Adaptation of innate lymphoid cells to a micronutrient deficiency promotes type 2 barrier immunity

The development of adult innate lymphoid cells

Innate lymphoid cells (ILC) are a specialized family of effector lymphocytes that transcriptionally and functionally mirror effector subsets of T cells, but differ from T cells in that they lack clonally distributed adaptive antigen receptors. Our understanding of this family of lymphocytes is still in its infancy. In this review, we summarize current understanding and discuss recent insights into the cellular and molecular events that occur during early ILC development in adult mice. We discuss how these events overlap and diverge with the early development of adaptive T cells, and how they may influence the molecular and functional properties of mature ILC.

Innate Lymphoid Cells Are Depleted Irreversibly during Acute HIV-1 Infection in the Absence of Viral Suppression

Innate lymphoid cells (ILCs) play a central role in the response to infection by secreting cytokines crucial for immune regulation, tissue homeostasis, and repair. Although dysregulation of these systems is central to pathology, the impact of HIV-1 on ILCs remains unknown. We found that human blood ILCs were severely depleted during acute viremic HIV-1 infection and that ILC numbers did not recover after resolution of peak viremia. ILC numbers were preserved by antiretroviral therapy (ART), but only if initiated during acute infection. Transcriptional profiling during the acute phase revealed upregulation of genes associated with cell death, temporally linked with a strong IFN acute-phase response and evidence of gut barrier breakdown. We found no evidence of tissue redistribution in chronic disease and remaining circulating ILCs were activated but not apoptotic. These data provide a potential mechanistic link between acute HIV-1 infection, lymphoid tissue breakdown, and persistent immune dysfunction.

Innate lymphoid cells in intestinal immunity and inflammation

Innate lymphoid cells (ILCs) are a new and distinct family of innate immune cells that play an important role in immunity and inflammation. In this review, we focus on the role of ILCs in mucosal tissues, especially in the gut, in health and disease. ILCs support intestinal homeostasis by protecting the intestine from pathogens, contributing to the development of gut lymphoid tissue, and helping to repair injuries. By cooperating with epithelial cells and other innate and adaptive immune cells, ILCs participate in the control of pathogens and tolerance of commensal bacteria. The development and maintenance of ILCs are influenced by nutrients and metabolites sourced from diet and/or gut bacteria. ILCs have been shown to be involved in host metabolism and to participate in various diseases of the intestine including infectious and chronic inflammatory diseases, and cancer. Thus, the elucidation of ILC biology provides an exciting potential for development of novel therapeutic means to modulate immune responses in various disease settings.

Migration and Tissue Tropism of Innate Lymphoid Cells

Innate lymphoid cell (ILCs) subsets differentially populate various barrier and non-barrier tissues, where they play important roles in tissue homeostasis and tissue-specific responses to pathogen attack. Recent findings have provided insight into the molecular mechanisms that guide ILC migration into peripheral tissues, revealing common features among different ILC subsets as well as important distinctions. Recent studies have also highlighted the impact of tissue-specific cues on ILC migration, and the importance of the local immunological milieu. We review these findings here and discuss how the migratory patterns and tissue tropism of different ILC subsets relate to the development and differentiation of these cells, and to ILC-mediated tissue-specific regulation of innate and adaptive immune responses. In this context we outline open questions and important areas of future research.

Immune dysregulation in the functional gastrointestinal disorders

Gastrointestinal conditions may be broadly classified into two: organic and functional disease, with functional disorders accounting for the majority of patients with chronic gastrointestinal symptoms. Functional gastrointestinal disorders (FGIDs) present with no obvious pathology or well-accepted biochemical mechanism and, as such, treatment strategies are limited and focus on symptoms rather than cure. Irritable bowel syndrome and functional dyspepsia are the most widely recognised FGIDs, and there is a growing body of evidence to suggest an underlying inflammatory phenotype in subsets with these conditions. Here, we discuss the current knowledge of immune involvement in FGIDs and the commonalities between the different manifestations of FGIDs and propose a new hypothesis, potentially defining an underlying immunopathological basis of these conditions.

Type 3 innate lymphoid cell depletion is mediated by TLRs in lymphoid tissues of simian immunodeficiency virus-infected macaques

Innate lymphoid cells (ILCs) type 3, also known as lymphoid tissue inducer cells, plays a major role in both the development and remodeling of organized lymphoid tissues and the maintenance of adaptive immune responses. HIV/simian immunodeficiency virus (SIV) infection causes breakdown of intestinal barriers resulting in microbial translocation, leading to systemic immune activation and disease progression. However, the effects of HIV/SIV infection on ILC3 are unknown. Here, we analyzed ILC3 from mucosal and systemic lymphoid tissues in chronically SIV-infected macaques and uninfected controls. ILC3 cells were defined and identified in macaque lymphoid tissues as non-T, non-B (lineage-negative), c-Kit(+)IL-7Rα(+) (CD117(+)CD127(+)) cells. These ILC3 cells highly expressed CD90 (∼ 63%) and aryl hydrocarbon receptor and produced IL-17 (∼ 63%), IL-22 (∼ 36%), and TNF-α (∼ 72%) but did not coexpress CD4 or NK cell markers. The intestinal ILC3 cell loss correlated with the reduction of total CD4(+) T cells and T helper (Th)17 and Th22 cells in the gut during SIV infection (P < 0.001). Notably, ILC3 could be induced to undergo apoptosis by microbial products through the TLR2 (lipoteichoic acid) and/or TLR4 (LPS) pathway. These findings indicated that persistent microbial translocation may result in loss of ILC3 in lymphoid tissues in SIV-infected macaques, further contributing to the HIV-induced impairment of gut-associated lymphoid tissue structure and function, especially in mucosal tissues.

