T-cell anergy

Up-regulation of FOXP3 and induction of suppressive function in CD4+ Jurkat T-cells expressing hepatitis C virus core protein

HCV (hepatitis C virus) infection is a serious health care problem that affects more than 170 million people worldwide. Viral clearance depends on the development of a successful cellular immune response against the virus. Interestingly, such a response is altered in chronically infected patients, leading to chronic hepatitis that can result in liver fibrosis, cirrhosis and hepatocellular carcinoma. Among the mechanisms that have been described as being responsible for the immune suppression caused by the virus, Treg-cells (regulatory T-cells) are emerging as an essential component. In the present work we aim to study the effect of HCV-core protein in the development of T-cells with regulatory-like function. Using a third-generation lentiviral system to express HCV-core in CD4+ Jurkat T-cells, we describe that HCV-core-expressing Jurkat cells show an up-regulation of FOXP3 (forkhead box P3) and CTLA-4 (cytotoxic T-lymphocyte antigen-4). Moreover, we show that HCV-core-transduced Jurkat cells are able to suppress CD4+ and CD8+ T-cell responses to anti-CD3 plus anti-CD28 stimulation.

Anergic CD8+ T lymphocytes have impaired NF-κB activation with defects in p65 phosphorylation and acetylation

Because of the cytotoxic potential of CD8(+) T cells, maintenance of CD8(+) peripheral tolerance is extremely important. A major peripheral tolerance mechanism is the induction of anergy, a refractory state in which proliferation and IL-2 production are inhibited. We used a TCR transgenic mouse model to investigate the signaling defects in CD8(+) T cells rendered anergic in vivo. In addition to a previously reported alteration in calcium/NFAT signaling, we also found a defect in NF-κB-mediated gene transcription. This was not due to blockade of early NF-κB activation events, including IκB degradation and NF-κB nuclear translocation, as these occurred normally in tolerant T cells. However, we discovered that anergic cells failed to phosphorylate the NF-κB p65 subunit at Ser(311) and also failed to acetylate p65 at Lys(310). Both of these modifications have been implicated as critical for NF-κB transactivation capacity, and thus, our results suggest that defects in key phosphorylation and acetylation events are important for the inhibition of NF-κB activity (and subsequent T cell function) in anergic CD8(+) T cells.

Role of dendritic cell maturity/costimulation for generation, homeostasis, and suppressive activity of regulatory T cells

Tolerogenicity of dendritic cells (DCs) has initially been attributed exclusively to immature/resting stages, while mature/activated DCs were considered strictly immunogenic. Later, all different subsets among the myeloid/conventional DCs and plasmacytoid DCs have been shown to bear tolerogenic potential, so that tolerogenicity could not be attributed to a specific subset. Immunosuppressive treatments of immature DC subsets could prevent re-programming into mature DCs or upregulated inhibitory surface markers or cytokines. Furthermore, the different T cell tolerance mechanisms anergy, deletion, immune deviation, and suppression require different quantities and qualities of costimulation by DCs. Since expansion of regulatory T cells (Tregs) has been shown to be promoted best by fully mature DCs the role of CD80/B7-1 and CD86/B7-2 as major costimulatory molecules for Treg biology is under debate. In this review, we discuss the role of these and other costimulatory molecules on myeloid DCs and their ligands CD28 and CD152/CTLA-4 on Tregs for peripheral conversion from naive CD4⁺ T cells into the major subsets of Foxp3⁺ Tregs and Foxp3⁻ IL-10⁺ regulatory type-1 T cells (Tr1) or Tr1-like cells and their role for peripheral maintenance in the steady state and after activation.

