Cutting edge: lymphoid tissue inducer cells maintain memory CD4 T cells within secondary lymphoid tissue

Innate Lymphoid Cells and Their Role in the Immune Response to Infections

Over the past decade, a group of lymphocyte-like cells called innate lymphoid cells (ILCs) has gained considerable attention due to their crucial role in regulating immunity and tissue homeostasis. ILCs, lacking antigen-specific receptors, are a group of functionally differentiated effector cells that act as tissue-resident sentinels against infections. Numerous studies have elucidated the characteristics of ILC subgroups, but the mechanisms controlling protective or pathological responses to pathogens still need to be better understood. This review summarizes the functions of ILCs in the immunology of infections caused by different intracellular and extracellular pathogens and discusses their possible therapeutic potential.

Lymphoid tissue inducer cells in cancer: a potential therapeutic target

Tumor cells are dynamic in nature; these cells first acquire immune surveillance and then escape from the immune system. Hence, progressed cancer cells distribute and metastasize to other organs via blood vessels as well as from the lymphatic system. Prognosis and treatment of metastatic cancer patients remain a major challenge nowadays. Till now, lots of target -based and immune checkpoint blocker therapies are used to treat disease patients. But these therapies fail to control the dissemination and metastasis of cancer. Before designing a treatment regimen for metastatic patients, understanding the mechanism of tumor cells spreading within lymph vessels remain undetermined. Construction of lymphoid structures since embryonic to adult stage are depend upon LTi. Foundation of lymph node, payer patches and TLO is initiated and regulated through these cells in any part of the body. During tumor growth, newly developed lymph node contained MDSCs and Treg cells which inhibit the immune response and promote tumor invasion and metastasis. LTi reconstituted lymph node can be used for both early and high risk detection of cancers. High and low risk of tumor growth and invasion depend upon the location and composition of immune cells within lymph nodes. However, LTi are not reported as predictive marker in cancer till date. Recent reports in cancer indicate that LTi cells are engaged in the spreading of tumor cells into a lymphatic vessel. Through this review we are trying to brief the development and role of the LTi in immune system during homeostasis and cancer.

Controversial role of ILC3s in intestinal diseases: A novelty perspective on immunotherapy

ILC3s have been identified as crucial immune regulators that play a role in maintaining host homeostasis and modulating the antitumor response. Emerging evidence supports the idea that LTi cells play an important role in initiating lymphoid tissue development, while other ILC3s can promote host defense and orchestrate adaptive immunity, mainly through the secretion of specific cytokines and crosstalk with other immune cells or tissues. Additionally, dysregulation of ILC3-mediated overexpression of cytokines, changes in subset abundance, and conversion toward other ILC subsets are closely linked with the occurrence of tumors and inflammatory diseases. Regulation of ILC3 cytokines, ILC conversion and LTi-induced TLSs may be a novel strategy for treating tumors and intestinal or extraintestinal inflammatory diseases. Herein, we discuss the development of ILCs, the biology of ILC3s, ILC plasticity, the correlation of ILC3s and adaptive immunity, crosstalk with the intestinal microenvironment, controversial roles of ILC3s in intestinal diseases and potential applications for treatment.

ILC3s select microbiota-specific regulatory T cells to establish tolerance in the gut

Microbial colonization of the mammalian intestine elicits inflammatory or tolerogenic T cell responses, but the mechanisms controlling these distinct outcomes remain poorly understood, and accumulating evidence indicates that aberrant immunity to intestinal microbiota is causally associated with infectious, inflammatory and malignant diseases1-8. Here we define a critical pathway controlling the fate of inflammatory versus tolerogenic T cells that respond to the microbiota and express the transcription factor RORγt. We profiled all RORγt+ immune cells at single-cell resolution from the intestine-draining lymph nodes of mice and reveal a dominant presence of T regulatory (Treg) cells and lymphoid tissue inducer-like group 3 innate lymphoid cells (ILC3s), which co-localize at interfollicular regions. These ILC3s are distinct from extrathymic AIRE-expressing cells, abundantly express major histocompatibility complex class II, and are necessary and sufficient to promote microbiota-specific RORγt+ Treg cells and prevent their expansion as inflammatory T helper 17 cells. This occurs through ILC3-mediated antigen presentation, αV integrin and competition for interleukin-2. Finally, single-cell analyses suggest that interactions between ILC3s and RORγt+ Treg cells are impaired in inflammatory bowel disease. Our results define a paradigm whereby ILC3s select for antigen-specific RORγt+ Treg cells, and against T helper 17 cells, to establish immune tolerance to the microbiota and intestinal health.

Divide and Conquer: Phenotypic and Temporal Heterogeneity Within CD8+ T Cell Responses

The advent of technologies that can characterize the phenotypes, functions and fates of individual cells has revealed extensive and often unexpected levels of diversity between cells that are nominally of the same subset. CD8⁺ T cells, also known as cytotoxic T lymphocytes (CTLs), are no exception. Investigations of individual CD8⁺ T cells both in vitro and in vivo have highlighted the heterogeneity of cellular responses at the levels of activation, differentiation and function. This review takes a broad perspective on the topic of heterogeneity, outlining different forms of variation that arise during a CD8⁺ T cell response. Specific attention is paid to the impact of T cell receptor (TCR) stimulation strength on heterogeneity. In particular, this review endeavors to highlight connections between variation at different cellular stages, presenting known mechanisms and key open questions about how variation between cells can arise and propagate.

