Identification of multiple isolated lymphoid follicles on the antimesenteric wall of the mouse small intestine

The expanding family of innate lymphoid cells: regulators and effectors of immunity and tissue remodeling

Research has identified what can be considered a family of innate lymphoid cells (ILCs) that includes not only natural killer (NK) cells and lymphoid tissue-inducer (LTi) cells but also cells that produce interleukin 5 (IL-5), IL-13, IL-17 and/or IL-22. These ILC subsets are developmentally related, requiring expression of the transcriptional repressor Id2 and cytokine signals through the common γ-chain of the IL-2 receptor. The functional differentiation of ILC subsets is orchestrated by distinct transcription factors. Analogous to helper T cell subsets, these evolutionarily conserved yet distinct ILCs seem to have important roles in protective immunity, and their dysregulation can promote immune pathology.

Secretory immunoglobulin A: well beyond immune exclusion at mucosal surfaces

At mucosal surfaces, secretory IgA (SIgA) antibodies serve as the first line of defense against microorganisms through a mechanism called immune exclusion that prevents interaction of neutralized antigens with the epithelium. In addition, SIgA plays a role in the immune balance of the epithelial barrier through selective adhesion to M cells in intestinal Peyer's patches. This mediates the transepithelial retro-transport of the antibody and associated antigens from the intestinal lumen to underlying gut-associated organized lymphoid tissue. In Peyer's patches, SIgA-based immune complexes are internalized by underlying antigen-presenting cells, leaving the antigen with masked epitopes, a form that limits the risk of overwhelming the local immune protection system with danger signals. This translates into the onset of mucosal and systemic responses associated with production of anti-inflammatory cytokines and limited activation of antigen-presenting cells. In the gastrointestinal tract, SIgA exhibits thus properties of a neutralizing agent (immune exclusion) and of an immunopotentiator inducing effector immune responses in a noninflammatory context favorable to preserve local homeostasis.

A new vision of immunity: homeostasis of the superorganism

The immune system is commonly perceived as an army of organs, tissues, cells, and molecules that protect from disease by eliminating pathogens. However, as in human society, a clear definition of good and evil might be sometimes difficult to achieve. Not only do we live in contact with a multitude of microbes, but we also live with billions of symbionts that span all the shades from mutualists to potential killers. Together, we compose a superorganism that is capable of optimal living. In that context, the immune system is not a killer, but rather a force that shapes homeostasis within the superorganism.

New insights into the development of lymphoid tissues

Secondary lymphoid organs are important locations for the initiation of adaptive immune responses. They develop before birth, and their formation requires interaction between lymphotoxin-α₁ß₂-expressing lymphoid-tissue inducer cells and lymphotoxin-ß receptor-expressing stromal organizer cells. Here, we discuss new insights into the earliest phases of peripheral lymph node and Peyer's patch formation that occur before lymphotoxin-ß receptor signalling and suggest a role for the developing nervous system. In addition, we discuss the differing requirements for the postnatal formation of mucosa-associated lymphoid tissues and tertiary lymphoid structures that develop at sites of chronic inflammation.

Function of mucosa-associated lymphoid tissue in antibody formation

Abundant evidence supports the notion that human intestinal plasma cells are largely derived from B cells initially activated in gut-associated lymphoid tissue (GALT). Nevertheless, insufficient knowledge exists about the uptake, processing, and presentation of luminal antigens occurring in GALT to accomplish priming and sustained expansion of mucosal B cells. Also, it is unclear how the germinal center reaction so strikingly promotes class switch to IgA and expression of J chain, although the commensal microbiota appears to contribute to both diversification and memory. B-cell migration from GALT to the intestinal lamina propria is guided by rather well-defined adhesion molecules and chemokines/chemokine receptors, but the cues directing homing to secretory effector sites beyond the gut require better definition. In this respect, the role of human Waldeyer's ring (including adenoids and the palatine tonsils) as a regional mucosa-associated lymphoid tissue must be better defined, although the balance of evidence suggests that it functions as nasopharynx-associated lymphoid tissue (NALT) like the characteristic NALT structures in rodents. Altogether, data suggest a remarkable compartmentalization of the mucosal immune system that must be taken into account in the development of effective local vaccines to protect specifically the airways, small and large intestines, and the female genital tract.

Role of gut-resident dendritic cells in inflammatory bowel disease

The gastrointestinal immune system, innate and adaptive, is continuously exposed to challenges provided by the enteric flora. In most cases, the result of mucosal immune responses is the development of tolerance. Mucosal dendritic cells initiate and regulate local immune responses. Uncontrolled local immune responses are thought to be a major factor in the development of inflammatory bowel disease, such as Crohn's disease and ulcerative colitis. This review will discuss the function of dendritic cells in the recognition of the enteric flora and their role in the development of intestinal inflammation.

Synergy of IL-23 and Th17 cytokines: new light on inflammatory bowel disease

Inflammatory bowel diseases (IBDs), including Crohn's disease and ulcerative colitis, involve an interplay between host genetics and environmental factors including intestinal microbiota. Animal models of IBD have indicated that chronic inflammation can result from over-production of inflammatory responses or deficiencies in key negative regulatory pathways. Recent research advances in both T-helper 1 (Th1) and T-helper 17 (Th17) effect responses have offered new insights on the induction and regulation of mucosal immunity which is linked to the development of IBD. Th17 cytokines, such as IL-17 and IL-22, in combination with IL-23, play crucial roles in intestinal protection and homeostasis. IL-23 is expressed in gut mucosa and tends to orchestrate T-cell-independent pathways of intestinal inflammation as well as T cell dependent pathways mediated by cytokines produced by Th1 and Th17 cells. Th17 cells, generally found to be proinflammatory, have specific functions in host defense against infection by recruiting neutrophils and macrophages to infected tissues. Here we will review emerging data on those cytokines and their related regulatory networks that appear to govern the complex development of chronic intestinal inflammation; we will focus on how IL-23 and Th17 cytokines act coordinately to influence the balance between tolerance and immunity in the intestine.

