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

Insertion of a ligand to HER2 in gB retargets HSV tropism and obviates the need for activation of the other entry glycoproteins

Herpes simplex virus (HSV) entry into the cells requires glycoproteins gD, gH/gL and gB, activated in a cascade fashion by conformational modifications induced by cognate receptors and intermolecular signaling. The receptors are nectin1 and HVEM (Herpes virus entry mediator) for gD, and αvβ6 or αvβ8 integrin for gH. In earlier work, insertion of a single chain antibody (scFv) to the cancer receptor HER2 (human epidermal growth factor receptor 2) in gD, or in gH, resulted in HSVs specifically retargeted to the HER2-positive cancer cells, hence in highly specific non-attenuated oncolytic agents. Here, the scFv to HER2 was inserted in gB (gBHER2). The insertion re-targeted the virus tropism to the HER2-positive cancer cells. This was unexpected since gB is known to be a fusogenic glycoprotein, not a tropism determinant. The gB-retargeted recombinant offered the possibility to investigate how HER2 mediated entry. In contrast to wt-gB, the activation of the chimeric gBHER2 did not require the activation of the gD and of gH/gL by their respective receptors. Furthermore, a soluble form of HER2 could replace the membrane-bound HER2 in mediating virus entry, hinting that HER2 acted by inducing conformational changes to the chimeric gB. This study shows that (i) gB can be modified and become the major determinant of HSV tropism; (ii) the chimeric gBHER2 bypasses the requirement for receptor-mediated activation of other essential entry glycoproteins.

Human cytomegalovirus glycoproteins gB and gH/gL mediate epithelial cell-cell fusion when expressed either in cis or in trans

Herpesviruses use a cascade of interactions with different cell surface molecules to gain entry into cells. In many cases, this involves binding to abundant glycosaminoglycans or integrins followed by interactions with more limited cell surface proteins, leading to fusion with cellular membranes. Human cytomegalovirus (HCMV) has the ability to infect a wide variety of human cell types in vivo. However, very little is known about which HCMV glycoproteins mediate entry into various cell types, including relevant epithelial and endothelial cells. For other herpesviruses, studies of cell-cell fusion induced by viral proteins have provided substantial information about late stages of entry. In this report, we describe the fusion of epithelial, endothelial, microglial, and fibroblast cells in which HCMV gB and gH/gL were expressed from nonreplicating adenovirus vectors. Fusion frequently involved the majority of cells, and gB and gH/gL were both necessary and sufficient for fusion, whereas no fusion occurred when either glycoprotein was omitted. Coexpression of UL128, UL130, and UL131 did not enhance fusion. We concluded that the HCMV core fusion machinery consists of gB and gH/gL. Coimmunoprecipitation indicated that HCMV gB and gH/gL can interact. Importantly, expression of gB and gH/gL in trans (gB-expressing cells mixed with other gH/gL-expressing cells) resulted in substantial fusion. We believe that this is the first description of a multicomponent viral fusion machine that can be split between cells. If gB and gH/gL must interact for fusion, then these molecules must reach across the space between apposing cells. Expression of gB and gH/gL in trans with different cell types revealed surface molecules that are required for fusion on HCMV-permissive cells but not on nonpermissive cells.

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.

Isolation of the enhanced neurovirulent HSV-1 strains from Korean patients

Herpes simplex virus type 1 (HSV-1) is a neurotropic DNA virus which has latency in human. In this study, we isolated various HSV-1 strains, named KHS, from the skin lesions of Korean patients and characterized the specific features of each strain. We found that KHS strains produced small, cell associated and nonsyncycial plaques in Vero cells. We classified KHS strains into two substrains, KHS 1 which had highly condensed plaques and KHS 2 which had less condensed plaques. Since gD protein of HSV-1 plays important roles in viral plaque formation, we determined the nucleotide sequences of gD genes of KHS strains. According to deduced amino acid sequences of gD protein in KHS strains compared with prototype strains KOS and F, we found that gD of KHS strains have more putative O-glycosidic sites, serine in KHS 1 and threonine in KHS 2, respectively. To find out the establishment of viral latency, we infected each virus strain into eyes of mice and carried out trigerminal ganglia explanting experiment. We found that both KHS strains established latent infections stably just as did the prototype KOS and F strains. The eye swab experiments were carried out to check the viral replication in vivo. KHS 1 exhibited a longer shedding time in eyes of mice. We also found that KHS 1 has a higher neurotropic affinity by determining the time it took for the virus to reach the trigerminal ganglia from the eyes. Currently, we are studying the possible mechanism of high neuroinvasiveness of KHS 1 strain.

A single amino acid substitution in the cytoplasmic tail of the glycoprotein B of herpes simplex virus 1 affects both syncytium formation and binding to intracellular heparan sulfate

Herpes simplex virus 1 (HSV-1) (S) is a spontaneous syncytial mutant derived from the prototype HSV-1(F) after extensive plaque purification, and produces large syncytial plaques on Vero cells. Marker transfer experiments and DNA sequence analysis mapped the syncytial phenotype to a T-C base substitution at codon 787 of the cytoplasmic domain of mature gB, that results in Leu to Pro substitution and consequently belongs to the syn 3 locus. Both the cytoplasmic and the extracellular domains of gB are active in the fusion event since the addition of anti-gB monoclonal antibodies that recognize the extracellular domain of gB prevent HSV-1(S) induced cell fusion. Similarly, gD also participates in cell fusion since addition of anti-gD monoclonal antibodies also prevent HSV-1(S) induced cell fusion. Furthermore the glycoproteins B and D formed complexes in cells infected with mutant or wild type viruses. The amount of gB bound to total heparan sulfate is lower in the mutant than in the wild type strain. This difference becomes particularly profound when gB is associated with a portion of heparan sulfate intercalated to the membranes. The discrepancy in the binding of the mutant and wild type gB to heparan sulfate may be related to the mechanism of cell fusion induced by HSV-1(S).

Roles for endocytosis and low pH in herpes simplex virus entry into HeLa and Chinese hamster ovary cells

Herpes simplex virus (HSV) infection of many cultured cells, e.g., Vero cells, can be initiated by receptor binding and pH-neutral fusion with the cell surface. Here we report that a major pathway for HSV entry into the HeLa and CHO-K1 cell lines is dependent on endocytosis and exposure to a low pH. Enveloped virions were readily detected in HeLa or receptor-expressing CHO cell vesicles by electron microscopy at <30 min postinfection. As expected, images of virus fusion with the Vero cell surface were prevalent. Treatment with energy depletion or hypertonic medium, which inhibits endocytosis, prevented uptake of HSV from the HeLa and CHO cell surface relative to uptake from the Vero cell surface. Incubation of HeLa and CHO cells with the weak base ammonium chloride or the ionophore monensin, which elevate the low pH of organelles, blocked HSV entry in a dose-dependent manner. Noncytotoxic concentrations of these agents acted at an early step during infection by HSV type 1 and 2 strains. Entry mediated by the HSV receptor HveA, nectin-1, or nectin-2 was also blocked. As analyzed by fluorescence microscopy, lysosomotropic agents such as the vacuolar H(+)-ATPase inhibitor bafilomycin A1 blocked the delivery of virus capsids to the nuclei of the HeLa and CHO cell lines but had no effect on capsid transport in Vero cells. The results suggest that HSV can utilize two distinct entry pathways, depending on the type of cell encountered.

