Regulatory T cell plasticity: beyond the controversies

Profiling of Lymphovascular Space Invasion in Cervical Cancer Revealed PI3K/Akt Signaling Pathway Overactivation and Heterogenic Tumor-Immune Microenvironments

Lymphovascular space invasion (LVSI) is the presence of tumor emboli in the endothelial-lined space at the tumor body's invasive edge. LVSI is one of three Sedlis criteria components-a prognostic tool for early cervical cancer (CC)-essential for indicating poor prognosis, such as lymph node metastasis, distant metastasis, or shorter survival rate. Despite its clinical significance, an in-depth comprehension of the molecular mechanisms or immune dynamics underlying LVSI in CC remains elusive. Therefore, this study investigated tumor-immune microenvironment (TIME) dynamics of the LVSI-positive group in CC. RNA sequencing included formalin-fixed paraffin-embedded (FFPE) slides from 21 CC patients, and differentially expressed genes (DEGs) were analyzed. Functional analysis and immune deconvolution revealed aberrantly enriched PI3K/Akt pathway activation and a heterogenic immune composition with a low abundance of regulatory T cells (Treg) between LVSI-positive and LVSI-absent groups. These findings improve the comprehension of LSVI TIME and immune mechanisms, benefiting targeted LVSI therapy for CC.

The role of transcription factors in shaping regulatory T cell identity

Forkhead box protein 3-expressing (FOXP3+) regulatory T cells (Treg cells) suppress conventional T cells and are essential for immunological tolerance. FOXP3, the master transcription factor of Treg cells, controls the expression of multiples genes to guide Treg cell differentiation and function. However, only a small fraction (<10%) of Treg cell-associated genes are directly bound by FOXP3, and FOXP3 alone is insufficient to fully specify the Treg cell programme, indicating a role for other accessory transcription factors operating upstream, downstream and/or concurrently with FOXP3 to direct Treg cell specification and specialized functions. Indeed, the heterogeneity of Treg cells can be at least partially attributed to differential expression of transcription factors that fine-tune their trafficking, survival and functional properties, some of which are niche-specific. In this Review, we discuss the emerging roles of accessory transcription factors in controlling Treg cell identity. We specifically focus on members of the basic helix-loop-helix family (AHR), basic leucine zipper family (BACH2, NFIL3 and BATF), CUT homeobox family (SATB1), zinc-finger domain family (BLIMP1, Ikaros and BCL-11B) and interferon regulatory factor family (IRF4), as well as lineage-defining transcription factors (T-bet, GATA3, RORγt and BCL-6). Understanding the imprinting of Treg cell identity and specialized function will be key to unravelling basic mechanisms of autoimmunity and identifying novel targets for drug development.

The Effects of Tamoxifen on Tolerogenic Cells in Cancer

Tamoxifen (TAM) is the most prescribed selective estrogen receptor modulator (SERM) to treat hormone-receptor-positive breast cancer patients and has been used for more than 20 years. Its role as a hormone therapy is well established; however, the potential role in modulating tolerogenic cells needs to be better clarified. Infiltrating tumor-microenvironment-regulatory T cells (TME-Tregs) are important as they serve a suppressive function through the transcription factor Forkhead box P3 (Foxp3). Abundant studies have suggested that Foxp3 regulates the expression of several genes (CTLA-4, PD-1, LAG-3, TIM-3, TIGIT, TNFR2) involved in carcinogenesis to utilize its tumor suppressor function through knockout models. TAM is indirectly concomitant via the Cre/loxP system by allowing nuclear translocation of the fusion protein, excision of the floxed STOP cassette and heritable expression of encoding fluorescent protein in a cohort of cells that express Foxp3. Moreover, TAM administration in breast cancer treatment has shown its effects directly through MDSCs by the enrichment of its leukocyte populations, such as NK and NKT cells, while it impairs the differentiation and activation of DCs. However, the fundamental mechanisms of the reduction of this pool by TAM are unknown. Here, we review the vital effects of TAM on Tregs for a precise mechanistic understanding of cancer immunotherapies.

Epigenetic Control of Regulatory T Cell Stability and Function: Implications for Translation

FoxP3+ regulatory T (Treg) cells maintain immune homeostasis, promote self-tolerance, and have an emerging role in resolving acute inflammation, providing tissue protection, and repairing tissue damage. Some data suggest that FoxP3+ T cells are plastic, exhibiting susceptibility to losing their function in inflammatory cytokine-rich microenvironments and paradoxically contributing to inflammatory pathology. As a result, plasticity may represent a barrier to Treg cell immunotherapy. Here, we discuss controversies surrounding Treg cell plasticity and explore determinants of Treg cell stability in inflammatory microenvironments, focusing on epigenetic mechanisms that clinical protocols could leverage to enhance efficacy and limit toxicity of Treg cell-based therapeutics.

Bone Morphogenic Proteins Are Immunoregulatory Cytokines Controlling FOXP3+ Treg Cells

Bone morphogenic proteins (BMPs) are members of the transforming growth factor β (TGF-β) cytokine family promoting differentiation, homeostasis, and self-renewal of multiple tissues. We show that signaling through the bone morphogenic protein receptor 1α (BMPR1α) sustains expression of FOXP3 in T_reg cells in peripheral lymphoid tissues. BMPR1α signaling promotes molecular circuits supporting acquisition and preservation of T_reg cell phenotype and inhibiting differentiation of pro-inflammatory effector Th1/Th17 CD4⁺ T cell. Mechanistically, increased expression of KDM6B (JMJD3) histone demethylase, an antagonist of the polycomb repressive complex 2, underlies lineage-specific changes of T cell phenotypes associated with abrogation of BMPR1α signaling. These results reveal that BMPs are immunoregulatory cytokines mediating maturation and stability of peripheral FOXP3⁺ regulatory T cells (T_reg cells) and controlling generation of iT_reg cells. Thus, we establish that BMPs, a large cytokine family, are an essential link between stromal tissues and the adaptive immune system involved in sustaining tissue homeostasis by promoting immunological tolerance.

Multiple developmental pathways lead to the generation of CD4 T-cell memory

Long-term immunological memory mediated by CD4 T cells provides a rapid protection against previously encountered pathogens or antigens. However, it is still controversial how memory CD4 T cells are generated and maintained. Unclear definitions of T-cell memory may be partially responsible for this controversy. It is becoming clear that diverse pathways are responsible for the differentiation and long-term persistence of memory T cells. We herein discuss the diversity of memory cell generation, describing a novel population of resting memory CD4 T cells and their precursors.

