Innate lymphoid cells: models of plasticity for immune homeostasis and rapid responsiveness in protection

A Membrane Lipid Signature Unravels the Dynamic Landscape of Group 1 ILCs

In an era where the established lines between cell identities are blurred by intra-lineage plasticity, distinguishing between stable and transitional states becomes imperative. This challenge is particularly pronounced within the Group 1 ILC lineage, where the similarity and plasticity between NK cells and ILC1s obscure their classification and the assignment of their unique contributions to immune regulation. This study exploits the unique property of Asialo-GM1 (AsGM1)-a membrane lipid associated with cytotoxic attributes absent in ILC1s-as a definitive criterion to distinguish between these cells. By prioritizing cytotoxic potential as the cardinal differentiator, our strategic use of the AsGM1 signature achieved precise delineation of NK cells and ILC1s across tissues, validated by RNA-seq analysis. This capability extends beyond steady-state classifications, adeptly capturing the binary classification of NK cells and ILC1s during acute liver injury. By leveraging two established models of NK-to-ILC1 plasticity driven by TGFβ and Toxoplasma gondii , we demonstrate the stability of the AsGM1 signature, which sharply contrasts with the loss of Eomes. This signature identified a spectrum of known and novel NK cell derivatives-ILC1-like entities that bridge traditional binary classifications in aging and infection. The early detection of the AsGM1 signature at the immature NK (iNK) stage, preceding Eomes, and its stability, unaffected by transcriptional reprogramming that typically alters Eomes, position AsGM1 as a unique, site-agnostic marker for fate mapping NK-to-ILC1 plasticity. This provides a powerful tool to explore the expanding heterogeneity within the Group 1 ILC landscape, effectively transcending the ambiguity inherent to the NK-to-ILC1 continuum.

IL-27-induced PD-L1highSca-1+ innate lymphoid cells suppress contact hypersensitivity in an IL-10-dependent manner

Innate lymphoid cells (ILCs) play an important role in maintaining tissue homeostasis and various inflammatory responses. ILCs are typically classified into three subsets, as is the case for T-cells. Recent studies have reported that IL-10-producing type 2 ILCs (ILC210s) have an immunoregulatory function dependent on IL-10. However, the surface markers of ILC210s and the role of ILC210s in contact hypersensitivity (CHS) are largely unknown. Our study revealed that splenic ILC210s are extensively included in PD-L1highSca-1+ ILCs and that IL-27 amplifies the development of PD-L1highSca-1+ ILCs and ILC210s. Adoptive transfer of PD-L1highSca-1+ ILCs suppressed oxazolone-induced CHS in an IL-10-dependent manner Taken together, our results demonstrate that ILC210s are critical for the control of CHS and suggest that ILC210s can be used as target cells for the treatment of CHS.

Regulation of the immune system by the insulin receptor in health and disease

The signaling pathways downstream of the insulin receptor (InsR) are some of the most evolutionarily conserved pathways that regulate organism longevity and metabolism. InsR signaling is well characterized in metabolic tissues, such as liver, muscle, and fat, actively orchestrating cellular processes, including growth, survival, and nutrient metabolism. However, cells of the immune system also express the InsR and downstream signaling machinery, and there is increasing appreciation for the involvement of InsR signaling in shaping the immune response. Here, we summarize current understanding of InsR signaling pathways in different immune cell subsets and their impact on cellular metabolism, differentiation, and effector versus regulatory function. We also discuss mechanistic links between altered InsR signaling and immune dysfunction in various disease settings and conditions, with a focus on age related conditions, such as type 2 diabetes, cancer and infection vulnerability.

ILC3-like ILC2 subset increases in minimal persistent inflammation after acute type II inflammation of allergic rhinitis and inhibited by Biminkang: Plasticity of ILC2 in minimal persistent inflammation

Minimal persistent inflammation (MPI), the local inflammation that occurs after an acute type II immune response in patients with allergic rhinitis (AR), is responsible for airway hyperreactivity and the recurrence of AR. Innate lymphoid cells (ILCs) play a crucial role in mucosal immune homeostasis, but the changes of ILC subsets in the MPI stage remain unclear. In this study, the levels of ILC-secreting cytokines in nasal lavages were analyzed from 19 AR patients and 8 healthy volunteers. AR and MPI model mice were established to study the ILC subsets. The results showed that IL-17A was significantly increased in nasal lavage of AR patients in the MPI stage by MSD technology. When compared with the AR model mice, the frequency of IL-13⁺ ILC2 in the nasal mucosa and lungs decreased, while IL-5⁺ ILC2 remain high in MPI model mice. A part of the IL-5⁺ ILC2 subset displayed ILC3-like characteristics with elevated RORγt, IL-17A and IL-23R expression. Especially, these ILC3-like ILC2 exhibited up-regulation of GATA3⁺ RORγt⁺ were increased in MPI model mice. After the treatment of Biminkang, the frequencies of IL-5⁺ ILC2, IL-17A⁺ ILC3, and GATA3⁺ RORγt⁺ ILC3-like ILC2 were significantly reduced, and IL-23R expression was also decreased on ILC3-like-ILC2 subset. These results suggested that the elevated IL-17A in the MPI stage has been related to or at least partly due to the increased of ILC3-like ILC2. Biminkang could effectively decrease IL-17A⁺ ILC3 and inhibit ILC3-like ILC2 subset in the MPI stage. Biminkang is effective in administrating MPI by regulating airway ILC homeostasis.

