The induction and maintenance of T cell anergy

Mesenchymal stem cell transplantation may be able to induce immunological tolerance in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a common, potentially fatal autoimmune disease involving a significant inflammatory response. SLE is characterised by failure of self-tolerance and activation of autoreactive lymphocytes, leading to persistent disease. Although current treatments achieve some improvement in patients, some SLE patients are refractory and others relapse after drug withdrawal. The toxicity of current drug regimens, with recurrent infections, together with ongoing inflammation, contribute significantly to the progressive decline in organ function. Therefore, the clinical management of SLE requires more effective and less toxic treatments, ideally inducing complete remission and self-tolerance. In this context, recently developed cell therapies based on mesenchymal stem cells (MSCs) represent a promising and safe strategy in SLE. MSCs inhibit the activation of B cells, prevent the differentiation of CD4⁺ T cells into autoreactive T cells, reprogram macrophages with anti-inflammatory effects and inhibit dendritic cells (DCs), limiting their activity as antigen-presenting cells. In addition, MSCs could induce antigen-specific tolerance by enhancing anergy processes in autoreactive cells - by inhibiting the maturation of antigen-presenting DCs, blocking the T cell receptor (TcR) pathway and secreting inhibitory molecules -, increasing apoptotic activity to eliminate them, and activating regulatory T cells (Tregs) to enhance their proliferation and induction of tolerogenic DCs. Thus, induction of self-tolerance leads to immune balance, keeping inflammation under control and reducing lupus flares.

Epigenetic Dysregulation in the Pathogenesis of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disease in which immune disorders lead to autoreactive immune responses and cause inflammation and tissue damage. Genetic and environmental factors have been shown to trigger SLE. Recent evidence has also demonstrated that epigenetic factors contribute to the pathogenesis of SLE. Epigenetic mechanisms play an important role in modulating the chromatin structure and regulating gene transcription. Dysregulated epigenetic changes can alter gene expression and impair cellular functions in immune cells, resulting in autoreactive immune responses. Therefore, elucidating the dysregulated epigenetic mechanisms in the immune system is crucial for understanding the pathogenesis of SLE. In this paper, we review the important roles of epigenetic disorders in the pathogenesis of SLE.

Bcl-3 regulates T cell function through energy metabolism

BACKGROUND

Bcl-3 is a member of the IκB protein family and an essential modulator of NF-κB activity. It is well established that Bcl-3 is critical for the normal development, survival and differentiation of adaptive immune cells, especially T cells. However, the regulation of immune cell function by Bcl-3 through metabolic pathways has rarely been studied.

RESULTS

In this study, we explored the role of Bcl-3 in the metabolism and function of T cells via the mTOR pathway. We verified that the proliferation of Bcl-3-deficient Jurkat T cells was inhibited, but their activation was promoted, and Bcl-3 depletion regulated cellular energy metabolism by reducing intracellular ATP and ROS production levels and mitochondrial membrane potential. Bcl-3 also regulates cellular energy metabolism in naive CD4+ T cells. In addition, the knockout of Bcl-3 altered the expression of mTOR, Akt, and Raptor, which are metabolism-related genes, in Jurkat cells.

CONCLUSIONS

This finding indicates that Bcl-3 may mediate the energy metabolism of T cells through the mTOR pathway, thereby affecting their function. Overall, we provide novel insights into the regulatory role of Bcl-3 in T-cell energy metabolism for the prevention and treatment of immune diseases.

Antitumor Peptide-Based Vaccine in the Limelight

The success of the immune checkpoint blockade has provided a proof of concept that immune cells are capable of attacking tumors in the clinic. However, clinical benefit is only observed in less than 20% of the patients due to the non-specific activation of immune cells by the immune checkpoint blockade. Developing tumor-specific immune responses is a challenging task that can be achieved by targeting tumor antigens to generate tumor-specific T-cell responses. The recent advancements in peptide-based immunotherapy have encouraged clinicians and patients who are struggling with cancer that is otherwise non-treatable with current therapeutics. By selecting appropriate epitopes from tumor antigens with suitable adjuvants, peptides can elicit robust antitumor responses in both mice and humans. Although recent experimental data and clinical trials suggest the potency of tumor reduction by peptide-based vaccines, earlier clinical trials based on the inadequate hypothesis have misled that peptide vaccines are not efficient in eliminating tumor cells. In this review, we highlighted the recent evidence that supports the rationale of peptide-based antitumor vaccines. We also discussed the strategies to select the optimal epitope for vaccines and the mechanism of how adjuvants increase the efficacy of this promising approach to treat cancer.

