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Monocytes and T helper (TH) cells rapidly infiltrate inflamed tissues where monocytes differentiate into inflammatory dendritic cells (DCs) through undefined mechanisms. Our studies indicate that TH cells frequently interact with monocytes in inflamed skin and elicit the differentiation of specialized DC subsets characteristic of these lesions. In psoriasis lesions, TH1 and TH17 cells interact with monocytes and instruct these cells to differentiate into TH1- and TH17-promoting DCs, respectively. Correspondingly, in acute atopic dermatitis, TH2 cells interact with monocytes and elicit the formation of TH2-promoting DCs. DC formation requires GM-CSF and cell contact, whereas TH subset specific cytokines dictate DC function and the expression of DC subset specific surface molecules. Moreover, the phenotypes of T cell–induced DC subsets are maintained after subsequent stimulation with a panel of TLR agonists, suggesting that TH-derived signals outweigh downstream TLR signals in their influence on DC function. These findings indicate that TH cells govern the formation and function of specialized DC subsets.

Circulating monocytes can differentiate into dendritic cells (moDCs), which are potent inducers of adaptive immune responses. Previous reports show that granulocyte macrophage–colony-stimulating factor (GM-CSF) and interleukin-4 induce monocyte differentiation into moDCs in vitro, but little is known about the physiological requirements that initiate moDC differentiation in vivo. Here we show that a unique natural killer (NK) cell subset (CD3−CD56bright) that accumulates in lymph nodes and chronically inflamed tissues triggers CD14+ monocytes to differentiate into potent T-helper-1 (TH1) promoting DC. This process requires direct contact of monocytes with NK cells and is mediated by GM-CSF and CD154 derived from NK cells. It is noteworthy that synovial fluid (SF) from patients with rheumatoid arthritis (RA) and psoriatic arthritis (PsA), but not osteoarthritis (OA), induces monocytes to differentiate into DC. However, this process occurs only in the presence of NK cells. We propose that NK cells play a role in the maintenance of TH1-mediated inflammatory diseases such as RA by providing a local milieu for monocytes to differentiate into DC.