Identification of Epstein-Barr virus genes expressed during the early phase of virus replication and during lymphocyte immortalization

Identification of protein tyrosine kinases required for B-cell- receptor-mediated activation of an Epstein-Barr Virus immediate-early gene promoter

Epstein-Barr Virus (EBV) is a potentially oncogenic herpesvirus that infects >90% of the world's population. EBV exists predominantly as a latent infection in B lymphocytes, with periodic lytic-cycle reactivation essential for cellular and host transmission. Viral reactivation can be stimulated by ligand-induced activation of B-cell-receptor (BCR)-coupled signaling pathways. The critical first step in the transition from latency to the lytic cycle is the expression of the viral immediate-early gene BZLF1 through the transcription activation of its promoter, Zp. However, the BCR-coupled signal transduction cascade(s) leading to the induction of Zp and the expression of the BZLF1 gene product, Zta, is currently unclear. A major obstacle to delineating the relevant signal transduction events has been the lack of a model of EBV infection that is amenable to genetic manipulation. The use of the avian B-cell line DT40 has proven to be a powerful tool for delineating BCR-mediated signal transduction pathways that appear to be highly conserved between avian and mammalian systems. We demonstrate that the DT40 cell line is a robust and genetically tractable system for the study of BCR-mediated signaling pathways leading to transcriptional activation of BZLF1. Using this system, we demonstrate that activation of Zp requires the BCR-coupled protein tyrosine kinases Syk and Btk and that it is positively regulated by Lyn. Thus, the use of DT40 cells has allowed us to delineate the early signaling components required for BCR-dependent reactivation of latent EBV, and this system is likely to prove useful for further dissection of the downstream signaling cascades involved.

Human B cell interleukin 10

Interleukin 10 (IL-10) is a novel lymphokine which exhibits strong DNA and amino acid sequence homology to BCRF1, an open reading frame in the Epstein-Barr virus genome. Using a wide panel of EBV positive and EBV negative cell lines, it has been shown that EBV positive B cell lines derived from patients with AIDS and Burkitt's lymphoma (AABCL) secrete large quantities of B cell IL-10, compared with EBV-positive B cell lines obtained from patients with undifferentiated lymphomas of Burkitt's and non-Burkitt's types. In contrast, EBV-negative B cell lines do not express IL-10 by Northern blot analysis, ELISA or even PCR. B cell IL-10 is confined to a narrow window in the B cell differentiation pathway, and whereas IL-10 expression is detected in mature and preplasmacytic stages, none of the pro-B, pre-B, or myeloma cell lines produce IL-10. EBV exerts direct effect on the production of B cell IL-10, and purified tonsillar B cells infected with EBV were triggered to secrete IL-10. The large amount of IL-10 secreted by B cells derived from AIDS-related lymphomas suggests that HIV-1 also exerts direct effect on IL-10 secretion. B cell IL-10 may function as autocrine growth factor for B cell lymphomas, and both IL-10 and BCRF1 seem to be involved in the pathophysiology of non-Hodgkin's lymphomas.

Identification of a protein encoded in the EB-viral open reading frame BMRF2

Using monospecific rabbit sera against a peptide derived from a potential antigenic region of the Epstein-Barr viral amino acid sequence encoded in the open reading frame BMRF2 we could identify a protein-complex of 53/55 kDa in chemically induced B95-8, P3HR1 and Raji cell lines. This protein could be shown to be membrane-associated, as predicted by previous computer analysis of the secondary structure and hydrophilicity pattern, and may be a member of EBV-induced membrane proteins in lytically infected cells.

Early events in Epstein-Barr virus genome expression after activation: regulation by second messengers of B cell activation

