Negative transcriptional regulation in anergic T cells

Transcription elements AREB6 and miR-34a affect apoptosis of PAMs by regulating the expression of SS2-related gene PPP1R11

In our previous work, gene PPP1R11 (protein phosphatase 1 regulatory subunit 11) was significantly expressed in pigs after Streptococcus suis 2 (SS2) challenged. This study firstly confirmed that SS2 induced significant expression of PPP1R11 gene in porcine alveolar macrophage (PAM) cells, and apoptosis of PAM cells were observed. After that, the core promoter of porcine PPP1R11 was identified and its transcription factor AREB6 which significantly regulated PPP1R11. We also characterized that the PPP1R11 gene is a target of miR-34a. Further, we found that PPP1R11 helped to inhibit apoptosis of PAM cells under SS2 infecting, through transcription factor AREB6 was negatively correlated with apoptosis whereas miR-34a was positively correlated. Those findings provide a functional connection among the transcription factor AREB6, miR-34a, PPP1R11 gene and apoptosis of PAM cells in the pathogenesis of the SS2 infection.

T cell ignorance is bliss: T cells are not tolerized by Langerhans cells presenting human papillomavirus antigens in the absence of costimulation

Human papillomavirus type 16 (HPV16) infections are intra-epithelial, and thus, HPV16 is known to interact with Langerhans cells (LCs), the resident epithelial antigen-presenting cells (APCs). The current paradigm for APC-mediated induction of T cell anergy is through delivery of T cell receptor signals via peptides on MHC molecules (signal 1), but without costimulation (signal 2). We previously demonstrated that LCs exposed to HPV16 in vitro present HPV antigens to T cells without costimulation, but it remained uncertain if such T cells would remain ignorant, become anergic, or in the case of CD4+ T cells, differentiate into Tregs. Here we demonstrate that Tregs were not induced by LCs presenting only signal 1, and through a series of in vitro immunizations show that CD8⁺ T cells receiving signal 1+2 from LCs weeks after consistently receiving signal 1 are capable of robust effector functions. Importantly, this indicates that T cells are not tolerized but instead remain ignorant to HPV, and are activated given the proper signals.

The role of indoleamine 2,3 dioxygenase in the induction of immune tolerance in organ transplantation

The aim of this review is to present current information on transplantation research regarding the role of indoleamine 2,3 dioxygenase in immune regulation. We present the basic mechanisms by which the enzyme is expressed, followed by tryptophan catabolism that leads to midg1 phase arrest and apoptosis. Other effects proposed, although not yet completely proven and generally accepted, include T-cell development suppression, secretion of regulatory cytokines such as IL10, and generation of new T regulatory cells. Clinical studies are being performed worldwide; thus, our goal is to focus on the clinical potential relevance of the enzyme rather than a presentation on a molecular basis so that health care providers concerning transplantation are aware of this promising field in immunology and therapeutics. We do emphasize the fact that information regarding the role of indoleamine 2,3 dioxygenase in human beings is still scarce.

Regulation of T-cell tolerance by calcium/NFAT signaling

Cells that escape negative selection in the thymus must be inactivated or eliminated in the periphery through a series of mechanisms that include the induction of anergy, dominant suppression by regulatory T cells, and peripheral deletion of self-reactive T cells. Calcium signaling plays a central role in the induction of anergy in T cells, which become functionally inactivated and incapable of proliferating and expressing cytokines following antigen re-encounter. Suboptimal stimulation of T cells results in the activation of a calcium/calcineurin/nuclear factor of activated T cells-dependent cell-intrinsic program of self-inactivation. The proteins encoded by those genes are required to impose a state of functional unresponsiveness through different mechanisms that include downregulation of T-cell receptor signaling and inhibition of cytokine transcription.

IDO-expressing regulatory dendritic cells in cancer and chronic infection

Immune evasion and T cell tolerance induction have been associated both with malignant disease and chronic infection. In recent years, increasing evidence has been accumulated that antigen-presenting cells such as dendritic cells (DC) play a major role in immune regulation. They are not only involved in the induction of immunity but also can inhibit immune responses. Interesting parallels for major molecular mechanisms involved in turning DC from stimulatory to regulatory cells have been uncovered between malignant disease and chronic infection. Apparently, not only inhibitory cytokines such as IL-10 seem to play a role, but also metabolic mechanisms dysregulating tryptophan metabolism, thereby, leading to inhibition of T cells and pathogens. We focus here on recent findings establishing the tryptophan catabolizing enzyme indoleamine-pyrrole 2,3 dioxygenase (IDO) as a central feature of DC with regulatory function both in cancer and chronic infection. Induction of enzymatically active IDO can be triggered by various soluble and membrane-bound factors, and in general, require interferon (IFN) signaling. In addition, based on the most recently established link between tumor necrosis factor alpha (TNFalpha), prostaglandin E2 and IDO, a new model of regulation of IDO in context of cancer and infection is proposed. In light of the increasing use of anti-TNFalpha drugs, these findings are also of great interest to the clinician scientist.

Transcriptional regulation of T cell tolerance

Self-reactive T cells that escape negative selection in the thymus must be kept under control in the periphery. Mechanisms of peripheral tolerance include deletion or functional inactivation of self-reactive T cells and mechanisms of dominant tolerance mediated by regulatory T cells. In the absence of costimulation, T cell receptor (TCR) engagement results in unopposed calcium signaling that leads to the activation of a cell-intrinsic program of inactivation, which makes T cells hyporesponsive to subsequent stimulations. The activation of this program in anergic T cells is a consequence of the induction of a nuclear factor of activated T cells (NFAT)-dependent program of gene expression. Recent studies have offered new insights into the mechanisms responsible for the implementation and maintenance of T cell anergy and have provided evidence that the proteins encoded by the genes upregulated in anergic T cells are responsible for the implementation of anergy by interfering with TCR signaling or directly inhibiting cytokine gene transcription.