Health-beneficial effects of probiotics: Its mode of action

It is now widely recognized that probiotics have health-beneficial effects on humans and animals. Probiotics should survive in the intestinal tract to exert beneficial effects on the host's health. To keep a sufficient level of probiotic bacteria in the gastrointestinal tract, a shorter interval between doses may be required. Although adherence to the intestinal epithelial cell and mucus is not a universal property of probiotics, high ability to adhere to the intestinal surface might strongly interfere with infection of pathogenic bacteria and regulate the immune system. The administration of probiotic Lactobacillus stimulated indigenous Lactobacilli and the production of short-chain fatty acids. This alteration of the intestinal environment should contribute to maintain the host's health. The immunomodulatory effects of probiotics are related to important parts of their beneficial effects. Probiotics may modulate the intestinal immune response through the stimulation of certain cytokine and IgA secretion in intestinal mucosa. The health-beneficial effects, in particular the immunomodulation effect, of probiotics depend on the strain used. Differences in indigenous intestinal microflora significantly alter the magnitude of the effects of a probiotic. Specific probiotic strains suitable for each animal species and their life stage as well as each individual should be found.

Adaptive immune regulation in the gut: T cell-dependent and T cell-independent IgA synthesis

In mammals, the gastrointestinal tract harbors an extraordinarily dense and complex community of microorganisms. The gut microbiota provide strong selective pressure to the host to evolve adaptive immune responses required for the maintenance of local and systemic homeostasis. The continuous antigenic presence in the gut imposes a dynamic remodeling of gut-associated lymphoid tissues (GALT) and the selection of multiple layered strategies for immunoglobulin (Ig) A production. The composite and dynamic gut environment also necessitates heterogeneous, versatile, and convertible T cells, capable of inhibiting (Foxp3(+) T cells) or helping (T(FH) cells) local immune responses. In this review, we describe recent advances in our understanding of dynamic pathways that lead to IgA synthesis, in gut follicular structures and in extrafollicular sites, by T cell-dependent and T cell-independent mechanisms. We discuss the finely tuned regulatory mechanisms for IgA production and emphasize the role of mucosal IgA in the selection and maintenance of the appropriate microbial composition that is necessary for immune homeostasis.

Dendritic cells produce CXCL13 and participate in the development of murine small intestine lymphoid tissues

In the adult intestine, luminal microbiota induce cryptopatches to transform into isolated lymphoid follicles (ILFs), which subsequently act as sites for the generation of IgA responses. The events leading to this conversion are incompletely understood. Dendritic cells (DCs) are components of cryptopatches (CPs) and ILFs and were therefore evaluated in this process. We observed that the adult murine intestine contains clusters of DCs restricted to the CP/ILF continuum. A numerical and cell associative hierarchy in the adult intestine and a chronologic hierarchy in the neonatal intestine demonstrated that these clusters form after the coalescence of CD90+ cells to form CPs and before the influx of B220+ B lymphocytes to form ILFs. Cluster formation was dependent on lymphotoxin and the lymphotoxin beta receptor and independent of lymphocytes. The ILF DC population was distinguished from that of the lamina propria by the absence of CD4+CD11c+ cells and an increased proportion of CD11c+B220+ cells. The formation of clusters was not limited by DC numbers but was induced by luminal microbiota. Moreover, in the absence of the chemokine CXCL13, CP transformation into ILF was arrested. Furthermore, ILF DCs express CXCL13, and depletion of DCs resulted in regression of ILFs and disorganization of CPs. These results reveal DC participation in ILF transformation and maintenance and suggest that in part this may be due to CXCL13 production by these cells.

Functional morphology of the gastrointestinal tract

The primary function of the gastrointestinal tract is water, electrolyte, and nutrient transport. To perform this function, the epithelium lining the gastrointestinal tract is in close contact with the gastrointestinal lumen. Because the lumen is connected to the external environment and, depending on the site, has a high bacterial and antigen load, the epithelium must also prevent pathogenic agents within the gastrointestinal lumen from gaining access to internal tissues. This creates a unique challenge for the gastrointestinal tract to balance the requirements of forming a barrier to separate the intestinal lumen from underlying tissue while simultaneously setting up a system for moving water, electrolytes, and nutrients across the barrier. In the face of this, the epithelial cells of the gastrointestinal tract form a selectively permeable barrier that is tightly regulated. In addition, the intestinal mucosa actively participates in host defense by engaging the mucosal immune system. Complex tissue organization and diverse cellular composition are necessary to achieve such a broad range of functions. In this chapter, the structure and function of the gastrointestinal tract and their relevance to infectious diseases are discussed.

Loss of CD103+ intestinal dendritic cells during colonic inflammation

AIM: To investigate possible differences in dendritic cells (DC) within intestinal tissue of mice before and after induction of colitis.

METHODS: Mucosal DC derived from intestinal tissue, as well as from mesenteric lymph nodes and spleen, were analyzed by fluorescence activated cell sorting (FACS) analysis. Supernatants of these cells were analyzed for secretion of different pro- and anti-inflammatory cytokines. Immunohistochemistry and immunofluorescence were performed on cryosections of mucosal tissue derived from animals with colitis as well as from healthy mice.

