Prenatal blockage of lymphotoxin beta receptor and TNF receptor p55 signaling cascade resulted in the acceleration of tissue genesis for isolated lymphoid follicles in the large intestine

Redefining the Role of Lymphotoxin Beta Receptor in the Maintenance of Lymphoid Organs and Immune Cell Homeostasis in Adulthood

Lymphotoxin beta receptor (LTβR) is a promising therapeutic target in autoimmune and infectious diseases as well as cancer. Mice with genetic inactivation of LTβR display multiple defects in development and organization of lymphoid organs, mucosal immune responses, IgA production and an autoimmune phenotype. As these defects are imprinted in embryogenesis and neonate stages, the impact of LTβR signaling in adulthood remains unclear. Here, to overcome developmental defects, we generated mice with inducible ubiquitous genetic inactivation of LTβR in adult mice (iLTβR^Δ/Δ mice) and redefined the role of LTβR signaling in organization of lymphoid organs, immune response to mucosal bacterial pathogen, IgA production and autoimmunity. In spleen, postnatal LTβR signaling is required for development of B cell follicles, follicular dendritic cells (FDCs), recruitment of neutrophils and maintenance of the marginal zone. Lymph nodes of iLTβR^Δ/Δ mice were reduced in size, lacked FDCs, and had disorganized subcapsular sinus macrophages. Peyer`s patches were smaller in size and numbers, and displayed reduced FDCs. The number of isolated lymphoid follicles in small intestine and colon were also reduced. In contrast to LTβR^-/- mice, iLTβR^Δ/Δ mice displayed normal thymus structure and did not develop signs of systemic inflammation and autoimmunity. Further, our results suggest that LTβR signaling in adulthood is required for homeostasis of neutrophils, NK, and iNKT cells, but is dispensable for the maintenance of polyclonal IgA production. However, iLTβR^Δ/Δ mice exhibited an increased sensitivity to C. rodentium infection and failed to develop pathogen-specific IgA responses. Collectively, our study uncovers new insights of LTβR signaling in adulthood for the maintenance of lymphoid organs, neutrophils, NK and iNKT cells, and IgA production in response to mucosal bacterial pathogen.

Berberine augments hypertrophy of colonic patches in mice with intraperitoneal bacterial infection

Colonic patches, the counterparts of Peyer's patches in the small intestine, are dynamically regulated lymphoid tissues in the colon that have an important role in defensing against microbial infections. Berberine is an isoquinoline alkaloid extracted from medicinal herbs including Rhizoma coptidis and has long been used for the treatment of infectious gastroenteritis, but its impact on the colonic lymphoid tissues (such as colonic patches) is unknown. In this study, we aimed to investigate whether berberine had any influences on the colonic patches in mice with bacterial infection. The results showed that oral berberine administration in bacterial infected mice substantially enhanced the hypertrophy of colonic patches, which usually possessed the features of two large B-cell follicles with a separate T-cell area. Moreover, the colonic patches displayed follicular dendritic cell networks within the B-cell follicles, indicative of mature colonic patches containing germinal centers. Concomitant with enlarged colonic patches, the cultured colon of infected mice treated with berberine secreted significantly higher levels of interleukin-1β (IL-1β), IL-6, TNF-α, and CCL-2, while NLRP3 inhibitor MMC950 or knockout of NLRP3 gene abrogated berberine-induced hypertrophy of colonic patches, suggesting the involvement of the NLRP3 signaling pathway in this process. Functionally, oral administration of berberine ameliorated liver inflammation and improved formed feces in the colon. Altogether, these results indicated that berberine was able to augment the hypertrophy of colonic patches in mice with bacterial infection probably through enhancing local inflammatory responses in the colon.

