Altered activation of AKT is required for the suppressive function of human CD4+CD25+ T regulatory cells

Selective inhibition of regulatory T cells by targeting the PI3K-Akt pathway

Despite the strides that immunotherapy has made in mediating tumor regression, the clinical effects are often transient, and therefore more durable responses are still needed. The temporary nature of the therapy-induced immune response can be attributed to tumor immune evasion mechanisms, mainly the effect of suppressive immune cells and, in particular, regulatory T cells (Treg). Although the depletion of Tregs has been shown to be effective in enhancing immune responses, selective depletion of these suppressive cells without affecting other immune cells has not been very successful, and new agents are sought. We found that PI3K-Akt pathway inhibitors selectively inhibit Tregs with minimal effect on conventional T cells (Tconv). Our results clearly show selective in vitro inhibition of activation (as represented by a decrease in downstream signaling) and proliferation of Tregs in comparison with Tconvs when treated with different Akt and PI3K inhibitors. This effect has been observed in both human and murine CD4 T cells. In vivo treatment with these inhibitors resulted in a significant and selective reduction in Tregs in both naïve and tumor-bearing mice. Furthermore, these PI3K-Akt inhibitors led to a significant therapeutic antitumor effect, which was shown to be Treg dependent. Here, we report the use of PI3K-Akt pathway inhibitors as potent agents for the selective depletion of suppressive Tregs. We show that these inhibitors are able to enhance the antitumor immune response and are therefore promising clinical reagents for Treg depletion.

Post-translational modification networks regulating FOXP3 function

Forkhead box (FOX)P3 is a requisite transcription factor for the development and maintenance of immunosuppressive function of regulatory T (Treg) cells, and therefore for immune homeostasis. Post-translational modifications (PTMs) can transiently alter the functionality of transcription factors, and recent evidence reveals that FOXP3 can be regulated by various PTMs including acetylation, ubiquitination, and phosphorylation. Here, we review the current understanding of how these modifications control FOXP3, including regulation of DNA binding, transactivation potential, and proteasomal degradation. We place these findings in the context of the biology of Treg cells, and discuss both limitations in translating biochemical findings into in vivo functions and the opportunities presented by a better understanding of the molecular mechanisms that can transiently control FOXP3 activity in response to environmental cues.

Complement regulation of T-cell alloimmunity

Complement proteins are generated both by the liver (systemic compartment) and by peripheral tissue-resident cells and migratory immune cells (local compartment). The immune cell-derived, alternative pathway complement components activate spontaneously, yielding local, but not systemic, production of C3a and C5a. These anaphylatoxins bind to their respective G-protein-coupled receptors, the C3a receptor and the C5a receptor, expressed on T cells and antigen-presenting cells, leading to their reciprocal activation and driving T-cell differentiation, expansion, and survival. Complement deficiency or blockade attenuates T-cell-mediated autoimmunity and delays allograft rejection in mice. Increasing complement activation, achieved by genetic removal of the complement regulatory protein decay accelerating factor, enhances murine T-cell immunity and accelerates allograft rejection. Signaling through the C3a receptor and the C5a receptor reduces suppressive activity of natural regulatory T cells and the generation and stability of induced regulatory T cells. The concepts, initially generated in mice, recently were confirmed in human immune cells, supporting the need for testing of complement targeting therapies in organ transplants patients.

Human CD4+ HLA-G+ regulatory T cells are potent suppressors of graft-versus-host disease in vivo

CD4(+) T cells expressing the immunotolerizing molecule HLA-G have been described as a unique human thymus-derived regulatory T (tTreg) cell subset involved in immunoregulation and parenchymal homeostasis during infectious and autoimmune inflammation. We compared properties and molecular characteristics of human CD4(+)HLA-G(+) with those of CD4(+)CD25(+)FoxP3-expressing tTreg cells using in vitro studies of T-cell receptor (TCR) signaling, single-cell electrophysiology, and functional in vivo studies. Both tTreg populations are characterized by alterations in proximal-signaling pathways on TCR stimulation and a hyperpolarization of the plasma membrane when compared to conventional CD4(+) T cells. However, both clearly differ in phenotype and pattern of secreted cytokines, which results in distinct mechanisms of suppression: While CD4(+)HLA-G(+) cells secrete high levels of inhibitory molecules (IL-10, soluble HLA-G, IL-35), CD4(+)CD25(+)FoxP3(+) cells express these molecules at significantly lower levels and seem to exert their function mainly in a contact-dependent manner via cyclic adenosine-monophosphate. Finally we demonstrate that human CD4(+)HLA-G(+) tTreg cells significantly ameliorated graft-versus-host disease in a humanized mouse model as a first proof of their in vivo relevance. Our data further characterize and establish CD4(+)HLA-G(+) cells as a potent human tTreg population that can modulate polyclonal adaptive immune responses in vivo and thus being a promising candidate for potential clinical applications in the future.

Tyrosine 201 of the cytoplasmic tail of CTLA-4 critically affects T regulatory cell suppressive function

Cytotoxic T lymphocyte antigen-4 (CTLA-4) is a major negative regulatory molecule for T-cell activation with a complex biology and function. CTLA-4 is known to regulate homeostatic lymphoproliferation as well as tolerance induction and has been proposed to be an important effector molecule by which Treg cells suppress immunity. The immunoregulatory properties of CTLA-4 are primarily mediated by competition with the costimulator CD28 for ligand binding but also by delivering negative signals to T cells through its cytoplasmic tail. In this study, we addressed the effect of directly mutating the amino acid residue, Tyrosine 201 (Tyr201), of the intracellular domain of CTLA-4 in situ and its implications in T-cell function in the context of autoimmunity. Therefore, a novel CTLA-4 knock-in mouse (Y201V KI) was generated, in which Tyr201 was replaced by a valine that could not be phosphorylated. Mice expressing the CTLA-4 mutant molecule were generally healthy and did not show signs of disruption of T-cell homeostasis under steady-state conditions seen in CTLA-4 deficient mice. However, T cells isolated from Y201V KI mice expressed higher levels of CTLA-4 on the cell surface and displayed a Th2-biased phenotype following TCR stimulation. Furthermore, Y201V KI mice developed exacerbated disease as compared to wild-type upon antigen-specific T-cell activation in an in vivo model of EAE. Importantly, the Y201V mutation resulted in impaired suppressive activity of Treg cells while T effector function remained intact. These data suggest that effects associated with and mediated through Tyr201 of CTLA-4s intracellular domain are critical for Treg-cell function.

Enhanced activity of Akt in Teff cells from children with lupus nephritis is associated with reduced induction of tumor necrosis factor receptor-associated factor 6 and increased OX40 expression

OBJECTIVE

The breakdown of peripheral tolerance mechanisms is central to the pathogenesis of systemic lupus erythematosus (SLE). Although true Treg cells in patients with SLE exhibit intact suppressive activity, Teff cells are resistant to suppression. The underlying mechanisms are incompletely understood. This study was undertaken to examine the Akt signaling pathway and molecules that may alter its activity in T cells in lupus patients.

METHODS

The Akt pathway and its regulators were analyzed in Teff and Treg cells from children with lupus nephritis and controls using flow cytometry and real-time quantitative polymerase chain reaction. T cell proliferation was assessed by analysis of 5,6-carboxyfluorescein succinimidyl ester dilution.

RESULTS

CD4+CD45RA-FoxP3(low) and FoxP3- Teff cells from children with lupus nephritis expressed high levels of activated Akt, resulting in the down-regulation of the proapoptotic protein Bim and an enhanced proliferative response. The induction of tumor necrosis factor receptor-associated factor 6 (TRAF6) was impaired, and TRAF6 levels inversely correlated with Akt activity. Although the expression of OX40 was enhanced on Teff cells from children with lupus nephritis compared to controls, OX40 stimulation failed to significantly increase TRAF6 expression in cells from patients, in contrast to those from healthy controls, but resulted in further increased Akt activation that was reversed by blockade of OX40 signaling. Moreover, inhibition of Akt signaling markedly decreased the proliferation of Teff cells from lupus patients.

