Strong selection for cells containing new ecotropic recombinant murine leukemia virus provirus after propagation of C57BL/6 radiation-induced thymoma cells in vitro or in vivo

A new leukemogenic retrovirus isolated from tumor cells derived from a radio-induced lymphoma of C57BL/6 mice: analysis of the env and LTR sequences

We report the cloning and characterization of a new ecotropic provirus encountered in a radio-induced thymic lymphoma of the C57BL/6 mouse. The provirus with an abnormally long LTR was inserted in the chromosomal DNA within the Pvt-1/MLVi-1/Mis-1 region which is a common integration site for MCF virus in mice and for Mo-MuLV in rats. This new ecotropic provirus was molecularly cloned and found to be infectious and competent for replication after transfection of murine cells. The recovered virus termed T3651/B was B-ecotropic, T-lymphotropic (in vivo) and highly leukemogenic for newborn C57BL/6 mice and for adult mice provided they were submitted to a subleukemogenic dose of irradiation. As compared to the AKV prototype N-ecotropic endogenous retrovirus, the T3651/B env proteins are only affected by few scattered point mutations. In contrast, the LTR has five repeats of enhancer sequences containing consensus motifs specific of the nuclear factors NF1-like, LVa, LVb and SEF1. Since a virus with such properties was encountered only once in 31 radio-induced tumors and isolated at a fourth tumor passage, a direct role of T3651/B virus in tumor genesis after irradiation is uncertain. Nevertheless, it is clear that T3651/B virus is a new leukemogenic retrovirus with a particular LTR structure which fits well with the model proposed by Rassart et al. (J. Virol. 58, 96-106, 1986) for the emergence of a thymotropic highly leukemogenic RadLV.

Evidence of recombinant ecotropic provirus integration in thymic lymphomas induced by direct or indirect radiation effects

Several investigators described the occurrence of ecotropic recombinant proviruses in the DNA of in-vivo or in-vitro propagated radio-induced lymphomas, but such proviruses were never detected in primary tumors. To assess their biological significance in the tumorigenic process, we reinvestigated the presence of new proviruses chiefly in primary radio-induced tumors and in models of radioleukemogenesis which could give additional support for their role. Such models included thymic lymphomas originating after (i) graft of non-irradiated thymuses in thymectomized irradiated mice and (ii) the injection of a B-ecotropic retrovirus (T1223/B) in association with a subleukemogenic dose of irradiation. We report for the first time that new ecotropic proviral sequences are encountered in a significant number (30%) of primary lymphomas induced directly by irradiation or indirectly in non-irradiated thymuses grafted in irradiated hosts. The existence of a 3.5-kbp Kpn1 restriction fragment with ecotropic sequences in the digested DNA of these tumor cells indicates that these new sequences belong to an ecotropic provirus recombinant in the gag-pol region. We observed that most of the primary radio-induced tumors in which novel recombinant provirus could be detected, displayed the integration at a single or at a few sites, demonstrating their clonality with respect to viral integration. The same was observed in thymic lymphomas arising after T1223/B virus injection and irradiation and in in-vivo or in-vitro propagated tumors. Altogether, these data bring the first evidence of the integration of ecotropic recombinant proviral genomes in a significant number of primary radiation induced thymic lymphomas and of their possible role in view of their frequent occurrence in grafted thymomas.

N-methyl-N-nitrosourea-induced T-lymphomas of AKR/J mice contain somatically acquired ecotropic-like murine leukemia proviruses

We have studied somatically murine leukemia proviral integrations in primary N-methyl-N-nitrosourea (MNU)-induced thymic lymphomas of AKR/J mice. The majority of MNU-induced lymphomas contain newly acquired murine leukemia proviral sequences. In contrast to spontaneous AKR/J lymphomas which contain multiple integrations of mink cell focus-forming recombinant proviruses, MNU-induced lymphomas contain ecotropic-related proviruses. This conclusion was based on the demonstration that EcoRI- and PvuII-digested DNA from MNU-induced lymphomas contains new 3' proviral-cellular junction fragments that hybridize with the ecotropic-specific pAKV-4 and pAKV-5 hybridization probes. Also, EcoRI/PstI double digests of DNA from MNU-induced lymphomas revealed that the acquired proviruses do not contain an internal 3' EcoRI site characteristic of mink cell focus-forming recombinant viruses. The proviral integration patterns suggest that MNU-induced lymphomas are clonal or oligoclonal in nature. This conclusion is supported by comparison of proviral integration patterns in lymphomas obtained from thymus and spleen of individual mice, and by analyses of T-cell receptor beta-chain gene rearrangements. The frequent occurrence of ecotropic-related proviral sequences in MNU-induced lymphomas suggests that these newly acquired proviruses may play a role in tumor development.