A structural perspective on the design of decoy immune modulators

Therapeutic mAbs have dominated the class of immunotherapeutics in general and immune checkpoint inhibitors in particular. The high specificity of mAbs to the target molecule as well as their extended half-life and (or) the effector functions raised by the Fc part are some of the important aspects that contribute to the success of this class of therapeutics. Equally potential candidates are decoys and their fusions that can address some of the inherent limitations of mAbs, like immunogenicity, resistance development, low bio-availability and so on, besides maintaining the advantages of mAbs. The decoys are molecules that trap the ligands and prevent them from interacting with the signaling receptors. Although a few FDA-approved decoy immune modulators are very successful, the potential of this class of drugs is yet to be fully realized. Here, we review various strategies employed in fusion protein therapeutics with a focus on the design of decoy immunomodulators from the structural perspective and discuss how the information on protein structure and function can strategically guide the development of next-generation immune modulators.

Targeting glutamine metabolism as an effective means to promote allograft acceptance while inhibit tumor growth

Recently the role of metabolic signaling pathways has emerged as playing a critical role in dictating the outcome of T cell responses. The uptake and metabolism of the amino acid glutamine is essential for effector T cell activation. Since the growth and expansion of tumor cells relies on similar anabolic and metabolic requirements, we hypothesized that glutamine blockage might represent a promising strategy to promote allograft survival while inhibit tumor growth. 6-Diazo-5-oxo-L-norleucine (DON) was used as a glutamine antagonist. First, an in vitro study of T cell proliferation was performed to examine the ability of glutamine antagonism to inhibit T cell proliferation. Then we investigated whether DON could prolong allograft survival and inhibit tumor growth by using a fully MHC-mismatched mice full thickness skin transplantation model and a mice TC-1 tumor-bearing model. The proliferation study demonstrated that DON inhibited effector T cells proliferation in a dose-dependent manner. We found a marked prolonged graft median survival time and significant tumor inhibition for mice that received DON compared to those that received no treatment. These results highlight that targeting glutamine metabolism can promote allograft acceptance in a long tumor-free period.

CD38: T Cell Immuno-Metabolic Modulator

Activation and subsequent differentiation of T cells following antigenic stimulation are triggered by highly coordinated signaling events that lead to instilling cells with a discrete metabolic and transcriptional feature. Compelling studies indicate that intracellular nicotinamide adenine dinucleotide (NAD⁺) levels have profound influence on diverse signaling and metabolic pathways of T cells, and hence dictate their functional fate. CD38, a major mammalian NAD⁺ glycohydrolase (NADase), expresses on T cells following activation and appears to be an essential modulator of intracellular NAD⁺ levels. The enzymatic activity of CD38 in the process of generating the second messenger cADPR utilizes intracellular NAD^+, and thus limits its availability to different NAD⁺ consuming enzymes (PARP, ART, and sirtuins) inside the cells. The present review discusses how the CD38-NAD⁺ axis affects T cell activation and differentiation through interfering with their signaling and metabolic processes. We also describe the pivotal role of the CD38-NAD⁺ axis in influencing the chromatin remodeling and rewiring T cell response. Overall, this review emphasizes the crucial contribution of the CD38^-NAD⁺ axis in altering T cell response in various pathophysiological conditions.

ASGR1 and Its Enigmatic Relative, CLEC10A

The large family of C-type lectin (CLEC) receptors comprises carbohydrate-binding proteins that require Ca²⁺ to bind a ligand. The prototypic receptor is the asialoglycoprotein receptor-1 (ASGR1, CLEC4H1) that is expressed primarily by hepatocytes. The early work on ASGR1, which is highly specific for N-acetylgalactosamine (GalNAc), established the foundation for understanding the overall function of CLEC receptors. Cells of the immune system generally express more than one CLEC receptor that serve diverse functions such as pathogen-recognition, initiation of cellular signaling, cellular adhesion, glycoprotein turnover, inflammation and immune responses. The receptor CLEC10A (C-type lectin domain family 10 member A, CD301; also called the macrophage galactose-type lectin, MGL) contains a carbohydrate-recognition domain (CRD) that is homologous to the CRD of ASGR1, and thus, is also specific for GalNAc. CLEC10A is most highly expressed on immature DCs, monocyte-derived DCs, and alternatively activated macrophages (subtype M2a) as well as oocytes and progenitor cells at several stages of embryonic development. This receptor is involved in initiation of T_H1, T_H2, and T_H17 immune responses and induction of tolerance in naïve T cells. Ligand-mediated endocytosis of CLEC receptors initiates a Ca²⁺ signal that interestingly has different outcomes depending on ligand properties, concentration, and frequency of administration. This review summarizes studies that have been carried out on these receptors.

The Regulatory Effects of mTOR Complexes in the Differentiation and Function of CD4+ T Cell Subsets

T cells are an important part of the adaptive immune system and play critical roles in the elimination of various pathogens. T cells could differentiate into distinct cellular subsets under different extracellular signals and then play different roles in maintaining host homeostasis and defense. The mechanistic target of rapamycin (mTOR) is a conserved intracellular serine/threonine kinase which belongs to the phosphoinositide 3-kinase- (PI3K-) related kinase family. The mTOR signaling pathway is closely involved in a variety of cell biological processes, including cell growth and cell metabolism, by senses and integrates various environmental cues. Recent studies showed that mTOR including mTORC1 and mTORC2 is closely involved in the development of T cell subpopulations such as Th1, Th2, Th9, Th17, follicular helper T cells (Tfh), and Treg cells through distinctive pathways. We herein mainly focused on the recent progress in understanding the roles of mTOR in regulating the development and differentiation of CD4⁺ T cell subsets.

