Frequent isolation of ecotropic murine leukemia virus after x-ray irradiation of C57BL/6 mice and establishment of producer lymphoid cell lines from radiation-induced lymphomas

Limiting dilution analysis of T-cell progenitors in the bone marrow of thymic lymphoma-susceptible B10 and -resistant C3H mice after fractionated whole-body X-irradiation

Earlier studies from this laboratory using Thy 1 congenic B10 strain mice suggested that a depletion of T cell progenitors (pre T cells) in the bone marrow in addition to the destruction of the thymus after fractionated whole body X-irradiation (IR) are the two main critical factors that cause differentiation arrest of initially repopulating intrathymic radio-resistant T cell progenitors, which then lead to the appearance of preneoplastic, prelymphoma cells, and eventually to highly neoplastic thymic lymphomas under the influence of the thymic environment. In order to explore the significance of the depletion (or reduction) of T cell progenitors in the bone marrow during pathogenesis of radiation-induced thymic lymphomas, we compared the pool size of pre T cells in the bone marrow and the spleens as well as the profiles of the regenerating thymocyte populations between thymic lymphoma induction-susceptible B10 and -resistant C3H strain mice following irradiation. The results indicated that irradiation severely depleted the pre T cells in the bone marrow and the spleens of both lymphoma induction-susceptible and -resistant mice. They also showed that in C3H mice the differentiation and maturation of intrathymic T cell progenitors which initially repopulated the depleted thymus seemed to proceed normally in spite of the poor cellularity, while this process was greatly suppressed in B10 mice. These data indicate that a depletion of pre T cells in the bone marrow combined with atrophy of the thymus in the irradiated mice is necessary, but not sufficient for development of thymic lymphoma. Implication of these findings on the possible mechanism of radiation-induced thymic lymphomagenesis is discussed.

Identification of a 16-kDa thymocyte membrane glycoprotein involved in the thymocyte/thymic medullary epithelial cell interaction

We have previously described a type of lymphoepithelial interaction involving CD4+ CD8+ thymocytes and a medullary epithelial cell line (E-5). This interaction is mediated by the recently described gp23/45 epithelial adhesion molecule and an as yet unknown thymocyte receptor. The present work describes a thymocyte surface glycoprotein of 16 kDa which binds both to E-5 cells and to the purified gp23/45 adhesion molecule. In addition, a thymic lymphoma cell line (Ti-6), which interacts with the E-5 cells via the gp23/45 receptor, also present a 16-kDa glycoprotein on its surface. Taken together, the data suggest that the 16-kDa thymocyte surface glycoprotein participates in the binding between these cells and the thymic epithelium.

Contributions to transcriptional activity and to viral leukemogenicity made by sequences within and downstream of the MCF13 murine leukemia virus enhancer

We have identified nucleotide sequences that regulate transcription in both a cell-type-specific and general manner in the long terminal repeat of the MCF13 murine leukemia virus. Besides the enhancer element, we have observed that the region between the enhancer and promoter (DEN) has a profound effect on transcription in different cell types. This effect, however, was dependent on the copy number of enhancer repeats and was detectable in the presence of a single repeat. When two enhancer repeats were present, the effect of DEN on transcription was abrogated except in T cells. DEN also makes a significant contribution to the leukemogenic property of the MCF13 retrovirus. Its deletion from the MCF13 virus dramatically reduced the incidence of thymic lymphoma and increased the latency of disease in comparison with the wild-type virus. This effect was most marked when one rather than two enhancer repeats was present in the mutant viruses. We also observed that the removal of one repeat alone remarkably reduced leukemogenicity by the MCF13 virus. A newly identified protein-binding site (MLPal) located within DEN affects transcription only in T cells, and its deletion attenuates the ability of an MCF13 virus with a single enhancer repeat to induce thymic lymphoma. This observation suggests that the MLPal protein-binding site contributes to the effect of the DEN region on T-cell-specific transcription and viral leukemogenicity. This study identifies the importance of nonenhancer sequences in the long terminal repeat for the oncogenesis of the MCF13 retrovirus.