RESULTS: It was shown that DC derived from healthy intestinal lamina propria (LP) represented an immature phenotype as characterized by low-level expression of costimulatory cytokines. In contrast to DC from spleen and mesenteric lymph nodes (MLN) that secreted proinflammatory cytokines, LP-DC produced high levels of the anti-inflammatory cytokine IL-10. After induction of murine colitis in a CD4⁺CD62L⁺ transfer model or in chronic dextran sulfate sodium-colitis, a marked increase of activated CD80⁺ DC could be observed within the inflamed colonic tissue. Interestingly, in contrast to splenic DC, a significant population of DC within MLN and colonic LP expressed the mucosal integrin CD103 which was lost during colitis.

CONCLUSION: The constitutive secretion of anti-inflammatory cytokines by immature DC within the intestinal LP might regulate the homeostatic balance between mucosal immunity and tolerance. CD103⁺ DC could mediate this important function.

Isolation of NK cells and NK-like cells from the intestinal lamina propria

Being exposed to food products, pathogens and harmless commensal bacteria, the mucosal immune system faces a constant challenge. Therefore, maintenance of a homeostatic balance is required to achieve tolerance to harmless bacteria and their products and to induce potent immunity to infection with pathogenic bacteria. Until recently, the literature on mucosal natural killer (NK) cells residing in the intestinal lamina propria was scarce and phenotype and function of gut mucosal NK cells did not receive much attention. Recently, data have become available identifying two distinct subsets of mucosal NKp46(+) lymphocytes based on the expression of the orphan transcription factor RORgammat. In many ways, the RORgammat(-) subset resembled "classical" NK cells in that it was developmentally dependent on IL-15 but not on RORgammat and displayed NK cell function (e.g., cell-mediated cytotoxicity, IFN-gamma production). In contrast, the RORgammat(+) subset developed independent of IL-15 but required RORgammat, suggesting that this subset may be related to lymphoid tissue inducer (LTi) cells. Interestingly, these RORgammat(+) NKp46(+) NK-LTi cells constitutively produced large amounts of IL-22, a cytokine regulating antimicrobial protection and regeneration of epithelial cells. In this chapter, we provide experimental procedures to isolate "classical" NK cells from the intestinal lamina propria as well as the newly described lymphoid tissue inducer-like (LTi-like) cells producing IL-22 and co-expressing NK cell receptors.

A functional role for CCR6 on proallergic T cells in the gastrointestinal tract

BACKGROUND & AIMS

CCL20 is a chemokine that regulates the homeostatic and inflammatory trafficking of leukocytes to the small intestine and regulates the development of the gastrointestinal lymphoid architecture. T cells expressing T helper cell (Th) 2 cytokines are critical for experimental food allergy, and we hypothesized that CCL20 is involved in the localization of these cells to the gut.

METHODS

We evaluated the role of CCR6 in allergic diarrhea induced by sensitization and oral challenge with ovalbumin (OVA) using CCR6(+/+) and CCR6(-/-) mice.

RESULTS

CCR6(-/-) mice were protected from OVA-induced diarrhea but surprisingly were not impaired in mastocytosis or allergen-specific immunoglobulin E. CCR6(-/-) mice were also protected from T cell-mediated diarrhea induced by anti-CD3 antibody. Allergic diarrhea was associated with an increased expression of Th2 cytokines within the intestinal mucosa that was significantly reduced in CCR6(-/-) mice. Inhibition of lymphocyte homing by treatment with FTY720 did not impair allergic diarrhea, indicating that reactivation of T cells could occur locally within the small intestine. Finally, T-cell transfer studies demonstrated that CCR6 was required both on the transferred T cells and in the recipient mouse to manifest allergic disease in the gastrointestinal tract.

CONCLUSIONS

These studies highlight a mast cell- and immunoglobulin E-independent role for CCR6-bearing T cells in the pathogenesis of gastrointestinal allergic disease.

Innate signaling networks in mucosal IgA class switching

The past 20 years have seen a growing interest over the control of adaptive immune responses by the innate immune system. In particular, considerable attention has been paid to the mechanisms by which antigen-primed dendritic cells orchestrate the differentiation of T cells. Additional studies have elucidated the pathways followed by T cells to initiate immunoglobulin responses in B cells. In this review, we discuss recent advances on the mechanisms by which intestinal bacteria, epithelial cells, dendritic cells, and macrophages cross talk with intestinal T cells and B cells to induce frontline immunoglobulin A class switching and production.

Opening the crypt: current facts and hypotheses on the function of cryptopatches

Cryptopatches, small aggregates of lymphoid cells found in the intestinal lamina propria, have been assigned many functions specific to gut immunity. Populated with seemingly immature lymphoid cells and dendritic cells, it has been suggested that cryptopatches maturate intraepithelial lymphocytes, Th17 cells, IL-22-producing NKp46(+) cells, and lymphoid tissues in response to the gut microbiota. Some of these issues, however, remain hotly debated. Therefore, cryptopatches are coming to the forefront of gut immunology and warrant a comprehensive discussion of their role in the development of the immune system.

