Expression of the syncytial (syn) phenotype in HSV-1, strain KOS: genetic and phenotypic studies of mutants in two syn loci

Herpesvirus Nuclear Egress across the Outer Nuclear Membrane

Herpesvirus capsids are assembled in the nucleus and undergo a two-step process to cross the nuclear envelope. Capsids bud into the inner nuclear membrane (INM) aided by the nuclear egress complex (NEC) proteins UL31/34. At that stage of egress, enveloped virions are found for a short time in the perinuclear space. In the second step of nuclear egress, perinuclear enveloped virions (PEVs) fuse with the outer nuclear membrane (ONM) delivering capsids into the cytoplasm. Once in the cytoplasm, capsids undergo re-envelopment in the Golgi/trans-Golgi apparatus producing mature virions. This second step of nuclear egress is known as de-envelopment and is the focus of this review. Compared with herpesvirus envelopment at the INM, much less is known about de-envelopment. We propose a model in which de-envelopment involves two phases: (i) fusion of the PEV membrane with the ONM and (ii) expansion of the fusion pore leading to release of the viral capsid into the cytoplasm. The first phase of de-envelopment, membrane fusion, involves four herpes simplex virus (HSV) proteins: gB, gH/gL, gK and UL20. gB is the viral fusion protein and appears to act to perturb membranes and promote fusion. gH/gL may also have similar properties and appears to be able to act in de-envelopment without gB. gK and UL20 negatively regulate these fusion proteins. In the second phase of de-envelopment (pore expansion and capsid release), an alpha-herpesvirus protein kinase, US3, acts to phosphorylate NEC proteins, which normally produce membrane curvature during envelopment. Phosphorylation of NEC proteins reverses tight membrane curvature, causing expansion of the membrane fusion pore and promoting release of capsids into the cytoplasm.

A synthetic glycosaminoglycan mimetic blocks HSV-1 infection in human iris stromal cells

Herpes simplex virus type-1 (HSV-1) is a significant pathogen that affects vision by targeting multiple regions in the human eye including iris. Using a focused library of synthetic non-saccharide glycosaminoglycan mimetics (NSGMs), we identified sulfated pentagalloylglucoside (SPGG) as a potent inhibitor of HSV-1 entry and cell-to-cell spread in the primary cultures of human iris stromal (HIS) cells isolated from eye donors. Using in vitro β-galactosidase reporter assay and plaque reduction assay, SPGG was found to inhibit HSV-1 entry in a dosage-dependent manner (IC₅₀ ∼6.0 μM). Interestingly, a pronounced inhibition in HSV-1 entry and spread was observed in HIS cells, or a cell line expressing specific gD-receptor, when virions were pre-treated with mimetics suggesting a possible interaction between SPGG and the HSV-1 glycoprotein. To examine the significance of gD-SPGG interaction, HIS cells were pretreated with SPGG, which showed a significant reduction in gD binding. Taken together, our results provide strong evidence of SPGG being a novel viral entry inhibitor against ocular HSV infection.

HSV-1 interaction to 3-O-sulfated heparan sulfate in mouse-derived DRG explant and profiles of inflammatory markers during virus infection

The molecular mechanism of herpes simplex virus (HSV) entry and the associated inflammatory response in the nervous system remain poorly understood. Using mouse-derived ex vivo dorsal root ganglia (DRG) explant model and single cell neurons (SCNs), in this study, we provided a visual evidence for the expression of heparan sulfate (HS) and 3-O-sulfated heparan sulfate (3-OS HS) followed by their interactions with HSV-1 glycoprotein B (gB) and glycoprotein D (gD) during cell entry. Upon heparanase treatment of DRG-derived SCN, a significant inhibition of HSV-1 entry was observed suggesting the involvement of HS role during viral entry. Finally, a cytokine array profile generated during HSV-1 infection in DRG explant indicated an enhanced expression of chemokines (LIX, TIMP-2, and M-CSF)—known regulators of HS. Taken together, these results highlight the significance of HS during HSV-1 entry in DRG explant. Further investigation is needed to understand which isoforms of 3-O-sulfotransferase (3-OST)-generated HS contributed during HSV-1 infection and associated cell damage.

Viral forensic genomics reveals the relatedness of classic herpes simplex virus strains KOS, KOS63, and KOS79

Herpes simplex virus 1 (HSV-1) is a widespread global pathogen, of which the strain KOS is one of the most extensively studied. Previous sequence studies revealed that KOS does not cluster with other strains of North American geographic origin, but instead clustered with Asian strains. We sequenced a historical isolate of the original KOS strain, called KOS63, along with a separately isolated strain attributed to the same source individual, termed KOS79. Genomic analyses revealed that KOS63 closely resembled other recently sequenced isolates of KOS and was of Asian origin, but that KOS79 was a genetically unrelated strain that clustered in genetic distance analyses with HSV-1 strains of North American/European origin. These data suggest that the human source of KOS63 and KOS79 could have been infected with two genetically unrelated strains of disparate geographic origins. A PCR RFLP test was developed for rapid identification of these strains.

Mutations within the pathogenic region of herpes simplex virus 1 gK signal sequences alter cell surface expression and neurovirulence

To investigate the role of the signal sequences of herpes simplex virus 1 (HSV-1) gK on virus replication and viral pathogenesis, we constructed recombinant viruses with or without mutations within the signal sequences of gK. These recombinant viruses expressed two additional copies of the mutated (MgK) or native (NgK) form of the gK gene in place of the latency-associated transcript with a myc epitope tag to facilitate detection at their 3' ends. The replication of MgK virus was similar to that of NgK both in vitro and in vivo, as well as in the trigeminal ganglia (TG) of latently infected mice. The levels of gB and gK transcripts in the corneas, TG, and brains of infected mice on days 3 and 5 postinfection were markedly virus and time dependent, as well as tissue specific. Mutation in the signal sequence of gK in MgK virus blocked cell surface expression of gK-myc in rabbit skin cells, increased 50% lethal dose, and decreased corneal scarring in ocularly infected mice compared to the NgK or revertant (RgK) virus. MgK and NgK viruses, and not the RgK virus, showed a reduced extent of explant reactivation at the lower dose of ocular infection but not at the higher dose. However, the time of reactivation was not affected by overexpression of the different forms of gK. Taken together, these results strongly suggest that the 8mer peptide (ITAYGLVL) within the signal sequence of gK promotes cell surface expression of gK in infected cells and ocular pathogenesis in infected mice.

IMPORTANCE

In this study, we show for the first time that mutations within the signal sequence of gK blocked cell surface expression of inserted recombinant gK in vitro. Furthermore, this blockage in cell surface expression was correlated with higher 50% lethal dose and less corneal scarring in vivo. Thus, these studies point to a key role for the 8mer within the signal sequence of gK in HSV-1-induced pathogenicity.

Overexpression of herpes simplex virus glycoprotein K (gK) alters expression of HSV receptors in ocularly-infected mice

PURPOSE

We have shown previously that HSV-1 glycoprotein K (gK) exacerbates corneal scarring (CS) in mice and rabbits. Here, we investigated the relative impact of gK overexpression on host responses during primary corneal infection and latency in trigeminal ganglia (TG) of infected mice.

METHODS

Mice were infected ocularly with HSV-gK(3) (expressing two extra copies of gK replacing latency associated transcript [LAT]), HSV-gK(3) revertant (HSV-gK(3)R), or wild-type HSV-1 strain McKrae. Individual corneas on day 5 post infection (PI) and TG on day 28 PI were isolated and used for detection of gB DNA in the TG, HSV-1 receptors in the cornea and TG, and inflammatory infiltrates in TG.