CONCLUSION

Our findings indicate that hyperactivation of the Akt pathway in Teff cells from children with lupus nephritis is associated with reduced induction of TRAF6 and up-regulation of OX40, which may cause Teff cell resistance to Treg cell-mediated suppression.

Transcriptional control of regulatory T cells

Regulatory T cells (Tregs) constitute unique T cell lineage that plays a key role for immunological tolerance. Tregs are characterized by the expression of the forkhead box transcription factor Foxp3, which acts as a lineage-specifying factor by determining the unique suppression profile of these immune cells. Here, we summarize the recent progress in understanding how Foxp3 expression itself is epigenetically and transcriptionally controlled, how the Treg-specific signature is achieved and how unique properties of Treg subsets are defined by other transcription factors. Finally, we will discuss recent studies focusing on the molecular targeting of Tregs to utilize the specific properties of this unique cell type in therapeutic settings.

Insulin inhibits IL-10-mediated regulatory T cell function: implications for obesity

Chronic inflammation is known to promote metabolic dysregulation in obesity and type 2 diabetes. Although the precise origin of the unchecked inflammatory response in obesity is unclear, it is known that overproduction of proinflammatory cytokines by innate immune cells affects metabolism. For example, TNF-α contributes to the inability of cells to respond to insulin and to the increase in levels of insulin. Whether this hyperinsulinemia itself is part of a feedback loop that affects the progression of chronic adipose inflammation is unknown. In this article, we show that regulatory T cells (Tregs) express the insulin receptor, and that high levels of insulin impair the ability of Tregs to suppress inflammatory responses via effects on the AKT/mTOR signaling pathway. Insulin activated AKT signaling in Tregs, leading to inhibition of both IL-10 production and the ability of Tregs to suppress the production of TNF-α by macrophages in a contact-independent manner. The effect of insulin on Treg suppression was limited to IL-10 production and it did not alter the expression of other proteins associated with Treg function, including CTLA-4, CD39, and TGF-β. In a model of diet-induced obesity, Tregs from the visceral adipose tissue of hyperinsulinemic, obese mice showed a similar specific decrease in IL-10 production, as well as a parallel increase in production of IFN-γ. These data suggest that hyperinsulinemia may contribute to the development of obesity-associated inflammation via a previously unknown effect of insulin on the IL-10-mediated function of Tregs.

Differential response of regulatory and conventional CD4⁺ lymphocytes to CD3 engagement: clues to a possible mechanism of anti-CD3 action?

Several clinical trials have shown anti-CD3 treatment to be a promising therapy for autoimmune diabetes, but its mechanism of action remains unclear. Foxp3(+) regulatory T cells (Tregs) are likely to be involved, but through unknown mechanistic pathways. We profiled the transcriptional consequences in CD4(+) Tregs and conventional T cells (Tconvs) in the first hours and days after anti-CD3 treatment of NOD mice. Anti-CD3 treatment led to a transient transcriptional response, terminating faster than most Ag-induced responses. Most transcripts were similarly induced in Tregs and Tconvs, but several were differential, in particular, those encoding the IL-7R and transcription factors Id2/3 and Gfi1, upregulated in Tregs but repressed in Tconvs. Because IL-7R was a plausible candidate for driving the homeostatic response of Tregs to anti-CD3, we tested its relevance by supplementation of anti-CD3 treatment with IL-7/anti-IL-7 complexes. Although ineffective alone, IL-7 significantly improved the rate of remission induced by anti-CD3. Four anti-human CD3 mAbs exhibited the same differential effect on IL-7R expression in human as in mouse cells, suggesting that the mechanism also underlies therapeutic effect in human cells, and perhaps a rationale for testing a combination of anti-CD3 and IL-7 for the treatment of recent-onset human type 1 diabetes. Thus, systems-level analysis of the response to anti-CD3 in the early phase of the treatment demonstrates different responses in Tregs and Tconvs, and provides new leads to a mechanistic understanding of its mechanism of action in reverting recent-onset diabetes.

Mitochondrial and cytosolic roles of PINK1 shape induced regulatory T-cell development and function

Mutations in PTEN-induced kinase 1 (PINK1), a serine/threonine kinase linked to familial early-onset Parkinsonism, compromise mitochondrial integrity and metabolism and impair AKT signaling. As the activation of a naïve T cell requires an AKT-dependent reorganization of a cell's metabolic machinery, we sought to determine if PINK1-deficient T cells lack the ability to undergo activation and differentiation. We show that CD4(+) T cells from PINK1 knockout mice fail to properly phosphorylate AKT upon activation, resulting in reduced expression of the IL-2 receptor subunit CD25. Following, deficient IL-2 signaling mutes the activation-induced increase in respiratory capacity and mitochondrial membrane potential. Under polarization conditions favoring the development of induced regulatory T cells, PINK1(-/-) T cells exhibit a reduced ability to suppress bystander T-cell proliferation despite normal FoxP3 expression kinetics. Our results describe a critical role for PINK1 in integrating extracellular signals with metabolic state during T-cell fate determination, and may have implications for the understanding of altered T-cell populations and immunity during the progression of active Parkinson's disease or other immunopathologies.

Stability and function of regulatory T cells is maintained by a neuropilin-1-semaphorin-4a axis

Regulatory T cells (Treg cells) have a crucial role in the immune system by preventing autoimmunity, limiting immunopathology, and maintaining immune homeostasis. However, they also represent a major barrier to effective anti-tumour immunity and sterilizing immunity to chronic viral infections. The transcription factor Foxp3 has a major role in the development and programming of Treg cells. The relative stability of Treg cells at inflammatory disease sites has been a highly contentious subject. There is considerable interest in identifying pathways that control the stability of Treg cells as many immune-mediated diseases are characterized by either exacerbated or limited Treg-cell function. Here we show that the immune-cell-expressed ligand semaphorin-4a (Sema4a) and the Treg-cell-expressed receptor neuropilin-1 (Nrp1) interact both in vitro, to potentiate Treg-cell function and survival, and in vivo, at inflammatory sites. Using mice with a Treg-cell-restricted deletion of Nrp1, we show that Nrp1 is dispensable for suppression of autoimmunity and maintenance of immune homeostasis, but is required by Treg cells to limit anti-tumour immune responses and to cure established inflammatory colitis. Sema4a ligation of Nrp1 restrained Akt phosphorylation cellularly and at the immunologic synapse by phosphatase and tensin homologue (PTEN), which increased nuclear localization of the transcription factor Foxo3a. The Nrp1-induced transcriptome promoted Treg-cell stability by enhancing quiescence and survival factors while inhibiting programs that promote differentiation. Importantly, this Nrp1-dependent molecular program is evident in intra-tumoral Treg cells. Our data support a model in which Treg-cell stability can be subverted in certain inflammatory sites, but is maintained by a Sema4a-Nrp1 axis, highlighting this pathway as a potential therapeutic target that could limit Treg-cell-mediated tumour-induced tolerance without inducing autoimmunity.