RNA transcription from the BamHI Z and BamHI R and HindIII G regions of the Epstein-Barr virus (EBV) genome was studied after treatment of Akata cells with anti-immunoglobulin G (IgG), with second messenger agonists or antagonists to determine how latent EBV activation is regulated by B cell second messengers. Northern gel analysis demonstrated that BZLF1, BZLF1 + BRLF1, and BMLF1 + BSLF2 transcripts were induced at 2 hr and increased in concentration at 4 hr after induction with anti-IgG; transcripts from BRRF1, BaRF1, BMLF1, and BMRF1 were initiated at 4 hr; a transcript from BRRF2 appeared at 6 hr. The patterns of transcription from these genes after repeated stimulations with calcium ionophore A23187 + dioctanoylglycerol paralleled those with anti-IgG except that times of initiation were delayed by about 2 hr. Nuclear run-off assay of BZLF1 gene showed rapid increases in their transcriptions from 30 to 60 min after anti-IgG treatment. The protein kinase C antagonist, staurosporine, completely blocked the appearance of these transcripts, while 8-bromo cAMP + theophylline suppressed the transcription by about 40%. The regulation of EBV activation in Akata cells with anti-IgG or with second messenger agonists or antagonists can be explained by regulation at the level of transcription of immediate-early genes of EBV.

Expression of interleukin-10 activity by Epstein-Barr virus protein BCRF1

Cytokine synthesis inhibitory factor (CSIF; interleukin-10), a product of mouse TH2 T cell clones that inhibits synthesis of cytokines by mouse TH1 T cell clones, exhibits extensive sequence similarity to an uncharacterized open reading frame in the Epstein-Barr virus BCRF1. Recombinant BCRF1 protein mimics the activity of interleukin-10, suggesting that BCRF1 may have a role in the interaction of the virus with the host's immune system.

Novel 12-O-tetradecanoylphorbol-13-acetate-responsive elements in the upstream sequence of the MS gene promoter of Epstein-Barr virus

We have demonstrated that gene expression from the promoter of the Epstein-Barr virus (EBV) MS gene with its upstream sequence is inducible by 12-O-tetradecanoylphorbol-13-acetate (TPA). By transfecting mammalian cells with plasmids in which the MS promoter and its upstream sequence are linked to the bacterial chloramphenicol acetyltransferase gene, we have shown that treatment of the cells with TPA stimulates the expression of chloramphenicol acetyltransferase activity in both EBV-negative and -positive cell lines. This TPA response requires the cis-acting sequence between nucleotides 84440 and 85046 of the EBV genome, located either upstream or downstream of the MS promoter. The TPA induction is at the transcriptional level. When this sequence is linked to the promoter of the human herpesvirus 1 thymidine kinase gene, it can also enhance the expression of, and confer TPA responsiveness on, the thymidine kinase promoter. By constructing and transfecting mutants with 5' and 3' deletions, we have identified two TPA-responsive elements, one located between -726 and -690 and the other located between -603 and -546 relative to the transcription start site. These two sequences do not contain any homology to the previously defined elements for TPA response and may play an important role in EBV induction by TPA.

The Epstein-Barr virus immediate-early gene product, BMLF1, acts in trans by a posttranscriptional mechanism which is reporter gene dependent

In DNA cotransfection experiments, the Epstein-Barr virus immediate-early gene product, BMLF1, stimulated the chloramphenicol acetyltransferase (CAT) activity of both latent and productive EBV promoters linked to CAT. This BMLF1-induced increase in CAT activity was out of proportion to the effect on CAT mRNA, suggesting a posttranscriptional mechanism. Furthermore, when growth hormone was used as a reporter gene instead of CAT, BMLF1 no longer functioned. Thus, the BMLF1 effect was reporter-gene dependent. The effect of the BMLF1 gene product does not then appear to be directed at promoter activation, but instead may function to increase the level of an as yet unidentified protein(s) required for Epstein-Barr virus infection.

Responsiveness of the Epstein-Barr virus NotI repeat promoter to the Z transactivator is mediated in a cell-type-specific manner by two independent signal regions

