Repertoire requirements of CD4+ T cells that prevent spontaneous autoimmune encephalomyelitis

Tregs self-organize into a computing ecosystem and implement a sophisticated optimization algorithm for mediating immune response

Regulatory T cells (Tregs) play a crucial role in mediating immune response. Yet an algorithmic understanding of the role of Tregs in adaptive immunity remains lacking. Here, we present a biophysically realistic model of Treg-mediated self-tolerance in which Tregs bind to self-antigens and locally inhibit the proliferation of nearby activated T cells. By exploiting a duality between ecological dynamics and constrained optimization, we show that Tregs tile the potential antigen space while simultaneously minimizing the overlap between Treg activation profiles. We find that for sufficiently high Treg diversity, Treg-mediated self-tolerance is robust to fluctuations in self-antigen concentrations but lowering the Treg diversity results in a sharp transition-related to the Gardner transition in perceptrons-to a regime where changes in self-antigen concentrations can result in an autoimmune response. We propose an experimental test of this transition in immune-deficient mice and discuss potential implications for autoimmune diseases.

Gut Microbiota Influences Neuropathic Pain Through Modulating Proinflammatory and Anti-inflammatory T Cells

BACKGROUND

Gut microbiota, a consortium of diverse microorganisms residing in the gastrointestinal tract, has emerged as a key player in neuroinflammatory responses, supporting the functional relevance of the "gut-brain axis." Chronic-constriction injury of the sciatic nerve (CCI) is a commonly used animal model of neuropathic pain with a major input from T cell-mediated immune responses. In this article, we sought to examine whether gut microbiota influences CCI neuropathic pain, and, if so, whether T-cell immune responses are implicated.

METHODS

We used a mixture of wide-spectrum oral antibiotics to perturbate gut microbiota in mice and then performed CCI in these animals. Nociceptive behaviors, including mechanical allodynia and thermal hyperalgesia, were examined before and after CCI. Additionally, we characterized the spinal cord infiltrating T cells by examining interferon (IFN)-γ, interleukin (IL)-17, and Foxp3. Using a Foxp3-GFP-DTR "knock-in" mouse model that allows punctual depletion of regulatory T cells, we interrogated the role of these cells in mediating the effects of gut microbiota in the context of CCI neuropathic pain.

RESULTS

We found that oral antibiotics induced gut microbiota changes and attenuated the development of CCI neuropathic pain, as demonstrated by dampened mechanical allodynia and thermal hyperalgesia. Percentages of IFN-γ-producing Th1 cells and Foxp3+ regulatory T cells were significantly different between animals that received oral antibiotics (Th1 mean = 1.0, 95% confidence interval [CI], 0.9-1.2; Foxp3 mean = 8.1, 95% CI, 6.8-9.3) and those that received regular water (Th1 mean = 8.4, 95% CI, 7.8-9.0, P < .01 oral antibiotics versus water, Cohen's d = 18.8; Foxp 3 mean = 2.8, 95% CI, 2.2-3.3, P < .01 oral antibiotics versus water, Cohen's d = 6.2). These T cells characterized a skewing from a proinflammatory to an anti-inflammatory immune profile induced by gut microbiota changes. Moreover, we depleted Foxp3+ regulatory T cells and found that their depletion reversed the protection of neuropathic pain mediated by gut microbiota changes, along with a dramatic increase of IFN-γ-producing Th1 cell infiltration in the spinal cord (before depletion mean = 2.8%, 95% CI, 2.2-3.5; after depletion mean = 9.1%, 95% CI, 7.2-11.0, p < .01 before versus after, Cohen's d = 5.0).

CONCLUSIONS

Gut microbiota plays a critical role in CCI neuropathic pain. This role is mediated, in part, through modulating proinflammatory and anti-inflammatory T cells.

Immunomodulatory effects of Trichinella spiralis-derived excretory-secretory antigens

Helminth-derived products, either released into the circulation during the course of the infection or isolated after in vitro cultivation of the parasite and applied by the injection, are able to suppress the host immune response to autoantigens and allergens, but mechanisms could differ. Prophylactic application of Trichinella spiralis excretory-secretory muscle larvae (ES L1) products ameliorates experimental autoimmune encephalomyelitis (EAE) with the same success as infection did. However, a shift to the Th2-type response in the periphery and in the central nervous system, accompanied by activation of regulatory mechanisms, had a striking, new feature of increased proportion of unconventional CD4(+)CD25(-)Foxp3(+) regulatory cells both in the periphery and in the central nervous system of animals treated with ES L1 before the induction of EAE.

On the origin of immunopathology

Stranded between medicine and experimental biology, immunology is buried in its own problems and remains distant from important areas of current biology, such as evolutionary theory, developmental biology and cognitive sciences. Immunology has treated the living system merely as the place or dimension in which immune activity takes place, inserted on a misleading axis (progressive responsiveness versus no response; memory versus tolerance) which neglects the analysis of a robustly stable dynamics which is always present and is neither tolerance nor immunity-a problem currently approached as one of "regulatory" activity. However, a regulatory response also demands regulation, leading to an endless recursion and the adoption of a stimulus-response framework inevitably drives us away from the physiological processes in which lymphocytes are involved. Herein, we propose that immunological physiology, like everything else in the body is dynamic and conservative. Immunopathology, including inherited immunodeficiencies, severe forms of infectious diseases, allergy and autoimmune diseases, are interferences with this stability which frequently include oligoclonal expansions of T lymphocytes. We suggest that this decrease in clonal diversity results from a loss of the stabilizing connectivity among lymphocytes and are not simply markers of immunopathology, but are rather expressions of basic pathogenic mechanisms. The so-called autoimmune diseases are examples of this disequilibrium. In the last decade the characterization of an enormous and diversified commensal microbiota has posed a new and pressing problem: how to explain the harmonic conviviality with trillions of foreign macromolecules. In addition, robustly stable relations towards macromolecular diet can be established by simple ingestion, a state presently labeled as "oral tolerance", a problem that has been buffered for decades as anti-inflammatory protection of the gut. A major change in terminology is necessary to describe this new panorama. We focus on two important gaps in immunological discussions: (a) the organism, seen simultaneously as the medium with which the immune system is constantly in touch and as the entity that mediates the contact with external materials; and (b) the observer, the immunologist, who operates as a human being in human languaging with other human beings, and characterizes immunological specificity. We acknowledge that we are proposing radical departures from current dogma and that we should justify them. Most of what we propose stem form a way of seeing called Biology of Cognition and Language, that derives from ideas of the neurobiologist/philosopher Humberto Maturana, also known as "autopoiesis theory".

