The HCMV gH/gL/UL128-131 complex triggers the specific cellular activation required for efficient viral internalization into target monocytes

Cell Surface THY-1 Contributes to Human Cytomegalovirus Entry via a Macropinocytosis-Like Process

Previously we showed that THY-1 has a critical role in the initial stage of infection of certain cell types with human cytomegalovirus (HCMV) and that THY-1 is important for HCMV-mediated activation of phosphatidylinositol 3-kinase (PI3K)/Akt during virus entry. THY-1 is known to interact with integrins and is a major cargo protein of clathrin-independent endocytic vesicles. Since macropinocytosis involves integrin signaling, is PI3K/Akt dependent, and is a clathrin-independent endocytic process, we determined whether THY-1 has a role in HCMV entry by macropinocytosis. Using electron microscopy in two cell lines that support HCMV infection in a THY-1-dependent manner, we found that HCMV enters these cells by a macropinocytosis-like process. THY-1 associated with HCMV virions on the cell surface and colocalized with virus inside macropinosomes. 5-(N-Ethyl-N-isopropyl)amiloride (EIPA) and soluble THY-1 blocked HCMV infection in the cell lines by ≥80% and 60%, respectively. HCMV entry into the cells triggered increased influx of extracellular fluid, a marker of macropinocytosis, and this increased fluid uptake was inhibited by EIPA and by soluble THY-1. Blocking actin depolymerization, Na⁺/H⁺ exchange, PI3K, and Pak1 kinase, which are critical for macropinocytosis, impaired HCMV infection. Neither internalized HCMV virions nor THY-1 in virus-infected cells colocalized with transferrin as determined by confocal microscopy, indicating that clathrin-mediated endocytosis was not involved in THY-1-associated virus entry. These results suggest that HCMV has adapted to utilize THY-1, a cargo protein of clathrin-independent endocytotic vesicles, to facilitate efficient entry into certain cell types by a macropinocytosis-like process.

IMPORTANCE

Human cytomegalovirus (HCMV) infects over half of the population and is the most common infectious cause of birth defects. The virus is the most important infection occurring in transplant recipients. The mechanism of how HCMV enters cells is controversial. In this study, we show that THY-1, a cell surface protein that is critical for the early stage of entry of HCMV into certain cell types, contributes to virus entry by macropinocytosis. Our findings suggest that HCMV has adapted to utilize THY-1 to facilitate entry of HCMV into macropinosomes in certain cell types. Further knowledge about the mechanism of HCMV entry into cells may facilitate the development of novel inhibitors of virus infection.

Integrins as Herpesvirus Receptors and Mediators of the Host Signalosome

The repertoire of herpesvirus receptors consists of nonintegrin and integrin molecules. Integrins interact with the conserved glycoproteins gH/gL or gB. This interaction is a conserved biology across the Herpesviridae family, likely directed to promote virus entry and endocytosis. Herpesviruses exploit this interaction to execute a range of critical functions that include (a) relocation of nonintegrin receptors (e.g., herpes simplex virus nectin1 and Kaposi's sarcoma-associated herpesvirus EphA2), or association with nonintegrin receptors (i.e., human cytomegalovirus EGFR), to dictate species-specific entry pathways; (b) activation of multiple signaling pathways (e.g., Ca²⁺ release, c-Src, FAK, MAPK, and PI3K); and (c) association with Rho GTPases, tyrosine kinase receptors, Toll-like receptors, which result in cytoskeletal remodeling, differential cell type targeting, and innate responses. In turn, integrins can be modulated by viral proteins (e.g., Epstein-Barr virus LMPs) to favor spread of transformed cells. We propose that herpesviruses evolved a multipartite entry system to allow interaction with multiple receptors, including integrins, required for their sophisticated life cycle.

