Signaling via LTbetaR on the lamina propria stromal cells of the gut is required for IgA production

Role of TNFR-related 2 mediated immune responses in dextran sulfate sodium-induced inflammatory bowel disease

Previous work has suggested that the LIGHT-TR2 costimulatory pathway plays a role in the acute and chronic stages of dextran sulfate sodium (DSS)-induced colitis [Steinberg et al. (2008); Wang et al. (2005)]. To clarify the role of TNFR-related 2 (TR2) signaling in the maintenance of intestinal homeostasis, we generated a TR2 knock-out (KO) mouse. Using DSS to induce colitis, we compared the colitic symptoms and pathological changes in wild type (WT) and TR2 KO mice, and the production of cytokines by the diseased colons. We also studied the role of TR2 in suppressing innate and adaptive immunity in the DSS model. TR2 deficient mice were characterized by reduced symptoms of intestinal inflammation compared with wild-type mice, and reduced production of cytokines. We therefore generated a monoclonal antibody against mouse TR2 which was specific to TR2 and capable of blocking TR2 signals. With this antibody, we demonstrated that antagonizing TR2 during the development of DSS-induced colitis reduced the symptoms of inflammation. Our findings suggest that TR2 is an important mediator in colitis, and may serve as a therapeutic target in inflammatory bowel disease.

Roles of B-1 and B-2 cells in innate and acquired IgA-mediated immunity

The gut harbors an extremely dense and complex community of microorganisms that are in constant dialog with our immune cells. The gut bacteria provide strong selective pressure to the host to evolve innate and adaptive immune responses required for the maintenance of local and systemic homeostasis. One of the most conspicuous responses of the gut immune system following microbial colonization is the production of immunoglobulin A (IgA). In this review, we discuss the roles of B-1 and B-2 cells in IgA-mediated immunity and present an updated view for the sites and mechanisms of IgA synthesis in the gut. We summarize the role of secretory IgAs for regulation of microbial communities and provide clues as to how the gut microbiota contributes to the development of the gut-associated lymphoid tissues.

Vaccination strategies to promote mucosal antibody responses

There are great interest and demand for the development of vaccines to prevent and treat diverse microbial infections. Mucosal vaccines elicit immune protection by stimulating the production of antibodies at mucosal surfaces and systemic districts. Being positioned in close proximity to a large community of commensal microbes, the mucosal immune system deploys a heterogeneous population of cells and a complex regulatory network to maintain the balance between surveillance and tolerance. A successful mucosal vaccine relies on leveraging the functions of these immune cells and regulatory components. We review the important cellular interactions and molecular pathways underlying the induction and regulation of mucosal antibody responses and discuss their implications on mucosal vaccination.

Lymphoid tissue inducer cells and the evolution of CD4 dependent high-affinity antibody responses

Phylogeny indicates that in mammals memory CD4-dependent antibody responses evolved after monotremes split from the common ancestor of marsupial and eutherian mammals. This was strongly associated with the development of segregated B and T cell areas and the development of a linked lymph node network. The evolution of the lymphotoxin beta receptor in these higher mammals was key to the development of these new functions. Here, we argue that lymphoid tissue inducer cells played a pivotal role not only in the development of organized lymphoid structures but also in the subsequent genesis of the CD4-dependent class-switched memory antibody responses that depend on an organized infrastructure to work. In this review, we concentrate on the role of this cell type in the making of a tolerant CD4 T cell repertoire and in the sustenance of CD4 T cell responses for protective immunity.

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.

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.

Lymphoid tissue inducer cells: bridges between the ancient innate and the modern adaptive immune systems

Phylogeny indicates that adaptive immunity evolved first in diffusely distributed lymphoid tissues found in the lamina propria (LP) of the gut. B follicular structures appeared later, probably initially in isolated lymphoid follicles in the LP and then in organized lymphoid tissues such as lymph nodes and Peyer's patches. The development of these new lymphoid structures was enabled by gene duplication and evolution of new tumor necrosis family members. Here, we argue that lymphoid tissue inducer cells (LTis) had a pivotal role, not only in the development of organized lymphoid structures, but also in the subsequent genesis of the CD4-dependent class-switched memory antibody responses. In this review, we concentrate on the latter function: the sustenance by LTis of CD4 T-cell responses for protective immunity.

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.

Regulating the mucosal immune system: the contrasting roles of LIGHT, HVEM, and their various partners

LIGHT and herpes virus entry mediator (HVEM) comprise a ligand-receptor pair in the tumor necrosis factor superfamily. These molecules play an important role in regulating immunity, particularly in the intestinal mucosa. LIGHT also binds the lymphotoxin beta receptor, and HVEM can act as a ligand for immunoglobulin family molecules, including B- and T-lymphocyte attenuator, which suppresses immune responses. Complexity in this pivotal system arises from several factors, including the non-monogamous pairing of ligands and receptors, and reverse signaling or the ability of some ligands to serve as receptors. As a result, recognition events in this fascinating network of interacting molecules can have pro- or anti-inflammatory consequences. Despite complexity, experiments we and others are carrying out are establishing rules for understanding when and in what cell types these molecules contribute to intestinal inflammation.

Decreased expression of intestinal chemokine TECK/CCL25 in experimental obstructive jaundice and its reversal following internal biliary drainage

BACKGROUND

Although bacterial translocation is a significant problem in patients with obstructive jaundice, how translocation is promoted in this situation is not clearly understood. We previously reported the recovery of gut mucosal T-lymphocyte numbers in jaundiced rats following internal biliary drainage. This suggests that bile in the intestinal lumen promotes T-lymphocyte redistribution into the gut mucosa. To test this hypothesis, we have examined the expression patterns of chemokines that play an important role in lymphocyte recruitment into the small intestine.

