The Epstein-Barr virus nuclear antigen 2 (EBNA2), a protein required for B lymphocyte immortalization, induces the synthesis of type I interferon in Burkitt's lymphoma cell lines

Research landmarks on the 60th anniversary of Epstein-Barr virus

Epstein-Barr virus (EBV), the first human oncovirus discovered in 1964, has become a focal point in virology, immunology, and oncology because of its unique biological characteristics and significant role in human diseases. As we commemorate the 60th anniversary of EBV's discovery, it is an opportune moment to reflect on the major advancements in our understanding of this complex virus. In this review, we highlight key milestones in EBV research, including its virion structure and life cycle, interactions with the host immune system, association with EBV-associated diseases, and targeted intervention strategies.

Building a Bridge Between the Mechanism of EBV Reactivation and the Treatment of EBV-Associated Cancers

Epstein-Barr virus (EBV) infection is closely associated with the development of various tumors such as lymphomas and epithelial cancers. EBV has a discrete life cycle with latency and lytic phases. In recent years, significant progress has been made in the understanding of the mechanism underlying the transition of EBV from latency to lytic replication. Multiple new lytic activation factors have been emerged and promoted our understanding of this field. In addition, we have comprehensively presented the existing therapeutic strategies and their relationship to the mechanism underlying the transition of EBV from latency to lytic replication in this review, such as lytic induction therapy and drugs to prevent EBV from entering the lytic phase fully utilize the EBV reactivation mechanisms. This year marks the 60th anniversary of the discovery of EBV, and building a bridge between the mechanism of EBV reactivation and the treatment may help us to design new approaches for treating EBV-associated diseases.

Epstein-Barr virus nuclear antigen EBNA3A modulates IRF3-dependent IFNβ expression

Epstein-Barr virus (EBV), the causative agent of infectious mononucleosis, persistently infects over 90% of the human adult population and is associated with several human cancers. To establish life-long infection, EBV tampers with the induction of type I interferon (IFN I)-dependent antiviral immunity in the host. How various EBV genes help orchestrate this crucial strategy is incompletely defined. Here, we reveal a mechanism by which the EBV nuclear antigen 3A (EBNA3A) may inhibit IFNβ induction. Using proximity biotinylation we identify the histone acetyltransferase P300, a member of the IFNβ transcriptional complex, as a binding partner of EBNA3A. We further show that EBNA3A also interacts with the activated IFN-inducing transcription factor interferon regulatory factor 3 that collaborates with P300 in the nucleus. Both events are mediated by the N-terminal domain of EBNA3A. We propose that EBNA3A limits the binding of interferon regulatory factor 3 to the IFNβ promoter, thereby hampering downstream IFN I signaling. Collectively, our findings suggest a new mechanism of immune evasion by EBV, affected by its latency gene EBNA3A.

Strategies of Epstein-Barr virus to evade innate antiviral immunity of its human host

Epstein-Barr virus (EBV) is a double-stranded DNA virus of the Herpesviridae family. This virus preferentially infects human primary B cells and persists in the human B cell compartment for a lifetime. Latent EBV infection can lead to the development of different types of lymphomas as well as carcinomas such as nasopharyngeal and gastric carcinoma in immunocompetent and immunocompromised patients. The early phase of viral infection is crucial for EBV to establish latency, but different viral components are sensed by cellular sensors called pattern recognition receptors (PRRs) as the first line of host defense. The efficacy of innate immunity, in particular the interferon-mediated response, is critical to control viral infection initially and to trigger a broad spectrum of specific adaptive immune responses against EBV later. Despite these restrictions, the virus has developed various strategies to evade the immune reaction of its host and to establish its lifelong latency. In its different phases of infection, EBV expresses up to 44 different viral miRNAs. Some act as viral immunoevasins because they have been shown to counteract innate as well as adaptive immune responses. Similarly, certain virally encoded proteins also control antiviral immunity. In this review, we discuss how the virus governs innate immune responses of its host and exploits them to its advantage.

