Differentiation of type 1 ILCs from a common progenitor to all helper-like innate lymphoid cell lineages

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.

Inflammatory group 2 innate lymphoid cells

Group 2 innate lymphoid cells (ILC2 cells) are able to produce type 2 cytokines and to mediate type 2 immune protection and tissue homeostasis. ILC2 cells have often been considered to be a single set of cells that respond to IL-33 and/or IL-25. Recent evidence now indicates that ILC2 cells can be grouped into two distinct subsets: homeostatic or natural ILC2s (nILC2 cells); and inflammatory ILC2 cells (iILC2 cells). nILC2 cells reside in barrier tissues and primarily respond to IL-33. They play critical roles not only in immune protection but also in tissue repair and beige fat biogenesis. iILC2 cells are not present in peripheral tissues in the steady state but can be elicited at many sites by helminth infection or IL-25 treatment. IL-25-elicited ilLC2 cells act as transient ILC progenitors with multipotency. They can be mobilized by distinct types of infections to develop into nILC2-like or ILC3-like cells, functioning in corresponding immune responses. The demonstration of the existence of iILC2 cells adds to our understanding of the complexity of ILC2 biology and makes necessary an analysis of the relationship between nILC2 cells and iILC2 cells.

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.

Innate Immune Defenses Mediated by Two ILC Subsets Are Critical for Protection against Acute Clostridium difficile Infection

Infection with the opportunistic enteric pathogen Clostridium difficile is an increasingly common clinical complication that follows antibiotic treatment-induced gut microbiota perturbation. Innate lymphoid cells (ILCs) are early responders to enteric pathogens; however, their role during C. difficile infection is undefined. To identify immune pathways that mediate recovery from C. difficile infection, we challenged C57BL/6, Rag1(-/-) (which lack T and B cells), and Rag2(-/-)Il2rg(-/-) (Ragγc(-/-)) mice (which additionally lack ILCs) with C. difficile. In contrast to Rag1(-/-) mice, ILC-deficient Ragγc(-/-) mice rapidly succumbed to infection. Rag1(-/-) but not Ragγc(-/-) mice upregulate expression of ILC1- or ILC3-associated proteins following C. difficile infection. Protection against infection was restored by transferring ILCs into Ragγc(-/-) mice. While ILC3s made a minor contribution to resistance, loss of IFN-γ or T-bet-expressing ILC1s in Rag1(-/-) mice increased susceptibility to C. difficile. These data demonstrate a critical role for ILC1s in defense against C. difficile.

Transcription Factor T-Bet in Atlantic Salmon: Characterization and Gene Expression in Mucosal Tissues during Aeromonas Salmonicida Infection

The T-box transcription factor T-bet is expressed in a number of hematopoietic cell types in mammals and plays an essential role in the lineage determination of Th1 T-helper cells and is considered as an essential feature for both innate and adaptive immune responses in higher vertebrates. In the present study, we have identified and characterized the full-length Atlantic salmon T-bet cDNA (3502 bp). The putative primary structure of the polypeptide deduced from the cDNA sequence contained 612 aa, which possessed a T-box DNA binding domain. Phylogenetic study and gene synteny revealed it is as a homolog to mammalian T-bet. Quantitative PCR analysis of different tissues in healthy fish showed that salmon T-bet gene was highly expressed in spleen, followed by head kidney, and was expressed in intestine, skin, and liver at lower levels. Moreover, the time-dependent expression profile of T-bet, interferon gamma (IFNγ), interleukin-22 (IL-22), and natural killer enhancement factor in mucosal tissues during water-borne infection with live Aeromonas salmonicida, indicated the involvement of T-bet in mucosal immune response in Atlantic salmon.

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.

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.

Innate lymphoid cells: the new kids on the block

PURPOSE OF REVIEW

The purpose of this article is to review recent advances in our understanding of innate lymphoid cell function and to speculate on how these cells may become activated and influence the immune response to allogeneic tissues and cells following transplantation.

