Antigenic domain 1 is required for oligomerization of human cytomegalovirus glycoprotein B

Antiviral Approach to Cytomegalovirus Infection: An Overview of Conventional and Novel Strategies

Human cytomegalovirus (HCMV) is a herpesvirus capable of establishing a lifelong persistence in the host through a chronic state of infection and remains an essential global concern due to its distinct life cycle, mutations, and latency. It represents a life-threatening pathogen for immunocompromised patients, such as solid organ transplanted patients, HIV-positive individuals, and hematopoietic stem cell recipients. Multiple antiviral approaches are currently available and administered in order to prevent or manage viral infections in the early stages. However, limitations due to side effects and the onset of antidrug resistance are a hurdle to their efficacy, especially for long-term therapies. Novel antiviral molecules, together with innovative approaches (e.g., genetic editing and RNA interference) are currently in study, with promising results performed in vitro and in vivo. Since HCMV is a virus able to establish latent infection, with a consequential risk of reactivation, infection management could benefit from preventive treatment for critical patients, such as immunocompromised individuals and seronegative pregnant women. This review will provide an overview of conventional antiviral clinical approaches and their mechanisms of action. Additionally, an overview of proposed and developing new molecules is provided, including nucleic-acid-based therapies and immune-mediated approaches.

A fully human neutralizing monoclonal antibody targeting a highly conserved epitope of the human cytomegalovirus glycoprotein B

Human cytomegalovirus causes severe diseases in children (by congenital infection) and immunocompromised patients. Treatment with antiviral agents, such as ganciclovir, is limited by their toxicity. In this study, we investigated the effectiveness of a fully human neutralizing monoclonal antibody to inhibit human cytomegalovirus infection and viral cell-to-cell spread. We isolated a potent neutralizing antibody, EV2038 (IgG1 lambda), targeting human cytomegalovirus glycoprotein B using Epstein-Barr virus transformation. This antibody inhibited human cytomegalovirus infection by all four laboratory strains and 42 Japanese clinical isolates, including ganciclovir-resistant isolates, with a 50% inhibitory concentration (IC50) ranging from 0.013 to 0.105 μg/mL, and 90% inhibitory concentration (IC90) ranging from 0.208 to 1.026 μg/mL, in both human embryonic lung fibroblasts (MRC-5) and human retinal pigment epithelial (ARPE-19) cells. Additionally, EV2038 prevented cell-to-cell spread of eight clinical viral isolates, with IC50 values ranging from 1.0 to 3.1 μg/mL, and IC90 values ranging from 13 to 19 μg/mL, in ARPE-19 cells. EV2038 recognized three discontinuous sequences on antigenic domain 1 of glycoprotein B (amino acids 549-560, 569-576, and 625-632), which were highly conserved among 71 clinical isolates from Japan and the United States. Pharmacokinetics study in cynomolgus monkeys suggested the potential efficacy of EV2038 in vivo, the concentration of which in serum remained higher than the IC90 values of cell-to-cell spread until 28 days after intravenous injection of 10 mg/kg EV2038. Our data strongly support EV2038 as a promising candidate and novel alternative for the treatment of human cytomegalovirus infection.

Protective anti-gB neutralizing antibodies targeting two vulnerable sites for EBV-cell membrane fusion

Epstein-Barr virus (EBV) accounts for 200,000 new epithelial and B cell malignancy cases and 140,000 deaths annually. Glycoprotein B (gB) is the sole fusogen that is highly conserved and essential for all herpesvirus entry into target cells and thus, is attracting attention to identify potent antibodies to neutralize viral infection. Here, we discovered two anti-EBV gB neutralizing antibodies, 3A3 and 3A5, that effectively neutralized EBV infection of both B and epithelial cells. They also potently protected against EBV-induced lymphoproliferative disorders in humanized mice. Importantly, the 3A3 and 3A5 epitopes identified here represent the neutralizing antigenic sites to block EBV infection and membrane fusion. They are major targets of protective gB-specific neutralizing antibodies elicited by natural EBV infection in humans.

