PGM-2 is a variant of the transplantable PGM-1 leukemia of strain C3H/HeJ. Freshly explanted cells had lymphoid morphology with a CD5+ CD45R (B220)- IgM- phenotype. They were not viable in unstimulated cultures, but formed IgM+ lymphoid colonies in response to interleukin- 2 (IL-2), IL-4, IL-5, IL-6, IL-7, and Steel factor, and macrophage colonies in response to IL-3. IL-3-stimulated colonies had no recloning potential, but colonies from IL-7 cultures gave rise to large numbers of secondary macrophage colonies in IL-3-stimulated cultures and secondary lymphoid colonies in IL-7-stimulated cultures. The latter ones could be serially transferred in vitro for several months, and formed typical PGM-2 tumors in vivo. IL-7-stimulated colonies could therefore be used to measure leukemic stem cells in vitro. Supramaximal IL-3 stimulation (2,500 U/mL) of suspension cultures was followed by an increase in overall cell numbers and a disappearance of leukemic stem cells, compatible with differentiation induction. This could not be counteracted by simultaneous stimulation with IL-7. However, lower IL-3 concentrations (500 U/mL) induced an expansion of the stem cell pool, possibly by facilitating density-dependent autostimulatory mechanisms involving endogenous production of IL-7. The system described is a simple in vitro model for differentiation therapy. It shows that leukemic stem cells can be induced by hematopoietic growth factors to undergo terminal differentiation, but the concentrations required for differentiation induction in stem cells are much higher than those required for other biologic effects. Submaximal stimulation may favor expansion rather than repression of the leukemic cell population.

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