Epstein-Barr virus gH is essential for penetration of B cells but also plays a role in attachment of virus to epithelial cells

Epstein Barr virus entry; kissing and conjugation

Epstein Barr virus (EBV) is a highly prevalent human gamma 1 lymphocryptovirus which infects both B lymphocytes and epithelial cells. In the healthy host, infection of these different cell lineages broadly reflects the different phases of the virus lifecycle. Memory B cells are the reservoir for latent EBV, in which viral gene expression is highly restricted to maintain an asymptomatic lifelong infection. In contrast, epithelial cells may be a major site of the virus lytic cycle, where infectious virus is propagated and transmitted via saliva to uninfected hosts. To achieve this dual tropism, EBV has evolved a unique set of glycoproteins in addition to a highly conserved set, which interact with cell lineage-specific receptors and switch cellular tropism during infection.

Tousled-like kinases modulate reactivation of gammaherpesviruses from latency

Kaposi's sarcoma-associated herpesvirus (KSHV) is linked to human malignancies. The majority of tumor cells harbor latent virus, and a small percentage undergo spontaneous lytic replication. Both latency and lytic replication are important for viral pathogenesis and spread, but the cellular players involved in the switch between the two viral life-cycle phases are not clearly understood. We conducted a small interfering RNA (siRNA) screen targeting the cellular kinome and identified Tousled-like kinases (TLKs) as cellular kinases that control KSHV reactivation from latency. Upon treatment of latent KSHV-infected cells with siRNAs targeting TLKs, we saw robust viral reactivation. Knockdown of TLKs in latent KSHV-infected cells induced expression of viral lytic proteins and production of infectious virus. TLKs were also found to play a role in regulating reactivation from latency of another related oncogenic gammaherpesvirus, Epstein-Barr virus. Our results establish the TLKs as cellular repressors of gammaherpesvirus reactivation.

Epstein-Barr virus-associated lymphoid malignancies: the expanding spectrum of hematopoietic neoplasms

Ubiquitous Epstein-Barr virus (EBV) infects not only B cells but also T and NK cells, and is associated with various lymphoid malignancies. The spectrum of EBV-associated lymphoid malignancies is expanding from Burkitt lymphoma to the newly defined systemic EBV+ T cell lymphoproliferative disease of childhood and hydroa vacciniforme-like lymphoma. However, some EBV-associated malignancies are not defined well and overlap other diseases. Furthermore, the role of EBV in tumorigenesis of lymphoid malignancies is still not clear. Further studies are necessary to clarify the pathogenesis of EBV-associated lymphoid malignancies for a better classification of each disease and for the establishment of effective treatment.

Hsp90 inhibitor 17-DMAG decreases expression of conserved herpesvirus protein kinases and reduces virus production in Epstein-Barr virus-infected cells

All eight human herpesviruses have a conserved herpesvirus protein kinase (CHPK) that is important for the lytic phase of the viral life cycle. In this study, we show that heat shock protein 90 (Hsp90) interacts directly with each of the eight CHPKs, and we demonstrate that an Hsp90 inhibitor drug, 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG), decreases expression of all eight CHPKs in transfected HeLa cells. 17-DMAG also decreases expression the of the endogenous Epstein-Barr virus protein kinase (EBV PK, encoded by the BGLF4 gene) in lytically infected EBV-positive cells and inhibits phosphorylation of several different known EBV PK target proteins. Furthermore, 17-DMAG treatment abrogates expression of the human cytomegalovirus (HCMV) kinase UL97 in HCMV-infected human fibroblasts. Importantly, 17-DMAG treatment decreased the EBV titer approximately 100-fold in lytically infected AGS-Akata cells without causing significant cellular toxicity during the same time frame. Increased EBV PK expression in 17-DMAG-treated AGS-Akata cells did not restore EBV titers, suggesting that 17-DMAG simultaneously targets multiple viral and/or cellular proteins required for efficient viral replication. These results suggest that Hsp90 inhibitors, including 17-DMAG, may be a promising group of drugs that could have profound antiviral effects on herpesviruses.

Epstein-Barr virus transcytosis through polarized oral epithelial cells

Although Epstein-Barr virus (EBV) is an orally transmitted virus, viral transmission through the oropharyngeal mucosal epithelium is not well understood. In this study, we investigated how EBV traverses polarized human oral epithelial cells without causing productive infection. We found that EBV may be transcytosed through oral epithelial cells bidirectionally, from both the apical to the basolateral membranes and the basolateral to the apical membranes. Apical to basolateral EBV transcytosis was substantially reduced by amiloride, an inhibitor of macropinocytosis. Electron microscopy showed that virions were surrounded by apical surface protrusions and that virus was present in subapical vesicles. Inactivation of signaling molecules critical for macropinocytosis, including phosphatidylinositol 3-kinases, myosin light-chain kinase, Ras-related C3 botulinum toxin substrate 1, p21-activated kinase 1, ADP-ribosylation factor 6, and cell division control protein 42 homolog, led to significant reduction in EBV apical to basolateral transcytosis. In contrast, basolateral to apical EBV transcytosis was substantially reduced by nystatin, an inhibitor of caveolin-mediated virus entry. Caveolae were detected in the basolateral membranes of polarized human oral epithelial cells, and virions were detected in caveosome-like endosomes. Methyl β-cyclodextrin, an inhibitor of caveola formation, reduced EBV basolateral entry. EBV virions transcytosed in either direction were able to infect B lymphocytes. Together, these data show that EBV transmigrates across oral epithelial cells by (i) apical to basolateral transcytosis, potentially contributing to initial EBV penetration that leads to systemic infection, and (ii) basolateral to apical transcytosis, which may enable EBV secretion into saliva in EBV-infected individuals.

Epstein-Barr Virus (EBV)-associated gastric carcinoma

The ubiquitous Epstein-Barr virus (EBV) is associated with several human tumors, which include lymphoid and epithelial malignancies. It is known that EBV persistently infects the memory B cell pool of healthy individuals by activating growth and survival signaling pathways that can contribute to B cell lymphomagenesis. Although the monoclonal proliferation of EBV-infected cells can be observed in epithelial tumors, such as nasopharyngeal carcinoma and EBV-associated gastric carcinoma, the precise role of EBV in the carcinogenic progress is not fully understood. This review features characteristics and current understanding of EBV-associated gastric carcinoma. EBV-associated gastric carcinoma comprises almost 10% of all gastric carcinoma cases and expresses restricted EBV latent genes (Latency I). Firstly, definition, epidemiology, and clinical features are discussed. Then, the route of infection and carcinogenic role of viral genes are presented. Of particular interest, the association with frequent genomic CpG methylation and role of miRNA for carcinogenesis are topically discussed. Finally, the possibility of therapies targeting EBV-associated gastric carcinoma is proposed.

Interactions of the Kaposi's Sarcoma-associated herpesvirus nuclear egress complex: ORF69 is a potent factor for remodeling cellular membranes

All herpesviruses encode a complex of two proteins, referred to as the nuclear egress complex (NEC), which together facilitate the exit of assembled capsids from the nucleus. Previously, we showed that the Kaposi's sarcoma-associated herpesvirus (KSHV) NEC specified by the ORF67 and ORF69 genes when expressed in insect cells using baculoviruses for protein expression forms a complex at the nuclear membrane and remodels these membranes to generate nuclear membrane-derived vesicles. In this study, we have analyzed the functional domains of the KSHV NEC proteins and their interactions. Site-directed mutagenesis of gammaherpesvirus conserved residues revealed functional domains of these two proteins, which in many cases abolish the formation of the NEC and remodeling of nuclear membranes. Small in-frame deletions within ORF67 in all cases result in loss of the ability of the mutant protein to induce cellular membrane proliferation as well as to interact with ORF69. Truncation of the C terminus of ORF67 that resides in the perinuclear space does not impair the functions of ORF67; however, deletion of the transmembrane domain of ORF67 produces a protein that cannot induce membrane proliferation but can still interact with ORF69 in the nucleus and can be tethered to the nuclear membrane by virtue of its interaction with the wild-type-membrane-anchored ORF67. In-frame deletions in ORF69 have varied effects on NEC formation, but all abolish remodeling of nuclear membranes into circular structures. One mutant interacts with ORF67 as well as the wild-type protein but cannot function in membrane curvature and fission events that generate circular vesicles. These studies genetically confirm that ORF67 is required for cellular membrane proliferation and that ORF69 is the factor required to remodel these duplicated membranes into circular-virion-size vesicles. Furthermore, we also investigated the NEC encoded by Epstein-Barr virus (EBV). The EBV complex comprised of BFRF1 and BFLF2 was visualized at the nuclear membrane using autofluorescent protein fusions. BFRF1 is a potent inducer of membrane proliferation; however, BFLF2 cannot remodel these membranes into circular structures. What was evident is the superior remodeling activity of ORF69, which could convert the host membrane proliferations induced by BFRF1 into circular structures.