Epstein-Barr virus (EBV) infects more than 90% of the world’s adult population and accounts for a significant cancer burden of epithelial and B cell origins. Glycoprotein B (gB) is the primary fusogen essential for EBV entry into host cells. Here, we isolated two EBV gB-specific neutralizing antibodies, 3A3 and 3A5; both effectively neutralized the dual-tropic EBV infection of B and epithelial cells. In humanized mice, both antibodies showed effective protection from EBV-induced lymphoproliferative disorders. Cryoelectron microscopy analyses identified that 3A3 and 3A5 bind to nonoverlapping sites on domains D-II and D-IV, respectively. Structure-based mutagenesis revealed that 3A3 and 3A5 inhibit membrane fusion through different mechanisms involving the interference with gB-cell interaction and gB activation. Importantly, the 3A3 and 3A5 epitopes are major targets of protective gB-specific neutralizing antibodies elicited by natural EBV infection in humans, providing potential targets for antiviral therapies and vaccines.

Pharmacokinetics, Safety, and Tolerability of NPC-21, an Anti-Cytomegalovirus Monoclonal Antibody, in Healthy Japanese and White Adult Men: A Randomized, Placebo-Controlled, First-in-Human Phase 1 Study

NPC-21 (EV2038) is a fully human monoclonal antibody that targets the antigenic domain 1 of glycoprotein B on the human cytomegalovirus (hCMV) envelope. NPC-21 has been shown to have broadly neutralizing activity and to inhibit cell-to-cell transmission of hCMV in preclinical studies. It is currently in development for the prophylactic or preemptive treatment of hCMV in patients receiving a solid-organ transplant or hematopoietic stem cell transplant. A first-in-human phase 1 study was conducted to assess the pharmacokinetics, safety, and tolerability of NPC-21 in healthy adult men. Forty participants (Japanese, n = 32; White, n = 8) were randomly assigned to receive a single intravenous dose of NPC-21 1, 3, 10, or 20 mg/kg or placebo. Six Japanese participants were included in each dose group and six White participants received a 10-mg/kg dose. The placebo group included 8 Japanese participants and 2 White participants. All 40 participants completed the study. Serum concentration, maximum serum concentration, area under the plasma concentration-time curve from time 0 to the last measurable concentration, and area under the plasma concentration-time curve from time 0 to infinity increased dose dependently; dose proportionality was linear. NPC-21 demonstrated a biphasic elimination pattern, with an estimated half-life between 612 and 790 hours. NPC-21 was safe and well tolerated up to 20 mg/kg. All adverse events were mild, and none led to treatment discontinuation or were considered related to the study drug. There were no differences in pharmacokinetics or safety between Japanese and White participants. These results support further investigation of NPC-21.

A trimeric capable gB CMV vaccine provides limited protection against a highly cell associated and epithelial tropic strain of cytomegalovirus in guinea pigs

Multiple strains of human cytomegalovirus (HCMV) can cause congenital cytomegalovirus (cCMV) by primary or secondary infection. The viral gB glycoprotein is a leading vaccine candidate, essential for infection of all cell-types, and immunodominant antibody target. Guinea pig cytomegalovirus (GPCMV) is the only small animal model for cCMV. Various gB vaccines have shown efficacy but studies have utilized truncated gB and protection against prototype strain 22122 with preferential tropism to fibroblasts despite encoding a gH-based pentamer complex for non-fibroblast infection. A highly cell-associated novel strain of GPCMV (TAMYC) with 99 % identity in gB sequence to 22122 exhibited preferred tropism to epithelial cells. An adenovirus vaccine encoding full-length gB (AdgB) was highly immunogenic and partially protected against 22122 strain challenge in vaccinated animals but not when challenged with TAMYC strain. GPCMV studies with AdgB vaccine sera on numerous cell-types demonstrated impaired neutralization (NA₅₀) compared to fibroblasts. GPCMV-convalescent sera including pentamer complex antibodies increased virus neutralization on non-fibroblasts and anti-gB depletion from GPCMV-convalescent sera had minimal impact on epithelial cell neutralization. GPCMV(PC+) 22122-convalescent animals challenged with TAMYC exhibited higher protection compared to AdgB vaccine. Overall, results suggest that antibody response to both gB and PC are important components of a GPCMV vaccine.

Neutralizing antibodies to gB based CMV vaccine requires full length antigen but reduced virus neutralization on non-fibroblast cells limits vaccine efficacy in the guinea pig model