Epstein-Barr Virus Early Protein BFRF1 Suppresses IFN-β Activity by Inhibiting the Activation of IRF3

Epstein-Barr virus (EBV) is the causative agent of infectious mononucleosis that is closely associated with several human malignant diseases, while type I interferon (IFN-I) plays an important role against EBV infection. As we all know, EBV can encode some proteins to inhibit the production of IFN-I, but it’s not clear whether other proteins also take part in this progress. EBV early lytic protein BFRF1 is shown to be involved in viral maturation, however, whether BFRF1 participates in the host innate immune response is still not well known. In this study, we found BFRF1 could down-regulate sendai virus-induced IFN-β promoter activity and mRNA expression of IFN-β and ISG54 during BFRF1 plasmid transfection and EBV lytic infection, but BFRF1 could not affect the promoter activity of NF-κB or IRF7. Specifically, BFRF1 could co-localize and interact with IKKi. Although BFRF1 did not interfere the interaction between IKKi and IRF3, it could block the kinase activity of IKKi, which finally inhibited the phosphorylation, dimerization, and nuclear translocation of IRF3. Taken together, BFRF1 may play a critical role in disrupting the host innate immunity by suppressing IFN-β activity during EBV lytic cycle.

Epstein-Barr Virus (EBV) Tegument Protein BGLF2 Suppresses Type I Interferon Signaling To Promote EBV Reactivation

Interferon alpha (IFN-α) and IFN-β are type I IFNs that are induced by virus infection and are important in the host's innate antiviral response. EBV infection activates multiple cell signaling pathways, resulting in the production of type I IFN which inhibits EBV infection and virus-induced B-cell transformation. We reported previously that EBV tegument protein BGLF2 activates p38 and enhances EBV reactivation. To further understand the role of BGLF2 in EBV infection, we used mass spectrometry to identify cellular proteins that interact with BGLF2. We found that BGLF2 binds to Tyk2 and confirmed this interaction by coimmunoprecipitation. BGLF2 blocked type I IFN-induced Tyk2, STAT1, and STAT3 phosphorylation and the expression of IFN-stimulated genes (ISGs) IRF1, IRF7, and MxA. In contrast, BGLF2 did not inhibit STAT1 phosphorylation induced by IFN-γ. Deletion of the carboxyl-terminal 66 amino acids of BGLF2 reduced the ability of the protein to repress type I IFN signaling. Treatment of gastric carcinoma and Raji cells with IFN-α blocked BZLF1 expression and EBV reactivation; however, expression of BGLF2 reduced the ability of IFN-α to inhibit BZLF1 expression and enhanced EBV reactivation. In summary, EBV BGLF2 interacts with Tyk2, inhibiting Tyk2, STAT1, and STAT3 phosphorylation and impairs type I IFN signaling; BGLF2 also counteracts the ability of IFN-α to suppress EBV reactivation.IMPORTANCE Type I interferons are important for controlling virus infection. We have found that the Epstein-Barr virus (EBV) BGLF2 tegument protein binds to a protein in the type I interferon signaling pathway Tyk2 and inhibits the expression of genes induced by type I interferons. Treatment of EBV-infected cells with type I interferon inhibits reactivation of the virus, while expression of EBV BGLF2 reduces the ability of type I interferon to inhibit virus reactivation. Thus, a tegument protein delivered to cells during virus infection inhibits the host's antiviral response and promotes virus reactivation of latently infected cells. Therefore, EBV BGLF2 might protect virus-infected cells from the type I interferon response in cells undergoing lytic virus replication.

Epstein-Barr Virus and Innate Immunity: Friends or Foes?

Epstein–Barr virus (EBV) successfully persists in the vast majority of adults but causes lymphoid and epithelial malignancies in a small fraction of latently infected individuals. Innate immunity is the first-line antiviral defense, which EBV has to evade in favor of its own replication and infection. EBV uses multiple strategies to perturb innate immune signaling pathways activated by Toll-like, RIG-I-like, NOD-like, and AIM2-like receptors as well as cyclic GMP-AMP synthase. EBV also counteracts interferon production and signaling, including TBK1-IRF3 and JAK-STAT pathways. However, activation of innate immunity also triggers pro-inflammatory response and proteolytic cleavage of caspases, both of which exhibit proviral activity under some circumstances. Pathogenic inflammation also contributes to EBV oncogenesis. EBV activates NFκB signaling and induces pro-inflammatory cytokines. Through differential modulation of the proviral and antiviral roles of caspases and other host factors at different stages of infection, EBV usurps cellular programs for death and inflammation to its own benefits. The outcome of EBV infection is governed by a delicate interplay between innate immunity and EBV. A better understanding of this interplay will instruct prevention and intervention of EBV-associated cancers.