Development and Organization of the Secondary and Tertiary Lymphoid Organs: Influence of Microbial and Food Antigens

Background:

Secondary lymphoid organs (SLO) are distributed in many districts of the body and, especially, lymph nodes, spleen and gut-associated lymphoid tissue are the main cellular sites. On the other hand, tertiary lymphoid organs (TLO) are formed in response to inflammatory, infectious, autoimmune and neoplastic events. </P><P> Developmental Studies: In the present review, emphasis will be placed on the developmental differences of SLO and TLO between small intestine and colon and on the role played by various chemokines and cell receptors. Undoubtedly, microbiota is indispensable for the formation of SLO and its absence leads to their poor formation, thus indicating its strict interaction with immune and non immune host cells. Furthermore, food antigens (for example, tryptophan derivatives, flavonoids and byphenils) bind the aryl hydrocarbon receptor on innate lymphoid cells (ILCs), thus promoting the development of postnatal lymphoid tissues. Also retinoic acid, a metabolite of vitamin A, contributes to SLO development during embryogenesis. Vitamin A deficiency seems to account for reduction of ILCs and scarce formation of solitary lymphoid tissue. </P><P> Translational Studies: The role of lymphoid organs with special reference to intestinal TLO in the course of experimental and human disease will also be discussed. </P><P> Future Perspectives: Finally, a new methodology, the so-called “gut-in-a dish”, which has facilitated the in vitro interaction study between microbe and intestinal immune cells, will be described.

Spleen development is modulated by neonatal gut microbiota

The full development of the mammalian immune system occurs after birth upon exposure to non self-antigens. The gut is the first site of bacterial colonization where it is crucial to create the appropriate microenvironment able to balance effector or tolerogenic responses to external stimuli. It is a well-established fact that at mucosal sites bacteria play a key role in developing the immune system but we ignore how colonising bacteria impact the maturation of the spleen. Here we addressed this issue. Taking advantage of the fact that milk SIgA regulates bacterial colonization of the newborn intestine, we generated immunocompetent mice born either from IgA pro-efficient or IgA deficient females. Having demonstrated that SIgA in maternal milk modulates neonatal gut microbiota by promoting an increased diversity of the colonizing species we also found that immunocompetent pups, not exposed to milk SIgA, fail to properly develop the FDC network and primary follicles in the spleen compromising the response to T-dependent antigens. The presence of a less diverse microbiota with a higher representation of pathogenic species leads to a fast replenishment of the marginal zone and the IgM plasma cell compartment of the spleen as well as IgA plasma cells in the gut.

Stromal Fibroblasts in Tertiary Lymphoid Structures: A Novel Target in Chronic Inflammation

Tertiary lymphoid structures (TLS) are organized aggregates of lymphocytes, myeloid, and stromal cells that provide ectopic hubs for acquired immune responses. TLS share phenotypical and functional features with secondary lymphoid organs (SLO); however, they require persistent inflammatory signals to arise and are often observed at target sites of autoimmune disease, chronic infection, cancer, and organ transplantation. Over the past 10 years, important progress has been made in our understanding of the role of stromal fibroblasts in SLO development, organization, and function. A complex and stereotyped series of events regulate fibroblast differentiation from embryonic life in SLOs to lymphoid organ architecture observed in adults. In contrast, TLS-associated fibroblasts differentiate from postnatal, locally activated mesenchyme, predominantly in settings of inflammation and persistent antigen presentation. Therefore, there are critical differences in the cellular and molecular requirements that regulate SLO versus TLS development that ultimately impact on stromal and hematopoietic cell function. These differences may contribute to the pathogenic nature of TLS in the context of chronic inflammation and malignant transformation and offer a window of opportunity for therapeutic interventions in TLS associated pathologies.

Development and Function of Secondary and Tertiary Lymphoid Organs in the Small Intestine and the Colon

