Retinoic acid induces homing of protective T and B cells to the gut after subcutaneous immunization in mice

Mucosal immunology of food allergy

Food allergies are increasing in prevalence at a higher rate than can be explained by genetic factors, suggesting a role for as yet unidentified environmental factors. In this review, we summarize the state of knowledge about the healthy immune response to antigens in the diet and the basis of immune deviation that results in immunoglobulin E (IgE) sensitization and allergic reactivity to foods. The intestinal epithelium forms the interface between the external environment and the mucosal immune system, and emerging data suggest that the interaction between intestinal epithelial cells and mucosal dendritic cells is of particular importance in determining the outcome of immune responses to dietary antigens. Exposure to food allergens through non-oral routes, in particular through the skin, is increasingly recognized as a potentially important factor in the increasing rate of food allergy. There are many open questions on the role of environmental factors, such as dietary factors and microbiota, in the development of food allergy, but data suggest that both have an important modulatory effect on the mucosal immune system. Finally, we discuss recent developments in our understanding of immune mechanisms of clinical manifestations of food allergy. New experimental tools, particularly in the field of genomics and the microbiome, are likely to shed light on factors responsible for the growing clinical problem of food allergy.

Tracking plasma cell differentiation and survival

Plasma cells play a crucial role for the humoral immune response as they represent the body's factories for antibody production. The differentiation from a B cell into a plasma cell is controlled by a complex transcriptional network and happens within secondary lymphoid organs. Based on their lifetime, two types of antibody secreting cells can be distinguished: Short-lived plasma cells are located in extrafollicular sites of secondary lymphoid organs such as lymph node medullary cords and the splenic red pulp. A fraction of plasmablasts migrate from secondary lymphoid organs to the bone marrow where they can become long-lived plasma cells. Bone marrow plasma cells reside in special microanatomical environments termed survival niches, which provide factors promoting their longevity. Reticular stromal cells producing the chemokine CXCL12, which is known to attract plasmablasts to the bone marrow but also to promote plasma cell survival, play a crucial role in the maintenance of these niches. In addition, hematopoietic cells are contributing to the niches by providing other soluble survival factors. Here, we review the current knowledge on the factors involved in plasma cell differentiation, their localization and migration. We also give an overview on what is known regarding the maintenance of long lived plasma cells in survival niches of the bone marrow.

Lung dendritic cells induce migration of protective T cells to the gastrointestinal tract

Developing efficacious vaccines against enteric diseases is a global challenge that requires a better understanding of cellular recruitment dynamics at the mucosal surfaces. The current paradigm of T cell homing to the gastrointestinal (GI) tract involves the induction of α4β7 and CCR9 by Peyer's patch and mesenteric lymph node (MLN) dendritic cells (DCs) in a retinoic acid-dependent manner. This paradigm, however, cannot be reconciled with reports of GI T cell responses after intranasal (i.n.) delivery of antigens that do not directly target the GI lymphoid tissue. To explore alternative pathways of cellular migration, we have investigated the ability of DCs from mucosal and nonmucosal tissues to recruit lymphocytes to the GI tract. Unexpectedly, we found that lung DCs, like CD103(+) MLN DCs, up-regulate the gut-homing integrin α4β7 in vitro and in vivo, and induce T cell migration to the GI tract in vivo. Consistent with a role for this pathway in generating mucosal immune responses, lung DC targeting by i.n. immunization induced protective immunity against enteric challenge with a highly pathogenic strain of Salmonella. The present report demonstrates novel functional evidence of mucosal cross talk mediated by DCs, which has the potential to inform the design of novel vaccines against mucosal pathogens.

Vitamin-mediated regulation of intestinal immunity

The intestine is exposed continuously to complex environments created by numerous injurious and beneficial non-self antigens. The unique mucosal immune system in the intestine maintains the immunologic homeostasis between the host and the external environment. Crosstalk between immunocompetent cells and endogenous (e.g., cytokines and chemokines) as well as exogenous factors (e.g., commensal bacteria and dietary materials) achieves the vast diversity of intestinal immune functions. In addition to their vital roles as nutrients, vitamins now also are known to have immunologically crucial functions, specifically in regulating host immune responses. In this review, we focus on the immunologic functions of vitamins in regulating intestinal immune responses and their roles in moderating the fine balance between physiologic and pathologic conditions of the intestine.

