Transcriptional mechanisms underlying lymphocyte tolerance

Lymphocyte calcium signaling from membrane to nucleus

Ca(2+) signals control a variety of lymphocyte responses, ranging from short-term cytoskeletal modifications to long-term changes in gene expression. The identification of molecules and channels that modulate Ca(2+) entry into T and B lymphocytes has both provided details of the molecular events leading to immune responses and raised controversy. Here we review studies of the pathways that allow Ca(2+) entry, the function of Ca(2+) in the regulation of cell polarity and motility and the principles by which Ca(2+)-dependent transcription regulates lymphocyte function.

Turning down the system: counter-regulatory mechanisms in bone and adaptive immunity

Major advances have been made in recent years toward the identification of transcription factors that control cell-type-specific gene expression in the skeletal and adaptive immune systems. However, the identification of factors necessary and sufficient to drive production of effector cell proteins such as matrix components and cytokines represents the first step toward understanding how cells in bone and the adaptive system achieve their highly specialized functions. Here, we provide selected examples of counter-regulatory mechanisms that serve to turn down cells involved in extracellular matrix biosynthesis and adaptive immunity at the level of the transcription factors Runx2 and nuclear factor for the activation of T cells.

Differential roles of PKC-theta in the regulation of intracellular calcium concentration in primary T cells

Activation of T lymphocytes requires protein kinase C theta (PKC-theta) and an appropriately elevated free intracellular Ca2+ concentration ([Ca2+]i). Here, we show that phorbol 12 myristate 13-acetate (PMA) inhibited Ca2+ influx in wild-type but not PKC-theta-/- T cells, suggesting that PKC-theta plays a role in PMA-mediated inhibition of Ca2+ influx. In contrast, T cell receptor (TCR) crosslinking in the same PKC-theta-/- T cells did result in significantly decreased [Ca2+]i compared to wild-type T cells, suggesting a positive role for PKC-theta in TCR-mediated Ca2+ mobilization. In PKC-theta-/- mice, peripheral mature T cells, but not developing thymocytes, displayed significantly decreased TCR-induced Ca2+ influx and nuclear factor of activated T cells (NFAT) translocation upon sub-optimal TCR crosslinking. The decreased intracellular free Ca2+ was due to changes in Ca2+ influx but not efflux, as observed in extracellular and intracellular Ca2+ mobilization studies. However, these differences in Ca2+ influx and nuclear factor of activated T cells (NFAT) translocation disappeared with increasing intensity of TCR crosslinking. The enhancing effect of PKC-theta on Ca2+ influx is not only dependent on the strength of TCR crosslinking but also on the developmental stage of T cells. The underlying mechanism involved phospholipase Cgamma1 activation and inositol triphosphate production. Furthermore, knockdown of endogenous PKC-theta expression in Jurkat cells resulted in significant inhibition of TCR-induced activation of NFAT, as evidenced from NFAT reporter studies. Forced expression of a constitutively active form of calcineurin in PKC-theta-/- Jurkat cells could readily overcome the above inhibition. Thus, PKC-theta can both positively and negatively regulate the Ca2+ influx that is critical for NFAT activity.

Differential regulation of the IL-10 gene in Th1 and Th2 T cells

Interleukin-10 (IL-10), an immunoregulatory cytokine, modulates the function of various immune and nonimmune cells, yet little information is available on the molecular mechanism of transcriptional regulation at the chromatin level. During T cell differentiation from naive T cells into Th1 and Th2 cells, the expression of IL-10 in Th1 cells slowly disappears, whereas Th2 cells produce more IL-10. We examined the chromatin structural changes associated with IL-10 gene transcription by naive and differentiated murine Th1 and Th2 cells. Naive T cells lack DNase I hypersensitivity (HS) sites in the vicinity of the IL-10 gene, whereas differentiated T cells display a strong 3' constitutive HS site as well as several inducible sites. In committed Th1 cells, the mechanism of IL-10 gene silencing is associated with a closed chromatin structure, the lack of an HS site at the promoter region, and the development of repressive histone modification near the IL-10 promoter and introns 3 and 4. We confirm that the majority of HS sites coincide with conserved noncoding sequences (CNSs) identified by comparative genomic sequence alignment between human and mouse genomes. Potential transcription factor binding sites were located by comparing CNSs with the TRANSFAC database. Predicted in vivo binding of specific factors on the CNS locus were confirmed by chromatin immunoprecipitation assays. Our results suggest that the combination of HS site and comparative genomic approaches allows identification of regulatory elements involved in differential IL-10 gene expression between Th1 and Th2 cells during T cell differentiation.

Oral tolerance

Multiple mechanisms of tolerance are induced by oral antigen. Low doses favor active suppression, whereas higher doses favor clonal anergy/deletion. Oral antigen induces T-helper 2 [interleukin (IL)-4/IL-10] and Th3 [transforming growth factor (TGF)-beta] T cells plus CD4+CD25+ regulatory cells and latency-associated peptide+ T cells. Induction of oral tolerance is enhanced by IL-4, IL-10, anti-IL-12, TGF-beta, cholera toxin B subunit, Flt-3 ligand, and anti-CD40 ligand. Oral (and nasal) antigen administration suppresses animal models of autoimmune diseases including experimental autoimmune encephalitis, uveitis, thyroiditis, myasthenia, arthritis, and diabetes in the non-obese diabetic (NOD) mouse, plus non-autoimmune diseases such as asthma, atherosclerosis, graft rejection, allergy, colitis, stroke, and models of Alzheimer's disease. Oral tolerance has been tested in human autoimmune diseases including multiple sclerosis (MS), arthritis, uveitis, and diabetes and in allergy, contact sensitivity to dinitrochlorobenzene (DNCB), and nickel allergy. Although positive results have been observed in phase II trials, no effect was observed in phase III trials of CII in rheumatoid arthritis or oral myelin and glatiramer acetate (GA) in MS. Large placebo effects were observed, and new trials of oral GA are underway. Oral insulin has recently been shown to delay onset of diabetes in at-risk populations, and confirmatory trials of oral insulin are being planned. Mucosal tolerance is an attractive approach for treatment of autoimmune and inflammatory diseases because of lack of toxicity, ease of administration over time, and antigen-specific mechanisms of action. The successful application of oral tolerance for the treatment of human diseases will depend on dose, developing immune markers to assess immunologic effects, route (nasal versus oral), formulation, mucosal adjuvants, combination therapy, and early therapy.

E3 ubiquitin ligases and their control of T cell autoreactivity

A loss of T cell tolerance underlies the development of most autoimmune diseases. The design of therapeutic strategies to reinstitute immune tolerance, however, is hampered by uncertainty regarding the molecular mechanisms involved in the inactivation of potentially autoreactive T cells. Recently, E3 ubiquitin ligases have been shown to mediate the development of a durable state of unresponsiveness in T cells called clonal anergy. In this review, we will discuss the mechanisms used by E3 ligases to control the activation of T cells and prevent the development of autoimmunity.

