The immune geography of IgA induction and function

Mucosal control of the intestinal microbial community

Although the knowledge of the effects of bacterial colonization on the immune system is rapidly expanding, surprisingly little is known about the immunological mechanisms that shape the intestinal microbial community. Specifically, the complexity of the intestinal microbiota and what constitutes a "healthy" microbial composition has only recently been addressed, facilitated by large-scale metagenomic screens. Containment of such a vast number of different microbes requires tight regulation at the mucosal surface. While beneficial relationships must not be compromised, invading pathogenic bacteria must be dealt with in order to maintain homeostasis. In this review, we will address the latest insights into the role of the mucosal immune system in the control of the microbiota.

Immunohistochemical characterization of selected cell markers for the detection of hematopoietic cells in formalin-fixed, paraffin wax-embedded lymphoid tissues of harbor seals (Phoca vitulina) and walruses (Odobenus rosmarus rosmarus)

To facilitate a detailed investigation of pinniped lymphoid organs, 30 monoclonal antibodies (mAb) as well as eight polyclonal antibodies (pAb) of different species specificities directed against cell antigens of the hematopoietic system were tested for immunohistochemical cross-reactivity on formalin-fixed, paraffin wax-embedded tissues of harbor seals (Phoca vitulina) and a walrus (Odobenus rosmarus rosmarus). Six monoclonal and eight polyclonal antibodies showed specific immunoreactivities. Lymphocytes were immunolabeled by an anti-CD3 pAb, anti-Foxp3 mAb and anti-CD79 alpha mAb, while plasma cell subpopulations were recognized by anti-IgA pAb, anti-IgG pAb and anti-IgM pAb as well as by anti-kappa- and anti-lambda light chain pAb. Cells of the histiocytic lineage were recognized by lysozyme-, myeloid/histiocyte antigen-, and CD68-specific markers. Furthermore, dendritic cell-like cells were detected by an anti-S100 protein pAb. The MHC class II antigen was labeled on the majority of immune cells of the harbor seal and walrus using a bovine mAb. Mast cells were stained by an anti-mast cell tryptase mAb. Thus, using these antibodies from various species, it is now possible to determine phenotypical changes in lymphoid organs and detect different leukocyte subsets involved in inflammatory responses in archived tissue samples of these pinniped species.

Probiotic bacteria and intestinal epithelial barrier function

The intestinal tract is a diverse microenvironment where more than 500 species of bacteria thrive. A single layer of epithelium is all that separates these commensal microorganisms and pathogens from the underlying immune cells, and thus epithelial barrier function is a key component in the arsenal of defense mechanisms required to prevent infection and inflammation. The epithelial barrier consists of a dense mucous layer containing secretory IgA and antimicrobial peptides as well as dynamic junctional complexes that regulate permeability between cells. Probiotics are live microorganisms that confer benefit to the host and that have been suggested to ameliorate or prevent diseases including antibiotic-associated diarrhea, irritable bowel syndrome, and inflammatory bowel disease. Probiotics likely function through enhancement of barrier function, immunomodulation, and competitive adherence to the mucus and epithelium. This review summarizes the evidence about effects of the many available probiotics with an emphasis on intestinal barrier function and the mechanisms affected by probiotics.

Origin and functional heterogeneity of non-lymphoid tissue dendritic cells in mice

Dendritic cells (DCs) have been extensively studied in mice lymphoid organs, but less is known about the origin and the mechanisms that regulate DC development and function in non-lymphoid tissues. Here, we discuss recent evidence establishing the contribution of the DC-restricted lineage to the non-lymphoid tissue DC pool and discuss the mechanisms that control the homeostasis of non-lymphoid tissue DCs. We also review recent results underlining the functional specialization of tissue DCs and discuss the potential implications of these findings in tissue immunity and in the development of novel vaccine strategies.

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.

Bronchial epithelial cells produce IL-5: implications for local immune responses in the airways

IL-5 is a pleiotropic cytokine that promotes eosinophil differentiation and survival. While naïve bronchial epithelial cells (BEC) produce low levels of IL-5, the role of BEC-derived IL-5 in allergic airway inflammation is unknown. We now show that BEC, isolated from mice with OVA-induced allergic airway disease (AAD), produced elevated levels of IL-5 mRNA and protein as compared to BEC from naïve mice. To determine the contribution of BEC-derived IL-5 to effector responses in the airways, IL-5 deficient bone marrow chimeric mice were generated in which IL-5 expression was restricted to stromal (e.g. BEC) or hematopoietic cells. When subjected to AAD, IL-5 produced by BECs contributed to mucous metaplasia, airway eosinophilia, and OVA-specific IgA levels. Thus, IL-5 production by BEC can impact the microenvironment of the lung, modifying pathologic and protective immune responses in the airways.

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.

