Innate lymphoid cells (ILCs) are lymphoid cells that do not express rearranged receptors and have important effector and regulatory functions in innate immunity and tissue remodeling. ILCs are categorized into 3 groups based on their distinct patterns of cytokine production and the requirement of particular transcription factors for their development and function. Group 1 ILCs (ILC1s) produce interferon γ and depend on Tbet, group 2 ILCs (ILC2s) produce type 2 cytokines like interleukin-5 (IL-5) and IL-13 and require GATA3, and group 3 ILCs (ILC3s) include lymphoid tissue inducer cells, produce IL-17 and/or IL-22, and are dependent on RORγt. Whereas ILCs play essential roles in the innate immune system, uncontrolled activation and proliferation of ILCs can contribute to inflammatory autoimmune diseases. In this review, we provide an overview of the characteristics of ILCs in the context of health and disease. We will focus on human ILCs but refer to mouse studies if needed to clarify aspects of ILC biology.

Introduction

Innate lymphoid cells (ILCs) constitute a recently identified family of mononuclear hematopoietic cells with key functions in the preservation of epithelial integrity and tissue immunity throughout the body. They are defined by their lymphoid morphology (Figure 1A) and the absence of rearranged antigen-specific receptors.1 

Figure 1

Morphology and phenotype of peripheral blood ILCs. (A) May-Grünwald-Giemsa staining (original magnification ×100), after cytospin, of human lineage CD127+ CRTH2+ ILC2s that were sort-purified from the peripheral blood. (B) Phenotype and gating strategy for ILC1s, ILC2s, and NCR and NCR+ ILC3s derived from tonsil (upper panels) and peripheral blood (lower panels) of healthy humans. The lineage cocktail contains markers for T cells (TCRαβ and TCRγδ), B cells (CD19), NK cells (CD94), myeloid and plasmacytoid dendritic cells (CD1a, CD11c, CD123, and BDCA2), monocytes and macrophages (CD14), mast cells (FcεR1), and stem cells (CD34). In the peripheral blood, NCR+ ILC3s are virtually absent in healthy individuals.

Figure 1

Morphology and phenotype of peripheral blood ILCs. (A) May-Grünwald-Giemsa staining (original magnification ×100), after cytospin, of human lineage CD127+ CRTH2+ ILC2s that were sort-purified from the peripheral blood. (B) Phenotype and gating strategy for ILC1s, ILC2s, and NCR and NCR+ ILC3s derived from tonsil (upper panels) and peripheral blood (lower panels) of healthy humans. The lineage cocktail contains markers for T cells (TCRαβ and TCRγδ), B cells (CD19), NK cells (CD94), myeloid and plasmacytoid dendritic cells (CD1a, CD11c, CD123, and BDCA2), monocytes and macrophages (CD14), mast cells (FcεR1), and stem cells (CD34). In the peripheral blood, NCR+ ILC3s are virtually absent in healthy individuals.

Two prototypic members of the ILC family are natural killer (NK) cells and lymphoid tissue inducer (LTi) cells. NK cells were discovered in the mouse in 19752  and are operationally defined by the capability to kill certain target cells in the absence of antigen-specific priming. LTi cells, identified in 1997,3  are essential for the formation of lymph nodes during embryogenesis and are also present in the postnatal gut, where they are important for the formation of cryptopatches and intestinal lymphoid structures, also called isolated lymphoid follicles. NK cells and LTi cells are developmentally related, as both cell types require the common γ (γc) chain of the interleukin-2 (IL-2) receptor and the transcriptional repressor Id2 for their development.4 

Recently, several distinct ILC populations have been identified that also depend on the γc chain5,6-7  and Id2 for their development.6,8  Each of these ILC populations show distinct patterns of cytokine production that mirror the cytokine-secreting profiles of helper T-cell subsets.9  Recent studies demonstrated that ILC populations have important effector functions during the early stages of the immune response against microbes,5,6  in tissue repair,10,11  in the anatomical containment of commensals,12  and in maintaining epithelial integrity at barrier surfaces.13  ILC function needs to be tightly regulated, as uncontrolled activation and proliferation can contribute to severe inflammation and damage in gut,14  lung,15,16  skin,17,18  and liver.19 

A group of researchers has proposed a classification of these ILC populations on the basis of their phenotypical (Figure 1B) and functional characteristics.20  The nomenclature is based on the helper T-cell nomenclature and categorizes the ILC subsets into 3 groups (Figure 2): group 1 ILCs (ILC1s) comprise ILCs that produce interferon γ (IFN-γ); group 2 ILCs (ILC2s) produce type 2 cytokines, in particular IL-5 and IL-13; and group 3 ILCs (ILC3s) produce Il-17 and/or IL-22. In this model, NK cells were classified in group 1 ILCs because of their capacity to produce IFN-γ.20  Recent information on the developmental pathways of mouse ILCs, however, suggests that NK cells and CD127+ ILCs should be considered as the innate forms of CD8+ and CD4+ T cells, respectively.21  Development of NK cells and CD127+ ILCs may be driven by the transcription factors Nfil322  and GATA3,23,24  respectively (Figure 2). In this review, we provide an overview of the characteristics of distinct ILC subsets, with emphasis on human ILCs. Because of space constraints, we will not extensively review human LTi cells but refer to reviews that are published elsewhere.1,25  In addition, some excellent recent reviews provide more detailed information on mouse ILC biology.26,27 

Figure 2

The developmental relationship of ILCs. The NK cell progenitor (pre-NK) and the ILC progenitor (pre-ILC) evolve from the common lymphoid progenitor (CLP), but the phenotype and developmental requirements of the pre-ILCs have not been defined in humans (dotted lines). ILC3s and ILC2s develop from pre-ILCs under the influence of the transcription factors RORγt and GATA3, respectively. CD127+ ILC1s may derive from pre-ILCs or may be developmentally separated as part of the NK branch together with conventional NK cells (cNK) and CD127low ILC1s. Inset: ILCs have plasticity, as RORγt+ NCR ILC3 can differentiate in vitro into ILC1s and into NCR+ ILC3s; the latter, in turn, can be induced into a NKp44 cKit CRTH2 ILC1s, and vice versa, depending on specific activation signals. Whether these ILC1s are similar to the NKp44 cKit CRTH2 ILC1s that can be found in human tissues and blood remains to be determined. During these processes, these cells downregulate RORγt and upregulate Tbet.

Figure 2

The developmental relationship of ILCs. The NK cell progenitor (pre-NK) and the ILC progenitor (pre-ILC) evolve from the common lymphoid progenitor (CLP), but the phenotype and developmental requirements of the pre-ILCs have not been defined in humans (dotted lines). ILC3s and ILC2s develop from pre-ILCs under the influence of the transcription factors RORγt and GATA3, respectively. CD127+ ILC1s may derive from pre-ILCs or may be developmentally separated as part of the NK branch together with conventional NK cells (cNK) and CD127low ILC1s. Inset: ILCs have plasticity, as RORγt+ NCR ILC3 can differentiate in vitro into ILC1s and into NCR+ ILC3s; the latter, in turn, can be induced into a NKp44 cKit CRTH2 ILC1s, and vice versa, depending on specific activation signals. Whether these ILC1s are similar to the NKp44 cKit CRTH2 ILC1s that can be found in human tissues and blood remains to be determined. During these processes, these cells downregulate RORγt and upregulate Tbet.

Key features of group 1, 2, and 3 ILCs

ILC1s are not yet well defined. Both in humans and mice, several populations have been described that produce IFN-γ but not other signature cytokines such as IL-17, IL-22, or IL-5. The developmental relationship of these ILC1s is not yet clarified, and the mouse equivalents of currently identified human ILC1s and vice versa are not precisely known. Human group 1 ILCs include 2 ILC1 subsets that can be distinguished from NK cells and each other based on their intestinal anatomical location and the expression of certain surface markers. Intraepithelial CD127low ILC1s express CD103, CD56, CD94, and NKp44 and are responsive to IL-12 and IL-15.28  The intraepithelial CD127low ILC1s bear resemblance to NK cells, as these cells are CD56+ and also express perforin, which is essential for cytotoxicity.28  The equivalent of CD127low ILC1s in the mouse are claimed to be intraepithelial CD160+ ILCs (CD160 is also expressed in human CD127low ILC1s), which, like NK cells, are dependent on the transcription factor Nfil3 but, in contrast to NK cells, are independent of IL-15 for their development.28  It has been argued that the CD127low ILC1s are the equivalents of intraepithelial CD8+ T cells,28  which would fit with the emerging concept that Nfil3-dependent ILCs are innate equivalents of CD8+ T cells.21  Human CD127high ILC1s are predominantly located in the lamina propria; lack CD56, CD94, and NKp44 expression; and respond to IL-12 and IL-18 by producing IFN-γ.29  It is likely that human CD127high ILC1s depend on IL7 for their development, but this has yet to be confirmed.

