Interactions between products of the mouse W locus, which encodes the c-kit tyrosine kinase receptor, and the Sl locus, which encodes a ligand for c-kit receptor, which we have designated stem cell factor (SCF), have a critical role in the development of mast cells. Mice homozygous for mutations at either locus exhibit several phenotypic abnormalities including a virtual absence of mast cells. Moreover, the c-kit ligand SCF can induce the proliferation and maturation of normal mast cells in vitro or in vivo, and also can result in repair of the mast cell deficiency of Sl/Sld mice in vivo. We now report that administration of SCF intradermally in vivo results in dermal mast cell activation and a mast cell-dependent acute inflammatory response. This effect is c-kit receptor dependent, in that it is not observed when SCF is administered to mice containing dermal mast cells expressing functionally inactive c-kit receptors, is observed with both glycosylated and nonglycosylated forms of SCF, and occurs at doses of SCF at least 10-fold lower on a molar basis than the minimally effective dose of the classical dermal mast cell-activating agent substance P. These findings represent the first demonstration in vivo that a c-kit ligand can result in the functional activation of any cellular lineage expressing the c-kit receptor, and suggest that interactions between the c-kit receptor and its ligand may influence mast cell biology through complex effects on proliferation, maturation, and function.

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