Interleukin-10 inhibits lipopolysaccharide induced miR-155 precursor stability and maturation

The anti-inflammatory cytokine interleukin-10 (IL-10) is essential for attenuating the inflammatory response, which includes reducing the expression of pro-inflammatory microRNA-155 (miR-155) in lipopolysaccharide (LPS) activated macrophages. miR-155 enhances the expression of pro-inflammatory cytokines such as TNFα and suppresses expression of anti-inflammatory molecules such as SOCS1. Therefore, we examined the mechanism by which IL-10 inhibits miR-155. We found that IL-10 treatment did not affect the transcription of the miR-155 host gene nor the nuclear export of pre-miR-155, but rather destabilized both pri-miR-155 and pre-miR-155 transcripts, as well as interfered with the final maturation of miR-155. This inhibitory effect of IL-10 on miR-155 expression involved the contribution of both the STAT3 transcription factor and the phosphoinositol phosphatase SHIP1. This is the first report showing evidence that IL-10 regulates miRNA expression post-transcriptionally.

Signaling via TLR2 and TLR4 Directly Down-Regulates T Cell Effector Functions: The Regulatory Face of Danger Signals

Toll-like receptors (TLRs) are widely expressed and play an essential role in the activation of innate immune cells. However, certain TLRs are also expressed on T cells, and TLR ligands can directly modulate T cell functions. Here, we discuss findings indicating that T cells directly respond to Heat Shock Protein (HSP) 60, a self molecule, or to the HSP60-derived peptide, p277, via a TLR2-dependent mechanism. HSP60 has been considered to be a “danger signal” for the immune system because of its ability to induce pro-inflammatory phenotypes in innate immune cells – in this case via TLR4 activation; nevertheless, TLR2 engagement by HSP60 on T cells can lead to resolution of inflammation by up-regulating the suppression function of regulatory T cells and shifting the resulting cytokine secretion balance toward a Th2 phenotype. Moreover, T cell TLR4 engagement by LPS leads to up-regulation of suppressor of cytokine signaling 3 expression and consequently down-regulates T cell chemotaxis. Thus, TLR2 and TLR4 activation can contribute to both induction and termination of effector immune responses by controlling the activities of both innate and adaptive immune cells.

Transiently reduced PI3K/Akt activity drives the development of regulatory function in antigen-stimulated Naïve T-cells

Regulatory T-cells (T_regs) are central for immune homeostasis and divided in thymus-derived natural T_regs and peripherally induced iT_reg. However, while phenotype and function of iT_regs are well known, a remarkable lack exists in knowledge about signaling mechanisms leading to their generation from naïve precursors in peripheral tissues. Using antigen specific naïve T-cells from mice, we investigated CD4+ CD25+ FoxP3- iT_reg induction during antigen-specific T-cell receptor (TCR) stimulation with weak antigen presenting cells (APC). We show that early signaling pathways such as ADAM-17-activation appeared similar in developing iT_reg and effector cells (T_eff) and both initially shedded CD62-L. But iT_reg started reexpressing CD62-L after 24 h while T_eff permanently downmodulated it. Furthermore, between 24 and 72 hours iT_reg presented with significantly lower phosphorylation levels of Akt-S473 suggesting lower activity of the PI3K/Akt-axis. This was associated with a higher expression of the Akt hydrophobic motif-specific phosphatase PHLPP1 in iT_reg. Importantly, the lack of costimulatory signals via CD28 from weak APC was central for the development of regulatory function in iT_reg but not for the reappearance of CD62-L. Thus, T-cells display a window of sensitivity after onset of TCR triggering within which the intensity of the PI3K/Akt signal controls entry into either effector or regulatory pathways.

Different modulation of Ptpn22 in effector and regulatory T cells leads to attenuation of autoimmune diabetes in transgenic nonobese diabetic mice

Ptpn22 encodes PEST domain-enriched tyrosine phosphatase (Pep), which negatively regulates TCR proximal signaling and is strongly associated with a variety of autoimmune diseases in humans. The net effect of Pep on the balance of immunity and tolerance is uncertain because of the simultaneous inhibition of TCR-mediated signaling of effector and regulatory T cells (T(regs)). In this study, we generated transgenic NOD mice that overexpressed Pep in T cells. The transgenic mice had a significantly lower incidence of spontaneous autoimmune diabetes, which was accompanied by fewer IFN-γ-producing T cells, and an increased ratio of CD4(+)Foxp3(+) T(regs)to CD4(+)IFN-γ(+) or to CD8(+)IFN-γ(+) T cells, respectively, in pancreatic islets. Transgenic T cells showed markedly decreased TCR-mediated effector cell responses such as proliferation and Th1 differentiation. By contrast, the inhibitory effect of transgenic Pep on TCR signaling did not affect the differentiation of T(regs) or their suppressive activity. Adoptive transfer experiments showed that transgenic splenocytes exhibited attenuated diabetogenic ability. To examine further the pathogenic features of transgenic T cells, we generated Ptpn22/BDC2.5 doubly transgenic mice and found reduced proliferation and Th1 differentiation in CD4(+) T lymphocytes with additional Pep in pancreatic lymph nodes but not in inguinal lymph nodes of NOD/SCID recipients. This finding indicates that transgenic Pep attenuates T cell functions in an islet Ag-driven manner. Taken together, our results demonstrate that Pep overexpression in T cells attenuates autoimmune diabetes in NOD mice by preferentially modulating TCR signaling-mediated functions in diabetogenic T cells but not in T(regs).

Immunosuppressive effects of the traditional Chinese herb Qu Mai on human alloreactive T cells

Current therapies for transplant rejection are suboptimally effective. In an effort to discover novel immunosuppressants we used cytokine ELISPOT and ELISAs to screen extracts from 53 traditional Chinese herbs for their ability to suppress human alloreactive T cells. We identified a dichloromethane-soluble fraction (Qu Mai fraction AD [QMAD]) of Qu Mai (Dianthus superbus) as a candidate. High-performance liquid chromatography (HPLC) analysis of QMAD revealed three dominant peaks, each with a MW ~600 Daltons and distinct from cyclosporine and rapamycin. When we added QMAD to human mixed lymphocyte cultures, we observed dose-dependent inhibition of proliferation and IFNγ production, by naïve and memory alloreactive T cells, and observed an increased frequency of Foxp3(+) CD4(+) T cells. To address whether QMAD induces regulatory T cells we added QMAD to anti-CD3/CD28-stimulated naïve CD4 T cells and observed a dose-dependent upregulation of Foxp3 associated with new suppressive capacity. Mechanistically, QMAD did not induce T cell IL-10 or TGFβ but blocked T cell AKT phosphorylation, a key signaling nexus required for T cell proliferation and expansion, that simultaneously prevents Foxp3 transcription. Our findings provide novel insight into the antiinflammatory effects of one traditional Chinese herb, and support the need for continued isolation, characterization and testing of QMAD-derived components as immune suppressants for transplant rejection.

Early in vivo signaling profiles in MUC1-specific CD4+ T cells responding to two different MUC1-targeting vaccines in two different microenvironments

Vaccines are beginning to be explored for as measures to prevent cancer. Since determining the efficacy of vaccines by evaluating disease outcome requires a long time, there is an urgent need for early predictive biomarkers. To this end, immunological endpoints that can be assessed weeks or months post-vaccination are currently being evaluated. However, when multiple vaccines are available, waiting for the development of humoral and cellular immunity could still cause delays, whereas early assessments would allow for a timely shift to more effective prevention modalities. Applying the phospho-flow technique to primary T cells, we examined the phosphorylation status of various proteins that shape the activation, proliferation, and differentiation of mucin 1 (MUC1)-specific CD4+ T cells within the first 24 hours post-immunization. It is known that a vaccine composed of a MUC1-derived peptide loaded on dendritic cells is more effective in eliciting T-cell responses than a vaccine including the same peptide plus an adjuvant. Both these vaccines stimulate T cells more effectively in wild-type (WT) than in MUC1-transgenic mice. We examined if the signaling events downstream of the TCR or linked to various proliferative and survival pathways, monitored in two different hosts as early as 3, 6, 12 and 24 hours post-immunization, could predict the differential potential of these two MUC1-targeting vaccines. The signaling signatures that we obtained primarily reflect differences between the vaccines rather than between the hosts. We demonstrate the feasibility of using a phospho-flow-based approach to evaluate the potential of a given vaccine to elicit a desired immune response.

