The IL-23–IL-17 immune axis: from mechanisms to therapeutic testing

T helper 17 (TH17) cells promote protective immune responses against infection, particularly at barrier sites, but they can also have pathogenic roles in inflammatory diseases. In this Review, the authors describe the factors that control the development and maintenance of TH17 cells, and discuss their diverse functions in both health and disease. Following the discovery of T helper 17 (TH17) cells, the past decade has witnessed a major revision of the TH subset paradigm and substantial progress has been made in deciphering the molecular mechanisms of T cell lineage commitment and function. In this Review, we focus on the recent advances that have been made regarding the transcriptional control of TH17 cell plasticity and stability, as well as the effector functions of TH17 cells, and we highlight the mechanisms of IL-17 signalling in mesenchymal and barrier epithelial tissues. We also discuss the emerging clinical data showing that IL-17-specific and IL-23-specific antibody treatments are remarkably effective for treating many immune-mediated inflammatory diseases.