Epigenetic mechanisms of regulation of Foxp3 expression

Epigenomics of idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a complex lung disease of unknown etiology. Development of IPF is influenced by both genetic and environmental factors. Gene-expression profiling studies have taught us quite a bit about the biology of this fatal disease, but epigenetic marks may be the missing link that connects the environmental exposure in genetically predisposed individuals to transcriptome changes associated with the development of IPF. This review will begin with an introduction to the disease, followed by brief summaries of studies of gene expression in IPF and epigenetic marks associated with exposures relevant to IPF. The majority of the discussion will focus on epigenetic studies conducted so far in IPF, the limitations, challenges nd future directions in this field.

Epigenetics mediate environment : gene effects on occupational sensitization

PURPOSE OF REVIEW

Epigenetics is the study of stable modifications of fixed genomes that direct which genes are expressed and which are silenced. Epigenetic changes are modulated by environmental exposures, making epigenetics the interface between genes and environment. This has particular relevance in understanding the effect of occupational exposures on the expression of allergic disease. The goal of this review is to describe how epigenetic changes affect transcription potential, and to examine more closely the effect of specific environmental and occupational exposures on epigenetic variations that alter allergy gene transcripts and the inflammatory milieu.

RECENT FINDINGS

Gene transcription is activated when specific CpG sites are demethylated and histones are acetylated, and, conversely, silenced when sites are methylated and histones deacetylated. The development of Th1 and Th2 phenotypes, and expression of Treg cells, are now known to be modulated by epigenetic mechanisms. Workplace exposures such as tobacco smoke, particulates, diesel exhaust, polyaromatic hydrocarbons, ozone, and endotoxin, among others, suppress Treg development, and enhance expression of inflammatory cytokines and allergic phenotypes by epigenetic means.

SUMMARY

Epigenetic manipulation to open and close transcription sites provides flexibility of gene expression in response to changing environmental cues. It may also be the window whereby allergic disease in the workplace can be reduced by targeted environmental interventions.

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.

Epigenetic modifications and improved regulatory T-cell function in subjects undergoing dual sublingual immunotherapy

BACKGROUND

Allergen-specific immunotherapy is the only mode of therapy that has been demonstrated to offer a cure in patients with IgE-mediated respiratory allergies.

OBJECTIVE

We sought to demonstrate the safety and efficacy of timothy grass (TG) and dust mite (DM) dual sublingual immunotherapy (SLIT) and to begin to investigate the immune mechanisms involved in successful immunotherapy with multiple allergens.

METHODS

The safety and efficacy of dual SLIT with TG and DM in children and adults with demonstrated allergies to TG and DM were investigated in a single-center, randomized, double-blind, controlled phase I study. Thirty subjects received either TG and DM dual SLIT (n= 20) or placebo (n = 10). Immune parameters were evaluated for differentiation of desensitized subjects from control subjects.

RESULTS

Subjects treated with dual SLIT had decreased rhinoconjunctivitis scores (P < .001) and medication use scores (P < .001) and reduced responses to TG and DM allergen based on results of skin prick tests or nasal disk challenges (P < .01 and P < .001, respectively) compared with placebo-treated control subjects. An increase in TG- and DM-specific IgG(4) levels, reduced allergen-specific IgE levels, and subsequent basophil activation were observed in the active treatment group. Dual SLIT promoted allergen-specific suppressive CD4(+)CD25(high)CD127(low)CD45RO(+) forkhead box protein 3 (Foxp3)(+) memory regulatory T cells with reduced DNA methylation of CpG sites within the Foxp3 locus.

CONCLUSION

The results of this pilot study suggest that dual SLIT could be an effective means to treat subjects with sensitivities to a variety of allergens and that long-term tolerance might be induced by epigenetic modifications of Foxp3 in memory regulatory T cells.

Role of regulatory T cells in the promotion of transplant tolerance

Liver transplantation is now recognized as the most effective therapy for patients with end-stage acute and chronic liver failure. Despite outstanding short-term graft and patient survival, liver transplantation continues to face several major challenges, including poor long-term graft survival due to chronic rejection and major side effects of long-term immunosuppressive therapy (which is required for the prevention of rejection). The ability to produce a state of tolerance after transplantation would potentially obviate long-term immunosuppression. Self-tolerance and immune homeostasis involve both central and peripheral immunoregulatory mechanisms. To date, studies have shown that many subsets of regulatory T cells (Tregs) control immune responses to foreign and alloantigens. The identification of Tregs that are positive for CD4, CD25, and the transcription factor forkhead box (Foxp3) has resulted in major advances in our understanding of the immunology of rejection and the development of transplant tolerance. In this article, we focus on the importance of Tregs in tolerance induction in experimental models of liver transplantation. Furthermore, we discuss the therapeutic potential of Tregs for the promotion of tolerance in transplant patients and highlight recent clinical trials of Treg-based therapies.

Epigenetics, asthma, and allergic diseases: a review of the latest advancements

Environmental epigenetic regulation in asthma and allergic disease is an exciting area that has gained a great deal of scientific momentum in recent years. Environmental exposures, including prenatal maternal smoking, have been associated with asthma-related outcomes that may be explained by epigenetic regulation. In addition, several known allergy and asthma genes have been found to be susceptible to epigenetic regulation. We review the latest experimental and translational studies that have been published this past year in several areas, including 1) characterization of environmental asthma triggers that induce epigenetic changes, 2) characterization of allergic immune and regulatory pathways important to asthma that undergo epigenetic regulation, 3) evidence of active epigenetic regulation in asthma experimental models and the production of asthma biomarkers, 4) evidence of transmission of an asthma-related phenotype across multiple generations, and 5) "pharmaco-epigenetics." The field has certainly advanced significantly in the past year.