METHODS

Four groups of rats receiving one of the following surgical procedures were studied: a sham operation (SHAM), common bile duct ligation (CBDL), CBDL followed by external drainage, or CBDL followed by internal drainage. Expression levels of intestinal mRNAs encoding TECK, MECK, and LARC chemokines were assessed using real-time polymerase chain reaction. Distribution of chemokine mRNA in the rat ileum was examined using in situ hybridization (ISH).

RESULTS

Following surgery, the expression levels of TECK mRNA decreased significantly in the CBDL group compared with in the SHAM group. While TECK expression did not recover after external drainage, it recovered to a near-normal level after internal drainage. Expression levels of MECK and LARC mRNAs were similar among all groups. ISH confirmed strong expression of TECK mRNA in the epithelial cells of the small intestine.

CONCLUSIONS

These results indicate that bile may contribute to high expression levels of TECK/CCL25 mRNA in the small intestine. Bile may also have a role in regulating the distribution of gut mucosal T lymphocytes by promoting TECK production from epithelial cells.

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

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

Role of mesenteric lymph nodes and aging in secretory IgA production in mice

Although it is known that Peyer's patches are the major inductive site for S-IgA production and B1 cells contribute to half of the IgA plasma cells detected in the gut lamina propria, the type of contribution of mesenteric lymph nodes to the process is still unclear. Cytokines such as TGF-beta, IL-10, IL-4, IL-5, and IL-6, are required to promote IgA class switching and IgA synthesis. Aging-related alterations in T and B cells and in cytokine production are already known. Some reports have also proposed that S-IgA production might be altered in aged animals. Herein, we investigated the role of MLN and aging in S-IgA production. Two- to 18-month-old BALB/c mice were used to evaluate aging-related alterations and MLN were removed to study its role in S-IgA production. We found that MLN are important, although not essential for S-IgA production. In addition, we showed that production of IgA-related cytokines are well preserved in MLN but not in PP of aged mice and that S-IgA levels are not affected by aging. Our results suggest that MLN may play a complementary role in S-IgA production mostly in aged animals.

The biology of intestinal immunoglobulin A responses

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

A unique B2 B cell subset in the intestine

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

Regulation of humoral and cellular gut immunity by lamina propria dendritic cells expressing Toll-like receptor 5

The intestinal cell types responsible for defense against pathogenic organisms remain incompletely characterized. Here we identify a subset of CD11c(hi)CD11b(hi) lamina propria dendritic cells (LPDCs) that expressed Toll-like receptor 5 (TLR5) in the small intestine. When stimulated by the TLR5 ligand flagellin, TLR5(+) LPDCs induced the differentiation of naive B cells into immunoglobulin A-producing plasma cells by a mechanism independent of gut-associated lymphoid tissue. In addition, by a mechanism dependent on TLR5 stimulation, these LPDCs promoted the differentiation of antigen-specific interleukin 17-producing T helper cells and type 1 T helper cells. Unlike spleen DCs, the LPDCs specifically produced retinoic acid, which, in a dose-dependent way, supported the generation and retention of immunoglobulin A-producing cells in the lamina propria and positively regulated the differentiation interleukin 17-producing T helper cells. Our findings demonstrate unique properties of LPDCs and the importance of TLR5 for adaptive immunity in the intestine.

The immune geography of IgA induction and function

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

Association of gamma interferon and interleukin-17 production in intestinal CD4+ T cells with protection against rotavirus shedding in mice intranasally immunized with VP6 and the adjuvant LT(R192G)

Mucosal immunization of mice with chimeric, Escherichia coli-expressed VP6, the protein that comprises the intermediate capsid layer of the rotavirus particle, together with attenuated E. coli heat-labile toxin LT(R192G) as an adjuvant, reduces fecal shedding of rotavirus antigen by >95% after murine rotavirus challenge, and the only lymphocytes required for protection are CD4+ T cells. Because these cells produce cytokines with antiviral properties, the cytokines whose expression is upregulated in intestinal memory CD4+ T cells immediately after rotavirus challenge of VP6/LT(R192G)-immunized mice may be directly or indirectly responsible for the rapid suppression of rotavirus shedding. This study was designed to identify which cytokines are significantly upregulated in intestinal effector sites and secondary lymphoid tissues of intranasally immunized BALB/c mice after challenge with murine rotavirus strain EDIM. Initially, this was done by using microarray analysis to quantify mRNAs for 96 murine common cytokines. With this procedure, the synthesis of mRNAs for gamma interferon (IFN-gamma) and interleukin-17 (IL-17) was found to be temporarily upregulated in intestinal lymphoid cells of VP6/LT(R192G)-immunized mice at 12 h after rotavirus challenge. These cytokines were also produced in CD4+ T cells obtained from intestinal sites specific to VP6/LT(R192G)-immunized mice after in vitro exposure to VP6 as determined by intracellular cytokine staining and secretion of cytokines. Although genetically modified mice that lack receptors for either IFN-gamma or IL-17 remained protected after immunization, these results provide suggestive evidence that these cytokines may play direct or indirect roles in protection against rotavirus after mucosal immunization of mice with VP6/LT(R192G).

