Lineage relationship analysis of RORgammat+ innate lymphoid cells

Immune modules shared by innate lymphoid cells and T cells

In recent years, innate lymphoid cells (ILCs) have emerged as innate correlates to T cells. The similarities between ILCs and T cells indicate that lymphocytes of fundamentally distinct lineages can share core "immune modules" that encompass transcriptional circuitry and effector functions while using nonredundant complementary mechanisms of pattern recognition to enact these functions. We review modules currently recognized to be shared between ILCs and T cells.

Tertiary Lymphoid Organs in Central Nervous System Autoimmunity

Multiple sclerosis (MS) is an autoimmune disease characterized by chronic inflammation in the central nervous system (CNS), which results in permanent neuronal damage and substantial disability in patients. Autoreactive T cells are important drivers of the disease; however, the efficacy of B cell depleting therapies uncovered an essential role for B cells in disease pathogenesis. They can contribute to inflammatory processes via presentation of autoantigen, secretion of pro-inflammatory cytokines, and production of pathogenic antibodies. Recently, B cell aggregates reminiscent of tertiary lymphoid organs (TLOs) were discovered in the meninges of MS patients, leading to the hypothesis that differentiation and maturation of autopathogenic B and T cells may partly occur inside the CNS. Since these structures were associated with a more severe disease course, it is extremely important to gain insight into the mechanism of induction, their precise function, and clinical significance. Mechanistic studies in patients are limited. However, a few studies in the MS animal model experimental autoimmune encephalomyelitis (EAE) recapitulate TLO formation in the CNS and provide new insight into CNS TLO features, formation, and function. This review summarizes what we know so far about CNS TLOs in MS and what we have learned about them from EAE models. It also highlights the areas that are in need of further experimental work, as we are just beginning to understand and evaluate the phenomenon of CNS TLOs.

Notch Signaling Contributes to Liver Inflammation by Regulation of Interleukin-22-Producing Cells in Hepatitis B Virus Infection

The mechanism of hepatitis B virus (HBV) induced liver inflammation is not fully elucidated. Notch signaling augmented interleukin (IL)-22 secretion in CD4+ T cells, and Notch-IL-22 axis fine-tuned inflammatory response. We previously demonstrated a proinflammatory role of IL-22 in HBV infection. Thus, in this study, we analyzed the role of Notch in development of IL-22-producing cells in HBV infection by inhibition of Notch signaling using γ-secretase inhibitor DAPT in both hydrodynamic induced HBV-infected mouse model and in peripheral blood cells isolated from patients with HBV infection. mRNA expressions of Notch1 and Notch2 were significantly increased in livers and CD4+ T cells upon HBV infection. Inhibition of Notch signaling in vivo leaded to the reduction in NKp46+ innate lymphoid cells 22 (ILC22) and lymphoid tissue inducer 4 (LTi4) cells in the liver. This process was accompanied by downregulating the expressions of IL-22 and related proinflammatory cytokines and chemokines in the liver, as well as blocking the recruitment of antigen-non-specific inflammatory cells into the liver and subsequent liver injury, but did not affect HBV antigens production and IL-22 secretion in the serum. Furthermore, IL-22 production in HBV non-specific cultured CD4+ T cells, but not HBV-specific CD4+ T cells, was reduced in response to in vitro inhibition of Notch signaling. In conclusion, Notch siganling appears to be an important mediator of the liver inflammation by modulating hepatic ILC22. The potential proinflammatory effect of Notch-mediated ILC22 may be significant for the development of new therapeutic approaches for treatment of hepatitis B.

Emerging concepts and future challenges in innate lymphoid cell biology

Innate lymphoid cells (ILCs) are innate immune cells that are ubiquitously distributed in lymphoid and nonlymphoid tissues and enriched at mucosal and barrier surfaces. Three major ILC subsets are recognized in mice and humans. Each of these subsets interacts with innate and adaptive immune cells and integrates cues from the epithelium, the microbiota, and pathogens to regulate inflammation, immunity, tissue repair, and metabolic homeostasis. Although intense study has elucidated many aspects of ILC development, phenotype, and function, numerous challenges remain in the field of ILC biology. In particular, recent work has highlighted key new questions regarding how these cells communicate with their environment and other cell types during health and disease. This review summarizes new findings in this rapidly developing field that showcase the critical role ILCs play in directing immune responses through their ability to interact with a variety of hematopoietic and nonhematopoietic cells. In addition, we define remaining challenges and emerging questions facing the field. Finally, this review discusses the potential application of basic studies of ILC biology to the development of new treatments for human patients with inflammatory and infectious diseases in which ILCs play a role.

Understanding Immune Cells in Tertiary Lymphoid Organ Development: It Is All Starting to Come Together

Tertiary lymphoid organs (TLOs) are frequently observed in tissues affected by non-resolving inflammation as a result of infection, autoimmunity, cancer, and allograft rejection. These highly ordered structures resemble the cellular composition of lymphoid follicles typically associated with the spleen and lymph node compartments. Although TLOs within tissues show varying degrees of organization, they frequently display evidence of segregated T and B cell zones, follicular dendritic cell networks, a supporting stromal reticulum, and high endothelial venules. In this respect, they mimic the activities of germinal centers and contribute to the local control of adaptive immune responses. Studies in various disease settings have described how these structures contribute to either beneficial or deleterious outcomes. While the development and architectural organization of TLOs within inflamed tissues requires homeostatic chemokines, lymphoid and inflammatory cytokines, and adhesion molecules, our understanding of the cells responsible for triggering these events is still evolving. Over the past 10–15 years, novel immune cell subsets have been discovered that have more recently been implicated in the control of TLO development and function. In this review, we will discuss the contribution of these cell types and consider the potential to develop new therapeutic strategies that target TLOs.

