Activation of CD25(+)CD4(+) regulatory T cells by oral antigen administration

Oral CD3-specific antibody suppresses autoimmune encephalomyelitis by inducing CD4+ CD25- LAP+ T cells

A major goal of immunotherapy for autoimmune diseases and transplantation is induction of regulatory T cells that mediate immunologic tolerance. The mucosal immune system is unique, as tolerance is preferentially induced after exposure to antigen, and induction of regulatory T cells is a primary mechanism of oral tolerance. Parenteral administration of CD3-specific monoclonal antibody is an approved therapy for transplantation in humans and is effective in autoimmune diabetes. We found that orally administered CD3-specific antibody is biologically active in the gut and suppresses autoimmune encephalomyelitis both before induction of disease and at the height of disease. Orally administered CD3-specific antibody induces CD4+ CD25- LAP+ regulatory T cells that contain latency-associated peptide (LAP) on their surface and that function in vitro and in vivo through a TGF-beta-dependent mechanism. These findings identify a new immunologic approach that is widely applicable for the treatment of human autoimmune conditions.

Animal models of allergen-induced tolerance in asthma: are T-regulatory-1 cells (Tr-1) the solution for T-helper-2 cells (Th-2) in asthma?

Non-specific anti-inflammatory medication is actually the treatment of choice for controlling the T-helper type 2 (Th-2) cell-driven airway inflammation in asthma. The induction of counterbalancing Th-1 cell clones, long considered a promising approach for immunotherapy, has failed to fulfil its promise because of potentially detrimental side-effects. This is therefore probably not a valid option for the treatment of asthma. With the increasing awareness that active immune mechanisms exist to control inflammatory responses, interest rises to investigate whether these can be exploited to control allergen-induced airway disease. The induction of antigen-specific T cells with suppressive characteristics (regulatory T cells) is therefore a potentially interesting approach. These regulatory T cells mediate tolerance in healthy, non-atopic individuals and have the potential of becoming an effective means of preventing allergen-induced airway inflammation and possibly of suppressing ongoing allergic immune responses. Here we review the available knowledge about allergen-induced suppressive immunity obtained from animal models taking into account the different developmental stages of allergic airway disease.

Immunology and homeopathy. 3. Experimental studies on animal models

A search of the literature and the experiments carried out by the authors of this review show that there are a number of animal models where the effect of homeopathic dilutions or the principles of homeopathic medicine have been tested. The results relate to the immunostimulation by ultralow doses of antigens, the immunological models of the ‘simile’, the regulation of acute or chronic inflammatory processes and the use of homeopathic medicines in farming. The models utilized by different research groups are extremely etherogeneous and differ as the test medicines, the dilutions and the outcomes are concerned. Some experimental lines, particularly those utilizing mice models of immunomodulation and anti-inflammatory effects of homeopathic complex formulations, give support to a real effect of homeopathic high dilutions in animals, but often these data are of preliminary nature and have not been independently replicated. The evidence emerging from animal models is supporting the traditional ‘simile’ rule, according to which ultralow doses of compounds, that in high doses are pathogenic, may have paradoxically a protective or curative effect. Despite a few encouraging observational studies, the effectiveness of the homeopathic prevention or therapy of infections in veterinary medicine is not sufficiently supported by randomized and controlled trials.

Antigen-induced, tolerogenic CD11c+,CD11b+ dendritic cells are abundant in Peyer's patches during the induction of oral tolerance to type II collagen and suppress experimental collagen-induced arthritis

OBJECTIVE

Although oral tolerance is a well-known phenomenon, the role of dendritic cells (DCs) is not well characterized. This study was conducted to better understand the differential role played by each Peyer's patch DC subset in the induction of oral tolerance to type II collagen (CII) in murine collagen-induced arthritis (CIA).

METHODS

CII was fed 6 times to DBA/1 mice beginning 2 weeks before immunization, and the effect on arthritis was assessed. We compared the proportion of CD11c+,CD11b+ DCs and CD11c+,CD8alpha+ DCs in the Peyer's patches of CII-fed tolerized and phosphate buffered saline-fed nontolerized mice after the induction of CIA. The immunosuppressive properties of each DC subset were determined using fluorescence-activated cell sorter analysis for intracellular interleukin-10 (IL-10) and IL-12 and mixed lymphocyte culture. The ability of each DC subset to induce CD4+,CD25+ T regulatory cells was also examined. Mice were injected with CII-pulsed CD11c+,CD11b+ DCs isolated from Peyer's patches of tolerized mice, and the effect on CIA was examined.

RESULTS

The severity of arthritis was significantly lower in tolerized mice. The proportion of CD11c+,CD11b+ DCs was increased in the Peyer's patches of tolerized mice and those DCs exhibited immunosuppressive characteristics, such as increased IL-10 production, inhibition of T cell proliferative responses to CII, and CD4+,CD25+ regulatory T cell induction. Furthermore, the CD11c+,CD11b+ DCs suppressed the severity of arthritis upon adoptive transfer.

CONCLUSION

Our observations demonstrate that CD11c+,CD11b+ DCs, which are abundant in Peyer's patches during the induction of oral tolerance to CII, are crucial for the suppression of CIA and could be exploited for immunotherapy of autoimmune diseases.

Intranasal Vaccination with Proinsulin DNA Induces Regulatory CD4+ T Cells That Prevent Experimental Autoimmune Diabetes

Insulin, an autoantigen in type 1 diabetes, when administered mucosally to diabetes-prone NOD mice induces regulatory T cells (T(reg)) that protect against diabetes. Compared with protein, Ag encoded as DNA has potential advantages as a therapeutic agent. We found that intranasal vaccination of NOD mice with plasmid DNA encoding mouse proinsulin II-induced CD4+ T(reg) that suppressed diabetes development, both after adoptive cotransfer with "diabetogenic" spleen cells and after transfer into NOD mice given cyclophosphamide to accelerate diabetes onset. In contrast to prototypic CD4+ CD25+ T(reg), CD4+ T(reg) induced by proinsulin DNA were both CD25+ and CD25- and not defined by markers such as glucocorticoid-induced TNFR-related protein (GITR), CD103, or Foxp3. Intriguingly, despite induction of T(reg) and reduced islet inflammation, diabetes incidence in proinsulin DNA-treated mice was unchanged. However, diabetes was prevented when DNA vaccination was performed under the cover of CD40 ligand blockade, known to prevent priming of CTL by mucosal Ag. Thus, intranasal vaccination with proinsulin DNA has therapeutic potential to prevent diabetes, as demonstrated by induction of protective T(reg), but further modifications are required to improve its efficacy, which could be compromised by concomitant induction of pathogenic immunity.

