Transcription Factor Bcl11b Controls Identity and Function of Mature Type 2 Innate Lymphoid Cells

The Innate Lymphoid Cell Precursor

The discovery of tissue-resident innate lymphoid cell populations effecting different forms of type 1, 2, and 3 immunity; tissue repair; and immune regulation has transformed our understanding of mucosal immunity and allergy. The emerging complexity of these populations along with compounding issues of redundancy and plasticity raise intriguing questions about their precise lineage relationship. Here we review advances in mapping the emergence of these lineages from early lymphoid precursors. We discuss the identification of a common innate lymphoid cell precursor characterized by transient expression of the transcription factor PLZF, and the lineage relationships of innate lymphoid cells with conventional natural killer cells and lymphoid tissue inducer cells. We also review the rapidly growing understanding of the network of transcription factors that direct the development of these lineages.

Priming of lineage-specifying genes by Bcl11b is required for lineage choice in post-selection thymocytes

T-lineage committed precursor thymocytes are screened by a fate-determination process mediated via T cell receptor (TCR) signals for differentiation into distinct lineages. However, it remains unclear whether any antecedent event is required to couple TCR signals with the transcriptional program governing lineage decisions. Here we show that Bcl11b, known as a T-lineage commitment factor, is essential for proper expression of ThPOK and Runx3, central regulators for the CD4-helper/CD8-cytotoxic lineage choice. Loss of Bcl11b results in random expression of these factors and, thereby, lineage scrambling that is disconnected from TCR restriction by MHC. Initial Thpok repression by Bcl11b prior to the pre-selection stage is independent of a known silencer for Thpok, and requires the last zinc-finger motif in Bcl11b protein, which by contrast is dispensable for T-lineage commitment. Collectively, our findings shed new light on the function of Bcl11b in priming lineage-specifying genes to integrate TCR signals into subsequent transcriptional regulatory mechanisms.

CD4 and CD8 T cells develop in the thymus with their transcription programs controlled by ThPOK and Runx3, respectively. Here the authors show that a pre-commitment event modulated by the transcription factor, Bcl11b, is required for the proper expression of ThPOK and Runx3 and correct CD4/CD8 lineage commitment.

An antigen-specific semi-therapeutic treatment with local delivery of tolerogenic factors through a dual-sized microparticle system blocks experimental autoimmune encephalomyelitis

Antigen-specific treatments are highly desirable for autoimmune diseases in contrast to treatments which induce systemic immunosuppression. A novel antigen-specific therapy has been developed which, when administered semi-therapeutically, is highly efficacious in the treatment of the mouse model for multiple sclerosis, experimental autoimmune encephalomyelitis (EAE). The treatment uses dual-sized, polymeric microparticles (dMPs) loaded with specific antigen and tolerizing factors for intra- and extra-cellular delivery, designed to recruit and modulate dendritic cells toward a tolerogenic phenotype without systemic release. This approach demonstrated robust efficacy and provided complete protection against disease. Therapeutic efficacy required encapsulation of the factors in controlled-release microparticles and was antigen-specific. Disease blocking was associated with a reduction of infiltrating CD4⁺ T cells, inflammatory cytokine-producing pathogenic CD4⁺ T cells, and activated macrophages and microglia in the central nervous system. Furthermore, CD4⁺ T cells isolated from dMP-treated mice were anergic in response to disease-specific, antigen-loaded splenocytes. Additionally, the frequency of CD86^hiMHCII^hi dendritic cells in draining lymph nodes of EAE mice treated with Ag-specific dMPs was reduced. Our findings highlight the efficacy of microparticle-based drug delivery platform to mediate antigen-specific tolerance, and suggest that such a multi-factor combinatorial approach can act to block autoimmunity.

