Mutants of Epstein-Barr virus with a selective marker disrupting the TP gene transform B cells and replicate normally in culture

EBV and Apoptosis: The Viral Master Regulator of Cell Fate?

Epstein-Barr virus (EBV) was first discovered in cells from a patient with Burkitt lymphoma (BL), and is now known to be a contributory factor in 1-2% of all cancers, for which there are as yet, no EBV-targeted therapies available. Like other herpesviruses, EBV adopts a persistent latent infection in vivo and only rarely reactivates into replicative lytic cycle. Although latency is associated with restricted patterns of gene expression, genes are never expressed in isolation; always in groups. Here, we discuss (1) the ways in which the latent genes of EBV are known to modulate cell death, (2) how these mechanisms relate to growth transformation and lymphomagenesis, and (3) how EBV genes cooperate to coordinately regulate key cell death pathways in BL and lymphoblastoid cell lines (LCLs). Since manipulation of the cell death machinery is critical in EBV pathogenesis, understanding the mechanisms that underpin EBV regulation of apoptosis therefore provides opportunities for novel therapeutic interventions.

C-terminal region of EBNA-2 determines the superior transforming ability of type 1 Epstein-Barr virus by enhanced gene regulation of LMP-1 and CXCR7

Type 1 Epstein-Barr virus (EBV) strains immortalize B lymphocytes in vitro much more efficiently than type 2 EBV, a difference previously mapped to the EBNA-2 locus. Here we demonstrate that the greater transforming activity of type 1 EBV correlates with a stronger and more rapid induction of the viral oncogene LMP-1 and the cell gene CXCR7 (which are both required for proliferation of EBV-LCLs) during infection of primary B cells with recombinant viruses. Surprisingly, although the major sequence differences between type 1 and type 2 EBNA-2 lie in N-terminal parts of the protein, the superior ability of type 1 EBNA-2 to induce proliferation of EBV-infected lymphoblasts is mostly determined by the C-terminus of EBNA-2. Substitution of the C-terminus of type 1 EBNA-2 into the type 2 protein is sufficient to confer a type 1 growth phenotype and type 1 expression levels of LMP-1 and CXCR7 in an EREB2.5 cell growth assay. Within this region, the RG, CR7 and TAD domains are the minimum type 1 sequences required. Sequencing the C-terminus of EBNA-2 from additional EBV isolates showed high sequence identity within type 1 isolates or within type 2 isolates, indicating that the functional differences mapped are typical of EBV type sequences. The results indicate that the C-terminus of EBNA-2 accounts for the greater ability of type 1 EBV to promote B cell proliferation, through mechanisms that include higher induction of genes (LMP-1 and CXCR7) required for proliferation and survival of EBV-LCLs.

Epstein-Barr virus (EBV) is a common human virus that is involved in several types of cancer and directly causes human B lymphocytes to proliferate when they become infected. EBV occurs naturally as two different viral types (type 1 and type 2). The genomes of these viruses are mostly very similar but they differ in a few genes, particularly the EBNA-2 gene. For many years it has been known that type 1 EBV is much more effective than type 2 EBV at causing B lymphocyte proliferation and this difference is mediated by the EBNA-2 gene. Here we have shown that the greater ability of type 1 EBNA-2 to cause B cell proliferation is due to superior induction of the EBV LMP-1 and the cell CXCR7 genes, both of which are required for growth of EBV-infected lymphocytes. We mapped the section of type 1 EBNA-2 responsible for this to the C-terminus of the protein, including the transactivation and EBNA-LP interaction domains. The results provide a mechanism for the long-standing question of the functional difference between these two major types of EBV and will be important in understanding the significance of the EBV types in human infection.

Regulation of intracellular signalling by the terminal membrane proteins of members of the Gammaherpesvirinae

The human gamma(1)-herpesvirus Epstein-Barr virus (EBV) and the gamma(2)-herpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV), rhesus rhadinovirus (RRV), herpesvirus saimiri (HVS) and herpesvirus ateles (HVA) all contain genes located adjacent to the terminal-repeat region of their genomes, encoding membrane proteins involved in signal transduction. Designated 'terminal membrane proteins' (TMPs) because of their localization in the viral genome, they interact with a variety of cellular signalling molecules, such as non-receptor protein tyrosine kinases, tumour-necrosis factor receptor-associated factors, Ras and Janus kinase (JAK), thereby initiating further downstream signalling cascades, such as the MAPK, PI3K/Akt, NF-kappaB and JAK/STAT pathways. In the case of TMPs expressed during latent persistence of EBV and HVS (LMP1, LMP2A, Stp and Tip), their modulation of intracellular signalling pathways has been linked to the provision of survival signals to latently infected cells and, hence, a contribution to occasional cellular transformation. In contrast, activation of similar pathways by TMPs of KSHV (K1 and K15) and RRV (R1), expressed during lytic replication, may extend the lifespan of virus-producing cells, alter their migration and/or modulate antiviral immune responses. Whether R1 and K1 contribute to the oncogenic properties of KSHV and RRV has not been established satisfactorily, despite their transforming qualities in experimental settings.

