Glycosylation pattern of herpes simplex virus type 2 glycoprotein G from precursor species to the mature form

A truncated glycoprotein G vaccine formulated with Advax-CpG adjuvant provides protection of mice against genital herpes simplex virus 2 infection

Herpes simplex virus type 2 (HSV-2) is a common sexually transmitted disease that affects approximately 500 million individuals globally. There is currently no approved vaccine to prevent HSV-2 infection. EXCT4 is a truncated form of the mature glycoprotein G-2 (mgG-2) that unlike full mature form is secreted by expressing cells enabling it to be rapidly scaled up for production. The current study examined whether EXCT4 immunity in mice could be further enhanced through use of adjuvants. EXCT4 formulated with Advax-CpG adjuvant induced a strong Th1-type immune response characterized by interferon gamma (IFN-γ) and protected animals against a lethal genital challenge with HSV-2. This response was associated with reduced viral load in vaginal washes, spinal cord, and dorsal root ganglia. Together the results provide proof of concept that EXCT4 formulated with Advax-CpG adjuvant is a promising HSV-2 vaccine candidate warranting further investigation.

Identification of the Cleavage Domain within Glycoprotein G of Herpes Simplex Virus Type 2

Glycoprotein G (gG) from herpes simplex virus type 1 and 2 (HSV-1 and HSV-2, respectively) functions as a viral chemokine binding protein (vCKBP). Soluble recombinant forms of gG of HSV-1 and HSV-2 (SgG1 and SgG2, respectively) enhance chemokine-mediated leukocyte migration, in contrast to most known vCKBPs, including those from animal alpha-herpesviruses. Furthermore, both proteins bind to nerve growth factor (NGF), but only SgG2 enhances NGF-dependent neurite outgrowth. The basis and implications of this functional difference between the two proteins are still unknown. While gG1 and gG2 are positional homologues in the genome, they share very limited sequence homology. In fact, US4, the open reading frame encoding gG is the most divergent genetic locus between these viruses. Full-length gG1 and gG2 are type I transmembrane proteins located on the plasma membrane of infected cells and at the viral envelope. However, gG2 is larger than gG1 and is cleaved during protein maturation, secreting the N-terminal domain to the supernatant of infected cells, whereas gG1 is not. The enzyme involved in gG2 cleavage and the functional relevance of gG2 cleavage and secretion are unknown. We aim to identify the gG2 sequence required for cleavage to determine its functional role in future experiments. Our results prove the existence of at least two cleavage motifs in gG2 within the amino acid region 314-343. Transfer of this sequence to a fusion protein results in cleavage. Finally, we show that propeptide convertases like furin are responsible for gG2 cleavage.

Characterization of the viral O-glycopeptidome: a novel tool of relevance for vaccine design and serodiagnosis

Viral envelope proteins mediate interactions with host cells, leading to internalization and intracellular propagation. Envelope proteins are glycosylated and are known to serve important functions in masking host immunity to viral glycoproteins. However, the viral infectious cycle in cells may also lead to aberrant glycosylation that may elicit immunity. Our knowledge of immunity to aberrant viral glycans and glycoproteins is limited, potentially due to technical limitations in identifying immunogenic glycans and glycopeptide epitopes. This work describes three different complementary methods for high-throughput screening and identification of potential immunodominant O-glycopeptide epitopes on viral envelope glycoproteins: (i) on-chip enzymatic glycosylation of scan peptides, (ii) chemical glycopeptide microarray synthesis, and (iii) a one-bead-one-compound random glycopeptide library. We used herpes simplex virus type 2 (HSV-2) as a model system and identified a simple O-glycopeptide pan-epitope, (501)PPA(GalNAc)TAPG(507), on the mature gG-2 glycoprotein that was broadly recognized by IgG antibodies in HSV-2-infected individuals but not in HSV-1-infected or noninfected individuals. Serum reactivity to the extended sialyl-T glycoform was tolerated, suggesting that self glycans can participate in immune responses. The methods presented provide new insight into viral immunity and new targets for immunodiagnostic and therapeutic measures.

