CD25- T cells generate CD25+Foxp3+ regulatory T cells by peripheral expansion

Adjusting to self in the thymus: CD4 versus CD8 lineage commitment and regulatory T cell development

During thymic development, thymocytes adjust their TCR response based on the strength of their reactivity to self-peptide MHC complexes. This tuning process allows thymocytes with a range of self-reactivities to survive positive selection and contribute to a diverse T cell pool. In this review, we will discuss recent advances in our understanding of how thymocytes tune their responsiveness during positive selection, and we present a "sequential selection" model to explain how MHC specificity influences lineage choice. We also discuss recent evidence for cell type diversity in the medulla and discuss how this heterogeneity may contribute to medullary niches for negative selection and regulatory T cell development.

Control of Asthma and Allergy by Regulatory T Cells

BACKGROUND

Epithelial barriers, such as the lungs and skin, face the challenge of providing the tissues' physiological function and maintaining tolerance to the commensal microbiome and innocuous environmental factors while defending the host against infectious microbes. Asthma and allergic diseases can result from maladaptive immune responses, resulting in exaggerated and persistent type 2 immunity and tissue inflammation.

SUMMARY

Among the diverse populations of tissue immune cells, CD4+ regulatory T cells (Treg cells) are central to controlling immune responses and inflammation and restoring tissue homeostasis. Humans and mice that are deficient in Treg cells experience extensive inflammation in their mucosal organs and skin. During past decades, major progress has been made toward understanding the immunobiology of Treg cells and the molecular and cellular mechanisms that control their differentiation and function. It is now clear that Treg cells are not a single cell type and that they demonstrate diversity and plasticity depending on their differentiation stages and tissue environment. They could also take on a proinflammatory phenotype in certain conditions.

KEY MESSAGES

Treg cells perform distinct functions, including the induction of immune tolerance, suppression of inflammation, and promotion of tissue repair. Subsets of Treg cells in mucosal tissues are regulated by their differentiation stage and tissue inflammatory milieu. Treg cell dysfunction likely plays roles in persistent immune responses and tissue inflammation in asthma and allergic diseases.

Oral tolerance to dietary antigens and Foxp3+ regulatory T cells

Immune tolerance to foods develops in the intestine upon food ingestion and is essential to prevent IgE-mediated food allergy and gut inflammation. In homeostasis, the intestine is a tolerogenic environment that favors the formation of food-specific Foxp3+ regulatory T cells. A tolerogenic intestinal environment depends on colonization by diverse microbiota and exposure to solid foods at a critical period in early life. These early immune responses lead to the induction of antigen-specific Foxp3+ regulatory T cells in draining mesenteric lymph nodes. These peripherally induced regulatory cells circulate and seed the lamina propria of the gut, exerting suppressive function systemically and locally in the intestine. Successful establishment of a tolerogenic intestinal environment in early life sets the stage for oral tolerance to new antigens in adult life.

Type 1 Diabetes Brazilian patients exhibit reduced frequency of recent thymic emigrants in regulatory CD4+CD25+Foxp3+T cells

To control immune responses, regulatory CD4+CD25+Foxp3+ T cells (Treg) maintain their wide and diverse repertoire through continuous arrival of recent thymic emigrants (RTE). However, during puberty, the activity of RTE starts to decline as a natural process of thymic involution, introducing consequences, not completely described, to the repertoire. Type 1 diabetes (T1D) patients show quantitative and qualitative impairments on the Treg cells. Our aim was to evaluate peripheral Treg and RTE cell frequencies, in T1D patients from two distinct age groups (young and adults) and verify if HLA phenotypes are concomitant associated. To this, blood samples from Brazilian twenty established T1D patients (12 young and 8 adults) and twenty-one healthy controls (11 young and 10 adults) were analyzed, by flow cytometry, to verify the percentages of CD4, Treg (CD4+CD25+Foxp3+) and the subsets of CD45RA+ (naive) and CD31+(RTE) within then. Furthermore, the HLA typing was also set. We observed that the young established T1D patients feature decreased frequencies in total Treg cells and naive RTE within Treg cells. Significant prevalence of HLA alleles, associated with risk, in T1D patients, was also identified. Performing a multivariate analysis, we confirmed that the cellular changes described offers significant variables that distinct T1D patients from the controls. Our data collectively highlight relevant aspects about homeostasis imbalances in the Treg cells of T1D patients, especially in young, and disease prognosis; that might contribute for future therapeutic strategies involving Treg cells manipulation.

Spectrum of Treg and self-reactive T cells: single cell perspectives from old friend HTLV-1

Despite extensive regulatory T cell (Treg) research, fundamental questions on in vivo dynamics remain to be answered. The current study aims to dissect several interwoven concepts in Treg biology, highlighting the 'self-reactivity' of Treg and their counterparts, namely naturally-arising memory-phenotype T-cells, as a key mechanism to be exploited by a human retroviral infection. We propose the novel key concept, Periodic T cell receptor (TCR)-signalled T-cells, capturing self-reactivity in a quantifiable manner using the Nr4a3-Timer-of-cell-kinetics-and-activity (Tocky) technology. Periodic and brief TCR signals in self-reactive T-cells contrast with acute TCR signals during inflammation. Thus, we propose a new two-axis model for T-cell activation by the two types of TCR signals or antigen recognition, elucidating how Foxp3 expression and acute TCR signals actively regulate Periodic TCR-signalled T-cells. Next, we highlight an underappreciated branch of immunological research on Human T-cell Leukemia Virus type 1 (HTLV-1) that precedes Treg studies, illuminating the missing link between the viral infection, CD25, and Foxp3. Based on evidence by single-cell analysis, we show how the viral infection exploits the regulatory mechanisms for T-cell activation and suggests a potential role of periodic TCR signalling in infection and malignant transformation. In conclusion, the new perspectives and models in this study provide a working framework for investigating Treg within the self-reactive T-cell spectrum, expected to advance understanding of HTLV-1 infection, cancer, and immunotherapy strategies for these conditions.

Beyond FOXP3: a 20-year journey unravelling human regulatory T-cell heterogeneity

The initial idea of a distinct group of T-cells responsible for suppressing immune responses was first postulated half a century ago. However, it is only in the last three decades that we have identified what we now term regulatory T-cells (Tregs), and subsequently elucidated and crystallized our understanding of them. Human Tregs have emerged as essential to immune tolerance and the prevention of autoimmune diseases and are typically contemporaneously characterized by their CD3+CD4+CD25high CD127lowFOXP3+ phenotype. It is important to note that FOXP3+ Tregs exhibit substantial diversity in their origin, phenotypic characteristics, and function. Identifying reliable markers is crucial to the accurate identification, quantification, and assessment of Tregs in health and disease, as well as the enrichment and expansion of viable cells for adoptive cell therapy. In our comprehensive review, we address the contributions of various markers identified in the last two decades since the master transcriptional factor FOXP3 was identified in establishing and enriching purity, lineage stability, tissue homing and suppressive proficiency in CD4+ Tregs. Additionally, our review delves into recent breakthroughs in innovative Treg-based therapies, underscoring the significance of distinct markers in their therapeutic utilization. Understanding Treg subsets holds the key to effectively harnessing human Tregs for immunotherapeutic approaches.

Sustained store-operated calcium entry utilizing activated chromatin state leads to instability in iTregs

Thymus-originated tTregs and in vitro induced iTregs are subsets of regulatory T cells. While they share the capacity of immune suppression, their stabilities are different, with iTregs losing their phenotype upon stimulation or under inflammatory milieu. Epigenetic differences, particularly methylation state of Foxp3 CNS2 region, provide an explanation for this shift. Whether additional regulations, including cellular signaling, could directly lead phenotypical instability requires further analysis. Here, we show that upon TCR (T cell receptor) triggering, SOCE (store-operated calcium entry) and NFAT (nuclear factor of activated T cells) nuclear translocation are blunted in tTregs, yet fully operational in iTregs, similar to Tconvs. On the other hand, tTregs show minimal changes in their chromatin accessibility upon activation, in contrast to iTregs that demonstrate an activated chromatin state with highly accessible T cell activation and inflammation related genes. Assisted by several cofactors, NFAT driven by strong SOCE signaling in iTregs preferentially binds to primed-opened T helper (TH) genes, resulting in their activation normally observed only in Tconv activation, ultimately leads to instability. Conversely, suppression of SOCE in iTregs can partially rescue their phenotype. Thus, our study adds two new layers, cellular signaling and chromatin accessibility, of understanding in Treg stability, and may provide a path for better clinical applications of Treg cell therapy.