Multiple facets of intestinal permeability and epithelial handling of dietary antigens

The intestinal epithelium, the largest interface between the host and environment, regulates fluxes of ions and nutrients and limits host contact with the massive load of luminal antigens. Local protective and tolerogenic immune responses toward luminal content depend on antigen sampling by the gut epithelial layer. Whether, and how exaggerated, the entrance of antigenic macromolecules across the gut epithelium might initiate and/or perpetuate chronic inflammation as well as the respective contribution of paracellular and transcellular permeability remains a matter of debate. To this extent, experimental studies involving the in vivo assessment of intestinal permeability using small inert molecules do not necessarily correlate with the uptake of larger dietary antigens. This review analyzes both the structural and functional aspects of intestinal permeability with special emphasis on antigen handling in healthy and diseased states and consequences on local immune responses to food antigens.

Hypersensitivity and oral tolerance in the absence of a secretory immune system

BACKGROUND

Mucosal immunity protects the epithelial barrier by immune exclusion of foreign antigens and by anti-inflammatory tolerance mechanisms, but there is a continuing debate about the role of secretory immunoglobulins (SIgs), particularly SIgA, in the protection against allergy and other inflammatory diseases. Lack of secretory antibodies may cause immune dysfunction and affect mucosally induced (oral) tolerance against food antigens.

METHODS

We used polymeric Ig receptor (pIgR) knockout (KO) mice, which cannot export SIgA or SIgM, to study oral tolerance induction by ovalbumin (OVA) feeding and for parenteral antigen sensitization in the same animal.

RESULTS

Remarkable systemic hyperreactivity was observed in pIgR KO mice, as 50% died after intradermal OVA challenge, which was not seen in similarly sensitized and challenged wild-type (WT) mice. Oral tolerance induced by OVA completely protected the sensitized pIgR KO mice against anaphylaxis and suppressed antibody levels (particularly IgG1) as well as delayed-type hypersensitivity (DTH) to OVA. Delayed-type hypersensitivity to a bystander antigen, human serum albumin, was also suppressed and T-cell proliferation against OVA in vitro was reduced in tolerized compared with non-tolerized pIgR KO mice. This effect was largely mediated by CD25+ T cells. Adoptive transfer of splenic putative regulatory T cells (CD4+ CD25+) obtained from OVA-fed pIgR KO mice to naïve WT mice mediated suppression of DTH against OVA after sensitization of the recipients.

CONCLUSION

Compensatory regulatory T-cell function becomes critical in pIgR-deficient mice to avoid the potentially catastrophic effects of systemic immune hyperreactivity, presumably resulting from defective secretory antibody-mediated immune exclusion of microbial components.

Clinical consequences of defects in B-cell development

Abnormalities in humoral immunity typically reflect a generalized or selective failure of effective B-cell development. The developmental processes can be followed through analysis of cell-surface markers, such as IgM, IgD, CD10, CD19, CD20, CD21, and CD38. Early phases of B-cell development are devoted to the creation of immunoglobulin and testing of B-cell antigen receptor signaling. Failure leads to the absence of B cells and immunoglobulin in the blood from birth. As the developing B cells begin to express a surface B-cell receptor, they become subject to negative and positive selection pressures and increasingly depend on survival signals. Defective signaling can lead to selective or generalized hypogammaglobulinemia, even in the presence of normal numbers of B cells. In the secondary lymphoid organs some B cells enter the splenic marginal zone, where preactivated cells lie ready to rapidly respond to T-independent antigens, such as the polysaccharides that coat some microorganisms. Other cells enter the follicle and, with the aid of cognate follicular T cells, divide to help form a germinal center (GC) after their interaction with antigen. In the GC B cells can undergo the processes of class switching and somatic hypermutation. Failure to properly receive T-cell signals can lead to hyper-IgM syndrome. B cells that leave the GC can develop into memory B cells, short-lived plasma cells, or long-lived plasma cells. The latter ultimately migrate back to the bone marrow, where they can continue to produce protective antigen-specific antibodies for decades.

Mucosal immunology, eosinophilic esophagitis, and other intestinal inflammatory diseases

The gastrointestinal mucosa constitutes the largest host-environment interface of the body. It uses both innate and adaptive immune mechanisms to provide protection from the diverse onslaught of foods, microbes, and other ingested products. The innate immune system is genetically encoded and evolutionarily ancient, possesses no memory, and lacks diversity. In contrast, the adaptive immune system is quite diverse, develops memory, and undergoes expansion after stimulation. The gastrointestinal mucosa is charged with the difficult task of mounting protective responses against invading microorganisms while simultaneously maintaining an overall state of nonresponsiveness or tolerance to innocuous substances, such as commensal bacteria and food antigens. Perturbation or malfunction of these complex protective mechanisms results in diseases, such as inflammatory bowel diseases, celiac disease, or eosinophilic gastrointestinal diseases.

