Several hematopoietic growth factors, including interleukin-10 (IL-10) and transforming growth factor-β1 (TGF-β1), promote the differentiation of tolerogenic dendritic cells (DCs). Hepatocyte growth factor (HGF) is a pleiotropic cytokine whose effects on human DC differentiation and function have not been investigated. Monocytes cultured with HGF (^HGFMo) differentiated into accessory cells with DC-like morphology, released low amounts of IL-12p70 and up-regulated IL-10 both at the mRNA and at the protein level. Upon activation with ^HGFMo, allogeneic CD4⁺CD25^- T cells expressed the T regulatory (Treg)-associated transcription factor FoxP3, proliferated poorly, and released high levels of IL-10. Interestingly, blockade of surface immunoglobulin-like transcript 3 (ILT3) on ^HGFMo or neutralization of secreted IL-10 translated into partial restoration of T-cell proliferation. Secondary stimulation of ^HGFMo-primed CD4⁺ T cells with immunogenic DCs differentiated with granulocyte-macrophage colony-stimulating factor (GM-CSF) and IL-4 from monocytes of the same donor resulted in measurable T-cell proliferation. ^HGFMo-primed CD4⁺ T cells significantly inhibited the proliferation of naive CD4⁺CD25^- T cells in a cell-contact-dependent manner. Finally, DNA microarray analysis revealed a unique gene-expression profile of HGF-activated monocytes. Collectively, our findings point to a novel role for HGF in the regulation of monocyte/DC functions that might be exploited therapeutically. (Blood. 2006;108:218-227)