Type two innate lymphoid cells: the Janus cells in health and disease

Innate lymphoid cells are functionally diverse subsets of immune cells including the conventional natural killer cells, lymphoid tissue inducers, type 1, 2, and 3 with significant roles in immunity and pathogenesis of inflammatory diseases. Type 2 innate lymphoid cells (ILC2s) resemble type 2 helper (Th2) cells in cytokine production and contribute to anti-helminth immunity, maintaining mucosal tissue integrity, and adipose tissue browning. ILC2s play important roles in the pathogenesis of allergic diseases and asthma. Studying the pathways of activation and regulation of ILC2s are currently a priority for giving a better understanding of pathogenesis of diseases with immunological roots. Recently, our laboratory and others have shown several pathways of regulation of ILC2s by co-stimulatory molecules such as ICOS, regulatory T cells and by compounds such as nicotine. In this review, we summarize the current understanding of the mechanisms of activation and regulation of ILC2s and the role of these cells in health and disease.

Dynamic balance between master transcription factors determines the fates and functions of CD4 T cell and innate lymphoid cell subsets

Fang and Zhu discuss similarities and differences between CD4 T cell and ILC subsets and the master transcription factors that determine the heterogeneity and plasticity of these subsets.

CD4 T cells, including T regulatory cells (Treg cells) and effector T helper cells (Th cells), and recently identified innate lymphoid cells (ILCs) play important roles in host defense and inflammation. Both CD4 T cells and ILCs can be classified into distinct lineages based on their functions and the expression of lineage-specific genes, including those encoding effector cytokines, cell surface markers, and key transcription factors. It was first recognized that each lineage expresses a specific master transcription factor and the expression of these factors is mutually exclusive because of cross-regulation among these factors. However, recent studies indicate that the master regulators are often coexpressed. Furthermore, the expression of master regulators can be dynamic and quantitative. In this review, we will first discuss similarities and differences between the development and functions of CD4 T cell and ILC subsets and then summarize recent literature on quantitative, dynamic, and cell type–specific balance between the master transcription factors in determining heterogeneity and plasticity of these subsets.

Transcriptome analysis reveals similarities between human blood CD3- CD56bright cells and mouse CD127+ innate lymphoid cells

For many years, human peripheral blood natural killer (NK) cells have been divided into functionally distinct CD3⁻ CD56^bright CD16⁻ and CD3⁻ CD56^dim CD16⁺ subsets. Recently, several groups of innate lymphoid cells (ILC), distinct from NK cells in development and function, have been defined in mouse. A signature of genes present in mouse ILC except NK cells, defined by Immunological Genome Project studies, is significantly over-represented in human CD56^bright cells, by gene set enrichment analysis. Conversely, the signature genes of mouse NK cells are enriched in human CD56^dim cells. Correlations are based upon large differences in expression of a few key genes. CD56^bright cells show preferential expression of ILC-associated IL7R (CD127), TNFSF10 (TRAIL), KIT (CD117), IL2RA (CD25), CD27, CXCR3, DPP4 (CD26), GPR183, and MHC class II transcripts and proteins. This could indicate an ontological relationship between human CD56^bright cells and mouse CD127⁺ ILC, or conserved networks of transcriptional regulation. In line with the latter hypothesis, among transcription factors known to impact ILC or NK cell development, GATA3, TCF7 (TCF-1), AHR, SOX4, RUNX2, and ZEB1 transcript levels are higher in CD56^bright cells, while IKZF3 (AIOLOS), TBX21 (T-bet), NFIL3 (E4BP4), ZEB2, PRDM1 (BLIMP1), and RORA mRNA levels are higher in CD56^dim cells.

Development of innate lymphoid cells

Innate lymphoid cells (ILCs) are a family of immune effector cells that have important roles in host defense, metabolic homeostasis and tissue repair but can also contribute to inflammatory diseases such as asthma and colitis. These cells can be categorized into three groups on the basis of the transcription factors that direct their function and the cytokines they produce, which parallel the effector functions of T lymphocytes. The hierarchy of cell-fate-restriction events that occur as common lymphoid progenitors become committed to each of the ILC lineages further underscores the relationship between these innate immune cells and T lymphocytes. In this Review we discuss the developmental program of ILCs and transcription factors that guide ILC lineage specification and commitment.

