Cutting Edge: Notch Signaling Promotes the Plasticity of Group-2 Innate Lymphoid Cells

Deciphering the Role of Innate Lymphoid Cells Group 3 in the Gut Microenvironment: A Narrative Review of Their Novel Contributions to Autoimmune Disease Pathogenesis

Type 3 Innate lymphoid cells (ILC3s) play a crucial role in intestinal immune function by serving as an innate effector that contributes to early-life defense against pathogens and helps protect the intestines from bacterial infections. ILC3s exert their immune function through cytokine secretion, patrolling actions and the generation of memory ILC3s that aid in repairing epithelial tissue and preserving mucosal barrier integrity. Moreover, dysregulation of ILC3s function has been implicated in the pathogenesis and progression of autoimmune diseases. This comprehensive review aims to explore the interactions between gut microbes, gut microbial metabolites, and diet in relation to ILC3s within the context of the gut microenvironment. Furthermore, the gut microenvironment has the potential to influence distant extra-intestinal sites through immunomodulation, thereby modifying their risk of inflammation. The gut has emerged as a significant focus of autoimmune disease research in recent years. However, the relationship between gut ILC3s and autoimmune diseases remains poorly understood. This paper aims to examine the potential association between ILC3s and autoimmune diseases.

Innate lymphoid cells, immune functional dynamics, epithelial parallels, and therapeutic frontiers in infections

Innate lymphoid cells (ILCs) have emerged as pivotal players in the field of immunology, expanding our understanding of innate immunity beyond conventional paradigms. This comprehensive review delves into the multifaceted world of ILCs, beginning with their serendipitous discovery and traversing their ontogeny and heterogeneity. We explore the distinct subsets of ILCs unraveling their intriguing plasticity, which adds a layer of complexity to their functional repertoire. As we journey through the functional activities of ILCs, we address their role in immune responses against various infections, categorizing their interactions with helminthic parasites, bacterial pathogens, fungal infections, and viral invaders. Notably, this review offers a detailed examination of ILCs in the context of specific infections, such as Mycobacterium tuberculosis, Citrobacter rodentium, Clostridium difficile, Salmonella typhimurium, Helicobacter pylori, Listeria monocytogenes, Staphylococcus aureus, Pseudomonas aeruginosa, Influenza virus, Cytomegalovirus, Herpes simplex virus, and severe acute respiratory syndrome coronavirus 2. This selection aimed for a comprehensive exploration of ILCs in various infectious contexts, opting for microorganisms based on extensive research findings rather than considerations of virulence or emergence. Furthermore, we raise intriguing questions about the potential immune functional resemblances between ILCs and epithelial cells, shedding light on their interconnectedness within the mucosal microenvironment. The review culminates in a critical assessment of the therapeutic prospects of targeting ILCs during infection, emphasizing their promise as novel immunotherapeutic targets. Nevertheless, due to their recent discovery and evolving understanding, effectively manipulating ILCs is challenging. Ensuring specificity and safety while evaluating long-term effects in clinical settings will be crucial.

Chronic Inflammation in Asthma: Looking Beyond the Th2 Cell

Asthma is a common chronic inflammatory disease of the airways. A substantial number of patients present with severe and therapy-resistant asthma, for which the underlying biological mechanisms remain poorly understood. In most asthma patients, airway inflammation is characterized by chronic activation of type 2 immunity. CD4+ T helper 2 (Th2) cells are the canonical producers of the cytokines that fuel type 2 inflammation: interleukin (IL)-4, IL-5, IL-9, and IL-13. However, more recent findings have shown that other lymphocyte subsets, in particular group 2 innate lymphoid cells (ILC2s) and type 2 CD8+ cytotoxic T (Tc2) cells, can also produce large amounts of type 2 cytokines. Importantly, a substantial number of severe therapy-resistant asthma patients present with chronic type 2 inflammation, despite the high sensitivity of Th2 cells for suppression by corticosteroids-the mainstay drugs for asthma. Emerging evidence indicates that ILC2s and Tc2 cells are more abundant in severe asthma patients and can adopt corticosteroid-resistance states. Moreover, many severe asthma patients do not present with overt type 2 airway inflammation, implicating non-type 2 immunity as a driver of disease. In this review, we will discuss asthma pathophysiology and focus on the roles played by ILC2s, Tc2 cells, and non-type 2 lymphocytes, placing special emphasis on severe disease forms.

Allergen-specific immunotherapy at the extremes of age: below 5 years and elderly: evidence beyond indications?

PURPOSE OF REVIEW

Allergen-specific immunotherapy (AIT) has been used in clinical practice to treat allergic diseases for over 100 years. The effectiveness and safety of AIT have been substantiated in numerous studies; however, children before 5 years of age and elderly are not encompassed generally. This review aims to present the current understanding of AIT in the extremes of age.

RECENT FINDINGS

Early allergen immunotherapy during infancy or early childhood may prevent the development of allergic sensitization to common allergens, thereby reducing the risk of developing allergic diseases later in life. In the elderly, improved symptoms and quality of life and reduced dependence on medication are indicated the importance on the implementation of AIT. Both clinical and immunological parameters demonstrated that the treatment was effective at the time of cessation and trend to sustained tolerance.

SUMMARY

There is no specific lower or upper age limit for initiating immunotherapy; however, it is important to thoroughly evaluate the severity of disease and the risks and benefits in each case.

Mechanics-activated fibroblasts promote pulmonary group 2 innate lymphoid cell plasticity propelling silicosis progression

Crystalline silica (CS) particle exposure leads to silicosis which is characterized as progressive fibrosis. Fibroblasts are vital effector cells in fibrogenesis. Emerging studies have identified immune sentinel roles for fibroblasts in chronic disease, while their immune-modulatory roles in silicosis remain unclear. Herein, we show that group 2 innate lymphoid cell (ILC2) conversion to ILC1s is closely involved in silicosis progression, which is mediated by activated fibroblasts via interleukin (IL)-18. Mechanistically, Notch3 signaling in mechanics-activated fibroblasts modulates IL-18 production via caspase 1 activity. The mouse-specific Notch3 knockout in fibroblasts retards pulmonary fibrosis progression that is linked to attenuated ILC conversion. Our results indicate that activated fibroblasts in silicotic lungs are regulators of ILC2-ILC1 conversion, associated with silicosis progression via the Notch3-IL-18 signaling axis. This finding broadens our understanding of immune-modulatory mechanisms in silicosis, and indicates potential therapeutic targets for lung fibrotic diseases.

Induction of a distinct macrophage population and protection from lung injury and fibrosis by Notch2 blockade

Macrophages are pleiotropic and diverse cells that populate all tissues of the body. Besides tissue-specific resident macrophages such as alveolar macrophages, Kupffer cells, and microglia, multiple organs harbor at least two subtypes of other resident macrophages at steady state. During certain circumstances, like tissue insult, additional subtypes of macrophages are recruited to the tissue from the monocyte pool. Previously, a recruited macrophage population marked by expression of Spp1, Cd9, Gpnmb, Fabp5, and Trem2, has been described in several models of organ injury and cancer, and has been linked to fibrosis in mice and humans. Here, we show that Notch2 blockade, given systemically or locally, leads to an increase in this putative pro-fibrotic macrophage in the lung and that this macrophage state can only be adopted by monocytically derived cells and not resident alveolar macrophages. Using a bleomycin and COVID-19 model of lung injury and fibrosis, we find that the expansion of these macrophages before lung injury does not promote fibrosis but rather appears to ameliorate it. This suggests that these damage-associated macrophages are not, by themselves, drivers of fibrosis in the lung.

Bimin Kang ameliorates the minimal persistent inflammation in allergic rhinitis by reducing BCL11B expression and regulating ILC2 plasticity

ETHNOPHARMACOLOGICAL RELEVANCE

Minimal persistent inflammation (MPI) is a major contributor to the recurrence of allergic rhinitis (AR). The traditional Chinese herbal medicine known as Bimin Kang Mixture (BMK) have been used in clinics for decades to treat AR, which can relieve AR symptoms, reduce inflammatory response and improve immune function. However, its mechanism in controlling MPI is still unclear.

