Identity, regulation and in vivo function of gut NKp46+RORγt+ and NKp46+RORγt- lymphoid cells

A vasculature-resident innate lymphoid cell population in mouse lungs

Tissue-resident immune cells such as innate lymphoid cells (ILC) are known to reside in the parenchymal compartments of tissues and modulate local immune protection. Here we use intravascular cell labeling, parabiosis and multiplex 3D imaging to identify a population of group 3 ILCs in mice that are present within the intravascular space of lung blood vessels (vILC3). vILC3s are distributed broadly in alveolar capillary beds from which inhaled pathogens enter the lung parenchyma. By contrast, conventional ILC3s in tissue parenchyma are enriched in lymphoid clusters in proximity to large veins. In a mouse model of pneumonia, Pseudomonas aeruginosa infection results in rapid vILC3 expansion and production of chemokines including CCL4. Blocking CCL4 in vivo attenuates neutrophil recruitment to the lung at the early stage of infection, resulting in prolonged inflammation and delayed bacterial clearance. Our findings thus define the intravascular space as a site of ILC residence in mice, and reveal a unique immune cell population that interfaces with tissue alarmins and the circulating immune system for timely host defense.

Hematopoietic MyD88 orchestrates the control of gut colonization by segmented filamentous bacteria

Host-microbiota cooperation is critical for successful intestinal homeostasis. The commensal segmented filamentous bacteria (SFB) are crucial for orchestrating the post-natal maturation of the host gut immune system and establishing a healthy state of physiological inflammation, which largely depends on their intimate attachment to the ileal mucosa. However, the signaling pathways used by SFB to induce gut immune responses and how such responses ultimately control SFB colonization remain controversial. Using gnotobiotic approaches, we showed that SFB load is controlled by complex interactions involving the gut microbiota and the host immune system. Therefore, to clearly determine the role of host immune responses induced by SFB in directly controlling their growth, immunodeficient mice monocolonized with SFB were used. Here, we show that in the absence of a complex microbiota, the humoral immune response is dispensable to control SFB growth in the jejunum and ileum, shortly and later after colonization. In contrast, MyD88 signaling in myeloid cells is critical for licensing interleukin (IL)-22 production by type 3 innate lymphoid cells (ILC3) and CD4+ T cells, which ultimately limits SFB expansion. Thus, by revisiting the hierarchy of immune mechanisms that directly control SFB growth, our results emphasize the necessary and sufficient role of a hematopoietic MyD88/IL-22 axis.

The renaissance of oral tolerance: merging tradition and new insights

Oral tolerance is the process by which feeding of soluble proteins induces antigen-specific systemic immune unresponsiveness. Oral tolerance is thought to have a central role in suppressing immune responses to 'harmless' food antigens, and its failure can lead to development of pathologies such as food allergies or coeliac disease. However, on the basis of long-standing experimental observations, the relevance of oral tolerance in human health has achieved new prominence recently following the discovery that oral administration of peanut proteins prevents the development of peanut allergy in at-risk human infants. In this Review, we summarize the new mechanistic insights into three key processes necessary for the induction of tolerance to oral antigens: antigen uptake and transport across the small intestinal epithelial barrier to the underlying immune cells; the processing, transport and presentation of fed antigen by different populations of antigen-presenting cells; and the development of immunosuppressive T cell populations that mediate antigen-specific tolerance. In addition, we consider how related but distinct processes maintain tolerance to bacterial antigens in the large intestine. Finally, we outline the molecular mechanisms and functional consequences of failure of oral tolerance and how these may be modulated to enhance clinical outcomes and prevent disease.

Group 3 innate lymphoid cells in intestinal health and disease

The gastrointestinal tract is an immunologically rich organ, containing complex cell networks and dense lymphoid structures that safeguard this large absorptive barrier from pathogens, contribute to tissue physiology and support mucosal healing. Simultaneously, the immune system must remain tolerant to innocuous dietary antigens and trillions of normally beneficial microorganisms colonizing the intestine. Indeed, a dysfunctional immune response in the intestine underlies the pathogenesis of numerous local and systemic diseases, including inflammatory bowel disease, food allergy, chronic enteric infections or cancers. Here, we discuss group 3 innate lymphoid cells (ILC3s), which have emerged as orchestrators of tissue physiology, immunity, inflammation, tolerance and malignancy in the gastrointestinal tract. ILC3s are abundant in the developing and healthy intestine but their numbers or function are altered during chronic disease and cancer. The latest studies provide new insights into the mechanisms by which ILC3s fundamentally shape intestinal homeostasis or disease pathophysiology, and often this functional dichotomy depends on context and complex interactions with other cell types or microorganisms. Finally, we consider how this knowledge could be harnessed to improve current treatments or provoke new opportunities for therapeutic intervention to promote gut health.

The Role of Gut-derived Short-Chain Fatty Acids in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic condition affecting the central nervous system (CNS), where the interplay of genetic and environmental factors influences its pathophysiology, triggering immune responses and instigating inflammation. Contemporary research has been notably dedicated to investigating the contributions of gut microbiota and their metabolites in modulating inflammatory reactions within the CNS. Recent recognition of the gut microbiome and dietary patterns as environmental elements impacting MS development emphasizes the potential influence of small, ubiquitous molecules from microbiota, such as short-chain fatty acids (SCFAs). These molecules may serve as vital molecular signals or metabolic substances regulating host cellular metabolism in the intricate interplay between microbiota and the host. A current emphasis lies on optimizing the health-promoting attributes of colonic bacteria to mitigate urinary tract issues through dietary management. This review aims to spotlight recent investigations on the impact of SCFAs on immune cells pivotal in MS, the involvement of gut microbiota and SCFAs in MS development, and the considerable influence of probiotics on gastrointestinal disruptions in MS. Comprehending the gut-CNS connection holds promise for the development of innovative therapeutic approaches, particularly probiotic-based supplements, for managing MS.

HOIL1 Regulates Group 3 Innate Lymphoid Cells in the Colon and Protects against Systemic Dissemination, Colonic Ulceration, and Lethality from Citrobacter rodentium Infection

Heme-oxidized IRP2 ubiquitin ligase-1 (HOIL1)-deficient patients experience chronic intestinal inflammation and diarrhea as well as increased susceptibility to bacterial infections. HOIL1 is a component of the linear ubiquitin chain assembly complex that regulates immune signaling pathways, including NF-κB-activating pathways. We have shown previously that HOIL1 is essential for survival following Citrobacter rodentium gastrointestinal infection of mice, but the mechanism of protection by HOIL1 was not examined. C. rodentium is an important murine model for human attaching and effacing pathogens, enteropathogenic and enterohemorrhagic Escherichia coli that cause diarrhea and foodborne illnesses and lead to severe disease in children and immunocompromised individuals. In this study, we found that C. rodentium infection resulted in severe colitis and dissemination of C. rodentium to systemic organs in HOIL1-deficient mice. HOIL1 was important in the innate immune response to limit early replication and dissemination of C. rodentium. Using bone marrow chimeras and cell type-specific knockout mice, we found that HOIL1 functioned in radiation-resistant cells and partly in radiation-sensitive cells and in myeloid cells to limit disease, but it was dispensable in intestinal epithelial cells. HOIL1 deficiency significantly impaired the expansion of group 3 innate lymphoid cells and their production of IL-22 during C. rodentium infection. Understanding the role HOIL1 plays in type 3 inflammation and in limiting the pathogenesis of attaching and effacing lesion-forming bacteria will provide further insight into the innate immune response to gastrointestinal pathogens and inflammatory disorders.