RECENT FINDINGS

Innate lymphoid cells encompass several novel cell types whose wide-ranging roles in the immune system are only now being uncovered. Through cytokine production, cross-talk with both haematopoietic and nonhaematopoietic populations and antigen presentation to T cells, these cells have been shown to be key regulators in maintaining tissue integrity, as well as initiating and then sustaining immune responses.

SUMMARY

It is now clear that innate lymphoid cells markedly contribute to immune responses and tissue repair in a number of disease contexts. Although experimental and clinical data on the behaviour of these cells following transplantation are scant, it is highly likely that innate lymphoid cells will perform similar functions in the alloimmune response following transplantation and therefore may be potential therapeutic targets for manipulation to prevent allograft rejection.

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.

Bone-Marrow-Resident NK Cells Prime Monocytes for Regulatory Function during Infection

Tissue-infiltrating Ly6C(hi) monocytes play diverse roles in immunity, ranging from pathogen killing to immune regulation. How and where this diversity of function is imposed remains poorly understood. Here we show that during acute gastrointestinal infection, priming of monocytes for regulatory function preceded systemic inflammation and was initiated prior to bone marrow egress. Notably, natural killer (NK) cell-derived IFN-γ promoted a regulatory program in monocyte progenitors during development. Early bone marrow NK cell activation was controlled by systemic interleukin-12 (IL-12) produced by Batf3-dependent dendritic cells (DCs) in the mucosal-associated lymphoid tissue (MALT). This work challenges the paradigm that monocyte function is dominantly imposed by local signals after tissue recruitment, and instead proposes a sequential model of differentiation in which monocytes are pre-emptively educated during development in the bone marrow to promote their tissue-specific function.

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.

T helper 2 (Th2) cell differentiation, type 2 innate lymphoid cell (ILC2) development and regulation of interleukin-4 (IL-4) and IL-13 production

Interleukin-4 (IL-4), IL-5 and IL-13, the signature cytokines that are produced during type 2 immune responses, are critical for protective immunity against infections of extracellular parasites and are responsible for asthma and many other allergic inflammatory diseases. Although many immune cell types within the myeloid lineage compartment including basophils, eosinophils and mast cells are capable of producing at least one of these cytokines, the production of these "type 2 immune response-related" cytokines by lymphoid lineages, CD4 T helper 2 (Th2) cells and type 2 innate lymphoid cells (ILC2s) in particular, are the central events during type 2 immune responses. In this review, I will focus on the signaling pathways and key molecules that determine the differentiation of naïve CD4 T cells into Th2 cells, and how the expression of Th2 cytokines, especially IL-4 and IL-13, is regulated in Th2 cells. The similarities and differences in the differentiation of Th2 cells, IL-4-producing T follicular helper (Tfh) cells and ILC2s as well as their relationships will also be discussed.

Cutting Edge: Developmental Regulation of IFN-γ Production by Mouse Neutrophil Precursor Cells

Neutrophils are an emerging cellular source of IFN-γ, a key cytokine that mediates host defense to intracellular pathogens. Production of IFN-γ by neutrophils, in contrast to lymphoid cells, is TLR- and IL-12-independent and the events associated with IFN-γ production by neutrophils are not understood. In this study, we show that mouse neutrophils express IFN-γ during their lineage development in the bone marrow niche at the promyelocyte stage independently of microbes. IFN-γ accumulates in primary neutrophilic granules and is released upon induction of degranulation. The developmental mechanism of IFN-γ production in neutrophils arms the innate immune cells prior to infection and assures the potential for rapid release of IFN-γ upon neutrophil activation, the first step during responses to many microbial infections.