AIM OF THE STUDY

This study aims to assess the therapeutic effect of BMK on MPI, and elaborate the mechanism involved in BMK intervention in BCL11B regulation of type 2 innate lymphoid cell (ILC2) plasticity in the treatment of MPI.

MATERIAL AND METHODS

The effect of BMK (9.1 ml/kg) and Loratadine (15.15 mg/kg) on MPI was evaluated based on symptoms, pathological staining, and ELISA assays. RT-qPCR and flow cytometry were also employed to assess the expression of BCL11B, IL-12/IL-12Rβ2, and IL-18/IL-18Rα signaling pathways associated with ILC2 plasticity in the airway tissues of MPI mice following BMK intervention.

RESULTS

BMK restored the airway epithelial barrier, and markedly reduced inflammatory cells (eosinophils, neutrophils) infiltration (P < 0.01) and goblet cells hyperplasia (P < 0.05). BCL11B expression positively correlated with the ILC2 proportion in the lungs and nasal mucosa of AR and MPI mice (P < 0.01). BMK downregulated BCL11B expression (P < 0.05) and reduced the proportion of ILC2, ILC3 and ILC3-like ILC2 subsets (P < 0.05). Moreover, BMK promoted the conversion of ILC2 into an ILC1-like phenotype through IL-12/IL-12Rβ2 and IL-18/IL-18Rα signaling pathways in MPI mice.

CONCLUSION

By downregulating BCL11B expression, BMK regulates ILC2 plasticity and decreases the proportion of ILC2, ILC3, and ILC3-like ILC2 subsets, promoting the conversion of ILC2 to ILC1, thus restoring balance of ILC subsets in airway tissues and control MPI.

The epithelial cell types and their multi-phased defenses against fungi and other pathogens

Mucus and its movements are essential to epithelial tissue immune defenses against pathogens, including fungal pathogens, which can infect respiratory, gastrointestinal or the genito-urinary tracts. Several epithelial cell types contribute to their immune defense. This review focuses on the respiratory tract because of its paramount importance, but the observations will apply to epithelial cell defenses of other mucosal tissue, including the gastrointestinal and genito-urinary tracts. Mucus and its movements can enhance or degrade the immune defenses of the respiratory tract, particularly the lungs. The enhancements include inhaled pathogen entrapments, including fungal pathogens, pollutants and particulates, for their removal. The detriments include smaller lung airway obstructions by mucus, impairing the physical removal of pathogens and impairing vital transfers of oxygen and carbon dioxide between the alveolar circulatory system and the pulmonary air. Inflammation, edema and/or alveolar cellular damage can also reduce vital transfers of oxygen and carbon dioxide between the lung alveolar circulatory system and the pulmonary air. Furthermore, respiratory tract defenses are affected by several fatty acid mediators which activate cellular receptors to manipulate neutrophils, macrophages, dendritic cells, various innate lymphoid cells including the natural killer cells, T cells, γδ T cells, mucosal-associated invariant T cells, NKT cells and mast cells. These mediators include the inflammatory and frequently immunosuppressive prostaglandins and leukotrienes, and the special pro-resolving mediators, which normally resolve inflammation and immunosuppression. The total effects on the various epithelial cell and immune cell types, after exposures to pathogens, pollutants or particulates, will determine respiratory tract health or disease.

Mef2d potentiates type-2 immune responses and allergic lung inflammation

Innate lymphoid cells (ILCs) and adaptive T lymphocytes promote tissue homeostasis and protective immune responses. Their production depends on the transcription factor GATA3, which is further elevated specifically in ILC2s and T helper 2 cells to drive type-2 immunity during tissue repair, allergic disorders, and anti-helminth immunity. The control of this crucial up-regulation is poorly understood. Using CRISPR screens in ILCs we identified previously unappreciated myocyte-specific enhancer factor 2d (Mef2d)-mediated regulation of GATA3-dependent type-2 lymphocyte differentiation. Mef2d-deletion from ILC2s and/or T cells specifically protected against an allergen lung challenge. Mef2d repressed Regnase-1 endonuclease expression to enhance IL-33 receptor production and IL-33 signaling and acted downstream of calcium-mediated signaling to translocate NFAT1 to the nucleus to promote type-2 cytokine-mediated immunity.

COPD with elevated sputum group 2 innate lymphoid cells is characterized by severe disease

Rationale

Pulmonary innate immune cells play a central role in the initiation and perpetuation of chronic obstructive pulmonary disease (COPD), however the precise mechanisms that orchestrate the development and severity of COPD are poorly understood.

Objectives

We hypothesized that the recently described family of innate lymphoid cells (ILCs) play an important role in COPD.

Methods

Subjects with COPD and healthy controls were clinically evaluated, and their sputum samples were assessed by flow cytometry. A mouse model of spontaneous COPD [genetically deficient in surfactant protein-D (SP-D -/- )] and ozone (O 3 ) exposure were used to examine the mechanism by which lack of functional SP-D may skew ILC2s to produce IL-17A in combination with IL-5 and IL-13, leading to a mixed inflammatory profile and more severe disease.

Measurements and Main Results

COPD was characterized by poor spirometry, sputum inflammation, and the emergence of sputum GATA3 + ILCs (ILC2s), but not T-bet + ILCs (ILC1s) nor RORγt + ILCs (ILC3s). COPD subjects with elevated sputum ILC2s (the ILC2 high group) had worse spirometry and sputum neutrophilia and eosinophilia than healthy and ILC2 low subjects. This was associated with the presence of dual-positive IL-5 + IL-17A + and IL-13 + IL-17A + ILCs and nonfunctional SP-D in the sputum in ILC2 high subjects. SP-D -/- mice showed spontaneous airway neutrophilia. Lack of SP-D in the mouse lung licensed ILC2s to produce IL-17A, which was dose-dependently inhibited by recombinant SP-D. SP-D -/- mice showed enhanced susceptibility to O 3 -induced airway neutrophilia, which was associated with the emergence of inflammatory IL-13 + IL-17A + ILCs.

Conclusions

We report that the presence of sputum ILC2s predicts the severity of COPD, and unravel a novel pathway of IL-17A plasticity in lung ILC2s, prevented by the immunomodulatory protein SP-D.

An Evaluation of Type 1 Interferon Related Genes in Male and Female-Matched, SARS-CoV-2 Infected Individuals Early in the COVID-19 Pandemic

SARS-CoV-2 infection has claimed just over 1.1 million lives in the US since 2020. Globally, the SARS-CoV-2 respiratory infection spread to 771 million people and caused mortality in 6.9 million individuals to date. Much of the early literature showed that SARS-CoV-2 immunity was defective in the early stages of the pandemic, leading to heightened and, sometimes, chronic inflammatory responses in the lungs. This lung-associated 'cytokine storm' or 'cytokine release syndrome' led to the need for oxygen supplementation, respiratory distress syndrome, and mechanical ventilation in a relatively high number of people. In this study, we evaluated circulating PBMC from non-hospitalized, male and female, COVID-19+ individuals over the course of infection, from the day of diagnosis (day 0) to one-week post diagnosis (day 7), and finally 4 weeks after diagnosis (day 28). In our early studies, we included hospitalized and critically care patient PBMC; however, most of these individuals were lymphopenic, which limited our assessments of their immune integrity. We chose a panel of 30 interferon-stimulated genes (ISG) to evaluate by PCR and completed flow analysis for immune populations present in those PBMC. Lastly, we assessed immune activation by stimulating PBMC with common TLR ligands. We identified changes in innate cells, primarily the innate lymphoid cells (ILC, NK cells) and adaptive immune cells (CD4+ and CD8+ T cells) over this time course of infection. We found that the TLR-7 agonist, Resiquimod, and the TLR-4 ligand, LPS, induced significantly better IFNα and IFNγ responses in the later phase (day 28) of SARS-CoV-2 infection in those non-hospitalized COVID-19+ individuals as compared to early infection (day 0 and day 7). We concluded that TLR-7 and TLR-4 agonists may be effective adjuvants in COVID-19 vaccines for mounting immunity that is long-lasting against SARS-CoV-2 infection.