Innate (and Innate-like) Lymphoid Cells: Emerging Immune Subsets With Multiple Roles Along Transplant Life

Transplant immunology is currently largely focused on conventional adaptive immunity, particularly T and B lymphocytes, which have long been considered as the only cells capable of allorecognition. In this vision, except for the initial phase of ischemia/reperfusion, during which the role of innate immune effectors is well established, the latter are largely considered as "passive" players, recruited secondarily to amplify graft destruction processes during rejection. Challenging this prevalent dogma, the recent progresses in basic immunology have unraveled the complexity of the innate immune system and identified different subsets of innate (and innate-like) lymphoid cells. As most of these cells are tissue-resident, they are overrepresented among passenger leukocytes. Beyond their role in ischemia/reperfusion, some of these subsets have been shown to be capable of allorecognition and/or of regulating alloreactive adaptive responses, suggesting that these emerging immune players are actively involved in most of the life phases of the grafts and their recipients. Drawing upon the inventory of the literature, this review synthesizes the current state of knowledge of the role of the different innate (and innate-like) lymphoid cell subsets during ischemia/reperfusion, allorecognition, and graft rejection. How these subsets also contribute to graft tolerance and the protection of chronically immunosuppressed patients against infectious and cancerous complications is also examined.

Multiplexed histology analyses for the phenotypic and spatial characterization of human innate lymphoid cells

Innate lymphoid cells (ILCs) emerge in the last few years as important regulators of immune responses and biological processes. Although ILCs are mainly known as tissue-resident cells, their precise localization and interactions with the microenvironment are still unclear. Here we combine a multiplexed immunofluorescence technique and a customized computational, open-source analysis pipeline to unambiguously identify CD127+ ILCs in situ and characterize these cells and their microenvironments. Moreover, we reveal the transcription factor IRF4 as a marker for tonsillar ILC3, and identify conserved stromal landmarks characteristic for ILC localization. We also show that CD127+ ILCs share tissue niches with plasma cells in the tonsil. Our works thus provide a platform for multiparametric histological analysis of ILCs to improve our understanding of ILC biology.

The interplay between innate lymphoid cells and T cells

ILCs and T cells are closely related functionally but they significantly differ in their ability to circulate, expand, and renew. Cooperation and reciprocal functional regulation suggest that these cell types are more complementary than simply redundant during immune responses. How ILCs shape T-cell responses is strongly dependent on the tissue and inflammatory context. Likewise, indirect regulation of ILCs by adaptive immunity is induced by environmental cues such as the gut microbiota. Here, we review shared requirements for the development and function of both cell types and divergences in the orchestration of prototypic immune functions. We discuss the diversity of functional interactions between T cells and ILCs during homeostasis and immune responses. Identifying the location and the nature of the tissue microenvironment in which these interactions are taking place may uncover the remaining mysteries of their close encounters.

Th1 responses in vivo require cell-specific provision of OX40L dictated by environmental cues

The OX40-OX40L pathway provides crucial co-stimulatory signals for CD4 T cell responses, however the precise cellular interactions critical for OX40L provision in vivo and when these occur, remains unclear. Here, we demonstrate that provision of OX40L by dendritic cells (DCs), but not T cells, B cells nor group 3 innate lymphoid cells (ILC3s), is critical specifically for the effector Th1 response to an acute systemic infection with Listeria monocytogenes (Lm). OX40L expression by DCs is regulated by cross-talk with NK cells, with IFNγ signalling to the DC to enhance OX40L in a mechanism conserved in both mouse and human DCs. Strikingly, DC expression of OX40L is redundant in a chronic intestinal Th1 response and expression by ILC3s is necessary. Collectively these data reveal tissue specific compartmentalisation of the cellular provision of OX40L and define a mechanism controlling DC expression of OX40L in vivo.

The OX40-OX40L axis is a crucial component of the costimulatory requirement of CD4 T cell responses. Here, the authors show context and cell type specific expression of OX40L for driving Th1 cell generation during acute and chronic models of infection.

Microbiota-induced tissue signals regulate ILC3-mediated antigen presentation

Although group 3 innate lymphoid cells (ILC3s) are efficient inducers of T cell responses in the spleen, they fail to induce CD4+ T cell proliferation in the gut. The signals regulating ILC3-T cell responses remain unknown. Here, we show that transcripts associated with MHC II antigen presentation are down-modulated in intestinal natural cytotoxicity receptor (NCR)- ILC3s. Further data implicate microbiota-induced IL-23 as a crucial signal for reversible silencing of MHC II in ILC3s, thereby reducing the capacity of ILC3s to present antigen to T cells in the intestinal mucosa. Moreover, IL-23-mediated MHC II suppression is dependent on mTORC1 and STAT3 phosphorylation in NCR- ILC3s. By contrast, splenic interferon-γ induces MHC II expression and CD4+ T cell stimulation by NCR- ILC3s. Our results thus identify biological circuits for tissue-specific regulation of ILC3-dependent T cell responses. These pathways may have implications for inducing or silencing T cell responses in human diseases.

Pleiotropic Role and Bidirectional Immunomodulation of Innate Lymphoid Cells in Cancer

Innate lymphoid cells (ILCs) are largely tissue resident and respond rapidly toward the environmental signals from surrounding tissues and other immune cells. The pleiotropic function of ILCs in diverse contexts underpins its importance in the innate arm of immune system in human health and disease. ILCs derive from common lymphoid progenitors but lack adaptive antigen receptors and functionally act as the innate counterpart to T-cell subsets. The classification of different subtypes is based on their distinct transcription factor requirement for development as well as signature cytokines that they produce. The discovery and subsequent characterization of ILCs over the past decade have mainly focused on the regulation of inflammation, tissue remodeling, and homeostasis, whereas the understanding of the multiple roles and mechanisms of ILCs in cancer is still limited. Emerging evidence of the potent immunomodulatory properties of ILCs in early host defense signifies a major advance in the use of ILCs as promising targets in cancer immunotherapy. In this review, we will decipher the non-exclusive roles of ILCs associated with both protumor and antitumor activities. We will also dissect the heterogeneity, plasticity, genetic evidence, and dysregulation in different cancer contexts, providing a comprehensive understanding of the complexity and diversity. These will have implications for the therapeutic targeting in cancer.

Identification of Murine and Human Innate Lymphoid Cells in Frozen Tissue Sections Using Immunofluorescence

ILCs interact with multiple cell types within their local environment to integrate a wealth of different signals into coordinated responses that regulate tissue homeostasis as well as immune responses upon challenge. While the development and function of ILCs has been extensively studied, principally using flow cytometry, there is limited understanding of the precise composition of cellular niches within which ILCs reside. While this might be optimally studied using dynamic live imaging approaches, immunofluorescence staining of tissue sections can provide fundamental basic information regarding the nature of these microenvironments. Here, a methodology enabling the identification of murine and human ILC populations in frozen tissue sections using immunofluorescence is described.

