The properties and sequence of glycoprotein H of herpes simplex virus type 1

Immunisation Using Novel DNA Vaccine Encoding Virus Membrane Fusion Complex and Chemokine Genes Shows High Protection from HSV-2

Herpes simplex virus 1 and 2 infections cause high unmet disease burdens worldwide. Mainly HSV-2 causes persistent sexually transmitted disease, fatal neonatal disease and increased transmission of HIV/AIDS. Thus, there is an urgent requirement to develop effective vaccines. We developed nucleic acid vaccines encoding a novel virus entry complex stabilising cell membrane fusion, 'virus-like membranes', VLM. Two dose intramuscular immunisations using DNA expression plasmids in a guinea pig model gave 100% protection against acute disease and significantly reduced virus replication after virus intravaginal challenge. There was also reduced establishment of latency within the dorsal root ganglia and spinal cord, but recurrent disease and recurrent virus shedding remained. To increase cellular immunity and protect against recurrent disease, cDNA encoding an inhibitor of chemokine receptors on T regulatory cells was added and compared to chemokine CCL5 effects. Immunisation including this novel human chemokine gene, newly defined splice variant from an endogenous virus genome, 'virokine immune therapeutic', VIT, protected most guinea pigs from recurrent disease and reduced recurrent virus shedding distinct from a gD protein vaccine similar to that previously evaluated in clinical trials. All DNA vaccines induced significant neutralising antibodies and warrant evaluation for new therapeutic treatments.

Gene expression and in vitro replication of bovine gammaherpesvirus type 4

In vitro cell cultures are widely used models for dissecting cellular and molecular mechanisms that lead to certain physiological conditions and diseases. The pathogenesis of BoHV-4 in the bovine reproductive tract has been studied by conducting tests on primary cultures. However, many questions remain to be answered about the role of BoHV-4 in endometrial cells. The aim of this study was to compare the replication and gene expression of BoHV-4 in cell lines and bovine reproductive tract primary cells as an in vitro model for the study of this virus. We demonstrated that BoHV-4 strains differ in their in vitro growth kinetics and gene expression but have the same cell type preference. Our results demonstrate that BoHV-4 replicates preferentially in bovine endometrial cells (BEC). However, its replication capacity extends to various cell types, since all cells that were tested were permissive to BoHV-4 infection. The highest virus titers were obtained in BEC cells. Nevertheless, virus replication efficiency could not be fully predicted from the mRNA expression profiles. This implies that there are multiple cell-type-dependent factors and strain properties that determine the level of BoHV-4 replication. The results of this study provide relevant information about the in vitro behavior of two field isolates of BoHV-4 in different cell cultures. These findings may be useful for the design of future in vitro experiments to obtain reliable results not only about the pathogenic role of BoHV-4 in the bovine female reproductive tract but also in the development of efficient antiviral strategies.

Protein- and Peptide-Based Virus Inactivators: Inactivating Viruses Before Their Entry Into Cells

Infectious diseases caused by human immunodeficiency virus (HIV) and other highly pathogenic enveloped viruses, have threatened the global public health. Most antiviral drugs act as passive defenders to inhibit viral replication inside the cell, while a few of them function as gate keepers to combat viruses outside the cell, including fusion inhibitors, e.g., enfuvirtide, and receptor antagonists, e.g., maraviroc, as well as virus inactivators (including attachment inhibitors). Different from fusion inhibitors and receptor antagonists that must act in the presence of target cells, virus inactivators can actively inactivate cell-free virions in the blood, through interaction with one or more sites in the envelope glycoproteins (Envs) on virions. Notably, a number of protein- and peptide-based virus inactivators (PPVIs) under development are expected to have a better utilization rate than the current antiviral drugs and be safer for in vivo human application than the chemical-based virus inactivators. Here we have highlighted recent progress in developing PPVIs against several important enveloped viruses, including HIV, influenza virus, Zika virus (ZIKV), dengue virus (DENV), and herpes simplex virus (HSV), and the potential use of PPVIs for urgent treatment of infection by newly emerging or re-emerging viruses.

The herpes simplex virus host shutoff (vhs) RNase limits accumulation of double stranded RNA in infected cells: Evidence for accelerated decay of duplex RNA

The herpes simplex virus virion host shutoff (vhs) RNase destabilizes cellular and viral mRNAs and blunts host innate antiviral responses. Previous work demonstrated that cells infected with vhs mutants display enhanced activation of the host double-stranded RNA (dsRNA)-activated protein kinase R (PKR), implying that vhs limits dsRNA accumulation in infected cells. Confirming this hypothesis, we show that partially complementary transcripts of the UL23/UL24 and UL30/31 regions of the viral genome increase in abundance when vhs is inactivated, giving rise to greatly increased levels of intracellular dsRNA formed by annealing of the overlapping portions of these RNAs. Thus, vhs limits accumulation of dsRNA at least in part by reducing the levels of complementary viral transcripts. We then asked if vhs also destabilizes dsRNA after its initial formation. Here, we used a reporter system employing two mCherry expression plasmids bearing complementary 3’ UTRs to produce defined dsRNA species in uninfected cells. The dsRNAs are unstable, but are markedly stabilized by co-expressing the HSV dsRNA-binding protein US11. Strikingly, vhs delivered by super-infecting HSV virions accelerates the decay of these pre-formed dsRNAs in both the presence and absence of US11, a novel and unanticipated activity of vhs. Vhs binds the host RNA helicase eIF4A, and we find that vhs-induced dsRNA decay is attenuated by the eIF4A inhibitor hippuristanol, providing evidence that eIF4A participates in the process. Our results show that a herpesvirus host shutoff RNase destabilizes dsRNA in addition to targeting partially complementary viral mRNAs, raising the possibility that the mRNA destabilizing proteins of other viral pathogens dampen the host response to dsRNA through similar mechanisms.

Essentially all viruses produce double-stranded RNA (dsRNA) during infection. Host organisms therefore deploy a variety of dsRNA receptors to trigger innate antiviral defenses. Not surprisingly, viruses in turn produce an array of antagonists to block this host response. The best characterized of the viral antagonists function by binding to and masking dsRNA and/or blocking downstream signaling events. Other less studied viral antagonists appear to function by reducing the levels of dsRNA in infected cells, but exactly how they do so remains unknown. Here we show that one such viral antagonist, the herpes simplex virus vhs ribonuclease, reduces dsRNA levels in two distinct ways. First, as previously suggested, it dampens the accumulation of partially complementary viral mRNAs, reducing the potential for generating dsRNA. Second, it helps remove dsRNA after its formation, a novel and surprising activity of a protein best known for its activity on single-stranded mRNA. Many other viral pathogens produce proteins that target mRNAs for rapid destruction, and it will be important to determine if these also limit host dsRNA responses in similar ways.

Herpes simplex virus Membrane Fusion

Herpes simplex virus mediates multiple distinct fusion events during infection. HSV entry is initiated by fusion of the viral envelope with either the limiting membrane of a host cell endocytic compartment or the plasma membrane. In the infected cell during viral assembly, immature, enveloped HSV particles in the perinuclear space fuse with the outer nuclear membrane in a process termed de-envelopment. A cell infected with some strains of HSV with defined mutations spread to neighboring cells by a fusion event called syncytium formation. Two experimental methods, the transient cell-cell fusion approach and fusion from without, are useful surrogate assays of HSV fusion. These five fusion processes are considered in terms of their requirements, mechanism, and regulation. The execution and modulation of these events require distinct yet often overlapping sets of viral proteins and host cell factors. The core machinery of HSV gB, gD, and the heterodimer gH/gL is required for most if not all of the HSV fusion mechanisms.

