Epstein-Barr virus LMP2A enhances B-cell responses in vivo and in vitro

Regulation of B cell receptor signalling by Epstein-Barr virus nuclear antigens

The cancer-associated Epstein-Barr virus (EBV) latently infects and immortalises B lymphocytes. EBV latent membrane protein 2A and EBV-encoded microRNAs are known to manipulate B cell receptor signalling to control cell growth and survival and suppress lytic replication. Here, we show that the EBV transcription factors EBNA2, 3A, 3B and 3C bind to genomic sites around multiple B cell receptor (BCR) pathway genes, regulate their expression and affect BCR signalling. EBNA2 regulates the majority of BCR pathway genes associated with binding sites, where EBNA3 proteins regulate only 42% of targets predicted by binding. Both EBNA2 and 3 proteins predominantly repress BCR pathway gene expression and target some common genes. EBNA2 and at least one EBNA3 protein repress the central BCR components CD79A and CD79B and the downstream genes BLNK, CD22, CD72, NFATC1, PIK3CG and RASGRP3. Studying repression of CD79B, we show that EBNA2 decreases transcription by disrupting binding of Early B cell Factor-1 to the CD79B promoter. Consistent with repression of BCR signalling, we demonstrate that EBNA2 and EBNA3 proteins suppress the basal or active BCR signalling that culminates in NFAT activation. Additionally, we show that EBNA2, EBNA3A and EBNA3C expression can result in reductions in the active serine 473 phosphorylated form of Akt in certain cell contexts, consistent with transcriptional repression of the PI3K-Akt BCR signalling arm. Overall, we identify EBNA2, EBNA3A and EBNA3C-mediated transcription control of BCR signalling as an additional strategy through which EBV may control the growth and survival of infected B cells and maintain viral latency.

The impact of Epstein-Barr virus latent membrane protein 2A on the production of B cell activating factor of the tumor necrosis factor family (BAFF), APRIL and their receptors

Introduction

Epstein‐Barr virus (EBV) establishes a lifelong infection in human B cells where the virus consistently expresses Latent Membrane Protein 2A (LMP2A) to promote B cell survival. A prior study indicates that LMP2A may increase the production of the pro‐survival factor, B cell Activating Factor of the tumor necrosis factor family (BAFF), which could also indirectly increase B cell survival. The current study sought to extend these findings and determine if LMP2A increased BAFF production and/or the responsiveness of LMP2A‐expressing cells to this cytokine.

Methods

Four independently derived LMP2A‐negative and ‐positive B cell lymphoma cell lines were analyzed for BAFF and APRIL levels by both ELISA and Western Blot analysis. Additionally, flow cytometric analysis measured any LMP2A‐dependent changes in the receptors for BAFF and APRIL (BAFF‐R, transmembrane activator and calcium‐modulator and cyclophilin ligand interactor [TACI], B cell maturation antigen [BCMA]) in both LMP2A‐negative and ‐positive B cell lymphoma cell lines.

Results

In contrast to previous reports, our data indicate that LMP2A does not increase the expression of BAFF or APRIL by Western blot analysis or ELISA. Additionally, flow cytometric analysis indicates that LMP2A does not influence the expression of the receptors for BAFF and APRIL: TACI, BAFF‐R, and BCMA.

Conclusion

Therefore, these data suggest that while EBV utilizes other latency proteins to regulate BAFF production, EBV does not appear to use LMP2A to enhance BAFF‐or APRIL‐dependent survival to promote EBV latency.

Functional interplay of Epstein-Barr virus oncoproteins in a mouse model of B cell lymphomagenesis

Epstein-Barr virus (EBV) is a B cell transforming virus that causes B cell malignancies under conditions of immune suppression. EBV orchestrates B cell transformation through its latent membrane proteins (LMPs) and Epstein-Barr nuclear antigens (EBNAs). We here identify secondary mutations in mouse B cell lymphomas induced by LMP1, to predict and identify key functions of other EBV genes during transformation. We find aberrant activation of early B cell factor 1 (EBF1) to promote transformation of LMP1-expressing B cells by inhibiting their differentiation to plasma cells. EBV EBNA3A phenocopies EBF1 activities in LMP1-expressing B cells, promoting transformation while inhibiting differentiation. In cells expressing LMP1 together with LMP2A, EBNA3A only promotes lymphomagenesis when the EBNA2 target Myc is also overexpressed. Collectively, our data support a model where proproliferative activities of LMP1, LMP2A, and EBNA2 in combination with EBNA3A-mediated inhibition of terminal plasma cell differentiation critically control EBV-mediated B cell lymphomagenesis.

