New insights into ILC2 memory.

Group 2 Innate Lymphoid Cells (ILC2s) are innate lymphocytes involved in type 2 immunity. ILC2s are abundant at the barrier tissues and upon allergen exposure, respond to epithelial-derived alarmins by producing type 2 cytokines (e.g., IL-5 and IL-13). Upon activation, some of these activated ILC2s acquire immunological memory and can mount enhanced responses upon further allergen encounters. Here, we review recent findings of the cellular and molecular mechanisms underlying immune memory in ILC2s both in mice and humans and discuss the implications of memory ILC2s in the context of allergic diseases.

Laboratory mice with a wild microbiota generate strong allergic immune responses.

Allergic disorders are caused by a combination of hereditary and environmental factors. The hygiene hypothesis postulates that early-life microbial exposures impede the development of subsequent allergic disease. Recently developed "wildling" mice are genetically identical to standard laboratory specific pathogen-free (SPF) mice but are housed under seminatural conditions and have rich microbial exposures from birth. Thus, by comparing conventional SPF mice with wildlings, we can uncouple the impact of lifelong microbial exposures from genetic factors on the allergic immune response. We found that wildlings developed larger populations of antigen-experienced T cells than conventional SPF mice, which included interleukin-10-producing CD4 T cells specific for commensal Lactobacilli strains and allergy-promoting T helper 2 (TH2) cells. In models of airway exposure to house dust mite (HDM), recombinant interleukin-33, or Alternaria alternata, wildlings developed strong allergic inflammation, characterized by eosinophil recruitment, goblet cell metaplasia, and antigen-specific immunoglobulin G1 (IgG1) and IgE responses. Wildlings developed robust de novo TH2 cell responses to incoming allergens, whereas preexisting TH2 cells could also be recruited into the allergic immune response in a cytokine-driven and TCR-independent fashion. Thus, wildling mice, which experience diverse and lifelong microbial exposures, were not protected from developing pathological allergic immune responses. Instead, wildlings mounted robust allergic responses to incoming allergens, shedding new light on the hygiene hypothesis.

Tissue Resident and Migratory Group 2 Innate Lymphoid Cells.

Group 2 innate lymphoid cells (ILC2s) are present in both mouse and human mucosal and non-mucosal tissues and implicated in initiating type 2 inflammation. ILC2s are considered to be tissue resident cells that develop in the perinatal period and persist throughout life with minimal turning over in adulthood. However, recent studies in animal models have shown their ability to circulate between different organs during inflammation and their potential functions in the destined organs, suggesting their roles in mediating multiple type 2 diseases. Here, we review recent findings on ILC2 migration, including migration within, into and out of tissues during inflammation.

Genome-Wide RNAi Screening Identifies Novel Pathways/Genes Involved in Oxidative Stress and Repurposable Drugs to Preserve Cystic Fibrosis Airway Epithelial Cell Integrity.

Recurrent infection-inflammation cycles in cystic fibrosis (CF) patients generate a highly oxidative environment, leading to progressive destruction of the airway epithelia. The identification of novel modifier genes involved in oxidative stress susceptibility in the CF airways might contribute to devise new therapeutic approaches. We performed an unbiased genome-wide RNAi screen using a randomized siRNA library to identify oxidative stress modulators in CF airway epithelial cells. We monitored changes in cell viability after a lethal dose of hydrogen peroxide. Local similarity and protein-protein interaction network analyses uncovered siRNA target genes/pathways involved in oxidative stress. Further mining against public drug databases allowed identifying and validating commercially available drugs conferring oxidative stress resistance. Accordingly, a catalog of 167 siRNAs able to confer oxidative stress resistance in CF submucosal gland cells targeted 444 host genes and multiple circuitries involved in oxidative stress. The most significant processes were related to alternative splicing and cell communication, motility, and remodeling (impacting cilia structure/function, and cell guidance complexes). Other relevant pathways included DNA repair and PI3K/AKT/mTOR signaling. The mTOR inhibitor everolimus, the α1-adrenergic receptor antagonist doxazosin, and the Syk inhibitor fostamatinib significantly increased the viability of CF submucosal gland cells under strong oxidative stress pressure. Thus, novel therapeutic strategies to preserve airway cell integrity from the harsh oxidative milieu of CF airways could stem from a deep understanding of the complex consequences of oxidative stress at the molecular level, followed by a rational repurposing of existing "protective" drugs. This approach could also prove useful to other respiratory pathologies.

