Monoclonal and polyclonal antibodies against Epstein-Barr virus nuclear antigen 5 (EBNA-5) detect multiple protein species in Burkitt's lymphoma and lymphoblastoid cell lines

Identification of major phosphorylation sites of Epstein-Barr virus nuclear antigen leader protein (EBNA-LP): ability of EBNA-LP to induce latent membrane protein 1 cooperatively with EBNA-2 is regulated by phosphorylation

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) is a phosphoprotein suggested to play important roles in EBV-induced immortalization of B cells. One of the potential functions of EBNA-LP is a cooperative induction with EBNA-2 of viral and cellular gene expression, including that of the genes for viral latent membrane protein 1 (LMP-1) and cellular cyclin D2. We report here that the phosphorylation of EBNA-LP by cellular kinase(s) is critical to its ability to cooperate with EBNA-2 in up-regulating the expression of LMP-1 in a B-lymphoma cell line. Our conclusion is based on the following observations. (i) Mass-spectrometric analysis of purified EBNA-LP and mutational analyses of EBNA-LP revealed that the serine residue at position 35 in the W2 repeat domain is the major phosphorylation site of EBNA-LP in vivo. (ii) Substitutions of this site in each W2 repeat domain with alanine markedly reduced the ability of the protein to induce LMP-1 expression in combination with EBNA-2 in Akata cells. (iii) Replacement at the major phosphorylation sites with glutamic acids restored the wild-type phenotype. It is well established that this substitution mimics constitutive phosphorylation. These results indicated that the coactivator function of EBNA-LP is regulated by phosphorylation.

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.

Direct and indirect regulation of cytokine and cell cycle proteins by EBNA-2 during Epstein-Barr virus infection

We have studied the pathways of regulation of cytokine and cell cycle control proteins during infection of human B lymphocytes by Epstein-Barr virus (EBV). Among 30 cytokine RNAs analyzed by the RNase protection assay, tumor necrosis factor alpha (TNF-alpha), granulocyte colony-stimulating factor, lymphotoxin (LT), and LTbeta were found to be regulated within 20 h of EBV infection of primary B cells. Similar results were obtained using the estrogen-regulated EBNA-2 cell line EREB2.5, in which RNAs for LT and TNF-alpha were induced within 6 h of activation of EBNA-2. Expression of Notch also caused an induction of TNF-alpha RNA. The induction of TNF-alpha RNA by EBNA-2 was indirect, and constitutive expression of either LMP-1 or c-myc proteins did not substitute for EBNA-2 in induction of TNF-alpha RNA. Cyclin D2 is also an indirect target of EBNA-2-mediated transactivation. EBNA-2 was found to activate the cyclin D2 promoter in a transient-transfection assay. A mutant of EBNA-2 that does not bind RBP-Jkappa retained some activity in this assay, and activation did not depend on the presence of B-cell-specific factors. Deletion analysis of the cyclin D2 promoter revealed that removal of sequences containing E-box c-myc consensus DNA binding sequences did not reduce EBNA-2-mediated activation of the cyclin D2 promoter in the transient-transfection assay. The results indicate that cytokines are an early target of EBNA-2 and that EBNA-2 can regulate cyclin D2 transcription in EBV-infected cells by mechanisms additional to the c-myc pathway.

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.

Proteasome inhibitor induces nucleolar translocation of Epstein-Barr virus-encoded EBNA-5

We have previously shown that Epstein-Barr virus (EBV)-encoded EBNA-5 is localized to PML bodies (PODs) in EBV-immortalized lymphoblastoid cell lines (LCLs). Here we have extended our study of the subnuclear localization of EBNA-5 and found a strict co-localization with PML in LCLs and in BL lines with an immunoblastic, LCL-like phenotype. Moreover, GFP-EBNA-5 accumulated in PML bodies upon transfection into LCLs. In contrast, transfection of cell lines of non-immunoblastic origin with an EBNA-5 expression construct showed preferential localization of the protein to the nucleoplasm. Since PML is involved in proteasome-dependent protein degradation, we investigated the total levels and sub-cellular localization of EBNA-5 upon inhibition of proteasome activity. We found that a proteasome inhibitor, MG132, induced the translocation of both endogenous and transfected EBNA-5 to the nucleoli in every cell line tested. The total EBNA-5 protein levels were not affected by the proteasomal block. EBNA-5 forms complexes with heat shock protein Hsp70. The proteasome inhibitor induced a rise in total levels of Hsp70 and dramatically changed its homogeneous nuclear and cytoplasmic distribution into nucleolar and cytoplasmic. This effect was EBNA-5-independent. The nucleolar localization of Hsp70 was enhanced by the presence of EBNA-5, however. EBNA-5 also enhanced the nucleolar translocation of a mutant p53 in a colon cancer line, SW480, treated with MG132. The coordinated changes in EBNA-5 and Hsp70 localization and the effect of EBNA-5 on mutant p53 distribution upon MG132 treatment might reflect the involvement of EBNA-5 in the regulation of intracellular protein trafficking associated with the proteasome-mediated degradation.

Methylation of transcription factor binding sites in the Epstein-Barr virus latent cycle promoter Wp coincides with promoter down-regulation during virus-induced B-cell transformation

Two Epstein-Barr virus latent cycle promoters for nuclear antigen expression, Wp and Cp, are activated sequentially during virus-induced transformation of B cells to B lymphoblastoid cell lines (LCLs) in vitro. Previously published restriction enzyme studies have indicated hypomethylation of CpG dinucleotides in the Wp and Cp regions of the viral genome in established LCLs, whereas these same regions appeared to be hypermethylated in Burkitt's lymphoma cells, where Wp and Cp are inactive. Here, using the more sensitive technique of bisulfite genomic sequencing, we reexamined the situation in established LCLs with the typical pattern of dominant Cp usage; surprisingly, this showed substantial methylation in the 400-bp regulatory region upstream of the Wp start site. This was not an artifact of long-term in vitro passage, since, in cultures of recently infected B cells, we found progressive methylation of Wp (but not Cp) regulatory sequences occurring between 7 and 21 days postinfection, coincident with the period in which dominant nuclear antigen promoter usage switches from Wp to Cp. Furthermore, in the equivalent in vivo situation, i.e., in the circulating B cells of acute infectious mononucleosis patients undergoing primary EBV infection, we again frequently observed selective methylation of Wp but not Cp sequences. An effector role for methylation in Wp silencing was supported by methylation cassette assays of Wp reporter constructs and by bandshift assays, where the binding of two sets of transcription factors important for Wp activation in B cells, BSAP/Pax5 and CREB/ATF proteins, was shown to be blocked by methylation of their binding sites.

Interaction of Epstein-Barr virus nuclear antigen leader protein (EBNA-LP) with HS1-associated protein X-1: implication of cytoplasmic function of EBNA-LP

Epstein-Barr virus (EBV) nuclear antigen leader protein (EBNA-LP) consists of W1W2 repeats and a unique C-terminal Y1Y2 domain and has been suggested to play an important role in EBV-induced transformation. To identify the cellular factors interacting with EBNA-LP, we performed a yeast two-hybrid screen, using EBNA-LP cDNA containing four W1W2 repeats as bait and an EBV-transformed human peripheral blood lymphocyte cDNA library as the source of cellular genes. Our results were as follows. (i) All three cDNAs in positive yeast colonies were found to encode the same cellular protein, HS1-associated protein X-1 (HAX-1), which is localized mainly in the cytoplasm and has been suggested to be involved in the regulation of B-cell signal transduction and apoptosis. (ii) Mutational analysis of EBNA-LP revealed that the association with HAX-1 is mediated by the W1W2 repeat domain. (iii) A purified chimeric protein consisting of glutathione S-transferase fused to EBNA-LP specifically formed complexes with HAX-1 transiently expressed in COS-7 cells. (iv) When EBNA-LP and HAX-1 were coexpressed in COS-7 cells, EBNA-LP was specifically coimmunoprecipitated with HAX-1. (v) Careful cell fractionation experiments of an EBV-infected lymphoblastoid cell line revealed that EBNA-LP is localized in the cytoplasm as well as in the nucleus. (vi) When EBNA-LP containing four W1W2 repeats was expressed in COS-7 cells, EBNA-LP was detected mainly in the nucleus by immunofluorescence assay. Interestingly, when EBNA-LP containing a single W1W2 repeat was expressed in COS-7 cells, EBNA-LP was localized predominantly in the cytoplasm and was colocalized with HAX-1. These results indicate that EBNA-LP is in fact present and may have a significant function in the cytoplasm, possibly by interacting with and affecting the function of HAX-1.