The immune system of the gut has evolved a number of specific lymphoid structures that contribute to homeostasis in the face of microbial colonization and food-derived antigenic challenge. These lymphoid organs encompass Peyer’s patches (PP) in the small intestine and their colonic counterparts that develop in a programed fashion before birth. In addition, the gut harbors a network of lymphoid tissues that is commonly designated as solitary intestinal lymphoid tissues (SILT). In contrast to PP, SILT develop strictly after birth and consist of a dynamic continuum of structures ranging from small cryptopatches (CP) to large, mature isolated lymphoid follicles (ILF). Although the development of PP and SILT follow similar principles, such as an early clustering of lymphoid tissue inducer (LTi) cells and the requirement for lymphotoxin beta (LTβ) receptor-mediated signaling, the formation of CP and their further maturation into ILF is associated with additional intrinsic and environmental signals. Moreover, recent data also indicate that specific differences exist in the regulation of ILF formation between the small intestine and the colon. Importantly, intestinal inflammation in both mice and humans is associated with a strong expansion of the lymphoid network in the gut. Recent experiments in mice suggest that these structures, although they resemble large, mature ILF in appearance, may represent de novo-induced tertiary lymphoid organs (TLO). While, so far, it is not clear whether intestinal TLO contribute to the exacerbation of inflammatory pathology, it has been shown that ILF provide the critical microenvironment necessary for the induction of an effective host response upon infection with enteric bacterial pathogens. Regarding the importance of ILF for intestinal immunity, interfering with the development and maturation of these lymphoid tissues may offer novel means for manipulating the immune response during intestinal infection or inflammation.

Epithelial Control of Gut-Associated Lymphoid Tissue Formation through p38α-Dependent Restraint of NF-κB Signaling

The protein kinase p38α mediates cellular responses to environmental and endogenous cues that direct tissue homeostasis and immune responses. Studies of mice lacking p38α in several different cell types have demonstrated that p38α signaling is essential to maintaining the proliferation-differentiation balance in developing and steady-state tissues. The mechanisms underlying these roles involve cell-autonomous control of signaling and gene expression by p38α. In this study, we show that p38α regulates gut-associated lymphoid tissue (GALT) formation in a noncell-autonomous manner. From an investigation of mice with intestinal epithelial cell-specific deletion of the p38α gene, we find that p38α serves to limit NF-κB signaling and thereby attenuate GALT-promoting chemokine expression in the intestinal epithelium. Loss of this regulation results in GALT hyperplasia and, in some animals, mucosa-associated B cell lymphoma. These anomalies occur independently of luminal microbial stimuli and are most likely driven by direct epithelial-lymphoid interactions. Our study illustrates a novel p38α-dependent mechanism preventing excessive generation of epithelial-derived signals that drive lymphoid tissue overgrowth and malignancy.

The Gut-Associated Lymphoid Tissues in the Small Intestine, Not the Large Intestine, Play a Major Role in Oral Prion Disease Pathogenesis

Prion diseases are infectious neurodegenerative disorders characterized by accumulations of abnormally folded cellular prion protein in affected tissues. Many natural prion diseases are acquired orally, and following exposure, the early replication of some prion isolates upon follicular dendritic cells (FDC) within gut-associated lymphoid tissues (GALT) is important for the efficient spread of disease to the brain (neuroinvasion). Prion detection within large intestinal GALT biopsy specimens has been used to estimate human and animal disease prevalence. However, the relative contributions of the small and large intestinal GALT to oral prion pathogenesis were unknown. To address this issue, we created mice that specifically lacked FDC-containing GALT only in the small intestine. Our data show that oral prion disease susceptibility was dramatically reduced in mice lacking small intestinal GALT. Although these mice had FDC-containing GALT throughout their large intestines, these tissues were not early sites of prion accumulation or neuroinvasion. We also determined whether pathology specifically within the large intestine might influence prion pathogenesis. Congruent infection with the nematode parasite Trichuris muris in the large intestine around the time of oral prion exposure did not affect disease pathogenesis. Together, these data demonstrate that the small intestinal GALT are the major early sites of prion accumulation and neuroinvasion after oral exposure. This has important implications for our understanding of the factors that influence the risk of infection and the preclinical diagnosis of disease.

IMPORTANCE

Many natural prion diseases are acquired orally. After exposure, the accumulation of some prion diseases in the gut-associated lymphoid tissues (GALT) is important for efficient spread of disease to the brain. However, the relative contributions of GALT in the small and large intestines to oral prion pathogenesis were unknown. We show that the small intestinal GALT are the essential early sites of prion accumulation. Furthermore, congruent infection with a large intestinal helminth (worm) around the time of oral prion exposure did not affect disease pathogenesis. This is important for our understanding of the factors that influence the risk of prion infection and the preclinical diagnosis of disease. The detection of prions within large intestinal GALT biopsy specimens has been used to estimate human and animal disease prevalence. However, our data suggest that using these biopsy specimens may miss individuals in the early stages of oral prion infection and significantly underestimate the disease prevalence.