Gut IgA class switch recombination in the absence of CD40 does not occur in the lamina propria and is independent of germinal centers

Conflicting findings have recently been presented as to the sites and sources of B cells that undergo class switch recombination (CSR) to IgA in the gut. In this study we provide compelling evidence in CD40(-/-) mice demonstrating that IgA CSR can be independent of CD40 signaling and germinal center formation and does not occur in the gut lamina propria (LP) itself. We found that CD40(-/-) mice had near normal levels of gut total IgA despite lacking germinal centers and completely failing to raise specific responses against the T cell-dependent Ags cholera toxin and keyhole limpet hemocyanin. The Peyer's patches in CD40(-/-) mice expressed unexpectedly high levels of activation-induced cytidine deaminase mRNA and germline alpha transcripts, but few postswitch circular DNA transcripts, arguing against significant IgA CSR. Moreover and more surprisingly, wild-type mice exhibited no to low IgA CSR in mesenteric lymph nodes or isolated lymphoid follicles. Importantly, both strains failed to demonstrate any of the molecular markers for IgA CSR in the gut LP itself. Whereas all of the classical sites for IgA CSR in the GALT in CD40(-/-) mice appeared severely compromised for IgA CSR, B cells in the peritoneal cavity demonstrated the expression of activation-induced cytidine deaminase mRNA comparable to that of wild-type mice. However, peritoneal cavity B cells in both strains expressed intermediate levels of the germinal center marker GL7 and exhibited no germline alpha transcripts, and only three of 51 mice analyzed showed the presence of postswitch circular DNA transcripts. Taken together, these findings strongly argue for alternative inductive sites for gut IgA CSR against T cell-independent Ags outside of the GALT and the nonorganized LP.

Enhanced IgA class switching in marginal zone and B1 B cells relative to follicular/B2 B cells

Mouse splenic marginal zone (MZ) B cells and B1 B cells enriched in the peritoneal cavity respond preferentially to T cell-independent Ags compared with follicular (FO)/B2 B cells. Despite the differential responses of B cell subsets to various stimuli, and despite the need for multiple stimuli to induce IgA class switching, the relative contribution of B cell subpopulations to IgA production is unknown. By culturing purified B cell populations, we find that MZ and peritoneal B1 cells switch more readily to IgA than do splenic FO or peritoneal B2 cells in BLyS/LPS/TGF-beta. Addition of IL-4, IL-5, and anti-IgD dextran to the cultures enhances IgA switching in FO/B2 and MZ B cells to a similar frequency, but this treatment suppresses IgA class switching in B1 cells. Thus, IgA switching differs among purified B cell subsets, suggesting that individual B cell populations could contribute differentially to IgA expression in vivo, depending on available stimuli.

Intestinal IgA synthesis: a primitive form of adaptive immunity that regulates microbial communities in the gut

Our intestine is colonized by an impressive community of bacteria, that has profound effects on the immune functions. The relationship between gut microbiota and the immune system is one of reciprocity: bacteria have important contribution in nutrient processing and education of the immune system and conversely, the immune system, particularly gut-associated lymphoid tissues (GALT) plays a key role in shaping the repertoire of gut microbiota. In this review we discuss new insights into the role of IgA in the maintenance of immune homeostasis and the reciprocal interactions between gut B cells and intestinal bacteria.

Activation of non-canonical NF-kappaB pathway mediated by STP-A11, an oncoprotein of Herpesvirus saimiri

Although Saimiri Transforming Protein (STP)-A11, an oncoprotein of Herpesvirus saimiri, has been known to activate NF-kappaB signaling pathway, the detailed mechanism has not been reported yet. We herein report that STP-A11 activates non-canonical NF-kappaB pathway, resulting in p100 processing to p52. In addition, translocation of p52 protein (NF-kappaB2) into the nucleus is observed by the expression of STP-A11. STP-A11-mediated processing of p100 to p52 protein requires proteosome-mediated proteolysis because MG132 treatment clearly blocked p52 production in spite of the expression of STP-A11. Analysis of STP-A11 mutants to activate NF-kappaB2 pathway discloses the requirement of TRAF6-binding site not Src-binding site for STP-A11-mediated NF-kappaB2 pathway. Blockage of STP-A11-mediated p52 production using siRNA against p52 enhanced a chemotherapeutic drug-mediated cell death, suggesting that p52 production induced by the expression of STP-A11 would contribute to cellular transformation, which results from a resistance to cell death.

Lymphoid Tissue Inducer Cells in Intestinal Immunity

During fetal development, lymphoid tissue inducer cells (LTis) seed the developing lymph node and Peyer's patch anlagen and initiate the formation of both types of lymphoid organs. In the adult, a similar population of cells, termed lymphoid tissue inducer-like cells (LTi-like cells), supports the formation of organized gut-associated lymphoid tissue (GALT) in the intestine, including both isolated lymphoid follicles (ILFs) and cryptopatches (CPs). Both LTi and LTi-like cells require expression of the transcription factor RORgammat for their differentiation and function, and mice lacking RORgammat lack lymph nodes, Peyer's patches, and other organized GALT. In ILFs and cryptopatches, LTi-like cells are in close contact with different populations of intestinal dendritic cells (DCs), including a subpopulation recently shown to extend dendrites and sample luminal microflora. This interaction may allow for communication between the intestinal lumen and the immune cells in the lamina propria, which is necessary for maintaining homeostasis between the commensal microflora and the intestinal immune system. The potential functional implications of the organization of LTi-like cells, DCs, and lymphocytes in the lamina propria are discussed in the context of maintenance of homeostasis and of infectious diseases, particularly HIV infection.

