Special regulatory T-cell review: A rose by any other name: from suppressor T cells to Tregs, approbation to unbridled enthusiasm

Inhibition of Immunological Suppression

This Pillars of Immunology article is a commentary on “Cytotoxic T lymphocyte-associated antigen 4 plays an essential role in the function of CD25+CD4+ regulatory cells that control intestinal inflammation,” a pivotal article written by S. Read, V. Malmström, and F. Powrie, and published in the Journal of Experimental Medicine, in 2000. https://doi.org/10.1084/jem.192.2.295.

Enhancing Regulatory T Cells to Treat Inflammatory and Autoimmune Diseases

Regulatory T cells (Tregs) control immune responses and are essential to maintain immune homeostasis and self-tolerance. Hence, it is no coincidence that autoimmune and chronic inflammatory disorders are associated with defects in Tregs. These diseases have currently no cure and are treated with palliative drugs such as immunosuppressant and immunomodulatory agents. Thereby, there is a great interest in developing medical interventions against these diseases based on enhancing Treg cell function and numbers. Here, we give an overview of Treg cell ontogeny and function, paying particular attention to mucosal Tregs. We review some notable approaches to enhance immunomodulation by Tregs with therapeutic purposes including adoptive Treg cell transfer therapy and discuss relevant clinical trials for inflammatory bowel disease. We next introduce ways to expand mucosal Tregs in vivo using microbiota and dietary products that have been the focus of clinical trials in various autoimmune and chronic-inflammatory diseases.

4-1BB and cytokines trigger human NK, γδ T, and CD8+ T cell proliferation and activation, but are not required for their effector functions

INTRODUCTION

This study was designed to compare the costimulatory molecules and cytokines required to trigger the proliferation and activation of natural killer (NK), γδ T, and CD8+ T cells, and gain in-depth insight into the mechanisms shifting tolerance to immunity.

METHODS

K562-derived artificial antigen-presenting cells (aAPCs); that is, K562 forced to express CD86 and 4-1BBL costimulatory receptors, in the presence of cytokines, were used to mimic dendritic cells (DCs) and provide signals to support the proliferation and activation of NK, γδ T, and CD8+ T cells.

RESULTS

Three signals are required to trigger optimal proliferation in MART-1-specific CD8+ T cells: activation of T-cell receptors (TCRs) by the major histocompatibility complex (MHC) I/peptide complexes (signal 1); 4-1BB engagement (signal 2); and IL-15 and IL-21 receptor co-signaling (signal 3). NK and γδ T cell proliferation also require three signals, but the precise nature of signal 1 involving cell-to-cell contact was not determined. Once they become effectors, only signal 1 determines the sensitivity or resistance of the target cells to cytolysis by killer lymphocytes. When freshly purified, none had effector functions, except the NK cells, which could be activated by CD16 engagement.

CONCLUSIONS

Therefore, lymphocytes committed to kill are produced as inactive precursors, and the license to kill is delivered by three signals, allowing for extensive proliferation and effector function acquisition. This data challenges the paradigm of anergy and supports the danger signal theory originally proposed by Polly Matzinger, which states that killer cells are tolerant by default, thereby protecting the mammalian body from autoimmunity.

CD8+ Regulatory T Cell - A Mystery to Be Revealed

Regulatory T cells (Treg) are essential to maintain immune homeostasis and prevent autoimmune disorders. While the function and molecular regulation of Foxp3+CD4+ Tregs are well established, much of CD8+ Treg biology remains to be revealed. Here, we will review the heterogenous subsets of CD8+ T cells have been named "CD8+ Treg" and mainly focus on CD122hiLy49+CD8+ Tregs present in naïve mice. CD122hiLy49+CD8+ Tregs, which depends on transcription factor Helios and homeostatic cytokine IL-15, have been established as a non-redundant regulator of germinal center (GC) reaction. Recently, we have demonstrated that TGF-β (Transforming growth factor-β) and transcription factor Eomes (Eomesodermin) are essential for the function and homeostasis of CD8+ Tregs. In addition, we will discuss several open questions regarding the differentiation, function and true identity of CD8+ Tregs as well as a brief comparison between two regulatory T cell subsets critical to control GC reaction, namely CD4+ TFR (follicular regulatory T cells) and CD8+ Tregs.

The Role of Regulatory T Cells in Epicutaneous Immunotherapy for Food Allergy

In recent decades, a rapid increase in the prevalence of food allergies has led to extensive research on novel treatment strategies and their mechanisms. Mouse models have provided preliminary insights into the mechanism of epicutaneous immunotherapy (EPIT)-induced immune tolerance. In EPIT, antigen applied on the skin surface can be captured, processed, and presented in the lymph nodes (LNs) by Antigen-presenting cells (APCs). In the LNs, induction of regulatory T cells (Treg cells) requires both direct contact during antigen presentation and indirect mechanisms such as cytokines. Foxp3+CD62L+ Treg cells can exhibit the characteristics of hypomethylation of Foxp3 TSDR and Foxp3-LAP+ Treg cells, which increase the expression of surface tissue-specific homing molecules to exert further sustained systemic immune tolerance. Studies have shown that EPIT is a potential treatment for food allergies and can effectively induce immune tolerance, but its mechanism needs further exploration. Here, we review Treg cells' role in immune tolerance induced by EPIT and provide a theoretical basis for future research directions, such as the mechanism of EPIT and the development of more effective EPIT treatments.