Macroautophagy regulates energy metabolism during effector T cell activation

Macroautophagy is a highly conserved mechanism of lysosomal-mediated protein degradation that plays a key role in maintaining cellular homeostasis by recycling amino acids, reducing the amount of damaged proteins, and regulating protein levels in response to extracellular signals. We have found that macroautophagy is induced after effector T cell activation. Engagement of the TCR and CD28 results in enhanced microtubule-associated protein 1 light chain 3 (LC3) processing, increased numbers of LC3-containing vesicles, and increased LC3 flux, indicating active autophagosome formation and clearance. The autophagosomes formed in stimulated T cells actively fuse with lysosomes to degrade their cargo. Using a conditional KO mouse model where Atg7, a critical gene for macroautophagy, is specifically deleted in T cells, we have found that macroautophagy-deficient effector Th cells have defective IL-2 and IFN-γ production and reduced proliferation after stimulation, with no significant increase in apoptosis. We have found that ATP generation is decreased when autophagy is blocked, and defects in activation-induced cytokine production are restored when an exogenous energy source is added to macroautophagy-deficient T cells. Furthermore, we present evidence showing that the nature of the cargo inside autophagic vesicles found in resting T cells differs from the cargo of autophagosomes in activated T cells, where mitochondria and other organelles are selectively excluded. These results suggest that macroautophagy is an actively regulated process in T cells that can be induced in response to TCR engagement to accommodate the bioenergetic requirements of activated T cells.

Physiological roles of the TRPM4 channel extracted from background currents

Calcium-activated nonselective cationic currents have been known for 30 years, but their physiological implications have remained unresolved until the recent cloning of the TRPM4 ion channel. Since then, TRPM4 has been identified as a key modulator of numerous calcium-dependent mechanisms such as the immune response, insulin secretion, cerebral artery constriction, respiratory rhythm, and cardiac conduction.

Sprouty proteins inhibit receptor-mediated activation of phosphatidylinositol-specific phospholipase C

PLCγ03B3 binds Spry1 and Spry2. Overexpression of Spry decreased PLCγ03B3 activity and IP₃ and DAG production, whereas Spry-deficient cells yielded more IP₃. Spry overexpression inhibited T-cell receptor signaling and Spry1 null T-cells hyperproliferated with TCR ligation. Through action of PLCγ03B3, Spry may influence signaling through multiple receptors.

Sprouty (Spry) proteins are negative regulators of receptor tyrosine kinase signaling; however, their exact mechanism of action remains incompletely understood. We identified phosphatidylinositol-specific phospholipase C (PLC)-γ as a partner of the Spry1 and Spry2 proteins. Spry–PLCγ interaction was dependent on the Src homology 2 domain of PLCγ and a conserved N-terminal tyrosine residue in Spry1 and Spry2. Overexpression of Spry1 and Spry2 was associated with decreased PLCγ phosphorylation and decreased PLCγ activity as measured by production of inositol (1,4,5)-triphosphate (IP₃) and diacylglycerol, whereas cells deficient for Spry1 or Spry1, -2, and -4 showed increased production of IP₃ at baseline and further increased in response to growth factor signals. Overexpression of Spry 1 or Spry2 or small-interfering RNA-mediated knockdown of PLCγ1 or PLCγ2 abrogated the activity of a calcium-dependent reporter gene, suggesting that Spry inhibited calcium-mediated signaling downstream of PLCγ. Furthermore, Spry overexpression in T-cells, which are highly dependent on PLCγ activity and calcium signaling, blocked T-cell receptor-mediated calcium release. Accordingly, cultured T-cells from Spry1 gene knockout mice showed increased proliferation in response to T-cell receptor stimulation. These data highlight an important action of Spry, which may allow these proteins to influence signaling through multiple receptors.

