Selection of self-reactive T cells in the thymus

SOCS1 and regulation of regulatory T cells plasticity

Several reports have suggested that natural regulatory T cells (Tregs) lose Forkhead box P3 (Foxp3) expression and suppression activity under certain inflammatory conditions. Treg plasticity has been studied because it may be associated with the pathogenesis of autoimmunity. Some studies showed that a minor uncommitted Foxp3⁺ T cell population, which lacks hypomethylation at Treg-specific demethylation regions (TSDRs), may convert to effector/helper T cells. Suppressor of cytokine signaling 1 (SOCS1), a negative regulator of cytokine signaling, has been reported to play an important role in Treg cell integrity and function by protecting the cells from excessive inflammatory cytokines. In this review, we discuss Treg plasticity and maintenance of suppression functions in both physiological and pathological settings. In addition, we discuss molecular mechanisms of maintaining Treg plasticity by SOCS1 and other molecules. Such information will be useful for therapy of autoimmune diseases and reinforcement of antitumor immunity.

Fine-tuning T cell receptor signaling to control T cell development

T cell development from immature CD4(+)CD8(+) double-positive (DP) thymocytes to the mature CD4 or CD8 single-positive (SP) stage requires proper T cell receptor (TCR) signaling. The current working model of thymocyte development is that the strength of the TCR-mediated signal - from little-or-none, through intermediate, to strong - received by the immature cells determines whether they will undergo death by neglect, positive selection, or negative selection, respectively. In recent years, several developmentally regulated, stage-specifically expressed proteins and miRNAs have been found that act like fine-tuners for signal transduction and propagation downstream of the TCR. This allows them to govern thymocyte positive selection. Here, we summarize recent findings on these molecules and suggest new concepts of TCR positive-selection signaling.

Costimulation via the tumor-necrosis factor receptor superfamily couples TCR signal strength to the thymic differentiation of regulatory T cells

Regulatory T cells (Treg cells) express members of the tumor-necrosis factor (TNF) receptor superfamily (TNFRSF), but the role of those receptors in the thymic development of Treg cells is undefined. We found here that Treg cell progenitors had high expression of the TNFRSF members GITR, OX40 and TNFR2. Expression of those receptors correlated directly with the signal strength of the T cell antigen receptor (TCR) and required the coreceptor CD28 and the kinase TAK1. The neutralization of ligands that are members of the TNF superfamily (TNFSF) diminished the development of Treg cells. Conversely, TNFRSF agonists enhanced the differentiation of Treg cell progenitors by augmenting responsiveness of the interleukin 2 receptor (IL-2R) and transcription factor STAT5. Costimulation with the ligand of GITR elicited dose-dependent enrichment for cells of lower TCR affinity in the Treg cell repertoire. In vivo, combined inhibition of GITR, OX40 and TNFR2 abrogated the development of Treg cells. Thus, expression of members of the TNFRSF on Treg cell progenitors translated strong TCR signals into molecular parameters that specifically promoted the development of Treg cells and shaped the Treg cell repertoire.

Self-reactive T cells: suppressing the suppressors

The immune system is a tightly regulated and complex system. An important part of this immune regulation is the assurance of tolerance toward self-antigens to maintain immune homeostasis. However, in recent years, antigen-specific cellular immune responses toward several normal self-proteins expressed in regulatory immune cells have been reported, especially in patients with cancer. The seemingly lack of tolerance toward such proteins is interesting, as it suggests a regulatory function of self-reactive T (srT) cells, which may be important for the fine tuning of the immune system. In particular, surprising has been the description of cytotoxic srT cells that are able to eliminate normal regulatory immune cells. Such srT cells may be important as effector cells that suppress regulatory suppressor cells. The current knowledge of the nature and function of srT cells is still limited. Still, the therapeutic targeting of srT cells offers a novel approach to harness immune-regulatory networks in cancer.

Intrinsic CD4+ T cell sensitivity and response to a pathogen are set and sustained by avidity for thymic and peripheral complexes of self peptide and MHC

Interactions of T cell antigen receptors (TCRs) with complexes of self peptide and major histocompatibility complex (MHC) are crucial to T cell development, but their role in peripheral T cell responses remains unclear. Specific and nonspecific stimulation of LLO56 and LLO118 T cells, which transgenically express a TCR specific for the same Listeria monocytogenes epitope, elicited distinct interleukin 2 (IL-2) and phosphorylated kinase Erk responses, the strength of which was set in the thymus and maintained in the periphery in proportion to the avidity of the binding of the TCR to the self peptide-MHC complex. Deprivation of self peptide-MHC substantially compromised the population expansion of LLO56 T cells in response to L. monocytogenes in vivo. Despite their very different self-reactivity, LLO56 T cells and LLO118 T cells bound cognate peptide-MHC with an identical affinity, which challenges associations made between these parameters. Our findings highlight a crucial role for selecting ligands encountered during thymic 'education' in determining the intrinsic functionality of CD4+ T cells.

From pre-DP, post-DP, SP4, and SP8 Thymocyte Cell Counts to a Dynamical Model of Cortical and Medullary Selection

Cells of the mature αβ T cell repertoire arise from the development in the thymus of bone marrow precursors (thymocytes). αβ T cell maturation is characterized by the expression of thousands of copies of identical αβ T cell receptors and the CD4 and/or CD8 co-receptors on the surface of thymocytes. The maturation stages of a thymocyte are: (1) double negative (DN) (TCR⁻, CD4⁻ and CD8⁻), (2) double positive (DP) (TCR⁺, CD4⁺ and CD8⁺), and (3) single positive (SP) (TCR⁺, CD4⁺ or CD8⁺). Thymic antigen presenting cells provide the appropriate micro-architecture for the maturation of thymocytes, which “sense” the signaling environment via their randomly generated TCRs. Thymic development is characterized by (i) an extremely low success rate, and (ii) the selection of a functional and self-tolerant T cell repertoire. In this paper, we combine recent experimental data and mathematical modeling to study the selection events that take place in the thymus after the DN stage. The stable steady state of the model for the pre-DP, post-DP, and SP populations is identified with the experimentally measured cell counts from 5.5- to 17-week-old mice. We make use of residence times in the cortex and the medulla for the different populations, as well as recently reported asymmetric death rates for CD4 and CD8 SP thymocytes. We estimate that 65.8% of pre-DP thymocytes undergo death by neglect. In the post-DP compartment, 91.7% undergo death by negative selection, 4.7% become CD4 SP, and 3.6% become CD8 SP. Death by negative selection in the medulla removes 8.6% of CD4 SP and 32.1% of CD8 SP thymocytes. Approximately 46.3% of CD4 SP and 27% of CD8 SP thymocytes divide before dying or exiting the thymus.

