Epstein-Barr virus latent membrane protein 2A mimics B-cell receptor-dependent virus reactivation

European Association of Neuro-Oncology (EANO) guidelines for treatment of primary central nervous system lymphoma (PCNSL)

The management of primary central nervous system (PCNSL) is one of the most controversial topics in neuro-oncology because of the complexity of the disease and the limited number of controlled studies available. In 2021, given recent advances and the publication of practice-changing randomized trials, the European Association of Neuro-Oncology (EANO) created a multidisciplinary task force to update the previously published evidence-based guidelines for immunocompetent adult patients with PCNSL and added a section on immunosuppressed patients. The guideline provides consensus considerations and recommendations for the treatment of PCNSL, including intraocular manifestations and specific management of the elderly. The main changes from the previous guideline include strengthened evidence for the consolidation with ASCT in first-line treatment, prospectively assessed chemotherapy combinations for both young and elderly patients, clarification of the role of rituximab even though the data remain inconclusive, of the role of new agents, and the incorporation of immunosuppressed patients and primary ocular lymphoma. The guideline should aid the clinicians in everyday practice and decision making and serve as a basis for future research in the field.

Calcium-dependent signalling in B-cell lymphomas

Induced waves of calcium fluxes initiate multiple signalling pathways that play an important role in the differentiation and maturation of B-cells. Finely tuned transient Ca⁺² fluxes from the endoplasmic reticulum in response to B-cell receptor (BCR) or chemokine receptor activation are followed by more sustained calcium influxes from the extracellular environment and contribute to the mechanisms responsible for the proliferation of B-cells, their migration within lymphoid organs and their differentiation. Dysregulation of these well-balanced mechanisms in B-cell lymphomas results in uncontrolled cell proliferation and resistance to apoptosis. Consequently, several cytotoxic drugs (and anti-proliferative compounds) used in standard chemotherapy regimens for the treatment of people with lymphoma target calcium-dependent pathways. Furthermore, ~10% of lymphoma associated mutations are found in genes with functions in calcium-dependent signalling, including those affecting B-cell receptor signalling pathways. In this review, we provide an overview of the Ca²⁺-dependent signalling network and outline the contribution of its key components to B cell lymphomagenesis. We also consider how the oncogenic Epstein-Barr virus, which is causally linked to the pathogenesis of a number of B-cell lymphomas, can modify Ca²⁺-dependent signalling.

Spleen Tyrosine Kinase Inhibitor TAK-659 Prevents Splenomegaly and Tumor Development in a Murine Model of Epstein-Barr Virus-Associated Lymphoma

The novel SYK and FLT3 inhibitor TAK-659 prevents the enlargement of spleen and tumor development in a mouse model of EBV-associated lymphoma by counteracting the activation of cellular kinase SYK through the viral LMP2A gene by inducing cell death in tumor cells but not in nontumor cells. These findings indicate that TAK-659 may be a very effective nontoxic therapeutic molecule especially for EBV-positive hematologic malignancies.

Epstein-Barr virus (EBV) is associated with several B and epithelial cell cancers. EBV-encoded latent membrane protein 2A (LMP2A) contributes to cellular transformation by mimicking B cell receptor signaling. LMP2A/MYC double transgenic mice develop splenomegaly and B cell lymphoma much faster than MYC transgenic mice do. In this study, we explored the potential therapeutic efficacy of a novel spleen tyrosine kinase (SYK) and FLT3 inhibitor TAK-659 for development of a treatment option for EBV-associated malignancies. In our transgenic model, TAK-659 treatment totally abrogated splenomegaly and tumor development in LMP2A/MYC mice in both pretumor and tumor cell transfer experiments. TAK-659 treatment killed tumor cells, but not host cells within the spleen and tumors. Furthermore, TAK-659 treatment abrogated metastasis of tumor cells into bone marrow. Our data also show that TAK-659 inhibits SYK phosphorylation and induces apoptosis in LMP2A/MYC tumor cells at low nanomolar concentrations. Therefore, TAK-659 may provide an effective therapeutic option for treatment of LMP2A-positive EBV-associated malignancies and should be explored further in clinical trials.