Interaction of calmodulin with Bcl10 modulates NF-kappaB activation

Calcium signals resulting from antigen receptor activation are important in determining the responses of a T or B lymphocyte to an antigen. Calmodulin (CaM), a multi-functional sensor of intracellular calcium (Ca(2+)) signals in cells, is required in the pathway from the T cell receptor (TCR) to activation of the key transcription factor NF-kappaB. Here we searched for a partner in direct interaction with CaM in the pathway, and found that CaM interacts specifically with the signaling adaptor Bcl10. The binding is Ca(2+) dependent and of high affinity, with a K(d) of approximately 160 nM. Proximity of CaM and Bcl10 in vivo is induced by increases in the intracellular Ca(2+) level. The interaction is localized to the CARD domain of Bcl10, which interacts with the CARD domain of the upstream signaling partner Carma1. Binding of CaM to Bcl10 is shown to inhibit the ability of Bcl10 to interact with Carma1, an interaction that is required for signaling from the TCR to NF-kappaB. Furthermore, a mutant of Bcl10 with reduced binding to CaM shows increased activation of an NF-kappaB reporter, which is further enhanced by activating stimuli. We propose a novel mechanism whereby the Ca(2+) sensor CaM regulates T cell responses to antigens by binding to Bcl10, thereby modulating its interaction with Carma1 and subsequent activation of NF-kappaB.

Anergic CD4+ T cells form mature immunological synapses with enhanced accumulation of c-Cbl and Cbl-b

CD4(+) T cell recognition of MHC:peptide complexes in the context of a costimulatory signal results in the large-scale redistribution of molecules at the T cell-APC interface to form the immunological synapse. The immunological synapse is the location of sustained TCR signaling and delivery of a subset of effector functions. T cells activated in the absence of costimulation are rendered anergic and are hyporesponsive when presented with Ag in the presence of optimal costimulation. Several previous studies have looked at aspects of immunological synapses formed by anergic T cells, but it remains unclear whether there are differences in the formation or composition of anergic immunological synapses. In this study, we energized primary murine CD4(+) T cells by incubation of costimulation-deficient, transfected fibroblast APCs. Using a combination of TCR, MHC:peptide, and ICAM-1 staining, we found that anergic T cells make mature immunological synapses with characteristic central and peripheral supramolecular activation cluster domains that were indistinguishable from control synapses. There were small increases in total phosphotyrosine at the anergic synapse along with significant decreases in phosphorylated ERK 1/2 accumulation. Most striking, there was specific accumulation of c-Cbl and Cbl-b to the anergic synapses. Cbl-b, previously shown to be essential in anergy induction, was found in both the central and the peripheral supramolecular activation clusters of the anergic synapse. This Cbl-b (and c-Cbl) accumulation at the anergic synapse may play an important role in anergy maintenance, induction, or both.

Biomarkers to discern transplantation tolerance after allogeneic hematopoietic cell transplantation

Although it is commonly accepted that allogeneic hematopoietic cell transplant (HCT) recipients develop transplantation tolerance and can quickly discontinue all immunosuppressive drugs, existing data does not support this concept. Most patients will require a prolonged duration of immunosuppression, lasting commonly several years. This has even greater importance, as the majority of transplants are now performed utilizing peripheral blood mobilized stem cells, which are associated with an increased risk of chronic graft-versus-host disease (cGVHD) and prolonged duration of immunosuppression. Despite these challenges, the approach to liberation from immunosuppression after HCT is empiric, and biomarkers of operational tolerance after HCT are lacking. Conversely, investigators in solid organ allografting have begun to examine tolerance associated gene expression in renal and hepatic allograft recipients. Significant challenges in the design and interpretation of these studies potentially limit comparisons. However, a relatively unified model is beginning to emerge, which largely recapitulates previously established mechanisms of immune tolerance. This evidence supports a state of immune quiescence with reduced expression of costimulation and immune response genes, and upregulation of cell cycle control genes. Data indirectly supports the importance of tumor growth factor (TGF)-beta, supports the role of CD4(+)CD25(+) regulatory T cells, and offers new insights into the role of natural killer (NK) cells. Distinct in hepatic allograft tolerance, emerging evidence highlights the importance of gammadeltaT cells, and selection of the Vgammadelta1+ subtype among the gammadeltaT cell population. The deficiencies in the current understanding of transplantation tolerance after HCT, as well as the inadequacies evident in the current empiric approach to immunosuppressive medication (IS) management after HCT make clear the rationale for investigation aimed at elucidating tolerance associated biomarkers after HCT.

