Stem Cell Factor and FLT3-Ligand Are Strictly Required to Sustain the Long-Term Expansion of Primitive CD34+DR− Dendritic Cell Precursors

Abstract

We studied cytokine-driven differentiation of primitive human CD34+HLA-DR− cells to myeloid dendritic cells (DC). Hemopoietic cells were grown in long-term cultures in the presence of various combinations of early acting cytokines such as FLT3-ligand (FLT3-L) and stem cell factor (SCF) and the differentiating growth factors GM-CSF and TNF-α. Two weeks of incubation with GM-CSF and TNF-α generated fully functional DC. However, clonogenic assays demonstrated that CFU-DC did not survive beyond 1 wk in liquid culture regardless of whether FLT3-L and/or SCF were added. FLT3-L or SCF alone did not support DC maturation. However, the combination of the two early acting cytokines allowed a 100-fold expansion of CFU-DC for >1 month. Phenotypic analysis demonstrated the differentiation of CD34+DR− cells into CD34−CD33+DR+CD14+ cells, which were intermediate progenitors capable of differentiating into functionally active DC upon further incubation with GM-CSF and TNF-α. As expected, GM-CSF and TNF-α generated DC from committed CD34+DR+ cells. However, only SCF, with or without FLT3-L, induced the expansion of DC precursors for >4 wk, as documented by secondary clonogenic assays. This demonstrates that although GM-CSF and TNF-α do not require additional cytokines to generate DC from primitive human CD34+DR− progenitor cells, they do force terminal differentiation of DC precursors. Conversely, FLT3-L and SCF do not directly affect DC differentiation, but instead sustain the long-term expansion of CFU-DC, which can be induced to produce mature DC by GM-CSF and TNF-α.