Regressing nude mouse grafts of Burkitt's lymphoma x lymphoblastoid cell hybrids show deregulation of the c-myc gene and expression of the EBV latent membrane protein

Cell fusion is not involved in the generation of giant cells in the Hodgkin-Reed Sternberg cell line L1236

The mechanism of multinucleated cell formation in Hodgkin's disease has not yet been elucidated. We asked whether the giant multinucleated cells of the H-RS cell line L1236 develop via fusion of the predominant smaller cells. As a positive control for the fusion assay, human B cells from the B-cell lymphoma cell line BJA-B were split into two fractions, stained with the fluorochromes CMTMR and CMFDA, respectively, and fused using the polyethylene glycol 1500 cell hybridization protocol. Double-stained cells indicating fusion of BJA-B cells were detectable for up to 5 days. In parallel, L1236 cells were split into two fractions, stained with the fluorochromes, and mixed. No double-stained L1236 cells were detected. The same result was obtained when using FACS-sorted small mononuclear L1236 cells. It is thus concluded that the large multinucleated cells of the monoclonal H-RS cell line L1236 have emerged by endomitosis rather than by spontaneous cell fusion.

Suppression of Burkitt's lymphoma tumorigenicity in nude mice by co-inoculation of EBV-immortalized lymphoblastoid cells

EBV-immortalized B-lymphoblastoid cell lines (LCL) inoculated s.c. into T-cell-deficient nude mice regress completely after a short initial growth period. We tested whether the putative host response underlying this phenomenon might also be directed against progressively growing Burkitt's lymphoma (BL) tumors in nude mice. Outgrowth of BL tumors was suppressed when cells of the highly tumorigenic BL cell line BL 60 were mixed with cells of the autologous LCL IARC 277 before s.c. inoculation into nude mice. Even when the cells were inoculated separately and simultaneously into contralateral flanks of the mice, regression of initially growing BL tumors could be observed, albeit with reduced frequency and dependent on the dose of LCL cells. Tumor growth of BL 60 cells could also be suppressed by co-inoculation with the non-autologous LCL IARC 174 and IARC 277 cells could suppress growth of the non-autologous BL cell line Eli. Pronounced infiltration with murine (m)CD-11b-positive mouse macrophages and mCD-8a-positive mouse lymphoid cells, most probably natural killer cells, was seen in histological tissue sections of regressing BL 60 tumors when LCL cells were inoculated contralaterally. In regressing BL tumors, these mouse cells were present not only in necrotic areas but also in vital BL tissue, indicating that infiltration of mouse cells had taken place before the development of necrosis. Since tumor-infiltrating mouse cells can be activated at least by some human cytokines, we measured cytokine production of BL 60 and IARC 277. High amounts of IL 6 and IL 10 were produced by the LCL cells, whereas IL-6 and IL-10 production by the BL 60 cells was beyond or close to the detection threshold. In addition, IL 8 was secreted up to 5-fold more by the LCL than by the BL cells. The results presented here thus suggest a host response of the nude mouse, which is triggered by cytokines released from the LCL but, once induced, is directed also against BL cells.

Integration of Epstein Barr virus near the breakpoint of a translocation 11;19 in a Burkitt's lymphoma cell line

The physical state of Epstein Barr virus (EBV) DNA was analyzed in the Burkitt's lymphoma (BL) cell line BL60 and in the autologous EBV-immortalized lymphoblastoid cell line (LCL) IARC 277 by Gardella gel analysis, Southern blotting, and nonradioactive in situ hybridization to metaphase chromosomes. Although only episomal viral DNA was detected in the LCL, the 6-12 copies of EBV DNA in the BL cell line are integrated in one site of the host cell genome. The integration site is located near the breakpoint of a translocation 11;19 which is present in this cell line in addition to the BL-specific t(8;22).

Down-regulation of integrated Epstein-Barr virus nuclear antigen 1 and 2 genes in a Burkitt lymphoma cell line after somatic cell fusion with autologous EBV-immortalized lymphoblastoid cells