Intestinal Immune Imbalance is an Alarm in the Development of IBD

Immune regulation plays a crucial role in human health and disease. Inflammatory bowel disease (IBD) is a chronic relapse bowel disease with an increasing incidence worldwide. Clinical treatments for IBD are limited and inefficient. However, the pathogenesis of immune-mediated IBD remains unclear. This review describes the activation of innate and adaptive immune functions by intestinal immune cells to regulate intestinal immune balance and maintain intestinal mucosal integrity. Changes in susceptible genes, autophagy, energy metabolism, and other factors interact in a complex manner with the immune system, eventually leading to intestinal immune imbalance and the onset of IBD. These events indicate that intestinal immune imbalance is an alarm for IBD development, further opening new possibilities for the unprecedented development of immunotherapy for IBD.

ILC3s restrict the dissemination of intestinal bacteria to safeguard liver regeneration after surgery

It is generally believed that environmental or cutaneous bacteria are the main origin of surgical infections. Therefore, measures to prevent postoperative infections focus on optimizing hygiene and improving asepsis and antisepsis. In a large cohort of patients with infections following major surgery, we identified that the causative bacteria are mainly of intestinal origin. Postoperative infections of intestinal origin were also found in mice undergoing partial hepatectomy. CCR6⁺ group 3 innate lymphoid cells (ILC3s) limited systemic bacterial spread. Such bulwark function against host invasion required the production of interleukin-22 (IL-22), which controlled the expression of antimicrobial peptides in hepatocytes, thereby limiting bacterial spread. Using genetic loss-of-function experiments and punctual depletion of ILCs, we demonstrate that the failure to restrict intestinal commensals by ILC3s results in impaired liver regeneration. Our data emphasize the importance of endogenous intestinal bacteria as a source for postoperative infection and indicate ILC3s as potential new targets.

Role of natural killer and B cell interaction in inducing pathogen specific immune responses

The innate lymphoid cell (ILC) system comprising of the circulating and tissue-resident cells is known to clear infectious pathogens, establish immune homeostasis as well as confer antitumor immunity. Human natural killer cells (hNKs) and other ILCs carry out mopping of the infectious pathogens and perform cytolytic activity regulated by the non-adaptive immune system. The NK cells generate immunological memory and rapid recall response tightly regulated by the adaptive immunity. The interaction of NK and B cell, and its role to induce the pathogen specific immunity is not fully understood. Hence, present article sheds light on the interaction between NK and B cells and resulting immune responses in the infectious diseases. The immune responses elicited by the NK-B cell interaction is of particular importance for developing therapeutic vaccines against the infectious pathogens. Further, experimental evidences suggest the immune-response driven by NK cell population elicits the host-specific antibodies and memory B cells. Also, recently developed humanized immune system (HIS) mice and their importance in to understanding the NK-B cell interaction and resulting pathogen specific immunity has been discussed.

Cell autonomous expression of BCL6 is required to maintain lineage identity of mouse CCR6+ ILC3s

Innate lymphoid cells (ILC) are similar to T helper (Th) cells in expression of cytokines and transcription factors. For example, RORγt is the lineage-specific transcription factor for both ILC3 and Th17 cells. However, the ILC counterpart for BCL6-expressing T follicular helper (Tfh) cells has not been defined. Here, we report that in the ILC compartment, BCL6 is selectively co-expressed with not only CXCR5 but also RORγt and CCR6 in ILC3 from multiple tissues. BCL6-deficient ILC3 produces enhanced levels of IL-17A and IL-22. More importantly, phenotypic and single-cell ATAC-seq analysis show that absence of BCL6 in mature ILC3 increases the numbers of ILC1 and transitional cells co-expressing ILC3 and ILC1 marker genes. A lineage-tracing experiment further reveals BCL6+ ILC3 to ILC1 trans-differentiation under steady state. Finally, microbiota promote BCL6 expression in colonic CCR6+ ILC3 and thus reinforce their stability. Collectively, our data have demonstrated that CCR6+ ILC3 have both Th17 and Tfh programs and that BCL6 expression in these cells functions to maintain their lineage identity.

Disturbed natural killer cell homeostasis in the salivary gland enhances autoimmune pathology via IFN-γ in a mouse model of primary Sjögren's syndrome

OBJECTIVE

Innate lymphoid cells (ILCs), including natural killer (NK) cells, ILC1, ILC2, lymphoid tissue-inducer (LTi) cells, and ILC3 cell, play a key role in various immune responses. Primary Sjögren's syndrome (pSS) is an autoimmune disease characterized by chronic inflammation of exocrine glands, such as the lacrimal and salivary glands (SGs). The role of NK cells among ILCs in the pathogenesis of pSS is still unclear. In this study, the characteristics and subsets of NK cells in the salivary gland (SG) tissue were analyzed using a murine model of pSS.

METHODS

Multiple phenotypes and cytotoxic signature of the SG NK cells in control and pSS model mice were evaluated by flow cytometric analysis. Intracellular expression of interferon-γ (IFN-γ) among T cells and NK cells from the SG tissues was compared by in vitro experiments. In addition, pathological analysis was performed using anti-asialo-GM1 (ASGM1) antibody (Ab)-injected pSS model mice.

RESULTS

The number of conventional NK (cNK) cells in the SG of pSS model mice significantly increased compared with that in control mice at 6 weeks of age. The production level of IFN-γ was significantly higher in SG NK cells than in SG T cells. The depletion of NK cells by ASGM1 Ab altered the ratio of tissue resident NK (rNK) cells to cNK cells, which inhibited the injury to SG cells with the recovery of saliva secretion in pSS model mice.

CONCLUSION

The results indicate that SG cNK cells may enhance the autoreactive response in the target organ by upregulating of IFN-γ, whereas SG rNK cells protect target cells against T cell cytotoxicity. Therefore, the activation process and multiple functions of NK cells in the target organ could be helpful to develop potential markers for determining autoimmune disease activity and target molecules for incurable immune disorders.