Innate immunity as the orchestrator of allergic airway inflammation and resolution in asthma

The respiratory system is constantly in direct contact with the environment and, has therefore, developed strong innate and adaptive immune responses to combat pathogens. Unlike adaptive immunity which is mounted later in the course of the immune response and is naive at the outset, innate immunity provides the first line of defense against microbial agents, while also promoting resolution of inflammation. In the airways, innate immune effector cells mainly consist of eosinophils, neutrophils, mast cells, basophils, macrophages/monocytes, dendritic cells and innate lymphoid cells, which attack pathogens directly or indirectly through the release of inflammatory cytokines and antimicrobial peptides, and coordinate T and B cell-mediated adaptive immunity. Airway epithelial cells are also critically involved in shaping both the innate and adaptive arms of the immune response. Chronic allergic airway inflammation and linked asthmatic disease is often considered a result of aberrant activation of type 2 T helper cells (Th2) towards innocuous environmental allergens; however, innate immune cells are increasingly recognized as key players responsible for the initiation and the perpetuation of allergic responses. Moreover, innate cells participate in immune response regulation through the release of anti-inflammatory mediators, and guide tissue repair and the maintenance of airway homeostasis. The scope of this review is to outline existing knowledge on innate immune responses involved in allergic airway inflammation, highlight current gaps in our understanding of the underlying molecular and cellular mechanisms and discuss the potential use of innate effector cells in new therapeutic avenues.

Transcriptional regulators dictate innate lymphoid cell fates

Research on innate lymphoid cells (ILC) has recently been a fast paced topic of immunological research. As ILCs are able to produce signature Th cytokine, ILCs have garnered considerable attention and have been described to represent the innate counterpart of the CD4+ T helper (Th) cells. The development and function of ILCs are precisely regulated by a network of crucial transcription factors, which are also involved in the development or differentiation of conventional natural killer (cNK) cells and T cells. In this review, we will summarize the key transcriptional regulators and their functions through each phases of ILC development. With the phase of ILC lineage commitment, we will focus in particular on the roles of the transcription regulators Id2 and GATA-3, which in collaboration with other transcriptional factors, are critically involved in the generation of ILC fate determined progenitors. Once an ILC lineage has been established, several other transcription factors are required for the specification and functional regulation of distinct mature ILC subsets. Thus, a comprehensive understanding of the interactions and regulatory mechanisms mediated by these transcription factors will help us to further understand how ILCs exert their helper-like functions and bridge the innate and adaptive immunity.

Developmental options and functional plasticity of innate lymphoid cells

Innate lymphoid cells (ILCs) are lineage- and antigen receptor-negative lymphocytes including natural killer (NK) cells and at least three distinguishable cell subsets (ILC1, ILC2, ILC3) that rapidly produce cytokines (IFN-γ, IL-5, IL-13, IL-17A, IL-22) upon activation. As such, ILCs can act as first-line defenders in the context of infection, inflammation and cancer. Because of the strong conservation between the expression of key transcription factors that can drive signature cytokine outputs in ILCs and differentiated helper T cells, it has been proposed that ILCs represent innate counterparts of the latter. Several distinct ILC precursors (ILCP) with pan-ILC (giving rise to all ILCs) or subset-restricted potentials have been described in both mouse and man. How and where these different ILCP give rise to more mature tissue-resident ILCs remains unclear. Recently, environmental signals have been shown to epigenetically influence canonical ILC differentiation pathways, generating substantial functional plasticity. These new results suggest that while ILC differentiation may be 'fixed' in principle, it remains 'flexible' in practice. A more comprehensive knowledge in the molecular mechanisms that regulate ILC development and effector functions may allow for therapeutic manipulation of ILCs for diverse disease conditions.