Interferon regulatory factor 1 (IRF-1) promotes intestinal group 3 innate lymphoid responses during Citrobacter rodentium infection

Group 3 innate lymphoid cells (ILC3s) are crucial mediators of immunity and epithelial barrier function during immune responses against extracellular bacteria. Here, we identify Interferon regulatory factor 1 (IRF-1), a transcription factor previously associated with type 1 immunity, as an essential regulator of intestinal ILC3 accumulation and effector cytokine production. We demonstrate that IRF-1 is upregulated in the context of infection with the enteropathogen Citrobacter rodentium and that its presence is central for anatomical containment and prevention of pathogen dissemination. We furthermore show that IRF-1 is required in order for intestinal ILC3s to produce large amounts of the protective effector cytokine IL-22 early in the course of infection. On a molecular level, our data indicate that IRF-1 controls ILC3 numbers and their activation by direct transcriptional regulation of the IL-12Rβ1 chain, thereby allowing ILCs to physiologically respond to IL-23 stimulation.

Innate lymphoid cells (ILC) are involved with different immune responses. Here the authors show that Interferon regulatory factor 1 (IRF1) is important for intestinal ILC3 accumulation during Citrobacter rodentium infection and promotes release of the protective cytokine IL-22 and response to IL-23.

Luteolin alleviates ulcerative colitis by restoring the balance of NCR-ILC3/NCR+ILC3 to repairing impaired intestinal barrier

Ulcerative colitis (UC) is a multifactorial, refractory chronic inflammatory disease. The primary factor leading to prolonged ulcerative colitis is the imbalance of the group 3 innate lymphoid cells (ILC3) subgroup resulting in the delayed reconstruction of damaged intestinal barrier. Previous studies show that luteolin had efficacy on UC, however, the potency of luteolin on restoring the balance of NCR^-ILC3/NCR⁺ILC3 to repairing impaired intestinal barrier remains unclear. In this study, to investigate the potential mechanism of luteolin on ILC3 subgroup, we first evidenced that luteolin could promote transformation NCR^-MNK3 to NCR⁺MNK3 in vitro. Then, a UC model was established in C57BL/6J mice to assess the efficacy of luteolin in restoring ILC3 subgroup balance and repairing intestinal barrier in chronic UC. Finally, the experiments in vitro validated the potential mechanism of luteolin in regulating ILC3 plasticity. The results showed that luteolin significantly alleviated the symptoms of DSS-induced UC in mice, including preventing body weight loss and decreasing the disease activity index (DAI) and intestinal damages. Additionally, luteolin increased NCR⁺ILC3 levels, promoted the production of IL-22 and decreased the levels of IL-17a and INF-γ in the intestine, and encourage intestinal barrier function recovery in UC mice by promoting the expression of ZO-1 and Occludin. Experiments in vitro revealed that luteolin facilitated the transformation of NCR^-MNK3 to NCR⁺MNK3 and promoted the secretion of IL-22, which was linked to the Notch pathway. All results revealed that luteolin restored the balance of NCR^-ILC3/NCR⁺ILC3 and contributed to repair of injured intestinal epithelium to alleviate ulcerative colitis.

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.

Types of necroinflammation, the effect of cell death modalities on sterile inflammation

Distinct types of immune responses are activated by infections, which cause the development of type I, II, or III inflammation, regulated by Th1, Th2, Th17 helper T cells and ILC1, ILC2 and ILC3 cells, respectively. While the classification of immune responses to different groups of pathogens is widely accepted, subtypes of the immune response elicited by sterile inflammation have not yet been detailed. Necroinflammation is associated with the release of damage-associated molecular patterns (DAMP) from dying cells. In this review, we present that the distinct molecular mechanisms activated during apoptosis, necroptosis, pyroptosis, and ferroptosis lead to the release of different patterns of DAMPs and their suppressors, SAMPs. We summarize the currently available data on how regulated cell death pathways and released DAMPs and SAMPs direct the differentiation of T helper and ILC cells. Understanding the subtypes of necroinflammation can be crucial in developing strategies for the treatment of sterile inflammatory diseases caused by cell death processes.