Non-alcoholic fatty liver disease and intestinal immune status: a narrative review

Background and objectives: Non-alcoholic fatty liver disease (NAFLD) interacts with the gut immunity. However, the mechanisms underlying alternations of intestinal immune system in NAFLD remains unclear. To date, no effective medical interventions exist that completely reverse the disease. In this review, we mainly elaborates on the impact of NAFLD on intestinal immune cells and briefly summarize the new treatment methods for NAFLD targeting at intestinal immune cells.Methods: We searched MEDLINE, EMBASE and Web of Science for English-language sources. The preferred citations were meta-analyses and systematic or narrative reviews. Citation tracking was completed for all identified studies included in the refined library, using Google Scholar. No restriction was placed on the year of publication for the included reports.Results: The intestinal immune imbalance promotes liver inflammation and fibrosis in the process of NAFLD, and meanwhile, NAFLD influences disorders of immune cells in the liver and intestinal tract. Biological agents targeting at intestinal immunity has been shown in preclinical studies to be an effective method for systemic immune modulation and alleviates immune-mediated injury.Conclusions: Intestinal immune disorder plays an important role in triggering and amplifying hepatic inflammation in NAFLD. Advances in knowledge of the gut-liver axis are driving the development of diagnostic, prognostic and therapeutic tools based on intestine immunity for the management of NAFLD.

Oral tolerance: an updated review

Oral tolerance (OT) has classically been defined as the specific suppression of cellular and/or humoral immune responses to an antigen by prior administration of the antigen through the oral route. Multiple mechanisms have been proposed to explain the induction of OT including T cell clonal depletion and anergy when high doses of antigens are fed, and regulatory T (Treg) cell generation following oral administration of low and repeated doses of antigens. Oral antigen administration suppresses the immune response in several animal models of autoimmune disease, including experimental autoimmune encephalomyelitis, uveitis, thyroiditis, myasthenia, arthritis and diabetes, but also non-autoimmune inflammatory conditions such as asthma, atherosclerosis, graft rejection, allergy and stroke. However, human trials have given mixed results and a great deal remains to be learned about the mechanisms of OT before it can be successfully applied to people. One of the possible mechanisms relates to the gut microbiota and in this review, we will explore the cellular components involved in the induction of OT and the role of the gut microbiota in contributing to OT development.

Innate Lymphoid Cells in Response to Intracellular Pathogens: Protection Versus Immunopathology

Innate lymphoid cells (ILCs) are a heterogeneous group of cytokine-producing lymphocytes which are predominantly located at mucosal barrier surfaces, such as skin, lungs, and gastrointestinal tract. ILCs contribute to tissue homeostasis, regulate microbiota-derived signals, and protect against mucosal pathogens. ILCs are classified into five major groups by their developmental origin and distinct cytokine production. A recently emerged intriguing feature of ILCs is their ability to alter their phenotype and function in response to changing local environmental cues such as pathogen invasion. Once the pathogen crosses host barriers, ILCs quickly activate cytokine production to limit the spread of the pathogen. However, the dysregulated ILC responses can lead to tissue inflammation and damage. Furthermore, the interplay between ILCs and other immune cell types shapes the outcome of the immune response. Recent studies highlighted the important role of ILCs for host defense against intracellular pathogens. Here, we review recent advances in understanding the mechanisms controlling protective and pathogenic ILC responses to intracellular pathogens. This knowledge can help develop new ILC-targeted strategies to control infectious diseases and immunopathology.

Innate Lymphoid Cells and Natural Killer Cells in Bacterial Infections: Function, Dysregulation, and Therapeutic Targets

Infectious diseases represent one of the largest medical challenges worldwide. Bacterial infections, in particular, remain a pertinent health challenge and burden. Moreover, such infections increase over time due to the continuous use of various antibiotics without medical need, thus leading to several side effects and bacterial resistance. Our innate immune system represents our first line of defense against any foreign pathogens. This system comprises the innate lymphoid cells (ILCs), including natural killer (NK) cells that are critical players in establishing homeostasis and immunity against infections. ILCs are a group of functionally heterogenous but potent innate immune effector cells that constitute tissue-resident sentinels against intracellular and extracellular bacterial infections. Being a nascent subset of innate lymphocytes, their role in bacterial infections is not clearly understood. Furthermore, these pathogens have developed methods to evade the host immune system, and hence permit infection spread and tissue damage. In this review, we highlight the role of the different ILC populations in various bacterial infections and the possible ways of immune evasion. Additionally, potential immunotherapies to manipulate ILC responses will be briefly discussed.

Maternal natural killer cells at the intersection between reproduction and mucosal immunity

Many maternal immune cells populate the decidua, which is the mucosal lining of the uterus transformed during pregnancy. Here, abundant natural killer (NK) cells and macrophages help the uterine vasculature adapt to fetal demands for gas and nutrients, thereby supporting fetal growth. Fetal trophoblast cells budding off the forming placenta and invading deep into maternal tissues come into contact with these and other immune cells. Besides their homeostatic functions, decidual NK cells can respond to pathogens during infection, but in doing so, they may become conflicted between destroying the invader and sustaining fetoplacental growth. We review how maternal NK cells balance their double duty both in the local microenvironment of the uterus and systemically, during toxoplasmosis, influenza, cytomegalovirus, malaria and other infections that threat pregnancy. We also discuss recent developments in the understanding of NK-cell responses to SARS-Cov-2 infection and the possible dangers of COVID-19 during pregnancy.

Mutual interaction of microbiota and host immunity during health and diseases

Microbiota-host interaction has attracted more and more attentions in recent years. The association between microbiota and host health is largely attributed to its influence on host immune system. Microbial-derived antigens and metabolites play a critical role in shaping the host immune system, including regulating its development, activation, and function. However, during various diseases the microbiota-host communication is frequently found to be disordered. In particular, gut microbiota dysbiosis associated with or led to the occurrence and progression of infectious diseases, autoimmune diseases, metabolic diseases, and neurological diseases. Pathogenic microbes and their metabolites disturb the protective function of immune system, and lead to disordered immune responses that usually correlate with disease exacerbation. In the other hand, the immune system also regulates microbiota composition to keep host homeostasis. Here, we will discuss the current advances of our knowledge about the interactions between microbiota and host immune system during health and diseases.

Innate lymphoid cell dysfunction during long-term suppressive antiretroviral therapy in an African cohort

BACKGROUND

Innate lymphoid cells (ILC) are lymphoid lineage innate immune cells that do not mount antigen-specific responses due to their lack of B and T-cell receptors. ILCs are predominantly found at mucosal surfaces, as gatekeepers against invading infectious agents through rapid secretion of immune regulatory cytokines. HIV associated destruction of mucosal lymphoid tissue depletes ILCs, among other immune dysfunctions. Studies have described limited restoration of ILCs during the first three years of combined antiretroviral therapy (cART). Little is known about restoration of ILCs during long-term cART, particularly in sub-Saharan Africa which hosts increasing numbers of adults with at least a decade of cART.