The Group 3 Innate Lymphoid Cell Defect in Aryl Hydrocarbon Receptor Deficient Mice Is Associated with T Cell Hyperactivation during Intestinal Infection

Intestinal infection with the intracellular parasite Toxoplasma gondii results in the translocation of commensal bacteria to peripheral organs and the development of a T cell response specific to the microbiota. In naïve mice, the recently described RORγt+ group 3 innate lymphoid cell (ILC) population plays a critical role in promoting intestinal barrier function and limiting responses to gut-resident commensal bacteria. Given this role for group 3 ILCs, studies were performed to evaluate whether these cells might influence the immune response to mucosal infection with T. gondii. Phenotypic characterization of RORγt+ ILCs in T. gondii infected mice revealed that this population decreased following challenge but the population that remained expressed costimulatory molecules and IL-22. One factor that influences the maintenance of RORγt+ ILCs is the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor, and Ahr-/- mice have a marked defect in the lamina propria group 3 ILC population. When Ahr-/- mice were challenged with T. gondii, they lost more weight than wild type controls. This disease course in Ahr-/- animals was associated with increased T cell responses to Toxoplasma antigen and crude commensal antigen preparations. Together, these data suggest that group 3 ILCs have a role in limiting T cell activation during intestinal infection.

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.

The Dialogue of the Host-Parasite Relationship: Leishmania spp. and Trypanosoma cruzi Infection

The intracellular protozoa Leishmania spp. and Trypanosoma cruzi and the causative agents of Leishmaniasis and Chagas disease, respectively, belong to the Trypanosomatidae family. Together, these two neglected tropical diseases affect approximately 25 million people worldwide. Whether the host can control the infection or develops disease depends on the complex interaction between parasite and host. Parasite surface and secreted molecules are involved in triggering specific signaling pathways essential for parasite entry and intracellular survival. The recognition of the parasite antigens by host immune cells generates a specific immune response. Leishmania spp. and T. cruzi have a multifaceted repertoire of strategies to evade or subvert the immune system by interfering with a range of signal transduction pathways in host cells, which causes the inhibition of the protective response and contributes to their persistence in the host. The current therapeutic strategies in leishmaniasis and trypanosomiasis are very limited. Efficacy is variable, toxicity is high, and the emergence of resistance is increasingly common. In this review, we discuss the molecular basis of the host-parasite interaction of Leishmania and Trypanosoma cruzi infection and their mechanisms of subverting the immune response and how this knowledge can be used as a tool for the development of new drugs.

Cutting edge: c-Kit signaling differentially regulates type 2 innate lymphoid cell accumulation and susceptibility to central nervous system demyelination in male and female SJL mice

Multiple sclerosis preferentially affects women, and this sexual dimorphism is recapitulated in the SJL mouse model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE). In this study, we demonstrate that signaling through c-Kit exerts distinct effects on EAE susceptibility in male and female SJL mice. Previous studies in females show that Kit mutant (W/W(v)) mice are less susceptible to EAE than are wild-type mice. However, male W/W(v) mice exhibit exacerbated disease, a phenotype independent of mast cells and corresponding to a shift from a Th2- to a Th17-dominated T cell response. We demonstrate a previously undescribed deficit in c-Kit(+) type 2 innate lymphoid cells (ILC2s) in W/W(v) mice. ILC2s are also significantly reduced in EAE-susceptible wild-type females, indicating that both c-Kit signals and undefined male-specific factors are required for ILC2 function. We propose that deficiencies in Th2-promoting ILC2s remove an attenuating influence on the encephalitogenic T cell response and therefore increases disease susceptibility.

The transcription factor Bcl11b is specifically expressed in group 2 innate lymphoid cells and is essential for their development

Yu et al. demonstrate that the transcription factor Bcl11b is specifically expressed in mouse innate lymphoid progenitors committed to the ILC2 lineage and is required for their development. Bcl11b-deficient mice exhibit a complete lack of ILC2 development, which is confirmed by immune challenges with either papain treatment or influenza virus infection.