Both human CD127low and CD127high ILC1 populations highly express Tbet, and it is likely that Tbet is important for development. Recently, a Tbet-dependent ILC1 population was identified in the mouse liver that displays some cytotoxic activity30  but differs from NK cells by lacking expression of the transcription factor Eomes and develops from a precursor that is unable to differentiate into NK cells.21,30,31  The human equivalent of these cells has yet to be identified.

ILC2s are responsive to IL-25, IL-33,6,32,33-34  and thymic stromal lymphopoietin (TSLP)15,35  and produce type 2 cytokines, predominantly IL-5 and IL-13, but also amphiregulin, which is important for tissue repair,11  and IL-9 and IL-4. GATA3 is essential for development and function of human ILC2, whereas Notch signaling stimulates development of human ILC2s.36  Likewise, mouse ILC2s are dependent on GATA335,37  and Notch in addition to TCF-1 and RORα38,39  for their development and function.

ILC3s respond to IL-235,7,40,41-42  and IL-1β43,44  by secreting IL-22. ILC3s can be divided into natural cytotoxicity receptor (NCR)+ (NKp46 in mice and NKp44 and NKp46 in humans) and NCR ILC3s. In the mouse, ILC3s depend on the transcription factors Notch, TCF-1, and RORγt for their development and function, but the requirement of these factors for human ILC3 development has yet to be confirmed. Mouse ILC3s that express the NCR NKp46 also require Tbet for their development,45,46-47  but it is unknown whether this is also the case for human NKp44+ ILC3s. It has been observed that upon stimulation with IL-1β and IL-23, human RORγt+ NKp44 ILC3s can differentiate in vitro into NCR+ ILC3s and under the influence of IL-12 into CD127+ ILC1s.29  NCR+ ILC3s can also differentiate into ILC1s upon stimulation with IL-12. During this process, these cells downregulate RORγt and upregulate Tbet.29  A similar transition of NCR+ ILC3s into INF-γ–producing Tbethigh RORγtlow ILCs has been observed in vivo in a mouse model.45  These data indicate that ILC3s are plastic cells that can adopt an ILC1 fate depending on environmental cues. An overview of the ILC subsets, their phenotype, the signature cytokines they produce, their critical transcription factors, and their developmental relationships is provided in Tables 1 and 2 and Figure 2.

Table 1

Phenotype of human ILCs

 NK cells ILC1 ILC2 ILC3 
CD127 CD127 CD127+ LTi NKp44 NKp44+ 
Lin − − − − − − − 
CD7  
CD16 +/− − − − − − − 
CD25 +/−  −  − 
CD56 − − − − +/− 
CD94 − − − − − − 
CD103      − 
CD117 (cKit) −  − +/− low 
CD127 − − 
CD161 +/−  +/− 
CCR6  − 
CRTH2 −  − − − − 
ICOS       
IL1bR − − 
IL12RB  − − − +/− 
IL15RA −     
IL17RB −  − −  − 
IL23R −  +/− − 
MHC-II +/−   +/−   +/− 
NKp44 +/− − − − − 
NKp46 −  − low low 
ST2 +/−  − −  − 
 NK cells ILC1 ILC2 ILC3 
CD127 CD127 CD127+ LTi NKp44 NKp44+ 
Lin − − − − − − − 
CD7  
CD16 +/− − − − − − − 
CD25 +/−  −  − 
CD56 − − − − +/− 
CD94 − − − − − − 
CD103      − 
CD117 (cKit) −  − +/− low 
CD127 − − 
CD161 +/−  +/− 
CCR6  − 
CRTH2 −  − − − − 
ICOS       
IL1bR − − 
IL12RB  − − − +/− 
IL15RA −     
IL17RB −  − −  − 
IL23R −  +/− − 
MHC-II +/−   +/−   +/− 
NKp44 +/− − − − − 
NKp46 −  − low low 
ST2 +/−  − −  − 

ILCs are categorized into CD127 NK cells, CD127 ILC1s and CD127+ ILC1s, ILC2s, and ILC3s, the latter including LTi, NCR ILCs, and NCR+ ILC3s. +/− denotes expression that is upregulated by ILCs after activation or by a nonactivated subset of ILCs.

Table 2

Signature cytokines of human ILCs

 NK cells ILC1 ILC2 ILC3 
CD127 CD127 CD127+ LTi NKp44 NKp44+ 
Amphiregulin − − − − − − 
BAFF       
CSF1       
GM-CSF  +/−    
Granzyme − − − − − 
IFN-γ − − − − 
IL-2       
IL-3       
IL-4 −  − − − − 
IL-5 −  − − − − 
IL-8       
IL-9 −  − − − − 
IL-13 −  − − − +/− 
IL-17 −  − − − 
IL-21       
IL-22 −  − +/− − 
LTα −      
LTβ −      
Perforin − − − − − 
TNFα       
 NK cells ILC1 ILC2 ILC3 
CD127 CD127 CD127+ LTi NKp44 NKp44+ 
Amphiregulin − − − − − − 
BAFF       
CSF1       
GM-CSF  +/−    
Granzyme − − − − − 
IFN-γ − − − − 
IL-2       
IL-3       
IL-4 −  − − − − 
IL-5 −  − − − − 
IL-8       
IL-9 −  − − − − 
IL-13 −  − − − +/− 
IL-17 −  − − − 
IL-21       
IL-22 −  − +/− − 
LTα −      
LTβ −      
Perforin − − − − − 
TNFα       

ILCs are categorized into CD127 NK cells, CD127 ILC1s and CD127+ ILC1s, ILC2s, and ILC3s, the latter including LTi, NCR ILCs, and NCR+ ILC3s. +/− denotes cytokine expression by a subset of ILCs after activation for example with phorbol 12-myristate 13-acetate (PMA)/ionomycin.

GM-CSF, granulocyte-macrophage colony-stimulating factor.

Developmental relationship of ILCs and NK cells

Because human CD127+ ILCs express many NK cell markers including CD56, NKp44, and CD161,5,40  questions about the relationship between NK cells and CD127+ ILCs have been raised. The developmental pathway of NK cells in the mouse has been studied in detail and has been reviewed extensively recently,48  but the developmental pathways of human NK cell development and the transcription factors that control this process are less well defined.49,50  Earlier studies have led to a model in which 4 stages of human NK cell development can be distinguished on the basis of expression of CD34, cKit, CD94, and CD56.51,52  Stage 1 cells are CD34+CD56 cells that most likely overlap with the human common lymphoid progenitor; stage 2 cells coexpress CD34 and CD117 (cKit); stage 3 cells, called immature NK cells (iNK), express CD117 and CD56; and stage 4 and 5 cells are more mature CD56+CD94+CD16 and CD56+CD94+CD16+ NK cells, respectively.