Signaling through C5a receptor and C3a receptor diminishes function of murine natural regulatory T cells

Blockade of C3aR/C5aR signaling in nT reg cells augments in vitro and in vivo suppression, abrogates autoimmune colitis, and prolongs allogeneic skin graft survival.

Thymus-derived (natural) CD4⁺ FoxP3⁺ regulatory T cells (nT reg cells) are required for immune homeostasis and self-tolerance, but must be stringently controlled to permit expansion of protective immunity. Previous findings linking signals transmitted through T cell–expressed C5a receptor (C5aR) and C3a receptor (C3aR) to activation, differentiation, and expansion of conventional CD4⁺CD25⁻ T cells (T conv cells), raised the possibility that C3aR/C5aR signaling on nT reg cells could physiologically modulate nT reg cell function and thereby further impact the induced strength of T cell immune responses. In this study, we demonstrate that nT reg cells express C3aR and C5aR, and that signaling through these receptors inhibits nT reg cell function. Genetic and pharmacological blockade of C3aR/C5aR signal transduction in nT reg cells augments in vitro and in vivo suppression, abrogates autoimmune colitis, and prolongs allogeneic skin graft survival. Mechanisms involve C3a/C5a-induced phosphorylation of AKT and, as a consequence, phosphorylation of the transcription factor Foxo1, which results in lowered nT reg cell Foxp3 expression. The documentation that C3a/C3aR and C5a/C5aR modulate nT reg cell function via controlling Foxp3 expression suggests targeting this pathway could be exploited to manipulate pathogenic or protective T cell responses.

Helios+ and Helios- cells coexist within the natural FOXP3+ T regulatory cell subset in humans

FOXP3-expressing T regulatory cells (Tregs) can be divided into two distinct subsets: naturally occurring Tregs (nTregs) that develop in the thymus, and induced Tregs (iTregs) that differentiate in peripheral tissues upon exposure to Ag in a tolerogenic environment. Recently it has been proposed that expression of Helios, an Ikaros family transcription factor, may specifically identify nTregs, allowing specific tracking of Tregs from different origins in health and disease. Surprisingly, we found that Helios(-) cells can be readily identified within naive (CD45RA(+)CD31(+)CCR7(+)CD62L(+)) FOXP3(+) Tregs, a finding inconsistent with the notion that lack of Helios expression identifies Ag-experienced iTregs that should express memory markers. To investigate the phenotype and function of naive Helios(+) and Helios(-) Tregs within the nTreg population, we isolated single-cell clones from each subset. We found that both Helios(+) and Helios(-) nTreg clones have a similar suppressive capacity, as well as expression of FOXP3 and cell surface proteins, including CD39 and CTLA-4. Helios(-) nTregs, however, produced significantly more CCL3 and IFN-γ compared with Helios(+) nTregs. Despite increased cytokine/chemokine production, Helios(-) FOXP3(+) nTreg clones were demethylated at the FOXP3 Treg-specific demethylated region, indicative of Treg lineage stability. When cultured under Th1-polarizing conditions, Helios(+) and Helios(-) nTreg clones had an equal ability to produce IFN-γ. Collectively, these data show that a lack of Helios expression does not exclusively identify human iTregs, and, to our knowledge, the data provide the first evidence for the coexistence of Helios(+) and Helios(-) nTregs in human peripheral blood.

MicroRNA-126 regulates the induction and function of CD4(+) Foxp3(+) regulatory T cells through PI3K/AKT pathway

Recent evidence showed that limited activation of PI3K/Akt pathway was critical for induction and function sustainment of CD4(+) Foxp3(+) regulatory T cells (Tregs). However, the underlying mechanism remains largely unknown. In this study, we reported that miR-126 was expressed in mouse and human Tregs. Further study showed that silencing of miR-126 using miR-126 antisense oligonucleotides (ASO) could significantly reduce the induction of Tregs in vitro. Furthermore, miR-126 silencing could obviously reduce the expression of Foxp3 on Tregs, which was accompanied by decreased expression of CTLA-4 and GITR, as well as IL-10 and TGF-β, and impair its suppressive function. Mechanistic evidence showed that silencing of miR-126 enhanced the expression of its target p85β and subsequently altered the activation of PI3K/Akt pathway, which was ultimately responsible for reduced induction and suppressive function of Tregs. Finally, we further revealed that miR-126 silencing could impair the suppressive function of Tregs in vivo and endow effectively antitumour effect of CD8(+) T cells in adoptive cell transfer assay using a murine breast cancer model. Therefore, our study showed that miR-126 could act as fine-tuner in regulation of PI3K-Akt pathway transduction in the induction and sustained suppressive function of Tregs and provided a novel insight into the development of therapeutic strategies for promoting T-cell immunity by regulating Tregs through targeting specific miRNAs.

HSP70 enhances immunosuppressive function of CD4(+)CD25(+)FoxP3(+) T regulatory cells and cytotoxicity in CD4(+)CD25(-) T cells

Human CD4⁺CD25⁺FoxP3⁺ T regulatory cells (Tregs) control effector T cells and play a central role in peripheral tolerance and immune homeostasis. Heat shock protein 70 (HSP70) is a major immunomodulatory molecule, but its effect on the functions of Tregs is not well understood. To investigate target-dependent and –independent Treg functions, we studied cytokine expression, regulation of proliferation and cytotoxicity after exposure of Tregs to HSP70. HSP70-treated Tregs significantly inhibited proliferation of CD4⁺CD25⁻ target cells and downregulated the secretion of the proinflammatory cytokines IFN-γ and TNF-α. By contrast, HSP70 increased the secretion of Treg suppressor cytokines IL-10 and TGF-β. Treatment with HSP70 enhanced the cytotoxic properties of Tregs only to a minor extent (4-fold), but led to stronger responses in CD4⁺CD25⁻ cells (42-fold). HSP70-induced modulation of T-cell responses was further enhanced by combined treatment with HSP70 plus IL-2. Treatment of Tregs with HSP70 led to phosphorylation of PI3K/AKT and the MAPKs JNK and p38, but not that of ERK1/2. Exposure of Tregs to specific inhibitors of PI3K/AKT and the MAPKs JNK and p38 reduced the immunosuppressive function of HSP70-treated Tregs as indicated by the modified secretion of specific target cell (IFN-γ, TNF-α) and suppressor cytokines (IL-10, TGF-β). Taken together, the data show that HSP70 enhances the suppressive capacity of Tregs to neutralize target immune cells. Thus HSP70-enhanced suppression of Tregs may prevent exaggerated immune responses and may play a major role in maintaining immune homeostasis.

FOXO transcription factors throughout T cell biology

The outcome of an infection with any given pathogen varies according to the dosage and route of infection, but, in addition, the physiological state of the host can determine the efficacy of clearance, the severity of infection and the extent of immunopathology. Here we propose that the forkhead box O (FOXO) transcription factor family--which is central to the integration of growth factor signalling, oxidative stress and inflammation--provides connections between physical well-being and the form and magnitude of an immune response. We present a case that FOXO transcription factors guide T cell differentiation and function in a context-driven manner, and might provide a link between metabolism and immunity.

