An EBV membrane protein expressed in immortalized lymphocytes transforms established rodent cells

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.

Activation and immortalization of leukaemic B cells by Epstein-Barr virus

We have studied the responses of chronic leukaemic B cells to infection by Epstein-Barr virus (EBV). Our results define one population of B lymphocytes, represented by prolymphocytic leukaemic (PLL) cells, which are highly susceptible to immortalization by EBV. Another B-cell type, represented by chronic lymphocytic leukaemic (CLL) cells, can be readily infected by the virus but is resistant to immortalization. Comparative studies of viral and cellular related events early after infection in these 2 cell types reveal that both express the EB viral nuclear antigen (EBNA) complex, but the immortalization-resistant CLL cells fail to express the latent membrane protein (LMP), which can be detected in PLL cells 48 hr after infection. Circularization of the linear viral genome could be detected at 7 days post infection in the PLL cells, but only in 2 out of 4 CLL cells tested. Both CLL and PLL cells show increased surface expression of CD23 and HLA-DR molecules after infection but, whereas PLL cells show an increase in size, together with RNA and DNA synthesis indicative of cell cycle progression, CLL cells appear to be arrested in the G1/S phase of the cycle. The results suggest that the outcome of infection by EBV is determined by the nature of the target cell rather than by random virus-related events. The correlation between the block in immortalization of CLL cells and their failure to express LMP suggests that expression of this protein is essential for in vitro immortalization of B cells. The failure to detect circularization in some EBV-infected CLL cells suggests that this, as well as LMP expression, may be dependent on prior activation of the B cell by EBV, an event which may vary between the different CLL samples tested.

Expression of Epstein-Barr virus transformation-associated genes in tissues of patients with EBV lymphoproliferative disease

Epstein-Barr virus (EBV) has been associated with serious or fatal lymphoproliferative disease in immunocompromised patients. EBV nuclear protein 2 and latent membrane protein are characteristically expressed in B lymphocytes proliferating in vitro in response to growth transformation by EBV. These two proteins are thought to be effectors of lymphocyte growth since they increase the expression of B-lymphocyte activation (CD23) and cell-adhesion (LFA 3 and ICAM 1) molecules in vitro. Using monoclonal antibody-immune microscopy, we have demonstrated that these two EBV proteins and their associated B-lymphocyte activation or adhesion molecules are expressed in the infiltrating B lymphocytes in immunocompromised patients with EBV lymphoproliferative disease. These monoclonal antibodies should be useful in the early diagnosis of EBV lymphoproliferative disease and in distinguishing it from other B-lymphocyte cancers associated with EBV, such as Burkitt's lymphoma. The finding of EBV nuclear protein 2 and latent membrane protein and their associated activation or adhesion molecules provides a further pathophysiologic link between EBV and the proliferation of B lymphocytes in immunocompromised patients.

Human herpesviruses: a consideration of the latent state

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Epstein-Barr virus latent membrane protein: induction of B-cell activation antigens and membrane patch formation does not require vimentin

The latent membrane protein (LMP) of Epstein-Barr virus (EBV) forms patches associated with the vimentin intermediate filament system in EBV-transformed lymphoblastoid cell lines, EBV-infected Burkitt's lymphoma cells, and LMP-transfected, EBV-negative Burkitt's lymphoma cells. By gene transfer, LMP induces the expression of vimentin and B-cell activation antigens in EBV-negative Burkitt's lymphoma cells. We have now expressed LMP in an EBV-positive Burkitt's lymphoma cell line, Daudi, which does not express any LMP or vimentin. In these Daudi transfectants, LMP still formed plasma membrane patches in the absence of vimentin. LMP did not resist nonionic detergent extraction in Daudi cells as it does in vimentin-expressing cells. LMP still retained functional activity as judged by induction of B-cell activation antigens. These data indicate that LMP can form plasma membrane patches and induce B-lymphocyte activation independent of vimentin association.

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

Latent Epstein-Barr virus (EBV) infection activates B-lymphocyte proliferation through mechanisms which are partially known. One approach to further delineate these mechanisms is to identify cellular genes whose expression is augmented in cells latently infected with EBV. Since EBV-negative Burkitt's lymphoma cells can be grown in continuous culture and EBV can establish growth-altering latent infection in these cells, some effects of EBV on B-lymphocyte gene expression can be studied by using this in vitro system. Pursuing this latter approach, we have used cDNA cloning and subtractive hybridization to identify a gene whose expression is increased after EBV infection. This gene encodes the cytoskeletal protein vimentin. Latent infection of established EBV-negative Burkitt's lymphoma cell lines with the transforming EBV strain, B95-8, resulted in dramatic increases in vimentin mRNA and protein levels, while infection with the nontransforming P3HR1 strain failed to do so. Vimentin induction was reproduced by the expression of the single EBV gene which encodes the latent infection membrane protein (LMP). An amino-terminal LMP deletion mutant did not induce vimentin. These results are of particular interest in light of the transforming potential of LMP, as demonstrated in rodent fibroblasts, and the interaction between vimentin and LMP observed in immunofluorescent colocalization and cell fractionation studies.

The Epstein-Barr virus immediate-early gene product, BMLF1, acts in trans by a posttranscriptional mechanism which is reporter gene dependent

In DNA cotransfection experiments, the Epstein-Barr virus immediate-early gene product, BMLF1, stimulated the chloramphenicol acetyltransferase (CAT) activity of both latent and productive EBV promoters linked to CAT. This BMLF1-induced increase in CAT activity was out of proportion to the effect on CAT mRNA, suggesting a posttranscriptional mechanism. Furthermore, when growth hormone was used as a reporter gene instead of CAT, BMLF1 no longer functioned. Thus, the BMLF1 effect was reporter-gene dependent. The effect of the BMLF1 gene product does not then appear to be directed at promoter activation, but instead may function to increase the level of an as yet unidentified protein(s) required for Epstein-Barr virus infection.

Deletion mutants of herpesvirus saimiri define an open reading frame necessary for transformation

Analysis of a 5,549-base-pair sequence at the left end of herpesvirus saimiri unique (L-) DNA revealed two open reading frames and genes for five small nuclear U RNAs (herpesvirus saimiri U RNAs). Replication-competent deletion mutants were constructed in order to assess the importance of these genetic features for transformation by this oncogenic herpesvirus. Although not required for replication, one of the open reading frames was found to be required for immortalization of marmoset T lymphocytes into continuously growing cell lines. The protein predicted by this reading frame (STP; saimiri transformation-associated protein) has a highly hydrophobic stretch of 26 amino acids sufficient for a membrane-spanning domain near its carboxy terminus; this domain is immediately preceded by a sequence appropriate for formation of a metal-binding domain (His X2 His X6 Cys X2 Cys, where Xs are other amino acids). One of two poly(A)+ RNAs that could encode STP is bicistronic, while the other has a long 5' untranslated region (approximately 1.5 kilobases). Although some analogies can be drawn between STP and LMP (lymphocyte membrane protein) of Epstein-Barr virus, STP is not related in sequence to LMP.