Turning the Tide Against Regulatory T Cells

Regulatory T (Treg) cells play crucial roles in health and disease through their immunosuppressive properties against various immune cells. In this review we will focus on the inhibitory role of Treg cells in anti-tumor immunity. We outline how Treg cells restrict T cell function based on our understanding of T cell biology, and how we can shift the equilibrium against regulatory T cells. To date, numerous strategies have been proposed to limit the suppressive effects of Treg cells, including Treg cell neutralization, destabilizing Treg cells and rendering T cells resistant to Treg cells. Here, we focus on key mechanisms which render T cells resistant to the suppressive effects of Treg cells. Lastly, we also examine current limitations and caveats of overcoming the inhibitory activity of Treg cells, and briefly discuss the potential to target Treg cell resistance in the context of anti-tumor immunity.

The Role of T Cells in Herpes Stromal Keratitis

The blinding inflammatory lesion stromal keratitis (SK), which occurs in some patients in response to ocular herpes simplex virus (HSV) infection, represents mainly an immune cell mediated inflammatory response to the virus infection. The principal orchestrators of the immunopathological lesions are T cells although additional events participate that include the extent of recruitment of non-lymphoid cells, the extent of neoangiogenesis, and the extent of damage to nerve function. This review focuses on evidence that the balance of the functional subsets of T cells has a major impact on lesion severity and duration. Accordingly, if proinflammatory Th1 and Th17 CD4 T cells, and perhaps in some cases CD8 T cells, predominate lesions occur earlier and are more severe. Lesions are diminished when cells with regulatory function predominate. Moreover, when regulatory cells acquire the property to produce Amphiregulin this may facilitate lesion resolution. An objective to controlling lesions is to learn how to manipulate the balance of T cells to favor the representation and function of regulatory T cells and their products over proinflammatory cells. In this review we emphasize how exploiting the differential metabolic requirements of immune cells could be a valuable approach to control SK.

Decreased CD4+CD25+CD127dim/- Regulatory T Cells and T Helper 17 Cell Responsiveness to Toll-Like Receptor 2 in Chronic Hepatitis C Patients with Daclatasvir Plus Asunaprevir Therapy

Direct-acting antivirals (DAAs) not only rapidly inhibited hepatitis C virus (HCV) replication but also modulated innate and adaptive immune response in chronic hepatitis C patients. However, the regulatory activity of DAAs to Toll-like receptor 2 (TLR2) stimulation on CD4⁺CD25⁺CD127^dim/- regulatory T cells (Tregs) and T helper (Th) 17 cells was not completely understood. In the present study, a total of 23 patients with chronic HCV genotype 1b infection were enrolled, and blood samples were collected at baseline (treatment naive), end of therapy (EOT), and 12 weeks after EOT (SVR12) with daclatasvir plus asunaprevir therapy. TLR2 expression on Tregs and Th17 cells was measured by flow cytometry. Cellular proliferation, cytokine production, and suppressive activity were also tested in purified CD4⁺CD25⁺CD127^dim/- Tregs in response to the stimulation of Pam3Csk4, an agonist of TLR2. Inhibition of HCV RNA by daclatasvir and asunaprevir did not affect either percentage of Tregs/Th17 cells or TLR2 expression on Tregs/Th17 cells. Pam3Csk4 stimulation also did not influence either cellular proliferation or Tregs/Th17 proportion at each time point. Stimulation with Pam3Csk4 only enhanced the suppressive function and interleukin (IL)-35 production by Tregs purified from baseline, but not those from EOT or SVR12. Similarly, Pam3Csk4 stimulation only elevated Th17 cell frequency of CD4⁺ T cells from baseline, but not those from EOT or SVR12. Moreover, daclatasvir and asunaprevir therapy did not promote TLR2-induced shift of Tregs toward Th17-like phenotype and function. These data suggested that daclatasvir plus asunaprevir therapy resulted in the decreased responsiveness of Tregs/Th17 cells to TLR2 stimulation in chronic hepatitis C patients, which might provide a novel mechanism underlying DAA-induced immunoregulation.

Regulatory T Cells As Supporters of Psychoimmune Resilience: Toward Immunotherapy of Major Depressive Disorder

There is growing evidence that inflammation plays a role in major depressive disorder (MDD). As the main role of regulatory T cells (Tregs) is to control inflammation, this might denote a Treg insufficiency in MDD. However, neither a qualitative nor a quantitative defect of Tregs has been ascertained and no causality direction between inflammation and depression has been established. Here, after reviewing the evidence supporting a relation between Treg insufficiency and MDD, we conclude that a novel therapeutic approach based on Treg stimulation could be valuable in at least the subset of patients with inflammatory MDD. Low-dose interleukin-2 appears to be a good candidate as it is not only a safe stimulator of Tregs in humans but also an inhibitor of pro-inflammatory Th17 lymphocytes. Here, we discuss that a thorough immune investigation as well as immunotherapy will be heuristic for deciphering the pathophysiology of MDD.

Regulatory T cells: tolerance induction in solid organ transplantation

The concept of regulatory T cell (T_reg ) therapy in transplantation is now a reality. Significant advances in science and technology have enabled us to isolate human T_regs , expand them to clinically relevant numbers and infuse them into human transplant recipients. With several Phase I/II trials under way investigating T_reg safety and efficacy it is now more crucial than ever to understand their complex biology. However, our journey is by no means complete; results from these trials will undoubtedly provoke both further knowledge and enquiry which, alongside evolving science, will continue to drive the optimization of T_reg therapy in the pursuit of transplantation tolerance. In this review we will summarize current knowledge of T_reg biology, explore novel technologies in the setting of T_reg immunotherapy and address key prerequisites surrounding the clinical application of T_regs in transplantation.

Increased peripheral CD4+ regulatory T cells persist after successful direct-acting antiviral treatment of chronic hepatitis C

BACKGROUND & AIMS

CD4⁺ regulatory T cells (Tregs) expand during chronic hepatitis C virus (HCV) infection, inhibit antiviral immunity and promote fibrosis. Direct-acting antiviral agents (DAA) have revolutionized HCV therapy. However, it is unclear if Tregs are normalized after DAA-induced HCV elimination.

METHODS

We analyzed Tregs before (baseline), at end of therapy (EOT), 12 and 24weeks (SVR12, SVR24) and long-term (51±14weeks) after EOT in 26 genotype-1-infected patients who were successfully treated with sofosbuvir (SOF) plus interferon (IFN)/ribavirin (n=12) and IFN-free DAA regimens (SOF plus daclatasvir or simeprevir; n=14). Frequency, phenotype and suppressor function of peripheral Foxp3⁺ CD25⁺ CD4⁺ T cells were studied by multi-color flow cytometry and co-culture inhibition assays.