Recognition of secretory IgA by DC-SIGN: implications for immune surveillance in the intestine

Secretory IgA (SIgA), the predominant class of antibody in intestinal secretions, serves as the first line of defense against enteric infections. SIgA has also been proposed to function in immune surveillance, given that both SIgA and SIgA-antigen complexes are actively transported by Peyer's patch M cells from the intestinal lumen to sub-epithelial dendritic cells (DCs). The goal of the present study was to identify the receptor(s) potentially utilized by mucosal DCs to recognize and internalize SIgA. We demonstrate that human colostral SIgA is recognized by purified recombinant human DC-specific ICAM-3 grabbing nonintegrin (DC-SIGN) in a solid phase binding assay, as well as by DC-SIGN ectopically expressed on the surface of Chinese hamster ovary (CHO-S) cells. The interaction between SIgA and DC-SIGN was specific, given that it was Ca(2+)-dependent and inhibited by mannan. Moreover, SIgA bound to, and was internalized by, endogenous DC-SIGN expressed on THP-1 cells following monocyte to macrophage-like cell differentiation by stimulation with phorbol ester and interleukin-4. These data identify DC-SIGN as a putative receptor for SIgA, and reveal a mechanism by which DCs could collaborate with M cells in immune surveillance at mucosal surfaces.

The immunopathogenesis of Entamoeba histolytica

Amebiasis is the disease caused by the enteric dwelling protozoan parasite Entamoeba histolytica. The WHO considers amebiasis as one of the major health problems in developing countries; it is surpassed by only malaria and schistosomiasis for death caused by parasitic infection. E. histolytica primarily lives in the colon as a harmless commensal, but is capable of causing devastating dysentery, colitis and liver abscess. What triggers the switch to a pathogenic phenotype and the onset of disease is unknown. We are becoming increasingly aware of the complexity of the host-parasite interaction. During chronic stages of amebiasis, the host develops an immune response that is incapable of eliminating tissue resident parasites, while the parasite actively immunosuppresses the host. However, most individuals with symptomatic infections succumb only to an episode of dysentery. Why most halt invasion and a minority progress to chronic disease remains poorly understood. This review presents a current understanding of the immune processes that shape the outcome of E. histolytica infections during its different stages.

Toll-like receptor signalling in the intestinal epithelium: how bacterial recognition shapes intestinal function

A single layer of epithelial cells lines the small and large intestines and functions as a barrier between commensal bacteria and the rest of the body. Ligation of Toll-like receptors (TLRs) on intestinal epithelial cells by bacterial products promotes epithelial cell proliferation, secretion of IgA into the gut lumen and expression of antimicrobial peptides. As described in this Review, this establishes a microorganism-induced programme of epithelial cell homeostasis and repair in the intestine. Dysregulation of this process can result in chronic inflammatory and over-exuberant repair responses, and it is associated with the development of colon cancer. Thus, dysregulated TLR signalling by intestinal epithelial cells may explain how colonic bacteria and inflammation promote colorectal cancer.

T cell-independent IgA class switch recombination is restricted to the GALT and occurs prior to manifest germinal center formation

Recently, we reported that CD40(-/-) mice, exhibiting exclusively T cell-independent IgA class switch recombination (CSR), demonstrated near normal levels of IgA plasma cells in the gut lamina propria (LP), despite the complete lack of germinal centers (GCs). In this study, we have extended our analysis focusing on how to reconcile these findings using flow cytometry and molecular markers for IgA CSR. In agreement with our previous results with small intestinal LP, the colon LP was found to host IgA CSR only when lymphoid follicles were present. Thus, no IgA CSR was observed in the nonorganized colon LP. By contrast, the Peyer's patch (PP) was the dominant IgA CSR site in both CD40(-/-) and wild type (WT) mice, and they both hosted similar levels of mRNA expression for B cell activating factor of the TNF family, a proliferation inducing ligand, and inducible NO synthase, potential switch-factors for IgA. Unexpectedly, we found that PP B cells undergoing IgA CSR were GL7-intermediate. These cells had not undergone somatic hypermutations (SHMs), whereas GL7-high cells in WT PP, which exhibited GCs, were heavily mutated. Moreover, IgA plasma cells in the LP of CD40(-/-) mice demonstrated few mutations in their Ig V regions, whereas WT LP B cells from different sites showed extensive SHMs, which were also clonally related. Therefore, IgA CSR can occur in PP at a stage preceding manifest GC (GL7-intermediate), whereas SHM require GC formations (GL7-high). These findings reconcile that IgA CSR can occur in PP in the absence of GC with the fact that CD40(-/-) mice host near normal levels of IgA plasma cells in the LP.

Requirement for Runx proteins in IgA class switching acting downstream of TGF-beta 1 and retinoic acid signaling

IgA is a specific isotype required for mucosal immunity and is the most abundant Ab produced in vivo. Recently, several inductive signals for IgA class switch recombination have been identified; however, the molecular details of the action of these signals and the specific factors acting in B cells remain elusive. In this study, we show that combination of retinoic acid (RA) and TGF-beta1 with other factors induced a much higher frequency of IgA-switched cells than reported previously. In addition, IgA production is severely impaired in Runx2-Runx3 double-deficient mice. In Runx2-Runx3-deficient B cells, both RA- and TGF-beta1-dependent inductions of alpha germline transcription are completely blocked. These data suggest that Runx proteins play an essential role in IgA class switching acting downstream of RA and TGF-beta1 signaling.

