T_H17 cells in the big picture of immunology

The pathogenesis of chronic inflammatory diseases is assumed to depend on activated T cells interacting with resident tissue cells or migratory inflammatory cells. The discovery of new T-cell subsets such as the IL-17-producing T_H17 and T-regulatory cells innovated our understanding of T-cell biology. Studies on new subsets confirm the important role of T cells in the instruction of tissue cells and also demonstrate the important role of feedback regulation for the polarization toward distinct T-cell subsets. The understanding of IL-17 and T_H17 differentiation pathways has also changed the perspective of immunologists regarding the basis of chronic tissue inflammation, particularly where T_H1 cells were considered as driving force of the pathology. This review summarizes the recent developments on T_H cell subsets and integrates these findings into existing concepts of immunopathologic mechanisms.