Microbiota-induced tertiary lymphoid tissues aggravate inflammatory disease in the absence of RORgamma t and LTi cells

Transition from an autoimmune-prone state to fatal autoimmune disease in CCR7 and RORγt double-deficient mice is dependent on gut microbiota

Autoimmunity is associated with a strong genetic component, but onset and persistence of clinically apparent autoimmune diseases often require an additional environmental trigger. The balance between immunity and tolerance is regulated by numerous molecular factors including nuclear hormone and homeostatic chemokine receptors. The nuclear hormone receptor RORγt and the chemokine receptor CCR7 are both essentially involved in functional lymphoid organogenesis and maintenance of lymphocyte homeostasis. Lack of one or the other impairs thymic T cell development and alters T cell homeostasis. Mice deficient for both, Ccr7(-/-)Rorγt(-/-), succumbed early to acute destructive inflammation, characterized by massive recruitment of inflammatory leukocytes, pro-inflammatory cytokine and autoantibody production, and wasting disease. Antibiotic-treatment of mice before disease onset reduced the overall gut microflora and abrogated the development of fatal mucosal inflammation. Hence, commensal bacteria and a confined tissue-specific inflammatory milieu serve as complementary trigger to initiate the lethal pathophysiologic process in Ccr7(-/-)Rorγt(-/-) mice.

Crosstalk between the intestinal microbiota and the innate immune system in intestinal homeostasis and inflammatory bowel disease

: Inflammatory bowel diseases are a set of complex and chronic disorders that arise in genetically predisposed individuals due to a lack of tolerance to the gut microflora. Although the intestinal microbiota is required for the proper development of the host and the maintenance of intestinal homeostasis, its dysbiosis is associated with inflammatory bowel diseases pathogenesis. In this review, we focus the discussion on the crosstalk between the innate immune system and the microbiota. We examine new findings from genetic and functional studies investigating the critical role of the intestinal epithelial cell layer and the processes that maintain its integrity in health and disease. We further explore the mechanisms of the mucosal innate immune system including dendritic cells, macrophages, and innate-like lymphocytes in mediating immunological tolerance at the steady state or pathogenic inflammatory responses in inflammatory bowel diseases.

Aryl hydrocarbon receptor promotes RORγt⁺ group 3 ILCs and controls intestinal immunity and inflammation

Unlike adaptive immune cells that require antigen recognition and functional maturation during infection, innate lymphoid cells (ILCs) usually respond to pathogens promptly and serve as the first line of defense in infectious diseases. RAR-related orphan receptor (RORγt)⁺ group 3 ILCs are one of the innate cell populations that have recently been intensively studied. During the fetal stage of development, RORγt⁺ group 3 ILCs (e.g., lymphoid tissue inducer cells) are required for lymphoid organogenesis. In adult mice, RORγt⁺ group 3 ILCs are abundantly present in the gut to exert immune defensive functions. Under certain circumstances, however, RORγt⁺ group 3 ILCs can be pathogenic and contribute to intestinal inflammation. Aryl hydrocarbon receptor (Ahr), a ligand-dependent transcriptional factor, is widely expressed by various immune and non-immune cells. In the gut, the ligand for Ahr can be derived/generated from diet, microflora, and/or host cells. Ahr has been shown to regulate different cell populations in the immune system including RORγt⁺ group 3 ILCs, T helper (Th)17/22 cells, γδT cells, regulatory T cells (Tregs), Tr1 cells, and antigen presenting cells. In this review, we will focus on the development and function of RORγt⁺ group 3 ILCs, and discuss the role of Ahr in intestinal immunity and inflammation in mice and in humans. A better understanding of the function of Ahr in the gut is important for developing new therapeutic means to target Ahr in future treatment of infectious and autoimmune diseases.

CXCL13 production in B cells via Toll-like receptor/lymphotoxin receptor signaling is involved in lymphoid neogenesis in chronic obstructive pulmonary disease

RATIONALE

Little is known about what drives the appearance of lymphoid follicles (LFs), which may function as lymphoid organs in chronic obstructive pulmonary disease (COPD). In animal infection models, pulmonary LF formation requires expression of homeostatic chemokines by stromal cells and dendritic cells, partly via lymphotoxin.

OBJECTIVES

To study the role of homeostatic chemokines in LF formation in COPD and to identify mechanism(s) responsible for their production.

METHODS

Peripheral lung homeostatic chemokine and lymphotoxin expression were visualized by immunostainings and quantified by ELISA/quantitative reverse transcriptase-polymerase chain reaction in patients with COPD with and without LFs. Expression of lymphotoxin and homeostatic chemokine receptors was investigated by flow cytometry. Primary lung cell cultures, followed by ELISA/quantitative reverse transcriptase-polymerase chain reaction/flow cytometry, were performed to identify mechanisms of chemokine expression. Polycarbonate membrane filters were used to assess primary lung cell migration toward lung homogenates.

