Effect of monoclonal antibodies on limited proteolysis of native glycoprotein gD of herpes simplex virus type 1

Herpes Simplex Virus 1 Envelope Glycoprotein C Shields Glycoprotein D to Protect Virions from Entry-Blocking Antibodies

Herpes simplex virus 1 (HSV-1) gD interaction with the host cell receptor nectin-1 triggers the membrane fusion cascade during viral entry. Potent neutralizing antibodies to gD prevent receptor-binding or prevent gD interaction with gH/gL critical for fusion. HSV has many strategies to evade host immune responses. We investigated the ability of virion envelope gC to protect envelope gD from antibody neutralization. HSV-1 lacking gC was more sensitive to neutralization by anti-gD monoclonal antibodies than a wild type rescuant virus. gD in the HSV-1 gC-null viral envelope had enhanced reactivity to anti-gD antibodies compared to wild type. HSV-1 ΔgC binding to the nectin-1 receptor was more readily inhibited by a neutralizing anti-gD monoclonal antibody. HSV-1 ΔgC was also more sensitive to inhibition by soluble nectin-1 receptor. The viral membrane protein composition of HSV-1 ΔgC was equivalent to that of wild type, suggesting that the lack of gC is responsible for the increased reactivity of gD-specific antibodies and the consequent increased susceptibility to neutralization by those antibodies. Together, the results suggest that gC in the HSV-1 envelope shields both receptor-binding domains and gH/gL-interacting domains of gD from neutralizing antibodies, facilitating HSV cell entry.

Human Antibodies against Herpes Simplex Virus 2 Glycoprotein G Do Not Neutralize but Mediate Antibody-Dependent Cellular Cytotoxicity

Herpes simplex virus 2 (HSV-2) is a sexually transmitted infection affecting 491 million individuals globally. Consequently, there is a great need for both prophylactic and therapeutic vaccines. Unfortunately, several vaccine clinical trials, primarily employing the glycoprotein D of HSV-2 (gD-2), have failed. The immune protection conferred by human anti-HSV-2 antibodies in genital infection and disease remains elusive. It is well-known that gD-2 elicits cross-reactive neutralizing antibodies, i.e., anti-gD-2 antibodies recognize gD in HSV-1 (gD-1). In contrast, anti-glycoprotein G in HSV-2 (mgG-2) antibodies are exclusively type-specific for HSV-2. In this study, truncated versions of gD-2 and mgG-2 were recombinantly produced in mammalian cells and used for the purification of anti-gD-2 and anti-mgG-2 antibodies from the serum of five HSV-2-infected subjects, creating a pool of purified antibodies. These antibody pools were utilized as standards together with purified mgG-2 and gD-2 antigens in ELISA to quantitatively estimate and compare the levels of cross-reactive anti-gD-1 and anti-gD-2 antibodies, as well as anti-mgG-2 antibodies in sera from HSV-1+2-, HSV-2-, and HSV-1-infected subjects. The median concentration of anti-mgG-2 antibodies was five times lower in HSV-1+2-infected subjects as compared with cross-reactive anti-gD-1 and anti-gD-2 antibodies, and three times lower in HSV-2 infected subjects as compared with anti-gD-2 antibodies. The pool of purified anti-gD-2 antibodies presented neutralization activity at low concentrations, while the pool of purified anti-mgG-2 antibodies did not. Instead, these anti-mgG-2 antibodies mediated antibody-dependent cellular cytotoxicity (ADCC) by human granulocytes, monocytes, and NK-cells, but displayed no complement-dependent cytotoxicity. These findings indicate that antibodies to mgG-2 in HSV-2-infected subjects are present at low concentrations but mediate the killing of infected cells via ADCC rather than by neutralizing free viral particles. We, and others, speculate that Fc-receptor mediated antibody functions such as ADCC following HSV-2 vaccination may serve as a better marker of protection correlate instead of neutralizing activity. In an mgG-2 therapeutic vaccine, our findings of low levels of anti-mgG-2 antibodies in HSV-2-infected subjects may suggest an opportunity to enhance the immune responses against mgG-2. In a prophylactic HSV-2 mgG-2 vaccine, a possible interference in cross-reactive immune responses in already infected HSV-1 subjects can be circumvented.

Localization of the Interaction Site of Herpes Simplex Virus Glycoprotein D (gD) on the Membrane Fusion Regulator, gH/gL

A cascade of protein-protein interactions between four herpes simplex virus (HSV) glycoproteins (gD, gH/gL, and gB) drive fusion between the HSV envelope and host membrane, thereby allowing for virus entry and infection. Specifically, binding of gD to one of its receptors induces a conformational change that allows gD to bind to the regulatory complex gH/gL, which then activates the fusogen gB, resulting in membrane fusion. Using surface plasmon resonance and a panel of anti-gD monoclonal antibodies (MAbs) that sterically blocked the interaction, we previously showed that gH/gL binds directly to gD at sites distinct from the gD receptor binding site. Here, using an analogous strategy, we first evaluated the ability of a panel of uncharacterized anti-gH/gL MAbs to block binding to gD and/or inhibit fusion. We found that the epitopes of four gD-gH/gL-blocking MAbs were located within flexible regions of the gH N terminus and the gL C terminus, while the fifth was placed around gL residue 77. Taken together, our data localized the gD binding region on gH/gL to a group of gH and gL residues at the membrane distal region of the heterodimer. Surprisingly, a second set of MAbs did not block gD-gH/gL binding but instead stabilized the complex by altering the kinetic binding. However, despite this prolonged gD-gH/gL interaction, "stabilizing" MAbs also inhibited cell-cell fusion, suggesting a unique mechanism by which the fusion process is halted. Our findings support targeting the gD-gH/gL interaction to prevent fusion in both therapeutic and vaccine strategies against HSV.IMPORTANCE Key to developing a human HSV vaccine is an understanding of the virion glycoproteins involved in entry. HSV employs multiple glycoproteins for attachment, receptor interaction, and membrane fusion. Determining how these proteins function was resolved, in part, by structural biology coupled with immunological and biologic evidence. After binding, virion gD interacts with a receptor to activate the regulator gH/gL complex, triggering gB to drive fusion. Multiple questions remain, one being the physical location of each glycoprotein interaction site. Using protective antibodies with known epitopes, we documented the long-sought interaction between gD and gH/gL, detailing the region on gD important to create the gD-gH/gL triplex. Now, we have identified the corresponding gD contact sites on gH/gL. Concurrently we discovered a novel mechanism whereby gH/gL antibodies stabilize the complex and inhibit fusion progression. Our model for the gD-gH/gL triplex provides a new framework for studying fusion, which identifies targets for vaccine development.