Tissue regulatory T cells

Foxp3+  CD4+ regulatory T cells (Tregs) are an immune cell lineage endowed with immunosuppressive functionality in a wide array of contexts, including both anti-pathogenic and anti-self responses. In the past decades, our understanding of the functional diversity of circulating or lymphoid Tregs has grown exponentially. Only recently, the importance of Tregs residing within non-lymphoid tissues, such as visceral adipose tissue, muscle, skin and intestine, has been recognized. Not only are Tregs critical for influencing the kinetics and strength of immune responses, but the regulation of non-immune or parenchymal cells, also fall within the purview of tissue-resident or infiltrating Tregs. This review focuses on providing a systematic and comprehensive comparison of the molecular maintenance, local adaptation and functional specializations of Treg populations operating within different tissues.

LPS-treated bone marrow-derived dendritic cells induce immune tolerance through modulating differentiation of CD4+ regulatory T cell subpopulations mediated by 3G11 and CD127

Intravenous transfer of LPS-treated bone marrow-derived dendritic cells blocks development of autoimmunity induced by CD4⁺ T cells in vivo. However, cellular mechanisms of dendritic cell-mediated immune tolerance have not yet been fully elucidated. Here, we report that there are two new subpopulations of CD4⁺CD25⁺FoxP3⁺GITR⁺ regulatory T cells (CD127⁺3G11⁺ and CD127⁺3G11^- cells). LPS-treated dendritic cells facilitate development of CD4⁺CD127⁺3G11^- regulatory T cells but inhibit that of CD4⁺CD127⁺3G11⁺ regulatory T cells. LPS-induced tolerogenic dendritic cells may cause immune tolerance through modulating balance of different subsets of CD4⁺ regulatory T cells mediated by CD127 and 3G11. Our results imply a new potential cellular mechanism of dendritic cell-mediated immune tolerance.

Therapeutic Potential of Gene-Modified Regulatory T Cells: From Bench to Bedside

Regulatory T cells (Tregs) are an important subset of adaptive immune cells and control immune reactions for maintaining homeostasis. Tregs are generated upon their encounter with self or non-self-antigen and mediate tolerance or suppress aberrant immune responses. A high level of specificity of Tregs to recognize antigen(s) suggested their instrumental potential to treat various inflammatory diseases. This review will first introduce seminal basic research findings in the field of Tregs over the last two decades pertinent to therapeutic approaches in progress. We will then discuss the previous approaches to use Tregs for therapeutic purposes and the more recent development of gene-modification approaches. The suppressive function of Tregs has been studied intensively in clinical settings, including cancer, autoimmunity, and allotransplantation. In cancer, Tregs are often aberrantly increased in their number, and their suppressor function inhibits mounting of effective antitumor immune responses. We will examine potential approaches of using gene-modified Tregs to treat cancer. In autoimmunity and allotransplantation, chronic inflammation due to inherent genetic defects in the immune system or mismatch between organ donor and recipient results in dysfunction of Tregs, leading to inflammatory diseases or rejection, respectively. Since the recognition of antigen is a central part in Treg function and their therapeutic use, the modulation of T cell receptor specificity will be discussed. Finally, we will focus on future novel strategies employing the therapeutic potential of Tregs using gene modification to broaden our perspective.

Restimulation-induced cell death: new medical and research perspectives

In the periphery, homeostasis of the immune system depends on the equilibrium of expanding and contracting T lymphocytes during immune response. An important mechanism of lymphocyte contraction is clonal depletion of activated T cells by cytokine withdrawal induced death (CWID) and TCR restimulation induced cell death (RICD). Deficiencies in signaling components for CWID and RICD leads to autoimmunune lymphoproliferative disorders in mouse and human. The most important feature of CWID and RICD is clonal specificity, which lends great appeal as a strategy for targeted tolerance induction and treatment of autoimmune diseases, allergic disorders, and graft rejection by depleting undesired disease-causing T cells while keeping the overall host immunity intact.