Conserved regions in the Epstein-Barr virus leader protein define distinct domains required for nuclear localization and transcriptional cooperation with EBNA2

Epstein-Barr virus (EBV) EBNA-LP is a latent protein whose function is not fully understood. Recent studies have shown that EBNA-LP may be an important EBNA2 cofactor by enhancing EBNA2 stimulation of the latency C and LMP-1 promoters. To further our understanding of EBNA-LP function, we have introduced a series of mutations into evolutionarily conserved regions and tested the mutant proteins for the ability to enhance EBNA2 stimulation of the latency C and LMP-1 promoters. Three conserved regions (CR1 to CR3) are located in the repeat domains that are essential for the EBNA2 cooperativity function. In addition, three serine residues are also well conserved in the repeat domains. Clustered alanine mutations were introduced into CR1 to CR3, and the conserved serines were also changed to alanine residues in an EBNA-LP with two repeats, which is the minimal protein able to cooperate with EBNA2. Mutations introduced into CR1a had no effect on EBNA-LP function, while mutations introduced into CR1b resulted in EBNA-LP with slightly decreased activity. Mutations in CR1c and CR2 resulted in proteins that no longer localized exclusively to the nucleus and also had no EBNA2 cooperation activity. Mutations introduced into conserved serines S5/71 resulted in proteins with slightly higher activity, while mutations introduced into conserved serines S35/101 or in CR3 (which contains S60/126) resulted in EBNA-LP proteins with substantially reduced activity. The potential karyophilic signals within EBNA-LP CR1c and CR2 were also examined by introducing oligonucleotides encoding these positively charged amino acid groupings into a cytoplasmic test protein, herpes simplex virus DeltaIE175, and by examining the intracellular localization of the resulting proteins. This assay identified a strong nuclear localization signal between EBNA-LP amino acids 43 and 50 (109 to 117 in the second W repeat) comprising CR2, while EBNA-LP amino acids 29 to 36 (91 to 98 in the second W repeat) were unable to function independently as a nuclear localization signal. However, a combination of amino acids 29 to 50 resulted in more efficient nuclear localization than with amino acids 43 to 50 alone. These results indicate that EBNA-LP has a bipartite nuclear localization signal and that efficient nuclear localization is essential for EBNA2 cooperativity function. Interestingly, EBNA-LP with only a single repeat localized exclusively to the cytoplasm, providing an explanation for why this isoform has no activity. In addition, two conserved serine residues which are distinct from nuclear import functions are important for EBNA2 cooperativity function.

Hairy leukoplakia: an unusual combination of transforming and permissive Epstein-Barr virus infections

Human herpesviruses are characterized by distinct states of infection. Typically in permissive herpesvirus infection, abundant virus production results in cell lysis. In latent transforming Epstein-Barr virus (EBV) infection, viral proteins that induce cell growth are expressed. The immunodeficiency-associated hairy leukoplakia (HLP) lesion is the only pathologic manifestation of permissive EBV infection; however, within HLP, viral proteins characteristic of latent infection have also been detected. In this study, we further analyzed expression of EBV latent genes and investigated their contribution to the unique histologic phenotype of HLP. Coexpression of lytic and transforming viral proteins was detected simultaneously within individual HLP keratinocytes. LMP1 has now been shown to be uniformly expressed in the affected tissue, and it is associated and colocalizes with tumor necrosis factor receptor-associated factor (TRAF) signaling molecules. Effects induced by activated TRAF signaling that were detected in HLP included activation of NF-kappaB and c-Jun terminal kinase 1 (JNK1) and upregulated expression of epidermal growth factor receptor (EGFR), CD40, A20, and TRAFs. This study identifies a novel state of EBV infection with concurrent expression of replicative and transforming proteins. It is probable that both replicative and latent proteins contribute to HLP development and induce many of the histologic features of HLP, such as acanthosis and hyperproliferation. In contrast to other permissive herpesvirus infections, expression of EBV transforming proteins within the permissively infected HLP tissue enables epithelial cell survival and may enhance viral replication.

Epstein-Barr virus suppresses a G(2)/M checkpoint activated by genotoxins

Several Epstein-Barr virus (EBV)-negative Burkitt lymphoma-derived cell lines (for example, BL41 and Ramos) are extremely sensitive to genotoxic drugs despite being functionally null for the tumor suppressor p53. They rapidly undergo apoptosis, largely from G(2)/M of the cell cycle. 5-bromo-2'-deoxyuridine labeling experiments showed that although the treated cells can pass through S phase, they are unable to complete cell division, suggesting that a G(2)/M checkpoint is activated. Surprisingly, latent infection of these genotoxin-sensitive cells with EBV protects them from both apoptosis and cell cycle arrest, allowing them to complete the division cycle. However, a comparison with EBV-immortalized B-lymphoblastoid cell lines (which have functional p53) showed that EBV does not block apoptosis per se but rather abrogates the activation of, or signalling from, the checkpoint in G(2)/M. Furthermore, analyses of BL41 and Ramos cells latently infected with P3HR1 mutant virus, which expresses only a subset of the latent viral genes, showed that LMP-1, the main antiapoptotic latent protein encoded by EBV, is not involved in the protection afforded here by viral infection. This conclusion was confirmed by analysis of clones of BL41 stably expressing LMP-1 from a transfected plasmid, which respond like the parental cell line. Although steady-state levels of Bcl-2 and related proteins varied between BL41 lines and clones, they did not change significantly during apoptosis, nor was the level of any of these anti- or proapoptotic proteins predictive of the outcome of treatment. We have demonstrated that a subset of EBV latent gene products can inactivate a cell cycle checkpoint for monitoring the fidelity and timing of cell division and therefore genomic integrity. This is likely to be important in EBV-associated growth transformation of B cells and perhaps tumorigenesis. Furthermore, this study suggests that EBV will be a unique tool for investigating the intimate relationship between cell cycle regulation and apoptosis.

Sequence and functional analysis of EBNA-LP and EBNA2 proteins from nonhuman primate lymphocryptoviruses

The Epstein-Barr virus (EBV) EBNA-LP and EBNA2 proteins are the first to be synthesized during establishment of latent infection in B lymphocytes. EBNA2 is a key transcriptional regulator of both viral and cellular gene expression and is essential for EBV-induced immortalization of B lymphocytes. EBNA-LP is also important for EBV-induced immortalization of B lymphocytes, but far less is known about the functional domains and cellular cofactors that mediate EBNA-LP function. While recent studies suggest that serine phosphorylation of EBNA-LP and coactivation of EBNA2-mediated transactivation are important, more detailed mutational and genetic studies are complicated by the repeat regions that comprise the majority of the EBNA-LP sequence. Therefore, we have used a comparative approach by studying the EBNA-LP homologues from baboon and rhesus macaque lymphocryptoviruses (LCVs) (baboon LCV and rhesus LCV). The predicted baboon and rhesus LCV EBNA-LP amino acid sequences are 61 and 64% identical to the EBV EBNA-LP W1 and W2 exons and 51% identical to the EBV EBNA-LP Y1 and Y2 exons. Five evolutionarily conserved regions can be defined, and four of eight potential serine residues are conserved among all three EBNA-LPs. The major internal repeat sequence also revealed a highly conserved Wp EBNA promoter with strong conservation of upstream activating sequences important for Wp transcriptional regulation. To test whether transcriptional coactivating properties were common to the rhesus LCV EBNA-LP, a rhesus LCV EBNA2 homologue was cloned and expressed. The rhesus LCV EBNA2 transcriptionally transactivates EBNA2-responsive promoters through a CBF1-dependent mechanism. The rhesus LCV EBNA-LP was able to further enhance rhesus LCV or EBV EBNA2 transactivation 5- to 12-fold. Thus, there is strong structural and functional conservation among the simian EBNA-LP homologues. Identification of evolutionarily conserved serine residues and regions in EBNA-LP homologues provides important clues for identifying the cellular cofactors and molecular mechanisms mediating these conserved viral functions.