Induction of thymic lymphosarcomas in C57BL/6 mice after inoculation of weakly oncogenic viruses associated with a sub-leukemogenic radiation exposure (1.75 Gy x 2)

B-ecotropic retroviruses arise frequently in old or irradiated C57BL/6 mice as a consequence of a genetic recombination between endogenous eco- and xenotropic retroviruses. They are weakly oncogenic and express a very low tropism for thymic cells. However, their activation by X-rays and the subsequent insertion of new proviral sequences in the cell genome of in vivo- and in vitro-passaged tumors suggest that they might play a role in radioleukemogenesis. To study this possibility, a cloned B-ecotropic virus (1223) was injected into C57BL/6 mice subjected to a subleukemogenenic irradiation which induces only 7% of thymic lymphosarcomas (TL). When it was injected prior to or after irradiation, 1223 induced respectively 31% and 19% of TL. The incidence of TL in the different groups closely correlated with virus expression in hematopoietic tissues during the preleukemic period. Thus, irradiation seems to amplify bone marrow (BM) and thymic cell population(s) which play a decisive role in viral expression. A recombinant provirus (presumably the injected 1223) was detected in the genomic DNA of all tumors tested irrespective of the inductive protocol. BM restoration, which does not inhibit TL produced by highly oncogenic passaged viruses, but prevents the development of TL induced by 4 doses of 1.75 Gy, also provided strong protection in the present experiments. The present data support the hypothesis whereby weakly oncogenic B-ecotropic viruses similar to those activated by radiation might be involved in the development of TL.

Studies on emerging radiation leukemia virus variants in C57BL/Ka mice

To analyze the emergence of radiation leukemia virus (RadLV) variants in primary X-ray-induced C57BL/Ka thymoma and to identify the virus responsible for the very high leukemogenic potential of passaged Kaplan strain BL/VL3 preparation, we cloned several primary and passaged ecotropic RadLV infectious genomes. By restriction analysis, we found that BL/VL3 cells harbor three related but different ecotropic RadLVs. Their restriction map differs significantly from those of primary RadLVs. Hybridization analysis also indicated that BL/VL3 and primary RadLVs differ in their p15E and long terminal repeat (LTR) regions. As compared with the LTR sequence of the putative parental endogenous ecotropic provirus, the LTR sequence of primary weakly leukemogenic RadLV has only one change, a C-rich sequence, generating a 6-base-pair direct repeat just in front of the promotor. The LTR of the primary nonleukemogenic RadLV only showed few base changes, mainly clustered in R and U5. The LTR from a moderately leukemogenic passaged BL/VL3 RadLV had conserved the C-rich sequence and acquired a 43-base-pair direct repeat in U3 and several other point mutations, small insertions, and deletions scattered in U3, R, and U5. All cloned primary RadLVs were fibrotropic, and some were weakly leukemogenic. All cloned BL/VL3 RadLVs were thymotropic and nonfibrotropic. The block of their replication was found to be after the synthesis of unintegrated linear and supercoiled viral DNA. Most of the BL/VL3 RadLVs were moderately leukemogenic, and one (V-13) was highly leukemogenic, being as virulent as the Moloney strain. We propose a model for the emergence of the RadLV variants and show that the virus responsible for the high leukemogenic potential of BL/VL3 preparation is a nondefective, ecotropic, lymphotropic, nonfibrotropic, unique retrovirus which most likely arose from a parental primary RadLV similar to those studied here.

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.

Molecular analysis of emerging radiation leukemia virus variants of C57BL/Ka mice

Molecular cloning of several primary or passaged RadLV variants and their biological characterization has allowed us to propose a model of their emergence following X-ray irradiation of C57BL/6 mouse.