Heterogeneity of type 2 innate lymphoid cells

More than a decade ago, type 2 innate lymphoid cells (ILC2s) were discovered to be members of a family of innate immune cells consisting of five subsets that form a first line of defence against infections before the recruitment of adaptive immune cells. Initially, ILC2s were implicated in the early immune response to parasitic infections, but it is now clear that ILC2s are highly diverse and have crucial roles in the regulation of tissue homeostasis and repair. ILC2s can also regulate the functions of other type 2 immune cells, including T helper 2 cells, type 2 macrophages and eosinophils. Dysregulation of ILC2s contributes to type 2-mediated pathology in a wide variety of diseases, potentially making ILC2s attractive targets for therapeutic interventions. In this Review, we focus on the spectrum of ILC2 phenotypes that have been described across different tissues and disease states with an emphasis on human ILC2s. We discuss recent insights in ILC2 biology and suggest how this knowledge might be used for novel disease treatments and improved human health.

Type 2 innate lymphoid cells (ILC2s) have diverse phenotypes across different tissues and disease states. Recent insights into ILC2 biology raise new possibilities for the improved treatment of cancer and of metabolic, infectious and chronic inflammatory diseases.

When a Friend Becomes Your Enemy: Natural Killer Cells in Atherosclerosis and Atherosclerosis-Associated Risk Factors

Atherosclerosis (ATS), the change in structure and function of arteries with associated lesion formation and altered blood flow, is the leading cause of cardiovascular disease, the number one killer worldwide. Beyond dyslipidemia, chronic inflammation, together with aberrant phenotype and function of cells of both the innate and adaptive immune system, are now recognized as relevant contributors to atherosclerosis onset and progression. While the role of macrophages and T cells in atherosclerosis has been addressed in several studies, Natural Killer cells (NKs) represent a poorly explored immune cell type, that deserves attention, due to NKs’ emerging contribution to vascular homeostasis. Furthermore, the possibility to re-polarize the immune system has emerged as a relevant tool to design new therapies, with some succesfull exmples in the field of cancer immunotherapy. Thus, a deeper knowledge of NK cell pathophysiology in the context of atherosclerosis and atherosclerosis-associated risk factors could help developing new preventive and treatment strategies, and decipher the complex scenario/history from “the risk factors for atherosclerosis” Here, we review the current knowledge about NK cell phenotype and activities in atherosclerosis and selected atherosclerosis risk factors, namely type-2 diabetes and obesity, and discuss the related NK-cell oriented environmental signals.

Natural Killer Cells and Type 1 Innate Lymphoid Cells in Hepatocellular Carcinoma: Current Knowledge and Future Perspectives

Natural killer (NK) cells and type 1 innate lymphoid cells (ILC1) are specific innate lymphoid cell subsets that are key for the detection and elimination of pathogens and cancer cells. In liver, while they share a number of characteristics, they differ in many features. These include their developmental pathways, tissue distribution, phenotype and functions. NK cells and ILC1 contribute to organ homeostasis through the production of key cytokines and chemokines and the elimination of potential harmful bacteria and viruses. In addition, they are equipped with a wide range of receptors, allowing them to detect “stressed cells’ such as cancer cells. Our understanding of the role of innate lymphoid cells in hepatocellular carcinoma (HCC) is growing owing to the development of mouse models, the progress in immunotherapeutic treatment and the recent use of scRNA sequencing analyses. In this review, we summarize the current understanding of NK cells and ILC1 in hepatocellular carcinoma and discuss future strategies to take advantage of these innate immune cells in anti-tumor immunity. Immunotherapies hold great promise in HCC, and a better understanding of the role and function of NK cells and ILC1 in liver cancer could pave the way for new NK cell and/or ILC1-targeted treatment.

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.

Dynamic regulation of innate lymphoid cells in the mucosal immune system

The mucosal immune system is considered a local immune system, a term that implies regional restriction. Mucosal tissues are continually exposed to a wide range of antigens. The regulation of mucosal immune cells is tightly associated with the progression of mucosal diseases. Innate lymphoid cells (ILCs) are abundant in mucosal barriers and serve as first-line defenses against pathogens. The subtype changes and translocation of ILCs are accompanied by the pathologic processes of mucosal diseases. Here, we review the plasticity and circulation of ILCs in the mucosal immune system under physiological and pathological conditions. We also discuss the signaling pathways involved in dynamic ILC changes and the related targets in mucosal diseases.

