T cell hyporesponsiveness induced by oral administration of ovalbumin is associated with impaired NFAT nuclear translocation and p27kip1 degradation

Antigen Uptake in the Gut: An Underappreciated Piece to the Puzzle?

The mammalian gut is a vast, diverse, and dynamic single-layer epithelial surface exposed to trillions of microbes, microbial products, and the diet. Underlying this epithelium lies the largest collection of immune cells in the body; these cells encounter luminal substances to generate antigen-specific immune responses characterized by tolerance at homeostasis and inflammation during enteric infections. How the outcomes of antigen-specific tolerance and inflammation are appropriately balanced is a central question in mucosal immunology. Furthermore, how substances large enough to generate antigen-specific responses cross the epithelium and encounter the immune system in homeostasis and during inflammation remains largely unexplored. Here we discuss the challenges presented to the gut immune system, the identified pathways by which luminal substances cross the epithelium, and insights suggesting that the pathways used by substances to cross the epithelium affect the ensuing immune response.

Targeting Transcriptional Regulators of CD8+ T Cell Dysfunction to Boost Anti-Tumor Immunity

Transcription is a dynamic process influenced by the cellular environment: healthy, transformed, and otherwise. Genome-wide mRNA expression profiles reflect the collective impact of pathways modulating cell function under different conditions. In this review we focus on the transcriptional pathways that control tumor infiltrating CD8+ T cell (TIL) function. Simultaneous restraint of overlapping inhibitory pathways may confer TIL resistance to multiple mechanisms of suppression traditionally referred to as exhaustion, tolerance, or anergy. Although decades of work have laid a solid foundation of altered transcriptional networks underlying various subsets of hypofunctional or “dysfunctional” CD8+ T cells, an understanding of the relevance in TIL has just begun. With recent technological advances, it is now feasible to further elucidate and utilize these pathways in immunotherapy platforms that seek to increase TIL function.

Molecular mechanisms of T-cell anergy

Anergy is a long-term stable state of T-lymphocyte unresponsiveness to antigenic stimulation associated with the blockade of IL-2 production and proliferation. Anergy is a pathway of peripheral tolerance formation. In this review, mechanisms underlying T-cell tolerization are considered in a classical in vitro model of clonal anergy, and these mechanisms are compared with different pathways of anergy induction in vivo. Special attention is given to regulatory T-lymphocytes because, on one hand, anergy is a specific feature of these cells, and on the other hand anergy is also a mechanism of their action on target cells - effector T-lymphocytes. The role of this phenomenon in the differentiation of regulatory T-cells and also in the development of activation-induced apoptosis in effector T-lymphocytes is discussed.

NFATc1/αA: The other Face of NFAT Factors in Lymphocytes

In effector T and B cells immune receptor signals induce within minutes a rise of intracellular Ca++, the activation of the phosphatase calcineurin and the translocation of NFAT transcription factors from cytosol to nucleus. In addition to this first wave of NFAT activation, in a second step the occurrence of NFATc1/αA, a short isoform of NFATc1, is strongly induced. Upon primary stimulation of lymphocytes the induction of NFATc1/αA takes place during the G1 phase of cell cycle. Due to an auto-regulatory feedback circuit high levels of NFATc1/αA are kept constant during persistent immune receptor stimulation. Contrary to NFATc2 and further NFATc proteins which dampen lymphocyte proliferation, induce anergy and enhance activation induced cell death (AICD), NFATc1/αA supports antigen-mediated proliferation and protects lymphocytes against rapid AICD. Whereas high concentrations of NFATc1/αA can also lead to apoptosis, in collaboration with NF-κB-inducing co-stimulatory signals they support the survival of mature lymphocytes in late phases after their activation. However, if dysregulated, NFATc1/αA appears to contribute to lymphoma genesis and - as we assume - to further disorders of the lymphoid system. While the molecular details of NFATc1/αA action and its contribution to lymphoid disorders have to be investigated, NFATc1/αA differs in its generation and function markedly from all the other NFAT proteins which are expressed in lymphoid cells. Therefore, it represents a prime target for causal therapies of immune disorders in future.