MEASUREMENTS AND MAIN RESULTS

LFs expressed the homeostatic chemokine CXCL13. Total CXCL13 levels correlated with LF density. Lung B cells of patients with COPD were important sources of CXCL13 and lymphotoxin and also expressed their receptors. Cigarette smoke extract, H2O2, and LPS exposure up-regulated B cell-derived CXCL13. The LPS-induced increase in CXCL13 was partly mediated via lymphotoxin. Notably, CXCL13 was required for efficient lung B-cell migration toward COPD lung homogenates and induced lung B cells to up-regulate lymphotoxin, which further promoted CXCL13 production, establishing a positive feedback loop.

CONCLUSIONS

LF formation in COPD may be driven by lung B cells via a CXCL13-dependent mechanism that involves toll-like receptor and lymphotoxin receptor signaling.

Innate lymphoid cells regulate CD4+ T-cell responses to intestinal commensal bacteria

Innate lymphoid cells (ILCs) are a recently characterized family of immune cells that have critical roles in cytokine-mediated regulation of intestinal epithelial cell barrier integrity. Alterations in ILC responses are associated with multiple chronic human diseases, including inflammatory bowel disease, implicating a role for ILCs in disease pathogenesis. Owing to an inability to target ILCs selectively, experimental studies assessing ILC function have predominantly used mice lacking adaptive immune cells. However, in lymphocyte-sufficient hosts ILCs are vastly outnumbered by CD4(+) T cells, which express similar profiles of effector cytokines. Therefore, the function of ILCs in the presence of adaptive immunity and their potential to influence adaptive immune cell responses remain unknown. To test this, we used genetic or antibody-mediated depletion strategies to target murine ILCs in the presence of an adaptive immune system. We show that loss of retinoic-acid-receptor-related orphan receptor-γt-positive (RORγt(+)) ILCs was associated with dysregulated adaptive immune cell responses against commensal bacteria and low-grade systemic inflammation. Remarkably, ILC-mediated regulation of adaptive immune cells occurred independently of interleukin (IL)-17A, IL-22 or IL-23. Genome-wide transcriptional profiling and functional analyses revealed that RORγt(+) ILCs express major histocompatibility complex class II (MHCII) and can process and present antigen. However, rather than inducing T-cell proliferation, ILCs acted to limit commensal bacteria-specific CD4(+) T-cell responses. Consistent with this, selective deletion of MHCII in murine RORγt(+) ILCs resulted in dysregulated commensal bacteria-dependent CD4(+) T-cell responses that promoted spontaneous intestinal inflammation. These data identify that ILCs maintain intestinal homeostasis through MHCII-dependent interactions with CD4(+) T cells that limit pathological adaptive immune cell responses to commensal bacteria.

Site-specific immunophenotyping of keloid disease demonstrates immune upregulation and the presence of lymphoid aggregates

BACKGROUND

Keloid disease (KD) is a common fibroproliferative disorder of unknown aetiology. T cells and macrophages are increased in KD and are thought to contribute to its pathogenesis. However, while a link between inflammation and fibrotic disorders is well known for other disorders, it remains undetermined in KD.

OBJECTIVES

Systematically to immunophenotype the inflammatory infiltrate of KD in situ in a site-specific manner, and to compare this with normal skin and scar tissue.

METHODS

Sixty-eight keloid cases were screened for the presence of all three (intralesional, perilesional and extralesional) keloid-associated specific tissue sites. Subsequently, a complete set of 25 keloid biopsies (from different patients) was compared with normal skin (n = 11) and normal scar (n = 11) samples and subjected to systematic, site-specific quantitative immunohistomorphometry and histochemistry, using a range of immunological markers of B cells, T cells, macrophages, mast cells (MCs) and Langerhans cells.

RESULTS

T cells, B cells, degranulated and mature MCs (coexpressing OX40 ligand) and alternative macrophages (M2) were all significantly increased in intralesional and perilesional KD sites compared with normal skin and scar tissue (P < 0·05). Additionally, 10 of 68 KD cases (15%) showed the presence of distinctive lymphoid aggregates, which resembled mucosa-associated lymphoid tissue (MALT).

CONCLUSIONS

The increased number and activity of MCs and M2 may implicate inflammation in the fibrotic process in KD. The distinct KD-associated lymphoid aggregate resembles MALT, for which we propose the term 'keloid-associated lymphoid tissue' (KALT). It may perpetuate inflammatory stimuli that promote KD growth. KALT, MCs and M2 are promising novel targets for future KD therapy.

Lymphotoxin signalling in immune homeostasis and the control of microorganisms

Lymphotoxin (LT) is a member of the tumour necrosis factor (TNF) superfamily that was originally thought to be functionally redundant to TNF, but these proteins were later found to have independent roles in driving lymphoid organogenesis. More recently, LT-mediated signalling has been shown to actively contribute to effector immune responses. LT regulates dendritic cell and CD4(+) T cell homeostasis in the steady state and determines the functions of these cells during pathogenic challenges. The LT receptor pathway is essential for controlling pathogens and even contributes to the regulation of the intestinal microbiota, with recent data suggesting that LT-induced changes in the microbiota promote metabolic disease. In this Review, we discuss these newly defined roles for LT, with a particular focus on how the LT receptor pathway regulates innate and adaptive immune responses to microorganisms.

