Epstein-Barr virus latent infection membrane protein increases vimentin expression in human B-cell lines

Proteomic Profiling and Functional Analysis of B Cell-Derived Exosomes upon Pneumocystis Infection

Pneumocystis is a life-threatening fungal pathogen that frequently causes fatal pneumonia (PCP) in immunocompromised individuals. Recently, B cells have been reported to play a crucial role in the pathogenesis of PCP through producing antibodies and activating CD4+ T cell response. Exosomes are nanoscale small extracellular vesicles abundant with protein cargo and can mediate immune response during infectious disease. In this study, using tandem mass tag-based quantitative proteomics coupled with bioinformatic analysis, we attempted to characterize exosomes derived from B lymphocytes in response to PCP. Several proteins were verified by parallel reaction monitoring (PRM) analysis. Also, the effects of B cell exosomes on CD4+ T cell response and phagocytic function of macrophages were clarified. Briefly, 1701 proteins were identified from B cell exosomes, and the majority of them were reported in Vesiclepedia. A total of 51 differentially expressed proteins of B cell exosomes were found in response to PCP. They were mainly associated with immune response and transcription regulation. PRM analysis confirmed the significantly changed levels of histone H1.3, vimentin, and tyrosine-protein phosphatase nonreceptor type 6 (PTPN6). Moreover, a functional study revealed the proinflammatory profile of B cell exosomes on CD4+ T cell response in PCP. Taken together, our results suggest the involvement of exosomes derived from B cells in cell-to-cell communication, providing new information on the function of B cells in response to PCP.

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.

The tumor marker Fascin is induced by the Epstein-Barr virus-encoded oncoprotein LMP1 via NF-κB in lymphocytes and contributes to their invasive migration

Background

The actin-bundling protein Fascin (FSCN1) is a tumor marker that is highly expressed in numerous types of cancer including lymphomas and is important for migration and metastasis of tumor cells. Fascin has also been detected in B lymphocytes that are freshly-infected with Epstein-Barr virus (EBV), however, both the inducers and the mechanisms of Fascin upregulation are still unclear.

Results

Here we show that the EBV-encoded oncoprotein latent membrane protein 1 (LMP1), a potent regulator of cellular signaling and transformation, is sufficient to induce both Fascin mRNA and protein in lymphocytes. Fascin expression is mainly regulated by LMP1 via the C-terminal activation region 2 (CTAR2). Block of canonical NF-κB signaling using a chemical inhibitor of IκB kinase β (IKKβ) or cotransfection of a dominant-negative inhibitor of IκBα (NFKBIA) reduced not only expression of p100, a classical target of the canonical NF-κB-pathway, but also LMP1-induced Fascin expression. Furthermore, chemical inhibition of IKKβ reduced both Fascin mRNA and protein levels in EBV-transformed lymphoblastoid cell lines, indicating that canonical NF-κB signaling is required for LMP1-mediated regulation of Fascin both in transfected and transformed lymphocytes. Beyond that, chemical inhibition of IKKβ significantly reduced invasive migration of EBV-transformed lymphoblastoid cells through extracellular matrix. Transient transfection experiments revealed that Fascin contributed to LMP1-mediated enhancement of invasive migration through extracellular matrix. While LMP1 enhanced the number of invaded cells, functional knockdown of Fascin by two different small hairpin RNAs resulted in significant reduction of invaded, non-attached cells.

Conclusions

Thus, our data show that LMP1-mediated upregulation of Fascin depends on NF-κB and both NF-κB and Fascin contribute to invasive migration of LMP1-expressing lymphocytes.

Epstein-Barr virus LMP1 modulates lipid raft microdomains and the vimentin cytoskeleton for signal transduction and transformation

The Epstein-Barr virus (EBV) is an important human pathogen that is associated with multiple cancers. The major oncoprotein of the virus, latent membrane protein 1 (LMP1), is essential for EBV B-cell immortalization and is sufficient to transform rodent fibroblasts. This viral transmembrane protein activates multiple cellular signaling pathways by engaging critical effector molecules and thus acts as a ligand-independent growth factor receptor. LMP1 is thought to signal from internal lipid raft containing membranes; however, the mechanisms through which these events occur remain largely unknown. Lipid rafts are microdomains within membranes that are rich in cholesterol and sphingolipids. Lipid rafts act as organization centers for biological processes, including signal transduction, protein trafficking, and pathogen entry and egress. In this study, the recruitment of key signaling components to lipid raft microdomains by LMP1 was analyzed. LMP1 increased the localization of phosphatidylinositol 3-kinase (PI3K) and its activated downstream target, Akt, to lipid rafts. In addition, mass spectrometry analyses identified elevated vimentin in rafts isolated from LMP1 expressing NPC cells. Disruption of lipid rafts through cholesterol depletion inhibited PI3K localization to membranes and decreased both Akt and ERK activation. Reduction of vimentin levels or disruption of its organization also decreased LMP1-mediated Akt and ERK activation and inhibited transformation of rodent fibroblasts. These findings indicate that LMP1 reorganizes membrane and cytoskeleton microdomains to modulate signal transduction.

Herpesviruses and intermediate filaments: close encounters with the third type

Intermediate filaments (IF) are essential to maintain cellular and nuclear integrity and shape, to manage organelle distribution and motility, to control the trafficking and pH of intracellular vesicles, to prevent stress-induced cell death, and to support the correct distribution of specific proteins. Because of this, IF are likely to be targeted by a variety of pathogens, and may act in favor or against infection progress. As many IF functions remain to be identified, however, little is currently known about these interactions. Herpesviruses can infect a wide variety of cell types, and are thus bound to encounter the different types of IF expressed in each tissue. The analysis of these interrelationships can yield precious insights into how IF proteins work, and into how viruses have evolved to exploit these functions. These interactions, either known or potential, will be the focus of this review.

Epstein-Barr virus EBNA-3C is targeted to and regulates expression from the bidirectional LMP-1/2B promoter

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA-3C) is essential for EBV-mediated immortalization of human B lymphocytes and regulates both the cell cycle and transcription. Transient reporter gene assays have implicated a pivotal role for EBNA-3C in the regulation of transcription of the majority of latency-associated genes expressed during the EBV growth program, including the viral oncoprotein LMP-1. To examine the regulation of latency gene expression by EBNA-3C, we generated an EBV-positive cell line that inducibly expresses EBNA-3C. This cell line allowed us to examine expression from the endogenous latency gene promoters in the context of an actual latent infection and the presence of other EBNA proteins, in particular EBNA-2, which is presumed to coregulate transcription with EBNA-3C. EBNA-3C induced the expression of both LMP-1 and LMP-2B mRNAs from the bidirectional LMP-1/LMP-2B promoter. In contrast, no effect was seen on expression from the common EBNA promoter Cp, which is responsive to EBNA-3C in reporter assays. Activation of LMP expression was not the consequence of increases in EBNA-2, PU.1 or Spi-B transcription factors, all of which are believed to be critical for activation of LMP-1. Chromatin immunoprecipitation assays furthermore indicated that EBNA-3C is present at the bidirectional LMP-1/LMP-2B promoter. These results indicate that EBNA-3C directly activates the expression of LMP-1 and LMP-2B but is unlikely to significantly regulate EBNA expression via Cp under normal growth conditions.