Cytomegalovirus is a leading cause of congenital disease and a vaccine is a high priority. The viral gB glycoprotein is essential for infection on all cell types. The guinea pig is the only small animal model for congenital CMV (cCMV), but requires guinea pig cytomegalovirus (GPCMV). Various GPCMV gB vaccine strategies have been investigated but not with a full length protein. Previous GPCMV gB vaccines have failed to fully protect against cCMV, with approximately 50% efficacy. In an effort to define the basis of GPCMV gB based vaccine failure, we evaluated recombinant defective Ad vectors encoding GPCMV gB full length (gBwt), or truncated protein lacking transmembrane domain (gBTMD). Both candidate vaccines evoked high anti-gB titers and neutralized virus infection on fibroblast cells but had varying weaker results on non-fibroblasts (renal epithelial and placental trophoblasts). Non-fibroblast cells are dependent upon the viral pentamer complex (PC) for endocytic pathway cell entry. In contrast, fibroblasts cells that express the viral receptor platelet derived growth factor receptor alpha (PDGFRA) to enable entry by direct cell fusion independent of the PC. Anti-gBwt sera was approximately 2-fold (renal epithelial) to 3-fold (fibroblasts) more effective at neutralizing virus compared to anti-gBTMD sera. Both gB vaccines were weakest against virus neutralization on trophoblasts. Knockout of PDGFRA cell receptor on fibroblast cells (GPKO) rendered virus dependent upon the PC pathway for cell entry and anti-gB GPCMV NA₅₀ was more similar to epithelial cells. In a gBwt vaccine protection study, vaccination of animals significantly reduced, but did not prevent dissemination of wild type GPCMV challenge virus to target organs. Depletion of complement in vivo had limited impact on vaccine efficacy. Overall, a full length gB antigen has the potential to improve neutralizing antibody titer but fails to fully prevent virus dissemination and likely congenital infection.

The Human Cytomegalovirus Nonstructural Glycoprotein UL148 Reorganizes the Endoplasmic Reticulum

Human cytomegalovirus (HCMV) encodes an endoplasmic reticulum (ER)-resident glycoprotein, UL148, which activates the unfolded protein response (UPR) but is fully dispensable for viral replication in cultured cells. Hence, its previously ascribed roles in immune evasion and modulation of viral cell tropism are hypothesized to cause ER stress. Here, we show that UL148 is necessary and sufficient to drive the formation of prominent ER-derived structures that on average occupy 5% of the infected cell cytoplasm. The structures are sites where UL148 coalesces with cellular proteins involved in ER quality control, such as HRD1 and EDEM1. Electron microscopy revealed that cells infected with wild-type but not UL148-null HCMV show prominent accumulations of densely packed ruffled ER membranes which connect to distended cisternae of smooth and partially rough ER. During ectopic expression of UL148-green fluorescent protein (GFP) fusion protein, punctate signals traffic to accumulate at conspicuous structures. The structures exhibit poor recovery of fluorescence after photobleaching, which suggests that their contents are poorly mobile and do not efficiently exchange with the rest of the ER. Small-molecule blockade of the integrated stress response (ISR) prevents the formation of puncta, leading to a uniform reticular fluorescent signal. Accordingly, ISR inhibition during HCMV infection abolishes the coalescence of UL148 and HRD1 into discrete structures, which argues that UL148 requires the ISR to cause ER reorganization. Given that UL148 stabilizes immature forms of a receptor binding subunit for a viral envelope glycoprotein complex important for HCMV infectivity, our results imply that stress-dependent ER remodeling contributes to viral cell tropism.IMPORTANCE Perturbations to endoplasmic reticulum (ER) morphology occur during infection with various intracellular pathogens and in certain genetic disorders. We identify that a human cytomegalovirus (HCMV) gene product, UL148, profoundly reorganizes the ER during infection and is sufficient to do so when expressed on its own. Our results reveal that UL148-dependent reorganization of the ER is a prominent feature of HCMV-infected cells. Moreover, we find that this example of virally induced organelle remodeling requires the integrated stress response (ISR), a stress adaptation pathway that contributes to a number of disease states. Since ER reorganization accompanies roles of UL148 in modulation of HCMV cell tropism and in evasion of antiviral immune responses, our results may have implications for understanding the mechanisms involved. Furthermore, our findings provide a basis to utilize UL148 as a tool to investigate organelle responses to stress and to identify novel drugs targeting the ISR.