Alterations in T cell signal transduction by M. leprae antigens is associated with downregulation of second messengers PKC, calcium, calcineurin, MAPK and various transcription factors in leprosy patients

Mycobacterium leprae, the causative agent of leprosy, challenges host defense mechanism by impairing the signal transduction of T cells which leads to downregulation of T cell proliferation, mainly as a consequence of interference with IL-2 production. In this study we sought to identify how soluble forms of M. leprae antigen(s) or particulate (liposome) delivery of the same antigens with two immunomodulators Murabutide and T cell peptide of Trat protein influence the transcription of IL-2 gene in anergic T cells of lepromatous patients. It was demonstrated that MLCwA/ManLAM stimulated cells of BL/LL patients showed defects in both jun-NH2-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) activities there by resulting in decreased AP-1 activity. Additionally these cells showed reduced calcium levels, PKC activity and calcineurin (CN) activity. This led to impaired nuclear translocation of NFkappaB and NFAT in these patients. In contrast, when same M. leprae antigen(s) were incorporated with the two immunomodulators in liposomal form, increased transcription of IL-2 gene was observed especially in BL/LL patients which appears to be due to, at least in part, to increased expression of AP-1 Fos and Jun family members, NFkappaB and NFAT1 proteins. The increased expression of these transcription factors correlated with increased ERK/JNK, PKC and CN activities in these patients. Since activation of ERK/JNK/PKC kinases and CN phosphatase are required for stimulation of IL-2 transcription, these data provide a molecular explanation for the block in IL-2 production by M. leprae antigens. Thus the above study revealed suppression of all the three distinct biochemical pathways, viz. Ca-CN-NFAT pathway, PKC-NF-kappaB pathway, and MAPK-AP-1 pathway by M. leprae antigen(s) in anergized T cells of lepromatous patients which were activated by liposomal delivery of M. leprae antigens containing the two immunomodulators leading to optimal induction of IL-2 gene expression, which was required for the activation, and proliferation of T cells in lepromatous patients.

Does our current understanding of the molecular basis of immune tolerance predict new therapies for autoimmune disease?

The creation of specific immune tolerance has often been referred to as the ultimate goal of immunotherapy, because it would allow autoimmune disease to be reversed without the need for nonspecific and potentially harmful immunosuppressive therapy. Studies performed during the past decade have been immensely fruitful in terms of advances in our understanding of the cellular and molecular mechanisms of immune tolerance, and have paved the way for successful exploitation of these mechanisms for therapeutic purposes. Important developments include an increased understanding of central and peripheral tolerance, and treatment strategies that mimic the mechanisms behind deletion of self-reactive cells, the identification of crucial gene products that are involved in the induction of anergy, and the characterization of regulatory T cells and protocols for their induction and expansion for therapeutic applications. These landmarks of immune-tolerance research are summarized and their potential use in the immunotherapy of autoimmune disease discussed.

In vivo anergized T cells form altered immunological synapses in vitro

T cells contact allogeneic antigen presenting cells (APCs) and assemble, at their contact interface, a molecular platform called the immunological synapse. Synapse-based molecules provide directional signals for the T cell--either positive signals, resulting in T-cell activation, or negative signals causing T-cell inactivation or anergy. To better understand the molecular basis of in vivo T-cell anergy we analyzed the contacts made between in vivo anergized T cells and APCs, and determined which signaling molecules were included or excluded from their immunological synapses. Anergy was induced in TCR transgenic mice by the intravenous injection of semiallogeneic donor spleen cells. T cells from anergized mice were mixed with APCs, the T-cell/APC synapses imaged using deconvolution microscopy, and their molecular compositions were determined. T cells from anergic mice formed unstable immunological synapses in vitro with allogeneic APCs and failed to recruit the signaling proteins necessary to initiate T-cell activation. These findings suggest that T-cell anergy induced by exposure to semiallogeneic donor cells is associated with defects in the earliest events of T-cell activation, immunological synapse formation and recruitment of TCR-mediated signaling proteins.

Both integrated and differential regulation of components of the IL-2/IL-2 receptor system

Interleukin-2 was discovered in 1976 as a T-cell growth factor. It was the first type I cytokine cloned and the first for which a receptor component was cloned. Its importance includes its multiple actions, therapeutic potential, and lessons for receptor biology, with three components differentially combining to form high, intermediate, and low-affinity receptors. IL-2Ralpha and IL-2Rbeta, respectively, are markers for double-negative thymocytes and regulatory T-cells versus memory cells. gamma(c), which is shared by six cytokines, is mutated in patients with X-linked severe-combined immunodeficiency. We now cover an under-reviewed area-the regulation of genes encoding IL-2 and IL-2R components, with an effort to integrate/explain this knowledge.

The induction and maintenance of T cell anergy

While the "true" role of T cell anergy in promoting peripheral tolerance continues to be debated, it is clear that studying the various models of anergy have led to important insight in terms of understanding the pathways and molecules responsible for T cell activation and inhibition. This review will examine our current understanding of CD4+ T cell anergy. In particular, it will focus on the signaling pathways responsible for both the induction and maintenance of anergy. Furthermore, it will examine how specifically targeting these pathways can be exploited clinically in terms of promoting tolerance in transplantation and autoimmunity and inhibiting tumor-induced tolerance in the case of tumor-immunotherapy.

Ectopic B-Raf expression enhances extracellular signal-regulated kinase (ERK) signaling in T cells and prevents antigen-presenting cell-induced anergy

T cells that receive stimulation through the T cell receptor (TCR) in the absence of costimulation become anergic and are refractory to subsequent costimulation. This unresponsiveness is associated with the constitutive activation of the small G protein, Rap1, and the lack of Ras-dependent activation of ERK. Recent studies suggest that Rap1 can activate the MAP kinase kinase kinase B-Raf that is either endogenously or ectopically expressed. Peripheral T cells generally do not express B-Raf; therefore, to test the hypothesis that ectopic expression of B-Raf could permit Rap1 to activate ERK signaling, we generated transgenic mice expressing B-Raf within peripheral T cells. This converted Rap1 into an activator of ERK, to enhance ERK activation and proliferation following TCR engagement in the absence of costimulation. When T cells were incubated with engineered APCs presenting antigen on I-Ek and expressing low levels of B7, they became anergic, displayed constitutive activation of Rap1, and were deficient in Ras and ERK activation. However, when incubated with the same APCs, T cells expressing the B-Raf transgene proliferated upon restimulation and displayed elevated ERK activation. Thus B-Raf expression and enhanced ERK activation is sufficient to prevent anergy in a model of APC-induced T cell anergy. However, studies using anti-TCR antibody-induced anergy showed that the ability of ERKs to reverse T cell anergy is dependent on the anergic model utilized.