Co-Expression of the Epstein-Barr Virus-Encoded Latent Membrane Proteins and the Pathogenesis of Classic Hodgkin Lymphoma

The Epstein-Barr virus (EBV) is present in the tumour cells of a subset of patients with classic Hodgkin lymphoma (cHL), yet the contribution of the virus to the pathogenesis of these tumours remains only poorly understood. The EBV genome in virus-associated cHL expresses a limited subset of genes, restricted to the non-coding Epstein-Barr virus-encoded RNAs (EBERs) and viral miRNA, as well as only three virus proteins; the Epstein-Barr virus nuclear antigen-1 (EBNA1), and the two latent membrane proteins, known as LMP1 and LMP2, the latter of which has two isoforms, LMP2A and LMP2B. LMP1 and LMP2A are of particular interest because they are co-expressed in tumour cells and can activate cellular signalling pathways, driving aberrant cellular transcription in infected B cells to promote lymphomagenesis. This article seeks to bring together the results of recent studies of the latent membrane proteins in different B cell systems, including experiments in animal models as well as a re-analysis of our own transcriptional data. In doing so, we summarise the potentially co-operative and antagonistic effects of the LMPs that are relevant to B cell lymphomagenesis.

Hypothesis: Multiple sclerosis is caused by three-hits, strictly in order, in genetically susceptible persons

Multiple Sclerosis is a chronic, progressive and debilitating neurological disease which, despite extensive study for over 100 years, remains of enigmatic aetiology. Drawn from the epidemiological evidence, there exists a consensus that there are environmental (possibly infectious) factors that contribute to disease pathogenesis that have not yet been fully elucidated. Here we propose a three-tiered hypothesis: 1) a clinic-epidemiological model of multiple sclerosis as a rare late complication of two sequential infections (with the temporal sequence of infections being important); 2) a proposal that the first event is helminthic infection with Enterobius Vermicularis, and the second is Epstein Barr Virus infection; and 3) a proposal for a testable biological mechanism, involving T-Cell exhaustion for Epstein-Barr Virus protein LMP2A. We believe that this model satisfies some of the as-yet unexplained features of multiple sclerosis epidemiology, is consistent with the clinical and neuropathological features of the disease and is potentially testable by experiment. This model may be generalizable to other autoimmune diseases.

Global Proteomic Changes Induced by the Epstein-Barr Virus Oncoproteins Latent Membrane Protein 1 and 2A

The Epstein-Barr virus (EBV) oncoproteins latent membrane protein 1 (LMP1) and LMP2A constitutively activate multiple signaling pathways, and both have been shown to interact with cellular ubiquitin ligases and affect cellular ubiquitination. To detect the LMP1- and LMP2A-mediated effects on the global cellular proteome, epithelial cell lines expressing LMP1 or LMP2A were analyzed using label-free quantitative proteomics. To identify proteins whose ubiquitination is affected by the viral proteins, the cells were cultured in the presence and absence of deubiquitinase (DUB) and proteasome inhibitors. More than 7,700 proteins were identified with high confidence and considerably more proteins showed significant differences in expression in the presence of inhibitors. Few of the differentially expressed proteins with or without inhibitors were common between LMP1 and LMP2A, confirming that the viral proteins induce unique changes in cell expression and function. However, ingenuity pathway analysis (IPA) of the data indicated that LMP1 and LMP2A modulate many of the same cellular regulatory pathways, including cell death and survival, cell movement, and actin filament dynamics. In addition, various proteasome subunits, ubiquitin-specific peptidases and conjugating enzymes, vesicle trafficking proteins, and NF-κB and mitogen-activated protein kinase signaling proteins were affected by LMP1 or LMP2A. These findings suggest that LMP1 and LMP2A may commonly target critical cell pathways through effects on distinct genes, with many cellular proteins modified by ubiquitination and/or degradation.

The Epstein-Barr virus proteins latent membrane protein 1 and 2 have potent effects on cell growth and signaling. Both proteins bind to specific ubiquitin ligases and likely modulate the cellular proteome through ubiquitin-mediated effects on stability and intracellular location. In this study, a comprehensive proteomic analysis of the effects of LMP1 and LMP2A revealed that both proteins affected proteasome subunits, ubiquitin-specific conjugases and peptidases, and vesical trafficking proteins. The data suggest that the effects of these proteins on the abundance and ubiquitination of cellular proteins are in part responsible for their effects on cell growth regulation.