New friendships and old feuds: relationships between innate lymphoid cells and microbial communities

Mammalian barrier surfaces, including the skin, lung and intestine, are constantly exposed to environmental stimuli, including beneficial and pathogenic microbes, dietary substances and non-organic materials. At these anatomical sites it is essential to maintain barrier integrity to promote tissue homeostasis and prevent local and systemic inflammation. As such, changes in the composition and localization of commensal bacteria are closely associated with inflammatory, metabolic and infectious disease in mammals. Cells of the innate and adaptive immune systems have a crucial role in the tight regulation of host-commensal relationships. A recently described family of immune cells, termed innate lymphoid cells (ILCs), contributes to inflammation, modulates adaptive immunity and regulates wound healing and tissue regeneration. ILCs are present at barrier surfaces, and thus are in close proximity to environmental antigens, including commensal bacteria. The composition and localization of microbial communities have a profound impact on immunity at barrier surfaces as well as at distant sites. This review will summarize the phenotypic characteristics of ILC family members and discuss recent findings about the interactions between ILCs and the microbiota in the contexts of homeostasis, immunity, inflammation and tissue organization and repair.

Innate lymphoid cells--how did we miss them?

Innate lymphoid cells (ILCs) are newly identified members of the lymphoid lineage that have emerging roles in mediating immune responses and in regulating tissue homeostasis and inflammation. Here, we review the developmental relationships between the various ILC lineages that have been identified to date and summarize their functions in protective immunity to infection and their pathological roles in allergic and autoimmune diseases.

Daclizumab therapy for multiple sclerosis

Daclizumab is a humanized monoclonal antibody of IgG1 subtype that binds to the Tac epitope on the interleukin-2 (IL-2) receptor α-chain (CD25), thus, effectively blocking the formation of the high-affinity IL-2 receptor. Because the high-affinity IL-2 receptor signaling promotes expansion of activated T cells in vitro, daclizumab was designed as a therapy that selectively inhibits T-cell activation. Assuming the previous statement, daclizumab received regulatory approval as add-on therapy to standard immunosuppressive regimen for the prevention of acute allograft rejection in renal transplantation. Based on its putative mechanism of action (MOA), daclizumab represented an ideal therapy for T-cell-mediated autoimmune diseases and was subsequently tested in inflammatory uveitis and multiple sclerosis (MS). In both of these diseases, daclizumab therapy significantly inhibited target organ inflammation. Mechanistic studies in MS demonstrated that the MOA of daclizumab is surprisingly broad and that the drug exerts unexpected effects on multiple components of the innate immune system. Specifically, daclizumab dramatically expands and activates immunoregulatory CD56(bright) NK cells, which gain access to the intrathecal compartment in MS and can kill autologous activated T cells. Daclizumab also blocks trans-presentation of IL-2 by mature dendritic cells to primed T cells, resulting in profound inhibition of antigen-specific T cells. Finally, daclizumab modulates the development of innate lymphoid cells. In conclusion, daclizumab therapy, which is currently in phase III testing for inflammatory MS, has a unique MOA that does not limit migration of immune cells into the intrathecal compartment, but rather provides multifactorial immunomodulatory effects with resultant inhibition of MS-related inflammation.

Ectopic lymphoid tissue alters the chemokine gradient, increases lymphocyte retention and exacerbates murine ileitis

BACKGROUND

The earliest endoscopically-evident lesion in Crohn's disease is the aphthous ulcer, which develops over ectopic lymphoid tissues (ie, inducible lymphoid follicles (ILF), tertiary lymphoid tissue (TLT)) in the chronically inflamed intestine. ILF/TLT are induced within effector sites by homeostatic lymphoid chemokines, but their role in the development of intestinal ILF/TLT and in the pathogenesis of Crohn's disease is poorly understood.

DESIGN

Using a mouse model of Crohn's-like ileitis (TNFARE) which develops florid induction of ILF/TLT within its terminal ileum, the contribution of the CCR7/CCL19/CCL21 chemokine axis during the development of TLT and its role in disease pathogenesis were assessed.

RESULTS

Both CCL19 and CCL21 were increased within the inflamed ileum of TNFARE mice, which resulted in CCR7 internalisation and impaired T cell chemotaxis. ILF/TLT were a major source of CCL19 and CCL21 and increased local synthesis, augmented recruitment/retention of effector, naïve and central memory T cell subsets within the inflamed ileum. Immunoblockade of CCR7 resulted in further effector T cell retention and exacerbation of ileitis.

CONCLUSIONS

Induction of ILF/TLT in the chronically inflamed intestine alters the homeostatic CCL19-CCL21 lymphoid-chemokine gradient and increases recruitment/retention of effector CCR7+ T cell subsets within the terminal ileum, contributing to the perpetuation of chronic inflammation. Thus, blockade of CCR7 or its ligands might result in deleterious consequences for subjects with chronic inflammatory diseases.