RESULTS

Frequencies and activation status of Foxp3⁺ CD25⁺ CD4⁺ T cells remained elevated above those of normal controls in both treatment groups even long-term after HCV elimination. Co-culture assays indicated a dose-response relationship for functional inhibition of autologous CD4⁺ effector T cells and confirmed that activation of Tregs remained largely unchanged over the observation period. Unlike IFN-free regimens, SOF plus IFN/ribavirin induced a transiently increased frequency of Foxp3⁺ CD25⁺ CD4⁺ T cells at EOT (5.0% at baseline to 6.1% at EOT; p=0.001). These Foxp3⁺ CD25⁺ CD4⁺ T cells co-expressed the activation markers glycoprotein A repetitions predominant (GARP; p=0.012) and tumor necrosis factor receptor superfamily, member 4 (OX-40; p=0.001) but showed unchanged in vitro inhibitory activity.

CONCLUSION

Although IFN-based DAA therapy induced transient expansion of activated Foxp3⁺ CD25⁺ CD4⁺ T cells, neither IFN-based nor IFN-free DAA regimens normalized frequencies and activation status of Tregs one year after viral elimination. Persistence of immunosuppressive Tregs may thus contribute to complications of liver disease even long-term after HCV cure.

LAY SUMMARY

In chronic hepatitis C virus (HCV) infection, CD4⁺ regulatory T cells (Tregs) can reduce antiviral immune responses, promote liver fibrosis and may increase the risk for liver cancer, because they gradually expand during disease. Modern direct-acting antiviral agents (DAA) can "cure" hepatitis C in almost all treated patients. However, our study shows that DAA do not normalize the increased frequency and activation status of Tregs even long-term after HCV elimination. Tregs may persistently modulate functions of the immune system even after "cure" of hepatitis C.

Ubiquitin Ligases and Deubiquitinating Enzymes in CD4+ T Cell Effector Fate Choice and Function

The human body is exposed to potentially pathogenic microorganisms at barrier sites such as the skin, lungs, and gastrointestinal tract. To mount an effective response against these pathogens, the immune system must recruit the right cells with effector responses that are appropriate for the task at hand. Several types of CD4(+) T cells can be recruited, including Th cells (Th1, Th2, and Th17), T follicular helper cells, and regulatory T cells. These cells help to maintain normal immune homeostasis in the face of constantly changing microbes in the environment. Because these cells differentiate from a common progenitor, the composition of their intracellular milieu of proteins changes to appropriately guide their effector function. One underappreciated process that impacts the levels and functions of effector fate-determining factors is ubiquitylation. This review details our current understanding of how ubiquitylation regulates CD4(+) T cell effector identity and function.

FcγRIIIa-Syk Co-signal Modulates CD4+ T-cell Response and Up-regulates Toll-like Receptor (TLR) Expression

CD4(+) T-cells in systemic lupus erythematosus (SLE) patients show altered T-cell receptor signaling, which utilizes Fc-receptor γ-chain FcRγ-Syk. A role for FcγRIIIa activation from immune complex (IC) ligation and sublytic terminal complement complex (C5b-9) in CD4(+) T-cell responses is not investigated. In this study, we show that the ICs present in SLE patients by ligating to FcγRIIIa on CD4(+) T-cells phosphorylate Syk and provide a co-stimulatory signal to CD4(+) T-cells in the absence of CD28 signal. This led to the development of pathogenic IL-17A(+) and IFN-γ(high) CD4(+) T-cells in vitro. Cytokines IL-1β, IL-6, TGF-β1, and IL-23 were the only requirement for the development of both populations. SLE patients CD4(+) T-cells that expressed CD25, CD69, and CD98 bound to ICs showed pSyk and produced IFN-γ and IL-17A. This FcγRIIIa-mediated co-signal differentially up-regulated the expression of IFN pathway genes compared with CD28 co-signal. FcγRIIIa-pSyk up-regulated several toll-like receptor genes as well as the HMGB1 and MyD88 gene transcripts. ICs co-localized with these toll-like receptor pathway proteins. These results suggest a role for the FcγRIIIa-pSyk signal in modulating adaptive immune responses.

The Macrophage Galactose-Type C-Type Lectin (MGL) Modulates Regulatory T Cell Functions

Regulatory T cells (Tregs) are physiologically designed to prevent autoimmune disease and maintain self-tolerance. In tumour microenvironments, their presence is related to a poor prognosis, and they influence the therapeutic outcome due to their capacity to suppress the immune response by cell-cell contact and to release immunosuppressive cytokines. In this study, we demonstrate that Treg immunosuppressive activity can be modulated by the cross-linking between the CD45RA expressed by Tregs and the C-type lectin MGL. This specific interaction strongly decreases the immunosuppressive activity of Tregs, restoring the proliferative capacity of co-cultured T lymphocytes. This effect can be attributed to changes in CD45RA and TCR signalling through the inhibition of Lck and inactivation of Zap-70, an increase in the Foxp3 methylation status and, ultimately, the reduced production of suppressive cytokines. These results indicate a role of MGL as an immunomodulator within the tumour microenvironment interfering with Treg functions, suggesting its possible use in the design of anticancer vaccines.

Regulatory Rebound in IL-12-Treated Tumors Is Driven by Uncommitted Peripheral Regulatory T Cells

IL-12 promotes a rapid reversal of immune suppression in the tumor microenvironment. However, the adjuvant activity of IL-12 is short-lived due to regulatory T cell (Treg) reinfiltration. Quantitative analysis of Treg kinetics in IL-12-treated tumors and tumor-draining lymph nodes revealed a transient loss followed by a rapid 4-fold expansion of tumor Treg between days 3 and 10. Subset-specific analysis demonstrated that the posttreatment rebound was driven by the CD4(+)CD25(+)Foxp3(+) neuropilin-1(low) peripheral Treg (pTreg), resulting in a 3-5-fold increase in the pTreg to CD4(+)CD25(+)Foxp3(+) neuropilin-1(high) thymic Treg ratio by day 10. The expanding pTreg displayed hypermethylation of the CpG islands in Treg-specific demethylated region, CTLA-4 exon 2, and glucocorticoid-induced TNFR exon 5, were phenotypically unstable, and exhibited diminished suppressive function consistent with an uncommitted in vitro-induced Treg-like phenotype. In vitro culture of posttherapy Treg populations under Th1-promoting conditions resulted in higher levels of IFN-γ production by pTreg compared with thymic Treg, confirming their transitional state. Blockade of selected molecular mechanisms that are known to promote Treg expansion identified IDO-positive dendritic cells as the primary mediator of post-IL-12 pTreg expansion. Clinical implications of these findings are discussed.

