Assembly and organization of glycoproteins B, C, D, and H in herpes simplex virus type 1 particles lacking individual glycoproteins: No evidence for the formation of a complex of these molecules

Toward the Eradication of Herpes Simplex Virus: Vaccination and Beyond

Herpes simplex virus (HSV) has coevolved with Homo sapiens for over 100,000 years, maintaining a tenacious presence by establishing lifelong, incurable infections in over half the human population. As of 2024, an effective prophylactic or therapeutic vaccine for HSV remains elusive. In this review, we independently screened PubMed, EMBASE, Medline, and Google Scholar for clinically relevant articles on HSV vaccines. We identified 12 vaccines from our literature review and found promising candidates across various classes, including subunit vaccines, live vaccines, DNA vaccines, and mRNA vaccines. Notably, several vaccines-SL-V20, HF10, VC2, and mRNA-1608-have shown promising preclinical results, suggesting that an effective HSV vaccine may be within reach. Additionally, several other vaccines such as GEN-003 (a subunit vaccine from Genocea), HerpV (a subunit vaccine from Agenus), 0ΔNLS/RVx201 (a live-attenuated replication-competent vaccine from Rational Vaccines), HSV 529 (a replication-defective vaccine from Sanofi Pasteur), and COR-1 (a DNA-based vaccine from Anteris Technologies) have demonstrated potential in clinical trials. However, GEN-003 and HerpV have not advanced further despite promising results. Continued progress with these candidates brings us closer to a significant breakthrough in preventing and treating HSV infections.

Herpes Simplex Virus Organizes Cytoplasmic Membranes To Form a Viral Assembly Center in Neuronal Cells

Herpes simplex virus (HSV) is a neuroinvasive virus that has been used as a model organism for studying common properties of all herpesviruses. HSV induces host organelle rearrangement and forms multiple, dispersed assembly compartments in epithelial cells, which complicates the study of HSV assembly. In this study, we show that HSV forms a visually distinct unitary cytoplasmic viral assembly center (cVAC) in both cancerous and primary neuronal cells that concentrates viral structural proteins and is a major site of capsid envelopment. The HSV cVAC also concentrates host membranes that are important for viral assembly, such as Golgi- and recycling endosome-derived membranes. Finally, we show that HSV cVAC formation and/or maintenance depends on an intact microtubule network and a viral tegument protein, pUL51. Our observations suggest that the neuronal cVAC is a uniquely useful model to study common herpesvirus assembly pathways and cell-specific pathways for membrane reorganization.IMPORTANCE Herpesvirus particles are complex and contain many different proteins that must come together in an organized and coordinated fashion. Many viruses solve this coordination problem by creating a specialized assembly factory in the host cell, and the formation of such factories provides a promising target for interfering with virus production. Herpes simplex virus 1 (HSV-1) infects several types of cells, including neurons, but has not previously been shown to form such an organized factory in the nonneuronal cells in which its assembly has been best studied. Here, we show that HSV-1 forms an organized assembly factory in neuronal cells, and we identify some of the viral and host cell factors that are important for its formation.

Herpesvirus membrane fusion - a team effort

One of the essential steps in every viral 'life' cycle is entry into the host cell. Membrane-enveloped viruses carry dedicated proteins to catalyse the fusion of the viral and cellular membrane. Herpesviruses feature a set of essential, structurally diverse glycoproteins on the viral surface that form a multicomponent fusion machinery, necessary for the entry mechanism. For Herpes simplex virus 1, these essential glycoproteins are gD, gH, gL and gB. In this review we describe the functions of the individual components, the potential interactions between them as well as the influence of post-translational modifications on the fusion mechanism.