Along the Axis between Type 1 and Type 2 Immunity; Principles Conserved in Evolution from Fish to Mammals

A phenomenon already discovered more than 25 years ago is the possibility of naïve helper T cells to polarize into TH1 or TH2 populations. In a simplified model, these polarizations occur at opposite ends of an "immune 1-2 axis" (i1-i2 axis) of possible conditions. Additional polarizations of helper/regulatory T cells were discovered later, such as for example TH17 and Treg phenotypes; although these polarizations are not selected by the axis-end conditions, they are affected by i1-i2 axis factors, and may retain more potential for change than the relatively stable TH1 and TH2 phenotypes. I1-i2 axis conditions are also relevant for polarizations of other types of leukocytes, such as for example macrophages. Tissue milieus with "type 1 immunity" ("i1") are biased towards cell-mediated cytotoxicity, while the term "type 2 immunity" ("i2") is used for a variety of conditions which have in common that they inhibit type 1 immunity. The immune milieus of some tissues, like the gills in fish and the uterus in pregnant mammals, probably are skewed towards type 2 immunity. An i2-skewed milieu is also created by many tumors, which allows them to escape eradication by type 1 immunity. In this review we compare a number of i1-i2 axis factors between fish and mammals, and conclude that several principles of the i1-i2 axis system seem to be ancient and shared between all classes of jawed vertebrates. Furthermore, the present study is the first to identify a canonical TH2 cytokine locus in a bony fish, namely spotted gar, in the sense that it includes RAD50 and bona fide genes of both IL-4/13 and IL-3/ IL-5/GM-CSF families.

Selective programming of CCR10(+) innate lymphoid cells in skin-draining lymph nodes for cutaneous homeostatic regulation

Innate lymphoid cells (ILCs) 'preferentially' localize into barrier tissues, where they function in tissue protection but can also contribute to inflammatory diseases. The mechanisms that regulate the establishment of ILCs in barrier tissues are poorly understood. Here we found that under steady-state conditions, ILCs in skin-draining lymph nodes (sLNs) were continuously activated to acquire regulatory properties and high expression of the chemokine receptor CCR10 for localization into the skin. CCR10(+) ILCs promoted the homeostasis of skin-resident T cells and, reciprocally, their establishment in the skin required T cell-regulated homeostatic environments. CD207(+) dendritic cells expressing the transcription factor Foxn1 were required for the proper generation of CCR10(+) ILCs. These observations reveal mechanisms that underlie the specific programming and priming of skin-homing CCR10(+) ILCs in the sLNs.

Nutritional control of immunity: Balancing the metabolic requirements with an appropriate immune function

The immune system is a highly integrated network of cells sensitive to a number of environmental factors. Interestingly, recent years have seen a dramatic increase in our understanding of how diet makes a crucial contribution to human health, affecting the immune system, secretion of adipocytokines and metabolic pathways. Recent experimental evidence indicates that diet and its components are able to profoundly influence immune responses, thus affecting the development of inflammatory and autoimmune diseases. This review aims to discuss some of the main topics concerning the impact of nutrients and their relative composition on immune cell development and function that may be particularly important for regulating the balance between inflammatory and tolerogenic processes. We also highlight the effects of diet on commensal bacteria and how changes in the composition of the microbiota alter intestinal and systemic immune homeostasis. Finally, we summarize the effects of dietary compounds on epigenetic mechanisms involved in the regulation of several immune related genes.

Type-2 innate lymphoid cells in asthma and allergy

Type-2 innate lymphoid cells (ILC2) belong to an expanding family of innate lymphocytes that provide a potent source of immune effector cytokines at the initiation of immune responses. ILC2 arise, under the control of the transcription factors RORα and GATA3, from lymphoid progenitors in the bone marrow, to secrete type-2 cytokines including IL-5 and IL-13. Using experimental models, ILC2 have been implicated in allergic diseases, such as asthma and atopic dermatitis, but also in metabolic homeostasis. Furthermore, recent reports have indicated that ILC2 not only play roles at the initiation of type-2 immunity but can also contribute to chronic pathology, such as fibrosis, and can impact on the priming of the adaptive T-cell response. The identification of ILC2 in patients with allergic dermatitis and allergic rhinitis indicates that these cells may represent new therapeutic targets.

The messenger between worlds: the regulation of innate and adaptive type-2 immunity by innate lymphoid cells

Although type-2 immune responses evolved primarily to defend against extracellular helminths, in part through the co-opting of tissue repair and remodeling mechanisms, they are often inappropriately directed towards relatively innocuous allergens resulting in conditions including asthma, allergic rhinitis, food allergy, and atopic dermatitis. The recent discovery of group 2 innate lymphoid cells (ILC2) has increased our understanding of the initiation of these responses and the roles played by CD4(+) T helper (Th) 2 cells in their modulation. This review focuses on the important messenger role of ILC2 in translating epithelial-derived alarmins into downstream adaptive type-2 responses via dendritic cells and T cells, with special emphasis on their roles in allergic disease.

Type 3 innate lymphoid cells producing IL-17 and IL-22 are expanded in the gut, in the peripheral blood, synovial fluid and bone marrow of patients with ankylosing spondylitis

BACKGROUND

The aim of the study was to better characterise the immunological origin and the behaviour of interleukin (IL)-23-responsive innate lymphoid cells (ILCs) in the gut, synovial fluid (SF) and bone marrow (BM) of patients with ankylosing spondylitis (AS).