Selective IgA deficiency

Introduction

Immunoglobulin A (IgA) deficiency is the most common primary immunodeficiency defined as decreased serum level of IgA in the presence of normal levels of other immunoglobulin isotypes. Most individuals with IgA deficiency are asymptomatic and identified coincidentally. However, some patients may present with recurrent infections of the respiratory and gastrointestinal tracts, allergic disorders, and autoimmune manifestations.

IgA and Its Functions

Although IgA is the most abundant antibody isotype produced in the body, its functions are not clearly understood. Subclass IgA1 in monomeric form is mainly found in the blood circulation, whereas subclass IgA2 in dimeric form is the dominant immunoglobulin in mucosal secretions. Secretory IgA appears to have prime importance in immune exclusion of pathogenic microorganisms and maintenance of intestinal homeostasis. Despite this critical role, there may be some compensatory mechanisms that would prevent disease manifestations in some IgA-deficient individuals.

Pathogenesis

In IgA deficiency, a maturation defect in B cells to produce IgA is commonly observed. Alterations in transmembrane activator and calcium modulator and cyclophilin ligand interactor gene appear to act as disease-modifying mutations in both IgA deficiency and common variable immunodeficiency, two diseases which probably lie in the same spectrum. Certain major histocompatibility complex haplotypes have been associated with susceptibility to IgA deficiency.

Conclusion

The genetic basis of IgA deficiency remains to be clarified. Better understanding of the production and function of IgA is essential in elucidating the disease mechanism in IgA deficiency.

Regulation of adaptive immunity by the innate immune system

Twenty years after the proposal that pattern recognition receptors detect invasion by microbial pathogens, the field of immunology has witnessed several discoveries that have elucidated receptors and signaling pathways of microbial recognition systems and how they control the generation of T and B lymphocyte-mediated immune responses. However, there are still many fundamental questions that remain poorly understood, even though sometimes the answers are assumed to be known. Here, we discuss some of these questions, including the mechanisms by which pathogen-specific innate immune recognition activates antigen-specific adaptive immune responses and the roles of different types of innate immune recognition in host defense from infection and injury.

Stress-related modulation of inflammation in experimental models of bowel disease and post-infectious irritable bowel syndrome: role of corticotropin-releasing factor receptors

The interaction between gut inflammatory processes and stress is gaining increasing recognition. Corticotropin-releasing factor (CRF)-receptor activation in the brain is well established as a key signaling pathway initiating the various components of the stress response including in the viscera. In addition, a local CRF signaling system has been recently established in the gut. This review summarize the present knowledge on mechanisms through which both brain and gut CRF receptors modulate intestinal inflammatory processes and its relevance towards increased inflammatory bowel disease (IBD) activity and post-infectious irritable bowel syndrome (IBS) susceptibility induced by stress.

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.

Repeated restraint stress increases IgA concentration in rat small intestine

The most abundant intestinal immunoglobulin and first line of specific immunological defense against environmental antigens is secretory immunoglobulin A. To better understand the effect of repeated stress on the secretion of intestinal IgA, the effects of restraint stress on IgA concentration and mRNA expression of the gene for the alpha-chain of IgA was assessed in both the duodenum and ileum of the rats. Restraint stress induced an increase in intestinal IgA, which was blocked by an adrenalectomy, suggesting a role of catecholamines and glucocorticoids. Whereas the blocking of glucocorticoid receptors by RU-486 did not affect the increased IgA concentration, it did reduce IgA alpha-chain mRNA expression in both segments, indicating a possible mediation on the part of glucocorticoids in IgA secretion by individual cells. Treatment with corticosterone significantly increased both the IgA concentration and IgA alpha-chain mRNA expression in ileum but not in duodenum, suggesting that glucocorticoids may act directly on IgA-antibody forming cells to increase IgA secretion in the former segment. A probable role by catecholamines was evidenced by the reduction in IgA concentration and IgA alpha-chain mRNA expression in both segments after a chemical sympathectomy with 6-hydroxydopamine (6-OHDA). Additionally, norepinephrine significantly reduced IgA alpha-chain mRNA levels but increased pIgR mRNA expression and IgA concentration in both intestinal segments. We propose that the increased intestinal IgA levels caused by repeated restraint stress is likely due to the effects of catecholamines on the transport of plgA across the epithelium.

Innate host barriers to viral trafficking and population diversity: lessons learned from poliovirus

Poliovirus is an error-prone enteric virus spread by the fecal-oral route and rarely invades the central nervous system (CNS). However, in the rare instances when poliovirus invades the CNS, the resulting damage to motor neurons is striking and often permanent. In the prevaccine era, it is likely that most individuals within an epidemic community were infected; however, only 0.5% of infected individuals developed paralytic poliomyelitis. Paralytic poliomyelitis terrified the public and initiated a huge research effort, which was rewarded with two outstanding vaccines. During research to develop the vaccines, many questions were asked: Why did certain people develop paralysis? How does the virus move from the gut to the CNS? What limits viral trafficking to the CNS in the vast majority of infected individuals? Despite over 100 years of poliovirus research, many of these questions remain unanswered. The goal of this chapter is to review our knowledge of how poliovirus moves within and between hosts, how host barriers limit viral movement, how viral population dynamics impact viral fitness and virulence, and to offer hypotheses to explain the rare incidence of paralytic poliovirus disease.