Characterizing Epitope Binding Regions of Entire Antibody Panels by Combining Experimental and Computational Analysis of Antibody: Antigen Binding Competition

Vaccines and immunotherapies depend on the ability of antibodies to sensitively and specifically recognize particular antigens and specific epitopes on those antigens. As such, detailed characterization of antibody-antigen binding provides important information to guide development. Due to the time and expense required, high-resolution structural characterization techniques are typically used sparingly and late in a development process. Here, we show that antibody-antigen binding can be characterized early in a process for whole panels of antibodies by combining experimental and computational analyses of competition between monoclonal antibodies for binding to an antigen. Experimental "epitope binning" of monoclonal antibodies uses high-throughput surface plasmon resonance to reveal which antibodies compete, while a new complementary computational analysis that we call "dock binning" evaluates antibody-antigen docking models to identify why and where they might compete, in terms of possible binding sites on the antigen. Experimental and computational characterization of the identified antigenic hotspots then enables the refinement of the competitors and their associated epitope binding regions on the antigen. While not performed at atomic resolution, this approach allows for the group-level identification of functionally related monoclonal antibodies (i.e., communities) and identification of their general binding regions on the antigen. By leveraging extensive epitope characterization data that can be readily generated both experimentally and computationally, researchers can gain broad insights into the basis for antibody-antigen recognition in wide-ranging vaccine and immunotherapy discovery and development programs.

Surface Plasmon Resonance Reveals Direct Binding of Herpes Simplex Virus Glycoproteins gH/gL to gD and Locates a gH/gL Binding Site on gD

Herpes simplex virus (HSV) requires fusion between the viral envelope and host membrane. Four glycoproteins, gD, gH/gL, and gB, are essential for this process. To initiate fusion, gD binds its receptor and undergoes a conformational change that hypothetically leads to activation of gH/gL, which in turn triggers the fusion protein gB to undergo rearrangements leading to membrane fusion. Our model predicts that gD must interact with both its receptor and gH/gL to promote fusion. In support of this, we have shown that gD is structurally divided into two "faces": one for the binding receptor and the other for its presumed interaction with gH/gL. However, until now, we have been unable to demonstrate a direct interaction between gD and gH/gL. Here, we used surface plasmon resonance to show that the ectodomain of gH/gL binds directly to the ectodomain of gD when (i) gD is captured by certain anti-gD monoclonal antibodies (MAbs) that are bound to a biosensor chip, (ii) gD is bound to either one of its receptors on a chip, and (iii) gD is covalently bound to the chip surface. To localize the gH/gL binding site on gD, we used multiple anti-gD MAbs from six antigenic communities and determined which ones interfered with this interaction. MAbs from three separate communities block gD-gH/gL binding, and their epitopes encircle a geographical area on gD that we propose comprises the gH/gL binding domain. Together, our results show that gH/gL interacts directly with gD, supporting a role for this step in HSV entry.IMPORTANCE HSV entry is a multistep process that requires the actions of four glycoproteins, gD, gH/gL, and gB. Our current model predicts that gD must interact with both its receptor and gH/gL to promote viral entry. Although we know a great deal about how gD binds its receptors, until now we have been unable to demonstrate a direct interaction between gD and gH/gL. Here, we used a highly sensitive surface plasmon resonance technique to clearly demonstrate that gD and gH/gL interact. Furthermore, using multiple MAbs with defined epitopes, we have delineated a domain on gD that is independent of that used for receptor binding and which likely represents the gH/gL interaction domain. Targeting this interaction to prevent fusion may enhance both therapeutic and vaccine strategies.

Global sensing of the antigenic structure of herpes simplex virus gD using high-throughput array-based SPR imaging

While HSV-2 typically causes genital lesions, HSV-1 is increasingly the cause of genital herpes. In addition, neonatal HSV infections are associated with a high rate of mortality and HSV-2 may increase the risk for HIV or Zika infections, reinforcing the need to develop an effective vaccine. In the GSK Herpevac trial, doubly sero-negative women were vaccinated with a truncated form of gD2 [gD2(284t)], then examined for anti-gD serum titers and clinical manifestations of disease. Surprisingly, few vaccinees were protected against genital HSV-2 but 86% were protected from genital HSV-1. These observations suggest that subtle differences in gD structure might influence a protective response. To better understand the antigenic structure of gD and how it impacts a protective response, we previously utilized several key anti-gD monoclonal antibodies (mAbs) to dissect epitopes in vaccinee sera. Several correlations were observed but the methodology limited the number of sera and mAbs that could be tested. Here, we used array-based surface plasmon imaging (SPRi) to simultaneously measure a larger number of protein-protein interactions. We carried out cross-competition or "epitope binning" studies with 39 anti-gD mAbs and four soluble forms of gD, including a form [gD2(285t)] that resembles the Herpevac antigen. The results from these experiments allowed us to organize the mAbs into four epitope communities. Notably, relationships within and between communities differed depending on the form of gD, and off-rate analysis suggested differences in mAb-gD avidity depending on the gD serotype and length. Together, these results show that gD1 and gD2 differ in their structural topography. Consistent with the Herpevac results, several mAbs that bind both gD1 and gD2 neutralize only HSV-1. Thus, this technology provides new insights into the antigenic structure of gD and provides a rationale as to how vaccination with a gD2 subunit may lead to protection from HSV-1 infection.

Prophylactic Herpes Simplex Virus 2 (HSV-2) Vaccines Adjuvanted with Stable Emulsion and Toll-Like Receptor 9 Agonist Induce a Robust HSV-2-Specific Cell-Mediated Immune Response, Protect against Symptomatic Disease, and Reduce the Latent Viral Reservoir

Several prophylactic vaccines targeting herpes simplex virus 2 (HSV-2) have failed in the clinic to demonstrate sustained depression of viral shedding or protection from recurrences. Although these vaccines have generated high titers of neutralizing antibodies (NAbs), their induction of robust CD8 T cells has largely been unreported, even though evidence for the importance of HSV-2 antigen-specific CD8 T cells is mounting in animal models and in translational studies involving subjects with active HSV-2-specific immune responses. We developed a subunit vaccine composed of the NAb targets gD and gB and the novel T cell antigen and tegument protein UL40, and we compared this vaccine to a whole-inactivated-virus vaccine (formaldehyde-inactivated HSV-2 [FI-HSV-2]). We evaluated different formulations in combination with several Th1-inducing Toll-like receptor (TLR) agonists in vivo In mice, the TLR9 agonist cytosine-phosphate-guanine (CpG) oligodeoxynucleotide formulated in a squalene-based oil-in-water emulsion promoted most robust, functional HSV-2 antigen-specific CD8 T cell responses and high titers of neutralizing antibodies, demonstrating its superiority to vaccines adjuvanted by monophosphoryl lipid A (MPL)-alum. We further established that FI-HSV-2 alone or in combination with adjuvants as well as adjuvanted subunit vaccines were successful in the induction of NAbs and T cell responses in guinea pigs. These immunological responses were coincident with a suppression of vaginal HSV-2 shedding, low lesion scores, and a reduction in latent HSV-2 DNA in dorsal root ganglia to undetectable levels. These data support the further preclinical and clinical development of prophylactic HSV-2 vaccines that contain appropriate antigen and adjuvant components responsible for programming elevated CD8 T cell responses.IMPORTANCE Millions of people worldwide are infected with herpes simplex virus 2 (HSV-2), and to date, an efficacious prophylactic vaccine has not met the rigors of clinical trials. Attempts to develop a vaccine have focused primarily on glycoproteins necessary for HSV-2 entry as target antigens and to which the dominant neutralizing antibody response is directed during natural infection. Individuals with asymptomatic infection have exhibited T cell responses against specific HSV-2 antigens not observed in symptomatic individuals. We describe for the first time the immunogenicity profile in animal models of UL40, a novel HSV-2 T cell antigen that has been correlated with asymptomatic HSV-2 disease. Additionally, vaccine candidates adjuvanted by a robust formulation of the CpG oligonucleotide delivered in emulsion were superior to unadjuvanted or MPL-alum-adjuvanted formulations at eliciting a robust cell-mediated immune response and blocking the establishment of a latent viral reservoir in the guinea pig challenge model of HSV-2 infection.