Entry of herpesviruses into cells: the enigma variations

The entry of herpesviruses into their target cells is complex at many levels. Virus entry proceeds by a succession of interactions between viral envelope glycoproteins and molecules on the cell membrane. The process is divided into distinct steps: attachment to the cell surface, interaction with a specific entry receptor, internalization of the particle (optional and cell specific), and membrane fusion. Several viral envelope glycoproteins are involved in one or several of these steps. The most conserved entry glycoproteins in the herpesvirus family (gB, gH/gL) are involved in membrane fusion. Around this functional core, herpesviruses have a variety of receptor binding glycoproteins, which interact with cell surface proteins often from different families. This interaction activates and controls the actual fusion machinery. Interactions with cellular receptors and between viral glycoproteins have to be tightly coordinated and regulated to guarantee successful entry. Although additional entry receptors for herpesviruses continue to be identified, the molecular interactions between viral glycoproteins remain mostly enigmatic. This chapter will review our current understanding of the molecular interactions that occur during herpesvirus entry from attachment to fusion. Particular emphasis will be placed on structure-based representation of receptor binding as a trigger of fusion during herpes simplex virus entry.

Vectored co-delivery of human cytomegalovirus gH and gL proteins elicits potent complement-independent neutralizing antibodies

Human cytomegalovirus (hCMV) is prevalent worldwide with infection generally being asymptomatic. Nevertheless, hCMV infection can lead to significant morbidity and mortality. Primary infection of seronegative women or reactivation/re-infection of seropositive women during pregnancy can result in transmission to the fetus, leading to severe neurological defects. In addition, hCMV is the most common viral infection in immunosuppressed organ transplant recipients and can produce serious complications. Hence, a safe and effective vaccine to prevent hCMV infection is an unmet medical need. Neutralizing antibodies to several hCMV glycoproteins, and complexes thereof, have been identified in individuals following hCMV infection. Interestingly, a portion of the CMV-specific neutralizing antibody responses are directed to epitopes found on glycoprotein complexes but not the individual proteins. Using an alphavirus replicon particle (VRP) vaccine platform, we showed that bicistronic VRPs encoding hCMV gH and gL glycoproteins produce gH/gL complexes in vitro. Furthermore, mice vaccinated with these gH/gL-expressing VRPs produced broadly cross-reactive complement-independent neutralizing antibodies to hCMV. These neutralizing antibody responses were of higher titer than those elicited in mice vaccinated with monocistronic VRPs encoding gH or gL antigens, and they were substantially more potent than those raised by VRPs encoding gB. These findings underscore the utility of co-delivery of glycoprotein components such as gH and gL for eliciting potent, broadly neutralizing immune responses against hCMV, and indicate that the gH/gL complex represents a potential target for future hCMV vaccine development.

Stuck in the middle: structural insights into the role of the gH/gL heterodimer in herpesvirus entry

Enveloped viruses enter cells by fusing the viral and cellular membranes, and most use a single viral envelope protein that combines receptor-binding and fusogenic functions. In herpesviruses, these functions are distributed among multiple proteins: the conserved fusion protein gB, various non-conserved receptor-binding proteins, and the conserved gH/gL heterodimer that curiously lacks an apparent counterpart in other enveloped viruses. Recent structural studies of gH/gL from HSV-2 and EBV revealed a unique complex with no structural or functional similarity to other viral proteins. Here we analyzed gH/gL structures and highlighted important functional regions. We propose that gH/gL functions as an adaptor that transmits the triggering signals from various non-conserved inputs to the highly conserved fusion protein gB.

Oral dysplasia and squamous cell carcinoma: correlation between increased expression of CD21, Epstein-Barr virus and CK19

OBJECTIVES

Epstein-Barr virus is an orally transmitted human gammaherpesvirus that infects B lymphocytes and epithelial cells. Although most primary infections are asymptomatic, long term carriage of the virus can be associated with either lymphoid or epithelial malignancies. The association of EBV with oral squamous cell carcinomas is sporadic and it is uncertain if the virus is involved in initiation of the tumor or, possibly, in its progression. Complement receptor type 2, CR2 or CD21, is a receptor for the major attachment protein of EBV, which significantly enhances epithelial cell infection, but its expression on normal tissues is restricted to tonsil and adenoid epithelium. As cells become dysplastic they are reported to express higher levels of CK19. We sought to evaluate whether CD21 and CK19 expression change as oral epithelial cells outside Waldeyer's ring become dysplastic.

MATERIALS AND METHODS

Epithelial cells were isolated by laser capture microdissection and levels of CD21, CK19 and EBV RNA were measured by quantitative reverse transcriptase PCR.

RESULTS

We report that expression of CD21 increases in frequency and intensity as oral epithelial cells become more dysplastic and that expression correlates with an increase in infection by EBV. Tumors or dysplastic lesions that carry EBV also generally express higher levels of CK19 than those that do not.

CONCLUSION

The findings suggest that dysplasia may make cells more susceptible to infection by EBV and that infection by the virus may alter the phenotype of the infected cell in a manner which could affect prognosis.

Infection of Epstein-Barr virus in a gastric carcinoma cell line induces anchorage independence and global changes in gene expression

Latent infection of EBV is linked to the development of multiple cancers that have distinct patterns of expression of viral proteins and microRNAs (miRNAs). In this study, we show that in vitro infection of a gastric epithelial cell line with EBV alters growth properties and induces growth in soft agar. The infected cells have high levels of expression of a large cluster of viral miRNAs, [the BamHI A rightward transcript (BART) miRNAs] and limited viral protein expression. Expression profile microarray analysis of this cell line revealed a large number of changes in cellular expression, with decreased expression of many genes. Inhibition of the trace-expressed levels of the viral oncoprotein, latent membrane protein 1, did not affect growth or alter the pattern of cellular expression. The expression changes are highly enriched for genes involved in cell motility and transformation pathways, suggesting these changes are important for the altered growth phenotype. Importantly, the transcripts decreased by microarray are significantly enriched in both experimentally and bioinformatically predicted BART miRNA targets. The absence of viral protein expression and the enrichment for viral miRNA targets in the modulated cell genes suggest that the BART miRNAs are major contributors to the transformed growth properties of the EBV-infected cells. The ability to affect cell growth through miRNA expression without viral protein expression would be a major factor in the development of cancer in individuals with functional immune systems.

Characterization of Epstein-Barr virus gp350/220 gene variants in virus isolates from gastric carcinoma and nasopharyngeal carcinoma

To characterize the sequence variation of the gp350/220 and explore its potential association with EBV-associated tumors, the gp350/220 gene was sequenced from 41 EBV-associated gastric carcinoma (EBVaGC) and 81 nasopharyngeal carcinoma (NPC) biopsies as well as 35 throat washing (TW) samples from healthy donors. Preferential linkages between variants of the N-terminus of gp350/220 and EBNA3C variants were detected, and type A/BLLF1-a was the dominant variant in this study. The dominant variant in the C-terminal region of gp350/220 was 9P. The similar distribution of gp350/220 variants in NPC, EBVaGC and healthy donors suggest that gp350/220 variations are geographically restricted rather than tumor-specific polymorphisms.

Important but differential roles for actin in trafficking of Epstein-Barr virus in B cells and epithelial cells

Epstein-Barr virus (EBV) uses different virus and cell proteins to enter its two major targets, B lymphocytes and epithelial cells. The routes that the virus takes into the two cell types are also different. To determine if these differences extend to movement from the cell surface to the nucleus, we examined the fate of incoming virus. Essentially all virus that entered a B cell remained stable for at least 8 h. In contrast, up to 80% of virus entering an epithelial cell was degraded in a compartment sensitive to inhibitors of components involved in autophagy. Inhibitors of actin remodeling blocked entry into a B cell but had no effect or enhanced entry into an epithelial cell. Inhibitors of the microtubule network reduced intracellular transport in both cell types, but movement to the nucleus in an epithelial cell also required involvement of the actin cytoskeleton. Deletion of the cytoplasmic tail of CR2, which in an epithelial cell interacts with the actin nucleator FHOS/FHOD when cross-linked by EBV, had no effect on infection. However, inhibitors of downstream signaling by integrins reduced intracellular transport. Cooperation of the microtubule and actin cytoskeletons, possibly activated by interaction with integrin binding proteins in the envelope of EBV, is needed for successful infection of an epithelial cell.