IMPORTANCE The novel SYK and FLT3 inhibitor TAK-659 prevents the enlargement of spleen and tumor development in a mouse model of EBV-associated lymphoma by counteracting the activation of cellular kinase SYK through the viral LMP2A gene by inducing cell death in tumor cells but not in nontumor cells. These findings indicate that TAK-659 may be a very effective nontoxic therapeutic molecule especially for EBV-positive hematologic malignancies.

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.

Latent Membrane Protein 1 (LMP1) and LMP2A Collaborate To Promote Epstein-Barr Virus-Induced B Cell Lymphomas in a Cord Blood-Humanized Mouse Model but Are Not Essential

Epstein-Barr virus (EBV) infection is associated with B cell lymphomas in humans. The ability of EBV to convert human B cells into long-lived lymphoblastoid cell lines (LCLs) in vitro requires the collaborative effects of EBNA2 (which hijacks Notch signaling), latent membrane protein 1 (LMP1) (which mimics CD40 signaling), and EBV-encoded nuclear antigen 3A (EBNA3A) and EBNA3C (which inhibit oncogene-induced senescence and apoptosis). However, we recently showed that an LMP1-deleted EBV mutant induces B cell lymphomas in a newly developed cord blood-humanized mouse model that allows EBV-infected B cells to interact with CD4 T cells (the major source of CD40 ligand). Here we examined whether the EBV LMP2A protein, which mimics constitutively active B cell receptor signaling, is required for EBV-induced lymphomas in this model. We find that the deletion of LMP2A delays the onset of EBV-induced lymphomas but does not affect the tumor phenotype or the number of tumors. The simultaneous deletion of both LMP1 and LMP2A results in fewer tumors and a further delay in tumor onset. Nevertheless, the LMP1/LMP2A double mutant induces lymphomas in approximately half of the infected animals. These results indicate that neither LMP1 nor LMP2A is absolutely essential for the ability of EBV to induce B cell lymphomas in the cord blood-humanized mouse model, although the simultaneous loss of both LMP1 and LMP2A decreases the proportion of animals developing tumors and increases the time to tumor onset. Thus, the expression of either LMP1 or LMP2A may be sufficient to promote early-onset EBV-induced tumors in this model.IMPORTANCE EBV causes human lymphomas, but few models are available for dissecting how EBV causes lymphomas in vivo in the context of a host immune response. We recently used a newly developed cord blood-humanized mouse model to show that EBV can cooperate with human CD4 T cells to cause B cell lymphomas even when a major viral transforming protein, LMP1, is deleted. Here we examined whether the EBV protein LMP2A, which mimics B cell receptor signaling, is required for EBV-induced lymphomas in this model. We find that the deletion of LMP2A alone has little effect on the ability of EBV to cause lymphomas but delays tumor onset. The deletion of both LMP1 and LMP2A results in a smaller number of lymphomas in infected animals, with an even more delayed time to tumor onset. These results suggest that LMP1 and LMP2A collaborate to promote early-onset lymphomas in this model, but neither protein is absolutely essential.

Latent Membrane Protein LMP2A Impairs Recognition of EBV-Infected Cells by CD8+ T Cells

The common pathogen Epstein-Barr virus (EBV) transforms normal human B cells and can cause cancer. Latent membrane protein 2A (LMP2A) of EBV supports activation and proliferation of infected B cells and is expressed in many types of EBV-associated cancer. It is not clear how latent EBV infection and cancer escape elimination by host immunity, and it is unknown whether LMP2A can influence the interaction of EBV-infected cells with the immune system. We infected primary B cells with EBV deleted for LMP2A, and established lymphoblastoid cell lines (LCLs). We found that CD8+ T cell clones showed higher reactivity against LMP2A-deficient LCLs compared to LCLs infected with complete EBV. We identified several potential mediators of this immunomodulatory effect. In the absence of LMP2A, expression of some EBV latent antigens was elevated, and cell surface expression of MHC class I was marginally increased. LMP2A-deficient LCLs produced lower amounts of IL-10, although this did not directly affect CD8+ T cell recognition. Deletion of LMP2A led to several changes in the cell surface immunophenotype of LCLs. Specifically, the agonistic NKG2D ligands MICA and ULBP4 were increased. Blocking experiments showed that NKG2D activation contributed to LCL recognition by CD8+ T cell clones. Our results demonstrate that LMP2A reduces the reactivity of CD8+ T cells against EBV-infected cells, and we identify several relevant mechanisms.