Stories From the Dendritic Cell Guardhouse

Phagocytic cells [dendritic cells (DCs), macrophages, monocytes, neutrophils, and mast cells] utilize C-type (Ca²⁺-dependent) lectin-like (CLEC) receptors to identify and internalize pathogens or danger signals. As monitors of environmental imbalances, CLEC receptors are particularly important in the function of DCs. Activation of the immune system requires, in sequence, presentation of antigen to the T cell receptor (TCR) by DCs, interaction of co-stimulatory factors such as CD40/80/86 on DCs with CD40L and CD28 on T cells, and production of IL-12 and/or IFN-α/β to amplify T cell differentiation and expansion. Without this sequence of events within an inflammatory environment, or in a different order, antigen-specific T cells become unresponsive, are deleted or become regulatory T cells. Thus, the mode by which CLEC receptors on DCs are engaged can either elicit activation of T cells to achieve an immune response or induce tolerance. This minireview illustrates these aspects with Dectin-1, DEC205, the mannose receptor and CLEC10A as examples.

Diacylglycerol Kinases in T Cell Tolerance and Effector Function

Diacylglycerol kinases (DGKs) are a family of enzymes that regulate the relative levels of diacylglycerol (DAG) and phosphatidic acid (PA) in cells by phosphorylating DAG to produce PA. Both DAG and PA are important second messengers cascading T cell receptor (TCR) signal by recruiting multiple effector molecules, such as RasGRP1, PKCθ, and mTOR. Studies have revealed important physiological functions of DGKs in the regulation of receptor signaling and the development and activation of immune cells. In this review, we will focus on recent progresses in our understanding of two DGK isoforms, α and ζ, in CD8 T effector and memory cell differentiation, regulatory T cell development and function, and invariant NKT cell development and effector lineage differentiation.

Impaired CD98 signaling protects against graft-versus-host disease by increasing regulatory T cells

Graft-versus-host disease (GvHD) is a major barrier to the broader use of allogenic hematopoietic stem cell transplantation for non-malignant clinical applications. A murine model of C57BL/6 to B6D2F1 acute GvHD was employed with T lymphocytes harboring a deletion of the CD98 heavy chain (CD98hc(-/-)) as donor cells. The CD98hc(-/-) resulted in lower responses to alloantigen stimulation in a mixed leukocyte reaction assay, and prevented the mortality associated with disease progression. The percentage of donor CD8 T lymphocytes was significantly decreased, while the percentage of Foxp3-positive regulatory T cells (Tregs) in recipients was increased by CD98hc(-/-). Decreased expression of FAS, FASL, ICOS, ICOSL, PD-1 and PD-L1 by donor CD8 T cells, and mRNA expression of cytotoxic T cell-related cytokines in the recipients were shown in those with CD98hc(-/-). Fewer infiltrated cells are found in the lungs, liver, tongue and skin of recipients with CD98hc(-/-) compared with the wild type recipients. Taken together, our data indicate that T cell-specific deletion of CD98hc can contribute to the prevention of GvHD development due to the attenuation of lymphocyte migration and by increasing the generation of Treg cells. These findings are expected to make it possible to develop novel approaches for the prevention of GvHD.

Multifactorial T-cell hypofunction that is reversible can limit the efficacy of chimeric antigen receptor-transduced human T cells in solid tumors

PURPOSE

Immunotherapy using vaccines or adoptively transferred tumor-infiltrating lymphocytes (TIL) is limited by T-cell functional inactivation within the solid tumor microenvironment. The purpose of this study was to determine whether a similar tumor-induced inhibition occurred with genetically modified cytotoxic T cells expressing chimeric antigen receptors (CAR) targeting tumor-associated antigens.

EXPERIMENTAL DESIGN

Human T cells expressing CAR targeting mesothelin or fibroblast activation protein and containing CD3ζ and 4-1BB cytoplasmic domains were intravenously injected into immunodeficient mice bearing large, established human mesothelin-expressing flank tumors. CAR TILs were isolated from tumors at various time points and evaluated for effector functions and status of inhibitory pathways.

RESULTS

CAR T cells were able to traffic into tumors with varying efficiency and proliferate. They were able to slow tumor growth, but did not cause regressions or cures. The CAR TILs underwent rapid loss of functional activity that limited their therapeutic efficacy. This hypofunction was reversible when the T cells were isolated away from the tumor. The cause of the hypofunction seemed to be multifactorial and was associated with upregulation of intrinsic T-cell inhibitory enzymes (diacylglycerol kinase and SHP-1) and the expression of surface inhibitory receptors (PD1, LAG3, TIM3, and 2B4).