Interleukin-23-Dependent γ/δ T Cells Produce Interleukin-17 and Accumulate in the Enthesis, Aortic Valve, and Ciliary Body in Mice

OBJECTIVE

The spondyloarthritides (SpA) are a group of rheumatic diseases characterized by ossification and inflammation of entheseal tissue, the region where tendon attaches to bone. Interleukin-23 (IL-23) is involved in the pathogenesis of SpA by acting on IL-23 receptor (IL-23R) expressed on enthesis-resident lymphocytes. Upon IL-23 binding, CD3+CD4-CD8- tissue-resident lymphocytes secrete IL-17A and IL-22, leading to inflammation, bone loss, and ossification. Knowledge about enthesis-resident lymphocytes remains fragmentary, and the contribution of entheseal γ/δ T cells in particular is not clear. This study was undertaken to investigate the presence of γ/δ T cells in the enthesis.

METHODS

We used 2-photon microscopy and flow cytometry to analyze entheseal lymphocytes from C57BL/6, Tcrd-H2BeGFP, Rorc-GFP, and IL-23R-eGFP mice. To analyze entheseal γ/δ T cells in IL-23-induced inflammation, Tcrd-H2BeGFP mice were crossed with mice of the susceptible B10.RIII background. Hydrodynamic injection of IL-23 minicircle DNA was performed for overexpression of IL-23 and induction of inflammation. Light-sheet fluorescence microscopy was used to visualize arthritic inflammation.

RESULTS

Activated Vγ6+CD27- γ/δ T cells were abundant in uninflamed entheseal tissue and constituted the large majority of retinoic acid receptor-related orphan nuclear receptor γt (RORγt)+IL-23R+ enthesis-resident lymphocytes. Fetal thymus-dependent γ/δ T cells were the main source of IL-17A at the enthesis. Under inflammatory conditions, γ/δ T cells increased in number at the Achilles tendon enthesis, aortic root, and adjacent to the ciliary body.

CONCLUSION

Entheseal γ/δ T cells are derived from fetal thymus and are maintained as self-renewing tissue-resident cells. As main IL-17A producers within tissues exposed to mechanical stress including enthesis, γ/δ T cells are key players in the pathogenesis of IL-23-induced local inflammation.

Colonization and effector functions of innate lymphoid cells in mucosal tissues

Innate lymphoid cells (ILCs) protect mucosal barrier tissues to fight infection and maintain tissue integrity. ILCs and their progenitors are developmentally programmed to migrate, differentiate and populate various mucosal tissues and associated lymphoid tissues. Functionally mature ILC subsets respond to diverse pathogens such as bacteria, viruses, fungi and parasites in subset-specific manners. In this review, we will discuss how ILCs populate mucosal tissues and regulate immune responses to distinct pathogens to protect the host and maintain tissue integrity.

Human innate lymphoid cells

Innate lymphoid cells (ILCs) are increasingly acknowledged as important mediators of immune homeostasis and pathology. ILCs act as early orchestrators of immunity, responding to epithelium-derived signals by expressing an array of cytokines and cell-surface receptors, which shape subsequent immune responses. As such, ILCs make up interesting therapeutic targets for several diseases. In patients with allergy and asthma, group 2 innate lymphoid cells produce high amounts of IL-5 and IL-13, thereby contributing to type 2-mediated inflammation. Group 3 innate lymphoid cells are implicated in intestinal homeostasis and psoriasis pathology through abundant IL-22 production, whereas group 1 innate lymphoid cells are accumulated in chronic inflammation of the gut (inflammatory bowel disease) and lung (chronic obstructive pulmonary disease), where they contribute to IFN-γ-mediated inflammation. Although the ontogeny of mouse ILCs is slowly unraveling, the development of human ILCs is far from understood. In addition, the growing complexity of the human ILC family in terms of previously unrecognized functional heterogeneity and plasticity has generated confusion within the field. Here we provide an updated view on the function and plasticity of human ILCs in tissue homeostasis and disease.

The orphan nuclear receptor ROR alpha and group 3 innate lymphoid cells drive fibrosis in a mouse model of Crohn's disease

Fibrosis is the result of dysregulated tissue regeneration and is characterized by excessive accumulation of matrix proteins that become detrimental to tissue function. In Crohn's disease, this manifests itself as recurrent gastrointestinal strictures for which there is no effective therapy beyond surgical intervention. Using a model of infection-induced chronic gut inflammation, we show that Rora-deficient mice are protected from fibrosis; infected intestinal tissues display diminished pathology, attenuated collagen deposition and reduced fibroblast accumulation. Although Rora is best known for its role in ILC2 development, we find that Salmonella-induced fibrosis is independent of eosinophils, STAT6 signaling and Th2 cytokine production arguing that this process is largely ILC2-independent. Instead, we observe reduced levels of ILC3- and T cell-derived IL-17A and IL-22 in infected gut tissues. Furthermore, using Rorasg/sg /Rag1-/- bone marrow chimeric mice, we show that restoring ILC function is sufficient to re-establish IL-17A and IL-22 production and a profibrotic phenotype. Our results show that RORα-dependent ILC3 functions are pivotal in mediating gut fibrosis and they offer an avenue for therapeutic intervention in Crohn's-like diseases.

