Dendritic cells govern induction and reprogramming of polarized tissue-selective homing receptor patterns of T cells: important roles for soluble factors and tissue microenvironments

25 years of UV-induced immunosuppression mediated by T cells-from disregarded T suppressor cells to highly respected regulatory T cells

For more than 25 years it is known that UV radiation, in particular the UVB range suppresses the immune system. In contrast to conventional immunosuppression by immunosuppressive drugs, UV radiation does not compromise the immune system in a general but rather in an antigen-specific fashion via induction of immunotolerance. This effect is mostly mediated via regulatory T cells (Treg) induced by UV. Several subtypes of UV-induced Treg may exist, the best characterized are those which inhibit contact hypersensitivity. Induction of these Tregs by UV radiation is an active process which requires antigen presentation by UV-damaged but still alive Langerhans cells (LC) in the lymph nodes. UV-induced Treg have recently been characterized as expressing CD4 and CD25 and as releasing upon activation the immunosuppressive cytokine interleukin (IL)-10. Once activated in an antigen-specific manner, they suppress immune responses in a general fashion via the release of IL-10, a phenomenon called bystander suppression. The further phenotypic and functional characterization of these cells will not only contribute to a better understanding of the impact of UV radiation on the immune system but will also determine whether they can be applied in the future therapeutically with the final aim of achieving specific immunosuppression.

Differentiation and homing of IgA-secreting cells

Most antibody-secreting cells (ASCs) in mucosal tissues produce immunoglobulin A (IgA), the most abundant immunoglobulin in the body and the main class of antibody found in secretions. IgA-ASCs differentiate in the mucosal-associated lymphoid tissues and are usually considered as a homogeneous population of cells. However, IgA-ASCs that travel to the small intestine have unique characteristics in terms of their migratory requirements. These IgA-ASCs require the homing molecules alpha4beta7 and CCR9 to interact with their ligands, mucosal addressin cell adhesion molecule-1 and CCL25, which are constitutively expressed in the small intestine. Indeed, recent work has shown that IgA-ASCs specific for the small bowel are generated under different conditions as compared with IgA-ASCs in other mucosal compartments. Moreover, the mechanisms inducing IgA class switching may also vary according to the tissue where IgA-ASCs differentiate. Here we describe the mechanisms involved in the differentiation of IgA-ASCs in mucosal compartments, in particular those involved in the generation of gut-homing IgA-ASCs.

Homing imprinting and immunomodulation in the gut: role of dendritic cells and retinoids

Lymphocyte migration is at the heart of chronic inflammatory ailments, including inflammatory bowel disease (IBD). Whereas naïve lymphocytes migrate to all secondary lymphoid organs, they are mostly excluded from nonlymphoid peripheral tissues. Upon activation, lymphocytes change their pattern of adhesion receptors and acquire the capacity to migrate to extralymphoid tissues. Antigen-experienced T cells are subdivided into different subsets based on their expression of homing receptors that favor their accumulation in specific tissues, such as the skin and the gut mucosa. B cells and antibody-secreting cells (ASC) also show tissue-tropism, which is somewhat correlated with the class of immunoglobulin that they produce. In fact, IgA-ASC are located in mucosal tissues, where they produce IgA, the main class of antibodies found in secretions. Although IgA-ASC are usually considered as a homogeneous pool of cells, those located in the small bowel have some unique migratory characteristics, suggesting that they are generated under different conditions as compared to IgA-ASC in other mucosal compartments. Foxp3(+) regulatory T cells (T(REG)) can also exhibit tissue-specific migratory potential and recent evidence suggests that T(REG) can be imprinted with gut-specific homing. Moreover, foxp3(+) T(REG) are enriched in the small bowel lamina propria, where they can be generated locally. The present review addresses our current understanding of how tissue-specific homing is acquired and modulated on T cells, B cells, and ASC, with a special emphasis on the intestinal mucosa. Harnessing these mechanisms could offer novel, effective, and more specific therapeutic strategies in IBD.

Organ-derived dendritic cells have differential effects on alloreactive T cells

Dendritic cells (DCs) are considered critical for the induction of graft-versus-host disease (GVHD) after bone marrow transplantation (BMT). In addition to their priming function, dendritic cells have been shown to induce organ-tropism through induction of specific homing molecules on T cells. Using adoptive transfer of CFSE-labeled cells, we first demonstrated that alloreactive T cells differentially up-regulate specific homing molecules in vivo. Host-type dendritic cells from the GVHD target organs liver and spleen or skin- and gut-draining lymph nodes effectively primed naive allogeneic T cells in vitro with the exception of liver-derived dendritic cells, which showed less stimulatory capacity. Gut-derived dendritic cells induced alloreactive donor T cells with a gut-homing phenotype that caused increased GVHD mortality and morbidity compared with T cells stimulated with dendritic cells from spleen, liver, and peripheral lymph nodes in an MHC-mismatched murine BMT model. However, in vivo analysis demonstrated that the in vitro imprinting of homing molecules on alloreactive T cells was only transient. In conclusion, organ-derived dendritic cells can efficiently induce specific homing molecules on alloreactive T cells. A gut-homing phenotype correlates with increased GVHD mortality and morbidity after murine BMT, underlining the importance of the gut in the pathophysiology of GVHD.