The requirement of varicella zoster virus glycoprotein E (gE) for viral replication and effects of glycoprotein I on gE in melanoma cells

The glycoprotein E (gE) of varicella zoster virus (VZV), encoded by ORF68, is the most abundant viral glycoprotein. In the current experiments, we demonstrated that ORF68 deletion was incompatible with recovery of infectious virus from VZV cosmids. Replacing ORF68 at a nonnative AvrII site in the genome restored infectivity. Further, we found that VZV gE could be expressed under the control of the Tet-On promoter in stably transfected melanoma cell lines (Met-gE cells) without evidence of toxicity. In these Met-gE cells, gE colocalized with gamma-adaptin, a trans Golgi network marker, in perinuclear sites, but did not reach plasma membranes. In order to investigate how infection altered gE localization, we made a recombinant virus, vOka-MSPgE, with ORF68 from the VZV MSP strain. VZV MSP encodes a mutant gE protein (D150N) that lacks the mAb epitope, 3B3 (Santos et al., Virology 275, 306-317, 2000), whereas Met-gE protein binds mAb 3B3. Within 48 h after Met-gE cells were infected with vOka-MSPgE, the steady-state distribution of Met-gE protein extended beyond the perinuclear areas to other cytoplasmic sites and to plasma membranes. A second recombinant, vOka-MSPgE without gI (vOka-MSPgEdeltagI), was constructed to investigate Met-gE protein distribution in the absence of gI. The redistribution of Met-gE protein which was observed by 48 h after vOka-MSPgE infection did not occur until 5 days (140 h) within vOka-MSPgEdeltagI infected cells. After vOka-MSPgE infection of Met-gE cells, most Met-gE protein was in the final 94K mature form by 72 h. However, progression to predominance of mature gE was delayed in Met-gE cells infected with vOka-MSPgEdeltagI. These observations confirm our hypothesis that VZV gE is essential, based upon the demonstration of restored infectivity after replacing ORF68 in a nonnative site in the genome, and provide further evidence of the role of gI in facilitating the maturation and intracellular distribution of this critical VZV glycoprotein.

Recovery of an unusual fusogenic herpes simplex virus type 2 strain from a clinical specimen

A highly unusual herpes simplex virus type 2 strain, strain Burr, was isolated from a female genital tract clinical specimen. This virus induced remarkably rapid and extensive syncytium formation in Vero cells involving hundreds of cells but was less fusion active in HEp-2 cells, MRC-5 cells, and mink lung cells. Virus-infected cells produced the glycoproteins gB, gC, gD and gE.

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

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

Pseudorabies virus glycoprotein M inhibits membrane fusion

A transient transfection-fusion assay was established to investigate membrane fusion mediated by pseudorabies virus (PrV) glycoproteins. Plasmids expressing PrV glycoproteins under control of the immediate-early 1 promoter-enhancer of human cytomegalovirus were transfected into rabbit kidney cells, and the extent of cell fusion was quantitated 27 to 42 h after transfection. Cotransfection of plasmids encoding PrV glycoproteins B (gB), gD, gH, and gL resulted in formation of polykaryocytes, as has been shown for homologous proteins of herpes simplex virus type 1 (HSV-1) (A. Turner, B. Bruun, T. Minson, and H. Browne, J. Virol. 72:873-875, 1998). However, in contrast to HSV-1, fusion was also observed when the gD-encoding plasmid was omitted, which indicates that PrV gB, gH, and gL are sufficient to mediate fusion. Fusogenic activity was enhanced when a carboxy-terminally truncated version of gB (gB-008) lacking the C-terminal 29 amino acids was used instead of wild-type gB. With gB-008, only gH was required in addition for fusion. A very rapid and extended fusion was observed after cotransfection of plasmids encoding gB-008 and gDH, a hybrid protein consisting of the N-terminal 271 amino acids of gD fused to the 590 C-terminal amino acids of gH. This protein has been shown to substitute for gH, gD, and gL function in the respective viral mutants (B. G. Klupp and T. C. Mettenleiter, J. Virol. 73:3014-3022, 1999). Cotransfection of plasmids encoding PrV gC, gE, gI, gK, and UL20 with gB-008 and gDH had no effect on fusion. However, inclusion of a gM-expressing plasmid strongly reduced the extent of fusion. An inhibitory effect was also observed after inclusion of plasmids encoding gM homologs of equine herpesvirus 1 or infectious laryngotracheitis virus but only in conjunction with expression of the gM complex partner, the gN homolog. Inhibition by PrV gM was not limited to PrV glycoprotein-mediated fusion but also affected fusion induced by the F protein of bovine respiratory syncytial virus, indicating a general mechanism of fusion inhibition by gM.

Cell fusion activity of hepatitis C virus envelope proteins

To examine the cell fusion activity of hepatitis C virus (HCV) envelope proteins (E1 and E2), we have established a sensitive cell fusion assay based on the activation of a reporter gene as described previously (O. Nussbaum, C. C. Broder, and E. A. Berger, J. Virol. 68:5411-5422, 1994). The chimeric HCV E1 and E2 proteins, each consisting of the ectodomain of the E1 and E2 envelope protein and the transmembrane and cytoplasmic domains of the vesicular stomatitis virus G glycoprotein, were expressed on the cell surface. Cells expressing the chimeric envelope proteins and T7 RNA polymerase were cocultured with the various target cell lines transfected with a reporter plasmid encoding the luciferase gene under the control of the T7 promoter. After cocultivation, the cell fusion activity was determined by the expression of luciferase in the cocultured cells. The induction of cell fusion requires both the chimeric E1 and E2 proteins and occurs in a low-pH-dependent manner. Although it has been shown that HCV E2 protein binds human CD81 (P. Pileri, Y. Uematsu, S. Campagnoli, G. Galli, F. Falugi, R. Petracca, A. J. Weiner, M. Houghton, D. Rosa, G. Grandi, and S. Abrignani, Science 282:938-941, 1998), the expression of human CD81 alone is not sufficient to confer susceptibility to cell fusion in the mouse cell line. Treatment of the target cells with pronase, heparinase, or heparitinase reduced the cell fusion activity induced by the chimeric envelope proteins. These results suggest (i) that both HCV E1 and E2 proteins are responsible for fusion with the endosomal membrane after endocytosis and (ii) that certain protein molecules other than human CD81 and some glycosaminoglycans on the cell surface are also involved in the cell fusion induced by HCV.