CD28- and CD28lowCD8+ Regulatory T Cells: Of Mice and Men

Since the rebirth of regulatory (formerly known as suppressor) T cells in the early 1990s, research in the field of immune-regulation by various T cell populations has quickly gained momentum. While T cells expressing the transcription factor Foxp3 are currently in the spotlight, several other T cell populations endowed with potent immunomodulatory capacities have been identified in both the CD8⁺ and CD4⁺ compartment. The fundamental difference between CD4⁺ and CD8⁺ T cells in terms of antigen recognition suggests non-redundant, and perhaps complementary, functions of regulatory CD4⁺ and CD8⁺ T cells in immunoregulation. This emphasizes the importance and necessity of continuous research on both subpopulations of regulatory T cells (Tregs) so as to decipher their complex physiological relevance and possible synergy. Two distinct CD8-expressing Treg populations can be distinguished based on expression of the co-stimulatory receptor CD28. Here, we review the literature on these (at least in part) thymus-derived CD28^low and peripherally induced CD28^-CD8⁺ Tregs.

Lymph node - an organ for T-cell activation and pathogen defense

The immune system is a multicentered organ that is characterized by intimate interactions between its cellular components to efficiently ward off invading pathogens. A key constituent of this organ system is the distinct migratory activity of its cellular elements. The lymph node represents a pivotal meeting point of immune cells where adaptive immunity is induced and regulated. Additionally, besides barrier tissues, the lymph node is a critical organ where invading pathogens need to be eliminated in order to prevent systemic distribution of virulent microbes. Here, we explain how the lymph node is structurally and functionally organized to fulfill these two critical functions - pathogen defense and orchestration of adaptive immunity. We will discuss spatio-temporal aspects of cellular immune responses focusing on CD8 T cells and review how and where these cells are activated in the context of viral infections, as well as how viral antigen expression kinetics and different antigen presentation pathways are involved. Finally, we will describe how such responses are regulated and 'helped', and discuss how this relates to intranodal positioning and cellular migration of the various cellular components that are involved in these processes.

Ontogeny of Tumor-associated CD4+CD25+Foxp3+ T-regulatory Cells

The critical contribution of CD4+CD25+Foxp3+ T-regulatory cells (Treg) to immune suppression in the tumor microenvironment is well-established. Whereas the mechanisms that drive the generation and accumulation of Treg in tumors have been an active area of study, the information on their origin and population dynamics remains limited. In this review, we discuss the ontogeny of tumor-associated Treg in light of the recently identified lineage markers.

Manipulation of regulatory T cells and antigen-specific cytotoxic T lymphocyte-based tumour immunotherapy

The most potent killing machinery in our immune system is the cytotoxic T lymphocyte (CTL). Since the possibility for self-destruction by these cells is high, many regulatory activities exist to prevent autoimmune destruction by these cells. A tumour (cancer) grows from the cells of the body and is tolerated by the body's immune system. Yet, it has been possible to generate tumour-associated antigen (TAA) -specific CTL that are also self-antigen specific in vivo, to achieve a degree of therapeutic efficacy. Tumour-associated antigen-specific T-cell tolerance through pathways of self-tolerance generation represents a significant challenge to successful immunotherapy. CD4(+)  CD25(+)  FoxP3(+) T cells, referred to as T regulatory (Treg) cells, are selected in the thymus as controllers of the anti-self repertoire. These cells are referred to as natural T regulatory (nTreg) cells. According to the new consensus (Nature Immunology 2013; 14:307-308) these cells are to be termed as (tTreg). There is another class of CD4(+) Treg cells also involved in regulatory function in the periphery, also phenotypically CD4(+)  CD25(±) , classified as induced Treg (iTreg) cells. These cells are to be termed as peripherally induced Treg (pTreg) cells. In vitro-induced Treg cells with suppressor function should be termed as iTreg. These different Treg cells differ in their requirements for activation and in their mode of action. The current challenges are to determine the degree of specificity of these Treg cells in recognizing the same TAA as the CTL population and to circumvent their regulatory constraints so as to achieve robust CTL responses against cancer.

Double negative (DN) αβ T cells: misperception and overdue recognition

CD4(-)CD8(-)double negative (DN) αβ T cells are legitimate components of the normal immune system. However, they are poorly understood and largely ignored by immunologists because of their historical association with the lymphoproliferation that occurs in mice (lpr and gld) and humans (autoimmune lymphoproliferative syndromes patients) with impaired Fas-mediated apoptosis where they are considered abnormal T cells. We believe that the traditional view that DN T cells that cause lymphoproliferation (hereafter referred to as lpr DN T cells) are CD4 and CD8 T cells that lost their coreceptor, conceived more than two decades ago, is flawed and that conflating lpr DN T cells with DN T cells found in normal immune system (hereafter referred to as nDN T cells) is unnecessarily dampening interest of this potentially important cell type. To begin rectifying these misperceptions, we will revisit the traditional view of lpr DN T cells and show that it does not hold true in light of recent immunological advances. In lieu of it, we offer a new model proposing that Fas-mediated apoptosis actively removes normally existing DN T cells from the periphery and that impaired Fas-mediated apoptosis leads to accumulation of these cells rather than de novo generation of DN T cells from activated CD4 or CD8 T cells. By doing so, we hope to provoke a new discussion that may lead to a consensus about the origin of lpr DN T cells and regulation of their homeostasis by the Fas pathway and reignite wider interest in nDN T cells.

