Herpes simplex virus type 1 glycoprotein E is not indispensable for viral infectivity

Improving antibody-mediated protection against HSV infection by eliminating interactions with the viral Fc receptor gE/gI

Herpes simplex virus (HSV) encodes surface glycoproteins that are host defense evasion molecules, allowing the virus to escape immune clearance. In addition to their role in neuropathogenesis and cell-cell spread, glycoproteins E and I (gE/gI) form a viral Fc receptor (vFcR) for most subclasses and allotypes of human IgG and promote evasion of humoral immune responses. While monoclonal antibodies (mAbs) protect mice from neonatal HSV (nHSV) infections, the impact of the vFcR on mAb-mediated protection by binding to IgG is unknown. Using HSV-1 with intact and ablated gE-mediated IgG Fc binding, and Fc-engineered antibodies with modified ability to interact with gE/gI, we investigated the role of the vFcR in viral pathogenesis and mAb-mediated protection from nHSV. The gD-specific human mAb HSV8 modified to lack binding to gE exhibited enhanced neutralization and in vivo protection compared to its native IgG1 form. This improved protection by the engineered mAbs was dependent on the presence of the vFcR. Human IgG3 allotypes lacking vFcR binding also exhibited enhanced antiviral activity in vivo, suggesting that vaccines that robustly induce IgG3 responses could show enhanced protection. suggesting the value of vaccination strategies that robustly induce this subclass. Lastly, analysis of longitudinal responses to acute primary genital infection in humans raised the possibility that unlike most viruses, HSV may exhibited slow induction of IgG3. In summary, this study demonstrates that mAbs lacking the ability to interact with the vFcR can exhibit improved protection from HSV-offering new prospects for antibody-based interventions.

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

Glycoprotein D of HSV-1 is dependent on tegument protein UL16 for packaging and contains a motif that is differentially required for syncytia formation

Glycoprotein D (gD) of herpes simplex virus type 1 (HSV-1) plays a key role in multiple events during infection including virus entry, cell-to-cell spread, and virus-induced syncytia formation. Here, we provide evidence that an arginine/lysine cluster located at the transmembrane-cytoplasm interface of gD critically contributes to viral spread and cell-cell fusion. Our studies began with the discovery that packaging of gD into virions is almost completely blocked in the absence of tegument protein UL16. We subsequently identified a novel, direct, and regulated interaction between UL16 and gD, but this was not important for syncytia formation. However, a mutational analysis of the membrane-proximal basic residues of gD revealed that they are needed for the gBsyn phenotype, salubrinal-induced fusion of HSV-infected cells, and cell-to-cell spread. Finally, we found that these same gD tail basic residues are not required for cell fusion induced by a gKsyn variant.

Herpes simplex virus Membrane Fusion

Herpes simplex virus mediates multiple distinct fusion events during infection. HSV entry is initiated by fusion of the viral envelope with either the limiting membrane of a host cell endocytic compartment or the plasma membrane. In the infected cell during viral assembly, immature, enveloped HSV particles in the perinuclear space fuse with the outer nuclear membrane in a process termed de-envelopment. A cell infected with some strains of HSV with defined mutations spread to neighboring cells by a fusion event called syncytium formation. Two experimental methods, the transient cell-cell fusion approach and fusion from without, are useful surrogate assays of HSV fusion. These five fusion processes are considered in terms of their requirements, mechanism, and regulation. The execution and modulation of these events require distinct yet often overlapping sets of viral proteins and host cell factors. The core machinery of HSV gB, gD, and the heterodimer gH/gL is required for most if not all of the HSV fusion mechanisms.

Roles of Us8A and Its Phosphorylation Mediated by Us3 in Herpes Simplex Virus 1 Pathogenesis

The herpes simplex virus 1 (HSV-1) Us8A gene overlaps the gene that encodes glycoprotein E (gE). Previous studies have investigated the roles of Us8A in HSV-1 infection using null mutations in Us8A and gE; therefore, the role of Us8A remains to be elucidated. In this study, we investigated the function of Us8A and its phosphorylation at serine 61 (Ser-61), which we recently identified as a phosphorylation site by mass spectrometry-based phosphoproteomic analysis of HSV-1-infected cells, in HSV-1 pathogenesis. We observed that (i) the phosphorylation of Us8A Ser-61 in infected cells was dependent on the activity of the virus-encoded Us3 protein kinase; (ii) the Us8A null mutant virus exhibited a 10-fold increase in the 50% lethal dose for virulence in the central nervous system (CNS) of mice following intracranial infection compared with a repaired virus; (iii) replacement of Ser-61 with alanine (S61A) in Us8A had little effect on virulence in the CNS of mice following intracranial infection, whereas it significantly reduced the mortality of mice following ocular infection to levels similar to the Us8A null mutant virus; (iv) the Us8A S61A mutation also significantly reduced viral yields in mice following ocular infection, mainly in the trigeminal ganglia and brains; and (v) a phosphomimetic mutation at Us8A Ser-61 restored wild-type viral yields and virulence. Collectively, these results indicate that Us8A is a novel HSV-1 virulence factor and suggest that the Us3-mediated phosphorylation of Us8A Ser-61 regulates Us8A function for viral invasion into the CNS from peripheral sites.

IMPORTANCE

The DNA genomes of viruses within the subfamily Alphaherpesvirinae are divided into unique long (UL) and unique short (Us) regions. Us regions contain alphaherpesvirus-specific genes. Recently, high-throughput sequencing of ocular isolates of HSV-1 showed that Us8A was the most highly conserved of 13 herpes simplex virus 1 (HSV-1) genes mapped to the Us region, suggesting Us8A may have an important role in the HSV-1 life cycle. However, the specific role of Us8A in HSV-1 infection remains to be elucidated. Here, we show that Us8A is a virulence factor for HSV-1 infection in mice, and the function of Us8A for viral invasion into the central nervous system from peripheral sites is regulated by Us3-mediated phosphorylation of the protein at Ser-61. This is the first study to report the significance of Us8A and its regulation in HSV-1 infection.

Immune response and cytokine production following immunization with experimental herpes simplex virus 1 (HSV-1) vaccines

Balb/c mice were immunized with the recombinant fusion protein gD1/313 (FpgD1/313 representing the ectodomain of HSV-1 gD), with the non-pathogenic ANGpath gE-del virus, with the plasmid pcDNA3.1-gD expressing full-length gD1 and with the recombinant immediate early (IE) HSV-1 protein ICP27. Specific antibodies against these antigens (as detected by ELISA) reached high titers with the exception of the DNA vaccine. High-grade protection against challenge with the virulent strain SC16 was found following immunization with the pcDNA3.1-gD plasmid and with the gE-del virus. Medium grade, but satisfactory protection developed after immunization with the FpgD1/313 and minimum grade protection was seen upon immunization with the IE/ICP27 polypeptide. A considerable response of peripheral blood cells (PBL) and splenocytes in the lymphocyte transformation test (LTT) was found in mice immunized with FpgD1/313, with the pcDNA3.1-gD plasmid and with the live ANGpathgE-del virus. For lymphocyte stimulation in vitro, the FpgD1/313 antigen was less effective than the purified gD1/313 polypeptide (cleaved off from the fusion protein); both proteins elicited higher proliferation at the 5 microg per 0.1 mL dose than at the 1 microg per 0.1 mL dose. The secretion of Th type 1 (TNF, IFN-gamma and IL-2) and Th type 2 (IL-4 and IL-6) cytokines was tested in the medium fluid of purified PBL and splenocyte cultures; their absolute values were expressed in relative indexes. The PBL from FpgD1/313 immunized mice showed increased secretion of both T(H)1 (TNF) as well as T(H)2 (IL-4) cytokines (7-10-fold, respectively). Splenocytes from FpgD1/313 immunized mice showed a significant (23-fold) increase in IL-4 production.