The kinetics of mTORC1 activation associates with FOXP3 expression pattern of CD4+ T cells and outcome of steroid-sensitive minimal change disease

Minimal change disease (MCD) usually responds to glucocorticoids (GCs) but relapses in most cases. Relapse pathogenesis after complete remission (CR) remains unclear. We hypothesized that FOXP3+ T regulatory cell (Treg) dysregulation may drive early relapses (ER). In this study, a cohort of 23 MCD patients were treated with a conventional GC regimen for the initial onset of nephrotic syndrome. Upon GC withdrawal, seven patients suffered from ER, while 16 patients sustained remission (SR) during the 12-month follow-up. Patients with ER had reduced FOXP3+ Treg proportions compared with healthy controls. Treg reduction, accompanied by IL-10 impairment, was ascribed to a proportional decline of FOXP3medium rather than FOXP3high cells. GC-induced CR was marked by a rise in the proportions of FOXP3+ and FOXP3medium cells compared to baseline levels. These increases declined in patients with ER. The expression level of phosphorylated ribosomal protein S6 was used to track the dynamic shifts in mTORC1 activity within CD4+ T cells of MCD patients at various stages of treatment. Baseline mTORC1 activity was inversely correlated with FOXP3+ and FOXP3medium Treg proportion. The mTORC1 activity in CD4+ T cells served as a reliable indicator for ER and demonstrated improved performance when paired with FOXP3 expression. Mechanically, targeting mTORC1 intervention by siRNAs sufficiently altered the conversion pattern of CD4+ T cell to FOXP3+ Treg. Taken together, the activity of mTORC1 in CD4+ T cells can act as a credible predictor for ER in MCD, especially when combined with FOXP3 expression, and may offer a potential therapeutic avenue for the treatment of podocytopathies.

Impact of in vitro HIV infection on human thymic regulatory T cell differentiation

Background

The differentiation and function of immunosuppressive regulatory T cells (Tregs) is dictated by the master transcription factor FoxP3. During HIV infection, there is an increase in Treg frequencies in the peripheral blood and lymphoid tissues. This accentuates immune dysfunction and disease progression. Expression of FoxP3 by thymic Tregs (tTregs) is partially controlled by TGF-β. This cytokine also contributes to Treg development in the peripheral blood and lymphoid tissues. Although TGF-β mediates lymphoid tissue fibrosis and peripheral Treg differentiation in HIV-infected individuals, its role in the induction and maintenance of Tregs within the thymus during HIV infection remains unclear.

Methods

Thymocytes were isolated from fresh human thymic tissues obtained from pediatric patients undergoing cardiac surgery. Infection by both R5- and X4-tropic HIV-1 strains and TGF-β treatment of human thymocytes was performed in an in vitro co-culture model with OP9-DL1 cells expressing Notch ligand delta-like 1 without T cell receptor (TCR) activation.

Results

Despite high expression of CCR5 and CXCR4 by tTregs, FoxP3 +  CD3highCD8- thymocytes were much less prone to in vitro infection with R5- and X4-tropic HIV strains compared to FoxP3-CD3highCD8- thymocytes. As expected, CD3highCD4+ thymocytes, when treated with TGF-β1, upregulated CD127 and this treatment resulted in increased FoxP3 expression and Treg differentiation, but did not affect the rate of HIV infection. FoxP3 expression and Treg frequencies remained unchanged following in vitro HIV infection alone or in combination with TGF-β1.

Conclusion

FoxP3 expression and tTreg differentiation is not affected by in vitro HIV infection alone or the combination of in vitro HIV infection and TGF-β treatment.

Long-term glucocorticoid exposure persistently impairs CD4+ T cell biology by epigenetically modulating the mTORC1 pathway

Conventional glucocorticoid (GC) treatment has a long-term influence on T-cell immunity, resulting in an increased risk of opportunistic infection after drug withdrawal. The underlying mechanisms remain ambiguous. This study demonstrated that long-term GC treatment induced persistent lymphopenia in patients with primary glomerular disease. GCs continuously suppressed the proportion of CD4+ T cells even after the daily dose was tapered down to the physiologic equivalences, leading to a significant decline of the CD4/CD8 ratio. Meanwhile, GCs impaired CD4+ T cell biology, leading to enhanced apoptotic cell death, reduced proliferative capacity, downregulated pro-inflammatory genes, and upregulated immunoregulatory genes. Specifically, GCs altered FOXP3 expression pattern in CD4+ T cells and favored their acquisition of an active T regulatory (Treg) cell phenotype with enhanced IL-10 production upon stimulation. Mechanistically, GCs tampered with the transcriptional regulation of mechanistic target of rapamycin complex 1 (mTORC1) pathway, resulting in an inhibitory impact on the signaling activity. Targeting mTORC1 signaling by siRNAs could sufficiently modify the viability of GC-exposed CD4+ T cells. By high-throughput sequencing of genome-wide DNA methylation and mRNA, we further uncovered a causal relationship between the altered DNA methylation level and transcription activity in a subset of mTORC1 pathway genes in long-term GC exposure. Taken together, this study reveals a novel regulation of mTORC1 signaling, which might dominate the long-term influence of GC on CD4+ T cell biology in a dose-independent manner.

Differentiation of Peripheral Treg

This chapter shows protocols for the differentiation of peripheral Treg (pTreg) from polyclonal and monoclonal CD4⁺ T cells. Polyclonal naïve CD4⁺ T cells can differentiate into pTreg upon adoptive transfer into Foxp3-diphtheria toxin receptor transgenic recipient mice in which endogenous Tregs are transiently depleted by administration of diphtheria toxin before adoptive transfer. Differentiation of monoclonal pTreg is induced through oral delivery of ovalbumin into RAG-deficient DO11.10 mice, in which T cells are ovalbumin specific. We show the isolation of naïve CD4⁺ T cells by flow cytometry, the administration of ovalbumin in drinking water, and the analysis tools, including an optional protocol for the enrichment of analysis samples in CD4⁺ T cells using a magnetic purification.

The ketone body β-hydroxybutyrate mitigates ILC2-driven airway inflammation by regulating mast cell function

Ketone bodies are increasingly understood to have regulatory effects on immune cell function, with β-hydroxybutyrate (BHB) exerting a predominantly anti-inflammatory response. Dietary strategies to increase endogenous ketone body availability such as the ketogenic diet (KD) have recently been shown to alleviate inflammation of the respiratory tract. However, the role of BHB has not been addressed. Here, we observe that BHB suppresses group 2 innate lymphoid cell (ILC2)-mediated airway inflammation. Central to this are mast cells, which support ILC2 proliferation through interleukin-2 (IL-2). Suppression of the mast cell/IL-2 axis by BHB attenuates ILC2 proliferation and the ensuing type 2 cytokine response and immunopathology. Mechanistically, BHB directly inhibits mast cell function in part through GPR109A activation. Similar effects are achieved with either the KD or 1,3-butanediol. Our data reveal the protective role of BHB in ILC2-driven airway inflammation, which underscores the potential therapeutic value of ketone body supplementation for the management of asthma.

