Airways epithelial exposure to Streptococcus pneumoniae in the presence of the alarmin IL-33 induces a novel subset of pro-inflammatory ILC2s promoting a mixed inflammatory response.

Abstract

Background: We have previously shown that asthma-like airways inflammation may be induced by topical exposure to respiratory tract pathogens such as S. pneumoniae (SP) in concert with epithelial alarmins such as IL-33. Details of the pathogenesis of this murine surrogate remain however unexplored.

Methods: Airways inflammation was induced by repeated, intranasal exposure of Il-4^-/-, Rag1^-/- and Rag2^-/-Il2rg^-/- mice (in which B lymphocyte IgE switching, adaptive and innate immunity are respectively ablated) as well as wild type mice to inactivated SP, IL-33 or both. Airways pathological changes were analysed, and the subsets and functions of locally accumulated ILC2s investigated by single cell RNA sequencing and flow cytometry.

Results: In the presence of IL-33, repeated exposure of the airways to inactivated SP caused marked eosinophil- and neutrophil-rich inflammation and local accumulation of ILC2s, which was retained in the Il-4^-/- and Rag1^-/- deficient mice but abolished in the Rag2^-/-Il2rg^-/- mice, an effect partly reversed by adoptive transfer of ILC2s. Single cell sequencing analysis of ILC2s recruited following SP and IL-33 exposure revealed a Klrg1⁺Ly6a⁺subset, expressing particularly elevated quantities of the pro-inflammatory cytokine IL-6, type 2 cytokines (IL-5 and IL-13) and MHC class II molecules, promoting type 2 inflammation as well as involved in neutrophil-mediated inflammatory responses.

Conclusion: Local accumulation of KLRG1⁺Ly6a⁺ ILC2s in the lung tissue is a critical aspect of the pathogenesis of airways eosinophilic and neutrophil-rich inflammation induced by repeated exposure to SP in the presence of the epithelial alarmin IL-33.