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.

Molecular Pharmacological Phenotyping of EBI2

Epstein-Barr virus (EBV)-induced receptor 2 (EBI2) is an orphan seven-transmembrane (7TM) receptor originally identified as the most up-regulated gene (>200-fold) in EBV-infected cells. Here we show that EBI2 signals with constitutive activity through Galpha(i) as determined by a receptor-mediated inhibition of forskolin-induced cAMP production and an induction of the serum response element-driven transcriptional activity in a pertussis toxin-sensitive manner. Galpha(s) and Galpha(q) were not activated constitutively as determined by the lack of cAMP production, the lack of inositol phosphate turnover, and the lack of activities of the transcription factors: cAMP response element-binding protein and nuclear factor-kappaB. Immunohistochemistry and confocal microscopy of FLAG- and green fluorescent protein-tagged EBI2 revealed cell-surface expression. A putative N-terminal truncated version of EBI2, delta4-EBI2, showed similar expression and signaling through Galpha(i) as full-length EBI2. By using a 32P-labeled EBI2 probe we found a very high expression in lymphoid tissue (spleen and lymph node) and peripheral blood mononuclear cells and a high expression in lung tissue. Real-time PCR of EBV-infected cells showed high expression of EBI2 during latent and lytic infection, in contrast to the EBV-encoded 7TM receptor BILF1, which was induced during lytic infection. EBI2 clustered with the orphan GPR18 by alignment analysis as well as by close proximity in the chromosomal region 13q32.3. Based on the constitutive signaling and cellular expression pattern of EBI2, it is suggested that it may function in conjunction with BILF1 in the reprogramming of the cell during EBV infection.

Role of NF-kappa B in cell survival and transcription of latent membrane protein 1-expressing or Epstein-Barr virus latency III-infected cells

Epstein-Barr virus (EBV) latency III infection converts B lymphocytes into lymphoblastoid cell lines (LCLs) by expressing EBV nuclear and membrane proteins, EBNAs, and latent membrane proteins (LMPs), which regulate transcription through Notch and tumor necrosis factor receptor pathways. The role of NF-kappa B in LMP1 and overall EBV latency III transcriptional effects was investigated by treating LCLs with BAY11-7082 (BAY11). BAY11 rapidly and irreversibly inhibited NF-kappa B, decreased mitochondrial membrane potential, induced apoptosis, and altered LCL gene expression. BAY11 effects were similar to those of an NF-kappa B inhibitor, Delta N-I kappa B alpha, in effecting decreased JNK1 expression and in microarray analyses. More than 80% of array elements that decreased with Delta N-I kappa B alpha expression decreased with BAY11 treatment. Newly identified NF-kappa B-induced, LMP1-induced, and EBV-induced genes included pleckstrin, Jun-B, c-FLIP, CIP4, and I kappa B epsilon. Of 776 significantly changed array elements, 134 were fourfold upregulated in EBV latency III, and 74 were fourfold upregulated with LMP1 expression alone, whereas only 28 were more than fourfold downregulated by EBV latency III. EBV latency III-regulated gene products mediate cell migration (EBI2, CCR7, RGS1, RANTES, MIP1 alpha, MIP1 beta, CXCR5, and RGS13), antigen presentation (major histocompatibility complex proteins and JAW1), mitogen-activated protein kinase pathway (DUSP5 and p62Dok), and interferon (IFN) signaling (IFN-gamma R alpha, IRF-4, and STAT1). Comparison of EBV latency III LCL gene expression to immunoglobulin M (IgM)-stimulated B cells, germinal-center B cells, and germinal-center-derived lymphomas clustered LCLs with IgM-stimulated B cells separately from germinal-center cells or germinal-center lymphoma cells. Expression of IRF-2, AIM1, ASK1, SNF2L2, and components of IFN signaling pathways further distinguished EBV latency III-infected B cells from IgM-stimulated or germinal-center B cells.

Epstein-Barr virus signal transduction and B-lymphocyte growth transformation

Latent EBV growth transformation of resting B-cells into indefinitely proliferating cell lines is a successful viral strategy for survival in its host and the basis of several human malignancies. EBV transforms cell growth through viral proteins that modify cell gene expression at the level of transcription or by appropriating signaling pathways. Analyses of the EBV-transforming protein LMP1 have begun to reveal that this receptor transduces critical signals by appropriating the TNF receptor signal transduction pathway to activate NF-kappaB and MAPK. While this has brought an important aspect into clearer focus, future progress in delineating the underlying mechanism of transformation, which will be essential to devising effective therapies to treat EBV-associated malignancies, will depend on resolving the intricacies of TRAF signal transduction. Since expression of cytokines, receptors, and anti-apoptotic proteins are regulated by TRAF signaling, another critical issue is delineating the genes that are specifically targeted by LMP1 in order to transform B-lymphocyte growth.

Alterations of biologic properties and gene expression in nasopharyngeal epithelial cells by the Epstein-Barr virus-encoded latent membrane protein 1

Undifferentiated nasopharyngeal carcinoma (NPC) is closely associated with EBV infection, and the EBV-encoded latent membrane protein 1 (LMP1) is frequently detected in NPC. However, little is known about the pathologic roles of LMP1 in this disease. Recently, we reported the morphologic transformation and increased expression of the LAMC2 and ITGalpha6 genes in LMP1-expressing NPC cell lines. In this study, we further examine the effects of LMP1 in an immortalized nasopharyngeal epithelial cell line called NP69. This cell line was established from primary nonmalignant nasopharyngeal epithelial cells and may represent a model of premalignant nasopharyngeal epithelial cells. LMP1 induced many phenotypic changes in NP69 cells. These include morphologic transformation, increased cell proliferation, anchorage-independent growth, resistance to serum free-induced cell death, and enhanced cell migration and invasion. In addition, expression array analysis identified 28 genes that demonstrated a more than 2-fold difference in expression of NP69 cells expressing LMP1 when compared with a vector control. Two of the up-regulated genes (VEGF and vimentin) identified have been previously reported as LMP1 targets. The majority of the identified genes are associated with cell growth, differentiation, cell shape, and invasion. The present findings support the proposed roles of LMP1 in promoting cell transformation, migration, and invasion in premalignant nasopharyngeal epithelial cells. The present study also indicates the activation of the Ras/MAPK pathway in LMP1-expressing cells, which may be involved in mediating some of the transforming effects of LMP1 observed in nasopharyngeal epithelial cells.

