IL-2 is essential for TGF-beta to convert naive CD4+CD25- cells to CD25+Foxp3+ regulatory T cells and for expansion of these cells

Levels of transforming growth factor-beta are low in systemic lupus erythematosus patients with active disease

OBJECTIVE

Cytokines are central regulators of the immune response but the workings of this complex network in systemic lupus erythematosus (SLE) are not fully understood. We investigated a range of inflammatory and immune-modulating cytokines to determine their value as biomarkers for disease subsets in SLE.

METHODS

This was a cross-sectional study in 102 patients with SLE (87% women, disease duration 10.6 yrs). Circulating concentrations of interleukin 1β (IL-1β), IL-4, IL-6, IL-10, IL-12, IL-17, monocyte chemotactic protein 1 (MCP-1), macrophage inflammatory protein 1 (MIP-1α), MIP-1β, interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α), and total transforming growth factor-β1 (TGF-β1) were related to disease activity (SLE Disease Activity Index; SLEDAI), lymphocyte subsets, autoantibody levels, accrued damage (Systemic Lupus International Collaborating Clinics/ACR Damage Index; SDI), and concomitant treatment.

RESULTS

Patients with SLE had lower levels of TGF-β1 (p = 0.01) and IL-1β (p = 0.0004) compared to controls. TGF-β1 levels were lower in patients with SLEDAI scores 1-10 and SDI > 3; and were correlated with CD4+, CD8+, and natural killer cell counts; and were independent of steroid or cytotoxic drug use. Treatment with cardiovascular drugs was associated with lower IL-12 levels. No consistent disease associations existed for the other cytokines investigated.

CONCLUSION

Lower TGF-β1 was the most consistent cytokine abnormality in patients with SLE. The associations with disease activity, lymphocyte subsets, and damage suggest that TGF-β1 may be a therapeutic target of interest in SLE.

Cutting edge: all-trans retinoic acid sustains the stability and function of natural regulatory T cells in an inflammatory milieu

Recent studies have demonstrated that plasticity of naturally occurring CD4(+)Foxp3(+) regulatory T cells (nTregs) may account for their inability to control chronic inflammation in established autoimmune diseases. All-trans retinoic acid (atRA), the active derivative of vitamin A, has been demonstrated to promote Foxp3(+) Treg differentiation and suppress Th17 development. In this study, we report a vital role of atRA in sustaining the stability and functionality of nTregs in the presence of IL-6. We found that nTregs treated with atRA were resistant to Th17 and other Th cell conversion and maintained Foxp3 expression and suppressive activity in the presence of IL-6 in vitro. atRA decreased IL-6R expression and signaling by nTregs. Of interest, adoptive transfer of nTregs even from arthritic mice treated with atRA suppressed progression of established collagen-induced arthritis. We suggest that nTregs treated with atRA may represent a novel treatment strategy to control established chronic immune-mediated inflammatory diseases.

IL-2 reverses established type 1 diabetes in NOD mice by a local effect on pancreatic regulatory T cells

Regulatory T cells (T reg cells) play a major role in controlling the pathogenic autoimmune process in type 1 diabetes (T1D). Interleukin 2 (IL-2), a cytokine which promotes T reg cell survival and function, may thus have therapeutic efficacy in T1D. We show that 5 d of low-dose IL-2 administration starting at the time of T1D onset can reverse established disease in NOD (nonobese diabetic) mice, with long-lasting effects. Low-dose IL-2 increases the number of T reg cells in the pancreas and induces expression of T reg cell–associated proteins including Foxp3, CD25, CTLA-4, ICOS (inducible T cell costimulator), and GITR (glucocorticoid-induced TNF receptor) in these cells. Treatment also suppresses interferon γ production by pancreas-infiltrating T cells. Transcriptome analyses show that low-dose IL-2 exerts much greater influence on gene expression of T reg cells than effector T cells (T eff cells), suggesting that nonspecific activation of pathogenic T eff cells is less likely. We provide the first preclinical data showing that low-dose IL-2 can reverse established T1D, suggesting that this treatment merits evaluation in patients with T1D.

Therapeutic potential of TGF-β-induced CD4(+) Foxp3(+) regulatory T cells in autoimmune diseases

Foxp3(+) T regulatory cell (Treg) subsets play a crucial role in the maintenance of immune homeostasis against self-antigens. The lack or dysfunction of these cells contributes to the pathogenesis and development of many autoimmune diseases. Therefore, manipulation of these cells may provide a novel therapeutic approach to treat autoimmune diseases. In this review, we provide current opinions concerning the classification, developmental, and functional characterization of Treg subsets. Particular emphasis will be focused on the therapeutic role of TGF-β-induced CD4M(+) Foxp3(+) cells (iTregs) in established autoimmune disease. Moreover, the similarity and diversity of iTregs and naturally occurring, thymus-derived CD4(+) CD25(+) Foxp3(+) regulatory T cells (nTregs) will be discussed, including the finding that the pro-inflammatory cytokine IL-6 can convert nTregs to IL-17-producing cells, whereas iTregs induced by TGF-β are resistant to the effects of this cytokine. Understanding these aspects may help to determine how Tregs can be used in the treatment of autoimmune diseases.

Regulatory T cells induced by Mycobacterium chelonae sensitization influence murine responses to bacille Calmette-Guerin

The efficacy of live Mycobacterium bovis BCG as a tuberculosis vaccine is highly varied globally. Differential sensitization to environmental mycobacteria prior to BCG vaccination may prime immune effects leading to this variation, but the precise immune mechanisms and cell types involved in this phenomenon are unknown. We hypothesized that pre-vaccination sensitization to environmental mycobacteria induces mycobacterium-specific Tregs that suppress responses to BCG. This was investigated by testing Treg responses following priming of BALB/c mice by i.p. immunization with heat-killed CHE. Such mice produced higher levels of IL-10 before and after intranasal, live BCG administration and had fewer lung inflammatory cells post-BCG, relative to nonsensitized mice. In CHE-sensitized mice, the percentage of splenic CD4+CD25+ cells expressing Foxp3 amongst total lymphocytes was not elevated significantly, but these cells limited nonspecific proliferation of CD4+CD25⁻ effector cells upon coculture and promoted higher expression levels of CD103 and Foxp3 in response to BCG antigen stimulation than CD4+CD25+ cells from nonsensitized mice. In adoptive transfer experiments, naïve, WT mice receiving CD4+CD25+ cells from CHE-sensitized mice and then given live BCG intranasally had significantly elevated lung IL-10 levels, reduced frequencies of lung IL-2-producing cells, and lower lymphocyte numbers in the BAL. Therefore, CHE sensitization induced CD4+CD25+ Tregs with functional, suppressive activity on BCG responses in vitro and in vivo. Treg induction could therefore be one mechanism underlying how environmental mycobacteria priming modulates host responses to the BCG vaccine.

