Epstein-Barr virus latent membrane protein 2A (LMP2A) employs the SLP-65 signaling module

E3 Ubiquitin Ligases in Gammaherpesviruses and HIV: A Review of Virus Adaptation and Exploitation

For productive infection and replication to occur, viruses must control cellular machinery and counteract restriction factors and antiviral proteins. Viruses can accomplish this, in part, via the regulation of cellular gene expression and post-transcriptional and post-translational control. Many viruses co-opt and counteract cellular processes via modulation of the host post-translational modification machinery and encoding or hijacking kinases, SUMO ligases, deubiquitinases, and ubiquitin ligases, in addition to other modifiers. In this review, we focus on three oncoviruses, Epstein-Barr virus (EBV), Kaposi's sarcoma herpesvirus (KSHV), and human immunodeficiency virus (HIV) and their interactions with the ubiquitin-proteasome system via viral-encoded or cellular E3 ubiquitin ligase activity.

Epstein-Barr virus and genetic risk variants as determinants of T-bet+ B cell-driven autoimmune diseases

B cells expressing the transcription factor T-bet are found to have a protective role in viral infections, but are also considered major players in the onset of different types of autoimmune diseases. Currently, the exact mechanisms driving such 'atypical' memory B cells to contribute to protective immunity or autoimmunity are unclear. In addition to general autoimmune-related factors including sex and age, the ways T-bet+ B cells instigate autoimmune diseases may be determined by the close interplay between genetic risk variants and Epstein-Barr virus (EBV). The impact of EBV on T-bet+ B cells likely relies on the type of risk variants associated with each autoimmune disease, which may affect their differentiation, migratory routes and effector function. In this hypothesis-driven review, we discuss the lines of evidence pointing to such genetic and/or EBV-mediated influence on T-bet+ B cells in a range of autoimmune diseases, including systemic lupus erythematosus (SLE) and multiple sclerosis (MS). We provide examples of how genetic risk variants can be linked to certain signaling pathways and are differentially affected by EBV to shape T-bet+ B-cells. Finally, we propose options to improve current treatment of B cell-related autoimmune diseases by more selective targeting of pathways that are critical for pathogenic T-bet+ B-cell formation.

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.

The role of PLCγ2 in immunological disorders, cancer, and neurodegeneration

Phosphatidylinositol-specific phospholipase Cγ2 (PLCγ2) is a critical signaling molecule activated downstream from a variety of cell surface receptors that contain an intracellular immunoreceptor tyrosine-based activation motif. These receptors recruit kinases such as Syk, BTK, and BLNK to phosphorylate and activate PLCγ2, which then generates 1D-myo-inositol 1,4,5-trisphosphate and diacylglycerol. These well-known second messengers are required for diverse membrane functionality including cellular proliferation, endocytosis, and calcium flux. As a result, PLCγ2 dysfunction is associated with a variety of diseases including cancer, neurodegeneration, and immune disorders. The diverse pathologies associated with PLCγ2 are exemplified by distinct genetic variants. Inherited mutations at this locus cause PLCγ2-associated antibody deficiency and immune dysregulation, in some cases with autoinflammation. Acquired mutations at this locus, which often arise as a result of BTK inhibition to treat chronic lymphocytic leukemia, result in constitutive downstream signaling and lymphocyte proliferation. Finally, a third group of PLCγ2 variants actually has a protective effect in a variety of neurodegenerative disorders, presumably by increased uptake and degradation of deleterious neurological aggregates. Therefore, manipulating PLCγ2 activity either up or down could have therapeutic benefit; however, we require a better understanding of the signaling pathways propagated by these variants before such clinical utility can be realized. Here, we review the signaling roles of PLCγ2 in hematopoietic cells to help understand the effect of mutations driving immune disorders and cancer and extrapolate from this to roles which may relate to protection against neurodegeneration.

Rewiring of B cell receptor signaling by Epstein-Barr virus LMP2A

Epstein-Barr virus (EBV) infects human B cells and reprograms them to allow virus replication and persistence. One key viral factor in this process is latent membrane protein 2A (LMP2A), which has been described as a B cell receptor (BCR) mimic promoting malignant transformation. However, how LMP2A signaling contributes to tumorigenesis remains elusive. By comparing LMP2A and BCR signaling in primary human B cells using phosphoproteomics and transcriptome profiling, we identified molecular mechanisms through which LMP2A affects B cell biology. Consistent with the literature, we found that LMP2A mimics a subset of BCR signaling events, including tyrosine phosphorylation of the kinase SYK, the calcium initiation complex consisting of BLNK, BTK, and PLCγ2, and its downstream transcription factor NFAT. However, the majority of LMP2A-induced signaling events markedly differed from those induced by BCR stimulation. These included differential phosphorylation of kinases, phosphatases, adaptor proteins, transcription factors such as nuclear factor κB (NF-κB) and TCF3, as well as widespread changes in the transcriptional output of LMP2A-expressing B cells. LMP2A affected apoptosis and cell-cycle checkpoints by dysregulating the expression of apoptosis regulators such as BCl-xL and the tumor suppressor retinoblastoma-associated protein 1 (RB1). LMP2A cooperated with MYC and mutant cyclin D3, two oncogenic drivers of Burkitt lymphoma, to promote proliferation and survival of primary human B cells by counteracting MYC-induced apoptosis and by inhibiting RB1 function, thereby promoting cell-cycle progression. Our results indicate that LMP2A is not a pure BCR mimic but rather rewires intracellular signaling in EBV-infected B cells that optimizes cell survival and proliferation, setting the stage for oncogenic transformation.