Transcriptional and epigenetic networks of helper T and innate lymphoid cells

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

Single-cell RNA sequencing reveals T helper cells synthesizing steroids de novo to contribute to immune homeostasis

T helper 2 (Th2) cells regulate helminth infections, allergic disorders, tumor immunity, and pregnancy by secreting various cytokines. It is likely that there are undiscovered Th2 signaling molecules. Although steroids are known to be immunoregulators, de novo steroid production from immune cells has not been previously characterized. Here, we demonstrate production of the steroid pregnenolone by Th2 cells in vitro and in vivo in a helminth infection model. Single-cell RNA sequencing and quantitative PCR analysis suggest that pregnenolone synthesis in Th2 cells is related to immunosuppression. In support of this, we show that pregnenolone inhibits Th cell proliferation and B cell immunoglobulin class switching. We also show that steroidogenic Th2 cells inhibit Th cell proliferation in a Cyp11a1 enzyme-dependent manner. We propose pregnenolone as a “lymphosteroid,” a steroid produced by lymphocytes. We speculate that this de novo steroid production may be an intrinsic phenomenon of Th2-mediated immune responses to actively restore immune homeostasis.

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Differential upregulation of the steroid biosynthetic pathway during Th2 differentiation

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T helper cells produce the steroid pregnenolone in vitro and in vivo

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Steroidogenic Th2 cells suppress Th cell proliferation in a Cyp11a1-dependent manner

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Pregnenolone inhibits B cell immunoglobulin class switching in vitro

Role of regulatory T cells during virus infection

The host response to viruses includes multiple cell types that have regulatory function. Most information focuses on CD4(+) regulatory T cells that express the transcription factor Foxp3(+) (Tregs), which are the topic of this review. We explain how viruses through specific and non-specific means can trigger the response of thymus-derived natural Tregs as well as induce Tregs. The latter derive under appropriate stimulation conditions either from uncommitted precursors or from differentiated cells that convert to become Tregs. We describe instances where Tregs appear to limit the efficacy of antiviral protective immunity and other, perhaps more common, immune-mediated inflammatory conditions, where the Tregs function to limit the extent of tissue damage that occurs during a virus infection. We discuss the controversial roles that Tregs may play in the pathogenesis of human immunodeficiency and hepatitis C virus infections. The issue of plasticity is discussed, as this may result in Tregs losing their protective function when present in inflammatory environments. Finally, we mention approaches used to manipulate Treg numbers and function and assess their current value and likely future success to manage the outcome of virus infection, especially those that are responsible for chronic tissue damage.

Exquisite selectivity for human toll-like receptor 8 in substituted furo[2,3-c]quinolines

Toll-like receptor (TLR)-8 agonists activate adaptive immune responses by inducing robust production of T helper 1-polarizing cytokines, suggesting that TLR8-active compounds may be promising candidate adjuvants. We synthesized and evaluated hitherto unexplored furo[2,3-c]quinolines and regioisomeric furo[3,2-c]quinolines derived via a tandem, one-pot Sonogashira coupling and intramolecular 5-endo-dig cyclization strategy in a panel of primary screens. We observed a pure TLR8-agonistic activity profile in select furo[2,3-c]quinolines, with maximal potency conferred by a C2-butyl group (EC50 = 1.6 μM); shorter, longer, or substituted homologues as well as compounds bearing C1 substitutions were inactive, which was rationalized by docking studies using the recently described crystal structure of human TLR8. The best-in-class compound displayed prominent proinflammatory cytokine induction (including interleukin-12 and interleukin-18), but was bereft of interferon-α inducing properties, confirming its high selectivity for human TLR8.

PLAU inferred from a correlation network is critical for suppressor function of regulatory T cells

Human FOXP3(+)CD25(+)CD4(+) regulatory T cells (Tregs) are essential to the maintenance of immune homeostasis. Several genes are known to be important for murine Tregs, but for human Tregs the genes and underlying molecular networks controlling the suppressor function still largely remain unclear. Here, we describe a strategy to identify the key genes directly from an undirected correlation network which we reconstruct from a very high time-resolution (HTR) transcriptome during the activation of human Tregs/CD4(+) T-effector cells. We show that a predicted top-ranked new key gene PLAU (the plasminogen activator urokinase) is important for the suppressor function of both human and murine Tregs. Further analysis unveils that PLAU is particularly important for memory Tregs and that PLAU mediates Treg suppressor function via STAT5 and ERK signaling pathways. Our study demonstrates the potential for identifying novel key genes for complex dynamic biological processes using a network strategy based on HTR data, and reveals a critical role for PLAU in Treg suppressor function.

Treg cells, life history, and diversity

Regulatory T cells expressing the FoxP3 transcription factor have a profound and nonredundant role in several aspects of immunological tolerance. We will review here the specification of this lineage, its population dynamics, and the diversity of subphenotypes that correlate with their diverse roles in controlling inflammation in a variety of settings.