Murine gammaherpesvirus M2 antigen modulates splenic B cell activation and terminal differentiation in vivo

Murine gammaherpesvirus 68 (MHV68) infection of laboratory strains of mice has provided a tractable small animal model for dissecting gammaherpesvirus pathogenesis. The MHV68 latency associated antigen M2 promotes viral latency establishment in germinal center (GC) B cells and plays an important role in virus infection of plasma cells (PCs), which is linked to virus reactivation. More recently, M2 has been highlighted as a potent immunomodulatory molecule capable of hindering both cell-mediated and humoral immunity to MHV68 infection and subsequent challenges. M2 expression in B cells results in activation of B cell receptor signaling pathways that promote proliferation, differentiation, and cytokine production—a hallmark of gammaherpesviruses. In this study, we utilized an adoptive transfer model to explore the biological consequence of M2 expression in activated B cells in vivo. Secondly, we engineered and validated two independent MHV68 M2 reporter viruses that track M2 protein expression in latently infected B cells during infection. Here we demonstrate that upon adoptive transfer into naive mice, M2 expression promotes activated primary B cells to competitively establish residency in the spleen as either a GC B cell or a PC, most notably in the absence of an ongoing GC reaction. Moreover, M2 antigen drives robust PC differentiation and IL10 production in vivo in the absence of other viral factors. Lastly, we confirm that M2 expression during MHV68 infection is localized to the GC compartment, which is a long term latency reservoir for gammaherpesviruses. Overall, these observations are consistent with, and extend upon previous reports of M2 function in B cells and within the context of MHV68 infection. Moreover, this work provides support for a model by which M2-driven dysregulation of B cell function compromises multiple aspects of antiviral immunity to achieve persistence within the infected host.

Gammaherpesvirus (GHVs), which primarily infect B cells, are capable of exploiting B cell biology to achieve a stable and persistent infection for the lifetime of the host. GHV infections traffick to germinal center (GC) B cells and plasma cells (PCs), which are important immune effectors that promote the generation of protective antibodies in response to pathogens. The mechanism by which murine gammaherpesvirus 68 (MHV68) M2 latency protein activates B cell receptor signaling pathways to modulate the immune response to infection and further promote viral pathogenesis within the GC B cell and PC compartments is not completely understood. Here we demonstrate that M2 expression alone, in the absence of other viral factors, drives robust PC differentiation and IL10 production in vivo. Moreover, M2 promotes the accumulation of splenic GC B cells, which was subsequently verified as the site for potent M2 expression during latent MHV68 infection. Our work further substantiates a model in which a viral protein dysregulates B cell activation, differentiation, and cytokine production to create a permissive environment for viral persistence in the infected host. This work justifies further investigations addressing the impact of GHV latency antigen function within the GC reaction and overall host response to infection.

Mouse model of Epstein-Barr virus LMP1- and LMP2A-driven germinal center B-cell lymphoproliferative disease

Epstein-Barr virus (EBV) is a major cause of immunosuppression-related B-cell lymphomas and Hodgkin lymphoma (HL). In these malignancies, EBV latent membrane protein 1 (LMP1) and LMP2A provide infected B cells with surrogate CD40 and B-cell receptor growth and survival signals. To gain insights into their synergistic in vivo roles in germinal center (GC) B cells, from which most EBV-driven lymphomas arise, we generated a mouse model with conditional GC B-cell LMP1 and LMP2A coexpression. LMP1 and LMP2A had limited effects in immunocompetent mice. However, upon T- and NK-cell depletion, LMP1/2A caused massive plasmablast outgrowth, organ damage, and death. RNA-sequencing analyses identified EBV oncoprotein effects on GC B-cell target genes, including up-regulation of multiple proinflammatory chemokines and master regulators of plasma cell differentiation. LMP1/2A coexpression also up-regulated key HL markers, including CD30 and mixed hematopoietic lineage markers. Collectively, our results highlight synergistic EBV membrane oncoprotein effects on GC B cells and provide a model for studies of their roles in immunosuppression-related lymphoproliferative diseases.