TNF-α impairs differentiation and function of TGF-β-induced Treg cells in autoimmune diseases through Akt and Smad3 signaling pathway

Deficiency in the TGF-β-induced regulatory T (iTreg) cell differentiation is associated with compromised immune homeostasis and plays a key role in many autoimmune diseases. Therapeutic intervention to enhance in situ iTreg differentiation has become a promising treatment modality for autoimmune diseases. Here we describe that the development of autoimmune inflammation in experimental autoimmune encephalomyelitis (EAE) is associated with selective impairment of iTreg differentiation largely due to the increased production of TNF-α. The neutralization of TNF-α markedly increases iTreg differentiation, leading to the amelioration of EAE, whereas the depletion of iTreg cells abolishes the therapeutic effect of an anti-TNF-α antibody. The inhibition of iTreg differentiation by TNF-α is mediated through a signaling cascade involving the induction of TNF receptor II (TNFR2) expression and the activation of Akt. The activated Akt in turn interacts with Smad3, resulting in the inhibition of TGF-β-induced Smad3 phosphorylation and consequently the reduction of p-Smad3 results in the decreased binding to the specific binding site of the foxp3 promoter, and finally foxp3 transcription itself. Interestingly, this regulatory pathway is iTreg cell specific as TNF-α does not activate Akt in naturally occurring regulatory T cells, therefore conferring a selective effect of TNF-α and its antagonism on iTreg cells. The study sheds new light on the critical role and underlying mechanism of TNF-α in the regulation of iTreg differentiation and provides a novel rationale for TNF-α antagonistic therapy for autoimmune diseases.

mTORC2-PKBα/Akt1 Serine 473 phosphorylation axis is essential for regulation of FOXP3 Stability by chemokine CCL3 in psoriasis

The connection between infections and acute guttate psoriasis (AGP) outbreaks/chronic plaque psoriasis (CPP) exacerbation has been known for years. Impaired function of FOXP3+Tregs in psoriasis has been identified. However, the mechanisms behind these two observations have not been fully interpreted. In the present study, we provide evidence to support chemokine CCL3 as one of the vital links between infections and FOXP3 stability in the psoriatic microenvironment. We found that serum CCL3, strongly induced by microorganism infections including streptococcus, was closely correlated with FOXP3 levels in CD4+CD25+T cells of patients with psoriasis. CCL3 manipulated FOXP3 stability in a concentration-dependent bidirectional manner. High-concentration CCL3 decreased FOXP3 stability by promoting FOXP3's degradation through K48-linkage ubiquitination. This degradation was mainly dependent on upregulation of Serine 473 phosphorylation of the PKBα/Akt1 isoform, and almost independent of mTORC1 (mammalian target of rapamycin complex 1) activity. On the other hand, low-concentration CCL3 could enhance FOXP3 stability by the maintenance of the PKC pathway and the restriction of the PKB/Akt pathway. We further demonstrated that enhancing FOXP3 stability by low-concentration CCL3 attributed, at least partly, to the prevention of cytoplasmic Sin1, a vital component of mTORC2, nuclear translocation. Our results suggest vital roles for CCL3-mTORC2-isoform PKB/Akt1 S473 phosphorylation axis in FOXP3+Tregs and the development of psoriasis.

Novel Foxo1-dependent transcriptional programs control T(reg) cell function

Regulatory T (T(reg)) cells, characterized by expression of the transcription factor forkhead box P3 (Foxp3), maintain immune homeostasis by suppressing self-destructive immune responses. Foxp3 operates as a late-acting differentiation factor controlling T(reg) cell homeostasis and function, whereas the early T(reg)-cell-lineage commitment is regulated by the Akt kinase and the forkhead box O (Foxo) family of transcription factors. However, whether Foxo proteins act beyond the T(reg)-cell-commitment stage to control T(reg) cell homeostasis and function remains largely unexplored. Here we show that Foxo1 is a pivotal regulator of T(reg )cell function. T(reg) cells express high amounts of Foxo1 and display reduced T-cell-receptor-induced Akt activation, Foxo1 phosphorylation and Foxo1 nuclear exclusion. Mice with T(reg)-cell-specific deletion of Foxo1 develop a fatal inflammatory disorder similar in severity to that seen in Foxp3-deficient mice, but without the loss of T(reg) cells. Genome-wide analysis of Foxo1 binding sites reveals ~300 Foxo1-bound target genes, including the pro-inflammatory cytokine Ifng, that do not seem to be directly regulated by Foxp3. These findings show that the evolutionarily ancient Akt-Foxo1 signalling module controls a novel genetic program indispensable for T(reg) cell function.

Does the PI3K pathway promote or antagonize regulatory T cell development and function?

Regulatory T cells (Tregs) prevent autoimmunity and inflammation by suppressing the activation of other T cells and antigen presenting cells. The role of phosphoinositide 3-kinase (PI3K) signaling in Treg is controversial. Some studies suggest that inhibition of the PI3K pathway is essential for the development of Tregs whereas other studies have shown reduced Treg numbers and function when PI3K activity is suppressed. Here we attempt to reconcile the different studies that have explored PI3K and the downstream effectors Akt, Foxo, and mTOR in regulatory T cell development and function and discuss the implications for health and therapeutic intervention.

The Role of the PI3K Signaling Pathway in CD4(+) T Cell Differentiation and Function

The relative activity of regulatory versus conventional CD4(+) T cells ultimately maintains the delicate balance between immune tolerance and inflammation. At the molecular level, the activity of phosphatidylinositol 3-kinase (PI3K) and its downstream positive and negative regulators has a major role in controlling the balance between immune regulation and activation of different subsets of effector CD4(+) T cells. In contrast to effector T cells which require activation of the PI3K to differentiate and mediate their effector function, regulatory T cells rely on minimal activation of this pathway to develop and maintain their characteristic phenotype, function, and metabolic state. In this review, we discuss the role of the PI3K signaling pathway in CD4(+) T cell differentiation and function, and focus on how modulation of this pathway in T cells can alter the outcome of an immune response, ultimately tipping the balance between tolerance and inflammation.

First insight into the kinome of human regulatory T cells

Regulatory T cells (Tregs) are essential for controlling peripheral tolerance by the active suppression of various immune cells including conventional T effector cells (Teffs). Downstream of the T cell receptor (TCR), more than 500 protein kinases encoded by the human genome have to be considered in signaling cascades regulating the activation of Tregs and Teffs, respectively. Following TCR engagement, Tregs posses a number of unique attributes, such as constitutive expression of Foxp3, hyporesponsiveness and poor cytokine production. Furthermore, recent studies showed that altered regulation of protein kinases is important for Treg function. These data indicate that signaling pathways in Tregs are distinctly organized and alterations at the level of protein kinases contribute to the unique Treg phenotype. However, kinase-based signaling networks in Tregs are poorly understood and necessitate further systematic characterization. In this study, we analyzed the differential expression of kinases in Tregs and Teffs by using a kinase-selective proteome strategy. In total, we revealed quantitative information on 185 kinases expressed in the human CD4(+) T cell subsets. The majority of kinases was equally abundant in both T cell subsets, but 11 kinases were differentially expressed in Tregs. Most strikingly, Tregs showed an altered expression of cell cycle kinases including CDK6. Quantitative proteomics generates first comparative insight into the kinase complements of the CD4(+) Teff and Treg subset. Treg-specific expression pattern of 11 protein kinases substantiate the current opinion that TCR-mediated signaling cascades are altered in Tregs and further suggests that Tregs exhibit significant specificities in cell-cycle control and progression.