Intestinal IgA synthesis: a primitive form of adaptive immunity that regulates microbial communities in the gut

Our intestine is colonized by an impressive community of commensals that has profound effects on the immune functions. The relationship between gut microbiota and the immune system is one of reciprocity: Commensals have important contributions in nutrient processing and education of the immune system, and, conversely, the immune system, particularly gut-associated lymphoid tissues (GALT), plays a key role in shaping the repertoire of gut microbiota. In this chapter we attempt to discuss the mechanisms that underlie this reciprocity and emphasize the key role of mucosal IgA in maintenance of an appropriate segmental distribution of microbiota, which is necessary for immune homeostasis.

BAFF induces a hyper-IgA syndrome in the intestinal lamina propria concomitant with IgA deposition in the kidney independent of LIGHT

BAFF is a peripheral B cell survival factor and can mediate antibody (Ab) class switching. Over-expression of BAFF in mice results in B cell hyperplasia, elevated serum immunoglobulin (Ig), spontaneous germinal centre (GC) reactions and mild glomerulonephritis (GN). Here we show that, in addition to driving excessive levels of serum IgA, BAFF over-expression results in increased IgA levels within the intestinal lamina propria (LP) and deposition of IgA immune complexes in the renal glomerular mesangium. LIGHT has been previously shown to mediate a similar phenotype via signaling through the lymphotoxin-beta receptor (LTbetaR). We evaluated if LIGHT and BAFF cooperate in the etiology of a hyper-IgA syndrome in BAFF-overexpressing transgenic (BAFF-Tg) mice. We find that LIGHT-deficient BAFF-Tg mice exhibit similar levels of IgA in the serum, gut and kidney and develop nephritis to the same degree as LIGHT-sufficient BAFF-Tg mice. Therefore, in the context of BAFF over-expression, LIGHT is dispensable for the generation of a hyper-IgA syndrome accompanied by nephritis.

Development of nephritis but not sialadenitis in autoimmune-prone BAFF transgenic mice lacking marginal zone B cells

B cell-activating factor belonging to the TNF family (BAFF) is a B cell survival factor required for B cell maturation. BAFF transgenic (Tg) mice develop autoimmune disorders characterized by autoantibody production, which leads to nephritis and salivary gland destruction (sialadenitis), features reminiscent of systemic lupus erythematosus and Sjögren's syndrome (SS), respectively. Disease in BAFF Tg mice correlates with the expansion of the marginal zone (MZ) B cell compartment and the abnormal presence of MZ-like B cells in the blood, LN and inflamed salivary glands, suggesting a role for these cells in BAFF-induced autoimmunity. Lymphotoxin-beta (LTbeta)-deficient mice show disrupted splenic architecture, lack MZ B cells and some peripheral LN, and are unable to mount T cell-dependent immune responses. BAFF Tg mice lacking LTbeta (LTbetaDelta-BTg) retained these defects, yet still developed nephritis associated with the presence of B-1 B cells in the kidneys. However, in contrast to old BAFF Tg mice, aging LTbetaDelta-BTg mice no longer developed sialadenitis. Thus, autoimmune disorders in BAFF Tg mice are possibly events coordinated by MZ and B-1 B cells at separate anatomical sites.

Inhibition of Th2-Mediated Allergic Airway Inflammatory Disease by CD137 Costimulation

The engagement of CD137 (4-1BB), an inducible T cell costimulatory receptor and member of the TNF receptor superfamily, by agonistic Abs can promote strong tumor and viral immunity mediated by CD8(+) T cells and stimulate IFN-gamma production. However, its role in Th2-mediated immune responses has not been well defined. To address this issue, we studied the function of CD137 engagement using an allergic airway disease model in which the mice were sensitized with inactivated Schistosoma mansoni eggs followed by S. mansoni egg Ag challenge directly in the airways and Th1/2 cytokine production was monitored. Interestingly, treatment of C57BL/6 mice with agonistic anti-CD137 (2A) during sensitization completely prevents allergic airway inflammation, as shown by a clear inhibition of T cell and eosinophil infiltration into the lung tissue and airways, accompanied by diminished Th2 cytokine production and reduced serum IgE levels, as well as a reduction of airway hyperresponsiveness. At various time points after immunization, restimulated splenocytes from 2A-treated mice displayed reduced proliferation and Th2 cytokine production. In accordance with this, agonistic Ab to CD137 can directly coinhibit Th2 responses in vitro although it costimulates Th1 responses. CD137-mediated suppression of Th2 response is independent of IFN-gamma and T regulatory cells. Our study has identified a novel pathway to inhibit Th2 responses in a CD137-dependent fashion.