EBNA2 amino acids 3 to 30 are required for induction of LMP-1 and immortalization maintenance

Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA2), a direct transcriptional activator of viral and cellular genes, is required for EBV-induced B-cell transformation. The functional role of conserved regions within the amino terminus of the protein preceding the poly-proline region has yet to be fully characterized. Thus, we tested whether the EBNA2 amino-terminal 30 amino acid residues, containing evolutionarily conserved region 1, are required for stimulating viral and cellular gene expression necessary for B-cell transformation in a viral transcomplementation assay. We found that these residues are required for its ability to induce LMP-1 expression in lymphoblastoid cell lines (LCLs), to stimulate LMP-1 promoter reporter plasmids in transient-cotransfection assays, and to rescue LCL growth following inactivation of endogenous wild-type EBNA2 protein. Deletion of amino acid residues 3 to 30 also impaired its ability to self-associate in coimmunoprecipitation assays. These data indicate that EBNA2 residues 3 to 30 comprise an essential domain required for induction of LMP-1 expression and, consequently, for maintenance of the immortalized phenotype of LCLs. The ability to self-associate into dimers or multimers conferred by this domain may be an important mechanism for these effects.

Epstein-Barr virus latent membrane protein 2B (LMP2B) co-localizes with LMP2A in perinuclear regions in transiently transfected cells

Epstein-Barr virus is a human gammaherpesvirus that infects and establishes latency in B lymphocytes in vivo. The latent membrane protein 2 (LMP2) gene is expressed in latently infected B cells and encodes two protein isoforms, LMP2A and LMP2B, that are identical except for an additional N-terminal 119 aa cytoplasmic domain which is present in the LMP2A isoform. A panel of fusion proteins was constructed in which the fluorescent enhanced green fluorescent protein and DsRed protein domains were fused to the N- and C-termini of LMP2A and LMP2B. By fluorescence microscopy, LMP2B localized to perinuclear regions of both live and fixed transiently transfected cells. Co-localization was detected with markers for the endoplasmic reticulum and the trans-Golgi network. No evidence of co-localization of LMP2B with endosomes or surface expression was obtained. Transiently expressed LMP2B co-localized with transiently or constitutively expressed LMP2A. Confocal microscopy confirmed that LMP2A proteins localized to intracellular perinuclear compartments with markers for the trans-Golgi network. Only LMP2A proteins with C-terminal truncations were detected in the plasma membrane with extracellular loop1 epitope tags. These results indicate that the transmembrane domain of LMP2 proteins possess intracellular retention signals and suggest that LMP2A-mediated signalling effects are likely to be ectopic, originating from sites inside the cell close to the nucleus.

Epstein-Barr virus infection and human malignancies

The Epstein-Barr virus (EBV) is a herpes virus which establishes a life-long persistent infection in over 90% of the human adult population world-wide. Based on its association with a variety of lymphoid and epithelial malignancies, EBV has been classified as a group 1 carcinogen by the International Agency for Research on Cancer. In this article we discuss the evidence supporting an aetiological role for EBV in the pathogenesis of human tumours. The biology of EBV infection will be described with special emphasis on viral transforming gene products. A brief survey of EBV-associated tumours is followed by a discussion of specific problems. Evidence is presented which suggests that failures of the EBV-specific immunity may play a role in the pathogenesis of EBV-associated tumours also in patients without clinically manifest immunodeficiencies. Finally, the timing of EBV infection in the pathogenesis of virus-associated malignancies is discussed. There is good evidence that EBV infection precedes expansion of the malignant cell populations in some virus-associated tumours. However, this is clearly not always the case and for some of these tumours there are indications that clonal genetic alterations may occur prior to EBV infection. Thus, whilst there is good evidence to suggest that EBV is a human carcinogen, its precise role(s) in the development of virus-associated human tumours requires clarification.

