Glycoprotein D homologs in herpes simplex virus type 1, pseudorabies virus, and bovine herpes virus type 1 bind directly to human HveC(nectin-1) with different affinities

The biology and development of vaccines for bovine alphaherpesvirus 1

Bovine alphaherpesvirus type 1 (BoAHV-1) infections lead to compromised herd health and significantly reduced productivity of affected cattle. While BoAHV-1 may cause rhinotracheitis, conjunctivitis, genital infections, and abortions, respiratory tract infections constitute the predominant clinical disease. Immune suppression induced by BoAHV-1 may contribute to co-infections initiating the bovine respiratory disease complex. In this review, the emphasis is to recapitulate the biology and the vaccine technologies currently in use and in development for BoAHV-1, and to discuss the major limitations. Studies on the life cycle and host interactions of BoAHV-1 have resulted in the identification of virulence factors. While several vaccine types, such as vectored vaccines and subunit vaccines, are under investigation, modified live and inactivated BoAHV-1 vaccines are still most frequently used in most areas of the world, whereas attenuated and inactivated marker vaccines are in use in Europe. The knowledge gained from studies on the biology of BoAHV-1 can form a basis for the rational design of future vaccines.

Crystal structures of glycoprotein D of equine alphaherpesviruses reveal potential binding sites to the entry receptor MHC-I

Cell entry of most alphaherpesviruses is mediated by the binding of glycoprotein D (gD) to different cell surface receptors. Equine herpesvirus type 1 (EHV-1) and EHV-4 gDs interact with equine major histocompatibility complex I (MHC-I) to initiate entry into equine cells. We have characterized the gD-MHC-I interaction by solving the crystal structures of EHV-1 and EHV-4 gDs (gD1, gD4), performing protein-protein docking simulations, surface plasmon resonance (SPR) analysis, and biological assays. The structures of gD1 and gD4 revealed the existence of a common V-set immunoglobulin-like (IgV-like) core comparable to those of other gD homologs. Molecular modeling yielded plausible binding hypotheses and identified key residues (F213 and D261) that are important for virus binding. Altering the key residues resulted in impaired virus growth in cells, which highlights the important role of these residues in the gD-MHC-I interaction. Taken together, our results add to our understanding of the initial herpesvirus-cell interactions and will contribute to the targeted design of antiviral drugs and vaccine development.

Membrane fusion, potential threats, and natural antiviral drugs of pseudorabies virus

Pseudorabies virus (PrV) can infect several animals and causes severe economic losses in the swine industry. Recently, human encephalitis or endophthalmitis caused by PrV infection has been frequently reported in China. Thus, PrV can infect animals and is becoming a potential threat to human health. Although vaccines and drugs are the main strategies to prevent and treat PrV outbreaks, there is no specific drug, and the emergence of new PrV variants has reduced the effectiveness of classical vaccines. Therefore, it is challenging to eradicate PrV. In the present review, the membrane fusion process of PrV entering target cells, which is conducive to revealing new therapeutic and vaccine strategies for PrV, is presented and discussed. The current and potential PrV pathways of infection in humans are analyzed, and it is hypothesized that PrV may become a zoonotic agent. The efficacy of chemically synthesized drugs for treating PrV infections in animals and humans is unsatisfactory. In contrast, multiple extracts of traditional Chinese medicine (TCM) have shown anti-PRV activity, exerting its effects in different phases of the PrV life-cycle and suggesting that TCM compounds may have great potential against PrV. Overall, this review provides insights into developing effective anti-PrV drugs and emphasizes that human PrV infection should receive more attention.

Host Cell Neddylation Facilitates Alphaherpesvirus Entry in a Virus-Specific and Cell-Dependent Manner

Herpes simplex virus 1 (HSV-1) commandeers the host cell proteasome at several steps of its replication cycle, including entry. Here we demonstrate that HSV-2, pseudorabies virus (PRV), and bovine herpesvirus 1 (BoHV-1) entry are blocked by bortezomib, a proteasome inhibitor that is an FDA-approved cancer drug. Proteasome-dependent entry of HSV-1 is thought to be ubiquitin-independent. To interrogate further the proteasomal mechanism of entry, we determined the involvement of the ubiquitin-like molecule NEDD8 and the neddylation cascade in alphaherpesvirus entry and infection. MLN4924 is a small-molecule inhibitor of neddylation that binds directly to the NEDD8-activating enzyme. Cell treatment with MLN4924 inhibited plaque formation and infectivity by HSV-1, PRV, and BoHV-1 at noncytotoxic concentrations. Thus, the neddylation pathway is broadly important for alphaherpesvirus infection. However, the neddylation inhibitor had little effect on entry of the veterinary viruses but had a significant inhibitory effect on entry of HSV-1 and HSV-2 into seven different cell types. Washout experiments indicated that MLN4924's effect on viral entry was reversible. A time-of-addition assay suggested that the drug was acting on an early step in the entry process. Small interfering RNA (siRNA) knockdown of NEDD8 significantly inhibited HSV entry. In probing the neddylation-dependent step in entry, we found that MLN4924 dramatically blocked endocytic uptake of HSV from the plasma membrane by >90%. In contrast, the rate of HSV entry into cells that support direct fusion of HSV with the cell surface was unaffected by MLN4924. Interestingly, proteasome activity was less important for the endocytic internalization of HSV from the cell surface. The results suggest that the NEDD8 cascade is critical for the internalization step of HSV entry. IMPORTANCE Alphaherpesviruses are ubiquitous pathogens of humans and veterinary species that cause lifelong latent infections and significant morbidity and mortality. Host cell neddylation is important for cell homeostasis and for the infection of many viruses, including HSV-1, HSV-2, PRV, and BoHV-1. Inhibition of neddylation by a pharmacologic inhibitor or siRNA blocked HSV infection at the entry step. Specifically, the NEDD8 pathway was critically important for HSV-1 internalization from the cell surface by an endocytosis mechanism. The results expand our limited understanding of cellular processes that mediate HSV internalization. To our knowledge, this is the first demonstration of a function for the neddylation cascade in virus entry.

Construction and characterisation of glycoprotein E and glycoprotein I deficient mutants of Australian strains of infectious laryngotracheitis virus using traditional and CRISPR/Cas9-assisted homologous recombination techniques

In alphaherpesviruses, glycoproteins E and I (gE and gI, respectively) form a heterodimer that facilitates cell-to-cell spread of virus. Using traditional homologous recombination techniques, as well as CRISPR/Cas9-assisted homologous recombination, we separately deleted gE and gI coding sequences from an Australian field strain (CSW-1) and a vaccine strain (A20) of infectious laryngotracheitis virus (ILTV) and replaced each coding sequence with sequence encoding green fluorescent protein (GFP). Virus mutants in which gE and gI gene sequences had been replaced with GFP were identified by fluorescence microscopy but were unable to be propagated separately from the wildtype virus in either primary chicken cells or the LMH continuous chicken cell line. These findings build on findings from a previous study of CSW-1 ILTV in which a double deletion mutant of gE and gI could not be propagated separately from wildtype virus and produced an in vivo phenotype of single-infected cells with no cell-to-cell spread observed. Taken together these studies suggest that both the gE and gI genes have a significant role in cell-to-cell spread in both CSW-1 and A20 strains of ILTV. The CRISPR/Cas9-assisted deletion of genes from the ILTV genome described in this study adds this virus to a growing list of viruses to which this approach has been used to study viral gene function.

Varicella-zoster virus: molecular controls of cell fusion-dependent pathogenesis

Varicella-zoster virus (VZV) is the causative agent of chicken pox (varicella) and shingles (zoster). Although considered benign diseases, both varicella and zoster can cause complications. Zoster is painful and can lead to post herpetic neuralgia. VZV has also been linked to stroke, related to giant cell arteritis in some cases. Vaccines are available but the attenuated vaccine is not recommended in immunocompromised individuals and the efficacy of the glycoprotein E (gE) based subunit vaccine has not been evaluated for the prevention of varicella. A hallmark of VZV pathology is the formation of multinucleated cells termed polykaryocytes in skin lesions. This cell-cell fusion (abbreviated as cell fusion) is mediated by the VZV glycoproteins gB, gH and gL, which constitute the fusion complex of VZV, also needed for virion entry. Expression of gB, gH and gL during VZV infection and trafficking to the cell surface enables cell fusion. Recent evidence supports the concept that cellular processes are required for regulating cell fusion induced by gB/gH-gL. Mutations within the carboxyl domains of either gB or gH have profound effects on fusion regulation and dramatically restrict the ability of VZV to replicate in human skin. This loss of regulation modifies the transcriptome of VZV infected cells. Furthermore, cellular proteins have significant effects on the regulation of gB/gH-gL-mediated cell fusion and the replication of VZV, exemplified by the cellular phosphatase, calcineurin. This review provides the current state-of-the-art knowledge about the molecular controls of cell fusion-dependent pathogenesis caused by VZV.