Identification and Functional Characterization of Murine Group 3 Innate Lymphoid Cell (ILC3) Subsets in the Intestinal Tract and Associated Lymphoid Tissues

Group 3 innate lymphoid cells (ILC3s) are critical mediators of innate immune responses at mucosal barriers in both health and disease. ILC3s rapidly respond to environmental cues to reinforce barrier function and foster a mutualistic microbiota. ILC3s are defined by the expression of the master transcription factor RORγt, but can be further subdivided by the surface expression of the chemokine receptor CCR6 or the natural killer cell-associated receptor NKp46, as well as through the coexpression of the transcription factor T-bet. Importantly, while these subsets exhibit overlapping functions such as the secretion of the cytokines IL-17A and IL-22, they also differ significantly transcriptionally, functionally and by their localization within tissues. Thus, it is critical that studies investigating ILC3 biology consider the heterogeneity and tissue specificities of these subsets. Here, we describe common tools used to dissect and characterize ILC3s subset phenotypes and functions by flow cytometry and strategies for cell sorting of these cells in both the gastrointestinal tract and associated lymph nodes. Together, these approaches provide a tool kit for researchers aiming to dissect ILC3 subset responses at homeostasis, during infection, or in the context of inflammation.

Innate lymphoid cells link gut microbes with mucosal T cell immunity

Despite continuous exposure to trillions of microbes, the intestinal immune system protects the mucosa by balancing barrier protection, tolerance, and immunity. As both sentinel and effector, the mucosal innate immune system plays a central role in coordinating these responses. By integrating signals from the intestinal microbiota, mononuclear phagocytes (MNPs) serve as a critical link in regulating effector functions of group 3 innate lymphoid cells (ILC3s). Our recent work identified the role for MNP production of the IBD-linked protein TNF-like ligand 1A (TL1A) in modulating microbial regulation of ILC3 barrier immunity. These findings highlight a broader role for ILC3s in local control of T cell immunity and their potential role in the pathogenesis and treatment of inflammatory disease.

Functional interactions between innate lymphoid cells and adaptive immunity

Innate lymphoid cells (ILCs) are enriched at barrier surfaces of the mammalian body where they rapidly respond to host, microbial or environmental stimuli to promote immunity or tissue homeostasis. Furthermore, ILCs are dysregulated in multiple human diseases. Over the past decade, substantial advances have been made in identifying the heterogeneity and functional diversity of ILCs, which have revealed striking similarities to T cell subsets. However, emerging evidence indicates that ILCs also have a complex role in directly influencing the adaptive immune response in the context of development, homeostasis, infection or inflammation. In turn, adaptive immunity reciprocally regulates ILCs, which indicates that these interactions are a crucial determinant of immune responses within tissues. Here, we summarize our current understanding of functional interactions between ILCs and the adaptive immune system, discuss limitations and future areas of investigation, and consider the potential for these interactions to be therapeutically harnessed to benefit human health.

Innate lymphoid cells, mediators of tissue homeostasis, adaptation and disease tolerance

Innate lymphoid cells (ILC) are a recently identified group of tissue-resident innate lymphocytes. Available data support the view that ILC or their progenitors are deposited and retained in tissues early during ontogeny. Thereby, ILC become an integral cellular component of tissues and organs. Here, we will review the intriguing relationships between ILC and basic developmental and homeostatic processes within tissues. Studying ILC has already led to the appreciation of the integral roles of immune cells in tissue homeostasis, morphogenesis, metabolism, regeneration, and growth. This area of immunology has not yet been studied in-depth but is likely to reveal important networks contributing to disease tolerance and may be harnessed for future therapeutic approaches.

The balance of power: innate lymphoid cells in tissue inflammation and repair

Over the last ten years, immunologists have recognized the central importance of an emerging group of innate lymphoid cells (ILCs) in health and disease. Characterization of these cells has provided a molecular definition of ILCs and their tissue-specific functions. Although the lineage-defining transcription factors, cytokine production, and nomenclature parallel those of T helper cells, ILCs do not require adaptive immune programming. Both environmental and host-derived signals shape the function of these evolutionarily ancient cells, which provide pathogen protection and promote tissue restoration. As such, ILCs function as a double-edged sword, balancing the inflammatory and reparative responses that arise during injury and disease. This Review highlights our recent understanding of tissue-resident ILCs and the signals that regulate their contribution to inflammation and tissue repair in health and disease.

Daclizumab Therapy for Multiple Sclerosis

Daclizumab is a humanized monoclonal antibody that prevents formation of high-affinity interleukin (IL)-2 receptor (IL-2R). Because activated T cells up-regulate high-affinity IL-2R and IL-2 is used to grow activated T cells in vitro, daclizumab was envisioned to selectively inhibit activated T cells. However, the mechanism of action (MOA) of daclizumab is surprisingly broad and it includes many unanticipated effects on innate immunity. Specifically, daclizumab modulates the development of innate lymphoid cells, leading to expansion of immunoregulatory CD56^bright natural killer (NK) cells. Activated CD56^bright NK cells migrate to the intrathecal compartment in multiple sclerosis (MS) and regulate autoreactive T cells via cytotoxicity. Finally, daclizumab also restricts initial steps of T-cell activation by blocking trans-presentation of IL-2 by dendritic cells to antigen-specific T cells. In conclusion, daclizumab has complex immunomodulatory effects with resultant inhibition of central nervous system inflammation in MS.

