A functionally specialized population of mucosal CD103+ DCs induces Foxp3+ regulatory T cells via a TGF-beta and retinoic acid-dependent mechanism

The interleukin-23 axis in intestinal inflammation

Immune responses in the intestine are tightly regulated to ensure host protective immunity in the absence of immune pathology. Interleukin-23 (IL-23) has recently been shown to be a key player in influencing the balance between tolerance and immunity in the intestine. Production of IL-23 is enriched within the intestine and has been shown to orchestrate T-cell-dependent and T-cell-independent pathways of intestinal inflammation through effects on T-helper 1 (Th1) and Th17-associated cytokines. Furthermore, IL-23 restrains regulatory T-cell responses in the gut, favoring inflammation. Polymorphisms in the IL-23 receptor have been associated with susceptibility to inflammatory bowel diseases (IBDs) in humans, pinpointing the IL-23 axis as a key, conserved pathway in intestinal homeostasis. In addition to its role in dysregulated inflammatory responses, there is also evidence that IL-23 and the Th17 axis mediate beneficial roles in host protective immunity and barrier function in the intestine. Here we discuss the dual roles of IL-23 in intestinal immunity and how IL-23 and downstream effector pathways may make novel targets for the treatment of IBD.

New insights into the roles of dendritic cells in intestinal immunity and tolerance

Dendritic cells (DCs) play a critical key role in the initiation of immune responses to pathogens. Paradoxically, they also prevent potentially damaging immune responses being directed against the multitude of harmless antigens, to which the body is exposed daily. These roles are particularly important in the intestine, where only a single layer of epithelial cells provides a barrier against billions of commensal microorganisms, pathogens, and food antigens, over a huge surface area. In the intestine, therefore, DCs are required to perform their dual roles very efficiently to protect the body from the dual threats of invading pathogens and unwanted inflammatory reactions. In this review, we first describe the biology of DCs and their interactions with other cells types, paying particular attention to intestinal DCs. We, then, examine the ways in which this biology may become misdirected, resulting in inflammatory bowel disease. Finally, we discuss how DCs potentiate immune responses against viral, bacterial, parasitic infections, and their importance in the pathogenesis of prion diseases. We, therefore, provide an overview of the complex cellular interactions that affect intestinal DCs and control the balance between immunity and tolerance.

Naturally occurring regulatory dendritic cells regulate murine cutaneous chronic graft-versus-host disease

Chronic graft-versus-host disease (cGVHD) is a limiting factor in allogeneic hematopoietic stem cell transplantation (alloHSCT) for the treatment of leukemia and other malignancies. Relative to the process that initiates and promotes cGVHD, the regulation is poorly understood. In this study, we examined the role of naturally occurring regulatory dendritic cells (DC(regs)) in murine major histocompatibility complex (MHC)-compatible and multiple minor histocompatibility antigen (miHAg)-incompatible model of cGVHD in alloHSCT. DC(regs) generated from bone marrow in vitro (BM-DC(regs)) exclusively expressed CD200 receptor 3 (CD200R3), which exerted a suppressive function in the Ag-specific CD4(+) T-cell response. CD49(+)CD200R3(+) cells showed similarities in phenotype and function to BM-DC(regs), which formally distinguishes them from other leukocytes, suggesting that they are the natural counterpart of BM-DC(regs). Treatment of the recipient mice after alloHSCT with the recipient-type CD49(+)CD200R3(+) cells as well as BM-DC(regs) protected against cGVHD, and the protection was associated with the generation of Ag-specific anergic CD4(+) T cells as well as CD4(+)CD25(+)Foxp3(+) regulatory T cells (T(regs)) from donor-derived alloreactive CD4(+)CD25(-)Foxp3(-) T cells. In addition, the depletion of CD49(+)CD200R3(+) cells before alloHSCT enhanced the progression of cGVHD. In conclusion, CD49(+)CD200R3(+) cells act as naturally occurring DC(regs) to regulate the pathogenesis of cGVHD in alloHSCT mediated through the control of the transplanted alloreactive CD4(+) T cells.

Early recruitment of natural CD4+ Foxp3+ Treg cells by infective larvae determines the outcome of filarial infection

Human helminth infections are synonymous with impaired immune responsiveness indicating suppression of host immunity. Using a permissive murine model of filariasis, Litomosoides sigmodontis infection of inbred mice, we demonstrate rapid recruitment and increased in vivo proliferation of CD4(+)Foxp3(+) Treg cells upon exposure to infective L3 larvae. Within 7 days post-infection this resulted in an increased percentage of CD4(+)T cells at the infection site expressing Foxp3. Antibody-mediated depletion of CD25(+) cells prior to infection to remove pre-existing 'natural' CD4(+)CD25(+)Foxp3(+) Treg cells, while not affecting initial larval establishment, significantly reduced the number of adult parasites recovered 60 days post-infection. Anti-CD25 pre-treatment also impaired the fecundity of the surviving female parasites, which had reduced numbers of healthy eggs and microfilaria within their uteri, translating to a reduced level of blood microfilaraemia. Enhanced parasite killing was associated with augmented in vitro production of antigen-specific IL-4, IL-5, IL-13 and IL-10. Thus, upon infection filarial larvae rapidly provoke a CD4(+)Foxp3(+) Treg-cell response, biasing the initial CD4(+) T-cell response towards a regulatory phenotype. These CD4(+)Foxp3(+) Treg cells are predominantly recruited from the 'natural' regulatory pool and act to inhibit protective immunity over the full course of infection.

It takes guts for tolerance: the phenomenon of oral tolerance and the regulation of autoimmune response

The intestinal tract is a peculiar environment due to its constant contact with the microbiota agents, food antigens and other molecules. Such exposure requires the establishment of important regulatory mechanisms in order to avoid inflammatory response and self aggression. In this context, the GALT plays a very relevant role due to the presence of several different cellular populations which are the main players in this phenomenon. Moreover, it was described a while ago that the oral ingestion of a given molecule is able to induce systemic tolerance to the same molecule when it is used as an immunogen by parenteral route, known as oral tolerance. This observation led researches to use these mechanisms to induce tolerance against cognate antigens of different autoimmune diseases. In this context, in this review we focused on several tolerance inducing mechanisms which are relevant not only for the maintenance of intestinal tract but also for the suppression of T effector cells, such as Th1, Th2 and the newly described Th17 cells. To name a few, CD103(+) dendritic cells, Tr1 cells derived IL-10 secretion, Foxp3 conversion and CD4(+)LAP(+) regulatory cells induction are among the recently described features of the tolerogenic environment of the intestinal tract.