A novel type-2 innate lymphoid cell-based immunotherapy for cancer

Cell-based cancer immunotherapy has achieved significant advancements, providing a source of hope for cancer patients. Notwithstanding the considerable progress in cell-based immunotherapy, the persistently low response rates and the exorbitant costs associated with their implementation still present a formidable challenge in clinical settings. In the landscape of cell-based cancer immunotherapies, an uncharted territory involves Type 2 innate lymphoid cells (ILC2s) and interleukin-33 (IL-33) which promotes ILC2 functionality, recognized for their inherent ability to enhance immune responses. Recent discoveries regarding their role in actuating cytolytic T lymphocyte responses, including curbing tumor growth rates and hindering metastasis, have added a new dimension to our understanding of the IL-33/ILC2 axis. These recent insights may hold significant promise for ILC2 cell-based immunotherapy. Nevertheless, the prospect of adoptively transferring ILC2s to confer immune protection against tumors has yet to be investigated. The present study addresses this hypothesis, revealing that ILC2s isolated from the lungs of tumor-bearing mice, and tumor infiltrating ILC2s when adoptively transferred after tumor establishment at a ratio of one ILC2 per sixty tumor cells, leads to an influx of tumor infiltrating CD4+ and CD8+ T lymphocytes as well as tumor infiltrating eosinophils resulting in a remarkable reduction in tumor growth. Moreover, we find that post-adoptive transfer of ILC2s, the number of tumor infiltrating ILC2s is inversely proportional to tumor size. Finally, we find corollaries of the IL-33/ILC2 axis enhancing the infiltration of eosinophils in human prostate carcinomas patients' expressing high levels of IL-33 versus those expressing low levels of IL-33. Our results underscore the heightened efficacy of adoptively transferred ILC2s compared to alternative approaches, revealing an approximately one hundred fifty-fold superiority on a cell-per-cell basis over CAR T-cells in the specific targeting and elimination of tumors within the same experimental model. Overall, this study demonstrates the functional significance of ILC2s in cancer immunosurveillance and provides the proof of concept of the potential utility of ILC2 cell-based cancer immunotherapies.

Combination of IL-33 with PD-1 blockade augment mILC2s-mediated anti-tumor immunity

BACKGROUND

Group 2 innate lymphoid cells (ILC2s) represent one of the main tissue-specific innate lymphoid cell populations, which are key drivers of cytokine secretion in their occupational niche. However, the precise involvement of ILC2s in cancer immunity and their potential impact on immunotherapeutic approaches remain poorly understood.

METHODS

The proportion of ILC2s originating from various tissue sources were quantified through flow cytometry, along with the determination of CD4+ T cell and CD8+ T cell percentages. Flow cytometry was also employed to assess IFN-γ production and programmed cell death protein-1 (PD-1) expression in T cells. Immunohistochemistry was utilized to detect IL-33 expression in tumor tissues, while immunofluorescence was employed to confirm the infiltration of ILC2s in both murine and human tumor tissues.

RESULTS

In this study, we provide evidence that intra-tumoral ILC2s in lung adenocarcinoma (LUAD) exist in a quiescent state. However, the activation of intra-tumoral ILC2s is induced by IL-33 specifically in a natural ILC2s (nILC2, ST2+KLRG1-) phenotype. Considering the pivotal role of PD-1 in cancer immunotherapy and its immunoregulatory functions, we investigated the synergistic effects of IL-33 and anti-PD-1 and found that their combination enhances anti-tumor immunity and improves the efficacy of immunotherapy. Moreover, this combination leads to the upregulation of activated mature ILC2s (mILC2, ST2+KLRG1+) phenotype, thereby highlighting the activated ILC2s as a novel enhancer of the immunoregulatory properties of anti-PD-1.

CONCLUSIONS

Collectively, these findings underscore the significance of ILC2s and their contribution to the anti-tumor response in the context of cancer immunotherapy. Consequently, the simultaneous targeting of ILC2s and T cells represents a potentially promising and widely applicable strategy for immunotherapeutic interventions.

Allergen immunotherapy combined with Notch pathway inhibitors improves HDM-induced allergic airway inflammation and inhibits ILC2 activation

Introduction

Group 2 innate lymphoid cells (ILC2s) play a crucial role in house dust mite (HDM)-induced allergic inflammation, and allergen immunotherapy (AIT) holds promise for treating the disease by reducing the frequency of ILC2s. Despite significant progress in AIT for allergic diseases, there remains a need to improve the control of allergic symptoms.

Methods

We investigated the synergistic effect of the Notch signaling pathway and subcutaneous immunotherapy (SCIT) in treating allergic airway inflammation in mice and their impact on the ratio of ILC2s in lung tissues. This was achieved by establishing the HDM-induced airway allergic disorders (HAAD) model and SCIT model. Additionally, we conducted in vitro investigations into the effect of the Notch signaling pathway on the secretory function of activated ILC2s using fluorescence-activated cell sorting. Furthermore, we explored the coactivation of the Notch signaling pathway with SCIT in vitro by sorting ILC2s from the lung tissues of mice after SCIT modeling.

Results

Previously, our group demonstrated that Notch signaling pathway inhibitors can reduce allergic airway inflammation in mice. Notch signaling induces lineage plasticity of mature ILC2s. In this study, we showed that AIT alleviates allergic airway inflammation and suppresses the frequency of ILC2s induced by HDM. Interestingly, AIT combined with a γ-secretase inhibitor (GSI), an inhibitor of the Notch signaling pathway, significantly inhibited the frequency of ILC2s, reduced airway inflammation, and suppressed Th2-type responses in a mouse model. Furthermore, lung ILC2s from HDM-challenged mice with or without AIT were treated with GSI in vitro, and we found that GSI dramatically reduced the secretion of type 2 inflammatory factors in ILC2s.

Discussion

These findings suggest that Notch signaling pathway inhibitors can be used as adjuvant therapy for AIT and may hold potential treatment value in the cooperative control of allergic airway inflammation during early AIT.

A diversity of novel type-2 innate lymphoid cell subpopulations revealed during tumour expansion

Type 2 innate lymphoid cells (ILC2s) perform vital functions in orchestrating humoral immune responses, facilitating tissue remodelling, and ensuring tissue homeostasis. Additionally, in a role that has garnered considerably less attention, ILC2s can also enhance Th1-related cytolytic T lymphocyte immune responses against tumours. Studies have thus far generally failed to address the mystery of how one ILC2 cell-type can participate in a multiplicity of functions. Here we utilized single cell RNA sequencing analysis to create the first comprehensive atlas of naïve and tumour-associated lung ILC2s and discover multiple unique subtypes of ILC2s equipped with developmental gene programs that become skewed during tumour expansion favouring inflammation, antigen processing, immunological memory and Th1-related anti-tumour CTL responses. The discovery of these new subtypes of ILC2s challenges current paradigms of ILC2 biology and provides an explanation for their diversity of function.

Retinoic acid drives intestine-specific adaptation of effector ILC2s originating from distant sites

Adaptation of immune cells to tissue-specific microenvironments is a crucial process in homeostasis and inflammation. Here, we show that murine effector type 2 innate lymphoid cells (ILC2s) from various organs are equally effective in repopulating ILC2 niches in other anatomical locations where they adapt tissue-specific phenotypes of target organs. Single-cell transcriptomics of ILC2 populations revealed upregulation of retinoic acid (RA) signaling in ILC2s during adaptation to the small intestinal microenvironment, and RA signaling mediated reprogramming of kidney effector ILC2s toward the small intestinal phenotype in vitro and in vivo. Inhibition of intestinal ILC2 adaptation by blocking RA signaling impaired worm expulsion during Strongyloides ratti infection, indicating functional importance of ILC2 tissue imprinting. In conclusion, this study highlights that effector ILC2s retain the ability to adapt to changing tissue-specific microenvironments, enabling them to exert tissue-specific functions, such as promoting control of intestinal helminth infections.

Plasticity between type 2 innate lymphoid cell subsets and amphiregulin expression regulates epithelial repair in biliary atresia

BACKGROUND AND AIMS

Although a dysregulated type 1 immune response is integral to the pathogenesis of biliary atresia, studies in both humans and mice have uncovered a type 2 response, primarily driven by type 2 innate lymphoid cells. In nonhepatic tissues, natural type 2 innate lymphoid cell (nILC2s) regulate epithelial proliferation and tissue repair, whereas inflammatory ILC2s (iIlC2s) drive tissue inflammation and injury. The aim of this study is to determine the mechanisms used by type 2 innate lymphoid cell (ILC2) subpopulations to regulate biliary epithelial response to an injury.