Mechanisms of autoregulation of C3G, activator of the GTPase Rap1, and its catalytic deregulation in lymphomas

C3G is a guanine nucleotide exchange factor (GEF) that regulates cell adhesion and migration by activating the GTPase Rap1. The GEF activity of C3G is stimulated by the adaptor proteins Crk and CrkL and by tyrosine phosphorylation. Here, we uncovered mechanisms of C3G autoinhibition and activation. Specifically, we found that two intramolecular interactions regulate the activity of C3G. First, an autoinhibitory region (AIR) within the central domain of C3G binds to and blocks the catalytic Cdc25H domain. Second, the binding of the protein's N-terminal domain to its Ras exchanger motif (REM) is required for its GEF activity. CrkL activated C3G by displacing the AIR/Cdc25HD interaction. Two missense mutations in the AIR found in non-Hodgkin's lymphomas, Y554H and M555K, disrupted the autoinhibitory mechanism. Expression of C3G-Y554H or C3G-M555K in Ba/F3 pro-B cells caused constitutive activation of Rap1 and, consequently, the integrin LFA-1. Our findings suggest that sustained Rap1 activation by deregulated C3G might promote progression of lymphomas and that designing therapeutics to target C3G might treat these malignancies.

EBV latent membrane protein 2A orchestrates p27kip1 degradation via Cks1 to accelerate MYC-driven lymphoma in mice

Epstein-Barr virus (EBV) establishes lifelong infection in B lymphocytes of most human hosts and is associated with several B lymphomas. During latent infection, EBV encodes latent membrane protein 2A (LMP2A) to promote the survival of B cells by mimicking host B-cell receptor signaling. By studying the roles of LMP2A during lymphoma development in vivo, we found that LMP2A mediates rapid MYC-driven lymphoma onset by allowing B cells to bypass MYC-induced apoptosis mediated by the p53 pathway in our transgenic mouse model. However, the mechanisms used by LMP2A to facilitate transformation remain elusive. In this study, we demonstrate a key role of LMP2A in promoting hyperproliferation of B cells by enhancing MYC expression and MYC-dependent degradation of the p27^kip1 tumor suppressor. Loss of the adaptor protein cyclin-dependent kinase regulatory subunit 1 (Cks1), a cofactor of the SCF^Skp2 ubiquitin ligase complex and a downstream target of MYC, increases p27^kip1 expression during a premalignant stage. In mice that express LMP2A, Cks1 deficiency reduces spleen weights, restores B-cell follicle formation, impedes cell cycle progression of pretumor B cells, and eventually prolongs MYC-driven tumor onset. This study demonstrates that LMP2A uses the role of MYC in the cell cycle, particularly in the p27^kip1 degradation process, to accelerate lymphomagenesis in vivo. Thus, our results reveal a novel mechanism of EBV in diverting the functions of MYC in malignant transformation and provide a rationale for targeting EBV's roles in cell cycle modulation.

Epstein-Barr virus Latent Membrane Protein 2A (LMP2A)-mediated changes in Fas expression and Fas-dependent apoptosis: Role of Lyn/Syk activation

Epstein-Barr virus Latent Membrane Protein 2A (LMP2A) is expressed in EBV-infected B cells in the germinal center, a site of significant apoptosis induced by engagement of Fas on activated B cells. Signals from the B cell receptor (BCR) protect germinal center B cells from Fas-mediated apoptosis, and since LMP2A is a BCR mimic, we hypothesized that LMP2A would also protect B cells from Fas-mediated apoptosis. Surprisingly, latently-infected human and murine B cell lines expressing LMP2A were more sensitive to Fas-mediated apoptosis, as determined by increases in Annexin-V staining, and cleavage of caspase-8, -3 and PARP. Additional studies show that LMP2A-expressing B cell lines demonstrate a Lyn- and Syk-dependent increase in sensitivity to Fas-mediated apoptosis, due to an LMP2A-dependent enhancement in Fas expression. These findings demonstrate the ability for LMP2A to directly increase a pro-apoptotic molecule and have implications for EBV latency as well as the treatment of EBV-associated malignancies.

K1 and K15 of Kaposi's Sarcoma-Associated Herpesvirus Are Partial Functional Homologues of Latent Membrane Protein 2A of Epstein-Barr Virus

The human herpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) are associated with Hodgkin's lymphoma (HL) and Primary effusion lymphomas (PEL), respectively, which are B cell malignancies that originate from germinal center B cells. PEL cells but also a quarter of EBV-positive HL tumor cells do not express the genuine B cell receptor (BCR), a situation incompatible with survival of normal B cells. EBV encodes LMP2A, one of EBV's viral latent membrane proteins, which likely replaces the BCR's survival signaling in HL. Whether KSHV encodes a viral BCR mimic that contributes to oncogenesis is not known because an experimental model of KSHV-mediated B cell transformation is lacking. We addressed this uncertainty with mutant EBVs encoding the KSHV genes K1 or K15 in lieu of LMP2A and infected primary BCR-negative (BCR(-)) human B cells with them. We confirmed that the survival of BCR(-) B cells and their proliferation depended on an active LMP2A signal. Like LMP2A, the expression of K1 and K15 led to the survival of BCR(-) B cells prone to apoptosis, supported their proliferation, and regulated a similar set of cellular target genes. K1 and K15 encoded proteins appear to have noncomplementing, redundant functions in this model, but our findings suggest that both KSHV proteins can replace LMP2A's key activities contributing to the survival, activation and proliferation of BCR(-) PEL cells in vivo.