When it was found that iNK cells produced IL-22,53  it was speculated that IL-22 production was a property of iNK cells, which then lose this capacity upon maturation to conventional NK cells. However, more recent analyses have revealed that stage 3 iNK cells comprise a heterogeneous population. A substantial proportion of these cells were RORγt+, expressed CD127, and produced IL-22 and were unable to differentiate into mature NK cells in vitro, strongly suggesting that these cells within the iNK population are in fact mature ILC3s and not immature NK cells.54  Concurrently, the minority of iNK cells lacked expression of CD127, and those cells were able to differentiate into mature NK cells in vitro.54  In another study, it was confirmed that human CD56+ ILC3s generated from cord blood hematopoietic stem cells are unable to differentiate into conventional NK cells.55  In line with these observations, experiments with RORγt fate-mapped mice demonstrated that RORγt is not expressed during the development of NK cells.56,57  In addition, whereas NK cells are IL-7 independent, both human and mouse ILC3s require IL-7 for optimal development.56,57  These data clearly demonstrate that RORγt+ ILCs and conventional NK cells belong to different lineages. Because 50% of all ILC3 cells express CD56,40  CD56 cannot be used as a defining marker for NK cells, as previously thought. Other markers such as KIRs, CD94, and CD16, which are not expressed on CD127+ ILCs, should be used to distinguish between CD127+CD56+ ILC3s and conventional NK cells. LFA-1 was found to be selectively expressed in human NK cells, but not in ILC3s, qualifying LFA-1 as another marker to distinguish these 2 populations.55 

With the recent identification in mice of committed precursor subsets that give rise to group 1, 2, and 3 ILCs but not to NK cells (or T and B lymphocytes), it becomes clear that NK cells indeed represent a subset of ILCs that are developmentally related to but distinct from ILC1s, ILC2s, and ILC3s.21,31  These ILC precursor subsets express the transcription factors PLZF, Id2, GATA3, and TOX and are phenotypically defined as cells that are lineage negative, IL7Rα+cKit+α4β7+. Human ILC progenitor subsets that are committed to develop into ILC1s, ILC2s, or ILC3 but have lost the potential to develop into NK cells have yet to be identified.

Tissue distribution of human ILCs

In healthy individuals, about 0.01% to 0.1% of circulating lymphocytes express a CD127+ ILC phenotype. The majority of CD127+ ILCs found in peripheral blood are group 2 ILCs,58  whereas NKp44+ ILC3s58  and CD127 ILC1s28  are nearly absent (Figure 1B). Peripheral blood ILC subsets from healthy individuals do not express cytokine transcripts, indicating that they are not activated. The composition of human ILC subsets in tissues depends on the tissue type. For instance, whereas group 2 ILCs and NKp44 ILC3s are the most prevalent ILC subsets in healthy human skin tissue,59,60  in other tissues such as thymus, tonsils, and bone marrow and in the gut, NKp44+ ILC3 is the prominent ILC subset. In the following paragraphs, we review our knowledge of human ILCs in intestine, lung, and skin. Mouse ILCs have also been identified and characterized in adipose tissue,6,61  where they promote accumulation of eosinophils and alternatively activated macrophages, which are implicated in metabolic homeostasis. Moreover, these cells are resident in the liver, and when stimulated with IL-33, they can mediate liver fibrosis.19  Thus far, there is no information available about ILCs in the human liver and adipose tissue. Recently, ILC3 were detected in the human spleen, where they interact with stromal cells for survival signals and with innate B cells to produce antibodies.62 

ILCs in the intestine

In 2009, in parallel with the identification of human fetal LTi cells and postnatal tonsillar LTi-like cells,40  the first report appeared demonstrating the presence of human IL-22–producing ILC3s in the healthy gut (Figure 3A).5  These NKp44+ ILC3s, which were originally called NK22 cells, produce IL-22 that signals to epithelial cells where it promotes proliferation, IL-10 and antimicrobial peptide production, and mucus production.5  In vitro, NKp44+ ILC3s were responsive to IL-23, IL-1β, IL-2, and IL-7 by producing IL-22,43,44,63  which is enhanced in the presence of TLR263  and NKp44 ligands.64  Furthermore, human gut ILC3s express transcripts for leukemia inhibitory factor,5  which induces proliferation of epithelial cells, and IL-26,5  a cytokine that, like IL-22, belongs to the IL-10 cytokine family. IL-26 negatively modulates proliferation of intestinal epithelial cells and induces secretion of proinflammatory cytokines tumor necrosis factor (TNF) and IL-8 by these cells.65  Also, gut resident NKp44+ ILC3s produce cytokines that act on T and B cells including IL-2,63  the B-cell activating factor BAFF that supports survival and expansion of mature B cells,44  and the chemokine CCL20 that directs the migration of T and B lymphocytes and ILCs into the gut.44 

Figure 3

ILCs in gut, lung, and skin. (A) In the healthy situation, ILC3s produce IL-22 to maintain the epithelial barrier, generate antimicrobial products (such as RegIIIβ, RegIIIγ, and β-defensins), and suppress the reactivity of commensal bacteria-specific T cells (left panel). Crohn disease is characterized by an accumulation of IFN-γ–producing ILC1s (middle panel). During intestinal inflammation, ILC3s produce IL-22 to maintain epithelial barrier homeostasis. CRC develops when this autoregenerative function is not switched off in time (right panel). (B) Airway hypersensitivity is characterized by stromal production of TSLP and IL-33 that induces IL-5 and IL-13 production by ILC2s and subsequent recruitment and activation of eosinophils and mast cells (left panel). Upon viral airway infection, however, ILC2s are induced to produce amphiregulin, which is involved in airway epithelium repair and maintenance, and thereby function as tissue-protective cells (right panel). (C) In the healthy skin, ILC2s maintain close interactions with mast cells, suppressing their proinflammatory function, whereas ILC3s are involved in wound repair (left panel). Atopic dermatitis is an ILC2-mediated disease (middle panel), whereas ILC3s are accumulated in psoriasis (right panel).

Figure 3

ILCs in gut, lung, and skin. (A) In the healthy situation, ILC3s produce IL-22 to maintain the epithelial barrier, generate antimicrobial products (such as RegIIIβ, RegIIIγ, and β-defensins), and suppress the reactivity of commensal bacteria-specific T cells (left panel). Crohn disease is characterized by an accumulation of IFN-γ–producing ILC1s (middle panel). During intestinal inflammation, ILC3s produce IL-22 to maintain epithelial barrier homeostasis. CRC develops when this autoregenerative function is not switched off in time (right panel). (B) Airway hypersensitivity is characterized by stromal production of TSLP and IL-33 that induces IL-5 and IL-13 production by ILC2s and subsequent recruitment and activation of eosinophils and mast cells (left panel). Upon viral airway infection, however, ILC2s are induced to produce amphiregulin, which is involved in airway epithelium repair and maintenance, and thereby function as tissue-protective cells (right panel). (C) In the healthy skin, ILC2s maintain close interactions with mast cells, suppressing their proinflammatory function, whereas ILC3s are involved in wound repair (left panel). Atopic dermatitis is an ILC2-mediated disease (middle panel), whereas ILC3s are accumulated in psoriasis (right panel).

The observation that TLR agonists can activate NKp44+ ILC3s raised the question whether microbiota regulate not only the function but also the development of these cells. However, analysis of ILC3s in the human fetal gut revealed that the development of NKp44+ ILC3s is a programmed event independent of commensals.66  These data are consistent with mouse studies demonstrating that IL-22–producing ILC3s are present in the gut during fetal life, before intestinal colonization.12,67 

Studies in mouse models showed the importance of IL-22–producing NCR+ ILC3s in the protection against colitis induced by the attaching and effacing pathogenic bacterium Citrobacter rodentium5,7,41  and in the anatomical containment of commensal intestinal bacteria.12  Recently, another role of ILC3s in the maintenance of gut homeostasis in mice was revealed. It was demonstrated that NCR ILC3s express major histocompatibility complex (MHC) class II antigens and present microbial antigens to gut CD4+ T cells. This did not result in activation, presumably because ILC3 lack costimulatory molecules, but did result in inactivation of gut commensal bacteria–specific T-cell responses.68  Of note, human ILC3s were also found to express MHC class II molecules in this study, suggesting that this class II MHC–dependent control of T cells is also operational in the human gut. ILC3s have in addition been shown to regulate Th17 cells via the aryl hydrocarbon receptor (AHR) in mice. AHR is a ligand-dependent transcription factor; the ligands include environmental toxins such as dioxin derivatives, dietary components, and endogenous ligands such as the tryptophan metabolite. Qiu et al observed that Th17 cells were strongly increased in Ahr−/− mice.69  This effect was indirect: the absence of AHR resulted in diminished IL-22 production, which in turn caused segmented filamentous bacteria, known to stimulate Th17 cells, to expand. Whether this type of control of Th17 cells is also functional in the human gut remains to be established.