The Fate of Activated Group 2 Innate Lymphoid Cells.

Group 2 innate lymphoid cells (ILC2s) reside in both mucosal and non-mucosal tissues and play critical roles in the first line of defense against parasites and irritants such as allergens. Upon activation by cytokines released from epithelial and stromal cells during tissue damage or stimulation, ILC2s produce copious amounts of IL-5 and IL-13, leading to type 2 inflammation. Over the past 10 years, ILC2 involvement in a variety of human diseases has been unveiled. However, questions remain as to the fate of ILC2s after activation and how that might impact their role in chronic inflammatory diseases such as asthma and fibrosis. Here, we review studies that have revealed novel properties of post-activation ILC2s including the generation of immunological memory, exhausted-like phenotype, transdifferentiation and activation-induced migration.

Correction: Female and male mouse lung group 2 innate lymphoid cells differ in gene expression profiles and cytokine production.

[This corrects the article DOI: 10.1371/journal.pone.0214286.].

Steroid-resistant human inflammatory ILC2s are marked by CD45RO and elevated in type 2 respiratory diseases.

Group 2 innate lymphoid cells (ILC2s) orchestrate protective type 2 immunity and have been implicated in various immune disorders. In the mouse, circulatory inflammatory ILC2s (iILC2s) were identified as a major source of type 2 cytokines. The human equivalent of the iILC2 subset remains unknown. Here, we identify a human inflammatory ILC2 population that resides in inflamed mucosal tissue and is specifically marked by surface CD45RO expression. CD45RO+ ILC2s are derived from resting CD45RA+ ILC2s upon activation by epithelial alarmins such as IL-33 and TSLP, which is tightly linked to STAT5 activation and up-regulation of the IRF4/BATF transcription factors. Transcriptome analysis reveals marked similarities between human CD45RO+ ILC2s and mouse iILC2s. Frequencies of CD45RO+ inflammatory ILC2 are increased in inflamed mucosal tissue and in the circulation of patients with chronic rhinosinusitis or asthma, correlating with disease severity and resistance to corticosteroid therapy. CD45RA-to-CD45RO ILC2 conversion is suppressed by corticosteroids via induction of differentiation toward an immunomodulatory ILC2 phenotype characterized by low type 2 cytokine and high amphiregulin expression. Once converted, however, CD45RO+ ILC2s are resistant to corticosteroids, which is associated with metabolic reprogramming resulting in the activation of detoxification pathways. Our combined data identify CD45RO+ inflammatory ILC2s as a human analog of mouse iILC2s linked to severe type 2 inflammatory disease and therapy resistance.

Single-cell analysis of RORα tracer mouse lung reveals ILC progenitors and effector ILC2 subsets.

Lung group 2 innate lymphoid cells (ILC2s) drive allergic inflammation and promote tissue repair. ILC2 development is dependent on the transcription factor retinoic acid receptor-related orphan receptor (RORα), which is also expressed in common ILC progenitors. To elucidate the developmental pathways of lung ILC2s, we generated RORα lineage tracer mice and performed single-cell RNA sequencing, flow cytometry, and functional analyses. In adult mouse lungs, we found an IL-18Rα+ST2- population different from conventional IL-18Rα-ST2+ ILC2s. The former was GATA-3intTcf7EGFP+Kit+, produced few cytokines, and differentiated into multiple ILC lineages in vivo and in vitro. In neonatal mouse lungs, three ILC populations were identified, namely an ILC progenitor population similar to that in adult lungs and two distinct effector ILC2 subsets that differentially produced type 2 cytokines and amphiregulin. Lung ILC progenitors might actively contribute to ILC-poiesis in neonatal and inflamed adult lungs. In addition, neonatal lung ILC2s include distinct proinflammatory and tissue-repairing subsets.