Interplay between the TH17 and TReg cell lineages: a (co-)evolutionary perspective

The origins of the adaptive immune system and the basis for its unique association with vertebrate species have been a source of considerable speculation. In light of recent advances in our understanding of the developmental and functional links between the induced regulatory T cell and T helper 17 cell lineages, and their specialized relationship to the gut, we speculate that the co-evolution of these adaptive immune pathways might have given primitive vertebrates a means to benefit from the diversification of their commensal microbiota.

Mucosal immunity: induction, dissemination, and effector functions

Prevention of infections by vaccination remains a compelling goal to improve public health. Most infections involve the mucosae, but the development of vaccines against many of these pathogens has yet to be successful. Mucosal vaccines would make immunization procedures easier, be better suited for mass administration, and most efficiently induce immune exclusion - a term coined for non-inflammatory antibody shielding of internal body surfaces - mediated principally by secretory immunoglobulin A (SIgA). The exported antibodies are polymeric, mainly IgA dimers (pIgA) - produced by local plasma cells stimulated by antigens that target the mucosae. SIgA was early shown to be complexed with an epithelial glycoprotein - the secretory component (SC). In 1974, a common SC-dependent transport of pIgA and pentameric IgM was proposed. From the basolateral surface, pIg-SC complexes are taken up by endocytosis and finally extruded into the lumen. Membrane SC is now referred to as polymeric Ig receptor (pIgR). In 1980, it was shown to be synthesized as a larger transmembrane protein - first cloned from rabbit and then from human. Mice deficient for pIgR showed that this is the only receptor responsible for epithelial transport of IgA and IgM. In the gut, induction of B cells occurs in gut-associated lymphoid tissue, particularly the Peyer's patches, but also in mesenteric lymph nodes. Plasma cell differentiation is accomplished in the lamina propria to which the memory/effector cells home. The airways also receive such cells from nasopharynx-associated lymphoid tissue - but by different homing receptors. Such compartmentalization is a challenge for development of mucosal vaccines.

The gut immune barrier and the blood-brain barrier: are they so different?

In order to protect itself from a diverse set of environmental pathogens and toxins, the body has developed a number of barrier mechanisms to limit the entry of potential hazards. Here, we compare two such barriers: the gut immune barrier, which is the primary barrier against pathogens and toxins ingested in food, and the blood-brain barrier, which protects the central nervous system from pathogens and toxins in the blood. Although each barrier provides defense in very different environments, there are many similarities in their mechanisms of action. In both cases, there is a physical barrier formed by a cellular layer that tightly regulates the movement of ions, molecules, and cells between two tissue spaces. These barrier cells interact with different cell types, which dynamically regulate their function, and with a different array of immune cells that survey the physical barrier and provide innate and adaptive immunity.

Mesenteric lymph nodes are not required for an intestinal immunoglobulin A response to oral cholera toxin

Stimulation of the adaptive immune system in the gut is thought to be mainly initiated in the Peyer's patches as well as in the mesenteric lymph nodes (mLNs) and results in immunoglobulin A (IgA) secretion by plasma cells in the lamina propria. However, the precise role of the mLNs in the development of IgA immune responses is poorly understood. Thus, cholera toxin (CT) was administered to mLN-resected and mLN-bearing animals and the IgA response to CT in the intestine and serum was examined. Levels of CT-specific IgA antibodies and the numbers of cells producing these antibodies in the intestine were increased in mLN-resected rats. Particularly in the distal parts of the intestine, the jejunum and the ileum, IgA responses to orally administered antigens developed were stronger in the intestine after removal of the mLNs. This strongly indicates that the mLNs play a critical role in modulating the expansion of specific IgA responses. After removal of the mLNs, the lymph from the gut flows directly into the blood. It was investigated whether the spleen is involved in the initiation of an immune response to orally administered CT after removal of the mLNs. In the spleens of mLN-resected animals, proliferation was up-regulated, and germinal centres were formed in the follicles. However, CT-specific IgM(+) cells, but no IgA(+) cells, developed. Additionally, an increase of CT-specific IgM in the serum was found in mLN-resected animals. Thus, the data indicate that the spleen is involved in the immune response to CT after mLN resection.

A dominant, coordinated T regulatory cell-IgA response to the intestinal microbiota

A T cell receptor transgenic mouse line reactive to a microbiota flagellin, CBir1, was used to define mechanisms of host microbiota homeostasis. Intestinal IgA, but not serum IgA, was found to block mucosal flagellin uptake and systemic T cell activation in mice. Depletion of CD4(+)CD25(+) Tregs decreased IgA(+) B cells, total IgA, and CBir1-specific IgA in gut within days. Repletion of T cell-deficient mice with either CD4(+)CD25(+) or CD4(+)foxp3(+) Tregs restored intestinal IgA to a much greater extent than their reciprocal CD4(+) subsets, indicating that Tregs are the major helper cells for IgA responses to microbiota antigens such as flagellin. We propose that the major role of this coordinated Treg-IgA response is to maintain commensalism with the microbiota.