Viral binding-induced signaling drives a unique and extended intracellular trafficking pattern during infection of primary monocytes

We initiated experiments to examine the infection of monocytes postentry. New data show that human cytomegalovirus (HCMV) DNA is detected in the nucleus beginning only at 3 d postinfection in monocytes, compared with 30 min postinfection in fibroblasts and endothelial cells, suggesting that HCMV nuclear translocation in monocytes is distinct from that seen in other cell types. We now show that HCMV is initially retained in early endosomes and then moves sequentially to the trans-Golgi network (TGN) and recycling endosomes before nuclear translocation. HCMV is retained initially as a mature particle before deenvelopment in recycling endosomes. Disruption of the TGN significantly reduced nuclear translocation of viral DNA, and HCMV nuclear translocation in infected monocytes was observed only when correct gH/gL/UL128-131/integrin/c-Src signaling occurred. Taken together, our findings show that viral binding of the gH/gL/UL128-131 complex to integrins and the ensuing c-Src signaling drive a unique nuclear translocation pattern that promotes productive infection and avoids viral degradation, suggesting that it represents an additional viral evasion/survival strategy.

A Homolog Pentameric Complex Dictates Viral Epithelial Tropism, Pathogenicity and Congenital Infection Rate in Guinea Pig Cytomegalovirus

In human cytomegalovirus (HCMV), tropism to epithelial and endothelial cells is dependent upon a pentameric complex (PC). Given the structure of the placenta, the PC is potentially an important neutralizing antibody target antigen against congenital infection. The guinea pig is the only small animal model for congenital CMV. Guinea pig cytomegalovirus (GPCMV) potentially encodes a UL128-131 HCMV PC homolog locus (GP128-GP133). In transient expression studies, GPCMV gH and gL glycoproteins interacted with UL128, UL130 and UL131 homolog proteins (designated GP129 and GP131 and GP133 respectively) to form PC or subcomplexes which were determined by immunoprecipitation reactions directed to gH or gL. A natural GP129 C-terminal deletion mutant (aa 107-179) and a chimeric HCMV UL128 C-terminal domain swap GP129 mutant failed to form PC with other components. GPCMV infection of a newly established guinea pig epithelial cell line required a complete PC and a GP129 mutant virus lacked epithelial tropism and was attenuated in the guinea pig for pathogenicity and had a low congenital transmission rate. Individual knockout of GP131 or 133 genes resulted in loss of viral epithelial tropism. A GP128 mutant virus retained epithelial tropism and GP128 was determined not to be a PC component. A series of GPCMV mutants demonstrated that gO was not strictly essential for epithelial infection whereas gB and the PC were essential. Ectopic expression of a GP129 cDNA in a GP129 mutant virus restored epithelial tropism, pathogenicity and congenital infection. Overall, GPCMV forms a PC similar to HCMV which enables evaluation of PC based vaccine strategies in the guinea pig model.

Virion Glycoprotein-Mediated Immune Evasion by Human Cytomegalovirus: a Sticky Virus Makes a Slick Getaway

The prototypic herpesvirus human cytomegalovirus (CMV) exhibits the extraordinary ability to establish latency and maintain a chronic infection throughout the life of its human host. This is even more remarkable considering the robust adaptive immune response elicited by infection and reactivation from latency. In addition to the ability of CMV to exist in a quiescent latent state, its persistence is enabled by a large repertoire of viral proteins that subvert immune defense mechanisms, such as NK cell activation and major histocompatibility complex antigen presentation, within the cell. However, dissemination outside the cell presents a unique existential challenge to the CMV virion, which is studded with antigenic glycoprotein complexes targeted by a potent neutralizing antibody response. The CMV virion envelope proteins, which are critical mediators of cell attachment and entry, possess various characteristics that can mitigate the humoral immune response and prevent viral clearance. Here we review the CMV glycoprotein complexes crucial for cell attachment and entry and propose inherent properties of these proteins involved in evading the CMV humoral immune response. These include viral glycoprotein polymorphism, epitope competition, Fc receptor-mediated endocytosis, glycan shielding, and cell-to-cell spread. The consequences of CMV virion glycoprotein-mediated immune evasion have a major impact on persistence of the virus in the population, and a comprehensive understanding of these evasion strategies will assist in designing effective CMV biologics and vaccines to limit CMV-associated disease.

