Inducible expression of herpes simplex virus type 2 glycoprotein gene gG-2 in a mammalian cell line

Novel deletion in glycoprotein G forms a cluster and causes epidemiologic spread of herpes simplex virus type 2 infection

The herpes simplex virus type 2 (HSV-2) glycoprotein G (gG-2) gene of 106 clinical isolates was analyzed and six isolates were identified with 63 nucleotides comprising 21 amino acids (aa) deleted in the immunodominant region. Compared with strain HG52, variations in the gG-2 gene were found at 276 and 27 sites in nucleotide and aa sequences, respectively, in the 106 strains. Significant variations in both nucleotides and aa were accumulated in the immunodominant region rather than in the other regions (P < 0.001), indicating that the immunodominant region might be indispensable in vivo and a hot spot for variation. The frequency of 21 aa-deleted strains (HSVΔ21/gG-2) among clinical isolates was 5%, indicating the advantage of this deletion of gG-2 for epidemiological expansion. Phylogenetic analysis of the 106 strains indicated that the HSVΔ21/gG-2 strains formed a cluster among the various variations but that their genomes showed different endonuclease digestion patterns. The antibody titers to total HSV antigens of patients infected with wild HSV-2 and HSVΔ21/gG-2 were similar, but patients with HSVΔ21/gG-2 had a lower antibody titer to gG-2 than those with wild HSV-2 (P < 0.001). HSVΔ21/gG-2 might be less immnunogenic and reduce antibody production to gG-2, while its pathogenicity in humans was not distinguished in its clinical manifestations. Thus, infection with HSVΔ21/gG-2 caused genital lesions similar to wild HSV-2 infection, but evaded the immune response to gG-2 to allow epidemiological spread, indicating the importance of this deletion in the immunodominant region of gG-2 in the pathogenesis and transmission of genital herpes.

Glycoprotein G of herpes simplex virus 2 as a novel vaccine antigen for immunity to genital and neurological disease

The envelope glycoproteins of herpes simplex virus 1 (HSV-1) and HSV-2, with the exception of glycoprotein G, elicit cross-reactive B- and T-cell responses. Human vaccine trials, using the cross-reactive glycoproteins B and D, have shown no protection against genital HSV-2 infection or disease. In this study, the mature form of glycoprotein G (mgG-2) of HSV-2 was used for immunization of mice, either alone or in combination with adjuvant CpG, followed by an intravaginal challenge with a lethal dose of a fully virulent HSV-2 strain. Mice immunized with mgG-2 plus CpG showed low disease scores and a significantly higher survival rate (73%) than mice immunized with mgG-2 alone (20%) or controls (0%). Accordingly, limited numbers of infectious HSV-2 particles were detected in the spinal cord of mice immunized with mgG-2 plus CpG. The observed protection was associated with a gamma interferon (IFN-γ) response by splenic CD4(+) T cells upon antigen restimulation in vitro and in vaginal washes 1 day postinfection. The majority of sera collected from mice immunized with mgG-2 plus CpG showed macrophage-mediated antibody-dependent cellular cytotoxicity and antibody-dependent complement-mediated cytolysis, while no neutralization activity was observed. In conclusion, we have shown that immunization with the type-specific mgG-2 protein in combination with CpG could elicit protective immunity against an otherwise lethal vaginal HSV-2 challenge. The mgG-2 protein may therefore constitute a promising HSV-2 vaccine antigen to be considered for future human trials.

Use of a fragment of glycoprotein G-2 produced in the baculovirus expression system for detecting herpes simplex virus type 2-specific antibodies

Fragments of glycoprotein G (gG-2(281-594His)), comprising residues 281 to 594 of herpes simplex virus type 2 (HSV-2), glycoprotein G of HSV-1 (gG-1(t26-189His)), and glycoprotein D of HSV-1 (gD-1(1-313)), were expressed in the baculovirus expression system to develop an assay for the detection of HSV-1 and HSV-2 type-specific antibodies. The expression of the gG-1(t26-189His) and gG-2(281-594His) fragments was analyzed by Western blotting using monoclonal antibodies LP10 and AP1, respectively. The molecular masses of the major products of gG-1(t26-189His) and the fragment of gG-2(281-594His) were 36 to 39 kDa and 64 to 72 kDa, respectively. Human sera positive for HSV-1 reacted with gG-1(t26-189His), sera positive for HSV-2 reacted with the gG-2(281-594His) fragment, and sera positive for both types reacted with gG-1(t26-189His) and gG-2(281-594His) in Western blotting. The human sera recognized polypeptides of gG-2(281-594His) with molecular masses of 57 to 67 and 120 to 150 kDa and additional faint bands of 21, 29, and 45 kDa. The recombinant gG-1(t26-189His) and the recombinant gG-2(281-594His) fragment were used as type-specific antigens for the detection of HSV-1- and HSV-2-specific antibody responses in human sera, respectively. As type-common antigens, an extract of HSV-1-infected Vero cells and recombinant gD-1(1-313) were used. An enzyme-linked immunosorbent assay to detect type-specific antibodies was developed, and the sensitivity and specificity were evaluated by comparison with commercial tests by using sera obtained from different sources. The sensitivity and specificity were 91.5 and 95.5%, respectively, compared to the Gull assay. The gG-2(281-594His) fragment can be obtained in relatively large quantities at low cost.