CXCL13 expression in the gut promotes accumulation of IL-22-producing lymphoid tissue-inducer cells, and formation of isolated lymphoid follicles

The chemokine CXCL13 is overexpressed in the intestine during inflammation. To mimic this condition, we created transgenic mice-expressing CXCL13 in intestinal epithelial cells. CXCL13 expression promoted a marked increase in the number of B cells in the lamina propria and an increase in the size and number of lymphoid follicles in the small intestine. Surprisingly, these changes were associated with a marked increase in the numbers of RORgammat(+)NKp46(-)CD3(-)CD4(+) and RORgammat(+)NKp46(+) cells. The RORgammat(+)NKp46(-)CD3(-)CD4(+) cells expressed CXCR5, the receptor for CXCL13, and other markers of lymphoid tissue-inducer cells, such as LTalpha, LTbeta, and TNF-related activation-induced cytokine (TRANCE). RORgammat(+)NKp46(-)CD3(-)CD4(+) gut LTi cells produced IL-22, a cytokine implicated in epithelial repair; and expressed the IL-23 receptor, a key regulator of IL-22 production. These results suggest that overexpression of CXCL13 in the intestine during inflammatory conditions favors mobilization of B cells and of LTi and NK cells with immunomodulatory and reparative functions.

The development of intestinal lymphoid tissues at the interface of self and microbiota

Intestinal lymphoid tissues face the challenging task of inducing adaptive immunity to pathogens, yet maintaining homeostasis with the enormous commensal microbiota. To that aim, the ancient partnership between self and flora has resulted in the generation of a unique set of lymphoid tissues capable of constant large-scale reformatting. A first set of lymphoid tissues, the mesenteric lymph nodes and Peyer's patches, are programmed to develop in the sterile environment of the fetus, whereas a second set of lymphoid tissues, the tertiary lymphoid tissues, are induced to form by the microbiota and inflammation. The diversity of intestinal lymphoid tissues confers the flexibility required to adapt the number of immune inductive sites to the size of the flora and the extent of the pathogenic threat. The result is a functional superorganism combining self and microbes for the best possible symbiosis.

Id2-, RORgammat-, and LTbetaR-independent initiation of lymphoid organogenesis in ocular immunity

The eye is protected by the ocular immunosurveillance system. We show that tear duct–associated lymphoid tissue (TALT) is located in the mouse lacrimal sac and shares immunological characteristics with mucosa-associated lymphoid tissues (MALTs), including the presence of M cells and immunocompetent cells for antigen uptake and subsequent generation of mucosal immune responses against ocularly encountered antigens and bacteria such as Pseudomonas aeruginosa. Initiation of TALT genesis began postnatally; it occurred even in germ-free conditions and was independent of signaling through organogenesis regulators, including inhibitor of DNA binding/differentiation 2, retinoic acid–related orphan receptor γt, lymphotoxin (LT) α1β2–LTβR, and lymphoid chemokines (CCL19, CCL21, and CXCL13). Thus, TALT shares immunological features with MALT but has a distinct tissue genesis mechanism and plays a key role in ocular immunity.

Induction of intestinal lymphoid tissue formation by intrinsic and extrinsic signals

Since the discovery of inducer cells as a separate lineage for organogenesis of Peyer's patches in the small intestine of fetal mice, a lot of progress has been made in understanding the molecular pathways involved in the generation of lymphoid tissue and the maintenance of the lymphoid architecture. The findings that inducer cells also exist in adult mice and in humans, have a lineage relationship to natural killer cells, and can be stimulated during infections highlight their possible role in establishing innate and adaptive immune responses. Novel concepts in the development of intestinal lymphoid tissues have been made in the past few years suggesting that lymphoid organs are more plastic as previously thought and depend on antigenic stimulation. In addition, the generation of novel lymphoid organs in the gut under inflammatory conditions indicates a function in chronic diseases. The present review summarizes current knowledge on the basic framework of signals required for developing lymphoid tissue under normal and inflammatory conditions.

How host-bacterial interactions lead to IgA synthesis in the gut

In mammals, the gut is populated with an extremely dense and diverse bacterial community. One response following intestinal colonization is the production of immunoglobulin (Ig) A by B cells present in the gut-associated lymphoid tissues (GALT). In this review, we summarize recent advances in our understanding of the sites, mechanisms, and functions of intestinal IgA synthesis. We discuss here the pathways leading to IgA production, in organized structures as well as nonorganized tissues, by T-dependent as well as T-independent mechanisms. In addition, we discuss new insights into the role of gut IgA in the regulation of bacterial communities and maintenance of immune homeostasis.

Generation of Immunoglobulin diversity in human gut-associated lymphoid tissue

The organised gut associated lymphoid tissue (GALT) exists adjacent to an extensive and diverse luminal flora. The follicle associated epithelium and associated dendritic cells and lymphocytes form a tightly fortified gateway between the flora and the host that permits connectivity between them and chronic activation of the lymphoid compartment. As a consequence, plasma cell precursors are generated continuously, and in abundance, in GALT by clonal proliferation. Clonal proliferation alone on this scale would reduce the spectrum of B cell specificity. To compensate, GALT also houses molecular machinery that diversifies the receptor repertoire by somatic hypermutation, class switch recombination and receptor revision. These three processes of enhancing the diversity of mature B cells ensure that although clonally related plasma cells may secrete immunoglobulin side by side in the mucosa they rarely have identical antigen binding sites.