Retinoic acid receptor signaling levels and antigen dose regulate gut homing receptor expression on CD8+ T cells

Recent studies have highlighted a central role for intestinal dendritic cells (DCs) and vitamin A metabolite retinoic acid (RA) in the generation of alpha4beta7(+) CCR9(+)"gut tropic" effector T cells. Here, using RA-responsive element reporter mice, we demonstrate that both splenic and mesenteric lymph node (MLN) DCs enhanced retinoic acid receptor (RAR) signaling in CD8(+) T cells; however, only a subset of MLN DCs, expressing the integrin alpha-chain CD103, induced an early RAR signal that is required for efficient CCR9 induction. MLN-primed CD8(+) T cells also received enhanced RAR-dependent signals compared with splenic-primed CD8(+) T cells in vivo. Further DC-mediated induction of gut homing receptors was inhibited at a high antigen dose without influencing RAR signaling events, and resulted in less efficient CD8(+) T-cell entry into the small intestinal mucosa. These results highlight a complex interplay between antigen dose and DC subset-induced RAR signaling events in the generation of tissue tropic effector T-cell subsets.

Th2 differentiation in distinct lymph nodes influences the site of mucosal Th2 immune-inflammatory responses

Allergic individuals rarely present with concurrent multiple-organ disease but, rather, with manifestations that privilege a specific site such as the lung, skin, or gastrointestinal tract. Whether the site of allergic sensitization influences the localization of Th2 immune-inflammatory responses and, ultimately, the organ-specific expression of disease, remains to be determined. In this study, we investigated whether both the site of initial Ag exposure and concomitant Th2 differentiation in specific lymph nodes (LNs) privileges Th2 memory responses to mucosal and nonmucosal sites, and whether this restriction is associated with a differential expression in tissue-specific homing molecules. In mice exposed to Ag (OVA) via the peritoneum, lung, or skin, we examined several local and distal LNs to determine the site of Ag-specific proliferation and Th2 differentiation. Whereas respiratory and cutaneous Ag exposure led to Ag-specific proliferation and Th2 differentiation exclusively in lung- and skin-draining LNs, respectively, Ag delivery to the peritoneum evoked responses in gut-associated, as well as distal thoracic, LNs. Importantly, only mice that underwent Th2 differentiation in thoracic- or gut-associated LNs mounted Th2 immune-inflammatory responses upon respiratory or gastric Ag challenge, respectively, whereas cutaneous Th2 recall responses were evoked irrespective of the site of initial sensitization. In addition, we observed the differential expression of gut homing molecules (CCR9, alpha(4), beta(7)) in gut-associated LNs and, unexpectedly, a universal induction of skin-related homing molecules (CCR4, CCR10) in all LNs. These data suggest that the site of initial Th2 differentiation and differential homing molecule expression restricts Th2 immune-inflammatory responses to mucosal, but not cutaneous, tissues.

Lactic acid bacterial colonization and human rotavirus infection influence distribution and frequencies of monocytes/macrophages and dendritic cells in neonatal gnotobiotic pigs

Despite accumulating knowledge of porcine macrophages and dendritic cells (DCs) from in vitro studies, information regarding monocytes/macrophages and DCs in lymphoid tissues of enteric pathogen-infected neonatal animals in vivo is limited. In this study we evaluated the influence of commensal bacterial [two strains of lactic acid bacteria (LAB), Lactobacillus acidophilus and L. reuteri] colonization and rotavirus infection on distribution and frequencies of monocytes/macrophages and conventional DCs (cDCs) in ileum, spleen and blood. Gnotobiotic pigs were inoculated with LAB and virulent Wa strain human rotavirus (HRV) (LAB+HRV+), HRV only (LAB−HRV+), LAB only (LAB+HRV−) or mock (LAB−HRV−). The cDCs were characterized as SWC3⁺CD11R1⁺, whereas monocytes/macrophages were identified as SWC3⁺CD11R1⁻ by flow cytometry in the gnotobiotic pigs at 10 days of age. Infection with HRV alone activated/recruited significantly more monocytes/macrophages to the intestine than LAB colonization and 56% versus 28% of these cells expressed CD14. Colonization with LAB alone also significantly increased the frequencies of monocytes/macrophages and cDCs and the CD14 expression on monocytes/macrophages in ileum and spleen compared to the controls. LAB colonization plus HRV infection significantly reduced macrophage and cDC frequencies in spleen compared to LAB colonization or HRV infection alone, suggesting that LAB colonization down-regulated HRV− infection-induced monocyte/macrophage activation/recruitment at the systemic lymphoid tissue. These results illustrated the distribution of porcine monocytes/macrophages and cDCs and the frequencies of CD14 expression on these cells in intestinal and systemic lymphoid tissues in the early stage of immune responses to intestinal colonization by LAB versus infection by an enteric pathogen HRV and will facilitate further in vivo studies on functional characterization of these immune cells in neonates.

