cause severe disease in immunocompromised hosts. Currently used drugs successfully target the viral replication within the host cell, but their use is restricted due to side effects and the development of resistance. An alternative approach is the inhibition of virus entry, for which understanding the details of the initial virus-cell interaction is desirable. As binding of the viral gH/gL/gO complex to the cellular PDGFRα drives infection of fibroblasts, this is a potential target for inhibition of infection. Our mutational mapping approach suggests the N terminus as the receptor binding portion of the protein. The respective mutants were partially resistant to inhibition by PDGFRα-Fc but also attenuated for infection of fibroblasts, indicating that such mutations have little if any benefit for the virus. These findings highlight the potential of targeting the interaction of gH/gL/gO with PDGFRα for therapeutic inhibition of HCMV.

HCMV Envelope Glycoprotein Diversity Demystified

Human cytomegalovirus (HCMV) is the leading viral cause of congenital birth defects and is responsible for morbidity and mortality in immunosuppressed individuals. Considerable efforts have been deployed over the last decade to develop a vaccine capable of preventing HCMV infection. However, in recent clinical trials, vaccines showed at best modest efficacy in preventing infection. These findings might be explained by the high level of sequence polymorphism at the genomic level. To investigate if genomic variation also leads to antigenic variation, we performed a bioinformatic sequence analysis and evaluated the percentage of conservation at the amino acid level of all the proteins present in the virion envelope. Using more than two hundred sequences per envelope glycoprotein and analyzing their degree of conservation, we observe that antigenic variation is in large part limited to three proteins. In addition, we demonstrate that the two leading vaccine candidates, the pentamer and gB complexes, are well conserved at the amino acid level. These results suggest that despite genomic polymorphism, antigenic variability is not involved in the modest efficacy observed in the recent clinical trials for a HCMV vaccine. We therefore propose that next-generation vaccines should focus on stabilizing and refining the gB domains needed to induce a protective humoral response.

Natural Inhibitor of Human Cytomegalovirus in Human Seminal Plasma

Human cytomegalovirus (HCMV) is the most frequent viral cause of congenital infections that can lead to severe birth defects. Although HCMV is frequently detected in semen and thus is potentially sexually transmitted, the role of semen in HCMV transmission is largely unclear. Here we describe that human seminal plasma (SP; the cell-free supernatant of semen) inhibits HCMV infection. The inhibition of HCMV infection was dose dependent and effective for different cell types, virus strains, and semen donors. This inhibitory effect was specific for HCMV, as herpes simplex virus 2 (HSV-2) and human immunodeficiency virus type 1 (HIV-1) infections were enhanced by SP. Mechanistically, SP inhibited infection by interfering with the attachment of virions to cells most likely via an interaction with the trimeric glycoprotein complex gH/gL/gO. Together, our findings suggest that semen contains a factor that potentially limits sexual transmission of HCMV.IMPORTANCE The role of semen in sexual transmission of human cytomegalovirus (HCMV) is currently unclear. This is surprising, as HCMV is frequently detected in this body fluid and infection is of high danger for neonates and pregnant women. In this study, we found that seminal plasma (SP) dose dependently inhibited HCMV infection. The infection inhibition was specific for HCMV, as other viruses, such as human immunodeficiency virus type 1 (HIV-1) and herpes simplex virus 2 (HSV-2), were not inhibited by SP. SP must contain a soluble, heat-resistant factor that limits attachment of HCMV particles to cells, probably by interaction with the trimeric glycoprotein complex gH/gL/gO. This novel virus-host interaction could possibly limit transmission of HCMV via semen during sexual intercourse.