Cutting Edge: Notch Signaling Promotes the Plasticity of Group-2 Innate Lymphoid Cells

The mechanisms underlying lymphocyte lineage stability and plasticity remain elusive. Recent work indicates that innate lymphoid cells (ILC) possess substantial plasticity. Whereas natural ILC2 (nILC2) produce type-2 cytokines, plastic inflammatory ILC2 (iILC2) can coproduce both type-2 cytokines and the ILC3-characteristic cytokine, IL-17. Mechanisms that elicit this lineage plasticity, and the importance in health and disease, remain unclear. In this study we show that iILC2 are potent inducers of airway inflammation in response to acute house dust mite challenge. We find that Notch signaling induces lineage plasticity of mature ILC2 and drives the conversion of nILC2 into iILC2. Acute blockade of Notch signaling abolished functional iILC2, but not nILC2, in vivo. Exposure of isolated nILC2 to Notch ligands induced Rorc expression and elicited dual IL-13/IL-17 production, converting nILC2 into iILC2. Together these results reveal a novel role for Notch signaling in eliciting ILC2 plasticity and driving the emergence of highly proinflammatory innate lymphocytes.

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.

Hematopoiesis and T-cell specification as a model developmental system

The pathway to generate T cells from hematopoietic stem cells guides progenitors through a succession of fate choices while balancing differentiation progression against proliferation, stage to stage. Many elements of the regulatory system that controls this process are known, but the requirement for multiple, functionally distinct transcription factors needs clarification in terms of gene network architecture. Here, we compare the features of the T-cell specification system with the rule sets underlying two other influential types of gene network models: first, the combinatorial, hierarchical regulatory systems that generate the orderly, synchronized increases in complexity in most invertebrate embryos; second, the dueling 'master regulator' systems that are commonly used to explain bistability in microbial systems and in many fate choices in terminal differentiation. The T-cell specification process shares certain features with each of these prevalent models but differs from both of them in central respects. The T-cell system is highly combinatorial but also highly dose-sensitive in its use of crucial regulatory factors. The roles of these factors are not always T-lineage-specific, but they balance and modulate each other's activities long before any mutually exclusive silencing occurs. T-cell specification may provide a new hybrid model for gene networks in vertebrate developmental systems.

Views on helper/cytotoxic lineage choice from a bottom-up approach

There has been speculation as to how bi-potent CD4(+)  CD8(+) double-positive precursor thymocytes choose their distinct developmental fate, becoming either CD4(+) helper or CD8(+) cytotoxic T cells. Based on the clear correlation of αβT cell receptor (TCR) specificity to major histocompatibility complex (MHC) classes with this lineage choice, various studies have attempted to resolve this question by examining the cellular signaling events initiated by TCR engagements, a strategy referred to as a 'top-down' approach. On the other hand, based on the other correlation of CD4/CD8 co-receptor expression with its selected fate, other studies have addressed this question by gradually unraveling the sequential mechanisms that control the phenotypic outcome of this fate decision, a method known as the 'bottom-up' approach. Bridging these two approaches will contribute to a more comprehensive understanding of how TCR signals are coupled with developmental programs in the nucleus. Advances made during the last two decades seemed to make these two approaches more closely linked. For instance, identification of two transcription factors, ThPOK and Runx3, which play central roles in the development of helper and cytotoxic lineages, respectively, provided significant insights into the transcriptional network that controls a CD4/CD8 lineage choice. This review summarizes achievements made using the 'bottom-up' approach, followed by a perspective on future pathways toward coupling TCR signaling with nuclear programs.