METHODS

ILC1, ILC2 and ILC3 cells were determined and characterised by confocal microscopy and flow cytometry in ileal and BM biopsies, in peripheral blood (PB) and SF mononuclear cells obtained from patients with AS and controls. Mucosal vascular addressin cell adhesion molecule 1 (MADCAM-1), IL-7, IL-15 and aggregates of lymphoid tissue inducer cells (LTi) were evaluated by immunohistochemistry. The in vitro ability of epithelial cells in driving the differentiation of ILC3 and the effect of tumour necrosis factor inhibitors (TNFi) on the frequency of ILC3 and the expression of MADCAM1 were also assessed.

RESULTS

ILC3 characterised as Lyn(-)RORc(-)Tbet(+) NKp44(+) cells were significantly expanded in the gut, SF and BM of patients with AS compared with controls, produced high levels of IL-17 and IL-22 and expressed α4β7. MADcAM1 was overexpressed in BM and ileal high endothelial venules. IL-7 was significantly increased in AS gut, especially in the context of Paneth cells, and accompanied by the presence of aggregates of c-kit/IL-7R(+) cells (LTi). In in vitro experiments, epithelial cells from patients with AS actively induced differentiation of ILC3 from LTi. TNFi efficacy was accompanied by a significant decrease in the percentage of intestinal and circulating ILC3 and in the expression of MADCAM1.

CONCLUSIONS

Gut-derived IL-17(+) and IL-22(+)ILC3 are expanded in the peripheral blood, SF and inflamed BM of patients with AS, suggesting the presence of an active homing axis between the gut and the inflamed sacroiliac joints.

Group 2 innate lymphoid cells in disease

Group 2 innate lymphoid cells (ILC2) are now recognized as an important innate source of type-2 effector cytokines. Although initially associated with mucosal tissues, it is clear that ILC2 are present in diverse anatomical locations. The function of ILC2 at these sites is equally varied, and although ILC2 represent a relatively minor population, they are fundamentally important regulators of innate and adaptive immune processes. As such, there is much interest to understand the role of ILC2 in diseases with a type-2 inflammatory component. This review explores the known roles of ILC2 in disease, and the diseases that show associations or other strong evidence for the involvement of ILC2.

Environmental enteric dysfunction: an overview

BACKGROUND

Environmental enteric dysfunction (EED) refers to an incompletely defined syndrome of inflammation, reduced absorptive capacity, and reduced barrier function in the small intestine. It is widespread among children and adults in low- and middle-income countries. Understanding of EED and its possible consequences for health is currently limited.

OBJECTIVE

A narrative review of the current understanding of EED: epidemiology, pathogenesis, therapies, and relevance to child health.

METHODS

Searches for key papers and ongoing trials were conducted using PUBMED 1966-June 2014; ClinicalTrials.gov; the WHO Clinical Trials Registry; the Cochrane Library; hand searches of the references of retrieved literature; discussions with experts; and personal experience from the field.

RESULTS

EED is established during infancy and is associated with poor sanitation, certain gut infections, and micronutrient deficiencies. Helicobacter pylori infection, small intestinal bacterial overgrowth (SIBO), abnormal gut microbiota, undernutrition, and toxins may all play a role. EED is usually asymptomatic, but it is important due to its association with stunting. Diagnosis is frequently by the dual sugar absorption test, although other biomarkers are emerging. EED may partly explain the reduced efficacy of oral vaccines in low- and middle-income countries and the increased risk of serious infection seen in children with undernutrition.

CONCLUSIONS

Despite its potentially significant impacts, it is currently unclear exactly what causes EED and how it can be treated or prevented. Ongoing trials involve nutritional supplements, water and sanitation interventions, and immunomodulators. Further research is needed to better understand this condition, which is of likely crucial importance for child health and development in low- and middle-income settings.

Breast Milk and Solid Food Shaping Intestinal Immunity

After birth, the intestinal immune system enters a critical developmental stage, in which tolerogenic and pro-inflammatory cells emerge to contribute to the overall health of the host. The neonatal health is continuously challenged by microbial colonization and food intake, first in the form of breast milk or formula and later in the form of solid food. The microbiota and dietary compounds shape the newborn immune system, which acquires the ability to induce tolerance against innocuous antigens or induce pro-inflammatory immune responses against pathogens. Disruption of these homeostatic mechanisms might lead to undesired immune reactions, such as food allergies and inflammatory bowel disease. Hence, a proper education and maturation of the intestinal immune system is likely important to maintain life-long intestinal homeostasis. In this review, the most recent literature regarding the effects of dietary compounds in the development of the intestinal immune system are discussed.

Maintenance of Immune Homeostasis through ILC/T Cell Interactions

Innate lymphoid cells (ILCs) have emerged as a new family of immune cells with crucial functions in innate and adaptive immunity. ILC subsets mirror the cytokine and transcriptional profile of CD4⁺ T helper (T_H) cell subsets. Hence, group 1 (ILC1), group 2 (ILC2), and group 3 (ILC3) ILCs can be distinguished by the production of T_H1, T_H2, and T_H17-type cytokines, respectively. Cytokine release by ILCs not only shapes early innate immunity but can also orchestrate T_H immune responses to microbial or allergen exposure. Recent studies have identified an unexpected effector function of ILCs as antigen presenting cells. Both ILC2s and ILC3s are able to process and present foreign antigens (Ags) via major histocompatibility complex class II, and to induce cognate CD4⁺ T cell responses. In addition, Ag-stimulated T cells promote ILC activation and effector functions indicating a reciprocal interaction between the adaptive and innate immune system. A fundamental puzzle in ILC function is how ILC/T cell interactions promote host protection and prevent autoimmune diseases. Furthermore, the way in which microenvironmental and inflammatory signals determine the outcome of ILC/T cell immune responses in various tissues is not yet understood. This review focuses on recent advances in understanding the mechanisms that coordinate the collaboration between ILCs and T cells under homeostatic and inflammatory conditions. We also discuss the potential roles of T cells and other immune cells to regulate ILC functions and to maintain homeostasis in mucosal tissues.