Immune Evasion by Epstein-Barr Virus

Epstein-Bar virus (EBV) is widespread within the human population with over 90% of adults being infected. In response to primary EBV infection, the host mounts an antiviral immune response comprising both innate and adaptive effector functions. Although the immune system can control EBV infection to a large extent, the virus is not cleared. Instead, EBV establishes a latent infection in B lymphocytes characterized by limited viral gene expression. For the production of new viral progeny, EBV reactivates from these latently infected cells. During the productive phase of infection, a repertoire of over 80 EBV gene products is expressed, presenting a vast number of viral antigens to the primed immune system. In particular the EBV-specific CD4+ and CD8+ memory T lymphocytes can respond within hours, potentially destroying the virus-producing cells before viral replication is completed and viral particles have been released. Preceding the adaptive immune response, potent innate immune mechanisms provide a first line of defense during primary and recurrent infections. In spite of this broad range of antiviral immune effector mechanisms, EBV persists for life and continues to replicate. Studies performed over the past decades have revealed a wide array of viral gene products interfering with both innate and adaptive immunity. These include EBV-encoded proteins as well as small noncoding RNAs with immune-evasive properties. The current review presents an overview of the evasion strategies that are employed by EBV to facilitate immune escape during latency and productive infection. These evasion mechanisms may also compromise the elimination of EBV-transformed cells, and thus contribute to malignancies associated with EBV infection.

Differential regulation of miR-21 and miR-146a by Epstein-Barr virus-encoded EBNA2

The discovery of microRNA (miR) represents a novel paradigm in RNA-based regulation of gene expression and their dysregulation has become a hallmark of many a tumor. In virally associated cancers, the host–pathogen interaction could involve alteration in miR expression. Epstein–Barr virus (EBV)-encoded EBNA2 is indispensable for the capacity of the virus to transform B cells in vitro. Here, we studied how it affects cellular miRs. Extensive miR profiling of the virus-infected and EBNA2-transfected B lymphoma cells revealed that oncomiR miR-21 is positively regulated by this viral protein. Conversely, Burkitt's lymphoma (BL) cell lines infected with EBNA2 lacking P3HR1 strain did not show any increase in miR-21. EBNA2 increased phosphorylation of AKT and this was directly correlated with increased miR-21. In contrast, miR-146a was downregulated by EBNA2 in B lymphoma cells. Low miR-146a expression correlates with an elevated level of IRAK1 and type I interferon in EBNA2 transfectants. Taken together, the present data suggest that EBNA2 might contribute to EBV-induced B-cell transformation by altering miR expression and in particular by increasing oncomiR-like miR-21 and by affecting the antiviral responses of the innate immune system through downregulation of its key regulator miR-146a.

Innate immune modulation in EBV infection

Epstein-Barr Virus (EBV) belongs to the gammaherpesvirus family, members of which are oncogenic. Compared with other closely related herpesviruses, EBV has developed much more elaborate and sophisticated strategies for subverting host immune system, which may account for its high prevalence in immune competent hosts. Thus, study of EBV-specific immune dysregulation is important for understanding EBV latency and oncogenesis, and will identify potential molecular targets for immunotherapeutic interventions. Here I summarize the recent findings of individual EBV products in regulating host immune responses, with emphasis on the innate immune modulation.

EBV Zta protein induces the expression of interleukin-13, promoting the proliferation of EBV-infected B cells and lymphoblastoid cell lines

Epstein-Barr virus (EBV) infection can modify the cytokine expression profiles of host cells and determine the fate of those cells. Of note, expression of interleukin-13 (IL-13) may be detected in EBV-associated Hodgkin lymphoma and the natural killer (NK) cells of chronic active EBV-infected patients, but its biologic role and regulatory mechanisms are not understood. Using cytokine antibody arrays, we found that IL-13 production is induced in B cells early during EBV infection. Furthermore, the EBV lytic protein, Zta (also known as the BZLF-1 product), which is a transcriptional activator, was found to induce IL-13 expression following transfection. Mechanistically, induction of IL-13 expression by Zta is mediated directly through its binding to the IL-13 promoter, via a consensus AP-1 binding site. Blockade of IL-13 by antibody neutralization showed that IL-13 is required at an early stage of EBV-induced proliferation and for long-term maintenance of the growth of EBV immortalized lymphoblastoid cell lines (LCLs). Thus, Zta-induced IL-13 production facilitates B-cell proliferation and may contribute to the pathogenesis of EBV-associated lymphoproliferative disorders, such as posttransplantation lymphoproliferative disease (PTLD) and Hodgkin lymphoma.