Autoimmunity-associated LYP-W620 does not impair thymic negative selection of autoreactive T cells

A C1858T (R620W) variation in the PTPN22 gene encoding the tyrosine phosphatase LYP is a major risk factor for human autoimmunity. LYP is a known negative regulator of signaling through the T cell receptor (TCR), and murine Ptpn22 plays a role in thymic selection. However, the mechanism of action of the R620W variant in autoimmunity remains unclear. One model holds that LYP-W620 is a gain-of-function phosphatase that causes alterations in thymic negative selection and/or thymic output of regulatory T cells (Treg) through inhibition of thymic TCR signaling. To test this model, we generated mice in which the human LYP-W620 variant or its phosphatase-inactive mutant are expressed in developing thymocytes under control of the proximal Lck promoter. We found that LYP-W620 expression results in diminished thymocyte TCR signaling, thus modeling a "gain-of-function" of LYP at the signaling level. However, LYP-W620 transgenic mice display no alterations of thymic negative selection and no anomalies in thymic output of CD4(+)Foxp3(+) Treg were detected in these mice. Lck promoter-directed expression of the human transgene also causes no alteration in thymic repertoire or increase in disease severity in a model of rheumatoid arthritis, which depends on skewed thymic selection of CD4(+) T cells. Our data suggest that a gain-of-function of LYP is unlikely to increase risk of autoimmunity through alterations of thymic selection and that LYP likely acts in the periphery perhaps selectively in regulatory T cells or in another cell type to increase risk of autoimmunity.

Chimeric DNA Vaccines: An Effective Way to Overcome Immune Tolerance

The fact that cancer immunotherapy is considered to be a safe and successful weapon for use in combination with surgery, radiation, and chemotherapy treatments means that it has recently been chosen as Breakthrough of the Year 2013 by Science editors. Anticancer vaccines have been extensively tested, in this field, both in preclinical cancer models and in the clinic. However, tumor-associated antigens (TAAs) are often self-tolerated molecules and cancer patients suffer from strong immunosuppressive effects, meaning that the triggering of an effective anti-tumor immune response is difficult. One possible means to overcome immunological tolerance to self-TAAs is of course the use of vaccines that code for xenogeneic proteins. However, a low-affinity antibody response against the self-homologous protein expressed by cancer cells is generally induced by xenovaccination. This issue becomes extremely limiting when working with tumors in which the contribution of the humoral rather than the cellular immune response is required if tumor growth is to be hampered. A possible way to avoid this problem is to use hybrid vaccines which code for chimeric proteins that include both homologous and xenogeneic moieties. In fact, a superior protective anti-tumor immune response against ErbB2⁺ transplantable and autochthonous mammary tumors was observed over plasmids that coded for the fully rat or fully human proteins when hybrid plasmids that coded for chimeric rat/human ErbB2 protein were tested in ErbB2 transgenic mice. In principle, these findings may become the basis for a new rational means of designing effective vaccines against TAAs.

Regulation of regulatory T cells: epigenetics and plasticity

Regulatory T (Treg) cells, as central mediators of immune suppression, play crucial roles in many aspects of immune system's physiology and pathophysiology. The transcription factor Foxp3 has been characterized as a master gene of Tregs. Yet Treg cells possess a distinct pattern of gene expression, including upregulation of immune-suppressive genes and silencing of inflammatory cytokine genes. Recent studies have revealed the molecular mechanisms that establish and maintain such gene regulation in Treg cells. This review discusses recent progress in our understanding of molecular features of Treg cells, with particular attention to Treg-cell lineage commitment and stability.

Radiological thymus alterations in systemic sclerosis: our experience and a review of the literature

OBJECTIVE

Thymus alterations have been related to several autoimmune disorders. In particular, previous studies identified a significant frequency of gland abnormalities by chest high-resolution CT (HRCT) in SSc patients. In this study we aimed to investigate the prevalence of radiological thymic alterations and their correlation with clinical and serological features in a large SSc series.

METHODS

We retrospectively evaluated thymic shape on CT scans of 200 consecutive, unselected SSc patients aged over 30 years The presence of radiological abnormalities, i.e. enlarged gland >13 mm or nodular lesions >7 mm, was correlated with SSc clinico-serological features. Moreover, the patients were also classified using a second thickness cut-off of 7 mm in order to identify incomplete thymic involution.

RESULTS

Twenty-four of 200 (12%) SSc patients presented an abnormal thymus at HRCT, including hyperplasic (19/24) and nodular (5/24) glands. Otherwise, using the cut-off of 7 mm for gland thickness and excluding subjects with nodular thymus, 50/195 (25.6%) patients presented an incomplete thymic involution. Thymic radiological alterations are significantly correlated with younger age and diffuse cutaneous SSc. Moreover, an abnormally enlarged thymus tended to be more common in patients with shorter disease duration.

CONCLUSION

The present report on a large series of SSc patients further reinforces previous data present in the literature that includes other cohort studies and a number of anecdotal observations. Even though the actual role of thymus radiological abnormalities remains unclear, possible involvement of the gland in the early phase of immune-mediated SSc pathogenesis might be supposed.