Functional characterization of the CCL25 promoter in small intestinal epithelial cells suggests a regulatory role for caudal-related homeobox (Cdx) transcription factors

The chemokine CCL25 is selectively and constitutively expressed in the small intestinal epithelium and plays an important role in mediating lymphocyte recruitment to this site. In this study, we demonstrate that CCL25 expression in murine small intestinal epithelial cells is independent of signaling through the lymphotoxin beta receptor and is not enhanced by inflammatory stimuli, pathways involved in driving the expression of most other chemokines. We define a transcriptional start site in the CCL25 gene and a region -141 to -5 proximal of exon 1 that is required for minimal promoter activity in the small intestinal epithelial cell lines, MODE-K and mICc12. These cell lines expressed far less CCL25 mRNA than freshly isolated small intestinal epithelial cells indicating that they are missing important factors driving CCL25 expression. The CCL25 promoter contained putative binding sites for the intestinal epithelial-associated Caudal-related homeobox (Cdx) transcription factors Cdx-1 and Cdx-2, and small intestinal epithelial cells but not MODE-K and mICc12 cells expressed Cdx-1 and Cdx-2. EMSA analysis demonstrated that Cdx proteins were present in nuclear extracts from freshly isolated small intestinal epithelial cells but not in MODE-K or mICcl2 cells, and bound to putative Cdx sites within the CCL25 promoter. Finally, cotransfection of MODE-K cells with Cdx transcription factors significantly increased CCL25 promoter activity as well as endogenous CCL25 mRNA levels. Together these results demonstrate a unique pattern of regulation for CCL25 and suggest a role for Cdx proteins in regulating CCL25 transcription.

The IgA system: a comparison of structure and function in different species

The predominant immunoglobulin isotype on most mucosal surfaces is secretory immunoglobulin A (SIgA), a polypeptide complex comprising two IgA monomers, the connecting J chain, and the secretory component. The molecular stability and strong anti-inflammatory properties make SIgA particularly well suited to provide protective immunity to the vulnerable mucosal surfaces by preventing invasion of inhaled and ingested pathogens. In contrast to SIgA, IgA in serum functions as an inflammatory antibody through interaction with FcalphaR on immune effector cells. Although IgA appears to share common features and protective functions in different species, significant variations exist within the IgA systems of different species. This review will give an overview of the basic concepts underlying mucosal IgA defence which will focus on the variations present among species in structure, antibody repertoire development, pIgR-mediated transport, colostral IgA content, hepatobiliary transport, and function with particular emphasis on the IgA system of the pig and dog. These interspecies variations emphasise the importance of elucidating and analysing the IgA system within the immune system of the species of interest rather than inferring roles from conclusions made in human and mouse studies.

Role of CXC chemokine ligand 13, CC chemokine ligand (CCL) 19, and CCL21 in the organization and function of nasal-associated lymphoid tissue

Nasal-associated lymphoid tissue (NALT) orchestrates immune responses to Ags in the upper respiratory tract. Unlike other lymphoid organs, NALT develops independently of lymphotoxin-alpha (LTalpha). However, the structure and function of NALT are impaired in Ltalpha(-/-) mice, suggesting a link between LTalpha and chemokine expression. In this study we show that the expression of CXCL13, CCL19, CCL21, and CCL20 is impaired in the NALT of Ltalpha(-/-) mice. We also show that the NALT of Cxcl13(-/-) and plt/plt mice exhibits some, but not all, of the structural and functional defects observed in the NALT of Ltalpha(-/-) mice. Like the NALT of Ltalpha(-/-) mice, the NALT in Cxcl13(-/-) mice lacks follicular dendritic cells, BP3(+) stromal cells, and ERTR7(+) lymphoreticular cells. However, unlike the NALT of Ltalpha(-/-) mice, the NALT of Cxcl13(-/-) mice has peripheral node addressin(+) high endothelial venules (HEVs). In contrast, the NALT of plt/plt mice is nearly normal, with follicular dendritic cells, BP3(+) stromal cells, ERTR7(+) lymphoreticular cells, and peripheral node addressin(+) HEVs. Functionally, germinal center formation and switching to IgA are defective in the NALT of Ltalpha(-/-) and Cxcl13(-/-) mice. In contrast, CD8 T cell responses to influenza are impaired in Ltalpha(-/-) mice and plt/plt mice. Finally, the B and T cell defects in the NALT of Ltalpha(-/-) mice lead to delayed clearance of influenza from the nasal mucosa. Thus, the B and T cell defects in the NALT of Ltalpha(-/-) mice can be attributed to the impaired expression of CXCL13 and CCL19/CCL21, respectively, whereas impaired HEV development is directly due to the loss of LTalpha.

Mucosal B cells: phenotypic characteristics, transcriptional regulation, and homing properties

Mucosal antibody defense depends on a complex cooperation between local B cells and secretory epithelia. Mucosa-associated lymphoid tissue gives rise to B cells with striking J-chain expression that are seeded to secretory effector sites. Such preferential homing constitutes the biological basis for local production of polymeric immunoglobulin A (pIgA) and pentameric IgM with high affinity to the epithelial pIg receptor that readily can export these antibodies to the mucosal surface. This ultimate functional goal of mucosal B-cell differentiation appears to explain why the J chain is also expressed by IgG- and IgD-producing plasma cells (PCs) occurring at secretory tissue sites; these immunocytes may be considered as 'spin-offs' from early effector clones that through class switch are on their way to pIgA production. Abundant evidence supports the notion that intestinal PCs are largely derived from B cells initially activated in gut-associated lymphoid tissue (GALT). Nevertheless, insufficient knowledge exists concerning the relative importance of M cells, major histocompatibility complex class II-expressing epithelial cells, and professional antigen-presenting cells for the uptake, processing, and presentation of luminal antigens in GALT to accomplish the extensive and sustained priming and expansion of mucosal B cells. Likewise, it is unclear how the germinal center reaction in GALT so strikingly can promote class switch to IgA and expression of J chain. Although B-cell migration from GALT to the intestinal lamina propria is guided by rather well-defined adhesion molecules and chemokines/chemokine receptors, the cues directing preferential homing to different segments of the gut require better definition. This is even more so for the molecules involved in homing of mucosal B cells to secretory effector sites beyond the gut, and in this respect, the role of Waldever's ring (including the palatine tonsils and adenoids) as a regional inductive tissue needs further characterization. Data suggest a remarkable compartmentalization of the mucosal immune system that must be taken into account in the development of effective local vaccines to protect specifically the airways, eyes, oral cavity, small and large intestines, and urogenital tract.