Regulatory T-cells: diverse phenotypes integral to immune homeostasis and suppression

Regulatory T-cells (T(REG)) are diverse populations of lymphocytes that regulate the adaptive immune response in higher vertebrates. T(REG) delete autoreactive T-cells, induce tolerance, and dampen inflammation. T(REG) cell deficiency in humans (i.e., IPEX [Immunodysregulation, Polyendocrinopathy and Enteropathy, X-linked syndrome]) and animal models (e.g., "Scurfy" mouse) is associated with multisystemic autoimmune disease. T(REG) in humans and laboratory animal species are similar in type and regulatory function. A molecular marker of and the cell lineage specification factor for T(REG) is FOXP3, a forkhead box transcription factor. CD4(+) T(REG) are either natural (nT(REG)), which are thymus-derived CD4(+)CD25(+)FOXP3(+) T-cells, or inducible (i.e., Tr1 cells that secrete IL-10, Th3 cells that secrete TGF-β and IL-10, and Foxp3(+) Treg). The proinflammatory Th17 subset has been a major focus of research. T(H)17 CD4(+) effector T-cells secrete IL-17, IL-21, and IL-22 in autoimmune and inflammatory disease, and are dynamically balanced with T(REG) cell development. Other lymphocyte subsets with regulatory function include: inducible CD8(+) T(REG), CD3(+)CD4(-)CD8(-) T(REG) (double-negative), CD4(+)Vα14(+) (NKT(REG)), and γδ T-cells. T(REG) have four regulatory modes of action: secretion of inhibitory cytokines (e.g., IL-10 and TGF-β), granzyme-perforin-induced apoptosis of effector lymphocytes, depriving effector T-cells of cytokines leading to apoptosis, or inhibition of dendritic cell function. The role of T(REG) in mucosal sites, inflammation/infection, pregnancy, and cancer as well as a review of T(REG) as a modulatory target in drug development will be covered.

Immunological tolerance during fetal development: from mouse to man

The development of the adaptive immune system has been studied in the mouse primarily because it is easier to access fetal tissues and because there exists a rich array of probes for analysis of various components of the immune system. While much has been learned from this exercise, it is also clear that different species show substantial temporal variation in the development of the immune system during early life. In mice, for instance, mature α/β T cells first appear in the periphery during the final stages of fetal gestation and only increase in number after birth (Friedberg and Weissman, 1974); in humans, on the other hand, the first mature α/β T cells are seen in peripheral tissues at 10-12 gestational weeks (g.w.) and are circulating in significant numbers by the end of the second trimester (Ceppellini et al., 1971; Haynes et al., 1988; Hayward and Ezer, 1974; Kay et al., 1970). Although the functional implications of these differences remain unclear, it is likely that there are significant biological consequences associated with the relatively early development of the peripheral adaptive immune system in humans, for example, with respect to the development of peripheral tolerance as well as to the response to antigens that might cross the placenta from the mother (e.g., cells bearing noninherited maternal alloantigens, infectious agents, food antigens, and the like). Here, we will review studies of immune system ontogeny in the mouse and in humans, and then focus on the possible functional roles of fetal T cell populations during development and later in life in humans.

Melatonin synthesized by T lymphocytes as a ligand of the retinoic acid-related orphan receptor

Melatonin modulates a wide array of physiological events with pleiotropic effects on the immune system. While the relevance of specific melatonin membrane receptors has been well established for several biological functions, retinoic acid-related orphan receptor alpha (RORα) has been suggested as a mediator of nuclear melatonin signalling by results obtained from pharmacological approaches. However, a melatonin-mediated downstream effect cannot be ruled out, and further evidence is needed to support a direct interaction between melatonin and RORα. Here, we show that RORα is mainly located in human Jurkat T-cell nucleus, and it is co-immunoprecipitated with melatonin. Moreover, immunocytochemistry studies confirmed the co-localization of melatonin and RORα. Melatonin promoted a time-dependent decrease in nuclear RORα levels, suggesting a role in the RORα transcriptional activity. Interestingly, RORα acts as a molecular switch implicated in the mutually exclusive generation of Th17 and Treg cells, both involved in the harm/protection balance of immune conditions such as autoimmunity or acute transplant rejection. Therefore, the identification of melatonin as a natural modulator of RORα gives it a tremendous therapeutic potential for a variety of clinical disorders.