Immune-complex mimics as a molecular platform for adjuvant-free vaccine delivery

Protein-based vaccine development faces the difficult challenge of finding robust yet non-toxic adjuvants suitable for humans. Here, using a molecular engineering approach, we have developed a molecular platform for generating self-adjuvanting immunogens that do not depend on exogenous adjuvants for induction of immune responses. These are based on the concept of Immune Complex Mimics (ICM), structures that are formed between an oligomeric antigen and a monoclonal antibody (mAb) to that antigen. In this way, the roles of antigens and antibodies within the structure of immune complexes are reversed, so that a single monoclonal antibody, rather than polyclonal sera or expensive mAb cocktails can be used. We tested this approach in the context of Mycobacterium tuberculosis (MTB) infection by linking the highly immunogenic and potentially protective Ag85B with the oligomeric Acr (alpha crystallin, HspX) antigen. When combined with an anti-Acr monoclonal antibody, the fusion protein formed ICM which bound to C1q component of the complement system and were readily taken up by antigen-presenting cells in vitro. ICM induced a strong Th1/Th2 mixed type antibody response, which was comparable to cholera toxin adjuvanted antigen, but only moderate levels of T cell proliferation and IFN-γ secretion. Unfortunately, the systemic administration of ICM did not confer statistically significant protection against intranasal MTB challenge, although a small BCG-boosting effect was observed. We conclude that ICM are capable of inducing strong humoral responses to incorporated antigens and may be a suitable vaccination approach for pathogens other than MTB, where antibody-based immunity may play a more protective role.

Transfusion of IgG-opsonized foreign red blood cells mediates reduction of antigen-specific B cell priming in a murine model

Hemolytic disease of the fetus and newborn can be effectively prevented by administration of anti-D to the mother. The administered IgG results in the attenuation of RBC-specific Ab production, a process termed Ab-mediated immune suppression (AMIS). Because in animal models of AMIS no major effect on T cell priming occurs, we hypothesized that the effect of the IgG on the immune system under AMIS conditions may involve a deficiency in B cell priming. We therefore challenged mice with either untreated RBCs or IgG-opsonized RBCs (AMIS) and assessed B cell priming. B cells from mice transfused with untreated RBCs, but not from mice treated under AMIS conditions, were primed as assessed by their ability to function as Ag-specific APCs to appropriate T cells. To our knowledge, this is the first report demonstrating that AMIS inhibits the appearance of Ag-primed RBC-specific B cells.

Highly immunogenic and protective recombinant vaccine candidate expressed in transgenic plants

Vaccine development has been hampered by difficulties in developing new and safe adjuvants, so alternative technologies that offer new avenues forward are urgently needed. The goal of this study was to express a monoclonal recombinant immune complex in a transgenic plant. A recombinant protein consisting of a tetanus toxin C fragment-specific monoclonal antibody fused with the tetanus toxin C fragment was designed and expressed. Immune complex formation occurred between individual fusion proteins to form immune complex-like aggregates that bound C1q and FcgammaRIIa receptor and could be targeted to antigen-presenting cells. Unlike antigen alone, the recombinant immune fusion complexes were highly immunogenic in mice and did not require coadministration of an adjuvant (when injected subcutaneously). Indeed, these complexes elicited antibody titers that were more than 10,000 times higher than those observed in animals immunized with the antigen alone. Furthermore, animals immunized with only 1 mug of recombinant immune complex without adjuvant were fully protected against lethal challenge. This the first report on the use of a genetic fusion between antigen and antibody to ensure an optimal expression ratio between the two moieties and to obtain fully functional recombinant immune complexes as a new vaccine model.

The morphogenesis of herpes simplex virus type 1 in infected parental mouse L fibroblasts and mutant gro29 cells

Mutants of cell lines and viruses are important biological tools. The pathway of herpesvirus particle maturation and egress are contentious issues. The mutant gro29 line of mouse L cells is defective for egress of herpes simplex virus type 1 (HSV-1) virions, and a candidate for studies of virus-cell interactions. The properties of uninfected and HSV-1-infected L fibroblasts and gro29 cells investigated by protein assay, immunoblot, titration assay, immunofluorescence light microscopy and immunogold cryosection electron microscopy are reported. The ultrastructure of both HSV-1-infected L and gro29 cells confirmed primary envelopment of virions at the nuclear membranes followed by maturing multiple de-envelopments and re-envelopments in the endoplasmic reticulum and in the Golgi complex. The gro29 cells presented changed cytoskeleton, abolished egress of virions, and were defective in the trafficking of glycoproteins, giving rise to accumulation of viral particles and glycoproteins in the endoplasmic reticulum and the Golgi complex. The results suggest that gro29 cells harbour a causal underlying defect of the cytoskeleton in addition to the HSV-1-induced cytoskeletal changes.

Morphologic, immunohistochemical, immunologic, ultrastructural, and time-related study of herpes simplex virus type 1-infected cultured human fibroblasts

Membrane glycoproteins of enveloped animal viruses are synthesized, processed, and transported inside infected cells. Expression of viral glycoproteins on the surface of viral particles and host cells are essential for many biologic functions. In the case of herpes simplex virus, the glycoprotein molecules may act as nucleation points for virus assembly and budding at the nuclear membrane. The temporal distribution of herpes simplex virus type 1 particles and glycoproteins in cultured human fibroblasts was studied by titration plaque assay, immunoblots, immunofluorescence light microscopy, and immunogold cryosection electron microscopy to describe the virus-cell interactions. These concordant analyses revealed significant release of infectious viral particles to the medium at 6 hours postinfection, that the capacity of the host cells to make infectious viral particles was complete at 18 hours postinfection, and that the infection brought time-related modifications of tubulin, cell morphology, and viral glycoproteins. The data presented is in accord with the theory of envelopment at the nuclear membranes containing immature glycoproteins followed by multiple deenvelopments and reenvelopments of the virus particles during the transport and maturation in the endoplasmic reticulum and the Golgi complex.