CONCLUSIONS

Advanced-generation human CAR T cells are reversibly inactivated within the solid tumor microenvironment of some tumors by multiple mechanisms. The model described here will be an important tool for testing T cell-based strategies or systemic approaches to overcome this tumor-induced inhibition. Our results suggest that PD1 pathway antagonism may augment human CAR T-cell function.

Regulation of Lipid Signaling by Diacylglycerol Kinases during T Cell Development and Function

Diacylglycerol (DAG) and phosphatidic acid (PA) are bioactive lipids synthesized when the T cell receptor binds to a cognate peptide-MHC complex. DAG triggers signaling by recruiting Ras guanyl-releasing protein 1, PKCθ, and other effectors, whereas PA binds to effector molecules that include mechanistic target of rapamycin, Src homology region 2 domain-containing phosphatase 1, and Raf1. While DAG-mediated pathways have been shown to play vital roles in T cell development and function, the importance of PA-mediated signals remains less clear. The diacylglycerol kinase (DGK) family of enzymes phosphorylates DAG to produce PA, serving as a molecular switch that regulates the relative levels of these critical second messengers. Two DGK isoforms, α and ζ, are predominantly expressed in T lineage cells and play an important role in conventional αβ T cell development. In mature T cells, the activity of these DGK isoforms aids in the maintenance of self-tolerance by preventing T cell hyper-activation and promoting T cell anergy. In this review, we discuss the roles of DAG-mediated pathways, PA-effectors, and DGKs in T cell development and function. We also highlight recent work that has uncovered previously unappreciated roles for DGK activity, for instance in invariant NKT cell development, anti-tumor and anti-viral CD8 responses, and the directional secretion of soluble effectors.

CRYAB modulates the activation of CD4+ T cells from relapsing-remitting multiple sclerosis patients

BACKGROUND

Suppression of activation of pathogenic CD4(+) T cells is a potential therapeutic intervention in multiple sclerosis (MS). We previously showed that a small heat shock protein, CRYAB, reduced T cell proliferation, pro-inflammatory cytokine production and clinical signs of experimental allergic encephalomyelitis, a model of MS.

OBJECTIVE

We assessed whether the ability of CRYAB to reduce the activation of T cells translated to the human disease.

METHODS

CD4(+) T cells from healthy controls and volunteers with MS were activated in vitro in the presence or absence of a CRYAB peptide (residues 73-92). Parameters of activation (proliferation rate, cytokine secretion) and tolerance (anergy, activation-induced cell death, microRNAs) were evaluated.

RESULTS

The secretion of pro-inflammatory cytokines by CD4(+) T cells was decreased in the presence of CRYAB in a subset of relapsing-remitting multiple sclerosis (RRMS) participants with mild disease severity while no changes were observed in healthy controls. Further, there was a correlation for higher levels of miR181a microRNA, a marker upregulated in tolerant CD8(+) T cells, in CD4(+) T cells of MS patients that displayed suppressed cytokine production (responders).

CONCLUSION

CRYAB may be capable of suppressing the activation of CD4(+) T cells from a subset of RRMS patients who appear to have less disability but similar age and disease duration.

Systemic delivery of Salmonella typhimurium transformed with IDO shRNA enhances intratumoral vector colonization and suppresses tumor growth

Generating antitumor responses through the inhibition of tumor-derived immune suppression represents a promising strategy in the development of cancer immunotherapeutics. Here, we present a strategy incorporating delivery of the bacterium Salmonella typhimurium (ST), naturally tropic for the hypoxic tumor environment, transformed with a small hairpin RNA (shRNA) plasmid against the immunosuppressive molecule indoleamine 2,3-dioxygenase 1 (shIDO). When systemically delivered into mice, shIDO silences host IDO expression and leads to massive intratumoral cell death that is associated with significant tumor infiltration by polymorphonuclear neutrophils (PMN). shIDO-ST treatment causes tumor cell death independently of host IDO and adaptive immunity, which may have important implications for use in immunosuppressed patients with cancer. Furthermore, shIDO-ST treatment increases reactive oxygen species (ROS) produced by infiltrating PMNs and, conversely, PMN immunodepletion abrogates tumor control. Silencing of host IDO significantly enhances S. typhimurium colonization, suggesting that IDO expression within the tumor controls the immune response to S. typhimurium. In summary, we present a novel approach to cancer treatment that involves the specific silencing of tumor-derived IDO that allows for the recruitment of ROS-producing PMNs, which may act primarily to clear S. typhimurium infection, but in the process also induces apoptosis of surrounding tumor tissue resulting in a vigorous antitumor effect.

Deletion of CD98 heavy chain in T cells results in cardiac allograft acceptance by increasing regulatory T cells

BACKGROUND

Little is known about the CD98 heavy chain (CD98hc) in the T lymphocyte-mediated immune response to alloantigen.

METHODS

We used an in vitro mixed leukocyte reaction assay and a cardiac transplantation model to evaluate the mechanisms of CD98hc in regulating alloimmune responses.

