Relationship between the amounts of EBV-DNA and EBNA per cell, clonability and tumorigenicity in two ebv-negative lymphoma lines and their EBV-converted sublines

Methods for assessing the in vitro transforming activity of NF-κB transcription factor c-Rel and related proteins

Among NF-κB transcription factors, c-Rel and c-Rel-derived proteins, including v-Rel, are the only ones that have shown consistent and frank transforming activity in cell culture. In particular, viral, chicken, mouse, and human Rel proteins can rapidly transform primary chicken spleen and bone marrow cells. Overexpression of a human Rel protein missing a C-terminal transactivation domain can also enhance the transformed state of the human B-lymphoma cell line BJAB. As described in this chapter, these in vitro assays can be used to quantitatively assess the transforming activity of Rel proteins.

EBV and genomic instability--a new look at the role of the virus in the pathogenesis of Burkitt's lymphoma

Epidemiological and molecular evidence links Epstein-Barr virus (EBV) carriage to the pathogenesis of human malignancies of lymphoid and epithelial cell origin but the mechanisms of viral oncogenesis are poorly understood. Burkitt's lymphoma, a tumor occurring in both EBV-positive and -negative forms, provides a convenient model for analysis of the relative contribution of genetic changes and viral products that are expressed in the malignant cells. Here we review recent findings that highlight several mechanisms by which EBV could play an important role in oncogenesis by promoting genomic instability.

Overexpression of an activated REL mutant enhances the transformed state of the human B-lymphoma BJAB cell line and alters its gene expression profile

The human REL proto-oncogene encodes a transcription factor in the NF-κB family. Overexpression of REL is acutely transforming in chicken lymphoid cells, but has not been shown to transform any mammalian lymphoid cell type. In this report, we show that overexpression of a highly transforming mutant of REL (RELΔTAD1) increases the oncogenic properties of the human B-cell lymphoma BJAB cell line, as demonstrated by increased colony formation in soft agar, tumor formation in SCID mice, and adhesion. BJAB-RELΔTAD1 cells also show decreased activation of caspase in response to doxorubicin. BJAB-RELΔTAD1 cells have increased levels of active nuclear REL protein as determined by immunofluorescence, subcellular fractionation, and electrophoretic mobility shift assay. Overexpression of RELΔTAD1 in BJAB cells has transformed the gene expression profile of BJAB cells from that of a germinal center B-cell subtype of diffuse large B-cell lymphoma (GCB-DLBCL) to that of an activated B-cell subtype (ABC-DLBCL), as evidenced by increased expression of many ABC-defining mRNAs. Up-regulated genes in BJAB-RELΔTAD1 cells include several NF-κB targets that encode proteins previously implicated in B-cell development or oncogenesis, including BCL2, IRF4, CD40 and VCAM1. The cell system we describe here may be valuable for further characterizing the molecular details of REL-induced lymphoma in humans.

Epstein-Barr virus (EBV) recombinants: use of positive selection markers to rescue mutants in EBV-negative B-lymphoma cells

The objective of these experiments was to develop strategies for creation and identification of recombinant mutant Epstein-Barr viruses (EBV). EBV recombinant molecular genetics has been limited to mutations within a short DNA segment deleted from a nontransforming EBV and an underlying strategy which relies on growth transformation of primary B lymphocytes for identification of recombinants. Thus, mutations outside the deletion or mutations which affect transformation cannot be easily recovered. In these experiments we investigated whether a toxic drug resistance gene, guanine phosphoribosyltransferase or hygromycin phosphotransferase, driven by the simian virus 40 promoter can be recombined into the EBV genome and can function to identify B-lymphoma cells infected with recombinant virus. Two different strategies were used to recombine the drug resistance marker into the EBV genome. Both utilized transfection of partially permissive, EBV-infected B95-8 cells and positive selection for cells which had incorporated a functional drug resistance gene. In the first series of experiments, B95-8 clones were screened for transfected DNA that had recombined into the EBV genome. In the second series of experiments, the transfected drug resistance marker was linked to the plasmid and lytic EBV origins so that it was maintained as an episome and could recombine with the B95-8 EBV genome during virus replication. The recombinant EBV from either experiment could be recovered by infection and toxic drug selection of EBV-negative B-lymphoma cells. The EBV genome in these B-lymphoma cells is frequently an episome. Virus genes associated with latent infection of primary B lymphocytes are expressed. Expression of Epstein-Barr virus nuclear antigen 2 (EBNA-2) and the EBNA-3 genes is variable relative to that of EBNA-1, as is characteristic of some naturally infected Burkitt tumor cells. Moreover, the EBV-infected B-lymphoma cells are often partially permissive for early replicative cycle gene expression and virus replication can be induced, in contrast to previously reported in vitro infected B-lymphoma cells. These studies demonstrate that dominant selectable markers can be inserted into the EBV genome, are active in the context of the EBV genome, and can be used to recover recombinant EBV in B-lymphoma cells. This system should be particularly useful for recovering EBV genomes with mutations in essential transforming genes.