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.

Immature and maturation-resistant human dendritic cells generated from bone marrow require two stimulations to induce T cell anergy in vitro

Immature dendritic cells (DC) represent potential clinical tools for tolerogenic cellular immunotherapy in both transplantation and autoimmunity. A major drawback in vivo is their potential to mature during infections or inflammation, which would convert their tolerogenicity into immunogenicity. The generation of immature DC from human bone marrow (BM) by low doses of GM-CSF (lowGM) in the absence of IL-4 under GMP conditions create DC resistant to maturation, detected by surface marker expression and primary stimulation by allogeneic T cells. This resistence could not be observed for BM-derived DC generated with high doses of GM-CSF plus IL-4 (highGM/4), although both DC types induced primary allogeneic T cell anergy in vitro. The estabishment of the anergic state requires two subsequent stimulations by immature DC. Anergy induction was more profound with lowGM-DC due to their maturation resistance. Together, we show the generation of immature, maturation-resistant lowGM-DC for potential clinical use in transplant rejection and propose a two-step-model of T cell anergy induction by immature DC.

Pattern recognition receptors and control of adaptive immunity

The mammalian immune system effectively fights infection through the cooperation of two connected systems, innate and adaptive immunity. Germ-line encoded pattern recognition receptors (PRRs) of the innate immune system sense the presence of infection and activate innate immunity. Some PRRs also induce signals that lead to the activation of adaptive immunity. Adaptive immunity is controlled by PRR-induced signals at multiple checkpoints dictating the initiation of a response, the type of response, the magnitude and duration of the response, and the production of long-term memory. PRRs thus instruct the adaptive immune system on when and how to best respond to a particular infection. In this review, we discuss the roles of various PRRs in control of adaptive immunity.

Tryptophan deprivation induces inhibitory receptors ILT3 and ILT4 on dendritic cells favoring the induction of human CD4+CD25+ Foxp3+ T regulatory cells

Tryptophan catabolism through IDO activity can cause nonresponsiveness and tolerance acting on T cells. Given the crucial importance of dendritic cells (DCs) in the initiation of a T cell response, surprisingly little is known about the impact of IDO activity and tryptophan deprivation on DCs themselves. In the present study, we show that human DCs differentiated under low-tryptophan conditions acquire strong tolerogenic capacity. This effect is associated with a markedly decreased Ag uptake as well as the down-regulation of costimulatory molecules (CD40, CD80). In contrast, the inhibitory receptors ILT3 and ILT4 are significantly increased. Functionally, tryptophan-deprived DCs show a reduced capacity to stimulate T cells, which can be restored by blockade of ILT3. Moreover, ILT3(high)ILT4(high) DCs lead to the induction of CD4(+)CD25(+) Foxp3(+) T regulatory cells with suppressive activity from CD4(+)CD25(-) T cells. The generation of ILT3(high)ILT4(high) DCs with tolerogenic properties by tryptophan deprivation is linked to a stress response pathway mediated by the GCN2 kinase. These results demonstrate that tryptophan degradation establishes a regulatory microenvironment for DCs, enabling these cells to induce T regulatory cells. The impact of IDO thus extends beyond local immune suppression to a systemic control of the immune response.

