Tuning immune tolerance with vasoactive intestinal peptide: a new therapeutic approach for immune disorders

The induction of immune tolerance is essential for the maintenance of immune homeostasis and to limit the occurrence of exacerbated inflammatory and autoimmune conditions. Multiple mechanisms act together to ensure self-tolerance, including central clonal deletion, cytokine deviation and induction of regulatory T cells. Identifying the factors that regulate these processes is crucial for the development of new therapies of autoimmune diseases and transplantation. The vasoactive intestinal peptide (VIP) is a well-characterized endogenous anti-inflammatory neuropeptide with therapeutic potential for a variety of immune disorders. Here, we examine the latest research findings, which indicate that VIP participates in maintaining immune tolerance in two distinct ways: by regulating the balance between pro-inflammatory and anti-inflammatory factors, and by inducing the emergence of regulatory T cells with suppressive activity against autoreactive T-cell effectors.     

Regulating the immune system: the induction of regulatory T cells in the periphery

The immune system has evolved a variety of mechanisms to achieve and maintain tolerance both centrally and in the periphery. Central tolerance is achieved through negative selection of autoreactive T cells, while peripheral tolerance is achieved primarily via three mechanisms: activation-induced cell death, anergy, and the induction of regulatory T cells. Three forms of these regulatory T cells have been described: those that function via the production of the cytokine IL-10 (T regulatory 1 cells), transforming growth factor beta (Th3 cells), and a population of T cells that suppresses proliferation via a cell-contact-dependent mechanism (CD4+CD25+ TR cells). The present review focuses on the third form of peripheral tolerance - the induction of regulatory T cells. The review will address the induction of the three types of regulatory T cells, the mechanisms by which they suppress T-cell responses in the periphery, the role they play in immune homeostasis, and the potential these cells have as therapeutic agents in immune-mediated disease.     

Autoimmunity: basic mechanisms and implications in endocrine diseases. Part II

Regulation of the immune response to self-antigens is a complex process that involves maintaining self-tolerance while preserving the capacity to exert an effective immune response. The primary mechanism that leads to self-tolerance is central tolerance. However, potential pathogenic autoreactive lymphocytes are normally present in the periphery of all individuals. This suggests the existence of mechanisms of peripheral tolerance that prevent the initiation of autoimmune diseases by limiting the activation of autoreactive lymphocytes. If these mechanisms of peripheral tolerance are impaired, the autoreactive lymphocytes may be activated and autoimmune diseases can develop. Several processes are involved in the maintenance of peripheral tolerance: the active suppression mediated by regulatory T cell populations, the different maturation state of antigen-presenting cells presenting the autoantigen to autoreactive lymphocytes, inducing tolerance instead of cell activation, the characteristics of B cell populations. A deeper comprehension of these mechanisms may lead to important therapeutic applications, such as the development of cellular vaccines for organ-specific autoimmune diseases. In addition, autoimmunity does not always have pathological consequences, but may exert a protective function, as suggested by several observations on the beneficial role of autoreactive T cells in central nervous system injury.     

The role of peripheral T-cell deletion in transplantation tolerance

The apoptotic deletion of thymocytes that express self-reactive antigen receptors is the basis of central (thymic) self-tolerance. However, it is clear that some autoreactive T cells escape deletion in the thymus and exist as mature lymphocytes in the periphery. Therefore, peripheral mechanisms of tolerance are also crucial, and failure of these peripheral mechanisms leads to autoimmunity. Clonal deletion, clonal anergy and immunoregulation and/or suppression have been suggested as mechanisms by which 'inappropriate' T-lymphocyte responses may be controlled in the periphery. Peripheral clonal deletion, which involves the apoptotic elimination of lymphocytes, is critical for T-cell homeostasis during normal immune responses, and is recognized as an important process by which self-tolerance is maintained. Transplantation of foreign tissue into an adult host represents a special case of 'inappropriate' T-cell reactivity that is subject to the same central and peripheral tolerance mechanisms that control reactivity against self. In this case, the unusually high frequency of naive T cells able to recognize and respond against non-self-allogeneic major histocompatibility complex (MHC) antigens leads to an exceptionally large pool of pathogenic effector lymphocytes that must be controlled if graft rejection is to be avoided. A great deal of effort has been directed toward understanding the role of clonal anergy and/or active immunoregulation in the induction of peripheral transplantation tolerance but, until recently, relatively little progress had been made towards defining the potential contribution of clonal deletion. Here, we outline recent data that define a clear requirement for deletion in the induction of peripheral transplantation tolerance across MHC barriers, and discuss the potential implications of these results in the context of current treatment modalities used in the clinical transplantation setting.     