PI3K signalling in B- and T-lymphocytes: new developments and therapeutic advances

Activation of PI3K (phosphoinositide 3-kinase) is a shared response to engagement of diverse types of transmembrane receptors. Depending on the cell type and stimulus, PI3K activation can promote different fates including proliferation, survival, migration and differentiation. The diverse roles of PI3K signalling are well illustrated by studies of lymphocytes, the cells that mediate adaptive immunity. Genetic and pharmacological experiments have shown that PI3K activation regulates many steps in the development, activation and differentiation of both B- and T-cells. These findings have prompted the development of PI3K inhibitors for the treatment of autoimmunity and inflammatory diseases. PI3K activation, however, has both positive and negative roles in immune system activation. Consequently, although PI3K suppression can attenuate immune responses it can also enhance inflammation, disrupt peripheral tolerance and promote autoimmunity. An exciting discovery is that a selective inhibitor of the p110δ catalytic isoform of PI3K, CAL-101, achieves impressive clinical efficacy in certain B-cell malignancies. A model is emerging in which p110δ inhibition disrupts signals from the lymphoid microenvironment, leading to release of leukaemia and lymphoma cells from their protective niche. These encouraging findings have given further momentum to PI3K drug development efforts in both cancer and immune diseases.

A randomized phase II study to evaluate tacrolimus in combination with sirolimus or methotrexate after allogeneic hematopoietic cell transplantation

BACKGROUND

There is evidence suggesting that sirolimus, in combination with tacrolimus, is active in the prevention of graft-versus-host disease. Sirolimus-based immune suppression may suppress alloreactive T cells, while sparing the survival and function of regulatory T cells.

DESIGN AND METHODS

We conducted a randomized trial to compare the impact of sirolimus/tacrolimus against that of methotrexate/tacrolimus on the prevention of graft-versus-host disease and regulatory T-cell reconstitution.

RESULTS

Seventy-four patients were randomized 1:1 to sirolimus/tacrolimus or methotrexate/tacrolimus, stratified for type of donor (sibling or unrelated) and the patients' age. The rate of grade II-IV acute graft-versus-host disease at 100 days was 43% (95% CI: 27-59%) in the sirolimus/tacrolimus group and 89% (95% CI: 72-96%) in the methotrexate/tacrolimus group (P<0.001). The rate of moderate/severe chronic graft-versus-host disease was 24% (95% CI: 7-47%) in the sirolimus/tacrolimus group and 64% (95% CI: 41-79%) in the methotrexate/tacrolimus group (P=0.008). Overall survival and patient-reported quality of life did not differ between the two groups. On days 30 and 90 post-transplant, sirolimus-treated patients had a significantly greater proportion of regulatory T cells among the CD4(+) cells in the peripheral blood, and isolated regulatory T cells were functional.

CONCLUSIONS

These data demonstrate that sirolimus/tacrolimus prevents grade II-IV acute graft-versus-host disease and moderate-severe chronic graft-versus-host disease more effectively than does methotrexate/tacrolimus, and supports regulatory T-cell reconstitution following allogeneic hematopoietic cell transplantation.

Requirements for growth and IL-10 expression of highly purified human T regulatory cells

Human regulatory T cells (T(R)) cells have potential for the treatment of a variety of immune mediated diseases but the anergic phenotype of these cells makes them difficult to expand in vitro. We have examined the requirements for growth and cytokine expression from highly purified human T(R) cells, and correlated these findings with the signal transduction events of these cells. We demonstrate that these cells do not proliferate or secrete IL-10 even in the presence of high doses of IL-2. Stimulation with a superagonistic anti-CD28 antibody (clone 9.3) and IL-2 partially reversed the proliferative defect, and this correlated with reversal of the defective calcium mobilization in these cells. Dendritic cells were effective at promoting T(R) cell proliferation, and under these conditions the proliferative capacity of T(R) cells was comparable to conventional CD4 lymphocytes. Blocking TGF-β activity abrogated IL-10 expression from these cells, while addition of TGF-β resulted in IL-10 production. These data demonstrate that highly purified populations of T(R) cells are anergic even in the presence of high doses of IL-2. Furthermore, antigen presenting cells provide proper co-stimulation to overcome the anergic phenotype of T(R) cells, and under these conditions they are highly sensitive to IL-2. In addition, these data demonstrate for the first time that TGF-β is critical to enable human T(R) cells to express IL-10.

Protein kinase C-θ inhibits inducible regulatory T cell differentiation via an AKT-Foxo1/3a-dependent pathway

Protein kinase C (PKC)-θ has been shown to be a critical TCR signaling molecule that promotes the activation and differentiation of naive T cells into inflammatory effector T cells. In this study, we demonstrate that PKC-θ-mediated signals inhibit inducible regulatory T cell (iTreg) differentiation via an AKT-Foxo1/3A pathway. TGF-β-induced iTreg differentiation was enhanced in PKC-θ(-/-) T cells or wild-type cells treated with a specific PKC-θ inhibitor, but was inhibited by the PKC-θ activator PMA, or by CD28 crosslinking, which enhances PKC-θ activation. PKC-θ(-/-) T cells had reduced activity of the AKT kinase, and the expression of a constitutively active form of AKT in PKC-θ(-/-) T cells restored the ability to inhibit iTreg differentiation. Furthermore, knockdown or overexpression of the AKT downstream targets Foxo1 and Foxo3a was found to inhibit or promote iTreg differentiation in PKC-θ(-/-) T cells accordingly, indicating that the AKT-Foxo1/3A pathway is responsible for the inhibition of iTreg differentiation of iTregs downstream of PKC-θ. We conclude that PKC-θ is able to control T cell-mediated immune responses by shifting the balance between the differentiation of effector T cells and inhibitory Tregs.

Harnessing regulatory T cells for transplant tolerance in the clinic through mTOR inhibition: myth or reality?

PURPOSE OF REVIEW

The inhibition of mTOR promotes immune tolerance in mouse models of transplantation, by favoring the expansion of regulatory T cells over effector T cells. However, attempts at inducing immune tolerance with the mTOR inhibitor (mTOR-I) in humans have so far failed. We herein review the immunological obstacles that need to be overcome in order to translate mTOR-I-related tolerogenic properties into the clinic.

RECENT FINDINGS

Our knowledge of regulatory T-cell biology has exploded over the past few years, providing clues to explain the complex impact of prolonged mTOR inhibition on the biology of regulatory T cells. Furthermore, recent data have shed light on the unexpected pro-inflammatory burst observed in some transplant recipients treated with mTOR-I. We propose that the exposure of an organism to pathogens determines the immunodominant effect of mTOR-I, altering the immune system from a state of tolerance in inbred animals to a state of infection-triggered enhanced inflammation in humans.

SUMMARY

Recent advances in the understanding of the pleiotropic effects of mTOR-I on the immune system are paving the way to new therapeutic avenues. Future mTOR-I-based tolerogenic protocols should counter the mTOR-I-related inflammation in order to selectively promote expansion of stable regulatory T cells. We herein envisage promising therapeutic perspectives.

Scaffold protein Disc large homolog 1 is required for T-cell receptor-induced activation of regulatory T-cell function

Foxp3(+)CD4(+)CD25(high) regulatory T cell (Treg) suppression of inflammation depends on T-cell receptor-mediated Nuclear Factor of Activated T cells c1 (NFATc1) activation with reduced Akt activity. We investigated the role of the scaffold protein Disc large homolog 1 (Dlgh1) in linking the T-cell receptor to this unique signaling outcome. The Treg immunological synapse (IS) recruited fourfold more Dlgh1 than conventional CD4(+) T-cell IS. Tregs isolated from patients with active rheumatoid arthritis, or treated with tumor necrosis factor-α, displayed reduced function and diminished Dlgh1 recruitment to the IS. Furthermore, Dlgh1 silencing abrogated Treg function, impaired NFATc1 activation, reduced phosphatase and tensin homolog levels, and increased Akt activation. Dlgh1 operates independently of the negative feedback pathway mediated by the related adapter protein Carma1 and thus presents an array of unique targets to selectively manipulate Treg function.