Innate lymphoid cells: models of plasticity for immune homeostasis and rapid responsiveness in protection

Innate lymphoid cells (ILCs) have stormed onto the immune landscape as "newly discovered" cell types. These tissue-resident sentinels are enriched at mucosal surfaces and engage in complex cross talk with elements of the adaptive immune system and microenvironment to orchestrate immune homeostasis. Many parallels exist between innate cells and T cells leading to the initial partitioning of ILCs into rather rigid subsets that reflect their "adaptive-like" effector cytokines profiles. ILCs themselves, however, have unique attributes that are only just beginning to be elucidated. These features result in complementarity with, rather than complete duplication of, functions of the adaptive immune system. Key transcription factors determine the pathway of differentiation of progenitors towards an ILC1, ILC2, or ILC3 subset. Once formed, flexibility in the responses of these subsets to stimuli unexpectedly allows transdifferentation between the different subsets and the acquisition of altered phenotypes and function. This provides a mechanism for rapid innate immune responsiveness. Here, we discuss the models of differentiation for maintenance and activation of tissue-resident ILCs in maintaining immune homeostasis and protection.

The origin and role of innate lymphoid cells in the lung

Innate lymphoid cells (ILCs), a newly identified member of the lymphoid population, play a critical role in the transition from innate to adaptive immunity in host defense. ILCs are important in mucosal barrier immunity, tissue homeostasis, and immune regulation throughout the body. Significant alterations in ILC responses in lung diseases have been observed and reported. Emerging evidence has shown that ILCs are importantly involved in the pathogenesis and development of a variety of lung diseases, i.e., helminth infections, allergic airway inflammation, and airway hyper-responsiveness. However, as a tissue-resident cell population, the role of ILCs in the lung remains poorly characterized. In this review, we discuss the role of ILCs in lung diseases, the mechanisms underlying the ILC-mediated regulation of immunity, and the therapeutic potential of modulating ILC responses.

Innate lymphoid cell regulation of adaptive immunity

Innate lymphoid cells (ILCs) were identified principally as non-T-cell sources of key cytokines, able to provide rapid and early production of these molecules in the support of tissue homeostasis, repair and response to infection. As our understanding of these cells has developed, it has become evident that ILCs can impact on lymphocytes through a range of mechanisms. Hence, an exciting area of research has evolved in determining the extent to which ILCs may regulate adaptive immune responses. This review will focus initially on our current understanding of where ILC populations are located and what this means for potential cellular interactions. Mechanisms underpinning such interactions and how they may contribute to controlling adaptive immunity will then be considered.

The microbiome and innate immunity

The intestinal microbiome is a signalling hub that integrates environmental inputs, such as diet, with genetic and immune signals to affect the host's metabolism, immunity and response to infection. The haematopoietic and non-haematopoietic cells of the innate immune system are located strategically at the host-microbiome interface. These cells have the ability to sense microorganisms or their metabolic products and to translate the signals into host physiological responses and the regulation of microbial ecology. Aberrations in the communication between the innate immune system and the gut microbiota might contribute to complex diseases.

Glial-cell-derived neuroregulators control type 3 innate lymphoid cells and gut defence

Group 3 innate lymphoid cells (ILC3) are major regulators of inflammation and infection at mucosal barriers. ILC3 development is thought to be programmed, but how ILC3 perceive, integrate and respond to local environmental signals remains unclear. Here we show that ILC3 in mice sense their environment and control gut defence as part of a glial–ILC3–epithelial cell unit orchestrated by neurotrophic factors. We found that enteric ILC3 express the neuroregulatory receptor RET. ILC3-autonomous Ret ablation led to decreased innate interleukin-22 (IL-22), impaired epithelial reactivity, dysbiosis and increased susceptibility to bowel inflammation and infection. Neurotrophic factors directly controlled innate Il22 downstream of the p38 MAPK/ERK-AKT cascade and STAT3 activation. Notably, ILC3 were adjacent to neurotrophic-factor-expressing glial cells that exhibited stellate-shaped projections into ILC3 aggregates. Glial cells sensed microenvironmental cues in a MYD88-dependent manner to control neurotrophic factors and innate IL-22. Accordingly, glial-intrinsic Myd88 deletion led to impaired production of ILC3-derived IL-22 and a pronounced propensity towards gut inflammation and infection. Our work sheds light on a novel multi-tissue defence unit, revealing that glial cells are central hubs of neuron and innate immune regulation by neurotrophic factor signals.

Innate lymphoid cells, precursors and plasticity

Innate lymphoid cells (ILC) have only recently been recognized as a separate entity of the lymphoid lineage. Their subpopulations share common characteristics in terms of early development and major transcriptional circuitry with their related cousins of the T cell world. It is currently hypothesized that ILCs constitute an evolutionary older version of the lymphoid immune system. They are found at all primary entry points for pathogens such as mucosal surfaces of the lung and gastrointestinal system, the skin and the liver, which is the central contact point for pathogens that breach the intestinal barrier and enter the circulation. There, ILC contribute to the first line defense as well as to organ homeostasis. However, ILC are not only involved in classical defense tasks, but also contribute to the organogenesis of lymphoid organs as well as tissue remodeling and even stem cell regeneration. ILC may, therefore, implement different functions according to their emergence in ontogeny, their development and their final tissue location. We will review here their early development from precursors of the fetal liver and the adult bone marrow as well as their late plasticity in adaptation to their environment.

Tolerance to the Intestinal Microbiota Mediated by ROR(γt)(+) Cells

Harmless microbes colonizing the gut require the establishment of a well-equilibrated symbiosis between this microbiota and its host. However, the immune system is primed to recognize both conserved microbial patterns and foreign antigens, and therefore developed strong tolerance mechanisms to prevent potential fatal immune reactivity to symbiotic microbes. The transcription factor RAR-related orphan-like γt [ROR(γt); encoded by Rorc] plays a key role in the gut for lymphoid tissue organogenesis, development of innate lymphoid cells type 3 (ILC3s) and proinflammatory type 17 T helper (Th17) cells. Surprisingly, recent research has revealed a contribution of ROR(γt)-expressing cells in a variety of tolerance mechanisms in both the innate and adaptive immune system.