Innate Lymphoid Cells: Important Regulators of Host-Bacteria Interaction for Border Defense

Innate lymphoid cells (ILCs) are a recently discovered type of innate immune lymphocyte. They include three different groups classified by the nature of the transcription factors required for their development and by the cytokines they produce. ILCs mainly reside in tissues close to the mucosal barrier such as the respiratory and gastrointestinal tracts. Due to their close proximity to the mucosal surface, ILCs are exposed to a variety of both commensal and pathogenic bacteria. Under non-pathological conditions, ILCs have been shown to be important regulators for the maintenance of tissue homeostasis by mutual interactions with the microbiome. Besides these important functions at homeostasis, several studies have also provided emerging evidence that ILCs contribute to defense against pathogenic bacterial infection by responding rapidly to the pathogens as well as orchestrating other immune cells. In this review, we summarize recent advances in our understanding of the interactions of ILCs and bacteria, with special focus on the function of the different ILC subsets in bacterial infections.

Plasticity and regulatory mechanisms of human ILC2 functions

Human group 2 innate lymphoid cells (ILC2) represent the innate counterpart of Th2 cells and cooperate with them in helminths protection and in the pathogenesis of allergic diseases. Some reports described ILC2 plasticity and few studies investigated the cellular and molecular mechanisms regulating human ILC2 functions. The aim of this study is to define how immune deviation and immune regulation control human ILC2-mediated immune response. Human circulating ILC2 were expanded in vitro and then cultured in presence of IL-12 or IL-1β plus IL-23 or co-coltured in presence of circulating CD4+CD25highFoxp3+Treg. IL-12 induces IFN-γ production and upregulation of T-bet mRNA level on human circulating ILC2 whereas IL-1β and IL-23 mediate IL-22 production and upregulation of RORC mRNA level. In all these conditions, GATA-3 mRNA level is not reduced and the typical type 2 cytokines are only partially reduced. Moreover, "modulated" ILC2 have reduced ability to induce IgE producing by B cells. ILC2 proliferation, cytokines production and CD154 expression were inhibited by CD4+CD25highFoxp3+ Treg cells. TGF-β reduced CD154 expression on ILC2 stimulated with IL-25/IL-33. This study defines possible cellular and molecular mechanisms responsible for modulation and inhibition of human ILC2 activity. These results may be useful in the development of strategies aimed to dampen ILC2 function in type-2 mediated diseases.

Type 2 Innate Lymphoid Cells Accumulate in the Brain After Hypoxia-Ischemia but Do Not Contribute to the Development of Preterm Brain Injury

Background

The immune system of human and mouse neonates is relatively immature. However, innate lymphoid cells (ILCs), commonly divided into the subsets ILC1, ILC2, and ILC3, are already present in the placenta and other fetal compartments and exhibit higher activity than what is seen in adulthood. Recent reports have suggested the potential role of ILCs, especially ILC2s, in spontaneous preterm labor, which is associated with brain damage and subsequent long-term neurodevelopmental deficits. Therefore, we hypothesized that ILCs, and especially ILC2s, play a role in preterm brain injury.

Methods

C57Bl/6J mice at postnatal day 6 were subjected to hypoxia-ischemia (HI) insult induced by left carotid artery ligation and subsequent exposure to 10% oxygen in nitrogen. The presence of ILCs and ILC2s in the brain was examined at different time points after HI. The contribution of ILC2s to HI-induced preterm brain damage was explored using a conditionally targeted ILC2-deficient mouse strain (Rorα ^fl/fl IL7r ^Cre ), and gray and white-matter injury were evaluated at 7 days post-HI. The inflammatory response in the injured brain was assessed using immunoassays and immunochemistry staining.

Results

Significant increases in ILCs and ILC2s were observed at 24 h, 3 days, and 7 days post-HI in the injured brain hemisphere compared with the uninjured hemisphere in wild-type mice. ILC2s in the brain were predominantly located in the meninges of the injured ipsilateral hemispheres after HI but not in the brain parenchyma. Overall, we did not observe changes in cytokine/chemokine levels in the brains of Rorα ^fl/fl IL7r ^Cre mice compared with wild type animals apart from IL-13. Gray and white-matter tissue loss in the brain was not affected after HI in Rorα ^fl/fl IL7r ^Cre mice. Correspondingly, we did not find any differences in reactive microglia and astrocyte numbers in the brain in Rorα ^fl/fl IL7r ^Cre mice compared with wild-type mice following HI insult.

Conclusion

After HI, ILCs and ILC2s accumulate in the injured brain hemisphere. However, ILC2s do not contribute to the development of brain damage in this mouse model of preterm brain injury.

