Variability of herpes simplex virus 1 gL and anti-gL antibodies that inhibit cell fusion but not viral infectivity

Herpes Simplex Virus Glycoprotein B Mutations Define Structural Sites in Domain I, the Membrane Proximal Region, and the Cytodomain That Regulate Entry

The viral fusion protein glycoprotein B (gB) is conserved in all herpesviruses and is essential for virus entry. During entry, gB fuses viral and host cell membranes by refolding from a prefusion to a postfusion form. We previously introduced three structure-based mutations (gB-I671A/H681A/F683A) into the domain V arm of the gB ectodomain that resulted in reduced cell-cell fusion. A virus carrying these three mutations (called gB3A) displayed a small-plaque phenotype and remarkably delayed entry into cells. To identify mutations that could counteract this phenotype, we serially passaged the gB3A virus and selected for revertant viruses with increased plaque sizes. Genomic sequencing revealed that the revertant viruses had second-site mutations in gB, including E187A, M742T, and S383F/G645R/V705I/V880G. Using expression constructs encoding these mutations, only gB-V880G was shown to enhance cell-cell fusion. In contrast, all of the revertant viruses showed enhanced entry kinetics, underscoring the fact that cell-cell fusion and virus-cell fusion are different. The results indicate that mutations in three different regions of gB (domain I, the membrane proximal region, and the cytoplasmic tail domain) can counteract the slow-entry phenotype of gB3A virus. Mapping these compensatory mutations to prefusion and postfusion structural models suggests sites of intramolecular functional interactions with the gB domain V arm that may contribute to the gB fusion function. IMPORTANCE The nine human herpesviruses are ubiquitous and cause a range of diseases in humans. Glycoprotein B (gB) is an essential viral fusion protein that is conserved in all herpesviruses. During host cell entry, gB mediates virus-cell membrane fusion by undergoing a conformational change. Structural models for the prefusion and postfusion forms of gB exist, but the details of how the protein converts from one to the other are unclear. We previously introduced structure-based mutations into gB that inhibited virus entry and fusion. By passaging this entry-deficient virus over time, we selected second-site mutations that partially restore virus entry. The locations of these mutations suggest regulatory sites that contribute to fusion and gB refolding during entry. gB is a target of neutralizing antibodies, and defining how gB refolds during entry could provide a basis for the development of fusion inhibitors for future research or clinical use.

Epstein-Barr Virus Fusion with Epithelial Cells Triggered by gB Is Restricted by a gL Glycosylation Site

Epstein-Barr virus (EBV) entry into epithelial cells is mediated by the conserved core fusion machinery, composed of the fusogen gB and the receptor-binding complex gH/gL. The heterodimeric gH/gL complex binds to the EBV epithelial cell receptor or gp42, which binds to the B-cell receptor, triggering gB-mediated fusion of the virion envelope with cellular membranes. Our previous study found that the gL glycosylation mutant N69L/S71V had an epithelial cell-specific hyperfusogenic phenotype. To study the influence of this gL mutant on the initiation and kinetics of gB-driven epithelial cell fusion, we established a virus-free split-green fluorescent protein cell-cell fusion assay that enables real-time measurements of membrane fusion using live cells. The gL_N69L/S71V mutant had a large increase in epithelial cell fusion activity of up to 300% greater than that of wild-type gL starting at early time points. The hyperfusogenicity of the gL mutant was not a result of alterations in complex formation with gH or alterations in cellular localization. Moreover, the hyperfusogenic phenotype of the gL mutant correlated with the formation of enlarged syncytia. In summary, our present findings highlight an important role of gL in the kinetics of gB-mediated epithelial cell fusion, adding to previous findings indicating a direct interaction between gL and gB in EBV membrane fusion.IMPORTANCE EBV predominantly infects epithelial cells and B lymphocytes, which are the cells of origin for the EBV-associated malignancies Hodgkin and Burkitt lymphoma as well as nasopharyngeal carcinoma. Contrary to the other key players of the core fusion machinery, gL has the most elusive role during EBV-induced membrane fusion. We found that the glycosylation site N69/S71 of gL is involved in restricting epithelial cell fusion activity, strongly correlating with syncytium size. Interestingly, our data showed that the gL glycosylation mutant increases the fusion activity of the hyperfusogenic gB mutants, indicating that this gL mutant and the gB mutants target different steps during fusion. Our studies on how gL and gB work together to modulate epithelial cell fusion kinetics are essential to understand the highly tuned tropism of EBV for epithelial cells and B lymphocytes and may result in novel strategies for therapies preventing viral entry into target host cells. Finally, making our results of particular interest is the absence of gL syncytial mutants in other herpesviruses.

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.

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.