Restriction of Human Cytomegalovirus Infection by Galectin-9

Human cytomegalovirus (HCMV) is a ubiquitous human herpesvirus. While HCMV infection is generally asymptomatic in the immunocompetent, it can have devastating consequences in those with compromised or underdeveloped immune systems, including transplant recipients and neonates. Galectins are a widely expressed protein family that have been demonstrated to modulate both antiviral immunity and regulate direct host-virus interactions. The potential for galectins to directly modulate HCMV infection has not previously been studied, and our results reveal that galectin-9 (Gal-9) can potently inhibit HCMV infection. Gal-9-mediated inhibition of HCMV was dependent upon its carbohydrate recognition domains and thus dependent on glycan interactions. Temperature shift studies revealed that Gal-9 specific inhibition was mediated primarily at the level of virus-cell fusion and not binding. Additionally, we found that during reactivation of HCMV in hematopoietic stem cell transplant (HSCT) patients soluble Gal-9 is upregulated. This study provides the first evidence for Gal-9 functioning as a potent antiviral defense effector molecule against HCMV infection and identifies it as a potential clinical candidate to restrict HCMV infections.IMPORTANCE Human cytomegalovirus (HCMV) continues to cause serious and often life-threatening disease in those with impaired or underdeveloped immune systems. This virus is able to infect and replicate in a wide range of human cell types, which enables the virus to spread to other individuals in a number of settings. Current antiviral drugs are associated with a significant toxicity profile, and there is no vaccine; these factors highlight a need to identify additional targets for the development of anti-HCMV therapies. We demonstrate for the first time that secretion of a member of the galectin family of proteins, galectin-9 (Gal-9), is upregulated during natural HCMV-reactivated infection and that this soluble cellular protein possesses a potent capacity to block HCMV infection by inhibiting virus entry into the host cell. Our findings support the possibility of harnessing the antiviral properties of Gal-9 to prevent HCMV infection and disease.

Pathogen at the Gates: Human Cytomegalovirus Entry and Cell Tropism

The past few years have brought substantial progress toward understanding how human cytomegalovirus (HCMV) enters the remarkably wide spectrum of cell types and tissues that it infects. Neuropilin-2 and platelet-derived growth factor receptor alpha (PDGFRα) were identified as receptors, respectively, for the trimeric and pentameric glycoprotein H/glycoprotein L (gH/gL) complexes that in large part govern HCMV cell tropism, while CD90 and CD147 were also found to play roles during entry. X-ray crystal structures for the proximal viral fusogen, glycoprotein B (gB), and for the pentameric gH/gL complex (pentamer) have been solved. A novel virion gH complex consisting of gH bound to UL116 instead of gL was described, and findings supporting the existence of a stable complex between gH/gL and gB were reported. Additional work indicates that the pentamer promotes a mode of cell-associated spread that resists antibody neutralization, as opposed to the trimeric gH/gL complex (trimer), which appears to be broadly required for the infectivity of cell-free virions. Finally, viral factors such as UL148 and US16 were identified that can influence the incorporation of the alternative gH/gL complexes into virions. We will review these advances and their implications for understanding HCMV entry and cell tropism.

The Human Cytomegalovirus Trimer and Pentamer Promote Sequential Steps in Entry into Epithelial and Endothelial Cells at Cell Surfaces and Endosomes

Human cytomegalovirus (HCMV) infects a wide variety of human cell types by different entry pathways that involve distinct envelope glycoprotein complexes that include gH/gL, a trimer complex consisting of gHgL/gO, and a pentamer complex consisting of gH/gL/UL128/UL130/UL131. We characterized the effects of soluble forms of these proteins on HCMV entry. Soluble trimer and pentamer blocked entry of HCMV into epithelial and endothelial cells, whereas soluble gH/gL did not. Trimer inhibited HCMV entry into fibroblast cells, but pentamer and gH/gL did not. Both trimer and pentamer bound to the surfaces of fibroblasts and epithelial cells, whereas gH/gL did not bind to either cell type. Cell surface binding of trimer and pentamer did not involve heparin sulfate moieties. The ability of soluble trimer to block entry of HCMV into epithelial cells did not involve platelet-derived growth factor PDGFRα, which has been reported as a trimer receptor for fibroblasts. Soluble trimer reduced the amount of virus particles that could be adsorbed onto the surface of epithelial cells, whereas soluble pentamer had no effect on virus adsorption. However, soluble pentamer reduced the ability of virus particles to exit from early endosomes into the cytoplasm and then travel to the nucleus. These studies support a model in which both the trimer and pentamer are required for HCMV entry into epithelial and endothelial cells, with trimer interacting with cell surface receptors other than PDGFR and pentamer acting later in the entry pathway to promote egress from endosomes.IMPORTANCE HCMV infects nearly 80% of the world's population and causes significant morbidity and mortality. The current antiviral agents used to treat HCMV infections are prone to resistance and can be toxic to patients, and there is no current vaccine against HCMV available. The data in this report will lead to a better understanding of how essential HCMV envelope glycoproteins function during infection of biologically important cell types and will have significant implications for understanding HCMV pathogenesis for developing new therapeutics.

