New Epstein-Barr virus variants from cellular subclones of P3J-HR-1 Burkitt lymphoma

Latent Epstein-Barr virus infection collaborates with Myc over-expression in normal human B cells to induce Burkitt-like Lymphomas in mice

Epstein-Barr virus (EBV) is an important cause of human lymphomas, including Burkitt lymphoma (BL). EBV+ BLs are driven by Myc translocation and have stringent forms of viral latency that do not express either of the two major EBV oncoproteins, EBNA2 (which mimics Notch signaling) and LMP1 (which activates NF-κB signaling). Suppression of Myc-induced apoptosis, often through mutation of the TP53 (p53) gene or inhibition of pro-apoptotic BCL2L11 (BIM) gene expression, is required for development of Myc-driven BLs. EBV+ BLs contain fewer cellular mutations in apoptotic pathways compared to EBV-negative BLs, suggesting that latent EBV infection inhibits Myc-induced apoptosis. Here we use an EBNA2-deleted EBV virus (ΔEBNA2 EBV) to create the first in vivo model for EBV+ BL-like lymphomas derived from primary human B cells. We show that cord blood B cells infected with both ΔEBNA2 EBV and a Myc-expressing vector proliferate indefinitely on a CD40L/IL21 expressing feeder layer in vitro and cause rapid onset EBV+ BL-like tumors in NSG mice. These LMP1/EBNA2-negative Myc-driven lymphomas have wild type p53 and very low BIM, and express numerous germinal center B cell proteins (including TCF3, BACH2, Myb, CD10, CCDN3, and GCSAM) in the absence of BCL6 expression. Myc-induced activation of Myb mediates expression of many of these BL-associated proteins. We demonstrate that Myc blocks LMP1 expression both by inhibiting expression of cellular factors (STAT3 and Src) that activate LMP1 transcription and by increasing expression of proteins (DNMT3B and UHRF1) known to enhance DNA methylation of the LMP1 promoters in human BLs. These results show that latent EBV infection collaborates with Myc over-expression to induce BL-like human B-cell lymphomas in mice. As NF-κB signaling retards the growth of EBV-negative BLs, Myc-mediated repression of LMP1 may be essential for latent EBV infection and Myc translocation to collaboratively induce human BLs.

The Epstein-Barr Virus Lytic Protein BMLF1 Induces Upregulation of GRP78 Expression through ATF6 Activation

The unfolded protein response (UPR) is an intracellular signaling pathway essential for alleviating the endoplasmic reticulum (ER) stress. To support the productive infection, many viruses are known to use different strategies to manipulate the UPR signaling network. However, it remains largely unclear whether the UPR signaling pathways are modulated in the lytic cycle of Epstein-Barr virus (EBV), a widely distributed human pathogen. Herein, we show that the expression of GRP78, a central UPR regulator, is up-regulated during the EBV lytic cycle. Our data further revealed that knockdown of GRP78 in EBV-infected cell lines did not substantially affect lytic gene expression; however, GRP78 knockdown in these cells markedly reduced the production of virus particles. Importantly, we identified that the early lytic protein BMLF1 is the key regulator critically contributing to the activation of the grp78 gene promoter. Mechanistically, we found that BMLF1 can trigger the proteolytic cleavage and activation of the UPR senor ATF6, which then transcriptionally activates the grp78 promoter through the ER stress response elements. Our findings therefore provide evidence for the connection between the EBV lytic cycle and the UPR, and implicate that the BMLF1-mediated ATF6 activation may play critical roles in EBV lytic replication.

EBNA2-deleted Epstein-Barr virus (EBV) isolate, P3HR1, causes Hodgkin-like lymphomas and diffuse large B cell lymphomas with type II and Wp-restricted latency types in humanized mice

EBV transforms B cells in vitro and causes human B-cell lymphomas including classical Hodgkin lymphoma (CHL), Burkitt lymphoma (BL) and diffuse large B-cell lymphoma (DLBCL). The EBV latency protein, EBNA2, transcriptionally activates the promoters of all latent viral protein-coding genes expressed in type III EBV latency and is essential for EBV's ability to transform B cells in vitro. However, EBNA2 is not expressed in EBV-infected CHLs and BLs in humans. EBV-positive CHLs have type II latency and are largely driven by the EBV LMP1/LMP2A proteins, while EBV-positive BLs, which usually have type I latency are largely driven by c-Myc translocations, and only express the EBNA1 protein and viral non-coding RNAs. Approximately 15% of human BLs contain naturally occurring EBNA2-deleted viruses that support a form of viral latency known as Wp-restricted (expressing the EBNA-LP, EBNA3A/3B/3C, EBNA1 and BHRF1 proteins), but whether Wp-restricted latency and/or EBNA2-deleted EBV can induce lymphomas in humanized mice, or in the absence of c-Myc translocations, is unknown. Here we show that a naturally occurring EBNA2-deleted EBV strain (P3HR1) isolated from a human BL induces EBV-positive B-cell lymphomas in a subset of infected cord blood-humanized (CBH) mice. Furthermore, we find that P3HR1-infected lymphoma cells support two different viral latency types and phenotypes that are mutually exclusive: 1) Large (often multinucleated), CD30-positive, CD45-negative cells reminiscent of the Reed-Sternberg (RS) cells in CHL that express high levels of LMP1 but not EBNA-LP (consistent with type II viral latency); and 2) smaller monomorphic CD30-negative DLBCL-like cells that express EBNA-LP and EBNA3A but not LMP1 (consistent with Wp-restricted latency). These results reveal that EBNA2 is not absolutely required for EBV to form tumors in CBH mice and suggest that P3HR1 virus can be used to model EBV positive lymphomas with both Wp-restricted and type II latency in vivo.

A single phosphoacceptor residue in BGLF3 is essential for transcription of Epstein-Barr virus late genes

Almost one third of herpesvirus proteins are expressed with late kinetics. Many of these late proteins serve crucial structural functions such as formation of virus particles, attachment to host cells and internalization. Recently, we and others identified a group of Epstein-Barr virus early proteins that form a pre-initiation complex (vPIC) dedicated to transcription of late genes. Currently, there is a fundamental gap in understanding the role of post-translational modifications in regulating assembly and function of the complex. Here, we used mass spectrometry to map potential phosphorylation sites in BGLF3, a core component of the vPIC module that connects the BcRF1 viral TATA box binding protein to other components of the complex. We identified threonine 42 (T42) in BGLF3 as a phosphoacceptor residue. T42 is conserved in BGLF3 orthologs encoded by other gamma herpesviruses. Abolishing phosphorylation at T42 markedly reduced expression of vPIC-dependent late genes and disrupted production of new virus particles, but had no effect on early gene expression, viral DNA replication, or expression of vPIC-independent late genes. We complemented failure of BGLF3(T42A) to activate late gene expression by ectopic expression of other components of vPIC. Only BFRF2 and BVLF1 were sufficient to suppress the defect in late gene expression associated with BGLF3(T42A). These results were corroborated by the ability of wild type BGLF3 but not BGLF3(T42A) to form a trimeric complex with BFRF2 and BVLF1. Our findings suggest that phosphorylation of BGLF3 at threonine 42 serves as a new checkpoint for subsequent formation of BFRF2:BGLF3:BVLF1; a trimeric subcomplex essential for transcription of late genes. Our findings provide evidence that post-translational modifications regulate the function of the vPIC nanomachine that initiates synthesis of late transcripts in herpesviruses.

Inhibition of Epstein-Barr Virus Lytic Reactivation by the Atypical Antipsychotic Drug Clozapine

Epstein–Barr virus (EBV), a member of the Herpesviridae family, maintains a lifelong latent infection in human B cells. Switching from the latent to the lytic phase of its lifecycle allows the virus to replicate and spread. The viral lytic cycle is induced in infected cultured cells by drugs such as sodium butyrate and azacytidine. Lytic reactivation can be inhibited by natural products and pharmaceuticals. The anticonvulsant drugs valproic acid and valpromide inhibit EBV in Burkitt lymphoma cells. Therefore, other drugs that treat neurological and psychological disorders were investigated for effects on EBV lytic reactivation. Clozapine, an atypical antipsychotic drug used to treat schizophrenia and bipolar disorder, was found to inhibit the reactivation of the EBV lytic cycle. Levels of the viral lytic genes BZLF1, BRLF1, and BMLF1 were decreased by treatment with clozapine in induced Burkitt lymphoma cells. The effects on viral gene expression were dependent on the dose of clozapine, yet cells were viable at an inhibitory concentration of clozapine. One metabolite of clozapine—desmethylclozapine—also inhibited EBV lytic reactivation, while another metabolite—clozapine-N-oxide—had no effect. These drugs may be used to study cellular pathways that control the viral lytic switch in order to develop treatments for diseases caused by EBV.