Peak inflammation in atherosclerosis, primary biliary cirrhosis and autoimmune arthritis is counter-intuitively associated with regulatory T cell enrichment

Regulatory T cells (Treg) influence the development of autoimmunity and their use is increasingly proposed for clinical applications. The well-characterized suppressive potential of Treg frequently leads to the assumption that Treg presence in prevailing numbers is indicative of immunosuppression. We hypothesized that this assumption may be false. We examined models of three different diseases caused by organ-specific autoimmune responses: primary biliary cirrhosis, atherosclerosis and rheumatoid arthritis (RA). We examined indicators of relative abundance of Treg compared to pro-inflammatory T cells, during peak inflammation. In all cases, the results were compatible with a relative enrichment of Treg at the site of inflammation or its most proximal draining lymph node. Conversely, in healthy mice or mice successfully protected from disease via a Treg-mediated mechanism, the data did not suggest that any Treg accumulation was occurring. This counter-intuitive finding may appear to be at odds with the immunosuppressive nature of Treg. Yet extensive previous studies in RA show that an accumulation of Treg occurs at peak inflammation, albeit without resulting in suppression, as the Treg suppressive function is overcome by the cytokine-rich environment. We suggest that this is a ubiquitous feature of autoimmune inflammation. Treg abundance in patient samples is increasingly used as an indicator of a state of immunosuppression. We conclude that this strategy should be revisited as it may potentially be a source of misinterpretation.

CD4 T-cell memory generation and maintenance

Immunologic memory is the adaptive immune system's powerful ability to remember a previous antigen encounter and react with accelerated vigor upon antigen re-exposure. It provides durable protection against reinfection with pathogens and is the foundation for vaccine-induced immunity. Unlike the relatively restricted immunologic purview of memory B cells and CD8 T cells, the field of CD4 T-cell memory must account for multiple distinct lineages with diverse effector functions, the issue of lineage commitment and plasticity, and the variable distribution of memory cells within each lineage. Here, we discuss the evidence for lineage-specific CD4 T-cell memory and summarize the known factors contributing to memory-cell generation, plasticity, and long-term maintenance.

Plasticity of human CD4 T cell subsets

Human beings are exposed to a variety of different pathogens, which induce tailored immune responses and consequently generate highly diverse populations of pathogen-specific T cells. CD4(+) T cells have a central role in adaptive immunity, since they provide essential help for both cytotoxic T cell- and antibody-mediated responses. In addition, CD4(+) regulatory T cells are required to maintain self-tolerance and to inhibit immune responses that could damage the host. Initially, two subsets of CD4(+) helper T cells were identified that secrete characteristic effector cytokines and mediate responses against different types of pathogens, i.e., IFN-γ secreting Th1 cells that fight intracellular pathogens, and IL-4 producing Th2 cells that target extracellular parasites. It is now well established that this dichotomy is insufficient to describe the complexity of CD4(+) T cell differentiation, and in particular the human CD4 compartment contains a myriad of T cell subsets with characteristic capacities to produce cytokines and to home to involved tissues. Moreover, it has become increasingly clear that these T cell subsets are not all terminally differentiated cells, but that the majority is plastic and that in particular central memory T cells can acquire different properties and functions in secondary immune responses. In addition, there is compelling evidence that helper T cells can acquire regulatory functions upon chronic stimulation in inflamed tissues. The plasticity of antigen-experienced human T cell subsets is highly relevant for translational medicine, since it opens new perspectives for immune-modulatory therapies for chronic infections, autoimmune diseases, and cancer.

Treg functional stability and its responsiveness to the microenvironment

Regulatory T cells (Tregs) prevent autoimmunity and tissue damage resulting from excessive or unnecessary immune activation through their suppressive function. While their importance for proper immune control is undeniable, the stability of the Treg lineage has recently become a controversial topic. Many reports have shown dramatic loss of the signature Treg transcription factor Forkhead box protein 3 (Foxp3) and Treg function under various inflammatory conditions. Other recent studies demonstrate that most Tregs are extremely resilient in their expression of Foxp3 and the retention of suppressive function. While this debate is unlikely to be settled in the immediate future, improved understanding of the considerable heterogeneity within the Foxp3(+) Treg population and how Treg subsets respond to ranging environmental cues may be keys to reconciliation. In this review, we discuss the diverse mechanisms responsible for the observed stability or instability of Foxp3(+) Treg identity and function. These include transcriptional and epigenetic programs, transcript targeting, and posttranslational modifications that appear responsive to numerous elements of the microenvironment. These mechanisms for Treg functional modulation add to the discussion of Treg stability.

Regulatory T-cell therapy in the induction of transplant tolerance: the issue of subpopulations

Clinical tolerance induction to permit minimization or cessation of immunosuppressive drugs is one of the key research goals in solid organ transplantation. The use of ex vivo expanded or manipulated immunologic cells, including CD4CD25FOXP3 regulatory T cells (Tregs), to achieve this aim is already a reality, with several trials currently recruiting patients. Tregs are a highly suppressive, nonredundant, population of regulatory cells that prevent the development of autoimmune diseases in mammals. Data from transplanted humans and animal models support the notion that Tregs can mediate both induction and adoptive transfer of transplantation tolerance. However, human Tregs are highly heterogeneous and include subpopulations with the potential to produce the proinflammatory cytokine interleukin-17, which has been linked to transplant rejection. Tregs are also small in number in the peripheral circulation, thus they require ex vivo expansion before infusion into man. Selection of the most appropriate Treg population for cell therapy is, therefore, a critical step in ensuring successful clinical outcomes. In this review, we discuss Treg subpopulations, their subdivision based on nonmutually exclusive criteria of origin, expression of immunologic markers and function, availability in the peripheral blood of patients awaiting transplantation, and their suitability for programs of cell-based therapy.

Transcriptional and epigenetic networks of helper T and innate lymphoid cells

The discovery of the specification of CD4(+) helper T cells to discrete effector 'lineages' represented a watershed event in conceptualizing mechanisms of host defense and immunoregulation. However, our appreciation for the actual complexity of helper T-cell subsets continues unabated. Just as the Sami language of Scandinavia has 1000 different words for reindeer, immunologists recognize the range of fates available for a CD4(+) T cell is numerous and may be underestimated. Added to the crowded scene for helper T-cell subsets is the continuously growing family of innate lymphoid cells (ILCs), endowed with common effector responses and the previously defined 'master regulators' for CD4(+) helper T-cell subsets are also shared by ILC subsets. Within the context of this extraordinary complexity are concomitant advances in the understanding of transcriptomes and epigenomes. So what do terms like 'lineage commitment' and helper T-cell 'specification' mean in the early 21st century? How do we put all of this together in a coherent conceptual framework? It would be arrogant to assume that we have a sophisticated enough understanding to seriously answer these questions. Instead, we review the current status of the flexibility of helper T-cell responses in relation to their genetic regulatory networks and epigenetic landscapes. Recent data have provided major surprises as to what master regulators can or cannot do, how they interact with other transcription factors and impact global genome-wide changes, and how all these factors come together to influence helper cell function.