Monoclonal antibodies and human sera directed to the secreted glycoprotein G of herpes simplex virus type 2 recognize type-specific antigenic determinants

Glycoprotein G-2 (gG-2) of herpes simplex virus type 2 (HSV-2) is cleaved to a secreted amino-terminal portion (sgG-2) and to a cell-associated carboxy-terminal portion which is further O-glycosylated to constitute the mature gG-2 (mgG-2). In contrast to mgG-2, which is known to elicit a type-specific antibody response in the human host, information on the immunogenic properties of sgG-2 is lacking. Here the sgG-2 protein was purified on a heparin column and used for production of monoclonal antibodies (mAbs). Four anti-sgG-2 mAbs were mapped using a Pepscan technique and identified linear epitopes which localized to the carboxy-terminal part of the protein. One additional anti-sgG-2 mAb, recognizing a non-linear epitope, was reactive to three discrete peptide stretches where the most carboxy-terminally located stretch was constituted by the amino acids (320)RRAL(323). Although sgG-2 is rapidly secreted into the cell-culture medium after infection, the anti-sgG-2 mAbs identified substantial amounts of sgG-2 in the cytoplasm of HSV-2-infected cells. All of the anti-sgG-2 mAbs were HSV-2 specific showing no cross-reactivity to HSV-1 antigen or to HSV-1-infected cells. Similarly, sera from 50 HSV-2 isolation positive patients were all reactive to sgG-2 in an enzyme immunoassay whilst no reactivity was seen in 25 sera from HSV-1 isolation positive patients or in 25 serum samples from HSV-negative patients suggesting that sgG-2 is a novel antigen potentially suitable for type-discriminating serodiagnosis.

Bovine herpesvirus 1 U(s) open reading frame 4 encodes a glycoproteoglycan

Sequence analysis of the short unique (Us) segment of the bovine herpesvirus 1 (BHV-1) genome predicted that the Us open reading frame (ORF) 4 encodes a protein with homology to glycoprotein G (gG) of other alpha-herpesviruses (P. Leung-Tack, J.-C. Audonnet, and M. Riviere, Virology 199:409-421, 1994). RNA analysis showed that the Us ORF4 is contained within two transcripts of 3.5 and 1.8 kb. The 3.5 kb RNA represents a structurally bicistronic RNA which encompasses the Us ORF3 and Us ORF4, whereas the 1.8-kb RNA constitutes the monocistronic Us ORF4 mRNA. To identify the predicted BHV-I gG, recombinant vaccinia virus expressing the Us ORF4 was used to raise specific antibodies in rabbits. The antiserum recognized a 65-kDa polypeptide and a very diffusely migrating species of proteins with an apparent molecular mass of between 90 and greater than 240 kDa in supernatants of BHV-1-infected cells which was also precipitated together with 61- and 70-kDa polypeptides from cell-associated proteins. The specificity of the reaction was demonstrated by the absence of these proteins from the supernatant of cells infected with the Us ORF4 deletion mutant BHV-l/gp1-8. Treatment of the immunoprecipitated proteins with glycosidases and chondroitinase AC showed that the 65-kDa protein constitutes gG, which contains both N- and O-linked carbohydrates, and that the high-molecular-mass proteins contain glycosaminoglycans linked to a 65-kDa glycoprotein that is antigenically related to gG. These molecules were therefore named glycoproteoglycan C (gpgG). Pulse chase experiments indicated that gG and gpgG were processed from a common precursor molecule with an apparent molecular mass of 61 kDa via a 70-kDa intermediate. Both gG and gpgG could not be found associated with purified virions. In summary, our results identify the BHV-I gG protein and demonstrate the presence of a form of posttranslational modification, glycosamino-glycosylation, that has not yet been described for a herpesvirus-encoded protein.