Epstein-Barr Virus-Induced Nodules on Viral Replication Compartments Contain RNA Processing Proteins and a Viral Long Noncoding RNA

Profound alterations in host cell nuclear architecture accompany the lytic phase of Epstein-Barr virus (EBV) infection. Viral replication compartments assemble, host chromatin marginalizes to the nuclear periphery, cytoplasmic poly(A)-binding protein translocates to the nucleus, and polyadenylated mRNAs are sequestered within the nucleus. Virus-induced changes to nuclear architecture that contribute to viral host shutoff (VHS) must accommodate selective processing and export of viral mRNAs. Here we describe additional previously unrecognized nuclear alterations during EBV lytic infection in which viral and cellular factors that function in pre-mRNA processing and mRNA export are redistributed. Early during lytic infection, before formation of viral replication compartments, two cellular pre-mRNA splicing factors, SC35 and SON, were dispersed from interchromatin granule clusters, and three mRNA export factors, Y14, ALY, and NXF1, were depleted from the nucleus. During late lytic infection, virus-induced nodular structures (VINORCs) formed at the periphery of viral replication compartments. VINORCs were composed of viral (BMLF1 and BGLF5) and cellular (SC35, SON, SRp20, and NXF1) proteins that mediate pre-mRNA processing and mRNA export. BHLF1 long noncoding RNA was invariably found in VINORCs. VINORCs did not contain other nodular nuclear cellular proteins (PML or coilin), nor did they contain viral proteins (BRLF1 or BMRF1) found exclusively within replication compartments. VINORCs are novel EBV-induced nuclear structures. We propose that EBV-induced dispersal and depletion of pre-mRNA processing and mRNA export factors during early lytic infection contribute to VHS; subsequent relocalization of these pre-mRNA processing and mRNA export proteins to VINORCs and viral replication compartments facilitates selective processing and export of viral mRNAs.IMPORTANCE In order to make protein, mRNA transcribed from DNA in the nucleus must enter the cytoplasm. Nuclear export of mRNA requires correct processing of mRNAs by enzymes that function in splicing and nuclear export. During the Epstein-Barr virus (EBV) lytic cycle, nuclear export of cellular mRNAs is blocked, yet export of viral mRNAs is facilitated. Here we report the dispersal and dramatic reorganization of cellular (SC35, SON, SRp20, Y14, ALY, and NXF1) and viral (BMLF1 and BGLF5) proteins that play key roles in pre-mRNA processing and export of mRNA. These virus-induced nuclear changes culminate in formation of VINORCs, novel nodular structures composed of viral and cellular RNA splicing and export factors. VINORCs localize to the periphery of viral replication compartments, where viral mRNAs reside. These EBV-induced changes in nuclear organization may contribute to blockade of nuclear export of host mRNA, while enabling selective processing and export of viral mRNA.

Mutant Cellular AP-1 Proteins Promote Expression of a Subset of Epstein-Barr Virus Late Genes in the Absence of Lytic Viral DNA Replication

Epstein-Barr virus (EBV) ZEBRA protein activates the EBV lytic cycle. Cellular AP-1 proteins with alanine-to-serine [AP-1(A/S)] substitutions homologous to ZEBRA(S186) assume some functions of EBV ZEBRA. These AP-1(A/S) mutants bind methylated EBV DNA and activate expression of some EBV genes. Here, we compare expression of 67 viral genes induced by ZEBRA versus expression induced by AP-1(A/S) proteins. AP-1(A/S) activated 24 genes to high levels and 15 genes to intermediate levels; activation of 28 genes by AP-1(A/S) was severely impaired. We show that AP-1(A/S) proteins are defective at stimulating viral lytic DNA replication. The impairment of expression of many late genes compared to that of ZEBRA is likely due to the inability of AP-1(A/S) proteins to promote viral DNA replication. However, even in the absence of detectable viral DNA replication, AP-1(A/S) proteins stimulated expression of a subgroup of late genes that encode viral structural proteins and immune modulators. In response to ZEBRA, expression of this subgroup of late genes was inhibited by phosphonoacetic acid (PAA), which is a potent viral replication inhibitor. However, when the lytic cycle was activated by AP-1(A/S), PAA did not reduce expression of this subgroup of late genes. We also provide genetic evidence, using the BMRF1 knockout bacmid, that these genes are true late genes in response to ZEBRA. AP-1(A/S) binds to the promoter region of at least one of these late genes, BDLF3, encoding an immune modulator.IMPORTANCE Mutant c-Jun and c-Fos proteins selectively activate expression of EBV lytic genes, including a subgroup of viral late genes, in the absence of viral DNA replication. These findings indicate that newly synthesized viral DNA is not invariably required for viral late gene expression. While viral DNA replication may be obligatory for late gene expression driven by viral transcription factors, it does not limit the ability of cellular transcription factors to activate expression of some viral late genes. Our results show that expression of all late genes may not be strictly dependent on viral lytic DNA replication. The c-Fos A151S mutation has been identified in a human cancer. c-Fos A151S in combination with wild-type c-Jun activates the EBV lytic cycle. Our data provide proof of principle that mutant cellular transcription factors could cause aberrant regulation of viral lytic cycle gene expression and play important roles in EBV-associated diseases.

The Epstein-Barr Virus Immunoevasins BCRF1 and BPLF1 Are Expressed by a Mechanism Independent of the Canonical Late Pre-initiation Complex

Subversion of host immune surveillance is a crucial step in viral pathogenesis. Epstein-Barr virus (EBV) encodes two immune evasion gene products, BCRF1 (viral IL-10) and BPLF1 (deubiquitinase/deneddylase); both proteins suppress antiviral immune responses during primary infection. The BCRF1 and BPLF1 genes are expressed during the late phase of the lytic cycle, an essential but poorly understood phase of viral gene expression. Several late gene regulators recently identified in beta and gamma herpesviruses form a viral pre-initiation complex for transcription. Whether each of these late gene regulators is necessary for transcription of all late genes is not known. Here, studying viral gene expression in the absence and presence of siRNAs to individual components of the viral pre-initiation complex, we identified two distinct groups of late genes. One group includes late genes encoding the two immunoevasins, BCRF1 and BPLF1, and is transcribed independently of the viral pre-initiation complex. The second group primarily encodes viral structural proteins and is dependent on the viral pre-initiation complex. The protein kinase BGLF4 is the only known late gene regulator necessary for expression of both groups of late genes. ChIP-seq analysis showed that the transcription activator Rta associates with the promoters of eight late genes including genes encoding the viral immunoevasins. Our results demonstrate that late genes encoding immunomodulatory proteins are transcribed by a mechanism distinct from late genes encoding viral structural proteins. Understanding the mechanisms that specifically regulate expression of the late immunomodulatory proteins could aid the development of antiviral drugs that impair immune evasion by the oncogenic EB virus.

Late proteins are expressed during the productive cycle of Epstein-Barr virus (EBV) after the onset of viral DNA replication. Many late proteins serve structural functions; they form the capsid shell around the viral genome or mediate attachment and fusion of the virus to the host cell. EBV also encodes two late proteins that suppress the immune system during primary infection. The current model suggests that transcription of all late genes is regulated by a common mechanism involving seven late gene regulators. Here, we demonstrate that late genes encoding two viral immune suppressants are transcribed by a mechanism different from that regulating late genes encoding structural proteins. Abolishing expression of the late immunomodulators without disrupting expression of the antigenic viral structural proteins could serve as an approach to block EBV de novo infection and its associated malignancies.

Valpromide Inhibits Lytic Cycle Reactivation of Epstein-Barr Virus

Reactivation of Epstein-Barr virus (EBV) from latency into the lytic phase of its life cycle allows the virus to spread among cells and between hosts. Valproic acid (VPA) inhibits initiation of the lytic cycle in EBV-infected B lymphoma cells. While VPA blocks viral lytic gene expression, it induces expression of many cellular genes, because it is a histone deacetylase (HDAC) inhibitor. Here we show, using derivatives of VPA, that blockade of EBV reactivation is separable from HDAC inhibition. Valpromide (VPM), an amide derivative of valproic acid that is not an HDAC inhibitor, prevented expression of two EBV genes, BZLF1 and BRLF1, that mediate lytic reactivation. VPM also inhibited expression of a viral late gene, but not early genes, when BZLF1 was exogenously expressed. Unlike VPA, VPM did not activate lytic expression of Kaposi's sarcoma-associated herpesvirus. Expression of cellular immediate-early genes, such as FOS and EGR1, is kinetically upstream of the EBV lytic cycle. VPM did not activate expression of these cellular immediate-early genes but decreased their level of expression when induced by butyrate, an HDAC inhibitor. VPM did not alter expression of several other cellular immediate-early genes, including STAT3, which were induced by the HDAC inhibitors in cells refractory to lytic induction. Therefore, VPM selectively inhibits both viral and cellular gene expression. VPA and VPM represent a new class of antiviral agents. The mechanism by which VPA and VPM block EBV reactivation may be related to their anticonvulsant activity.

IMPORTANCE

Epstein-Barr virus, (EBV), a human tumor virus, establishes a life-long latent infection. Reactivation of EBV into the lytic phase of its life cycle allows the virus to spread. Previously, we showed that EBV reactivation was blocked by valproic acid (VPA), an inhibitor of cellular histone deacetylases (HDACs). VPA alters the expression of thousands of cellular genes. In this study, we demonstrate that valpromide (VPM), an amide derivative of valproic acid that is not an HDAC inhibitor, prevented initiation of the EBV lytic cycle. VPA induced lytic reactivation of Kaposi's sarcoma-associated herpesvirus (KSHV), but VPM did not. Unlike VPA, VPM did not activate cellular immediate-early gene expression. VPM is a new type of antiviral agent. VPM will be useful in probing the mechanism of EBV lytic reactivation and may have therapeutic application.