IMPORTANCE

Several herpesviruses encode oncogenes that are receptor-like proteins. Often, they are constitutively active providing important functions to the latently infected cells. LMP2A of Epstein-Barr virus (EBV) is such a receptor that mimics an activated B cell receptor, BCR. K1 and K15, related receptors of Kaposi's sarcoma-associated herpesvirus (KSHV) expressed in virus-associated tumors, have less obvious functions. We found in infection experiments that both viral receptors of KSHV can replace LMP2A and deliver functions similar to the endogenous BCR. K1, K15, and LMP2A also control the expression of a related set of cellular genes in primary human B cells, the target cells of EBV and KSHV. The observed phenotypes, as well as the known characteristics of these genes, argue for their contributions to cellular survival, B cell activation, and proliferation. Our findings provide one possible explanation for the tumorigenicity of KSHV, which poses a severe problem in immunocompromised patients.

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.

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.

Epstein-Barr virus and the origin of Hodgkin lymphoma

Although Epstein-Barr virus (EBV) is present in the malignant Hodgkin/Reed-Sternberg (HRS) cells of a proportion of cases of classical Hodgkin lymphoma (cHL), how the virus contributes to the pathogenesis of this disease remains poorly defined. It is clear from the studies of other EBV-associated cancers that the virus is usually not sufficient for tumor development and that other oncogenic co-factors are required. This article reviews what is known about the contribution of EBV to the pathogenesis of cHL and focuses on emerging evidence implicating chronic inflammation as a potential oncogenic co-factor in this malignancy.

Dasatinib therapy results in decreased B cell proliferation, splenomegaly, and tumor growth in a murine model of lymphoma expressing Myc and Epstein-Barr virus LMP2A

Epstein-Barr virus (EBV) infection and latency has been associated with malignant diseases including nasopharyngeal carcinoma, Hodgkin lymphoma, Burkitt lymphoma, and immune deficiency associated lymphoproliferative diseases. EBV-encoded latent membrane protein 2A (LMP2A) recruits Lyn and Syk kinases via its SH2-domain binding motifs, and modifies their signaling pathways. LMP2A transgenic mice develop hyperproliferative bone marrow B cells and immature peripheral B cells through modulation of Lyn kinase signaling. LMP2A/λ-MYC double transgenic mice develop splenomegaly and cervical lymphomas starting at 8 weeks of age. We reasoned that targeting Lyn in LMP2A-expressing B cells with dasatinib would provide a therapeutic option for EBV-associated malignancies. Here, we show that dasatinib inhibits B cell colony formation by LMP2A transgenic bone marrow cells, and reverses splenomegaly and tumor growth in both a pre-tumor and a syngeneic tumor transfer model of EBV-associated Burkitt lymphoma. Our data support the idea that dasatinib may prove to be an effective therapeutic molecule for the treatment of EBV-associated malignancies.

Epstein-Barr virus LMP2A signaling in statu nascendi mimics a B cell antigen receptor-like activation signal

Background

The latent membrane protein (LMP) 2A of Epstein-Barr virus (EBV) is expressed during different latency stages of EBV-infected B cells in which it triggers activation of cytoplasmic protein tyrosine kinases. Early studies revealed that an immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic N-terminus of LMP2A can trigger a transient increase of the cytosolic Ca²⁺ concentration similar to that observed in antigen-activated B cells when expressed as a chimeric transmembrane receptor. Even so, LMP2A was subsequently ascribed an inhibitory rather than an activating function because its expression seemed to partially inhibit B cell antigen receptor (BCR) signaling in EBV-transformed B cell lines. However, the analysis of LMP2A signaling has been hampered by the lack of cellular model systems in which LMP2A can be studied without the influence of other EBV-encoded factors.

Results

We have reanalyzed LMP2A signaling using B cells in which LMP2A is expressed in an inducible manner in the absence of any other EBV signaling protein. This allowed us for the first time to monitor LMP2A signaling in statu nascendi as it occurs during the EBV life cycle in vivo. We show that mere expression of LMP2A not only stimulated protein tyrosine kinases but also induced phospholipase C-γ2-mediated Ca²⁺ oscillations followed by activation of the extracellular signal-regulated kinase (Erk) mitogen-activated protein kinase pathway and induction of the lytic EBV gene bzlf1. Furthermore, expression of the constitutively phosphorylated LMP2A ITAM modulated rather than inhibited BCR-induced Ca²⁺ mobilization.

Conclusion

Our data establish that LMP2A expression has a function beyond the putative inhibition of the BCR by generating a ligand-independent cellular activation signal that may provide a molecular switch for different EBV life cycle stages and most probably contributes to EBV-associated lymphoproliferative disorders.

Epstein-Barr virus in Burkitt's lymphoma: a role for latent membrane protein 2A

Burkitt's lymphoma (BL) is characterized by translocation of the MYC gene to an immunoglobulin locus. Transgenic mouse models have been used to study the molecular changes that are necessary to bypass tumor suppression in the presence of translocated MYC. Inactivation of the p53 pathway is a major step to tumor formation in mouse models that is also seen in human disease. Human BL is often highly associated with Epstein-Barr virus (EBV). The EBV latency protein latent membrane protein 2A (LMP2A) is known to promote B cell survival by affecting levels of pro-survival factors. Using LMP2A transgenic mouse models, we have identified a novel mechanism that permits lymphomagenesis in the presence of an intact p53 pathway. This work uncovers a contribution of EBV to molecular events that have documented importance in BL pathogenesis, and may underlie the poorly understood link between EBV and BL.

The c-Cbl proto-oncoprotein downregulates EBV LMP2A signaling

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) plays a key role in regulating viral latency and EBV pathogenesis by functionally mimicking signals induced by the B-cell receptor (BCR) altering normal B cell development. As c-Cbl ubiquitin ligase (E3) is a critical negative regulator in the BCR signal pathway, the role of c-Cbl in the function and formation of the LMP2A signalosome was examined. c-Cbl promoted LMP2A degradation through ubiquitination, specifically degraded the Syk protein tyrosine kinase in the presence of LMP2A, and inhibited LMP2A induction of the EBV lytic cycle. Our earlier studies indicated that LMP2A-dependent Lyn degradation was mediated by Nedd4-family E3s in LMP2A expressing cells. Combine with these new findings, we propose a model in which c-Cbl and Nedd4-family E3s cooperate to degrade target proteins at discrete steps in the function of the LMP2A signalosome.

