Metabolic Regulation of Innate Lymphoid Cell-Mediated Tissue Protection-Linking the Nutritional State to Barrier Immunity

Innate lymphoid cells (ILC) are a recently described group of tissue-resident immune cells that play essential roles in maintaining and protecting the tissue barrier against invading pathogens. Extensive research has revealed that ILC-mediated immune responses are controlled by dietary components and metabolites. An additional role of ILC as important direct regulators of host metabolism and glucose tolerance is emerging. This suggests that ILC may act as key dietary sensors integrating nutritional and metabolic stress to facilitate both maintenance of barrier sites and a coordinated immune response protecting these tissues. In this respect, investigations have begun to determine how different ILC responses are metabolically fueled and the impact of nutrient availability on the regulation of ILC function. Here, we discuss the current literature concerning dietary and metabolic control of ILC. In particular, we address whether the dietary and metabolic control of ILC and their simultaneous influence on host metabolism may function as a coordinated program of barrier defense.

Interleukin-33-Activated Islet-Resident Innate Lymphoid Cells Promote Insulin Secretion through Myeloid Cell Retinoic Acid Production

Pancreatic-islet inflammation contributes to the failure of β cell insulin secretion during obesity and type 2 diabetes. However, little is known about the nature and function of resident immune cells in this context or in homeostasis. Here we show that interleukin (IL)-33 was produced by islet mesenchymal cells and enhanced by a diabetes milieu (glucose, IL-1β, and palmitate). IL-33 promoted β cell function through islet-resident group 2 innate lymphoid cells (ILC2s) that elicited retinoic acid (RA)-producing capacities in macrophages and dendritic cells via the secretion of IL-13 and colony-stimulating factor 2. In turn, local RA signaled to the β cells to increase insulin secretion. This IL-33-ILC2 axis was activated after acute β cell stress but was defective during chronic obesity. Accordingly, IL-33 injections rescued islet function in obese mice. Our findings provide evidence that an immunometabolic crosstalk between islet-derived IL-33, ILC2s, and myeloid cells fosters insulin secretion.

Deciphering the Innate Lymphoid Cell Transcriptional Program

Innate lymphoid cells (ILCs) are enriched at mucosal surfaces, where they provide immune surveillance. All ILC subsets develop from a common progenitor that gives rise to pre-committed progenitors for each of the ILC lineages. Currently, the temporal control of gene expression that guides the emergence of these progenitors is poorly understood. We used global transcriptional mapping to analyze gene expression in different ILC progenitors. We identified PD-1 to be specifically expressed in PLZF⁺ ILCp and revealed that the timing and order of expression of the transcription factors NFIL3, ID2, and TCF-1 was critical. Importantly, induction of ILC lineage commitment required only transient expression of NFIL3 prior to ID2 and TCF-1 expression. These findings highlight the importance of the temporal program that permits commitment of progenitors to the ILC lineage, and they expand our understanding of the core transcriptional program by identifying potential regulators of ILC development.

Innate Lymphoid Cell Immunometabolism

Innate lymphoid cells (ILCs) are tissue-resident "first responders" of the immune system that function to protect epithelial barriers against pathogens and maintain tissue homeostasis. However, because ILCs are finely tuned to perturbations within tissue microenvironments, they can also contribute to host pathology when upstream activating signals are dysregulated. Recent work has demonstrated that the crosstalk between ILCs and their environment has a significant impact on host metabolism in health and disease. In this brief review, we summarize recent studies that demonstrate the ability of ILCs to influence tissue and systemic metabolism, as well as how ILC biology can be regulated by environmental changes in host metabolism. We also highlight studies showing how ILC-intrinsic metabolism influences their activation, proliferation, and homeostasis. Finally, this review discusses the challenges and open questions in the rapidly expanding field of immunometabolism.

The biology of natural killer cells during sepsis

Natural killer (NK) cells are large granular lymphocytes largely recognized for their importance in tumour surveillance and the host response to viral infections. However, as the major innate lymphocyte population, NK cells also coordinate early responses to bacterial infections by amplifying the antimicrobial functions of myeloid cells, especially macrophages, by production of interferon-γ (IFN-γ). Alternatively, excessive NK cell activation and IFN-γ production can amplify the systemic inflammatory response during sepsis resulting in increased physiological dysfunction and organ injury. Our understanding of NK cell biology during bacterial infections and sepsis is mostly derived from studies performed in mice. Human studies have demonstrated a correlation between altered NK cell functions and outcomes during sepsis. However, mechanistic understanding of NK cell function during human sepsis is limited. In this review, we will review the current understanding of NK cell biology during sepsis and discuss the challenges associated with modulating NK cell function during sepsis for therapeutic benefit.