Monoclonal antibodies to distinct sites on herpes simplex virus (HSV) glycoprotein D block HSV binding to HVEM

HVEM (for herpesvirus entry mediator) is a member of the tumor necrosis factor receptor superfamily and mediates entry of many strains of herpes simplex virus (HSV) into normally nonpermissive Chinese hamster ovary (CHO) cells. We used sucrose density centrifugation to demonstrate that purified HSV-1 KOS virions bind directly to a soluble, truncated form of HVEM (HVEMt) in the absence of any other cell-associated components. Therefore, HVEM mediates HSV entry by serving as a receptor for the virus. We previously showed that soluble, truncated forms of HSV glycoprotein D (gDt) bind to HVEMt in vitro. Here we show that antibodies specific for gD, but not the other entry glycoproteins gB, gC, or the gH/gL complex, completely block HSV binding to HVEM. Thus, virion gD is the principal mediator of HSV binding to HVEM. To map sites on virion gD which are necessary for its interaction with HVEM, we preincubated virions with gD-specific monoclonal antibodies (MAbs). MAbs that recognize antigenic sites Ib and VII of gD were the only MAbs which blocked the HSV-HVEM interaction. MAbs from these two groups failed to coprecipitate HVEMt in the presence of soluble gDt, whereas the other anti-gD MAbs coprecipitated HVEMt and gDt. Previous mapping data indicated that site VII includes amino acids 11 to 19 and site Ib includes 222 to 252. The current experiments indicate that these sites contain residues important for HSV binding to HVEM. Group Ib and VII MAbs also blocked HSV entry into HVEM-expressing CHO cells. These results suggest that the mechanism of neutralization by these MAbs is via interference with the interaction between gD in the virus and HVEM on the cell. Group Ia and II MAbs failed to block HSV binding to HVEM yet still neutralized HVEM-mediated entry, suggesting that these MAbs block entry at a step other than HVEM binding.

Herpes simplex virus type 1-infected human embryonic lung cells studied by optimized immunogold cryosection electron microscopy

Herpes simplex virus type 1 (HSV-1) is a common human pathogen of skin and mucous membranes and is potentially dangerous when the infection is disseminated. Viral morphogenesis, especially the mechanism of viral envelopment and the exact pathway for processing and transport of HSV-1 glycoproteins, is still unclear. We report the results of optimized immunogold-labeled cryosection electron microscopy of HSV-1-infected cultured human fibroblasts (MRC-5). The simplified method presented has proved necessary to obtain reproducible results on cellular distribution of viral glycoproteins. It is now possible to demonstrate the viral glycoprotein gD-1, but not gC-1, in the nuclear membranes and to demonstrate gD-1- and gC-1-labeled viral particles in the perinuclear space, and to show the fate of the viral particles in the endoplasmic reticulum and Golgi area in infected cells.

Cryptic antigenic determinants on the extracellular pyruvate dehydrogenase complex/mimeotope found in primary biliary cirrhosis. A probe by affinity mass spectrometry

Affinity mass spectrometry (AMS) was used to evaluate the structural diversity of the E2 component of pyruvate dehydrogenase complex (PDC) in normal and diseased liver cells, including those from patients with the autoimmune disease primary biliary cirrhosis (PBC). Two different antibodies to PDC-E2, the immunodominant mitochondrial autoantigen in patients with PBC, were used. AMS was performed directly on frozen liver sections and purified bile duct epithelial cells. Mass spectrometric signals associated with the molecular recognition of PBC-specific antigenic determinants were enhanced by an in situ enzyme-linked signal amplification process. Samples from patients with PBC gave strong positive signals for the antigen(s) recognized by the monoclonal antibody C355.1. Conversely, tissues from normal and disease controls showed only a minimal signal. AMS was used to identify specific antigenic determinants within the E2 component of PDC for comparison with unknown antigenic determinants observed by affinity capture with C355.1 monoclonal antibody from PBC samples. PDC components bound to C355.1 were mapped and identified by mass before dissociation from the E2 component. A similar approach was used to identify unknown antigenic determinants associated with PBC. We believe AMS may be an important new approach with wide application to the identification of molecules associated with a number of disease states.

Kinetic analysis of synthetic analogues of linear-epitope peptides of glycoprotein D of herpes simplex virus type 1 by surface plasmon resonance

The interaction between mAb A16 and glycoprotein D (gD) of herpes simplex virus type 1 was analyzed by studying the kinetics of binding with a surface-plasmon-resonance biosensor. mAb A16 belongs to group VII antibodies, which recognize residues 11-19 of gD. In a previous study, three critical residues, Asp13, Arg16 and Phe17, of this epitope were identified by screening a phage display library that contained a random 15-amino-acid insert with the antibody. The contribution to binding of these residues in the motif DXXRF was further analyzed by an amino-acid-replacement study of the epitope gD-(9-19)-peptide and of a gD-(9-19)-peptide mimotope, previously obtained by screening the phage display library. Amino acid residues of the motif were replaced by a neutral amino acid residue, an amino acid residue with opposite charge and a corresponding D-amino acid residue. Kinetic parameters of peptide analogues were determined with a surface plasmon-resonance biosensor. The kinetic parameters of the peptide analogues were compared with the kinetic parameters of the interaction between mAb A16 and the epitope gD-(9-19)-peptide. The minimal size of the gD epitope for mAb A16 was also determined in this study. The kinetic constants of the resulting gD-(11-17)-peptide were found to be similar to those of entire gD. The kinetic analysis precisely defined the epitope on gD for mAb A16 to residues 11-17, identified Arg16 as an essential residue and suggested that Asp13 and Phe17 are mainly involved in stabilization of the secondary structure of the peptide.

Modulation of antigen processing by bound antibodies can boost or suppress class II major histocompatibility complex presentation of different T cell determinants

Bound antibodies can modulate antigen processing but it is not clear to what extent this affects antigen presentation. Here we show that presentation of T cell determinants in tetanus toxin can be either enhanced or suppressed as a direct consequence of antibody modulation of antigen processing in human B lymphoblastoid cells. Remarkably, a single bound antibody or its Fab fragment can simultaneously enhance the presentation of one T cell determinant by more than 10-fold while strongly suppressing the presentation of a different T cell determinant. Biochemical analysis demonstrates that both the suppressed and boosted determinants fall within an extended domain of antigen stabilized or "footprinted" by this antibody during proteolysis. These results demonstrate that bound antibodies can modulate the capture of peptides by class II major histocompatibility complex (MHC), thus manipulating the T cell response towards or away from particular determinants. Altered processing of protein-protein complexes leading to enhanced loading of class II MHC and substantially lowered threshold for T cell activation suggests a novel mechanism that might reveal "cryptic" self determinants.

