Innate lymphoid cells (ILCs) are emerging subsets of immune cells that produce large amounts of cytokines upon cytokine and/or alarmin stimulation. Recent studies have shown that T-bet plays pivotal roles in the development of ILC3s and type 1 ILCs; however, the roles of T-bet in lung type 2 innate lymphoid cells (ILC2s) remain unknown.

We sought to determine the role of T-bet in ILC2-mediated airway inflammation.

The expression of T-bet in lung ILCs (defined as Thy1.2⁺ Lin⁻ cells) was examined. The roles of T-bet in the development of lung ILC2s and airway inflammation induced by IL-33 administration were examined by using T-bet–deficient (T-bet^−/−) mice. Gene expression profiles of T-bet^−/− lung ILCs were analyzed by RNA sequencing.

T-bet was expressed in lung ILC2s (defined as Thy1.2⁺ Lin⁻ cells expressing ST2 or CD25) and IFN-γ enhanced its expression. Although the development of lung ILC2s at steady-state conditions was normal in T-bet^−/− mice, IL-33–induced accumulation of lung ILC2s and eosinophilic airway inflammation were exacerbated in T-bet^−/− mice. The exacerbated accumulation of ILC2s and eosinophilic airway inflammation by the absence of T-bet were evident even in a RAG2^−/− background, suggesting that T-bet expressed in non–T/non–B population is involved in the suppression of IL-33–induced eosinophilic airway inflammation. Transcriptome analysis revealed that IL-9 expression in IL-33–stimulated lung ILCs was upregulated in T-bet^−/− mice compared with that in wild-type mice. Importantly, neutralization of IL-9 markedly attenuated IL-33–induced accumulation of lung ILC2s and eosinophilic inflammation in T-bet^−/− mice.

T-bet suppresses IL-9 production from lung ILC2s and thereby inhibits IL-33–induced eosinophilic airway inflammation.