Characterization of the Cyno-EBV LMP1 homologue and comparison with LMP1s of EBV and other EBV-like viruses

EBV latent membrane protein 1 (LMP1) is essential for EBV-mediated transformation and has been associated with several cases of malignancies. EBV-like viruses in Cynomolgus monkeys (Macaca fascicularis) have been associated with high lymphoma rates in immunosuppressed monkeys. In the study, the entire coding region of the Cyno-EBV LMP1 gene was cloned, sequenced and expressed in human embryonic kidney (HEK) cells 293. The Cyno-EBV LMP1 homologue sequence predicted a 588 amino acid (a.a.) protein with a short 19 a.a. N-terminus, six transmembrane domains and a long carboxy tail of 404 a.a. The protein contained a series of seven 9 a.a.-tandem repeats and two 20 a.a.-repeats, which harbored two potential TRAF binding motifs, PxQxT/S. These repeats shared no homology with the repeats in any other LMP1. However, the proline-rich sequence GPxxPx(6) found within the 11 a.a.-repeats of EBV LMP1 was conserved in Cyno-EBV carboxy tail and contained two consensus JAK/STAT sequences PxxPxP. A cluster of eight histidine residues was found in proximity to the last transmembrane domain of Cyno-EBV LMP1 and was exploited as a natural protein tag in expression studies. Western blot analysis revealed a major polypeptide of 110 kDa. Comparative functional studies showed that Cyno-EBV LMP1 expressed in HEK 293 cells shares the same ability as EBV LMP1 to induce NFkappaB driven CAT activity.

Induction of inducible nitric oxide synthase by Epstein-Barr virus B95-8-derived LMP1 in Balb/3T3 cells promotes stress-induced cell death and impairs LMP1-mediated transformation

The latent membrane protein 1 (LMP1) of Epstein-Barr virus (EBV) causes cellular transformation and activation of several intracellular signaling events. In this report, we show that BLMP1 (encoded by the LMP1 gene derived from the B95-8 strain of EBV) triggers the expression of inducible nitric oxide synthase (iNOS) in Balb/3T3 fibroblasts. Intriguingly, NLMP1, a natural sequence variant of LMP1 identified in EBV-positive nasopharyngeal carcinoma biopsy, does not similarly induce iNOS expression. BLMP1-induced iNOS in Balb/3T3 cells is active to produce nitric oxide (NO), and NO production can be blocked by several iNOS inhibitors. When subjected to environmental stress, Balb/3T3 cells that produce NO lose viability more rapidly than non NO-producing cells. Blockage of NO generation by iNOS inhibitors enhances the viability of NO-producing cells under stress conditions. The activities of caspase-3 and c-Jun N-terminal kinase, two important regulators mediating stress-induced apoptosis, are significantly potentiated following heat shock treatment of BLMP1-expressing/NO-producing cells, compared to parental and NLMP1-expressing cells. Furthermore, treatment with iNOS inhibitor augmented the cloning efficiency (in culture) and tumor growth (in nude mice) of BLMP1-expressing/NO-producing cells. Collectively, the results demonstrate that BLMP1 induces iNOS expression and NO production in Balb/3T3 cells, which leads to the alteration of cell functions, including sensitivity to environmental stress, capability to colonize independent of anchorage and tumorigenicity in nude mice. Our data additionally implicate that the differential iNOS induction potential of the two LMP1 forms may represent the basis of a functional difference between the two LMP1 proteins.

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

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

Preferential localization of the Epstein-Barr virus (EBV) oncoprotein LMP-1 to nuclei in human T cells: implications for its role in the development of EBV genome-positive T-cell lymphomas

The Epstein-Barr virus (EBV)-encoded latent membrane protein-1 (LMP-1) is thought to play a role in the EBV-induced B-cell transformation and immortalization. EBV has also been implicated in certain human T-cell lymphomas; however, the phenotypic effects of the expression of this oncoprotein in T cells are not known. To learn whether LMP-1 also induces phenotypic changes in T cells, we stably expressed it in human cell lines of T and B lineages and 25 LMP-1-expressing T-cell clones and 7 B-cell clones were examined. Our results show for the first time that, in sharp contrast to B cells, LMP-1 preferentially localizes to nuclei in T cells and does not induce the phenotypic changes in these cells that it induces in B cells, does not associate with TRAF proteins, and does not arrest the cell cycle in the G2/M phase. A computer-assisted analysis revealed that LMP-1 lacks the canonical nuclear localization signal. Our results suggest that this oncoprotein may not play the same role in the lymphomagenesis of T cells as it does in B cells.

Pathogenetic and histogenetic features of HIV-associated Hodgkin's disease

Compared with the cases in the general population, Hodgkin's disease (HD) arising in the HIV setting shows distinctive features in terms of epidemiology, aetiopathogenesis, histopathology and clinical behaviour. Although HD does not represent an AIDS-defining condition, recent evidence consistently indicates that HIV-infected individuals have a significantly increased risk of developing HD. HIV-related HD is characterised by the preponderance of aggressive histological subtypes, advanced stage at presentation, and highly malignant clinical course. Moreover, unlike HD in the general population, the large majority of HIV-related HD cases are pathogenetically linked to Epstein-Barr virus (EBV), with rates of EBV positivity ranging from 80 to 100%. Hodgkin and Reed-Sternberg cells of these cases invariably show a strong expression of the EBV-encoded latent membrane protein-1 (LMP-1), which functions as a constitutively activated tumour necrosis factor (TNF) receptor-like molecule. Usurpation of physiologically relevant pathways by LMP-1 may lead to the simultaneous or sequential activation of signalling pathways involved in the promotion of cell activation, growth, and survival, contributing thus to most of the features of HIV-related HD.

Epstein-Barr virus in the pathogenesis of Hodgkin's disease

Epidemiologic and clinico-pathologic features of Hodgkin's disease suggest that an infectious agent may be involved in the pathogenesis of this puzzling disorder. Recently accumulated data provide direct evidence supporting a causal role of Epstein-Barr virus in a significant proportion of cases. In addition to allowing a better understanding of the complex pathogenesis of Hodgkin's disease, these virological advances, briefly reviewed herein, also constitute an important basis for the development of new therapeutic strategies.