Once a Treg, always a Treg?

Regulatory T cells (Tregs) prevail as a specialized cell lineage that has a central role in the dominant control of immunological tolerance and maintenance of immune homeostasis. Thymus-derived Tregs (tTregs) and their peripherally induced counterparts (pTregs) are imprinted with unique Forkhead box protein 3 (Foxp3)-dependent and independent transcriptional and epigenetic characteristics that bestows on them the ability to suppress disparate immunological and non-immunological challenges. Thus, unidirectional commitment and the predominant stability of this regulatory lineage is essential for their unwavering and robust suppressor function and has clinical implications for the use of Tregs as cellular therapy for various immune pathologies. However, recent studies have revealed considerable heterogeneity or plasticity in the Treg lineage, acquisition of alternative effector or hybrid fates, and promotion rather than suppression of inflammation in extreme contexts. In addition, the absolute stability of Tregs under all circumstances has been questioned. Since these observations challenge the safety and efficacy of human Treg therapy, the issue of Treg stability versus plasticity continues to be enthusiastically debated. In this review, we assess our current understanding of the defining features of Foxp3(+) Tregs, the intrinsic and extrinsic cues that guide development and commitment to the Treg lineage, and the phenotypic and functional heterogeneity that shapes the plasticity and stability of this critical regulatory population in inflammatory contexts.

Harnessing FOXP3+ regulatory T cells for transplantation tolerance

Early demonstrations that mice could be tolerized to transplanted tissues with short courses of immunosuppressive therapy and that with regard to tolerance to self, CD4+FOXP3+ regulatory T cells (Tregs) appeared to play a critical role, have catalyzed strategies to harness FOXP3-dependent processes to control rejection in human transplantation. This review seeks to examine the scientific underpinning for this new approach to finesse immunosuppression.

Helper T cell plasticity: impact of extrinsic and intrinsic signals on transcriptomes and epigenomes

CD4(+) helper T cells are crucial for autoimmune and infectious diseases; however, the recognition of the many, diverse fates available continues unabated. Precisely what controls specification of helper T cells and preserves phenotypic commitment is currently intensively investigated. In this review, we will discuss the major factors that impact helper T cell fate choice, ranging from cytokines and the microbiome to metabolic control and epigenetic regulation. We will also discuss the technological advances along with the attendant challenges presented by "big data," which allow the understanding of these processes on comprehensive scales.

Forkhead box P3: the peacekeeper of the immune system

Ten years ago Forkhead box P3 (FOXP3) was discovered as master gene driving CD4(+)CD25(+) T cell regulatory (Treg) function. Since then, several layers of complexity have emerged in the regulation of its expression and function, which is not only exerted in Treg cells. While the mechanisms leading to the highly selective expression of FOXP3 in thymus-derived Treg cells still remain to be elucidated, we review here the current knowledge on the role of FOXP3 in the development of Treg cells and the direct and indirect consequences of FOXP3 mutations on multiple arms of the immune response. Finally, we summarize the newly acquired knowledge on the epigenetic regulation of FOXP3, still largely undefined in human cells.

Selective depletion of FOXP3(high) cells by Fas-Fas-L-induced apoptosis occurs in CD4(+)CD25(+)-enriched populations during repeated expansion

BACKGROUND AIMS

Expansion of anti-CD25 bead-isolated human Tregs culture has paradoxically resulted in reduced suppressive activity, but the mechanism(s) responsible for these observations are poorly defined.

METHODS

Magnetic-bead isolated human CD25(+) cells were expanded with anti-CD3/CD28 beads and high doses of rhIL-2. Detection of Fas and Fas ligand (Fas-L) expression, activation of Caspase 8, cell proliferation and cytokine production was evaluated by multi-color fluorescence-activated cell sorting analysis. The role of Fas-Fas-L-mediated cell death was dissected through the use of agonist or antagonist monoclonal antibodies directed at Fas and Fas-L.

RESULTS

Repeated expansion of bead-enriched CD4(+)CD25(+) cells generated a cellular product with markedly reduced suppressive activity and with significantly increased CD8(+) T cells and CD4(+) T cells producing interferon-γ and/or interleukin-2. We showed that Fas-Fas-L-mediated apoptosis of CD4(+)FOXP3(high) cells and rapid cell-cycling of CD8(+) T cells were collectively responsible for the reduced proportion of CD4(+)FOXP3(high) cells in expanded cultures. The depletion of CD4(+)FOXP3(high) cells and activation of Caspase 8 in CD4(+)FOXP3(high) cells was attenuated by Fas antagonist antibody, ZB4, in short-term culture. However, the loss of CD4(+)FOXP3(high) cells during expansion was not prevented by either Fas or Fas-L antagonist antibodies.

CONCLUSIONS

Taken together, the data show that Fas-Fas-L-mediated apoptosis may limit the expansion of anti-CD25 bead-isolated cells in vitro.

The interplay between regulatory and effector T cells in autoimmune hepatitis: Implications for innovative treatment strategies

Autoimmune hepatitis is an immuno-mediated inflammatory liver disorder of unknown etiology and is characterized by hypergammaglobulinaemia, circulating autoantibodies and interface hepatitis. The disease may often present as an acute icteric hepatitis, or run an insidious and progressive course, and in most of the cases it is expected to evolve toward liver cirrhosis and end-stage liver failure, without prompt and appropriate treatment with steroids and other immunosuppressive drugs. Nonetheless, several patients are non-responsive or become non-tolerant to conventional therapy with prednisone/prednisolone with or without azathioprine. Recent findings highlight the role of the interplay between CD4+CD25+ regulatory T cells and Th17 cells in the pathogenesis of autoimmune hepatitis. A numerical and functional imbalance between regulatory and effector cells in favor of the latter appears to be pivotal in the progression of the disease. In addition, the intra-hepatic microenvironment of autoimmune hepatitis is particularly rich in pro-inflammatory cytokines such as IL-6, IL-17, IL-23, IL-1β which play a crucial role in perpetuating and expanding effector cells and subsequent liver damage, whereas regulatory T cells are greatly disadvantaged and inhibited in such polarized habitat. Novel therapeutic interventions should aim at modulating the intra-hepatic pro-inflammatory milieu while favoring the expansion of regulatory T cells. Liver autoantigen-specific regulatory T cells generated and expanded in vitro from patients' own cells might offer a potentially curative approach to autoimmune hepatitis by inhibiting effector cells of the same specificity without inducing pan-immunosuppression.

