Further studies of the effect of bone marrow cells on chemically induced lymphoma in C57BL-6 mice

Radiation carcinogenesis in mouse thymic lymphomas

Ionizing radiation is a well-known carcinogen for various human tissues and a complete carcinogen that is able to initiate and promote neoplastic progression. Studies of radiation-induced mouse thymic lymphomas, one of the classic models in radiation carcinogenesis, demonstrated that even the unirradiated thymus is capable of developing into full malignancy when transplanted into the kidney capsule or subcutaneous tissue of irradiated mice. This suggests that radiation targets tissues other than thymocytes to allow expansion of cells with tumorigenic potential in the thymus. The idea is regarded as the 'indirect mechanism' for tumor development. This paper reviews the indirect mechanism and genes affecting the development of thymic lymphomas that we have analyzed. One is the Bcl11b/Rit1 tumor suppressor gene and the other is Mtf-1 gene affecting tumor susceptibility.

The D4Mit12 locus on mouse chromosome 4 provides susceptibility to both gamma-ray-induced and N-methyl-N-nitrosourea-induced thymic lymphomas

Low-penetrance genes control different susceptibilities to gamma-ray-induced thymic lymphomas in mouse strains. Our previous genetic analyses with backcross mice between BALB/c and MSM strains and congenic lines localized one such gene near the D4Mit12 locus on chromosome 4. N-Methyl-N-nitrosourea (MNU) is a guanine base-alkylating agent and differs from gamma-radiation in its mechanism of mutagenic action. Accordingly, in this study, we examined whether or not the locus also provides susceptibility to MNU-induced thymic lymphomas using 84 offsprings derived from congenic mice for D4Mit12. Association analysis provided a suggestive linkage at D4Mit12 (P = 0.0075) and the linkage was sustained by the peak of likelihood ratio statistical values being at the same position as that for the gamma-ray-induced lymphomas. The results strongly suggest that the BALB/c allele near D4Mit12 is associated with susceptibility to lymphomas induced by two carcinogenic agents having different mechanisms of mutagenic action.

The role of prothymocytes in radiation-induced leukemogenesis in C57BL/Rij mice

Following transplantation of bone marrow into recipient mice subjected to irradiation with four weekly fractions of 1.8 Gy, donor progenitor cells were found to compete with host elements for colonization of the thymus. The capacity for thymus repopulation of bone marrow cells from 4 X 1.8 Gy irradiated mice is decreased to a similar low level as that of regenerating bone marrow derived from lethally irradiated mice reconstituted with isogeneic marrow. It was calculated that the prothymocyte content of both types of regenerating marrow was less than 0.1% of that of normal marrow. Long-term cultures of C57BL bone marrow were found to contain the same numbers of prothymocytes per CFU-S as normal bone marrow, in contrast to cultured marrow from (C3H X AKR)F1 mice, that shows a striking decrease of prothymocytes. Normal bone marrow cells and long-term cultured bone marrow cells of C57BL mice were about equally effective on the basis of numbers of CFU-S injected in protecting 4 X 1.8 Gy irradiated syngeneic recipients from developing thymic lymphomas, while regenerating bone marrow was virtually nonprotective. These data are interpreted as supporting the notion that prothymocytes from the bone marrow play a crucial role in the prevention of radiation-induced thymic lymphomas.

Effects of Syngeneic Bone Marrow Cells on Mice Given a Leukemogenic Treatment with Urethan

A leukemogenic treatment with urethan in infant (SWR x C57BL)F1 mice (1 mg/g body weight, 5 times, once every second day) induced a marked reduction of the thymus and spleen weight, of the number of RBC, WBC, and bone marrow cells, with an inverted lymphoid/myeloid cell ratio in the bone marrow. An analysis of the bone marrow population showed that the carcinogen acted selectively on the lymphoid cells and, to a lesser extent, on the erythroblastic ones, while the myeloid cells did not appear to be involved. The damage was still present 10 days after the end of treatment. A single intracardiac inoculum of 15–20 × 106 syngeneic normal bone marrow cells, 3 days after the last urethan injection, determined a recovery of the spleen weight and of the cell ratio in the bone marrow, whereas the thymus weight and the blood and bone marrow cell counts were uneffected. Moreover the bone marrow inoculum failed to inhibit the induction of thymic lymphosarcomas.