ILC3-derived OX40L is essential for homeostasis of intestinal Tregs in immunodeficient mice

OX40L is one of the co-stimulatory molecules that can be expressed by splenic lymphoid tissue inducer (Lti) cells, a subset of group 3 innate lymphoid cells (ILC3s). OX40L expression in subsets of intestinal ILC3s and the molecular regulation of OX40L expression in ILC3s are unknown. Here, we showed intestinal ILC3s marked as an OX40L^high population among all the intestinal leukocytes and were the dominant source of OX40L in Rag1^-/- mice. All ILC3 subsets expressed OX40L, and NCR^-ILC3s were the most abundant source of OX40L. The expression of OX40L in ILC3s could be upregulated during inflammation. In addition to tumor necrosis factor (TNF)-like cytokine 1A (TL1A), which has been known as a trigger for OX40L, we found that Poly (I:C) representing viral stimulus promoted OX40L expression in ILC3s via a cell-autonomous manner. Furthermore, we demonstrated that IL-7-STAT5 signaling sustained OX40L expression by ILC3s. Intestinal regulatory T cells (Tregs), most of which expressed OX40, had defective expansion in chimeric mice, in which ILC3s were specifically deficient for OX40L expression. Consistently, co-localization of Tregs and ILC3s was found in the cryptopatches of the intestine, which suggests the close interaction between ILC3s and Tregs. Our study has unveiled the crosstalk between Tregs and ILC3s in mucosal tissues through OX40-OX40L signaling, which is crucial for the homeostasis of intestinal Tregs.

Innate Lymphoid Cells in Inflammatory Bowel Disease

Inflammatory bowel disease (IBD), including Crohn's disease and ulcerative colitis, is a complex chronic inflammatory condition of the human gut of unknown causes. Traditionally, dysregulated adaptive immune responses are thought to play a major role; however, accumulating evidence suggests that innate immunity also contributes to this process. Innate lymphoid cells (ILCs) are recently identified important components of innate immunity. They have critical roles in immunity, tissue development and remodeling. Numerous researchers have linked ILCs to the pathogenesis of IBD. In this review, we describe recent progress in our understanding about the phenotype and function alterations of ILCs as well as its interactions with other key mucosal cells in the gut of IBD patients. A better delineation of the ILCs' behavior in the human intestine will contribute to our understanding of ILCs biology and provide valuable insights for potential therapeutic target selection for IBD patients.

Memory responses of innate lymphocytes and parallels with T cells

Natural killer (NK) cells are classified as innate immune cells, given their ability to rapidly respond and kill transformed or virally infected cells without prior sensitization. Recently, accumulating evidence suggests that NK cells also exhibit many characteristics similar to cells of the adaptive immune system. Analogous to T cells, NK cells acquire self-tolerance during development, express antigen-specific receptors, undergo clonal-like expansion, and can become long-lived, self-renewing memory cells with potent effector function providing potent protection against reappearing pathogens. In this review, we discuss the requirements for memory NK cell generation and highlight the similarities with the formation of memory T cells.

Cross-Tissue Transcriptomic Analysis of Human Secondary Lymphoid Organ-Residing ILC3s Reveals a Quiescent State in the Absence of Inflammation

A substantial number of human and mouse group 3 innate lymphoid cells (ILC3s) reside in secondary lymphoid organs, yet the phenotype and function of these ILC3s is incompletely understood. Here, we employed an unbiased cross-tissue transcriptomic approach to compare human ILC3s from non-inflamed lymph nodes and spleen to their phenotypic counterparts in inflamed tonsils and from circulation. These analyses revealed that, in the absence of inflammation, lymphoid organ-residing ILC3s lack transcription of cytokines associated with classical ILC3 functions. This was independent of expression of the natural cytotoxicity receptor NKp44. However, and in contrast to ILC3s from peripheral blood, lymphoid organ-residing ILC3s express activating cytokine receptors and have acquired the ability to be recruited into immune responses by inflammatory cytokines. This comprehensive cross-tissue dataset will allow for identification of functional changes in human lymphoid organ ILC3s associated with human disease.

Orchestration of intestinal homeostasis and tolerance by group 3 innate lymphoid cells

The gastrointestinal tract is the primary site of exposure to a multitude of microbial, environmental, and dietary challenges. As a result, immune responses in the intestine need to be tightly regulated in order to prevent inappropriate inflammatory responses to exogenous stimuli. Intestinal homeostasis and tolerance are mediated through a multitude of immune mechanisms that act to reinforce barrier integrity, maintain the segregation and balance of commensal microbes, and ensure tissue health and regeneration. Here, we discuss the role of group 3 innate lymphoid cells (ILC3) as key regulators of intestinal health and highlight how increasing evidence implicates dysregulation of this innate immune cell population in the onset or progression of a broad range of clinically relevant pathologies. Finally, we discuss how the next generation of immunotherapeutics may be utilized to target ILC3 in disease and restore gastrointestinal tolerance and tissue health.

The role of IL-22 in the resolution of sterile and nonsterile inflammation

In a broad sense, inflammation can be conveniently characterised by two phases: the first phase, which is a pro-inflammatory, has evolved to clear infection and/or injured tissue; and the second phase concerns regeneration of normal tissue and restitution of normal physiology. Innate immune cell-derived pro-inflammatory cytokines and chemokines activate and recruit nonresident immune cells to the site of infection, thereby amplifying the inflammatory responses to clear infection or injury. This phase is followed by a cytokine milieu that promotes tissue regeneration. There is no absolute temporal distinction between these two phases, and cytokines may have dual pleiotropic effects depending on the timing of release, inflammatory microenvironment or concentrations. IL-22 is a cytokine with reported pro- and anti-inflammatory roles; in this review, we contend that this protein has primarily a function in restitution of normal tissue and physiology.

