Effects of IL-6 on SCF-dependent mast cell development. (A) Dose response to IL-6 or IL-4 of mast cell growth supported by SCF. Ten-week cultured mast cells (1 × 10⁴) were incubated in wells containing either IL-6 at 0.01 to 100 ng/mL or IL-4 at 0.01 to 80 ng/mL with SCF at 100 ng/mL. After 2 weeks, the number of viable cells was determined. Significantly different from SCF alone (*P < .0005, **P < .0001). (B) Effects of IL-6 on the formation of mast cell colonies and clusters supported by SCF. Five thousand 10-week cultured mast cells were plated per dish containing serum-deprived methylcellulose culture medium supplemented with SCF or SCF+IL-6. After 14 days, aggregates consisting of 30 or more cells were scored as mast cell colonies and those of 10 to 29 cells were scored as mast cell clusters. SCF, 100 ng/mL; IL-6, 50 ng/mL. Numbers of mast cell colonies (▪) and mast cell clusters (▩). Results shown are the mean ± SD of three experiments. Significantly different from SCF alone (*P < .0001). (C) Effects of IL-6 on the cell growth by CD34⁺ cord blood cells under stimulation with SCF. CD34⁺ cord blood cells (1 × 10⁴) were cultured with 10 ng/mL of SCF or 10 ng/mL of SCF + 50 ng/mL of IL-6. The number of viable cells was determined every week. Significantly different from SCF alone (*P < .0001).