Innate lymphoid cell and adaptive immune cell cross-talk: A talk meant not to forget

Innate lymphoid cells (ILCs) are a relatively new class of innate immune cells with phenotypical characters of lymphocytes but genotypically or functionally behave as typical innate immune cells. They have been classically divided into 3 groups (group 1 ILCs or ILC1s, group 2 ILCs or ILC2s, and group 3 ILCs or ILC3s). They serve as the first line of defense against invading pathogens and allergens at mucosal surfaces. The adaptive immune response works effectively in association with innate immunity as innate immune cells serve as APCs to directly stimulate the adaptive immune cells (various sets of T and B cells). Additionally, innate immune cells also secrete various effector molecules, including cytokines or chemokines impacting the function, differentiation, proliferation, and reprogramming among adaptive immune cells to maintain immune homeostasis. Only superantigens do not require their processing by innate immune cells as they are recognized directly by T cells and B cells. Thus, a major emphasis of the current article is to describe the cross-talk between different ILCs and adaptive immune cells during different conditions varying from normal physiological situations to different infectious diseases to allergic asthma.

Pleiotropic Role and Bidirectional Immunomodulation of Innate Lymphoid Cells in Cancer

Innate lymphoid cells (ILCs) are largely tissue resident and respond rapidly toward the environmental signals from surrounding tissues and other immune cells. The pleiotropic function of ILCs in diverse contexts underpins its importance in the innate arm of immune system in human health and disease. ILCs derive from common lymphoid progenitors but lack adaptive antigen receptors and functionally act as the innate counterpart to T-cell subsets. The classification of different subtypes is based on their distinct transcription factor requirement for development as well as signature cytokines that they produce. The discovery and subsequent characterization of ILCs over the past decade have mainly focused on the regulation of inflammation, tissue remodeling, and homeostasis, whereas the understanding of the multiple roles and mechanisms of ILCs in cancer is still limited. Emerging evidence of the potent immunomodulatory properties of ILCs in early host defense signifies a major advance in the use of ILCs as promising targets in cancer immunotherapy. In this review, we will decipher the non-exclusive roles of ILCs associated with both protumor and antitumor activities. We will also dissect the heterogeneity, plasticity, genetic evidence, and dysregulation in different cancer contexts, providing a comprehensive understanding of the complexity and diversity. These will have implications for the therapeutic targeting in cancer.

Tumor progression mechanisms: Insights from the central immune regulation of tissue homeostasis

Knowledge of the mechanisms underlying the spread of cancer at the cellular and molecular levels is expanding rapidly. However, the central regulators governing the initiation and the rate of tumor growth remain poorly established. The fundamental principles of innate and adaptive immunity may explain how immune cells generate a specific response to tumor tissue. In the current review, the functional features of the immune system that contribute to the maintenance of normal tissue homeostasis, as well as their disruption in malignant transformations, were analyzed. Experimental and clinical studies previously demonstrated the involvement of regulatory T-cells in the process of tumor metastasis in a tissue-specific manner. An understanding of the cross talk between lymphoid and tumor cells may provide an insight into cancer evolution in terms of the mechanisms of T-cell competency formation. Elucidating the mechanisms of tumor progression via central immune regulation has implications for the development of novel therapeutic agents that target immune checkpoints.

Spatial and Temporal Mapping of Human Innate Lymphoid Cells Reveals Elements of Tissue Specificity

Innate lymphoid cells (ILC) play critical roles in regulating immunity, inflammation, and tissue homeostasis in mice. However, limited access to non-diseased human tissues has hindered efforts to profile anatomically-distinct ILCs in humans. Through flow cytometric and transcriptional analyses of lymphoid, mucosal, and metabolic tissues from previously healthy human organ donors, here we have provided a map of human ILC heterogeneity across multiple anatomical sites. In contrast to mice, human ILCs are less strictly compartmentalized and tissue localization selectively impacts ILC distribution in a subset-dependent manner. Tissue-specific distinctions are particularly apparent for ILC1 populations, whose distribution was markedly altered in obesity or aging. Furthermore, the degree of ILC1 population heterogeneity differed substantially in lymphoid versus mucosal sites. Together, these analyses comprise a comprehensive characterization of the spatial and temporal dynamics regulating the anatomical distribution, subset heterogeneity, and functional potential of ILCs in non-diseased human tissues.

Social Networking of Group Two Innate Lymphoid Cells in Allergy and Asthma

Allergic diseases including asthma, chronic rhinosinusitis, and atopic dermatitis are common conditions worldwide. While type 2 immune responses induced by T-cells significantly cause allergic inflammation, the recently identified group two innate lymphoid cells (ILC2s) are emerging as critical players in the development of allergy. Upon allergen exposure, ILC2s are rapidly activated by cytokines released by epithelial cells. Activated ILC2s release various effector cytokines altogether contributing to the pathogenesis of allergy and can even cause inflammation in the absence of T-cells, as observed in asthma. Although the factors inducing ILC2 activation have been identified, evidence suggests that multiple factors can enhance or repress ILC2 proliferation, trafficking, or secretion of effector cytokines upon allergic inflammation. In this review, we discuss the recent findings that influence ILC2 activation and the resulting effects on the pathogenesis of allergy. A better understanding of how ILC2s are modulated will open the door to the development of new therapeutic strategies against allergic diseases.