Epstein-Barr virus infection and human malignancies

The Epstein-Barr virus (EBV) is a herpes virus which establishes a life-long persistent infection in over 90% of the human adult population world-wide. Based on its association with a variety of lymphoid and epithelial malignancies, EBV has been classified as a group 1 carcinogen by the International Agency for Research on Cancer. In this article we discuss the evidence supporting an aetiological role for EBV in the pathogenesis of human tumours. The biology of EBV infection will be described with special emphasis on viral transforming gene products. A brief survey of EBV-associated tumours is followed by a discussion of specific problems. Evidence is presented which suggests that failures of the EBV-specific immunity may play a role in the pathogenesis of EBV-associated tumours also in patients without clinically manifest immunodeficiencies. Finally, the timing of EBV infection in the pathogenesis of virus-associated malignancies is discussed. There is good evidence that EBV infection precedes expansion of the malignant cell populations in some virus-associated tumours. However, this is clearly not always the case and for some of these tumours there are indications that clonal genetic alterations may occur prior to EBV infection. Thus, whilst there is good evidence to suggest that EBV is a human carcinogen, its precise role(s) in the development of virus-associated human tumours requires clarification.

Epstein-Barr virus infection and human malignancies

The Epstein-Barr virus (EBV) is a herpes virus which establishes a life-long persistent infection in over 90% of the human adult population world-wide. Based on its association with a variety of lymphoid and epithelial malignancies, EBV has been classified as a group 1 carcinogen by the International Agency for Research on Cancer. In this article we discuss the evidence supporting an aetiological role for EBV in the pathogenesis of human tumours. The biology of EBV infection will be described with special emphasis on viral transforming gene products. A brief survey of EBV-associated tumours is followed by a discussion of specific problems. Evidence is presented which suggests that failures of the EBV-specific immunity may play a role in the pathogenesis of EBV-associated tumours also in patients without clinically manifest immunodeficiencies. Finally, the timing of EBV infection in the pathogenesis of virus-associated malignancies is discussed. There is good evidence that EBV infection precedes expansion of the malignant cell populations in some virus-associated tumours. However, this is clearly not always the case and for some of these tumours there are indications that clonal genetic alterations may occur prior to EBV infection. Thus, whilst there is good evidence to suggest that EBV is a human carcinogen, its precise role(s) in the development of virus-associated human tumours requires clarification.

Molecular virology of Epstein-Barr virus

Epstein-Barr virus (EBV) interacts with its host in three distinct ways in a highly regulated fashion: (i) EBV infects human B lymphocytes and induces proliferation of the infected cells, (ii) it enters into a latent phase in vivo that follows the proliferative phase, and (iii) it can be reactivated giving rise to the production of infectious progeny for reinfection of cells of the same type or transmission of the virus to another individual. In healthy people, these processes take place simultaneously in different anatomical and functional compartments and are linked to each other in a highly dynamic steady-state equilibrium. The development of a genetic system has paved the way for the dissection of those processes at a molecular level that can be studied in vitro, i.e. B-cell immortalization and the lytic cycle leading to production of infectious progeny. Polymerase chain reaction analyses coupled to fluorescent-activated cell sorting has on the other hand allowed a descriptive analysis of the virus-host interaction in peripheral blood cells as well as in tonsillar B cells in vivo. This paper is aimed at compiling our present knowledge on the process of B-cell immortalization in vitro as well as in vivo latency, and attempts to integrate this knowledge into the framework of the viral life cycle in vivo.

EBNA-LP associates with cellular proteins including DNA-PK and HA95

EBNA-LP-associated proteins were identified by sequencing proteins that immunoprecipitated with Flag epitope-tagged EBNA-LP (FLP) from lymphoblasts in which FLP was stably expressed. The association of EBNA-LP with Hsp70 (72/73) was confirmed, and sequences of DNA-PK catalytic subunit (DNA-PKcs), HA95, Hsp27, prolyl 4-hydroxylase alpha-1 subunit, alpha-tubulin, and beta-tubulin were identified. The fraction of total cellular HA95 that associated with FLP was very high, while progressively lower fractions of the total DNA-PKcs, Hsp70, Hsp 27, alpha-tubulin, and beta-tubulin specifically associated with EBNA-LP as determined by immunoblotting with antibodies to these proteins. EBNA-LP bound to two domains in the DNA-PKcs C terminus and DNA-PKcs associated with the EBNA-LP repeat domain. DNA-PKcs that was bound to EBNA-LP phosphorylated p53 or EBNA-LP in vitro, and the phosphorylation of EBNA-LP was inhibited by Wortmannin, a specific in vitro inhibitor of DNA-PKcs.