Regulation of herpes simplex virus gB-induced cell-cell fusion by mutant forms of gH/gL in the absence of gD and cellular receptors

Herpesvirus entry requires the viral glycoprotein triad of gB and gH/gL to carry out fusion between the virion envelope and a cellular membrane in order to release the nucleocapsid into the target cell. Herpes simplex virus (HSV) also requires glycoprotein gD to initiate the fusion cascade by binding a cell receptor such as nectin 1 or herpesvirus entry mediator (HVEM). While the structure of gB is that of a class III fusion protein, gH/gL has no features that resemble other viral fusion proteins. Instead, it is suggested that gH/gL acts as a regulator of gB. The crystal structure of HSV-2 gH/gL was obtained with a functional protein that had a deletion of 28 residues at the gH N terminus (gHΔ48/gL). Unexplainably, monoclonal antibodies (MAbs) with virus-neutralizing activity map to these residues. To reconcile these two disparate observations, we studied the ability of gHΔ48/gL to regulate fusion. Here, we show that the protein induces low (constitutive) levels of fusion by gB in the absence of gD and/or receptor. However, when gD and receptor are present, this mutant functions as well as does wild-type (wt) gH/gL for fusion. We propose that gHΔ48/gL has an intermediate structure on the pathway leading to full regulatory activation. We suggest that a key step in the pathway of fusion is the conversion of gH/gL to an activated state by receptor-bound gD; this activated gH/gL resembles gHΔ48/gL.

IMPORTANCE

Herpes simplex viruses (HSVs) cause many human diseases, from mild cold sores to lethal neonatal herpes. As an enveloped virus, HSV must fuse its membrane with a host membrane in order for replication to take place. The virus uses four glycoproteins for this process, gD, gB, and gH/gL, and either of two cell receptors, herpesvirus entry mediator (HVEM) and nectin 1. Although the virus can enter the cell by direct fusion at the plasma membrane or via endocytosis, the same four glycoproteins are involved. The absence of any of these proteins abolishes the entry process. Here, we show that a mutant form of gH/gL, gHΔ48/gL, can induce fusion of gB-expressing cells in the absence of gD and a gD receptor. Our study supports the concept that gB is the HSV fusogen and its activity is regulated by gH/gL.

Insertional mutations in herpes simplex virus type 1 gL identify functional domains for association with gH and for membrane fusion

Glycoprotein L (gL) is one of four glycoproteins required for the entry of herpes simplex virus (HSV) into cells and for virus-induced cell fusion. This glycoprotein oligomerizes with gH to form a membrane-bound heterodimer but can be secreted when expressed without gH. Twelve unique gL linker-insertion mutants were generated to identify regions critical for gH binding and gH/gL processing and regions essential for cell fusion and viral entry. All gL mutants were detected on the cell surface in the absence of gH, suggesting incomplete cleavage of the signal peptide or the presence of a cell surface receptor for secreted gL. Coexpression with gH enhanced the levels of cell surface gL detected by antibodies for all gL mutants except those that were defective in their interactions with gH. Two insertions into a conserved region of gL abrogated the binding of gL to gH and prevented gH expression on the cell surface. Three other insertions reduced the cell surface expression of gH and/or altered the properties of gH/gL heterodimers. Altered or absent interaction of gL with gH was correlated with reduced or absent cell fusion activity and impaired complementation of virion infectivity. These results identify a conserved domain of gL that is critical for its binding to gH and two noncontiguous regions of gL, one of which contains the conserved domain, that are critical for the gH/gL complex to perform its role in membrane fusion.

Functional analysis of glycoprotein L (gL) from rhesus lymphocryptovirus in Epstein-Barr virus-mediated cell fusion indicates a direct role of gL in gB-induced membrane fusion

Glycoprotein L (gL), which complexes with gH, is a conserved herpesvirus protein that is essential for Epstein-Barr virus (EBV) entry into host cells. The gH/gL complex has a conserved role in entry among herpesviruses, yet the mechanism is not clear. To gain a better understanding of the role of gL in EBV-mediated fusion, chimeric proteins were made using rhesus lymphocryptovirus (Rh-LCV) gL (Rh gL), which shares a high sequence homology with EBV gL but does not complement EBV gL in mediating fusion with B cells. A reduction in fusion activity was observed with chimeric gL proteins that contained the amino terminus of Rh gL, although they retained their ability to process and transport gH/gL to the cell surface. Amino acids not conserved within this region in EBV gL when compared to Rh gL were further analyzed, with the results mapping residues 54 and 94 as being functionally important for EBV-mediated fusion. All chimeras and mutants displayed levels of cell surface expression similar to that of wild-type gL and interacted with gH and gp42. Our data also suggest that the role of gL involves the activation or recruitment of gB with the gH/gL complex, as we found that reduced fusion of Rh gL, EBV/Rh-LCV chimeras, and gL point mutants could be restored by replacing EBV gB with Rh gB. These observations demonstrate a distinction between the role of gL in the processing and trafficking of gH to the cell surface and a posttrafficking role in cell-cell fusion.

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.

Herpes simplex virus type 1 mediates fusion through a hemifusion intermediate by sequential activity of glycoproteins D, H, L, and B

Virus-induced membrane fusion can be subdivided into three phases defined by studies of class I and class II fusion proteins. During Phase I, two membranes are brought into close apposition. Phase II marks the mixing of the outer membrane leaflets leading to formation of a hemifusion intermediate. A fusion pore stably forms and expands in Phase III, thereby completing the fusion process. Herpes simplex virus type 1 (HSV-1) requires four glycoproteins to complete membrane fusion, but none has been defined as class I or II. Therefore, we investigated whether HSV-1-induced membrane fusion occurred following the same general phases as those described for class I and II proteins. In this study we demonstrate that glycoprotein D (gD) and the glycoprotein H and glycoprotein L complex (gHL) mediated lipid mixing indicative of hemifusion. However, content mixing and full fusion required glycoprotein B (gB) to be present along with gD and gHL. Our results indicate that, like class I and II fusion proteins, fusion mediated by HSV-1 glycoproteins occurred through a hemifusion intermediate. In addition, both gB and gHL are probably directly involved in the fusion process. From this, we propose a sequential model for fusion via HSV-1 glycoproteins whereby gD is required for Phase I, gHL is required for Phase II, and gB is required for Phase III. We further propose that glycoprotein H and gB are likely to function sequentially to promote membrane fusion in other herpesviruses such as Epstein-Barr virus and human herpesvirus 8.