Themis sets the signal threshold for positive and negative selection in T-cell development

Development of a self-tolerant T-cell receptor (TCR) repertoire with the potential to recognize the universe of infectious agents depends on proper regulation of TCR signalling. The repertoire is whittled down during T-cell development in the thymus by the ability of quasi-randomly generated TCRs to interact with self-peptides presented by major histocompatibility complex (MHC) proteins. Low-affinity TCR interactions with self-MHC proteins generate weak signals that initiate 'positive selection', causing maturation of CD4- or CD8αβ-expressing 'single-positive' thymocytes from CD4(+)CD8αβ(+) 'double-positive' precursors. These develop into mature naive T cells of the secondary lymphoid organs. TCR interaction with high-affinity agonist self-ligands results in 'negative selection' by activation-induced apoptosis or 'agonist selection' of functionally differentiated self-antigen-experienced T cells. Here we show that positive selection is enabled by the ability of the T-cell-specific protein Themis to specifically attenuate TCR signal strength via SHP1 recruitment and activation in response to low- but not high-affinity TCR engagement. Themis acts as an analog-to-digital converter translating graded TCR affinity into clear-cut selection outcome. By dampening mild TCR signals Themis increases the affinity threshold for activation, enabling positive selection of T cells with a naive phenotype in response to low-affinity self-antigens.

Stimulation of natural killer T cells by glycolipids

Natural killer T (NKT) cells are a subset of T cells that recognize glycolipid antigens presented by the CD1d protein. The initial discovery of immunostimulatory glycolipids from a marine sponge and the T cells that respond to the compounds has led to extensive research by chemists and immunologists to understand how glycolipids are recognized, possible responses by NKT cells, and the structural features of glycolipids necessary for stimulatory activity. The presence of this cell type in humans and most mammals suggests that it plays critical roles in antigen recognition and the interface between innate and adaptive immunity. Both endogenous and exogenous natural antigens for NKT cells have been identified, and it is likely that glycolipid antigens remain to be discovered. Multiple series of structurally varied glycolipids have been synthesized and tested for stimulatory activity. The structural features of glycolipids necessary for NKT cell stimulation are moderately well understood, and designed compounds have proven to be much more potent antigens than their natural counterparts. Nevertheless, control over NKT cell responses by designed glycolipids has not been optimized, and further research will be required to fully reveal the therapeutic potential of this cell type.

TGF-β: guardian of T cell function

A fundamental aspect of the adaptive immune system is the generation and maintenance of a diverse and self-tolerant T cell repertoire. Through its regulation of T cell development, homeostasis, tolerance, and differentiation, the highly evolutionarily conserved cytokine TGF-β critically supports a functional T cell pool. The pleiotropic nature of this regulation is likely due to the elaborate control of TGF-β production and activation in the immune system, and the intricacy of TGF-β signaling pathways. In this review we discuss the current understanding of TGF-β regulation of T cells.

Extra-adrenal glucocorticoid synthesis: immune regulation and aspects on local organ homeostasis

Systemic glucocorticoids (GCs) mainly originate from de novo synthesis in the adrenal cortex under the control of the hypothalamus-pituitary-adrenal (HPA)-axis. However, research during the last 1-2 decades has revealed that additional organs express the necessary enzymes and have the capacity for de novo synthesis of biologically active GCs. This includes the thymus, intestine, skin and the brain. Recent research has also revealed that locally synthesized GCs most likely act in a paracrine or autocrine manner and have significant physiological roles in local homeostasis, cell development and immune cell activation. In this review, we summarize the nature, regulation and known physiological roles of extra-adrenal GC synthesis. We specifically focus on the thymus in which GC production (by both developing thymocytes and epithelial cells) has a role in the maintenance of proper immunological function.

Regulatory T cells and the immune pathogenesis of prenatal infection

Pregnancy in placental mammals offers exceptional comprehensive benefits of in utero protection, nutrition, and metabolic waste elimination for the developing fetus. However, these benefits also require durable strategies to mitigate maternal rejection of fetal tissues expressing foreign paternal antigens. Since the initial postulate of expanded maternal immune tolerance by Sir Peter Medawar 60 years ago, an amazingly elaborate assortment of molecular and cellular modifications acting both locally at the maternal-placental interface and systemically have been shown to silence potentially detrimental maternal immune responses. In turn, simultaneously maintaining host defense against the infinite array of potential pathogens during pregnancy is equally important. Fortunately, resistance against most infections is preserved seamlessly throughout gestation. On the other hand, recent studies on pathogens with unique predisposition for prenatal infections have uncovered distinctive holes in host defense associated with the reproductive process. Using these infections to probe the response during pregnancy, the immune suppressive regulatory subset of maternal CD4 T cells has been increasingly shown to dictate the inter-workings between prenatal infection susceptibility and pathogenesis of ensuing pregnancy complications. Herein, the recent literature suggesting a necessity for maternal regulatory T cells (Tregs) in pregnancy-induced immunological shifts that sustain fetal tolerance is reviewed. Additional discussion is focused on how expansion of maternal Treg suppression may become exploited by pathogens that cause prenatal infections and the perilous potential of infection-induced immune activation that may mitigate fetal tolerance and inadvertently inject hostility into the protective in utero environment.

Lineage tracing and cell ablation identify a post-Aire-expressing thymic epithelial cell population

Thymic epithelial cells in the medulla (mTECs) play a critical role in enforcing central tolerance through expression and presentation of tissue-specific antigens (TSAs) and deletion of autoreactive thymocytes. TSA expression requires autoimmune regulator (Aire), a transcriptional activator present in a subset of mTECs characterized by high CD80 and major histocompatibility complex II expression and a lack of potential for differentiation or proliferation. Here, using an Aire-DTR transgenic line, we show that short-term ablation specifically targets Aire(+) mTECs, which quickly undergo RANK-dependent recovery. Repeated ablation also affects Aire(-) mTECs, and using an inducible Aire-Cre fate-mapping system, we find that this results from the loss of a subset of mTECs that showed prior expression of Aire, maintains intermediate TSA expression, and preferentially migrates toward the center of the medulla. These results clearly identify a distinct stage of mTEC development and underscore the diversity of mTECs that play a key role in maintaining tolerance.

