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

KSHV (HHV8) vaccine: promises and potential pitfalls for a new anti-cancer vaccine

Seven viruses cause at least 15% of the total cancer burden. Viral cancers have been described as the "low-hanging fruit" that can be potentially prevented or treated by new vaccines that would alter the course of global human cancer. Kaposi sarcoma herpesvirus (KSHV or HHV8) is the sole cause of Kaposi sarcoma, which primarily afflicts resource-poor and socially marginalized populations. This review summarizes a recent NIH-sponsored workshop's findings on the epidemiology and biology of KSHV as an overlooked but potentially vaccine-preventable infection. The unique epidemiology of this virus provides opportunities to prevent its cancers if an effective, inexpensive, and well-tolerated vaccine can be developed and delivered.

Viral glycoproteomes: technologies for characterization and outlook for vaccine design

It has long been known that surface proteins of most enveloped viruses are covered with glycans. It has furthermore been demonstrated that glycosylation is essential for propagation and immune evasion for many viruses. The recent development of high-resolution mass spectrometry techniques has enabled identification not only of the precise structures but also the positions of such post-translational modifications on viruses, revealing substantial differences in extent of glycosylation and glycan maturation for different classes of viruses. In-depth characterization of glycosylation and other post-translational modifications of viral envelope glycoproteins is essential for rational design of vaccines and antivirals. In this Review, we provide an overview of techniques used to address viral glycosylation and summarize information on glycosylation of enveloped viruses representing ongoing public health challenges. Furthermore, we discuss how knowledge on glycosylation can be translated to means to prevent and combat viral infections.

Global aspects of viral glycosylation

Enveloped viruses encompass some of the most common human pathogens causing infections of different severity, ranging from no or very few symptoms to lethal disease as seen with the viral hemorrhagic fevers. All enveloped viruses possess an envelope membrane derived from the host cell, modified with often heavily glycosylated virally encoded glycoproteins important for infectivity, viral particle formation and immune evasion. While N-linked glycosylation of viral envelope proteins is well characterized with respect to location, structure and site occupancy, information on mucin-type O-glycosylation of these proteins is less comprehensive. Studies on viral glycosylation are often limited to analysis of recombinant proteins that in most cases are produced in cell lines with a glycosylation capacity different from the capacity of the host cells. The glycosylation pattern of the produced recombinant glycoproteins might therefore be different from the pattern on native viral proteins. In this review, we provide a historical perspective on analysis of viral glycosylation, and summarize known roles of glycans in the biology of enveloped human viruses. In addition, we describe how to overcome the analytical limitations by using a global approach based on mass spectrometry to identify viral O-glycosylation in virus-infected cell lysates using the complex enveloped virus herpes simplex virus type 1 as a model. We underscore that glycans often pay important contributions to overall protein structure, function and immune recognition, and that glycans represent a crucial determinant for vaccine design. High throughput analysis of glycosylation on relevant glycoprotein formulations, as well as data compilation and sharing is therefore important to identify consensus glycosylation patterns for translational applications.

The Neutralizing Linear Epitope of Human Herpesvirus 6A Glycoprotein B Does Not Affect Virus Infectivity

Human herpesvirus 6A (HHV-6A) glycoprotein B (gB) is a glycoprotein consisting of 830 amino acids and is essential for the growth of the virus. Previously, we reported that a neutralizing monoclonal antibody (MAb) called 87-y-13 specifically reacts with HHV-6A gB, and we identified its epitope residue at asparagine (Asn) 347 on gB. In this study, we examined whether the epitope recognized by the neutralizing MAb is essential for HHV-6A infection. We constructed HHV-6A bacterial artificial chromosome (BAC) genomes harboring substitutions at Asn347, namely, HHV-6A BACgB(N347K) and HHV-6A BACgB(N347A). These mutant viruses could be reconstituted and propagated in the same manner as the wild type and their revertants, and MAb 87-y-13 could not inhibit infection by either mutant. In a cell-cell fusion assay, Asn at position 347 on gB was found to be nonessential for cell-cell fusion. In addition, in building an HHV-6A gB homology model, we found that the epitope of the neutralizing MAb is located on domain II of gB and is accessible to solvents. These results indicate that Asn at position 347, the linear epitope of the neutralizing MAb, does not affect HHV-6A infectivity.IMPORTANCE Glycoprotein B (gB) is one of the most conserved glycoproteins among all herpesviruses and is a key factor for virus entry. Therefore, antibodies targeted to gB may neutralize virus entry. Human herpesvirus 6A (HHV-6A) encodes gB, which is translated to a protein of about 830 amino acids (aa). Using a monoclonal antibody (MAb) for HHV-6A gB, which has a neutralizing linear epitope, we analyzed the role of its epitope residue, N347, in HHV-6A infectivity. Interestingly, this gB linear epitope residue, N347, was not essential for HHV-6A growth. By constructing a homology model of HHV-6A gB, we found that N347 was located in the region corresponding to domain II. Therefore, with regard to its neutralizing activity against HHV-6A infection, the epitope on gB might be exposed to solvents, suggesting that it might be a target of the immune system.