The herpesvirus glycoproteins B and H.L are sequentially recruited to the receptor-bound gD to effect membrane fusion at virus entry

Four glycoproteins (gD, gB, gH, and gL) are required for herpes simplex virus entry into the cell or for cell-cell fusion in transfected cells. gD serves as the receptor-binding glycoprotein and as the trigger of fusion; the other three execute fusion between the viral envelope and the plasma and endocytic membranes or the membranes of adjacent cells and are highly conserved among members of the herpesvirus family. Details of the interaction of gD with gB, gH, and gL were not known. Here, we report that the four glycoproteins assemble into a complex initiated by the interaction of gD with its cellular receptor. gB is recruited to the gD-receptor complex next, even in the absence of gH.gL. gH.gL is recruited next, but only to the receptor-gD-gB ensemble. A complex with the composition receptor-gD-gB-gH.gL is assembled transiently with a life span of 15-30 min in cells exposed to virus but can also be found in infected cells and in cells committed to form polykaryocytes after transfection of the glycoprotein quartet. The results indicate that the complex assembly is a critical step in the process of virus entry and fusion, and that no viral protein other than those that participate in the complex itself is required for complex assembly. These findings imply critical protein-protein interactions among the quartet as herpes simplex virions enter the cells and at cell-cell fusion, define a specific order of recruitment, and place gH.gL as the last link in the process of glycoprotein recruitment to the complex.

The requirements for herpes simplex virus type 1 cell-cell spread via nectin-1 parallel those for virus entry

Herpes simplex virus type 1 (HSV-1) spreads from an infected cell to an uninfected cell by virus entry, virus-induced cell fusion, and cell-cell spread. The three forms of virus spread require the viral proteins gB, gD, and gH-gL, as well as a cellular gD receptor. The mutual requirement for the fusion glycoproteins and gD receptor suggests that virus entry, cell fusion, and cell-cell spread occur by a similar mechanism. The goals of this study were to examine the role of the nectin-1alpha transmembrane domain and cytoplasmic tail in cell-cell spread and to obtain a better understanding of the receptor-dependent events occurring at the plasma membrane during cell-cell spread. We determined that an intact nectin-1alpha V-like domain was required for cell-cell spread, while a membrane-spanning domain and cytoplasmic tail were not. Chimeric forms of nectin-1 that were non-functional for virus entry did not mediate cell-cell spread regardless of whether they could mediate cell fusion. Also, cell-cell spread of syncytial isolates was dependent upon nectin-1alpha expression and occurred through a nectin-1-dependent mechanism. Taken together, our results indicate that nectin-1-dependent events occurring at the plasma membrane during cell-cell spread were equivalent to those for virus entry.

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.

Epitope mapping of herpes simplex virus type 2 gH/gL defines distinct antigenic sites, including some associated with biological function

The gH/gL complex plays an essential role in virus entry and cell-cell spread of herpes simplex virus (HSV). Very few immunologic reagents were previously available to either identify important functional regions or gain information about structural features of this complex. Therefore, we generated and characterized a panel of 31 monoclonal antibodies (MAbs) against HSV type 2 (HSV-2) gH/gL. Fourteen MAbs bound to a conformation-dependent epitope of the gH2/gL2 complex, and all blocked virus spread. The other 17 MAbs recognized linear epitopes of gH (12) or gL (5). Interestingly, two of the gL MAbs and six of the gH MAbs were type common. Overlapping synthetic peptides were used to map MAbs against linear epitopes. These data, along with results of competition analyses and functional assays, assigned the MAbs to groups representing eight distinct antigenic sites on gH (I to VIII) and three sites on gL (A, B, and C). Of most importance, the MAbs with biological activity mapped either to site I of gH2 (amino acids 19 to 38) or to sites B and C of gL2 (residues 191 to 210). Thus, these MAbs constitute a novel set of reagents, including the first such reagents against gH2 and gL2 as well as some that recognize both serotypes of each protein. Several recognize important functional domains of gH2, gL2, or the complex. We suggest a common grouping scheme for all of the known MAbs against gH/gL of both HSV-1 and HSV-2.

Comparative analysis of Marek’s disease virus (MDV) glycoprotein-, lytic antigen pp38- and transformation antigen Meq-encoding genes: association of meq mutations with MDVs of high virulence