Modulation of regulatory T cell function by monocyte-derived dendritic cells matured through electroporation with mRNA encoding CD40 ligand, constitutively active TLR4, and CD70

Regulatory T cells (Tregs) counteract anticancer immune responses through a number of mechanisms, limiting dendritic cell (DC)-based anticancer immunotherapy. In this study, we investigated the influence of various DC activation stimuli on the Treg functionality. We compared DCs activated by electroporation with mRNA encoding constitutively active TLR4 (caTLR4) and CD40 ligand (DiMix-DCs), or these factors together with mRNA encoding the costimulatory molecule CD70 (TriMix-DCs) with DCs maturated in the presence of a mixture of inflammatory cytokines (DCs maturated with a combination of the cytokines IL-1β, IL-6, TNF-α, and PGE2) for their ability to counteract Tregs on different levels. We first demonstrated that there was no difference in the extent of Treg induction starting from CD4(+)CD25(-) T cells under the influence of the different DC maturation stimuli. Second, we showed that both DiMix- and TriMix-DCs could partly alleviate Treg inhibition of CD8(+) T cells. Third, we observed that CD8(+) T cells that had been precultured with DiMix-DCs or TriMix-DCs were partially protected against subsequent Treg suppression. Finally, we showed that Tregs cocultured in the presence of TriMix-DCs, but not DiMix-DCs, partially lost their suppressive capacity. This was accompanied by a decrease in CD27 and CD25 expression on Tregs, as well as an increase in the expression of T-bet and secretion of IFN-γ, TNF-α, and IL-10, suggesting a shift of the Treg phenotype toward a Th1 phenotype. In conclusion, these data suggest that TriMix-DCs are not only able to suppress Treg functions, but moreover could be able to reprogram Tregs to Th1 cells under certain circumstances.

CD161 expression characterizes a subpopulation of human regulatory T cells that produces IL-17 in a STAT3-dependent manner

Treg cells are critical for the prevention of autoimmune diseases and are thus prime candidates for cell-based clinical therapy. However, human Treg cells are “plastic”, and are able to produce IL-17 under inflammatory conditions. Here, we identify and characterize the human Treg subpopulation that can be induced to produce IL-17 and identify its mechanisms. We confirm that a subpopulation of human Treg cells produces IL-17 in vitro when activated in the presence of IL-1β, but not IL-6. “IL-17 potential” is restricted to population III (CD4⁺CD25^hiCD127^loCD45RA⁻) Treg cells expressing the natural killer cell marker CD161. We show that these cells are functionally as suppressive and have similar phenotypic/molecular characteristics to other subpopulations of Treg cells and retain their suppressive function following IL-17 induction. Importantly, we find that IL-17 production is STAT3 dependent, with Treg cells from patients with STAT3 mutations unable to make IL-17. Finally, we show that CD161⁺ population III Treg cells accumulate in inflamed joints of patients with inflammatory arthritis and are the predominant IL-17-producing Treg-cell population at these sites. As IL-17 production from this Treg-cell subpopulation is not accompanied by a loss of regulatory function, in the context of cell therapy, exclusion of these cells from the cell product may not be necessary.

Regulatory T cells and transplantation tolerance

The success of clinical organ transplantation relies on life-long use of immunosuppressive drugs that target immune responses associated with graft rejection. Preclinical studies in mice have convincingly demonstrated that robust, long-term transplantation tolerance can be achieved after a short-term treatment with T-cell coreceptor and costimulation blockade even for a fully mismatched graft. Such therapeutically induced tolerance requires the induction of Foxp3+ Tregs, which are essential for both the development and maintenance of the tolerant state. Recent advances in understanding the molecular and epigenetic mechanisms underlying the induction and stabilization of Foxp3 expression, thus guiding Foxp3+ Treg differentiation, have revealed novel therapeutic targets in animal models that can be translated to harness Foxp3+ Tregs from within the patient. Such in vivo induced Foxp3+ Tregs can also induce the tolerant state. Pharmacological compounds are available to exploit these targets and their further development holds great promise for clinical translation.

Thymic versus induced regulatory T cells - who regulates the regulators?

Physiological health must balance immunological responsiveness against foreign pathogens with tolerance toward self-components and commensals. Disruption of this balance causes autoimmune diseases/chronic inflammation, in case of excessive immune responses, and persistent infection/immunodeficiency if regulatory components are overactive. This homeostasis occurs at two different levels: at a resting state to prevent autoimmune disease, as autoreactive effector T-cells (Teffs) are only partially deleted in the thymus, and during inflammation to prevent excessive tissue injury, contract the immune response, and enable tissue repair. Adaptive immune cells with regulatory function (“regulatory T-cells”) are essential to control Teffs. Two sets of regulatory T cell are required to achieve the desired control: those emerging de novo from embryonic/neonatal thymus (“thymic” or tTregs), whose function is to control autoreactive Teffs to prevent autoimmune diseases, and those induced in the periphery (“peripheral” or pTregs) to acquire regulatory phenotype in response to pathogens/inflammation. The differentiation mechanisms of these cells determine their commitment to lineage and plasticity toward other phenotypes. tTregs, expressing high levels of IL-2 receptor alpha chain (CD25), and the transcription factor Foxp3, are the most important, since mutations or deletions in these genes cause fatal autoimmune diseases in both mice and men. In the periphery, instead, Foxp3⁺ pTregs can be induced from naïve precursors in response to environmental signals. Here, we discuss molecular signatures and induction processes, mechanisms and sites of action, lineage stability, and differentiating characteristics of both Foxp3⁺ and Foxp3⁻ populations of regulatory T cells, derived from the thymus or induced peripherally. We relate these predicates to programs of cell-based therapy for the treatment of autoimmune diseases and induction of tolerance to transplants.

DGCR8-mediated production of canonical microRNAs is critical for regulatory T cell function and stability

Regulatory T cells (Treg) are integral for immune homeostasis. Here we demonstrate that canonical microRNAs (miRNAs) are required for Treg function because mice with DGCR8-deficient Treg cells spontaneously develop a scurfy-like disease. Using genetic lineage marking we show that absence of miRNAs leads to reduced FoxP3 expression in Treg cells in vivo. In vitro culture of purified DGCR8-deficient Treg leads to a loss of FoxP3 expression. We conclude that canonical miRNAs are essential to maintain stable FoxP3 expression and Treg function. Thus, signals interfering with miRNA homeostasis might contribute to autoimmune diseases.