Functional analysis of FOXP3

Tolerance to self antigens is established in two ways: first in the thymus through the deletion of thymocytes expressing self-reactive T cell receptors; and second, in the periphery through multiple mechanisms involving deletion, anergy, and suppression. Dominant tolerance to self antigens in the periphery is primarily the function of the CD4(+)CD25(+)FOXP3(+) subset of T cells, which have the capability of suppressing autoreactive T cells that have escaped deletion during thymic selection. The essential role of the transcription factor FOXP3 in the development and function of these cells has been well documented. However, the underlying mechanisms by which FOXP3 controls this process are less well understood. This review will focus on the role of FOXP3 in regulating CD4 T cell function in both humans and mice, with an emphasis on recent work in human systems.

Retinoic acid in the immune system

On occasion, emerging scientific fields intersect and great discoveries result. In the last decade, the discovery of regulatory T cells (T(reg)) in immunity has revolutionized our understanding of how the immune system is controlled. Intersecting the rapidly emerging field of T(reg) function, has been the discovery that retinoic acid (RA) controls both the homing and differentiation of T(reg). Instantly, the wealth and breadth of knowledge of the molecular basis for RA action, its receptors, and how it controls cellular differentiation can and will be exploited to understand its profound effects on T(reg). Historically, vitamin A deprivation and repletion and RA agonists have been shown to profoundly affect immunity. Now these findings can be interpreted in light of the revelations that RA controls leukocyte homing and T(reg) function.

Expansion of peripheral naturally occurring T regulatory cells by Fms-like tyrosine kinase 3 ligand treatment

Fms-like tyrosine kinase 3 ligand (FLT3L) plays a major role in dendritic cell (DC) biology. Deficiency of FLT3L causes a dramatic decrease in DC numbers, whereas increasing its availability (by repetitive injections for 7-10 days) leads to a 10-fold increase in DC numbers. In this study, we show that FLT3L treatment indirectly leads to an expansion of peripheral naturally occurring T regulatory cells (NTregs). The FLT3L-induced increase in NTregs was still observed in thymectomized mice, ruling out the role of the thymus in this mechanism. Instead, the increased number of NTregs was due to proliferation of preexisting NTregs, most likely due to favored interactions with increased number of DCs. In vitro, we show that DCs induce regulatory T-cell (Treg) proliferation by direct cell contact and in an interleukin-2-dependent, T-cell receptor-independent manner. FLT3L could prevent death induced by acute graft-versus-host disease (GVHD). This study demonstrates unique aspects in the regulation of Treg homeostasis by DCs, which were unappreciated until now. It also reinforces the relevance of FLT3L treatment in GVHD by its ability to increase both the number of tolerizing DCs and NTregs.

Contrasting roles for all-trans retinoic acid in TGF-beta-mediated induction of Foxp3 and Il10 genes in developing regulatory T cells

Extrathymic induction of regulatory T (T reg) cells is essential to the regulation of effector T cell responses in the periphery. In addition to Foxp3, T reg cell expression of suppressive cytokines, such as IL-10, is essential for peripheral tolerance, particularly in the intestines. TGF-β has been shown to induce expression of Foxp3 as well as IL10 and the vitamin A metabolite; all-trans retinoic acid (RA [at-RA]) has been found to enhance the former. We report that in contrast to its enhancement of TGF-β–mediated Foxp3 induction, at-RA potently inhibits the TGF-β–mediated induction of Il10 in naive CD4 T cells. Thus, mucosal DC subsets that are active producers of at-RA inhibit induction of Il10 in naive CD4 T cells while promoting induction of Foxp3. Accordingly, mice with vitamin A deficiency have increased numbers of IL-10–competent T reg cells. Activation of DCs by certain Toll-like receptors (TLRs), particularly TLR9, suppresses T cell induction of Foxp3 and enables induction of Il10. Collectively, our data indicate that at-RA has reciprocal effects on the induction of Foxp3 and Il10 in developing CD4⁺ T reg cells and suggest that TLR9-dependent inhibition of at-RA production by antigen-presenting cells might represent one mechanism to promote the development of IL-10–expressing T cells.

Vitamin effects on the immune system: vitamins A and D take centre stage

Vitamins are essential constituents of our diet that have long been known to influence the immune system. Vitamins A and D have received particular attention in recent years as these vitamins have been shown to have an unexpected and crucial effect on the immune response. We present and discuss our current understanding of the essential roles of vitamins in modulating a broad range of immune processes, such as lymphocyte activation and proliferation, T-helper-cell differentiation, tissue-specific lymphocyte homing, the production of specific antibody isotypes and regulation of the immune response. Finally, we discuss the clinical potential of vitamin A and D metabolites for modulating tissue-specific immune responses and for preventing and/or treating inflammation and autoimmunity.