RESULTS

During primary HSV-1 infection, gK overexpression resulted in altered expression of herpesvirus entry mediator (HVEM), 3-O-sulfated heparin sulfate (3-OS-HS), paired immunoglobulin-like type 2 receptor-α (PILR-α), nectin-1, and nectin-2 in cornea of BALB/c, but not C57BL/6 mice. However, gK overexpression did have an effect on 3-OS-HS, PILR-α, nectin-1, and nectin-2 expression (but not HVEM expression) in TG of C57BL/6 mice during latency. These differences did not affect the level of latency, but instead were correlated with the presence of CS. The presence of LAT increased HVEM expression and this effect was enhanced further by the presence of CS in latently-infected mice. Finally, the presence of LAT, but not overexpression of gK, affected CD4, CD8, TNF-α, Tim-3, PD-1, IL-21, IL-2, and IFN-γ expression in TG.

CONCLUSIONS

We demonstrate a novel link between gK exacerbation of CS and HSV-1 receptors, suggesting a gK-induced molecular route for the pathogenesis as well as selective advantage of these entry routes for the pathogen during latency-reactivation cycle.

Binding of HSV-1 glycoprotein K (gK) to signal peptide peptidase (SPP) is required for virus infectivity

Glycoprotein K (gK) is a virion envelope protein of herpes simplex virus types 1 (HSV-1) and 2 (HSV-2), which plays important roles in virion entry, morphogenesis and egress. Two-hybrid and pull-down assays were utilized to demonstrate that gK and no other HSV-1 genes specifically binds to signal peptide peptidase (SPP), also known as minor histocompatibility antigen H13. SPP dominant negative mutants, shRNA against SPP significantly reduced HSV-1 replication in vitro. SPP also affected lysosomes and ER responses to HSV-1 infection. Thus, in this study we have shown for the first time that gK, despite its role in fusion and egress, is also involved in binding the cytoplasmic protein SPP. These results also suggest that SPP plays an important role in viral replication and possibly virus pathogenesis. This makes SPP unique in that its function appears to be required by the virus as no other protein can compensate its loss in terms of viral replication.

Susceptibility of human iris stromal cells to herpes simplex virus 1 entry

Ocular herpes simplex virus 1 (HSV-1) infection can lead to multiple complications, including iritis, an inflammation of the iris. Here, we use human iris stroma cells as a novel in vitro model to demonstrate HSV-1 entry and the inflammatory mediators that can damage the iris. The upregulated cytokines observed in this study provide a new understanding of the intrinsic immune mechanisms that can contribute to the onset of iritis.

Herpes simplex virus type-1 (HSV-1) entry into human mesenchymal stem cells is heavily dependent on heparan sulfate

Hematopoietic stem cells recipients remain susceptible to opportunistic viral infections including herpes simplex virus type-1 (HSV-1). The purpose of this investigation was to analyze susceptibility of human mesenchymal stem cells (hMSCs) to HSV-1 infection and identify the major entry receptor. Productive virus infection in hMSCs was confirmed by replication and plaque formation assays using a syncytial HSV-1 KOS (804) virus. To examine the significance of entry receptors, RT-PCR and antibody-blocking assays were performed. RT-PCR data showed the expression of gD receptors: nectin-1, 3-O sulfotransferase-3 (3-OST-3), and HVEM. Antibody-blocking assay together with heparinase treatment suggested an important role for HS and 3-O-sulfated heparan sulfate (3-OS HS), but not nectin-1 or HVEM, in mediating HSV-1 entry and spread in hMSCs. Taken together, our results provide strong evidence demonstrating that HSV-1 is capable of infecting hMSCs and HS and 3-OS HS serve as its entry receptors during this process.

Immunization with different viral antigens alters the pattern of T cell exhaustion and latency in herpes simplex virus type 1-infected mice

We have shown previously that immunization with herpes simplex virus type 1 (HSV-1) glycoprotein K (gK) exacerbated corneal scarring (CS) in ocularly infected mice. In this study, we investigated whether higher levels of CS were correlated with higher levels of latency and T cell exhaustion in gK-immunized mice. BALB/c mice were vaccinated with baculovirus-expressed gK or gD or mock immunized. Twenty-one days after the third immunization, mice were ocularly infected with 2 × 10(4) PFU/eye of virulent HSV-1 strain McKrae. On day 5 postinfection, virus replication in the eye was measured, and on day 30 postinfection, infiltration of the trigeminal ganglia (TG) by CD4, CD8, programmed death 1 (PD-1), and T cell immunoglobulin and mucin domain-containing protein 3 (Tim-3) was monitored by immunohistochemistry and quantitative real-time PCR (qRT-PCR). This study demonstrated that higher levels of CS were correlated with higher levels of latency, and this was associated with the presence of significantly higher numbers of CD4(+)PD-1(+) and CD8(+)PD-1(+) cells in the TG of the gK-immunized group than in both the gD- and mock-immunized groups. Levels of exhaustion associated with Tim-3 were the same among gK- and mock-vaccinated groups but higher than levels in the gD-vaccinated group. In this study, we have shown for the first time that both PD-1 and Tim-3 contribute to T cell exhaustion and an increase of latency in the TG of latently infected mice.

Priscilla Schaffer (1941-2009): a Stalwart Herpesvirologist

Priscilla Ann Schaffer died from complications of Parkinson's Disease on 18 November 2009. Priscilla was a colleague and a friend, as well as being a stellar member of the virology community. She was a longtime Journal of Virology reviewer, a member of the editorial board, and a frequent contributor. Her energy and enthusiasm for virology, her students, her colleagues, and life in general were exceptional. She will be missed. Her colleague Donald Coen has prepared a memorial celebrating her life and accomplishments. Lynn W. Enquist Editor in Chief, Journal of Virology

A recombinant herpes simplex virus type 1 expressing two additional copies of gK is more pathogenic than wild-type virus in two different strains of mice

The effect of glycoprotein K (gK) overexpression on herpes simplex virus type 1 (HSV-1) infection in two different strains of mice was evaluated using a recombinant HSV-1 virus that expresses two additional copies of the gK gene in place of the latency-associated transcript (LAT). This mutant virus (HSV-gK3) expressed higher levels of gK than either the wild-type McKrae virus or the parental dLAT2903 virus both in vitro (in cultured cells) and in vivo (in infected mouse corneas and trigeminal ganglia [TG] of BALB/c and C57BL/6 mice). gK transcripts were detected in the TG of both HSV-gK3-infected mouse strains on day 30 postinfection (p.i.), while gB transcripts were detected only in the TG of the HSV-gK3-infected C57BL/6 mice, a finding that suggests that increased gK levels promote chronic infection. C57BL/6 mice infected with HSV-gK3 also contained free virus in their TG on day 30 p.i. Both HSV-gK3-infected mouse strains had significantly higher corneal scarring (CS) than did McKrae-infected mice. T-cell depletion studies in C57BL/6 mice suggested that this CS enhancement in the HSV-gK3-infected mice was mediated by a CD8+ T-cell response. Taken together, these results strongly suggest that increased gK levels promote eye disease and chronic infection in infected mice.

