Similar to the antagonistic relationship between Th1 and Th2 types, Th17 cells can be inhibited by cytokines of other T Helper classes. IFNγ (Th1) and IL-4 (Th2) have both been shown to block development of Th17 cells. Interestingly, low doses of TGFβ induce Th17s, while high levels block their development and favor Treg induction, even if IL-6 is present. The relationship between Tregs and Th17s is often inverse, as FOXP3 (Treg transcription factor) will inhibit RORγt, and IL-2 will prevent activation of the IL-17 promoter.
While the establishment of Th17 cells has revealed a great deal of information , it should be noted that Th17 cells typically display a high level of plasticity, both in mice and humans. Th17 cells have been shown to convert into Th1 cells both in vitro and in vivo. This trend is particularly evident in lymphopenic mouse models, as highly purified Th17 cells produced IFNγ following transfer into these mice. There has been evidence of Th17 conversion into Th2 types as well. It seems this is a one-way street, however, as there is little evidence of Th1 or Th2 conversion into Th17 cells. Recently, FOXP3+ RORγt+ cells have been found and exhibit regulatory functions.
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