Programming of neuroendocrine self in the thymus and its defect in the development of neuroendocrine autoimmunity

For centuries after its first description by Galen, the thymus was considered as only a vestigial endocrine organ until the discovery in 1961 by Jacques FAP Miller of its essential role in the development of T (thymo-dependent) lymphocytes. A unique thymus first appeared in cartilaginous fishes some 500 million years ago, at the same time or shortly after the emergence of the adaptive (acquired) immune system. The thymus may be compared to a small brain or a computer highly specialized in the orchestration of central immunological self-tolerance. This was a necessity for the survival of species, given the potent evolutionary pressure imposed by the high risk of autotoxicity inherent in the stochastic generation of the diversity of immune cell receptors that characterize the adaptive immune response. A new paradigm of “neuroendocrine self-peptides” has been proposed, together with the definition of “neuroendocrine self.” Neuroendocrine self-peptides are secreted by thymic epithelial cells (TECs) not according to the classic model of neuroendocrine signaling, but are processed for presentation by, or in association with, the thymic major histocompatibility complex (MHC) proteins. The autoimmune regulator (AIRE) gene/protein controls the transcription of neuroendocrine genes in TECs. The presentation of self-peptides in the thymus is responsible for the clonal deletion of self-reactive T cells, which emerge during the random recombination of gene segments that encode variable parts of the T cell receptor for the antigen (TCR). At the same time, self-antigen presentation in the thymus generates regulatory T (Treg) cells that can inhibit, in the periphery, those self-reactive T cells that escaped negative selection in the thymus. Several arguments indicate that the origin of autoimmunity directed against neuroendocrine glands results primarily from a defect in the intrathymic programming of self-tolerance to neuroendocrine functions. This defect may be genetic or acquired, for example during an enteroviral infection. This novel knowledge of normal and pathologic functions of the thymus constitutes a solid basis for the development of a novel type of tolerogenic/negative self-vaccination against type 1 diabetes (T1D).

Distinct temporal patterns of T cell receptor signaling during positive versus negative selection in situ

The recognition by the T cell receptor (TCR) of self-peptides presented by the major histocompatibility complex (MHC) on antigen-presenting cells, such as dendritic cells and thymic epithelial cells, controls T cell fate in the thymus, with weak TCR signals inducing survival (positive selection) and stronger signals inducing death (negative selection). In vitro studies indicate that peptide ligands that induce positive selection stimulate a low, but sustained, pattern of TCR signaling; however, the temporal pattern of TCR signaling in MHC class I-restricted thymocytes (thymocytes that are presented with peptides by MHC class I) in the thymus, under conditions that support positive selection, is unknown. We addressed this question by examining intracellular Ca(2+) dynamics and migratory changes in thymocytes undergoing positive and negative selection in thymic slices. Brief, serial signaling events that were separated by migratory periods and low cytosolic Ca(2+) concentrations correlated with the positive selection of MHC class I-restricted thymocytes, whereas sustained Ca(2+) signaling and the arrest of thymocytes were associated with negative selection. Low-avidity peptides and the presentation of peptides by cortical thymic epithelial cells, rather than dendritic cells, failed to induce strong migratory arrest of thymocytes, which led to transient TCR signaling. Thus, we provide a comparison of positive and negative selection signals in situ and suggest that the absence of strong stop signals distinguishes between positive and negative selection.

Established thymic epithelial progenitor/stem cell-like cell lines differentiate into mature thymic epithelial cells and support T cell development

Common thymic epithelial progenitor/stem cells (TEPCs) differentiate into cortical and medullary thymic epithelial cells (TECs), which are required for the development and selection of thymocytes. Mature TEC lines have been widely established. However, the establishment of TEPC lines is rarely reported. Here we describe the establishment of thymic epithelial stomal cell lines, named TSCs, from fetal thymus. TSCs express some of the markers present on tissue progenitor/stem cells such as Sca-1. Gene expression profiling verifies the thymic identity of TSCs. RANK stimulation of these cells induces expression of autoimmune regulator (Aire) and Aire-dependent tissue-restricted antigens (TRAs) in TSCs in vitro. TSCs could be differentiated into medullary thymic epithelial cell-like cells with exogenously expressed NF-κB subunits RelB and p52. Importantly, upon transplantation under the kidney capsules of nude mice, TSCs are able to differentiate into mature TEC-like cells that can support some limited development of T cells in vivo. These findings suggest that the TSC lines we established bear some characteristics of TEPC cells and are able to differentiate into functional TEC-like cells in vitro and in vivo. The cloned TEPC-like cell lines may provide useful tools to study the differentiation of mature TEC cells from precursors.

In silico modeling of Itk activation kinetics in thymocytes suggests competing positive and negative IP4 mediated feedbacks increase robustness

The inositol-phosphate messenger inositol(1,3,4,5)tetrakisphosphate (IP₄) is essential for thymocyte positive selection by regulating plasma-membrane association of the protein tyrosine kinase Itk downstream of the T cell receptor (TCR). IP₄ can act as a soluble analog of the phosphoinositide 3-kinase (PI3K) membrane lipid product phosphatidylinositol(3,4,5)trisphosphate (PIP₃). PIP₃ recruits signaling proteins such as Itk to cellular membranes by binding to PH and other domains. In thymocytes, low-dose IP₄ binding to the Itk PH domain surprisingly promoted and high-dose IP₄ inhibited PIP₃ binding of Itk PH domains. However, the mechanisms that underlie the regulation of membrane recruitment of Itk by IP₄ and PIP₃ remain unclear. The distinct Itk PH domain ability to oligomerize is consistent with a cooperative-allosteric mode of IP₄ action. However, other possibilities cannot be ruled out due to difficulties in quantitatively measuring the interactions between Itk, IP₄ and PIP₃, and in generating non-oligomerizing Itk PH domain mutants. This has hindered a full mechanistic understanding of how IP₄ controls Itk function. By combining experimentally measured kinetics of PLCγ1 phosphorylation by Itk with in silico modeling of multiple Itk signaling circuits and a maximum entropy (MaxEnt) based computational approach, we show that those in silico models which are most robust against variations of protein and lipid expression levels and kinetic rates at the single cell level share a cooperative-allosteric mode of Itk regulation by IP₄ involving oligomeric Itk PH domains at the plasma membrane. This identifies MaxEnt as an excellent tool for quantifying robustness for complex TCR signaling circuits and provides testable predictions to further elucidate a controversial mechanism of PIP₃ signaling.