Distinct patterns of viral antigen expression in Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus coinfected body-cavity-based lymphoma cell lines: potential switches in latent gene expression due to coinfection

Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV), also referred to as human herpesvirus 8 (HHV-8), are human gammaherpesviruses associated with numerous lymphomas and proliferative diseases in humans. We were interested in the protein expression patterns of specific latent and lytic proteins from the EBV genome in two body-cavity-based lymphoma cell lines, BC-1 and BC-2, which are coinfected with EBV and KSHV. BC-1 and BC-2 were analyzed using specific antibodies to latent proteins known to be essential for EBV immortalization of human primary B-lymphocytes in vitro and lytic antigens important for EBV replication and production of viral progeny. The coinfected cell lines are compared with two singly infected KSHV cell lines to determine whether antibodies against EBV-specific proteins cross-reacted against KSHV antigens. All the KSHV-infected cell lines express the KSHV-specific latency-associated nuclear antigen (LANA) with a specific pattern in the nucleus. This staining was distinct from that seen for EBNA1 in the EBV coinfected lines BC-1 and BC-2 staining the nucleus as a diffused pattern throughout the nucleus with denser staining in some regions. The coinfected cell lines all express EBNA1 and LMP1 at lower levels compared with singly infected EBV lymphoblastoid cell lines (LCLs). However, the essential latent antigens EBNA2, EBNA3A, and EBNA3C are not expressed in BC-1 and BC-2. This indicates potential regulation of EBV latent gene expression by KSHV-encoded viral or KSHV-induced cellular gene products. Additionally, lytic gene expression analysis demonstrated that BZLF1 and BMRF1 are expressed along with other early antigens (EA-D). A specific protein is detected in a singly infected KSHV cell line with cross-reactivity to antibodies that detected the EA-D complex. Moreover, in all the cell lines infected with EBV, KSHV, or EBV and KSHV, human serum with antibodies against KSHV antigens recognizes specific viral antigens approximately 110 and 41-42 kDa, suggesting that human antibodies against KSHV-specific antigens can cross-react with similar EBV antigens. Therefore these data suggest that the EBV pattern of gene expression in the coinfected cell lines is a type II pattern of latency also seen in other human tumors including nasopharyngeal carcinoma and Hodgkin's lymphoma. This distinct pattern of latent and lytic gene expression in these cell lines may provide clues as to the selection for coinfection in these body cavity based lymphomas in immunocompromised hosts.

Control of cell cycle entry and apoptosis in B lymphocytes infected by Epstein-Barr virus

Infection of human B cells with Epstein-Barr virus (EBV) results in activation of the cell cycle and cell growth. To interpret the mechanisms by which EBV activates the cell, we have assayed many proteins involved in control of the G0 and G1 phases of the cell cycle and regulation of apoptosis. In EBV infection most of the changes, including the early induction of cyclin D2, are dependent on expression of EBV genes, but an alteration in the E2F-4 profile was partly independent of viral gene expression, presumably occurring in response to signal transduction activated when the virus binds to its receptor, CD21. By comparing the expression of genes controlling apoptosis, including those encoding several members of the BCL-2 family of proteins, the known relative resistance of EBV-immortalized B-cell lines to apoptosis induced by low serum was found to correlate with expression of both BCL-2 and A20. A20 can be regulated by the NF-kappaB transcription factor, which is known to be activated by the EBV LMP-1 protein. Quantitative assays demonstrated a direct temporal relationship between LMP-1 protein levels and active NF-kappaB during the time course of infection.

Multiple functions within the Epstein-Barr virus EBNA-3A protein

Two regions of the EBNA-3A protein of Epstein-Barr virus were shown to be capable of binding to the cell protein RBP-Jk (also known as CBF-1), a component of the Notch signaling pathway. Consistent with this binding, EBNA-3A inhibited reporter gene expression from plasmids containing RBP-Jk DNA binding sites within their promoters, including the Cp promoter. When EBNA-3A was linked to a GAL4 DNA binding domain, it repressed the activity of a promoter containing GAL4 binding sites at all plasmid concentrations tested. However, a deletion mutant of EBNA-3A lacking amino acids 100 to 364 showed a biphasic response in the GAL4 assay: it inhibited transcription at low DNA concentrations but activated it at high DNA concentrations. There appears to be a gene activation function within EBNA-3A that is masked in the full-length protein in this assay. Current models for EBNA-3 function have stressed transcription repression through binding to RBP-Jk, but we consider an alternative scheme in which the role of the binding of EBNA-3A, -3B, and -3C to RBP-Jk is to buffer the levels of active EBNA-3 protein. We have also found that the behavior of EBNA-3A in a cell fractionation procedure that distinguishes insoluble matrix from soluble cell fractions is modified by EBNA-LP, indicating a further novel level of interplay between the EBNA proteins.

The macrophage migration inhibitory factor MIF is a phenylpyruvate tautomerase

A macrophage migration inhibitory factor (MIF), originally described as a product of activated lymphocytes, has been defined as a 12 kDa protein, expressed in a wide variety of tissues. Here MIF is identified as a phenylpyruvate tautomerase (EC 5.3.2.1) having p-hydroxyphenylpyruvate and phenylpyruvate as its natural substrates. The definition of MIF as an enzyme may yield insight into the mechanism of action of this proinflammatory and immunomodulating cytokine.

Epstein-Barr virus leader protein enhances EBNA-2-mediated transactivation of latent membrane protein 1 expression: a role for the W1W2 repeat domain

The Epstein-Barr virus (EBV)-encoded leader protein EBNA-LP is made up of several 66-amino-acid repeats (the W1W2 domains) linked to a unique 45-amino-acid C-terminal sequence (the Y1Y2 domain). This protein is highly expressed along with a second nuclear antigen, EBNA-2, during the initial stages of virus-induced B-cell transformation. While EBNA-2's essential role in transformation as a transcriptional activatory is well documented, very little is known about EBNA-LP function except that recombinant viruses lacking the EBNA-LP Y1Y2 exons show reduced, but still detectable, transforming ability. This was taken as evidence that EBNA-LP plays an auxiliary role but is not essential for transformation. A recent study showed that EBNA-LP could cooperate with EBNA-2 in activating cyclin D2 transcription in resting B cells (A.J. Sinclair, L Palmero, G. Peters, and P.J. Farrell, EMBO J. 13:3321-3328, 1994). Here we report that EBNA-LP can also cooperate with EBNA-2 in up-regulating expression of the major EBV effector protein of B-cell transformation, latent membrane protein 1 (LMP1). In transient-transfection assays, EBNA-LP enhanced the level of EBNA-2-induced LMP1 expression by 5- to 10-fold in one Latency I Burkitt's lymphoma cell line, Eli-BL, and was absolutely required, along with EBNA-2, to induce LMP1 in a second line, Akata-BL. These changes in LMP1 protein expression appeared to be reflected at the transcriptional level. A study of EBNA-LP mutants showed that this cooperative function mapped to the W1W2 repeat domain rather than to Y1Y2. Because a Y1Y2-deleted form of EBNA-LP may therefore retain some aspects of wild-type function, the original data from virus recombinants leave open the possibility that EBNA-LP is actually an essential transforming gene.

