Recombinant human stem cell factor stimulates differentiation of mast cells from dispersed human fetal liver cells

We have previously shown the development in vitro of tryptase⁺ human mast cells from fetal liver cells cocultured with murine 3T3 fibroblasts. In this study, recombinant human stem cell factor (rhuSCF), the ligand for the c-kit proto-oncogene product called Kit, stimulated the growth and differentiation primarily of mast cells from dispersed fetal liver cells, whereas recombinant human interleukin-3 (rhuIL-3) stimulated the differentiation of basophils along with other cell types. Cultures of fetal liver cells were initiated and maintained in the presence of rhuSCF or rhuIL-3 for up to 6 weeks. Metachromatic cells in cytospins were identified as mast cells primarily on the basis of tryptase expression, and as MC_T or MC_TC by immunohistochemistry using monoclonal antibodies against tryptase and chymase, whereas basophils were metachromatic, polymorphonuclear, and lacked these proteases. Levels of tryptase and histamine were measured by radioimmunoassay, tryptase and chymase activities by peptide hydrolysis, and cell surface Kit by flow cytometry with the monoclonal antibody YB5.B8. The predominant presence of mast cells occurred only in the cultures supplemented with rhuSCF. The percentage and total number of mast cells increased over time with increasing concentrations of rhuSCF and reached a plateau at 55 ng/mL. At this concentration of rhuSCF, mast cells first appeared by day 7; by day 42, 106% of the starting number of cells were present and 85% of these were tryptase⁺, 31% being weakly chymase⁺. These mast cells appeared immature by ultrastructural criteria; most cells were mononuclear, but some had nuclei with deeply divided lobes. DNA synthesis in tryptase⁺ mast cells at days 21 and 28 of culture with rhuSCF was demonstrated by incorporation of bromodeoxyuridine. Calculated levels of histamine (1.2 pg/mast cell) and tryptase (0.9 pg/mast cell) were similar to those determined previously in coculture experiments with murine 3T3 fibroblasts. Chymase activity was undetectable in most cell extracts. On day 0, 4% to 20% of fetal liver cells expressed cell surface Kit. In the presence of rhuSCF, the percentages and total numbers of Kit⁺ cells and the apparent concentration of Kit per cell increased along with the number of tryptase⁺ cells. In the presence of rhuIL-3, toluidine blue⁺, tryptase^- cells first and maximally appeared at day 14 (11% ± 2.5%). The percentage of these toluidine blue⁺ cells then declined to about 6% by days 21 and 35, while the total number of positive cells declined over 10-fold. Kit⁺ cells in the presence of rhuIL-3 declined from 9% on day 3 to 2% on day 35. Thus, rhuSCF induces the differentiation of Kit⁺, tryptase⁺, toluidine blue⁺ mast cells, whereas rhuIL-3 induces the differentiation of Kit^-, tryptase^-, toluidine blue⁺ basophils. In conclusion, human SCF appears to be a major growth factor for the differentiation of human mast cells.