RESULTS

A T cell-specific deficiency of CD98hc resulted in lower responses to alloantigen stimulation in a mixed leukocyte reaction assay, and CD98hc-deficient mice accepted full major histocompatibility complex-mismatched cardiac allografts. Consistent with graft survival, the infiltration of the graft by immune cells in CD98hc-deficient mice was significantly lower than that in wild-type mice. A chemotaxis assay revealed the migration of CD98hc-deficient lymphocytes to decrease in the presence of CCL5 compared with wild-type cells. Moreover, the proportion of CD4/Foxp3-positive cells and Foxp3 messenger RNA increased significantly in CD98hc-deficient recipients, consistent with the down-regulation of mammalian target of rapamycin and PS6 kinase; and allograft permanent acceptance was shortened by the depletion of antibody-induced regulatory T cells. Finally, neutralizing antibody against CD98hc prolonged the cardiac allograft survival.

CONCLUSIONS

Taken together, our data indicate that T cell-specific deficiency in CD98hc can contribute to cardiac allograft permanent acceptance correlating with the attenuation of lymphocyte migration and by increasing the generation of regulatory T cells. These findings are expected to make it possible to develop novel approaches for treating allograft rejection and promoting transplantation tolerance.

FoxO-dependent regulation of diacylglycerol kinase α gene expression

Diacylglycerol kinase α (DGKα) regulates diacylglycerol levels, catalyzing its conversion into phosphatidic acid. The α isoform is central to immune response regulation; it downmodulates Ras-dependent pathways and is necessary for establishment of the unresponsive state termed anergy. DGKα functions are regulated in part at the transcriptional level although the mechanisms involved remain poorly understood. Here, we analyzed the 5' end structure of the mouse DGKα gene and detected three binding sites for forkhead box O (FoxO) transcription factors, whose function was confirmed using luciferase reporter constructs. FoxO1 and FoxO3 bound to the 5' regulatory region of DGKα in quiescent T cells, as well as after interleukin-2 (IL-2) withdrawal in activated T cells. FoxO binding to this region was lost after complete T cell activation or IL-2 addition, events that correlated with FoxO phosphorylation and a sustained decrease in DGKα gene expression. These data strongly support a role for FoxO proteins in promoting high DGKα levels and indicate a mechanism by which DGKα function is downregulated during productive T cell responses. Our study establishes a basis for a causal relationship between DGKα downregulation, IL-2, and anergy avoidance.

Regulation of Lymphocyte Function by PPARgamma: Relevance to Thyroid Eye Disease-Related Inflammation

Thyroid eye disease (TED) is an autoimmune condition in which intense inflammation leads to orbital tissue remodeling, including the accumulation of extracellular macromolecules and fat. Disease progression depends upon interactions between lymphocytes and orbital fibroblasts. These cells engage in a cycle of reciprocal activation which produces the tissue characteristics of TED. Peroxisome proliferator-activated receptor-gamma (PPARgamma) may play divergent roles in this process, both attenuating and promoting disease progression. PPARgamma has anti-inflammatory activity, suggesting that it could interrupt intercellular communication. However, PPARgamma activation is also critical to adipogenesis, making it a potential culprit in the pathological fat accumulation associated with TED. This review explores the role of PPARgamma in TED, as it pertains to crosstalk between lymphocytes and fibroblasts and the development of therapeutics targeting cell-cell interactions mediated through this signaling pathway.

T cell anergy as a strategy to reduce the risk of autoimmunity

Some self-reactive immature T cells escape negative selection in the thymus and may cause autoimmune diseases later. In the periphery, if T cells are stimulated insufficiently by peptide-major histocompatibility complex, they become inactive and their production of cytokines changes, a phenomenon called "T cell anergy". In this paper, we explore the hypothesis that T cell anergy may function to reduce the risk of autoimmunity. The underlying logic is as follows: Since those self-reactive T cells that receive strong stimuli from self-antigens are eliminated in the thymus, T cells that receive strong stimuli in the periphery are likely to be non-self-reactive. As a consequence, when a T cell receives a weak stimulus, the likelihood that the cell is self-reactive is higher than in the case that it receives a strong stimulus. Therefore, inactivation of the T cell may reduce the danger of autoimmunity. We consider the formalism in which each T cell chooses its response depending on the strength of stimuli in order to reduce the risk of autoimmune diseases while maintaining its ability to attack non-self-antigens effectively. The optimal T cell responses to a weak and a strong stimulus are obtained both when the cells respond in a deterministic manner and when they respond in a probabilistic manner. We conclude that T cell anergy is the optimal response when a T cell meets with antigen-presenting cells many times in its lifetime, and when the product of the autoimmunity risk and the number of self-reactive T cells has an intermediate value.