Characterization of Vesicular Stomatitis Virus Pseudotypes Bearing Essential Entry Glycoproteins gB, gD, gH, and gL of Herpes Simplex Virus 1

Herpes simplex viruses (HSVs) are unusual in that unlike most enveloped viruses, they require at least four entry glycoproteins, gB, gD, gH, and gL, for entry into target cells in addition to a cellular receptor for gD. The dissection of the herpes simplex virus 1 (HSV-1) entry mechanism is complicated by the presence of more than a dozen proteins on the viral envelope. To investigate HSV-1 entry requirements in a simplified system, we generated vesicular stomatitis virus (VSV) virions pseudotyped with HSV-1 essential entry glycoproteins gB, gD, gH, and gL but lacking the native VSV fusogen G. These virions, referred to here as VSVΔG-BHLD virions, infected a cell line expressing a gD receptor, demonstrating for the first time that the four essential entry glycoproteins of HSV-1 are not only required but also sufficient for cell entry. To our knowledge, this is the first time the VSV pseudotyping system has been successfully extended beyond two proteins. Entry of pseudotyped virions required a gD receptor and was inhibited by HSV-1 specific anti-gB or anti-gH/gL neutralizing antibodies, which suggests that membrane fusion during the entry of the pseudotyped virions shares common requirements with the membrane fusion involved in HSV-1 entry and HSV-1-mediated syncytium formation. The HSV pseudotyping system established in this study presents a novel tool for systematic exploration of the HSV entry and membrane fusion mechanisms.

IMPORTANCE

Herpes simplex viruses (HSVs) are human pathogens that can cause cold sores, genital herpes, and blindness. No vaccines or preventatives are available. HSV entry into cells-a prerequisite for a successful infection-is a complex process that involves multiple viral and host proteins and occurs by different routes. Detailed mechanistic knowledge of the HSV entry is important for understanding its pathogenesis and would benefit antiviral and vaccine development, yet the presence of more than a dozen proteins on the viral envelope complicates the dissection of the HSV entry mechanisms. In this study, we generated heterologous virions displaying the four essential entry proteins of HSV-1 and showed that they are capable of cell entry and, like HSV-1, require all four entry glycoproteins along with a gD receptor. This HSV pseudotyping system pioneered in this work opens doors for future systematic exploration of the herpesvirus entry mechanisms.

Dissociation of HSV gL from gH by αvβ6- or αvβ8-integrin promotes gH activation and virus entry

Herpes simplex virus (HSV) is an important human pathogen. It enters cells through an orchestrated process that requires four essential glycoproteins, gD, gH/gL, and gB, activated in cascade fashion by receptor-binding and signaling. gH/gL heterodimer is conserved across the Herpesviridae family. HSV entry is enabled by gH/gL interaction with αvβ6- or αvβ8-integrin receptors. We report that the interaction of virion gH/gL with integrins resulted in gL dissociation and its release in the medium. gL dissociation occurred if all components of the entry apparatus-receptor-bound gD and gB-were present and was prevented if entry was blocked by a neutralizing monoclonal antibody to gH or by a mutation in gH. We propose that (i) gL dissociation from gH/gL is part of the activation of HSV glycoproteins, critical for HSV entry; and (ii) gL is a functional inhibitor of gH and maintains gH in an inhibited form until receptor-bound gD and integrins signal to gH/gL.

Dissection of the antibody response against herpes simplex virus glycoproteins in naturally infected humans

Relatively little is known about the extent of the polyclonal antibody (PAb) repertoire elicited by herpes simplex virus (HSV) glycoproteins during natural infection and how these antibodies affect virus neutralization. Here, we examined IgGs from 10 HSV-seropositive individuals originally classified as high or low virus shedders. All PAbs neutralized virus to various extents. We determined which HSV entry glycoproteins these PAbs were directed against: glycoproteins gB, gD, and gC were recognized by all sera, but fewer sera reacted against gH/gL. We previously characterized multiple mouse monoclonal antibodies (MAbs) and mapped those with high neutralizing activity to the crystal structures of gD, gB, and gH/gL. We used a biosensor competition assay to determine whether there were corresponding human antibodies to those epitopes. All 10 samples had neutralizing IgGs to gD epitopes, but there were variations in which epitopes were seen in individual samples. Surprisingly, only three samples contained neutralizing IgGs to gB epitopes. To further dissect the nature of these IgGs, we developed a method to select out gD- and gB-specific IgGs from four representative sera via affinity chromatography, allowing us to determine the contribution of antibodies against each glycoprotein to the overall neutralization capacity of the serum. In two cases, gD and gB accounted for all of the neutralizing activity against HSV-2, with a modest amount of HSV-1 neutralization directed against gC. In the other two samples, the dominant response was to gD.

IMPORTANCE

Antibodies targeting functional epitopes on HSV entry glycoproteins mediate HSV neutralization. Virus-neutralizing epitopes have been defined and characterized using murine monoclonal antibodies. However, it is largely unknown whether these same epitopes are targeted by the humoral response to HSV infection in humans. We have shown that during natural infection, virus-neutralizing antibodies are principally directed against gD, gB, and, to a lesser extent, gC. While several key HSV-neutralizing epitopes within gD and gB are commonly targeted by human serum IgG, others fail to induce consistent responses. These data are particularly relevant to the design of future HSV vaccines.

Mutations in the amino terminus of herpes simplex virus type 1 gL can reduce cell-cell fusion without affecting gH/gL trafficking

The gH/gL heterodimer represents two of the four herpes simplex virus glycoproteins necessary and sufficient for membrane fusion. We generated deletions and point mutations covering gL residues 24 to 43 to investigate that region's role in gH/gL intracellular trafficking and in membrane fusion. Multiple mutants displayed a 40 to 60% reduction in cell fusion with no effect on gH/gL trafficking. The amino terminus of gL plays an important role in the gH/gL contribution to membrane fusion.

The role of glycoprotein H of equine herpesviruses 1 and 4 (EHV-1 and EHV-4) in cellular host range and integrin binding

Equine herpesvirus type 1 and 4 (EHV-1 and EHV-4) glycoprotein H (gH) has been hypothesized to play a role in direct fusion of the virus envelope with cellular membranes. To investigate gH’s role in infection, an EHV-1 mutant lacking gH was created and the gH genes were exchanged between EHV-1 and EHV-4 to determine if gH affects cellular entry and/or host range. In addition, a serine-aspartic acid-isoleucine (SDI) integrin-binding motif present in EHV-1 gH was mutated as it was presumed important in cell entry mediated by binding to α4β1 or α4β7 integrins. We here document that gH is essential for EHV-1 replication, plays a role in cell-to-cell spread and significantly affects plaque size and growth kinetics. Moreover, we could show that α4β1 and α4β7 integrins are not essential for viral entry of EHV-1 and EHV-4, and that viral entry is not affected in equine cells when the integrins are inaccessible.

Herpes virus fusion and entry: a story with many characters

Herpesviridae comprise a large family of enveloped DNA viruses all of whom employ orthologs of the same three glycoproteins, gB, gH and gL. Additionally, herpesviruses often employ accessory proteins to bind receptors and/or bind the heterodimer gH/gL or even to determine cell tropism. Sorting out how these proteins function has been resolved to a large extent by structural biology coupled with supporting biochemical and biologic evidence. Together with the G protein of vesicular stomatitis virus, gB is a charter member of the Class III fusion proteins. Unlike VSV G, gB only functions when partnered with gH/gL. However, gH/gL does not resemble any known viral fusion protein and there is evidence that its function is to upregulate the fusogenic activity of gB. In the case of herpes simplex virus, gH/gL itself is upregulated into an active state by the conformational change that occurs when gD, the receptor binding protein, binds one of its receptors. In this review we focus primarily on prototypes of the three subfamilies of herpesviruses. We will present our model for how herpes simplex virus (HSV) regulates fusion in series of highly regulated steps. Our model highlights what is known and also provides a framework to address mechanistic questions about fusion by HSV and herpesviruses in general.

