Origin and T cell receptor diversity of Foxp3+CD4+CD25+ T cells

The possible anti-tumor effects of regulatory T cells plasticity / IL-35 in the tumor microenvironment of the major three cancer types

T lymphocytes are among the immunological cells that make up the tumor microenvironment (TME), and they are essential in the growth of tumors and anti-tumor reactions. Regulatory T cells (Treg cells) are a subset of CD4+ T cells in the immune system that suppress the immune system. They are distinguished by their expression of the master transcription factor forkhead box protein P3 (FOXP3). Furthermore, Treg cells are essential for maintaining immunological homeostasis, inhibiting inflammation, and maintaining self-tolerance. In a variety of malignancies within the TME, Treg cells demonstrate notable flexibility and functional diversity. Highly plastic Treg cells can change into Th-like Treg cells in specific circumstances, which allow them to secrete particular pro-inflammatory cytokines. Interleukin 35 (IL-35) is a part of the immunosuppressive cytokines that belong to the IL-12 family. Treg cells release IL-35, which was elevated in the peripheral blood and TME of numerous cancer patients, implying that IL-35 in the TME may be an intriguing target for cancer therapy. In cancer, IL-35 is a two-edged sword; it promotes tumorigenicity in cancer cells while shielding them from apoptosis. Nonetheless, other investigations have mentioned its conflicting effects on cancer prevention. Herein, we provide an updated understanding of the critical mechanisms behind the anticancer immunity mediated by Treg cells plasticity, the role of IL-35, and tactics to strengthen the immune response against malignancies, outlining major clinical trials that used Treg cells/IL-35 therapies in the three main cancer types (lung, breast, and colorectal cancers).

Bulk T-cell receptor sequencing confirms clonality in obstetric antiphospholipid syndrome and may as a potential biomarker

The heterogeneity of the T cell receptor (TCR) repertoire critically influences the autoimmune response in obstetric antiphospholipid syndrome (OAPS) and is intimately associated with the prophylaxis of autoimmune disorders. Investigating the TCR diversity patterns in patients with OAPS is thus of paramount clinical importance. This investigation procured peripheral blood specimens from 31 individuals with OAPS, 21 patients diagnosed with systemic lupus erythematosus (SLE), and 22 healthy controls (HC), proceeding with TCR repertoire sequencing. Concurrently, adverse pregnancy outcomes in the OAPS cohort were monitored and documented over an 18-month timeframe. We paid particular attention to disparities in V/J gene utilisation and the prevalence of shared clonotypes amongst OAPS patients and the comparative groups. When juxtaposed with observations from healthy controls and SLE patients, immune repertoire sequencing disclosed irregular T- and B-cell profiles and a contraction of diversity within the OAPS group. Marked variances were found in the genomic rearrangements of the V gene, J gene, and V/J combinations. Utilising a specialised TCRβ repertoire, we crafted a predictive model for OAPS classification with robust discriminative capability (AUC = 0.852). Our research unveils alterations in the TCR repertoire among OAPS patients for the first time, positing potential covert autoimmune underpinnings. These findings nominate the TCR repertoire as a prospective peripheral blood biomarker for the clinical diagnosis of OAPS and may offer valuable insights for advancing the understanding of OAPS immunologic mechanisms and prognostic outcomes.

Defective autophagy and autophagy activators in myasthenia gravis: a rare entity and unusual scenario

Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction (NMJ) that results from autoantibodies against nicotinic acetylcholine receptors (nAchRs) at NMJs. These autoantibodies are mainly originated from autoreactive B cells that bind and destroy nAchRs at NMJs preventing nerve impulses from activating the end-plates of skeletal muscle. Indeed, immune dysregulation plays a crucial role in the pathogenesis of MG. Autoreactive B cells are increased in MG due to the defect in the central and peripheral tolerance mechanisms. As well, autoreactive T cells are augmented in MG due to the diversion of regulatory T (Treg) cells or a defect in thymic anergy leading to T cell-mediated autoimmunity. Furthermore, macroautophagy/autophagy, which is a conserved cellular catabolic process, plays a critical role in autoimmune diseases by regulating antigen presentation, survival of immune cells and cytokine-mediated inflammation. Abnormal autophagic flux is associated with different autoimmune disorders. Autophagy regulates the connection between innate and adaptive immune responses by controlling the production of cytokines and survival of Tregs. As autophagy is involved in autoimmune disorders, it may play a major role in the pathogenesis of MG. Therefore, this mini-review demonstrates the potential role of autophagy and autophagy activators in MG.Abbreviations: Ach, acetylcholine; Breg, regulatory B; IgG, immunoglobulin G; MG, myasthenia gravis; NMJ, neuromuscular junction; ROS, reactive oxygen species; Treg, regulatory T; Ubl, ubiquitin-like.

MicroRNAs targeting TGF-β signaling exacerbate central nervous system autoimmunity by disrupting regulatory T cell development and function

Transforming growth factor beta (TGF-β) signaling is essential for a balanced immune response by mediating the development and function of regulatory T cells (Tregs) and suppressing autoreactive T cells. Disruption of this balance can result in autoimmune diseases, including multiple sclerosis (MS). MicroRNAs (miRNAs) targeting TGF-β signaling have been shown to be upregulated in naïve CD4 T cells in MS patients, resulting in a limited in vitro generation of human Tregs. Utilizing the murine model experimental autoimmune encephalomyelitis, we show that perinatal administration of miRNAs, which target the TGF-β signaling pathway, enhanced susceptibility to central nervous system (CNS) autoimmunity. Neonatal mice administered with these miRNAs further exhibited reduced Treg frequencies with a loss in T cell receptor repertoire diversity following the induction of experimental autoimmune encephalomyelitis in adulthood. Exacerbated CNS autoimmunity as a result of miRNA overexpression in CD4 T cells was accompanied by enhanced Th1 and Th17 cell frequencies. These findings demonstrate that increased levels of TGF-β-associated miRNAs impede the development of a diverse Treg population, leading to enhanced effector cell activity, and contributing to an increased susceptibility to CNS autoimmunity. Thus, TGF-β-targeting miRNAs could be a risk factor for MS, and recovering optimal TGF-β signaling may restore immune homeostasis in MS patients.

Decoding the potential role of regulatory T cells in sepsis-induced immunosuppression

Sepsis, a multiorgan dysfunction with high incidence and mortality, is caused by an imbalanced host-to-infection immune response. Organ-support therapy improves the early survival rate of sepsis patients. In the long term, those who survive the "cytokine storm" and its secondary damage usually show higher susceptibility to secondary infections and sepsis-induced immunosuppression, in which regulatory T cells (Tregs) are evidenced to play an essential role. However, the potential role and mechanism of Tregs in sepsis-induced immunosuppression remains elusive. In this review, we elucidate the role of different functional subpopulations of Tregs during sepsis and then review the mechanism of sepsis-induced immunosuppression from the aspects of regulatory characteristics, epigenetic modification, and immunometabolism of Tregs. Thoroughly understanding how Tregs impact the immune system during sepsis may shed light on preclinical research and help improve the translational value of sepsis immunotherapy.

Tipping the balance in autoimmunity: are regulatory t cells the cause, the cure, or both?