Epitope mapping of HSV-1 glycoprotein K (gK) reveals a T cell epitope located within the signal domain of gK

Glycoprotein K (gK) is a virion envelope component of herpes simplex virus types 1 (HSV-1) and 2 (HSV-2), which plays an important role in virion morphogenesis and egress. We previously demonstrated that immunization of mice with gK, but not with any of the 10 other HSV-1 glycoproteins, resulted in exacerbation of corneal scarring and herpetic dermatitis following ocular HSV-1 infection. However, little is known about the gK epitope(s) that is (are) involved in T cell activities in vitro or in vivo. Thus, epitope mapping of gK was performed using a panel of 15-mer peptides with five-amino acid overlaps spanning the full-length gK, and four expressed gK recombinant proteins representing different regions of gK. Epitope mapping within the gK polypeptide defined the amino acid sequence STVVLITAYGLVLVW as the predominant CD4(+) and CD8(+) T cell stimulatory region both in vitro and in vivo. IFN-gamma expression by CD4(+) T cells was CD8(+) T cells-dependent. This immunodominant epitope is located within the signal sequence of the gK polypeptide and is highly conserved in HSV-1 and HSV-2 strains. Using prediction algorithms, the peptide is predicted to bind to numerous MHC class I and class II molecules.

High-molecular-weight protein (pUL48) of human cytomegalovirus is a competent deubiquitinating protease: mutant viruses altered in its active-site cysteine or histidine are viable

We show here that the high-molecular-weight protein (HMWP or pUL48; 253 kDa) of human cytomegalovirus (HCMV) is a functionally competent deubiquitinating protease (DUB). By using a suicide substrate probe specific for ubiquitin-binding cysteine proteases (DUB probe) to screen lysates of HCMV-infected cells, we found just one infected-cell-specific DUB. Characteristics of this protein, including its large size, expression at late times of infection, presence in extracellular virus particles, and reactivity with an antiserum to the HMWP, identified it as the HMWP. This was confirmed by constructing mutant viruses with substitutions in two of the putative active-site residues, Cys24Ile and His162Ala. HMWP with these mutations either failed to bind the DUB probe (C24I) or had significantly reduced reactivity with it (H162A). More compellingly, the deubiquitinating activity detected in wild-type virus particles was completely abolished in both the C24I and H162A mutants, thereby directly linking HMWP with deubiquitinating enzyme activity. Mutations in these active-site residues were not lethal to virus replication but slowed production of infectious virus relative to wild type and mutations of other conserved residues. Initial studies, by electron microscopy, of cells infected with the mutants revealed no obvious differences at late times of replication in the general appearance of the cells or in the distribution, relative numbers, or appearance of virus particles in the cytoplasm or nucleus.

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.

Contributions of gD receptors and glycosaminoglycan sulfation to cell fusion mediated by herpes simplex virus 1

Two cell surface proteins (nectin-1/HveC and nectin-2/HveB) shown previously to serve as receptors for the entry of herpes simplex virus 1 (HSV-1) wild-type and/or mutant strains were found to serve also as receptors for HSV-1-induced cell fusion. Transfection with genomic DNA from a syncytial HSV-1 strain encoding wild-type gD resulted in fusion of Chinese hamster ovary (CHO) cells expressing nectin-1 but not of cells expressing nectin-2. In contrast, transfection with DNA from a related HSV-1 strain encoding the mutant Rid1 form of gD resulted in fusion of CHO cells expressing either receptor but not of control cells. These results are consistent with the ability of each receptor to mediate entry of viruses expressing wild-type or Rid1 gD and with results obtained previously with HVEM (HveA), a third HSV-l entry receptor. Undersulfation of GAGs in receptor-expressing cell lines predictably reduced susceptibility to HSV-l infection. In contrast, susceptibility to cell fusion mediated by HVEM or nectin-1 was not reduced. Undersulfation of GAGs partially inhibited cell fusion mediated by nectin-2. We conclude that HSV-1-induced cell fusion requires a gD-binding entry receptor, that ability of an HSV-1 strain to use HVEM, nectin-2 or nectin-1 for cell fusion depends on the allele of gD expressed and that GAGs may influence cell fusion, dependent on the gD-binding receptor used, but are less important for cell fusion mediated by HVEM, nectin-2 or nectin-l than for viral entry.

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.

Conservation of the architecture of the Golgi apparatus related to a differential organization of microtubules in polykaryocytes induced by syn- mutants of herpes simplex virus 1

Infection of Vero and HEp-2 but not of 143TK- cells with herpes simplex virus 1 results in fragmentation and dispersal of the Golgi apparatus. Concurrently, in all three infected cell lines the microtubular network is disrupted, suggesting that the disruption of microtubules is essential but not sufficient to induce the fragmentation of the Golgi apparatus. We now report the following: (i) In polykaryocytes formed in Vero cells infected with HSV-1 syn- mutant viruses, intact Golgi stacks were readily detected by electron microscopy. These aggregated in the center of large polykaryocytes. (ii) The distribution of viral glycoprotein D, examined in both fixed and nonfixed cells, appeared to match the distribution of the Golgi stacks, suggesting that the aggregated Golgi stacks funnel viral glycoproteins and viral particles to a limited region of the plasma membrane of the polykaryocytes rather than directing exocytic flow in a more dispersed fashion as seen in syn+ virus-infected cells exhibiting fragmented and dispersed Golgi. (iii) In most polykaryocytes, the microtubules formed parallel bundles extending along the axis of recruitment of new cells. (iv) Fragmentation of the microtubules at the periphery of the cell near the plasma membrane was observed in untreated or cycloheximide-treated cells 2 h after infection with syn- virus HSV-1(MP) or syn+ HSV-1(mP) but not in mock-infected cells. These observations suggest that peripheral depolymerization is initiated at the time of infection and that a factor which determines the syn- or syn+ phenotype is whether the microtubular network regenerates concomitant with cell fusion or reorganizes to form a collapsed network surrounding nuclei of syn+ infected cells.

Determination of the transmembrane topology of herpes simplex virus type 1 glycoprotein K

Herpes simplex virus type 1 glycoprotein K (gK) plays an essential role in viral replication and cell fusion. gK is a very hydrophobic membrane protein that contains a signal sequence and several hydrophobic regions. It has been shown that mutations inducing cell fusion map to two distinct domains of gK, suggesting that these domains are functionally important. To understand the transmembrane topology of gK and the localization of these functional domains, we constructed a set of gK deletion, insertion, and truncation mutants and expressed these by in vitro translation in the presence of microsomal membranes. The transmembrane topology of gK was determined by examination of the post-translational processing and protease sensitivity of the mutant proteins. Our data demonstrate that gK contains three transmembrane domains (amino acids 125-139, 226-239, and 311-325). Another hydrophobic domain (amino acids 241-265), which is relatively less hydrophobic and much longer compared with the transmembrane sequences, is located in the extracellular loop. The analysis showed that the domains containing syncytial mutations are both ectodomains. They may interact with each other to form a complex tertiary structure that is critical for the biological function of gK.

Herpes simplex virus-1 entry into cells mediated by a novel member of the TNF/NGF receptor family

We identified and cloned a cellular mediator of herpes simplex virus (HSV) entry. Hamster and swine cells resistant to viral entry became susceptible upon expression of a human cDNA encoding this protein, designated HVEM (for herpesvirus entry mediator). HVEM was shown to mediate the entry of several wild-type HSV strains of both serotypes. Anti-HVEM antibodies and a soluble hybrid protein containing the HVEM ectodomain inhibited HVEM-dependent infection but not virus binding to cells. Mutations in the HSV envelope glycoprotein gD significantly reduced HVEM-mediated entry. The contribution of HVEM to HSV entry into human cells was demonstrable in activated T cells. HVEM, the first identified mediator of HSV entry, is a new member of the TNF/NGF receptor family.