Epitope-Specific Humoral Responses to Human Cytomegalovirus Glycoprotein-B Vaccine With MF59: Anti-AD2 Levels Correlate With Protection From Viremia

The human cytomegalovirus (HCMV) virion envelope protein glycoprotein B (gB) is essential for viral entry and represents a major target for humoral responses following infection. Previously, a phase 2 placebo-controlled clinical trial conducted in solid organ transplant candidates demonstrated that vaccination with gB plus MF59 adjuvant significantly increased gB enzyme-linked immunosorbent assay (ELISA) antibody levels whose titer correlated directly with protection against posttransplant viremia. The aim of the current study was to investigate in more detail this protective humoral response in vaccinated seropositive transplant recipients. We focused on 4 key antigenic domains (AD) of gB (AD1, AD2, AD4, and AD5), measuring antibody levels in patient sera and correlating these with posttransplant HCMV viremia. Vaccination of seropositive patients significantly boosted preexisting antibody levels against the immunodominant region AD1 as well as against AD2, AD4, and AD5. A decreased incidence of viremia correlated with higher antibody levels against AD2 but not with antibody levels against the other 3 ADs. Overall, these data support the hypothesis that antibodies against AD2 are a major component of the immune protection of seropositives seen following vaccination with gB/MF59 vaccine and identify a correlate of protective immunity in allograft patients.

HCMV glycoprotein B is expressed in primary glioblastomas and enhances growth and invasiveness via PDGFR-alpha activation

Our laboratory first demonstrated that human cytomegalovirus (HCMV) is associated with the most deadly form of primary brain tumor, glioblastoma (GBM). We showed that HCMV glycoprotein B (gB) mediates viral cellular entry via the receptor tyrosine kinase PDGFR-alpha (PDGFRα), resulting in activation of the PI3K/Akt pathway, a critical signaling axis gliomagenesis. Here, we investigated the effects of gB overexpression on glioma progression. We demonstrate that gB is endogenously expressed in primary GBM samples and show that ectopic gB expression in glioma cells induced sustained phosphorylation of PDGFRα, Akt, and Src. Recombinant gB protein and the whole virus enhanced invasion of primary glioblastoma cells into Matrigel and rat brain slices, and this effect was specifically inhibited by neutralizing antibodies to either gB or PDGFRα. Importantly, neutralizing antibodies to gB significantly inhibited the invasiveness of patient-derived HCMV-positive glioblastoma cells, suggesting that functional inhibition of this viral protein could hinder glioblastoma progression. gB overexpression promoted in vivo glioma growth and enhanced phosphor-Akt levels and tumor cell dispersal relative to controls. Taken together, our results demonstrate that HCMV gB promotes key hallmarks of glioblastoma and suggest that targeting gB may have therapeutic benefits for patients with HCMV -positive gliomas.

Human cytomegalovirus vaccine based on the envelope gH/gL pentamer complex

Human Cytomegalovirus (HCMV) utilizes two different pathways for host cell entry. HCMV entry into fibroblasts requires glycoproteins gB and gH/gL, whereas HCMV entry into epithelial and endothelial cells (EC) requires an additional complex composed of gH, gL, UL128, UL130, and UL131A, referred to as the gH/gL-pentamer complex (gH/gL-PC). While there are no established correlates of protection against HCMV, antibodies are thought to be important in controlling infection. Neutralizing antibodies (NAb) that prevent gH/gL-PC mediated entry into EC are candidates to be assessed for in vivo protective function. However, these potent NAb are predominantly directed against conformational epitopes derived from the assembled gH/gL-PC. To address these concerns, we constructed Modified Vaccinia Ankara (MVA) viruses co-expressing all five gH/gL-PC subunits (MVA-gH/gL-PC), subsets of gH/gL-PC subunits (gH/gL or UL128/UL130/UL131A), or the gB subunit from HCMV strain TB40/E. We provide evidence for cell surface expression and assembly of complexes expressing full-length gH or gB, or their secretion when the corresponding transmembrane domains are deleted. Mice or rhesus macaques (RM) were vaccinated three times with MVA recombinants and serum NAb titers that prevented 50% infection of human EC or fibroblasts by HCMV TB40/E were determined. NAb responses induced by MVA-gH/gL-PC blocked HCMV infection of EC with potencies that were two orders of magnitude greater than those induced by MVA expressing gH/gL, UL128-UL131A, or gB. In addition, MVA-gH/gL-PC induced NAb responses that were durable and efficacious to prevent HCMV infection of Hofbauer macrophages, a fetal-derived cell localized within the placenta. NAb were also detectable in saliva of vaccinated RM and reached serum peak levels comparable to NAb titers found in HCMV hyperimmune globulins. This vaccine based on a translational poxvirus platform co-delivers all five HCMV gH/gL-PC subunits to achieve robust humoral responses that neutralize HCMV infection of EC, placental macrophages and fibroblasts, properties of potential value in a prophylactic vaccine.