Immunogenicity and protective efficacy induced by an mRNA vaccine encoding gD antigen against pseudorabies virus infection

Messenger RNA-based vaccines represent new tools with prophylactic and therapeutic potential characterized by high flexibility of application for infectious diseases. Pseudorabies virus (PRV) is one of the major viruses affecting the pig industry. PRV has serious effects in piglets, sows, and growing-fattening pigs and can lead to huge economic losses. In this study, an envelope glycoprotein D (gD) gene-based specific mRNA vaccine was generated, and a mouse model was used to investigate the protective efficacy of the vaccine. The gD mRNA vaccine and the recombinant plasmid pVAX-gD were transfected into BHK21 cells, and the antigenicity of the expressed proteins was detected by Western blot analysis. Groups of mice were vaccinated with the gD mRNA vaccine, pVAX-gD, and PBS. T cell immune responses were measured by flow cytometry or ELISA and serum neutralization tests every two weeks. The challenge with the PRV-XJ strain was performed eight weeks after the primary immunization, and the response was monitored for 15 days. The levels of specific and neutralizing antibodies in the gD mRNA vaccine group were significantly increased in 8 weeks compared to those in the control group, and cytokine levels, including that of IFN-γ/IL-2, were considerably higher than those in the control animal. Additionally, the proportion of CD4+/CD8+ cells in peripheral lymphocytes was remarkably increased. Our data demonstrate that mRNA is a promising and effective tool for the development of vaccines. The PRV-gD-based mRNA vaccine can elicit an efficient neutralizing antibody response and induce effective protection in mice in defense against PRV infection.

The structural basis of herpesvirus entry

Herpesviruses are ubiquitous, double-stranded DNA, enveloped viruses that establish lifelong infections and cause a range of diseases. Entry into host cells requires binding of the virus to specific receptors, followed by the coordinated action of multiple viral entry glycoproteins to trigger membrane fusion. Although the core fusion machinery is conserved for all herpesviruses, each species uses distinct receptors and receptor-binding glycoproteins. Structural studies of the prototypical herpesviruses herpes simplex virus 1 (HSV-1), HSV-2, human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV) entry glycoproteins have defined the interaction sites for glycoprotein complexes and receptors, and have revealed conformational changes that occur on receptor binding. Recent crystallography and electron microscopy studies have refined our model of herpesvirus entry into cells, clarifying both the conserved features and the unique features. In this Review, we discuss recent insights into herpesvirus entry by analysing the structures of entry glycoproteins, including the diverse receptor-binding glycoproteins (HSV-1 glycoprotein D (gD), EBV glycoprotein 42 (gp42) and HCMV gH-gL-gO trimer and gH-gL-UL128-UL130-UL131A pentamer), as well gH-gL and the fusion protein gB, which are conserved in all herpesviruses.

Localization of the Interaction Site of Herpes Simplex Virus Glycoprotein D (gD) on the Membrane Fusion Regulator, gH/gL

A cascade of protein-protein interactions between four herpes simplex virus (HSV) glycoproteins (gD, gH/gL, and gB) drive fusion between the HSV envelope and host membrane, thereby allowing for virus entry and infection. Specifically, binding of gD to one of its receptors induces a conformational change that allows gD to bind to the regulatory complex gH/gL, which then activates the fusogen gB, resulting in membrane fusion. Using surface plasmon resonance and a panel of anti-gD monoclonal antibodies (MAbs) that sterically blocked the interaction, we previously showed that gH/gL binds directly to gD at sites distinct from the gD receptor binding site. Here, using an analogous strategy, we first evaluated the ability of a panel of uncharacterized anti-gH/gL MAbs to block binding to gD and/or inhibit fusion. We found that the epitopes of four gD-gH/gL-blocking MAbs were located within flexible regions of the gH N terminus and the gL C terminus, while the fifth was placed around gL residue 77. Taken together, our data localized the gD binding region on gH/gL to a group of gH and gL residues at the membrane distal region of the heterodimer. Surprisingly, a second set of MAbs did not block gD-gH/gL binding but instead stabilized the complex by altering the kinetic binding. However, despite this prolonged gD-gH/gL interaction, "stabilizing" MAbs also inhibited cell-cell fusion, suggesting a unique mechanism by which the fusion process is halted. Our findings support targeting the gD-gH/gL interaction to prevent fusion in both therapeutic and vaccine strategies against HSV.IMPORTANCE Key to developing a human HSV vaccine is an understanding of the virion glycoproteins involved in entry. HSV employs multiple glycoproteins for attachment, receptor interaction, and membrane fusion. Determining how these proteins function was resolved, in part, by structural biology coupled with immunological and biologic evidence. After binding, virion gD interacts with a receptor to activate the regulator gH/gL complex, triggering gB to drive fusion. Multiple questions remain, one being the physical location of each glycoprotein interaction site. Using protective antibodies with known epitopes, we documented the long-sought interaction between gD and gH/gL, detailing the region on gD important to create the gD-gH/gL triplex. Now, we have identified the corresponding gD contact sites on gH/gL. Concurrently we discovered a novel mechanism whereby gH/gL antibodies stabilize the complex and inhibit fusion progression. Our model for the gD-gH/gL triplex provides a new framework for studying fusion, which identifies targets for vaccine development.

Crystal structure of bovine herpesvirus 1 glycoprotein D bound to nectin-1 reveals the basis for its low-affinity binding to the receptor

Bovine herpesvirus 1 (BHV-1) has received increasing attention for its potential oncolytic applications. BHV-1 recognizes nectin-1 for cell entry via viral glycoprotein D (gD) but represents a low-affinity nectin-1 binding virus. The molecular basis underlying this low receptor-binding affinity, however, remains unknown. Here, the crystal structures of BHV-1 gD in the free and nectin-1-bound forms are presented. While showing an overall resembled nectin-1 binding mode to other alphaherpesvirus gDs, BHV-1 gD has a unique G-strand/α2-helix interloop that disturbs gD/nectin-1 interactions. Residue R188 residing in this loop is observed to otherwise cause strong steric hindrance with the bound receptor, making a large conformational change of the loop a prerequisite for nectin-1 engagement. Subsequently, substitution of R188 with glycine markedly enhances the affinity of the BHV-1-gD/nectin-1 interaction (by about fivefold). These structural and functional data delineate the receptor-recognition basis for BHV-1, which might facilitate BHV-1-based oncolytic design in the future.

Mapping a highly conserved linear neutralizing epitope at the N-terminus of the gD glycoprotein of bovine herpesvirus type I using a monoclonal antibody

Bovine herpesvirus type 1 (BoHV-1), a member of the Alphaherpesvirinae subfamily, causes significant economic losses to the cattle industry worldwide. Envelope glycoprotein D (gD) of BoHV-1 plays an essential role in the viral entry into permissive cells and possibly cooperates with other envelope glycoproteins. The herpesvirus gD induces a protective immune response against diseases in cattle or animal models. Mapping epitopes on gD will facilitate the understanding of the BoHV-1 pathogenesis and development of alternative vaccines against various diseases associated with the virus. In this study, a monoclonal antibody (MAb), designated as 3C1, was generated using naive BoHV-1 in vaccination of mice, demonstrating that 3C1 was specific to gD and represents a neutralizing activity against BoHV-1 infection in Madin-Darby bovine kidney cells. Panels of overlapping gD recombinant proteins with glutathione S-transferase tag were prepared to define the epitope recognized by 3C1. The data demonstrated that the N-terminus of gD ²³APRVTVYVD³¹ was recognized by 3C1. Furthermore, the ²⁶VTVYVD³¹ motif was the minimal amino acid sequence for the recognition. The epitope identified in this study is highly conserved among the typical strains of BoHV-1 and BoHV-5, suggesting that this epitope may be useful in the diagnosis of diseases. In addition, the defined region on gD of BoHV-1 might be essential in viral entry upon comparison with the prototype virus in herpes simplex virus (Alphaherpesvirinae). The data will elucidate the roles of gD of BoHV-1 in viral entry and pathogenesis and its potential application for the development of vaccine candidates and diagnostic techniques based on the conserved epitopes on gD or in combination with those of other herpesvirus glycoproteins.