The Mechanisms of Effector Th Cell Responses Contribute to Treg Cell Function: New Insights into Pathogenesis and Therapy of Asthma

CD4 ⁺ helper T (Th) cell subsets are critically involved in the pathogenesis of asthma. Naive Th cells differentiate into different subsets under the stimulation of different sets of cytokines, and the differentiation process is dominantly driven by lineage specific transcription factors, such as T-bet (Th1), GATA3 (Th2), RORγt (Th17) and Foxp3 (Treg). The differentiation mechanisms driven by these transcription factors are mutually exclusive, resulting in functional inhibition of these Th subsets to each other, particularly prominent between effector Th cells and Treg cells, such as Th2 versus Treg cells and Th17 versus Treg cells. Being of significance in maintaining immune homeostasis, the balance between effector Th cell response and Treg cell immunosuppression provides an immunological theoretical basis for us to understand the immunopathological mechanism and develop the therapy strategies of asthma. However, recent studies have found that certain factors involved in effector Th cells response, such as cytokines and master transcription factors (IL-12 and T-bet of Th1, IL-4 and GATA3 of Th2, IL-6 and RORγt of Th17), not only contribute to immune response of effector Th cells, but also promote the development and function of Treg cells, therefore bridging the interplay between effector Th cell immune responses and Treg cell immunosuppression. Although we have an abundant knowledge concerning the role of these cytokines and transcription factors in effector Th cell responses, our understanding on their role in Treg cell development and function is scattered thus need to be summarized. This review summarized the role of these cytokines and transcription factors involved in effector Th cell responses in the development and function of Treg cells, in the hope of providing new insights of understanding the immunopathological mechanism and seeking potential therapy strategies of asthma.

Double Negative T Regulatory Cells: An Emerging Paradigm Shift in Reproductive Immune Tolerance?

Immune regulation of female reproductive function plays a crucial role in fertility, as alterations in the relationship between immune and reproductive processes result in autoimmune subfertility or infertility. The breakdown of immune tolerance leads to ovulation dysfunction, implantation failure, and pregnancy loss. In this regard, immune cells with regulatory activities are essential to restore self-tolerance. Apart from regulatory T cells, double negative T regulatory cells (DNTregs) characterized by TCRαβ+/γδ+CD3+CD4–CD8– (and negative for natural killer cell markers) are emerging as effector cells capable of mediating immune tolerance in the female reproductive system. DNTregs are present in the female reproductive tract of humans and murine models. However, their full potential as immune regulators is evolving, and studies so far indicate that DNTregs exhibit features that can also maintain tolerance in the female reproductive microenvironment. This review describes recent progress on the presence, role and mechanisms of DNTregs in the female reproductive system immune regulation and tolerance. In addition, we address how DNTregs can potentially provide a paradigm shift from the known roles of conventional regulatory T cells and immune tolerance by maintaining and restoring balance in the reproductive microenvironment of female fertility.

Cytokine and Lymphocyte Profiles in Dogs with Atopic Dermatitis after Allergen-Specific Immunotherapy

Canine atopic dermatitis (cAD) is a chronic and recurrent inflammatory and pruritic skin disease in dogs. Currently, allergen-specific immunotherapy (ASIT) is the only identified disease-modifying intervention for allergic diseases. It decreases the symptoms triggered by allergens and prevents recurrence of the disease in the long-term. The aim of our research was to determine how immunotherapy changes the proportion of lymphocyte subsets in dog peripheral blood and the levels of cytokines secreted by these cells during therapy. ASIT was applied for 6 months. Blood samples for further analyses were collected from patients in the third and sixth month of immunotherapy. Six out of seven dogs receiving ASIT showed a positive effect. A reduction in cytokine levels (IL-13, TNF-α) in peripheral blood of cAD patients and changes in the number of specific T cell subpopulations—reduction of Tc cells (CD8+) and increase of activated T cells (CD3+CD25+)—confirmed the beneficial effect of the applied ASIT. In addition, a significantly higher percentage of Treg cells (CD4+CD25+FOXP3+) was noted in cAD patients before treatment compared to healthy dogs. After 3 months of therapy, the percentage of Tregs significantly decreased, and after 6 months, it increased significantly again.

Modulation of the Response to Mycobacterium leprae and Pathogenesis of Leprosy

The initial infection by the obligate intracellular bacillus Mycobacterium leprae evolves to leprosy in a small subset of the infected individuals. Transmission is believed to occur mainly by exposure to bacilli present in aerosols expelled by infected individuals with high bacillary load. Mycobacterium leprae-specific DNA has been detected in the blood of asymptomatic household contacts of leprosy patients years before active disease onset, suggesting that, following infection, the bacterium reaches the lymphatic drainage and the blood of at least some individuals. The lower temperature and availability of protected microenvironments may provide the initial conditions for the survival of the bacillus in the airways and skin. A subset of skin-resident macrophages and the Schwann cells of peripheral nerves, two M. leprae permissive cells, may protect M. leprae from effector cells in the initial phase of the infection. The interaction of M. leprae with these cells induces metabolic changes, including the formation of lipid droplets, that are associated with macrophage M2 phenotype and the production of mediators that facilitate the differentiation of specific T cells for M. leprae-expressed antigens to a memory regulatory phenotype. Here, we discuss the possible initials steps of M. leprae infection that may lead to active disease onset, mainly focusing on events prior to the manifestation of the established clinical forms of leprosy. We hypothesize that the progressive differentiation of T cells to the Tregs phenotype inhibits effector function against the bacillus, allowing an increase in the bacillary load and evolution of the infection to active disease. Epigenetic and metabolic mechanisms described in other chronic inflammatory diseases are evaluated for potential application to the understanding of leprosy pathogenesis. A potential role for post-exposure prophylaxis of leprosy in reducing M. leprae-induced anti-inflammatory mediators and, in consequence, Treg/T effector ratios is proposed.

Role of CNSs Conserved Distal Cis-Regulatory Elements in CD4 + T Cell Development and Differentiation

Naïve CD4⁺ T cells differentiate into diverse subsets of effector cells and perform various homeostatic and immune functions. The differentiation and maintenance of these different subsets are controlled through the upregulation and silencing of master genes. Mechanistic studies of the regulation of these master genes identified conserved and distal intronic regulatory elements, which are accessible subsets of conserved non-coding sequences (CNSs), acting as cis-regulatory elements in a lineage-specific manner that controls the function of CD4⁺ T cells. Abnormal CNS activity is associated with incorrect expression of master genes and development of autoimmune diseases or immune suppression. Here, we describe the function of several conserved, distal cis-regulatory elements at the Foxp3, Rorc, Il-4, Il-10 and Il-17 gene locus were shown to play important roles in CD4⁺ T cells differentiation. Together, this review briefly outlines currently known CNSs, with a focus on their regulations and functions in complexes modulating the differentiation and maintenance of various CD4⁺ T cells subsets, in health and disease contexts, as well as during the conversion of T regulatory cells to T helper 17 cells. This article will provide a comprehensive view of CNSs conserved distal cis-regulatory elements at a few loci that control aspects of CD4⁺ T cells function.

Research Progress on the Role of Regulatory T Cell in Tumor Microenvironment in the Treatment of Breast Cancer

The tumor microenvironment (TME) is a complex ecosystem comprised of cancer cells, stromal cells, and immune cells. Analysis of the composition of TME is essential to assess the prognosis of patients with breast cancer (BC) and the efficacy of different regimes. Treg plays a crucial role in the microenvironment of breast cancer subtypes, and its function contributes to the development and progression of BC by suppressing anti-tumor immunity directly or indirectly through multiple mechanisms. In addition, conventional treatments, such as anthracycline-based neoadjuvant chemotherapy, and neo-therapies, such as immune-checkpoint blockades, have a significant impact on the absence of Tregs in BC TME, thus gaining additional anti-tumor effect to some extent. Strikingly, Treg in BC TME revealed the predicted efficacy of some therapeutic strategies. All these results suggest that we can manipulate the abundance of Treg to achieve the ultimate effect of both conventional and novel treatments. In this review, we discuss new insights into the characteristics of Treg in BC TME, the impact of different regiments on Treg, and the possibilities of Treg as a predictive marker of efficacy for certain treatments.

