Differential induction of the NF-AT complex during restimulation and the induction of T-cell anergy

Up-regulation of proliferative and migratory genes in regulatory T cells from patients with metastatic castration-resistant prostate cancer

A higher frequency of regulatory T cells (Tregs) has been observed in peripheral blood mononuclear cells (PBMC) of patients with different types of solid tumors and hematological malignancies as compared to healthy donors. In prostate cancer patients, Tregs in PBMC have been shown to have increased suppressive function. Tumor-induced biological changes in Tregs may enable tumor cells to escape immunosurveillance. We performed genome-wide expression analyses comparing the expression levels of more than 38,500 genes in Tregs with similar suppressive activity, isolated from the peripheral blood of healthy donors and patients with metastatic castration-resistant prostate cancer (mCRPC). The differentially expressed genes in mCRPC Tregs are involved in cell cycle processes, cellular growth and proliferation, immune responses, hematological system development and function and the interleukin-2 (IL-2) and transforming growth factor-β (TGF-β) pathways. Studies revealed that the levels of expression of genes responsible for T-cell proliferation (C-FOS, C-JUN and DUSP1) and cellular migration (RGS1) were greater in Tregs from mCRPC patients as compared to values observed in healthy donors. Increased RGS1 expression in Tregs from mCRPC patients suggests a decrease in these Tregs' migratory ability. In addition, the higher frequency of CD4(+) CD25(high) CD127(-) Tregs in the peripheral blood of mCRPC patients may be the result of an increase in Treg proliferation capacity. Results also suggest that the alterations observed in gene expression profiles of Tregs in mCRPC patients may be part of the mechanism of tumor escape from host immune surveillance.

FoxP3 maintains Treg unresponsiveness by selectively inhibiting the promoter DNA-binding activity of AP-1

Regulatory T cells (Tregs) have been shown to play a crucial role in maintaining self-tolerance and suppressing autoimmunity. The forkhead transcription factor, FoxP3, is a key molecule necessary and sufficient for Tregs development and function. However, the molecular mechanisms by which FoxP3 regulates the phenotypic (anergic) and the functional (suppressive) characteristics of Tregs are not well defined. Here we found that the promoter DNA-binding activity of AP-1 transcription factors is selectively inhibited in the naturally occurring CD4+ CD25+ Tregs from mice. The impaired AP-1 DNA binding is not the result of the decreased nuclear translocation of AP-1 family transcription factors, including c-Jun, JunB, and c-Fos. FoxP3 significantly suppresses both the transcriptional activity and promoter DNA-binding of AP-1 by interacting with c-Jun. The N-terminus of FoxP3, but not its C-terminus forkhead domain, specifically interacts with phosphorylated c-Jun and alters c-Jun subnuclear distribution. This N-terminus of FoxP3 with nuclear localization signals (FoxP3N/NLS) is able to suppress AP-1 transcriptional activity. Ectopic expression of FoxP3N/NLS sufficiently induces the unresponsiveness of mouse primary CD4+ CD25- T cells, whereas the full-length FoxP3 is required for the suppressive functions of Tregs. These findings uncover one of the mechanisms underlying how FoxP3 maintains the unresponsiveness of Tregs.

Adaptive tolerance and clonal anergy are distinct biochemical states

Adaptive tolerance is a process by which T cells become desensitized when Ag stimulation persists following an initial immune response in vivo. To examine the biochemical changes in TCR signaling present in this state, we used a mouse model in which Rag2(-/-) TCR-transgenic CD4(+) T cells were transferred into CD3epsilon(-/-) recipients expressing their cognate Ag. Compared with naive T cells, adaptively tolerant T cells had normal levels of TCR and slightly increased levels of CD4. Following activation with anti-TCR and anti-CD4 mAbs, the predominant signaling block in the tolerant cells was at the level of Zap70 kinase activity, which was decreased 75% in vitro. Phosphorylations of the Zap70 substrates (linker of activated T cells and phospholipase Cgamma1 were also profoundly diminished. This proximal defect impacted mostly on the calcium/NFAT and NF-kappaB pathways, with only a modest decrease in ERK1/2 phosphorylation. This state was contrasted with T cell clonal anergy in which the RAS/MAPK pathway was preferentially impaired and there was much less inhibition of Zap70 kinase activity. Both hyporesponsive states manifested a block in IkappaB degradation. These results demonstrate that T cell adaptive tolerance and clonal anergy are distinct biochemical states, possibly providing T cells with two molecular mechanisms to curtail responsiveness in different biological circumstances.