Crystal structure of the conserved herpesvirus fusion regulator complex gH-gL

Herpesviruses, which cause many incurable diseases, infect cells by fusing viral and cellular membranes. Whereas most other enveloped viruses use a single viral catalyst called a fusogen, herpesviruses, inexplicably, require two conserved fusion-machinery components, gB and the heterodimer gH-gL, plus other nonconserved components. gB is a class III viral fusogen, but unlike other members of its class, it does not function alone. We determined the crystal structure of the gH ectodomain bound to gL from herpes simplex virus 2. gH-gL is an unusually tight complex with a unique architecture that, unexpectedly, does not resemble any known viral fusogen. Instead, we propose that gH-gL activates gB for fusion, possibly through direct binding. Formation of a gB-gH-gL complex is critical for fusion and is inhibited by a neutralizing antibody, making the gB-gH-gL interface a promising antiviral target.

Mutagenic analysis of herpes simplex virus type 1 glycoprotein L reveals the importance of an arginine-rich region for function

Herpes simplex virus type 1 (HSV-1) glycoproteins H and L (gH and gL) are required for virus-induced membrane fusion. Expression of gH at the virion or infected cell surface is mediated by the chaperone-like activity of gL. We have previously shown that a region between amino acids 155 and 161 is critical for gL chaperone-like activity. Here, we conducted Ala substitution mutagenesis of residues in this region and found that substitution of Cys160, Arg156, Arg158, or Arg156/158/159 with Ala resulted in a gL mutant that bound gH but displayed a reduced ability in gH trafficking and membrane fusion. Substitution of Arg156 with another positively charged amino acid, Lys, restored function. Substitution of Arg158 with Lys restored function in gH trafficking and cell fusion but not virus entry. These results indicate that an arginine-rich region of gL is critical for function.

Understanding HSV-1 entry glycoproteins

Herpes Simplex Virus‐1 is a common infectious agent, but the precise detail of entry and infection of cells has only now begun to be clarified. Four viral surface glycoproteins (gB, gD, gH and gL) are required. This review summarises the known structure and function of each of these essential viral envelope glycoproteins, and explores what is known about their close cooperation with each other in mediating cellular membrane fusion. It is suggested that, following gD binding to one of its entry receptors, membrane fusion is mediated by gB and the heterodimer gH/gL. Significantly, these four entry glycoproteins also play a key role in the interaction between HSV and the host immune system. The glycoproteins serve an important role as targets of adaptive immunity. However, recent studies have demonstrated that the same proteins also play a key role in initiating the early innate immune response to HSV. Understanding the complex functions of these HSV proteins may be essential for successful development of vaccines for HSV. Copyright © 2007 John Wiley & Sons, Ltd.

Herpes simplex virus type 1 glycoprotein L mutants that fail to promote trafficking of glycoprotein H and fail to function in fusion can induce binding of glycoprotein L-dependent anti-glycoprotein H antibodies

The herpes simplex virus type 1 (HSV-1) glycoproteins H (gH) and L (gL) form a heterodimer and efficient expression of gH at the virion or cell surface is dependent upon gL. Five carboxy-terminal deletion mutants of gL were created and their ability to interact with and mediate cell-surface expression of gH, to promote binding of gL-dependent anti-gH antibodies and to contribute to cell fusion was analysed. All of the gL mutants bound gH, but only two mutants, containing the amino-terminal 161 or 168 aa of gL, mediated cell-surface expression of gH, and only gL161 and gL168 functioned in cell fusion. The binding of gL to gH, therefore, was not sufficient to ensure gH cell-surface expression and it was not possible to separate the gH-trafficking role of gL from gL function in fusion. Co-expression of gH with any gL mutant conferred binding of the anti-gH mAbs 53S and LP11. If the acquisition of 53S and LP11 binding to gH reflects a gL-induced conformational change, such a change is not sufficient to mediate trafficking of the gH-gL heterodimer.

Human herpesvirus 1 UL24 gene encodes a potential PD-(D/E)XK endonuclease

Using Meta-BASIC, a highly sensitive method for detection of distant similarity between proteins, we have identified another potential PD-(D/E)XK endonuclease in human herpesvirus 1 (HHV-1) encoded by the UL24 gene. The universal presence of UL24 in completed herpesviral genomes of three major subfamilies, Alphaherpesvirinae, Betaherpesvirinae, and Gammaherpesvirinae, suggests a fundamental role for this predicted PD-(D/E)XK endonuclease activity in the viral life cycle.

Heptad repeat 2 in herpes simplex virus 1 gH interacts with heptad repeat 1 and is critical for virus entry and fusion

Herpes simplex virus 1 (HSV-1) entry into cells and cell-cell fusion mediated by HSV-1 glycoproteins require four glycoproteins, gD, gB, gH, gL. Of these, gH is the only one that so far exhibits structural-functional features typical of viral fusion glycoproteins, i.e., a candidate fusion peptide and, downstream of it, a heptad repeat (HR) segment able to form a coiled coil, named HR-1. Here, we show that gH carries a functional HR-2 capable of physical interaction with HR-1. Specifically, mutational analysis of gH aimed at increasing or decreasing the ability of HR-2 to form a coiled coil resulted in an increase or decrease of fusion activity, respectively. HSV infection was modified accordingly. A mimetic peptide with the HR-2 sequence inhibited HSV-1 infection in a specific and dose-dependent manner. Circular dichroism spectroscopy showed that both HR-2 and HR-1 mimetic peptides adopt mainly random conformation in aqueous solution, while a decrease in peptide environmental polarity determines a conformational change, with a significant increase of the alpha-helical conformation content, in particular, for the HR-1 peptide. Furthermore, HR-1 and HR-2 mimetic peptides formed a stable complex, as revealed in nondenaturing electrophoresis and by circular dichroism. The mixture of HR-1 and HR-2 peptides reversed the inhibition of HSV infection exerted by the single peptides. Complex formation between HR-1 and HR-2 was independent of the presence of adjacent gH sequences and of additional glycoproteins involved in entry and fusion. Altogether, HR-2 adds to the features typical of class 1 fusion glycoproteins exhibited by HSV-1 gH.

A randomized controlled trial of a replication defective (gH deletion) herpes simplex virus vaccine for the treatment of recurrent genital herpes among immunocompetent subjects

BACKGROUND

A replication incompetent herpes virus lacking the glycoprotein H gene has been developed as a potential therapeutic vaccine for genital herpes.

GOAL

To determine vaccine efficacy on reducing HSV reactivation and clinical disease among immunocompetent persons with recurrent genital HSV-2 infection.

STUDY DESIGN

Randomized multicenter placebo-controlled trial. Healthy volunteers who had six or more recurrences of genital herpes per year were randomized to receive injections of vaccine at 0 and 8 or 0, 4, and 8 or 0, 2, 4, and 8 weeks or placebo and were followed for subsequent recurrences for 1 year.

RESULTS

The median times to first recurrence of genital herpes (40 days versus 30 days versus 37 days versus 42 days, respectively), mean number of recurrences (3 versus 3 versus 2.4 versus 1.9, respectively), and time to lesion healing of the first recurrence (8 days versus 7.8 days versus 7.4 days versus 7.5 days, respectively), were similar for all treatment groups. Asymptomatic viral shedding was detected by PCR in 61/74 (82%) persons performing daily sample collection following completion of the vaccination series. No differences were noted in the proportion of days with shedding between treatment groups (11.9% versus 17.2% versus 13.1% versus 16.4%, respectively).