HCMV Induces Macropinocytosis for Host Cell Entry in Fibroblasts

Human cytomegalovirus (HCMV) is an important and widespread pathogen in the human population. While infection by this β-herpesvirus in endothelial, epithelial and dendritic cells depends on endocytosis, its entry into fibroblasts is thought to occur by direct fusion of the viral envelope with the plasma membrane. To characterize individual steps during entry in primary human fibroblasts, we employed quantitative assays as well as electron, fluorescence and live cell microscopy in combination with a variety of inhibitory compounds. Our results showed that while infectious entry was pH- and clathrin-independent, it required multiple, endocytosis-related factors and processes. The virions were found to undergo rapid internalization into large vacuoles containing internalized fluid and endosome markers. The characteristics of the internalization process fulfilled major criteria for macropinocytosis. Moreover, we found that soon after addition to fibroblasts the virus rapidly triggered the formation of circular dorsal ruffles in the host cell followed by the generation of large macropinocytic vacuoles. This distinctive form of macropinocytosis has been observed especially in primary cells but has not previously been reported in response to virus stimulation.

Human Cytomegalovirus Promotes Survival of Infected Monocytes via a Distinct Temporal Regulation of Cellular Bcl-2 Family Proteins

Monocytes play a key role in the hematogenous dissemination of human cytomegalovirus (HCMV) to target organ systems. To infect monocytes and reprogram them to deliver infectious virus, HCMV must overcome biological obstacles, including the short life span of monocytes and their antiviral proapoptotic response to infection. We have shown that virally induced upregulation of cellular Mcl-1 promotes early survival of HCMV-infected monocytes, allowing cells to overcome an early apoptotic checkpoint at around 48 h postinfection (hpi). Here, we demonstrate an HCMV-dependent shift from Mcl-1 as the primary antiapoptotic player to the related protein, Bcl-2, later during infection. Bcl-2 was upregulated in HCMV-infected monocytes beginning at 48 hpi. Treatment with the Bcl-2 antagonist ABT-199 only reduced the prosurvival effects of HCMV in target monocytes beginning at 48 hpi, suggesting that Mcl-1 controls survival prior to 48 hpi, while Bcl-2 promotes survival after 48 hpi. Although Bcl-2 was upregulated following viral binding/signaling through cellular integrins (compared to Mcl-1, which is upregulated through binding/activation of epidermal growth factor receptor [EGFR]), it functioned similarly to Mcl-1, adopting the early role of Mcl-1 in preventing caspase-3 cleavage/activation. This distinct, HCMV-induced shift from Mcl-1 to Bcl-2 occurs in response to a cellular upregulation of proapoptotic Bax, as small interfering RNA (siRNA)-mediated knockdown of Bax reduced the upregulation of Bcl-2 in infected monocytes and rescued the cells from the apoptotic effects of Bcl-2 inhibition. Our data demonstrate a distinct survival strategy whereby HCMV induces a biphasic regulation of cellular Bcl-2 proteins to promote host cell survival, leading to viral dissemination and the establishment of persistent HCMV infection.

IMPORTANCE

Hematogenous dissemination of HCMV via infected monocytes is a crucial component of the viral survival strategy and is required for the establishment of persistent infection and for viral spread to additional hosts. Our system of infected primary human blood monocytes provides us with an opportunity to answer specific questions about viral spread and persistence in in vivo-relevant myeloid cells that cannot be addressed with the more traditionally used replication-permissive cells. Our goal in examining the mechanisms whereby HCMV reprograms infected monocytes to promote viral dissemination is to uncover new targets for therapeutic intervention that would disrupt key viral survival and persistence strategies. Because of this important role in maintaining survival of HCMV-infected monocytes, our new data on the role of Bcl-2 regulation during viral infection represents a promising molecular target for mitigating viral spread and persistence.