Regulatory T cells (Tregs) are a specialized subgroup of T-cell lymphocytes that is crucial for maintaining immune homeostasis and preventing excessive immune responses. Depending on their differentiation route, Tregs can be subdivided into thymically derived Tregs (tTregs) and peripherally induced Tregs (pTregs), which originate from conventional T cells after extrathymic differentiation at peripheral sites. Although the regulatory attributes of tTregs and pTregs partially overlap, their modes of action, protein expression profiles, and functional stability exhibit specific characteristics unique to each subset. Over the last few years, our knowledge of Treg differentiation, maturation, plasticity, and correlations between their phenotypes and functions has increased. Genetic and functional studies in patients with numeric and functional Treg deficiencies have contributed to our mechanistic understanding of immune dysregulation and autoimmune pathologies. This review provides an overview of our current knowledge of Treg biology, discusses monogenetic Treg pathologies and explores the role of Tregs in various other autoimmune disorders. Additionally, we discuss novel approaches that explore Tregs as targets or agents of innovative treatment options.

Mechanism study of ubiquitination in T cell development and autoimmune disease

T cells play critical role in multiple immune processes including antigen response, tumor immunity, inflammation, self-tolerance maintenance and autoimmune diseases et. Fetal liver or bone marrow-derived thymus-seeding progenitors (TSPs) settle in thymus and undergo T cell-lineage commitment, proliferation, T cell receptor (TCR) rearrangement, and thymic selections driven by microenvironment composed of thymic epithelial cells (TEC), dendritic cells (DC), macrophage and B cells, thus generating T cells with diverse TCR repertoire immunocompetent but not self-reactive. Additionally, some self-reactive thymocytes give rise to Treg with the help of TEC and DC, serving for immune tolerance. The sequential proliferation, cell fate decision, and selection during T cell development and self-tolerance establishment are tightly regulated to ensure the proper immune response without autoimmune reaction. There are remarkable progresses in understanding of the regulatory mechanisms regarding ubiquitination in T cell development and the establishment of self-tolerance in the past few years, which holds great potential for further therapeutic interventions in immune-related diseases.

Interleukin-2 signaling in the regulation of T cell biology in autoimmunity and cancer

Interleukin-2 (IL-2) is a critical cytokine for T cell peripheral tolerance and immunity. Here, we review how IL-2 interaction with the high-affinity IL-2 receptor (IL-2R) supports the development and homeostasis of regulatory T cells and contributes to the differentiation of helper, cytotoxic, and memory T cells. A critical element for each T cell population is the expression of CD25 (Il2rα), which heightens the receptor affinity for IL-2. Signaling through the high-affinity IL-2R also reinvigorates CD8+ exhausted T (Tex) cells in response to checkpoint blockade. We consider the molecular underpinnings reflecting how IL-2R signaling impacts these various T cell subsets and the implications for enhancing IL-2-dependent immunotherapy of autoimmunity, other inflammatory disorders, and cancer.

Evolutionary trade-offs constraining the MHC gene expansion: beyond simple TCR depletion model

The immune system is as much shaped by the pressure of pathogens as it is by evolutionary trade-offs that constrain its structure and function. A perfect example comes from the major histocompatibility complex (MHC), molecules that initiate adaptive immune response by presentation of foreign antigens to T cells. The remarkable, population-level polymorphism of MHC genes is assumed to result mainly from a co-evolutionary arms race between hosts and pathogens, while the limited, within-individual number of functional MHC loci is thought to be the consequence of an evolutionary trade-off between enhanced pathogen recognition and excessive T cell depletion during negative selection in the thymus. Certain mathematical models and infection studies suggest that an intermediate individual MHC diversity would thus be optimal. A recent, more direct test of this hypothesis has shown that the effects of MHC diversity on T-cell receptor (TCR) repertoires may differ between MHC classes, supporting the depletion model only for MHC class I. Here, we used the bank vole (Myodes=Cletronomys glareolus), a rodent species with variable numbers of expressed MHC genes, to test how an individual MHC diversity influences the proportions and TCR repertoires of responding T cell subsets. We found a non-linear relationship between MHC diversity and T cell proportions (with intermediate MHC numbers coinciding with the largest T cell proportions), perhaps reflecting an optimality effect of balanced positive and negative thymic selection. The association was strongest for the relationship between MHC class I and splenic CD8+ T cells. The CD8+ TCR richness alone was unaffected by MHC class I diversity, suggesting that MHC class I expansion may be limited by decreasing T cell counts, rather than by direct depletion of TCR richness. In contrast, CD4+ TCR richness was positively correlated with MHC class II diversity, arguing against a universal TCR depletion. It also suggests that different evolutionary forces or trade-offs may limit the within-individual expansion of the MHC class II loci.

Regulatory T Cells for Control of Autoimmunity

Regulatory T (Treg) cells, which specifically express the master transcription factor FoxP3, are indispensable for the maintenance of immunological self-tolerance and homeostasis. Their functional or numerical anomalies can be causative of autoimmune and other inflammatory diseases. Recent advances in the research of the cellular and molecular basis of how Treg cells develop, exert suppression, and maintain their function have enabled devising various ways for controlling physiological and pathological immune responses by targeting Treg cells. It is now envisaged that Treg cells as a "living drug" are able to achieve antigen-specific immune suppression of various immune responses and reestablish immunological self-tolerance in the clinic.

The Function of Ubiquitination in T-Cell Development

Thymus is an important primary lymphoid organ for T cell development. After T-lineage commitment, the early thymic progenitors (ETPs) develop into CD4-CD8- (DN), CD4+CD8+ (DP) and further CD4+ SP or CD8+ SP T cells. Under the help of thymic epithelial cells (TEC), dendritic cell (DC), macrophage, and B cells, ETPs undergo proliferation, T cell receptor (TCR) rearrangement, β-selection, positive selection, and negative selection, and thus leading to the generation of T cells that are diverse repertoire immunocompetent but not self-reactive. Additionally, some self-reactive thymocytes give rise to Treg under the help of TEC and DC. The regulation of T cell development is complicated. As a post-translational modification, ubiquitination regulates signal transduction in diverse biological processes. Ubiquitination functions in T cell development through regulating key signal pathway or maturation and function of related cells. In this review, the regulation of T cell development by ubiquitination is summarized and discussed.

Selective reprogramming of regulatory T cells in solid tumors can strongly enhance or inhibit tumor growth

Folate receptor delta (FRδ) has been used as a biomarker for regulatory T cells (Tregs), because its expression is limited to Tregs and ovum. Although FRδ is unable to bind folate, we have used molecular docking software to identify a folate congener that binds FRδ with high affinity and have exploited this FRδ-specific ligand to target attached drugs (imaging agents, immune activators, and immune suppressors) specifically to Tregs in murine tumor xenografts. Analysis of treated tumors demonstrates that targeting of a Toll-like receptor 7 agonist inhibits Treg expression of FOXP3, PD-1, CTLA4, and HELIOS, resulting in 40-80% reduction in tumor growth and repolarization of other tumor-infiltrating immune cells to more inflammatory phenotypes. Targeting of the immunosuppressive drug dexamethasone, in contrast, promotes enhanced tumor growth and shifts the tumor-infiltrating immune cells to more anti-inflammatory phenotypes. Since Tregs comprise <1% of cells in the tumor masses examined, and since the targeted drugs are not internalized by cancer cells, these data demonstrate that Tregs exert a disproportionately large effect on tumor growth. Because the targeted drug did not bind to Tregs or other immune cells in healthy tissues, the data demonstrate that the immunosuppressive properties of Tregs in tumors can be manipulated without causing systemic toxicities associated with global reprogramming of the immune system.

