IL-2 induces in vivo suppression by CD4(+)CD25(+)Foxp3(+) regulatory T cells

Regulating the adaptive immune response to blood-stage malaria: role of dendritic cells and CD4⁺Foxp3⁺ regulatory T cells

Although a clearer understanding of the underlying mechanisms involved in protection and immunopathology during blood-stage malaria has emerged, the mechanisms involved in regulating the adaptive immune response especially those required to maintain a balance between beneficial and deleterious responses remain unclear. Recent evidence suggests the importance of CD11c⁺ dendritic cells (DC) and CD4⁺Foxp3⁺ regulatory T cells in regulating immune responses during infection and autoimmune disease, but information concerning the contribution of these cells to regulating immunity to malaria is limited. Here, we review recent findings from our laboratory and others in experimental models of malaria in mice and in Plasmodium-infected humans on the roles of DC and natural regulatory T cells in regulating adaptive immunity to blood-stage malaria.

Inflammatory cytokines in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease of unknown origin affecting virtually all organ systems. Beyond genetic and environmental factors, cytokine imbalances contribute to immune dysfunction, trigger inflammation, and induce organ damage. The key cytokine that is involved in SLE pathogenesis is interferon alpha. Interferon secretion is induced by immune complexes and leads to upregulation of several inflammatory proteins, which account for the so-called IFN signature that can be found in the majority of SLE PBMCs. Additionally IL-6 and IFN-y as well as T-cell-derived cytokines like IL-17, IL-21, and IL-2 are dysregulated in SLE. The latter induce a T-cell phenotype that is characterized by enhanced B-cell help and enhanced secretion of proinflammatory cytokines but reduced induction of suppressive T cells and activation-induced cell death. This paper will focus on these cytokines and highlights pathophysiological approaches and therapeutic potential.

Local delivery of IL-2 reduces atherosclerosis via expansion of regulatory T cells

OBJECTIVE

Recent studies indicate that regulatory T cells (Tregs) attenuate murine atherosclerosis. Since interleukin (IL)-2 induces Tregs proliferation, we tested the impact of L19-IL2, a fusion antibody specific to extra-domain B of fibronectin (ED-B) containing an active human IL-2 molecule, in experimental atherosclerosis.

METHODS AND RESULTS

L19-IL2 or appropriate controls were given intravenously to 6 month old Western diet-fed apoE(-/-) mice on day 1, 3, and 5. Human IL-2 was detected on day 7 within atherosclerotic plaques of L19-IL2-treated mice, and magnetic resonance imaging of the plaques showed a significant adventitial gadolinium enhancement on day 7 and 13, suggesting microvascular leakage as a result of the pharmacodynamic activity of L19-IL2. Treatment with L19-IL2 significantly reduced the size of pre-established atherosclerotic plaques at the thoracic aorta (Sudan III stained area) and in the aortic root area (microscopic, morphometric analysis) on day 7 as compared to controls (L19, D1.3-IL2, NaCl) as well as compared to baseline (day 0). Tregs markers Foxp3 and CTLA4 were highly increased in plaques after L19-IL2 treatment compared to controls (p<0.01), whereas the macrophage marker Mac3 was significantly reduced (p<0.03). Co-treatment with IL-2-receptor blocking antibody PC61 abrogated L19-IL2-induced plaque reduction compared with IgG control (p<0.03).

CONCLUSION

L19-IL2 delivers functional IL-2 to pre-established atherosclerotic plaques of WD-fed apoE(-/-) mice resulting in significant plaque size reduction mediated by local Tregs.

Opposing functions of IL-2 and IL-7 in the regulation of immune responses

Regulation of the magnitude and quality of immune responses is dependent on the integration of multiple signals which typically operate through positive and negative feedback loops. Cytokines that promote or limit T cell expansion and differentiation are often both present in the complex lymphoid environment where antigen-initiated T cell responses take place. The nature and strength of the cytokine signal received by the responding cell, as well as by surrounding regulatory cells, will determine the extent of clonal expansion and the progression towards effector and memory cell differentiation. The mechanisms that determine how much cytokine is produced and how cytokine activities are controlled by receptor expression and intracellular regulators of signaling are not fully understood. Here we discuss the opposing functions of two members of the common receptor gamma chain (γc) cytokines, IL-2 and IL-7 in the generation and regulation of immune responses in vivo.

Modulation of tumor immunity by therapeutic monoclonal antibodies

The surveillance of tumors by the immune system of cancer patients and its impact on disease progression and patient survival have been largely documented over the last years. In parallel, the use of therapeutic monoclonal antibodies (mAbs) in oncology has gained a widespread recognition as it has made it possible to increase patient survival and to ameliorate the quality of life in a number of cancers. However, the clinical responses observed following mAb treatment remain largely heterogeneous and their duration is still highly unpredictable. Recently, the concept that the injection of therapeutic antibodies not only triggers early anti-tumor events such as receptor blockade, cytostasis, apoptosis, complement-dependent cytotoxicity and/or antibody-dependent cytotoxicity but also allows the host immune system to fight tumor cells through the development of a long-lasting adaptive immunity has emerged. In the present review, we will examine the arguments that support this concept by detailing the cellular and molecular events likely to underlie the induction of an efficient anti-tumor adaptive immune response by mAbs. We will also discuss the consequences of this induction on the way therapeutic antibodies can be used and inserted in a more global immunotherapeutic approach aiming at strengthening the adaptive anti-tumor immune response developed by cancer patients.

