Domain structure of herpes simplex virus 1 glycoprotein B: neutralizing epitopes map in regions of continuous and discontinuous residues

Targeting Herpes Simplex Virus Glycoprotein D with Bispecific Antibodies: Expanding Therapeutic Horizons by Searching for Synergy

Herpes simplex viruses (HSV-1 and HSV-2), which can be transmitted both orally and sexually, cause lifelong morbidity and in some cases, meningitis and encephalitis. While both the passive transfer of neutralizing antibodies and placental transfer of anti-HSV monoclonal antibodies (Mabs) have shown therapeutic promise in animal models, clinical trials have yet to identify approved immunotherapeutics for herpes infection. Here, we present strategies for the generation of recombinant bispecific antibodies (BsAbs) that target different domains of glycoprotein D (gD), crucial for HSV entry, that have the potential to outperform the effect of individual Mabs to curb herpes infection. Specifically, we selected three pairs of Mabs from our extensive panel for BsAb design and production based on their binding site and ability to block virus entry. Actual binding of BsAbs to gD and epitope availability on gD after BsAb binding were characterized using surface plasmon resonance (SPR) and inhibition by IgG Fab fragments generated from selected Mabs. While one BsAb exhibited an additive effect similar to that observed using a combination of the Mabs utilized for its generation, two showed antagonistic effects, suggesting that the simultaneous engagement of two epitopes or selective binding to one affected their activity against HSV. One BsAb (DL11/1D3) targeting the binding site for both nectin-1 and HVEM receptors demonstrated synergistic inhibitory activity against HSV, outperforming the effect of the individual antibodies. Recombinant DL11/1D3 antibody variants, in which the size of one or both paratopes was decreased to single chains (scFv-Fc), highlighted differences in potency depending on antibody size and format. We propose that BsAbs to individual glycoproteins offer a potential avenue for herpes therapeutics, but their design, mechanism of action, antibody format, and epitope engagement require careful consideration of structure for optimal efficacy.

Herpes Simplex Virus 1 Glycoprotein B from a Hyperfusogenic Virus Mediates Enhanced Cell-Cell Fusion

Herpes simplex virus 1 (HSV-1) causes significant morbidity and death in humans worldwide. Herpes simplex virus 1 has a complex fusion mechanism that is incompletely understood. The HSV-1 strain ANG has notable fusion and entry activities that distinguish it from wild type. HSV-1 ANG virions fused with the Vero cell surface at 4 °C and also entered cells more efficiently at 15 °C, relative to wild type HSV-1 strain KOS virions, consistent with a hyperfusogenic phenotype. Understanding the molecular basis for the unique entry and fusion activities of HSV-1 strain ANG will help decipher the HSV fusion reaction and entry process. Sequencing of HSV-1 ANG genes revealed multiple changes in gB, gC, gD, gH, and gL proteins relative to wild type HSV-1 strains. The ANG UL45 gene sequence, which codes for a non-essential envelope protein, was identical to wild type KOS. HSV-1 ANG gB, gD, and gH/gL were necessary and sufficient to mediate cell-cell fusion in a virus-free reporter assay. ANG gB, when expressed with wild type KOS gD and gH/gL, increased membrane fusion, suggesting that ANG gB has hyperfusogenic cell-cell fusion activity. Replacing the KOS gD, gH, or gL with the corresponding ANG alleles did not enhance cell-cell fusion. The novel mutations in the ANG fusion and entry glycoproteins provide a platform for dissecting the cascade of interactions that culminate in HSV fusion and entry.

Membrane fusion activity of herpes simplex virus 1 glycoproteins from a hyperfusogenic virus

Herpes simplex virus 1 (HSV-1) causes significant morbidity and death in humans worldwide. Herpes simplex virus 1 has a complex fusion mechanism that is incompletely understood. The HSV-1 strain ANG has notable fusion and entry activities that distinguish it from wild type. HSV-1 ANG virions fused with the Vero cell surface at 4°C and also entered cells more efficiently at 15°C relative to wild type virions, consistent with a hyperfusogenic phenotype. Understanding the molecular basis for the unique entry and fusion activities of HSV-1 strain ANG will help decipher the HSV fusion reaction and entry process. Sequencing of HSV-1 ANG genes revealed multiple changes in gB, gC, gD, gH, and gL proteins relative to wild type HSV-1 strains. The ANG UL45 gene sequence, which codes for a non-essential envelope protein, was identical to wild type. HSV-1 ANG gB, gD, and gH/gL were necessary and sufficient to mediate cell-cell fusion in a virus-free reporter assay. ANG gB, when expressed with wild type gD and gH/gL, increased membrane fusion, suggesting that ANG gB has hyperfusogenic cell-cell fusion activity. Replacing the wild type gD, gH, or gL with the corresponding ANG alleles did not enhance cell-cell fusion. Wild type gC is proposed to facilitate fusion and entry into epithelial cells by optimizing conformational changes in the fusion protein gB. ANG gC substitution or addition also had no effect on cell-cell fusion. The novel mutations in the ANG fusion and entry glycoproteins provide a platform for dissecting the cascade of interactions that culminate in HSV fusion and entry.

The N-terminus of varicella-zoster virus glycoprotein B has a functional role in fusion

Varicella-zoster virus (VZV) is a medically important alphaherpesvirus that induces fusion of the virion envelope and the cell membrane during entry, and between cells to form polykaryocytes within infected tissues during pathogenesis. All members of the Herpesviridae, including VZV, have a conserved core fusion complex composed of glycoproteins, gB, gH and gL. The ectodomain of the primary fusogen, gB, has five domains, DI-V, of which DI contains the fusion loops needed for fusion function. We recently demonstrated that DIV is critical for fusion initiation, which was revealed by a 2.8Å structure of a VZV neutralizing mAb, 93k, bound to gB and mutagenesis of the gB-93k interface. To further assess the mechanism of mAb 93k neutralization, the binding site of a non-neutralizing mAb to gB, SG2, was compared to mAb 93k using single particle cryogenic electron microscopy (cryo-EM). The gB-SG2 interface partially overlapped with that of gB-93k but, unlike mAb 93k, mAb SG2 did not interact with the gB N-terminus, suggesting a potential role for the gB N-terminus in membrane fusion. The gB ectodomain structure in the absence of antibody was defined at near atomic resolution by single particle cryo-EM (3.9Å) of native, full-length gB purified from infected cells and by X-ray crystallography (2.4Å) of the transiently expressed ectodomain. Both structures revealed that the VZV gB N-terminus (aa72-114) was flexible based on the absence of visible structures in the cryo-EM or X-ray crystallography data but the presence of gB N-terminal peptides were confirmed by mass spectrometry. Notably, N-terminal residues 109KSQD112 were predicted to form a small α-helix and alanine substitution of these residues abolished cell-cell fusion in a virus-free assay. Importantly, transferring the 109AAAA112 mutation into the VZV genome significantly impaired viral propagation. These data establish a functional role for the gB N-terminus in membrane fusion broadly relevant to the Herpesviridae.

Glycoprotein C of Herpes Simplex Virus 1 Shields Glycoprotein B from Antibody Neutralization

Viruses have evolved strategies to avoid neutralization by the host antibody response. Herpes simplex virus (HSV) glycoprotein C (gC) functions in viral entry and binds to complement component C3b, inhibiting complement-mediated immunity. We investigated whether gC protects HSV from antibody neutralization. HSV-1 that lacks gC was more sensitive to complement-independent neutralization by a panel of gB monoclonal antibodies than a wild-type gC rescuant virus. The presence of gC decreased neutralization by 2- to 16-fold. The gB in the native envelope of HSV-1 had reduced reactivity with antibodies in comparison to gB from the gC-null virus, suggesting that gC hampers the recognition of gB epitopes in the viral particle. The protein composition of the gC-null virus, including the surface glycoproteins essential for entry, was equivalent to that of the wild type, suggesting that gC is directly responsible for the reduced antibody recognition and neutralization. The neutralizing activity of antibodies to gD and gH antibodies was also increased in HSV lacking gC. Together, the data suggest that HSV-1 gC protects the viral envelope glycoproteins essential for entry, including gB, by shielding them from neutralization as a potential mechanism of immune evasion.IMPORTANCE HSV-1 causes lifelong infection in the human population and can be fatal in neonatal and immunocompromised individuals. There is no vaccine or cure, in part due to the ability of HSV to escape the host immune response by various mechanisms. The HSV particle contains at least 15 envelope proteins, four of which are required for entry and replication. This work suggests a novel role for gC in shielding the HSV entry glycoproteins. gC may function to help HSV escape neutralization by antibodies.