Novel highly sensitive IL-10-beta-lactamase reporter mouse reveals cells of the innate immune system as a substantial source of IL-10 in vivo

In this study, we report on a novel, highly sensitive IL-10 reporter mouse based on the reporter enzyme β-lactamase and the fluorescence resonance energy transfer substrate coumarin-cephalosporin-fluorescein (4). In contrast to an IL-10 reporter mouse model that we generated by using enhanced GFP as reporter and allowed tracking IL-10 expression only in T cells, the IL-10-β-lactamase reporter (ITIB) mouse enables us to easily analyze and quantify IL-10 production at the single-cell level in all myeloid and lymphoid cell types. Furthermore, the ITIB mouse allows studying of the kinetics of IL-10 expression on a single-cell basis and provides a valuable tool for in vivo screening of cell type-specific IL-10-modulating drugs. Remarkably, the ITIB mouse revealed that, although a significant portion of each myeloid and lymphoid cell type produces IL-10, macrophages represent the major IL-10 producer population in several organs of naive mice. Moreover, using the examples of bacterial infection and transplantable skin melanoma models, we demonstrate the exceptional applicability of the ITIB mouse for the identification of IL-10-producing cells during immune responses in vivo. In this study, we identified tumor-infiltrating F4/80(+) macrophages as the major source for IL-10 in B16-F10 melanoma in vivo. During systemic infection with Yersinia enterocolitica, although the proportion of IL-10(+) cells increased in each myeloid and lymphoid cell type population, infiltrating CD11b(+)Ly6G(+) neutrophils represent a majority among IL-10-producing cells at the site of infection. We conclude that cells of the innate immune system that are involved in immune homeostasis or immune responses are substantial sources of IL-10.

Induction of self-antigen-specific Foxp3+ regulatory T cells in the periphery by lymphodepletion treatment with anti-mouse thymocyte globulin in mice

Lymphodepletion therapies are increasingly tested for controlling immune damage. One appealing premise for such a therapy is that it may 'reboot' the immune system and restore immune tolerance. However, the tolerogenic potential of lymphodepletion therapies remains controversial. The debate is exemplified by conflicting evidence from the studies of anti-thymocyte globulin (ATG), a prototype of immunodepleting drugs, in particular on whether it induces CD4(+) CD25(+) Foxp3(+) regulatory T (Treg) cells. To understand the impact of ATG on T cells at a clonal level in vivo, we studied the effect of anti-mouse thymocyte globulin (mATG) in a reductionist model in which the T-lymphocyte repertoire consists of a single clone of pathogenic T effector (Teff) cells specific to a physiological self-antigen. The mATG treatment led to peripheral induction of antigen-specific Treg cells from an otherwise monoclonal Teff repertoire, independent of thymic involvement. The de novo induction of Treg cells occurred consistently in local draining lymph nodes, and persistence of induced Treg cells in blood correlated with long-term protection from autoimmune destruction. This study provides in vivo evidence for clonal conversion from a pathogenic self-antigen-specific Teff cell to a Treg cell in the setting of immunodepletion therapies.

Imunopatologia da dermatite de contato alérgica

A dermatite de contato alérgica é consequência de uma reação imune mediada por células T contra químicos de baixo peso molecular, denominados haptenos. É uma condição frequente que ocorre em todas as raças e faixas etárias e afeta a qualidade de vida de seus portadores. O mecanismo imunológico desta doença vem sendo revisto nas últimas décadas com significativo avanço no seu entendimento. A metabolização e o caminho dos haptenos, bem como a formação e o mecanismo de ação das células responsáveis tanto pela reação quanto pelo seu término, são discutidos neste artigo

Regulatory T-cell differentiation versus clonal deletion of autoreactive thymocytes

The concept of clonal deletion of immune cells that carry an autoreactive antigen receptor was a central prediction of Burnet's clonal selection theory. A series of classical experiments in the late 1980s revealed that certain immature thymocytes upon encounter of 'self' are indeed removed from the T-cell repertoire before their release into the blood circulation. A second essential cornerstone of immunological tolerance, not anticipated by Burnett, has more recently surfaced through the discovery of Foxp3(+) regulatory T cells (Treg). Intriguingly, it appears that the expression of an autoreactive T-cell receptor is a shared characteristic of T cells that are subject to clonal deletion as well as of those deviated into the Treg lineage. This is all the more striking as Treg differentiation for the most part branches off from mainstream CD4T cell development during thymocyte maturation in the thymus, that is, it may neither temporally nor spatially be separated from clonal deletion. This raises the question of how an apparently identical stimulus, namely the encounter of 'self' during thymocyte development, can elicit fundamentally different outcomes such as apoptotic cell death on the one hand or differentiation into a highly specialized T-cell lineage on the other hand. Here, we will review the T-cell intrinsic and extrinsic factors that have been implicated in intrathymic Treg differentiation and discuss how these parameters may determine whether an autoreactive major histocompatibility complex class II-restricted thymocyte is deviated into the Treg lineage or subject to clonal deletion.