Evasion of affinity-based selection in germinal centers by Epstein-Barr virus LMP2A

Epstein-Barr virus (EBV) infects germinal center (GC) B cells and establishes persistent infection in memory B cells. EBV-infected B cells can cause B-cell malignancies in humans with T- or natural killer-cell deficiency. We now find that EBV-encoded latent membrane protein 2A (LMP2A) mimics B-cell antigen receptor (BCR) signaling in murine GC B cells, causing altered humoral immune responses and autoimmune diseases. Investigation of the impact of LMP2A on B-cell differentiation in mice that conditionally express LMP2A in GC B cells or all B-lineage cells found LMP2A expression enhanced not only BCR signals but also plasma cell differentiation in vitro and in vivo. Conditional LMP2A expression in GC B cells resulted in preferential selection of low-affinity antibody-producing B cells despite apparently normal GC formation. GC B-cell-specific LMP2A expression led to systemic lupus erythematosus-like autoimmune phenotypes in an age-dependent manner. Epigenetic profiling of LMP2A B cells found increased H3K27ac and H3K4me1 signals at the zinc finger and bric-a-brac, tramtrack domain-containing protein 20 locus. We conclude that LMP2A reduces the stringency of GC B-cell selection and may contribute to persistent EBV infection and pathogenesis by providing GC B cells with excessive prosurvival effects.

Nasopharyngeal Carcinoma: An Evolving Role for the Epstein-Barr Virus

The Epstein-Barr herpesvirus (EBV) is an important human pathogen that is closely linked to several major malignancies including the major epithelial tumor, undifferentiated nasopharyngeal carcinoma (NPC). This important tumor occurs with elevated incidence in specific areas, particularly in southern China but also in Mediterranean Africa and some regions of the Middle East. Regardless of tumor prevalence, undifferentiated NPC is consistently associated with EBV. The consistent detection of EBV in all cases of NPC, the maintenance of the viral genome in every cell, and the continued expression of viral gene products suggest that EBV is a necessary factor for the malignant growth in vivo. However, the molecular characterization of the infection and identification of critical events have been hampered by the difficulty in developing in vitro models of NPC. Epithelial cell infection is difficult in vitro and in contrast to B-cell infection does not result in immortalization and transformation. Cell lines established from NPC usually do not retain the genome, and the successful establishment of tumor xenografts is difficult. However, critical genetic changes that contribute to the onset and progression of NPC and key molecular properties of the viral genes expressed in NPC have been identified. In some cases, viral expression becomes increasingly restricted during tumor progression and tumor cells may express only the viral nuclear antigen EBNA1 and viral noncoding RNAs. As NPC develops in the immunocompetent, the continued progression of deregulated growth likely reflects the combination of expression of viral oncogenes in some cells and viral noncoding RNAs that likely function synergistically with changes in cellular RNA and miRNA expression.

Latent Membrane Protein 2 (LMP2)

LMP2A is an EBV-encoded protein with three domains: (a) an N-terminal cytoplasmic domain, which has PY motifs that bind to WW domain-containing E3 ubiquitin ligases and an ITAM that binds to SH2 domain-containing proteins, (b) a transmembrane domain with 12 transmembrane segments that localizes LMP2A in cellular membranes, and (c) a 27-amino acid C-terminal domain which mediates homodimerization and heterodimerization of LMP2 protein isoforms. The most prominent two isoforms of the protein are LMP2A and LMP2B. The LMP2B isoform lacks the 19-amino acid N-terminal domain found in LMP2A, which modulates cellular signaling resulting in a baseline activation of B cells and degradation of cellular kinases leading to the downregulation of normal B cell signaling pathways. These two seemingly contradictory processes allow EBV to establish and maintain latency. LMP2 is expressed in many EBV-associated malignancies. While its antigenic properties may be useful in developing LMP2-specific immunity, the LMP2A N-terminal motifs also provide a basis to target LMP2A-modulated cellular kinases for the development of treatment strategies.

Linking complement and anti-dsDNA antibodies in the pathogenesis of systemic lupus erythematosus

Systemic lupus erythematosus is a severe autoimmune disease that affects multiple organ systems resulting in diverse symptoms and outcomes. It is characterized by antibody production to a variety of self-antigens, but it is specifically associated with those against anti-dsDNA. Anti-dsDNA antibodies are present before the onset of clinical disease and are associated with severe manifestations of lupus such as glomerulonephritis. Their levels fluctuate with changes in disease activity and, in combination with the levels of complement proteins C3 and C4, are strong indicators of disease flare and treatment response in patients with lupus. The decreased complement levels that are noted during flares of lupus activity are believed to be secondary to increased autoantibody production and immune complex formation that results in tissue damage; however, recent data suggest that complement activation can also drive development of these pathogenic autoantibodies. This review will explore the various roles of complement in the development and pathogenesis of anti-dsDNA antibodies.

