Multiphase process involved in radiation induced murine AML.

Exposure of 3 month old SJL/J mice to a single dose of 300 r yielded 15-30% acute myelomonocytic leukemia (AML) development at a mean latency of 1 year. Additional treatment with dexamethasone shortly after irradiation increased leukemia incidence to 50%. All tumors were characterized by a partial deletion of one allele of chromosome 2 and the same deletion was detected in bone marrow and spleen cells of most irradiated mice, irrespective of the development of the disease. The presence of potential leukemic cells (PLC) in mice 4 months after the leukemogenic treatment was confirmed by transplantation studies. In these experiments PLC transition into overt AML seemed to be dependent on their transfer into irradiated recipients. Thus, exposure to 300 r results in the initiation of potential leukemic cells. Experiments were conducted in order to explore the possible role of radiation, cytokines and different hemopoietic growth factors on PLC promotion to overt leukemia. Exposure to 300 r, beside PLC initiation, was found to trigger the production of IL-6 and CSF-1; the additional administration of dexamethasone further increased CSF-1 levels. In vivo administration of CSF-1 into mice carrying radiation-induced PLC was most effective in PLC promotion to overt AML development.