Cellular Protein WDR11 Interacts with Specific Herpes Simplex Virus Proteins at the trans-Golgi Network To Promote Virus Replication

It has recently been proposed that the herpes simplex virus (HSV) protein ICP0 has cytoplasmic roles in blocking antiviral signaling and in promoting viral replication in addition to its well-known proteasome-dependent functions in the nucleus. However, the mechanisms through which it produces these effects remain unclear. While investigating this further, we identified a novel cytoplasmic interaction between ICP0 and the poorly characterized cellular protein WDR11. During an HSV infection, WDR11 undergoes a dramatic change in localization at late times in the viral replication cycle, moving from defined perinuclear structures to a dispersed cytoplasmic distribution. While this relocation was not observed during infection with viruses other than HSV-1 and correlated with efficient HSV-1 replication, the redistribution was found to occur independently of ICP0 expression, instead requiring viral late gene expression. We demonstrate for the first time that WDR11 is localized to the trans-Golgi network (TGN), where it interacts specifically with some, but not all, HSV virion components, in addition to ICP0. Knockdown of WDR11 in cultured human cells resulted in a modest but consistent decrease in yields of both wild-type and ICP0-null viruses, in the supernatant and cell-associated fractions, without affecting viral gene expression. Although further study is required, we propose that WDR11 participates in viral assembly and/or secondary envelopment.

IMPORTANCE

While the TGN has been proposed to be the major site of HSV-1 secondary envelopment, this process is incompletely understood, and in particular, the role of cellular TGN components in this pathway is unknown. Additionally, little is known about the cellular functions of WDR11, although the disruption of this protein has been implicated in multiple human diseases. Therefore, our finding that WDR11 is a TGN-resident protein that interacts with specific viral proteins to enhance viral yields improves both our understanding of basic cellular biology as well as how this protein is co-opted by HSV.

VirD: a virion display array for profiling functional membrane proteins

To facilitate high-throughput biochemical analyses of membrane proteins, we have developed a novel display technology in a microarray format. Both single-pass (cluster of differentiation 4, CD4) and multiple-pass (G protein-coupled receptor 77, GPR77) human transmembrane proteins were engineered to be displayed in the membrane envelop of herpes simplex virions. These viruses produce large spherical virions displaying multiple copies of envelop proteins. Our aim was to engineer this virus to express these human proteins during the virus productive cycle and incorporate the human proteins into the virion during the assembly process. Another strategy presented includes engineering a fusion of glycoprotein C (gC), a major constituent of herpes simplex virus type 1 (HSV-1) virions, by hijacking the cis-acting signals to direct incorporation of the chimeric protein into the virion. The expression of the human proteins in infected cells, at the cell surface and in purified virions, is in the correct transmembrane orientation, and the proteins are biochemically functional. Purified virions printed on glass slides form a high-density Virion Display (VirD) Array, and the displayed proteins were demonstrated to retain their native conformations and interactions on the VirD Array judging by similar assays, such as antibody staining, as well as lectin and ligand binding. This method can be readily scaled or tailored for different modalities including a high-content, high-throughput platform for screening ligands and drugs of human membrane proteins.

Glycoprotein N of human cytomegalovirus protects the virus from neutralizing antibodies

Herpes viruses persist in the infected host and are transmitted between hosts in the presence of a fully functional humoral immune response, suggesting that they can evade neutralization by antiviral antibodies. Human cytomegalovirus (HCMV) encodes a number of polymorphic highly glycosylated virion glycoproteins (g), including the essential envelope glycoprotein, gN. We have tested the hypothesis that glycosylation of gN contributes to resistance of the virus to neutralizing antibodies. Recombinant viruses carrying deletions in serine/threonine rich sequences within the glycosylated surface domain of gN were constructed in the genetic background of HCMV strain AD169. The deletions had no influence on the formation of the gM/gN complex and in vitro replication of the respective viruses compared to the parent virus. The gN-truncated viruses were significantly more susceptible to neutralization by a gN-specific monoclonal antibody and in addition by a number of gB- and gH-specific monoclonal antibodies. Sera from individuals previously infected with HCMV also more efficiently neutralized gN-truncated viruses. Immunization of mice with viruses that expressed the truncated forms of gN resulted in significantly higher serum neutralizing antibody titers against the homologous strain that was accompanied by increased antibody titers against known neutralizing epitopes on gB and gH. Importantly, neutralization activity of sera from animals immunized with gN-truncated virus did not exhibit enhanced neutralizing activity against the parental wild type virus carrying the fully glycosylated wild type gN. Our results indicate that the extensive glycosylation of gN could represent a potentially important mechanism by which HCMV neutralization by a number of different antibody reactivities can be inhibited.