Impaired secretion of interleukin-4 and interleukin-13 by allergen-specific T cells correlates with defective nuclear expression of NF-AT2 and jun B: relevance to immunotherapy

BACKGROUND

Allergen immunotherapy (IT) is a successful treatment associated with decreased Th2 cytokine production by allergen-specific T cells. We have previously demonstrated (Faith et al., J Immunol 1997; 159:53-57) that inhibition of Th2 cytokine production in vitro correlates with impaired tyrosine kinase activity through the TCR. The transcription factor complex, nuclear factor of activated T cells (NF-AT), which regulates Th2 cytokine production is controlled by the activity of tyrosine kinases.

OBJECTIVE

To address whether decreased Th2 cytokine production by allergen-specific CD4+ T cells following IT is correlated with altered translocation and nuclear expression of the NF-AT family member, NF-AT2, and the activator protein 1 (AP1) component of NF-AT, jun B.

METHODS

T cell lines specific for insect venom phospholipase A2 (PLA) were derived from patients prior to and during conventional venom IT. Nuclear expressions of NF-AT and jun B were assessed following stimulation through the TCR. Th1 and Th2 cytokine and IL-10 production by insect venom-specific T cells were also determined. Results were compared with a well-established model system in which anergy was induced in cloned, allergen-specific Th2 cells.

RESULTS

Impaired translocation and decreased expression of NF-AT2 and jun B were detected in PLA-specific T cell lines derived from bee venom-allergic individuals following 16 weeks treatment compared to pre-treatment. These results correlated with significantly reduced production of IL-4 and IL-13 and significantly increased production of IFN-gamma and IL-10 by PLA-specific T cells. Impaired IL-4 and IL-13 production also correlated with defective nuclear expression of NF-AT2/jun B in cloned, anergic allergen-specific Th2 cells.

CONCLUSION

These results suggested that optimal production of IL-4 and IL-13 by allergen-specific T cells is dependent on the nuclear expression of NF-AT2 and jun B. Thus, specific inhibition of NF-AT2/jun B might be an option in novel and improved forms of allergen IT.

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.

Cord blood serum affects T cells ability to produce and respond to IL-2

The current literature suggests that cord blood (CB) cells are functionally immature. We previously reported that CB sera inhibit T cell proliferation and suggested that the microenvironment in which CB T cells reside may be, in part, responsible for their reduced function. In this study we have tried to explain some of the actions of the CB sera on peripheral blood mononuclear cells (PBMC). We showed that, as expected CB sera decreased the anti-CD3 and anti-CD28-induced proliferative response of PBMC (p < 0.01) but unexpectedly, increased the interleukin-2 (IL-2) specific proliferation of both a human T cell line (p < 0.005) and T cells within a mononuclear cell population (p < 0.05). These findings prompted us to analyse the effect of CB sera on the T cell ability to make and respond to IL-2. Stimulation of T cells in the presence of CB sera increased the frequency of IL-2 producing cells (p < 0.005) (but not the amount of IL-2 secreted) and resulted in a higher expression of CD25 (p < 0.05). Furthermore CB sera (in the presence and absence of IL-2) made the cells apoptose less (p < 0.005) than adult sera. Our results go some way to explaining the effect of the CB microenvironment on CB cellular function.

P59(fyn) is upregulated in anergic CD8+ T cells

The ultimate goal in clinical transplantation is achievement of graft tolerance. Despite long-term immunosuppression, alloantigens on transplants elicit alloresponses that can initiate organ rejection. Acute rejection is mediated by CD8(+) cytotoxic T cells, whereas chronic rejection is a result of many factors including non-immunological events. The aim of this study was to examine the molecular requirements of T cell anergy, a cellular state that is an integral component of tolerance in vivo. In vitro, the tolerant state is usually best represented by T cell anergy, which is defined by loss of the ability of T cells to produce and secrete interleukin-2 upon restimulation. In the literature, molecular changes in anergic CD4(+) T cells have been studied in great detail, but only little is known about functional and biochemical characteristics of anergic CD8(+) T lymphocytes. In this study, we demonstrate, that CD8(+) T cells are rendered anergic by TCR stimulation without costimulation. They exhibit impaired interleukin-2 production and tyrosine-phosphorylation, but markedly upregulated p59(fyn) expression, which could be shown to be an early event during anergization. Anergic CD8(+) T lymphocytes show elevated surface expression of early activation markers as well as costimulatory molecules, especially that of CTLA4. These results, are an important component for the discovery of potential molecular targets, which contribute to the development and maintenance of tolerance.

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.