In Vitro Induction of Mucosa-Type Dendritic Cells by All-Trans Retinoic Acid

Efficient induction of mucosal immunity usually employs nasal or oral vaccination while parenteral immunization generally is ineffective at generating mucosal immune responses. This relates to the unique ability of resident mucosal dendritic cells (DC) to induce IgA switching and to imprint mucosa-specific homing receptors on lymphocytes. Based on the well-established plasticity of the DC system, this study sought to investigate whether peripheral DC could be modulated toward "mucosa-type" DC by treatment with immunomodulatory, and therefore potentially adjuvant-like, factors. In this study, we show that monocyte-derived DCs pretreated with the vitamin A derivative all-trans retinoic acid (RA) indeed acquired several attributes characteristic of mucosal DC: secretion of TGF-beta and IL-6 and the capacity to augment mucosal homing receptor expression and IgA responses in cocultured lymphocytes. Addition of a TGF-beta-neutralizing Ab to cocultures significantly inhibited alpha4beta7 integrin, but not CCR9 mRNA expression by the lymphocytes. Both alpha4beta7 integrin and CCR9 mRNA expression, but not IgA production, were suppressed in the presence of a RA receptor antagonist. None of the observed effects on the lymphocytes were influenced by citral, a retinal dehydrogenase inhibitor, arguing against a role for de novo-synthesized RA. Collectively, our findings identified a novel role for RA as a mucosal immune modulator targeting DC. Our results further demonstrate that DC can act as efficient carriers of RA at least in vitro. Consequently, RA targeting of DC shows potential for promoting vaccine-induced mucosal immune responses via a parenteral route of immunization.

Effector and memory CTL differentiation

Technological advances in recent years have allowed for an ever-expanding ability to analyze and quantify in vivo immune responses. MHC tetramers, intracellular cytokine staining, an increasing repertoire of transgenic and "knockout" mice, and the detailed characterization of a variety of infectious models have all facilitated more precise and definitive analyses of the generation and function of cytotoxic T lymphocytes (CTL). Understanding the mechanisms behind the differentiation of effector and memory CTL is of increasing importance to develop vaccination strategies against a variety of established and emerging infectious diseases. This review focuses on recent advances in our understanding of how effector and memory CTL differentiate and survive in vivo in response to viral or bacterial infection.

Long-Term Commitment to Inflammation-Seeking Homing in CD4+ Effector Cells

Access of T effector cells to sites of inflammation is a prerequisite for an efficient action in immune defense and is mediated by different, partly tissue-specific sets of adhesion molecules. To what extent lymphocytes memorize the site of initial priming and develop organ-specific homing properties is still a matter of debate. Notably, data on the stability of homing receptor expression on T cells in vivo are largely lacking. We approached this question by the adoptive transfer of CD4(+) T cells sorted for the expression of P-selectin ligands, which contribute to migration into inflamed sites in skin and other tissues. We observed long-term expression of P-selectin ligands on roughly one-third of effector cells. On those cells that had lost P-selectin ligands, re-expression upon Ag challenge was observed but only within pLNs, similar to the organ-selective induction upon the primary activation of naive T cells. The frequency of cells stably expressing P-selectin ligands was higher when cells were repeatedly stimulated under permissive conditions in the presence of IL-12, indicating a gradual fixation of this phenotype. In line with that finding, isolated P-selectin ligand positive memory T cells showed the highest frequency of long-term expressing cells. A tissue-specific environment was not required for the long-term maintenance of P-selectin ligand expression on the subfraction of effector cells. These data indicate that the expression of selectin ligands can become clonally imprinted under certain conditions, but also that a major fraction of the cells remains flexible and subject to environmental modulation upon restimulation.