Herpes Simplex Virus Glycoprotein C Regulates Low-pH Entry

Herpes simplex viruses (HSVs) cause significant morbidity and mortality in humans worldwide. Herpesviruses mediate entry by a multicomponent virus-encoded machinery. Herpesviruses enter cells by endosomal low-pH and pH-neutral mechanisms in a cell-specific manner. HSV mediates cell entry via the envelope glycoproteins gB and gD and the heterodimer gH/gL regardless of pH or endocytosis requirements. Specifics concerning HSV envelope proteins that function selectively in a given entry pathway have been elusive. Here, we demonstrate that gC regulates cell entry and infection by a low-pH pathway. Conformational changes in the core herpesviral fusogen gB are critical for membrane fusion. The presence of gC conferred a higher pH threshold for acid-induced antigenic changes in gB. Thus, gC may selectively facilitate low-pH entry by regulating conformational changes in the fusion protein gB. We propose that gC modulates the HSV fusion machinery during entry into pathophysiologically relevant cells, such as human epidermal keratinocytes.IMPORTANCE Herpesviruses are ubiquitous pathogens that cause lifelong latent infections and that are characterized by multiple entry pathways. We propose that herpes simplex virus (HSV) gC plays a selective role in modulating HSV entry, such as entry into epithelial cells, by a low-pH pathway. gC facilitates a conformational change of the main fusogen gB, a class III fusion protein. We propose a model whereby gC functions with gB, gD, and gH/gL to allow low-pH entry. In the absence of gC, HSV entry occurs at a lower pH, coincident with trafficking to a lower pH compartment where gB changes occur at more acidic pHs. This report identifies a new function for gC and provides novel insight into the complex mechanism of HSV entry and fusion.

Acidic pH Mediates Changes in Antigenic and Oligomeric Conformation of Herpes Simplex Virus gB and Is a Determinant of Cell-Specific Entry

Herpes simplex virus (HSV) is an important human pathogen with a high worldwide seroprevalence. HSV enters epithelial cells, the primary site of infection, by a low-pH pathway. HSV glycoprotein B (gB) undergoes low pH-induced conformational changes, which are thought to drive membrane fusion. When neutralized back to physiological pH, these changes become reversible. Here, HSV-infected cells were subjected to short pulses of radiolabeling, followed by immunoprecipitation with a panel of gB monoclonal antibodies (MAbs), demonstrating that gB folds and oligomerizes rapidly and cotranslationally in the endoplasmic reticulum. Full-length gB from transfected cells underwent low-pH-triggered changes in oligomeric conformation in the absence of other viral proteins. MAbs to gB neutralized HSV entry into cells regardless of the pH dependence of the entry pathway, suggesting a conservation of gB function in distinct fusion mechanisms. The combination of heat and acidic pH triggered irreversible changes in the antigenic conformation of the gB fusion domain, while changes in the gB oligomer remained reversible. An elevated temperature alone was not sufficient to induce gB conformational change. Together, these results shed light on the conformation and function of the HSV-1 gB oligomer, which serves as part of the core fusion machinery during viral entry.IMPORTANCE Herpes simplex virus (HSV) causes infection of the mouth, skin, eyes, and genitals and establishes lifelong latency in humans. gB is conserved among all herpesviruses. HSV gB undergoes reversible conformational changes following exposure to acidic pH which are thought to mediate fusion and entry into epithelial cells. Here, we identified cotranslational folding and oligomerization of newly synthesized gB. A panel of antibodies to gB blocked both low-pH and pH-neutral entry of HSV, suggesting conserved conformational changes in gB regardless of cell entry route. Changes in HSV gB conformation were not triggered by increased temperature alone, in contrast to results with EBV gB. Acid pH-induced changes in the oligomeric conformation of gB are related but distinct from pH-triggered changes in gB antigenic conformation. These results highlight critical aspects of the class III fusion protein, gB, and inform strategies to block HSV infection at the level of fusion and entry.

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

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

Mildly Acidic pH Triggers an Irreversible Conformational Change in the Fusion Domain of Herpes Simplex Virus 1 Glycoprotein B and Inactivation of Viral Entry

Herpes simplex virus (HSV) entry into a subset of cells requires endocytosis and endosomal low pH. Preexposure of isolated virions to mildly acidic pH of 5 to 6 partially inactivates HSV infectivity in an irreversible manner. Acid inactivation is a hallmark of viruses that enter via low-pH pathways; this occurs by pretriggering conformational changes essential for fusion. The target and mechanism(s) of low-pH inactivation of HSV are unclear. Here, low-pH-treated HSV-1 was defective in fusion activity and yet retained normal levels of attachment to cell surface heparan sulfate and binding to nectin-1 receptor. Low-pH-triggered conformational changes in gB reported to date are reversible, despite irreversible low-pH inactivation. gB conformational changes and their reversibility were measured by antigenic analysis with a panel of monoclonal antibodies and by detecting changes in oligomeric conformation. Three-hour treatment of HSV-1 virions with pH 5 or multiple sequential treatments at pH 5 followed by neutral pH caused an irreversible >2.5 log infectivity reduction. While changes in several gB antigenic sites were reversible, alteration of the H126 epitope was irreversible. gB oligomeric conformational change remained reversible under all conditions tested. Altogether, our results reveal that oligomeric alterations and fusion domain changes represent distinct conformational changes in gB, and the latter correlates with irreversible low-pH inactivation of HSV. We propose that conformational change in the gB fusion domain is important for activation of membrane fusion during viral entry and that in the absence of a host target membrane, this change results in irreversible inactivation of virions.IMPORTANCE HSV-1 is an important pathogen with a high seroprevalence throughout the human population. HSV infects cells via multiple pathways, including a low-pH route into epithelial cells, the primary portal into the host. HSV is inactivated by low-pH preexposure, and gB, a class III fusion protein, undergoes reversible conformational changes in response to low-pH exposure. Here, we show that low-pH inactivation of HSV is irreversible and due to a defect in virion fusion activity. We identified an irreversible change in the fusion domain of gB following multiple sequential low-pH exposures or following prolonged low-pH treatment. This change appears to be separable from the alteration in gB quaternary structure. Together, the results are consistent with a model by which low pH can have an activating or inactivating effect on HSV depending on the presence of a target membrane.

Neutralizing human recombinant antibodies against herpes simplex virus type 1 glycoproteins B from a phage-displayed scFv antibody library

The HSV-1 envelope glycoprotein B (gB) plays a critical role in virus entry into host cells. Neutralizing antibodies can therefore potentially prevent virus entry into target cells and cell-to-cell spread of infection. Our present study focused on the selection of neutralizing single-chain Fv (scFv) antibodies of a phage-displayed nonimmune human scFv antibody library against gB of HSV-1. To enrich specific scFvs, two phage antibodies were isolated against amino acid residues 31–43 derived from the N-terminal part of gB using panning technique. Two scFvs, scFv-gB₁ and scFv-gB₂, with frequencies of 45% and 20% were obtained from scFv clones after performing PCR and MvaI fingerprinting. In phage ELISA analysis, both gB₁ and gB₂ scFvs demonstrated high reactivity with the gB peptide. In the neutralization assay, scFv-gB₁ and scFv-gB₂ represented neutralizing effects of 55% and 59%, respectively. Upon further enhancement of the neutralizing effects of these antibodies, they can be considered as new potential alternatives in the treatment and prophylaxis of HSV-1 infections.

Herpesvirus Entry into Host Cells Mediated by Endosomal Low pH

Herpesviral pathogenesis stems from infection of multiple cell types including the site of latency and cells that support lytic replication. Herpesviruses utilize distinct cellular pathways, including low pH endocytic pathways, to enter different pathophysiologically relevant target cells. This review details the impact of the mildly acidic milieu of endosomes on the entry of herpesviruses, with particular emphasis on herpes simplex virus 1 (HSV-1). Epithelial cells, the portal of primary HSV-1 infection, support entry via low pH endocytosis mechanisms. Mildly acidic pH triggers reversible conformational changes in the HSV-1 class III fusion protein glycoprotein B (gB). In vitro treatment of herpes simplex virions with a similar pH range inactivates infectivity, likely by prematurely activating the viral entry machinery in the absence of a target membrane. How a given herpesvirus mediates both low pH and pH-independent entry events is a key unresolved question.