Cells latently infected with Epstein-Barr virus (EBV) can be activated to express lytic-cycle polypeptides by the introduction of the EBV-encoded Z transactivator, indicating that this protein has a pivotal role in virus reactivation. We examined the target specificity of the Z transactivator in short-term contransfection assays and found that the most responsive target to Z transactivation was the divergent NotI repeat promoter, located within the EBV BamHI H fragment. In contrast, target plasmids containing the cat gene linked to heterologous viral promoters were not activated by cotransfection with the Z gene. S1 nuclease analysis of RNA from chemically induced B95-8 cells and from Vero cells cotransfected with NotI repeat promoter-CAT and Z showed that Z transactivation increased the level of correctly initiated, stable RNA transcripts. The NotI repeat gene (ntr) gives rise to a highly abundant mRNA species after chemical induction of lytic virus replication, but no protein product had been previously identified. Using monospecific antiserum raised against a synthetic peptide from the BHLF1 open reading frame, we demonstrated that the ntr gene encodes a protein product that is found in nuclear patches colocalizing with nucleoli. A series of deletions introduced into the upstream sequences of the NotI-repeat-promoter revealed two separate Z-response regions. The minimal promoter region between -7 and -155 of the leftward RNA cap site and an upstream region between -644 and -902 were both independently capable of conferring Z responsiveness. However, the minimal region, which was activated by Z cotransfection in Vero cells, was poorly responsive in lymphocytes, whereas the response of the far-upstream region to Z cotransfection was lymphocyte specific. In its human host, EBV infects both epithelial and lymphocyte populations. This dual lifestyle may have led to the evolution of multiple Z-response signals that enable the Z transactivator to interact with both cell-specific promoter and enhancer factors.

Cycloheximide-resistant gene of Epstein-Barr virus in freshly infected B lymphocytes

The transcription of latent Epstein-Barr viral (EBV) genes, i.e., genes encoding EBNA-1, EBNA-2, EBNA-3A, -3B, -3C, and LMP, were detected in human tonsillar lymphocytes early after infection with EBV. Transcription of the BHLF-1 open reading frame was also detected at this initial phase of immortalization. Cycloheximide treatment inhibited the transcription of all the latent EBV genes but not BHLF-1. These results suggest that BHLF-1 might be considered an immediate-early gene of EBV. Cycloheximide treatment of EBV-infected cells reduced not only the degree of the transcription but also the size of the transcript for EBNA-2. We hypothesize that the immediate-early expression of BHLF-1 may be required for the enhanced transcription of the viral genes in lymphocytes early after infection with EBV.

A new Epstein-Barr virus transactivator, R, induces expression of a cytoplasmic early antigen

Several Epstein-Barr virus (EBV) early promoters respond to a new EBV transactivator encoded by BRLF1, designated R. Transactivation was measured in chloramphenicol acetyltransferase assays on Raji, BHK, and Vero cells that were cotransfected with the transactivator and target promoters linked to the cat gene. The divergent promoter of BamHI-H was particularly responsive to R transactivation. This large promoter region consists of a leftward TATA box for the NotI repeat gene (BHLF1) and a probable rightward TATA box for the EA-R gene (BHRF1) separated by 940 base pairs of unusual sequence complexity. Sequences within this divergent promoter region appear to confer inducibility by EBV transactivators R and Z (BZLF1). The Z transactivator stimulated expression in both the leftward and rightward directions, and R stimulated expression primarily in the rightward direction, but the MS transactivator (BMLF1) had no activity in either direction. The adenovirus E3 promoter also responded to the R transactivator, but several other herpesvirus and human promoters were nonresponsive. When the divergent promoter was linked to the EA-R gene as it is in the EBV genome, the R and Z transactivators also induced the expression of EA-R in cotransfected cells. This cytoplasmic early antigen is encoded by BHRF1 and may be anchored in intracellular membranes by a carboxy-terminal transmembrane region.

TPA-inducible Epstein-Barr virus genes in Raji cells and their regulation

A number of agents including the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) can induce an abortive virus cycle in the EBV nonproducer Burkitt's lymphoma line Raji. We describe the pattern of viral RNAs transcribed in uninduced cells and in cells treated with TPA for 8 hr, as analyzed by Northern blotting. By comparing the patterns of RNAs observed in cells treated with TPA, TPA plus cycloheximide, or cycloheximide alone, we have tested whether any EBV gene in TPA-treated Raji cells would be inducible directly by TPA in the presence of protein synthesis inhibitors, similarly to immediate-early genes induced by superinfection of Raji cells with P3HR-1 virus in the presence of cycloheximide. We demonstrate here that induction of all early EBV genes is dependent on ongoing protein synthesis. The experiments do not provide an answer to whether TPA acts by activating an initial step in the cascade of virus production or whether TPA has a simultaneous pleiotropic effect on the regulation of a large number of viral genes.