T cell anergy

T cell anergy is a tolerance mechanism in which the lymphocyte is intrinsically functionally inactivated following an antigen encounter, but remains alive for an extended period of time in a hyporesponsive state. Models of T cell anergy affecting both CD4(+) and CD8(+) cells fall into two broad categories. One, clonal anergy, is principally a growth arrest state, whereas the other, adaptive tolerance or in vivo anergy, represents a more generalized inhibition of proliferation and effector functions. The former arises from incomplete T cell activation, is mostly observed in previously activated T cells, is maintained by a block in the Ras/MAP kinase pathway, can be reversed by IL-2 or anti-OX40 signaling, and usually does not result in the inhibition of effector functions. The latter is most often initiated in naïve T cells in vivo by stimulation in an environment deficient in costimulation or high in coinhibition. Adaptive tolerance can be induced in the thymus or in the periphery. The cells proliferate and differentiate to varying degrees and then downregulate both functions in the face of persistent antigen. The state involves an early block in tyrosine kinase activation, which predominantly inhibits calcium mobilization, and an independent mechanism that blocks signaling through the IL-2 receptor. Adaptive tolerance reverses in the absence of antigen. Aspects of both of the anergic states are found in regulatory T cells, possibly preventing them from dominating initial immune responses to foreign antigens and shutting down such responses prematurely.

Proteome analysis reveals caspase activation in hyporesponsive CD4 T lymphocytes induced in vivo by the oral administration of antigen

The oral administration of antigen can lead to systemic antigen-specific hyporesponsiveness, also known as oral tolerance. This phenomenon is a representative form of immune tolerance to exogenous antigen under physiological conditions. We have previously reported that long term feeding of dietary antigen to ovalbumin-specific T cell receptor (TCR) transgenic mice induced oral tolerance of peripheral T cells with impairment in their TCR-induced calcium-signaling pathway. In this study, we utilized two-dimensional electrophoresis to compare intracellular protein expression patterns of orally tolerant and unsensitized CD4 T cells. We detected 26 increased and 16 decreased protein spots and identified 35 of these by mass spectrometry. The results indicated that the expression of caspases was up-regulated and that the protein levels of intact proteins susceptible to caspase cleavage, such as Grb2-related adaptor downstream of Shc (GADS), were decreased in orally tolerant CD4 T cells. Western blotting experiments confirmed that expression of the active form of caspase-3 and the antiapoptotic factor, X-linked inhibitor of apoptosis, were both up-regulated in orally tolerant CD4 T cells, which were found to be nonapoptotic. We further demonstrated that orally tolerant CD4 T cells could not form normal TCR signaling complexes associated with GADS and showed down-regulated phospholipase C-gamma1 activation, which is likely to contribute to the impairment of TCR-induced calcium signaling. Our findings indicate that orally tolerant CD4 T cells up-regulate caspase activation and show decreased levels of caspase-targeted proteins, including TCR signaling-associated molecules, while up-regulating antiapoptotic factors, all of which appear to contribute to their unique tolerant characteristics.

T-cell activation through the antigen receptor. Part 2: role of signaling cascades in T-cell differentiation, anergy, immune senescence, and development of immunotherapy

Part 2 of this review on cellular activation by the T-cell antigen receptor (TCR) will highlight how TCR signaling pathways are adapted to achieve specific biologic outcomes, including different states of T-cell differentiation and the induction of T-cell tolerance. We will also explore how treatment with altered peptide ligands affects TCR signaling to change T-cell differentiation or to induce an anergy state. These changes are accomplished through alteration of protein tyrosine kinase activity, the stoichiometry of phosphorylation of immunoreceptor tyrosine-based activation motifs, intracellular free ionized calcium flux, mitogen-activated protein kinase activity, and transcriptional activation of key cytokine promoters. The CTLA-4 plays an important role in the induction and maintenance of anergy. The second theme will highlight how altered TCR signal transduction, including changes in the compartmentalization of signaling components at the TCR synapse, contributes to decreased T-cell activation during immune senescence. Finally, we will illustrate how the molecular details of TCR activation can be used to modify the function of the immune system. This includes a description of the mechanism of action of altered peptide ligands, CTLA-4Ig, and pharmacologic inhibitors of mitogen-activated protein kinases, nuclear factor kappaB, and protein kinase C cascades.

T-cell tolerance and autoimmune diabetes

Herein we describe the major signaling events that occur in T-cells upon T-cell receptor (TCR) engagement, and the mechanisms responsible for the induction of T-cell anergy that may ultimately lead to the development of immunospecific therapies in T-cell mediated autoimmune diseases. A new type of antigen presenting molecule (dimeric MHC class-II/peptide, DEF) endowed with antigen-specific immunomodulatory effects such as induction of Th2 polarization and T-cell anergy is also described as a potential antidiabetogenic agent. According to our preliminary results, the MHC II/peptide-based approach may provide rational grounds for further development of antigen-specific immunotherapeutic agents such as human-like MHC lI/peptide chimeras endowed with efficient down-regulatory effects in CD4 T-cell-mediated autoimmune diseases such as Type 1 diabetes, multiple sclerosis, primary biliary cirrhosis, and rheumatoid arthritis.

Effects of cadmium and vanadium ions on antigen-induced signaling in CD4(+) T cells

Heavy metal environmental pollutants modulate antigen-directed responses by T lymphocytes, but the molecular mechanisms by which certain metal ions exert their effects are only poorly understood. We tested the hypothesis that cadmium and vanadium ions alter antigen-induced T cell signal transduction pathways in CD4(+) T helper cells. We used CD4(+) primary T lymphocytes and splenic T cells from DO.11.10 T cell receptor transgenic mice. We determined the effects of cadmium chloride and sodium orthovanadate at concentrations that did not induce apoptotic cell death, but affected cytokine or proliferation responses to antigenic stimulation. We used electrophoretic mobility shift assays to measure effects of cadmium and vanadium ions on antigen-induced activation of the nuclear transcriptional regulator proteins, nuclear factor-kappaB, cyclic AMP response element binding protein, nuclear factor of activated T cells, and activator protein-1. Different signaling pathways lead to activation of these transcription factors. Our results suggest that the two heavy metal ions differentially affect signaling pathways. This knowledge will help in the development of molecular epidemiological assays.