Comparative analysis identifies conserved tumor necrosis factor receptor-associated factor 3 binding sites in the human and simian Epstein-Barr virus oncogene LMP1

Nonhuman primates are naturally infected with a B-lymphotropic herpesvirus closely related to Epstein-Barr virus (EBV). These simian EBV share considerable genetic, biologic, and epidemiologic features with human EBV, including virus-induced tumorigenesis. However, latent, transformation-associated viral genes demonstrate marked sequence divergence among species despite the conserved functions. We have cloned the latent membrane protein 1 (LMP1) homologs from the simian EBV naturally infecting baboons (cercopithicine herpesvirus 12, herpesvirus papio) and rhesus monkeys (cercopithicine herpesvirus 15) for a comparative study with the human EBV oncogene. The transmembrane domains are well conserved, but there is striking sequence divergence of the carboxy-terminal cytoplasmic domain essential for B-cell immortalization and interaction with the tumor necrosis factor receptor signaling pathway. Nevertheless, the simian EBV LMP1s retain most functions in common with EBV LMP1, including the ability to induce NF-(kappa)B activity in human cells, to bind the tumor necrosis factor-associated factor 3 (TRAF3) in vitro, and to induce expression of tumor necrosis factor-responsive genes, such as ICAM1, in human B lymphocytes. Multiple TRAF3 binding sites containing a PXQXT/S core sequence can be identified in the simian EBV LMP1s by an in vitro binding assay. A PXQXT/S-containing sequence is also present in the cytoplasmic domain of the Hodgkin's disease marker, CD30, and binds TRAF3 in vitro. The last 13 amino acids containing a PXQXT/S sequence are highly conserved in human and simian EBV LMP1 but do not bind TRAF3, suggesting a distinct role for this conserved region of LMP1. The conserved TRAF3 binding sites in LMP1 and the CD30 Hodgkin's disease marker provides further evidence that a TRAF3-mediated signal transduction pathway may be important in malignant transformation.

A novel interleukin-12 p40-related protein induced by latent Epstein-Barr virus infection in B lymphocytes

We have isolated a cDNA encoding a novel hematopoietin receptor family member related to the p40 subunit of interleukin-12 and to the ciliary neurotrophic factor receptor, whose expression is induced in B lymphocytes by Epstein-Barr virus (EBV) infection. This gene, which we have designated EBV-induced gene 3 (EBI3), encodes a 34-kDa glycoprotein which lacks a membrane-anchoring motif and is secreted. Despite the absence of a membrane-anchoring motif and of cysteines likely to mediate covalent linkage to an integral membrane protein, EBI3 is also present on the plasma membrane of EBV-transformed B lymphocytes and of transfected cells. Most newly synthesized EBI3 is retained in the endoplasmic reticulum in an endoglycosidase H-sensitive form associated with the molecular chaperone calnexin and with a novel 60-kDa protein. EBI3 is expressed in vivo by scattered cells in interfollicular zones of tonsil tissue, by cells associated with sinusoids in perifollicular areas of spleen tissue, and at very high levels by placental syncytiotrophoblasts. EBI3 expression in vitro is induced in EBV-negative cell lines by expression of the EBV latent infection membrane protein-1 and in peripheral blood mononuclear cells by pokeweed mitogen stimulation. EBI3 maps to chromosome 19p13.2/3, near genes encoding the erythropoietin receptor and the cytokine receptor-associated kinase, Tyk2. EBI3 synthesis by trophoblasts and by EBV-transformed cells and similarities to interleukin-12 p40 are compatible with a role for EBI3 in regulating cell-mediated immune responses.

CD44 isoforms containing exons V6 and V7 are differentially expressed on mitogenically stimulated normal and Epstein-Barr virus-transformed human B cells

CD44, a cell adhesion molecule, exists in multiple isoforms that are generated by RNA alternative splicing. CD44 isoforms containing exon V6 (CD44 V6) have been associated with tumorigenesis and metastasis. We investigated the association between human B-cell activation and CD44 V6 isoform expression by analysing its expression in resting and mitogenically stimulated B cells. Results showed that resting B cells expressed the CD44 H (no variable exon) isoform alone. Activation of B cells [phorbol myristate acetate (PMA), surface immunoglobulin cross-linking alone or in the presence of interleukin-2 (IL-2)] induced CD44E (variable exon V8-10), R2 (VIO) and CD44 isoforms containing exons V6 and/or V7 (CD44 V6/V7). Epstein-Barr virus (EBV) infection of B cells, an alternative method of B-cell activation, induced the expression of CD44 E and R2 but not CD44 V6/V7. These results indicate that CD44 V6/V7 expression depends on the mode of activation. CD44 isoform expression was also investigated in a panel of EBV-negative and EBV-positive Burkitt's lymphoma (BL) B-cell lines. EBV-negative BL cells did not express CD44. In contrast, EBV-positive BL cells expressed CD44 H, R2 and E but not CD44 V6/V7 isoforms, suggesting an association between EBV infection and CD44 isoform induction. To determine directly the role of EBV in CD44 isoform induction, an EBV-negative BL cell line, BL30 (negative for all isoforms of CD44), BL30 infected in vitro with the EBNA-2-defective P3HR1 (BL30/P3HR1), and the wild-type B95-8 strain of EBV (BL30/B95-8) were examined. The parental BL30 cells infected with the wild-type EBV strain, but not with the P3HR-1 strain, expressed CD44 H, R2 and E isoforms, as seen in EBV-immortalized B cells. These studies suggest that (1) alternative splicing of CD44 isoforms is differentially regulated depending on the mode and state of cell activation, and that (2) the CD44 V6/V7 isoforms may represent B-cell activation antigens that are induced by mitogenic stimulation but not following EBV infection.

