Figure 1.

The protective and pathogenic functions of Th17 cells. Depending on the local cytokine environment, different subsets of Th17 cells arise and mediate distinct effector function. In the presence of IL-23, Th17 cells seem to be pro-inflammatory and can either make IFN-γ themselves or work in concert with Th1 cells to drive a positive feedback pathway of tissue damage such as that seen in psoriasis or RA. In Crohn’s disease, Th17 cells can differentiate in the gut into protective or pathogenic Th17 cells. In the presence of TGF-β and IL-21, Th17 are re-programmed into “regulatory” Th17 cells which seem to protect from intestinal inflammation.