GATA3 Regulates the Development and Functions of Innate Lymphoid Cell Subsets at Multiple Stages

Innate lymphoid cells (ILCs) are regarded as the innate counterpart of effector CD4 T helper (Th) cells. Just as Th cells, ILCs are classified into distinct subsets based on their functions that are delivered mainly through effector cytokine production. Both ILCs and Th cells play critical roles in various protective immune responses and inflammatory diseases. Similar to Th cell differentiation, the development of ILC subsets depends on several master transcription factors, among which GATA3 is critical for the development and maintenance of type 2 ILCs (ILC2s). However, GATA3 is expressed by all ILC subsets and ILC progenitors, albeit at different levels. In a striking parallel with GATA3 function in T cell development and differentiation, GATA3 also has multiple functions in different ILCs at various stages. In this review, I will discuss how quantitative and dynamic expression of GATA3 regulates the development and functions of ILC subsets.

Epigenomic Views of Innate Lymphoid Cells

The discovery of innate lymphoid cells (ILCs) with selective production of cytokines typically attributed to subsets of T helper cells forces immunologists to reassess the mechanisms by which selective effector functions arise. The parallelism between ILCs and T cells extends beyond these two cell types and comprises other innate-like T lymphocytes. Beyond the recognition of specialized effector functionalities in diverse lymphocytes, features typical of T cells, such as plasticity and memory, are also relevant for innate lymphocytes. Herein, we review what we have learned in terms of the molecular mechanisms underlying these shared functions, focusing on insights provided by next generation sequencing technologies. We review data on the role of lineage-defining- and signal-dependent transcription factors (TFs). ILC regulomes emerge developmentally whereas the much of the open chromatin regions of T cells are generated acutely, in an activation-dependent manner. And yet, these regions of open chromatin in T cells and ILCs have remarkable overlaps, suggesting that though accessibility is acquired by distinct modes, the end result is that convergent signaling pathways may be involved. Although much is left to be learned, substantial progress has been made in understanding how TFs and epigenomic status contribute to ILC biology in terms of differentiation, specification, and plasticity.

Interactions between Innate Lymphoid Cells and Cells of the Innate and Adaptive Immune System

Type 2 innate lymphoid cells (ILC2s) are a major source of cytokines, which are also produced by Th2 cells and several cell types of the innate immune system. Work over the past few years indicates that ILC2s play a central role in regulating type 2 immune responses against allergens and helminths. ILC2s can interact with a variety of cells types of the innate and adaptive immune system by cell–cell contacts or by communication via soluble factors. In this review, we provide an overview about recent advances in our understanding how ILC2s orchestrate type 2 immune responses with focus on direct interactions between ILC2s and other cells of the immune system.

Alterations in circulating lymphoid cell populations in systemic small vessel vasculitis are non-specific manifestations of renal injury

Innate lymphocyte populations, such as innate lymphoid cells (ILCs), γδ T cells, invariant natural killer T (iNK T) cells and mucosal-associated invariant T (MAIT) cells are emerging as important effectors of innate immunity and are involved in various inflammatory and autoimmune diseases. The aim of this study was to assess the frequencies and absolute numbers of innate lymphocytes as well as conventional lymphocytes and monocytes in peripheral blood from a cohort of anti-neutrophil cytoplasm autoantibody (ANCA)-associated vasculitis (AAV) patients. Thirty-eight AAV patients and 24 healthy and disease controls were included in the study. Patients with AAV were sampled both with and without immunosuppressive treatment, and in the setting of both active disease and remission. The frequencies of MAIT and ILC2 cells were significantly lower in patients with AAV and in the disease control group compared to healthy controls. These reductions in the AAV patients remained during remission. B cell count and frequencies were significantly lower in AAV in remission compared to patients with active disease and disease controls. Despite the strong T helper type 2 (Th) preponderance of eosinophilic granulomatosis with polyangiitis, we did not observe increased ILC2 frequency in this cohort of patients. The frequencies of other cell types were similar in all groups studied. Reductions in circulating ILC2 and MAIT cells reported previously in patients with AAV are not specific for AAV, but are more likely to be due to non-specific manifestations of renal impairment and chronic illness. Reduction in B cell numbers in AAV patients experiencing remission is probably therapy-related.

