Background: Innate lymphoid cells (ILCs) are classically divided into 3 groups-ILC1, ILC2 and ILC3 to reflect their functional analogy to Th1, Th2, Th17. ILC2s play an essential role in the pathogenesis of allergic diseases by the production of IL-9 and IL-13. There is also an IL-9 single producing T cell subset, namely the Th9 cells and the counterpart subset of Th9 in ILCs has not been reported. Accordingly, we aimed to determine the group 9 innate lymphoid cells (ILC9)-inducing conditions and the cytokines and surface markers of ILC9.
Methods: Circulating ILCs were isolated and expanded in Yssel’s medium. Then, ILCs were treated with IL-2, IL-4, transforming growth factor beta (TGF-β), epidermal growth factor (EGF), and amphiregulin (AREG). Intracellular IL-9, IL-13, IL-17A and IFN-γ expressions were determined by flow cytometry. The expression of chemokine receptors on ILCs in human blood was assessed and their correlation with the induction of ILC9s was analyzed.
Results: IL-4 and TGF- β induced the IL-9 secretion by human ILCs in a time-dependent manner. There was no effect of EGF and AREG on the production of IL-9. Interestingly, the synthesis of IL-9 was suppressed by increased doses of TGF- β. The expressions of CXCR3 and CCR9 were significantly up-regulated under the ILC9-inducing condition. Additionally, the expressions of CXCR3 and CCR9 have positively correlated to the proportion of IL-9+ILC9. Thus, ILC9s were defined as CRTH2-CXCR3+CCR9+ ILC.
Conclusions: Our study identified a new functional subset of ILC and shed a light on new mechanism in the pathogenesis of allergic inflammation in which IL-4 and TGF-β promote the differentiation of ILCs into the novel ILC9 subset.