Microparticles for oral delivery of vaccines

Nonreplicating antigens are poorly immunogenic when given orally, either due to their degradation in the gastrointestinal tract or because they are not efficiently taken up in the gut. Studies in laboratory animals have clearly demonstrated that microparticles can significantly improve the immunogenicity of orally administered antigens. However, the oral delivery of vaccines using microparticles has not been explored extensively in humans and large animals. In this article the progress in oral microparticle antigen delivery will be reviewed and, where possible, studies in humans and large animals will be highlighted. In addition, possible approaches that have the potential to significantly improve microparticle delivery of oral vaccines will be suggested.

Tumor necrosis factor family members and inflammatory bowel disease

Tumor necrosis factor (TNF) is one of the most potent effector cytokines in the pathogenesis of inflammatory bowel disease (IBD). Previous studies strongly implicate the critical involvement of several TNF family members in human IBD. This review focuses on the recent studies of TNF family members in IBD development. In particular, we discuss the findings about LIGHT (homologous to lymphotoxins, inducible expression, competes with herpes simplex virus glycoprotein D for herpes viral entry mediator, a receptor expressed on T lymphocytes) in the pathogenesis of IBD, and the potential mechanisms by which LIGHT induces IBD. Such mechanisms may also apply to other TNF family members.

Organizing a mucosal defense

Gastrointestinal associated lymphoid tissue can be divided into loosely organized effector sites, which include the lamina propria and intraepithelial lymphocytes, and more organized structures, such as mesenteric lymph nodes (LNs), Peyer's patches (PPs), isolated lymphoid follicles, and cryptopatches (CPs). These organized structures in the gastrointestinal tract have been hypothesized to play the role of primary lymphoid organ, supporting the extrathymic development of T lymphocytes (CPs), secondary lymphoid organs involved in the induction of the mucosal immune response (PPs), and tertiary lymphoid structures whose function is still under debate (isolated lymphoid follicles). The most widely studied lymphoid structure found in the small intestine is the PP. PPs are secondary lymphoid structures, and their development and function have been extensively investigated. However, single lymphoid aggregates resembling PPs have been also described in humans and in the murine small intestines. These isolated lymphoid follicles have both germinal centers and an overlying follicle-associated epithelium, suggesting that they also can function as inductive sites for the mucosal immune response. This review compares and contrasts the development and function of the four main organized gastrointestinal lymphoid tissues: CPs, isolated lymphoid follicles, PPs, and mesenteric LNs.

Network communications: lymphotoxins, LIGHT, and TNF

Lymphotoxins (LT) provide essential communication links between lymphocytes and the surrounding stromal and parenchymal cells and together with the two related cytokines, tumor necrosis factor (TNF) and LIGHT (LT-related inducible ligand that competes for glycoprotein D binding to herpesvirus entry mediator on T cells), form an integrated signaling network necessary for efficient innate and adaptive immune responses. Recent studies have identified signaling pathways that regulate several genes, including chemokines and interferons, which participate in the development and function of microenvironments in lymphoid tissue and host defense. Disruption of the LT/TNF/LIGHT network alleviates inflammation in certain autoimmune disease models, but decreases resistance to selected pathogens. Pharmacological disruption of this network in human autoimmune diseases such as rheumatoid arthritis alleviates inflammation in a significant number of patients, but not in other diseases, a finding that challenges our molecular paradigms of autoimmunity and perhaps will reveal novel roles for this network in pathogenesis.

The Critical Role of LIGHT in Promoting Intestinal Inflammation and Crohn’s Disease

Crohn's disease (CD) is a type of inflammatory bowel disease associated with increased Th1 cytokines and unique pathological features. However, its pathogenesis has not been fully understood. Previous studies showed that homologous to lymphotoxin, exhibits inducible expression, competes with herpesvirus glycoprotein D for HVEM on T cells (LIGHT) transgenic (Tg) mice develop autoimmunity including intestinal inflammation with a variable time course. In this study, we establish an experimental model for CD by adoptive transfer of Tg mesenteric lymph node cells into RAG(-/-) mice. The recipients of Tg lymphocytes rapidly develop a disease strikingly similar to the key pathologic features and cytokine characterization observed in CD. We demonstrate that, as a costimulatory molecule, LIGHT preferentially drives Th1 responses. LIGHT-mediated intestinal disease is dependent on both of its identified signaling receptors, lymphotoxin beta receptor and herpes virus entry mediator, because LIGHT Tg mesenteric lymph node cells do not cause intestinal inflammation when transferred into the lymphotoxin beta receptor-deficient mice, and herpes virus entry mediator on donor T cells is required for the full development of disease. Furthermore, we demonstrated that up-regulation of LIGHT is associated with active CD. These data establish a new mouse model resembling CD and suggest that up-regulation of LIGHT may be an important mediator of CD pathogenesis.