Reversible expression of CD138 on mature follicular B cells is downregulated by IL-4

CD138, known as a marker of plasma cells, was reported to be expressed to an intermediate level in the murine bone marrow precursor B cells. Here an intermediate level of CD138 expression was also noted in a subpopulation of splenic follicular B cells, which were distinguishable from CD138(high) plasma cells, whereas the majority of transitional or marginal zone B cells did not express CD138. These CD138(int) B cells were IgM(low)IgD(high) mature B cells, located within follicular B cell zone, and expressed a lower level of CD21 than CD138(-) follicular B cells. During in vitro culture of splenic cells, the proportion of CD138(int) B cells increased, which was noticeably reversed by the addition of IL-4 to the culture. The experiments with sorted CD138(int) cells showed that IL-4-mediated regulation of the CD138 expression was B cell-intrinsic and independent of in vitro B cell death. Our results demonstrate that mouse CD138(int) B cells characterize a subpopulation of IgM(low)IgD(high) mature follicular B cells. The CD138 expression on follicular B cells may represent a reversible status, reflecting a dynamic state probably influenced by IL-4.

Peripheral and thymic foxp3(+) regulatory T cells in search of origin, distinction, and function

Over the past decade, much has been learnt and much more to discover about Foxp3(+) regulatory T cells (Tregs). Initially, it was thought that Tregs were a unique entity that originates in the thymus. It is now recognized that there is a fraternal twin sibling that is generated in the periphery. The difficulty is in the distinction between these two subsets. The ability to detect, monitor, and analyze these two subsets in health and disease will provide invaluable insights into their functions and purposes. The plasticity and mechanisms of action can be unique and not overlapping within these subsets. Therefore, the therapeutic targeting of a particular subset of Tregs might be more efficacious. In the past couple of years, a vast amount of data have provided a better understanding of the cellular and molecular components essential for their development and stability. Many studies are implicating their preferential involvement in certain diseases and immunologic tolerance. However, it remains controversial as to whether any phenotypic markers have been identified that can differentiate thymic versus peripheral Tregs. This review will address the validity and controversy regarding Helios, Lap/Garp and Neuropilin-1 as markers of thymic Tregs. It also will discuss updated information on distinguishing features of these two subsets and their critical roles in maternal-fetal tolerance and transplantation.

Inhibitory role of the transcription repressor Gfi1 in the generation of thymus-derived regulatory T cells

Foxp3(+) regulatory T (T(reg)) cells are essential for the maintenance of self-tolerance and immune homeostasis. The majority of T(reg) cells is generated in the thymus as a specific subset of CD4(+) T cells, known as thymus-derived or natural T(reg) (nT(reg)) cells, in response to signals from T-cell receptors, costimulatory molecules, and cytokines. Recent studies have identified intracellular signaling and transcriptional pathways that link these signals to Foxp3 induction, but how the production of these extrinsic factors is controlled remains poorly understood. Here, we report that the transcription repressor growth factor independent 1 (Gfi1) has a key inhibitory role in the generation of nT(reg) cells by a noncell-autonomous mechanism. T cell-specific deletion of Gfi1 results in aberrant expansion of thymic nT(reg) cells and increased production of cytokines. In particular, IL-2 overproduction plays an important role in driving the expansion of nT(reg) cells. In contrast, although Gfi1 deficiency elevated thymocyte apoptosis, Gfi1 repressed nT(reg) generation independently of its prosurvival effect. Consistent with an inhibitory role of Gfi1 in this process, loss of Gfi1 dampens antitumor immunity. These data point to a previously unrecognized extrinsic control mechanism that negatively shapes thymic generation of nT(reg) cells.

Regulatory T cells and immune tolerance in the intestine

A fundamental role of the mammalian immune system is to eradicate pathogens while minimizing immunopathology. Instigating and maintaining immunological tolerance within the intestine represents a unique challenge to the mucosal immune system. Regulatory T cells are critical for continued immune tolerance in the intestine through active control of innate and adaptive immune responses. Dynamic adaptation of regulatory T-cell populations to the intestinal tissue microenvironment is key in this process. Here, we discuss specialization of regulatory T-cell responses in the intestine, and how a breakdown in these processes can lead to chronic intestinal inflammation.

Elimination of self-reactive T cells in the thymus: a timeline for negative selection

Two-photon microscopy and flow cytometry reveal the timing of thymocyte death and the surprisingly close coupling between cell death and phagocytosis during negative selection in thymic slices.

The elimination of autoreactive T cells occurs via thymocyte apoptosis and removal by thymic phagocytes, but the sequence of events in vivo, and the relationship between thymocyte death and phagocytic clearance, are unknown. Here we address these questions by following a synchronized cohort of thymocytes undergoing negative selection within a three-dimensional thymic tissue environment, from the initial encounter with a negative selecting ligand to thymocyte death and clearance. Encounter with cognate peptide–MHC complexes results in rapid calcium flux and migratory arrest in auto-reactive thymocytes over a broad range of peptide concentrations, followed by a lag period in which gene expression changes occurred, but there was little sign of thymocyte death. Caspase 3 activation and thymocyte loss were first detectable at 2 and 3 hours, respectively, and entry of individual thymocytes into the death program occurred asynchronously over the next 10 hours. Two-photon time-lapse imaging revealed that thymocyte death and phagocytosis occurred simultaneously, often with thymocytes engulfed prior to changes in chromatin and membrane permeability. Our data provide a timeline for negative selection and reveal close coupling between cell death and clearance in the thymus.

As an important safeguard against autoimmunity, T cells bearing autoreactive T cell antigen receptors are eliminated during their development in the thymus, a process known as negative selection. Although much is known about the molecular events involved in negative selection, surprisingly little is known about the dynamic aspects of the process. Here we examine a synchronized population of developing T cells (thymocytes) undergoing negative selection within three-dimensional living thymic tissue. We show that the initial encounter with negative selecting ligands results in migratory arrest, but in spite of this synchronous early response, individual thymocytes then undergo delayed and asynchronous entry into the death program between 2 and 12 hours thereafter. Using time-lapse two-photon imaging, we reveal that thymocyte death and the clearance of the dead cells invariably occur together, with many thymocytes already engulfed by a macrophage before the cell death-related changes in chromatin and membrane permeability are evident. These data provide a timeline of the major events during negative selection, and suggest close coupling between thymocyte death and clearance by macrophages.