Scanning Mutagenesis of Human Cytomegalovirus Glycoprotein gH/gL

The core, conserved function of the herpesvirus gH/gL is to promote gB-mediated membrane fusion during entry, although the mechanism is poorly understood. The human cytomegalovirus (HCMV) gH/gL can exist as either the gH/gL/gO trimer or the gH/gL/UL128/UL130/UL131 (gH/gL/UL128-131) pentamer. One model suggests that gH/gL/gO provides the core fusion role during entry into all cells within the broad tropism of HCMV, whereas gH/gL/UL128-131 acts at an earlier stage, by a distinct receptor-binding mechanism to enhance infection of select cell types, such as epithelial cells, endothelial cells, and monocytes/macrophages. To further study the distinct functions of these complexes, mutants with individual charged cluster-to-alanine (CCTA) mutations of gH and gL were combined to generate a library of 80 mutant gH/gL heterodimers. The majority of the mutant gH/gL complexes were unable to facilitate gB-mediated membrane fusion in transient-expression cell-cell fusion experiments. In contrast, these mutants supported the formation of gH/gL/UL128-131 complexes that could block HCMV infection in receptor interference experiments. These results suggest that receptor interactions with gH/gL/UL128-131 involve surfaces contained on the UL128-131 proteins but not on gH/gL. gH/gL/UL128-131 receptor interference could be blocked with anti-gH antibodies, suggesting that interference is a cell surface phenomenon and that anti-gH antibodies can block gH/gL/UL128-131 in a manner that is distinct from that for gH/gL/gO.

IMPORTANCE

Interest in the gH/gL complexes of HCMV (especially gH/gL/UL128-131) as vaccine targets has far outpaced our understanding of the mechanism by which they facilitate entry and contribute to broad cellular tropism. For Epstein-Barr virus (EBV), gH/gL and gH/gL/gp42 are both capable of promoting gB fusion for entry into epithelial cells and B cells, respectively. In contrast, HCMV gH/gL/gO appears to be the sole fusion cofactor that promotes gB fusion activity, whereas gH/gL/UL128-131 expands cell tropism through a distinct yet unknown mechanism. This study suggests that the surfaces of HCMV gH/gL are critical for promoting gB fusion but are dispensable for gH/gL/UL128-131 receptor interaction. This underscores the importance of gH/gL/gO in HCMV entry into all cell types and reaffirms the complex as a candidate target for vaccine development. The two functionally distinct forms of gH/gL present in HCMV make for a useful model with which to study the fundamental mechanisms by which herpesvirus gH/gL regulates gB fusion.

Cellular defense against latent colonization foiled by human cytomegalovirus UL138 protein

Intrinsic immune defenses mediated by restriction factors inhibit productive viral infections. Select viruses rapidly establish latent infections and, with gene expression profiles that imply cell-autonomous intrinsic defenses, may be the most effective immune control measure against latent reservoirs. We illustrate that lysine-specific demethylases (KDMs) are restriction factors that prevent human cytomegalovirus from establishing latency by removing repressive epigenetic modifications from histones associated with the viral major immediate early promoter (MIEP), stimulating the expression of a viral lytic phase target of cell-mediated adaptive immunity. The viral UL138 protein negates this defense by preventing KDM association with the MIEP. The presence of an intrinsic defense against latency and the emergence of a cognate neutralizing viral factor indicate that "arms races" between hosts and viruses over lifelong colonization exist at the cellular level.

Decoding protein networks during virus entry by quantitative proteomics

Virus entry into host cells relies on interactions between viral and host structures including lipids, carbohydrates and proteins. Particularly, protein–protein interactions between viral surface proteins and host proteins as well as secondary host protein–protein interactions play a pivotal role in coordinating virus binding and uptake. These interactions are dynamic and frequently involve multiprotein complexes. In the past decade mass spectrometry based proteomics methods have reached sensitivities and high throughput compatibilities of genomics methods and now allow the reliable quantitation of proteins in complex samples from limited material. As proteomics provides essential information on the biologically active entity namely the protein, including its posttranslational modifications and its interactions with other proteins, it is an indispensable method in the virologist's toolbox. Here we review protein interactions during virus entry and compare classical biochemical methods to study entry with novel technically advanced quantitative proteomics techniques. We highlight the value of quantitative proteomics in mapping functional virus entry networks, discuss the benefits and limitations and illustrate how the methodology will help resolve unsettled questions in virus entry research in the future.