Responses of herpes simplex virus type 1-infected cells to the presence of extracellular antibodies: gE-dependent glycoprotein capping and enhancement in cell-to-cell spread

Binding of anti-herpes simplex virus (HSV) immunoglobulin G (IgG) to HSV type 1 (HSV-1)-infected HEL and HEp-2 cells causes changes in surface viral glycoprotein distribution, resulting in a capping of all viral glycoproteins towards one pole of the cell. This occurs in a gE-dependent manner. In HEL cells, low concentrations of anti-HSV IgG also enhance cell-to-cell spread of wild-type HSV-1 but not of gE deletion mutant HSV-1. These observations raised the possibility that gE-dependent mechanisms exist that allow some HSV-1-infected cells to respond to the presence of extracellular antibodies by enhancing the antibody-resistant mode of virus transmission.

Identification and expression of human cytomegalovirus transcription units coding for two distinct Fcgamma receptor homologs

Cellular receptors for the Fc domain of immunoglobulin G (IgG) (FcgammaRs) comprise a family of surface receptors on immune cells connecting humoral and cellular immune responses. Several herpesviruses induce FcgammaR activities in infected cells. Here we identify two distinct human cytomegalovirus (HCMV)-encoded vFcgammaR glycoproteins of 34 and 68 kDa. A panel of HCMV strains exhibited a slight molecular microheterogeneity between Fcgamma-binding proteins, suggesting their viral origin. To locate the responsible genes within the HCMV genome, a large set of targeted HCMV deletion mutants was constructed. The mutant analysis allowed the identification of a spliced UL119-UL118 mRNA to encode vFcgammaR gp68 and TRL11/IRL11 to encode vFcgammaR gp34. Both vFcgammaRs are surface resident type I transmembrane glycoproteins. Significant relatedness of sequences in the extracellular chain of gpUL119-118 and gpTRL11 with particular immunoglobulin supergene family domains present in FcgammaR I and FcgammaRs II/III, respectively, indicates a different ancestry and function of gpUL119-118 and gpTRL11. The HCMV-encoded vFcgammaRs highlight an impressive diversification and redundancy of FcgammaR structures.

Transcriptional analysis of Marek's disease virus glycoprotein D, I, and E genes: gD expression is undetectable in cell culture

The various alphaherpesviruses, including Marek's disease virus (MDV), have both common and unique features of gene content and expression. The entire MDV U(s) region has been sequenced in our laboratory (P. Brunovskis and L. F. Velicar, Virology 206:324-338, 1995). Genes encoding the MDV glycoprotein D (gD), glycoprotein I (gI), and glycoprotein E (gE) homologs have been found in this region, although no gG homolog was found. In this work, transcription of the tandem MDV gD, gI, and gE genes was studied and found to have both unique characteristics and also features in common with other alphaherpesviruses. MDV gD could not be immunoprecipitated from MDV GA-infected duck embryo fibroblast cells by antisera reactive to its TrpE fusion proteins, while gI and gE could be. When the gD gene was subjected to in vitro-coupled transcription-translation, the precursor polypeptide was produced and could be immunoprecipitated by anti-gD. Northern blot, reverse transcriptase PCR, and RNase protection analyses have shown that (i) no mRNA initiating directly from the gD gene could be detected; (ii) a large but low-abundance 7.5-kb transcript spanning five genes, including the one encoding gD, was seen on longer exposure; and (iii) transcription of the gI and gE genes formed an abundant bicistronic 3.5-kb mRNA, as well as an abundant 2.0-kb gE-specific mRNA. Therefore, the MDV gD gene expression is down-regulated at the transcription level in MDV-infected cell culture, which may be related to the cell-associated nature of MDV in fibroblast cells. Compared to the highly gD-dependent herpes simplex virus and the other extreme of the varicella-zoster virus which lacks the gD gene, MDV is an intermediate type of alphaherpesvirus.

Herpes simplex virus gE/gI sorts nascent virions to epithelial cell junctions, promoting virus spread

Alphaherpesviruses spread rapidly through dermal tissues and within synaptically connected neuronal circuitry. Spread of virus particles in epithelial tissues involves movement across cell junctions. Herpes simplex virus (HSV), varicella-zoster virus (VZV), and pseudorabies virus (PRV) all utilize a complex of two glycoproteins, gE and gI, to move from cell to cell. HSV gE/gI appears to function primarily, if not exclusively, in polarized cells such as epithelial cells and neurons and not in nonpolarized cells or cells that form less extensive cell junctions. Here, we show that HSV particles are specifically sorted to cell junctions and few virions reach the apical surfaces of polarized epithelial cells. gE/gI participates in this sorting. Mutant HSV virions lacking gE or just the cytoplasmic domain of gE were rarely found at cell junctions; instead, they were found on apical surfaces and in cell culture fluids and accumulated in the cytoplasm. A component of the AP-1 clathrin adapter complexes, mu1B, that is involved in sorting of proteins to basolateral surfaces was involved in targeting of PRV particles to lateral surfaces. These results are related to recent observations that (i) HSV gE/gI localizes specifically to the trans-Golgi network (TGN) during early phases of infection but moves out to cell junctions at intermediate to late times (T. McMillan and D. C. Johnson, J. Virol., in press) and (ii) PRV gE/gI participates in envelopment of nucleocapsids into cytoplasmic membrane vesicles (A. R. Brack, B. G. Klupp, H. Granzow, R. Tirabassi, L. W. Enquist, and T. C. Mettenleiter, J. Virol. 74:4004-4016, 2000). Therefore, interactions between the cytoplasmic domains of gE/gI and the AP-1 cellular sorting machinery cause glycoprotein accumulation and envelopment into specific TGN compartments that are sorted to lateral cell surfaces. Delivery of virus particles to cell junctions would be expected to enhance virus spread and enable viruses to avoid host immune defenses.

A deletion in the gI and gE genes of equine herpesvirus type 4 reduces viral virulence in the natural host and affects virus transmission during cell-to-cell spread

In order to identify the role of the equine herpesvirus type 4 (EHV-4) glycoprotein I (gI) and E (gE) genes in determining viral virulence and their affect on the infection cycle, we constructed an EHV-4 recombinant strain containing a deletion in both gI and gE genes and its revertant. The recombinant was assayed in vitro in order to compare its growth kinetics with the parent and revertant viruses. Our results indicated that a deletion in the genes encoding gI and gE affected cell-to-cell spread of the virus in vitro. In order to assess the pathogenicity and vaccine efficacy of the recombinant in a natural host, colostrum-deprived foals were inoculated intranasally with the recombinant. Clinical signs obtained in foals upon the inoculation with the recombinant were milder than that for the revertant. This suggests that intact gI and/or gE genes are important factors in the expression of virulence in EHV-4 as in seen in the case of other herpesviruses. In addition, full protection against challenge infection was observed in foals, which had undergone a previous inoculation of the recombinant.

Bovine herpesvirus 5 glycoprotein E is important for neuroinvasiveness and neurovirulence in the olfactory pathway of the rabbit

Glycoprotein E (gE) is important for full virulence potential of the alphaherpesviruses in both natural and laboratory hosts. The gE sequence of the neurovirulent bovine herpesvirus 5 (BHV-5) was determined and compared with that of the nonneurovirulent BHV-1. Alignment of the predicted amino acid sequences of BHV-1 and BHV-5 gE open reading frames showed that they had 72% identity and 77% similarity. To determine the role of gE in the differential neuropathogenesis of BHV-1 and BHV-5, we have constructed BHV-1 and BHV-5 recombinants: gE-deleted BHV-5 (BHV-5gEDelta), BHV-5 expressing BHV-1 gE (BHV-5gE1), and BHV-1 expressing BHV-5 gE (BHV-1gE5). Neurovirulence properties of these recombinant viruses were analyzed using a rabbit seizure model (S. I. Chowdhury et al., J. Comp. Pathol. 117:295-310, 1997) that distinguished wild-type BHV-1 and -5 based on their differential neuropathogenesis. Intranasal inoculation of BHV-5 gEDelta and BHV-5gE1 produced significantly reduced neurological signs that affected only 10% of the infected rabbits. The recombinant BHV-1gE5 did not invade the central nervous system (CNS). Virus isolation and immunohistochemistry data suggest that these recombinants replicate and spread significantly less efficiently in the brain than BHV-5 gE revertant or wild-type BHV-5, which produced severe neurological signs in 70 to 80% rabbits. Taken together, the results of neurological signs, brain lesions, virus isolation, and immunohistochemistry indicate that BHV-5 gE is important for efficient neural spread and neurovirulence within the CNS and could not be replaced by BHV-1 gE. However, BHV-5 gE is not required for initial viral entry into olfactory pathway.