Structure-function analysis of varicella-zoster virus glycoprotein H identifies domain-specific roles for fusion and skin tropism

Enveloped viruses require membrane fusion for cell entry and replication. For herpesviruses, this event is governed by the multiprotein core complex of conserved glycoproteins (g)B and gH/gL. The recent crystal structures of gH/gL from herpes simplex virus 2, pseudorabies virus, and Epstein-Barr virus revealed distinct domains that, surprisingly, do not resemble known viral fusogens. Varicella-zoster virus (VZV) causes chicken pox and shingles. VZV is an α-herpesvirus closely related to herpes simplex virus 2, enabling prediction of the VZV gH structure by homology modeling. We have defined specific roles for each gH domain in VZV replication and pathogenesis using structure-based site-directed mutagenesis of gH. The distal tip of domain (D)I was important for skin tropism, entry, and fusion. DII helices and a conserved disulfide bond were essential for gH structure and VZV replication. An essential (724)CXXC(727) motif was critical for DIII structural stability and membrane fusion. This assignment of domain-dependent mechanisms to VZV gH links elements of the glycoprotein structure to function in herpesvirus replication and virulence.

Fusion of Epstein-Barr virus with epithelial cells can be triggered by αvβ5 in addition to αvβ6 and αvβ8, and integrin binding triggers a conformational change in glycoproteins gHgL

Fusion of herpesviruses with their target cells requires a minimum of three glycoproteins, namely, gB and a complex of gH and gL. Epstein-Barr virus (EBV) fusion with an epithelial cell requires no additional virus glycoproteins, and we have shown previously that it can be initiated by an interaction between integrin αvβ6 or αvβ8 and gHgL. We now report that integrin αvβ5 can also bind to gHgL and trigger fusion. Binding of gHgL to integrins is a two-step reaction. The first step, analyzed by surface plasmon resonance, was fast, with high association and low dissociation rate constants. The second step, detected by fluorescence spectroscopy of gHgL labeled at cysteine 153 at the domain I-domain II interface with the environmentally sensitive probes acrylodan and IANBD, involved a slower conformational change. Interaction of gHgL with neutralizing monoclonal antibodies or Fab' fragments was also consistent with a two-step reaction involving fast high-affinity binding and a subsequent slower conformational change. None of the antibodies bound to the same epitope, and none completely inhibited integrin binding. However, binding of each decreased the rate of conformational change induced by integrin binding, suggesting that neutralization might involve a conformational change that precludes fusion. Overall, the data are consistent with the interaction of gHgL with an integrin inducing a functionally important rearrangement at the domain I-domain II interface.

Epstein-Barr virus infection of polarized epithelial cells via the basolateral surface by memory B cell-mediated transfer infection

Epstein Barr virus (EBV) exhibits a distinct tropism for both B cells and epithelial cells. The virus persists as a latent infection of memory B cells in healthy individuals, but a role for infection of normal epithelial is also likely. Infection of B cells is initiated by the interaction of the major EBV glycoprotein gp350 with CD21 on the B cell surface. Fusion is triggered by the interaction of the EBV glycoprotein, gp42 with HLA class II, and is thereafter mediated by the core fusion complex, gH/gL/gp42. In contrast, direct infection of CD21-negative epithelial cells is inefficient, but efficient infection can be achieved by a process called transfer infection. In this study, we characterise the molecular interactions involved in the three stages of transfer infection of epithelial cells: (i) CD21-mediated co-capping of EBV and integrins on B cells, and activation of the adhesion molecules, (ii) conjugate formation between EBV-loaded B cells and epithelial cells via the capped adhesion molecules, and (iii) interaction of EBV glycoproteins with epithelial cells, with subsequent fusion and uptake of virions. Infection of epithelial cells required the EBV gH and gL glycoproteins, but not gp42. Using an in vitro model of normal polarized epithelia, we demonstrated that polarization of the EBV receptor(s) and adhesion molecules restricted transfer infection to the basolateral surface. Furthermore, the adhesions between EBV-loaded B cells and the basolateral surface of epithelial cells included CD11b on the B cell interacting with heparan sulphate moieties of CD44v3 and LEEP-CAM on epithelial cells. Consequently, transfer infection was efficiently mediated via CD11b-positive memory B cells but not by CD11b-negative naïve B cells. Together, these findings have important implications for understanding the mechanisms of EBV infection of normal and pre-malignant epithelial cells in vivo.

Fusing structure and function: a structural view of the herpesvirus entry machinery

Herpesviruses are double-stranded DNA, enveloped viruses that infect host cells through fusion with either the host cell plasma membrane or endocytic vesicle membranes. Efficient infection of host cells by herpesviruses is remarkably more complex than infection by other viruses, as it requires the concerted effort of multiple glycoproteins and involves multiple host receptors. The structures of the major viral glycoproteins and a number of host receptors involved in the entry of the prototypical herpesviruses, the herpes simplex viruses (HSVs) and Epstein-Barr virus (EBV), are now known. These structural studies have accelerated our understanding of HSV and EBV binding and fusion by revealing the conformational changes that occur on virus-receptor binding, depicting potential sites of functional protein and lipid interactions, and identifying the probable viral fusogen.

Rhesus and human cytomegalovirus glycoprotein L are required for infection and cell-to-cell spread of virus but cannot complement each other

Rhesus cytomegalovirus (RhCMV), the homolog of human cytomegalovirus (HCMV), serves as a model for understanding the pathogenesis of HCMV and for developing candidate vaccines. In order to develop a replication-defective virus as a vaccine candidate, we constructed RhCMV with glycoprotein L (gL) deleted. RhCMV gL was essential for viral replication, and virus with gL deleted could only replicate in cells expressing RhCMV gL. Noncomplementing cells infected with RhCMV with gL deleted released intact, noninfectious RhCMV particles that were indistinguishable from wild-type RhCMV by electron microscopy and could be rescued by treatment of cells with polyethylene glycol. In addition, noncomplementing cells infected with RhCMV with gL deleted produced levels of gB, the major target of neutralizing antibodies, at levels similar to those observed in cells infected with wild-type RhCMV. Since RhCMV and HCMV gL share 53% amino acid identity, we determined whether the two proteins could complement the heterologous virus. Cells transfected with an HCMV bacterial artificial chromosome with gL deleted yielded virus that could replicate in human cells expressing HCMV gL. This is the second HCMV mutant with an essential glycoprotein deleted that has been complemented in cell culture. Finally, we found that HCMV gL could not complement the replication of RhCMV with gL deleted and that RhCMV gL could not complement the replication of HCMV with gL deleted. These data indicate that RhCMV and HCMV gL are both essential for replication of their corresponding viruses and, although the two gLs are highly homologous, they are unable to complement each another.

The bovine herpesvirus 4 Bo10 gene encodes a nonessential viral envelope protein that regulates viral tropism through both positive and negative effects

All gammaherpesviruses encode a glycoprotein positionally homologous to the Epstein-Barr virus gp350 and the Kaposi's sarcoma-associated herpesvirus (KSHV) K8.1. In this study, we characterized the positional homologous glycoprotein of bovine herpesvirus 4 (BoHV-4), encoded by the Bo10 gene. We identified a 180-kDa gene product, gp180, that was incorporated into the virion envelope. A Bo10 deletion virus was viable but showed a growth deficit associated with reduced binding to epithelial cells. This seemed to reflect an interaction of gp180 with glycosaminoglycans (GAGs), since compared to the wild-type virus, the Bo10 mutant virus was both less infectious for GAG-positive (GAG(+)) cells and more infectious for GAG-negative (GAG(-)) cells. However, we could not identify a direct interaction between gp180 and GAGs, implying that any direct interaction must be of low affinity. This function of gp180 was very similar to that previously identified for the murid herpesvirus 4 gp150 and also to that of the Epstein-Barr virus gp350 that promotes CD21(+) cell infection and inhibits CD21(-) cell infection. We propose that such proteins generally regulate virion attachment both by binding to cells and by covering another receptor-binding protein until they are displaced. Thus, they regulate viral tropism both positively and negatively depending upon the presence or absence of their receptor.

