Response to self antigen imprints regulatory memory in tissues

Delayed anti-CD3 therapy results in depletion of alloreactive T cells and the dominance of Foxp3+ CD4+ graft infiltrating cells

The engineered Fc-nonbinding (crystallizable fragment-nonbinding) CD3 antibody has lower mitogenicity and a precise therapeutic window for disease remission in patients with type 1 diabetes. Before anti-CD3 can be considered for use in transplantation, the most effective timing of treatment relative to transplantation needs to be elucidated. In this study anti-CD3F(ab')2 fragments or saline were administered intravenously for 5 consecutive days (early: d1-3 or delayed: d3-7) to mice transplanted with a cardiac allograft (H2(b)-to-H2(k); d0). Survival of allografts was prolonged in mice treated with the early protocol (MST = 48 days), but most were rejected by d100. In contrast, in mice treated with the delayed protocol allografts continued to survive long term. The delayed protocol significantly inhibited donor alloreactivity at d30 as compared to the early protocol. A marked increase in Foxp3(+) T cells (50.3 ± 1.6%) infiltrating the allografts in mice treated with the delayed protocol was observed (p < 0.0001 vs. early (24.9 ± 2.1%)) at d10; a finding that was maintained in the accepted cardiac allografts at d100. We conclude that the timing of treatment with anti-CD3 therapy is critical for inducing long-term graft survival. Delaying administration effectively inhibits the alloreactivity and promotes the dominance of intragraft Foxp3(+) T cells allowing long-term graft acceptance.

Fat-resident Tregs: an emerging guard protecting from obesity-associated metabolic disorders

Obesity, a chronic, low-grade inflammatory disease, is associated with alterations of multiple immune-cell components in the visceral adipose tissue (VAT), in which CD4(+) Foxp3(+) regulatory T cells (Tregs) have been suggested to be a critical regulator. This review focuses on the current understanding of VAT-resident Tregs (VAT Tregs) and their intriguing roles in modulating fat tissue inflammation and obesity-associated metabolic disorders. The potential mechanisms for the regulation of VAT Tregs in lean vs. obese individuals are also discussed. Accumulating evidence reveals that the modulation of VAT Tregs may offer a potential novel strategy for treating obesity-related metabolic disorders, such as insulin resistance and type 2 diabetes.

Diversification and senescence of Foxp3+ regulatory T cells during experimental autoimmune encephalomyelitis

The fate of Foxp3(+) regulatory T (Treg) cells responding during autoimmunity is not well defined. We observed a marked elevation in KLRG1(+) (where KLRG1 stands for killer cell lectin-like receptor G1) CNS-infiltrating Treg cells in experimental autoimmune encephalomyelitis (EAE), and assessed their origin and properties. KLRG1(+) Treg cells showed increased activation marker expression, Foxp3 and CD25 levels, and more rapid cell cycling than KLRG1(-) cells. KLRG1(-) Treg cells converted into KLRG1(+) cells and this was increased in autoimmune inflammation. Conversion was unidirectional; KLRG1(+) Treg cells did not revert to a KLRG1(-) state. KLRG1(+) but notKLRG1(-) Treg cells survived poorly, indicative of terminal differentiation. This was associated with diminished BCL2 and increased apoptosis of isolated cells. KLRG1 was also upregulated on iTreg cells after transfer and EAE induction or on iTreg cells developing spontaneously during EAE. KLRG1(+) Treg cells produced more IL-10 and had altered effector cytokine production compared with their KLRG1(-) counterparts. Despite their differences, KLRG1(+) and KLRG1(-) Treg cells proved similarly potent in suppressing EAE. KLRG1(+) and KLRG1(-) populations were phenotypically heterogeneous, with the extent and pattern of activation marker expression dependent both on cellular location and inflammation. Our results support an extensive diversification of Treg cells during EAE, and associate KLRG1 with altered Treg-cell function and senescence.

Cutting Edge: memory regulatory t cells require IL-7 and not IL-2 for their maintenance in peripheral tissues

Thymic Foxp3-expressing regulatory T cells are activated by peripheral self-antigen to increase their suppressive function, and a fraction of these cells survive as memory regulatory T cells (mTregs). mTregs persist in nonlymphoid tissue after cessation of Ag expression and have enhanced capacity to suppress tissue-specific autoimmunity. In this study, we show that murine mTregs express specific effector memory T cell markers and localize preferentially to hair follicles in skin. Memory Tregs express high levels of both IL-2Rα and IL-7Rα. Using a genetic-deletion approach, we show that IL-2 is required to generate mTregs from naive CD4(+) T cell precursors in vivo. However, IL-2 is not required to maintain these cells in the skin and skin-draining lymph nodes. Conversely, IL-7 is essential for maintaining mTregs in skin in the steady state. These results elucidate the fundamental biology of mTregs and show that IL-7 plays an important role in their survival in skin.