Rapamycin and IL-2 reduce lethal acute graft-versus-host disease associated with increased expansion of donor type CD4+CD25+Foxp3+ regulatory T cells

Previous work has demonstrated that both rapamycin (RAPA) and IL-2 enhance CD4⁺CD25⁺Foxp3⁺ regulatory T-cell (Treg) proliferation and function in vitro. We investigated whether the combination of RAPA plus IL-2 could impact acute GVHD induction after bone marrow transplantation (BMT). RAPA plus IL-2 resulted in improved survival and a reduction in acute GVHD lethality associated with an increased expansion of donor type CD4⁺Foxp3⁺ Tregs and reduced CD4⁺CD25⁻ conventional T cells (Tcons). RAPA plus IL-2, but not either drug alone, increased both expansion of donor natural Tregs and conversion of induced Tregs from donor CD25⁻ Tcons while IL-2 alone increased conversion of Tregs from CD25⁻ Tcon. RAPA plus IL-2 treatment resulted in less production of IFN-γ and TNF, cytokines known to be important in the initiation of acute GVHD. These studies indicate that the pharmacologic stimulation of T cells with IL-2 and the suppression of Tcon proliferation with RAPA result in a selective expansion of functional Tregs and suppression of acute GVHD.

CD4+CD25(high) Treg cells in peripheral blood during remission and exacerbation of allergic asthma in children

AIM

To determine the percentage of CD4+CD25(high) Treg cells in peripheral bloodCD4+ T cells of allergic asthmatic children during disease remission and exacerbation.

METHODS

Peripheral blood mononuclear cells (PBMC) and serum samples were collected from 6- to 11-year-old children with mild-to-moderate allergic asthma (n = 34)and from healthy controls (n = 15). CD4+CD25(high) T cells in PBMC were detected by flow cytometry. Total and specific IgE in serum were analysed by enzyme-amplified chemiluminescence, and IL-2 was measured by ELISA.

RESULTS

There was no significant difference in CD4+CD25(high) T-cell proportions between asthmatic children in exacerbation and remission as compared with controls.CD4+CD25(high) T-cell percentages were not correlated with total and specific IgE. IL-2 was elevated in both disease remission and exacerbation but did not correlate significantly with CD4+CD25(high) T-cell percentages.

CONCLUSION

CD4+CD25(high) T-cell proportion in the peripheral blood of total CD4+T cells is not reduced in children with allergic IgE-mediated asthma and does not differ between disease remission and exacerbation.

CD4+CD25+ regulatory T cells control CD8+ T-cell effector differentiation by modulating IL-2 homeostasis

CD4(+)CD25(+) regulatory T cells (Treg) play a crucial role in the regulation of immune responses. Although many mechanisms of Treg suppression in vitro have been described, the mechanisms by which Treg modulate CD8(+) T cell differentiation and effector function in vivo are more poorly defined. It has been proposed, in many instances, that modulation of cytokine homeostasis could be an important mechanism by which Treg regulate adaptive immunity; however, direct experimental evidence is sparse. Here we demonstrate that CD4(+)CD25(+) Treg, by critically regulating IL-2 homeostasis, modulate CD8(+) T-cell effector differentiation. Expansion and effector differentiation of CD8(+) T cells is promoted by autocrine IL-2 but, by competing for IL-2, Treg limit CD8(+) effector differentiation. Furthermore, a regulatory loop exists between Treg and CD8(+) effector T cells, where IL-2 produced during CD8(+) T-cell effector differentiation promotes Treg expansion.

Changes in peripheral blood level of regulatory T cells in patients with malignant melanoma during treatment with dendritic cell vaccination and low-dose IL-2

In this study, changes in peripheral blood regulatory T cell (Treg) levels were evaluated in 46 progressive patients with melanoma treated with a dendritic cell-based vaccine and concomitant low-dose IFN-α and IL-2. The regulatory subset of CD4 T cells, characterized by CD25(high) , was prospectively analysed in fresh blood, and treatment-associated quantitative and qualitative changes were analysed. By the 4th vaccine, patients showed a marked increase in CD4+ CD25(high) T cell subset from 6% to 22% (P<0.001). At the 6th vaccine, a general decline was observed and a significantly (P=0.01) lower level of CD4+ CD25(high) Treg cells was reached in the group of patients who attained disease stabilization (9.5%) compared to patients with continued progressive disease (14.5%). However, when FoxP3 was employed for retrospective analysis of Tregs on frozen blood, this difference did not reach significance (P=0.09). The vast majority of the Treg produced IL-10 and, to a varying extent, TGF-β. In addition, sorted CD4+ CD25(high) CD127⁻ Tregs were able to suppress proliferation of peripheral blood mononuclear cells in a dose-dependent manner, thus suggesting a regulatory functionality. These findings emphasize the need for strategies to effectively eliminate Treg cells to optimize the clinical effectiveness of cancer immunotherapy.

Single-cell quantification of IL-2 response by effector and regulatory T cells reveals critical plasticity in immune response

The sensitivity of T cells to interleukin-2 (IL-2) can vary by three orders of magnitude and is determined by the surface densities of the IL-2 receptor α subunits.