Group 2 innate lymphoid cells (ILCs), or ILC2s, are a subset of recently identified ILCs, which play important roles in innate immunity by producing type 2 effector cytokines. Several transcription factors have been found to have critical functions in the development of both ILC2s and T cells. We report here that Bcl11b, a transcription factor essential in T cell lineage commitment and maintenance, is specifically expressed in progenitors committed to the ILC2 lineage and is required for ILC2 development. The Bcl11b gene is expressed in ∼28% of ILC progenitors (ILCPs; common helper innate lymphoid progenitors or ILCPs expressing either ID2 or promyelocytic leukemia zinc finger, respectively). Both in vitro and in vivo, these Bcl11b-expressing early ILCPs generate only ILC2s. Inactivation of Bcl11b causes a complete loss of ILC2 development from hematopoietic progenitors, which is confirmed upon immune challenge with either papain administration or influenza virus infection.

Staying innate: transcription factor maintenance of innate lymphoid cell identity

Innate and adaptive lymphocytes are characterized by phenotypic and functional characteristics that result from genomic rearrangements (in the case of antigen-specific B and T cells) coupled with selective gene expression patterns that are generated in a context-dependent fashion. Cell-intrinsic expression of transcription factors (TFs) play a critical role in the regulation of gene expression that establish the distinct lymphoid subsets but also have been proposed to play an ongoing role in the maintenance of lineage-associated transcriptional signatures that comprise lymphocyte identity. This is the case for CD19(+) B cells that require Pax5 expression throughout their lifespan, as well as for diverse T-helper subsets that have specialized immune functions. Innate lymphoid cells (ILCs) comprise diverse effectors cells that differentiate under TF control and have critical roles in the early stages of immune responses. In this review, ILC development is reviewed and the requirement for persistent TF expression in the maintenance of transcriptional signatures that define ILC identity is explored.

ID'ing innate and innate-like lymphoid cells

The immune system can be divided into innate and adaptive components that differ in their rate and mode of cellular activation, with innate immune cells being the first responders to invading pathogens. Recent advances in the identification and characterization of innate lymphoid cells have revealed reiterative developmental programs that result in cells with effector fates that parallel those of adaptive lymphoid cells and are tailored to effectively eliminate a broad spectrum of pathogenic challenges. However, activation of these cells can also be associated with pathologies such as autoimmune disease. One major distinction between innate and adaptive immune system cells is the constitutive expression of ID proteins in the former and inducible expression in the latter. ID proteins function as antagonists of the E protein transcription factors that play critical roles in lymphoid specification as well as B- and T-lymphocyte development. In this review, we examine the transcriptional mechanisms controlling the development of innate lymphocytes, including natural killer cells and the recently identified innate lymphoid cells (ILC1, ILC2, and ILC3), and innate-like lymphocytes, including natural killer T cells, with an emphasis on the known requirements for the ID proteins.

Persistence of Toxoplasma gondii in the central nervous system: a fine-tuned balance between the parasite, the brain and the immune system

Upon infection of humans and animals with Toxoplasma gondii, the parasites persist as intraneuronal cysts that are controlled, but not eliminated by the immune system. In particular, intracerebral T cells are crucial in the control of T. gondii infection and are supported by essential functions from other leukocyte populations. Additionally, brain-resident cells including astrocytes, microglia and neurons contribute to the intracerebral immune response by the production of cytokines, chemokines and expression of immunoregulatory cell surface molecules, such as major histocompatibility (MHC) antigens. However, the in vivo behaviour of these individual cell populations, specifically their interaction during cerebral toxoplasmosis, remains to be elucidated. We discuss here what is known about the function of T cells, recruited myeloid cells and brain-resident cells, with particular emphasis on the potential cross-regulation of these cell populations, in governing cerebral toxoplasmosis.

Local communication among mucosal immune cells in patients with celiac disease

In patients with celiac disease, gluten consumption causes inflammation of the duodenum, and, to a lesser extent, the proximal jejunum. Immune-dominant gluten peptides are modified by the enzyme TG2, leading to their high-affinity binding to HLA-DQ2 or HLA-DQ8 molecules, present in people with a predisposition to celiac disease. Gluten peptide-loaded HLA-DQ2 or HLA-DQ8 molecules are recognized by highly conserved receptors on CD4(+) T cells in the lamina propria. B cells specific for TG2 and modified gluten peptides are also abundant in the lamina propria of patients with celiac disease. In the epithelium, interleukin-15 activates intraepithelial lymphocytes that promote destruction of epithelial cells. However, it is not clear how the immune responses in the lamina propria and the epithelium, separated by a basement membrane, are linked. We review the immune processes that occur in the lamina propria and their potential effects on epithelial pathology in celiac disease.