Drug resistance in glioblastoma: from chemo- to immunotherapy

As the most common and aggressive type of primary brain tumor in adults, glioblastoma is estimated to end over 10,000 lives each year in the United States alone. Stand treatment for glioblastoma, including surgery followed by radiotherapy and chemotherapy (i.e., Temozolomide), has been largely unchanged since early 2000. Cancer immunotherapy has significantly shifted the paradigm of cancer management in the past decade with various degrees of success in treating many hematopoietic cancers and some solid tumors, such as melanoma and non-small cell lung cancer (NSCLC). However, little progress has been made in the field of neuro-oncology, especially in the application of immunotherapy to glioblastoma treatment. In this review, we attempted to summarize the common drug resistance mechanisms in glioblastoma from Temozolomide to immunotherapy. Our intent is not to repeat the well-known difficulty in the area of neuro-oncology, such as the blood-brain barrier, but to provide some fresh insights into the molecular mechanisms responsible for resistance by summarizing some of the most recent literature. Through this review, we also hope to share some new ideas for improving the immunotherapy outcome of glioblastoma treatment.

Obesity-induced thymic involution and cancer risk

Declining thymic functions associated either with old age (i.e., age-related thymic involution), or with acute involution as a result of stress, infectious disease, or cytoreductive therapies (e.g., chemotherapy/radiotherapy), have been associated with cancer development. A key mechanism underlying such increased cancer risk is the thymus-dependent debilitation of adaptive immunity, which is responsible for orchestrating immunoediting mechanisms and tumor immune surveillance. In the past few years, a blooming set of evidence has intriguingly linked obesity with cancer development and progression. The majority of such studies has focused on obesity-driven chronic inflammation, steroid/sex hormone and adipokine production, and hyperinsulinemia, as principal factors affecting the tumor microenvironment and driving the development of primary malignancy. However, experimental observations about the negative impact of obesity on T cell development and maturation have existed for more than half a century. Here, we critically discuss the molecular and cellular mechanisms of obesity-driven thymic involution as a previously underrepresented intermediary pathology leading to cancer development and progression. This knowledge could be especially relevant in the context of childhood obesity, because impaired thymic function in young individuals leads to immune system abnormalities, and predisposes to various pediatric cancers. A thorough understanding behind the molecular and cellular circuitries governing obesity-induced thymic involution could therefore help towards the rationalized development of targeted thymic regeneration strategies for obese individuals at high risk of cancer development.

The role of regulatory T cells and follicular T helper cells in HBV infection

Hepatitis B has become one of the major global health threats, especially in developing countries and regions. Hepatitis B virus infection greatly increases the risk for liver diseases such as cirrhosis and cancer. However, treatment for hepatitis B is limited when considering the huge base of infected people. The immune response against hepatitis B is mediated mainly by CD8+ T cells, which are key to fighting invading viruses, while regulatory T cells prevent overreaction of the immune response process. Additionally, follicular T helper cells play a key role in B-cell activation, proliferation, differentiation, and formation of germinal centers. The pathogenic process of hepatitis B virus is generally the result of a disorder or dysfunction of the immune system. Therefore, we present in this review the critical functions and related biological processes of regulatory T cells and follicular T helper cells during HBV infection.

Regulatory T cells in peripheral tissue tolerance and diseases

Maintenance of peripheral tolerance by CD4+Foxp3+ regulatory T cells (Tregs) is essential for regulating autoreactive T cells. The loss of function of Foxp3 leads to autoimmune disease in both animals and humans. An example is the rare, X-linked recessive disorder known as IPEX (Immune Dysregulation, Polyendocrinopathy, Enteropathy X-linked) syndrome. In more common human autoimmune diseases, defects in Treg function are accompanied with aberrant effector cytokines such as IFNγ. It has recently become appreciated that Tregs plays an important role in not only maintaining immune homeostasis but also in establishing the tissue microenvironment and homeostasis of non-lymphoid tissues. Tissue resident Tregs show profiles that are unique to their local environments which are composed of both immune and non-immune cells. Core tissue-residence gene signatures are shared across different tissue Tregs and are crucial to homeostatic regulation and maintaining the tissue Treg pool in a steady state. Through interaction with immunocytes and non-immunocytes, tissue Tregs exert a suppressive function via conventional ways involving contact dependent and independent processes. In addition, tissue resident Tregs communicate with other tissue resident cells which allows Tregs to adopt to their local microenvironment. These bidirectional interactions are dependent on the specific tissue environment. Here, we summarize the recent advancements of tissue Treg studies in both human and mice, and discuss the molecular mechanisms that maintain tissue homeostasis and prevent pathogenesis.

Human T follicular helper clones seed the germinal center-resident regulatory pool

The mechanisms by which FOXP3+ T follicular regulatory (Tfr) cells simultaneously steer antibody formation toward microbe or vaccine recognition and away from self-reactivity remain incompletely understood. To explore underappreciated heterogeneity in human Tfr cell development, function, and localization, we used paired TCRVA/TCRVB sequencing to distinguish tonsillar Tfr cells that are clonally related to natural regulatory T cells (nTfr) from those likely induced from T follicular helper (Tfh) cells (iTfr). The proteins iTfr and nTfr cells differentially expressed were used to pinpoint their in situ locations via multiplex microscopy and establish their divergent functional roles. In silico analyses and in vitro tonsil organoid tracking models corroborated the existence of separate Treg-to-nTfr and Tfh-to-iTfr developmental trajectories. Our results identify human iTfr cells as a distinct CD38+, germinal center-resident, Tfh-descended subset that gains suppressive function while retaining the capacity to help B cells, whereas CD38- nTfr cells are elite suppressors primarily localized in follicular mantles. Interventions differentially targeting specific Tfr cell subsets may provide therapeutic opportunities to boost immunity or more precisely treat autoimmune diseases.

Principles of regulatory T cell function

Regulatory T (Treg) cells represent a distinct lineage of cells of the adaptive immune system indispensable for forestalling fatal autoimmune and inflammatory pathologies. The role of Treg cells as principal guardians of the immune system can be attributed to their ability to restrain all currently recognized major types of inflammatory responses through modulating the activity of a wide range of cells of the innate and adaptive immune system. This broad purview over immunity and inflammation is afforded by the multiple modes of action Treg cells exert upon their diverse molecular and cellular targets. Beyond the suppression of autoimmunity for which they were originally recognized, Treg cells have been implicated in tissue maintenance, repair, and regeneration under physiologic and pathologic conditions. Herein, we discuss the current and emerging understanding of Treg cell effector mechanisms in the context of the basic properties of Treg cells that endow them with such functional versatility.

Comprehensive analysis of T-cell receptor repertoires reveals antigen-driven T-cell clusters in patients with Behçet's syndrome

T lymphocytes are the major components of adaptive immunity in Behçet's syndrome (BS) pathology. However, the precise mechanism of T-cell-induced inflammatory condition remains to be determined. We applied bulk sequencing of the T-cell receptor (TCR) β chain in peripheral blood samples from 45 patients with BS and 10 healthy donors as controls. TCR repertoires in BS patients displayed more clonality and less diversity than in healthy donors. Male patients exhibited lower diversity metrics of TCR and had a larger proportion in the top 10 clones than females (p = 0.016). There were no TCR clonality differences in other clinical features, such as age, disease duration, organ involvement, disease severity, and activity. By "Grouping of Lymphocyte Interactions by Paratope Hotspots" (GLIPH2) for antigen prediction, we found distinct 2477 clusters of TCR-β sequences that potentially recognize similar antigens shared between BS patients. We observed clonal T-cell expansion in BS patients. Sexual differences in TCR clonal expansion and public TCR groups deserve further study to reveal the underline T-cell-mediated immunity in BS.