Regulatory T cells and mechanisms of immune system control

The immune system evolved to protect the host against the attack of foreign, potentially pathogenic, microorganisms. It does so by recognizing antigens expressed by those microorganisms and mounting an immune response against all cells expressing them, with the ultimate aim of their elimination. Various mechanisms have been reported to control and regulate the immune system to prevent or minimize reactivity to self-antigens or an overexuberant response to a pathogen, both of which can result in damage to the host. Deletion of autoreactive cells during T- and B-cell development allows the immune system to be tolerant of most self-antigens. Peripheral tolerance to self was suggested several years ago to result from the induction of anergy in peripheral self-reactive lymphocytes. More recently, however, it has become clear that avoidance of damage to the host is also achieved by active suppression mediated by regulatory T (T(reg)) cell populations. We discuss here the varied mechanisms used by T(reg) cells to suppress the immune system.     

Antigen-based immunointervention in human autoimmune diseases

Antigen administration can ameliorate autoimmune disease via various mechanisms, including deletion of autoreactive cells, induction of regulatory T cells, and deviation to non-pathogenic or protective responses. All these mechanisms of immunointervention have been successfully used to prevent and sometimes treat experimental models of autoimmune diseases. Based on these results, expectations have been raised for exploiting similar strategies to inhibit pathogenic autoreactive T cells in human autoimmune diseases. Among them, mucosal administration of autoantigen is an attractive mode of immunointervention still awaiting demonstration of clinical efficacy in human autoimmune diseases. A further step in this direction is now provided by the clear-cut immune deviation observed following oral administration of a disease-related peptide to rheumatoid arthritis patients, leading to inhibition of Th1 while enhancing Th2 and possibly Foxp3-positive regulatory T cells.     

Myeloid-derived suppressor cells prevent type 1 diabetes in murine models

Effective immunotherapy for type 1 diabetes (T1D) relies on active induction of peripheral tolerance. Myeloid-derived suppressor cells (MDSCs) play a critical role in suppressing immune responses in various pathologic settings via multiple mechanisms, including expansion of regulatory T cells (Tregs). In this study, we investigated whether MDSCs could act as APCs to induce expansion of Ag-specific Tregs, suppress T cell proliferation, and prevent autoimmune T1D development. We found that MDSC-mediated expansion of Tregs and T cell suppression required MHC-dependent Ag presentation. A murine T1D model was established in INS-HA/RAG(-/-) mice in which animals received CD4-HA-TCR transgenic T cells via adoptive transfer. We found a significant reduction in the incidence of diabetes in recipients receiving MDSC plus HA, but not OVA peptide, leading to 75% diabetes-free mice among the treated animals. To test further whether MDSCs could prevent diabetes onset in NOD mice, nondiabetic NOD/SCID mice were injected with inflammatory T cells from diabetic NOD mice. MDSCs significantly prevented diabetes onset, and 60% of MDSC-treated mice remained diabetes free. The pancreata of treated mice showed significantly lower levels of lymphocyte infiltration in islet and less insulitis compared with that of the control groups. The protective effects of MDSCs might be mediated by inducing anergy in autoreactive T cells and the development of CD4(+)CD25(+)Foxp3(+) Tregs. Thist study demonstrates a remarkable capacity of transferred MDSCs to downregulate Ag-specific autoimmune responses and prevent diabetes onset, suggesting that MDSCs possess great potential as a novel cell-based tolerogenic therapy in the control of T1D and other autoimmune diseases.     