Marek's disease (MD) is a highly contagious lymphoproliferative and demyelinating disorder of chickens. MD is caused by Marek's disease virus (MDV), a cell-associated, acute-transforming alphaherpesvirus. For three decades, losses to the poultry industry due to MD have been greatly limited through the use of live vaccines. MDV vaccine strains are comprised of antigenically related, apathogenic MDVs originally isolated from chickens (MDV-2), turkeys (herpesvirus of turkeys, HVT) or attenuated-oncogenic strains of MDV-1 (CVI-988). Since the inception of high-density poultry production and MD vaccination, there have been two discernible increases in the virulence of MDV field strains. Our objectives were to determine if common mutations in the major glycoprotein genes, a major lytic antigen phosphoprotein 38 (pp38) or a major latency/transformation antigen Meq (Marek's EcoRI-Q-encoded protein) were associated with enhanced MDV virulence. To address this, we cloned and sequenced the major surface glycoprotein genes (gB, gC, gD, gE, gH, gI, and gL) of five MDV strains that were representative of the virulent (v), very virulent (vv) and very virulent plus (vv+) pathotypes of MDV. We found no consistent mutations in these genes that correlated strictly with virulence level. The glycoprotein genes most similar among MDV-1, MDV-2 and HVT (gB and gC, approximately 81 and 75%, respectively) were among the most conserved across pathotype. We found mutations mapping to the putative signal cleavage site in the gL genes in four out of eleven vv+MDVs, but this mutation was also identified in one vvMDV (643P) indicating that it did not correlate with enhanced virulence. In further analysis of an additional 12 MDV strains, we found no gross polymorphism in any of the glycoprotein genes. Likewise, by PCR and RFLP analysis, we found no polymorphism at the locus encoding the pp38 gene, an early lytic-phase gene associated with MDV replication. In contrast, we found distinct mutations in the latency and transformation-associated Marek's EcoRI-Q-encoded protein, Meq. In examination of the DNA and deduced amino acid sequence of meq genes from 26 MDV strains (9 m/vMDV, 5 vvMDV and 12 vv+MDVs), we found distinct polymorphism and point mutations that appeared to correlate with virulence. Although a complex trait like MDV virulence is likely to be multigenic, these data describe the first sets of mutations that appear to correlate with MDV virulence. Our conclusion is that since Meq is expressed primarily in the latent/transforming phase of MDV infection, and is not encoded by MDV-2 or HVT vaccine viruses, the evolution of MDV virulence may be due to selection on MDV-host cell interactions during latency and may not be mediated by the immune selection against virus lytic antigens such as the surface glycoproteins.

Coexpression of UL20p and gK inhibits cell-cell fusion mediated by herpes simplex virus glycoproteins gD, gH-gL, and wild-type gB or an endocytosis-defective gB mutant and downmodulates their cell surface expression

Syncytium formation in cells that express herpes simplex virus glycoprotein B (gB), gD, gH, and gL is blocked by gK (E. Avitabile, G. Lombardi, and G. Campadelli-Fiume, J. Virol. 77:6836-6844, 2003). Here, we report the results of two series of experiments. First, UL20 protein (UL20p) expression weakly inhibited cell-cell fusion. Coexpression of UL20p and gK drastically reduced fusion in a cell-line-dependent manner, with the highest inhibition in BHK cells. Singly expressed UL20p and gK localized at the endoplasmic reticulum and nuclear membranes. When they were coexpressed, both proteins relocalized to the Golgi apparatus. Remarkably, in cells that coexpressed UL20p and gK, the antifusion activity correlated with a downmodulation of gD, gB, gH, and gL cell surface expression. Second, gB(Delta867) has a partial deletion in the cytoplasmic tail that removed endocytosis motifs. Whereas wild-type (wt) gB was internalized in vesicles lined with the endosomal marker Rab5, gB(delta867) was not internalized, exhibited enhanced cell surface expression, and was more efficient in mediating cell-cell fusion than wt gB. The antifusion activity of UL20p and gK was also exerted when gB(delta867) replaced wt gB in the cell fusion assay. These studies show that the gB C tail carries a functional endocytosis motif(s) and that the removal of the motif correlated with increased gB surface expression and increased fusion activity. We conclude that cell-cell fusion in wt-virus-infected cells is negatively controlled by at least two mechanisms. The novel mechanism described here involves the concerted action of UL20p and gK and correlates with a moderate but consistent reduction in the cell surface expression of the fusion glycoproteins. This mechanism is independent of the one exerted through endocytosis-mediated downmodulation of gB from the plasma membrane.

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.

Fusion activity of lipid-anchored envelope glycoproteins of herpes simplex virus type 1

Expression of the herpes simplex virus type 1 (HSV-1) glycoproteins gB, gD, gH, and gL is necessary and sufficient to cause cell fusion. To identify the requirements for a membrane-spanning domain in HSV-1 glycoprotein-induced cell fusion, we created gB, gD, and gH mutants with transmembrane and cytoplasmic domains replaced by a glycosylphosphatidylinositol (gpi)-addition sequence. The corresponding gBgpi, gDgpi, and gHgpi proteins were expressed with wild-type efficiency at the cell surface and were linked to the plasma membrane via a gpi anchor. The gDgpi mutant promoted cell fusion near wild-type gD levels when co-expressed with gB, gH, and gL in a cell-mixing fusion assay, indicating that the gD transmembrane and cytoplasmic domains were not required for fusion activity. A plasma membrane link was required for fusion because a gD mutant lacking a transmembrane and cytoplasmic domain was nonfunctional for fusion. The gDgpi mutant was also able to cooperate with wild-type gB, gH, and gL to form syncytia, albeit at a size smaller than those formed in the wild-type situation. The gBgpi and gHgpi mutants were unable to promote fusion when expressed with the other wild-type viral glycoproteins, highlighting the requirement of the specific transmembrane and cytoplasmic domains for gB and gH function.