Conservation of the glycoprotein B homologs of the Kaposi׳s sarcoma-associated herpesvirus (KSHV/HHV8) and old world primate rhadinoviruses of chimpanzees and macaques

The envelope-associated glycoprotein B (gB) is highly conserved within the Herpesviridae and plays a critical role in viral entry. We analyzed the evolutionary conservation of sequence and structural motifs within the Kaposi׳s sarcoma-associated herpesvirus (KSHV) gB and homologs of Old World primate rhadinoviruses belonging to the distinct RV1 and RV2 rhadinovirus lineages. In addition to gB homologs of rhadinoviruses infecting the pig-tailed and rhesus macaques, we cloned and sequenced gB homologs of RV1 and RV2 rhadinoviruses infecting chimpanzees. A structural model of the KSHV gB was determined, and functional motifs and sequence variants were mapped to the model structure. Conserved domains and motifs were identified, including an "RGD" motif that plays a critical role in KSHV binding and entry through the cellular integrin αVβ3. The RGD motif was only detected in RV1 rhadinoviruses suggesting an important difference in cell tropism between the two rhadinovirus lineages.

KSHV 2.0: a comprehensive annotation of the Kaposi's sarcoma-associated herpesvirus genome using next-generation sequencing reveals novel genomic and functional features

Productive herpesvirus infection requires a profound, time-controlled remodeling of the viral transcriptome and proteome. To gain insights into the genomic architecture and gene expression control in Kaposi's sarcoma-associated herpesvirus (KSHV), we performed a systematic genome-wide survey of viral transcriptional and translational activity throughout the lytic cycle. Using mRNA-sequencing and ribosome profiling, we found that transcripts encoding lytic genes are promptly bound by ribosomes upon lytic reactivation, suggesting their regulation is mainly transcriptional. Our approach also uncovered new genomic features such as ribosome occupancy of viral non-coding RNAs, numerous upstream and small open reading frames (ORFs), and unusual strategies to expand the virus coding repertoire that include alternative splicing, dynamic viral mRNA editing, and the use of alternative translation initiation codons. Furthermore, we provide a refined and expanded annotation of transcription start sites, polyadenylation sites, splice junctions, and initiation/termination codons of known and new viral features in the KSHV genomic space which we have termed KSHV 2.0. Our results represent a comprehensive genome-scale image of gene regulation during lytic KSHV infection that substantially expands our understanding of the genomic architecture and coding capacity of the virus.

Kaposi's sarcoma-associated herpesvirus (KSHV) is a cancer-causing agent in immunocompromised patients that establishes long-lasting infections in its hosts. Initially described in 1994 and extensively studied ever since, KSHV molecular biology is understood in broad outline, but many detailed questions are still to be resolved. After almost two decades, specific aspects pertaining to the organization of the KSHV genome as well as the fate of the viral transcripts during the productive stages of infection remain unexplored. Here we use a systematic genome-wide approach to investigate changes in gene and protein expression during the productive stage of infection known as the lytic cycle. We found that the viral genome has a large coding capacity, capable of generating at least 45% more products than initially anticipated by bioinformatic analyses alone, and that it uses multiple strategies to expand its coding capacity well beyond what is determined solely by the DNA sequence of its genome. We also provide an expanded and highly detailed annotation of known and new genomic features in KSHV. We have termed this new architectural and functional annotation KSHV 2.0. Our results indicate that viral genomes are more complex than anticipated, and that they are subject to tight mechanisms of regulation to ensure correct gene expression.