CPHEN-016: Comprehensive phenotyping of human regulatory T cells

Peripheral immunological tolerance is mainly maintained by regulatory T (Treg) cells, a specific CD4 T cells subset that expresses the transcription factor Foxp3. Treg cells are crucial to control autoimmunity and inflammation and to limit tissue destruction arising from inflammatory responses. Loss of functions mutations in FOXP3 in humans induces a fatal autoimmune lymphoproliferative disorder, known as Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked (IPEX). Specific Treg cell differentiation and activation states have been linked to several human diseases. Indeed, Treg cells play a crucial role in different diseases including colitis, multiple sclerosis, autoimmunity, and infection. Characterization of Treg cell functions and understanding the role of different Treg cell subsets are crucial to the development of novel Treg cell-specific therapeutics for inflammatory diseases. In this phenotype report, we will describe laboratory methods to effectively study and characterize human Treg cells.

Thymic epithelial cells require lipid kinase Vps34 for CD4 but not CD8 T cell selection

The generation of a functional, self-tolerant T cell receptor (TCR) repertoire depends on interactions between developing thymocytes and antigen-presenting thymic epithelial cells (TECs). Cortical TECs (cTECs) rely on unique antigen-processing machinery to generate self-peptides specialized for T cell positive selection. In our current study, we focus on the lipid kinase Vps34, which has been implicated in autophagy and endocytic vesicle trafficking. We show that loss of Vps34 in TECs causes profound defects in the positive selection of the CD4 T cell lineage but not the CD8 T cell lineage. Utilizing TCR sequencing, we show that T cell selection in conditional mutants causes altered repertoire properties including reduced clonal sharing. cTECs from mutant mice display an increased abundance of invariant chain intermediates bound to surface MHC class II molecules, indicating altered antigen processing. Collectively, these studies identify lipid kinase Vps34 as an important contributor to the repertoire of selecting ligands processed and presented by TECs to developing CD4 T cells.

A hierarchy of selection pressures determines the organization of the T cell receptor repertoire

We systematically examine the receptor repertoire in T cell subsets in young, adult, and LCMV-infected mice. Somatic recombination generates diversity, resulting in the limited overlap between nucleotide sequences of different repertoires even within the same individual. However, statistical features of the repertoire, quantified by the V gene and CDR3 k-mer frequency distributions, are highly conserved. A hierarchy of immunological processes drives the evolution of this structure. Intra-thymic divergence of CD4+ and CD8+ lineages imposes subtle but dominant differences observed across repertoires of all subpopulations in both young and adult mice. Differentiation from naive through memory to effector phenotype imposes an additional gradient of repertoire diversification, which is further influenced by age in a complex and lineage-dependent manner. The distinct repertoire of CD4+ regulatory T cells is more similar to naive cells in young mice and to effectors in adults. Finally, we describe divergent (naive and memory) and convergent (CD8+ effector) evolution of the repertoire following acute infection with LCMV. This study presents a quantitative framework that captures the structure of the repertoire in terms of its fundamental statistical properties and describes how this structure evolves as individual T cells differentiate, migrate and mature in response to antigen exposure.

A Novel GMP Protocol to Produce High-Quality Treg Cells From the Pediatric Thymic Tissue to Be Employed as Cellular Therapy

Due to their suppressive capacity, the adoptive transfer of regulatory T cells (Treg) has acquired a growing interest in controlling exacerbated inflammatory responses. Limited Treg recovery and reduced quality remain the main obstacles in most current protocols where differentiated Treg are obtained from adult peripheral blood. An alternate Treg source is umbilical cord blood, a promising source of Treg cells due to the higher frequency of naïve Treg and lower frequency of memory T cells present in the fetus’ blood. However, the Treg number isolated from cord blood remains limiting. Human thymuses routinely discarded during pediatric cardiac surgeries to access the retrosternal operative field has been recently proposed as a novel source of Treg for cellular therapy. This strategy overcomes the main limitations of current Treg sources, allowing the obtention of very high numbers of undifferentiated Treg. We have developed a novel good manufacturing practice (GMP) protocol to obtain large Treg amounts, with very high purity and suppressive capacity, from the pediatric thymus (named hereafter thyTreg). The total amount of thyTreg obtained at the end of the procedure, after a short-term culture of 7 days, reach an average of 1,757 x10⁶ (range 50 x 10⁶ – 13,649 x 10⁶) cells from a single thymus. The thyTreg product obtained with our protocol shows very high viability (mean 93.25%; range 83.35% – 97.97%), very high purity (mean 92.89%; range 70.10% – 98.41% of CD25⁺FOXP3⁺ cells), stability under proinflammatory conditions and a very high suppressive capacity (inhibiting in more than 75% the proliferation of activated CD4⁺ and CD8⁺ T cells in vitro at a thyTreg:responder cells ratio of 1:1). Our thyTreg product has been approved by the Spanish Drug Agency (AEMPS) to be administered as cell therapy. We are recruiting patients in the first-in-human phase I/II clinical trial worldwide that evaluates the safety, feasibility, and efficacy of autologous thyTreg administration in children undergoing heart transplantation (NCT04924491). The high quality and amount of thyTreg and the differential features of the final product obtained with our protocol allow preparing hundreds of doses from a single thymus with improved therapeutic properties, which can be cryopreserved and could open the possibility of an “off-the-shelf” allogeneic use in another individual.

Essential Roles of the Transcription Factor NR4A1 in Regulatory T Cell Differentiation under the Influence of Immunosuppressants

Calcineurin inhibitors (CNIs), used as immunosuppressants, have revolutionized transplantation medicine with their strong suppressive activity on alloreactive T lymphocytes; however, they may also cause various adverse effects, including an increased risk for infection and nephrotoxicity. Regulatory T (Treg) cells can complement the deleterious side effects of CNIs with their effective Ag-specific suppressive activities. However, several studies have shown that CNIs suppress Treg cell differentiation. Therefore, an understanding of the mechanisms by which CNIs suppress Treg cell differentiation, as well as an approach for promoting the differentiation of Treg cells in the presence of CNIs, has significant clinical value. In this article, we report that the nuclear orphan receptor Nr4a1 plays a pivotal role in Treg cell differentiation in the presence of CNIs. Unlike that of its family members, Nr4a2 and Nr4a3, the expression of Nr4a1 was not suppressed by CNI treatment, thereby mediating Treg cell differentiation in the presence of CNIs. In a mouse allogeneic graft-versus-host disease model, Nr4a1 mediated tolerance by promoting Treg cell differentiation in mice administered cyclosporine A, prolonging the survival of recipients. Furthermore, activation of Nr4a1 via its agonist partially restored Treg cell differentiation, which was suppressed by cyclosporine A treatment. Finally, we found that the rs2701129 single-nucleotide polymorphism, which was shown to downregulate NR4A1 expression, showed a trend toward a higher incidence of chronic graft-versus-host disease in patients undergoing hematopoietic stem cell transplantation. Therefore, our study will be of clinical significance because we demonstrated the role of Nr4a1 in Treg cell differentiation in the presence of CNIs.