Surface Plasmon Resonance Reveals Direct Binding of Herpes Simplex Virus Glycoproteins gH/gL to gD and Locates a gH/gL Binding Site on gD

Herpes simplex virus (HSV) requires fusion between the viral envelope and host membrane. Four glycoproteins, gD, gH/gL, and gB, are essential for this process. To initiate fusion, gD binds its receptor and undergoes a conformational change that hypothetically leads to activation of gH/gL, which in turn triggers the fusion protein gB to undergo rearrangements leading to membrane fusion. Our model predicts that gD must interact with both its receptor and gH/gL to promote fusion. In support of this, we have shown that gD is structurally divided into two "faces": one for the binding receptor and the other for its presumed interaction with gH/gL. However, until now, we have been unable to demonstrate a direct interaction between gD and gH/gL. Here, we used surface plasmon resonance to show that the ectodomain of gH/gL binds directly to the ectodomain of gD when (i) gD is captured by certain anti-gD monoclonal antibodies (MAbs) that are bound to a biosensor chip, (ii) gD is bound to either one of its receptors on a chip, and (iii) gD is covalently bound to the chip surface. To localize the gH/gL binding site on gD, we used multiple anti-gD MAbs from six antigenic communities and determined which ones interfered with this interaction. MAbs from three separate communities block gD-gH/gL binding, and their epitopes encircle a geographical area on gD that we propose comprises the gH/gL binding domain. Together, our results show that gH/gL interacts directly with gD, supporting a role for this step in HSV entry.IMPORTANCE HSV entry is a multistep process that requires the actions of four glycoproteins, gD, gH/gL, and gB. Our current model predicts that gD must interact with both its receptor and gH/gL to promote viral entry. Although we know a great deal about how gD binds its receptors, until now we have been unable to demonstrate a direct interaction between gD and gH/gL. Here, we used a highly sensitive surface plasmon resonance technique to clearly demonstrate that gD and gH/gL interact. Furthermore, using multiple MAbs with defined epitopes, we have delineated a domain on gD that is independent of that used for receptor binding and which likely represents the gH/gL interaction domain. Targeting this interaction to prevent fusion may enhance both therapeutic and vaccine strategies.

Common characteristics and unique features: A comparison of the fusion machinery of the alphaherpesviruses Pseudorabies virus and Herpes simplex virus

Membrane fusion is a fundamental biological process that allows different cellular compartments delimited by a lipid membrane to release or exchange their respective contents. Similarly, enveloped viruses such as alphaherpesviruses exploit membrane fusion to enter and infect their host cells. For infectious entry the prototypic human Herpes simplex viruses 1 and 2 (HSV-1 and -2, collectively termed HSVs) and the porcine Pseudorabies virus (PrV) utilize four different essential envelope glycoproteins (g): the bona fide fusion protein gB and the regulatory heterodimeric gH/gL complex that constitute the "core fusion machinery" conserved in all members of the Herpesviridae; and the subfamily specific receptor binding protein gD. These four components mediate attachment and fusion of the virion envelope with the host cell plasma membrane through a tightly regulated sequential activation process. Although PrV and the HSVs are closely related and employ the same set of glycoproteins for entry, they show remarkable differences in the requirements for fusion. Whereas the HSVs strictly require all four components for membrane fusion, PrV can mediate cell-cell fusion without gD. Moreover, in contrast to the HSVs, PrV provides a unique opportunity for reversion analyses of gL-negative mutants by serial cell culture passaging, due to a limited cell-cell spread capacity of gL-negative PrV not observed in the HSVs. This allows a more direct analysis of the function of gH/gL during membrane fusion. Unraveling the molecular mechanism of herpesvirus fusion has been a goal of fundamental research for years, and yet important mechanistic details remain to be uncovered. Nevertheless, the elucidation of the crystal structures of all key players involved in PrV and HSV membrane fusion, coupled with a wealth of functional data, has shed some light on this complex puzzle. In this review, we summarize and discuss the contemporary knowledge on the molecular mechanism of entry and membrane fusion utilized by the alphaherpesvirus PrV, and highlight similarities but also remarkable differences in the requirements for fusion between PrV and the HSVs.

Exploring First Interactions Between Ostreid Herpesvirus 1 (OsHV-1) and Its Host, Crassostrea gigas: Effects of Specific Antiviral Antibodies and Dextran Sulfate

Viral entry mechanisms of herpesviruses constitute a highly complex process which implicates several viral glycoproteins and different receptors on the host cell surfaces. This initial infection stage was currently undescribed for Ostreid herpes virus 1 (OsHV-1), a herpesvirus infecting bivalves including the Pacific oyster, Crassostrea gigas. To identify OsHV-1 glyproteins implicated in the attachment of the virus to oyster cells, three viral putative membrane proteins, encoded by ORF 25, 41, and 72, were selected and polyclonal antibodies against these targets were used to explore first interactions between the virus and host cells. In addition, effects of dextran sulfate, a negative charged sulfated polysaccharide, were investigated on OsHV-1 infection. Effects of antiviral antibodies and dextran sulfate were evaluated by combining viral DNA and RNA detection in spat (in vivo trials) and in oyster hemolymph (in vitro trials). Results showed that viral protein encoded by ORF 25 appeared to be involved in interaction between OsHV-1 and host cells even if other proteins are likely implicated, such as proteins encoded by ORF 72 and ORF 41. Dextran sulfate at 30 μg/mL significantly reduced the spat mortality rate in the experimental conditions. Taken together, these results contribute to better understanding the pathogenesis of the viral infection, especially during the first stage of OsHV-1 infection, and open the way toward new approaches to control OsHV-1 infection in confined facilities.

Analysis of herpes simplex type 1 gB, gD, and gH/gL on production of infectious HIV-1: HSV-1 gD restricts HIV-1 by exclusion of HIV-1 Env from maturing viral particles

BACKGROUND

We previously showed that the gM of HSV-1 could restrict the release of infectious HIV-1 from cells. In this study, we analyzed if the four HSV-1 glycoproteins (gD, gB, and gH/gL), which are the minimum glycoproteins required for HSV-1 entry, restricted the release of infectious HIV-1.

RESULTS

Of these four glycoproteins, gD and gH/gL restricted the production of infectious HIV-1 from cells transfected with an infectious molecular clone of HIV-1 (strain NL4-3) while gB had no significant effect. Pulse-chase analyses indicated that gD did not affect the biosynthesis and processing of gp160 into gp120/gp41, the transport of the gp120/gp41 to the cell surface, or the release of HIV-1 particles from the cell surface. Our analyses revealed that gD was incorporated into HIV-1 virus particles while gp120/gp41 was excluded from released virus particles. Truncated mutants of gD revealed that the cytoplasmic domain was dispensable but that a membrane bound gD was required for the restriction of release of infectious HIV-1. Finally, cell lines expressing gD also potently restricted the release of infectious virus.

CONCLUSIONS

Due to its ability to exclude HIV-1 gp120/gp41 from maturing virus, gD may provide a useful tool in deciphering mechanisms of Env incorporation into maturing virus particles.

Functional Relevance of the Transmembrane Domain and Cytoplasmic Tail of the Pseudorabies Virus Glycoprotein H for Membrane Fusion

Herpesvirus membrane fusion depends on the core fusion machinery, comprised of glycoproteins B (gB) and gH/gL. Although gB structurally resembles autonomous class III fusion proteins, it strictly depends on gH/gL to drive membrane fusion. Whether the gH/gL complex needs to be membrane anchored to fulfill its function and which role the gH cytoplasmic (CD) and transmembrane domains (TMD) play in fusion is unclear. While the gH CD and TMD play an important role during infection, soluble gH/gL of herpes simplex virus 1 (HSV-1) seems to be sufficient to mediate cell-cell fusion in transient assays, arguing against an essential contribution of the CD and TMD. To shed more light on this apparent discrepancy, we investigated the role of the CD and TMD of the related alphaherpesvirus pseudorabies virus (PrV) gH. For this purpose, we expressed C-terminally truncated and soluble gH and replaced the TMD with a glycosylphosphatidylinositol (gpi) anchor. We also generated chimeras containing the TMD and/or CD of PrV gD or HSV-1 gH. Proteins were characterized in cell-based fusion assays and during virus infection. Although truncation of the CD resulted in decreased membrane fusion activity, the mutant proteins still supported replication of gH-negative PrV, indicating that the PrV gH CD is dispensable for viral replication. In contrast, PrV gH lacking the TMD, membrane-anchored via a lipid linker, or comprising the PrV gD TMD were nonfunctional, highlighting the essential role of the gH TMD for function. Interestingly, despite low sequence identity, the HSV-1 gH TMD could substitute for the PrV gH TMD, pointing to functional conservation.IMPORTANCE Enveloped viruses depend on membrane fusion for virus entry. While this process can be mediated by only one or two proteins, herpesviruses depend on the concerted action of at least three different glycoproteins. Although gB has features of bona fide fusion proteins, it depends on gH and its complex partner, gL, for fusion. Whether gH/gL prevents premature fusion or actively triggers gB-mediated fusion is unclear, and there are contradictory results on whether gH/gL function requires stable membrane anchorage or whether the ectodomains alone are sufficient. Our results show that in pseudorabies virus gH, the transmembrane anchor plays an essential role for gB-mediated fusion while the cytoplasmic tail is not strictly required.