Regulation of nuclear Prointerleukin-16 and p27Kip1 in primary human T lymphocytes

Prointerleukin-16 (Pro-IL-16) is an abundant, PDZ domain-containing protein expressed in the nucleus and cytoplasm of resting human T lymphocytes. We have previously shown that ectopic expression of Pro-IL-16 in Pro-IL-16-negative human Jurkat cells represses transcription of the F-box protein, Skp2, resulting in accumulation of the cyclin-dependent kinase inhibitor, p27(Kip1), and G0/G1 cell cycle arrest. The current studies demonstrate the kinetics of Pro-IL-16 and p27(Kip1) expression in activated normal human T lymphocytes. We correlate nuclear Pro-IL-16 loss with decreased p27(Kip1) expression, increased cell cycle progression, and proliferation. Conversely, we show that constitutive expression of Pro-IL-16 by retroviral infection of activated human T lymphocytes induces G0/G1 cell cycle arrest, inhibits proliferation, and is associated with increased levels of p27(Kip1). These findings implicate nuclear Pro-IL-16 as a cell cycle regulatory protein for human T lymphocytes.

Hepatitis C virus core protein induces an anergic state characterized by decreased interleukin-2 production and perturbation of mitogen-activated protein kinase responses

Alterations of cytokine responses are thought to favor the establishment of persistent hepatitis C virus (HCV) infections, enhancing the risk of liver cirrhosis and hepatocellular carcinoma. Here we demonstrate that the expression of the HCV core (C) protein in stably transfected T cells correlates with a selective reduction of interleukin-2 (IL-2) promoter activity and IL-2 production in response to T-cell receptor triggering, whereas the activation of IL-4, IL-10, gamma interferon, and tumor necrosis factor alpha was moderately increased. This altered cytokine expression profile was associated with a perturbation of mitogen-activated protein (MAP) kinase responses. Extracellular regulated kinase and p38 were constitutively phosphorylated in C-expressing cells, while triggering of the costimulatory c-Jun N-terminal kinase (JNK) signaling cascade and activation of the CD28 response element within the IL-2 promoter appeared to be impaired. The perturbations of MAP kinase phosphorylation could be eliminated by cyclosporine A-mediated inhibition of nuclear factor of activated T cells, suggesting that the inactivation of JNK signaling and hyporesponsiveness to IL-2 induction were downstream consequences of C-induced Ca(2+) flux in a manner that mimics the induction of clonal anergy.

Cyclosporine Preserves the Anergic State of Human T Cells Induced by Costimulation Blockade In Vitro

BACKGROUND

Costimulation blockade based tolerance-inducing therapies might be disrupted by adjunct conventional immunosuppressive drug use. In the current study, we evaluated the compatibility of various immunosuppressive agents on costimulation blockade-based immunosuppression and T-cell anergy induction of human alloreactive T-cells in vitro. T-cell anergy is crucial in transplantation tolerance.

METHODS

T cell anergy was induced in human mixed lymphocyte cultures in vitro, by monoclonal antibodies directed against the costimulatory ligands CD40 and CD86. The effect of coadministration of conventional immunosuppressive drugs (CsA, rapamycin or FK506) on the inhibitory potential of costimulation blockade and the induction and maintenance of T cell anergy was analyzed.

RESULTS

We found that monoclonal antibodies against CD40 and CD86 and the simultaneous use of conventional immunosuppressive drugs resulted in strong immunosuppression of proliferation and cytokine production. Rapamycin, in contrast to FK506 and CsA, facilitated T-cell apoptosis. However, drug cotreatment prevented costimulation blockade induced T-cell anergy. Induction of human T-cell anergy in vitro required approximately 5 days of culture. Coadministration of drugs at day 5 after the start of mAb treatment, when anergy was established, did not increase the immunosuppressive effect of mAb treatment. But interestingly, in the majority of experiments, in contrast to rapamycin and FK506, CsA did not affect the anergic state when given after T-cell anergy induction. Moreover, the cell death facilitating potential of rapamycin vanished when used later after T-cell activation.

CONCLUSIONS

Timing and choice of conventional drug are crucial in the success of costimulation blockade-based tolerance induction therapies.

A pivotal role for the multifunctional calcium/calmodulin-dependent protein kinase II in T cells: from activation to unresponsiveness

Stimulation of the TCR leads to an oscillatory release of free calcium that activates members of the calcium/calmodulin-dependent protein kinase II (CaMKII) family. The CaMKII molecules have profound and lasting effects on cellular signaling in several cell types, yet the role of CaMKII in T cells is still poorly characterized. In this report we describe a splice variant of CaMKIIbeta, CaMKIIbeta'e, in mouse T cells. We have determined its function, along with that of CaMKIIgamma, by introducing the active and kinase-dead mutants into activated P14 TCR transgenic T cells using retroviral transduction. Active CaMKII enhanced the proliferation and cytotoxic activity of T cells while reducing their IL-2 production. Furthermore, it induced a profound state of unresponsiveness that could be overcome only by prolonged culture in IL-2. These results indicate that members of the CaMKII family play an important role in regulation of CD8 T cell proliferation, cytotoxic effector function, and the response to restimulation.

NFAT proteins: key regulators of T-cell development and function

Since the discovery of the first nuclear factor of activated T cells (NFAT) protein more than a decade ago, the NFAT family of transcription factors has grown to include five members. It has also become clear that NFAT proteins have crucial roles in the development and function of the immune system. In T cells, NFAT proteins not only regulate activation but also are involved in the control of thymocyte development, T-cell differentiation and self-tolerance. The functional versatility of NFAT proteins can be explained by their complex mechanism of regulation and their ability to integrate calcium signalling with other signalling pathways. This Review focuses on the recent advances in our understanding of the regulation, mechanism of action and functions of NFAT proteins in T cells.

Transcriptional Regulation of the HumanCD3γ Gene: The TATA-LessCD3γ Promoter Functions via an Initiator and Contiguous Sp-Binding Elements

Growing evidence that the CD3gamma gene is specifically targeted in some T cell diseases focused our attention on the need to identify and characterize the elusive elements involved in CD3gamma transcriptional control. In this study, we show that while the human CD3gamma and CD3delta genes are oriented head-to-head and separated by only 1.6 kb, the CD3gamma gene is transcribed from an independent but weak, lymphoid-specific TATA-less proximal promoter. Using RNA ligase-mediated rapid amplification of cDNA ends, we demonstrate that a cluster of transcription initiation sites is present in the vicinity of the primary core promoter, and the major start site is situated in a classical initiator sequence. A GT box immediately upstream of the initiator binds Sp family proteins and the general transcription machinery, with the activity of these adjacent elements enhanced by the presence of a second GC box 10 nt further upstream. The primary core promoter is limited to a sequence that extends upstream to -15 and contains the initiator and GT box. An identical GT box located approximately 50 nt from the initiator functions as a weak secondary core promoter and likely generates transcripts originating upstream from the +1. Finally, we show that two previously identified NFAT motifs in the proximal promoter positively (NFATgamma(1)) or negatively (NFATgamma(1) and NFATgamma(2)) regulate expression of the human CD3gamma gene by their differential binding of NFATc1 plus NF-kappaB p50 or NFATc2 containing complexes, respectively. These data elucidate some of the mechanisms controlling expression of the CD3gamma gene as a step toward furthering our understanding of how its transcription is targeted in human disease.