IL-27 suppresses type 2 immune responses in vivo via direct effects on group 2 innate lymphoid cells

Group 2 innate lymphoid cells (ILC2) were recently characterized by their ability to produce significant amounts of type-2 signature cytokines and drive central beneficial and pathological features of type-2 immune responses. Although factors such as IL-33 and IL-25 were shown to have ILC2 activating capacity, it is not well understood, how ILC2 responses are regulated in vivo. Here we provide compelling evidence that IL-27-signalling directly inhibits ILC2 responses and reveal a novel mechanism for negative regulation of the innate arm of type-2 immunity. We demonstrate that IL-27-deficiency is linked to increased mucosal presence of ILC2 in a model of inflammatory lung disease. Moreover, IL-27-treatment inhibited ILC2 proliferation and cytokine production and significantly reduced their accumulation in vivo. During helminth infection, regulation of ILC2 by IL-27 directly impacted anti-parasitic immunity. Thus, therapeutic modulation of the IL-27/IL-27R axis may be relevant in a number of inflammatory conditions associated with dysregulated type-2 responses.

Transcription Factor Bcl11b Controls Effector and Memory CD8 T cell Fate Decision and Function during Poxvirus Infection

CD8⁺ T cells play an important role in host resistance to many viral infections, but the underlying transcriptional mechanisms governing their differentiation and functionality remain poorly defined. By using a highly virulent systemic and respiratory poxvirus infection in mice, we show that the transcription factor Bcl11b provides a dual trigger that sustains the clonal expansion of virus-specific effector CD8⁺ T cells, while simultaneously suppressing the expression of surface markers associated with short-lived effector cell (SLEC) differentiation. Additionally, we demonstrate that Bcl11b supports the acquisition of memory precursor effector cell (MPEC) phenotype and, thus, its absence causes near complete loss of lymphoid and lung-resident memory cells. Interestingly, despite having normal levels of T-bet and Eomesodermin, Bcl11b-deficient CD8⁺ T cells failed to execute effector differentiation needed for anti-viral cytokine production and degranulation, suggesting a non-redundant role of Bcl11b in regulation of this program. Thus, Bcl11b is a critical player in fate decision of SLECs and MPECs, as well as effector function and memory formation.

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.

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.

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.

Transcription factor Bcl11b sustains iNKT1 and iNKT2 cell programs, restricts iNKT17 cell program, and governs iNKT cell survival

Invariant natural killer T (iNKT) cells are innate-like T cells that recognize glycolipid antigens and play critical roles in regulation of immune responses. Based on expression of the transcription factors (TFs) Tbet, Plzf, and Rorγt, iNKT cells have been classified in effector subsets that emerge in the thymus, namely, iNKT1, iNKT2, and iNKT17. Deficiency in the TF Bcl11b in double-positive (DP) thymocytes has been shown to cause absence of iNKT cells in the thymus and periphery due to defective self glycolipid processing and presentation by DP thymocytes and undefined intrinsic alterations in iNKT precursors. We used a model of cre-mediated postselection deletion of Bcl11b in iNKT cells to determine its intrinsic role in these cells. We found that Bcl11b is expressed equivalently in all three effector iNKT subsets, and its removal caused a reduction in the numbers of iNKT1 and iNKT2 cells, but not in the numbers of iNKT17 cells. Additionally, we show that Bcl11b sustains subset-specific cytokine production by iNKT1 and iNKT2 cells and restricts expression of iNKT17 genes in iNKT1 and iNKT2 subsets, overall restraining the iNKT17 program in iNKT cells. The total numbers of iNKT cells were reduced in the absence of Bcl11b both in the thymus and periphery, associated with the decrease in iNKT1 and iNKT2 cell numbers and decrease in survival, related to changes in survival/apoptosis genes. Thus, these results extend our understanding of the role of Bcl11b in iNKT cells beyond their selection and demonstrate that Bcl11b is a key regulator of iNKT effector subsets, their function, identity, and survival.

G9a regulates group 2 innate lymphoid cell development by repressing the group 3 innate lymphoid cell program

Antignano, Zaph, and collaborators show that the lysine methyltransferase G9a plays a critical role in determining the developmental programs of group 2 and 3 innate lymphoid cells.