Epstein-Barr virus nuclear protein LP stimulates EBNA-2 acidic domain-mediated transcriptional activation

Epstein-Barr virus (EBV) nuclear proteins EBNA-LP and EBNA-2 are the first two proteins expressed in latent infection of primary B lymphocytes. EBNA-2 is essential for lymphocyte transformation, and EBNA-LP is at least critical. While EBNA-2 activates specific viral and cellular promoters, EBNA-LP's role has been obscure. We now show that EBNA-LP stimulates EBNA-2 activation of the LMP1 promoter and of the LMP1/LMP2B bidirectional transcriptional regulatory element. EBNA-LP alone has only a negative effect. EBNA-LP also stimulates EBNA-2 activation of a multimerized regulatory element from the BamC EBNA promoter. Since both viral regulatory elements can bind the EBNA-2-associated cell protein RBPJ kappa, consensus RBPJ kappa binding sites were positioned upstream of the herpes simplex virus type 1 thymidine kinase promoter and were found to be sufficient for EBNA-LP and EBNA-2 coactivation. EBNA-LP strongly stimulated activation of an adenovirus E1b promoter with upstream Gal4 binding sites by a Gal4 DNA binding domain/ EBNA-2 acidic domain fusion protein, indicating that EBNA-LP coactivation requires only the EBNA-2 acidic domain to be localized near a promoter. The EBNA-LP stimulatory activity resides in the amino-terminal 66-amino-acid repeat domain. The carboxyl-terminal unique 45 amino acids appear to regulate EBNA-LP's effects. The first 11 amino acids of the 45 have a strong negative effect, while the last 10 are critical for the ability of the last 34 to relieve the negative effect. These results indicate that EBNA-LP's critical role in EBV-mediated cell growth transformation is in stimulating (and probably regulating) EBNA-2-mediated transcriptional activation.

Differential immunogenicity of Epstein-Barr virus (EBV) encoded growth transformation-associated antigens in a murine model system

The strong immunosurveillance of humans against EBV transformed immunoblasts, mediated by CD8+ cytotoxic T cells, is based on the recognition of peptides derived from eight of the nine growth transformation-associated proteins, the nuclear antigens EBNA2-6 and the membrane proteins LMP1, -2A and -2B. The ninth protein, EBNA1, required for maintenance of the viral episomes, and expressed in a cell phenotype independent manner, has not been found to generate a cytotoxic lymphocyte (CTL) response in humans. We tested whether EBNA1 has a similar immunologically privileged status in a species that has not encountered the virus in nature, the mouse. Non-immunogenic murine mammary carcinoma cells were transfected with the appropriate viral gene. Rejection responses were assayed in syngeneic mice following repeated immunisation with irradiated cells. Previously, we found that LMP1 expression in S6C, a murine mammary carcinoma of ACA (H-2f) origin, induces high rejectability, whereas corresponding EBNA1 transfectants remained non-immunogenic. In order to test whether this finding could be reproduced on another MHC class 1 background, we expressed LMP1 and EBNA1 in another non-immunogenic mammary carcinoma, SBfnHd of CBA (H-2k) origin. LMP1 but not EBNA1 transfectants were immunogenic in this system. In order to investigate whether other growth transformation-associated EBV proteins were immunogenic in the mouse, we also transfected the S6C cells with EBNA4, EBNA5, LMP2A and -2B. All four proteins induced strong rejection reactions. These findings are fully consistent with corresponding findings in the human system. They also show that the immunologically privileged status of EBNA1 is not due to some peculiarity of the long-standing co-existence between EBV and the human species, nor to any specific features of the human MHC class I system. They are consistent with the suggestion that EBNA1 may not be properly processed and/or transported, due to specific features of the protein itself.

Cell cycle stage-specific phosphorylation of the Epstein-Barr virus immortalization protein EBNA-LP

EBNA-LP is a viral nuclear oncoprotein implicated in the immortalization of B lymphocytes by Epstein-Barr virus. An analysis of EBNA-LP migration on polyacrylamide gels was performed with protein derived from the X50-7 lymphoblastoid cell line blocked by hydroxyurea or aphidicolin at the G1/S phase of the cell cycle or by nocodazole at the G2/M phase. More slowly migrating species of EBNA-LP were detected in G2/M phase-arrested cell extracts. Release from nocodazole G2/M block or treatment with phosphatase caused the more slowly migrating species of EBNA-LP to disappear. Analyses of 32PO(4)(3-)-labeled EBNA-LP protein immunoprecipitated from the drug-synchronized cells showed that phosphorylated EBNA-LP was present throughout the cell cycle but that phosphorylation increased in G2 and was maximal at G2/M. Phosphoamino acid analysis revealed that all phosphorylation was on serine residues only. The ability of EBNA-LP to be phosphorylated by p34(cdc2) kinase and casein kinase II exclusively on serines implicates these enzymes as being potentially involved in EBNA-LP phosphorylation.

Isolation of intertypic recombinants of Epstein-Barr virus from T-cell-immunocompromised individuals

All wild-type isolates of Epstein-Barr virus (EBV) analyzed to date for allelic polymorphisms of the nuclear antigen EBNA2 gene (in the BamHI YH region of the genome) and of the EBNA3A,-3B, -3C genes (tandemly arranged in the BamHI E region) have proved either uniformly type 1 or uniformly type 2 at all four loci. The absence of detectable intertypic recombination in the wild probably reflects the rarity with which individual carriers, and certainly individual target cells, become coinfected with both virus types. Studying a group of human immunodeficiency virus-positive T-cell-immunocompromised patients known to be at enhanced risk of multiple EBV infections, we have isolated intertypic EBV recombinants from 2 of 40 patients analyzed. These recombinants, whose in vitro transforming capacity appeared at least equal to that of type 1 strains, carried a type 1 EBNA2 allele and type 2 EBNA3A,-3B, and -3C alleles. This was clearly demonstrable at the DNA level by PCR amplification using type-specific primer-probe combinations and was confirmed at the protein level (for EBNA2 and EBNA3C) by immunoblotting with type-specific antibodies. In one patient, the recombinant appeared to be the predominant strain, being the virus most commonly rescued by in vitro transformation both from the blood and from the throat washings on two separate occasions 20 months apart. A regular type 1 virus strain was also present in this individual, but this was not related to the recombinant since the two viruses carried type 1 EBNA2 genes with different patterns of variance from the B95.8 prototype sequence. In the other patient, recombinants were isolated on one occasion from the blood and on a separate occasion, 21 months later, from the throat; these recombinants were almost certainly related, being identical at several genomic polymorphisms and differing only in one facet of the "EBNAprint," the size of the EBNA1 protein. Three different type 1 viruses were also isolated from this patient, two of which carried EBNA2 genes with the same pattern of sequence variation from B95.8 as the recombinant; however, since this is a fairly common pattern of variance, the relationship of these viruses to the recombinant remains an open question. We infer that intertypic recombinants of EBV are not uncommon in HIV-positive T-cell-immunocompromised patients, that they arise in such individuals as a consequence of their increased frequency of mixed-type infections, and that they will prove capable of efficient transmission in the human population.

EBNA-2 and EBNA-3C extensively and mutually exclusively associate with RBPJkappa in Epstein-Barr virus-transformed B lymphocytes

Although genetic and biochemical data indicate that the cell protein RBPJkappa is a mediator of EBNA-2 and EBNA-3C effects on transcriptional regulatory elements, the extent of association of these Epstein-Barr virus nuclear proteins with RBPJkappa in transformed B lymphocytes has not been determined. We now report that most of the EBNA-2 and at least 20% of the EBNA-3C coimmunoprecipitated with RBPJkappa from extracts of transformed B lymphocytes that contained most of the cellular EBNA-2 and EBNA3C. Both proteins are associated preferentially with the smaller of the two RBPJkappa isoforms. EBNA-2-RBPJkappa complexes do not contain EBNA-3C, and EBNA-3C-RBPJkappa complexes do not contain EBNA-2. Although EBNA-2 and EBNA-3C are extensively associated with RBPJkappa, a fraction of RBPJkappa appears to be free of EBNAs after repeated immunoprecipitations with anti-EBNA, Epstein-Barr virus-immune, human antibody. Promoters with RBPJkappa sites in their regulatory elements are likely to be differentially regulated by these RBPJkappa-EBNA-2 and RBPJkappa-EBNA-3 complexes.