Suppressive activity and altered conventional phenotype markers/mediators of regulatory T cells in patients with self-limiting hepatitis E

Hepatitis E virus (HEV) is a major cause of self-limiting acute viral hepatitis in several developing countries. Elevated levels of peripheral CD4(+) CD25(+) Foxp3(+) , CD4(+) CD25(-) Foxp3(+) and rise in IL-10 in hepatitis E have been associated with the involvement of regulatory T cells (Treg). The functional role of the same is yet elusive. In the current study, we have assessed (i) Foxp3 expression by real-time PCR and by flow cytometry, (ii) the levels of antigen-specific IL-10 and TGF-β by ELISA, (iii) functional analysis of Treg cells and (iv) expression of Treg-associated conventional phenotypes by flow cytometry in 54 acute patients, 44 recovered individuals from hepatitis E and in 33 healthy controls. Foxp3 mRNA elevation in the acute compared with recovered group and elevation in Foxp3(+) cells in both patient groups were significantly elevated. The levels of IL-10 and TGF-β in the acute patients and TGF-β in the recovered individuals were elevated. Significantly higher expression of CTLA-4, PD1, GITR, CD95, CD103 and CD73 on Treg and T effector (Teff) cells was detected in the patient groups. Treg cells of acute patients and recovered individuals exhibited suppressive activity indicating that the Treg cells of hepatitis E patients are functional. The suppressive capacity of Treg cells in acute hepatitis E patients was significantly higher compared with the recovered individuals. Based on our findings, the suppressive functionality of these key markers associated with hepatitis E Treg function need further exploration to get a better understanding of the mechanisms of Treg-mediated suppression.

High TCR diversity ensures optimal function and homeostasis of Foxp3+ regulatory T cells

Dominant tolerance to self-antigen requires the presence of sufficient numbers of CD4(+) Foxp3(+) Treg cells with matching antigen specificity. However, the size and role of TCR repertoire diversity for antigen-specific immuno-regulation through Treg cells is not clear. Here, we developed and applied a novel high-throughput (HT) TCR sequencing approach to analyze the TCR repertoire of Treg cells and revealed the importance of high diversity for Treg-cell homeostasis and function. We found that highly polyclonal Treg cells from WT mice vigorously expanded after adoptive transfer into non-lymphopenic TCR-transgenic recipients with low Treg-cell diversity. In that system, we identified specific Treg-cell TCR preferences in distinct anatomic locations such as the mesenteric LN indicating that Treg cells continuously compete for MHC class-II-presented self-, food-, or flora-antigen. Functionally, we showed that high TCR diversity was required for optimal suppressive function of Treg cells in experimental acute graft versus host disease (GvHD). In conclusion, we suggest that efficient immuno-regulation by Treg cells requires high TCR diversity. Thereby, continuous competition of peripheral Treg cells for limited self-antigen shapes an organ-optimized, yet highly diverse, local TCR repertoire.

Resolving the identity myth: key markers of functional CD4+FoxP3+ regulatory T cells

Authenticating markers for the functional suppressive CD4(+)FoxP3(+) regulatory T cells (Tregs) are important for the quantitative identification and enrichment of viable Tregs for possible therapeutic use. CD25 as a surrogate marker of Tregs has some limitations, which prompted investigators to identify more specific marker(s) of Tregs. The search for a firm molecular definition of Tregs resulted in the identification of FoxP3 as a better marker of this subset of CD4 cells. Nevertheless, FoxP3(+) Tregs are phenotypically and functionally heterogeneous. Even in normal mice, only a minority of FoxP3(+) T cells are potent suppressor cells. Therefore, additional marker(s) are required for delineation of truly functional Tregs. In this review, the studies identifying markers of functional Tregs, both in mouse and in human, and their functional implications are discussed. Our finding that TNFR2, which mediates the effect of TNF on the activation of Tregs, is a superb marker of the most suppressive subset of mouse Tregs and its application in the identification of functional human Tregs will also be reviewed.

Thymic and peripheral differentiation of regulatory T cells

The development of regulatory T (Treg) cells is essential for the maintenance of immune tolerance and homeostasis. Here, we review recent studies that have advanced our understanding of Treg cell differentiation. In the thymus, TCR specificity to self-antigen appears to be a primary determinant for Treg cell lineage commitment, with c-Rel being an important factor that links T cell receptor (TCR) engagement and Foxp3 expression, along with cytokines and costimulatory molecules. It is also clear that postthymic events shape the peripheral Treg cell population. This includes preferential maintenance of Treg cells specific to self-antigens presented in the periphery, as well as the de novo generation of Treg cells from conventional Foxp3(-) T cells. The process of peripheral Treg cell differentiation shares some features with thymic Treg cell development, but there are notable differences. Together, thymic and peripheral Treg cell differentiation appear to generate an "imprint" of both self- and foreign antigens in the peripheral Treg cell population to provide dominant tolerance.

Rapamycin inhibits relapsing experimental autoimmune encephalomyelitis by both effector and regulatory T cells modulation

Rapamycin is an oral immunosuppressant drug previously reported to efficiently induce naturally occurring CD4(+)CD25(+)FoxP3(+) regulatory T ((n)T(reg)) cells re-establishing long-term immune self-tolerance in autoimmune diseases. We investigated the effect of rapamycin administration to SJL/j mice affected by PLP(139-151)-induced relapsing-remitting experimental autoimmune encephalomyelitis (RR-EAE). We found that oral or intraperitoneal treatment at the peak of disease or at the end of the first clinical attack, dramatically ameliorated the clinical course of RR-EAE. Treatment suspension resulted in early reappearance of disease. Clinical response was associated with reduced central nervous system demyelination and axonal loss. Rapamycin induced suppression of IFN-gamma, and IL-17 release from antigen-specific T cells in peripheral lymphoid organs. While CD4(+)FoxP3(+) cells were unaffected, we observed disappearance of CD4(+)CD45RB(high) effector T (T(eff)) cells and selective expansion of T(reg) cells bearing the CD4(+)CD45RB(low)FoxP3(+)CD25(+)CD103(+) extended phenotype. Finally, the dual action of rapamycin on both T(eff) and T(reg) cells resulted in modulation of their ratio that closely paralleled disease course. Our data show that rapamycin inhibits RR-EAE, provide evidence for the immunological mechanisms, and indicate this compound as a potential candidate for the treatment of multiple sclerosis.

