A role for IL-25 and IL-33-driven type-2 innate lymphoid cells in atopic dermatitis

Group 2 innate lymphoid cells: new players in human allergic diseases

Allergic diseases are characterized by tissue eosinophilia, mucus secretion, IgE production, and activation of mast cells and TH2 cells. Production of TH2 cytokines including IL-4, IL-5, IL-9, and IL-13 has mainly been attributed to CD4+T(H)2 cells. However, the recent discovery of group 2 innate lymphoid cells (ILC2s) in humans and findings from experimental disease models have challenged conventional concepts associated with the contribution of specific cells to type 2 inflammation in allergic diseases. ILC2s produce high levels of T(H)2 cytokines and have been detected in human lung tissue, peripheral blood, the gastrointestinal tract, skin, and sinonasal tissue, suggesting that ILC2s could contribute to chronic rhinosinusitis, asthma, atopic dermatitis, and gastrointestinal allergic disease. Moreover, depletion of ILC2s in animal models suggests a role for these cells in atopic dermatitis and asthma. This review will focus on the role of ILC2s in human allergy and asthma and provide a mechanistic insight from animal models.

IL-25-responsive, lineage-negative KLRG1(hi) cells are multipotential 'inflammatory' type 2 innate lymphoid cells

Innate lymphoid cells (ILCs) are lymphocyte-like cells lacking T or B cell receptors that mediate protective and repair functions through cytokine secretion. Among them, type 2 ILC (ILC2) cells are capable of producing type 2 cytokines. We report the existence of an inflammatory (i) ILC2 population responsive to IL-25 that complements IL-33-responsive natural (n) ILC2 cells. iILC2 cells developed into nILC2-like cells in vitro and in vivo, contributing to expulsion of Nippostrongylus brasiliensis. They also acquired IL-17-producing capacity, providing partial protection against Candida albicans. We propose that iILC2 cells are transient ILC progenitors mobilized by inflammation and infection that develop into nILC2-like cells or ILC3-like cells and contribute to immunity to both helminths and fungi.

Innate lymphoid cells in type 2 immune responses

In recent years, several distinct innate lymphoid cell populations (ILC) have been characterized in mice and humans. Group 2 ILC function as a rapid responder population in type 2 immune responses. Thus, a wealth of data has implicated an important role for ILC2 in immunity to parasitic infection and in immune pathology in inflammatory and allergic responses. In this review, we describe recent progress in our understanding of the development and ontogeny of ILC2 populations and the mechanisms by which these cells function in a variety of infection and disease settings. Finally, we emphasize recent findings indicating functional interactions between these innate cells and their adaptive CD4(+) Th2 cell counterparts.

A new player on the psoriasis block: IL-17A- and IL-22-producing innate lymphoid cells

Innate lymphoid cells (ILCs) are a recently discovered family of innate immune cells belonging to the lymphoid lineage, yet lacking antigen-specific receptors. ILCs were first identified in the intestinal tract, where they contribute to epithelial barrier integrity and host responses to commensal microbes. Teunissen et al. (in the current issue) and Villanova et al. (2014) now suggest an important role for type 3 ILCs (ILC3s) in the skin, particularly in psoriasis. Both groups found an increased frequency of IL-22- and/or IL-17A-producing ILCs in psoriatic skin and blood. These cells are activated in response to IL-1β and IL-23, correlate with disease severity, and are decreased following antitumor necrosis factor-α (anti-TNFα) treatment. The presence of a novel ILC population in psoriatic skin, one that responds to biologic therapeutics, suggests that dysregulation of ILCs is a contributing factor to psoriasis pathogenesis.

At the bedside: the emergence of group 2 innate lymphoid cells in human disease

ILC2s have been primarily identified at environmental-mucosal interfaces and can be activated quickly by environmental antigens and pathogens to produce large quantities of IL-5 and IL-13. As a result of the production of these cytokines, ILC2s have been implicated in the host response to allergens, viruses, and parasites. However, the exact role of ILC2s in any human disease state is presently unknown, as specifically eliminating these cells is not possible, given that potentially targetable cell-surface markers are shared with other immune cells. Likewise, selectively and completely inhibiting ILC2 activation is also not currently possible, as several activating cytokines, IL-25, IL-33, and TSLP, act in redundancy or are not specific for ILC2 stimulation. Therefore, at this point, we can only identify the relative abundance of ILC2s in organs and tissue identified as being involved in specific diseases, and the contribution of ILC2s in human disease can only be inferred from mouse studies. Given these limitations, in this article, we will review the studies that have examined the presence of ILC2s in human disease states and speculate on their possible role in disease pathogenesis. The intent of the review is to identify priority areas for basic research based on clinical research insights.