Intracellular traffic of herpes simplex virus glycoprotein gE: characterization of the sorting signals required for its trans-Golgi network localization

Herpes simplex virus (HSV) and varicella-zoster virus (VZV) are two pathogenic human alphaherpesviruses whose intracellular assembly is thought to follow different pathways. VZV presumably acquires its envelope in the trans-Golgi network (TGN), and it has recently been shown that its major envelope glycoprotein, VZV-gE, accumulates in this compartment when expressed alone. In contrast, the envelopment of HSV has been proposed to occur at the inner nuclear membrane, although to which compartment the gE homolog (HSV-gE) is transported is unknown. For this reason, we have studied the intracellular traffic of HSV-gE and have found that this glycoprotein accumulates at steady state in the TGN, both when expressed from cloned cDNA and in HSV-infected cells. In addition, HSV-gE cycles between the TGN and the cell surface and requires a conserved tyrosine-containing motif within its cytoplasmic tail for proper trafficking. These results show that VZV-gE and HSV-gE have similar intracellular trafficking pathways, probably reflecting the presence of similar sorting signals in the cytoplasmic domains of both molecules, and suggest that the respective viruses, VZV and HSV, could use the same subcellular organelle, the TGN, for their envelopment.

The herpes simplex virus gE-gI complex facilitates cell-to-cell spread and binds to components of cell junctions

The herpes simplex virus (HSV) glycoprotein complex gE-gI mediates the spread of viruses between adjacent cells, and this property is especially evident for cells that form extensive cell junctions, e.g., epithelial cells, fibroblasts, and neurons. Mutants lacking gE or gI are not compromised in their ability to enter cells as extracellular viruses. Therefore, gE-gI functions specifically in the movement of virus across cell-cell contacts and, as such, provides a molecular handle on this poorly understood process. We expressed gE-gI in human epithelial cells by using replication-defective adenovirus (Ad) vectors. gE-gI accumulated at lateral surfaces of the epithelial cells, colocalizing with the adherens junction protein beta-catenin but was not found on either the apical or basal plasma membranes and did not colocalize with ZO-1, a component of tight junctions. In subconfluent monolayers, gE-gI was found at cell junctions but was absent from those lateral surfaces not in contact with another cell, as was the case for beta-catenin. Similar localization of gE-gI to cell junctions was observed in HSV-infected epithelial cells. By contrast, HSV glycoprotein gD, expressed using a recombinant Ad vectors, was found primarily along the apical surfaces of cells, with little or no protein found on the basal or lateral surfaces. Expression of gE-gI without other HSV polypeptides did not cause redistribution of either ZO-1 or beta-catenin or alter tight-junction functions. Together these results support a model in which gE-gI accumulates at sites of cell-cell contact by interacting with junctional components. We hypothesize that gE-gI mediates transfer of HSV across cell junctions by virtue of these interactions with cell junction components.

The disulfide-bonded structure of feline herpesvirus glycoprotein I

Alphaherpesvirus glycoproteins E and I (gE and gI, respectively) assemble into a hetero-oligomeric complex which promotes cell-to-cell transmission, a determining factor of virulence. Focusing on gI of feline herpesvirus (FHV), we examined the role of disulfide bonds during its biosynthesis, its interaction with gE, and gE-gI-mediated spread of the infection in vitro. The protein's disulfide linkage pattern was determined by single and pairwise substitutions for the four conserved cysteine residues in the ectodomain. The resulting mutants were coexpressed with gE in the vaccinia virus-based vTF7-3 system, and the formation and endoplasmic reticulum (ER)-to-Golgi transport of the hetero-oligomeric complex were monitored. The results were corroborated biochemically by performing an endoproteinase Lys-C digestion of a [35S]Cys-labeled secretory recombinant form of gI followed by tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the peptides under reducing and nonreducing conditions. We found that (i) gI derivatives lacking Cys79 (C1) and/or Cys223 (C4) still assemble with gE into transport-competent complexes, (ii) mutant proteins lacking Cys91 (C2) and/or Cys102 (C3) bind to gE but are retained in the ER, (iii) radiolabeled endoproteinase Lys-C-generated peptide species containing C1 and C4 are linked through disulfide bonds, and (iv) peptides containing both C2 and C3 are not disulfide linked to any other peptide. From these findings emerges a model in which C1 and C4 as well as C2 and C3 form intramolecular disulfide bridges. Since the cysteines in the ectodomain have been conserved during alphaherpesvirus divergence, we postulate that the model applies for all gI proteins. Analysis of an FHV recombinant with a C1-->S substitution confirmed that the C1-C4 disulfide bond is not essential for the formation of a transport-competent gE-gI complex. The mutation affected the posttranslational modification of gI and caused a slight cold-sensitivity defect in the assembly or the intracellular transport of the gE-gI complex but did not affect plaque size. Thus, C1 and the C1-C4 bond are not essential for gE-gI-mediated cell-to-cell spread, at least not in vitro.

Intracellular transport of the glycoproteins gE and gI of the varicella-zoster virus. gE accelerates the maturation of gI and determines its accumulation in the trans-Golgi network

The varicella-zoster virus (VZV) is the etiological agent of two different human pathologies, chickenpox (varicella) and shingles (zoster). This alphaherpesvirus is believed to acquire its lipidic envelope in the trans-Golgi network (TGN). This is consistent with previous data showing that the most abundant VZV envelope glycoprotein gE accumulates at steady-state in this organelle when expressed from cloned cDNA. In the present study, we have investigated the intracellular trafficking of gI, another VZV envelope glycoprotein. In transfected cells, this protein shows a very slow biosynthetic transport to the cell surface where it accumulates. However, upon co-expression of gE, gI experiences a dramatic increase in its exit rate from the endoplasmic reticulum, it accumulates in a sialyltransferase-positive compartment, presumably the TGN, and cycles between this compartment and the cell surface. This differential behavior results from the ability of gE and gI to form a complex in the early stages of the biosynthetic pathway whose intracellular traffic is exclusively determined by the sorting information in the tail of gE. Thus, gI provides the first example of a molecule localized to the TGN by means of its association with another TGN protein. We also show that, during the early stages of VZV infection, both proteins are also found in the TGN of the host cell. This suggests the existence of an intermediate stage during VZV biogenesis in which the envelope glycoproteins, transiently arrested in the TGN, could promote the envelopment of newly synthesized nucleocapsids into this compartment and, therefore, the assembly of infective viruses.