T cell metabolism drives immunity

Buck et al. discuss the role of lymphocyte metabolism on immune cell development and function.

Lymphocytes must adapt to a wide array of environmental stressors as part of their normal development, during which they undergo a dramatic metabolic remodeling process. Research in this area has yielded surprising findings on the roles of diverse metabolic pathways and metabolites, which have been found to regulate lymphocyte signaling and influence differentiation, function and fate. In this review, we integrate the latest findings in the field to provide an up-to-date resource on lymphocyte metabolism.

Group 3 innate lymphoid cells (ILC3s): Origin, differentiation, and plasticity in humans and mice

Since their discovery, innate lymphoid cells (ILCs) have been the subject of intense research. As their name implies, ILCs are innate cells of lymphoid origin, and can be grouped into subsets based on their cytotoxic activity, cytokine profile, and the transcriptional requirements during ILC differentiation. The main ILC groups are "killer" ILCs, comprising NK cells, and "helper-like" ILCs (including ILC1s, ILC2s, and ILC3s). This review examines the origin, differentiation stages, and plasticity of murine and human ILC3s. ILC3s express the retinoic acid receptor (RAR) related orphan receptor RORγt and the signature cytokines IL-22 and IL-17. Fetal ILC3s or lymphoid tissue inducer cells are required for lymphoid organogenesis, while postnatally developing ILC3s are important for the generation of intestinal cryptopatches and isolated lymphoid follicles as well as for the defence against pathogens and epithelial homeostasis. Here, we discuss the transcription factors and exogenous signals (including cytokines, nutrients and cell-to-cell interaction) that drive ILC3 lineage commitment and acquisition of their distinctive effector program.

Interleukin-12 and -23 Control Plasticity of CD127(+) Group 1 and Group 3 Innate Lymphoid Cells in the Intestinal Lamina Propria

Human group 1 ILCs consist of at least three phenotypically distinct subsets, including NK cells, CD127(+) ILC1, and intraepithelial CD103(+) ILC1. In inflamed intestinal tissues from Crohn's disease patients, numbers of CD127(+) ILC1 increased at the cost of ILC3. Here we found that differentiation of ILC3 to CD127(+) ILC1 is reversible in vitro and in vivo. CD127(+) ILC1 differentiated to ILC3 in the presence of interleukin-2 (IL-2), IL-23, and IL-1β dependent on the transcription factor RORγt, and this process was enhanced in the presence of retinoic acid. Furthermore, we observed in resection specimen from Crohn's disease patients a higher proportion of CD14(+) dendritic cells (DC), which in vitro promoted polarization from ILC3 to CD127(+) ILC1. In contrast, CD14(-) DCs promoted differentiation from CD127(+) ILC1 toward ILC3. These observations suggest that environmental cues determine the composition, function, and phenotype of CD127(+) ILC1 and ILC3 in the gut.

MUCOSAL IMMUNOLOGY. The microbiota regulates type 2 immunity through RORγt⁺ T cells

Changes to the symbiotic microbiota early in life, or the absence of it, can lead to exacerbated type 2 immunity and allergic inflammations. Although it is unclear how the microbiota regulates type 2 immunity, it is a strong inducer of proinflammatory T helper 17 (T(H)17) cells and regulatory T cells (T(regs)) in the intestine. Here, we report that microbiota-induced T(regs) express the nuclear hormone receptor RORγt and differentiate along a pathway that also leads to T(H)17 cells. In the absence of RORγt(+) T(regs), T(H)2-driven defense against helminths is more efficient, whereas T(H)2-associated pathology is exacerbated. Thus, the microbiota regulates type 2 responses through the induction of type 3 RORγt(+) T(regs) and T(H)17 cells and acts as a key factor in balancing immune responses at mucosal surfaces.

Innate lymphocyte cells in asthma phenotypes

T helper type 2 (T_H2) cells were previously thought to be the main initiating effector cell type in asthma; however, exaggerated T_H2 cell activities alone were insufficient to explain all aspects of asthma. Asthma is a heterogeneous syndrome comprising different phenotypes that are characterized by their different clinical features, treatment responses, and inflammation patterns. The most-studied subgroups of asthma include T_H2-associated early-onset allergic asthma, late-onset persistent eosinophilic asthma, virus-induced asthma, obesity-related asthma, and neutrophilic asthma. The recent discovery of human innate lymphoid cells capable of rapidly producing large amounts of cytokines upon activation and the mouse data pointing to an essential role for these cells in asthma models have emphasized the important role of the innate immune system in asthma and have provided a new means of better understanding asthma mechanisms and differentiating its phenotypes.

Vitamins A and D have antagonistic effects on expression of effector cytokines and gut-homing integrin in human innate lymphoid cells

BACKGROUND

Retinoic acid (RA), the main biologically active metabolite of vitamin A, is known to promote gut homing of lymphocytes, as well as various regulatory and effector immune responses. In contrast, the active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25D3), is predominantly immunosuppressive. Little is known about the direct effects of these vitamins on the recently identified innate lymphoid cells (ILCs).

OBJECTIVE

We sought to characterize the effects of RA and 1,25D3 on human ILCs.