Roles of cytotoxic and helper innate lymphoid cells in cancer

Natural killer (NK) cells have long been recognized for their anti-cancer activity and are now included in the large family of innate lymphoid cells (ILCs). The discovery of new ILC subsets that, similarly to NK cells, are able to kill tumor cells encourages us to redefine NK cell role in anti-tumor immunity. Conventional NK cells circulate through the blood and screen the body for "stressed" cells. Therefore, NK cells are believed to play a key role in cancer immunosurveillance by the early elimination of cells undergoing malignant transformation. Tissue-resident ILCs might play a similar role since they are ideally located to detect the early signs of malignant transformation in their organ of residence. We are only beginning to appreciate the importance of the whole ILC family in cancer. Confusingly, these cells have been reported to both inhibit and fuel cancer progression and the factors regulating these dual functions remain unclear. Here, I review the recent advances in our understanding of cytotoxic and cytokine-producing helper ILC subsets in cancer.

Mouse models for the study of fate and function of innate lymphoid cells

Natural killer (NK) cells and lymphoid tissue inducer (LTi) cells were discovered more than 40 and 20 years ago, respectively. These two cell types were initially studied for their unique functions in the elimination of infected or transformed cells, and in the development of lymphoid tissues. It took an additional 10 years to realize that NK cells and LTi cells were members of a larger family of innate lymphoid cells (ILCs), whose phenotypes and functions mirror those of T cells. Many mouse models have since been developed to identify and isolate ILCs, map their developmental pathways and characterize their functions. Because of the similarity between ILCs and T cells, this exploration remains a challenge. In spite of this, a broad range of mouse models available to researchers has lead to significant progress in untangling the unique roles of ILCs early in defense, regulation of adaptive immunity and homeostasis. Here, we review these mouse models, and discuss their strengths and limitations.

Influenza-Activated ILC1s Contribute to Antiviral Immunity Partially Influenced by Differential GITR Expression

Innate lymphoid cells (ILCs) represent diversified subsets of effector cells as well as immune regulators of mucosal immunity and are classified into group 1 ILCs, group 2 ILCs, and group 3 ILCs. Group 1 ILCs encompass natural killer (NK) cells and non-NK ILCs (ILC1s) and mediate their functionality via the rapid production of IFN-γ and TNF-α. The current knowledge of ILC1s mainly associates them to inflammatory processes. Much less is known about their regulation during infection and their capacity to interact with cells of the adaptive immune system. The present study dissected the role of ILC1s during early influenza A virus infection, thereby revealing their impact on the antiviral response. Exploiting in vitro and in vivo H1N1 infection systems, a cross-talk of ILC1s with cells of the innate and the adaptive immunity was demonstrated, which contributes to anti-influenza immunity. A novel association of ILC1 functionality and the expression of the glucocorticoid-induced TNFR-related protein (GITR) was observed, which hints toward a so far undescribed role of GITR in regulating ILC1 responsiveness. Overexpression of GITR inhibits IFN-γ production by ILC1s, whereas partial reduction of GITR expression can reverse this effect, thereby regulating ILC1 functionality. These new insights into ILC1 biology define potential intervention targets to modulate the functional properties of ILC1s, thus contributing toward the development of new immune interventions against influenza.

Effects of HIV infection and ART on phenotype and function of circulating monocytes, natural killer, and innate lymphoid cells

HIV infection causes upregulation of markers of inflammation, immune activation and apoptosis of host adaptive, and innate immune cells particularly monocytes, natural killer (NK) and innate lymphoid cells (ILCs). Although antiretroviral therapy (ART) restores CD4 T-cell counts, the persistent aberrant activation of monocytes, NK and ILCs observed likely contributes to the incomplete recovery of T-cell effector functions. A better understanding of the effects of HIV infection and ART on the phenotype and function of circulating monocytes, NK, and ILCs is required to guide development of novel therapeutic interventions to optimize immune recovery.

Re-thinking our understanding of immunity: Robustness in the tissue reconstruction system

Robustness, understood as the maintenance of specific functionalities of a given system against internal and external perturbations, is pervasive in today's biology. Yet precise applications of this notion to the immune system have been scarce. Here we show that the concept of robustness sheds light on tissue repair, and particularly on the crucial role the immune system plays in this process. We describe the specific mechanisms, including plasticity and redundancy, by which robustness is achieved in the tissue reconstruction system (TRS). In turn, tissue repair offers a very important test case for assessing the usefulness of the concept of robustness, and identifying different varieties of robustness.