Pre-existing central nervous system lesions negate cytokine requirements for regional experimental autoimmune encephalomyelitis development

In region-specific forms of experimental autoimmune encephalomyelitis (EAE), lesion initiation is regulated by T-cell-produced interferon-γ (IFN-γ) resulting in spinal cord disease in the presence of IFN-γ and cerebellar disease in the absence of IFN-γ. Although this role for IFN-γ in regional disease initiation is well defined, little is known about the consequences of previous tissue inflammation on subsequent regional disease, information vital to the development of therapeutics in established disease states. This study addressed the hypothesis that previous establishment of regional EAE would determine subsequent tissue localization of new T-cell invasion and associated symptoms regardless of the presence or absence of IFN-γ production. Serial transfer of optimal or suboptimal doses of encephalitogenic IFN-γ-sufficient or -deficient T-cell lines was used to examine the development of new clinical responses associated with the spinal cord and cerebellum at various times after EAE initiation. Previous inflammation within either cerebellum or spinal cord allowed subsequent T-cell driven inflammation within that tissue regardless of IFN-γ presence. Further, T-cell IFN-γ production after initial lesion formation exacerbated disease within the cerebellum, suggesting that IFN-γ plays different roles at different stages of cerebellar disease. For the spinal cord, IFN-γ-deficient cells (that are ordinarily cerebellum disease initiators) were capable of driving new spinal-cord-associated clinical symptoms more than 60 days after the initial acute EAE resolution. These data suggest that previous inflammation modulates the molecular requirements for new neuroinflammation development.

The receptor PD-1 controls follicular regulatory T cells in the lymph nodes and blood

CD4(+)CXCR5(+)Foxp3(+) follicular regulatory T cells (T(FR) cells) inhibit humoral immunity mediated by CD4(+)CXCR5(+)Foxp3(-) follicular helper T cells (T(FH) cells). Although the inhibitory receptor PD-1 is expressed by both cell types, its role in the differentiation of T(FR) cells is unknown. Here we found that mice deficient in PD-1 and its ligand PD-L1 had a greater abundance of T(FR) cells in the lymph nodes and that those T(FR) cells had enhanced suppressive ability. We also found substantial populations of T(FR) cells in mouse blood and demonstrated that T(FR) cells in the blood homed to lymph nodes and potently inhibited T(FH) cells in vivo. T(FR) cells in the blood required signaling via the costimulatory receptors CD28 and ICOS but were inhibited by PD-1 and PD-L1. Our findings demonstrate mechanisms by which the PD-1 pathway regulates antibody production and help reconcile inconsistencies surrounding the role of this pathway in humoral immunity.

Regulatory T cells in HIV infection: can immunotherapy regulate the regulator?

Regulatory T cells (Tregs) have a dominant role in self-tolerance and control of autoimmune diseases. These cells also play a pivotal role in chronic viral infections and cancer by limiting immune activation and specific immune response. The role of Tregs in HIV pathogenesis remains poorly understood as their function, changes according to the phases of infection. Tregs can suppress anti-HIV specific responses and conversely can have a beneficial role by reducing the deleterious impact of immune activation. We review the frequency, function and homing potential of Tregs in the blood and lymphoid tissues as well as their interaction with dendritic cells in the context of HIV infection. We also examine the new insights generated by recombinant IL-2 and IL-7 clinical trials in HIV-infected adults, including the immunomodulatory effects of Tregs. Based on their detrimental role in limiting anti-HIV responses, we propose Tregs as potential targets for immunotherapeutic strategies aimed at decreasing Tregs frequency and/or immunosuppressive function. However, such approaches require a better understanding of the time upon infection when interfering with Treg function may not cause a deleterious state of hyperimmune activation.