Glucuronoxylomannan promotes the generation of antigen-specific T regulatory cell that suppresses the antigen-specific Th2 response upon activation

T regulatory cells (Treg) have the capability to suppress the skewed immune response, but the generation of antigen (Ag)-specific Treg for therapeutic purpose is a challenge; the mechanism of Ag-specific Treg activation remains obscure. Here, we report that glucuronoxylomannan (GXM) is capable of promoting the development of human tolerogenic dendritic cells (DC). GXM-pulsed DCs increased the expression of forkhead box P3 (Foxp3) in naïve human CD4(+)CD25(-) T cells via activating Fc gamma receptor IIb and activator protein-1 and promoting the expression of transforming growth factor beta in dendritic cells. Furthermore, the conjugated complex of house dust mite Ag, Dermatophagoides pteronyssinus (Der p) 1, and GXM-pulsed DCs to drive the naïve human CD4(+)CD25(-) T cells to develop into the Der p 1-specific Tregs, which efficiently suppressed the Ag-specific Th2 responses. We conclude that GXM-conjugated specific Ag have the capacity to up-regulate the tolerogenic property of DCs and promote the generation of Ag-specific Tregs; the latter can be activated upon the re-exposure to specific Ag and suppress the skewed Ag-specific T helper (Th)2 responses.

[Role of Th17 cells in rheumatoid arthritis]

Th17 cells are the newly described subset of the CD4(+) T lymphocytes. Activated Th17 cells are characterized by their ability to produce IL-17A and other pro-inflammatory cytokines. IL-17A regulates immune function through its cell-surface receptor expressed on epithelial-and endothelial cells, fibroblasts and leukocytes by promoting neutrophil recruitment and releasing further pro-inflammatory mediators. Failures of the susceptible balance of the immunoregulation may lead to unchecked immune response and autoimmune diseases. The central role of Th17 cells and cytokines produced by Th17 cells were confirmed in a wide variety of human autoimmune diseases, including rheumatoid arthritis. Recently Th17 cells and its cytokines come into the focus of immunological research as potential therapeutic targets.

The imbalance between regulatory and IL-17-secreting CD4+ T cells in lupus patients

It has been well recognized that a deficit of numbers and function of CD4+CD25+Foxp3+ cells (Treg) is attributed to the development of some autoimmune diseases; however, there are controversial data regarding the suppressive effect of Treg cells on the T cell response in systemic lupus erythematosus (SLE). Additionally, IL-17-producing cells (Th17) have been recently emerged as a new pathogenic cell, but their role in lupus remains unclear. In this study, we studied the connection between Treg and Th17 cells in lupus patients. We observed that, while Treg or Th17 cells alone were not correlated to SLE development, the ratio of Treg to Th17 cells in active SLE patients is significantly lower than that in inactive SLE patients and healthy controls, and we also found corticosteroid treatment increased the ratio of Treg to Th17 cells in active SLE patients. Moreover, this ratio is inversely correlated with the severity of active SLE. The present study indicates that active SLE appears to exist as an imbalance between Treg and Th17 cells. Correction of this Treg/Th17 imbalance may have therapeutic impact for patients with SLE.

Distinct gene signature revealed in white blood cells, CD4(+) and CD8(+) T cells in (NZBx NZW) F1 lupus mice after tolerization with anti-DNA Ig peptide

Tolerizing mice polygenically predisposed to lupus-like disease (NZB/NZW F1 females) with a peptide mimicking anti-DNA IgG sequences containing MHC class I and class II T cell determinants (pConsensus, pCons) results in protection from full-blown disease attributable in part to the induction of CD4(+)CD25(+)Foxp3+ and CD8(+)Foxp3+ regulatory T cells. We compared 45 000 murine genes in total white blood cells (WBC), CD4(+) T cells, and CD8(+) T cells from splenocytes of (NZBxNZW) F1 lupus-prone mice tolerized with pCons vs untreated naïve mice and found two-fold or greater differential expression for 448 WBC, 174 CD4, and 60 CD8 genes. We identified differentially expressed genes that played roles in the immune response and apoptosis. Using real-time PCR, we validated differential expression of selected genes (IFI202B, Bcl2, Foxp3, Trp-53, CCR7 and IFNar1) in the CD8(+)T cell microarray and determined expression of selected highly upregulated genes in different immune cell subsets. We also determined Smads expression in different immune cell subsets, including CD4(+) T cells and CD8(+) T cells, to detect the effects of TGF-beta, known to be the major cytokine that accounts for the suppressive capacity of CD8(+) Treg in this system. Silencing of anti-apoptotic gene Bcl2 or interferon genes (IFI202b and IFNar1 in combination) in CD8(+) T cells from tolerized mice did not affect the expression of the other selected genes. However, silencing of Foxp3 reduced expression of Foxp3, Ifi202b and PD1-all of which are involved in the suppressive capacity of CD8(+) Treg in this model.

"Default" generation of neonatal regulatory T cells

CD4(+)Foxp3(+) regulatory T (Treg) cells were shown to control all aspects of immune responses. How these Treg cells develop is not fully defined, especially in neonates during development of the immune system. We studied the induction of Treg cells from neonatal T cells with various TCR stimulatory conditions, because TCR stimulation is required for Treg cell generation. Independent of the types of TCR stimulus and without the addition of exogenous TGF-beta, up to 70% of neonatal CD4(+)Foxp3(-) T cells became CD4(+)Foxp3(+) Treg cells, whereas generally <10% of adult CD4(+)Foxp3(-) T cells became CD4(+)Foxp3(+) Treg cells under the same conditions. These neonatal Treg cells exert suppressive function and display relatively stable Foxp3 expression. Importantly, this ability of Treg cell generation gradually diminishes within 2 wk of birth. Consistent with in vitro findings, the in vivo i.p. injection of anti-CD3 mAb to stimulate T cells also resulted in a >3-fold increase in Treg cells in neonates but not in adults. Furthermore, neonatal or adult Foxp3(-) T cells were adoptively transferred into Rag1(-/-) mice. Twelve days later, the frequency of CD4(+)Foxp3(+) T cells converted from neonatal cells was 6-fold higher than that converted from adult cells. Taken together, neonatal CD4(+) T cells have an intrinsic "default" mechanism to become Treg cells in response to TCR stimulations. This finding provides intriguing implications about neonatal immunity, Treg cell generation, and tolerance establishment early in life.

Dendritic cell-directed CTLA-4 engagement during pancreatic beta cell antigen presentation delays type 1 diabetes

The levels of expression of alternatively spliced variants of CTLA-4 and insufficient CTLA-4 signaling have been implicated in type 1 diabetes. Hence, we hypothesized that increasing CTLA-4-specific ligand strength on autoantigen-presenting dendritic cells (DCs) can enhance ligation of CTLA-4 on T cells and lead to modulation of autoreactive T cell response. In this study, we show that DC-directed enhanced CTLA-4 engagement upon pancreatic beta cell Ag presentation results in the suppression of autoreactive T cell response in NOD mice. The T cells from prediabetic NOD mice treated with an agonistic anti-CTLA-4 Ab-coated DC (anti-CTLA-4-Ab DC) showed significantly less proliferative response and enhanced IL-10 and TGF-beta1 production upon exposure to beta cell Ags. Furthermore, these mice showed increased frequency of Foxp3+ and IL-10+ T cells, less severe insulitis, and a significant delay in the onset of hyperglycemia compared with mice treated with control Ab-coated DCs. Further analyses showed that diabetogenic T cell function was modulated primarily through the induction of Foxp3 and IL-10 expression upon Ag presentation by anti-CTLA-4-Ab DCs. The induction of Foxp3 and IL-10 expression appeared to be a consequence of increased TGF-beta1 production by T cells activated using anti-CTLA-4-Ab DCs, and this effect could be enhanced by the addition of exogenous IL-2 or TGF-beta1. Collectively, this study demonstrates the potential of a DC-directed CTLA-4 engagement approach not only in treating autoimmunity in type 1 diabetes, but also in altering diabetogenic T cell function ex vivo for therapy.