5-Azacytidine up regulates the expression of Epstein-Barr virus nuclear antigen 2 (EBNA-2) through EBNA-6 and latent membrane protein in the Burkitt's lymphoma line rael

Nonproductive infection of B lymphocytes by Epstein-Barr virus (EBV) is associated with a highly restricted expression of viral genes. In growth-transformed lymphoblastoid cell lines, the products of these genes include a complex of at least six EBV nuclear antigens (EBNAs) (EBNA-1 through EBNA-6) and one membrane protein (latent membrane protein [LMP]). EBV-carrying Burkitt's lymphoma (BL) biopsies and derived cell lines that have retained a representative phenotype (group I BL lines) express only EBNA-1 (M. Rowe, D. T. Rowe, C. D. Gregory, L. S. Young, P. J. Farrell, H. Rupani, and A. B. Rickinson, EMBO J. 6:2743-2751, 1987). We have found that EBNA-2 through EBNA-6 and LMP can be up regulated by treating the group I BL line Rael with the DNA-demethylating agent 5-azacytidine (5-AzaC). The drug acted in a time- and dose-dependent manner. EBNA-2-positive cells were detected by anti-complement immunofluorescence staining just 12 h after addition of 4 microM 5-AzaC and reached a maximum number at 72 h, when up to 75% of the population was positive. EBNA-2, EBNA-3, EBNA-4, EBNA-4, EBNA-6, and LMP were demonstrated immunoblots starting at 48 h. The EBV-encoded early antigens and viral capsid antigens were also induced but at a lower level. EBNA-2 and the lytic cycle-associated antigens appeared with a different time course and in largely nonoverlapping cell subpopulations, as demonstrated by double fluorescence staining. Thus, EBNA-2 expression was not restricted to lytically infected cells, nor was EBNA-2 required for entry into the lytic cycle. The coding and regulatory sequences of EBNA-2 and LMP were found to be highly methylated in Rael cells and were, as expected, demethylated after 5-AzaC treatment. These findings suggest that DNA methylation may participate in the regulation of growth transformation-associated viral genes in BL cells.

The transforming domain alone of the latent membrane protein of Epstein-Barr virus is toxic to cells when expressed at high levels

A previously unrecognized activity has been associated with the product of the BNLF-1 gene of Epstein-Barr virus. This gene encodes the latent membrane protein of Epstein-Barr virus. When the gene was expressed at high levels, it was toxic to all cell lines tested, which included six human B-lymphoid lines as well as BALB/3T3, 143/EBNA-1, and HEp-2 cells. The BNLF-1 gene was previously shown to induce anchorage-independent and tumorigenic growth in Rat-1 and BALB/3T3 cells. We demonstrate here that only those mutations in the BNLF-1 gene that score positively in the anchorage-independent growth assay were cytotoxic when expressed at high levels. It is therefore possible that the same activities of the latent membrane protein that are necessary to induce anchorage-independent growth of some rodent cell lines also confer toxicity to many cell lines when expressed at high levels.

Host cell-dependent regulation of growth transformation-associated Epstein-Barr virus antigens in somatic cell hybrids

We have analyzed the expression of the three major known growth transformation-associated Epstein-Barr virus (EBV) proteins, EBNA-1, EBNA-2, and latent membrane protein (LMP), in a series of somatic cell hybrids derived from the fusion of EBV-carrying Burkitt lymphoma (BL) lines with EBV-positive or EBV-negative B-cell lines. Independently of the cell phenotype, EBNA-1 was invariably coexpressed in all EBV-carrying hybrids. In hybrids between EBV-carrying, LMP-positive and LMP-negative Burkitt lymphoma lines, LMP was expressed, indicating positive control. Two EBV-negative lymphoma lines, Ramos and BJAB, differed in their ability to express LMP after B95-8 virus-induced conversion and after hybridization with Raji cells. BJAB was permissive while Ramos was nonpermissive for LMP, although both expressed EBNA-2. The EBNA-2-deleted P3HR-1 virus gave the same pattern of LMP expression in these two cells. Our findings indicate that the expression of EBNA-1, EBNA-2, and LMP is regulated by independent mechanisms.

Epstein-Barr virus (EBV) antigen-specific leukocyte migration inhibition in infectious mononucleosis. II. Kinetics of sensitization against five EBV-encoded nuclear proteins and the latent membrane protein

The T cell-mediated immune response of infectious mononucleosis (IM) patients to five Epstein-Barr virus (EBV)-determined nuclear antigens, EBNAs, and to the membrane antigen associated with growth-transformed cells (latent membrane protein, LMP) was measured by the leukocyte migration inhibition (LMI) assay. Two different antigen sources were used: extracts from cells that only expressed EBNA-1, EBNA-2, or LMP after transfection with the corresponding EBV-DNA fragment, and synthetic peptides deduced from the corresponding genes. Patients in the acute phase of the disease failed to respond to EBNA-1, -5, -6, and LMP, but became responsive during convalescence. The majority of the patients responded to EBNA-2 and/or EBNA-3 in the acute phase (9/15 and 12/15, respectively). The response to EBNA-2 and/or EBNA-3 in the acute phase (9/15 and 12/15, respectively). The response to EBNA-3 disappeared more often in convalescence than the response to EBNA-2: 6 of 15 patients were negative to EBNA-2 and 12 of 15 to EBNA-3 during recovery. In addition to its value in the assessment of host sensitization to virus EBV antigens, these studies and the derived hypotheses also provide certain predictions about the predominant antigen expression in the EBV-infected host under normal and pathological conditions that can be subjected to direct experimental tests.