Inhibition of Tetraspanin Functions Impairs Human Papillomavirus and Cytomegalovirus Infections

Tetraspanins are suggested to regulate the composition of cell membrane components and control intracellular transport, which leaves them vulnerable to utilization by pathogens such as human papillomaviruses (HPV) and cytomegaloviruses (HCMV) to facilitate host cell entry and subsequent infection. In this study, by means of cellular depletion, the cluster of differentiation (CD) tetraspanins CD9, CD63, and CD151 were found to reduce HPV16 infection in HeLa cells by 50 to 80%. Moreover, we tested recombinant proteins or peptides of specific tetraspanin domains on their effect on the most oncogenic HPV type, HPV16, and HCMV. We found that the C-terminal tails of CD63 and CD151 significantly inhibited infections of both HPV16 and HCMV. Although CD9 was newly identified as a key cellular factor for HPV16 infection, the recombinant CD9 C-terminal peptide had no effect on infection. Based on the determined half-maximal inhibitory concentration (IC₅₀), we classified CD63 and CD151 C-terminal peptides as moderate to potent inhibitors of HPV16 infection in HeLa and HaCaT cells, and in EA.hy926, HFF (human foreskin fibroblast) cells, and HEC-LTT (human endothelial cell-large T antigen and telomerase) cells for HCMV, respectively. These results indicate that HPV16 and HCMV share similar cellular requirements for their entry into host cells and reveal the necessity of the cytoplasmic CD151 and CD63 C-termini in virus infections. Furthermore, this highlights the suitability of these peptides for functional investigation of tetraspanin domains and as inhibitors of pathogen infections.

Role of PDGF receptor-α during human cytomegalovirus entry into fibroblasts

Human CMV (HCMV) exhibits a broad cell tropism that depends on two virion glycoprotein complexes: a trimeric complex (gH/gL/gO) that facilitates viral infection primarily in fibroblasts and a pentameric complex (gH/gL/pUL128-pUL130-pUL131A) that mediates infection in epithelial and endothelial cells. We performed genome-wide CRISPR screens in which the PDGF receptor-α (PDGFRα) was identified as the most significant cellular gene product essential for infection by HCMV virions containing only trimeric complex (trimer-only virus). Trimer-only virus did not enter PDGFRα knockout fibroblasts. By using knockout fibroblasts, the extracellular domain of PDGFRα required for virus entry was mapped, and the intracellular tyrosine kinase domain was shown to be nonessential. In addition, direct cell-to-cell spread of virus from knockout cells transfected with trimer-only viral DNA was blocked, despite the production of infectious virus in the transfected cells. In contrast to trimer-only virus, wild-type HCMV virions containing both trimeric and pentameric complexes entered PDGFRα knockout cells, reinforcing the view that fibroblasts contain a second, independent receptor for the pentameric complex. Importantly, however, wild-type virus entered the knockout fibroblasts at reduced efficiency compared with parental fibroblasts, arguing that the cellular receptor for the virion pentameric complex is limiting or that virions are produced containing different relative amounts of the two glycoprotein complexes. Finally, ectopic expression of PDGFRα in ARPE-19 epithelial cells and THP-1 monocytic cells, which have little to no endogenous PDGFRα expression, markedly enhanced their susceptibility to trimer-only virions. In sum, our data clarify several key determinants of HCMV tropism.