TCR-dependent differentiation of thymic Foxp3+ cells is limited to small clonal sizes

Numerous studies have highlighted the importance of high-affinity interactions between T cell receptors (TCRs) and their ligands in the selection of Foxp3⁺ regulatory T cells (T reg cells). To determine the role of the TCR in directing T cells into the Foxp3⁺ lineage, we generated transgenic (Tg) mice expressing TCRs from Foxp3⁺ cells. Initial analyses of the TCR Tg mice crossed with RAG-deficient mice showed that the percentage of Foxp3⁺ cells was very low. However, intrathymic injection and bone marrow chimera experiments showed a saturable increase of the Foxp3⁺ population when T reg TCR Tg cells were present in low numbers. Furthermore, when analyzing whole thymi of T reg TCR Tg RAG-deficient mice, we found significantly more Foxp3⁺ cells than in conventional T cell TCR Tg mice. Our results indicate that although the TCR has an instructive role in determining Foxp3 expression, selection of Foxp3⁺ individual clones in the thymus is limited by a very small niche.

Steroid treatments in mice do not alter the number and function of regulatory T cells, but amplify cyclophosphamide-induced autoimmune disease

Corticosteroids are commonly used in the therapy of autoimmune disease (AID), although they are rarely, if ever, curative. This failure may result from their deleterious effects on regulatory T cells (Treg). In this work, we directly tested the effects of hydrocortisone (HC) administration on Treg number and function in established mouse models of multiple sclerosis and colitis. Treatment with pertussis toxin (Ptx) or Cyclophosphamide (Cyp), two compounds known to affect Treg function served as controls. We first show that contrarily to Ptx, HC administration to mice transgenic for a TCR specific to myelin basic protein induces a mild lymphopenia, without selective depletion of Treg, nor induction of experimental autoimmune encephalomyelitis (EAE). We next report that HC administration to normal mice has no effect on Treg suppressive function tested in vitro. Moreover, we document that Treg isolated from HC-treated animals maintain their capacity to prevent T cell-induced colitis. In contrast, the combined administration of HC and Cyp, as is frequently used in the therapy of severe AID, dramatically enhanced the deleterious effect of Cyp on Treg number and function. Our analysis indicates that while a short course of corticosteroids alone is not deleterious to immune regulation, combined therapies, notably with Cyp, should be avoided.

The T-cell receptor repertoire of regulatory T cells

The CD4(+) CD25(+) regulatory population of T cells (Treg cells), which expresses the forkhead family transcription factor (Foxp3), is the key component of the peripheral tolerance mechanism that protects us from a variety of autoimmune diseases. Experimental evidence shows that Treg cells recognize a wide range of antigenic specificities with increased reactivity to self antigens, although the affinity of these interactions remains to be further defined. The Treg repertoire is highly diverse with a distinct set of T-cell receptors (TCRs), and yet is overlapping to some extent with the repertoire of conventional T cells (Tconv cells). The majority of Treg cells are generated in the thymus. However, the role of the TCR specificity in directing thymic precursors to become Treg or Tconv cells remains unclear. On the one hand, the higher self reactivity of Treg cells and utilization of different TCRs in Treg and Tconv repertoires suggest that in TCR interactions an initial decision is made about the 'suitability' of a developing thymocyte to become a Treg cell. On the other hand, as Treg cells can recognize a wide range of foreign antigens, have a diverse TCR repertoire, and show some degree of overlap with Tconv cells, the signals through the TCR may be complementary to the TCR-independent process that generates precursors of Treg cells. In this review, we discuss how different features of the Treg repertoire influence our understanding of Treg specificities and the role of self reactivity in the generation of this population.

Critical evaluation of regulatory T cells in autoimmunity: are the most potent regulatory specificities being ignored?

The identification of CD4+ CD25+ Foxp3+ regulatory T (Treg) cells as natural regulators of immunity in the periphery and tissues has stimulated tremendous interest in developing therapeutic strategies for autoimmune diseases. In this review, the site of origin, antigen specificity, homing markers and cytokine profiles of Treg cells were evaluated in autoimmune colitis and type 1 diabetes, two examples in which Treg cells were effective as therapy. These studies were compared with studies of Treg cells in experimental autoimmune encephalomyelitis and multiple sclerosis, where successful therapy has not yet been achieved. Antigen-specific Treg cells appear to have more potent activity than polyclonal Treg cells and therefore hold more promise as therapeutic agents. However, Treg cells specific for the pathogenic T effector cells themselves have largely been overlooked and deserve consideration in future studies.

Control of experimental autoimmune encephalomyelitis by CD4+ suppressor T cells: peripheral versus in situ immunoregulation

The pathogenesis of experimental autoimmune encephalomyelitis (EAE) can be efficiently kept under control by specialized subsets of CD4+ T lymphocytes able to negatively regulate the function of T cells with encephalitogenic potential. A number of observations support a role for such suppressor T cells in controlling early phases of disease development at the level of peripheral lymphoid organs but there is also evidence suggesting immunoregulation within the central nervous system (CNS) microenvironment itself. This review evaluates the sites of regulation based on available data from distinct experimental models. We then discuss these aspects with reference to suppressor CD4+ T cells induced through the epicutaneous application of pure CNS antigens that confer long term protection against EAE. Finally, we give an overview of genes recently discovered to be important in regulation of the immune system that may also prove to be key players in the modulation of EAE and MS.

Modulation of microglial/macrophage activation by macrophage inhibitory factor (TKP) or tuftsin (TKPR) attenuates the disease course of experimental autoimmune encephalomyelitis

Background

Myelin Oligodendrocyte Glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE) is the most commonly used mouse model for multiple sclerosis (MS). During the of progression of EAE, microglia, the immunocompetent cells of the brain, become activated and accumulate around demyelinated lesions. Microglial activation is mediated by the extracellular protease tissue Plasminogen Activator (tPA), and mice lacking tPA display altered EAE progression. In this study, we have used pharmacological inhibitors and stimulators of microglial/macrophage activation to examine the temporal requirement for microglial activation in EAE progression and to determine whether such approaches might potentially be of therapeutic value.

