In vivo growth of Epstein-Barr virus transformed B cells with mutations in latent membrane protein 2 (LMP2)

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.

Epstein-Barr virus latent genes

Latent Epstein–Barr virus (EBV) infection has a substantial role in causing many human disorders. The persistence of these viral genomes in all malignant cells, yet with the expression of limited latent genes, is consistent with the notion that EBV latent genes are important for malignant cell growth. While the EBV-encoded nuclear antigen-1 (EBNA-1) and latent membrane protein-2A (LMP-2A) are critical, the EBNA-leader proteins, EBNA-2, EBNA-3A, EBNA-3C and LMP-1, are individually essential for in vitro transformation of primary B cells to lymphoblastoid cell lines. EBV-encoded RNAs and EBNA-3Bs are dispensable. In this review, the roles of EBV latent genes are summarized.

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.

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.

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.

EBV LMP2A affects LMP1-mediated NF-kappaB signaling and survival of lymphoma cells by regulating TRAF2 expression

A mechanism used by Epstein-Barr virus (EBV) for in vitro transformation of B cells into lymphoblastoid cell lines (LCLs) is activation of the NF-kappaB pathway, which is largely mediated by the EBV latent membrane protein 1 (LMP1). LMP1 is coexpressed with LMP2A in many EBV-associated lymphoid malignancies. Since inhibition of NF-kappaB leads to apoptosis of EBV-infected LCLs and lymphoma cell lines, we sought to determine whether LMP1 alone, or in combination with other viral proteins, is responsible for initiating NF-kappaB activation in these cells, thereby playing a role in cell survival. We found that suppression of LMP1 by RNA interference results in inhibition of basal NF-kappaB and induction of apoptosis. Unexpectedly, knockdown of LMP2A also resulted in comparable decrease of NF-kappaB activity and apoptosis. We report that LMP2A protein controls the expression of TRAF2 mRNA, which in turn is necessary for signaling by LMP1. Our data contrast with previous studies showing that transfected LMP1 can signal in the absence of LMP2A or TRAF2, and demonstrate that both LMP2A and TRAF2 are required for survival in naturally infected lymphoma cells and LCLs. These results also support LMP1, LMP2A, and TRAF2 as potential therapeutic targets in a subset of EBV-associated lymphoid malignancies.

Minimal protein domain requirements for the intracellular localization and self-aggregation of Epstein-Barr Virus Latent Membrane Protein 2

The EBV Latent Membrane Protein 2 (LMP2) may have a role in the establishment and maintenance of in vivo latency. The gene is transcribed into two mRNAs that produce two LMP2 protein isoforms. The LMP2a protein isoform has 12 transmembrane segments (TMs) and an amino terminal cytoplasmic signaling domain (CSD) while the LMP2b isoform is identical but lacks the CSD. There has not been a consensus on the cellular membrane localization being sometimes ascribed to either a plasma membrane or an intracellular location [M. Rovedo, R. Longnecker, J. Virol. 81:89-94, 2007; D. Lynch, J. Zimmerman, D.T. Rowe, J. Gen. Virol. 83:1025-1035, 2002; C. Dawson, J. George, S. Blake, R. Longnecker, L.S. Young, Virology 289:192-207, 2001]. Fluorescent marker and epitope tagged LMP2b truncation mutants progressively removing TMs from the N and C termini were used to assess the localization and aggregation properties of LMP2b. wtLMP2b had an exclusively intracellular perinuclear localization, while all truncations of the protein resulted in localization to the cell surface. By epitope loop-tagging, all the truncated LMP2b proteins were verified to be in the predicted membrane orientation. In co-transfection experiments, the C-terminal region was implicated in the self-aggregation properties of LMP2b. Thus, an intact 12 TM domain was required for intracellular localization and protein-protein interaction while a C-terminal region was responsible for auto-aggregative properties.