Reciprocal regulation of lymphoid tissue development in the large intestine by IL-25 and IL-23

Isolated lymphoid follicles (ILFs) develop after birth in the small and large intestines (SI and LI) and represent a dynamic response of the gut immune system to the microbiota. Despite their similarities, ILF development in the SI and LI differs on a number of levels. We show that unlike ILF in the SI, the microbiota inhibits ILF development in the colon as conventionalization of germ-free mice reduced colonic ILFs. From this, we identified a novel mechanism regulating colonic ILF development through the action of interleukin (IL)-25 on IL-23 and its ability to modulate T regulatory cell (Treg) differentiation. Colonic ILF develop in the absence of a number of factors required for the development of their SI counterparts and can be specifically suppressed by factors other than IL-25. However, IL-23 is the only factor identified that specifically promotes colonic ILFs without affecting SI-ILF development. Both IL-23 and ILFs are associated with inflammatory bowel disease, suggesting that disruption to this pathway may have an important role in the breakdown of microbiota-immune homeostasis.

Examination of the role of galectins in intestinal inflammation

The intestine, which provides the first line of defense against over trillion of enteric microorganisms, suffers from broad range of inflammatory conditions caused by infectious, autoimmune, allergic, neurological, and ischemic mechanisms. Recent data have suggested dual roles (protective versus deleterious) for galectins in the pathogenesis of some intestinal inflammations, highlighting the importance of this area of research. A potential problem with the research of intestinal inflammation may be the requirement of some unique techniques. Therefore, we herein describe how to induce intestinal inflammation and how to isolate lymphocyte, myeloid cell, follicular cell, and epithelial cell populations separately from the intestine for the study of intestinal inflammations.

Colonic patch and colonic SILT development are independent and differentially regulated events

Intestinal lymphoid tissues have to simultaneously ensure protection against pathogens and tolerance toward commensals. Despite such vital functions, their development in the colon is poorly understood. Here, we show that the two distinct lymphoid tissues of the colon-colonic patches and colonic solitary intestinal lymphoid tissues (SILTs)-can easily be distinguished based on anatomical location, developmental timeframe, and cellular organization. Furthermore, whereas colonic patch development depended on CXCL13-mediated lymphoid tissue inducer (LTi) cell clustering followed by LTα-mediated consolidation, early LTi clustering at SILT anlagen did not require CXCL13, CCR6, or CXCR3. Subsequent dendritic cell recruitment to and gp38(+)VCAM-1(+) lymphoid stromal cell differentiation within SILTs required LTα; B-cell recruitment and follicular dendritic cell differentiation depended on MyD88-mediated signaling, but not the microflora. In conclusion, our data demonstrate that different mechanisms, mediated mainly by programmed stimuli, induce the formation of distinct colonic lymphoid tissues, therefore suggesting that these tissues may have different functions.

Distinct developmental requirements for isolated lymphoid follicle formation in the small and large intestine: RANKL is essential only in the small intestine

Cryptopatches (CPs) and isolated lymphoid follicles (ILFs) are organized intestinal lymphoid tissues that develop postnatally in mice and include stromal cells expressing the receptor activator of nuclear factor kappa-B ligand (RANKL). We investigated how stromal RANKL influences the development and differentiation of CPs and ILFs by analyzing the development of these lymphoid structures in knockout mice lacking RANKL. We found that RANKL(-/-) mice had a fourfold reduction in the overall density of CPs in the small intestine compared to control mice, with the largest decrease in the proximal small intestine. No B cells were present in CPs from the small intestine of RANKL(-/-) mice and ILF formation was completely blocked. In sharp contrast, colonic ILFs containing B cells were present in RANKL(-/-) mice. Stromal cells within CPs in the small intestine of RANKL(-/-) mice did not express CXCL13 (originally called B lymphocyte chemoattractant) and often lacked other normally expressed stromal cell antigens, whereas colonic lymphoid aggregates in RANKL(-/-) mice retained stromal CXCL13 expression. The CXCL13-dependent maturation of precursor CPs into ILFs is differentially regulated in the small intestine and colon, with an absolute requirement for RANKL only in the small intestine.