RESULTS

We examined phenotypes and function of ILCs from peripheral blood mononuclear cells after 12 years of suppressive cART. We report that ILC1 frequencies (T-BET + CD127 + and CD161 +) were higher in cART-treated HIV-infected relative to age-matched health HIV-negative adults; P = 0.04 whereas ILC precursors (ILCP) were comparable in the two groups (P = 0.56). Interferon gamma (IFN-γ) secretion by ILC1 was higher among cART-treated HIV-infected relative to HIV-negative adults (P = 0.03).

CONCLUSION

HIV associated alteration of ILC persisted during cART and may likely affect the quality of host innate and adaptive immune responses during long-term cART.

Innate Lymphoid Cells in Intestinal Homeostasis and Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a heterogeneous state of chronic intestinal inflammation of unknown cause encompassing Crohn’s disease (CD) and ulcerative colitis (UC). IBD has been linked to genetic and environmental factors, microbiota dysbiosis, exacerbated innate and adaptive immunity and epithelial intestinal barrier dysfunction. IBD is classically associated with gut accumulation of proinflammatory Th1 and Th17 cells accompanied by insufficient Treg numbers and Tr1 immune suppression. Inflammatory T cells guide innate cells to perpetuate a constant hypersensitivity to microbial antigens, tissue injury and chronic intestinal inflammation. Recent studies of intestinal mucosal homeostasis and IBD suggest involvement of innate lymphoid cells (ILCs). These lymphoid-origin cells are innate counterparts of T cells but lack the antigen receptors expressed on B and T cells. ILCs play important roles in the first line of antimicrobial defense and contribute to organ development, tissue protection and regeneration, and mucosal homeostasis by maintaining the balance between antipathogen immunity and commensal tolerance. Intestinal homeostasis requires strict regulation of the quantity and activity of local ILC subpopulations. Recent studies demonstrated that changes to ILCs during IBD contribute to disease development. A better understanding of ILC behavior in gastrointestinal homeostasis and inflammation will provide valuable insights into new approaches to IBD treatment. This review summarizes recent research into ILCs in intestinal homeostasis and the latest advances in the understanding of the role of ILCs in IBD, with particular emphasis on the interaction between microbiota and ILC populations and functions.

Chronic Stress-Induced Depression and Anxiety Priming Modulated by Gut-Brain-Axis Immunity

Chronic stress manifests as depressive- and anxiety-like behavior while recurrent stress elicits disproportionate behavioral impairments linked to stress-induced immunological priming. The gut-brain-microbiota-axis is a promising therapeutic target for stress-induced behavioral impairments as it simultaneously modulates peripheral and brain immunological landscapes. In this study, a combination of probiotics and prebiotics, known as a synbiotic, promoted behavioral resilience to chronic and recurrent stress by normalizing gut microbiota populations and promoting regulatory T cell (Treg) expansion through modulation of ileal innate lymphoid cell (ILC)3 activity, an impact reflecting behavioral responses better than limbic brain region neuroinflammation. Supporting this conclusion, a multivariate machine learning model correlatively predicted a cross-tissue immunological signature of stress-induced behavioral impairment where the ileal Treg/T helper17 cell ratio associated to hippocampal chemotactic chemokine and prefrontal cortex IL-1β production in the context of stress-induced behavioral deficits. In conclusion, stress-induced behavioral impairments depend on the gut-brain-microbiota-axis and through ileal immune regulation, synbiotics attenuate the associated depressive- and anxiety-like behavior.

Innate lymphoid cell development

Innate lymphoid cells (ILCs) mainly reside at barrier surfaces and regulate tissue homeostasis and immunity. ILCs are divided into 3 groups, group 1 ILCs, group 2 ILCs, and group 3 ILC3, on the basis of their similar effector programs to T cells. The development of ILCs from lymphoid progenitors in adult mouse bone marrow has been studied in detail, and multiple ILC progenitors have been characterized. ILCs are mostly tissue-resident cells that develop in the perinatal period. More recently, ILC progenitors have also been identified in peripheral tissues. In this review, we discuss the stepwise transcription factor-directed differentiation of mouse ILC progenitors into mature ILCs, the critical time windows in ILC development, and the contribution of bone marrow versus tissue ILC progenitors to the pool of mature ILCs in tissues.

Innate Lymphoid Cells and Adaptive Immune Cells Cross-Talk: A Secret Talk Revealed in Immune Homeostasis and Different Inflammatory Conditions

The inflammatory immune response has evolved to protect the host from different pathogens, allergens, and endogenous death or damage-associated molecular patterns. Both innate and adaptive immune components are crucial in inducing an inflammatory immune response depending on the stimulus type and its duration of exposure or the activation of the primary innate immune response. As the source of inflammation is removed, the aggravated immune response comes to its homeostatic level. However, the failure of the inflammatory immune response to subside to its normal level generates chronic inflammatory conditions, including autoimmune diseases and cancer. Innate lymphoid cells (ILCs) are newly discovered innate immune cells, which are present in abundance at mucosal surfaces, including lungs, gastrointestinal tract, and reproductive tract. Also, they are present in peripheral blood circulation, skin, and lymph nodes. They play a crucial role in generating the pro-inflammatory immune response during diverse conditions. On the other hand, adaptive immune cells, including different types of T and B cells are major players in the pathogenesis of autoimmune diseases (type 1 diabetes mellitus, rheumatoid arthritis, psoriasis, and systemic lupus erythematosus, etc.) and cancers. Thus the article is designed to discuss the immunological role of different ILCs and their interaction with adaptive immune cells in maintaining the immune homeostasis, and during inflammatory autoimmune diseases along with other inflammatory conditions (excluding pathogen-induced inflammation), including cancer, graft-versus-host diseases, and human pregnancy.

Innate immune responses to Listeria in vivo

Listeria monocytogenes (Lm) is a foodborne bacterial pathogen that causes listeriosis, a severe infection that manifests as bacteremia and meningo-encephalitis mostly in immunocompromised individuals, and maternal-fetal infection. A critical pathogenic determinant of Lm relies on its ability to actively cross the intestinal barrier, disseminate systemically and cross the blood-brain and placental barriers. Here we illustrate how Lm both evades innate immunity, favoring its dissemination in host tissues, and triggers innate immune defenses that participate to its control.