The syn3 strain HSZP of herpes simplex virus type 1 (HSV-1) is not pathogenic for mice and shows limited neural spread

Strain HSZP of the herpes simplex virus type 1 (HSV-1) forms large giant cells in vitro. This property was found associated with a mutation that alters the codon CGC (in the strain KOS or 17 sequence) to CAC (in the HSZP sequence), changing the amino acid 857 from arginine to histidine in the cytoplasmic domain of the glycoprotein B (gB) polypeptide chain. Giant cell formation by ANGpath was attributed to a mutation that alters the codon GCC (in KOS and strain 17 sequences) to GTC (in ANGpath sequence) changing the amino acid 854 in the same (syn3) region of the gB molecule. In contrast to the ANGpath virus, which is pathogenic (1 LD50 < 1 x 10(4) PFU) for adult DBA/2 mice after peripheral inoculation, strain HSZP was never found to be lethal for adult mice. Whereas ANGpath-infected mice which survived acute infection frequently (79%) developed latency in the regional sensory ganglion (as proved by virus reactivation during explantation), latent HSZP reactivated in ganglion culture at a considerably reduced rate (21%). Only 10-day-old DBA/2 mice were sensitive to HSZP infection. In these, HSZP spread from the site of peripheral administration mainly by hematogenous route. The neural spread of HSZP in suckling DBA/2 mice was manifested by the involvement of vegetative neurons in the wall of the small intestine and in the retroperitoneal vegetative ganglia. We conclude that HSZP, a polykaryocyte-forming strain with a mutation in the syn3 region II, shows limited neuroinvasity for mice after peripheral administration.

Herpes simplex virus glycoprotein K promotes egress of virus particles

Herpes simplex virus (HSV) glycoprotein K (gK) is thought to be intimately involved in the process by which infected cells fuse because HSV syncytial mutations frequently alter the gK (UL53) gene. Previously, we characterized gK produced in cells infected with wild-type HSV or syncytial HSV mutants and found that the glycoprotein was localized to nuclear and endoplasmic reticulum membranes and did not reach the cell surface (L. Hutchinson, C. Roop, and D. C. Johnson, J. Virol. 69:4556-4563, 1995). In this study, we have characterized a mutant HSV type 1, denoted F-gK beta, in which a lacZ gene cassette was inserted into the gK coding sequences. Since gK was found to be essential for virus replication, F-gK beta was propagated on complementing cells which can express gK. F-gK beta produced normal plaques bounded by nonfused cells when plated on complementing cells, although syncytia were observed when the cells produced smaller amounts of gK. In contrast, F-gK beta produced only microscopic plaques on Vero cells and normal human fibroblasts (which do not express gK) and these plaques were reduced by 10(2) to 10(6) in number. Further, large numbers of nonenveloped capsids accumulated in the cytoplasm of F-gK beta-infected Vero cells, virus particles did not reach the cell surface, and the few enveloped particles that were produced exhibited a reduced capacity to enter cells and initiate an infection of complementing cells. Overexpression of gK in HSV-infected cells also caused defects in virus egress, although particles accumulated in the perinuclear space and large multilamellar membranous structures juxtaposed with the nuclear envelope were observed. Together, these results demonstrate that gK regulates or facilitates egress of HSV from cells. How this property is connected to cell fusion is not clear. In this regard, gK may alter cell surface transport of viral particles or other viral components directly involved in the fusion process.

Syncytial mutations in the herpes simplex virus type 1 gK (UL53) gene occur in two distinct domains

Syncytial (syn) mutants of herpes simplex virus cause cell fusion. Many syn mutations map to the syn1 locus, which has been identified with the gK (UL53) gene. In this work, the gK genes of eight syn mutants derived from the KOS strain were sequenced to identify residues and, possibly, domains important for the fusion activity of mutant gK. DNA sequencing showed that six mutants (syn30, syn31, syn32, syn102, syn103, and syn105) had single missense mutations in the gK gene. Two of these, syn31 and syn32, had identical mutations that caused the introduction of a potential site for N-linked glycosylation. syn31 gK was analyzed by in vitro translation and found to utilize the novel glycosylation site. Two other mutants, syn8 and syn33, had three mutations each, resulting in three amino acid substitutions in syn8 and two substitutions in syn33. Of the 10 gK syn mutant sequences known, 8 have mutations in the N-terminal domain of gK, suggesting that this domain, which is likely to be an ectodomain, is important for the function of the protein. The other two mutants, syn30 and syn103, have mutations near the C terminus of gK.

Mutations in the cytoplasmic tail of herpes simplex virus glycoprotein H suppress cell fusion by a syncytial strain

We have developed a complementation assay, using transiently transfected COS cells, to facilitate a molecular analysis of the herpes simplex virus type 1 glycoprotein gH. When infected by a gH-null syncytial virus, COS cells expressing wild-type gH generate infectious progeny virions and form a syncytium with neighboring cells. By deletion and point mutagenesis, we have found particular residues in the gH cytoplasmic tail to be essential for generation of a syncytium but apparently dispensable for production of infectious virions. This study emphasizes the different requirements for cell-cell and cell-envelope fusion and demonstrates that changes in the non-syn locus UL22-gH can reverse the syncytial phenotype.

Analysis of the transcription pattern of HSV-1 UL52 and UL53 genes

The UL52 and UL53 genes of herpes simplex virus type-1 are both located in the BamHI-L DNA fragment, with an overlap of 14 amino acids. An RNase protection experiment was designed to determine the 5' termini of both the UL52 and UL53 mRNAs. The 5' end of the UL52 mRNA was found to be located 100 bp upstream of its ATG initiation codon. Surprisingly, the 5' terminus of the UL53 gene was found to be downstream of its putative initiation codon. Therefore, it was suggested that the translation of the UL53 open reading frame (ORF) starts at an internal initiation codon that is located 55 codons downstream of the putative one. A hybrid selection experiment was performed in which the UL53-specific mRNA was selected from BSC-1 cells infected with HSV-1 KOS and translated in vitro. The translation product of the UL53 message was found to be 32 kD (shorter than the original 37.5 kD ORF). The size of the protein obtained corresponds with the expected translation product starting at the downstream initiation codon. Analysis of the sequence upstream of this initiation codon reveals the presence of a promotor sequence. Therefore, we suggest that the UL53 protein is 54 amino acids shorter than was previously suggested and is located at coordinates 112,341-113,193.

Characterization of baculovirus-expressed herpes simplex virus type 1 glycoprotein K

The DNA region encoding the complete herpes simplex virus type 1 (HSV-1) glycoprotein K (gK) was inserted into a baculovirus transfer vector, and recombinant viruses expressing gK were isolated. Four gK-related recombinant baculovirus-expressed peptides of 29, 35, 38, and 40 kDa were detected with polyclonal antibody to gK. The 35-, 38-, and 40-kDa species were susceptible to tunicamycin treatment, suggesting that they were glycosylated. The 38- and 40-kDa species corresponded to partially glycosylated precursor gK (pgK) and mature gK, respectively. The 29-kDa peptide probably represented a cleaved, unglycosylated peptide. The 35-kDa peptide probably represented a cleaved, glycosylated peptide that may be a precursor to pgK. Indirect immunofluorescence with polyclonal antibody to gK peptides indicated that the recombinant baculovirus-expressed gK was abundant on the surface of the insect cells in which it was expressed. Mice vaccinated with the baculovirus-expressed gK produced very low levels (< 1:10) of HSV-1 neutralizing antibody. Nonetheless, these mice were partially protected from lethal challenge with HSV-1 (75% survival). This protection was significant (P = 0.02). Despite some protection against death, gK-vaccinated mice showed no protection against the establishment of latency. Surprisingly, gK-vaccinated mice that were challenged ocularly with a stromal disease-producing strain of HSV-1 had significantly higher levels of ocular disease (herpes stromal keratitis) than did mock-vaccinated mice. In summary, this is the first report to show that vaccination with HSV-1 gK can provide protection against lethal HSV-1 challenge and that vaccination with an HSV-1 glycoprotein can significantly increase the severity of HSV-1-induced ocular disease.

