Granulocyte-macrophage colony-stimulating factor modulation of the inhibitory effect of transforming growth factor-beta on normal and leukemic human hematopoietic progenitor cells.

Experiments were undertaken to investigate the molecular basis of primitive hematopoietic progenitor cell regulation in both the long-term culture system and in methylcellulose, particularly with a view to characterizing factors either able or unable to influence the behaviour of primitive leukemic cells from patients with chronic myeloid leukemia (CML). Long-term cultures of CML cells with or without irradiated normal marrow feeder layers were initiated from peripheral blood cells of CML patients with high white blood cell counts. Three weeks later the effect of exogenously added transforming growth factor-beta 1 (TGF-beta 1) on progenitor cycling status was examined. A single addition of 5 ng/ml TGF-beta 1 was able to reversibly arrest the otherwise uninterrupted turnover of primitive leukemic erythroid and granulopoietic progenitors for a period of up to 7 days both in the presence and absence of a normal adherent cell population. When TGF-beta 1 was incorporated into methylcellulose cultures, its ability to inhibit colony formation by CML progenitors showed the same differential activity on primitive cell types exhibited by normal progenitors. Dose-response curves for analogous populations of normal and leukemic cells were indistinguishable. Increasing the concentration of granulocyte-macrophage colony-stimulating factor (GM-CSF) in methylcellulose colony assays decreased the sensitivity displayed by normal clonogenic cells to TGF-beta 1 and no differences were detectable when CML cells were used in such regulator competition experiments. These findings support a general model of primitive hematopoietic cell regulation in which entry into S-phase is determined at the intracellular level by multiple convergent pathways that may deliver either positive or negative signals from activated cell surface receptors for distinct extracellular factors. The present study shows for the first time that primitive CML progenitors exposed to TGF-beta 1 in vitro can be transiently blocked in a noncycling state for several days without loss of viability and that the mechanisms responsible for the emergence and maintenance of a clonal population of CML cells in vivo do not appear to involve changes in their sensitivity to TGF-beta 1. It is thus unlikely that the heightened proliferative activity exhibited by primitive CML progenitors both in vivo and in long-term culture can be explained by an abnormality in the intracellular mechanisms normally activated by TGF-beta 1 receptor-ligand binding. We suggest that primitive CML cells are either defective in their ability to see (or activate) endogenously produced TGF-beta 1, or are defective in their responsiveness to another, undefined, regulator.