Initial Contact: The First Steps in Herpesvirus Entry

The entry process of herpesviruses into host cells is complex and highly variable. It involves a sequence of well-orchestrated events that begin with virus attachment to glycan-containing proteinaceous structures on the cell surface. This initial contact tethers virus particles to the cell surface and results in a cascade of molecular interactions, including the tight interaction of viral envelope glycoproteins to specific cell receptors. These interactions trigger intracellular signaling and finally virus penetration after fusion of the viral envelope with cellular membranes. Based on the engaged cellular receptors and co-receptors, and the subsequent signaling cascades, the entry pathway will be decided on the spot. A number of viral glycoproteins and many cellular receptors and molecules have been identified as players in one or several of these events during virus entry. This chapter will review viral glycoproteins, cellular receptors and signaling cascades associated with the very first interactions of herpesviruses with their target cells.

Structural basis of nectin-1 recognition by pseudorabies virus glycoprotein D

An early and yet indispensable step in the alphaherpesvirus infection is the engagement of host receptors by the viral envelope glycoprotein D (gD). Of the thus-far identified gD receptors, nectin-1 is likely the most effective in terms of its wide usage by multiple alphaherpesviruses for cell entry. The molecular basis of nectin-1 recognition by the gD protein is therefore an interesting scientific question in the alphaherpesvirus field. Previous studies focused on the herpes simplex virus (HSV) of the Simplexvirus genus, for which both the free gD structure and the gD/nectin-1 complex structure were reported at high resolutions. The structural and functional features of other alphaherpesviral gDs, however, remain poorly characterized. In the current study, we systematically studied the characteristics of nectin-1 binding by the gD of a Varicellovirus genus member, the pseudorabies virus (PRV). We first showed that PRV infects host cells via both human and swine nectin-1, and that its gD exhibits similar binding affinities for nectin-1 of the two species. Furthermore, we demonstrated that removal of the PRV gD membrane-proximal residues could significantly increase its affinity for the receptor binding. The structures of PRV gD in the free and the nectin-1-bound states were then solved, revealing a similar overall 3D fold as well as a homologous nectin-1 binding mode to its HSV counterpart. However, several unique features were observed at the binding interface of PRV gD, enabling the viral ligand to utilize different gD residues (from those of HSV) for nectin-1 engagement. These observed binding characteristics were further verified by the mutagenesis study using the key-residue mutants of nectin-1. The structural and functional data obtained in this study, therefore, provide the basis of receptor recognition by PRV gD.

Protein Composition of the Bovine Herpesvirus 1.1 Virion

Bovine herpesvirus (BoHV) type 1 is an important agricultural pathogen that infects cattle and other ruminants worldwide. Acute infection of the oro-respiratory tract leads to immune suppression and allows commensal bacteria to infect an otherwise healthy lower respiratory tract. This condition is known as the Bovine Respiratory Disease (BRD). BoHV-1 latently infects the host for life and periodical stress events re-initiate BRD, translating into high morbidity and large economic losses. To gain a better understanding of the biology of BoHV-1 and the disease it causes, we elucidated the protein composition of extracellular virions using liquid chromatography-mass spectrometry analysis. We detected 33 viral proteins, including the expected proteins of the nucleocapsid and envelope as well as other regulatory proteins present in the viral tegument. In addition to viral proteins, we have also identified packaged proteins of host origin. This constitutes the first proteomic characterization of the BoHV virion.

Bovine herpesvirus glycoprotein D: a review of its structural characteristics and applications in vaccinology

The viral envelope glycoprotein D from bovine herpesviruses 1 and 5 (BoHV-1 and -5), two important pathogens of cattle, is a major component of the virion and plays a critical role in the pathogenesis of herpesviruses. Glycoprotein D is essential for virus penetration into permissive cells and thus is a major target for virus neutralizing antibodies during infection. In view of its role in the induction of protective immunity, gD has been tested in new vaccine development strategies against both viruses. Subunit, DNA and vectored vaccine candidates have been developed using this glycoprotein as the primary antigen, demonstrating that gD has the capacity to induce robust virus neutralizing antibodies and strong cell-mediated immune responses, as well as protection from clinical symptoms, in target species. This review highlights the structural and functional characteristics of BoHV-1, BoHV-5 and where appropriate, Human herpesvirus gD, as well as its role in viral entry and interactions with host cell receptors. Furthermore, the interactions of gD with the host immune system are discussed. Finally, the application of this glycoprotein in new vaccine design is reviewed, taking its structural and functional characteristics into consideration.

Genetic diversity of 3' region of glycoprotein D gene of bovine herpesvirus 1 and 5

Bovine herpesviruses 1 (BoHV-1) and 5 (BoHV-5) are closely related alphaherpesviruses of cattle. While BoHV-1 is mainly associated with respiratory/genital disease and rarely associated with neurological disease, BoHV-5 is the primary agent of meningoencephalitis in cattle. The envelope glycoprotein D of alphaherpesviruses (BoHV-1/gD1 and BoHV-5/gD5) is involved in the early steps of virus infection and may influence virus tropism and neuropathogenesis. This study performed a sequence analysis of the 3' region of gD gene (gD3') of BoHV-1 isolates recovered from respiratory/genital disease (n = 6 and reference strain Cooper) or from neurological disease (n = 7); and from seven typical neurological BoHV-5 isolates. After PCR amplification, nucleotide (nt) sequencing, and aminoacid (aa) sequence prediction; gD3' sequences were compared, identity levels were calculated, and selective pressure was analyzed. The phylogenetic reconstruction based on nt and aa sequences allowed for a clear differentiation of BoHV-1 (n = 14) and BoHV-5 (n = 7) clusters. The seven BoHV-1 isolates from neurological disease are grouped within the BoHV-1 branch. A consistent alignment of 346 nt revealed a high similarity within each viral species (gD1 = 98.3 % nt and aa; gD5 = 97.8 % nt and 85.8 % aa) and an expected lower similarity between gD1 and gD5 (73.7 and 64.1 %, nt and aa, respectively). The analysis of molecular evolution revealed an average negative selection at gD3'. Thus, the phylogeny and similarity levels allowed for differentiation of BoHV-1 and BoHV-5 species, but not further division in subspecies. Sequence analysis did not allow for the identification of genetic differences in gD3' potentially associated with the respective clinical/pathological phenotypes, yet revealed a lower level of gD3' conservation than previously reported.

Induction of conformational changes at the N-terminus of herpes simplex virus glycoprotein D upon binding to HVEM and nectin-1

Herpes simplex virus entry is initiated by glycoprotein D (gD) binding to a cellular receptor, such as HVEM or nectin-1. gD is activated by receptor-induced displacement of the C-terminus from the core of the glycoprotein. Binding of HVEM requires the formation of an N-terminal hairpin loop of gD; once formed this loop masks the nectin-1 binding site on the core of gD. We found that HVEM and nectin-1 exhibit non-reciprocal competition for binding to gD. The N-terminus of gD does not spontaneously form a stable hairpin in the absence of receptor and HVEM does not appear to rely on a pre-existing hairpin for binding to gD(3C-38C) mutants. However, HVEM function is affected by mutations that impair optimal hairpin formation. Furthermore, nectin-1 induces a new conformation of the N-terminus of gD. We conclude that the conformation of the N-terminus of gD is actively modified by the direct action of both receptors.