Manipulation of the toll-like receptor 7 signaling pathway by Epstein-Barr virus

Epstein-Barr virus (EBV) infection of primary B cells causes B-cell activation and proliferation. Activation of B cells requires binding of antigen to the B-cell receptor and a survival signal from ligand-bound CD40, signals that are provided by the EBV LMP1 and LMP2A latency proteins. Recently, Toll-like receptor (TLR) signaling has been reported to provide a third B-cell activation stimulus. The interaction between the EBV and TLR pathways was therefore investigated. Both UV-inactivated and untreated EBV upregulated the expression of TLR7 and downregulated the expression of TLR9 in naive B cells. UV-inactivated virus transiently stimulated naive B-cell proliferation in the presence of the TLR7 ligand R837, while addition of the TLR7 antagonist IRS 661 impaired cell growth induced by untreated EBV. Interferon regulatory factor 5 (IRF-5) is a downstream mediator of TLR7 signaling. IRF-5 was induced following EBV infection, and IRF-5 was expressed in B-cell lines with type III latency. Expression of IRF-5 in this setting is surprising since IRF-5 has tumor suppressor and antiviral properties. B-cell proliferation assays provided evidence that EBV modulates TLR7 signaling responses. Examination of IRF-5 transcripts identified a novel splice variant, V12, that was induced by EBV infection, was constitutively nuclear, and acted as a dominant negative form in IRF-5 reporter assays. IRF-4 negatively regulates IRF-5 activation, and IRF-4 was also present in type III latently infected cells. EBV therefore initially uses TLR7 signaling to enhance B-cell proliferation and subsequently modifies the pathway to regulate IRF-5 activity.

The Epstein-Barr virus-encoded LMP-1 oncoprotein negatively affects Tyk2 phosphorylation and interferon signaling in human B cells

Epstein-Barr virus (EBV) establishes a persistent infection in the human host and is associated with a variety of human cancers. Persistent infection results from a balance between the host immune response and viral immune evasion mechanisms. EBV infection is controlled initially by the innate immune response and later by T-cell-mediated adaptive immunity. EBV has evolved mechanisms to evade the host immune response so that it can persist for the lifetime of the host. Latent membrane protein 1 (LMP-1) is the EBV oncoprotein essential for B-cell immortalization by EBV. We show here that LMP-1 interacts with Tyk2, a signaling intermediate in the alpha interferon (IFN-alpha) signaling pathway, via a previously uncharacterized LMP-1 signaling domain. LMP-1 prevents Tyk2 phosphorylation and inhibits IFN-alpha-stimulated STAT2 nuclear translocation and interferon-stimulated response element transcriptional activity. Long-term culture of EBV+ lymphoblastoid cells in IFN-alpha is associated with outgrowth of a population expressing elevated LMP-1 protein levels, suggesting that cells expressing higher levels of LMP-1 survive the antiproliferative selective pressure imposed by IFN-alpha. These results show that LMP-1 can protect EBV+ cells from the IFN-alpha-stimulated antiviral/antiproliferative response and suggest that chronic IFN-alpha treatment may encourage the outgrowth of cells expressing elevated, and therefore potentially oncogenic, LMP-1 levels in EBV+ individuals.

