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A number of growth factors acting on hematopoietic stem cells have now been purified and characterized. These include erythropoietin, granulocyte-macrophage colony-stimulating activity (GM-CSA), granulocyte colony-stimulating activity and colony-stimulating factor-1 (CSF-1). Factors which act in concert with these defined factors and appear to act relatively early in the hematopoietic stem cell lineage are currently under study. Interleukin 3 appears to have both the characteristics of a differentiating hormone and the ability to generate proliferation of relatively early stem cells. Interleukin 3 acts in concert with at least CSF-1 and erythropoietin to enhance their effect on stem cell proliferation and differentiation. A new class of hematopoietic growth factor activities termed synergizing activities also exist. These activities appear to have no intrinsic capacity to stimulate hematopoietic colony formation by themselves but enhance the effects of other differentiating hormones such as GM-CSA and CSF-1. Activities which appear to represent synergizing activities have now been found to evolve from a human bladder carcinoma line, a cell line derived from murine marrow adherent cells and normal murine marrow and thymic cells. These activities may act on very primitive hematopoietic progenitors to allow them to express receptors to various differentiating hormones or alternatively they may act as commitment factors in a commitment-progression model of stem cell regulation.
Peter J. Quesenberry (Wed,) studied this question.