Epstein-Barr virus gene expression in malignant lymphomas induced by experimental virus infection of cottontop tamarins

Inoculation of cottontop tamarins with a large dose of Epstein-Barr virus (EBV) leads to the induction of multiple EBV genome-positive lymphomas. These tumors have been characterized as oligoclonal or monoclonal large-cell malignant lymphomas that closely resemble the EBV genome-positive B-cell lymphomas that arise in human allograft recipients. The expression of latent and lytic EBV-encoded proteins was investigated in these virus-induced tamarin lymphomas and in derived cell lines. The tamarin tumors were found to express EBV nuclear antigen 1 (EBNA 1), EBNA 2, EBNA leader protein, and the latent membrane protein (LMP) as determined both by immunohistochemical staining and by immunoblotting. However, within the limits of the immunoblotting assays, no expression of the EBNA 3a protein family could be detected. Assays for lytic-cycle proteins by using both polyclonal human sera and monoclonal antibodies against viral capsid antigen, early antigen, and membrane antigen (gp340/220) showed minimal, if any, expression of these antigens in the lymphoma biopsies. In contrast, the cell lines derived from these lymphomas, even in early passage, expressed abundant levels of the lytic-cycle antigens and also expressed the EBNA 3a protein as well as EBNA 1, EBNA 2, EBNA leader protein, and LMP. This finding suggests that the virus-lymphoma cell interaction, in particular the switch to lytic cycle, is subject to some form of host control in vivo. The expression of EBNA 2 and LMP in these tamarin lymphomas strengthens their resemblance to posttransplant lymphomas in humans, since these human tumors are also EBNA 2 and LMP positive (L. S. Young, C. Alfieri, K. Hennessy, H. Evans, C. O'Hara, K. Anderson, A. Rickinson, E. Kieff, and J. I. Cohen, submitted for publication). Since both proteins are known to be important effector molecules of virus-induced B-cell growth transformation in vitro, their expression in these lymphomas constitutes the best evidence for a direct oncogenic role for EBV in vivo.

Epstein-Barr virus and a tumour-promoting phorbol ester use similar mechanisms in the stimulation of human B-cell proliferation

In marked contrast to ligands which activate B cells via their physiological receptors for antigen, transforming Epstein-Barr virus (EBV) was found to be mitogenic for human B lymphocytes without increasing inositol phospholipid hydrolysis. B-cell stimulation by EBV showed similar characteristics to those achieved by the tumour-promoting phorbol ester TPA, in terms of the temporal appearance of surface activation antigens, the induction of RNA and DNA synthesis and the lower requirement for medium Ca++ in comparison to agonists that lead to an increase in inositol phospholipid hydrolysis. The calcium- and phospholipid-dependent kinase, protein kinase C (PKC), is activated by TPA and a proteolytically cleaved fragment (PKM) results. EBV induced the appearance of a calcium- and phospholipid-independent activity that was chromatographically inseparable from PKM and this activity was capable of phosphorylating vimentin, a cell component that is thought to participate in the signal transduction cascade. These findings are discussed with special reference to the biochemical signalling pathways on which EBV might impinge to usurp growth control in B lymphocytes.

Influence of Burkitt's lymphoma and primary B cells on latent gene expression by the nonimmortalizing P3J-HR-1 strain of Epstein-Barr virus

The Epstein-Barr virus (EBV) genes expressed in B lymphocytes immortalized in vitro or in Burkitt's lymphoma (BL) cells infected in vivo have been characterized previously; however, the viral products which are essential for immortalization or for establishment of EBV latency are still not known. To approach this question, we compared the kinetics of expression of EBV nuclear antigens and the two EBV-encoded small RNAs, EBER1 and EBER2, after infection of primary B cells or EBV genome-negative BL cells with either an immortalizing EBV strain (B95-8) or the nonimmortalizing deletion mutant (HR-1). Following infection of primary cells with B95-8 virus, EBV nuclear antigen (EBNA)-2 was expressed first, followed by EBNA-1, -3, and -4 (also called leader protein [LP]) and the two small RNAs. Infection of EBV genome-negative BL cells with the same strain of virus resulted in a similar pattern of gene expression, except that the EBNAs appeared together and more rapidly. EBERs were not apparent in one BL cell line converted by B95-8. The only products detected after infection of primary B lymphocytes with the HR-1 deletion mutant were the EBNA-4 (LP) family and trace amounts of EBER1. Although HR-1 could express neither EBNA-1, EBNA-3, nor EBER2 in primary cells, all these products were expressed rapidly after HR-1 infection of EBV genome-negative BL cell lines. The results indicate that the mutation in HR-1 virus affects immortalization not only through failure to express EBNA-2, a gene which is deleted, but also indirectly by curtailing expression of several other EBV genes whose coding regions are intact in the HR-1 virus and normally expressed during latency. The pattern of latent EBV gene expression after HR-1 infection is dependent on the host cell, perhaps through products specific for the cell cycle or the state of B-cell differentiation.

The Epstein-Barr virus (EBV) BZLF1 immediate-early gene product differentially affects latent versus productive EBV promoters

The Epstein-Barr virus (EBV) BZLF1 gene product is thought to mediate the disruption of latent EBV infection. We have examined the regulatory effects of BZLF1 by studying its transactivating effects on seven different EBV promoters. We find that whereas the BZLF1 gene product increases the activity of the two early promoters, BMLF1 and BMRF1, it decreases the activity of three latent promoters (the BamHI-C and BamHI-W Epstein-Barr nuclear antigen promoters and the latent membrane protein promoter). The BZLF1-induced changes in promoter-directed chloramphenicol acetyltransferase activity occur in EBV-negative as well as EBV-positive cell lines and are accompanied by a similar change in chloramphenicol acetyltransferase mRNA. Deletion analysis of the BamHI Z fragment indicates that in a portion of the amino-terminal half of the BZLF1 gene product (amino acids 24 to 86) is not essential for positive transactivating effects but is required for down-regulating effects. Thus, different domains of the same EBV immediate-early gene product can either increase the function of EBV promoters active in productive infection or decrease the function of key promoters active in latent infection.

The human DNA tumor viruses: human papilloma virus and Epstein-Barr virus

HPV and EBV are common infectious agents that persist after primary infection in a latent state with occasional shedding of virus. Therefore, one of the fundamental questions in the etiology of those cancers that are linked to infection with such ubiquitous viruses is why cancer develops in a few people when many are infected. Because only a small subset of infected people will develop specific cancers, it has been suggested that the presence of the viral genomes in the malignancies merely indicates a persistent or latent infection. However, if the viral infection was not an etiologic factor in the development of the specific cancers, then one would predict that the proportion of cancers that contained the viral genome would reflect the proportion of infected people and that the same cancers could develop in uninfected people. The sporadic detection from nonendemic areas of Burkitt's lymphoma without EBV initially suggested that EBV infection was not etiologic. However, the rate of incidence of BL in infected populations of children is disproportionately greater than the very low incidence in uninfected children, which suggests that EBV infection is an important contributing factor. Moreover, the development of EBV-induced lymphomas in the immunocompromised and the consistent detection of EBV in specific epithelial malignancies such as NPC suggest that EBV infection is essential in the induction of specific cancers. Similarly, the consistent detection of particular HPV types in certain types of cancer suggests that HPV is also an etiologic factor. There are several strikingly similar aspects of infection with HPV and EBV. In latent infection, both of the viral genomes persist as an extrachromosomal episome with an origin of replication that is activated by binding to a virally encoded polypeptide. The state of viral infection appears to be linked with the state of cellular differentiation such that latent infections are activated into a replicative state as the cells differentiate. Moreover, elevated levels of expression of the putative transforming genes are linked to transformation. However, perhaps most importantly, the malignancies are clonal with regard to the viral infection; HPV-associated malignancies have unique integrative events and EBV-associated malignancies have clonal episomal forms. This reveals that the specific cancers are clonal cellular proliferations that developed after viral infection. In vitro, the initially polyclonal cell lines produced by EBV infection rapidly evolve to oligoclonality or monoclonality. This could be due to a slightly faster rate of growth such that the progeny of one clone rapidly predominate.(ABSTRACT TRUNCATED AT 400 WORDS)

