Maintenance and differentiation of hematopoietic stem and progenitor cells are controlled by complex interactions with the stroma microenvironment. Stroma-cell interactions can be supported by locally expressed membrane-spanning cell-surface (cs) growth factors. CSF-1 is expressed by stroma as a soluble glycoprotein, as proteoglycan, or as a membrane-spanning cs glycoprotein. CSF-1 regulates the survival, proliferation, and differentiation of mononuclear phagocytes. Whereas the biological role of soluble CSF-1 is well characterized, the function of the membrane-spanning cell-surface CSF-1 (csCSF-1) remains unclear. To analyze the biological significance of csCSF-1 in vitro, we used an epithelial cell line to ectopically express the different CSF-1 isoforms. In co-cultures of CSF-1 transduced epithelial cells with primary, early hematopoietic progenitor cells we examined whether interaction between csCSF-1 and its receptor mediates cell proliferation, self-renewal, or differentiation. csCSF-1 induces long-lasting proliferation of stimulated cells and furthermore supports self-renewal. Ectopic secretion of soluble CSF-1 does not permit long-term growth of progenitor cells but induces differentiation of monocytes into macrophages. Previously, we showed that the soluble and cs isoforms of stroma-encoded SCF differently affect the development of hematopoietic cells. Cell-surface SCF (csSCF) promotes self-renewal of stimulated cells whereas soluble SCF causes clonal extinction. These results and those presented here for CSF-1 provide evidence for diverse functions of the isoforms of the ligands SCF and CSF-1 for two tyrosine kinase receptors of the subclass III both regulating hematopoiesis on stroma.
(c) 2004 Wiley-Liss, Inc.