Collectively, these studies show that in mice, ILC3s play a crucial role in gut immunity by directly inducing epithelial cell proliferation, promoting epithelial-derived production of anti-inflammatory cytokines and antimicrobial peptides, preventing dissemination of commensal bacteria, and suppressing microbiota-specific proinflammatory CD4+ T-cell responses and suggest that in humans, gut ILC3s may have similar functions.

The human gut also contains CD127high ILC1s and CD127low ILC1s.28,29  CD127low ILC1s respond to danger signals originating from epithelial cells and myeloid cells, suggesting that they play a role in the immune response against pathogens that elicit these danger signals,28  but the specific pathogens that provoke a response of these cells have yet to be identified. Also, the function of human CD127high ILC1 cells has yet to be firmly established. Because there is uncertainty about the exact equivalent of these cells in mice, a precise determination of the in vivo function of CD127high ILCs is not yet possible. However, because in mice Tbet-dependent, IFN-γ–producing ILCs were found to be important in the immune response against Salmonella enterica,45  we speculate that human CD127high Tbethigh ILC1s also play a role in the immune response against pathogenic gut bacteria. This hypothesis is supported by the observation that in contrast to ILC3s and ILC2s, CD127high ILC1s are not present in the human fetal gut, suggesting that colonization with bacteria triggers the appearance of these cells.29  Interestingly, a pronounced accumulation of IFN-γ–producing CD127low ILC1s and CD127high ILC1s was observed in inflamed intestinal tissues of Crohn disease patients, whereas the frequency of NKp44+ ILC3s was diminished, possibly through IL-12–dependent differentiation of ILC3s toward ILC1.28,29  Of note, other ILC subsets such as IL-17–producing CD56 ILC3s70  are also expanded in Crohn disease patients. The importance of IL-12– and IL-23–responsive lymphocytes in the pathobiology of Crohn disease has been emphasized recently by the observation that blockade of the IL-12/IL-23 axis by ustekinumab, an inhibitor of the subunit dimer p40 that is shared between IL-12 and IL-23, led to disease remission in TNF-antagonist–resistant patients.71 

The role of ILC2s in gut homeostasis and immunity has received considerably less attention. ILC2s are also present in the human gut, most prevalently in the fetal gut58 ; in adult human intestinal tissues, CRTH2+ ILC2s account for only a very small minority of the total CD127+ ILC pool. Fetal gut ILC2s express transcripts for IL-5 and IL-13 in situ, but the role of these cytokines in the fetal gut is unknown. It is possible that ILC2s play important roles in tissue generation in the gut. In the mouse, ILC2s are the source of IL-5 that regulate eosinophil homeostasis72  and IL-13 that may induce alternatively activated macrophages. Upon infection with nematodes, murine gut ILC2s contribute to clearance of worms by producing IL-973  and IL-13.32  The antihelminth and eosinophil- and alternatively activated macrophage–regulating activities of ILC2 in the human gut remain to be confirmed.

ILCs in the respiratory system

ILC2s are most thoroughly studied in the context of lung immune cell homeostasis, immunity, and inflammation (Figure 3B). Studies in the mouse have demonstrated that ILC2s are involved in the immune response against nematodes such as Nippostrogylus brasiliensis6,32,34  and in the repair of lung tissue damage inflicted by infection with pathogens through the production of the epidermal growth factor family member amphiregulin.11  CRTH2+ ST2+ ILC2s have been identified in healthy human lung,11,58  but the role of human ILC2s in lung homeostasis is currently unknown.

Inflammatory conditions of the lung are characterized by a type 2 signature. It has been recognized that type 2 cytokines are critical for pulmonary recruitment of type 2 effector cells, such as eosinophils (IL-5 and granulocyte-macrophage colony-stimulating factor), mast cells (IL-9), and immunoglobulin E–producing B cells (IL-4 and IL-13), and cytokines that directly affect target tissue (eg, IL-13–induced fibrosis). It is now recognized that not only Th2 cells but also ILC2s are the cellular source of type 2 cytokines in the lung. In humans, IL-5– and IL-13–producing innate cells that resemble ILC2s have been found in the sputum of asthma patients,74  in the lung parenchyma and bronchoalveolar lavage fluids of lung transplantation patients,11  and in patients with idiopathic pulmonary fibrosis.75  We identified CD34 CRTH2+ ILC2s in nasal polyp tissues of patients suffering from chronic rhinosinusitis (CRS), a typical type 2 inflammatory disease characterized by eosinophilia and high immunoglobulin E levels.58  Interestingly, TSLP activates human ILC2 by directly upregulating GATA3 via STAT5, resulting in the production of high amounts of type 2 cytokines.58  This observation is highly relevant in the context of CRS and also asthma, because TSLP protein expression was significantly increased in epithelial cells derived from nasal polyps of CRS patients58  and in the airway epithelium and lamina propria of asthmatic patients, particularly in patients with severe asthma.76  TSLP immunostaining in both compartments correlated with the severity of airflow obstruction. This study also provided evidence that majority of leukocytes expressing IL-13 were ILC2s.76  Taken together, these data indicate that human ILC2s are involved in lung inflammation and pathology. This conclusion is confirmed by numerous studies in mouse models of type 2 inflammatory diseases, in particular of allergic asthma15,16,77,78-79  (reviewed by Walker and colleagues26,80 ). Given its potential role in inflammatory diseases of the lung, such as asthma, it is highly relevant to understand how lung-residing ILC2s are regulated in healthy humans and asthmatics. Recently it was reported that lipoxin A4, a member of a class of pro-resolving lipid mediators, decreases IL-13 production by ILC2 by interacting with ALX/FPR2 receptors.81  Lipoxin A4 also decreased numbers of eosinophils by promoting NK cell mediated apoptosis of these cells. The same study also documented that ILC2s are located in close proximity of mast cells in the lung and that the mast cell product Prostaglandin D2 (PGD2) increases IL-13 production by ILC2 through its receptor CRTH2.81  Thus the balance of lipoxin A4 and PGD2 may regulate ILC2 activity and perhaps affecting this balance by drugs may present a new therapeutic option to treat lung inflammations caused by uncontrolled ILC2 activities.

Little is known about the role of other ILC subsets in the lung. Besides ILC2s, we have also detected ILC1s and ILC3s in the human lung, but their function is unknown. In a mouse model of lung inflammation induced by ovalbumin, IL-22–producing ILC3s reduced airway inflammation by lowering the production of proinflammatory cytokines such as IL-33.82 

ILCs in the skin

Several ILC subsets have been characterized in the skin of healthy wild-type mice and humans (Figure 3C). Kim et al were the first to identify an ILC subset (ILC2) in human skin, which expressed ST2, a component of the IL-33 receptor, but not CRTH2.17  The presence of ILC2 in healthy skin was confirmed by us60  and another group, but in these studies the dermal ILC2s were found to express CRTH2.83  We also detected ILC1 and NCR ILC3s but not NCR+ ILC3s in human skin. A proportion of circulating ILC1s, CRTH2+ ILC2s, and NKp44 ILC3s expressed the skin-homing markers CLA and CCR10, suggesting that the dermal ILC populations are derived from circulating ILCs that migrate to the skin.60  In mouse skin, dermal ILC2s express CD103 and are the major cellular source of IL-13 under homeostatic conditions.18  Both human and mouse dermal ILC2s produce IL-4 when activated with IL-3384  and TSLP,18  respectively, whereas mouse lung ILC2s do not produce IL-4. It is unclear whether this reflects a differential signaling in the lung and skin or whether there is a distinct IL-4–producing ILC2 subset in the skin. Interestingly, mouse dermal ILC2s most strongly interact with mast cells, and ILC2-produced IL-13 may moderate mast cell responses.18  Whether ILC2s also interact with mast cells in human skin remains to be determined. Mast cells are able to produce prostaglandin D2 (PGD2), which is the ligand for CRTH2. PGD2 strongly enhanced production of IL-13 by human CRTH2+ ILC2s, an effect mimicked by supernatant-activated human mast cells.84  In addition, PGD2 induced migration of ILC2. Interestingly, CRTH2 antagonists strongly inhibited PGD2-mediated migration and cytokine production by ILC2.84  These data suggest an attenuating role of mast cells through its product PGD2 and raise the possibility that small molecular CRTH2 antagonists modify the function of human ILC2s in vivo.