Epstein-Barr virus gene expression in P3HR1-superinfected Raji cells

The pattern of Epstein-Barr virus (EBV) RNAs expressed in Raji cells superinfected with P3HR1 EBV was examined. RNAs whose expression was of an immediate-early type (resistant to treatment of the cells with anisomycin) were identified. These RNAs, encoding the EBV reading frames BZLF1 and BRLF1, were probably expressed from defective virus within the P3HR1 preparation, and some of them were responsible for the induction of the EBV productive cycle in the Raji cells. The structures of the B95-8 RNAs equivalent to the anisomycin-resistant RNAs were determined. The RNA encoding the BZLF1 reading frame contained two splices which extended and modified the reading frame from that previously described.

Both the rightward and the leftward open reading frames within the BamHI M DNA fragment of Epstein-Barr virus act as trans-activators of gene expression

The BamHI M DNA fragment of Epstein-Barr virus was shown to activate transcription of the cotransfected chloramphenicol acetyltransferase gene under the control of the simian virus 40 early promoter. Both the BamHI-BglII and the HindIII-BamHI subfragments of the BamHI M fragment, corresponding to the rightward reading frame BMRF1 and the leftward reading frame BMLF1, respectively, had the ability to activate transcription from the simian virus 40 promoter. The trans-activating function was well correlated with the expression of nuclear early antigens, which suggests that early antigens encoded by BMRF1 and BMLF1 are responsible for trans-activation and possibly play a role in regulated expression of virus genomes.

Characterization of a cDNA clone corresponding to a transcript from the Epstein-Barr virus BamHI M fragment: evidence for overlapping mRNAs

A 1.95-kilobase cDNA clone was isolated by screening a size-selected lambda gt10 cDNA library prepared from an Epstein-Barr virus-transformed B-cell line, IB4, with the Epstein-Barr virus BamHI M fragment. Sequence analysis revealed that this clone contains about 75% of the BMRF1 and the complete BMRF2 open reading frames. The transcript is not spliced, and the polyadenylation signal at base pair 2641 of the BamHI M fragment is used. Northern blots (RNA blots) indicate that this polyadenylation signal is used for three overlapping mRNAs. The sizes of these transcripts are 3.5, 2.6, and 1.5 kilobases.

Isolation and characterization of cDNA clones corresponding to transcripts from the BamHI H and F regions of the Epstein-Barr virus genome

The Epstein-Barr virus (EBV) mutant P3HR1 is incapable of immortalizing B lymphocytes because of a 6.8-kilobase deletion in the BamHI W, Y, and H regions of the viral genome (M. Rabson, L. Gradoville, L. Heston, and G. Miller, J. Virol. 44:834-844, 1982). To characterize transcripts that are encoded in this region, poly(A)+ RNA from the EBV-transformed lymphoblastoid cell line JY was isolated, and this RNA was used to generate a cDNA library in lambda gt10. By screening 500,000 recombinant bacteriophages with the BamHI H fragment, we isolated 10 cDNA clones and characterized them in detail. One group of six cDNA clones was derived from a 2.9-kilobase early transcript encoded by the IR2 repeat element and showed restriction site polymorphism for the enzyme SmaI. The second group consisted of four cDNA clones, all of which contained the BamHI-H right reading frame (BHRF1), and used the polyadenylation signal at base pair 662 in the BamHI F fragment. Computer analysis of the hydrophobicity of the BHRF1 protein revealed that it is likely to be a membrane protein. Northern blotting experiments with RNA from an EBV producer line, B95-8, and a tightly latent lymphoblastoid B-cell line, IB4, revealed that BHRF1 is contained in at least two different mRNA species which can be detected during the latent cycle of EBV. These data and the recent characterization of a spliced transcript (containing five exons in common with other known latent messages [M. Bodescot and M. Perricaudet, Nucleic Acids Res. 14:7103-7113, 1986]) suggest that alternative splicing is used to generate transcripts containing BHRF1, as for the EBV nuclear antigen 1 transcripts. Furthermore, the observation that a potential oncogene activated in human follicular lymphomas is homologous to the BHRF1-encoded polypeptide (M. L. Cleary, S.D. Smith, and J. Sklar, Cell 47:19-28, 1986) suggests a possible role for this putative viral protein in EBV-induced growth transformation of B lymphocytes.