The transcriptional landscape of αβ T cell differentiation

αβT cell differentiation from thymic precursors is a complex process, explored here with the breadth of ImmGen expression datasets, analyzing how differentiation of thymic precursors gives rise to transcriptomes. After surprisingly gradual changes though early T commitment, transit through the CD4⁺CD8⁺ stage involves a shutdown or rare breadth, and correlating tightly with MYC. MHC-driven selection promotes a large-scale transcriptional reactivation. We identify distinct signatures that mark cells destined for positive selection versus apoptotic deletion. Differential expression of surprisingly few genes accompany CD4 or CD8 commitment, a similarity that carries through to peripheral T cells and their activation, revealed by mass cytometry phosphoproteomics. The novel transcripts identified as candidate mediators of key transitions help define the “known unknown” of thymocyte differentiation.

Agonist-selected T cell development requires strong T cell receptor signaling and store-operated calcium entry

T cell receptor (TCR) signaling driven by interaction of the TCR with specific complexes of self-peptide and the major histocompatibility complex determines T cell fate in thymic development. However, the signaling pathway through which TCR signal strength regulates distinct T cell lineages remains unknown. Here we have used mice lacking the endoplasmic reticulum Ca2+ sensors stromal interaction molecule 1 (STIM1) and STIM2 to show that STIM-induced store-operated Ca2+ entry is not essential for thymic development of conventional TCRαβ+ T cells but is specifically required for the development of agonist-selected T cells (regulatory T cells, invariant natural killer T cells, and TCRαβ+ CD8αα+ intestinal intraepithelial lymphocytes). The severe impairment of agonist-selected T cell development is mainly due to a defect in interleukin-2 (IL-2) or IL-15 signaling. Thus, STIM1 and STIM2-mediated store-operated Ca2+ influx, leading to efficient activation of NFAT (nuclear factor of activated T cells), is critical for the postselection maturation of agonist-selected T cells.

Differential requirement for RhoH in development of TCRαβ CD8αα IELs and other types of T cells

RhoH is a new member of the atypical G proteins exclusively expressed in hematopoietic lineage cells. It has been shown to act as an adaptor for ZAP70, Syk, Lck and Csk kinases in signal transduction, and is required for positive selection of thymocytes as well as activation of peripheral T cells and mast cells. In the present study, we showed that RhoH is required not only for positive selection but also for negative selection of thymocytes. Regarding development of unconventional T cell subsets, development of NKT and regulatory T cells was also inhibited, whereas development of TCRαβ CD8αα intestinal intraepithelial lymphocytes (IEL) was not affected by the absence of RhoH. TCR-dependent in vitro activation of TCRαβ CD8αα IEL required RhoH, suggesting that overall development of IEL does not critically depend on TCR signaling but more on cytokine-dependent expansion and survival in the periphery. Our current results indicate differential requirements for RhoH in the development of TCRαβ CD8αα IELs compared to other subsets of T cells including agonist selected T cells.

Murine thymic selection quantified using a unique method to capture deleted T cells

Thymic positive and negative selection events generate a T-cell repertoire that is MHC restricted and self-tolerant. The number of T cells undergoing positive and negative selection in normal mice has never been firmly established. We generated mice that lack the proapoptotic molecule Bim (bcl2l11) together with a Nur77(GFP) transgene, which allowed the identification and enumeration of T cells that would normally undergo clonal deletion. Using this method, we report the striking observation that six times more cells undergo negative selection than complete positive selection. Seventy-five percent of the negatively selected cells are deleted at the double positive stage in the thymic cortex, compared with 25% at the single positive stage in the medulla. The fact that more thymocytes are highly reactive to MHC than are weakly reactive is inconsistent with a random model of recognition and suggests that T-cell recognition is MHC biased. Furthermore, Bim(-/-) mice had an increased number of GFP(hi) cells in the peripheral lymphoid tissue and a corresponding increase in antigen experienced or anergic cell phenotype. Our data also show that the CD4+ T cells that are clonally deleted experienced only slightly stronger T-cell receptor signaling than those that developed into regulatory T cells.

Th17 cell development: from the cradle to the grave

T cells surviving the clonal selection process emigrate from the thymus to the periphery as immature naive T cells. In the periphery, upon activation under specific cytokine milieus, naive T cells adopt specific effector phenotypes, e.g. T-helper 1 (Th1), Th2, or Th17, and acquire diverse functions to control a myriad of pathogens, tissue injuries, and other immunological insults. Interleukin-23 (IL-23) is one of the key cytokines that shapes the development and function of Th17 cells with characteristic expression of retinoic acid receptor-related orphan receptor γ-t (RORγt), IL-17, IL-22, and granulocyte macrophage colony-stimulating factor (GM-CSF). More recently, emerging data suggest that IL-23 also promotes development of 'natural Th17' (nTh17) cells that arise from the thymus, analogous to natural regulatory T cells (nTreg). We are just beginning to understand the unique thymic developmental path of nTh17 cells, which are distinct from antigen-experienced memory Th17 cells. In this review, we explore the differentiation and function of inducible, natural, and memory Th17 subsets, which encompass a broad range of immune functions while maintaining tissue hemostasis, and highlight the participation of IL-23 during the life cycle of Th17 cells.

Antidiabetogenic MHC class II promotes the differentiation of MHC-promiscuous autoreactive T cells into FOXP3+ regulatory T cells

Polymorphisms in MHC class II molecules, in particular around β-chain position-57 (β57), afford susceptibility/resistance to multiple autoimmune diseases, including type 1 diabetes, through obscure mechanisms. Here, we show that the antidiabetogenic MHC class II molecule I-A(b) affords diabetes resistance by promoting the differentiation of MHC-promiscuous autoreactive CD4(+) T cells into disease-suppressing natural regulatory T cells, in a β56-67-regulated manner. We compared the tolerogenic and antidiabetogenic properties of CD11c promoter-driven transgenes encoding I-A(b) or a form of I-A(b) carrying residues 56-67 of I-Aβ(g7) (I-A(b-g7)) in wild-type nonobese diabetic (NOD) mice, as well as NOD mice coexpressing a diabetogenic and I-A(g7)-restricted, but MHC-promiscuous T-cell receptor (4.1). Both I-A transgenes protected NOD and 4.1-NOD mice from diabetes. However, whereas I-A(b) induced 4.1-CD4(+) thymocyte deletion and 4.1-CD4(+)Foxp3(+) regulatory T-cell development, I-A(b-g7) promoted 4.1-CD4(+)Foxp3(+) Treg development without inducing clonal deletion. Furthermore, non-T-cell receptor transgenic NOD.CD11cP-I-A(b) and NOD.CD11cP-IA(b-g7) mice both exported regulatory T cells with superior antidiabetogenic properties than wild-type NOD mice. We propose that I-A(b), and possibly other protective MHC class II molecules, afford disease resistance by engaging a naturally occurring constellation of MHC-promiscuous autoreactive T-cell clonotypes, promoting their deviation into autoregulatory T cells.