An equine herpesvirus type 1 recombinant with a deletion in the gE and gI genes is avirulent in young horses

The cell culture-adapted KyA strain of equine herpesvirus type 1 (EHV-1) has been found to be attenuated in young horses (Matsumura et al., 1996, Vet. Microbiol. 48, 353-365). The KyA strain lacks at least six genes in its genome, including those encoding glycoproteins gE and gI. To elucidate whether EHV-1 glycoproteins gE and gI play a role in viral virulence, we have constructed an EHV-1 recombinant that has the genes encoding both gE and gI deleted from its genome and its revertant. Growth properties of the deletion mutant virus in vitro were compared with those of the parent and the revertant viruses. Plaque size of the mutant virus in fetal horse kidney (FHK) cells was significantly smaller than those of the parent and the revertant viruses. In one-step growth experiments, however, the yields of infectious virus from FHK cells infected with the deletion mutant, the parent, or the revertant virus were approximately the same. The results suggested that gE and/or gI of EHV-1 promoted cell-to-cell spread of the virus, but that these glycoproteins were not involved in the process of virus maturation and release or in virus attachment and penetration. Subsequently, the virulence of mutant and revertant viruses was examined in young horses. No clinical signs were observed in six horses, including three colostrum-deprived foals inoculated intranasally with the deletion mutant virus, whereas three colostrum-deprived foals inoculated intranasally with the revertant virus manifested clinical signs typical for EHV-1 respiratory infection (i.e., pyrexia, nasal discharge, and swelling of submandibular lymph nodes). The results obtained from in vivo studies revealed that the EHV-1 mutant defective in both gE and gI genes was avirulent in young horses, suggesting that gE and/or gI of the EHV-1 have an important role in EHV-1 virulence. However, the EHV-1 mutant defective in both gE and gI genes induced only a partial protectivity in inoculated foals from manifestation of respiratory symptoms after challenge infection.

Nucleotide sequences of glycoprotein I and E genes of equine herpesvirus type 4

The nucleotide sequences of the glycoprotein I (gI) and E (gE) genes of equine herpesvirus type 4 (EHV-4) strain TH20 were determined. The predicted region encoding the EHV-4 gI gene is 1,263 nucleotides, corresponding to a polypeptide of 420 amino acids in length. The predicted region encoding the EHV-4 gE gene is 1,647 nucleotides, corresponding to a polypeptide of 548 amino acids in length. The EHV-4 gI and gE genes show 74% and 85% identity at the amino acid level with those of equine herpesvirus type 1 (EHV-1), respectively. Furthermore, we have found an open reading frame homologous to the EHV-1 gene 75, which overlaps in part with the 3' end of EHV-4 gE gene. These sequence data will be useful for development of a modified live vaccine against equine herpesvirus type 1 and 4 infections.

Structure-function analysis of the gE-gI complex of feline herpesvirus: mapping of gI domains required for gE-gI interaction, intracellular transport, and cell-to-cell spread

Alphaherpesvirus glycoproteins gE and gI form a noncovalently associated hetero-oligomeric complex, which is involved in cell-to-cell spread. In the absence of gI, feline herpesvirus (FHV) gE is transport incompetent and fully retained in the endoplasmic reticulum. Here, we assess the effect of progressive C-terminal truncations of FHV gI on the biosynthesis, intracellular transport, and function of the gE-gI complex. The truncated gI proteins were coexpressed with gE in the vaccinia virus-based vTF7-3 expression system. The results were corroborated and extended by studying FHV recombinants expressing truncated gI derivatives. The following conclusions can be drawn. (i) Deletion of the cytoplasmic tail, the transmembrane region plus the C-terminal half of the ectodomain of gI, does not affect intracellular transport of gE. Apparently, the N-terminal 166 residues of gI constitute a domain involved in gE-gI interaction. (ii) A region mediating stable association with gE is located within the N-terminal 93 residues of gI. (iii) The cytoplasmic domain of gI is not essential for gE-gI-mediated cell-to-cell transmission of FHV, as judged from plaque morphology. Deletion of the cytoplasmic tail of gI reduced plaque size by only 35%. (iv) Recombinants expressing the N-terminal 166 residues of gI display a small-plaque phenotype but produce larger plaques than recombinants with a disrupted gI gene. Thus, a complex consisting of gE and the N-terminal half of the gI ectodomain may retain residual biological activity. The implications of these findings for gE-gI interaction and function are discussed.

The UL3 open reading frame of herpes simplex virus type 1 codes for a phosphoprotein

Based on sequence analysis, the protein encoded by the UL3 open reading frame (ORF) of herpes simplex virus type 1 (HSV-1) was predicted to contain an N glycosylation site and to be a glycoprotein. To determine if this prediction was correct, we cloned and expressed the DNA encoding the complete sequence of the UL3 ORF in a baculovirus expression system. Western blotting was done using polyclonal antibody raised against synthetic UL3 peptides. Two major baculovirus-UL3 expressed protein bands with apparent molecular weights of 30 kDa and 31 kDa, and two minor protein bands with apparent molecular weights of 29 kDa and 33 kDa were detected. None of the expressed UL3 protein species were susceptible to tunicamycin treatment, suggesting that they were not N-linked glycosylated. Cell fractionation studies indicated that the UL3 protein was localized in the cytoplasmic and nuclear portion of the cells, rather than the cell membrane, again suggesting a lack of glycosylation. In contrast, the baculovirus expressed UL3 protein was phosphorylated as judged by 32Pi-labeling. Immunoprecipitation followed by SDS-PAGE demonstrated a single 32Pi-labeled UL3 related band with an apparent molecular weight of 33 kDa, indicating that the UL3 protein was a phosphoprotein. Antibodies produced in mice vaccinated with baculovirus-UL3 protein reacted with two UL3 related HSV-1 bands on Western blots. These protein bands had apparent molecular weights of 27 and 33 kDa and presumably represent the unphosphorylated and phosphorylated forms of UL3.

Biosynthesis of glycoproteins E and I of feline herpesvirus: gE-gI interaction is required for intracellular transport

The biosynthesis of glycoproteins E and I of feline herpesvirus was studied by using the vaccinia virus vTF7-3 expression system. gE and gI were synthesized as N-glycosylated, endoglycosidase H (EndoH)-sensitive precursors with Mrs of 83,000 and 67,000, respectively. When coexpressed, gE and gI formed sodium dodecyl sulfate-sensitive hetero-oligomeric complexes that were readily transported from the endoplasmic reticulum (ER). Concomitantly, the glycoproteins acquired extensive posttranslational modifications, including O glycosylation, leading to an increase in their apparent molecular weights to 95,000 and 80,000 to 100,000 for gE and gI, respectively. In the absence of gE, most gI remained EndoH sensitive. Only a minor population became EndoH resistant, but these molecules were processed aberrantly as indicated by their Mrs (100,000 to 120,000). By immunofluorescence microscopy, gI was detected primarily in the ER but also at the plasma membrane. gE, when expressed by itself, remained EndoH sensitive and was found only in the ER and the nuclear envelope. These results were corroborated by studying the biosynthesis of gE in feline herpesvirus (FHV)-infected cells. In cells infected with wild-type FHV, gE acquired the same co- and posttranslational modifications as during vTF7-3-driven expression. However, an FHV mutant lacking gI failed to produce mature gE. We conclude that gE is retained in the ER, presumably by associating with molecular chaperones, and becomes transport competent only when in a complex with gI.

Construction of bovine herpesvirus-1 (BHV-1) recombinants which express pseudorabies virus (PRV) glycoproteins gB, gC, gD, and gE

We have improved the method for constructing recombinants of bovine herpesvirus type-1 (BHV-1). Using this method, we constructed three recombinants in which the pseudorabies virus (PRV) thymidine kinase (tk) gene was inserted at three different sites in the unique short region of BHV-1. These three sites are located in the open reading frame of gE, gG and gI genes. Previously, two sites (tk and gC) had been used to insert foreign DNA fragments to BHV-1 genome. Therefore we now have 5 sites in BHV-1 where DNA can be inserted. The gB, gC, gD, gE and gI genes of PRV were successfully inserted at the tk or the gC gene of BHV-1 genome and Western blot analyses confirmed that the recombinants express PRV gB, gC, gD and gE. Anti-PRV gB and gC antibodies as well as anti-PRV polyclonal serum neutralized BHV-1 recombinants which express PRV gB and gC. The latter was neutralized more strongly. However, anti-gD monoclonal antibody and anti-PRV polyclonal serum failed to neutralize gD-expressing recombinants. This suggests that PRV gC and some gB are integrated into the viral envelope of the recombinants, but very little gD is present in the viral envelope.