Plasticity of DNA methylation in mouse T cell activation and differentiation

Background

Circulating CD4⁺ T helper cells are activated through interactions with antigen presenting cells and undergo differentiation into specific T helper cell subsets depending on the type of antigen encountered. In addition, the relative composition of the circulating CD4⁺ T cell population changes as animals mature with an increased percentage of the population being memory/effector type cells.

Results

Here, we report on the highly plastic nature of DNA methylation at the genome-wide level as T cells undergo activation, differentiation and aging. Of particular note were the findings that DNA demethylation occurred rapidly following T cell activation and that all differentiated T cell populations displayed lower levels of global methylation than the non-differentiated population. In addition, T cells from older mice had a reduced level of DNA methylation, most likely explained by the increase in the memory/effector cell fraction. Although significant genome-wide changes were observed, changes in DNA methylation at individual genes were restricted to specific cell types. Changes in the expression of enzymes involved in DNA methylation and demethylation reflect in most cases the changes observed in the genome-wide DNA methylation status.

Conclusion

We have demonstrated that DNA methylation is dynamic and flexible in CD4+ T cells and changes rapidly both in a genome-wide and in a targeted manner during T cell activation, differentiation. These changes are accompanied by parallel changes in the enzymatic complexes that have been implicated in DNA methylation and demethylation implying that the balance between these opposing activities may play a role in the maintaining the methylation profile of a given cell type but also allow flexibility in a cell population that needs to respond rapidly to environmental signals.

Are we ready for the use of foxp3(+) regulatory T cells for immunodiagnosis and immunotherapy in kidney transplantation?

The existence of T-cell subsets naturally committed to perform immunoregulation has led to enthusiastic efforts to investigate their role in the immunopathogenesis of transplantation. Being able to modulate alloresponses, regulatory T cells could be used as an immunodiagnostic tool in clinical kidney transplantation. Thus, the measurement of Foxp3 transcripts, the presence of regulatory T cells in kidney biopsies, and the phenotypic characterisation of the T-cell infiltrate could aid in the diagnosis of rejection and the immune monitoring and prediction of outcomes in kidney transplantation. Interestingly, the adoptive transfer of regulatory T cells in animal models has been proven to downmodulate powerful alloresponses, igniting translational research on their potential use as an immunomodulatory therapy. For busy transplant clinicians, the vast amount of information in the literature on regulatory T cells can be overwhelming. This paper aims to highlight the most applicable research findings on the use of regulatory T cells in the immune diagnosis and potential immunomodulatory therapy of kidney transplant patients. However, can we yet rely on differential regulatory T-cell profiles for the identification of rejection or to tailor patient's immunosuppression? Are we ready to administer regulatory T cells as inductive or adjunctive therapy for kidney transplantation?

Epigenetics in asthma and other inflammatory lung diseases

Asthma is a chronic inflammatory disease of the airways. The causes of asthma and other inflammatory lung diseases are thought to be both environmental and heritable. Genetic studies do not adequately explain the heritability and susceptabilty to the disease, and recent evidence suggests that epigentic changes may underlie these processes. Epigenetics are heritable noncoding changes to DNA and can be influenced by environmental factors such as smoking and traffic pollution, which can cause genome-wide and gene-specific changes in DNA methylation. In addition, alterations in histone acetyltransferase/deacetylase activities can be observed in the cells of patients with lung diseases such as severe asthma and chronic obstructive pulmonary disease, and are often linked to smoking. Drugs such as glucocorticoids, which are used to control inflammation, are dependent on histone deacetylase activity, which may be important in patients with severe asthma and chronic obstructive pulmonary disease who do not respond well to glucocorticoid therapy. Future work targeting specific histone acetyltransferases/deacetylases or (de)methylases may prove to be effective future anti-inflammatory treatments for patients with treatment-unresponsive asthma.

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.

The ambiguity in immunology

In the present article, we discuss the various ambiguous aspects of the immune system that render this complex biological network so highly flexible and able to defend the host from different external invaders. This ambiguity stems mainly from the property of the immune system to be both protective and harmful. Immunity cannot be fully protective without producing a certain degree of damage (immunopathology) to the host. The balance between protection and tissue damage is, therefore, critical for the establishment of immune homeostasis and protection. In this review, we will consider as ambiguous, various immunological tactics including: (a) the opposing functions driving immune responses, immune-regulation, and contra-regulation, as well as (b) the phenomenon of chronic immune activation as a result of a continuous cross-presentation of apoptotic T cells by dendritic cells. All these plans participate principally to maintain a state of chronic low-level inflammation during persisting infections, and ultimately to favor the species survival.