Induction of organ-selective CD4+ regulatory T cell homing

Compelling evidence suggests that Foxp3(+)CD25(+)CD4(+) Treg play a fundamental role in immunoregulation. We have previously demonstrated that Treg have to enter peripheral tissues to suppress ongoing inflammation. However, relatively little is known about how Treg acquire the expression of homing receptors required for tissue- or inflammation-specific migration. Migratory properties of conventional naïve T cells are shaped by the tissue microenvironment and organ-specific dendritic cells during priming. Here, we show that this basic concept also holds true for CD25(+)CD4(+) Treg: Priming of Treg within peripheral LN led to the expression of selectin ligands, which facilitate migration into inflamed skin, whereas activation within mesenteric LN led to induction of the integrin alpha(4)beta(7), which is required for migration into mucosal tissues. Furthermore, we could establish in vitro culture systems containing either dendritic cells from mesenteric and peripheral LN, or retinoic acid and IL-12 as polarizing compounds to induce mucosa- and skin-seeking Treg, respectively. Together, our results demonstrate that Treg, similarly to conventional T cells, can be configured with organ-selective homing properties allowing efficient targeting into distinct tissues.

Immunological principles of T-cell-mediated adverse drug reactions in skin

Drug hypersensitivity reactions in skin are an immune-mediated phenomenon associated with significant patient mortality and morbidity. Antigen-specific T cells, which have been isolated from the peripheral circulation and target organs of hypersensitive patients, are thought to propagate and regulate the development of clinical symptoms. The investigation of clinical cases with respect to the basic cellular and chemical mechanisms that underpin drug hypersensitivity has resulted in: i) the need to redress some aspects of present immunological dogma; and ii) additional fundamental immunological questions. Thus, the aim of this review article is to summarise present opinion on how and why drugs initiate a pathogenic T-cell response in a small section of the population and subsequently reflect on gaps in basic immunology and where future research might lead.

Initiation of adaptive immune responses by transcutaneous immunization

The development of new, effective, easy-to-use and lower-cost vaccination approaches for the combat against malignant and infectious diseases is a pre-eminent need: cancer is a leading cause of morbidity in the Western World; there are numerous pathogenic diseases for which we still have no protective or therapeutic cure; and the financial limitations of developing countries to fight these diseases. In this mini-review we focus on transcutaneous immunization (TCI), a relatively new route for antigen delivery. TCI protocols appear to be particularly promising by gaining access to skin resident APC, which are highly efficient for the initiation of humoral and/or cellular immune responses. Consisting of an adjuvant as a stimulus in combination with an antigen which defines the target, TCI offers a most attractive immunization strategy to mount highly specific full-blown adaptive immune responses. As a topically applicable cell-free adjuvant/antigen mixture, TCI might be suitable to improve patient compliance, as well as feasible economically for the use in Third World countries. In addition, this non-invasive procedure might increase the safety of vaccinations by eliminating the risk of infections related to the recycling and improper disposal of needles. The dissection of antigen and adjuvant is important because it allows "free" combinations in contrast to classical immunizations which are based on application of the pathogen of interest. The most relevant ways and means to find new, effective pathogenic target antigens are "reverse vaccinology" and the direct peptide-epitope identification from MHC molecules with mass-spectrometry. Due to these efficient approaches the variety of antigenic epitopes for potential protective/therapeutic use is perpetually expanding. The most studied adjuvants in TCI approaches are cholera toxin (CT) and its less toxic relative, the heat-labile enterotoxin (LT). Both CT and LT can serve as antigen as well. In contrast to these large proteins, which can only penetrate "pre-treated" skin barrier, the immune response modifier, TLR7 agonist R-837 (Imiquimod) is a small compound adjuvant that easily passages non-disrupted epidermis. It remains currently elusive which cells of the complex-structured "skin-associated lymphoid tissue" (SALT) respond to the adjuvant and which APC carries the antigen to the draining lymphnodes for subsequent initiation of adaptive immune responses.

CD18 Is Required for Intestinal T Cell Responses at Multiple Immune Checkpoints

The intestinal immune response to oral Ags involves a complex multistep process. The requirements for optimal intestinal T cell responses in this process are unclear. LFA-1 plays a critical role in peripheral T cell trafficking and activation, however, its role in intestinal immune responses has not been precisely defined. To dissect the role of LFA-1 in intestinal immune responses, we used a system that allows for segregation of T cell migration and activation through the adoptive transfer of LFA-1-deficient (CD18(-/-)) CD4(+) T cells from DO11.10 TCR transgenic mice into wild-type BALB/c mice. We find that wild-type mice adoptively transferred with CD18(-/-) DO11.10 CD4(+) T cells demonstrate decreases in the numbers of Ag-specific T cells in the intestinal lamina propria after oral Ag administration. We also find that in addition to its role in trafficking to intestinal secondary lymphoid organs, LFA-1 is required for optimal CD4(+) T cell proliferation in vivo upon oral Ag immunization. Furthermore, CD18(-/-) DO11.10 CD4(+) T cells primed in the intestinal secondary lymphoid organs demonstrate defects in up-regulation of the intestinal-specific trafficking molecules, alpha(4)beta(7) and CCR9. Interestingly, the defect in trafficking of CD18(-/-) DO11.10 CD4(+) T cells to the intestinal lamina propria persists even under conditions of equivalent activation and intestinal-tropic differentiation, implicating a role for CD18 in the trafficking of activated T cells into intestinal tissues independent of the earlier defects in the intestinal immune response. This argues for a complex role for CD18 in the early priming checkpoints and ultimately in the trafficking of T cells to the intestinal tissues during an intestinal immune response.

