E3 ligases in T cell anergy--turning immune responses into tolerance

Establishing an evaluation system for T cell activation and anergy based on CD25 expression levels as an indicator

T cell anergy refers to a state where T cells become unresponsive, playing an important role in several types of immune tolerance, such as oral tolerance. This tolerance is vital for preventing some diseases, including food allergies. Understanding the mechanism underlying T cell anergy is essential to addressing food allergies. Previous studies often identified anergic T cells by their decreased ability to produce cytokine compared to the control cells. In the studies, unstimulated or naïve T cells were commonly used as the control cells. These systems could evaluate the hyporesponsiveness of anergic T cells; however, it was challenging to distinguish whether the decrease in cytokine production by anergic T cells was owing to anergy induction or merely a temporarily response to a certain stimulation. This complexity arises because some T cell responses are temporarily suppressed, even by activating stimuli. Therefore, this study aims to explore a new evaluation index that can differentiate the responsiveness of activated T cells from that of anergic T cells compared to the control cells. It was demonstrated that CD25 expression levels serve as an appropriate indicator for distinguishing between T-cell activation and anergy. Conversely, cytokine-producing ability proved inadequate for this purpose. It was found that CD25 expression increased in activated T cells than in naïve T cells, whereas it decreased in anergic T cells after restimulation. This occurred despite decreased cytokine production in the activated and anergic T cells than in the naïve T cells. This new evaluation system, centered on CD25 expression, may help in identifying the mechanism for determining T cell activation and anergy.

Negative intracellular regulators of T-cell receptor (TCR) signaling as potential antitumor immunotherapy targets

Immunotherapy strategies aim to mobilize immune defenses against tumor cells by targeting mainly T cells. Co-inhibitory receptors or immune checkpoints (ICPs) (such as PD-1 and CTLA4) can limit T cell receptor (TCR) signal propagation in T cells. Antibody-based blocking of immune checkpoints (immune checkpoint inhibitors, ICIs) enable escape from ICP inhibition of TCR signaling. ICI therapies have significantly impacted the prognosis and survival of patients with cancer. However, many patients remain refractory to these treatments. Thus, alternative approaches for cancer immunotherapy are needed. In addition to membrane-associated inhibitory molecules, a growing number of intracellular molecules may also serve to downregulate signaling cascades triggered by TCR engagement. These molecules are known as intracellular immune checkpoints (iICPs). Blocking the expression or the activity of these intracellular negative signaling molecules is a novel field of action to boost T cell-mediated antitumor responses. This area is rapidly expanding. Indeed, more than 30 different potential iICPs have been identified. Over the past 5 years, several phase I/II clinical trials targeting iICPs in T cells have been registered. In this study, we summarize recent preclinical and clinical data demonstrating that immunotherapies targeting T cell iICPs can mediate regression of solid tumors including (membrane associated) immune-checkpoint inhibitor refractory cancers. Finally, we discuss how these iICPs are targeted and controlled. Thereby, iICP inhibition is a promising strategy opening new avenues for future cancer immunotherapy treatments.

NR4A nuclear receptors in T and B lymphocytes: Gatekeepers of immune tolerance. Immunol Rev 2022;307:116-133. [PMID: 35174510 DOI: 10.1111/imr.13072]

Random VDJ recombination early in T and B cell development enables the adaptive immune system to recognize a vast array of evolving pathogens via antigen receptors. However, the potential of such randomly generated TCRs and BCRs to recognize and respond to self-antigens requires layers of tolerance mechanisms to mitigate the risk of life-threatening autoimmunity. Since they were originally cloned more than three decades ago, the NR4A family of nuclear hormone receptors have been implicated in many critical aspects of immune tolerance, including negative selection of thymocytes, peripheral T cell tolerance, regulatory T cells (Treg), and most recently in peripheral B cell tolerance. In this review, we discuss important insights from many laboratories as well as our own group into the function and mechanisms by which this small class of primary response genes promotes self-tolerance and immune homeostasis to balance the need for host defense against the inherent risks posed by the adaptive immune system.