Epstein-Barr virus infection induces expression in B lymphocytes of a novel gene encoding an evolutionarily conserved 55-kilodalton actin-bundling protein

A novel human mRNA whose expression is induced over 200-fold in B lymphocytes by latent Epstein-Barr virus (EBV) infection was reverse transcribed, cloned, and sequenced. The mRNA is predicted to encode a protein containing four peptides which precisely match amino acid sequences from a previously identified 55-kDa actin-bundling protein, p55. In vitro translation of the cDNA results in a 55-kDa protein which binds to actin filaments in the presence of purified p55 from HeLa cells. The p55 mRNA is undetectable in non-EBV-infected B- and T-cell lines or in a myelomonocytic cell line (U937). Newly infected primary human B lymphocytes, EBV-transformed B-cell lines, latently infected Burkitt tumor cells expressing EBNA2 and LMP1, a chronic myelogenous leukemia cell line (K562), and an osteosarcoma cell line (TK143) contain high levels of p55 mRNA or protein. In EBV-transformed B cells, p55 localizes to perinuclear cytoplasm and to cell surface processes that resemble filopodia. The p55 mRNA is detected at high levels in spleen and brain tissues, at moderate levels in lung and placenta tissues, and at low levels in skeletal muscle, liver, and tonsil tissues and is undetectable in heart, kidney, pancreas, and bone marrow tissues. Immunohistochemical staining of human brain tissue demonstrates p55 localization to the perinuclear cytoplasm and dendritic processes of many, but not all, types of cortical or cerebellar neurons, to glial cells, and to capillary endothelial cells. In cultured primary rat neurons, p55 is distributed throughout the perinuclear cytoplasm and in subcortical filamentous structures of dendrites and growth cones. p55 is highly evolutionarily conserved since it shows 40% amino acid sequence identity to the Drosophila singed gene product and 37% identity to fascin, an echinoderm actin-bundling protein. The evolutionary conservation of p55 and its lack of extensive homology to other actin-binding proteins suggest that p55 has specific microfilament-associated functions in cells in which it is differentially expressed, including neural cells and EBV-transformed B lymphocytes.

Epstein-Barr virus, infectious mononucleosis, and posttransplant lymphoproliferative disorders

PTLD may be considered as an "opportunistic cancer" in which the immunodeficiency state of the host plays a key role in fostering the environment necessary for abnormal lymphoproliferation. The following discussion reflects our own current thoughts regarding events which may result in PTLD and its sequelae. Many of the individual steps have not been rigorously proved or disproved at this point in time. Following transplantation and iatrogenic immunosuppression, the host:EBV equilibrium is shifted in favor of the virus. Most seronegative patients will become infected either via the graft or through natural means; seropositive patients will begin to shed higher levels of virus and may become secondarily superinfected via the graft. There is a "grace" period of approximately one month posttransplant before increased viral shedding begins. PTLD is almost never seen during this interval. In many cases infection continues to be silent whereas in rare individuals there is an overwhelming polyclonal proliferation of infected B lymphocytes. This is the parallel of infectious mononucleosis occurring in patients with a congenital defect in virus handling (X-linked lymphoproliferative disorder). It is possible that transplant patients with this presentation also suffer a defect in virus handling. In other cases excessive iatrogenic immunosuppression may paralyze their ability to respond to the infection. With CsA and FK506 regimens, individual tumors may occur within a matter of months following transplant. The short time of incubation suggests that these are less than fully developed malignancies. It may be that local events conspire to allow outgrowth of limited numbers of B-lymphocyte clones. A cytokine environment favoring B-lymphocyte growth may be one factor and differential inhibition by the immuno-suppressive drugs of calcium-dependent and -independent B-cell stimulation may be another. In addition, there is some evidence that CsA itself may inhibit apoptosis within B cells. Since most patients do not develop PTLDs, an additional signal(s) for B-cell stimulation may be required. Indeed, it is possible that the virus may simply serve to lower the threshold for B-cell activation and/or provide a survival advantage to these cells. The ability of individual cell clones to evade a weakened immune system may set into play a Darwinian type of competition in which the most rapidly proliferating cells with the least number of antigenic targets predominate. In this regard, differences in host HLA types may determine the repertoire of viral antigens which are subject to attack.(ABSTRACT TRUNCATED AT 400 WORDS)

Epstein-Barr virus-induced genes: first lymphocyte-specific G protein-coupled peptide receptors

Since Epstein-Barr virus (EBV) infection of Burkitt's lymphoma (BL) cells in vitro reproduces many of the activation effects of EBV infection of primary B lymphocytes, mRNAs induced in BL cells have been cloned and identified by subtractive hybridization. Nine genes encode RNAs which are 4- to > 100-fold more abundant after EBV infection. Two of these, the genes for CD21 and vimentin, were previously known to be induced by EBV infection. Five others, the genes for cathepsin H, annexin VI (p68), serglycin proteoglycan core protein, CD44, and the myristylated alanine-rich protein kinase C substrate (MARCKS), are genes which were not previously known to be induced by EBV infection. Two novel genes, EBV-induced genes 1 and 2 (EBI 1 and EBI 2, respectively) can be predicted from their cDNA sequences to encode G protein-coupled peptide receptors. EBI 1 is expressed exclusively in B- and T-lymphocyte cell lines and in lymphoid tissues and is highly homologous to the interleukin 8 receptors. EBI 2 is most closely related to the thrombin receptor. EBI 2 is expressed in B-lymphocyte cell lines and in lymphoid tissues but not in T-lymphocyte cell lines or peripheral blood T lymphocytes. EBI 2 is also expressed at lower levels in a promyelocytic and a histiocytic cell line and in pulmonary tissue. These predicted G protein-coupled peptide receptors are more likely to be mediators of EBV effects on B lymphocytes or of normal lymphocyte functions than are genes previously known to be up-regulated by EBV infection.

Modulation of expression of insulin and IGF-I receptor by Epstein-Barr virus and its gene products LMP and EBNA-2 in lymphocyte cell lines

The receptors for insulin and insulin-like growth factor I (IGF-I) are two closely related integral membrane glycoproteins involved in signalling of cell growth and metabolism. We have used the unique paradigm of pairs of Burkitt lymphoma cell lines (BLO2, BL30, BL41) with or without Epstein-Barr Virus (EBV) infection and cells transfected with EBV-related genes to examine effects of EBV on expression of these receptors at the gene and protein functional level. In BL30 and BL41 cells, EBV infection increased surface insulin binding and total receptor number by 2- and 18-fold, respectively. By contrast, EBV infection decreased total IGF-I receptors by 29 to 87% in all three cell lines. In general, there was a correlation between total receptor concentration and the level of insulin or IGF-I receptor mRNAs, although in one cell line insulin binding increased while receptor mRNA levels decreased slightly, suggesting posttranslational effects. BL41 cells transfected with a vector expressing the EBV latent membrane protein (LMP) exhibited a 2.6- to 3.2-fold increase in insulin receptor expression, whereas cells transfected with EBNA-2 (one of the EBV nuclear antigens) alone exhibited no effect. However, EBNA-2 appears to be required for the EBV effect on insulin receptor expression since cells infected with a mutant virus, P3JHRI, which lacks the EBNA-2 gene failed to show an increase in insulin receptor number. These data indicate that EBV infection of lymphocytes increases expression of insulin receptors while simultaneously decreasing expression of IGF-I receptors. The magnitude and sometimes even the direction of change, depends on host cell factors. A maximal increase in insulin receptors appears to require the coordinate action of several of the EBV proteins including LMP and EBNA-2.

