The biology of innate lymphoid cells

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

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

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

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

The Bug Stops Here: Innate Lymphoid Cells in Clostridium difficile Infection

The contribution of the innate immune response to the resolution of Clostridium difficile infection (CDI) remains incompletely defined. In this issue of Cell Host & Microbe, Abt et al. demonstrated that innate lymphoid cells and the effector cytokine IFN-γ are important for recovery from the acute phase of CDI.

Innate lymphocyte cells in asthma phenotypes

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

The Etiology and Pathogenesis of Chronic Rhinosinusitis: a Review of Current Hypotheses

Chronic rhinosinusitis (CRS) is a broad clinical syndrome that is characterized by prolonged mucosal inflammation of the nose and paranasal sinuses, and is typically divided into two subtypes based on the presence or absence of nasal polyps. The etiology and pathogenesis of both forms remain areas of active research. Over the last 15 years, a number of hypotheses have been proposed to explain all or part of the clinical CRS spectrum. These hypotheses reflect the concept that CRS results from a dysfunctional interplay between individual host characteristics and factors exogenous to the host. Six broad theories on CRS etiology and pathogenesis are discussed as follows: (1) the "fungal hypothesis," (2) the "superantigen hypothesis," (3) the "biofilm hypothesis," and (4) the "microbiome hypothesis," all of which emphasize key environmental factors, and (5) the "eicosanoid hypothesis" and (6) the "immune barrier hypothesis," which describe specific host factors. These theories are reviewed, and the evidence supporting them is critically appraised.

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

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

Copycat innate lymphoid cells dampen gut inflammation

The mechanisms whereby the gut mucosa tolerates trillions of commensal bacteria without developing inflammation remain poorly understood. A recent Science article reveals that gut innate lymphoid cells constrain inflammatory T cell responses to commensal bacteria by adopting a strategy usually deployed by thymic epithelial cells to negatively select self-reactive T cells.

Innate lymphoid cells: the new kids on the block

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Novel molecular mechanism for generating NK-cell fitness and memory

The mammalian immune system has been traditionally subdivided into two compartments known as the innate and the adaptive. T cells and B cells, which rearrange their antigen-receptor genes using the RAG recombinase, comprise the adaptive arm of immunity. Meanwhile, every other white blood cell has been grouped together under the broad umbrella of innate immunity, including NK cells. NK cells are considered innate lymphocytes because of their rapid responses to stressed cells and their ability to develop without receptor gene rearrangement (i.e. in RAG-deficient mice). However, new findings implicate a critical function for RAG proteins during NK-cell ontogeny, and suggest a novel mechanism by which controlled DNA breaks during NK-cell development dictate the fitness, function, and longevity of these cells. This review highlights recent work describing how DNA break events can impact cellular differentiation and fitness in a variety of cell types and settings.

Role of HLA-B associated transcript 3 in immune diseases

HLA-B associated transcript 3 (BAT3/Scythe/BAG6) is a member of the BAG protein family which can regulate the cell cycle. Recently, BAT3 has also been identified to have immunoregulatory function through kinds of mechanisms. First, BAT3 can promote the maturation of dendritic cells (DCs), the activity of macrophages and the expression of major histocompatibility complex (MHC)-Ⅱ on antigen presenting cells (APCs) to regulate chronic inflammation. Second, BAT3 can suppress T cell immunoglobulin and mucin domain 3 (Tim-3)-mediated cell death and exhaustion of T helper cell type 1 (Th1) to exacerbate autoimmune diseases. Finally, BAT3 can regulate the cytotoxicity of natural killer cells (NKs) in a NKp30-dependent manner to play a part in tumor immune evasion and tumor rejection. Further details about BAT3 and its involvement in immunity and immunity-associated diseases will benefit the novel strategy for treatment of immune diseases.

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

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

Emerging role of long noncoding RNAs in autoimmune diseases

Long noncoding RNA (lncRNA), with size larger than 200 nucleotides, is a new class of noncoding RNA. Emerging evidence has revealed that lncRNAs play a key role in the regulation of immunological functions and autoimmunity. Herein, we review the recent findings of lncRNA regulation in immune functions and in the development of autoimmunity and autoimmune disease. In addition, we focus on the involvement of lncRNA regulation in innate and adaptive immune responses, immune cell development, and differential expression of lncRNAs in autoimmune diseases, including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), type 1 diabetes mellitus (T1DM), multiple sclerosis (MS), autoimmune thyroid disease (AITD), psoriasis, polymyositis/dermatomyositis (PM/DM) and Crohn's disease (CD).