Identification of the core residues of the epitope of a monoclonal antibody raised against glycoprotein D of herpes simplex virus type 1 by screening of a random peptide library

Random peptide libraries (RPL) displayed on the surface of a filamentous bacteriophage can be used to identify peptide ligands that interact with target molecules. We have used a 15-amino acid residue RPL displayed on bacteriophage M13 to identify the core residues within the epitope of a monoclonal antibody (mAb) A16 which interacts with a continuous epitope restricted to amino acid residues 9 to 19 in the N-terminal region of glycoprotein D of herpes simplex virus type 1 (gD-1). The single peptide sequence obtained after three rounds of selection contained identical residues at three positions compared to the authentic gD-1 sequence. Synthetic peptides were prepared based on the sequence of the original epitope and the phage-derived epitope. The binding constants (Ka) with mAb A16 were determined using surface plasmon resonance (SPR) biosensor technology. The RPL-derived peptide and peptide 9-19 of gD-1 had approximately the same affinity for mAb A16. This suggests that those residues within the epitope that are essential for binding were identified. The synthesis of shorter versions of the RPL-derived peptide restricted the binding region to seven amino acid residues. These results show that minimal information retrieved from the screening of an RPL combined with peptide synthesis can characterize the epitope of an mAb with high resolution. Immunization of mice with the phage-derived peptide protected against a challenge with a lethal dose of herpes simplex virus type 1 equally well as the gD-1 derived peptide.

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

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

Anergic B cells constitutively present self antigen: enhanced immunoglobulin receptor-mediated presentation of antigenic determinants by B cells is hierarchical

Presentation of hen egg lysozyme (HEL) by HEL-specific B cells was studied in transgenic mice expressing anti-HEL immunoglobulin (Ig-transgenic). In T hybridoma assays, presentation of the HEL46-61 determinant by B cells from Ig-transgenic mice required 10(3)-10(4)-fold lower concentrations of HEL than were required for presentation by B cells from non-transgenic mice. In contrast, presentation of the HEL determinants 112-129 and 25-43 by HEL-specific B cells was either not significantly enhanced, or enhanced only 10-fold compared with B cells from non-transgenic mice. Enhanced presentation of HEL determinants by B cells from Ig-transgenic donors was specific for HEL, since keyhole limpet hemocyanin or synthetic HEL46-61 peptide were presented comparably by B cells from Ig-transgenic mice and non-transgenic littermates. A minimum of 1-4% Ig-transgenic B cells was required to detect enhanced presentation of HEL46-61 in vitro. Constitutive presentation of the HEL46-61 determinant, but not the HEL25-43 or HEL112-129 determinants, was detectable on anergic HEL-specific B cells from double (HEL/Ig)-transgenic mice. In the presence of exogenously added HEL, anergic B cells presented all three HEL determinants. Constitutively presented HEL46-61 was not due to endogenous synthesis of HEL antigen by anergic B cells from double-transgenic mice, as comparable levels of the HEL46-61 determinant were constitutively presented by B cells from Ig-Tg-->HEL-Tg irradiation bone marrow chimeric mice. Firstly, these results indicate that the enhanced antigen presentation mediated by Ig receptors on B cells is not equivalent for all antigenic determinants. Secondly, the data demonstrate that anergic, autoreactive B cells efficiently process and present nominal antigens in addition to constitutively presenting specific self antigen in vivo.

The herpes simplex virus type 1 particle: structure and molecular functions. Review article

This review is a summary of our present knowledge with respect to the structure of the virion of herpes simplex virus type 1. The virion consists of a capsid into which the DNA is packaged, a tegument and an external envelope. The protein compositions of the structures outside the genome are described as well as the functions of individual proteins. Seven capsid proteins are identified, and two of them are mainly present in precursors of mature DNA-containing capsids. The protein components of the 150 hexamers and 12 pentamers in the icosahedral capsid are known. These capsomers all have a central channel and are connected by Y-shaped triplexes. In contrast to the capsid, the tegument has a less defined structure in which 11 proteins have been identified so far. Most of them are phosphorylated. Eleven virus-encoded glycoproteins are present in the envelope, and there may be a few more membrane proteins not yet identified. Functions of these glycoproteins include attachment to and penetration of the cellular membrane. The structural proteins, their functions, coding genes and localizations are listed in table form.

High-level expression and purification of secreted forms of herpes simplex virus type 1 glycoprotein gD synthesized by baculovirus-infected insect cells

Two forms of herpes simplex virus glycoprotein gD were recombined into Autographa californica nuclear polyhedrosis virus (baculovirus) and expressed in infected Spodoptera frugiperda (Sf9) cells. Each protein was truncated at residue 306 of mature gD. One form, gD-1(306t), contains the coding sequence of Patton strain herpes simplex virus type 1 gD; the other, gD-1(QAAt), contains three mutations which eliminate all signals for addition of N-linked oligosaccharides. Prior to recombination, each gene was cloned into the baculovirus transfer vector pVT-Bac, which permits insertion of the gene minus its natural signal peptide in frame with the signal peptide of honeybee melittin. As in the case with many other baculovirus transfer vectors, pVT-Bac also contains the promoter for the baculovirus polyhedrin gene and flanking sequences to permit recombination into the polyhedrin site of baculovirus. Each gD gene was engineered to contain codons for five additional histidine residues following histidine at residue 306, to facilitate purification of the secreted protein on nickel-containing resins. Both forms of gD-1 were abundantly expressed and secreted from infected Sf9 cells, reaching a maximum at 96 h postinfection for gD-1(306t) and 72 h postinfection for gD-1(QAAt). Secretion of the latter protein was less efficient than gD-1(306t), possibly because of the absence of N-linked oligosaccharides from gD-1(QAAt). Purification of the two proteins by a combination of immunoaffinity chromatography, nickel-agarose chromatography, and gel filtration yielded products that were > 99% pure, with excellent recovery. We are able to obtain 20 mg of purified gD-1(306t) and 1 to 5 mg of purified gD-1(QAAt) per liter of infected insect cells grown in suspension. Both proteins reacted with monoclonal antibodies to discontinuous epitopes, indicating that they retain native structure. Use of this system for gD expression makes crystallization trials feasible.

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

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

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

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

Immunoselection of recombinant baculoviruses expressing high levels of biologically active herpes simplex virus type 1 glycoprotein D

The DNA sequence encoding the complete herpes simplex virus type 1 (HSV-1) glycoprotein D (gD) was inserted into a baculovirus transfer vector under control of the polyhedrin gene promoter of the baculovirus Autographa california nuclear polyhedrosis virus (AcNPV). After co-transfection of Spodoptera frugiperda (Sf9) insect cells with wild-type AcNPV DNA and the recombinant transfer vector DNA, polyhedrin-negative recombinants that expressed high levels of HSV-1 gD were isolated using immunoaffinity selection with antibody coated magnetic particles followed by plaque purification. These recombinant baculoviruses expressed a protein that was slightly smaller than virion HSV-1 gD made in Vero cells. This recombinant protein was expressed at high levels. The expressed protein was glycosylated, was found on the membrane of Sf9 cells, and reacted with gD specific antibodies. Antibodies raised in mice to the recombinant gD neutralized HSV-1 as measured by plaque reduction assays. Mice inoculated with the recombinant baculovirus were completely protected from lethal challenge with HSV-1.