Co-expression of Epstein-Barr virus latent membrane protein and vimentin in "aggressive" histological subtypes of Hodgkin's disease

The presence of Epstein-Barr virus (EBV) genome in Hodgkin's and Reed-Sternberg (HRS) cells, as detected using in situ hybridization (ISH) with biotinylated BamHI "V" probes, along with the expression of EBV-encoded latent membrane protein (LMP) and vimentin was examined in paraffin-embedded sections of 39 immunomorphologically characterized cases of Hodgkin's disease (HD). ISH demonstrated EBV in HRS cells in 15 of 39 cases, whereas LMP expression was detected in 11 of 39 cases, only in the presence of EBV genome detection. With the exception of 1 case, in which HRS cells expressed B-cell-associated antigens, the LMP-positive cases included specimens in which HRS cells were of non-B, non-T phenotype. LMP expression showed a stronger association with lymphocyte depletion (LD) (3/3) and mixed cellularity (MC) (6/11) than with lymphocyte predominance (0/5) or nodular sclerosis (2/20) subtypes. Vimentin expression on HRS cells was found in all the LMP-expressing cases and only in a fraction (13/28) of LMP-negative cases. This study supports the view that HD represents a heterogeneous group of diseases also in terms of EBV association, LMP expression being strongly related to the "aggressive" LD and MC histological subtypes. In light of the supposed interactions between vimentin and LMP, their co-expression on HRS cells, as detected in this study, provides further evidence for a significant role of EBV in the development of a proportion of HD cases.

Epstein-Barr virus nuclear protein 2 is a critical determinant for tumor growth in SCID mice and for transformation in vitro

Injection of Epstein-Barr virus (EBV)-transformed human lymphoblastoid B cells into immunodeficient SCID mice results in the appearance of rapidly growing, fatal human B-cell tumors. To evaluate the role of EBV nuclear protein 2 (EBNA-2) in this process, we generated lymphoblastoid cell lines transformed by several EBV mutants which were identical except for deletions in the EBNA-2 gene (J. I. Cohen, F. Wang, and E. Kieff, J. Virol. 65:2545-2554, 1991). These cell lines were injected intraperitoneally into SCID mice, and the interval until tumor detection was determined. Cell lines transformed with EBV type 1 (strain W91) or with EBV type 2 (strain P3HR-1) with an inserted type 1 EBNA-2 gene grew at the same rapid rate, indicating the potential importance of EBNA-2 for tumor formation in vivo. Cell lines derived from three different EBV mutants with deletions in the amino half of EBNA-2 produced tumors more slowly than cell lines transformed by wild-type W91 virus. In contrast, a cell line transformed with an EBV mutant with a deletion in the carboxy terminus of EBNA-2 grew more rapidly than cell lines transformed by wild-type virus. EBV mutants with deletions in the amino half of EBNA-2 had had reduced transforming activity in vitro, while the carboxy-terminal EBNA-2 mutant had had transforming activity greater than or equal to that of the wild type. These data indicate that EBNA-2 plays a critical role both for B-cell tumor growth in SCID mice and for B-lymphocyte transformation in vitro.

A selectable marker allows investigation of a nontransforming Epstein-Barr virus mutant

The derivation of specifically mutated Epstein-Barr virus (EBV) recombinants is dependent on strategies to identify, enumerate, and clone infected B lymphocytes. In recent experiments, EBV recombinants containing a positive selection marker were identified and cloned in B-lymphoma (BL) cells infected and then plated under selective conditions (F. Wang, A. Marchini, and E. Kieff, J. Virol. 65:1701-1709, 1991). We now use BL cells, for the first time, as hosts for assaying and cloning otherwise isogenic EBV recombinants carrying a hygromycin phosphotransferase (HYG) gene linked to either a nontransforming deletion mutant or a transforming wild-type EBV nuclear antigen 2 (EBNA-2) gene. Both types of recombinants converted BL cells to hygromycin resistance with similar efficiency, formed episomes, and usually expressed only EBNA-1. Only the wild-type EBNA-2 HYG gene EBV recombinant transformed primary B lymphocytes. This strategy of assaying virus on BL and primary B lymphocytes makes possible the direct assessment of the transforming efficiency of an EBV recombinant. The resultant infected BL cells are also useful for the characterization of the nontransforming recombinant EBV genomes. The HYG gene insertion in the BHLF1 open reading frame eliminated BHLF1 protein expression. The insertion and resulting BHLF1 mutation did not interfere with primary B-lymphocyte infection, growth transformation, induction of lytic infection, or virus production. Thus, these experiments also indicate that neither the BHLF1 open reading frame nor the HYG gene insertion critically affects B-lymphocyte infection in vitro.

Immunohistochemical demonstration of the Epstein-Barr virus-encoded latent membrane protein in paraffin sections of Hodgkin's disease

Paraffin sections from 46 cases of Hodgkin's disease were examined for the presence of the Epstein-Barr virus (EBV)-encoded latent membrane protein (LMP) using a sensitive (double layer alkaline phosphatase-anti-alkaline phosphatase) immunohistochemical method. LMP was detected in 22 cases, the majority of positive cases being of nodular sclerosis (12/24), mixed cellularity (6/7), and lymphocyte depletion (3/3) subtypes. Only one of 12 cases of lymphocyte predominant disease was positive. In all cases, reactivity was confined to Hodgkin's and Reed-Sternberg cells. These results provide further evidence for an association between EBV and Hodgkin's disease and indicate that LMP may be readily detected in archival material.

The Epstein-Barr virus BNLF-1 membrane protein LMP1 induces homotypic adhesion mediated by CD11a/CD18 in a murine B-cell line, mimicking the action of phorbol ester

Expression of the Epstein-Barr virus BNLF1 gene (LMP1) or treatment with 4B-phorbol-12,13-dibutyrate in the murine pro-B-cell line A/J-95 leads to a five- to sevenfold enhancement of homotypic adhesion without significant changes in adhesion molecule expression. Antibody to CD11a inhibits aggregation, indicating that CD11a/CD18-mediated adhesion is a common target for both agents.