Innate lymphoid cells, possible interaction with microbiota

Recent studies have identified novel lymphocyte subsets named innate lymphoid cells (ILCs) lacking antigen-specific receptors. ILCs are present in a wide variety of epithelial compartments and occupy an intermediate position between acquired immune cells and myeloid cells. ILCs are now classified into three groups: group 1 ILC, group 2 ILC, and group 3 ILC based on their cytokine production patterns that correspond to the helper T cell subsets Th1, Th2, and Th17, respectively. ILCs play important roles in protection against various invading microbes including multicellular parasites, and in the maintenance of homeostasis and repair of epithelial layers. Excessive activation of ILCs, however, leads to various inflammatory disease conditions. ILCs have thus attracted interests of many researchers in the fields of infectious immunity, inflammatory diseases, and allergic diseases. Because epithelial cells sense alterations in environmental cues, it is important to understand the functional interaction between epithelial cells, ILCs, and environmental factors such as commensal microbiota. We discuss in this review developmental pathways of ILCs, their functions, and contribution of commensal microbiota to the differentiation and function of ILCs.

Innate lymphoid cells and the skin

Innate lymphoid cells are an emerging family of effector cells that contribute to lymphoid organogenesis, metabolism, tissue remodelling and protection against infections. They maintain homeostatic immunity at barrier surfaces such as lung, skin and gut (Nature 464:1367–1371, 2010, Nat Rev Immunol 13: 145–149, 2013). Several human and mouse studies suggest a role for innate lymphoid cells in inflammatory skin conditions including atopic eczema and psoriasis. Here we review the innate lymphoid cell family and discuss their function in the skin and during inflammation.

The route to pathologies in chronic inflammatory diseases characterized by T helper type 2 immune cells

T helper type 2 (Th2)-characterized inflammatory responses are highly dynamic processes initiated by epithelial cell damage resulting in remodelling of the tissue architecture to prevent further harm caused by a dysfunctional epithelial barrier or migrating parasites. This process is a temporal and spatial response which requires communication between immobile cells such as epithelial, endothelial, fibroblast and muscle cells and the highly mobile cells of the innate and adaptive immunity. It is further characterized by a high cellular plasticity that enables the cells to adapt to a specific inflammatory milieu. Incipiently, this milieu is shaped by cytokines released from epithelial cells, which stimulate Th2, innate lymphoid and invariant natural killer (NK) T cells to secrete Th2 cytokines and to activate dendritic cells which results in the further differentiation of Th2 cells. This milieu promotes wound-healing processes which are beneficial in parasitic infections or toxin exposure but account for increasingly dysfunctional vital organs, such as the lung in the case of asthma and the colon in ulcerative colitis. A better understanding of the dynamics underlying relapses and remissions might lead ultimately to improved therapeutics for chronic inflammatory diseases adapted to individual needs and to different phases of the inflammation.

Innate lymphoid cells in inflammation and immunity

Innate lymphoid cells (ILCs) were first described as playing important roles in the development of lymphoid tissues and more recently in the initiation of inflammation at barrier surfaces in response to infection or tissue damage. It has now become apparent that ILCs play more complex roles throughout the duration of immune responses, participating in the transition from innate to adaptive immunity and contributing to chronic inflammation. The proximity of ILCs to epithelial surfaces and their constitutive strategic positioning in other tissues throughout the body ensures that, in spite of their rarity, ILCs are able to regulate immune homeostasis effectively. Dysregulation of ILC function might result in chronic pathologies such as allergies, autoimmunity, and inflammation. A new role for ILCs in the maintenance of metabolic homeostasis has started to emerge, underlining their importance in fundamental physiological processes beyond infection and immunity.