DNA Damage Signaling Is Induced in the Absence of Epstein-Barr Virus (EBV) Lytic DNA Replication and in Response to Expression of ZEBRA

Epstein Barr virus (EBV), like other oncogenic viruses, modulates the activity of cellular DNA damage responses (DDR) during its life cycle. Our aim was to characterize the role of early lytic proteins and viral lytic DNA replication in activation of DNA damage signaling during the EBV lytic cycle. Our data challenge the prevalent hypothesis that activation of DDR pathways during the EBV lytic cycle occurs solely in response to large amounts of exogenous double stranded DNA products generated during lytic viral DNA replication. In immunofluorescence or immunoblot assays, DDR activation markers, specifically phosphorylated ATM (pATM), H2AX (γH2AX), or 53BP1 (p53BP1), were induced in the presence or absence of viral DNA amplification or replication compartments during the EBV lytic cycle. In assays with an ATM inhibitor and DNA damaging reagents in Burkitt lymphoma cell lines, γH2AX induction was necessary for optimal expression of early EBV genes, but not sufficient for lytic reactivation. Studies in lytically reactivated EBV-positive cells in which early EBV proteins, BGLF4, BGLF5, or BALF2, were not expressed showed that these proteins were not necessary for DDR activation during the EBV lytic cycle. Expression of ZEBRA, a viral protein that is necessary for EBV entry into the lytic phase, induced pATM foci and γH2AX independent of other EBV gene products. ZEBRA mutants deficient in DNA binding, Z(R183E) and Z(S186E), did not induce foci of pATM. ZEBRA co-localized with HP1β, a heterochromatin associated protein involved in DNA damage signaling. We propose a model of DDR activation during the EBV lytic cycle in which ZEBRA induces ATM kinase phosphorylation, in a DNA binding dependent manner, to modulate gene expression. ATM and H2AX phosphorylation induced prior to EBV replication may be critical for creating a microenvironment of viral and cellular gene expression that enables lytic cycle progression.

Cellular STAT3 functions via PCBP2 to restrain Epstein-Barr Virus lytic activation in B lymphocytes

A major hurdle to killing Epstein-Barr virus (EBV)-infected tumor cells using oncolytic therapy is the presence of a substantial fraction of EBV-infected cells that does not support the lytic phase of EBV despite exposure to lytic cycle-promoting agents. To determine the mechanism(s) underlying this refractory state, we developed a strategy to separate lytic from refractory EBV-positive (EBV(+)) cells. By examining the cellular transcriptome in separated cells, we previously discovered that high levels of host STAT3 (signal transducer and activator of transcription 3) curtail the susceptibility of latently infected cells to lytic cycle activation signals. The goals of the present study were 2-fold: (i) to determine the mechanism of STAT3-mediated resistance to lytic activation and (ii) to exploit our findings to enhance susceptibility to lytic activation. We therefore analyzed our microarray data set, cellular proteomes of separated lytic and refractory cells, and a publically available STAT3 chromatin immunoprecipitation sequencing (ChIP-Seq) data set to identify cellular PCBP2 [poly(C)-binding protein 2], an RNA-binding protein, as a transcriptional target of STAT3 in refractory cells. Using Burkitt lymphoma cells and EBV(+) cell lines from patients with hypomorphic STAT3 mutations, we demonstrate that single cells expressing high levels of PCBP2 are refractory to spontaneous and induced EBV lytic activation, STAT3 functions via cellular PCBP2 to regulate lytic susceptibility, and suppression of PCBP2 levels is sufficient to increase the number of EBV lytic cells. We expect that these findings and the genome-wide resources that they provide will accelerate our understanding of a longstanding mystery in EBV biology and guide efforts to improve oncolytic therapy for EBV-associated cancers.

IMPORTANCE

Most humans are infected with Epstein-Barr virus (EBV), a cancer-causing virus. While EBV generally persists silently in B lymphocytes, periodic lytic (re)activation of latent virus is central to its life cycle and to most EBV-related diseases. However, a substantial fraction of EBV-infected B cells and tumor cells in a population is refractory to lytic activation. This resistance to lytic activation directly and profoundly impacts viral persistence and the effectiveness of oncolytic therapy for EBV(+) cancers. To identify the mechanisms that underlie susceptibility to EBV lytic activation, we used host gene and protein expression profiling of separated lytic and refractory cells. We find that STAT3, a transcription factor overactive in many cancers, regulates PCBP2, a protein important in RNA biogenesis, to regulate susceptibility to lytic cycle activation signals. These findings advance our understanding of EBV persistence and provide important leads on devising methods to improve viral oncolytic therapies.

Epstein-Barr Virus-Specific Humoral Immune Responses in Health and Disease

Epstein-Barr virus (EBV) is widely distributed in the world and associated with a still increasing number of acute, chronic, malignant and autoimmune disease syndromes. Humoral immune responses to EBV have been studied for diagnostic, pathogenic and protective (vaccine) purposes. These studies use a range of methodologies, from cell-based immunofluorescence testing to antibody-diversity analysis using immunoblot and epitope analysis using recombinant or synthetic peptide-scanning. First, the individual EBV antigen complexes (VCA , MA, EA(D), EA(R) and EBNA) are defined at cellular and molecular levels, providing a historic overview. The characteristic antibody responses to these complexes in health and disease are described, and differences are highlighted by clinical examples. Options for EBV vaccination are briefly addressed. For a selected number of immunodominant proteins, in particular EBNA1, the interaction with human antibodies is further detailed at the epitope level, revealing interesting insights for structure, function and immunological aspects, not considered previously. Humoral immune responses against EBV-encoded tumour antigens LMP1, LMP2 and BARF1 are addressed, which provide novel options for targeted immunotherapy. Finally, some considerations on EBV-linked autoimmune diseases are given, and mechanisms of antigen mimicry are briefly discussed. Further analysis of humoral immune responses against EBV in health and disease in carefully selected patient cohorts will open new options for understanding pathogenesis of individual EBV-linked diseases and developing targeted diagnostic and therapeutic approaches.

Activation and repression of Epstein-Barr Virus and Kaposi's sarcoma-associated herpesvirus lytic cycles by short- and medium-chain fatty acids

The lytic cycles of Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) are induced in cell culture by sodium butyrate (NaB), a short-chain fatty acid (SCFA) histone deacetylase (HDAC) inhibitor. Valproic acid (VPA), another SCFA and an HDAC inhibitor, induces the lytic cycle of KSHV but blocks EBV lytic reactivation. To explore the hypothesis that structural differences between NaB and VPA account for their functional effects on the two related viruses, we investigated the capacity of 16 structurally related short- and medium-chain fatty acids to promote or prevent lytic cycle reactivation. SCFAs differentially affected EBV and KSHV reactivation. KSHV was reactivated by all SCFAs that are HDAC inhibitors, including phenylbutyrate. However, several fatty acid HDAC inhibitors, such as isobutyrate and phenylbutyrate, did not reactivate EBV. Reactivation of KSHV lytic transcripts could not be blocked completely by any fatty acid tested. In contrast, several medium-chain fatty acids inhibited lytic activation of EBV. Fatty acids that blocked EBV reactivation were more lipophilic than those that activated EBV. VPA blocked activation of the BZLF1 promoter by NaB but did not block the transcriptional function of ZEBRA. VPA also blocked activation of the DNA damage response that accompanies EBV lytic cycle activation. Properties of SCFAs in addition to their effects on chromatin are likely to explain activation or repression of EBV. We concluded that fatty acids stimulate the two related human gammaherpesviruses to enter the lytic cycle through different pathways. Importance: Lytic reactivation of EBV and KSHV is needed for persistence of these viruses and plays a role in carcinogenesis. Our direct comparison highlights the mechanistic differences in lytic reactivation between related human oncogenic gammaherpesviruses. Our findings have therapeutic implications, as fatty acids are found in the diet and produced by the human microbiota. Small-molecule inducers of the lytic cycle are desired for oncolytic therapy. Inhibition of viral reactivation, alternatively, may prove useful in cancer treatment. Overall, our findings contribute to the understanding of pathways that control the latent-to-lytic switch and identify naturally occurring molecules that may regulate this process.