Regulation of thymic T regulatory cell differentiation by TECs in health and disease

The thymus produces self-limiting and self-tolerant T cells through the interaction between thymocytes and thymus epithelial cells (TECs), thereby generating central immune tolerance. The TECs are composed of cortical and medullary thymic epithelial cells, which regulate the positive and negative selection of T cells, respectively. During the process of negative selection, thymocytes with self-reactive ability are deleted or differentiated into regulatory T cells (Tregs). Tregs are a subset of suppressor T cells that are important for maintaining immune homeostasis. The differentiation and development of Tregs depend on the development of TECs and other underlying molecular mechanisms. Tregs regulated by thymic epithelial cells are closely related to human health and are significant in autoimmune diseases, thymoma and pregnancy. In this review, we summarize the current molecular and transcriptional regulatory mechanisms by which TECs affect the development and function of thymic Tregs. We also review the pathophysiological models of thymic epithelial cells regulating thymic Tregs in human diseases and specific physiological conditions.

The Impact of Graft CD3 Cell/Regulatory T Cell Ratio on Acute Graft-versus-Host Disease and Post-Transplantation Outcome: A Prospective Multicenter Study of Patients with Acute Leukemia Undergoing Allogeneic Peripheral Blood Stem Cell Transplantation

Although it is well known that tumor site- or bone marrow-infiltrating regulatory T cells (Tregs) might be correlated with worse outcomes in solid tumors and acute leukemias by promoting immune surveillance escape, their contribution to the immediate post-allogeneic transplantation phase by peripheral blood (PB) allografts remains unclear. Moreover, the Treg content in stem cells harvested from PB has been suggested to be correlated with acute graft versus-host-disease (aGVHD) and immunologic recovery after allogeneic PB stem cell transplantation (allo-PBSCT). This study aimed to investigate the impact of the graft content of Tregs, as graft CD3⁺/Tregs ratio (gCD3/TregsR), on acute GVHD and post-allo-PBSCT outcomes. We prospectively enrolled 94 consecutive patients at 9 Italian centers of the Gruppo Italiano Trapianto di Midollo Osseo (GITMO) with acute myelogenous (n = 71; 75%) or lymphoblastic (n = 23; 25%) leukemia in complete remission who underwent matched related donor (n = 35; 37%) or unrelated donor (n = 59; 63%) allo-PBSCT. The median graft CD3⁺ cell, Treg, and gCD3/TregsR values were 196 × 10⁶/kg body weight (range, 17 to 666 × 10⁶/kg), 3 × 10⁶/kg (range, 0.1 to 35 × 10⁶/kg), and 71 (range, 1 to 1883), respectively. The discriminatory power of the gCD3/TregsR value to predict grade ≥II aGVHD was assessed by estimating the area under the receiver operating characteristic (ROC) curve (AUC). Any grade and grade ≥II aGVHD occurred in 24 (26%) and 17 (18%) allo-PBSCT recipients, respectively. By ROC analysis, AUC (0.74; 95% confidence interval [CI], 0.608 to 0.866; P = .002) identified 70 as the optimal gCD3/TregsR cutoff value predicting the appearance of grade ≥II aGVHD with 76% sensitivity and 71% specificity. Patients were subdivided into a high (ROC curve value ≥70) gCD3/TregsR group (HR; n = 48) and a low (ROC curve value <70) gCD3/TregsR group (LR; n = 46). The incidence of grade II-IV aGVHD was lower in the LR group compared with the HR group (9% [4 of 46] versus 27% [13 of 48]) in both univariate analysis (odds ratio [OR], 4.8; 95% CI, 1.44 to 16.17; P = .015) and multivariate analysis (OR, 5.0; 95% CI, 1.34 to 18.93; P = .017), whereas no differences were documented taking into account aGVHD of any grade. The overall survival, disease-free survival, nonrelapse mortality, and relapse rates at 2 and 3 years were 61% and 54%, 62% and 55%, 15% and 23%, and 27% and 30%, respectively. Of note, gCD3/TregsR did not significantly correlate with relapse (P = .135). Taken together, our data from this prospective multicenter study confirm the value of Tregs in preventing aGVHD while maintaining the graft-versus-leukemia effect. © 2021 American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc.

TCR repertoire characteristics predict clinical response to adoptive CTL therapy against nasopharyngeal carcinoma

The past decade has witnessed the gradual and steady progress of adoptive T cell therapy in treating various types of cancer. In combination with gemcitabine and carboplatin chemotherapy, we previously conducted a clinical trial, NCT00690872, to treat Epstein-Barr virus (EBV)-positive nasopharyngeal carcinoma (NPC) patients with autologous EBV-expanded cytotoxic T lymphocytes (CTLs). While achieving a 2-year overall survival rate of 62.9%, this trial failed to induce an anti-tumor response in a sizable fraction of patients. Thus, the identification of benchmarks capable of evaluating CTL products and predicting clinical immunotherapeutic efficacy remains an urgent need. We conducted T cell receptor (TCR) repertoire sequencing to assess EBV-expanded infusion-ready CTL products. To depict the overall repertoire landscape, we evaluated the individual repertoire diversity by Shannon entropy, and, compared the inter-patient CDR3 similarity to estimate T cells expanded by common antigens. With a recently developed bioinformatics algorithm, termed Motif Analysis, we made a machine-learning prediction of structural regions within the CDR3 of TCRβ that associate with CTL therapy prognosis. We found that long term survivors, defined as patients surviving longer than two years, had a higher CTL repertoire diversity with reduced inter-patient similarity. Furthermore, TCR Motif Analysis identified 11 structural motifs distinguishing long term survivors from short term survivors. Specifically, two motifs with a high area under the curve (AUC) values were identified as potential predictive benchmarks for efficacious CTL production. Together, these results reveal that the presence of diverse TCR sequences containing a common core motif set is associated with a favorable response to CTL immunotherapy against EBV-positive NPC.

Role of IL-6 in the commitment of T cell subsets

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IL-6 is a non-redundant differentiation factor for Th17 cells and Tfh cells.

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The induction of ROR-γt⁺ Treg cells in the lamina propria depends on IL-6.

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Generation of distinct T helper cell subsets might depend on different IL-6 signaling modalities.

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IL-6-directed therapies must consider the disease-relevant IL-6 signaling modality.

IL-6 gained much attention with the discovery that this cytokine is a non-redundant differentiation factor for Th17 cells and T follicular helper cells. Adaptive immune responses to fungi and extracellular bacteria are impaired in the absence of IL-6. IL-6 is also required for the induction of ROR-γt⁺ Treg cells, which are gatekeepers of homeostasis in the gut lamina propria in the presence of commensal bacteria. Conversely, severe immunopathology in T cell-mediated autoimmunity is mediated by Th17 cells that rely on IL-6 for their generation and maintenance. Recently, it has been discovered that the differentiation of these distinct T helper cell subsets may be linked to distinct signaling modalities of IL-6. Here, we summarize the current knowledge on the mode of action of IL-6 in the differentiation and maintenance of T cell subsets and propose that a context-dependent understanding of the impact of IL-6 on T cell subsets might inform rational IL-6-directed interventions in autoimmunity and chronic inflammation.

Antigen-Specific Regulatory T Cell Therapy in Autoimmune Diseases and Transplantation

Regulatory T (Treg) cells are a heterogenous population of immunosuppressive T cells whose therapeutic potential for the treatment of autoimmune diseases and graft rejection is currently being explored. While clinical trial results thus far support the safety and efficacy of adoptive therapies using polyclonal Treg cells, some studies suggest that antigen-specific Treg cells are more potent in regulating and improving immune tolerance in a disease-specific manner. Hence, several approaches to generate and/or expand antigen-specific Treg cells in vitro or in vivo are currently under investigation. However, antigen-specific Treg cell therapies face additional challenges that require further consideration, including the identification of disease-relevant antigens as well as the in vivo stability and migratory behavior of Treg cells following transfer. In this review, we discuss these approaches and the potential limitations and describe prospective strategies to enhance the efficacy of antigen-specific Treg cell treatments in autoimmunity and transplantation.