The effects of the Epstein-Barr virus latent membrane protein 2A on B cell function

Epstein-Barr Virus (EBV) infects B-lymphocytes circulating through the oral epithelium and establishes a lifelong latent infection in a subset of mature-memory B cells. In these latently infected B cells, EBV exhibits limited gene expression with the latent membrane protein 2A (LMP2A) being the most consistently detected transcript. This persistent expression, coupled with many studies ofthe function of LMP2A in vitro and invivo, indicates that LMP2A is functioning to control some aspect of viral latency. Establishment and maintenance of viral latency requires exquisite manipulation of normal B cell signaling and function. LMP2A is capable of blocking normal B cell signal transduction in vitro, suggesting that LMP2A may act to regulate lytic activation from latency in vivo. Furthermore, LMP2A is capable of providing B cells with survival signals in the absence of normal BCR signaling. These data show that LMP2A may help EBV-infected cells to persist in vivo. This review discusses the advances that have been made in our understanding of LMP2A and the effects it has on B cell development, activation, and viral latency.

Smokeless tobacco extracts modulate exogenous gene expression in early passage cultured human oral epithelial cells: an in vitro system to study chemical and viral enhancer/promoter interaction

The increased risk for cancers of the oral cavity from smokeless tobacco use may reflect the interaction of tobacco with genetic factors, such as oncogenes, and other exogenous factors, such as viruses. An in vitro system was developed based on expression of the chloramphenicol acetyltransferase (CAT) reporter gene to study interactions of chemical treatments with viral enhancer/promoters in early passage cell cultures of oral cavity-derived epithelial cells. Expression of CAT in transfected cells was significantly greater with CAT under the control of the cytomegalovirus immediate early enhancer/promoter (pCEP4/CAT) compared to the Rous sarcoma virus long terminal repeat enhancer/promoter (pRSV-cat) and the simian virus 40 (SV40) early promoter (pSV2-cat). No CAT expression was detected using corresponding control plasmids without the CAT reporter gene. Using this system, smokeless tobacco extracts prepared from either dry snuff or moist snuff delayed maximum CAT expression from Day 4 to Day 5, with sustained, significantly increased CAT expression at 12 days compared to the declining CAT expression observed in untreated control cells. Smokeless tobacco extracts can modulate intracellular gene expression. This system provides an in vitro model to test specificity of toxic agents on enhancer/promoter activity and the interaction on exogenous gene expression.

The genetic approach to the Epstein-Barr virus: from basic virology to gene therapy

The Epstein-Barr virus (EBV) infects humans and the genome of this infectious agent has been detected in several tumour types, ranging from lymphomas to carcinomas. The analysis of the functions of the numerous viral proteins encoded by EBV has been impeded by the large size of the viral genome, which renders the construction of viral mutants difficult. To overcome these limitations, several genetic systems have been developed that allow the modification of the viral genome. Two different approaches, depending on the host cell type in which the viral mutants are generated, have been used in the past. Traditionally, mutants were constructed in EBV infected eukaryotic cells, but more recently, approaches that make use of a recombinant EBV cloned in Escherichia coli have been proposed. The phenotype associated with the inactivation or modification of nearly 20 of the 100 EBV viral genes has been reported in the literature. In most of the reported cases, the EBV latent genes that mediate the ability of EBV to immortalize infected cells were the targets of the genetic analysis, but some virus mutants in which genes involved in DNA lytic replication or infection were disrupted have also been reported. The ability to modify the viral genome also opens the way to the construction of viral strains with medical relevance. A cell line infected by a virus that lacks the EBV packaging sequences can be used as a helper cell line for the encapsidation of EBV based viral vectors. This cell line will allow the evaluation of EBV as a gene transfer system with applications in gene therapy. Finally, genetically modified non-pathogenic strains will provide a basis for the design of an attenuated EBV live vaccine.

Evolutionary aspects of oncogenic herpesviruses

Several of the gamma-herpesviruses are known to have cellular transforming and oncogenic properties. The genomes of eight distinct gamma-herpesviruses have been sequenced, and the resulting database of information has enabled the identification of genetic similarities and differences between evolutionarily closely related and distant viruses of the subfamily and between the gamma-herpesviruses and other members of the herpesvirus family. The recognition of coincident loci of genetic divergence between individual gamma-herpesviruses and the identification of novel genes and cellular gene homologues in these genomic regions has delineated a subset of genes that are likely to contribute to the unique biological properties of these viruses. These genes, together with gamma-herpesvirus conserved genes not found in viruses outside the family, might be responsible for virus specific pathogenicity and pathogenic effects, such as viral associated neoplasia, characteristic of the subfamily. The presence of the gamma-herpesvirus major divergent genomic loci and the apparent increased mutational frequencies of homologous genes (where they occur) within these regions, indicates that these loci possess particular features that drive genetic divergence. Whatever the mechanisms underlying this phenomenon, it potentially provides the basis for the relatively rapid adaptation and evolution of gamma-herpesviruses and the diversity of biological and pathogenic properties.