Optimized Protocol for Characterization of Mouse Gut Innate Lymphoid Cells

Since their discovery, innate lymphoid cells (ILCs) have gradually been gaining greater relevance in the field of immunology due to their multiple functions in the innate immune response. They can mainly be found in mucosal and barrier organs like skin, gut, and lungs, and have been classified into five main types (NKs, ILC1s, ILC2s, ILC3s, and Lti cells) according to their function and development. They all play major roles in functions such as tissue homeostasis, early pathogen defense, regulation of inflammation, or tissue remodeling. ILCs are mostly tissue-resident cells tightly bound to the tissue structure, a fact that requires long and complex protocols that do not always provide sufficient yield for analysis. This suggests the need for optimized approaches aimed at ensuring that enriched and viable ILC samples are obtained, in order to furnish quality results. Herein a detailed protocol is established for obtaining a single-cell suspension highly enriched in lymphoid cells from mouse gut in order to identify the different subsets of ILCs by means of flow cytometry. The cell marker panel and flow cytometry gating strategies for identification and quantification of all the different ILC populations are provided for simultaneous analysis. Moreover, the protocol described includes a procedure for studying the different cytokines produced by ILC3s involved in maintaining the integrity of the gut barrier and defending against extracellular pathogens. As a result, herein an efficient method is presented for studying mouse ILCs within the lamina propria of the small intestine and colon; this can constitute a useful tool for future investigations in the field.

Type 1 innate lymphoid cells: Soldiers at the front line of immunity

Innate lymphoid cells (ILCs) are tissue-resident innate lymphocytes that have functions to protect the hosts against pathogens and that regulate tissue inflammation and homeostasis. ILC subsets rapidly produce particular cytokines in response to infection, inflammation, and tissue injury at the local environment. Type 1 ILCs (ILC1s) promptly and abundantly produce interferon (IFN)-γ but lack appreciable cytotoxic activity. ILC1s share many phenotypic, developmental, and functional characteristics with natural killer (NK) cells, which are circulating innate lymphocytes with potent natural cytotoxicity. However, recent studies have established ILC1s as distinct from NK cells. ILC1s predominantly reside in the liver—they initially were discovered as a liver-resident ILC subset—as well as in other lymphoid and non-lymphoid tissues. Accumulating evidence has demonstrated that ILC1s play an important and unique role in host protection and in immunomodulation in their resident organs. However, the pathophysiological role of tissue-resident ILC1s remains largely unclear. In this review, we summarize emerging evidence showing that ILC1s not only contribute to inflammation to protect against pathogens but also promote tissue protection and metabolism. We highlight a unique function of ILC1s in their resident tissues.

Acute neonatal Listeria monocytogenes infection causes long-term, organ-specific changes in immune cell subset composition

Listeria monocytogenes (Lm) is a food-borne pathogen with a high chance of infecting neonates, pregnant women, elderly and immunocompromised individuals. Lm infection in neonates can cause neonatal meningitis and sepsis with a high risk of severe neurological and developmental sequelae and high mortality rates. However, whether an acute neonatal Lm infection causes long-term effects on the immune system persisting until adulthood has not been fully elucidated. Here, we established a neonatal Lm infection model and monitored the composition of major immune cell subsets at defined time points post infection (p.i.) in secondary lymphoid organs and the intestine. Twelve weeks p.i., the CD8⁺ T cell population was decreased in colon and mesenteric lymph nodes (mLNs) with an opposing increase in the spleen. In the colon, we observed an accumulation of CD4⁺ and CD8⁺ effector/memory T cells with an increase of T-bet⁺ T helper 1 (Th1) cells. In addition, 12 weeks p.i. an altered composition of innate lymphoid cell (ILC) and dendritic cell (DC) subsets was still observed in colon and mLNs, respectively. Together, these findings highlight organ-specific long-term consequences of an acute neonatal Lm infection on both the adaptive and innate immune system.

Innate lymphoid cell and adaptive immune cell cross-talk: A talk meant not to forget

Innate lymphoid cells (ILCs) are a relatively new class of innate immune cells with phenotypical characters of lymphocytes but genotypically or functionally behave as typical innate immune cells. They have been classically divided into 3 groups (group 1 ILCs or ILC1s, group 2 ILCs or ILC2s, and group 3 ILCs or ILC3s). They serve as the first line of defense against invading pathogens and allergens at mucosal surfaces. The adaptive immune response works effectively in association with innate immunity as innate immune cells serve as APCs to directly stimulate the adaptive immune cells (various sets of T and B cells). Additionally, innate immune cells also secrete various effector molecules, including cytokines or chemokines impacting the function, differentiation, proliferation, and reprogramming among adaptive immune cells to maintain immune homeostasis. Only superantigens do not require their processing by innate immune cells as they are recognized directly by T cells and B cells. Thus, a major emphasis of the current article is to describe the cross-talk between different ILCs and adaptive immune cells during different conditions varying from normal physiological situations to different infectious diseases to allergic asthma.

The link "Cancer and autoimmune diseases" in the light of microbiota: Evidence of a potential culprit

Evidence establishes that chronic inflammation and autoimmunity are associated with cancer development and patients with a primary malignancy may develop autoimmune-like diseases. Despite immune dysregulation is a common feature of both cancer and autoimmune diseases, precise mechanisms underlying this susceptibility are not clarified and different hypotheses have been proposed, starting from genetic and environmental common features, to intrinsic properties of immune system. Moreover, as the development and use of immunomodulatory therapies for cancer and autoimmune diseases are increasing, the elucidation of this relationship must be investigated in order to offer the best and most secure therapeutic options. The microbiota could represent a potential link between autoimmune diseases and cancer. The immunomodulation role of microbiota is widely recognized and under eubiosis, it orchestrates both the innate and adaptive response of immunity, in order to discriminate and modulate the immune response itself in the most appropriate way. Therefore, a dysbiotic status can alter the immune tonus rendering the host prone to exogenous or endogenous infections, breaking the tolerance against self-components and activating the immune responses in an excessive (i.e. chronic inflammation) or deficient way, favoring the onset of neoplastic and autoimmune diseases.

Innate lymphoid cells control signaling circuits to regulate tissue-specific immunity

The multifaceted organization of the immune system involves not only patrolling lymphocytes that constantly monitor antigen-presenting cells in secondary lymphoid organs but also immune cells that establish permanent tissue-residency. The integration in the respective tissue and the adaption to the organ milieu enable tissue-resident cells to establish signaling circuits with parenchymal cells to coordinate immune responses and maintain tissue homeostasis. Innate lymphoid cells (ILCs) are tissue-resident innate immune cells that have a similar functional diversity to T cells including lineage-specifying transcription factors that drive certain effector programs. Since their formal discovery 10 years ago, it has become clear that ILCs are present in almost every tissue but strongly enriched at barrier surfaces, where they regulate immunity to infection, chronic inflammation, and tissue maintenance. In this context, recent research has identified ILCs as key in orchestrating tissue homeostasis through their ability to sustain bidirectional interactions with epithelial cells, neurons, stromal cells, adipocytes, and many other tissue-resident cells. In this review, we provide a comprehensive discussion of recent studies that define the development and heterogeneity of ILC populations and their impact on innate and adaptive immunity. Further, we discuss emerging research on the influence of the nervous system, circadian rhythm, and developmental plasticity on ILC function. Uncovering the signaling circuits that control development and function of ILCs will provide an integrated view on how immune responses in tissues are synchronized with functional relevance far beyond the classical view of the role of the immune system in discrimination between self/non-self and host defense.