T follicular helper cells during immunity and tolerance

Helper T cells are required for the generation of a potent immune response to foreign antigens. Amongst them, T follicular helper (Tfh) cells are specialized in promoting protective, long-lived antibody responses that arise from germinal centers. Within these structures, the specificity of B cell receptors may change, due to the process of random somatic hypermutation aimed at increasing the overall affinity of the antibody response. The danger of emerging self-reactive specificities is offset by a stringent selection mechanism delegated in great part to Tfh cells. Only those B cells receiving survival signals from Tfh cells can exit the germinal centers to join the long-lived pools of memory B cells and bone marrow-homing plasma cells. Thus, a crucial immune tolerance checkpoint to prevent long-term autoantibody production lies in the ability to tolerize Tfh cells and to control positive and negative selection signals delivered by this subset. This review tackles the known mechanisms that ensure Tfh tolerance, many of them shared by other T helper subsets during thymic development and priming, but others unique to Tfh cells. Amongst the latter are checkpoints at the stages of Tfh differentiation, follicular migration, growth, longevity, and quality control of selection signals. Finally, we also discuss the consequences of a breakdown in Tfh tolerance.

The epithelial gatekeeper against food allergy

The rapid rise of allergic disorders in developed countries has been attributed to the hygiene hypothesis, implicating that increased environmental sanitation in early childhood may be associated with higher incidence of hypersensitivity. Intestinal epithelial barriers play a crucial role in the maintenance of gut homeostasis by limiting penetration of luminal bacteria and dietary allergens, yet allowing antigen sampling via the follicle-associated epithelium for generation of tolerance. However, this intricate balance is upset in allergic intestines, whereby luminal proteins with antigenic properties gain access to the subepithelial compartment and stimulate mast cell degranulation. Recent studies demonstrated that food allergens were protected from lysosomal degradation, and were transported in large quantities across the epithelium by binding to cell surface IgE/CD23 (FcepsilonRII) that prevented the antigenic protein from lysosomal degradation in enterocytes. IL-4 (a Th2-type cytokine) not only increased production of IgE from B cells, but also upregulated the expression of CD23 on intestinal epithelial cells. Further studies indicated that CD23 was responsible for the bidirectional transport of IgE across epithelium. The presence of IgE/CD23 opens a gate for intact dietary allergens to transcytose across the epithelial cells, and thus foments the mast cell-dependent anaphylactic responses. The understanding of the molecular mechanism responsible for epithelial barrier defects may be helpful in designing novel therapies to treat food allergy and other allergic diseases.

The molecular signature of CD8+ T cells undergoing deletional tolerance

Peripheral tolerance induction is critical for the maintenance of self-tolerance and can be mediated by immunoregulatory T cells or by direct induction of T-cell anergy or deletion. Although the molecular processes underlying anergy have been extensively studied, little is known about the molecular basis for peripheral T-cell deletion. Here, we determined the gene expression signature of peripheral CD8(+) T cells undergoing deletional tolerance, relative to those undergoing immunogenic priming or lymphopenia-induced proliferation. From these data, we report the first detailed molecular signature of cells undergoing deletion. Consistent with defective cytolysis, these cells exhibited deficiencies in granzyme up-regulation. Furthermore, they showed antigen-driven Bcl-2 down-regulation and early up-regulation of the proapoptotic protein Bim, consistent with the requirement of this BH3-only protein for peripheral T-cell deletion. Bim up-regulation was paralleled by defective interleukin-7 receptor alpha (IL-7Ralpha) chain reexpression, suggesting that Bim-dependent death may be triggered by loss of IL-7/IL-7R signaling. Finally, we observed parallels in molecular signatures between deletion and anergy, suggesting that these tolerance pathways may not be as molecularly distinct as previously surmised.

Identification of the genes specifically expressed in orally tolerized T cells

Oral tolerance is the systemic immunological unresponsiveness that occurs after feeding protein antigens. Its physiological role is thought to be the prevention of hypersensitivity to food antigens, and its therapeutic use to treat inflammatory diseases has been suggested. Although it has been shown that CD4(+) T cells mediate oral tolerance, the precise molecular mechanisms remain unclear. In the present study, we employed suppression subtractive hybridization and identified 10 genes specifically expressed in orally tolerized T cells. These included genes that were interesting in terms of their putative functions in the negative regulation of T cell activation, e.g. Culin 1, LAX, and Zfhx1b, as well as four genes that encoded unknown proteins. We further investigated the expression of these genes in hyporesponsive T cells induced in vitro (in vitro anergized T cells). We found that six of the 10 genes were highly expressed in these cells, and kinetic studies suggested that one was associated with the induction of anergy, while the other five were associated with the maintenance of anergy. The remaining 4 genes that were not expressed in in vitro anergized T cells are also of interest as they may play a specific role in in vivo T cell tolerance. Functional analysis of these genes should help to understand the complex mechanisms underlying the induction and maintenance of oral tolerance, and moreover, in vivo immune tolerance in general.