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.

Exoenzyme T Plays a Pivotal Role in the IFN-γ Production after Pseudomonas Challenge in IL-12 Primed Natural Killer Cells

Pseudomonas aeruginosa (PA) expresses the type III secretion system (T3SS) and effector exoenzymes that interfere with intracellular pathways. Natural killer (NK) cells play a key role in antibacterial immunity and their activation is highly dependent on IL-12 produced by myeloid cells. We studied PA and NK cell interactions and the role of IL-12 using human peripheral blood mononuclear cells, sorted human NK cells, and a human NK cell line (NK92). We used a wild-type (WT) strain of PA (PAO1) or isogenic PA-deleted strains to delineate the role of T3SS and exoenzymes. Our hypotheses were tested in vivo in a PA-pneumonia mouse model. Human NK cells or NK92 cell line produced low levels of IFN-γ in response to PA without IL-12 stimulation, whereas PA significantly increased IFN-γ after IL-12 priming. The modulation of IFN-γ production by PA required bacteria-to-cell contact. Among T3SS effectors, exoenzyme T (ExoT) upregulates IFN-γ production and control ERK activation. In vivo, ExoT also increases IFN-γ levels and the percentage of IFN-γ⁺ NK cells in lungs during PA pneumonia, confirming in vitro data. In conclusion, our results suggest that T3SS could modulate the production of IFN-γ by NK cells after PA infection through ERK activation.

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.

Innate lymphoid cells at the interface between obesity and asthma

Obesity and asthma prevalence has dramatically and concomitantly increased over the last 25 years, and many epidemiological studies have highlighted obesity as an important risk factor for asthma. Although many studies have been performed, the underlying mechanisms remain poorly understood. Innate mechanisms have been involved in both diseases, in particular through the recently described innate lymphoid cells (ILCs). ILCs are subdivided into three groups that are defined by their cytokine production and by their master transcription factor expression, in sharp correlation with their T helper counterparts. However, unlike T helper cells, ILCs do not express antigen-specific receptors, but respond to damage-induced signals. ILCs have been found in target tissues of both diseases, and data have implicated these cells in the pathogenesis of both diseases. In particular group 2 ILCs (ILC2) are activated in both the adipose and lung tissues under the effect of interleukin-33 and interleukin-25 expression. However, counter-intuitively to the well-known association between obesity and asthma, ILC2 are beneficial for obesity but deleterious for asthma. This review will examine the roles of ILCs in each disease and recent data highlighting ILCs as a putative link between obesity and asthma.

Allergen-induced Increases in Sputum Levels of Group 2 Innate Lymphoid Cells in Subjects with Asthma

RATIONALE

Group 2 innate lymphoid cells (ILC2), a major source of type 2 cytokines, initiate eosinophilic inflammatory responses in murine models of asthma.

OBJECTIVES

To investigate the role of ILC2 in allergen-induced airway eosinophilic responses in subjects with atopy and asthma.

METHODS

Using a diluent-controlled allergen challenge crossover study, where all subjects (n = 10) developed allergen-induced early and late responses, airway eosinophilia, and increased methacholine airway responsiveness, bone marrow, blood, and sputum samples were collected before and after inhalation challenge.

MEASUREMENTS AND MAIN RESULTS

ILC2 (lin^-FcεRI^-CD45⁺CD127⁺ST2⁺) and CD4⁺T lymphocytes were enumerated by flow cytometry, as well as intracellular IL-5 and IL-13 expression. Steroid sensitivity of ILC2 and CD4⁺ T cells was investigated in vitro. A significant increase in total, IL-5⁺, IL-13⁺, and CRTH2⁺ ILC2 was found in sputum, 24 hours after allergen, coincident with a significant decrease in blood ILC2. Total, IL-5⁺, and IL-13⁺, but not CRTH2⁺, CD4⁺ T cells significantly increased at 24 and 48 hours after allergen in sputum. In blood and bone marrow, only CD4⁺ cells demonstrated increased activation after allergen. Airway eosinophilia correlated with IL-5⁺ ILC2 at all time points and allergen-induced changes in IL-5⁺ CD4⁺ cells at 48 hours after allergen. Dexamethasone significantly attenuated IL-2- and IL-33-stimulated IL-5 and IL-13 production by both cell types.