Analysis of the steps involved in Dengue virus entry into host cells

The initial steps of dengue viral entry have been divided into adsorption and penetration using acid glycine treatment to inactivate extracellular virus after attachment to baby hamster kidney (BHK) cells but prior to penetration. First, we showed that virus infection was accomplished within 2 h after adsorption. Second, the assay was used to examine the properties of dengue envelope E protein-specific monoclonal antibodies (MAbs), lectins, and heparin. We found that three MAbs, 17-2, 46-9, and 51-3, may neutralize dengue 2 virus (DEN-2) through inhibition of not only viral attachment but also of penetration. However, one MAb, 56-3.1, interfered specifically with attachment. Therefore, the functional domains of E protein involved in attachment and penetration may be different. Moreover, studies with lectins indicated that carbohydrates, especially alpha-mannose residues, present on the virion glycoproteins may contribute to binding and penetration of the virus into BHK and mosquito C6/36 cells. Finally, virus infectivity was inhibited by heparin through its blocking effects at both virus attachment and penetration. This suggests that cell surface heparan sulfate functions in both viral attachment and penetration of DEN-2 virus. In conclusion, our results further elucidated some aspects of the dengue virus entry process.

Recombinant herpes simplex virus type 1 engineered for targeted binding to erythropoietin receptor-bearing cells

The utility of recombinant herpes simplex virus type 1 (HSV-1) vectors may be expanded by manipulation of the virus envelope to achieve cell-specific gene delivery. To this end, an HSV-1 mutant virus deleted for glycoprotein C (gC) and the heparan sulfate binding domain of gB (KgBpK-gC-) was engineered to encode different chimeric proteins composed of N-terminally truncated forms of gC and the full-length erythropoietin hormone (EPO). Biochemical analyses demonstrated that one gC-EPO chimeric molecule (gCEPO2) was posttranslationally processed, incorporated into recombinant HSV-1 virus (KgBpK-gCEPO2), and neutralized with antibodies directed against gC or EPO in a complement-dependent manner. Moreover, KgBpK-gCEPO2 recombinant virus was specifically retained on a soluble EPO receptor column, was neutralized by soluble EPO receptor, and stimulated proliferation of FD-EPO cells, an EPO growth-dependent cell line. FD-EPO cells were nevertheless refractory to productive infection by both wild-type HSV-1 and recombinant KgBpK-gCEPO2 virus. Transmission electron microscopy of FD-EPO cells infected with KgBpK-gCEPO2 showed virus endocytosis leading to aborted infection. Despite the lack of productive infection, these data provide the first evidence of targeted HSV-1 binding to a non-HSV-1 cell surface receptor.

Herpes simplex virus proteins are damaged following photodynamic inactivation with phthalocyanines

The photodynamic inactivation of herpes simplex virus type 1 (HSV-1) by two phthalocyanines (Pcs), the cationic dye HOSi-PcOSi(CH3)2(CH2)3N+(CH3)3I-(Pc5) and the amphiphilic dye aluminum dibenzodisulfophthalocyanine hydroxide (AlN2SB2POH), has been compared with that by the anionic dye, Merocyanine 540 (Mc540). Both Pc derivatives demonstrate a remarkable virucidal activity upon light activation even 3 h after the onset of HSV-1 adsorption, while Mc540 is effective for only 30 min after adsorption. Since fusion and virus penetration are promoted by membrane glycoproteins, we have studied the damage to viral proteins following photodynamic treatment (PDT) of HSV-1 and its relation to inactivation. The effect of AlN2SB2POH PDT is assessed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Major changes are found in the protein profile of PDT-treated HSV-1. A reduced ability of specific antibodies to react with HSV-1 major envelope proteins is detected by employing the Western blot assay. In particular, we demonstrate the related changes of glycoprotein D (gD), a structural protein of the HSV envelope. Since the envelope proteins participate in viral entry into the host cell, these alterations to viral envelope proteins may impair their ability to participate in early events of viral entry, leading to reduced infectivity of HSV-1. In contrast, no significant changes in the proteins' electrophoretic mobility could be seen after PDT with Mc540 or with Pc5. When HSV-1 purified proteins are subjected to combined electrophoretic and electro osmotic forces on cellulose acetate, there is a shift in their cathode mobility, which may indicate changes in the protein mass and protein net charges following AlN2SB2POH photosensitization. There are only minor changes in the virus proteins, assayed as above, when HSV-1 is treated with Pc5.

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

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

Functional analysis of the transmembrane anchor region of bovine herpesvirus 1 glycoprotein gB

In herpesviruses, homologues of glycoprotein B (gB) are essential membrane proteins which are involved in fusion. However, there is no clear evidence regarding the location of the fusogenic domain on gB. By using bovine herpesvirus 1 (BHV-1) as a model, we studied the relationship between the structure and the fusogenic activity of gB. This was achieved by expressing genes of different gB derivatives containing specific truncations at the end of segments 2 or 3 of the transmembrane region in Madin-Darby bovine kidney cells under the control of the bovine heat-shock protein hsp70A gene promoter. All expressed gB products were structurally similar to authentic gB. One truncated form of gB, gBt, which contains residues 1-763, was efficiently secreted. However, gBtM (residues 1-807), which includes the first two segments at the carboxyl terminus, showed unstable retention on the cell surface, whereas gBtMA (residues 1 829), which contains all three membrane-spanning segments, was mostly intracellularly retained with some unstable surface anchorage. Another truncated gB, gBtDAF, which has gB residues 1-763 (gBt) and a human decay-accelerating factor (DAF) carboxyl tail, was also expressed. The DAF fragment provided a signal for the addition of a glycosyl phosphatidylinositol-based membrane anchor, which could target the gBt chimeric protein on the cell membrane. Immunofluorescence staining and pulse-chase kinetic studies support the theory that gBtM, gBtMA, and gBtDAF are retained on nuclear and cellular membranes via different segments of the transmembrane region or the DAF fragment, respectively. For the cells expressing gBt or gBtM, no cell fusion was observed, whereas cells expressing gBtMA clearly showed fusion. However, in gBtDAF cells, the overexpression and cellular accumulation of recombinant gB products did not cause fusion either, which supports our contention that the fusion phenomenon in gBtMA cells is caused by the fusogenic activity of the expressed gBtMA. With the help of sequence analysis, our results indicate that segment 2 of the transmembrane anchor region might be a fusogenic domain, whereas the real anchor is segment 3.

Expression and characterization of equine herpesvirus 1 glycoprotein D in mammalian cell lines

Equine herpesvirus 1 glycoprotein D (EHV-1 gD) expressed constitutively in mammalian cell lines had similar electrophoretic mobility to gD produced in EHV-1 infected cells but lacked a possibly complexed higher molecular weight form seen in the latter. Recombinant gD was N-terminally cleaved at the same site as gD in EHV-1 infected cells and expression was associated with enhanced levels of cell-cell fusion, indicating a role for EHV-1 gD in cell-to-cell transmission of virus.

