Model how fibroblasts could stimulate IL-23 secretion from preactivated DCs resulting in expansion of Th17 cells. (1) Preactivation of DC (LPS). (2) Then, DCs secrete IL-1β and TNF-α, which stimulate fibroblasts. (3) Stimulated fibroblasts secrete PGE₂, which acts on DCs and therewith increases their IL-23 production. (4) Through the action of elevated IL-23, CD3⁺ (Pan), CD4⁺, and CD4⁺CD45RO⁺ (memory) TCs are supported to produce IL-17A, IL-22, and to express RORγt as well as to secrete IFN-γ.