Nr4a receptors are essential for thymic regulatory T cell development and immune homeostasis

Regulatory T cells (T(reg) cells) develop from progenitor thymocytes after the engagement of T cell antigen receptors (TCRs) with high-affinity ligands, but the underlying molecular mechanisms are still unclear. Here we show that the Nr4a nuclear receptors, which are encoded by immediate-early genes upregulated by TCR stimulation in thymocytes, have essential roles in T(reg) cell development. Mice that lacked all Nr4a factors could not produce T(reg) cells and died early owing to systemic autoimmunity. Nr4a receptors directly activated the promoter of the gene encoding the transcription factor Foxp3, and forced activation of Nr4a receptors bypassed low-strength TCR signaling to drive the T(reg) cell developmental program. Our results suggest that Nr4a receptors have key roles in determining CD4(+) T cell fates in the thymus and thus contribute to immune homeostasis.

Helios marks strongly autoreactive CD4+ T cells in two major waves of thymic deletion distinguished by induction of PD-1 or NF-κB

Expression of the transcription factor Helios identifies thymocyte divergence during positive and negative selection.

Acquisition of self-tolerance in the thymus requires T cells to discriminate strong versus weak T cell receptor binding by self-peptide–MHC complexes. We find this discrimination is reported by expression of the transcription factor Helios, which is induced during negative selection but decreases during positive selection. Helios and the proapoptotic protein Bim were coinduced in 55% of nascent CCR7⁻ CD4⁺ CD69⁺ thymocytes. These were short-lived cells that up-regulated PD-1 and down-regulated CD4 and CD8 during Bim-dependent apoptosis. Helios and Bim were also coinduced at the subsequent CCR7⁺ CD4⁺ CD69⁺ CD8⁻ stage, and this second wave of Bim-dependent negative selection involved 20% of nascent cells. Unlike CCR7⁻ counterparts, Helios⁺ CCR7⁺ CD4⁺ cells mount a concurrent Card11- and c-Rel–dependent activation response that opposes Bim-mediated apoptosis. This “hollow” activation response consists of many NF-κB target genes but lacks key growth mediators like IL-2 and Myc, and the thymocytes were not induced to proliferate. These findings identify Helios as the first marker known to diverge during positive and negative selection of thymocytes and reveal the extent, stage, and molecular nature of two distinct waves of clonal deletion in the normal thymus.

Nr4a receptors are essential for thymic regulatory T cell development and immune homeostasis

Regulatory T cells (T(reg) cells) develop from progenitor thymocytes after the engagement of T cell antigen receptors (TCRs) with high-affinity ligands, but the underlying molecular mechanisms are still unclear. Here we show that the Nr4a nuclear receptors, which are encoded by immediate-early genes upregulated by TCR stimulation in thymocytes, have essential roles in T(reg) cell development. Mice that lacked all Nr4a factors could not produce T(reg) cells and died early owing to systemic autoimmunity. Nr4a receptors directly activated the promoter of the gene encoding the transcription factor Foxp3, and forced activation of Nr4a receptors bypassed low-strength TCR signaling to drive the T(reg) cell developmental program. Our results suggest that Nr4a receptors have key roles in determining CD4(+) T cell fates in the thymus and thus contribute to immune homeostasis.

B-Raf is required for positive selection and survival of DP cells, but not for negative selection of SP cells

The duration of signaling through the MAP kinase (or ERK pathway) cascade has been implicated in thymic development, particularly positive and negative selection. In T cells, two isoforms of the MAP kinase kinase kinase Raf function to transmit signals from the T-cell receptor to ERK: C-Raf and B-Raf. In this study, we conditionally ablated B-Raf expression within thymocytes to assess the effects on ERK activation and thymocyte development. The complete loss of B-Raf is accompanied by a dramatic loss of ERK activation in both the double positive (DP) and single positive (SP) thymocytes, as well as peripheral splenocytes. There was a significant decrease in the cellularity of KO thymi, largely due to a loss of pre-selected DP cells, a decrease in DP cells undergoing positive selection, and a defect in SP maturation. B-Raf plays significant roles in survival of DP thymocytes and function of SP cells in the periphery. Surprisingly, we saw no effect of B-Raf deficiency on negative selection of autoreactive SP thymocytes, despite the greatly reduced ERK activation in these cells.

Self-peptides in TCR repertoire selection and peripheral T cell function

The vertebrate antigen receptors are anticipatory in their antigen recognition and display a vast diversity. Antigen receptors are assembled through V(D)J recombination, in which one of each Variable, (Diverse), and Joining gene segment are randomly utilized and recombined. Both gene rearrangement and mutational insertion are generated through randomness; therefore, the process of antigen receptors generation requires a rigorous testing system to select every receptor which is useful to recognize foreign antigens, but which would cause no harm to self cells. In the case of T cell receptors (TCR), such a quality control responsibility rests in thymic positive and negative selection. In this review, we focus on the critical involvement of self-peptides in the generation of a T cell repertoire, discuss the role of T cell thymic development in shaping the specificity of TCR repertoire, and directing function fitness of mature T cells in periphery. Here, we consider thymic positive selection to be not merely a one-time maturing experience for an individual T cell, but a life-long imprinting which influences the function of each individual T cell in periphery.