Orally administered Bifidobacterium triggers immune responses following capture by CD11c(+) cells in Peyer's patches and cecal patches

We have investigated the immunomodulatory mechanisms of Bifidobacterium pseudocatenulatum JCM7041 (Bp) as model of probiotics following oral administration to mice. This study was conducted with the aim of clarifying the mechanism of immunomodulation induced by oral administration of probiotic bacteria through elucidation of the detailed mechanism of transfer of orally administered bacterial cells within the body and the interaction between bacterial cells and cells of the immune tissues. We observed the localization of Bp in mice following oral administration, showing that Bp was surrounded by CD11c(+) cells in Peyer's patches (PP) and cecal patches (CP). These results indicated that Bp might induce CD11c(+) cell-mediated immune responses directly. Furthermore, IL-10 and IL-12p40 production by Thy1.2(-) cells, including CD11c(+) cells, increased significantly. Production of IL-10 and IL-12p40 by bone marrow-derived dendritic cells (BMDC) was significantly increased by Bp stimulation. These results suggest that oral administration of Bp induces immune responses directly following capture by CD11c(+) dendritic cells (DCs). Subsequently, we observed oral administration of Bp for 1 week induced IgA and IgA-associated cytokine production by CP and PP cells, suggesting that Bp induced DC-mediated immune responses on CP as well as PP.

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.

Rapid dendritic cell mobilization to the large intestinal epithelium is associated with resistance to Trichuris muris infection

The large intestine is a major site of infection and disease, yet little is known about how immunity is initiated within this site and the role of dendritic cells (DCs) in this process. We used the well-established model of Trichuris muris infection to investigate the innate response of colonic DCs in mice that are inherently resistant or susceptible to infection. One day postinfection, there was a significant increase in the number of immature colonic DCs in resistant but not susceptible mice. This increase was sustained at day 7 postinfection in resistant mice when the majority of the DCs were mature. There was no increase in DC numbers in susceptible mice until day 13 postinfection. In resistant mice, most colonic DCs were located in or adjacent to the epithelium postinfection. There were also marked differences in the expression of colonic epithelial chemokines in resistant mice and susceptible mice. Resistant mice had significantly increased levels of epithelium-derived CCL2, CCL3, CCL5, and CCL20 compared with susceptible mice. Furthermore, administering neutralizing CCL5 and CCL20 Abs to resistant mice prevented DC recruitment. This study provides clear evidence of differences in the kinetics of DC responses in hosts inherently resistant and susceptible to infection. DC responses in the colon correlate with resistance to infection. Differences in the production of DC chemotactic chemokines by colonic epithelial cells in response to infection in resistant vs susceptible mice may explain the different kinetics of the DC response.

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.

Development of secondary lymphoid organs

Secondary lymphoid organs develop during embryogenesis or in the first few weeks after birth according to a highly coordinated series of interactions between newly emerging hematopoietic cells and immature mesenchymal or stromal cells. These interactions are orchestrated by homeostatic chemokines, cytokines, and growth factors that attract hematopoietic cells to sites of future lymphoid organ development and promote their survival and differentiation. In turn, lymphotoxin-expressing hematopoietic cells trigger the differentiation of stromal and endothelial cells that make up the scaffolding of secondary lymphoid organs. Lymphotoxin signaling also maintains the expression of adhesion molecules and chemokines that govern the ultimate structure and function of secondary lymphoid organs. Here we describe the current paradigm of secondary lymphoid organ development and discuss the subtle differences in the timing, molecular interactions, and cell types involved in the development of each secondary lymphoid organ.

