TGF-β1 is synthesized in cells as a 390-amino acid. Furin cleaves the protein at residue 278, yielding an N-terminal cleavage product which corresponds to the latency-associated peptide (LAP), and the 25-kD C-terminal portion of the precursor constitutes the mature TGF-β1. TGF-β activators can release TGF-β from LAP. These activators include proteases that degrade LAP, thrombospondin-1, reactive oxygen species, and integrins avb6 and avb8. Mouse TGF-β converts na?ve T cells into regulatory T (Treg) cells that prevent autoimmunity. Although human TGF-β1 is widely used for inducing FOXP3+ in vitro, it might not be an essential factor for human Treg differentiation. Th17 murine can be induced from na?ve CD4+ T cells by the combination of TGF-β1 and IL-6 or IL-21. Nevertheless, the regulation of human Th17 differentiation is distinct. TGF-β1 seems to have dual effects on human Th17 differentiation in a dose-dependent manner. While TGF-β1 is required for the expression of RORγt, in human naive CD4+ T cells from cord blood, TGF-β1 can inhibit the function of RORγt at high doses. By using serum-free medium, it has been clarified that the optimum conditions for human Th17 differentiation areTGF-β1, IL-1β, and IL-2 in combination with IL-6, IL-21 or IL-23.