CONCLUSION

This replication incompetent HSV-2 vaccine lacking the glycoprotein H gene was safe but had no clinical or virologic benefit in the amelioration of genital HSV-2 disease among immunocompetent men and women.

The pro-fusion domain of herpes simplex virus glycoprotein D (gD) interacts with the gD N terminus and is displaced by soluble forms of viral receptors

Entry of herpes simplex virus into the cell requires the interaction of gD with one of its receptors, herpesvirus entry mediator or nectin 1, and the intervention of gB, gH, or gL, required to execute fusion of the virion envelope with cell membranes. The gD ectodomain is organized in two structurally and functionally differentiated regions. The N terminus (residues 1-260) carries the receptor binding sites, and the C terminus (residues 260-310) functions as the pro-fusion domain (PFD), which is required for viral infectivity and fusion but not for receptor binding. The objective of our studies is to elucidate how gD links receptor recognition to the triggering of fusion. Here, we show that PFD is made of subdomains 1 and 2 (amino acids 260-285 and 285-310). Each one partially contributed to herpes simplex virus infectivity. By means of glutathione S-transferase (GST) fusion proteins, we show that PFD bound soluble forms of gD, truncated at residue 260 (gD260t) or downstream. Both PFD subdomains bound gD260t, highlighting multiple contact sites between the N and C termini of gD. When gD260t was in complex with either receptor, it failed to bind GST-PFD. In turn, the receptors did not bind GST-PFD, irrespective of whether they were in complex with gD. Thus, gD260t interacted with the C terminus only if unbound to the receptor. We propose that (i) before receptor binding, gD adopts a "closed" conformation in which the N and C termini interact; and (ii) on encounter with a receptor, gD modifies its conformation and the N and C termini are released from reciprocal interactions ("opened" conformation) and enabled to trigger fusion.

A heptad repeat in herpes simplex virus 1 gH, located downstream of the alpha-helix with attributes of a fusion peptide, is critical for virus entry and fusion

Entry of herpes simplex virus 1 (HSV-1) into cells occurs by fusion with cell membranes; it requires gD as the receptor binding glycoprotein and the trigger of fusion, and the trio of the conserved glycoproteins gB, gH, and gL to execute fusion. Recently, we reported that the ectodomain of HSV-1 gH carries a hydrophobic alpha-helix (residues 377 to 397) with attributes of an internal fusion peptide (T. Gianni, P. L. Martelli, R. Casadio, and G. Campadelli-Fiume, J. Virol. 79:2931-2940, 2005). Downstream of this alpha-helix, a heptad repeat (HR) with a high propensity to form a coiled coil was predicted between residues 443 and 471 and was designated HR-1. The simultaneous substitution of two amino acids in HR-1 (E450G and L453A), predicted to abolish the coiled coil, abolished the ability of gH to complement the infectivity of a gH-null HSV mutant. When coexpressed with gB, gD, and gL, the mutant gH was unable to promote cell-cell fusion. These defects were not attributed to a defect in heterodimer formation with gL, the gH chaperone, or in trafficking to the plasma membrane. A 25-amino-acid synthetic peptide with the sequence of HR-1 (pep-gH(wt25)) inhibited HSV replication if present at the time of virus entry into the cell. A scrambled peptide had no effect. The effect was specific, as pep-gH(wt25) did not reduce HSV-2 and pseudorabies virus infection. The presence of a functional HR in the HSV-1 gH ectodomain strengthens the view that gH has attributes typical of a viral fusion glycoprotein.

The ectodomain of herpes simplex virus glycoprotein H contains a membrane alpha-helix with attributes of an internal fusion peptide, positionally conserved in the herpesviridae family

Human herpesviruses enter cells by fusion with target membranes, a process that requires three conserved glycoproteins: gB, gH, and gL. How these glycoproteins execute fusion is unknown. Neural network bioinformatics predicted a membrane alpha-helix contained within the ectodomain of herpes simplex virus (HSV) gH, positionally conserved in the gH of all examined herpesviruses. Evidence that it has attributes of an internal fusion peptide rests on the following lines of evidence. (i) The predicted membrane alpha-helix has the attribute of a membrane segment, since it transformed a soluble form of gD into a membrane-bound gD. (ii) It represents a critical domain of gH. Its partial or entire deletion, or substitution of critical residues inhibited HSV infectivity and fusion in the cell-cell fusion assay. (iii) Its replacement with the fusion peptide from human immunodeficiency virus gp41 or from vesicular stomatitis virus G partially rescued HSV infectivity and cell-cell fusion. The corresponding antisense sequences did not. (iv) The predicted alpha-helix located in the varicella-zoster virus gH ectodomain can functionally substitute the native HSV gH membrane alpha-helix, suggesting a conserved function in the human herpesviruses. We conclude that HSV gH exhibits features typical of viral fusion glycoproteins and that this property is likely conserved in the Herpesviridae family.

Intracellular processing of human herpesvirus 6 glycoproteins Q1 and Q2 into tetrameric complexes expressed on the viral envelope

Human herpesvirus 6 (HHV-6) glycoproteins H and L (gH and gL, respectively) and the 80-kDa form of glycoprotein Q (gQ-80K) form a heterotrimeric complex that is found on the viral envelope and that is a viral ligand for human CD46. Besides gQ-80K, the gQ gene encodes an additional product whose mature molecular mass is 37 kDa (gQ-37K) and which is derived from a different transcript. Therefore, we designated gQ-80K as gQ1 and gQ-37K as gQ2. We show here that gQ2 also interacts with the gH-gL-gQ1 complex in HHV-6-infected cells and in virions. To examine how these components interact in HHV-6-infected cells, we performed pulse-chase studies. The results demonstrated that gQ2-34K, which is endo-beta-N-acetylglucosaminidase H sensitive and which is the precursor form of gQ2-37K, associates with gQ1-74K, which is the precursor form of gQ1-80K, within 30 min of the pulse period. After a 1-h chase, these precursor forms had associated with the gH-gL dimer. Interestingly, an anti-gH monoclonal antibody coimmunoprecipitated mainly gQ1-80K and gQ2-37K, with little gQ1-74K or gQ2-34K. These results indicate that although gQ2-34K and gQ1-74K interact in the endoplasmic reticulum, the gH-gL-gQ1-80K-gQ2-37K heterotetrameric complex arises in the post-endoplasmic reticulum compartment. The mature complex is subsequently incorporated into viral particles.

Cellular and viral requirements for rapid endocytic entry of herpes simplex virus

It was recently demonstrated that herpes simplex virus (HSV) successfully infects Chinese hamster ovary (CHO) cells expressing glycoprotein D (gD) receptors and HeLa cells by an endocytic mechanism (A. V. Nicola, A. M. McEvoy, and S. E. Straus, J. Virol. 77:5324-5332, 2003). Here we define cellular and viral requirements of this pathway. Uptake of intact, enveloped HSV from the cell surface into endocytic vesicles was rapid (t(1/2) of 8 to 9 min) and independent of the known cell surface gD receptors. Following uptake from the surface, recovery of intracellular, infectious virions increased steadily up to 20 min postinfection (p.i.), which corresponds to accumulation of enveloped virus in intracellular compartments. There was a sharp decline in recovery by 30 min p.i., suggesting loss of the virus envelope as a result of capsid penetration from endocytic organelles into the cytosol. In the absence of gD receptors, endocytosed virions did not successfully penetrate into the cytosol but were instead transported to lysosomes for degradation. Inhibitors of phosphatidylinositol (PI) 3-kinase, such as wortmannin, blocked transport of incoming HSV to the nuclear periphery and virus-induced gene expression but had no effect on virus binding or uptake. This suggests a role for PI 3-kinase activity in trafficking of HSV through the cytosol. Viruses that lack viral glycoproteins gB, gD, or gH-gL were defective in transport to the nucleus and had reduced infectivity. Thus, similar to entry via direct penetration at the cell surface, HSV entry into cells by wortmannin-sensitive endocytosis is efficient, involves rapid cellular uptake of viral particles, and requires gB, gD, and gH-gL.