Colonic patches direct the cross-talk between systemic compartments and large intestine independently of innate immunity

Although the mucosal and the systemic immune compartments are structurally and functionally independent, they engage in cross-talk under specific conditions. To investigate this cross-talk, we vaccinated mice with tetanus toxoid together with cholera toxin with s.c. priming followed by intrarectal (IR) boosting. Interestingly, higher numbers of Ag-specific IgA and IgG Ab-secreting cells (ASCs) were detected in the lamina propria of the large intestine of mice vaccinated s.c.-IR. Ag-specific ASCs from the colon migrated to SDF-1alpha/CXCL12 and mucosae-associated epithelial chemokine/CCL28, suggesting that CXCR4(+) and/or CCR10(+) IgA ASCs found in the large intestine after s.c.-IR are of systemic origin. In the colonic patches-null mice, IgA ASCs in the large intestine were completely depleted. Furthermore, the accumulation of IgA ASCs in the colonic patches by inhibition of their migration with FTY720 revealed that colonic patches are the IgA class-switching site after s.c.-IR. Most interestingly, s.c.-IR induced numbers of Ag-specific IgA ASCs in the large intestine of TLR2(-/-), TLR4(-/-), MyD88(-/-), and TRIF(-/-) mice that were comparable with those of wild-type mice. Taken together, our results suggest the possibility that cross-talk could occur between the large intestine and the systemic immune compartments via the colonic patches without the assistance of innate immunity.

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.

CC chemokine receptor 6 expression by B lymphocytes is essential for the development of isolated lymphoid follicles

Isolated lymphoid follicles (ILFs) are organized lymphoid structures that facilitate the efficient interaction of antigen, antigen-presenting cells, and lymphocytes to generate controlled adaptive immune responses within the intestine. Because CC chemokine receptor 6 (CCR6) deficiency affects the generation of mucosal immune responses, we evaluated a potential role for CCR6 in the development of ILFs. We observed that CCR6 and its ligand CCL20 are highly expressed within ILFs and that B lymphocytes are the largest CCR6-expressing population within ILFs. ILF development was profoundly arrested in the absence of CCR6. Concordant with a block in ILF development at a stage corresponding to the influx of B lymphocytes, we observed that CCR6-deficient mice had a diminished population of intestinal B lymphocytes. Bone marrow reconstitution studies demonstrated that ILF development is dependent on CCR6-sufficient B lymphocytes, and adoptive transfers demonstrated that CCR6(-/-) B lymphocytes were inefficient at localizing to intestinal lymphoid structures. Paralleling these findings, we observed that CCR6-deficient mice had a reduced proportion of Peyer's patch B lymphocytes and an associated re-duction in the number and size of Peyer's patch follicular domes. These observations define an essential role for CCR6 expression by B lymphocytes in localizing to intestinal lymphoid structures and in ILF development.

Regulatory role of Peyer’s patches for the inhibition of OVA-induced allergic diarrhea

Intestinal allergic diseases are initiated by aberrant Th2-type immune responses, including elevation of IgE antibodies (Abs) and infiltration of eosinophils. However, little is known about the role of Peyer's patches (PP) in the control of allergic diseases. Using a mouse model for food allergy, we here show that mice lacking PP are more susceptible to disease development and show higher levels of antigen-specific IgE Abs than do PP-intact mice. In our study, we noted that high numbers of eosinophils infiltrated into the small intestine of PP-null mice. In contrast, the PP of intact mice contained regulatory CD4+CD25+ Foxp3+ T cells (Treg) that are known to produce high levels of IL-10, and inhibited the development of allergic diarrhea. PP-intact mice thus developed allergic diarrhea when treated with anti-CD25 or anti-IL-10 monoclonal antibody (mAb) in vivo. These studies demonstrate that PP, as the site where IL-10-producing Treg cells are created, mediate the mucosal regulatory network for the control of undesired allergic responses in the intestine.

