Schematic illustration of CD4+ T cell differentiation into effector or regulatory T cells. Depending on the cytokine milieu (shown above the arrows) present at the time of the initial engagement of their TCR, naive CD4+ T cells can differentiate into various subsets of T-helper cells (Th1, Th2, Th9, and Th17). However, in the presence of TGF-1, naive T cells convert into foxp3-expressing induced Treg (iTreg) cells. For each T-helper cell differentiation program, specific transcription factors (shown below the arrows) have been identified as master regulators (T-bet, GATA3 and RORt) for Th1, Th2, and Th17, respectively. IRF4, PU.1, and RBP-Jκ transcription factors have been shown recently to contribute to the induction to Th9 cells.