IgA production requires B cell interaction with subepithelial dendritic cells in Peyer's patches

Immunoglobulin A (IgA) induction primarily occurs in intestinal Peyer's patches (PPs). However, the cellular interactions necessary for IgA class switching are poorly defined. Here we show that in mice, activated B cells use the chemokine receptor CCR6 to access the subepithelial dome (SED) of PPs. There, B cells undergo prolonged interactions with SED dendritic cells (DCs). PP IgA class switching requires innate lymphoid cells, which promote lymphotoxin-β receptor (LTβR)-dependent maintenance of DCs. PP DCs augment IgA production by integrin αvβ8-mediated activation of transforming growth factor-β (TGFβ). In mice where B cells cannot access the SED, IgA responses against oral antigen and gut commensals are impaired. These studies establish the PP SED as a niche supporting DC-B cell interactions needed for TGFβ activation and induction of mucosal IgA responses.

Thymus: the next (re)generation

As the primary site of T-cell development, the thymus plays a key role in the generation of a strong yet self-tolerant adaptive immune response, essential in the face of the potential threat from pathogens or neoplasia. As the importance of the role of the thymus has grown, so too has the understanding that it is extremely sensitive to both acute and chronic injury. The thymus undergoes rapid degeneration following a range of toxic insults, and also involutes as part of the aging process, albeit at a faster rate than many other tissues. The thymus is, however, capable of regenerating, restoring its function to a degree. Potential mechanisms for this endogenous thymic regeneration include keratinocyte growth factor (KGF) signaling, and a more recently described pathway in which innate lymphoid cells produce interleukin-22 (IL-22) in response to loss of double positive thymocytes and upregulation of IL-23 by dendritic cells. Endogenous repair is unable to fully restore the thymus, particularly in the aged population, and this paves the way toward the need for exogenous strategies to help regenerate or even replace thymic function. Therapies currently in clinical trials include KGF, use of the cytokines IL-7 and IL-22, and hormonal modulation including growth hormone administration and sex steroid inhibition. Further novel strategies are emerging in the preclinical setting, including the use of precursor T cells and thymus bioengineering. The use of such strategies offers hope that for many patients, the next regeneration of their thymus is a step closer.

Phenotypic and Functional Plasticity of Murine Intestinal NKp46+ Group 3 Innate Lymphoid Cells

Group 3 innate lymphoid cells (ILC3) actively participate in mucosal defense and homeostasis through prompt secretion of IL-17A, IL-22, and IFN-γ. Reports identify two ILC3 lineages: a CCR6(+)T-bet(-) subset that appears early in embryonic development and promotes lymphoid organogenesis and a CCR6(-)T-bet(+) subset that emerges after microbial colonization and harbors NKp46(+) ILC3. We demonstrate that NKp46 expression in the ILC3 subset is highly unstable. Cell fate mapping using Ncr1(CreGFP) × Rosa26(RFP) mice revealed the existence of an intestinal RFP(+) ILC3 subset (Ncr1(FM)) lacking NKp46 expression at the transcript and protein levels. Ncr1(FM) ILC3 produced more IL-22 and were distinguishable from NKp46(+) ILC3 by differential CD117, CD49a, DNAX accessory molecule-1, and, surprisingly, CCR6 expression. Ncr1(FM) ILC3 emerged after birth and persisted in adult mice following broad-spectrum antibiotic treatment. These results identify an unexpected phenotypic instability within NKp46(+) ILC3 that suggests a major role for environmental signals in tuning ILC3 functional plasticity.

Cutting Edge: Innate Lymphoid Cells Suppress Homeostatic T Cell Expansion in Neonatal Mice

In adult mice, lymphopenia-induced proliferation (LIP) leads to T cell activation, memory differentiation, tissue destruction, and a loss of TCR diversity. Neonatal mice are lymphopenic within the first week of life. This enables some recent thymic emigrants to undergo LIP and convert into long-lived memory T cells. Surprisingly, however, most neonatal T cells do not undergo LIP. We therefore asked whether neonate-specific mechanisms prevent lymphopenia-driven T cell activation. In this study, we show that IL-7R-dependent innate lymphoid cells (ILCs) block LIP of CD8(+) T cells in neonatal but not adult mice. Importantly, CD8(+) T cell responses against a foreign Ag are not inhibited by neonatal ILCs. This ILC-based inhibition of LIP ensures the generation of a diverse naive T cell pool in lymphopenic neonates that is mandatory for the maintenance of T cell homeostasis and immunological self-tolerance later in life.

Transient inhibition of ROR-γt therapeutically limits intestinal inflammation by reducing TH17 cells and preserving group 3 innate lymphoid cells

RAR-related orphan receptor-γt (ROR-γt) directs differentiation of proinflammatory T helper 17 (TH17) cells and is a potential therapeutic target in chronic autoimmune and inflammatory diseases. However, ROR-γt-dependent group 3 innate lymphoid cells ILC3s provide essential immunity and tissue protection in the intestine, suggesting that targeting ROR-γt could also result in impaired host defense after infection or enhanced tissue damage. Here, we demonstrate that transient chemical inhibition of ROR-γt in mice selectively reduces cytokine production from TH17 but not ILCs in the context of intestinal infection with Citrobacter rodentium, resulting in preserved innate immunity. Temporal deletion of Rorc (encoding ROR-γt) in mature ILCs also did not impair cytokine response in the steady state or during infection. Finally, pharmacologic inhibition of ROR-γt provided therapeutic benefit in mouse models of intestinal inflammation and reduced the frequency of TH17 cells but not ILCs isolated from primary intestinal samples of individuals with inflammatory bowel disease (IBD). Collectively, these results reveal differential requirements for ROR-γt in the maintenance of TH17 cell and ILC3 responses and suggest that transient inhibition of ROR-γt is a safe and effective therapeutic approach during intestinal inflammation.