Identification of a neutralizing epitope within antigenic domain 5 of glycoprotein B of human cytomegalovirus

Human cytomegalovirus (HCMV) is an important, ubiquitous pathogen that causes severe clinical disease in immunocompromised individuals, such as organ transplant recipients and infants infected in utero. The envelope glycoprotein B (gB) of HCMV is a major antigen for the induction of virus-neutralizing antibodies. We have begun to define target structures within gB that are recognized by virus-neutralizing antibodies. Antigenic domain 5 (AD-5) of gB has been identified as an important target for neutralizing antibodies in studies using human monoclonal antibodies (MAbs). Anti-AD-5 MAbs share a target site on gB, despite originating from different, healthy, HCMV-infected donors. Mutational analysis of AD-5 identified tyrosine 280 in combination with other surface-exposed residues (the YNND epitope) as critical for antibody binding. The YNND epitope is strictly conserved among different HCMV strains. Recombinant viruses carrying YNND mutations in AD-5 were resistant to virus-neutralizing MAbs. Competition enzyme-linked immunosorbent assays (ELISAs) with human HCMV-convalescent-phase sera from unselected donors confirmed the conserved antibody response for the YNND epitope in HCMV-infected individuals and, because a significant fraction of the gB AD-5 response was directed against the YNND epitope, further argued that this epitope is a major target of anti-AD-5 antibody responses. In addition, affinity-purified polyclonal anti-AD-5 antibodies prepared from individual sera showed reactivity to AD-5 and neutralization activity toward gB mutant viruses that were similar to those of AD-5-specific MAbs. Taken together, our data indicate that the YNND epitope represents an important target for anti-gB antibody responses as well as for anti-AD-5 virus-neutralizing antibodies.

IMPORTANCE

HCMV is a major global health concern, and a vaccine to prevent HCMV disease is a widely recognized medical need. Glycoprotein B of HCMV is an important target for neutralizing antibodies and hence an interesting molecule for intervention strategies, e.g., vaccination. Mapping the target structures of neutralizing antibodies induced by naturally occurring HCMV infection can aid in defining the properties required for a protective capacity of vaccine antigens. The data presented here extend our knowledge of neutralizing epitopes within gB to include AD-5. Collectively, our data will contribute to optimal vaccine design and development of antibody-based therapies.

Functional fluorescent protein insertions in herpes simplex virus gB report on gB conformation before and after execution of membrane fusion

Entry of herpes simplex virus (HSV) into a target cell requires complex interactions and conformational changes by viral glycoproteins gD, gH/gL, and gB. During viral entry, gB transitions from a prefusion to a postfusion conformation, driving fusion of the viral envelope with the host cell membrane. While the structure of postfusion gB is known, the prefusion conformation of gB remains elusive. As the prefusion conformation of gB is a critical target for neutralizing antibodies, we set out to describe its structure by making genetic insertions of fluorescent proteins (FP) throughout the gB ectodomain. We created gB constructs with FP insertions in each of the three globular domains of gB. Among 21 FP insertion constructs, we found 8 that allowed gB to remain membrane fusion competent. Due to the size of an FP, regions in gB that tolerate FP insertion must be solvent exposed. Two FP insertion mutants were cell-surface expressed but non-functional, while FP insertions located in the crown were not surface expressed. This is the first report of placing a fluorescent protein insertion within a structural domain of a functional viral fusion protein, and our results are consistent with a model of prefusion HSV gB constructed from the prefusion VSV G crystal structure. Additionally, we found that functional FP insertions from two different structural domains could be combined to create a functional form of gB labeled with both CFP and YFP. FRET was measured with this construct, and we found that when co-expressed with gH/gL, the FRET signal from gB was significantly different from the construct containing CFP alone, as well as gB found in syncytia, indicating that this construct and others of similar design are likely to be powerful tools to monitor the conformation of gB in any model system accessible to light microscopy.

Viral fusion proteins undergo complicated conformational changes in order to fuse viral and host membranes during viral entry. Conformational changes between prefusion and postfusion states also allow the virus to hide critical regions of the fusion machinery from the immune system. The structure of herpes simplex virus fusion protein gB is known only in its postfusion state, while the prefusion structure is unknown. To study the prefusion state, we created fluorescent protein (FP) insertions within gB and tested them for fusion activity. Due to the size of the fluorescent protein insertion, regions in gB that tolerate this insertion must be solvent exposed, thereby describing structural features of the prefusion structure. We created functional gB constructs with FP insertions in two of the three globular domains of gB, while non-functional insertions in the third domain suggested that it may be buried in the prefusion structure. Additionally, we created a dual-labeled FP gB construct which we found to report on the conformation of gB before and after fusion. Using this dual-labeled gB construct, we have demonstrated how fluorescence-based methods can be used to directly study dynamics of viral fusion proteins in living cells.

Dissection of the antibody response against herpes simplex virus glycoproteins in naturally infected humans

Relatively little is known about the extent of the polyclonal antibody (PAb) repertoire elicited by herpes simplex virus (HSV) glycoproteins during natural infection and how these antibodies affect virus neutralization. Here, we examined IgGs from 10 HSV-seropositive individuals originally classified as high or low virus shedders. All PAbs neutralized virus to various extents. We determined which HSV entry glycoproteins these PAbs were directed against: glycoproteins gB, gD, and gC were recognized by all sera, but fewer sera reacted against gH/gL. We previously characterized multiple mouse monoclonal antibodies (MAbs) and mapped those with high neutralizing activity to the crystal structures of gD, gB, and gH/gL. We used a biosensor competition assay to determine whether there were corresponding human antibodies to those epitopes. All 10 samples had neutralizing IgGs to gD epitopes, but there were variations in which epitopes were seen in individual samples. Surprisingly, only three samples contained neutralizing IgGs to gB epitopes. To further dissect the nature of these IgGs, we developed a method to select out gD- and gB-specific IgGs from four representative sera via affinity chromatography, allowing us to determine the contribution of antibodies against each glycoprotein to the overall neutralization capacity of the serum. In two cases, gD and gB accounted for all of the neutralizing activity against HSV-2, with a modest amount of HSV-1 neutralization directed against gC. In the other two samples, the dominant response was to gD.

IMPORTANCE

Antibodies targeting functional epitopes on HSV entry glycoproteins mediate HSV neutralization. Virus-neutralizing epitopes have been defined and characterized using murine monoclonal antibodies. However, it is largely unknown whether these same epitopes are targeted by the humoral response to HSV infection in humans. We have shown that during natural infection, virus-neutralizing antibodies are principally directed against gD, gB, and, to a lesser extent, gC. While several key HSV-neutralizing epitopes within gD and gB are commonly targeted by human serum IgG, others fail to induce consistent responses. These data are particularly relevant to the design of future HSV vaccines.

Mechanism of neutralization of herpes simplex virus by antibodies directed at the fusion domain of glycoprotein B

Glycoprotein B (gB), the fusogen of herpes simplex virus (HSV), is a class III fusion protein with a trimeric ectodomain of known structure for the postfusion state. Seen by negative-staining electron microscopy, it presents as a rod with three lobes (base, middle, and crown). gB has four functional regions (FR), defined by the physical location of epitopes recognized by anti-gB neutralizing monoclonal antibodies (MAbs). Located in the base, FR1 contains two internal fusion loops (FLs) and is the site of gB-lipid interaction (the fusion domain). Many of the MAbs to FR1 are neutralizing, block cell-cell fusion, and prevent the association of gB with lipid, suggesting that these MAbs affect FL function. Here we characterize FR1 epitopes by using electron microscopy to visualize purified Fab-gB ectodomain complexes, thus confirming the locations of several epitopes and localizing those of MAbs DL16 and SS63. We also generated MAb-resistant viruses in order to localize the SS55 epitope precisely. Because none of the epitopes of our anti-FR1 MAbs mapped to the FLs, we hyperimmunized rabbits with FL1 or FL2 peptides to generate polyclonal antibodies (PAbs). While the anti-FL1 PAb failed to bind gB, the anti-FL2 PAb had neutralizing activity, implying that the FLs become exposed during virus entry. Unexpectedly, the anti-FL2 PAb (and the anti-FR1 MAbs) bound to liposome-associated gB, suggesting that their epitopes are accessible even when the FLs engage lipid. These studies provide possible mechanisms of action for HSV neutralization and insight into how gB FR1 contributes to viral fusion.

IMPORTANCE

For herpesviruses, such as HSV, entry into a target cell involves transfer of the capsid-encased genome of the virus to the target cell after fusion of the lipid envelope of the virus with a lipid membrane of the host. Virus-encoded glycoproteins in the envelope are responsible for fusion. Antibodies to these glycoproteins are important biological tools, providing a way of examining how fusion works. Here we used electron microscopy and other techniques to study a panel of anti-gB antibodies. Some, with virus-neutralizing activity, impair gB-lipid association. We also generated a peptide antibody against one of the gB fusion loops; its properties provide insight into the way the fusion loops function as gB transits from its prefusion form to an active fusogen.