Epstein-Barr virus (EBV) is carried by most humans. It can cause several types of cancer. In healthy infected people, EBV persists for life in a "latent" state in white blood cells called B cells. For infected persons to remain healthy, it is crucial that they harbor CD8-positive "killer" T cells that recognize and destroy precancerous EBV-infected cells. However, this protection is imperfect, because the virus is not eliminated from the body, and the danger of EBV-associated cancer remains. How does the virus counteract CD8+ T cell control? Here we study the effects of latent membrane protein 2A (LMP2A), which is an important viral molecule because it is present in several types of EBV-associated cancers, and in latently infected cells in healthy people. We show that LMP2A counteracts the recognition of EBV-infected B cells by antiviral killer cells. We found a number of mechanisms that are relevant to this effect. Notably, LMP2A disturbs expression of molecules on B cells that interact with NKG2D, a molecule on the surface of CD8+ T cells that aids their activation. In this way, LMP2A weakens important immune responses against EBV. Similar mechanisms may operate in different types of LMP2A-expressing cancers caused by EBV.

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.

Epstein-Barr virus, the germinal centre and the development of Hodgkin's lymphoma

The relationship between Epstein-Barr virus (EBV) and the germinal centre (GC) of the asymptomatic host remains an enigma. The occasional appearance of EBV-positive germinal centres in some patients, particularly those with a history of immunosuppression, suggests that EBV numbers in the GC are subject to immune control. The relationship, if any, between lymphoid hyperplasia with EBV-positive germinal centres and subsequent or concurrent lymphomagenesis remains to be clarified. As far as the development of EBV-associated Hodgkin's lymphoma is concerned, the suppression of virus replication, mediated by LMP1 on the one hand, and the loss of B-cell receptor signalling on the other, appears to be an important pathogenic mechanism. A further important emerging concept is that alterations in the microenvironment of the EBV-infected B-cell may be important for lymphomagenesis.

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.

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.

Molecular mimicry in systemic lupus erythematosus

Systemic lupus erythematosus is a multi-systemic autoimmune disease distinguished by the presence of various autoantibodies. Like most autoimmune diseases, systemic lupus erythematosus is believed to be induced by a combination of genetic, immunologic, and environmental factors, mainly infectious agents. Molecular mimicry between an infectious antigen and self-components is implicated as a pivotal mechanism by which autoimmune diseases such as systemic lupus erythematosus are triggered. Here we review the current evidence of molecular mimicry between different infectious agents and systemic lupus erythematosus.

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.

Latent membrane protein 2B regulates susceptibility to induction of lytic Epstein-Barr virus infection

The B-lymphotropic Epstein-Barr virus (EBV) encodes two isoforms of latent membrane protein 2 (LMP2), LMP2A and LMP2B, which are expressed during latency in B cells. The function of LMP2B is largely unknown, whereas LMP2A blocks B-cell receptor (BCR) signaling transduction and induction of lytic EBV infection, thereby promoting B-cell survival. Transfection experiments on LMP2B in EBV-negative B cells and the silencing of LMP2B in EBV-harboring Burkitt's lymphoma-derived Akata cells suggest that LMP2B interferes with the function of LMP2A, but the role of LMP2B in the presence of functional EBV has not been established. Here, LMP2B, LMP2A, or both were overexpressed in EBV-harboring Akata cells to study the function of LMP2B. The overexpression of LMP2B increased the magnitude of EBV switching from its latent to its lytic form upon BCR cross-linking, as indicated by a more-enhanced upregulation and expression of EBV lytic genes and significantly increased production of transforming EBV compared to Akata vector control cells or LMP2A-overexpressing cells. Moreover, LMP2B lowered the degree of BCR cross-linking required to induce lytic EBV infection. Finally, LMP2B colocalized with LMP2A as demonstrated by immunoprecipitation and immunofluorescence and restored calcium mobilization upon BCR cross-linking, a signaling process inhibited by LMP2A. Thus, our findings suggest that LMP2B negatively regulates the function of LMP2A in preventing the switch from latent to lytic EBV replication.