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.

Re-examining the origin and function of liver-resident NK cells

Recent studies have identified a population of liver-resident innate lymphoid cells (ILCs) that, based on the expression of certain phenotypic markers, were termed 'liver-resident NK cells' and considered to be a new subset of conventional natural killer (cNK) cells. However, different transcriptional networks control the development of liver-resident NK cells and cNK cells and, furthermore, these cells exhibit features that characterize mucosal ILC1s. Here, we review findings providing insight into the origin, phenotype, and function of liver-resident NK cells, and discuss these in the context of the current understanding of lineage relations of ILC subsets. We propose that the similarities between liver-resident NK cells and mucosal ILC1s should be considered when revising the categorization framework for these cells, and discuss implications of this revision for other tissue-specific NK cells.

PLZF expression maps the early stages of ILC1 lineage development

Among the variety of tissue-resident NK-like populations recently distinguished from recirculating classical NK (cNK) cells, liver innate lymphoid cells (ILC) type 1 (ILC1s) have been shown to represent a distinct lineage that originates from a novel promyelocytic leukaemia zinc finger (PLZF)-expressing ILC precursor (ILCP) strictly committed to the ILC1, ILC2, and ILC3 lineages. Here, using PLZF-reporter mice and cell transfer assays, we studied the developmental progression of ILC1s and demonstrated substantial overlap with stages previously ascribed to the cNK lineage, including pre-pro-NK, pre-NK precursor (pre-NKP), refined NKP (rNKP), and immature NK (iNK). Although they originated from different precursors, the ILC1 and cNK lineages followed a parallel progression at early stages and diverged later at the iNK stage, with a striking predominance of ILC1s over cNKs early in ontogeny. Although a limited set of ILC1 genes depended on PLZF for expression, characteristically including Il7r, most of these genes were also differentially expressed between ILC1s and cNKs, indicating that PLZF together with other, yet to be defined, factors contribute to the divergence between these lineages.

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.

Transcription factors controlling innate lymphoid cell fate decisions

The mucosal epithelium is in direct contact with symbiotic and pathogenic microorganisms. Therefore, the mucosal surface is the principal portal of entry for invading pathogens and immune cells accumulated in the intestine to prevent infections. In addition to these conventional immune system functions, it has become clear that immune cells during steady-state continuously integrate microbial and nutrient-derived signals from the environment to support organ homeostasis. A major role in both processes is played by a recently discovered group of lymphocytes referred to as innate lymphoid cells (ILCs) Innate lymphoid cells (ILCs) that are specifically enriched at mucosal surfaces but are rather rare in secondary lymphoid organs. In analogy to the dichotomy between CD8 and CD4 T cells, we propose to classify ILCs into interleukin-7 receptor α-negative cytotoxic ILCs and IL-7Rα(+) helper-like ILCs. Dysregulated immune responses triggered by the various ILC subsets have been linked to inflammatory diseases such as inflammatory bowel disease, atopic dermatitis and airway hyperresponsiveness. Here, we will review recent progress in determining the transcriptional and developmental programs that control ILC fate decisions.

NK cells link obesity-induced adipose stress to inflammation and insulin resistance

An important cause of obesity-induced insulin resistance is chronic systemic inflammation originating in visceral adipose tissue (VAT). VAT inflammation is associated with the accumulation of proinflammatory macrophages in adipose tissue, but the immunological signals that trigger their accumulation remain unknown. We found that a phenotypically distinct population of tissue-resident natural killer (NK) cells represented a crucial link between obesity-induced adipose stress and VAT inflammation. Obesity drove the upregulation of ligands of the NK cell-activating receptor NCR1 on adipocytes; this stimulated NK cell proliferation and interferon-γ (IFN-γ) production, which in turn triggered the differentiation of proinflammatory macrophages and promoted insulin resistance. Deficiency of NK cells, NCR1 or IFN-γ prevented the accumulation of proinflammatory macrophages in VAT and greatly ameliorated insulin sensitivity. Thus NK cells are key regulators of macrophage polarization and insulin resistance in response to obesity-induced adipocyte stress.