Innate lymphoid cell interactions with microbiota: implications for intestinal health and disease

The mammalian intestine harbors trillions of beneficial commensal bacteria that are essential for the development of the immune system and for maintenance of physiologic processes in multiple organs. However, numerous chronic infectious, inflammatory, and metabolic diseases in humans have been associated with alterations in the composition or localization of commensal bacteria that result in dysregulated host-commensal bacteria relationships. The mammalian immune system plays an essential role in regulating the acquisition, composition, and localization of commensal bacteria in the intestine. Emerging research has implicated innate lymphoid cells (ILCs) as a critical immune cell population that orchestrates some of these host-commensal bacteria relationships that can impact immunity, inflammation, and tissue homeostasis in the intestine. This review will discuss reciprocal interactions between intestinal commensal bacteria and ILCs in the context of health and disease.

Innate lymphoid cell interactions with microbiota: implications for intestinal health and disease

The mammalian intestine harbors trillions of beneficial commensal bacteria that are essential for the development of the immune system and for maintenance of physiologic processes in multiple organs. However, numerous chronic infectious, inflammatory, and metabolic diseases in humans have been associated with alterations in the composition or localization of commensal bacteria that result in dysregulated host-commensal bacteria relationships. The mammalian immune system plays an essential role in regulating the acquisition, composition, and localization of commensal bacteria in the intestine. Emerging research has implicated innate lymphoid cells (ILCs) as a critical immune cell population that orchestrates some of these host-commensal bacteria relationships that can impact immunity, inflammation, and tissue homeostasis in the intestine. This review will discuss reciprocal interactions between intestinal commensal bacteria and ILCs in the context of health and disease.

Loss of mucosal CD103+ DCs and IL-17+ and IL-22+ lymphocytes is associated with mucosal damage in SIV infection

Human immunodeficiency virus (HIV) and Simian immunodeficiency virus (SIV) disease progression is associated with multifocal damage to the gastrointestinal tract epithelial barrier that correlates with microbial translocation and persistent pathological immune activation, but the underlying mechanisms remain unclear. Investigating alterations in mucosal immunity during SIV infection, we found that damage to the colonic epithelial barrier was associated with loss of multiple lineages of interleukin (IL)-17-producing lymphocytes, cells that microarray analysis showed expressed genes important for enterocyte homeostasis, including IL-22. IL-22-producing lymphocytes were also lost after SIV infection. Potentially explaining coordinate loss of these distinct populations, we also observed loss of CD103+ dendritic cells (DCs) after SIV infection, which associated with the loss of IL-17- and IL-22-producing lymphocytes. CD103+ DCs expressed genes associated with promotion of IL-17/IL-22+ cells, and coculture of CD103+ DCs and naïve T cells led to increased IL17A and RORc expression in differentiating T cells. These results reveal complex interactions between mucosal immune cell subsets providing potential mechanistic insights into mechanisms of mucosal immune dysregulation during HIV/SIV infection, and offer hints for development of novel therapeutic strategies to address this aspect of AIDS virus pathogenesis.

Aryl hydrocarbon receptor: a molecular link between postnatal lymphoid follicle formation and diet

Intestinal homeostasis results from a complex mutualism between gut microbiota and host cells. Defining the molecular network regulating such mutualism is currently of increasing interest, as its deregulation is reported to lead to increased susceptibility to infections, chronic inflammatory bowel diseases and cancer. Until now, the focus has been on the mechanism, by which the composition of indigenous microbiota shapes the immune system. In a recent study, we have shown that dietary compounds have also the ability to affect innate immune system. This regulation involves aryl hydrocarbon receptor (AhR), a sensor of plant-derived phytochemicals, which mediates the maintenance of Retinoic acid related orphan receptor γ t-expressing innate lymphoid cells (RORγt(+) ILC) in the gut and consequently formation of postnatal lymphoid follicles. Thus, AhR represents the first evidence of a molecular link between diet and immunity at intestinal mucosal surfaces.

Inhibition of LTi cell development by CD25 blockade is associated with decreased intrathecal inflammation in multiple sclerosis

Genetic polymorphisms in the interleukin-2 receptor α (IL-2Rα) chain (CD25) locus are associated with several human autoimmune diseases, including multiple sclerosis (MS). Blockade of CD25 by the humanized monoclonal antibody daclizumab decreases MS-associated inflammation but has surprisingly limited direct inhibitory effects on activated T cells. The present study describes unexpected effects of daclizumab therapy on innate lymphoid cells (ILCs). The number of circulating retinoic acid receptor-related orphan receptor γt-positive ILCs, which include lymphoid tissue inducer (LTi) cells, was found to be elevated in untreated MS patients compared to healthy subjects. Daclizumab therapy not only decreased numbers of ILCs but also modified their phenotype away from LTi cells and toward a natural killer (NK) cell lineage. Mechanistic studies indicated that daclizumab inhibited differentiation of LTi cells from CD34⁺ hematopoietic progenitor cells or c-kit⁺ ILCs indirectly, steering their differentiation toward immunoregulatory CD56(bright) NK cells through enhanced intermediate-affinity IL-2 signaling. Because adult LTi cells may retain lymphoid tissue-inducing capacity or stimulate adaptive immune responses, we indirectly measured intrathecal inflammation in daclizumab-treated MS patients by quantifying the cerebrospinal fluid chemokine (C-X-C motif) ligand 13 and immunoglobulin G index. Both of these inflammatory biomarkers were inhibited by daclizumab treatment. Our study indicates that ILCs are involved in the regulation of adaptive immune responses, and their role in human autoimmunity should be investigated further, including their potential as therapeutic targets.