Valproic acid antagonizes the capacity of other histone deacetylase inhibitors to activate the Epstein-barr virus lytic cycle

Diverse stimuli reactivate the Epstein-Barr virus (EBV) lytic cycle in Burkitt lymphoma (BL) cells. In HH514-16 BL cells, two histone deacetylase (HDAC) inhibitors, sodium butyrate (NaB) and trichostatin A (TSA), and the DNA methyltransferase inhibitor azacytidine (AzaCdR) promote lytic reactivation. Valproic acid (VPA), which, like NaB, belongs to the short-chain fatty acid class of HDAC inhibitors, fails to induce the EBV lytic cycle in these cells. Nonetheless, VPA behaves as an HDAC inhibitor; it causes hyperacetylation of histone H3 (J. K. Countryman, L. Gradoville, and G. Miller, J. Virol. 82:4706-4719, 2008). Here we show that VPA blocked the induction of EBV early lytic proteins ZEBRA and EA-D in response to NaB, TSA, or AzaCdR. The block in lytic activation occurred prior to the accumulation of BZLF1 transcripts. Reactivation of EBV in Akata cells, in response to anti-IgG, and in Raji cells, in response to tetradecanoyl phorbol acetate (TPA), was also inhibited by VPA. MS-275 and apicidin, representing two additional classes of HDAC inhibitors, and suberoylanilide hydroxamic acid (SAHA) reactivated EBV in HH514-16 cells; this activity was also inhibited by VPA. Although VPA potently blocked the expression of viral lytic-cycle transcripts, it did not generally block the transcription of cellular genes and was not toxic. The levels and kinetics of specific cellular transcripts, such as Stat3, Frmd6, Mad1, Sepp1, c-fos, c-jun, and egr1, which were activated by NaB and TSA, were similar in HH514-16 cells treated with VPA. When combined with NaB or TSA, VPA did not inhibit the activation of these cellular genes. Changes in cellular gene expression in response to VPA, NaB, or TSA were globally similar as assessed by human genome arrays; however, VPA selectively stimulated the expression of some cellular genes, such as MEF2D, YY1, and ZEB1, that could repress the EBV lytic cycle. We describe a novel example of functional antagonism between HDAC inhibitors.

Epstein-Barr virus-encoded Bcl-2 homologue functions as a survival factor in Wp-restricted Burkitt lymphoma cell line P3HR-1

Burkitt lymphoma (BL) is etiologically associated with Epstein-Barr virus (EBV). EBV-positive BL tumors display two latent forms of infection. One is referred to as latency I infection, in which EBV expresses the virus genome maintenance protein EBNA1 as the only viral protein. The other is referred to as Wp-restricted latency and was recently identified in a subset of BL tumors. In these tumors, EBV expresses EBNA1, EBNA3A, EBNA3B, EBNA3C, a truncated form of EBNA-LP, and the viral Bcl-2 homologue BHRF1, all of which are driven by the BamHI W promoter (Wp). To investigate the role of EBV in Wp-restricted BL, we conditionally expressed a dominant-negative EBNA1 (dnEBNA1) mutant which interrupts the virus genome maintenance function of EBNA1 in the P3HR-1 BL cell line. Induction of dnEBNA1 expression caused loss of the EBV genome and resulted in apoptosis of P3HR-1 cells in the absence of exogenous apoptosis inducers, indicating that P3HR-1 cells cannot survive without EBV. Stable transfection of the BHRF1 gene into P3HR-1 cells rescued the cells from the apoptosis induced by dnEBNA1 expression, whereas stable transfection of truncated EBNA-LP, EBNA3A, or EBNA3C did not. Moreover, knockdown of BHRF1 expression in P3HR-1 cells resulted in increased cell death. These results indicate that EBV is essential for the survival of P3HR-1 cells and that BHRF1 functions as a survival factor. Our finding implies a critical contribution of BHRF1 to the pathogenesis of Wp-restricted BLs.

Upregulation of STAT3 marks Burkitt lymphoma cells refractory to Epstein-Barr virus lytic cycle induction by HDAC inhibitors

A fundamental problem in studying the latent-to-lytic switch of Epstein-Barr virus (EBV) and the viral lytic cycle itself is the lack of a culture system fully permissive to lytic cycle induction. Strategies to target EBV-positive tumors by inducing the viral lytic cycle with chemical agents are hindered by inefficient responses to stimuli. In vitro, even in the most susceptible cell lines, more than 50% of cells latently infected with EBV are refractory to induction of the lytic cycle. The mechanisms underlying the refractory state are not understood. We separated lytic from refractory Burkitt lymphoma-derived HH514-16 cells after treatment with an HDAC inhibitor, sodium butyrate. Both refractory- and lytic-cell populations responded to the inducing stimulus by hyperacetylation of histone H3. However, analysis of host cell gene expression showed that specific cellular transcripts Stat3, Fos, and interleukin-8 (IL-8) were preferentially upregulated in the refractory-cell population, while IL-6 was upregulated in the lytic population. STAT3 protein levels were also substantially increased in refractory cells relative to untreated or lytic cells. This increase in de novo expression resulted primarily in unphosphorylated STAT3. Examination of single cells revealed that high levels of STAT3 were strongly associated with the refractory state. The refractory state is manifest in a unique subpopulation of cells that exhibits different cellular responses than do lytic cells exposed to the same stimulus. Identifying characteristics of cells refractory to lytic induction relative to cells that undergo lytic activation will be an important step in developing a better understanding of the regulation of the EBV latent to lytic switch.

Stimulus duration and response time independently influence the kinetics of lytic cycle reactivation of Epstein-Barr virus

Epstein-Barr virus (EBV) can be reactivated from latency into the lytic cycle by many stimuli believed to operate by different mechanisms. Cell lines containing EBV differ in their responses to inducing stimuli, yet all stimuli require de novo protein synthesis (44). A crucial step preliminary to identifying these proteins and determining when they are required is to measure the duration of stimulus and response time needed for activation of expression of EBV BRLF1 and BZLF1, the earliest viral indicators of reactivation. Here we show, with four EBV-containing cell lines that respond to different inducing agents, that stimuli that are effective at reactivating EBV can be divided into two main groups. The histone deacetylase inhibitors sodium butyrate and trichostatin A require a relatively long period of exposure, from 2 to 4 h or longer. Phorbol esters, anti-immunoglobulin G (anti-IgG), and, surprisingly, 5-aza-2'-deoxycytidine require short exposures of 15 min or less. The cell/virus background influences the response time. Expression of the EBV BZLF1 and BRLF1 genes can be detected before 2 h in Akata cells treated with anti-IgG, but both long- and short-duration stimuli required 4 or more hr to activate BZLF1 and BRLF1 expression in HH514-16, Raji, or B95-8 cells. Thus, stimulus duration and response time are independent variables. Neither stimulus duration nor response time can be predicted by the number of cells activated into the lytic cycle. These experiments shed new light on the earliest events leading to lytic cycle reactivation of EBV.

Epstein-Barr virus WZhet DNA can induce lytic replication in epithelial cells in vitro, although WZhet is not detectable in many human tissues in vivo

OBJECTIVE

WZhet is a rearranged and partially deleted form of the Epstein-Barr virus (EBV) genome in which the BamH1W region becomes juxtaposed with and activates BZLF1, resulting in constitutive viral replication. We tested whether WZhet induces viral replication in epithelial cells, and we studied its prevalence in a wide range of lesional tissues arising in vivo.

METHODS

A quantitative real-time PCR assay targeting EBV WZhet DNA was developed to measure this recombinant form of the EBV genome.

RESULTS

WZhet DNA was undetectable in any of 324 plasma or paraffin-embedded tissue samples from patients with EBV-associated and EBV-negative disorders. These included specimens from patients with Hodgkin or non-Hodgkin lymphoma, post-transplant lymphoproliferation, nasopharyngeal or gastric adenocarcinoma, and infectious mononucleosis. However, WZhet DNA was detected in vitro in EBV-infected AGS gastric cancer cells. Additionally, transient transfection of infected AGS gastric cancer cells showed that viral replication could be induced by a WZhet plasmid.

CONCLUSION

This is the first evidence that WZhet induces the EBV lytic cycle in an epithelial cell line. Our negative findings in natural settings suggest that WZhet is a defective viral product that thrives in the absence of a host immune system but is rarely present in vivo.

Two phenylalanines in the C-terminus of Epstein-Barr virus Rta protein reciprocally modulate its DNA binding and transactivation function

The Rta (R transactivator) protein plays an essential role in the Epstein-Barr viral (EBV) lytic cascade. Rta activates viral gene expression by several mechanisms including direct and indirect binding to target viral promoters, synergy with EBV ZEBRA protein, and stimulation of cellular signaling pathways. We previously found that Rta proteins with C-terminal truncations of 30 aa were markedly enhanced in their capacity to bind DNA (Chen, L.W., Chang, P.J., Delecluse, H.J., and Miller, G., (2005). Marked variation in response of consensus binding elements for the Rta protein of Epstein-Barr virus. J. Virol. 79(15), 9635-9650.). Here we show that two phenylalanines (F600 and F605) in the C-terminus of Rta play a crucial role in mediating this DNA binding inhibitory function. Amino acids 555 to 605 of Rta constitute a functional DNA binding inhibitory sequence (DBIS) that markedly decreased DNA binding when transferred to a minimal DNA binding domain of Rta (aa 1-350). Alanine substitution mutants, F600A/F605A, abolished activity of the DBIS. F600 and F605 are located in the transcriptional activation domain of Rta. Alanine substitutions, F600A/F605A, decreased transcriptional activation by Rta protein, whereas aromatic substitutions, such as F600Y/F605Y or F600W/F605W, partially restored transcriptional activation. Full-length Rta protein with F600A/F605A mutations were enhanced in DNA binding compared to wild-type, whereas Rta proteins with F600Y/F605Y or F600W/F605W substitutions were, like wild-type Rta, relatively poor DNA binders. GAL4 (1-147)/Rta (416-605) fusion proteins with F600A/F605A mutations were diminished in transcriptional activation, relative to GAL4/Rta chimeras without such mutations. The results suggest that, in the context of a larger DBIS, F600 and F605 play a role in the reciprocal regulation of DNA binding and transcriptional activation by Rta. Regulation of DNA binding by Rta is likely to be important in controlling its different modes of action.