Induction and regulation of IFNs during viral infections

Interferons (IFN)s are involved in numerous immune interactions during viral infections and contribute to both induction and regulation of innate and adaptive antiviral mechanisms. IFNs play a pivotal rule in the outcome of a viral infection, as demonstrated by the impaired resistance against different viruses in mice deficient for the receptors IFNAR-2 and IFNGR. During viral infections, IFNs are involved in numerous immune interactions as inducers, regulators, and effectors of both innate and adaptive antiviral mechanisms. IFN-alpha/beta is produced rapidly when viral factors, such as envelope glycoproteins, CpG DNA, or dsRNA, interact with cellular pattern-recognition receptors (PRRs), such as mannose receptors, toll-like receptors (TLRs), and cytosolic receptors. These host-virus interactions signal downstream to activate transcription factors needed to achieve expression from IFN-alpha/beta genes. These include IFN regulatory factor-3 (IRF-3), IRF-5, IRF-7, c-Jun/ATF-2, and NF-kappaB. In contrast, IFN-gamma is induced by receptor-mediated stimulation or in response to early produced cytokines, including interleukin-2 (IL-12), IL-18, and IFN-alpha/beta, or by stimulation through T cell receptors (TCRs) or natural killer (NK) cell receptors. IFNs signal through transmembrane receptors, activating mainly Jak-Stat pathways but also other signal transduction pathways. Cytokine and TCR-induced IFN-gamma expression uses distinct signal transduction pathways involving such transcription factors as NFAT, Stats and NF-kappaB. This results in induction and activation of numerous intrinsic antiviral factors, such as RNA-activated protein kinase (PKR), the 2-5A system, Mx proteins, and several apoptotic pathways. In addition, IFNs modulate distinct aspects of both innate and adaptive immunity. Thus, IFN-alpha/beta and IFN-gamma affect activities of macrophages, NK cells, dendritic cells (DC), and T cells by enhancing antigen presentation, cell trafficking, and cell differentiation and expression profiles, ultimately resulting in enhanced antiviral effector functions. This review focuses on the latest findings regarding induction and regulation of IFNs, primarily during the early phase of an antiviral immune response. Both cellular and molecular aspects are discussed from the perspective of host-virus interactions.

Viral interactions with the Notch pathway

The Notch signaling pathway influences cell fate decisions, proliferation versus differentiation and cell survival. Viruses both utilize and manipulate the differentiation state of infected cells, promote or block cell cycling and employ a variety of mechanisms to evade innate cellular anti-viral responses and promote cell survival. In light of these commonalities, it is perhaps not surprising that several viruses have tapped into the Notch pathway to advance their own life cycles. This first became apparent from studies showing targeting of Epstein-Barr virus proteins to the nuclear effector of Notch signaling CSL (CBF1/RBPJk). More recently the Kaposi's sarcoma-associated herpesvirus RTA protein has been found to bind CSL. Notch pathway interactions have also been described for adenovirus SV40 and human papilloma virus. This review focuses on the herpesvirus protein interactions with the Notch pathway and the insights that these interactions have provided.

Epstein-barr virus-induced changes in B-lymphocyte gene expression

To elucidate the mechanisms by which Epstein-Barr virus (EBV) latency III gene expression transforms primary B lymphocytes to lymphoblastoid cell lines (LCLs), the associated alterations in cell gene expression were assessed by using 4,146 cellular cDNAs arrayed on nitrocellulose filters and real-time reverse transcription-PCR (RT-PCR). A total of 1,405 of the 4,146 cDNAs were detected using cDNA probes from poly(A)(+) RNA of IB4 LCLs, a non-EBV-infected Burkitt's lymphoma (BL) cell line, BL41, or EBV latency III-converted BL41 cells (BL41EBV). Thirty-eight RNAs were consistently twofold more abundant in the IB4 LCL and BL41EBV than in BL41 by microarray analysis. Ten of these are known to be EBV induced. A total of 23 of 28 newly identified EBV-induced genes were confirmed by real-time RT-PCR. In addition, nine newly identified genes and CD10 were EBV repressed. These EBV-regulated genes encode proteins involved in signal transduction, transcription, protein biosynthesis and degradation, and cell motility, shape, or adhesion. Seven of seven newly identified EBV-induced RNAs were more abundant in newly established LCLs than in resting B lymphocytes. Surveys of eight promoters of newly identified genes implicate NF-kappaB or PU.1 as potentially important mediators of EBV-induced effects through LMP1 or EBNA2, respectively. Thus, examination of the transcriptional effects of EBV infection can elucidate the molecular mechanisms by which EBV latency III alters B lymphocytes.

Alterations in NF-kappaB and RBP-Jkappa by arenavirus infection of macrophages in vitro and in vivo