Herpes simplex virion entry into and intracellular transport within mammalian cells

Alphaherpesviruses, membrane-enveloped DNA viruses that are responsible for a host of human ailments, bind to, enter and are directly targeted to specific intracellular domains within their mammalian host cells. This review emphasizes recent work on the best studied of the alphaherpesviruses, Herpes simplex virus type 1 (HSV1). One area of focus is on recent work that has identified viral glycoproteins that are important in binding and internalization of the virus to the host cell. Complementary work on the receptors for those viral glycoproteins that reside on the host cell surface is also presented, with some discussion of how receptor variety might lead to the tissue tropism demonstrated by alphaherpes viruses. An additional area of focus in this review is how HSV uses the host cell transport systems to achieve intracellular targeting of the incoming virion toward the cell nucleus, and, after production of newly synthesized and assembled viral progeny, targeting them toward the plasma membrane for release.

Herpes simplex virus triggers activation of calcium-signaling pathways

The cellular pathways required for herpes simplex virus (HSV) invasion have not been defined. To test the hypothesis that HSV entry triggers activation of Ca2+-signaling pathways, the effects on intracellular calcium concentration ([Ca2+]i) after exposure of cells to HSV were examined. Exposure to virus results in a rapid and transient increase in [Ca2+]i. Pretreatment of cells with pharmacological agents that block release of inositol 1,4,5-triphosphate (IP3)-sensitive endoplasmic reticulum stores abrogates the response. Moreover, treatment of cells with these pharmacological agents inhibits HSV infection and prevents focal adhesion kinase (FAK) phosphorylation, which occurs within 5 min after viral infection. Viruses deleted in glycoprotein L or glycoprotein D, which bind but do not penetrate, fail to induce a [Ca2+]i response or trigger FAK phosphorylation. Together, these results support a model for HSV infection that requires activation of IP3-responsive Ca2+-signaling pathways and that is associated with FAK phosphorylation. Defining the pathway of viral invasion may lead to new targets for anti-viral therapy.

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.

Herpes simplex virus glycoprotein K, but not its syncytial allele, inhibits cell-cell fusion mediated by the four fusogenic glycoproteins, gD, gB, gH, and gL

A Myc epitope was inserted at residue 283 of herpes simplex virus type 1 (HSV-1) glycoprotein K (gK), a position previously shown not to interfere with gK activity. The Myc-tagged gK localized predominantly to the endoplasmic reticulum, both in uninfected and in HSV-infected cells. gK, coexpressed with the four HSV fusogenic glycoproteins, gD, gB, gH, and gL, inhibited cell-cell fusion. The effect was partially dose dependent and was observed both in baby hamster kidney (BHK) and in Vero cells, indicating that the antifusion activity of gK may be cell line independent. The antifusion activity of gK did not require viral proteins other than the four fusogenic glycoproteins. A syncytial (syn) allele of gK (syn-gK) carrying the A40V substitution present in HSV-1(MP) did not block fusion to the extent seen with the wild-type (wt) gK, indicating that the syn mutation ablated, at least in part, the antifusogenic activity of wt gK. We conclude that gK is part of the mechanism whereby HSV negatively regulates its own fusion activity. Its effect accounts for the notion that cells infected with wt HSV do not fuse with adjacent, uninfected cells into multinucleated giant cells or syncytia. gK may also function to preclude fusion between virion envelope and the virion-encasing vesicles during virus transport to the extracellular compartment, thus preventing nucleocapsid de-envelopment in the cytoplasm.

Human herpesvirus 6 variant A glycoprotein H-glycoprotein L-glycoprotein Q complex associates with human CD46

Human CD46 is a cellular receptor for human herpesvirus 6 (HHV-6). Virus entry into host cells requires a glycoprotein H (gH)-glycoprotein L (gL) complex. We show that the CD46 ectodomain blocked HHV-6 infection and bound a complex of gH-gL and the 80-kDa U100 gene product, designated glycoprotein Q, indicating that the complex is a viral ligand for CD46.

The interaction between dendritic cells and herpes simplex virus-1

Dendritic cells (DCs) are the most potent antigen-presenting cells, because they are also able to induce native T cells. Thus they are crucial in the induction of antiviral immune responses. Several viral immune escape mechanisms have been described; here we concentrate on the interaction between DCs and herpes simplex virus type 1 (HSV-1). DCs can be infected by HSV-1; however, only immature DCs generate infectious viral particles, whereas mature DCs do not support virus production and only immediate-early and early viral transcripts are generated. To induce potent immune responses DCs must mature. Interestingly, HSV-1 interferes with this maturation process, thus inhibiting antiviral T cell stimulation. Furthermore, HSV-1 strongly interferes with DC-mediated T cell proliferation. A striking finding was the complete degradation of CD83, the best-known marker for mature DC, after HSV-1 infection in lysosomal compartments. This CD83 degradation coincided with a clearly reduced T cell stimulation representing an additional new escape strategy. The functional role and the importance of CD83 are discussed in detail.