Pregnancy imprints regulatory memory that sustains anergy to fetal antigen

Pregnancy is an intricately orchestrated process where immune effector cells with fetal specificity are selectively silenced. This requires the sustained expansion of immune-suppressive maternal FOXP3(+) regulatory T cells (T(reg) cells), because even transient partial ablation triggers fetal-specific effector T-cell activation and pregnancy loss. In turn, many idiopathic pregnancy complications proposed to originate from disrupted fetal tolerance are associated with blunted maternal T(reg) expansion. Importantly, however, the antigen specificity and cellular origin of maternal T(reg) cells that accumulate during gestation remain incompletely defined. Here we show that pregnancy selectively stimulates the accumulation of maternal FOXP3(+) CD4 cells with fetal specificity using tetramer-based enrichment that allows the identification of rare endogenous T cells. Interestingly, after delivery, fetal-specific T(reg) cells persist at elevated levels, maintain tolerance to pre-existing fetal antigen, and rapidly re-accumulate during subsequent pregnancy. The accelerated expansion of T(reg) cells during secondary pregnancy was driven almost exclusively by proliferation of fetal-specific FOXP3(+) cells retained from prior pregnancy, whereas induced FOXP3 expression and proliferation of pre-existing FOXP3(+) cells each contribute to T(reg) expansion during primary pregnancy. Furthermore, fetal resorption in secondary compared with primary pregnancy becomes more resilient to partial maternal FOXP3(+) cell ablation. Thus, pregnancy imprints FOXP3(+) CD4 cells that sustain protective regulatory memory to fetal antigen. We anticipate that these findings will spark further investigation on maternal regulatory T-cell specificity that unlocks new strategies for improving pregnancy outcomes and novel approaches for therapeutically exploiting T(reg) cell memory.

T lymphocytes and normal tissue responses to radiation

There is compelling evidence that lymphocytes are a recurring feature in radiation damaged normal tissues, but assessing their functional significance has proven difficult. Contradictory roles have been postulated in both tissue pathogenesis and protection, although these are not necessarily mutually exclusive as the immune system can display what may seem to be opposing faces at any one time. While the exact role of T lymphocytes in irradiated normal tissue responses may still be obscure, their accumulation after tissue damage suggests they may be critical targets for radiotherapeutic intervention and worthy of further study. This is accentuated by recent findings that pathologically damaged “self,” such as occurs after exposure to ionizing radiation, can generate danger signals with the ability to activate pathways similar to those that activate adoptive immunity to pathogens. In addition, the demonstration of T cell subsets with their recognition radars tuned to “self” moieties has revolutionized our ideas on how all immune responses are controlled and regulated. New concepts of autoimmunity have resulted based on the dissociation of immune functions between different subsets of immune cells. It is becoming axiomatic that the immune system has the power to regulate radiation-induced tissue damage, from failure of regeneration to fibrosis, to acute and chronic late effects, and even to carcinogenesis. Our understanding of the interplay between T lymphocytes and radiation-damaged tissue may still be rudimentary but this is a good time to re-examine their potential roles, their radiobiological and microenvironmental influences, and the possibilities for therapeutic manipulation. This review will discuss the yin and yang of T cell responses within the context of radiation exposures, how they might drive or protect against normal tissue side effects and what we may be able do about it.

Regulatory T cells in radiotherapeutic responses

Radiation therapy (RT) can extend its influence in cancer therapy beyond what can be attributed to in-field cytotoxicity by modulating the immune system. While complex, these systemic effects can help tip the therapeutic balance in favor of treatment success or failure. Engagement of the immune system is generally through recognition of damage-associated molecules expressed or released as a result of tumor and normal tissue radiation damage. This system has evolved to discriminate pathological from physiological forms of cell death by signaling "danger." The multiple mechanisms that can be evoked include a shift toward a pro-inflammatory, pro-oxidant microenvironment that can promote maturation of dendritic cells and, in cancer treatment, the development of effector T cell responses to tumor-associated antigens. Control over these processes is exerted by regulatory T cells (Tregs), suppressor macrophages, and immunosuppressive cytokines that act in consort to maintain tolerance to self, limit tissue damage, and re-establish tissue homeostasis. Unfortunately, by the time RT for cancer is initiated the tumor-host relationship has already been sculpted in favor of tumor growth and against immune-mediated mechanisms for tumor regression. Reversing this situation is a major challenge. However, recent data show that removal of Tregs can tip the balance in favor of the generation of radiation-induced anti-tumor immunity. The clinical challenge is to do so without excessive depletion that might precipitate serious autoimmune reactions and increase the likelihood of normal tissue complications. The selective modulation of Treg biology to maintain immune tolerance and control of normal tissue damage, while releasing the "brakes" on anti-tumor immune responses, is a worthy aim with promise for enhancing the therapeutic benefit of RT for cancer.

