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Th17 cells are a subset of CD4+ T helper cells that play an important role in host defense and have been strongly associated with the pathogenesis of autoimmunity. The immense research effort on Th17 cells has benefited in recent years from major breakthroughs in genomic profiling technologies. The picture emerging from these studies has led us away from thinking about T cell differentiation in terms of rigid, separate pathways that give rise to canonical 'types'. Instead, it has encouraged us to understand T cell differentiation and function through a complex network of transcriptional regulators that can lead to different and potentially plastic cell states. This review summarizes some of the lessons we have learned from these studies about the identity, interplay, and function of the factors that are involved in Th17 cell differentiation, effector functions and plasticity. It also highlights some applications, challenges, and limitations of large-scale systematic analyses of T cell function.