O-GlcNAc transferase inhibits KSHV propagation and modifies replication relevant viral proteins as detected by systematic O-GlcNAcylation analysis

O-GlcNAcylation is an inducible, highly dynamic and reversible post-translational modification, mediated by a unique enzyme named O-linked N-acetyl-d-glucosamine (O-GlcNAc) transferase (OGT). In response to nutrients, O-GlcNAc levels are differentially regulated on many cellular proteins involved in gene expression, translation, immune reactions, protein degradation, protein-protein interaction, apoptosis and signal transduction. In contrast to eukaryotic cells, little is known about the role of O-GlcNAcylation in the viral life cycle. Here, we show that the overexpression of the OGT reduces the replication efficiency of Kaposi's sarcoma-associated herpesvirus (KSHV) in a dose-dependent manner. In order to investigate the global impact of O-GlcNAcylation in the KSHV life cycle, we systematically analyzed the 85 annotated KSHV-encoded open reading frames for O-GlcNAc modification. For this purpose, an immunoprecipitation (IP) strategy with three different approaches was carried out and the O-GlcNAc signal of the identified proteins was properly controlled for specificity. Out of the 85 KSHV-encoded proteins, 18 proteins were found to be direct targets for O-GlcNAcylation. Selected proteins were further confirmed by mass spectrometry for O-GlcNAc modification. Correlation of the functional annotation and the O-GlcNAc status of KSHV proteins showed that the predominant targets were proteins involved in viral DNA synthesis and replication. These results indicate that O-GlcNAcylation plays a major role in the regulation of KSHV propagation.

Bovine herpesvirus 4 glycoprotein B is indispensable for lytic replication and irreplaceable by VSVg

BACKGROUND

Bovine herpesvirus 4 (BoHV-4) is a gammaherpesvirus, belonging to Rhadinovirus genus, with no clear association with disease. However, there is increasing evidence of its secondary pathogenic role in cases of post-partum metritis in cattle. BoHV-4 Open Reading Frame 8 (ORF8) codifies for glycoprotein B (gB) that shows a heterodimeric structure, composed of two subunits and covalently linked by disulfide bonds and responsible for host cell adhesion through binding to heparan sulfates associated with cellular proteoglycans. Here we describe the generation of several tagged soluble forms of gB ectodomain, in order to test their ability to neutralize BoHV-4 infection.

RESULTS

The results show, however, that none of these soluble forms are able to block viral infectivity. To better understand the role of gB during BoHV-4 lytic replication, a recombinant BoHV-4 was generated by homologous recombination from a BoHV-4 cloned as a Bacterial artificial chromosome (BAC) (pBAC-BoHV-4-A), in which most of the BoHV-4 gB ORF was substituted by the insertion of a DNA stuffer selectable cassette. The resulting recombinant BoHV-4 genome (pBAC-BoHV-4-AΔgB-KanaGalK) was completely unable to reconstitute infectious replicating viral particles (Infectious Replicating Viral Particles, IRVPs) and to replicate when transfected in permissive cell lines in comparison to its revertant clone (pBAC-BoHV-4-ΔgB-Rev) or pBAC-BoHV-4-A parental clone.

CONCLUSION

This demonstrates that the BoHV-4 replicating cycle is dependent on gB. Moreover, when gB was deleted from a recombinant BoHV-4 genome delivering an heterologous glycoprotein, Vesicular Stomatitis Virus Glycoprotein (VSVg), VSVg was unable to complement gB. This study provides direct evidence that gB is necessary for BoHV-4 lytic replication.