Herpes viruses are transmitted between individuals in cell free form and successful spread benefits from mechanisms that limit the loss of infectivity by the activity of virus neutralizing antibodies. Human cytomegalovirus (HCMV) is an important pathogen and understanding how the virus can evade antiviral antibodies may be clinically relevant. HCMV particles contain a number of highly polymorphic, extensively glycosylated envelope proteins, one of which is glycoprotein N (gN). This protein is essential for replication of HCMV. We have hypothesized that the extensive glycosylation of gN may serve as a tool to evade neutralization by antiviral antibodies. Recombinant viruses were generated expressing gN proteins with reduced glycan modification. The loss of glycan modification had no detectable influence on the in vitro replication of the respective viruses. However, the recombinant viruses containing under-glycosylated forms of gN were significantly more susceptible to neutralization by a diverse array of antibody reactivities. Immunization of mice with viruses carrying fewer glycan modification induced significantly higher antibody titers against the homologous virus; however, the neutralization titers against the fully glycosylated virions, were not enhanced. Our results indicate that glycosylation of gN of HCMV represents a potentially important mechanism for evasion of antibody-mediated neutralization by a number of different antibody specificities.

Glycoprotein M is important for the efficient incorporation of glycoprotein H–L into herpes simplex virus type 1 particles

Herpes simplex virus type 1 glycoprotein M (gM) is a type III membrane protein conserved throughout the family Herpesviridae. However, despite this conservation, gM is classed as a non-essential protein in most alphaherpesviruses. Previous data have suggested that gM is involved in secondary envelopment, although how gM functions in this process is unknown. Using transfection-based assays, we have previously shown that gM is able to mediate the internalization and subcellular targeting of other viral envelope proteins, suggesting a possible role for gM in localizing herpesvirus envelope proteins to sites of secondary envelopment. To investigate the role of gM in infected cells, we have now analysed viral envelope protein localization and virion incorporation in cells infected with a gM-deletion virus or its revertant. In the absence of gM expression, we observed a substantial inhibition of glycoprotein H–L (gH–L) internalization from the surface of infected cells. Although deletion of gM does not affect expression of gH and gL, virions assembled in the absence of gM demonstrated significantly reduced levels of gH–L, correlating with defects of the gM-negative virus in entry and cell-to-cell spread. These data suggest an important role of gM in mediating the specific internalization and efficient targeting of gH–L to sites of secondary envelopment in infected cells.

Binding of herpes simplex virus type-1 virions leads to the induction of intracellular signalling in the absence of virus entry

The envelope of HSV-1 contains a number of glycoproteins, four of which are essential for virus entry. Virus particles lacking gB, gD, gH or gL are entry-defective, although these viruses retain the ability to bind to the plasma membrane via the remaining glycoproteins. Soluble forms of gD have been shown to trigger the nuclear translocation of the NF-κB transcriptional complex in addition to stimulating the production of Type I interferon. By taking advantage of the entry-defective phenotype of glycoprotein-deficient HSV-1 virus particles, the results presented here show that binding of virions to cellular receptors on the plasma membrane is sufficient to stimulate a change in cellular gene expression. Preliminary microarray studies, validated by quantitative real-time PCR, identified the differential expression of cellular genes associated with the NF-κB, PI3K/Akt, Jak/Stat and related Jak/Src pathways by virions lacking gB or gH but not gD. Gene induction occurred at a few particles per cell, corresponding to physiological conditions during primary infection. Reporter assay studies determined that NF-κB transcriptional activity is stimulated within an hour of HSV-1 binding, peaks between two and three hours post-binding and declines to background levels by five hours after induction. The immediate, transient nature of these signalling events suggests that HSV-1 glycoproteins, particularly gD, may alter the cellular environment pre-entry so as to condition the cell for viral replication.