Cyclosporin A resistance of herpes simplex virus-induced "fusion from within" as a phenotypical marker of mutations in the Syn 3 locus of the glycoprotein B gene

We here report research in which nine strains of Herpes simplex virus (HSV) with fusing activity were investigated in order to establish precise phenotypical markers of mutations in the carboxy terminus of glycoprotein B (gB). The gene region encoding the carboxy terminus of gB was isolated, then cloned, and finally sequenced. Our investigation showed that seven strains have different mutations in the syn 3 locus. We observed no base difference in the gB gene region encoding the carboxy terminus of gB of two other strains. Strains with a mutation in the carboxy terminus of gB induced fusion from within (FFWI) in the presence of Cyclosporin A (CyA) at a concentration up to 150 microM. There are two clusters of mutations correlated with the syn 3 locus and selected in the presence of CyA: One group comprised of amino acid substitutions at position 816, the other of changes at positions 853, 854, and 857. In contrast, the fusion induced by strains with mutations in other syn loci is CyA sensitive. CyA inhibits the FFWI at concentrations of 20-60 microM. The results demonstrate the CyA resistance of HSV-induced FFWI should serve as a phenotypical marker of mutations in the carboxy terminus of gB. Moreover, our investigations revealed that fusion from without (FFWO) does not always serve as a phenotypical marker of mutations in the syn 3 locus. On the one hand, all FFWO-positive strains possess a syn 3 locus mutation, whilst, on the other hand, five strains with mutations in the carboxy terminus of gB are FFWO negative.(ABSTRACT TRUNCATED AT 250 WORDS)

Syncytium-inducing mutations localize to two discrete regions within the cytoplasmic domain of herpes simplex virus type 1 glycoprotein B

Herpes simplex virus type 1 glycoprotein B (gB) is essential for virus entry, an event involving fusion of the virus envelope with the cell surface membrane, and virus-induced cell-cell fusion, resulting in polykaryocyte, or syncytium, formation. The experiments described in this report employed a random mutagenesis strategy to develop a more complete genetic map of mutations resulting in the syn mutant phenotype. The results indicate that syn mutations occur within two essential and highly conserved hydrophilic, alpha-helical regions of the gB cytoplasmic domain. Region I is immediately proximal to the transmembrane domain and includes residues R796 to E816/817. Region II is localized centrally in the cytoplasmic domain and includes residues A855 and R858. Positively charged residues were particularly affected in both regions, suggesting that charge interactions may be required to suppress the syn mutant phenotype. No syn mutations were identified within the transmembrane domain. A virus containing a rate of entry (roe) mutation at residue A851, either within or immediately proximal to syn region II, was isolated. Since roe mutations have also been discovered in the external domain of gB, it appears likely that the external and cytoplasmic domains cooperate in virus penetration. Moreover, the observation that both roe and syn mutations occur in the cytoplasmic domain further suggests that gB functions in an analogous manner in both membrane fusion events. It might be predicted from these observations that membrane fusion involves transduction of a fusion signal along the gB molecule through the transmembrane domain. Communication between the external and cytoplasmic domain may thus be required for gB-mediated membrane fusion events.

A mutant herpes simplex virus type 1 unable to express glycoprotein L cannot enter cells, and its particles lack glycoprotein H

Herpes simplex virus type 1 (HSV-1) glycoprotein H (gH) is essential for virus entry into cells and forms a hetero-oligomer with a newly described viral glycoprotein, gL. Normal folding, posttranslational processing, and intracellular transport of both gH and gL depend upon the coexpression of gH and gL in cells infected with vaccinia virus vectors (L. Hutchinson, H. Browne, V. Wargent, N. Davis-Poynter, S. Primorac, K. Goldsmith, A. C. Minson, and D. C. Johnson, J. Virol. 66:2240-2250, 1992). Homologs of gH and gL have been found in herpesviruses of all subgroups, and thus it appears likely that the gH-gL complex serves a highly conserved function during herpesvirus penetration into cells. To examine the role of gL in the infectious cycle of HSV-1, a mutant HSV-1 unable to express gL was constructed by inserting a lacZ gene cassette into the coding sequences of the UL1 (gL) gene. Because gL was found to be essential for virus replication, cell lines capable of expressing gL were constructed to complement the virus mutant. In the absence of gL, virus particles were produced, and these particles reached the cell surface; however, gL-negative particles purified from infected cells were also deficient in gH. Mutant virions lacking gH and gL were able to adsorb onto cells but were unable to enter cells and initiate an infection. Further, the role of gL in fusion of infected cells was reexamined. A mutation in HSV-1 (804) which produces the syncytial phenotype had previously been mapped to a region of the HSV-1 genome which includes the UL1 gene and no other open reading frame. However, in contrast to this previous report, we found that the syncytial mutation in 804 affects the UL53 gene, which encodes gK, a gene commonly mutated in syncytial viruses.

Identification and characterization of a novel herpes simplex virus glycoprotein, gK, involved in cell fusion

Antipeptide sera were used to identify a novel glycoprotein encoded by the UL53 gene of herpes simplex virus type 1 (HSV-1). The UL53 gene product is thought to play a central role in regulating membrane fusion because mutations giving rise to the syncytial phenotype, wherein cells are extensively fused, frequently map to this gene. A single 40-kDa protein, designated gK (the ninth HSV-1 glycoprotein to be described), was detected with antipeptide sera in cells infected with both wild-type and syncytial strains of HSV-1 which were labelled with [35S]methionine and [35S]cysteine or with [3H]glucosamine, and this protein was sensitive to treatment of cells with tunicamycin. With all other HSV glycoproteins studied to date, at least two glycosylated species, often differing substantially in electrophoretic mobility, have been observed in infected cells; thus, gK is unusual in this respect. The 40-kDa gK protein was also immunoprecipitated from cells infected with a recombinant adenovirus vector carrying the UL53 gene. Two glycosylated species of 39 and 41 kDa were produced when UL53 mRNA was translated in vitro in the presence of microsomes, and these proteins differed from gK produced in infected cells not only because they possessed different electrophoretic mobilities but also because they were unable to enter gels after being heated. In addition, a 36-kDa protein was detected in extracts from cells infected with HSV-2 with use of these sera.

