The initiation of immune responses is associated with the maturation of dendritic cells (DCs) and their migration to draining lymph nodes. En route activated DCs encounter cells of the tissue microenvironment, such as fibroblasts. Because we have shown that DCs interact with fibroblasts during immune responses, we studied the impact of skin fibroblasts on human monocyte-derived DC function and subsequent human T-cell (TC) differentiation. We show that fibroblasts support interleukin-23 (IL-23) secretion from DCs preactivated by lipopolysaccharide (DC(act)) compared with lipopolysaccharide-activated DCs alone. The underlying complex feedback-loop mechanism involves IL-1β/tumor necrosis factor-α (from DC(act)), which stimulate fibroblasts prostaglandin E(2) production. Prostaglandin E(2), in turn, acts on DC(act) and increases their IL-23 release. Furthermore, fibroblast-stimulated DC(act) are far superior to DC(act) alone, in promoting the expansion of Th17 cells in a Cox-2-, IL-23-dependent manner. Using CD4(+)CD45RO(+) memory TCs and CD4(+)CD45RA(+) naive TCs, we showed that fibroblasts induce a phenotype of DC(act) that promotes the expansion of Th17 cells. Moreover, in psoriasis, a prototypic immune response in which the importance of IL-23/Th17 is known, high expression of Cox-2 in fibroblasts was observed. In conclusion, skin fibroblasts are involved in regulation of IL-23 production in DCs and, as a result, of Th17 expansion.