Beyond the Cell Surface: Targeting Intracellular Negative Regulators to Enhance T cell Anti-Tumor Activity

It is well established that extracellular proteins that negatively regulate T cell function, such as Cytotoxic T-Lymphocyte-Associated protein 4 (CTLA-4) and Programmed Cell Death protein 1 (PD-1), can be effectively targeted to enhance cancer immunotherapies and Chimeric Antigen Receptor T cells (CAR-T cells). Intracellular proteins that inhibit T cell receptor (TCR) signal transduction, though less well studied, are also potentially useful therapeutic targets to enhance T cell activity against tumor. Four major classes of enzymes that attenuate TCR signaling include E3 ubiquitin kinases such as the Casitas B-lineage lymphoma proteins (Cbl-b and c-Cbl), and Itchy (Itch), inhibitory tyrosine phosphatases, such as Src homology region 2 domain-containing phosphatases (SHP-1 and SHP-2), inhibitory protein kinases, such as C-terminal Src kinase (Csk), and inhibitory lipid kinases such as Src homology 2 (SH2) domain-containing inositol polyphosphate 5-phosphatase (SHIP) and Diacylglycerol kinases (DGKs). This review describes the mechanism of action of eighteen intracellular inhibitory regulatory proteins in T cells within these four classes, and assesses their potential value as clinical targets to enhance the anti-tumor activity of endogenous T cells and CAR-T cells.

NFAT2 is a critical regulator of the anergic phenotype in chronic lymphocytic leukaemia

Chronic lymphocytic leukaemia (CLL) is a clonal disorder of mature B cells. Most patients are characterised by an indolent disease course and an anergic phenotype of their leukaemia cells, which refers to a state of unresponsiveness to B cell receptor stimulation. Up to 10% of CLL patients transform from an indolent subtype to an aggressive form of B cell lymphoma over time (Richter´s syndrome) and show a significantly worse treatment outcome. Here we show that B cell-specific ablation of Nfat2 leads to the loss of the anergic phenotype culminating in a significantly compromised life expectancy and transformation to aggressive disease. We further define a gene expression signature of anergic CLL cells consisting of several NFAT2-dependent genes including Cbl-b, Grail, Egr2 and Lck. In summary, this study identifies NFAT2 as a crucial regulator of the anergic phenotype in CLL.NFAT2 is a transcription factor that has been linked with chronic lymphocytic leukaemia (CLL), but its functions in CLL manifestation are still unclear. Here the authors show, by analysing mouse CLL models and characterising biopsies from CLL patients, that NFAT2 is an important regulator for the anergic phenotype of CLL.

Destabilizing the autoinhibitory conformation of Zap70 induces up-regulation of inhibitory receptors and T cell unresponsiveness

Hsu et al. show that a hypermorphic allele of Zap70, characterized by reduced autoinhibition, is associated with increased TCR signaling and triggers regulatory mechanisms by which negative selection and inhibitory receptors restrain TCR signaling to enforce T cell tolerance.

Zap70 plays a critical role in normal T cell development and T cell function. However, little is known about how perturbation of allosteric autoinhibitory mechanisms in Zap70 impacts T cell biology. Here, we analyze mice with a hypermorphic Zap70 mutation, W131A, which destabilizes the autoinhibitory conformation of Zap70, rendering the kinase in a semiactive state. W131A mutant mice with wild-type T cell receptor (TCR) repertoires exhibited relatively normal T cell development. However, crossing the W131A mutant mice to OTII TCR transgenic mice resulted in increased negative selection of OTII⁺ thymocytes and in increased thymic and peripheral T regulatory cells. Strikingly, increased basal TCR signaling was associated with a marked increase in inhibitory receptor expression and with T cells that were relatively refractory to TCR stimulation. PD-1 inhibitory receptor blockade partially reversed T cell unresponsiveness. Collectively, disruption of normal Zap70 autoinhibition engaged negative feedback mechanisms by which negative selection and inhibitory receptors restrain TCR signaling to enforce both central and peripheral tolerance.

