T(H)1, T(H)2, and T(H)17 cells instruct monocytes to differentiate into specialized dendritic cell subsets

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.