Bimolecular complementation reveals that glycoproteins gB and gH/gL of herpes simplex virus interact with each other during cell fusion

Herpes simplex virus entry into cells requires four glycoproteins, gB, gD, gH, and gL. Binding of gD to one of its receptors triggers steps requiring the core fusion proteins, gB and the gH/gL heterodimer. There is evidence that gH/gL initiates hemifusion of cells, but whether this complex interacts physically with gB to cause complete fusion is unknown. We used bimolecular complementation (BiMC) of enhanced yellow fluorescent protein (EYFP) to detect glycoprotein interactions during cell-cell fusion. The N- or C-terminal half of EYFP was fused to the C terminus of gD, gB, and gH to form six chimeric proteins (Dn, Dc, Bn, Bc, Hn, and Hc). BiMC was detected by confocal microscopy. Receptor-bearing (C10) cells cotransfected with Dn and Bc or Dn, Hc, and untagged gL exhibited EYFP fluorescence, indicative of interactions between gD and gB and between gD and gH/gL. EYFP complementation did not occur in cells transfected with gL, Bc, and Hn. However, when gD was coexpressed with these other three proteins, cell-cell fusion occurred and the syncytia exhibited bright EYFP fluorescence. To separate glycoprotein expression from fusion, we transfected C10 cells with gL, Bc, and Hn for 20 h and then added soluble gD to trigger fusion. We detected fluorescent syncytia within 10 min, and both their number and size increased with exposure time to gD. Thus, when gD binds its receptor, the core fusion machinery is triggered to form a multiprotein complex as a step in fusion and possibly virus entry.

Evidence for a multiprotein gamma-2 herpesvirus entry complex

Herpesviruses use multiple virion glycoproteins to enter cells. How these work together is not well understood: some may act separately or they may form a single complex. Murine gammaherpesvirus 68 (MHV-68) gB, gH, gL, and gp150 all participate in entry. gB and gL are involved in binding, gB and gH are conserved fusion proteins, and gp150 inhibits cell binding until glycosaminoglycans are engaged. Here we show that a gH-specific antibody coprecipitates gB and thus that gH and gB are associated in the virion membrane. A gH/gL-specific antibody also coprecipitated gB, implying a tripartite complex of gL/gH/gB, although the gH/gB association did not require gL. The association was also independent of gp150, and gp150 was not demonstrably bound to gB or gH. However, gp150 incorporation into virions was partly gL dependent, suggesting that it too contributes to a single entry complex. gp150- and gL- gp150- mutants bound better than the wild type to B cells and readily colonized B cells in vivo. Thus, gp150 and gL appear to be epithelial cell-adapted accessories of a core gB/gH entry complex. The cell binding revealed by gp150 disruption did not require gL and therefore seemed most likely to involve gB.

Two-color fluorescence analysis of individual virions determines the distribution of the copy number of proteins in herpes simplex virus particles

We present a single virion method to determine absolute distributions of copy number in the protein composition of viruses and apply it to herpes simplex virus type 1. Using two-color coincidence fluorescence spectroscopy, we determine the virion-to-virion variability in copy numbers of fluorescently labeled tegument and envelope proteins relative to a capsid protein by analyzing fluorescence intensity ratios for ensembles of individual dual-labeled virions and fitting the resulting histogram of ratios. Using EYFP-tagged capsid protein VP26 as a reference for fluorescence intensity, we are able to calculate the mean and also, for the first time to our knowledge, the variation in numbers of gD, VP16, and VP22 tegument. The measurement of the number of glycoprotein D molecules was in good agreement with independent measurements of average numbers of these glycoproteins in bulk virus preparations, validating the method. The accuracy, straightforward data processing, and high throughput of this technique make it widely applicable to the analysis of the molecular composition of large complexes in general, and it is particularly suited to providing insights into virus structure, assembly, and infectivity.