Dendritic cell immunization route determines integrin expression and lymphoid and nonlymphoid tissue distribution of CD8 T cells

Exogenous dendritic cells (bone marrow-derived dendritic cell (BMDC)) display restricted trafficking in vivo after injection into mice, but the route(s) by which they generate gut-homing effector cells is unclear. Mesenteric lymph nodes (LN) and spleen were differentially targeted by i.p. and i.v. administration of BMDC, respectively, whereas mediastinal LN were targeted by both routes. BMDC injected by either route activated CD8(+) T cells to up-regulate both alpha(4)beta(1) and alpha(4)beta(7) integrins. However, the lymphoid compartment in which activation occurred determined their expression kinetics, magnitude, and population distribution. Only T cells activated in mesenteric LN after i.p. immunization expressed high levels of alpha(4)beta(7), which also correlated with localization to small intestine. These alpha(4)beta(7)(high) cells also redistributed to mediastinal LN in a manner sensitive to treatment with alpha(4)beta(7) blocking Abs, but not to mucosal addressin cell adhesion molecule-1 blocking Abs. Our results demonstrate the importance of lymphoid compartment, as dictated by immunization route, in determining integrin expression on activated T cells and their distribution in lymphoid and nonlymphoid tissues.

Generation of gut-homing T cells and their localization to the small intestinal mucosa

The intestinal mucosa represents the largest body surface toward the external environment and harbors numerous T lymphocytes that take up resident within the intestinal epithelium or in the underlying lamina propria (LP). The intraepithelial lymphocytes include subsets of 'unconventional' T cells with unclear ontogeny and reactivity that localize to this site independently of antigen-specific activation in secondary lymphoid organs. In contrast, the majority of the 'conventional' gut T cells are recruited into the intestinal mucosa subsequent to their activation in intestinal inductive sites, including Peyer's patches (PPs) and mesenteric lymph nodes (MLNs). T cells homing to the small intestine express a distinct pattern of homing molecules, allowing them to interact with and transmigrate across intestinal postcapillary endothelium. At least some of these homing molecules, including the integrin alpha(4)beta(7) and the chemokine receptor CCR9, are induced on T cells during their activation in PPs or MLNs. Mucosal dendritic cells (DCs) play a key role in this process, but not all intestinal DCs possess the ability to confer a gut-homing capacity to T cells. Instead, functionally specialized CD103(+) DCs derived from the small intestinal LP appear to selectively regulate T-cell homing to the small intestine.

Alteration of the Migratory Behavior of UV-Induced Regulatory T Cells by Tissue-Specific Dendritic Cells

UV radiation-induced regulatory T cells (UV-Treg) inhibit the sensitization but not the elicitation of contact hypersensitivity when injected i.v. Because UV-Treg express the lymph node homing receptor CD62 ligand, upon i.v. injection they migrate into the lymph nodes but not into the periphery and therefore inhibit sensitization but not elicitation. We tried to modify the migratory behavior of UV-Treg with the aim to get them into the periphery and thereby to suppress the effector phase of immune reactions. Because the tissue selective homing of T effector cells is determined by tissue-specific dendritic cells (DC), we attempted to reprogram the migratory behavior of UV-Treg by DC. 2,4-Dinitrofluorobencene (DNFB)-specific UV-Treg coincubated with epidermal Langerhans cells (LC) blocked the elicitation upon i.v. injection into DNFB-sensitized mice. In contrast, i.v. injection of UV-Treg not incubated with LC did not inhibit the ear challenge. The same negative effect was observed for UV-Treg coincubated with DC from bone marrow, spleen, or lymph nodes. This effect was not due to different maturation stages as checked by MHC class II expression of the different DC types. Incubation with LC but not with bone marrow-derived DC down-regulated the expression of CD62 ligand on UV-Treg. Accordingly, CFDA-SE labeled UV-Treg coincubated with LC were found in the ears but not in the lymph nodes upon i.v. injection. This finding shows that the migratory behavior can be reprogrammed by tissue-specific DC and may have input on strategies trying to use Treg not only for the prevention but also for the treatment of immune-mediated diseases.