Notch: a unique therapeutic target for immunomodulation

Under normal circumstances, the adaptive immune response to either self or harmless antigens is kept under tight control by a combination of deletion mechanisms in the central immune system, and by a system of regulatory cells in the periphery. Together, these control mechanisms enforce a state referred to as immunological tolerance. Breakdown of these mechanisms lead to a variety of immunological disease states involving persistent immune-mediated pathologies. Whereas the processes inducing central tolerance in the immune system are well documented, the mechanisms by which peripheral regulatory cells function are still unclear. Recent publications have reported an unexpected role for the Notch pathway, itself a classical regulator of cell fate, in the development of regulatory T cells. These exciting data demonstrate that Notch signals modulate events downstream of the T cell receptor, diverting T cell differentiation into alternative fates which regulate immune responses in an antigen-specific manner. The Notch pathway is, therefore, uniquely positioned in the developmental pathways leading to regulatory T cells. In this review, the authors discuss the data surrounding the role of Notch in the peripheral immune system, and discuss how this pathway might be manipulated for the treatment of immunological disorders.

T cell tolerance and autoimmunity

CD4 T cells are the master controllers of immune responses to protein antigens, and many autoimmune diseases are thought to arise from a breakdown of immunological tolerance in CD4 cells. Peripheral tolerance in CD4 T cells is maintained by several mechanisms, including functional anergy, deletion (death) by apoptosis and suppression by regulatory T lymphocytes (Treg). Using transgenic mouse models, we have explored the roles of these mechanisms in tolerance to cell-associated tissue-restricted self-antigens and secreted systemic self-antigens. Tolerance to a membrane form of the antigen expressed in islet beta cells is maintained by Treg, which block T cell differentiation into pathogenic effectors, and by CTLA-4, which increases the activation threshold of T cells and prevents responses to the self-antigen. A systemically produced soluble form of the antigen induces rapid T cell anergy followed by deletion. The induction of anergy does not require either CTLA-4 or Treg, although in the absence of Treg tolerance can be broken more readily by potent immunogenic signals. Encounter with circulating antigen in T cells induces a state of antigen receptor "desensitization" that is associated with a block in proximal receptor-triggered signals. Thus, different mechanisms play dominant roles in T cell tolerance to different types of self-antigens.

Egr-2 and Egr-3 are negative regulators of T cell activation

T cell receptor engagement in the absence of proper accessory signals leads to T cell anergy. E3 ligases are involved in maintaining the anergic state. However, the specific molecules responsible for the induction of anergy have yet to be elucidated. Using microarray analysis we have identified here early growth response gene 2 (Egr-2) and Egr-3 as key negative regulators of T cell activation. Overexpression of Egr2 and Egr3 was associated with an increase in the E3 ubiquitin ligase Cbl-b and inhibition of T cell activation. Conversely, T cells from Egr3(-/-) mice had lower expression of Cbl-b and were resistant to in vivo peptide-induced tolerance. These data support the idea that Egr-2 and Egr-3 are involved in promoting a T cell receptor-induced negative regulatory genetic program.

T-cell tolerance and autoimmunity to systemic and tissue-restricted self-antigens

We have used transgenic mouse models to examine the mechanisms of tolerance in CD4(+) T lymphocytes to soluble, systemic and cell-associated, tissue-restricted self-antigens. Anergy to an islet antigen, as a model of a tissue antigen, is dependent on the inhibitory receptor cytotoxic T-lymphocyte antigen-4 (CTLA-4), and tissue-restricted autoimmunity is inhibited by regulatory T lymphocytes. Anergy to a circulating systemic antigen can occur independently of CTLA-4 signals, and it is induced primarily by a block in proximal receptor-initiated signals. CD4(+)CD25(+) regulatory T cells are generated in response to both forms of self-antigens, but the induction is much more efficient with the tissue antigen. Receptor desensitization can be induced by the systemic antigen even in the absence of regulatory T cells, but tolerance can be broken by immunization much more easily if these cells are absent. Deletion of mature T cells is striking with the systemic antigen; there is little evidence to support peripheral deletion as a mechanism of tolerance to the tissue antigen. Thus, both distinct and overlapping mechanisms account for unresponsiveness to different forms of self-antigens. These results establish a foundation for searching for genetic influences and pathogenic mechanisms in organ-specific and systemic autoimmune diseases.

Ca2+ signaling down-regulates TGF-beta1 gene expression in CD4+ T cells

In the immune system, TGF-beta1 exerts two major functions, anti-inflammatory and immuno-suppressive effects. This work aims to investigate the molecular mechanisms involved in the regulation of the TGF-beta1 gene expression in CD4(+) T cells. The TGF-beta1 gene expresses three transcripts of 2.5, 1.9, and 1.4kb. The 1.9kb mRNA which has the highest translation activity was the major transcript. The relationship between T cell receptor (TCR) stimulation and the expression of the gene was investigated. TCR stimulation with a low dose of antigen peptide enhanced the gene expression, whereas a higher dose suppressed the expression. TCR stimulation activates PKC/MAPK and Ca(2+) signaling pathways. PMA increased the gene expression, whereas ionomycin decreased the gene expression, markedly. The results indicate that Ca(2+) signaling down-regulates TGF-beta1 gene expression. The molecular regulation of TGF-beta1 gene expression is unique when comparing to other cytokine genes which are generally activated by Ca(2+) signaling.

Peripheral Tolerance of CD8 T Lymphocytes

Whereas high-avidity recognition of peptide-MHC complexes by developing T cells in the thymus results in deletion and promotes self-tolerance, such recognition by mature T cells in the periphery results in activation and clonal expansion. This dichotomy represents the basis of a dilemma that has stumped immunologists for many years, how are self-specific T cells tolerized in the periphery? There appear to be two important criteria used to achieve this goal. The first is that in the absence of inflammatory pathogens, tolerance is promoted when T cells recognize antigen presented by quiescent dendritic cells (DCs) expressing low levels of costimulatory molecules. A second critical factor that defines "self" and drives tolerance through deletion, anergy, or suppression is the persistence of antigen.