TGF-beta-induced myelin peptide-specific regulatory T cells mediate antigen-specific suppression of induction of experimental autoimmune encephalomyelitis

The low number of natural regulatory T cells (nTregs) in the circulation specific for a particular Ag and concerns about the bystander suppressive capacity of expanded nTregs presents a major clinical challenge for nTreg-based therapeutic treatment of autoimmune diseases. In the current study, we demonstrate that naive CD4+CD25-Foxp3- T cells specific for the myelin proteolipid protein (PLP)139-151 peptide can be converted into CD25+Foxp3+ induced Treg cells (iTregs) when stimulated in the presence of TGF-beta, retinoic acid, and IL-2. These PLP139-151-specific iTregs (139-iTregs) have a phenotype similar to nTregs, but additionally express an intermediate level of CD62L and a high level of CD103. Upon transfer into SJL/J mice, 139-iTregs undergo Ag-driven proliferation and are effective at suppressing induction of experimental autoimmune encephalomyelitis induced by the cognate PLP139-151 peptide, but not PLP178-191 or a mixture of the two peptides. Furthermore, 139-iTregs inhibit delayed-type hypersensitivity responses to PLP139-151, but not PLP178-191, myelin oligodendrocyte glycoprotein (MOG)35-55, or OVA323-339 in mice primed with a mixture of PLP139-151 and the other respective peptides. Additionally, 139-iTregs suppress the proliferation and activation of PLP139-151-, but not MOG35-55-specific CD4+ T cells in SJL/B6 F1 mice primed with a combination of PLP139-151 and MOG35-55. These findings suggest that Ag-specific iTregs are amplified in vivo when exposed to cognate Ag under inflammatory conditions, and these activated iTregs suppress CD4+ responder T cells in an Ag-specific manner.

Molecular mechanisms regulating TGF-beta-induced Foxp3 expression

Molecular mechanisms regulating transforming growth factor-beta (TGF-beta) induction of Foxp3 (forkhead box P3) expression and thus induction of induced regulatory T cells (Tregs) have been the focus of a great deal of study in recent years. It has become clear that this process is influenced by a number of factors as perhaps might be predicted by the fact that there is an overarching need of the immune system to finetune response to environmental antigens. In this review we discuss these mechanisms, with the aim of presenting a broad picture of how the various observations fit together to form an integrated regulatory regime.

Isolation of purified and live Foxp3+ regulatory T cells using FACS sorting on scatter plot

There are no ideal ways to identify and isolate viable and purified Foxp3(+) regulatory T cells so far. Here we developed a novel procedure for the isolation of highly purified Foxp3(+) cells using flow cytometry. This method relies on an identification and sorting of the lymphoblast cell population identified on a scatter plot using flow cytometry. We confirmed that greater than 98% of the cells sorted using this technique expressed Foxp3 and displayed a potent suppressive activity. This method provides a valuable tool for the study of the T regulatory cell biology and their therapeutic manipulation.

Role of SMAD and non-SMAD signals in the development of Th17 and regulatory T cells

Whereas TGF-beta is essential for the development of peripherally induced Foxp3(+) regulatory T cells (iTreg cells) and Th17 cells, the intracellular signaling mechanism by which TGF-beta regulates development of both cell subsets is less understood. In this study, we report that neither Smad2 nor Smad3 gene deficiency abrogates TGF-beta-dependent iTreg induction by a deacetylase inhibitor trichostatin A in vivo, although the loss of the Smad2 or Smad3 gene partially reduces iTreg induction in vitro. Similarly, SMAD2 and SMAD3 have a redundant role in development of Th17 in vitro and in experimental autoimmune encephalomyelitis. In addition, ERK and/or JNK pathways were shown to be involved in regulating iTreg cells, whereas the p38 pathway predominately modulated Th17 and experimental autoimmune encephalomyelitis induction. Therefore, selective targeting of these intracellular TGF-beta signaling pathways during iTreg and Th17 cell development might lead to the development of therapies in treating autoimmune and other chronic inflammatory diseases.

A role for the transcription factor Helios in human CD4(+)CD25(+) regulatory T cells

Relative upregulation of the Ikaros family transcription factor Helios in natural regulatory T cells (Tregs) has been reported by several groups. However, a role for Helios in regulatory T cells has not yet been described. Here, we show that Helios is upregulated in CD4(+)CD25(+) regulatory T cells. Chromatin-immunoprecipitation (ChIP) experiments indicated that Helios binds to the FoxP3 promoter. These data were further corroborated by experiments showing that knocking-down Helios with siRNA oligonucleotides results in down-regulation of FoxP3. Functionally, we found that suppression of Helios message in CD4(+)CD25(+) T cells significantly attenuates their suppressive function. Taken together, these data suggest that Helios may play an important role in regulatory T cell function and support the concept that Helios may be a novel target to manipulate Treg activity in a clinical setting.

Defects in IL-2R signaling contribute to diminished maintenance of FOXP3 expression in CD4(+)CD25(+) regulatory T-cells of type 1 diabetic subjects

OBJECTIVE

In humans, multiple genes in the interleukin (IL)-2/IL-2 receptor (IL-2R) pathway are associated with type 1 diabetes. However, no link between IL-2 responsiveness and CD4(+)CD25(+)FOXP3(+) regulatory T-cells (Tregs) has been demonstrated in type 1 diabetic subjects despite the role of these IL-2-dependent cells in controlling autoimmunity. Here, we address whether altered IL-2 responsiveness impacts persistence of FOXP3 expression in Tregs of type 1 diabetic subjects.

RESEARCH DESIGN AND METHODS

Persistence of Tregs was assessed by culturing sorted CD4(+)CD25(hi) natural Tregs with IL-2 and measuring FOXP3 expression over time by flow cytometry for control and type 1 diabetic populations. The effects of IL-2 on FOXP3 induction were assessed 48 h after activation of CD4(+)CD25(-) T-cells with anti-CD3 antibody. Cytokine receptor expression and signaling upon exposure to IL-2, IL-7, and IL-15 were determined by flow cytometry and Western blot analysis.