Kaposi's sarcoma-associated herpesvirus glycoproteins B and K8.1 regulate virion egress and synthesis of vascular endothelial growth factor and viral interleukin-6 in BCBL-1 cells

Kaposi's sarcoma-associated herpesvirus (KSHV) viral glycoproteins play important roles in the infectious life cycle and have been implicated in KSHV-associated endothelial cell transformation, angiogenesis, and KS-induced malignancies. KSHV-associated primary effusion lymphomas (PELs) secrete high levels of vascular endothelial growth factor (VEGF) and viral interleukin-6 (vIL-6) in vitro and VEGF, vIL-6, and basic-fibroblast growth factor (b-FGF) in mouse xenografts. KSHV-encoded glycoproteins B (gB) and K8.1 stimulate VEGF secretion, most likely mediated by direct or indirect binding to cell surface receptors, including the gB-specific alphaVbeta3 and alpha3beta1 integrins. In this study, the short interfering RNA (siRNA)-mediated inhibition of either gB or K8.1 transcription by anti-gB or -K8.1 siRNAs caused a substantial reduction in virion egress and a decrease in both vIL-6 and VEGF production. Similarly, the treatment of BCBL-1 cells with anti-gB or anti-K8.1 antibodies caused a substantial reduction in vIL-6 and VEGF production. Codon-optimized versions of either wild-type gB, mutant gB having the RGD amino acid motif changed to RAA, or K8.1 efficiently rescued virion egress and VEGF and vIL-6 production. These results suggest that the binding of gB via its RGD motif to integrin receptors was not responsible for the observed gB-associated regulation of VEGF and vIL-6 transcription. Conditioned medium collected from BCBL-1 cells transfected with anti-gB and anti-K8.1 siRNAs or treated with anti-gB and anti-K8.1 antibodies exhibited a significantly reduced ability to induce the formation of the capillary network of endothelial cells compared to the ability of medium from mock-infected BCBl-1 cells. Furthermore, medium obtained from BCBL-1 cells expressing smaller amounts of gB and K8.1 produced a substantial reduction in endothelial cell migration in a vertical migration assay compared to that of control medium containing wild-type levels of gB and K8.1. These results suggest a functional linkage between gB/K8.1 synthesis and VEGF/vIL-6 transcriptional regulation via paracrine and/or autocrine signaling pathways.

The cytoplasmic terminus of Kaposi's sarcoma-associated herpesvirus glycoprotein B is not essential for virion egress and infectivity

Kaposi's sarcoma-associated herpesvirus (KSHV)-encoded glycoprotein B (gB) is an important determinant of viral infectivity and virion egress. A small interfering RNA (siRNA)-based strategy was devised to inhibit KSHV gB gene expression. Transient cotransfection of plasmids constitutively expressing gB and anti-gB siRNAs in 293 cells substantially inhibited gB mRNA levels and protein production. Similarly, transient expression of siRNAs into the primary effusion lymphoma cell line BCBL-1 caused a substantial reduction of gB transcripts and protein synthesis. TaqMan real-time PCR assays against the lytic KSHV gene ORF59 and infectivity assays on 293 cells were employed to assess the effect of inhibiting gB synthesis on virion egress from BCBL-1 cells and infectivity on 293 cells, respectively. These experiments showed that gB was essential for virion egress and infectivity. Transfection of a codon-optimized gB gene with the first 540 nucleotides altered, and therefore not recognized by anti-gB siRNAs that target the native but not the codon-optimized sequence, efficiently rescued virion egress and infectivity in BCBL-1 cells in the presence of siRNAs inhibiting wild-type gB expression. To assess the role of the cytoplasmic domain of gB in virion egress, mutant gB genes were generated specifying carboxyl terminal truncations of 25 and 58 amino acids disrupting two prominent predicted alpha-helical domains associated with virus-induced cell fusion. A third truncation removed the entire predicted cytoplasmic terminus of 84 amino acids, while a fourth truncation removed 110 amino acids, including the terminal most hydrophobic, intramembrane anchoring sequence. Virion egress experiments revealed that all truncated gBs facilitated virion egress from BCBL-1 cells, with the exception of the largest 110-amino-acid truncation, which removed the gB anchoring sequence. Importantly, the gB truncation that removed the entire predicted cytoplasmic domain increased virion egress, suggesting the presence of a egress regulation domain located proximal to the intramembrane sequence within the cytoplasmic domain of gB. All supernatant virions were infectious on 293 cells, indicating that the carboxyl terminus of gB is not essential for either virion egress or virus infectivity.