Immunity, thyroid function and pregnancy: molecular mechanisms

Pregnancy and the postpartum period have a profound effect on autoimmune thyroid disease. Graves disease ameliorates during pregnancy, only to relapse postpartum, whereas postpartum thyroiditis is caused by destructive thyroiditis during the first few months after delivery. The immunology of pregnancy underlies these changes: the mother must maintain tolerance of the fetal semi-allograft while not suppressing her own immune system and exposing herself and the fetus to infection. Nonspecific factors, including hormonal changes, trophoblast expression of key immunomodulatory molecules and a switch to a predominantly T-helper-2-type pattern of cytokines, play some part in the maintenance of transient tolerance to paternal antigens in pregnancy; however, the generation of specific regulatory T (T(REG)) cells is key to this maintenance. T(REG) cells preferentially accumulate in the decidua but may also be present in the mother's circulation and are thus capable of regulating coincidental autoimmune responses through the phenomenon of linked suppression. In turn, this suppression may explain why thyroid autoantibody levels decline during pregnancy, which leads to remission of Graves disease. Postpartum exacerbation of autoimmunity may reflect an imbalance in T(REG) cells, which is caused by the rapid fall in the numbers of these cells after delivery.

CD11c+CD8+ T cells: two-faced adaptive immune regulators

Regulatory cells, important controllers of immune homeostasis, carry out a multi-pronged attack by deleting overactive pathogenic immune cells, by supporting anergy, and by blocking effector functions, thereby contributing to the amelioration of disease. CD8+ T cells co-expressing CD11c are a new addition to the growing list of regulatory cells. Naïve mice harbor CD11c-expressing CD8+ T cells (<3%) that expand further in an antigen-dependent manner. Although activated CD11c+CD8+ T cells express suppressive cytokines such as IL-10 and TGF-beta, their production of IFN-gamma is central to their immune suppressive potential. The CD11c+CD8+ T cells target pathogenic CD4+ T cells in a cell-cell contact dependent manner via IDO- and GCN2-dependent mechanisms. Adoptive transfer of activated CD11c+CD8+ T cells halts the progression of autoimmune rheumatoid arthritis and colitis. However, in certain virus and cancer models the CD11c+CD8+ T cells assume the role of immune effectors, boosting immune potential. This seemingly dual nature of these cells--exerting regulatory vs. effector activities--makes them an attractive therapeutic target. In this review, we discuss the discovery, origins and developmental requirements of CD11c+CD8+cells, and the basis of their immuno-suppressive and effector potentials.

A hassle a day may keep the pathogens away: The fight-or-flight stress response and the augmentation of immune function

Stress is known to suppress or dysregulate immune function and increase susceptibility to disease. Paradoxically, the short-term fight-or-flight stress response is one of nature's fundamental defense mechanisms that galvanizes the neuroendocrine, cardiovascular, and musculoskeletal systems into action to enable survival. Therefore, it is unlikely that short-term stress would suppress immune function at a time when it may be critically required for survival (e.g., in response to wounding and infection by a predator or aggressor). In fact, studies have shown that stress can enhance immune function under certain conditions. Several factors influence the direction (enhancing versus suppressive) of the effects of stress on immune function: (1) DURATION: acute or short-term stress experienced at the time of activation of an immune response enhances innate and adaptive immune responses. Chronic or long-term stress can suppress or dysregulate immune function. (2) Leukocyte distribution: compartments (e.g., skin), that are enriched with immune cells during acute stress show immuno-enhancement, while those that are depleted of leukocytes (e.g., blood), show immuno-suppression. (3) The differential effects of physiologic versus pharmacologic stress hormones: Endogenous hormones in physiological concentrations can have immuno-enhancing effects. Endogenous hormones at pharmacologic concentrations, and synthetic hormones, are immuno-suppressive. (4) Timing: immuno-enhancement is observed when acute stress is experienced during the early stages of an immune response while immuno-suppression may be observed at late stages. The type of immune response (protective, regulatory/inhibitory, or pathological) that is affected determines whether the effects of stress are ultimately beneficial or harmful for the organism. Arguments based on conservation of energy have been invoked to explain potential adaptive benefits of stress-induced immuno-suppression, but generally do not hold true because most mechanisms for immuno-suppression expend, rather than conserve, energy. We propose that it is important to study, and if possible, to clinically harness, the immuno-enhancing effects of the acute stress response that evolution has finely sculpted as a survival mechanism, just as we study its maladaptive ramifications (chronic stress) that evolution has yet to resolve.

T regulatory cells in contact hypersensitivity

PURPOSE OF REVIEW

The review summarizes the recent investigations focused on T regulatory cells in hapten diseases.

RECENT FINDINGS

Multiple mechanisms ensure tolerance to small chemicals penetrating the skin. Among these, specific T regulatory cells play a major role in controlling harmful immune responses to environmental antigens. Most of the T regulatory cells involved in this process belongs to the CD4 subset and suppress hapten-specific immune response through the release of IL-10 and through direct interaction with effector T cells, blocking their function.

SUMMARY

Methods for in-vitro and in-vivo expansion of specific T regulatory cells may represent an innovative approach for the cure of contact hypersensitivity.

