Interferon-γ switches monocyte differentiation from dendritic cells to macrophages

Human monocytes differentiate into dendritic cells (DCs) or macrophages according to the nature of environmental signals. Monocytes stimulated with granulocyte-macrophage colony-stimulating factor (GM-CSF) plus interleukin 4 (IL-4) yield DCs. We tested here whether interferon-γ (IFN-γ), a potent activator of macrophages, may modulate monocyte differentiation. Addition of IFN-γ to IL-4 plus GM-CSF–stimulated monocytes switches their differentiation from DCs to CD14−CD64+macrophages. IFN-γ increases macrophage colony-stimulating factor (M-CSF) and IL-6 production by IL-4 plus GM-CSF–stimulated monocytes by acting at the transcriptional level and acts together with IL-4 to up-regulate M-CSF but not IL-6 production. IFN-γ also increases M-CSF receptor internalization. Results from neutralizing experiments show that both M-CSF and IL-6 are involved in the ability of IFN-γ to skew monocyte differentiation from DCs to macrophages. Finally, this effect of IFN-γ is limited to early stages of differentiation. When added to immature DCs, IFN-γ up-regulates IL-6 but not M-CSF production and does not convert them to macrophages, even in the presence of exogenous M-CSF. In conclusion, IFN-γ shifts monocyte differentiation to macrophages rather than DCs through autocrine M-CSF and IL-6 production. These data show that IFN-γ controls the differentiation of antigen-presenting cells and thereby reveals a new mechanism by which IFN-γ orchestrates the outcome of specific immune responses.