RESULTS

Maintenance of FOXP3 expression in CD4(+)CD25(+) Tregs of type 1 diabetic subjects was diminished in the presence of IL-2, but not IL-7. Impaired responsiveness was not linked to altered expression of the IL-2R complex. Instead, IL-2R signaling was reduced in Tregs and total CD4(+) T-cells of type 1 diabetic subjects. In some individuals, decreased signal transducer and activator of transcription 5 phosphorylation correlated with significantly higher expression of protein tyrosine phosphatase N2, a negative regulator of IL-2R signaling.

CONCLUSIONS

Aberrant IL-2R signaling in CD4(+) T-cells of type 1 diabetic subjects contributes to decreased persistence of FOXP3 expression that may impact establishment of tolerance. These findings suggest novel targets for treatment of type 1 diabetes within the IL-2R pathway and suggest that an altered IL-2R signaling signature may be a biomarker for type 1 diabetes.

Harnessing regulatory T cells to suppress asthma: from potential to therapy

Regulatory T cells (Tregs) play an essential role in maintaining the homeostatic balance of immune responses. Asthma is an inflammatory condition of the airways that is driven by dysregulated immune responses toward normally innocuous antigens. Individuals with asthma have fewer and less functional Tregs, which may lead to uncontrolled effector cell responses and promote proasthmatic responses of T helper type 2, T helper 17, natural killer T, antigen-presenting, and B cells. Tregs have the capacity to either directly or indirectly suppress these responses. Hence, the induced expansion of functional Tregs in predisposed or individuals with asthma is a potential approach for the prevention and treatment of asthma. Infection by a number of micro-organisms has been associated with reduced prevalence of asthma, and many infectious agents have been shown to induce Tregs and reduce allergic airways disease in mouse models. The translation of the regulatory and therapeutic properties of infectious agents for use in asthma requires the identification of key modulatory components and the development and trial of effective immunoregulatory therapies. Further translational and clinical research is required for the induction of Tregs to be harnessed as a therapeutic strategy for asthma.

Synergistic effect of TGF-beta superfamily members on the induction of Foxp3+ Treg

TGF-beta plays an important role in the induction of Treg and maintenance of immunologic tolerance, but whether other members of TGF-beta superfamily act together or independently to achieve this effect is poorly understood. Although others have reported that the bone morphogenetic proteins (BMP) and TGF-beta have similar effects on the development of thymocytes and T cells, in this study, we report that members of the BMP family, BMP-2 and -4, are unable to induce non-regulatory T cells to become Foxp3+ Treg. Neutralization studies with Noggin have revealed that BMP-2/4 and the BMP receptor signaling pathway is not required for TGF-beta to induce naïve CD4+CD25- cells to express Foxp3; however, BMP-2/4 and TGF-beta have a synergistic effect on the induction of Foxp3+ Treg. BMP-2/4 affects non-Smad signaling molecules including phosphorylated ERK and JNK, which could subsequently promote the differentiation of Foxp3+ Treg induced by TGF-beta. Data further advocate that TGF-beta is a key signaling factor for Foxp3+ Treg development. In addition, the synergistic effect of BMP-2/4 and TGF-beta indicates that the simultaneous manipulation of TGF-beta and BMP signaling might have considerable effects in the clinical setting for the enhancement of Treg purity and yield.

Ex vivo generation of regulatory T cells: characterization and therapeutic evaluation in a model of chronic colitis

Naturally occurring regulatory T cells (nTregs; CD4(+)CD25(+)Foxp3(+)) are capable of suppressing the chronic inflammation observed in a variety of different animal models of autoimmune and chronic inflammatory diseases such as inflammatory bowel diseases, diabetes, and arthritis. A major limitation in exploring how and where nTregs exert their suppression in vivo is the relative paucity of these regulatory cells. Although several laboratories have described different methods to expand flow-purified nTregs or convert conventional/naïve T cells (CD4(+)Foxp3(-)) to Foxp3-expressing "induced" Tregs (iTregs; CD4(+)Foxp3(+)) ex vivo, we have found that many of these approaches are encumbered with their own limitations. Therefore, we sought to develop a relatively simple ex vivo method to generate large numbers of Foxp3-expressing iTregs that can be used to evaluate their trafficking properties, suppressive activity, and therapeutic efficacy in a mouse model of chronic gut inflammation in vivo. We present a detailed protocol demonstrating that polyclonal activation of conventional CD4(+) T cells in the presence of IL-2, TGFβ, and all trans retinoic acid induces >90% conversion of these T cells to Foxp3-expressing iTregs as well as promotes a three- to fourfold increase in proliferation following a 4-day incubation period in vitro. This protocol enhances modestly the surface expression of the gut-homing adhesion molecule CCR9 but not α(4)β(7). Furthermore, we provide preliminary data demonstrating that these iTregs are significantly more potent at suppressing T-cell activation in vitro and are equally effective as freshly isolated nTregs at attenuating chronic colitis in vivo. Finally, we report that this protocol has the potential to generate 30-40 million iTregs from one healthy mouse spleen.

1,25-Dihydroxyvitamin D3 and IL-2 combine to inhibit T cell production of inflammatory cytokines and promote development of regulatory T cells expressing CTLA-4 and FoxP3

The active form of vitamin D, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), has potent immunomodulatory properties that have promoted its potential use in the prevention and treatment of infectious disease and autoimmune conditions. A variety of immune cells, including macrophages, dendritic cells, and activated T cells express the intracellular vitamin D receptor and are responsive to 1,25(OH)(2)D(3.) Despite this, how 1,25(OH)(2)D(3) regulates adaptive immunity remains unclear and may involve both direct and indirect effects on the proliferation and function of T cells. To further clarify this issue, we have assessed the effects of 1,25(OH)(2)D(3) on human CD4(+)CD25(-) T cells. We observed that stimulation of CD4(+)CD25(-) T cells in the presence of 1,25(OH)(2)D(3) inhibited production of proinflammatory cytokines including IFN- gamma, IL-17, and IL-21 but did not substantially affect T cell division. In contrast to its inhibitory effects on inflammatory cytokines, 1,25(OH)(2)D(3) stimulated expression of high levels of CTLA-4 as well as FoxP3, the latter requiring the presence of IL-2. T cells treated with 1,25(OH)(2)D(3) could suppress proliferation of normally responsive T cells, indicating that they possessed characteristics of adaptive regulatory T cells. Our results suggest that 1,25(OH)(2)D(3) and IL-2 have direct synergistic effects on activated T cells, acting as potent anti-inflammatory agents and physiologic inducers of adaptive regulatory T cells.

Mechanistic insights into immunomodulation by hepatic stellate cells in mice: a critical role of interferon-gamma signaling

The liver is considered to be an immune-privileged organ that favors the induction of tolerance. The underlying mechanisms are not completely understood. Interestingly, liver transplants are spontaneously accepted in several animal models, but hepatocyte transplants are acutely rejected, suggesting that liver nonparenchymal cells may effectively protect the parenchymal cells from immune attack. We have shown the profound T cell inhibitory activity of hepatic stellate cells (HSCs). Thus, cotransplantation with HSCs effectively protects islet allografts from rejection in mice. In this study, using T cell receptor transgenic and gene knockout approaches, we provided definitive evidence that HSCs protected cotransplanted islet allografts by exerting comprehensive inhibitory effects on T cells, including apoptotic death in graft-infiltrating antigen-specific effector T cells and marked expansion of CD4(+) Forkhead box protein (Foxp)3(+) T regulatory (Treg) cells. All these effects required an intact interferon-gamma (IFN-gamma) signaling in HSCs, demonstrated by using HSCs isolated from IFN-gamma receptor 1 knockout mice. B7-H1 expression on HSCs, a product molecule of IFN-gamma signaling, was responsible for induction of T cells apoptosis, but had no effect on expansion of Treg cells, suggesting that undetermined effector molecules produced by IFN-gamma signaling is involved in this process.