Autoimmune T cells lured to a FASL web of death by MSCs

Achieving immune tolerance through cell transplantation is a promising approach for treating autoimmune disease. In this issue of Cell Stem Cell, Akiyama et?al. (2012) demonstrate that human and mouse mesenchymal stem cells can induce immune suppression by attracting and killing autoreactive T?cells, which stimulates TGFb production by macrophages and generates regulatory T?cells.     

CD4+ CD25+ suppressor T cells: more questions than answers

Several mechanisms control discrimination between self and non-self, including the thymic deletion of autoreactive T cells and the induction of anergy in the periphery. In addition to these passive mechanisms, evidence has accumulated for the active suppression of autoreactivity by a population of regulatory or suppressor T cells that co-express CD4 and CD25 (the interleukin-2 receptor alpha-chain). CD4+ CD25+ T cells are powerful inhibitors of T-cell activation both in vivo and in vitro. The enhancement of suppressor-cell function might prove useful for the treatment of immune-mediated diseases, whereas the downregulation of these cells might be beneficial for the enhancement of the immunogenicity of vaccines that are specific for tumour antigens.     

Requirement for T-cell apoptosis in the induction of peripheral transplantation tolerance.

The mechanisms of allograft tolerance have been classified as deletion, anergy, ignorance and suppression/regulation. Deletion has been implicated in central tolerance, whereas peripheral tolerance has generally been ascribed to clonal anergy and/or active immunoregulatory states. Here, we used two distinct systems to assess the requirement for T-cell deletion in peripheral tolerance induction. In mice transgenic for Bcl-xL, T cells were resistant to passive cell death through cytokine withdrawal, whereas T cells from interleukin-2-deficient mice did not undergo activation-induced cell death. Using either agents that block co-stimulatory pathways or the immunosuppressive drug rapamycin, which we have shown here blocks the proliferative component of interleukin-2 signaling but does not inhibit priming for activation-induced cell death, we found that mice with defective passive or active T-cell apoptotic pathways were resistant to induction of transplantation tolerance. Thus, deletion of activated T cells through activation-induced cell death or growth factor withdrawal seems necessary to achieve peripheral tolerance across major histocompatibility complex barriers.     

E3 ubiquitin ligases and their control of T cell autoreactivity

A loss of T cell tolerance underlies the development of most autoimmune diseases. The design of therapeutic strategies to reinstitute immune tolerance, however, is hampered by uncertainty regarding the molecular mechanisms involved in the inactivation of potentially autoreactive T cells. Recently, E3 ubiquitin ligases have been shown to mediate the development of a durable state of unresponsiveness in T cells called clonal anergy. In this review, we will discuss the mechanisms used by E3 ligases to control the activation of T cells and prevent the development of autoimmunity.     

Oral tolerance: immune mechanisms and the generation of Th3-type TGF-beta-secreting regulatory cells

Oral tolerance is a long recognized method to induce peripheral immune tolerance. Oral tolerance has been used successfully to treat animal models of autoimmune diseases and is being tested in human diseases. Low doses of oral antigen induce active suppression, whereas high doses induce clonal anergy and deletion. Oral antigen preferentially generates a Th2(IL-4/IL-10)- or a Th3(TGF-beta)-type response. Th3-type cells are a unique T-cell subset which primarily secrete TGF-beta, provide help for IgA and have suppressive properties for Th1 and other immune cells. Th3-type cells appear distinct from the Th2 cells as CD4(+) TGF-beta-secreting cells with suppressive properties in the gut have been generated from IL-4-deficient animals. In vitro differentiation of Th3-type cells from Th0 precursors from TCR transgenic mice is enhanced by culture with TGF-beta, IL-4, IL-10 and anti-IL-12. Because regulatory T cells generated by oral antigen are triggered in an antigen-specific fashion but suppress in an antigen-nonspecific fashion, they mediate bystander suppression when they encounter the fed autoantigen at the target organ. Thus, mucosal tolerance can be used to treat inflammatory processes that are not autoimmune in nature. Mucosal antigen has also been used to treat animal models of stroke and of Alzheimer's disease. Induction of low-dose oral tolerance is enhanced by oral administration of IL-4 and IL-10. Coupling antigen to CTB or administration of Flt-3 ligand enhances oral tolerance. Anti-B7.2 but not anti-B7.1 blocks low-dose, but not high-dose oral tolerance. High-dose oral tolerance is blocked by anti-CTLA-4. CD25(+) CD4(+) regulatory T-cell function also appears to be related to TFG-beta.     

