HUMAN AND MURINE RECOMBINANT C-KIT LIGANDS SUPPORT THE DEVELOPMENT OF HUMAN MAST-CELLS FROM UMBILICAL-CORD BLOOD-CELLS - ULTRASTRUCTURAL IDENTIFICATION

The recently identified ligand for c-kit, a protooncogene encoded by the W locus in mice, is a member of the tyrosine kinase receptor family with growth factor activity for mouse mast cells. Mature human mast cells regularly develop from agranular precursors in cord blood in long-term cocultures of cord blood and murine fibroblasts. Since the c-kit ligand is a product of murine fibroblasts, we examined the growth effect of recombinant human c-kit ligand (stem cell factor), of recombinant murine c-kit ligand (mast cell growth factor), and of a partially purified fraction derived from mouse fibroblast culture supernatant on the mast cell lineage of humans by electron microscopy in 8-week cultures of cord blood cells. We found that immature mast cells which developed in cultures containing the recombinant ligand for c-kit of human or murine origin as well as the naturally occurring c-kit ligand in 3T3 fibroblast supernatants were identical. Thus, each of these sources of the c-kit ligand exerted identical effects on the ontogeny of human mast cells as they develop from their agranular precursors in cord blood. Full maturity of factor-supported mast cells did not occur.