Autoantibodies in nonautoimmune individuals during infections

Infections can act as environmental triggers inducing or promoting autoimmune disease in genetically predisposed individuals. Identification of microbial peptides similar to self-tissues may by molecular mimicry, provide the inducing mechanism for an immune response. The aim of this study was to identify autoantibodies (autoAbs) in nonautoimmune individuals during acute bacterial, viral, or parasitic infections. Specific Abs or specific infections with an increased autoAb load may shed insight into the mechanisms of autoimmune disease. Sera from 88 patients with acute infections (41 bacterial, 23 viral, 17 parasitic, and 7 rickettsial) were tested by the ELISA method for antinuclear antibodies (ANA) 8 Pro, and Abs to thyroid peroxidase (TPO), thyroglobulin, phospholipids, annexin-V, laminin, anti-Saccharomyces cervisiae (ASCA), and prothrombin, along with 80 normal controls. Elevated titers of Abs to annexin-V and prothrombin were the most prevalent in viral, parasitic, and rickettsial infections and to laminin in viral and parasitic infections. Elevated titers of ASCA and ANA were found in viral and bacterial infections. Antiphospholipid Abs were found in parasitic and Q-fever infections. Thirty-four individuals harbored elevated titers of at least two Abs. An autoAb burden was detected in individuals with hepatitis A, hepatitis B, toxoplasma or Q-fever infections. In nonautoimmune individuals with various (bacterial, viral, parasitic, and rickettsial) infections, elevated titers of Abs to annexin-V, prothrombin, laminin, ASCA, ANA, and phospholipids were most frequently detected.

Molecular mimicry and auto-immunity

The term "molecular mimicry" was coined by R. Damian in 1964, who was first to suggest that antigenic determinants of micro-organisms may resemble antigenic determinants of their host. Damian suggested that this similarity served as a defense mechanism of a microorganism from the host's immune system and prevented the development of immune response to the micro-organism, thereby protecting it from host defense. Years later, the term "molecular mimicry" was attributed a different meaning-namely, antigenic determinants of microorganisms might elicit an auto-immune response that harms the host. The concept of molecular mimicry is based on a structural similarity between a pathogen or metabolite and self-structures. The similarity could be expressed as shared amino acid sequences (linear or mimotope) or similar conformational structure between a pathogen and self-antigen. "Molecular mimicry" has become a very popular explanation for the frequent association of infection with auto-immune disease.

Epstein-Barr virus latent membrane protein 2A is a B-cell receptor mimic and essential for B-cell survival

Many cells latently infected with Epstein-Barr virus (EBV), including certain virus-associated tumors, express latent membrane protein 2A (LMP2A), suggesting an important role for this protein in viral latency and oncogenesis. LMP2A mimics B-cell receptor signaling but can also act as a decoy receptor blocking B-cell receptor (BCR) activation. Studies of peripheral B cells have not resolved this apparent contradiction because LMP2A seems to be dispensable for EBV-induced transformation of these B cells in vitro. We show here that LMP2A is essential for growth transformation of germinal center B cells, which do not express the genuine BCR because of deleterious somatic hypermutations in their immunoglobulin genes. BCR-positive (BCR(+)) and BCR-negative (BCR(-)) B cells are readily transformed with a recombinant EBV encoding a conditional, floxed LMP2A allele, but the survival and continued proliferation of both BCR(+) and BCR(-) B cells is strictly dependent on LMP2A. These findings indicate that LMP2A has potent, distinct antiapoptotic and/or transforming characteristics and point to its role as an indispensable BCR mimic in certain B cells from which human B-cell tumors such as Hodgkin lymphoma originate.

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.

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.