Effect of antigen/antibody ratio on macrophage uptake, processing, and presentation to T cells of antigen complexed with polyclonal antibodies

Activation of a galactosidase-specific murine T hybridoma clone and of a human tetanus toxoid-specific T clone by antigen-presenting cells (APC) was used to evaluate the regulatory function of antibodies complexed with the relevant antigen. Complexed antigen, in fact, is taken up with high efficiency thanks to Fc receptors borne by APC. Antibody/antigen ratio in the complexes proved to be a critical parameter in enhancing antigen presentation. Complexes in moderate antibody excess provided optimal T cell activation independently of the physical state of the complexes (precipitated by a second antibody or solubilized by complement). Complexes in extreme antibody excess, on the contrary, did not yield T cell activation although taken up by APC efficiently. The effect of antibodies at extreme excess was observed with substimulatory dose of antigen (loss of potentiation) and with optimal dose of antigen (loss of stimulation). An excess of specific polyclonal antibodies hampers proteolytic degradation of antigen in vitro, supporting the view that a similar mechanism may operate within the APC that have internalized immune complexes in extreme antibody excess. The possibility that immune complex forming in extreme antibody excess may turn off the T cell response is proposed as a regulatory mechanism.

Molecular epitope identification by limited proteolysis of an immobilized antigen-antibody complex and mass spectrometric peptide mapping

Sequences of antigenic determinants were identified by limited proteolysis of peptide antigens bound to an immobilized monoclonal antibody and direct molecular weight determination of the monoclonal antibody-bound peptide fragments by 252Cf plasma desorption mass spectrometry. The epitope peptides to the monoclonal antibody h453 [Burger, R., Zilow, G., Bader, A., Friedlein, A. & Naser, W. (1988) J. Immunol. 141, 553-558] were isolated from immobilized antigen-antibody complexes by partial trypsin digestion. A synthetic eicosapeptide comprised of the C-terminal sequence of the human complement component polypeptide des-Arg77-C3a as well as guinea pig des-Arg78-C3a was used as an antigen. Conditions were developed under which trypsin specifically degraded the antigens without inactivation of the immobilized antibody. After proteolysis, epitope peptides were dissociated from the antibody with 4 M MgCl2. The antigenic peptides were purified by HPLC and identified by 252Cf plasma desorption mass spectrometry. The epitope recognized by h453 resides on the C-terminal tryptic peptides of human (residues 70-76) and guinea pig (residues 70-77) C3a. As an estimation of accuracy this method is able to provide, trypsin digestion of immune complexes caused cleavage of the antigen within a distance of two amino acid residues upstream from the epitope.

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

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

Serologic type conversion of a herpes simplex virus type 1 (HSV-1) to an HSV-2 epitope caused by a single amino acid substitution in glycoprotein C

A monoclonal antibody to herpes simplex virus type 2 glycoprotein C (gC-2) did not recognize wild-type herpes simplex virus type 1 gC (gC-1) but did recognize a mutant gC-1 molecule. This conversion from a type 1 to a type 2 epitope was shown to be due to a single amino acid substitution in gC-1.

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.

Deletions in herpes simplex virus glycoprotein D define nonessential and essential domains

Herpes simplex virus glycoprotein D (gD) is a major component of the virion envelope and infected cell membranes and is essential for virus entry into cells. We have recently shown that gD interacts with a limited number of cell surface receptors which are required for virus penetration into cells. To define domains of gD which are required for aspects of virus replication including receptor binding, deletion mutations of 5 to 14 amino acids were constructed by using oligonucleotide-directed mutagenesis. Plasmids containing mutant genes for gD were assayed for the ability to rescue a recombinant virus, F-gD beta, in which beta-galactosidase sequences replace gD-coding sequences. Effects on global folding and posttranslational processing of the molecules were assessed by using a panel of monoclonal antibodies which recognize both continuous and discontinuous epitopes. A region near the amino terminus (residues 7 to 21) of gD which is recognized by monoclonal antibodies able to neutralize herpes simplex virus in the absence of complement was not essential for function. In addition, virtually all of the cytoplasmic domain of gD and an extracellular domain close to the membrane were dispensable. In contrast, deletion mutations in the central region of the molecule, save for one exception, led to alterations in global folding of the molecule and maturation of the protein was inhibited.

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

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

Influence of peptide acylation, liposome incorporation, and synthetic immunomodulators on the immunogenicity of a 1-23 peptide of glycoprotein D of herpes simplex virus: implications for subunit vaccines

A peptide corresponding to residues 1 to 23 of glycoprotein D of herpes simplex virus type 1 was chemically synthesized and coupled to a fatty acid carrier by standard Merrifield synthesis procedures. The resulting peptide-palmitic acid conjugate (acylpeptide) exhibited enhanced immunogenicity in mice as compared with that exhibited by the free form of the peptide. Incorporation of the acylpeptide into liposomes further increased the immunogenicity of the peptide, while inclusion of the immunomodulators muramyl tripeptide phosphatidylethanolamine and monophosphoryl lipid A into the same liposome stimulated the strongest response. The humoral immune responses induced by the acylpeptide-liposome construct were greater than those induced by peptide in Freund complete adjuvant, and cellular responses were equal. The acylpeptide-immunomodulator-liposome formulation also induced significant levels of protective immunity, although the immunity was less than that induced by herpes simplex virus infection. Acylated peptides, especially in liposomes, were taken up more effectively by draining lymph nodes, which possibly accounts in part for the enhanced immunogenicity of the peptides. Since the acylpeptide-immunoliposome formulation used was nontoxic, it could represent a useful way to enhance immunogenicity of subunit peptides used for vaccine purpose in humans and animals.