Pivotal roles of ILCs in hepatic diseases

Innate lymphoid cells (ILCs) are involved in the development of mucosal-associated lymphoid tissues and serve as a rapid and early source of the effector cytokines that are typically associated with the T helper cell subsets in response to pathogen-induced changes in the microenvironment. Recent research has implicated ILCs as potential contributing factors to the spectrum of inflammation-related hepatic diseases, particularly hepatitis, fibrosis and carcinoma. In this review, we summarize the current knowledge on the roles of ILCs in these hepatic pathogeneses, providing insights into the underlying cellular and signaling mechanisms to help guide the future research to elucidate the ILCs' characters under normal and diseased conditions and provide interventional targets with therapeutic potential.

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.

Transcription factor c-Rel plays a crucial role in driving anti-CD40-mediated innate colitis

Genetic and environmental factors, including the commensal microbiota, have a crucial role in the development of inflammatory bowel disease. Aberrant activation of the transcription factor NF-κB is associated with chronic intestinal inflammation in mice and humans. Recently, an emerging family of innate lymphoid cells (ILCs) has been identified at mucosal sites contributing to the maintenance of gut homeostasis and intestinal immunopathology. Here, we show that the NF-κB protein c-Rel regulates the inflammatory potential of colonic IFN-γ(+)Thy1(+) ILCs to induce anti-CD40-mediated colitis in rag1(-/-) mice. Stimulation of dendritic cells (DCs) with anti-CD40 or CD40L led to translocation of c-Rel into the nucleus resulting in induction of expression of interleukin-12 (IL-12) and IL-23, key regulators of innate cell-induced colitis. While c-Rel deficiency completely abrogated anti-CD40-induced colitis, adoptively transferred wild-type DCs were able to induce pronounced colonic inflammation in rag1(-/-)rel(-/-) mice. In summary, these results suggest that the expression of c-Rel in DCs is essential for initiating anti-CD40-mediated intestinal pathogenesis.

Lung ILC2s link innate and adaptive responses in allergic inflammation

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

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.

Transcription factor networks directing the development, function, and evolution of innate lymphoid effectors

Mammalian lymphoid immunity is mediated by fast and slow responders to pathogens. Fast innate lymphocytes are active within hours after infections in mucosal tissues. Slow adaptive lymphocytes are conventional T and B cells with clonal antigen receptors that function days after pathogen exposure. A transcription factor (TF) regulatory network guiding early T cell development is at the core of effector function diversification in all innate lymphocytes, and the kinetics of immune responses is set by developmental programming. Operational units within the innate lymphoid system are not classified by the types of pathogen-sensing machineries but rather by discrete effector functions programmed by regulatory TF networks. Based on the evolutionary history of TFs of the regulatory networks, fast effectors likely arose earlier in the evolution of animals to fortify body barriers, and in mammals they often develop in fetal ontogeny prior to the establishment of fully competent adaptive immunity.

Role of group 3 innate lymphoid cells in antibody production

Innate lymphoid cells (ILCs) constitute a heterogeneous family of effector lymphocytes of the innate immune system that mediate lymphoid organogenesis, tissue repair, immunity and inflammation. The initial view that ILCs exert their protective functions solely during the innate phase of an immune response has been recently challenged by evidence indicating that ILCs shape adaptive immunity by establishing both contact-dependent and contact-independent interactions with multiple hematopoietic and non-hematopoietic cells, including B cells. Some of these interactions enhance antibody responses both systemically and at mucosal sites of entry.

