Granulocyte-macrophage colony stimulating factor (GM-CSF) stimulates the in vitro proliferation and differentiation of granulocytic and macrophage cells. This regulator is now known to act at other levels of hemopoietic regulation. The heterogeneity of GM-CSFs is not only related to the tissue of origin and the in vitro production method, but also to functional subclasses of the molecule that have distinct biologic specificities. Most adult mouse organs produce GM-CSF (mol wt 23,000), but a macrophage (M)-CSF has been detected in fetal conditioned medium (CM) and isolated from L-cell CM. Murine endotoxin serum appears to contain a M-CSF, GM-CSF, and G-CSF, the last of which cofractionates with a differentiation factor active on leukemic cells. Human GM-CSFs, G-CSF, and EO-CSFs active on human cells have been detected in a variety of CM, but as yet none have been purified. Again, there are subclasses of progenitor cells that respond to particular forms of human active CSFs. GM-CSF isolated from mouse lung CM stimulates multipotential progenitor cells, the initial proliferatin of progenitors in the erythroid, eosinophil, and megakaryocyte series, as well as mature cells in the GM series. While GM-CSF is also able to stimulate the differentiation of myeloid leukemic cells, other factors appear to be more potent in this respect. Information on the regulation of GM-CSF production, on the modulators of its action on specific target cells, and on its role in vivo will be required before the physiologic function of this molecule can be properly assessed.

Granulocyte-macrophage colony formation by C57BL bone marrow cells was initiated in agar cultures either by the granulocyte-macrophage stimulus, GM-CSF, or by the predominantly macrophage stimulus, M-CSF. After 24 hours, paired daughter cells of granulocyte-macrophage colony-forming cells (GM-CFC) were separated by micromanipulation and one cultured in GM-CSF, the other in M-CSF. From the differentiation pattern of the resulting colonies, irreversible commitment of some cells occurred during the first 24 hours and completion of the first cell division. A similar result was obtained using granddaughter cells present after 24 hours of incubation. However, when intact developing day 2 and day 3 clones were cross-transferred to GM-CSF or M-CSF recipient cultures, irreversible commitment was more obvious. Most M-CSF-initiated clones exhibited irreversible commitment to macrophage formation in GM-CSF cultures and a high proportion of GM-CSF-initiated clones continued to produce granulocyte progeny after transfer to M-CSF. The results indicated that GM-CSF and M-CSF can irreversibly commit the progeny of GM-CFC respectively to granulocyte or macrophage production. While for women GM-CFC this occurs within 24 hours and one cell division, for many cells, the process is slower and requires an incubation period of up to 48 hours and/or several cell divisions. Calculations from the data indicated that two-thirds of GM-CFC in adult C57BL marrow are biresponsive and respond to stimulation both by GM-CSF and M-CSF.

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