CD4(+)CD25(+) T cells facilitate the induction of T cell anergy

T cell anergy is characterized by the inability of the T cell to produce IL-2 and proliferate. It is reversible by the addition of exogenous IL-2. A similar state of unresponsiveness is observed when the proliferative response of murine CD4(+)CD25(-) T cells is suppressed in vitro by coactivated CD4(+)CD25(+) T cells. We have developed a suppression system that uses beads coated with anti-CD3 and anti-CD28 Abs as surrogate APCs to study the interaction of CD4(+)CD25(+) and CD4(+)CD25(-) T cells in vitro. CD4(+)CD25(+) T cell-induced suppression, in this model, was not abrogated by blocking the B7-CTLA-4 pathway. When the CD4(+)CD25(-) T cells were separated from the CD4(+)CD25(+) suppressor cells after 24 h of coactivation by the Ab-coated beads, the CD4(+)CD25(-) T cells were unable to proliferate or to produce IL-2 upon restimulation. The induction of this anergic phenotype in the CD4(+)CD25(-) T cells correlated with the up-regulated expression of the gene related to anergy in lymphocytes (GRAIL), a novel anergy-related gene that acts as a negative regulator of IL-2 transcription. This system constitutes a novel mechanism of anergy induction in the presence of costimulation.

T-cell anergy and peripheral T-cell tolerance

The discovery that T-cell recognition of antigen can have distinct outcomes has advanced understanding of peripheral T-cell tolerance, and opened up new possibilities in immunotherapy. Anergy is one such outcome, and results from partial T-cell activation. This can arise either due to subtle alteration of the antigen, leading to a lower-affinity cognate interaction, or due to a lack of adequate co-stimulation. The signalling defects in anergic T cells are partially defined, and suggest that T-cell receptor (TCR) proximal, as well as downstream defects negatively regulate the anergic T cell's ability to be activated. Most importantly, the use of TCR-transgenic mice has provided compelling evidence that anergy is an in vivo phenomenon, and not merely an in vitro artefact. These findings raise the question as to whether anergic T cells have any biological function. Studies in rodents and in man suggest that anergic T cells acquire regulatory properties; the regulatory effects of anergic T cells require cell to cell contact, and appear to be mediated by inhibition of antigen-presenting cell immunogenicity. Close similarities exist between anergic T cells, and the recently defined CD4+ CD25+ population of spontaneously arising regulatory cells that serve to inhibit autoimmunity in mice. Taken together, these findings suggest that a spectrum of regulatory T cells exists. At one end of the spectrum are cells, such as anergic and CD4+ CD25+ T cells, which regulate via cell-to-cell contact. At the other end of the spectrum are cells which secrete antiinflammatory cytokines such as interleukin 10 and transforming growth factor-beta. The challenge is to devise strategies that reliably induce T-cell anergy in vivo, as a means of inhibiting immunity to allo- and autoantigens.

Cutting edge: cell autonomous rather than environmental factors control bacterial superantigen-induced T cell anergy in vivo

Anergic T cells display a marked decrease in their ability to produce IL-2 and to proliferate in the presence of an appropriate antigenic signal. Two nonmutually exclusive classes of models have been proposed to explain the persistence of T cell anergy in vivo. While some reports indicate that anergic T cells have intrinsic defects in signaling pathways or transcriptional activities, other studies suggest that anergy is maintained by environmental "suppressor" factors such as cytokines or Abs. To distinguish between these conflicting hypotheses, we employed the well-characterized bacterial superantigen model system to evaluate in vivo the ability of a trace population of adoptively transferred naive or anergized T cells to proliferate in a naive vs anergic environment upon subsequent challenge. Our data clearly demonstrate that bacterial superantigen-induced T cell anergy is cell autonomous and independent of environmental factors.

CD40-ligated dendritic cells effectively expand melanoma-specific CD8+ CTLs and CD4+ IFN-gamma-producing T cells from tumor-infiltrating lymphocytes

Professional APC, notably dendritic cells (DC), are necessary for stimulation and expansion of naive T cells. By means of murine models, the interaction between CD40 on DC and its ligand CD154 has been recognized as an important element for conditioning of DC to prime and expand CTL. We translated these findings into the human system, scrutinizing the ability of DC to initiate clonal expansion of single T cells. DC generated under completely autologous conditions from peripheral blood monocytes were cocultured at a rate of 0.3 cell/well with melanoma-infiltrating T cells; this procedure guaranteed that either a CD4+ or a CD8+ cell interacted with the DC, thus avoiding the contact of more than one T cell to the DC. In the absence of further stimulation, this cloning protocol yielded almost exclusively CD4+ T cell clones that predominantly exhibited a Th2 phenotype. However, cross-linking of CD40 on DC resulted in the induction of IFN-gamma-producing Th1 CD4+ T cell clones. In addition, CD40-activated DC were capable of expanding CD8+ CTL clones. The ratio of CD4 to CD8 T cell clones corresponded to the ratio present in the initial tumor-infiltrating lymphocyte preparation. The CTL clones efficiently lysed autologous tumor cells whereas autologous fibroblasts or MHC-mismatched melanoma cells were not killed. Our findings support the critical role of CD40/CD154 interactions for the induction of cellular immune responses.

Impaired Ca/calcineurin pathway in in vivo anergized CD4 T cells

Clonal anergy is one of the mechanisms that may account for self tolerance induced in T cells in the periphery. In this study we used the well-documented system of in vivo administration of a superantigen, staphylococcal enterotoxin B (SEB), to induce a state of hyporesponsiveness (anergy) in murine peripheral T cells to decipher the intracellular biochemical basis for this process. The TCR-induced Ca response of in vitro activated T cells was found to be impaired with significant defects in the phosphorylation of phospholipase C-gamma 1. Experiments with calcium ionophore and newly established transgenic mouse lines that express an active form of calcineurin suggested that in vivo SEB-induced anergy is established and/or maintained by a selective impairment in the TCR-induced activation of the Ca/calcineurin pathway.