Reciprocal NFAT1 and NFAT2 nuclear localization in CD8+ anergic T cells is regulated by suboptimal calcium signaling

Anergy is an important mechanism of maintaining peripheral immune tolerance. T cells rendered anergic are refractory to further stimulation and are characterized by defective proliferation and IL-2 production. We used a model of in vivo anergy induction in murine CD8+ T cells to analyze the initial signaling events in anergic T cells. Tolerant T cells displayed reduced phospholipase Cgamma activation and calcium mobilization, indicating a defect in calcium signaling. This correlated with a block in nuclear localization of NFAT1 in anergic cells. However, we found that stimulation of anergic, but not naive T cells induced nuclear translocation of NFAT2. This suggested that NFAT2 is activated preferentially by reduced calcium signaling, and we confirmed this hypothesis by stimulating naive T cells under conditions of calcium limitation or partial calcineurin inhibition. Thus, our work provides new insight into how T cell stimulation conditions might dictate specific NFAT isoform activation and implicates NFAT2 involvement in the expression of anergy-related genes.

Molecular mechanisms for adaptive tolerance and other T cell anergy models

Since the original description of T cell anergy in CD4 clones from mice and humans, a number of different unresponsive states have been described, both in vivo and in vitro, that have been called anergic. While initial attempts were made to understand the similarities between the different models, it has now become clear from biochemical experiments that many of them have different molecular mechanisms underlying their unresponsiveness. In this review we will detail our own work on the in vivo model referred to as adaptive tolerance and then attempt to compare this biochemical state to the multitude of other states that have been described in the literature.

mTOR at the crossroads of T cell proliferation and tolerance

Several events control the activation, proliferation, and the continued Ag responsiveness of naïve and memory T lymphocytes. Here we review the individual contributions of TCR, CD28, and IL-2-driven signaling to T cell proliferation and anergy avoidance. The role of mTOR as a rheostat capable of integrating extracellular, plasma membrane-associated, and intracellular signals with relevance to T cell priming and tolerance is discussed.

The cyclin-dependent kinase inhibitor p27kip1 is required for transplantation tolerance induced by costimulatory blockade

The cyclin-dependent kinase (CDK) inhibitor p27kip1 is an important negative regulator of the cell cycle that sets a threshold for mitogenic signals in T lymphocytes, and is required for T cell anergy in vitro. To determine whether p27(kip1) is required for tolerance in vivo, we performed cardiac allograft transplantation under conditions of combined CD28/CD40L costimulatory blockade. Although this treatment induced long-term allograft survival in wild-type recipients, costimulatory blockade was no longer sufficient to induce tolerance in mice lacking p27kip1. Rejected allografts from p27kip1-/- mice contained more CD4+ T lymphocytes and exhibited more tissue damage than allografts from tolerant, wild-type mice. Infiltrating p27kip1-deficient T cells, but not wild-type T cells, exhibited nuclear expression of cyclins E and A, indicating uncontrolled T cell cycle progression in the graft. The failure of tolerance in p27kip1-/- mice was also accompanied by markedly increased numbers of allospecific, IFN-gamma-producing cells in the periphery, and occurred despite apparently normal regulatory T cell activity. These data demonstrate that the CDK inhibitor p27kip1 enforces the costimulatory requirement for the expansion and differentiation of alloimmune effector T lymphocytes in vivo, and point to CDKs as novel targets for immunosuppressive or tolerance-inducing therapies.

Distinctly different sensitivity in the induction and reversal of anergy of Th1 and Th2 cells

T cell anergy is one of the mechanisms of immunological tolerance. We examined in this study the distinct responses of Th1 and Th2 cells to in vitro anergic stimulation using Th1 and Th2 cells from two strains of T cell receptor transgenic mice. Proliferation of the Th2 cells was difficult to suppress by anergic stimulation, while that of Th1 cells was significantly inhibited even by weak stimulation. However, IL-4 production by Th2 cells was definitely reduced by anergic stimulation, although the inhibition level of IL-4 was lower than that of IFN-gamma production by Th1 cells. We also examined the reversal of anergy in both subsets. While both the anergized Th1 and Th2 cells responded to IL-2 stimulation, only the anergy of the Th2 cells could be reversed. This result indicates that progression of the cell cycle was not sufficient for anergy reversal in Th1 cells. Our findings indicate that the induction and reversal of T cell anergy might be affected by the distinct signaling features of Th1 and Th2 cells.