The Epstein-Barr virus-encoded nuclear antigen EBNA-5 accumulates in PML-containing bodies

EBNA-5 is one of the Epstein-Barr virus (EBV)-encoded nuclear proteins required for immortalization of human B lymphocytes. In the nuclei of EBV-transformed lymphoblastoid cell lines EBNA-5 is preferentially targetted to distinct nuclear foci. Previously we have shown (W.Q. Jiang, L. Szekely, V. Wendel-Hansen, N. Ringertz, G. Klein, and A. Rosen, Exp. Cell Res. 197:314-318, 1991) that the same foci also contained the retinoblastoma (Rb) protein. Using a similar double immunofluorescence technique, we now show that these foci colocalize with nuclear bodies positive for PML, the promyelocytic leukemia-associated protein. Artificial spreading of the chromatin by exposure to the forces of fluid surface tension disrupts this colocalization gradually, suggesting that the bodies consist of at least two subcomponents. Heat shock or metabolic stress induced by high cell density leads to the release of EBNA-5 from the PML-positive nuclear bodies and induces it to translocate to the nucleoli. In addition to their presence in nuclear bodies, both proteins are occasionally present in nuclear aggregates and doughnut-like structures in which PML is concentrated in an outer shell. Nuclear bodies with prominent PML staining are seen in resting B lymphocytes. This staining pattern does not change upon EBV infection. In freshly infected cells EBNA-5 antigens are first distributed throughout the nucleoplasm. After a few days intensely staining foci develop. These foci coincide with PML-positive nuclear bodies. At a later stage and in established lymphoblastoid cell lines EBNA-5 is almost exclusively present in the PML-positive nuclear foci. The colocalization is restricted to EBV-infected human lymphoblasts. The data presented indicate that the distinct EBNA-5 foci are not newly formed structures but the result of translocation of the viral protein to a specialized domain present already in the nuclei of uninfected cells.

Molecular genetic analysis of Epstein-Barr virus Cp promoter function

The Cp promoter of Epstein-Barr virus (EBV) directs most transcription of the EBNA genes in lymphoblastoid cell lines. The functions of two control regions in the Cp promoter have been studied by construction of recombinant EBV strains containing specific mutations in these elements. Mutation of the RBP-Jk (CBF1) binding site reduced but did not completely abolish EBNA-2-dependent Cp activity in transient transfection assays. The same mutation in recombinant virus gave only a modest average reduction in Cp function, ranging from full activity to almost no activity in different isolates. Separate deletion of a 262-bp region containing glucocorticoid response elements had little effect in a transient assay but caused a fivefold increase in the steady-state level of Cp RNA in recombinant virus. The results indicate that other elements in addition to the intensively studied RBP-Jk site are important in determining Cp activity in the whole virus. Clonal EBV-infected cell lines expressed RNA from both the Cp and Wp promoters, but the level of Wp RNA did not simply compensate for changes in the level of Cp RNA. The levels of EBNA proteins varied much less than the levels of Cp and Wp RNA, suggesting other types of control in addition to initiation of transcription. A survey of RNAs derived from the internal repeat region of the virus indicated that gene expression from this region of EBV in lymphoblastoid cell lines is accounted for by the known transcripts.

The Epstein-Barr virus nuclear antigen leader protein associates with hsp72/hsc73

Epstein-Barr virus nuclear antigen leader protein (EBNA-LP) is important for primary B-lymphocyte growth transformation. We now demonstrate that the W repeat-encoded domain of EBNA-LP significantly associates with proteins of the heat shock protein 70 family (hsp72/hsc73). hsp72/hsc73 may mediate the previously observed interaction between EBNA-LP and the retinoblastoma protein or p53.

Host cell requirements for efficient infection of quiescent primary B lymphocytes by Epstein-Barr virus

Quiescent primary B lymphocytes are efficiently immortalized by Epstein-Barr virus (EBV). This process requires both the delivery and expression of the viral genome and results in activation of the cell division cycle. Infection of B lymphocytes depends on a direct interaction between the viral glycoprotein gp340/220 and CD21, the C3dg complement receptor. This interaction is required for the adsorption of EBV. In addition, several lines of evidence suggest that the interaction of EBV with CD21 modulates the phenotype of cells. CD21 forms part of a multimeric signal transduction complex with CD19, TAPA-1, and Leu-13. In normal B lymphocytes, CD19 becomes tyrosine phosphorylated following stimulation of the antigen receptor and recruits the signal-transducing enzyme phosphatidylinositol 3-kinase kinase. Here, we investigated the involvement of signal transduction pathways in efficient infection. Protein synthesis is not required for events leading to the transcription of the viral genome, suggesting that the early stages of infection do not depend on the expression of novel cell genes and consistent with the Wp promoter being the first viral promoter used upon infection. Since the stimulation of cells with gp340/220 leads to an increase in the level of CD19 tyrosine phosphorylation, we investigated the potential contribution of both tyrosine and phosphatidylinositol 3-kinase kinases to efficient infection. Both kinases contribute to the posttranscriptional control of viral gene expression following infection, but neither is required for the entry or initial transcription of the virus. Thus, it appears that EBV exploits a host signal transduction pathway to efficiently infect primary cells.

Expression of cyclin D2 in Epstein-Barr virus-positive Burkitt's lymphoma cell lines is related to methylation status of the gene

The cyclin D2 gene is not expressed in resting primary B lymphocytes or in group I Burkitt's lymphoma (BL) cell lines that retain the characteristics of authentic BL cells. Expression of cyclin D2 is induced in primary B lymphocytes following infection with Epstein-Barr virus (EBV) or transfection of the EBV genes EBNA-LP and EBNA-2. However, attempts to induce cyclin D2 expression in BL cell lines by the enforced expression of EBV genes were unsuccessful. Since the demethylation agent 5-azacytidine has been shown to modulate viral gene expression in BL cells, we explored the possibility that methylation plays a significant role in the control of cyclin D2 expression. We show that 5-azacytidine treatment of the Mutu CI 179 BL cell line led to expression of cyclin D2 RNA and that expression correlated with differences in the methylation status of a CCGG restriction enzyme site near the transcription initiation region of the cyclin D2 gene. Thus, methylation appears to play a direct role in the regulation of the cyclin D2 locus in BL.

Gastric carcinoma: monoclonal epithelial malignant cells expressing Epstein-Barr virus latent infection protein

In 1000 primary gastric carcinomas, 70 (7.0%) contained Epstein-Barr virus (EBV) genomic sequences detected by PCR and Southern blots. The positive tumors comprised 8 of 9 (89%) undifferentiated lymphoepithelioma-like carcinomas, 27 of 476 (5.7%) poorly differentiated adenocarcinomas, and 35 of 515 (6.8%) moderately to well-differentiated adenocarcinomas. In situ EBV-encoded small RNA 1 hybridization and hematoxylin/eosin staining in adjacent sections showed that the EBV was present in every carcinoma cell but was not significantly present in lymphoid stroma and in normal mucosa. Two-color immunofluorescence and hematoxylin/eosin staining in parallel sections revealed that every keratin-positive epithelial malignant cell expressed EBV-determined nuclear antigen 1 (EBNA1) but did not significantly express CD45+ infiltrating leukocytes. A single fused terminal fragment was detected in each of the EBNA1-expressing tumors, thereby suggesting that the EBV-carrying gastric carcinomas represent clonal proliferation of cells infected with EBV. The carcinoma cells had exclusively EBNA1 but not EBNA2, -3A, -3B, and -3C; leader protein; and latent membrane protein 1 because of methylation. The patients with EBV-carrying gastric carcinoma had elevated serum EBV-specific antibodies. The EBV-specific cellular immunity was not significantly reduced; however, the cytotoxic T-cell target antigens were not expressed. These findings strongly suggest a causal relation between a significant proportion of gastric carcinoma and EBV, and the virus-carrying carcinoma cells may evade immune surveillance.

EBNA-2 and EBNA-LP cooperate to cause G0 to G1 transition during immortalization of resting human B lymphocytes by Epstein-Barr virus

Epstein-Barr virus (EBV) is unusual among DNA tumour viruses in that the virus particle is able to infect and immortalize resting cells with very high efficiency. Mutation of the viral genome has indicated that at least six viral genes (LMP-1 and EBNAs 1, 2, 3A, 3C and LP) are essential for immortalization. We demonstrate that the activation of a G1 cyclin, cyclin D2, is an early event following infection with EBV and that cyclin D2 activation is dependent on the expression of viral genes. The different levels of cyclin D2 transcripts in Burkitt's lymphoma cell lines expressing different subsets of EBV immortalizing genes suggest an involvement of EBNA-2 or EBNA-LP in cyclin D2 regulation. By exposing resting primary B cells to a purified preparation of the EBV surface glycoprotein gp340, we have been able to achieve efficient expression of plasmid DNAs introduced by electroporation. Vectors encoding two viral genes, EBNA-2 and EBNA-LP, are sufficient to activate the expression of cyclin D2 in this system. Thus, the progression of resting B lymphocytes into the G1 phase of the cell cycle can be reconstituted in the absence of virus by the cooperation of two of the six viral genes required for immortalization.