Human CD4low CD25high regulatory T cells indiscriminately kill autologous activated T cells

The interest of the scientific community in regulatory CD4(+) T cells has reached an enormously high level. Common agreement is that they inhibit not only the proliferation of CD4 and CD8 lymphocytes, but also the activities of natural killer cells and macrophages. However, very important issues concerning actual mechanism(s) and specificity of the action of regulatory T cells (Tregs) upon responder cells are still unsolved or vague. The best known marker for Tregs is the expression of transcription factor FoxP3, widely used for their enumeration. It is known that FoxP3 inhibits cytokine production so the most probable action of Tregs is direct. However, FoxP3 expression cannot be used for functional studies in humans. Therefore we identified human peripheral blood Tregs as a distinct, very well-defined population of peripheral blood T cells with reduced CD4 and high CD25 expression (CD4(low) CD25(high)), which fulfils the current phenotypic criteria identifying the Tregs by simultaneously expressing high amounts of FoxP3. We conclude that the definition of a CD4(low) CD25(high) phenotype is enough to unambiguously detect and study the regulatory function of these cells. On the functional level, the CD4(low) Tregs are able to non-specifically suppress the proliferation of autologous, previously polyclonally activated CD4(+) and CD4(-) lymphocytes and to kill them by direct contact, probably utilizing intracellular granzyme B and perforin.

Murine CD8+ regulatory T lymphocytes: the new era

Regulatory T lymphocytes unequivocally play a major role in the maintenance of immunologic homeostasis. The first descriptions of regulatory T lymphocytes concerned CD8(+) cells, but this field was brought into discredit when some of its central tenets turned out to be erroneous. CD4(+) regulatory T cells took over and, with the help of newly developed molecular tools, rapidly were phenotypically and functionally characterized. We now know that these cells control a large variety of immune responses. However some observations of in vitro or in vivo immune regulation could not be explained with CD4(+) regulatory T cell activity and depended on the action of a variety of CD8(+) T cell populations. In recent years, substantial progress has been made in the phenotypic and functional characterization of CD8(+) regulatory T cells. These cells play a role in the control of intestinal immunity, immunopathology, and autoimmunity, as well as in immune privilege of the eye, in oral tolerance, and in prevention of graft-versus-host disease and graft-rejection. The suppressor effector mechanisms used by these cells are in part shared with CD4(+) regulatory T cells and in part unique to this population. We here review the current literature on naturally occurring and experimentally induced murine CD8(+) regulatory T-cell populations.

Early growth response gene 2 (Egr-2) controls the self-tolerance of T cells and prevents the development of lupuslike autoimmune disease

Maintaining tolerance of T cells to self-antigens is essential to avoid autoimmune disease. How self-reactive T cells are kept functionally inactive is, however, unknown. In this study, we show that early growth response gene 2 (Egr-2), a zinc-finger transcription factor, is expressed in CD44^high T cells and controls their proliferation and activation. In the absence of Egr-2, CD44^high, but not CD44^low T cells, are hyperreactive and hyperproliferative in vivo. The accumulation of activated CD4⁺CD44^high T cells leads to the development of a late onset lupuslike autoimmune disease characterized by the accumulation of interferon (IFN)-γ and interleukin (IL)-17–producing CD4⁺ T cells, loss of tolerance to nuclear antigens, massive infiltration of T cells into multiple organs and glomerulonephritis. We found that the expression of cyclin-dependent kinase inhibitor p21cip1 was impaired in Egr-2–deficient T cells, whereas the expression of IFN-γ and IL-17 in response to T cell receptor ligation was significantly increased, suggesting that Egr-2 activates the expression of genes involved in the negative regulation of T cell proliferation and inflammation. These results demonstrate that Egr-2 is an intrinsic regulator of effector T cells and controls the expansion of self-reactive T cells and development of autoimmune disease.

Mechanisms of regulatory T-cell suppression - a diverse arsenal for a moving target

Naturally-occurring regulatory T cells (Tregs) are emerging as key regulators of immune responses to self-tissues and infectious agents. Insight has been gained into the cell types and the cellular events that are regulated by Tregs. Indeed, Tregs have been implicated in the control of initial activation events, proliferation, differentiation and effector function. However, the mechanisms by which Tregs disable their cellular targets are not well understood. Here we review recent advances in the identification of distinct mechanisms of Treg action and of signals that enable cellular targets to escape regulation. Roles for inhibitory cytokines, cytotoxic molecules, modulators of cAMP and cytokine competition have all been demonstrated. The growing number of inhibitory mechanisms ascribed to Tregs suggests that Tregs take a multi-pronged approach to immune regulation. It is likely that the relative importance of each inhibitory mechanism is context dependent and modulated by the inflammatory milieu and the magnitude of the immune response. In addition, the target cell may be differentially susceptible or resistant to distinct Treg mechanisms depending on their activation or functional status at the time of the Treg encounter. Understanding when and where each suppressive tool is most effective will help to fine tune therapeutic strategies to promote or constrain specific arms of Treg suppression.