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

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

PREPs: herpes simplex virus type 1-specific particles produced by infected cells when viral DNA replication is blocked

Herpes simplex virus (HSV)-infected cells produce not only infectious nucleocapsid-containing virions but also virion-related noninfectious light particles (L-particles) composed of the envelope and tegument components of the virus particle (J. F. Szilágyi and C. Cunningham, J. Gen. Virol. 62:661-668, 1991). We show that BHK and MeWO cells infected either with wild-type (WT) HSV type 1 (HSV-1) in the presence of viral DNA replication inhibitors (cytosine-beta-D-arabinofuranoside, phosphonoacetic acid, and acycloguanosine) or with a viral DNA replication-defective mutant of HSV-1 (ambUL8) synthesize a new type of virus-related particle that is morphologically similar to an L-particle but differs in its relative protein composition. These novel particles we term pre-viral DNA replication enveloped particles (PREPs). The numbers of PREPs released into the culture medium were of the same order as those of L-particles from control cultures. The particle/PFU ratios of different PREP stocks ranged from 6 x 10(5) to 3.8 x 10(8), compared with ratios of 3 x 10(3) to 1 x 10(4) for WT L-particle stocks. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western immunoblot analyses revealed that true late proteins, such as 273K (VP1-2), 82/81K (VP13/14), and gC (VP8), were greatly reduced or absent in PREPs and that gD (VP17) and 40K proteins were also underrepresented. In contrast, the amounts of proteins 175K (VP4; IE3), 92/91K (VP11/12), 38K (VP22), and gE (with BHK cells) were increased. The actual protein composition of PREPs showed some cell line-dependent differences, particularly in the amount of gE. PREPs were biologically competent and delivered functional Vmw65 (VP16; alpha TIF) to target cells, but the efficiency of complementation of the HSV-1 (strain 17) mutant in1814 was 10 to 30% of that of WT L-particles.

Characterization of a Marek's disease virus mutant containing a lacZ insertion in the US6 (gD) homologue gene

We report the construction of a Marek's disease virus (MDV) mutant containing the lacZ gene of Escherichia coli inserted into a homologue of the US6 (glycoprotein D, gD) gene of herpes simplex virus. The mutant was constructed using the high-passage GAatt85 MDV strain as the parent virus, since that strain grows readily in chicken embryo fibroblasts using culture conditions conducive to mutant virus construction. The lacZ insertion site was positioned one third of the way into the US6 (gD) open reading frame. Insertion of the lacZ gene disrupted a major 6.2 kb transcript that initiated approximately 2.5 kb upstream of the gD homologue gene in the vicinity of the US3 homologue and sorf4 genes, and extended into the US7 (gI) homologue gene. The mutant virus (US6lac) and the parent virus had similar growth kinetics in cell culture at 37 degrees C and 41 degrees C. Furthermore, the US6lac mutant could be reisolated from the spleens and peripheral blood of infected chickens with a frequency comparable to that of the parent virus. Our results indicate that the gene encoding the gD homologue is nonessential for growth in cell culture or for infection of chickens following intra-abdominal inoculation with an attenuated serotype-1 MDV.

Transformed cells producing the glycoprotein D of HSV-1 are resistant to infection with clinical strains of HSV

The generality of the resistance exhibited by gD producing cells to HSV-1 infection was tested. We tested three different cell lines producing various amounts of gD for resistance against three HSV-1 strains. The strains used were the prototype laboratory F strain and two recently isolated low passage local clinical strains, VG and VD. The results indicate that: (i) the resistance of the cell lines is directly related to the amount of gD they produce, (ii) the cell lines showed greater resistance against the two local clinical HSV-1 strains than against the laboratory strain, and (iii) the resistance is not mediated at the level of virus adsorption to the cell membranes.

Analogues of peptide 9-21 of glycoprotein D of herpes simplex virus and their binding to group VII monoclonal antibodies

Several analogues of the amino acid sequence of peptide 9-21 of glycoprotein D of herpes simplex virus type 1 (HSV-1) were synthesized and investigated for reactivity with different group VII monoclonal antibodies, Mabs LP14, ID3, 170, HD4, A16, EII-24 and Ev-10, in a competition enzyme-linked immunosorbent assay (ELISA). Replacement of Arg at position 16 by His resulted in a loss of binding with the group VII Mabs. Substitution of Pro at residue 14 by Leu gave a reduced binding for a number of Mabs and loss of binding for Mab A16. Substitution of Lys at position 10 by Glu gave reduced binding for three out of the seven Mabs. In addition substitutions of Met at position 11 by norleucine and oxidized Met were studied. The boundaries of the epitope cluster were mapped by studying synthetic variants of peptide 9-21 which were shorter either at the C-terminus or at the N-terminus, or both. Peptide 10-18 and peptide 9-17 were the shortest peptides, which were still reactive with the group VII Mabs. Mab HD4 requires the N-terminus of peptide 9-21 for effective binding, while for binding of other Mabs contribution of the residues in the C-terminal part of this peptide is more important.

Mapping of herpes simplex virus 1 genes with mutations which overcome host restrictions to infection

Earlier studies have shown that the thymidine kinase-negative baby hamster kidney (BHKTK-) cell lines expressing constitutively the herpes simplex virus 1 (HSV-1) glycoprotein D (gD), designated BJ, restrict infection by HSV-1 at the level of virus entry. U10, a HSV-1 mutant not restricted by the BJ cells, carried the substitution of proline for Leu25 in the gD gene, suggesting that gD encodes a specialized domain which precludes virus entry into cells expressing gD. Analyses of a new series of 36 unrestricted viral mutants showed the following. (i) Only two mutants contained mutations at a site which did not overlap with the previously reported mutation. A representative of a previously mapped mutant and one of the two new mutants were examined in detail. Thus, in the gD of mutant U30 Ala185 was replaced by threonine, whereas in gD of U21, Ala185 and Leu25 were replaced with threonine and proline, respectively. U30 and U21 multiplied better than the wild-type parent virus in the parental BHKTK- cells. (ii) Transfer of the gD gene from U21 or U30 to wild-type parent virus or to the gD- virus FgD beta yielded recombinants which, while capable of infecting BJ cells, were considerably less efficient than the parent unrestricted mutants, suggesting that the latter contained additional mutations which were responsible in part for the unrestricted phenotype. Conversely, marker rescue of mutant viruses with wild-type gD reduced but did not abrogate entirely the unrestricted phenotype. (iii) Mutations in gD which conferred the unrestricted phenotype were not random. (iv) gD plays a role in the restriction, inasmuch as preincubation of cells expressing gD with antibodies to gD abolished restriction. (v) In mutant R5000, the gD substitution Ser140 to Asn was capable of overcoming a restriction of a BHKTK- clonal line which does not express gD but conferred very low ability to replicate on BJ cells. We conclude that (a) uncloned stocks of BHKTK- cells exhibit a low level restriction to infection with wild-type virus, (b) clonal lines of BHKTK- cells which vary with respect to the stringency of restriction express either allelic genes differing in the properties of their products or products of different genes, and (c) both the restricted and unrestricted phenotypes reflect the interactions of gD with these cellular products. The implications of these conclusions with respect to the restriction imposed on BHK cells by the expression of gD are discussed.