Diverse regulatory pathways for IgA synthesis in the gut

Immunoglobulin (Ig) A is the most abundant antibody isotype in mucosal secretions. In this study we summarize recent advances in our understanding of the compartments and mechanisms of intestinal IgA synthesis. We discuss the pathways leading to the generation of IgA(+) B cells in follicular and extra-follicular structures, by T-cell-dependent and T-cell-independent mechanisms.

A rice-based oral cholera vaccine induces macaque-specific systemic neutralizing antibodies but does not influence pre-existing intestinal immunity

We previously showed that oral immunization of mice with a rice-based vaccine expressing cholera toxin (CT) B subunit (MucoRice-CT-B) induced CT-specific immune responses with toxin-neutralizing activity in both systemic and mucosal compartments. In this study, we examined whether the vaccine can induce CT-specific Ab responses in nonhuman primates. Orally administered MucoRice-CT-B induced high levels of CT-neutralizing serum IgG Abs in the three cynomolgus macaques we immunized. Although the Ab level gradually decreased, detectable levels were maintained for at least 6 mo, and high titers were rapidly recovered after an oral booster dose of the rice-based vaccine. In contrast, no serum IgE Abs against rice storage protein were induced even after multiple immunizations. Additionally, before immunization the macaques harbored intestinal secretory IgA (SIgA) Abs that reacted with both CT and homologous heat-labile enterotoxin produced by enterotoxigenic Escherichia coli and had toxin-neutralizing activity. The SIgA Abs were present in macaques 1 mo to 29 years old, and the level was not enhanced after oral vaccination with MucoRice-CT-B or after subsequent oral administration of the native form of CT. These results show that oral MucoRice-CT-B can effectively induce CT-specific, neutralizing, serum IgG Ab responses even in the presence of pre-existing CT- and heat-labile enterotoxin-reactive intestinal SIgA Abs in nonhuman primates.

Role of IgA and IgA fc receptors in inflammation

INTRODUCTION

Signals delivered by serum monomeric IgA (mIgA) are essential in controlling the immune system by preventing the development of autoimmunity and inflammation. However, IgA can also, when aggregated, be deleterious to the host, inducing inflammatory diseases. This Janus-like nature of IgA is mainly due to their heterogeneity in molecular forms and their interaction with IgA receptors.

DISCUSSION

While serum mIgA are mainly involved in FcalphaRI-mediated inhibition of immune responses, macromolecular serum IgA or circulating IgA immune complexes are often deleterious to the host by inducing sustained activation through IgA receptors including FcalphaRI and transferrin receptor.

CONCLUSION

FcalphaRI-mediated inhibitory function is able to suppress several inflammatory diseases in mice including asthma and glomerulonephritis. Intravenous mIgA (mIgAIV) and anti-FcalphaR monovalent antibodies represent thus promising tools for immunotherapy.

Alpha versus beta: are we on the way to resolve the mystery as to which is the endogenous ligand for natural killer T cells?

Natural killer T (NKT) lymphocytes are a unique subset of cells that play a role in regulating the immune system. For the past decade, studies have focused upon attempts to define these cells and to determine the ligand(s) that are required for their development and peripheral activation. Many research groups have focused upon determining the mechanisms for activating or inhibiting NKT cells in an attempt to control immune-mediated disorders as well as infectious and malignant conditions by using different ligand structures. Alpha-anomeric glycolipids and phospholipids derived from mammalian, bacterial, protozoan and plant species have been suggested as potential ligands for these lymphocytes. Some of these ligands were structured in forms that can bind to CD1d molecules. The lack of alpha-anomeric glycosphingolipids in mammals and the modest effect of these ligands in human studies, along with recent data from animal models and humans on the NKT-dependent immunomodulatory effect of beta-glycosphingolipids, suggest that the beta-anomeric ligands have the potential to be the endogenous NKT ligand.

Immune responses to the microbiota at the intestinal mucosal surface

The mammalian intestinal mucosal surface is continuously exposed to a complex and dynamic community of microorganisms. These microbes establish symbiotic relationships with their hosts, making important contributions to metabolism and digestive efficiency. The intestinal epithelial surface is the primary interface between the vast microbiota and internal host tissues. Given the enormous numbers of enteric bacteria and the persistent threat of opportunistic invasion, it is crucial that mammalian hosts monitor and regulate microbial interactions with intestinal epithelial surfaces. Here we discuss recent insights into how the innate and adaptive arms of the immune system collaborate to maintain homeostasis at the luminal surface of the intestinal host-microbial interface. These findings are also yielding a better understanding of how symbiotic host-microbial relationships can break down in inflammatory bowel disease.