Characterization and whole genome sequences of the Rhodococcus bacteriophages RGL3 and RER2

We report here the isolation and genome sequences of two novel phages, lytic for Rhodococcus and Nocardia species. Named RER2 and RGL3, both are members of the family Siphoviridae, and each possesses a novel genome of 46,586 bp and 48,072 bp, respectively. RER2 and RGL3 phages share a modular genome organization, as seen in other sequenced Siphoviridae phage genomes, and appear to share a common evolutionary origin. The genomes of these phages share no similarity with other Rhodococcus or Nocardia phages but are related to Mycobacterium phages. The data presented here extend our understanding of Rhodococcus phage genomics.

Herpes virus fusion and entry: a story with many characters

Herpesviridae comprise a large family of enveloped DNA viruses all of whom employ orthologs of the same three glycoproteins, gB, gH and gL. Additionally, herpesviruses often employ accessory proteins to bind receptors and/or bind the heterodimer gH/gL or even to determine cell tropism. Sorting out how these proteins function has been resolved to a large extent by structural biology coupled with supporting biochemical and biologic evidence. Together with the G protein of vesicular stomatitis virus, gB is a charter member of the Class III fusion proteins. Unlike VSV G, gB only functions when partnered with gH/gL. However, gH/gL does not resemble any known viral fusion protein and there is evidence that its function is to upregulate the fusogenic activity of gB. In the case of herpes simplex virus, gH/gL itself is upregulated into an active state by the conformational change that occurs when gD, the receptor binding protein, binds one of its receptors. In this review we focus primarily on prototypes of the three subfamilies of herpesviruses. We will present our model for how herpes simplex virus (HSV) regulates fusion in series of highly regulated steps. Our model highlights what is known and also provides a framework to address mechanistic questions about fusion by HSV and herpesviruses in general.

Glycoprotein B of herpes simplex virus 2 has more than one intracellular conformation and is altered by low pH

The crystal structure of herpes simplex virus (HSV) gB identifies it as a class III fusion protein, and comparison with other such proteins suggests this is the postfusion rather than prefusion conformation, although this is not proven. Other class III proteins undergo a pH-dependent switch between pre- and postfusion conformations, and a low pH requirement for HSV entry into some cell types suggests that this may also be true for gB. Both gB and gH undergo structural changes at low pH, but there is debate about the extent and significance of the changes in gB, possibly due to the use of different soluble forms of the protein and different assays for antigenic changes. In this study, a complementary approach was taken, examining the conformations of full-length intracellular gB by quantitative confocal microscopy with a panel of 26 antibodies. Three conformations were distinguished, and low pH was found to be a major influence. Comparison with previous studies indicates that the intracellular conformation in low-pH environments may be the same as that of the soluble form known as s-gB at low pH. Interestingly, the antibodies whose binding was most affected by low pH both have neutralizing activity and consequently must block either the function of a neutral pH conformation or its switch from an inactive form to an activated form. If one of the intracellular conformations is the fusion-active form, another factor required for fusion is presumably absent from wherever that conformation is present in infected cells so that inappropriate fusion is avoided.

Residues within the C-terminal arm of the herpes simplex virus 1 glycoprotein B ectodomain contribute to its refolding during the fusion step of virus entry

Herpesvirus entry into cells requires coordinated interactions among several viral glycoproteins. The final membrane fusion step of entry is executed by glycoprotein B (gB), a class III viral fusion protein that is conserved across all herpesviruses. Fusion proteins are metastable proteins that mediate fusion by inserting into a target membrane and refolding from a prefusion to postfusion conformation to bring the viral and cell membranes together. Although the structure of gB has been solved in a conformation that likely represents its postfusion form, its prefusion structure and the details of how it refolds to execute fusion are unknown. The postfusion gB structure contains a trimeric coiled-coil at its core and a long C-terminal arm within the ectodomain packs against this coil in an antiparallel manner. This coil-arm complex is reminiscent of the six-helix bundle that provides the energy for fusion in class I fusogens. To determine the role of the coil-arm complex, we individually mutated residues in the herpes simplex virus 1 gB coil-arm complex to alanine and assessed the contribution of each residue to cell-cell and virus-cell fusion. Several coil mutations resulted in a loss of cell surface expression, indicating that the coil residues are important for proper processing of gB. Three mutations in the arm region (I671A, H681A, and F683A) reduced fusion without affecting expression. Combining these three arm mutations drastically reduced the ability of gB to execute fusion; however, fusion function could be restored by adding known hyperfusogenic mutations to the arm mutant. We propose that the formation of the coil-arm complex drives the gB transition to a postfusion conformation and the coil-arm complex performs a function similar to that of the six-helix bundle in class I fusion. Furthermore, we suggest that these specific mutations in the arm may energetically favor the prefusion state of gB.

Impact of valency of a glycoprotein B-specific monoclonal antibody on neutralization of herpes simplex virus

Herpes simplex virus (HSV) glycoprotein B (gB) is an integral part of the multicomponent fusion system required for virus entry and cell-cell fusion. Here we investigated the mechanism of viral neutralization by the monoclonal antibody (MAb) 2c, which specifically recognizes the gB of HSV type 1 (HSV-1) and HSV-2. Binding of MAb 2c to a type-common discontinuous epitope of gB resulted in highly efficient neutralization of HSV at the postbinding/prefusion stage and completely abrogated the viral cell-to-cell spread in vitro. Mapping of the antigenic site recognized by MAb 2c to the recently solved crystal structure of the HSV-1 gB ectodomain revealed that its discontinuous epitope is only partially accessible within the observed multidomain trimer conformation of gB, likely representing its postfusion conformation. To investigate how MAb 2c may interact with gB during membrane fusion, we characterized the properties of monovalent (Fab and scFv) and bivalent [IgG and F(ab')(2)] derivatives of MAb 2c. Our data show that the neutralization capacity of MAb 2c is dependent on cross-linkage of gB trimers. As a result, only bivalent derivatives of MAb 2c exhibited high neutralizing activity in vitro. Notably, bivalent MAb 2c not only was capable of preventing mucocutaneous disease in severely immunodeficient NOD/SCID mice upon vaginal HSV-1 challenge but also protected animals even with neuronal HSV infection. We also report for the first time that an anti-gB specific monoclonal antibody prevents HSV-1-induced encephalitis entirely independently from complement activation, antibody-dependent cellular cytotoxicity, and cellular immunity. This indicates the potential for further development of MAb 2c as an anti-HSV drug.

Characterisation of the epitope for a herpes simplex virus glycoprotein B-specific monoclonal antibody with high protective capacity

Monoclonal antibody (MAb) 2c, specific for glycoprotein B of herpes simplex virus (HSV), had been shown to mediate clearance of infection from the mucous membranes of mice, thereby completely inhibiting mucocutaneous inflammation and lethality, even in mice depleted of both CD4(+) and CD8(+) cells. Additionally, ganglionic infection was highly restricted. In vitro, MAb 2c exhibits a potent complement-independent neutralising activity against HSV type 1 and 2, completely inhibits the viral cell-to-cell spread as well as the syncytium formation induced by syncytial HSV strains (Eis-Hübinger et al. in Intervirology 32:351-360, 1991; Eis-Hübinger et al. in J Gen Virol 74:379-385, 1993). Here, we describe the mapping of the epitope for MAb 2c. The antibody was found to recognise a discontinuous epitope comprised of the HSV type 1 glycoprotein B residues 299 to 305 and one or more additional discontinuous regions that can be mimicked by the sequence FEDF. Identification of the epitope was confirmed by loss of antibody binding to mutated glycoprotein B with replacement of the epitopic key residues, expressed in COS-1 cells. Similarly, MAb 2c was not able to neutralise HSV mutants with altered key residues, and MAb 2c was ineffective in mice inoculated with such mutants. Interestingly, identification and fine-mapping of the discontinuous epitope was not achieved by binding studies with truncated glycoprotein B variants expressed in COS cells but by peptide scanning with synthetic overlapping peptides and peptide key motif analysis. Reactivity of MAb 2c was immensely increased towards a peptide composed of the glycoprotein B residues 299 to 305, a glycine linker, and a C-terminal FEDF motif. If it could be demonstrated that antibodies of the specificity and bioactivity of MAb 2c can be induced by the epitope or a peptide mimicking the epitope, strategies for active immunisation might be conceivable.