Novel Epstein-Barr virus immunoglobulin G-based approach for the specific detection of nasopharyngeal carcinoma

PURPOSE

This study was designed to find a reliable Epstein-Barr virus (EBV) immunoglobulin (Ig) G-based diagnostic/screening test for nasopharyngeal carcinoma (NPC) able to demarcate between the NPC-related seropositivity of EBV IgG antibodies and that of other head and neck cancer (HNCA) and control groups. The NPC-associated immunosuppression affects EBV IgA much more than IgG, leading to inconsistent detection of NPC using EBV IgA antibodies.

MATERIALS AND METHODS

One hundred twenty-two HNCA patients, 42 NPC, 66 laryngeal carcinoma, and 14 hypopharyngeal carcinoma and 3 groups of 100 control subjects were enrolled in this study. Enzyme-linked immunosorbent assay (ELISA) was used to find a specific cutoff value for the NPC-related seropositivity of EBV IgG antibodies.

RESULTS

NPC group showed higher serum level of EBV IgG antibodies than control and other HNCA groups (P < .05). However, the traditional cutoff value, mean + 2 SDs of control subjects, failed to demarcate the seropositives of NPC patients from those of healthy population (P > .05). The new cutoff value, mean + 2 SDs of the seropositives group of control subjects who had already been grouped by the traditional cutoff value, proved successful. It succeeded to demarcate between the NPC-related EBV IgG seropositivity and that issued from the persistent, latent, or reactivated EBV infection in the population (P < .05). The sensitivity/specificity of NPC detection by the new cutoff-based ELISA kit, 76.19% and 86%, was close or higher than that of EBV IgA antibodies.

CONCLUSION

EBV IgG-based ELISA could be used for the diagnosis of NPC using a new cutoff threshold that excludes the population baseline of EBV IgG seropositivity.

Biophysical investigations of complement receptor 2 (CD21 and CR2)-ligand interactions reveal amino acid contacts unique to each receptor-ligand pair

Human complement receptor type 2 (CR2 and CD21) is a cell membrane receptor, with 15 or 16 extracellular short consensus repeats (SCRs), that promotes B lymphocyte responses and bridges innate and acquired immunity. The most distally located SCRs, SCR1-2, mediate the interaction of CR2 with its four known ligands (C3d, EBV gp350, IFNalpha, and CD23). To ascertain specific interacting residues on CR2, we utilized NMR studies wherein gp350 and IFNalpha were titrated into (15)N-labeled SCR1-2, and chemical shift changes indicative of specific inter-molecular interactions were identified. With backbone assignments made, the chemical shift changes were mapped onto the crystal structure of SCR1-2. With regard to gp350, the binding region of CR2 is primarily focused on SCR1 and the inter-SCR linker, specifically residues Asn(11), Arg(13), Ala(22), Arg(28), Ser(32), Arg(36), Lys(41), Lys(57), Tyr(64), Lys(67), Tyr(68), Arg(83), Gly(84), and Arg(89). With regard to IFNalpha, the binding is similar to the CR2-C3d interaction with specific residues being Arg(13), Tyr(16), Arg(28), Ser(42), Lys(48), Lys(50), Tyr(68), Arg(83), Gly(84), and Arg(89). We also report thermodynamic properties of each ligand-receptor pair determined using isothermal titration calorimetry. The CR2-C3d interaction was characterized as a two-mode binding interaction with K(d) values of 0.13 and 160 microm, whereas the CR2-gp350 and CR2-IFNalpha interactions were characterized as single site binding events with affinities of 0.014 and 0.035 microm, respectively. The compilation of chemical binding maps suggests specific residues on CR2 that are uniquely important in each of these three binding interactions.

The Epstein-Barr virus latent membrane protein 1 and transforming growth factor--β1 synergistically induce epithelial--mesenchymal transition in lung epithelial cells

The histopathology of idiopathic pulmonary fibrosis (IPF) includes the presence of myofibroblasts within so-called fibroblastic foci, and studies suggest that lung myofibroblasts may be derived from epithelial cells through epithelial--mesenchymal transition (EMT). Transforming growth factor (TGF)-β1 is expressed and/or activated in fibrogenesis, and induces EMT in lung epithelial cells in a dose-dependent manner. A higher occurrence of Epstein-Barr virus (EBV) has been reported in the lung tissue of patients with IPF. EBV expresses latent membrane protein (LMP) 1 during the latent phase of infection, and may play a role in the pathogenesis of pulmonary fibrosis inasmuch as LMP-1 may act as a constitutively active TNF-α receptor. Our data show a remarkable increase in mesenchymal cell markers, along with a concurrent reduction in the expression of epithelial cell markers in lung epithelial cells cotreated with LMP-1, and very low doses of TGF-β1. This effect was mirrored in lung epithelial cells infected with EBV expressing LMP1 and cotreated with TGF-β1. LMP1 pro-EMT signaling was identified, and occurs primarily through the nuclear factor-κB pathway and secondarily through the extracellular signal--regulated kinase (ERK) pathway. Activation of the ERK pathway was shown to be critical for aspects of TGF-β1-induced EMT. LMP1 accentuates the TGF-β1 activation of ERK. Together, these data demonstrate that the presence of EBV-LMP1 in lung epithelial cells synergizes with TGF-β1 to induce EMT. Our in vitro data may help to explain the observation that patients with IPF demonstrating positive staining for LMP1 in lung epithelial cells have a more rapid demise than patients in whom LMP1 is not detected.

Mapping the N-terminal residues of Epstein-Barr virus gp42 that bind gH/gL by using fluorescence polarization and cell-based fusion assays

Epstein-Barr virus (EBV) requires at a minimum membrane-associated glycoproteins gB, gH, and gL for entry into host cells. B-cell entry additionally requires gp42, which binds to gH/gL and triggers viral entry into B cells. The presence of soluble gp42 inhibits membrane fusion with epithelial cells by forming a stable heterotrimer of gH/gL/gp42. The interaction of gp42 with gH/gL has been previously mapped to residues 36 to 81 at the N-terminal region of gp42. In this study, we further mapped this region to identify essential features for binding to gH/gL by use of synthetic peptides. Data from fluorescence polarization, cell-cell fusion, and viral infection assays demonstrated that 33 residues corresponding to 44 to 61 and 67 to 81 of gp42 were indispensable for maintaining low-nanomolar-concentration gH/gL binding affinity and inhibiting B-cell fusion and epithelial cell fusion as well as viral infection. Overall, specific, large hydrophobic side chain residues of gp42 appeared to provide critical interactions, determining the binding strength. Mutations of these residues also diminished the inhibition of B-cell and epithelial cell fusions as well as EBV infection. A linker region (residues 62 to 66) between two gH/gL binding regions served as an important spacer, but individual amino acids were not critical for gH/gL binding. Probing the binding site of gH/gL and gp42 with gp42 peptides is critical for a better understanding of the interaction of gH/gL with gp42 as well as for the design of novel entry inhibitors of EBV and related human herpesviruses.

Shedding dynamics of Epstein-Barr virus: A type 1 carcinogen

Epstein Barr virus (EBV) is one of the ubiquitous viral carcinogens found in humans and successfully infects more than 90% of the world population. The spectrum of EBV-related pathology ranges from asymptomatic primary infection to grave B- and T-cell malignancies. EBV triggers lymphoproliferative disorders after allogeneic stem cell transplantation, which contributes to higher mortality rates. Although the transmission of EBV primarily occurs from an infected host to a naive host through viral shedding from the oropharynx, increasing evidence points to considerable amount of shedding in other anatomical sites such as cervix, anal mucosa, breast milk and respiratory tract. It is impossible to eradicate the prevalence of EBV-related malignancies and other pathologies without preventing viral shedding. However, a detail analysis of the multifaceted nature of EBV shedding is not available in the literature. Thus, this review focuses on elucidating the key elements of the shedding dynamics of this carcinogenic virus.

Characteristics of Epstein-Barr virus envelope protein gp42

Epstein-Barr virus (EBV) glycoprotein 42 (gp42) is a membrane protein essential for fusion and entry of EBV into host B-lymphocytes. Gp42 is a member of the protein-fold family C-type lectin or lectin-like domains (CLECT or CTLD) and specifically is classified as a natural-killer receptor (NKR)-like CLECT. Literature review and phylogenetic comparison show that EBV gp42 shares a common structure with other NKR-like CLECTs and possibly with many viral CTLDs, but does not appear to exhibit some common binding characteristics of many CTLDs, such as features required for calcium binding. The flexible N-terminal region adjacent to the CTLD fold is important for binding to other EBV glycoproteins and for a cleavage site that is necessary for infection of host cells. From structural studies of gp42 unbound and bound to receptor and extensive mutational analysis, a general model of how gp42 triggers membrane fusion utilizing both the flexible N-terminal region and the CTLD domain has emerged.