Points of recombination in Epstein-Barr virus (EBV) strain P3HR-1-derived heterogeneous DNA as indexes to EBV DNA recombinogenic events in vivo

Deletions and rearrangements in the genome of Epstein-Barr virus (EBV) strain P3HR-1 generate subgenomic infectious particles that, unlike defective interfering particles in other viral systems, enhance rather than restrict EBV replication in vitro. Reports of comparable heterogeneous (het) DNA in EBV-linked human diseases, based on detection of an abnormal juxtaposition of EBV DNA fragments BamHI W and BamHI Z that disrupts viral latency, prompted us to determine at the nucleotide level all remaining recombination joints formed by the four constituent segments of P3HR-1-derived het DNA. Guided by endonuclease restriction maps, we chose PCR primer pairs that approximated and framed junctions creating the unique BamHI M/B1 and E/S fusion fragments. Sequencing of PCR products revealed points of recombination that lacked regions of extensive homology between constituent fragments. Identical recombination junctions were detected by PCR in EBV-positive salivary samples from human immunodeficiency virus-infected donors, although the W/Z rearrangement that induces EBV reactivation was frequently found in the absence of the other two. In vitro infection of lymphoid cells similarly indicated that not all three het DNA rearrangements need to reside on a composite molecule. These results connote a precision in the recombination process that dictates both composition and regulation of gene segments altered by genomic rearrangement. Moreover, the apparent frequency of het DNA at sites of EBV replication in vivo is consistent with a likely contribution to the pathogenesis of EBV reactivation.

Phosphoacceptor site S173 in the regulatory domain of Epstein-Barr Virus ZEBRA protein is required for lytic DNA replication but not for activation of viral early genes

The Epstein-Barr virus ZEBRA protein controls the viral lytic cycle. ZEBRA activates the transcription of viral genes required for replication. ZEBRA also binds to oriLyt and interacts with components of the viral replication machinery. The mechanism that differentiates the roles of ZEBRA in regulation of transcription and initiation of lytic replication is unknown. Here we show that S173, a residue in the regulatory domain, is obligatory for ZEBRA to function as an origin binding protein but is dispensable for its role as a transcriptional activator of early genes. Serine-to-alanine substitution of this residue, which prevents phosphorylation of S173, resulted in a threefold reduction in the DNA binding affinity of ZEBRA for oriLyt, as assessed by chromatin immunoprecipitation. An independent assay based on ZEBRA solubility demonstrated a marked defect in DNA binding by the Z(S173A) mutant. The phenotype of a phosphomimetic mutant, the Z(S173D) mutant, was similar to that of wild-type ZEBRA. Our findings suggest that phosphorylation of S173 promotes viral replication by enhancing ZEBRA's affinity for DNA. The results imply that stronger DNA binding is required for ZEBRA to activate replication than that required to activate transcription.

Cells lytically infected with Epstein-Barr virus are detected and separable by immunoglobulins from EBV-seropositive individuals

The role of Epstein-Barr virus (EBV) lytic cycle gene expression in lymphocytes in the pathogenesis of EBV-associated diseases is incompletely understood. The ability to physically separate lytically induced from latently infected cells from the same population and to examine them in parallel would significantly enhance understanding of the viral, cellular, and environmental factors that govern susceptibility of an EBV-infected cell to lytic cycle induction stimuli at the single cell level. This study demonstrates, using a flow cytometry-based system, that human serum immunoglobulins from individuals immune to EBV reproducibly discriminate between and can be used to physically separate lytically induced from latently infected B cells. Using this new quantitative and sensitive technique, two novel observations about lytic cycle activation were made. First, the kinetics of lytic cycle activation by histone deacetylase inhibitors is more rapid than induction by a DNA methyl transferase inhibitor. Second, butyrate-treated cells which are initially refractory to lytic cycle activation can be induced upon subsequent exposure to the inducing agent. Therefore, susceptibility to lytic cycle induction of a latently infected B cell is governed by environmental and physiologic factors and not by permanent cellular or viral genetic changes.

Identification of constitutive phosphorylation sites on the Epstein-Barr virus ZEBRA protein

ZEBRA, the product of the Epstein-Barr virus gene bzlf1, and a member of the AP-1 subfamily of basic zipper (bZIP) transcription factors, is necessary and sufficient to disrupt viral latency and to initiate the viral lytic cycle. Two serine residues of ZEBRA, Ser167 and Ser173, are substrates for casein kinase 2 (CK2) and are constitutively phosphorylated in vivo. Phosphorylation of ZEBRA at its CK2 sites is required for proper temporal regulation of viral gene expression. Phosphopeptide analysis indicated that ZEBRA contains additional constitutive phosphorylation sites. Here we employed a co-migration strategy to map these sites in vivo. The cornerstone of this strategy was to correlate the migration of 32P- and 35S-labeled tryptic peptides of ZEBRA. The identity of the peptides was revealed by mutagenesis of methionine and cysteine residues present in each peptide. Phosphorylation sites within the peptide were identified by mutagenesis of serines and threonines. ZEBRA was shown to be phosphorylated at serine and threonine residues, but not tyrosine. Two previously unrecognized phosphorylation sites of ZEBRA were identified in the NH2-terminal region of the transactivation domain: a cluster of weak phosphorylation sites at Ser6, Thr7, and Ser8 and a strong phosphorylation site at Thr14. Thr14 was embedded in a MAP kinase consensus sequence and could be phosphorylated in vitro by JNK, despite the absence of a canonical JNK docking site. Thus ZEBRA is now known to be constitutively phosphorylated at three distinct sites.

Epstein-Barr virus nuclear antigen 2 (EBNA2) gene deletion is consistently linked with EBNA3A, -3B, and -3C expression in Burkitt's lymphoma cells and with increased resistance to apoptosis

Most Epstein-Barr virus (EBV)-positive Burkitt's lymphomas (BLs) carry a wild-type EBV genome and express EBV nuclear antigen 1 (EBNA1) selectively from the BamHI Q promoter (latency I). Recently we identified a distinct subset of BLs carrying both wild-type and EBNA2 gene-deleted (transformation-defective) viral genomes. The cells displayed an atypical "BamHI W promoter (Wp)-restricted" form of latency where Wp (rather than Qp) was active and EBNA1, -3A, -3B, -3C, and -LP were expressed in the absence of EBNA2 or latent membrane proteins 1 and 2. Here we present data strongly supporting the view that the EBNA2-deleted genome is transcriptionally active in these cells and the wild-type genome is silent. Single-cell cloning of three parental Wp-restricted BL lines generated clones carrying either both viral genomes or the EBNA2-deleted genome only, never clones with the wild-type genome only. All rescued clones displayed the Wp-restricted form of latency characteristic of the parent line and retained the original parent cell phenotype. Interestingly, Wp-restricted parent lines and derived clones were markedly more resistant to inducers of apoptosis than standard latency I BL lines. Furthermore, in vitro infection of EBV-negative BL lines with an EBNA2 gene-deleted virus generated EBV-positive converts with Wp-restricted latency and a similarly marked apoptosis resistance. We postulate that, in the subset of BLs displaying Wp-restricted latency, infection of a tumor progenitor cell with an EBNA2 gene-deleted virus has provided that cell with a survival advantage through broadening antigen expression to include the EBNA3 proteins.

The Epstein-Barr virus BILF1 gene encodes a G protein-coupled receptor that inhibits phosphorylation of RNA-dependent protein kinase

Epstein-Barr virus (EBV) infection is associated with many lymphoproliferative diseases, such as infectious mononucleosis and Burkitt's lymphoma. Consequently, EBV is one of the most extensively studied herpesviruses. Surprisingly, a putative G protein-coupled receptor (GPCR) gene of EBV, BILF1, has hitherto escaped attention, yet BILF1-like genes are conserved among all known lymphocryptovirus species, suggesting that they play a pivotal role in viral infection. To determine the function of EBV BILF1, the activity of this gene and its products was studied. BILF1-specific mRNA was detected in various EBV-positive cell types and found to be expressed predominantly during the immediate early and early phases of infection in vitro. Interestingly, in COS-7 cells transfected with BILF1 expression constructs, a decrease in forskolin-induced CRE-mediated transcription was measured, as well as an increase in NF-kappaB-mediated transcription. In contrast, CRE-mediated transcription was increased in EBV-positive Burkitt's lymphoma cells as well as EBV-positive lymphoblastoid B cells transfected with BILF1, whereas NF-kappaB-mediated transcription levels remained unaffected in these cells. All observed activities were sensitive to treatment with pertussis toxin, indicating that the BILF1-encoded protein mediates these activities by coupling to G proteins of the G(i/o) class. Finally, reduced levels of phosphorylated RNA-dependent antiviral protein kinase were observed in COS-7 and Burkitt's lymphoma cells transfected with BILF1. Neither of the observed effects required a ligand to interact with the BILF1 gene product, suggesting that BILF1 encodes a constitutively active GPCR capable of modulating various intracellular signaling pathways.