Initiation of DNA replication within oriP is dispensable for stable replication of the latent Epstein-Barr virus chromosome after infection of established cell lines

The 165-kb circularized chromosome of Epstein-Barr virus (EBV) is replicated in latently infected cells once per cell cycle by host proteins during S phase. Replication initiates at multiple sites on latent EBV chromosomes, including within a 1.8-kb region called oriP, which can provide both replication and stabilization for recombinant plasmids in the presence of the EBV-encoded protein, EBNA-1. Replication initiates at or near the dyad symmetry component (DS) of oriP, which depends on multiple EBNA-1 binding sites for activity. To test the importance of the replication function of oriP, the DS was deleted from the viral genome. EBV mutants lacking the DS and carrying a selectable gene could establish latent infections in BL30 cells, in which circular, mutant viral chromosomes were stably maintained. Analysis of replication fork movement using two-dimensional gel electrophoresis showed that the deletion of the DS reduced the initiation events to an undetectable level within the oriP region so that this segment was replicated exclusively by forks entering the region from either direction. A significant slowing or stalling of replication forks that occurs normally at the approximate position of the DS was also eliminated by deletion of the DS. The results confirm the DS as both a replication origin and a place where replication forks pause. Since the replication function of oriP is dispensable at least in certain cell lines, the essential role of EBNA-1 for infection of these cell lines is likely to be that of stabilizing the EBV chromosome by associating with the 30-bp repeats of oriP. The results also imply that in established cell lines, the EBV chromosome can be efficiently replicated entirely from origins that are activated by cellular factors. Presumably, initiation of replication at the DS, mediated by EBNA-1, is important for the natural life cycle of EBV, perhaps in establishing latent infections of normal B cells.

Epstein-Barr virus latency: LMP2, a regulator or means for Epstein-Barr virus persistence?

Like other herpesviruses, Epstein-Barr virus (EBV) persists in its host through an ability to establish a latent infection that periodically reactivates, producing infectious virus that infects naïve hosts. Disease syndromes in humans caused by EBV reflect the cell types that EBV infects, being primarily of lymphoid or epithelial origin. The most notable lymphoid disease, infectious mononucleosis, is a self-limiting lymphoproliferative disease that occurs in normal adolescents on primary infection. Children are normally able to resolve primary EBV infection with few or no symptoms. By the age of 25 most individuals are EBV seropositive. EBV is associated with a variety of hematopoietic cancers such as African Burkitt's lymphoma, Hodgkin's, and adult T-cell leukemia. EBV-associated lymphoproliferative disease occurs in individuals with congenital or acquired cellular immune deficiencies. The two notable epithelial diseases associated with EBV infection are nasopharyngeal cancer, a malignancy endemic to southern China, and oral hairy leukoplakia, an epithelial hyperplasia of the lingual squamous epithelium in AIDS patients. Latent membrane protein 2 (LMP2) is expressed both in normal EBV latency and EBV-associated pathologies. LMP2 may regulate reactivation from latency by interfering with normal B-cell signal transduction processes and in doing so may also provide a survival signal that could be important for viral persistence. Current knowledge about the function of LMP2 is described, defining a new class of regulators of herpesvirus latency.

Identification of a spliced gene from Kaposi's sarcoma-associated herpesvirus encoding a protein with similarities to latent membrane proteins 1 and 2A of Epstein-Barr virus

Kaposi's sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8 (HHV-8) is a novel herpesvirus implicated as the causative agent of Kaposi's sarcoma (KS), primary effusion lymphoma, and some cases of multicentric Castleman's disease. KSHV persists in the majority of KS spindle (endothelial tumor) cells and lymphoid cells in a latent form, with only a limited set of viral genes expressed in a tissue-specific manner. Here, we report the identification of a family of alternatively-spliced transcripts of approximately 7.5 kb expressed in latently infected body cavity-based lymphoma (BCBL) cell lines which are predicted to encode membrane proteins with similarities to the LMP2A and LMP1 proteins of Epstein-Barr virus. In two highly divergent sequence variants of the right end of the KSHV genome, alternative splicing of eight exons located between KSHV ORF 75 and the terminal repeats yields transcripts appropriate for proteins with up to 12 transmembrane domains, followed by a hydrophilic C-terminal, presumably cytoplasmic, domain. This C-terminal domain contains several YxxI/L motifs reminiscent of LMP2A and a putative TRAF binding site as in LMP1. In latently (persistently) infected BCBL cells the predominant transcript utilizes all eight exons, whereas in phorbol-ester-induced cells, a shorter transcript, lacking exons 4 and 5, is also abundant. We also found evidence for an alternative use of exon 1. Transfection of an epitope-tagged cDNA construct containing all exons indicates that the encoded protein is localized on cell surface and intracellular membranes, and glutathione S-transferase pull-down experiments indicate that its cytoplasmic domain, like that of LMP1, interacts with TRAF1, -2, and -3. Two of 20 KS patients had antibodies to the hydrophilic C-terminal domain, suggesting that the protein is expressed in vivo.