Lymphoid tissue inducer-A divergent member of the ILC family

Innate lymphoid cells (ILCs) that are capable of producing effector cytokines reminiscent of CD4⁺ T helper (Th) cells during infections and tissue inflammations have drawn much attention in the immunology field in recent years. Within the ILCs, the lymphoid tissue inducer (LTi) cells that play a critical role in lymphoid organogenesis were identified long before the establishment of the ILC concept. LTi cells, developed and functioning mainly at the fetal stage, and LTi-like cells, presumably generated during the adulthood, are regarded as a subset of type 3 ILCs (ILC3s) because they express the ILC3 lineage-defining transcription factor RORγt, and like other ILC3s, can produce an ILC3 signature cytokine IL-22 and initiate protective immune responses against extracellular bacteria. However, LTi/LTi-like cells have a unique gene expression pattern, and they develop from a progenitor that is distinct from the progenitor of all other ILCs and the progenitor of conventional natural killer (cNK) cells. There are also several other unique features of LTi/LTi-like cells comparing to non-LTi ILC3s. In addition to their classical function in lymphoid organogenesis, LTi/LTi-like cells also have specialized functions in association with the adaptive immune system, which include their effects on T and B cell development, activation and function. In this review, we summarize these specific features of LTi/LTi-like cells and propose that these cells should be considered as a separated innate lymphoid lineage in parallel with other non-LTi ILCs and cNK cells.

Group 3 innate lymphoid cells in tissue damage and graft-versus-host disease pathogenesis

PURPOSE OF REVIEW

Innate lymphoid cells (ILC) have emerged as modulators of conditioning-induced tissue damage and development of graft-versus-host disease (GVHD) in the context of allogeneic hematopoietic stem cell transplantation (HSCT). This review highlights experimental and clinical evidence for a role of ILC in GVHD pathogenesis.

RECENT FINDINGS

ILC are well known for their role in epithelial homeostasis and innate immunity. In addition, recent studies identified ILC as architects of intestinal responses to tissue damage after experimental radio and chemotherapy. Group 3 ILC, and their signature cytokine IL-22, can enhance intestinal stem cell regeneration and protect the stem cell niche from damage during experimental HSCT. Moreover, in leukemia patients undergoing HSCT conditioning, appearance of activated group 3 ILC prior to transplant is correlated to reduced incidence of acute GVHD.

SUMMARY

ILC have a profound impact on the recovery from tissue damage and severity of GVHD in experimental models. Together with the available data from leukemia patients, this argues for in-depth analysis of the mechanisms of ILC function and the translation of experimental findings to clinical application. Ultimately, control of ILC activation, or of the cytokines they produce, could be employed to reduce GVHD lesion in patients receiving allogeneic HSCT.

Group 3 Innate Lymphoid Cells: Communications Hubs of the Intestinal Immune System

The maintenance of mammalian health requires the generation of appropriate immune responses against a broad range of environmental and microbial challenges, which are continually encountered at barrier tissue sites including the skin, lung, and gastrointestinal tract. Dysregulated barrier immune responses result in inflammation, both locally and systemically in peripheral organs. Group 3 innate lymphoid cells (ILC3) are constitutively present at barrier sites and appear to be highly specialized in their ability to sense a range of environmental and host-derived signals. Under homeostatic conditions, ILC3 respond to local cues to maintain tissue homeostasis and restrict inflammatory responses. In contrast, perturbations in the tissue microenvironment resulting from disease, infection, or tissue damage can drive dysregulated pro-inflammatory ILC3 responses and contribute to immunopathology. The tone of the ILC3 response is dictated by a balance of “exogenous” signals, such as dietary metabolites and commensal microbes, and “endogenous” host-derived signals from stromal cells, immune cells, and the nervous system. ILC3 must therefore have the capacity to simultaneously integrate a wide array of complex and dynamic inputs in order to regulate barrier function and tissue health. In this review, we discuss the concept of ILC3 as a “communications hub” in the intestinal tract and associated lymphoid tissues and address the variety of signals, derived from multiple biological systems, which are interpreted by ILC3 to modulate the release of downstream effector molecules and regulate cell–cell crosstalk. Successful integration of environmental cues by ILC3 and downstream propagation to the broader immune system is required to maintain a tolerogenic and anti-inflammatory tone and reinforce barrier function, whereas dysregulation of ILC3 responses can contribute to the onset or progression of clinically relevant chronic inflammatory diseases.

Enrichment of IL-17A+ IFN-γ+ and IL-22+ IFN-γ+ T cell subsets is associated with reduction of NKp44+ ILC3s in the terminal ileum of Crohn's disease patients

Crohn's disease (CD) is a chronic inflammatory condition of the human gastrointestinal tract whose aetiology remains largely unknown. Dysregulated adaptive immune responses and defective innate immunity both contribute to this process. In this study, we demonstrated that the interleukin (IL)-17A+ interferon (IFN)-γ+ and IL-22+ IFN-γ+ T cell subsets accumulated specifically in the inflamed terminal ileum of CD patients. These cells had higher expression of Ki-67 and were active cytokine producers. In addition, their proportions within both the IL-17A-producer and IL-22-producer populations were increased significantly. These data suggest that IL-17A+ IFN-γ+ and IL-22+ IFN-γ+ T cell subsets might represent the pathogenic T helper type 17 (Th17) population in the context of intestinal inflammation for CD patients. In the innate immunity compartment we detected a dramatic alteration of both phenotype and function of the intestinal innate lymphoid cells (ILCs), that play an important role in the maintenance of mucosal homeostasis. In the inflamed gut the frequency of the NKp44- CD117- ILC1s subset was increased significantly, while the frequency of NKp44+ ILC3s was reduced. Furthermore, the frequency of human leucocyte antigen D-related (HLA-DR)-expressing-NKp44+ ILC3s was also reduced significantly. Interestingly, the decrease in the NKp44+ ILC3s population was associated with an increase of pathogenic IL-17A+ IFN-γ+ and IL-22+ IFN-γ+ T cell subsets in the adaptive compartment. This might suggest a potential link between NKp44+ ILC3s and the IL-17A+ IFN-γ+ and IL-22+ IFN-γ+ T cell subsets in the terminal ileum of CD patients.

Brief Report: Altered Innate Lymphoid Cell Subsets in Human Lymph Node Biopsy Specimens Obtained During the At-Risk and Earliest Phases of Rheumatoid Arthritis

Objective

Innate lymphoid cells (ILCs) are emerging mediators of immunity, and accumulation of inflammatory ILC populations can occur in inflammatory‐mediated conditions. Since early lymph node (LN) activation has been shown in rheumatoid arthritis (RA), we aimed to investigate the frequency and distribution of ILCs in LN biopsy specimens obtained during the earliest phases of RA.