Tyrosine kinase activation in thymic epithelial cells: necessity of thymocyte contact through the gp23/45/90 adhesion complex

Interactions between thymocytes and thymic stromal cells are necessary for T cell differentiation, maturation and proliferation. The signals required for these events to occur often necessitate close contact, and indeed adhesion, between the cell types involved. While the transmission of signals from stromal cells to thymocytes has been well documented, there is little evidence that binding of thymocytes to stromal cells can result in stromal cell activation. We have recently identified a novel thymic epithelial adhesion complex composed of three non-covalently associated glycoproteins (gp23, gp45 and gp90). While gp23 and gp45 are jointly required for adhesion to thymocytes, the function of gp90 is unknown. In the present work, we show that gp23/45-mediated contact with thymocytes induces de novo tyrosine phosphorylation of gp90. Furthermore, the protein tyrosine kinase responsible for gp90 neophosphorylation is itself an integral part of the adhesion complex.

The molecular biology of radiation-induced carcinogenesis: thymic lymphoma, myeloid leukaemia and osteosarcoma

In mice, external X- or gamma-irradiation may induce thymic lymphomas or myeloid leukaemias, while bone-seeking alpha-emitters may induce osteosarcomas and, to a lesser extent, acute myeloid leukaemia. The present paper aims to review briefly some of the experimental data with respect to the molecular mechanisms underlying these radiation-induced carcinogenic processes. Thymic lymphomagenesis proceeds through an indirect mechanism. Recombinant proviruses often occur in the tumour cell DNA, favouring the idea that they might be involved. However, there are indications that they might mediate tumour growth rather than induction. It is plausible that activation of ras oncogenes by somatic point mutations might play a role in the carcinogenic process, although at a yet undetermined stage. Myeloid leukaemogenesis is characterized by a very early, putative initiating event, consisting of non-random rearrangements and/or deletions of chromosome 2. These may be related to deletions in the developmentally important homeobox gene clusters and to rearrangements of the sequences flanking the IL-1 beta gene. Either a gene of the homeobox family or IL-1 beta might be considered as potentially involved in the induction process. Osteosarcomagenesis in mice is often associated with the expression of proviruses, and the tumours often contain somatically acquired proviruses. These viruses may contribute to tumour development by affecting various growth-suppressor genes. Viruses isolated from bone tumours, although non-sarcomagenic, induce osteopetrosis, osteomas and lymphomas upon infection of newborn mice. Osteogenic tumours frequently display amplification of a region on mouse chromosome 15, which encompasses c-myc and Mlvi-1 sequences. Enhanced transcription of various oncogenes is found in individual tumours, but no specificity for osteosarcomas has been identified. In vitro systems of skeletoblast differentiation are being developed to study tumour induction in vitro.

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.

Endogenous murine leukemia viruses: frequency of radiation-activation and novel pathogenic effects of viral isolates

Female C57BL/6 and BALB/c mice were injected i.p. with 0.06 microCi/kg or 0.5 microCi/kg of the short-lived alpha-emitting radionuclide 224radium at 3-day intervals. Infectious N-ecotropic XC+, and xenotropic C-type retroviruses were activated in several tissues in both strains. In C57BL/6 mice the activation of ecotropic and xenotropic virus was dose-dependent as observed 4 weeks after the start of irradiation. In BALB/c mice a few animals showed activation of ecotropic virus after four weeks of irradiation. The expression of xenotropic virus was similar in irradiated mice and controls. Viral antigen, indicative for viraemia, was not detected in irradiated or control animals. Antiviral antibodies were found in both control and irradiated mice but higher titers were found in the irradiated mice. Bone tissue-derived N-tropic XC+ virus isolates were found to be non-oncogenic in newborn mice of the parental strain. In contrast, the same virus isolates induced a novel pattern of disease, such as osteopetrosis and osteomas together with malignant lymphomas in NMRI mice. The data indicate that the pattern of endogenous murine leukemia virus activation by internal alpha-irradiation is dependent on the dose rate, and on the genetics of the mouse strain.