B cell activation and the establishment of Epstein-Barr virus latency

Linear EBV genomes undergo a transition to the circular form characteristic of latency by 16-20 h post-infection. This transition requires that the infected cells be activated to the G1 stage of the cell cycle. Cellular proliferation and expression of the activation marker CD23 were not required. Nevertheless, 36 h post-infection, only cells expressing CD23 contained covalently closed, circular episomes (CCC), at an average of one copy per cell. Since the presence of CD23 at this time is predictive that a cell will immortalize, we suggest that the presence of CCC is required for CD23 expression and subsequent immortalization. The one CCC present in each CD23+ cell did not undergo amplification until well after the cells had acquired all of the characteristic phenotypic markers of immortalization. Therefore, while amplification is not necessary for proliferation and immortalization, circularization of a single genome is crucial to the establishment and maintenance of latency by EBV.

Epstein-Barr virus latent infection membrane protein alters the human B-lymphocyte phenotype: deletion of the amino terminus abolishes activity

A latent infection membrane protein (LMP) encoded by the Epstein-Barr virus (EBV) genome in latently infected, growth-transformed lymphocytes alters the phenotype of a human EBV-negative B-lymphoma cell line (Louckes) when introduced by gene transfer. These LMP-expressing cells exhibit increased homotypic adhesion due to increased expression of the adhesion molecules LFA-1 and ICAM-1. Increased homotypic adhesion could foster B-cell growth by facilitating autocrine growth factor effects. LFA-3 expression is also induced. The induction of LFA-3 and ICAM-1 results in increased heterotypic adhesion to T lymphocytes. This could result in more effective T-cell immune surveillance. Since LMP is expressed in EBV-transformed lymphocytes and has been demonstrated to transform rodent fibroblasts in vitro, a wide range of possible effects on B-lymphoma cell growth were assayed. In the Louckes B-lymphoma cell line, EBV LMP causes increased cell size, acid production, plasma membrane ruffling, and villous projections. Although cell proliferation rate was not greatly affected, the steady-state intracellular free calcium level, transforming growth factor beta responsiveness, and expression of the lymphocyte activation markers (CD23 and transferrin receptor) were increased. Thus, LMP appears to be a mediator of EBV effects on B-cell transformation. In transfected lymphoma cells, LMP localizes to patches at the cell periphery and associates with the cytoskeleton as it does in EBV-transformed B lymphocytes or in rodent fibroblasts. A partially deleted form of LMP (D1LMP) does not aggregate in patches or associate with the cytoskeleton and had little effect on B-cell growth. Thus, cytoskeletal association may be integral to LMP activity.

Epstein-Barr virus-specific cytotoxic T-cell recognition of transfectants expressing the virus-coded latent membrane protein LMP

Cytotoxic T cells from Epstein-Barr virus (EBV)-immune individuals specifically kill EBV-transformed B cells from HLA class I antigen-matched donors even though the latently infected cells express only a restricted set of virus genes. The virus-induced target antigens recognized by these immune T cells have not been identified. In our experiments, EBV DNA sequences encoding the virus latent gene products Epstein-Barr nuclear antigen (EBNA)1, EBNA 2, and EBNA-LP and the latent membrane protein (LMP) were individually expressed in a virus-negative human B-lymphoma cell line, Louckes. Transfected clones expressing LMP were killed by EBV-specific cytotoxic T-cell preparations from each of three virus-immune donors HLA matched with Louckes through HLA-A2, B44 antigens; control transfectants or clones expressing one of the EBNA proteins were not recognized. Expression of LMP in a second virus-negative B-cell line, BL41, sensitized these cells to EBV-specific cytolysis restricted through the HLA-A11 antigen. To distinguish between the viral protein and an induced human B-cell activation antigen as the target for T-cell recognition, LMP was then expressed in a murine mastocytoma cell line, P815-A11-restricted human T cells. The LMP-expressing P815-A11 transfectants were susceptible to lysis by EBV-specific cytotoxic T cells from three HLA-A11-positive individuals. Both Louckes and P815-A11 cells were also transfected with constructs capable of encoding a truncated form of LMP (Tr-LMP) which lacks the N-terminal 128 amino acids of the full-length protein. Tr-LMP-expressing transfectants were not recognized by the above T-cell preparations. The results suggest that LMP, and, in particular, epitopes derived from the N-terminal region of the protein, provides one of the target antigens for the EBV-induced human cytotoxic T-cell response.

Expression of Epstein-Barr virus-encoded proteins in nasopharyngeal carcinoma

Expression of the Epstein-Barr virus (EBV) encoded nuclear antigens (EBNA 1 to 6) and membrane-associated protein (LMP) was investigated by immunoblotting in 83 nasopharyngeal carcinoma (NPC) biopsies and 25 other tumor and normal tissue specimens from the head and neck region. Fifty-eight of the 83 NPC biopsies were large enough to yield parallel data on virus DNA and viral expression. All 16 cases of clinically diagnosed and histologically confirmed NPCs from North Africa contained EBV DNA and expressed EBNA-1. Of 31 clinically diagnosed NPCs from China, 29 contained EBV DNA and 25 of these expressed EBNA-1. One control tissue biopsy from the oropharynx of NPC patients contained EBV DNA, but none expressed EBNA-1. The latent membrane protein (LMP) was detected in 22/31 of the Chinese and in 10/16 of the North African NPC biopsies. None of the NPC biopsies or control tissues expressed detectable amounts of EBNA 2 or any of the other 4 nuclear antigens which are invariably expressed in EBV-transformed B cells. A smaller number of tumors from Malaysia and East Africa exhibited a similar pattern of expression. EBV was rescued from a nude-mouse-passaged North African NPC tumor by co-cultivation of the tumor cells with umbilical cord blood lymphocytes. The tumor expressed EBNA 1 and LMP, but not EBNA 2 or the other 4 EBNAs. The resulting LCLs expressed all 6 nuclear antigens, EBNA 1 to 6 and LMP. Our data suggest that expression of the EBV genome is regulated in a tissue-specific fashion.