Expression of CD94/NKG2 subtypes on tumor-infiltrating lymphocytes in primary and metastatic melanoma

Natural killer receptors are expressed both on natural killer populations and subpopulations of T cells, mainly alpha/beta TCR+CD8+ T cells. We have characterized the expression of the C-type lectin natural killer receptor CD94/NKG2 on tumor-infiltrating lymphocytes in primary and metastatic melanoma lesions. By immunohistochemistry, 5-10% of the tumor-infiltrating lymphocytes, both in primary and metastatic lesions, expressed CD94. More than 95% of these CD94+ cells coexpressed CD8 and the percentage of CD94 expression within the CD8+ cell population ranged from 5 to 20% with a higher expression in metastatic lesions. CD94/NKG2 exists both in an inhibitory and an activating form; thus, it was necessary to determine whether the inhibitory CD94/NKG2-A/B, the activating CD94/NKG2-C/E, or both were expressed on tumor-infiltrating lymphocytes. Reverse transcription-polymerase chain reaction using specific primers for NKG2-A/B and C/E mRNA revealed the presence of NKG2-C/E in all primary and metastatic lesions. In contrast, the inhibitory NKG2-A/B was only present in 50% of primary tumors whereas 80% of tumor-infiltrating lymphocytes in metastatic lesions expressed these transcripts. In healthy humans, the mean number of inhibitory natural killer receptors is higher than that of activating receptors, but the opposite was true for tumor-infiltrating lymphocytes in melanoma. The reversal of the ratio of inhibitory to activating natural killer receptors among tumor-infiltrating lymphocytes suggests a regulated event due to either specific factors within the tumor microenvironment, preferential homing of T cell subsets, or certain stages of T cell activation.

Anergic T lymphocytes selectively express an integrin regulatory protein of the cytohesin family

It has been proposed that the maintenance of T cell anergy depends on the induction of negative regulatory factors. Differential display of reverse transcribed RNA was used to identify novel genes that might mediate this function in anergic Th1 clones. We report that anergic Th1 clones do indeed express a genetic program different from that of responsive T cells. Moreover, one gene, the general receptor of phosphoinositides 1 (GRP1), was selectively induced in anergic T cells. The GRP1, located in the plasma membrane, regulated integrin-mediated adhesion and was invariably associated with unresponsiveness in multiple models of anergy. T cells expressing retrovirally transduced GRP1 exhibited normal proliferation and cytokine production. However, GRP1-transduced T cells were not stable and rapidly lost GRP1 expression. Thus, although GRP1 may not directly mediate T cell anergy, it regulates cell expansion and survival, perhaps through its integrin-associated activities.

Increased c-Fos/activator protein-1 confers resistance against anergy induction on antigen-specific T cell

We have studied the contribution of c-Fos/activator protein-1 (AP-1) to antigen-specific T cell response with reference to T cell anergy by increasing c-Fos/AP-1 in vivo and in vitro. First, after injection of a high dose of staphylococcus enterotoxin B (SEB), clonal deletion of SEB-reactive V(beta)8(+) CD4 T cells occurred both in control B6 and H2-c-fos transgenic (fos) mice, whereas proliferation of T cells against SEB was profoundly depressed in B6 but not in fos mice. Second, the keyhole limpet hemocyanin-specific CD4 T(h)1 cell clone produced decreasing amounts of IL-2 in response to increasing amounts of concanavalin A (Con A) in vitro, whereas the decrease was less significant in the T(h)1 clones stably transfected with c-fos gene. Electrophoretic mobility shift assay with nuclear protein from the transformants showed that overexpression of the c-fos gene compensated the amounts of AP-1 in the nuclei of Con A-treated T(h)1 clones. Thus, increased c-Fos/AP-1 confers resistance against anergy induction on antigen-specific T cells.

Negative regulation of CD4 expression in T cells by the transcriptional repressor ZEB

ZEB, an E-box binding transcriptional repressor, is an important regulator of T cell and muscle development. Targeted disruption of ZEB in mice resulted in a strong reduction of thymocytes and the few T cells that reached the mature stage were predominantly CD4(+). CD4 expression during the various stages of T cell differentiation is controlled at the transcriptional level by a complex array of regulatory elements in the CD4 gene locus, consisting of at least three enhancers, one promoter and one silencer. Here we present evidence that CD4 gene expression is negatively regulated by ZEB. We show that ZEB binds to the 5'E-box in the CD4-3 element of the proximal CD4 enhancer in competition with the transcriptional activators E12 and HEB, thereby reducing CD4 expression on CD4 single-positive but not CD4/CD8 double-positive T cells. The conversion of the CD4 proximal enhancer into a potential silencer element by the transcriptional repressor ZEB offers an additional concept of CD4 gene regulation in T cells.

CD28-Mediated Regulation of mRNA Stability Requires Sequences Within the Coding Region of the IL-2 mRNA

Using sequence-tagged genomic reporter constructs, we investigated the contribution of IL-2 sequences to CD28-mediated regulation of mRNA stability. We find that CD28 signaling acts transiently to stabilize the IL-2 mRNA following T cell activation. Such stabilization requires sequences within both exon 2 and the coding region of exon 4. Unexpectedly, CD28 signaling at later times enhances the decay of the IL-2 mRNA. This CD28-dependent decay of IL-2 mRNA requires sequences localized between exon 3 and the stop codon. Our findings demonstrate that the coding region of the IL-2 mRNA contains previously undefined CD28-responsive sequence elements that are critical for the regulation of mRNA stability.

T cell costimulatory blockade: new therapies for transplant rejection

Optimal T cell responses occur when T cells receive both antigen-specific signals through the T cell receptor and non-antigen-specific costimulatory signals through accessory cell surface molecules. The best understood costimulatory receptor is CD28. Signals through the T cell receptor and CD28 cooperatively induce cytokine gene expression and promote T cell proliferation and survival. Negative signals delivered through a related cell surface receptor, cytotoxic T lymphocyte antigen (CTLA-4), act to terminate immune responses and are required for normal immune homeostasis. This article reviews T cell costimulation, including the CD28/CTLA-4 system and other potential costimulatory pathways (such as CD40/CD154), the role of these pathways in normal immune responses, and the potential for the inhibition of these pathways to induce transplantation tolerance.