α-Helix peptides designed from EBV-gH protein display higher antigenicity and induction of monocyte apoptosis than the native peptide

We tested the hypothesis that stabilizing α-helix of Epstein-Barr virus gH-derived peptide 11438 used for binding human cells will increase its biological activity. Non-stable α-helix of peptide 11438 was unfolded in an entropy-driven process, despite the opposing effect of the enthalpy factor. Adding and/or changing amino acids in peptide 11438 allowed the designing of peptides 33207, 33208 and 33210; peptides 33208 and 33210 displayed higher helical content due to a decreased unfolding entropy change as was determined by AGADIR, molecular dynamics and circular dichroism analysis. Peptides 33207, 33208 and 33210 inhibited EBV invasion of peripheral blood mononuclear cells and displayed epitopes more similar to native protein than peptide 11438; these peptides could be useful for detecting antibodies induced by native gH protein since they displayed high reactivity with anti-EBV antibodies. Anti-peptide 33207 antibodies showed higher reactivity with EBV than anti-peptide 11438 antibodies being useful for inducing antibodies against EBV. Anti-peptide 33210 antibodies inhibit EBV invasion of epithelial cells better than anti-peptide 11438 antibodies. Peptide 33210 bound to normal T lymphocytes and Raji cells stronger than peptide 11438 and also induced apoptosis of monocytes and Raji cells but not of normal T cells in a similar way to EBV-gH. Peptide 33210 inhibited the monocytes' development toward dendritic cells better than EBV and peptide 11438. In conclusion, stabilizing the α-helix in peptides 33208 and 33210 designed from peptide 11438 increased the antigenicity and the ability of the antibodies induced by peptides of inhibiting EBV invasion of host cells.

Suberoylanilide hydroxamic acid induces viral lytic cycle in Epstein-Barr virus-positive epithelial malignancies and mediates enhanced cell death

In Epstein-Barr virus (EBV)-associated malignancies, the virus is harbored in every tumor cell and persists in tightly latent forms expressing a very limited number of viral latent proteins. Induction of EBV lytic cycle leads to expression of a much larger number of viral proteins, which may serve as potential therapeutic targets. We found that 4 histone deacetylase inhibitors, trichostatin A (TSA), sodium butyrate (SB), valproic acid (VPA) and suberoylanilide hydroxamic acid (SAHA), all significantly induced EBV lytic cycle in EBV-positive gastric carcinoma cells (AGS/BX1, latency II) but only weakly induced in Burkitt lymphoma cells (AK2003, latency I) and did not induce in lymphoblastoid cells (LCLs, latency III). Interestingly, SAHA potently induced viral lytic cycle in AGS/BX1 cells at micromolar concentrations (evidenced by 8-fold increase in viral DNA replication, strong expression of viral lytic proteins and production of infectious virus particles) and mediated enhanced cell death of EBV-positive AGS/BX1 cells when compared with that of EBV-negative AGS cells, possibly related to cell cycle arrest at G2/M phase. Furthermore, SAHA effected strong induction of EBV lytic cycle in nasopharyngeal carcinoma but not in NK lymphoma cells (both expressing EBV latency II pattern), indicating preferential viral lytic induction in epithelial rather than lymphoid malignancies. In conclusion, SAHA is found to be a potent EBV lytic cycle inducing agent, which warrants further investigation into its potential application as a novel virus-targeted drug for treatment of EBV-associated epithelial malignancies.

The Epstein-Barr virus (EBV)-encoded protein kinase, EBV-PK, but not the thymidine kinase (EBV-TK), is required for ganciclovir and acyclovir inhibition of lytic viral production

Ganciclovir (GCV) and acyclovir (ACV) are guanine nucleoside analogues that inhibit lytic herpesvirus replication. GCV and ACV must be monophosphorylated by virally encoded enzymes to be converted into nucleotides and incorporated into viral DNA. However, whether GCV and/or ACV phosphorylation in Epstein-Barr virus (EBV)-infected cells is mediated primarily by the EBV-encoded protein kinase (EBV-PK), the EBV-encoded thymidine kinase (EBV-TK), or both is controversial. To examine this question, we constructed EBV mutants containing stop codons in either the EBV-PK or EBV-TK open reading frame and selected for stable 293T clones latently infected with wild-type EBV or each of the mutant viruses. Cells were induced to the lytic form of viral replication with a BZLF1 expression vector in the presence and absence of various doses of GCV and ACV, and infectious viral titers were determined by a green Raji cell assay. As expected, virus production in wild-type EBV-infected 293T cells was inhibited by both GCV (50% inhibitory concentration [IC(50)] = 1.5 microM) and ACV (IC(50) = 4.1 microM). However, the EBV-PK mutant (which replicates as well as the wild-type (WT) virus in 293T cells) was resistant to both GCV (IC(50) = 19.6 microM) and ACV (IC(50) = 36.4 microM). Expression of the EBV-PK protein in trans restored GCV and ACV sensitivity in cells infected with the PK mutant virus. In contrast, in 293T cells infected with the TK mutant virus, viral replication remained sensitive to both GCV (IC(50) = 1.2 microM) and ACV (IC(50) = 2.8 microM), although susceptibility to the thymine nucleoside analogue, bromodeoxyuridine, was reduced. Thus, EBV-PK but not EBV-TK mediates ACV and GCV susceptibilities.

Human cytomegalovirus TR strain glycoprotein O acts as a chaperone promoting gH/gL incorporation into virions but is not present in virions

Human cytomegalovirus (HCMV) produces the following two gH/gL complexes: gH/gL/gO and gH/gL/UL128-131. Entry into epithelial and endothelial cells requires gH/gL/UL128-131, and we have provided evidence that gH/gL/UL128-131 binds saturable epithelial cell receptors to mediate entry. HCMV does not require gH/gL/UL128-131 to enter fibroblasts, and laboratory adaptation to fibroblasts results in mutations in the UL128-131 genes, abolishing infection of epithelial and endothelial cells. HCMV gO-null mutants produce very small plaques on fibroblasts yet can spread on endothelial cells. Thus, one prevailing model suggests that gH/gL/gO mediates infection of fibroblasts, while gH/gL/UL128-131 mediates entry into epithelial/endothelial cells. Most biochemical studies of gO have involved the HCMV lab strain AD169, which does not assemble gH/gL/UL128-131 complexes. We examined gO produced by the low-passage clinical HCMV strain TR. Surprisingly, TR gO was not detected in purified extracellular virus particles. In TR-infected cells, gO remained sensitive to endoglycosidase H, suggesting that the protein was not exported from the endoplasmic reticulum (ER). However, TR gO interacted with gH/gL in the ER and promoted export of gH/gL from the ER to the Golgi apparatus. Pulse-chase experiments showed that a fraction of gO remained bound to gH/gL for relatively long periods, but gO eventually dissociated or was degraded and was not found in extracellular virions or secreted from cells. The accompanying report by P. T. Wille et al. (J. Virol., 84:2585-2596, 2010) showed that a TR gO-null mutant failed to incorporate gH/gL into virions and that the mutant was unable to enter fibroblasts and epithelial and endothelial cells. We concluded that gO acts as a molecular chaperone, increasing gH/gL ER export and incorporation into virions. It appears that gO competes with UL128-131 for binding onto gH/gL but is released from gH/gL, so that gH/gL (lacking UL128-131) is incorporated into virions. Thus, our revised model suggests that both gH/gL and gH/gL/UL128-131 are required for entry into epithelial and endothelial cells.

