Activities of granulocyte-macrophage colony-stimulating factor and interleukin-3 on monocytes.

We examined the actions of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) on human monocytes, using a serum-free culture system. GM-CSF and IL-3 did not promote the differentiation of monocytes into macrophages but rather into cells with a phenotype compatible with that of immature dendritic cells (DCs). The addition of fetal bovine serum to serum-free cultures with GM-CSF or IL-3 restored the differentiation of monocytes into macrophages. Cells generated with GM-CSF or IL-3 elicited phagocytic activity. Cells generated in the presence of GM-CSF or IL-3, followed by the addition of tumor necrosis factor-alpha, displayed a phenotype of mature DCs, and primed and stimulated immunogenic peptide-specific T lymphocytes. Surprisingly, GM-CSF and IL-3 inhibited macrophage colony-stimulating factor (M-CSF)-dependent differentiation of monocytes into macrophages and induced differentiation into immature DCs. We asked if the inhibition of M-CSF-dependent differentiation into macrophages by GM-CSF or IL-3 was associated with the expression of M-CSF receptors (M-CSFR). GM-CSF or IL-3 down-regulated the expression of M-CSFR. These data demonstrate that GM-CSF and IL-3 primarily support the differentiation of monocytes into DCs and inhibit M-CSF-dependent differentiation into macrophages by suppressing the expression of M-CSFR, thereby promoting differentiation into DCs. Copyright 2004 Wiley-Liss, Inc.