Large-Scale Screening of HCMV-Seropositive Blood Donors Indicates that HCMV Effectively Escapes from Antibodies by Cell-Associated Spread

Immunoglobulins are only moderately effective for the treatment of human cytomegalovirus (HCMV) infections, possibly due to ineffectiveness against cell-associated virus spread. To overcome this limitation, we aimed to identify individuals with exceptional antibodies in their plasma that can efficiently block the cell-associated spread of HCMV. A Gaussia luciferase-secreting mutant of the cell-associated HCMV strain Merlin was generated, and luciferase activity evaluated as a readout for the extent of cell-associated focal spread. This reporter virus-based assay was then applied to screen plasma samples from 8400 HCMV-seropositive individuals for their inhibitory effect, including direct-acting antiviral drugs as positive controls. None of the plasmas reduced virus spread to the level of these controls. Even the top-scoring samples that partially reduced luciferase activity in the screening assay failed to inhibit focal growth when reevaluated with a more accurate, immunofluorescence-based assay. Selected sera with high neutralizing capacity against free viruses were analyzed separately, and none of them prevented the focal spread of three recent clinical HCMV isolates nor reduced the number of particles transmitted, as demonstrated with a fluorescent Merlin mutant. We concluded that donors with cell-to-cell-spread-inhibiting plasma are nonexistent or extremely rare, emphasizing cell-associated spread as a highly efficient immune escape mechanism of HCMV.

Human Cytomegalovirus Tropism Modulator UL148 Interacts with SEL1L, a Cellular Factor That Governs Endoplasmic Reticulum-Associated Degradation of the Viral Envelope Glycoprotein gO

UL148 is a viral endoplasmic reticulum (ER)-resident glycoprotein that contributes to human cytomegalovirus (HCMV) cell tropism. The influence of UL148 on tropism correlates with its potential to promote the expression of glycoprotein O (gO), a viral envelope glycoprotein that participates in a heterotrimeric complex with glycoproteins H and L that is required for infectivity. In an effort to gain insight into the mechanism, we used mass spectrometry to identify proteins that coimmunoprecipitate from infected cells with UL148. This approach led us to identify an interaction between UL148 and SEL1L, a factor that plays key roles in ER-associated degradation (ERAD). In pulse-chase experiments, gO was less stable in cells infected with UL148-null mutant HCMV than during wild-type infection, suggesting a potential functional relevance for the interaction with SEL1L. To investigate whether UL148 regulates gO abundance by influencing ERAD, small interfering RNA (siRNA) silencing of either SEL1L or its partner, Hrd1, was carried out in the context of infection. Knockdown of these ERAD factors strongly enhanced levels of gO but not other viral glycoproteins, and the effect was amplified in the presence of UL148. Furthermore, pharmacological inhibition of ERAD showed similar results. Silencing of SEL1L during infection also stabilized an interaction of gO with the ER lectin OS-9, which likewise suggests that gO is an ERAD substrate. Taken together, our results identify an intriguing interaction of UL148 with the ERAD machinery and demonstrate that gO behaves as a constitutive ERAD substrate during infection. These findings have implications for understanding the regulation of HCMV cell tropism.IMPORTANCE Viral glycoproteins in large part determine the cell types that an enveloped virus can infect and hence play crucial roles in transmission and pathogenesis. The glycoprotein H/L heterodimer (gH/gL) is part of the conserved membrane fusion machinery that all herpesviruses use to enter cells. In human cytomegalovirus (HCMV), gH/gL participates in alternative complexes in virions, one of which is a trimer of gH/gL with glycoprotein O (gO). Here, we show that gO is constitutively degraded during infection by the endoplasmic reticulum-associated degradation (ERAD) pathway and that UL148, a viral factor that regulates HCMV cell tropism, interacts with the ERAD machinery and slows gO decay. Since gO is required for cell-free virus to enter new host cells but dispensable for cell-associated spread that resists antibody neutralization, our findings imply that the posttranslational instability of a viral glycoprotein provides a basis for viral mechanisms to modulate tropism and spread.