The herpes simplex virus JMP mutant enters receptor-negative J cells through a novel pathway independent of the known receptors nectin1, HveA, and nectin2

The herpes simplex virus type 1(JMP) [HSV-1(JMP)] mutant was selected for its ability to grow and form plaques in receptor-negative J cells. It enters J cells through a novel gD-dependent pathway, independent of all known HSV receptors, nectin1, nectin2, and HveA. Evidence that the pathway is dependent on a nectin3 binding site on HSV-1(JMP) and requires three mutations in gD rests on the following. We derived monoclonal antibodies to nectin3 and show that J cells express nectin3. HSV-1(JMP) entry and cell-to-cell spread were inhibited by soluble nectin3-Fc, demonstrating that virions carry a binding site for nectin3. The site is either directly involved in HSV-1(JMP) entry, or nectin3 binding to its site affects the gD domains involved in entry (entry site). HSV-1(JMP) entry and cell-to-cell spread in J cells were also inhibited by soluble nectin1-Fc, showing that the nectin1 binding site on gD(JMP) overlaps with the entry site or that nectin1 binding to gD affects the entry site. gD(JMP) carries three mutations, S140N, R340H, and Q344R. The latter two lie in the C tail and are present in the parental HSV-1(MP). HSV-1 strain R5000 carrying the S140N substitution was not infectious in J cells, indicating that this substitution was not sufficient. We constructed two recombinants, one carrying the three substitutions and the other carrying the two C-tail substitutions. Only the first recombinant infected J cells with an efficiency similar to that of HSV-1(JMP), indicating that the three mutations are required for the novel entry pathway. The results highlight plasticity in gD which accounts for changes in receptor usage.

Structure-function analysis of herpes simplex virus type 1 gD and gH-gL: clues from gDgH chimeras

In alphaherpesviruses, glycoprotein B (gB), gD, gH, and gL are essential for virus entry. A replication-competent gL-null pseudorabies virus (PrV) (B. G. Klupp and T. C. Mettenleiter, J. Virol. 73:3014-3022, 1999) was shown to express a gDgH hybrid protein that could replace gD, gH, and gL in cell-cell fusion and null virus complementation assays. To study this phenomenon in herpes simplex virus type 1 (HSV-1), we constructed four gDgH chimeras, joining the first 308 gD amino acids to various gH N-terminal truncations. The chimeras were named for the first amino acid of gH at which each was truncated: 22, 259, 388, and 432. All chimeras were immunoprecipitated with both gD and gH antibodies to conformational epitopes. Normally, transport of gH to the cell surface requires gH-gL complex formation. Chimera 22 contains full-length gH fused to gD308. Unlike PrV gDgH, chimera 22 required gL for transport to the surface of transfected Vero cells. Interestingly, although chimera 259 failed to reach the cell surface, chimeras 388 and 432 exhibited gL-independent transport. To examine gD and gH domain function, each chimera was tested in cell-cell fusion and null virus complementation assays. Unlike PrV gDgH, none of the HSV-1 chimeras substituted for gL for fusion. Only chimera 22 was able to replace gH for fusion and could also replace either gH or gD in the complementation assay. Surprisingly, this chimera performed very poorly as a substitute for gD in the fusion assay despite its ability to complement gD-null virus and bind HSV entry receptors (HveA and nectin-1). Chimeras 388 and 432, which contain the same portion of gD as that in chimera 22, substituted for gD for fusion at 25 to 50% of wild-type levels. However, these chimeras functioned poorly in gD-null virus complementation assays. The results highlight the fact that these two functional assays are measuring two related but distinct processes.

Gene delivery using herpes simplex virus vectors

Herpes simplex virus (HSV) is a neurotropic DNA virus with many favorable properties as a gene delivery vector. HSV is highly infectious, so HSV vectors are efficient vehicles for the delivery of exogenous genetic material to cells. Viral replication is readily disrupted by null mutations in immediate early genes that in vitro can be complemented in trans, enabling straightforward production of high-titre pure preparations of non-pathogenic vector. The genome is large (152 Kb) and many of the viral genes are dispensable for replication in vitro, allowing their replacement with large or multiple transgenes. Latent infection with wild-type virus results in episomal viral persistence in sensory neuronal nuclei for the duration of the host lifetime. Transduction with replication-defective vectors causes a latent-like infection in both neural and non-neural tissue; the vectors are non-pathogenic, unable to reactivate and persist long-term. The latency active promoter complex can be exploited in vector design to achieve long-term stable transgene expression in the nervous system. HSV vectors transduce a broad range of tissues because of the wide expression pattern of the cellular receptors recognized by the virus. Increasing understanding of the processes involved in cellular entry has allowed preliminary steps to be taken towards targeting the tropism of HSV vectors. Using replication-defective HSV vectors, highly encouraging results have emerged from recent pre-clinical studies on models of neurological disease, including glioma, peripheral neuropathy, chronic pain and neurodegeneration. Consequently, HSV vectors encoding appropriate transgenes to tackle these pathogenic processes are poised to enter clinical trials.

A role for human cytomegalovirus glycoprotein O (gO) in cell fusion and a new hypervariable locus

A cell fusion assay using fusion-from-without (FFWO) recombinant adenoviruses (RAds) and specific antibody showed a role in fusion modulation for glycoprotein gO, the recently identified third component of the gH/gL gCIII complex of human cytomegalovirus (HCMV). As in HCMV, RAd gO expressed multiple glycosylated species with a mature product of 125 kDa. Coexpression with gH/gL RAds showed gCIII reconstitution in the absence of other HCMV products and stabilisation by intermolecular disulfide bonds. Properties of HCMV clinical isolate, Pt, also implicated gO in cell spread. Compared to laboratory strain AD169, Pt was resistant to gH antibody plaque inhibition, but mature gH was identical. However, the gO sequences were highly divergent (20%), with further variation in laboratory strain Towne gO (34%). Thus, gO forms gCIII with gH/gL, performs in cell fusion, and is a newly identified HCMV hypervariable locus which may influence gCIII's function in mediating infection.

Targeting gene expression using HSV vectors

Herpes simplex virus (HSV) is an encapsulated DNA virus, with many favourable properties for use as a gene transfer vector. For gene therapy applications, it may be desirable to restrict transgene expression to pre-defined subsets of cells. One potential method for achieving targeted transgene expression using the HSV vector system might involve dictating the cell types to which the vector will transfer the therapeutic transgene of interest. HSV delivers its genetic payload to cells directly through the plasmalemma; the mechanisms are complex and involve multiple viral and cell surface determinants. We have investigated several ways in which each component of the cell entry cascade may be manipulated in order to restrict viral DNA and transgene delivery to particular cellular populations. Our results indicate that targeted transduction may be a viable approach to achieving our goal of targeted HSV-mediated transgene expression.