A herpes simplex virus 1 US11-expressing cell line is resistant to herpes simplex virus infection at a step in viral entry mediated by glycoprotein D

A baby hamster kidney [BHK(tk-)] cell line (US11cl19) which stably expresses the US11 and alpha 4 genes of herpes simplex virus 1 strain F [HSV-1(F)] was found to be resistant to infection with HSV-1. Although wild-type HSV-1(F) attached with normal kinetics to the surface of US11cl19 cells, most cells showed no evidence of infection and failed to accumulate detectable amounts of alpha mRNAs. The relationship between the expression of UL11 and resistance to HSV infection in US11cl19 cells has not been defined, but the block to infection with wild-type HSV-1 was overcome by exposing cells with attached virus on their surface to the fusogen polyethylene glycol, suggesting that the block to infection preceded the fusion of viral and cellular membranes. An escape mutant of HSV-1(F), designated R5000, that forms plaques on US11cl19 cells was selected. This mutant was found to contain a mutation in the glycoprotein D (gD) coding sequence that results in the substitution of the serine at position 140 in the mature protein to asparagine. A recombinant virus, designated R5001, was constructed in which the wild-type gD gene was replaced with the R5000 gD gene. The recombinant formed plaques on US11cl19 cells with an efficiency comparable to that of the escape mutant R5000, suggesting that the mutation in gD determines the ability of the mutant R5000 to grow on US11cl19 cells. The observation that the US11cl19 cells were slightly more resistant to fusion by polyethylene glycol than parental BHK(tk-) cells led to the selection and testing of clonal lines from unselected and polyethylene glycol-selected BHK(tk-) cells. The results were that 16% of unselected to as much as 36% of the clones selected for relative resistance to polyethylene glycol fusion exhibited various degrees of resistance to infection. The exact step at which the infection was blocked is not known, but the results illustrate the ease of selection of cell clones with one or more sites at which infection could be blocked.

Photodynamic inactivation of herpes viruses with phthalocyanine derivatives

The antiviral photosensitization capacity of 11 different phthalocyanine (Pc) derivatives was examined using herpes simplex virus-1, herpes simplex virus-2 and varicella zoster virus in the search for the most potent sensitizers for viral decontamination of blood. The kinetics of viral photoinactivation were resolved during the stages of viral adsorption and penetration into the host cells. The capacity of Pc in the photodynamic inactivation of viruses was compared with that of merocyanine 540 (MC540), another widely studied photosensitizer. Sensitivity to photoinactivation decreased progressively with time after addition of viruses to their host cells. The viruses were most sensitive to photodynamic inactivation up to 30 min from the initiation of adsorption. Cell-associated viruses, 45-60 min after the onset of adsorption, are highly resistant to photodynamic treatment by most photosensitizers, with the exception of amphiphilic Pc derivatives. Thus the mixed sulfonated Pc-naphthalocyanine derivatives AlNSB3P and AlN2SB2P demonstrated a remarkable decontamination activity even 60 min after the onset of adsorption. Ultrastructural examination of these photosensitized viruses demonstrated damage to the viral envelope which prevented viral adsorption and/or penetration. The non-enveloped adenovirus was found to be resistant to all the dyes tested.

Involvement of membrane-bound viral glycoproteins in adhesion of pseudorabies virus-infected cells

Cell-associated spread of pseudorabies virus (PrV) plays an important role in the pathogenesis of the disease. Besides the already known direct cell-to-cell spread of the virus in monolayers, adhesion and subsequent fusion of suspended PrV infected cells to monolayers of uninfected cells are thought to occur. To study the adhesion of PrV-infected cells, an in vitro model was developed in SK-6 cells. Specific adhesion of PrV-infected cells to an uninfected monolayer started 5 h after infection of the cells and reached a maximum 6 h later. A correlation was found between the surface expression of PrV glycoproteins on the infected cells and the adhesion of these cells. PrV hyperimmune serum completely inhibited binding of the infected cells. To investigate which PrV envelope glycoproteins were responsible for the cell adhesion, the infected cells were incubated with antisera against glycoproteins gII, gIII, and gp50. Antiserum against either gII or gIII inhibited cell adhesion, and antisera against gII and gIII together had a cooperative effect. Antiserum against gp50 had no effect on binding when used alone but enhanced the inhibition induced by gII and gIII antisera. Heparin and neomycin inhibited adhesion, showing that the receptor for adhesion was a heparinlike substance. SK-6 cells infected with a gIII deletion mutant of PrV exhibited a much lower adhesion. This binding was heparin and neomycin independent and was not blocked by anti-gII serum. Nevertheless, it was completely inhibited with PrV hyperimmune serum and with anti-gp50 serum. This finding demonstrates that the ligand for adhesion of gIII(-)-infected cells is glycoprotein gp50. These results strongly suggest that the mechanism for adhesion of a PrV-infected cell to an uninfected monolayer is similar to the mechanism of adsorption and penetration of a PrV virion to a host cell.

Equine herpesvirus 1 glycoprotein D: mapping of the transcript and a neutralization epitope

Studies with molecular and immunological techniques identified and mapped the transcript encoding glycoprotein D (gD) of equine herpesvirus 1 KyA, as well as two continuous gD antigenic determinants. Three mRNA species of 5.5, 3.8, and 1.7 kb overlap the gD open reading frame and are transcribed from the DNA strand encoding gD. Northern (RNA) blot hybridization with both DNA clones and riboprobes, as well as S1 nuclease analyses, showed the 3.8-kb mRNA to encode gD and to be synthesized as a late (beta-gamma) transcript. The 3.8-kb gD mRNA initiates within the US segment 91 and 34 nucleotides downstream of the CCAAT and TATA elements, respectively, and encodes a potential polypeptide of 392 amino acids. The termination site of this transcript maps within the terminal repeat at a site also used by the 5.5-kb mRNA and the IR6-encoded 1.2-kb mRNA, such that these three transcripts form a 3'-coterminal nested set. The extended size (2,250 nucleotides) of the 3' untranslated region of the gD transcript and its termination within the terminal repeat may result from the deletion of 3,859 bp, which eliminates two consensus polyadenylation signals downstream of the gD open reading frame of EHV-1 KyA. Use of antisera to synthetic peptides of 19 amino acids (residues 4 to 22) and 20 amino acids (residues 267 to 285) in Western immunoblot analyses revealed that gD is present in EHV-1 virions as a 55-kDa polypeptide. In addition, these antisera detected the 55-kDa protein as well as 58- and 47-kDa polypeptides in infected-cell extracts at late times of infection. Residues 4 to 22 make up a continuous neutralizing epitope of gD, since incubation of equine herpesvirus 1 with the anti-19-mer serum prior to infection results in reduced numbers of plaques and reduced levels of virus-encoded thymidine kinase. Complement is not required for neutralization mediated by the anti-19-mer serum.

