AbstractWe have recently demonstrated that G‐CSF promotes the generation of human T regulatory (TREG) type 1 cells. In this study, we investigated whether the immunomodulatory effects of G‐CSF might be mediated by DC. CD14+ monocytes were cultured with serum collected after clinical administration of G‐CSF (post‐G), which contained high amounts of IL‐10 and IFN‐α. Similar to incompletely matured DC, monocytes nurtured with post‐G serum acquired a DC‐like morphology, expressed high levels of costimulatory molecules and HLA‐DR, and exhibited diminished IL‐12p70 release and poor allostimulatory capacity. Importantly, post‐G DC‐like cells were insensitive to maturation stimuli. As shown by neutralization studies, IFN‐α and, even more pronounced, IL‐10 contained in post‐G serum inhibited IL‐12p70 release by post‐G DC‐like cells. Furthermore, phenotypic and functional features of post‐G DC‐like cells were replicated by culturing post‐G monocytes with exogenous IL‐10 and IFN‐α. Post‐G DC‐like cells promoted Ag‐specific hyporesponsiveness in naive allogeneic CD4+ T cells and orchestrated a TREG response that was dependent on secreted TGFβ1 and IL‐10. Finally, neutralization of IL‐10 and IFN‐α contained in post‐G serum translated into abrogation of the regulatory features of post‐G DC‐like cells. This novel mechanism of immune regulation effected by G‐CSF might be therapeutically exploited for tolerance induction in autoimmune disorders.