Epstein-Barr virus lacking latent membrane protein 2 immortalizes B cells with efficiency indistinguishable from that of wild-type virus

Epstein-Barr virus (EBV) is a human herpesvirus that efficiently transforms and immortalizes human primary B lymphocytes. In this study, the role of latent membrane protein 2 (LMP2) in EBV growth transformation was investigated. LMP2 is a virally encoded membrane protein expressed in EBV-immortalized B cells previously shown to be nonessential for EBV transformation. However, a recent study reported that LMP2 may be an important determinant for efficient B cell transformation (Brielmeier et al., Journal of General Virology 77, 2807-2818, 1996). In this study a deletion mutation was introduced into the LMP2 gene using an E. coli mini-EBV construct containing sufficient EBV DNA to result in growth transformation of primary B cells. In an alternative approach, the introduction of the gene encoding the enhanced green fluorescent protein (EGFP) by homologous recombination into the LMP2 gene of EBV strain B95-8, generating the same LMP2 deletion mutation is reported. Careful quantification of B cell transformation using the EGFP+ LMP2- recombinant virus determined that in liquid culture medium or in culture medium containing soft agarose there was no difference in the ability of LMP2- virus to immortalize primary human B cells when compared to that of wild-type virus.

Mechanisms that regulate Epstein-Barr virus EBNA-1 gene transcription during restricted latency are conserved among lymphocryptoviruses of Old World primates

Epstein-Barr virus (EBV), the only known human lymphocryptovirus (LCV), displays a remarkable degree of genetic and biologic identity to LCVs that infect Old World primates. Within their natural hosts, infection by these viruses recapitulates many key aspects of EBV infection, including the establishment of long-term latency within B lymphocytes, and is therefore a potentially valuable animal model of EBV infection. However, it is unclear whether these LCVs have adopted or maintained the same mechanisms used by EBV to express essential viral proteins, such as EBNA-1, in the face of cell-mediated repression of EBV gene expression that occurs upon establishment of the asymptomatic carrier state. To address this issue, we determined whether the endogenous LCVs of baboon (Cercopithecine herpesvirus 12) and rhesus macaque (Cercopithecine herpesvirus 15) have the functional equivalent of the EBV promoter Qp, which mediates exclusive expression of EBNA-1 during the restricted programs of EBV latency associated with the carrier state. Our results indicate that (i) both the baboon and rhesus macaque LCVs have a genomic locus that is highly homologous to the EBV Qp region, (ii) key cis-regulatory elements of Qp are conserved in these LCV genomes and compose promoters that are functionally indistinguishable from EBV Qp, and (iii) EBNA-1 transcripts identical in structure to EBV Qp-specific EBNA-1 mRNAs are present in nonhuman LCV-infected cells, demonstrating that these Qp homologs are indeed utilized as alternative EBNA-1 promoters. These observations indicate that the molecular mechanisms which regulate EBV gene expression during restricted latency have been conserved among the LCVs. The contribution of these mechanisms to viral persistence in vivo can now be experimentally tested in nonhuman primate models of LCV infection.

Epstein-Barr virus latent membrane protein 2 associates with and is a substrate for mitogen-activated protein kinase

The latent membrane protein 2 (LMP2) of Epstein-Barr virus interferes with B-lymphocyte signal transduction through the immunoglobulin (Ig) receptor. Two isoforms of LMP2 exist and differ only in that one isoform (LMP2a) contains an N-terminal cytoplasmic domain that the other isoform does not. LMP2a is a phosphoprotein that is phosphorylated on tyrosines and serines in the cytoplasmic domain. GST1-119, a glutathione S-transferase (GST) fusion protein containing the 119 amino acids of the cytoplasmic domain, affinity precipitated serine kinase activity from BJAB cell extracts. The affinity-precipitated kinase phosphorylated LMP2a sequences, and kinase activity was increased following induction. Probing of Western immunoblots of affinity-precipitated proteins showed that the Erk1 form of mitogen-activated protein kinase (MAPK) was present. Purified MAPK phosphorylated GST fusion proteins containing the cytoplasmic domain of LMP2a and mutational analyses were used to identify S15 and S102 as the sites of in vitro phosphorylation. A polyclonal rabbit antiserum was prepared against a maltose binding protein-LMP2a cytoplasmic domain fusion protein (MBP1-119) and used to immunoprecipitate LMP2a from the in vitro-immortalized lymphoblastoid B-cell line B95-8CR. LMP2a immunoprecipitates from B95-8CR contained MAPK as a coprecipitated protein. Cross-linking surface Ig on B95-8CR cells failed to induce MAPK activity within the cells. Treatment of B95-8CR with phorbol myristate acetate (PMA) was able to bypass the Ig receptor block and activate MAPK activity. Phosphorylation of LMP2a on serine residues increased after PMA induction. The possible role for LMP2a serine phosphorylation by MAPK in the control of latency is discussed.

