Involvement of CSF-1 in generating a stroma-independent hematopoietic stem cell line

The hematopoietic stem cell line, Myl-D7, is maintained by a self-renewing stem cell population that spontaneously generates myeloid, lymphoid, and erythroid progeny. MS-5 stromal cells are necessary for the growth of Myl-D7 cells. One component of the Myl-D7 cells proliferation activity released by MS-5 stromal cells was enriched by Q sepharose fractionation and shown to be colony stimulating factor-1 (CSF-1) by Western blotting, BAC1.2F5 cell bioassay and inhibition of Myl-D7 proliferation by CSF-1 antibody. The requirement of Myl-D7 cells for CSF-1 was also demonstrated independently by selecting for rare, stroma-independent Myl-D7 mutant clones able to grow without stroma and additional factors. Eighty-nine stroma-independent mutant clones were obtained and belonged to two classes. The majority of mutants did not secrete any growth promoting activity. The second, rarer class of mutants releases a factor that stimulates proliferation/survival for up to several months and approximately half of the secretors express high levels of CSF-1 mRNA. Wild type Myl-D7 grown with supernatants from the secretor cells retained the stem cell phenotype. These data suggest that CSF-1 may act as a key factor in stroma-regulated hematopoiesis and cell-cell interaction.