Potential Targets to Mitigate Trauma- or Sepsis-Induced Immune Suppression

In sepsis and trauma, pathogens and injured tissue provoke a systemic inflammatory reaction which can lead to overwhelming inflammation. Concurrent with the innate hyperinflammatory response is adaptive immune suppression that can become chronic. A current key issue today is that patients who undergo intensive medical care after sepsis or trauma have a high mortality rate after being discharged. This high mortality is thought to be associated with persistent immunosuppression. Knowledge about the pathophysiology leading to this state remains fragmented. Immunosuppressive cytokines play an essential role in mediating and upholding immunosuppression in these patients. Specifically, the cytokines Interleukin-10 (IL-10), Transforming Growth Factor-β (TGF-β) and Thymic stromal lymphopoietin (TSLP) are reported to have potent immunosuppressive capacities. Here, we review their ability to suppress inflammation, their dynamics in sepsis and trauma and what drives the pathologic release of these cytokines. They do exert paradoxical effects under certain conditions, which makes it necessary to evaluate their functions in the context of dynamic changes post-sepsis and trauma. Several drugs modulating their functions are currently in clinical trials in the treatment of other pathologies. We provide an overview of the current literature on the effects of IL-10, TGF-β and TSLP in sepsis and trauma and suggest therapeutic approaches for their modulation.

Increased Frequencies of Myeloid-Derived Suppressor Cells Precede Immunodiscordance in HIV-Infected Subjects

Background

We have previously observed increased levels of inflammatory biomarkers and Th17 as well as Treg cells, but not other T-cell specific alterations, preceding immunodiscordance of successfully-treated HIV-infected subjects. Our hypothesis is that this could be related with potential alterations in myeloid-derived suppressor cells (MDSCs) and/or monocyte subsets.

Methods

We determined the frequencies of MDSCs and monocyte subsets and the expression of several functional markers (CCR2, β7-integrin, IDO, PDL1, CD11b) in HIV-infected subjects before treatment. We additionally analyzed follow-up samples after 24 months of suppressive cART in a subgroup of subjects. Bivariate regressions were performed, and correlations with soluble proinflammatory and bacterial translocation biomarkers, as well as with Th17/Treg ratio and anti-CMV titers were explored.

Results

Increased frequencies of MDSCs, but normal distribution of monocyte subsets, preceded immunodiscordance. The expression of several functional markers, such as CCR2, CD16, CD11b and PDL1, on MDSCs and monocyte subsets was altered in this scenario. MDSC and monocyte-related functional markers were associated with soluble biomarkers and T-cell parameters. Several of these cellular alterations were not restored after 24 months of suppressive cART.

Conclusion

An early immunosuppressive environment, characterized by the expansion of MDSCs and Tregs, precedes immunodiscordance and is related with a highly inflammatory status.

Solanine Inhibits Immune Escape Mediated by Hepatoma Treg Cells via the TGFβ/Smad Signaling Pathway

Objective

To observe the inhibitory effect of solanine on regulatory T cells (Treg) in transplanted hepatoma mice and to study the mechanism of solanine inhibiting tumor growth.

Methods

The levels of Treg cells and IL-2, IL-10, and TGFβ in the blood of patients with liver cancer were detected by flow cytometry and ELISA, respectively. A mouse hepatocellular carcinoma (HCC) graft model was established and randomly divided into four groups: control group, solanine group, TGFβ inhibitor group (SB-431542), and solanine ⁺TGFβ inhibitor combined group. Tumor volume of each group was recorded, tumor inhibition rate was calculated, and tumor metastasis was counted. The proportion of CD4⁺CD25⁺Foxp3⁺ Treg in transplanted tumor tissues was detected by flow cytometry. The expression levels of Foxp3 and TGFβ in transplanted tumor tissues were detected by quantitative fluorescence PCR.

Results

Compared with healthy people, Treg cells and IL-2, IL-10, and TGFβ contents in peripheral blood of liver cancer patients were increased. The results of the transplanted tumor model in mice showed that the tumor volume of the transplanted mice in the solanine group and the TGFβ inhibitor mice was reduced compared with the control group. The combined group had the smallest tumor volume. The proportion of CD4⁺CD25⁺Foxp3⁺ Treg in the transplanted tumor tissues of mice in the solanine treatment group was significantly lower than that in the control group. The expressions of Foxp3 and TGFβ in the transplanted tumor tissues of mice in the solanine group were significantly lower than those in the control group.

Conclusion

Solanine may enhance the antitumor immune response by downregulating the proportion of CD4⁺CD25⁺ Treg and the expression of Foxp3 and TGFβ in tumor tissues.

Control of regulatory T cell homeostasis

CD4⁺ Foxp3⁺ T Regulatory (Treg) cells play a critical role in the homeostasis and maintenance of the immune system. The understanding of different aspects of Treg cells biology remains an intensively investigated subject as altering their generation, stability, or function by drugs or biologics may have therapeutic value in the treatment of autoimmune and inflammatory diseases as well as cancers. This review will focus on recent studies on the role of cytokines, T Cell Receptor (TCR) and co-stimulatory/co-inhibitory molecules signaling, location and metabolism on the homeostasis and stability of Treg cells. The potential for therapeutic manipulation of each of these factors will be discussed.

Teriflunomide inhibits activation-induced CD25 expression on T cells and may affect Foxp3-expressing regulatory T cells

Teriflunomide (TER) is an immunomodulatory agent. Although the first reports on the use of TER in dogs have already appeared, immune mechanisms underlying the immunomodulatory effect of TER do not seem to have been fully elucidated yet. There were two aspects of this study. First, further insight into the mode of action of TER was gained by investigating its effect on the expression of IL-2 receptor α-chain (CD25) and Forkhead box P3 (Foxp3) by CD4⁺ and CD8⁺ T cells and apoptosis of these cells. Second, in view in the earlier lack of data on the effect of TER on T cells in dogs, the results of this study filled in this gap. TER at a concentration which can be achieved in vivo prevented or reduced the activation-induced CD25 expression on CD4⁺ and CD8⁺ T cells, respectively. Taking into consideration the role of CD25 in T cell proliferation, this effect may constitute an additional mechanism responsible for the antiproliferative effect of the drug. Under stimulation conditions, TER induced Foxp3 expression in Foxp3-negative CD4⁺ and CD8⁺ T cells, while down-regulating it under unstimulated conditions. These results suggest that TER may generate iTreg cells, but this process requires cell activation. TER was not found to affect on the absolute count and apoptosis of CD4⁺ and CD8⁺ T cells. The results suggest that the impairment of CD25 expression during T cell activation and generation of iTreg cells may constitute additional mechanisms, besides the principal one, underlying the immunomodulatory effect of TER.

A prenatally disrupted airway epithelium orchestrates the fetal origin of asthma in mice

BACKGROUND

Prenatal challenges such as maternal stress perception increase the risk and severity of asthma during childhood. However, insights into the trajectories and targets underlying the pathogenesis of prenatally triggered asthma are largely unknown. The developing lung and immune system may constitute such targets.

OBJECTIVE

Here we have aimed to identify the differential sex-specific effects of prenatal challenges on lung function, immune response, and asthma severity in mice.

METHODS

We generated bone marrow chimeric (BMC) mice harboring either prenatally stress-exposed lungs or a prenatally stress-exposed immune (hematopoietic) system and induced allergic asthma via ovalbumin. Next-generation sequencing (RNA sequencing) of lungs and assessment of airway epithelial barrier function in ovalbumin-sensitized control and prenatally stressed offspring was also performed.

RESULTS

Profoundly enhanced airway hyperresponsiveness, inflammation, and fibrosis were exclusively present in female BMC mice with prenatally stress-exposed lungs. These effects were significantly perpetuated if both the lungs and the immune system had been exposed to prenatal stress. A prenatally stress-exposed immune system alone did not suffice to increase the severity of these asthma features. RNA sequencing analysis of lungs from prenatally stressed, non-BMC, ovalbumin-sensitized females unveiled a deregulated expression of genes involved in asthma pathogenesis, tissue remodeling, and tight junction formation. It was also possible to independently confirm a tight junction disruption. In line with this, we identified an altered perinatal and/or postnatal expression of genes involved in lung development along with an impaired alveolarization in female prenatally stressed mice.

CONCLUSION

Here we have shown that the fetal origin of asthma is orchestrated by a disrupted airway epithelium and further perpetuated by a predisposed immune system.