A proinflammatory peptide from herpes simplex virus type 2 glycoprotein G affects neutrophil, monocyte, and NK cell functions

We have identified a synthetic peptide derived from the secreted portion of HSV type 2 glycoprotein G, denoted gG-2p20, which has proinflammatory properties in vitro. The gG-2p20 peptide, corresponding to aa 190-205 of glycoprotein G-2, was a chemoattractant for both monocytes and neutrophils in a dose-dependent fashion, and also induced the release of reactive oxygen from these cells. The receptor mediating the responses was identified as the formyl peptide receptor. The gG-2p20-induced activation of phagocytes had a profound impact on NK cell functions. The reactive oxygen species produced by gG-2p20-activated phagocytes both inhibited NK cell cytotoxicity and accelerated the apoptotic cell death in NK cell-enriched lymphocyte populations. Hence, we have for the first time been able to identify a potential function of the secreted portion of HSV-2 glycoprotein G. We propose that the proinflammatory gG-2p20 peptide identified could contribute to a reduced function and viability of NK cells during HSV-2 infection due to its ability to recruit and activate phagocytic cells.

CD4+ T-cell responses to herpes simplex virus type 2 (HSV-2) glycoprotein G are type specific and differ in symptomatic and asymptomatic HSV-2-infected individuals

T-cell recognition of the secreted and membrane-bound portions of the herpes simplex virus type 2 (HSV-2) glycoprotein G (sgG-2 and mgG-2, respectively) was compared in symptomatic and asymptomatic HSV-2-infected individuals and in HSV-2-seronegative controls and the responses with HSV-1 glycoproteins C and E (gC-1 and gE-1) were compared. CD4+ T cells from HSV-2-infected individuals specifically recognized both sgG-2 and mgG-2, whereas HSV-1-infected and HSV-seronegative controls did not respond to these glycoproteins. The responses to gC-1 and gE-1, on the other hand, were not type specific, as blood mononuclear cells from both HSV-1- and HSV-2-infected individuals responded in vitro. There was an association between the status of the infection (symptomatic versus asymptomatic) and the CD4+ T-cell responsiveness. Symptomatic HSV-2-seropositive individuals responded with significantly lower Th1 cytokine production to sgG-2 and mgG-2 than did asymptomatic HSV-2-infected carriers, especially within the HSV-1-negative cohort. No differences in T-cell proliferation were observed between asymptomatic and symptomatic individuals. The results have implications for studies of HSV-2-specific CD4+ T-cell reactivity in general and for analysis of immunological differences between asymptomatic and symptomatic individuals in particular.

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.

Conservation of type-specific B-cell epitopes of glycoprotein G in clinical herpes simplex virus type 2 isolates

Glycoprotein G (gG-2) of herpes simplex virus type 2 (HSV-2) is cleaved to a secreted amino-terminal portion and to a cell-associated, heavily O-glycosylated carboxy-terminal portion that constitutes the mature gG-2 (mgG-2). The mgG-2 protein is commonly used as a type-specific antigen in the serodiagnosis of HSV-2 infection. As the amino acid sequence variability of mgG-2 in clinical isolates may affect the performance of such assays, the gG-2 gene was sequenced from 15 clinical HSV-2 isolates. Few mutations were identified, and these were mostly localized outside the epitope regions described earlier. Five isolates were identical to different laboratory strains, indicating that the gG-2 gene is highly conserved over time. In the search for HSV-2 isolates harboring mutations within the immunodominant region of mgG-2, a pool of 2,400 clinical HSV-2 isolates was tested for reactivity with two anti-mgG-2 monoclonal antibodies (MAbs). Ten MAb escape HSV-2 mutants, which all harbored structurally restricted single- or dual-point mutations within the respective epitopes explaining the loss of binding, were identified. Sera from corresponding patients were reactive to mgG-2, as well as to a peptide representing the immunodominant region, suggesting that the point mutations detected did not diminish seroreactivity to mgG-2. The conservation of the gG-2 gene reported here further supports the use of mgG-2 as a type-specific antigen in the diagnosis of HSV-2 infections.