Inhibitor of PI3Kγ ameliorates TNBS-induced colitis in mice by affecting the functional activity of CD4+CD25+FoxP3+ regulatory T cells

BACKGROUND AND PURPOSE

Phosphoinositide 3-kinase-γ (PI3Kγ) is implicated in many pathophysiological conditions, and recent evidence has suggested its involvement in colitis. In the present study, we investigated the effects of AS605240, a relatively selective PI3Kγ inhibitor, in experimental colitis and its underlying mechanisms.

EXPERIMENTAL APPROACH

Acute colitis was induced in mice by treatment with trinitrobenzene sulphonic acid (TNBS), and the effect of AS605240 on colonic injury was assessed. Pro-inflammatory mediators and cytokines were measured by immunohistochemistry, elisa, real time-polymerase chain reaction and flow cytometry.

KEY RESULTS

Oral administration of AS605240 significantly attenuated TNBS-induced acute colitis and diminished the expression of matrix metalloproteinase-9 and vascular endothelial growth factor. The colonic levels and expression of IL-1β, CXCL-1/KC, MIP-2 and TNF-α were also reduced following therapeutic treatment with AS605240. Moreover, AS605240 reduced MIP-2 levels in a culture of neutrophils stimulated with lipopolysaccharide. The mechanisms underlying these actions of AS605240 are related to nuclear factor-κ (NF-κB) inhibition. Importantly, the PI3Kγ inhibitor also up-regulated IL-10, CD25 and FoxP3 expression. In addition, a significant increase in CD25 and FoxP3 expression was found in isolated lamina propria CD4+ T cells of AS605240-treated mice. The effect of AS605240 on Treg induction was further confirmed by showing that concomitant in vivo blockade of IL-10R significantly attenuated its therapeutic activity.

CONCLUSIONS AND IMPLICATIONS

These results suggest that AS605240 protects mice against TNBS-induced colitis by inhibiting multiple inflammatory components through the NF-κB pathway while simultaneously inducing an increase in the functional activity of CD4+CD25+ Treg. Thus, AS605240 may offer a promising new therapeutic strategy for the treatment of inflammatory bowel diseases.

IRX-2, a novel biologic, favors the expansion of T effector over T regulatory cells in a human tumor microenvironment model

IRX-2, a natural cytokine biological with multiple components, has been used in preclinical and clinical studies to promote antitumor activity of T lymphocytes. To define cellular mechanisms responsible for antitumor effects of IRX-2, its ability to induce effector T cells (Teff) was examined in a model simulating the tumor microenvironment. An in vitro model containing conventional CD4⁺CD25⁻ cells co-cultured with autologous immature dendritic cells, irradiated tumor cells, and cytokines was used to study differentiation and expansion of regulatory T cells (Treg) and Teff in the presence and absence of IRX-2. Phenotype, suppressor function, signaling, and cytokine production were serially measured using flow cytometry, Western blots, CFSE-based suppressor assays, and Luminex-based analyses. The presence of IRX-2 in the co-cultures promoted the induction and expansion of IFN-γ⁺Tbet⁺ Teff and significantly (p < 0.01) decreased the induction of inducible IL-10⁺TGF-β⁺ Treg. The responsible mechanism involved IFN-γ-driven T cell polarization towards Teff and suppression of Treg differentiation. In an in vitro model simulating the human tumor microenvironment, IRX-2 promoted Teff expansion and antitumor activity without inducing Treg. Thus, IRX-2 could be considered as a promising component of future antitumor therapies.

Phospho-flow cytometry based analysis of differences in T cell receptor signaling between regulatory T cells and CD4+ T cells

CD4+ T regulatory cells (Tregs) are activated during auto-immune, injury, and inflammatory responses, however, the molecular events that trigger Treg activation are poorly understood. The purpose of this study was to investigate whether Tregs (FoxP3+ CD4+ T cells) and non-Treg CD4+ T cells might display differences in T cell receptor (TCR) dependent signaling responses following in vitro or in vivo stimulation. This study used phospho-flow cytometry as a tool to profile the kinetics and extent of TCR signaling (ZAP-70 and PKC-θ phosphorylation and expression) in Tregs and non-Tregs. We found that in vitro stimulation with anti-CD3ε induces early and transient activation of ZAP-70 and PKC-θ in both Tregs and non-Tregs. However, the response in Tregs was more rapid and higher in magnitude than responses seen in non-Tregs. In contrast, bacterial superantigen or antigen-specific TCR stimulation did not significantly activate these signaling pathways in Tregs or non-Tregs. Additional experiments tested the kinetics of in vivo TCR signaling in Tregs and non-Tregs in mice challenged with bacterial superantigen. The results of these experiments showed that superantigen rapidly activated ZAP-70 and PKC-θ in lymph node Tregs, but not in non-Tregs. In summary, we demonstrate the versatility of using phospho-flow cytometry to measure cell signaling in CD4+ T cells. The results of these in vitro and in vivo studies demonstrate that Tregs and non-Treg CD4+ T cells show marked differences in their reactivity to TCR-dependent stimulation and contribute new insights into basic mechanisms that lead to Treg activation.

Functional human regulatory T cells fail to control autoimmune inflammation due to PKB/c-akt hyperactivation in effector cells

During the last decade research has focused on the application of FOXP3(+) regulatory T cells (Tregs) in the treatment of autoimmune disease. However, thorough functional characterization of these cells in patients with chronic autoimmune disease, especially at the site of inflammation, is still missing. Here we studied Treg function in patients with juvenile idiopathic arthritis (JIA) and observed that Tregs from the peripheral blood as well as the inflamed joints are fully functional. Nevertheless, Treg-mediated suppression of cell proliferation and cytokine production by effector cells from the site of inflammation was severely impaired, because of resistance to suppression. This resistance to suppression was not caused by a memory phenotype of effector T cells or activation status of antigen presenting cells. Instead, activation of protein kinase B (PKB)/c-akt was enhanced in inflammatory effector cells, at least partially in response to TNFα and IL-6, and inhibition of this kinase restored responsiveness to suppression. We are the first to show that PKB/c-akt hyperactivation causes resistance of effector cells to suppression in human autoimmune disease. Furthermore, these findings suggest that for a Treg enhancing strategy to be successful in the treatment of autoimmune inflammation, resistance because of PKB/c-akt hyperactivation should be targeted as well.

Injury induces early activation of T-cell receptor signaling pathways in CD4+ regulatory T cells

Although it is known that injury enhances the regulatory activity of CD4 regulatory T cells (Tregs), the cellular and molecular mechanisms responsible for injury-induced Treg activation remain unclear. This study was designed to investigate and compare injury-induced T-cell receptor (TCR) signaling in Tregs, non-Tregs, and CD8 T cells. Specifically, we used phospho-flow cytometry to measure the expression and phosphorylation of ZAP-70, protein kinase C θ, nuclear factor of activated T cells, and glycogen synthase kinase 3β in FoxP3 Tregs versus FoxP3 non-Tregs versus CD8 T cells. Groups of male C57BL/6J mice underwent burn or sham injury, and lymph nodes and spleens were harvested at early time points-15, 30, 60, 120, and 240 min-to measure TCR signaling. As early as 15 min after burn injury, we observed a significant upregulation and phosphorylation of ZAP-70, protein kinase C θ, nuclear factor of activated T cells, and glycogen synthase kinase 3β in Tregs prepared from injury-site-draining lymph nodes. Burn injury did not activate TCR signaling in Tregs from the spleen or in CD4 non-Tregs and CD8 T cells. In conclusion, the results of this study demonstrate that burn injury activates TCR signaling in Tregs, but not non-Tregs or CD8 T cells. These findings suggest that injury provides an early TCR-activating signal to Tregs and supply new insights into how injury influences the adaptive immune system.

