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.