Methods

Twelve patients with early RA, 12 individuals with IgM rheumatoid factor and/or anti–citrullinated protein antibodies without arthritis (RA risk group), and 7 healthy controls underwent ultrasound‐guided inguinal LN biopsy. ILC subsets and the expression of vascular cell adhesion molecule (VCAM) and intercellular adhesion molecule (ICAM) by LN endothelial cells and fibroblasts were analyzed by flow cytometry.

Results

Although no differences in the frequencies of total ILCs (Lin−CD45^+/lowCD127+) were found, the distribution of the ILC subpopulations differed among groups. RA patients showed lower numbers of lymphoid tissue–inducer (LTi) cells (c‐Kit+NKp44− ILCs) and increased ILC1 (c‐Kit−NKp44− ILCs) and ILC3 (c‐Kit+NKp44+ ILCs) numbers compared with controls (P < 0.001, P < 0.050, and P < 0.050, respectively). Individuals at risk of RA exhibited an increased frequency of ILC1 compared with controls (P < 0.01). LTi cells paralleled the expression of adhesion molecules on endothelial cells and fibroblasts.

Conclusion

Our findings indicate that during the at‐risk and earliest phases of RA, the ILC distribution in LN changes from a homeostatic profile toward a more inflammatory profile, thereby providing evidence of a role for ILCs in RA pathogenesis.

Maintenance of pathogenic Th2 cells in allergic disorders

Immunological memory is an important protective mechanism that enables host organisms to respond rapidly and vigorously to pathogens that have been previously encountered. In addition to the protective function, memory CD4⁺ T helper (Th) cells play a central role in the pathogenesis of chronic inflammatory disorders, including asthma. Recently, several investigators have identified phenotypically and functionally distinct memory Th2 cell subsets that produce IL-5. These memory Th2 cell subsets play an important role in the pathology of allergic inflammation and function as memory-type "pathogenic Th2 (Tpath2) cells" both in mice and humans. We review the role of lung Tpath2 cells in the development of allergic inflammation and, in the context of recent findings, propose a mechanism by which Tpath2 cells not only survive but also continue to function at the sites where antigens were encountered. A greater understanding of the functional molecules or signaling pathways that regulate the inflammatory niche for Tpath2 cells may aid in the design of more effective treatments for chronic inflammatory disorders.

RORγt+ Innate Lymphoid Cells Promote Lymph Node Metastasis of Breast Cancers

Cancer cells tend to metastasize first to tumor-draining lymph nodes, but the mechanisms mediating cancer cell invasion into the lymphatic vasculature remain little understood. Here, we show that in the human breast tumor microenvironment (TME), the presence of increased numbers of RORγt+ group 3 innate lymphoid cells (ILC3) correlates with an increased likelihood of lymph node metastasis. In a preclinical mouse model of breast cancer, CCL21-mediated recruitment of ILC3 to tumors stimulated the production of the CXCL13 by TME stromal cells, which in turn promoted ILC3-stromal interactions and production of the cancer cell motile factor RANKL. Depleting ILC3 or neutralizing CCL21, CXCL13, or RANKL was sufficient to decrease lymph node metastasis. Our findings establish a role for RORγt+ILC3 in promoting lymphatic metastasis by modulating the local chemokine milieu of cancer cells in the TME. Cancer Res; 77(5); 1083-96. ©2017 AACR.

Functional and phenotypic heterogeneity of group 3 innate lymphoid cells

Group 3 innate lymphoid cells (ILC3), defined by expression of the transcription factor retinoid-related orphan receptor γt, play key roles in the regulation of inflammation and immunity in the gastrointestinal tract and associated lymphoid tissues. ILC3 consist largely of two major subsets, NCR⁺ ILC3 and LTi-like ILC3, but also demonstrate significant plasticity and heterogeneity. Recent advances have begun to dissect the relationship between ILC3 subsets and to define distinct functional states within the intestinal tissue microenvironment. In this review we discuss the ever-expanding roles of ILC3 in the context of intestinal homeostasis, infection and inflammation - with a focus on comparing and contrasting the relative contributions of ILC3 subsets.

Innate lymphoid cells in secondary lymphoid organs

The family of innate lymphoid cells (ILCs) has attracted attention in recent years as its members are important regulators of immunity, while they can also cause pathology. In both mouse and man, ILCs were initially discovered in developing lymph nodes as lymphoid tissue inducer (LTi) cells. These cells form the prototypic members of the ILC family and play a central role in the formation of secondary lymphoid organs (SLOs). In the absence of LTi cells, lymph nodes (LN) and Peyer's Patches (PP) fail to form in mice, although the splenic white pulp can develop normally. Besides LTi cells, the ILC family encompasses helper-like ILCs with functional distinctions as seen by T-helper cells, as well as cytotoxic natural killer (NK) cells. ILCs are still present in adult SLOs where they have been shown to play a role in lymphoid tissue regeneration. Furthermore, ILCs were implicated to interact with adaptive lymphocytes and influence the adaptive immune response. Here, we review the recent literature on the role of ILCs in secondary lymphoid tissue from the formation of SLOs to mature SLOs in adults, during homeostasis and pathology.

Cellular Innate Immunity: An Old Game with New Players

Innate immunity is a rapidly evolving field with novel cell types and molecular pathways being discovered and paradigms changing continuously. Innate and adaptive immune responses are traditionally viewed as separate from each other, but emerging evidence suggests that they overlap and mutually interact. Recently discovered cell types, particularly innate lymphoid cells and myeloid-derived suppressor cells, are gaining increasing attention. Here, we summarize and highlight current concepts in the field, focusing on innate immune cells as well as the inflammasome and DNA sensing which appear to be critical for the activation and orchestration of innate immunity, and may provide novel therapeutic opportunities for treating autoimmune, autoinflammatory, and infectious diseases.