Retinol concentrations after birth are inversely associated with atopic manifestations in children and young adults

BACKGROUND

Vitamin A has anti-inflammatory and immunomodulatory effects, and its deficiency results in impaired specific and innate immunity. Vitamin A is essential for inducing the gut-homing specificity on T cells.

OBJECTIVE

As an impaired gut immune response in early infancy may contribute to the development of atopic sensitization, we looked for an association of plasma retinol concentrations and the subsequent development of allergic symptoms in healthy infants.

METHODS

A cohort of 200 unselected, full-term newborns were followed up from birth to age 20 years. The plasma retinol concentration was determined in cord blood (n=97), at ages of 2, 4 and 12 months (n=95), and at ages 5 years (n=155) and 11 years (n=151). The subjects were re-examined at the ages of 5, 11 and 20 years with assessment of the occurrence of allergic symptoms during the preceding year, skin prick testing and measurement of serum total IgE.

RESULTS

subjects with allergic symptoms or a positive skin prick test (SPT) in childhood or adolescence had lower retinol concentrations in infancy and childhood than symptom-free subjects. The difference was most pronounced at age 2 months. Retinol concentration at 2 months correlated inversely with positive SPT at ages of 5 and 20 years, and with allergic symptoms at age 20 years.

CONCLUSION

Retinol concentration in young infants is inversely associated with the subsequent development of allergic symptoms. We propose that an inborn regulation of retinol may play a role in atopic sensitization, possibly through regulating the intestinal T cell responses.

The importance of regional lymph nodes for mucosal tolerance

The immune system is organized as a number of distinct lymphoid organs interconnected by recirculating lymphocytes. These organs, such as lymph nodes, spleen, and gut-associated Peyer's patches, are compartmentalized, providing separate niches for T and B cells. In addition, regional compartmentalization of lymphoid organs themselves exists, leading to the distinction between the mucosal and the systemic immune systems. This distinction not only reflects the anatomical localization but also is based on functional differences, with predominant tolerance induction via mucosal routes and immunity seen after systemic antigen exposure. These differences are associated with regional differences in the lymphoid organs and with environmental conditions of the tissues in which the immune system functions. Recirculation patterns of lymphocytes differ between mucosal and systemic lymphoid organs, and more insight into the mechanisms that imprint this behavior has been generated recently. Differences in dendritic cells have been observed between mucosal and systemic sites, and knowledge on how local factors contribute to the immune system is emerging. From our studies on mucosal tolerance in mouse models, it has become evident that regional lymph nodes draining the mucosa are important sites to direct immune responses. Here, we discuss the way regional lymph nodes contribute to the direction of immune responses and what is known about the local factors and cell behavior that form the basis for these differences.

Stromal-cell regulation of dendritic-cell differentiation and function

Dendritic cells (DCs) are the ubiquitous sentinels of the immune system, instructing and shaping the adaptive immune response. As such, DCs are often targeted directly by pathogens as a means of immune evasion. Although DCs in different anatomical locations originate from common bone-marrow-derived progenitors and, hence, share several characteristics, microenvironmental factors have an important influence on DC biology under both steady-state and inflammatory conditions. A growing body of literature suggests that these instructive processes are mediated by tissue stromal cells, empowering these cells with a decisive role in local immune regulation. Here, we review recent progress in this area, focussing on the role of stromal cells in supporting the generation of regulatory DCs, and propose that tissue stromal cells provide an alternate avenue whereby pathogens can influence DC function.

Generation of Gut-Homing IgA-Secreting B Cells by Intestinal Dendritic Cells

Normal intestinal mucosa contains abundant immunoglobulin A (IgA)-secreting cells, which are generated from B cells in gut-associated lymphoid tissues (GALT). We show that dendritic cells (DC) from GALT induce T cell-independent expression of IgA and gut-homing receptors on B cells. GALT-DC-derived retinoic acid (RA) alone conferred gut tropism but could not promote IgA secretion. However, RA potently synergized with GALT-DC-derived interleukin-6 (IL-6) or IL-5 to induce IgA secretion. Consequently, mice deficient in the RA precursor vitamin A lacked IgA-secreting cells in the small intestine. Thus, GALT-DC shape mucosal immunity by modulating B cell migration and effector activity through synergistically acting mediators.

Dynamic programming of CD8+ T cell trafficking after live viral immunization

After viral infection, activated T cells are present in multiple tissues regardless of the infection route. How these cells acquire pleiotropic homing ability is unclear. By using a cutaneous vaccinia virus infection model, we demonstrate that regulation of T cell trafficking is multiphasic. Upon completion of three cell divisions, CD8+ T cells upregulated specific skin-homing molecules within draining lymph nodes (LN). By 60 hr after infection, some activated T cells reached the infected tissue, while others entered distant antigen-free LN. These latter cells continued to divide and acquire additional tissue-homing molecules in this new setting, independent of antigen presentation. After viral clearance, the initial skin-homing imprint became the predominant homing phenotype on memory cells and provided superior protection against secondary cutaneous challenge. These observations demonstrate a mechanism by which T cells provide both immediate tissue-specific immune control at the pathogen entry site and a more flexible systemic protection against pathogen dissemination.