Molecular Mechanisms Controlling Foxp3 Expression in Health and Autoimmunity: From Epigenetic to Post-translational Regulation

The discovery of the transcription factor Forkhead box-p3 (Foxp3) has shed fundamental insights into the understanding of the molecular determinants leading to generation and maintenance of T regulatory (Treg) cells, a cell population with a key immunoregulatory role. Work over the past few years has shown that fine-tuned transcriptional and epigenetic events are required to ensure stable expression of Foxp3 in Treg cells. The equilibrium between phenotypic plasticity and stability of Treg cells is controlled at the molecular level by networks of transcription factors that bind regulatory sequences, such as enhancers and promoters, to regulate Foxp3 expression. Recent reports have suggested that specific modifications of DNA and histones are required for the establishment of the chromatin structure in conventional CD4⁺ T (Tconv) cells for their future differentiation into the Treg cell lineage. In this review, we discuss the molecular events that control Foxp3 gene expression and address the associated alterations observed in human diseases. Also, we explore how Foxp3 influences the gene expression programs in Treg cells and how unique properties of Treg cell subsets are defined by other transcription factors.

Regulatory T Cell Development

Foxp3-expressing CD4⁺ regulatory T (Treg) cells play key roles in the prevention of autoimmunity and the maintenance of immune homeostasis and represent a major barrier to the induction of robust antitumor immune responses. Thus, a clear understanding of the mechanisms coordinating Treg cell differentiation is crucial for understanding numerous facets of health and disease and for developing approaches to modulate Treg cells for clinical benefit. Here, we discuss current knowledge of the signals that coordinate Treg cell development, the antigen-presenting cell types that direct Treg cell selection, and the nature of endogenous Treg cell ligands, focusing on evidence from studies in mice. We also highlight recent advances in this area and identify key unanswered questions.

Lineage Tracking the Generation of T Regulatory Cells From Microbial Activated T Effector Cells in Naïve Mice

Regulatory T cells (Tregs) are essential for the maintenance of gut homeostasis by suppressing conventional CD4⁺ helper T cells (Tconvs) that are activated by microbial antigens. Although thymus is the major source of the peripheral Tregs, peripheral conversion from Tconvs to Tregs have also been shown to occur under various experimental conditions. It remains less clear about the frequency of lineage conversion from Tconvs to Tregs in naïve animals. Here we used a newly established reporter system to track a group of post expansion Tregs (eTregs), which exhibited a stronger suppressive ability than the non-lineage marked Tregs. Notably, microbial antigens are the primary driver for the formation of eTregs. TCR repertoire analysis of Peyer's patch T cells revealed that eTregs are clonally related to Tconvs, but not to the non-lineage tracked Tregs. Adoptive transfer of Tconvs into lymphopenic hosts demonstrated a conversion from Tconvs to eTregs. Thus, our lineage tracking method was able to capture the lineage conversion from microbial activated effector T cells to Tregs in naïve animals. This study suggests that a fraction of clonally activated T cells from the natural T cell repertoire exhibits lineage conversion to Tregs in response to commensal microbes under homeostatic conditions.

Depletion of CD40 on CD11c+ cells worsens the metabolic syndrome and ameliorates hepatic inflammation during NASH

The co-stimulatory CD40-CD40L dyad plays a central role in fine-tuning immune reactions, including obesity-induced inflammation. Genetic ablation of CD40L reduced adipose tissue inflammation, while absence of CD40 resulted in aggravated metabolic dysfunction in mice. During obesity, CD40 expressing CD11c⁺ dendritic cells (DC) and macrophages accumulate in adipose tissue and liver. We investigated the role of CD40⁺CD11c⁺ cells in the metabolic syndrome and nonalcoholic steatohepatitis (NASH). DC-CD40-ko mice (CD40^fl/flCD11c^cre) mice were subjected to obesity or NASH. Obesity and insulin resistance were induced by feeding mice a 54% high fat diet (HFD). NASH was induced by feeding mice a diet containing 40% fat, 20% fructose and 2% cholesterol. CD40^fl/flCD11c^cre mice fed a HFD displayed increased weight gain, increased adipocyte size, and worsened insulin resistance. Moreover, CD40^fl/flCD11c^cre mice had higher plasma and hepatic cholesterol levels and developed profound liver steatosis. Overall, regulatory T cell numbers were decreased in these mice. In NASH, absence of CD40 on CD11c⁺ cells slightly decreased liver inflammation but did not affect liver lipid accumulation. Our experiments suggest that CD40 expressing CD11c⁺ cells can act as a double-edged sword: CD40 expressing CD11c⁺ cells contribute to liver inflammation during NASH but are protective against the metabolic syndrome via induction of regulatory T cells.

Cell-based high throughput screening identified a novel compound that promotes regulatory T cells and prevents autoimmune colitis

Regulatory T cells (T_R) show great promise for treating autoimmune diseases, allergies and preventing transplant rejection; however, their clinical application has been hampered by the lack of efficient ex vivo or in vivo expansion strategies. Here we report screening data on 130,000 low molecular weight (LMW) compounds for their T_R promoting potential using a self-developed high-throughput cell-based assay. One of the lead compounds, an isoxazolecarboxamide designated as TRP38, efficiently converts naïve CD4⁺ T cells to T_R cells in vitro and protects mice from autoimmune colitis in vivo. In addition, TRP38 can synergize with other compounds and/or cytokines such as rapamycin and TGFβ for T_R conversion, probably via directly inhibiting P70s6 phosphorylation without affecting mTOR expression, underscoring the importance of complementary and coordinated activity of multiple signaling pathways for the increased level of stable T_R cell production.

Blockade of TGF-β signaling to enhance the antitumor response is accompanied by dysregulation of the functional activity of CD4+CD25+Foxp3+ and CD4+CD25-Foxp3+ T cells

Background

The pleiotropic cytokine, transforming growth factor (TGF)-β, and CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs) play a critical role in actively suppressing antitumor immune responses. Evidence shows that TGF-β produced by tumor cells promotes tolerance via expansion of Tregs. Our group previously demonstrated that blockade of TGF-β signaling with a small molecule TGF-β receptor I antagonist (SM16) inhibited primary and metastatic tumor growth in a T cell dependent fashion. In the current study, we evaluated the effect of SM16 on Treg generation and function.

Methods

Using BALB/c, FoxP3eGFP and Rag^−/− mice, we performed FACS analysis to determine if SM16 blocked de novo TGF-β-induced Treg generation in vitro and in vivo. CD4⁺ T cells from lymph node and spleen were isolated from control mice or mice maintained on SM16 diet, and flow cytometry analysis was used to detect the frequency of CD4⁺CD25⁻FoxP3⁺ and CD4⁺CD25⁺FoxP3⁺ T cells. In vitro suppression assays were used to determine the ability to suppress naive T cell proliferation in vitro of both CD4⁺CD25⁺FoxP3⁺ and CD4⁺CD25⁻FoxP3⁺ T cell sub-populations. We then examined whether SM16 diet exerted an inhibitory effect on primary tumor growth and correlated with changes in FoxP3⁺expression. ELISA analysis was used to measure IFN-γ levels after 72 h co-culture of CD4⁺CD25⁺ T cells from tumor-bearing mice on control or SM16 diet with CD4⁺CD25⁻ T cells from naive donors.

Results

SM16 abrogates TGF-β-induced Treg generation in vitro but does not prevent global homeostatic expansion of CD4⁺ T cell sub-populations in vivo. Instead, SM16 treatment causes expansion of a population of CD4⁺CD25⁻Foxp3⁺ Treg-like cells without significantly altering the overall frequency of Treg in lymphoreplete naive and tumor-bearing mice. Importantly, both the CD4⁺CD25⁻Foxp3⁺ T cells and the CD4⁺CD25⁺Foxp3⁺ Tregs in mice receiving SM16 diet exhibited diminished ability to suppress naive T cell proliferation in vitro compared to Treg from mice on control diet.