Epstein-Barr virus latent membrane protein LMP-2A is sufficient for transactivation of the human endogenous retrovirus HERV-K18 superantigen

Superantigens are microbial proteins that strongly stimulate T cells. We described previously that the Epstein-Barr virus (EBV) transactivates a superantigen encoded by the human endogenous retrovirus, HERV-K18. We now report that the transactivation is dependent upon the EBV latent cycle proteins. Moreover, LMP-2A is sufficient for induction of HERV-K18 superantigen activity.

Phosphorylation of Epstein-Barr virus ZEBRA protein at its casein kinase 2 sites mediates its ability to repress activation of a viral lytic cycle late gene by Rta

ZEBRA, a member of the bZIP family, serves as a master switch between latent and lytic cycle Epstein-Barr virus (EBV) gene expression. ZEBRA influences the activity of another viral transactivator, Rta, in a gene-specific manner. Some early lytic cycle genes, such as BMRF1, are activated in synergy by ZEBRA and Rta. However, ZEBRA suppresses Rta's ability to activate a late gene, BLRF2. Here we show that this repressive activity is dependent on the phosphorylation state of ZEBRA. We find that two residues of ZEBRA, S167 and S173, that are phosphorylated by casein kinase 2 (CK2) in vitro are also phosphorylated in vivo. Inhibition of ZEBRA phosphorylation at the CK2 substrate motif, either by serine-to-alanine substitutions or by use of a specific inhibitor of CK2, abolished ZEBRA's capacity to repress Rta activation of the BLRF2 gene, but did not alter its ability to initiate the lytic cycle or to synergize with Rta in activation of the BMRF1 early-lytic-cycle gene. These studies illustrate how the phosphorylation state of a transcriptional activator can modulate its behavior as an activator or repressor of gene expression. Phosphorylation of ZEBRA at its CK2 sites is likely to play an essential role in proper temporal control of the EBV lytic life cycle.

Genome re-arrangements associated with loss of pathogenicity of the gamma-herpesvirus alcelaphine herpesvirus-1

The alcelaphine herpesvirus 1 (AlHV-1) causes malignant catarrhal fever in ruminants. Previous work had shown that serial passage of AlHV-1 in culture resulted in genome alterations that are associated with a loss in pathogenicity. Here we have analysed the re-arrangements that occur in more detail. None of the observed re-arrangements was entirely consistent. However, they did all involve translocation of a similar region of DNA from around the centre of the genome to areas either next to or in between terminal repeat elements at either end of the genome. There was also a concomitant loss of the wild-type locus. These re-arrangements appeared to be associated with the loss of virulence and the appearance of cell-free virus.

Down-regulation of spontaneous Epstein-Barr virus reactivation in the P3HR-1 cell line by L-arginine

We tested the hypothesis that inhibition of Epstein-Barr virus (EBV) reactivation is controlled in part by nitric oxide (NO) generated from L-arginine (Arg). The spontaneous reactivation of EBV in the Burkitt's lymphoma (BL) cell line P3HR-1 was inhibited when the cells were cultured in L-Arg-supplemented medium. The expression of EBV early antigen (EA), immediate-early BZLF1 mRNA and the protein ZEBRA, and production of infectious virus were reduced by L-Arg supplementation in a dose-dependent manner. We demonstrated that inducible NO synthase (iNOS) mRNA was constitutively expressed in P3HR-1 cells, as quantitated by the reverse transcription-polymerase chain reaction. L-Arg supplementation enhanced iNOS and NOx expression in the cells. A specific NOS inhibitor, NG-monomethyl-L-Arg enhanced the expression of ZEBRA and early BMRF1 protein EA-D in the cells. L-Arg supplementation also inhibited the spontaneous EBV reactivation in another BL cell line EB1 and a B lymphoblastoid cell line OB. These results indicated that L-Arg induces iNOS and generates NO, which inhibits EBV reactivation in EBV-positive cells.

A rearranged form of Epstein-Barr virus DNA is associated with idiopathic pulmonary fibrosis

An association between idiopathic pulmonary fibrosis (IPF) and productive Epstein-Barr virus (EBV) infection has been found previously. Productive EBV replication can be associated with a rearrangement in EBV genomes termed WZhet. We hypothesized that WZhet genomes might be present in patients with IPF. Thirty-nine patients with IPF, 26 lung transplant recipients, and 24 normal subjects were studied. When EBV DNA-positive lung tissue biopsies from IPF patients were analyzed, 11 of 18 (61%) were positive for WZhet. Buffy coat DNA analysis showed that 75-85% were EBV DNA-positive in both IPF and control groups. Buffy coat analysis for WZhet was positive in 16 of 27 (59%) IPF patients, compared with none of 32 lung transplant recipients and 1 of 24 (4%) normal blood donors (p < or = 0.001). There was thus a good correlation between the presence of WZhet in lung tissue and peripheral blood. However, there was no significant association between the presence of WZhet and immunosuppressive therapy. These data further confirm the association between active EBV infection and IPF and provide a potential marker in the peripheral blood for the tracking of EBV in this disease.

Protein kinase C-independent activation of the Epstein-Barr virus lytic cycle

The protein kinase C (PKC) pathway has been considered to be essential for activation of latent Epstein-Barr virus (EBV) into the lytic cycle. The phorbol ester tetradecanoyl phorbol acetate (TPA), a PKC agonist, is one of the best understood activators of EBV lytic replication. Zp, the promoter of the EBV immediate-early gene BZLF1, whose product, ZEBRA, drives the lytic cycle, contains several phorbol ester response elements. We investigated the role of the PKC pathway in lytic cycle activation in prototype cell lines that differed dramatically in their response to inducing agents. We determined whether PKC was involved in lytic cycle induction by histone deacetylase (HDAC) inhibitors. Consistent with prevailing views, B95-8 cells were activated into the lytic cycle by the phorbol ester TPA, via a PKC-dependent mechanism. B95-8 was not inducible by HDAC inhibitors such as n-butyrate and trichostatin A (TSA). Bisindolylmaleimide I, a selective PKC inhibitor, blocked lytic cycle activation in B95-8 cells in response to TPA. In marked contrast, in HH514-16 cells, the immediate-early promoters Zp and Rp were simultaneously activated by the HDAC inhibitors; TPA by itself failed to activate lytic gene expression. Inhibition of PKC activity by bisindolylmaleimide I did not block lytic cycle activation in HH514-16 cells by n-butyrate or TSA. In an extensive exploration of the mechanism underlying these different responses we found that the variable role of the PKC pathway in the two cell lines could not be accounted for by significant polymorphisms in the promoters of the immediate-early genes, by differences in the start sites of immediate-early gene transcription, or by differences in the nucleosomal organization of EBV DNA in the region of Zp or Rp. While B95-8 cells contained more total PKC activity than did HH514-16 cells in an in vitro assay, another EBV-transformed marmoset lymphoblastoid cell line, FF41, in which the lytic cycle was not inducible by TPA, contained comparably high levels of PKC activity. Moreover, two marmoset lymphoblastoid cells lines in which the lytic cycle could not be triggered by TPA maintained the same profile of EBV latency proteins as B95-8 cells. Thus, the profile of EBV latency proteins did not account for susceptibility to induction by PKC agonists. PKC activation is neither obligatory nor sufficient for the switch between latency and lytic cycle gene expression of EBV in many cell backgrounds. Lytic cycle induction by HDAC inhibitors proceeds by a PKC-independent mechanism.

Infectious delivery of 120-kilobase genomic DNA by an epstein-barr virus amplicon vector

It has been shown in a wide variety of contexts that persistent gene expression can best be obtained by using the genomic locus of a transgene. However, the size of most genomic loci precludes their use in current viral gene therapy vectors. Large transgene capacity and extrachromosomal persistence make Epstein-Barr virus (EBV) a promising vector to deliver genomic transgenes for gene therapy. We constructed an EBV amplicon vector that contains the EBV lytic origin of replication, the terminal repeats for viral packaging, and the EBV latent origin of replication for episomal persistence. This vector was able to deliver inserts of 60-123 kb to B-cell lines in culture in three steps. First, clonal packaging cells lines were generated that produce infectious amplicons at a titer of approximately 3-4x10(6) transducing units/ml after concentration. Second, we show infectious vector delivery to the Loukes B-cell line and three different EBV-immortalized lymphoblastoid cell lines. This infectious delivery system was 2000 times more efficient than transfection in B cells. Third, clonal cell lines from infection of Loukes contained persistent episomes of recircularized infectious vector. This first demonstration of infectious delivery of 120 kb of genomic DNA shows the potential of this high-capacity vector system.

