IL-9 Enhances the Growth of Human Mast Cell Progenitors Under Stimulation with Stem Cell Factor

We examined the effects of IL-9 on human mast cell development from CD34 + cord blood (CB) and peripheral blood cells in serum-deprived cultures. IL-9 apparently enhanced cell production under stimulation with stem cell factor (SCF) from CD34 + CB cells. A great majority of the cultured cells grown with SCF + IL-9 became positive for tryptase at 4 wk. In methylcellulose cultures of CD34 + CB cells, IL-9 increased both the number and size of mast cell colonies grown with SCF. Furthermore, SCF + IL-9 caused an exclusive expansion of mast cell colony-forming cells in a 2-wk liquid culture of CD34 + CB cells, at a level markedly greater than for SCF alone. Clonal cell cultures and RT-PCR analysis showed that the targets of SCF + IL-9 were the CD34 + CD38 + CB cells rather than the CD34 + CD38 − CB cells. IL-9 neither augmented the SCF-dependent generation of progeny nor supported the survival of 6-wk-cultured mast cells. Moreover, there was no difference in the appearance of tryptase + cells and histamine content in the cultured cells between SCF and SCF + IL-9. The addition of IL-9 increased numbers of mast cell colonies grown with SCF from CD34 + peripheral blood cells in children with or without asthma. It is of interest that mast cell progenitors of asthmatic patients responded to SCF + IL-9 to a greater extent than those of normal controls. Taken together, IL-9 appears to act as a potent enhancer for the SCF-dependent growth of mast cell progenitors in humans, particularly asthmatic patients.