Epstein-Barr virus and virus human protein interaction maps

A comprehensive mapping of interactions among Epstein-Barr virus (EBV) proteins and interactions of EBV proteins with human proteins should provide specific hypotheses and a broad perspective on EBV strategies for replication and persistence. Interactions of EBV proteins with each other and with human proteins were assessed by using a stringent high-throughput yeast two-hybrid system. Overall, 43 interactions between EBV proteins and 173 interactions between EBV and human proteins were identified. EBV-EBV and EBV-human protein interaction, or "interactome" maps provided a framework for hypotheses of protein function. For example, LF2, an EBV protein of unknown function interacted with the EBV immediate early R transactivator (Rta) and was found to inhibit Rta transactivation. From a broader perspective, EBV genes can be divided into two evolutionary classes, "core" genes, which are conserved across all herpesviruses and subfamily specific, or "noncore" genes. Our EBV-EBV interactome map is enriched for interactions among proteins in the same evolutionary class. Furthermore, human proteins targeted by EBV proteins were enriched for highly connected or "hub" proteins and for proteins with relatively short paths to all other proteins in the human interactome network. Targeting of hubs might be an efficient mechanism for EBV reorganization of cellular processes.

The Shb signalling scaffold binds to and regulates constitutive signals from the Epstein–Barr virus LMP2A membrane protein

The Epstein-Barr virus latency-associated membrane protein LMP2A has been shown to activate the survival kinase Akt in epithelial and B cells in a phosphoinositide 3-kinase-dependent fashion. In this study, we demonstrate that the signalling scaffold Shb associates through SH2 and PTB domain interactions with phosphorylated tyrosine motifs in the LMP2A N-terminal tail. Additionally, we show that mutation of tyrosines in these motifs as well as shRNA-mediated downregulation of Shb leads to a loss of constitutive Akt-activation in LMP2A-expressing cells. Furthermore, utilization by Shb of the LMP2A ITAM motif regulates stability of the Syk tyrosine kinase in LMP2A-expressing cells. Our data set the precedent for viral utilization of the Shb signalling scaffold and implicate Shb as a regulator of LMP2A-dependent Akt activation.

Influence of Epstein-Barr virus latent membrane protein 2A on the proliferation of human gastric cancer cells

AIM: To construct recombinant adenovirus expression vector containing Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) and investigate the effects of LMP2A on the proliferation of human gastric cancer cell line.

METHODS: The target gene LMP2A was amplified by reverse transcription-polymerase chain reaction (RT-PCR), and the recombinant adenovirus vector carrying LMP2A gene was constructed with AdEasy system. The recombinant adenovirus vector pAd-2A was transfected into HEK293 cells with lipofectamine 2000 to package recombinant adenovirus, and then gastric cancer cells SGC with negative EBV were infected by recombinant adenovirus. The expression of LMP2A and its effect on the proliferation of SGC cells were analyzed by PT-PCR, MTT assay, fluorescence activated cell sorting (FACS) and confocal microscopy.

RESULTS: The recombinant adenovirus vector pAd-2A was successfully constructed and identified by PCR, restrictive enzyme analysis, and DNA sequencing. The recombinant adenovirus vAd-2A packaged by HEK293 cells had stable infectivity. The virus titer was 3.16×10¹² pfu/L. MTT assay, FACS and confocal microscopy showed that vAd-2A transfection significantly increased the proliferation of SGC cells (P < 0.01). The ratio of S-phase cells was markedly elevated after vAd-2A transfection in comparison with that after vAd transfection and non-transfection (24 h: 35.2% ± 5.1% vs 14.0% ± 3.4%, 13.2% ± 4.6%, P < 0.01; 48 h: 25.6% ± 4.1% vs 12.9% ± 2.6%, 12.5% ± 3.2%, P < 0.01). The expression of Cyclin E protein was also induced by LMP2A.

CONCLUSION: The recombinant adenovirus vector carrying EBV LMP2A gene is constructed successfully. Meanwhile, it is confirmed that LMP2A may promote the proliferation of SGC cells by induction of Cyclin E expression.

Epstein-barr virus latent membrane protein 2B (LMP2B) modulates LMP2A activity

Latent membrane protein 2A (LMP2A) and LMP2B are viral proteins 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 (BCR) signal transduction by associating with the cellular tyrosine kinases Lyn and Syk via specific phosphotyrosine motifs found within the LMP2A N-terminal tail domain. LMP2A has been shown to alter normal BCR signal transduction in B cells by reducing levels of Lyn and by blocking tyrosine phosphorylation and calcium mobilization following BCR cross-linking. Although little is currently known about the function of LMP2B in B cells, the similarity in structure between LMP2A and LMP2B suggests that they may localize to the same cellular compartments. To investigate the function of LMP2B, B-cell lines expressing LMP2A, LMP2B, LMP2A/LMP2B, and the relevant vector controls were analyzed. As was previously shown, cells expressing LMP2A had a dramatic block in normal BCR signal transduction as measured by calcium mobilization and tyrosine phosphorylation. There was no effect on BCR signal transduction in cells expressing LMP2B. Interestingly, when LMP2B was expressed in conjunction with LMP2A, there was a restoration of normal BCR signal transduction upon BCR cross-linking. The expression of LMP2B did not alter the cellular localization of LMP2A but did bind to and prevent the phosphorylation of LMP2A. A restoration of Lyn levels, but not a change in LMP2A levels, was also observed in cells coexpressing LMP2B with LMP2A. From these results, we conclude that LMP2B modulates LMP2A activity.