Cigarette tar phenols impede T cell cycle progression by inhibiting cyclin-dependent kinases

Cigarette smoking causes profound suppression of pulmonary T cell responses, which is associated with increased susceptibility to respiratory tract infections and decreased tumor surveillance. We previously demonstrated that the phenolic compounds in cigarette tar inhibit blastogenesis and interfere with human T cell cycle progression. To identify the mechanism by which cell cycle arrest occurs, we examined the effects of these compounds on cyclin-dependent kinases (Cdk) that control the G0/G1 transition. We found that hydroquinone inhibited induction of Cdk4 and Cdk6 kinase activities by >80%, while catechol and phenol were markedly less potent. HQ did not affect mitogenic induction of the Cdk6 protein, but inhibited expression of cyclin D3 by >90% resulting in a dramatic reduction in proper Cdk6/Cyclin D3 complex formation.

Food antigen causes TH2-dependent enteropathy followed by tissue repair in T-cell receptor transgenic mice

BACKGROUND

Clarification of the mechanisms underlying the development of food-sensitive intestinal inflammation will provide an important clue to combating food allergies.

OBJECTIVE

To establish a model of intestinal inflammation caused by oral administration of antigen without additional treatments, we focused on the ovalbumin (OVA) 23-3 T-cell receptor transgenic mouse, which had been reported to have high serum antigen-specific IgE responses to the feeding of an egg white diet.

METHODS

Changes in body weight of mice fed an egg white diet were monitored throughout the 28-day experimental period. After the 28-day feeding, intestinal tissues were harvested for histologic examination. Endogenous production of cytokines and histamine in the jejunum, and production of cytokines secreted by OVA-specific CD4+ T cells purified from mesenteric lymph nodes, were analyzed.

RESULTS

Egg white diet-fed OVA23-3 mice developed weight loss and inflammation with villous atrophy and goblet cell hyperplasia, especially in the jejunum. A further characteristic feature was evidence of weight recovery and tissue repair. Jejunal inflammation was also observed in egg white diet-fed recombination activating gene (RAG)-2-deficient OVA23-3 mice. In addition, tissue sections revealed significant infiltration of specific IgE-positive cells and IgE-positive degranulating mast cells. Higher levels of IL-4 and significant levels of histamine were detected in the tissues. In the supernatant of OVA-stimulated T cells, IL-10 levels were also markedly elevated.

CONCLUSION

We report that high-dose and continuous intake of primitive OVA alone induces enteropathy containing regions under repair in OVA23-3 mice. Antigen-specific T cells and inflammatory cells primed by T(H)2 responses play important roles in regulation of development and improvement of the disease.

CLINICAL IMPLICATIONS

Long-term antigen intake causes T(H)2-dependent and food-sensitive enteropathy followed by tissue repair.

Impaired Activation and Localization of LAT in Anergic T Cells as a Consequence of a Selective Palmitoylation Defect

The molecular basis of T cell anergy is not completely understood. We show that in antigen-primed anergic murine CD4(+) T cells the linker for activation of T cells (LAT) is hypophosphorylated upon CD3/CD28 restimulation. Signaling events downstream of LAT (PLCgamma1 phosphorylation and p85 [PI3-K] association) were impaired, whereas upstream events (CD3zeta and ZAP-70 phosphorylation) remained intact. LAT recruitment to the immunological synapse and its localization in detergent-resistant membrane (DRM) fractions were defective in anergic T cells. These defects resulted from impaired palmitoylation of LAT and were selective since the DRM localization and palmitoylation of Fyn were intact. This LAT defect was independent of Cbl-b and did not reflect enhanced LAT degradation. These results identify LAT as the most upstream target of anergy induction; moreover, they suggest that regulation of the amount of LAT in the immunological synapse and DRM by posttranslational palmitoylation contributes to the induction of T cell anergy.