Cutting edge: requirement for TRAF6 in the induction of T cell anergy

TRAF6, TNFR-associated factor 6, is a key adaptor downstream from the TNF receptor and TLR superfamily members. T cell-specific deletion of TRAF6 (TRAF6-DeltaT) was recently shown to result in the development of multiorgan inflammatory disease and the resistance of responder T cells to suppression by CD4+CD25+ regulatory T cells. In this study we examined the role of TRAF6 in an additional mechanism of peripheral tolerance, anergy. We have determined that the loss of TRAF6 restores the ability of CD28-/- T cells to proliferate and produce IL-2. Consistent with this, TRAF6-DeltaT T cells were resistant to anergizing signals both in vitro and in vivo. Resistance to anergy was correlated with decreased expression of Cbl-b. These findings reveal that in addition to its role in rendering T cells susceptible to control by CD4+CD25+ regulatory T cells, TRAF6 is essential for the induction of T cell anergy, implicating TRAF6 as a critical mediator of peripheral tolerance.     

Preferential recognition of self antigens despite normal thymic deletion of CD4(+)CD25(+) regulatory T cells

T cell tolerance to self Ags is in part established in the thymus by induction of apoptosis or anergy of potentially autoreactive thymocytes. Some autospecific T cells nevertheless migrate to peripheral lymphoid organs but are kept under control by the recently identified CD4(+)CD25(+) regulatory T cell subset. Because these cells inhibit autoimmunity more efficiently than useful non-self Ag-specific immune responses, they are probably autospecific, posing important questions as to how they develop in the thymus. In this study we show that significantly more peripheral CD4(+)CD25(+) regulatory T cells recognize self than non-self Ags. However, we also show for a large panel of endogenous superantigens as well as for self peptide/MHC complexes that autospecific CD4(+)CD25(+) thymocyte precursors are normally deleted during ontogeny. Combined, our data firmly establish that the repertoire of regulatory T cells is specifically enriched in autospecific cells despite the fact that their precursors are normally susceptible to thymic deletion.     

Dendritic cell-regulatory T-cell interactions control self-directed immunity

In addition to their immunostimulatory capacity, dendritic cells (DCs) play a crucial role in central and peripheral tolerance mechanisms. In the absence of an infection, immature DCs constantly take up, process and present self-antigens to specific T cells, which leads to the induction of T-cell anergy or deletion. In recent years, several additional mechanisms have been identified by which DCs constantly downregulate immune responses to maintain immunological tolerance. Among these are the complex interactions between several DC subtypes and different types of regulatory T cells. In this review, we summarize recent key findings and concepts in this field.     

Regulatory CD8+ T cells control neonatal tolerance to a Th2-mediated autoimmunity

Exposure of newborn animals to a foreign Ag may result in immunological tolerance to that specific Ag, a phenomenon called neonatal tolerance. We have previously reported that neonatal administration to Brown-Norway rats of mercury, a heavy metal toxicant, induces a dominant tolerance, specific for the chemical otherwise responsible for Th2 cell-mediated autoimmune responses in this susceptible strain of rats. Neonatal exposure to Ags can prime immunity, rather than inactivate or delete responses, and sustain regulatory functions effective against autoreactive T cells. Here, we address whether such a tolerant response is due to the generation of regulatory cells. The results suggest that the CD8(+) T cell subset is involved in neonatal tolerance to mercuric salt-induced Th2 autoimmune disease. Thus, we demonstrate that in vivo CD8 depletion breaks tolerance following mercury recall in animals under a neonatal tolerance protocol. Furthermore, adoptive cotransfer of splenocytes from naive and tolerant rats as well as transfer of CD8(+) T cells from tolerant animals prevent naive syngeneic rats from developing pathologic Th2 immune responses. These observations indicate that CD8(+) T cells are endowed with regulatory functions in neonatal tolerance and mediate active suppression. Moreover, neonatal tolerance induced the expansion of CD8(+)CD45RC(high) T cells and the emergence of a high percentage of IFN-gamma-synthesizing CD8(+) T cells, which probably reflects the implication of regulatory Tc1 cells. Thus, in vivo induction of neonatal tolerance suppresses Th2 autoimmune responses via generation of a CD8(+) cell-mediated regulatory response.     

