Leukemic cells cultured in the presence of various conditioned media differentiate into macrophages. This finding suggested that the maintenance of undifferentiated state and self-renewal in vivo may be related to the inability of the host to generate an appropriate level of differentiation factor (DF). Evidence for this gypothesis was derived from experiments in vitro and in vivo with myeloid leukemia of rat. The following results were obtained: (i) in vitro, the percentage of cell differentiation at a fixed concentration of DF was inversely related to the concentration of cels; (ii) leukemic cell inoculates that were lethal to 7-day-old rats were rejected by 21-day-old rats;iiiiiiiiii leukemic cells in diffusion chambers underwent differentiation in 21-day-old rats but not in 7-day-old rats; (v) treatment of rats with DF in diffusion chambers resulted in leukemic cell differentiation inside the chamber; and (iv) the development of leukemia in 7-day-old rats was aborted by treatment with DF. These results show that the differentiation of rat leukemia cells requires theappropriate lvel of DF. The proliferation of transplanted leukemia cells in 7-day-old rats goes unchecked because of inadequate generation of DF. Conversely, in the 21-day-old rats, rejection is accomplished by differentiation of the transplanted cells.
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