Immune Regulation by Ubiquitin Tagging as Checkpoint Code

The immune system is equipped with effective machinery to mobilize its activation to defend invading microorganisms, and at the same time, to refrain from attacking its own tissues to maintain immune tolerance. The balance of activation and tolerance is tightly controlled by diverse mechanisms, since breakdown of tolerance could result in disastrous consequences such as the development of autoimmune diseases. One of the mechanisms is by the means of protein ubiquitination, which involves the process of tagging a small peptide ubiquitin to protein substrates. E3 ubiquitin ligases are responsible for catalyzing the final step of ubiquitin-substrate conjugation by specifically recognizing substrates to determine their fates of degradation or functional modification. The ubiquitination process is reversible, which is carried out by deubiquitinating enzymes to release the ubiquitin molecule from the conjugated substrates. Protein ubiquitination and deubiquitination serve as checkpoint codes in many key steps of lymphocyte regulation including the development, activation, differentiation, and tolerance induction. In this chapter, we will discuss a few E3 ligases and deubiquitinating enzymes that are important in controlling immune responses, with emphasis on their roles in T cells.

Cbl-b Deficiency in Mice Results in Exacerbation of Acute and Chronic Stages of Allergic Asthma

Mice sensitized to ovalbumin (OVA) develop allergic airway disease (AAD) with short-term daily OVA aerosol challenge; inflammation resolves with long-term OVA aerosol exposure, resulting in local inhalational tolerance (LIT). Cbl-b is an E3 ubiquitin ligase involved with CD28 signaling; Cbl-b^−/− effector T cells are resistant to regulatory T cell-mediated suppression in vitro and in vivo. The present study utilized Cbl-b^−/− mice to investigate the role of Cbl-b in the development of AAD and LIT. Cbl-b^−/− mice exhibited increased airway inflammation during AAD, which failed to resolve with long-term OVA aerosol exposure. Exacerbation of inflammation in Cbl-b^−/− mice correlated with increased proinflammatory cytokine levels and expansion of effector T cells in the BAL during AAD, but did not result in either a modulation of lymphocyte subsets in systemic tissues or in OVA-specific IgE in serum. These results implicate a role for Cbl-b in the resolution of allergic airway inflammation.

Modulation of Immune Cell Functions by the E3 Ligase Cbl-b

Maintenance of immunological tolerance is a critical hallmark of the immune system. Several signaling checkpoints necessary to balance activating and inhibitory input to immune cells have been described so far, among which the E3 ligase Cbl-b appears to be a central player. Cbl-b is expressed in all leukocyte subsets and regulates several signaling pathways in T cells, NK cells, B cells, and different types of myeloid cells. In most cases, Cbl-b negatively regulates activation signals through antigen or pattern recognition receptors and co-stimulatory molecules. In line with this function, cblb-deficient immune cells display lower activation thresholds and cblb knockout mice spontaneously develop autoimmunity and are highly susceptible to experimental autoimmunity. Interestingly, genetic association studies link CBLB-polymorphisms with autoimmunity also in humans. Vice versa, the increased activation potential of cblb-deficient cells renders them more potent to fight against malignancies or infections. Accordingly, several reports have shown that cblb knockout mice reject tumors, which mainly depends on cytotoxic T and NK cells. Thus, targeting Cbl-b may be an interesting strategy to enhance anti-cancer immunity. In this review, we summarize the findings on the molecular function of Cbl-b in different cell types and illustrate the potential of Cbl-b as target for immunomodulatory therapies.

Grail controls Th2 cell development by targeting STAT6 for degradation

T helper (Th)-2 cells are the major players in allergic asthma; however, the mechanisms that control Th2-mediated inflammation are poorly understood. Here we find that enhanced expression of Grail, an E3 ubiquitin ligase, in Th2 cells depends on IL-4-signaling components, Stat6 and Gata3 that bind to and transactivate the Grail promoter. Grail-deficiency in T cells leads to increased expression of Th2 effector cytokines in vitro and in vivo and Grail deficient mice are more susceptible to allergic asthma. Mechanistically, the enhanced effector function of Grail-deficient Th2 cells is mediated by increased expression of Stat6 and IL-4 receptor α-chain. Grail interacts with Stat6 and targets it for ubiquitination and degradation. Thus, our results indicate that Grail plays a critical role in controlling Th2 development through a negative feedback loop.

