IL-21 induces pyroptosis of Treg cells via Akt-mTOR-NLRP3-caspase 1 axis in eosinophilic chronic rhinosinusitis

Abstract

Background: Regulatory T (Treg) cells, which prevent inflammation-induced eosinophil infiltration, are deficient in nasal polyps (NPs) in patients with eosinophilic chronic rhinosinusitis (ECRS). It is concomitant with loss of Foxp3 after certain inflammatory stimuli.

Objective: We sought to determine the inflammatory cytokines involved in inducing the loss of Treg cells in NPs.

Methods: The abundance of cytokines in ECRS patients or mice were tested using ELISA, immunochemistry, immunofluorescence, quantitative reverse transcription PCR (qPCR), and/or flow cytometry. Expression of eosinophil cationic protein (ECP), CD4⁺ T cells, IL-4, and IL-17A and eosinophils in nasal mucosa of mouse model was investigated by immunochemistry, immunofluorescence, and hematoxylin and eosin staining. The percentage and death of induced Treg (iTreg) cells, source of IL-21 in NPs from ECRS and non-ECRS patients, and abundance of different systemic phenotypes of CD4⁺ T cells in a mouse model were studied by flow cytometry. Western blot analysis, scanning, and transmission electronic microscopy were used to detect pyroptosis of iTreg cells.

Results: IL-21 was highly expressed in nasal mucosa of ECRS patients and mice, causing pyroptosis and preventing development of iTreg cells in vitro. The elevated IL-21 in NPs from ECRS patients was mainly produced by CD3⁺ T cells, including T follicular helper, T peripheral helper, T_H2, and T_H17 cells and CD3⁺CD4^- T cells. T peripheral helper cells and CD3⁺CD4^- T cells were the predominant source of IL-21 in NPs from non-ECRS patients. Blocking IL-21/IL-21R signaling significantly reduced the number of eosinophils and CD4⁺ T cells along with ECP, IL-4, and IL-17A expression in the nasal mucosa of ECRS mice. It also increased Treg cell percentage and systemically decreased T_H2 and T_H17 ratios. Akt-mTOR inhibition prevented IL-21-induced pyroptosis in human and mouse iTreg cells.

Conclusion: Elevated IL-21 drives pyroptosis and prevents Treg cell development in ECRS patients. IL-21 induced pyroptosis via activating Akt-mTOR-NLRP3-caspase 1 signaling.

Keywords: Eosinophilic chronic rhinosinusitis; nasal polyps; pyroptosis; regulatory T cells; type 2 cytokines.