A human cytomegalovirus gO-null mutant fails to incorporate gH/gL into the virion envelope and is unable to enter fibroblasts and epithelial and endothelial cells

Human cytomegalovirus (HCMV) depends upon a five-protein complex, gH/gL/UL128-131, to enter epithelial and endothelial cells. A separate HCMV gH/gL-containing complex, gH/gL/gO, has been described. Our prevailing model is that gH/gL/UL128-131 is required for entry into biologically important epithelial and endothelial cells and that gH/gL/gO is required for infection of fibroblasts. Genes encoding UL128-131 are rapidly mutated during laboratory propagation of HCMV on fibroblasts, apparently related to selective pressure for the fibroblast entry pathway. Arguing against this model in the accompanying paper by B. J. Ryckman et al. (J. Virol., 84:2597-2609, 2010), we describe evidence that clinical HCMV strain TR expresses a gO molecule that acts to promote endoplasmic reticulum (ER) export of gH/gL and that gO is not stably incorporated into the virus envelope. This was different from results involving fibroblast-adapted HCMV strain AD169, which incorporates gO into the virion envelope. Here, we constructed a TR gO-null mutant, TRDeltagO, that replicated to low titers, spread poorly among fibroblasts, but produced normal quantities of extracellular virus particles. TRDeltagO particles released from fibroblasts failed to infect fibroblasts and epithelial and endothelial cells, but the chemical fusogen polyethylene glycol (PEG) could partially overcome defects in infection. Therefore, TRDeltagO is defective for entry into all three cell types. Defects in entry were explained by observations showing that TRDeltagO incorporated about 5% of the quantities of gH/gL in extracellular virus particles compared with that in wild-type virions. Although TRDeltagO particles could not enter cells, cell-to-cell spread involving epithelial and endothelial cells was increased relative to TR, apparently resulting from increased quantities of gH/gL/UL128-131 in virions. Together, our data suggest that TR gO acts as a chaperone to promote ER export and the incorporation of gH/gL complexes into the HCMV envelope. Moreover, these data suggest that it is gH/gL, and not gH/gL/gO, that is present in virions and is required for infection of fibroblasts and epithelial and endothelial cells. Our observations that both gH/gL and gH/gL/UL128-131 are required for entry into epithelial/endothelial cells differ from models for other beta- and gammaherpesviruses that use one of two different gH/gL complexes to enter different cells.

Fusion of epithelial cells by Epstein-Barr virus proteins is triggered by binding of viral glycoproteins gHgL to integrins alphavbeta6 or alphavbeta8

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that is causally implicated in the development of lymphoid and epithelial tumors. Entry of virus requires fusion of virus envelopes and cell membranes. Fusion with B lymphocytes requires virus glycoprotein gB and a 3-part complex of glycoproteins, gHgLgp42. It is triggered by interactions between glycoprotein 42 (gp42) and HLA class II. However, fusion with epithelial cells is impeded by gp42 and instead is triggered by interactions between an unknown epithelial protein and a 2-part complex of gHgL. We report here that gHgL binds with high affinity to epithelial cells and that affinity of binding is increased by 3 orders of magnitude in the presence of Mn(2+). Binding and infection can be reduced by fibronectin and vitronectin, by down-regulation of integrin alphav, or by a peptide corresponding to 13 aa of gH which include a KGDE motif. Fusion of cells expressing gB and gHgL can be blocked by vitronectin or triggered by addition of soluble truncated integrins alphavbeta6 and alphavbeta8. We conclude that the direct interaction between EBV gHgL and integrins alphavbeta6 and alphavbeta8 can provide the trigger for fusion of EBV with an epithelial cell.

Epstein-Barr viruses that express a CD21 antibody provide evidence that gp350's functions extend beyond B-cell surface binding

The gp350 glycoprotein encoded by BLLF1 is crucial for efficient Epstein-Barr virus (EBV) infection of resting B cells. Gp350 binds to CD21, but whether this interaction sums up its functions remains unknown. We generated gp350-null EBVs that display CD19-, CD21-, or CD22-specific antibodies at their surface (designated as DeltaBLLF1-Ab). Gp350-complemented (DeltaBLLF1-C) and DeltaBLLF1-Ab were found to bind equally well to B cells. Surprisingly, DeltaBLLF1 binding was reduced only 1.7-fold relative to its complemented counterparts. Furthermore, B cells exposed to DeltaBLLF1-Ab or DeltaBLLF1 viruses presented structural antigens with comparable efficiency and achieved 25 to 80% of the T-cell activation elicited by DeltaBLLF1-C. These findings show that the gp350-CD21 interaction pair plays only a modest role during virus transfer to the endosomal compartment. However, primary B cells or Raji B cells infected with DeltaBLLF1-C viruses displayed a 35- to 70-fold higher infection rates than those exposed to DeltaBLLF1, DeltaBLLF1-CD22Ab, or DeltaBLLF1-CD19Ab viruses. Complementation of the gp350 knockout phenotype with CD21Ab substantially enhanced infection rates relative to DeltaBLLF1 but remained sevenfold (Raji B-cell line) to sixfold (primary B cells) less efficient than with gp350. We therefore infer that gp350 mainly exerts its functions after the internalization step, presumably during release of the viral capsid from the endosomal compartment, and that CD21-dependent but also CD21-independent molecular mechanisms are involved in this process. The latter appear to be characteristic of B-cell infection since transfection of CD21 in 293 cells improved the infection rates with both DeltaBLLF1-CD21Ab and DeltaBLLF1-C to a similar extent.

Maribavir inhibits epstein-barr virus transcription in addition to viral DNA replication

Although many drugs inhibit the replication of Epstein-Barr virus (EBV) in cell culture systems, there is still no drug that is effective and approved for use in primary EBV infection. More recently, maribavir (MBV), an l-ribofuranoside benzimidazole, has been shown to be a potent and nontoxic inhibitor of EBV replication and to have a mode of action quite distinct from that of acyclic nucleoside analogs such as acyclovir (ACV) that is based primarily on MBV's ability to block the phosphorylation of target proteins by EBV and human cytomegalovirus protein kinases. However, since the antiviral mechanisms of the drug are complex, we have carried out a comprehensive analysis of the effects of MBV on the RNA expression levels of all EBV genes with a quantitative real-time reverse transcription-PCR-based array. We show that in comparisons with ACV, the RNA expression profiles produced by the two drugs are entirely different, with MBV causing a pronounced inhibition of multiple viral mRNAs and with ACV causing virtually none. The results emphasize the different modes of action of the two drugs and suggest that the action of MBV may be linked to indirect effects on the transcription of EBV genes through the interaction of BGLF4 with multiple viral proteins.

Self-assembly of Epstein-Barr virus capsids

Epstein-Barr virus (EBV), a member of the Gammaherpesvirus family, primarily infects B lymphocytes and is responsible for a number of lymphoproliferative diseases. The molecular genetics of the assembly pathway and high-resolution structural analysis of the capsid have not been determined for this lymphocryptovirus. As a first step in studying EBV capsid assembly, the baculovirus expression vector (BEV) system was used to express the capsid shell proteins BcLF1 (major capsid protein), BORF1 (triplex protein), BDLF1 (triplex protein), and BFRF3 (small capsid protein); the internal scaffold protein, BdRF1; and the maturational protease (BVRF2). Coinfection of insect cells with the six viruses expressing these proteins resulted in the production of closed capsid structures as judged by electron microscopy and sedimentation methods. Therefore, as shown for other herpesviruses, only six proteins are required for EBV capsid assembly. Furthermore, the small capsid protein of EBV (BFRF3), like that of Kaposi's sarcoma-associated herpesvirus, was found to be required for assembly of a stable structure. Localization of the small capsid protein to nuclear assembly sites required both the major capsid (BcLF1) and scaffold proteins (BdRF1) but not the triplex proteins. Mutational analysis of BFRF3 showed that the N-terminal half (amino acids 1 to 88) of this polypeptide is required and sufficient for capsid assembly. A region spanning amino acids 65 to 88 is required for the concentration of BFRF3 at a subnuclear site and the N-terminal 65 amino acids contain the sequences required for interaction with major capsid protein. These studies have identified the multifunctional role of the gammaherpesvirus small capsid proteins.

Epstein-Barr virus in lymphoproliferative processes: an update for the diagnostic pathologist

The Epstein-Barr virus is an orally transmitted herpesvirus that is widespread in human populations and exhibits marked B-cell tropism. It is associated with more human neoplasms than any other known virus, and its role in the pathogenesis of such neoplasms has been the subject of intense investigation. This review presents an overview and update of the biology of Epstein-Barr virus and the diagnostic features of lymphoproliferative disorders associated with this intriguing human pathogen.

The BDLF2 protein of Epstein-Barr virus is a type II glycosylated envelope protein whose processing is dependent on coexpression with the BMRF2 protein

Epstein-Barr virus has been documented to encode for ten envelope glycoproteins, gB, gH, gL, gM, gN, gp350, gp42, gp78, gp150 and BMRF2. The BDLF2 open reading frame is also predicted to encode a type II membrane protein but, although found in the virion, it has been described as a component of the tegument. We show here that, as predicted, it is the eleventh envelope glycoprotein of the virus. The full length 65 kDa glycoprotein formed a complex with BMRF2 and, as its homologs in other gammaherpesviruses, was dependent on BMRF2, for authentic processing and transport. Two cleavage products of BDLF2 were also identified in cells and in purified virion particles, one corresponding approximately to the aminoterminal half of the protein, that remained associated with the full length form, and one corresponding to the carboxyterminal glycosylated portion of the protein which did not.