Inflammatory bowel disease pathogenesis: what is new?

PURPOSE OF REVIEW

Knowledge on and understanding of the pathophysiology of inflammatory bowel disease (IBD) is continuously growing. Important insights from the last years are summarized in this review.

RECENT FINDINGS

Further genetic risk factors for IBD have been identified and confirmed. Novel studies analyzing the function of these susceptibility factors have improved our understanding of specific pathophysiological pathways. Both the innate and the adaptive immune systems appear to be deregulated. The current notion that only about 25% of genetic heritability is explained by the published findings is being challenged. Epigenetic changes triggered by environmental factors probably contribute to heritability. Such environmental factors have been shown not only to influence immunological function and the intestinal barrier, but they also affect the composition of the gut microbiome and its interaction with the mucosal immune system. The gut microbiome, innate defense mechanisms and barrier function regulate each other, contributing to a balance that determines physiological or pathological inflammation.

SUMMARY

New therapies will emerge from the concept of a multidirectional interplay between environment and microbiome on one hand and defense mechanisms on the other.

Role of the Aryl Hydrocarbon Receptor in Controlling Maintenance and Functional Programs of RORγt(+) Innate Lymphoid Cells and Intraepithelial Lymphocytes

Mucosal retinoic receptor-related orphan receptor (ROR)γt-expressing innate lymphoid cells (ILC) play an important role in the defense against intestinal pathogens and in promoting epithelial homeostasis and adaptation, thereby effectively protecting the vertebrate host against intestinal inflammatory disorders. The functional activity of RORγt(+) ILC is under the control of environmental cues. However, the molecular sensors for such environmental signals are largely unknown. Recently, the aryl hydrocarbon receptor (AhR) has emerged as a master regulator for the postnatal maintenance of intestinal RORγt(+) ILC and intraepithelial lymphocytes. AhR is a highly conserved transcription factor whose activity is regulated by environmental and dietary small molecule ligands. Here, we review the role of AhR signaling for the maintenance of intestinal immune cells and its impact on the immunological protection against intestinal infections and debilitating chronic inflammatory disorders.

Tertiary lymphoid organs in infection and autoimmunity

The lymph nodes (LNs) and spleen have an optimal structure that allows the interaction between T cells, B cells and antigen-presenting dendritic cells (DCs) on a matrix made up by stromal cells. Such a highly organized structure can also be formed in tertiary lymphoid organs (TLOs) at sites of infection or chronic immune stimulation. This review focuses on the molecular mechanisms of TLO formation and maintenance, the controversies surrounding the nature of the inducing events, and the functions of these structures in infection, transplantation and autoimmunity.

Lymphoid microenvironments and innate lymphoid cells in the gut

Gut-associated lymphoid tissue (GALT) is a sensor region for luminal content and plays an important role in lymphoid maturation, activation and differentiation. It comprises isolated and aggregated lymphoid follicles, cryptopatches (CPs) and tertiary lymphoid tissue. Innate lymphoid cells (ILCs) play a central role within GALT. Prenatal GALT development is dependent on ILC lymphoid-inducer function. Postnatally, these cells rapidly respond to commensal and pathogenic intestinal bacteria, parasites and food components by polarized cytokine production [such as interleukin (IL)-22, IL-17 or IL-13] and further contribute to GALT formation and function. Here, we discuss how ILCs shape lymphoid intestinal microenvironments and act as amplifier cells for innate and adaptive immune responses.

Lymphotoxin and the amazing technicolor circus of intestinal homeostasis

Recently a new lymphocyte subset called innate lymphoid cells has emerged and it includes key producers of interleukin (IL)-17 and IL-22 in the mucosal environment. Using Citrobacter rodentium infection to deliver a pathogenic insult to the colon, two studies have revealed an underlying role for lymphotoxin-β receptor signaling in the generation of IL-22 by these cells. This observation links a system well known for its ability to organize lymphoid microenvironments into a basic mucosal response.

Development and evolution of RORγt+ cells in a microbe's world

The nuclear hormone receptor retinoid-related orphan receptor γt (RORγt) induces a pro-inflammatory program in lymphoid cells, culminating in the expression of interleukin-6 (IL-6), IL-17, IL-22, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor. During ontogeny, the first type of cells expressing RORγt are lymphoid tissue inducer cells, a type of innate lymphoid cell (ILC) generated in mammalian fetuses to induce the development of lymph nodes and Peyer's patches. After birth, RORγt(+) ILCs and RORγt(+) T cells are involved in the defense of epithelial surfaces against extracellular microbes and play an important role in the intestinal homeostasis with symbiotic microbiota. The development and evolution of RORγt(+) cells is intimately associated with the construction of a stable host-microbe interface.

The development of LTi cells

Lymphoid tissue inducer (LTi) cells are programmed by the mammalian fetus to induce the development of lymph nodes and Peyer's patches. LTi cells share a pro-inflammatory profile with Th17 cells, as well as their requirement for the transcription factor RORγt. We discuss here the latest data on the fetal and post-natal development of LTi cells, and their relationship with the larger family of innate lymphoid cells (ILCs). We suggest that the re-programming of RORγt in a subset of common lymphoid progenitors allowed mammals to develop lymphoid organs before birth, whereas other vertebrates only develop such organs in response to infection or injury.