Regulatory T cells inflict a double hit on effector T cells by lowering the bulk IL-2 concentration as well as the sensitivity of effector T cells to this crucial cytokine.

This double hit deprives weakly activated effector T cells of pSTAT5 survival signals while having only minimal effects on strongly activated effector cells that express increased levels of the IL-2 receptor.

Short-term signaling differences lead to a differential functional in terms of proliferation and cell division: regulatory T cell specifically suppress weakly activated effector T cells even at large numbers; small numbers of strongly activated effector T cells overcome the suppression.

Self-/non-self-discrimination in the adaptive immune system relies, to a large extent, on distinctions between self-antigens and foreign antigens as made by individual T cells. As such, single-cell decisions are prone to errors a reliable immune response can be expected to incorporate further proofreading schemes. One such scheme involves long time scale, population-level interactions between effector (T_eff) and regulatory (T_reg) T cells. T_reg cells are often described as immune suppressors; their role as immune regulators can be understood by mapping out the scenarios in which T_reg suppression is either significant or insignificant.

In this study, we have focused on one mechanism that allows T_reg cells to suppress T_eff survival, namely, interleukin-2 (IL-2) deprivation. Following antigen activation, T_eff cells secrete IL-2 and express the α subunit of the IL-2 receptor (IL-2r). The binding of extracellular IL-2 to the IL-2r is crucial for T_eff survival and proliferation and consequently for a full-blown immune response. T_reg cells deplete this IL-2 from the environment and deprive the T_eff cells of this important survival signal. In this tug-of-war for IL-2, we sought to quantitatively describe those scenarios in which IL-2 uptake by T_reg cells suffices to suppress T_eff cell activation and those where it does not.

The core of this competition for IL-2 lies in the fact that IL-2rα is expressed on both T_eff and T_reg cells. To understand how IL-2 binds to its receptor, we measured IL-2r subunit levels on single cells, together with STAT5 phosphorylation as evoked by varied IL-2. Contrary to previous descriptions that set the EC₅₀ of IL-2/IL-2r interaction at 10 pM, we found that the sensitivity of T cells to IL-2 varies over three orders of magnitude concentrations (Figure 1E, experiment). T_eff cells with higher levels of IL-2rα receptor subunit are more sensitive to IL-2, T_reg cells with higher levels of IL-2rα are more efficient in the scavenging of IL-2. IL-2rβ levels, on the other hand, determine response amplitudes. We describe a short time scale, two-step model to quantitatively describe IL-2 binding onto individual cells (Figure 1E, theory).

IL-2r expression levels are therefore a crucial parameter for determining the outcome of the competition for IL-2. We measured the regulation of IL-2r subunits on longer time scales in cultures of either T_eff or T_reg cells. For both cell types, IL-2r levels depend on the exposure to IL-2. For T_eff cells, there is a further dependence on the concentration of antigen by which they were activated.

We then measured IL-2r expression in cocultures of T_reg and T_eff cells. We show how IL-2 secreted by activated T_eff cells suffices in inducing IL-2rα upregulation in the T_reg population. We further show that the presence of T_reg cells decreased IL-2r upregulation in cocultured weakly activated T_eff cells. T_reg cells thus inflict a double hit on T_eff cells by reducing not only extracellular IL-2 concentrations but also the T_eff cells' ability to sense IL-2. T_eff cells activated by high-antigen concentrations exhibit sustained IL-2rα expression that is less prone to this effect. We compared IL-2r levels on T_reg cells and T_eff activated by varied antigen concentrations and found a critical crossover: at low-antigen concentrations T_reg cells have higher IL-2rα than T_eff cells, but this is reversed at high-antigen concentrations.

We constructed a long time scale computational model to quantify the significance of this crossover. The model describes IL-2/IL-2r binding and the regulation of IL-2 and IL-2r expression in populations of T_reg and T_eff cells. For a pure T_eff population, our model predicted a ‘quorum-sensing' threshold implying that sustained pSTAT5 signaling requires a minimal concentration of cells that increases with decreasing activation strength. The model further predicts that the addition of T_reg cells will greatly increase the quorum concentration for weakly activated T_eff cells but have no effect on strongly activated T_eff cells.

We validated the model's predictions in vitro. We show a quorum-sensing threshold for activated T_eff cells. We also show that the presence of a T_reg population suppressed pSTAT5 signaling in a large number of weakly but has little effect on even a few strongly activated T_eff cells (Figure 6C and D). On longer time scales, this translates to the suppression of cell division (Figure 6G and H) and proliferation (Figure 6I) in a manner that discriminates between strongly and weakly activated cells.

We then went to demonstrate that IL-2 deprivation by T_reg cells takes place in vivo. We used IL-2 injections to upregulate IL-2rα levels in T_reg cells. As predicted by our in vitro results, such treatment leads to a suppressive environment in which T_eff cells activated by subsequent antigen/LPS immunization proliferate to a lesser extent. We were able to reverse this suppressive effect by continuing IL-2 treatment post-immunization. This highlights IL-2 as a limiting factor for T_eff proliferation and renders its scavenging by T_reg cells an important mechanism of suppression in vivo.