KLRG1 and NKp46 discriminate subpopulations of human CD117+CRTH2- ILCs biased toward ILC2 or ILC3.

Recently, human ILCs that express CD117 and CD127 but lack CRTH2 and NKp44 have been shown to contain precursors of ILC1, ILC2, and ILC3. However, these ILCs have not been extensively characterized. We performed an unbiased hierarchical stochastic neighbor embedding (HSNE) analysis of the phenotype of peripheral blood CD117+ ILCs, which revealed the presence of three major subsets: the first expressed NKp46, the second expressed both NKp46 and CD56, and the third expressed KLRG1, but not NKp46 or CD56. Analysis of their cytokine production profiles and transcriptome revealed that NKp46+ ILCs predominantly develop into ILC3s; some of them can differentiate into ILC1/NK-like cells, but they are unable to develop into ILC2s. In contrast, KLRG1+ ILCs predominantly differentiate into ILC2s. Single-cell cultures demonstrate that KLRG1+ ILCs can also differentiate into other ILC subsets depending on the signals they receive. Epigenetic profiling of KLRG1+ ILCs is consistent with the broad differentiation potential of these cells.

Female and male mouse lung group 2 innate lymphoid cells differ in gene expression profiles and cytokine production.

Epidemiological studies have shown sex differences in prevalence of non-allergic asthma. Recent reports demonstrated negative effects of androgen signaling on group 2 innate lymphoid cells (ILC2s), explaining a potential mechanism behind sex bias in asthma prevalence. To further understand sex-related differences in ILC2 functions and ILC2 intrinsic or lung environmental mechanisms behind it, we have investigated the effects of sex and age on lung ILC2 function, the amounts of ILC2-activating cytokines in the lung and gene expression profiles of male and female ILC2s. Flow cytometric analyses of naive male and female mouse lung ILC2s showed no difference in their numbers. However, upon three daily intranasal IL-33 injections, lung ILC2s in postpubertal female mice expanded to a greater degree than male counterpart. In line with in vivo results, purified female mouse lung ILC2s produced more cytokines than male ILC2s upon in vitro stimulation. Gene expression profiles of purified naïve male and female ILC2s differed in 4% of the genes, and gene set enrichment analysis showed that female ILC2s are enriched for gene signatures of memory T cells. We did not observe similar degree of differences between female and male ILC2s after IL-33 stimulation. ILC2-activating cytokines including IL-33, IL-7 and TSLP were more highly expressed in whole lung homogenate samples prepared from naïve post pubertal female mouse lung than male mouse lung. Moreover, the differences in responsiveness of male and female ILC2s to IL-33 were not affected in IL-33-deficient mice. These results suggest that female ILC2s are more readily activated than male ILC2s due to their gene expression at the naïve state, which is potentially influenced by the lung environment.

ILC2 memory: Recollection of previous activation.

Immunological memory, traditionally thought to belong to T and B cells, has now been extended to innate lymphocytes, including NK cells and ILC2s, myeloid cells such as macrophages, also termed "trained immunity" and more recently to epithelial stem cells. In this review, we discuss the mechanisms underlying memory generation on ILC2s and speculate about their potential role in human allergic diseases, such as asthma. Moreover, we examine the relevance of the spontaneous ILC2 activation in the lung during the neonatal period in order to efficiently respond to stimuli later in life. These "training" of neonatal ILC2s may have an impact on the generation of memory ILC2s in the adulthood.

Type 2 innate lymphoid cells disrupt bronchial epithelial barrier integrity by targeting tight junctions through IL-13 in asthmatic patients.