BCL-2 induction is part of the strategy of Epstein-Barr virus

Epstein Barr virus can infect B lymphocytes and epithelial cells. Epithelial cells present the natural reservoir for the virus in man. In vitro, infected cells harbor the virus predominantly in a latent state with the expression of a set of nuclear (EBNA 1-6) and latent membrane genes (LMP 1-2) and virus-transformed B cells grow as permanently immortalized lymphoblastoid cell lines, that show increased resistance to various growth inhibiting factors. Here we show that the lymphoma-associated oncogene BCL-2 is upregulated by different latent Epstein-Barr virus genes in B lymphocytes as well as keratinocyte cell lines. Thus, the induction of BCL-2 gene expression seems to be part of the survival strategy of the virus independently of the host cell infected.

Expression of Epstein-Barr-virus-related nuclear antigens and B-cell markers in lymphomas of SIV-immunosuppressed monkeys

Simian-immunodeficiency-virus(SIV)-infected cynomolgus monkeys develop B-cell lymphomas in approximately one third of the cases. We have now studied the expression of cynomolgus-Epstein-Barr-virus(cyno-EBV) nuclear antigens in 13 cyno-EBV-carrying SIVsm-associated monkey lymphomas and established cell lines from 3 of these tumors. Immunoblots of cell lysates were probed with polyspecific and monospecific reagents directed against human EB-virus EBNAI-6, and against the membrane protein LMPI. An EBNA2-cross-reacting protein was demonstrated in 8 lymphoma tissues (8/13) and in the 3 cell lines derived from the tumors. All tumors expressed a polypeptide with 50 to 55 kDa molecular weight, which cross-reacted with some antibodies to EBNAI. Absorption experiments with normal monkey tissue showed that this polypeptide was specific for the cyno-EBV-carrying lymphoma cells. Equivalents of EBNA3-6 and LMPI could not be detected. Immunophenotypical characterization showed that the monkey lymphomas were similar to human HIV-associated B-cell lymphomas. Malignant B-cell lymphomas in experimentally SIVsm-infected cynomolgus monkeys can be a model for EBV-associated lymphomagenesis in immunodeficiency states.

Presentation of endogenous viral peptide epitopes by anti-CD40 stimulated human B cells following recombinant vaccinia infection

Cytotoxic T lymphocytes (CTLs) recognising viral antigens are an important host defence mechanism in restricting the proliferation of virus-infected cells. Previously, lymphoblastoid cell lines (LCLs) infected with vaccinia recombinants encoding viral proteins have been used to identify specific CTL epitopes. However, to localise the EBV CTL epitopes encoded by Epstein-Barr virus (EBV) transformants, LCLs are an inappropriate host for vaccinia recombinants. In the present study, an alternative host cell for vaccinia infection is described. Initial studies demonstrated that anti-CD40 stimulated human B cells replicated vaccinia virus and expressed EBV nuclear antigen(s) (EBNA) following infection with recombinant vaccinia encoding the appropriate region of the EBV genome. Recombinant vaccinia-infected anti-CD40 stimulated B cell lines were then used to localise target epitopes for a panel of EBV-specific CTL clones. Most importantly, in vitro stimulation of unfractionated mononuclear cells with recombinant vaccinia-infected anti-CD40 B cells activated a memory CTL response. Based on the vaccinia results, screening of peptides from EBNA6 defined the epitope for the EBNA6-specific CTL clones to the sequence KEHVIQNAFRK. This work clearly demonstrates that anti-CD40 stimulated B cell lines not only provide an efficient tool for localising CTL epitopes but also presents an alternative mechanism of reactivating a memory T cell response to any gene product expressed by recombinant vaccinia.

EBNA-5, an Epstein-Barr virus-encoded nuclear antigen, binds to the retinoblastoma and p53 proteins

Epstein-Barr virus (EBV) immortalized human lymphoblastoid cell lines express six virally encoded nuclear proteins, designated EBV nuclear antigens 1-6 (EBNA-1-6). We show that the EBNA-5 protein (alternatively designated EBNA-LP) that is required for B-cell transformation can form a molecular complex with the retinoblastoma (RB) and p53 tumor suppressor proteins. Using EBNA-5 deletion mutants, we have found that a 66-amino acid-long peptide, encoded by the W repeat of the EBV genome, is sufficient for binding. Point mutations of RB and p53 that inhibit their complexing with other DNA viral oncoproteins do not affect their binding to EBNA-5. p53 competes with RB for EBNA-5 binding. Our data suggest that the mechanisms involved in EBV transformation may include impairment of RB and p53 function.

The EBNA2-related resistance towards alpha interferon (IFN-alpha) in Burkitt's lymphoma cells effects induction of IFN-induced genes but not the activation of transcription factor ISGF-3

Transfection of a plasmid encoding the Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA2) gene confers resistance to the antiproliferative effect of alpha interferon (IFN-alpha) in EBV-negative U968 cells (P. Aman and A. von Gabain, EMBO J. 9:147-152, 1990). We studied the expression of IFN-stimulated genes (ISGs) in two pairs of Burkitt's lymphoma cell lines, differing in the expression of the putative immortalizing gene of EBV, EBNA2. In EBNA2-expressing cells, the induction of four ISGs by IFN-alpha was strongly reduced or, in some cases, abolished. Chloramphenicol acetyltransferase reporter gene constructs containing different IFN-stimulated response elements were transfected into EBNA2-negative and EBNA2-positive cells. Induction of chloramphenicol acetyltransferase activity by IFN was impaired in EBNA2-positive cells. Also, a reporter gene construct driven by an IFN-gamma-sensitive promoter element was affected. However, as revealed by gel shift assays, EBNA2-positive and EBNA2-negative cells exhibited a nearly identical pattern of IFN-stimulated response element-binding proteins. Most important, activation of the factor ISGF-3, which previously has been shown to be required and sufficient for transcriptional activation of IFN-induced genes, was not inhibited in IFN-resistant cells expressing EBNA2. The mechanism of the EBNA2-related IFN resistance seems to be distinct both from the resistance mediated by hepatitis virus and adenovirus gene products and from the IFN resistance in Daudi cell variants. In these three cases, the transcriptional block of IFN-induced genes is due to inhibition of ISGF-3 activation and binding. Our data suggest that the EBNA2-related IFN resistance in Burkitt's lymphoma cells acts downstream of the activation of ISGF-3.

The EBNA2-related resistance towards alpha interferon (IFN-alpha) in Burkitt's lymphoma cells effects induction of IFN-induced genes but not the activation of transcription factor ISGF-3

Transfection of a plasmid encoding the Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA2) gene confers resistance to the antiproliferative effect of alpha interferon (IFN-alpha) in EBV-negative U968 cells (P. Aman and A. von Gabain, EMBO J. 9:147-152, 1990). We studied the expression of IFN-stimulated genes (ISGs) in two pairs of Burkitt's lymphoma cell lines, differing in the expression of the putative immortalizing gene of EBV, EBNA2. In EBNA2-expressing cells, the induction of four ISGs by IFN-alpha was strongly reduced or, in some cases, abolished. Chloramphenicol acetyltransferase reporter gene constructs containing different IFN-stimulated response elements were transfected into EBNA2-negative and EBNA2-positive cells. Induction of chloramphenicol acetyltransferase activity by IFN was impaired in EBNA2-positive cells. Also, a reporter gene construct driven by an IFN-gamma-sensitive promoter element was affected. However, as revealed by gel shift assays, EBNA2-positive and EBNA2-negative cells exhibited a nearly identical pattern of IFN-stimulated response element-binding proteins. Most important, activation of the factor ISGF-3, which previously has been shown to be required and sufficient for transcriptional activation of IFN-induced genes, was not inhibited in IFN-resistant cells expressing EBNA2. The mechanism of the EBNA2-related IFN resistance seems to be distinct both from the resistance mediated by hepatitis virus and adenovirus gene products and from the IFN resistance in Daudi cell variants. In these three cases, the transcriptional block of IFN-induced genes is due to inhibition of ISGF-3 activation and binding. Our data suggest that the EBNA2-related IFN resistance in Burkitt's lymphoma cells acts downstream of the activation of ISGF-3.