Analysis of Epstein-Barr virus glycoprotein B functional domains via linker insertion mutagenesis

Epstein-Barr Virus (EBV) glycoprotein B (gB) is essential for viral fusion events with epithelial and B cells. This glycoprotein has been studied extensively in other herpesvirus family members, but functional domains outside of the cytoplasmic tail have not been characterized in EBV gB. In this study, a total of 28 linker insertion mutations were generated throughout the length of gB. In general, the linker insertions did not disrupt intracellular expression and variably altered cell surface expression. Oligomerization was disrupted by insertions located between residues 561 and 620, indicating the location of a potential site of oligomer contacts between EBV gB monomers. In addition, a novel N-glycosylated form of wild-type gB was identified under nonreducing Western blot conditions that likely represents a mature form of the protein. Fusion activity was abolished in all but three variants containing mutations in the N-terminal region (gB30), within the ectodomain (gB421), and in the intracellular C-terminal domain (gB832) of the protein. Fusion activity with variants gB421 and gB832 was comparable to that of the wild type with epithelial and B cells, and only these two mutants, but not gB30, were able to complement gB-null virus and subsequently function in virus entry. The mutant gB30 exhibited a low level of fusion activity with B cells and was unable to complement gB-null virus. The mutations generated here indicate important structural domains, as well as regions important for function in fusion, within EBV gB.

Measuring Epstein-Barr virus (EBV) load: the significance and application for each EBV-associated disease

Because Epstein-Barr virus (EBV) is ubiquitous and persists latently in lymphocytes, simply detecting EBV is insufficient to diagnose EBV-associated diseases. Therefore, measuring the EBV load is necessary to diagnose EBV-associated diseases and to explore EBV pathogenesis. Due to the diverse biology of EBV, the significance of measuring EBV DNA and the optimal type of specimen differ among EBV-associated diseases. Recent advances in molecular technology have enabled the EBV genome to be quantitated rapidly and accurately. Real-time polymerase chain reaction (PCR) is a rapid and reliable method to quantify DNA and is widely used not only as a diagnostic tool, but also as a management tool for EBV-associated diseases. However, each laboratory currently measures EBV load with its own "homebrew" system, and there is no consensus on sample type, sample preparation protocol, or assay units. The EBV real-time PCR assay system must be standardised for large-scale studies and international comparisons.

Epstein-Barr Virus (EBV): infection, propagation, quantitation, and storage

Epstein-Barr virus (EBV) was first reported as the etiological agent of Burkitt's lymphoma in 1964. Since then, EBV has also been associated with nasopharyngeal carcinoma, which is highly prevalent in Southeast Asia, as well as infectious mononucleosis, complications of AIDS, and transplant-related B cell lymphomas. This virus has further been linked with T cell lymphomas and Hodgkin's disease, establishing the concept of a wide spectrum of EBV-associated malignant disorders. So far, there are a number of EBV-infected cell lines established that can be induced for production of infectious viral progeny and that facilitate the study of the mechanism of EBV-related infection, transformation, and oncogenesis. This unit describes procedures for the preparation of EBV virion particles and in vitro infection of cells with EBV. In addition, procedures for quantitation and storage of the virus are provided.

Molecular basis of the interaction between complement receptor type 2 (CR2/CD21) and Epstein-Barr virus glycoprotein gp350

The binding of the Epstein-Barr virus glycoprotein gp350 by complement receptor type 2 (CR2) is critical for viral attachment to B lymphocytes. We set out to test hypotheses regarding the molecular nature of this interaction by developing an enzyme-linked immunosorbent assay (ELISA) for the efficient analysis of the gp350-CR2 interaction by utilizing wild-type and mutant forms of recombinant gp350 and also of the CR2 N-terminal domains SCR1 and SCR2 (designated CR2 SCR1-2). To delineate the CR2-binding site on gp350, we generated 17 gp350 single-site substitutions targeting an area of gp350 that has been broadly implicated in the binding of both CR2 and the major inhibitory anti-gp350 monoclonal antibody (MAb) 72A1. These site-directed mutations identified a novel negatively charged CR2-binding surface described by residues Glu-21, Asp-22, Glu-155, Asp-208, Glu-210, and Asp-296. We also identified gp350 amino acid residues involved in non-charge-dependent interactions with CR2, including Tyr-151, Ile-160, and Trp-162. These data were supported by experiments in which phycoerythrin-conjugated wild-type and mutant forms of gp350 were incubated with CR2-expressing K562 cells and binding was assessed by flow cytometry. The ELISA was further utilized to identify several positively charged residues (Arg-13, Arg-28, Arg-36, Lys-41, Lys-57, Lys-67, Arg-83, and Arg-89) within SCR1-2 of CR2 that are involved in the binding interaction with gp350. These experiments allowed a comparison of those CR2 residues that are important for binding gp350 to those that define the epitope for an effective inhibitory anti-CR2 MAb, 171 (Asn-11, Arg-13, Ser-32, Thr-34, Arg-36, and Tyr-64). The mutagenesis data were used to calculate a model of the CR2-gp350 complex using the soft-docking program HADDOCK.

Laser-capture microdissection of oropharyngeal epithelium indicates restriction of Epstein-Barr virus receptor/CD21 mRNA to tonsil epithelial cells

BACKGROUND

Epstein-Barr virus colonizes the oropharynx of a majority of individuals. It infects B lymphocytes and epithelial cells and can contribute to the development of both lymphoid and epithelial tumors. The virus uses CD21 for attachment to B cells which constitutively express the protein. Infection of epithelial cells in vitro is also more efficient if CD21 is available. However, its potential contribution to infection in vivo has been difficult to evaluate as discrepant results with antibodies have made it difficult to determine which, if any, epithelial cells in the oropharynx express CD21.

METHODS

To reevaluate CD21 expression by an alternative method, epithelial cells were isolated by laser-capture microdissection from formalin-fixed sections of tissues from various parts of the oropharynx and mRNA was amplified with primers specific for the exons of CD21 which code for the Epstein-Barr virus binding site.

RESULTS

CD21 mRNA was expressed in tonsil epithelium, but not in epithelium from buccal mucosa, uvula, soft palate or tongue.

CONCLUSIONS

CD21 does not contribute to infection of most normal epithelial tissues in the oropharynx, but may contribute to infection of epithelial cells in the tonsil, where virus has been demonstrated in healthy carriers.

Isolating the Epstein-Barr virus gp350/220 binding site on complement receptor type 2 (CR2/CD21)

Complement receptor type 2 (CR2/CD21) is essential for the attachment of Epstein-Barr virus (EBV) to the surface of B-lymphocytes in an interaction mediated by the viral envelope glycoprotein gp350. The heavily glycosylated structure of EBV gp350 has recently been elucidated by x-ray crystallography, and the CR2 binding site on this protein has been characterized. To identify the corresponding gp350 binding site on CR2, we have undertaken a site-directed mutagenesis study targeting regions of CR2 that have previously been implicated in the binding of CR2 to the C3d/C3dg fragments of complement component C3. Wild-type or mutant forms of CR2 were expressed on K562 cells, and the ability of these CR2-expressing cells to bind gp350 was measured using flow cytometry. Mutations directed toward the two N-terminal extracellular domains of CR2 (SCR1-2) reveal that a large contiguous surface of CR2 SCR1-2 is involved in gp350 binding, including a number of positively charged residues (Arg-13, (Arg-28, (Arg-36, Lys-41, Lys-57, Lys-67, and Arg-83). These data appear to complement the CR2 binding site on gp350, which is characterized by a preponderance of negative charge. In addition to identifying the importance of charge in the formation of a CR2-gp350 complex, we also provide evidence that both SCR1 and SCR2 make contact with gp350. Specifically, two anti-CR2 monoclonal antibodies, designated as monoclonal antibodies 171 and 1048 whose primary epitopes are located within SCR2, inhibit binding of wild-type CR2 to EBV gp350; with regard to SCR1, both K562 cells expressing an S15P mutation and recombinant S15P CR2 proteins exhibit diminished gp350 binding.