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 multipartite system that mediates entry of herpes simplex virus into the cell

The multipartite entry-fusion system of herpes simplex virus is made of a quartet of glycoproteins-gD, gB, gH.gL-and three alternative gD receptors, herpesvirus entry mediator (HVEM), nectin1 and modified sites on heparan sulphate. This multipartite system recapitulates the basic steps of virus-cell fusion, i.e. receptor recognition, triggering of fusion and fusion execution. Specifically, in addition to serving as the receptor-binding glycoprotein, gD triggers fusion through a specialised domain, named pro-fusion domain (PFD), located C-terminally in the ectodomain. In the unliganded gD the C-terminal region folds around the N-terminal region, such that gD adopts a closed autoinhibited conformation. In HVEM- and nectin1-bound gD the C-terminal region is displaced (opened conformation). gD is the tool for modification of HSV tropism, through insertion of ligands to heterologous tumour-specific receptors. It is discussed whether gD responds to the interaction with the natural and the heterologous receptors by adopting similar conformations, and whether the closed-to-open switch in conformation is a generalised mechanism of activation. A peculiar recombinant highlighted that the central Ig-folded core of gD may not encode executable functions for entry and that the 219-314 aa segment may be sufficient to trigger fusion. With respect to fusion execution, gB appears to be a prospective fusogen based on its coiled-coil trimeric structure, similar to that of another fusion glycoprotein. On the other hand, gH exhibits molecular elements typical of class 1 fusion glycoproteins, in particular heptad repeats and strong tendency to interact with lipids. Whether fusion execution is carried out by gB or gH.gL, or both glycoproteins in complex or sequentially remains to be determined.

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.

Molecule by molecule direct and quantitative counting of antibody-protein complexes in solution

We have used two-color fluorescence coincidence detection to directly count individual protein-antibody complexes of protein G or herpes simplex virus labeled with one or more red- and blue-excited antibodies. This allowed quantitative measurement of the concentration of the protein-antibody complexes over 3 orders of magnitude down to the femtomolar level. Single molecule measurements in diluted serum are also possible. The sample preparation is simple, takes place in solution, and requires no separation. Both the antibody affinity and complex dissociation rate are important in determining the sensitivity of the method. At present, the sensitivity limit of 50 fM is determined by the encounter rate of the labeled analyte with the probe volume. This method can be used to detect and quantitate proteins and to measure the stoichiometry, equilibrium constant, and dissociation rate of protein-protein complexes at low concentrations.

Herpes simplex virus entry mediator associates in infected cells in a complex with viral proteins gD and at least gH

We examined herpes simplex virus (HSV)-infected human HEp-2 cells or porcine cells that express herpes virus entry mediator (HVEM) for virus and receptor protein interactions. Antibody to HVEM, or its viral ligand gD, coimmunoprecipitated several similar proteins. A prominent 110-kDa protein that coprecipitated was identified as gH. The HVEM/gD/gH complex was detected with mild or stringent cell lysis conditions. It did not form in cells infected with HSV-1(KOS)Rid1 virus or with null virus lacking gD, gH, or gL. Thus, in cells a complex forms through physical associations of HVEM, gD, and at least gH.

Analysis of the requirement for glycoprotein m in herpes simplex virus type 1 morphogenesis

A mutant of herpes simplex virus type 1 lacking both glycoprotein M and glycoprotein E was marginally compromised in terms of its in vitro growth characteristics. This finding is in marked contrast to a similar mutant of the related alphaherpesvirus, pseudorabies virus (A. R. Brack, J. M. Dijkstra, H. Granzow, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 73:5364-5372, 1999), and suggests that the glycoprotein requirements for virion assembly may vary among different members of this family of viruses.