Characterization of the Epstein-Barr virus-induced early polypeptide complex p50/58 EA-D using rabbit antisera, a monoclonal antibody, and human antibodies

A polypeptide complex (p52) belonging to the D-subspecificity of the EBV-induced early antigens (EA-D) was purified from chemically induced P3HR-1 cells. Rabbit antisera raised against the isolated polypeptides reacted with components of EA-D as could be shown by indirect immunofluorescence and immunoperoxidase staining of IdU-induced EA positive Raji cells, ELISA, and immunoblotting. In one-dimensional immunoblots the rabbit antisera detected a predominant polypeptide complex of 52 kDa. Two-dimensional immunoblots prepared with proteins from IdU-induced Raji cells showed that the rabbit sera detect three series of polypeptides of 52 kDa (pl 8.5-6.2), 55-58 kDa (pl 6.2-4.5), and 48-50 kDa (pl 6.0-4.5). These three groups of polypeptides could also be identified by 50 high titered anti-EA-D positive human sera and a specific monoclonal antibody (R3) as being the main components of EA-D in Raji and B95-8 cells. All polypeptides of the p50/58 complex showed DNA binding properties either by themselves or by an interaction with other proteins. When TPA or IdU-induced Raji cells were labeled with 2Pi, two phosphorylated polypeptides pp50 and pp58 could be immunoprecipitated with the rabbit sera and a high anti-EA titered human serum. The time course of the synthesis of polypeptides associated with the EA-D complex was studied by 2-D immunoblots: EA polypeptides of 52 kDa appeared as early as 6 hr after the addition of IdU to Raji cells in culture, polypeptides of 55-58 and 48-50 kDa after 18 and 25 hr, respectively. The coordinated appearance of these groups of polypeptides and their similar size and reactivity with human sera and rabbit antisera produced against the isolated p52 as well as with a monoclonal antibody (R3) suggested that most of these polypeptides are derived from post-translational modifications of one or a few initially synthesized polypeptides, possibly p52. Phosphorylation seems to be at least one possibility of post-translational modification.

Mapping of genes in BamHI fragment M of Epstein-Barr virus DNA that may determine the fate of viral infection

We used nuclease digestion to map RNA transcripts encoded in the BamHI M fragment of the Epstein-Barr virus (EBV) genome (strain B95-8). Of the five RNAs, three are rightwardly transcribed, have different cap sites but common 3' termini, and are unspliced. The two remaining RNAs are leftwardly transcribed and are 5' and 3' coterminal. One of these transcripts is spliced, resulting in the removal of a small intron from the 5' region of this RNA. We have previously published data which indicated that the BamHI M region is the first actively transcribed region of the viral genome during the replicative cycle, suggesting that one or more genes in this region is important in the initiation of EBV replication. We have now mapped two large EcoRI restriction fragments which span approximately 75% of the P3HR-1 defective genome and which contain DNA from the BamHI M region of the standard genome. The data indicate that only the coding and 5' flanking sequences for the leftwardly transcribed RNAs are intact within the defective genome. Fewer than 500 bases coding for the 3'-most regions of the rightwardly transcribed RNAs are intact, and it is unlikely that these encode functional native polypeptides. Therefore, it seems that transcriptional activation of the BamHI M-region genes is not mediated directly by the rearrangement of M genes in defective P3HR-1 EBV.

Sequence-specific DNA binding of the Epstein-Barr virus nuclear antigen (EBNA-1) to clustered sites in the plasmid maintenance region

Latently infected B lymphocytes continuously express an Epstein-Barr Virus nuclear antigen (EBNA-1) required in trans for maintenance of the plasmid state of the EBV genome. Filter binding assays and DNAase I footprinting analyses revealed that the carboxy-terminal domain of EBNA-1 protects binding sites at three different loci in the 172,000 bp EBV genome. Two of these loci correspond to essential elements within an 1800 bp segment defined as the minimal region required for plasmid maintenance (ori-P). Binding to each of 20 X 30 bp tandem repeats in the "sink" locus protects 25 bp centered over a 12 bp palindromic consensus sequence TAGCATATGCTA. The nearby dyad symmetry "origin" locus contains two 46 bp protected regions each encompassing two paired core binding sites. The demonstration of sequence-specific binding at multiple loci suggests that EBNA-1 has pleiotropic functions, which may include control of copy number and segregation of the EBV plasmids as well as initiation of replication.