Glycoproteins E and I facilitate neuron-to-neuron spread of herpes simplex virus

Two herpes simplex virus (HSV) glycoproteins E and I (gE and gI) form a heterooligomer which acts as an Fc receptor and also facilitates cell-to-cell spread of virus in epithelial tissues and between certain cultured cells. By contrast, gE-gI is not required for infection of cells by extracellular virus. HSV glycoproteins gD and gJ are encoded by neighboring genes, and gD is required for both virus entry into cells and cell-to-cell spread, whereas gJ has not been shown to influence these processes. Since HSV infects neurons and apparently spreads across synaptic junctions, it was of interest to determine whether gD, gE, gI and gJ are also important for interneuronal transfer of virus. We tested the roles of these glycoproteins in neuron-to-neuron transmission of HSV type 1 (HSV-1) by injecting mutant viruses unable to express these glycoproteins into the vitreous body of the rat eye. The spread of virus infection was measured in neuron-rich layers of the retina and in the major retinorecipient areas of the brain. Wild-type HSV-1 and a gJ- mutant spread rapidly between synaptically linked retinal neurons and efficiently infected major retinorecipient areas of the brain. gD mutants, derived from complementing cells, infected only a few neurons and did not spread in the retina or brain. Mutants unable to express gE or gI were markedly restricted in their ability to spread within the retina, produced 10-fold-less virus in the retina, and spread inefficiently to the brain. Furthermore, when compared with wild-type HSV-1, gE- and gI- mutants spread inefficiently from cell to cell in cultures of neurons derived from rat trigeminal ganglia. Together, our results suggest that the gE-gI heterooligomer is required for efficient neuron-to-neuron transmission through synaptically linked neuronal pathways.

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.

Mapping, cloning and sequencing of a glycoprotein-encoding gene from bovine herpesvirus type 1 homologous to the gE gene from HSV-1

In order to map and identify the glycoprotein-encoding gene from bovine herpesvirus type 1 (BHV-1), homologous to the gE glycoprotein from herpes simplex virus type 1 (HSV-1), a region of the unique short sequence from the BHV-1 genome has been sequenced. The sequenced region contains an ORF coding for a polypeptide of 575 amino acids (aa). The aa sequence presents substantial similarity to that of the glycoprotein gE from HSV-1 and to homologous proteins of related viruses such as pseudorabies virus, equine herpesvirus type 1 and varicella zoster virus. The aa sequence presents additional characteristics compatible with the structure of a viral glycoprotein: signal peptide, putative glycosylation sites and a long C-terminal transmembrane alpha-helix.

Analysis of the contributions of herpes simplex virus type 1 membrane proteins to the induction of cell-cell fusion

The contributions of a set of herpes simplex virus type 1 membrane proteins towards the process of cell-cell fusion were examined with a series of deletion mutants into which a syncytial mutation had been introduced at codon 855 of the glycoprotein B (gB) gene. Analysis of the fusion phenotypes of these recombinant viruses in Vero cells revealed that while gC, gG, US5, and UL43 are dispensable for syncytium formation at both high and low multiplicities of infection, gD, gHgL, gE, gI, and gM were all required for the fusion of cellular membranes. These data confirm that the requirements for virion entry and cell-cell fusion are not identical. gD and gHgL, like gB, are essential for both processes. gG, gI, and gM, on the other hand, are dispensable for virus penetration, yet play a role in cell-to-cell spread by the direct contact route, at least on an SC16 gBANG background.

Glycoprotein E of pseudorabies virus and homologous proteins in other alphaherpesvirinae

This paper reviews biological properties of glycoprotein E (gE) of pseudorabies virus (Aujeszky's disease virus) and homologous proteins in other alphaherpesvirinae. It focuses on the gene encoding gE, conserved regions in the gE protein and its homologs, the complex of gE and gI, biological functions of gE in vitro and in vivo, the role of gE in latency and the role of gE in the induction of humoral and cellular immune responses. Special emphasis is placed on the use of gE as a marker protein in the control and eradication of pseudorabies virus.

Herpes simplex virus glycoproteins E and I facilitate cell-to-cell spread in vivo and across junctions of cultured cells

Herpes simplex virus (HSV) glycoproteins E and I (gE and gI) can act as a receptor for the Fc domain of immunoglobulin G (IgG). To examine the role of HSV IgG Fc receptor in viral pathogenesis, rabbits and mice were infected by the corneal route with HSV gE- or gI- mutants. Wild-type HSV-1 produced large dendritic lesions in the corneal epithelium and subsequent stromal disease leading to viral encephalitis, whereas gE- and gI- mutant viruses produced microscopic punctate or small dendritic lesions in the epithelium and no corneal disease or encephalitis. These differences were not related to the ability of the gE-gI oligomer to bind IgG because the differences were observed before the appearance of anti-HSV IgG and in mice, in which IgG binds to the Fc receptor poorly or not at all. Mutant viruses produced small plaques on monolayers of normal human fibroblasts and epithelial cells. Replication of gE- and gI- mutant viruses in human fibroblasts were normal, and the rates of entry of mutant and wild-type viruses into fibroblasts were similar; however, spread of gE- and gI- mutant viruses from cell to cell was significantly slower than that of wild-type HSV-1. In experiments in which fibroblast monolayers were infected with low multiplicities of virus and multiple rounds of infection occurred, the presence of neutralizing antibodies in the culture medium caused the yields of mutant viruses to drop dramatically, whereas there was a lesser effect on the production of wild-type HSV. It appears that cell-to-cell transmission of wild-type HSV-1 occurs by at least two mechanisms: (i) release of virus from cells and entry of extracellular virus into a neighboring cell and (ii) transfer of virus across cell junctions in a manner resistant to neutralizing antibodies. Our results suggest that gE- and gI- mutants are defective in the latter mechanism of spread, suggesting the possibility that the gE-gI complex facilitates virus transfer across cell junctions, a mode of spread which may predominate in some tissues. It is ironic that the gE-gI complex, usually considered an IgG Fc receptor, may, through its ability to mediate cell-to-cell spread, actually protect HSV from IgG in a manner different than previously thought.

Analysis of the nucleotide sequence of five genes at the left end of the unique short region of the equine herpesvirus 4 genome

Eco RI fragment G of equine herpesvirus 4 strain 405/76 (EHV 4.405/76) is located at the left end of the unique short region close to or extending into the internal repeat region of the prototypic arrangement of the genome. The nucleotide sequence of two subclones designated HS and G 19, contiguous within Eco RI fragment G, was determined for each strand by obtaining a nested set of deletion clones of these double-stranded DNA plasmids. Analysis of the nucleotide sequence revealed that the two subclones contain 5449 base pairs with four complete open reading frames (ORFs) and part of a fifth ORF. Comparison of the predicted amino acid sequences of these reading frames showed that they correspond to ORFs 67, 68, 69, 70, and 71 of equine herpesvirus type 1 (EHV 1) [41], of which ORFs 68, 69, and 70 are homologous to human herpes simplex virus (HSV) genes in the unique short (US) region, i.e., US 2, US 3, and US 4. ORF 67' of EHV 4 and ORF 67 of EHV 1 are homologous (65.7%) but these genes have no homologue in HSV 1.