Down-regulation of proteolytic complexes following EBV activation in BL cells

In Burkitt's lymphoma cells, Epstein Barr virus (EBV) latency products interact with the ubiquitin-proteasome system to promote episomal maintenance and immunological evasion while the tripeptidylpeptidase II (TPPII) functions as an alternative protease. In the present study, we have examined the activities and levels of the proteasome and TPPII complex in Raji and in Akata cells after induction of EBV lytic cycle. The results show that the chymotrypsin-like and caspase-like activities of the proteasome were substantially reduced in Raji and Akata cells. Similarly, TPPII activity was diminished in both cell lines but was recovered in Akata cells at longer time after induction. Protein levels of the alpha/beta subunits of the 20S proteasome and TPPII concentration decreased to different extents after EBV activation, whereas the ubiquitin binding S6' subunit of the 19S regulatory complex increased three to fourfold along with the levels of ubiquitin-conjugates. Collectively, these observations demonstrate impairment of two major cellular proteolytic systems at the onset of EBV lytic infection.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.

Human SLP-65 isoforms contribute differently to activation and apoptosis of B lymphocytes

The SH2 domain-containing leukocyte adaptor protein of 65 kDa (SLP-65) is the key effector for signaling downstream of the B-cell antigen receptor (BCR). SLP-65 controls not only B lymphopoiesis and humoral immunity but also possesses a yet poorly defined tumor suppressor activity that is lost in many cases of acute lymphoblastic leukemia. We found that the 2 isoforms of human SLP-65 are differentially involved in positive and negative B-cell signaling. Reconstitution experiments revealed that an atypical SH3 domain-binding motif, which is present in the long but not in the short SLP-65 isoform, mediates association to Grb2 and suppresses activation of mitogen-activated protein kinases p38 and JNK as well as up-regulation of c-Fos expression. In turn, the short isoform activates not only AP1-driven but also NF-kappaB-driven gene transcription more potently than the long isoform. Conversely, the long rather than the short SLP-65 isoform promotes BCR-induced B-cell apoptosis. Our data further delineate the structural requirements of positive and negative SLP-65 signal transduction in normal and neoplastic cells.

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.

Pathogenesis of Hodgkin's lymphoma

In Hodgkin's lymphoma (HL), the B cell origin of the tumour cells, the Hodgkin and Reed-Sternberg (HRS) cells, has been disclosed by molecular single cell analysis about 10 yr ago. This finding formed the basis for various studies aimed to better understand the pathogenesis of this peculiar malignancy and the pathophysiology of the HRS cells. Work of our groups in this regard was focussed recently on two main topics, namely the study of differential gene expression in HRS cells and the pathogenesis of composite lymphomas. Composite lymphomas are combinations of HL and B cell non-Hodgkin lymphomas, that turned out to be often clonally related. By molecular analysis of several composite lymphomas for potential transforming events, we identified examples of both shared as well as distinct transforming events. Comparing gene expression profiles of HL-derived cell lines with the corresponding profiles from other B cell lymphomas and normal B cell subsets revealed a global down-regulation of the B cell-specific gene expression signature in HRS cells. Moreover, we identifed aberrant expression and activity of multiple receptor tyrosine kinases in HRS cells of classical and to a lesser extend lymphocyte predominant HL, which appears to be a unique feature of HL, and may offer novel strategies for treatment.

Epstein-Barr virus LMP2A alters in vivo and in vitro models of B-cell anergy, but not deletion, in response to autoantigen

A significant percentage of the population latently harbors Epstein-Barr virus (EBV) in B cells. One EBV-encoded protein, latent membrane protein 2A (LMP2A), is expressed in tissue culture models of EBV latent infection, in human infections, and in many of the EBV-associated proliferative disorders. LMP2A constitutively activates proteins involved in the B-cell receptor (BCR) signal transduction cascade and inhibits the antigen-induced activation of these proteins. In the present study, we investigated whether LMP2A alters B-cell receptor signaling in primary B cells in vivo and in vitro. LMP2A does not inhibit antigen-induced tolerance in response to strong stimuli in an in vivo tolerance model in which B cells are reactive to self-antigen. In contrast, LMP2A bypasses anergy induction in response to low levels of soluble hen egg lysozyme (HEL) both in vivo and in vitro as determined by the ability of LMP2A-expressing HEL-specific B cells to proliferate and induce NF-kappaB nuclear translocation after exposure to low levels of antigen. Furthermore, LMP2A induces NF-kappaB nuclear translocation independent of BCR cross-linking. Since NF-kappaB is required to bypass tolerance induction, this LMP2A-dependent NF-kappaB activation may complete the tolerogenic signal induced by low levels of soluble HEL. Overall, the findings suggest that LMP2A may not inhibit BCR-induced signals under all conditions as previously suggested by studies with EBV immortalized B cells.

Mechanisms of B-cell lymphoma pathogenesis

Chromosomal translocations involving the immunoglobulin loci are a hallmark of many types of B-cell lymphoma. Other factors, however, also have important roles in the pathogenesis of B-cell malignancies. Most B-cell lymphomas depend on the expression of a B-cell receptor (BCR) for survival, and in several B-cell malignancies antigen activation of lymphoma cells through BCR signalling seems to be an important factor for lymphoma pathogenesis. Recent insights into the lymphomagenic role of factors supplied by the microenvironment also offer new therapeutic strategies.