Tissue-tropic effector T cells: generation and targeting opportunities

The localization of effector T cells to extralymphoid tissues is crucial for the generation of an effective immune response, but it also underlies many autoimmune and inflammatory disorders. Recent studies have highlighted a central role for draining lymph nodes and environmentally imprinted dendritic cells in the generation of tissue-tropic effector T cells. Here, I outline our current understanding of the mechanisms that regulate the generation and localization of tissue-tropic effector T cells, and the potential ways in which these pathways can be exploited for immunotherapeutic purposes.

Regulatory T-cell physiology and application to treat autoimmunity

Endowed with the ability to actively suppress an immune response, regulatory T cells (Tregs) hold the promise of halting ongoing pathogenic autoimmunity and restoring self-tolerance in patients suffering from autoimmune diseases. Through many in vitro and in vivo studies, we have learned that Tregs can function in the lymph nodes as well as in the peripheral tissues. In vivo, Tregs act through dendritic cells to limit autoreactive T-cell activation, thus preventing their differentiation and acquisition of effector functions. By limiting the supply of activated pathogenic cells, Tregs prevent or slow down the progression of autoimmune diseases. However, this protective mechanism appears insufficient in autoimmune individuals, likely because of a shortage of Tregs cells and/or the development and accumulation of Treg-resistant pathogenic T cells over the long disease course. Thus, restoration of self-tolerance in these patients will likely require purging of pathogenic T cells along with infusion of Tregs with increased ability to control ongoing tissue injury. In this review, we highlight advances in dissecting Treg function in vivo in autoimmune settings and summarize multiple studies that have overcome the limitations of the low abundance of Tregs and their hypoproliferative phenotype to develop Treg-based therapies.

Active participation of antigen-nonspecific lymphoid cells in immune-mediated inflammation

The pathogenic process of tissue-specific autoimmune disease depends to a large extent on recruitment of Ag-nonspecific cells into the target tissue. Little is known, however, about the recruitment process and the features that characterize the recruited cells. In this study, we analyzed the recruitment of Ag-nonspecific lymphoid cells into an inflammatory site by using an experimental system in which TCR-transgenic Th1 cells are adoptively transferred to induce ocular inflammation in recipient mice that express the target Ag in their eyes. A sharp increase in number of all host cell populations was observed in the recipient spleen, reaching a peak on day 4 postcell transfer and declining thereafter. A large portion of the host's spleen CD4 cells underwent phenotypic changes that facilitate their migration into the target organ, the eye. These changes included increased expression of the chemokine receptor CXCR3, and the adhesion molecule CD49d, as well as a decline in expression of CD62L. The host lymphocytes migrated into the recipient mouse eye more slowly than the donor cells, but became the great majority of the infiltrating cells at the peak of inflammation on day 7 postcell injection. Interestingly, the mass migration of host T cells was preceded by an influx of host dendritic cells, that reached their peak on day 4 postcell injection. The eye-infiltrating host CD4 lymphocytes underwent additional changes, acquiring a profile of activated lymphocytes, i.e., up-regulation of CD25 and CD69. Our results thus provide new information about the active participation of Ag-nonspecific lymphoid cells in immune-mediated inflammation.

Aberrant homing of mucosal T cells and extra-intestinal manifestations of inflammatory bowel disease

Active inflammatory bowel disease (IBD) is often associated with simultaneous inflammation in the skin, eyes and joints. Inflammatory disease in the liver can also occur in patients with IBD but seems to be independent of inflammation in the bowel. In this Opinion article, we propose that the hepatic complications of IBD are mediated by long-lived mucosal T cells that are recruited to the liver in response to aberrantly expressed endothelial-cell adhesion molecules and chemokines that are normally restricted to the gut. Similar mechanisms might explain why certain diseases are associated with site-specific tissue distributions and might point to new therapeutic strategies that are based on modulating tissue-specific lymphocyte homing.

Immune response to xenobiotics in the skin: from contact sensitivity to drug allergy

Skin is the most frequent target of adverse drug reactions. These cutaneous drug reactions (CDRs) show varied clinical manifestations ranging from mildly discomforting rashes to life-threatening Stevens-Johnson syndrome or toxic epidermal necrolysis. Most CDRs appear to be immune mediated, although the mechanism by which they are initiated remains unclear. In this review, current knowledge of the mechanisms by which xenobiotics provoke immune responses in the skin after epicutaneous administration and how similar reactions may occur after systemic routes are summarised. This review also discusses a variety of genetic or environmental factors that may determine the susceptibility of individuals towards immune responses in skin following drug exposure.