Feeding-dependent VIP neuron-ILC3 circuit regulates the intestinal barrier

The intestinal mucosa serves both as a conduit for the uptake of food-derived nutrients and microbiome-derived metabolites, and as a barrier that prevents tissue invasion by microorganisms and tempers inflammatory responses to the myriad contents of the lumen. How the intestine coordinates physiological and immune responses to food consumption to optimize nutrient uptake while maintaining barrier functions remains unclear. Here we show in mice how a gut neuronal signal triggered by food intake is integrated with intestinal antimicrobial and metabolic responses that are controlled by type-3 innate lymphoid cells (ILC3)1-3. Food consumption rapidly activates a population of enteric neurons that express vasoactive intestinal peptide (VIP)4. Projections of VIP-producing neurons (VIPergic neurons) in the lamina propria are in close proximity to clusters of ILC3 that selectively express VIP receptor type 2 (VIPR2; also known as VPAC2). Production of interleukin (IL)-22 by ILC3, which is upregulated by the presence of commensal microorganisms such as segmented filamentous bacteria5-7, is inhibited upon engagement of VIPR2. As a consequence, levels of antimicrobial peptide derived from epithelial cells are reduced but the expression of lipid-binding proteins and transporters is increased8. During food consumption, the activation of VIPergic neurons thus enhances the growth of segmented filamentous bacteria associated with the epithelium, and increases lipid absorption. Our results reveal a feeding- and circadian-regulated dynamic neuroimmune circuit in the intestine that promotes a trade-off between innate immune protection mediated by IL-22 and the efficiency of nutrient absorption. Modulation of this pathway may therefore be effective for enhancing resistance to enteropathogens2,3,9 and for the treatment of metabolic diseases.

Type 1 Innate Lymphoid Cells Protect Mice from Acute Liver Injury via Interferon-γ Secretion for Upregulating Bcl-xL Expression in Hepatocytes

Although type 1 innate lymphoid cells (ILC1s) have been originally found as liver-resident ILCs, their pathophysiological role in the liver remains poorly investigated. Here, we demonstrated that carbon tetrachloride (CCl₄) injection into mice activated ILC1s, but not natural killer (NK) cells, in the liver. Activated ILC1s produced interferon-γ (IFN-γ) and protected mice from CCl₄-induced acute liver injury. IFN-γ released from activated ILC1s promoted the survival of hepatocytes through upregulation of Bcl-xL. An activating NK receptor, DNAM-1, was required for the optimal activation and IFN-γ production of liver ILC1s. Extracellular adenosine triphosphate accelerated interleukin-12-driven IFN-γ production by liver ILC1s. These findings suggest that ILC1s are critical for tissue protection during acute liver injury.

Natural Killer Cell-Derived IL-10 Prevents Liver Damage During Sustained Murine Cytomegalovirus Infection

Natural Killer (NK) cells are lymphocytes of the innate immune response that play a vital role in controlling infections and cancer. Their pro-inflammatory role has been well-established; however, less is known about the regulatory functions of NK cells, in particular, their production of the anti-inflammatory cytokine IL-10. In this study, we investigated the immunoregulatory function of NK cells during MCMV infection and demonstrated that NK cells are major producers of IL-10 during the early stage of infection. To investigate the effect of NK cell-derived IL-10, we have generated NK cell-specific IL-10-deficient mice (NKp46-Cre-Il10^fl/fl) displaying no signs of age-related spontaneous inflammation, with NK cells that show no detectable IL-10 production upon in vitro stimulation. In NKp46-Cre-Il10^fl/fl mice, the levels of IL-10 and IFNγ, viral burdens and T cell activation were similar between NKp46-Cre-Il10^fl/fl mice and their control littermates, suggesting that NK cell-derived IL-10 is dispensable during acute MCMV infection in immunocompetent hosts. In perforin-deficient mice that show a more sustained infection, NK cells produce more sustained levels of IL-10. By crossing NKp46-Cre-Il10^fl/fl mice with perforin-deficient mice, we demonstrated that NK cell-derived IL-10 regulates T cell activation, prevents liver damage, and allows for better disease outcome. Taken together, NK cell-derived IL-10 can be critical in regulating the immune response during early phases of infection and therefore protecting the host from excessive immunopathology.

The role of helper innate lymphoid cells in cancer

Innate lymphoid cells (ILCs) are an emerging family of innate immune cells and have been found to have an important role in infection, inflammation and tissue repair. In particular, recent work has identified significant alterations of ILC responses in tumor patients, suggesting potential roles of ILCs in tumor development. In this paper, we have focused on the basic features of ILCs and their interaction with other immune cells. Importantly, as the role of cytotoxic natural killer cells, assigned to ILC1 family, in cancer has been well established, we have summarized the new findings that showcase the potential role and mechanism of helper ILCs in different tumors. Helper ILCs might promote or inhibit tumor growth and metastasis, which depends on tumor type and ILC subset.

Deconstructing deployment of the innate immune lymphocyte army for barrier homeostasis and protection

The study of the immune system has shifted from a purely dichotomous separation between the innate and adaptive arms to one that is now highly complex and reshaping our ideas of how steady‐state health is assured. It is now clear that immune cells do not neatly fit into these two streams and immune homeostasis depends on continual dialogue between multiple lineages of the innate (including dendritic cells, innate lymphoid cells, and unconventional lymphocytes) and adaptive (T and B lymphocytes) arms together with a finely tuned synergy between the host and microbes which is essential to ensure immune homeostasis. Innate lymphoid cells are critical players in this new landscape. Here, we discuss recent studies that have elucidated in detail the development of ILCs from their earliest progenitors and examine factors that influence their identification and ability to drive immune homeostasis and long‐term immune protection.

An Essential Role of Innate Lymphoid Cells in the Pathophysiology of Graft-vs.-Host Disease

Allogeneic hematopoietic stem cell transplantation (Allo-HSCT) is the only curative treatment for multiple hematologic malignancies and non-malignant hematological diseases. However, graft-vs.-host disease (GVHD), one of the main complications after allo-HSCT, remains the major reason for morbidity and non-relapse mortality. Emerging evidence has demonstrated that innate lymphoid cells (ILCs) play a non-redundant role in the pathophysiology of GVHD. In this review, we will summarize previously published data regarding the role of ILCs in the pathogenesis of GVHD.