Conclusions

These findings suggest that blockade of TGF-β signaling is a potentially useful strategy for blunting Treg function to enhance the anti-tumor response. Our data further suggest that the overall dampening of Treg function may involve the expansion of a quiescent Treg precursor population, which is CD4⁺CD25⁻Foxp3⁺.

Helios Deficiency Predisposes the Differentiation of CD4+Foxp3- T Cells into Peripherally Derived Regulatory T Cells

The transcription factor Helios is expressed in a large percentage of Foxp3⁺ regulatory T (Treg) cells and is required for the maintenance of their suppressive phenotype, as mice with a selective deficiency of Helios in Treg cells spontaneously develop autoimmunity. However, mice with a deficiency of Helios in all T cells do not exhibit autoimmunity, despite the defect in the suppressor function of their Treg cell population, suggesting that Helios also functions in non-Treg cells. Although Helios is expressed in a small subset of CD4⁺Foxp3^- and CD8⁺ T cells and its expression is upregulated upon T cell activation, its function in non-Treg cells remains unknown. To examine the function of Helios in CD4⁺Foxp3^- T cells, we transferred Helios-sufficient or -deficient naive CD4⁺Foxp3^- TCR transgenic T cells to normal recipients and examined their capacity to respond to their cognate Ag. Surprisingly, Helios-deficient CD4⁺ T cells expanded and differentiated into Th1 or Th2 cytokine-producing effectors in a manner similar to wild-type TCR transgenic CD4⁺ T cells. However, the primed Helios-deficient cells failed to expand upon secondary challenge with Ag. The tolerant state of the Helios-deficient memory T cells was not cell-intrinsic but was due to a small population of Helios-deficient naive T cells that had differentiated into Ag-specific peripheral Treg cells that suppressed the recall response in an Ag-specific manner. These findings demonstrate that Helios plays a role in the determination of CD4⁺ T cell fate.

Protein Tyrosine Phosphatases: Regulators of CD4 T Cells in Inflammatory Bowel Disease

Protein tyrosine phosphatases (PTPs) play a critical role in co-ordinating the signaling networks that maintain lymphocyte homeostasis and direct lymphocyte activation. By dephosphorylating tyrosine residues, PTPs have been shown to modulate enzyme activity and both mediate and disrupt protein-protein interactions. Through these molecular mechanisms, PTPs ultimately impact lymphocyte responses to environmental cues such as inflammatory cytokines and chemokines, as well as antigenic stimulation. Mouse models of acute and chronic intestinal inflammation have been shown to be exacerbated in the absence of PTPs such as PTPN2 and PTPN22. This increase in disease severity is due in part to hyper-activation of lymphocytes in the absence of PTP activity. In accordance, human PTPs have been linked to intestinal inflammation. Genome wide association studies (GWAS) identified several PTPs within risk loci for inflammatory bowel disease (IBD). Therapeutically targeting PTP substrates and their associated signaling pathways, such as those implicated in CD4⁺ T cell responses, has demonstrated clinical efficacy. The current review focuses on the role of PTPs in controlling CD4⁺ T cell activity in the intestinal mucosa and how disruption of PTP activity in CD4⁺ T cells can contribute to intestinal inflammation.

Development and Functional Modulation of Regulatory T Cells by Transcription Factors and Epigenetics

Regulatory T cells (Tregs) are essential for the maintenance of immune homeostasis. Studies of Treg are not only necessary for understanding the mechanism of immune homeostasis but also extremely useful for the development of treatments of various immune diseases. Forkhead box P3 (Foxp3) was identified as the master gene responsible for the immune-suppressing activity of Tregs. The promoter region and several intronic enhancers, designated conserved noncoding sequence (CNS) 0, 1, 2, and 3, at the Foxp3 gene locus have important roles in Foxp3 expression and Treg development. We demonstrated that transcription factors Nr4a and Smad2/3 are required for development of thymic Tregs and induced Tregs, respectively. In addition to transcription factors, Treg-specific DNA demethylation has been shown to be important for Treg stability. In particular, DNA demethylation of CNS2 was implicated in Treg stability, and members of the ten-eleven translocation family of demethylation factors were recently demonstrated to have important roles in 5'-C-phosphate-G-3' demethylation at CNS2. This article summarizes recent findings regarding the roles of transcription factors and epigenetic modifications in the differentiation, maintenance, and function of Tregs. This review will facilitate clinical application of Tregs to diseases in the field of ophthalmology, including uveitis and age-related macular degeneration.

From stability to dynamics: understanding molecular mechanisms of regulatory T cells through Foxp3 transcriptional dynamics

Studies on regulatory T cells (T_reg) have focused on thymic T_reg as a stable lineage of immunosuppressive T cells, the differentiation of which is controlled by the transcription factor forkhead box protein 3 (Foxp3). This lineage perspective, however, may constrain hypotheses regarding the role of Foxp3 and T_regin vivo, particularly in clinical settings and immunotherapy development. In this review, we synthesize a new perspective on the role of Foxp3 as a dynamically expressed gene, and thereby revisit the molecular mechanisms for the transcriptional regulation of Foxp3. In particular, we introduce a recent advancement in the study of Foxp3‐mediated T cell regulation through the development of the Timer of cell kinetics and activity (Tocky) system, and show that the investigation of Foxp3 transcriptional dynamics can reveal temporal changes in the differentiation and function of T_regin vivo. We highlight the role of Foxp3 as a gene downstream of T cell receptor (TCR) signalling and show that temporally persistent TCR signals initiate Foxp3 transcription in self‐reactive thymocytes. In addition, we feature the autoregulatory transcriptional circuit for the Foxp3 gene as a mechanism for consolidating T_reg differentiation and activating their suppressive functions. Furthermore, we explore the potential mechanisms behind the dynamic regulation of epigenetic modifications and chromatin architecture for Foxp3 transcription. Lastly, we discuss the clinical relevance of temporal changes in the differentiation and activation of T_reg.

HIV-Infected Subjects With Poor CD4 T-Cell Recovery Despite Effective Therapy Express High Levels of OX40 and α4β7 on CD4 T-Cells Prior Therapy Initiation

Background

HIV-infected subjects with suboptimal CD4 restoration despite suppressive combined antiretroviral treatment (cART) (immunodiscordant subjects) have been classically characterized after a variable period of time under cART. Recently, we have reported that an increased frequency of proliferating CD4 T-cells in these subjects is already present before the cART onset. The potential contribution of peripheral compensatory homeostatic proliferation (HP) is yet unknown. We aimed to analyze the expression of HP-related cellular markers on CD4 T-cells of immunodiscordant subjects before cART.

Methods

We analyzed the expression of OX40 and α4β7 on peripheral CD4 T-cells from immunodiscordant and control subjects (n = 21 each group) before cART initiation, and also on available follow-up samples (after 24 month of suppressive cART). Additionally, we tested the expression of these markers in an in vitro system for the study of human HP processes.

Results

Immunodiscordant subjects showed increased levels of OX40 and α4β7 on CD4 T-cells before cART initiation. While the cART tended to reduce these levels, immunodiscordant subjects still maintained comparatively higher levels of OX40 and α4β7 after 24 months under suppressive cART. These HP-related markers were upregulated in vitro during the human HP, especially during the fast HP.

Conclusion

Our results are compatible with exacerbated HP processes in immunodiscordant subjects, already before the cART onset.

Elucidating T Cell Activation-Dependent Mechanisms for Bifurcation of Regulatory and Effector T Cell Differentiation by Multidimensional and Single-Cell Analysis

In T cells, T cell receptor (TCR) signaling initiates downstream transcriptional mechanisms for T cell activation and differentiation. Foxp3-expressing regulatory T cells (Treg) require TCR signals for their suppressive function and maintenance in the periphery. It is, however, unclear how TCR signaling controls the transcriptional program of Treg. Since most of studies identified the transcriptional features of Treg in comparison to naïve T cells, the relationship between Treg and non-naïve T cells including memory-phenotype T cells (Tmem) and effector T cells (Teff) is not well understood. Here, we dissect the transcriptomes of various T cell subsets from independent datasets using the multidimensional analysis method canonical correspondence analysis (CCA). We show that at the cell population level, resting Treg share gene modules for activation with Tmem and Teff. Importantly, Tmem activate the distinct transcriptional modules for T cell activation, which are uniquely repressed in Treg. The activation signature of Treg is dependent on TCR signals and is more actively operating in activated Treg. Furthermore, by using a new CCA-based method, single-cell combinatorial CCA, we analyzed unannotated single-cell RNA-seq data from tumor-infiltrating T cells, and revealed that FOXP3 expression occurs predominantly in activated T cells. Moreover, we identified FOXP3-driven and T follicular helper-like differentiation pathways in tumor microenvironments, and their bifurcation point, which is enriched with recently activated T cells. Collectively, our study reveals the activation mechanisms downstream of TCR signals for the bifurcation of Treg and Teff differentiation and their maturation processes.