Identification and functional analysis of membrane proteins gD, gE, gI, and pUS9 of Infectious laryngotracheitis virus

Herpesvirus envelope proteins are of particular interest for development of attenuated live, marker, and subunit vaccines, as well as development of diagnostic tools. The unique short genome region of the chicken pathogen infectious laryngotracheitis virus (ILTV, Gallid herpesvirus 1) contains a cluster of six conserved alphaherpesvirus genes encoding membrane proteins, of which up to now only glycoproteins gG and gJ have been analyzed in detail. We have now prepared monospecific rabbit antisera against ILTV gD, gE, and gI, and the ILTV type II membrane protein pUS9, each of which showed specific immunofluorescence reactions, and detected proteins of approximately 65 and 70 kDa (gD), 62 kDa (gI), 75 kDa (gE), or 37 kDa (pUS9) in western blot analyses of infected chicken cells. The proteins gD, gI, and gE, but not pUS9, were identified as abundant virion proteins, and gE and gI were shown to be N-glycosylated. We also isolated gE-, gI-, and pUS9-deleted ILTV recombinants, whereas it was not possible to purify gD-negative ILTV to homogeneity, indicating that gD, like in other alphaherpesviruses, is essential for receptor binding and virus entry. The pUS9-deleted ILTV exhibited almost wild-type-like replication properties in cell culture. The gE- and gI-negative viruses showed significantly reduced plaque sizes, whereas virus titers were barely affected. Since homologous gene-deletion mutants of other alphaherpesviruses are in use as live vaccines, the generated ILTV recombinants might be also suitable for this application.

Immunity to Bovine Herpesvirus 1: I. Viral lifecycle and innate immunity

Bovine herpesvirus 1 (BHV-1) causes a variety of diseases and is globally distributed. It infects via mucosal epithelium, leading to rapid lytic replication and latent infection, primarily in sensory ganglia. Large amounts of virus can be excreted by the host on primary infection or upon recrudescence of latent infection, resulting in disease spread. The bovine immune response to BHV-1 is rapid, robust, balanced, and long-lasting. The innate immune system is the first to respond to the infection, with type I interferons (IFNs), inflammatory cytokines, killing of infected host cells, and priming of a balanced adaptive immune response. The virus possesses a variety of immune evasion strategies, including inhibition of type I IFN production, chemokine and complement binding, infection of macrophages and neutrophils, and latency. BHV-1 immune suppression contributes to the severity of its disease manifestations and to the bovine respiratory disease complex, the leading cause of cattle death loss in the USA.

Directional spread of alphaherpesviruses in the nervous system

Alphaherpesviruses are pathogens that invade the nervous systems of their mammalian hosts. Directional spread of infection in the nervous system is a key component of the viral lifecycle and is critical for the onset of alphaherpesvirus-related diseases. Many alphaherpesvirus infections originate at peripheral sites, such as epithelial tissues, and then enter neurons of the peripheral nervous system (PNS), where lifelong latency is established. Following reactivation from latency and assembly of new viral particles, the infection typically spreads back out towards the periphery. These spread events result in the characteristic lesions (cold sores) commonly associated with herpes simplex virus (HSV) and herpes zoster (shingles) associated with varicella zoster virus (VZV). Occasionally, the infection spreads transsynaptically from the PNS into higher order neurons of the central nervous system (CNS). Spread of infection into the CNS, while rarer in natural hosts, often results in severe consequences, including death. In this review, we discuss the viral and cellular mechanisms that govern directional spread of infection in the nervous system. We focus on the molecular events that mediate long distance directional transport of viral particles in neurons during entry and egress.

Entry of herpesviruses into cells: the enigma variations

The entry of herpesviruses into their target cells is complex at many levels. Virus entry proceeds by a succession of interactions between viral envelope glycoproteins and molecules on the cell membrane. The process is divided into distinct steps: attachment to the cell surface, interaction with a specific entry receptor, internalization of the particle (optional and cell specific), and membrane fusion. Several viral envelope glycoproteins are involved in one or several of these steps. The most conserved entry glycoproteins in the herpesvirus family (gB, gH/gL) are involved in membrane fusion. Around this functional core, herpesviruses have a variety of receptor binding glycoproteins, which interact with cell surface proteins often from different families. This interaction activates and controls the actual fusion machinery. Interactions with cellular receptors and between viral glycoproteins have to be tightly coordinated and regulated to guarantee successful entry. Although additional entry receptors for herpesviruses continue to be identified, the molecular interactions between viral glycoproteins remain mostly enigmatic. This chapter will review our current understanding of the molecular interactions that occur during herpesvirus entry from attachment to fusion. Particular emphasis will be placed on structure-based representation of receptor binding as a trigger of fusion during herpes simplex virus entry.

In vitro antiviral activity of neem (Azardirachta indica L.) bark extract against herpes simplex virus type-1 infection

Herpes simplex virus type 1 (HSV-1) causes significant health problems from periodical skin and corneal lesions to encephalitis. We report here that an aqueous extract preparation from the barks of neem plant Azardirachta indica acts as a potent entry inhibitor against HSV-1 infection into natural target cells. The neem bark extract (NBE) significantly blocked HSV-1 entry into cells at concentrations ranging from 50 to 100 microg/ml. The blocking activity of NBE was observed when the extract was pre-incubated with the virus but not with the target cells, suggesting a direct antiHSV-1 property of the neem bark. Further, virions treated with NBE failed to bind the cells which implicate a role of NBE as an attachment step blocker. Cells treated with NBE also inhibited HSV-1 glycoprotein-mediated cell-cell fusion and polykaryocytes formation suggesting an additional role of NBE at the viral fusion step. These findings open a potential new avenue for the development of NBE as a novel antiherpetic microbicide.

Herpes simplex virus glycoprotein D interferes with binding of herpesvirus entry mediator to its ligands through downregulation and direct competition

To initiate membrane fusion and virus entry, herpes simplex virus (HSV) gD binds to a cellular receptor such as herpesvirus entry mediator (HVEM). HVEM is a tumor necrosis factor (TNF) receptor family member with four natural ligands that either stimulate (LIGHT and LTα) or inhibit (BTLA and CD160) T cell function. We hypothesized that the interaction of gD with HVEM affects the binding of natural ligands, thereby modulating the immune response during infection. Here, we investigated the effect that gD has on the interaction of HVEM with its natural ligands. First, HSV gD on virions or cells downregulates HVEM from the cell surface. Similarly, trans-interaction with BTLA or LIGHT also downregulates HVEM from the cell surface, suggesting that HSV may subvert a natural mechanism for regulating HVEM activity. Second, we showed that wild-type gD had the lowest affinity for HVEM compared with the four natural ligands. Moreover, gD directly competed for binding to HVEM with BTLA but not LTα or LIGHT, indicating the possibility that gD selectively controls HVEM signals. On the other hand, natural ligands influence the use of HVEM by HSV. For instance, soluble BTLA, LTα, and LIGHT inhibited the binding of wild-type gD to HVEM, and soluble BTLA and LTα blocked HSV infection of HVEM-expressing cells. Thus, gD is at the center of the interplay between HVEM and its ligands. It can interfere with HVEM function in two ways, by competing with the natural ligands and by downregulating HVEM from the cell surface.

Glycoprotein D of bovine herpesvirus 5 (BoHV-5) confers an extended host range to BoHV-1 but does not contribute to invasion of the brain

Bovine herpesvirus 1 (BoHV-1) and BoHV-5 are closely related pathogens of cattle, but only BoHV-5 is considered a neuropathogen. We engineered intertypic gD exchange mutants with BoHV-1 and BoHV-5 backbones in order to address their in vitro and in vivo host ranges, with particular interest in invasion of the brain. The new viruses replicated in cell culture with similar dynamics and to titers comparable to those of their wild-type parents. However, gD of BoHV-5 (gD5) was able to interact with a surprisingly broad range of nectins. In vivo, gD5 provided a virulent phenotype to BoHV-1 in AR129 mice, featuring a high incidence of neurological symptoms and early onset of disease. However, only virus with the BoHV-5 backbone, independent of the gD type, was detected in the brain by immunohistology. Thus, gD of BoHV-5 confers an extended cellular host range to BoHV-1 and may be considered a virulence factor but does not contribute to the invasion of the brain.

Critical role of cholesterol in bovine herpesvirus type 1 infection of MDBK cells

Cholesterol is involved in the life cycle of many viruses. Here, we examined the role of cholesterol for both viral envelope and target cell membrane for bovine herpesvirus type 1 (BoHV-1) infection. Cholesterol depletion by pretreatment of Madin-Darby bovine kidney (MDBK) cells with a cholesterol-sequestering drug methyl-beta-cyclodextrin (MbetaCD), inhibited the production of BoHV-1 in a dose-dependent manner. This inhibitory effect was partially reversed by cholesterol replenishment, indicating that the reduction was caused by cholesterol depletion. Cholesterol depletion at the post-entry stage only had a mild effect on the virus production. However, cell membrane cholesterol depletion did not reduce the virus attachment. In addition, treatment of BoHV-1 particles with MbetaCD also reduced the virus infectivity significantly and the effect was partially reversed by addition of exogenous cholesterol. Taken together, these data implicated that cell membrane cholesterol mainly contributed to BoHV-1 entry into MDBK cells and the viral envelope cholesterol was also essential for the virus infectivity.