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)

The average number of molecules of Epstein-Barr nuclear antigen 1 per cell does not correlate with the average number of Epstein-Barr virus (EBV) DNA molecules per cell among different clones of EBV-immortalized cells

Epstein-Barr nuclear antigen 1 (EBNA-1) is the only viral protein required to support latent replication of Epstein-Barr virus (EBV). To assess the likelihood that EBNA-1 regulates the amount of EBV DNA in a cell, we measured the average numbers of EBNA-1 molecules and EBV DNA molecules per cell in different clones of cells. The amount of EBNA-1 protein present in recently established lymphoblastoid cell lines was measured with affinity-purified anti-EBNA-1 antibodies, and viral DNA was measured by nucleic acid hybridization. The average levels of EBNA-1 protein varied little between these cell lines, whereas the average amount of viral DNA present varied substantially; consequently, these numbers were not correlated. There is no apparent relationship between amounts of EBNA-1 and viral DNA.

Induction of B cell responsiveness to growth factors by Epstein-Barr virus conversion: comparison of endogenous factors and interleukin-1

Immortalized B lymphocytes produce a factor(s) that stimulates growth of B cell lines carrying Epstein-Barr virus (EBV). Stimulatory supernatants derived from B cells also exhibit interleukin-1 (IL-1) activity in costimulator assays with the D10.G4.1 helper T cell line. Experiments with purified macrophage-derived IL-1 and recombinant IL-1 beta demonstrate that IL-1 stimulates proliferation of the cell lines that respond to the factors from B lymphocyte lines. One B cell line, Ramos, an EBV-Burkitt's lymphoma, contrasts with other B cell lines in that it is refractory to the growth enhancing effects of B cell conditioned medium and macrophage-derived IL-1. When EBV was introduced into Ramos cells, growth was enhanced by the factor(s) in B cell conditioned medium (six out of seven lines); growth of EBV-converted Ramos lines (six out of seven lines) also was enhanced by IL-1. These findings demonstrate that infection of a non-responsive transformed B lymphocyte by EBV induces cellular responsiveness to factor-mediated growth stimulation.

Reversion of tumorigenicity and decreased agarose clonability after EBV conversion of an IgH/myc translocation-carrying BL line

The Epstein-Barr virus (EBV)-negative Burkitt lymphoma (BL) line BL-41, and 5 independently established EBV-converted sublines, derived by infection with a transforming (B95-8) or a nontransforming (P3HR1) strain of EBV, were compared for clonability in semi-solid agarose and for tumorigenicity in immuno-suppressed mice. One P3HR1 viral convertant and 3 out of 4 B95-8 virus-converted sublines had a high (greater than 40%) agarose clonability, like the BL 41 parent, and were slightly more tumorigenic than BL-41. In contrast, the fourth B95-8 converted subline, BL-41/95, was virtually non-tumorigenic and its agarose clonability was much lower (3-23%). It showed a more drastic shift towards an LCL-like phenotype than the other convertants as reflected by high HLA class-I and EBV-encoded latent membrane protein (LMP) expression. BL 41/95 still contains the 8;14 IgH/myc translocation, carried by the parental line, and maintains the same relatively high steady-state level of c-myc mRNA and protein as the highly tumorigenic convertants. We conclude that the tumorigenicity of BL41/95 has been suppressed by a gene that acts at a level beyond the expression of the activated oncogene, in the same way as the revertants isolated from ras and SV-40-transformed cultures (Klein, 1987b; Bassin and Noda, 1987).