A novel herpes simplex virus glycoprotein, gL, forms a complex with glycoprotein H (gH) and affects normal folding and surface expression of gH

A glycoprotein encoded by the UL1 gene of herpes simplex virus type 1 (HSV-1) was detected in infected cells with antipeptide sera. The UL1 gene has previously been implicated in virus-induced cell fusion (S. Little and P. A. Schaffer, Virology 112:686-697, 1981). Two protein species, a 30-kDa precursor form and a 40-kDa mature form of the glycoprotein, both of which were modified with N-linked oligosaccharides, were observed. This novel glycoprotein is the 10th HSV-1 glycoprotein to be described and was named glycoprotein L (gL). A complex was formed between gL and gH, a glycoprotein known to be essential for entry of HSV-1 into cells and for virus-induced cell fusion. Previously, it had been reported that gH expressed in the absence of other viral proteins was antigenically abnormal, not processed, and not expressed at the cell surface (U.A. Gompels and A. C. Minson, J. Gen. Virol. 63:4744-4755, 1989; A. J. Forrester, V. Sullivan, A. Simmons, B. A. Blacklaws, G. L. Smith, A. A. Nash, and A. C. Minson, J. Gen. Virol. 72:369-375, 1991). However, gH coexpressed with gL by using vaccinia virus recombinants was antigenically normal, processed normally, and transported to the cell surface. Similarly, gL was dependent on gH for proper posttranslational processing and cell surface expression. These results suggest that it is a hetero-oligomer of gH and gL which is incorporated into virions and transported to the cell surface and which acts during entry of virus into cells.

Relationship between HLA I surface expression and different cytopathic effects produced after herpes simplex virus infection in vitro

In the present study, we investigated the effects of herpes simplex virus (HSV) infection on the expression of HLA class I antigens and beta 2-microglobulin in human fibroblasts. The mRNA abundance for HLA class I was shown to be strongly reduced after infection with HSV strains either producing cell rounding or fusion from within (FFWI), however, HLA class I expression on the surface of cells is strongly reduced only after appearance of FFWI. Using a ts mutant (ts 78R) or CyA in combination with a fusion from without (FFWO) inducing strain of HSV, this loss of HLA class I antigens is assumed to be correlated to the rearrangement of the cell membrane during the fusion process itself as a late event of cytopathogenicity.

In vitro characterization of the HSV-1 UL53 gene product

The UL53 gene is the locus altered in many syncytial mutants of herpes simplex virus type 1 (HSV-1). However, the protein encoded by this gene has not been characterized. In this study, the UL53 protein was produced by in vitro translation of in vitro-transcribed UL53 RNA. Post-translational processing of the protein was studied by translation in the presence of pancreatic microsomal membranes. These microsomes carry out the processing steps that normally occur in the rough endoplasmic reticulum. The unprocessed protein had an apparent molecular weight of 27K, whereas the microsomally processed form had an apparent molecular weight of 36K. Two types of post-translational modification were detected: Addition of N-linked oligosaccharides and cleavage of an N-terminal signal sequence. N-linked glycosylation occurred in the first 112 residues of the protein, consistent with the presence of N-linked glycosylation signals at residues 48 and 58. Signal sequence cleavage occurred after residue 30. A membrane-binding, possibly transmembrane, domain was found between residues 113 and 170, probably consisting of the hydrophobic sequence 125-139. These results establish that the N-terminal domain of the UL53 protein, which is the site of those syncytial mutations that have been sequenced, is on the interior side of the microsomal membranes, which is topologically equivalent to the lumen of the rough endoplasmic reticulum and to the extracellular side of the plasma membrane. Additional hydrophobic, possibly transmembrane, domains exist nearer the C-terminus of the protein. It also was found that the in vitro-translated UL53 protein aggregated when heated, even in the presence of SDS. This property was mapped to the C-terminal one-third of the protein.

The UL20 gene of herpes simplex virus 1 encodes a function necessary for viral egress

A recombinant virus from which the start codon and 53% of the UL20 open reading frame had been deleted was constructed and characterized. We report the following: (i) The UL20- mutant formed small plaques in 143 tk- cells but failed to form plaques in Vero cells. Virus yields were approximately 10- to 100-fold lower than those of wild-type virus in all cell lines tested. (ii) Electron microscopic examination of Vero cells infected with the UL20- mutant revealed that enveloped and unenveloped capsids accumulated in the cytoplasm, possibly in the space between the inner and outer lamellae of the nuclear membrane, and that virtually no virus was present in the extracellular space. (iii) Glycoproteins B, C, D, E, H, and I recovered from lysates of cells infected with the UL20- mutant could not be differentiated from those present in lysates of cells infected with the wild-type parent virus with respect to the electrophoretic mobility of mature and precursor forms. (iv) Repair of the deleted sequences restored the wild-type phenotype. (v) The gene product of the UL20 gene was shown to be associated with cellular membranes and to possess characteristics of integral membrane proteins. We conclude that the UL20 gene encodes an integral membrane protein with a hitherto unrecognized function in that it enables the transit of virions to the extracellular space. The function of the UL20 gene product is complemented by some cell lines but not by Vero cells. The vesicles which serve to transport virions may have an origin different from those associated with transport of normal cellular proteins.

Characterization of glycoprotein C-negative mutants of herpes simplex virus type 1 isolated from a patient with keratitis

Recently three strains of herpes simplex virus type 1 (HSV-1), which did not react with Micro Trak Herpes (Syva Co.), were isolated by us from a patient with recurrent herpetic keratitis. In this study we characterized these strains of HSV-1 and found them to be HSV-1 gC- mutants which are very rare isolates from humans. The properties of the HSV-1 strains regarding plaque morphology on Vero cells and chick embryo fibroblasts and viral DNA analysis were the same as those of the usual HSV-1 strains. An immunofluorescence study using anti-gC-1 monoclonal antibody and SDS-PAGE analysis of radiolabeled viral glycoproteins showed that these strains are deficient in gC-1. They were virulent for mice and sensitive to acyclovir and bromovinyldeoxyuridine. Furthermore the infectivity of the strains was inactivated by complement though the phenomenon was not observed in the usual HSV-1 strains. This finding suggests that protection from damages by complement is an important function of gC. In keratitis the effects of complement are thought to be minimal because of the scanty blood supply and this may be the reason why these strains were isolated from the cornea.

Nucleotide sequence of the herpes simplex virus type-2 syn gene that causes cell fusion

The nucleotide (nt) sequence of the entire coding region of the syn gene of Herpes simplex virus type 2 (HSV-2) is reported. The amino acid (aa) sequence deduced from the nt sequence has been compared to analogous Herpes simplex virus type 1 (HSV-1) syn gene that causes cell fusion. The aa sequences are 82.8% homologous and contain a highly conserved region of 90 aa. These results identify some of the conserved regions in the coding sequences between the two types.

Antibody-resistant mutations in cross-reactive and type-specific epitopes of herpes simplex virus 1 glycoprotein B map in separate domains

To characterize the domains of HSV-1 glycoprotein B (gB), we isolated mutants resistant to monoclonal antibodies with potent neutralizing activity. Partial nucleotide sequencing of the mutations revealed that gB contains two domains comprising discontinuous and continuous amino acids that bind cross-reactive and type-specific neutralizing antibodies. Four mutations in a discontinuous domain, R1435, R233, R1375, and R126, contained substitutions of Tyr278 for His278, His298 for Arg298, Gln274 for Arg274, and Asn273 for Tyr273, respectively. Two mutations in a continuous domain, R1392 and R1397, contained substitutions of Thr32 for Ala32 and Thr47 for Asn47, respectively, and overlapped two other type-specific epitopes. Analysis of the nucleotide sequence of strain KOS showed differences from strain F at four residues proximal to the R1392 mutation and one residue proximal to the R1397 mutation, which explains the failure of HSV-1(F)-specific antibodies to these epitopes to react with KOS. One target site for proteolytic cleavage of gB by cellular enzymes maps at the amino terminus, partially overlapping four HSV-1-specific epitopes.

