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

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.

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.