First do no harm: uterine natural killer (NK) cells in assisted reproduction

Natural killer (NK) cells are a type of lymphocyte circulating in peripheral blood named because of their effector functions in killing target cells. Immune cells that share similar phenotypic characteristics but are poor killers populate the uterine lining at implantation and during early pregnancy when the placenta is established. The functions of these uterine NK (uNK) cells are essentially unknown but available data point to a role in regulating placentation in concert with other elements of the decidua and invading trophoblast cells. Despite the lack of scientific rationale and advice from clinical governing bodies, such as the Human Fertilisation and Embryology Authority, an increasing range of tests and therapies are still offered to women undergoing IVF or attending recurrent miscarriage clinics based on the myth that uterine NK cells need suppressing to prevent damage to the embryo. New treatments can be introduced at whim with subsequent demands for expensive trials to prove/disprove their efficacy. The evidence that targeting uNK or peripheral blood NK cells assists women with recurrent pregnancy failure is lacking. Healthcare professionals and patients should very carefully evaluate the practice of immunomodulation to enhance pregnancy outcome. A discussion on how to move towards stricter regulation of immunotherapy in non-hospital settings is now needed because it is clear that the potential risks and costs of these therapies outweigh any benefits.

Lessons from NK Cell Deficiencies in the Mouse

Since their discovery in the late 1970s, in vivo studies on mouse natural killer (NK) cell almost entirely relied on the use of depleting antibodies and were associated with significant limitations. More recently, large-scale gene-expression analyses allowed the identification of NKp46 as one of the best markers of NK cells across mammalian species. Since then, NKp46 has been shown to be expressed on other subsets of innate lymphoid cells (ILCs) such as the closely related ILC1 and the mucosa-associated NCR(+) ILC3. Based on this marker, several mouse models specifically targeting NKp46-expressing cell have recently been produced. Here, we review recent advances in the generation of models of deficiency in NKp46-expressing cells and their use to address the role of NK cells in immunity, notably on the regulation of adaptive immune responses.

Diversification and Functional Specialization of Human NK Cell Subsets

Natural killer (NK) cells are lymphocytes that participate in different facets of immunity. They can act as innate sentinels through recognition and eradication of infected or transformed target cells, so-called immunosurveillance. In addition, they can contain immune responses through the killing of other activated immune cells, so-called immunoregulation. Furthermore, they instruct and regulate immune responses by producing pro-inflammatory cytokines such as IFN-γ, either upon direct target cell recognition or by relaying cytokine cues from various cell types. Recent studies in mouse and man have uncovered infection-associated expansions of NK cell subsets with specific receptor repertoires and diverse patterns of intracellular signaling molecule expression. Moreover, distinct attributes of NK cells in tissues, including tissue-resident subsets, are being further elucidated. Findings support an emerging theme of ever-increasing diversification and functional specialization among different NK cell subsets, with a functional dichotomy between subsets involved in immunoregulation or immunosurveillance. The epigenetic landscapes and transcriptional profiles of different NK cell subsets are providing insights into the molecular regulation of effector functions. Here, we review phenotypic, functional, and developmental characteristics of a spectrum of human NK cell subsets. We also discuss the molecular underpinnings of different NK cell subsets and their potential contributions to immunity as well as disease susceptibility.

Transcriptional Control of NK Cells

Natural killer (NK) cells are innate lymphocytes that survey the environment and protect the host from infected and cancerous cells. As their name implies, NK cells represent an early line of defense during pathogen invasion by directly killing infected cells and secreting inflammatory cytokines. Although the function of NK cells was first described more than four decades ago, the development of this cytotoxic lineage is not well understood. In recent years, we have begun to identify specific transcription factors that control each stage of development and maturation, from ontogeny of the NK cell progenitor to the effector functions of activated NK cells in peripheral organs. This chapter highlights the transcription factors that are unique to NK cells, or shared between NK cells and other hematopoietic cell lineages, but govern the biology of this cytolytic lymphocyte.