Epstein-Barr virus (EBV) LMP2A mediates B-lymphocyte survival through constitutive activation of the Ras/PI3K/Akt pathway

Epstein-Barr virus (EBV) establishes a lifelong latent infection in host B cells and is associated with the development of a variety of malignancies. The viral LMP2A protein mediates viral latency by mimicking a constitutively activated B-cell receptor (BCR). In vivo LMP2A provides developmental and survival signals to BCR-negative B cells, allowing them to survive in peripheral lymphoid organs. In this study, we have demonstrated that Ras is constitutively active in peripheral, BCR-negative B cells from LMP2A transgenic mice. Furthermore, increased expression of activated Ras correlated with elevated levels of Bcl-xL expression and a slower migrating, band-shifted form of Bcl-2. B cells from LMP2A transgenic mice were sensitive to apoptosis induction in the presence of specific inhibitors of Ras, phosphatidylinositol 3-kinase (PI3K), and Akt, indicating that LMP2A activates the Ras/PI3K/Akt pathway to mediate B-cell survival. Increased B-cell apoptosis correlated with reduced expression of Bcl-xL, suggesting that this Bcl-2 family member may be involved in apoptosis inhibition mediated by LMP2A. The ability of LMP2A to activate constitutively the Ras pathway, a common event during tumorigenesis, suggests that this viral protein plays an active role in the development of EBV-associated malignancies.

LMP2A does not require palmitoylation to localize to buoyant complexes or for function

Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) is expressed constitutively in lipid rafts in latently infected B lymphocytes. Lipid rafts are membrane microdomains enriched in cholesterol and sphingolipids selective for specific protein association. Lipid rafts have been shown to be necessary for B-cell receptor (BCR) signal transduction. LMP2A prevents BCR recruitment to lipid rafts, thereby abrogating BCR function. As LMP2A is palmitoylated, whether this fatty acid modification is necessary for LMP2A to localize to lipid rafts and for protein function was investigated. LMP2A palmitoylation was confirmed in latently infected B cells. LMP2A was found to be palmitoylated on multiple cysteines only by S acylation. An LMP2A mutant that was not palmitoylated was identified and functioned similar to wild-type LMP2A; unmodified LMP2A localized to lipid rafts, was tyrosine phosphorylated, was associated with LMP2A-associated proteins, was ubiquitinated, and was able to block calcium mobilization following BCR cross-linking. Therefore, palmitoylation of LMP2A is not required for LMP2A targeting to buoyant complexes or for function.

Latent membrane protein 2A, a viral B cell receptor homologue, induces CD5+ B-1 cell development

The latent membrane protein 2A (LMP2A) of EBV plays a key role in regulating viral latency and EBV pathogenesis by functionally mimicking a constitutively active B cell Ag receptor. When expressed as a B cell-specific transgene in mice, LMP2A drives B cell development, resulting in the bypass of normal developmental checkpoints. In this study, we have demonstrated that expression of LMP2A in transgenic mice results in B cell development that exclusively favors B-1 cells. This switch to B-1 cell development occurs at the pre-B-cell stage of normal B cell development in the bone marrow, a B cell stage much earlier than appreciated for B-1 commitment. This finding indicates that all pre-B cells have the capacity to assume a B-1 cell phenotype if they encounter the appropriate signal during normal development. Furthermore, these studies offer insight into EBV latency and pathogenesis in the human host.

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.

Epstein-Barr virus (EBV) LMP2A alters normal transcriptional regulation following B-cell receptor activation

The latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) is an important mediator of viral latency in infected B-lymphocytes. LMP2A inhibits B-cell receptor (BCR) signaling in vitro and allows for the survival of BCR-negative B cells in vivo. In this study, we compared gene transcription in BCR-activated B cells from non-transgenic and LMP2A Tg6 transgenic mice. We found that the transcriptional induction and down-regulation of many genes that normally occurs in B cells following BCR activation did not occur in B cells from LMP2A Tg6 transgenic mice. Furthermore, LMP2A induced the expression of various transcription factors and genes associated with DNA/RNA metabolism, which may allow for the altered transcriptional regulation observed in BCR-activated B cells from LMP2A Tg6 mice. These results suggest that LMP2A may inhibit the downstream effects of BCR signaling by directly or indirectly altering gene transcription to ensure EBV persistence in infected B cells.

Epstein–Barr virus oncogenesis and the ubiquitin–proteasome system

Epstein-Barr virus (EBV), a human herpesvirus associated with lymphoid and epithelial cell tumors, encodes several proteins that exploit the ubiquitin-proteasome system to regulate latency and allow the persistence of infected cells in immunocompetent hosts. Further modifications of ubiquitin-dependent proteolysis by activated cellular oncogenes contribute to malignant transformation. A detailed understanding of these processes may lead to the development of new therapeutic strategies for EBV-associated cancers.

B cell receptor signal strength determines B cell fate

B cell receptor (BCR)-mediated antigen recognition is thought to regulate B cell differentiation. BCR signal strength may also influence B cell fate decisions. Here, we used the Epstein-Barr virus protein LMP2A as a constitutively active BCR surrogate to study the contribution of BCR signal strength in B cell differentiation. Mice carrying a targeted replacement of Igh by LMP2A leading to high or low expression of the LMP2A protein developed B-1 or follicular and marginal zone B cells, respectively. These data indicate that BCR signal strength, rather than antigen specificity, determines mature B cell fate. Furthermore, spontaneous germinal centers developed in gut-associated lymphoid tissue of LMP2A mice, indicating that microbial antigens can promote germinal centers independently of BCR-mediated antigen recognition.