Inflammatory lesion and parasite load are inversely associated in Leishmania amazonensis infected mice genetically selected according to oral tolerance susceptibility

Two strains of mice selected according to extreme phenotypes of susceptibility and resistance to oral tolerance (TS and TR mice, respectively) were infected with 1 x 10(7) Leishmania amazonensis promastigotes and studied comparatively. TS mice developed a minor pathology while permitting parasite growth with the presence of increased IL-4, IL-10 and IFN-gamma, and lower NO and IL-2 levels and delayed-type hypersensitivity (DTH). In contrast, in TR mice, footpad swelling was increased but parasite growth was reduced. They produced lower IL-4, IL-10 and IFN-gamma but increased NO, IL-2 levels, DTH, activated spleen macrophages and periarteriolar lymphoid sheaths. The results suggest that the tolerogenic TS mouse profile, with higher IL-10 production, impaired lesion development but also avoided macrophage leishmanicidal activity, maintaining in this manner a silent parasite load. On the other hand, the TR mouse profile contributed to lesion progression with controlled parasite load. To directly address the influence of oral tolerance on infection, mice were gavaged with OVA, and 7 days afterwards were infected and challenged to bystander suppression with OVA in the same footpad. In TR mice gavaged with 25 mg OVA the inflammatory lesion was largely enhanced, while with 5 mg OVA the lesion was diminished. In TS mice the footpad swelling was always lower. However, the bystander effect did not modify the establishment of infection; and similarly to the control non-bystander mice, parasite clearance was maintained in TR and prevented in TS mice. Therefore, a better comprehension of immunoregulation of innate and adaptive immunity in the early stages of infection is necessary for the development of protocols preventing inflammation and contributing to the elimination of parasites.

T cell-mediated oral tolerance is intact in germ-free mice

Commensal enteric bacteria stimulate innate immune cells and increase numbers of lamina propria and mesenteric lymph node (MLN) T and B lymphocytes. However, the influence of luminal bacteria on acquired immune function is not understood fully. We investigated the effects of intestinal bacterial colonization on T cell tolerogenic responses to oral antigen compared to systemic immunization. Lymphocytes specific for ovalbumin-T cell receptor (OVA-TCR Tg(+)) were transplanted into germ-free (GF) or specific pathogen-free (SPF) BALB/c mice. Recipient mice were fed OVA or immunized subcutaneously with OVA peptide (323-339) in complete Freund's adjuvant (CFA). Although the efficiency of transfer was less in GF recipients, similar proportions of cells from draining peripheral lymph node (LN) or MLN were proliferating 3-4 days later in vivo in GF and SPF mice. In separate experiments, mice were fed tolerogenic doses of OVA and then challenged with an immunogenic dose of OVA 4 days later. Ten days after immunization, lymphocytes were restimulated with OVA in vitro to assess antigen-specific proliferative responses. At both high and low doses of OVA, cells from both SPF and GF mice fed OVA prior to immunization had decreased proliferation compared to cells from control SPF or GF mice. In addition, secretion of interferon (IFN)-gamma and interleukin (IL)-10 by OVA-TCR Tg(+) lymphocytes was reduced in both SPF and GF mice fed OVA compared to control SPF or GF mice. Unlike previous reports indicating defective humoral responses to oral antigen in GF mice, our results indicate that commensal enteric bacteria do not enhance the induction of acquired, antigen-specific T cell tolerance to oral OVA.

CD18 is required for optimal development and function of CD4+CD25+ T regulatory cells

CD4+CD25+ T regulatory (Treg) cells inhibit immunopathology and autoimmune disease in vivo. CD4+CD25+ Treg cells' capacity to inhibit conventional T cells in vitro is dependent upon cell-cell contact; however, the cell surface molecules mediating this cell:cell contact have not yet been identified. LFA-1 (CD11a/CD18) is an adhesion molecule that plays an established role in T cell-mediated cell contact and in T cell activation. Although expressed at high levels on murine CD4+CD25+ Treg cells, the role of LFA-1 in these cells has not been defined previously. We hypothesized that LFA-1 may play a role in murine CD4+CD25+ Treg function. To evaluate this, we analyzed LFA-1-deficient (CD18-/-) CD4+CD25+ T cells. We show that CD18-/- mice demonstrate a propensity to autoimmunity. Absence of CD18 led to diminished CD4+CD25+ T cell numbers and affected both thymic and peripheral development of these cells. LFA-1-deficient CD4+CD25+ T cells were deficient in mediating suppression in vitro and in mediating protection from colitis induced by the transfer of CD4+CD25- T cells into lymphopenic hosts. Therefore, we define a crucial role for CD18 in optimal CD4+CD25+ Treg development and function.

Immune deviation by mucosal antigen administration suppresses gene-transfer-induced inhibitor formation to factor IX

Formation of inhibitory antibodies is a serious complication of protein or gene replacement therapy for hemophilias, congenital X-linked bleeding disorders. In hemophilia B (coagulation factor IX [F.IX] deficiency), lack of endogenous F.IX antigen expression and other genetic factors may increase the risk of antibody formation to functional F.IX. Here, we developed a protocol for reducing inhibitor formation in gene therapy by prior mucosal (intranasal) administration of a peptide representing a human F.IX-specific CD4(+) T-cell epitope in hemophilia B mice. C3H/HeJ mice with a F.IX gene deletion produced inhibitory IgG to human F.IX after hepatic gene transfer with an adeno-associated viral vector. These animals subsequently lost systemic F.IX expression. In contrast, repeated intranasal administration of the specific peptide resulted in reduced inhibitor formation, sustained circulating F.IX levels, and sustained partial correction of coagulation following hepatic gene transfer. This was achieved through immune deviation to a T-helper-cell response with increased IL-10 and TGF-beta production and activation of regulatory CD4(+)CD25(+) T cells.

Recovery from experimental allergic encephalomyelitis is TGF-β dependent and associated with increases in CD4+LAP+ and CD4+CD25+ T cells

SJL mice are highly susceptible to proteolipid protein (PLP) 139-151-induced experimental allergic encephalomyelitis (EAE). The disease is characterized by a relapsing-remitting type of paralysis. However, the mechanism by which animals recover from EAE is poorly understood. Here, we investigated the role of regulatory T cells in the recovery from disease. We found that Forkhead box P3-expressing CD4+CD25+ T cells were increased in the blood, draining lymph node and spleen of EAE-recovered SJL mice. These cells were anergic and inhibited proliferation of CD4+CD25- T cells to PLP 139-151 or anti-CD3 antibody stimulation. Depletion of CD4+CD25+ T cells during the recovery phase exacerbated disease, resulted in the expansion of IA(s)/PLP 139-151-tetramer-positive cells and enhanced IFN-gamma production. In addition, transforming growth factor-beta (TGF-beta) was shown to be involved in the recovery from EAE as the percentage of CD4+ cells expressing TGF-beta latency-associated peptide (LAP) on the cell surface increased significantly in blood and spleen of EAE-recovered mice as compared with the naive mice and in vivo neutralization of TGF-beta abolished recovery from disease. Taken together, our results demonstrate that both CD4+CD25+ and CD4+LAP+ regulatory T cells mediate recovery from PLP 139-151-induced EAE in SJL mice in which TGF-beta plays an important role.

Achieving antigen-specific tolerance in diabetes: regulating specifically

Autoreactive T cells that escape negative selection in the thymus do not normally cause productive immune responses to self-antigens because of a number of regulatory mechanisms. Studies with anti-CD3 monoclonal antibodies (mAbs) have suggested that immune regulatory mechanisms are induced by drug treatments that are able to stop on-going unwanted immune responses, such as type 1 diabetes, involving induction of regulatory T cells. TGF-beta dependent and independent mechanisms have been described involving CD4(+) as well as CD8(+) T cells. The challenge is now to apply these mechanisms in an antigen-specific manner and so that lasting tolerance to the autoimmune responses can be maintained. We discuss recent data concerning the mechanisms of anti-CD3 mAb treatment and the ways in which our understanding of these mechanisms can be used to develop adoptive immune therapy with regulatory T cells to treat patients with type 1 diabetes or other autoimmune diseases.

