Biology and potential clinical applications of flt3 ligand.

The flt3 ligand is a member of a small family of growth factors that stimulate the proliferation of hematopoietic cells. Other members of this family include Steel factor (also known as mast cell growth factor, stem cell factor, and kit ligand) and colony-stimulating factor 1. These proteins function by binding to and activating unique tyrosine kinase receptors. Both flt3 ligand and Steel factor stimulate the proliferation of early progenitor or stem cells. Neither of these factors exhibits much biologic activity by itself, but each factor can synergize with a wide range of other colony-stimulating factors and interleukins. One major difference between the two factors appears to be their effect on mast cells, which Steel factor stimulates, but flt3 ligand does not. Although flt3 ligand and Steel factor each act on early hematopoietic cells, differences in their activities suggest that they are not redundant and are both required for normal hematopoiesis. There are a number of clinical settings in which the flt3 ligand may prove quite useful.