Spontaneous reduction in Epstein-Barr virus (EBV) DNA copy number in EBV-infected epithelial cell lines

We found that spontaneous and 12-0-tetradecanoylphorbol-13-acetate-induced Epstein-Barr virus (EBV) reactivation occurred in short-term (ST)-cultured EBV-infected epithelial cell lines GT38 and GT39 after their establishment; however, it diminished in the long-term (LT)-cultured cells passaged for more than 2 years from ST-cultured cells. We hypothesized that the EBV reactivation may be related to the EBV DNA copy number in the cells. A higher level of EBV DNA content was detected in ST-cultured cells than in LT-cultured cells by Southern hybridization using an EBV DNA XhoI probe. Fluorescence in situ hybridization using EBV DNA BamHI W fragments showed that ST-cultured cells contained a higher EBV DNA copy number than that of LT-cultured cells. EBV DNA-negative cells were detected in small proportions in LT-cultured cells, but were undetected in ST-cultured cells. These results demonstrate that EBV genomes are not maintained stably in the cell lines, and some of them are lost in continuous passages of the cells. We discuss the mechanisms of reduction of EBV reactivation and EBV DNA in the cell lines.

Autostimulation of the Epstein-Barr virus BRLF1 promoter is mediated through consensus Sp1 and Sp3 binding sites

As an essential step in the lytic cascade, the Rta homologues of gammaherpesviruses all activate their own expression. Consistent with this biologic function, the Epstein-Barr virus (EBV) Rta protein powerfully stimulates the promoter of its own gene, Rp, in EBV-positive B cells in transient-transfection reporter-based assays. We analyzed the activity of RpCAT in response to Rta by deletional and site-directed mutagenesis. Two cognate Sp1 binding sites located at -279 and -45 relative to the transcriptional start site proved crucial for Rta-mediated activation. Previously described binding sites for the cellular transcription factor Zif268 and the viral transactivator ZEBRA were found to be dispensable for activation of RpCAT by Rta. Gel shift analysis, using extracts of B cells in latency or induced into the lytic cycle, identified Sp1 and Sp3 as the predominant cellular proteins bound to Rp near -45. During the lytic cycle, ZEBRA bound Rp near the Sp1/Sp3 site. The binding of Sp1 and Sp3 to Rp correlated with the reporter activities in the mutagenesis study, establishing a direct link between transcriptional activation of Rp by Rta and DNA binding by Sp1 and/or Sp3. The relative abundance or functional state of the cellular Sp1 and Sp3 transcription factors may be altered in response to stimuli that induce the BRLF1 promoter and thereby contribute to the activation of the viral lytic cycle.

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.

Epstein-Barr virus that lacks glycoprotein gN is impaired in assembly and infection

The Epstein-Barr virus (EBV) glycoproteins N and M (gN and gM) are encoded by the BLRF1 and BBRF3 genes. To examine the function of the EBV gN-gM complex, recombinant virus was constructed in which the BLRF1 gene was interrupted with a neomycin resistance cassette. Recombinant virus lacked not only gN but also detectable gM. A significant proportion of the recombinant virus capsids remained associated with condensed chromatin in the nucleus of virus-producing cells, and cytoplasmic vesicles containing enveloped virus were scarce. Virus egress was impaired, and sedimentation analysis revealed that the majority of the virus that was released lacked a complete envelope. The small amount of virus that could bind to cells was also impaired in infectivity at a step following fusion. These data are consistent with the hypothesis that the predicted 78-amino-acid cytoplasmic tail of gM, which is highly charged and rich in prolines, interacts with the virion tegument. It is proposed that this interaction is important both for association of capsids with cell membrane to assemble and release enveloped particles and for dissociation of the capsid from the membrane of the newly infected cell on its way to the cell nucleus. The phenotype of EBV lacking the gN-gM complex is more striking than that of most alphaherpesviruses lacking the same complex but resembles in many respects the phenotype of pseudorabies virus lacking glycoproteins gM, gE, and gI. Since EBV does not encode homologs for gE and gI, this suggests that functions that may have some redundancy in alphaherpesviruses have been concentrated in fewer proteins in EBV.

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

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

Tyrosines 60, 64, and 101 of Epstein-Barr virus LMP2A are not essential for blocking B cell signal transduction

Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) is expressed on the membrane of B-lymphocytes and blocks B cell receptor (BCR) signaling in EBV-transformed B-lymphocytes in vitro. The LMP2A amino-terminal domain, which is essential for the LMP2A-mediated block of B cell signal transduction, contains eight tyrosine residues. Three of these tyrosine residues (Y74, Y85, and Y112) have been demonstrated to be essential for the LMP2A-mediated block on protein tyrosine phosphorylation, calcium mobilization, and induction of BZLF1 expression after BCR activation. To investigate the importance of tyrosines at positions 60, 64, and 101 on B cell signaling, EBV recombinants were constructed containing a tyrosine-to-phenylalanine point mutation at amino acid 60, 64, or 101 within LMP2A. Tyrosine phosphorylation, calcium mobilization, and induction of BZLF1 expression were not affected by any of the tyrosine point mutations after BCR activation. In addition, constitutive phosphorylation of LMP2A was unaffected by any of the tyrosine point mutations. These data indicate that tyrosines 60, 64, and 101 are not essential for the LMP2A-mediated block of B cell signal transduction in transformed cell lines.

Expression of mucin (MUC-1) from a mini-Epstein-Barr virus in immortalized B-cells to generate tumor antigen specific cytotoxic T cells

BACKGROUND

EBV immortalized B-cells can be used as antigen presenting cells (APC) to stimulate specific T-cell responses. Mini-Epstein-Barr virus (mini-EBV) plasmids contain all functional elements of Epstein-Barr virus (EBV) necessary to immortalize B-cells in vitro. These immortalized B-cells are incapable of releasing infectious virus in contrast to cells immortalized by wildtype EBV. In addition, mini-EBVs can be modified in E. coli to alter their genetic composition or adopt new genes.

METHODS

We constructed a mini-EBV plasmid carrying an expression cassette for the human tumor antigen mucin encoded by the gene MUC-1. Primary human B-cells were infected with the MUC-1 carrying mini-EBV plasmid packaged into an EBV coat and immortalized B-cell clones were expanded in vitro. These B-cells were analyzed by FACS analyses for the expression of mucin and co-stimulatory molecules and were subsequently used as antigen presenting cells (APC) to stimulate peripheral blood mononuclear cells from healthy donors.

RESULTS

Several B-cell lines were established that were shown to be free of helper virus or wildtype EBV. These B-cells expressed the relevant tumor-specific epitopes of mucin and the co-stimulatory ligands B7.1 and B7.2 necessary for efficient T-cell activation. Using the mucin expressing B-cells as antigen presenting cells (APC) mucin-epitope specific cytotoxic T-cells were established.

CONCLUSIONS

Virus-free B-cell lines expressing tumor-associated epitopes such as mucin or other antigens of interest provide an unlimited and safe source of APC to generate antigen specific T-cells which could be used for clinical trials in adoptive immune therapy or cancer vaccines.

CD21-Dependent infection of an epithelial cell line, 293, by Epstein-Barr virus

Epstein-Barr virus (EBV) is invariably present in undifferentiated nasopharyngeal carcinomas, is found sporadically in other carcinomas, and replicates in the differentiated layer of the tongue epithelium in lesions of oral hairy leukoplakia. However, it is not clear how frequently or by what mechanism EBV infects epithelial cells normally. Here, we report that a human epithelial cell line, 293, can be stably infected by EBV that has been genetically marked with a selectable gene. We show that 293 cells express a relatively low level of CD21, that binding of fluorescein-labeled EBV to 293 cells can be detected, and that both the binding of virus to cells and infection can be blocked with antibodies specific for CD21. Two proteins known to form complexes with CD21 on the surface of lymphoid cells, CD35 and CD19, could not be detected at the surface of 293 cells. All infected clones of 293 cells exhibited tight latency with a pattern of gene expression similar to that of type II latency, but productive EBV replication and release of infectious virus could be induced inefficiently by forced expression of the lytic transactivators, R and Z. Low levels of mRNA specific for the transforming membrane protein of EBV, LMP-1, as well as for LMP-2, were detected; however, LMP-1 protein was either undetectable or near the limit of detection at less than 5% of the level typical of EBV-transformed B cells. A slight increase in expression of the receptor for epidermal growth factor, which can be induced in epithelial cells by LMP-1, was detected at the cell surface with two EBV-infected 293 cell clones. These results show that low levels of surface CD21 can support infection of an epithelial cell line by EBV. The results also raise the possibility that in a normal infection of epithelial cells by EBV, the LMP-1 protein is not expressed at levels that are high enough to be oncogenic and that there might be differences in the cells of EBV-associated epithelial cancers that have arisen to allow for elevated expression of LMP-1.

Persistent Hz-1 virus infection in insect cells: evidence for insertion of viral DNA into host chromosomes and viral infection in a latent status

Persistent/latent viral infections of insect cells are a prominent though poorly understood phenomenon. In this study, the long-term association between the Hz-1 virus and insect host cells, conventionally referred to as persistent viral infection, is described. With the aid of a newly developed fluorescent cell-labeling system, we found that productive viral replication occurs by spontaneous viral reactivation in fewer than 0.2% of persistently infected cell lines over a 5-day period. Once viral reactivation takes place, the host cell dies. The persistently infected cells contain various amounts of viral DNA, and, in an extreme case, up to 16% of the total DNA isolated from infected cells could be of viral origin. Both pulsed-field gel electrophoresis and in situ hybridization experiments showed that some of these viral DNA molecules are inserted into the host chromosomes but that the rest of viral DNA copies are free from host chromosomes. Thus, Hz-1 virus is the first nonretroviral insect virus known to insert its genome into the host chromosome during the infection process. These data also suggest that the previously described persistent infection of Hz-1 virus in insect cells should be more accurately referred to as latent viral infection.