Molecular mechanisms underlying FOXP3 induction in human T cells

FOXP3 is playing an essential role for T regulatory cells and is involved in the molecular mechanisms controlling immune tolerance. Although the biological relevance of this transcription factor is well documented, the pathways responsible for its induction are still unclear. The current study reveals structure and function of the human FOXP3 promoter, revealing essential molecular mechanisms of its induction. The FOXP3 promoter was defined by RACE, cloned, and functionally analyzed using reporter-gene constructs in primary human T cells. The analysis revealed the basal, T cell-specific promoter with a TATA and CAAT box 6000 bp upstream the translation start site. The basal promoter contains six NF-AT and AP-1 binding sites, which are positively regulating the trans activation of the FOXP3 promoter after triggering of the TCR. The chromatin region containing the FOXP3 promoter was bound by NF-ATc2 under these conditions. Furthermore, FOXP3 expression was observed following TCR engagement. Promoter activity, mRNA, and protein expression of T cells were suppressed by addition of cyclosporin A. Taken together, this study reveals the structure of the human FOXP3 promoter and provides new insights in mechanisms of addressing T regulatory cell-inducing signals useful for promoting immune tolerance. Furthermore, the study identifies essential, positive regulators of the FOXP3 gene and highlights cyclosporin A as an inhibitor of FOXP3 expression contrasting other immunosuppressants such as steroids or rapamycin.

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.

Orally tolerized T cells can form conjugates with APCs but are defective in immunological synapse formation

Oral tolerance is systemic immune hyporesponsiveness induced by the oral administration of soluble Ags. Hyporesponsiveness of Ag-specific CD4 T cells is responsible for this phenomenon. However, the molecular mechanisms underlying the hyporesponsive state of these T cells are not fully understood. In the present study, we investigated the ability of orally tolerized T cells to form conjugates with Ag-bearing APCs and to translocate TCR, protein kinase C-theta (PKC-theta), and lipid rafts into the interface between T cells and APCs. Orally tolerized T cells were prepared from the spleens of OVA-fed DO11.10 mice. Interestingly, the orally tolerized T cells did not show any impairment in the formation of conjugates with APCs. The conjugates were formed in a LFA-1-dependent manner. Upon antigenic stimulation, the tolerized T cells could indeed activate Rap1, which is critical for LFA-1 activation and thus cell adhesion. However, orally tolerized T cells showed defects in the translocation of TCR, PKC-theta, and lipid rafts into the interface between T cells and APCs. Translocation of TCR and PKC-theta to lipid raft fractions upon antigenic stimulation was also impaired in the tolerized T cells. Ag-induced activation of Vav, Rac1, and cdc42, which are essential for immunological synapse and raft aggregation, were down-regulated in orally tolerized T cells. These results demonstrate that orally tolerized T cells can respond to specific Ags in terms of conjugate formation but not with appropriate immunological synapse formation. This may account for the hyporesponsive state of orally tolerized T cells.

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.

Ovalbumin-specific IgE modulates ovalbumin-specific T-cell response after repetitive oral antigen administration

BACKGROUND

Some patients outgrow their food allergies even though their serum antigen-specific IgE levels remain high.

OBJECTIVE

To elucidate the role of T cells in outgrowing food allergies in the presence of antigen-specific IgE, we tracked antigen-specific T-cell responses after oral antigen administration.

METHODS

Ovalbumin (OVA)-specific T-cell receptor (TCR) and OVA-specific IgE transgenic (Tg) mice (OVA-TCR/IgE-Tg) and OVA-specific TCR Tg (OVA-TCR-Tg) mice were fed with high doses of OVA or PBS every other day. After 7 administrations, OVA-specific proliferation and cytokine production of mononuclear cells of the spleen, mesenteric lymph nodes, and Peyer's patches and the number of splenic CD4 + CD25 + T cells were analyzed.

RESULTS

Without OVA administration, the splenocytes from OVA-TCR/IgE-Tg mice exhibited a higher proliferative response and produced more IL-4 and IL-10 and less IFN-gamma than those from OVA-TCR-Tg mice. The proliferative responses of the splenocytes from either OVA-TCR/IgE-Tg mice or OVA-TCR-Tg mice fed with OVA were significantly reduced compared with those from PBS-fed mice. The number of OVA-specific TCR + T cells decreased in the spleen from OVA-fed mice, whereas the number of CD4 + CD25 + T cells increased. The suppressed proliferation of splenocytes of OVA-fed mice was partially resumed by neutralization of TGF-beta1, but not of IL-10.