Self and microbiota-derived epitopes induce CD4+ T cell anergy and conversion into CD4+Foxp3+ regulatory cells

The physiological role of T cell anergy induction as a key mechanism supporting self-tolerance remains undefined, and natural antigens that induce anergy are largely unknown. In this report, we used TCR sequencing to show that the recruitment of CD4⁺CD44⁺Foxp3^-CD73⁺FR4⁺ anergic (Tan) cells expands the CD4⁺Foxp3⁺ (Tregs) repertoire. Next, we report that blockade in peripherally-induced Tregs (pTregs) formation due to mutation in CNS1 region of Foxp3 or chronic exposure to a selecting self-peptide result in an accumulation of Tan cells. Finally, we show that microbial antigens from Akkermansia muciniphila commensal bacteria can induce anergy and drive conversion of naive CD4⁺CD44^-Foxp3^- T (Tn) cells to the Treg lineage. Overall, data presented here suggest that Tan induction helps the Treg repertoire to become optimally balanced to provide tolerance toward ubiquitous and microbiome-derived epitopes, improving host ability to avert systemic autoimmunity and intestinal inflammation.

Regulatory T Cell Heterogeneity in the Thymus: Impact on Their Functional Activities

Foxp3⁺ regulatory T cells (Treg) maintain the integrity of the organism by preventing excessive immune responses. These cells protect against autoimmune diseases but are also important regulators of other immune responses including inflammation, allergy, infection, and tumors. Furthermore, they exert non-immune functions such as tissue repair and regeneration. In the periphery, Foxp3⁺ Treg have emerged as a highly heterogeneous cell population with distinct molecular and functional properties. Foxp3⁺ Treg mainly develop within the thymus where they receive instructive signals for their differentiation. Recent studies have revealed that thymic Treg are also heterogeneous with two distinct precursors that give rise to mature Foxp3⁺ Treg exhibiting non-overlapping regulatory activities characterized by a differential ability to control different types of autoimmune reactions. Furthermore, the thymic Treg cell pool is not only composed of newly developing Treg, but also contain a large fraction of recirculating peripheral cells. Here, we review the two pathways of thymic Treg cell differentiation and their potential impact on Treg activity in the periphery. We also summarize our current knowledge on recirculating peripheral Treg in the thymus.

Regulatory T Cells in the Tumor Microenvironment

Regulatory T cells (T_regs) are an immunosuppressive subpopulation of CD4⁺ T cells that are endowed with potent suppressive activity and function to limit immune activation and maintain homeostasis. These cells are identified by the hallmark transcription factor FOXP3 and the high-affinity interleukin-2 (IL-2) receptor chain CD25. T_regs can be recruited to and persist within the tumor microenvironment (TME), acting as a potent barrier to effective antitumor immunity. This chapter will discuss [i] the history and hallmarks of T_regs; [ii] the recruitment, development, and persistence of T_regs within the TME; [iii] T_reg function within TME; asnd [iv] the therapeutic targeting of T_regs in the clinic. This chapter will conclude with a discussion of likely trends and future directions.

Tregs in Autoimmune Uveitis

Uveitis is a chronic disease with relapsing and remitting ocular attack, which requires corticosteroids and systemic immunosuppression to prevent severe vision loss. Classically, uveitis is referred to an autoimmune disease, mediated by pro-inflammatory Th17 cells and immunosuppressive CD4+CD25+FoxP3+ T-regulatory cells (Tregs). More and more evidence indicates that Tregs are involved in development, resolution, and remission of uveitis. Clinically, many researchers have conducted quantitative and functional analyses of peripheral blood from patients with different subtypes of uveitis, in an attempt to find the changing rules of Tregs. Consistently, using the experimental autoimmune uveitis (EAU) model, researchers have explored the development and resolution mechanism of uveitis in many aspects. In addition, many drug and Tregs therapy investigations have yielded encouraging results. In this chapter, we introduced the current understanding of Tregs, summarized the clinical changes in the number and function of patients with uveitis and the immune mechanism of Tregs involved in EAU model, as well as discussed the progress and shortcomings of Tregs-related drug therapy and Tregs therapy. Although the exact mechanism of Tregs-mediated uveitis protection remains to be elucidated, the strategy of Tregs regulation may provide a specific and meaningful way for the prevention and treatment of uveitis.

Functional analysis of clinical response to low-dose IL-2 in patients with refractory chronic graft-versus-host disease

Patients with chronic graft-versus-host disease (cGVHD) have a paucity of regulatory CD4 T cells (CD4Tregs) that mediate peripheral tolerance. In clinical trials, daily low-dose interleukin-2 (IL-2) has been administered safely for prolonged periods in patients with steroid-refractory cGVHD. Peripheral CD4Tregs expand dramatically in all patients during IL-2 therapy but clinical improvement was observed in ∼50% of patients. Here, we examined the impact of low-dose IL-2 therapy on functional T-cell markers and the T-cell repertoire within CD4Tregs, conventional CD4 T cells (CD4Tcons), and CD8⁺ T cells. IL-2 had profound effects on CD4Tregs homeostasis in both response groups including selective expansion of the naive subset, improved thymic output, and increased expression of Ki67, FOXP3, and B-cell lymphoma 2 within CD4Tregs. Similar changes were not seen in CD4Tcons or CD8 T cells. Functionally, low-dose IL-2 enhanced, in vitro, CD4Treg-suppressive activity in both response groups, and all patient CD4Tcons were similarly suppressed by healthy donor CD4Tregs. High-throughput sequencing of the T-cell receptor β (TCRβ) locus demonstrated that low-dose IL-2 therapy increased TCR repertoire diversity and decreased evenness within CD4Tregs without affecting CD4Tcons or CD8 T cells. Using clone-tracking analysis, we observed rapid turnover of highly prevalent clones in CD4Tregs as well as the conversion of CD4Tcons to CD4Tregs. After 12 weeks of daily IL-2, clinical responders had a greater influx of novel clones within the CD4Treg compartment compared with nonresponders. Further studies to define the function and specificity of these novel CD4Treg clones may help establish the mechanisms whereby low-dose IL-2 therapy promotes immune tolerance.

Developmental and cellular age direct conversion of CD4+ T cells into RORγ+ or Helios+ colon Treg cells

RORγ+ and Helios+ Treg cells in the colon are phenotypically and functionally distinct, but their origins and relationships are poorly understood. In monocolonized and normal mice, single-cell RNA-seq revealed sharing of TCR clonotypes between these Treg cell populations, potentially denoting a common progenitor. In a polyclonal Treg cell replacement system, naive conventional CD4+ (Tconv) cells, but not pre-existing tTregs, could differentiate into RORγ+ pTregs upon interaction with gut microbiota. A smaller proportion of Tconv cells converted into Helios+ pTreg cells, but these dominated when the Tconv cells originated from preweaning mice. T cells from infant mice were predominantly immature, insensitive to RORγ-inducing bacterial cues and to IL6, and showed evidence of higher TCR-transmitted signals, which are also characteristics of recent thymic emigrants (RTEs). Correspondingly, transfer of adult RTEs or Nur77high Tconv cells mainly yielded Helios+ pTreg cells, recapitulating the infant/adult difference. Thus, CD4+ Tconv cells can differentiate into both RORγ+ and Helios+ pTreg cells, providing a physiological adaptation of colonic Treg cells as a function of the age of the cell or of the individual.