Antigen-specific peripheral tolerance induced by topical application of NF-kappaB decoy oligodeoxynucleotide

Activation and maturation of dendritic cells (DC) are crucial for the establishment of delayed-type hypersensitivity (DTH). However, antigen presentation by immature DC (iDC) might lead to antigen-specific peripheral tolerance. NF-kappaB plays significant roles in upregulation of co-stimulatory molecules and cytokines in DC and therefore we investigated whether NF-kappaB decoy oligodeoxynucleotide (ODN) might induce tolerance to DTH. NF-kappaB decoy ODN suppressed ovalbumin (OVA)-induced DTH responses not only in naïve but also in presensitized mice. The suppressive effect was found to be antigen-specific. NF-kappaB decoy ODN-induced tolerance involved CD4(+)CD25(+) regulatory T cells (Treg), because in vivo depletion of CD25(+) T cells abrogated the tolerance, whereas adoptive transfer of such T cell population from tolerant mice induced tolerance. Furthermore, the induction of Treg was related to insufficient migration and/or maturation of DC, because a sizable DC population still remained in peripheral tissue even after exposure to exogenous antigen in NF-kappaB decoy ODN-treated mice. Even if they migrated into lymph nodes, they showed insufficient upregulation of co-stimulatory molecules and impaired antigen-specific activation of T cells. Topical application of NF-kappaB decoy ODN might thus be a new approach to induce antigen-specific peripheral tolerance.

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.

Defect of oral tolerance in NC/Nga mice

The NC/Nga mouse is a model animal for human atopic dermatitis. In this study, we investigated oral tolerance induction in NC/Nga mice. In BALB/c mice, oral administration of ovalbumin (OVA) resulted in suppression of both OVA-specific T and B cell responses induced by OVA immunization. In NC/Nga mice, OVA-induced antigen (Ag) -specific T and B cell responses were significantly less than those in BALB/c mice. Furthermore, oral administration of OVA did not suppress OVA-specific immunoresponses in NC/Nga mice. We further examined antibody (Ab) response against food Ag by feeding mice an experimental diet that contained OVA or casein as a protein source. The level of serum OVA or casein-specific IgG was significantly higher in NC/Nga mice than in BALB/c mice. These results indicate that NC/Nga mice have a defect in the induction of oral tolerance. NC/Nga mice can therefore be used as a model for investigating the mechanism of oral tolerance.

Selection and behavior of CD4+ CD25+ T cells in vivo: lessons from T cell receptor transgenic models

Despite great interest in CD4+ CD25+ suppressor T cells, many of the fundamental properties of these cells remain enigmatic. This is in part due to experimental limitations inherent to the study of polyclonal suppressor T cells, and the extensive use of in vitro assays. This review article intends to outline recent advances in our understanding of the biology of suppressor T cells that have emerged from the analysis of T cell receptor (TCR) transgenic models. Several laboratories have taken advantage of model systems in which suppressor T cells of defined antigen-specificity are naturally selected in order to characterize the selection and behavior of these cells in vivo. In addition to providing valuable insights into the mechanism of differentiation of suppressor T cells, these systems now offer new possibilities for understanding the mode of action of suppressor T cells. For example, adoptive transfer of small numbers of ex vivo isolated TCR transgenic suppressor T cells allows for the visualization of the fate of such cells when confronted with cognate antigen in a quasi-normal, nonlymphopenic environment. Characteristic features of the currently available TCR transgenic models of suppressor T cells will be highlighted, and particular issues pertaining to the differentiation, function, and homeostasis of this T cell subset that have emerged from these models will be discussed.

Induction of antigen-specific regulatory T cells in the liver-draining celiac lymph node following oral antigen administration

Regulatory T cells are induced by oral administration of an antigen, but the physiological requirements and localization of the inductive sites are largely unknown. Using an adoptive transfer system of cells transgenic for ovalbumin T-cell receptor (OVA TCR tg), we found that antigen-specific CD4+ T cells were activated in the liver-draining celiac lymph node (CLN) shortly after ovalbumin feeding, and that a significantly higher proportion of the T cells in the CLN developed into the putative regulatory phenotype [co-expressing CD25 with the glucocortico-induced tumour necrosis factor (TNF) receptor family related gene (GITR), cytotoxic T-lymphocyte antigen (CTLA)-4 and CD103] than in Peyer's patches, the mesenteric and peripheral lymph nodes and the spleen. In addition, a particularly high level of expression of CD103 on the OVA-specific T cells in the CLN may favour homing to the epithelium of the intestine. While equally suppressive, OVA tg T cells isolated from the CLN of OVA-fed DO11.10 mice were less dependent on transforming growth factor (TGF)-beta for suppression than cells isolated from the peripheral and mesenteric lymph nodes, which indicates the involvement of an additional suppressive mechanism. The expression of FoxP3 was not up-regulated in any of the lymph node compartments studied. Our phenotypic and functional findings suggest that the induction of regulatory T cells in the CLN may be relevant in the control of the immune response to dietary antigens.

Prevention of allergen-specific, Th2-biased immune responses in vivo: role of increased IL-12 and IL-18 responsiveness

The factors that control development of adaptive responses to exogenous Ag remain incompletely understood. An ability to selectively direct immunity toward a specific phenotype would be of clinical benefit in numerous immunological disorders. Administration of chemically modified allergen glutaraldehyde-polymerized OVA (OA-POL) leads to >90% reductions in murine IgE and >500-fold increases in IgG2c responses that develop upon subsequent immunization with native Ag. In the present study, we examine the mechanisms underlying this reorientation of the type 2 dominant response that would normally develop. Lack of endogenous IL-12 or IFN-gamma results in markedly reduced induction of IgG2c responses following OA-POL treatment, but only IFN-gamma(-/-) mice demonstrate reduced capacity to prevent IgE induction. This indicates that while both IL-12 and IFN-gamma are critical promoters of type 1 immunity, only IFN-gamma is required to maximally inhibit development of type 2 immune responses. Compared with OVA-immunized mice, CD69(+) T cells from OA-POL-immunized mice demonstrate elevated IL-12Rbeta(2), IL-18Ralpha, and IL-18Rbeta mRNA levels, as well as increased IFN-gamma production in response to rIL-12 or rIL-18 stimulation. Collectively, these data indicate that preventing induction of type 2 immune responses is critically dependent on altered T cell responsiveness to these cytokines. The finding that targeted, Ag-specific manipulation of IL-12 and IL-18 responsiveness can be used to shape the phenotype of the dominant immune response that develops suggests that specifically targeting IL-12 and IL-18 receptor expression may offer clinical options for clinical prophylaxis or intervention.

Oral tolerance: lessons on treatment of food allergy

Oral tolerance is the active non-response by the immune system to an antigen administered through the oral route. It is postulated that food hypersensitivity results from a breakdown in oral tolerance induction, and the importance of oral tolerance in food hypersensitivity can be traced back to classic experiments from 1911 in which guinea pigs were protected from anaphylaxis by prior feeding of antigen. Host and antigenic factors play a role in determining the pathways and mechanisms to which a fed antigen can gain tolerance. Recent studies have demonstrated the potential of using oral tolerance to treat food allergies, and additional studies are necessary to further our understanding of mechanisms of oral tolerance induction.