Herpes simplex virus glycoprotein D is sufficient to induce spontaneous pH-independent fusion in a cell line that constitutively expresses the glycoprotein

Spontaneous small polykaryocytes were detected in a cell line designated BJ-o that harbors the BamHI J fragment of herpes simplex virus 1 DNA and expresses constitutively glycoprotein D (gD). The fusion activity of BJ-o cells correlated with gD production and was drastically reduced following exposure of the cells to monoclonal antibody HD1 to gD. Studies on the characteristics and requirements of cell fusion dependent on gD led to the conclusion that the characteristics and requirements for gD-mediated fusion activity of BJ-o cells are similar to those previously reported for cell fusion induced by the virus in that (i) polykaryocytosis was not augmented by exposure to medium of low pH with or without prior exposure to trypsin, (ii) the number of polykaryocytes was reduced following removal of terminal sialic acid residues by neuraminidase, and (iii) the number of polykaryocytes was augmented by masking of high-mannose N-linked oligosaccharides with concanavalin A or with its reduced form, succinyl concanavalin A. This effect was reversed by competition with mannose.

A herpes simplex virus type 1 mutant resistant to benzhydrazone, an inhibitor of glycoprotein synthesis in herpesvirus-infected cells. Preliminary mapping of benzhydrazone-resistance and of a novel syncytial mutation

Benzhydrazone (BH) is an inhibitor of glycoprotein biosynthesis. It acts selectively in Herpes simplex virus (HSV)-infected cells and does not significantly affect glycoprotein synthesis in uninfected cells and in cells infected with other viruses. Previously, we reported on a syncytial (syn) mutant, designated HSV-1(13)S11, resistant to BH, and showed that BH-resistance is encoded in the mutant virus DNA and therefore can be transferred into the genome of wild type HSVs. The present paper reports on a preliminary mapping in HSV-1(13)S11 genome of the loci which confer resistance to BH and of three distinct syn mutations present simultaneously in this mutant. Two of them were mapped in previously described syn loci localized in BamHI fragment L (map units 0.707-0.745) (locus syn 1) and BamHI fragment Q (map units 0.296-0.317) (locus syn 5). A third mutation not described before and mapping in BamHI fragment SP (c.a. map units 0.81-0.85) conferred the syn phenotype to both HEp-2 and Vero cells. This novel mutation has been designated herein locus syn 6. Transfer of BH-resistance could be achieved in cotransfection experiments involving two HSV-1(13)S11 fragments, BamHIL and BamHISP.

Common epitopes of glycoprotein B map within the major DNA-binding proteins of bovine herpesvirus type 2 (BHV-2) and herpes simplex virus type 1 (HSV-1)

Bovine herpesvirus 2 (BHV-2) specifies a glycoprotein of 130 kDa (gB BHV-2) which shows extensive homology to glycoprotein B (gB-1) of herpes simplex virus 1 (HSV-1). The BHV-2-specific 130-kDa glycoprotein is able to induce cross-reacting antibodies, some of which even cross-neutralize HSV-1. In order to determine the genome localization of gB BHV-2 and in order to identify conserved antigenic domains in both glycoproteins, we established libraries of subgenic fragments of BHV-2 and HSV-1 DNA in the prokaryotic expression vector lambda gt11 and screened them with cross-reacting monoclonal antibodies which allowed us to identify recombinant lambda gt11 clones expressing gB fusion protein. Nucleotide sequencing of inserted DNA fragments within these recombinant lambda gt11 clones revealed that they originated from the carboxy-terminal part of the major DNA-binding proteins (dbp) of BHV-2 (dbp BHV-2) and its counterpart ICP8 in HSV-1. Antisera raised against the beta-galactosidase fusion protein of recombinant phage lambda-113/2 coding for an 84 amino acid (aa) polypeptide originating from dbp BHV-2 neutralized infectivity of BHV-2 and HSV-1 in the presence of complement and precipitated [3H] glucosamine-labeled gB BHV-2 and gB-1. This antiserum also reacts with ICP8 and presumably with dbp BHV-2. Two hypotheses are discussed to explain this unexpected result: (i) epitopes in the carboxy-terminal part of gB BHV-2 and gB-1 are similar to antigenic determinants in the amino-terminal region of the gBs, thus providing cross-reacting antibody-binding sites; (iii) during gene expression a carboxy-terminal part of dbp BHV-2 and ICP8 genes might be spliced to the amino-terminal region of the glycoproteins gB BHV-2 and gB-1.

Role of glycoprotein B of herpes simplex virus type 1 in viral entry and cell fusion

Glycoprotein B (gB) of herpes simplex virus type 1 is an envelope protein that is essential for viral growth. We previously reported the isolation of two gB-null viruses, which form gB-free virions in nonpermissive cells. In the present study, these gB-free virions were shown to bind to the cell surface at the same rate as the wild-type virus. They failed, however, to form plaques and to synthesize virus-specific proteins upon infection. Their plating efficiency was significantly enhanced by treatment with polyethylene glycol, a membrane fusion agent. Therefore, gB is required in a stage after viral attachment but before the expression of the virus-specific proteins. A gB-null syncytial virus was isolated, which contained a gB defect and a syncytial mutation in another genetic locus. It caused complete fusion of gB-transformed cells but no fusion on untransformed cells, indicating the essential role of gB in virus-induced cell fusion. Mutations located at two independent sites in the cytoplasmic domain of gB were transferred to viral DNA and shown to confer a syncytial phenotype to the virus. A transient-expression assay was developed to determine the ability of a set of plasmids containing addition and nonsense mutations in the gB gene to complement the cell-fusion defect in the gB-null syncytial virus. Mutations in plasmids, including those located in the extracytoplasmic domain of gB, were identified that reduced the fusion activity of gB. Therefore, gB contains different functional regions responsible for fusion induction and its inhibition.

Abundant expression of herpes simplex virus glycoprotein gB using an adenovirus vector

Herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is a major component of infected cell membranes and virion envelopes. Glycoprotein B is known to be essential for entry of viruses into cells and may play important roles in virus-induced cell fusion and other alterations in cell morphology. In order to study the biochemical and immunological properties of gB in isolation from other HSV-1 polypeptides we have constructed human adenovirus vectors capable of expressing high levels of gB. The gB gene was coupled to the SV40 early promoter and inserted into the E3 region of two adenovirus vectors, one in which the E1 region was deleted (AdgB-1) and another which contained E1 sequences (AdgB-2). In AdgB-1 the orientation of the chimeric gB-SV40 gene was right to left, i.e., opposite to the direction of late and E3 mRNA transcription, whereas in AdgB-2 the orientation was left to right. Human 293 cells which express E1 functions supported replication of AdgB-1 and gB was expressed in these cells but not in mouse cells and only at very low levels in human cells other than 293. Replication of AdgB-2 was not limited to 293 cells and the virus was able to induce synthesis of gB at levels equal to or higher than those expressed in HSV-1-infected human or mouse cells. Microscopic examination of AdgB-2-infected cells revealed extensive vacuolization in a manner completely uncharacteristic of adenovirus-infected cells, and fluorescent antibody staining indicated that gB was not only present at the cell surface but also concentrated in the cytoplasmic vacuoles.

Entry of herpes simplex virus 1 in BJ cells that constitutively express viral glycoprotein D is by endocytosis and results in degradation of the virus

The BJ cell line which constitutively expresses herpes simplex virus 1 glycoprotein D is resistant to infection with herpes simplex viruses. Analysis of clonal lines indicated that resistance to superinfecting virus correlates with the expression of glycoprotein D. Resistance was not due to a failure of attachment to cells, since the superinfecting virus absorbed to the BJ cells. Electron microscopic studies showed that the virions are juxtaposed to coated pits and are then taken up into endocytic vesicles. The virus particles contained in the vesicles were in various stages of degradation. Viral DNA that reached the nucleus was present in fewer copies per BJ cell than that in the parental BHKtk- cells infected at the same multiplicity. Moreover, unlike the viral DNA in BHKtk- cells which was amplified, that in BJ cells decreased in copy number. The results suggest that the glycoprotein D expressed in the BJ cell line interfered with fusion of the virion envelope with the plasma membrane but not with the adsorption of the virus to cells and that the viral proteins that mediate adsorption to and fusion of membranes appear to be distinct.