Deciphering the transcriptional switches of innate lymphoid cell programming: the right factors at the right time

Innate lymphoid cells (ILCs) are increasingly recognised as an innate immune counterpart of adaptive T_H cells. In addition to their similar effector cytokine production, there is a strong parallel between the transcription factors that control the differentiation of T_H1, T_H2 and T_H17 cells and ILC Groups 1, 2 and 3, respectively. Here, we review the transcriptional circuit that specifies the development of a common ILC progenitor and its subsequent programming into distinct ILC groups. Notch, GATA-3, Nfil3 and Id2 are identified as early factors that suppress B and T cell potentials and are turned on in favour of ILC commitment. Natural killer cells, which are the cytotoxic ILCs, develop along a pathway distinct from the rest of the helper-like ILCs that are derived from a common progenitor to all helper-like innate lymphoid cells (CHILPs). PLZF⁻ CHILPs give rise to lymphoid tissue inducer cells while PLZF⁺ CHILPs have multi-lineage potential and could give rise to ILCs 1, 2 and 3. Such lineage specificity is dictated by the controlled expression of T-bet, RORα, RORγt and AHR. In addition to the type of transcription factors, the developmental stages at which these factors are expressed are crucial in specifying the fate of the ILCs.

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.

CCR7-dependent trafficking of RORγ⁺ ILCs creates a unique microenvironment within mucosal draining lymph nodes

Presentation of peptide:MHCII by RORγ-expressing group 3 innate lymphoid cells (ILC3s), which are enriched within gut tissue, is required for control of CD4 T-cell responses to commensal bacteria. It is not known whether ILC populations migrate from their mucosal and peripheral sites to local draining secondary lymphoid tissues. Here we demonstrate that ILC3s reside within the interfollicular areas of mucosal draining lymph nodes, forming a distinct microenvironment not observed in peripheral lymph nodes. By photoconverting intestinal cells in Kaede mice we reveal constitutive trafficking of ILCs from the intestine to the draining mesenteric lymph nodes, which specifically for the LTi-like ILC3s was CCR7-dependent. Thus, ILC populations traffic to draining lymph nodes using different mechanisms.

Innate lymphoid cells have an important role in mucosal immunity and present peptide:MHCII to CD4 T cells. Here the authors show that innate lymphoid cell subsets migrate from the gut mucosa to the draining lymph nodes via different mechanisms, where they form distinct microenvironments.

Group 2 innate lymphoid cells in health and disease

Group 2 innate lymphoid cells (ILC2s) play critical roles in anti-helminth immunity, airway epithelial repair, and metabolic homeostasis. Recently, these cells have also emerged as key players in the development of allergic inflammation at multiple barrier surfaces. ILC2s arise from common lymphoid progenitors in the bone marrow, are dependent on the transcription factors RORα, GATA3, and TCF-1, and produce the type 2 cytokines interleukin (IL)-4, IL-5, IL-9, and/or IL-13. The epithelial cell-derived cytokines IL-25, IL-33, and TSLP regulate the activation and effector functions of ILC2s, and recent studies suggest that their responsiveness to these cytokines and other factors may depend on their tissue environment. In this review, we focus on recent advances in our understanding of the various factors that regulate ILC2 function in the context of immunity, inflammation, and tissue repair across multiple organ systems.

2014: signaling breakthroughs of the year

The 2014 breakthroughs fell into four main areas: innate immunity, host-microbe interactions, cell death signaling, and methodological advances in the study of cell signaling. Nominations included new discoveries about signaling in innate immune cells, innate immune functions for lymphoid and nonhematological cells, and the importance of host-microbe interactions for the regulation of host physiology. Also this year, we received nominations highlighting molecular mechanisms by which p53 contributes to the pathology of chronic inflammation and how signaling pathways mediate programmed necrotic cell death. Finally, 2014 saw the use of new techniques to study cell signaling and identify drug targets, such as the in vivo use of RNA interference to study signaling in T cells and new computational methods to study large datasets of different data types.