The IL-4 receptor alpha-chain-binding cytokines, IL-4 and IL-13, induce forkhead box P3-expressing CD25+CD4+ regulatory T cells from CD25-CD4+ precursors

The mechanisms underlying the extrathymic generation of CD25+CD4 regulatory T cells (Tregs) are largely unknown. In this study the IL-4R alpha-chain-binding cytokines, IL-4 and IL-13, were identified as inducers of CD25+ Tregs from peripheral CD25-CD4 naive T cells. IL-4-induced CD25+ Tregs phenotypically and functionally resemble naturally occurring Tregs in that they are anergic to mitogenic stimulation, inhibit the proliferation of autologous responder T cells, express high levels of the Forkhead box P3 and the surface receptors glucocorticoid-induced TNFR family-related protein and CTLA-4, and inhibit effector T cells in a contact-dependent, but cytokine-independent, manner. The IL-4-induced generation of peripheral Tregs was independent of the presence of TGF-beta or IL-10, but was dependent on Ag-specific stimulation and B7 costimulation. The significance of the IL-4Ralpha-binding cytokines in the generation of Ag-specific Tregs was emphasized in a mouse model of oral tolerance, in which neutralization of IL-4 and IL-13 in mice transgenic for the TCR specific for OVA completely inhibited the expansion of OVA-specific Tregs that can be induced in untreated mice by feeding the nominal Ag. Together, our results demonstrate that IL-4 and IL-13 play an important role in generating Forkhead box P3-expressing CD25+ Tregs extrathymically in an Ag-dependent manner and therefore provide an intriguing link between the well-established immunoregulatory capacity of Th2 cells and the powerful CD25+ Treg population. Moreover, our findings might provide the basis for the design of novel therapeutic approaches for targeted immunotherapy with Tregs to known Ags in autoimmune diseases or graft-vs-host reactions.

The ICOS Molecule Plays a Crucial Role in the Development of Mucosal Tolerance

The ICOS molecule stimulates production of the immunoregulatory cytokine IL-10, suggesting an important role for ICOS in controlling IL-10-producing regulatory T cells and peripheral T cell tolerance. In this study we investigate whether ICOS is required for development of oral, nasal, and high dose i.v. tolerance. Oral administration of encephalitogenic myelin oligodendrocyte glycoprotein (MOG) 35-55 peptide to ICOS-deficient (ICOS-/-) mice did not inhibit experimental autoimmune encephalomyelitis (EAE), T cell proliferation, or IFN-gamma production, in striking contrast to wild-type mice. Similarly, intranasal administration of MOG(35-55) before EAE induction suppressed EAE and T cell responses in wild-type, but not in ICOS-/-, mice. In contrast, ICOS-/- mice were as susceptible as wild-type mice to high dose tolerance. These results indicate that ICOS plays an essential and specific role in mucosal tolerance and that distinct costimulatory pathways differentially regulate different forms of peripheral tolerance. Surprisingly, CD4+ cells from MOG-fed wild-type and ICOS-/- mice could transfer suppression to wild-type recipients, indicating that functional regulatory CD4+ cells can develop in the absence of ICOS. However, CD4+ T cells from MOG-fed wild-type mice could not transfer suppression to ICOS-/- recipients, suggesting that ICOS may have a key role in controlling the effector functions of regulatory T cells. These results suggest that stimulating ICOS may provide an effective therapeutic approach for promoting mucosal tolerance.

Tolerance induction by viral in vivo gene transfer

Treatment of genetic disease by protein or gene replacement therapy is hampered by immune responses to the therapeutic protein. An excellent example is formation of inhibitory antibodies to coagulation factors in treatment of the X-linked bleeding disorder hemophilia. Experiments in murine and canine models of hemophilia B (deficiency in factor IX) have demonstrated sustained therapeutic levels of factor IX transgene expression following hepatic adeno-associated viral gene transfer in animals with deletion and nonsense mutations in the factor IX gene. This article reviews experimental evidence for induction of immune tolerance to the factor IX transgene product by hepatic adeno-associated viral gene transfer, which has been shown to limit T helper cell responses and to substantially reduce the risk of antibody responses. Tolerance induction is associated with activation of regulatory CD4(+) T cells capable of suppressing antibody formation to factor IX protein. Hepatic administration of adeno-associated viral vector expressing ovalbumin in mice transgenic for a T cell receptor specific for this antigen provided direct evidence for induction of CD4(+) T cell tolerance, including T cell anergy and clonal deletion. Taken together, these data indicate the potential for viral in vivo gene transfer not only to provide sustained systemic expression, but moreover to induce immunological hypo-responsiveness to the therapeutic gene product.

Oral tolerance

Multiple mechanisms of tolerance are induced by oral antigen. Low doses favor active suppression, whereas higher doses favor clonal anergy/deletion. Oral antigen induces T-helper 2 [interleukin (IL)-4/IL-10] and Th3 [transforming growth factor (TGF)-beta] T cells plus CD4+CD25+ regulatory cells and latency-associated peptide+ T cells. Induction of oral tolerance is enhanced by IL-4, IL-10, anti-IL-12, TGF-beta, cholera toxin B subunit, Flt-3 ligand, and anti-CD40 ligand. Oral (and nasal) antigen administration suppresses animal models of autoimmune diseases including experimental autoimmune encephalitis, uveitis, thyroiditis, myasthenia, arthritis, and diabetes in the non-obese diabetic (NOD) mouse, plus non-autoimmune diseases such as asthma, atherosclerosis, graft rejection, allergy, colitis, stroke, and models of Alzheimer's disease. Oral tolerance has been tested in human autoimmune diseases including multiple sclerosis (MS), arthritis, uveitis, and diabetes and in allergy, contact sensitivity to dinitrochlorobenzene (DNCB), and nickel allergy. Although positive results have been observed in phase II trials, no effect was observed in phase III trials of CII in rheumatoid arthritis or oral myelin and glatiramer acetate (GA) in MS. Large placebo effects were observed, and new trials of oral GA are underway. Oral insulin has recently been shown to delay onset of diabetes in at-risk populations, and confirmatory trials of oral insulin are being planned. Mucosal tolerance is an attractive approach for treatment of autoimmune and inflammatory diseases because of lack of toxicity, ease of administration over time, and antigen-specific mechanisms of action. The successful application of oral tolerance for the treatment of human diseases will depend on dose, developing immune markers to assess immunologic effects, route (nasal versus oral), formulation, mucosal adjuvants, combination therapy, and early therapy.

Orally tolerized T cells can form conjugates with APCs but are defective in immunological synapse formation

Oral tolerance is systemic immune hyporesponsiveness induced by the oral administration of soluble Ags. Hyporesponsiveness of Ag-specific CD4 T cells is responsible for this phenomenon. However, the molecular mechanisms underlying the hyporesponsive state of these T cells are not fully understood. In the present study, we investigated the ability of orally tolerized T cells to form conjugates with Ag-bearing APCs and to translocate TCR, protein kinase C-theta (PKC-theta), and lipid rafts into the interface between T cells and APCs. Orally tolerized T cells were prepared from the spleens of OVA-fed DO11.10 mice. Interestingly, the orally tolerized T cells did not show any impairment in the formation of conjugates with APCs. The conjugates were formed in a LFA-1-dependent manner. Upon antigenic stimulation, the tolerized T cells could indeed activate Rap1, which is critical for LFA-1 activation and thus cell adhesion. However, orally tolerized T cells showed defects in the translocation of TCR, PKC-theta, and lipid rafts into the interface between T cells and APCs. Translocation of TCR and PKC-theta to lipid raft fractions upon antigenic stimulation was also impaired in the tolerized T cells. Ag-induced activation of Vav, Rac1, and cdc42, which are essential for immunological synapse and raft aggregation, were down-regulated in orally tolerized T cells. These results demonstrate that orally tolerized T cells can respond to specific Ags in terms of conjugate formation but not with appropriate immunological synapse formation. This may account for the hyporesponsive state of orally tolerized T cells.