T cell receptor repertoire as a prognosis marker for heat shock protein peptide complex-96 vaccine trial against newly diagnosed glioblastoma

Glioblastoma multiforme (GBM) is the most common primary malignant brain tumor in adults with a dismal prognosis. We previously reported that vaccination with heat shock protein peptide complex-96 (HSPPC-96) improves survival in patients with newly diagnosed GBM (NCT02122822). Especially for patients with a strong antitumor immune response after vaccination, a durable survival benefit can be achieved. Here, we conducted T cell receptor (TCR) sequencing to retrospectively examine the TCR repertoires of tumor-infiltrating lymphocytes in long-term survivors (LTS) and short-term survivors (STS). We found that LTS exhibit lower TCR repertoire diversity compared with STS, indicating the prevalence of dominant TCR clones in LTS tumors. Accordingly, the LTS group showed increased inter-patient similarity, especially among high-frequency TCR clones, implying some of these dominant clones are shared among LTS. Indeed, we discovered four TCR clones significantly enriched in the LTS group: the presence of these clones has predictive value for stratifying patients prior to vaccination. Together, these findings uncover a group of preexisting TCR clones shared in LTS that can be utilized as candidate biomarkers to select GBM patients most likely to durably respond to HSPPC-96 treatment.

Effects of Mitophagy on Regulatory T Cell Function in Patients With Myasthenia Gravis

Objective: This study was conducted to determine whether regulatory T cells (CD4⁺CD25⁺T, Tregs) show abnormal mitophagy as well as the function of Tregs in patients with myasthenia gravis (MG).

Methods: CD4⁺T cells and CD4⁺CD25⁺Treg cells were obtained from 15 patients with MG (MG group) and 15 controls (N group). Tregs from the MG group were subjected to rapamycin-induced culture for 48 h (Rapa group) and 3-methyladenine-induced culture for 48 h (3-MA group). The levels of mitophagy in Tregs were then observed through electron and confocal microscopy. Expression of the autophagy-related protein LC3-II was detected by western blotting, and mitochondrial function in each group was evaluated by flow cytometry. Inhibition of Treg cell proliferation was detected by flow cytometry.

Results: Mitophagy in the MG group was lower than that in the N group; it was higher in the Rapa group compared to that in the MG group and lowered in the 3-MA group than in the MG group. Expression of the autophagy-related protein LC3-II was lower in the MG group than in the N group, higher in the Rapa group than in the MG group, and lower in the 3-MA group than in the MG group. The mitochondrial membrane potential was lower in the MG group compared to that in the N group; it was higher in the Rapa group than in the MG group and lowered in the 3-MA group than in the MG group. Inhibition of Treg proliferation was lower in the MG group than in the N group; it was higher in the Rapa group than in the MG group and lowered in the 3-MA group than in the MG group.

Conclusion: The decreased mitochondrial membrane potential and mitophagy in Tregs in the MG group may be related to a decreased inhibition of Treg proliferation. The mitochondrial membrane potential was increased after adding the autophagy agent Rapa to enhance mitophagy, and the proliferation inhibition function of Tregs was also enhanced. The autophagy agent 3-MA down-regulated mitophagy, which decreased the mitochondrial membrane potential and inhibitory effect of Tregs. These results reveal the possible cellular immune mechanism of Treg dysfunction in MG.

Commensal epitopes drive differentiation of colonic Tregs

The gut microbiome is the largest source of intrinsic non-self-antigens that are continuously sensed by the immune system but typically do not elicit lymphocyte responses. CD4⁺ T cells are critical to sustain uninterrupted tolerance to microbial antigens and to prevent intestinal inflammation. However, clinical interventions targeting commensal bacteria-specific CD4⁺ T cells are rare, because only a very limited number of commensal-derived epitopes have been identified. Here, we used a new approach to study epitopes and identify T cell receptors expressed by CD4⁺Foxp3⁺ (T_reg) cells specific for commensal-derived antigens. Using this approach, we found that antigens from Akkermansia muciniphila reprogram naïve CD4⁺ T cells to the T_reg lineage, expand preexisting microbe specific T_regs, and limit wasting disease in the CD4⁺ T cell transfer model of colitis. These data suggest that the administration of specific commensal epitopes may help to widen the repertoire of specific T_regs that control intestinal inflammation.

Recent Progress in Treg Biology and Transplant Therapeutics

Purpose of Review

Regulatory T cell (Treg) biology continues to evolve at a rapid pace. The role of Tregs in solid organ transplantation offers a unique window into Treg ontogeny and function as well as limitless possibilities for clinical application. Here we review recent significant discoveries and key translational work.

Recent Findings

Advances in transplantation deepen understanding of Treg differentiation, expansion, transcription, co-stimulation, and signaling. T cell receptor (TCR) sequencing and single-cell analytics allow unprecedented insight into Treg repertoire diversity and phenotypic heterogeneity. Efforts to replace conventional immunosuppression with Treg adoptive immunotherapy are underway and coalescing around strategies to increase efficiency through development of donor-reactive Tregs.

Summary

Adoptive immunotherapy with Tregs is a leading tolerogenic strategy. Early clinical trials suggest that Treg infusion is safe and reports on efficacy will soon follow.

Treg cell-based therapies: challenges and perspectives

Cellular therapies using regulatory T (Treg) cells are currently undergoing clinical trials for the treatment of autoimmune diseases, transplant rejection and graft-versus-host disease. In this Review, we discuss the biology of Treg cells and describe new efforts in Treg cell engineering to enhance specificity, stability, functional activity and delivery. Finally, we envision that the success of Treg cell therapy in autoimmunity and transplantation will encourage the clinical use of adoptive Treg cell therapy for non-immune diseases, such as neurological disorders and tissue repair.

Comprehensive TCR repertoire analysis of CD4+ T-cell subsets in rheumatoid arthritis

The pathogenesis of rheumatoid arthritis (RA), a systemic autoimmune disease characterized by autoreactive T-cell accumulation and pro-inflammatory cytokine overproduction, is unclear. Systematically addressing T-cell receptor (TCR) repertoires of different CD4⁺ T-cell subsets could help understand RA pathogenesis. Here, peripheral CD4⁺ T cells from treatment-naïve RA patients and healthy controls were sorted into seven subsets including naïve, effector, central memory, effector memory (EMT), Th1, Th17, and regulatory T cells. T-cell receptor β chain repertoires were then analyzed by next-generation sequencing. We identified T-cell clonal expansion in EMT and Th17 cells of RA patients, with highly similar TCR repertoires. Ex vivo experiments demonstrated the preferred differentiation from EMT to Th17 cells in RA. Notably, we showed that TCR diversity and abundance of differentiated T cells of Th17 were significantly correlated with RA disease activity. Based on these observations, we propose that abnormal differentiation from EMT to Th17 and expansion of Th17 play pivotal role in RA pathogenesis.