METHODS

Peripheral blood mononuclear cells were isolated from 27 non-selected blood donor buffy coats, and ILCs were sorted by FACS. ILC1, ILC2, and ILC3 cells were cultured for 5 days with RA, 1,25D3, and various cytokines known to activate ILCs (IL-2, IL-7, IL-12, thymic stromal lymphopoietin (TSLP), IL-25, and IL-33). Cytokines produced by ILCs were measured in culture supernatants, and surface receptor expression was analysed by flow cytometry.

RESULTS

Retinoic acid acted synergistically with IL-2 and other activating cytokines to induce expression of the gut-homing integrin α4β7 in ILCs, as well as production of IL-5 and IL-13 in ILC2 cells, and IFN-γ in ILC1 and ILC3 cells. Expression of integrin α4β7 and cytokine production in ILCs stimulated with RA + IL-2 was increased at least fourfold as compared to ILCs cultured with RA or IL-2 alone. In contrast, RA completely inhibited the IL-2-induced expression of cutaneous lymphocyte antigen (CLA) in ILCs. Moreover, addition of 1,25D3 to ILCs cultured with RA + IL-2 inhibited cytokine production and expression of integrin α4β7 by at least 30%.

CONCLUSIONS

Retinoic acid and 1,25D3 have antagonistic effects on the expression of effector cytokines and gut-homing integrin by human ILCs. The balance between these vitamins may be an important factor in the functioning of ILCs and the diseases in which ILCs are implicated, such as allergic inflammation.

Retinoic Acid Differentially Regulates the Migration of Innate Lymphoid Cell Subsets to the Gut

Distinct groups of innate lymphoid cells (ILCs) such as ILC1, ILC2, and ILC3 populate the intestine, but how these ILCs develop tissue tropism for this organ is unclear. We report that prior to migration to the intestine ILCs first undergo a "switch" in their expression of homing receptors from lymphoid to gut homing receptors. This process is regulated by mucosal dendritic cells and the gut-specific tissue factor retinoic acid (RA). This change in homing receptors is required for long-term population and effector function of ILCs in the intestine. Only ILC1 and ILC3, but not ILC2, undergo the RA-dependent homing receptor switch in gut-associated lymphoid tissues. In contrast, ILC2 acquire gut homing receptors in a largely RA-independent manner during their development in the bone marrow and can migrate directly to the intestine. Thus, distinct programs regulate the migration of ILC subsets to the intestine for regulation of innate immunity.

The impact of diet on asthma and allergic diseases

The incidence of allergic diseases is increasing, both in developed and developing countries, concomitantly with the rise in living standards and the adoption of a 'western lifestyle'. For two decades, the hygiene hypothesis - which proposes that the lack of early childhood exposure to infectious agents increases susceptibility to allergic diseases in later life - provided the conceptual framework for unravelling the mechanisms that could account for the increased incidence of allergic diseases. In this Review, we discuss recent evidence that highlights the role of diet as a key factor influencing immune homeostasis and the development of allergic diseases through a complex interplay between nutrients, their metabolites and immune cell populations. Although further investigations are still required to understand these complex relationships, recent data have established a possible connection between metabolic homeostasis and allergic diseases.

Innate lymphoid cells in the initiation, regulation and resolution of inflammation

A previously unappreciated cell type of the innate immune system, termed innate lymphoid cells (ILCs), has been characterized in mice and humans and found to influence the induction, regulation and resolution of inflammation. ILCs have an important role in these processes in mouse models of infection, inflammation and tissue repair. Further, disease-association studies in defined patient populations have identified significant alterations in ILC responses, suggesting a potential role for these cell populations in human health and disease. In this review we discuss the emerging family of ILCs, the role of ILCs in inflammation, and how current or novel therapeutic strategies could be used to selectively modulate ILC responses and limit chronic inflammatory diseases.

Transcriptional regulation of innate lymphoid cell fate

Innate lymphoid cells (ILCs) are a recently described family of lymphoid effector cells that have important roles in immune defence, inflammation and tissue remodelling. It has been proposed that ILCs represent 'innate' homologues of differentiated effector T cells, and they have been categorized into three groups — namely, ILC1s, ILC2s and ILC3s — on the basis of their expression of cytokines and transcription factors that are typically associated with T helper 1 (T(H)1)-, T(H)2- and T(H)17-type immune responses, respectively. Indeed, remarkable similarity is seen between the specific transcription factors required for the development and diversification of different ILC groups and those that drive effector T cell differentiation. The recent identification of dedicated ILC precursors has provided a view of the mechanisms that control this first essential stage of ILC development. Here, we discuss the transcriptional mechanisms that regulate ILC development and diversification into distinct effector subsets with key roles in immunity and tissue homeostasis. We further caution against the current distinction between 'helper' versus 'killer' subsets in the evolving area of ILC nomenclature.

The development of innate lymphoid cells requires TOX-dependent generation of a common innate lymphoid cell progenitor

Diverse innate lymphoid cell (ILC) subtypes have been defined on the basis of effector function and transcription factor expression. ILCs derive from common lymphoid progenitors, although the transcriptional pathways that lead to ILC-lineage specification remain poorly characterized. Here we found that the transcriptional regulator TOX was required for the in vivo differentiation of common lymphoid progenitors into ILC lineage-restricted cells. In vitro modeling demonstrated that TOX deficiency resulted in early defects in the survival or proliferation of progenitor cells, as well as ILC differentiation at a later stage. In addition, comparative transcriptome analysis of bone marrow progenitors revealed that TOX-deficient cells failed to upregulate many genes of the ILC program, including genes that are targets of Notch, which indicated that TOX is a key determinant of early specification to the ILC lineage.