HSV-1 virulence for mice by the intracerebral route is encoded by the BamHI-L DNA fragment containing the cell fusion gene

The phenotype of pathogenicity by direct intracerebral inoculation of herpes simplex virus type 1 (HSV-1) was mapped in the viral genome. This phenotype could be rescued by cotransfection of unit length HSV-1 DNA of an avirulent strain with the BamHI fragment L (0.70-0.738 map units) cloned from a virulent strain. The virulence function was localized in the 2.0 Kb NruI-BamHI fragment in the right-hand side of BamHI-L, the same region that encodes a virus cell-fusion gene (3). Transduction of virulence was linked with the phenotype of a larger plaque size. It is concluded that a neurovirulence function resides in the BamHI-L fragment of the HSV-1 genome, closely linked to the viral gene for cell fusion.

The single base pair substitution responsible for the Syn phenotype of herpes simplex virus type 1, strain MP

Nucleotide sequences were determined for portions of the genomes of the syncytial (Syn) mutant of herpes simplex virus type 1, strain MP, and the related wild-type strain mP. Comparisons of the nucleotide sequences showed only 1 bp difference between the DNAs of strains MP and mP in the region to which the Syn mutation of MP had previously been mapped. This base pair substitution in MP (at map coordinate 0.737) eliminates a ThaI restriction endonuclease recognition site that is present in mP DNA. Analyses of MP X mP recombinant viruses showed that presence of the ThaI site correlates with the Syn+ phenotype and absence of the ThaI site correlates with the Syn phenotype as predicted. We conclude that the base pair substitution at map coordinate 0.737 is responsible for the Syn phenotype of MP. This mutation could alter translation in four of the six reading frames, causing amino acid substitutions. From only one of these reading frames is a product likely to be expressed. The 338-amino acid polypeptide that could be expressed has features characteristic of membrane-associated proteins, including hydrophobic domains, potential sites for the attachment of N-linked carbohydrate, and a potential cleavable signal sequence.

Expression of cell-associated and secreted forms of herpes simplex virus type 1 glycoprotein gB in mammalian cells

The gene for glycoprotein gB1 of herpes simplex virus type 1 strain Patton was expressed in stable Chinese hamster ovary cell lines. Expression vectors containing the dihydrofolate reductase (dhfr) cDNA plus the complete gB1 gene or a truncated gene lacking the 194 carboxyl-terminal amino acids of gB1 were transfected into CHO DHFR-deficient cells. Radioimmunoprecipitation demonstrated that the complete gB1 protein expressed in CHO cell lines was cell associated, whereas the truncated protein was secreted from the cells due to deletion of the transmembrane and C-terminal domains of gB1. Cells expressing the truncated gB1 protein were subjected to stepwise methotrexate selection, and a cell line was isolated in which the gB1 gene copy number had been amplified 10-fold and the level of expression of gB1 had increased over 60-fold. The truncated gB1 protein was purified from medium conditioned by the amplified cell line. N-terminal amino acid sequence analysis of this purified protein identified the signal peptide cleavage site and predicted the cleavage of a 30-amino-acid signal sequence from the primary protein. The immunogenicity of the truncated gB1 protein was also tested in mice, and high levels of antibody and protection from virus challenge were observed.

Multiple adjacent or overlapping loci affecting the level of gC and cell fusion mapped by intratypic recombinants of HSV-1

We have prepared and analyzed 40 HSV-1 intratypic recombinants with regard to plaque morphology and glycoprotein C(gC) phenotypes. Vero cells have been cotransfected with the intact genome of HSV-1(F) and cloned or uncloned DNA fragments from HSV-1(MP) and recombinants inducing the fusion of Vero cells [syncytial (Syn) recombinants] have been selected and purified. Marker transfer of the Syn phenotype has been observed with the cloned BamHI L and B fragments (0.706-0.745 and 0.745-0.810 map units, respectively) as well as with the uncloned HpaI TXO fragment (0.710-0.761) from MP DNA. No marker transfer has been observed with F DNA alone or with the cloned BamHI N fragment (0.863-0.898 map units). When viruses expressing the Syn phenotype in Vero cells were tested in HEp-2 cells, three kinds of recombinants were observed. Members of the first class expressed a wild type, cytoaggregating (Syn+), plaque morphology in these cells. Members of the second class induced the complete fusion (Syn phenotype) of the cells. Members of the third class induced an intermediate plaque morphology, characterized by the formation of groups of polykaryocytes (fused cells) but without formation of a complete syncytium. All recombinants expressing the Syn+ phenotype in HEp-2 cells were also gC+, whereas recombinants expressing the Syn phenotype in these cells were gC- with one exception, in which low levels of gC could be detected (but clearly less than with HSV-1(F]. Concerning polykaryocytic class of recombinants, some of them were gC+ while others expressed only low amounts of gC; no gC- virus was observed within this class of recombinants. The three classes of recombinants were observed with each of the cloned BamHI L and B fragments and also with the HpaI TXO fragment, suggesting the existence of multiple adjacent or overlapping loci affecting plaque morphology and the control of the accumulation or the synthesis of gC at both sides of 0.745 map units.

Pathogenicity in mice of herpes simplex virus type 2 mutants unable to express glycoprotein C

Herpes simplex virus type 2 (HSV-2) mutants that were unable to express glycoprotein C (gC-2) were isolated. Deletions were made in a cloned copy of the gC-2 gene, and recombinant viruses containing these deletions were screened by using an immunoreactive plaque selection protocol. The viruses did not display a syncytial phenotype. Intravaginal inoculation of BALB/cJ mice with one of the HSV-2 gC-2- viruses produced local inflammation followed by a lethal spread of the viral infection into the nervous system in a manner identical to that produced by parental HSV-2 strain 333. Similarly, intracerebral inoculation of DBA-2 mice with the gC-2- virus produced a lethal neurological disease paralleling that caused by HSV-2 strain 333. These results indicate that gC-2 is not required for the spread of HSV-2 infections in mice.

Nucleotide sequence of a herpes simplex virus type 1 gene that causes cell fusion

The nucleotide sequence (2041 nucleotides) of a genomic region of herpes simplex virus type 1 (KOS strain) associated with virus-induced cell fusion has been determined. The sequence is bounded by a NruI site at 0.732 and a BamHI site at 0.745 prototypic map units. An open reading frame in the left-to-right orientation specifies a protein of 338 amino acids. The protein is positively charged. Since secondary structure analysis predicts four extensive hydrophobic domains the protein is probably a membrane-associated or a transmembrane protein. Transcription of the putative fusion gene is dependent on viral DNA synthesis, characteristic of the late (gamma) viral gene class. Two syncytia-inducing mutations, syn20 and MP, have been previously mapped to a 504-base pair PstI fragment within these genomic coordinates (V. C. Bond and S. Person (1984), Virology 132, 368-376). The nucleotide sequence of the PstI fragment was determined for the two mutants. Both were shown to have an amino acid substitution at residue 40 of the fusion protein. A second change at residue 101 for MP is probably unrelated to the fusion phenotype.