Everolimus and basiliximab permit suppression by human CD4+CD25+ cells in vitro

Immunosuppressive drugs are essential for the prevention of acute transplant rejection but some may not promote long-term tolerance. Tolerance is dependent on the presence and regulatory function of CD4(+)CD25(+) T cells in a number of animal models. The direct effects of immunosuppressive drugs on CD4(+)CD25(+) cells, particularly those that interfere with IL-2 signaling are uncertain. We studied the effects of the rapamycin derivative everolimus and the anti-CD25 monoclonal antibody basiliximab on the regulatory capacity of human CD4(+)CD25(+) cells in vitro. Both drugs permitted the suppression of proliferation and IFN-gamma secretion by CD4(+)CD25(-) cells responding to allogeneic and other polyclonal stimuli; CTLA-4 expression was abolished on CD4(+)CD25(+) cells without compromising their suppressive ability. Everolimus reduced IFN-gamma secretion by CD4(+)CD25(-) cells before the anti-proliferative effect: this is a novel finding. Exogenous IL-2 and IL-15 could prevent the suppression of proliferation by CD4(+)CD25(+) cells and the drugs could not restore suppression. By contrast, suppression of IFN-gamma secretion was only slightly impeded with the exogenous cytokines. Finally, CD4(+)CD25(+) cells were more resistant than CD4(+)CD25(-) cells to the pro-apoptotic action of the drugs. Together these data suggest that CD4(+)CD25(+) cells may still exert their effects in transplant patients taking immunosuppression that interferes with IL-2 signaling.

Pro-IL-16 Regulation in Activated Murine CD4+ Lymphocytes

Prior DNA microarray studies suggested that IL-16 mRNA levels decrease following T cell activation, a property unique among cytokines. We examined pro-IL-16 mRNA and protein expression in resting and anti-CD3 mAb-activated primary murine CD4(+) T cells. Consistent with the microarray reports, pro-IL-16 mRNA levels fell within 4 h of activation, and this response is inhibited by cyclosporin A. Total cellular pro-IL-16 protein also fell, reaching a nadir at 48 h. Pro-IL-16 comprises a C-terminal cytokine domain and an N-terminal prodomain that are cleaved by caspase-3. Pro-IL-16 expressed in transfected tumor cells was previously shown to translocate to the nucleus and to promote G(0)/G(1) arrest by stabilizing the cyclin-dependent kinase inhibitor p27(Kip1). In the present study, we observed increased S-phase kinase-associated protein 2 mRNA expression in IL-16 null mice, but basal expression and activation-dependent regulation of p27(Kip1) were no different from wild-type mice. Stimulation with anti-CD3 mAb induced transiently greater thymidine incorporation in IL-16-deficient CD4(+) T cells than wild-type controls, but there was no difference in cell survival or in the CFSE dilution profiles. Analysis of CD4(+) T cell proliferation in vivo using BrdU labeling similarly failed to identify a hyperproliferative phenotype in T cells lacking IL-16. These data demonstrate that pro-IL-16 mRNA and protein expression are dynamically regulated during CD4(+) T cell activation by a calcineurin-dependent mechanism, and that pro-IL-16 might influence T cell cycle regulation, although not in a dominant manner.

Two-photon analysis of calcium signals in T lymphocytes of intact lamina propria from human intestine

Lamina propria (LP) T cells of the human intestinal mucosa usually do not develop systemic immune responses despite permanent exposure to foreign antigens. The mechanisms maintaining this hyporeactivity in the normal gut are poorly understood. It is, at present, not clear what role the microenvironment of the mucosa plays for low T cell reactivity and in the pathogenesis of mucosal inflammation. Despite the importance of cytosolic Ca(2+) signals for T lymphocyte activation, intracellular Ca(2+) concentration measurements have so far only been performed in dissociated T cells, following disruption of the microenvironment. We used two-photon technology to measure Ca(2+) signals in identified T lymphocytes within the intact mucosa to minimize impact on tissue integrity while preserving the cellular microenvironment. We show that Ca(2+) signals in LP T cells correlate with the hyporeactivity of T cells in the intestinal immune system and furthermore link Ca(2+) signals with inflammatory bowel disease. Our data implicate that Ca(2+) signals in LP T cells do not depend on the microenvironment of the intact mucosa, since they are very similar to Ca(2+) signals in dissociated LP T cells.

Regulation of phospholipase C-gamma2 networks in B lymphocytes

The modulation of inositol-1,4,5-trisphosphate (IP3), a product of phospholipase C (PLC) activity, is one of a common signaling mechanism used in many biological systems. B lymphocytes also rely on IP3 and subsequent calcium signaling to ensure appropriate developmental outcomes, as well as antigen-specific responses. In establishing the optimal intensity and duration of the PLC-gamma activity, an important role has emerged for adaptor molecules, which direct the appropriate subcellular localization of PLC-gamma and induce its conformational changes. Generated IP3 binds to IP3 receptors located on the endoplasmic reticulum (ER), which in turn is essential for triggering calcium release from the ER and subsequent entry of extracellular calcium by so-called Ca2+ entry channels. Recent data has begun to shed new light on the connection between the calcium release and the influx of extracellular calcium, and the molecular identity of the Ca2+ entry channels.

The Transcription Factor NFAT3 Mediates Neuronal Survival

Neuronal apoptosis is critical for normal development of the mammalian nervous system and also contributes to the pathogenesis of ischemic and degenerative diseases of the brain. Apoptosis of neurons is tightly regulated by extrinsic signals including growth factors and neuronal activity, but the intracellular mechanisms by which these signals promote neuronal survival are incompletely understood. We report that the transcription factor NFAT3 plays a critical role in mediating survival of granule neurons of the developing cerebellum. NFAT3 accumulated in the nucleus of primary granule neurons under survival conditions of serum growth factors and neuronal activity that was elicited by depolarization with high K(+). In contrast, deprivation of serum and K(+), which leads to neuronal apoptosis, triggered NFAT3 nuclear export. Treatment of granule neurons with Li(+), an inhibitor of the NFAT export kinase GSK3, prevented the nuclear export of NFAT3 and increased granule cell survival even under pro-apoptotic conditions. Thus, the nuclear localization of NFAT3 correlated tightly with granule neuron survival. Consistent with a pro-survival function for NFAT3, genetic knockdown of NFAT3 by RNA interference in primary granule neurons led to increased apoptosis even in neurons cultured under survival conditions. Conversely, expression of a constitutively active form of NFAT protected neurons against apoptosis induced by serum withdrawal and low K(+). Taken together, these results reveal an essential function for NFAT3-mediated transcription in neuronal survival that may play important roles in the developing and mature brain.