Gene mapping and sequence analysis of the unique short region of the simian herpesvirus SA 8 genome

A 10.5 kbp BamHI restriction fragment representing most of the unique short (Us) region of the genome of the simian alpha-herpesvirus SA8 was identified and cloned. Partial sequencing of this DNA fragment identified regions of sequence homology with eight open reading frames (ORFs) of HSV1 and/or HSV2. Sequence and size analysis of subcloned fragments of the SA8 Us region and comparison with homologous HSV Us sequences determined that the number, order, size, and orientation of SA8 Us ORFs are comparable to those of HSV. Based on the location of transcriptional control elements, transcription of SA8 Us genes appears to be organized into 3' co-terminal mRNA sets as in HSV, although the grouping of the gene sets is different. The SA8 US4 (gG) ORF is more similar to that of HSV2 than HSV1, both in size and predicted amino acid sequence. Complete sequences were determined for five SA8 genes which represent homologs of the HSV gD, gE, gI, US5, and US9 genes. The predicted polypeptides encoded by SA8 are similar to the corresponding HSV polypeptides. All SA8 Us genes were more closely related to those of HSV than to related gene homologs of other mammalian alpha-herpesviruses.

Orientation of the cleavage site of the herpes simplex virus glycoprotein G-2

During the synthesis of glycoprotein G-2 (gG-2) of herpes simplex virus type 2, the 104,000-Da gG-2 precursor (104K precursor) is cleaved to generate the 72K and the 31K intermediates. The 72K product is processed to generate the mature gG-2 (molecular mass, 108,000 Da), while the 31K product is additionally processed and secreted into the extracellular medium as the 34K component (H. K. Su, R. Eberle, and R. J. Courtney, J. Virol. 61:1735-1737, 1987). In this study, the orientations of the 31K and 72K products on the 104K precursor were determined by using two antipeptide sera produced in rabbits and a monoclonal antibody, 13 alpha C6, directed against gG-2. The sera prepared against synthetic peptides corresponding to the terminal amino acid residues 67 to 78 and an internal peptide at amino acids 247 to 260 of gG-2 recognized the 104K precursor and the 31K cleavage product but not the 72K intermediate. In contrast, 13 alpha C6 detected the 72K cleavage product and the uncleaved precursor but not the 31K cleavage component. The epitope recognized by 13 alpha C6 was mapped within amino acids 486 to 566. These results suggest that the 31K cleavage product is derived from the amino-terminal portion of the 104K precursor molecule and that the 72K intermediate is derived from the carboxyl terminus. In support of our model described above for the synthesis of gG-2, antibodies recognizing either of the cleavage products reacted with the uncleaved precursor but not with the other cleavage product. By using partial endo-beta-N-acetylglucosaminidase H analysis, two N-linked glycosylation sites were found on each of the cleavage products. The distribution of the N-linked glycosylation sites and the reactivities of the antipeptide sera allowed the cleavage region on the precursor to be mapped to within amino acids 260 to 437.

Detection of antibodies to herpes simplex virus type 2 with a mammalian cell line expressing glycoprotein gG-2

The gene (US4) coding for herpes simplex virus type 2 (HSV-2) glycoprotein G (gG-2) was cloned and constitutively expressed in Chinese hamster ovary (CHO) cells. The expression vector containing the dihydrofolate reductase (dhfr) gene, and the HSV-2 US4 gene under the control of the Simian virus 40 early promoter (SV40 EP), was transfected into dhfr-deficient CHO cells. The transfected cells were selected and amplified using methotrexate (MTX). To demonstrate that the gG-2 produced in these transformed cells had antigenic determinants in common with the native glycoprotein, CHO cells expressing gG-2 were used in an immunofluorescent assay (IFA) for the detection of HSV-2 type-specific antibodies in human serum samples. Seven of eight serum samples from adults with prior episodes of culture proven HSV-2 infections were found to be positive by the IFA method whereas none of seven serum samples from young children with culture documented HSV-1 infections were positive by IFA. Thus the recombinant CHO : gG-2 cells have diagnostic utility in an HSV-2 specific serologic assay.