Regulatory T cells that recognize a ubiquitous stress-inducible self-antigen are long-lived suppressors of autoimmune arthritis

Reestablishing self-tolerance in autoimmunity is thought to depend on self-reactive regulatory T cells (Tregs). Exploiting these antigen-specific regulators is hampered by the obscure nature of disease-relevant autoantigens. We have uncovered potent disease-suppressive Tregs recognizing Heat Shock Protein (Hsp) 70 self-antigens, enabling selective activity in inflamed tissues. Hsp70 is a major contributor to the MHC class II ligandome. Here we show that a conserved Hsp70 epitope (B29) is present in murine MHC class II and that upon transfer, B29-induced CD4(+)CD25(+)Foxp3(+) T cells suppress established proteoglycan-induced arthritis in mice. These self-antigen-specific Tregs were activated in vivo, and when using Lymphocyte Activation Gene-3 as a selection marker, as few as 4,000 cells sufficed. Furthermore, depletion of transferred Tregs abrogated disease suppression. Transferred cells exhibited a stable phenotype and were found in joints and draining lymph nodes up to 2 mo after transfer. Given that (i) B29 administration by itself suppressed disease, (ii) our findings were made with wild-type (T-cell receptor nontransgenic) Tregs, and (iii) the B29 human homolog is presented by HLA class II, we are nearing translation of antigen-specific Treg activation as a promising intervention for chronic inflammatory diseases.

Immunology of hepatitis C virus infections

The immune response in patients chronically infected with HCV plays a unique role during the infection because of its potential to contribute not only to viral clearance and, in some cases, protective immunity, but also to liver injury. A detailed understanding of the immunological mechanisms involved in persistence to HCV is essential to fully appreciate the complexity of the disease. In recent years, enormous progress has been made to characterize the dysfunctional natural killer cells and T cells during the chronic phase of infection. This information is important to further optimize treatment strategies based on the strengthening antiviral and immunomodulatory activities in patients chronically infected with HCV.

IL-2 receptor signaling is essential for the development of Klrg1+ terminally differentiated T regulatory cells

Thymic-derived natural T regulatory cells (Tregs) are characterized by functional and phenotypic heterogeneity. Recently, a small fraction of peripheral Tregs has been shown to express Klrg1, but it remains unclear as to what extent Klrg1 defines a unique Treg subset. In this study, we show that Klrg1(+) Tregs represent a terminally differentiated Treg subset derived from Klrg1(-) Tregs. This subset is a recent Ag-responsive and highly activated short-lived Treg population that expresses enhanced levels of Treg suppressive molecules and that preferentially resides within mucosal tissues. The development of Klrg1(+) Tregs also requires extensive IL-2R signaling. This activity represents a distinct function for IL-2, independent from its contribution to Treg homeostasis and competitive fitness. These and other properties are analogous to terminally differentiated short-lived CD8(+) T effector cells. Our findings suggest that an important pathway driving Ag-activated conventional T lymphocytes also operates for Tregs.

Treating human autoimmunity: current practice and future prospects

Autoimmune diseases are caused by immune cells attacking the host tissues they are supposed to protect. Recent advances suggest that maintaining a balance of effector and regulatory immune function is critical for avoiding autoimmunity. New therapies, including costimulation blockade, regulatory T cell therapy, antigen-specific immunotherapy, and manipulating the interleukin-2 pathway, attempt to restore this balance. This review discusses these advances as well as the challenges that must be overcome to target these therapies to patients suffering from autoimmune disease while avoiding the pitfalls of general immunosuppression.

Layers of dendritic cell-mediated T cell tolerance, their regulation and the prevention of autoimmunity

The last decades of Nobel prize-honored research have unequivocally proven a key role of dendritic cells (DCs) at controlling both T cell immunity and tolerance. A tight balance between these opposing DC functions ensures immune homeostasis and host integrity. Its perturbation could explain pathological conditions such as the attack of self tissues, chronic infections, and tumor immune evasion. While recent insights into the complex DC network help to understand the contribution of individual DC subsets to immunity, the tolerogenic functions of DCs only begin to emerge. As these consist of many different layers, the definition of a “tolerogenic DC” is subjected to variation. Moreover, the implication of DCs and DC subsets in the suppression of autoimmunity are incompletely resolved. In this review, we point out conceptual controversies and dissect the various layers of DC-mediated T cell tolerance. These layers include central tolerance, Foxp3⁺ regulatory T cells (Tregs), anergy/deletion and negative feedback regulation. The mode and kinetics of antigen presentation is highlighted as an additional factor shaping tolerance. Special emphasis is given to the interaction between layers of tolerance as well as their differential regulation during inflammation. Furthermore, potential technical caveats of DC depletion models are considered. Finally, we summarize our current understanding of DC-mediated tolerance and its role for the suppression of autoimmunity. Understanding the mechanisms of DC-mediated tolerance and their complex interplay is fundamental for the development of selective therapeutic strategies, e.g., for the modulation of autoimmune responses or for the immunotherapy of cancer.