Reevaluating herpes simplex virus hemifusion

Membrane fusion induced by enveloped viruses proceeds through the actions of viral fusion proteins. Once activated, viral fusion proteins undergo large protein conformational changes to execute membrane fusion. Fusion is thought to proceed through a "hemifusion" intermediate in which the outer membrane leaflets of target and viral membranes mix (lipid mixing) prior to fusion pore formation, enlargement, and completion of fusion. Herpes simplex virus type 1 (HSV-1) requires four glycoproteins-glycoprotein D (gD), glycoprotein B (gB), and a heterodimer of glycoprotein H and L (gH/gL)-to accomplish fusion. gD is primarily thought of as a receptor-binding protein and gB as a fusion protein. The role of gH/gL in fusion has remained enigmatic. Despite experimental evidence that gH/gL may be a fusion protein capable of inducing hemifusion in the absence of gB, the recently solved crystal structure of HSV-2 gH/gL has no structural homology to any known viral fusion protein. We found that in our hands, all HSV entry proteins-gD, gB, and gH/gL-were required to observe lipid mixing in both cell-cell- and virus-cell-based hemifusion assays. To verify that our hemifusion assay was capable of detecting hemifusion, we used glycosylphosphatidylinositol (GPI)-linked hemagglutinin (HA), a variant of the influenza virus fusion protein, HA, known to stall the fusion process before productive fusion pores are formed. Additionally, we found that a mutant carrying an insertion within the short gH cytoplasmic tail, 824L gH, is incapable of executing hemifusion despite normal cell surface expression. Collectively, our findings suggest that HSV gH/gL may not function as a fusion protein and that all HSV entry glycoproteins are required for both hemifusion and fusion. The previously described gH 824L mutation blocks gH/gL function prior to HSV-induced lipid mixing.

Bimolecular complementation defines functional regions of Herpes simplex virus gB that are involved with gH/gL as a necessary step leading to cell fusion

Herpes simplex virus (HSV) entry into cells requires four membrane glycoproteins: gD is the receptor binding protein, and gB and gH/gL constitute the core fusion machinery. Crystal structures of gD and its receptors have provided a basis for understanding the initial triggering steps, but how the core fusion proteins function remains unknown. The gB crystal structure shows that it is a class III fusion protein, yet unlike other class members, gB itself does not cause fusion. Bimolecular complementation (BiMC) studies have shown that gD-receptor binding triggers an interaction between gB and gH/gL and concurrently triggers fusion. Left unanswered was whether BiMC led to fusion or was a by-product of it. We used gB monoclonal antibodies (MAbs) to block different aspects of these events. Non-virus-neutralizing MAbs to gB failed to block BiMC or fusion. In contrast, gB MAbs that neutralize virus blocked fusion. These MAbs map to three functional regions (FR) of gB. MAbs to FR1, which contains the fusion loops, and FR2 blocked both BiMC and fusion. In contrast, MAbs to FR3, a region involved in receptor binding, blocked fusion but not BiMC. Thus, FR3 MAbs separate the BiMC interaction from fusion, suggesting that BiMC occurs prior to fusion. When substituted for wild-type (wt) gB, fusion loop mutants blocked fusion and BiMC, suggesting that loop insertion precedes BiMC. Thus, we postulate that each of the gB FRs are involved in different aspects of the path leading to fusion. Upon triggering by gD, gB fusion loops are inserted into target lipid membranes. gB then interacts with gH/gL, and this interaction is eventually followed by fusion.

Low pH-induced conformational change in herpes simplex virus glycoprotein B

Herpesviruses can enter host cells using pH-dependent endocytosis pathways in a cell-specific manner. Envelope glycoprotein B (gB) is conserved among all herpesviruses and is a critical component of the complex that mediates membrane fusion and entry. Here we demonstrate that mildly acidic pH triggers specific conformational changes in herpes simplex virus (HSV) gB. The antigenic structure of gB was specifically altered by exposure to low pH both in vitro and during entry into host cells. The oligomeric conformation of gB was altered at a similar pH range. Exposure to acid pH appeared to convert virion gB into a lower-order oligomer. The detected conformational changes were reversible, similar to those in other class III fusion proteins. Exposure of purified, recombinant gB to mildly acidic pH resulted in similar changes in conformation and caused gB to become more hydrophobic, suggesting that low pH directly affects gB. We propose that intracellular low pH induces alterations in gB conformation that, together with additional triggers such as receptor binding, are essential for virion-cell fusion during herpesviral entry by endocytosis.

Structure-function analysis of herpes simplex virus glycoprotein B with fusion-from-without activity

Fusion-from-without (FFWO) is the rapid induction of cell fusion by virions in the absence of viral protein synthesis. The combination of two amino acid mutations in envelope glycoprotein B (gB), one in the ectodomain and one in the cytoplasmic tail, can confer FFWO activity to wild type herpes simplex virus (HSV). In this report, we analyzed the entry and cell fusion phenotypes of HSV that contains FFWO gB, with emphasis on the cellular receptors for HSV, nectin-1, nectin-2 and HVEM. The ability of an HSV strain with FFWO gB to efficiently mediate FFWO via a specific gD-receptor correlated with its ability to mediate viral entry by that receptor. A FFWO form of gB was not sufficient to switch the entry of HSV from a pH-dependent, endocytic pathway to a direct fusion, pH-independent pathway. The conformation of gB with FFWO activity was not globally altered relative to wild type. However, distinct monoclonal antibodies had reduced reactivity with FFWO gB, suggesting an altered antigenic structure relative to wild type. FFWO was blocked by preincubation of virions with neutralizing antibodies to gB or gD. Together with previous studies, the results indicate that the roles of gB in FFWO and in virus-cell fusion during entry are related but not identical. This study also suggests that the FFWO function of gB is not a specific determinant for the selection of HSV entry pathway and that antigenic differences in FFWO gB may reflect its enhanced fusion activity.

Antigenic and mutational analyses of herpes simplex virus glycoprotein B reveal four functional regions

Glycoprotein B (gB), along with gD, gH, and gL, is essential for herpes simplex virus (HSV) entry. The crystal structure of the gB ectodomain revealed it to be an elongated multidomain trimer. We generated and characterized a panel of 67 monoclonal antibodies (MAbs). Eleven of the MAbs had virus-neutralizing activity. To organize gB into functional regions within these domains, we localized the epitopes recognized by the entire panel of MAbs and mapped them onto the crystal structure of gB. Most of the MAbs were directed to continuous or discontinuous epitopes, but several recognized discontinuous epitopes that showed some resistance to denaturation, and we refer to them as pseudo-continuous. Each category contained some MAbs with neutralizing activity. To map continuous epitopes, we used overlapping peptides that spanned the gB ectodomain and measured binding by enzyme-linked immunosorbent assay. To identify discontinuous and pseudocontinuous epitopes, a purified form of the ectodomain of gB, gB(730t), was cleaved by alpha-chymotrypsin into two major fragments comprising amino acids 98 to 472 (domains I and II) and amino acids 473 to 730 (major parts of domains III, IV, and V). We also constructed a series of gB truncations to augment the other mapping strategies. Finally, we used biosensor analysis to assign the MAbs to competition groups. Together, our results identified four functional regions: (i) one formed by residues within domain I and amino acids 697 to 725 of domain V; (ii) a second formed by residues 391 to 410, residues 454 to 475, and a less-defined region within domain II; (iii) a region containing residues of domain IV that lie close to domain III; and (iv) the first 12 residues of the N terminus that were not resolved in the crystal structure. Our data suggest that multiple domains are critical for gB function.

Murine gammaherpesvirus-68 glycoprotein B presents a difficult neutralization target to monoclonal antibodies derived from infected mice

Persistent viruses disseminate from immune hosts. They must therefore resist neutralization by antibody. Murine gammaherpesvirus-68 (MHV-68) represents an accessible model with which to address how resistance to neutralization is achieved and how overcoming it might improve infection control. The MHV-68 glycoprotein B (gB), like that of other herpesviruses, is a virion protein that is essential for infectivity. As such, it presents a potential neutralization target. In order to test whether virus-induced antibodies reduce virion infectivity by binding to gB, monoclonal antibodies (mAbs) were derived from MHV-68-infected mice. gB-specific mAbs were common, but only an IgM specific for the gB N terminus reduced virion infectivity significantly. It inhibited MHV-68 entry into BHK-21 cells at a post-binding step that was linked closely to membrane fusion. Reducing the mAb to IgM monomers compromised neutralization severely, suggesting that a pentameric structure was crucial to its function. Antibody treatment never blocked BHK-21 cell infection completely and blocked the infection of NMuMG epithelial cells hardly at all. Virions saturated with antibody also remained infectious to mice. Thus, the MHV-68 gB presents at best a very difficult target for antibody-mediated neutralization.

Developments in herpes simplex virus vaccines: old problems and new challenges

Vaccination has remained the best method for preventing virus spread. The herpes simplex virus (HSV) candidate vaccines tested till now were mostly purified subunit vaccines and/or recombinant envelope glycoproteins (such as gB and gD). In many experiments performed in mice, guinea pigs and rabbits, clear-cut protection against acute virus challenge was demonstrated along with the reduction of the extent of latency, when established in the immunized host. The immunotherapeutic effect of herpes vaccines seems less convincing. However, introduction of new adjuvants, which shift the cytokine production of helper T-cells toward stimulation of cytotoxic T-cells (TH1 type cytokine response), reveals a promising development. Mathematical analysis proved that overall prophylactic vaccination of seronegative women, even when eliciting 40-60 % antibody response only, would reduce the frequency of genital herpes within the vaccinated population. Even when partially effective, immunotherapeutic vaccination might represent a suitable alternative of chronic chemotherapy in recurrent labial and genital herpes.