Varicella-zoster Virus gB and gE coexpression, but not gB or gE alone, leads to abundant fusion and syncytium formation equivalent to those from gH and gL coexpression

Varicella-zoster virus (VZV) is distinguished from herpes simplex virus type 1 (HSV-1) by the fact that cell-to-cell fusion and syncytium formation require only gH and gL within a transient-expression system. In the HSV system, four glycoproteins, namely, gH, gL, gB, and gD, are required to induce a similar fusogenic event. VZV lacks a gD homologous protein. In this report, the role of VZV gB as a fusogen was investigated and compared to the gH-gL complex. First of all, the VZV gH-gL experiment was repeated under a different set of conditions; namely, gH and gL were cloned into the same vaccinia virus (VV) genome. Surprisingly, the new expression system demonstrated that a recombinant VV-gH+gL construct was even more fusogenic than seen in the prior experiment with two individual expression plasmids containing gH and gL (K. M. Duus and C. Grose, J. Virol. 70:8961-8971, 1996). Recombinant VV expressing VZV gB by itself, however, effected the formation of only small syncytia. When VZV gE and gB genes were cloned into one recombinant VV genome and another fusion assay was performed, extensive syncytium formation was observed. The degree of fusion with VZV gE-gB coexpression was comparable to that observed with VZV gH-gL: in both cases, >80% of the cells in a monolayer were fused. Thus, these studies established that VZV gE-gB coexpression greatly enhanced the fusogenic properties of gB. Control experiments documented that the fusion assay required a balance between the fusogenic potential of the VZV glycoproteins and the fusion-inhibitory effect of the VV infection itself.

An alpha-helical domain within the carboxyl terminus of herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is associated with cell fusion and resistance to heparin inhibition of cell fusion

Previous studies from our laboratory indicated that a 28-amino-acid carboxyl-terminal truncation of gB caused extensive virus-induced cell fusion (Baghian et al., 1993, J Virol 67, 2396-2401). We tested the ability of additional truncations and mutations within gB to cause cell fusion in the recently established virus-free cell fusion assay (Turner et al., 1998, J. Virol. 72, 873-875). Deletion of the carboxyl-terminal 28 amino acids of gB (gBDelta28), which removed part of the predicted alpha-helical structure H17b, caused extensive cell fusion. A gB truncation specified by gBDelta36, which removed the entire H17b domain, caused as much cell fusion as the gBDelta28 truncation. Similarly, gB(A874P) containing a substitution of an Ala with Pro within H17b caused cell fusion. Heparin, a gB-specific inhibitor of virus-induced cell fusion, inhibited both wild-type gB and gB(syn3)-mediated cell fusion. In contrast, fusion of cells transfected with gB(Delta28), gB(Delta36), or gB(A874P) was resistant to heparin inhibition of cell fusion. We concluded the following: (1) The predicted alpha-helical structure of H17b within the carboxyl terminus of gB is involved in both virus-induced and virus-free cell fusion. (2) Heparin is a specific inhibitor of gB-mediated fusion in both systems. (3) Resistance to heparin inhibition of gB-mediated cell fusion is associated with the predicted alpha-helical structure H17b within the carboxyl terminus of gB.

Parameters that determine virus adsorption kinetics: toward the design of better infection strategies for the insect cell - baculovirus expression system

High productivities of bioprocesses involving viruses can be attained through infection strategies based on adequate understanding of parameters ruling cell-virus interactions. Two factors that affect virus binding and infection efficiency were studied: the utilization of an adsorption step, where infection volume at constant cell/virus ratio was varied; and the concentration of fetal bovine serum (FBS). The insect cell-baculovirus expression system and recombinant protein VP4 of rotavirus were used as models. Virus binding kinetics were adequately described by a sigmoidal response curve. The adsorption step, with or without FBS, increased virus attachment rate, whereas it increased bound virus at equilibrium only in FBS-free infections. A first-order dependance of virus attachment on cell concentration was found above 5 x 10(6) cell/mL in infections with 10% FBS. Addition of 10% FBS decreased maximum bound baculovirus and binding rate by as much as 3 times and VP4 concentration up to 4 times. In contrast, heat inactivation of FBS increased bound virus from 20% to over 90%, an increase of 1.4 times compared to FBS-free infections. A direct linear relation was found between attached virus and maximum VP4 concentration for the different FBS concentrations tested, indicating that baculovirus-cell attachment was the limiting step for recombinant protein production. Interestingly, virus progeny accumulation was not affected by differences in virus binding. In conclusion, infection strategies aimed at increasing productivity should be performed at high cell concentrations and without FBS, or with heat-inactivated FBS.

Herpes simplex virus type 1 entry is inhibited by the cobalt chelate complex CTC-96

The CTC series of cobalt chelates display in vitro and in vivo activity against herpes simplex virus types 1 and 2 (HSV-1 and HSV-2). The experiments described here identify the stage in the virus life cycle where CTC-96 acts and demonstrate that the drug inhibits infection of susceptible cells. CTC-96 at 50 microg/ml has no effect on adsorption of virions to Vero cell monolayers. Penetration assays reveal that CTC-96 inhibits entry of the virus independent of gC and cellular entry receptors. This observation was supported by the failure to detect the accumulation of virus-specified proteins and alpha mRNA transcripts when CTC-96 is present at the onset of infection. Moreover, virion-associated alphaTIF does not accumulate in the nucleus of cells infected in the presence of CTC-96. CTC-96 targets the initial fusion event between the virus and the cell and also inhibits cell-to-cell spread and syncytium formation. Furthermore, CTC-96 inhibits plaque formation by varicella-zoster virus and vesicular stomatitis virus as efficiently as by HSV-1. Collectively, these experiments suggest that CTC-96 is a broad-spectrum inhibitor of infection by enveloped viruses and that it inhibits HSV-1 infection at the point of membrane fusion independent of the type of virus and cellular receptors present.