Human Cytomegalovirus gH/gL/gO Promotes the Fusion Step of Entry into All Cell Types, whereas gH/gL/UL128-131 Broadens Virus Tropism through a Distinct Mechanism

Interaction between gH/gL and the fusion protein gB is likely a conserved feature of the entry mechanism for all herpesviruses. Human cytomegalovirus (HCMV) gH/gL can be bound by gO or by the set of proteins UL128, UL130, and UL131, forming gH/gL/gO and gH/gL/UL128-131. The mechanisms by which these complexes facilitate entry are poorly understood. Mutants lacking UL128-131 replicate well on fibroblasts but fail to enter epithelial/endothelial cells, and this has led to the general assumption that gH/gL/UL128-131 promotes gB-mediated fusion on epithelial/endothelial cells whereas gH/gL/gO provides this function on fibroblasts. This was challenged by observations that gO-null mutants were defective on all of these cell types, suggesting that entry into epithelial/endothelial cells requires both of the gH/gL complexes, but the severe replication defect of the gO mutants precluded detailed analysis. We previously reported that the ratio of gH/gL/gO and gH/gL/UL128-131 in the virion envelope varied dramatically among HCMV strains. Here, we show that strains not only differ in the ratio, but also vary in the total amount of gH/gL in the virion. Cell-type-specific particle-to-PFU ratios of HCMV strains that contained different amounts of gH/gL/gO and gH/gL/UL128-131 were determined. Infection of both fibroblasts and epithelial cells was generally correlated with the abundance of gH/gL/gO, but not with that of gH/gL/UL128-131. The low infectivity of virions rich in gH/gL/UL128-131 but low in gH/gL/gO could be overcome by treatment with the chemical fusogen polyethylene glycol (PEG), strongly arguing that gH/gL/gO provides the conserved herpesvirus gH/gL entry function of promoting gB-mediated fusion for entry into all cell types, whereas gH/gL/UL128-131 acts through a distinct mechanism to allow infection of select cell types.

IMPORTANCE

The functions of HCMV gH/gL complexes in entry are unclear. Unlike the well-studied Epstein-Barr virus (EBV), where gH/gL and gH/gL/gp42 complexes both seem capable of promoting gB fusion during entry into different cell types, our studies here suggest that for HCMV, gH/gL/gO promotes gB fusion on all cell types, whereas gH/gL/UL128-131 broadens virus tropism through a distinct, as yet unknown mechanism. To our knowledge, this is the first suggestion of a herpesvirus gH/gL that does not act by promoting gB fusion, which might make HCMV a useful model to study the fundamental mechanisms by which herpesvirus gH/gL regulates gB fusion. Moreover, gH/gL/UL128-131 is a candidate vaccine target. Our findings help to explain the cell-type-dependent virus neutralization exhibited by anti-gH/gL/UL128-131 antibodies and underscore the importance of gH/gL/gO as another important part of vaccine or therapeutic strategies.

Human Cytomegalovirus UL135 and UL136 Genes Are Required for Postentry Tropism in Endothelial Cells

Endothelial cells (ECs) are a critical target of viruses, and infection of the endothelium represents a defining point in viral pathogenesis. Human cytomegalovirus (HCMV), the prototypical betaherpesvirus, encodes proteins specialized for entry into ECs and delivery of the genome to the nuclei of ECs. Virus strains competent to enter ECs replicate with differing efficiencies, suggesting that the virus encodes genes for postentry tropism in ECs. We previously reported a specific requirement for the UL133/8 locus of HCMV for replication in ECs. The UL133/8 locus harbors four genes: UL133, UL135, UL136, and UL138. In this study, we find that while UL133 and UL138 are dispensable for replication in ECs, both UL135 and UL136 are important. These genes are not required for virus entry or the expression of viral genes. The phenotypes associated with disruption of either gene reflect phenotypes observed for the UL133/8NULL virus, which lacks the entire UL133/8 locus, but are largely distinct from one another. Viruses lacking UL135 fail to properly envelop capsids in the cytoplasm, produce fewer dense bodies (DB) than the wild-type (WT) virus, and are unable to incorporate viral products into multivesicular bodies (MVB). Viruses lacking UL136 also fail to properly envelop virions and produce larger dense bodies than the WT virus. Our results indicate roles for the UL135 and UL136 proteins in commandeering host membrane-trafficking pathways for virus maturation. UL135 and UL136 represent the first HCMV genes crucial for early- to late-stage tropism in ECs.