Inhalation Tolerance Is Induced Selectively in Thoracic Lymph Nodes but Executed Pervasively at Distant Mucosal and Nonmucosal Tissues

Under immunogenic conditions, both the site of initial Ag exposure and consequent T cell priming in specific draining lymph nodes (LNs) imprint the ensuing immune response with lasting tissue-selective tropism. With respect to immune tolerance, whether the site of tolerance induction leads to compartmentalized or, alternatively, pervasive tolerance has not been formally investigated. Using a murine model of inhalation tolerance, we investigated whether the induction of respiratory mucosal tolerance precludes the development of de novo Th2 sensitization upon subsequent exposure to the same Ag at distant mucosal (gut) and nonmucosal (cutaneous) sites. By tracking the proliferation of CFSE-labeled OVA-TCR transgenic CD4(+) T cells upon OVA inhalation in vivo, we defined the site of tolerance induction to be restricted to the thoracic LNs. Expectedly, inhalation tolerance prevented de novo Th2 sensitization upon subsequent exposure to the same Ag at the same site. Importantly, although gut- and skin-draining LNs were not used during tolerance induction, de novo Ag-specific proliferation and Th2 differentiation in these LNs, as well as memory/effector Th2 responses in the gut (allergic diarrhea) and skin (late-phase cutaneous responses) were inhibited upon immunogenic challenge to the same Ag. Interestingly, this pervasive tolerogenic phenotype was not associated with the presence of suppressive activity throughout the lymphatics; indeed, potent suppressive activity was detected solely in the spleen. These data indicate that while inhalation tolerance is selectively induced in local thoracic LNs, its tolerogenic activity resides systemically and leads to pervasive immune tolerance in distant mucosal and nonmucosal sites.

Essential role for CD103 in the T cell-mediated regulation of experimental colitis

The integrin CD103 is highly expressed at mucosal sites, but its role in mucosal immune regulation remains poorly understood. We have analyzed the functional role of CD103 in intestinal immune regulation using the T cell transfer model of colitis. Our results show no mandatory role for CD103 expression on T cells for either the development or CD4⁺CD25⁺ regulatory T (T reg) cell–mediated control of colitis. However, wild-type CD4⁺CD25⁺ T cells were unable to prevent colitis in immune-deficient recipients lacking CD103, demonstrating a nonredundant functional role for CD103 on host cells in T reg cell–mediated intestinal immune regulation. Non–T cell expression of CD103 is restricted primarily to CD11c^highMHC class II^high dendritic cells (DCs). This DC population is present at a high frequency in the gut-associated lymphoid tissue and appears to mediate a distinct functional role. Thus, CD103⁺ DCs, but not their CD103⁻ counterparts, promoted expression of the gut-homing receptor CCR9 on T cells. Conversely, CD103⁻ DCs promoted the differentiation of IFN-γ–producing T cells. Collectively, these data suggest that CD103⁺ and CD103⁻ DCs represent functionally distinct subsets and that CD103 expression on DCs influences the balance between effector and regulatory T cell activity in the intestine.

Functional specialization of gut CD103+ dendritic cells in the regulation of tissue-selective T cell homing

Gut-associated lymphoid tissue (GALT) dendritic cells (DCs) display a unique ability to generate CCR9⁺α₄β₇⁺ gut-tropic CD8⁺ effector T cells. We demonstrate efficient induction of CCR9 and α₄β₇ on CD8⁺ T cells in mesenteric lymph nodes (MLNs) after oral but not intraperitoneal (i.p.) antigen administration indicating differential targeting of DCs via the oral route. In vitro, lamina propria (LP)–derived DCs were more potent than MLN or Peyer's patch DCs in their ability to generate CCR9⁺α₄β₇⁺ CD8⁺ T cells. The integrin α chain CD103 (α_E) was expressed on almost all LP DCs, a subset of MLN DCs, but on few splenic DCs. CD103⁺ MLN DCs were reduced in number in CCR7^−/− mice and, although CD8⁺ T cells proliferated in the MLNs of CCR7^−/− mice after i.p. but not oral antigen administration, they failed to express CCR9 and had reduced levels of α₄β₇. Strikingly, although CD103⁺ and CD103⁻ MLN DCs were equally potent at inducing CD8⁺ T cell proliferation and IFN-γ production, only CD103⁺ DCs were capable of generating gut-tropic CD8⁺ effector T cells in vitro. Collectively, these results demonstrate a unique function for LP-derived CD103⁺ MLN DCs in the generation of gut-tropic effector T cells.