CONCLUSION

Upon inflammatory stimulation, specific organ stromal cells (such as HSCs in the liver) demonstrate potent immune regulatory activity. Understanding of the mechanisms involved may lead to development of novel strategies for clinical applications in transplantation and autoimmune diseases.

Induction of granzyme B expression in T-cell receptor/CD28-stimulated human regulatory T cells is suppressed by inhibitors of the PI3K-mTOR pathway

Background

Regulatory T cells (Tregs) can employ a cell contact- and granzyme B-dependent mechanism to mediate suppression of bystander T and B cells. Murine studies indicate that granzyme B is involved in the Treg-mediated suppression of anti-tumor immunity in the tumor microenvironment and in the Treg-mediated maintenance of allograft survival. In spite of its central importance, a detailed study of granzyme B expression patterns in human Tregs has not been performed.

Results

Our data demonstrated that natural Tregs freshly isolated from the peripheral blood of normal adults lacked granzyme B expression. Tregs subjected to prolonged TCR and CD28 triggering, in the presence of IL-2, expressed high levels of granzyme B but CD3 stimulation alone or IL-2 treatment alone failed to induce granzyme B. Treatment of Tregs with the mammalian target of rapamycin (mTOR) inhibitor, rapamycin or the PI3 kinase (PI3K) inhibitor LY294002 markedly suppressed granzyme B expression. However, neither rapamycin, as previously reported by others, nor LY294002 inhibited Treg proliferation or induced significant cell death in TCR/CD28/IL-2 stimulated cells. The proliferation rate of Tregs was markedly higher than that of CD4+ conventional T cells in the setting of rapamycin treatment. Tregs expanded by CD3/CD28/IL-2 stimulation without rapamycin demonstrated increased in vitro cytotoxic activity compared to Tregs expanded in the presence of rapamycin in both short term (6 hours) and long term (48 hours) cytotoxicity assays.

Conclusion

TCR/CD28 mediated activation of the PI3K-mTOR pathway is important for granyzme B expression but not proliferation in regulatory T cells. These findings may indicate that suppressive mechanisms other than granzyme B are utilized by rapamycin-expanded Tregs.

Cytokine levels correlate with immune cell infiltration after anti-VEGF therapy in preclinical mouse models of breast cancer

The effect of blocking VEGF activity in solid tumors extends beyond inhibition of angiogenesis. However, no studies have compared the effectiveness of mechanistically different anti-VEGF inhibitors with respect to changes in tumor growth and alterations in the tumor microenvironment. In this study we use three distinct breast cancer models, a MDA-MB-231 xenograft model, a 4T1 syngenic model, and a transgenic model using MMTV-PyMT mice, to explore the effects of various anti-VEGF therapies on tumor vasculature, immune cell infiltration, and cytokine levels. Tumor vasculature and immune cell infiltration were evaluated using immunohistochemistry. Cytokine levels were evaluated using ELISA and electrochemiluminescence. We found that blocking the activation of VEGF receptor resulted in changes in intra-tumoral cytokine levels, specifically IL-1β, IL-6 and CXCL1. Modulation of the level these cytokines is important for controlling immune cell infiltration and ultimately tumor growth. Furthermore, we demonstrate that selective inhibition of VEGF binding to VEGFR2 with r84 is more effective at controlling tumor growth and inhibiting the infiltration of suppressive immune cells (MDSC, Treg, macrophages) while increasing the mature dendritic cell fraction than other anti-VEGF strategies. In addition, we found that changes in serum IL-1β and IL-6 levels correlated with response to therapy, identifying two possible biomarkers for assessing the effectiveness of anti-VEGF therapy in breast cancer patients.

Cytokine levels correlate with immune cell infiltration after anti-VEGF therapy in preclinical mouse models of breast cancer

The effect of blocking VEGF activity in solid tumors extends beyond inhibition of angiogenesis. However, no studies have compared the effectiveness of mechanistically different anti-VEGF inhibitors with respect to changes in tumor growth and alterations in the tumor microenvironment. In this study we use three distinct breast cancer models, a MDA-MB-231 xenograft model, a 4T1 syngenic model, and a transgenic model using MMTV-PyMT mice, to explore the effects of various anti-VEGF therapies on tumor vasculature, immune cell infiltration, and cytokine levels. Tumor vasculature and immune cell infiltration were evaluated using immunohistochemistry. Cytokine levels were evaluated using ELISA and electrochemiluminescence. We found that blocking the activation of VEGF receptor resulted in changes in intra-tumoral cytokine levels, specifically IL-1beta, IL-6 and CXCL1. Modulation of the level these cytokines is important for controlling immune cell infiltration and ultimately tumor growth. Furthermore, we demonstrate that selective inhibition of VEGF binding to VEGFR2 with r84 is more effective at controlling tumor growth and inhibiting the infiltration of suppressive immune cells (MDSC, Treg, macrophages) while increasing the mature dendritic cell fraction than other anti-VEGF strategies. In addition, we found that changes in serum IL-1beta and IL-6 levels correlated with response to therapy, identifying two possible biomarkers for assessing the effectiveness of anti-VEGF therapy in breast cancer patients.

The regulatory function of umbilical cord blood CD4(+) CD25(+) T cells stimulated with anti-CD3/anti-CD28 and exogenous interleukin (IL)-2 or IL-15

The abundance of CD4(+) CD25(+) regulatory T cells in umbilical cord blood (UCB) might contribute to the decreased severity of graft-vs.-host disease (GVHD) for UCB transplantation. This study aims to characterize the phenotypes and suppressive function of UCB CD4(+) CD25(+) T cells under the influence of anti-CD3/anti-CD28 (CD3/CD28) and exogenous interleukin (IL)-2 or IL-15. Higher percentages of CD4(+) CD25(high) and FoxP3(+) cells were detected in UCB compared to their adult counterparts. IL-15 was as effective as IL-2 in enhancing the proliferation of CD3/CD28 stimulated UCB CD4(+) CD25(+) T cells. Phenotypically, IL-2/IL-15-stimulated UCB CD4(+) CD25(+) T cells expressed higher level of CTLA-4, GITR, membrane bound transforming growth factor-beta (mTGF-beta), and especially Foxp-3 than controls. IL-2/IL-15-stimulated UCB CD4(+) CD25(+) T cells also produced much higher IL-10 and TGF-beta than controls; while IL-2/IL-15-stimulated UCB CD4(+) CD25(-) T cells showed increased TGF-beta, but not IL-10 production. IL-2/IL-15-cultured UCB CD4(+) CD25(+) T cells showed comparable suppressor activity on allogeneic adult CD4(+) T-cell proliferation compared to controls, partly through a contact-dependent fashion. Taken together, IL-2/IL-15-stimulated UCB CD4(+) CD25(+) T cells show distinct regulatory T-cell phenotypic and functional features, and may be applied for the alleviation of GVHD severity following UCB transplantation.