In conclusion, we formulated a quantitative description of IL-2/IL-2r regulation in mixed population of T_reg and T_eff cells. Population feedback loops that depend on cell numbers, molecular cell surface densities, free molecular densities and timing critically affect the outcome of the competition for IL-2. Such a description allows us to precisely identify the scenarios in which IL-2 deprivation by T_reg cells has a major suppressive role in vitro and better understand the role of this mechanism in vivo.

Understanding how the immune system decides between tolerance and activation by antigens requires addressing cytokine regulation as a highly dynamic process. We quantified the dynamics of interleukin-2 (IL-2) signaling in a population of T cells during an immune response by combining in silico modeling and single-cell measurements in vitro. We demonstrate that IL-2 receptor expression levels vary widely among T cells creating a large variability in the ability of the individual cells to consume, produce and participate in IL-2 signaling within the population. Our model reveals that at the population level, these heterogeneous cells are engaged in a tug-of-war for IL-2 between regulatory (T_reg) and effector (T_eff) T cells, whereby access to IL-2 can either increase the survival of T_eff cells or the suppressive capacity of T_reg cells. This tug-of-war is the mechanism enforcing, at the systems level, a core function of T_reg cells, namely the specific suppression of survival signals for weakly activated T_eff cells but not for strongly activated cells. Our integrated model yields quantitative, experimentally validated predictions for the manipulation of T_reg suppression.

CD28 costimulation Impairs the efficacy of a redirected t-cell antitumor attack in the presence of regulatory t cells which can be overcome by preventing Lck activation

Adoptive T-cell transfer showed promising efficacy in recent trials raising interest in T cells with redirected specificity against tumors. T cells were engineered with a chimeric antigen receptor (CAR) with predefined binding and CD3ζ signaling to initiate T-cell activation. CD28 costimulation provided by a CD28-CD3ζ signaling CAR moreover improved T cell activation and persistence; however, it failed to meet the expectations with respect to mounting attacks against solid tumors infiltrated with regulatory T (Treg) cells. We revealed that a CD28 CAR-redirected T-cell attack is accompanied by higher numbers of Treg cells infiltrating the tumor and is less efficient against cancer cells in presence of Treg cells than a CD3ζ CAR T-cell attack. Deletion of the lck binding moiety in the CD28 CAR endodomain, however, improved redirected anti-tumor activity in presence of Treg cells without impairing interferon-γ (IFN-γ) secretion, proliferation, and cytolysis. CD28 modification abrogated interleukin-2 (IL-2) induction upon CAR engagement which in turn is no longer available to sustain Treg cell persistence. CARs with the modified CD28 endodomain thereby expedite the implementation of adoptive T-cell therapy in patients with a variety of cancer types that are heavily infiltrated by Treg cells.

Interleukin-2 receptor signaling: at the interface between tolerance and immunity

Interleukin-2 receptor (IL-2R) signaling regulates tolerance and immunity. Here, we review recent work concerning the structure, signaling, and function of the IL-2R, emphasizing the contribution of IL-2 for T cell-dependent activity in vivo. IL-2R signaling influences two discrete aspects of immune responses by CD8(+) T cells, terminal differentiation of effector cells in primary responses, and aspects of memory recall responses. IL-2 also delivers essential signals for thymic development of regulatory T (Treg) cells and later to promote their homeostasis and function. Each of these outcomes on T effector and Treg cells requires distinct amounts of IL-2R signaling, with low IL-2R signaling sufficient for many key aspects of Treg cells. Thus, tolerance is readily maintained and favored with limited IL-2.

Foxp3 expressing regulatory T-cells in allergic disease

Allergic diseases such as asthma, rhinitis and eczema are increasing in prevalence worldwide, in particular in industrialised countries affecting up to 20% of the population. Regulatory T-cells (Tregs) have been shown to be critical in T-cell homeostasis and in the maintenance of immune responses, such as prevention of autoimmunity and hampering allergic diseases. The so-called 'natural' CD4+CD25+ Tregs and/or IL-10-producing Tr1 cells have been shown to be responsible for the protection of immune tolerance and intact immune reactions following exposure to allergens such as aeroallergens or food allergens. In this regard, both cell-cell contact (through membrane bound TGF-beta or via suppressive molecules such as CLTA-4) and soluble cytokine-(TGF-beta and IL-10) dependent mechanisms have been shown to contribute to the ability of Tregs to operate effectively. The transcription factor Foxp3, a member of the forkhead-winged helix family, appears to be critical in the suppressive abilities of regulatory T-cells. Adoptive transfer of CD4+CD25+ Tregs from healthy to diseased animals corroborated and provided further evidence of the vital role of these populations in the prevention or cure of certain autoimmune conditions. Clinical improvement seen after allergen immunotherapy for allergic diseases such as rhinitis and asthma has also been associated with the induction of IL-10 and TGF-beta producing Trl cells as well as Foxp3 expressing CD4+CD25+ T-cells, resulting in the suppression ofTh2 cytokine milieu. Activation and expansion ofantigen-specific CD4+CD25+ Tregs in vivo using adjuvants such as IL-10 or pharmacological agents such as low dose steroids or vitamin D3 could represent novel approaches to induce antigen-specific tolerance in immune-mediated conditions such as allergic asthma, autoimmune disease and the rejection of transplanted organs in man.