BACKGROUND: Bronchial epithelial barrier leakiness and type 2 innate lymphoid cells (ILC2s) have been separately linked to asthma pathogenesis; however, the influence of ILC2s on the bronchial epithelial barrier has not been investigated previously. OBJECTIVE: We investigated the role of ILC2s in the regulation of bronchial epithelial tight junctions (TJs) and barrier function both in bronchial epithelial cells of asthmatic patients and healthy subjects and general innate lymphoid cell- and ILC2-deficient mice. METHODS: Cocultures of human ILC2s and bronchial epithelial cells were used to determine transepithelial electrical resistance, paracellular flux, and TJ mRNA and protein expressions. The effect of ILC2s on TJs was examined by using a murine model of IL-33-induced airway inflammation in wild-type, recombination-activating gene 2 (Rag2)-/-, Rag2-/-Il2rg-/-, and Rorasg/sg mice undergoing bone marrow transplantation to analyze the in vivo relevance of barrier disruption by ILC2s. RESULTS: ILC2s significantly impaired the epithelial barrier, as demonstrated by reduced transepithelial electrical resistance and increased fluorescein isothiocyanate-dextran permeability in air-liquid interface cultures of human bronchial epithelial cells. This was in parallel to decreased mRNAs and disrupted protein expression of TJ proteins and was restored by neutralization of IL-13. Intranasal administration of recombinant IL-33 to wild-type and Rag2-/- mice lacking T and B cells triggered TJ disruption, whereas Rag2-/-Il2rg-/- and Rorasg/sg mice undergoing bone marrow transplantation that lack ILC2s did not show any barrier leakiness. Direct nasal administration of IL-13 was sufficient to induce deficiency in the TJ barrier in the bronchial epithelium of mice in vivo. CONCLUSION: These data highlight an essential mechanism in asthma pathogenesis by demonstrating that ILC2s are responsible for bronchial epithelial TJ barrier leakiness through IL-13.

Immunological Memory of Group 2 Innate Lymphoid Cells.

Immunological memory has long been described as a property of the adaptive immune system that results in potent responses on exposure to an antigen encountered previously. While this definition appears to exclude cells that do not express antigen receptors, recent studies have shown that innate immune cells, including natural killer (NK) cells, macrophages, and, more recently, group 2 innate lymphoid cells (ILC2s) can record previous activations and respond more vigorously on reactivation. Here we review the similarities and differences between these forms of memory and the underlying mechanisms. Based on these insights, we propose to revise the definition of immunological memory, as the capacity to remember being previously activated and respond more efficiently on reactivation regardless of antigen specificity.

Allergen-Experienced Group 2 Innate Lymphoid Cells Acquire Memory-like Properties and Enhance Allergic Lung Inflammation.

Group 2 innate lymphoid cells (ILC2s) in the lung are stimulated by inhaled allergens. ILC2s do not directly recognize allergens but they are stimulated by cytokines including interleukin (IL)-33 released by damaged epithelium. In response to allergens, lung ILC2s produce T helper 2 cell type cytokines inducing T cell-independent allergic lung inflammation. Here we examined the fate of lung ILC2s upon allergen challenges. ILC2s proliferated and secreted cytokines upon initial stimulation with allergen or IL-33, and this phase was followed by a contraction phase as cytokine production ceased. Some ILC2s persisted long after the resolution of the inflammation as allergen-experienced ILC2s and responded to unrelated allergens more potently than naive ILC2s, mediating severe allergic inflammation. The allergen-experienced ILC2s exhibited a gene expression profile similar to that of memory T cells. The memory-like properties of allergen-experienced ILC2s may explain why asthma patients are often sensitized to multiple allergens.

Common-Lymphoid-Progenitor-Independent Pathways of Innate and T Lymphocyte Development.

