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Summary: Regulatory T (Treg) cells expressing the transcription factor forkhead box P3 (FoxP3) control immune responses and prevent autoimmunity. Treatment with glucocorticoids (GCs) has been shown to increase Treg cell frequency, but the mechanisms of their action on Treg cell induction are largely unknown. Here, we report that glucocorticoid-induced leucine zipper (GILZ), a protein induced by GCs, promotes Treg cell production. In mice, GILZ overexpression causes an increase in Treg cell number, whereas GILZ deficiency results in impaired generation of peripheral Treg cells (pTreg), associated with increased spontaneous and experimental intestinal inflammation. Mechanistically, we found that GILZ is required for GCs to cooperate with TGF-β in FoxP3 induction, while it enhances TGF-β signaling by binding to and promoting Smad2 phosphorylation and activation of FoxP3 expression. Thus, our results establish an essential GILZ-mediated link between the anti-inflammatory action of GCs and the regulation of TGF-β-dependent pTreg production. : Peripherally induced Treg cells (pTreg) are generated outside of the thymus and regulate responses to foreign antigens. In this manuscript, Riccardi and colleagues demonstrate that glucocorticoid-induced protein GILZ controls generation of pTreg cells and colon homeostasis. GILZ promotes TGF-β-induced phosphorylation of Smad2 and the expression of FoxP3. Thus, GILZ mediates a synergy between glucocorticoids and TGF-β in pTreg cell induction. GILZ is essential for Treg induction by glucocorticoids and their anti-inflammatory activity.