STIM1-independent T cell development and effector function in vivo

Store-operated Ca(2+) entry (SOCE) is believed to be of pivotal importance in T cell physiology. To test this hypothesis, we generated mice constitutively lacking the SOCE-regulating Ca(2+) sensor stromal interaction molecule 1 (STIM1). In vitro analyses showed that SOCE and Ag receptor complex-triggered Ca(2+) flux into STIM1-deficient T cells is virtually abolished. In vivo, STIM1-deficient mice developed a lymphoproliferative disease despite normal thymic T cell maturation and normal frequencies of CD4(+)Foxp3(+) regulatory T cells. Unexpectedly, STIM1-deficient bone marrow chimeric mice mounted humoral immune responses after vaccination and STIM1-deficient T cells were capable of inducing acute graft-versus-host disease following adoptive transfer into allogeneic hosts. These results demonstrate that STIM1-dependent SOCE is crucial for homeostatic T cell proliferation, but of much lesser importance for thymic T cell differentiation or T cell effector functions.

PD-1/PD-L blockade prevents anergy induction and enhances the anti-tumor activities of glycolipid-activated invariant NKT cells

Invariant NKT (iNKT) cells recognize glycolipid Ags, such as the marine sponge-derived glycosphingolipid alpha-galactosylceramide (alphaGalCer) presented by the CD1d protein. In vivo activation of iNKT cells with alphaGalCer results in robust cytokine production, followed by the acquisition of an anergic phenotype. Here we have investigated mechanisms responsible for the establishment of alphaGalCer-induced iNKT cell anergy. We found that alphaGalCer-activated iNKT cells rapidly up-regulated expression of the inhibitory costimulatory receptor programmed death (PD)-1 at their cell surface, and this increased expression was retained for at least one month. Blockade of the interaction between PD-1 and its ligands, PD-L1 and PD-L2, at the time of alphaGalCer treatment prevented the induction iNKT cell anergy, but was unable to reverse established iNKT cell anergy. Consistently, injection of alphaGalCer into PD-1-deficient mice failed to induce iNKT cell anergy. However, blockade of the PD-1/PD-L pathway failed to prevent bacterial- or sulfatide-induced iNKT cell anergy, suggesting additional mechanisms of iNKT cell tolerance. Finally, we showed that blockade of PD-1/PD-L interactions enhanced the antimetastatic activities of alphaGalCer. Collectively, our findings reveal a critical role for the PD-1/PD-L costimulatory pathway in the alphaGalCer-mediated induction of iNKT cell anergy that can be targeted for the development of immunotherapies.

Transcriptional complexes formed by NFAT dimers regulate the induction of T cell tolerance

In T cells, anergy can be induced after T cell receptor engagement in the absence of costimulation. Under these conditions, the expression of a specific set of anergy-associated genes is activated. Several lines of evidence suggest that nuclear factor of activated T cells (NFAT) proteins may regulate the expression of many of those genes; however, the nature of the complexes responsible for the induction of this new program of gene expression is unknown. Here, we show that transcriptional complexes formed by NFAT homodimers are directly responsible for the activation of at least two anergy-inducing genes, Grail and Caspase3. Our data shows that Grail expression is activated by direct binding of NFAT dimers to the Grail promoter at two different sites. Consequently, a mutant NFAT protein with impaired ability to dimerize is not able to induce an unresponsive state in T cells. Our results not only identify a new biological function for NFAT dimers but also reveal the different nature of NFAT-containing complexes that induce anergy versus those that are activated during a productive immune response. These data also establish a basis for the design of immunomodulatory strategies that specifically target each type of complex.

Microarrays: monitoring for transplant tolerance and mechanistic insights

With recent advances in immunology and a growing understanding of transplantation biology, the development of reliable assays that may be used for identification and prediction of the current state of an immune response (rejection and tolerance) are urgently needed to allow us to predict the development of immunologic graft injury, individualize immunosuppression, rationally minimize immunosuppressive drug toxicity, promote a better understanding of the mechanisms underlying stable graft acceptance, and aid in the design of tolerance-inducing clinical transplantation trials. Microarrays can provide nonbiased, simultaneous global expression patterns for more than 40,000 human genes across different experiments. High throughput microarray technology offers a means to study disease-specific transcriptional changes in tissue biopsy, peripheral blood, and biofluids.