Human cytomegalovirus (HCMV) fetal infection during pregnancy and infection of immunocompromised patients are both clinical problems considered extremely important by the Institute of Medicine. Limited efficacy against primary HCMV infection was found using a subunit vaccine based on glycoprotein B, an important neutralizing antibody determinant blocking HCMV entry into fibroblasts. The HCMV field has been transformed by the discovery that a five-member (pentamer) protein complex is a required factor for epithelial and endothelial cell entry and indispensable for transmission as shown in non-human primates. Targeting HCMV with antibodies specific to the pentamer may interrupt horizontal and vertical transmission. We describe an innovative vaccine strategy to induce serum neutralizing antibodies of impressive magnitude against HCMV in two animal models. Using an attenuated poxvirus vector system, we demonstrate that co-expression of all five pentamer components is significantly more potent to induce serum neutralizing antibodies than subunit subsets of the complex or glycoprotein B, reaching peak levels comparable to HCMV hyperimmune globulin. A vaccine that elicits systemic and mucosal antibody responses that prevents infection of multiple cell types crucial to natural history of HCMV infection could play a role in preventing congenital HCMV infection and control of infection in immunocompromised patients.

HCMV gB shares structural and functional properties with gB proteins from other herpesviruses

Glycoprotein B (gB) facilitates HCMV entry into cells by binding receptors and mediating membrane fusion. The crystal structures of gB ectodomains from HSV-1 and EBV are available, but little is known about the HCMV gB structure. Using multiangle light scattering and electron microscopy, we show here that HCMV gB ectodomain is a trimer with the overall shape similar to HSV-1 and EBV gB ectodomains. HCMV gB ectodomain forms rosettes similar to rosettes formed by EBV gB and the postfusion forms of other viral fusogens. Substitution of several bulky hydrophobic residues within the putative fusion loops with more hydrophilic residues reduced rosette formation and abolished cell fusion. We propose that like gB proteins from HSV-1 and EBV, HCMV gB has two internal hydrophobic fusion loops that likely interact with target membranes. Our work establishes structural and functional similarities between gB proteins from three subfamilies of herpesviruses.

Diagnostic consequences of cytomegalovirus glycoprotein B polymorphisms

Failure of a cytomegalovirus (CMV) real-time PCR assay targeting glycoprotein B (gB) was investigated. A multiplex assay targeting gB and immediate-early 2 (IE2) genes showed discordant results (gB negative and IE positive or a >10-fold-higher viral load with IE primers) in saliva from 14.6% of CMV-infected newborns. Sequencing revealed 3 patterns of gB variations.

Update on the current status of cytomegalovirus vaccines

Human cytomegalovirus (HCMV) is ubiquitous in all populations, and is the most commonly recognized cause of congenital viral infection in developed countries. On the basis of the economic costs saved and the improvement in quality of life that could potentially be conferred by a successful vaccine for prevention of congenital HCMV infection, the Institute of Medicine has identified HCMV vaccine development as a major public health priority. An effective vaccine could potentially also be beneficial in preventing or ameliorating HCMV disease in immunocompromised individuals. Although there are no licensed HCMV vaccines currently available, enormous progress has been made in the last decade, as evidenced by the recently reported results of a Phase II trial of a glycoprotein B vaccine for the prevention of HCMV infection in seronegative women of childbearing age. HCMV vaccines currently in clinical trials include: glycoprotein B subunit vaccines; alphavirus replicon particle vaccines; DNA vaccines; and live-attenuated vaccines. A variety of vaccine strategies are also being examined in preclinical systems and animal models of infection. These include: recombinant vesicular stomatitis virus vaccines; recombinant modified vaccinia virus Ankara; replication-deficient adenovirus-vectored vaccines; and recombinant live-attenuated virus vaccines generated by mutagenesis of cloned rodent CMV genomes maintained as bacterial artificial chromosomes in Escherichia coli. In this article, we provide an overview of the current state of clinical trials and preclinical development of vaccines against HCMV, with an emphasis on studies that have been conducted in the past 5 years. We also summarize a number of recent advances in the study of the biology of HCMV, particularly with respect to epithelial and endothelial cell entry of the virus, which have implications for future vaccine design.