Peripheral expression of rod photoreceptor arrestin induces an epitope-specific, protective response against experimental autoimmune uveoretinitis

PURPOSE

To examine the immunological basis for reduced susceptibility to experimental autoimmune uveoretinitis (EAU) in rats expressing retinal photoreceptor cell arrestin in the periphery.

METHODS

Peripheral expression of arrestin in Lewis rats was achieved by engraftment of syngeneic bone marrow (BM) transduced with retroviruses encoding wild-type arrestin or a mutant arrestin lacking the immunodominant epitope Arr(273 - 289) (Delta273-Arr). EAU was induced by immunization with arrestin peptides Arr(273-289) or Arr(343-362). Cultured splenocytes and/or lymphocytes from immunized rats were assayed for antigen-induced proliferation, antibody production, and cytokines.

RESULTS

Rats expressing Delta273-Arr were not protected from Arr(273 - 289)-induced EAU, showing that protection was epitope specific. Proliferation assays found little difference in the ability of draining lymph node cells from arrestin-transduced rats to proliferate in response to the antigen, indicating that antigen-responsive T cells were not deleted in BM recipients. Only rats immunized with Arr(343 - 362) elicited antibodies, but no difference in titer was found between transduced and control animals. Higher levels of IFN-gamma mRNA were made by Arr(273 - 289)-immunized rats than Arr(343 - 366)-immunized rats, but in either case, the levels did not correlate with chimeric status or EAU susceptibility. Arr(273 - 289)-immunized rats had higher levels of IL-10 mRNA than Arr(343 - 362)-immunized rats, and those levels were decreased in arrestin chimeric rats. Overall, immunization with the more potently uveitogenic Arr(343 - 362) induced lower levels of IL-10 and IFN-gamma than the less uveitogenic Arr(273 - 289). A strong correlation was found between the ability of lymphocytes to make IL-4 in the arrestin-chimeric animals and inhibition of EAU.

CONCLUSIONS

Peripheral expression of arrestin in a regenerating immune system induces an epitope-specific protective response to EAU induced by arrestin peptides. Although IL-4 and IL-10 levels were altered in arrestin-chimeric mice, the outcome was not consistently T(H)2-like. Only IL-4 production was clearly associated with reduced susceptibility to EAU.

CD4CD25 regulatory T lymphocytes in allergy and asthma

Allergic asthma is characterized by airway hyper-responsiveness and chronic mucosal inflammation mediated by CD4(+) Th2 lymphocytes. Regulatory CD4(+)CD25(+) T cells are important components of the homeostasis of the immune system, as impaired CD4(+)CD25(+) T cell activity can cause autoimmune diseases and allergy. The mechanism of suppression by CD4(+)CD25(+) T cells remains controversial; different in vivo and in vitro studies raise possible roles for the immunosuppressive cytokines interleukin-10 and transforming growth factor-beta, forkhead transcription factor Foxp3, glucocorticoid-induced tumor necrosis factor receptor, cytotoxic lymphocyte associated antigen-4, 4-1BB costimulator receptor, a CD4-related molecule LAG-3, and neuropilin-1. Current data suggest that Th2 responses to allergens are normally suppressed by CD4(+)CD25(+) T cells. Suppression by CD4(+)CD25(+) T cells is decreased in allergic individuals. Furthermore, CD4(+)CD25(+) T cells play a key role in regulating airway eosinophilic inflammation. The immunomodulatory properties of CD4(+)CD25(+) T cells do extend to Th2 responses, most notably by limiting the development of a proinflammatory CD4(+) Th2 phenotype characterized by reduced cytokine production. An understanding of the roles of CD4(+)CD25(+) T cells in vivo could provide better insight into the design of novel approaches to modulate the chronic airway inflammatory reaction evident in bronchial asthma.

Oral tolerance in the absence of naturally occurring Tregs

Mucosal tolerance prevents pathological reactions against environmental and food antigens, and its failure results in exacerbated inflammation typical of allergies and asthma. One of the proposed mechanisms of oral tolerance is the induction of Tregs. Using a mouse model of hyper-IgE and asthma, we found that oral tolerance could be effectively induced in the absence of naturally occurring thymus-derived Tregs. Oral antigen administration prior to i.p. immunization prevented effector/memory Th2 cell development, germinal center formation, class switching to IgE, and lung inflammation. Oral exposure to antigen induced development of antigen-specific CD4CD25Foxp3CD45RB cells that were anergic and displayed suppressive activity in vivo and in vitro. Oral tolerance to the Th2 allergic response was in large part dependent on TGF-beta and independent of IL-10. Interestingly, Tregs were also induced by single i.p. immunization with antigen and adjuvant. However, unlike oral administration of antigen, which induced Tregs but not effector T cells, i.p. immunization led to the simultaneous induction of Tregs and effector Th2 cells displaying the same antigen specificity.

Preventing intolerance: the induction of nonresponsiveness to dietary and microbial antigens in the intestinal mucosa

The gut-associated lymphoid tissue (GALT) is constantly exposed to a variety of Ags and must therefore decipher a large number of distinct signals at all times. Responding correctly to each set of signals is crucial. When the GALT receives signals from the intestinal flora or food Ags, it must induce a state of nonresponsiveness (mucosal tolerance). In contrast, when pathogenic bacteria invade the intestinal mucosa, it is necessary to elicit strong T and B cell responses. The GALT is therefore in the position of constantly fighting intolerance to food and the commensal flora while effectively battling infectious microbes. Determining precisely which type of response to generate in each case is key to the prevention of immune dysregulation and tissue damage.

TGF-beta1 maintains suppressor function and Foxp3 expression in CD4+CD25+ regulatory T cells

Transforming growth factor (TGF)-β1 is a major pluripotential cytokine with a pronounced immunosuppressive effect and its deficiency results in lethal autoimmunity in mice. However, mechanisms of its immunosuppressive action are not completely understood. Here, we report that TGF-β1 supports the maintenance of Foxp3 expression, regulatory function, and homeostasis in peripheral CD4⁺CD25⁺ regulatory T (T reg) cells, but is not required for their thymic development. We found that in 8–10-d-old TGF-β1–deficient mice, peripheral, but not thymic, T reg cells are significantly reduced in numbers. Moreover, our experiments suggest that a defect in TGF-β–mediated signaling in T reg cells is associated with a decrease in Foxp3 expression and suppressor activity. Thus, our results establish an essential link between TGF-β1 signaling in peripheral T reg cells and T reg cell maintenance in vivo.