Blocking costimulatory pathways: prospects for inducing transplantation tolerance

Tolerance induction to alloantigens is a major challenge in transplant immunology. Whereas conventional immunosuppression inhibits the immune system in a nonspecific way, thereby also undermining an appropriate immune response towards potentially harmful infectious organisms, tolerance in a transplantation setting is restricted to alloantigens, while protective immunity is preserved. Moreover, tolerance implies an immunological status that is preserved after withdrawal of the tolerance-inducing therapy. Among the most promising strategies to induce immunological tolerance are costimulation blockade and establishment of mixed chimerism. Despite significant advances, we still know little about the mechanisms responsible for such tolerance. In this article, we discuss tolerance induction to transplantation antigens by costimulation blockade.

Anergic T cells are metabolically anergic

Full T cell activation requires TCR engagement (signal 1) in the context of costimulation (signal 2). Costimulation is required for maximal expression of effector cytokines and prevention of T cell anergy. It has become increasingly clear that another major function of costimulation is to up-regulate the metabolic machinery necessary for T cell function. In this report we demonstrate that anergic T cells are metabolically anergic, in that upon full stimulation (signals 1 plus 2) they fail to up-regulate the machinery necessary to support increased metabolism. These findings suggest that one mechanism responsible for the maintenance of T cell anergy is failure to up-regulate the metabolic machinery. Furthermore, we demonstrate that by blocking leucine, glucose, and energy metabolism, T cell activation is mitigated. Additionally, inhibition of these metabolic pathways during T cell activation leads to anergy in Th1-differentiated cells. Overall, our findings extend the role of T cell metabolism in regulating T cell function.

The mTOR kinase differentially regulates effector and regulatory T cell lineage commitment

Effector T cell differentiation requires the simultaneous integration of multiple, and sometimes opposing, cytokine signals. We demonstrated mTOR's role in dictating the outcome of T cell fate. mTOR-deficient T cells displayed normal activation and IL-2 production upon initial stimulation. However, such cells failed to differentiate into T helper 1 (Th1), Th2, or Th17 effector cells. The inability to differentiate was associated with decreased STAT transcription factor activation and failure to upregulate lineage-specific transcription factors. Under normally activating conditions, T cells lacking mTOR differentiated into Foxp3(+) regulatory T cells. This was associated with hyperactive Smad3 activation in the absence of exogenous TGF-beta. Surprisingly, T cells selectively deficient in TORC1 do not divert to a regulatory T cell pathway, implicating both TORC1 and TORC2 in preventing the generation of regulatory T cells. Overall, our studies suggest that mTOR kinase signaling regulates decisions between effector and regulatory T cell lineage commitment.

Adenosine and anergy

T cells must integrate multiple environmental cues when deciding whether to mount an immunogenic or tolerogenic response. Since not all self-reactive T cells are eliminated during thymic development, mechanisms of peripheral tolerance such as T cell anergy contribute to preventing autoimmunity. Recent studies have implicated extracellular adenosine and the adenosine A(2A) receptor as playing an important role in inhibiting T cell effector function. Herein, we review the current literature regarding T cell anergy and the emerging literature implicating the A(2A) receptor as critical regulator of immune activation. Finally, we present evidence to suggest a possible role for adenosine A(2A) receptor signaling in T cell anergy.

Naive CD4 t cell proliferation is controlled by mammalian target of rapamycin regulation of GRAIL expression

In this study, we demonstrate that the E3 ubiquitin ligase gene related to anergy in lymphocytes (GRAIL) is expressed in quiescent naive mouse and human CD4 T cells and has a functional role in inhibiting naive T cell proliferation. Following TCR engagement, CD28 costimulation results in the expression of IL-2 whose signaling through its receptor activates the Akt-mammalian target of rapamycin (mTOR) pathway. Activation of mTOR allows selective mRNA translation, including the epistatic regulator of GRAIL, Otubain-1 (Otub1), whose expression results in the degradation of GRAIL and allows T cell proliferation. The activation of mTOR appears to be the critical component of IL-2R signaling regulating GRAIL expression. CTLA4-Ig treatment blocks CD28 costimulation and resultant IL-2 expression, whereas rapamycin and anti-IL-2 treatment block mTOR activation downstream of IL-2R signaling. Thus, all three of these biotherapeutics inhibit mTOR-dependent translation of mRNA transcripts, resulting in blockade of Otub1 expression, maintenance of GRAIL, and inhibition of CD4 T cell proliferation. These observations provide a mechanistic pathway sequentially linking CD28 costimulation, IL-2R signaling, and mTOR activation as important requirements for naive CD4 T cell proliferation through the regulation of Otub1 and GRAIL expression. Our findings also extend the role of GRAIL beyond anergy induction and maintenance, suggesting that endogenous GRAIL regulates general cell cycle and proliferation of primary naive CD4 T cells.