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

Epstein-Barr virus (EBV) establishes latent infections in a significant percentage of the population. Latent membrane protein 2A (LMP2A) is an EBV protein expressed during latency that inhibits B-cell receptor signaling in lymphoblastoid cell lines. In the present study, we have utilized a transgenic mouse system in which LMP2A is expressed in B cells that are specific for hen egg lysozyme (E/HEL-Tg). To determine if LMP2A allows B cells to respond to antigen, E/HEL-Tg mice were immunized with hen egg lysozyme. E/HEL-Tg mice produced antibody in response to antigen, indicating that LMP2A allows B cells to respond to antigen. In addition, E/HEL-Tg mice produced more antibody and an increased percentage of plasma cells after immunization compared to HEL-Tg littermates, suggesting that LMP2A increased the antibody response in vivo. Finally, in vitro studies determined that LMP2A acts directly on the B cell to increase antibody production by augmenting the expansion and survival of the activated B cells, as well as increasing the percentage of plasma cells generated. Taken together, these data suggest that LMP2A enhances, not diminishes, B-cell-specific antibody responses in vivo and in vitro in the E/HEL-Tg system.

Regulation of intracellular signalling by the terminal membrane proteins of members of the Gammaherpesvirinae

The human gamma(1)-herpesvirus Epstein-Barr virus (EBV) and the gamma(2)-herpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV), rhesus rhadinovirus (RRV), herpesvirus saimiri (HVS) and herpesvirus ateles (HVA) all contain genes located adjacent to the terminal-repeat region of their genomes, encoding membrane proteins involved in signal transduction. Designated 'terminal membrane proteins' (TMPs) because of their localization in the viral genome, they interact with a variety of cellular signalling molecules, such as non-receptor protein tyrosine kinases, tumour-necrosis factor receptor-associated factors, Ras and Janus kinase (JAK), thereby initiating further downstream signalling cascades, such as the MAPK, PI3K/Akt, NF-kappaB and JAK/STAT pathways. In the case of TMPs expressed during latent persistence of EBV and HVS (LMP1, LMP2A, Stp and Tip), their modulation of intracellular signalling pathways has been linked to the provision of survival signals to latently infected cells and, hence, a contribution to occasional cellular transformation. In contrast, activation of similar pathways by TMPs of KSHV (K1 and K15) and RRV (R1), expressed during lytic replication, may extend the lifespan of virus-producing cells, alter their migration and/or modulate antiviral immune responses. Whether R1 and K1 contribute to the oncogenic properties of KSHV and RRV has not been established satisfactorily, despite their transforming qualities in experimental settings.

AIDS-related systemic non-Hodgkin's lymphoma at a large community program

The introduction of triple antiretroviral therapy has led to reductions in opportunistic diseases in HIV-infected patients. However, little is known of the effect of this therapy on the clinical and pathological features and the outcome of patients with AIDS-related systemic non-Hodgkin's lymphoma (NHL). We examined the incidence and clinical manifestations of HIV-infected patients with systemic NHL at the Harris County Hospital District and Veterans Affairs Medical Center (Houston, TX). Between January 1996 and December 2000, 76 cases of systemic NHL were diagnosed in 3655 HIV-infected patients. Three groups of patients diagnosed with systemic NHL were identified according to their history of antiretroviral therapy: treatment naive (n = 20), dual nucleoside (n = 22), and triple antiretroviral drug-treated patients (n = 34). The median duration of antiretroviral therapy before the diagnosis of systemic NHL in the triple antiretroviral and dual nucleoside treatment groups was 12 versus 8 months (p < 0.0004). Thirty-five percent of patients (12 of 34) in the triple treatment group had an HIV RNA viral load of <400 copies/ml and their median CD4+ cell count was 301 cells/mm(3) (range, 46 to 667 cells/mm(3)) at the time of diagnosis of systemic NHL. More patients treated with triple antiretroviral therapy received complete courses of chemotherapy as compared with the other two groups (p = 0.013). However, the overall survival did not differ significantly among the three groups of patients. These data suggest that AIDS-related systemic NHL continues to occur even in patients treated with triple antiretroviral therapy. In addition, this opportunistic malignancy is associated with significant mortality. Therefore, it is necessary to develop a better understanding of the pathogenesis of this disease.