Development, differentiation, and diversity of innate lymphoid cells

Recent years have witnessed the discovery of an unprecedented complexity in innate lymphocyte lineages, now collectively referred to as innate lymphoid cells (ILCs). ILCs are preferentially located at barrier surfaces and are important for protection against pathogens and for the maintenance of organ homeostasis. Inappropriate activation of ILCs has been linked to the pathogenesis of inflammatory and autoimmune disorders. Recent evidence suggests that ILCs can be grouped into two separate lineages, cytotoxic ILCs represented by conventional natural killer (cNK) cells and cytokine-producing helper-like ILCs (i.e., ILC1s, ILC2s, ILC3s). We will focus here on current work in humans and mice that has identified core transcriptional circuitry required for the commitment of lymphoid progenitors to the ILC lineage. The striking similarities in transcriptional control of ILC and T cell lineages reveal important insights into the evolution of transcriptional programs required to protect multicellular organisms against infections and to fortify barrier surfaces.

The role of ILC2 in pathology of type 2 inflammatory diseases

Group 2 innate lymphoid cells (ILC2) which are strategically interspersed throughout barrier surfaces are important regulators of type 2 immune reactions, particularly against helminthic parasites. ILC2 are also involved in tissue homeostasis and repair. Studies in a variety of animal models have demonstrated that when dysregulated or chronically activated, ILC2 can contribute to the pathology of allergic inflammatory diseases such as allergic asthma and atopic dermatitis. Here we discuss new findings of the cross talk of ILC2 with other hematopoietic cells, in particular T cells, and review recent information on the role of ILC2 in type 2 inflammatory diseases.

Innate lymphoid cells in the skin

Innate lymphoid cells (ILCs) are part of a heterogeneous family of innate immune cells with newly identified roles in mediating immunity, tissue homeostasis and pathologic inflammation. Here, we review recent studies delineating the roles of ILCs in the pathogenesis of multiple inflammatory skin disorders and their unique effector functions. Finally, we address how these studies have informed our understanding of the regulation of ILCs and the therapeutic potential of targeting these cells in the context of skin inflammation.

Increased frequencies of nuocytes in peripheral blood from patients with Graves' hyperthyroidism

Newly identified nuocytes play an important role in Th2 cell mediated immunity such as protective immune responses to helminth parasites, allergic asthma and chronic rhinosinusitis. However, the contributions of nuocytes in the occurrence and development of Graves' hyperthyroidism remains unknown. Previous studies found that there was a predominant Th2 phenotype in patients with Graves' hyperthyroidism, it might relate to polarization of nuocytes. Nuocytes were defined by transcription factor RORα, various cell surface markers (T1/ST2, IL-17RB, ICOS, CD45) and associated cytokines. In this study, these cells related genes or molecules in PBMC from patients with Graves' hyperthyroidism were measured, and the potential correlation between them was analyzed. The expression levels of T1/ST2, IL-17RB, ICOS, IL-5 and IL-13, which represented nuocytes associated molecules were significantly increased in patients, meanwhile, the RORα mRNA also had a tendency to increase. In addition, IFN-γ and T-bet (Th1 related cytokine and transcription factor) were obviously decreased, and there was a positive correlation between IL-17RB and IL-13. These results suggested that there were polarized nuocytes in Graves' hyperthyroidism patients, and which closely related to the down-regulation of Th1 cells or relatively advantage of Th2 differentiation.

Central domain of IL-33 is cleaved by mast cell proteases for potent activation of group-2 innate lymphoid cells