Migratory, and not lymphoid-resident, dendritic cells maintain peripheral self-tolerance and prevent autoimmunity via induction of iTreg cells

There is evidence that dendritic cells (DCs) induce peripheral tolerance. Nevertheless, it is not known whether immature DCs in general are able to tolerize CD4(+) T cells or if this is a prerogative of specialized subtypes. Here we show that, when autoantigen presentation is extended to all conventional mouse DCs, immature lymphoid tissue resident DCs are unable to induce autoantigen-specific regulatory T (iTreg) cell conversion. In contrast, this is an exclusive prerogative of steady-state migratory DCs. Because only lymph nodes host migratory DCs, iTreg cells develop and are retained solely in lymph nodes, and not in the spleen. Mechanistically, in cutaneous lymph nodes, DC-derived CCL22 contributes to the retention of iTreg cells. The importance of the local generation of iTreg cells is emphasized by their essential role in preventing autoimmunity.

Transcutaneous gene gun delivery of hNC16A Induces BPAG2-specific tolerance

Immune recognition and rejection of tissues expressing transfected genes is a major complication of gene replacement therapy for inherited genetic disorders. Owing to the high immunogenicity of human bullous pemphigoid antigen 2 (hBPAG2), the induction and maintenance of tolerance to this neo-antigen is essential to deliver the gene product to patients with epidermolysis bullosa junctionalis. In a skin grafting mouse model, we used gene gun transfection with a construct encoding hNC16A, the immunodominant domain of hBPAG2, to induce antigen-specific immune tolerance. Eighty percent of wild-type mice transfected with hNC16A showed long-term survival of skin grafts expressing hBPAG2. Tolerance was stable and transferable by T cells but not by B cells of tolerant mice to naive hosts. A dense Foxp3(+) regulatory T-cell (T(reg)) infiltrate was noticed in grafts of tolerant mice and depletion of these cells resulted in a loss of tolerance. Taken together, we show that long-lasting hBPAG2-specific tolerance was induced with gene gun delivery of hNC16A through a T(reg)-dependent mechanism. This is of relevance to patients undergoing gene therapy and has broader implications for the treatment of antigen-specific autoimmune diseases.

Human epidermal Langerhans cells maintain immune homeostasis in skin by activating skin resident regulatory T cells

Recent studies have demonstrated that the skin of a normal adult human contains 10-20 billion resident memory T cells, including various helper, cytotoxic, and regulatory T cell subsets, that are poised to respond to environmental antigens. Using only autologous human tissues, we report that both in vitro and in vivo, resting epidermal Langerhan cells (LCs) selectively and specifically induced the activation and proliferation of skin resident regulatory T (Treg) cells, a minor subset of skin resident memory T cells. In the presence of foreign pathogen, however, the same LCs activated and induced proliferation of effector memory T (Tem) cells and limited Treg cells' activation. These underappreciated properties of LCs, namely maintenance of tolerance in normal skin, and activation of protective skin resident memory T cells upon infectious challenge, help clarify the role of LCs in skin.