CONCLUSIONS

Innate and adaptive immune cells are increased in the airways associated with allergic asthmatic responses. Total and type 2 cytokine-positive ILC2 are increased only within the airways, whereas CD4⁺ T lymphocytes demonstrated local and systemic increases. Steroid sensitivity of both cells may explain effectiveness of this therapy in those with mild asthma.

Group 1 Innate Lymphoid Cell Lineage Identity Is Determined by a cis-Regulatory Element Marked by a Long Non-coding RNA

Commitment to the innate lymphoid cell (ILC) lineage is determined by Id2, a transcriptional regulator that antagonizes T and B cell-specific gene expression programs. Yet how Id2 expression is regulated in each ILC subset remains poorly understood. We identified a cis-regulatory element demarcated by a long non-coding RNA (lncRNA) that controls the function and lineage identity of group 1 ILCs, while being dispensable for early ILC development and homeostasis of ILC2s and ILC3s. The locus encoding this lncRNA, which we termed Rroid, directly interacted with the promoter of its neighboring gene, Id2, in group 1 ILCs. Moreover, the Rroid locus, but not the lncRNA itself, controlled the identity and function of ILC1s by promoting chromatin accessibility and deposition of STAT5 at the promoter of Id2 in response to interleukin (IL)-15. Thus, non-coding elements responsive to extracellular cues unique to each ILC subset represent a key regulatory layer for controlling the identity and function of ILCs.

Innate Lymphoid Cells Mediate Pulmonary Eosinophilic Inflammation, Airway Mucous Cell Metaplasia, and Type 2 Immunity in Mice Exposed to Ozone

Exposure to elevated levels of ambient ozone in photochemical smog is associated with eosinophilic airway inflammation and nonatopic asthma in children. In the present study, we determined the role of innate lymphoid cells (ILCs) in the pathogenesis of ozone-induced nonatopic asthma by using lymphoid cell-sufficient C57BL/6 mice, ILC-sufficient Rag2^-/- mice (devoid of T and B cells), and ILC-deficient Rag2^-/-Il2rg^-/- mice (depleted of all lymphoid cells including ILCs). Mice were exposed to 0 or 0.8 parts per million ozone for 1 day or 9 consecutive weekdays (4 hr/day). A single exposure to ozone caused neutrophilic inflammation, airway epithelial injury, and reparative DNA synthesis in all strains of mice, irrespective of the presence or absence of ILCs. In contrast, 9-day exposures induced eosinophilic inflammation and mucous cell metaplasia only in the lungs of ILC-sufficient mice. Repeated ozone exposures also elicited increased messenger RNA expression of transcripts associated with type 2 immunity and airway mucus production in ILC-sufficient mice. ILC-deficient mice repeatedly exposed to ozone had no pulmonary pathology or increased gene expression related to type 2 immunity. These results suggest a new paradigm for the biologic mechanisms underlying the development of a phenotype of childhood nonatopic asthma that has been linked to ambient ozone exposures.

Human CD5+ Innate Lymphoid Cells Are Functionally Immature and Their Development from CD34+ Progenitor Cells Is Regulated by Id2

Innate lymphoid cells (ILCs) have emerged as a key cell type involved in surveillance and maintenance of mucosal tissues. Mouse ILCs rely on the transcriptional regulator Inhibitor of DNA-binding protein 2 (Id2) for their development. Here, we show that Id2 also drives development of human ILC because forced expression of Id2 in human thymic progenitors blocked T cell commitment, upregulated CD161 and promyelocytic leukemia zinc finger (PLZF), and maintained CD127 expression, markers that are characteristic for human ILCs. Surprisingly CD5 was also expressed on these in vitro generated ILCs. This was not an in vitro artifact because CD5 was also found on ex vivo isolated ILCs from thymus and from umbilical cord blood. CD5 was also expressed on small proportions of ILC2 and ILC3. CD5⁺ ILCs were functionally immature, but could further differentiate into mature CD5⁻ cytokine-secreting ILCs. Our data show that Id2 governs human ILC development from thymic progenitor cells toward immature CD5⁺ ILCs.