Identification of equine herpesvirus 4 glycoprotein G: a type-specific, secreted glycoprotein

Equine herpesvirus 4 (EHV4) glycoproteins of M(r) 63K and 250K were identified in the supernatant of infected cell cultures. The 63K glycoprotein was type-specific; that is, it reacted with monospecific sera from horses that had been immunized or infected with EHV4, but not with monospecific sera from horses immunized or infected with EHV1, a closely related alphaherpesvirus. It was postulated that the secreted protein may be the homologue of similarly secreted glycoproteins of herpes simplex virus 2 glycoprotein G (HSV2 gG) and pseudorabies virus (PRV) gX, which is the homologue of HSV2 gG. The US region of the EHV4 genome, toward the internal repeat structure, was sequenced. Four open reading frames (ORFs) were identified of which ORF4 showed 52% similarity to the gene-encoding PRV gX in a 650-nucleotide region. ORF4 coded for a primary translational product of 405 amino acids which has a predicted size of 44K. The amino acid sequence of ORF4 showed 28% identity with PRV gX and 16% identity with HSV2 gG, although significantly greater identity was observed in the N-terminal region including the conservation of 4 cysteine residues. Accordingly, we designate ORF4 as EHV4 gG. The predicted amino acid sequence of the EHV4 gG showed characteristics of an envelope glycoprotein. Expression of the entire EHV4 gG gene in the bacterial expression vector pGEX-3X produced a type-specific fusion protein of M(r) 70K of which the gG portion composes 43K. Antibody that was affinity purified from selected portions of Western blots containing the 70K gG fusion protein reacted with the 63K secreted glycoprotein. Conversely, antibody affinity purified to the 63K secreted product reacted with the 70K gG fusion protein. These results showed that the EHV4 63K secreted glycoprotein was EHV4 gG, the third alphaherpesvirus gG homologue known to be, at least in part, secreted. The type-specificity of this glycoprotein provides, for the first time, the opportunity to differentiate between antibodies present in polyclonal sera from EHV4, EHV1, and dual-infected horses and this has important implications for understanding the epidemiology of these viruses.

Apical expression of herpes simplex virus type 2 glycoproteins in human neuroblastoma cells

The expression of herpes simplex virus type 2 (HSV-2) glycoproteins on the surface of human neuroblastoma cells has been investigated using Millipore Millicell culture plate inserts. Utilizing a modified radioimmunoassay, we learned that glycoproteins B, C, D, E, and I were expressed predominantly on the apical membrane domain of the infected neuroblastoma cells. The unidirectional transport of HSV-2 glycoproteins was substantiated by the analysis of extracellular glycoproteins released from neuroblastoma cells. The results suggest that the evaluated HSV-2 glycoproteins were transported primarily to the apical plasma membrane domain of human neuroblastoma cells.

In vitro synthesis and processing of herpes simplex virus type 2 gG-2, using cell-free transcription and translation systems

Translation of in vitro-synthesized herpes simplex virus type 2 (HSV-2) gG-2 mRNA in a reticulocyte lysate system was used to study the processing of HSV-2 gG-2. In the presence of canine pancreatic microsomal membranes, a single species that is protected from trypsin digestion was detected. This product comigrates with the 104,000-Mr (104K) high mannose intermediate seen in HSV-2-infected-cell lysates. Endo-beta-N-acetylglucosaminidase H treatment of the in vitro-synthesized 104K protein yielded a single product migrating at 100 K. The 72K and 31K cleavage products of gG-2 were not observed in the in vitro system. These data show that the molecular weight of the nonglycosylated form of the gG-2 protein is 100,000 and that the cotranslational processing of this protein in the endoplasmic reticulum yields the 104K high-mannose intermediate.

Antigenic properties and cellular localization of herpes simplex virus glycoprotein H synthesized in a mammalian cell expression system

Herpes simplex virus type 1 glycoprotein H (HSV-1 gH) was synthesized in an inducible mammalian cell expression system, and its properties were examined. The gH coding sequence, together with the stable 5' untranslated leader sequence from xenopus beta-globin, was placed under control of the strong promoter from the human cytomegalovirus major immediate-early gene in an amplifiable plasmid which contains the simian virus 40 (SV40) virus origin for replication (ori). This expression vector was transfected into ts COS cells constitutively expressing a temperature-sensitive SV40 T antigen which allows utilization of the SV40 ori at permissive temperatures. The results of transient expression assays at the permissive temperature showed that HSV-1 gH could be synthesized in greater amounts than those produced by a high-multiplicity virus infection. The proteins produced were detected in Western blots (immunoblots) with a HSV-1 gH-specific polyclonal serum raised against a TrpE-gH fusion protein. The transfected gH had an apparent molecular weight of approximately 105,000, intermediate in size to those of the precursor (100,000) and fully processed forms (110,000) of HSV-1 gH from infections. Antigenicity was investigated by reactions with three virus-neutralizing monoclonal antibodies specific for conformational epitopes on gH. Only one of these monoclonal antibodies could immunoprecipitate the synthesized gH. However, equal recognition of the transfected gH was achieved by superinfection with virus. In addition, detectable amounts of gH were not expressed on the cell surface unless the cells were superinfected with virus. Studies with a temperature-sensitive mutant, ts1201, defective in encapsidation showed that the changes in antigenic structure and cell surface expression caused by superinfection with virus were not due simply to incorporation of gH into virions. These results suggest that gH requires additional virus gene products for cell surface localization and formation of an antigenic structure important for its function in mediating infectivity.