Innate lymphoid cells: emerging insights in development, lineage relationships, and function

Innate lymphoid cells (ILCs) are immune cells that lack a specific antigen receptor yet can produce an array of effector cytokines that in variety match that of T helper cell subsets. ILCs function in lymphoid organogenesis, tissue remodeling, antimicrobial immunity, and inflammation, particularly at barrier surfaces. Their ability to promptly respond to insults inflicted by stress-causing microbes strongly suggests that ILCs are critical in first-line immunological defenses. Here, we review current data on developmental requirements, lineage relationships, and effector functions of two families of ILCs: (a) Rorγt-expressing cells involved in lymphoid tissue formation, mucosal immunity, and inflammation and (b) type 2 ILCs that are important for helminth immunity. We also discuss the potential roles of ILCs in the pathology of immune-mediated inflammatory and infectious diseases including allergy.

Microbial translocation across the GI tract

The lumen of the gastrointestinal (GI) tract is home to an enormous quantity of different bacterial species, our microbiota, that thrive in an often symbiotic relationship with the host. Given that the healthy host must regulate contact between the microbiota and its immune system to avoid overwhelming systemic immune activation, humans have evolved several mechanisms to attenuate systemic microbial translocation (MT) and its consequences. However, several diseases are associated with the failure of one or more of these mechanisms, with consequent immune activation and deleterious effects on health. Here, we discuss the mechanisms underlying MT, diseases associated with MT, and therapeutic interventions that aim to decrease it.

Effect of probiotic bacteria on microbial host defense, growth, and immune function in human immunodeficiency virus type-1 infection

The hypothesis that probiotic administration protects the gut surface and could delay progression of Human Immunodeficiency Virus type1 (HIV-1) infection to the Acquired Immunodeficiency Syndrome (AIDS) was proposed in 1995. Over the last five years, new studies have clarified the significance of HIV-1 infection of the gut associated lymphoid tissue (GALT) for subsequent alterations in the microflora and breakdown of the gut mucosal barrier leading to pathogenesis and development of AIDS. Current studies show that loss of gut CD4+ Th17 cells, which differentiate in response to normal microflora, occurs early in HIV-1 disease. Microbial translocation and suppression of the T regulatory (Treg) cell response is associated with chronic immune activation and inflammation. Combinations of probiotic bacteria which upregulate Treg activation have shown promise in suppressing pro inflammatory immune response in models of autoimmunity including inflammatory bowel disease and provide a rationale for use of probiotics in HIV-1/AIDS. Disturbance of the microbiota early in HIV-1 infection leads to greater dominance of potential pathogens, reducing levels of bifidobacteria and lactobacillus species and increasing mucosal inflammation. The interaction of chronic or recurrent infections, and immune activation contributes to nutritional deficiencies that have lasting consequences especially in the HIV-1 infected child. While effective anti-retroviral therapy (ART) has enhanced survival, wasting is still an independent predictor of survival and a major presenting symptom. Congenital exposure to HIV-1 is a risk factor for growth delay in both infected and non-infected infants. Nutritional intervention after 6 months of age appears to be largely ineffective. A meta analysis of randomized, controlled clinical trials of infant formulae supplemented with Bifidobacterium lactis showed that weight gain was significantly greater in infants who received B. lactis compared to formula alone. Pilot studies have shown that probiotic bacteria given as a supplement have improved growth and protected against loss of CD4+ T cells. The recognition that normal bacterial flora prime neonatal immune response and that abnormal flora have a profound impact on metabolism has generated insight into potential mechanisms of gut dysfunction in many settings including HIV-1 infection. As discussed here, current and emerging studies support the concept that probiotic bacteria can provide specific benefit in HIV-1 infection. Probiotic bacteria have proven active against bacterial vaginosis in HIV-1 positive women and have enhanced growth in infants with congenital HIV-1 infection. Probiotic bacteria may stabilize CD4+ T cell numbers in HIV-1 infected children and are likely to have protective effects against inflammation and chronic immune activation of the gastrointestinal immune system.

Natural killer cell receptor-expressing innate lymphocytes: more than just NK cells

Recently, additional subsets that extend the family of innate lymphocytes have been discovered. Among these newly identified innate lymphoid cells is a subset sharing phenotypic characteristics of natural killer cells and lymphoid tissue inducer cells. These cells co-express the transcription factor RORγt and activating NK cell receptors (NKR), but their lineage and functional qualities remain poorly defined. Here, we discuss recent proposals to place these NKR(+)RORγt(+) innate lymphocytes on hematopoietic lineage maps. An overview of the transcriptional circuitry determining fate decisions of innate lymphocytes and a summary of current concepts concerning plasticity and stability of innate lymphocyte effector fates are provided. We will conclude by discussing the function of RORγt-expressing innate lymphocytes during inflammatory bowel diseases and in the immune response to tumors.

