ABSTRACT
Haemopoietic cell growth factors are normally assayed using unfractionated marrow cells (NBM). However, using this population it is difficult to distinguish between direct versus indirect effects, because of the low incidence of colony forming cells (CFC) and the presence of possible accessory cells (which may themselves be acted upon by the growth factors and stimulated to produce other growth stimulatory or inhibitory molecules that influence the development of the CFC). Furthermore, NBM contain the whole spectrum of multipotent and lineage-restricted CFC and it is often difficult to determine precisely which populations are being stimulated to develop. This latter problem can be solved, in part, by using marrow from mice previously treated with 5-fluorouracil (5-FU): an agent that preferentially kills the more mature, actively cycling CFC but spares the proliferatively quiescent multipotent stem cells. Since the 5-FU-treated marrow also contains many possible accessory cells, however, it is again not clear whether or not the responses elicited by growth factors are due to direct or indirect effects upon the CFC. To circumvent this problem we have obtained a highly enriched population of multipotent stem cells (FACS-BM) that is free of accessory cells, and have compared the responses of these cells to NBM and to 5-FU-BM in the presence of a variety of growth factors. The data demonstrate that interleukin-1 (IL-1), which is not a growth factor by itself, can act synergistically with other growth factors on FACS-BM and 5-FU-BM but not on NBM. With FACS-BM, IL-1 can synergize with GM-CSF and with M-CSF but not with IL-3 or G-CSF. With 5-FU-BM, IL-1 can synergize with GM-CSF, M-CSF and IL-3. Furthermore, G-CSF, a poor growth stimulus when used alone, can synergize with GM-CSF and M-CSF, using either FACS-BM or 5-FU-BM, and can also synergize with IL-3 when using 5-FU-BM but not with FACS-BM. The data further indicate that extensive overlap occurs between the multipotential cells that respond to IL-3 alone and those that can respond to a combination of other growth-promoting stimuli such as IL-1 plus CSF-1 or G-CSF and CSF-1. This indicates that at least some multipotential cells have receptors for all the growth factors used in this study, but that combinations of growth factors sometimes need to be present in order to facilitate proliferation and development. Furthermore, the outcome of the response in terms of mature cell lineages produced is a reflection of the stimuli to which the multipotential cells are exposed. The developmental implications of these findings are discussed.