Epstein-Barr virus that lacks glycoprotein gN is impaired in assembly and infection

The Epstein-Barr virus (EBV) glycoproteins N and M (gN and gM) are encoded by the BLRF1 and BBRF3 genes. To examine the function of the EBV gN-gM complex, recombinant virus was constructed in which the BLRF1 gene was interrupted with a neomycin resistance cassette. Recombinant virus lacked not only gN but also detectable gM. A significant proportion of the recombinant virus capsids remained associated with condensed chromatin in the nucleus of virus-producing cells, and cytoplasmic vesicles containing enveloped virus were scarce. Virus egress was impaired, and sedimentation analysis revealed that the majority of the virus that was released lacked a complete envelope. The small amount of virus that could bind to cells was also impaired in infectivity at a step following fusion. These data are consistent with the hypothesis that the predicted 78-amino-acid cytoplasmic tail of gM, which is highly charged and rich in prolines, interacts with the virion tegument. It is proposed that this interaction is important both for association of capsids with cell membrane to assemble and release enveloped particles and for dissociation of the capsid from the membrane of the newly infected cell on its way to the cell nucleus. The phenotype of EBV lacking the gN-gM complex is more striking than that of most alphaherpesviruses lacking the same complex but resembles in many respects the phenotype of pseudorabies virus lacking glycoproteins gM, gE, and gI. Since EBV does not encode homologs for gE and gI, this suggests that functions that may have some redundancy in alphaherpesviruses have been concentrated in fewer proteins in EBV.

The novel receptors that mediate the entry of herpes simplex viruses and animal alphaherpesviruses into cells

An extended array of cell surface molecules serve as receptors for HSV entry into cells. In addition to the heparan sulphate glycosaminoglycans, which mediate the attachment of virion to cells, HSV requires an entry receptor. The repertoire of entry receptors into human cells includes molecules from three structurally unrelated molecular families. They are (i) HveA (herpesvirus entry mediator A), (ii) members of the nectin family, (iii) 3-O-sulphated heparan sulphate. The molecules have different attributes and play potentially different roles in HSV infection and spread to human tissues. All the human entry receptors interact physically with the virion envelope glycoprotein D (gD). (i) HveA is a member of the TNF-receptor family. It mediates entry of a restricted range of HSV strains. Its expression is restricted to few lineages (e.g. T-lymphocytes). (ii) The human nectin1alpha (HIgR), nectin1delta (PRR1-HveC), and the nectin2alpha (PRR2alpha-HveB) and nectin2delta (PRR2delta) belong to the immunoglobulin superfamily. They are homologues of the poliovirus receptor (CD155), with which they share the overall structure of the ectodomain. The human nectin1alpha-delta are broadly expressed in cell lines of different lineages, are expressed in human tissue targets of HSV infection, serve as receptors for all HSV-1 and HSV-2 strains tested and mediate entry not only of free virions, but also cell-to-cell spread of virus. (iii) The 3-O-sulphated heparan sulphate is expressed in some selected human cell lines (e.g. endothelial and mast cells) and human tissues, and mediates entry of HSV-1, but not HSV-2. The human nectin2alpha and nectin2delta serve as receptors for a narrow range of viruses. A characteristic of the human nectin1alpha-delta is the promiscuous species non-specific receptor activity towards the animal alphaherpesviruses, pseudorabies virus (PrV) and bovine herpesvirus 1 (BHV-1). By contrast with the human nectin1delta, its murine homologue (mNectin1delta) does not bind gD at detectable level, yet it mediates entry of HSV, as well as of PrV and BHV-1. This provides the first example of a mediator of HSV entry independent of a detectable interaction with gD.

Cell-to-cell spread of wild-type herpes simplex virus type 1, but not of syncytial strains, is mediated by the immunoglobulin-like receptors that mediate virion entry, nectin1 (PRR1/HveC/HIgR) and nectin2 (PRR2/HveB)

The immunoglobulin-like receptors that mediate entry of herpes simplex virus type 1 (HSV-1) into human cells were found to mediate the direct cell-to-cell spread of wild-type virus. The receptors here designated Nectin1alpha and -delta and Nectin2alpha were originally designated HIgR, PRR1/HveC, and PRR2alpha/HveB, respectively. We report the following. (i) Wild-type HSV-1 spreads from cell to cell in J cells expressing nectin1alpha or nectin1delta but not in parental J cells that are devoid of entry receptors. A monoclonal antibody to nectin1, which blocks entry, also blocked cell-to-cell spread in nectin1-expressing J cells. Moreover, wild-type virus did not spread from a receptor-positive to a receptor-negative cell. (ii) The antibody to nectin1 blocked transmission of wild-type virus in a number of human cell lines, with varying efficiencies, suggesting that nectin1 is the principal mediator of wild-type virus spread in a variety of human cell lines. (iii) Nectin1 did not mediate cell fusion induced by the syncytial strains HSV-1(MP) and HFEM-syn. (iv) Nectin2alpha could serve as a receptor for spread of a mutant virus carrying the L25P substitution in glycoprotein D, but not of wild-type virus, in agreement with its ability to mediate entry of the mutant but not of wild-type virus.

Trafficking to the plasma membrane of the seven-transmembrane protein encoded by human herpesvirus 6 U51 gene involves a cell-specific function present in T lymphocytes

The sequence of human herpesvirus 6 (HHV-6) U51 open reading frame predicts a protein of 301 amino acid residues with seven transmembrane domains. To identify and characterize U51, we derived antipeptide polyclonal antibodies and developed a transient expression assay. We ascertained that U51 was synthesized in cord blood mononuclear cells infected with either variant A- or variant B-HHV-6 and was transported to the surface of productively infected cells. When synthesized in transient expression systems, U51 intracellular trafficking was regulated in a cell-type-dependent fashion. In human monolayer HEK-293 and 143tk- cells, U51 accumulated predominantly in the endoplasmic reticulum and failed to be transported to the cell surface. In contrast, in T-lymphocytic cell lines J-Jhan, Molt-3, and Jurkat, U51 was successfully transported to the plasma membrane. We infer that transport of U51 to the cell surface requires a cell-specific function present in activated T lymphocytes and T-cell lines.

Structural and antigenic analysis of a truncated form of the herpes simplex virus glycoprotein gH-gL complex

The herpes simplex virus (HSV) gH-gL complex is essential for virus infectivity and is a major antigen for the host immune system. The association of gH with gL is required for correct folding, cell surface trafficking, and membrane presentation of the complex. Previously, a mammalian cell line was constructed which produces a secreted form of gHt-gL complex lacking the transmembrane and cytoplasmic tail regions of gH. gHt-gL retains a conformation similar to that of its full-length counterpart in HSV-infected cells. Here, we examined the structural and antigenic properties of gHt-gL. We first determined its stoichiometry and carbohydrate composition. We found that the complex consists of one molecule each of gH and gL. The N-linked carbohydrate (N-CHO) site on gL and most of the N-CHO sites on gH are utilized, and both proteins also contain O-linked carbohydrate and sialic acid. These results suggest that the complex is processed to the mature form via the Golgi network prior to secretion. To determine the antigenically active sites of gH and gL, we mapped the epitopes of a panel of gH and gL monoclonal antibodies (MAbs), using a series of gH and gL C-terminal truncation variant proteins produced in transiently transfected mammalian cells. Sixteen gH MAbs (including H6 and 37S) reacted with the N-terminal portion of gH between amino acids 19 and 276. One of the gH MAbs, H12, reacted with the middle portion of gH (residues 476 to 678). Nine gL MAbs (including 8H4 and VIII 62) reacted with continuous epitopes within the C-terminal portion of gL, and this region was further mapped within amino acids 168 to 178 with overlapping synthetic peptides. Finally, plasmids expressing the gH and gL truncations were employed in cotransfection assays to define the minimal regions of both gH and gL required for complex formation and secretion. The first 323 amino acids of gH and the first 161 amino acids of gL can form a stable secreted hetero-oligomer with gL and gH792, respectively, while gH323-gL168 is the smallest secreted hetero-oligomer. The first 648 amino acids of gH are required for reactivity with MAbs LP11 and 53S, indicating that a complex of gH648-gL oligomerizes into the correct conformation. The data suggest that both antigenic activity and oligomeric structure require the amino-terminal portions of gH and gL.