Bicaudal D1-dependent trafficking of human cytomegalovirus tegument protein pp150 in virus-infected cells

Human cytomegalovirus (HCMV) virion assembly takes place in the nucleus and cytoplasm of infected cells. The HCMV virion tegument protein pp150 (ppUL32) is an essential protein of HCMV and has been suggested to play a role in the cytoplasmic phase of HCMV assembly. To further define its role in viral assembly and to identify host cell proteins that interact with pp150 during viral assembly, we utilized yeast two-hybrid analyses to detect an interaction between pp150 and Bicaudal D1 (BicD1), a protein thought to play a role in trafficking within the secretory pathway. BicD1 is known to interact with the dynein motor complex and the Rab6 GTPase. The interaction between pp150 and BicD1 was confirmed by coimmunoprecipitation and fluorescence resonance energy transfer. Depletion of BicD1 with short hairpin RNA (shRNA) caused decreased virus yield and a defect in trafficking of pp150 to the cytoplasmic viral assembly compartment (AC), without altering trafficking to the AC of another essential tegument protein, pp28, or the viral glycoprotein complex gM/gN. The C terminus of BicD1 has been previously shown to interact with the GTPase Rab6, suggesting a potential role for Rab6-mediated vesicular trafficking in HCMV assembly. Finally, overexpression of the N terminus of truncated BicD1 acts in a dominant-negative manner and leads to disruption of the AC and a decrease in the assembly of infectious virus. This phenotype was similar to that observed following overexpression of dynamitin (p50) and provided additional evidence that morphogenesis of the AC and virus assembly were dynein dependent.

Analysis of Epstein-Barr virus glycoprotein B functional domains via linker insertion mutagenesis

Epstein-Barr Virus (EBV) glycoprotein B (gB) is essential for viral fusion events with epithelial and B cells. This glycoprotein has been studied extensively in other herpesvirus family members, but functional domains outside of the cytoplasmic tail have not been characterized in EBV gB. In this study, a total of 28 linker insertion mutations were generated throughout the length of gB. In general, the linker insertions did not disrupt intracellular expression and variably altered cell surface expression. Oligomerization was disrupted by insertions located between residues 561 and 620, indicating the location of a potential site of oligomer contacts between EBV gB monomers. In addition, a novel N-glycosylated form of wild-type gB was identified under nonreducing Western blot conditions that likely represents a mature form of the protein. Fusion activity was abolished in all but three variants containing mutations in the N-terminal region (gB30), within the ectodomain (gB421), and in the intracellular C-terminal domain (gB832) of the protein. Fusion activity with variants gB421 and gB832 was comparable to that of the wild type with epithelial and B cells, and only these two mutants, but not gB30, were able to complement gB-null virus and subsequently function in virus entry. The mutant gB30 exhibited a low level of fusion activity with B cells and was unable to complement gB-null virus. The mutations generated here indicate important structural domains, as well as regions important for function in fusion, within EBV gB.

Platelet-derived growth factor-alpha receptor activation is required for human cytomegalovirus infection

Human cytomegalovirus (HCMV) is a ubiquitous human herpesvirus that can cause life-threatening disease in the fetus and the immunocompromised host. Upon attachment to the cell, the virus induces robust inflammatory, interferon- and growth-factor-like signalling. The mechanisms facilitating viral entry and gene expression are not clearly understood. Here we show that platelet-derived growth factor-alpha receptor (PDGFR-alpha) is specifically phosphorylated by both laboratory and clinical isolates of HCMV in various human cell types, resulting in activation of the phosphoinositide-3-kinase (PI(3)K) signalling pathway. Upon stimulation by HCMV, tyrosine-phosphorylated PDGFR-alpha associated with the p85 regulatory subunit of PI(3)K and induced protein kinase B (also known as Akt) phosphorylation, similar to the genuine ligand, PDGF-AA. Cells in which PDGFR-alpha was genetically deleted or functionally blocked were non-permissive to HCMV entry, viral gene expression or infectious virus production. Re-introducing human PDGFRA gene into knockout cells restored susceptibility to viral entry and essential viral gene expression. Blockade of receptor function with a humanized PDGFR-alpha blocking antibody (IMC-3G3) or targeted inhibition of its kinase activity with a small molecule (Gleevec) completely inhibited HCMV viral internalization and gene expression in human epithelial, endothelial and fibroblast cells. Viral entry in cells harbouring endogenous PDGFR-alpha was competitively inhibited by pretreatment with PDGF-AA. We further demonstrate that HCMV glycoprotein B directly interacts with PDGFR-alpha, resulting in receptor tyrosine phosphorylation, and that glycoprotein B neutralizing antibodies inhibit HCMV-induced PDGFR-alpha phosphorylation. Taken together, these data indicate that PDGFR-alpha is a critical receptor required for HCMV infection, and thus a target for novel anti-viral therapies.