Expression Levels of Glycoprotein O (gO) Vary between Strains of Human Cytomegalovirus, Influencing the Assembly of gH/gL Complexes and Virion Infectivity

The tropism of human cytomegalovirus (HCMV) is influenced by the envelope glycoprotein complexes gH/gL/gO and gH/gL/UL128-131. During virion assembly, gO and the UL128-131 proteins compete for binding to gH/gL in the endoplasmic reticulum (ER). This assembly process clearly differs among strains, since Merlin (ME) virions contain abundant gH/gL/UL128-131 and little gH/gL/gO, whereas TR contains much higher levels of total gH/gL, mostly in the form of gH/gL/gO, but much lower levels of gH/gL/UL128-131 than ME. Remaining questions include (i) what are the mechanisms behind these assembly differences, and (ii) do differences reflect in vitro culture adaptations or natural genetic variations? Since the UL74(gO) open reading frame (ORF) differs in 25% of amino acids between TR and ME, we analyzed recombinant viruses in which the UL74(gO) ORF was swapped. TR virions were >40-fold more infectious than ME. Transcriptional repression of UL128-131 enhanced the infectivity of ME to the level of TR, despite still far lower levels of gH/gL/gO. Swapping the UL74(gO) ORF had no effect on either TR or ME. A quantitative immunoprecipitation approach revealed that gH/gL expression levels were within 4-fold between TR and ME, but the gO expression level was 20-fold lower for ME, which suggested differences in mRNA transcription, translation, or rapid ER-associated degradation of gO. trans-Complementation of gO expression during ME replication gave a 6-fold enhancement of infectivity beyond the 40-fold effect of UL128-131 repression alone. Overall, strain variations in the assembly of gH/gL complexes result from differences in the expression of gO and UL128-131, and selective advantages for reduced UL128-131 expression during fibroblast propagation are much stronger than those for higher gO expression.IMPORTANCE Specific genetic differences between independently isolated HCMV strains may result from purifying selection on de novo mutations arising during propagation in culture or random sampling among the diversity of genotypes present in clinical specimens. Results presented indicate that while reduced UL128-131 expression may confer a powerful selective advantage during cell-free propagation of HCMV in fibroblast cultures, selective pressures for increased gO expression are much weaker. Thus, variation in gO expression among independent strains may represent natural genotype variability present in vivo This may have important implications for virus-host interactions, such as immune recognition, and underscores the value of studying molecular mechanisms of replication using multiple HCMV strains.

Repurposing Nilotinib for Cytomegalovirus Infection Prophylaxis after Allogeneic Hematopoietic Stem Cell Transplantation: A Single-Arm, Phase II Trial

Platelet-derived growth factor receptor-alpha (PDGFRa) is a critical receptor for cytomegalovirus (CMV) entry into cells, leading to subsequent infection. This trial tested whether PDGFRa inhibition by nilotinib could prevent CMV infection in patients after allogeneic stem cell transplantation (allo-HSCT). Nilotinib (200 mg/day) was given continuously after engraftment, and plasma CMV DNA levels were monitored weekly. The primary endpoint was successful prophylaxis of CMV infection, defined as plasma CMV DNA copies less than 10,000 copies/mL, no anti-CMV treatment initiated, and no clinical CMV disease by day 100. All 37 enrolled recipients and their donors were CMV seropositive. Thirty patients received matched sibling transplants, 15 received nonmyeloablative conditioning regimens, and 15 received antithymocyte globulin as a part of graft-versus-host disease prophylaxis. The median interval from transplantation to nilotinib treatment was 23 days, and the median duration of administration was 76 days. None of the 31 assessable patients had nilotinib-associated grade 3/4 adverse events or nilotinib discontinuation. Twenty-five of 31 assessable patients (80.6%) fulfilled the predefined criteria for successful CMV prophylaxis, and none of them had clinical CMV disease. Only 1 of 6 failed patients developed CMV colitis. Nilotinib is well tolerated in allo-HSCT recipients, and its preliminary efficacy results suggest that blocking CMV entry to prevent CMV infection may warrant further exploration. (ClinicalTrials.gov identifier: NCT01252017.).

From recognition to execution-the HCMV Pentamer from receptor binding to fusion triggering

The β-herpesvirus human cytomegalovirus (HCMV) is the leading viral cause of neonatal developmental disabilities. In HCMV, the conserved herpesvirus glycoprotein B (gB) mediates membrane fusion between the viral and host cell membranes, whereas the trimeric gH/gL/gO or the pentameric gH/gL/UL128/UL130/UL31A complexes (Pentamer) bind to cell-specific receptors and provide the triggering signal to gB. Recent structural and functional studies have provided new insights into Pentamer structure, conformational flexibility, location of epitopes for neutralizing antibodies and potential binding sites for cell surface receptors. Together, these data suggest a model where receptor binding triggers a conformational change in Pentamer, allowing it to interact with gB and initiate the membrane fusion process.