In the diseased skin of human atopic dermatitis patients, increased numbers of ILC2s were observed compared with healthy controls,17,83  suggesting that ILC2s play a role in atopic dermatitis. Interestingly, interaction of ILC2-expressed killer lectin-like receptor G1 with E-cadherin widely expressed on keratinocytes and Langerhans cells suppressed IL-33–induced production of IL-5 and IL-13 by dermal ILC2s. This is suggestive for an involvement of ILC2s in atopic dermatitis, because interrupted E-cadherin signaling may be a key factor in the development of atopic dermatitis.83  Consistently, increased numbers of ILC2s were also found in a mouse model of atopic dermatitis. These ILC2s induced skin inflammation when stimulated with IL-2 or with the dermatitis-causing vitamin D analog calcipotriol in RAG2-deficient mice.17,18,83  In addition, transgenic overexpression of IL-33 in keratinocytes results in an atopic dermatitis-like syndrome that correlated strong infiltration of ILC2s.85 

The skin also contains ILC3s, which in mice have been shown to interact with fibroblasts via the production of IL-22 to mediate wound healing.86  Dermal ILC3 may also mediate pathology. Psoriasis is an autoimmune disease of the skin that is driven by IL-17A, IL-17F, and IL-22. Topical exposure of mouse skin to the TLR7 agonist imiquod causes skin inflammation that bears some similarity to psoriasis and therefore has been used as an experimental model for psoriatic skin disease. In this model, IL-17A, IL-17F, and IL-22 contribute to disease, and these cytokines were shown to be produced by γδ-T cells and RORγt+ ILC3s.87  In patients with psoriasis, an accumulation of NCR+ ILC3s was observed in affected skin, suggesting that these IL-22–producing innate cells may be involved in the pathology of psoriasis.59,60  Interestingly, we and others have observed significantly elevated numbers of NKp44+ ILC3 not only in the diseased skin of psoriasis patients but also in the peripheral blood, whereas these cells are hardly present in the peripheral blood of healthy individuals.60,88  Interestingly, a favorable response to treatment of psoriasis with the anti-TNF antibody adalimumab in 1 patient was associated with a significant reduction of NKp44+ ILC3s in the peripheral blood.78  Future studies are needed to determine whether the number of NKp44+ ILC3s can indeed be used as a biomarker for psoriasis.

ILCs and cancer

The correlation between chronic inflammatory responses and an increased susceptibility to develop cancer has long been recognized. With the accumulating evidence that ILCs play pivotal roles in autoimmunity and inflammation as discussed in the foregoing paragraphs, it can be postulated that ILCs may also be involved in the development of malignancies. In colorectal carcinoma (CRC) patients, IL-22 was found to be highly expressed by tumor-infiltrating lymphocytes, which turned out to comprise IL-22–producing CD3+ and CD3 lymphocytes.89  Moreover, IL-22 production was significantly higher in cancerous tissue than in nontumor tissue sections of the same patients.89  Using an established mouse model of microbe-induced inflammatory bowel disease–associated CRC, it was found that IL-22–producing ILCs drove the induction and maintenance of CRC.89  Thus, CRC did not develop in mice that were depleted of IL-22–producing ILC3, and treatment with IL-22–blocking agents did protect against the development of CRC in these mice. Interestingly, neutralization of ILC3-derived IL-17 in the colon of mice did lead to a reduction in inflammation but did not prevent CRC, suggesting that the oncogenic effect of ILCs may be specifically attributable to IL-22. Another study confirmed the association of IL-22 with the occurrence of CRC in a mouse model.90  In other malignancies such as cutaneous T-cell lymphoma91  and hepatic carcinoma,92  IL-22 has been shown to play a key role in humans. In these malignancies, IL-22 is produced not only by T cells but also by non–T cells, and it will be of interest to determine whether ILC3s are that cellular source.

Hematopoietic stem cell transplantation

Considering the involvement of ILCs in inflammatory responses and tissue repair, and given the critical effects of chemotherapy, radiotherapy, and alloimmune responses on epithelial barriers and mucosal tissues, it can be postulated that ILCs are important modulators of pre- and postallogeneic hematopoietic stem cell transplantation (HSCT) immunity.

In a mouse model of acute graft-versus-host disease (GVHD), IL-22 that is produced by IL-23–responsive, radiotherapy-resistant recipient ILC3s protects intestinal stem cells against the detrimental effects of GVHD, because GVHD severity was significantly increased in the absence of ILC3s.93  The same group showed that IL-22–producing ILC3s are essential in the recovery of thymic epithelial cells after radiation-induced damage,94  suggesting that ILC3s, which should be radioresistant, may be important in post-HSCT T-lymphocyte reconstitution in these mice. We have longitudinally studied ILC recovery in a group of acute myeloid leukemia patients following induction chemotherapy and after allogeneic HSCT. Reconstituting ILCs were activated and expressed tissue-homing molecules and after allogeneic HSCT were of donor origin. Interestingly, patients with high proportions of CD69+ gut-homing ILC2s, NCR ILC3s, and NCR+ ILCs had less mucositis and GVHD. In addition, following induction chemotherapy and after allogeneic HSCT, a large number of NKp44+ ILC3s appeared in the circulation, which was associated with an absence of GVHD.95 

The post-HSCT period is characterized by a significant susceptibility to develop opportunistic infections. This is generally attributed to the absence of a full T-cell repertoire, in particular during the first 1 or 2 years after stem cell transplantation, and the frequent use of immunosuppressants including steroids, to prevent or treat GVHD. However, recent data suggest that ILCs may also be involved here, because it was shown in a mouse model that RORγt-dependent, IL-23–responsive, IL-17–producing ILCs are imperative for the clearance of fungal infections such as Candida albicans.96  In another paper, it was shown that IL-22 is important in clearance of Aspergillus fumigatus infection,97  but the source of IL-22 production in this mouse model was not specified.

Hematologic malignancies arising from ILCs or ILC progenitors

ILCs may transform into malignant cells. To identify malignancies that are derived from ILC progenitors, a more profound understanding of the developmental pathways of ILCs is needed. However, it can be speculated from data available in the literature that ILC progenitor malignancies do exist. About 4% of all acute leukemias are of ambiguous lineage (World Health Organization, 2008), including acute undifferentiated leukemias (AULs) that do not express any lineage specific antigens and mixed-phenotype leukemias coexpressing antigens of myeloid and lymphoid lineages. A proportion of the AULs are now thought to represent leukemias of NK cell progenitors. Because certain human NK cell progenitor subsets such as stage III NK cell progenitor cells recently have been shown to include ILC,53,54-55  it is tempting to postulate that AULs may include ILC-lineage–derived leukemias. Several series of NK cell precursor malignancies such as myeloid/NK cell precursor acute leukemia and blastic NK cell lymphoma/leukemia have been described in the past decades. Leukemic blasts in the earliest of these series were characterized by a lymphoblastic morphology and coexpression of lymphoid markers CD7 and CD56 and myeloid markers such as CD33 and CD34.98,99  More recently, the healthy counterparts of myeloid/NK cell precursor acute leukemia were proposed to be stage 1 pro–NK cell progenitors and stage 2 pre–NK cell progenitors. In particular, stage 2 pre–NK cells are characterized by a CD34+ CD33+ CD117+ phenotype with a variable expression of CD161 and CD56100  and could therefore very well include ILCs. Precursor NK lymphoblastic lymphomas/leukemias are derived from CD34 CD33+ CD117+ CD161+ CD56+ or CD56 stage III progenitor NK cells,99  cells that include ILC3s.40,54,55,101  More extensive immunophenotyping, that includes CD161 and the IL-7 receptor CD127, in combination with analyses of transcription factor expression, is needed to further characterize these rare type leukemias and to determine to what extent they include malignantly transformed ILC progenitors.