Three novel acetylation sites in the Foxp3 transcription factor regulate the suppressive activity of regulatory T cells

The Foxp3 transcription factor is the master regulator of regulatory T cell (Treg) differentiation and function. Its activity is regulated by reversible acetylation. Using mass spectrometry of immunoprecipitated proteins, we identify three novel acetylation sites in murine Foxp3 (K31, K262, and K267) and the corresponding sites in human FOXP3 proteins. Newly raised modification-specific Abs against acetylated K31 and K267 confirm acetylation of these residues in murine Tregs. Mutant Foxp3 proteins carrying arginine substitutions at the three acetylation sites (3KR) accumulate in T cells to higher levels than wild-type Foxp3 and exert better suppressive activity in coculture experiments. Acetylation and stability of wild-type, but not mutant, Foxp3 is enhanced when cells are treated with Ex-527, an inhibitor of the NAD(+)-dependent deacetylase SIRT1. Treatment with Ex-527 promotes Foxp3 expression during induced Treg differentiation, enhances Foxp3 levels in natural Tregs, and prevents loss of Foxp3 expression in adoptively transferred Tregs in mice. Our data identify SIRT1 as a negative regulator of Treg function via deacetylation of three novel target sites in Foxp3. SIRT1 inhibitors strengthen the suppressive activity of Tregs and may be useful in enhancing Treg-based therapeutic approaches to autoimmune diseases or graft rejections.

Demethylation analysis of the FOXP3 locus shows quantitative defects of regulatory T cells in IPEX-like syndrome

Immune dysregulation, Polyendocrinopathy, Enteropathy X-linked (IPEX) syndrome is a unique example of primary immunodeficiency characterized by autoimmune manifestations due to defective regulatory T (Treg) cells, in the presence of FOXP3 mutations. However, autoimmune symptoms phenotypically resembling IPEX often occur in the absence of detectable FOXP3 mutations. The cause of this “IPEX-like” syndrome presently remains unclear.

To investigate whether a defect in Treg cells sustains the immunological dysregulation in IPEX-like patients, we measured the amount of peripheral Treg cells within the CD3⁺ T cells by analysing demethylation of the Treg cell-Specific-Demethylated-Region (TSDR) in the FOXP3 locus and demethylation of the T cell-Specific-Demethylated-Region (TLSDR) in the CD3 locus, highly specific markers for stable Treg cells and overall T cells, respectively.

TSDR demethylation analysis, alone or normalized for the total T cells, showed that the amount of peripheral Treg cells in a cohort of IPEX-like patients was significantly reduced, as compared to both healthy subjects and unrelated disease controls. This reduction could not be displayed by flow cytometric analysis, showing highly variable percentages of FOXP3⁺ and CD25⁺FOXP3⁺ T cells. These data provide evidence that a quantitative defect of Treg cells could be considered a common biological hallmark of IPEX-like syndrome. Since Treg cell suppressive function was not impaired, we propose that this reduction per se could sustain autoimmunity.

Retinoic acid generates regulatory T cells in experimental transplantation

Regulatory T cells play a key role to inhibit effector lymphocytes, avoid, autoimmunity, and restrain allogeneic immunity. Retinoic acid is an important cofactor that stimulates the generation and expansion of regulatory T cells. Naive T cells, coincubated with allogeneic antigen-presenting cells and retinoic acid, in conjunction with transforming growth factor (TGF) β and interleukin (IL) 2, generated allogeneic regulatory T cells de novo. These cells were able to inhibit skin rejection in adoptive transfer experiments. The generation of regulatory T cells ex vivo with retinoic acid, TGF-β, and IL-2 represents a new step toward specific regulation of allogeneic immune responses.

Nature and nurture in Foxp3(+) regulatory T cell development, stability, and function

Foxp3(+) regulatory T lymphocytes (Treg) are critical homeostatic regulators of immune and inflammatory responses. Their absence leads to fulminant multiorgan autoimmunity. This review explores recent studies that have altered our emerging view of the development, stability, and plasticity of these cells. Treg appear not to be a single entity, but a family of immunomodulatory cell types with shared capabilities. On a first level, Treg may alternatively form in response to developmental cues in the thymus as a distinct lineage of CD4(+) T cells or adaptively, in response to environmental cues received by mature conventional CD4(+) T lymphocytes. These 2 populations bear distinct specificity, stability, and genetic profiles and are differentially used in immune responses. Secondarily, in a manner analogous to the generation of T helper (Th)-1, Th2, and other T cell subsets, Treg may further specialize, adapting to the needs of their immunologic surroundings. Treg therefore comprise developmentally distinct, functionally overlapping cell populations that are uniquely designed to preserve immunologic homeostasis. They combine an impressive degree of both stability and adaptability.

Clinical perspectives for regulatory T cells in transplantation tolerance

Three main types of CD4+ regulatory T cells can be distinguished based upon whether they express Foxp3 and differentiate naturally in the thymus (natural Tregs) or are induced in the periphery (inducible Tregs); or whether they are FoxP3 negative but secrete IL-10 in response to antigen (Tregulatory type 1, Tr1 cells). Adoptive transfer of each cell type has proven highly effective in mouse models at preventing graft vs. host disease (GVHD) and autoimmunity. Although clinical application was initially hampered by low Treg frequency and unfavorable ex vivo expansion properties, several phase I trials are now being conducted to assess their effect on GVHD following hematopoietic stem cell transplantation (HSCT) and in type I diabetes. Human Treg trials for HSCT recipients have preceded other indications because GVHD onset is precisely known, the time period needed for prevention relatively short, initial efficacy is likely to provide life-long protection, and complications of GVHD can be lethal. This review will summarize the clinical trials conducted to date that have employed Tregs to prevent GVHD following HSCT and discuss recent advances in Treg cellular therapy.