A sugar binding protein cyanovirin-N blocks herpes simplex virus type-1 entry and cell fusion

Herpes simplex virus type-1 (HSV-1) causes significant health problems from periodic skin and corneal lesions to encephalitis. It is also considered a cofactor in the development of age-related secondary glaucoma. Inhibition of HSV-1 at the stage of viral entry generates a unique opportunity for preventative and/or therapeutic intervention. Here we provide evidence that a sugar binding antiviral protein, cyanovirin-N (CV-N), can act as a potent inhibitor of HSV-1 entry into natural target cells. Inhibition of entry was independent of HSV-1 gD receptor usage and it was observed in transformed as well as primary cell cultures. Evidence presented herein suggests that CV-N can not only block virus entry to cells but also, it is capable of significantly inhibiting membrane fusion mediated by HSV glycoproteins. While CV-N treated virions were significantly deficient in entering into cells, HSV-1 glycoproteins-expressing cells pretreated with CV-N demonstrated reduced cell-to-cell fusion and polykaryocytes formation. The observation that CV-N can block both entry as well as membrane fusion suggests a stronger potential for this compound in antiviral therapy against HSV-1.

Glycoprotein D-independent spread of pseudorabies virus infection in cultured peripheral nervous system neurons in a compartmented system

The molecular mechanisms underlying the directional neuron-to-epithelial cell transport of herpesvirus particles during infection are poorly understood. To study the role of the viral glycoprotein D (gD) in the directional spread of herpes simplex virus (HSV) and pseudorabies virus (PRV) infection, a culture system consisting of sympathetic neurons or epithelial cells in different compartments was employed. We discovered that PRV infection could spread efficiently from neurons to cells and back to neurons in the absence of gD, the viral ligand required for entry of extracellular particles. Unexpectedly, PRV infection can also spread transneuronally via axo-axonal contacts. We show that this form of interaxonal spread between neurons is gD independent and is not mediated by extracellular virions. We also found that unlike PRV gD, HSV-1 gD is required for neuron-to-cell spread of infection. Neither of the host cell gD receptors (HVEM and nectin-1) is required in target primary fibroblasts for neuron-to-cell spread of HSV-1 or PRV infection.

Role for 3-O-sulfated heparan sulfate as the receptor for herpes simplex virus type 1 entry into primary human corneal fibroblasts

Herpes simplex virus type 1 (HSV-1) infection of the corneal stroma remains a major cause of blindness. Primary cultures of corneal fibroblasts (CF) were tested and found susceptible to HSV-1 entry, which was confirmed by deconvolution imaging of infected cells. Plaque assay and real-time PCR demonstrated viral replication and hence a productive infection of CF by HSV-1. A role for glycoprotein D (gD) receptors in cultured CF was determined by gD interference assay. Reverse transcription-PCR analysis indicated expression of herpesvirus entry mediator and 3-O-sulfated (3-OS) heparan sulfate (HS)-generating enzyme 3-O sulfotransferase 3 (3-OST-3) but not nectin-1 or nectin-2. Subsequently, HS isolated from these cells was found to contain two distinct disaccharides (IdoUA2S-AnMan3S and IdoUA2S-AnMan3S6S) that are representative of 3-OST-3 activity. The following lines of evidence supported the important role of 3-OS HS as the mediator of HSV-1 entry into CF. (i) Blockage of entry was observed in CF treated with heparinases. The same enzymes had significantly less effect on HeLa cells that use nectin-1 as the entry receptor. (ii) Enzymatic removal of cell surface HS also removed the major gD-binding receptor, as evident from the reduced binding of gD to cells. (iii) Spinoculation assay demonstrated that entry blockage by heparinase treatment included the membrane fusion step. (iv) HSV-1 glycoprotein-induced cell-to-cell fusion was inhibited by either prior treatment of cells with heparinases or by HS preparations enriched in 3-OS HS. Taken together, the data in this report provide novel information on the role of 3-OS HS in mediating infection of CF, a natural target cell type.

Alpha-herpesvirus glycoprotein D interaction with sensory neurons triggers formation of varicosities that serve as virus exit sites

α-Herpesviruses constitute closely related neurotropic viruses, including herpes simplex virus in man and pseudorabies virus (PRV) in pigs. Peripheral sensory neurons, such as trigeminal ganglion (TG) neurons, are predominant target cells for virus spread and lifelong latent infections. We report that in vitro infection of swine TG neurons with the homologous swine α-herpesvirus PRV results in the appearance of numerous synaptophysin-positive synaptic boutons (varicosities) along the axons. Nonneuronal cells that were juxtaposed to these varicosities became preferentially infected with PRV, suggesting that varicosities serve as axonal exit sites for the virus. Viral envelope glycoprotein D (gD) was found to be necessary and sufficient for the induction of varicosities. Inhibition of Cdc42 Rho GTPase and p38 mitogen-activated protein kinase signaling pathways strongly suppressed gD-induced varicosity formation. These data represent a novel aspect of the cell biology of α-herpesvirus infections of sensory neurons, demonstrating that virus attachment/entry is associated with signaling events and neuronal changes that may prepare efficient egress of progeny virus.

Prevention of the interaction between HVEM, herpes virus entry mediator, and gD, HSV envelope protein, by a Keggin polyoxotungstate, PM-19

One of the Keggin-type heteropolyoxotungstates (K7[PTi2W10O40]6H2O:PM-19) is a potent inhibitor of the replication of herpes simplex virus (HSV) both standard strain 169 and the thymidine kinase-defective strain YS-4C-1 in vitro and in vivo. HSV envelope protein, gD, is necessary for virus entry into the cells. Some cellular molecules, such as HVEM, were reported to act as cofactors during the viral entry step. We determined whether PM-19 prevents these interactions between HSV-gD and HVEM. These activities were investigated using the Ciphergen and BIACORE system. Using a protein chip, many kinds of gD-specific binding proteins were captured, but these proteins could not be identified. Several proteins in these gD-binding proteins were inhibited its interaction with gD due to the presence of PM-19. Using the BIACORE system, the affinity of PM-19 to gD was low, because PM-19 has no direct inactivation activity against the virion. The specific binding of HVEM to the gD was shown as KD of 1.1e-9. The affinity of PM-19 for HVEM was high (KD:2e-9). To determine the competitive binding, the PM-19 (10 microg/ml) and several concentrations of HVEM solution mixtures were injected over the gD-fixed sensor surface. Each binding signal was stable in the range of approximately 270-300 RU. In the case of the addition of PM-19 to HVEM solution, the binding signals were elevated by PM-19 dose dependently. These results suggest that the bindings of PM-19 to gD are not disturbed by the presence of HVEM. PM-19 prevents the interaction between HVEM and gD.

Soluble V domain of Nectin-1/HveC enables entry of herpes simplex virus type 1 (HSV-1) into HSV-resistant cells by binding to viral glycoprotein D

Interaction of herpes simplex virus (HSV) glycoprotein D (gD) with specific cellular receptors is essential for HSV infection of susceptible cells. Virus mutants that lack gD can bind to the cell surface (attachment) but do not enter, implying that interaction of gD with its receptor(s) initiates the postattachment (entry) phase of HSV infection. In this report, we have studied HSV entry in the presence of the gD-binding variable (V) domain of the common gD receptor nectin-1/HveC to determine whether cell association of the gD receptor is required for HSV infection. In the presence of increasing amounts of the soluble nectin-1 V domain (sNec1(123)), increasing viral entry into HSV-resistant CHO-K1 cells was observed. At a multiplicity of 3 in the presence of optimal amounts of sNec1(123), approximately 90% of the cells were infected. The soluble V domain of nectin-2, a strain-specific HSV entry receptor, promoted entry of the HSV type 1 (HSV-1) Rid-1 mutant strain, but not of wild-type HSV-1. Preincubation and immunofluorescence studies indicated that free or gD-bound sNec1(123) did not associate with the cell surface. sNec1(123)-mediated entry was highly impaired by interference with the cell-binding activities of viral glycoproteins B and C. While gD has at least two functions, virus attachment to the cell and initiation of the virus entry process, our results demonstrate that the attachment function of gD is dispensable for entry provided that other means of attachment are available, such as gB and gC binding to cell surface glycosaminoglycans.