IL-2 is a critical regulator of group 2 innate lymphoid cell function during pulmonary inflammation

BACKGROUND

Group 2 innate lymphoid cells (ILC2) have been implicated in the pathogenesis of allergic lung diseases. However, the upstream signals that regulate ILC2 function during pulmonary inflammation remain poorly understood. ILC2s have been shown to respond to exogenous IL-2, but the importance of endogenous IL-2 in ILC2 function in vivo remains unclear.

OBJECTIVE

We sought to understand the role of IL-2 in the regulation of ILC2 function in the lung.

METHODS

We used histology, flow cytometry, immunohistochemistry, ELISA, and quantitative PCR with knockout and reporter mice to dissect pulmonary ILC2 function in vivo. We examined the role of ILC2s in eosinophilic crystalline pneumonia, an idiopathic type 2 inflammatory lung condition of mice, and the effect of IL-2 deficiency on this disease. We determined the effect of IL-2 administration on pulmonary ILC2 numbers and function in mice in the steady state and after challenge with IL-33.

RESULTS

We discovered an unexpected role for innate cell-derived IL-2 as a major cofactor of ILC2 function during pulmonary inflammation. Specifically, we found that IL-2 was essential for the development of eosinophilic crystalline pneumonia, a type 2 disease characterized by increased numbers of activated ILC2s. We show that IL-2 signaling serves 2 distinct functions in lung ILC2s, namely promoting cell survival/proliferation and serving as a cofactor for the production of type 2 cytokines. We further demonstrate that group 3 innate lymphoid cells are an innate immune source of IL-2 in the lung.

CONCLUSION

Innate cell-derived IL-2 is a critical cofactor in regulating ILC2 function in pulmonary type 2 pathology.

Innate lymphoid cells. Innate lymphoid cells: a new paradigm in immunology

Innate lymphoid cells (ILCs) are a growing family of immune cells that mirror the phenotypes and functions of T cells. However, in contrast to T cells, ILCs do not express acquired antigen receptors or undergo clonal selection and expansion when stimulated. Instead, ILCs react promptly to signals from infected or injured tissues and produce an array of secreted proteins termed cytokines that direct the developing immune response into one that is adapted to the original insult. The complex cross-talk between microenvironment, ILCs, and adaptive immunity remains to be fully deciphered. Only by understanding these complex regulatory networks can the power of ILCs be controlled or unleashed in order to regulate or enhance immune responses in disease prevention and therapy.

Vitamin A-Deficient Hosts Become Nonsymptomatic Reservoirs of Escherichia coli-Like Enteric Infections

Vitamin A deficiency (A(-)) remains a public health concern in developing countries and is associated with increased susceptibility to infection. Citrobacter rodentium was used to model human Escherichia coli infections. A(-) mice developed a severe and lethal (40%) infection. Vitamin A-sufficient (A(+)) mice survived and cleared the infection by day 25. Retinoic acid treatment of A(-) mice at the peak of the infection eliminated C. rodentium within 16 days. Inflammation levels were not different between A(+) and A(-) mouse colons, although the A(-) mice were still infected at day 37. Increased mortality of A(-) mice was not due to systemic cytokine production, an inability to clear systemic C. rodentium, or increased pathogenicity. Instead, A(-) mice developed a severe gut infection with most of the A(-) mice surviving and resolving inflammation but not eliminating the infection. Improvements in vitamin A status might decrease susceptibility to enteric pathogens and prevent potential carriers from spreading infection to susceptible populations.

Seeing through the dark: New insights into the immune regulatory functions of vitamin A

The importance of vitamin A for host defense is undeniable and the study of its mechanisms is paramount. Of the estimated 250 million preschool children who are vitamin A-deficient (VAD), 10% will die from their increased susceptibility to infectious disease. Vitamin A supplementation was established in the 1980s as one of the most successful interventions in the developing world. Understanding how vitamin A controls immunity will help curb the mortality and morbidity associated with vitamin A deficiency and exploit the immune-enhancing capacity of vitamin A to heighten host resistance to infectious disease. The discoveries that retinoic acid (RA) imprints the homing of leukocytes to the gut and enhances the induction of regulatory T cells, highlighted a potential role for RA in mucosal tolerance. However, more recently emerging data tell of a more profound systemic impact of RA on leukocyte function and commitment. In animal models using genetic manipulation of RA signaling, we learned when and how RA controls T cell fate. Here, we review the role for RA as a critical checkpoint regulator in the differentiation of CD4(+) T cells within the immune system.

The unusual suspects--innate lymphoid cells as novel therapeutic targets in IBD

Innate lymphoid cells (ILCs) are a family of immune cells that selectively accumulate in mucosal tissues serving as sentinels at the vanguard of host protective immunity. However, they are also implicated as cellular mediators of immune-mediated diseases, most notably IBD. ILCs are subdivided into distinct lineages based on the expression of effector cytokines and master transcription factors that programme their differentiation and inflammatory behaviour. Strikingly, these subsets closely resemble CD4(+) T-cell lineages, including T helper (TH)1, TH2 and TH17 cells that are similarly implicated in immune-mediated diseases. However, ILCs that promote the maintenance of intestinal epithelial cells, mostly through production of IL-22, also exist. ILCs rapidly respond to environmental cues, including cytokines, metabolic signals and luminal bacteria. They are potent and immediate producers of key cytokines linked to IBD pathogenesis, including TNF, IL-17, IL-22 and IFN-γ. Some subsets are implicated as mediators of chronic intestinal inflammation, whereas others might provide protective functions. They are present in the gut of patients with IBD and, intriguingly, closer scrutiny of IBD susceptibility loci shows that many of these genes are either expressed by, or are intimately linked to, ILC function. Looking forward, targeting ILCs could represent a new IBD treatment paradigm.