Maintenance of growth transformation with Epstein-Barr virus is mediated by secretion of autocrine growth factors in two serum-free B-cell lines

The characteristics of two tamarin (Saguinus oedipus) B-cell lines (sfBIT and sfBT) growth-transformed by Epstein-Barr virus (EBV) that proliferate continuously in serum-free medium are described. sfBIT was established by selecting cells for growth in RPMI 1640 supplemented with insulin, transferrin, and selenium (J. E. Shaw, R. G. Petit, and K. Leung, J. Virol. 61:4033-4037, 1987). sfBT, a subline of sfBIT cells reported here for the first time, required transferrin as the only protein supplement for continuous growth in RPMI 1640. Growth of sfBT cells was linear with human transferrin at 10(-2) to 10 micrograms/ml. Transferrin at 5 micrograms/ml yielded a culture density of 5 X 10(5) to 1 X 10(6) cells per ml, a cell doubling time of 2 to 3 days, and a culture viability greater than 95%. sfBIT and sfBT cells released transforming virus during continuous growth in serum-free culture medium without EBV-inducing agents. The spent medium of both serum-free lines supported cell growth at low culture density (1 x 10(4) to 5 X 10(4) cells per ml), but growth was arrested at low culture density with fresh serum-free medium. A procedure to measure growth-promoting activity (GPA) was established, and it revealed that the GPA of spent medium was greater than that of fresh medium for both serum-free cell lines. When fresh and spent media were dialyzed (molecular weight cutoff, 3,500) and subsequently concentrated by lyophilization, only the GPA of spent medium increased. We conclude that maintenance of growth transformation of tamarin cells latently infected with EBV is mediated by growth factors that are entirely autocrine in origin.

Isolation and characterization of the genes for two small RNAs of herpesvirus papio and their comparison with Epstein-Barr virus-encoded EBER RNAs

Genes for the Epstein-Barr virus-encoded RNAs (EBERs), two low-molecular-weight RNAs encoded by the human gammaherpesvirus Epstein-Barr virus (EBV), hybridize to two small RNAs in a baboon cell line that contains a similar virus, herpesvirus papio (HVP). The genes for the HVP RNAs (HVP-1 and HVP-2) are located together in the small unique region at the left end of the viral genome and are transcribed by RNA polymerase III in a rightward direction, similar to the EBERs. There is significant similarity between EBER1 and HVP-1 RNA, except for an insert of 22 nucleotides which increases the length of HVP-1 RNA to 190 nucleotides. There is less similarity between the sequences of EBER2 and HVP-2 RNA, but both have a length of about 170 nucleotides. The predicted secondary structure of each HVP RNA is remarkably similar to that of the respective EBER, implying that the secondary structures are important for function. Upstream from the initiation sites of all four RNA genes are several highly conserved sequences which may function in the regulation of transcription. The HVP RNAs, together with the EBERs, are highly abundant in transformed cells and are efficiently bound by the cellular La protein.

Proteins in normal and malignant cells, cross-reacting with the latent membrane protein encoded by Epstein-Barr virus

Human B lymphocytes transformed by infection with the Epstein-Barr virus (EBV) express a new membrane protein of 63 kDa (latent membrane protein, LMP) encoded by the virus. The function of this protein in the virus-cell interaction is not known. In this work we have identified in EBV- human and mouse cell molecules which cross-react with LMP. Two types of reagents were employed: (a) antibodies against LMP-derived synthetic peptides, affinity purified from antisera against a fusion protein containing the carboxy half of the LMP molecule and (b) antisera prepared by immunizing rabbits directly with the peptide conjugates. Cross-reactions were determined by radioimmunoblotting experiments. At least six molecules (Mr = 110, 85, 63, 53, 45 and 23 kDa), present in a variety of human cells (peripheral blood lymphocytes, B cell lines and epithelial cell lines) were found to cross-react with the LMP-derived peptides. Cross-reacting proteins were also identified in normal mouse tissues. The specificity of the cross-reacting antibodies was confirmed by inhibition experiments with the corresponding peptide. Furthermore, antibodies eluted from individual bands were shown to bind to the same band when reacted with new blots of the same extracts. Our data suggest that normal cells contain a family of highly conserved proteins cross-reacting with the LMP molecule. If, indeed, these proteins share common functions, their study may lead the way to unraveling the function of LMP.

The truncated form of the Epstein-Barr virus latent-infection membrane protein expressed in virus replication does not transform rodent fibroblasts

The gene encoding the Epstein-Barr virus membrane protein LMP, expressed in latent infection, is known to induce morphologic changes and some loss of contact inhibition in NIH 3T3 cells as well as profound loss of contact inhibition and of anchorage dependence in Rat-1 cells. Another form of LMP (D1LMP), deleted for the amino terminus and first four putative transmembrane domains of LMP, was recently shown to be expressed late in Epstein-Barr virus replication. We now demonstrate that D1LMP has no transformation-associated phenotypic effect in Rat-1 cells and does not significantly affect LMP-induced Rat-1 cell transformation. LMP activity and D1LMP inactivity in inducing anchorage-independent growth are not restricted to Rat-1 cells, but are also evident in BALB/c 3T3 cells. In both cell types, loss of contact inhibition and anchorage independence are acutely evident after LMP expression. Although newly transfected polyclonal Rat-1 or BALB/c cells have a lower agar cloning efficiency than established LMP-expressing clones, this is attributable, at least in part, to their lower average LMP expression, since among clones of transfected cells, higher cloning efficiencies correlated with higher levels of LMP. LMP is bound to the vimentin cytoskeletal network in rodent fibroblasts as it is in transformed lymphocytes, whereas D1LMP showed no detectable cytoskeletal binding, suggesting that cytoskeletal association may be integral to LMP-mediated cell transformation. LMP association with the cytoskeleton in latently infected, growth-transformed lymphocytes and LMP-transformed rodent fibroblasts, correlated with the lack of both rodent cell-transforming activity and cytoskeletal association of D1LMP supports working hypothesis that cytoskeletal association is important in LMP transforming activity.

A sixth Epstein-Barr virus nuclear protein (EBNA3B) is expressed in latently infected growth-transformed lymphocytes

In the Epstein-Barr virus BamHI E genomic fragment, there are three distantly homologous long open reading frames, BERF1, BERF2b, and BERF4, each of which is preceded by a short open reading frame. The most leftward and most rightward short and long open reading frame pairs encode 145- and 155-kilodalton proteins in latently infected cells (EBNA3A and EBNA3C, respectively). In this report, we demonstrate that the middle long open reading frame, BERF2b, encodes part of a 165-kilodalton nuclear protein in every latently infected cell. Therefore, this protein is designated EBNA3B.