Use of a glucocorticoid-inducible promoter for expression of herpes simplex virus type 1 glycoprotein gC1, a cytotoxic protein in mammalian cells

Abundant expression of herpes simplex virus type 1 glycoprotein gC (gC1) in transfected mammalian cells has not previously been achieved, possibly because gC1 protein is toxic to cells. To approach this problem, the gC1 coding sequence was placed under the control of the weak but inducible glucocorticoid-responsive promoter from the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). As controls to evaluate for gC1 cytotoxicity, the MMTV LTR promoter was used to express glycoprotein gD1, and a strong, constitutive promoter from the Moloney murine sarcoma virus LTR was used to express gC1. L cells were transfected with these constructs, and a clone expressing gC1 from the inducible MMTV LTR promoter was analyzed. In the absence of glucocorticoid (dexamethasone) stimulation, only a low level of gC1 mRNA expression was detected; after overnight stimulation with dexamethasone, transcription increased approximately 200-fold. Abundant gC1 protein that was functionally active in that it bound complement component C3b, was produced. From passages 5 through 26 (70 cell population doublings), the gC1-producing clone became less responsive to overnight dexamethasone stimulation. The block to gC1 expression occurred at the level of transcription and was associated with hypermethylation of the MMTV LTR DNA. Treatment of the clone with 5-aza-2'-deoxycytidine partially reversed the block in gC1 protein production. Late-passage cells assumed a gC1-negative phenotype that appeared to offer a selective growth advantage, which suggested that gC1 was cytotoxic. Several findings support this view: (i) some cells expressing gC1 after overnight stimulation with dexamethasone assumed bizarre, syncytial shapes; (ii) continuous stimulation with dexamethasone for 5 weeks resulted in death of most cells; (iii) cells transfected with gC1 under the control of the strong Moloney murine sarcoma virus promoter assumed bizarre shapes, and stable gC1-expressing clones could not be established; and (iv) cells induced to express gD1 retained a normal appearance after overnight stimulation or 15 weeks of continuous stimulation with dexamethasone. The inducible MMTV LTR promoter is useful for expressing gC1 and may have applications for expressing other cytotoxic proteins.

Use of a glucocorticoid-inducible promoter for expression of herpes simplex virus type 1 glycoprotein gC1, a cytotoxic protein in mammalian cells

Abundant expression of herpes simplex virus type 1 glycoprotein gC (gC1) in transfected mammalian cells has not previously been achieved, possibly because gC1 protein is toxic to cells. To approach this problem, the gC1 coding sequence was placed under the control of the weak but inducible glucocorticoid-responsive promoter from the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). As controls to evaluate for gC1 cytotoxicity, the MMTV LTR promoter was used to express glycoprotein gD1, and a strong, constitutive promoter from the Moloney murine sarcoma virus LTR was used to express gC1. L cells were transfected with these constructs, and a clone expressing gC1 from the inducible MMTV LTR promoter was analyzed. In the absence of glucocorticoid (dexamethasone) stimulation, only a low level of gC1 mRNA expression was detected; after overnight stimulation with dexamethasone, transcription increased approximately 200-fold. Abundant gC1 protein that was functionally active in that it bound complement component C3b, was produced. From passages 5 through 26 (70 cell population doublings), the gC1-producing clone became less responsive to overnight dexamethasone stimulation. The block to gC1 expression occurred at the level of transcription and was associated with hypermethylation of the MMTV LTR DNA. Treatment of the clone with 5-aza-2'-deoxycytidine partially reversed the block in gC1 protein production. Late-passage cells assumed a gC1-negative phenotype that appeared to offer a selective growth advantage, which suggested that gC1 was cytotoxic. Several findings support this view: (i) some cells expressing gC1 after overnight stimulation with dexamethasone assumed bizarre, syncytial shapes; (ii) continuous stimulation with dexamethasone for 5 weeks resulted in death of most cells; (iii) cells transfected with gC1 under the control of the strong Moloney murine sarcoma virus promoter assumed bizarre shapes, and stable gC1-expressing clones could not be established; and (iv) cells induced to express gD1 retained a normal appearance after overnight stimulation or 15 weeks of continuous stimulation with dexamethasone. The inducible MMTV LTR promoter is useful for expressing gC1 and may have applications for expressing other cytotoxic proteins.