Dichotomous role of TGF-β controls inducible regulatory T-cell fate in allergic airway disease through Smad3 and TGF-β-activated kinase 1

Abstract

Background: Inducible CD4⁺CD25⁺ regulatory T (iTreg) cells can become pathogenic effector cells, enhancing lung allergic responses.

Objective: We aimed to define the underlying cellular and molecular pathways activated by TGF-β, which determine the suppressor or enhancing activities of iTreg cells.

Methods: Sensitized wild-type and CD8-deficient (CD8^-/-) mice were challenged with allergen. Isolated CD4⁺CD25^- T cells were activated by using anti-CD3/anti-CD28. To generate suppressor iTreg cells, cells were then differentiated in the presence of TGF-β, whereas IL-17-producing effector T cells were additionally exposed to IL-6. After TGF-β, Smad3 and TGF-β-activated kinase 1 (TAK1) kinase levels were monitored. The consequences of inhibiting either kinase were determined in vitro and after transfer into CD8^-/- recipients. Quantitative PCR and chromatin immunoprecipitation were used to monitor gene expression and histone modifications at the retinoic acid-related orphan receptor γt (Rorγt) locus.

Results: In wild-type mice, iTreg cells suppressed lung allergic responses linked to Smad3-dependent forkhead box P3 (Foxp3) expression and IL-10 production. In the presence of IL-6, iTreg cells converted to T_H17 cells, mediating a neutrophil-dependent enhancement of lung allergic responses in CD8^-/- mice. Conversion was regulated by TAK1. Inhibition or silencing of TAK1 prevented expression of Rorγt and T_H17 differentiation through histone modifications of Rorγt; Foxp3 expression and iTreg cell-mediated suppression remained intact. In the same cell, TGF-β induced coexpression of Smad3 and TAK1 proteins; in the presence of IL-6, expression of Smad3 and Foxp3 but not TAK1 decreased.

Conclusion: TGF-β regulates iTreg cell outcomes through 2 distinct signal transduction pathways: one Smad3 dependent and the other TAK1 dependent. The balance of these pathways has important implications in T_H17-mediated autoimmune diseases and neutrophil-dependent asthma.

Keywords: IL-17; Inducible CD4(+)CD25(+) regulatory T cells; Smad3; T(H)17 cells; TGF-β–activated kinase 1; allergic lung; epigenetics; forkhead box P3; retinoic acid–related orphan receptor γt.