A transendocytosis perspective on the CD28/CTLA-4 pathway

T cell activation is a key event in the adaptive immune response and vital to the generation of both cellular and humoral immunity. Activation is required not only for effective CD4 T cell responses but also to provide help for B cells and the generation of cytotoxic T cell responses. Unsurprisingly, impaired T cell activation results in infectious pathology, whereas dysregulated activation can result in autoimmunity. The decision to activate is therefore tightly regulated and the CD28/CTLA-4 pathway represents this apical decision point at the molecular level. In particular, CTLA-4 (CD152) is an essential checkpoint control for autoimmunity; however, the molecular mechanism(s) by which CTLA-4 achieves its regulatory function are not well understood, especially how it functionally intersects with the CD28 pathway. In this chapter, we review the established molecular and cellular concepts relating to CD28 and CTLA-4 biology, and attempt to integrate these by discussing the transendocytosis of ligands as a new model of CTLA-4 function.

T-cell tolerance in cancer

T cells are the master regulators of adaptive immune responses and maintenance of their tolerance is critical to prevent autoimmunity. However, in the case of carcinogenesis, the tumor microenvironment aids T-cell tolerance, which contributes to uncontrolled tumor growth. Recently, there has been significant progress in understanding the intrinsic extracellular (positive and negative costimulatory molecules on APCs) and intracellular mechanisms (E3 ubiquitin ligases, transcriptional and epigenetic repressors), as well as extrinsic mechanisms (Tregs and tolerogenic dendritic cells) that are required for the implementation and maintenance of T-cell tolerance. Ultimately, understanding and manipulating T-cell tolerance will help to break the tolerance state in cancer.

IκB kinase β is required for activation of NF-κB and AP-1 in CD3/CD28-stimulated primary CD4(+) T cells

Engagement of the TCR and CD28 coreceptor by their respective ligands activates signal transduction cascades that ultimately lead to the activation of the transcription factors NFAT, AP-1, and NF-κB, which are required for the expression of cytokines and T cell clonal expansion. Previous studies have demonstrated that in mature T cells, activation of AP-1 and NF-κB is dependent on protein kinase C θ, suggesting the existence of a common signaling pathway. In this study, we show that in human primary CD4(+) T cells, exposure to the cell-permeable IKKβ inhibitor PS-1145 or genetic ablation of IKKβ abrogates cell proliferation and impairs the activation of NF-κB and AP-1 transcription factors in response to engagement of CD3 and CD28 coreceptor. In addition, we show that stimulation of T cells in the absence of IKKβ activity promotes the time-dependent and cyclosporine-sensitive expression of negative regulators of T cell signaling leading to a hyporesponsive state of T cells.

Defects in the Bcl-2-regulated apoptotic pathway lead to preferential increase of CD25 low Foxp3+ anergic CD4+ T cells

Defects in the Bcl-2-regulated apoptotic pathway inhibit the deletion of self-reactive T cells. What is unresolved, however, is the nature and fate of such self-reactive T cells escaping deletion. In this study, we report that mice with such defects contained increased numbers of CD25(low)Foxp3(+) cells in the thymus and peripheral lymph tissues. The increased CD25(low)Foxp3(+) population contained a large fraction of cells bearing self-reactive TCRs, evident from a prominent increase in self-superantigen-specific Foxp3(+)Vβ5(+)CD4(+) T cells in BALB/c Bim(-/-) mice compared with control animals. The survival rate of the expanded CD25(low)Foxp3(+) cells was similar to that of CD25(high)Foxp3(+) CD4 T cells in vitro and in vivo. IL-2R stimulation, but not TCR ligation, upregulated CD25 on CD25(low)Foxp3(+)CD4(+) T cells in vitro and in vivo. The expanded CD25(low)Foxp3(+)CD4(+) T cells from Bim(-/-) mice were anergic but also had weaker regulatory function than CD25(high)Foxp3(+) CD4(+) T cells from the same mice. Analysis of Bim(-/-) mice that also lacked Fas showed that the peripheral homeostasis of this expanded population was in part regulated by this death receptor. In conclusion, these results show that self-reactive T cell escapes from thymic deletion in mice defective in the Bcl-2-regulated apoptotic pathway upregulate Foxp3 and become unresponsive upon encountering self-Ag without necessarily gaining potent regulatory function. This clonal functional diversion may help to curtail autoaggressiveness of escaped self-reactive CD4(+) T cells and thereby safeguard immunological tolerance.