Intracellular localization map of human herpesvirus 8 proteins

Human herpesvirus 8 (HHV-8) is the etiological agent of Kaposi's sarcoma. We present a localization map of 85 HHV-8-encoded proteins in mammalian cells. Viral open reading frames were cloned with a Myc tag in expression plasmids, confirmed by full-length sequencing, and expressed in HeLa cells. Protein localizations were analyzed by immunofluorescence microscopy. Fifty-one percent of all proteins were localized in the cytoplasm, 22% were in the nucleus, and 27% were found in both compartments. Surprisingly, we detected viral FLIP (v-FLIP) in the nucleus and in the cytoplasm, whereas cellular FLIPs are generally localized exclusively in the cytoplasm. This suggested that v-FLIP may exert additional or alternative functions compared to cellular FLIPs. In addition, it has been shown recently that the K10 protein can bind to at least 15 different HHV-8 proteins. We noticed that K10 and only five of its 15 putative binding factors were localized in the nucleus when the proteins were expressed in HeLa cells individually. Interestingly, in coexpression experiments K10 colocalized with 87% (13 of 15) of its putative binding partners. Colocalization was induced by translocation of either K10 alone or both proteins. These results indicate active intracellular translocation processes in virus-infected cells. Specifically in this framework, the localization map may provide a useful reference to further elucidate the function of HHV-8-encoded genes in human diseases.

Kaposi's sarcoma-associated herpesvirus ORF57 protein enhances mRNA accumulation independently of effects on nuclear RNA export

The ORF57 protein expressed by Kaposi's sarcoma-associated herpesvirus (KSHV) during lytic replication is essential for KSHV virion production. ORF57 enhances gene expression by increasing accumulation of target gene mRNAs. ORF57 interacts with the cellular export factor REF and with RNA, suggesting that it may provide target mRNAs with access to REF, which mediates nuclear RNA export by binding to TAP/NXF1. A mutational analysis of ORF57 was performed to study the role of REF binding, RNA interaction, and multimerization in ORF57 function. ORF57 was shown to directly bind RNA. The ability to bind REF did not correlate with ORF57 function in enhancing mRNA accumulation. ORF57 enhanced the nuclear levels of mRNA and PAN, a nuclear KSHV RNA, and the activity of various ORF57 mutants on the levels of mRNA paralleled their ability to enhance nuclear PAN accumulation, suggesting that ORF57 may also act on messenger RNAs by export-independent effects on RNA stability. Finally, an ORF57 mutant lacking a region homologous to a nucleolar localization signal in herpesvirus saimiri was constructed. This mutant retained function, demonstrating that, unlike the ORF57 homolog in herpesvirus saimiri, nucleolar trafficking is not required for ORF57 function in enhancing mRNA accumulation.

Hydrophobic residues that form putative fusion loops of Epstein-Barr virus glycoprotein B are critical for fusion activity

To test the importance of the hydrophobic residues within the putative Epstein-Barr virus (EBV) glycoprotein B (gB) fusion loops in membrane fusion, WY(112-113) and WLIW(193-196) were mutated into alanine, glutamic acid, or the analogous residues from herpes simplex virus type 1 (HSV-1) gB (HR and RVEA). All gB variants exhibited cell surface expression, demonstrating that the substitutions did not perturb gB trafficking. None of six gB variants was, however, capable of mediating fusion with either epithelial or B cells. These data demonstrate that the bulky and hydrophobic EBV loop residues, which differ from the more hydrophilic HSV-1 residues and appear more compatible with membrane insertion, are essential for EBV gB-dependent fusion.

Functional homology of gHs and gLs from EBV-related gamma-herpesviruses for EBV-induced membrane fusion

Epstein-Barr virus (EBV) is a human gamma-herpesvirus that primarily infects B lymphocytes and epithelial cells. Entry of EBV into B cells requires the viral glycoproteins gp42, gH/gL and gB, while gp42 is not necessary for infection of epithelial cells. In EBV, gH and gL form two distinct complexes, a bipartite complex that contains only gH and gL, used for infection of epithelial cells, and a tripartite complex that additionally includes gp42, used for infection of B cells. The gH/gL complex is conserved within the herpesvirus family, but its exact role in entry and mechanism of fusion is not yet known. To understand more about the functionality of EBVgH/gL, we investigated the functional homology of gHs and gLs from human herpesvirus 8 (HHV8) and two primate (rhesus and marmoset) gamma-herpesviruses in EBV-mediated virus-free cell fusion assay. Overall, gHs and gLs from the more homologous primate herpesviruses were better at complementing EBV gH and gL in fusion than HHV8 gH and gL. Interestingly, marmoset gH was able to complement fusion with epithelial cells, but not B cells. Further investigation of this led to the discovery that EBVgH is the binding partner of gp42 in the tripartite complex and the absence of fusion with B cells in the presence of marmoset gH/gL is due to its inability to bind gp42.

