INTRODUCTION
Allergic airway diseases such as allergic rhinitis (AR) and asthma are a group of chronic immunopathological conditions affecting more than 20% of the world’s population. As the most common clinical presentation of allergy, AR affects up to 400 million individuals worldwide and afflicts substantial health and economic morbidity. In many regions, AR and other allergic diseases are still increasing. [1]
The immunopathogenesis of eosinophilic AR and asthma is typically associated with a type 2 response, involving cytokines such as interleukin-5 (IL-5), IL-4 and IL-13. Importantly, IL-5 plays a key role in promoting the differentiation and survival of eosinophils. The main producers of IL-5 are type 2 helper T (TH2) cells and mast cells. In RAG-/- mice lacking both T and B lymphocytes, IL-5 is released by tissue resident Innate Lymphoid Cells type 2 cells (ILC2), which is associated with eosinophilia in the pancreas, lung, and the spleen after IL-2 treatment. [2] In patients with eosinophilic gastrointestinal disease (EGID), IL-5 is preferentially produced by a subset of TH2 cells which expresses CRTH2, CD161 and hematopoietic prostaglandin synthase (HPGDS). As TH2 cell counts positively correlated with blood eosinophilia, they are also termed pathogenic effector (peTH2) cells. [3] HPGDS catalyses the synthesis of prostaglandin D2 (PGD2), a powerful lipid mediator for allergic inflammation. PGD2 exerts its effect by promoting the recruitment of CRTH2 expressing cells to induce airway hyperreactivity.[4-6] Given that CRTH2 is found on most of the cell types involved in the eosinophilic response pathway including eosinophils, basophils, ILC2, type 2 CD8+ T cells (TC2), macrophages and non-classical monocytes [2, 3, 7-9] , we sought to determine the contribution of each cell type in mediating pathogenesis of AR.
In a prior study, we established a sensitisation rate of approximately 80% of the local population in Singapore to house dust mite (HDM) allergens. [10] This results in an incidence rate of about 40% for AR. In order to identify the key driver behind airway allergies, we analysed the CRTH2+ subset in the PBMCs of our Singapore Systems Immunology Cohort (SSIC) longitudinal cohort. [10, 11]
Here, we employed unsupervised PhenoGraph clustering to analyse whole blood and PBMCs from the SSIC cohort. Flow cytometry and ultra-sensitive cytokine arrays revealed positive associations between circulating eosinophil numbers and plasma IL-5, confirming the eosinophilic nature of AR. Furthermore, supervised cluster analysis of PBMCs revealed strong correlation between CD161+ TH2 and self-reported symptoms of AR as well as between eosinophil numbers and IgE titres. Notably, IL-5 was found to be exclusively produced by CD45RB low expressing (CD45RBlo) CD161+ TH2 subset, suggesting the role of this population in driving eosinophilia in AR. Finally, in-depth analysis in an active disease cohort by functional mass cytometry further established that a specific subset of CD45RBloHPGDS+ CD161+ TH2 is directly associated with AR. [12, 13] In particular, this subset co-expressed CD27, KLRG1, CD38, ICOS, C45RO and TSLP-R. Upon stimulation, a complex set of cytokines comprising IL-5, IL-13, IL-4, IL-2, IL-3 and IL-9 is concomitantly released from our identified disease-driving subset, contributing towards inflammation in AR.