Inhibition of bovine sperm–zona binding by bovine herpesvirus-1

The purpose of the present study was to identify a potential interference of bovine herpesvirus-1 (BoHV-1) with sperm–oocyte interactions during bovinein vitrofertilization. An inhibition of almost 70% of sperm–zona binding was observed when bovine cumulus-denuded oocytes were inseminated in the presence of 10750% tissue culture infective dose/ml BoHV-1. The inhibitory effect of BoHV-1 on sperm–zona binding was mediated by an interaction of the virus with spermatozoa, but not with oocytes. Treatment of spermatozoa with BoHV-1, however, did not affect sperm motility and acrosomal status. Antiserum against BoHV-1 prevented the virus-induced inhibition of sperm–zona binding, indicating that BoHV-1 itself affects the fertilization process. In order to investigate which BoHV-1 glycoprotein(s) are responsible for the virus–sperm interaction, BoHV-1 was treated with monoclonal antibodies against the viral glycoproteins gB, gC, gD and gH prior to insemination. Anti-gC completely prevented the inhibitory effect of BoHV-1 on sperm–zona binding, while anti-gD caused a reduction of this inhibition. Further evidence for the involvement of gC and gD in the virus–sperm interaction was provided by the fact that purified gC and gD decreased sperm–zona binding in a dose-dependent way with gC being more effective than gD. These results indicated that BoHV-1 inhibits bovine sperm–zona binding by interacting with spermatozoa. The binding of BoHV-1 to a spermatozoon is mediated by the viral glycoproteins gC and gD, and therefore seems to be comparable with the mechanisms of BoHV-1 attachment to its natural host cell.

Potential nectin-1 binding site on herpes simplex virus glycoprotein d

Four glycoproteins (gD, gB, gH, and gL) are essential for herpes simplex virus (HSV) entry into cells. An early step of fusion requires gD to bind one of several receptors, such as nectin-1 or herpesvirus entry mediator (HVEM). We hypothesize that a conformational change in gD occurs upon receptor binding that triggers the other glycoproteins to mediate fusion. Comparison of the crystal structures of gD alone and gD bound to HVEM reveals that upon HVEM binding, the gD N terminus transitions from a flexible stretch of residues to a hairpin loop. To address the contribution of this transition to the ability of gD to trigger fusion, we attempted to "lock" the gD N terminus into a looped conformation by engineering a disulfide bond at its N and C termini. The resulting mutant (gD-A3C/Y38C) failed to trigger fusion in the absence of receptor, suggesting that formation of the loop is not the sole fusion trigger. Unexpectedly, although gD-A3C/Y38C bound HVEM, it failed to bind nectin-1. This was due to the key role played by Y38 in interacting with nectin-1. Since tyrosines are often "hot spot" residues at the center of protein-protein interfaces, we mutated residues that surround Y38 on the same face of gD and tested their binding and functional properties. Our results suggest that this region of gD is important for nectin-1 interaction and is distinct from but partially overlaps the site of HVEM binding. Unique gD mutants with altered receptor usage generated in this study may help dissect the roles played by various HSV receptors during infection.

Herpes simplex virus and varicella-zoster virus: why do these human alphaherpesviruses behave so differently from one another?

Members of the Herpesviridae family of viruses are classified into the alpha, beta and gamma subfamilies. The alpha subfamily is estimated to have diverged from the beta and gamma subfamilies 200-220 million years ago. The ancestors of the herpes simplex virus (HSV) and the varicella-zoster virus (VZV), two ubiquitous and clinically important human pathogens, appeared 70-80 million years ago. As these viruses coevolved with their specific primate hosts, genetic rearrangements led to the development of the contemporary alphaherpesviruses and their distinct complement of genes. Here the distinct features of HSV and VZV are discussed in terms of their transmissibility, clinical picture, tissue tropism, establishment of latency/reactivation and immune evasion, which can, at least in part, be explained by differences in their genomes.

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.

Function of herpes simplex virus type 1 gD mutants with different receptor-binding affinities in virus entry and fusion

We have studied the receptor-specific function of four linker-insertion mutants of herpes simplex virus type 1 glycoprotein D (gD) representing each of the functional regions of gD. We used biosensor analysis to measure binding of the gD mutants to the receptors HVEM (HveA) and nectin-1 (HveC). One of the mutants, gD(inverted Delta 34t), failed to bind HVEMt but showed essentially wild-type (WT) affinity for nectin-1t. The receptor-binding kinetics and affinities of the other three gD mutants varied over a 1,000-fold range, but each mutant had the same affinity for both receptors. All of the mutants were functionally impaired in virus entry and cell fusion, and the levels of activity were strikingly similar in these two assays. gD(inverted Delta 34)-containing virus was defective on HVEM-expressing cells but did enter nectin-1-expressing cells to about 60% of WT levels. This showed that the defect of this form of gD on HVEM-expressing cells was primarily one of binding and that this was separable from its later function in virus entry. gD(inverted Delta 243t) showed WT binding affinity for both receptors, but virus containing this form of gD had a markedly reduced rate of entry, suggesting that gD(inverted Delta 243) is impaired in a postbinding step in the entry process. There was no correlation between gD mutant activity in fusion or virus entry and receptor-binding affinity. We conclude that gD functions in virus entry and cell fusion regardless of its receptor-binding kinetics and that as long as binding to a functional receptor occurs, entry will progress.

Cellular localization of nectin-1 and glycoprotein D during herpes simplex virus infection

During viral entry, herpes simplex virus (HSV) glycoprotein D (gD) interacts with a specific cellular receptor such as nectin-1 (PRR1/HveC/CD111) or the herpesvirus entry mediator A (HVEM/HveA). Nectin-1 is involved in cell-to-cell adhesion. It is located at adherens junctions, where it bridges cells through homophilic or heterophilic interactions with other nectins. Binding of HSV gD prevents nectin-1-mediated cell aggregation. Since HSV gD affects the natural function of nectin-1, we further investigated the effects of gD expression on nectin-1 during HSV infection or in transfected cells. We also studied the importance of the interaction between nectin-1 and the cytoplasmic protein afadin for HSV entry and spread as well as the effects of infection on this interaction. In these investigations, we used a panel of cells expressing nectin-1 or nectin-1-green fluorescent protein fusions as the only mediators of HSV entry. During HSV infection, nectin-1 localization at adherens junction was dramatically altered in a manner dependent on gD expression. Nectin-1 and gD colocalized at cell contact areas between infected and noninfected cells and at the edges of plaques. This specific accumulation of gD at junctions was driven by expression of nectin-1 in trans on the surface of adjacent cells. Reciprocally, nectin-1 was maintained at junctions by the trans expression of gD in the absence of a cellular natural ligand. Our observations indicate that newly synthesized gD substitutes for nectin-1 of infected cells at junctions with noninfected cells. We propose that gD attracts and maintains the receptor at junctions where it can be used for virus spread.

Structure-based mutagenesis of herpes simplex virus glycoprotein D defines three critical regions at the gD-HveA/HVEM binding interface

Herpes simplex virus (HSV) entry into cells requires the binding of glycoprotein D (gD) to one of several cell surface receptors. The crystal structure of gD bound to one of these receptors, HveA/HVEM, reveals that the core of gD comprises an immunoglobulin fold flanked by a long C-terminal extension and an N-terminal hairpin loop. HveA is a member of the tumor necrosis factor receptor family and contains four cysteine-rich domains (CRDs) characteristic of this family. Fourteen amino acids within the gD N-terminal loop comprise the entire binding site for HveA. To determine the contribution of each gD contact residue to virus entry, we constructed gD molecules mutated in these amino acids. We determined the abilities of the gD mutants to bind receptors, facilitate virus entry, and mediate cell-cell fusion. Seven of the gD mutants exhibited wild-type levels of receptor binding and gD function. Results from the other seven gD mutants revealed three critical regions at the gD-HveA interface. (i) Several gD residues that participate in an intermolecular beta-sheet with HveA were found to be crucial for HveA binding and entry into HveA-expressing cells. (ii) Two gD residues that contact HveA-Y23 contributed to HveA binding but were not required for mediating entry into cells. HveA-Y23 fits into a crevice on the surface of gD and was previously shown to be essential for gD binding. (iii) CRD2 was previously shown to contribute to gD binding, and this study shows that one gD residue that contacts CRD2 contributes to HveA binding. None of the gD mutations prevented interaction with nectin-1, another gD receptor. However, when cotransfected with the other glycoproteins required for fusion, two gD mutants gained the ability to mediate fusion of cells expressing nectin-2, a gD receptor that interacts with several laboratory-derived gD mutants but not with wild-type gD. Thus, results from this panel of gD mutants as well as those of previous studies (A. Carfi, S. H. Willis, J. C. Whitbeck, C. Krummenacher, G. H. Cohen, R. J. Eisenberg, and D. C. Wiley, Mol. Cell 8:169-179, 2001, and S. A. Connolly, D. J. Landsburg, A. Carfi, D. C. Wiley, R. J. Eisenberg, and G. H. Cohen, J. Virol. 76:10894-10904, 2002) provide a detailed picture of the gD-HveA interface and the contacts required for functional interaction. The results demonstrate that of the 35 gD and HveA contact residues that comprise the gD-HveA interface, only a handful are critical for complex formation.