Learning to live together: harnessing regulatory T cells to induce organ transplant tolerance

The discovery of immune cells with regulatory effects has created considerable excitement for their potential use in inducing tolerance to transplanted tissues. Despite the fact that these cells possess essential functions in vivo, attempts to translate them into effective clinical therapies has proved challenging due to a number of unanticipated complexities in their behavior. This article provides a broad summary of research done to understand the largest of the regulatory cell subtypes, namely CD4+Foxp3+ Regulatory T cells (T(Regs)). Special attention will be paid to current and future difficulties in using T(Regs) clinically, as well as room for improvement and innovation in this field.

Zinc and its role in age-related inflammation and immune dysfunction

Zinc is an essential micronutrient required for many cellular processes, especially for the normal development and function of the immune system. Zinc homeostasis and signaling are critical in immune activation, and an imbalance in zinc homeostasis is associated with the development of chronic diseases. Zinc deficiency causes significant impairment in both adaptive and innate immune responses, and promotes systemic inflammation. The elderly are a population particularly susceptible to zinc deficiency. National surveys indicate that a significant portion of the aged population has inadequate zinc intake, and a decline in zinc status is observed with age. There are remarkable similarities between the hallmarks of zinc deficiency and immunological dysfunction in aged individuals. Both zinc deficiency and the aging process are characterized by impaired immune responses and systemic low grade chronic inflammation. It has been hypothesized that age-related zinc deficiency may be an important factor contributing to immune dysfunction and chronic inflammation during the aging process. In this review, we discuss the effects of zinc status on aging, potential molecular and epigenetic mechanisms contributing to age-related decline in zinc status, and the role of zinc in age-related immune dysfunction and chronic inflammation.

Maternal exposure to air pollution before and during pregnancy related to changes in newborn's cord blood lymphocyte subpopulations. The EDEN study cohort

Background

Toxicants can cross the placenta and expose the developing fetus to chemical contamination leading to possible adverse health effects, by potentially inducing alterations in immune competence. Our aim was to investigate the impacts of maternal exposure to air pollution before and during pregnancy on newborn's immune system.

Methods

Exposure to background particulate matter less than 10 μm in diameter (PM₁₀) and nitrogen dioxide (NO₂) was assessed in 370 women three months before and during pregnancy using monitoring stations. Personal exposure to four volatile organic compounds (VOCs) was measured in a subsample of 56 non-smoking women with a diffusive air sampler during the second trimester of pregnancy. Cord blood was analyzed at birth by multi-parameter flow cytometry to determine lymphocyte subsets.

Results

Among other immunophenotypic changes in cord blood, decreases in the CD4+CD25+ T-cell percentage of 0.82% (p = 0.01), 0.71% (p = 0.04), 0.88% (p = 0.02), and 0.59% (p = 0.04) for a 10 μg/m³ increase in PM₁₀ levels three months before and during the first, second and third trimester of pregnancy, respectively, were observed after adjusting for confounders. A similar decrease in CD4+CD25+ T-cell percentage was observed in association with personal exposure to benzene. A similar trend was observed between NO₂ exposure and CD4+CD25+ T-cell percentage; however the association was stronger between NO₂ exposure and an increased percentage of CD8+ T-cells.

Conclusions

These data suggest that maternal exposure to air pollution before and during pregnancy may alter the immune competence in offspring thus increasing the child's risk of developing health conditions later in life, including asthma and allergies.

B7-H4 Pathway in Islet Transplantation and β-Cell Replacement Therapies

Type 1 diabetes (T1D) is a chronic autoimmune disease and characterized by absolute insulin deficiency. β-cell replacement by islet cell transplantation has been established as a feasible treatment option for T1D. The two main obstacles after islet transplantation are alloreactive T-cell-mediated graft rejection and recurrence of autoimmune diabetes mellitus in recipients. T cells play a central role in determining the outcome of both autoimmune responses and allograft survival. B7-H4, a newly identified B7 homolog, plays a key role in maintaining T-cell homeostasis by reducing T-cell proliferation and cytokine production. The relationship between B7-H4 and allograft survival/autoimmunity has been investigated recently in both islet transplantation and the nonobese diabetic (NOD) mouse models. B7-H4 protects allograft survival and generates donor-specific tolerance. It also prevents the development of autoimmune diabetes. More importantly, B7-H4 plays an indispensable role in alloimmunity in the absence of the classic CD28/CTLA-4 : B7 pathway, suggesting a synergistic/additive effect with other agents such as CTLA-4 on inhibition of unwanted immune responses.

Histone/protein deacetylases control Foxp3 expression and the heat shock response of T-regulatory cells

Lysine ɛ-acetylation is a post-translational modification that alters the biochemical properties of many proteins. The reaction is catalyzed by histone/protein acetyltransferases (HATs), and is reversed by histone/protein deacetylases (HDACs). As a result, HATs and HDACs constitute an important, though little recognized, set of proteins that control the functions of T-regulatory (Treg) cells. Targeting certain HDACs, especially HDAC6, HDAC9, and Sirtuin-1 (Sirt1), can augment Treg suppressive potency by several distinct and potentially additive mechanisms. These involve promoting Forkhead box p3 (Foxp3) gene expression and preserving Foxp3 lysine ɛ-acetylation, which infers resistance to ubiquitination and proteasomal degradation, and increases DNA binding. Moreover, depleting certain HDAC can enhance the heat shock response, which increases the tenacity of Treg to survive under stress, and helps preserve a suppressive phenotype. As a result, HDAC inhibitor therapy can be used to enhance Treg functions in vivo and have beneficial effects on allograft survival and autoimmune diseases.