Innate lymphoid cells (ILCs) are emerging as important regulators of homeostatic and disease-associated immune processes. Despite recent advances in defining the molecular pathways that control development and function of ILCs, the epigenetic mechanisms that regulate ILC biology are unknown. Here, we identify a role for the lysine methyltransferase G9a in regulating ILC2 development and function. Mice with a hematopoietic cell–specific deletion of G9a (Vav.G9a^−/− mice) have a severe reduction in ILC2s in peripheral sites, associated with impaired development of immature ILC2s in the bone marrow. Accordingly, Vav.G9a^−/− mice are resistant to the development of allergic lung inflammation. G9a-dependent dimethylation of histone 3 lysine 9 (H3K9me2) is a repressive histone mark that is associated with gene silencing. Genome-wide expression analysis demonstrated that the absence of G9a led to increased expression of ILC3-associated genes in developing ILC2 populations. Further, we found high levels of G9a-dependent H3K9me2 at ILC3-specific genetic loci, demonstrating that G9a-mediated repression of ILC3-associated genes is critical for the optimal development of ILC2s. Together, these results provide the first identification of an epigenetic regulatory mechanism in ILC development and function.

Inflammatory triggers associated with exacerbations of COPD orchestrate plasticity of group 2 innate lymphoid cells in the lungs

Innate lymphoid cells (ILCs) are critical mediators of mucosal immunity, and group 1 ILCs (ILC1 cells) and group 3 ILCs (ILC3 cells) have been shown to be functionally plastic. Here we found that group 2 ILCs (ILC2 cells) also exhibited phenotypic plasticity in response to infectious or noxious agents, characterized by substantially lower expression of the transcription factor GATA-3 and a concomitant switch to being ILC1 cells that produced interferon-γ (IFN-γ). Interleukin 12 (IL-12) and IL-18 regulated this conversion, and during viral infection, ILC2 cells clustered within inflamed areas and acquired an ILC1-like phenotype. Mechanistically, these ILC1 cells augmented virus-induced inflammation in a manner dependent on the transcription factor T-bet. Notably, IL-12 converted human ILC2 cells into ILC1 cells, and the frequency of ILC1 cells in patients with chronic obstructive pulmonary disease (COPD) correlated with disease severity and susceptibility to exacerbations. Thus, functional plasticity of ILC2 cells exacerbates anti-viral immunity, which may have adverse consequences in respiratory diseases such as COPD.

IL-1 is a critical regulator of group 2 innate lymphoid cell function and plasticity

Group 2 innate lymphoid cells (ILC2 cells) are important for type 2 immune responses and are activated by the epithelial cytokines interleukin 33 (IL-33), IL-25 and thymic stromal lymphopoietin (TSLP). Here we demonstrated that IL-1β was a critical activator of ILC2 cells, inducing proliferation and cytokine production and regulating the expression of epithelial cytokine receptors. IL-1β also governed ILC2 plasticity by inducing low expression of the transcription factor T-bet and the cytokine receptor chain IL-12Rβ2, which enabled the conversion of these cells into an ILC1 phenotype in response to IL-12. This transition was marked by an atypical chromatin landscape characterized by the simultaneous transcriptional accessibility of the locus encoding interferon-γ (IFN-γ) and the loci encoding IL-5 and IL-13. Finally, IL-1β potentiated ILC2 activation and plasticity in vivo, and IL-12 acted as the switch that determined an ILC2-versus-ILC1 response. Thus, we have identified a previously unknown role for IL-1β in facilitating ILC2 maturation and plasticity.

The ETS1 transcription factor is required for the development and cytokine-induced expansion of ILC2

Zook et al. use a novel mouse model to demonstrate a requirement for the transcription factor ETS1 in the development and function of group 2 innate lymphoid cells.