Interleukin-33 (IL-33) is an alarmin cytokine from the IL-1 family. IL-33 activates many immune cell types expressing the interleukin 1 receptor-like 1 (IL1RL1) receptor ST2, including group-2 innate lymphoid cells (ILC2s, natural helper cells, nuocytes), the major producers of IL-5 and IL-13 during type-2 innate immune responses and allergic airway inflammation. IL-33 is likely to play a critical role in asthma because the IL33 and ST2/IL1RL1 genes have been reproducibly identified as major susceptibility loci in large-scale genome-wide association studies. A better understanding of the mechanisms regulating IL-33 activity is thus urgently needed. Here, we investigated the role of mast cells, critical effector cells in allergic disorders, known to interact with ILC2s in vivo. We found that serine proteases secreted by activated mast cells (chymase and tryptase) generate mature forms of IL-33 with potent activity on ILC2s. The major forms produced by mast cell proteases, IL-33(95-270), IL-33(107-270), and IL-33(109-270), were 30-fold more potent than full-length human IL-33(1-270) for activation of ILC2s ex vivo. They induced a strong expansion of ILC2s and eosinophils in vivo, associated with elevated concentrations of IL-5 and IL-13. Murine IL-33 is also cleaved by mast cell tryptase, and a tryptase inhibitor reduced IL-33-dependent allergic airway inflammation in vivo. Our study identifies the central cleavage/activation domain of IL-33 (amino acids 66-111) as an important functional domain of the protein and suggests that interference with IL-33 cleavage and activation by mast cell and other inflammatory proteases could be useful to reduce IL-33-mediated responses in allergic asthma and other inflammatory diseases.

IL-33: an alarmin cytokine with crucial roles in innate immunity, inflammation and allergy

IL-33 is a nuclear cytokine from the IL-1 family constitutively expressed in epithelial barrier tissues and lymphoid organs, which plays important roles in type-2 innate immunity and human asthma. Recent studies indicate that IL-33 induces production of large amounts of IL-5 and IL-13 by group 2 innate lymphoid cells (ILC2s), for initiation of allergic inflammation shortly after exposure to allergens or infection with parasites or viruses. IL-33 appears to function as an alarmin (alarm signal) rapidly released from producing cells upon cellular damage or cellular stress. In this review, we discuss the cellular sources, mode of action and regulation of IL-33, and we highlight its crucial roles in vivo with particular emphasis on results obtained using IL33-deficient mice.

Basophils promote innate lymphoid cell responses in inflamed skin

Type 2 inflammation underlies allergic diseases such as atopic dermatitis, which is characterized by the accumulation of basophils and group 2 innate lymphoid cells (ILC2s) in inflamed skin lesions. Although murine studies have demonstrated that cutaneous basophil and ILC2 responses are dependent on thymic stromal lymphopoietin, whether these cell populations interact to regulate the development of cutaneous type 2 inflammation is poorly defined. In this study, we identify that basophils and ILC2s significantly accumulate in inflamed human and murine skin and form clusters not observed in control skin. We demonstrate that murine basophil responses precede ILC2 responses and that basophils are the dominant IL-4-enhanced GFP-expressing cell type in inflamed skin. Furthermore, basophils and IL-4 were necessary for the optimal accumulation of ILC2s and induction of atopic dermatitis-like disease. We show that ILC2s express IL-4Rα and proliferate in an IL-4-dependent manner. Additionally, basophil-derived IL-4 was required for cutaneous ILC2 responses in vivo and directly regulated ILC2 proliferation ex vivo. Collectively, these data reveal a previously unrecognized role for basophil-derived IL-4 in promoting ILC2 responses during cutaneous inflammation.

Lessons learned from mice and man: mimicking human allergy through mouse models

The relevance of using mouse models to represent human allergic pathologies is still unclear. Recent studies suggest the limitations of using models as a standard for assessing immune response and tolerance mechanisms, as mouse models often do not sufficiently depict human atopic conditions. Allergy is a combination of aberrant responses to innocuous environmental agents and the subsequent TH2-mediated inflammatory responses. In this review, we will discuss current paradigms of allergy - specifically, TH2-mediated and IgE-associated immune responses - and current mouse models used to recreate these TH2-mediated pathologies. Our overall goal is to highlight discrepancies that exist between mice and men by examining the advantages and disadvantages of allergic mouse models with respect to the human allergic condition.

MHCII-mediated dialog between group 2 innate lymphoid cells and CD4(+) T cells potentiates type 2 immunity and promotes parasitic helminth expulsion

Group 2 innate lymphoid cells (ILC2s) release interleukin-13 (IL-13) during protective immunity to helminth infection and detrimentally during allergy and asthma. Using two mouse models to deplete ILC2s in vivo, we demonstrate that T helper 2 (Th2) cell responses are impaired in the absence of ILC2s. We show that MHCII-expressing ILC2s interact with antigen-specific T cells to instigate a dialog in which IL-2 production from T cells promotes ILC2 proliferation and IL-13 production. Deletion of MHCII renders IL-13-expressing ILC2s incapable of efficiently inducing Nippostrongylus brasiliensis expulsion. Thus, during transition to adaptive T cell-mediated immunity, the ILC2 and T cell crosstalk contributes to their mutual maintenance, expansion and cytokine production. This interaction appears to augment dendritic-cell-induced T cell activation and identifies a previously unappreciated pathway in the regulation of type-2 immunity.