Moving to tolerance: clinical application of T regulatory cells

Decreasing the incidence of chronic rejection and reducing the need for life-long immunosuppression remain important goals in clinical transplantation. In this article, we will review how regulatory T cells (Treg) came to be recognized as an attractive way to prevent or treat allograft rejection, the ways in which Treg can be manipulated or expanded in vivo, and the potential of in vitro expanded/generated Treg for cellular therapy. We will describe the first regulatory T cell therapies that have been or are in the process of being conducted in the clinic as well as the safety concerns of such therapies and how outcomes may be measured.

TLR2 stimulation drives human naive and effector regulatory T cells into a Th17-like phenotype with reduced suppressive function

Naturally occurring CD4(+)CD25(+)FOXP3(+) regulatory T cells suppress the activity of pathogenic T cells and prevent development of autoimmune responses. There is growing evidence that TLRs are involved in modulating regulatory T cell (Treg) functions both directly and indirectly. Specifically, TLR2 stimulation has been shown to reduce the suppressive function of Tregs by mechanisms that are incompletely understood. The developmental pathways of Tregs and Th17 cells are considered divergent and mutually inhibitory, and IL-17 secretion has been reported to be associated with reduced Treg function. We hypothesized that TLR2 stimulation may reduce the suppressive function of Tregs by regulating the balance between Treg and Th17 phenotype and function. We examined the effect of different TLR2 ligands on the suppressive functions of Tregs and found that activation of TLR1/2 heterodimers reduces the suppressive activity of CD4(+)CD25(hi)FOXP3(low)CD45RA(+) (naive) and CD4(+)CD25(hi)FOXP3(hi)CD45RA(-) (memory or effector) Treg subpopulations on CD4(+)CD25(-)FOXP3(-)CD45RA(+) responder T cell proliferation while at the same time enhancing the secretion of IL-6 and IL-17, increasing RORC, and decreasing FOXP3 expression. Neutralization of IL-6 or IL-17 abrogated Pam3Cys-mediated reduction of Treg suppressive function. We also found that, in agreement with recent observations in mouse T cells, TLR2 stimulation can promote Th17 differentiation of human T helper precursors. We conclude that TLR2 stimulation, in combination with TCR activation and costimulation, promotes the differentiation of distinct subsets of human naive and memory/effector Tregs into a Th17-like phenotype and their expansion. Such TLR-induced mechanism of regulation of Treg function could enhance microbial clearance and increase the risk of autoimmune reactions.

Single-cell analysis of the human T regulatory population uncovers functional heterogeneity and instability within FOXP3+ cells

Natural FOXP3(+)CD4(+)CD25(High) regulatory T cells are critical in immunological self-tolerance. Their characterization in humans is hindered by the failure to discriminate these cells from activated effector T cells in inflammation. To explore the relationship between FOXP3 expression and regulatory function at the clonal level, we used a single-cell cloning strategy of CD25-expressing CD4(+) T cell subsets from healthy human donors. Our approach unveils a functional heterogeneity nested within CD4(+)CD25(High)FOXP3(+) T cells, and typically not revealed by conventional bulk assays. Whereas most cells display the canonical regulatory T (T(reg)) cell characteristics, a significant proportion of FOXP3(+) T cells is compromised in its suppressive function, despite the maintenance of other phenotypic and functional regulatory T hallmark features. In addition, these nonsuppressive FOXP3(+) T cells preferentially emerge from the CD45RO(+) memory pool, and arise as a consequence of a rapid downregulation of FOXP3 expression upon T cell reactivation. Surprisingly, these dysfunctional T(reg) cells with unstable FOXP3 expression do not manifest overt plasticity in terms of inflammatory cytokine secretion. These results open a path to an extensive study of the functional heterogeneity of CD4(+)CD25(High)FOXP3(+) T(reg) cells and warrant caution in the sole use of FOXP3 as a clinical marker for monitoring of immune regulation in humans.

Stability of human rapamycin-expanded CD4+CD25+ T regulatory cells

BACKGROUND

The clinical use of ex vivo-expanded T-regulatory cells for the treatment of T-cell-mediated diseases has gained increasing momentum. However, the recent demonstration that FOXP3(+) T-regulatory cells may contain interleukin-17-producing cells and that they can convert into effector cells once transferred in vivo raises significant doubts about their safety. We previously showed that rapamycin permits the ex vivo expansion of FOXP3(+) T-regulatory cells while impairing the proliferation of non-T-regulatory cells. Here we investigated the Th17-cell content and the in vivo stability of rapamycin-expanded T-regulatory cells as pertinent aspects of cell-based therapy.

DESIGN AND METHODS

T-regulatory-enriched cells were isolated from healthy volunteers and were expanded ex vivo with rapamycin with a pre-clinical applicable protocol. T-regulatory cells cultured with and without rapamycin were compared for their regulatory activity, content of pro-inflammatory cells and stability.

RESULTS

We found that CD4(+)CCR6(+)CD161(+) T cells (i.e., precursor/committed Th17 cells) contaminate the T-regulatory cells cultured ex vivo in the absence of rapamycin. In addition, Th17 cells do not expand when rapamycin-treated T-regulatory cells are exposed to a "Th17-favorable" environment. Rapamycin-expanded T-regulatory cells maintain their in vitro regulatory phenotype even after in vivo transfer into immunodeficient NOD-SCID mice despite being exposed to the irradiation-induced pro-inflammatory environment. Importantly, no additional rapamycin treatment, either in vitro or in vivo, is required to keep their phenotype fixed.

CONCLUSIONS

These data demonstrate that rapamycin secures ex vivo-expanded human T-regulatory cells and provide additional justification for their clinical use in future cell therapy-based trials.

Foxo transcription factors control regulatory T cell development and function

Foxo transcription factors integrate extrinsic signals to regulate cell division, differentiation and survival, and specific functions of lymphoid and myeloid cells. Here, we showed the absence of Foxo1 severely curtailed the development of Foxp3(+) regulatory T (Treg) cells and those that developed were nonfunctional in vivo. The loss of function included diminished CTLA-4 receptor expression as the Ctla4 gene was a direct target of Foxo1. T cell-specific loss of Foxo1 resulted in exocrine pancreatitis, hind limb paralysis, multiorgan lymphocyte infiltration, anti-nuclear antibodies and expanded germinal centers. Foxo-mediated control over Treg cell specification was further revealed by the inability of TGF-β cytokine to suppress T-bet transcription factor in the absence of Foxo1, resulting in IFN-γ secretion. In addition, the absence of Foxo3 exacerbated the effects of the loss of Foxo1. Thus, Foxo transcription factors guide the contingencies of T cell differentiation and the specific functions of effector cell populations.

Intrinsic and extrinsic control of peripheral T-cell tolerance by costimulatory molecules of the CD28/ B7 family

Positive and negative costimulation by members of the CD28 family is critical for the development of productive immune responses against foreign pathogens and their proper termination to prevent inflammation-induced tissue damage. In addition, costimulatory signals are critical for the establishment and maintenance of peripheral tolerance. This paradigm has been established in many animal models and has led to the development of immunotherapies targeting costimulation pathways for the treatment of cancer, autoimmune disease, and allograft rejection. During the last decade, the complexity of the biology of costimulatory pathways has greatly increased due to the realization that costimulation does not affect only effector T cells but also influences regulatory T cells and antigen-presenting cells. Thus, costimulation controls T-cell tolerance through both intrinsic and extrinsic pathways. In this review, we discuss the influence of costimulation on intrinsic and extrinsic pathways of peripheral tolerance, with emphasis on members of the CD28 family, CD28, cytotoxic T-lymphocyte antigen-4 (CTLA-4), and programmed death-1 (PD-1), as well as the downstream cytokine interleukin-1 (IL-2).