T helper 17 cell

T helper 17 cells (Th17) are a subset of interleukin 17-producing T helper cells. They are considered developmentally distinct from Th1 and Th2 cells and are thought to play a key role in autoimmune disease.

Differentiation
Activation of precursor T helper cells in the presence of TGF-beta and IL-6 is thought to drive differentiation of Th17 cells. Aside from cytokine environment, it is unclear whether any other elements of the initial activation of Th17 cells differ from those of other T helper cells. It has been suggested that IL-23 is involved in the expansion of established Th17 populations, but that cytokine alone does not induce differentiation of naive T-cell precursors into that cell type. IL-21, a cytokine produced by Th17 cells themselves, has also been shown to initiate an alternative route for the activation of Th17 populations. Both Interferon gamma and IL-4, the main stimulators of Th1 and Th2 differentiation respectively, have been shown to negatively regulate Th17 differentiation.

Functions
On initial characterisation, Th17 cells were broadly implicated in autoimmune disease and auto-specific Th17 were shown to be highly pathogenic. A more natural role for Th17 cells is suggested by studies which have demonstrated preferential induction of IL-17 in cases of host infection with various bacterial and fungal species. Th17 primarily produce two main members of the IL-17 family; IL-17A and IL-17F which are involved in the recruitment, activation and migration of neutrophils.