Natural aryl hydrocarbon receptor ligands control organogenesis of intestinal lymphoid follicles

Innate lymphoid cells (ILC) expressing the transcription factor RORγt induce the postnatal formation of intestinal lymphoid follicles and regulate intestinal homeostasis. RORγt(+) ILC express the aryl hydrocarbon receptor (AhR), a highly conserved, ligand-inducible transcription factor believed to control adaptation of multicellular organisms to environmental challenges. We show that AhR is required for the postnatal expansion of intestinal RORγt(+) ILC and the formation of intestinal lymphoid follicles. AhR activity within RORγt(+) ILC could be induced by dietary ligands such as those contained in vegetables of the family Brassicaceae. AhR-deficient mice were highly susceptible to infection with Citrobacter rodentium, a mouse model for attaching and effacing infections. Our results establish a molecular link between nutrients and the formation of immune system components required to maintain intestinal homeostasis and resistance to infections.

Rorγt+ innate lymphoid cells in intestinal homeostasis and immunity

Innate lymphoid cells (ILC) combine innate and adaptive immune functions and are part of the first line of defense against mucosal infections. ILC are set apart from adaptive lymphocytes by their independence on RAG genes and the resulting absence of specific antigen receptors. In this review, we will discuss the biology and function of intestinal ILC that express the nuclear hormone receptor Rorγt (encoded by the Rorc gene) and highlight their role in intestinal homeostasis and immunity.

Lymphotoxin controls the IL-22 protection pathway in gut innate lymphoid cells during mucosal pathogen challenge

Innate lymphoid cells (ILCs) have emerged as important players, regulating the balance between protective immunity and immunopathology at mucosal surfaces. However, mechanisms that regulate ILCs' effector functions during mucosal pathogenic challenge are poorly defined. Using mice infected with the natural mouse enteric pathogen Citrobacter rodentium, we demonstrate that lymphotoxin (LT) is essential for IL-22 production by intestinal ILCs. Blocking of LTβR signaling dramatically reduced intestinal IL-22 production after C. rodentium infection. Conversely, stimulating LTβR signaling induced an IL-22 protection pathway in LT-deficient mice. Furthermore, exogenous IL-22 expression rescued LTβR-deficient mice. IL-22-producing ILCs were predominantly located in lymphoid follicles in the colon and interacted closely with dendritic cells (DCs). We find that an LT-driven positive feedback loop controls IL-22 production by RORγt(+) ILCs via LTβR signaling in DCs. Taken together, our data show that LTβR signaling in gut lymphoid follicles regulates IL-22 production by ILCs in response to mucosal pathogen challenge.

Manifestation of spontaneous and early autoimmune gastritis in CCR7-deficient mice

Autoimmune gastritis is a common autoimmune disorder characterized by chronic inflammatory cell infiltrates, atrophy of the corpus and fundus, and the occurrence of autoantibodies to parietal cell antigen. In CCR7-deficient mice, autoimmune gastritis developed spontaneously and was accompanied by metaplasia of the gastric mucosa and by the formation of tertiary lymphoid organs at gastric mucosal sites. T cells of CCR7-deficient mice showed an activated phenotype in the gastric mucosa, mesenteric lymph nodes, and peripheral blood. In addition, elevated serum IgG levels specific to gastric parietal cell antigen were detected. Because the role of organized lymphocytic aggregates at this inflammatory site is not completely understood, we first analyzed the cellular requirements for the formation of these structures. Autoreactive CD4(+) T cells were pivotal for tertiary lymphoid follicle formation, most likely in cooperation with dendritic cells, macrophages, and B cells. Second, we analyzed the necessity of secondary lymph nodes and tertiary lymphoid organs for the development of autoimmune gastritis using CCR7 single- and CCR7/lymphotoxin α double-deficient mice. Strikingly, manifestation of autoimmune gastritis was observed in the absence of secondary lymph nodes and preceded the development of tertiary lymphoid organs. Taken together, these findings identify an inflammatory process where gastric autoreactive T cells independent of organized tertiary lymphoid organs and classic lymph nodes can induce and maintain autoimmune gastritis.

The development of inducible bronchus-associated lymphoid tissue depends on IL-17

Ectopic or tertiary lymphoid tissues, such as inducible bronchus-associated lymphoid tissue (iBALT), form in nonlymphoid organs after local infection or inflammation. However, the initial events that promote this process remain unknown. Here we show that iBALT formed in mouse lungs as a consequence of pulmonary inflammation during the neonatal period. Although we found CD4(+)CD3(-) lymphoid tissue-inducer cells (LTi cells) in neonatal lungs, particularly after inflammation, iBALT was formed in mice that lacked LTi cells. Instead, we found that interleukin 17 (IL-17) produced by CD4(+) T cells was essential for the formation of iBALT. IL-17 acted by promoting lymphotoxin-α-independent expression of the chemokine CXCL13, which was important for follicle formation. Our results suggest that IL-17-producing T cells are critical for the development of ectopic lymphoid tissues.