Disulfide bond structure of glycoprotein D of herpes simplex virus types 1 and 2

Glycoprotein D (gD) is a structural component of the herpes simplex virus envelope which is essential for virus penetration. The function of this protein is highly dependent on its structure, and its structure is dependent on maintenance of three intact disulfide bonds. gD contains six cysteines in its ectodomain whose spacing is conserved among all its homologs in other alphaherpesviruses as well as Marek's disease virus. For other proteins, conservation of cysteine spacing correlates with conservation of disulfide bond structure. We have now solved the disulfide bond structure of gD-1 and gD-2 of herpes simplex virus types 1 and 2, respectively. Two approaches were used. First, we constructed 15 double-Cys mutants of gD-1, representing all possible disulfide pairs. In each case, codons for cysteines were changed to serine. We reasoned that if two cysteines normally form a disulfide bond, double mutations which eliminate one proper bond should be less harmful to gD structure than double mutations which eliminate two disulfide bonds. The mutated genes were cloned into a eucaryotic expression vector, and the proteins were expressed in transiently transfected cells. Three double mutations, Cys-1,5, Cys-2,6, and Cys-3,4 permitted gD-1 folding, processing, transport to the cell surface, and function in virus infection, whereas 12 other double mutations each produced a malfolded and nonfunctional protein. Thus, the three functional double-Cys mutants may represent the actual partners in disulfide bond linkages. The second approach was to define the actual disulfide bond structure of gD by biochemical means. Purified native gD-2 was cleaved by CNBr and proteases, and the peptides were separated by high-performance liquid chromatography. Disulfide-linked peptides were subjected to N-terminal amino acid sequencing. The results show that cysteine 1 (amino acid [aa] 66) is bonded to cysteine 5 (aa 189), cysteine 2 (aa 106) is bonded to cysteine 6 (aa 202), and cysteine 3 (aa 118) is bonded to cysteine 4 (aa 127). Thus, the biochemical analysis of gD-2 agrees with the genetic analysis of gD-1. A similar disulfide bond arrangement is postulated to exist in other gD homologs.

Herpes simplex virus type 1 entry through a cascade of virus-cell interactions requires different roles of gD and gH in penetration

We examined the entry process of herpes simplex virus type 1 (HSV-1) by using infectious virus and previously characterized noninfectious viruses that can bind to cells but cannot penetrate as a result of inactivation of essential viral glycoprotein D (gD) or H (gH). After contact of infectious virus with the cell plasma membrane, discernible changes of the envelope and tegument could be seen by electron microscopy. Noninfectious virions were arrested at distinct steps in interactions with cells. Viruses inactivated by anti-gD neutralizing antibodies attached to cells but were arrested prior to initiation of a visible fusion bridge between the virus and cell. As judged from its increased sensitivity to elution, virus lacking gD was less stably bound to cells than was virus containing gD. Moreover, soluble gD could substantially reduce virus attachment when added to cells prior to or with the addition of virus. Virus inactivated by anti-gH neutralizing antibodies attached and could form a fusion bridge but did not show expansion of the fusion bridge or extensive rearrangement of the envelope and tegument. We propose a model for infectious entry of HSV-1 by a series of interactions between the virion envelope and the cell plasma membrane that trigger virion disassembly, membrane fusion, and capsid penetration. In this entry process, gD mediates a stable attachment that is likely required for penetration, and gH seems to participate in fusion initiation or expansion.

Glycoprotein gI of pseudorabies virus promotes cell fusion and virus spread via direct cell-to-cell transmission

Mutants of pseudorabies virus defective in either glycoprotein gI or gIII are only slightly less virulent for mice and chickens than is wild-type virus, while mutants defective in both gI and gIII are avirulent. To clarify the reason for the lack of virulence of the gI- gIII- mutants, we have analyzed in some detail the interactions of these mutants with their hosts. The results obtained showed that the gI glycoprotein is an accessory protein that promotes cell fusion. This conclusion is based on the findings that in some cell types, syncytium formation is significantly reduced in mutants deficient in gI. Furthermore, despite efficient replication, gI- mutants form significantly smaller plaques on some cell types. Finally, while wild-type and gI- virus are neutralized similarly by antisera, the size of the plaques formed by gI- mutants, but not by wild-type virus, is reduced by the presence of neutralizing antibodies in the overlay. Passive immunization of mice with neutralizing antipseudorabies virus sera is also considerably more effective in protecting them against challenge with gI- mutants than in protecting them against challenge with wild-type virus. These results show that gI- mutants are deficient in their ability to form syncytia and to spread directly by cell-to-cell transmission and that these mutants spread mainly by adsorption of released virus to uninfected cells. Wild-type virus and gIII- mutants, however, spread mainly via direct cell-to-cell transmission both in vivo and in vitro. We postulate that the lack of virulence of the gIII- gI- virus is attributable to its inability to spread by either mode, the defect in gIII affecting virus spread by adsorption of released virus and the defect in gI affecting cell-to-cell spread. Although a gI- gIII- mutant replicates as well as a gIII- mutant, it will be amplified much less well. Our results with in vitro systems show that this is indeed the case.

Pseudorabies virus envelope glycoproteins gp50 and gII are essential for virus penetration, but only gII is involved in membrane fusion

To investigate the function of the envelope glycoproteins gp50 and gII of pseudorabies virus in the entry of the virus into cells, we used linker insertion mutagenesis to construct mutant viruses that are unable to express these proteins. In contrast to gD mutants of herpes simplex virus, gp50 mutants, isolated from complementing cells, were able to form plaques on noncomplementing cells. However, progeny virus released from these cells was noninfectious, although the virus was able to adsorb to cells. Thus, the virus requires gp50 to penetrate cells but does not require it in order to spread by cell fusion. This finding indicates that fusion of the virus envelope with the cell membrane is not identical to fusion of the cell membranes of infected and uninfected cells. In contrast to the gp50 mutants, the gII mutant was unable to produce plaques on noncomplementing cells. Examination by electron microscopy of cells infected by the gII mutant revealed that enveloped virus particles accumulated between the inner and outer nuclear membranes. Few noninfectious virus particles were released from the cell, and infected cells did not fuse with uninfected cells. These observations indicate that gII is involved in several membrane fusion events, such as (i) fusion of the viral envelope with the cell membrane during penetration, (ii) fusion of enveloped virus particles with the outer nuclear membrane during the release of nucleocapsids into the cytoplasm, and (iii) fusion of the cell membranes of infected and uninfected cells.

Glycoprotein IV of bovine herpesvirus 1-expressing cell line complements and rescues a conditionally lethal viral mutant

Glycoprotein IV (gIV) of bovine herpesvirus 1 (BHV-1), a homolog of herpes simplex virus glycoprotein D, represents a major component of the viral envelope and a dominant immunogen. To analyze the functional role of gIV during BHV-1 replication, cell line BUIV3-7, which constitutively expresses gIV, was constructed and used for the isolation of gIV- BHV-1 mutant 80-221, in which the gIV gene was replaced by a lacZ expression cassette. On complementing gIV-expressing cells, the gIV- BHV-1 replicated normally but was unable to form plaques and infectious progeny on noncomplementing cells. Further analysis showed that gIV is essential for BHV-1 entry into target cells, whereas viral gene expression, DNA replication, and envelopment appear unchanged in both noncomplementing and complementing cells infected with phenotypically complemented gIV- BHV-1. The block in entry could be overcome by polyethylene glycol-induced membrane fusion. After passaging of gIV- BHV-1 on complementing cells, a rescued variant, BHV-1res, was isolated and shown to underexpress gIV in comparison with its wild-type parent. Comparison of the penetration kinetics of BHV-1 wild type, phenotypically complemented gIV- BHV-1, and BHV-1res indicated that penetration efficiency correlated with the amount of gIV present in virus particles. In conclusion, we show that gIV of BHV-1 is an essential component of the virion involved in virus entry and that the amount of gIV in the viral envelope modulates the penetration efficiency of the virus.