Burkitt's lymphoma cell lines reveal different degrees of tumorigenicity in nude mice

A quantitative in vivo assay for evaluating the tumorigenicity of Burkitt's lymphoma (BL) cell lines in nude mice is described. It is based on the dose-response kinetics of BL cell lines in pre-irradiated (480 rad) nude mice following the s.c. injection of 4 different cell doses. This model system was used to estimate the xenografting potential of 26 BL cell lines derived from BL patients of different geographic and ethnic origins, as well as lymphoblastoid cell lines (LCLs) established by Epstein-Barr virus (EBV) immortalization of normal B lymphocytes. The results indicate that most BL cell lines are tumorigenic, but LCLs fail to produce progressively growing tumours in nude mice. However, BL cell lines revealed individual degrees of tumorigenicity and accordingly could be divided into 4 groups with high, moderate, low or no tumorigenicity. Preliminary attempts to correlate the xenografting phenotype of BL lines with other characteristics indicate that: (1) the aberrations of chromosome 1 are more often encountered in cell lines with high and moderate tumorigenicity; (2) EBV-positive BL lines do not reveal a higher tumorigenic phenotype in comparison with EBV-negative ones; (3) cell lines carrying translocations t(8;22) and t(2;8) might fall more frequently in the group of lines with high and moderate tumorigenicity; and (4) when LCLs grow in nu/nu mice, rejection always occurs and is associated with massive tumour necrosis. These findings suggest that the tumorigenicity of BL cell lines in immunosuppressed animals is not related with EBV, but with certain chromosomal abnormalities (BL-specific and non-specific) indicating that this in vivo model system can be instrumental for the identification of other factors or stages involved in BL development.

Paired Epstein-Barr virus (EBV)-negative and EBV-converted Burkitt lymphoma lines: stimulatory capacity in allogeneic mixed lymphocyte cultures

Epstein-Barr virus (EBV)-negative Burkitt lymphoma lines (BLE-) and their in vitro EBV-converted sublines (BLEc), obtained by infection with the P3HRI and B95-8 strains of EBV, were compared for their capacity to induce T-lymphocyte proliferation in allogeneic mixed lymphocyte cultures (MLC). Regardless of the virus strain used for conversion, the BLEc lines induced a considerably stronger primary MLC response than their EBV-negative parentals. Only the BLEc lines were able to maintain T-lymphocyte proliferation in repeated stimulations. The low proliferative response observed in cultures stimulated with BLE- cells was not due to the generation of suppressor cells or to the release of inhibitory factors. The increased stimulatory capacity of BLEc lines was unrelated to changes in expression of MHC class-I and class-II antigen, or of B-cell activation markers, and was not due to the reactivation of EBV-specific memory T cells, since lymphocytes from EBV-seropositive and seronegative donors responded similarly. The results indicate that the capacity of BL cells to elicit cellular immune responses may be influenced by their EBV-carrying status.

Conversion of the lymphoma line "BJAB" by Epstein-Barr virus into phenotypically altered sublines is accompanied by increased c-myc mRNA levels

The steady-state level of c-myc proto-oncogene mRNA was investigated in the EBV-negative human B-lymphoma line BJAB and 2 sublines that have been converted by EBV into stable EBV-genome-carrying and EBNA-positive status. The EBV-converted sublines expressed c-myc at a 2- to 6-fold higher level than the original BJAB during exponential growth. The EBV-positive BJAB lines are known to differ from the parent line in several phenotypic characteristics, including increased agarose clonability, lower serum requirement and, in one case, increased tumorigenicity in nude mice. The pattern of increased c-myc expression accompanying EBV conversion was not observed in the myc/Ig translocation-carrying Burkitt lymphoma line RAMOS or in 2 of its EBV-converted sublines.

Epstein-Barr virus-related ultrastructural modifications of plasma membrane during B-cell transformation

Ultrastructural modifications are described in the plasma membrane of in vitro established human B cells. By the freeze-fracture technique, intramembrane particles (IMPs) are quantified in B lymphocytes following Epstein-Barr virus (EBV) transformation in vitro, and in B-lymphoma (Burkitt-type) cells, either positive or negative for EBV genome. Analysis shows an overall increase in IMP density as compared to normal controls. Differences are observed between the protoplasmic and exoplasmic faces of fractured membranes as well as among in vitro transformed and clearly neoplastic cells. Results indicate that conformational changes in IMP distribution parallel neoplastic evolution of transformed cells.