The anti-inflammatory mechanisms of Hsp70

Immune responses to heat shock proteins (Hsp) develop in virtually all inflammatory diseases; however, the significance of such responses is only now becoming clear. In experimental disease models, Hsp administration can prevent or arrest inflammatory damage, and in initial clinical trials in patients with chronic inflammatory diseases, Hsp peptides have been shown to promote the production of anti-inflammatory cytokines, indicating immunoregulatory potential of Hsp. Therefore, the presence of immune responses to Hsp in inflammatory diseases can be seen as an attempt of the immune system to correct the inflammatory condition. Hsp70 can modulate inflammatory responses in models of arthritis, colitis and graft rejection, and the mechanisms underlying this effect are now being elucidated. Incubation with microbial Hsp70 was seen to induce tolerogenic dendritic cells (DCs) and to promote a suppressive phenotype in myeloid-derived suppressor cells and monocytes. These DC could induce regulatory T cells (Tregs), independently of the antigens they presented. Some Hsp70 family members are associated with autophagy, leading to a preferential uploading of Hsp70 peptides in MHC class II molecules of stressed cells. Henceforth, conserved Hsp70 peptides may be presented in these situations and constitute targets of Tregs, contributing to downregulation of inflammation. Finally, an interfering effect in multiple intracellular inflammatory signaling pathways is also known for Hsp70. Altogether it seems attractive to use Hsp70, or its derivative peptides, for modulation of inflammation. This is a physiological immunotherapy approach, without the immediate necessity of defining disease-specific auto-antigens. In this article, we present the evidence on anti-inflammatory effects of Hsp70 and discuss the need for experiments that will be crucial for the further exploration of the immunosuppressive potential of this protein.

CD14 and NFAT mediate lipopolysaccharide-induced skin edema formation in mice

Inflammation is a multistep process triggered when innate immune cells - for example, DCs - sense a pathogen or injured cell or tissue. Edema formation is one of the first steps in the inflammatory response; it is fundamental for the local accumulation of inflammatory mediators. Injection of LPS into the skin provides a model for studying the mechanisms of inflammation and edema formation. While it is known that innate immune recognition of LPS leads to activation of numerous transcriptional activators, including nuclear factor of activated T cells (NFAT) isoforms, the molecular pathways that lead to edema formation have not been determined. As PGE2 regulates many proinflammatory processes, including swelling and pain, and it is induced by LPS, we hypothesized that PGE2 mediates the local generation of edema following LPS exposure. Here, we show that tissue-resident DCs are the main source of PGE2 and the main controllers of tissue edema formation in a mouse model of LPS-induced inflammation. LPS exposure induced expression of microsomal PGE synthase-1 (mPGES-1), a key enzyme in PGE2 biosynthesis. mPGES-1 activation, PGE2 production, and edema formation required CD14 (a component of the LPS receptor) and NFAT. Therefore, tissue edema formation induced by LPS is DC and CD14/NFAT dependent. Moreover, DCs can regulate free antigen arrival at the draining lymph nodes by controlling edema formation and interstitial fluid pressure in the presence of LPS. We therefore suggest that the CD14/NFAT/mPGES-1 pathway represents a possible target for antiinflammatory therapies.

Selection of regulatory T cells in the thymus

The generation of regulatory T (T(Reg)) cells in the thymus is crucial for immune homeostasis and self-tolerance. Recent discoveries have revealed the cellular and molecular mechanisms that govern the differentiation of a subset of developing thymocytes into natural T(Reg) cells. Several models, centred on the self-reactivity of the T cell receptor (TCR), have been proposed to explain the generation of a T(Reg) cell population that is cognizant of self. Several molecular pathways link TCR and cytokine signalling with the expression of the T(Reg) cell-associated transcription factor forkhead box P3 (FOXP3). Moreover, interplay between thymocytes and thymic antigen-presenting cells is also involved in T(Reg) cell generation.

Preferential control of induced regulatory T cell homeostasis via a Bim/Bcl-2 axis

Apoptosis has an essential role in controlling T cell homeostasis, especially during the contraction phase of an immune response. However, its contribution to the balance between effector and regulatory populations remains unclear. We found that Rag1^−/− hosts repopulated with Bim^−/− conventional CD4⁺ T cells (Tconv) resulted in a larger induced regulatory T cell (iTreg) population than mice given wild-type (WT) Tconv. This appears to be due to an increased survival advantage of iTregs compared with activated Tconv in the absence of Bim. Downregulation of Bcl-2 expression and upregulation of Bim expression were more dramatic in WT iTregs than activated Tconv in the absence of IL-2 in vitro. The iTregs generated following Tconv reconstitution of Rag1^−/− hosts exhibited lower Bcl-2 expression and higher Bim/Bcl-2 ratio than Tconv, which indicates that iTregs were in an apoptosis-prone state in vivo. A significant proportion of the peripheral iTreg pool exhibits low Bcl-2 expression indicating increased sensitivity to apoptosis, which may be a general characteristic of certain Treg subpopulations. In summary, our data suggest that iTregs and Tconv differ in their sensitivity to apoptotic stimuli due to their altered ratio of Bim/Bcl-2 expression. Modulating the apoptosis pathway may provide novel therapeutic approaches to alter the balance between effector T cells and Tregs.