GM-CSF and IL-4 synergistically trigger dendritic cells to acquire retinoic acid-producing capacity

Retinoic acid (RA) produced by intestinal dendritic cells (DCs) imprints gut-homing specificity on lymphocytes and enhances Foxp3⁺ regulatory T-cell differentiation. The expression of aldehyde dehydrogenase (ALDH) 1A in these DCs is essential for the RA production. However, it remains unclear how the steady-state ALDH1A expression is induced under specific pathogen-free (SPF) conditions. Here, we found that bone marrow-derived dendritic cells (BM-DCs) generated with granulocyte-macrophage colony-stimulating factor (GM-CSF) expressed Aldh1a2, an isoform of Aldh1a, but that fms-related tyrosine kinase 3 ligand-generated BM-DCs did not. DCs from mesenteric lymph nodes (MLN) and Peyer's patches (PP) of normal SPF mice expressed ALDH1A2, but not the other known RA-producing enzymes. Employing a flow cytometric method, we detected ALDH activities in 10–30% of PP-DCs and MLN-DCs. They were CD11c^highCD4^−/lowCD8α^intermediateCD11b^−/low F4/80^{low/intermediate}CD45RB^lowCD86^highMHC class II^highB220⁻CD103⁺. Equivalent levels of aldehyde dehydrogenase activity (ALDHact) and ALDH1A2 expression were induced synergistically by GM-CSF and IL-4 in splenic DCs in vitro. In BM-DCs, however, additional signals via Toll-like receptors or RA receptors were required for inducing the equivalent levels. The generated ALDH1A2⁺ DCs triggered T cells to express gut-homing receptors or Foxp3. GM-CSF receptor-deficient or vitamin A-deficient mice exhibited marked reductions in the ALDHact in intestinal DCs and the T cell number in the intestinal lamina propria, whereas IL-4 receptor-mediated signals were dispensable. GM-CSF⁺CD11c⁻F4/80⁺ cells existed constitutively in the intestinal tissues. The results suggest that GM-CSF and RA itself are pivotal among multiple microenvironment factors that enable intestinal DCs to produce RA.

CD4+ regulatory T cells require CTLA-4 for the maintenance of systemic tolerance

Cytotoxic T lymphocyte antigen-4 (CTLA-4) plays a critical role in negatively regulating T cell responses and has also been implicated in the development and function of natural FOXP3⁺ regulatory T cells. CTLA-4–deficient mice develop fatal, early onset lymphoproliferative disease. However, chimeric mice containing both CTLA-4–deficient and –sufficient bone marrow (BM)–derived cells do not develop disease, indicating that CTLA-4 can act in trans to maintain T cell self-tolerance. Using genetically mixed blastocyst and BM chimaeras as well as in vivo T cell transfer systems, we demonstrate that in vivo regulation of Ctla4^−/− T cells in trans by CTLA-4–sufficient T cells is a reversible process that requires the persistent presence of FOXP3⁺ regulatory T cells with a diverse TCR repertoire. Based on gene expression studies, the regulatory T cells do not appear to act directly on T cells, suggesting they may instead modulate the stimulatory activities of antigen-presenting cells. These results demonstrate that CTLA-4 is absolutely required for FOXP3⁺ regulatory T cell function in vivo.

From the diet to the nucleus: vitamin A and TGF-beta join efforts at the mucosal interface of the intestine

The vitamin A metabolites, including retinoic acid (RA), form ligands for retinoic acid-related nuclear receptors and together they play pleiotropic roles in various biological processes. Recently, we described that RA also functions as a key modulator of transforming growth factor-beta (TGF-beta)-driven immune deviation, capable of suppressing the differentiation of interleukin-17 secreting T helper cells (T(H)17) and conversely promoting the generation of Foxp3(+) T regulatory (Treg) cells. This review will focus on the role of RA in the reciprocal TGF-beta-driven differentiation of T(H)17 and Treg and on the importance of such regulatory mechanism to control a functional immune system, in particular at the mucosal interface of the intestine.

Prevention of allograft rejection by amplification of Foxp3(+)CD4(+)CD25(+) regulatory T cells

CD4(+)CD25(+) T cells were identified originally as potent suppressors of autoimmunity and were later termed "natural regulatory T cells" or nTreg cells. Subsequently, a transcription factor called forkhead box protein 3 (Foxp3) was identified to be a critical regulator for Treg differentiation and function. Foxp3(+)CD4(+)CD25(+) Treg cells have been increasingly documented to suppress allograft rejection and to mediate allograft tolerance in transplantation. In this article, the authors review current approaches for amplification of allo-specific Foxp3(+)CD4(+)CD25(+) Treg cells for prevention of allograft rejection and induction of allo-specific transplant tolerance.

Suppression of experimental colitis in mice by CD11c+ dendritic cells

BACKGROUND

The innate immune system serves a critical role in homeostasis of the gastrointestinal (GI) tract. Both macrophages (MØs) and dendritic cells (DCs) have been shown to have pathogenic roles in animal models of inflammatory bowel disease. However, studies by several labs have established that resident MØs and DCs within the normal GI tract maintain an immunosuppressive phenotype compared to that seen in other peripheral sites. Recent studies by our lab demonstrated that the depletion of both MØs and DCs before the initiation of dextran sodium sulfate (DSS)-induced colitis resulted in exacerbation of disease, partly caused by increased neutrophil influx.

METHODS/RESULTS

In this current report, DSS-induced colitis was shown to be significantly more severe when DCs were selectively depleted in mice as indicated by changes in weight loss, stool consistency, rectal bleeding, and histopathology. In contrast to enhanced colitis in MØ/DC-depleted mice, which was associated with increased neutrophil influx, increased colitis in DC-depleted mice was not associated with an increase in neutrophils in the colon, as shown by CXCL1 chemokine levels and myeloperoxidase (MPO) activity. However, increased IL-6 gene and protein expression in colon tissues correlated positively with increased colitis severity in DC-depleted mice compared to colitis in DC-intact mice.

CONCLUSIONS

This study demonstrates that resident DCs can suppress the severity of acute DSS colitis and that regulation of IL-6 production may contribute to DC-mediated control of intestinal inflammation.