Chemokine receptor CXCR5 supports solitary intestinal lymphoid tissue formation, B cell homing, and induction of intestinal IgA responses

Solitary intestinal lymphoid tissue (SILT) comprises a spectrum of phenotypically diverse lymphoid aggregates interspersed throughout the small intestinal mucosa. Manifestations of SILT range from tiny lymphoid aggregates almost void of mature lymphocytes to large structures dominated by B cells. Large SILT phenotypically resemble a single Peyer's patch follicle, suggesting that SILT might contribute to intestinal humoral immune responses. In this study, we track the fate of individual SILT in vivo over time and analyze SILT formation and function in chemokine receptor CXCR5-deficient mice. We show that, in analogy to Peyer's patches, formation of SILT is invariantly determined during ontogeny and depends on CXCR5. Young CXCR5-deficient mice completely lack SILT, suggesting that CXCR5 is essential for SILT formation during regular postnatal development. However, microbiota and other external stimuli can induce the formation of aberrant SILT distinguished by impaired development of B cell follicles in CXCR5-deficient mice. Small intestinal transplantation and bone marrow transplantation reveal that defect follicle formation is due to impaired B cell homing. Moreover, oral immunization with cholera toxin or infection with noninvasive Salmonella fail to induce efficient humoral immune responses in CXCR5-deficient mice. Bone marrow transplantation of CXCR5-deficient recipients with wild-type bone marrow rescued B cell follicle formation in SILT but failed to restore full humoral immune responses. These results reveal an essential role of CXCR5 in Peyer's patch and SILT development and function and indicate that SILT do not fully compensate for the lack of Peyer's patches in T cell-dependent humoral immune responses.

Involvement of dendritic cells in the pathogenesis of inflammatory bowel disease

In conclusion, during inflammation, DCs are likely activated by inflammatory signals and induced to migrate to T cell zones of organized lymphoid tissues where the cells induce T cell responses. In addition to their established role in T cell priming and the induction of tolerance, DCs may act to enhance (or possibly suppress) T cell responses at sites of mucosal inflammation. Determining the importance of DCs in this regard, as well as establishing a potential role for DCs in continuous activation of naive or central memory cells in lymph nodes draining inflammatory sites, will elucidate the role of DCs as a potential therapeutic target for chronic inflammatory diseases, like IBD. Resident intestinal macrophages are noninflammatory and do not efficiently present antigens to intestinal T cells, yet are avidly phagocytic and able to kill internalized organisms. During intestinal inflammation, monocytes are recruited from the blood, become inflammatory macrophages in the inflamed tissue, and are major contributors to tissue destruction and perpetuation of inflammation via their production of chemokines and pro-inflammatory cytokines. Macrophages may also contribute directly to DC activation and maturation, which would drive DCs to present antigens from the bacterial flora to T cells locally within tissue or to more efficiently traffic to T cell zones of lymphoid tissue. Thus, DCs and macrophages have evolved functional niches that promote cooperation in the prevention of untoward intestinal inflammation in the steady state and in the eradication of invasive microorganisms during infection. The balance between suppressing inflammation and promoting host defense is altered in humans with IBD allowing a persistent inflammatory response to commensal bacteria. Based on studies from animal models, the pathogenesis of IBD likely involves either the lack of appropriate regulation from T cells, or an over-production of effector T cells. The end result of these potential mechanisms is the abnormal induction and/or survival of effector T cells and the production of factors such as cytokines by inflammatory macrophages and neutrophils that result in tissue destruction. The destructive process likely involves normally tolerizing DCs, which in the microenvironment of the inflamed mucosa activate T cell responses to normal flora in both draining lymphoid tissues and at sites of inflammation, with macrophages and neutrophils contributing the bulk of inflammatory and destructive cytokines.

The "Microflora Hypothesis" of allergic disease

Predisposition to allergic disease is a complex function of an individual's genetic background and, as is the case with multi-gene traits, environmental factors have important phenotypic consequences. Over a span of decades, a dramatic increase in the prevalence of allergic disease in westernized populations suggests the occurrence of critical changes in environmental pressures. Recently, it has been shown that the microbiota (i.e. microflora) of allergic individuals differs from that of non-allergic ones and that differences are detectable prior to the onset of atopy, consistent with a possible causative role. Features of the westernized lifestyle that are known to alter the microbiota, such as antibiotics and diet, are also associated with allergy in humans. In this chapter, we discuss the "Microflora Hypothesis" for allergy which predicts that an "unhealthy" microbiota composition, now commonly found within westernized communities, contributes to the development of allergy and conversely, that restoring a "healthy" microbiota, perhaps through probiotic supplementation, may prevent the development of allergy or even treat existing disease. In testing this hypothesis, our laboratory has recently reported that mice can develop allergic airway responses if their microbiota is altered at the time of first allergen exposure.

Cellular assessment of crop lymphoid tissue from specific-pathogen-free white leghorn chickens after Salmonella enteritidis challenge

The crop may be an important site along the upper alimentary tract in which a humoral immune response against Salmonella Enteritidis (SE) is elicited locally. The mucosal immune response within the crop (ingluvies) of specific-pathogen-free (SPF) white leghorn (WL) chickens against SE was investigated. Three trials were conducted using SPF WL pullets at age 5-6 wk. Trial 1 consisted of 77 birds evaluated for 10 wk post-SE infection (pi), trial 2 was composed of 72 birds monitored through 8 wk pi, and trial 3 was made up of 30 birds assessed for 5 wk pi. Birds were challenged per os with 10(8) colony-forming units/ml SE phage type 13. Crop lavage samples, crop tissues, ceca, and/or liver-spleen were collected preinfection and then at weekly intervals post-SE infection. Bacteriologic examination of cecal contents and/or liver-spleen occurred weekly to monitor progression of SE infection. Crop lavages were analyzed for SE-lipopolysaccharide (LPS)-specific immunoglobulin A (IgA) by enzyme-linked immunosorbent assay to assess humoral immune response. General histologic staining (hematoxylin and eosin [H&E] and methyl green-pyronin [MGP]) and immunohistochemical (IHC) staining (monoclonal antibodies CD45 and Bu-1) were applied to serial sections of crop to evaluate lymphoid tissue via light microscopy, to grade isolated lymphoid follicles (ILFs) by using score 0 (minimal, < 50 microm in diameter) to score 5 (sizable, > 200 microm in diameter) scale, and to characterize the cellular population of ILFs. Results revealed that cecum samples and liver-spleen samples were 100% SE culture positive at 1 wk pi, and then the percentage of SE positives progressively declined over time. Markedly increased crop SE-LPS-specific IgA antibodies were detected in crop samples by 2-3 wk pi, and the humoral response remained elevated above week 0 baseline for the duration of each trial. Crop ILFs of score 3 to 5 were observed in H&E-stained tissues, with an increased proportion of ILFs in post-SE-infected crops vs. uninfected. MGP staining showed plasma cells scattered within and at the periphery of ILFs. IHC staining revealed CD45 (pan-leukocyte) and Bu-1 (B-lymphocyte)-positive cells within crop ILFs. The chicken crop seems to be an organ in which lymphoid tissue may arise in response to enteric SE infection, and a site in which a humoral response may be generated against the SE pathogen.