The domains of glycoprotein D required to block apoptosis induced by herpes simplex virus 1 are largely distinct from those involved in cell-cell fusion and binding to nectin1

Glycoprotein D (gD) interacts with two alternative protein receptors, nectin1 and HveA, to mediate herpes simplex virus (HSV) entry into cells. Fusion of the envelope with the plasma membrane requires, in addition to gD, glycoproteins gB, gH, and gL. Coexpression of the four glycoproteins (gD, gB, gH, and gL) promotes cell-cell fusion. gD delivered in trans is also capable of blocking the apoptosis induced by gD deletion viruses grown either in noncomplementing cells (gD(-/-)) or in complementing cells (gD(-/+)). While ectopic expression of cation-independent mannose-6 phosphate receptor blocks apoptosis induced by both stocks, other requirements differ. Thus, apoptosis induced by gD(-/-) virus is blocked by full-length gD (or two gD fragments reconstituting a full-length molecule), whereas ectopic expression of the gD ectodomain is sufficient to block apoptosis induced by gD(-/+) virus. In this report we took advantage of a set of gD insertion-deletion mutants to map the domains of gD required to block apoptosis by gD(-/-) and gD(-/+) viruses and those involved in cell-cell fusion. The mutations that resulted in failure to block apoptosis were the same for gD(-/-) and gD(-/+) viruses and were located in three sites, one within the immunoglobulin-type core region (residues 125, 126, and 151), one in the upstream connector region (residues 34 and 43), and one in the C-terminal portion of the ectodomain (residue 277). A mutant that carried amino acid substitutions at the three glycosylation sites failed to block apoptosis but behaved like wild-type gD in all other assays. The mutations that inhibited polykaryocyte formation were located in the upstream connector region (residues 34 and 43), at the alpha1 helix (residue 77), in the immunoglobulin core and downstream regions (residue 151 and 187), and at the alpha3 helix (residues 243 and 246). Binding of soluble nectin1-Fc to cells expressing the mutant gDs was generally affected by the same mutations that affected fusion, with one notable exception (Delta277-310), which affected fusion without hampering nectin1 binding. This deletion likely identifies a region of gD involved in fusion activity at a post-nectin1-binding step. We conclude that whereas mutations that affected all functions (e.g., upstream connector region and residue 151) may be detrimental to overall gD structure, the mutations that affect specific activities identify domains of gD involved in the interactions with entry receptors and fusogenic glycoproteins and with cellular proteins required to block apoptosis. The evidence that glycosylation of gD is required for blocking apoptosis supports the conclusion that the interacting protein is the mannose-6 phosphate receptor.

Mechanism of the protective effect of heteropolyoxotungstate against herpes simplex virus type 2

The effects of heteropolyoxotungstate (K(7)[PTi(2)W(10)O(40)]. 6H(2)O; PM-19) on the replication of herpes simplex virus type 2 (HSV-2) were examined using a semiquantitative polymerase chain reaction of intracellular viral DNA established by us and also other methods. Vero cells were infected with HSV-2 strains: either the standard strain 169, or the acyclovir-resistant strain YS-4C-1. PM-19 was added at various stages during the replication of HSV-2. PM-19 strongly inhibited the synthesis of viral genomic DNA when it was added at the time of infection. The addition of PM-19 60-90 min after viral inoculation time-dependently decreased the antiviral activity and increased the relative yield of viral DNA, and the addition of PM-19 was completely ineffective at times later than 90 min. These results suggested that PM-19 inhibited viral penetration but did not affect the synthesis of viral DNA. Furthermore, PM-19 strongly inhibited a second round of infection.

Syncytiotrophoblast is a barrier to maternal-fetal transmission of herpes simplex virus

Herpes simplex virus (HSV)-1 has been discovered in placental tissue from spontaneous miscarriages, but reports of transplacental transmission and fetal infection are extremely rare. Previously, we demonstrated that the villous syncytiotrophoblast, which forms a continuous layer between the maternal and fetal circulation, is resistant to HSV entry. Here, we tested our hypothesis that the villous syncytiotrophoblast prevents transplacental transmission of HSV secondary to decreased expression of HSV entry mediators (HveA, HveB, and HveC). In addition, we investigated the ability of HSV to infect extravillous trophoblast cells, which mediate placental attachment to the uterine wall, and the expression of HSV receptors in these cells. We performed fluorescence-activated cell sorting (FACS) analyses and immunostaining to demonstrate that HveA, HveB, and HveC were not expressed in third-trimester villous trophoblast cells. Consequently, villous explants obtained from third-trimester placentas were resistant to infection by a recombinant HSV-1 vector, HSV-1 KOS, but approximately 20% of mesenchymal cells within the villous core were infected when villous explants were pretreated with trypsin to disrupt the villous trophoblast layer. Conversely, FACS analysis and immunostaining demonstrated that extravillous trophoblast cells expressed HveA, HveB, and HveC, and these cells were efficiently infected by HSV vectors. Infection of extravillous trophoblast cells by HSV-1 was not reduced when the cells were pretreated with an antibody against HveA but was partially reduced when the cells were pretreated with antibodies directed against HveB and HveC. Thus, the decreased expression of herpesvirus entry mediators in villous syncytiotrophoblast prevents placental villous infection, thereby limiting maternal-fetal transmission of HSV.

Structure-based analysis of the herpes simplex virus glycoprotein D binding site present on herpesvirus entry mediator HveA (HVEM)

Binding of herpes simplex virus (HSV) envelope glycoprotein D (gD) to a cell surface receptor is an essential step of virus entry. We recently determined the crystal structure of gD bound to one receptor, HveA. HveA is a member of the tumor necrosis factor receptor family and contains four characteristic cysteine-rich domains (CRDs). The first two CRDs of HveA are necessary and sufficient for gD binding. The structure of the gD-HveA complex reveals that 17 amino acids in HveA CRD1 and 4 amino acids in HveA CRD2 directly contact gD. To determine the contribution of these 21 HveA residues to virus entry, we constructed forms of HveA mutated in each of these contact residues. We determined the ability of the mutant proteins to bind gD, facilitate virus entry, and form HveA oligomers. Our results point to a binding hot spot centered around HveA-Y23, a residue that protrudes into a crevice on the surface of gD. Both the hydroxyl group and phenyl group of HveA-Y23 contribute to HSV entry. Our results also suggest that an intermolecular beta-sheet formed between gD and HveA residues 35 to 37 contributes to binding and that a C37-C19 disulfide bond in CRD1 is a critical component of HveA structure necessary for gD binding. The results argue that CRD2 is required for gD binding mainly to provide structural support for a gD binding site in CRD1. Only one mutant, HveA-R75A, exhibited enhanced gD binding. While some mutations influenced complex formation, the majority did not affect HSV entry, suggesting that most contact residues contribute to HveA receptor function collectively rather than individually. This structure-based dissection of the gD-HveA binding site highlights the contribution of key residues within HveA to gD binding and HSV entry and defines a target region for the design of small-molecule inhibitors.

The memory effect of heteropolyoxotungstate (PM-19) pretreatment on infection by herpes simplex virus at the penetration stage

The keggin-type heteropolyoxotungstate K(7)[PTi(2)W(10)O(40)].6H(2)O (PM-19) is a potent polyoxometalate (PM) inhibitor of the replication of herpes simplex virus (HSV). Pretreatment of Vero cells with PM-19 prior to HSV-2 infection enhanced the antiviral potency of PM-19 almost 10-fold compared with treatment of the cells only after infection. The pretreatment effect of PM-19 is called "the memory effect". The memory effect was reflected by inhibition of plaque formation and decrease of intracellular virus DNA quantity, and was strongest when PM-19 was present during the penetration stage of HSV-2 infection. The effect was maintained under conditions of fusion induced by polyethyleneglycol treatment. This suggests that PM-19 does not act at the fusion stage of infection. Using the infectious center assay method, it was clarified that a second round of infection was inhibited by about 30% in the presence of PM-19 at the penetration stage compared with the virus control in nontreated cells. The inhibition was enhanced to about 60% by PM-19 pretreatment prior to infection. This suggests that PM-19 pretreatment of the cells protects them against HSV-2 infection.