Antagonism of airway tolerance by endotoxin/lipopolysaccharide through promoting OX40L and suppressing antigen-specific Foxp3+ T regulatory cells

Respiratory exposure to allergens can lead to airway tolerance. Factors that antagonize tolerance mechanisms in the lung might result in susceptibility to diseases such as asthma. We show that inhalation of endotoxin/LPS with Ag prevented airway tolerance and abolished protection from T cell-driven asthmatic lung inflammation. Under conditions leading to tolerance, adaptive Ag-specific CD4(+)Foxp3(+) T regulatory cells (Treg) were generated following exposure to intranasal Ag and outnumbered IL-4- and IFN-gamma-producing CD4 T cells by 100:1 or greater. Inhaled LPS altered the ratio of Treg to IL-4(+) or IFN-gamma(+) T cells by concomitantly suppressing Treg generation and promoting effector T cell generation. LPS induced OX40L expression on dendritic cells and B cells that resulted in a synergistic activity between TLR4 and OX40 signals, leading to production of IL-4, IFN-gamma, and IL-6, which blocked Treg development. Furthermore, inhibiting OX40/OX40L interactions prevented LPS from suppressing tolerance, and resulted in the generation of greater numbers of adaptive Treg. Thus, cooperation between TLR4 and OX40 controls susceptibility to developing airway disease via modulating the balance between adaptive Treg and IL-4(+) or IFN-gamma(+) T cells. Targeting OX40L then has the potential to improve the efficacy of Ag immunotherapy to promote tolerance.

CD103 is dispensable for anti-viral immunity and autoimmunity in a mouse model of virally-induced autoimmune diabetes

Recent studies suggest a beneficial role for blocking CD103 signaling in preventing islet allograft rejection and thus Type 1 diabetes (T1D) in non-obese diabetic (NOD) mice. However, antibody blockade approaches generally raise anti-microbial safety issues, necessitating additional studies to address the possible adverse effects of antibody therapy. Here we report that CD103 had no significant impact on the development of primary and memory CD8(+) or CD4(+) responses after acute lymphocytic choriomeningitis virus (LCMV) infection. In addition, CD103 was found to be dispensable for T1D progression in a rapid, CD8-mediated virally-induced T1D model (the rat insulin promoter [RIP]-LCMV), suggesting that its previous efficacy in the NOD mouse model may not be related to its effect on the generation, memory conversion and/or effector function of CD8(+) or CD4(+) T cells. While the data does not preclude a role for CD103 in T1D in its entirety, the current study does provide much evidence to suggest that CD103 blockade may prove to be a safe intervention for autoimmunity and allo-transplantation. While in cases of rapid microbial (CD8)-driven T1D CD103 antibody blockade may not limit disease progression or severity, in mucosally-driven cases of T1D anti-CD103 antibody treatment may provide a new and safe therapeutic avenue.

CX3CR1+ c-kit+ bone marrow cells give rise to CD103+ and CD103- dendritic cells with distinct functional properties

Dendritic cells (DC) represent a rather heterogeneous cell population with regard to morphology, phenotype, and function and, like most cells of the immune system, are subjected to a continuous renewal process. CD103(+) (integrin alpha(E)) DC have been identified as a major mucosal DC subset involved in the induction of tissue-specific homing molecules on T cells, but little is known about progenitors able to replenish this DC subset. Herein we report that lineage (lin)(-)CX(3)CR1(+)c-kit(+) (GFP(+)c-kit(+)) bone marrow cells can differentiate to either CD11c(+)CD103(-) or CD11c(+)CD103(+) DC in vitro and in vivo. Gene expression as well as functional assays reveal distinct phenotypical and functional properties of both subsets generated in vitro. CD103(-) DC exhibit enhanced phagocytosis and respond to LPS stimulation by secreting proinflammatory cytokines, whereas CD103(+) DC express high levels of costimulatory molecules and efficiently induce allogeneic T cell proliferation. Following adoptive transfer of GFP(+)c-kit(+) bone marrow cells to irradiated recipients undergoing allergic lung inflammation, we identified donor-derived CD103(+) DC in lung and the lung-draining bronchial lymph node. Collectively, these data indicate that GFP(+)c-kit(+) cells contribute to the replenishment of CD103(+) DC in lymphoid and nonlymphoid organs.

Gut commensal flora: tolerance and homeostasis

Commensal microorganisms are not ignored by the intestinal immune system. Recent evidence shows that commensals actively participate in maintaining intestinal immune homeostasis by interacting with intestinal epithelial cells and delivering tolerogenic signals that are transmitted to the underlying cells of the immune system.

Dysrulation of T cell peripheral tolerance in type 1 diabetes

Type 1 diabetes (T1D) is a T cell-mediated autoimmune disease in which the insulin producing beta cells are destroyed. The breakdown of beta cell-specific self-tolerance by T cells involves a number of dysregulated events intrinsic and extrinsic to T cells. Herein, we review the key mechanisms that drive beta cell autoimmunity, with an emphasis on events that influence the expansion and differentiation of pathogenic T cells in the periphery.

Mucosal T-cell responses to HIV: responding at the front lines

Mucosal surfaces of the body serve as the major portal of entry for human immunodeficiency virus (HIV). These tissues also house a majority of the body's lymphocytes, including the CD4(+) T cells that are the major cellular target for HIV infection. Mucosal surfaces are defended by innate and adaptive immune mechanisms, including secreted antibodies and CD8(+) cytotoxic T cells (CTL). CTL in mucosal lymphoid tissues may serve to limit viral replication, decreasing the host's viral burden as well as reducing the likelihood of sexual transmission to a naïve host. This review summarizes recent literature on HIV-specific T-cell responses in mucosal tissues, with an emphasis on the gastrointestinal tract.

FOXP3 and its role in the immune system

FOXP3 is a member of the forkhead transcription factor family. Unlike other members, it is mainly expressed in a subset of CD4+ T-cells that play a suppressive role in the immune system. A function of FOXP3 is to suppress the function of NFAT and NFkappaB and this leads to suppression ofexpression of many genes including IL-2 and effector T-cell cytokines. FOXP3 acts also as a transcription activator for many genes induding CD2S, Cytotoxic T-Lymphocyte Antigen 4 (CTLA4), glucocorticoid-induced TNF receptorfamily gene (GITR) andfolate receptor 4. FOXP3+ T-cells are made in the thymus and periphery. The FOXP3+ T-cells made in the thymus migrate to secondary lymphoid tissues and suppress antigen priming of lymphocytes. Antigen priming of naive FOXP3 T-cdlls and naive FOXP3 T-cells leads to generation of memory FOXP3+ T-cells which are efficient in migration to nonlymphoid tissues. Memory FOXP3+ T-cells are, therefore, effective in suppression of effector T-cell function, while naive FOXP3 T-cells are adept at suppressing the early immune responses in lymphoid tissues. Both naive and memory FOXP3 T-cells are required for effective maintenance of tolerance and prevention of autoimmune diseases throughout the body. Many factors such as cytokines and noncytokine factors regulate the generation of FOXP3 T-cells. For example, retinoic acid, produced by the dendritic cells and epithelial cells in the intestine, works together with TGF-beta1 and promotes generation of small intestine-homing FOXP3 T-cells by upregulating the expression ofFOXP3 and gut homing receptors. FOXP3+ T-cells can be produced in vitro from autologous naive T-cells and, therefore, have great therapeutic potentials in treating a number of inflammatory diseases and grafi rejection.