Board-invited review: porcine mucosal immunity of the gastrointestinal tract

The gastrointestinal tract (GIT) constitutes one of the largest immunological organs of the body. The GIT must permit absorption of nutrients while also maintaining the ability to respond appropriately to a diverse milieu of dietary and microbial antigenic components. Because of the diverse population of antigenic components within the GIT, a sophisticated mucosal immune system has evolved that relies on collaboration between the innate and adaptive arms of immunity. The collaborative, mucosal immune effort offers protection from harmful pathogens while also being tolerant of dietary antigens and normal microbial flora. Knowledge with respect to porcine mucosal immunity is important as we strive to understand the interrelationships among GIT physiology, immunology, and the resident microbiota. The aim of this review is to provide a descriptive overview of GIT immunity and components of the mucosal immune system and to highlight differences that exist between the porcine species and other mammals.

Lymphoid tissue genesis induced by commensals through NOD1 regulates intestinal homeostasis

Intestinal homeostasis is critical for efficient energy extraction from food and protection from pathogens. Its disruption can lead to an array of severe illnesses with major impacts on public health, such as inflammatory bowel disease characterized by self-destructive intestinal immunity. However, the mechanisms regulating the equilibrium between the large bacterial flora and the immune system remain unclear. Intestinal lymphoid tissues generate flora-reactive IgA-producing B cells, and include Peyer's patches and mesenteric lymph nodes, as well as numerous isolated lymphoid follicles (ILFs). Here we show that peptidoglycan from Gram-negative bacteria is necessary and sufficient to induce the genesis of ILFs in mice through recognition by the NOD1 (nucleotide-binding oligomerization domain containing 1) innate receptor in epithelial cells, and beta-defensin 3- and CCL20-mediated signalling through the chemokine receptor CCR6. Maturation of ILFs into large B-cell clusters requires subsequent detection of bacteria by toll-like receptors. In the absence of ILFs, the composition of the intestinal bacterial community is profoundly altered. Our results demonstrate that intestinal bacterial commensals and the immune system communicate through an innate detection system to generate adaptive lymphoid tissues and maintain intestinal homeostasis.

Alpha4beta7/MAdCAM-1 interactions play an essential role in transitioning cryptopatches into isolated lymphoid follicles and a nonessential role in cryptopatch formation

The alpha(4) integrins alpha(4)beta(7) and alpha(4)beta(1), and their ligands mucosal vascular addressin cell adhesion molecule 1 (MAdCAM-1) and VCAM-1, have diverse functions, including roles in the formation of secondary lymphoid tissues at early time points during the colonization and clustering of the fetal lymphoid tissue inducer (LTi) cells and at later time points during the recruitment of lymphocytes. In this study, we evaluated the role of alpha(4) integrins in the development of a recently appreciated class of intestinal lymphoid tissues, isolated lymphoid follicles (ILFs). We observed that diverse ILF cellular populations express alpha(4)beta(7) and alpha(4)beta(1), including the LTi-like cells and lymphocytes, while ILF stromal cells and vessels within ILFs express VCAM-1 and MAdCAM-1, respectively. Evaluation of adult and neonatal beta(7)(-/-) mice and adult and neonatal mice given blocking Abs to alpha(4)beta(7), MAdCAM-1, or VCAM-1 did not identify a role for alpha(4) integrins in cryptopatch (CP) development; however, these studies demonstrated that alpha(4)beta(7) and MAdCAM-1 are required for the transitioning of CP into lymphoid tissues containing lymphocytes or ILFs. Competitive bone marrow transfers demonstrated that beta(7)(-/-) LTi-like cells had a reduced but not significantly impaired ability to localize to CP. Bone marrow transfers and adoptive transfers of B lymphocytes revealed that beta(7) expression by B lymphocytes was essential for their entry into the developing ILFs. These findings demonstrate an essential role for alpha(4)beta(7)/MAdCAM-1 in ILF development corresponding to the influx of beta(7)-expressing lymphocytes and a nonessential role for beta(7)-localizing LTi-like cells to the small intestine.

Role of mucosal dendritic cells in inflammatory bowel disease

The gastrointestinal innate and adaptive immune system continuously faces the challenge of potent stimuli from the commensal microflora and food constituents. These local immune responses require a tight control, the outcome of which is in most cases the induction of tolerance. Local T cell immunity is an important compartment of the specific intestinal immune system. T cell reactivity is programmed during the initial stage of its activation by professional presenting cells. Mucosal dendritic cells (DCs) are assumed to play key roles in regulating immune responses in the antigen-rich gastrointestinal environment. Mucosal DCs are a heterogeneous population that can either initiate (innate and adaptive) immune responses, or control intestinal inflammation and maintain tolerance. Defects in this regulation are supposed to lead to the two major forms of inflammatory bowel disease (IBD), Crohn’s disease (CD) and ulcerative colitis (UC). This review will discuss the emerging role of mucosal DCs in regulating intestinal inflammation and immune responses.

