The IL-23/IL-17 axis as a therapeutic target

T helper (Th) cells are known to differentiate into several distinct subsets depending on their environment. The Interleukin 17 (IL-17)-producing subset Th17, which was reported and named in 2005, has been extensively analyzed because a strong association with inflammatory disorders has been suggested. The condition in which Th17 cells are differentiated, critical transcription factors, and similarity and difference between mouse and human Th17 cells have been reported one after the other. IL-23, which is believed to be important for the activation of Th17 cells, and IL-17 became promising targets for drug development. In fact, biological drugs that block the IL-23/IL-17 axis have demonstrated to be highly effective against the inflammatory skin disease psoriasis and have been approved for clinical use. Here an overview and brief history of the basic research on Th17 cells are provided, with the latter full of surprises and unexpected twists in the plot. The current state of biological drug development is also described.