Peripheral tolerance induction by lymph node stroma

In this review we have highlighted the role of LNSCs in the regulation of CD8+ T cell immune responses in peripheral lymph nodes, thereby adding another layer of protection, in addition to the role of resting DCs, against autoimmunity. LNSCs have recently been implicated in the induction of peripheral CD8+ T cell tolerance due to their ability to endogenously express, process, and present PTAs. Furthermore, LNSCs express surface molecules, such as MHC class II and PD-L1, similar to those expressed by mTECs in the thymus and APCs. For future studies it will be important to address some of the new questions that have emerged with respect to the biology and function of LNSCs. Further work will help us to (1) dissect the specific roles that DCs and LNSCs have in the induction and maintenance of tolerance to intestinal antigens, (2) gain a more in-depth understanding of the molecular mechanisms underlying self-tolerance induction by LNSCs and the impact of inflammation on this function, (3) evaluate the relationship of LNSCs to the FRN, and (4) determine if the APC function of LNSCs extends to the acquisition and presentation of exogenous antigens. Finally, it is important to mention that so far the studies done on LNSCs have focused on their role in CD8+ T cell tolerance. At the moment, we do not know if presentation of PTAs by LNSCs can also induce tolerance of CD4+ T cells. Based on the finding that LNSCs express MHC class II (I-A(b)) molecules it is possible that they may present self-antigens to CD4+ T cells and induce tolerance. However, this has yet to be elucidated.

IL-17 and Th17 Cells

CD4+ T cells, upon activation and expansion, develop into different T helper cell subsets with different cytokine profiles and distinct effector functions. Until recently, T cells were divided into Th1 or Th2 cells, depending on the cytokines they produce. A third subset of IL-17-producing effector T helper cells, called Th17 cells, has now been discovered and characterized. Here, we summarize the current information on the differentiation and effector functions of the Th17 lineage. Th17 cells produce IL-17, IL-17F, and IL-22, thereby inducing a massive tissue reaction owing to the broad distribution of the IL-17 and IL-22 receptors. Th17 cells also secrete IL-21 to communicate with the cells of the immune system. The differentiation factors (TGF-beta plus IL-6 or IL-21), the growth and stabilization factor (IL-23), and the transcription factors (STAT3, RORgammat, and RORalpha) involved in the development of Th17 cells have just been identified. The participation of TGF-beta in the differentiation of Th17 cells places the Th17 lineage in close relationship with CD4+CD25+Foxp3+ regulatory T cells (Tregs), as TGF-beta also induces differentiation of naive T cells into Foxp3+ Tregs in the peripheral immune compartment. The investigation of the differentiation, effector function, and regulation of Th17 cells has opened up a new framework for understanding T cell differentiation. Furthermore, we now appreciate the importance of Th17 cells in clearing pathogens during host defense reactions and in inducing tissue inflammation in autoimmune disease.

Intrarectal vaccination with recombinant vaccinia virus expressing carcinoembronic antigen induces mucosal and systemic immunity and prevents progression of colorectal cancer

The gastrointestinal mucosa contains an intact immune system that protects the host from pathogens and communicates with the systemic immune system. Absorptive epithelial cells in the mucosa give rise to malignant tumors although the interaction between tumor cells and the mucosal immune system is not well defined. The pathophysiology of colorectal cancer has been elucidated through studies of hereditary syndromes, such as familial adenomatous polyposis, a cancer predisposition syndrome caused by germline mutations in the adenomatous polyposis coli tumor suppressor gene. Patients with FAP develop adenomas and inevitably progress to invasive carcinomas by the age of 40. To better delineate the role of mucosal immunity in colorectal cancer, we evaluated the efficacy of intrarectal recombinant vaccinia virus expressing the human carcinoembryonic Ag (CEA) in a murine FAP model in which mice are predisposed to colorectal cancer and also express human CEA in the gut. Mucosal vaccination reduced the incidence of spontaneous adenomas and completely prevented progression to invasive carcinoma. The therapeutic effects were associated with induction of mucosal CEA-specific IgA Ab titers and CD8(+) CTLs. Mucosal vaccination was also associated with an increase in systemic CEA-specific IgG Ab titers, CD4(+) and CD8(+) T cell responses and resulted in growth inhibition of s.c. implanted CEA-expressing tumors suggesting communication between mucosal and systemic immune compartments. Thus, intrarectal vaccination induces mucosal and systemic antitumor immunity and prevents progression of spontaneous colorectal cancer. These results have implications for the prevention of colorectal cancer in high-risk individuals.

Functional maturation of lamina propria dendritic cells by activation of NKT cells mediates the abrogation of oral tolerance

We previously showed that although systemic administration of alpha-galactosylceramide (alphaGalCer) or agonistic anti-CD40 induced functional maturation of dendritic cells (DC) in mesenteric lymph nodes, only the former treatment succeeded in breaking the induction of oral tolerance. In this study, we looked for the essential factor responsible for the disruption of oral tolerance. We found that lamina propria (LP)-DC was responsible for the oral OVA presentation and that Peyer's patch was not essential for the induction of oral tolerance. Therefore, we investigated the role of LP-DC. Treatment with alphaGalCer but not with anti-CD40 induced the full maturation of LP-DC at an early time point. This functional activation of LP-DC was mediated by strong activation of NKT cells that reside abundantly in the small intestinal lamina propria (SI-LP) and interferon-gamma partially contributed to the LP-DC activation. LP-DC isolated from alphaGalCer-treated OVA-fed mice induced the differentiation of naïve CD4+ T cells into Th1 and Th2 and was associated with the reduced Foxp3+ population. In contrast, LP-DC isolated from anti-CD40-treated OVA-fed mice failed to generate Th cell differentiation but induced more Foxp3+ CD4+ T cells. Our results demonstrate that triggered by NKT cells in SI-LP, functional maturation of Ag-capturing DC from SI-LP is necessary for the abrogation of oral tolerance induction.