Competing feedback loops shape IL-2 signaling between helper and regulatory T lymphocytes in cellular microenvironments

Cytokines are pleiotropic and readily diffusible messenger molecules, raising the question of how their action can be confined to specific target cells. The T cell cytokine interleukin-2 (IL-2) is essential for the homeostasis of regulatory T (Treg) cells that suppress (auto)immunity and stimulates immune responses mediated by conventional T cells. We combined mathematical modeling and experiments to dissect the dynamics of the IL-2 signaling network that links the prototypical IL-2 producers, conventional T helper (Th) cells, and Treg cells. We show how the IL-2-induced upregulation of high-affinity IL-2 receptors (IL-2R) establishes a positive feedback loop of IL-2 signaling. This feedback mediates a digital switch for the proliferation of Th cells and functions as an analog amplifier for the IL-2 uptake capacity of Treg cells. Unlike other positive feedbacks in cell signaling that augment signal propagation, the IL-2/IL-2R loop enhances the capture of the signal molecule and its degradation. Thus Treg and Th cells can compete for IL-2 and restrict its range of action through efficient cellular uptake. Depending on activation status and spatial localization of the cells, IL-2 may be consumed exclusively by Treg or Th cells, or be shared between them. In particular, a Treg cell can deprive a stimulated Th cell of its IL-2, but only when the cells are located in close proximity, within a few tens of micrometers. The present findings explain how IL-2 can play two distinct roles in immune regulation and point to a hitherto largely unexplored spatiotemporal complexity of cytokine signaling.

Homeostatic imbalance of regulatory and effector T cells due to IL-2 deprivation amplifies murine lupus

The origins and consequences of a regulatory T cell (Treg) disorder in systemic lupus erythematosus (SLE) are poorly understood. In the (NZBxNZW) F(1) mouse model of lupus, we found that CD4(+)Foxp3(+) Treg failed to maintain a competitive pool size in the peripheral lymphoid organs resulting in a progressive homeostatic imbalance of CD4(+)Foxp3(+) Treg and CD4(+)Foxp3(-) conventional T cells (Tcon). In addition, Treg acquired phenotypic changes that are reminiscent of IL-2 deficiency concomitantly to a progressive decline in IL-2-producing Tcon and an increase in activated, IFN-gamma-producing effector Tcon. Nonetheless, Treg from lupus-prone mice were functionally intact and capable to influence the course of disease. Systemic reduction of IL-2 levels early in disease promoted Tcon hyperactivity, induced the imbalance of Treg and effector Tcon, and strongly accelerated disease progression. In contrast, administration of IL-2 partially restored the balance of Treg and effector Tcon by promoting the homeostatic proliferation of endogenous Treg and impeded the progression of established disease. Thus, an acquired and self-amplifying disruption of the Treg-IL-2 axis contributed essentially to Tcon hyperactivity and the development of murine lupus. The reversibility of this homeostatic Treg disorder provides promising approaches for the treatment of SLE.

Effect of in vitroexpanded CD4(+)CD25(+)Foxp3(+) regulatory T cell therapy combined with lymphodepletion in murine skin allotransplantation

A promising approach for preventing allograft rejection involves shifting the balance between cytopathic and regulatory T cells to dominance of the latter cell type. Nonspecific lymphodepletion was conducted by administration of depleting anti-CD4 and anti-CD8 antibodies to reduce effector T cells and adoptive transfer of ex vivo-expanded host Treg cells by stimulation with donor dendritic cells to augment the Treg cell compartment. Evaluation of an MHC-mismatched skin allograft model revealed that combined therapy with these two protocols consistently induced modest prolongation of allograft survival, although all skin grafts were eventually rejected. The administration of IL-2/anti-IL-2 complexes significantly improved the efficacy of combination therapy via promoting the expansion of adoptively transferred Treg cells as well as endogenous recipient Treg cells. We conclude that Treg cell therapy combined with lymphodepletion is of practical benefit for the control of allograft rejection, but requires supplementary measures to promote immune tolerance.

The emergence of immunomodulation: combinatorial immunochemotherapy opportunities for the next decade

In the past decade we have witnessed important advances in the treatment of gynecological cancers and have recognized their potential immunogenicity. This has opened the door to explore immune therapy not only in HPV-induced cancers but also in ovarian and endometrial cancers. Here we will review the off-target immune effects of select chemotherapy drugs commonly used to treat gynecologic cancers and novel tools that can stimulate both the adaptive and innate immune mechanisms such as novel pleiotropic cytokines, Toll-like receptors, and powerful antibodies that can target inhibitory checkpoints, thereby activating effector cellular immune mechanisms and neutralizing suppressor cells. We will also review how existing drugs can be used for combinatorial tumor therapy.