Requirement for lymphoid tissue-inducer cells in isolated follicle formation and T cell-independent immunoglobulin A generation in the gut

Immunoglobulin A (IgA) is generated in the gut by both T cell-dependent and T cell-independent processes. The sites and the mechanisms for T cell-independent IgA synthesis remain elusive. Here we show that isolated lymphoid follicles (ILFs) were sites where induction of activation-induced cytidine deaminase (AID) and IgA class switching of B cells took place in the absence of T cells. We also show that formation of ILFs was regulated by interactions between lymphoid tissue-inducer cells expressing the nuclear receptor ROR gamma t (ROR gamma t(+)LTi cells) and stromal cells (SCs). Activation of SCs by ROR gamma t(+)LTi cells through lymphotoxin (LT)-beta receptor (LT beta R) and simultaneously by bacteria through TLRs induced recruitment of dendritic cells (DCs) and B cells and formation of ILFs. These findings provide insight into the crosstalk between bacteria, ROR gamma t(+)LTi cells, SCs, DCs, and B cells required for ILF formation and establish a critical role of ILFs in T cell-independent IgA synthesis in gut.

A unique B2 B cell subset in the intestine

Over 80% of the body's activated B cells are located in mucosal sites, including the intestine. The intestine contains IgM⁺ B cells, but these cells have not been characterized phenotypically or in terms of their developmental origins. We describe a previously unidentified and unique subset of immunoglobulin M⁺ B cells that present with an AA4.1⁻CD21⁻CD23⁻ major histocompatibility complex class II^bright surface phenotype and are characterized by a low frequency of somatic hypermutation and the potential ability to produce interleukin-12p70. This B cell subset resides within the normal mucosa of the large intestine and expands in response to inflammation. Some of these intestinal B cells originate from the AA4.1⁺ immature B2 cell pool in the steady state and are also recruited from the recirculating naive B cell pool in the context of intestinal inflammation. They develop in an antigen-independent and BAFF-dependent manner in the absence of T cell help. Expansion of these cells can be induced in the absence of the spleen and gut-associated lymphoid tissues. These results describe the existence of an alternative pathway of B cell maturation in the periphery that gives rise to a tissue-specific B cell subset.

The biology of intestinal immunoglobulin A responses

The gut mucosa is exposed to a large community of commensal bacteria that are required for the processing of nutrients and the education of the local immune system. Conversely, the gut immune system generates innate and adaptive responses that shape the composition of the local microbiota. One striking feature of intestinal adaptive immunity is its ability to generate massive amounts of noninflammatory immunoglobulin A (IgA) antibodies through multiple follicular and extrafollicular pathways that operate in the presence or absence of cognate T-B cell interactions. Here we discuss the role of intestinal IgA in host-commensal mutualism, immune protection, and tolerance and summarize recent advances on the role of innate immune cells in intestinal IgA production.

Evolution, development, mechanism and function of IgA in the gut

Since its discovery as the most abundant Ig produced at mucosal surfaces, IgA has been the subject of continuous studies. The concepts emerged were that the precursors for IgA plasma cells are efficiently generated in follicular organized structures in the gut with the help of CD4 T cells and that secretory IgA provides protection against mucosal pathogens. Novel conceptual advances have been made in the past few years in describing new sites, mechanisms and functions of mucosal IgA synthesis.

ROR gamma t is dispensable for the development of intestinal mucosal T cells

To examine the origin of intestinal mucosal T cells and, in particular, unconventional CD8 alpha alpha(+) T cells, we have undertaken a thorough analysis of the gut immune compartment in euthymic and athymic mice carrying either wild-type or mutant transcription factor retinoic acid-related orphan receptor-gamma t (ROR gamma t). We identified a previously unrealized complexity of gut cryptopatch (CP) cells that challenges the previous assertion that CP cells comprise ROR gamma t-expressing adult counterparts of fetal lymphoid tissue inducer (Lti) cells. We showed that many CP cells express intermediate T cell differentiation markers, whether or not they express ROR gamma t, and found that CPs are not completely dependent on ROR gamma t, as previously reported, but merely fewer in number in the ROR gamma t-deficient condition. Indeed, c-kit(+)IL-7R(+)Lin(-)ROR gamma t(-) cells inside the CP and c-kit(+)IL-7R(+)Lin(-)ROR gamma t(-) and c-kit(+)IL-7R(+)Lin(-)ROR gamma t(low) cells outside the CP basically remain in the gut mucosa of ROR gamma t-deficient ROR gamma t(EGFP/EGFP) mice. Consistent with these non-Lti-like c-kit(+)IL-7R(+)Lin(-) cells being gut T cell progenitors, ROR gamma t-deficient mice develop the normal number of intestinal mucosal T cells. These results clearly reassert the intraintestinal differentiation of the body's largest peripheral T cell subpopulation.

Homeostatic chemokines in development, plasticity, and functional organization of the intestinal immune system

In the past decade accumulating evidence supported the view that the immune system should be regarded as trust consisting of several branches. In this review, we will first introduce the architectural features comprising the intestinal immune system emphasising its plasticity and subsequently discuss the concepts describing its development. We then focus on the chemokine/receptor system as a key integrator managing coordinated migration of and communication among the cells mediating intestinal immunity. Thus, chemokines control development and maintain functionality of the intestinal immune system that is required to perform the unique balancing act between tolerating food, curtailing commensals activities and eliminating pathogenic infections.