Immunogenicity and protective efficacy offered by a ribosomal-based vaccine from Shigella flexneri 2a

Shigellosis is a major form of bacillary dysentery caused by Shigella infection. Shigella ribosome-based vaccines (SRV), considered among the potent vaccine candidates, are composed of O-antigen and ribosome isolated from S. flexneri 2a. To investigate the immunogenicity and protective efficacy of SRV, mice were vaccinated with SRV via the intranasal (i.n.) route. Interestingly, robust levels of Shigella-derived LPS-specific IgG and IgA Abs and antibody-forming cells were elicited in systemic and mucosal compartments following two i.n. administrations of SRV. Groups of mice receiving i.n. SRV developed milder pulmonary pneumonia upon challenge with virulent S. flexneri 2a than did those receiving parenteral SRV. We further found that the MyD88-dependent TLR2 signal partially mediates SRV-induced mucosal immunity, with the exception of TLR4- and TLR5-governed innate immunity. Most importantly, polymeric immunoglobulin receptor knockout (pIgR-/-) mice, which lack secretory IgA Ab, were afforded less protective efficacy than were wild-type mice. It can be concluded then that SRV is immunogenic and provides protective efficacy in mice. It can also be surmised that a mucosal SRV vaccine would be particularly relevant in targeting shigellosis, which provokes inflammation in the human colon.

New Animal Model of Shigellosis in the Guinea Pig: Its Usefulness for Protective Efficacy Studies

It has been difficult to evaluate the protective efficacy of vaccine candidates against shigellosis, a major form of bacillary dysentery caused by Shigella spp. infection, because of the lack of suitable animal models. To develop a proper animal model representing human bacillary dysentery, guinea pigs were challenged with virulent Shigella flexneri serotype 2a (strains 2457T or YSH6000) or S. flexneri 5a (strain M90T) by the intrarectal (i.r.) route. Interestingly, all guinea pigs administered these Shigella strains developed severe and acute rectocolitis. They lost approximately 20% of their body weight and developed tenesmus by 24 h after Shigella infection. Shigella invasion and colonization of the distal colon were seen at 24 h but disappeared by 48 h following i.r. infection. Histopathological approaches demonstrated significant damage and destruction of mucosal and submucosal layers, thickened intestinal wall, edema, erosion, infiltration of neutrophils, and depletion of goblet cells in the distal colon. Furthermore, robust expression of IL-8, IL-1beta, and inducible NO synthase mRNA was detected in the colon from 6 to 24 h following Shigella infection. Most importantly, in our new shigellosis model, guinea pigs vaccinated with an attenuated S. flexneri 2a SC602 strain possessing high levels of mucosal IgA Abs showed milder symptoms of bacillary dysentery than did animals receiving PBS alone after Shigella infection. In the guinea pig, administration of Shigella by i.r. route induces acute inflammation, making this animal model useful for assessing the protective efficacy of Shigella vaccine candidates.

Isolated lymphoid follicle maturation induces the development of follicular dendritic cells

Isolated lymphoid follicles (ILFs) are recently identified lymphoid structures in the small intestine with features similar to Peyer's patches (PPs). Using immunohistochemistry we characterized the composition of ILFs in the small intestines of immunocompetent mice and of mice that lacked PPs as a result of either genetic deficiency of lymphotoxin or temporary in utero lymphotoxin-beta receptor-signalling blockade. We showed that although both immature and mature ILFs were present in the intestines of immunocompetent mice, PP-deficiency induced a significantly greater number of mature ILFs. We found that in addition to B-lymphocyte-containing germinal centres, mature ILFs also possessed large networks of follicular dendritic cells (FDCs). These features were not detected within immature ILFs. Indeed, the presence of FDCs could be used to reliably distinguish ILF maturity. Further analysis revealed that the area occupied by the FDCs within mature ILFs was substantial. The total area occupied by FDCs in all the mature ILFs in mice lacking PPs was equivalent to the total area occupied by FDCs in all the PPs and the few mature ILFs in immunocompetent mice. Based on these data we reasoned that in the absence of PPs, mature ILFs are important inductive sites for intestinal immune responses. Indeed, in mice that lacked PPs, ILF maturation coincided with a restoration of faecal immunoglobulin A levels to values that were comparable to those found in immunocompetent mice. Taken together, these data imply that the induction of germinal centres and FDC networks within mature ILFs in response to PP deficiency provides an important compensatory mechanism.