The same holds true for difficult-to-categorize, NK cell–like lymphomas that may be derived from more mature ILC subsets. For example, refractory celiac disease is characterized by the presence of lymphocytes with an unusual phenotype that have a tendency to develop into lymphomas. It was recently observed that the nonmalignant counterparts of these aberrant lymphocytes include lineage CD127+ CD7+ CD56+ or CD56 lymphocytes that express CD122, the IL-2/IL-15Rβ subunit, which are very similar to CD127low ILC1s described by Fuchs et al,28  and it was suggested that under the influence of chronic stimulation with IL-15, these cells may undergo malignant tranformation.102 

Concluding remarks

Over the past 6 years, ILC subsets have been discovered that play important roles in innate immunity, homeostasis of a variety of cell types, and tissue (re)modeling. ILCs show a remarkable similarity with helper T-cell subsets, which has aided the rapid identification of networks of transcription factors that drive the development and function of these cells. Understandably, the knowledge of the mechanisms underlying ILC function and development in humans lags behind that of mice, but the ILC system is conserved in mice and humans, which helps in the translation of fundamental findings in ILC biology in animal model systems to that of humans. Experiments in mice have laid the groundwork for our understanding of the developmental pathways, but these pathways have yet to be fully deciphered in humans.

Studies in mouse models of inflammatory diseases indicate that ILCs can be involved in inflammatory diseases. Moreover, changes in composition of ILCs have been observed in inflamed tissues in a number of inflammatory diseases in humans. In-depth comparisons of the characteristics of ILCs in diseased tissues with those in healthy tissues will help to further delineate their possible roles in disease and to determine whether targeting ILCs will help to prevent or treat these diseases. ILCs have also been associated with cancer in mouse models. Elucidating their role in human cancer will be a challenge for the future. It seems obvious that ILCs and their precursors can undergo malignant transformation. It would be highly interesting to see whether some of the yet-undefined leukemias and lymphomas are in fact derived from (pre-)ILCs.

There is an Inside Blood Commentary on this article in this issue.

Acknowledgments

The authors would like to thank Jenny Mjösberg, Tom Cupedo, Jochem Bernink, Kristine Germar, and Marius Munneke for their critical input and Johan Dobber for technical assistance. M.D.H. is supported by The Netherlands Organization for Scientific Research (NWO-ZonMW). H.S. is supported by an advanced grant of the European Research Council.

Authorship

Contribution: H.S. and M.D.H. wrote the manuscript and designed the figures.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Hergen Spits, Department of Cell Biology and Histology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands; e-mail: hergen.spits@amc.uva.nl.