Rheumatoid arthritis synovial T cells regulate transcription of several genes associated with antigen-induced anergy

Rheumatoid arthritis (RA) is a chronic, inflammatory synovitis whose pathogenesis may involve autoimmune mechanisms. Anergy is a state of T-cell nonresponsiveness characterized by downregulated IL-2 production. Paradoxically, RA T cells are hyporesponsive and proliferate poorly to antigens and mitogens, thus sharing some characteristics with anergic T cells. We analyzed the molecular basis of anergy in cloned human CD4+ T cells using differential display RT-PCR and subsequently examined the levels of differentially expressed transcripts in RA and, as control, reactive arthritis (ReA) synovium. Several transcriptional events were common to anergic T cells and RA synovium. These included downregulation of CALMODULIN:, which is critical to T-cell activation, and of cellular apoptosis susceptibility protein, which may mediate resistance to apoptosis in RA. Transcription of CALMODULIN: in RA synovium was less than 1% of that in ReA and was lower in RA synovial fluid mononuclear cells than in paired PBMCs. Following anti-TNF-alpha therapy in vivo, RA PBMC CALMODULIN: transcripts increased five- to tenfold. Pharmacological calmodulin blockade in vitro impaired antigen-specific proliferation. These data provide a link between reduced CALMODULIN: transcription and impaired T-cell responsiveness in RA. The identification of transcriptional changes common to anergic and RA synovial T cells should help interpret some of the characteristic RA cellular defects.

A population of in vivo anergized T cells with a lower activation threshold for the induction of CD25 exhibit differential requirements in mobilization of intracellular calcium and mitogen-activated protein kinase activation

Chronic exposure of mature T cells with specificity for self-Ags can lead to the induction of a nonfunctional state which is referred to as T cell anergy. It is unclear whether anergic T cells are destined for cell death and thereby harmless or whether they can contribute to the induction of autoimmunity and/or regulation of anti-self reactivity. We have begun to address this issue. In a recent study, we showed that a population of mature CD4-CD8- T cells that express a transgenic TCR specific for the Ld MHC class I molecule are rendered anergic in Ld-expressing mice. In this study, we show that this population of anergic T cells possess a lower activation threshold for the induction of CD25 and CD69 in response to stimulation by antigenic ligands. Furthermore, these anergic T cells undergo extensive proliferation when stimulated with a low-affinity ligand in the presence of an exogenous source of IL-2. Biochemical analysis of the early intracellular signaling events of these in vivo anergized T cells showed that they have a signaling defect at the level of ZAP-70 and linker for the activation of T cell (LAT) phosphorylation. They also exhibit a defect in mobilization of intracellular calcium in response to TCR signaling. However, these anergic T cells demonstrate no defect in SLP-76 phosphorylation and extracellular signal-regulated kinase 1/2 activation. These biochemical characteristics of the anergic T cells were associated with an elevated level of Fyn, but not Lck expression. The potential contributions of these anergic T cells in the induction and/or regulation of autoimmune responses are discussed.

Signalling into the T-cell nucleus: NFAT regulation

The nuclear factor of activated T cells (NFAT) plays an important role in T-cell biology. Activation of T cells results in the rapid calcineurin-dependent translocation of NFAT transcription factors from the cytoplasm to the nucleus. This translocation process coupled to the subsequent active maintenance of NFAT in the nucleus compartment is critical for the induction of expression of several genes encoding cytokines and membrane proteins that modulate immune responses. The molecular cloning of the NFAT family of transcription factors has facilitated rapid progress in the understanding of the signalling mechanisms that control the activity of NFAT.

Induction of T cell anergy by high concentrations of immunodominant native peptide is accompanied by IL-10 production and a block in JNK activity

The ability to induce anergy in antigen-specific T cells has potential therapeutic value for altering pathologic immune responses. This study was undertaken to further analyze changes in cytokine production and intracellular signaling during anergy induction using high concentrations of native peptide ligand of tetanus toxoid (TT)- and myelin basic protein (MBP)-specific human T cell lines. The TT-selected T cell line could be rendered unresponsive to its dominant epitope in a dose-dependent manner (IC50 = 0.03 microg/ml). The TT-selected line, as well as three T cell clones established from this line, continued to produce IFN-gamma and significantly increased IL-4 and IL-10 production when anergy was induced with high concentrations of the immunodominant epitope. JNK enzymatic activity was blocked in anergized T cells. The MBP-selected line could likewise be rendered unresponsive by incubation with supraoptimal concentrations of immunodominant peptide and anergy induction was accompanied by IL-10 release. Both T cell lines could be anergized by the autopresentation of native peptide since anergy was induced in cultures lacking fresh antigen-presenting cells. This study shows that the mitogen-activated protein kinase cascade is blocked when anergy is induced to high concentrations of soluble peptide.

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.

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.