FOXP3+ regulatory T cells: from suppression of rejection to induction of renal allograft tolerance

Naturally occurring and induced regulatory T cells (Tregs) can become hyporesponsive and anergic to antigen stimulation in autoimmune diseases and allograft rejection. The mechanisms of suppression of effector T cells by Tregs remain unclear, but there are in vitro and in vivo evidences showing that these cells are able to suppress antigen-specific responses via direct cell-to-cell contact, secrete anti-inflammatory cytokines such as TGF-β and IL-10, and inhibit the generation of memory T cells, among others. The transcription factor FOXP3 is a specific marker of Tregs and its deficiency is associated with autoimmune diseases and inflammation. During acute rejection of kidney allografts, an augmented FOXP3 gene expression as well as increased CD4(+)CD25(+)FOXP3(+) and other cell populations are observed in graft biopsies. However, it is not clear whether Tregs migrate into the graft and are retained there to suppress the inflammatory process, or whether they are directly associated with more complex mechanisms to induce immune tolerance. FOXP3(+) Tregs may direct the immune response toward a graft acceptance program, potentially affecting the long-term survival of transplanted organs and tissues. Immunosuppressive drugs modulate the number and function of circulating Tregs and FOXP3 expression. Experimental and clinical studies have shown that mTOR inhibitors have positive and calcineurin inhibitors negative effects on Tregs, but it is difficult to set apart the effect of multiple other factors known to be associated with short- and long-term renal graft outcomes. This review aimed to describe the functions of Tregs and its transcription factor FOXP3 in suppression of immune response during rejection and in induction of kidney graft tolerance, as well as to review the individual effects of immunosuppressive drugs on Tregs.

Tregs and infections: on the potential value of modifying their function

CD4(+) T cells, which express a master transcription factor, Foxp3, have been recognized as bona fide Tregs. These cells are essential to maintain immune homeostasis in healthy as well as infected mice and humans. Extensive investigations in the last decade have provided ways to manipulate the Foxp3(+) Treg response therapeutically so the role of such cells in microbe-induced inflammatory reactions can be evaluated. This review focuses on our current understanding of the mechanisms required for the generation and sustenance of Tregs in vivo and the potential value of modulating Tregs to control microbe-induced immunopathological responses.

Rapamycin-sensitive signals control TCR/CD28-driven Ifng, Il4 and Foxp3 transcription and promoter region methylation

The mammalian target of rapamycin (mTOR) controls T-cell differentiation in response to polarizing cytokines. We previously found that mTOR blockade by rapamycin (RAPA) delays the G1-S cell cycle transition and lymphocyte proliferation. Here, we report that both mTOR complex 1 and mTOR complex 2 are readily activated following TCR/CD28 engagement and are critical for early expression of Ifng, Il4 and Foxp3, and for effector T cell differentiation in the absence of polarizing cytokines. While inhibition of mTOR complex 1 and cell division were evident at low doses of RAPA, inhibition of mTOR complex 2, Ifng, Il4 and Foxp3 expression, and T-cell polarization required higher doses and more prolonged treatments. We found that while T-bet and GATA3 were readily induced following TCR/CD28 engagement, administration of RAPA delayed their expression, and interfered with the loss of DNA methylation within Ifng and Il4 promoter regions. In contrast, RAPA prevented activation-dependent DNA methylation of the Foxp3 promoter favoring Foxp3 expression. As a result, RAPA-cultured cells lacked immediate effector functions and instead were enriched for IL-2+ cells. We propose that mTOR-signaling, by timing the expression of critical transcription factors and DNA methylation of proximal promoter regions, regulates transcriptional competence at immunologically relevant sites and hence lymphocyte differentiation.

Regulatory T-cells in systemic lupus erythematosus and rheumatoid arthritis

Regulatory T-cells (Tregs) are the guardians of peripheral tolerance acting to prevent autoimmune diseases such as systemic lupus erythomatosus (SLE) and rheumatoid arthritis (RA). Defects in Tregs have been reported in these two diseases despite significant differences in their clinical phenotype and pathogenesis. In both diseases the potency of Treg fails to keep pace with the activation of effector cells and are unable to resist the ensuing inflammation. This review will discuss the phenotypic, numeric, and functional abnormalities in Tregs and their role in patients and murine models of SLE and RA.

Challenges in the pursuit of immune tolerance

Strategies for inducing immune tolerance are fundamentally similar across a spectrum of immune-mediated disorders, including allergic disease, autoimmunity, and rejection of allografts. In each case, the objective of establishing an immunoregulatory balance is challenged by variable upswings in effector cell populations and proinflammatory mediators of immunity, requiring careful, and innovative therapeutic intervention to restore stability. The Immune Tolerance Network, an international consortium sponsored by the National Institutes of Health, seeks to advance both the scientific understanding and the clinical success of immune therapies for these disorders, through an innovative and collaborative effort involving clinical trials and mechanistic studies. Over the last decade, scientists have evaluated cell-based ablation and deviation strategies in trials using lymphocyte-specific targeting, induction of host-donor hematopoietic chimerism, induction of antigen-specific immune regulation, and a variety of antigen desensitization approaches. In this article, we review some of the highlights of this experience and discuss the potential for progress, utilizing new insights into regulatory mechanisms and biomarker signatures of tolerance.