Influence of asparagine-linked oligosaccharides on antigenicity, processing, and cell surface expression of herpes simplex virus type 1 glycoprotein D

Glycoprotein D (gD) is an envelope component of herpes simplex virus types 1 and 2. gD-1 contains three sites for the addition of N-linked carbohydrate (N-CHO), all of which are used. Three mutants were constructed by site-directed mutagenesis, each of which altered one N-CHO addition site from Asn-X-Thr/Ser to Asn-X-Ala. A fourth mutant was altered at all three sites. The mutant genes were inserted into an expression vector, and the expressed protein was analyzed in transiently transfected COS-1 cells. The mutant protein lacking N-CHO at site 1 (Asn-94) had a reduced affinity for monoclonal antibodies (MAbs) to discontinuous epitopes, suggesting that the conformation of the protein had been altered. However, the protein was processed and transported to the cell surface. The absence of N-CHO at site 2 (Asn-121) had no apparent effect on processing or transport of gD-1 but resulted in reduced binding of two MAbs previously shown to be in group VI. Binding of other MAbs to discontinuous epitopes (including other group VI MAbs) was not affected. The absence of N-CHO at site 3 (Asn-262) had no effect on processing, transport, or conformation of the gD-1 protein. The absence of N-CHO from site 1 or from all three sites resulted in the formation of high-molecular-weight aggregates or complexes and a reduction in MAb binding. However, these proteins were modified by the addition of O-glycans and transported to the cell surface. We conclude that the absence of the first or all N-linked carbohydrates alters the native conformation of gD-1 but does not prevent its transport to the cell surface.

Immunological properties of multiple repeats of a linear epitope of herpes simplex virus type 1 glycoprotein D

Several peptides containing the amino acid sequence 9-21 of glycoprotein D of herpes simplex virus type 1 (HSV-1) were synthesized and investigated for reactivity with monoclonal antibody LP14 in a competition enzyme-linked immunosorbent assay (ELISA). Peptides containing two or four repeats of sequence 9-21 reacted at least one order of magnitude better with LP14 than with the monomeric form of sequence 9-21. Dimers in which one of the repeats of one or more essential residues were absent did not show this increased reactivity. Antisera obtained from rabbits immunized with a peptide containing two repeats of sequence 9-21 coupled to bovine serum albumin showed high antipeptide antibody titers with this peptide and were able to neutralize virus infectivity in vitro. Sera obtained from rabbits immunized with the free dimer could not neutralize virus infectivity.

Fine mapping of antigenic site II of herpes simplex virus glycoprotein D

Glycoprotein D (gD) is a virion envelope component of herpes simplex virus types 1 (HSV-1) and 2 (HSV-2) which plays an important role in viral infection and pathogenesis. Previously, anti-gD monoclonal antibodies (MAbs) were arranged into groups which recognize distinct type-common and type-specific sites on HSV-1 gD (gD-1) and HSV-2 gD (gD-2). Several groups recognize discontinuous epitopes which are dependent on tertiary structure. Three groups, VII, II, and V, recognize continuous epitopes present in both native and denatured gD. Previously, group II consisted of a single MAb, DL6, whose epitope was localized between amino acids 268 and 287. In the study reported here, we extended our analysis of the antigenic structure of gD, concentrating on continuous epitopes. The DL6 epitope was localized with greater precision to residues 272 to 279. Four additional MAbs including BD78 were identified, each of which recognizes an epitope within residues 264 to 275. BD78 and DL6 blocked each other in binding to gD. In addition, a mutant form of gD was constructed in which the proline at 273 was replaced by serine. This change removes a predicted beta turn in gD. Neither antibody reacted with this mutant, indicating that the BD78 and DL6 epitopes overlap and constitute an antigenic site (site II) within residues 264 to 279. A separate antigenic site (site XI) was recognized by MAb BD66 (residues 284 to 301). This site was only six amino acids downstream of site II, but was distinct as demonstrated by blocking studies. Synthetic peptides mimicking these and other regions of gD were screened with polyclonal antisera to native gD-1 or gD-2. The results indicate that sites II, V, VII, and XI, as well as the carboxy terminus, are the major continuous antigenic determinants on gD. In addition, the results show that the region from residues 264 through 369, except the transmembrane anchor, contains a series of continuous epitopes.

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

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

Antigenic analysis of a major neutralization site of herpes simplex virus glycoprotein D, using deletion mutants and monoclonal antibody-resistant mutants

Herpes simplex virus glycoprotein D is a component of the virion envelope and appears to be involved in attachment, penetration, and cell fusion. Monoclonal antibodies against this protein can be arranged in groups on the basis of a number of biological and biochemical properties. Group I antibodies are type common, have high complement-independent neutralization titers, and recognize discontinuous (conformational) epitopes; they are currently being used in several laboratories to study the functions of glycoprotein D. We have used a panel of neutralization-resistant mutants to examine the relationships between these antibodies in detail. We found that they can be divided into two subgroups, Ia and Ib, such that mutations selected with Ia antibodies have little or no effect on binding and neutralization by Ib antibodies and vice versa. In addition, Ia antibodies are able to bind deletion and truncation mutants of glycoprotein D that Ib antibodies do not recognize, suggesting that their epitopes are physically distinct. However, with one exception, Ia and Ib antibodies block each other strongly in binding assays with purified glycoprotein D, whereas antibodies from other groups have no effect. We have therefore defined the sum of the Ia and Ib epitopes as antigenic site 1.

Immunological properties of an N-terminal fragment of herpes simplex virus type 1 glycoprotein D expressed in Escherichia coli

The N-terminal fragment, comprising residues -5 to 55 of herpes simplex virus type 1 glycoprotein D was expressed as a beta-galactosidase fusion protein in Escherichia coli. This gD-fusion protein reacts with monoclonal antibody LP 14 directed against glycoprotein D of HSV. Antisera obtained after immunization of rabbits with purified gD-fusion protein react with HSV-1 gD in a Western blot and with N-terminal synthetic peptides of gD. In addition, these antisera are able to neutralize viral infectivity in vitro.

Expression of herpes simplex virus type 1 glycoprotein D deletion mutants in mammalian cells

Glycoprotein D (gD) is a viron envelope component of herpes simplex virus types 1 and 2. We have previously defined seven monoclonal antibody (MAb) groups which recognize distinct epitopes on the mature gD-1 protein of 369 amino acids. MAb groups VII, II, and V recognize continuous epitopes at residues 11-19, 272-279, and 340-356, respectively. MAb groups I, III, IV, and VI recognize discontinuous epitopes. Recent studies have focused on epitopes I, III, and VI. Using truncated forms of gD generated by recombinant DNA methods and proteolysis, epitopes III, IV, and VI were located within amino acids 1-233. A portion of discontinuous epitope I was located in a region within residues 233-275. For this study, we used recombinant DNA methods to create mutations in the gD-1 gene and studied the effects of those mutations on gD as expressed in mammalian cells. Plasmid pRE4, containing the coding sequence of gD-1 and the Rous sarcoma virus long terminal repeat promoter, was transfected into mammalian cells. The expressed protein, gD-1-(pRE4), was identical in size and antigenic properties to gD-1 from infected cells. Six in-frame deletion mutations were subsequently constructed by using restriction enzymes to excise portions of the gD-1 gene. Plasmids carrying these mutated forms were transfected into cells, and the corresponding proteins were examined at 48 h posttransfection for antigenicity and glycosylation patterns. Three deletions of varying size were located downstream of residue 233. Analysis of these mutants showed that amino acids within the region 234-244 were critical for binding of DL11 (group I), but not for other MAb groups. Three other deletion mutants lost all ability to bind MAbs which recognize discontinuous epitopes. In addition, much of the gD expressed by these mutants was observed to migrate as high-molecular-weight aggregated forms in nondenaturing gels. Each of these mutations involved the loss of a cysteine residue, suggesting that disulfide linkages play an essential role in the formation of discontinuous epitopes. The extent of glycosylation of the mutant gD molecules accumulated at 48 h posttransfection suggested altered carbohydrate processing. In one case, there was evidence for increased O-linked glycosylation. Those proteins which had lost a cysteine residue as part of the deletion did not accumulate molecules processed beyond the high-mannose stage. The results suggest that carbohydrate processing during synthesis of gD is very sensitive to alterations in structure, particularly changes involving cysteine residues.