Trafficking to the thymus

The continuous production of T lymphocytes requires that hematopoietic progenitors developing in the bone marrow migrate to the thymus. Rare progenitors egress from the bone marrow into the circulation, then traffic via the blood to the thymus. It is now evident that thymic settling is tightly regulated by selectin ligands, chemokine receptors, and integrins, among other factors. Identification of these signals has enabled progress in identifying specific populations of hematopoietic progenitors that can settle the thymus. Understanding the nature of progenitor cells and the molecular mechanisms involved in thymic settling may allow for therapeutic manipulation of this process, and improve regeneration of the T lineage in patients with impaired T cell numbers.

Distribution patterns of stromal eosinophil cells in chick thymus during postnatal development

Eosinophils are a type of thymic stromal cell that are present in the thymus of both humans and mice. They participate in regulating T-cell development under non-pathological conditions. However, studies are scarce regarding the role of eosinophils in the development of the thymus in chickens. Therefore, this study investigated the distribution of eosinophils in normal chicken thymi at different stages of development. Seven thymi were obtained from chickens at days 1, 21 and 35 of development. The distribution of eosinophils in the thymi was analyzed by histological and immunohistochemical techniques using Lendrum's chromotrope 2R method and an antibody against eosinophilic cationic protein (ECP), respectively. Eosinophils were constitutively located in the chick thymus. They were mainly distributed in the thymic corticomedullary junction and medulla, especially around vessels and Hassall's corpuscles, and only a few were in the trabeculae among thymic lobules and around vessels. There were none in the cortex. The number of thymic eosinophils decreased with increasing age (P<0.01). These results indicated that eosinophils comprise a type of thymic stromal cells in the chick, which may regulate thymic development, especially during the early stages of development.

Harnessing DNA-induced immune responses for improving cancer vaccines

DNA vaccines have emerged as an attractive strategy to promote protective cellular and humoral immunity against the encoded antigen. DNA vaccines are easy to generate, inexpensive to produce and purify at large-scale, highly stable and safe. In addition, plasmids used for DNA vaccines act as powerful "danger signals" by stimulating several DNA-sensing innate immune receptors that promote the induction of protective adaptive immunity. The induction of tumor-specific immune responses represents a major challenge for DNA vaccines because most of tumor-associated antigens are normal non-mutated self-antigens. As a consequence, induction of potentially self-reactive T cell responses against such poorly immunogenic antigens is controlled by mechanisms of central and peripheral tolerance as well as tumor-induced immunosuppression. Although several DNA vaccines against cancer have reached clinical testing, disappointing results have been observed. Therefore, the development of new adjuvants that strongly stimulate the induction of antitumor T cell immunity and counteract immune-suppressive regulation is an attractive approach to enhance the potency of DNA vaccines and overcome tumor-associated tolerance. Understanding the DNA-sensing signaling pathways of innate immunity that mediate the induction of T cell responses elicited by DNA vaccines represents a unique opportunity to develop novel adjuvants that enhance vaccine potency. The advance of DNA adjuvants needs to be complemented with the development of potent delivery systems, in order to step toward successful clinical application. Here, we briefly discuss recent evidence showing how to harness DNA-induced immune response to improve the potency of cancer vaccines and counteract tumor-associated tolerance.

On the role of CD8 T cells in the control of persistent infections

The control of pathogen density during infections is typically assumed to be the result of a combination of resource limitation (loss of target cells that the pathogen can infect), innate immunity, and specific immunity. The contributions of these factors have been considered in acute infections, which are characterized by having a short duration. What controls the pathogen during persistent infections is less clear, and is complicated by two factors. First, specific immune responses become exhausted if they are subject to chronic stimulation. Exhaustion has been best characterized for CD8 T cell responses, and occurs through a combination of cell death and loss of functionality of surviving cells. Second, new nonexhausted T cells can immigrate from the thymus during the infection, and may play a role in the control of the infection. In this article, we formulate a partial-differential-equation model to describe the interaction between these processes, and use this model to explore how thymic influx and exhaustion might affect the ability of CD8 T cell responses to control persistent infections. We find that although thymic influx can play a critical role in the maintenance of a limited CD8 T cell response during persistent infections, this response is not sufficiently large to play a significant role in controlling the infection. In doing so, our results highlight the importance of resource limitation and innate immunity in the control of persistent infections.

Autoreactive T cells bypass negative selection and respond to self-antigen stimulation during infection

Autoimmunity occurs because central and peripheral tolerance mechanisms fail to tolerize T cells with weak self-reactivity to tissue-restricted antigen.

Central and peripheral tolerance prevent autoimmunity by deleting the most aggressive CD8⁺ T cells but they spare cells that react weakly to tissue-restricted antigen (TRA). To reveal the functional characteristics of these spared cells, we generated a transgenic mouse expressing the TCR of a TRA-specific T cell that had escaped negative selection. Interestingly, the isolated TCR matches the affinity/avidity threshold for negatively selecting T cells, and when developing transgenic cells are exposed to their TRA in the thymus, only a fraction of them are eliminated but significant numbers enter the periphery. In contrast to high avidity cells, low avidity T cells persist in the antigen-positive periphery with no signs of anergy, unresponsiveness, or prior activation. Upon activation during an infection they cause autoimmunity and form memory cells. Unexpectedly, peptide ligands that are weaker in stimulating the transgenic T cells than the thymic threshold ligand also induce profound activation in the periphery. Thus, the peripheral T cell activation threshold during an infection is below that of negative selection for TRA. These results demonstrate the existence of a level of self-reactivity to TRA to which the thymus confers no protection and illustrate that organ damage can occur without genetic predisposition to autoimmunity.

Diversity and specificity of the mitogen-activated protein kinase phosphatase-1 functions

The balance of protein phosphorylation is achieved through the actions of a family of protein serine/threonine kinases called the mitogen-activated protein kinases (MAPKs). The propagation of MAPK signals is attenuated through the actions of the MAPK phosphatases (MKPs). The MKPs specifically inactivate the MAPKs by direct dephosphorylation. The archetypal MKP family member, MKP-1 has garnered much of the attention amongst its ten other MKP family members. Initially viewed to play a redundant role in the control of MAPK signaling, it is now clear that MKP-1 exerts profound regulatory functions on the immune, metabolic, musculoskeletal and nervous systems. This review focuses on the physiological functions of MKP-1 that have been revealed using mouse genetic approaches. The implications from studies using MKP-1-deficient mice to uncover the role of MKP-1 in disease will be discussed.