An Epstein-Barr virus protein associated with cell growth transformation interacts with a tyrosine kinase

Epstein-Barr virus (EBV) encodes two integral membrane proteins in latently infected growth-transformed cells. One of these, LMP1, can transform rodent fibroblasts and induce markers of B-lymphocyte activation. The second, LMP2, colocalizes with LMP1 in a constitutive patch in the EBV-transformed B-lymphocyte plasma membrane. The experiments reported here demonstrate that LMP2 may biochemically interact with LMP1 and that LMP2 closely associates with and is an important substrate for a B-lymphocyte tyrosine kinase in EBV-transformed B lymphocytes or in B-lymphoma cells in which LMP2 is expressed by gene transfer. LMP2 is also serine and threonine phosphorylated. LMP2 localizes to a peripheral membrane (presumably plasma membrane) patch in transfected B-lymphoma cells and colocalizes with much of the cellular tyrosine-phosphorylated proteins. LMP2 undergoes tyrosine phosphorylation in anti-LMP2 or antiphosphotyrosine immunoprecipitates from transfected B-lymphoma cells or EBV-transformed B lymphocytes. The first 167 of the 497 amino acids of LMP2 retain full ability to associate with and act as a substrate for a tyrosine kinase. A 70-kDa phosphotyrosine cell protein associates with LMP2 in transfected cells or in EBV-transformed B lymphocytes and could be a mediator of the effects of LMP2.

EBNA-2 transactivates a lymphoid-specific enhancer in the BamHI C promoter of Epstein-Barr virus

Among the few Epstein-Barr virus (EBV) genes expressed during latency are the Epstein-Barr nuclear antigens (EBNAs), at least one of which contributes to the ability of the virus to transform B lymphocytes. We have analyzed a promoter located in the BamHI-C fragment of EBV which is responsible for the expression of EBNA-1 in some cell lines. Deletion analysis of a 1.4-kb region 5' of the RNA start site has identified a 700-bp fragment that is required for optimal promoter activity in latently infected B lymphocytes, as shown by promoter constructs linked to the chloramphenicol acetyltransferase reporter gene. This fragment is also able to enhance activity, in an orientation-independent manner, of the simian virus 40 early promoter linked to the chloramphenicol acetyltransferase gene. The enhancer element has some constitutive activity in EBV-negative lymphoid cells, which is increased in the presence of the EBNA-2 gene product. Further deletions have shown that the EBNA-2-responsive region requires a 98-bp region that contains a degenerate octamer-binding motif. In epithelial cells there was no enhancer activity regardless of the presence of EBNA-2. These results demonstrate that BamHI-C promoter activity may be dependent not on an enhancer contained in the ori-P, as was previously assumed, but rather on EBNA-2 transactivation of this more proximal enhancer located in the upstream region of the BamHI C promoter itself.

Early events in Epstein-Barr virus infection of human B lymphocytes

The sequence of Epstein-Barr virus (EBV) and B lymphocyte changes in the 3 days following acute infection was analyzed. By 16 hr the average infected lymphocyte had 1 EBV episome. Nuclear protein-2 (EBNA-2) and EBNA-leader protein (-LP) were detected by 12 hr, and by 32 hr were at the levels of stable EBV infection in lymphoblastoid cell lines (LCLs). At 12 hr, all EBNA-LP and EBNA-2 RNAs were initiated from the Pw promoter. By 36 hr a significant EBNA-LP and EBNA-2 RNA fraction initiated from the upstream Pc promoter. Throughout acute infection, a similar fraction of potentially bicistronic EBNA-LP mRNAs had first exon splices which would result in EBNA-LP translation. By 36 hr c-myc RNA was transiently induced, and CD21 and CD23 RNAs were beginning to increase. This coincided with low-level EBNA-1, EBNA-3A, B, and C, and latent membrane protein-1 (LMP-1) expression. By 46 hr, EBNA-1, the EBNA-3s, and LMP-1 were near the levels ordinarily found in LCLs and a substantial fraction of lymphocytes were in S phase. These results are compatible with a key role for EBNA-2 (or EBNA-LP) in regulating virus and cell gene expression. High-level expression of the EBV-encoded small RNAs, EBERs, was delayed beyond 36 hr and may, therefore, be activated by other virus or cell genes. A 65-kDa virion protein persisted in acutely infected cells. This protein could be a mediator of virus or cell gene expression.

Modulation of cellular gene expression in B lymphoma cells following in vitro infection by Epstein-Barr virus (EBV)

In vitro infection of EBV-negative lymphoma cell lines with immortalizing strains of Epstein-Barr virus induces the cell-surface expression of B-cell markers, such as the EBV receptor/CR2 (CD21) and the CD23 antigen. The non-immortalizing EBV variant, P3HRI, which carries a deletion encompassing the EBV nuclear antigen 2 (EBNA2) gene, fails to induce any such expression. We show here that the EBV-mediated up-regulation of cell-surface expression of these molecules is associated with an increased level of the specific steady-state RNA corresponding to these 2 genes. These results suggest that the role of EBNA2 in B-cell growth and immortalization may be related to its role in transactivation of cellular genes. In order to identify other cellular genes whose expression may be modulated by EBV, we analyzed the level of transcription of a set of genes possibly involved in Burkitt's lymphoma pathogenesis. The level of the c-myc oncogene transcript was not significantly affected by in vitro EBV infection. The c-fgr oncogene, thought to be specifically activated in EBV-infected cells, was found to be expressed in some EBV-negative lymphoma cells and also to be activated by both non-immortalizing and immortalizing strains of EBV. The expression of vimentin, the major 56-kDa polypeptide of mesenchymal cell intermediate filaments, was altered by all EBV isolates, in either a negative or a positive way, depending on the cell line. Expression of lymphocyte-function-associated antigens, LFA-1 alpha/beta (CD11 a/18) and LFA-3 (CD58), involved in intercellular adhesion and the T-cytotoxic pathway, were differentially regulated by EBV; a crucial observation was the activation by immortalizing EBV isolates of LFA-1 beta chain (CD18) and of LFA-3 (CD58). The EBV-and possibly EBNA2-associated modulation of cellular genes, such as CR2 (CD21), CD23 and LFAs, probably represents key events for EBV-induced B-cell proliferation, and also for in vivo immune control of EBV-infected B cells.