Protein kinase C signaling during T cell activation induces the endoplasmic reticulum stress response

T cell receptor (TCR) ligation (signal one) in the presence of co-stimulation (signal two) results in downstream signals that increase protein production enabling naïve T cells to fully activate and gain effector function. Enhanced production of proteins by a cell requires an increase in endoplasmic reticulum (ER) chaperone expression, which is accomplished through activation of a cellular mechanism known as the ER stress response. The ER stress response is initiated during the cascade of events that occur for the activation of many cells; however, this process has not been comprehensively studied for T cell function. In this study, we used primary T cells and mice circulating TCR transgenic CD8(+) T cells to investigate ER chaperone expression in which TCR signaling was initiated in the presence or absence of co-stimulation. In the presence of both signals, in vitro and in vivo analyses demonstrated induction of the ER stress response, as evidenced by elevated expression of GRP78 and other ER chaperones. Unexpectedly, ER chaperones were also increased in T cells exposed only to signal one, a treatment known to cause T cells to enter the 'nonresponsive' states of anergy and tolerance. Treatment of T cells with an inhibitor to protein kinase C (PKC), a serine/threonine protein kinase found downstream of TCR signaling, indicated PKC is involved in the induction of the ER stress response during the T cell activation process, thus revealing a previously unknown role for this signaling protein in T cells. Collectively, these data suggest that induction of the ER stress response through PKC signaling is an important component for the preparation of a T cell response to antigen.

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

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

The calcium-activated nonselective cation channel TRPM4 is essential for the migration but not the maturation of dendritic cells

Dendritic cell (DC) maturation and migration are events critical for the initiation of immune responses. After encountering pathogens, DCs upregulate the expression of costimulatory molecules and subsequently migrate to secondary lymphoid organs. Calcium (Ca(2+)) entry governs the functions of many hematopoietic cell types, but the role of Ca(2+) entry in DC biology remains unclear. Here we report that the Ca(2+)-activated nonselective cation channel TRPM4 was expressed in and controlled the Ca(2+) homeostasis of mouse DCs. The absence of TRPM4, which elicited Ca(2+) overload, did not influence DC maturation but did considerably impair chemokine-dependent DC migration. Our results establish TRPM4-regulated Ca(2+) homeostasis as crucial for DC mobility but not maturation and emphasize that DC maturation and migration are independently regulated.

Extrathymic mechanisms of T cell tolerance: lessons from autoimmune gastritis

While the thymus plays a key role in the prevention of many autoimmune phenomena it is clear that robust mechanisms external to the thymus are also vital in controlling self-reactive T cells. Here we review the current concepts in the field of extrathymic tolerance and use recent studies of autoimmune gastritis to illustrate how T cells directed to a prominent, clinically relevant autoantigen, namely the gastric proton pump, can be silenced with little or no thymic involvement. Autoimmune gastritis represents one of the most thoroughly characterised autoimmune systems and the knowledge and tools available to study this disease will continue to allow a thorough assessment of the genetic, cellular and molecular events that underlie tolerance and autoimmunity.

Blockade of CD11a by efalizumab in psoriasis patients induces a unique state of T-cell hyporesponsiveness

Efalizumab (anti-CD11a) interferes with LFA-1/ICAM-1 binding and inhibits several key steps in psoriasis pathogenesis. This study characterizes the effects of efalizumab on T-cell activation responses and expression of surface markers on human circulating psoriatic T cells during a therapeutic trial. Our data suggest that efalizumab may induce a unique type of T-cell hyporesponsiveness, directly induced by LFA-1 binding, which is distinct from conventional anergy described in animal models. Direct activation of T cells through different activating receptors (CD2, CD3, CD3/28) is reduced, despite T cells being fully viable. This hyporesponsiveness was spontaneously reversible after withdrawal of the drug, and by IL-2 in vitro. In contrast to the state of anergy, Ca(+2) release is intact during efalizumab binding. Furthermore, lymphocyte function-associated antigen-1 (LFA-1) blockade resulted in an unexpected downregulation of a broad range of surface molecules, including the T-cell receptor complex, co-stimulatory molecules, and integrins unrelated to LFA-1, both in the peripheral circulation and in diseased skin tissue. These observations provide evidence for the mechanism of action of efalizumab. The nature of this T-cell hyporesponsiveness suggests that T-cell responses may be reduced during efalizumab therapy, but are reversible after ceasing efalizumab treatment.