Th17 cells induce colitis and promote Th1 cell responses through IL-17 induction of innate IL-12 and IL-23 production

Both Th1 and Th17 cells have been implicated in the pathogenesis of inflammatory bowel disease and experimental colitis. However, the complex relationship between Th1 and Th17 cells and their relative contributions to the pathogenesis of inflammatory bowel disease have not been completely analyzed. Although it has been recently shown that Th17 cells can convert into Th1 cells, the underlying in vivo mechanisms and the role of Th1 cells converted from Th17 cells in the pathogenesis of colitis are still largely unknown. In this study, we report that Th17 cells from CBir1 TCR transgenic mice, which are specific for an immunodominant microbiota Ag, are more potent than Th1 cells in the induction of colitis, as Th17 cells induced severe colitis, whereas Th1 cells induced mild colitis when transferred into TCRβxδ(-/-) mice. High levels of IL-12 and IL-23 and substantial numbers of IFN-γ(+) Th1 cells emerged in the colons of Th17 cell recipients. Administration of anti-IL-17 mAb abrogated Th17 cell-induced colitis development, blocked colonic IL-12 and IL-23 production, and inhibited IFN-γ(+) Th1 cell induction. IL-17 promoted dendritic cell production of IL-12 and IL-23. Furthermore, conditioned media from colonic tissues of colitic Th17 cell recipients induced IFN-γ production by Th17 cells, which was inhibited by blockade of IL-12 and IL-23. Collectively, these data indicate that Th17 cells convert to Th1 cells through IL-17 induction of mucosal innate IL-12 and IL-23 production.

Regulation of innate and adaptive immunity by the commensal microbiota

The microbial communities that inhabit the intestinal tract are essential for mammalian health. Communication between the microbiota and the host establishes and maintains immune homeostasis, enabling protective immune responses against pathogens while preventing adverse inflammatory responses to harmless commensal microbes. Specific bacteria, such as segmented filamentous bacteria, Clostridium species, and Bacteroides fragilis, are key contributors to immune homeostasis in the gut. The cellular and molecular interactions between intestinal microbes and the immune system are rapidly being elucidated. Here, we review advances in our understanding of the microbial populations that shape the mucosal immune system and create a protective defense that prevents infection while tolerating friendly commensals.

RORγt+ innate lymphoid cells regulate intestinal homeostasis by integrating negative signals from the symbiotic microbiota

Lymphoid cells that express the nuclear hormone receptor RORγt are involved in containment of the large intestinal microbiota and defense against pathogens through the production of interleukin 17 (IL-17) and IL-22. They include adaptive IL-17-producing helper T cells (T(H)17 cells), as well as innate lymphoid cells (ILCs) such as lymphoid tissue-inducer (LTi) cells and IL-22-producing NKp46+ cells. Here we show that in contrast to T(H)17 cells, both types of RORγt+ ILCs constitutively produced most of the intestinal IL-22 and that the symbiotic microbiota repressed this function through epithelial expression of IL-25. This function was greater in the absence of adaptive immunity and was fully restored and required after epithelial damage, which demonstrates a central role for RORγt+ ILCs in intestinal homeostasis. Our data identify a finely tuned equilibrium among intestinal symbionts, adaptive immunity and RORγt+ ILCs.

Intestinal microbiota, evolution of the immune system and the bad reputation of pro-inflammatory immunity

The mammalian intestine provides a unique niche for a large community of bacterial symbionts that complements the host in digestive and anabolic pathways, as well as in protection from pathogens. Only a few bacterial phyla have adapted to this predominantly anaerobic environment, but hundreds of different species create an ecosystem that affects many facets of the host's physiology. Recent data show how particular symbionts are involved in the maturation of the immune system, in the intestine and beyond, and how dysbiosis, or alteration of that community, can deregulate immunity and lead to immunopathology. The extensive and dynamic interactions between the symbionts and the immune system are key to homeostasis and health, and require all the blends of so-called regulatory and pro-inflammatory immune reactions. Unfortunately, pro-inflammatory immunity leading to the generation of Th17 cells has been mainly associated with its role in immunopathology. Here we discuss the view that the immune system in general, and type 17 immunity in particular, develop to maintain the equilibrium of the host with its symbionts.

Human T cell cytokine responses are dependent on multidrug resistance protein-1

Multidrug resistance protein-1 (MRP1) belongs to subfamily C of the ATP-binding cassette transporters, and exports leukotriene C(4) and organic anions including the fluorescent calcium indicator indo-1. The observation that leukocytes from patients with an autoimmune disease exported indo-1 at a higher rate than controls prompted the hypothesis that MRP1 contributes to the function of activated cells. To test this, we defined the expression of MRP1 on resting and activated human T cells, and determined whether T cell activation is dependent upon MRP1 function. MRP1 is expressed on resting memory but not on naive CD4 and CD8 T cells. After activation through the TCR, cord blood CD4 T cells express high levels of MRP1. Blockade of MRP1 with the specific inhibitor MK-571 abrogated superantigen-induced expression of IFN-gamma, tumor necrosis factor-alpha, IL-10, IL-2, IL-4 and CD69 by T cells without affecting their viability, and was reversible upon removal of MK-571 from the culture media. Electrophoretic mobility shift assays demonstrate that MRP1 blockade with MK-571 induces activation of the transcriptional repressor peroxisome proliferator-activated receptor-gamma in CD4 T cells, thus providing insight into the potential mechanism by which their responses are abrogated.