Herpes simplex virus type 2 glycoprotein G-negative clinical isolates are generated by single frameshift mutations

Herpes simplex virus (HSV) codes for several envelope glycoproteins, including glycoprotein G-2 (gG-2) of HSV type 2 (HSV-2), which are dispensable for replication in cell culture. However, clinical isolates which are deficient in such proteins occur rarely. We describe here five clinical HSV-2 isolates which were found to be unreactive to a panel of anti-gG-2 monoclonal antibodies and therefore considered phenotypically gG-2 negative. These isolates were further examined for expression of the secreted amino-terminal and cell-associated carboxy-terminal portions of gG-2 by immunoblotting and radioimmunoprecipitation. The gG-2 gene was completely inactivated in four isolates, with no expression of the two protein products. For one isolate a normally produced secreted portion and a truncated carboxy-terminal portion of gG-2 were detected in virus-infected cell medium. Sequencing of the complete gG-2 gene identified a single insertion or deletion of guanine or cytosine nucleotides in all five strains, resulting in a premature termination codon. The frameshift mutations were localized within runs of five or more guanine or cytosine nucleotides and were dispersed throughout the gene. For the isolate for which a partially inactivated gG-2 gene was detected, the frameshift mutation was localized upstream of but adjacent to the nucleotides coding for the transmembranous region. Thus, this study demonstrates the existence of clinical HSV-2 isolates which do not express an envelope glycoprotein and identifies the underlying molecular mechanism to be a single frameshift mutation.

Herpesvirus papio 2, an SA8-like alpha-herpesvirus of baboons

Several SA8 isolates obtained from baboons were compared to the prototype SA8 herpesvirus of African green monkeys. SDS-PAGE and restriction enzyme analyses revealed definite differences between green monkey and baboon isolates. DNA and amino acid sequences of the gB, gD and gJ glycoprotein genes exhibited substantial differences in variable regions. For the gB and gD, the amount of amino acid substitutions between SA8 and the baboon viruses was comparable to levels observed between analogous genes of SA8 & B virus or HSV1 & HSV2. Although a high degree of antigenic cross-reactivity was apparent, virus-specific antigenic determinants were also readily detected. Phylogenetic analyses supported separation of the baboon isolates and SA8 as distinct viruses. Taken together these results suggest that although closely related to SA8, the baboon viruses represent a distinct simian alpha-herpesvirus which we propose be designated Herpesvirus papio 2.

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.

Herpes simplex virus (HSV) glycoprotein H is partially processed in a cell line that expresses the glycoprotein and fully processed in cells infected with deletion or ts mutants in the known HSV glycoproteins

Cell lines that constitutively express herpes simplex virus 1 (HSV-1) glycoprotein H (gH-1) failed to synthesize the mature form of gH and accumulated a precursor-like form of the glycoprotein, which was retained intracellularly, most likely in RER. Fine-structure analysis of the oligosaccharides present in recombinant gH revealed oligosaccharides processed by RER enzymes; sialylated complex-type and biantennary oligosaccharides, which are assembled in the trans-Golgi, were absent. A small fraction had the characteristics of oligosaccharides processed by the early mannosidases of the Golgi. These findings suggest that a defect in the transport out of RER to the Golgi may account for the intracellular retention of the immature form of gH in cells that express the glycoprotein constitutively. Upon superinfection of cells expressing gH-1 with HSV-2, recombinant gH-1 underwent maturation, indicating that a viral function is required to attain full processing of gH. The known HSV glycoproteins do not appear to carry out this function, since in cells infected with deletion mutants in gD, gG, gE, and gE-gI, with a spontaneous gC- mutant, or with a temperature-sensitive mutant in gB, maturation of gH occurred independently of the presence or of the maturation of the single glycoproteins tested. The present findings together with previous observations on HSV, human CMV, and the EBV homologue of gH suggest that inability of gH to undergo full processing in the absence of viral protein(s) is a property of gH.