IL-2 Overcomes the Unresponsiveness but Fails to Reverse the Regulatory Function of Antigen-Induced T Regulatory Cells

Intranasal administration of peptide Ac1-9[4Y], based on the N-terminal epitope of myelin basic protein, can induce CD4(+) T cell tolerance, and suppress experimental autoimmune encephalomyelitis induction. The peptide-induced regulatory T (PI-T(Reg)) cells failed to produce IL-2, but expressed IL-10 in response to Ag and could suppress naive T cell responses in vitro. Analysis of Jak-STAT signaling pathways revealed that the activation of Jak1, STAT3, and STAT5 were induced in tolerant T cells after Ag stimulation in vivo. In addition, the expression of suppressor of cytokine signaling 3 was induced in tolerant T cells, suggesting that cytokines regulate the tolerant state of the PI-T(Reg) cells. Stimulation of PI-T(Reg) cells in vitro with IL-10 induced Jak1 and STAT3 activation, but not STAT5, suggesting that IL-10 is important, but not the only cytokine involved in the development of T cell tolerance. Although IL-2 expression was deficient, stimulation with IL-2 in vitro induced Jak1 and STAT5 activation in PI-T(Reg) cells, restored their proliferative response to antigenic stimulation, and abrogated PI-T(Reg)-mediated suppression in vitro. However, the addition of IL-2 could not suppress IL-10 expression, and the IL-2 gene remained inactive. After withdrawal of IL-2, the PI-T(Reg) cells regained their nonproliferative state and suppressive ability. These results underline the ability of the immune system to maintain tolerance to autoantigens, but at the same time having the ability to overcome the suppressive phenotype of tolerant T cells by cytokines, such as IL-2, during the protective immune response to infection.

The quantal theory of how the immune system discriminates between "self and non-self"

In the past 50 years, immunologists have accumulated an amazing amount of information as to how the immune system functions. However, one of the most fundamental aspects of immunity, how the immune system discriminates between self vs. non-self, still remains an enigma. Any attempt to explain this most intriguing and fundamental characteristic must account for this decision at the level of the whole immune system, but as well, at the level of the individual cells making up the immune system. Moreover, it must provide for a molecular explanation as to how and why the cells behave as they do. The "Quantal Theory", proposed herein, is based upon the "Clonal Selection Theory", first proposed by Sir McFarland Burnet in 1955, in which he explained the remarkable specificity as well as diversity of recognition of everything foreign in the environment. The "Quantal Theory" is built upon Burnet's premise that after antigen selection of cell clones, a proliferative expansion of the selected cells ensues. Furthermore, it is derived from experiments which indicate that the proliferation of antigen-selected cell clones is determined by a quantal, "all-or-none", decision promulgated by a critical number of cellular receptors triggered by the T Cell Growth Factor (TCGF), interleukin 2 (IL2). An extraordinary number of experiments reported especially in the past 20 years, and detailed herein, indicate that the T cell Antigen Receptor (TCR) behaves similarly, and also that there are several critical numbers of triggered TCRs that determine different fates of the T cells. Moreover, the fates of the cells appear ultimately to be determined by the TCR triggering of the IL2 and IL2 receptor (IL2R) genes, which are also expressed in a very quantal fashion. The "Quantal Theory" states that the fundamental decisions of the T cell immune system are dependent upon the cells receiving a critical number of triggered TCRs and IL2Rs and that the cells respond in an all-or-none fashion. The "Quantal Theory" accounts fully for the development of T cells in the thymus, and such fundamental cellular fates as both "positive" and "negative" selection, as well as the decision to differentiate into a "Regulatory T cell" (T-Reg). In the periphery, the "Quantal Theory" accounts for the decision to proliferate or not in response to the presence of an antigen, either non-self or self, or to differentiate into a T-Reg. Since the immune system discriminates between self and non-self antigens by the accumulated number of triggered TCRs and IL2Rs, therapeutic manipulation of the determinants of these quantal decisions should permit new approaches to either enhance or dampen antigen-specific immune responses.

Early Growth Response Gene-2, a Zinc-Finger Transcription Factor, Is Required for Full Induction of Clonal Anergy in CD4+ T Cells

Ag-specific immune tolerance results from the induction of cellular mechanisms that limit T cell responses to selective Ags. One of these mechanisms is characterized by attenuated proliferation and decreased IL-2 production in fully stimulated CD4(+) Th cells and is denoted T cell anergy. We report the identification of the early growth response gene (Egr-2; Krox-20), a zinc-finger transcription factor, as a key protein required for induction of anergy in cultured T cells. Gene array screening revealed high Egr-2 expression distinctly persists in anergized but not proliferating murine A.E7 T cells. In contrast, Egr-1, a related family member induced upon costimulation, displays little or no expression in the anergic state. IL-2-mediated abrogation of anergy causes rapid depletion of Egr-2 protein. Full stimulation of anergic A.E7 T cells fails to enhance IL-2 and Egr-1 expression, whereas Egr-2 expression is greatly increased. Silencing Egr-2 gene expression by small interfering RNA treatment of cultured A.E7 T cells before incubation with anti-CD3 alone prevents full induction of anergy. However, small interfering RNA-mediated depletion of Egr-2 5 days after anergy induction does not appear to abrogate hyporesponsiveness to stimulation. These data indicate that sustained Egr-2 expression is necessary to induce a full anergic state through the actions of genes regulated by this transcription factor.

Calcium-dependent activation of T-lymphocytes

Activation of T-lymphocytes requires stimulation of T-cell receptors (TCR) and co-stimulatory signals. Among different signalling cascades, TCR engagement induces Ca(2+) entry through plasma membrane Ca(2+) channels, which is an indispensable step for T-cells to expand clonally and to acquire effector functions. The Ca(2+) channels are activated by depletion of Ca(2+) stores and are called Ca(2+) release-activated Ca(2+) (CRAC) channels. Ca(2+) influx through CRAC channels is also controlled, directly or indirectly, by K(+) channels, Ca(2+)-ATPases, mitochondria, endoplasmic reticulum and Ca(2+) buffers. We review the functional implications of these transporters, organelles and buffers and develop a model of Ca(2+) signal generation that depends mainly on their relative mutual localization. This model offers the possibility of controlling amplitude and kinetics of Ca(2+) signals in T-cells. Decoding of various Ca(2+) signals allows differential activation of the transcription factor families nuclear factor of activated T-cells (NFAT), nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1). Variation of amplitude and kinetics of Ca(2+) signals thus is an important mechanism for modulating the specificity of T-cell responses.

Fas deficiency prevents type 1 diabetes by inducing hyporesponsiveness in islet beta-cell-reactive T-cells

Type 1 diabetes is an autoimmune disease wherein autoreactive T-cells promote the specific destruction of pancreatic islet beta-cells. Evidence for a crucial role for Fas/FasL interactions in this destruction has been highly controversial because of the pleiotropic effects of Fas deficiency on the lymphoid and other systems. Fas-deficient mice are protected from spontaneous development of diabetes not because Fas has a role in the destruction of beta-cells, but rather because insulitis is abrogated. Fas may somehow be involved in the series of events provoking insulitis; for example, it may play a role in the physiological wave of beta-cell death believed to result in the export of pancreatic antigens to the pancreatic lymph nodes and, thereby, to circulating, naive, diabetogenic T-cells for the first time. To explore the implication of Fas in these events, we crossed the lpr mutation into the BDC2.5 model of type 1 diabetes to make it easier to monitor direct effects on the pathogenic specificity. We demonstrated that BDC2.5/NOD(lpr/lpr) mice have qualitatively and quantitatively less aggressive insulitis than do BDC2.5/NOD mice. In vitro proliferation assays showed that BDC2.5/NOD(lpr/lpr) splenocytes proliferated less vigorously than those from control mice in the presence of islet extracts, which reflects their inability to produce interleukin-2, resulting in weaker pathogenicity.