Mass spectrometric analyses of purified rhesus monkey rhadinovirus reveal 33 virion-associated proteins

The repertoire of proteins that comprise intact gammaherpesviruses, including the human pathogen Kaposi's sarcoma-associated herpesvirus (KSHV), is likely to have critical functions not only in viral structure and assembly but also in the early stages of infection and evasion of the host's rapidly deployed antiviral defenses. To develop a better understanding of these proteins, we analyzed the composition of rhesus monkey rhadinovirus (RRV), a close phylogenetic relative of KSHV. Unlike KSHV, RRV replicates to high titer in cell culture and thus serves as an effective model for studying primate gammaherpesvirus structure and virion proteomics. We employed two complementary mass spectrometric approaches and found that RRV contains at least 33 distinct virally encoded proteins. We have assigned 7 of these proteins to the capsid, 17 to the tegument, and 9 to the envelope. Of the five gammaherpesvirus-specific tegument proteins, three have no known function. We also found three proteins not previously associated with a purified herpesvirus and an additional seven that represent new findings for a member of the gamma-2 herpesviruses. Detergent extraction resulted in particles that contained six distinct tegument proteins in addition to the expected capsid structural proteins, suggesting that this subset of tegument components may interact more directly with or with higher affinity for the underlying capsid and, in turn, may play a role in assembly or transport of viral or subviral particles during entry or egress.

Kaposi's sarcoma-associated herpesvirus (human herpesvirus 8) infection of human fibroblast cells occurs through endocytosis

Kaposi's sarcoma (KS)-associated herpesvirus or human herpesvirus 8 (HHV-8) DNA and transcripts have been detected in the B cells, macrophages, keratinocytes, and endothelial and epithelial cells of KS patients. In vitro, HHV-8 infects human B, endothelial, epithelial, and fibroblast cells, as well as animal cells, and the infection is characterized by (i) absence of lytic replication by the input virus and (ii) latent infection. For its initial binding to target cells, HHV-8 uses ubiquitous heparan sulfate molecules via its envelope-associated glycoproteins gB and gpK8.1A. HHV-8 also interacts with the alpha3beta1 integrin via its glycoprotein gB, and virus binding studies suggest that alpha3beta1 is one of the HHV-8 entry receptors (S. M. Akula, N. P. Pramod, F. Z. Wang, and B. Chandran, Cell 108:407-419, 2002). In this study, morphological and biochemical techniques were used to examine the entry of HHV-8 into human foreskin fibroblasts (HFF). HHV-8 was detected in coated vesicles and in large, smooth-surfaced endocytic vesicles. Fusion of viral envelope with the vesicle wall was also observed. In immune electron microscopy, anti-HHV-8 gB antibodies colocalized with virus-containing endocytic vesicles. In fluorescence microscopic analyses, transferrin was colocalized with HHV-8. HHV-8 infection was significantly inhibited by preincubation of cells with chlorpromazine HCl, which blocks endocytosis via clathrin-coated pits, but not by nystatin and cholera toxin B, which blocks endocytosis via caveolae and induces the dissociation of lipid rafts, respectively. Infection was also inhibited by blocking the acidification of endosomes by NH(4)Cl and bafilomycin A. Inhibition of HHV-8 open reading frame 73 gene expression by chlorpromazine HCl, bafilomycin A, and NH(4)Cl demonstrated that the virions in the vesicles could proceed to cause an infection. Taken together, these findings suggest that for its infectious entry into HFF, HHV-8 uses clathrin-mediated endocytosis and a low-pH intracellular environment.