Prophylactic ciprofloxacin treatment prevented high mortality, and modified systemic and intestinal immune function in tumour-bearing rats receiving dose-intensive CPT-11 chemotherapy

Infectious complications are a major cause of morbidity and mortality from dose-intensive cancer chemotherapy. In spite of the importance of intestinal bacteria translocation in these infections, information about the effect of high-dose chemotherapy on gut mucosal immunity is minimal. We studied prophylactic ciprofloxacin (Cipro) treatment on irinotecan (CPT-11) toxicity and host immunity in rats bearing Ward colon tumour. Cipro abolished chemotherapy-related mortality, which was 45% in animals that were not treated with Cipro. Although Cipro reduced body weight loss and muscle wasting, it was unable to prevent severe late-onset diarrhoea. Seven days after CPT-11, splenocytes were unable to proliferate (stimulation index=0.10+/-0.02) and produce proliferative and inflammatory cytokines (i.e., Interleukin (IL)-2, interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha) IL-1beta, IL-6) on mitogen stimulation in vitro (P<0.05 vs controls), whereas mesenteric lymph node (MLN) cells showed a hyper-proliferative response and a hyper-production of pro-inflammatory cytokines on mitogen stimulation. This suggests compartmentalised effects by CPT-11 chemotherapy on systemic and intestinal immunity. Cipro normalised the hyper-responsiveness of MLN cells, and in the spleen, it partially restored the proliferative response and normalised depressed production of IL-1beta and IL-6. Taken together, Cipro prevented infectious challenges associated with immune hypo-responsiveness in systemic immune compartments, and it may also alleviate excessive pro-inflammatory responses mediating local gut injury.

IL-2 signaling prevents T cell anergy by inhibiting the expression of anergy-inducing genes

T cell responses are determined by the environment in which antigen is encountered. In the absence of proper costimulation, anergizing stimuli induce the activation of a specific program of gene expression. Proteins encoded by these genes impose a state of functional unresponsiveness in anergic T cells through the activation of different mechanisms that include dampening of the T cell receptor signaling and direct inhibition of cytokine expression. Anergy can be reversed by stimulating T cells in the presence of interleukin (IL-)2. Signaling through the IL-2 receptor has been shown to activate mTOR, which plays an important role in the integration of signals that determine the fate of T cells. The mechanisms underlying the IL-2-dependent regulation of T cell tolerance are still not fully elucidated. In this study we show that IL-2 receptor signaling mediated through JAK3 and mTOR inhibits the expression of anergy-inducing genes independently of any effect on cell cycle progression. Interestingly, we also show that this effect is likely due to changes on the levels of AP-1 activation induced by IL-2 receptor signaling in T cells. Our data identifies a mechanism that can explain how IL-2 may prevent or reverse the establishment of anergy in T cells and, therefore, helps to understand how the cytokine environment can be determinant to shape the outcome of T cell responses - tolerance or activation - when antigen is encountered.

Immune mechanisms in thyroid eye disease

Thyroid eye disease (TED) is an inflammatory condition of the orbit closely associated with Graves' disease. During the course of TED, fibrosis can develop around the extraocular muscles, and excess extracellular matrix and fat accumulates in the periorbital space. This dramatic remodeling results in protrusion of the eye, also known as exophthalmos. Current treatments are sometimes effective in alleviating the symptoms of the disease, but there remains a demand for treatments that prevent or reverse the pathological alterations of orbital tissues. Such treatments may become available as a result of research aimed at understanding the mechanism by which Graves' disease leads to specific remodeling of orbital tissues. Recent findings have uncovered the importance of intercellular communication between autoreactive T cells and orbital fibroblasts. When orbital fibroblasts are activated, possibly by Graves' disease-related autoantibodies, they release T cell chemoattractants, initiating an interaction in which these cells activate each other. These interactions ultimately result in fibroblasts expressing extracellular matrix molecules, proliferating and differentiating into myofibroblasts or lipofibroblasts. Although the mechanisms underlying these processes are not completely understood, several currently available therapeutic strategies might interrupt the signaling between B and T cells and fibroblasts, thereby treating the clinical manifestations of TED.

Opposing regulation of T cell function by Egr-1/NAB2 and Egr-2/Egr-3

TCR-induced NF-AT activation leads to the up-regulation of multiple genes involved in T cell anergy. Since NF-AT is also involved in T cell activation, we have endeavored to dissect TCR-induced activating and inhibitory genetic programs. This approach revealed roles for the early growth response (Egr) family of transcription factors and the Egr coactivator/corepressor NGFI-A-binding protein (NAB)2 in regulating T cell function. TCR-induced Egr-1 and NAB2 enhance T cell function, while Egr-2 and Egr-3 inhibit T cell function. In this report, we demonstrate that Egr-2 and Egr-3 are induced by NF-AT in the absence of AP-1, while Egr-1 and NAB2 both require AP-1-mediated transcription. Our data suggest that Egr-3 is upstream of Egr-2, and that mechanistically Egr-2 and Egr-3 suppress Egr-1 and NAB2 expression. Functionally, T cells from Egr-2 and Egr-3 null mice are hyperresponsive while T cells from Egr-3 transgenic, overexpressing mice are hyporesponsive. Furthermore, an in vivo model of autoimmune pneumonitis reveals that T cells from Egr-3 null mice hasten death while Egr-3-overexpressing T cells cause less disease. Overall, our data suggest that just as the Egr/NAB network of genes control cell fate in other systems, TCR-induced Egr-1, 2, 3 and NAB2 control the fate of antigen recognition in T cells.