Manifestations of human cytomegalovirus infection: proposed mechanisms of acute and chronic disease

Infections with human cytomegalovirus (HCMV) are a major cause of morbidity and mortality in humans with acquired or developmental deficits in innate and adaptive immunity. In the normal immunocompetent host, symptoms rarely accompany acute infections, although prolonged virus shedding is frequent. Virus persistence is established in all infected individuals and appears to be maintained by both a chronic productive infections as well as latency with restricted viral gene expression. The contributions of the each of these mechanisms to the persistence of this virus in the individual is unknown but frequent virus shedding into the saliva and genitourinary tract likely accounts for the near universal incidence of infection in most populations in the world. The pathogenesis of disease associated with acute HCMV infection is most readily attributable to lytic virus replication and end organ damage either secondary to virus replication and cell death or from host immunological responses that target virus-infected cells. Antiviral agents limit the severity of disease associated with acute HCMV infections, suggesting a requirement for virus replication in clinical syndromes associated with acute infection. End organ disease secondary to unchecked virus replication can be observed in infants infected in utero, allograft recipients receiving potent immunosuppressive agents, and patients with HIV infections that exhibit a loss of adaptive immune function. In contrast, diseases associated with chronic or persistent infections appear in normal individuals and in the allografts of the transplant recipient. The manifestations of these infections appear related to chronic inflammation, but it is unclear if poorly controlled virus replication is necessary for the different phenotypic expressions of disease that are reported in these patients. Although the relationship between HCMV infection and chronic allograft rejection is well known, the mechanisms that account for the role of this virus in graft loss are not well understood. However, the capacity of this virus to persist in the midst of intense inflammation suggests that its persistence could serve as a trigger for the induction of host-vs-graft responses or alternatively host responses to HCMV could contribute to the inflammatory milieu characteristic of chronic allograft rejection.

Multimerization of tegument protein pp28 within the assembly compartment is required for cytoplasmic envelopment of human cytomegalovirus

Human cytomegalovirus (HCMV) UL99-encoded pp28 is an essential tegument protein required for envelopment and production of infectious virus. Nonenveloped virions accumulate in the cytoplasm of cells infected with recombinant viruses with the UL99 gene deleted. Previous results have suggested that a key function of pp28 in the envelopment of infectious HCMV is expressed after the protein localizes in the assembly compartment (AC). In this study, we investigated the potential role of pp28 multimerization in the envelopment of the infectious virion. Our results indicated that pp28 multimerized during viral infection and that interacting domains responsible for self-interaction were localized in the amino terminus of the protein (amino acids [aa] 1 to 43). The results from transient-expression and/or infection assays indicated that the self-interaction took place in the AC. A mutant pp28 molecule containing only the first 35 aa failed to accumulate in the AC, did not interact with pp28 in the AC, and could not support virus replication. In contrast, the first 50 aa of pp28 was sufficient for the self-interaction within the AC and the assembly of infectious virus. Recombinant viruses encoding an in-frame deletion of aa 26 to 33 of pp28 were replication competent, whereas infectious virus was not recovered from HCMV BACs lacking aa 26 to 43. These findings suggested that the accumulation of pp28 was a prerequisite for multimerization of pp28 within the AC and that pp28 multimerization in the AC represented an essential step in the envelopment and production of infectious virions.

Location, location, timing: analysis of cytomegalovirus epitopes for neutralizing antibodies

Attempts to produce a vaccine to human cytomegalovirus (HCMV) have failed. The principal target of the humoral immune response to HCMV is the viral envelope glycoprotein gB, which contains several immunodominant epitopes. Here we discuss human antibodies reacting with gB and offer an explanation as to why most humans make antibodies to an epitope that does not always elicit neutralizing antibodies. We suggest modifications to gB for an improved HCMV vaccine design.

The cytoplasmic tail of glycoprotein M (gpUL100) expresses trafficking signals required for human cytomegalovirus assembly and replication