Modulation of dendritic cell alternative activation and function by the vitamin A metabolite retinoic acid

Retinoic acid modulates the functions of IL-4 in alternatively activated DCs

The archetypal T_h2 cytokine IL-4 has previously been shown to alternatively activate murine macrophages and, more recently, dendritic cells (DCs) both in vitro and in vivo. IL-4 has also been shown to induce Aldh1a2 (aldehyde dehydrogenase 1a2) expression in murine macrophages recruited to the peritoneal cavity. However, the influence of IL-4 on DC Aldh1a2 induction in vivo has not yet been addressed. In this work, we found that DCs show enhanced aldehyde dehydrogenase enzyme activity in vivo, which led us to investigate the impact of the vitamin A metabolite all-trans retinoic acid (RA) on DC alternative activation and function. Antagonism of RA receptors reduced production of resistin-like molecule alpha by DCs responding to IL-4, while addition of exogenous RA enhanced production of this marker of alternative activation. Functionally, RA increased DC induction of CD4⁺ T-cell IL-10, while reducing CD4⁺ T-cell IL-4 and IL-13, revealing a previously unidentified role for RA in regulating the ability of alternatively activated DCs to influence T_h2 polarization.

Control of adaptive immunity by the innate immune system

Microbial infections are recognized by the innate immune system both to elicit immediate defense and to generate long-lasting adaptive immunity. To detect and respond to vastly different groups of pathogens, the innate immune system uses several recognition systems that rely on sensing common structural and functional features associated with different classes of microorganisms. These recognition systems determine microbial location, viability, replication and pathogenicity. Detection of these features by recognition pathways of the innate immune system is translated into different classes of effector responses though specialized populations of dendritic cells. Multiple mechanisms for the induction of immune responses are variations on a common design principle wherein the cells that sense infections produce one set of cytokines to induce lymphocytes to produce another set of cytokines, which in turn activate effector responses. Here we discuss these emerging principles of innate control of adaptive immunity.

NFIL3 orchestrates the emergence of common helper innate lymphoid cell precursors

Innate lymphoid cells (ILCs) are a family of effectors that originate from a common innate lymphoid cell progenitor. However, the transcriptional program that sets the identity of the ILC lineage remains elusive. Here, we show that NFIL3 is a critical regulator of the common helper-like innate lymphoid cell progenitor (CHILP). Cell-intrinsic Nfil3 ablation led to variably impaired development of fetal and adult ILC subsets. Conditional gene targeting demonstrated that NFIL3 exerted its function prior to ILC subset commitment. Accordingly, NFIL3 ablation resulted in loss of ID2(+) CHILP and PLZF(+) ILC progenitors. Nfil3 expression in lymphoid progenitors was under the control of the mesenchyme-derived hematopoietin IL-7, and NFIL3 exerted its function via direct Id2 regulation in the CHILP. Moreover, ectopic Id2 expression in Nfil3-null precursors rescued defective ILC lineage development in vivo. Our data establish NFIL3 as a key regulator of common helper-like ILC progenitors as they emerge during early lymphopoiesis.

Retinoic acid is essential for Th1 cell lineage stability and prevents transition to a Th17 cell program

CD4(+) T cells differentiate into phenotypically distinct T helper cells upon antigenic stimulation. Regulation of plasticity between these CD4(+) T-cell lineages is critical for immune homeostasis and prevention of autoimmune disease. However, the factors that regulate lineage stability are largely unknown. Here we investigate a role for retinoic acid (RA) in the regulation of lineage stability using T helper 1 (Th1) cells, traditionally considered the most phenotypically stable Th subset. We found that RA, through its receptor RARα, sustains stable expression of Th1 lineage specifying genes, as well as repressing genes that instruct Th17-cell fate. RA signaling is essential for limiting Th1-cell conversion into Th17 effectors and for preventing pathogenic Th17 responses in vivo. Our study identifies RA-RARα as a key component of the regulatory network governing maintenance and plasticity of Th1-cell fate and defines an additional pathway for the development of Th17 cells.

Transcriptional programs define molecular characteristics of innate lymphoid cell classes and subsets

The recognized diversity of innate lymphoid cells (ILCs) is rapidly expanding. Three ILC classes have emerged, ILC1, ILC2 and ILC3, with ILC1 and ILC3 including several subsets. The classification of some subsets is unclear, and it remains controversial whether natural killer (NK) cells and ILC1 cells are distinct cell types. To address these issues, we analyzed gene expression in ILCs and NK cells from mouse small intestine, spleen and liver, as part of the Immunological Genome Project. The results showed unique gene-expression patterns for some ILCs and overlapping patterns for ILC1 cells and NK cells, whereas other ILC subsets remained indistinguishable. We identified a transcriptional program shared by small intestine ILCs and a core ILC signature. We revealed and discuss transcripts that suggest previously unknown functions and developmental paths for ILCs.

Feeding immunity: skepticism, delicacies and delights

Immunologists studying the relationship between nutrition and immunological function face many challenges. We discuss here some of the historical skepticism with which nutritional research has often been faced and the complexities that need to be overcome in order to provide meaningful mechanistic insights.