Sequence analysis of Raji Epstein-Barr virus DNA

The DNA sequence of the EcoRI Dhet and part of the BamHI E fragments of Raji EBV has been determined. Precise locations of two deletions in Raji DNA have been identified and their consequences for gene structure evaluated. The deletion in Raji of reading frames BALF1, BARF1, and BZLF2 and truncation of BALF2 and BERF5 probably account for the replication defect in this strain. The degree of sequence variation between B95-8 and Raji has been examined and shows considerable variation between genes. The latent membrane protein gene is exceptionally polymorphic and the initiator methionine for the late productive cycle protein overlapping the latent membrane protein is absent in Raji.

Mutational analysis of Epstein-Barr virus nuclear antigen 1 (EBNA 1)

We have constructed a set of nonsense mutants in the EBNA 1 gene of Epstein-Barr virus by inserting a synthetic oligonucleotide, which has translational termination codons in all three reading frames, at various positions in a cloned copy of the EBNA 1 gene. The EBNA 1 proteins encoded by these mutants and three deletion mutants were analyzed using several functional assays. It was determined that there are two separable phosphorylation domains in the carboxy half of the molecule. The carboxy half of the molecule was also found to contain a region between the unique Sac I and Sac II sites that is required for transactivation of the EBNA 1-specific enhancer element found within ori P. The mutants also served to identify a 248 bp region that affects the pattern of intranuclear localization of the protein. Correlations between the functional domains established by these studies and other properties of EBNA 1 are discussed.

A fifth Epstein-Barr virus nuclear protein (EBNA3C) is expressed in latently infected growth-transformed lymphocytes

Three distantly homologous neighboring long open reading frames in the Epstein-Barr virus (EBV) genome are preceded by short open reading frames. The leftmost short and long open reading frames encode EBNA3, a nuclear protein which is slightly smaller (145 kilodaltons [kDa]) than two other nuclear proteins (150 to 155 kDa) detected in Western blots (immunoblots) of latently infected cell protein (K. Hennessy, F. Wang, E. Woodland-Bushman, and E. Kieff, Proc. Natl. Acad. Sci. USA 83:5693-5697, 1986; I. Joab, D. T. Rowe, M. Bodescot, J.-C. Nicolas, P. J. Farrell, and M. Perricaudet, J. Virol. 61:3340-3344, 1987). We have demonstrated that the most rightward short (BERF3) and long (BERF4) open reading frames are spliced in frame at the 3' end of a 5-kilobase latently infected cell RNA and that this RNA begins within or upstream of the EBV long internal repeat. EBV-immune human antibodies specific for the long open reading frame translation product identified a 155-kDa protein on Western blots of latently infected cell protein and specifically reacted with large nonnucleolar nuclear granules in every latently infected cell. Expression of the cDNA in BALB/c 3T3 cells resulted in translation of full-size EBNA3C but had no effect on cell morphology, contact inhibition, or serum independence.

B-cell lymphoproliferation and lymphomagenesis are associated with clonotypic intracellular terminal regions of the Epstein-Barr virus

We analyzed 17 B-cell lineages cloned from two patients with infectious mononucleosis and found that different B-cell lineages exhibited notable variation in the length of the fused Epstein-Barr virus (EBV) terminal region on intracellular EBV episomes. EBV termini in different B-cell clones from the same person differed by as many as 15 to 20 reiterations of the ca. 500-base-pair terminal repeat sequence. In contrast, analysis of seven B-cell lineages cloned from a patient with a fatal, oligoclonal lymphoma revealed that three of the cell clones had the same-sized EBV terminal region. These three clones had previously been shown, by immunoglobulin gene analysis, to be metastatic daughter cells descended from a common progenitor. Similarity of the EBV terminal regions in the three daughter clones suggested that EBV infected the progenitor cell before proliferation and metastasis. Individual, EBV-infected cells from a single individual showed sufficient heterogeneity in their EBV termini to allow use of terminal fragment size as a clonal marker in studies addressing the contribution of EBV to the clonal pathogenesis of tumors with which this virus has been associated.

A promoter for the highly spliced EBNA family of RNAs of Epstein-Barr virus

A transcription start for the highly spliced EBNA group of RNAs in B95-8 cells has been identified in the short unique region of the virus genome. This promoter is used in many (but not all) human cell lines carrying Epstein-Barr virus, including a tightly latent human lymphoblastoid cell line. Another promoter for the EBNA RNAs was described previously in the internal repeat region of the virus genome. The existence of these alternative promoters may be important for differential control of EBNA gene expression.

An Epstein-Barr virus transforming protein associates with vimentin in lymphocytes

The Epstein-Barr virus (EBV) latent infection membrane protein (LMP) is likely to be an important mediator of EBV-induced cell proliferation, since it is one of the few proteins encoded by the virus in latent infection and since production of this protein in Rat-1 cells results in their conversion to a fully transformed phenotype. LMP was previously noted to localize to patches at the cell periphery. In this paper we examine the basis of LMP patching in EBV-infected, transformed lymphocytes. Our data indicate that LMP is associated with the cytoskeletal protein vimentin. Although LMP is fully soluble in isotonic Triton X-100 buffer, only 50% of it is extracted from cells in this solution. The rest remains bound to the cytoskeleton. LMP undergoes phosphorylation, and phosphorylated LMP is preferentially associated with the cytoskeleton. As judged by both immunofluorescence and immunoelectron microscopy, the vimentin network in EBV-transformed lymphocytes or EBV-infected Burkitt tumor lymphocytes is abnormal. Vimentin and LMP often colocalize in a single patch near the plasma membrane. In response to Colcemid treatment of EBV-infected cells, vimentin reorganizes into perinuclear rings, as it does in uninfected cells. LMP is associated with these perinuclear rings. Vimentin (or a vimentin-associated protein) may be a transducer of an LMP transmembrane effect in lymphoproliferation.

Stable transfection of a human lymphoma line by sub-genomic fragments of Epstein-Barr virus DNA to measure humoral and cellular immunity to the corresponding proteins

An Epstein-Barr virus (EBV)-negative human lymphoid B-cell line, DG75, was stably transfected with recombinant selection vectors that carry a subfragment of the BamHI WYH region (nucleotides 44664 to 50628), the BamHI K fragment, or a subfragment of the EcoRI D region (nucleotides 166614 to 170149) of B95-8 EBV DNA. These fragments contain the coding exons for the EBV-determined nuclear antigens EBNA2 and EBNA1, and the membrane antigen LMP, respectively. Antigen expression of the cells was detected by immunofluorescence. EBNA2 was expressed in 80-100% of the transfected cells, in contrast to EBNA1 which was expressed in only 25%, and LMP in only about 5% of the cells. Humoral antibody responses were measured by immunofluorescence and compared to cellular immunity as determined by the leukocyte migration inhibition (LMI) technique. Extracts from transfected cell lines expressing EBNA1, EBNA2 or LMP elicited an LMI response with cells from healthy EBV-seropositive individuals whereas the extract from the parental DG75 cell line did not. The results demonstrate the value of stably transfected cell lines expressing a defined EBV antigen for the monospecific analysis of host responses to the EBV-encoded antigen complex in growth-transformed cells.