RDH10, RALDH2, and CRABP2 are required components of PPARγ-directed ATRA synthesis and signaling in human dendritic cells

All-trans retinoic acid (ATRA) has a key role in dendritic cells (DCs) and affects T cell subtype specification and gut homing. However, the identity of the permissive cell types and the required steps of conversion of vitamin A to biologically active ATRA bringing about retinoic acid receptor-regulated signaling remains elusive. Here we present that only a subset of murine and human DCs express the necessary enzymes, including RDH10, RALDH2, and transporter cellular retinoic acid binding protein (CRABP)2, to produce ATRA and efficient signaling. These permissive cell types include CD103(+) DCs, granulocyte-macrophage colony-stimulating factor, and interleukin-4-treated bone marrow-derived murine DCs and human monocyte-derived DCs (mo-DCs). Importantly, in addition to RDH10 and RALDH2, CRABP2 also appears to be regulated by the fatty acid-sensing nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) and colocalize in human gut-associated lymphoid tissue DCs. In our model of human mo-DCs, all three proteins (RDH10, RALDH2, and CRABP2) appeared to be required for ATRA production induced by activation of PPARγ and therefore form a linear pathway. This now functionally validated PPARγ-regulated ATRA producing and signaling axis equips the cells with the capacity to convert precursors to active retinoids in response to receptor-activating fatty acids and is potentially amenable to intervention in diseases involving or affecting mucosal immunity.

Epicutaneous sensitization results in IgE-dependent intestinal mast cell expansion and food-induced anaphylaxis

BACKGROUND

Sensitization to food antigen can occur through cutaneous exposure.

OBJECTIVE

We sought to test the hypothesis that epicutaneous sensitization with food antigen predisposes to IgE-mediated anaphylaxis on oral allergen challenge.

METHODS

BALB/c mice were epicutaneously sensitized by repeated application of ovalbumin (OVA) to tape-stripped skin over 7 weeks or orally immunized with OVA and cholera toxin (CT) weekly for 8 weeks and then orally challenged with OVA. Body temperature was monitored, and serum mouse mast cell protease 1 levels were determined after challenge. Tissue mast cell (MC) counts were examined by using chloroacetate esterase staining. Levels of serum OVA-specific IgE and IgG(1) antibodies and cytokines in supernatants of OVA-stimulated splenocytes were measured by means of ELISA. Serum IL-4 levels were measured by using an in vivo cytokine capture assay.

RESULTS

Epicutaneously sensitized mice exhibited expansion of connective tissue MCs in the jejunum, increased serum IL-4 levels, and systemic anaphylaxis after oral challenge, as evidenced by decreased body temperature and increased serum mouse mast cell protease 1 levels. Intestinal MC expansion and anaphylaxis were IgE dependent because they did not occur in epicutaneously sensitized IgE(-/-) mice. Mice orally immunized with OVA plus CT did not have increased serum IL-4 levels, expanded intestinal MCs, or anaphylaxis after oral challenge, despite OVA-specific IgE levels and splenocyte cytokine production in response to OVA stimulation, which were comparable with those of epicutaneously sensitized mice.

CONCLUSION

Epicutaneously sensitized mice, but not mice orally immunized with antigen plus CT, have expansion of intestinal MCs and IgE-mediated anaphylaxis after single oral antigen challenge. IgE is necessary but not sufficient for food anaphylaxis, and MC expansion in the gut can play an important role in the development of anaphylaxis.

The early activation marker CD69 regulates the expression of chemokines and CD4 T cell accumulation in intestine

Migration of naïve and activated lymphocytes is regulated by the expression of various molecules such as chemokine receptors and ligands. CD69, the early activation marker of C-type lectin domain family, is also shown to regulate the lymphocyte migration by affecting their egress from the thymus and secondary lymphoid organs. Here, we aimed to investigate the role of CD69 in accumulation of CD4 T cells in intestine using murine models of inflammatory bowel disease. We found that genetic deletion of CD69 in mice increases the expression of the chemokines CCL-1, CXCL-10 and CCL-19 in CD4⁺ T cells and/or CD4⁻ cells. Efficient in vitro migration of CD69-deficient CD4 T cells toward the chemokine stimuli was the result of increased expression and/or affinity of chemokine receptors. In vivo CD69^−/− CD4 T cells accumulate in the intestine in higher numbers than B6 CD4 T cells as observed in competitive homing assay, dextran sodium sulphate (DSS)-induced colitis and antigen-specific transfer colitis. In DSS colitis CD69^−/− CD4 T cell accumulation in colonic lamina propria (cLP) was associated with increased expression of CCL-1, CXCL-10 and CCL-19 genes. Furthermore, treatment of DSS-administrated CD69^−/− mice with the mixture of CCL-1, CXCL-10 and CCL-19 neutralizing Abs significantly decreased the histopathological signs of colitis. Transfer of OT-II×CD69^−/− CD45RB^high CD4 T cells into RAG^−/− hosts induced CD4 T cell accumulation in cLP. This study showed CD69 as negative regulator of inflammatory responses in intestine as it decreases the expression of chemotactic receptors and ligands and reduces the accumulation of CD4 T cells in cLP during colitis.