Inhibition of Cell Cycle Progression by Rapamycin Induces T Cell Clonal Anergy Even in the Presence of Costimulation

Costimulation (signal 2) has been proposed to inhibit the induction of T cell clonal anergy by either directly antagonizing negative signals arising from TCR engagement (signal 1) or by synergizing with signal 1 to produce IL-2, which in turn leads to proliferation and dilution of negative regulatory factors. To better define the cellular events that lead to the induction of anergy, we used the immunosuppressive agent rapamycin, which blocks T cell proliferation in late G1 phase but does not affect costimulation-dependent IL-2 production. Our data demonstrate that full T cell activation (signal 1 plus 2) in the presence of rapamycin results in profound T cell anergy, despite the fact that these cells produce copious amounts of IL-2. Similar to conventional anergy (induction by signal 1 alone), the rapamycin-induced anergic cells show a decrease in mitogen-activated protein kinase activation, and these cells can be rescued by culture in IL-2. Interestingly, the rapamycin-induced anergic cells display a more profound block in IL-3 and IFN-γ production upon rechallenge. Finally, in contrast to rapamycin, full T cell activation in the presence of hydroxyurea (which inhibits the cell cycle in early S phase) did not result in anergy. These data suggest that it is neither the direct effect of costimulation nor the subsequent T cell proliferation that prevents anergy induction, but rather the biochemical events that occur upon progression through the cell cycle from G1 into S phase.

CD4-Mediated Inhibiton of IL-2 Production in Activated T Cells

The role of CD4 in T cell activation has been attributed to its capacity to increase the avidity of interaction with APC and to shuttle associated Lck to the TCR/CD3 activation complex. The results presented in this study demonstrate that ligation of CD4 inhibits ongoing responses of preactivated T cells. Specifically, delayed addition of CD4-specific mAb is shown to inhibit Ag- or mAb-induced responses of both primary T cells and T cell clonal variants. The Ag responses of the latter are independent of the adhesion provided by CD4; thus the observed inhibition is not due to blocking CD4-MHC interactions. Further, analysis of the clonal variants demonstrates that CD4-associated Lck is not essential for the inhibition observed, as anti-CD4 inhibits responses of clonal variants, expressing a form of CD4 unable to associate with Lck (double cysteine-mutated CD4). The inhibition is counteracted by the addition of exogenous IL-2, demonstrating that the block is not due to a lesion in IL-2 utilization, rather its production. It is demonstrated that the delayed addition of anti-CD4 results in a rapid reduction in steady-state levels of IL-2 mRNA in both primary T cells and clonal variants.

Evidence for Repression of IL-2 Gene Activation in Anergic T Cells

The induction of clonal anergy in a T cell inhibits IL-2 secretion because of the development of a proximal signal transduction defect. Fusion of anergic murine T cells to human Jurkat T leukemia cells and formation of heterokaryons failed to result in a complementation of this signaling defect and restoration of murine IL-2 mRNA inducibility. Instead, signal transduction to the human IL-2 gene became disrupted. Heterokaryons formed by the fusion of anergic murine T cells to normal murine T cells also failed to accumulate intracellular IL-2 protein in response to stimulation either with the combination of CD3 and CD28 mAbs or with ionomycin plus a protein kinase C-activating phorbol ester. The results argue against a loss-of-function signaling defect as the sole basis for clonal anergy induction and document the presence of a dominant-acting repressor molecule that inhibits signal transduction to the IL-2 gene within viable anergic T cells.

Molecular regulation of interleukin-2 expression by CD28 co-stimulation and anergy

The consequences of T-cell receptor engagement (signal 1) are profoundly affected by the presence or absence of co-stimulation (signal 2). T-cell receptor (TCR) stimulation in the absence of CD28-mediated co-stimulation not only results in little interleukin (IL)-2 production, but induces a long lasting hyporesponsive state known as T-cell clonal anergy. The addition of CD28 ligation to signal 1, on the other hand, results in the production of copious amounts of IL-2. Our laboratory has utilized CD4+ Th 1 clones in an effort to understand the molecular events resulting in enhanced IL-2 production by co-stimulation and the inhibition of IL-2 production in anergy. Our current studies have focused on defining the post-transcriptional effects of CD28-enhanced IL-2 production. The data suggest that a major component of CD28's ability to regulate IL-2 production occurs at the level of message stability and involves the 3'-untranslated region of the message. In terms of anergy, our recent studies support the notion that it is not the result of TCR engagement in the absence of co-stimulation, but rather signal 1 in the absence of IL-2 receptor signaling and proliferation. Furthermore, T-cell anergy appears to be an active negative state in which IL-2 production is inhibited both at the level of signal transduction and by cis-dominant repression at the level of the IL-2 promoter.

Transcriptional Repression of the IL-2 Gene in Th Cells by ZEB

Th1- and Th2-type cells mediate distinct effector functions via cytokine secretion in response to immunologic challenge. Precursor Th cells transcribe IFN-γ, IL-2, and IL-4 upon activation. Repeated stimulation of Th precursor cells in the presence of IL-4 leads to terminally differentiated Th2 cells that have lost the ability to transcribe the IL-2 gene. We provide evidence that repression of IL-2 gene expression in Th2 cells and partial repression in Th1 cells are mediated by ZEB, a zinc finger, E box-binding transcription factor. This factor binds to a negative regulatory element, NRE-A, in the IL-2 promoter, thereby acting as a potent repressor of IL-2 transcription.

Modifications of CD8+ T cell function during in vivo memory or tolerance induction

Naive monoclonal T cells specific for the male antigen can be stimulated in vivo to eliminate male cells and become memory cells or to permit survival of male cells and become tolerant. Memory cells responded to TCR ligation by cyclic oscillations of calcium levels and immediate secretion of very high levels of IL-2 and interferon-gamma. Tolerant cells did not proliferate in response to ionomycin and phorbol myristate acetate, failing to mobilize calcium to produce IL-2 or express IL-2R, but survived for long time periods in vivo and secreted IL-10. These results emphasize that tolerance is not an absence of all functional activity and may be associated with modifications of behavior conferring important regulatory functions on tolerant T cells.