Signaling through TLR7 enhances the immunosuppressive activity of murine CD4+CD25+ T regulatory cells

Although signaling through certain TLRs is known to modulate the function of T lymphocytes, the effect of TLR7 stimulation on CD4(+)CD25(+) T(reg) cell activity has not yet been elucidated. In this study, we show that mouse CD4(+)CD25(+) T(reg) cells express TLR7 mRNA and protein. We therefore used the TLR7 agonists imiquimod, gardiquimod, and single-stranded poly(U) to show that TLR7 stimulation enhanced the ability of murine T(reg) cells to suppress anti-CD3/anti-CD28 mAb-coated bead-stimulated proliferation of syngeneic CD4(+)CD25(-) T(resp) cells. In contrast, imiquimod failed to enhance the suppressor function of T(reg) cells from mice deficient in the MyD88 adaptor protein involved in TLR7 and other TLR signal transduction. Imiquimod increased murine T(reg) cell-mediated suppression of T(resp) cell proliferation induced by anti-TCRbeta mAb in the presence of syngeneic BMDCs, and T(reg) cells from gardiquimod-treated mice exhibited enhanced in vitro suppressor function. Moreover, levels of T(resp) cell-secreted IL-2 and IFN-gamma were reduced further in the presence of T(reg) cells plus imiquimod in comparison with T(reg) cells alone. In addition, imiquimod treatment increased CD25 expression by T(reg) cells and caused exogenous IL-2 to enhance T(reg) cell suppressor function. Furthermore, combined treatment with imiquimod and IL-2 increased Foxp3 expression by T(reg) cells. Collectively, these findings suggest that TLR7 signaling enhanced the suppressor function of T(reg) cells by sensitizing T(reg) cells to IL-2-induced activation. We speculate that TLR7-stimulated enhancement of T(reg) cell suppressor function may modulate host T cell responses against ssRNA viruses.

A review of immunomodulators with reference to Canova

Immunomodulators are substances which modify the immunity of an individual to favour a particular immunological response. The immune response and the function of the immune response regulation process are described, with special reference to cancer and autoimmune disease. Homeopathy and its role in immune regulation are discussed with special reference to Canova. Canova is a homeopathic product produced, according to the Hahnemannian homeopathic method, in Brazil. Its role in cancer, bone marrow and haematopoiesis as well as macrophage and monocyte activation is reviewed. Canova seems to stabilize platelet morphology in human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS). The data suggest that the future of immunomodulators and homeopathic products which appear to have an effect on the immune response requires a better understanding of the relative need for immune activation versus immune modulation. Homeopathic products specifically need more attention.

Human regulatory T cell suppressive function is independent of apoptosis induction in activated effector T cells

Background

CD4⁺CD25⁺FOXP3⁺ Regulatory T cells (Treg) play a central role in the immune balance to prevent autoimmune disease. One outstanding question is how Tregs suppress effector immune responses in human. Experiments in mice demonstrated that Treg restrict effector T cell (Teff) responses by deprivation of the growth factor IL-2 through Treg consumption, resulting in apoptosis of Teff.

Principal Findings

In this study we investigated the relevance of Teff apoptosis induction to human Treg function. To this end, we studied naturally occurring Treg (nTreg) from peripheral blood of healthy donors, and, to investigate Treg function in inflammation in vivo, Treg from synovial fluid of Juvenile Idiopathic Arthritis (JIA) patients (SF-Treg). Both nTreg and SF-Treg suppress Teff proliferation and cytokine production efficiently as predicted. However, in contrast with murine Treg, neither nTreg nor SF-Treg induce apoptosis in Teff. Furthermore, exogenously supplied IL-2 and IL-7 reverse suppression, but do not influence apoptosis of Teff.

Significance

Our functional data here support that Treg are excellent clinical targets to counteract autoimmune diseases. For optimal functional outcome in human clinical trials, future work should focus on the ability of Treg to suppress proliferation and cytokine production of Teff, rather than induction of Teff apoptosis.

Mast cells down-regulate CD4+CD25+ T regulatory cell suppressor function via histamine H1 receptor interaction

Mast cells promote both innate and acquired immune responses, but little is known about the effect of mast cells on T regulatory (T(reg)) cell function. In this study, we show for the first time that the capacity of murine CD4(+)CD25(+) T(reg) cells to suppress in vitro proliferation by CD4(+)CD25(-) T responder (T(resp)) cells in response to anti-CD3/anti-CD28 mAb-coated beads was reduced in the presence of syngeneic bone marrow-derived mast cells (BMMC) activated by FcepsilonR cross-linking. Activated BMMC culture supernatants or exogenous histamine also inhibited T(reg) cell suppressor function while the histamine H1 receptor-specific antagonist loratadine, but not the H2 receptor-specific antagonist famotidine, restored T(reg) cell suppressor function in the presence of activated BMMC or activated BMMC culture supernatants. Moreover, treatment of T(reg) cells with loratadine, but not famotidine, rescued T(reg) cell suppressor function in the presence of exogenous histamine. In addition, the H1 receptor-specific agonist 2-pyridylethylamine dihydrochloride inhibited T(reg) cell suppressor function to an extent that was comparable to histamine, whereas the H2 receptor-specific agonist amthamine dihydrobromide was without effect. Both T(reg) cells and T(resp) cells expressed H1 receptors. Exposure to histamine caused T(reg) cells to express lower levels of CD25 and the T(reg) cell-specific transcription factor Foxp3. Taken together, these data indicate that BMMC-elaborated histamine inhibited T(reg) cell suppressor function by signaling through the H1 receptor. We suggest that histamine released as a result of mast cell activation by microbial products might cause a transient decrease in T(reg) cell suppressor function, thereby enhancing the development of protective immunity.