The Epstein-Barr virus BMRF-2 protein facilitates virus attachment to oral epithelial cells

We previously reported that BMRF-2, an Epstein-Barr virus (EBV) glycoprotein, binds to beta1 family integrins and is important for EBV infection of polarized oral epithelial cells. To further study the functions of BMRF-2, we constructed a recombinant EBV that lacks BMRF-2 expression by homologous recombination in B95-8 cells. We found that lack of BMRF-2 resulted in about 50% reduction of EBV attachment to oral epithelial cells, but not to B lymphocytes, suggesting that BMRF-2 is critical for EBV infection in oral epithelial cells, but not in B lymphocytes. In polarized oral epithelial cells, infection rate of the recombinant EBV virus was about 4- to 8-fold lower than the wild-type B95-8 virus. Cell adhesion assays using the BMRF-2 RGD peptide and its RGE and AAA mutants showed that the RGD motif is critical for BMRF-2 binding to integrins. These data are consistent with our previous observation that interactions between EBV BMRF-2 and integrins are critical for infection of oral epithelial cells with EBV.

A molecular link between malaria and Epstein-Barr virus reactivation

Although malaria and Epstein–Barr (EBV) infection are recognized cofactors in the genesis of endemic Burkitt lymphoma (BL), their relative contribution is not understood. BL, the most common paediatric cancer in equatorial Africa, is a high-grade B cell lymphoma characterized by c-myc translocation. EBV is a ubiquitous B lymphotropic virus that persists in a latent state after primary infection, and in Africa, most children have sero-converted by 3 y of age. Malaria infection profoundly affects the B cell compartment, inducing polyclonal activation and hyper-gammaglobulinemia. We recently identified the cystein-rich inter-domain region 1α (CIDR1α) of the Plasmodium falciparum membrane protein 1 as a polyclonal B cell activator that preferentially activates the memory compartment, where EBV is known to persist. Here, we have addressed the mechanisms of interaction between CIDR1α and EBV in the context of B cells. We show that CIDR1α binds to the EBV-positive B cell line Akata and increases the number of cells switching to the viral lytic cycle as measured by green fluorescent protein (GFP) expression driven by a lytic promoter. The virus production in CIDR1α-exposed cultures was directly proportional to the number of GFP-positive Akata cells (lytic EBV) and to the increased expression of the EBV lytic promoter BZLF1. Furthermore, CIDR1α stimulated the production of EBV in peripheral blood mononuclear cells derived from healthy donors and children with BL. Our results suggest that P. falciparum antigens such as CIDR1α can directly induce EBV reactivation during malaria infection that may increase the risk of BL development for children living in malaria-endemic areas. To our knowledge, this is the first report to show that a microbial protein can drive a latently infected B cell into EBV replication.

Malaria and Epstein–Barr virus (EBV) infections are recognized cofactors in the genesis of endemic Burkitt lymphoma, the most common paediatric cancer in equatorial Africa. EBV is a ubiquitous virus residing in B lymphocytes that establishes a lifelong persistence in the host after primary infection. EBV has two lifestyles: latent infection (non-productive), and lytic replication (productive). Children living in malaria-endemic areas exhibit an elevated viral load, and acute malaria infection increases the levels of circulating EBV. The mechanisms leading to viral reactivation during Plasmodium falciparum malaria infection are not well understood. Cystein-rich inter-domain region 1α (CIDR1α) is a domain of a large protein expressed at the surface of P. falciparum–infected red blood cells. Based on previous findings showing that CIDR1α activates and expands the B cells compartment where EBV persists, we assessed the impact of CIDR1α on viral reactivation. Here, we identify CIDR1α as the first microbial protein able to drive a latently EBV-infected B cell (no virus production) into lytic replication (virus production). Our results suggest that P. falciparum–derived proteins can lead to a direct reactivation of EBV during acute malaria infection, increasing the risk of Burkitt lymphoma development for children living in malaria-endemic areas.

Compatibility of the gH homologues of Epstein-Barr virus and related lymphocryptoviruses

Glycoprotein gH, together with its chaperone gL and a third glycoprotein gB, is essential for cell-cell fusion and virus-cell fusion mediated by herpesviruses. Epstein-Barr virus (EBV), the prototype human lymphocryptovirus, requires a fourth glycoprotein gp42 to support fusion with B cells in addition to epithelial cells. Two other lymphocryptoviruses, the rhesus lymphocryptovirus (Rh-LCV) and the common marmoset lymphocryptovirus (CalHV3), have been sequenced in their entirety and each has a gp42 homologue. Combinations of proteins from EBV, Rh-LCV and CalHV3 were able to mediate fusion of epithelial cells, but, even when complexed with EBV gp42, only Rh-LCV and not CalHV3 proteins were able to mediate fusion with human B cells. CalHV3 gL was also unable to function effectively as a chaperone for EBV or Rh-LCV gH. The Rh-LCV gH homologue supported more fusion than EBV gH with an epithelial cell and supported the highest levels of fusion with a B cell. Chimeric constructs made from Rh-LCV gH and EBV gH that have 85.4 % sequence identity should prove useful for mapping the regions of gH that are of importance to fusion as a whole and to B-cell fusion in particular.

Characterization of EBV gB indicates properties of both class I and class II viral fusion proteins

To gain insight into Epstein-Barr virus (EBV) glycoprotein B (gB), recombinant, secreted variants were generated. The role of putative transmembrane regions, the proteolytic processing and the oligomerization state of the gB variants were investigated. Constructs containing 2 of 3 C-terminal hydrophobic regions were secreted, indicating that these do not act as transmembrane anchors. The efficiency of cleavage of the gB furin site was found to depend on the nature of C-terminus. All of the gB constructs formed rosette structures reminiscent of the postfusion aggregates formed by other viral fusion proteins. However, substitution of putative fusion loop residues, WY(112-113) and WLIY(193-196), with less hydrophobic amino acids from HSV-1 gB, produced trimeric protein and abrogated the ability of the EBV gB ectodomains to form rosettes. These data demonstrate biochemical features of EBV gB that are characteristic of other class I and class II viral fusion proteins, but not of HSV-1 gB.

Binding-site interactions between Epstein-Barr virus fusion proteins gp42 and gH/gL reveal a peptide that inhibits both epithelial and B-cell membrane fusion

Herpesviruses require membrane-associated glycoproteins gB, gH, and gL for entry into host cells. Epstein-Barr virus (EBV) gp42 is a unique protein also required for viral entry into B cells. Key interactions between EBV gp42 and the EBV gH/gL complex were investigated to further elucidate their roles in membrane fusion. Deletion and point mutants within the N-terminal region of gp42 revealed residues important for gH/gL binding and membrane fusion. Many five-residue deletion mutants in the N-terminal region of gp42 that exhibit reduced membrane fusion activity retain binding with gH/gL but map out two functional stretches between residues 36 and 96. Synthetic peptides derived from the gp42 N-terminal region were studied in in vitro binding experiments with purified gH/gL and in cell-cell fusion assays. A peptide spanning gp42 residues 36 to 81 (peptide 36-81) binds gH/gL with nanomolar affinity, comparable to full-length gp42. Peptide 36-81 efficiently inhibits epithelial cell membrane fusion and competes with soluble gp42 to inhibit B-cell fusion. Additionally, this peptide at low nanomolar concentrations inhibits epithelial cell infection by intact virus. Shorter gp42 peptides spanning the two functional regions identified by deletion mutagenesis had little or no binding to soluble gH/gL and were also unable to inhibit epithelial cell fusion, nor could they complement gp42 deletion mutants in B-cell fusion. These studies identify key residues of gp42 that are essential for gH/gL binding and membrane fusion activation, providing a nanomolar inhibitor of EBV-mediated membrane fusion.

Cellular responses to viral infection in humans: lessons from Epstein-Barr virus

Epstein-Barr virus (EBV) provides a useful model to study cellular immunity to a genetically stable, persistent human virus. Different sets of proteins expressed during EBV's lytic and cell transforming infections induce qualitatively different cellular immune responses. The factors governing immunodominance hierarchies and the biological effectiveness of these different responses are now being revealed. Analysis of infectious mononucleosis (IM), a clinical syndrome that can arise during primary EBV infection, has allowed the evolution of the responses to be tracked over time, giving an understanding of the immune response kinetics and of those determinants affecting selection into memory. Furthermore, following IM, expression of the receptor for the homeostatic cytokine IL-15 on NK and T cells is lost within these individuals. This experiment of nature provides a system to advance understanding of immunological homeostasis in humans, illustrating how data obtained from the study of EBV have wider significance to the immunological community.