Epstein-Barr virus uses different complexes of glycoproteins gH and gL to infect B lymphocytes and epithelial cells

The Epstein-Barr virus (EBV) gH-gL complex includes a third glycoprotein, gp42. gp42 binds to HLA class II on the surfaces of B lymphocytes, and this interaction is essential for infection of the B cell. We report here that, in contrast, gp42 is dispensable for infection of epithelial cell line SVKCR2. A soluble form of gp42, gp42.Fc, can, however, inhibit infection of both cell types. Soluble gp42 can interact with EBV gH and gL and can rescue the ability of virus lacking gp42 to transform B cells, suggesting that a gH-gL-gp42.Fc complex can be formed by extrinsic addition of the soluble protein. Truncated forms of gp42.Fc that retain the ability to bind HLA class II but that cannot interact with gH and gL still inhibit B-cell infection by wild-type virus but cannot inhibit infection of SVKCR2 cells or rescue the ability of recombinant gp42-negative virus to transform B cells. An analysis of wild-type virions indicates the presence of more gH and gL than gp42. To explain these results, we describe a model in which wild-type EBV virions are proposed to contain two types of gH-gL complexes, one that includes gp42 and one that does not. We further propose that these two forms of the complex have mutually exclusive abilities to mediate the infection of B cells and epithelial cells. Conversion of one to the other concurrently alters the ability of virus to infect each cell type. The model also suggests that epithelial cells may express a molecule that serves the same cofactor function for this cell type as HLA class II does for B cells and that the gH-gL complex interacts directly with this putative epithelial cofactor.

HveA (herpesvirus entry mediator A), a coreceptor for herpes simplex virus entry, also participates in virus-induced cell fusion

The purpose of this study was to determine whether a cell surface protein that can serve as coreceptor for herpes simplex virus type 1 (HSV-1) entry, herpesvirus entry mediator (previously designated HVEM but renamed HveA), also mediates HSV-1-induced cell-cell fusion. We found that transfection of DNA from KOS-804, a previously described HSV-1 syncytial (Syn) strain whose Syn mutation was mapped to an amino acid substitution in gK, induced numerous large syncytia on HveA-expressing Chinese hamster ovary cells (CHO-HVEM12) but not on control cells (CHO-C8). Antibodies specific for gD as well as for HveA were effective inhibitors of KOS-804-induced fusion, consistent with previously described direct interactions between gD and HveA. Since mutations in gD determine the ability of HSV-1 to utilize HveA for entry, we examined whether the form of virally expressed gD also influenced the ability of HveA to mediate fusion. We produced a recombinant virus carrying the KOS-804 Syn mutation and the KOS-Rid1 gD mutation, which significantly reduces viral entry via HveA, and designated it KOS-SR1. KOS-SR1 DNA had a markedly reduced ability to induce syncytia on CHO-HVEM12 cells and a somewhat enhanced ability to induce syncytia on CHO-C8 cells. These results support previous findings concerning the relative abilities of KOS and KOS-Rid1 to infect CHO-HVEM12 and CHO-C8 cells. Thus, HveA mediates cell-cell fusion as well as viral entry and both activities of HveA are contingent upon the form of gD expressed by the virus.

Accumulation of viral transcripts and DNA during establishment of latency by herpes simplex virus

Latent infection of mice with wild-type herpes simplex virus is established during an acute phase of ganglionic infection in which there is abundant viral replication and productive-cycle gene expression. Thymidine kinase-negative mutants establish latent infections but are severely impaired for acute ganglionic replication and productive-cycle gene expression. Indeed, by in situ hybridization assays, acute infection by these mutants resembles latency. To assess events during establishment of latency by wild-type and thymidine kinase-negative viruses, we quantified specific viral nucleic acid sequences in mouse trigeminal ganglia during acute ganglionic infection by using sensitive PCR-based assays. Through 32 h postinfection, viral DNA and transcripts representative of the three kinetic classes of productive-cycle genes accumulated to comparable levels in wild-type- and mutant-infected ganglia. At 48 and 72 h, although latency-associated transcripts accumulated to comparable levels in ganglia infected with wild-type or mutant virus, levels of DNA accumulating in wild-type-infected ganglia exceeded those in mutant-infected ganglia by 2 to 3 orders of magnitude. Coincident with this increase in DNA, wild-type-infected ganglia exhibited abundant expression of productive-cycle genes and high titers of infectious progeny. Nevertheless, the levels of productive-cycle RNAs expressed by mutant virus during acute infection greatly exceeded those expressed by wild-type virus during latency. The results thus distinguish acute infection of ganglia by a replication-compromised mutant from latent infection and may have implications for mechanisms of latency.

Epstein-Barr virus lacking glycoprotein gp42 can bind to B cells but is not able to infect

The Epstein-Barr virus gH-gL complex includes a third glycoprotein, gp42, which is the product of the BZLF2 open reading frame (ORF). gp42 has been implicated as critical to infection of the B lymphocyte by virtue of its interaction with HLA class II on the B-cell surface. A neutralizing antibody that reacts with gp42 inhibits virus-cell fusion and blocks binding of gp42 to HLA class II; antibody to HLA class II can inhibit infection, and B cells that lack HLA class II can only be infected if HLA class II expression is restored. To confirm whether gp42 is an essential component of the virion, we derived a recombinant virus with a selectable marker inserted into the BZLF2 ORF to interrupt expression of the protein. A complex of gH and gL was expressed by the recombinant virus in the absence of gp42. Recombinant virus egressed from the cell normally and could bind to receptor-positive cells. It had, however, lost the ability to infect or transform B lymphocytes. Treatment with polyethylene glycol restored the infectivity of recombinant virus, confirming that gp42 is essential for penetration of the B-cell membrane.

Identification and DNA sequence analysis of the Marek's disease virus serotype 2 gene homologous to the herpes simplex virus type 1 glycoprotein H

Marek's disease virus (MDV) serotype 2 (MDV2) gene homologous to the glycoprotein H (gH) gene of herpes simplex virus type 1 was identified and sequenced. The predicted region encoding for the MDV2 gH gene was 2436 nucleotide and the primary translation product was 812 amino acids with a molecular weight of 89.4 kDa. The protein encoded by MDV2 gH gene has a number of features characteristic of a membrane-associated glycoprotein. First, there are 9 potential N-linked glycosylation sites and 11 cysteine residues, and 6 of the sites and 8 of the residues were conserved among all of the three MDV serotypes. Second, this protein had N-terminal and C-terminal hydrophobic regions, which were a signal sequence and a transmembrane-anchor domain, respectively. From the northern blot analysis, it was suggested that a transcript encoding MDV2 gH and a poly-cistronic transcript encoding MDV2 thymidine kinase, gH, and possibly other genes of downstream on this strand existed. Alignment of the amino acid sequences of the gH homologues among the three MDV serotypes showed 57.5% (MDV1 and MDV2), 56.2% (MDV1 and HVT), and 50.1% (MDV2 and HVT) identities.

Epstein-Barr virus uses HLA class II as a cofactor for infection of B lymphocytes

Infection of B lymphocytes by Epstein-Barr virus (EBV) requires attachment of virus via binding of viral glycoprotein gp350 to CD21 on the cell surface. Penetration of the cell membrane additionally involves a complex of three glycoproteins, gH, gL, and gp42. Glycoprotein gp42 binds to HLA-DR. Interference with this interaction with a soluble form of gp42, with a monoclonal antibody (MAb) to gp42, or with a MAb to HLA-DR inhibited virus infection. It was not possible to superinfect cells that failed to express HLA-DR unless expression was restored by transfection or creation of hybrid cell lines with complementing deficiencies in expression of HLA class II. HLA class II molecules thus serve as cofactors for infection of human B cells.

