A comparative analysis of radiation- and virus-induced leukemias in BALB/c mice

T-cell lymphoma lines derived from rat thymomas induced by Moloney murine leukemia virus: phenotypic diversity and its implications

The phenotype of 27 Moloney murine leukemia virus-induced rat thymic lymphomas and 36 cell lines derived from these tumors was determined by using 18 monoclonal antibodies directed against hematopoietic cell surface determinants. The cell lines and the primary tumors from which they were derived were clonally related as determined by the pattern of provirus integration and the pattern of rearrangement of the T-cell receptor beta and delta and Igh loci. The differentiation phenotype of the primary tumors and the cell lines derived from them were related. The differences observed between the primary tumors and the cell lines could be explained either by the selection of subpopulations of tumor cells during establishment in culture or by the phenotypic instability of the tumor cells. One cell line (LE3Sp) underwent the transition from a CD4+ CD8+ to a CD4+ CD8- phenotype following exposure to interleukin-2 in culture. Both the primary tumors and the cell lines derived from them express a wide range of phenotypes which correspond to multiple stages in T-cell development. This observation suggests that the pleiomorphism of retrovirus-induced lymphomas, which had been suggested previously from the analysis of mouse tumors, is an intrinsic property of the process of oncogenesis and is not due to the transformation of different types of cells by spontaneously arising leukemogenic variants of the inoculated virus. The wide spectrum of phenotypes expressed by these tumors suggests that Moloney murine leukemia virus may infect and transform T cells at various stages of development. Alternatively, the target cells may be immature T-cell precursors which, following transformation, continue to differentiate. A host of early findings, suggesting that the repertoire of target cells is restricted to poorly differentiated hematopoietic progenitors, and the ability of the LE3Sp cell line to differentiate in culture indicate that the latter possibility may be more likely. The data in this report address the extent and mechanism of the phenotypic variability of retrovirus-induced rodent T-cell lymphomas. In addition, they demonstrate the potential usefulness of the T-cell lymphoma lines we have established in studies of oncogenesis and T-cell differentiation.

Amplification of endogenous proviral MuLV sequences in radiation-induced osteosarcomas

The endogenous ecotropic provirus of BALB/c mice was found to be amplified in 17 out of 29 radiation-induced osteosarcomas. In contrast, 19 clonal cell lines established from bone-marrow cells of a tumor-bearing mouse, which were used as controls, did not reveal newly acquired ecotropic proviruses. Ecotropic viral RNA was expressed in tumors that showed reintegrated proviruses. DNA probes from 2 tumors, derived from cellular sequences flanking the newly integrated proviruses, did not detect DNA rearrangements in any of the other tumors. The possible role of activated endogenous retroviruses in the development of radiation-induced osteosarcomas is discussed.

Trisomy 15 as a regular finding in chemically induced murine T-cell leukemogenesis

Trisomy 15 is described as a common finding in all T-cell leukemias induced by a single dose of methylnitrosourea (MNU) in BDF1 mice and in the leukemias induced by 7 doses of benzo(a)pyrene. Additional trisomies were found in about half of the leukemias. The organ distribution suggests that the leukemic cells with trisomy 15 originate in the thymus. Trisomy 15 was detected in the thymus as early as 6 weeks after the application of MNU, i.e. during the latency period.

Ecotropic and mink cell focus-forming murine leukemia viruses integrate in mouse T, B, and non-T/non-B cell lymphoma DNA

Structures of somatically acquired murine leukemia virus (MuLV) genomes present in the DNA of a large panel of MuLV-induced C57BL and BALB/c B and non-T/non-B cell lymphomas were compared with those present in MuLV-induced T-cell lymphomas induced in the same low-"spontaneous"-lymphoma-incidence mice. Analyses were performed with probes specific for the gp70, p15E, and U3-long terminal repeat (LTR) regions of ecotropic AKV MuLV and a mink cell focus-forming virus (MCF)-LTR probe annealing with U3-LTR sequences of a unique endogenous xenotropic MuLV, which also hybridizes with U3-LTR sequences of a substantial portion of somatically acquired MCF genomes in spontaneous AKR thymomas. The DNAs of both T- and B-cell tumors induced by neonatal inoculation with the highly oncogenic C57BL-derived MCF 1233 virus predominantly contain integrated MCF proviruses. In contrast, the DNAs of more slowly developing B and non-T/non-B cell lymphomas induced by poorly oncogenic ecotropic or MCF C57BL MuLV isolates mostly contain somatically acquired ecotropic MuLV genomes. Approximately 50% of the spontaneous C57BL lymphoma DNAs contain somatically acquired MuLV genomes. None of the integrated MuLV proviruses annealed with the MCF-LTR probe, which indicates a clear difference in LTR structure with a substantial portion of the somatically acquired MuLV genomes present in the DNA of spontaneous AKR thymomas. This study stresses a dominant role of MuLV with ecotropic gp70 and LTR sequences in the development of slowly arising MuLV-induced B and non-T/non-B cell lymphomas.