Peripheral and intestinal regulatory CD4+ CD25(high) T cells in inflammatory bowel disease

BACKGROUND & AIMS

Regulatory CD25+ T cells (T(reg)) are effective in the prevention and down-regulation of inflammatory bowel disease (IBD) in animal models. Functional T(reg) cells are characterized by the expression of the transcription factor FOXP3 and show a CD4+ CD25(high) phenotype in humans. The aim of this study was to determine whether disease activity in IBD correlates with changes in frequency of T(reg) cells and their distribution in the intestinal mucosa.

METHODS

T(reg) cells were analyzed from peripheral blood and from biopsy specimens of IBD patients, inflammatory controls, and healthy volunteers by flow cytometry (CD4+ CD25(high)), immunochemistry (FOXP3), and real-time PCR (FOXP3). Regulatory properties of purified peripheral CD4+ CD25(high) T(reg) cells were determined by their suppressive effect on the proliferation of CD4+ CD25- T cells.

RESULTS

In peripheral blood, CD4+ CD25(high) T cells from IBD patients retain their suppressive activity. CD4+ CD25(high) and FOXP3+ T(reg) cells are increased during remission but decreased during active disease. This contrasts with their strong increase in peripheral blood of patients with acute diverticulitis. Different than peripheral blood, inflamed IBD mucosa contains an increased number of CD4+ CD25(high) T cells, FOXP3+ T cells, and transcripts for FOXP3 compared with noninflamed mucosa. However, the increase of FOXP3+ T cells in IBD lesions is significantly lower compared with inflammatory controls.

CONCLUSIONS

The frequency of CD4+ CD25+ T(reg) cells varies with IBD activity. Active IBD is not associated with a functional defect but with a contraction of the peripheral blood T reg pool and an only moderate expansion in intestinal lesions. Thus, compensatory mechanisms, numerically, are not successfully achieved in these diseases.

Ovalbumin-specific IgE modulates ovalbumin-specific T-cell response after repetitive oral antigen administration

BACKGROUND

Some patients outgrow their food allergies even though their serum antigen-specific IgE levels remain high.

OBJECTIVE

To elucidate the role of T cells in outgrowing food allergies in the presence of antigen-specific IgE, we tracked antigen-specific T-cell responses after oral antigen administration.

METHODS

Ovalbumin (OVA)-specific T-cell receptor (TCR) and OVA-specific IgE transgenic (Tg) mice (OVA-TCR/IgE-Tg) and OVA-specific TCR Tg (OVA-TCR-Tg) mice were fed with high doses of OVA or PBS every other day. After 7 administrations, OVA-specific proliferation and cytokine production of mononuclear cells of the spleen, mesenteric lymph nodes, and Peyer's patches and the number of splenic CD4 + CD25 + T cells were analyzed.

RESULTS

Without OVA administration, the splenocytes from OVA-TCR/IgE-Tg mice exhibited a higher proliferative response and produced more IL-4 and IL-10 and less IFN-gamma than those from OVA-TCR-Tg mice. The proliferative responses of the splenocytes from either OVA-TCR/IgE-Tg mice or OVA-TCR-Tg mice fed with OVA were significantly reduced compared with those from PBS-fed mice. The number of OVA-specific TCR + T cells decreased in the spleen from OVA-fed mice, whereas the number of CD4 + CD25 + T cells increased. The suppressed proliferation of splenocytes of OVA-fed mice was partially resumed by neutralization of TGF-beta1, but not of IL-10.

CONCLUSION

The presence of OVA-specific IgE modulated the OVA-specific responses of the splenocytes. Irrespective of the presence of OVA-specific IgE, repetitive oral administration of OVA induced tolerance, which seems to be composed of clonal deletion/anergy and TGF-beta1-mediated active suppression.

Current issues in the treatment of human diseases by mucosal tolerance

Tolerance has been defined as a lack of response to self but a more appropriate definition of tolerance is "any mechanism by which a potentially injurious immune response is prevented, suppressed, or shifted to a non-injurious class of immune response." Thus, tolerance is related to productive self-recognition, rather than blindness of the immune system to its auto-components. Oral tolerance, in this sense, is of unique immunologic importance, as it is a continuous natural immunologic event driven by exogenous antigen. Because of their privileged access to the internal milieu, antigens that are continuously in contact with the mucosa are a frontier between foreign and self-components. Thus, oral tolerance is an immunological mechanism that evolved to treat external agents that gain access to the body via a natural route as internal components that then become part of self. Given this, it would seem logical that autoimmune diseases caused by an inappropriate response to self-antigens might ultimately be treated by presenting such autoantigens to the mucosal surface where they can be dealt with in a non-injurious (noninflammatory) immunologic environment. Furthermore, mucosal tolerance as a treatment for autoimmune diseases is an attractive concept, as antigen-specific therapy is the most physiologic means to manipulate immune responses, and mucosal antigen is nontoxic and can be given on a chronic basis. The efficacy of mucosal tolerance has been clearly demonstrated in several animal models.

Regulation of autoreactive T cell function by oral tolerance to self-antigens

The oral administration of neuroantigens can suppress as well as treat autoimmune disease. Using EAE as a model system, we examined the antigen-presenting cell in oral tolerance. Expansion of dendritic cells (DCs) prior to or after disease is established facilitated oral tolerance. Transfer of oral antigen-loaded DCs resulted in protection from EAE by induction of IL-4 and IL-5 in recipient animals. LPS treatment of donors abrogated the ability of DCs to transfer protection from EAE, emphasizing the importance of the DC activation state. T cells exposed to orally administered antigen were monitored in TCR transgenic mice and found to undergo activation followed by deletion. The thymus plays a critical role in oral tolerance since thymectomized mice could not be tolerized. The thymus is postulated to be a site for deletion of autoreactive T cells or a site for generation of regulatory T cells.

Retroviral Foxp3 gene transfer ameliorates liver granuloma pathology in Schistosoma mansoni infected mice

Schistosomiasis mansoni, a tropical helminthic disease, is caused by disseminated worm eggs that induce CD4(+) T-cell mediated granulomatous inflammation and fibrosis. T suppressor cell activity has been proposed as one of the mechanisms active in the down-modulation of the murine disease during the chronic stage (16-20 weeks of the infection). In recent years a new category of the CD4(+) CD25(+) T regulatory (Treg) lymphocyte has been identified that maintains immune tolerance to self, and also functions in the regulation of parasite-induced immunopathology. The Foxp3 gene which encodes the transcription factor Scurfin was found to be expressed by and required for the generation of CD4(+) CD25(+) T reg. At 8 weeks of the infection Foxp3 gene expression of splenocytes was similar to that of naive mice, but increased fourfold by 16 weeks. In contrast, granulomatous livers at 8 and 16 weeks showed 10- and 30-fold increases, respectively, in gene expression compared with normal liver. The percentage of granuloma CD4(+) CD25(+) T cells rose from 12% at 8 weeks to 88% at 16 weeks of the infection. Foxp3 expression was 3.5-fold higher in the CD4(+) CD25(+) versus the CD4(+) CD25(-) T cells in the 8 week infection granulomas. As a novel observation neuropilin-1 membrane expression, a recently identified marker for Treg, was correlated with Foxp3 expression in the granuloma CD4(+) CD25(+) but not the CD25(-) cells. Co-incubation with polyclonal stimulation of CD4(+) CD25(+) splenic cells with CD4(+) CD25(-) cells suppressed proliferation of the latter. Retroviral transfer of the Foxp3 gene at the onset of granuloma formation enhanced fourfold Foxp3 expression in the granuloma CD4(+) CD25(+) T cells and strongly suppressed full granuloma development. Gene transfer also significantly enhanced transforming growth factor-beta, interferon-gamma and interleukin-4 but not interleukin-10 expression. It is concluded, that CD4(+) CD25(+), Foxp3(+) Treg cells also regulate schistosome egg-induced immunopathology.