Baculovirus-expressed glycoprotein G of herpes simplex virus type 1 partially protects vaccinated mice against lethal HSV-1 challenge

The DNA sequence encoding the complete HSV-1 glycoprotein G (gG) was inserted into a baculovirus transfer vector and recombinant viruses expressing gG were isolated. Three gG-related recombinant baculovirus expressed peptides of 37, 42, and 44 kDa were detected by Western blotting using monoclonal antibody to gG. The 42- and 44-kDa species were susceptible to tunicamycin, Endoglycosidase H (Endo-H), and N-glycosidase F (PNGase F) treatments, suggesting that they were glycosylated. Although only very low levels (approximately 1:10) of HSV-1-neutralizing antibody were produced in mice vaccinated with the baculovirus gG, these mice were partially protected from lethal challenge with HSV-1 (75-78% survival) and this level of protection was highly significant (P = 0.002). This is the first report to show that vaccination with HSV-1 gG can provide mice with any level of protection against lethal HSV-1 challenge.

The equine herpesvirus type 1 (EHV-1) homolog of herpes simplex virus type 1 US9 and the nature of a major deletion within the unique short segment of the EHV-1 KyA strain genome

The DNA sequence of the short (S) genomic component of the equine herpesvirus type 1 (EHV-1)KyA strain has been determined recently in our laboratory. Analysis of a 1353-bp BamHI/PvuII clone mapping at the unique short/terminal inverted repeat (Us/TR) junction revealed 507 bp of Us and 846 bp of TR sequences as well as an open reading frame (ORF) that is contained entirely within the Us. This ORF encodes a potential polypeptide of 219 amino acids that shows significant homology to the US9 proteins of herpes simplex virus type 1 (HSV-1), EHV-4, pseudorabies virus (PRV), and varicella zoster virus (VZV). The US9 polypeptides of the two equine herpesviruses exhibit 50% identity but are twice as large as their counterparts in HSV-1, PRV, and VZV. All five US9 proteins are enriched for serine and threonine residues and share a conserved domain of highly basic residues followed by a region of nonpolar amino acids. DNA sequence and Southern blot hybridization analyses revealed that the Us of EHV-1 KyA differs from the Us of EHV-1 KyD and AB1 in that the ORFs encoding glycoproteins I and E and a unique 10-kDa polypeptide are deleted from the KyA genome. These data demonstrate that the predicted 10-kDa protein unique to EHV-1 is nonessential for replication in vitro and that EHV-1 glycoproteins I and E, like their equivalents in HSV-1 and PRV, are also nonessential. These findings and those reported previously by this laboratory and others reveal that the Us segment of EHV-1 comprises nine ORFs, two of which, US4 and 10-kDa ORF, are unique to EHV-1. The gene order of the Us is US2, protein kinase, gG, US4, gD, gI, gE, 10 kDa, and US9.

Direct evidence for antibody bipolar bridging on herpes simplex virus-infected cells

Cells infected with herpes simplex virus type 1 (HSV-1) express a cell-surface receptor able to bind the Fc portion of immunoglobulin G (IgG). In this study we provide direct evidence that bipolar bridging of antibodies, bound to the surface antigens on HSV-infected cells and to the Fc-receptor through the Fc part, offers the virus a survival advantage. Evidence was obtained by comparing the binding of FITC-labelled protein A, which has a similar binding site on IgG as the HSV-FcR, to cell-bound antibodies on HSV-infected cells and non-infected cells. The effectiveness of antibody bipolar bridging was dependent on the concentration of cell-bound IgG. At low concentrations of serum (0.1%) an 80% reduction in protein A-FITC binding to HSV-infected cells compared to non-infected cells was found. Even at higher concentrations of serum, antibody bipolar bridging resulted in a 40% reduction in the number of 'free' available Fc parts on HSV-infected cells compared to non-infected cells. Furthermore, these findings could be confirmed in a functional assay. The Fc-mediated attachment of granulocytes was significantly lower in HSV-infected cells compared to non-infected cells. From this study we conclude that HSV-FcR, by binding immune IgG in a bipolar fashion, provides the virus with an effective defence mechanism.

Involvement of glycoprotein C (gC) in adsorption of herpes simplex virus type 1 (HSV-1) to the cell

Results demonstrating involvement of glycoprotein C (gC) of herpes simplex type 1 virus (HSV-1) in attachment of the virus to the cell are presented. Monoclonal antibodies against gC-1 inhibited adsorption of gC(+)-strains. The gC(-)-mutant, MP, attached to cells but at a reduced rate. Attachment of the MP-mutant was unaffected by presence of anti-gC-1 antibody. Purified truncated gC-1 adsorbed to cells at a rate essentially the same as that of gC(+)-virus. Glycoprotein C-1 pretreated with heparin did not adsorb to cells. The results are compatible with a suggested role for gC in HSV attachment.

Isolation of a viable herpesvirus (pseudorabies virus) mutant specifically lacking all four known nonessential glycoproteins

Recently we described the isolation and characterization of a pseudorabies virus (PrV) mutant lacking the nonessential glycoproteins gI, gp63, and gIII. Using insertional mutagenesis with a functional gX-beta-galactosidase fusion gene we describe here the isolation of a PrV mutant specifically lacking all four known nonessential glycoproteins, gI, gp63, gIII, and gX. The quadruple mutant did not show any significant alterations in the vitro growth characteristics compared to its triple mutant parent. These results prove that PrV nonessential glycoproteins are dispensable for viral replication in cell culture altogether.

Herpes simplex virus IgG Fc receptors induced using recombinant adenovirus vectors expressing glycoproteins E and I

Evidence has been presented that herpes simplex virus (HSV) immunoglobulin (IgG) Fc receptors are composed of a complex of two glycoproteins, gE and gI. In previous studies, cells infected with HSV-1 mutants lacking either gE or gI bound lower levels of soluble IgG than cells infected with wild-type viruses suggesting that both gE and gI were required for IgG binding. We have reevaluated the Fc receptor activity of these mutants using a more sensitive assay involving IgG-coated erythrocytes and have found that cells infected with a gE- mutant HSV-1 did not bind IgG-coated erythrocytes whereas cells infected with a gI- mutant retained some Fc binding activity. To further study HSV-induced Fc receptors recombinant adenovirus vectors expressing gE or gI were constructed. Cells expressing gE alone bound both soluble IgG and IgG-coated red cells, although the binding was consistently lower than that observed with HSV-infected cells or cells expressing both gE and gI. Cells expressing only gI were unable to bind either soluble IgG or IgG-coated erythrocytes. These results support the conclusion that both gE and gI are required for full Fc receptor activity, although gE alone can bind IgG to a lesser extent.

Identification of herpes simplex virus type 1 glycoproteins interacting with the cell surface

To investigate the interaction of herpes simplex virus type 1 (HSV-1) with the cell surface, we studied the formation of complexes by HSV-1 virion proteins with biotinylated cell membrane components. HSV-1 virion proteins reactive with surface components of HEp-2 and other cells were identified as gC, gB, and gD. Results from competition experiments suggested that binding of gC, gB, and gD occurred in a noncooperative way. The observed complex formation could be specifically blocked by monospecific rabbit antisera against gB and gD. The interaction of gD with the cell surface was also inhibited by monoclonal antibody IV3.4., whereas other gD-specific monoclonal antibodies, despite their high neutralizing activity, were not able to inhibit this interaction. Taken together, these data provide direct evidence that at least three of the seven known HSV-1 glycoproteins are able to form complexes with cellular surface structures.