References

References
1
Spits
H
Cupedo
T
Innate lymphoid cells: emerging insights in development, lineage relationships, and function.
Annu Rev Immunol
2012
30
647
675
2
Kiessling
R
Klein
E
Pross
H
Wigzell
H
“Natural” killer cells in the mouse. II. Cytotoxic cells with specificity for mouse Moloney leukemia cells. Characteristics of the killer cell.
Eur J Immunol
1975
5
2
117
121
3
Mebius
RE
Rennert
P
Weissman
IL
Developing lymph nodes collect CD4+CD3- LTbeta+ cells that can differentiate to APC, NK cells, and follicular cells but not T or B cells.
Immunity
1997
7
4
493
504
4
Yokota
Y
Mansouri
A
Mori
S
et al. 
Development of peripheral lymphoid organs and natural killer cells depends on the helix-loop-helix inhibitor Id2.
Nature
1999
397
6721
702
706
5
Cella
M
Fuchs
A
Vermi
W
et al. 
A human natural killer cell subset provides an innate source of IL-22 for mucosal immunity.
Nature
2009
457
7230
722
725
6
Moro
K
Yamada
T
Tanabe
M
et al. 
Innate production of T(H)2 cytokines by adipose tissue-associated c-Kit(+)Sca-1(+) lymphoid cells.
Nature
2010
463
7280
540
544
7
Satoh-Takayama
N
Vosshenrich
CAJ
Lesjean-Pottier
S
et al. 
Microbial flora drives interleukin 22 production in intestinal NKp46+ cells that provide innate mucosal immune defense.
Immunity
2008
29
6
958
970
8
Cherrier
M
Sawa
S
Eberl
G
Notch, Id2, and RORγt sequentially orchestrate the fetal development of lymphoid tissue inducer cells.
J Exp Med
2012
209
4
729
740
9
Spits
H
Di Santo
JP
The expanding family of innate lymphoid cells: regulators and effectors of immunity and tissue remodeling.
Nat Immunol
2011
12
1
21
27
10
Scandella
E
Bolinger
B
Lattmann
E
et al. 
Restoration of lymphoid organ integrity through the interaction of lymphoid tissue-inducer cells with stroma of the T cell zone.
Nat Immunol
2008
9
6
667
675
11
Monticelli
LA
Sonnenberg
GF
Abt
MC
et al. 
Innate lymphoid cells promote lung-tissue homeostasis after infection with influenza virus.
Nat Immunol
2011
12
11
1045
1054
12
Sonnenberg
GF
Monticelli
LA
Alenghat
T
et al. 
Innate lymphoid cells promote anatomical containment of lymphoid-resident commensal bacteria.
Science
2012
336
6086
1321
1325
13
Sonnenberg
GF
Fouser
LA
Artis
D
Border patrol: regulation of immunity, inflammation and tissue homeostasis at barrier surfaces by IL-22.
Nat Immunol
2011
12
5
383
390
14
Buonocore
S
Ahern
PP
Uhlig
HH
et al. 
Innate lymphoid cells drive interleukin-23-dependent innate intestinal pathology.
Nature
2010
464
7293
1371
1375
15
Halim
TYF
Krauss
RH
Sun
AC
Takei
F
Lung natural helper cells are a critical source of Th2 cell-type cytokines in protease allergen-induced airway inflammation.
Immunity
2012
36
3
451
463
16
Chang
Y-J
Kim
HY
Albacker
LA
et al. 
Innate lymphoid cells mediate influenza-induced airway hyper-reactivity independently of adaptive immunity.
Nat Immunol
2011
12
7
631
638
17
Kim
BS
Siracusa
MC
Saenz
SA
et al. 
TSLP elicits IL-33-independent innate lymphoid cell responses to promote skin inflammation. Sci Transl Med. 2013;5(170):170ra16
18
Roediger
B
Kyle
R
Yip
KH
et al. 
Cutaneous immunosurveillance and regulation of inflammation by group 2 innate lymphoid cells.
Nat Immunol
2013
14
6
564
573
19
McHedlidze
T
Waldner
M
Zopf
S
et al. 
Interleukin-33-dependent innate lymphoid cells mediate hepatic fibrosis.
Immunity
2013
39
2
357
371
20
Spits
H
Artis
D
Colonna
M
et al. 
Innate lymphoid cells—a proposal for uniform nomenclature.
Nat Rev Immunol
2013
13
2
145
149
21
Klose
CSN
Flach
M
Möhle
L
et al. 
Differentiation of type 1 ILCs from a common progenitor to all helper-like innate lymhpoid lineages.
Cell
2014
157
2
340
356
22
Male
V
Nisoli
I
Kostrzewski
T
et al. 
The transcription factor E4bp4/Nfil3 controls commitment to the NK lineage and directly regulates Eomes and Id2 expression.
J Exp Med
2014
211
4
635
642
23
Serafini
N
Klein Wolterink
RGJ
Satoh-Takayama
N
et al. 
Gata3 drives development of RORγt+ group 3 innate lymphoid cells.
J Exp Med
2014
211
2
199
208
24
Yagi
R
Zhong
C
Northrup
DL
et al. 
The transcription factor GATA3 is critical for the development of all IL-7Rα-expressing innate lymphoid cells.
Immunity
2014
40
3
378
388
25
van de Pavert
SA
Mebius
RE
New insights into the development of lymphoid tissues.
Nat Rev Immunol
2010
10
9
664
674
26
Walker
JA
Barlow
JL
McKenzie
ANJ
Innate lymphoid cells—how did we miss them?
Nat Rev Immunol
2013
13
2
75
87
27
Sonnenberg
GF
Artis
D
Innate lymphoid cell interactions with microbiota: implications for intestinal health and disease.
Immunity
2012
37
4
601
610
28
Fuchs
A
Vermi
W
Lee
JS
et al. 
Intraepithelial type 1 innate lymphoid cells are a unique subset of IL-12- and IL-15-responsive IFN-γ-producing cells.
Immunity
2013
38
4
769
781
29
Bernink
JH
Peters
CP
Munneke
M
et al. 
Human type 1 innate lymphoid cells accumulate in inflamed mucosal tissues.
Nat Immunol
2013
14
3
221
229
30
Daussy
C
Faure
F
Mayol
K
et al. 
T-bet and Eomes instruct the development of two distinct natural killer cell lineages in the liver and in the bone marrow.
J Exp Med
2014
211
3
563
577
31
Constantinides
MG
McDonald
BD
Verhoef
PA
Bendelac
A
A committed precursor to innate lymphoid cells.
Nature
2014
508
7496
397
401
32
Neill
DR
Wong
SH
Bellosi
A
et al. 
Nuocytes represent a new innate effector leukocyte that mediates type-2 immunity.
Nature
2010
464
7293
1367
1370
33
Saenz
SA
Siracusa
MC
Perrigoue
JG
et al. 
IL25 elicits a multipotent progenitor cell population that promotes T(H)2 cytokine responses.
Nature
2010
464
7293
1362
1366
34
Price
AE
Liang
H-E
Sullivan
BM
et al. 
Systemically dispersed innate IL-13-expressing cells in type 2 immunity.
Proc Natl Acad Sci USA
2010
107
25
11489
11494
35
Mjösberg
J
Bernink
J
Golebski
K
et al. 
The transcription factor GATA3 is essential for the function of human type 2 innate lymphoid cells.
Immunity
2012
37
4
649
659
36
Gentek
R
Munneke
JM
Helbig
C
et al. 
Modulation of signal strength switches Notch from an inducer of T cells to an inducer of ILC2.
Front Immunol
2013
4
334
37
Hoyler
T
Klose
CSN
Souabni
A
et al. 
The transcription factor GATA-3 controls cell fate and maintenance of type 2 innate lymphoid cells.
Immunity
2012
37
4
634
648
38
Wong
SH
Walker
JA
Jolin
HE
et al. 
A role for IL-25 and IL-33-driven type-2 innate lymphoid cells in atopic dermatitis.
Nat Immunol
2012
210
13
2939
2950
39
Halim
TYF
MacLaren
A
Romanish
MT
Gold
MJ
McNagny
KM
Takei
F
Retinoic-acid-receptor-related orphan nuclear receptor alpha is required for natural helper cell development and allergic inflammation.
Immunity
2012
37
3
463
474
40
Cupedo
T
Crellin
NK
Papazian
N
et al. 
Human fetal lymphoid tissue-inducer cells are interleukin 17-producing precursors to RORC+ CD127+ natural killer-like cells.
Nat Immunol
2009
10
1
66
74
41
Sanos
SL
Bui
VL
Mortha
A
et al. 
RORgammat and commensal microflora are required for the differentiation of mucosal interleukin 22-producing NKp46+ cells.
Nat Immunol
2009
10
1
83
91
42
Luci
C
Reynders
A
Ivanov
II
et al. 
Influence of the transcription factor RORgammat on the development of NKp46+ cell populations in gut and skin.
Nat Immunol
2009
10
1
75
82
43
Hughes
T
Becknell
B
Freud
AG
et al. 
Interleukin-1beta selectively expands and sustains interleukin-22+ immature human natural killer cells in secondary lymphoid tissue.
Immunity
2010
32
6
803
814
44
Cella
M
Otero
K
Colonna
M
Expansion of human NK-22 cells with IL-7, IL-2, and IL-1beta reveals intrinsic functional plasticity.
Proc Natl Acad Sci USA
2010
107
24
10961
10966
45
Klose
CSN
Kiss
EA
Schwierzeck
V
et al. 
A T-bet gradient controls the fate and function of CCR6-RORγt+ innate lymphoid cells.
Nature
2013
494
7436
261
265
46
Sciumé
G
Hirahara
K
Takahashi
H
et al. 
Distinct requirements for T-bet in gut innate lymphoid cells.
J Exp Med
2012
209
13
2331
2338
47
Rankin
LC
Groom
JR
Chopin
M
et al. 
The transcription factor T-bet is essential for the development of NKp46+ innate lymphocytes via the Notch pathway.
Nat Immunol
2013
14
4
389
395
48
Vosshenrich
CAJ
Di Santo
JP
Developmental programming of natural killer and innate lymphoid cells.
Curr Opin Immunol
2013
25
2
130
138
49
Blom
B
Spits
H
Development of human lymphoid cells.
Annu Rev Immunol
2006
24
287
320
50
Caligiuri
MA
Human natural killer cells.
Blood
2008
112
3
461
469
51
Freud
AG
Becknell
B
Roychowdhury
S
et al. 
A human CD34(+) subset resides in lymph nodes and differentiates into CD56bright natural killer cells.
Immunity
2005
22
3
295
304
52
Freud
AG
Yokohama
A
Becknell
B
et al. 
Evidence for discrete stages of human natural killer cell differentiation in vivo.
J Exp Med
2006
203
4
1033
1043
53
Hughes
T
Becknell
B
McClory
S
et al. 
Stage 3 immature human natural killer cells found in secondary lymphoid tissue constitutively and selectively express the TH 17 cytokine interleukin-22.