APPROACH AND RESULTS

Using Spearman correlation analysis, nILC2 transcripts, but not those of iILC2s, are positively associated with cholangiocyte abundance in biliary atresia patients at the time of diagnosis. nILC2s are identified in the mouse liver through flow cytometry. They undergo expansion and increase amphiregulin production after IL-33 administration. This drives epithelial proliferation dependent on the IL-13/IL-4Rα/STAT6 pathway as determined by decreased nILC2s and reduced epithelial proliferation in knockout strains. The addition of IL-2 promotes inter-lineage plasticity towards a nILC2 phenotype. In experimental biliary atresia induced by rotavirus, this pathway promotes epithelial repair and tissue regeneration. The genetic loss or molecular inhibition of any part of this circuit switches nILC2s to inflammatory type 2 innate lymphoid cell-like, resulting in decreased amphiregulin production, decreased epithelial proliferation, and the full phenotype of experimental biliary atresia.

CONCLUSIONS

These findings identify a key function of the IL-13/IL-4Rα/STAT6 pathway in ILC2 plasticity and an alternate circuit driven by IL-2 to promote nILC2 stability and amphiregulin expression. This pathway induces epithelial homeostasis and repair in experimental biliary atresia.

The modulation of pulmonary group 2 innate lymphoid cell function in asthma: from inflammatory mediators to environmental and metabolic factors

A dysregulated type 2 immune response is one of the fundamental causes of allergic asthma. Although Th2 cells are undoubtedly central to the pathogenesis of allergic asthma, the discovery of group 2 innate lymphoid cells (ILC2s) has added another layer of complexity to the etiology of this chronic disease. Through their inherent innate type 2 responses, ILC2s not only contribute to the initiation of airway inflammation but also orchestrate the recruitment and activation of other members of innate and adaptive immunity, further amplifying the inflammatory response. Moreover, ILC2s exhibit substantial cytokine plasticity, as evidenced by their ability to produce type 1- or type 17-associated cytokines under appropriate conditions, underscoring their potential contribution to nonallergic, neutrophilic asthma. Thus, understanding the mechanisms of ILC2 functions is pertinent. In this review, we present an overview of the current knowledge on ILC2s in asthma and the regulatory factors that modulate lung ILC2 functions in various experimental mouse models of asthma and in humans.

Tissue-resident innate lymphoid cells in asthma

Asthma is a chronic airway inflammatory disease whose global incidence increases annually. The role of innate lymphoid cells (ILCs) is a crucial aspect of asthma research with respect to different endotypes of asthma. Based on its pathological and inflammatory features, asthma is divided into type 2 high and type 2 low endotypes. Type-2 high asthma is distinguished by the activation of type 2 immune cells, including T helper 2 (Th2) cells and ILC2s; the production of cytokines interleukin (IL)-4, IL-5 and IL-13; eosinophilic aggregation; and bronchial hyper-responsiveness. Type-2 low asthma represents a variety of endotypes other than type 2 high endotype such as the IL-1β/ILC3/neutrophil endotype and a paucigranulocytic asthma, which may be insensitive to corticosteroid treatment and/or associated with obesity. The complexity of asthma is due to the involvement of multiple cell types, including tissue-resident ILCs and other innate immune cells including bronchial epithelial cells, dendritic cells, macrophages and eosinophils, which provide immediate defence against viruses, pathogens and allergens. On this basis, innate immune cells and adaptive immune cells combine to induce the pathological condition of asthma. In addition, the plasticity of ILCs increases the heterogeneity of asthma. This review focuses on the phenotypes of tissue-resident ILCs and their roles in the different endotypes of asthma, as well as the mechanisms of tissue-resident ILCs and other immune cells. Based on the phenotypes, roles and mechanisms of immune cells, the therapeutic strategies for asthma are reviewed.

m6A RNA modification regulates innate lymphoid cell responses in a lineage-specific manner

Innate lymphoid cells (ILCs) can quickly switch from a quiescent state to an active state and rapidly produce effector molecules that provide critical early immune protection. How the post-transcriptional machinery processes different stimuli and initiates robust gene expression in ILCs is poorly understood. Here, we show that deletion of the N6-methyladenosine (m6A) writer protein METTL3 has little impact on ILC homeostasis or cytokine-induced ILC1 or ILC3 responses but significantly diminishes ILC2 proliferation, migration and effector cytokine production and results in impaired antihelminth immunity. m6A RNA modification supports an increase in cell size and transcriptional activity in activated ILC2s but not in ILC1s or ILC3s. Among other transcripts, the gene encoding the transcription factor GATA3 is highly m6A methylated in ILC2s. Targeted m6A demethylation destabilizes nascent Gata3 mRNA and abolishes the upregulation of GATA3 and ILC2 activation. Our study suggests a lineage-specific requirement of m6A for ILC2 responses.

Phenotype overlap in the natural history of asthma

The heterogeneity of asthma makes it challenging to unravel the pathophysiologic mechanisms of the disease. Despite the wealth of research identifying diverse phenotypes, many gaps still remain in our knowledge of the disease's complexity. A crucial aspect is the impact of airborne factors over a lifetime, which often results in a complex overlap of phenotypes associated with type 2 (T2), non-T2 and mixed inflammation. Evidence now shows overlaps between the phenotypes associated with T2, non-T2 and mixed T2/non-T2 inflammation. These interconnections could be induced by different determinants such as recurrent infections, environmental factors, T-helper plasticity and comorbidities, collectively resulting in a complex network of distinct pathways generally considered as mutually exclusive. In this scenario, we need to abandon the concept of asthma as a disease characterised by distinct traits grouped into static segregated categories. It is now evident that there are multiple interplays between the various physiologic, cellular and molecular features of asthma, and the overlap of phenotypes cannot be ignored.

Butyrate inhibits iILC2-mediated lung inflammation via lung-gut axis in chronic obstructive pulmonary disease (COPD)

BACKGROUND

The study investigated the effects and underlying mechanisms of intestinal flora metabolite butyrate on inflammatory ILC2 cells (iILC2s)-mediated lung inflammation in chronic obstructive pulmonary disease (COPD).

METHODS

Mouse models of COPD and acute exacerbation of COPD (AECOPD) were established. Flow cytometry was used to detect natural ILC2 cells (nILC2s) and iILC2s in lung and colon tissues. The 16s rRNA and GC-MS were used to detect microbial flora and short chain fatty acids (SCFAs) in feces. ELISA was used to detect IL-13 and IL-4. Western blot and qRT-PCR were used to detect the relative protein and mRNA levels, respectively. In vitro experiments were performed with sorted ILC2s from colon tissues of control mice. Mice with AECOPD were treated with butyrate.

RESULTS

The nILC2s and iILC2s in lung and colon tissues of AECOPD mice were significantly higher than control groups. The abundance of the flora Clostridiaceae was significantly reduced, and the content of SCFAs, including acetate and butyrate, was significantly reduced. The in vitro experiments showed that butyrate inhibited iILC2 cell phenotype and cytokine secretion. Butyrate treatment reduced the proportion of iILC2 cells in the colon and lung tissues of mice with AECOPD.

CONCLUSIONS

The nILC2s and iILC2s in the colon tissues are involved in the course of COPD. Decreased Clostridiaceae and butyrate in AECOPD mice caused the accumulation of iILC2 cells in the intestines and lungs. Supplementation of butyrate can reduce iILC2 in the intestine and lung tissues. Our data may provide new ideas for prevention and treatment of COPD.