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

Signaling by lymphotoxin (LT) and TNF is essential for the organogenesis of secondary lymphoid tissues in systemic and mucosal compartments. In this study, we demonstrated that the progeny of mice treated with fusion protein of LTbetaR and IgGFc (LTbetaR-Ig) or LTbetaR-Ig plus TNFR55-Ig (double Ig) showed significantly increased numbers of isolated lymphoid follicles (ILF) in the large intestine. Interestingly, double Ig treatment accelerated the maturation of large intestinal ILF. Three-week-old progeny of double Ig-treated mice showed increased numbers of ILF in the large intestine, but not in the small intestine. Furthermore, alteration of intestinal microflora by feeding of antibiotic water did not affect the increased numbers of ILF in the large intestine of double Ig-treated mice. Most interestingly, mice that developed numerous ILF also had increased levels of activation-induced cytidine deaminase expression and numbers of IgA-expressing cells in the lamina propria of the large intestine. Taken together, these results suggest that ILF formation in the large intestine is accelerated by blockage of LTbetaR and TNFR55 signals in utero, and ILF, like colonic patches, might play a role in the induction of IgA response in the large intestine.

CD70+ antigen-presenting cells control the proliferation and differentiation of T cells in the intestinal mucosa

One unresolved issue in gut immunity is how mucosal T lymphocytes are activated and which antigen-presenting cell (APC) is critical for the regulation of this process. We have identified a unique population of APCs that is exclusively localized in the lamina propria. These APCs constitutively expressed the costimulatory molecule CD70 and had antigen-presenting functions. After oral infection of mice with Listeria monocytogenes, proliferation and differentiation of antigen-specific T cells occurred in the gut mucosa in situ and blockade of CD70 costimulation abrogated the mucosal T cell proliferation and effector functions. Thus, a potent CD70-dependent stimulation via specialized tissue-specific APCs is required for the proliferation and differentiation of gut mucosal T cells after oral infection.

The intriguing biology of the tumour necrosis factor/tumour necrosis factor receptor superfamily: players, rules and the games

The members of the tumour necrosis factor (TNF)/tumour necrosis factor receptor (TNFR) superfamily are critically involved in the maintenance of homeostasis of the immune system. The biological functions of this system encompass beneficial and protective effects in inflammation and host defence as well as a crucial role in organogenesis. At the same time, members of this superfamily are responsible for host damaging effects in sepsis, cachexia, and autoimmune diseases. This review summarizes recent progress in the immunobiology of the TNF/TNFR superfamily focusing on results obtained from animal studies using gene targeted mice. The different modes of signalling pathways affecting cell proliferation, survival, differentiation, apoptosis, and immune organ development as well as host defence are reviewed. Molecular and cellular mechanisms that demonstrate a therapeutic potential by targeting individual receptors or ligands for the treatment of chronic inflammatory or autoimmune diseases are discussed.

Intratumor depletion of CD4+ cells unmasks tumor immunogenicity leading to the rejection of late-stage tumors

Tumor environment can be critical for preventing the immunological destruction of antigenic tumors. We have observed a selective accumulation of CD4(+)CD25(+) T cells inside tumors. In a murine fibrosarcoma L(d)-expressing Ag104, these cells made up the majority of tumor-infiltrating lymphocytes at the late stage of tumor progression, and their depletion during the effector phase, rather than priming phase, successfully enhanced antitumor immunity. We show here that CD4(+)CD25(+) T cells suppressed the proliferation and interferon-gamma production of CD8(+) T cells in vivo at the local tumor site. Blockade of the effects of IL-10 and TGF-beta partially reversed the suppression imposed by the CD4(+) cells. Furthermore, local depletion of CD4(+) cells inside the tumor resulted in a change of cytokine milieu and led to the eradication of well-established highly aggressive tumors and the development of long-term antitumor memory. Therefore, CD4(+)CD25(+) T cells maintained an environment in the tumor that concealed the immunogenicity of tumor cells to permit progressive growth of antigenic tumors. Our study illustrates that the suppression of antitumor immunity by regulatory T cells occurs predominantly at the tumor site, and that local reversal of suppression, even at a late stage of tumor development, can be an effective treatment for well-established cancers.

Lymphotoxin and LIGHT signaling pathways and target genes

Lymphotoxins (LT alpha and LT beta), LIGHT [homologous to LT, inducible expression, competes with herpes simplex virus (HSV) glycoprotein D for HSV entry mediator (HVEM), a receptor expressed on T lymphocytes], tumor necrosis factor (TNF), and their specific receptors LT beta R, HVEM, and TNF receptor 1 (TNFR1) and TNFR2, form the immediate family of the larger TNF superfamily. These cytokines establish a critical communication system required for the development of secondary lymphoid tissues; however, knowledge of the target genes activated by these signaling pathways is limited. Target genes regulated by the LT alpha beta-LT beta R pathway include the tissue-organizing chemokines, CXCL13, CCL19, and CCL21, which establish cytokine circuits that regulate LT expression on lymphocytes, leading to organized lymphoid tissue. Infectious disease models have revealed that LT alpha beta pathways are also important for innate and adaptive immune responses involved in host defense. Here, regulation of interferon-beta by LT beta R and TNFR signaling may play a crucial role in certain viral infections. Regulation of autoimmune regulator in the thymus via LT beta R implicates LT/LIGHT involvement in central tolerance. Dysregulated expression of LIGHT overrides peripheral tolerance leading to T-cell-driven autoimmune disease. Blockade of TNF/LT/LIGHT pathways as an intervention in controlling autoimmune diseases is attractive, but such therapy may have risks. Thus, identifying and understanding the target genes may offer an opportunity to fine-tune inhibitory interventions.