Monoclonal antibodies define a domain on herpes simplex virus glycoprotein B involved in virus penetration

In an earlier report (S.D. Marlin, S.L. Highlander, T.C. Holland, M. Levine, and J.C. Glorioso, J. Virol. 59: 142-153), we described the production and use of complement-dependent virus-neutralizing monoclonal antibodies (MAbs) and MAb-resistant (mar) mutants to identify five antigenic sites (I to V) on herpes simplex virus type 1 glycoprotein B (gB). In the present study, the mechanism of virus neutralization was determined for a MAb specific for site III (B4), the only site recognized by MAbs which exhibited complement-independent virus-neutralizing ability. This antibody had no detectable effect on virus attachment but neutralized viruses after adsorption to cell monolayers. These findings implied that the mechanism of B4 neutralization involved blocking of virus penetration. The remaining antibodies, which recognized sites I, II, and IV, required active complement for effective neutralization. These were further studied for their ability to impede virus infectivity in the absence of complement. Antibodies to sites I (B1 and B3) and IV (B6) slowed the rate at which viruses penetrated cell surfaces, supporting the conclusion that antibody binding to gB can inhibit penetration by a virus. The data suggest that MAbs can interfere with penetration by a virus by binding to a domain within gB which is involved in this process. In another assay of virus infection, MAb B6 significantly reduced plaque development, indicating that antibody binding to gB expressed on infected-cell surfaces can also interfere with the ability of a virus to spread from cell to cell. In contrast to these results, antibodies to site II (B2 and B5) had no effect on virus infectivity; this suggests that they recognized structures which do not play a direct role in the infectious process. To localize regions of gB involved in these phenomena, antibody-binding sites were operationally mapped by radioimmunoprecipitation of a panel of truncated gB molecules produced in transient-expression assays. Residues critical to recognition by antibodies which affect penetration by a virus (sites I, III, and IV) mapped to a region of the molecule (amino acid residues 241 to 441) which is centrally located within the external domain. Antibodies which had no effect on penetration (site II) recognized sequences distal to this region (residues 596 to 737) near the transmembrane domain. The data suggest that these gB-specific MAbs recognize two major antigenic sites which reside in physically distinct components of the external domain of gB.(ABSTRACT TRUNCATED AT 400 WORDS)

The contribution of cysteine residues to antigenicity and extent of processing of herpes simplex virus type 1 glycoprotein D

Glycoprotein D (gD) is an envelope component of herpes simplex virus types 1 (gD-1) and 2 (gD-2). The gD-1 polypeptide contains seven cysteine residues among its 369 amino acids; six are located on the N-terminal or luminal portion of the glycoprotein, and a seventh is located in the transmembrane region. Previous studies used a panel of monoclonal antibodies (MAbs) to define gD epitopes as continuous or discontinuous. Purified gD, denatured by reduction and alkylation, loses discontinuous epitopes, whereas continuous epitopes are retained. The contribution of disulfide bonds to maintenance of discontinuous epitopes is, therefore, significant. In the present study, our objective was to determine the contribution of individual cysteine residues to folding of gD-1 into its native conformation. Site-directed oligonucleotide mutagenesis was used to create seven mutants, each with a serine residue replacing a cysteine. The mutated genes were cloned into a eucaryotic expression vector and transfected into COS-1 cells, and the proteins were separated by nondenaturing polyacrylamide gel electrophoresis, followed by immunoblotting. Replacement of cysteine 7 (residue 333) had only a minimal effect on the antigenic properties of gD-1. In contrast, replacement of any one of the other six cysteine residues resulted in either a major reduction or a complete loss of binding of those MAbs that recognize discontinuous epitopes, with no effect on the binding of MAbs which recognize continuous epitopes. These mutations also had profound effects on the extent of oligosaccharide processing of gD-1. This was determined by digestion of the expressed proteins with various endoglycosidases, followed by electrophoresis and Western blotting (immunoblotting) to observe any mobility changes. Three mutant gD proteins which did not express discontinuous epitopes contained only high-mannose-type oligosaccharides, suggesting that processing had not proceeded beyond the precursor stage. Two mutant forms of gD exhibited reduced binding of MAbs to discontinuous epitopes. A small proportion of the molecules which accumulated at 48 h posttransfection contained complex oligosaccharides. One mutant exhibited reduced binding of MAbs to discontinuous epitopes, but was present at 48 h posttransfection only in the precursor form. The cysteine 7 mutant was processed to the same extent as wild-type gD. We conclude that the first six cysteine residues are critical to the correct folding, antigenic structure, and processing of gD-1, and we speculate that they form three disulfide-bonded pairs.

Point mutations in glycoprotein gene of vesicular stomatitis virus (New Jersey serotype) selected by resistance to neutralization by epitope-specific monoclonal antibodies

Antigenic variants of the New Jersey serotype of vesicular stomatitis virus (VSV-NJ) were isolated and cloned by selecting virus plaques resistant to neutralization by high-titered monoclonal antibodies (MAbs) directed to glycoprotein (G) epitopes V, VI, VII, or VIII. The G proteins of each neutralization-resistant virus variant also exhibited markedly reduced antigenic reactivity with each corresponding epitope-specific MAb as determined by enzyme-linked immuno-absorbent assay and by Western blot analysis. Loss of antigenic reactivity of certain mutant G proteins to a MAb other than the one used to select the mutant virus suggested close antigenic proximity, particularly for epitopes VI and VII. The virion RNAs coding for the entire G gene of the wild-type virus and 10 MAb-induced mutants were sequenced by primer DNA extension using the dideoxy method. Each mutant G gene exhibited only a single nucleotide change, leading in each case to a single amino acid substitution, as follows: Glu210----Lys for all three mutants selected by MAb14 (epitope VII); Pro268----Thr for one mutant selected by MAb12 (epitope VI); Ser277----Lys for all three mutants selected by MAb15 (epitope VIII); and Glu364----Lys for all three mutants selected by MAb11 (epitope V). These neutralizing MAb-selected mutations are clustered in the middle third of the 517-amino acid VSV-NJ G protein, presumably resulting in conformational changes that alter recognition of one or more antigenic determinants by a specific monoclonal antibody.