Results

Intervention using the tripeptide macrophage/microglia inhibitory factor MIF (TKP) and the tetrapeptide macrophage/microglial stimulator tuftsin (TKPR) attenuated EAE symptoms and revealed that the timing of macrophage/microglial activation is critical for the clinical outcome of EAE. We show that the disease progression can potentially be manipulated favorably at early stages by altering the timing of microglial activation, which in turn alters the systemic immune response to favor upregulation of T helper cell 2 genes that promote recovery from EAE.

Conclusion

Preventative and therapeutic modulation of macrophage/microglial activity significantly alters the outcome of EAE at symptomatic stages. Specific molecular targets have been identified that represent potential avenues of exploration for the treatment and prevention of MS.

Pertussis Toxin Reduces the Number of Splenic Foxp3+Regulatory T Cells

Pertussis toxin (PTx) is a bacterial toxin used to enhance the severity of experimental autoimmune diseases such as experimental autoimmune encephalomyelitis. It is known to promote permeabilization of the blood-brain barrier, maturation of APC, activation of autoreactive lymphocytes and alteration of lymphocyte migration. In this study, we show that i.v. injection of PTx in mice induces a decrease in the number of splenic CD4(+)CD25(+) regulatory T cells (Treg cells). Furthermore, PTx not only induces a depletion of the dominant CD4(+)CD25(+)Foxp3(+) subpopulation of splenic Treg cells, but also reduces to a similar extent the CD4(+)CD25(-)Foxp3(+) subpopulation. On a per cell basis, the suppressive properties of the remaining Treg cells are not modified by PTx treatment. The reduction in splenic Treg cells is associated with preferential migration of these cells to the liver. Additionally, Treg cells exhibit a high sensitivity to PTx-mediated apoptosis in vitro. Finally, in vivo depletion of Treg cells by injection of an anti-CD25 Ab, and PTx treatment, present synergistic experimental autoimmune encephalomyelitis exacerbating effects. Therefore, we identify a new effect of PTx and provide an additional illustration of the influence of microbial components on the immune system affecting the balance between tolerance, inflammation and autoimmunity.

Innate and adaptive immunity and the pathophysiology of psoriasis

Psoriasis is considered to be a genetically programmed disease of dysregulated inflammation, which is driven and maintained by multiple components of the immune system. The pathologic collaboration between innate immunity (mediated by antigen-presenting cells and natural killer T lymphocytes) and acquired immunity (mediated by T lymphocytes) results in the production of cytokines, chemokines, and growth factors that contribute to the inflammatory infiltrate seen in psoriatic plaques. This overview of the pathophysiology of psoriasis describes these events, and recent developments that have contributed to our understanding of the role of immune function in psoriasis. These developments include the creation of useful animal models and identification of new receptors and lymphocyte subtypes that may participate in the development of this chronic disease.

Regulatory T cells

Immunologic self-tolerance is critically dependent on the induction but also on the downregulation of immune responses. Though ignored and neglected for many years, suppressor T cells, now renamed regulatory T cells (Tregs), play an important role in the negative regulation of immune responses. Several subsets of Tregs have been described. Naturally occurring CD4(+)CD25(+) Tregs are important in the prevention of autoimmune diseases. Type 1 Tregs, another subtype of Treg that is inducible, exert their suppressive activity primarily via the release of IL-10. Detailed knowledge about the phenotype and mode of action of these cells will significantly increase our understanding of the pathogenesis of autoimmune diseases and will also help to identify new therapeutic strategies.

Control of homeostatic proliferation by regulatory T cells

Homeostatic proliferation of T cells leads to the generation of effector/memory cells, which have the potential to cause harm to the host. The role of Tregs in the control of homeostatic proliferation is unclear. In this study we utilized mice that either harbor or lack Tregs as recipients of monoclonal or polyclonal T cells. We observed that while Tregs completely prevented cell division of T cells displaying low affinity for self ligands, they had a less marked, albeit significant, effect on cell cycle entry of T cells displaying higher affinity. The presence of Tregs resulted in a lower accumulation of T cells, enhanced apoptosis, and impaired differentiation to a cytokine-producing state. We conclude that Tregs play a major role in the control of homeostatic proliferation.

Do regulatory T cells play a role in the control of homeostatic proliferation?

The control of peripheral lymphocyte numbers is a fundamental aspect of the immune system. Regulatory T cells are involved in the suppression of autoimmune, antitumor, allergic, and other inflammatory responses, as well as in facilitating graft acceptance. In this paper, we discuss whether the control of homeostatic proliferation is another facet of the immune system that is controlled by regulatory T cells. A review of the published data connecting regulatory T cells with the control of homeostatic proliferation indicates that several key questions remain open. One of these relates to the stage at which regulatory T cells could play a role (i.e., T-cell proliferation vs. survival).

High incidence of spontaneous autoimmune thyroiditis in immunocompetent self-reactive human T cell receptor transgenic mice

Autoantigen-specific TCR transgenic mice allow us to assess the role of T cells in autoimmunity. We have recently generated humanized TAZ10 transgenic mice expressing the human TCR specific for the immunodominant epitope of thyroid peroxidase (TPO). We have shown that these transgenic mice do not undergo tolerance in vivo and that on Rag deficient background they are susceptible to spontaneous autoimmune thyroiditis. Here we show that, in contrast to other transgenic models of autoimmunity, almost all TCR(+)Rag1+ (T+R+) T cells are activated in vivo leading to the development of spontaneous autoimmune thyroiditis. In these mice, disease is also accompanied by a significant reduction of CD4+CD25hi regulatory T cells. These data indicate that the pathogenic activity of the self-reactive TCR can circumvent the regulatory function operated by the non-transgenic T cells that are normally present in T+R+ mice, leading to autoimmunity.

Immune regulatory mechanisms influence early pathology in spinal cord injury and in spontaneous autoimmune encephalomyelitis

Injuries to the central nervous system (CNS) trigger an inflammatory reaction with potentially devastating consequences. In this report we compared the characteristics of the inflammatory response on spinal cord injury (SCI) caused by a stab wound between the T7 and T9 vertebrae and spontaneous experimental autoimmune encephalomyelitis (EAE). SCI and EAE were compared in two types of myelin basic protein Ac1-11-specific T-cell receptor transgenic mice: T/R+ mice harbor regulatory T cells, and T/R- mice lack regulatory T cells. Our results show that 8 days after SCI, T/R- mice developed a strong T-cell infiltrate in the spinal cord, with remarkable down-modulation of CD4 expression that was accompanied by a local increase in Mac-3+ and F4/80+ reactivity and diffuse local and distal astrogliosis. In contrast, T/R+ mice exhibited a modest increase in CD4+ cells localized to the site of injury, without CD4 down-modulation; focal astrogliosis was restricted to the site of the lesion, although Mac-3+ and F4/80+ cells were also present. Similarly to T/R- mice that underwent SCI, T cells displaying down-modulated CD4 expression were found in the CNS of older T/R- mice afflicted by spontaneous EAE. Overall, our results suggest that common mechanisms regulate T-cell accumulation in CNS lesions of different causes, such as mechanic lesion or autoimmune-mediated damage.