Adoptively transferred dendritic cells restore primary cell-mediated inflammatory competence to acutely malnourished weanling mice

Immune depression associated with prepubescent malnutrition underlies a staggering burden of infection-related morbidity. This investigation centered on dendritic cells as potentially decisive in this phenomenon. C57BL/6J mice, initially 19 days old, had free access for 14 days to a complete diet or to a low-protein formulation that induced wasting deficits of protein and energy. Mice were sensitized by i.p. injection of sheep red blood cells on day 9, at which time one-half of the animals in each dietary group received a simultaneous injection of 10(6) syngeneic dendritic cells (JAWS II). All mice were challenged with the immunizing antigen in the right hind footpad on day 13, and the 24-hour delayed hypersensitivity response was assessed as percentage increase in footpad thickness. The low-protein diet reduced the inflammatory immune response, but JAWS cells, which exhibited immature phenotypic and functional characteristics, increased the response of both the malnourished group and the controls. By contrast, i.p. injection of 10(6) syngeneic T cells did not influence the inflammatory immune response of mice subjected to the low-protein protocol. Antigen-presenting cell numbers limited primary inflammatory cell-mediated competence in this model of wasting malnutrition, an outcome that challenges the prevailing multifactorial model of malnutrition-associated immune depression. Thus, a new dendritic cell-centered perspective emerges regarding the cellular mechanism underlying immune depression in acute pediatric protein and energy deficit.

Regulatory T cells prevent CD8 T cell maturation by inhibiting CD4 Th cells at tumor sites

Natural regulatory T cells (Tregs) are present in high frequencies among tumor-infiltrating lymphocytes and in draining lymph nodes, supposedly facilitating tumor development. To investigate their role in controlling local immune responses, we analyzed intratumoral T cell accumulation and function in the presence or absence of Tregs. Tumors that grew in normal BALB/c mice injected with the 4T1 tumor cell line were highly infiltrated by Tregs, CD4 and CD8 cells, all having unique characteristics. Most infiltrating Tregs expressed low levels of CD25Rs and Foxp3. They did not proliferate even in the presence of IL-2 but maintained a strong suppressor activity. CD4 T cells were profoundly anergic and CD8 T cell proliferation and cytotoxicity were severely impaired. Depletion of Tregs modified the characteristics of tumor infiltrates. Tumors were initially invaded by activated CD4(+)CD25(-) T cells, which produced IL-2 and IFN-gamma. This was followed by the recruitment of highly cytotoxic CD8(+) T cells at tumor sites leading to tumor rejection. The beneficial effect of Treg depletion in tumor regression was abrogated when CD4 helper cells were also depleted. These findings indicate that the massive infiltration of tumors by Tregs prevents the development of a successful helper response. The Tregs in our model prevent Th cell activation and subsequent development of efficient CD8 T cell activity required for the control of tumor growth.

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.

Hepatitis C virus core protein up-regulates anergy-related genes and a new set of genes, which affects T cell homeostasis