Microbiota, immunity and the liver

The gut harbors a complex community of over 100 trillion microbial cells known to exist in symbiotic harmony with the host influencing human physiology, metabolism, nutrition and immune function. It is now widely accepted that perturbations of this close partnership results in the pathogenesis of several major diseases with increasing evidence highlighting their role outside of the intestinal tract. The intimate proximity and circulatory loop of the liver and the gut has attracted significant attention regarding the role of the microbiota in the development and progression of liver disease. Here we give an overview of the interaction between the microbiota and the immune system and focus on their convincing role in both the propagation and treatment of liver disease.

NCR1+ cells appear early in GALT development of the ovine foetus and acquire a c-kit+ phenotype towards the end of gestation

The amount, distribution and phenotype of ovine NCR1+ cells were investigated during developing GALT from day 70 of gestation. Antibodies against CD3 and CD79 were used to identify the compartments of GALT, and the localization of NCR1+ cells were correlated within these structures. Markers CD34 and c-kit, in addition to Ki67, were used to investigate possible origin and the stage of development of the NCR1+ cells. NCR1+ cells were present as single cells in the subepithelial tissue as early as 70 days of gestation, and were predominantly present in the T cell rich IFAs and domes as these intestinal wall compartments developed. While NCR1+ cells proliferated more intensively at mid-gestation (70-104 days), the number of NCR1+ cells also expressing c-kit, increased at the end of gestation. In conclusion, NCR1+ cells appeared early in T cell areas of the gut and displayed a phenotype consistent with intermediate stages of cNK cells and/or a subpopulation of ILC22.

Antibiotics, pediatric dysbiosis, and disease

Antibiotics are by far the most common medications prescribed for children. Recent epidemiological data suggests an association between early antibiotic use and disease phenotypes in adulthood. Antibiotic use during infancy induces imbalances in gut microbiota, called dysbiosis. The gut microbiome's responses to antibiotics and its potential link to disease development are especially complex to study in the changing infant gut. Here, we synthesize current knowledge linking antibiotics, dysbiosis, and disease and propose a framework for studying antibiotic-related dysbiosis in children. We recommend future studies into the microbiome-mediated effects of antibiotics focused on four types of dysbiosis: loss of keystone taxa, loss of diversity, shifts in metabolic capacity, and blooms of pathogens. Establishment of a large and diverse baseline cohort to define healthy infant microbiome development is essential to advancing diagnosis, interpretation, and eventual treatment of pediatric dysbiosis. This approach will also help provide evidence-based recommendations for antibiotic usage in infancy.

Group 3 innate lymphoid cells continuously require the transcription factor GATA-3 after commitment

The transcription factor GATA-3 is indispensable for the development of all innate lymphoid cells (ILCs) that express the interleukin 7 receptor α-chain (IL-7Rα). However, the function of low GATA-3 expression in committed group 3 ILCs (ILC3 cells) has not been identified. We found that GATA-3 regulated the homeostasis of ILC3 cells by controlling IL-7Rα expression. In addition, GATA-3 served a critical function in the development of the NKp46(+) ILC3 subset by regulating the balance between the transcription factors T-bet and RORγt. Among NKp46(+) ILC3 cells, although GATA-3 positively regulated genes specific to the NKp46(+) ILC3 subset, it negatively regulated genes specific to lymphoid tissue-inducer (LTi) or LTi-like ILC3 cells. Furthermore, GATA-3 was required for IL-22 production in both ILC3 subsets. Thus, despite its low expression, GATA-3 was critical for the homeostasis, development and function of ILC3 subsets.

Interleukin-22-Induced Antimicrobial Phospholipase A2 Group IIA Mediates Protective Innate Immunity of Nonhematopoietic Cells against Listeria monocytogenes

Listeria monocytogenes is a bacterial pathogen which establishes intracellular parasitism in various cells, including macrophages and nonhematopoietic cells, such as hepatocytes. It has been reported that several proinflammatory cytokines have pivotal roles in innate protection against L. monocytogenes infection. We found that a proinflammatory cytokine, interleukin 22 (IL-22), was expressed by CD3(+) CD4(+) T cells at an early stage of L. monocytogenes infection in mice. To assess the influence of IL-22 on L. monocytogenes infection in hepatocytes, cells of a human hepatocellular carcinoma line, HepG2, were treated with IL-22 before L. monocytogenes infection in vitro. Gene expression analysis of the IL-22-treated HepG2 cells identified phospholipase A2 group IIA (PLA2G2A) as an upregulated antimicrobial molecule. Addition of recombinant PLA2G2A to the HepG2 culture significantly suppressed L. monocytogenes infection. Culture supernatant of the IL-22-treated HepG2 cells contained bactericidal activity against L. monocytogenes, and the activity was abrogated by a specific PLA2G2A inhibitor, demonstrating that HepG2 cells secreted PLA2G2A, which killed extracellular L. monocytogenes. Furthermore, colocalization of PLA2G2A and L. monocytogenes was detected in the IL-22-treated infected HepG2 cells, which suggests involvement of PLA2G2A in the mechanism of intracellular killing of L. monocytogenes by HepG2 cells. These results suggest that IL-22 induced at an early stage of L. monocytogenes infection enhances innate immunity against L. monocytogenes in the liver by stimulating hepatocytes to produce an antimicrobial molecule, PLA2G2A.