Nasal Administration of Recombinant Rat α3(IV)NC1 Prevents the Development of Experimental Autoimmune Glomerulonephritis in the WKY Rat

Experimental autoimmune glomerulonephritis (EAG), an animal model of Goodpasture's disease, can be induced in Wistar Kyoto (WKY) rats by immunization with either collagenase-solubilized rat glomerular basement membrane (GBM) or the recombinant NC1 domain of the alpha3 chain of type IV collagen [alpha3(IV)NC1]. EAG is characterized by circulating and deposited anti-glomerular basement membrane antibodies, focal necrotizing glomerulonephritis with crescent formation, and glomerular infiltration by T cells and macrophages. Previous studies have demonstrated that oral administration of collagenase-solubilized GBM to WKY rats prevented the development of EAG. Nasal administration of specific autoantigens has been reported to be more effective than oral administration in other models of autoimmune disease. The main aim of this study was to investigate further the concept of mucosal tolerance in EAG by examining the effect of nasal administration of recombinant rat alpha3(IV)NC1. Groups of WKY rats with EAG, induced by immunization with recombinant rat alpha3(IV)NC1, were given alpha3(IV)NC1 nasally on 3 consecutive days before immunization, at total cumulative doses of 25, 100, or 250 microg per rat. A dose-dependent effect was observed on the development of EAG. A dose of 25 microg had no effect on disease; 100 microg resulted in a moderate reduction in the severity of nephritis; and 250 microg led to a marked reduction in circulating and deposited antibodies, albuminuria, severity of glomerular abnormalities, and numbers of glomerular CD8+ T cells and macrophages. In addition, there was a reduction in the proliferative response of splenocytes from rats in the high dose group (250 microg) to alpha3(IV)NC1 in vitro. The results from this study clearly demonstrate for the first time that mucosal tolerance in EAG can be induced by nasal administration of recombinant rat alpha3(IV)NC1 and that this approach is effective in the prevention of crescentic glomerulonephritis. Further work using new antigen-specific treatment strategies may provide a novel approach to the treatment of patients with anti-glomerular basement membrane disease.

Complementary role of CD4+CD25+ regulatory T cells and TGF-beta in oral tolerance

CD4(+)CD25(+) regulatory T cells are thought to be generated in the periphery as well as in the thymus. We sought to determine the roles played by CD4(+)CD25(+) T cells and transforming growth factor-beta (TGF-beta) in the induction and maintenance of tolerance generated by oral antigens in BALB/c mice. We found that oral administration of a high dose of ovalbumin (OVA) suppressed OVA-specific proliferation and antibody production in BALB/c mice depleted of CD25(+) cells. In contrast, the unresponsiveness induced by lower doses of OVA was only partially blocked by CD25 depletion prior to feeding. Depletion of CD4(+)CD25(+) cells after mice were orally tolerized did not reverse the tolerant status, indicating that these cells were not required to maintain the established tolerance. Furthermore, the induction of oral tolerance was not hampered by the administration of TGF-beta-neutralizing antibodies. However, in mice depleted of CD25(+) cells, anti-TGF-beta-neutralizing antibodies blocked the induction of tolerance, regardless of whether the mice followed the high- or low-dose regimens of oral OVA. CD25 depletion together with TGF-beta neutralization led the expansion of OVA-specific CD4 T cells against the subsequent antigen challenge, and each treatment alone did not. Our findings indicate that CD4(+)CD25(+) T cells and TGF-beta play a complementary role in the induction of oral tolerance, at least in part, by regulating the expansion of antigen-specific CD4 T cells.

Does the microbiota regulate immune responses outside the gut?

Perturbations in the gastrointestinal (GI) microbiota composition that occur as a result of antibiotics and diet in "westernized" countries are strongly associated with allergies and asthma ("hygiene hypothesis"). The microbiota ("microflora") plays a crucial role in the development of mucosal tolerance, including the airways. Significant attention has been focused on the role of the microbiota in GI development, immune adaptation and initiation of GI inflammatory diseases. This review covers the post-developmental functions that the microbiota plays in regulating immunological tolerance to allergen exposure outside the GI tract and proposes the question: is the microbiota a major regulator of the immune system?

Inflammatory bowel disease: dysfunction of GALT and gut bacterial flora (I)

Gut-associated lymphoid tissue (GALT) is the largest lymphoid organ in the body. This is not surprising considering the huge load of antigens (Ags) from food and commensal bacteria with which it interacts on a daily basis. Gut-associated lymphoid tissue has to recognise and allow the transfer of beneficial Ags whilst concurrently dealing with and successfully removing putative and overtly harmful Ags. This distinctive biological feature of GALT is believed to be crucial to good health. Deregulation or dysfunction of GALT is thought to predispose to inflammatory bowel diseases (IBD) such as ulcerative colitis and Crohn's disease. The exact mechanism(s) underlying the pathogenesis of IBD is (are) poorly understood and the immunological defects in GALT are poorly documented. Advances in immunology have highlighted the importance of dendritic cells (DCs), which are the key Ag presenting cells in tissues and lymphoid compartments. Their crucial role in GALT, in health and disease is discussed in this review. Interaction of DCs with T cells in the gut produces a subset of T lymphocytes, which have immunosuppressive function. Inappropriate Ag uptake and presentation to naïve T cells in mesenteric lymph nodes may lead to T cell tolerance in GALT. These various complex factors in the gut are discussed and their possible relevance to IBD evaluated.

Transforming growth factor-beta-induced regulatory T cells referee inflammatory and autoimmune diseases

Naturally occurring CD4+CD25+ regulatory T cells mediate immune suppression to limit immunopathogenesis associated with chronic inflammation, persistent infections and autoimmune diseases. Their mode of suppression is contact-dependent, antigen-nonspecific and involves a nonredundant contribution from the cytokine transforming growth factor (TGF)-beta. Not only can TGF-beta mediate cell-cell suppression between the regulatory T cells and CD4+CD25- or CD8+ T cells, but new evidence also reveals its role in the conversion of CD4+CD25- T cells, together with TCR antigen stimulation, into the regulatory phenotype. Elemental to this conversion process is induction of expression of the forkhead transcription factor, Foxp3. This context-dependent coercion of naive CD4+ T cells into a powerful subset of regulatory cells provides a window into potential manipulation of these cells to orchestrate therapeutic intervention in diseases characterized by inadequate suppression, as well as a promising means of controlling pathologic situations in which excessive suppression dominates.