Analysis of the role of the membrane-spanning and cytoplasmic tail domains of herpes simplex virus type 1 glycoprotein D in membrane fusion

Glycoprotein D (gD) of herpes simplex virus type 1 is a type 1 membrane protein in the virus envelope that binds to receptor molecules on the cell surface and which induces cell-cell fusion when co-expressed with gB, gH and gL. A chimeric gD molecule in which the membrane anchor and cytoplasmic tail domains were replaced with analogous regions from the human CD8 molecule was as competent as wild-type gD at mediating membrane fusion and virus entry. However, when gD was tethered to the membrane by means of a glycosylphosphatidylinositol (gpi)-anchor sequence, which binds only to the outer leaflet of the lipid bilayer, it was unable to function in cell-cell fusion assays. This chimera was incorporated into virions as efficiently as wild-type gD and yet virus particles containing gpi-linked gD entered cells more slowly than virions containing wild-type gD in their envelopes, suggesting that gD must be anchored in both leaflets of a lipid bilayer for it to function in both cell fusion and virus entry.

The transmembrane domain and cytoplasmic tail of herpes simplex virus type 1 glycoprotein H play a role in membrane fusion

Herpes simplex virus glycoprotein H (gH) is one of the four virion envelope proteins which are required for virus entry and for cell-cell fusion in a transient system. In this report, the role of the transmembrane and cytoplasmic tail domains of gH in membrane fusion was investigated by generating chimeric constructs in which these regions were replaced with analogous domains from other molecules and by introducing amino acid substitutions within the membrane-spanning sequence. gH molecules which lack the authentic transmembrane domain or cytoplasmic tail were unable to mediate cell-cell fusion when coexpressed with gB, gD, and gL and were unable to rescue the infectivity of a gH-null virus as efficiently as a wild-type gH molecule. Many amino acid substitutions of specific amino acid residues within the transmembrane domain also affected cell-cell fusion, in particular, those introduced at a conserved glycine residue. Some gH mutants that were impaired in cell-cell fusion were nevertheless able to rescue the infectivity of a gH-negative virus, but these pseudotyped virions entered cells more slowly than wild-type virions. These results indicate that the fusion event mediated by the coexpression of gHL, gB, and gD in cells shares common features with the fusion of the virus envelope with the plasma membrane, they point to a likely role for the membrane-spanning and cytoplasmic tail domains of gH in both processes, and they suggest that a conserved glycine residue in the membrane-spanning sequence is crucial for efficient fusion.

Restoration of function of carboxy-terminally truncated pseudorabies virus glycoprotein B by point mutations in the ectodomain

Glycoprotein B (gB) of pseudorabies virus (PrV) is essential for virus entry into target cells and direct viral cell-to-cell spread. Recently, we described a carboxy-terminally truncated derivative of PrV gB, gB-007, which was inefficiently incorporated into virions, was unable to complement infectivity, but was fully capable of restoring direct viral cell-to-cell spread of gB-negative PrV (R. Nixdorf, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 74:7137-7145, 2000). Since recombinant PrV-007, which expresses gB-007 instead of wild-type gB, was able to spread directly from cell to cell, we attempted to obtain compensatory mutations leading to restoration of the entry defect by performing serial passages in cell culture. This procedure has previously been used to successfully restore entry defects in gD- or gL-deficient PrV mutants. From an initial titer of 100 PFU per ml in the supernatant, titers increased, reaching wild-type levels of up to 10(7) PFU after ca. 20 passages. One single-plaque isolate of the passaged mutant, designated PrV-007Pass, was further characterized. PrV-007Pass gB was efficiently incorporated into the viral envelope and restored infectivity to a gB-negative PrV mutant, PrV-gB(-). Interestingly, localization of PrV-007Pass gB in the plasma membrane was similar to that of PrV-007. In contrast, wild-type gB is mainly found in intracellular vesicles. Marker rescue experiments and trans-complementation assays demonstrated the presence of compensatory mutations within the gB gene of PrV-007Pass. DNA sequencing revealed two point mutations in the gB open reading frame of PrV-007Pass, resulting in amino acid substitutions at positions 305 and 744 of gB, both of which are required for compensation of the defect in PrV-007. Our data again demonstrate the power of reversion analysis of herpesviruses and suggest that cytosolic and ectodomains play a role in incorporation of gB into virions.