Natural Killer Cells and Type 1 Innate Lymphoid Cells Are New Actors in Non-alcoholic Fatty Liver Disease

Obesity and associated liver diseases (Non Alcoholic Fatty Liver Disease, NAFLD) are a major public health problem with increasing incidence in Western countries (25% of the affected population). These complications develop from a fatty liver (steatosis) to an inflammatory state (steatohepatitis) evolving toward fibrosis and hepatocellular carcinoma. Lipid accumulation in the liver contributes to hepatocyte cell death and promotes liver injury. Local immune cells are activated either by Danger Associated Molecular Patterns (DAMPS) released by dead hepatocytes or by bacterial products (PAMPS) reaching the liver due to increased intestinal permeability. The resulting low-grade inflammatory state promotes the progression of liver complications toward more severe grades. Innate lymphoid cells (ILC) are an heterogeneous family of five subsets including circulating Natural Killer (NK) cells, ILC1, ILC2, ILC3, and lymphocytes tissue-inducer cells (LTi). NK cells and tissue-resident ILCs, mainly located at epithelial surfaces, are prompt to rapidly react to environmental changes to mount appropriate immune responses. Recent works have demonstrated the interplay between ILCs subsets and the environment within metabolic active organs such as liver, adipose tissue and gut during diet-induced obesity leading or not to hepatic abnormalities. Here, we provide an overview of the newly roles of NK cells and ILC1 in metabolism focusing on their contribution to the development of NAFLD. We also discuss recent studies that demonstrate the ability of these two subsets to influence tissue-specific metabolism and how their function and homeostasis are affected during metabolic disorders.

Innate Lymphoid Cells in Mucosal Immunity

Innate lymphoid cells (ILCs) are innate counterparts of T cells that contribute to immune responses by secreting effector cytokines and regulating the functions of other innate and adaptive immune cells. ILCs carry out some unique functions but share some tasks with T cells. ILCs are present in lymphoid and non-lymphoid organs and are particularly abundant at the mucosal barriers, where they are exposed to allergens, commensal microbes, and pathogens. The impact of ILCs in mucosal immune responses has been extensively investigated in the gastrointestinal and respiratory tracts, as well as in the oral cavity. Here we review the state-of-the-art knowledge of ILC functions in infections, allergy and autoimmune disorders of the mucosal barriers.

Commensal and Pathogenic Bacteria Indirectly Induce IL-22 but Not IFNγ Production From Human Colonic ILC3s via Multiple Mechanisms

Innate lymphoid cells (ILCs) are a diverse family of cells that play critical roles in mucosal immunity. One subset of the ILC family, Group 3 ILCs (ILC3s), has been shown to aid in gut homeostasis through the production of IL-22. IL-22 promotes gut homeostasis through its functional effect on the epithelial barrier. When gut epithelial barrier integrity is compromised, such as in Human Immunodeficiency Virus (HIV) infection and inflammatory bowel disease (IBD), microbes from the gut lumen translocate into the lamina propria, inducing a multitude of potentially pathogenic immune responses. In murine models of bacterial infection, there is evidence that bacteria can induce pro-inflammatory IFNγ production in ILC3s. However, the impact of diverse translocating bacteria, particularly commensal bacteria, in dictating IFNγ versus IL-22 production by human gut ILC3s remains unclear. Here, we utilized an in vitro human lamina propria mononuclear cell (LPMC) model to evaluate ILC3 cytokine production in response to a panel of enteric Gram-positive and Gram-negative commensal and pathogenic bacteria and determined potential mechanisms by which these cytokine responses were induced. The percentages of IL-22-producing ILC3s, but not IFNγ-producing ILC3s, were significantly increased after LPMC exposure to both Gram-positive and Gram-negative commensal or pathogenic bacterial stimuli. Stimulation of IL-22 production from ILC3s was not through direct recognition of bacterial antigen by ILC3s, but rather required the help of accessory cells within the LPMC population. CD11c+ myeloid dendritic cells generated IL-23 and IL-1β in response to enteric bacteria and contributed to ILC3 production of IL-22. Furthermore, ligation of the natural cytotoxicity receptor NKp44 on ILC3s in response to bacteria stimulation also significantly increased the percentage of IL-22-producing ILC3s. Overall, these data demonstrate that human gut microbiota, including commensal bacteria, indirectly modulate colonic ILC3 function to induce IL-22, but additional signals are likely required to induce IFNγ production by colonic ILC3s in the setting of inflammation and microbial translocation.

The impacts of natural polysaccharides on intestinal microbiota and immune responses – a review

This paper presents a comprehensive review of the impacts of natural polysaccharides on gut microbiota and immune responses as well as their interactions.

Peyer's patch myeloid cells infection by Listeria signals through gp38+ stromal cells and locks intestinal villus invasion

The foodborne pathogen Listeria monocytogenes (Lm) crosses the intestinal villus epithelium via goblet cells (GCs) upon the interaction of Lm surface protein InlA with its receptor E-cadherin. Here, we show that Lm infection accelerates intestinal villus epithelium renewal while decreasing the number of GCs expressing luminally accessible E-cadherin, thereby locking Lm portal of entry. This novel innate immune response to an enteropathogen is triggered by the infection of Peyer's patch CX3CR1⁺ cells and the ensuing production of IL-23. It requires STAT3 phosphorylation in epithelial cells in response to IL-22 and IL-11 expressed by lamina propria gp38⁺ stromal cells. Lm-induced IFN-γ signaling and STAT1 phosphorylation in epithelial cells is also critical for Lm-associated intestinal epithelium response. GC depletion also leads to a decrease in colon mucus barrier thickness, thereby increasing host susceptibility to colitis. This study unveils a novel innate immune response to an enteropathogen, which implicates gp38⁺ stromal cells and locks intestinal villus invasion, but favors colitis.

Interplay of innate lymphoid cells and the microbiota

Innate lymphoid cells (ILC) are a recently identified group of innate lymphocytes that are preferentially located at barrier surfaces. Barrier surfaces are in direct contact with complex microbial ecosystems, collectively referred to as the microbiota. It is now believed that the interplay of the microbiota with host components (i.e. epithelial cells and immune cells) promotes host fitness by regulating organ homeostasis, metabolism, and host defense against pathogens. In this review, we will give an overview of this multifaceted interplay between ILC and components of the microbiota.