Regulatory and pro-inflammatory phenotypes of myelin basic protein-autoreactive T cells in multiple sclerosis

MBP-specific autoreactive T cells are considered pro-inflammatory T cells and thought to play an important role in the pathogenesis of multiple sclerosis (MS). Here, we report that MBP(83-99)-specific T cells generated from MS patients (n = 7) were comprised of pro-inflammatory and regulatory subsets of distinct phenotypes. The pro-inflammatory phenotype was characterized by high production of IFN-gamma, IL-6, IL-21 and IL-17 and low expression of FOXP3, whereas the regulatory subset expressed high levels of FOXP3 and exhibited potent regulatory functions. The regulatory subset of MBP-specific T cells appeared to expand from the CD4(+)CD25(-) T-cell pool. Their FOXP3 expression was stable, independent of the activation state and it correlated with suppressive function and inversely with the production of IFN-gamma, IL-6, IL-21 and IL-17. In contrast, the phenotype and function of FOXP3(low) MBP-specific T cells were adaptive and dependent on IL-6. The higher frequency of FOXP3(high) MBP-specific T cells was observed when IL-6 was neutralized in the culture of PBMC with MBP. The study provides new evidence that MBP-specific T cells are susceptible to pro-inflammatory cytokine milieu and act as either pro-inflammatory or regulatory T cells.

Signal transducer and activator of transcription 4 limits the development of adaptive regulatory T cells

T-cell responses to a cytokine milieu instruct the development of multiple effector phenotypes. While transforming growth factor-beta(1) (TGF-beta(1)) inhibits the development of T helper type 1 (Th1) and Th2 cells, we demonstrate that like interleukin-6 (IL-6) and IL-4, IL-12 can inhibit the development of TGF-beta(1)-induced Foxp3-expressing adaptive T regulatory (aTreg) cells. Signal transducer and activator of transcription 4 (STAT4) is critical for the response to IL-12, although there is a parallel pathway involving T box expressed in T cells (T-bet), and cells from mice double-deficient in STAT4 and T-bet are refractory to the inhibition of aTreg-cell development by IL-12. While the ability of these cytokines to promote Th differentiation may contribute to this effect, we observe that culture with IL-12, or other instructive cytokines, results in an increase in repressive chromatin modifications at the Foxp3 locus that limit STAT5 binding to Foxp3, without observed effects on IL-2 signalling pathways. In a model of allergic lung inflammation there are increased percentages of Treg cells in the lungs of Stat4(-/-) mice, compared with wild-type mice, and increases in Treg cells correlate with decreased allergic inflammation. Overall, these results suggest an important role for STAT4 in regulating Treg-cell development.

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.

The role of NF-kappaB and Smad3 in TGF-beta-mediated Foxp3 expression

The transcription factor Foxp3 is essential for the development of functional, natural Treg (nTreg), which plays a prominent role in self-tolerance. Suppressive Foxp3(+) Treg cells can be generated from naïve T cells ex vivo, following TCR and TGF-beta1 stimulations. However, the molecular contributions from the different arms of these pathways leading to Foxp3 expression are not fully understood. TGF-beta1-activated Smad3 plays a major role in the expression of Foxp3, since TGF-beta1-induced-Treg generation from Smad3(-/-) mice is markedly reduced and abolished by inactivating Smad2. In the TCR pathway, deletion of Bcl10, which activates NF-kappaB, markedly reduces both IL-2 and Foxp3 production. However, partial rescue of Foxp3 expression occurs on addition of exogenous IL-2. TGF-beta1 significantly attenuates NF-kappaB binding to the Foxp3 promoter, while inducing Foxp3 expression. Furthermore, deletion of p50, a NF-kappaB subunit, results in increased Foxp3 expression despite a decline in the IL-2 production. We posit several TCR-NF-kappaB pathways, some increasing (Bcl10-IL-2-Foxp3) while others decreasing (p50-Foxp3) Foxp3 expression, with the former predominating. A better understanding of Foxp3 regulation could be useful in dissecting the cause of Treg dysfunction in several autoimmune diseases and for generating more potent TGF-beta1-induced-Treg cells for therapeutic purposes.

Regulatory T cells negatively regulate neovasculature of airway remodeling via DLL4-Notch signaling

Regulatory T cells (Treg) have been shown to prevent the development of allergic asthma; however, the role of Treg in asthma with established airway remodeling is unknown. To address this, we exploited an OVA-induced chronic asthma mouse model wherein Treg were adoptively transferred to the mice at chronic stage of the model. We found that among the structural alterations of airway remodeling, Treg selectively reduced the vessel numbers in both peritracheal and peribronchial regions and the lung parenchyma. Extracellular matrix deposition, mucus metaplasia, muscular hyperplasia, and vasodilation, as were also induced by chronic allergen challenge, were not affected by Treg. TUNEL staining of the lung sections revealed an increased endothelial cell (EC) apoptosis in mice receiving Treg transfers compared with their asthmatic counterparts. By using Matrigel angiogenesis assays, we showed that Treg inhibited EC angiogenesis both in vitro and in vivo. Treg preferentially expressed Notch ligand DLL4, and an anti-DLL4 blocking Ab abrogated the inhibitory effect of Treg on EC tube formation. In vivo, decreased airway and lung vessel numbers as well as ameliorated airway hyperresponsiveness after Treg transfers were reverted when Treg-derived DLL4 signal was blocked by the anti-DLL4 Ab. Our findings demonstrate a novel function of Treg whereby Treg down-regulate remodeling angiogenesis via proapoptotic DLL4-Notch signaling, and suggest a therapeutic potential of Treg in alleviating airway hyperresponsiveness of chronic asthma.

Disruption of TGF-beta signaling prevents the generation of tumor-sensitized regulatory T cells and facilitates therapeutic antitumor immunity

Regulatory T (Treg) cells represent a major roadblock to the induction of antitumor immunity through vaccine approaches. TGF-beta is a cytokine implicated in the generation and maintenance of Treg cells, as well as in their suppressive function. These experiments examined whether the generation of tumor-sensitized Treg cells was TGF-beta dependent and evaluated whether TGF-beta produced by Treg cells blocked the priming of tumor-specific T cells in vaccinated reconstituted lymphopenic mice. We show that tumor-sensitized Treg cells (CD25(+)/FoxP3(+)) obtained from tumor-bearing mice block the generation of tumor-specific T cells in reconstituted lymphopenic mice. Strikingly, this suppression is absent if tumor-sensitized Treg cells are acquired from tumor-bearing mice expressing the dominant-negative TGFbetaRII in T cells. This loss of suppression was a result of the crucial role of TGF-beta in generating tumor-sensitized Treg cells, and not due to the insensitivity of naive or tumor-primed effector T cells to the direct suppressive influence of TGF-beta. We conclude that blocking TGF-beta in a tumor-bearing host can inhibit the induction of highly suppressive tumor-sensitized Treg cells. These data suggest that an integrative strategy combining "up-front" Treg cell ablation followed by vaccination and TGF-beta blockade may limit generation of new tumor-sensitized Treg cells and improve the generation of therapeutic immune responses in patients with cancer.