Activation of interferon response factor-3 in human cells infected with herpes simplex virus type 1 or human cytomegalovirus

Activation of cellular interferon-stimulated genes (ISGs) after infection with herpes simplex virus type 1 (HSV-1) or human cytomegalovirus (HCMV) was investigated. The level of ISG54-specific RNA in human fetal lung (HFL) or human foreskin (BJ) fibroblasts increased substantially after infection with either virus in the presence of cycloheximide. HSV-1 particles lacking glycoprotein D or glycoprotein H failed to induce ISG54-specific RNA synthesis, demonstrating that entry of virus particles rather than binding of virions to the cell surface was required for the effect. A DNA-binding complex that recognized an interferon-responsive sequence motif was induced upon infection with HSV-1 or HCMV in the presence of cycloheximide, and the complex was shown to contain the cell proteins interferon response factor 3 (IRF-3) and CREB-binding protein. IRF-3 was modified after infection with HSV-1 or HCMV to a form of lower electrophoretic mobility, consistent with phosphorylation. De novo transcription of viral or cellular genes was not required for the activation of IRF-3, since the effect was not sensitive to inhibition by actinomycin D. Infection of HFL fibroblasts with HSV-1 under conditions in which viral replication proceeded normally resulted in severely reduced levels of the IRF-3-containing complex, defining the activation of IRF-3 as a target for viral interference with ISG induction. In BJ fibroblasts, however, significant activation of IRF-3 was detected even when the viral gene expression program progressed to later stages, demonstrating that the degree of inhibition of the response was dependent on host cell type. As a consequence of IRF-3 activation, endogenous interferon was released from BJ cells and was capable of triggering the appropriate signal transduction pathway in both infected and uninfected cells. Activation of ISG54-specific RNA synthesis was not detected after infection of human U-373MG glioblastoma cells, showing that the induction of the response by infection is cell type dependent.

Nuclear translocation and activation of the transcription factor NFAT is blocked by herpes simplex virus infection

Transcription factors of the NFAT (nuclear factor of activated T cells) family are expressed in most immune system cells and in a range of other cell types. Signaling through NFAT is implicated in the regulation of transcription for the immune response and other processes, including differentiation and apoptosis. NFAT normally resides in the cytoplasm, and a key aspect of the NFAT activation pathway is the regulation of its nuclear import by the Ca(2+)/calmodulin-dependent phosphatase calcineurin. In a cell line stably expressing green fluorescent protein (GFP)-NFAT, this import can be triggered by elevation of intracellular calcium and visualized in live cells. Here we show that the inducible nuclear import of GFP-NFAT is efficiently blocked at early stages of herpes simplex virus (HSV) infection. This is a specific effect, since we observed abundant nuclear accumulation of a test viral protein and no impediment to general nuclear localization signal-dependent nuclear import and retention in infected cells. We show that virus binding at the cell surface is not itself sufficient to inhibit the signaling that induces NFAT nuclear translocation. Since the block occurs following infection in the presence of phosphonoacetic acid but not cycloheximide, we infer that the entry of the virion and early gene transcription are required but the effect is independent of DNA replication or late virus gene expression. A consequence of the block to GFP-NFAT import is a reduction in NFAT-dependent transcriptional activation from the interleukin-2 promoter in infected cells. This HSV-mediated repression of the NFAT pathway may constitute an immune evasion strategy or subversion of other NFAT-dependent cellular processes to promote viral replication.

Plasma membrane requirements for cell fusion induced by herpes simplex virus type 1 glycoproteins gB, gD, gH and gL

Herpes simplex virus type 1 (HSV-1) glycoproteins gB, gD and gHL are capable of inducing cell fusion when expressed from plasmid vectors in the absence of any other virus components. Fusion requires the expression of all four glycoproteins on the same membrane, since they are unable to cooperate in trans to induce syncytium formation. In addition, the fusion event is dependent on the expression of a gD receptor on target cell membranes and does not require the presence of cell-surface glycosaminoglycans.