CD147 Promotes Entry of Pentamer-Expressing Human Cytomegalovirus into Epithelial and Endothelial Cells

Human cytomegalovirus (HCMV) replicates in many diverse cell types in vivo, and entry into different cells involves distinct entry mechanisms and different envelope glycoproteins. HCMV glycoprotein gB is thought to act as the virus fusogen, apparently after being triggered by different gH/gL proteins that bind distinct cellular receptors or entry mediators. A trimer of gH/gL/gO is required for entry into all cell types, and entry into fibroblasts involves trimer binding to platelet-derived growth factor receptor alpha (PDGFRα). HCMV entry into biologically relevant epithelial and endothelial cells and monocyte-macrophages also requires a pentamer, gH/gL complexed with UL128, UL130, and UL131, and there is evidence that the pentamer binds unidentified receptors. We screened an epithelial cell cDNA library and identified the cell surface protein CD147, which increased entry of pentamer-expressing HCMV into HeLa cells but not entry of HCMV that lacked the pentamer. A panel of CD147-specific monoclonal antibodies inhibited HCMV entry into epithelial and endothelial cells, but not entry into fibroblasts. shRNA silencing of CD147 in endothelial cells inhibited HCMV entry but not entry into fibroblasts. CD147 colocalized with HCMV particles on cell surfaces and in endosomes. CD147 also promoted cell-cell fusion induced by expression of pentamer and gB in epithelial cells. However, soluble CD147 did not block HCMV entry and trimer and pentamer did not bind directly to CD147, supporting the hypothesis that CD147 acts indirectly through other proteins. CD147 represents the first HCMV entry mediator that specifically functions to promote entry of pentamer-expressing HCMV into epithelial and endothelial cells.IMPORTANCE Human cytomegalovirus infects nearly 80% of the world's population and causes significant morbidity and mortality. The current method of treatment involves the use of antiviral agents that are prone to resistance and can be highly toxic to patients; currently, there is no vaccine against HCMV available. HCMV infections involve virus dissemination throughout the body, infecting a wide variety of tissues; however, the mechanism of spread is not well understood, particularly with regard to which cellular proteins are utilized by HCMV to establish infection. This report describes the characterization of a newly identified cellular molecule that affects HCMV entry into epithelial and endothelial cells. These results will lead to a better understanding of HCMV pathogenesis and have implications for the development of future therapeutics.

Navigating the Host Cell Response during Entry into Sites of Latent Cytomegalovirus Infection

The host cell represents a hostile environment that viruses must counter in order to establish infection. Human cytomegalovirus (HCMV) is no different and encodes a multitude of functions aimed at disabling, re-directing or hijacking cellular functions to promulgate infection. However, during the very early stages of infection the virus relies on the outcome of interactions between virion components, cell surface receptors and host signalling pathways to promote an environment that supports infection. In the context of latent infection—where the virus establishes an infection in an absence of many gene products specific for lytic infection—these initial interactions are crucial events. In this review, we will discuss key host responses triggered by viral infection and how, in turn, the virus ameliorates the impact on the establishment of non-lytic infections of cells. We will focus on strategies to evade intrinsic antiviral and innate immune responses and consider their impact on viral infection. Finally, we will consider the hypothesis that the very early events upon viral infection are important for dictating the outcome of infection and consider the possibility that events that occur during entry into non-permissive cells are unique and thus contribute to the establishment of latency.