Immune tolerance therapies for autoimmune diseases based on heat shock protein T-cell epitopes

Experimental models of autoimmune diseases have revealed the disease protective role of heat shock proteins (HSPs). Both the administration of exogenous extracellular, mostly recombinant, HSP and the experimental co-induction of endogenous intracellular HSP in models have been shown to lead to production of disease protective regulatory T cells (Tregs). Similar to HSP taken up from extracellular bodily fluids, due to stress-related autophagy upregulated HSP also from intracellular sources is a major provider for the major histocompatibility class II (MHCII) ligandome; therefore, both extracellular and intracellular HSP can be prominent targets of Treg. The development of therapeutic peptide vaccines for the restoration of immune tolerance in inflammatory diseases is an area of intensive research. In this area, HSPs are a target for tolerance-inducing T-cell therapy, because of their wide expression in inflamed tissues. In humans, in whom the actual disease trigger is frequently unknown, HSP peptides offer chances for tolerance-promoting interventions through induction of HSP-specific Treg. Recently, we have shown the ability of a bacterial HSP70-derived peptide, HSP70-B29, to induce HSP-specific Tregs that suppressed arthritis by cross-recognition of their mammalian HSP70 homologues, abundantly present in the MHCII ligandome of stressed mouse and human antigen-presenting cells in inflamed tissues.This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.

Antigen-specific Helios- , Neuropilin-1- Tregs induce apoptosis of autoreactive B cells via PD-L1

Regulatory T cells (Tregs) maintain self-tolerance and prevent autoimmunity by controlling autoreactive T cells. We recently demonstrated in vivo that Tregs can directly suppress auto-reactive B cells via programmed death ligand 1 (PD-L1) that ligated PD-1 on B cells and caused them to undergo apoptosis. Here, we asked whether this mechanism is utilized by thymus-derived natural Tregs and/or by peripheral lymphoid tissue-induced Tregs. We first demonstrated that antigen-specific PD-L1-expressing Tregs were induced in the draining lymph node of autoantigen-expressing tissue and characterized them by their lack of the transcription factor Helios and of the surface marker Neuropilin-1 (Nrp-1). Next, we established an in vitro co-culture system to study the interaction between B cells and Treg subsets under controlled conditions. We found that Nrp^- Treg, but not Nrp⁺ Treg suppressed autoreactive B cells, whereas both were able to suppress T-helper cells. Such suppression was antigen-specific and was facilitated by PD-L1/PD-1-induced apoptosis. Furthermore, it required physical cell contact and was MHC II-restricted, providing an explanation for the antigen-specificity of peripherally-induced Tregs. These findings identify a role for peripherally induced Helios^- Nrp-1^- inducible Treg in controlling peripheral B-cell tolerance against tissue auto-antigens.

An in vitro system of autologous lymphocytes culture that allows the study of homeostatic proliferation mechanisms in human naive CD4 T-cells

The size of peripheral T-cell pool is kept constant throughout life. However, a decline in lymphocyte numbers is a feature of several human disorders, in which fast and slow homeostatic proliferation play a crucial role. Several in vitro and in vivo models have been developed to study such processes. Nevertheless, self- and commensal- antigens, well-known triggers of homeostatic proliferation, have not been examined in these models. We have designed an in vitro culture of human T-cells exposed to rIL7 and autologous antigen-presenting cells (aAPC) that allows the simultaneous characterization of the different types of homeostatic proliferation. Using our model, we first confirmed that both rIL7 and aAPC are survival signals ultimately leading to homeostatic proliferation. In addition, we explored the modulation of different anti-apoptotic, proliferative, activation and homing markers during fast and slow homeostatic proliferation. Finally, different subsets of Treg were generated during homeostatic proliferation in our model. In summary, our in vitro system is able to simultaneously reproduce both types of homeostatic proliferation of human naive CD4 T-cells, and allows the characterization of these processes. Our in vitro system is a useful tool to explore specific features of human homeostatic proliferation in different human lymphopenia-related disorders and could be used as a cell therapy approach.

Critical Role of TGF-β and IL-2 Receptor Signaling in Foxp3 Induction by an Inhibitor of DNA Methylation

Under physiological conditions, CD4⁺ regulatory T (Treg) cells expressing the transcription factor Foxp3 are generated in the thymus [thymus-derived Foxp3⁺ Treg (tTregs) cells] and extrathymically at peripheral sites [peripherally induced Foxp3⁺ Treg (pTreg) cell], and both developmental subsets play non-redundant roles in maintaining self-tolerance throughout life. In addition, a variety of experimental in vitro and in vivo modalities can extrathymically elicit a Foxp3⁺ Treg cell phenotype in peripheral CD4⁺Foxp3⁻ T cells, which has attracted much interest as an approach toward cell-based therapy in clinical settings of undesired immune responses. A particularly notable example is the in vitro induction of Foxp3 expression and Treg cell activity (iTreg cells) in initially naive CD4⁺Foxp3⁻ T cells through T cell receptor (TCR) and IL-2R ligation, in the presence of exogenous TGF-β. Clinical application of Foxp3⁺ iTreg cells has been hampered by the fact that TGF-β-driven Foxp3 induction is not sufficient to fully recapitulate the epigenetic and transcriptional signature of in vivo induced Foxp3⁺ tTreg and pTreg cells, which includes the failure to imprint iTreg cells with stable Foxp3 expression. This hurdle can be potentially overcome by pharmacological interference with DNA methyltransferase activity and CpG methylation [e.g., by the cytosine nucleoside analog 5-aza-2′-deoxycytidine (5-aza-dC)] to stabilize TGF-β-induced Foxp3 expression and to promote a Foxp3⁺ iTreg cell phenotype even in the absence of added TGF-β. However, the molecular mechanisms of 5-aza-dC-mediated Foxp3⁺ iTreg cell generation have remained incompletely understood. Here, we show that in the absence of exogenously added TGF-β and IL-2, efficient 5-aza-dC-mediated Foxp3⁺ iTreg cell generation from TCR-stimulated CD4⁺Foxp3⁻ T cells is critically dependent on TGF-βR and IL-2R signaling and that this process is driven by TGF-β and IL-2, which could either be FCS derived or produced by T cells on TCR stimulation. Overall, these findings contribute to our understanding of the molecular mechanisms underlying the process of Foxp3 induction and may provide a rational basis for generating phenotypically and functionally stable iTreg cells.

Harnessing the power of regulatory T-cells to control autoimmune diabetes: overview and perspective

Type 1 diabetes (T1D) is a T-cell-mediated autoimmune disease resulting in islet β-cell destruction, hypoinsulinaemia and severely altered glucose homeostasis. Although the mechanisms that initiate T1D still remain elusive, a breakdown of immune tolerance between effector T-cells (T_eff ) and regulatory T-cells (T_reg ) is considered to be the crucial component leading to autoimmunity. As such, strategies have been developed to boost the number and/or function of T_reg in the hope of specifically hampering the pathogenic T_eff activity. In this review, we will summarize the current understanding of biomarkers and functions of both forkhead box protein 3 (FoxP3)⁺ T_reg and type 1 regulatory T (Tr1) cells in health and in T1D, examine the outcome of experimental therapies in both animal models and humans via manipulation of T_reg responses and also provide an outlook on the potential of T_reg -based immunotherapies in the prevention and treatment of this disease. Discussed immunotherapies include adoptive transfer of ex-vivo expanded FoxP3⁺ T_reg , manipulation of T_reg cells via the interleukin (IL)-2/IL-2R pathway and induction of T_reg by tolerogenic peptides, tolerogenic dendritic cells or altered gut microbiota.