Retinoic acid, acting as a highly specific IgA isotype switch factor, cooperates with TGF-β1 to enhance the overall IgA response

The present study demonstrates that RA has activity of an IgA switch factor and is more specific than TGF-β1. RA independently caused only IgA switching, whereas TGF-β1 caused IgA and IgG2b switching. We found that RA increased IgA production and that this was a result of its ability to increase the frequency of IgA-secreting B cell clones. Increased IgA production was accompanied by an increase of GLTα. RA activity was abrogated by an antagonist of the RAR. Additionally, RA affected intestinal IgA production in mice. Surprisingly, RA, in combination with TGF-β1, notably enhanced not only IgA production and GLTα expression but also CCR9 and α4β7 expression on B cells. These results suggest that RA selectively induces IgA isotype switching through RAR and that RA and TGF-β have important effects on the overall gut IgA antibody response.

Intestinal DC in migrational imprinting of immune cells

Dendritic cells (DCs) have a pivotal role in instructing antigen-specific immune responses, processing and presenting antigens to CD4(+) and CD8(+) T cells and producing factors capable to modulate the quality of T-cell responses. In this review, we will provide an historic overview on the identification of the mechanisms controlling lymphocyte migration into the largest immune organ of the body: the gut, and we will describe how in recent years an unexpected role for DCs has emerged as the architects in programming gut-homing immune cells. Specifically, we will review how intestinal DCs utilize the dietary vitamin A metabolite retinoic acid (RA) to program gut-homing lymphocytes and how intestinal DCs acquire the unique capacity to become RA producers.

Lymph node dissection--understanding the immunological function of lymph nodes

Lymph nodes (LN) are one of the important sites in the body where immune responses to pathogenic antigens are initiated. This immunological function induced by cells within the LN is an extensive area of research. To clarify the general function of LN, to identify cell populations within the lymphatic system and to describe the regeneration of the lymph vessels, the experimental surgical technique of LN dissection has been established in various animal models. In this review different research areas in which LN dissection is used as an experimental tool will be highlighted. These include regeneration studies, immunological analysis and studies with clinical questions. LN were dissected in order to analyse the different cell subsets of the incoming lymph in detail. Furthermore, LN were identified as the place where the induction of an antigen-specific response occurs and, more significantly, where this immune response is regulated. During bacterial infection LN, as a filter of the lymph system, play a life-saving role. In addition, LN are essential for the induction of tolerance against harmless antigens, because tolerance could not be induced in LN-resected animals. Thus, the technique of LN dissection is an excellent and simple method to identify the important role of LN in immune responses, tolerance and infection.

Dietary and commensal derived nutrients: shaping mucosal and systemic immunity

The intestine serves as the primary site of nutrient absorption in the body while also harboring the highest burden of commensal microflora and representing a major portal of pathogen exposure. As such, the immune network of the intestine relies on both dietary and commensal derived signals to guide appropriate function. Recent advances highlight the role of dietary derived nutrients and commensal derived metabolites in shaping gastrointestinal immunity. In particular, vitamin A has been shown to have dominant and pleiotropic effects in the intestine. In addition, dietary derived AHR ligands and commensal derived metabolites are now emerging as important players in mucosal immunity. Thus nutrition, commensal microflora and the mucosal immune system are all intimately connected.

Immunomodulation for gastrointestinal infections

The intestinal epithelium provides a barrier between a variety of luminal antigens and provides the components of intestinal innate and adaptive immunity. It is crucial that at this interface, the epithelial cell layer and the components of the intestinal immunity interact with dietary and bacterial antigens in a regulated way to maintain homeostasis. Failure to tightly control immune reactions can be detrimental and result in inflammation. In the current review, we described the regulatory mechanisms controlling host-immune homeostasis and the role of regulatory CD4(+) T cells, with a special emphasis in the regulatory T-cell subsets (Tregs). Furthermore, the participation of innate cell cross-talk in the polarization of intestinal immune responses is also evaluated. Finally, the recent characterization of host responses to normal commensal flora, the role of bacteria and bacterial factors in the maintenance of immunomodulation, and the disruption of this balance by bacterial enteric pathogens is also summarized.