Single-Cell Gene Expression Analyses Reveal Heterogeneous Responsiveness of Fetal Innate Lymphoid Progenitors to Notch Signaling

T and innate lymphoid cells (ILCs) share some aspects of their developmental programs. However, although Notch signaling is strictly required for T cell development, it is dispensable for fetal ILC development. Constitutive activation of Notch signaling, at the common lymphoid progenitor stage, drives T cell development and abrogates ILC development by preventing Id2 expression. By combining single-cell transcriptomics and clonal culture strategies, we characterize two heterogeneous α4β7-expressing lymphoid progenitor compartments. αLP1 (Flt3(+)) still retains T cell potential and comprises the global ILC progenitor, while αLP2 (Flt3(-)) consists of ILC precursors that are primed toward the different ILC lineages. Only a subset of αLP2 precursors is sensitive to Notch signaling required for their proliferation. Our study identifies, in a refined manner, the diversity of transitional stages of ILC development, their transcriptional signatures, and their differential dependence on Notch signaling.

Epithelial Control of Gut-Associated Lymphoid Tissue Formation through p38α-Dependent Restraint of NF-κB Signaling

The protein kinase p38α mediates cellular responses to environmental and endogenous cues that direct tissue homeostasis and immune responses. Studies of mice lacking p38α in several different cell types have demonstrated that p38α signaling is essential to maintaining the proliferation-differentiation balance in developing and steady-state tissues. The mechanisms underlying these roles involve cell-autonomous control of signaling and gene expression by p38α. In this study, we show that p38α regulates gut-associated lymphoid tissue (GALT) formation in a noncell-autonomous manner. From an investigation of mice with intestinal epithelial cell-specific deletion of the p38α gene, we find that p38α serves to limit NF-κB signaling and thereby attenuate GALT-promoting chemokine expression in the intestinal epithelium. Loss of this regulation results in GALT hyperplasia and, in some animals, mucosa-associated B cell lymphoma. These anomalies occur independently of luminal microbial stimuli and are most likely driven by direct epithelial-lymphoid interactions. Our study illustrates a novel p38α-dependent mechanism preventing excessive generation of epithelial-derived signals that drive lymphoid tissue overgrowth and malignancy.

The Gut Epithelial Receptor LRRC19 Promotes the Recruitment of Immune Cells and Gut Inflammation

Commensal microbes are necessary for a healthy gut immune system. However, the mechanism involving these microbes that establish and maintain gut immune responses is largely unknown. Here, we have found that the gut immune receptor leucine-rich repeat (LRR) C19 is involved in host-microbiota interactions. LRRC19 deficiency not only impairs the gut immune system but also reduces inflammatory responses in gut tissues. We demonstrate that the LRRC19-associated chemokines CCL6, CCL9, CXCL9, and CXCL10 play a critical role in immune cell recruitment and intestinal inflammation. The expression of these chemokines is associated with regenerating islet-derived (REG) protein-mediated microbiotas. We also found that the expression of REGs may be regulated by gut Lactobacillus through LRRC19-mediated activation of NF-κB. Therefore, our study establishes a regulatory axis of LRRC19, REGs, altered microbiotas, and chemokines for the recruitment of immune cells and the regulation of intestinal inflammation.

•

The gut immune receptor LRRC19 is involved in host-microbiota interactions

•

LRRC19-associated chemokines control immune cell recruitment and gut inflammation

•

Chemokines are regulated by REG protein-mediated gut microbiotas

•

Lactobacillus may modulate the expression of REG proteins through LRRC19

Heterogeneity and diversity of group 3 innate lymphoid cells: new cells on the block

Innate lymphoid cells (ILCs) are a newly identified subset of innate cells that play fundamentally crucial roles for early immune defense at mucosal and non-mucosal sites. ILCs consist of ILC1s, ILC2s and ILC3s, which each have distinct transcription factors controlling their development and function. Interestingly, each of the ILC subsets represents the innate counterparts of CD4(+) helper T-cell subsets T(h1), T(h2) and T(h17) on the basis of transcriptional regulation. ILC1s that produce IFN-γ or TNF-α, ILC2s that produce T(h2)-type cytokines mainly such as IL-5 or IL-13 and ILC3s have been recently reported and reviewed in terms of IL-22- or IL-17-producing function and cell development. However, in this relatively new field, it remains likely that additional functional and regulatory mechanisms remain to be explored. More recent findings show that ILC3s are regulated by RORγt, which plays an important role for the mucosal barrier and surface protection against pathogenic bacterial infection. ILC3s might cooperate with other cells (e.g. T cells or dendritic cells) directly or indirectly, and subsequently ILC3s have impact on tissues with prompt regulation. Especially, ILC3s in mucosal site are well known to protect the intestinal surface barrier through inducible anti-microbial peptides via IL-22. Here, I will summarize and discuss the roles, function and heterogeneity of ILC3s in mucosal tissues.

