Cytokine mediators of Th17 function

Th17 cells were identified as an independent lineage of CD4(+) T cells that secrete a distinctive set of immunoregulatory cytokines, including IL-17A, IL-17F, IL-22, and IL-21. These cytokines collectively play roles in inflammation and autoimmunity and in response to extracellular pathogens. The expression of the lineage-specific transcription factor retinoic acid receptor-related orphan receptor (ROR)gamma t leads to Th17 lineage commitment; however, it has become increasingly clear that the population of cells designated as Th17 cells is not homogeneous. Although these cells collectively produce characteristic Th17 cytokines, not all are produced by each individual cell in the population. The cytokines produced by individual cells are presumably affected in part by the specific local cytokine milieu. In this review, we discuss the current understanding of the specific functional characteristics and regulation of Th17 cytokines and clarify how they mediate the actions of Th17 cells.