Homeostatic regulation of T effector to Treg ratios in an area of seasonal malaria transmission

An important aspect of clinical immunity to malaria is the ability to down-regulate inflammatory responses, once parasitaemia is under control, in order to avoid immune-mediated pathology. The role of classical (CD4(+)CD25(+)CD127(lo/-)FOXP3(+)) Treg in this process, however, remains controversial. Thus, we have characterized the frequency, phenotype and function of Treg populations, over time, in healthy individuals in The Gambia. We observed that both the percentage and the absolute number of CD4(+)FOXP3(+)CD127(lo/-) T cells were higher among individuals living in a rural village with highly seasonal malaria transmission than among individuals living in an urban area where malaria rarely occurs. These CD4(+)FOXP3(+)CD127(lo/-) T cells exhibited an effector memory and apoptosis-prone phenotype and suppressed cytokine production in response to malaria antigen. Cells from individuals exposed to malaria expressed significantly higher levels of mRNA for forkhead box P3 and T-box 21 (T-BET) at the end of the malaria transmission season than at the end of the non-transmission season. Importantly, the ratio of T-BET to forkhead box P3 was remarkably consistent between populations and over time, indicating that in healthy individuals, a transient increase in Th1 responses during the malaria transmission season is balanced by a commensurate Treg response, ensuring that immune homeostasis is maintained.

Multiple sclerosis association study with the TENR-IL2-IL21 region in a Spanish population

Polymorphisms from the TENR-IL2-IL21 block in the 4q27 chromosome were recently associated with type 1 diabetes, celiac disease, rheumatoid arthritis and psoriasis. We undertook this study to investigate the potential role of polymorphisms rs3136534, rs6822844 and rs2069762 (-330 T/G IL2) in multiple sclerosis (MS) (805 patients of Spanish Caucasian origin and 952 health controls). We did not find evidence for association with any single nucleotide polymorphisms (SNPs) tested. Allele and genotype frequencies of the SNPs, which were studied, were similar in DRB1*15-positive or DRB1*15-negative patients. After stratification of MS patients by clinical course, a weak association was observed with rs2069762 G allele and haplotype bearing this allele with secondary progressive MS, although these cases represent 22% of the MS cases. Our results did not show major influence of TENR-IL2-IL21 locus on susceptibility or disease progression in MS. However, we could not exclude completely the effect in MS for this region. Additional studies, using much larger sample sizes and analysis of additional polymorphisms in the gene and its flanking region, will be required to ascertain their contributions to MS susceptibility.

Regulatory T cells suppress innate immunity in kidney ischemia-reperfusion injury

Both innate and adaptive mechanisms participate in the pathogenesis of kidney ischemia-reperfusion injury (IRI), but the role of regulatory immune mechanisms is unknown. We hypothesized that the anti-inflammatory effects of CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs) protect against renal IRI. Partial depletion of Tregs with an anti-CD25 mAb potentiated kidney damage induced by IRI. Reducing the number of Tregs resulted in more neutrophils, macrophages, and innate cytokine transcription in the kidney after IRI but did not affect CD4(+) T cells or B cells. We performed adoptive transfer of lymph node cells from wild-type mice or FoxP3-deficient Scurfy mice into T cell- and B cell-deficient RAG-1 knockout mice to generate mice with and without FoxP3(+) Tregs, respectively. FoxP3(+) Treg-deficient mice accumulated a greater number of inflammatory leukocytes after renal IRI than mice containing Tregs. To confirm that a lack of Tregs potentiated renal injury, we co-transferred isolated Tregs and Scurfy lymph node cells; Treg repletion significantly attenuated IRI-induced renal injury and leukocyte accumulation. Furthermore, although adoptive transfer of wild-type Tregs into RAG-1 knockout mice was sufficient to prevent kidney IRI, transfer of IL-10-deficient Tregs was not. Taken together, these results demonstrate that Tregs modulate injury after kidney IRI through IL-10-mediated suppression of the innate immune system.

Expanding and converting regulatory T cells: a horizon for immunotherapy

The human immune system is a myriad of diverse cellular populations, each contributing to maintaining an effective and optimal immune response against infectious agents. It is important to maintain a "self-check" in the immune system so that responses do not go haywire, leading to the development of autoimmune diseases. Regulatory/suppressor T (Treg) cells are a specialized subpopulation of T cells that suppress the activation, expansion, and function of other T cells, thereby maintaining homeostasis through a fine balance between reactivity to foreign and self antigens. Tregs are characterized by surface expression of interleukin (IL)-2 receptor alpha chain (CD25) and intracellular expression of forkhead box protein P3 (FoxP3). There are at least two important functional populations of Treg cells, namely natural Treg (nTreg), which are continuously derived from the thymus, and induced Treg (iTreg), which are converted from naive T cells. The development and function of both nTreg and iTreg cells are regulated by several factors, such as antigen T-cell receptor, co-stimulatory receptors (i.e., cytotoxic T lymphocyte-associated antigen, or CTLA-4), and cytokines (IL-2, IL-10, and tumor growth factor-beta, or TGF-beta). In addition, the TGF-beta inhibitor ALK5, retinoid acid, and rapamycin influence the expansion of nTreg cells and the conversion of iTreg cells in vitro and in vivo. The heightening of Treg expansion may be harnessed to therapeutic methods for the treatment of autoimmune diseases and the induction of transplantation tolerance.