[Epigenetic therapies, a step beyond biologics for rheumatoid arthritis]

Over the last decade, the management of rheumatoid arthritis has evolved as a result of both the understanding of disease-related processes and the availability of the necessary high-throughput technology to provide patients with molecule-based therapies. New therapies allow the classification of patients into subsets as regards clinical response, at the same time adding to our knowledge of rheumatoid arthritis pathogenesis. New generations of molecules will likely soon be ready for "a la carte" treatment of patients. A promising field of research is epigenetics. Epigenetic regulatory mechanisms switch on and off the transcription of specific genes in individual cells. Acting as observers on non-adequate gene expression, these mechanisms yield protection against the development of tumours. The major achievement of epigenetic therapies could be their selective action on cells with altered epigenetic programs, and it is our challenge to recognize these alterations among patients with rheumatoid arthritis. Although safety concerns may arise, epigenetic drugs will likely be used to treat autoimmune diseases.

Regulatory T cells in stem cell transplantation: strategies and first clinical experiences

The adoptive transfer of donor-type CD4(+)CD25(+)FOXP3(+) regulatory T cells (Treg) protects from graft-versus-host disease in murine bone marrow transplantation models. Results from first clinical trials exploring such strategies have recently been presented and seem to confirm the efficacy of Treg for the prevention of this severe complication after allogeneic stem cell transplantation. Further improvements in Treg isolation and in vitro expansion technologies will facilitate the broader exploration of Treg therapies, for example, for the treatment of ongoing graft-versus-host disease or the prevention of graft rejection after solid organ transplantation.

Epigenetic control of gene expression in the lung

Epigenetics is traditionally defined as the study of heritable changes in gene expression caused by mechanisms other than changes in the underlying DNA sequence. There are three main classes of epigenetic marks--DNA methylation, modifications of histone tails, and noncoding RNAs--each of which may be influenced by the environment, diet, diseases, and ageing. Importantly, epigenetic marks have been shown to influence immune cell maturation and are associated with the risk of developing various forms of cancer, including lung cancer. Moreover, there is emerging evidence that these epigenetic marks affect gene expression in the lung and are associated with benign lung diseases, such as asthma, chronic obstructive pulmonary disease, and interstitial lung disease. Technological advances have made it feasible to study epigenetic marks in the lung, and it is anticipated that this knowledge will enhance our understanding of the dynamic biology in the lung and lead to the development of novel diagnostic and therapeutic approaches for our patients with lung disease.

IL-2 controls the stability of Foxp3 expression in TGF-beta-induced Foxp3+ T cells in vivo

Stimulation of naive mouse CD4(+)Foxp3(-) T cells in the presence of TGF-β results in the induction of Foxp3 expression and T suppressor function. However, Foxp3 expression in these induced regulatory T cells (iTreg) is unstable, raising the possibility that iTreg would not be useful for treatment of autoimmune diseases. To analyze the factors that control the stability of Foxp3 expression in iTreg, we generated OVA-specific iTreg from OT-II Foxp3-GFP knockin mice. Following transfer to normal C57BL/6 mice, OT-II GFP(+) cells maintained high levels of Foxp3 expression for 8 d. However, they rapidly lost Foxp3 expression upon stimulation with OVA in IFA in vivo. This unstable phenotype was associated with a strong methylation of the Treg-specific demethylated region within the Foxp3 locus. Administration of IL-2/anti-IL-2 complexes expanded the numbers of transferred Foxp3(+) iTreg in the absence of Ag challenge. Notably, when the iTreg were stimulated with Ag, treatment with IL-2/anti-IL-2 complexes stabilized Foxp3 expression and resulted in enhanced demethylation of the Treg-specific demethylated region. Conversely, neutralization of IL-2 or disruption of its signaling by deletion of Stat5 diminished the level of Foxp3 expression resulting in decreased suppressor function of the iTreg in vivo. Our data suggest that stimulation with TGF-β in vitro is not sufficient for imprinting T cells with stable expression of Foxp3. Administration of IL-2 in vivo results in stabilization of Foxp3 expression and may prove to be a valuable adjunct for the use of iTreg for the treatment of autoimmune diseases.

Induction of regulatory T cells by green tea polyphenol EGCG

Regulatory T cells (Treg) are critical in maintaining immune tolerance and suppressing autoimmunity. The transcription factor Foxp3 serves as a master switch that controls the development and function of Treg. Foxp3 expression is epigenetically regulated by DNA methylation, and DNA methyltransferase (DNMT) inhibitors can induce Foxp3 expression in naive CD4(+) T cells. We showed that EGCG, a major green tea polyphenol, could act as a dietary DNMT inhibitor, and induced Foxp3 and IL-10 expression in CD4(+) Jurkat T cells at physiologically relevant concentrations in vitro. We further showed that mice treated with EGCG in vivo had significantly increased Treg frequencies and numbers in spleen and lymph nodes and had inhibited T cell response. Induction of Foxp3 expression correlated with a concomitant reduction in DNMT expression and a decrease in global DNA methylation. Our data suggested that EGCG can induce Foxp3 expression and increase Treg frequency via a novel epigenetic mechanism. While the DNMT inhibitory effects of EGCG was not as potent as pharmacologic agents such as 5-aza-2'-deoxycytidine, the ability of dietary agents to target similar mechanisms offers opportunities for potentially sustained and longer-term exposures with lower toxicity. Our work provides the foundation for future studies to further examine and evaluate dietary strategies to modulate immune function.