Molecular Networks Orchestrating GALT Development

During evolution, the development of secondary lymphoid organs has evolved as a strategy to promote adaptive immune responses at sites of antigen sequestration. Mesenteric lymph nodes (LNs) and Peyer's patches (PPs) are localized in proximity to mucosal surfaces, and their development is coordinated by a series of temporally and spatially regulated molecular events involving the collaboration between hematopoietic, mesenchymal, and, for PPs, epithelial cells. Transcriptional control of cellular differentiation, production of cytokines as well as adhesion molecules are mandatory for organogenesis, recruitment of mature leukocytes, and lymphoid tissue organization. Similar to fetal and neonatal organogenesis, lymphoid tissue neoformation can occur in adult individuals at sites of chronic stimulation via cytokines and TNF-family member molecules. These molecules represent new therapeutic targets to manipulate the microenvironment during autoimmune diseases.

Murine isolated lymphoid follicles contain follicular B lymphocytes with a mucosal phenotype

Isolated lymphoid follicles (ILFs) are organized intestinal lymphoid structures whose formation can be induced by luminal stimuli. ILFs have been demonstrated to act as inductive sites for the generation of immune responses directed toward luminal stimuli; however, the phenotype of the immune response initiated within ILFs has largely been uninvestigated. To gain a better understanding of the immune responses initiated within ILFs, we examined phenotypic and functional aspects of the largest cellular component of the murine ILF lymphocyte population, B lymphocytes. We observed that murine ILF B lymphocytes are composed of a relatively homogenous population of follicular B-2 B lymphocytes. Consistent with their proximity to multiple stimuli, ILF B lymphocytes displayed a more activated phenotype compared with their counterparts in the spleen and Peyer's patch (PP). ILF B lymphocytes also expressed higher levels of immunomodulatory B7 and CD28 family members B7X and programmed death-1 compared with their counterparts in the spleen and PP. ILF B lymphocytes preferentially differentiate into IgA-producing plasma cells and produce more IL-4 and IL-10 and less interferon-gamma compared with their counterparts in the spleen. Immunoglobulin repertoire analysis from individual ILFs demonstrated that ILFs contain a polyclonal population of B lymphocytes. These findings indicate that murine ILFs contain a polyclonal population of follicular B-2 B lymphocytes with a phenotype similar to PP B lymphocytes and that, in unchallenged animals, ILFs promote immune responses with a homeostatic phenotype.

The characterization of intraepithelial lymphocytes, lamina propria leukocytes, and isolated lymphoid follicles in the large intestine of mice infected with the intestinal nematode parasite Trichuris muris

Despite a growing understanding of the role of cytokines in immunity to the parasitic helminth Trichuris muris, the local effector mechanism culminating in the expulsion of worms from the large intestine is not known. We used flow cytometry and immunohistochemistry to characterize the phenotype of large intestinal intraepithelial lymphocytes (IEL) and lamina propria leukocytes (LPL) from resistant and susceptible strains of mouse infected with T. muris. Leukocytes accumulated in the epithelium and lamina propria after infection, revealing marked differences between the different strains of mouse. In resistant mice, which mount a Th2 response, the number of infiltrating CD4+, CD8+, B220+, and F4/80+ IEL and LPL was generally highest around the time of worm expulsion from the gut, at which point the inflammation was dominated by CD4+ IEL and F4/80+ LPL. In contrast, in susceptible mice, which mount a Th1 response, the number of IEL and LPL increased more gradually and was highest after a chronic infection had developed. At this point, CD8+ IEL and F4/80+ LPL were predominant. Therefore, this study reveals the local immune responses underlying the expulsion of worms or the persistence of a chronic infection in resistant and susceptible strains of mouse, respectively. In addition, for the first time, we illustrate isolated lymphoid follicles in the large intestine, consisting of B cells interspersed with CD4+ T cells and having a central zone of rapidly proliferating cells. Furthermore, we demonstrate the organogenesis of these structures in response to T. muris infection.