Tumor regulatory T cells potently abrogate antitumor immunity

Regulatory T cell (Treg) from mice bearing a breast tumor were elevated (tumor Treg). In vitro, whereas tumor Treg ability to inhibit tumor-primed CD4(+) T cell activity is comparable to Treg from naive mice (naive Treg), only tumor Treg suppress naive CD8(+) T cell activation and DC function. Neither tumor Treg nor naive Treg can suppress antitumor immunity at the effector phase of the immune response induced by adoptively transferred tumor-primed CD4(+) T cells. This is consistent with the observation that, in this model, neither tumor Treg nor naive Treg can inhibit effectors in vitro or in vivo. However, tumor Treg abrogate tumor-specific CD8(+) T cell responses in tumor-draining lymph nodes and antitumor immunity at the early stage of the immune response induced by adoptively transferred tumor-primed CD4(+) T cells. These data indicate that, in this model, tumor Treg potently abrogate tumor-specific CD8(+) T cell responses in tumor-draining lymph nodes, thereby suppressing antitumor immunity at the early stage of the immune response induced by adoptively transferred tumor-primed CD4(+) T cells.

Interleukin-2 in the development and control of inflammatory disease

Interleukin-2 (IL-2) has multiple, sometimes opposing, functions during an inflammatory response. It is a potent inducer of T-cell proliferation and T-helper 1 (Th1) and Th2 effector T-cell differentiation and provides T cells with a long-lasting competitive advantage resulting in the optimal survival and function of memory cells. In a regulatory role, IL-2 is important for the development, survival, and function of regulatory T cells, it enhances Fas-mediated activation-induced cell death, and it inhibits the development of inflammatory Th17 cells. Thus, in its dual and contrasting functions, IL-2 contributes to both the induction and the termination of inflammatory immune responses.

Distinct regulatory roles of transforming growth factor-beta and interleukin-4 in the development and maintenance of natural and induced CD4+ CD25+ Foxp3+ regulatory T cells

The development and function of CD4(+) CD25(+) Foxp3(+) regulatory T cells (Tregs) are strictly regulated by cytokines. Here we show that transforming growth factor-beta (TGF-beta) and interleukin-4 (IL-4) play a crucial and antagonistic role in the development of Tregs. Additionally, these cytokines also have distinct effects on the maintenance of natural (nTregs) and antigen-induced (iTregs) Tregs. Using double-staining and tracking of proliferation of purified and carboxyflourescein succinimidyl ester (CFSE)-labelled mouse T-cell subpopulations we demonstrated that CD4(+) CD25(+) Foxp3(+) iTregs develop upon alloantigenic stimulation in the presence of TGF-beta exclusively from CD4(+) CD25(-) Foxp3(-) precursors. Both the induction of Foxp3 expression and Treg proliferation were prevented when the cells were stimulated in the presence of IL-4. By contrast, nTregs did not proliferate in the presence of the antigen and TGF-beta, and partially lost their Foxp3 expression. IL-4 not only prevented the development of iTregs, but also down-regulated the level of Foxp3 mRNA and decreased the number of Foxp3(+) cells in a population of iTregs. Further analyses proved that IL-4 decreased the expression of Foxp3 only in a population of iTregs, whereas it substantially supported the survival of nTregs. Functional experiments showed that Tregs induced in the presence of alloantigen and TGF-beta inhibited, on a per-cell basis, cell proliferation comparably to nTregs, and their suppressive capacity was not modulated by IL-4. These data suggest that TGF-beta and IL-4 differentially regulate the development of Tregs and distinctly sustain Foxp3 expression and the number of nTregs and iTregs, but have no influence on the suppressive activity of Tregs on a per-cell basis.

Immunoregulatory profiles in liver transplant recipients on different immunosuppressive agents

We compared peripheral blood immunophenotyping in 31 adult liver transplant recipients on differing long-term immunosuppressive (IS) monotherapy with and without peri-transplantation alemtuzumab (AL) induction. All patients had been stable on monotherapy with either sirolimus (SRL) (n = 10) or without SRL (tacrolimus (TAC) (n = 10), mycophenolate mofetil (MMF) (n = 11)) for more than 6 months. Five-color flow cytometry for putative "regulatory" T and dendritic cells as well as serum assays for soluble HLA-G (sHLA-G) were performed. The SRL monotherapy group had significantly higher percentages of CD4+CD25(high+)Foxp3+ T cells (1.3 +/- 1.0) compared with the non-SRL group (0.7 +/- 0.6) (p = 0.04). The SRL effect was even higher in a subset with prior AL induction and no prior hepatitis C or rejection (1.7 +/- 0.2) compared with all other subgroups (0.7 +/- 0.6) (p = 0.02). TAC patients showed significantly higher "regulatory" DC2:DC1 ratios (10 +/- 7.6) compared with non-TAC patients (4.1 +/- 2.3) (p = 0.04). Although sHLA-G levels appeared higher in TAC patients, the differences were not statistically significant. In conclusion, IS monotherapy provides an opportunity to investigate regulatory roles of individual agents. SRL maintenance and prior AL induction in subsets of patients appeared to show a regulatory T cell immunophenotype. However, TAC patients may have other regulatory characteristics, supporting the need for larger, prospective studies to clarify differences.