Identification of C3b-binding regions on herpes simplex virus type 2 glycoprotein C

Glycoprotein C from herpes simplex viruses types 1 and 2 (gC-1 and gC-2) acts as a receptor for the C3b fragment of the third component of complement. Our goal is to identify domains on gC involved in C3b receptor activity. Here, we used in-frame linker-insertion mutagenesis of the cloned gene for gC-2 to identify regions of the protein involved in C3b binding. We constructed 41 mutants of gC-2, each having a single, double, or triple insertion of four amino acids at sites spread across the protein. A transient transfection assay was used to characterize the expressed mutant proteins. All of the proteins were expressed on the transfected cell surface, exhibited processing of N-linked oligosaccharides, and bound one or more monoclonal antibodies recognizing distinct antigenic sites on native gC-2. This suggested that each of the mutant proteins was folded into a native structure and that a loss of C3b binding by any of the mutants could be attributed to the disruption of a specific functional domain. When the panel of insertion mutants was assayed for C3b receptor activity, we identified three distinct regions that are important for C3b binding, since an insertion within those regions abolished C3b receptor activity. Region I was located between amino acids 102 and 107, region II was located between residues 222 and 279, and region III was located between residues 307 and 379. In addition, region III has some structural features similar to a conserved motif found in complement receptor 1, the human C3b receptor. Finally, blocking experiments indicated that gC-1 and gC-2 bind to similar locations on the C3b molecule.

Localization on the herpes simplex virus type 1 genome of a region encoding proteins involved in adsorption to the cellular receptor

We have previously shown that aminoglycosides such as neomycin and the polyamino acids polylysine and polyarginine selectively inhibit the binding of herpes simplex virus type 1 (HSV-1) to the cellular receptor, whereas HSV-2 infection is unaffected. In the present study we took advantage of this difference between HSV-1 and HSV-2 by using HSV(-1)-HSV(-2) intertypic recombinants to locate a region on the HSV-1 genome encoding proteins affecting the binding of the virion to the cellular receptor. The results were consistent with those obtained by marker rescue experiments. The identified region, which mapped between coordinates 0.580 and 0.687, contains two partial and eight complete genes, including the glycoprotein C (gC) gene and two others with potential transmembrane sequences. Various gC monoclonal antibody-resistant mutants of HSV-1 as well as a mutant completely lacking gC were found to be fully sensitive to neomycin, suggesting that gC is not the site of drug sensitivity and is not essential for adsorption of virus to the cellular receptor. However, the rate of adsorption was reduced in the absence of gC, indicating a facilitating function of the glycoprotein. The universal nature of this HSV-1 receptor binding was revealed by the similarity in drug sensitivity of infectivity in four different cell lines from various tissues and species.

A novel function of the herpes simplex virus type 1 Fc receptor: participation in bipolar bridging of antiviral immunoglobulin G

We describe a novel function of the Fc receptor of herpes simplex virus type 1 (HSV-1), its ability to participate in antibody bipolar bridging. This refers to the binding of a single immunoglobulin G (IgG) molecule by its Fab end to its antigenic target and by its Fc end to an Fc receptor (FcR). We demonstrate that various immune IgG antibodies, including polyclonal rabbit antibodies to HSV-1 glycoproteins gC1 and gD1 and monoclonal human antibody to gD1 blocked rosetting of IgG-coated erythrocytes at IgG concentrations 100- to 2,000-fold lower than required for rosette inhibition with nonimmune IgG. Steric hindrance did not account for the observed differences between immune and nonimmune IgG since rabbit anti-gC1 F(ab')2 fragments did not block rosetting. Murine anti-gC1 or anti-gD1 IgG, a species of IgG incapable of binding by its Fc end to the HSV-1 FcR, also did not block rosetting. When cells were infected with a gC1-deficient mutant, anti-gC1 IgG inhibited rosetting to the same extent as nonimmune IgG. This indicates that binding by the Fab end of the IgG molecule was required for maximum inhibition of rosetting. Bipolar bridging was shown to occur even when small concentrations of immune IgG were present in physiologic concentrations of nonimmune IgG. The biologic relevance of antibody bipolar bridging was evaluated by comparing antibody- and complement-dependent virus neutralization of an FcR-negative mutant and its parent HSV-1 strain. By engaging the Fc end of antiviral IgG, the parent strain resisted neutralization mediated by the classical complement pathway. These observations provide insight into the role of the HSV-1 FcR in pathogenesis and may help explain the function of FcR detected on other microorganisms.

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.

A viable HSV-1 mutant deleted in two nonessential major glycoproteins

A HSV-1 recombinant lacking the two major glycoproteins gC and gE was isolated from cells co-infected with mutants negative for only one of these glycoproteins. The deletions of the appropriate genes were shown to be the same as on the respective parental genomes. In cell culture, the gC/gE minus recombinant virus replicated to titers similar to those obtained with the gC minus parental strain.

Herpes simplex virus glycoprotein D mediates interference with herpes simplex virus infection

We showed that the expression of a single protein, glycoprotein D (gD-1), specified by herpes simplex virus type 1 (HSV-1) renders cells resistant to infection by HSV but not to infection by other viruses. Mouse (LMtk-) and human (HEp-2) cell lines containing the gene for gD-1 under control of the human metallothionein promoter II expressed various levels of gD-1 constitutively and could be induced to express higher levels with heavy metal ions. Radiolabeled viruses bound equally well to gD-1-expressing and control cell lines. Adsorbed viruses were unable to penetrate cells expressing sufficient levels of gD-1, based on lack of any cytopathic effects of the challenge virus and on failure to detect either the induction of viral protein synthesis or the shutoff of host protein synthesis normally mediated by a virion-associated factor. The resistance to HSV infection conferred by gD-1 expression was not absolute and depended on several variables, including the amount of gD-1 expressed, the dosage of the challenge virus, the serotype of the challenge virus, and the properties of the cells themselves. The interference activity of gD-1 is discussed in relation to the role of gD-1 in virion infectivity and its possible role in permitting escape of progeny HSV from infected cells.

Initial interaction of herpes simplex virus with cells is binding to heparan sulfate

We have shown that cell surface heparan sulfate serves as the initial receptor for both serotypes of herpes simplex virus (HSV). We found that virions could bind to heparin, a related glycosaminoglycan, and that heparin blocked virus adsorption. Agents known to bind to cell surface heparan sulfate blocked viral adsorption and infection. Enzymatic digestion of cell surface heparan sulfate but not of dermatan sulfate or chondroitin sulfate concomitantly reduced the binding of virus to the cells and rendered the cells resistant to infection. Although cell surface heparan sulfate was required for infection by HSV types 1 and 2, the two serotypes may bind to heparan sulfate with different affinities or may recognize different structural features of heparan sulfate. Consistent with their broad host ranges, the two HSV serotypes use as primary receptors ubiquitous cell surface components known to participate in interactions with the extracellular matrix and with other cell surfaces.

Neutralizing activity of antibodies against the major herpes simplex virus type 1 glycoproteins

The specificity and neutralizing activity of antibodies against the major herpes simplex virus type 1 (HSV-1) glycoproteins were tested in serum samples of patients with a history of HSV-1 infection. By preabsorption of sera to preparations of native and denatured HSV-1 proteins, followed by immunoblotting and microneutralization, it was shown that the majority of neutralizing antibodies are directed against denaturation-sensitive epitopes. Furthermore, preabsorption of sera to proteins of viral ts and deletion mutants revealed that antibodies specific for gB, gC, and gE had a low neutralizing activity. These results suggest a major role of anti-gD in neutralization of viral infectivity. In addition, it was shown that antibodies directed against the gB monomer were distinct from antibodies against the gB homodimers. The latter, however, did not reveal any measurable neutralizing activity.