The impact of T cell intrinsic antigen adaptation on peripheral immune tolerance

Overlapping roles have been ascribed for T cell anergy, clonal deletion, and regulation in the maintenance of peripheral immunological tolerance. A measurement of the individual and additive impacts of each of these processes on systemic tolerance is often lacking. In this report we have used adoptive transfer strategies to tease out the unique contribution of T cell intrinsic receptor calibration (adaptation) in the maintenance of tolerance to a systemic self-antigen. Adoptively transferred naïve T cells stably calibrated their responsiveness to a persistent self-antigen in both lymphopenic and T cell–replete hosts. In the former, this state was not accompanied by deletion or suppression, allowing us to examine the unique contribution of adaptation to systemic tolerance. Surprisingly, adapting T cells could chronically help antigen-expressing B cells, leading to polyclonal hypergammaglobulinemia and pathology, in the form of mild arthritis. The helper activity mediated by CD40L and cytokines was evident even if the B cells were introduced after extended adaptation of the T cells. In contrast, in the T cell–replete host, neither arthritis nor autoantibodies were induced. The containment of systemic pathology required host T cell–mediated extrinsic regulatory mechanisms to synergize with the cell intrinsic adaptation process. These extrinsic mechanisms prevented the effector differentiation of the autoreactive T cells and reduced their precursor frequency, in vivo.     

Anti‐inflammatory and immune‐modulatory impacts of berberine on activation of autoreactive T cells in autoimmune inflammation

Autoreactive inflammatory CD4⁺ T cells, such as T helper (Th)1 and Th17 subtypes, have been found to associate with the pathogenesis of autoimmune disorders. On the other hand, CD4⁺ Foxp3⁺ T regulatory (Treg) cells are crucial for the immune tolerance and have a critical role in the suppression of the excessive immune and inflammatory response promoted by these Th cells. In contrast, dendritic cells (DCs) and macrophages are immune cells that through their inflammatory functions promote autoreactive T‐cell responses in autoimmune conditions. In recent years, there has been increasing attention to exploring effective immunomodulatory or anti‐inflammatory agents from the herbal collection of traditional medicine. Berberine, an isoquinoline alkaloid, is one of the main active ingredients extracted from medicinal herbs and has been shown to exert various biological and pharmacological effects that are suggested to be mainly attributed to its anti‐inflammatory and immunomodulatory properties. Several lines of experimental study have recently investigated the therapeutic potential of berberine for treating autoimmune conditions in animal models of human autoimmune diseases. Here, we aimed to seek mechanisms underlying immunomodulatory and anti‐inflammatory effects of berberine on autoreactive inflammatory responses in autoimmune conditions. Reported data reveal that berberine can directly suppress functions and differentiation of pro‐inflammatory Th1 and Th17 cells, and indirectly decrease Th cell‐mediated inflammation through modulating or suppressing other cells assisting autoreactive inflammation, such as Tregs, DCs and macrophages.     