Nicotine activates nuclear factor of activated T cells c2 (NFATc2) and prevents cell cycle entry in T cells

We used primary peripheral blood T cells, a population that exists in G(0) and can be stimulated to enter the cell cycle synchronously, to define more precisely the effects of nicotine on pathways that control cell cycle entry and progression. Our data show that nicotine decreased the ability of T cells to transit through the G(0)/G(1) boundary (acquire competence) and respond to progression signals. These effects were due to nuclear factor of activated T cells c2 (NFATc2)-dependent repression of cyclin-dependent kinase 4 (CDK4) expression. Growth arrest at the G(0)/G(1) boundary was further enforced by inhibition of cyclin D2 expression and by increased expression and stabilization of p27Kip1. Intriguingly, T cells from habitual users of tobacco products and from NFATc2-deficient mice constitutively expressed CDK4 and were resistant to the antiproliferative effects of nicotine. These results indicate that nicotine impairs T cell cycle entry through NFATc2-dependent mechanisms and suggest that, in the face of chronic nicotine exposure, selection may favor cells that can evade these effects. We postulate that cross talk between nicotinic acetylcholine receptors and growth factor receptor-activated pathways offers a novel mechanism by which nicotine may directly impinge on cell cycle progression. This offers insight into possible reasons that underlie the unique effects of nicotine on distinct cell types and identifies new targets that may be useful control tobacco-related diseases.

Treatment of refractory autoimmune diseases with ablative immunotherapy

Immunological manipulations are the basis of modern therapy for refractory autoimmune diseases (AID). Ablative chemotherapy with stem cell support (autotransplant) as well as targeted immunotherapy using specific monoclonal antibodies, such as rituximab and campath 1-H have become acceptable second line therapy for severe refractory AID. Until now, more than 500 autotransplants have been performed worldwide for various autoimmune disorders, including multiple sclerosis (MS), systemic sclerosis (SSc), systemic lupus erythematosis (SLE) and rheumatoid arthritis (RA) with encouraging results, although transplant related mortality (TRM) in the range between 2 and 17% still remains one of the major limitations of the procedure. Immunotherapy is a relatively safe approach associating with sustained remissions in a considerable proportion of treated patients. Better selection of patients and earlier immunotherapy, preceded an irreversible organ damage might further improve the clinical outcome of patients with AID.

Cyclic adenosine 5'-monophosphate response element modulator is responsible for the decreased expression of c-fos and activator protein-1 binding in T cells from patients with systemic lupus erythematosus

T cells from patients with systemic lupus erythematosus express increased levels of the cAMP response element modulator (CREM) that has been shown to bind to the IL-2 promoter and suppress its activity. In this study, we demonstrate that CREM binds to the proximal promoter of the c-fos proto-oncogene in live systemic lupus erythematosus T cells and represses its expression following stimulation in vitro. Decreased levels of c-fos protein result in decreased AP-1 activity, as determined in shift assays. Blockade of the translation of CREM mRNA with an antisense CREM vector increases the expression of c-fos and the AP-1 activity. The levels of c-fos mRNA vary with disease activity. We conclude that CREM represses the expression of c-fos and limits the activity of the enhancer AP-1. Thus, CREM is involved indirectly in the modulation of transcriptional regulation of multiple genes including IL-2.

Regulation of the maintenance of peripheral T-cell anergy by TAB1-mediated p38 alpha activation

In anergic T cells, T-cell receptor (TCR)-mediated responses are functionally inactivated by negative regulatory signals whose mechanisms are poorly understood. Here, we show that CD4(+) T cells anergized in vivo by superantigen Mls-1(a) express a scaffolding protein, transforming growth factor beta-activated protein kinase 1-binding protein 1 (TAB1), that negatively regulates TCR signaling through the activation of mitogen-activated protein kinase p38 alpha. TAB1 was not expressed in naive and activated CD4(+) T cells. Inhibition of p38 activity in anergic T cells by a chemical inhibitor resulted in the recovery of interleukin 2 (IL-2) and the inhibition of IL-10 secretion. T-cell hybridoma 2B4 cells transduced with TAB1-containing retrovirus (TAB1-2B4 cells) showed activated p38 alpha, inhibited extracellular signal-regulated kinase (ERK) activity, culminating in reduced IL-2 levels and increased IL-10 production. The use of a p38 inhibitor or cotransfection of a dominant-negative form of p38 in TAB1-2B4 cells resulted in the recovery of ERK activity and IL-2 production. These results imply that TAB1-mediated activation of p38 alpha in anergic T cells regulates the maintenance of T-cell unresponsiveness both by inhibiting IL-2 production and by promoting IL-10 production.

Critical roles of c-Jun signaling in regulation of NFAT family and RANKL-regulated osteoclast differentiation

Receptor activator of NF-kappaB ligand (RANKL) plays an essential role in osteoclast formation and bone resorption. Although genetic and biochemical studies indicate that RANKL regulates osteoclast differentiation by activating receptor activator of NF-kappaB and associated signaling molecules, the molecular mechanisms of RANKL-regulated osteoclast differentiation have not yet been fully established. We investigated the role of the transcription factor c-Jun, which is activated by RANKL, in osteoclastogenesis using transgenic mice expressing dominant-negative c-Jun specifically in the osteoclast lineage. We found that the transgenic mice manifested severe osteopetrosis due to impaired osteoclastogenesis. Blockade of c-Jun signaling also markedly inhibited soluble RANKL-induced osteoclast differentiation in vitro. Overexpression of nuclear factor of activated T cells 1 (NFAT1) (NFATc2/NFATp) or NFAT2 (NFATc1/NFATc) promoted differentiation of osteoclast precursor cells into tartrate-resistant acid phosphatase-positive (TRAP-positive) multinucleated osteoclast-like cells even in the absence of RANKL. Overexpression of NFAT1 also markedly transactivated the TRAP gene promoter. These osteoclastogenic activities of NFAT were abrogated by overexpression of dominant-negative c-Jun. Importantly, osteoclast differentiation and induction of NFAT2 expression by NFAT1 overexpression or soluble RANKL treatment were profoundly diminished in spleen cells of the transgenic mice. Collectively, these results indicate that c-Jun signaling in cooperation with NFAT is crucial for RANKL-regulated osteoclast differentiation.