Controversial role of ILC3s in intestinal diseases: A novelty perspective on immunotherapy

ILC3s have been identified as crucial immune regulators that play a role in maintaining host homeostasis and modulating the antitumor response. Emerging evidence supports the idea that LTi cells play an important role in initiating lymphoid tissue development, while other ILC3s can promote host defense and orchestrate adaptive immunity, mainly through the secretion of specific cytokines and crosstalk with other immune cells or tissues. Additionally, dysregulation of ILC3-mediated overexpression of cytokines, changes in subset abundance, and conversion toward other ILC subsets are closely linked with the occurrence of tumors and inflammatory diseases. Regulation of ILC3 cytokines, ILC conversion and LTi-induced TLSs may be a novel strategy for treating tumors and intestinal or extraintestinal inflammatory diseases. Herein, we discuss the development of ILCs, the biology of ILC3s, ILC plasticity, the correlation of ILC3s and adaptive immunity, crosstalk with the intestinal microenvironment, controversial roles of ILC3s in intestinal diseases and potential applications for treatment.

Transcriptional control of ILC identity

Innate lymphoid cells (ILCs) are heterogeneous innate immune cells which participate in host defense, mucosal repair and immunopathology by producing effector cytokines similarly to their adaptive immune cell counterparts. The development of ILC1, 2, and 3 subsets is controlled by core transcription factors: T-bet, GATA3, and RORγt, respectively. ILCs can undergo plasticity and transdifferentiate to other ILC subsets in response to invading pathogens and changes in local tissue environment. Accumulating evidence suggests that the plasticity and the maintenance of ILC identity is controlled by a balance between these and additional transcription factors such as STATs, Batf, Ikaros, Runx3, c-Maf, Bcl11b, and Zbtb46, activated in response to lineage-guiding cytokines. However, how interplay between these transcription factors leads to ILC plasticity and the maintenance of ILC identity remains hypothetical. In this review, we discuss recent advances in understanding transcriptional regulation of ILCs in homeostatic and inflammatory conditions.

Natural killer cells and type 1 innate lymphoid cells in cancer

Innate lymphoid cells (ILCs) are a group of innate lymphocytes that do not express RAG-dependent rearranged antigen-specific cell surface receptors. ILCs are classified into five groups according to their developmental trajectory and cytokine production profile. They encompass NK cells, which are cytotoxic, helper-like ILCs 1-3, which functionally mirror CD4⁺ T helper (Th) type 1, Th2 and Th17 cells respectively, and lymphoid tissue inducer (LTi) cells. NK cell development depends on Eomes (eomesodermin), whereas the ILC1 program is regulated principally by the transcription factor T-bet (T-box transcription factor Tbx21), that of ILC2 is regulated by GATA3 (GATA-binding protein 3) and that of ILC3 is regulated by RORγt (RAR-related orphan receptor γ). NK cells were discovered close to fifty years ago, but ILC1s were first described only about fifteen years ago. Within the ILC family, NK and ILC1s share many similarities, as witnessed by their cell surface phenotype which largely overlap. NK cells and ILC1s have been reported to respond to tissue inflammation and intracellular pathogens. Several studies have reported an antitumorigenic role for NK cells in both humans and mice, but data for ILC1s are both scarce and contradictory. In this review, we will first describe the different NK cell and ILC1 subsets, their effector functions and development. We will then discuss their role in cancer and the effects of the tumor microenvironment on their metabolism.

Innate Lymphoid Cell Plasticity in Mucosal Infections

Mucosal tissue homeostasis is a dynamic process that involves multiple mechanisms including regulation of innate lymphoid cells (ILCs). ILCs are mostly tissue-resident cells which are critical for tissue homeostasis and immune response against pathogens. ILCs can sense environmental changes and rapidly respond by producing effector cytokines to limit pathogen spread and initiate tissue recovery. However, dysregulation of ILCs can also lead to immunopathology. Accumulating evidence suggests that ILCs are dynamic population that can change their phenotype and functions under rapidly changing tissue microenvironment. However, the significance of ILC plasticity in response to pathogens remains poorly understood. Therefore, in this review, we discuss recent advances in understanding the mechanisms regulating ILC plasticity in response to intestinal, respiratory and genital tract pathogens. Key transcription factors and lineage-guiding cytokines regulate this plasticity. Additionally, we discuss the emerging data on the role of tissue microenvironment, gut microbiota, and hypoxia in ILC plasticity in response to mucosal pathogens. The identification of new pathways and molecular mechanisms that control functions and plasticity of ILCs could uncover more specific and effective therapeutic targets for infectious and autoimmune diseases where ILCs become dysregulated.

Regulation of Notch Signaling Pathway to Innate Lymphoid Cells in Patients with Acute Myocardial Infarction

The Notch signaling pathway is an important regulator in fate decisions and immune responses of innate lymphoid cells (ILCs). However, the function of Notch signaling in ILCs in acute coronary syndrome is still not fully elucidated. Thirty-one unstable angina pectoris (UAP) patients, 21 acute myocardial infarction (AMI) patients, and 20 controls were included in this study. Peripheral blood mononuclear cells (PBMCs) were isolated. The mRNA expression levels of Notch receptors and ligands were measured by real-time PCR, while ILC subsets were measured by flow cytometry. Lin^- cells were purified and stimulated with γ-secretase inhibitor (GSI). ILC subsets, transcription factors, and secreted cytokines were assessed. Notch receptor and ligand mRNA levels were elevated in PBMCs and peripheral lin^- cells from AMI patients. There was no significant difference in total lin^-CD45⁺CD161⁺CD127⁺ ILC frequency among three groups. The CRTH2^-CD117^- ILC1 subset was down-regulated, while the CRTH2⁺ ILC2 subset was up-regulated in AMI patients. The CRTH2^-CD117⁺ ILC3 subpopulation was comparable among the three groups. ILC1% was negatively correlated with Notch1 and Notch2 in AMI patients. Inhibition of Notch signaling pathway by GSI induced elevations in ILC1 frequency, T-bet mRNA expression, and interferon-γ secretion and reduced ILC2 frequency, GATA3 mRNA levels, and interleukin-5/interleukin-13 production by lin^- cells from AMI patients. The current data indicated that activation of Notch signaling pathway might contribute to ILC1-to-ILC2 shift in peripheral blood in AMI patients.

Liver group 2 innate lymphoid cells regulate blood glucose levels through IL-13 signaling and suppression of gluconeogenesis

The liver stores glycogen and releases glucose into the blood upon increased energy demand. Group 2 innate lymphoid cells (ILC2) in adipose and pancreatic tissues are known for their involvement in glucose homeostasis, but the metabolic contribution of liver ILC2s has not been studied in detail. Here we show that liver ILC2s are directly involved in the regulation of blood glucose levels. Mechanistically, interleukin (IL)-33 treatment induces IL-13 production in liver ILC2s, while directly suppressing gluconeogenesis in a specific Hnf4a/G6pc-high primary hepatocyte cluster via Stat3. These hepatocytes significantly interact with liver ILC2s via IL-13/IL-13 receptor signaling. The results of transcriptional complex analysis and GATA3-ChIP-seq, ATAC-seq, and scRNA-seq trajectory analyses establish a positive regulatory role for the transcription factor GATA3 in IL-13 production by liver ILC2s, while AP-1 family members are shown to suppress IL-13 release. Thus, we identify a regulatory role and molecular mechanism by which liver ILC2s contribute to glucose homeostasis.

Besides hepatocytes, resident immune cells of the liver are also contributing to the body’s energy homeostasis. Here authors show that group 2 innate lymphoid cells interact with a specific set of hepatocytes in suppressing gluconeogenesis and regulate blood glucose levels via Interleukin-13 signalling.

Impact of MyD88, Microbiota, and Location on Type 1 and Type 3 Innate Lymphoid Cells during Toxoplasma gondii Infection

Toxoplasma gondii induces strong IFN-γ-based immunity. Innate lymphoid cells (ILC), in particular ILC1, are an important innate source of this protective cytokine during infection. Our objective was to determine how MyD88-dependent signaling influences ILC function during peroral compared with i.p. infection with T. gondii. MyD88 +/+ and MyD88 -/- mice were orally inoculated with ME49 cysts, and small intestinal lamina propria ILC were assessed using flow cytometry. We observed T-bet+ ILC1, retinoic acid-related orphan receptor γt+ ILC3, and a population of T-bet+retinoic acid-related orphan receptor γt+ double-positive ILC. In MyD88 -/- mice, IFN-γ-producing T-bet+ ILC1 frequencies were reduced compared with wild-type. Treatment of MyD88 -/- mice with an antibiotic mixture to deplete microflora reduced IFN-γ+ ILC1 frequencies. To examine ILC responses outside of the mucosal immune system, peritoneal exudate cells were collected from wild-type and knockout mice after i.p. inoculation with ME49 cysts. In this compartment, ILC were highly polarized to the ILC1 subset that increased significantly and became highly positive for IFN-γ over the course of infection. Increased ILC1 was associated with expression of the Ki67 cell proliferation marker, and the response was driven by IL-12p40. In the absence of MyD88, IFN-γ expression by ILC1 was not maintained, but proliferation remained normal. Collectively, these data reveal new aspects of ILC function that are influenced by location of infection and shaped further by MyD88-dependent signaling.