Depletion of Regulatory T Cells in HIV Infection Is Associated with Immune Activation

Immune activation during chronic HIV infection is a strong clinical predictor of death and may mediate CD4(+) T cell depletion. Regulatory T cells (Tregs) are CD4(+)CD25(bright)CD62L(high) cells that actively down-regulate immune responses. We asked whether loss of Tregs during HIV infection mediates immune activation in a cross-sectional study of 81 HIV-positive Ugandan volunteers. We found that Treg number is strongly correlated with both CD4(+) and CD8(+) T cell activation. In multivariate modeling, this relationship between Treg depletion and CD4(+) T cell activation was stronger than any other clinical factor examined, including viral load and absolute CD4 count. Tregs appear to decline at different rates compared with other CD4(+) T cells, resulting in an increased regulator to helper ratio in many patients with advanced disease. We hypothesize that this skewing may contribute to T cell effector dysfunction. Our findings suggest Tregs are a major contributor to the immune activation observed during chronic HIV infection.

T cells from epicutaneously immunized mice are prone to T cell receptor revision

Epicutaneous immunization of T cell receptor (TCR) transgenic (Tg) mice whose CD4(+) T cells are specific for the Ac1-11 fragment of myelin basic protein (MBP) with Ac1-11 elicits T cells with dominant suppressor/regulatory activity that confers protection against Ac1-11-induced experimental autoimmune encephalomyelitis. We now report that such disease-resistant MBP TCR Tg mice also harbor a sizeable fraction of peripheral CD4(+) T cells lacking surface expression of the Tg TCR beta chain and expressing diverse, endogenously rearranged TCR beta chains. Ex vivo incubation at physiological temperature caused the loss of neo-beta-chain expression and reversion to the MBP alphabeta TCR(+) phenotype. The presence of recombination activating gene 1 and 2 proteins in CD4(+) T cells with revised TCRs was consistent with effective V(D)J recombination activity. The emergence of these cells did not depend on the thymic compartment. We conclude that in mice epicutaneously immunized with an autoantigen, peripheral specific T cells are susceptible to multiple mechanisms of tolerance.

CD25- T cells generate CD25+Foxp3+ regulatory T cells by peripheral expansion

Naturally occurring CD4(+) regulatory T cells are generally identified through their expression of CD25. However, in several experimental systems considerable T(reg) activity has been observed in the CD4(+)CD25(-) fraction. Upon adoptive transfer, the expression of CD25 in donor-derived cells is not stable, with CD4(+)CD25(+) cells appearing in CD4(+)CD25(-) T cell-injected animals and vice versa. We show in this study that CD25(+) cells arising from donor CD25(-) cells upon homeostatic proliferation in recipient mice express markers of freshly isolated T(reg) cells, display an anergic state, and suppress the proliferation of other cells in vitro. The maintenance of CD25 expression by CD4(+)CD25(+) cells depends on IL-2 secreted by cotransferred CD4(+)CD25(-) or by Ag-stimulated T cells in peripheral lymphoid organs.

Regulatory CD4+ T cells and the control of autoimmune disease

The immune system is a delicately balanced network of interacting cells. In recent years, the concept of immune regulation/suppression has been firmly established, and both natural and induced regulatory cells play vital roles in protection from autoimmune disease. Recent work has revealed the diverse nature of regulatory CD4+ T (Treg) cells and the molecules involved in their function. Innate and adaptive responses to infection are able to override the suppressive properties of such regulatory cells, whereas several reports point to deficiencies in regulatory cell function in autoimmune disease. Protocols have been developed that allow the expansion of Treg cells in vitro and their antigen-specific induction in vivo. A full understanding of Treg differentiation and function will facilitate the development of improved strategies for prevention and treatment of autoimmune diseases.

Regulatory potential and control of Foxp3 expression in newborn CD4+ T cells

Thymectomy at day 3 after birth leads to autoimmune disease in some genetic backgrounds. Disease is thought to be caused by the lack/paucity of regulatory T cells. We show that 3-day-old mice already contain a significant compartment of Foxp3-expressing CD25(+)CD4(+) splenocytes. Whereas, in adult spleen, the subsets of regulatory T cells (CD25(+) and/or CD103(+)) express high amounts of Foxp3 mRNA, in 3-day-old mice, both thymic and splenic CD25(+)CD4(+) T cell subsets express lower amounts of Foxp3 mRNA, and CD103(+) cells are barely detected. In adult day 3-thymectomized mice, the CD25(+)CD4(+) T cell subset is overrepresented (most of the cells being CD103(+)) and expresses high amounts of Foxp3 mRNA, independent of the development of autoimmune gastritis. These cells control inflammatory bowel disease and the homeostatic expansion of lymphocytes. This study demonstrates that the peripheral immune system of newborn mice is endowed of a remarkable regulatory potential, which develops considerably in the absence of thymic supply.

CD4 T-cell memory

CD4 T-cell memory is in some ways more enigmatic than CD8 T-cell memory. This is mostly due to the fact that CD4 T cells tend to expand far less in response to antigenic stimuli, thereby thwarting attempts at their detection during the course of an immune response. Nevertheless, there is a wide range of experimental models that have provided information regarding the survival and maintenance of CD4 memory cells, their functional capacities, their differentiation states and program of development following activation. The emerging picture is one of great versatility and functional heterogeneity as befits their central position within the immune system.

In vivo instruction of suppressor commitment in naive T cells

The induction of antigen-specific tolerance in the mature immune system of the intact organism has met with limited success. Therefore, nonspecific immunosuppression has been the treatment of choice to prevent unwanted immunity. Here, it is shown that prolonged subcutaneous infusion of low doses of peptide by means of osmotic pumps transforms mature T cells into CD4⁺25⁺ suppressor cells that can persist for long periods of time in the absence of antigen and confer specific immunologic tolerance upon challenge with antigen. The described procedure resembles approaches of tolerance induction used decades ago, induces tolerance in the absence of immunity, and holds the promise to become an effective means of inducing antigen-specific tolerance prospectively, whereas its power to suppress already ongoing immune responses remains to be determined.