Epstein-Barr virus LMP2A increases IL-10 production in mitogen-stimulated primary B-cells and B-cell lymphomas

Epstein-Barr virus (EBV) latently infected B-cells are the precursors of EBV-associated malignancies. EBV-infection induces the production of pro-survival and anti-inflammatory cytokines that may be important in the transition between latency and malignancy. One EBV protein, LMP2A, can be detected in both latently infected resting B-cells and in EBV-associated malignancies. Therefore, we tested the ability of LMP2A to influence cytokine production using both LMP2A-Tg primary B-cells and LMP2A-expressing B-cell lines. Our data demonstrate that LMP2A does not globally alter B-cell-produced cytokine levels, but specifically targets IL-10. Additional studies using ELISA and real-time-RT-PCR confirm that LMP2A utilizes PI3-kinase to increase IL-10 levels. Finally, the data demonstrate that LMP2A-expressing B-cell lines are more dependent on IL-10 for survival in comparison to LMP2A-negative B-cell lines. These data identify a novel function of LMP2A in the alteration of a cytokine that is important for both tumour survival and anti-tumour responses.

Transcriptome changes induced by Epstein-Barr virus LMP1 and LMP2A in transgenic lymphocytes and lymphoma

Latent membrane protein 1 (LMP1) and LMP2A affect cell growth in both epithelial cells and lymphocytes. In this study, the effects on cellular gene expression were determined by microarray analysis of transgenic mice expressing LMP1, LMP2A, or both using the immunoglobulin heavy chain promoter and enhancer. Large differential changes were detected, indicating that LMP1 and LMP2A can both potently affect host gene transcription, inducing distinct transcriptional profiles. Seventy percent of the changes detected in LMP1/2A doubly transgenic lymphocytes were also modulated by LMP1 or LMP2A alone. These common and unique expression changes indicate that the combined effects of LMP1 and LMP2A may be additive, synergistic, or inhibitory. Using significant pathway analysis, the expression changes detected in LMP1, LMP2A, and LMP1/2A transgenic B lymphocytes were predicted to commonly target cancer and inflammatory pathways. Additionally, using the correlation coefficient to calculate the regulation of known c-Rel and Stat3 transcriptional targets, both were found to be enhanced in LMP1 lymphocytes and lymphomas, and a selection of Stat3 targets was further evaluated and confirmed using quantitative reverse transcription-PCR (RT-PCR). Analyses of the effects on cell growth and viability revealed that LMP2A transgenic lymphocytes had the greatest enhanced viability in vitro; however, doubly transgenic lymphocytes (LMP1/2A) did not have enhanced survival in culture and these mice were similar to negative littermates. These findings indicate that the combined expression of LMP1 and LMP2A has potentially different biological outcomes than when the two proteins are expressed individually.

The Epstein-Barr virus proteins latent membrane protein 1 (LMP1) and LMP2A have potent effects on cell growth. In transgenic mice that express these proteins in B lymphocytes, the cell growth and survival properties are also affected. LMP1 transgenic mice have increased development of lymphoma, and the LMP1 lymphocytes have increased viability in culture. LMP2A transgenic lymphocytes have altered B cell development and enhanced survival. In this study, analysis of the cellular gene expression changes in transgenic LMP1 and LMP2A lymphocytes and LMP1 lymphomas revealed that both transgenes individually and in combination affected pathways important for the development of cancer and inflammation. Importantly, the combined expression of the two proteins had unique effects on cellular expression and cell viability. This is the first study to look at the combined effects of LMP1 and LMP2A on global changes in host gene expression.

Epstein-Barr virus LMP2A reduces hyperactivation induced by LMP1 to restore normal B cell phenotype in transgenic mice

Epstein-Barr virus (EBV) latently infects most of the human population and is strongly associated with lymphoproliferative disorders. EBV encodes several latency proteins affecting B cell proliferation and survival, including latent membrane protein 2A (LMP2A) and the EBV oncoprotein LMP1. LMP1 and LMP2A signaling mimics CD40 and BCR signaling, respectively, and has been proposed to alter B cell functions including the ability of latently-infected B cells to access and transit the germinal center. In addition, several studies suggested a role for LMP2A modulation of LMP1 signaling in cell lines by alteration of TRAFs, signaling molecules used by LMP1. In this study, we investigated whether LMP1 and LMP2A co-expression in a transgenic mouse model alters B cell maturation and the response to antigen, and whether LMP2A modulates LMP1 function. Naïve LMP1/2A mice had similar lymphocyte populations and antibody production by flow cytometry and ELISA compared to controls. In the response to antigen, LMP2A expression in LMP1/2A animals rescued the impairment in germinal center generation promoted by LMP1. LMP1/2A animals produced high-affinity, class-switched antibody and plasma cells at levels similar to controls. In vitro, LMP1 upregulated activation markers and promoted B cell hyperproliferation, and co-expression of LMP2A restored a wild-type phenotype. By RT-PCR and immunoblot, LMP1 B cells demonstrated TRAF2 levels four-fold higher than non-transgenic controls, and co-expression of LMP2A restored TRAF2 levels to wild-type levels. No difference in TRAF3 levels was detected. While modulation of other TRAF family members remains to be assessed, normalization of the LMP1-induced B cell phenotype through LMP2A modulation of TRAF2 may be a pathway by which LMP2A controls B cell function. These findings identify an advance in the understanding of how Epstein-Barr virus can access the germinal center in vivo, a site critical for both the genesis of immunological memory and of virus-associated tumors.