“Just one word, plastic!”: Controversies and caveats in innate lymphoid cell plasticity

Innate lymphoid cells (ILCs) are frontline immune effectors involved in the early stages of host defense and maintenance of tissue homeostasis, particularly at mucosal surfaces such as the intestine, lung, and skin. Canonical ILCs are described as tissue-resident cells that populate peripheral tissues early in life and respond appropriately based on environmental exposure and their anatomical niche and tissue microenvironment. Intriguingly, there are accumulating reports of ILC “plasticity” that note the existence of non-canonical ILCs that exhibit distinct patterns of master transcription factor expression and cytokine production profiles in response to tissue inflammation. Yet this concept of ILC-plasticity is controversial due to several confounding caveats that include, among others, the independent large-scale recruitment of new ILC subsets from distal sites and the local, in situ, differentiation of uncommitted resident precursors. Nevertheless, the ability of ILCs to acquire unique characteristics and adapt to local environmental cues is an attractive paradigm because it would enable the rapid adaptation of innate responses to a wider array of pathogens even in the absence of pre-existing ‘prototypical’ ILC responder subsets. Despite the impressive recent progress in understanding ILC biology, the true contribution of ILC plasticity to tissue homeostasis and disease and how it is regulated remains obscure. Here, we detail current methodologies used to study ILC plasticity in mice and review the mechanisms that drive and regulate functional ILC plasticity in response to polarizing signals in their microenvironment and different cytokine milieus. Finally, we discuss the physiological relevance of ILC plasticity and its implications for potential therapeutics and treatments.

RNA-binding protein RBM3 intrinsically suppresses lung innate lymphoid cell activation and inflammation partially through CysLT1R

Innate lymphoid cells (ILC) promote lung inflammation in asthma through cytokine production. RNA-binding proteins (RBPs) are critical post-transcriptional regulators, although less is known about RBPs in ILC biology. Here, we demonstrate that RNA-binding motif 3 (RBM3) is highly expressed in lung ILCs and is further induced by alarmins TSLP and IL-33. Rbm3^-/- and Rbm3^-/-Rag2^-/- mice exposed to asthma-associated Alternaria allergen develop enhanced eosinophilic lung inflammation and ILC activation. IL-33 stimulation studies in vivo and in vitro show that RBM3 suppressed lung ILC responses. Further, Rbm3^-/- ILCs from bone marrow chimeric mice display increased ILC cytokine production suggesting an ILC-intrinsic suppressive function of RBM3. RNA-sequencing of Rbm3^-/- lung ILCs demonstrates increased expression of type 2/17 cytokines and cysteinyl leukotriene 1 receptor (CysLT1R). Finally, Rbm3^-/-Cyslt1r^-/- mice show dependence on CysLT1R for accumulation of ST2⁺IL-17⁺ ILCs. Thus, RBM3 intrinsically regulates lung ILCs during allergen-induced type 2 inflammation that is partially dependent on CysLT1R.

Crosstalk between macrophages and innate lymphoid cells (ILCs) in diseases

Innate lymphoid cells (ILCs) and macrophages are tissue-resident cells that play important roles in tissue-immune homeostasis and immune regulation. ILCs are mainly distributed on the barrier surfaces of mammals to ensure immunity or tissue homeostasis following host, microbial, or environmental stimulation. Their complex relationships with different organs enable them to respond quickly to disturbances in environmental conditions and organ homeostasis, such as during infections and tissue damage. Gradually emerging evidence suggests that ILCs also play complex and diverse roles in macrophage development, homeostasis, polarization, inflammation, and viral infection. In turn, macrophages also determine the fate of ILCs to some extent, which indicates that network crossover between these interactions is a key determinant of the immune response. More work is needed to better define the crosstalk of ILCs with macrophages in different tissues and demonstrate how it is affected during inflammation and other diseases. Here, we summarize current research on the functional interactions between ILCs and macrophages and consider the potential therapeutic utility of these interactions for the benefit of human health.

Role of ILC2s in Solid Tumors: Facilitate or Inhibit?

Group 2 innate lymphoid cells (ILC2s) are important mediators of type 2 immunity and play an important role in allergic diseases, helminth infections, and tissue fibrosis. However, the role of ILC2s in tumor immunity requires further elucidation. Studies over the past decade have reported that ILC2s play a promoting or suppressing role in different tumors. Here we reviewed the role of ILC2s in solid tumors demonstrating that ILC2s act as a crucial regulator in tumor immunity. We proposed that ILC2s could be an important predictor for tumor prognosis and a new therapeutic target after immunotherapy resistance. In conclusion, our study shed new light on modifying and targeting ILC2s for anti-tumor immunotherapy.

Plasticity of Innate Lymphoid Cells in Cancer

Innate lymphoid cells (ILCs) are a heterogenous population of the innate immune system, enriched at mucosal surfaces and are pivotal regulators of immune homeostasis. ILCs are the innate counterpart of T cells. Like T cells, ILC subsets are highly plastic with their composition and function controlled by alterations in their microenvironment. This plasticity allows for the trans-differentiation between the subsets to rapidly respond to their immune environment. The tumor microenvironment (TME) is a heterogeneous milieu characterized by different cytokines and growth factors. Through interaction with the tumor microenvironment, ILCs can transdifferentiate into different subsets resulting in pro or anti-tumor immunity. Thus, studying ILC plasticity might result in new therapeutic approaches for cancer therapy. In this review, we summarize current findings of the functional and plastic heterogeneity of ILCs in homeostasis as well as disease settings with a specific focus on cancer. We specifically highlight tumor-driven plasticity and how ILC-induced inflammation can impact the tumor microenvironment and anti-tumor immunity.

Crosstalk between ILC2s and Th2 CD4+ T Cells in Lung Disease

Cytokine secretion, such as interleukin-4 (IL-4), IL-5, IL-9, IL-13, and amphiregulin (Areg), by type 2 innate lymphoid cells (ILC2s) is indispensable for homeostasis, remodeling/repairing tissue structure, inflammation, and tumor immunity. Often viewed as the innate cell surrogate of T helper type 2 (Th2) cells, ILC2s not only secrete the same type 2 cytokines, but are also inextricably related to CD4⁺T cells in terms of cell origin and regulatory factors, bridging between innate and adaptive immunity. ILC2s interact with CD4⁺T cells to play a leading role in a variety of diseases through secretory factors. Here, we review the latest progress on ILC2s and CD4⁺T cells in the lung, the close relationship between the two, and their relevance in the lung disease and immunity. This literature review aids future research in pulmonary type 2 immune diseases and guides innovative treatment approaches for these diseases.

Signaling Pathways Tuning Innate Lymphoid Cell Response to Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the deadliest cancers worldwide and its incidence continues to rise globally. Various causes can lead to its development such as chronic viral infections causing hepatitis, cirrhosis or nonalcoholic steatohepatitis (NASH). The contribution of immune cells to HCC development and progression has been extensively studied when it comes to adaptive lymphocytes or myeloid populations. However, the role of the innate lymphoid cells (ILCs) is still not well defined. ILCs are a family of lymphocytes comprising five subsets including circulating Natural Killer (NK) cells, ILC1s, ILC2s, ILC3s and lymphocytes tissue-inducer cells (LTi). Mostly located at epithelial surfaces, tissue-resident ILCs and NK cells can rapidly react to environmental changes to mount appropriate immune responses. Here, we provide an overview of their roles and actions in HCC with an emphasis on the importance of diverse signaling pathways (Notch, TGF-β, Wnt/β-catenin…) in the tuning of their response to HCC.