The balance of deletion and regulation in allograft tolerance

Although the precise mechanisms involved in the establishment and maintenance of peripheral allograft tolerance are still not fully understood, it is now clear that acquisition of transplantation tolerance is an active, highly regulated, multistep process. According to the pool size model of allograft tolerance, the allograft outcome, rejection, or tolerance, often depends on the balance between cytopathic and regulatory T cells (Tregs). Although both deletion and regulation play important roles in allograft tolerance, our recent studies showed that the quantitative details for each mechanism differ from model to model. Therefore, we hypothesize that there is a delicate balance between deletion and regulation in allograft tolerance. In a model of allograft tolerance in which the deletional mechanism plays a dominant role, e.g. tolerance produced via creation of mixed chimeras, the regulatory mechanism, albeit sometimes present, is far less important. Whilst in a model in which the regulation mechanism plays a critical role, e.g. donor-specific transfusion plus MR1-induced allograft tolerance, a deletional mechanism lowers the threshold for effective Treg action.

Expression of dual TCR on DO11.10 T cells allows for ovalbumin-induced oral tolerance to prevent T cell-mediated colitis directed against unrelated enteric bacterial antigens

The triggering Ag for inflammatory bowel disease and animal models of colitis is not known, but may include gut flora. Feeding OVA to DO11.10 mice with OVA-specific transgenic (Tg) TCR generates Ag-specific immunoregulatory CD4(+) T cells (Treg) cells. We examined the ability of oral Ag-induced Treg cells to suppress T cell-mediated colitis in mice. SCID-bg mice given DO11.10 CD4(+)CD45RB(high) T cells developed colitis, and cotransferring DO11.10 CD45RB(low)CD4(+) T cells prevented CD4(+)CD45RB(high) T cell-induced colitis in the absence of OVA. The induction and prevention of disease by DO11.10 CD4(+) T cell subsets were associated with an increase in endogenous TCRalpha chain expression on Tg T cells. Feeding OVA to SCID-bg mice reconstituted with DO11.10 CD4(+)CD45RB(high) attenuated the colitis in association with increased TGF-beta and IL-10 secretion, and decreased proliferative responses to both OVA and cecal bacteria Ag. OVA feeding also attenuated colitis in SCID-bg mice reconstituted with a mix of BALB/c and DO11.10 CD45RB(high) T cells, suggesting that OVA-induced Treg cells suppressed BALB/c effector cells. The expression of endogenous non-Tg TCR allowed for DO11.10-derived T cells to respond to enteric flora Ag. Furthermore, feeding OVA-induced Treg cells prevented colitis by inducing tolerance in both OVA-reactive and non-OVA-reactive T cells and by inducing Ag-nonspecific Treg cells. Such a mechanism might allow for Ag-nonspecific modulation of intestinal inflammation in inflammatory bowel disease.

CD4+ CD25+ Treg: divide and rule?

Research on CD4+ CD25+ regulatory T cells (Treg) has gathered momentum over the last five years but many aspects of their fundamental biology remain elusive. Treg have been considered to be 'naturally anergic' based on their failure to proliferate in response to T-cell receptor ligation in vitro. Several recent studies challenge this view and demonstrate a robust proliferative capacity for CD25+ cells. The significance of this finding for Treg homeostasis and function is considered below.

T-cell receptor transgenic mice in the study of autoimmune diseases

T cell receptor transgenic mice have been a valuable tool in the study of the immune system, from development to selection to tolerance or pathogenesis. In this manuscript we review the T cell receptor transgenic mouse lines with specificity for self antigens that have been reported before August 2003. Many such lines have been generated, which have been instrumental in our understanding of, among other aspects, the role regulatory T cells in preventing autoimmunity, the role of microbes in modifying its outcome, the influence of the genetic background, the importance of regional differences in self-antigen concentration, and the importance of differences in antigen deposition between different tissues.

Multiple sclerosis: etiological mechanisms and future directions

Multiple sclerosis (MS) is a complex human autoimmune-type disease with a predominantly unknown etiology. Immunologic destruction of myelin basic protein (MBP) throughout the nervous system is the major pathology of multiple sclerosis. This review will attempt to update new information about basic mechanisms and therapeutic management of the disease. The significance of the structure of MBP is discussed with respect to the contribution of such structures to the disease process. A number of MBP peptides that serve as the immunodominant antigens in MS patients have been identified. These peptides have been studied in animal models for their antigenic characteristics and ability to induce disease. Evidence for genetic contributions is reviewed with multigenerational twin studies providing the best evidence for susceptible haplotypes. The role of microorganisms/viruses and environmental agents are discussed as potential etiological factors but are now thought to be of minor importance to the primary causal development of the disease. Of major consideration are immunological mechanisms that contribute to the development of autoimmunity. In particular, antigen expression, cytokine and leukocyte interactions, and regulatory T-cells are discussed. Particular attention is given to regulatory T-cells (Treg), which help balance/modulate other T-cells such as Th1 and Th2 cells, and how such Treg regulate autoimmunity is addressed. The importance of the role of Tregs is exemplified by the demonstration that administration of oral antigens can induce specific Tregs that counteract experimental autoimmune encephalomyelitis in animal models. The significance of animal studies to human multiple sclerosis is discussed. A potential role for natural antibodies and innate immune mechanisms to help provide resistance to disease development is also reviewed. Finally, a variety of therapeutic agents that have been and continue to be utilized for multiple sclerosis is reviewed. Trials with oral antigens, such as glatirmer acetate (copolymer 1) especially in combination with interferon-beta, have shown promise. Antibody therapy and bone marrow transplantation are also briefly discussed.

Immune tolerance to myelin proteins

Multiple sclerosis (MS) is a demyelinating disorder of the central nervous system. It is believed to be an autoimmune disease arising from a breakdown of immune tolerance in T cells specific for myelin antigens. The heterogeneity in clinical signs and pathology observed in MS patients suggests a complex pathogenesis in which the specificity of the pathogenic T cells and the tolerance mechanisms that are compromised vary among individual patients. In this review, we summarize some of the features of the diverse immune pathology observed in MS and the animal models used to study this disease. We then describe the current state of knowledge regarding the expression of the major myelin protein antigens believed to be targeted in MS and the mechanisms of immune tolerance that operate on T cells that recognize these antigens.