Programmed death one homolog maintains the pool size of regulatory T cells by promoting their differentiation and stability

Programmed death one homolog (PD-1H) is an immunoglobulin superfamily molecule and primarily acts as a coinhibitor in the initiation of T cell response to antigens. Here, we report that genetic ablation of PD-1H in mice blocks the differentiation of naive T cells to Foxp3⁺ inducible Treg cells (iTreg) with a significant decrease of iTreg in lymphoid organs. This effect of PD-1H is highly specific for iTreg because both naturally generated iTreg in gut-related tissues and in vitro induced iTreg by TGF-β were decreased whereas the genesis of natural Treg (nTreg) remains normal. The suppressive function of both iTreg and nTreg, however, is not affected by the loss of PD-1H. In addition to decreased production, PD-1H deficient iTreg could also rapidly convert to CD4⁺ T helper 1 or T helper 17 cells in an inflammatory environment. Our results indicate that PD-1H is required for maintenance of iTreg pool size by promoting its differentiation and preventing its conversion to other CD4⁺ T cell subsets. These findings may have important implications for manipulating Tregs to control inflammation.

Induction and maintenance of regulatory T cells by transcription factors and epigenetic modifications

Regulatory T cells (Tregs) are an essential cell subset for the maintenance of immune homeostasis. Foxp3 (Forkhead box P3) is the Treg master gene which is essential for immune suppressing activity. In addition, Tregs are characterized by a distinct pattern of gene expression, including upregulation of immune-suppressive genes and silencing of inflammatory genes. The molecular mechanisms of Treg development and maintenance have been intensively investigated. Tregs are characterized by expression of the transcription factor Foxp3. Several intronic enhancers and a promoter at the Foxp3 gene locus were shown to play important roles in Treg differentiation. The enhancers have been designated as conserved non-coding sequences (CNSs) 0, 1, 2, and 3. We showed that the transcription factors Nr4a and Smad2/3 are essential for the development of thymic Tregs and induced Tregs, respectively. Recently, Treg-specific DNA demethylation has been shown to play an important role in Treg stability. DNA demethylation of CNS2 has been implicated in Treg stability, and recent reports have revealed that the ten-eleven translocation (Tet) family of demethylation factor plays an important role in CpG demethylation at CNS2. This article reviews the recent progress on the roles of transcription factors and epigenetic modifications in the differentiation, maintenance, and function of Tregs.

Critical influence of the thymus on peripheral T cell homeostasis

Introduction

A tight balance between regulatory CD4⁺Foxp3⁺ (Treg) and conventional CD4⁺Foxp3⁻ (Tconv) T cell subsets in the peripheral compartment, maintained stable throughout most of lifetime, is essential for preserving self‐tolerance along with efficient immune responses. An excess of Treg cells, described for aged individuals, may critically contribute to their reported immunodeficiency. In this work, we investigated if quantitative changes in thymus emigration may alter the Treg/Tconv homeostasis regardless of the aging status of the peripheral compartment.

Methods

We used two different protocols to modify the rate of thymus emigration: thymectomy of adult young (4–6 weeks old) mice and grafting of young thymus onto aged (18 months old) hosts. Additionally, lymphoid cells from young and aged B6 mice were intravenously transferred to B6.RAG2^−/− mice. Alterations in Treg and Tconv peripheral frequencies following these protocols were investigated after 30 days by flow cytometry.

Results

Thymectomized young mice presented a progressive increase in the Treg cell frequency, while the grafting of a functional thymus in aged mice restored the young‐like physiological Treg/Tconv proportion. Strikingly, T cells derived from young or aged splenocytes colonized the lymphopenic periphery of RAG^−/− hosts to the same extent, giving rise to similarly elevated Treg cell levels irrespective of the age of the donor population. In the absence of thymus output, the Treg subset seems to survive longer, as confirmed by their lower proportion of Annexin‐V⁺ cells.

Conclusions

Our data suggest that the thymus‐emigrating population, harboring an adequate proportion of Treg/Tconv lymphocytes, may be essential to keep the Treg cell balance, independently of age‐related shifts intrinsic to the peripheral environment or to the T cell biology.

Influenza and Memory T Cells: How to Awake the Force

Annual influenza vaccination is an effective way to prevent human influenza. Current vaccines are mainly focused on eliciting a strain-matched humoral immune response, requiring yearly updates, and do not provide protection for all vaccinated individuals. The past few years, the importance of cellular immunity, and especially memory T cells, in long-lived protection against influenza virus has become clear. To overcome the shortcomings of current influenza vaccines, eliciting both humoral and cellular immunity is imperative. Today, several new vaccines such as infection-permissive and recombinant T cell inducing vaccines, are being developed and show promising results. These vaccines will allow us to stay several steps ahead of the constantly evolving influenza virus.

Amelioration of experimental colitis after short-term therapy with glucocorticoid and its relationship to the induction of different regulatory markers

The clinical benefits of short-term therapy with glucocorticoids (GC) in patients with inflammatory bowel disease (IBD) are widely known. However, the effects of this treatment towards the re-establishment of the regulatory network in IBD are not fully explored. We have evaluated the immunological effects of the abbreviated GC therapy in experimental colitis induced by 3% dextran sulphate sodium in C57BL/6 mice. Treatment with GC improved disease outcome, constrained circulating leucocytes and ameliorated intestinal inflammation. The control of the local inflammatory responses involved a reduction in the expression of interferon-γ and interleukin-1β, associated with augmented mRNA levels of peroxisome proliferator-activated receptors (α and γ) in intestine. Furthermore, there was a reduction of CD4⁺ T cells producing interferon-γ, together with an increased frequency of the putative regulatory population of T cells producing interleukin-10, in spleen. These systemic alterations were accompanied by a decrease in the proliferative potential of splenocytes of mice treated in vivo with GC. Notably, treatment with GC also led to an increase in the frequency of the regulatory markers GITR, CTLA-4, PD-1, CD73 and FoxP3, more prominently in spleen. Taken together, our results pointed to a role of GC in the control of leucocyte responsiveness and re-establishment of a regulatory system, which probably contributed to disease control and the restoration of immune balance. Finally, this is the first time that GC treatment was associated with the modulation of a broad number of regulatory markers in an experimental model of colitis.

Tissue-specific emergence of regulatory and intraepithelial T cells from a clonal T cell precursor

Peripheral Foxp3⁺ regulatory T cells (pT_regs) maintain immune homeostasis by controlling potentially harmful effector T cell responses toward dietary and microbial antigens. Although the identity of the T cell receptor (TCR) can impose commitment and functional specialization of T cells, less is known about how TCR identity governs pT_reg development from conventional CD4⁺ T cells. To investigate the extent to which TCR identity dictates pT_reg fate, we used somatic cell nuclear transfer to generate a transnuclear (TN) mouse carrying a monoclonal TCR from a pT_reg (pT_reg TN mice). We found that the pT_reg TCR did not inevitably predispose T cells to become pT_reg but instead allowed for differentiation of noninflammatory CD4⁺CD8αα⁺ intraepithelial lymphocytes (CD4_IELs) in the small intestine. Only when we limited the number of T cell precursors that carried the TN pT_reg TCR did we observe substantial pT_reg development in the mesenteric lymph nodes and small intestine lamina propria of mixed bone marrow chimeras. Small clonal sizes and therefore decreased intraclonal competition were required for pT_reg development. Despite bearing the same TCR, small intestine CD4_IEL developed independently of precursor frequency. Both pT_reg and CD4_IEL development strictly depended on the resident microbiota. A single clonal CD4⁺ T cell precursor can thus give rise to two functionally distinct and anatomically segregated T cell subsets in a microbiota-dependent manner. Therefore, plasticity of the CD4 T cell compartment depends not only on the microbiota but also on specialized environmental cues provided by different tissues.