Epstein-Barr virus nuclear antigen 2 transactivates latent membrane protein LMP1

Several lines of evidence are compatible with the hypothesis that Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA-2) or leader protein (EBNA-LP) affects expression of the EBV latent infection membrane protein LMP1. We now demonstrate the following. (i) Acute transfection and expression of EBNA-2 under control of simian virus 40 or Moloney murine leukemia virus promoters resulted in increased LMP1 expression in P3HR-1-infected Burkitt's lymphoma cells and the P3HR-1 or Daudi cell line. (ii) Transfection and expression of EBNA-LP alone had no effect on LMP1 expression and did not act synergistically with EBNA-2 to affect LMP1 expression. (iii) LMP1 expression in Daudi and P3HR-1-infected cells was controlled at the mRNA level, and EBNA-2 expression in Daudi cells increased LMP1 mRNA. (iv) No other EBV genes were required for EBNA-2 transactivation of LMP1 since cotransfection of recombinant EBNA-2 expression vectors and genomic LMP1 DNA fragments enhanced LMP1 expression in the EBV-negative B-lymphoma cell lines BJAB, Louckes, and BL30. (v) An EBNA-2-responsive element was found within the -512 to +40 LMP1 DNA since this DNA linked to a chloramphenicol acetyltransferase reporter gene was transactivated by cotransfection with an EBNA-2 expression vector. (vi) The EBV type 2 EBNA-2 transactivated LMP1 as well as the EBV type 1 EBNA-2. (vii) Two deletions within the EBNA-2 gene which rendered EBV transformation incompetent did not transactivate LMP1, whereas a transformation-competent EBNA-2 deletion mutant did transactivate LMP1. LMP1 is a potent effector of B-lymphocyte activation and can act synergistically with EBNA-2 to induce cellular CD23 gene expression. Thus, EBNA-2 transactivation of LMP1 amplifies the biological impact of EBNA-2 and underscores its central role in EBV-induced growth transformation.

Epstein-Barr virus latent membrane protein (LMP1) and nuclear proteins 2 and 3C are effectors of phenotypic changes in B lymphocytes: EBNA-2 and LMP1 cooperatively induce CD23

Latent Epstein-Barr virus (EBV) infection and growth transformation of B lymphocytes is characterized by EBV nuclear and membrane protein expression (EBV nuclear antigen [EBNA] and latent membrane protein [LMP], respectively). LMP1 is known to be an oncogene in rodent fibroblasts and to induce B-lymphocyte activation and cellular adhesion molecules in the EBV-negative Burkitt's lymphoma cell line Louckes. EBNA-2 is required for EBV-induced growth transformation; it lowers rodent fibroblast serum dependence and specifically induces the B-lymphocyte activation antigen CD23 in Louckes cells. These initial observations are now extended through an expanded study of EBNA- and LMP1-induced phenotypic effects in a different EBV-negative B-lymphoma cell line, BJAB. LMP1 effects were also evaluated in the EBV-negative B-lymphoma cell line BL41 and the EBV-positive Burkitt's lymphoma cell line, Daudi (Daudi is deleted for EBNA-2 and does not express LMP). Previously described EBNA-2- and LMP1-transfected Louckes cells were studied in parallel. EBNA-2, from EBV-1 strains but not EBV-2, induced CD23 and CD21 expression in transfected BJAB cells. In contrast, EBNA-3C induced CD21 but not CD23, while no changes were evident in vector control-, EBNA-1-, or EBNA-LP-transfected clones. EBNAs did not affect CD10, CD30, CD39, CD40, CD44, or cellular adhesion molecules. LMP1 expression in all cell lines induced growth in large clumps and expression of the cellular adhesion molecules ICAM-1, LFA-1, and LFA-3 in those cell lines which constitutively express low levels. LMP1 expression induced marked homotypic adhesion in the BJAB cell line, despite the fact that there was no significant increase in the high constitutive BJAB LFA-1 and ICAM-1 levels, suggesting that LMP1 also induces an associated functional change in these molecules. LMP1 induction of these cellular adhesion molecules was also associated with increased heterotypic adhesion to T lymphocytes. The Burkitt's lymphoma marker, CALLA (CD10), was uniformly down regulated by LMP1 in all cell lines. In contrast, LMP1 induced unique profiles of B-lymphocyte activation antigens in the various cell lines. LMP1 induced CD23 and CD39 in BJAB; CD23 in Louckes; CD39 and CD40 in BL41; and CD21, CD40, and CD44 in Daudi. In BJAB, CD23 surface and mRNA expression were markedly increased by EBNA-2 and LMP1 coexpression, compared with EBNA-2 or LMP1 alone. This cooperative effect was CD23 specific, since no such effect was observed on another marker, CD21.(ABSTRACT TRUNCATED AT 400 WORDS)

A second Epstein-Barr virus membrane protein (LMP2) is expressed in latent infection and colocalizes with LMP1

Recent cDNA cloning and sequencing of two Epstein-Barr virus (EBV)-specific mRNAs from latently infected cultures revealed that these RNAs are encoded across the fused terminal repeats of the viral genome and that they are likely to encode two nearly identical proteins with the same transmembrane domains. The smaller predicted protein (LMP2B) lacks 119 amino-terminal amino acids found in the larger one (LMP2A). To test whether these proteins are expressed in latently infected lymphocytes, antibodies to the LMP2 proteins were derived by immunizing rabbits with TrpE-LMP2A fusion proteins. Affinity-purified LMP2-specific antibodies recognized 54- and 40-kilodalton proteins, corresponding to LMP2A and LMP2B, in immunoblots of rodent fibroblasts stably transfected with eucaryotic expression plasmids containing either the LMP2A or LMP2B cDNA. Similar-size proteins were also identified in immunoblots of latently infected lymphocytes. LMP2A localized to membranes in cellular fractionation studies. In immunofluorescent studies, LMP2 localized in the plasma membrane of EBV-infected lymphocytes, with the majority of reactivity confined to the region of the LMP1 patch. This reactivity was detected in almost all lymphoblastoid cells latently infected with EBV.

Epstein-Barr virus nuclear protein 2 is a key determinant of lymphocyte transformation

Epstein-Barr virus (EBV) efficiently transforms B lymphocytes to perpetual proliferation. The EBV laboratory strain P3HR-1 is transformation-incompetent and lacks a DNA segment that includes the EBV nuclear antigen 2 (EBNA-2) gene and a portion of the EBNA leader protein (EBNA-LP) gene. These two genes are expressed in transformed B lymphocytes. Recombinant transformation-competent EBVs were produced by transfecting P3HR-1-infected cells with a cosmid containing the DNA deleted in P3HR-1. Deletion of 105 nucleotides from the middle of the EBNA-2 gene had no discernible affect on transformation. Two larger EBNA-2 deletions abolished transformation but did not affect EBNA-2 nuclear localization. Two naturally occurring EBV variants (EBV types 1 and 2) differ extensively in their growth-transformation phenotype and in their EBNA-LP, EBNA-2, and EBNA-3A, -3B, and -3C genes. Recombinant P3HR-1 carrying EBV-1 EBNA-2 has many of the EBV-1 in vitro growth-transforming effects; recombinant P3HR-1, isogenic except for EBV-2 EBNA-2, has many of the EBV-2 growth-transforming effects including slow emergence of transformants, growth in tight clumps with few surrounding viable cells, and early sensitivity to dilution with fresh medium. Thus, EBNA-2 is an essential molecule in lymphocyte growth transformation by EBV and a major determinant of the differences between EBV-1 and EBV-2 in lymphocyte growth transformation.