Interleukin-15 is not required for the induction or maintenance of orally induced peripheral tolerance

Orally induced tolerance is a physiologically relevant form of peripheral tolerance, which is believed to be important for the prevention of pathological immune responses in the gut. Of several mechanisms proposed to mediate oral tolerance, one that has received much attention recently is the concept of regulatory CD4+ T cells. As recent studies have suggested that interleukin (IL)-15 may be important for the differentiation and maintenance of regulatory CD4+ T cells, we have examined the role of IL-15 in oral tolerance, using a soluble form of the IL-15 receptor (sIL-15R) which blocks the biological effects of IL-15 in vivo. Oral tolerance induced by feeding mice ovalbumin (OVA) in a low-dose regimen believed to induce regulatory T cell activity was not affected by the administration of sIL-15R during either the induction or maintenance phase of tolerance. Thus, oral tolerance does not involve an IL-15-dependent mechanism.

CD4+ Tregs and immune control

Recent years have seen Tregs become a popular subject of immunological research. Abundant experimental data have now confirmed that naturally occurring CD25+CD4+ Tregs in particular play a key role in the maintenance of self tolerance, with their dysfunction leading to severe or even fatal immunopathology. The sphere of influence of Tregs is now known to extend well beyond just the maintenance of immunological tolerance and to impinge on a host of clinically important areas from cancer to infectious diseases. The identification of specific molecular markers in both human and murine immune systems has enabled the unprecedented investigation of these cells and should prove key to ultimately unlocking their clinical potential.

The Thymus Plays a Role in Oral Tolerance Induction in Experimental Autoimmune Encephalomyelitis

Mice are protected from experimental autoimmune encephalomyelitis (EAE) when fed myelin basic protein (MBP). Thymectomized mice do not exhibit oral tolerance. We found evidence for two mechanisms to explain the role of the thymus in oral tolerance: a site for deletion of autoreactive T cells and a source of regulatory T cells.

Involvement of Dendritic Cell Subsets in the Induction of Oral Tolerance and Immunity

Dendritic cells (DCs) play a central role in the generation of immune responses in the intestine. DCs induce differentiation and tolerance of T cells, and may have a direct role in B cell switching to IgA. Four distinct subsets of CD11c(+) DCs are present in murine Peyer's patches, which represent primary sites for the induction of mucosal T and B cell responses. Studies suggest that CD11b(+) DCs or plasmacytoid DCs may be specialized for the induction of regulatory T cells, and CD8alpha(+) DCs for the induction of clonal deletion in response to soluble oral antigen, while all DC subsets (including CD8alpha(-)/CD11b(-) DCs) may be involved in responses to pathogens. We are currently using reovirus type-1 Lang (TIL) to explore the role of DC populations in mucosal immunity in vivo, as oral administration of live T1L to mice induces strong mucosal and systemic antiviral immune responses, whereas oral administration of inactivated T1L results in tolerance to viral proteins. We found that primary infection with T1L occurs in epithelial cells of the PP follicle-associated epithelium, but that CD8alpha(-)/CD11b(-) DCs in the subepithelial dome region (SED) are loaded with T1L antigens in the absence of active DC infection. At least a portion of this antigen is associated with cell fragments from apoptotic epithelial cells, demonstrating that SED DCs cross-present antigens from apoptotic epithelial cells. In vitro, in contrast to exposure to several TLR-ligands or anti-CD40, exposure to T1L does not activate DCs to mature or to produce cytokines, despite clear loading of the DCs with viral antigens. These data suggest that T1L is taken up by a "silent" receptor on DCs, and that the induction of immunity to T1L is dependent on signals from non-DCs following active viral infection that induce DC maturation. Thus, the decision between tolerance and immunity to inactive and live virus, respectively, likely depends on whether there is active infection of epithelial cells by T1L, which results in the elaboration of molecules, such as cytokines, that induce DC maturation.

Oral Tolerance: Overview and Historical Perspectives

Oral tolerance was first detailed almost 100 years ago, and since then, it has been shown repeatedly that feeding a wide variety of nonpathogenic antigens can inhibit subsequent systemic immune responses. All systemic immune responses are susceptible, but the degree and scope of the suppression depends on the nature and dose of the fed antigen. Oral tolerance has been described in most mammals, including humans, and it may be the homeostatic mechanism that prevents hypersensitivity to food antigens, as is found in celiac disease. A similar process may prevent the aberrant immune responses to commensal bacteria that occur in inflammatory bowel disease. The ability of oral tolerance to modulate experimental models of autoimmune and inflammatory disease has led to clinical trials in such diseases as rheumatoid arthritis, multiple sclerosis, and type I diabetes, with only variable success. Despite intense research, the exact mechanisms responsible for the systemic tolerance and the reasons why tolerance is the default response to many fed antigens remain controversial. Early studies suggested that CD8(+) "suppressor" T cells were important, but it is now accepted that it may involve either anergy/deletion of CD4(+) T cells, or the induction of regulatory CD4(+) T cells that produce IL-10 and/or TGFbeta. There may also be a role for CD4(+) CD25(+) T(reg), but how and when all these different mechanisms operate is still unclear. The ability of fed antigens to induce tolerance probably reflects their uptake by "quiescent" antigen-presenting cells in the intestine, with presentation to specific CD4(+) T cells in the absence of costimulation, or with the involvement of inhibitory costimulatory molecules. Dendritic cells in the Peyer's patches or mucosal lamina propria are the most likely APCs involved, but it remains to be determined exactly where these interactions occur and what the precise nature of the relevant dendritic cells is.

CD4+ Tregs and immune control

Recent years have seen Tregs become a popular subject of immunological research. Abundant experimental data have now confirmed that naturally occurring CD25+CD4+ Tregs in particular play a key role in the maintenance of self tolerance, with their dysfunction leading to severe or even fatal immunopathology. The sphere of influence of Tregs is now known to extend well beyond just the maintenance of immunological tolerance and to impinge on a host of clinically important areas from cancer to infectious diseases. The identification of specific molecular markers in both human and murine immune systems has enabled the unprecedented investigation of these cells and should prove key to ultimately unlocking their clinical potential.

Therapeutic vaccines in autoimmunity

Similarly to prophylactic vaccines whose purpose is to prevent infectious diseases, therapeutic vaccines against autoimmune diseases are based on their similarity to the putative causes of the disease. We shall describe here two such examples: a copolymer of amino acids related to myelin basic protein, in the case of multiple sclerosis, and a peptide derived from the nicotinic acetylcholine receptor (AChR), in the case of myasthenia gravis (MG). Copolymer 1 (Cop 1, glatiramer acetate, Copaxone) is a synthetic amino acid random copolymer, immunologically cross-reactive with myelin basic protein and suppresses experimental allergic encephalomyelitis in several animal species. Cop 1 slows the progression of disability and reduces relapse rate in exacerbating-remitting multiple sclerosis patients. It was approved by the Food and Drug Administration in 1996, and today is used by tens of thousands of patients. Cop 1 is a potent inducer of T helper 2 (Th2) regulatory cells in mice and humans, and Th2 cells are found both in the brains and spinal cords of Cop 1-treated mice. MG and experimental autoimmune MG are T cell-regulated, antibody-mediated autoimmune diseases. Two peptides, representing sequences of the human AChR alpha-subunit, p195-212 and p259-271, are immunodominant T cell epitopes in MG patients and in two strains of mice. Altered peptide ligand, composed of the tandemly arranged two single amino acid analogs, inhibits in vitro and in vivo MG-associated autoimmune responses. The active suppression is mediated by the CD4(+)CD25(+) immunoregulatory cells and is associated with the down-regulation of Th1-type cytokines and the up-regulation of the secretion of IL-10 and the immunosuppressive cytokine, transforming growth factor beta.