IMPORTANCE

Human cytomegalovirus (HCMV) persists in the majority of the world's population. While typically asymptomatic in healthy hosts, HCMV can cause significant morbidity and mortality in immunocompromised or naïve individuals, particularly transplant patients and patients with congenital infections, respectively. Lifelong persistence of the virus may also contribute to age-related pathologies, such as vascular disease. One aspect of HCMV infection contributing to complex and varied pathogenesis is the diverse array of cell types that this virus infects in the host. The vascular endothelium is a particularly important target of infection, contributing to viral dissemination and likely leading to CMV complications following transplantation. In this work, we identify two viral gene products required for postentry tropism in endothelial cells. Identifying tropism factors required for replication in critical cell targets of infection is important for the development of strategies to restrict virus replication.

A viral regulator of glycoprotein complexes contributes to human cytomegalovirus cell tropism

Viral glycoproteins mediate entry of enveloped viruses into cells and thus play crucial roles in infection. In herpesviruses, a complex of two viral glycoproteins, gH and gL (gH/gL), regulates membrane fusion events and influences virion cell tropism. Human cytomegalovirus (HCMV) gH/gL can be incorporated into two different protein complexes: a glycoprotein O (gO)-containing complex known as gH/gL/gO, and a complex containing UL128, UL130, and UL131 known as gH/gL/UL128-131. Variability in the relative abundance of the complexes in the virion envelope correlates with differences in cell tropism exhibited between strains of HCMV. Nonetheless, the mechanisms underlying such variability have remained unclear. We have identified a viral protein encoded by the UL148 ORF (UL148) that influences the ratio of gH/gL/gO to gH/gL/UL128-131 and the cell tropism of HCMV virions. A mutant disrupted for UL148 showed defects in gH/gL/gO maturation and enhanced infectivity for epithelial cells. Accordingly, reintroduction of UL148 into an HCMV strain that lacked the gene resulted in decreased levels of gH/gL/UL128-131 on virions and, correspondingly, decreased infectivity for epithelial cells. UL148 localized to the endoplasmic reticulum, but not to the cytoplasmic sites of virion envelopment. Coimmunoprecipitation results indicated that gH, gL, UL130, and UL131 associate with UL148, but that gO and UL128 do not. Taken together, the findings suggest that UL148 modulates HCMV tropism by regulating the composition of alternative gH/gL complexes.

A role for 3-O-sulfated heparan sulfate in promoting human cytomegalovirus infection in human iris cells

Human cytomegalovirus (HCMV) has emerged as a clinically opportunistic pathogen that targets multiple types of ocular cells and tissues, including the iris region of the uveal tract during anterior uveitis. In this report, we used primary cultures of human iris stroma (HIS) cells derived from human eye donors to investigate HCMV entry. The following lines of evidence suggested the role of 3-O-sulfated heparan sulfate (3-OS HS) during HCMV-mediated entry and cell-to-cell fusion in HIS cells. First, 3-O-sulfotransferase-3 (3-OST-3) expression in HIS cells promoted HCMV internalization, while pretreatment of HIS cells with heparinase enzyme or with anti-3-OS HS (G2) peptide significantly reduced the HCMV-mediated formation of plaques/foci. Second, coculture of the HCMV-infected HIS cells with CHO-K1 cells expressing 3-OS HS significantly enhanced cell fusion. Finally, a similar trend of enhanced fusion was observed with cells expressing HCMV glycoproteins (gB, gO, and gH-gL) cocultured with 3-OS HS cells. Taken together, these results highlight the role of 3-OS HS during HCMV plaque formation and cell-to-cell fusion and identify a novel target for future therapeutic interventions.

Non-redundant and redundant roles of cytomegalovirus gH/gL complexes in host organ entry and intra-tissue spread

Herpesviruses form different gH/gL virion envelope glycoprotein complexes that serve as entry complexes for mediating viral cell-type tropism in vitro; their roles in vivo, however, remained speculative and can be addressed experimentally only in animal models. For murine cytomegalovirus two alternative gH/gL complexes, gH/gL/gO and gH/gL/MCK-2, have been identified. A limitation of studies on viral tropism in vivo has been the difficulty in distinguishing between infection initiation by viral entry into first-hit target cells and subsequent cell-to-cell spread within tissues. As a new strategy to dissect these two events, we used a gO-transcomplemented ΔgO mutant for providing the gH/gL/gO complex selectively for the initial entry step, while progeny virions lack gO in subsequent rounds of infection. Whereas gH/gL/gO proved to be critical for establishing infection by efficient entry into diverse cell types, including liver macrophages, endothelial cells, and hepatocytes, it was dispensable for intra-tissue spread. Notably, the salivary glands, the source of virus for host-to-host transmission, represent an exception in that entry into virus-producing cells did not strictly depend on either the gH/gL/gO or the gH/gL/MCK-2 complex. Only if both complexes were absent in gO and MCK-2 double-knockout virus, in vivo infection was abolished at all sites.

