Cofactors are essential for stem cell factor-dependent growth and maturation of mast cell progenitors: comparative effects of interleukin-3 (IL-3), IL-4, IL-10, and fibroblasts.

Stem cell factor (SCF) possesses many mast cell-stimulating activities, including the ability to support the growth of mucosal-like mast cells (MMCs) and connective tissue mast cells (CTMCs). However, this study shows that, in the absence of accessory cells, SCF does not stimulate the clonal growth of primitive mast cell progenitors. Nevertheless, SCF exhibited potent growth-promoting effects when combined with the cytokines interleukin-3 (IL-3), interleukin-4 (IL-4), and interleukin-10 (IL-10). Our comparative studies have shown that optimal mast cell colony formation occurs when both IL-4 and IL-10 are combined with SCF. However, in the presence of SCF, these two cofactors appear to mediate different effects. IL-4 was more efficient than IL-10 in costimulating the initiation of SCF-dependent colony formation by mast cell progenitors and in sustaining the proliferation of newly generated progeny. On the other hand, IL-4 was less efficient than IL-10 in supporting mast cell differentiation, as evidenced by morphology, cell enlargement, and granule production. Although the actions of IL-4 and IL-10 were not equivalent, additional experiments indicated that their ability to serve as early- and late-acting factors, respectively, were complimentary. We have also found that the mast cells generated in colonies stimulated by IL-4, IL-10, and SCF produced high levels of histamine (6-8 pg per cell). None of the mast cells generated in our cultures synthesized heparin. A phenotypic change from safranin-negative to safranin-positive cells associated with heparin-producing CTMCs was accomplished after coculture of the mast cells with fibroblast cell lines derived from normal mice or from SI/SId mice plus soluble factors. Collectively, our observations demonstrate that SCF acts as a competence factor for mast cell progenitor growth. In addition, the ability of SCF to support certain stages of mast cell differentiation is profoundly influenced by interactions with specific cofactors.