Crystal structure of glycoprotein B from herpes simplex virus 1

Glycoprotein B (gB) is the most conserved component of the complex cell-entry machinery of herpes viruses. A crystal structure of the gB ectodomain from herpes simplex virus type 1 reveals a multidomain trimer with unexpected homology to glycoprotein G from vesicular stomatitis virus (VSV G). An alpha-helical coiled-coil core relates gB to class I viral membrane fusion glycoproteins; two extended beta hairpins with hydrophobic tips, homologous to fusion peptides in VSV G, relate gB to class II fusion proteins. Members of both classes accomplish fusion through a large-scale conformational change, triggered by a signal from a receptor-binding component. The domain connectivity within a gB monomer would permit such a rearrangement, including long-range translocations linked to viral and cellular membranes.

Identification of syncytial mutations in a clinical isolate of herpes simplex virus 2

Small polykaryocytes resulting from cell fusion are found in herpes simplex virus (HSV) lesions in patients, but their significance for viral spread and pathogenesis is unclear. Although syncytial variants causing extensive fusion in tissue culture can be readily isolated from laboratory strains, they are rarely found in clinical isolates, suggesting that extensive cell fusion may be deleterious in vivo. Syncytial mutations have previously been identified for several laboratory strains, but not for clinical isolates of HSV type 2. To address this deficiency, we studied a recent syncytial clinical isolate, finding it to be a mixture of two syncytial and one nonsyncytial strain. The two syncytial strains have novel mutations in glycoprotein B, and in vitro cell fusion assays confirmed that they are responsible for syncytium formation. This panel of clinical strains may be ideal for examining the effect of increased cell fusion on pathogenesis.

Immobilized cobalt affinity chromatography provides a novel, efficient method for herpes simplex virus type 1 gene vector purification

Herpes simplex virus type 1 (HSV-1) is a promising vector for gene therapy applications, particularly at peripheral nerves, the natural site of virus latency. Many gene vectors require large particle numbers for even early-phase clinical trials, emphasizing the need for high-yield, scalable manufacturing processes that result in virus preparations that are nearly free of cellular DNA and protein contaminants. HSV-1 is an enveloped virus that requires the development of gentle purification methods. Ideally, such methods should avoid centrifugation and may employ selective purification processes that rely on the recognition of a unique envelope surface chemistry. Here we describe a novel method that fulfills these criteria. An immobilized metal affinity chromatography (IMAC) method was developed for the selective purification of vectors engineered to display a high-affinity binding peptide. Feasibility studies involving various transition metal ions (Cu2+, Zn2+, Ni2+, and Co2+) showed that cobalt had the most desirable features, which include a low level of interaction with either the normal virus envelope or contaminating DNA and proteins. The introduction of a cobalt-specific recognition element into the virus envelope may provide a suitable target for cobalt-dependent purification. To test this possibility, we engineered a peptide with affinity for immobilized cobalt in frame in the heparan sulfate binding domain of HSV-1 glycoprotein B, which is known to be exposed on the surface of the virion particle and recombined into the viral genome. By optimizing the IMAC loading conditions and reducing cobalt ion leakage, we recovered 78% of the tagged HSV-1 recombinant virus, with a >96% reduction in contaminating proteins and DNA.

Fusion activity of lipid-anchored envelope glycoproteins of herpes simplex virus type 1

Expression of the herpes simplex virus type 1 (HSV-1) glycoproteins gB, gD, gH, and gL is necessary and sufficient to cause cell fusion. To identify the requirements for a membrane-spanning domain in HSV-1 glycoprotein-induced cell fusion, we created gB, gD, and gH mutants with transmembrane and cytoplasmic domains replaced by a glycosylphosphatidylinositol (gpi)-addition sequence. The corresponding gBgpi, gDgpi, and gHgpi proteins were expressed with wild-type efficiency at the cell surface and were linked to the plasma membrane via a gpi anchor. The gDgpi mutant promoted cell fusion near wild-type gD levels when co-expressed with gB, gH, and gL in a cell-mixing fusion assay, indicating that the gD transmembrane and cytoplasmic domains were not required for fusion activity. A plasma membrane link was required for fusion because a gD mutant lacking a transmembrane and cytoplasmic domain was nonfunctional for fusion. The gDgpi mutant was also able to cooperate with wild-type gB, gH, and gL to form syncytia, albeit at a size smaller than those formed in the wild-type situation. The gBgpi and gHgpi mutants were unable to promote fusion when expressed with the other wild-type viral glycoproteins, highlighting the requirement of the specific transmembrane and cytoplasmic domains for gB and gH function.

Effects of mutations in the cytoplasmic domain of herpes simplex virus type 1 glycoprotein B on intracellular transport and infectivity

Herpes simplex virus type 1 (HSV-1) is a human pathogen of the alphaherpesvirus family which infects and spreads in the nervous system. Glycoproteins play a key role in the process of assembly and maturation of herpesviruses, which is essential for neuroinvasion and transneuronal spread. Glycoprotein B (gB) is a main component of the HSV-1 envelope and is necessary for the production of infectious particles. The cytoplasmic domain of gB, the longest one among HSV-1 glycoproteins, contains several highly conserved peptide sequences homologous to motifs involved in intracellular sorting. To determine the specific roles of these motifs in processing, subcellular localization, and the capacity of HSV-1 gB to complement a gB-null virus, we generated truncated or point mutated forms of a green fluorescent protein (GFP)-tagged gB. GFP-gB with a deletion in the acidic cluster DGDADEDDL (amino acids [aa] 896 to 904) behaved the same as the parental form. Deletion or disruption of the YTQV motif (aa 889 to 892) abolished internalization and reduced complementation by 60%. Disruption of the LL motif (aa 871 to 872) impaired the return of the protein to the trans-Golgi network (TGN) while enhancing its recycling to the plasma membrane. Truncations from residue E 857 abolished transport and processing of the truncated proteins, which had null complementation activity, through the Golgi complex. Altogether, our results favor a model in which HSV-1 gets its final envelope in the TGN, and they suggest that endocytosis, albeit not necessary, might play a role in infectivity.

Immunological properties of a DNA plasmid encoding a chimeric protein of herpes simplex virus type 2 glycoprotein B and glycoprotein D

A DNA plasmid containing a chimeric sequence encoding both herpes simplex virus type 2 (HSV-2) glycoprotein B (gB) and glycoprotein D (gD) external domains (pcgDB) was used to immunize BALB/c mice against genital HSV-2 infection. To determine the efficacy of this vaccine, groups of mice immunized with the pcgDB plasmid were compared with animals immunized with plasmids corresponding to the individual proteins (pcgBt or pcgDt), administered separately or in combination (pcgBt + pcgDt). We studied the response of the different mouse groups to viral challenge by analyzing clinical disease (vaginitis), serum antibody levels, as well as lymphoproliferative responses and cytokine production by spleen cells. Increased IFN-gamma levels correlated with prolonged survival in mice immunized with the plasmid pcgDB, relative to mice immunized with plasmids coding for the individual proteins alone or in combination. Our results show that immunization with the plasmid encoding the chimeric protein is advantageous over separate proteins. These findings may have important implications for the development of multivalent DNA vaccines against HSV and other complex pathogens.

Characterization of strain HSZP of herpes simplex virus type 1 (HSV1)

The genetic background of HSZP virus, an HSV1 strain with extensive passage history, was analyzed by parallel comparative sequencing of four relevant genes (UL27/gB, UL41/vhs, UL44/gC and UL53/gK) of HSZP and additional three selected viruses [strains ANGpath, strains KOS(a) and KOS(b) and the prototype strain 17]. Mutation at position 858 (His for Arg) in gB of HSZP was found to be responsible for giant cell formation (syn3gB mutation) similarly as the 855 mutation (Val for Ala) in the gB of ANGpath. No syn1gK mutations were detected in the UL53 gene either of HSZP or of ANGpath viruses. The reduced virulence of HSZP for adult mice after peripheral inoculation, similarly as that of KOS virus, seems to be related (at least in part) to numerous mutations in the gB ectodomain. Of these, two mutations located in the antigenic domain IV were the same in gBHSZP as well as in gBKOS (at amino acids 59 and 79), at least two (amino acids 313 and 553) were specific for gBKOS, while one mutation (Ser for Ala at position 108) was specific for gBHSZP. The abolished shutoff function of the HSZP virus was related to at least four out of six specific mutations seen in the vhs polypeptide (vhsHSZP) encoded by the UL41 gene, of which three (amino acids 374, 386, 392) were clustered in the semiconservative box A of vhsHSZP (the truncation of which abrogates the inhibition provided by this protein) and one mutation (at amino acid 18) was situated in the highly conservative locus I of vhsHSZP. In addition, the two vhsKOS specific mutations (amino acids 19 and 317) not found in vhsHSZP, enhanced the early host shutoff function of the vhsKOS protein. Finally, gCHSZP had two specific mutations (amino acids 137 and 147) located in the antigenic domain II of gC, which is responsible for binding of HSV1 virions to the glycosoaminoglycan (GAG) receptor. When expressed in Sf21 cells using the recombinant baculovirus system (Bac-to-Bac), gCHSZP and gCKOS showed no essential antigenic differences.