Myeloid Dendritic Cells Repress Human Cytomegalovirus Gene Expression and Spread by Releasing Interferon-Unrelated Soluble Antiviral Factors

Cytomegalovirus (CMV) is a betaherpesvirus that latently infects most adult humans worldwide and is a major cause of morbidity and mortality in immunocompromised hosts. Latent human CMV (HCMV) is believed to reside in precursors of myeloid-lineage leukocytes and monocytes, which give rise to macrophages and dendritic cells (DC). We report here that human monocyte-derived DC (mo-DC) suppress HCMV infection in coculture with infected fibroblast target cells in a manner dependent on the effector-to-target ratio. Intriguingly, optimal activation of mo-DC was achieved under coculture conditions and not by direct infection with HCMV, implying that mo-DC may recognize unique molecular patterns on, or within, infected fibroblasts. We show that HCMV is controlled by secreted factors that act by priming defenses in target cells rather than by direct viral neutralization, but we excluded a role for interferons (IFNs) in this control. The expression of lytic viral genes in infected cells and the progression of infection were significantly slowed, but this effect was reversible, indicating that the control of infection depended on the transient induction of antiviral effector molecules in target cells. Using immediate early or late-phase reporter HCMVs, we show that soluble factors secreted in the cocultures suppress HCMV replication at both stages of the infection and that their antiviral effects are robust and comparable in numerous batches of mo-DC as well as in primary fibroblasts and stromal cells.IMPORTANCE Human cytomegalovirus is a widespread opportunistic pathogen that can cause severe disease and complications in vulnerable individuals. This includes newborn children, HIV AIDS patients, and transplant recipients. Although the majority of healthy humans carry this virus throughout their lives without symptoms, it is not exactly clear which tissues in the body are the main reservoirs of latent virus infection or how the delicate balance between the virus and the immune system is maintained over an individual's lifetime. Here, for the first time, we provide evidence for a novel mechanism of direct virus control by a subset of human innate immune cells called dendritic cells, which are regarded as a major site of virus latency and reactivation. Our findings may have important implications in HCMV disease prevention as well as in development of novel therapeutic approaches.

Mechanisms of human cytomegalovirus infection with a focus on epidermal growth factor receptor interactions

Human cytomegalovirus (HCMV) is a widespread opportunistic herpesvirus that causes severe diseases in immunocompromised individuals. It has a high prevalence worldwide that is linked with socioeconomic factors. Similar to other herpesviruses, HCMV has the ability to establish lifelong persistence and latent infection following primary exposure. HCMV infects a broad range of cell types. This broad tropism suggests that it may use multiple receptors for host cell entry. The identification of receptors used by HCMV is essential for understanding viral pathogenesis, because these receptors mediate the early events necessary for infection. Many cell surface components have been identified as virus receptors, such as epidermal growth factor receptor (EGFR), which is characterized by tyrosine kinase activity and plays a crucial role in the control of key cellular transduction pathways. EGFR is essential for HCMV binding, signaling, and host cell entry. This review focuses on HCMV infection via EGFR on different cell types and its implications for the cellular environment, viral persistence, and infection.

The pentameric complex of human Cytomegalovirus: cell tropism, virus dissemination, immune response and vaccine development

Between the 1980s and 1990s, three assays were developed for diagnosis of human cytomegalovirus (HCMV) infections: leuko (L)-antigenemia, l-viremia and l-DNAemia, detecting viral protein pp65, infectious virus and viral DNA, respectively, in circulating leukocytes Repeated initial attempts to reproduce the three assays in vitro using laboratory-adapted strains and infected cell cultures were consistently unsuccessful. Results were totally reversed when wild-type HCMV strains were used to infect either fibroblasts or endothelial cells. Careful analysis and sequencing of plaque-purified viruses from recent clinical isolates drew attention to the ULb' region of the HCMV genome. Using bacterial artificial chromosome technology, it was shown by both gain-of-function and loss-of-function experiments that UL131-128 genes are indispensable for virus growth in endothelial cells and virus transfer to leukocytes. In addition, a number of clinical isolates passaged in human fibroblasts had lost both properties (leuko-tropism and endothelial cell-tropism) when displaying a mutation in the UL131-128 locus (referred to as UL128L). In the following years, it was shown that pUL128L was complexed with gH and gL to form the pentameric complex (PC), which is required to infect endothelial, epithelial and myeloid cells. The immune response to PC was studied extensively, particularly its humoral component, showing that the great majority of the neutralizing antibody response is directed to PC. Although anti-HCMV antibodies may act with other mechanisms than mere neutralizing activity, these findings definitely favour their protective activity, thus paving the way to the development of a potentially protective HCMV vaccine.