Glycoprotein H of herpes simplex virus type 1 requires glycoprotein L for transport to the surfaces of insect cells

In mammalian cells, formation of heterooligomers consisting of the glycoproteins H and L (gH and gL) of herpes simplex virus type 1 is essential for the cell-to-cell spread of virions and for the penetration of virions into cells. We examined whether formation of gH1/gL1 heterooligomers and cell surface expression of the complex occurs in insect cells. Three recombinant baculoviruses, expressing gL1, gH1, and truncated gH1 (gH1t), which lacks the transmembrane region, were constructed. It was shown that recombinant gH1/gL1 and gH1t/gL1 heterooligomers were produced in insect cells. As in mammalian cells, gH1 and gH1t were not detected on the surfaces of insect cells in the absence of gL1. When coexpressed with gL1, recombinant gH1 was displayed on the surfaces of insect cells. Coexpression of gH1t and gL1 resulted in secretion of the gH1t/gL1 complex into the cell culture medium, indicating that gH1t is also transported to the surfaces of insect cells. Our results indicate that the process of folding and intracellular transport of gH1 and gL1 is comparable in insect cells and mammalian cells and that the baculovirus expression system can be used to examine the complex formation and the intracellular transport of gH1 and gL1. The availability of secreted gH1t/gL1 complex offers the opportunity to further investigate the immunological properties of this complex.

Site-directed and linker insertion mutagenesis of herpes simplex virus type 1 glycoprotein H

The gH-gL complex of herpes simplex virus type 1 (HSV-1) is essential for virion infectivity and virus-induced cell fusion, but functional domains of the gH molecule remain to be defined. We have addressed this question by mutagenesis. A set of linker insertion mutants in HSV-1 gH was generated and tested in transient assays for their ability to complement a gH-negative virus. Insertions at three sites in the C-terminal third of the external domain affected the ability of gH to function in cell-cell fusion and virus entry, while insertions at six sites in the N-terminal half of the external domain induced conformational changes in gH such that it was not recognized by monoclonal antibody LP11, although expression at the cell surface was unchanged. A recombinant virus in which a potential integrin-binding motif, RGD, in gH was changed to the triplet RGE entered cells as efficiently as the wild type, indicating that HSV-1 entry is not mediated by means of the gH-RGD motif binding to cell surface integrins. Furthermore, mutagenesis of the glycosylation site which is positionally conserved in all herpesvirus gH sequences in close proximity to the transmembrane domain generated a recombinant virus that grew in vitro with wild-type single-step kinetics.

Chaperone functions common to nonhomologous Epstein-Barr virus gL and Varicella-Zoster virus gL proteins

Herpesviruses encode the complex-forming, essential glycoproteins gH and gL. Maturation and transport of gH are dependent on coexpression of its chaperone, gL. The gL proteins of alpha herpesviruses and gamma herpesviruses do not have a significant percentage of amino acid sequence homology. Yet, as we report herein, the diverse gL glycoproteins of Epstein-Barr virus (EBV) and varicella-zoster virus (VZV) were functionally interchangeable, although membrane expression and maturation of gH were separate functions for these viruses. In VZV both functions were performed by a single protein. EBV required two separate glycoproteins, one of which can be replaced by its homologous protein from VZV, a distant relative of EBV. Collectively, these results suggested that VZV gL is a simpler form of the gL chaperone protein than EBV gL.

Intercellular trafficking and protein delivery by a herpesvirus structural protein

We show that the HSV-1 structural protein VP22 has the remarkable property of intercellular transport, which is so efficient that following expression in a subpopulation the protein spreads to every cell in a monolayer, where it concentrates in the nucleus and binds chromatin. VP22 movement was observed both after delivery of DNA by transfection or microinjection and during virus infection. Moreover, we demonstrate that VP22 trafficking occurs via a nonclassical Golgi-independent mechanism. Sensitivity to cytochalasin D treatment suggests that VP22 utilizes a novel trafficking pathway that involves the actin cytoskeleton. In addition, we demonstrate intercellular transport of a VP22 fusion protein after endogenous synthesis or exogenous application, indicating that VP22 may have potential in the field of protein delivery.

Evidence for involvement of equine herpesvirus 1 glycoprotein B in cell-cell fusion

Monoclonal antibodies specific for equine herpesvirus 1 (EHV-1) glycoproteins (gB, gD, gp2 and a cleaved translation product of gene 71) were tested for ability to inhibit cell-cell fusion as measured by syncytium formation in EHV-1 infected cell cultures. Syncytium formation was inhibited by a complement-dependent neutralising antibody (7B10) which recognised the large subunit of EHV-1 gB. This indicated that EHV-1 gB, in common with gB homologues of herpes simplex virus and other herpesviruses, plays a role in the cell-cell fusion process.

Directed selection of recombinant human monoclonal antibodies to herpes simplex virus glycoproteins from phage display libraries

Human monoclonal antibodies have considerable potential in the prophylaxis and treatment of viral disease. However, only a few such antibodies suitable for clinical use have been produced to date. We have previously shown that large panels of human recombinant monoclonal antibodies against a plethora of infectious agents, including herpes simplex virus types 1 and 2, can be established from phage display libraries. Here we demonstrate that facile cloning of recombinant Fab fragments against specific viral proteins in their native conformation can be accomplished by panning phage display libraries against viral glycoproteins "captured" from infected cell extracts by specific monoclonal antibodies immobilized on ELISA plates. We have tested this strategy by isolating six neutralizing recombinant antibodies specific for herpes simplex glycoprotein gD or gB, some of which are against conformationally sensitive epitopes. By using defined monoclonal antibodies for the antigen-capture step, this method can be used for the isolation of antibodies to specific regions and epitopes within the target viral protein. For instance, monoclonal antibodies to a nonneutralizing epitope can be used in the capture step to clone antibodies to neutralizing epitopes, or antibodies to a neutralizing epitope can be used to clone antibodies to a different neutralizing epitope. Furthermore, by using capturing antibodies to more immunodominant epitopes, one can direct the cloning to less immunogenic ones. This method should be of value in generating antibodies to be used both in the prophylaxis and treatment of viral infections and in the characterization of the mechanisms of antibody protective actions at the molecular level.

Herpes simplex virus glycoprotein K is known to influence fusion of infected cells, yet is not on the cell surface

Syncytial mutants of herpes simplex virus (HSV) cause extensive fusion of cultured cells, whereas wild-type HSV primarily causes cell rounding and aggregation. A large fraction of syncytial viruses contain mutations in the UL53 gene, which encodes glycoprotein K (gK). Previously, we demonstrated that wild-type and syncytial forms of gK are expressed at similar levels and possess identical electrophoretic mobilities. Using immunofluorescence, we show that gK is not transported to the surfaces of cells infected with either wild-type or syncytial HSV. Instead, gK accumulates in the perinuclear and nuclear membranes of cells. This finding is in contrast to the behavior of all other HSV glycoproteins described to date, which reach the cell surface. When gK was expressed in the absence of other HSV proteins, using a recombinant adenovirus vector, a similar perinuclear and nuclear pattern was observed. In addition, gK remained sensitive to endoglycosidase H, consistent with the hypothesis that gK does not reach the Golgi apparatus and is retained in the endoplasmic reticulum and nuclear envelope. Therefore, although gK mutations promote fusion between the surface membranes of HSV-infected cells, the glycoprotein does not reach the plasma membrane and, thus, must influence fusion indirectly.