Migration and Tissue Tropism of Innate Lymphoid Cells

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

Ectopic lymphoid follicles: inducible centres for generating antigen-specific immune responses within tissues

Lymphoid neogenesis is traditionally viewed as a pre‐programmed process that promotes the formation of lymphoid organs during development. Here, the spatial organization of T and B cells in lymph nodes and spleen into discrete structures regulates antigen‐specific responses and adaptive immunity following immune challenge. However, lymphoid neogenesis is also triggered by chronic or persistent inflammation. Here, ectopic (or tertiary) lymphoid organs frequently develop in inflamed tissues as a response to infection, auto‐immunity, transplantation, cancer or environmental irritants. Although these structures affect local immune responses, the contribution of these lymphoid aggregates to the underlining pathology are highly context dependent and can elicit either protective or deleterious outcomes. Here we review the cellular and molecular mechanisms responsible for ectopic lymphoid neogenesis and consider the relevance of these structures in human disease.

Runx3 specifies lineage commitment of innate lymphoid cells

Subsets of innate lymphoid cells (ILCs) reside in the mucosa and regulate immune responses to external pathogens. While ILCs can be phenotypically classified into ILC1, ILC2 and ILC3 subsets, the transcriptional control of commitment to each ILC lineage is incompletely understood. Here we report that the transcription factor Runx3 was essential for the normal development of ILC1 and ILC3 cells but not of ILC2 cells. Runx3 controlled the survival of ILC1 cells but not of ILC3 cells. Runx3 was required for expression of the transcription factor RORγt and its downstream target, the transcription factor AHR, in ILC3 cells. The absence of Runx3 in ILCs exacerbated infection with Citrobacter rodentium. Therefore, our data establish Runx3 as a key transcription factor in the lineage-specific differentiation of ILC1 and ILC3 cells.

Cross-talk between type 3 innate lymphoid cells and the gut microbiota in inflammatory bowel disease

PURPOSE OF REVIEW

Innate lymphoid cells (ILCs) are a newly-identified population of immune cells prevalent in, but not limited to, mucosal tissues that not only play a significant role in immune homeostasis and host defense, but also in disease pathogenesis. This review highlights the importance of type 3 ILCs (ILC3s) and their interactions with the intestinal microflora, both in maintaining gut health and in the development of inflammatory bowel disease (IBD).

RECENT FINDINGS

Distinct lineages of ILCs are defined based on the presence of cell surface proteins, secretion of effector cytokines and expression of master transcription factors that determine their differentiation and inflammatory behavior. These ILC subgroups mirror corresponding CD4 T-cell subsets, with which they share many phenotypic, morphologic and functional attributes. ILC3s, in particular, through direct and indirect interactions with the gut microbiota, have been identified to promote protection and maintenance of epithelial integrity, as well as to regulate intestinal inflammation and fibrosis, such as that observed in IBD.

SUMMARY

Gut mucosal ILCs respond to environmental cues, such as diet and microflora composition, which can shape downstream immune function. As such, ILCs represent attractive targets for the development of therapeutic modalities to maintain gut health and to potentially treat IBD.

Type 3 innate lymphoid cells maintain intestinal epithelial stem cells after tissue damage

Disruption of the intestinal epithelial barrier allows bacterial translocation and predisposes to destructive inflammation. To ensure proper barrier composition, crypt-residing stem cells continuously proliferate and replenish all intestinal epithelial cells within days. As a consequence of this high mitotic activity, mucosal surfaces are frequently targeted by anticancer therapies, leading to dose-limiting side effects. The cellular mechanisms that control tissue protection and mucosal healing in response to intestinal damage remain poorly understood. Type 3 innate lymphoid cells (ILC3s) are regulators of homeostasis and tissue responses to infection at mucosal surfaces. We now demonstrate that ILC3s are required for epithelial activation and proliferation in response to small intestinal tissue damage induced by the chemotherapeutic agent methotrexate. Multiple subsets of ILC3s are activated after intestinal tissue damage, and in the absence of ILC3s, epithelial activation is lost, correlating with increased pathology and severe damage to the intestinal crypts. Using ILC3-deficient Lgr5 reporter mice, we show that maintenance of intestinal stem cells after damage is severely impaired in the absence of ILC3s or the ILC3 signature cytokine IL-22. These data unveil a novel function of ILC3s in limiting tissue damage by preserving tissue-specific stem cells.

CXCR6 Expression Is Important for Retention and Circulation of ILC Precursors

Innate lymphoid cells are present at mucosal sites and represent the first immune barrier against infections, but what contributes to their circulation and homing is still unclear. Using Rag2^−/−Cxcr6^Gfp/+ reporter mice, we assessed the expression and role of CXCR6 in the circulation of ILC precursors and their progeny. We identify CXCR6 expressing ILC precursors in the bone marrow and characterize their significant increase in CXCR6-deficient mice at steady state, indicating their partial retention in the bone marrow after CXCR6 ablation. Circulation was also impaired during embryonic life as fetal liver from CXCR6-deficient embryos displayed decreased numbers of ILC3 precursors. When injected, fetal CXCR6-deficient ILC3 precursors also fail to home and reconstitute ILC compartments in vivo. We show that adult intestinal ILC subsets have heterogeneous expression pattern of CXCR6, integrin α₄β₇, CD62L, CD69, and CD44, with ILC1 and ILC3 being more likely tissue resident lymphocytes. Intestinal ILC subsets were unchanged in percentages and numbers in both mice. We demonstrate that the ILC frequency is maintained due to a significant increase of ILC peripheral proliferation, as well as an increased proliferation of the in situ ILC precursors to compensate their retention in the bone marrow.