The virion envelope of human cytomegalovirus (HCMV) is complex and consists of an incompletely defined number of glycoproteins. The gM/gN protein complex is the most abundant protein component of the envelope. Studies have indicated that deletion of the viral gene encoding either gM or gN is a lethal mutation. Analysis of the amino acid sequence of gM disclosed a C-terminal acidic cluster of amino acids and a tyrosine-containing trafficking motif, both of which are well-described trafficking/sorting signals in the cellular secretory pathway. To investigate the roles of these signals in the trafficking of the gM/gN complex during virus assembly, we made a series of gM (UL100 open reading frame) mutants in the AD169 strain of HCMV. Mutant viruses that lacked the entire C-terminal cytoplasmic tail of gM were not viable, suggesting that the cytoplasmic tail of gM is essential for virus replication. In addition, the gM mutant protein lacking the cytoplasmic domain exhibited decreased protein stability. Mutant viruses with a deletion of the acidic cluster or alanine substitutions in tyrosine-based motifs were viable but exhibited a replication-impaired phenotype suggestive of a defect in virion assembly. Analysis of these mutant gMs using static immunofluorescence and fluorescence recovery after photobleaching demonstrated delayed kinetics of intracellular localization of the gM/gN protein to the virus assembly compartment compared to the wild-type protein. These data suggest an important role of the glycoprotein gM during virus assembly, particularly in the dynamics of gM trafficking during viral-particle assembly.

Sequence requirements for localization of human cytomegalovirus tegument protein pp28 to the virus assembly compartment and for assembly of infectious virus

The human cytomegalovirus UL99 open reading frame encodes a 190-amino-acid (aa) tegument protein, pp28, that is myristoylated and phosphorylated. pp28 is essential for assembly of infectious virus, and nonenveloped virions accumulate in the cytoplasm of cells infected with recombinant viruses with a UL99 deletion. pp28 is localized to the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) in transfected cells, while in infected cells, it is localized together with other virion proteins in a juxtanuclear compartment termed the assembly compartment (AC). We investigated the sequence requirements for pp28 trafficking to the AC and assembly of infectious virus. Our studies indicated that the first 30 to 35 aa were required for localization of pp28 to the ERGIC in transfected cells. Mutant forms of pp28 containing only the first 35 aa localized with other virion structural proteins to cytoplasmic compartments early in infection, but localization to the AC at late times required a minimum of 50 aa. In agreement with previous reports, we demonstrated that the deletion of a cluster of acidic amino acids (aa 44 to 59) prevented wild-type trafficking of pp28 and recovery of infectious virus. A recombinant virus expressing only the first 50 aa was replication competent, and this mutant, pp28, localized to the AC in cells infected with this virus. These findings argued that localization of pp28 to the AC was essential for assembly of infectious virus and raised the possibility that amino acids in the amino terminus of pp28 have additional roles in the envelopment and assembly of the virion other than simply localizing pp28 to the AC.

Incorporation of a lambda phage recombination system and EGFP detection to simplify mutagenesis of Herpes simplex virus bacterial artificial chromosomes

Background

Targeted mutagenesis of the herpesvirus genomes has been facilitated by the use of bacterial artificial chromosome (BAC) technology. Such modified genomes have potential uses in understanding viral pathogenesis, gene identification and characterization, and the development of new viral vectors and vaccines. We have previously described the construction of a herpes simplex virus 2 (HSV-2) BAC and the use of an allele replacement strategy to construct HSV-2 recombinants. While the BAC mutagenesis procedure is a powerful method to generate HSV-2 recombinants, particularly in the absence of selective marker in eukaryotic culture, the mutagenesis procedure is still difficult and cumbersome.

Results

Here we describe the incorporation of a phage lambda recombination system into an allele replacement vector. This strategy enables any DNA fragment containing the phage attL recombination sites to be efficiently inserted into the attR sites of the allele replacement vector using phage lambda clonase. We also describe how the incorporation of EGFP into the allele replacement vector can facilitate the selection of the desired cross-over recombinant BACs when the allele replacement reaction is a viral gene deletion. Finally, we incorporate the lambda phage recombination sites directly into an HSV-2 BAC vector for direct recombination of gene cassettes using the phage lambda clonase-driven recombination reaction.

Conclusion

Together, these improvements to the techniques of HSV BAC mutagenesis will facilitate the construction of recombinant herpes simplex viruses and viral vectors.

Crystal structure of glycoprotein B from herpes simplex virus 1

Glycoprotein B (gB) is the most conserved component of the complex cell-entry machinery of herpes viruses. A crystal structure of the gB ectodomain from herpes simplex virus type 1 reveals a multidomain trimer with unexpected homology to glycoprotein G from vesicular stomatitis virus (VSV G). An alpha-helical coiled-coil core relates gB to class I viral membrane fusion glycoproteins; two extended beta hairpins with hydrophobic tips, homologous to fusion peptides in VSV G, relate gB to class II fusion proteins. Members of both classes accomplish fusion through a large-scale conformational change, triggered by a signal from a receptor-binding component. The domain connectivity within a gB monomer would permit such a rearrangement, including long-range translocations linked to viral and cellular membranes.