T helper 17 cells: discovery, function, and physiological trigger

In the few years since their discovery, T helper 17 cells (TH17) have been shown to play an important role in host defense against infections, and in tissue inflammation during autoimmunity. TH17 cells produce IL-17, IL-21, IL-10, and IL-22 cytokines, and thus have broad effects on a variety of tissues. Notably, the requirement for the immunosuppressive cytokine TGF-β along with the pro-inflammatory cytokine IL-6 for TH17 differentiation supports the intimate relationship between the TH17 subset and FOXP3+ regulatory T cells. Here, we discuss current knowledge on effector functions and differentiation of the TH17 lineage. Furthermore, we now know of a physiological stimulus for TH17 differentiation: innate immune recognition of cells undergoing apoptosis as a direct result of infection induces unique development of this subset. As our knowledge of TH17 and T regulatory cells grows, we are building on a new framework for the understanding of effector T cell differentiation and the biology of CD4+ T cell adaptive immune responses.