Mck2-dependent infection of alveolar macrophages promotes replication of MCMV in nodular inflammatory foci of the neonatal lung

Infection with cytomegalovirus (CMV) shows a worldwide high prevalence with only immunocompromised individuals or newborns to become symptomatic. The host's constitution and the pathogen's virulence determine whether disease occurs after infection. Mouse CMV (MCMV) is an appreciated pathogen for in vivo investigation of host-pathogen interactions. It has recently been reported that a single base pair deletion can spontaneously occur in the open reading frame of MCMV-encoded chemokine 2 (MCK2), preventing the expression of the full-length gene product. To study the consequences of this mutation, we compared the Mck2-defective reporter virus MCMV-3D with the newly generated repaired Mck2(+) mutant MCMV-3DR. Compared with MCMV-3D, neonatal mice infected with MCMV-3DR showed severe viral disease after lung infection. Viral disease coincided with high viral activity in multiple organs and increased virus replication in previously described nodular inflammatory foci (NIF) in the lung. Notably, MCMV-3DR showed tropism for alveolar macrophages in vitro and in vivo, whereas MCMV-3D did not infect this cell type. Moreover, in vivo depletion of alveolar macrophages reduced MCMV-3DR replication in the lung. We proposed an Mck2-mediated mechanism by which MCMV exploits alveolar macrophages to increase replication upon first encounter with the host's lung mucosa.

Pseudorabies virus triggers glycoprotein gE-mediated ERK1/2 activation and ERK1/2-dependent migratory behavior in T cells

The interaction between viruses and immune cells of the host may lead to modulation of intracellular signaling pathways and to subsequent changes in cellular behavior that are of benefit for either virus or host. ERK1/2 (extracellular signal regulated kinase 1/2) signaling represents one of the key cellular signaling axes. Here, using wild-type and gE null virus, recombinant gE, and gE-transfected cells, we show that the gE glycoprotein of the porcine Varicellovirus pseudorabies virus (PRV) triggers ERK1/2 phosphorylation in Jurkat T cells and primary porcine T lymphocytes. PRV-induced ERK1/2 signaling resulted in homotypic T cell aggregation and increased motility of T lymphocytes. Our study reveals a new function of the gE glycoprotein of PRV and suggests that PRV, through activation of ERK1/2 signaling, has a substantial impact on T cell behavior.

IMPORTANCE

Herpesviruses are known to be highly successful in evading the immune system of their hosts, subverting signaling pathways of the host to their own advantage. The ERK1/2 signaling pathway, being involved in many cellular processes, represents a particularly attractive target for viral manipulation. Glycoprotein E (gE) is an important virulence factor of alphaherpesviruses, involved in viral spread. In this study, we show that gE has the previously uncharacterized ability to trigger ERK1/2 phosphorylation in T lymphocytes. We also show that virus-induced ERK1/2 signaling leads to increased migratory behavior of T cells and that migratory T cells can spread the infection to susceptible cells. In conclusion, our results point to a novel function for gE and suggest that virus-induced ERK1/2 activation may trigger PRV-carrying T lymphocytes to migrate and infect other cells susceptible to PRV replication.

Rapid and sensitive identification of glycoprotein H genotypes in clinical human cytomegalovirus samples

Human cytomegalovirus (HCMV) is a common pathogen that causes persistent infections in immune deficient patients and results in significant morbidity and mortality, particularly among transplant recipients and children. Different HCMV glycoprotein H (gH) genotypes may cause different diseases and affect the severity of these diseases. To develop a sensitive quantitative real-time PCR assay that could rapidly distinguish between two HCMV gH genotypes, primers were designed to target the conserved region of the gH gene. gH1 and gH2 probes were designed to target the two variable regions. Standard HCMV strains (AD169 and TOWNE) and 203 clinical urine samples from HCMV infected children were used for the present study. Based on the primer-probe set used to detect the target gH gene segment of HCMV, our quantitative real-time PCR assay specifically discriminated between HCMV gH1 and gH2 with a detection limit of approximately 10(2) viral copies/ml. Among the 203 clinical urine samples tested, 145 were gH1 positive, 56 were gH2 positive, and 2 were positive for both. Thus, we developed a gH gene-based real time-PCR method that could rapidly, stably, and specifically distinguish between two HCMV gH genotypes. We found HCMV gH1 to be common among children examined in Zhejiang, China.