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Genetic ablation of ILC2s impairs type-2 immunity

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MHCII-expressing ILC2s potentiate Th2 responses

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IL-2 from T cells promotes ILC2 proliferation and expression of type-2 cytokines

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MHCII and IL-13 expression by ILC2s is important for N. brasiliensis expulsion

Causes of epidermal filaggrin reduction and their role in the pathogenesis of atopic dermatitis

The epidermis protects human subjects from exogenous stressors and helps to maintain internal fluid and electrolyte homeostasis. Filaggrin is a crucial epidermal protein that is important for the formation of the corneocyte, as well as the generation of its intracellular metabolites, which contribute to stratum corneum hydration and pH. The levels of filaggrin and its degradation products are influenced not only by the filaggrin genotype but also by inflammation and exogenous stressors. Pertinently, filaggrin deficiency is observed in patients with atopic dermatitis regardless of filaggrin mutation status, suggesting that the absence of filaggrin is a key factor in the pathogenesis of this skin condition. In this article we review the various causes of low filaggrin levels, centralizing the functional and morphologic role of a deficiency in filaggrin, its metabolites, or both in the etiopathogenesis of atopic dermatitis.

Beneficial autoimmunity at body surfaces - immune surveillance and rapid type 2 immunity regulate tissue homeostasis and cancer

Epithelial cells (ECs) line body surface tissues and provide a physicochemical barrier to the external environment. Frequent microbial and non-microbial challenges such as those imposed by mechanical disruption, injury or exposure to noxious environmental substances including chemicals, carcinogens, ultraviolet-irradiation, or toxins cause activation of ECs with release of cytokines and chemokines as well as alterations in the expression of cell-surface ligands. Such display of epithelial stress is rapidly sensed by tissue-resident immunocytes, which can directly interact with self-moieties on ECs and initiate both local and systemic immune responses. ECs are thus key drivers of immune surveillance at body surface tissues. However, ECs have a propensity to drive type 2 immunity (rather than type 1) upon non-invasive challenge or stress – a type of immunity whose regulation and function still remain enigmatic. Here, we review the induction and possible role of type 2 immunity in epithelial tissues and propose that rapid immune surveillance and type 2 immunity are key regulators of tissue homeostasis and carcinogenesis.

Complexity of cytokine network regulation of innate lymphoid cells in protective immunity

The body's surface provides a critical barrier shielding us from various mechanical and pathogenic insults by virtue of the physical protection it provides and the presence of specialized populations of innate lymphoid cells (ILCs) that sense inflammatory signals induced by pathogens. This response plays a central role in the development and activation of early immune responses. While ILCs depend on common γ-chain cytokine signaling for their development, an essential component of the armory of these cells is their capacity to produce defensive cytokines when activated by viruses, microbes and other parasites. In this review, we describe the multiple intrinsic and extrinsic pathways that comprise the cytokine circuitry regulating the development and function of ILC necessary for protective immunity.

Beyond NK cells: the expanding universe of innate lymphoid cells

For a long time, natural killer (NK) cells were thought to be the only innate immune lymphoid population capable of responding to invading pathogens under the influence of changing environmental cues. In the last few years, an increasing amount of evidence has shown that a number of different innate lymphoid cell (ILC) populations found at mucosal sites rapidly respond to locally produced cytokines in order to establish or maintain homeostasis. These ILC populations closely mirror the phenotype of adaptive T helper subsets in their repertoire of secreted soluble factors. Early in the immune response, ILCs are responsible for setting the stage to mount an adaptive T cell response that is appropriate for the incoming insult. Here, we review the diversity of ILC subsets and discuss similarities and differences between ILCs and NK cells in function and key transcriptional factors required for their development.