Spotlight on daclizumab: its potential in the treatment of multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory-demyelinating disease of the central nervous system of a putative autoimmune etiology. Although the exact pathogenic mechanisms underlying demyelination and axonal damage in MS are not fully understood, T-cells are believed to play a central role in the pathogenesis of the disease. Daclizumab is a humanized binding monoclonal antibody that binds to the Tac epitope on the α-subunit (CD25) of the interleukin-2 (IL-2) receptor, thus effectively blocking the formation of the high-affinity IL-2 receptor, which is expressed mainly on T-cells. A series of clinical trials in patients with relapsing MS demonstrated a profound effect of daclizumab on inflammatory disease activity and improved clinical outcomes compared with placebo or interferon-β, which led to the recent approval of daclizumab (Zinbryta™) for the treatment of relapsing forms of MS. Enhancement of endogenous mechanisms of immune regulation rather than inhibition of effector T-cells might explain the effects of daclizumab in MS. These include expansion and improved function of regulatory CD56bright NK cells, inhibition of the early activation of T-cells through blockade of IL-2 transpresentation by dendritic cells and reduction in the number of intrathecal proinflammatory lymphoid tissue inducer cells. The enhanced efficacy of daclizumab is accompanied by an increased frequency of adverse events and risks of serious adverse events, thus placing it as a second-line therapy and calling for the implementation of a strict risk management program. This review details the mechanisms of action of daclizumab, discusses its efficacy and safety in patients with MS, and provides an insight into the place of this novel therapy in the treatment of MS.

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.

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.

Emerging roles for antigen presentation in establishing host-microbiome symbiosis

Trillions of beneficial bacteria inhabit the intestinal tract of healthy mammals from birth. Accordingly, mammalian hosts have evolved a series of complementary and redundant pathways to limit pathologic immune responses against these bacteria, while simultaneously protecting against enteric pathogen invasion. These pathways can be generically responsive to the presence of any commensal bacteria and innate in nature, as for IL-22-related pathways. Alternatively, specific bacterial antigens can drive a distinct set of adaptive immune cell responses, including IgA affinity maturation and secretion, and a recently described pathway of intestinal selection whereby MHCII(+) ILC3 deletes commensal bacteria-reactive CD4 T cells. These pathways can either promote or inhibit colonization by specific subsets of commensal bacteria, and cooperatively maintain intestinal homeostasis. In this review, we will highlight recent developments in understanding how these diverse pathways complement each other to cooperatively shape the symbiotic relationship between commensal bacteria and mammalian hosts.

Group 3 innate lymphoid cells: regulating host-commensal bacteria interactions in inflammation and cancer

A delicate balance exists between the mammalian immune system and normally beneficial commensal bacteria that colonize the gastrointestinal tract, which is necessary to maintain tissue homeostasis. Dysregulation of these interactions between the host and commensal bacteria is causally associated with chronic inflammation and the development of cancer. In contrast, recent reports have highlighted that commensal bacteria also play an essential role in promoting anti-tumor immune responses in several contexts, highlighting a paradox whereby interactions between the host and commensal bacteria can influence both pro- and anti-tumor immunity. Given the critical roles for group 3 innate lymphoid cells (ILC3s) in regulating inflammation, tissue repair and host-microbe interactions in the intestine, here we discuss new evidence that ILC3s may profoundly influence the development, progression and control of tumors. In this review, we provide an overview of recent advances in understanding the impact of commensal bacteria on tumorigenesis, discuss recent findings identifying ILC3s as critical regulators of host-microbe interactions and highlight the emerging role of this immune cell population in cancer and their potential implication as a therapeutic target.

Innate lymphoid cells in intestinal immunity and inflammation

Innate lymphoid cells (ILCs) are a new and distinct family of innate immune cells that play an important role in immunity and inflammation. In this review, we focus on the role of ILCs in mucosal tissues, especially in the gut, in health and disease. ILCs support intestinal homeostasis by protecting the intestine from pathogens, contributing to the development of gut lymphoid tissue, and helping to repair injuries. By cooperating with epithelial cells and other innate and adaptive immune cells, ILCs participate in the control of pathogens and tolerance of commensal bacteria. The development and maintenance of ILCs are influenced by nutrients and metabolites sourced from diet and/or gut bacteria. ILCs have been shown to be involved in host metabolism and to participate in various diseases of the intestine including infectious and chronic inflammatory diseases, and cancer. Thus, the elucidation of ILC biology provides an exciting potential for development of novel therapeutic means to modulate immune responses in various disease settings.

Type 3 innate lymphoid cell depletion is mediated by TLRs in lymphoid tissues of simian immunodeficiency virus-infected macaques

Innate lymphoid cells (ILCs) type 3, also known as lymphoid tissue inducer cells, plays a major role in both the development and remodeling of organized lymphoid tissues and the maintenance of adaptive immune responses. HIV/simian immunodeficiency virus (SIV) infection causes breakdown of intestinal barriers resulting in microbial translocation, leading to systemic immune activation and disease progression. However, the effects of HIV/SIV infection on ILC3 are unknown. Here, we analyzed ILC3 from mucosal and systemic lymphoid tissues in chronically SIV-infected macaques and uninfected controls. ILC3 cells were defined and identified in macaque lymphoid tissues as non-T, non-B (lineage-negative), c-Kit(+)IL-7Rα(+) (CD117(+)CD127(+)) cells. These ILC3 cells highly expressed CD90 (∼ 63%) and aryl hydrocarbon receptor and produced IL-17 (∼ 63%), IL-22 (∼ 36%), and TNF-α (∼ 72%) but did not coexpress CD4 or NK cell markers. The intestinal ILC3 cell loss correlated with the reduction of total CD4(+) T cells and T helper (Th)17 and Th22 cells in the gut during SIV infection (P < 0.001). Notably, ILC3 could be induced to undergo apoptosis by microbial products through the TLR2 (lipoteichoic acid) and/or TLR4 (LPS) pathway. These findings indicated that persistent microbial translocation may result in loss of ILC3 in lymphoid tissues in SIV-infected macaques, further contributing to the HIV-induced impairment of gut-associated lymphoid tissue structure and function, especially in mucosal tissues.