Brief Report: Inflammatory Colonic Innate Lymphoid Cells Are Increased During Untreated HIV-1 Infection and Associated With Markers of Gut Dysbiosis and Mucosal Immune Activation

BACKGROUND

HIV-1 infection is associated with intestinal inflammation, changes in the enteric microbiota (dysbiosis), and intestinal epithelial cell damage. NKp44 innate lymphoid cells (ILCs) play an important role in epithelial barrier maintenance through the production of interleukin (IL)-22 but also display functional plasticity and can produce inflammatory cytokines [eg, interferon gamma (IFNγ)] in response to cytokine milieu and stimulatory signals. The objective of this pilot study was to enumerate frequencies of IL-22 and IFNγ-expressing colonic NKp44 ILCs during untreated, chronic HIV-1 infection.

SETTING

A cross-sectional study was performed to compare numbers of cytokine-expressing ILCs in colonic biopsies of untreated, chronic HIV-1 infected (n = 22), and uninfected (n = 10) study participants. Associations between cytokine ILC and previously established measures of virological, immunological, and microbiome indices were analyzed.

METHODS

Multicolor flow cytometry was used to measure the absolute number of colonic CD3NKp44CD56 ILCs expressing IL-22 or IFNγ after in vitro mitogenic stimulation.

RESULTS

Numbers of colonic NKp44 ILCs that expressed IFNγ were significantly higher in HIV-1 infected versus uninfected persons and positively correlated with relative abundances of dysbiotic bacterial species in the Xanthomonadaceae and Prevotellaceae bacterial families and with colonic myeloid dendritic cell and T-cell activation.

CONCLUSION

Higher numbers of inflammatory colonic ILCs during untreated chronic HIV-1 infection that associated with dysbiosis and colonic myeloid dendritic cell and T-cell activation suggest that inflammatory ILCs may contribute to gut mucosal inflammation and epithelial barrier breakdown, important features of HIV-1 mucosal pathogenesis.

The good, the (not so) bad and the ugly of immune homeostasis in melanoma

Within the immune system multiple mechanisms balance the need for efficient pathogen recognition and destruction with the prevention of tissue damage by excessive, inappropriate or even self-targeting (auto)immune reactions. This immune homeostasis is a tightly regulated system which fails during tumor development, often due to the hijacking of its essential self-regulatory mechanisms by cancer cells. It is facilitated not only by tumor intrinsic properties, but also by the microbiome, host genetics and other factors. In certain ways many cancers can therefore be considered a rare failure of immune control rather than an uncommon or rare disease of the tissue of origin, as the acquisition of potentially oncogenic traits through mutation occurs constantly in most tissues during proliferation. Normally, aberrant cells are well-controlled by cell intrinsic (repair or apoptosis) and extrinsic (immune) mechanisms. However, occasionally oncogenic cells survive and escape control. Melanoma is one of the first cancer types where treatments aimed at restoring and enhancing an immune response to regain control over the tumor have been used with various success rates. With the advent of "modern" immunotherapeutics such as anti-CTLA-4 or anti-PD-1 antibodies that both target negative immune-regulatory pathways on immune cells resulting in durable responses in a proportion of patients, the importance of the interplay between the immune system and cancer has been established beyond doubt.

Group 3 Innate Lymphoid Cells: Communications Hubs of the Intestinal Immune System

The maintenance of mammalian health requires the generation of appropriate immune responses against a broad range of environmental and microbial challenges, which are continually encountered at barrier tissue sites including the skin, lung, and gastrointestinal tract. Dysregulated barrier immune responses result in inflammation, both locally and systemically in peripheral organs. Group 3 innate lymphoid cells (ILC3) are constitutively present at barrier sites and appear to be highly specialized in their ability to sense a range of environmental and host-derived signals. Under homeostatic conditions, ILC3 respond to local cues to maintain tissue homeostasis and restrict inflammatory responses. In contrast, perturbations in the tissue microenvironment resulting from disease, infection, or tissue damage can drive dysregulated pro-inflammatory ILC3 responses and contribute to immunopathology. The tone of the ILC3 response is dictated by a balance of “exogenous” signals, such as dietary metabolites and commensal microbes, and “endogenous” host-derived signals from stromal cells, immune cells, and the nervous system. ILC3 must therefore have the capacity to simultaneously integrate a wide array of complex and dynamic inputs in order to regulate barrier function and tissue health. In this review, we discuss the concept of ILC3 as a “communications hub” in the intestinal tract and associated lymphoid tissues and address the variety of signals, derived from multiple biological systems, which are interpreted by ILC3 to modulate the release of downstream effector molecules and regulate cell–cell crosstalk. Successful integration of environmental cues by ILC3 and downstream propagation to the broader immune system is required to maintain a tolerogenic and anti-inflammatory tone and reinforce barrier function, whereas dysregulation of ILC3 responses can contribute to the onset or progression of clinically relevant chronic inflammatory diseases.