Nucleotide sequence of a radiation leukemia virus genome

The complete nucleotide sequence of an infectious molecular clone of a radiation murine leukemia proviral DNA RadLV/VL3(T+L+) has been determined. The sequence of the RNA genome is 8318 nucleotides long and contains three large open reading frames encoding the gag, pol, and env gene products. With the exception of a xenotropiclike R peptide and the LTR which bears structural similarities to a xenotropic LTR, displaying typical enhancerlike sequences, the remaining sequences are strikingly similar to the endogenous, ecotropic Akv murine leukemia virus. Therefore, it could be postulated that the leukemogenic properties of RadLV/VL3(T+L+) were generated by a recombination event between a xenotropic virus and an Akv-like ecotropic virus.

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.

Thymic lymphosarcomas obtained by X-rays in association with a weakly leukemogenic virus: cellular studies

The association in C57BL/6 mice of a subleukemogenic radiation dose (1.75 Gy X 2) which induces 7% of thymic lymphosarcomas (TL) with the injection of a weakly oncogenic B-tropic retrovirus responsible for 5% of TL resulted in a higher incidence of TL (31%) than expected from a simple cumulative effect when the viral injection preceded the irradiations (VX protocol). When virus was injected after irradiation (XV protocol) TL incidence (19%) was not significantly different from that of a cumulative phenomenon. The B-tropic virus used (1223) was isolated from RadLV-Rs extract and cloned. The TL incidence correlates with the presence of virus firstly in the thymus and bone marrow (BM) during the preleukemic period, secondly in the cell lines established in vitro from TL obtained in both protocols. This suggests that B-tropic viruses derepressed by 4 radiation doses of 1.75 Gy might be similarly implicated in the mechanism of radio-induced TL. This hypothesis is further supported by the evidence that BM restoration inhibited leukemogenesis in processes induced either by 4 radiation doses of 1.75 Gy or by the association of 2 radiation doses and viral injection whereas it has no effect on TL induced by highly oncogenic thymotropic viruses. Transplantation of BM cells from animals which had been submitted shortly before to leukemogenic VX protocol failed to induce donor type TL or leukemias in irradiated recipients suggesting that preleukemic cells either are not present or cannot be detected. However a high incidence of recipient TL was observed indicating that viruses were transferred with the grafted cells.

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.

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.

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.

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

Using the Southern procedure, we have studied the presence of ecotropic-specific murine leukemia viral sequences in genomic DNA isolated from primary X-ray-induced thymomas, from lymphoid cell lines established from them, or from secondary tumors passaged in vivo. We found that primary radiation-induced thymomas and infiltrated spleens do not harbor newly acquired ecotropic provirus. However, additional ecotropic proviruses (which appear recombinant in the gagpol region) could be detected in most of the tumorigenic cell lines established in vitro from them and in tumors arising from subcutaneous transplantation of the primary thymomas. These results suggest that primary radiation-induced thymomas may not be clonal. They also indicate a strong correlation between the presence of ecotropic recombinant proviruses in the genome and the growth ability, both in vitro and in vivo, of specific cells within these thymomas, suggesting a possible mitogenic function for murine leukemia virus.

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

Using the Southern procedure, we have studied the presence of ecotropic-specific murine leukemia viral sequences in genomic DNA isolated from primary X-ray-induced thymomas, from lymphoid cell lines established from them, or from secondary tumors passaged in vivo. We found that primary radiation-induced thymomas and infiltrated spleens do not harbor newly acquired ecotropic provirus. However, additional ecotropic proviruses (which appear recombinant in the gagpol region) could be detected in most of the tumorigenic cell lines established in vitro from them and in tumors arising from subcutaneous transplantation of the primary thymomas. These results suggest that primary radiation-induced thymomas may not be clonal. They also indicate a strong correlation between the presence of ecotropic recombinant proviruses in the genome and the growth ability, both in vitro and in vivo, of specific cells within these thymomas, suggesting a possible mitogenic function for murine leukemia virus.