As a ubiquitous human pathogen, Epstein-Barr virus (EBV) infection is associated with several human B cell diseases characterized by inappropriate B cell activation and function, including infectious mononucleosis and certain cancers. EBV latent membrane protein 1 (LMP1) and 2A (LMP2A) hijack cell signaling pathways to alter B cell activation and function, and are detected in EBV-associated diseases. Defining the effect on B cell function when LMP1 and LMP2A are expressed together in the same cell is critical to understanding how EBV subverts normal B cell behavior before disease develops. Using transgenic mice, we have demonstrated that LMP2A dampens cellular proliferation and activation induced by LMP1, which may be due to the LMP2A-associated decrease in the levels of TRAF2, a signaling protein used by LMP1. LMP2A also allows B cells carrying LMP1 to enter the germinal center during an immune response, a site that gives rise to EBV-associated tumors in humans. In sum, this study highlights the biological outcomes of LMP1 and LMP2A co-expression in B cells and contributes to the knowledge of how EBV subverts normal B cell behavior before disease develops.

Epstein-Barr virus latent membrane protein 2A exacerbates experimental autoimmune encephalomyelitis and enhances antigen presentation function

Multiple sclerosis (MS) is an inflammatory, autoimmune disease of the central nervous system. The cause of MS is still unknown but epidemiological and immunological studies have implicated Epstein-Barr virus (EBV), which infects B cells, as a possible etiological agent involved in disease. Of particular interest is EBV latent membrane protein 2A (LMP2A) because previous studies have demonstrated that LMP2A enhances the expansion and differentiation of B cells upon antigen stimulation, revealing a potential contribution of this protein in autoimmunity. Since B cells are thought to contribute to MS, we examined the role of LMP2A in the animal model experimental autoimmune encephalomyelitis (EAE). In this model, transgenic mice in which B cells express LMP2A show increased severity and incidence of disease. This difference was not due to lymphocyte recruitment into the CNS or differences in T cell activation, rather, we show that LMP2A enhances antigen presentation function.

NF-κB as a target for oncogenic viruses

NF-κB is a pivotal transcription factor that controls cell survival and proliferation in diverse physiological processes. The activity of NF-κB is tightly controlled through its cytoplasmic sequestration by specific inhibitors, IκBs. Various cellular stimuli induce the activation of an IκB kinase, which phosphorylates IκBs and triggers their proteasomal degradation, causing nuclear translocation of activated NF-κB. Under normal conditions, the activation of NF-κB occurs transiently, thus ensuring rapid but temporary induction of target genes. Deregulated NF-κB activation contributes to the development of various diseases, including cancers and immunological disorders. Accumulated studies demonstrate that the NF-κB signaling pathway is a target of several human oncogenic viruses, including the human T cell leukemia virus type 1, the Kaposi sarcoma-associated herpesvirus, and the Epstein-Bar virus. These viruses encode specific oncoproteins that target different signaling components of the NF-κB pathway, leading to persistent activation of NF-κB. This chapter will discuss the molecular mechanisms by which NF-κB is activated by the viral oncoproteins.

The viral latency-associated nuclear antigen augments the B-cell response to antigen in vivo

Gammaherpesviruses, including Kaposi sarcoma-associated herpesvirus (KSHV), establish latency in B cells. We hypothesized that the KSHV latency-associated nuclear antigen (LANA/orf73) provides a selective advantage to infected B cells by driving proliferation in response to antigen. To test this, we used LANA B-cell transgenic mice. Eight days after immunization with antigen without adjuvant, LANA mice had significantly more activated germinal center (GC) B cells (CD19(+) PNA(+) CD71(+)) than controls. This was dependent upon B-cell receptor since LANA did not restore the GC defect of CD19 knockout mice. However, LANA was able to restore the marginal zone defect in CD19 knockout mice.