Quantitation of Epstein-Barr virus (EBV)-determined nuclear antigen (EBNA) by a two-site enzyme immunoassay, in parallel with EBV-DNA

A two-site enzyme (TSE) immunoassay was developed for the quantitation of the Epstein-Barr virus (EBV)-determined nuclear antigen (EBNA) using a rabbit serum raised against a synthetic peptide derived from the BamHI K region of the viral genome. Comparison of 12 EBNA-positive and 3 negative cell lines proved that the test was EBV-specific. A dot-blot assay utilizing cloned and nick translated EBV-DNA BamHI M fragment confirmed the EBV-carrier status of the EBNA-positive lines. The results obtained with both the TSE immunoassay and dot-blot assay were in agreement with published values. In contrast to earlier reports, we could not demonstrate any correlation between the content of EBNA and the number of viral genome copies.

Differential recognition of tumor-derived and in vitro Epstein-Barr virus-transformed B-cell lines by fetal calf serum-specific T4-positive cytotoxic T-lymphocyte clones

Two interleukin-2 (IL-2)-dependent cytotoxic T-cell clones were obtained by limiting dilution from a lymphocyte culture stimulated in vitro with the autologous Epstein-Barr virus-transformed lymphoblastoid cell line (LCL) in the presence of fetal calf serum (FCS). Both clones uniformly had a T3+, T4+, Dr+ phenotype and lysed autologous B blasts, the autologous LCL, and allogeneic B cell lines sharing major histocompatibility complex (MHC) class II antigens. The cytotoxic function was triggered by FCS-derived components. There was no killing if the sensitive targets were cultured in serum-free medium or in medium supplemented with human serum. Sensitivity to lysis could be restored by exposing the targets to FCS for at least 6 hr at 37 degrees C. Monoclonal antibodies directed to T-cell-specific surface antigens and MHC class II antigens inhibited lysis with different efficiencies depending on the target cell origin. Killing of Burkitt's lymphoma (BL)-derived cell lines was blocked more easily than killing of LCLs. LCLs but not BL lines induced proliferation of the T-cell clones in the absence of exogenous IL-2. The differences were not related to quantitative variations in the expression of MHC class II antigens, indicating that BL lines differ from LCLs in other cell membrane properties that may influence antigen presentation. The results suggest that the affinity of effector/target binding, which is probably influenced by the concentration of antigenic determinants expressed on the target cell membrane, determines whether proliferative responses or cytotoxicity are induced in the antigen-recognizing T cells.

Natural killer cell sensitivity of human lymphoid lines of B-cell origin does not correlate with tumorigenicity or with the expression of certain differentiation markers

Human B-cell lines derived from normal donors (LCL) or from Burkitt lymphomas (BL) were compared for their sensitivity to natural (NK) and interferon (IFN)-activated (IAK) cytotoxicity, mediated by effector cells from normal human blood. In four cases, a BL and an LCL line were derived from the same donor and had been kept in culture for the same period of time. The BL series included both Epstein-Barr virus (EBV)-carrying and EBV-negative lymphoma lines. The latter were compared with their own EBV-converted, Epstein-Barr nuclear antigen (EBNA)- and EBV-DNA-positive sublines, established by in vitro infection with two different viral substrains. LCL and BL lines from the same donor were lysed with equal efficiency by both NK and IAK effectors. There was no relationship between the NK sensitivity and the nude mouse tumorigenicity of different EBV-converted Ramos sublines, or the expression of differentiation markers such as insulin receptor, surface IgD, and the B2 surface antigen. Moreover, EBV-converted sublines of BJAB differed in their NK sensitivity, in spite of closely similar expression of these markers. NK-sensitive Ramos and BJAB sublines induced a stronger proliferative response upon confrontation with allogeneic lymphocytes than their NK-resistant counterparts. This suggests that the target cell may play an active role in triggering the lytic interaction. There was no correlation between this property and any of the other parameters studied.

Differential cytotoxicity of tumour promoter TPA for EBV-negative human lymphoma cell lines and their EBV-converted sublines

The tumour-promoting phorbol diester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) is toxic at nanomolar concentrations for Epstein-Barr virus (EBV)-negative human lymphoma cell lines BJAB and Ramos. Sublines that have been converted to Epstein-Barr nuclear antigen (EBNA)-positivity by infection with EBV survive and grow in TPA concentrations that are lethal for their non-converted parental lines. Resistance to the growth-inhibiting effect of TPA is higher in sublines of BJAB and Ramos that have been converted by the transforming B95-8 strain of EBV than in those converted by the non-transforming P3-HRI strain. The loss of TPA-sensitivity may reflect a change in the transformation or differentiation state of lymphoma cells as a result of EBV-conversion.