An anergic cytotoxic T lymphocyte clone exhibits granule exocytosis-mediated cytotoxicity

T cell receptor (TCR) occupancy in the absence of a costimulatory signal transforms T helper (Th) cells or cytotoxic T lymphocytes (CTL) into a state of anergy. The anergic T cells are unable to produce cytokines; nevertheless, they maintain their killing activity. We investigated the mechanisms through which anergic CTL causes lysis of target cells. Treatment of a CTL clone with phorbol myristate acetate and calcium ionophore A23187 (P/A) transformed these cells to anergic cells. While the anergic CTL clones failed to secrete TNF-alpha in the culture supernatant, they were still able to kill antigen-specific target cells via a granule exocytosis-mediated pathway. This was evident by the synthesis of perforin mRNA and release of N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester esterase by these cells. The anergic CTL clone also showed a low degree of Fas-mediated lysis of normal target cells. In addition, we generated anergic bulk CTL by treatment with P/A and observed that the anergic bulk CTL failed to produce TNF upon antigen stimulation, but retained target killing activity via a granule exocytosis mechanism. Our results suggest that the killing mechanisms of anergic CTL are mediated to a large extent by a granule exocytosis-mediated pathway.

Biochemical features of anergic T cells

T cell anergy is a functionally defined state of hyporesponsiveness in which T cells neither proliferate nor produce IL2 following subsequent TCR ligation. Recent biochemical data from in vitro studies suggest that anergic cells do not utilize all of the signaling pathways normally initiated by TCR triggering. These findings appear to hold true for T cells rendered anergic in vivo, as well; however, biochemical studies on clonal anergy in vivo have been limited by the inability to recover a homogeneous population of anergic T cells. Here we review progress on TCR mediated signaling pathways as well as the description of surface marker phenotypes specific to T cell anergy.

Oral tolerance as therapy for experimental autoimmune encephalomyelitis and multiple sclerosis: demonstration of T cell anergy

Experimental autoimmune encephalomyelitis (EAE) is an important model for developing therapies for multiple sclerosis (MS). The oral administration of the central nervous system antigen, myelin basic protein (MBP), to Lewis rats and susceptible mouse strains prior to MBP immunization prevents the induction of EAE. Clinical trials administering myelin orally to MS patients have met with only partial success, and thus require that oral tolerance be further studied to improve this treatment strategy. Clonal anergy, clonal deletion, immune deviation from Th1 to Th2 T cell subsets, and active suppression by TGF-beta-secreting T cells have all been implicated as possible mechanisms in oral tolerance. Which mechanism predominates depends on antigen dosage, frequency of feeding, and timing of antigen administration. In this study, we have characterized T cells derived from MBP-fed rats and determined the level of their unresponsiveness. Myelin basic protein-specific T cells are indeed present although in reduced numbers in lymphoid tissue of orally tolerized animals. Following several cell divisions in the presence of IL-2, these MBP-specific T cells undergo a dramatic reversal of unresponsiveness, proliferate in response to MBP and are capable of transferring EAE. These results support clonal anergy as an important mechanism for oral tolerance. Recent developments in clinical trials of oral tolerance are described.

Interleukin 10 secretion and impaired effector function of major histocompatibility complex class II-restricted T cells anergized in vivo

Continuous antigenic stimulation in vivo can result in the generation of so-called "anergic" CD4(+) or CD8(+) T cells that fail to proliferate upon antigenic stimulation and fail to develop cytolytic effector functions. Here we show that class II major histocompatibility complex-restricted T cells specific for influenza hemagglutinin (HA) that become anergic in mice expressing HA under control of the immunoglobulin kappa promoter exhibit an impaired effector function in causing diabetes in vivo, as compared to their naive counterparts, when transferred into immunodeficient recipients expressing HA under the control of the insulin promoter. Furthermore, HA-specific T cells anergized in vivo contain higher levels of interleukin (IL)-4 messenger RNA (mRNA) than naive and recently activated T cells with the same specificity and more than a 100-fold higher levels of IL-10 mRNA. The higher expression of the IL-10 gene is also evident at the protein level. These findings raise the interesting possibility that T cells rendered anergic in vivo have in fact become regulatory T cells that may influence neighboring immune responses through the release of IL-10.

Impaired Induction of the CD28-Responsive Complex in Granulocyte Colony-Stimulating Factor Mobilized CD4 T Cells

Use of the CD28/B7 costimulatory signal for T-cell activation was analyzed in granulocyte colony-stimulating factor (G-CSF) mobilized peripheral blood mononuclear cells (G-PBMCs) and in peripheral blood mononuclear cells obtained before administration of G-CSF (preG-PBMCs). CTLA4Ig inhibition of OKT3-stimulated proliferation was significantly lower in G-PBMCs compared with preG-PBMCs (39.9% ± 5.6% and 72.2% ± 5.4%, respectively; P < .001). Furthermore, as shown in electrophoretic mobility-shift assays, the inducible level of the T-cell transcription factor CD28 responsive complex (CD28RC) was suppressed in CD4 cells derived from G-PBMC. However, depletion of CD14 cells from G-PBMCs restored CD28RC induction to normal levels. Taken together, these findings suggest that the large number of CD14 monocytes in G-PBMCs may limit T-cell responsiveness by suppressing the induction of the CD28RC.

Impaired Induction of the CD28-Responsive Complex in Granulocyte Colony-Stimulating Factor Mobilized CD4 T Cells

Use of the CD28/B7 costimulatory signal for T-cell activation was analyzed in granulocyte colony-stimulating factor (G-CSF) mobilized peripheral blood mononuclear cells (G-PBMCs) and in peripheral blood mononuclear cells obtained before administration of G-CSF (preG-PBMCs). CTLA4Ig inhibition of OKT3-stimulated proliferation was significantly lower in G-PBMCs compared with preG-PBMCs (39.9% ± 5.6% and 72.2% ± 5.4%, respectively; P < .001). Furthermore, as shown in electrophoretic mobility-shift assays, the inducible level of the T-cell transcription factor CD28 responsive complex (CD28RC) was suppressed in CD4 cells derived from G-PBMC. However, depletion of CD14 cells from G-PBMCs restored CD28RC induction to normal levels. Taken together, these findings suggest that the large number of CD14 monocytes in G-PBMCs may limit T-cell responsiveness by suppressing the induction of the CD28RC.

