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.