Epstein-Barr virus LMP2A imposes sensitivity to apoptosis

In cell lines, the Epstein–Barr virus (EBV)-encoded protein latent membrane protein 2A (LMP2A) protects B-cells from apoptosis by blocking B-cell receptor (BCR) signalling. However, EBV-infected B-cells in vivo are extremely different from cell lines. This study used a murine transgenic model in which B-cells express LMP2A and a BCR specific for hen egg lysozyme to determine whether LMP2A protects resting and antigen-activated B-cells from apoptosis. LMP2A allows BCR signal transduction and induces constitutive activation of NF-κB to increase Bcl-2 levels that afford LMP2A-mediated protection from apoptosis in the absence or presence of antigen. In contrast, low levels of NF-κB inhibitor only affected Bcl-2 and Bcl-xL levels and increased apoptosis in LMP2A-negative B-cells after BCR cross-linking. These data suggest that LMP2A uniquely makes resting B-cells sensitive to NF-κB inhibition and apoptosis and suggest that NF-κB may be a novel target to eradicate latently EBV-infected B-cells.

Cellular factors associated with latency and spontaneous Epstein-Barr virus reactivation in B-lymphoblastoid cell lines

EBV-immortalized B-lymphoblastoid cell lines are used as models for cellular transformation and as antigen-presenting cells in immunological assays. LCLs vary in surface markers and other phenotypic properties, but it is not known how this heterogeneity relates to the EBV life cycle. To explore correlations, we examined 62 LCLs for cellular and viral phenotypes. LCLs generated from pediatric and adult donors could similarly be categorized as either low in EBV copy number or fluctuating within a high range. High-copy status accompanied higher lytic viral gene expression and lower latent gene expression. Inhibiting lytic EBV replication did not affect cellular phenotype or lytic switch protein expression, indicating that an LCL's lytic permissivity was a stable property. Among the cellular genes overexpressed in permissive LCLs were unfolded protein response genes and plasma cell markers. Among genes overexpressed in non-permissive LCLs were transcription factors involved in maintaining B cell lineage, in particular EBF1. This study suggests previously undetected mechanisms by which cellular pathways influence the lytic reactivation of EBV.

Epstein-Barr virus LMP2A accelerates MYC-induced lymphomagenesis

Despite the identification of Epstein-Barr virus (EBV) in tumors of Burkitt's lymphoma (BL) over 40 years ago, the exact contribution of EBV to BL is undefined. EBV encodes for multiple proteins in latent B cells that affect B cell survival and activation. One such protein, latent membrane protein 2A (LMP2A), protects B cells from numerous pro-apoptotic stimuli. Therefore, we tested whether LMP2A protects B cells from apoptosis induced by aberrant c-MYC expression that precedes and dominates BL. We crossed LMP2A-transgenic mice (LMP2A-Tg), in which all B cells express LMP2A, to a transgenic mouse that expresses a BL translocation of myc (lambda-MYC-Tg mice). LMP2A promotes proliferation and protects B cells from MYC-induced apoptosis in lambda-MYC-Tg mice. LMP2A also accelerates the development of lymphoma in LMP2A/lambda-MYC-Tg mice. Finally, LMP2A increases the expression of Bcl-X(L) in both pre-tumor B cells and tumor cells, suggesting a mechanism for LMP2A-mediated B cell survival in the presence of MYC. These results support a hypothesis that EBV LMP2A promotes tumor development by protecting pre-tumor B cells that would normally apoptose after the c-myc translocation.

The curious case of the tumour virus: 50 years of Burkitt's lymphoma

Burkitt's lymphoma (BL) was first described 50 years ago, and the first human tumour virus Epstein-Barr virus (EBV) was discovered in BL tumours soon after. Since then, the role of EBV in the development of BL has become more and more enigmatic. Only recently have we finally begun to understand, at the cellular and molecular levels, the complex and interesting interaction of EBV with B cells that creates a predisposition for the development of BL. Here, we discuss the intertwined histories of EBV and BL and their relationship to the cofactors in BL pathogenesis: malaria and the MYC translocation.

Role of latent membrane protein 2 isoforms in Epstein-Barr virus latency

The oncogenic Epstein-Barr virus (EBV) infects the majority of the human population without doing harm and establishes a latent infection in the memory B-cell compartment. To accomplish this, EBV hijacks B-cell differentiation pathways and uses its own viral genes to interfere with B-cell signalling to achieve life-long persistence. EBV latent membrane protein 2A (LMP2A) provides a surrogate B-cell receptor signal essential for cell survival and is believed to have a crucial role in the maintenance of latency by blocking B-cell activation which would otherwise lead to lytic EBV infection. These two functions demand tight control of LMP2A activity and expression levels. Based on recent insights in the function of LMP2B, an isoform of LMP2A, we propose a model for how LMP2B modulates the activity of LMP2A contributing to maintenance of EBV latency.