Development and maintenance of regulatory T cells

Regulatory T (Treg) cells are a developmentally and functionally distinct T cell subpopulation that is engaged in sustaining immunological self-tolerance and homeostasis. The transcription factor Foxp3 plays a key role in Treg cell development and function. However, expression of Foxp3 alone is not sufficient for conferring and maintaining Treg cell function and phenotype. Complementing the insufficiency, Treg-cell-specific epigenetic changes are also critical in the process of Treg cell specification, in regulating its potential plasticity, and hence in establishing a stable lineage. Understanding how epigenetic alterations and Foxp3 expression coordinately control Treg-cell-specific gene regulation will enable better control of immune responses by targeting the generation and maintenance of Treg cells.

Effects of cyclophosphamide and IL-2 on regulatory CD4+ T cell frequency and function in melanoma patients vaccinated with HLA-class I peptides: impact on the antigen-specific T cell response

The frequency and function of regulatory T cells (Tregs) were studied in stage II–III melanoma patients who were enrolled in a phase II randomized trial of vaccination with HLA-A*0201-modified tumor peptides versus observation. The vaccinated patients received low-dose cyclophosphamide (CTX) and low-dose interleukin-2 (IL-2). Tregs were analyzed in the lymph nodes (LNs) of stage III patients who were undergoing complete lymph node dissection and in peripheral blood mononuclear cells (PBMCs) collected before vaccination and at different time points during the vaccination period. The LNs of the vaccinated patients, which were surgically removed after two rounds of vaccination and one dose of CTX, displayed a low frequency of Tregs and a less immunosuppressive environment compared with those of the untreated patients. The accurate time-course analysis of the PBMCs of patients enrolled in the vaccination arm indicated a limited and transient modulation in the frequencies of Tregs in PBMCs collected after low-dose CTX administration and a strong Treg boost in those PBMCs collected after low-dose IL-2 administration. However, a fraction of the IL-2-boosted Tregs was functionally modulated to a Th-1-like phenotype in the vaccinated patients. Moreover, low-dose IL-2 promoted the concomitant expansion of conventional activated CD4⁺ T cells. Despite the amplification of Tregs, IL-2 administration maintained or further increased the number of antigen-specific CD8⁺ T cells that were induced by vaccination as demonstrated by the ex vivo human leukocyte antigen-multimer staining and IFN-γ ELISpot assays. Our study suggests that the use of CTX as a Treg modulator should be revised in terms of the administration schedule and of patients who may benefit from this drug treatment. Despite the Treg expansion that was observed in this study, low-dose IL-2 is not detrimental to the functional activities of vaccine-primed CD8⁺ T cell effectors when used in the inflammatory environment of vaccination.

Comparison of regulatory T cells in hemodialysis patients and healthy controls: implications for cell therapy in transplantation

BACKGROUND AND OBJECTIVES

Cell-based therapy with natural (CD4(+)CD25(hi)CD127(lo)) regulatory T cells to induce transplant tolerance is now technically feasible. However, regulatory T cells from hemodialysis patients awaiting transplantation may be functionally/numerically defective. Human regulatory T cells are also heterogeneous, and some are able to convert to proinflammatory Th17 cells. This study addresses the suitability of regulatory T cells from hemodialysis patients for cell-based therapy in preparation for the first clinical trials in renal transplant recipients (the ONE Study).

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Healthy controls and age- and sex-matched hemodialysis patients without recent illness/autoimmune disease on established, complication-free hemodialysis for a minimum of 6 months were recruited. Circulating regulatory T cells were studied by flow cytometry to compare the regulatory T cell subpopulations. Regulatory T cells from members of each group were compared for suppressive function and plasticity (IL-17-producing capacity) before and after in vitro expansion with and without Rapamycin, using standard assays.

RESULTS

Both groups had similar total regulatory T cells and subpopulations I and III. In each subpopulation, regulatory T cells expressed similar levels of the function-associated markers CD27, CD39, HLA-DR, and FOXP3. Hemodialysis regulatory T cells were less suppressive, expanded poorly compared with healthy control regulatory T cells, and produced IL-17 in the absence of Rapamycin. However, Rapamycin efficiently expanded hemodialysis regulatory T cells to a functional and stable cell product.

CONCLUSIONS

Rapamycin-based expansion protocols should enable clinical trials of cell-based immunotherapy for the induction of tolerance to renal allografts using hemodialysis regulatory T cells.

Regulation of nuclear factor-κB in autoimmunity

Nuclear factor (NF)-κB transcription factors are pivotal regulators of innate and adaptive immune responses, and perturbations of NF-κB signaling contribute to the pathogenesis of immunological disorders. NF-κB is a well-known proinflammatory mediator, and its deregulated activation is associated with the chronic inflammation of autoimmune diseases. Paradoxically, NF-κB plays a crucial role in the establishment of immune tolerance, including both central tolerance and the peripheral function of regulatory T (Treg) cells. Thus, defective or deregulated activation of NF-κB may contribute to autoimmunity and inflammation, highlighting the importance of tightly controlled NF-κB signaling. This review focuses on recent progress regarding NF-κB regulation and its association with autoimmunity.

Immunoregulation in cutaneous allergy: prevention and control

PURPOSE OF REVIEW

The cutaneous surface is exposed to a myriad of encounters with chemicals, allergens and microbes. Nevertheless, it withstands these environmental assaults without overt inflammation. We will discuss the role of T regulatory cells in a situation where this tissue homeostasis fails - cutaneous allergy, in particular contact hypersensitivity.

RECENT FINDINGS

Immune regulation is a complex process that is mediated by many cellular players. T regulatory cells have risen to particular prominence as potent immunosuppressors because their absence results in inflammation including skin allergy. Recent findings revealed that T regulatory cells comprise a heterogeneous group of subpopulations with specialized homing capabilities and suppressor functions. The stability of the T regulatory cell subset in proinflammatory microenvironments is controversially discussed. In addition, it has recently been shown that mechanisms by which T regulatory cells exert their immunosuppressive functions can be adopted by pathogenic effector T cells in certain situations.

SUMMARY

In cutaneous allergy, immunoregulatory mechanisms are dysfunctional. The cellular players comprise classical T regulatory cells as well as effector T cells with regulatory activities. Understanding their role in skin homeostasis and the mechanisms by which their regulatory functions are abrogated will yield novel therapeutic targets for the treatment of cutaneous allergies.

