Murine myeloid cells transformed by myb require fibroblast-derived or autocrine growth factors in addition to granulocyte-macrophage colony- stimulating factor for proliferation

Murine myeloid cells can be transformed in vitro by infection with recombinant retroviruses carrying activated myb genes. While these myb-transformed hematopoietic cells (MTHCs) can proliferate continuously in culture, they exhibit several characteristics of progenitor cells of the granulocyte-macrophage (GM) lineage, including an absolute dependence on hematopoietic growth factors (HGFs) such as GM colony-stimulating factor (GM-CSF) for survival and growth. Whereas we have previously shown that MTHCs respond synergistically to certain combinations of HGFs, we report here that MTHCs apparently require two HGFs for proliferation, because GM-CSF alone appears insufficient to promote growth when MTHCs are cultured at very low densities. However, proliferation can be stimulated by either increasing the density at which MTHCs are cultured (implying the production of an autocrine growth factor) or by the presence of a feeder layer of irradiated fibroblasts. We find that the activity of such feeder layers is greatest when the MTHCs are allowed to contact them directly; and by using mutant fibroblast lines, that it depends on the production of CSF-1, but not Steel factor (SLF). In contrast, the autocrine factor appears not to be either CSF-1 or SLF, and the possibility is raised that it may represent a novel HGF activity. Potential implications of these results for normal and leukemic hematopoiesis are discussed.