Human herpesvirus 8 glycoprotein B (gB), gH, and gL can mediate cell fusion

Herpesvirus entry into cells and herpesvirus-induced cell fusion are related processes in that virus penetration proceeds by fusion of the viral envelope and cell membrane. To characterize the human herpesvirus 8 (HHV-8) glycoproteins that can mediate cell fusion, a luciferase reporter gene activation assay was used. Chinese hamster ovary (CHO) cells expressing the HHV-8 glycoproteins of interest along with a luciferase reporter gene under the control of the T7 promoter were cocultivated with human cells transfected with T7 RNA polymerase. Because HHV-8 glycoprotein B (gB) expressed in CHO cells localizes to the perinuclear region, a truncated form of gB (designated gB(MUT)) that lacks putative endocytosis signals was constructed by deletion of the distal 58 amino acids of the cytoplasmic tail. HHV-8 gB(MUT) was expressed efficiently on the surface of CHO cells. HHV-8 gB, gH, and gL could mediate the fusion of CHO cells with two different human cell types, embryonic kidney cells and B lymphocytes. Substituting gB(MUT) for gB significantly enhanced the fusion of CHO cells with human embryonic kidney cells but not B lymphocytes. Thus, two human cell types known to be susceptible to HHV-8 entry were also suitable targets for cell fusion induced by HHV-8 gB, gH, and gL. For human embryonic kidney cells and B cells at least, optimal fusion was noted with the expression of all three HHV-8 glycoproteins.

Human herpesvirus 8 envelope-associated glycoprotein B interacts with heparan sulfate-like moieties

Cell-surface heparan sulfate (HS) serves as an initial attachment receptor for several herpesviruses. The gamma2-human herpesvirus-8 (HHV-8) or Kaposi's sarcoma associated herpesvirus DNA and transcripts have been detected in B cells, endothelial cells, macrophages, and epithelial cells. HHV-8 infects a variety of human and animal cell lines leading to latent or abortive infection. Our studies showed that this broad cellular tropism may be in part due to HHV-8's interaction with the ubiquitous host cell-surface HS-like molecules. HHV-8 binding to the target cells and the infection were inhibited by soluble heparan, a glycosaminoglycan (GAG) closely related to HS. Since HHV-8 gB possess a putative heparan-binding domain (HBD) in the extracellular domain, the interaction of gB with HS-like moieties was examined. Unlike gB of gamma1-Epstein-Barr virus and gamma2-murine herpesvirus 68, HHV-8 gB was expressed on the surface of the infected cell membranes and virion envelopes. Envelope-associated gB was made up of 75 and 54 kDa polypeptides forming disulfide-linked heterodimers and multimers. Rabbit anti-gB antibodies neutralized HHV-8 infection. Virion envelope-associated gB specifically bound to heparan-agarose, which was eluted by high concentration of soluble heparan, but not by chondroitin sulfates. In vitro transcribed and translated products of gB gene specifically bound to heparan-agarose beads, which was blocked by HS and heparan, but not by other GAGs such as chondroitin sulfates (A, B, and C), N-acetyl heparan, and de-N-sulfated heparan. Biotinylated gB peptide corresponding to the putative HBD also bound to heparan. These results suggest that gB plays an important role in the infectious process of HHV-8 and virus interaction with cell-surface HS-like moieties could be in part mediated by the envelope-associated gB.

Glycoprotein B of human herpesvirus 8 is a component of the virion in a cleaved form composed of amino- and carboxyl-terminal fragments

Human herpesvirus 8 (HHV-8) or Kaposi's sarcoma-associated herpesvirus (KSHV) is the only known human member of the Rhadinovirus genus of the gammaherpesvirus subfamily. Antibodies against peptides representing portions of the amino- and carboxyl-termini of HHV-8 gB were produced and used to detect gB expression in Vero cells transfected with the gB gene, in the HHV-8-harboring cell line, BCBL-1, and in purified virions. Expression of gB was detected in approximately 3% of uninduced BCBL-1 cells, while up to 30% of the cells expressed gB after 12-O-tetradecanoylphorbol-13-acetate (TPA) induction of virus replication. Indirect immunofluorescence assays and confocal microscopy showed that gB was distributed throughout the cytoplasm of BCBL-1 cells and transfected Vero cells. Immunoblot analyses of virion preparations revealed the presence of full-length as well as two smaller than full-length gB-derived species corresponding to the amino- and carboxy-terminal portions of gB, respectively. Biochemical analysis of the gB carbohydrate moieties using glycosylation inhibitors revealed that gB contained N-linked oligosaccharides of the high-mannose type, characteristic of precursor carbohydrate chains added in the endoplasmic reticulum.