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

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

Herpes simplex virus 1 (HSV-1) strain HSZP glycoprotein B gene: comparison of mutations among strains differing in virulence

The nonpathogenic HSZP strain of HSV-1 induces large polykaryocytes due to a syn3 mutation (His for Arg at residue 858) in the C-terminal endodomain of glycoprotein B (gB) (40). We determined the nucleotide (nt) sequence of the UL27 gene specifying the gB polypeptide of HSZP (gBHSZP) and found 3 mutations in its ectodomain at aminoacids (aa) 59, 79 and 108. The ANGpath virus, which also has a syn3 mutation in the C-terminal endodomain of gB (Val for Ala at residue 855) is pathogenic for adult mice (39), but can be made nonpathogenic by replacing the gBANGpath gene by the corresponding gBKOS sequence (21). The gBANGpath had three ectodomain mutations (at aa 62, 77 and 285), while gBKOS had at least four ectomain mutations (aa 59, 79, 313, and 553). Two mutations (aa 59 and 79) in the latter, located in the variable antigenic site IV/D1 were common for gBKOS and gBHSZP. These together with the gBANGpath mutations at aa 62 and 77 create a cluster of 4 mutations in diverse region of the N-terminal part of gB (between aa 59-79), in which the gBs of pathogenic ANGpath and 17 viruses differ from the gBs of nonpathogenic HSZP and KOS viruses. The lower pathogenicity of KOS as related to gBKOS, is furthermore associated with the change of Ser to Thr at aa 313 (locus III/D2). The possibility is discussed that mutations in both above mentioned antigenic loci could result in higher immunogenicity of the corresponding antigenic epitopes, which, in turn, would contribute to the decreased virulence of HSZP and KOS viruses.

Human herpesvirus-8 glycoprotein B interacts with Epstein-Barr virus (EBV) glycoprotein 110 but fails to complement the infectivity of EBV mutants

To characterize human herpesvirus 8 (HHV-8) gB, the open reading frame was PCR amplified from the HHV-8-infected cell line BCBL-1 and cloned into an expression vector. To facilitate detection of expressed HHV-8 gB, the cytoplasmic tail of the glycoprotein was tagged with the influenza hemagglutinin (HA) epitope. Expression of tagged HHV-8 gB (gB-HA), as well as the untagged form, was readily detected in CHO-K1 cells and several lymphoblastoid cell lines (LCLs). HHV-8 gB-HA was sensitive to endoglycosidase H treatment, and immunofluorescence revealed that HHV-8 gB-HA was detectable in the perinuclear region of CHO-K1 cells. These observations suggest that HHV-8 gB is not processed in the Golgi and localizes to the endoplasmic reticulum or nuclear membrane. Because both HHV-8 and EBV are gamma-herpesviruses, the ability of HHV-8 gB to interact with and functionally complement EBV gp110 was examined. HHV-8 gB-HA and EBV gp110 co-immunoprecipitated, indicating formation of hetero-oligomers. However, HHV-8 gB-HA and HHV-8 gB failed to restore the infectivity of gp110-negative EBV mutants. These findings indicate that although HHV-8 gB and EBV gp110 have similar patterns of intracellular localization and can interact, there is not sufficient functional homology to allow efficient complementation.

Use of a neural network secondary structure prediction to define targets for mutagenesis of herpes simplex virus glycoprotein B

Herpes simplex virus glycoprotein B (HSV gB) is essential for penetration of virus into cells, for cell-to-cell spread of virus, and for cell-cell fusion. Every member of the family Herpesviridae has a gB homolog, underlining its importance. The antigenic structure of gB has been studied extensively, but little is known about which regions of the protein are important for its roles in virus entry and spread. In contrast to successes with other HSV glycoproteins, attempts to map functional domains of gB by insertion mutagenesis have been largely frustrated by the misfolding of most mutants. The present study shows that this problem can be overcome by targeting mutations to the loop regions that connect alpha-helices and beta-strands, avoiding the helices and strands themselves. The positions of loops in the primary sequence were predicted by the PHD neural network procedure, using a multiple sequence alignment of 19 alphaherpesvirus gB sequences as input. Comparison of the prediction with a panel of insertion mutants showed that all mutants with insertions in predicted alpha-helices or beta-strands failed to fold correctly and consequently had no activity in virus entry; in contrast, half the mutants with insertions in predicted loops were able to fold correctly. There are 27 predicted loops of four or more residues in gB; targeting of mutations to these regions will minimize the number of misfolded mutants and maximize the likelihood of identifying functional domains of the protein.

The syn3 strain HSZP of herpes simplex virus type 1 (HSV-1) is not pathogenic for mice and shows limited neural spread

Strain HSZP of the herpes simplex virus type 1 (HSV-1) forms large giant cells in vitro. This property was found associated with a mutation that alters the codon CGC (in the strain KOS or 17 sequence) to CAC (in the HSZP sequence), changing the amino acid 857 from arginine to histidine in the cytoplasmic domain of the glycoprotein B (gB) polypeptide chain. Giant cell formation by ANGpath was attributed to a mutation that alters the codon GCC (in KOS and strain 17 sequences) to GTC (in ANGpath sequence) changing the amino acid 854 in the same (syn3) region of the gB molecule. In contrast to the ANGpath virus, which is pathogenic (1 LD50 < 1 x 10(4) PFU) for adult DBA/2 mice after peripheral inoculation, strain HSZP was never found to be lethal for adult mice. Whereas ANGpath-infected mice which survived acute infection frequently (79%) developed latency in the regional sensory ganglion (as proved by virus reactivation during explantation), latent HSZP reactivated in ganglion culture at a considerably reduced rate (21%). Only 10-day-old DBA/2 mice were sensitive to HSZP infection. In these, HSZP spread from the site of peripheral administration mainly by hematogenous route. The neural spread of HSZP in suckling DBA/2 mice was manifested by the involvement of vegetative neurons in the wall of the small intestine and in the retroperitoneal vegetative ganglia. We conclude that HSZP, a polykaryocyte-forming strain with a mutation in the syn3 region II, shows limited neuroinvasity for mice after peripheral administration.

Locations of herpes simplex virus type 2 glycoprotein B epitopes recognized by human serum immunoglobulin G antibodies

Herpes simplex virus type 2 (HSV-2) glycoprotein B (gB-2) gene segments were expressed as recombinant proteins in Escherichia coli. gB-2 recombinant proteins were reacted with human serum immunoglobulin G (IgG) antibodies in Western immunoblot assays. Initially, samples were tested for the presence of HSV-1-specific antibodies and HSV-2-specific antibodies by using HSV-infected cell lysates as antigen targets in Western blot assays. Serum samples that contained HSV-2-specific IgG (n = 58), HSV-1-specific IgG (n = 33), or no detectable HSV antibodies (n = 31) were tested for reactivities with the gB-2 recombinant proteins. In 58 of 58 samples that contained HSV-2-specific IgG, antibodies were present that reacted strongly with a gB-2 amino-proximal segment between amino acids (aa) 18 and 75. Three of 33 serum samples that contained HSV-1- and not HSV-2-specific IgG (as defined by the HSV lysate Western blot assay) reacted with this segment. Both HSV-2 antibodies and HSV-1 antibodies reacted strongly with a carboxy-terminal gB-2 segment between aa 819 and 904; a second minor cross-reactive region was mapped to a gB-2 segment between aa 564 and 626. The gB-2 segment from aa 18 to 75 may constitute a useful reagent for the virus type-specific serodiagnosis of HSV-2 infections. Further studies will be required to determine the relative sensitivities and specificities of the assay for gB-2 aa 18 to 75, HSV gG assays, and HSV lysate Western blot assays for detecting virus type-specific antibody responses in acute and chronic HSV-2 infections.