A2A receptor signaling promotes peripheral tolerance by inducing T-cell anergy and the generation of adaptive regulatory T cells

Tissue-derived adenosine, acting via the adenosine A(2A) receptor (A(2A)R), is emerging as an important negative regulator of T-cell function. In this report, we demonstrate that A(2A)R stimulation not only inhibits the generation of adaptive effector T cells but also promotes the induction of adaptive regulatory T cells. In vitro, antigen recognition in the setting of A(2A)R engagement induces T-cell anergy, even in the presence of costimulation. T cells initially stimulated in the presence of an A(2A)R agonist fail to proliferate and produce interleukin-2 and interferon (IFN)-gamma when rechallenged in the absence of A(2A)R stimulation. Likewise, in an in vivo model of autoimmunity, tissue-derived adenosine promotes anergy and abrogates tissue destruction. Indeed, A(2A)R stimulation inhibits interleukin-6 expression while enhancing the production of transforming growth factor-beta. Accordingly, treating mice with A(2A)R agonists not only inhibits Th1 and Th17 effector cell generation but also promotes the generation of Foxp3(+) and LAG-3(+) regulatory T cells. In this regard, A(2A)R agonists fail to prevent autoimmunity by LAG-3(-/-) clonotypic T cells, implicating an important role for LAG-3 in adenosine-mediated peripheral tolerance. Overall, our findings demonstrate that extracellular adenosine stimulates the A(2A)R to promote long-term T-cell anergy and the generation of adaptive regulatory T cells.

Cyclin-dependent kinases: molecular switches controlling anergy and potential therapeutic targets for tolerance

A large body of research has established the importance of costimulatory signals and proliferation for the generation of productive T cell immune responses. While costimulation and cell cycle progression are each individually necessary for CD4+ effector T cell differentiation, it has become clear that neither of these processes alone is sufficient to avoid anergy. This review outlines the links between T cell differentiation, tolerance, and the cell cycle, and highlights recent work that has implicated cyclin-dependent kinases as important regulators and potential targets for modulation of T cell immunity and tolerance.

Regulation of the T cell response

The T cell branch of the immune system can respond to a virtually infinite variety of exogenous antigens, thus including the possibility of self-antigen recognition and dangerous autoimmune reactions. Therefore, regulatory mechanisms operate both during ontogeny within the thymus and after birth in the periphery. The control of self-reactive T cells occurs through a process of negative selection that results in apoptosis of T cells showing high affinity for self-peptides expressed at the thymic level by means of promiscuous gene expression. Self-reactive T cells escaped to negative selection are controlled in the periphery by other regulatory mechanisms, the most important being natural Foxp3+ T regulatory (Treg) cells. Regulation is also required to control excessive effector T cell responses against exogenous antigens, when they become dangerous for the body. Three types of effector T cells have been recognized: T helper 1 (Th1) cells, which are protective against intracellular bacteria; Th2 cells, which play some role in the protection against nematodes, but are responsible for allergic reactions; Th17 cells, which are probably effective in the protection against extracellular bacteria, but also play a role in the amplification of autoimmune disorders. Abnormal or excessive Th effector responses are regulated by different mechanisms. Redirection or immune deviation of Th1- or Th2-dominated responses is provided by cytokines [interferon-gamma (IFN-gamma) vs. interleukin-4 (IL-4)] produced by the same cell types and by the CXCR3-binding chemokines CXCL4 and CXCL10. Moreover, both Th1 and Th2 responses can be suppressed by adaptive Treg cells through contact-dependent mechanisms and/or the production of IL-10 and transforming growth factor-beta (TGF-beta). Finally, TGF-beta1 can promote the development of both Th17 effector and adaptive Treg cells, while the contemporaneous production of IL-6 contributes to the development of Th17 cells, but inhibits Treg cells. The development of Th17 cells is also down-regulated by IL-4 produced by Th2 cells and by IFN-gamma produced by Th1 cells.

Immunological tolerance and autoimmunity

Immunological tolerance is a complex series of mechanisms that impair the immune system to mount responses against self antigens. Central tolerance occurs when immature lymphocytes encounter self antigens in the primary lymphoid organs, and consequently they die or become unreactive. Peripheral tolerance occurs when mature lymphocytes, escaped from negative selection during ontogeny, encounter self antigens in secondary lymphoid organs and undergo anergy, deletion or suppression. A heterogeneous family of T regulatory cells has recently been identified, which have been found to play an important role in suppressing immune responses against self. Failure or breakdown of immunological tolerance results in autoimmunity and autoimmune diseases. Such events are related to both genetic and environmental factors, the latter being mainly represented by infections. Infectious agents can indeed promote autoimmune responses either by inducing tissue inflammation and therefore an unintended bystander activation of autoreactive T cells, or by promoting T cell responses to microbial epitopes that cross react against self peptides.