Molecular mechanisms of T-cell tolerance

CD4(+) T cells are the master regulators of adaptive immune responses, and many autoimmune diseases arise due to a breakdown of self-tolerance in CD4(+) T cells. Activation of CD4(+) T cells is regulated by not only the binding of peptide-major histocompatibility complexes to T-cell receptor but also costimulatory signals from antigen-presenting cells. Recently, there has been progress in understanding the extracellular and intracellular mechanisms that are required for implementation and maintenance of T-cell tolerance. Understanding of the molecular mechanisms underlying T-cell tolerance will lead to development of pharmacological approaches either to promote the tolerance state in terms of autoimmunity or to break tolerance in cancer.

The E3 ubiquitin ligase GRAIL regulates T cell tolerance and regulatory T cell function by mediating T cell receptor-CD3 degradation

T cell activation is tightly regulated to avoid autoimmunity. Gene related to anergy in lymphocytes (GRAIL, encoded by Rnf128) is an E3 ubiquitin ligase associated with T cell tolerance. Here, we generated and analyzed GRAIL-deficient mice and found they were resistant to immune tolerance induction and exhibited greater susceptibility to autoimmune diseases than wild-type mice. GRAIL-deficient naive T cells, after activation, exhibited increased proliferation and cytokine expression than controls and did not depend on costimulation for effector generation. Moreover, GRAIL-deficient regulatory T (Treg) cells displayed reduced suppressive function, associated with increased Th17 cell-related gene expression. GRAIL-deficient naive and Treg cells were less efficient in downregulating T cell receptor (TCR)-CD3 expression after activation and exhibited increased NFATc1 transcription factor expression; GRAIL expression promoted CD3 ubiquitinylation. Our results indicate that GRAIL, by mediating TCR-CD3 degradation, regulates naive T cell tolerance induction and Treg cell function.

Yin-Yang of costimulation: crucial controls of immune tolerance and function

SUMMARY

In addition to signals from the T-cell receptor complex, it has been recognized for many years that a 'second' signal, most notably from CD28, is also important in T-cell activation. In the recent years, many new members of CD28 family as well as the molecules that share structural homology to CD28 ligands CD80 and CD86 have been discovered. Interestingly, some of these proteins function to dampen T-cell activation and regulate the induction of T-cell tolerance. Therefore, positive and negative costimulation are the two sides of the coin to fine tune T-cell receptor signaling to determine the outcome of T-cell receptor engagement-tolerance versus function.

Molecular mechanisms of T-cell receptor and costimulatory molecule ligation/blockade in autoimmune disease therapy

SUMMARY

Pro-inflammatory CD4(+) T-cell-mediated autoimmune diseases, such as multiple sclerosis and type 1 diabetes, are hypothesized to be initiated and maintained by activated antigen-presenting cells presenting self antigen to self-reactive interferon-gamma and interleukin-17-producing CD4(+) T-helper (Th) type 1/Th17 cells. To date, the majority of Food and Drug Administration-approved therapies for autoimmune disease primarily focus on the global inhibition of immune inflammatory activity. The goal of ongoing research in this field is to develop both therapies that inhibit/eliminate activated autoreactive cells as well as antigen-specific treatments, which allow for the directed blockade of the deleterious effects of self-reactive immune cell function. According to the two-signal hypothesis, activation of a naive antigen-specific CD4(+) T cell requires both stimulation of the T-cell receptor (TCR) (signal 1) and stimulation of costimulatory molecules (signal 2). There also exists a balance between pro-inflammatory and anti-inflammatory immune cell activity, which is regulated by the type and strength of the activating signal as well as the local cytokine milieu in which the naive CD4(+) T cell is activated. To this end, the majority of ongoing research is focused on the delivery of suboptimal TCR stimulation in the absence of costimulatory molecule stimulation, or potential blockade of stimulatory accessory molecules. Therefore, the signaling pathways involved in the induction of CD4(+) T-cell anergy, as apposed to activation, are topics of intense interest.

Cbl- and Nedd4-family ubiquitin ligases: balancing tolerance and immunity

Engagement of the T cell receptor (TCR) with its cognate peptide/MHC initiates a cascade of signaling events that results in T cell activation. Limiting the extent and duration of TCR signaling ensures a tightly constrained response, protecting cells from the deleterious impact of chronic activation. In order to limit the duration of activation, T cells must adjust levels of key signaling proteins. This can be accomplished by altering protein synthesis or by changing the rate of protein degradation. Ubiquitination is a process of 'tagging' a protein with ubiquitin and is one means of initiating protein degradation. This process is activated when an E3 ubiquitin ligase mediates the transfer of ubiquitin to a target protein. Accordingly, E3 ubiquitin ligases have recently emerged as key regulators of immune cell function. This review will explore how a small group of E3 ubiquitin ligases regulate T cell responses and thus direct adaptive immunity.