Cell fusion induced by herpes simplex virus glycoproteins gB, gD, and gH-gL requires a gD receptor but not necessarily heparan sulfate

To characterize cellular factors required for herpes simplex virus type 1 (HSV-1)-induced cell fusion, we used an efficient and quantitative assay relying on expression of HSV-1 glycoproteins in transfected cells. We showed the following: (1) Cell fusion depended not only on expression of four viral glycoproteins (gB, gD, and gH-gL), as previously shown, but also on expression of cell surface entry receptors specific for gD. (2) Cell fusion required expression of all four glycoproteins in the same cell. (3) Heparan sulfate was not required for cell fusion. (4) Coexpression of receptor with the four glycoproteins in the same cell reduced fusion activity, indicating that interaction of gD and receptor can limit polykaryocyte formation. Overall, the viral and cellular determinants of HSV-1-induced cell fusion are similar to those for viral entry, except that HSV-1 entry is significantly enhanced by binding of virus to cell surface heparan sulfate.

Mutations in herpes simplex virus glycoprotein D distinguish entry of free virus from cell-cell spread

Herpes simplex virus type 1 (HSV-1) glycoprotein D (gD) is an essential component of the entry apparatus that is responsible for viral penetration and subsequent cell-cell spread. To test the hypothesis that gD may serve distinguishable functions in entry of free virus and cell-cell spread, mutants were selected for growth on U(S)11cl19.3 cells, which are resistant to both processes due to the lack of a functional gD receptor, and then tested for their ability to enter as free virus and to spread from cell to cell. Unlike their wild-type parent, HSV-1(F), the variants that emerged from this selection, which were named SP mutants, are all capable of forming macroscopic plaques on the resistant cells. This ability is caused by a marked increase in cell-cell spread without a concomitant increase in efficiency of entry of free virus. gD substitutions that arose within these mutants are sufficient to mediate cell-cell spread in U(S)11cl19.3 cells but are insufficient to overcome the restriction to entry of free virions. These results suggest that mutations in gD (i) are sufficient but not necessary to overcome the block to cell-cell spread exhibited by U(S)11cl19.3 cells and (ii) are insufficient to mediate entry of free virus in the same cells.

Characterization of cell-cell fusion mediated by herpes simplex virus 2 glycoproteins gB, gD, gH and gL in transfected cells

The mechanisms by which herpes simplex viruses (HSV) mediate fusion between their envelope and the plasma membrane during entry into cells, and between the plasma membranes of adjacent infected and uninfected cells to form multinucleated giant cells, are poorly understood. Four viral glycoproteins (gB, gD, gH and gL) are required for virus-cell fusion, whereas these plus several others are required for cell-cell fusion (syncytium formation). A better understanding would be aided by the availability of a model system, whereby fusion could be induced with a minimal set of proteins, in the absence of infection. A suitable system has now been developed for HSV-2, using transfected COS7, 293 or HEp-2 cells. Insofar as the minimal set of HSV-2 proteins required to cause cell-cell fusion in this system is gB, gD, gH and gL, it would appear to resemble virus-cell fusion rather than syncytium formation. However, the ability of a mutation in gB to enhance the fusion of both transfected cells and infected cells, while having no effect on virus-cell fusion, points to the opposite conclusion. The differential effects of a panel of anti-HSV antibodies, and of the fusion-inhibitor cyclosporin A, confirm that the fusion of transfected cells shares some properties with virus-cell fusion and others with syncytium formation. It may therefore prove useful for determining how these processes differ, and for testing the hypothesis that some viral proteins prevent membrane fusion until the appropriate point in the virus life-cycle, with other proteins then overcoming this block.

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.

Domains of glycoprotein H of herpes simplex virus type 1 involved in complex formation with glycoprotein L

The complex formation between glycoproteins H (gH) and L (gL) of herpes simplex virus type 1 (HSV-1) was studied by using five recombinant baculoviruses expressing open reading frames that contain deletions in the coding region of the extracellular domain of gH. In addition, the gH-deletion mutants contained a C-terminal tag. Complex formation of gL and the gH-deletion mutants was studied by immunoprecipitations with anti-tag monoclonal antibody (MAb) A16 and with the gH-specific MAbs 37S, 46S, and 52S. All gH-deletion mutants were complexed to gL when analyzed by MAb A16. MAb 37S precipitated complexes between gL and the two gH-deletion mutants that contain the epitope of this MAb. When the gH conformation-dependent MAbs 46S and 52S were used, gL was coprecipitated together with the gH-deletion mutant lacking amino acids 31-299, but gL was not coprecipitated with the gH-deletion mutant lacking amino acids 31-473. The data from the precipitation studies do allow at least two interpretations. There is either one site for gL binding on gH (residue 300-473) or gL contacts multiple regions of gH. We were unable to demonstrate gL-dependent cell surface expression of either of the gH-deletion mutants. This suggests that the coassociation of gH with gL is necessary but not sufficient for transport of gH to the cell surface.