Differences in B cell growth phenotype reflect novel patterns of Epstein-Barr virus latent gene expression in Burkitt's lymphoma cells

Recently established Epstein-Barr virus (EBV)-positive Burkitt's lymphoma (BL) cell lines, carrying chromosomal translocations indicative of their malignant origin, have been monitored for their degree of in vitro progression towards a more 'lymphoblastoid' cell surface phenotype and growth pattern, and for their expression of three EBV latent gene products which are constitutively present in all virus-transformed normal lymphoblastoid cell lines (LCLs). BL cell lines which stably retained the original tumour biopsy phenotype on serial passage were all positive for the nuclear antigen EBNA 1 but did not express detectable amounts of two other 'transforming' proteins, EBNA 2 and the latent membrane protein (LMP). This novel pattern of EBV gene expression was also observed on direct analysis of BL biopsy tissue. All three viral proteins became detectable, however, in BL cell lines which had progressed towards a more LCL-like phenotype in vitro. This work establishes a link between B cell phenotype and the accompanying pattern of EBV latent gene expression, and identifies a novel type of EBV:cell interaction which may be unique to BL cells.

Isolation and characterization of cDNA clones corresponding to transcripts from the BamHI H and F regions of the Epstein-Barr virus genome

The Epstein-Barr virus (EBV) mutant P3HR1 is incapable of immortalizing B lymphocytes because of a 6.8-kilobase deletion in the BamHI W, Y, and H regions of the viral genome (M. Rabson, L. Gradoville, L. Heston, and G. Miller, J. Virol. 44:834-844, 1982). To characterize transcripts that are encoded in this region, poly(A)+ RNA from the EBV-transformed lymphoblastoid cell line JY was isolated, and this RNA was used to generate a cDNA library in lambda gt10. By screening 500,000 recombinant bacteriophages with the BamHI H fragment, we isolated 10 cDNA clones and characterized them in detail. One group of six cDNA clones was derived from a 2.9-kilobase early transcript encoded by the IR2 repeat element and showed restriction site polymorphism for the enzyme SmaI. The second group consisted of four cDNA clones, all of which contained the BamHI-H right reading frame (BHRF1), and used the polyadenylation signal at base pair 662 in the BamHI F fragment. Computer analysis of the hydrophobicity of the BHRF1 protein revealed that it is likely to be a membrane protein. Northern blotting experiments with RNA from an EBV producer line, B95-8, and a tightly latent lymphoblastoid B-cell line, IB4, revealed that BHRF1 is contained in at least two different mRNA species which can be detected during the latent cycle of EBV. These data and the recent characterization of a spliced transcript (containing five exons in common with other known latent messages [M. Bodescot and M. Perricaudet, Nucleic Acids Res. 14:7103-7113, 1986]) suggest that alternative splicing is used to generate transcripts containing BHRF1, as for the EBV nuclear antigen 1 transcripts. Furthermore, the observation that a potential oncogene activated in human follicular lymphomas is homologous to the BHRF1-encoded polypeptide (M. L. Cleary, S.D. Smith, and J. Sklar, Cell 47:19-28, 1986) suggests a possible role for this putative viral protein in EBV-induced growth transformation of B lymphocytes.

Enhancement of Epstein-Barr virus membrane protein (LMP) expression by serum, TPA, or n-butyrate in latently infected Raji cells

The BamHI Nhet region of the EBV DNA is known to code for two proteins. One is a membrane protein (LMP) with an apparent molecular weight of 60,000 on SDS-PAGE which is expressed in latently EBV infected cells. The second protein, so far unidentified, is presumably a late protein with a calculated molecular weight of 28,000 Da. Antisera against both proteins were generated by immunizing rabbits with either a fusion protein containing 155 amino acids of the C-terminus of LMP and a 37,000 mol wt piece of the bacterial anthranilate synthase or with a C-terminal synthetic peptide of 7 amino acids. These sera reacted with a protein varying in size between 60,000 and 65,000 mol wt on SDS-PAGE, found in all cell lines harboring EBV. In addition, these sera identified a second protein with an apparent molecular weight of 49,000 on SDS-PAGE in B95-8, P3HR-1, and M-ABA cells, which is presumably identical with the 28,000-Da protein mentioned above. Furthermore, with these sera a positive cytoplasmic immunofluorescence in 1 to 10% of the cells was obtained, depending on the cell line examined. Analyzing the nonproducer Raji cell line, the number of immunofluorescence-positive cells and the amount of the 60,000 protein, as judged by immunoblotting, was rapidly increased by addition of fresh medium with 10% fetal calf serum as well as by the tumor promoter TPA or to an even higher extend by n-butyrate. The kinetics of induction reached a maximum 24 hr after addition of medium plus 10% fresh serum or TPA or n-butyrate and decreased after 24 to 48 hr. Since the induction of the EBV early antigen (EA) associated proteins by TPA or n-butyrate exhibits a diverse kinetic with a maximum at 72 hr, the regulation of the 60,000 protein synthesis appears to be different from known EA-associated proteins.

Posttranslational processing of the Epstein-Barr virus-encoded p63/LMP protein

In this paper we describe the posttranslational processing of the p63/LMP (latent membrane protein) encoded by Epstein-Barr virus in transformed B cells. Specifically, we show that after synthesis, free LMP disappeared with a half-life of about 0.5 h. This was caused by the association of LMP with an insoluble complex. All detectable LMP in the plasma membrane was insoluble. This interaction was resistant to nondenaturing detergents but readily dissociated with 8 M urea or by boiling in 0.5% sodium dodecyl sulfate, suggesting that LMP may be associated with cytoskeletal elements. Most of the Nonidet P-40-insoluble LMP was phosphorylated (ppLMP) primarily on serine but also on threonine residues. No phosphotyrosine was detected. Furthermore, greater than 90% of the ppLMP resided in the Nonidet P-40-insoluble fraction, suggesting a strong correlation between complexing and phosphorylation. Additionally, ppLMP was found to be associated with a 53,000-molecular-weight phosphoprotein (pp53) of unknown origin. Finally, LMP turned over extremely rapidly, with a half-life of about 2 h. Taken together, these properties suggest that although LMP falls broadly within the category of phosphorylated, cytoskeleton-associated oncoproteins, it is nevertheless clearly different from any previously described member of this family.