Therapeutic blockade of T-cell antigen receptor signal transduction and costimulation in autoimmune disease

CD4+ T-cell-mediated autoimmune diseases are initiated and maintained by the presentation of self-antigen by antigen-presenting cells (APCs) to self-reactive CD4+ T-cells. According to the two-signal hypothesis, activation of a naive antigen-specific CD4+ T-cell requires stimulation of both the T-cell antigen receptor (signal 1) and costimulatory molecules such as CD28 (signal 2). To date, the majority of therapies for autoimmune diseases approved by the Food and Drug Administration primarily focus on the global inhibition of immune inflammatory activity. The goal of ongoing research in this field is to develop antigen-specific treatments which block the deleterious effects of self-reactive immune cell function while maintaining the ability of the immune system to clear nonself antigens. To this end, the signaling pathways involved in the induction of CD4+ T-cell anergy, as apposed to activation, are a topic of intense interest. This chapter discusses components of the CD4+ T-cell activation pathway that may serve as therapeutic targets for the treatment of autoimmune disease.

Nedd4 augments the adaptive immune response by promoting ubiquitin-mediated degradation of Cbl-b in activated T cells

Nedd4 and Itch are E3 ubiquitin ligases that ubiquitinate similar targets in vitro and thus are thought to function similarly. T cells lacking Itch show spontaneous activation and T helper type 2 polarization. To test whether loss of Nedd4 affects T cells in the same way, we generated Nedd4(+/+) and Nedd4(-/-) fetal liver chimeras. Nedd4(-/-) T cells developed normally but proliferated less, produced less interleukin 2 and provided inadequate help to B cells. Nedd4(-/-) T cells contained more of the E3 ubiquitin ligase Cbl-b, and Nedd4 was required for polyubiquitination of Cbl-b induced by CD28 costimulation. Our data demonstrate that Nedd4 promotes the conversion of naive T cells into activated T cells. We propose that Nedd4 and Itch ubiquitinate distinct target proteins in vivo.

T-cell regulator RNF125/TRAC-1 belongs to a novel family of ubiquitin ligases with zinc fingers and a ubiquitin-binding domain

The recently identified RNF125 [RING (really interesting new gene) finger protein 125], or TRAC-1 (T-cell RING protein in activation 1), is unique among ubiquitin ligases in being a positive regulator of T-cell activation. In addition, TRAC-1 has been shown to down-modulate HIV replication and to inhibit pathogen-induced cytokine production. However, apart from the presence of an N-terminal C3HC4 (Cys(3)-His-Cys(4)) RING domain, the TRAC-1 protein remains uncharacterized. In the present paper, we report novel interactions and modifications for TRAC-1, and elucidate its domain organization. Specifically, we determine that TRAC-1 associates with membranes and is excluded from the nucleus through myristoylation. Our data are further consistent with a crucial role for the C-terminus in TRAC-1 function. In this region, novel domains were recognized through the identification of three closely related proteins: RNF114, RNF138 and RNF166. TRAC-1 and its relatives were found to contain, apart from the RING domain, a C2HC (Cys(2)-His-Cys)- and two C2H2 (Cys(2)-His(2))-type zinc fingers, as well as a UIM (ubiquitin-interacting motif). The UIM of TRAC-1 binds Lys(48)-linked polyubiquitin chains and is, together with the RING domain, required for auto-ubiquitination. As a consequence of auto-ubiquitination, the half-life of TRAC-1 is shorter than 30 min. The identification of these novel modifications, interactions, domains and relatives significantly widens the contexts for investigating TRAC-1 activity and regulation.