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.

Monoclonal proliferation of Friend murine leukemia virus-transformed myeloblastic cells occurs early in the leukemogenic process

Integrated Friend murine leukemia virus copies were analyzed by the Southern blotting procedure in myeloblastic cell lines obtained after in vitro infection of long-term mouse bone marrow cultures. Several steps leading to the generation of malignant myeloblastic cells after a long latency period were observed in the evolution of infected cultures. Shortly after infection, a random distribution of integrated provirus copies was observed in the DNA of normally differentiating myeloid cells. In contrast, a distinct pattern of integrated Friend murine leukemia virus copies was evident in the first non-differentiating immature myeloblastic cells appearing in cultures, suggesting a monoclonal origin of these cells. For each cell line, characteristic hybridizing fragments were conserved during the 1-year culture period necessary for the acquisition of tumorigenic properties and were also observed in tumors grafted in vivo. We can conclude that monoclonality is effective very early in the myeloid transformation process, as soon as the precursor cells are blocked in their differentiation.

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

Endogenous murine leukemia virus (MuLV) proviral copies were analyzed in thymomas induced in normal BALB/c (Fv-1b) and in Fv-1n congenic mice by X-irradiation. Both strains of mice developed leukemia with similar kinetics, indicating that N-tropism of endogenous MuLV was not a rate-limiting factor in development of disease. Southern blot analysis, using a probe specific for ecotropic virus and for ecotropic-specific sequences retained in pathogenic, env-recombinant viruses, showed that the majority of radiation leukemias lacked newly acquired, clonally integrated, proviruses. This was in contrast to virus-induced leukemias, which routinely exhibited several new proviral integration sites. When an internal proviral DNA restriction fragment was monitored, some radiation leukemias showed evidence of nonclonal infection, accounting for more frequent isolation of infectious virus from such leukemias. Differences in expression of T-cell surface antigens were found in X-ray-induced and virus-induced leukemias. All radiation leukemias were TL positive, whereas virus-induced leukemias were primarily negative for TL. Some differences were also found in Lyt-1 and Lyt-2 expression. The data as a whole suggest that, in the majority of cases, radiation leukemogenesis is not initiated by a viral route--that is, the sort of viral mechanism for which exogenous infection by known pathogenic MuLV is the paradigm.

Patterns of thymocyte differentiation markers on virus and radiation induced lymphomas of C57BL/Ka mice

To better understand the biology of tumorigenesis in virus and radiation lymphomas of C57Bl/Ka mice, we have examined the cell surface phenotypes of a large series of primary tumors induced by both agents. Data derived using flow cytometry and recently available monoclonal antibodies to thymocyte differentiation antigens supports three major conclusions. First, tumor cell populations are unimodal for staining with most antibodies and are probably of clonal origin. Second, many, but not all, tumor cells show surface phenotypes similar to those of previously defined subpopulations of normal thymocytes. Third, at the cell surface level, no major differences between virus- and radiation-induced lymphomas can be discerned. Our data thus further define the relationship between thymomas induced by these two agents.

Methylation of human T-cell leukemia virus proviral DNA and viral RNA expression in short- and long-term cultures of infected cells

Leukemic peripheral blood lymphocytes from individuals infected with the human T-cell leukemia/lymphoma virus (HTLV) were found to express little or no viral RNA before being put into tissue culture. Within 24-48 hr, viral RNA expression increased at least four- to eightfold. Established HTLV-infected cell lines constitutively express viral RNA. Southern blots of DNA from HTLV-infected cells digested with the methylation-sensitive restriction enzyme HpaII showed that the proviral DNA was methylated in all of the uncultured peripheral blood cells tested. In contrast, no proviral methylation was detected in any of the cell lines examined, suggesting a functional correlation between methylation and viral RNA expression. However, DNA from HTLV-infected lymphocytes cultured for 48 hr (by which time increases in viral RNA expression are evident) did not differ detectably with respect to proviral DNA methylation from uncultured cells, suggesting that the increase in viral RNA expression after short-term culture is mediated by mechanisms independent of changes in DNA methylation.