Crystal structure of a mucus-binding protein repeat reveals an unexpected functional immunoglobulin binding activity

Lactobacillus reuteri mucus-binding protein (MUB) is a cell-surface protein that is involved in bacterial interaction with mucus and colonization of the digestive tract. The 353-kDa mature protein is representative of a broadly important class of adhesins that have remained relatively poorly characterized due to their large size and highly modular nature. MUB contains two different types of repeats (Mub1 and Mub2) present in six and eight copies, respectively, and shown to be responsible for the adherence to intestinal mucus. Here we report the 1.8-A resolution crystal structure of a type 2 Mub repeat (184 amino acids) comprising two structurally related domains resembling the functional repeat found in a family of immunoglobulin (Ig)-binding proteins. The N-terminal domain bears striking structural similarity to the repeat unit of Protein L (PpL) from Peptostreptococcus magnus, suggesting binding in a non-immune Fab-dependent manner. A distorted PpL-like fold is also seen in the C-terminal domain. As with PpL, Mub repeats were able to interact in vitro with a large repertoire of mammalian Igs, including secretory IgA. This hitherto undetected activity is consistent with the current model that antibody responses against commensal flora are of broad specificity and low affinity.

25-Hydroxycholesterol secreted by macrophages in response to Toll-like receptor activation suppresses immunoglobulin A production

25-Hydroxycholesterol is produced in mammalian tissues. The function of this oxysterol is unknown. Here we describe a central role for 25-hydroxycholesterol in regulating the immune system. In initial experiments, we found that stimulation of macrophage Toll-like receptors (TLR) induced expression of cholesterol 25-hydroxylase and the synthesis of 25-hydroxycholesterol. Treatment of naïve B cells with nanomolar concentrations of 25-hydroxycholesterol suppressed IL-2-mediated stimulation of B cell proliferation, repressed activation-induced cytidine deaminase (AID) expression, and blocked class switch recombination, leading to markedly decreased IgA production. Consistent with these findings, deletion of the mouse cholesterol 25-hydroxylase gene caused an increase in serum IgA. Conversely, inactivation of the CYP7B1 oxysterol 7alpha-hydroxylase, which degrades 25-hydroxycholesterol, decreased serum IgA. The suppression of IgA class switching in B cells by a macrophage-derived sterol in response to TLR activation provides a mechanism for local and systemic negative regulation of the adaptive immune response by the innate immune system.

Mucosal addressin cell-adhesion molecule-1 controls plasma-cell migration and function in the small intestine of mice

BACKGROUND & AIMS

Immunoglobulin (Ig) A secretion into the intestinal lumen is an important immune mechanism of the gastrointestinal (GI) tract. B cells proliferate and differentiate into IgA-secreting plasma cells (PC) within lymphoid organs then migrate directly into the intestinal lamina propria. We aimed to elucidate the in vivo role of the mucosal addressin cell-adhesion molecule-1 (MAdCAM-1), which is constitutively expressed in the GI-associated lymphoid tissue, in B-cell migration.

METHODS

We generated MAdCAM-1-deficient mice (MAdCAM(Delta)) and evaluated the B-cell compartment of the GI-associated lymphoid tissue. We also assessed PC migration to the small intestine and the intestinal immune response after oral immunization.

RESULTS

In MAdCAM(Delta) mice, the size of Peyer's patches was drastically reduced, compared with that of wild-type mice; this difference was detectable by 3 days after birth, indicating that MAdCAM-1 is dispensable for embryonic Peyer's patch development but mediates recruitment of lymphocytes into this lymphoid organ at later stages. Moreover, antigen-specific, IgA-positive PC were severely compromised in their migration to the small intestine; accordingly, there was a reduced number of IgA-secreting PC in the lamina propria of the small intestine. The MAdCAM(Delta) mice were unable to mount a normal intestinal IgA response after oral immunization with cholera toxin.

CONCLUSION

These data provide in vivo evidence that MAdCAM-1 is required for the localization and function of IgA-secreting PC in the intestine.

T-bet-/- RAG2-/- ulcerative colitis: the role of T-bet as a peacekeeper of host-commensal relationships

Inflammatory bowel disease is a disease that reflects a disequilibrium in host-commensal homeostasis. T-bet-/-xRAG2-/- deficient mice develop a spontaneous juvenile ulcerative colitis resulting from a pro-inflammatory response to the commensal microbiota that is dendritic cell and TNF-alpha driven [schematized in Fig. 1]. The TRUC (T-bet-/- RAG2-/- ulcerative colitis) model is discussed in the broader context of the adaptive and innate immune mechanisms that regulate host-commensal relationships within the intestine.

Immune and non-immune functions of the (not so) neonatal Fc receptor, FcRn

Careful regulation of the body's immunoglobulin-G (IgG) and albumin concentrations is necessitated by the importance of their respective functions. As such, the neonatal Fc receptor (FcRn) which, as a single receptor, is capable of regulating both of these molecules, has become an important focus of investigation. In addition to these essential protection functions, FcRn possesses a host of other functions that are equally as critical. During the very first stages of life, FcRn mediates the passive transfer of IgG from mother to offspring both before and after birth. In the adult, FcRn regulates the persistence of both IgG and albumin in the serum as well as the movement of IgG, and any bound cargo, between different compartments of the body. This shuttling allows for the movement not only of monomeric ligand but also of antigen/antibody complexes from one cell type to another in such a way as to facilitate the efficient initiation of immune responses towards opsonized pathogens. As such, FcRn continues to play the role of an immunological sensor throughout adult life, particularly in regions such as the gut which are exposed to a large number of infectious antigens. Increasing appreciation for the contributions of FcRn to both homeostatic and pathological states is generating an intense interest in the potential for therapeutic modulation of FcRn binding. A greater understanding of FcRn's pleiotropic roles is thus imperative for a variety of therapeutic purposes.