Natural killer cells in inflammation and autoimmunity

First described 40 years ago, natural killer (NK) cells represent the founding members of the innate lymphoid cell (ILC) family. They were initially defined by their ability to kill cancer cells of hematopoietic origin. More recently, NK cells are recognized not only for their ability to kill infected or malignant cells, but also for mediating cytotoxicity against a range of normal immune cells. They thereby play an important physiological role in controlling immune responses and maintaining homeostasis. Besides cytotoxic activity, NK cells activation is accompanied by secretion of pro-inflammatory cytokines. Hence, NK cells have the potential to act both in driving inflammation and in restricting adaptive immune responses that may otherwise lead to excessive inflammation or even autoimmunity. Here, we highlight how NK cell activity is linked to inflammasome activation and review new molecular insights to the roles of NK cells in inflammation and autoimmunity. Furthermore, in light of new insights to NK cell differentiation and memory, we deliberate on how distinct NK cell subsets may impact immunoregulatory functions. Hypothetically, memory-like or adaptive NK cells could drive NK cell-mediated autoreactive diseases. Together, new findings underscore the complex yet important physiological roles of NK cells in both promoting inflammation and exerting immunoregulation and maintenance of immune homeostasis. Insights raise intriguing questions as to how NK cells themselves maintain self-tolerance.

Interferon-gamma (IFN-γ): Exploring its implications in infectious diseases

A key player in driving cellular immunity, IFN-γ is capable of orchestrating numerous protective functions to heighten immune responses in infections and cancers. It can exhibit its immunomodulatory effects by enhancing antigen processing and presentation, increasing leukocyte trafficking, inducing an anti-viral state, boosting the anti-microbial functions and affecting cellular proliferation and apoptosis. A complex interplay between immune cell activity and IFN-γ through coordinated integration of signals from other pathways involving cytokines and Pattern Recognition Receptors (PRRs) such as Interleukin (IL)-4, TNF-α, Lipopolysaccharide (LPS), Type-I Interferons (IFNS) etc. leads to initiation of a cascade of pro-inflammatory responses. Microarray data has unraveled numerous genes whose transcriptional regulation is influenced by IFN-γ. Consequently, IFN-γ stimulated cells display altered expression of many such target genes which mediate its downstream effector functions. The importance of IFN-γ is further reinforced by the fact that mice possessing disruptions in the IFN-γ gene or its receptor develop extreme susceptibility to infectious diseases and rapidly succumb to them. In this review, we attempt to elucidate the biological functions and physiological importance of this versatile cytokine. The functional implications of its biological activity in several infectious diseases and autoimmune pathologies are also discussed. As a counter strategy, many virulent pathogenic species have devised ways to thwart IFN-γ endowed immune-protection. Thus, IFN-γ mediated host-pathogen interactions are critical for our understanding of disease mechanisms and these aspects also manifest enormous therapeutic importance for the annulment of various infections and autoimmune conditions.

Gut Immunity: Passing on the Baton from Innate to Adaptive Immunity

Innate and adaptive immunity are two complementary systems that work together to protect the host organism. A new study unravels how innate lymphoid cells and adaptive T lymphocytes act sequentially to establish microbial commensalism and ensure tissue and metabolic homeostasis.

Lymphocyte integration of complement cues

We address current data, views and puzzles on the emerging topic of regulation of lymphocytes by complement proteins or fragments. Such regulation is believed to take place through complement receptors (CR) and membrane complement regulators (CReg) involved in cell function or protection, respectively, including intracellular signalling. Original observations in B cells clearly support that complement cues through CR improve their performance. Other lymphocytes likely integrate complement-derived signals, as most lymphoid cells constitutively express or regulate CR and CReg upon activation. CR-induced signals, particularly by anaphylatoxins, clearly regulate lymphoid cell function. In contrast, data obtained by CReg crosslinking using antibodies are not always confirmed in human congenital deficiencies or knock-out mice, casting doubts on their physiological relevance. Unsurprisingly, human and mouse complement systems are not completely homologous, adding further complexity to our still fragmentary understanding of complement-lymphocyte interactions.

Immunological pathogenesis of inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic inflammatory state of the gastrointestinal tract and can be classified into 2 main clinical phenomena: Crohn's disease (CD) and ulcerative colitis (UC). The pathogenesis of IBD, including CD and UC, involves the presence of pathogenic factors such as abnormal gut microbiota, immune response dysregulation, environmental changes, and gene variants. Although many investigations have tried to identify novel pathogenic factors associated with IBD that are related to environmental, genetic, microbial, and immune response factors, a full understanding of IBD pathogenesis is unclear. Thus, IBD treatment is far from optimal, and patient outcomes can be unsatisfactory. As result of massive studying on IBD, T helper 17 (Th17) cells and innate lymphoid cells (ILCs) are investigated on their effects on IBD. A recent study of the plasticity of Th17 cells focused primarily on colitis. ILCs also emerging as novel cell family, which play a role in the pathogenesis of IBD. IBD immunopathogenesis is key to understanding the causes of IBD and can lead to the development of IBD therapies. The aim of this review is to explain the pathogenesis of IBD, with a focus on immunological factors and therapies.

Innate Lymphoid Cells in HIV/SIV Infections

Over the past several years, new populations of innate lymphocytes have been described in mice and primates that are critical for mucosal homeostasis, microbial regulation, and immune defense. Generally conserved from mice to humans, innate lymphoid cells (ILC) have been divided primarily into three subpopulations based on phenotypic and functional repertoires: ILC1 bear similarities to natural killer cells; ILC2 have overlapping functions with TH2 cells; and ILC3 that share many functions with TH17/TH22 cells. ILC are specifically enriched at mucosal surfaces and are possibly one of the earliest responders during viral infections besides being involved in the homeostasis of gut-associated lymphoid tissue and maintenance of gut epithelial barrier integrity. Burgeoning evidence also suggests that there is an early and sustained abrogation of ILC function and numbers during HIV and pathogenic SIV infections, most notably ILC3 in the gastrointestinal tract, which leads to disruption of the mucosal barrier and dysregulation of the local immune system. A better understanding of the direct or indirect mechanisms of loss and dysfunction will be critical to immunotherapeutics aimed at restoring these cells. Herein, we review the current literature on ILC with a particular emphasis on ILC3 and their role(s) in mucosal immunology and the significance of disrupting the ILC niche during HIV and SIV infections.

Recent advances in gut immunology

In recent years, there have been significant advances in our understanding of the mucosal immune system. In addition to unravelling some of the complexities of this system, including the discovery of completely new cells types, further insights into the three-way interactions between mucosal immune cells, the intestinal epithelium and the microbial communities colonizing the GI tract promise to redefine our understanding of how intestinal homeostasis is maintained, but also how dysregulation of these highly integrated interactions conspires to cause disease. In this review, we will discuss major recent advances in the role of key immune players in the gut, including innate lymphoid cells (ILCs), mucosa-associated invariant T cells (MAIT cells) and cells of the mononuclear phagocyte system (MPS), including how these cells interact with the intestinal epithelial and their crosstalk with components of the intestinal microbiota, and how these interactions shape host health.

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.