Identification of a novel protein encoded by the BamHI A region of the Epstein-Barr virus

An unusual set of 3' coterminal, spliced mRNAs transcribed through the BamHI A fragment have been previously identified in nasopharyngeal carcinoma (NPC) tissues. These RNAs have also been detected at low levels in Burkitt's lymphoma (BL) cell lines and Epstein-Barr virus (EBV)-transformed lymphocytes. Sequence analyses of clones from a cDNA library derived from the C15 NPC tumor indicated that the primary transcripts are differentially spliced, giving rise to a family of related transcripts, all of which encompass the BARF0 open reading frame (ORF) at the 3' end of the transcripts. One cDNA was identified that extended the BARF0 ORF at the 5' end, forming the RK-BARF0 ORF. In this study, a rabbit antiserum to a synthetic peptide representing an amino acid sequence encoded by the BARF0 ORF was prepared. This antiserum detected a glutathione S-transferase-BARF0 fusion protein and both BARF0 and RK-BARF0 proteins expressed from transfected constructs in H1299 cells. The serum also immunoprecipitated the 20-kDa BARF0 and 30-kDa RK-BARF0 in vitro-translated proteins. Immunoblot analyses identified a protein doublet of 30 and 35 kDa in all of the EBV-infected cell lines tested. Cellular fractionation studies revealed that the proteins were membrane associated. The sizes of the proteins detected in cell lines and their association with membranes suggests that they are likely encoded by the RK-BARF0 transcript, which is predicted to contain a membrane localization signal. The proteins were also detected in protein extracts prepared from NPC biopsies and a BL biopsy but not from hairy leukoplakia, a permissive EBV infection. These results reveal that the rightward RNA transcripts from the BamHI A region of EBV encode one or more proteins that are expressed in latently infected cells and in tumor tissue.

Identification of latent membrane protein 2A (LMP2A) domains essential for the LMP2A dominant-negative effect on B-lymphocyte surface immunoglobulin signal transduction

Epstein-Barr virus (EBV) recombinants which carry three different deletion mutations in the LMP2A cytoplasmic amino-terminal domain were constructed. The presence of each mutation, LMP2A delta 21-36, LMP2A delta 21-64, and LMP2A delta 21-85, in EBV-infected transformed lymphoblastoid cell lines was confirmed by PCR analysis and Southern blot hybridization. Confirmation of mutant LMP2A protein expression was by immunofluorescence and immunoblotting with a newly identified rat monoclonal antibody that recognizes each of the LMP2A deletion mutations. Lymphoblastoid cell lines infected with recombinant EBV DNAs containing the mutations were analyzed for loss of LMP2A's dominant-negative effect on surface immunoglobulin signal transduction by monitoring induction of tyrosine phosphorylation, calcium mobilization, and activation of lytic replication following surface immunoglobulin cross-linking. Domains of LMP2A important for induction of tyrosine phosphorylation, calcium mobilization, and activation of lytic replication were identified.

5' Coding and regulatory region sequence divergence with conserved function of the Epstein-Barr virus LMP2A homolog in herpesvirus papio