Current and Future Immunomodulation Strategies to Restore Tolerance in Autoimmune Diseases

Autoimmune diseases reflect a breakdown in self-tolerance that results from defects in thymic deletion of potentially autoreactive T cells (central tolerance) and in T-cell intrinsic and extrinsic mechanisms that normally control potentially autoreactive T cells in the periphery (peripheral tolerance). The mechanisms leading to autoimmune diseases are multifactorial and depend on a complex combination of genetic, epigenetic, molecular, and cellular elements that result in pathogenic inflammatory responses in peripheral tissues driven by self-antigen-specific T cells. In this article, we describe the different checkpoints of tolerance that are defective in autoimmune diseases as well as specific events in the autoimmune response which represent therapeutic opportunities to restore long-term tolerance in autoimmune diseases. We present evidence for the role of different pathways in animal models and the therapeutic strategies targeting these pathways in clinical trials in autoimmune diseases.Autoimmune diseases are debilitating conditions that affect a large and growing portion of the population (∼3%–5% in the United States) (Jacobson et al. 1997). Autoimmune diseases take a devastating toll on affected families and have a considerable economic impact. Thus, improving the understanding of autoimmune diseases and developing novel therapies have been significant goals in public health. The development of autoimmune diseases reflects a loss of tolerance of the immune system for self-antigens. With the exception of a few rare monogenic diseases such as immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX), and autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) syndrome, the development of autoimmunity is a complex and multifactorial process. This process usually involves genetic predispositions and poorly defined environmental factors that result in slight alterations in many different checkpoints, which in turn tilts the balance toward autoreactivity and away from immunoregulation. Although clearly there are key roles for B cells, antigen-presenting cells (APCs), and the innate immune response in the development and progression of autoimmune diseases, this article will focus on autoreactive T cells and potential targets of tolerogenic treatments (Fig. 1). In addition, we will discuss selected strategies currently available or being developed in the clinic as well as future opportunities to prevent and treat these diseases. Finally, current clinical strategies available as the standard of care for autoimmune diseases rely on immunosuppressive and anti-inflammatory treatments that curtail the pathological events, alleviate symptoms, and provide short-term relief in some patients. Thus, we will focus for the most part on immunotherapies aimed at reestablishing long-term tolerance.Open in a separate windowFigure 1.Development of the pathogenic autoimmune response and targets for immunotherapy. Autoreactive T cells that escape thymic negative selection are usually controlled by intrinsic (inhibitory receptors) and extrinsic (regulatory cell populations) mechanisms of tolerance in the periphery. In individuals genetically prone to autoimmunity, one or several of these checkpoints are defective, resulting in expansion of autoreactive T cells that cannot be controlled by Tregs (red, autoreactive effector T cells; green, Tregs; gray, polyclonal conventional T cells). Autoreactive T cells migrate to their targeted tissue where cytotoxic mechanisms and uncontrolled inflammation mediated by soluble mediators released by T cells and innate cells result in tissue damage. Various immunotherapeutic strategies target different steps in this process. (A) The ultimate goal of immunotherapy is to alter the balance of pathogenic versus regulatory T cells to restore tolerance, as detailed in Figure 2. (B) Anti-CD3 mAbs, antigen-specific therapies, and costimulation blockade alter the interactions between autoreactive T cells and antigen-presenting cells (APCs) and/or the signaling pathways resulting from productive T-cell receptor (TCR) ligation after presentation of cognate self-peptide/MHC (major histocompatibility complexes) in the presence of costimulatory signals, leading to deletion, anergy, immune deviation, or induction of Tregs. (C) Many strategies aim at boosting Tregs, either by concomitantly deleting Teff and promoting Tregs, and thus resetting the immune system to various degrees, such as antithymocyte globulin (ATG), rapamycin plus IL-2, and autologous hematopoietic stem cell transplantation (HSCT), or directly providing Tregs through cellular therapy. (D,E) Some therapies target populations of APCs, such as depletion of B cells by rituximab or the promotion of self-antigen presentation specifically by tolerogenic dendritic cells (DCs). (F) The migration of autoreactive T cells to their target tissue is being altered by inhibitors of leukocyte trafficking such as natalizumab and fingolimod. These drugs may further promote tolerance by keeping autoreactive T cells in the lymph nodes (LN) during immunosuppression, a prerequisite for efficient immunomodulation in some cases. (G) Anti-inflammatory therapies such as tumor necrosis factor (TNF) antagonists reduce tissue damage but also create an immunological environment more favorable to the induction of Tregs and restoration of tolerance.