Development of a high-content screen for the identification of inhibitors directed against the early steps of the cytomegalovirus infectious cycle

Human cytomegalovirus (CMV) is a latent and persistent virus whose proliferation increases morbidity and mortality of immune-compromised individuals. The current anti-CMV therapeutics targeting the viral DNA polymerase or the major immediate-early (MIE) gene locus are somewhat effective at limiting CMV-associated disease. However, due to low bioavailability, severe toxicity, and the development of drug resistant CMV strains following prolonged treatment, current anti-CMV therapeutics are insufficient. To help address this shortfall, we established a high-content assay to identify inhibitors targeting CMV entry and the early steps of infection. The infection of primary human fibroblasts with a variant of the CMV laboratory strain AD169 expressing a chimeric IE2-yellow fluorescence protein (YFP) (AD169IE2-YFP) provided the basis for the high-content assay. The localization of IE2-YFP to the nucleus shortly following an AD169IE2-YFP infection induced a robust fluorescent signal that was quantified using confocal microscopy. The assay was optimized to achieve outstanding assay fitness and high Z' scores. We then screened a bioactive chemical library consisting of 2080 compounds and identified hit compounds based on the decrease of fluorescence signal from IE2-YFP nuclear expression. The hit compounds likely target various cellular processes involved in the early steps of infection including capsid transport, chromatin remodeling, and viral gene expression. Extensive secondary assays confirmed the ability of a hit compound, convallatoxin, to inhibit infection of both laboratory and clinical CMV strains and limit virus proliferation. Collectively, the data demonstrate that we have established a robust high-content screen to identify compounds that limit the early steps of the CMV life cycle, and that novel inhibitors of early infection events may serve as viable CMV therapeutics.

Rho'ing in and out of cells: viral interactions with Rho GTPase signaling

Rho GTPases are key regulators of actin and microtubule dynamics and organization. Increasing evidence shows that many viruses have evolved diverse interactions with Rho GTPase signaling and manipulate them for their own benefit. In this review, we discuss how Rho GTPase signaling interferes with many steps in the viral replication cycle, especially entry, replication, and spread. Seen the diversity between viruses, it is not surprising that there is considerable variability in viral interactions with Rho GTPase signaling. However, several largely common effects on Rho GTPases and actin architecture and microtubule dynamics have been reported. For some of these processes, the molecular signaling and biological consequences are well documented while for others we just begin to understand them. A better knowledge and identification of common threads in the different viral interactions with Rho GTPase signaling and their ultimate consequences for virus and host may pave the way toward the development of new antiviral drugs that may target different viruses.

HCMV reprogramming of infected monocyte survival and differentiation: a Goldilocks phenomenon

The wide range of disease pathologies seen in multiple organ sites associated with human cytomegalovirus (HCMV) infection results from the systemic hematogenous dissemination of the virus, which is mediated predominately by infected monocytes. In addition to their role in viral spread, infected monocytes are also known to play a key role in viral latency and life-long persistence. However, in order to utilize infected monocytes for viral spread and persistence, HCMV must overcome a number of monocyte biological hurdles, including their naturally short lifespan and their inability to support viral gene expression and replication. Our laboratory has shown that HCMV is able to manipulate the biology of infected monocytes in order to overcome these biological hurdles by inducing the survival and differentiation of infected monocytes into long-lived macrophages capable of supporting viral gene expression and replication. In this current review, we describe the unique aspects of how HCMV promotes monocyte survival and differentiation by inducing a “finely-tuned” macrophage cell type following infection. Specifically, we describe the induction of a uniquely polarized macrophage subset from infected monocytes, which we argue is the ideal cellular environment for the initiation of viral gene expression and replication and, ultimately, viral spread and persistence within the infected host.