Villous B cells of the small intestine are specialized for invariant NK T cell dependence

B cells are important in mucosal microbial homeostasis through their well-known role in secretory IgA production and their emerging role in mucosal immunoregulation. Several specialized intraintestinal B cell compartments have been characterized, but the nature of conventional B cells in the lamina propria is poorly understood. In this study, we identify a B cell population predominantly composed of surface IgM(+) IgD(+) cells residing in villi of the small intestine and superficial lamina propria of the large intestine, but distinct from the intraepithelial compartment or organized intestinal lymphoid structures. Small intestinal (villous) B cells are diminished in genotypes that alter the strength of BCR signaling (Bruton tyrosine kinase(xid), Galphai2(-/-)), and in mice lacking cognate BCR specificity. They are not dependent on enteric microbial sensing, because they are abundant in mice that are germfree or genetically deficient in TLR signaling. However, villous B cells are reduced in the absence of invariant NK T cells (Jalpha18(-/-) or CD1d(-/-) mice). These findings define a distinct population of conventional B cells in small intestinal villi, and suggest an immunologic link between CD1-restricted invariant NK T cells and this B cell population.

Intestinal lymphoid tissues: is variety an asset or a liability?

PURPOSE OF REVIEW

Advances in our understanding of lymphoid tissue development has led to an appreciation of the variety of these structures in the intestinal mucosa. This knowledge has translated into more critical analysis of the function of mucosal lymphoid tissues and may lead to manipulation of development of these structures as a therapeutic modality. This review summarizes recent progress in understanding the variety, formation, and function of these structures.

RECENT FINDINGS

The intestinal mucosa and submucosa contain three types of lymphoid tissues ranging from the developmentally determined Peyer's patches to the inflammatory derived tertiary lymphoid tissues. Intermediate to these is a unique lymphoid tissue, isolated lymphoid follicles, which shares properties of both Peyer's patches and tertiary lymphoid tissues. In the healthy intestine, Peyer's patches and isolated lymphoid follicles generate protective and homeostatic immune responses. During chronic inflammation, the function of the more inducible lymphoid tissues, isolated lymphoid follicles and tertiary lymphoid tissues, is unclear, but may include an ominous role propagating inappropriate responses and predisposing to malignancy.

SUMMARY

Understanding the variety of lymphoid tissues, how they function, and how they develop may offer strategies to manipulate these structures to optimize mucosal vaccines and treat intestinal inflammatory diseases.

Ectopic lymphoid tissues and local immunity

Ectopic or tertiary lymphoid tissues develop at sites of inflammation or infection in peripheral, non-lymphoid organs. These tissues are architecturally similar to conventional secondary lymphoid organs, with separated B and T cell areas, specialized populations of dendritic cells, well-differentiated stromal cells and high endothelial venules. Ectopic lymphoid tissues are often associated with the local pathology that results from chronic infection or chronic inflammation. However, there are also examples in which ectopic lymphoid tissues appear to contribute to local protective immune responses. Here we review how ectopic lymphoid structures develop and function in the context of local immunity and pathology.

CD4+CD25+ regulatory T cells in the small intestinal lamina propria show an effector/memory phenotype

CD4(+)CD25(+) regulatory T cells (Tregs) have been implicated in the suppression of pathogenic responses to both self- and non-self-antigens in the intestine. However, their precise properties and functions in the gut, as well as the molecular basis of their recruitment to the gut, are poorly understood. Here, we found that most of the CD4(+)CD25(+) T cells in the small intestinal lamina propria (LP) express Foxp3 and exhibit an 'effector/memory' phenotype, CD44(hi)CD45RB(lo)CD62L(-), whereas only a minority of the Foxp3(+)CD4(+)CD25(+) T cells in the spleen and mesenteric lymph nodes showed this phenotype. The Tregs in the small intestinal LP (LP-Tregs) expressed higher levels of CCR4 and CCR9 and a substantially lower level of CCR7 than the Tregs in the spleen. In vitro, the LP-Tregs showed chemotaxis to CCL25/thymus-expressed chemokine. In addition, they showed efficient chemotaxis to the CCR4 ligands, CCL17/thymus and activation-regulated chemokine and CCL22/macrophage-derived chemokine, which are abundantly expressed by dendritic cells (DCs) in the small intestinal LP. In vivo, approximately 50% of the LP-Tregs were closely associated or in direct contact with LP-DCs. These findings demonstrate that LP-Tregs are phenotypically and functionally unique and raise the possibility that they are retained in the small intestinal LP through the action of CCL17 and CCL22, which are locally produced by LP-DCs.

The immune geography of IgA induction and function

The production of immunoglobulin A (IgA) in mammals exceeds all other isotypes, and it is mostly exported across mucous membranes. The discovery of IgA and the realization that it dominates humoral mucosal immunity, in contrast to the IgG dominance of the systemic immune system, was early evidence for the distinct nature of mucosal immunology. It is now clear that IgA can function in high-affinity modes for neutralization of toxins and pathogenic microbes, and as a low-affinity system to contain the dense commensal microbiota within the intestinal lumen. The basic map of induction of IgA B cells in the Peyer's patches, which then circulate through the lymph and bloodstream to seed the mucosa with precursors of plasma cells that produce dimeric IgA for export through the intestinal epithelium, has been known for more than 30 years. In this review, we discuss the mechanisms underlying selective IgA induction of mucosal B cells for IgA production and the immune geography of their homing characteristics. We also review the functionality of secretory IgA directed against both commensal organisms and pathogens.