Retroviruses in radiation-induced lymphomas

The nucleotide sequence of RadLV/VL3 (T+L+), the thymotropic and leukemogenic entity of the in-vitro propagated radiation leukemia virus complex (RadLV/VL3), is that of a recombinant retrovirus. The gag, pol and most of the env gene are very similar to the homologous regions of Akv MuLV. The 3' end of the env gene and the LTR appear to have derived from a xenotropic MuLV. However, the LTR has acquired a feature shared by other lymphomagenic MuLVs. This feature consists in sequence rearrangements resulting in the generation of presumed enhancer elements. RadLV/VL3(T+L+)-specific proviral sequences were found adjacent to the c-myc gene in several virus-induced thymic lymphomas of the rat, suggesting that the enhancer elements might play a role in lymphomagenesis. However, we found that the presence of a provirus at a specific DNA site can lead to an in-vitro growth advantage and to clonal cell selection independently of a lymphomagenic process. We conclude from this observation that clonal appearance of an integrated provirus in cultured radiogenic lymphoma cells does not necessarily reflect a viral induction of radiation-induced leukemogenesis.

Murine thymic lymphomas after infection with a B-ecotropic murine leukemia virus and/or X-irradiation: proviral organization and RNA expression

The role of retroviruses in murine radioleukemogenesis was reinvestigated using a protocol associating the injection of a non-pathogenic retrovirus (T1223/B virus) and a subleukemogenic dose of X-radiation (2 X 1.75 Gy). Using the Southern blotting technique we studied MuLV proviral organization and RNA expression in thymic lymphomas induced by the combined effect of virus and irradiation or irradiation alone. A recombinant provirus was detected in the chromosomal DNA of every tumor induced by associating virus and radiation whereas it was unconstantly found in radio-induced tumors. In every instance, the provirus was not integrated at a common site. No relationship was observed between viral RNA expression and tumor induction. Trisomy 15 was observed in all metaphases irrespective of the protocol of tumor induction. The G-banding technique revealed an extra-band in several thymic lymphomas induced by irradiation and T1223/B virus injection.

Activation and biological properties of endogenous retroviruses in radiation osteosarcomagenesis

The activation of endogenous retroviruses (MuLV) by internal irradiation and the presence of activated retroviruses in radiation-induced murine osteosarcomas as well as their biological properties in vivo and in vitro were studied. Ecotropic and xenotropic MuLV were expressed dependent on the radiation dose in spleen, bone marrow and bone tissues of C57Bl/6 mice after 224Ra treatment. Radiation-induced osteosarcomas of BALB/c, C57Bl/6 and C3H X 101/F1 mice harboured infectious ecotropic and/or xenotropic viruses whereas in osteosarcomas of NMRI mice predominantly virus protein could be detected. In about 50% of the radiation-induced osteosarcomas of BALB/c mice an amplification of ecotropic proviruses could be detected. This was not found in clonally grown cells from non-tumorous tissues. MuLV from radiation-induced osteosarcomas induced osteopetrosis, osteomas and lymphomas after infection of newborn NMRI mice. In differentiating bone tissue the viruses were found to infect predominantly osteoblast precursor cells suggesting that virus infection results in increased growth and metabolic activity of these cells, which may be a possible mechanism for their pathogenic action in bone tissues.

Oncogene activation and surface markers in mouse lymphomas induced by radiation and nitrosomethylurea

Thymic lymphomas have been induced by gamma-radiation and treatment with the chemical nitrosomethylurea in different mice strains. As indicated by the NIH 3T3 focus forming assay, a significant percentage of the tumors contain activated oncogenes of the ras family (K or N). Cloning and sequencing has enabled us to identify single base mutations as the only significant alteration present in the activated oncogenes. These alterations result in the substitution of amino-acid 12 or 61 of the p21 product of the ras genes. With the use of synthetic oligonucleotides it has been found that the tumors do not all contain the same mutation and in one case so far the normal allele is absent.

Phenotype of thymic lymphomas in the mouse

The MP2 cell line was established from a murine leukemia virus-induced thymic lymphoma. Half of the cells were consistently L3T4 positive and less than 5% of the cells were Lyt-2 positive. Single cell cloning on the basis of the presence or absence of Lyt-2 allowed the isolation of four clones with stable phenotypes: (1) Lyt-2-, L3T4-; (2) Lyt-2+, L3T4+; (3) Lyt-2-, L3T4+; (4) Lyt2+, L3T4-. These data are discussed in relation to tumour cell heterogeneity and to normal T-cell differentiation pathways.