The genetic approach to the Epstein-Barr virus: from basic virology to gene therapy

The Epstein-Barr virus (EBV) infects humans and the genome of this infectious agent has been detected in several tumour types, ranging from lymphomas to carcinomas. The analysis of the functions of the numerous viral proteins encoded by EBV has been impeded by the large size of the viral genome, which renders the construction of viral mutants difficult. To overcome these limitations, several genetic systems have been developed that allow the modification of the viral genome. Two different approaches, depending on the host cell type in which the viral mutants are generated, have been used in the past. Traditionally, mutants were constructed in EBV infected eukaryotic cells, but more recently, approaches that make use of a recombinant EBV cloned in Escherichia coli have been proposed. The phenotype associated with the inactivation or modification of nearly 20 of the 100 EBV viral genes has been reported in the literature. In most of the reported cases, the EBV latent genes that mediate the ability of EBV to immortalize infected cells were the targets of the genetic analysis, but some virus mutants in which genes involved in DNA lytic replication or infection were disrupted have also been reported. The ability to modify the viral genome also opens the way to the construction of viral strains with medical relevance. A cell line infected by a virus that lacks the EBV packaging sequences can be used as a helper cell line for the encapsidation of EBV based viral vectors. This cell line will allow the evaluation of EBV as a gene transfer system with applications in gene therapy. Finally, genetically modified non-pathogenic strains will provide a basis for the design of an attenuated EBV live vaccine.

The immunosuppressive macrolide RAD inhibits growth of human Epstein-Barr virus-transformed B lymphocytes in vitro and in vivo: A potential approach to prevention and treatment of posttransplant lymphoproliferative disorders

Whereas the standard immunosuppressive agents foster development of posttransplant lymphoproliferative disorders (PTLDs), the impact of RAD, a macrolide with potent immunosuppressive properties, and other immunosuppressive macrolides on these disorders remains undetermined. We found that RAD had a profound inhibitory effect on in vitro growth of six different PTLD-like Epstein-Barr virus+ lymphoblastoid B cell lines. Similar to normal T cells, RAD blocked cell-cycle progression in PTLD-like B cells in the early (G(0)/G(1)) phase. Furthermore, RAD increased the apoptotic rate in such cells. The drug also had a profound inhibitory effect on the growth of PTLD-like Epstein-Barr virus+ B cells xenotransplanted s.c. into SCID mice. The degree of the RAD effect varied among the three B cell lines tested and was proportional to its effects on the cell lines in vitro. In this in vivo xenotransplant model, RAD markedly delayed growth or induced regression of the established tumors. In one line, it was able to eradicate the tumor in four of eight mice. When RAD treatment was initiated before tumor cell injection, a marked inhibition of tumor growth was seen in all three lines. In two of them, the drug prevented tumor establishment in approximately 50% of mice (5/11 and 5/8). In summary, RAD is a potent inhibitor of PTLD-like cells in vitro and in vivo. These findings indicate that, in contrast to the standard immunosuppressive agents, macrolides such as RAD may be effective in prevention and treatment of PTLDs.

Inhibition of RNA polymerase II transcription in human cells by synthetic DNA-binding ligands

Sequence-specific DNA-binding small molecules that can permeate human cells potentially could regulate transcription of specific genes. Multiple cellular DNA-binding transcription factors are required by HIV type 1 for RNA synthesis. Two pyrrole-imidazole polyamides were designed to bind DNA sequences immediately adjacent to binding sites for the transcription factors Ets-1, lymphoid-enhancer binding factor 1, and TATA-box binding protein. These synthetic ligands specifically inhibit DNA-binding of each transcription factor and HIV type 1 transcription in cell-free assays. When used in combination, the polyamides inhibit virus replication by >99% in isolated human peripheral blood lymphocytes, with no detectable cell toxicity. The ability of small molecules to target predetermined DNA sequences located within RNA polymerase II promoters suggests a general approach for regulation of gene expression, as well as a mechanism for the inhibition of viral replication.