Epstein-Barr Virus (EBV) LMP2A induces alterations in gene transcription similar to those observed in Reed-Sternberg cells of Hodgkin lymphoma

Epstein-Barr virus (EBV) is associated with the development of a variety of malignancies, including Hodgkin lymphoma. One of the few viral transcripts expressed in EBV-positive Hodgkin/Reed-Sternberg (HRS) cells of Hodgkin lymphoma is latent membrane protein 2A (LMP2A). This viral protein blocks B-cell receptor (BCR)-signaling in vitro. Furthermore, expression of LMP2A in developing B cells in vivo induces a global down-regulation of genes necessary for proper B-cell development. In this study we have analyzed gene transcription in primary B cells from LMP2A transgenic mice, LMP2A-expressing human B-cell lines, and LMP2A-positive and -negative EBV-infected lymphoblastoid cell lines (LCLs). We demonstrate that LMP2A increases the expression of genes associated with cell cycle induction and inhibition of apoptosis, alters the expression of genes involved in DNA and RNA metabolism, and decreases the expression of B-cell-specific factors and genes associated with immunity. Furthermore, many alterations in gene expression induced by LMP2A are similar to those recently described in HRS cells of Hodgkin lymphoma and activated, proliferating germinal center centroblasts/centrocytes. These correlations suggest that LMP2A expression in EBV-infected B cells may lead to the induction and maintenance of an activated, proliferative state that could ultimately result in the development of Hodgkin lymphoma.

B cells under influence: transformation of B cells by Epstein-Barr virus

Epstein-Barr virus (EBV) is an extremely successful virus, infecting more than 90% of the human population worldwide. After primary infection, the virus persists for the life of the host, usually as a harmless passenger residing in B cells. However, EBV can transform B cells, which can result in the development of malignant lymphomas. Intriguingly, the three main types of EBV-associated B-cell lymphoma - that is, Burkitt lymphoma, Hodgkin lymphoma and post-transplant lymphomas - seem to derive from germinal-centre B cells or atypical survivors of the germinal-centre reaction in most, if not all, cases, indicating that EBV-infected germinal-centre B cells are at particular risk for malignant transformation.

Identification of gene products suppressed by human immunodeficiency virus type 1 infection or gp120 exposure of primary human astrocytes by rapid subtraction hybridization

Neurodegeneration and human immunodeficiency virus type 1 (HIV-1)-associated dementia (HAD) are the major disease manifestations of HIV-1 colonization of the central nervous system (CNS). In the brain, HIV-1 replicates in microglial cells and infiltrating macrophages and it persists in a low-productive, noncytolytic state in astrocytes. Astrocytes play critical roles in the maintenance of the brain microenvironment, responses to injury, and in neuronal signal transmission, and disruption of these functions by HIV-1 could contribute to HAD. To better understand the potential effects of HIV-1 on astrocyte biology, the authors investigated changes in gene expression using an efficient and sensitive rapid subtraction hybridization approach, RaSH. Primary human astrocytes were isolated from abortus brain tissue, low-passage cells were infected with HIV-1 or mock infected, and total cellular RNAs were isolated at multiple time points over a period of 1 week. This approach is designed to identify gene products modulated early and late after HIV-1 infection and limits the cloning of genes displaying normal cell-cycle fluctuations in astrocytes. By subtracting temporal cDNAs derived from HIV-1-infected astrocytes from temporal cDNAs made from uninfected cells, 10 genes displaying reduced expression in infected cells, termed astrocyte suppressed genes (ASGs), were identified and their suppression was confirmed by Northern blot hybridization. Both known and novel ASGs, not reported in current DNA databases, that are down-regulated by HIV-1 infection are described. Northern blotting confirms suppression of the same panel of ASGs by treatment of astrocytes with recombinant HIV-1 envelope glycoprotein, gp120. These results extend our previous analysis of astrocyte genes induced or enhanced by HIV-1 infection and together they suggest that HIV-1 and viral proteins have profound effects on astrocyte physiology, which may influence their function in the CNS.

Itchy, a Nedd4 ubiquitin ligase, downregulates latent membrane protein 2A activity in B-cell signaling

Nedd4 family ubiquitin protein ligases (E3s) specifically associate with latent membrane protein 2A (LMP2A) of Epstein-Barr virus. Our previous studies analyzing LMP2A function in vitro have suggested that Nedd4 family E3s regulate LMP2A function. To determine the role of Nedd4 family E3s in LMP2A B-cell signaling, LMP2A transgenic (LMP2A(+)) mice were crossed with mice with the Itch-deficient (Itch(-/-)) background. Itchy, a mouse homologue of human AIP4, is a Nedd4 family E3 and is also the most abundant Nedd4 family E3 found in LMP2A affinity precipitates from B cells. There were significantly fewer B-cell receptor-positive B cells in spleen and bone marrow B cells in LMP2A(+) Itch(-/-) mice than in LMP2A(+) mice. In addition, LMP2A(+) Itch(-/-) bone marrow B cells formed larger colonies in cultures treated with interleukin-7 (IL-7) than control bone marrow B cells did. Finally, there was a dramatic increase in tyrosine phosphorylation of LMP2A and Syk in IL-7-cultured LMP2A(+) Itch(-/-) B cells. These results indicate that Nedd4 family E3s, in particular Itchy, downmodulate LMP2A activity in B-cell signaling.

The ubiquitin/proteasome system in Epstein-Barr virus latency and associated malignancies

Epstein-Barr virus (EBV) is a human herpesvirus associated with lymphoid and epithelial malignancies. Three viral proteins, the EBV nuclear antigen 1 and the latent membrane proteins-1 and -2A, regulate viral latency by manipulating ubiquitin-dependent proteolysis. The activation of cellular oncogenes is instrumental for the progression of EBV infected cells to full malignancy. Constitutively activation of c-myc correlates with decreased proteasomal activity and upregulation of compensatory proteolytic pathways in Burkitt's lymphomas. Knowledge of these multiple strategies of interference with regulated proteolysis may provide new clues for the treatment of EBV-associated malignancies.