Domains of herpes simplex virus I glycoprotein B that function in virus penetration, cell-to-cell spread, and cell fusion

Herpes simplex virus 1 glycoprotein B (gB) is one of 10 glycoproteins in the virion envelope and in the membranes of infected cells. It is required for infection of cells in culture and functions in penetration of the cell by fusing the virion envelope with the plasma membrane. In studies to map the functional domains on HSV-1 gB, we reported that epitopes of potent neutralizing antibodies cluster in three major antigenic domains, D1, D2, and D5a. D1 contains continuous epitopes in the very amino terminus of gB. D2 comprises discontinuous epitopes that are assembled on gB derivatives 457 amino acids in length. D5a contains discontinuous epitopes that map between amino acids 600 and 690. We have now analyzed the function of these domains in virion infectivity by a detailed examination of the effects of 16 neutralizing antibodies on virion adsorption, penetration, plaque development, and cell fusion. Our results are as follows. (i) Ten antibodies with complement-independent neutralizing activity blocked penetration of virions into cells but not their adsorption to the cell surface. Treating cell-bound, neutralized virus with the fusogenic agent polyethylene glycol promoted their entry into cells. (ii) Ten antibodies with complement-dependent and -independent neutralizing activity interfered with plaque development by preventing spread of virus from infected to neighboring uninfected cells. (iii) Nine neutralizing antibodies, all complement-independent, prevented cell fusion induced by strain HFEM syn. We conclude that domains mapping in three regions of gB function in penetration of virions into cells, and that most neutralizing antibodies to these domains also block cell-to-cell spread of virus and cell fusion. The findings that three complement-independent neutralizing antibodies that blocked penetration did not inhibit plaque development, and that only one of these blocked cell fusion, indicate that the cell-to-cell spread of virus and cell fusion are related processes, but not identical to the penetration function.

Glycoprotein C of herpes simplex virus type 1 plays a principal role in the adsorption of virus to cells and in infectivity

The purpose of this study was to identify the herpes simplex virus glycoprotein(s) that mediates the adsorption of virions to cells. Because heparan sulfate moieties of cell surface proteoglycans serve as the receptors for herpes simplex virus adsorption, we tested whether any of the viral glycoproteins could bind to heparin-Sepharose in affinity chromatography experiments. Two glycoproteins, gB and gC, bound to heparin-Sepharose and could be eluted with soluble heparin. In order to determine whether virions devoid of gC or gB were impaired for adsorption, we quantitated the binding of wild-type and mutant virions to cells. We found that at equivalent input concentrations of purified virions, significantly fewer gC-negative virions bound to cells than did wild-type or gB-negative virions. In addition, the gC-negative virions that bound to cells showed a significant delay in penetration compared with wild-type virus. The impairments in adsorption and penetration of the gC-negative virions can account for their reduced PFU/particle ratios, which were found to be about 5 to 10% that of wild-type virions, depending on the host cell. Although gC is dispensable for replication of herpes simplex virus in cell culture, it clearly facilitates virion adsorption and enhances infectivity by about a factor of 10.

Glycoprotein D of herpes simplex virus encodes a domain which precludes penetration of cells expressing the glycoprotein by superinfecting herpes simplex virus

Earlier studies have shown that herpes simplex viruses adsorb to but do not penetrate permissive baby hamster kidney clonal cell lines designated the BJ series and constitutively expressing the herpes simplex virus 1 (HSV-1) glycoprotein D (gD). To investigate the mechanism of the restriction, the following steps were done. First, wild-type HSV-1 strain F [HSV-1(F)] virus was passaged blindly serially on clonal line BJ-1 and mutant viruses [HSV-1(F)U] capable of penetration were selected. The DNA fragment capable of transferring the capacity to infect BJ cells by marker transfer contains the gD gene. The mutant gD, designated gDU, differed from wild-type gD only in the substitution of Leu-25 by proline. gDU reacted with monoclonal antibodies which neutralize virus and whose epitopes encompass known functional domains involved in virus entry into cells. It did not react with the monoclonal antibody AP7 previously shown to react with an epitope which includes Leu-25. Second, cell lines expressing gDU constitutively were constructed and cloned. Unlike the clonal cell lines constitutively expressing gD (e.g., the BJ cell line), those expressing gDU were infectable by both HSV-1(F) and HSV-1(F)U. Lastly, exposure of BJ cells to monoclonal antibody AP7 rendered the cells capable of being infected with HSV-1(F). The results indicate that (i) gD expresses a specific function, determined by sequences at or around Leu-25, which blocks entry of virus into cells synthesizing gD, (ii) the gD which blocks penetration by superinfecting virus is located in the plasma membrane, (iii) the target of the restriction to penetration is the identical domain of the gD molecule contained in the envelope of the superinfecting virus, and (iv) the molecular basis of the restriction does not involve competition for a host protein involved in entry, as was previously thought.

Cysteine mutants of herpes simplex virus type 1 glycoprotein D exhibit temperature-sensitive properties in structure and function

We previously constructed seven mutations in the gene for glycoprotein D (gD) of herpes simplex virus type 1 in which the codon for one of the cysteine residues was replaced by a serine codon. Each of the mutant genes was cloned into a eucaryotic expression vector, and the proteins were transiently expressed in mammalian cells. We found that alteration of any of the first six cysteine residues had profound effects on protein conformation and oligosaccharide processing. In this report, we show that five of the mutant proteins exhibit temperature-sensitive differences in such properties as aggregation, antigenic conformation, oligosaccharide processing, and transport to the cell surface. Using a complementation assay, we have now assessed the ability of the mutant proteins to function in virus infection. This assay tests the ability of the mutant proteins expressed from transfected plasmids to rescue production of infectious virions of a gD-minus virus, F-gD beta, in Vero cells. Two mutant proteins, Cys-2 (Cys-106 to Ser) and Cys-4 (Cys-127 to Ser), were able to complement F-gD beta at 31.5 degrees C but not at 37 degrees C. The rescued viruses, designated F-gD beta(Cys-2) and F-gD beta(Cys-4), were neutralized as efficiently as wild-type virus by anti-gD monoclonal antibodies, indicating that gD was present in the virion envelope in a functional form. Both F-gD beta(Cys-2) and F-gD beta(Cys-4) functioned normally in a penetration assay. However, the infectivity of these viruses was markedly reduced compared with that of the wild type when they were preincubated at temperatures above 37 degrees C. The results suggest that mutations involving Cys-106 or Cys-127 in gD-1 confer a temperature-sensitive phenotype on herpes simplex virus. These and other properties of the cysteine-to-serine mutants allowed us to predict a disulfide bonding pattern for gD.

