An Indispensable Role of GM-CSF in the Differentiation of Human Blood Monocytes toward Langerhans Cells.

We recently demonstrated that GM-CSF, TGFβ1, and Notch ligand Delta-1, all of which are synthesized in the milieu of the skin, instruct human blood monocytes to differentiate into Langerhans cells (LCs) that are characterized by the expression of CD1a, Langerin, CLA, CCR6, and E-cadherin and the presence of Birbeck granules. Previous studies have shown similar biologic activities of GM-CSF and IL-3 on human blood monocytes. In the present study, we investigated whether GM-CSF and IL-3 have a similar action on blood monocytes with respect to their differentiation into LCs. In contrast to CD14⁺ monocytes cultured for 7 days in the presence of GM-CSF, TGF-β1, and Delta-1, the cells obtained from culture with IL-3, TGF-β1, and Delta-1 were negative for CLA, CCR6, Langerin, and E-cadherin. These cells expressed HLA-ABC, HLA-DR, CD80, CD86, CD40, CD54, and CD11c but not DC-SIGN. The expression level of CD1a declined, while CD14 was upregulated. The difference between cells cultured with GM-CSF, TGF-β1, and Delta-1 and those with IL-3, TGF-β1, and Delta-1 was corroborated by microarray analysis of the gene expression profiles. Thus, GM-CSF and IL-3 exert distinct effects on human blood monocytes in the presence of TGF-β1 and Delta-1. When GM-CSF was added to cultures containing IL-3, TGF-β1, and Delta-1, the phenotype of cultured cells closely resembled that observed with GM-CSF, TGF-β1, and Delta-1. As GM-CSF and IL-3 share common receptor β subunits, the signals mediated through GM-CSF receptor α subunits appear to be required for the development of LCs from monocytes. We also compared the action of GM-CSF with that of IL-3 in terms of the differentiation of monocytes into macrophages and dendritic cells (DCs). When CD14⁺ monocytes were cultured in the presence of GM-CSF or IL-3 for 7 days, the resulting macrophages obtained from both cultures were phenotypically indistinguishable. Next, we examined whether IL-3 as well as GM-CSF induces CD14⁺ monocytes to differentiate into DCs in the presence of IL-4. The phenotypic profiles in cells cultured with IL-3 plus IL-4 were parallel to those seen with DCs cultured in the presence of GM-CSF plus IL-4. These data suggest that although IL-3 can substitute for GM-CSF in the differentiation pathway of CD14⁺ monocytes toward macrophages or DCs, GM-CSF is indispensable for their LC development.