Tyrosine 112 of latent membrane protein 2A is essential for protein tyrosine kinase loading and regulation of Epstein-Barr virus latency

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) is expressed on the plasma membrane of B lymphocytes latently infected with EBV and blocks B-cell receptor (BCR) signal transduction in EBV-immortalized B cells in vitro. The LMP2A amino-terminal domain that is essential for the LMP2A-mediated block on BCR signal transduction contains eight tyrosine residues. Association of Syk protein tyrosine kinase (PTK) with LMP2A occurs at the two tyrosines of the LMP2A immunoreceptor tyrosine-based activation motif, and it is hypothesized that Lyn PTK associates with the YEEA amino acid motif at LMP2A tyrosine 112 (Y112). To examine the specific association of Lyn PTK to LMP2A, a panel of LMP2A cDNA expression vectors containing LMP2A mutations were transfected into an EBV-negative B-cell line and analyzed for Lyn and LMP2A coimmunoprecipitation. Lyn associates with wild-type LMP2A and other LMP2A mutant constructs, but Lyn association is lost in the LMP2A construct containing a tyrosine (Y)-to-phenylalanine (F) mutation at LMP2A residue Y112 (LMP2AY112F). Next, the LMP2AY112F mutation was recombined into the EBV genome to generate stable lymphoblastoid cell lines (LCLs) transformed with the LMP2AY112F mutant virus. Analysis of BCR-mediated signal transduction in the LMP2AY112F LCLs revealed loss of the LMP2A-mediated block in BCR signal transduction. In addition, LMP2A was not tyrosine phosphorylated in LMP2AY112F LCLs. Together these data indicate the importance of the LMP2A Y112 residue in the ability of LMP2A to block BCR-mediated signal transduction and place the role of this residue and its interaction with Lyn PTK as essential to LMP2A phosphorylation, PTK loading, and down-modulation of PTKs involved in BCR-mediated signal transduction.

Epstein-Barr virus recombinant lacking expression of glycoprotein gp150 infects B cells normally but is enhanced for infection of epithelial cells

Glycoprotein gp150 is a highly glycosylated protein encoded by the BDLF3 open reading frame of Epstein-Barr virus (EBV). It does not have a homolog in the alpha- and betaherpesviruses, and its function is not known. To determine whether the protein is essential for replication of EBV in vitro, a recombinant virus which lacked its expression was made. The recombinant virus had no defects in assembly, egress, binding, or infectivity for B cells or epithelial cells. Infection of epithelial cells was, however, enhanced. The glycoprotein was sensitive to digestion with a glycoprotease that digests sialomucins, but no adhesion to cells that express selectins that bind to sialomucin ligands could be detected.

Epstein-Barr virus-infected marmoset cells transfected with c-myc do not form lymphomas in mice with severe combined immunodeficiency

Epstein-Barr virus (EBV) has been associated with several malignant processes in man, most notably Burkitt lymphoma in previously healthy individuals and lesions resembling large cell non-Hodgkin lymphomas in organ transplant recipients. Mice with the severe combined immunodeficiency phenotype (SCID mice) are exquisitely susceptible to the development of EBV-associated lymphoproliferative lesions following the intraperitoneal (ip) inoculation of EBV-infected human lymphocytes. Recently, we reported that EBV-infected marmoset lymphocytes do not form lymphomas in SCID mice following ip injection, while human lymphocytes infected with the same EBV strains do. On the assumption that the EBV-infected marmoset cells were lacking a factor necessary for tumor formation, we transfected a plasmid containing c-myc into EBV-infected marmoset cells (B95-8, FF41, and W91 cells). Despite expression of the c-myc protein as determined by immunoblot and flow cytometry when probed with a monoclonal antibody, no increase over baseline lesion development was seen in SCID mice inoculated with 5 x 10(6) c-myc-expressing marmoset lymphoblastoid cells. Thus, cells that express c-myc and harbor EBV are not sufficient to form lymphomas in certain immunocompromised hosts.

Epstein-Barr virus uses different complexes of glycoproteins gH and gL to infect B lymphocytes and epithelial cells

The Epstein-Barr virus (EBV) gH-gL complex includes a third glycoprotein, gp42. gp42 binds to HLA class II on the surfaces of B lymphocytes, and this interaction is essential for infection of the B cell. We report here that, in contrast, gp42 is dispensable for infection of epithelial cell line SVKCR2. A soluble form of gp42, gp42.Fc, can, however, inhibit infection of both cell types. Soluble gp42 can interact with EBV gH and gL and can rescue the ability of virus lacking gp42 to transform B cells, suggesting that a gH-gL-gp42.Fc complex can be formed by extrinsic addition of the soluble protein. Truncated forms of gp42.Fc that retain the ability to bind HLA class II but that cannot interact with gH and gL still inhibit B-cell infection by wild-type virus but cannot inhibit infection of SVKCR2 cells or rescue the ability of recombinant gp42-negative virus to transform B cells. An analysis of wild-type virions indicates the presence of more gH and gL than gp42. To explain these results, we describe a model in which wild-type EBV virions are proposed to contain two types of gH-gL complexes, one that includes gp42 and one that does not. We further propose that these two forms of the complex have mutually exclusive abilities to mediate the infection of B cells and epithelial cells. Conversion of one to the other concurrently alters the ability of virus to infect each cell type. The model also suggests that epithelial cells may express a molecule that serves the same cofactor function for this cell type as HLA class II does for B cells and that the gH-gL complex interacts directly with this putative epithelial cofactor.

Epstein-Barr virus lacking glycoprotein gp42 can bind to B cells but is not able to infect

The Epstein-Barr virus gH-gL complex includes a third glycoprotein, gp42, which is the product of the BZLF2 open reading frame (ORF). gp42 has been implicated as critical to infection of the B lymphocyte by virtue of its interaction with HLA class II on the B-cell surface. A neutralizing antibody that reacts with gp42 inhibits virus-cell fusion and blocks binding of gp42 to HLA class II; antibody to HLA class II can inhibit infection, and B cells that lack HLA class II can only be infected if HLA class II expression is restored. To confirm whether gp42 is an essential component of the virion, we derived a recombinant virus with a selectable marker inserted into the BZLF2 ORF to interrupt expression of the protein. A complex of gH and gL was expressed by the recombinant virus in the absence of gp42. Recombinant virus egressed from the cell normally and could bind to receptor-positive cells. It had, however, lost the ability to infect or transform B lymphocytes. Treatment with polyethylene glycol restored the infectivity of recombinant virus, confirming that gp42 is essential for penetration of the B-cell membrane.

The immunoreceptor tyrosine-based activation motif of Epstein-Barr virus LMP2A is essential for blocking BCR-mediated signal transduction

The Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) blocks B-cell receptor (BCR) signal transduction in EBV-immortalized B lymphocytes in vitro. The cytoplasmic amino-terminal domain of LMP2A contains an immunoreceptor tyrosine activation motif (ITAM). ITAMs consist of paired tyrosine and leucine residues and play a central role in signal transduction of the BCR and the T-cell receptor (TCR). To investigate the importance of the LMP2A ITAM, two EBV recombinants were constructed, each containing a tyrosine-to-phenylalanine point mutation at amino acid 74 or 85 within the ITAM of LMP2A. Tyrosine phosphorylation, calcium mobilization, and induction of BZLF1 expression were no longer blocked in the LMP2A ITAM mutant LCLs following BCR cross-linking. In addition, the Syk protein tyrosine kinase (PTK) was unable to bind LMP2A in unstimulated LCLs infected with either of the LMP2A ITAM mutants. Analysis of Syk phosphorylation before and after BCR cross-linking in the LMP2A mutant ITAM LCLs compared with wild-type EBV LCLs indicates a specific role of the LMP2A ITAM on the LMP2A-mediated negative effect on the Syk PTK. These data indicate the importance of the LMP2A ITAM motif in the LMP2A-mediated block on BCR signal transduction and position the role of the Syk PTK as being central to the function of LMP2A.

An animal model for acute and persistent Epstein-Barr virus infection

Epstein-Barr virus (EBV) is a human lymphocryptovirus that causes infectious mononucleosis, persists asymptomatically for life in nearly all adults, and is associated with the development of B cell lymphomas and nasopharyngeal carcinomas. A highly similar rhesus lymphocryptovirus naturally endemic in rhesus monkeys was used to orally infect naïve animals from a pathogen-free colony. This animal model reproduced key aspects of human EBV infection, including oral transmission, atypical lymphocytosis, lymphadenopathy, activation of CD23(+) peripheral blood B cells, sustained serologic responses to lytic and latent EBV antigens, latent infection in the peripheral blood, and virus persistence in oropharyngeal secretions. This system may be useful for studying the pathogenesis, prevention, and treatment of EBV infection and associated oncogenesis.