Expression and identification of the feline herpesvirus type 1 glycoprotein B (gp143/108)

The gene for feline herpesvirus type 1 (FHV-1) glycoprotein B (gB) has been cloned into an expression vector, pRVSVneo, containing the long terminal repeat of Rous sarcoma virus and polyadenylation signal of SV40. This expression vector containing FHV-1 gB gene, pRVSVgBneo, was transfected into Crandell feline kidney (CRFK) cells which are susceptible to FHV-1 infection. By indirect immunofluorescence analysis, the expressed gB was recognized with a panel of monoclonal antibodies (MAbs) against FHV-1 gp143/108. Immunoprecipitation analysis using a MAb 34H12 showed that molecular weights of the gB were 143 and 108 kDa under non-denaturing conditions that 108, 70, 64, and 58 kDa under denaturing conditions. The molecular weights were similar to those of the gB expressed in FHV-1-infected CRFK cells. In addition, when plasmid DNAs were injected into mice to obtain gB-monospecific serum, the pooled serum from mice inoculated with pRVSVgBneo, but not with pRVSVgDneo or pRVSVneo, recognized the FHV-1 gB polypeptides.

Conformation-defective herpes simplex virus 1 glycoprotein B activates the promoter of the grp94 gene that codes for the 94-kD stress protein in the endoplasmic reticulum

GRP94 is a major glycoprotein in the endoplasmic reticulum with calcium-binding properties. Recently, GRP94 has been shown to bind to unassembled forms of multimeric proteins and peptides. We report here that GRP94 forms a stable association with the mutated form of the herpes simplex type virus 1 (HSV-1) glycoprotein B, but not with the fully processed viral protein. Both the glycosylated and unglycosylated forms of GRP94 are capable of complexing with the mutated, conformation-defective viral glycoprotein. Cotransfection of expression vectors for gB and grp94 promoter fusion genes revealed that the grp94 promoter is strongly activated by the mutant form of gB. Analysis of the grp94 promoter mutants showed that two regions in the promoter, a highly conserved element referred to as grp core and the CCAAT element most proximal to the TATA element (C1), mediate the induction of grp94 by malfolded protein. We further determined that the grp94 core and C1 element bind to common as well distinct nuclear factors from grp78, a commonly coregulated gene. Through UV cross-linking, site competition, and immunocross-reactivity, we identified that the heteromeric CCAAT-binding protein (CBF) is one component of the grp94 C1 complex.

Virus-neutralization domains on the oligomeric (230 kDa) forms of antigen B of herpesvirus of turkeys and Marek's disease virus differ in cross-serotypic activity

Herpesvirus of turkeys (HVT) is frequently used to protect chickens against Marek's disease (MD). The HVT and MDV native antigen B complex shares common epitopes. To determine whether these oligomers present virus-neutralizing domains, monospecific antibodies to the HVT and MDV native 230 kDa oligomers were produced. The monospecific antibody immunopurified from an anti-HVT avian serum neutralized the in vitro infectivity of the oncogenic isolate MDV-B and the vaccine strains CVI988, SB1 and HVT and immunoblotted the 230 kDa oligomers of HVT and CVI988. As a result of the immunofluorescence analysis on infected cells, the monospecific antibody revealed foci of diffuse cytoplasmic immunofluorescence. A second monospecific antibody to the heat-stable 130 kDa monomer of HVT had limited neutralizing activity against HVT and CVI988 only, immunoblotted only the native HVT oligomer, and was not active in immunofluorescence. The monospecific antibody to the MDV-B 230 kDa oligomer neutralized and immunoblotted only the two MDV-1 strains but stained cells infected with MDVs of the three serotypes in immunofluorescence. It is concluded that the cross-protective neutralizing epitopes of HVT are located on heat-labile oligomeric forms of antigen B.

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.

Common antigenic epitopes are present on heat-labile oligomers of MDV glycoprotein B and on HSV glycoprotein B

The antigenic cross-reactivity between the Marek's disease virus glycoprotein B (MDV gB) and glycoprotein B (gB) of herpes simplex virus type 1 and 2 (HSV1 and HSV2) was analysed by the immunoblotting method. We studied cell lysates in both denatured and in undenatured form (i.e., unheated) and reacted them with convalescent sera from chickens infected with the RBIB MDV strain and with human anti-HSV1 gB. Both sera detected the heat-labile MDV gB and the HSV gB oligomers. In addition, monospecific antibodies to the MDV gB 230 kDa oligomer (strain CVI988) were immunoaffinity purified from both the chicken and the human sera. The chicken and human monospecific antibodies detected the homologous and the heterologous gB oligomers in native MDV- and HSV1-infected cell lysates. 15 human sera were tested by immunoblotting and by immunofluorescence on HSV1-, CVI988-and herpes virus of turkeys (HVT)-infected cells. By both assays about half of the human sera reacted with MDV-infected cells. This study demonstrates that the MDV gB heat-labile oligomers possess conformational epitopes shared with the human alpha-herpes virus HSV1 and HSV2 gB heat-labile oligomers.

Glycoprotein B of bovine herpesvirus type 4: its phylogenetic relationship to gB equivalents of the herpesviruses

In order to estimate the phylogenetic relationship of BHV-4 among the herpesviruses, we have cloned and sequenced its glycoprotein B (gB). The 2.6 kb open reading frame codes for a 874 amino acid long protein. The comparison of its deduced amino acid sequence with those of its counterparts in 19 distinct herpesviruses groups BHV-4 into the gamma-herpesvirinae. The calculation of an evolutionary tree emphasized that BHV-4 is more closely related to herpesvirus saimiri (HVS) than to Epstein-Barr virus (EBV). However, in contrast to EBV and HVS, the gB of BHV-4 contains a putative protease cleavage site and 20 potential N-glycosylation sites. The alignment of the amino acid sequences revealed that 10 cysteine and 7 proline residues, as well as the motifs SPF and GQLG, were completely conserved among the 20 investigated gBs.

Truncation of the carboxy-terminal 28 amino acids of glycoprotein B specified by herpes simplex virus type 1 mutant amb1511-7 causes extensive cell fusion

Three amber mutations were introduced proximal to the syn3 locus of the herpes simplex virus type 1 glycoprotein B (gB) gene specifying gB derivatives lacking the carboxy-terminal 28, 49, or 64 amino acids. A complementation system that utilized gBs expressed in COS cells to complement gB-null virus K delta T was established. The 49- or 64-amino-acid-truncated gBs failed to complement gB-null virus K delta T, while the 28-amino-acid-truncated gB complemented K delta T efficiently. Mutant herpes simplex virus type 1 KOS (amb1511-7) specifying the 28-amino-acid-truncated gB fused Vero cells extensively.

Computer prediction of antigenic and topogenic domains in HSV-1 and HSV-2 glycoprotein B (gB)

The envelope glycoprotein B (gB) coded for by the herpes simplex virus type 1 (HSV-1) UL27 gene is similar to the amino acid (aa) sequence of the gB coded by a homologous gene in HSV-2 DNA. The putative antigenic domains in HSV-1 and HSV-2 gB glycoproteins were analyzed on a comparative basis by suitable computer programs, which allowed the prediction of putative antigenic and topogenic domains. The computer-derived domains were compared to experimentally reported antigenic domains in HSV-1 gB glycoprotein. The computer-predicted antigenic domains in the HSV-1 gB glycoprotein matched well with the reported experimentally derived antigenic domains. The aa sequence of antigenic domain 1 was noted to resemble the amino acid sequence in ApoE that is involved in the attachment of this protein to LDL receptors. The clusters of hydrophobic aa domains are conserved in the two viral glycoproteins and are signals for transfer of the viral proteins through the cellular membrane.

The dominant linear neutralizing antibody-binding site of glycoprotein gp86 of human cytomegalovirus is strain specific

Bacterial fusion proteins, constructed from overlapping fragments of the open reading frame coding for gp86 of human cytomegalovirus (HCMV) strain AD169, were used to localize antigenic regions recognized by antibodies from human convalescent sera. A major domain for binding of conformation-independent antibodies was localized on fusion protein AP86, containing amino acids 15 to 142 of gp86. Human antibodies, affinity purified on AP86, neutralized infectious virus in tissue culture. In addition, a mouse monoclonal antibody (AP86-SA4), raised against AP86, also neutralized HCMV. AP86-SA4 was reactive with viral gp86 in immunoblot assays and showed a plasma membrane staining on intact HCMV-infected fibroblasts late in infection. After exonuclease III deletions of the viral gene, the binding site of neutralizing human as well as mouse antibodies was localized between amino acid residues 34 and 43. The domain has sequence variation between laboratory strains AD169 and Towne, and binding of the antibodies was strain specific. To our knowledge, this is the first characterization of a strain-specific neutralizing epitope on HCMV.