T cell receptor stimulation-induced epigenetic changes and Foxp3 expression are independent and complementary events required for Treg cell development

The transcription factor Foxp3 is essential for the development of regulatory T (Treg) cells, yet its expression is insufficient for establishing the Treg cell lineage. Here we showed that Treg cell development was achieved by the combination of two independent processes, i.e., the expression of Foxp3 and the establishment of Treg cell-specific CpG hypomethylation pattern. Both events were induced by T cell receptor stimulation. The Treg cell-type CpG hypomethylation began in the thymus and continued to proceed in the periphery and could be fully established without Foxp3. The hypomethylation was required for Foxp3(+) T cells to acquire Treg cell-type gene expression, lineage stability, and full suppressive activity. Thus, those T cells in which the two events have concurrently occurred are developmentally set into the Treg cell lineage. This model explains how Treg cell fate and plasticity is controlled and can be exploited to generate functionally stable Treg cells.

Inflammatory and regulatory T cells contribute to a unique immune microenvironment in tumor tissue of colorectal cancer patients

Colorectal cancer is one of the five leading causes of cancer mortality worldwide. The mechanisms of pathogen clearance, inflammation and regulation by T cells in the healthy bowel are also important in controlling tumor growth. The majority of studies analyzing T cells and their relationship to colorectal tumor growth have focused on individual T cell markers or gene clusters and thus the complexity of the T cell response contributing to the growth of the tumor is not clear. We have studied the T cells in colorectal cancer patients and have defined a unique T cell signature for colorectal tumor tissue. Using a novel analytical flow cytometric approach in concert with confocal microscopy, we have shown that the tumor has a lower frequency of effector T cells (CD69+), but a higher frequency of both regulatory (CD25hi Foxp3+) and inflammatory T cells (IL-17+) compared with associated nontransformed bowel tissue. We have also identified minor populations of T cells expressing conventional markers of both inflammatory and regulatory T cells (CD4+IL-17+Foxp3+) in the tumor tissue. These cells may represent intermediate populations or they may dictate an inflammatory versus regulatory function in surrounding T cells. Together, these data describe an immune microenvironment in colorectal cancer unique to the tumor tissue and distinct from the surrounding healthy bowel tissue, and this distinct environment is reflected by a gradient of T cells expressing markers of multiple T cell populations. These findings may be used to improve diagnosis and prognosis of colorectal cancer patients.

Immunization strategies for Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative movement disorder. Currently, no curative treatments or treatments that interdict disease progression are available. Over the past decade, immunization strategies were developed in our laboratories to combat disease progression. These strategies were developed in laboratory and animal models of human disease. Induction of humoral immune responses can be elicited against misfolded protein aggregates. Robust cell-mediated immunity against nitrated misfolded protein(s) accelerates disease progression through effector T cell responses that facilitate neuronal death. We propose that shifting the balance between effector and regulatory T cell activity can attenuate neurotoxic inflammatory events. We now summarize our works that support immune regulation in PD with the singular goal of restoring homeostatic glial responses. New methods to optimize immunization schemes and measure their clinical efficacy are discussed.

Functional stability of Foxp3+ regulatory T cells

Significant evidence demonstrates that CD4(+) regulatory T cells (T(reg)) expressing the Forkhead box P3 (Foxp3) transcription factor are a distinct lineage of CD4(+) T cells that are essential for maintaining self-tolerance and modulating immunity to various nonself-antigens under changing inflammatory settings. Stable Foxp3 expression ensures T(reg) function in a variety of inflammatory contexts. However, the model of T(reg) cells as a stable, long-lived lineage is controversial. Whereas some studies have observed long-lived T(reg) function, recent studies suggest that T(reg) cells adapt to microenvironmental changes and consequently manifest functional plasticity by reprogramming into inflammatory T cells. Here, we review the evidence addressing the functional stability or plasticity of Foxp3(+) T(reg) cells and the implications for immune homeostasis and disease.

Multiple treg suppressive modules and their adaptability

Foxp3⁺ regulatory T cells (Tregs) are a constitutively immunosuppressive cell type critical for the control of autoimmunity and inflammatory pathology. A range of mechanisms of Treg suppression have been identified and it has not always been clear how these different mechanisms interact in order to properly suppress autoimmunity and excessive inflammation. In recent years it has become clear that, while all Tregs seem to share some core suppressive mechanisms, they are also able to adapt to their surroundings in response to a variety of stimuli by homing to the sites of inflammation and exerting ancillary suppressive functions. In this review, we discuss the relevance and possible modes of Treg adaptability and put forward a modular model of Treg suppressive function. Understanding this flexibility may hold the key to understanding the full spectrum of Treg suppressive behavior.

Ubc13 maintains the suppressive function of regulatory T cells and prevents their conversion into effector-like T cells

Maintenance of immune homeostasis requires regulatory T (T_reg) cells. Here we show that T_reg-specific ablation of Ubc13, a lysine 63-specific ubiquitin-conjugating enzyme, caused aberrant T cell activation and autoimmunity. Although Ubc13 deficiency did not affect T_reg cell survival or Foxp3 expression, it impaired the in vivo suppressive function of T_reg cells and rendered them sensitive for acquiring T helper (T_H) 1- and T_H17-like effector T cell phenotypes. This function of Ubc13 involved its downstream target, IκB kinase (IKK). The Ubc13-IKK signaling axis controlled the expression specific T_reg effector molecules, including interleukin 10 (IL-10) and SOCS1. Collectively, these findings suggest that the Ubc13-IKK signaling axis regulates the molecular program that maintains T_reg function and prevents T_reg cells from acquiring inflammatory phenotypes.

Stability of regulatory T-cell lineage

Regulatory T (Treg) cells expressing the transcription factor Foxp3 constitute a unique T-cell lineage committed to suppressive functions and play a central role in maintaining self-tolerance and immune homeostasis. While their differentiation state is remarkably stable in the face of various perturbations from the extracellular environment, recent studies have also revealed their adaptability to the changing environment; in response to extrinsic cues, Treg cells differentiate further into distinct substates to regulate different classes of immune responses effectively. In contrast, some other recent studies have challenged this notion of a committed Treg cell lineage and suggested that Treg cells might lose their identity and be reprogrammed to various effector helper T cells under certain circumstances, although this issue of environment-induced Treg cell reprogramming remains highly controversial. This review will focus on recent advances in our understanding of how the stability and adaptability of Treg cell lineage is regulated and how it might be perturbed in a changing environment.