Foxp3 controls autoreactive T cell activation through transcriptional regulation of early growth response genes and E3 ubiquitin ligase genes, independently of thymic selection

To elucidate the mechanisms of autoreactive T cell activation and expansion, we used endogenous viral superantigens (VSAg)-reactive T cells as a model of self-antigens in two strains of Foxp3-mutant mice. These two strains, together with wild-type mice, provided us with an advantage to simultaneously study the positively and negatively selected as well as rescued autoreactive T cells. We show here that while both VSAg-reactive and non-VSAg-reactive T cells are equally activated in Foxp3-mutant mice, only the VSAg-reactive T cells are preferentially expanded independently of their selected states in the thymus. The T cell activation appears to be controlled by Foxp3 through transcriptional regulation of early growth response (Egr) genes Egr-2 and Egr-3, and E3 ubiquitin (Ub) ligase genes Cblb, Itch and GRAIL, subsequently affecting degradation of two key signaling proteins, PLCgamma1 and PKC-theta. Physiologically, the positively, but not negatively selected VSAg-reactive T cells are spontaneously activated without significant expansion. The results suggest that autoreactive T cell activation is controlled by Foxp3 through transcriptional regulation of early growth response genes and E3 ubiquitin ligase genes, independently of thymic selection.

T-cell tolerance or function is determined by combinatorial costimulatory signals

Activated in immune responses, T lymphocytes differentiate into effector cells with potent immune function. CD28 is the most prominent costimulatory receptor for T-cell activation. However, absence of CD28 costimulation did not completely impair effector function of CD4 or CD8 T cells. Moreover, increasing number of costimulatory molecules are recently found on antigen-presenting cells to regulate T-cell activation. To understand the molecular mechanisms that determine T-cell function or tolerance, we have collectively examined the roles of positive and negative costimulatory molecules. Antigen-specific naïve CD4 and CD8 T cells, only when activated in the absence of both CD28 and ICOS pathways, were completely impaired in effector function. These tolerant T cells not only were anergic with profound defects in TcR signal transduction but also completely lacked expression of effector-specific transcription factors. T-cell tolerance induction in this system requires the action by negative costimulatory molecules; T-cell proliferation and function was partially restored by inhibiting PD-1, B7-H3 or B7S1. This work demonstrates that T-cell function or tolerance is controlled by costimulatory signals.

Role of Ubiquitylation in Cellular Membrane Transport

Ubiquitylation of membrane proteins has gained considerable interest in recent years. It has been recognized as a signal that negatively regulates the cell surface expression of many plasma membrane proteins both in yeast and in mammalian cells. Moreover, it is also involved in endoplasmic reticulum-associated degradation of membrane proteins, and it acts as a sorting signal both in the secretory pathway and in endosomes, where it targets proteins into multivesicular bodies in the lumen of vacuoles/lysosomes. In this review we discuss the progress in understanding these processes, achieved during the past several years.

AIRE and APECED: molecular insights into an autoimmune disease

Mutations in the autoimmune regulator (AIRE) protein are the causative factor in development of the human disease autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). In mice, the absence of the analogous protein aire influences ectopic expression of peripheral tissue antigens in thymic medullary epithelial cells (MECs), resulting in the development of an autoimmune disorder similar to APECED and establishing aire/AIRE as an important player in the induction of central tolerance. However, the molecular mechanism of AIRE's function, in particular its ability to specifically control the expression of peripheral tissue antigens in MECs, is still unclear. Here, we review current evidence relating to the molecular mechanism of AIRE.

The Yins of T Cell Activation

Transcription factors activated in response to T cell receptor (TCR) signaling include nuclear factor of activated T cells (NFAT) family, which is highly phosphorylated and thereby maintained in the cytoplasm of resting T cells, the nuclear factor NF-kappaB, which is kept in the cytoplasm of resting cells through its association with the inhibitor protein IkappaB, and activating protein-1 (AP-1), which is only transcribed after TCR stimulation. Negative regulators of TCR signaling can be divided into two groups: Class 1 regulators help maintain the quiescent state of unstimulated T cells, whereas class 2 regulators are themselves transcriptionally induced in response to TCR signaling and serve to limit and terminate the activating signal. Class 1 regulators include the autoinhibitory domain of the phosphatase calcineurin; IkappaB and its transcriptional activators Foxj1 and Foxo3a; and various transcriptional coregulators that inhibit interleukin-2 (IL-2) production. Class 2 regulators include the calcipressins, which, like NFATp and NFAT4 are feedback inhibitors of calcineurin-NFAT signaling, IkappaB, and the mitogen-activated protein kinase (MAPK) phosphatases, which inhibit MAPK signaling and thus the nuclear localization of AP-1 components.