B-lymphotropic herpesviruses naturally infecting Old World primates share biologic, epidemiologic, pathogenic, and molecular features with the human pathogen Epstein-Barr virus (EBV). These related gammaherpesviruses have colinear genomes with considerable nucleotide homology. The replicative cycle genes share a high degree of homology across species, whereas the transformation-associated EBV latent genes appear to be much more divergent. For example, the EBV BamHI Nhet fragment, which encodes all or part of the EBV latent infection membrane proteins, cross-hybridizes poorly to DNA from nonhuman primate B-lymphotropic herpesviruses. A viral DNA fragment corresponding to this region of the EBV genome was isolated from the baboon B-lymphotropic herpesvirus, herpesvirus papio, and used to clone a herpesvirus papio cDNA corresponding to EBV LMP2A. At least three tyrosine kinase interaction motifs are conserved despite significant amino acid divergence of the herpesvirus papio LMP2A first exon from the EBV homolog. Functionally, the herpesvirus papio LMP2A is tyrosine phosphorylated and induces tyrosine phosphorylation of cell proteins similar to EBV LMP2A. The 12 hydrophobic LMP2 transmembrane domains are well conserved. Two CBP (Jk) binding sites important for EBNA-2-induced transactivation of the LMP2A promoter are also present in the herpesvirus papio LMP2A promoter, and the simian LMP2A promoter is also responsive to EBV EBNA-2-induced transactivation in human B cells. Thus, transcriptional regulation, splicing, kinase interaction sites, and tyrosine phosphorylation of the LMP2A homologs have been conserved despite significant sequences heterogeneity in the preterminal repeat regions of these human and nonhuman primate EBVs. The conservation of the LMP2 gene, despite its apparent nonessential role for in vitro EBV infection, suggests an important role for LMP2A in vivo. The similarities between these human and simian B-lymphotropic herpesviruses, and the LMP2 genes in particular, suggest that the function of LMP2 in vivo could be addressed by using recombinant LMP2A-mutant simian viruses and experimental infection of Old World primates.

Immortalization of human primary B lymphocytes in vitro with DNA

Epstein-Barr virus (EBV) is a human DNA tumor virus that efficiently immortalizes human primary B lymphocytes in vitro. Although viral genes that are expressed in latently infected B lymphocytes have been shown to function in cellular growth control, their detailed genetic analysis has been cumbersome for two reasons. The viral genome is too large to permit genetic engineering and human primary B lymphocytes, the only targets for infection by EBV in vitro, are both intractable in culture and recalcitrant to DNA transfection. To overcome these obstacles, we have assembled all the essential genes of EBV on a single recombinant vector molecule in Escherichia coli. We show here that this mini-EBV plasmid can yield immortalized B cells upon transfer of its naked DNA into human primary B lymphocytes. Established cell lines carry recombinant vector DNA and cannot support virus production. Because this DNA can be easily manipulated in E. coli, mutant mini-EBVs as well as foreign genes can now be introduced and studied successfully in recipient B lymphocytes from any human donors. These mini-EBVs therefore are potentially useful for human gene therapy.

Integral membrane protein 2 of Epstein-Barr virus regulates reactivation from latency through dominant negative effects on protein-tyrosine kinases

An Epstein-Barr virus-encoded protein, LMP2, blocks the effects of surface immunoglobulin (slg) cross-linking on calcium mobilization and on lytic reactivation of EBV in latently infected and growth-transformed primary human B lymphocytes. In wild-type EBV-transformed cells, LMP2 is constitutively tyrosine phosphorylated and is associated with Lyn and Syk protein-tyrosine kinases (PTKs). Baseline Lyn PTK activity is substantially reduced, and slg cross-linking fails to activate Lyn, Syk, Pl3-K, PLC gamma 2, Vav, Shc, and MAPK. Syk, Pl3-K, PLC gamma 2, and Vav are constitutively tyrosine phosphorylated, and their tyrosine phosphorylation does not change following slg cross-linking. In contrast, cross-linking slg on cells transformed by LMP2 null mutant EBV recombinants triggers the same protein tyrosine kinase cascade as in noninfected B lymphocytes. These data are consistent with a model in which LMP2 is a constitutive dominant negative modulator of slg receptor signaling through its effects on Lyn, Syk, or regulators of these kinases.

Immortalization of human B lymphocytes by a plasmid containing 71 kilobase pairs of Epstein-Barr virus DNA

We have assembled derivatives of Epstein-Barr Virus (EBV) that include 71 kbp of noncontiguous DNA sequences cloned into a prokaryotic F-factor plasmid. These mini-EBVs, when introduced into an EBV-containing lymphoblastoid cell, can be packaged by the endogenous helper virus. One such mini-EBV was found to have a single C residue deleted from its EBNA3a open reading frame. When packaged, this mini-EBV initiates proliferation of infected primary human B lymphocytes only in conjunction with a complementing helper virus. Proliferation of the infected cells, however, was maintained either alone by the mini-EBV containing the mutated EBNA3a open reading frame or alone by its derivative in which the EBNA3a open reading frame had been healed of its lesion by recombination with the helper virus. The mini-EBV with a wild-type EBNA3a open reading frame when packaged alone can both initiate and maintain proliferation upon infection of primary human B lymphocytes. These findings identify 41% of EBV DNA which is sufficient to immortalize primary human B lymphocytes and provide an assay to distinguish virus contributions to initiation or maintenance of cell proliferation or both. They also identify EBNA3a as a transforming gene, which contributes primarily to the initiation of cell proliferation.