TCR Repertoires of Thymic Conventional and Regulatory T Cells: Identification and Characterization of Both Unique and Shared TCR Sequences

Thymic regulatory T cells (tTreg) are critical in the maintenance of normal T cell immunity and tolerance. The role of TCR in tTreg selection remains incompletely understood. In this study, we assessed TCRα and TCRβ sequences of mouse tTreg and thymic conventional CD4⁺ T cells (Tconv) by high-throughput sequencing. We identified αβ TCR sequences that were unique to either tTreg or Tconv and found that these were distinct as recognized by machine learning algorithm and by preferentially used amino acid trimers in αβ CDR3 of tTreg. In addition, a proportion of αβ TCR sequences expressed by tTreg were also found in Tconv, and machine learning classified the great majority of these shared αβ TCR sequences as characteristic of Tconv and not tTreg. These findings identify two populations of tTreg, one in which the regulatory T cell fate is associated with unique properties of the TCR and another with TCR properties characteristic of Tconv for which tTreg fate is determined by factors beyond TCR sequence.

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.

Treg Heterogeneity, Function, and Homeostasis

T-regulatory cells (Tregs) represent a unique subpopulation of helper T-cells by maintaining immune equilibrium using various mechanisms. The role of T-cell receptors (TCR) in providing homeostasis and activation of conventional T-cells is well-known; however, for Tregs, this area is understudied. In the last two decades, evidence has accumulated to confirm the importance of the TCR in Treg homeostasis and antigen-specific immune response regulation. In this review, we describe the current view of Treg subset heterogeneity, homeostasis and function in the context of TCR involvement. Recent studies of the TCR repertoire of Tregs, combined with single-cell gene expression analysis, revealed the importance of TCR specificity in shaping Treg phenotype diversity, their functions and homeostatic maintenance in various tissues. We propose that Tregs, like conventional T-helper cells, act to a great extent in an antigen-specific manner, which is provided by a specific distribution of Tregs in niches.

Dormant pathogenic CD4+ T cells are prevalent in the peripheral repertoire of healthy mice

Thymic central tolerance eliminates most immature T cells with autoreactive T cell receptors (TCR) that recognize self MHC/peptide complexes. Regardless, an unknown number of autoreactive CD4+Foxp3- T cells escape negative selection and in the periphery require continuous suppression by CD4+Foxp3+ regulatory cells (Tregs). Here, we compare immune repertoires of Treg-deficient and Treg-sufficient mice to find Tregs continuously constraining one-third of mature CD4+Foxp3- cells from converting to pathogenic effectors in healthy mice. These dormant pathogenic clones frequently express TCRs activatable by ubiquitous autoantigens presented by class II MHCs on conventional dendritic cells, including self-peptides that select them in the thymus. Our data thus suggest that identification of most potentially autoreactive CD4+ T cells in the peripheral repertoire is critical to harness or redirect these cells for therapeutic advantage.

Regulatory T Cell Development in the Thymus

Development of a comprehensive regulatory T (Treg) cell compartment in the thymus is required to maintain immune homeostasis and prevent autoimmunity. In this study, we review cellular and molecular determinants of Treg cell development in the thymus. We focus on the evidence for a self-antigen-focused Treg cell repertoire as well as the APCs responsible for presenting self-antigens to developing thymocytes. We also cover the contribution of different cytokines to thymic Treg development and the cellular populations that produce these cytokines. Finally, we update the originally proposed "two-step" model of thymic Treg differentiation by incorporating new evidence demonstrating that Treg cells develop from two Treg progenitor populations and discuss the functional importance of Treg cells generated via either progenitor pathway.

Helios: still behind the clouds

Regulatory T (Treg) cells are a subset of CD4⁺ T cells that are critical for the maintenance of self-tolerance. The forkhead box transcription factor Foxp3 is a master regulator for the Treg phenotype and function and its expression is essential in Treg cells, as the loss of Foxp3 results in lethal autoimmunity. Two major subsets of Treg cells have been described in vivo; thymus-derived Treg (tTreg) cells that develop in the thymus and peripherally induced Treg (pTreg) cells that are derived from conventional CD4⁺  Foxp3^- T cells and are converted in peripheral tissues to cells that express Foxp3 and acquire suppressive ability. The transcription factor Helios, a member of the Ikaros transcription factor family, is expressed in 60-70% of Treg cells in both mouse and man, and is believed to be a marker of tTreg cells. In this review, we discuss the role and function of Helios in Treg cells, the controversy surrounding the use of Helios as a marker of tTreg cells, and how Helios controls specific aspects of the Treg cell program.

Regulatory T Cells: the Many Faces of Foxp3

Regulatory T (Treg) cells expressing the transcription factor forkhead box P3 (Foxp3) play a requisite role in the maintenance of immunological homeostasis and prevention of peripheral self-tolerance breakdown. Although Foxp3 by itself is neither necessary nor sufficient to specify many aspects of the Treg cell phenotype, its sustained expression in Treg cells is indispensable for their phenotypic stability, metabolic fitness, and regulatory function. In this review, we summarize recent advances in Treg cell biology, with a particular emphasis on the role of Foxp3 as a transcriptional modulator and metabolic gatekeeper essential to an effective immune regulatory response. We discuss these findings in the context of human inborn errors of immune dysregulation, with a focus on FOXP3 mutations, leading to Treg cell deficiency. We also highlight emerging concepts of therapeutic Treg cell reprogramming to restore tolerance in the settings of immune dysregulatory disorders.

Inventories of naive and tolerant mouse CD4 T cell repertoires reveal a hierarchy of deleted and diverted T cell receptors

Deletion or T_reg cell differentiation are alternative fates of autoreactive MHCII-restricted thymocytes. How these different modes of tolerance determine the size and composition of polyclonal cohorts of autoreactive T cells with shared specificity is poorly understood. We addressed how tolerance to a naturally expressed autoantigen of the central nervous system shapes the CD4 T cell repertoire. Specific cells in the tolerant peripheral repertoire either were Foxp3⁺ or displayed anergy hallmarks and, surprisingly, were at least as frequent as in the nontolerant repertoire. Despite this apparent lack of deletional tolerance, repertoire inventories uncovered that some T cell receptors (TCRs) were lost from the CD4 T cell pool, whereas others mediated T_reg cell differentiation. The antigen responsiveness of these TCRs supported an affinity model of central tolerance. Importantly, the contribution of different diverter TCRs to the nascent thymic T_reg cell population reflected their antigen reactivity rather than their frequency among precursors. This reveals a multilayered TCR hierarchy in CD4 T cell tolerance that separates deleted and diverted TCRs and assures that the T_reg cell compartment is filled with cells of maximal permissive antigen reactivity.

Pregnancy Enables Expansion of Disease-Specific Regulatory T Cells in an Animal Model of Multiple Sclerosis

Disease activity of autoimmune disorders such as multiple sclerosis and its mouse model experimental autoimmune encephalomyelitis (EAE) is temporarily suppressed by pregnancy. However, whether disease amelioration is due to nonspecific immunomodulation or mediated by Ag-specific regulation of disease-causing conventional T cells (Tcon) and immunosuppressive regulatory T cells (Tregs) remains elusive. In the current study, we systematically analyzed changes of the TCRβ repertoire driven by EAE and pregnancy using TCR sequencing. We demonstrate that EAE, but not pregnancy, robustly increased TCR repertoire clonality in both peripheral Tcon and Treg. Notably, pregnancy was required for the expansion of Treg harboring the dominant EAE-associated TRBV13-2 chain and increased the frequency of EAE-associated clonotypes within the Treg compartment. Our findings indicate that pregnancy supports the expansion of Treg clonotypes that are equipped to recognize EAE-associated Ags. These Treg are thereby particularly suited to control corresponding encephalitogenic Tcon responses and likely contribute to pregnancy-associated protection in autoimmunity.