Plasmodium falciparum-mediated induction of human CD25Foxp3 CD4 T cells is independent of direct TCR stimulation and requires IL-2, IL-10 and TGFbeta

CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) regulate disease-associated immunity and excessive inflammatory responses, and numbers of CD4(+)CD25(+)Foxp3(+) Tregs are increased during malaria infection. The mechanisms governing their generation, however, remain to be elucidated. In this study we investigated the role of commonly accepted factors for Foxp3 induction, TCR stimulation and cytokines such as IL-2, TGFbeta and IL-10, in the generation of human CD4(+)CD25(+)Foxp3(+) T cells by the malaria parasite Plasmodium falciparum. Using a co-culture system of malaria-infected red blood cells (iRBCs) and peripheral blood mononuclear cells from healthy individuals, we found that two populations of Foxp3(hi) and Foxp3(int) CD4(+)CD25(hi) T cells with a typical Treg phenotype (CTLA-4(+), CD127(low), CD39(+), ICOS(+), TNFRII(+)) were induced. Pro-inflammatory cytokine production was confined to the Foxp3(int) subset (IFNgamma, IL-4 and IL-17) and inversely correlated with high relative levels of Foxp3(hi) cells, consistent with Foxp3(hi) CD4 T cell-mediated inhibition of parasite-induced effector cytokine T cell responses. Both Foxp3(hi) and Foxp3(int) cells were derived primarily from proliferating CD4(+)CD25(-) T cells with a further significant contribution from CD25(+)Foxp3(+) natural Treg cells to the generation of the Foxp3(hi) subset. Generation of Foxp3(hi), but not Foxp3(int), cells specifically required TGFbeta1 and IL-10. Add-back experiments showed that monocytes expressing increased levels of co-stimulatory molecules were sufficient for iRBC-mediated induction of Foxp3 in CD4 T cells. Foxp3 induction was driven by IL-2 from CD4 T cells stimulated in an MHC class II-dependent manner. However, transwell separation experiments showed that direct contact of monocytes with the cells that acquire Foxp3 expression was not required. This novel TCR-independent and therefore antigen-non specific mechanism for by-stander CD4(+)CD25(hi)Foxp3(+) cell induction is likely to reflect a process also occurring in vivo as a consequence of immune activation during malaria infection, and potentially a range of other infectious diseases.

Anti-inflammatory and pro-inflammatory roles of TGF-beta, IL-10, and IL-22 in immunity and autoimmunity

Cytokines play a major role in maintaining lymphocyte homeostasis under both steady-state and inflammatory conditions. Unregulated lymphocytes in steady-state conditions can lead to autoimmunity, whereas during inflammation they can cause excessive tissue damage. Regulatory cytokines function in combination with other environmental signals to properly modulate the function and the extent of lymphocyte activation. Many recent studies have highlighted the importance of regulatory cytokines in controlling the differentiation and function of lymphocytes under steady-state and inflammatory conditions, as well as minimizing tissue damage.

TGF-beta and 'adaptive' Foxp3(+) regulatory T cells

In naïve T cells transforming growth factor-beta (TGF-beta) induces Foxp3, a transcription factor essential for programming and developing T regulatory cells (Treg cells). This finding reveals a physiological factor which can turn on the Foxp3 gene and establishes an experimental approach to induce antigen-specific Treg cells as a potential therapy for human diseases. While this role for TGF-beta is well confirmed, several critical questions remain largely unanswered and await further investigation. In this regard, it is imperative to understand the molecular pathways by which TGF-beta signaling initiates and regulates Foxp3 expression. It is also important to elucidate which factors and/or cytokines influence the TGF-beta-mediated conversion of naïve T cells and how to create an immunologically regulatory milieu to facilitate Treg cell generation in vivo. In this short article, we will highlight the key findings and recent progress in the field, discuss the molecular mechanisms underlying the TGF-beta-mediated induction of Foxp3, and attempt to outline the challenges ahead.

STAT6 activation confers upon T helper cells resistance to suppression by regulatory T cells

Recent studies have highlighted characteristics of T regulatory cells (Tregs) that underlie their suppressive function. However, mechanisms that override their suppressive function in the context of an adaptive immune response are not well understood. In the lungs of mice undergoing allergic inflammation, appreciable numbers of Tregs were identified that possessed suppressive function when assayed ex vivo. We investigated whether the Th2-promoting cytokine IL-4 played a permissive role that superseded Treg function, thereby allowing the development of allergic inflammation. IL-4 signaling via the IL-4Ralpha-STAT6 axis was required to maintain Foxp3 expression in Tregs and promote their proliferation. However, the results of both in vivo experiments involving adoptive transfer of Tregs into Ag-sensitized vs naive animals and in vitro suppression assays performed with or without exogenous IL-4 showed the ability of IL-4 to compromise Treg-mediated suppression. Use of retrovirally expressed, constitutively active STAT6 revealed that the underlying mechanism was not IL-4-mediated dysfunction of Tregs but involved the resistance of Th cells to Treg-mediated suppression that would permit the development of an adaptive immune response. Our data suggest that infectious tolerance, mediated by membrane-bound TGF-beta expressed by Tregs, is compromised by the competing effects of IL4-induced signaling in naive CD4(+) Th cells.

Inhibition of TGFbeta1 makes nonimmunogenic tumor cells effective for therapeutic vaccination

We have previously demonstrated that transplanted cells from the SW1 clone of the K1735 melanoma and the Ag104 sarcoma grow progressively in syngeneic C3H mice even after transfection to engage CD137, a procedure that increases the immunogenicity of many other tumors. We now show that SW1 and Ag104 cells produce high levels of transforming growth factor (TGF) beta1, and that they can induce an antitumor response if they are transfected with a nonreplicating lentivirus vector to "silence" the TGFbeta1 gene via short hairpin RNA. Importantly, vaccination with SW1 or Ag104 cells, which do not make TGFbeta1, is therapeutically efficacious against small wild type tumors, including SW1 micrometastases in the lung. An analogous approach may be applicable to human tumors that produce TGFbeta1 or other immunosuppressive molecules to improve the efficacy of tumor cell-based therapeutic vaccines.

Natural and adaptive foxp3+ regulatory T cells: more of the same or a division of labor?

