Interactions between leukemia cells and bone marrow stromal cells: stroma-supported growth vs. serum dependence and the roles of TGF-beta and M-CSF.

Leukemia cell lines that do not proliferate in the absence of serum grow well when cultured with stromal cells. To study this growth dependence on stroma, we selected the M1 myeloblast clone, since its stroma dependence is reminiscent of that exhibited by hematopoietic stem cells. Conditioned medium form a stromal cell line, prepared under serum-free conditions, contained an activity that induced the proliferation of M1 cells and was therefore designated M1 myeloid activity (MMA). Among the various cytokines tested for MMA-like activity, only transforming growth factor-beta (TGF-beta) and macrophage colony-stimulating factor (M-CSF) were found to affect M1 cell survival, and the two cytokines acted synergistically to induce M1 cell growth. Antibodies to both TGF-beta and M-CSF abolished most, but not all, of the MMA in the medium conditioned by stromal cells, indicating that additional factors contribute to MMA. A subclone of M1 cells, M1/M2, selected in medium conditioned by stroma, was found to respond to stromal stimulation but was unable to proliferate in fetal calf serum (FCS). Neutralization experiments indicated that M1/2 cell growth depended mainly on M-CSF and also partially on TGF-beta. By contrast, the same neutralizing antibodies did not affect the ability of serum to support M1 cell growth. The molecules that promoted leukemia cell growth in serum seemed therefore to differ from those provided by stroma. This model system may offer novel information on the interactions of normal and leukemic hematopoietic cells with their stromal microenvironment.