Bovine cells expressing bovine herpesvirus 1 (BHV-1) glycoprotein IV resist infection by BHV-1, herpes simplex virus, and pseudorabies virus

We expressed the bovine herpesvirus 1 (BHV-1) glycoprotein IV (gIV) in bovine cells. The protein expressed was identical in molecular mass and antigenic reactivity to the native gIV protein but was localized in the cytoplasm. Expressing cells were partially resistant to BHV-1, herpes simplex virus, and pseudorabies virus, as shown by a 10- to 1,000-fold-lower number of plaques forming on these cells than on control cells. The level of resistance depended on the level of gIV expression and the type and amount of challenge virus. These data are consistent with previous reports by others that cellular expression of the BHV-1 gIV homologs, herpes simplex virus glycoprotein D, and pseudorabies virus glycoprotein gp50 provide partial resistance against infection with these viruses. We have extended these findings by showing that once BHV-1 enters gIV-expressing cells, it replicates and spreads normally, as shown by the normal size of BHV-1 plaques and the delayed but vigorous synthesis of viral proteins. Our data are consistent with the binding of BHV-1 gIV to a cellular receptor required for initial penetration by all three herpesviruses and interference with the function of that receptor molecule.

Molecular cloning, sequencing, and expression of functional bovine herpesvirus 1 glycoprotein gIV in transfected bovine cells

The gene encoding bovine herpesvirus 1 (BHV-1) glycoprotein gIV was mapped, cloned, and sequenced. The gene is situated between map units 0.892 and 0.902 and encodes a predicted protein of 417 amino acids with a signal sequence cleavage site between amino acids 18 and 19. Comparison of the BHV-1 amino acid sequence with the homologous glycoproteins of other alphaherpesviruses, including herpes simplex virus type 1 glycoprotein gD, revealed significant homology in the amino-terminal half of the molecules, including six invariant cysteine residues. The identity of the open reading frame was verified by expression of the authentic recombinant BHV-1 gIV in bovine cells by using eucaryotic expression vectors pRSDneo (strong, constitutive promoter) and pMSG (weak, dexamethasone-inducible promoter). Constitutive expression of gIV proved toxic to cells, since stable cell lines could only be established when the gIV gene was placed under the control of an inducible promoter. Expression of gIV was cell associated and localized predominantly in the perinuclear region, although nuclear and plasma membrane staining was also observed. Radioimmunoprecipitation revealed that the recombinant glycoprotein was efficiently processed and had a molecular weight similar to that of the native form of gIV expressed in BHV-1-infected bovine cells. Recombinant gIV produced in the transfected bovine cells induced cell fusion, polykaryon formation, and nuclear fusion. In addition, expression of gIV interfered with BHV-1 replication in the transfected bovine cells.

Antiviral effects of apolipoprotein A-I and its synthetic amphipathic peptide analogs

Apolipoprotein A-I (apo A-I), the major protein component of serum high density lipoproteins, was found to inhibit herpes simplex virus (HSV)-induced cell fusion at physiological (approximately 1 microM) concentrations. An 18 amino acid-long synthetic amphipathic alpha-helical peptide analog of apo A-I (18A) was also found to inhibit HSV-induced cell fusion at similar concentration (approximately 2 microM). Dimers of 18A connected via a proline (37pA) or an alanine (37aA) residue also inhibited virus-induced cell fusion at similar concentration, suggesting that the presence of a proline turn does not influence the antiviral activity of the amphipathic peptides. However, a peptide analog 18R, in which the distribution of charged residues was reversed, inhibited virus-induced cell fusion only at a higher (approximately 125 microM) concentration, suggesting that the anti-viral activity of the amphipathic peptide is strongly influenced by the nature of the charge distribution at the polar-nonpolar interface. Consistent with their ability to inhibit virus-induced cell fusion, the peptides inhibited the spread of HSV infection as demonstrated by a 10-fold reduction in the virus yield, when virus-infected cells were maintained in the presence of amphipathic peptides. The amphipathic peptides also inhibited penetration of virus into cells, but did not exert any effect on virus adsorption. A nearly complete inhibition of virus penetration was observed when the virus, or both virus and cells, was pretreated with the peptide, suggesting that the peptides may have a direct effect on the virus. The results indicate that amphipathic helices may be useful in designing novel antiviral agents that inhibit penetration and spread of enveloped viruses.

Antigenic and functional analysis of a neutralization site of HSV-1 glycoprotein D

Herpes simplex virus glycoprotein D is a component of the virion envelope and appears to be involved in attachment, penetration, and cell fusion. Monoclonal antibodies (MAbs) against this protein can be arranged in groups, on the basis of a number of biological and biochemical properties. Group I antibodies are type-common, have high complement-independent neutralization titers, recognize discontinuous (conformational) epitopes, and block each other in a binding assay. The sum of their epitopes constitutes antigenic site I of gD. Using a panel of neutralization-resistant mutants, we previously found that group I MAbs can be divided into two subgroups, Ia and Ib, such that mutations selected with Ia antibodies have little or no effect on binding and neutralization by Ib antibodies, and vice versa. Antigenic site I therefore consists of two parts, Ia and Ib. We have now identified the point mutations which prevent neutralization. Two Ib MAbs (DL11 and 4S) selected a Ser to Asn change at residue 140; this alteration creates a new N-linked glycosylation site, which is used. A third Ib MAb (D2) selected a Gln to Leu change at 132. The mutation selected by the Ia MAb HD1 (Ser to Asn at residue 216) is identical to that selected by MAb LP2, another Ia antibody. By using oligonucleotide-directed mutagenesis, we have produced gD genes with combinations of the above mutations. Attempts to recombine these genes into the virus genome were unsuccessful, suggesting that the combinations are lethal. This was confirmed by a complementation assay which measures the ability of gD transiently expressed in transfected Vero cells to rescue the production of infectious virus by the gD-minus mutant F-gD beta.

Neutralizing antibodies specific for glycoprotein H of herpes simplex virus permit viral attachment to cells but prevent penetration

Monoclonal antibodies specific for gH of herpes simplex virus were shown previously to neutralize viral infectivity. Results presented here demonstrate that these antibodies (at least three of them) block viral penetration without inhibiting adsorption of virus to cells. Penetration of herpes simplex virus is by fusion of the virion envelope with the plasma membrane of a susceptible cell. Electron microscopy of thin sections of cells exposed to virus revealed that neutralized virus bound to the cell surface but did not fuse with the plasma membrane. Quantitation of virus adsorption by measuring the binding of purified radiolabeled virus to cells revealed that the anti-gH antibodies had little or no effect on adsorption. Monitoring cell and viral protein synthesis after exposure of cells to infectious and neutralized virus gave results consistent with the electron microscopic finding that the anti-gH antibodies blocked viral penetration. On the basis of the results presented here and other information published elsewhere, it is suggested that gH is one of three glycoproteins essential for penetration of herpes simplex virus into cells.