Intratypic variation of herpes simplex virus type 2 isolates detected by monoclonal antibodies against viral glycoproteins

Monoclonal antibodies (MAbs) to herpes simplex virus (HSV) glycoproteins gD, gG, gB, and gE were used to analyze antigenic variations of 128 genital HSV-2 isolates by an indirect enzyme-linked immunosorbent assay (ELISA). Isolates were considered significantly different from the standard HSV-2 strain 186 when their optical density (OD) in ELISA was less than half that of strain 186. This criterion gave 30 patterns of reactivity among the genital HSV-2 isolates. The MAbs to gB, gG, and 2 of the gD antibodies reacted with more than 90% of the isolates, suggesting that these MAbs recognized highly conserved epitopes. However, the gE MAb reacted with only 47% of the isolates, and one of the gD antibodies with only 39%. Thus, HSV-2 can readily tolerate modifications in some parts of the gD and gE molecules while remaining infectious.

Temporal aspects of O-glycosylation of glycoprotein C from herpes simplex virus type-1

Herpes simplex virus type-1 glycoprotein C (gC1) contains several O-linked oligosaccharides clustered near N-linked chains, and Pronase digestion produces glycopeptides carrying both oligosaccharide types. We have taken advantage of this fact to investigate the temporal relationship between the initiation of O-linked chains and the processing of N-linked oligosaccharides. gC1 was isolated from herpes-simplex-virus-infected BHK (baby-hamster kidney) cells after short labelling periods with [3H]glucosamine, and the labelled Pronase-cleaved glycopeptides fractionated on concanavalin A-Sepharose. N-[3H]Acetylgalactosamine, mostly convertible into free N-[3H]acetylgalactosaminitol on mild alkaline-borohydride treatment, was found in glycopeptides with an affinity to concanavalin A-Sepharose corresponding to that of glycopeptides carrying Man8GlcNAc2 or larger N-linked chains. Since there is evidence that the processing of N-linked chains up to Man8GlcNAc2 involves enzymes located in the rough endoplasmic reticulum, current results strongly suggest that gC1 acquires O-linked N-acetylgalactosamine before the glycoprotein routing to the Golgi apparatus. The addition of the second sugar to the nascent O-linked chain appeared to occur after a relatively long lag time.

Characterization of N- and O-linked oligosaccharides of glycoprotein 350 from Epstein-Barr virus

Glycoprotein 350 (gp350), the major Epstein-Barr Virus (EBV) envelope glycoprotein, has extensive N- and O-linked oligosaccharide chains. To characterize these oligosaccharide chains, [3H]glucosamine-labeled gp350 was isolated from an EBV transformed marmoset lymphoblastoid cell line (B95-8) induced to replicate EBV. Radiolabeled pronase-glycopeptides were fractionated by serial affinity chromatography and O-linked oligosaccharides released by mild alkaline borohydride treatment. Virtually all (99%) N-linked oligosaccharides were of complex type, with a predominance of tri-tetraantennary versus diantennary chains. A significant portion (28%, in term of radioactivity) of the tri-tetraantennary chains bound to leucoagglutinin-agarose, indicating an additional branch in beta(1-6)-linkage to the trimannosyl core. N-linked oligosaccharides with such a branching pattern have not been previously described in any herpesvirus glycoprotein, but have been associated with neoplastic transformation. Half of [3H]glucosamine incorporated into gp350 was recovered in O-linked oligosaccharides. The smallest chains have a core beta Gal-GalNAc disaccharide structure. Most O-linked chains have two to three N-acetylglucosamine and one N-acetylgalactosamine residues, besides the N-acetylgalactosamine residue located at the terminal reducing end, suggesting a di- or tri- N-acetyllactosamine structure. Consistent with such a structure, the size of these chains, after sialic acid removal, was that of an heptasaccharide or larger.

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

The gG-2 glycoprotein gene of herpes simplex virus type 2 (HSV-2) was cloned into the mammalian expression vector pMSG under the control of the inducible mouse mammary tumor virus promoter. Transfection of this cloned gG-2 construct into NIH 3T3 cells resulted in the stable expression of gG-2 upon induction with dexamethasone. In addition, the 104,000-molecular-weight (104K) and 72K gG-2 precursors as well as the 34K secreted component were generated in the transformed cells. The synthesis of gG-2 in these transformed cells appeared to follow the same cleavage-processing pathway as gG-2 synthesis during an HSV-2 infection. These results indicate that the processing of gG-2 can occur in the absence of an HSV-2 infection.