Stem cell factor, but not flt3 ligand, induces differentiation and activation of human mast cells.

We have examined the effect of human ligand for the flt3/flk2 tyrosine kinase receptor on the differentiation of human mast cells in suspension cultures. We also explored the effect of flt3 ligand (FL) on the human mast cell line HMC-1 and mRNA expression of flt3/flk2 on in vitro developed human mast cells and HMC-1. The growth of cord blood mononuclear cells in suspension cultures was increased when cells were cultured in the presence of FL compared with cells cultured in the presence of stem cell factor (SCF). When SCF and FL were combined, the total cell growth was increased further. Our data show that Fl by itself neither induced differentiation of mast cells nor acted in the SCF-dependent differentiation of human cord blood-derived mast cells (CBMC). Furthermore, no effects of FL were found on the proliferation of HMC-1 cells or the induction of early-immediate response genes in HMC-1 cells. In addition, neither HMC-1 cells nor CBMC expressed mRNA for flt3. As has been shown before, SCF and FL have little biological effect on their own but synergize well with a number of other hematopoietic growth factors. This study shows that a major difference between FL and SCF is that only SCF affects the differentiation and activation of mast cells.