C3b receptor activity on transfected cells expressing glycoprotein C of herpes simplex virus types 1 and 2

Glycoprotein C from herpes simplex virus type 1 (gC-1 from HSV-1) acts as a receptor for the C3b fragment of the third component of complement on HSV-1-infected cell surfaces. Direct binding assays with purified gC-1 and C3b demonstrate that other viral and cellular proteins are not required for this interaction. Although C3b receptor activity is not expressed on HSV-2-infected cell surfaces, purified gC-2 specifically binds C3b in direct binding assays, suggesting that gC-1 and gC-2 are functionally similar. Here, we used a transient transfection system to further characterize the role of gC-1 and gC-2 as C3b receptors and to localize the site(s) on gC involved in C3b binding. The genes for gC-1 and gC-2 were each cloned into a eucaryotic expression vector containing the Rous sarcoma virus long terminal repeat as the promoter and transfected into NIH 3T3 cells. The expressed proteins were similar in molecular size, extent of carbohydrate processing, and antigenic properties to gC-1 and gC-2 purified from infected cells. Using a double-label immunofluorescence assay, we found that both gC-1 and gC-2 were expressed on the surfaces of transfected cells and bound C3b. These results suggest that other proteins expressed during HSV-2 infection prevent receptor activity. We constructed three in-frame deletion mutants of gC-2 to identify domains on the protein important for C3b receptor activity. These mutants lacked amino acids 26 to 73, 219 to 244, or 318 to 346. The mutant protein lacking residues 26 to 73 was reactive with two monoclonal antibodies recognizing distinct epitopes, showed a wild-type pattern of carbohydrate processing, and bound C3b on the transfected cell surface. These results suggest that residues 26 to 73 are not involved in C3b binding. The other two mutant proteins were present on the cell surface, but did not bind C3b. In addition, these mutant proteins showed altered patterns of carbohydrate processing, formed aggregates, and were no longer recognized by the monoclonal antibodies. These properties indicate that removal of residues 219 to 244 or 318 to 346 disrupted the native conformation of gC-2, possibly owing to an alteration in the spacing between critical cysteine residues.

Recognition of similar epitopes on varicella-zoster virus gpI and gpIV by monoclonal antibodies

Two monoclonal antibodies, MAb43.2 and MAb79.0, prepared against varicella-zoster virus (VZV) proteins were selected to analyze VZV gpIV and gpI, respectively. MAb43.2 reacted only with cytoplasmic antigens, whereas MAb79.0 recognized both cytoplasmic and membrane antigens in VZV-infected cells. Immunoprecipitation of in vitro translation products with MAb43.2 revealed only proteins encoded by the gpIV gene, whereas MAb79.0 precipitated proteins encoded by the gpIV and gpI genes. Pulse-chase analysis followed by immunoprecipitation of VZV-infected cells indicated reactivity of MAb43.2 with three phosphorylated precursor species of gpIV and reactivity of MAb79.0 with the precursor and mature forms of gpI and gpIV. These results indicated that (i) MAb43.2 and MAb79.0 recognize different epitopes on VZV gpIV, (ii) glycosylation of gpIV ablates recognition by MAb43.2, and (iii) gpIV is phosphorylated. To map the binding site of MAb79.0 on gpI, the pGEM transcription vector, containing the coding region of the gpI gene, was linearized, and three truncated gpI DNA fragments were generated. RNA was transcribed from each truncated fragment by using SP6 RNA polymerase, translated in vitro in a rabbit reticulocyte lysate, and immunoprecipitated with MAb79.0 and human sera. The results revealed the existence of an antibody-binding site within 14 amino acid residues located between residues 109 to 123 on the predicted amino acid sequences of gpI. From the predicted amino acid sequences, 14 residues on gpI (residues 107 to 121) displayed a degree of similarity (36%) to two regions (residues 55 to 69 and 245 to 259) of gp IV. Such similarities may account for the binding of MAb79.0 to both VZV gpI and gpIV.

The herpes simplex virus type I DNA polymerase. Polypeptide structure and antigenic domains

Polyclonal antibodies responding specifically to the N-terminal, central and C-terminal polypeptide domains of the herpes simplex virus type I (HSV-1) DNA polymerase of strain Angelotti were generated. Each of the five different rabbit antisera reacted specifically with a viral 132 +/- 5-kDa polypeptide as shown by immunoblot analysis. Enzyme binding and inhibition studies revealed that antibodies raised to the central and the C-terminal domains of the protein inhibited the polymerizing activity by 70-90%, respectively, which is well in line with the proposed site of the catalytic center of the enzyme and with the possible involvement of these polypeptide chains in DNA-protein interactions. In agreement with this, antibodies directed towards the N-terminal domain bound to the enzyme without effecting the enzymatic activity. The strong binding but low inhibitory properties of antibodies directed to the polypeptide region between residues 1072 and 1146 confirms previous suggestions that these C-terminal sequences, which share no homology to the Epstein-Barr virus DNA polymerase, are less likely involved in the building of the polymerase catalytic site. Antibodies, raised to the very C terminus of the polymerase (EX3), were successfully used to identify a single 132 +/- 5-kDa polypeptide, which coeluted with the HSV DNA polymerase activity during DEAE-cellulose chromatography, and were further shown to precipitate a major viral polypeptide of identical size. From the presented data it can be concluded that the native enzyme consists of a single polypeptide with a size predicted from the long open reading frame of the HSV-1 DNA polymerase gene.

A herpes simplex virus mutant in which glycoprotein D sequences are replaced by beta-galactosidase sequences binds to but is unable to penetrate into cells

Herpes simplex virus (HSV) glycoprotein gD is a major component of the virion envelope and is thought to play an important role in the initial stages of viral infection and stimulates the production of high titers of neutralizing antibodies. We assumed that gD plays an essential role in virus replication, and so to complement viruses with mutations in the gD gene we constructed a cell line, denoted VD60, which is capable of expressing high levels of gD after infection with HSV. A recombinant virus, designated F-gD beta, in which sequences encoding gD and a nonessential glycoprotein, gI, were replaced by Escherichia coli beta-galactosidase sequences, was selected on the basis that it produced blue plaques on VD60 cell monolayers under agarose overlays containing 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal). F-gD beta was able to replicate normally on complementing VD60 cells. However, F-gD beta was unable to form plaques on noncomplementing Vero cells. Virions lacking gD were produced in normal amounts by Vero cells infected with F-gD beta, and the virus particles were distributed throughout the cytoplasm and on the cell surface, suggesting that gD is not essential for HSV envelopment and egress. Virions lacking gD were able to bind to cells, but were unable to initiate synthesis of viral early polypeptides. Plaque production of F-gD beta particles lacking gD was enhanced by polyethylene glycol treatment, suggesting that gD is essential for penetration of HSV into cells. Other HSV glycoproteins have been implicated in the entry of virus into cells, and thus this process appears to involve multiple interactions at the cell surface.

Herpes simplex virus immunoglobulin G Fc receptor activity depends on a complex of two viral glycoproteins, gE and gI

Evidence was recently presented that herpes simplex virus type 1 (HSV-1) immunoglobulin G (IgG) Fc receptors are composed of a complex containing a previously described glycoprotein, gE, and a novel virus-induced polypeptide, provisionally named g70 (D. C. Johnson and V. Feenstra, J. Virol. 61:2208-2216, 1987). Using a monoclonal antibody designated 3104, which recognizes g70, in conjunction with antipeptide sera and virus mutants unable to express g70 or gE, we have mapped the gene encoding g70 to the US7 open reading frame of HSV-1 adjacent to the gE gene. Therefore, g70 appears to be identical to a recently described polypeptide which was named gI (R. Longnecker, S. Chatterjee, R. J. Whitley, and B. Roizman, Proc. Natl. Acad. Sci. USA 84:147-151, 1987). Under mildly denaturing conditions, monoclonal antibody 3104 precipitated both gI and gE from extracts of HSV-1-infected cells. In addition, rabbit IgG precipitated the gE-gI complex from extracts of cells transfected with a fragment of HSV-1 DNA containing the gI, gE, and US9 genes. Cells infected with mutant viruses which were unable to express gE or gI did not bind radiolabeled IgG; however, cells coinfected with two viruses, one unable to express gE and the other unable to express gI, bound levels of IgG approaching those observed with wild-type viruses. These results further support the hypothesis that gE and gI form a complex which binds IgG by the Fc domain and that neither polypeptide alone can bind IgG.