Multipotent mesenchymal stromal cells express FoxP3: a marker for the immunosuppressive capacity?

Multipotent mesenchymal stromal cells (MSCs) have immunosuppressive capacity but the exact mechanism by which they suppress proliferation of T lymphocytes is not fully understood. Recently, the characteristics and function of regulatory T lymphocytes (Tregs) have become better defined. Tregs and MSCs have immunosuppressive features in common. Here, we looked for a common basis for immunosuppression in these distinct cell types. Forkhead box P3 (FoxP3) and CD39 expression in MSCs was measured by flow cytometry and real-time quantitative polymerase chain reaction. The importance of FoxP3 in MSC-mediated immunosuppression was investigated by siRNA technology and mixed lymphocyte culture (MLC). The effect of 5-azacytidine and other immunosuppressive drugs on FoxP3 expression and immunosuppression by MSCs was explored by flow cytometry, MLC, and real-time quantitative polymerase chain reaction. MSCs express FoxP3 at variable levels, but they do not express CD39. FoxP3 MSCs suppress MLC to a greater extent than cells with lower FoxP3 expression. However, FoxP3-decreased MSCs were found to retain their immunosuppressive properties. 5-azacytitine had no effect on FoxP3 expression or MLC suppression by MSCs. However, immunosuppressive drugs led to increased FoxP3 levels and MLC inhibition in FoxP3 MSCs. This is the first demonstration of FoxP3 expression by MSCs. Although MSCs share several features with Tregs, and FoxP3 MSCs tend to be more immunosuppressive, MSCs do not require functional FoxP3 for their immunosuppressive activity. The increased MSC-mediated suppression of immune responses by immunosuppressive drugs deserves further investigation.

Epigenetic reprogramming of the RORC locus during in vitro expansion is a distinctive feature of human memory but not naïve Treg

The adoptive transfer of in vitro expanded Treg is a promising treatment option for autoimmune as well as alloantigen-induced diseases. Yet, concerns about the phenotypic and functional stability of Tregs upon in vitro culture command both careful selection of the starting population and thorough characterization of the final cell product. Recently, a high degree of developmental plasticity has been described for murine Treg and Th17 cells. Similarly, IL-17-producing FOXP3(+) cells have been detected among the CD45RA(-) memory-type subpopulation of human Tregs ex vivo. This prompted us to investigate the predisposition of human naïve and memory Tregs to develop into Th17 cells during polyclonal in vitro expansion. Here, we show that stimulation-induced DNA demethylation of RORC, which encodes the lineage-defining transcription factor for Th17 cells, occurs selectively in CD45RA(-) memory-type Tregs, irrespective of their FOXP3 expression level. On the contrary, naïve CD45RA(+) Tregs retain stable CpG methylation across the RORC locus even upon prolonged ex vivo expansion and in consequence show only a marginal tendency to express RORC and develop into IL-17-producing cells. These findings are highly relevant for the generation of therapeutic Treg products.

The dual role of the X-linked FoxP3 gene in human cancers

The FoxP3 (forkhead box P3) gene is an X-linked gene that is submitted to inactivation. It is an essential transcription factor in CD4(+)CD25(+)FoxP3 regulatory T cells, which are therapeutic targets in disseminated cutaneous melanoma. Moreover, FoxP3 is an important tumor suppressor gene in carcinomas and has putative cancer suppressor gene function in cutaneous melanoma as well. Therefore understanding the structure and function of the FoxP3 gene is crucial to gaining insight into the biology of melanoma to better develop immunotherapeutics and future therapeutic strategies.

Regulatory T cells in colorectal cancer: from biology to prognostic relevance

Regulatory T cells (Tregs) were initially described as "suppressive" lymphocytes in the 1980s. However, it took almost 20 years until the concept of Treg-mediated immune control in its present form was finally established. Tregs are obligatory for self-tolerance and defects within their population lead to severe autoimmune disorders. On the other hand Tregs may promote tolerance for tumor antigens and even hamper efforts to overcome it. Intratumoral and systemic accumulation of Tregs has been observed in various types of cancer and is often linked to worse disease course and outcome. Increase of circulating Tregs, as well as their presence in mesenteric lymph nodes and tumor tissue of patients with colorectal cancer de facto suggests a strong involvement of Tregs in the antitumor control. This review will focus on the Treg biology in view of colorectal cancer, means of Treg accumulation and the controversies regarding their prognostic significance. In addition, a concise overview will be given on how Tregs and their function can be targeted in cancer patients in order to bolster an inherent immune response and/or increase the efficacy of immunotherapeutic approaches.

