Allergic airway inflammation stimulates a dynamic ILC2 phenotype
medwireNews: Researchers have found that the group 2 innate lymphoid cell (ILC2) phenotype is more heterogeneous than previously thought, with a surface marker and gene expression profile that is highly dynamic.
“This phenotype is dependent on the mode of cellular activation, changed over time during stimulation, exhibits differences across tissues, and is reversible,” report Rudi Hendriks (Erasmus University Medical Center, Rotterdam, the Netherlands) and colleagues.
The researchers compared the dynamics and kinetics of the ILC2 phenotype in two scenarios: T-cell-independent inflammation, stimulated by interleukin (IL)-33, and T-cell-dependent inflammation, induced by house dust mites (HDMs).
They used a novel mouse strain called GATA3 IRES Reporter (GATIR) that had a yellow fluorescent protein (YFP) sequence inserted into the Gata3 gene, a central ILC2 marker. This allowed the YFP to be produced as a separate protein concomitant with Gata3 transcription, without affecting GATA3 protein levels or function. The expression of this YFP was then used to identify ILC2s in the mice.
Under the different inflammation scenarios, the cell surface marker composition of ILC2s varied. In an acute HDM-driven airway inflammation model, ILC2s in bronchoalveolar lavage (BAL) fluid, lungs, and draining lymph nodes were more heterogeneous than ILC2s activated by IL-33 stimulation.
Stimulation by IL-33 resulted in high surface expression of CD25, ICOS, KLRG1, and T1/ST2 on BAL fluid ILC2s, whereas HDM-activated ILC2s in BAL fluid showed reduced levels of all these markers at a comparable time point, with levels lower even than on ILC2s from wild-type control mice. A similar pattern was seen for CD25, ICOS, and KLRG1 expression on ICL2s in lung and mediastinal lymph nodes.
The researchers note in Frontiers in Immunology, however, that the microenvironment and stage of inflammation were important factors, with variability in surface marker expression differing substantially across different tissues and according to the time of analysis.
In spite of the low expression of CD25 on HDM-activated ILC2s, they were still able to produce a large amount of type 2 cytokines, comparable to that of IL-33-activated ILC2s, the team points out.
Also, low expression of CD25 could be reversed with a single dose of IL-33 stimulation. This induced upregulation of CD25 on the existing ILC2s, rather than an influx or generation of new high-CD25 expressing ILC2s. The other ILC2 markers were also upregulated, although more slowly.
Given the proposed role of ILC2s in the maintenance of chronic asthma, the team looked at their accumulation in GATIR mice chronically exposed to HDM for 5 weeks.
During this chronic inflammation exposure, the proportion of ILC2s, relative to all cytokine-producing cells, was greater than during acute HDM exposure. The majority of ILC2s had low expression of CD25, with this phenotype persisting into the resolution phase of inflammation. Irrespective of CD25 expression levels, HDM-induced ILC2s were prominent producers of IL-5 and IL-13.
The duration of stimulus as well as the type also appeared to influence the role of ILC2s, the researchers report. Genome-wide transcription profiling of BAL ILC2s showed more than 1600 differentially expressed genes, of which 915 were upregulated in IL-33-stimulated ILC2s and 708 in chronic HDM-stimulated ILC2s, including some implicated in asthma.
Pathway analysis of the genes showed that cell proliferation and division were highly active after IL-33 stimulation, while immune system regulation and activation were associated with HDM stimulation, including modulation of T- cell activity and chemoattraction of T and B cells.
ILC2s tended to accumulate in cellular infiltrates underneath the lung epithelium following either IL-33 or HDM stimulation, although T- and B-cell numbers were higher in the infiltrates following HDM stimulation. Chronic HDM stimulation resulted in ILC2s locating mainly in the submucosa close to epithelial cells, but also inside organized cellular infiltrates close to epithelial cells and T cells but not B cells.
Hendriks and colleagues acknowledge that ILC2s expressing high and low CD25 could represent two separate subsets, but they believe that the differences and changes over time that they have observed “point to a different activation status of a single ILC2 type.”
And they conclude that their “data suggest a more complex ILC2 phenotype than is currently appreciated.”
By Lucy Piper
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