Shift of graft-versus-host-disease target organ tropism by dietary vitamin A

Gut-homing of donor T cells is causative for the development of intestinal GvHD in recipients of allogeneic hematopoietic stem cell transplantation (HSCT). Expression of the gut-specific homing receptors integrin-α4β7 and chemokine receptor CCR9 on T cells is imprinted in gut-associated lymphoid tissues (GALT) under the influence of the vitamin A metabolite retinoic acid. Here we addressed the role of vitamin A deficiency in HSCT-recipients for donor T cell migration in the course of experimental GvHD. Vitamin A-deficient (VAD) mice were prepared by feeding them a vitamin A-depleted diet. Experiments were performed in a C57BL/6 into BALB/c model of acute GvHD. We found that expression of integrin-α4β7 and CCR9 in GALT was reduced in VAD recipients after HSCT. Competitive in vivo homing assays showed that allogeneic T cells primed in VAD mice did not home as efficiently to the intestine as T cells primed in mice fed with standard diet (STD). The course of GvHD was ameliorated in VAD HSCT-recipients and, consequently, their survival was prolonged compared to recipients receiving STD. However, VAD-recipients were not protected and died of clinical GvHD. We found reduced numbers of donor T cells in the intestine but increased cell counts and tissue damage in other organs of VAD-recipients. Furthermore, we observed high IFN-γ⁺CD4⁺ and low FoxP3⁺CD4⁺ frequencies of total donor CD4⁺ T cells in VAD as compared to STD recipients. Taken together, these results indicate that dietary vitamin A deficiency in HSCT-recipients changed target organ tropism in GvHD but also resulted in fatal inflammation after HSCT.

Effect of vitamin A deficiency on the immune response in obesity

Obesity has been associated with low-grade systemic inflammation and with micronutrient deficiencies. Obese individuals have been found to have lower vitamin A levels and lower vitamin A intake compared with normal-weight individuals. Vitamin A plays a major role in the immune function, including innate immunity, cell-mediated immunity and humoral antibody immunity. It has also been recognised recently that vitamin A has important regulatory functions. Vitamin A status has an important effect on the chronic inflammatory response. Vitamin A deficiency increases a T-helper type 1 (Th1) response, elevates levels of pro-inflammatory cytokines, increases the expression of leptin, resistin and uncoupling proteins (UCP) and promotes adipogenesis. The effect of vitamin A deficiency on obesity might be increasing the risk of fat deposition and also the risk of chronic inflammation associated with obesity. Supplementation with vitamin A in vitro and in animal models has been found to reduce concentrations of adipocytokines, such as leptin and resistin. In conclusion, vitamin A deficiency increases a Th1 response in the presence of obesity and thus, increases the inflammatory process involved in chronic inflammation and fat deposition. The metabolism of leptin and other adipocytokines may play a critical role in the effect of vitamin A deficiency in the inflammatory response observed in obesity.

Cryptosporidiosis: host immune responses and the prospects for effective immunotherapies

Cryptosporidium spp. that develop in intestinal epithelial cells are responsible for the diarrhoeal disease cryptosporidiosis, which is common in humans of all ages and in neonatal livestock. Following infection, parasite reproduction increases for a number of days before it is blunted and then impeded by innate and adaptive immune responses. Immunocompromised hosts often cannot establish strong immunity and develop chronic infections that can lead to death. Few drugs consistently inhibit parasite reproduction in the host, and chemotherapy might be ineffective in immunodeficient hosts. Future options for prevention or treatment of cryptosporidiosis might include vaccines or recombinant immunological molecules, but this will probably require a better understanding of both the mucosal immune system and intestinal immune responses to the parasite.

Vitamin A and immune regulation: role of retinoic acid in gut-associated dendritic cell education, immune protection and tolerance

The vitamin A (VA) metabolite all-trans retinoic acid (RA) plays a key role in mucosal immune responses. RA is produced by gut-associated dendritic cells (DC) and is required for generating gut-tropic lymphocytes and IgA-antibody-secreting cells (IgA-ASC). Moreover, RA modulates Foxp3(+) regulatory T cell (T(REG)) and Th17 effector T cell differentiation. Thus, although RA could be used as an effective "mucosal adjuvant" in vaccines, it also appears to be required for establishing intestinal immune tolerance. Here we discuss the roles proposed for RA in shaping intestinal immune responses and tolerance at the gut mucosal interface. We also focus on recent data exploring the mechanisms by which gut-associated DC acquire RA-producing capacity.

Exploiting mucosal surfaces for the development of mucosal vaccines

Mucosal immunity covers a variety of mucosal surfaces susceptible to different pathogens. This review highlights the diversity of mucosal tissues and the unique microenvironments in which an immune response is generated. It argues that tissue-specific factors present throughout mucosal tissues and lymph nodes determine the differentiation into IgA-producing B cells, which in turn determines their migration patterns. Mucosal immunity can therefore be induced when antigen is delivered at any mucosal tissue without the need for specific 'mucosal adjuvants' or targeting to specialised lymphoid structures. Non-oral vaccination strategies directed at alternative and more accessible mucosal tissue sites, may provide new avenues for both mucosal and systemic immunization, and will be greatly facilitated by the use of large animal models.