Increased Innate Lymphoid Cells in Periodontal Tissue of the Murine Model of Periodontitis: The Role of AMP-Activated Protein Kinase and Relevance for the Human Condition

Innate lymphoid cells (ILCs) are master regulators of immune and inflammatory responses, but their own regulatory mechanisms and functional roles of their subtypes (i.e., ILC1s-ILC3s) remain largely unresolved. Interestingly, AMP-activated protein kinase (AMPK), influences inflammatory responses, but its role in modulation of ILCs is not known. Periodontitis is a prevalent disorder with impairment of immune and inflammatory responses contributing importantly to its pathogenesis; however, neither the role of ILCs nor AMPK has been explored in this condition. We tested the hypotheses that (a) periodontitis increases ILCs and expression of relevant cytokines thereby contributing to inflammation and (b) knockdown of AMPK worsens indices of periodontitis in association with further increases in subtypes of ILCs and cytokine expression. The studies utilized wild-type (WT) and AMPK knockout (KO) mice, subjected to ligature-induced periodontitis or sham operation, in association with the use of micro-CT for assessment of bone loss, immunogold electron microscopy to show presence of ILCs in periodontal tissues, flow cytometry for quantitative assessment of subtypes of ILCs and RT-polymerase chain reaction analyses to measure mRNA expression of several relevant cytokines. The results for the first time show (a) presence of each subtype of ILCs in periodontal tissues of sham control and periodontitis animals, (b) that periodontitis is associated with increased frequencies of ILC1s-ILC3s with the effect more marked for ILC2s and differential phenotypic marker expression for ILC3s, (c) that AMPK KO mice display exacerbation of indices of periodontitis in association with further increases in the frequency of subtypes of ILCs with persistence of ILC2s effect, and (d) that periodontitis increased mRNA for interleukin (IL)-33, but not IL-5 or IL-13, in WT mice but expression of these cytokines was markedly increased in AMPK KO mice with periodontitis. Subsequently, we showed that human periodontitis is associated with increases in each ILCs subtype with the effect more marked for ILC2s and that mRNA expressions for IL-33 and IL-5 are markedly greater for sites affected by periodontitis than healthy sites. Collectively, these novel observations indicate a pivotal role for ILCs in pathogenesis of periodontitis and that AMPK is a regulator of their phenotype expression in this condition.

Regulatory T cells and type 2 innate lymphoid cell-dependent asthma

Group 2 innate lymphoid cells (ILC2s) are a recently identified group of cells with the potent capability to produce Th2-type cytokines such as interleukin (IL)-5 and IL-13. Several studies suggest that ILC2s play an important role in the development of allergic diseases and asthma. Activation of pulmonary ILC2s in murine models lacking T and B cells induces eosinophilia and airway hyper-reactivity (AHR), which are cardinal features of asthma. More importantly, numerous recent studies have highlighted the role of ILC2s in asthma persistence and exacerbation among human subjects, and thus, regulation of pulmonary ILC2s is a major area of investigation aimed at curbing allergic lung inflammation and exacerbation. Emerging evidence reveals that a group of regulatory T cells, induced Tregs (iTregs), effectively suppress the production of ILC2-driven, pro-inflammatory cytokines IL-5 and IL-13. The inhibitory effects of iTregs are blocked by preventing direct cellular contact or by inhibiting the ICOS-ICOS-ligand (ICOSL) pathway, suggesting that both direct contact and ICOS-ICOSL interaction are important in the regulation of ILC2 function. Also, cytokines such as IL-10 and TGF-β1 significantly reduce cytokine secretion by ILC2s. Altogether, these new findings uncover iTregs as potent regulators of ILC2 activation and implicate their utility as a therapeutic approach for the treatment of ILC2-mediated allergic asthma and respiratory disease.

Zbtb1 controls NKp46+ ROR-gamma-T+ innate lymphoid cell (ILC3) development

Innate lymphoid cells (ILCs) play a central role conferring protection at the mucosal frontier. In this study, we have identified a requirement of the transcription factor Zbtb1 for the development of RORγt⁺ ILCs (ILC3s). Zbtb1-deficient mice lacked NKp46⁺ ILC3 cells in the lamina propria of the small and large intestine. This requirement of Zbtb1 was cell intrinsic, as NKp46⁺ ILC3s were not generated from Zbtb1-deficient progenitors in bone marrow chimeras and Zbtb1-deficient RORγt⁺ CCR6⁻NKp46⁻ ILC3s didn't generate NKp46⁺ ILC3s in co-cultures with OP9-DL1 stroma. In correlation with this impairment, Zbtb1-deficient ILC3 cells failed to upregulate T-bet expression, and to acquire IFN-γ production characteristic of NKp46⁺ cells. Finally, absence of NKp46⁺ILC3 cells combined with the absence of T-cells in Zbtb1-deficient mice, led to a transient susceptibility to C. rodentium infections. Altogether, these results establish that Zbtb1 is essential for the development of NKp46⁺ ILC3 cells.

