RESPONSES OF THE MURINE MYELOID CELL-LINE FDC-P1 TO SOLUBLE AND MEMBRANE-BOUND FORMS OF STEEL FACTOR (SLF)

Cells of the murine interleukin-3 (IL-3) or granulocyte-macrophage col ony-stimulating factor (GM-CSF) factor dependent line, FDC-P1, express the tyrosine kinase receptor, c-kit. The ligand for c-kit steel facto r (SLF), encoded by the steel (SI) locus, is produced as both membrane -bound and soluble forms by fibroblastoid cells. Fibroblasts derived f rom normal (+/+) WCB6F(1) mice are known to produce both forms of SLF and were able to support FDC-P1 cells in a contact-dependent manner in the presence of neutralizing anti-GM-CSF antiserum. In contrast, Sl/S l(d) mutant fibroblasts, which produce only a soluble form of SLF, wer e incapable of supporting FDC-P1 cells in the presence of GM-CSF antis erum. These results suggested that FDC-P1 cells were being supported o n fibroblast layers by membrane-bound SLF. Attempts to grow FDC-P1 cel ls in high levels of soluble recombinant SLF to mimic the SLF-dependen t response seen in co-culture experiments showed that cells which had been previously grown in GM-CSF or IL-3 were minimally responsive to S LF at concentrations up to 100 ng/mL. Although these cultures were not supported by SLF alone, the cells showed synergistic proliferative re sponses to SLF combined with suboptimal levels of GM-CSF or IL-3. FDC- P1 cells could, however, be adapted to grow in SLF alone by gradual su bstitution of SLF for GM-CSF over a period of 3 weeks. These cells sho wed 5.6- to 8.4-fold and 2.5-fold higher levels of c-kif mRNA than cel ls grown in GM-CSF or IL-3, respectively. Downregulation of surface c- kit protein was also seen in FDC-P1 cells grown in GMCSF or IL-3 compa red with cells grown in SLF. Although FDC-P1 cells propagated in SLF w ere more responsive to SLF, they were still able to proliferate as wel l in GM-CSF and IL-3 as the cells originally grown in the latter facto rs. Thus, functional downregulation of c-kit by GM-CSF and IL-3 was un idirectional.