CD8+ CD205+ splenic dendritic cells are specialized to induce Foxp3+ regulatory T cells

Foxp3(+)CD25(+)CD4(+) regulatory T cells (Treg) mediate immunological self-tolerance and suppress immune responses. A subset of dendritic cells (DCs) in the intestine is specialized to induce Treg in a TGF-beta- and retinoic acid-dependent manner to allow for oral tolerance. In this study we compare two major DC subsets from mouse spleen. We find that CD8(+) DEC-205/CD205(+) DCs, but not the major fraction of CD8(-) DC inhibitory receptor-2 (DCIR2)(+) DCs, induce functional Foxp3(+) Treg from Foxp3(-) precursors in the presence of low doses of Ag but without added TGF-beta. CD8(+)CD205(+) DCs preferentially express TGF-beta, and the induction of Treg by these DCs in vitro is blocked by neutralizing Ab to TGF-beta. In contrast, CD8(-)DCIR2(+) DCs better induce Foxp3(+) Treg when exogenous TGF-beta is supplied. In vivo, CD8(+)CD205(+) DCs likewise preferentially induce Treg from adoptively transferred, Ag-specific DO11.10 RAG(-/-) Foxp3(-)CD4(+) T cells, whereas the CD8(-)DCIR2(+) DCs better stimulate natural Foxp3(+) Treg. These results indicate that a subset of DCs in spleen, a systemic lymphoid organ, is specialized to differentiate peripheral Foxp3(+) Treg, in part through the endogenous formation of TGF-beta. Targeting of Ag to these DCs might be useful for inducing Ag-specific Foxp3(+) Treg for treatment of autoimmune diseases, transplant rejection, and allergy.

Dendritic cells in the thymus contribute to T-regulatory cell induction

Central tolerance is established through negative selection of self-reactive thymocytes and the induction of T-regulatory cells (T(R)s). The role of thymic dendritic cells (TDCs) in these processes has not been clearly determined. In this study, we demonstrate that in vivo, TDCs not only play a role in negative selection but in the induction of T(R)s. TDCs include two conventional dendritic cell (DC) subtypes, CD8(lo)Sirpalpha(hi/+) (CD8(lo)Sirpalpha(+)) and CD8(hi)Sirpalpha(lo/-) (CD8(hi)Sirpalpha(-)) [corrected] which have different origins. We found that the CD8(hi)Sirpalpha(+) DCs represent a conventional DC subset that originates from the blood and migrates into the thymus. Moreover, we show that the CD8(lo)Sirpalpha(+) DCs demonstrate a superior capacity to induce T(R)s in vitro. Finally, using a thymic transplantation system, we demonstrate that the DCs in the periphery can migrate into the thymus, where they efficiently induce T(R) generation and negative selection.

Lack of conventional dendritic cells is compatible with normal development and T cell homeostasis, but causes myeloid proliferative syndrome

Dendritic cells are critically involved in the promotion and regulation of T cell responses. Here, we report a mouse strain that lacks conventional CD11c(hi) dendritic cells (cDCs) because of constitutive cell-type specific expression of a suicide gene. As expected, cDC-less mice failed to mount effective T cell responses resulting in impaired viral clearance. In contrast, neither thymic negative selection nor T regulatory cell generation or T cell homeostasis were markedly affected. Unexpectedly, cDC-less mice developed a progressive myeloproliferative disorder characterized by prominent extramedullary hematopoiesis and increased serum amounts of the cytokine Flt3 ligand. Our data identify a critical role of cDCs in the control of steady-state hematopoiesis, revealing a feedback loop that links peripheral cDCs to myelogenesis through soluble growth factors, such as Flt3 ligand.

Antigen-specific peripheral shaping of the natural regulatory T cell population

Although regulatory T (T reg) cells are thought to develop primarily in the thymus, the peripheral events that shape the protective T reg cell population are unclear. We analyzed the peripheral CD4(+) T cell receptor (TCR) repertoire by cellular phenotype and location in mice with a fixed TCRbeta chain. We found that T reg (Foxp3(+)) cells showed a marked skewing of TCR usage by anatomical location in a manner similar to antigen-experienced (CD44(hi)Foxp3(-)) but not naive (CD44(lo)Foxp3(-)) cells, even though CD44(hi) and T reg cells used mostly dissimilar TCRs. This was likely unrelated to peripheral conversion, which we estimate generates only a small percentage of peripheral T reg cells in adults. Conversion was readily observed, however, during the immune response induced by Foxp3(-) cells in lymphopenic hosts. Interestingly, the converted Foxp3(+) and expanded Foxp3(-) TCR repertoires were different, suggesting that generation of Foxp3(+) cells is not an automatic process upon antigen activation of Foxp3(-) T cells. Retroviral expression of these TCRs in primary monoclonal T cells confirmed that conversion did not require prior cellular conditioning. Thus, these data demonstrate that TCR specificity plays a crucial role in the process of peripheral conversion and in shaping the peripheral T reg cell population to the local antigenic landscape.

Leukocyte adhesion molecules in animal models of inflammatory bowel disease

The dysregulated recruitment of leukocytes into the intestine is required for the initiation and maintenance of inflammatory bowel disease (IBD). Several families of molecules regulate the influx of these cells into sites of inflammation. Interference with some of these molecules has already shown efficacy in the clinics and antibodies that target the molecules involved have been approved by the FDA for use in Crohn's disease (CD), multiple sclerosis (i.e., natalizumab), and psoriasis (i.e., efalizumab). Here, we discuss basic aspects of the different families of relevant molecules and compile a large body of preclinical studies that supported the targeting of specific steps of the leukocyte adhesion cascade for therapeutic purposes in colitis and in novel models of CD-like ileitis.