EBV LMP2A provides a surrogate pre-B cell receptor signal through constitutive activation of the ERK/MAPK pathway

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) provides developmental and survival signals that mimic those of a B-cell receptor (BCR). Expression of LMP2A during B-cell development results in the ability of B cells to exit the bone marrow in the absence of a BCR and persist in the periphery, where they would normally undergo apoptosis. This study extends the current knowledge of LMP2A function by examining the growth properties of bone marrow B cells from TgE LMP2A mice. Despite the lack of pre-BCR expression, bone marrow B cells from TgE LMP2A mice proliferate and survive in low concentrations of interleukin 7, similar to wild-type cells. Constitutive phosphorylation of ERK/MAPK and PI3K/Akt in TgE LMP2A bone marrow B cells is also reminiscent of signalling through the pre-BCR, altogether demonstrating that LMP2A provides a pre-BCR-like signal to developing B cells.

Epstein-Barr virus latent membrane protein 2A preferentially signals through the Src family kinase Lyn

Latent membrane protein 2A (LMP2A) is a viral protein expressed during Epstein-Barr virus (EBV) latency in EBV-infected B cells both in cell culture and in vivo. LMP2A has important roles in modulating B-cell receptor signal transduction and provides survival and developmental signals to B cells in vivo. Although Lyn has been shown to be important in mediating LMP2A signaling, it is still unclear if Lyn is used preferentially or if LMP2A associates promiscuously with other Src family kinase (SFK) members. To investigate the role of various SFKs in LMP2A signaling, we crossed LMP2A transgenic mice (TgE) with Lyn(-/-), Fyn(-/-), or Blk(-/-) mice. TgE Lyn(-/-) mice had a larger immunoglobulin M (IgM)-positive B-cell population than TgE mice, suggesting that the absence of Lyn prevents LMP2A from delivering survival and developmental signals to the B cells. Both TgE Fyn(-/-) and TgE Blk(-/-) mice have an IgM-negative population of splenic B cells, similar to the TgE mice. LMP2A was also transiently transfected into the human EBV-negative B-cell line BJAB to determine which SFK members associate with LMP2A. Lyn was detected in LMP2A immunoprecipitates, whereas Fyn was not. Both Lyn and Fyn were able to bind to an LMP2A mutant which contained a sequence shown previously to bind tightly to the SH2 domain of multiple SFK members. From these results, we conclude that LMP2A preferentially associates with and signals through Lyn compared to its association with other SFKs. This preferential association is due in part to the SH2 domain of Lyn associating with LMP2A.

Influence of EBV on the Peripheral Blood Memory B Cell Compartment

Peripheral blood memory B cells latently infected with EBV bear somatic mutations and are typically isotype switched consistent with being classical Ag-selected memory B cells. In this work, we performed a comparative analysis of the expressed Ig genes between large sets of EBV-infected and uninfected peripheral blood B cells, isolated from the same infectious mononucleosis patients, to determine whether differences exist that could reveal the influence of EBV on the production and maintenance of these cells. We observed that EBV(+) cells on average accumulated more somatic hypermutations than EBV(-) cells. In addition, they had more replacement mutations and a higher replacement-silent ratio of mutations in their CDRs. We also found that EBV occupies a skewed niche within the memory compartment, due to its exclusion from the CD27(+)IgD(+)IgM(+) subset, but this skewing does not affect the overall structure of the compartment. These results indicate that EBV impacts the mutation and selection process of infected cells but that once they enter memory they cannot be distinguished from uninfected cells by host homeostasis mechanisms.

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.

Epstein–Barr virus latent membrane protein 2A and autoimmunity

Epstein-Barr virus (EBV) has been associated with autoimmune diseases for over 40 years. However, the mechanisms by which EBV might promote autoimmune development remain elusive. Many of the hypotheses for the means by which EBV might achieve this incorporate the idea that autoimmune responses are initially immune responses against EBV proteins that crossreact with endogenous human proteins. However, recent evidence using transgenic mouse models suggests that B cells expressing the EBV-encoded protein latent membrane protein 2A (LMP2A) bypasses normal tolerance checkpoints and enhances the development of autoimmune diseases. Evidence from transgenic mouse models supports a paradigm in which LMP2A could promote autoimmune development. This novel model provides a framework to test potential mechanisms by which EBV could promote the development of autoimmune responses and might enable the identification of strategies to treat EBV-associated autoimmune diseases.