Recognition of herpes simplex virus type 2 tegument proteins by CD4 T cells infiltrating human genital herpes lesions

The local cellular immune response to herpes simplex virus (HSV) is important in the control of recurrent HSV infection. The antiviral functions of infiltrating CD4-bearing T cells may include cytotoxicity, inhibition of viral growth, lymphokine secretion, and support of humoral and CD8 responses. The antigens recognized by many HSV-specific CD4 T cells localizing to genital HSV-2 lesions are unknown. T cells recognizing antigens encoded within map units 0. 67 to 0.73 of HSV DNA are frequently recovered from herpetic lesions. Expression cloning with this region of DNA now shows that tegument protein VP22 and the viral dUTPase, encoded by genes UL49 and UL50, respectively, are T-cell antigens. Separate epitopes in VP22 were defined for T-cell clones from each of three patients. Reactivity with the tegument protein encoded by UL21 was identified for an additional patient. Three new epitopes were identified in VP16, a tegument protein associated with VP22. Some tegument-specific CD4 T-cell clones exhibited cytotoxic activity against HSV-infected cells. These results suggest that herpes simplex tegument proteins are processed for antigen presentation in vivo and are possible candidate compounds for herpes simplex vaccines.

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.

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.

The gH-gL complex of herpes simplex virus (HSV) stimulates neutralizing antibody and protects mice against HSV type 1 challenge

The herpes simplex virus type 1 (HSV-1) gH-gL complex which is found in the virion envelope is essential for virus infectivity and is a major antigen for the host immune system. However, little is known about the precise role of gH-gL in virus entry, and attempts to demonstrate the immunologic or vaccine efficacy of gH and gL separately or as the gH-gL complex have not succeeded. We constructed a recombinant mammalian cell line (HL-7) which secretes a soluble gH-gL complex, consisting of gH truncated at amino acid 792 (gHt) and full-length gL. Purified gHt-gL reacted with gH- and gL-specific monoclonal antibodies, including LP11, which indicates that it retains its proper antigenic structure. Soluble forms of gD (gDt) block HSV infection by interacting with specific cellular receptors. Unlike soluble gD, gHt-gL did not block HSV-1 entry into cells, nor did it enhance the blocking capacity of gD. However, polyclonal antibodies to the complex did block entry even when added after virus attachment. In addition, these antibodies exhibited high titers of complement-independent neutralizing activity against HSV-1. These sera also cross-neutralized HSV-2, albeit at low titers, and cross-reacted with gH-2 present in extracts of HSV-2-infected cells. To test the potential for gHt-gL to function as a vaccine, BALB/c mice were immunized with the complex. As controls, other mice were immunized with gD purified from HSV-infected cells or were sham immunized. Sera from the gD- or gHt-gL-immunized mice exhibited high titers of virus neutralizing activity. Using a zosteriform model of infection, we challenged mice with HSV-1. All animals showed some evidence of infection at the site of virus challenge. Mice immunized with either gD or gHt-gL showed reduced primary lesions and exhibited no secondary zosteriform lesions. The sham-immunized control animals exhibited extensive secondary lesions. Furthermore, mice immunized with either gD or gHt-gL survived virus challenge, while many control animals died. These results suggest that gHt-gL is biologically active and may be a candidate for use as a subunit vaccine.

Glycoproteins gB, gD, and gHgL of herpes simplex virus type 1 are necessary and sufficient to mediate membrane fusion in a Cos cell transfection system

Herpes simplex virus type 1 glycoproteins gB, gD, and gHgL were expressed by transient transfection of Cos cells. Polykaryocyte formation above the background level seen in untransfected controls was observed only if all three components were expressed. Thus, gB, gD, and gHgL are necessary and sufficient to induce membrane fusion.

Pseudorabies virus glycoprotein L is necessary for virus infectivity but dispensable for virion localization of glycoprotein H

Herpesviruses contain a number of envelope glycoproteins which play important roles in the interaction between virions and target cells. Although several glycoproteins are not present in all herpesviruses, others, including glycoproteins H and L (gH and gL), are conserved throughout the Herpesviridae. To elucidate common properties and differences in herpesvirus glycoprotein function, corresponding virus mutants must be constructed and analyzed in different herpesvirus backgrounds. Analysis of gH- mutants of herpes simplex virus type 1 (HSV-1) and pseudorabies virus (PrV) showed that in both viruses gH is essential for penetration and cell-to-cell spread and that its presence is required for virion localization of gL. Since gH homologs are found complexed with gL, it was of interest to assess the phenotype of gL- mutant viruses. By using this approach, HSV-1 gL has been shown to be required for entry and for virion localization of gH (C. Roop, L. Hutchinson, and D. Johnson, J. Virol. 67:2285-2297, 1993). To examine whether a similar phenotype is associated with lack of gL in another alphaherpesvirus, PrV, we constructed two independent gL- PrV mutants by insertion and deletion-insertion mutagenesis. The salient findings are as follows: (i) PrV gL is required for penetration of virions and cell-to-cell spread; (ii) unlike HSV-1, PrV gH is incorporated into the virion in the absence of gL; (iii) virion localization of gH in the absence of gL is not sufficient for infectivity; (iv) in the absence of gL, N-glycans on PrV gH are processed to a greater extent than in the presence of gL, indicating masking of N-glycans by association with gL; and (v) an anti-gL polyclonal antiserum is able to neutralize virion infectivity but did not inhibit cell-to-cell spread. Thus, whereas PrV gL is essential for virus replication, as is HSV-1 gL, gL- PrV mutants exhibit properties strikingly different from those of HSV-1. In conclusion, our data show an important functional role for PrV gL in the viral entry process, which is not explained by a chaperone-type mechanism in gH maturation and processing.