Characterization of env gene recombination in x-ray--induced thymomas of C57BL/6 mice

Scrapie infection activates the replication of ecotropic, xenotropic, and polytropic murine leukemia virus (MuLV) in brains and spinal cords of senescence-accelerated mice: implication of MuLV in progression of scrapie pathogenesis

Senescence-accelerated mice (SAMP8) have a short life span, whereas SAMR1 mice are resistant to accelerated senescence. Previously it has been reported that the Akv strain of ecotropic murine leukemia virus (E-MuLV) was detected in brains of SAMP8 mice but not in brains of SAMR1 mice. In order to determine the change of MuLV levels following scrapie infection, we analyzed the E-MuLV titer and the RNA expression levels of E-MuLV, xenotropic MuLV, and polytropic MuLV in brains and spinal cords of scrapie-infected SAM mice. The expression levels of the 3 types of MuLV were increased in scrapie-infected mice compared to control mice; E-MuLV expression was detected in infected SAMR1 mice, but only in the terminal stage of scrapie disease. We also examined incubation periods and the levels of PrPSc in scrapie-infected SAMR1 (sR1) and SAMP8 (sP8) mice. We confirmed that the incubation period was shorter in sP8 (210+/-5 days) compared to sR1 (235+/-10 days) after intraperitoneal injection. The levels of PrPSc in sP8 were significantly greater than sR1 at 210+/-5 days, but levels of PrPSc at the terminal stage of scrapie in both SAM strains were virtually identical. These results show the activation of MuLV expression by scrapie infection and suggest acceleration of the progression of scrapie pathogenesis by MuLV.

TL antigen is not linked to radioinduced thymic lymphoma

X irradiation of C57BL/Ka mice induces thymic lymphoma after a period of 8 to 36 weeks. This latency period represents an ideal time window in which to follow the development of prelymphoma cells that give rise to overt thymic lymphoma. Several attempts have been made to identify an unequivocal prelymphoma cell marker but these efforts have so far been unsuccessful. We monitored the evolution of thymocyte populations containing prelymphoma cells during the latency period, using CD3 and TL as markers, in a transfer assay. We demonstrated that: (1) particular cell populations could appear or disappear; (2) there were at least two prelymphoma phenotypes: CD3loTL+ and CD3hiTL-; (3) TL could be present transiently; and (4) TL could be absent throughout the latency period. We conclude that split-dose irradiation may induce both TL gene expression and a prelymphoma state but that the two are not necessarily related.

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.