Control of autoimmunity by naturally arising regulatory CD4+ T cells

Naturally acquired immunological self-tolerance is not entirely accounted for by clonal deletion, anergy, and ignorance. It is now well established that the T cell-repertoire of healthy individuals harbors self-reactive lymphocytes with a potential to cause autoimmune disease and these lymphocytes are under dominant control by a unique subpopulation of CD4+ T cells now called regulatory T cells. Efforts to delineate these Treg cells naturally present in normal individuals have revealed that they are enriched in the CD25+ CD4+ population. The identification of the CD25 molecule as a useful marker for naturally arising CD4+ regulatory T cells has made it possible to investigate many key aspects of their immunobiology, including their antigen specificities and the cellular/molecular pathways involved in their development and their mechanisms of action. Furthermore, reduction or dysfunction of the CD25+ CD4+ regulatory T cell population can be responsible for certain autoimmune diseases in humans.

Epicutaneous Immunization with Autoantigenic Peptides Induces T Suppressor Cells that Prevent Experimental Allergic Encephalomyelitis

Information on how suppressor/regulatory T cells can be generated directly in vivo and prevent autoimmunity remains fragmentary. We show here that epicutaneous immunization (ECi) with the immunodominant peptide of myelin basic protein (MBP), Ac1-11, protects mice that are transgenic for an Ac1-11-specific T cell receptor against both the induced and spontaneous forms of experimental allergic encephalomyelitis (EAE). This protection was antigen specific and antigen dose dependent, and was mediated by CD4(+)/CD25(-) T cells whose suppressive activity required cell-cell contact and could transfer protection to naive recipients. These ECi-induced suppressor T cells controlled naive MBP-specific CD4 T cells by inhibiting both their activation and their capacity to secrete IFN-gamma. There was no CD4 T cell infiltration in the brain of protected mice. Finally, ECi with autoantigenic peptides protected two nontransgenic models from relapsing-remitting EAE in an antigen-specific and antigen dose-dependent manner.

In vivo dynamics of antigen-specific regulatory T cells not predicted from behavior in vitro

Adoptive transfer of antigen-specific CD25+CD4+ regulatory T cells was used to analyze the stability of their phenotype, their behavior after immunization, and their mode of suppressing cotransferred naive T cells in vivo. We found that regulatory T cells maintained their phenotype in the absence of antigen, were not anergic in vivo, and proliferated as extensively as naive CD4+ T cells after immunization without losing their suppressive function in vivo and in vitro. In vivo, the expansion of cotransferred naive T cells was suppressed relatively late in the response such that regulatory T cells expressing mostly IL-10 but not IL-2 or IFN-gamma represented the dominant subset of cells. Our results reveal properties of regulatory T cells that were not predicted from in vitro studies.

Antigen-induced regulatory T cells in autoimmunity

The ultimate goal of any treatment for autoimmune diseases is antigen- and/or site-specific suppression of pathology. Autoaggressive lymphocytes need to be eliminated or controlled to prevent tissue damage and halt the progression of clinical disease. Strong evidence is emerging that the induction of regulatory T (T(Reg)) cells by autoantigens can suppress disease, even if the primary, initiating autoantigens are unknown and if inflammation is progressive. An advantage of these autoreactive T(Reg) cells is their ability to act as bystander suppressors and dampen inflammation in a site-specific manner in response to cognate antigen expressed locally by affected tissues. In this review, we consider the nature and function of such antigen-specific T(Reg) cells, and strategies for their therapeutic induction are discussed.

T cell regulation as a side effect of homeostasis and competition

We have previously hypothesized that maintaining a balanced peripheral immune system may not be the sole responsibility of a specialized subset of T cells dedicated to immune regulation, but also a side effect of normal competition for shared resources within an intact immune system. Here we show that regulatory activity is correlated with high homeostatic expansion potential, reflecting the avidity for self-peptide:MHC complexes. Monoclonal transgenic T cells with high homeostatic expansion potential and lacking characteristics previously associated with regulatory function were able to regulate wasting disease induced by transfer of a small number of naive CD45RB(hi) CD4 T cells into lymphopenic hosts. Self-regulatory function is also found in the naive polyclonal T cell repertoire depleted of CD25(+) T cells. T cells capable of preventing immune pathology, like the transgenic T cells, express higher than average levels of CD5, an indicator of avidity for self:MHC peptide complexes. We therefore propose that dysregulated expansion of potentially pathogenic T cells in a lymphopenic environment can be prevented by members of the naive T cell repertoire, irrespective of their specificity, as a side effect of their response to homeostatic and antigenic stimulation.

CD4+CD25- T cells in aged mice are hyporesponsive and exhibit suppressive activity

Studies on humans and rodents have established that functional deterioration of CD4 T cells occurs with aging. We report in this study that approximately 70% of CD4(+)CD25(-) T cell preparations from individual 24-mo-old mice are hyporesponsive to in vitro stimulation with anti-CD3 Ab. The remaining 30% of CD4(+)CD25(-) T cell preparations showing the intermediate or normal responsiveness in the primary stimulation also exhibit the hyporesponsive properties after primary stimulation. Both of these hyporesponsive aged CD4(+)CD25(-) T cells could inhibit the proliferation of cocultured CD4(+)CD25(-) T cells from young mice, like CD4(+)CD25(+) T cells, which have recently been demonstrated as an immune regulator in young mice. Another experiment revealed that hyporesponsive aged CD4(+)CD25(-) T cells arrest the cell division of cocultured young CD4(+)CD25(-) T cells. The suppressive activity observed in aged CD4(+)CD25(-) T cells is aging-dependent, not mediated by humoral factors, cell-contact dependent, and broken by the addition of IL-2 or anti-GITR Ab, but not by anti-CTLA-4 Ab. These studies show that aging not only leads to a decline in the ability to mount CD4(+)CD25(-) T cell responses, but at the same time, also renders these aged CD4(+)CD25(-) T cells suppressive.