An African perspective on mucosal immunity and HIV-1

HIV prevention mandates an understanding of the mechanisms of mucosal immunity with attention to some unique features of the epidemic and mucosal environment in the developing world. An effective vaccine will have to induce mucosal protection against a highly diverse virus, which is equipped with a number of immune evasion strategies. Its development will require assessment of mucosal immune responses, and it will have to protect a mucosal environment where inflammation and altered immune responses are common because of the presence of other mucosal infections, such as sexually transmitted infections and parasites, and where nutritional status may also be compromised. Ideally, not only prevention methods would protect adults but also provide cover against gastrointestinal transmission through maternal milk. Prevention might also be complemented by microbicides and circumcision, two alternative approaches to mucosal protection. It seems unlikely that a single solution will work in all instances and intervention might have to act at multiple levels and be tailored to local circumstances. We review here some of the mucosal events associated with HIV infection that are most relevant in an African setting.

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.

Intestinal T cells: facing the mucosal immune dilemma with synergy and diversity

The epithelium of the gastrointestinal tract, which represents the greatest body surface area exposed to the outside environment, is confronted with a plethora of foreign and potentially harmful antigens. Consequently, the immune system of the gut faces the daunting task of distinguishing harmless dietary proteins and commensal bacteria from potentially dangerous pathogens, and of then responding accordingly. Mucosal T cells play a central role in maintaining barrier function and controlling the delicate balance between immune activation and immune tolerance. This review will focus on the unique features of mucosal T cell subsets that reside in the epithelium and lamina propria of the gut.

Innate immunity mediated by MyD88 signal is not essential for induction of lipopolysaccharide-specific B cell responses but is indispensable for protection against Salmonella enterica serovar Typhimurium infection

Salmonella organisms are Gram negative and facultative anaerobic bacteria that cause typhoid fever in humans. In this study, we evaluated LPS-specific adaptive immunity in innate immune-deficient mice after oral administration of attenuated Salmonella enterica serovar Typhimurium (S. Typhimurium) strains. Of interest, identical levels of LPS-specific IgG and IgA Abs were elicited in the systemic (i.e., serum and spleen) and mucosal (i.e., fecal extract and small intestine) compartments of wild-type, TLR4(-/-), and MyD88(-/-) mice following oral vaccination with recombinant attenuated S. Typhimurium (RASV). Depletion of CD4(+) T cells during RASV vaccination completely abrogated the generation of LPS-specific Abs in MyD88(-/-) mice. In addition, mRNA expression levels of a B cell-activating factor of the TNF family were significantly increased in the spleens of MyD88(-/-) mice after oral administration, implying that T cell-independent B cell switching might be also enhanced in the MyD88 signal-deficient condition. Of most interest, orally vaccinated MyD88(-/-) mice that possessed high levels of LPS-specific IgG and IgA, which had a neutralizing effect against Salmonella, died earlier than nonvaccinated wild-type mice following lethal oral challenge with virulent Salmonella species. These results suggest that innate immunity mediated by MyD88 signal is dispensable for induction of LPS-specific Ab responses following oral administration of attenuated Salmonella strains but indispensable for efficient protection.

Effect of Lactobacillus casei Shirota strain intraperitoneal administration in CD1 mice on the establishment of Trichinella spiralis adult worms and on IgA anti-T. spiralis production

The effect of the intraperitoneal (ip) administration of Lactobacillus casei Shirota strain (LcS) in CD1 mice on the establishment of Trichinella spiralis adult worms (TSAW), and on the generation of intestinal IgA anti-T. spiralis after challenge (AC) were evaluated. One hundred and twenty mice were allocated at random into two groups of 60 mice each: Treated group (T) and Non-treated group (NT). Each mouse in T group was inoculated with LcS at days -21, -14, and -7. On day 0 each mouse was challenged with 200 larvae of T. spiralis. At days 3, 5, 7, 10, 12, 14, 17, 19, 21, and 28 AC, six mice from each group were sacrificed to obtain TSAW. At days 7, 14, 21, and 28 IgA-s anti-T. spiralis levels in intestinal washings were evaluated by ELISA. From day five on AC, mice in LcS group showed significantly less TSAW (P<0.05) than animals from NT group. At days 7, 14, 21, and 28 AC IgA anti-T. spiralis levels were higher in mice from T group (P<0.05) than in the NT group. The results indicate that LcS inoculated into mice induces protection against T. spiralis and an increase in the production of IgA anti-T. spiralis.