HIV-specific regulatory T cells are associated with higher CD4 cell counts in primary infection

OBJECTIVE

Expansion of regulatory T (Treg) cells has been described in chronically HIV-infected individuals. We investigated whether HIV-suppressive Treg could be detected during primary HIV infection (PHI).

METHODS

Seventeen patients diagnosed early after PHI (median: 13 days; 1-55) were studied. Median CD4 cell count was 480 cells/microl (33-1306) and plasma HIV RNA levels ranged between 3.3 and 5.7 log10 copies/ml. Suppressive capacity of blood purified CD4CD25 was evaluated in a coculture assay. Fox-p3, IL-2 and IL-10 were quantified by reverse transcriptase (RT)-PCR and intracellular staining of ex vivo and activated CD4+CD25 T cells.

RESULTS

The frequency of CD4CD127CD25 T cells among CD4 T cells was lower in patients with PHI compared with chronic patients (n = 19). They exhibited a phenotype of memory T cells and expressed constitutively FoxP3. Similar to chronic patients, Treg from patients with PHI inhibited the proliferation of purified tuberculin (PPD) and HIV p24 activated CD4CD25 T cells. CD4CD25 T cells from patients with PHI responded specifically to p24 stimulation by expressing IL-10. In untreated patients with PHI, the frequency as well as HIV-specific activity of Treg decreased during a 24-month follow-up. A positive correlation between percentages of Treg and both CD4 cell counts and the magnitude of p24-specific suppressive activity at diagnosis of PHI was found.

CONCLUSION

Our data showed that HIV drives Treg, as PHI and these cells persist throughout the course of the infection. A correlation between the frequency of Treg and CD4 T-cell counts suggest that these cells may impact on the immune activation set point at PHI diagnosis.

Local IL-21 promotes the therapeutic activity of effector T cells by decreasing regulatory T cells within the tumor microenvironment

The eradication of tumors by the immune system depends on the generation of antigen-specific T cells which can migrate to sites of tumor growth and maintain their effector functions despite local tumor-derived T-cell inhibitory factors. Interleukin-21 (IL-21) is an IL-2-related cytokine that has shown limited evidence of antitumor activity in murine models and early phase clinical trials. Effect of local IL-21 on T-cell responses within the tumor microenvironment, however, has not been extensively evaluated. Thus, we developed a stably transfected IL-21-secreting B16 melanoma cell line to test the effects of local IL-21 on endogenous and adoptively transferred T-cell responses. Tumors expressing IL-21 exhibited delayed growth in vivo, which was associated with an increase in activated systemic effector and memory CD8(+) T-cell responses. Local IL-21 also enhanced the therapeutic effects of adoptively transferred gp100-specific T cells and was synergistic with IL-2. The effect was also associated with an increased proliferation of local CD8(+) T cells and decreased accumulation of regulatory CD4(+)FOXP3(+) T cells within the tumor microenvironment. These data suggest that local IL-21 enhances endogenous and adoptively transferred T-cell immunity through increased effector CD8(+) T cells and decreased CD4(+) regulatory T cells in the tumor microenvironment.

High sensitivity of intestinal CD8+ T cells to nucleotides indicates P2X7 as a regulator for intestinal T cell responses

The purinoreceptor P2X7 is expressed on subsets of T cells and mediates responses of these cells to extracellular nucleotides such as ATP or NAD(+). We identified P2X7 as a molecule highly up-regulated on conventional CD8alphabeta(+) and unconventional CD8alphaalpha(+) T cells of the intestinal epithelium of mice. In contrast, CD8(+) T cells derived from spleen, mesenteric lymph nodes, and liver expressed only marginal levels of P2X7. However, P2X7 was highly up-regulated on CD8(+) T cells from spleen and lymph nodes when T cells were activated in the presence of retinoic acid. High P2X7 expression on intestinal CD8(+) T cells as well as on CD8(+) T cells incubated with retinoic acid resulted in enhanced sensitivity of cells to extracellular nucleotides. Both cell populations showed a high level of apoptosis following incubation with NAD(+) and the ATP derivative 2',3'-O-(benzoyl-4-benzoyl)-ATP, and injection of NAD(+) caused selective in vivo depletion of intestinal CD8(+) T cells. Following oral infection with Listeria monocytogenes, P2X7-deficient mice showed similar CD8(+) T cell responses in the spleen, but enhanced responses in the intestinal mucosa, when compared with similarly treated wild-type control mice. Overall, our observations define P2X7 as a new regulatory element in the control of CD8(+) T cell responses in the intestinal mucosa.

IL-6 controls Th17 immunity in vivo by inhibiting the conversion of conventional T cells into Foxp3+ regulatory T cells

The conditions leading to the induction of adaptive Foxp3(+) regulatory T cells (T-regs) from peripheral T cells in vivo are incompletely understood. Here, we show that unresponsiveness of T cells to IL-6 by T cell-selective deletion of gp130 or immunization of wild-type mice with antigen in incomplete Freund's adjuvant (IFA), which fails to induce IL-6, promotes the conversion of peripheral CD4(+) T cells into adaptive Foxp3(+) T-regs. Thus, both T cell-conditional gp130 knockout (KO) mice immunized with MOG35-55 in complete Freund's adjuvant (CFA) and wild-type mice immunized with MOG35-55 in IFA develop overwhelming antigen-specific T-reg responses and are protected from experimental autoimmune encephalomyelitis (EAE). Depletion of T-regs restores T helper (Th)17 responses and clinical EAE in MOG/CFA-immunized T cell-conditional gp130 KO mice, but not in MOG/IFA-immunized wild-type mice. We conclude that in the absence of T-regs, IL-6 signaling is dispensable for the induction of Th17 cells, and alternative pathways exist to induce Th17 cells and EAE in the absence of IL-6 signaling. However, IL-6 signaling is dominant in inhibiting the conversion of conventional T cells into Foxp3(+) T-regs in vivo, and in the absence of IL-6 signaling, no other cytokine can substitute in inhibiting T-reg conversion. These data identify IL-6 as an important target to modulate autoimmune responses and chronic inflammation.