CONCLUSION

The presence of OVA-specific IgE modulated the OVA-specific responses of the splenocytes. Irrespective of the presence of OVA-specific IgE, repetitive oral administration of OVA induced tolerance, which seems to be composed of clonal deletion/anergy and TGF-beta1-mediated active suppression.

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.

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.

Hyporesponsiveness of CD4 T cells induced in oral tolerance is maintained by selective impairment in the TCR-induced calcium/NFAT signaling pathway resulting from caspase activation

We examined intracellular signaling of hyporesponsive CD4 T cells induced by continuous feeding with high-dose antigen in a TCR transgenic mouse system. The results demonstrated a selective impairment in their TCR-induced calcium/NFAT-signaling pathway. Proteomic analysis revealed caspase activation in these cells, which resulted in cleavage of GADS. Further analysis of the TCR-signaling complex showed that GADS-LAT-SLP-76-associated PLC-gamma1 was decreased in both phosphorylation and association. Thus, as a consequence of caspase activation, orally tolerant CD4 T cells could not form normal TCR signaling complexes associated with GADS and showed downregulated PLC-gamma1 activation, which resulted in impairment of TCR-induced calcium signaling.

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.

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.

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.

Oral tolerance

Autoimmune conditions caused by injurious immune responses against self-antigens can be ameliorated if the inappropriate responses to self-components that cause tissue injury can be modulated by regulatory cells or shut off via the induction of anergy or via deletion of pathogenic immune responses. Antigen encounter at the gut mucosa can lead to suppression of injurious immune responses to self-antigen via these mechanisms. This type of immunological event is termed oral tolerance. In this review, we examine the mechanisms behind the induction of oral tolerance and provide findings from its use as a form of treatment for autoimmune diseases.

Ca2+ Signaling in Identified T-lymphocytes from Human Intestinal Mucosa

Ca2+ entry across the plasma membrane is necessary for the activation and proliferation of T-lymphocytes. Human intestinal lamina propria lymphocytes physiologically exhibit minimal proliferation in response to antigen receptor stimulation when compared with peripheral blood T-lymphocytes. This hyporeactivity is partially abolished in inflammatory bowel disease. We hypothesized that differences in Ca2+ signaling could be related to the disease. To test this possibility, we measured Ca2+ signals in identified lymphocytes from human blood and human intestinal mucosa. Ca2+ signals in lamina propria T-lymphocytes from non-inflamed tissue were drastically reduced when compared with Ca2+ signals of blood T-lymphocytes from the same persons. However, Ca2+ signals in T-lymphocytes from inflamed intestinal mucosa were much higher than the ones from non-inflamed mucosa and almost reached levels of Ca2+ signals in peripheral blood T-cells. Furthermore, Ca2+ influx was closely linked to cell proliferation in both peripheral blood T-lymphocytes and lamina propria lymphocytes cells. We conclude that differences in Ca2+ signaling can explain the differences of T-lymphocyte reactivity in blood versus lamina propria and, importantly, also between T-lymphocytes from inflamed and non-inflamed intestinal mucosa. Ca2+ channels in the plasma membrane of T-lymphocytes might thus prove an excellent target to screen for immunosuppressiva to potentially treat the symptoms of inflammatory bowel disease.

Scaffolding of antigen receptors for immunogenic versus tolerogenic signaling

Lymphocyte antigen receptors are responsible for inducing the opposite responses of immunity or tolerance. How the correct polarity of antigen receptor signaling is encoded has been an enduring enigma. Here we summarize recent advances defining key scaffolding molecules, CARMA1 (also known as CARD11) and the Cbl family of ubiquitin ligases, required for either immunogenic or tolerogenic signaling by antigen receptors. These scaffolding proteins may determine the polarity of response to antigen by promoting assembly around antigen receptors of competing multiprotein signal complexes: immunosomes versus tolerosomes. Each of the factors that influence immunogenicity or tolerogenicity--stage of lymphocyte differentiation, concurrent engagement of inhibitory or costimulatory receptors, extent of receptor crosslinking, and prior antigen experience--may be integrated in lymphocytes through their capacity to influence the probability of assembling immunosomes versus tolerosomes.

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.