An Epstein-Barr virus transforming protein associates with vimentin in lymphocytes

The Epstein-Barr virus (EBV) latent infection membrane protein (LMP) is likely to be an important mediator of EBV-induced cell proliferation, since it is one of the few proteins encoded by the virus in latent infection and since production of this protein in Rat-1 cells results in their conversion to a fully transformed phenotype. LMP was previously noted to localize to patches at the cell periphery. In this paper we examine the basis of LMP patching in EBV-infected, transformed lymphocytes. Our data indicate that LMP is associated with the cytoskeletal protein vimentin. Although LMP is fully soluble in isotonic Triton X-100 buffer, only 50% of it is extracted from cells in this solution. The rest remains bound to the cytoskeleton. LMP undergoes phosphorylation, and phosphorylated LMP is preferentially associated with the cytoskeleton. As judged by both immunofluorescence and immunoelectron microscopy, the vimentin network in EBV-transformed lymphocytes or EBV-infected Burkitt tumor lymphocytes is abnormal. Vimentin and LMP often colocalize in a single patch near the plasma membrane. In response to Colcemid treatment of EBV-infected cells, vimentin reorganizes into perinuclear rings, as it does in uninfected cells. LMP is associated with these perinuclear rings. Vimentin (or a vimentin-associated protein) may be a transducer of an LMP transmembrane effect in lymphoproliferation.

Epstein-Barr virus nuclear antigen 2 specifically induces expression of the B-cell activation antigen CD23

Epstein-Barr virus (EBV) infection of EBV-negative Burkitt lymphoma (BL) cells induces some changes similar to those seen in normal B lymphocytes that have been growth transformed by EBV. The role of individual EBV genes in this process was evaluated by introducing each of the viral genes that are normally expressed in EBV growth-transformed and latently infected lymphoblasts into an EBV-negative BL cell line, using recombinant retrovirus-mediated transfer. Clones of cells were derived that stably express the EBV nuclear antigen 1 (EBNA-1), EBNA-2, EBNA-3, EBNA-leader protein, or EBV latent membrane protein (LMP). These were compared with control clones infected with the retrovirus vector. All 10 clones converted to EBNA-2 expression differed from control clones or clones expressing other EBV proteins by growth in tight clumps and by markedly increased expression of one particular surface marker of B-cell activation, CD23. Other activation antigens were unaffected by EBNA-2 expression, as were markers already expressed on the parent BL cell line, including BL markers (cALLA and BLA), proliferation markers (transferrin receptor and BK19.9), and cell adhesion-related molecules (LFA-1 and LFA-3). Increased CD23 expression in cells expressing EBNA-2 was apparent from monoclonal anti-CD23 antibody binding to the cell surface, from immunoprecipitation of the 45-kDa and 90-kDa CD23 proteins with monoclonal antibody, and from RNA blots probed with labeled CD23 DNA. The results indicate that EBNA-2 is a specific direct or indirect trans-activator of CD23. This establishes a link between an EBV gene and cell gene expression. Since CD23 has been implicated in the transduction of B-cell growth signals, its specific induction by EBNA-2 could be important in EBV induction of B-lymphocyte transformation.

Pathogenesis of Burkitt lymphoma: expression of an activated c-myc oncogene causes the tumorigenic conversion of EBV-infected human B lymphoblasts

To study the pathogenesis of Burkitt lymphoma, we introduced activated c-myc genes into human EBV-infected lymphoblastoid cells derived from in vitro infection of normal cord blood or directly from infected peripheral blood from AIDS patients. In both cell types the constitutive expression of exogenous c-myc caused negative regulation of endogenous c-myc expression, changes in growth properties typical of transformed cells, and acquisition of tumorigenicity in immunodeficient mice. In all myc-transfected populations the degree of malignancy directly correlated with the level of c-myc mRNA. EBV infection and c-myc activation are thus sufficient for the tumorigenic conversion of human B cells in vitro, strongly supporting the hypothesis that these same two pathogenetic steps may be involved in the in vivo development of Burkitt lymphoma.

A bicistronic Epstein-Barr virus mRNA encodes two nuclear proteins in latently infected, growth-transformed lymphocytes

EBNA2 is a nuclear protein expressed in all cells latently infected with and growth transformed by Epstein-Barr virus (EBV) infection (K. Hennessy and E. Kieff, Science 227:1230-1240, 1985). The nucleotide sequence of the EBNA2 mRNA (J. Sample, M. Hummel, D. Braun, M. Birkenbach, and E. Kieff, Proc. Natl. Acad. Sci. USA 83:5096-5100, 1986) revealed that it begins with a 924-base open reading frame that has an unusual potential translational initiation site (CAAATGG). This open reading frame is followed by 138 nucleotides with only one highly unlikely translational initiation site (TACATGC), which would translate a pentapeptide before the next stop codon. The last part of the mRNA is the open reading frame which encodes EBNA2. In this paper, we demonstrate that the 924-base open reading frame translates a 40-kilodalton protein in vitro or in murine cells transfected with the EBNA2 cDNA under control of the murine leukemia virus long terminal repeat. A protein of identical size was detected in EBV-transformed, latently infected human lymphocyte nuclei by using antibody specific for the leader open reading frame expressed in bacteria. Therefore, this is a rare example of a mRNA which translates two proteins from nonoverlapping open reading frames. Since the protein encoded by the leader of the EBNA mRNA is expressed in all nuclei of a latently infected cell line, it was designated EBNA-LP. EBNA-LP localizes to small intranuclear particles and differs in this respect from EBNA1, EBNA2, or EBNA3. EBNA-LP is not expressed in an EBV-transformed marmoset lymphocyte cell (B95-8) or in one EBV-infected Burkitt tumor cell line (Raji) but is expressed in three other Burkitt tumor cell lines (Namalwa, P3HR-1, and Daudi).

The Florey lecture, 1986. Vaccine prevention of virus-induced human cancers

Carcinogenic viruses have been discovered in numerous animal species over the last 80 years but their role in human cancer has only recently become an important issue. With EB virus involved with endemic Burkitt's lymphoma and undifferentiated nasopharyngeal carcinoma, hepatitis B virus with primary liver cancer, papilloma viruses with carcinoma of the cervix, and T-cell leukaemia virus with adult T leukaemia, 20-25% of all human cancer appears to have a virus component in its causation. By analogy with certain virus-induced animal cancers, vaccine prevention of infection should greatly reduce subsequent tumour development; vaccines against hepatitis B virus are already on trial for this purpose in populations at risk. Experiments are described in which an EB virus subunit vaccine consisting of the virus-determined membrane antigen glycoprotein molecule of molecular mass 340 kDa (MA gp340) has been prepared by two purification methods. Material from one of these has successfully protected cotton-top tamarins against a 100% lymphomagenic dose of challenge virus and investigations are under way to identify an immunogen, based on MA gp340, suitable for use in man. Genetically engineered bacterial, yeast, and mammalian cells expressing the gp340 gene are already available; this gene has also been inserted into vaccinia and varicella virus vectors. Powerful new adjuvants are also considered, together with future strategies for human vaccine studies.