All lymphocytes are thought to develop from common lymphoid progenitors (CLPs). However, lymphoid-primed multipotent progenitors (LMPPs) are more efficient than CLPs in differentiating into T cells and group 2 innate lymphoid cells (ILC2s). Here, we have divided LMPPs into CD127(-) (LMPP-s) and CD127(+) (LMPP+s) subsets and compared them with Ly6D(-) and Ly6D(+) CLPs. Adult LMPP+s differentiated into T cells and ILCs more rapidly and efficiently than other progenitors in transplantation assays. The development of T cells and ILC2s is highly active in the neonatal period. Neonatal CLPs are rare and, unlike prominent neonatal LMPP+s, do not efficiently differentiate into T cells and ILC2s. ILC2s generated in the neonatal period are long lived and persist in adult tissues. These results suggest that some ILCs and T cells may develop from LMPP+s via CLP-independent pathways.

Lung ILC2s link innate and adaptive responses in allergic inflammation.

How allergens trigger the T helper 2 (Th2) response that characterizes allergic lung inflammation is not well understood. Epithelium-derived alarmins released after an allergen encounter activate the innate immune system, including group 2 innate lymphoid cells (ILC2s) which produce the type 2 interleukins IL-5 and IL-13. It has been recently shown that ILC2-derived cytokines are responsible not only for the innate responses underlying allergic inflammation but also for the initiation of the adaptive Th2 response. We review the role of lung ILC2s in the development of allergic inflammation and, in the context of recent findings, propose a common pathway wherein ILC2s, activated by the epithelium-derived cytokine IL-33, link the innate and the adaptive responses after allergen encounter in the lung.

Human mesenchymal stem cells resolve airway inflammation, hyperreactivity, and histopathology in a mouse model of occupational asthma.

Occupational asthma (OA) is characterized by allergic airway inflammation and hyperresponsiveness, leading to progressive airway remodeling and a concomitant decline in lung function. The management of OA remains suboptimal in clinical practice. Thus, establishing effective therapies might overcome the natural history of the disease. We evaluated the ability of human adipose-tissue-derived mesenchymal stem cells (hASCs), either unmodified or engineered to secrete the IL-33 decoy receptor sST2, to attenuate the inflammatory and respiratory symptoms in a previously validated mouse model of OA to ammonium persulfate (AP). Twenty-four hours after a dermal AP sensitization and intranasal challenge regimen, the animals received intravenously 1 × 10(6) cells (either hASCs or hASCs overexpressing sST2) or saline and were analyzed at 1, 3, and 6 days after treatment. The infused hASCs induced an anti-inflammatory and restorative program upon reaching the AP-injured, asthmatic lungs, leading to early reduction of neutrophilic inflammation and total IgE production, preserved alveolar architecture with nearly absent lymphoplasmacytic infiltrates, negligible smooth muscle hyperplasia/hypertrophy in the peribronchiolar areas, and baseline airway hyperreactivity (AHR) to methacholine. Local sST2 overexpression barely increased the substantial efficacy displayed by unmodified hASCs. Thus, hASCs may represent a viable multiaction therapeutic capable to adequately respond to the AP-injured lung environment by resolving inflammation, tissue remodeling, and bronchial hyperresponsiveness typical of OA.

Group 2 innate lymphoid cells are critical for the initiation of adaptive T helper 2 cell-mediated allergic lung inflammation.

Naive CD4(+) T cell differentiation into distinct subsets of T helper (Th) cells is a pivotal process in the initiation of the adaptive immune response. Allergens predominantly stimulate Th2 cells, causing allergic inflammation. However, why allergens induce Th2 cell differentiation is not well understood. Here we show that group 2 innate lymphoid cells (ILC2s) are required to mount a robust Th2 cell response to the protease-allergen papain. Intranasal administration of papain stimulated ILC2s and Th2 cells, causing allergic lung inflammation and elevated immunoglobulin E titers. This process was severely impaired in ILC2-deficient mice. Whereas interleukin-4 (IL-4) was dispensable for papain-induced Th2 cell differentiation, ILC2-derived IL-13 was critical as it promoted migration of activated lung dendritic cells into the draining lymph node where they primed naive T cells to differentiate into Th2 cells. Papain-induced ILC2 activation and Th2 cell differentiation was IL-33-dependent, suggesting a common pathway in the initiation of Th2 cell responses to allergen.