Characteristics of innate lymphoid cells (ILCs) and their role in immunological disorders (an update)

Innate lymphoid cells (ILCs) are a novel family of hematopoietic effectors and regulators of innate immunity. Although these cells are morphologically similar to B cells and T cells, however they do not express antigen receptors. ILCs seems to have emerging roles in innate immune responses against infectious or non-infectious microorganisms, protection of the epithelial barrier, lymphoid organogenesis and inflammation, tissue remodeling and regulating homeostasis of tissue stromal cells. In addition, it has recently been reported that ILCs have a crucial role in several disorders such as allergy and autoimmunity. Based on their phenotype and functions, ILCs are classified into three major groups called ILCs1, ILCs2, and ILCs3. Here we reviewed the most recent data concerning diverse ILC phenotypes, subclasses, functions in immune responses as well as in immune mediated disorders.

Intestinal Microbiota and the Innate Immune System - A Crosstalk in Crohn's Disease Pathogenesis

Crohn's disease (CD) is a chronic, relapsing inflammatory disorder that can occur anywhere along the gastrointestinal tract. The precise etiology of CD is still unclear but it is widely accepted that a complex series of interactions between susceptibility genes, the immune system and environmental factors are implicated in the onset and perpetuation of the disease. Increasing evidence from experimental and clinical studies implies the intestinal microbiota in disease pathogenesis, thereby supporting the hypothesis that chronic intestinal inflammation arises from an abnormal immune response against the microorganisms of the intestinal flora in genetically susceptible individuals. Given that CD patients display changes in their gut microbiota composition, collectively termed "dysbiosis," the question raises whether the altered microbiota composition is a cause of disease or rather a consequence of the inflammatory state of the intestinal environment. This review will focus on the crosstalk between the gut microbiota and the innate immune system during intestinal inflammation, thereby unraveling the role of the microbiota in CD pathogenesis.

Differentiation and function of group 3 innate lymphoid cells, from embryo to adult

Group 3 innate lymphoid cells (ILC3) represent a heterogeneous population of cells that share the nuclear hormone receptor RORγt (retinoic acid receptor-related orphan receptor γt) as a master regulator for differentiation and function. ILC3 can be divided into two major subsets based on the cell surface expression of the natural cytotoxicity receptor (NCR), NKp46. A subset of NCR(-) ILC3 includes the previously known lymphoid-tissue inducer cells that are essential for the embryonic formation of peripheral lymph nodes and Peyer's patches. After birth, the NCR(-) and NCR(+) ILC3 contribute to the maintenance of health but also to inflammation in mucosal tissues. This review will describe the differentiation pathways of ILC3, their involvement in the development of the adaptive immune system and their role in the establishment and maintenance of gut immunity.

Differentiation and diversity of subsets in group 1 innate lymphoid cells

NK cells were first identified in 1975 and represent the prototypical group 1 innate lymphoid cell (ILC). More recently, the discovery of new members of the ILC family has highlighted the complexity of this innate lymphoid lineage. Importantly, it has been recognized that different subsets exist within the group 1 ILC, which have potential roles in mediating immune protection and immunosurveillance, and in regulating tissue homeostasis and inflammation. Here, we review the developmental relationships between the different group 1 ILC, which have been identified to date and discuss how heterogeneity within this expanding family may have arisen.

RORC1 Regulates Tumor-Promoting "Emergency" Granulo-Monocytopoiesis

Cancer-driven granulo-monocytopoiesis stimulates expansion of tumor promoting myeloid populations, mostly myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages (TAMs). We identified subsets of MDSCs and TAMs based on the expression of retinoic-acid-related orphan receptor (RORC1/RORγ) in human and mouse tumor bearers. RORC1 orchestrates myelopoiesis by suppressing negative (Socs3 and Bcl3) and promoting positive (C/EBPβ) regulators of granulopoiesis, as well as the key transcriptional mediators of myeloid progenitor commitment and differentiation to the monocytic/macrophage lineage (IRF8 and PU.1). RORC1 supported tumor-promoting innate immunity by protecting MDSCs from apoptosis, mediating TAM differentiation and M2 polarization, and limiting tumor infiltration by mature neutrophils. Accordingly, ablation of RORC1 in the hematopoietic compartment prevented cancer-driven myelopoiesis, resulting in inhibition of tumor growth and metastasis.

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

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

Influence of nutrient-derived metabolites on lymphocyte immunity

Organisms need to protect themselves against potential dangers from their surroundings, yet they require constant and intimate interactions with the same environment for their survival. The immune system is instrumental for protection against invading organisms and their toxins. The immune system consists of many cell types and is highly integrated within other tissues. Immune activity is particularly enriched at surfaces that separate the host from its environment, such as the skin and the gastrointestinal tract. This enables protection at sites directly at risk but also enables environmental factors to influence the maturation and function of immune structures and cells. Recent work has indicated that the diet in particular is able to influence the immune system and thus affect the development of inflammatory disease. This review aims to highlight recent work on how external factors, with a focus on those derived from the diet such as vitamin A, can have a direct or indirect deterministic influence on the activity and function of immunity.

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

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

Transcriptional regulation of innate lymphoid cell fate

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

The human microbiome in hematopoiesis and hematologic disorders

Humans are now understood to be in complex symbiosis with a diverse ecosystem of microbial organisms, including bacteria, viruses, and fungi. Efforts to characterize the role of these microorganisms, commonly referred as the microbiota, in human health have sought to answer the fundamental questions of what organisms are present, how are they functioning to interact with human cells, and by what mechanism are these interactions occurring. In this review, we describe recent efforts to describe the microbiota in healthy and diseased individuals, summarize the role of various molecular technologies (ranging from 16S ribosomal RNA to shotgun metagenomic sequencing) in enumerating the community structure of the microbiota, and explore known interactions between the microbiota and humans, with a focus on the microbiota's role in hematopoiesis and hematologic diseases.