STEEL FACTOR (C-KIT LIGAND) STIMULATES THE IN-VITRO GROWTH OF IMMATURE CD3-/CD4-/CD8- 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(-)/CD8(-) 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(+)/CD4(-)/CD8(-)/CD3(-) 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. (C) 1994 Academic Press, Inc.