Respiratory viral infection, epithelial cytokines, and innate lymphoid cells in asthma exacerbations

Exacerbations of asthma are most commonly triggered by viral infections, which amplify allergic inflammation. Cytokines released by virus-infected AECs may be important in driving this response. This review focuses on accumulating evidence in support of a role for epithelial cytokines, including IL-33, IL-25, and TSLP, as well as their targets, type 2 innate lymphoid cells (ILC2s), in the pathogenesis of virus-induced asthma exacerbations. Production and release of these cytokines lead to recruitment and activation of ILC2s, which secrete mediators, including IL-5 and IL-13, which augment allergic inflammation. However, little information is currently available about the induction of these responses by the respiratory viruses that are strongly associated with exacerbations of asthma, such as rhinoviruses. Further human studies, as well as improved animal experimental models, are needed to investigate appropriately the pathogenetic mechanisms in virus-induced exacerbations of asthma, including the role of ILCs.

MicroRNAs in allergy and asthma

microRNAs (miRNAs) are short, single-stranded RNA molecules that function together with the partner proteins and cause degradation of target mRNAs or inhibit their translation. A particular miRNA can have hundreds of targets; therefore, miRNAs cumulatively influence the expression of a large proportion of genes. The functions of miRNAs in human diseases have been studied since their discovery in mammalian cells approximately 12 years ago. However, the role of miRNAs in allergic disease has only very recently begun to be uncovered. The purpose of this review is to provide an overview of the functions of miRNAs involved in the development of allergic diseases. We describe here the functions of miRNAs that regulate Th2 polarization and influence general inflammatory and tissue responses. In addition, we will highlight findings about the functions of extracellular miRNAs as possible noninvasive biomarkers of diseases with heterogeneous phenotypes and complex mechanisms and briefly discuss advances in the development of miRNA-based therapeutics.

Human innate lymphoid cells

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.

Mechanisms regulating skin immunity and inflammation

Immune responses in the skin are important for host defence against pathogenic microorganisms. However, dysregulated immune reactions can cause chronic inflammatory skin diseases. Extensive crosstalk between the different cellular and microbial components of the skin regulates local immune responses to ensure efficient host defence, to maintain and restore homeostasis, and to prevent chronic disease. In this Review, we discuss recent findings that highlight the complex regulatory networks that control skin immunity, and we provide new paradigms for the mechanisms that regulate skin immune responses in host defence and in chronic inflammation.

Significance of interleukin-33 and its related cytokines in patients with breast cancers

Interleukin-33 (IL-33) is a recently identified cytokine, an important member of the interleukin-1 family. IL-33 binds to its receptor ST2 to induce type 2 cytokines and exert both pro-inflammatory and protective functions in host defense and disease. Murine breast carcinoma models suggest disruption of ST2 signaling may enhance the anti-tumor immune response, suggesting IL-33 impedes anti-tumor immunity. However, the role of IL-33 in patients with breast cancers (BC) is not elucidated. We detected the expression of IL-33 in tumor tissue, and IL-33 and its related cytokines in serum from BC patients. Using Luminex and immunohistochemistry methods, we found that serum levels of IL-33 were nearly twofold higher in patients with BC, compared to patients with benign breast diseases. In cancer tissues, expression of IL-33 was higher than matched normal breast tissues from the same patients, and was also associated with a well-differentiated phenotype, HER2 overexpression, more lymph nodes involvement, and a family history of malignant carcinoma. These results suggest that IL-33 may play an important role in the progress of BC and may be a useful biomarker for predicting the progress and metastasis of BC.

Innate Lymphoid Cells: Immunoregulatory Cells of Mucosal Inflammation

Inflammation is a very complex immunopathological process occurring due to exaggerated activation of immune system in response to various inflammatory stimuli (i.e. bacterial, viral, fungal or parasitic antigens, xenobiotics, autoantigens and sterile inflammation of unknown cause (i.e. tumor associated inflammation), traumatic inflammation or allergic inflammation etc.). Innate lymphoid cells (ILCs) are particular newly discovered immune cells, which have characteristics of both innate and adaptive immune cells. These cells have shown very significant roles in the pathogenesis of inflammatory disorders at mucosal surfaces (i.e. respiratory tract, gastrointestinal tract and mucosal skin surfaces or barriers). The present review, explores their role in pathogenesis of inflammation at mucosal sites.