New class of leukemogenic ecotropic recombinant murine leukemia virus isolated from radiation-induced thymomas of C57BL/6 mice

We previously reported the establishment of several lymphoid cell lines from X-ray-induced thymomas of C57BL/Ka mice, and all, except one, produce retroviruses (P. Sankar-Mistry and P. Jolicoeur, J. Virol.35:270-275, 1980). Biological characterization of five of these new primary radiation leukemia viruses (RadLVs) indicated that they had a B-tropic, fibrotropic, and ecotropic host range and were leukemogenic when reinjected into C57BL/Ka newborn mice. The leukemogenic potential of one isolate (G(6)T(2)) was further assessed and shown to be retained after prolonged passaging on fibroblasts in vitro. Restriction endonuclease analysis of the DNA of four of our new RadLV isolates (G(6)T(2), Ti-7, Ti-8, and Ti-9) revealed that G(6)T(2) and Ti-7 murine leukemia virus (MuLV) genomes had identical restriction maps, whereas Ti-8 and Ti-9 genomes were different from each other and from the G(6)T(2) and Ti-7 genomes. The physical maps of these genomes were similar to that of known ecotropic MuLV genomes (including the C57BL/Ka endogenous ecotropic MuLV) within their long terminal repeats, env, the right portion of pol, and the left portion of gag. However, a region covering the end of gag and the beginning of pol was different and showed several similarities with xenotropic MuLV genomes of BALB/c, AKR, and C58 mice previously mapped. Our results suggest that these primary RadLV genomes are recombinants between the parental ecotropic MuLV genome and a nonecotropic (xenotropic) sequence. This nonecotropic gag-pol region might be important in conferring the leukemogenic potential to these isolates. Therefore, these RadLVs appear to form a new class of leukemogenic recombinant MuLVs recovered from leukemic tissues of mice. They appear to be distinct from the recombinant AKR mink cell focus-inducing MuLVs which have a dual-tropic host range and harbor xenotropic env sequences. To further study the leukemogenic potential of these RadLVs, the genome of one of them (G(6)T(2)) was cloned in Charon 21A as an infectious molecule.

Induction of leukemia by both fractionated x-irradiation and radiation leukemia virus involves loci in the chromosome 2 segment H-30-A

A common link between the induction of leukemia by (i) fractionated doses of x-irradiation and (ii) radiation leukemia virus in mice may be established by the observation that the segment of chromosome 2 between the loci for the minor histocompatibility antigen H-30 and color coat agouti (H-30-A) includes distinct loci involved in susceptibility to leukemogenesis induced by factors i and ii.

Influence of serum thymic factor (FTS) on radiation-induced leukaemogenesis in thymectomized AKR mice

The influence of serum thymic factor (FTS) on extrathymic leukaemogenesis induced in thymectomized AKR mice by fractionated sub-lethal irradiation was studied. Thirty-day-old thymectomized mice were submitted to four doses at weekly intervals (1.75 Gy) and thereafter treated with FTS (1 ng) for 9 days. Two groups were restored with either bone marrow or spleen cells prior to FTS treatment. In another group mice were treated with FTS (12 ng) for 1 month after thymectomy and prior to irradiation. Results indicated that primarily irradiation and FTS, and, to a lesser extent, restoration and age of the mice at the time of their first radiation exposure influenced leukaemogenesis. Irradiation increased the spontaneous low incidence of extrathymic leukaemia (5%) up to 50%. Although nothing is yet known about the expression of endogenous retroviruses in thymectomized AKR mice, the possible expression of leukaemogenic recombinants, either identical to or different from mink-cell-focus-inducing viruses (MCF), might explain this enhancing effect of radiation on leukaemogenesis. Mice developed two types of leukaemias: "null" leukaemias whose cells bore no detectable theta antigen or surface immunoglobulin, and B leukaemias. Among the null leukaemias, two kinds could be distinguished: "early" ones which were observed before 450 days, and in which BM and spleen were similarly involved, and "late" ones which displayed the same characteristics as B leukaemias, i.e. delayed appearance (after 450 days) and splenic origin. Although FTS did not modify the overall frequency of leukaemias, it increased significantly the incidence of the "early" null ones. It had no effect on the frequency or the latency of late null and B leukaemias. Any population of theta-negative T cells sensitive to FTS could be an acceptable candidate for "early" null leukaemogenesis. The origin of "late" null leukaemias remains an open question.