Characterization of Human Group 9 Innate Lymphoid Cells in Response to Allergen Immunotherapy in Patients With Allergic Rhinitis

Background Innate lymphoid cells (ILCs) are classically divided into three groups: ILC1, ILC2, and ILC3 to reflect their functional analogy to Th1, Th2, and Th17. There is also an IL‐9 single‐producing T cell subset, namely the Th9 cell, which plays a dominant role in the onset of allergic diseases compared with traditional Th2 cells. Although a corresponding cell subset of ILCs to different Th cell subsets exists, so far the counterpart of Th9 cells in ILCs has not been reported. Objective In this study, we aimed to report the existence of group 9 innate lymphoid cells (ILC9s) and characterize them in allergic rhinitis (AR) and in response to allergen immunotherapy. Methods The expressions and characterization of ILC9s were investigated in purified ILCs cultures, PBMCs from patients with AR and responder subcutaneous immunotherapy (SCIT) patients by flow cytometry, scRNA‐seq transcriptome, qRT‐PCR, siRNA knockdown, and immunofluorescence staining. Results IL‐9‐expressing cells were observed in the nasal mucosa of patients with AR without the co‐expression of IL‐5 and IL‐13. IL‐4 and TGF‐β induce IL‐9 secretion by human ILCs. scRNA‐seq of whole ILCs defines an H1R $^{+}$ OX‐40L $^{−}$ ILC subset as ILC9 expressing high levels of IL‐9 and low levels of the ILC2 transcription factor GATA3. Instead, this new ILC9 subset displays Bach2 as a transcription factor, and IL‐9 expression decreases after siRNA inhibition of Bach2. Histamine is an important regulator of ILC9 because ILC9 production increases in response to histamine, and IL‐9 levels in ILCs positively correlate with the expression of histamine 1R. An up‐regulation of PPARγ was observed in ILCs in response to IL‐4 and TGF‐β, and ILC9 differentiation was suppressed by the PPARγ antagonist. Conclusion ILC9s are highly expressed in the nasal mucosa and PBMCs of patients with AR and were decreased in response to house dust mite–SCIT. Our study sheds light on the newly discovered ILC9 subset and demonstrates a potential target in allergen immunotherapy.