E3 ubiquitin ligases as T cell anergy factors

E3 ubiquitin ligases have emerged as key molecular regulators of immune cell function. Three families of proteins with ubiquitin ligase activity have been described (the HECT, RING and U-box proteins), and each may be involved in the regulation of immune responses during infection by targeting specific inhibitory molecules for proteolytic destruction. Several HECT and RING E3 proteins have now also been linked to the induction and maintenance of immune self-tolerance: c-Cbl, Cbl-b, GRAIL, Itch and Nedd4 each negatively regulate T cell growth factor production and proliferation. This review will discuss the relationship between the ubiquitination of select components of the antigen-sensing signaling apparatus in T cells and the development and maintenance of the clonal anergy state.

Schnurri-3 (KRC) interacts with c-Jun to regulate the IL-2 gene in T cells

The activator protein 1 (AP-1) transcription factor is a key participant in the control of T cell proliferation, cytokine production, and effector function. In the immune system, AP-1 activity is highest in T cells, suggesting that a subset of T cell–specific coactivator proteins exist to selectively potentiate AP-1 function. Here, we describe that the expression of Schnurri-3, also known as κ recognition component (KRC), is induced upon T cell receptor signaling in T cells and functions to regulate the expression of the interleukin 2 (IL-2) gene. Overexpression of KRC in transformed and primary T cells leads to increased IL-2 production, whereas dominant-negative KRC, or loss of KRC protein in KRC-null mice, results in diminished IL-2 production. KRC physically associates with the c-Jun transcription factor and serves as a coactivator to augment AP-1–dependent IL-2 gene transcription.

The magnitude of TCR engagement is a critical predictor of T cell anergy or activation

Fast dissociation rate of peptide-MHC complexes from TCR has commonly been accepted to cause T cell anergy. In this study, we present evidence that peptides that form transient complexes with HLA-DR1 induce anergy in T cell clones in vitro and specific memory T cells in vivo. We demonstrate that similar to the low densities of long-lived agonist peptide-MHC, short-lived peptide-MHC ligands induce anergy by engagement of approximately 1000 TCR and activation of a similar pattern of intracellular signaling events. These data strongly suggest that short-lived peptides induce anergy by presentation of low densities of peptide-MHC complexes. Moreover, they suggest that the traditional antagonist peptides might also trigger anergy by a similar molecular mechanism. The use of short-lived peptides to induce T cells anergy is a potential strategy for the prevention or treatment of autoimmune diseases.

Ubiquitin ligases and the immune response

Ubiquitin (Ub)-protein conjugation represents a novel means of posttranscriptional modification in a proteolysis-dependent or -independent manner. E3 Ub ligases play a key role in governing the cascade of Ub transfer reactions by recognizing and catalyzing Ub conjugation to specific protein substrates. The E3s, which can be generally classified into HECT-type and RING-type families, are involved in the regulation of many aspects of the immune system, including the development, activation, and differentiation of lymphocytes, T cell-tolerance induction, antigen presentation, immune evasion, and virus budding. E3-promoted ubiquitination affects a wide array of biological processes, such as receptor downmodulation, signal transduction, protein processing or translocation, protein-protein interaction, and gene transcription, in addition to proteasome-mediated degradation. Deficiency or mutation of some of the E3s like Cbl, Cbl-b, or Itch, causes abnormal immune responses such as autoimmunity, malignancy, and inflammation. This review discusses our current understanding of E3 Ub ligases in both innate and adaptive immunity. Such knowledge may facilitate the development of novel therapeutic approaches for immunological diseases.

Essential Role of the E3 Ubiquitin Ligase Cbl-b in T Cell Anergy Induction

Antigen-specific immunotolerance limits the expansion of self-reactive T cells involved in autoimmune diseases. Here, we show that the E3 ubiquitin ligase Cbl-b is upregulated in T cells after tolerizing signals. Loss of Cbl-b in mice results in impaired induction of T cell tolerance both in vitro and in vivo. Importantly, rechallenge of Cbl-b mutant mice with the tolerizing antigen results in massive lethality. Moreover, ablation of Cbl-b resulted in exacerbated autoimmunity. Mechanistically, loss of Cbl-b rescues reduced calcium mobilization of anergic T cells, which was attributed to Cbl-b-mediated regulation of PLCgamma-1 phosphorylation. Our results show a critical role for Cbl-b in the regulation of peripheral tolerance and anergy of T cells.

The novel cyclophilin binding compound, sanglifehrin A, disassociates G1 cell cycle arrest from tolerance induction

T cell anergy has been demonstrated to play a role in maintaining peripheral tolerance to self Ags as well as a means by which tumors can evade immune destruction. Although the precise pathways involved in anergy induction have yet to be elucidated, it has been linked to TCR engagement in the setting of cell cycle arrest. Indeed, rapamycin, which inhibits T cell proliferation in G(1), has the ability to promote tolerance even in the presence of costimulation. To better define the role of the cell cycle in regulating anergy induction, we used the novel cyclophilin-binding ligand, sanglifehrin A (SFA). We demonstrate that SFA can inhibit TCR-induced cytokine and chemokine production without preventing TCR-induced anergy. Our data also indicate that despite its ability to induce G(1) arrest, SFA does not induce anergy in the presence of costimulation. Furthermore, although SFA blocks proliferation to exogenous IL-2, it does not prevent IL-2-induced reversal of anergy. When we examined the phosphorylation of 4EBP-1, a downstream substrate of the mammalian target of rapamycin, we found that rapamycin, but not SFA, inhibited the mammalian target of rapamycin activity. Based on these data, we propose that the decision as to whether TCR engagement will lead to productive activation or tolerance is dictated by a rapamycin -inhibitable pathway, independent of the G(1)-->S phase cell cycle progression.

Differences in the Kinetics, Amplitude, and Localization of ERK Activation in Anergy and Priming Revealed at the Level of Individual Primary T Cells by Laser Scanning Cytometry

One of the potential mechanisms of peripheral tolerance is the unresponsiveness of T cells to secondary antigenic stimulation as a result of the induction of anergy. It has been widely reported that antigenic unresponsiveness may be due to uncoupling of MAPK signal transduction pathways. However, such signaling defects in anergic T cell populations have been mainly identified using immortalized T cell lines or T cell clones, which do not truly represent primary Ag-specific T cells. We have therefore attempted to quantify signaling events in murine primary Ag-specific T cells on an individual cell basis, using laser-scanning cytometry. We show that there are marked differences in the amplitude and cellular localization of phosphorylated ERK p42/p44 (ERK1/2) signals when naive, primed and anergic T cells are challenged with peptide-pulsed dendritic cells. Primed T cells display more rapid kinetics of phosphorylation and activation of ERK than naive T cells, whereas anergic T cells display a reduced ability to activate ERK1/2 upon challenge. In addition, the low levels of pERK found in anergic T cells are distributed diffusely throughout the cell, whereas in primed T cells, pERK appears to be targeted to the same regions of the cell as the TCR. These data suggest that the different consequences of Ag recognition by T cells are associated with distinctive kinetics, amplitude, and localization of MAPK signaling.