Retinoic acid enhances Foxp3 induction indirectly by relieving inhibition from CD4+CD44hi Cells

CD4(+)Foxp3(+) regulatory T (Treg) cells originate primarily from thymic differentiation, but conversion of mature T lymphocytes to Foxp3 positivity can be elicited by several means, including in vitro activation in the presence of TGF-beta. Retinoic acid (RA) increases TGF-beta-induced expression of Foxp3, through unknown molecular mechanisms. We showed here that, rather than enhancing TGF-beta signaling directly in naive CD4(+) T cells, RA negatively regulated an accompanying population of CD4(+) T cells with a CD44(hi) memory and effector phenotype. These memory cells actively inhibited the TGF-beta-induced conversion of naive CD4(+) T cells through the synthesis of a set of cytokines (IL-4, IL-21, IFN-gamma) whose expression was coordinately curtailed by RA. This indirect effect was evident in vivo and required the expression of the RA receptor alpha. Thus, cytokine-producing CD44(hi) cells actively restrain TGF-beta-mediated Foxp3 expression in naive T cells, and this balance can be shifted or fine-tuned by RA.

Division of labor between dendritic cell subsets of the lung

Dendritic cells (DCs) are a heterogenous population of antigen-presenting cells, of which conventional DCs and plasmacytoid DCs are the main subsets. Like DC subsets in the central lymphoid organs, DC subsets in the lungs exert specific functions that can be associated with distinct expression of endocytic receptors, cell-surface molecules, and anatomical location within the lung. In recent years, DC populations are increasingly split up into a seemingly endless number of defined sub-populations. We argue that this is not a "stamp-collecting" activity but essential for a deeper understanding of the immune response to pathogens like respiratory viruses or tolerance to harmless antigens. In homeostatic conditions, a fine-tuned balance exists between the various functions of lung DC subsets, which is necessary for maintaining immune homeostasis in the lung. However, infectious or inflammatory conditions can profoundly alter the functions of steady-state DC subsets and recruit inflammatory type DCs to the lung. This might be important for clearing the inflicting pathogenic stimulus, but could at the same time also be involved in causing immune pathology.

Recent progress in understanding the phenotype and function of intestinal dendritic cells and macrophages

Mucosal immune responses must be tightly controlled, particularly in the intestine. As members of the mononuclear phagocyte family, dendritic cells (DCs) and macrophages are well represented in intestinal tissues and have developed unique functional niches. This review will focus on recent findings on antigen uptake and processing in the intestine and the role of DCs in the imprinting of homing receptors on T and B cells, the induction of immunoglobulin A B-cell responses, and the differentiation of regulatory T cells. It will also address the unique phenotype of intestinal macrophages and briefly what is known regarding the relationships between these cell types.

Regulation and function of proinflammatory TH17 cells

Naïve CD4(+) helper T (TH) cells, upon activation by antigen-presenting cells (APC), differentiate into different types of effector cells that are characterized by their distinct cytokine production profiles and immune regulatory functions. In addition to TH1 and TH2 cells, a third subset of effector TH cells has recently been described and termed TH17. Since their identification, TH17 cells have emerged as crucial players in infectious, inflammatory, and autoimmune diseases, and cancer. In this review, we summarize the latest discoveries on the cytokine-mediated regulation and transcriptional programming of TH17 cells and their roles in different immune responses and diseases.

Regulatory T cells in inflammatory bowel disease

PURPOSE OF REVIEW

The intestinal immune system must orchestrate a complex balance between proinflammatory and anti-inflammatory responses to luminal antigens, and disruptions in this balance can result in inflammatory bowel disease (IBD). This review explores recent data that elucidate the role of regulatory T cells (Tregs) in the pathogenesis of IBD in mice and humans.

RECENT FINDINGS

Data from murine models of colitis implicate several novel mechanisms critical to Treg function and generation including the inhibitory cytokine interleukin-35, pericellular adenosine generation and cytokine deprivation-induced apoptosis. Although Tregs are essential in mice for the maintenance of intestinal homeostasis, their role in human IBD remains unclear. Patients with IBD appear to have relatively reduced numbers of Tregs in the blood and colon; however, Tregs from these patients are functional in vitro.

SUMMARY

Tregs are important for the maintenance of intestinal self-tolerance and will likely prove to be an important avenue for therapeutic manipulation in IBD.

Carotenoids, retinol, and intestinal barrier function in children from northeastern Brazil

OBJECTIVES

To investigate the association of carotenoids and retinol (vitamin A) with intestinal barrier function in children in an urban community in Fortaleza, northeastern Brazil.

METHODS

Descriptive analysis of serum carotenoids and retinol concentrations with intestinal barrier function in 102 children from an urban community, July 2000 to August 2001.

RESULTS

The weight for height z score (wasting) showed that 19.6% (20/102) had mild malnutrition (-1 to -2 z score). All of the children's serum retinol concentrations were determined and none were severely deficient (< or =0.35 micromol/L), 2.9% (3/102) were moderately (0.36-0.70 micromol/L) deficient, 20.6% (21/102) were mildly (0.71-1.05 micromol/L) deficient; 76.5% (78/102) were vitamin A sufficient (>1.05 micromol/L). The lactulose:mannitol (L/M) ratio was elevated (> or =0.0864) in 49% (47/97) of children when compared with healthy children with normal L/M ratio (<0.0864) in the same geographic area. Serum carotenoids, lutein, beta-cryptoxanthin and beta-carotene showed significant inverse correlations with the L/M ratio, but not lutein after adjusting for age. Acute phase proteins (C-reactive protein and alpha-acid glycoprotein) were significantly inversely correlated with retinol but not with carotenoids. Retinol and retinol-binding protein were not significantly associated with L/M ratio.

CONCLUSIONS

These data suggest a disruption of intestinal barrier function in the paracellular pathway with low serum concentrations of carotenoids. Carotenoids may provide a better marker for disrupted intestinal barrier function than retinol-binding protein or retinol.