Recognition of the Epstein-Barr virus-encoded nuclear antigens EBNA-4 and EBNA-6 by HLA-A11-restricted cytotoxic T lymphocytes: implications for down-regulation of HLA-A11 in Burkitt lymphoma

Evasion from cytotoxic T-lymphocyte (CTL) surveillance may be an important step in the pathogenesis of Epstein-Barr virus (EBV)-carrying Burkitt lymphoma (BL) as suggested by the consistent down-regulation of all transformation-associated viral antigens, except EBV nuclear antigen 1 (EBNA-1), and of certain HLA class I alleles in BL biopsies and cell lines that maintain the tumor cell phenotype in vitro. The most common HLA class I defect recorded in BL lines is a selective down-regulation of HLA-A11. To gain some insight into the role of HLA-A11 down-regulation in pathogenesis of BL, we have investigated the target specificity of HLA-A11-restricted CTLs derived by stimulation of lymphocytes from three EBV-seropositive individuals with autologous EBV-transformed lymphoblastoid cell lines. Recombinant vaccinia viruses carrying the coding sequences for EBNA-1, -2A, -2B, -5, -3, -4, and -6 (also known as EBNA-1, -2A, -2B, -LP, -3a, -3b, and -3c, respectively) and EBV latent membrane protein 1 were used to induce high levels of expression of the relevant EBV antigen in fibroblasts derived from HLA class I-matched individuals. EBNA-4-expressing fibroblasts were the predominant target of HLA-A11-restricted CTLs in all three donors. A less pronounced and less regular EBNA-6-specific cytotoxic component was found in two of the donors.

Infection of the HTLV-I-harbouring T-lymphoblastoid line MT-2 by Epstein-Barr virus

Epstein-Barr virus (EBV), a ubiquitous human B-lymphotropic virus, is associated with certain lymphoproliferative diseases of T-cell lineage. To understand the mechanism by which EBV infects T cells, we have tested the susceptibility of various human T-cell lines to the virus. We report here that the HTLV-I-harbouring T-lymphoblastoid line MT-2 carries a high level of CD21/EBV receptors on their surface, adsorbs fluorescein isothiocyanate-labeled EBV, and synthesizes virus latent antigens (EBNA-1 and LMP) following EBV infection. Pretreatment of MT-2 cells with anti-CD21 monoclonal antibody OKB7 inhibited the virus binding as well as the synthesis of virus latent antigen. These data suggest that human T-cells can be infected with EBV via functionally active virus receptors.

Three transcriptionally distinct forms of Epstein-Barr virus latency in somatic cell hybrids: cell phenotype dependence of virus promoter usage

Phenotypically distinct human B cell lines display two transcriptionally distinct forms of Epstein-Barr virus (EBV) latency. Latency I (Lat I) in group I Burkitt's lymphoma (BL) cell lines is characterized by selective expression of the virus-coded nuclear antigen EBNA 1 from a uniquely spliced mRNA driven by the Fp promoter. Latency III (Lat III) in group III BL and EBV-transformed lymphoblastoid cell lines (LCLs) is characterized by expression of EBNAs 1, 2, 3a, 3b, 3c, and -LP from mRNAs driven by the Cp or Wp promoter and of the latent membrane proteins (LMPs 1, 2A, and 2B) from mRNAs driven by the LMP promoters. Here we have altered the group I BL and LCL phenotypes by cell hybridization and screened for attendant changes in EBV latency by PCR analysis of viral mRNAs and immunoblotting of viral proteins. Fusion of group I BL cells with LCLs activated the BL virus genome from a Lat I to Lat III pattern of gene expression. Fusion of LCLs with nonlymphoid lines repressed virus gene expression from Lat III either to Lat I or to another form of latency (Lat II) hitherto not seen in vitro and characterized by selective expression of the Fp-driven EBNA 1 mRNA and of the LMP 1, 2A, and 2B transcripts. There are therefore three forms of EBV latency which can be interconverted by altering cellular phenotype and thereby virus promoter usage.

Co-localization of the retinoblastoma protein and the Epstein-Barr virus-encoded nuclear antigen EBNA-5

A monoclonal antibody (aRB1C1) raised against an Rb fusion protein detects a limited number (4-10) of relatively large intranuclear foci in an EBV-immortalized cord blood cell line (IB4). These domains also bind an anti-EBNA-5 monoclonal antibody. The Rb antibody reactive sites also co-localize with the SV40 T antigen in transformed monkey cells (COS). The nuclear structures stained by aRB1C1 and EBNA-5 antibodies are distinct from the structures detected with antibodies against centromeric proteins and certain snRNP epitopes. EBNA-5/Rb-positive domains do not selectively react with antibodies against the La antigen known to associate with the small EBV-encoded nuclear RNA species designated as the EBERs.

Expression of Epstein-Barr virus nuclear antigens in anti-IgM-stimulated B cells following recombinant vaccinia infection and their recognition by human cytotoxic T cells

Cytotoxic T lymphocytes (CTL) recognizing Epstein-Barr virus (EBV) nuclear antigens (EBNA) are an important host defence mechanism in restricting the proliferation of EBV-infected B cells. Previously, B-type lymphoblastoid cell lines (LCL) infected with vaccinia recombinants encoding for the EBNA proteins have been used to identify A-type-specific CTL epitopes. However, to localize the CTL epitopes encoded by both A- and B-type transformants, B-type LCL are an inappropriate host for vaccinia. In the present study, an alternative host cell for vaccinia infection is described. Initial studies demonstrated that anti-IgM (mu-chain specific)-stimulated human B cells allowed vaccinia virus to replicate more efficiently than either phytohaemagglutinin-stimulated lymphocytes (PHA blasts) or CTL and expressed EBNA proteins following recombinant vaccinia infection. Furthermore, the presentation and recognition of target epitopes expressed on vaccinia-infected anti-mu-stimulated B cell blasts were comparable to that on similarly infected LCL. Anti-mu-stimulated B cells were used to define the CTL epitopes recognized by a panel of CTL clones from an EBV-immune donor. Using recombinant vaccinia-infected anti-mu-stimulated B cells, the CTL response from this donor was mapped to the EBNA6 protein. Most importantly, in vitro stimulation of unfractionated mononuclear cells with vaccinia-infected anti-mu B cells activated a memory CTL response. Based on the vaccinia results, screening of peptides from EBNA6 localized the epitope for the majority of the EBNA6-specific CTL clones to the sequence EENLLDFVRFM, apparently in association with HLA-B44. This work clearly demonstrates that anti-mu-stimulated B cells not only provide an efficient model for localizing the CTL epitope(s) but also raises the possibility of reactivating a memory T-cell response to any gene product expressed by recombinant vaccinia.

An improved dot immunobinding assay for screening hybridoma supernatants. Non-purified antigen immobilized on nitrocellulose paper discs

This report describes a modified dot immunobinding assay (DIA) in microplates using a crude mixture of non-purified antigen. Nitrocellulose filter paper discs exposed to the antigen mixture were inserted into the wells and kept in place by a specially constructed device. To test the efficiency of the modification a set of monoclonal antibodies from a mouse immunized with 58 kDa trpE-Bmyc fusion protein were screened. The advantage of this modified method over conventional ELISA is that it permits the use of non-purified antigen for screening large numbers of monoclonal antibodies.