Mucosal Immune Development in Early Life: Setting the Stage

Our environment poses a constant threat to our health. To survive, all organisms must be able to discriminate between good (food ingredients and microbes that help digest our food) and bad (pathogenic microbes, viruses and toxins). In vertebrates, discrimination between beneficial and harmful antigens mainly occurs at the mucosal surfaces of the respiratory, digestive, urinary and genital tract. Here, an extensive network of cells and organs form the basis of what we have come to know as the mucosal immune system. The mucosal immune system is composed of a single epithelial cell layer protected by a mucus layer. Different immune cells monitor the baso-lateral side of the epithelial cells and dispersed secondary lymphoid organs, such as Peyer’s patches and isolated lymphoid follicles are equipped with immune cells able to mount appropriate and specific responses. This review will focus on the current knowledge on host, dietary and bacterial-derived factors that shape the mucosal immune system before and after birth. We will discuss current knowledge on fetal immunity (both responsiveness and lymphoid organ development) as well as the impact of diet and microbial colonization on neonatal immunity and disease susceptibility. Lastly, inflammatory bowel disease will be discussed as an example of how the composition of the microbiota might predispose to disease later in life. A fundamental understanding of the mechanisms involved in mucosal immune development and tolerance will aid nutritional intervention strategies to improve health in neonatal and adult life.

Interleukin-22: immunobiology and pathology

Interleukin-22 (IL-22) is a recently described IL-10 family cytokine that is produced by T helper (Th) 17 cells, γδ T cells, NKT cells, and newly described innate lymphoid cells (ILCs). Knowledge of IL-22 biology has evolved rapidly since its discovery in 2000, and a role for IL-22 has been identified in numerous tissues, including the intestines, lung, liver, kidney, thymus, pancreas, and skin. IL-22 primarily targets nonhematopoietic epithelial and stromal cells, where it can promote proliferation and play a role in tissue regeneration. In addition, IL-22 regulates host defense at barrier surfaces. However, IL-22 has also been linked to several conditions involving inflammatory tissue pathology. In this review, we assess the current understanding of this cytokine, including its physiologic and pathologic effects on epithelial cell function.

Transcriptional regulation of innate and adaptive lymphocyte lineages

The lymphocyte family has expanded significantly in recent years to include not only the adaptive lymphocytes (T cells, B cells) and NK cells, but also several additional innate lymphoid cell (ILC) types. ILCs lack clonally distributed antigen receptors characteristic of adaptive lymphocytes and instead respond exclusively to signaling via germline-encoded receptors. ILCs resemble T cells more closely than any other leukocyte lineage at the transcriptome level and express many elements of the core T cell transcriptional program, including Notch, Gata3, Tcf7, and Bcl11b. We present our current understanding of the shared and distinct transcriptional regulatory mechanisms involved in the development of adaptive T lymphocytes and closely related ILCs. We discuss the possibility that a core set of transcriptional regulators common to ILCs and T cells establish enhancers that enable implementation of closely aligned effector pathways. Studies of the transcriptional regulation of lymphopoiesis will support the development of novel therapeutic approaches to correct early lymphoid developmental defects and aberrant lymphocyte function.

Immunity to gastrointestinal nematodes: mechanisms and myths

Immune responses to gastrointestinal nematodes have been studied extensively for over 80 years and intensively investigated over the last 30–40 years. The use of laboratory models has led to the discovery of new mechanisms of protective immunity and made major contributions to our fundamental understanding of both innate and adaptive responses. In addition to host protection, it is clear that immunoregulatory processes are common in infected individuals and resistance often operates alongside modulation of immunity. This review aims to discuss the recent discoveries in both host protection and immunoregulation against gastrointestinal nematodes, placing the data in context of the specific life cycles imposed by the different parasites studied and the future challenges of considering the mucosal/immune axis to encompass host, parasite, and microbiome in its widest sense.

Innate immunity in the lung regulates the development of asthma

The lung, while functioning as a gas exchange organ, encounters a large array of environmental factors, including particulate matter, toxins, reactive oxygen species, chemicals, allergens, and infectious microbes. To rapidly respond to and counteract these elements, a number of innate immune mechanisms have evolved that can lead to lung inflammation and asthma, which is the focus of this review. These innate mechanisms include a role for two incompletely understood cell types, invariant natural killer T (iNKT) cells and innate lymphoid cells (ILCs), which together produce a wide range of cytokines, including interleukin-4 (IL-4), IL-5, IL-13, interferon-γ, IL-17, and IL-22, independently of adaptive immunity and conventional antigens. The specific roles of iNKT cells and ILCs in immunity are still being defined, but both cell types appear to play important roles in the lungs, particularly in asthma. As we gain a better understanding of these innate cell types, we will acquire great insight into the mechanisms by which allergic and non-allergic asthma phenotypes develop.

Innate lymphoid cells: new insights into function and development

Here, we illustrate the complexity of ILC subsets, we discuss novel functions, focusing on emerging ILCs crosstalk with other immune cells and the microbiota. Furthermore, we highlight recent insights into the development of ILCs, the common pathways they share as well as points of divergence between ILC groups and subsets.

The biology of innate lymphoid cells

The innate immune system is composed of a diverse array of evolutionarily ancient haematopoietic cell types, including dendritic cells, monocytes, macrophages and granulocytes. These cell populations collaborate with each other, with the adaptive immune system and with non-haematopoietic cells to promote immunity, inflammation and tissue repair. Innate lymphoid cells are the most recently identified constituents of the innate immune system and have been the focus of intense investigation over the past five years. We summarize the studies that formally identified innate lymphoid cells and highlight their emerging roles in controlling tissue homeostasis in the context of infection, chronic inflammation, metabolic disease and cancer.