Maintenance of Immune Homeostasis through ILC/T Cell Interactions

Innate lymphoid cells (ILCs) have emerged as a new family of immune cells with crucial functions in innate and adaptive immunity. ILC subsets mirror the cytokine and transcriptional profile of CD4⁺ T helper (T_H) cell subsets. Hence, group 1 (ILC1), group 2 (ILC2), and group 3 (ILC3) ILCs can be distinguished by the production of T_H1, T_H2, and T_H17-type cytokines, respectively. Cytokine release by ILCs not only shapes early innate immunity but can also orchestrate T_H immune responses to microbial or allergen exposure. Recent studies have identified an unexpected effector function of ILCs as antigen presenting cells. Both ILC2s and ILC3s are able to process and present foreign antigens (Ags) via major histocompatibility complex class II, and to induce cognate CD4⁺ T cell responses. In addition, Ag-stimulated T cells promote ILC activation and effector functions indicating a reciprocal interaction between the adaptive and innate immune system. A fundamental puzzle in ILC function is how ILC/T cell interactions promote host protection and prevent autoimmune diseases. Furthermore, the way in which microenvironmental and inflammatory signals determine the outcome of ILC/T cell immune responses in various tissues is not yet understood. This review focuses on recent advances in understanding the mechanisms that coordinate the collaboration between ILCs and T cells under homeostatic and inflammatory conditions. We also discuss the potential roles of T cells and other immune cells to regulate ILC functions and to maintain homeostasis in mucosal tissues.

Group 3 innate lymphoid cells accumulate and exhibit disease-induced activation in the meninges in EAE

Innate lymphoid cells are immune cells that reside in tissues that interface with the external environment and contribute to the first line defense against pathogens. However, they also have roles in promoting chronic inflammation. Here we demonstrate that group 3 ILCs, (ILC3s - CD45+Lin-IL-7Rα+RORγt+), are normal residents of the meninges and exhibit disease-induced accumulation and activation in EAE. In addition to production of the pro-inflammatory cytokines IL-17 and GM-CSF, ILC3s constitutively express CD30L and OX40L, molecules required for memory T cell survival. We show that disease-induced trafficking of transferred wild type T cells to the meninges is impaired in ILC3-deficient Rorc-/- mice. Furthermore, lymphoid tissue inducer cells, a c-kit+ ILC3 subset that promotes ectopic lymphoid follicle development, a hallmark of many autoimmune diseases, are reduced in the meninges of EAE-resistant c-kit mutant Kit(W/Wv) mice. We propose that ILC3s sustain neuroinflammation by supporting T cell survival and reactivation in the meninges.

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.

Immune tolerance. Group 3 innate lymphoid cells mediate intestinal selection of commensal bacteria-specific CD4⁺ T cells

Inflammatory CD4(+) T cell responses to self or commensal bacteria underlie the pathogenesis of autoimmunity and inflammatory bowel disease (IBD), respectively. Although selection of self-specific T cells in the thymus limits responses to mammalian tissue antigens, the mechanisms that control selection of commensal bacteria-specific T cells remain poorly understood. Here, we demonstrate that group 3 innate lymphoid cell (ILC3)-intrinsic expression of major histocompatibility complex class II (MHCII) is regulated similarly to thymic epithelial cells and that MHCII(+) ILC3s directly induce cell death of activated commensal bacteria-specific T cells. Further, MHCII on colonic ILC3s was reduced in pediatric IBD patients. Collectively, these results define a selection pathway for commensal bacteria-specific CD4(+) T cells in the intestine and suggest that this process is dysregulated in human IBD.

The unusual suspects--innate lymphoid cells as novel therapeutic targets in IBD

Innate lymphoid cells (ILCs) are a family of immune cells that selectively accumulate in mucosal tissues serving as sentinels at the vanguard of host protective immunity. However, they are also implicated as cellular mediators of immune-mediated diseases, most notably IBD. ILCs are subdivided into distinct lineages based on the expression of effector cytokines and master transcription factors that programme their differentiation and inflammatory behaviour. Strikingly, these subsets closely resemble CD4(+) T-cell lineages, including T helper (TH)1, TH2 and TH17 cells that are similarly implicated in immune-mediated diseases. However, ILCs that promote the maintenance of intestinal epithelial cells, mostly through production of IL-22, also exist. ILCs rapidly respond to environmental cues, including cytokines, metabolic signals and luminal bacteria. They are potent and immediate producers of key cytokines linked to IBD pathogenesis, including TNF, IL-17, IL-22 and IFN-γ. Some subsets are implicated as mediators of chronic intestinal inflammation, whereas others might provide protective functions. They are present in the gut of patients with IBD and, intriguingly, closer scrutiny of IBD susceptibility loci shows that many of these genes are either expressed by, or are intimately linked to, ILC function. Looking forward, targeting ILCs could represent a new IBD treatment paradigm.

ICOS is required for the generation of both central and effector CD4(+) memory T-cell populations following acute bacterial infection

Interactions between ICOS and ICOS ligand (ICOSL) are essential for the development of T follicular helper (Tfh) cells and thus the formation and maintenance of GC reactions. Given the conflicting reports on the requirement of other CD4⁺ T‐cell populations for ICOS signals, we have employed a range of in vivo approaches to dissect requirements for ICOS signals in mice during an endogenous CD4⁺ T‐cell response and contrasted this with CD28 signals. Genetic absence of ICOSL only modestly reduced the total number of antigen‐specific CD4⁺ T cells at the peak of the primary response, but resulted in a severely diminished number of both T central memory and T effector memory cells. Treatment with blocking anti‐ICOS mAb during the primary response recapitulated these effects and caused a more substantial reduction than blocking CD28 signals with CTLA4Ig. During the memory phase of the response further signals through ICOS or CD28 were not required for survival. However, upon secondary challenge only Tfh cell expansion remained heavily ICOS‐dependent, while CD28 signals were required for optimal expansion of all subsets. These data demonstrate the importance of ICOS signals specifically for memory CD4⁺ T‐cell formation, while highlighting the potential of therapeutically targeting this pathway.