The IDO-AhR Axis Controls Th17/Treg Immunity in a Pulmonary Model of Fungal Infection

In infectious diseases, the enzyme indoleamine 2,3 dioxygenase-1 (IDO1) that catalyzes the tryptophan (Trp) degradation along the kynurenines (Kyn) pathway has two main functions, the control of pathogen growth by reducing available Trp and immune regulation mediated by the Kyn-mediated expansion of regulatory T (Treg) cells via aryl hydrocarbon receptor (AhR). In pulmonary paracoccidioidomycosis (PCM) caused by the dimorphic fungus Paracoccidioides brasiliensis, IDO1 was shown to control the disease severity of both resistant and susceptible mice to the infection; however, only in resistant mice, IDO1 is induced by TGF-β signaling that confers a stable tolerogenic phenotype to dendritic cells (DCs). In addition, in pulmonary PCM, the tolerogenic function of plasmacytoid dendritic cells was linked to the IDO1 activity. To further evaluate the function of IDO1 in pulmonary PCM, IDO1-deficient (IDO1^-/-) C57BL/6 mice were intratracheally infected with P. brasiliensis yeasts and the infection analyzed at three postinfection periods regarding several parameters of disease severity and immune response. The fungal loads and tissue pathology of IDO1^-/- mice were higher than their wild-type controls resulting in increased mortality rates. The evaluation of innate lymphoid cells showed an upregulated differentiation of the innate lymphoid cell 3 phenotype accompanied by a decreased expansion of ILC1 and NK cells in the lungs of infected IDO1^-/- mice. DCs from these mice expressed elevated levels of costimulatory molecules and cytokine IL-6 associated with reduced production of IL-12, TNF-α, IL-1β, TGF-β, and IL-10. This response was concomitant with a marked reduction in AhR production. The absence of IDO1 expression caused an increased influx of activated Th17 cells to the lungs with a simultaneous reduction in Th1 and Treg cells. Accordingly, the suppressive cytokines IL-10, TGF-β, IL-27, and IL-35 appeared in reduced levels in the lungs of IDO1^-/- mice. In conclusion, the immunological balance mediated by the axis IDO/AhR is fundamental to determine the balance between Th17/Treg cells and control the severity of pulmonary PCM.

Inflammation and its resolution and the musculoskeletal system

Inflammation, an essential tissue response to extrinsic/intrinsic damage, is a very dynamic process in terms of complexity and extension of cellular and metabolic involvement. The aim of the inflammatory response is to eliminate the pathogenic initiator with limited collateral damage of the inflamed tissue, followed by a complex tissue repair to the preinflammation phenotype. Persistent inflammation is a major contributor to the pathogenesis of many musculoskeletal diseases including ageing-related pathologies such as osteoporosis, osteoarthritis, and sarcopaenia. Understanding the mechanisms of inflammation and its resolution is therefore critical for the development of effective regenerative, and therapeutic strategies in orthopaedics.

The Role of Innate Lymphoid Cells in Immune-Mediated Liver Diseases

Innate lymphoid cells (ILCs) are a recently identified group of innate immune cells lacking antigen-specific receptors that can mediate immune responses and regulate tissue homeostasis and inflammation. ILCs comprise group 1 ILCs, group 2 ILCs, and group 3 ILCs. These ILCs usually localize at mucosal surfaces and combat pathogens by the rapid release of certain cytokines. However, the uncontrolled activation of ILCs can also lead to damaging inflammation, especially in the gut, lung, and skin. Although the physiological and pathogenic roles of ILCs in liver diseases have been attracting increasing attention recently, there has been no systematic review regarding the roles of ILCs in immune-mediated liver diseases. Here, we review the relationships between the ILC subsets and their functions in immune-mediated liver diseases, and discuss their therapeutic potential based on current knowledge about the functional roles of these cells in liver diseases.