Epstein-Barr virus LMP2A interferes with global transcription factor regulation when expressed during B-lymphocyte development

Epstein-Barr virus (EBV) is associated with the development of malignant lymphomas and lymphoproliferative disorders in immunocompromised individuals. The LMP2A protein of EBV is thought to play a central role in this process by allowing the virus to persist in latently infected B lymphocytes. We have demonstrated that LMP2A, when expressed in B cells of transgenic mice, allows normal B-cell developmental checkpoints to be bypassed. To identify cellular genes targeted by LMP2A that are involved in this process, we have utilized DNA microarrays to compare gene transcription in B cells from wild-type versus LMP2A transgenic mice. In B cells from LMP2A transgenic mice, we observed decreased expression of many genes associated with normal B-cell development as well as reduced levels of the transcription factors that regulate their expression. In particular, expression of the transcription factor E2A was down-regulated in bone marrow and splenic B cells. Furthermore, E2A activity was inhibited in these cells as determined by decreased DNA binding and reduced expression of its target genes, including the transcription factors early B-cell factor and Pax-5. Expression of two E2A inhibitors, Id2 and SCL, was up-regulated in splenic B cells expressing LMP2A, suggesting a possible mechanism for E2A inhibition. These results indicate that LMP2A deregulates transcription factor expression and activity in developing B cells, and this likely allows for a bypass of normal signaling events required for proper B-cell development. The ability of LMP2A to interfere with B-cell transcription factor regulation has important implications regarding its role in EBV latency.

Viruses and Hodgkin's lymphoma

Hodgkin's lymphoma (HL) is unusual among human malignancies in that the epidemiology suggests an infectious aetiology. The Epstein-Barr virus (EBV) is associated with a proportion of cases and this association is believed to be causal. In these cases the Hodgkin and Reed-Sternberg (HRS) cells express the EBV-encoded proteins LMP1 and LMP2, which can mimic CD40 and the B cell receptor, respectively, and therefore may play a critical role in facilitating the survival of HRS cells. EBV-associated and non-EBV-associated HL cases have different epidemiological features and recent data suggest that delayed exposure to EBV is a risk factor for the development of EBV-associated HL in young adults. We suggest that HL can be divided into four entities on the basis of EBV status and age at presentation, with three groups of EBV-associated cases and a single group of EBV-negative cases. The aetiology of the latter cases is obscure although involvement of an infectious agent(s) is suspected.

LMP-1's transmembrane domains encode multiple functions required for LMP-1's efficient signaling

The latent membrane protein-1 (LMP-1) of Epstein-Barr virus (EBV) contributes to the proliferation of infected B lymphocytes by signaling through its binding to cellular signaling molecules. It apparently mimics members of the tumor necrosis factor receptor family, in particular, CD40, by binding a similar set of cellular molecules as does CD40. LMP-1 differs dramatically in its structure from CD40. LMP-1 has six membrane-spanning domains as opposed to CD40's one. LMP-1 also differs from CD40 in its apparent independence of a ligand for its signaling. We have examined the role of LMP-1's membrane-spanning domains in its signaling. Their substitution with six membrane-spanning domains from the LMP-2A protein of EBV yields a derivative which neither coimmunoprecipitates with LMP-1 nor signals to increase the activity of NF-kappaB as does wild-type LMP-1. These observations indicate that LMP-1 has specific sequences in its membrane-spanning domains required for these activities. LMP-1's first and sixth membrane-spanning domains have multiple leucine residues potentially similar to leucine-heptad motifs that can mediate protein-protein interactions in membranes (Gurezka et al., J. Biol. Chem. 274:9265-9270, 1999). Substitution of seven leucines in LMP-1's sixth membrane-spanning domain has no effect on its function, whereas similar substitutions in its first membrane-spanning domain yielded a derivative which aggregates as does wild-type LMP-1 but has only 3% of wild-type's ability to signal through NF-kappaB. Importantly, this derivative complements a mutant of LMP-1 with wild-type membrane-spanning domains but no carboxy-terminal signaling domain. These findings together indicate that the membrane-spanning domains of LMP-1 contribute multiple functions to its signaling.

Molecular mechanisms of B-lymphocyte transformation by Epstein–Barr virus

The B-lymphotropic virus Epstein-Barr virus (EBV) has been implicated in the pathogenesis of B-cell malignancies, particularly in immunodeficient individuals. This review provides a brief overview of the EBV-encoded proteins involved in B-cell transformation, and the current state of knowledge about their roles in this process.

The effects of the Epstein-Barr virus latent membrane protein 2A on B cell function

Epstein-Barr Virus (EBV) infects B-lymphocytes circulating through the oral epithelium and establishes a lifelong latent infection in a subset of mature-memory B cells. In these latently infected B cells, EBV exhibits limited gene expression with the latent membrane protein 2A (LMP2A) being the most consistently detected transcript. This persistent expression, coupled with many studies ofthe function of LMP2A in vitro and invivo, indicates that LMP2A is functioning to control some aspect of viral latency. Establishment and maintenance of viral latency requires exquisite manipulation of normal B cell signaling and function. LMP2A is capable of blocking normal B cell signal transduction in vitro, suggesting that LMP2A may act to regulate lytic activation from latency in vivo. Furthermore, LMP2A is capable of providing B cells with survival signals in the absence of normal BCR signaling. These data show that LMP2A may help EBV-infected cells to persist in vivo. This review discusses the advances that have been made in our understanding of LMP2A and the effects it has on B cell development, activation, and viral latency.