Group 2 innate lymphoid cells (ILC2s) are a subset of ILCs that play a protective role in the response to helminth infection, but they also contribute to allergic lung inflammation. Here, we report that the deletion of the ETS1 transcription factor in lymphoid cells resulted in a loss of ILC2s in the bone marrow and lymph nodes and that ETS1 promotes the fitness of the common progenitor of all ILCs. ETS1-deficient ILC2 progenitors failed to up-regulate messenger RNA for the E protein transcription factor inhibitor ID2, a critical factor for ILCs, and these cells were unable to expand in cytokine-driven in vitro cultures. In vivo, ETS1 was required for the IL-33–induced accumulation of lung ILC2s and for the production of the T helper type 2 cytokines IL-5 and IL-13. IL-25 also failed to elicit an expansion of inflammatory ILC2s when these cells lacked ETS1. Our data reveal ETS1 as a critical regulator of ILC2 expansion and cytokine production and implicate ETS1 in the regulation of Id2 at the inception of ILC2 development.

The development of adult innate lymphoid cells

Innate lymphoid cells (ILC) are a specialized family of effector lymphocytes that transcriptionally and functionally mirror effector subsets of T cells, but differ from T cells in that they lack clonally distributed adaptive antigen receptors. Our understanding of this family of lymphocytes is still in its infancy. In this review, we summarize current understanding and discuss recent insights into the cellular and molecular events that occur during early ILC development in adult mice. We discuss how these events overlap and diverge with the early development of adaptive T cells, and how they may influence the molecular and functional properties of mature ILC.

Macrophage and Innate Lymphoid Cell Interplay in the Genesis of Fibrosis

Fibrosis is a characteristic pathological feature of an array of chronic diseases, where development of fibrosis in tissue can lead to marked alterations in the architecture of the affected organs. As a result of this process of sustained attrition to organs, many diseases that involve fibrosis are often progressive conditions and have a poor long-term prognosis. Inflammation is often a prelude to fibrosis, with innate and adaptive immunity involved in both the initiation and regulation of the fibrotic process. In this review, we will focus on the emerging roles of the newly described innate lymphoid cells (ILCs) in the generation of fibrotic disease with an examination of the potential interplay between ILC and macrophages and the adaptive immune system.

ILC Lineage Specification: To Be or Not 11b, That Is the Question

The transcription factor Bcl11b is important for T cell development and maintaining their phenotype. In this issue of Immunity, Califano et al. (2015) show that Bcl11b has a role in specifying type II innate lymphoid cell (ILC2) identity and blocks their conversion to ILC3s.

Forging T-Lymphocyte Identity: Intersecting Networks of Transcriptional Control

T-lymphocyte development branches off from other lymphoid developmental programs through its requirement for sustained environmental signals through the Notch pathway. In the thymus, Notch signaling induces a succession of T-lineage regulatory factors that collectively create the T-cell identity through distinct steps. This process involves both the staged activation of T-cell identity genes and the staged repression of progenitor-cell-inherited regulatory genes once their roles in self-renewal and population expansion are no longer needed. With the recent characterization of innate lymphoid cells (ILCs) that share transcriptional regulation programs extensively with T-cell subsets, T-cell identity can increasingly be seen as defined in modular terms, as the processes selecting and actuating effector function are potentially detachable from the processes generating and selecting clonally unique T-cell receptor structures. The developmental pathways of different classes of T cells and ILCs are distinguished by the numbers of prerequisites of gene rearrangement, selection, and antigen contact before the cells gain access to nearly common regulatory mechanisms for choosing effector function. Here, the major classes of transcription factors that interact with Notch signals during T-lineage specification are discussed in terms of their roles in these programs, the evidence for their spectra of target genes at different stages, and their cross-regulatory and cooperative actions with each other. Specific topics include Notch modulation of PU.1 and GATA-3, PU.1-Notch competition, the relationship between PU.1 and GATA-3, and the roles of E proteins, Bcl11b, and GATA-3 in guiding acquisition of T-cell identity while avoiding redirection to an ILC fate.