Epigenetic regulation of immune cell functions during post-septic immunosuppression

Studies in humans and animal models indicate that profound immunosuppression is one of the chronic consequences of severe sepsis. This immune dysfunction encompasses deficiencies in activation of cells in both the myeloid and lymphoid cell lineages. As a result, survivors of severe sepsis are at risk of succumbing to infections perpetrated by opportunistic pathogens that are normally controlled by a fully functioning immune system. Recent studies have indicated that epigenetic mechanisms may be one driving force behind this immunosuppression, through suppression of proinflammatory gene production and subsequent immune cell activation, proliferation and effector function. A better understanding of epigenetics and post-septic immunosuppression can improve our diagnostic tools and may be an important potential source of novel molecular targets for new therapies. This review will discuss important pathways of immune cell activation affected by severe sepsis, and highlight pathways of epigenetic regulation that may be involved in post-septic immunosuppression.

Epigenetics meets endocrinology

Although genetics determines endocrine phenotypes, it cannot fully explain the great variability and reversibility of the system in response to environmental changes. Evidence now suggests that epigenetics, i.e. heritable but reversible changes in gene function without changes in nucleotide sequence, links genetics and environment in shaping endocrine function. Epigenetic mechanisms, including DNA methylation, histone modification, and microRNA, partition the genome into active and inactive domains based on endogenous and exogenous environmental changes and developmental stages, creating phenotype plasticity that can explain interindividual and population endocrine variability. We will review the current understanding of epigenetics in endocrinology, specifically, the regulation by epigenetics of the three levels of hormone action (synthesis and release, circulating and target tissue levels, and target-organ responsiveness) and the epigenetic action of endocrine disruptors. We will also discuss the impacts of hormones on epigenetics. We propose a three-dimensional model (genetics, environment, and developmental stage) to explain the phenomena related to progressive changes in endocrine functions with age, the early origin of endocrine disorders, phenotype discordance between monozygotic twins, rapid shifts in disease patterns among populations experiencing major lifestyle changes such as immigration, and the many endocrine disruptions in contemporary life. We emphasize that the key for understanding epigenetics in endocrinology is the identification, through advanced high-throughput screening technologies, of plasticity genes or loci that respond directly to a specific environmental stimulus. Investigations to determine whether epigenetic changes induced by today's lifestyles or environmental 'exposures' can be inherited and are reversible should open doors for applying epigenetics to the prevention and treatment of endocrine disorders.

Distinct inflammatory signals have physiologically divergent effects on epigenetic regulation of Foxp3 expression and Treg function

Foxp3 expression in regulatory T cells (Treg) is required for their development and suppressive function. How different inflammatory signals affect Foxp3 chromatin structure, expression and Tregs plasticity are not completely known. In the present study, the Toll-like receptor 2 (TLR2) ligand peptidoglycan inhibited Foxp3 expression in both natural Treg (nTreg) and TGFβ-driven adaptive Treg (aTreg). Inhibition was independent of paracrine Th1, Th2 and Th17 cytokines. PGN-induced T cell-intrinsic TLR2-Myd88-dependent IFR1 expression and induced IRF1 bound to IRF1 response elements (IRF-E) in the Foxp3 promoter and intronic enhancers, and negatively regulated Foxp3 expression. Inflammatory IL-6 and TLR2 signals induced divergent chromatin changes at the Foxp3 locus and regulated Treg suppressor function, and in an islet transplant model resulted in differences in their ability to prolong graft survival. These findings are important for understanding how different inflammatory signals can affect the transplantation tolerance and immunity.

Retinoic acid and rapamycin differentially affect and synergistically promote the ex vivo expansion of natural human T regulatory cells

Natural T regulatory cells (Tregs) are challenging to expand ex vivo, and this has severely hindered in vivo evaluation of their therapeutic potential. All trans retinoic acid (ATRA) plays an important role in mediating immune homeostasis in vivo, and we investigated whether ATRA could be used to promote the ex vivo expansion of Tregs purified from adult human peripheral blood. We found that ATRA helped maintain FOXP3 expression during the expansion process, but this effect was transient and serum-dependent. Furthermore, natural Tregs treated with rapamycin, but not with ATRA, suppressed cytokine production in co-cultured effector T cells. This suppressive activity correlated with the ability of expanded Tregs to induce FOXP3 expression in non-Treg cell populations. Examination of CD45RA+ and CD45RA− Treg subsets revealed that ATRA failed to maintain suppressive activity in either population, but interestingly, Tregs expanded in the presence of both rapamycin and ATRA displayed more suppressive activity and had a more favorable epigenetic status of the FOXP3 gene than Tregs expanded in the presence of rapamycin only. We conclude that while the use of ATRA as a single agent to expand Tregs for human therapy is not warranted, its use in combination with rapamycin may have benefit.

CD4+ regulatory T cells in solid organ transplantation

PURPOSE OF REVIEW

Solid organ transplantation is the most effective treatment for end-stage organ failure, but the long-term outcomes remain suboptimal. CD4 regulatory T cells (Tregs) are emerging as a potential therapy to facilitate long-term allograft survival. This review provides a general overview of the biology of CD4 Tregs and then goes on to discuss the most relevant and recent experimental and clinical evidence for their therapeutic use in solid organ transplantation.

RECENT FINDINGS

There have been major advances in our understanding of Tregs, including improvements in methods for their isolation and expansion. Experimental models are providing very important data on the in-vitro and in-vivo behavior of Tregs in transplantation, while recent clinical trials of Treg cellular therapy in graft-versus-host disease are offering a valuable insight into the efficacy of Treg adoptive cellular therapy.

SUMMARY

Data in favor of Treg cellular therapy in transplantation are mounting, and we predict that their use in clinical trials is on the horizon.