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

Peripheral tolerance is an important strategy used by the immune system to prevent self-reactive lymphocytes from attacking host tissues. A variety of mechanisms contribute to peripheral tolerance, among them activation-induced cell death, suppression by regulatory T cells, and T cell anergy or unresponsiveness. Recent work has led to a better understanding of the cell-intrinsic program that establishes T cell anergy. A major insight is that during the induction phase of anergy, incomplete stimulation (T cell receptor stimulation without costimulation) leads via calcium influx to an altered gene expression program that includes up-regulation of several E3 ubiquitin ligases. When the anergic T cells contact antigen-presenting cells, intracellular signaling proteins are monoubiquitinated and targeted for lysosomal degradation, thus decreasing intracellular signaling and also resulting in decreased stability of the T cell-antigen-presenting cell contact. We propose a molecular program leading to T cell anergy and discuss other proteins that may play a role.

The Cbl Family and Other Ubiquitin Ligases

Regulation of tyrosine kinase-mediated cellular activation through antigen receptors is of great biological and practical significance. The evolutionarily conserved Cbl family ubiquitin ligases have emerged as key negative regulators of activated tyrosine kinase-coupled receptors, and their impaired function switches a normal immune response into autoimmunity. Cbl proteins facilitate the ubiquitinylation of activated tyrosine kinases and other signaling proteins and of the signaling chains of receptors themselves; monoubiquitin tag promotes sorting of activated receptors and associated proteins into internal vesicles of the multivesicular body, facilitating their lysosomal degradation, whereas polyubiquitin tag promotes proteasomal degradation. Notably, increased expression of Cbl proteins and other ubiquitin ligases is a component of anergic signaling program in T cells. Thus, controlled destruction of the signaling apparatus has emerged as a key to fine-tuning antigen receptor signaling. Further studies of this pathway are likely to elucidate the pathogenesis of autoimmune diseases and offer new therapeutic targets.

Role of NFAT proteins in IL13 gene transcription in mast cells

Th2 and mast cells are participants in the asthmatic response to allergens, and both cell types produce the cytokines interleukin (IL)-4 and IL-13. IL-13 in particular is both necessary and sufficient for experimental models of asthma. The transcription factor NFAT plays a central role in cytokine transcriptional regulation in both cell types. Here, we analyze the molecular basis of IL13 gene transcription in Th2 and mast cells. We show that NFAT1 is the major NFAT protein involved in regulating IL13 transcription in mast cells. Although NFAT2 is correctly expressed and regulated in mast cells, it does not contribute to IL13 gene transcription as shown by analysis of cells lacking NFAT2 and cells expressing a constitutively active version of NFAT2. The difference between NFAT1 and NFAT2 appears to be due to a preferential synergistic interaction of NFAT1 with GATA proteins at the IL13 promoter. We suggest that mast cells lack a co-activator protein that stabilizes the binding of NFAT2 to the IL13 promoter by interacting either with NFAT2 itself or with a DNA-bound complex of NFAT2 and GATA proteins.

The Gene Related to Anergy in Lymphocytes, an E3 Ubiquitin Ligase, Is Necessary for Anergy Induction in CD4 T Cells

Acquisition of the anergy phenotype in T cells is blocked by inhibitors of protein synthesis and calcineurin activity, suggesting that anergic T cells may have a unique genetic program. Retroviral transduction of hemopoietic stem cells from TCR transgenic mice and subsequent reconstitution of syngeneic mice to express the E3 ubiquitin ligase, gene related to anergy in lymphocytes (GRAIL), or an enzymatically inactive form, H2N2 GRAIL, allowed analysis of the role of GRAIL in T cell anergy in vivo. Constitutive expression of GRAIL was sufficient to render naive CD4 T cells anergic, however, when the enzymatically inactive form H2N2 GRAIL was expressed, it functioned as a dominant negative of endogenous GRAIL and blocked the development of anergy. These data provide direct evidence that a biochemical pathway composed of GRAIL and/or GRAIL-interacting proteins is important in the development of the CD4 T cell anergic phenotype in vivo.

Dual role of sumoylation in the nuclear localization and transcriptional activation of NFAT1

The nuclear import of nuclear factor of activated T cells (NFAT) transcription factors is critical for regulating NFAT activity. Here we demonstrate that the sumoylation of NFAT1 defines a novel mechanism of the nuclear anchorage and transcriptional activation downstream from the known mechanism of calcineurin-mediated dephosphorylation and nuclear import. We show that Lys(684) and Lys(897) of NFAT1 can be sumoylated. The sumoylation at Lys(684) is required for NFAT1 transcriptional activity and subsequent sumoylation of Lys(897), whereas the sumoylation of Lys(897) is only required for nuclear anchorage. Because Lys(897) of NFAT1 is not conserved among other members of the NFAT family, we propose that sumoylation of Lys(897) may provide a mechanism for NFAT1 isotype-specific regulation of nuclear anchorage and transcriptional activation. Furthermore, we found that treatment with both ionomycin and phorbol 12-myristate 13-acetate ensured efficient nuclear anchorage with the recruitment of NFAT1 into the SUMO-1 bodies, whereas treatment with ionomycin alone induced nuclear translocation of NFAT1 but not recruitment into the SUMO-1 bodies. Our results suggest that the recruitment of NFAT1 into SUMO-1 bodies may be required for the progressive transcriptional activity of NFAT1 upon co-stimulation with ionomycin and phorbol 12-myristate 13-acetate, whereas anergic transcription stimulated by ionomycin alone may occur without recruitment into the SUMO-1 bodies.

T-cell anergy

Self-reactive T cells that escape negative selection in the thymus must be inactivated in the periphery. Anergy constitutes one means of imposing peripheral tolerance. Anergic T cells are functionally inactivated and unable to initiate a productive response even when antigen is encountered in the presence of full co-stimulation. Recent studies have provided new insights into the mechanisms responsible for the induction and maintenance of T-cell anergy. These studies have helped clarify the nature of the signals that induce tolerance, the cells able to deliver them and the molecular processes that underlie the unresponsive state.

A B cell receptor with two Igalpha cytoplasmic domains supports development of mature but anergic B cells

B cell receptor (BCR) signaling is mediated through immunoglobulin (Ig)alpha and Igbeta a membrane-bound heterodimer. Igalpha and Igbeta are redundant in their ability to support early B cell development, but their roles in mature B cells have not been defined. To examine the function of Igalpha-Igbeta in mature B cells in vivo we exchanged the cytoplasmic domain of Igalpha for the cytoplasmic domain of Igbeta by gene targeting (Igbetac-->alphac mice). Igbetac-->alphac B cells had lower levels of surface IgM and higher levels of BCR internalization than wild-type B cells. The mutant B cells were able to complete all stages of development and were long lived, but failed to differentiate into B1a cells. In addition, Igbetac-->alphac B cells showed decreased proliferative and Ca2+ responses to BCR stimulation in vitro, and were anergic to T-independent and -dependent antigens in vivo.