Epstein-Barr virus (EBV)-associated lymphoproliferative disease in the SCID mouse model: implications for the pathogenesis of EBV-positive lymphomas in man

When human peripheral blood lymphocytes (PBLs) from Epstein-Barr virus (EBV)-seropositive donors are injected intraperitoneally into SCID mice, EBV+ B cell tumors develop within weeks. A preliminary report (Mosier, D. E., R. J. Gulizia, S. M. Baird, D. D. Richman, D. B. Wilson, R. I. Fox, and T. J. Kipps, 1989. Blood. 74(Suppl. 1):52a) has suggested that such tumors resemble the EBV-positive malignancy, Burkitt's lymphoma. The present work shows that generally the human (hu) PBL-SCID tumors are distinct from Burkitt's lymphoma and instead resemble lymphoblastoid cell lines (LCLs) generated by EBV-infection of normal B cells in vitro in terms of: (a) their cell surface phenotype, with expression of B cell activation antigens and adhesion molecules, (b) normal karyotype, and (c) viral phenotype, with expression of all the transformation-associated EBV latent proteins and, in a minority of cells, productive cycle antigens. Indeed, in vitro-transformed LCLs also grow when inoculated into SCID mice, the frequency of tumor outgrowth correlating with the in vitro growth phenotype of the LCL which is itself determined by the identity of the transforming virus (i.e., type 1 or type 2 EBV). Histologically the PBL-derived hu-SCID tumors resemble the EBV+ large cell lymphomas that develop in immuno-suppressed patients and, like the human tumors, often present at multiple sites as individual monoclonal or oligoclonal foci. The remarkable efficiency of tumor development in the hu-SCID model suggests that lymphomagenesis involves direct outgrowth of EBV-transformed B cells without requirement for secondary genetic changes, and that selection on the basis of cell growth rate alone is sufficient to explain the monoclonal/oligoclonal nature of tumor foci. EBV+ large cell lymphoma of the immunosuppressed may arise in a similar way.

Up regulation of the Epstein-Barr virus (EBV)-encoded membrane protein LMP in the Burkitt's lymphoma line Daudi after exposure to n-butyrate and after EBV superinfection

The Burkitt's lymphoma line Daudi carries a nontransforming Epstein-Barr virus (EBV) strain that has a deletion in the BamHI WYH region of the genome coding for the EBV nuclear antigen 2 (EBNA-2). Daudi cells fail to express the EBV-encoded latent membrane protein (LMP) (D. Ghosh and E. Kieff, J. Virol. 64:1855-1858, 1990). We show that LMP expression can be up regulated by exposure to n-butyrate and by superinfection with the B95-8 (B virus)- and P3HR1 (P virus)-derived EBV strains. Two LMP polypeptides of 60 and 48 kilodaltons (kDa) were detected in immunoblots of Daudi cells that had been exposed to 3 mM n-butyrate for 24 h. The intensity of the 48-kDa LMP increased during 72 h, in parallel with the appearance of early antigen-positive cells. The 60-kDa LMP was expressed at a low level and remained constant. Superinfection of Daudi cells with B and P virus induced the 60-kDa LMP within 3 h. In addition, P virus induced the 48-kDa LMP at a low level. The B virus-encoded EBNA-2 and EBNA-5 were detected 12 h after superinfection. The B virus-encoded 63-kDa LMP was coexpressed with the endogenous LMP after 48 h. Inactivation of the virus by UV illumination abolished the expression of the B virus-encoded antigens but did not affect the induction of the endogenous LMP. The B-cell activation marker CD23 was up regulated by B virus superinfection but not by n-butyrate exposure. CD23 was also expressed at a higher level in a stable B virus-converted subline, E95A-Daudi, that was EBNA-2 positive and coexpressed the Daudi virus- and B virus-encoded LMP. The results suggest that LMP expression is regulated by the interaction of cellular and viral factors. Binding of the virus to its membrane receptor might be involved in the triggering of cellular control mechanisms. Viral gene products are not directly involved in this function but may contribute to create a permissive cellular environment for LMP expression.

EBNA size polymorphism can be used to trace Epstein-Barr virus spread within families

The Epstein-Barr virus (EBV)-determined nuclear antigens EBNA 1, 2, 3, 4, and 6, regularly expressed in EBV-transformed lymphoblastoid cell lines, vary in size among viral strains. We have used this characteristic to trace the spread of the virus within seven families by using an approach called Ebnotyping. Among 33 evaluable individuals, 3 were EBV seronegative, and 17 different EBV strains could be isolated from the peripheral blood or throat washes of the remaining 30. All unrelated persons carried different strains. The EBV strain carried by 19 persons was also found in 1 or more family members. The same viral strain was carried by two members in five families, by three members in the sixth, and by five members in the seventh. The paternal strain was isolated from one child in two families, and the maternal strain was isolated from one or more children in three families. EBV was isolated from both blood and throat wash in six individuals. The Ebnotypes of both derived lymphoblastoid cell lines were identical within each individual. These results indicate that spread within families may be a relatively common route of EBV transmission. The number of horizontal transmission events required to generate diversification of the Ebnotype will require larger epidemiological studies.

Epstein-Barr virus gene expression in interferon-treated cells. Implications for the regulation of protein synthesis and the antiviral state

This paper presents data on the effects of interferon treatment on Epstein-Barr virus (EBV) gene expression in latently infected Daudi Burkitt's lymphoma cells, and reviews the possible role of viral gene products in the regulation of translation. In Daudi cells the main virally coded RNAs are the small untranslated RNAs EBER-1 and EBER-2, two mRNAs for the DNA binding protein EBNA-1, and a number of small RNAs containing sequences from the BamHI W repeat region of the viral genome. Interferon treatment does not change the qualitative pattern of EBV gene expression but decreases the levels of the EBNA-1 mRNAs. The chromatographic behaviour of EBV-encoded RNAs on CF11-cellulose indicates that many contain double-stranded regions; these RNAs co-purify with RNA that activates the interferon-induced, dsRNA-sensitive protein kinase DAI. Computer analysis indicates that the exons transcribed from the BamHI W repeats have the potential for formation of very stable secondary structures. Many viruses can counteract the inhibition of protein synthesis mediated by the DAI-catalysed phosphorylation of initiation factor eIF-2 and our data suggest that the small RNA EBER-1 may fulfil this function in the EBV system. During the infection and immortalization of B lymphocytes by EBV the synthesis of large amounts of EBER-1 RNA might thus allow the virus to circumvent one of the interferon-mediated mechanisms of host cell defence.

Epstein-Barr virus nuclear antigen 2 induces expression of the virus-encoded latent membrane protein

Infection of Epstein-Barr virus-negative human B-lymphoma cell lines with the fully transforming B95.8 Epstein-Barr virus strain was associated with complete virus latent gene expression and a change in the cell surface and growth phenotype toward that of in vitro-transformed lymphoblastoid cell lines. In contrast, the cells infected with the P3HR1 Epstein-Barr virus strain, a deletion mutant that cannot encode Epstein-Barr nuclear antigen 2 (EBNA2) or a full-length EBNA-LP, expressed EBNAs1, 3a, 3b, and 3c but were negative for the latent membrane protein (LMP) and showed no change in cellular phenotype. This suggests that EBNA2 and/or EBNA-LP may be required for subsequent expression of LMP in Epstein-Barr virus-infected B cells. Recombinant vectors capable of expressing the B95.8 EBNA2A protein were introduced by electroporation into two P3HR1-converted B-lymphoma cell lines, BL30/P3 and BL41/P3. In both cases, stable expression of EBNA2A was accompanied by activation of LMP expression from the resident P3HR1 genome; control transfectants that did not express the EBNA2A protein never showed induction of LMP. In further experiments, a recombinant vector capable of expressing the full-length B95.8 EBNA-LP was introduced into the same target lines. Strong EBNA-LP expression was consistently observed in the transfected clones but was never accompanied by induction of LMP. The EBNA2A gene transfectants expressing EBNA2A and LMP showed a dramatic change in cell surface and growth phenotype toward a pattern like that of lymphoblastoid cell lines; some but not all of these changes could be reproduced in the absence of EBNA2A by transfection of P3HR1-converted cell lines with a recombinant vector expressing LMP. These studies suggest that EBNA2 plays an important dual role in the process of B-cell activation to the lymphoblastoid phenotype; the protein can have a direct effect upon cellular gene expression and is also involved in activating the expression of a second virus-encoded effector protein, LMP.