Blood
2009
113
17
4008
4010
54
Crellin
NK
Trifari
S
Kaplan
CD
Cupedo
T
Spits
H
Human NKp44+IL-22+ cells and LTi-like cells constitute a stable RORC+ lineage distinct from conventional natural killer cells.
J Exp Med
2010
207
2
281
290
55
Ahn
Y-O
Blazar
BR
Miller
JS
Verneris
MR
Lineage relationships of human interleukin-22-producing CD56+ RORγt+ innate lymphoid cells and conventional natural killer cells.
Blood
2013
121
12
2234
2243
56
Vonarbourg
C
Mortha
A
Bui
VL
et al. 
Regulated expression of nuclear receptor RORγt confers distinct functional fates to NK cell receptor-expressing RORγt(+) innate lymphocytes.
Immunity
2010
33
5
736
751
57
Satoh-Takayama
N
Lesjean-Pottier
S
Vieira
P
et al. 
IL-7 and IL-15 independently program the differentiation of intestinal CD3-NKp46+ cell subsets from Id2-dependent precursors.
J Exp Med
2010
207
2
273
280
58
Mjösberg
JM
Trifari
S
Crellin
NK
et al. 
Human IL-25- and IL-33-responsive type 2 innate lymphoid cells are defined by expression of CRTH2 and CD161.
Nat Immunol
2011
12
11
1055
1062
59
Dyring-Andersen
B
Geisler
C
Agerbeck
C
et al. 
Increased number and frequency of group 3 innate lymphoid cells in nonlesional psoriatic skin.
Br J Dermatol
2014
170
3
609
616
60
Teunissen
MBM
Munneke
JM
Bernink
JH
et al. 
Composition of Innate lymphoid cell subsets in the human skin: enrichment of NCR(+) ILC3 in lesional skin and blood of psoriasis patients [published online ahead of print March 21, 2014].
J Invest Dermatol
61
Molofsky
AB
Nussbaum
JC
Liang
H-E
et al. 
Innate lymphoid type 2 cells sustain visceral adipose tissue eosinophils and alternatively activated macrophages.
J Exp Med
2013
210
3
535
549
62
Magri
G
Miyajima
M
Bascones
S
et al. 
Innate lymphoid cells integrate stromal and immunological signals to enhance antibody production by splenic marginal zone B cells.
Nat Immunol
2014
15
4
354
364
63
Crellin
NK
Trifari
S
Kaplan
CD
Satoh-Takayama
N
Di Santo
JP
Spits
H
Regulation of cytokine secretion in human CD127(+) LTi-like innate lymphoid cells by Toll-like receptor 2.
Immunity
2010
33
5
752
764
64
Glatzer
T
Killig
M
Meisig
J
et al. 
RORγt⁺ innate lymphoid cells acquire a proinflammatory program upon engagement of the activating receptor NKp44.
Immunity
2013
38
6
1223
1235
65
Dambacher
J
Beigel
F
Zitzmann
K
et al. 
The role of the novel Th17 cytokine IL-26 in intestinal inflammation.
Gut
2009
58
9
1207
1217
66
Hoorweg
K
Peters
CP
Cornelissen
F
et al. 
Functional differences between human NKp44(-) and NKp44(+) RORC(+) innate lymphoid cells.
Front Immunol
2012
3
72
67
Sawa
S
Cherrier
M
Lochner
M
et al. 
Lineage relationship analysis of RORgammat+ innate lymphoid cells.
Science
2010
330
6004
665
669
68
Hepworth
MR
Monticelli
LA
Fung
TC
et al. 
Innate lymphoid cells regulate CD4+ T-cell responses to intestinal commensal bacteria.
Nature
2013
498
7452
113
117
69
Qiu
J
Guo
X
Chen
Z-ME
et al. 
Group 3 innate lymphoid cells inhibit T-cell-mediated intestinal inflammation through aryl hydrocarbon receptor signaling and regulation of microflora.
Immunity
2013
39
2
386
399
70
Geremia
A
Arancibia-Cárcamo
CV
Fleming
MPP
et al. 
IL-23-responsive innate lymphoid cells are increased in inflammatory bowel disease.
J Exp Med
2011
208
6
1127
1133
71
Sandborn
WJ
Gasink
C
Gao
L-L
et al. 
CERTIFI Study Group
Ustekinumab induction and maintenance therapy in refractory Crohn’s disease.
N Engl J Med
2012
367
16
1519
1528
72
Nussbaum
JC
Van Dyken
SJ
von Moltke
J
et al. 
Type 2 innate lymphoid cells control eosinophil homeostasis.
Nature
2013
502
7470
245
248
73
Licona-Limón
P
Henao-Mejia
J
Temann
AU
et al. 
Th9 cells drive host immunity against gastrointestinal worm infection.
Immunity
2013
39
4
744
757
74
Allakhverdi Z, Comeau MR, Smith DE, et al. CD34+ hemopoietic progenitor cells are potent effectors of allergic inflammation. J Allergy Clin Immunol. 2009;123(2):472-478
75
Hams
E
Armstrong
ME
Barlow
JL
et al. 
IL-25 and type 2 innate lymphoid cells induce pulmonary fibrosis.
Proc Natl Acad Sci USA
2014
111
1
367
372
76
Shikotra
A
Choy
DF
Ohri
CM
et al. 
Increased expression of immunoreactive thymic stromal lymphopoietin in patients with severe asthma. J Allergy Clin Immunol. 2012;129(1):104-111
77
Barlow
JL
Bellosi
A
Hardman
CS
et al. 
Innate IL-13-producing nuocytes arise during allergic lung inflammation and contribute to airways hyperreactivity. J Allergy Clin Immunol. 2012;129(1):191-8.e1-4
78
Klein Wolterink
RGJ
Kleinjan
A
van Nimwegen
M
et al. 
Pulmonary innate lymphoid cells are major producers of IL-5 and IL-13 in murine models of allergic asthma.
Eur J Immunol
2012
42
5
1106
1116
79
Liang
H-E
Reinhardt
RL
Bando
JK
Sullivan
BM
Ho
IC
Locksley
RM
Divergent expression patterns of IL-4 and IL-13 define unique functions in allergic immunity.
Nat Immunol
2012
13
1
58
66
80
Walker
JA
McKenzie
A
Innate lymphoid cells in the airways.
Eur J Immunol
2012
42
6
1368
1374
81
Barnig
C
Cernadas
M
Dutile
S
et al. 
Lipoxin A4 regulates natural killer cell and type 2 innate lymphoid cell activation in asthma.
Sci Transl Med.
2013
5
174
174ra26
82
Taube
C
Tertilt
C
Gyülveszi
G
et al. 
IL-22 is produced by innate lymphoid cells and limits inflammation in allergic airway disease.
PLoS ONE
2011
6
7
e21799
83
Salimi
M
Barlow
JL
Saunders
SP
et al. 
A role for IL-25 and IL-33-driven type-2 innate lymphoid cells in atopic dermatitis.
J Exp Med
2013
210
13
2939
2950
84
Xue
L
Salimi
M
Panse
I
et al. 
Prostaglandin D2 activates group 2 innate lymphoid cells through chemoattractant receptor-homologous molecule expressed on TH2 cells. J Allergy Clin Immunol. 2014;133(4):1184-1194
85
Imai
Y
Yasuda
K
Sakaguchi
Y
et al. 
Skin-specific expression of IL-33 activates group 2 innate lymphoid cells and elicits atopic dermatitis-like inflammation in mice.
Proc Natl Acad Sci USA
2013
110
34
13921
13926
86
McGee
HM
Schmidt
BA
Booth
CJ
et al. 
IL-22 promotes fibroblast-mediated wound repair in the skin.
J Invest Dermatol
2013
133
5
1321
1329
87
Pantelyushin
S
Haak
S
Ingold
B
et al. 
Rorγt+ innate lymphocytes and γδ T cells initiate psoriasiform plaque formation in mice.
J Clin Invest
2012
122
6
2252
2256
88
Villanova
F
Flutter
B
Tosi
I
et al. 
Characterization of innate lymphoid cells in human skin and blood demonstrates increase of NKp44+ ILC3 in psoriasis.
J Invest Dermatol
2014
134
4
984
991
89
Kirchberger
S
Royston
DJ
Boulard
O
et al. 
Innate lymphoid cells sustain colon cancer through production of interleukin-22 in a mouse model.
J Exp Med
2013
210
5
917
931
90
Huber
S
Gagliani
N
Zenewicz
LA
et al. 
IL-22BP is regulated by the inflammasome and modulates tumorigenesis in the intestine.
Nature
2012
491
7423
259
263
91
Miyagaki
T
Sugaya
M
Suga
H
et al. 
IL-22, but not IL-17, dominant environment in cutaneous T-cell lymphoma.
Clin Cancer Res
2011
17
24
7529
7538
92
Jiang
R
Tan
Z
Deng
L
et al. 
Interleukin-22 promotes human hepatocellular carcinoma by activation of STAT3.
Hepatology
2011
54
3
900
909
93
Hanash
AM
Dudakov
JA
Hua
G
et al. 
Interleukin-22 protects intestinal stem cells from immune-mediated tissue damage and regulates sensitivity to graft versus host disease.
Immunity
2012
37
2
339
350
94
Dudakov
JA
Hanash
AM
Jenq
RR
et al. 
Interleukin-22 drives endogenous thymic regeneration in mice.
Science
2012
336
6077
91
95
95
Munneke
JM
Björklund
AT
Mjösberg
JM
et al. 
Activated innate lymphoid cells are associated with a reduced susceptibility to graft versus host disease [published online ahead of print on May 22, 2014]. Blood
96
Gladiator
A
Wangler
N
Trautwein-Weidner
K
LeibundGut-Landmann
S
Cutting edge: IL-17-secreting innate lymphoid cells are essential for host defense against fungal infection.
J Immunol
2013
190
2
521
525
97
Gessner
MA
Werner
JL
Lilly
LM
et al. 
Dectin-1-dependent interleukin-22 contributes to early innate lung defense against Aspergillus fumigatus.
Infect Immun
2012
80
1
410
417
98
Suzuki
R
Yamamoto
K
Seto
M
et al. 
CD7+ and CD56+ myeloid/natural killer cell precursor acute leukemia: a distinct hematolymphoid disease entity.
Blood
1997
90
6
2417
2428
99
Suzuki
R
Nakamura
S
Malignancies of natural killer (NK) cell precursor: myeloid/NK cell precursor acute leukemia and blastic NK cell lymphoma/leukemia.
Leuk Res
1999
23
7
615
624
100
Oshimi
K
Progress in understanding and managing natural killer-cell malignancies.
Br J Haematol
2007
139
4
532
544
101
Tang
Q
Ahn
Y-O
Southern
P
Blazar
BR
Miller
JS
Verneris
MR
Development of IL-22-producing NK lineage cells from umbilical cord blood hematopoietic stem cells in the absence of secondary lymphoid tissue.
Blood
2011
117
15
4052
4055
102
Schmitz
F
Tjon
JML
Lai
Y
et al. 
Identification of a potential physiological precursor of aberrant cells in refractory coeliac disease type II.
Gut
2013
62
4
509
519