Transcription factor-driven regulation of ILC1 and ILC3

Mammalian innate lymphoid cells (ILCs) have functional relevance under both homeostatic and disease settings, such as inflammatory bowel disease (IBD), particularly in the context of maintaining the integrity of mucosal surfaces. Early reports highlighted group 1 and 3 ILC regulatory transcription factors (TFs), T-box expressed in T cells (T-bet; Tbx21) and RAR-related orphan nuclear receptor γt (RORγt; Rorc), as key regulators of ILC biology. Since then, other canonical TFs have been shown to have a role in the development and function of ILC subsets. In this review, we focus on recent insights into the balance between mature ILC1 and ILC3 based on these TFs and how they interact with other key cell-intrinsic molecular pathways. We outline how this TF interplay might be explored to identify novel candidate therapeutic avenues for human diseases.

c-Maf enforces cytokine production and promotes memory-like responses in mouse and human type 2 innate lymphoid cells

Group‐2 innate lymphoid cells (ILC2s), which are involved in type 2 inflammatory diseases such as allergy, can exhibit immunological memory, but the basis of this ILC2 "trained immunity" has remained unclear. Here, we found that stimulation with IL‐33/IL‐25 or exposure to the allergen papain induces the expression of the transcription factor c‐Maf in mouse ILC2s. Chronic papain exposure results in high production of IL‐5 and IL‐13 cytokines and lung eosinophil recruitment, effects that are blocked by c‐Maf deletion in ILCs. Transcriptomic analysis revealed that knockdown of c‐Maf in ILC2s suppresses expression of type 2 cytokine genes, as well as of genes linked to a memory‐like phenotype. Consistently, c‐Maf was found highly expressed in human adult ILC2s but absent in cord blood and required for cytokine production in isolated human ILC2s. Furthermore, c‐Maf‐deficient mouse or human ILC2s failed to exhibit strengthened (“trained”) responses upon repeated challenge. Thus, the expression of c‐Maf is indispensable for optimal type 2 cytokine production and proper memory‐like responses in group‐2 innate lymphoid cells.

Innate lymphoid cells and cancer

The innate lymphoid cell (ILC) family is composed of natural killer (NK) cells, ILC1, ILC2 and ILC3, which participate in immune responses to virus, bacteria, parasites and transformed cells. ILC1, ILC2 and ILC3 subsets are mostly tissue-resident, and are profoundly imprinted by their organ of residence. They exhibit pleiotropic effects, driving seemingly paradoxical responses such as tissue repair and, alternatively, immunopathology toward allergens and promotion of tumorigenesis. Despite this, a trickle of studies now suggests that non-NK ILCs may not be overwhelmingly tumorigenic and could potentially be harnessed to drive anti-tumor responses. Here, we examine the pleiotropic behavior of ILCs in cancer and begin to unravel the gap in our knowledge that exposes a new horizon for thinking about modifying ILCs and targeting them for immunotherapy.

Innate Lymphoid Cells and Intestinal Inflammatory Disorders

Innate lymphoid cells (ILCs) are a population of lymphoid cells that do not express T cell or B cell antigen-specific receptors. They are largely tissue-resident and enriched at mucosal sites to play a protective role against pathogens. ILCs mimic the functions of CD4 T helper (Th) subsets. Type 1 innate lymphoid cells (ILC1s) are defined by the expression of signature cytokine IFN-γ and the master transcription factor T-bet, involving in the type 1 immune response; ILC2s are characterized by the expression of signature cytokine IL-5/IL-13 and the master transcription factor GATA3, participating in the type 2 immune response; ILC3s are RORγt-expressing cells and are capable of producing IL-22 and IL-17 to maintain intestinal homeostasis. The discovery and investigation of ILCs over the past decades extends our knowledge beyond classical adaptive and innate immunology. In this review, we will focus on the roles of ILCs in intestinal inflammation and related disorders.

HOIL1 regulates group 2 innate lymphoid cell numbers and type 2 inflammation in the small intestine

Patients with mutations in HOIL1 experience a complex immune disorder including intestinal inflammation. To investigate the role of HOIL1 in regulating intestinal inflammation, we employed a mouse model of partial HOIL1 deficiency. The ileum of HOIL1-deficient mice displayed features of type 2 inflammation including tuft cell and goblet cell hyperplasia, and elevated expression of Il13, Il5 and Il25 mRNA. Inflammation persisted in the absence of T and B cells, and bone marrow chimeric mice revealed a requirement for HOIL1 expression in radiation-resistant cells to regulate inflammation. Although disruption of IL-4 receptor alpha (IL4Rα) signaling on intestinal epithelial cells ameliorated tuft and goblet cell hyperplasia, expression of Il5 and Il13 mRNA remained elevated. KLRG1^hi CD90^lo group 2 innate lymphoid cells were increased independent of IL4Rα signaling, tuft cell hyperplasia and IL-25 induction. Antibiotic treatment dampened intestinal inflammation indicating commensal microbes as a contributing factor. We have identified a key role for HOIL1, a component of the Linear Ubiquitin Chain Assembly Complex, in regulating type 2 inflammation in the small intestine. Understanding the mechanism by which HOIL1 regulates type 2 inflammation will advance our understanding of intestinal homeostasis and inflammatory disorders and may lead to the identification of new targets for treatment.

ILC2s induce adaptive Th2-type immunity in different stages of tuberculosis through the Notch-GATA3 pathway

The study is to investigate the roles of group 2 innate lymphoid cells (ILC2s) in different courses of tuberculosis (TB). Serum and PBMCs were respectively isolated from the TST negative, LTBI (latent TB infection), ATB (active TB) and RTB (recurrent TB) patients. Flow cytometry was used to detect Th1, Th2 and ILC2s in the peripheral blood. The mRNA and protein levels of GATA3, RORα, CRTH2, Hes1, Notch1, NF-κB, and ID2 were detected by qRT-PCR and Western blotting. ILC2 cells from ATB and RTB patients were stimulated with rJagged2 or DAPT in vitro, and co-cultured with CD4⁺ T cells from TST negative group. ELISA was used to detect cytokine levels. The results showed that compared with the TST negative or LTBI group, Th2 cells and serum IL-4 in ATB group increased dramatically, accompanied by an increase of Th2/Th1 ratio in ATB patients, especially in RTB group. However, ILC2s in the ATB and RTB group increased significantly, along with increased GATA3, RORα, and CRTH2 levels. After rJagged2 stimulation in vitro, the levels of Hes1, Notch1, NF-κB, RORα, GATA3 and ID2 and those of IL-4, IL-5 and IL-13 were significantly increased. These effects were abrogated by DAPT treatment. Then, ILC2s, especially those from RTB patients, induced Th2-type immune response after co-culturing with CD4⁺ T cells. In conclusion, our results suggest that ILC2s may promote Th2-type immune response in different stages of TB via the Notch-GATA3 pathway.

Group-2 Innate Lymphoid Cells Promote HCC Progression Through CXCL2-Neutrophil-Induced Immunosuppression

BACKGROUND AND AIMS

Due to their inherent characteristics, the function of group-2 innate lymphoid cells (ILC2s) varies in a context-dependent manner. ILC2s are involved in certain liver diseases; however, their involvement in HCC is unknown. In the present study, we assessed the role of an HCC-derived ILC2 population in tumor progression.

APPROACH AND RESULTS

Through FACS and single-cell RNA sequencing, we discovered that ILC2s were highly enriched in human HCC and correlated significantly with tumor recurrence and worse progression-free survival as well as overall survival in patients. Mass cytometry identified a subset of HCC-derived ILC2s that had lost the expression of killer cell lectin-like receptor subfamily G, member 1 (KLRG1). Distinct from their circulating counterparts, these hepatic ILC2s highly expressed CD69 and an array of tissue resident-related genes. Furthermore, reduction of E-cadherin in tumor cells caused the loss of KLRG1 expression in ILC2s, leading to their increased proliferation and subsequent accumulation in HCC sites. The KLRG1^- ILC2 subset showed elevated production of chemotaxis factors, including C-X-C motif chemokine (C-X-C motif) ligand (CXCL)-2 and CXCL8, which in turn recruited neutrophils to form an immunosuppressive microenvironment, leading to tumor progression. Accordingly, restoring KLRG1 in ILC2s, inhibiting CXCL2 in ILC2s, or depleting neutrophils inhibited tumor progression in a murine HCC model.

