Steel factor (c-kit ligand) stimulates the in vitro growth of immature CD3 super(-)/CD4 super(-)/CD8 super(-) thymocytes: Synergy with IL-7

The ability of Steel factor (SLF) to stimulate the growth of immature thymocytes alone and in combination with IL-7 was assessed. In suspension cultures of CD4 super(-)/CD8 super(-) thymocytes, SLF itself did not induce significant proliferation, but in combination with IL-7, it induced a greater response than did IL-7 alone. In fetal thymus lobe submersion cultures, SLF stimulated the growth of cells to a magnitude similar to that of IL-7 and the combination resulted in a synergistic response. Phenotypic analysis of these cells revealed that SLF in comparison to IL-7 stimulated the growth of a less mature population. The synergistic growth stimulated by the combination of IL-7 and SLF occurred in a population of IL-2R super(+)/CD4 super(-)/CD8 super(-)/CD3 super(-) cells. Cells grown in SLF expressed low levels of IL-2R and overnight culture in IL-7 dramatically increased IL-2R expression. Thus, these results are consistent with the hypothesis that SLF stimulates the growth of a less mature thymocyte population than does IL-7.