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The Epstein-Barr virus (EBV)-encoded nuclear antigen (EBNA1) is expressed in latently EBV-infected B lymphocytes that persist for life in healthy virus carriers, and is the only viral protein regularly detected in all malignancies associated with EBV. Major histocompatibility complex (MHC) class I-restricted, EBNA1-specific cytotoxic T lymphocyte (CTL) responses have not been demonstrated. Using recombinant vaccinia viruses encoding chimaeric proteins containing an immunodominant human leukocyte antigen A11-restricted CTL epitope, amino acids 416-424 of the EBNA4 protein, inserted within the intact EBNA1, or within an EBNA1 deletion mutant devoid of the internal Gly-Ala repetitive sequence, we demonstrate that the Gly-Ala repeats generate a cis-acting inhibitory signal that interferes with antigen processing and MHC class I-restricted presentation. Insertion of the Gly-Ala repeats downstream of the 416-424 epitope inhibited CTL recognition of a chimaeric EBNA4 protein. The results highlight a previously unknown mechanism of viral escape from CTL surveillance, and support the view that the resistance of cells expressing EBNA1 to rejection mediated by CTL is a critical requirement for EBV persistence and pathogenesis.

The association of Epstein-Barr virus (EBV) with nasopharyngeal carcinoma (NPC) has been known for some time, but the precise role of EBV in this cancer is poorly understood, due partly to the lack of an in vitro system for studying NPC cells and the effect of EBV on epithelial cells. Biopsies of NPC tumours have revealed expression of the EBV latent membrane protein (LMP) in 65% of cases, suggesting that in at least some NPC tumours LMP may contribute to cell transformation. Here we address the question of the effect of LMP expression on epithelial cells. Transfection of an immortalized, non-tumorigenic keratinocyte cell line (RHEK-1) with the LMP gene causes a striking morphological transformation: the originally flat, polygonal colonies change to bundles of spindle-shaped cells that form multilayer foci, and cytokeratin expression is down-regulated. Our results suggest that LMP expression may be an important causal factor in the development of NPC.

Tumour biopsies from Burkitt lymphoma patients, as well as human nasopharyngeal carcinoma cells growing in athymic mice, contain Epstein-Barr virus DNA as covalently closed circular DNA. In addition integrated viral DNA sequences seem to be present.

During attempts to select nonsecretory variants from 0.467.3, and Epstein-Barr virus-transformed human lymphoblastoid cell line that secretes small amounts of IgM lambda, we exposed the cells to UV light. Cells that survived the irradiation were subcultured and their supernatants were screened for immunoglobulin production by an enzyme-linked immunosorbent assay (ELISA). Although stable nonsecretory variants were not isolated, we report here that an immunoglobulin class switch occurred in the UV-treated cell population. All survivors were found to produce large quantities of IgG lambda. Some cell cultures also produced the original IgM lambda. The UV-light-induced class switch was regularly reproducible with this target cell line.

Two B-cell-derived tumours, human Burkitt's lymphoma (BL) and murine plasmacytoma (MPC), are regularly associated with a distinctive form of chromosomal translocation (for reviews see refs 1, 2). In BL, the distal portion of chromosome 8 breaks off and is transposed, in most cases, to chromosome 14, known to carry the immunoglobulin heavy-chain locus. In about 5% of the cases the same distal part of the chromosome 8 has moved to either chromosome 2 or 22, to the neighbourhood of the kappa or the lambda locus, respectively. In MPC the distal region of chromosome 15 is transposed to the chromosome 12, known to carry the immunoglobulin heavy-chain locus, or enters into reciprocal exchange with the kappa locus-carrying chromosome 6 (ref. 7). Several laboratories have located c-myc, the cellular homologue of the MC29 retroviral oncogene v-myc, to human chromosome 8 (refs 8-10) and mouse chromosome 15 (refs 11-13). It has also been shown that the BL- and MPC-associated translocations remove the c-myc gene from its original site and transpose it into or close to one of the immunoglobulin gene clusters. In view of the above findings we also looked for possible involvement of the c-myc gene in a B-cell-derived tumour of a third species, the rat. Rat immunocytomas of spontaneous origin carry a reciprocal translocation between chromosomes 6 and 7 (ref. 17). Here we have localized the c-myc locus to chromosome 7 of the rat. Moreover, we have found that the c-myc gene was rearranged in four of five immunocytomas carrying the characteristic chromosomal translocation.

The v-myc oncogene can induce tumours in haematopoietic, mesenchymal and epithelial tissues. The corresponding c-myc proto-oncogene can contribute to the genesis and/or the progression of an equally wide variety of tumours when activated by retroviral insertions, chromosomal translocations or gene amplification. The c-myc gene product is a DNA-binding, nuclear phosphoprotein that is involved in the control of cell proliferation and possibly in DNA synthesis. The replication of Simian virus 40 (SV40) is a useful model system to study eukaryotic DNA replication as the virus relies almost entirely on cellular DNA replication apparatus. The SV40-based vector, pSVEpR4, replicates poorly in the human BJAB lymphoma line and in most human cells, but replicates well in Burkitt lymphoma lines, which have fused immunoglobulin and c-myc genes, resulting in high c-myc expression. Cotransfection of the BJAB cells with a c-myc-expressing construct (pI4-P6) increased the replication of pSVEpR4 tenfold. Our findings indicate that overexpression of the c-myc gene product allows the replication of SV40 in human lymphoma cells, suggesting that c-myc is involved in the control of replication.

It is generally accepted that tumours arise through the accumulation of several changes affecting the control of cell growth. Recent advances in molecular biology have made it possible to define some of these changes in molecular terms and to trace the steps by which certain tumours evolve.