Involvement of negative cofactor NC2 in active repression by zinc finger-homeodomain transcription factor AREB6

The transcription factor AREB6 contains a homeodomain flanked by two clusters of Krüppel type C2H2 zinc fingers. AREB6 binds to the E-box consensus sequence, CACCTGT, through either the N- or the C-terminal zinc finger cluster. To gain insights into the molecular mechanism by which AREB6 activates and represses gene expression, we analyzed the domain structure of AREB6 in the context of a heterologous DNA-binding domain by transient-transfection assays. The C-terminal region spanning amino acids 1011 to 1124 was identified as a conventional acidic activation domain. The region containing amino acids 754 to 901, which was identified as a repression domain, consists of 40% hydrophobic amino acids displaying no sequence similarities to other known repression domains. This region repressed transcription in vitro in a HeLa nuclear extract but not in reconstituted transcription systems consisting of transcription factor IID (TFIID), TFIIB, TFIIE, TFIIH/F, and RNA polymerase II. The addition of recombinant negative cofactor NC2 (NC2alpha/DRAP1 and NC2beta/Dr1) to the reconstituted transcription system restored the activity of the AREB6 repression domain. We further demonstrated interactions between the AREB6 repression domain and NC2alpha in yeast two-hybrid assay. Our findings suggest a mechanism of transcriptional repression that is mediated by the general cofactor NC2.

T cell clonal anergy

Recent experiments have elucidated two molecular mechanisms that may account for the failure of anergic T cell clones to initiate IL-2 gene transcription following TCR stimulation. First, a block has been identified in the ERK and JNK mitogen-activated protein kinase pathways; the block results from a failure to activate p21ras. It leads to reduced induction of c-Fos and JunB proteins and to a failure to form and phosphorylate the activator protein (AP)-1 heterodimers required for IL-2 gene transcriptional activation. Second, repressor molecules (Nil-2-a and a molecule related to AP-1) have been characterized that dominantly inhibit IL-2 gene transcription.

Translational control of interleukin 2 messenger RNA as a molecular mechanism of T cell anergy

T cell stimulation by triggering through the T cell receptor (TCR) in the absence of costimulatory signals or by calcium ionophore induces unresponsiveness in T cells to further stimulation, a phenomenon known as anergy. In freshly isolated T cells, calcium ionophore induces expression of interleukin (IL)-2 messenger (mRNA), but this mRNA is not translated and not loaded with ribosomes. In addition, while plate-bound anti-CD3 stimulation of resting T cells leads to IL-2 mRNA expression and IL-2 secretion, in cells pretreated with calcium ionophore before anti-CD3 stimulation, the IL-2 mRNA remains polysome unloaded and no IL-2 is produced. These observations show that IL-2 expression is controlled at the translational level, by differential ribosome loading. Furthermore, our data suggest that translational control of IL-2 mRNA may be a molecular mechanism by which anergy is attained.

Induction of alloantigen-specific hyporesponsiveness in vitro by n-butyrate: antagonistic effect of cyclosporin A

The short-chain fatty acid n-butyrate has recently been shown in vitro to specifically downregulate T cell reactivity to nominal antigen or to alloantigen, which possibly results from inhibition of cell cycle progression in early G1 phase during antigen contact. In the present study, we investigated the effect of cyclosporin A (CyA) on the modulation of alloreactivity in human mixed lymphocyte culture (MLC) by n-butyrate. Whereas in primary culture, CyA additively enhanced inhibition of DNA synthesis by n-butyrate, the effect of this agent on secondary T cell reactivity was clearly antagonized by CyA. Thus, specific downregulation of proliferative responsiveness to restimulation with antigen from the original donor, observed in cultures pretreated with n-butyrate alone, was at least partially prevented by the addition of CyA to the primary culture. Our in vitro finding indicates that specific downregulation of T cell alloreactivity by n-butyrate might depend on a calcium-dependent T cell receptor (TCR)-mediated signal sensitive to the immunosuppressive action of CyA.

Autoreactive T cells in healthy individuals show tolerance in vitro with characteristics similar to but distinct from clonal anergy

Peripheral tolerance to self antigens has been suspected to play an important role in the regulation of the immune response in humans since autoreactive T cells can be isolated from the peripheral blood of healthy individuals. The mechanism of this tolerance is not known, but a number of groups have shown that autoreactive T cells can be induced to proliferate in vitro by the addition of their specific antigen and exogenous interleukin (IL)-2. In this report, we present the analysis of autoreactive T cells, isolated from healthy individuals, to the autoantigen GpIIb-IIIa present on circulating bone-marrow-derived cells and on thymic epithelial cells. We found that the response of GpIIb-IIIa autoreactive T cells in vitro, when stimulated with GpIIb-IIIa, shares characteristics with the response found for anergic T cells. In response to GpIIb-IIIa, the GpIIb-IIIa-autoreactive T cells are neither able to proliferate nor produce IL-2 on their own, but do express IL-2 receptors alpha on their cell surface and produce IFN-gamma. This state of unresponsiveness can be broken by the addition of exogenous IL-2 and IL-7, as in the case of anergic T cells. However, GpIIb-IIIa-autoreactive T cells differ from anergic T cells in their capacity to be stimulated by IL-12 and by their production of IL-2 mRNA. Interestingly, once the unresponsive state to GpIIb-IIIa has been broken by the addition of IL-2, GpIIb-IIIa autoreactive T cells can produce IL-2 and proliferate when restimulated by GpIIb-IIIa alone. Altogether, these results suggest that the tolerance of GpIIb-IIIa autoreactive T cells from healthy individuals could involve post-transcriptional regulation of IL-2 expression.

DNA loops induced by cooperative binding of transcriptional activator proteins and preinitiation complexes

DNA conformational changes are essential for the assembly of multiprotein complexes that contact several DNA sequence elements. An approach based on atomic force microscopy was chosen to visualize specific protein-DNA interactions occurring on eukaryotic class II nuclear gene promoters. Here we report that binding of the transcription regulatory protein Jun to linearized plasmid DNA containing the consensus AP-1 binding site upstream of a class II gene promoter leads to bending of the DNA template. This binding of Jun was found to be essential for the formation of preinitiation complexes (PICs). The cooperative binding of Jun and PIC led to looping of DNA at the protein binding sites. These loops were not seen in the absence of either PICs, Jun, or the AP-1 binding site, suggesting a direct interaction between DNA-bound Jun homodimers and proteins bound to the core promoter. This direct visualization of functional transcriptional complexes confirms the theoretical predictions for the mode of gene regulation by trans-activating proteins.