Adaptive Foxp3(+)CD4(+) regulatory T (iTreg) cells develop outside the thymus under subimmunogenic antigen presentation, during chronic inflammation, and during normal homeostasis of the gut. iTreg cells are essential in mucosal immune tolerance and in the control of severe chronic allergic inflammation, and most likely are one of the main barriers to the eradication of tumors. The Foxp3(+) iTreg cell repertoire is drawn from naive conventional CD4(+) T cells, whereas natural Treg (nTreg) cells are selected by high-avidity interactions in the thymus. The full extent of differences and similarities between iTreg and nTreg cells is yet to be defined. We speculate that iTreg cell development is driven by the need to maintain a noninflammatory environment in the gut, to suppress immune responses to environmental and food allergens, and to decrease chronic inflammation, whereas nTreg cells prevent autoimmunity and raise the activation threshold for all immune responses.

The Nature of Increased Circulating CD4CD25Foxp3 T Cells in Patients with Systemic Lupus Erythematosus: A Novel Hypothesis

The forkhead family transcriptional factor (Foxp3) is an important lineage marker for regulatory T (Treg) cells. Foxp3 expression is primarily restricted to CD4+CD25+ cell population. Recently, an intriguing phenomenon is highlighted that there is a considerable amount of CD4⁺CD25^-Foxp3⁺ T cells present in the peripheral blood of patients with systemic lupus erythematosus (SLE). Up to now, it is still an open question as to the nature of this cell subset. Following an analyses of the available phenotypic characteristics of CD4⁺CD25^-Foxp3⁺ T cell subset along with some new findings in research of Treg in human SLE, we propose the hypothesis: the increased circulating CD4⁺CD25^-Foxp3⁺ T cells in patients with SLE may constitute a peripheral reservoir of CD4⁺CD25⁺ Foxp3⁺ Treg cells. Under the condition of autoimmune response reactivated, CD4⁺CD25^-Foxp3⁺ T cells could be recruited to expand the Treg pool upon CD25 regaining, for the effort to try to reverse a homeostatic imbalance shift to more aggressive expansion of autoreactive T cells and B cells. This hypothesis, if confirmed, would provide a new strategy for the treatment of SLE via the generation of therapeutic regulatory T cells.

More stories on Th17 cells

For more than two decades, immunologists have been using the so-called Th1/Th2 paradigm to explain most of the phenomena related to adaptive immunity. The Th1/Th2 paradigm implied the existence of two different, mutually regulated, CD4(+) T helper subsets: Th1 cells, driving cell-mediated immune responses involved in tissue damage and fighting infection against intracellular parasites; and Th2 cells that mediate IgE production and are particularly involved in eosinophilic inflammation, allergy and clearance of helminthic infections. A third member of the T helper set, IL-17-producing CD4(+) T cells, now called Th17 cells, was recently described as a distinct lineage that does not share developmental pathways with either Th1 or Th2 cells. The Th17 subset has been linked to autoimmune disorders, being able to produce IL-17, IL-17F and IL-21 among other inflammatory cytokines. Interestingly, it has been reported that there is not only a cross-regulation among Th1, Th2 and Th17 effector cells but there is also a dichotomy in the generation of Th17 and T regulatory cells. Therefore, Treg and Th17 effector cells arise in a mutually exclusive fashion, depending on whether they are activated in the presence of TGF-beta or TGF-beta plus inflammatory cytokines such as IL-6. This review will address the discovery of the Th17 cells, and recent progress on their development and regulation.

Regulatory T cells: key players in tolerance and autoimmunity

CD4 + CD25 + regulatory T (TR) lymphocytes are essential to the maintenance of immunologic tolerance in the host. The discovery of Foxp3 as a transcription factor essential to the differentiation of TR ushered in detailed studies of the molecular mechanisms of TR cell development, peripheral homeostasis, and effector functions. In humans, loss of function mutations in genes that regulate T-cell development and function have been associated with TR cell deficiency or dysfunction and syndromes of autoimmunity and immune dysregulation. Augmentation of TR cells by immunotherapy and pharmacologic agents is a promising strategy for the treatment of allergic and autoimmune diseases.

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.

A central role for induced regulatory T cells in tolerance induction in experimental colitis

In addition to thymus-derived or natural T regulatory (nT(reg)) cells, a second subset of induced T regulatory (iT(reg)) cells arises de novo from conventional CD4(+) T cells in the periphery. The function of iT(reg) cells in tolerance was examined in a CD45RB(high)CD4(+) T cell transfer model of colitis. In situ-generated iT(reg) cells were similar to nT(reg) cells in their capacity to suppress T cell proliferation in vitro and their absence in vivo accelerated bowel disease. Treatment with nT(reg) cells resolved the colitis, but only when iT(reg) cells were also present. Although iT(reg) cells required Foxp3 for suppressive activity and phenotypic stability, their gene expression profile was distinct from the established nT(reg) "genetic signature," indicative of developmental and possibly mechanistic differences. These results identified a functional role for iT(reg) cells in vivo and demonstrated that both iT(reg) and nT(reg) cells can act in concert to maintain tolerance.

Regulating the regulators: costimulatory signals control the homeostasis and function of regulatory T cells

SUMMARY

Costimulation is a concept that goes back to the early 1980s when Lafferty and others hypothesized that cell surface and soluble molecules must exist that are essential for initiating immune responses subsequent to antigen exposure. The explosion in this field of research ensued as over a dozen molecules have been identified to function as second signals following T-cell receptor engagement. By 1994, it seemed clear that the most prominent costimulatory pathway CD28 and functionally related costimulatory molecules, such as CD154, were the major drivers of a positive immune response. Then the immunology world turned upside down. CD28 knockout mice, which were, in most cases, immunodeficient, led to increased autoimmunity when bred into the non-obese diabetic background. Another CD28 family member, cytotoxic T-lymphocyte-associated protein 4, which was presumed to be a costimulatory molecule on activated T cells, turned out to be critical in downregulating immunity. These results, coupled with the vast suppressor cell literature which had been largely rebuked, suggested that the immune system was not poised for response but controlled in such a way that regulation was dominant. Over the last decade, we have learned that these costimulatory molecules play a key role in the now classical CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) that provide critical control of unwanted autoimmune responses. In this review, we discuss the connections between costimulation and Tregs that have changed the costimulation paradigm.

Cyclooxygenase-2 in mucosal DC mediates induction of regulatory T cells in the intestine through suppression of IL-4

Oral intake of protein leads to tolerance through the induction of regulatory T cells (Tr cells) in mesenteric lymph nodes (MLNs). Here we show that the inhibition of cyclooxygenase-2 (COX-2) in vivo suppressed oral tolerance and was associated with enhanced differentiation of interleukin (IL)-4-producing T cells and reduced Foxp3(+) Tr-cell differentiation in MLN. As a result, the functional suppressive capacity of these differentiated mucosal T cells was lost. IL-4 was causally related to loss of tolerance as treatment of mice with anti-IL-4 antibodies during COX-2 inhibition restored tolerance. Dendritic cells (DCs) in the MLN differentially expressed COX-2 and reductionist experiments revealed that selective inhibition of the enzyme in these cells inhibited Foxp3(+) Tr-cell differentiation in vitro. Importantly, the inhibition of COX-2 in MLN-DC caused increased GATA-3 expression and enhanced IL-4 release by T cells, which was directly related to impaired Tr-cell differentiation. These data provide crucial insights into the mechanisms driving de novo Tr-cell induction and tolerance in the intestine.