Pichinde virus is an arenavirus that infects guinea pigs and serves as an animal model for human Lassa fever. An attenuated Pichinde virus variant (P2) and a virulent variant (P18) are being used to delineate pathogenic mechanisms that culminate in shock. In guinea pigs, the infection has been shown to begin in peritoneal macrophages following intraperitoneal inoculation and then spreads to the spleen and other reticuloendothelial organs. We show here that infection of the murine monocytic cell line P388D1 with either Pichinde virus variant resulted in the induction of inflammatory cytokines and effectors, including interleukin-6 and tumor necrosis factor alpha. Since these genes are regulated in part by the cellular transcription factors NF-kappaB and RBP-Jkappa, we compared the activities of NF-kappaB and RBP-Jkappa in P388D1 cells following infection with Pichinde virus. The attenuated P2 virus inhibited NF-kappaB activation and caused a shift in the size of the RBP-Jkappa complex. The virulent P18 virus showed less inhibition of NF-kappaB and failed to alter the size of the RBP-Jkappa complex. Peritoneal cells from P2-infected guinea pigs showed induction of NF-kappaB RelA/p50 heterodimer and p50/p50 homodimer and manifested an increase in the size of RBP-Jkappa. By contrast, P18 induced large amounts of the NF-kappaB p50/p50 dimer but failed to induce RelA/p50 or to cause an increase in the RBP-Jkappa size. Taken together, these changes suggest that the attenuated viral strain induces an "activation" of macrophages, while the virulent form of the virus does not.

Molecular pathways in virus-induced cytokine production

Virus infections induce a proinflammatory response including expression of cytokines and chemokines. The subsequent leukocyte recruitment and antiviral effector functions contribute to the first line of defense against viruses. The molecular virus-cell interactions initiating these events have been studied intensively, and it appears that viral surface glycoproteins, double-stranded RNA, and intracellular viral proteins all have the capacity to activate signal transduction pathways leading to the expression of cytokines and chemokines. The signaling pathways activated by viral infections include the major proinflammatory pathways, with the transcription factor NF-kappaB having received special attention. These transcription factors in turn promote the expression of specific inducible host proteins and participate in the expression of some viral genes. Here we review the current knowledge of virus-induced signal transduction by seven human pathogenic viruses and the most widely used experimental models for viral infections. The molecular mechanisms of virus-induced expression of cytokines and chemokines is also analyzed.

Differential expression of human major histocompatibility class I loci: HLA-A, -B, and -C

Expression of the human class I MHC loci, HLA-A, -B, and -C, was examined by reverse transcription and competitive PCR with locus-specific primers. This approach allows unambiguous identification of target sequences by analysis of the amplified products. JY and Pala lymphoblastoid B cells express more HLA-A than HLA-B mRNAs and little HLA-C mRNA. Raji Burkitt lymphoma and HeLa carcinoma cells express approximately equal amounts of HLA-A and HLA-C mRNAs but less HLA-B mRNA. Jar trophoblast cells express no HLA class I mRNAs. Surprisingly, K562 leukemia cells express significant amounts of HLA-C mRNA. However, K562 cells contain no detectable HLA-A or -B mRNAs, suggesting that these loci are regulated independently. Furthermore, cultured endothelial cells and smooth muscle cells express low, approximately equal amounts of HLA-A, -B, and -C mRNAs, whereas donor-matched, EBV transformed B cells express much more HLA-B mRNA, suggesting that cell type dependent regulation underlies differential locus expression. Finally, expression of HLA class I molecules on the cell surface correlates with total HLA mRNAs but not with mRNAs encoded by any one locus. Differential expression of these HLA class I loci may contribute to cell-type dependent immune reactions by preferentially presenting distinct peptides to T cells.

A role for SKIP in EBNA2 activation of CBF1-repressed promoters

EBNA2 is essential for Epstein-Barr virus (EBV) immortalization of B lymphocytes. EBNA2 functions as a transcriptional activator and targets responsive promoters through interaction with the cellular DNA binding protein CBF1. We have examined the mechanism whereby EBNA2 overcomes CBF1-mediated transcriptional repression. A yeast two-hybrid screen performed using CBF1 as the bait identified a protein, SKIP, which had not previously been recognized as a CBF1-associated protein. Protein-protein interaction assays demonstrated contacts between SKIP and the SMRT, CIR, Sin3A, and HDAC2 proteins of the CBF1 corepressor complex. Interestingly, EBNA2 also interacted with SKIP in glutathione S-transferase affinity and mammalian two-hybrid assays and colocalized with SKIP in immunofluorescence assays. Interaction with SKIP was not affected by mutation of EBNA2 conserved region 6, the CBF1 interaction region, but was abolished by mutation of conserved region 5. Mutation of conserved region 5 also severely impaired EBNA2 activation of a reporter containing CBF1 binding sites. Thus, interaction with both CBF1 and SKIP is necessary for efficient promoter activation by EBNA2. A model is presented in which EBNA2 competes with the SMRT-corepressor complex for contacts on SKIP and CBF1.