CONCLUSIONS

We identified HCC-associated ILC2s as an immune regulatory cell type that promotes tumor development, suggesting that targeting these ILC2s might lead to new treatments for HCC.

The Role of Innate Lymphoid Cells in Chronic Respiratory Diseases

The lung is a vital mucosal organ that is constantly exposed to the external environment, and as such, its defenses are continuously under threat. The pulmonary immune system has evolved to sense and respond to these danger signals while remaining silent to innocuous aeroantigens. The origin of the defense system is the respiratory epithelium, which responds rapidly to insults by the production of an array of mediators that initiate protection by directly killing microbes, activating tissue-resident immune cells and recruiting leukocytes from the blood. At the steady-state, the lung comprises a large collection of leukocytes, amongst which are specialized cells of lymphoid origin known as innate lymphoid cells (ILCs). ILCs are divided into three major helper-like subsets, ILC1, ILC2 and ILC3, which are considered the innate counterparts of type 1, 2 and 17 T helper cells, respectively, in addition to natural killer cells and lymphoid tissue inducer cells. Although ILCs represent a small fraction of the pulmonary immune system, they play an important role in early responses to pathogens and facilitate the acquisition of adaptive immunity. However, it is now also emerging that these cells are active participants in the development of chronic lung diseases. In this mini-review, we provide an update on our current understanding of the role of ILCs and their regulation in the lung. We summarise how these cells and their mediators initiate, sustain and potentially control pulmonary inflammation, and their contribution to the respiratory diseases chronic obstructive pulmonary disease (COPD) and asthma.

The role of meningeal populations of type II innate lymphoid cells in modulating neuroinflammation in neurodegenerative diseases

Recent research into meningeal lymphatics has revealed a never-before appreciated role of type II innate lymphoid cells (ILC2s) in modulating neuroinflammation in the central nervous system (CNS). To date, the role of ILC2-mediated inflammation in the periphery has been well studied. However, the exact distribution of ILC2s in the CNS and therefore their putative role in modulating neuroinflammation in neurodegenerative diseases such as Alzheimer's disease (AD), multiple sclerosis (MS), Parkinson's disease (PD), and major depressive disorder (MDD) remain highly elusive. Here, we review the current evidence of ILC2-mediated modulation of neuroinflammatory cues (i.e., IL-33, IL-25, IL-5, IL-13, IL-10, TNFα, and CXCL16-CXCR6) within the CNS, highlight the distribution of ILC2s in both the periphery and CNS, and discuss some challenges associated with cell type-specific targeting that are important for therapeutics. A comprehensive understanding of the roles of ILC2s in mediating and responding to inflammatory cues may provide valuable insight into potential therapeutic strategies for many dementia-related disorders.

PD-1 Blockade on Tumor Microenvironment-Resident ILC2s Promotes TNF-α Production and Restricts Progression of Metastatic Melanoma

While pulmonary ILC2s represent one of the major tissue-resident innate lymphoid cell populations at steady state and are key drivers of cytokine secretion in their occupational niche, their role in pulmonary cancer progression remains unclear. As the programmed cell death protein-1 (PD-1) plays a major role in cancer immunotherapy and immunoregulatory properties, here we investigate the specific effect of PD-1 inhibition on ILC2s during pulmonary B16 melanoma cancer metastasis. We demonstrate that PD-1 inhibition on ILC2s suppresses B16 tumor growth. Further, PD-1 inhibition upregulates pulmonary ILC2-derived TNF-α production, a cytotoxic cytokine that directly induces cell death in B16 cells, independent of adaptive immunity. Together, these results highlight the importance of ILC2s and their anti-tumor role in pulmonary B16 cancer progression during PD-1 inhibitory immunotherapy.

Group 2 innate lymphoid cells are numerically and functionally deficient in the triple transgenic mouse model of Alzheimer's disease

Background

The immune pathways in Alzheimer’s disease (AD) remain incompletely understood. Our recent study indicates that tissue-resident group 2 innate lymphoid cells (ILC2) accumulate in the brain barriers of aged mice and that their activation alleviates aging-associated cognitive decline. The regulation and function of ILC2 in AD, however, remain unknown.

Methods

In this study, we examined the numbers and functional capability of ILC2 from the triple transgenic AD mice (3xTg-AD) and control wild-type mice. We investigated the effects of treatment with IL-5, a cytokine produced by ILC2, on the cognitive function of 3xTg-AD mice.

Results

We demonstrate that brain-associated ILC2 are numerically and functionally defective in the triple transgenic AD mouse model (3xTg-AD). The numbers of brain-associated ILC2 were greatly reduced in 7-month-old 3xTg-AD mice of both sexes, compared to those in age- and sex-matched control wild-type mice. The remaining ILC2 in 3xTg-AD mice failed to efficiently produce the type 2 cytokine IL-5 but gained the capability to express a number of proinflammatory genes. Administration of IL-5, a cytokine produced by ILC2, transiently improved spatial recognition and learning in 3xTg-AD mice.

Conclusion

Our results collectively indicate that numerical and functional deficiency of ILC2 might contribute to the cognitive impairment of 3xTg-AD mice.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02202-2.

The dual function of ILC2: From host protection to pathogenic players in type 2 asthma

Innate lymphoid cells type 2 (ILC2) are considered the innate counterpart of Th2 cells and cooperate with them in host protection against helminths and in the pathogenesis of allergic diseases. ILC2 are characterized by type 2 cytokines production (IL-13, IL-4 and IL-5) and by GATA-3 transcription factor expression. Belonging to innate immune system, ILC2 lack of antigen specific receptor and their activation is controlled mainly by epithelial derived cytokines, such as TSLP, IL-25, and IL-33. ILC2 are located in a strategic position in the airway mucosa and are important to patrol the airways, to recruit other immune system cells and to activate resident cells in response to pathogens injury and/or tissue damage. In the last decade, many studies, in both humans and mice, focused on ILC2, fully investigating their main features such as the development from the precursor, the stimuli for their activation or inhibition, their plasticity, their classification in different subsets, and finally, their pathogenetic role in type 2 immune-mediated disorders. In this review we performed an excursus on phenotypical and functional properties on both human and mouse ILC2, in physiological and pathological conditions (mainly in type 2 asthma), considering this cell subset as target for specific therapeutic strategies.

ILC2 Cells Promote Th2 Cell Differentiation in AECOPD Through Activated Notch-GATA3 Signaling Pathway

This study is to investigate the capacity of type 2 innate lymphoid cells (ILC2s) in regulating the Th2 type adaptive immune response of acute exacerbation of chronic obstructive pulmonary disease (AECOPD). The study enrolled healthy people, stable chronic obstructive pulmonary disease (COPD) patients, and AECOPD patients. Flow cytometry was used to detect Th2 and ILC2 cells in the peripheral blood. In addition, ILC2s from the peripheral blood of AECOPD patients were stimulated with PBS, IL-33, Jagged1, DAPT, IL-33+Jagged1, IL-33+DAPT, and IL-33+Jagged-1+DAP in vitro. The levels of cytokines in the culture supernatant were detected by ELISA and the culture supernatant was used to culture CD4 ⁺ T cells. The mRNA and protein levels of Notch1, hes1, GATA3, RORα, and NF-κB of ILC2s were detected by real-time PCR and Western blot. The proportion of Th2 and ILC2s was significantly increased in the peripheral blood of AECOPD patients, alone with the increased Notch1, hes1, and GATA3 mRNA levels. In vitro results showed that the mRNA and protein levels of Notch1, hes1, GATA3 and NF-κB were significantly increased after stimulation with Notch agonist, meanwhile, the level of type 2 cytokines were increased in the supernatant of cells stimulated with Notch agonist, and significantly promoted differentiation of Th2 cells in vitro. Disruption of Notch pathway weakened GATA3 expression and cytokine production, and ultimately affected the differentiation of Th2 cells. In conclusion, our results suggest that ILC2s can promote Th2 cell differentiation in AECOPD via activated Notch-GATA3 signal pathway.