Gut microbiota and IBD: causation or correlation?

A general consensus exists that IBD is associated with compositional and metabolic changes in the intestinal microbiota (dysbiosis). However, a direct causal relationship between dysbiosis and IBD has not been definitively established in humans. Findings from animal models have revealed diverse and context-specific roles of the gut microbiota in health and disease, ranging from protective to pro-inflammatory actions. Moreover, evidence from these experimental models suggest that although gut bacteria often drive immune activation, chronic inflammation in turn shapes the gut microbiota and contributes to dysbiosis. The purpose of this Review is to summarize current associations between IBD and dysbiosis, describe the role of the gut microbiota in the context of specific animal models of colitis, and discuss the potential role of microbiota-focused interventions in the treatment of human IBD. Ultimately, more studies will be needed to define host-microbial relationships relevant to human disease and amenable to therapeutic interventions.

Innate immune cells for immunotherapy of autoimmune and cancer disorders

Modulation of the immune system has been widely targeted for the treatment of several immune-related diseases, such as autoimmune disorders and cancer, due to its crucial role in these pathologies. Current available therapies focus mainly on symptomatic treatment and are often associated with undesirable secondary effects. For several years, remission of disease and subsequently recovery of immune homeostasis has been a major goal for immunotherapy. Most current immunotherapeutic strategies are aimed to inhibit or potentiate directly the adaptive immune response by modulating antibody production and B cell memory, as well as the effector potential and memory of T cells. Although these immunomodulatory approaches have shown some success in the clinic with promising therapeutic potential, they have some limitations related to their effectiveness in disease models and clinical trials, as well as elevated costs. In the recent years, a renewed interest has emerged on targeting innate immune cells for immunotherapy, due to their high plasticity and ability to exert a potent and extremely rapid response, which can influence the outcome of the adaptive immune response. In this review, we discuss the immunomodulatory potential of several innate immune cells, as well as they use for immunotherapy, especially in autoimmunity and cancer.

Affective immunology: where emotions and the immune response converge

Affect and emotion are defined as “an essential part of the process of an organism's interaction with stimuli.” Similar to affect, the immune response is the “tool” the body uses to interact with the external environment. Thanks to the emotional and immunological response, we learn to distinguish between what we like and what we do not like, to counteract a broad range of challenges, and to adjust to the environment we are living in. Recent compelling evidence has shown that the emotional and immunological systems share more than a similarity of functions. This review article will discuss the crosstalk between these two systems and the need for a new scientific area of research called affective immunology. Research in this field will allow a better understanding and appreciation of the immunological basis of mental disorders and the emotional side of immune diseases.

Immune-Mediated Repair: A Matter of Plasticity

Though the immune system is generally defined as a system of defense, it is increasingly recognized that the immune system also plays a crucial role in tissue repair and its potential dysregulations. In this review, we explore how distinct immune cell types are involved in tissue repair and how they interact in a process that is tightly regulated both spatially and temporally. We insist on the concept of immune cell plasticity which, in recent years, has proved fundamental for the success/understanding of the repair process. Overall, the perspective presented here suggests that the immune system plays a central role in the physiological robustness of the organism, and that cell plasticity contributes to the realization of this robustness.

Innate Immune Cytokines, Fibroblast Phenotypes, and Regulation of Extracellular Matrix in Lung

Chronic inflammation can be caused by adaptive immune responses in autoimmune and allergic conditions, driven by a T lymphocyte subset balance (TH1, TH2, Th17, Th22, and/or Treg) and skewed cellular profiles in an antigen-specific manner. However, several chronic inflammatory diseases have no clearly defined adaptive immune mechanisms that drive chronicity. These conditions include those that affect the lung such as nonatopic asthma or idiopathic pulmonary fibrosis comprising significant health problems. The remodeling of extracellular matrix (ECM) causes organ dysfunction, and it is largely generated by fibroblasts as the major cell controlling net ECM. As such, these are potential targets of treatment approaches in the context of ECM pathology. Fibroblast phenotypes contribute to ECM and inflammatory cell accumulation, and they are integrated into chronic disease mechanisms including cancer. Evidence suggests that innate cytokine responses may be critical in nonallergic/nonautoimmune disease, and they enable environmental agent exposure mechanisms that are independent of adaptive immunity. Innate immune cytokines derived from macrophage subsets (M1/M2) and innate lymphoid cell (ILC) subsets can directly regulate fibroblast function. We also suggest that STAT3-activating gp130 cytokines can sensitize fibroblasts to the innate cytokine milieu to drive phenotypes and exacerbate existing adaptive responses. Here, we review evidence exploring innate cytokine regulation of fibroblast behavior.