CD4+CD25+ T(R) cells suppress innate immune pathology through cytokine-dependent mechanisms

CD4(+)CD25(+) regulatory T (T(R)) cells can inhibit a variety of autoimmune and inflammatory diseases, but the precise mechanisms by which they suppress immune responses in vivo remain unresolved. Here, we have used Helicobacter hepaticus infection of T cell-reconstituted recombination-activating gene (RAG)(-/-) mice as a model to study the ability of CD4(+)CD25(+) T(R) cells to inhibit bacterially triggered intestinal inflammation. H. hepaticus infection elicited both T cell-mediated and T cell-independent intestinal inflammation, both of which were inhibited by adoptively transferred CD4(+)CD25(+) T(R) cells. T cell-independent pathology was accompanied by activation of the innate immune system that was also inhibited by CD4(+)CD25(+) T(R) cells. Suppression of innate immune pathology was dependent on T cell-derived interleukin 10 and also on the production of transforming growth factor beta. Thus, CD4(+)CD25(+) T(R) cells do not only suppress adaptive T cell responses, but are also able to control pathology mediated by innate immune mechanisms.

Development of CD4+ T cell lines that suppress an antigen-specific immune response in vivo

It has been suggested for many years that the regulation of the immune system for the maintenance of peripheral tolerance may involve regulatory/suppressor T cells. In the past few years, several investigators have demonstrated that these cells can be generated in vitro. It has also been shown that they can inhibit the progression of various autoimmune disease models when infused into susceptible mice. We have generated two murine T cell lines in the presence of KLH-specific T cell clones from BALB/c or DBA2 mice. The lines are characterized by a low proliferative response to mitogens, the capacity to secrete high amounts of IL-10 and TGF-beta, and small amounts of IFN-gamma. Interestingly, these cells are unable to produce IL-2, IL-4 or IL-5. The study of the surface phenotype of both lines revealed CD4+, CD25high, CD44low and CTLA-4- cells. When injected intravenously in (CBy.D2) F1 mice, these cells were able to inhibit 50-100% of the TNP-specific antibody production, when the hapten was coupled to KLH. In the present study we offer another evidence for the existence of regulatory T cells in the T lymphocyte repertoire, suggesting that they can also regulate immune responses to foreign antigens. Furthermore, we demonstrate an alternative pathway to generate these cells different from approaches used thus far.

Peripheral expansion of thymus-derived regulatory cells in anti-myelin basic protein T cell receptor transgenic mice

CD4+ regulatory T cells (Treg) play an indispensable role in tolerance to peripheral antigens, but the origin of the Treg pool in the adult remains unclear. Thus, while thymic commitment of Treg has been demonstrated, evidence also exists for the peripheral recruitment of naïve tissue-specific T cells into Treg functions. Anti-myelin basic protein TCR transgenic mice spontaneously develop autoimmune encephalomyelitis when "monoclonal", but are protected by adoptive transfer of CD4+ cells from wild-type donors. We have now used this transfer system to investigate whether previously infused Treg can recruit transgenic T cells to regulatory functions. The results show that transgenic T cells from protected animals did not transfer tolerance to secondary recipients, and that elimination of donor Treg in protected recipients resulted in rapid onset of disease. In addition, Treg-containing T cell susbsets were highly enriched for proliferating cells in vivo, which was also the case for CD4+CD25+ T cells in normal animals. These observations thus exclude peripheral differentiation of Treg in this particular system, and indicate that expansion of thymically committed cells ensures the maintenance of the peripheral Treg pool in the adult.

CD4(+) regulatory T cells in autoimmunity and allergy

Regulatory T cells (also referred to as suppressor T cells) are important components of the homeostasis of the immune system, as impaired regulatory T cell activity can cause autoimmune diseases and atopy. It is now clear that the phrase 'regulatory T cells' encompasses more than one cell type. For instance, CD4(+)CD25(+) regulatory T cells have received attention due to their immunosuppressive properties in vitro and in vivo, but in several instances it has been shown that CD4(+)CD25(-) T cell populations also contain potent regulatory activity. Recent progress in the field of regulatory T cells includes the discovery of the role of two tumor necrosis factor receptor (TNFR) family members (GITR and TRANCE-R/RANK) in Treg biology, the improved understanding of the role of co-stimulatory molecules and cytokines IL-10 and IL-2 in the induction and function of Tregs, and the generation of CD25(+) and CD25(-) regulatory T cells in vivo through high-avidity T cell receptor interactions.

Expression of the integrin alpha Ebeta 7 identifies unique subsets of CD25+ as well as CD25- regulatory T cells

Regulatory CD25(+)CD4(+) T cells are considered as important players in T cell homeostasis and self-tolerance. Here we report that the integrin alpha(E)beta(7), which recognizes epithelial cadherin, identifies the most potent subpopulation of regulatory CD25(+) T cells. Strikingly, CD25-negative alpha(E)+CD4(+) T cells displayed regulatory activity. Both alpha(E)+ subsets, CD25(+) and CD25(-), express CTLA-4, suppress T cell proliferation in vitro, and protect mice from colitis in the severe combined immunodeficient model (SCID) in vivo. Whereas alpha(E)+CD25(+) T cells produce almost no cytokines, alpha(E)+CD25(-) T cells represent a unique subset in which high IL-2, IFN-gamma and T helper 2-cytokine production is linked with suppressive function. Thus, the integrin alpha(E)beta(7) can be regarded as a novel marker for subsets of highly potent, functionally distinct regulatory T cells specialized for crosstalk with epithelial environments.

Interleukin 2 signaling is required for CD4(+) regulatory T cell function

Mice deficient in interleukin (IL)-2 production or the IL-2 receptor alpha or beta chains develop a lethal autoimmune syndrome. CD4(+) regulatory T cells have been shown to prevent autoimmune diseases, allograft rejection, and to down-regulate antibody responses against foreign antigens. To assess the role of IL-2 in the generation and function of regulatory T cells, we transferred CD4(+) T cells from mice genetically deficient in IL-2 or IL-2R(alpha) (CD25) expression. A small number of splenic or thymic CD4(+) T cells from IL-2 knockout mice can protect mice from spontaneous experimental autoimmune encephalomyelitis (EAE). In contrast, splenic or thymic CD4(+) T cells from CD25 knockout donor mice conferred little or no protection. We conclude that T cells with regulatory potential can develop, undergo thymic selection, and migrate to the peripheral lymphoid organs in the absence of IL-2, and do not protect from disease by means of IL-2 secretion. However, IL-2 signaling in regulatory T cells is essential for their protective function. Altogether, our results favor a model whereby IL-2 induces regulatory T cell activity.