Restricted IgA repertoire in both B-1 and B-2 cell-derived gut plasmablasts

Mucosal IgA is the most abundantly produced Ig upon colonization of the intestinal tract with commensal organisms in the majority of mammals. The repertoire of these IgA molecules is still largely unknown; a large amount of the mucosal IgA cannot be shown to react with the inducing microorganisms. Analysis of the repertoire of used H chain Ig (V(H)) genes by H-CDR3 spectrotyping, cloning, and sequencing of V(H) genes from murine intestinal IgA-producing plasma cells reveals a very restricted usage of V(H) genes and multiple clonally related sequences. The restricted usage of V(H) genes is a very consistent observation, and is observed for IgA plasma cells derived from B-1 or conventional B-2 cells from different mouse strains. Clonal patterns from all analyzed V(H) gene sequences show mainly independently acquired somatic mutations in contrast to the clonal evolution patterns often observed as a consequence of affinity maturation in germinal center reactions in peripheral lymphoid organs and Peyer's patches. Our data suggest a model of clonal expansion in which many mucosal IgA-producing B cells develop in the absence of affinity maturation. The affinity of most produced IgA might not be the most critical factor for its possible function to control the commensal organisms, but simply the abundance of large amounts of IgA that can bind with relatively unselected affinity to redundant epitopes on such organisms.

Two distinctive pathways for recruitment of naive and primed IgM+ B cells to the gut lamina propria

Intestinal IgA+ B cells are generated from IgM+ B cells by in situ class switching in two separate gut microenvironments: organized follicular structures and lamina propria (LP). However, the origin of IgM+ B cells in the gut LP is unknown. Transfer experiments to reconstitute IgM+ B cells and IgA plasma cells in LP of aly/aly mice, which are defective in all organized follicular structures because of an NF-kappaB-inducing kinase (NIK) mutation, revealed that naive B cells can directly migrate to the LP. This migration requires NIK-dependent activation of gut stromal cells. By contrast, the entry of gut-primed IgM+ B cells to the LP is independent of stromal cells with functional NIK. Our results indicate that naive B cells directly migrate to the LP by a distinct pathway from gut-primed B cells.

Differential lymphotoxin-beta and interferon gamma signaling during mouse liver regeneration induced by chronic and acute injury

The liver regenerates after acute injury via hepatocyte cell division; during chronic injury, when hepatocyte replication is impaired or blocked, liver progenitor oval cells mediate liver regeneration. If both regeneration options are blocked in animal models, then liver failure and death ensues. The mechanisms underlying oval cell induction, proliferation, and subsequent liver regeneration remain poorly characterized. In particular, cell-signaling pathways that distinguish the alternative pathways are unknown. This study shows that in a mouse model, hepatic expression of lymphotoxin-beta (LTbeta) and interferon gamma (IFNgamma) transcripts is increased in response to the choline-deficient, ethionine-supplemented (CDE) diet, which induces oval cell-mediated liver regeneration. Oval cells express LTbeta and IFNgamma transcripts, contributing to the increased expression in the liver of mice fed the CDE diet. An attenuated oval cell response to such a diet was observed in LTbeta receptor-, LTbeta-, and IFNgamma-gene targeted mice. Loss of LTbeta and LTbeta receptor signaling reduced the number of oval cells expressing A6 and muscle pyruvate kinase. The lack of IFNgamma signaling reduced muscle pyruvate kinase(+), but not A6(+), oval cells. In contrast, partial hepatectomy suppressed LTbeta and IFNgamma transcripts. We also show that IFNgamma induces STAT-3 phosphorylation in an oval cell line. In conclusion, LTbeta, LTbeta receptor, and IFNgamma are involved in oval cell-mediated, but not hepatocyte-mediated, liver regeneration, and the absence of these pathways impairs the oval cell-dependent regenerative response.

VDUP1 Is Required for the Development of Natural Killer Cells

Vitamin D3 upregulated protein 1 (VDUP1) is a stress-response gene that is upregulated by 1,25(OH)2D3 in tumor cells. The in vivo roles of VDUP1 were investigated by producing mice lacking VDUP1 (VDUP1-/- mice). VDUP1-/- mice showed minimal changes in the development of T and B cells, but there was a profound reduction in the numbers of natural killer (NK) cells. As well, these mice showed decreased NK activity. In the VDUP1-/- mice, the expression of CD122 was reduced, demonstrating that VDUP1 is required for CD122 expression and NK maturation. In addition, severe lymphoid hyperplasia in the small intestine was observed in VDUP1-/- mice. Taken together, these results suggest that VDUP1 is a critical factor for the development and function of NK cells in vivo.

Regulation of IgA synthesis at mucosal surfaces

Immunoglobulin A is the main element of the humoral immune response that has been selected through evolution, together with innate mucosal defences, to provide protection against microbial antigens at mucosal surfaces. IgA responses are initiated in organized inductive structures, such as Peyer's patches and nasal-associated lymphoid tissues, as well as diffuse effector tissues, such as gut lamina propria and nasal mucosa. Hypermutated secretory IgAs play a critical role in regulating the composition of the intestinal microflora. Dysregulation of gut homeostasis in IgA-deficient gut causes a continuous activation of the immune cells and induces inflammatory processes leading to lymphoneogenesis. Recent advances in this field include new insights into the role of IgA in the maintenance of gut homeostasis and the proposal of an updated model for the induction of IgA responses in the gut.