Epstein Barr virus (EBV) latent gene expression was analyzed in somatic cell hybrids between an EBV-positive Burkitt lymphoma (BL) cell line (BL 60) and an autologous EBV-immortalized lymphoblastoid cell line (LCL, IARC 277). The EBV genomes carried by the parental cell lines differ in sequence and in their physical state. The BL 60 EBV genome is integrated into the host cell genome whereas the LCL IARC 277 carries exclusively episomal EBV molecules. The hybrid cells contain both EBV genomes and display the differentiation phenotype of the parental LCL with regard to growth characteristics and cell-surface antigen expression in vitro and in vivo. While the EBNA 1 and EBNA 2 gene expression of the LCL-derived EBV is maintained in these hybrid cells, the BL-60-derived EBNA 1 and EBNA 2 genes are transcriptionally down-regulated. Mapping of the genomic region surrounding the latent Cp promoter of the BL-60-derived EBV revealed an extensive deletion upstream of the Cp promoter including the enhancer element in the ori P region, the origin of latent viral replication (ori P), the coding sequences for the EBV latent membrane protein (LMP) and the EBV terminal protein (TP), and suggested that one viral-cellular junction sequence is located near the Cp promoter. Integration of EBV into the host cell genome together with the extensive deletion might be causally related with the altered latent gene expression pattern after introduction of a lymphoblastoid host-cell background by somatic cell fusion. Down-regulation of the BL-60-derived EBNA genes could be due to loss of regulatory sequences in the BL-derived EBV necessary for EBNA 1 and EBNA 2 transcription in the lymphoblastoid hybrid cells, but not in the parental BL cells.

Metastatic capacity of Burkitt's lymphoma cell lines in nude mice

Burkitt's lymphoma (BL) cell lines estimated in a previous study as having a high, low and no tumourigenicity (7) were intravenously (i.v.) injected into preirradiated (480 rad) nude mice. BL cell lines with a high tumourigenic potential produced metastatic tumours in the brain, spinal cord, bone marrow, stomach and kidney, but did not disseminate into the lung, liver, ovary and spleen. The survival time of the tumour bearing animals ranged from 2 to 10 weeks. The majority of mice i.v. injected with highly tumourigenic BL cell lines showed paresis or paralysis of the hind legs. This was associated with the presence of neoplastic nodules either in the brain and/or in the spinal cord. In animals with metastasis to the stomach and kidney progressive cachexia was observed. The described experimental model of metastatic BL tumours in nude mice can effectively be used for the in vivo study of new therapeutic molecules such as monoclonal antibodies coupled or not to substances, toxic to tumour cells. This model can also be useful for the identification and analysis of homing properties of BL cells and their implication in BL pathogenesis.

Three transcriptionally distinct forms of Epstein-Barr virus latency in somatic cell hybrids: cell phenotype dependence of virus promoter usage

Phenotypically distinct human B cell lines display two transcriptionally distinct forms of Epstein-Barr virus (EBV) latency. Latency I (Lat I) in group I Burkitt's lymphoma (BL) cell lines is characterized by selective expression of the virus-coded nuclear antigen EBNA 1 from a uniquely spliced mRNA driven by the Fp promoter. Latency III (Lat III) in group III BL and EBV-transformed lymphoblastoid cell lines (LCLs) is characterized by expression of EBNAs 1, 2, 3a, 3b, 3c, and -LP from mRNAs driven by the Cp or Wp promoter and of the latent membrane proteins (LMPs 1, 2A, and 2B) from mRNAs driven by the LMP promoters. Here we have altered the group I BL and LCL phenotypes by cell hybridization and screened for attendant changes in EBV latency by PCR analysis of viral mRNAs and immunoblotting of viral proteins. Fusion of group I BL cells with LCLs activated the BL virus genome from a Lat I to Lat III pattern of gene expression. Fusion of LCLs with nonlymphoid lines repressed virus gene expression from Lat III either to Lat I or to another form of latency (Lat II) hitherto not seen in vitro and characterized by selective expression of the Fp-driven EBNA 1 mRNA and of the LMP 1, 2A, and 2B transcripts. There are therefore three forms of EBV latency which can be interconverted by altering cellular phenotype and thereby virus promoter usage.

Viruses in human cancers

Viruses may contribute to the development of human tumors by different mechanisms: indirectly by inducing immunosuppression or by modifying the host cell genome without persistence of viral DNA; directly by inducing oncoproteins or by altering the expression of host cell proteins at the site of viral DNA integration. Human cancers associated with papillomavirus, hepatitis B virus, Epstein-Barr virus, and human T cell leukemia-lymphoma virus infections are responsible for approximately 15 percent of the worldwide cancer incidence. Cancer of the cervix and hepatocellular carcinoma account for about 80 percent of virus-linked cancers. Because experimental and epidemiologic data imply a causative role for viruses, particularly in cervical and liver cancer, viruses must be thought of as the second most important risk factor for cancer development in humans, exceeded only by tobacco consumption.