Hepatitis C virus (HCV) infection is the main cause for chronic hepatitis, leading to cirrhosis and hepatic carcinoma. Virally induced immune dysfunction has been called as the cause for viral persistence. Previous results demonstrate that CD4 Jurkat cells stably expressing the HCV core protein show an increased activation of NFAT transcription factor and an impaired IL-2 promoter activity, affecting intracellular signaling pathways in a manner that mimics clonal anergy. We had shown previously that NFAT activates a transcriptional program, ensuing in immunological tolerance. In the present work, we have engineered lentiviral vectors expressing the HCV core to analyze the events, which unfold in the initial phase of HCV core-induced anergy. We show that genes initially described to be up-regulated by ionomycin-induced anergy in mice are also up-regulated in humans, not only by ionomycin but also by HCV core expression. We also show that HCV core is sufficient to cause NFAT nuclear translocation and a slow-down in cell-cycle progression, and using whole genome microarrays, we identify novel genes up-regulated in Jurkat cells expressing HCV core. The relevance of our results is highlighted by the presence of HCV in CD4 T cells from HCV chronically infected patients.

Modulation of NFAT-dependent gene expression by the RhoA signaling pathway in T cells

We have reported previously that p115Rho guanine nucleotide exchange factor, its upstream activator Galpha13, and its effector RhoA are able to inhibit HIV-1 replication. Here, we show that RhoA is able to inhibit HIV-1 gene expression through the NFAT-binding site in the HIV long-terminal repeat. Constitutively active NFAT counteracts the inhibitory activity of RhoA, and inhibition of NFAT activation also inhibits HIV-1 gene expression. We have shown further that RhoA inhibits NFAT-dependent transcription and IL-2 production in human T cells. RhoA does not inhibit nuclear localization of NFAT but rather, inhibits its transcriptional activity. In addition, RhoA decreases the level of acetylated histone H3, but not NFAT occupancy, at the IL-2 promoter. These data suggest that activation of RhoA can modulate IL-2 gene expression by inhibiting the transcriptional activity of NFAT and chromatin structure at the IL-2 promoter during T cell activation.

Response of T cells in vivo induced by repeated superantigen treatments at different time intervals

We have investigated the response of T cells to staphylococcal enterotoxin A (SEA) injections in vivo. We found that a single injection of SEA with an optimal dose of 10 microg increased the expression of both CD4 and CD8 significantly. There was expansion of SEA-reactive T cells in vivo after SEA re-injection and the time interval between injections strongly influenced the responsiveness of CD4+ and CD8+ T cells. Anergy of T cells was observed after three SEA treatments. The time interval between injections mainly affected the unresponsiveness of CD4+ T cells, not CD8+ T cells. Marked deletion followed by anergy of CD4+ T cells was induced at short intervals, and anergy without obvious deletion of CD4+ T cells was induced at long intervals. We also found that the anergic state was reversible in vivo. Repeated SEA stimulation led to down-regulation of interleukin (IL)-2, and high levels of IL-10. This study showed that both CD4+ and CD8+ SEA-primed T cells were responsive to SEA rechallenge in vivo, and a third injection was needed to induce the anergy of T cells.

The in vivo response of invariant natural killer T cells to glycolipid antigens

Invariant natural killer T (iNKT) cells are a subset of T lymphocytes that recognizes glycolipid antigens presented by the major histocompatibility complex class I-related protein CD1d. Although iNKT cells have received a lot of attention as targets for the development of immunotherapies, few studies have investigated the in vivo response of iNKT cells to glycolipid antigen activation. Accumulating evidence indicates that iNKT cells generate a dynamic response to in vivo activation by glycolipid antigens that is characterized by surface receptor downmodulation, expansion, cytokine production, cross talk with other cells, homeostatic contraction, and acquisition of an anergic phenotype. These studies provide new insight into the biology of iNKT cells and have important implications for designing safe and effective iNKT cell-based vaccines and therapies.

Transcriptional basis of lymphocyte tolerance

Signaling through lymphocyte antigen receptors has the potential to initiate several distinct outcomes: proliferation, differentiation, apoptosis, or functional unresponsiveness. Expansion and differentiation of effector T cells is required for defense against foreign antigens, whereas functional unresponsiveness, termed anergy, is a cell-intrinsic mechanism that contributes to peripheral self-tolerance. Other mechanisms of peripheral tolerance include the 'dominant' tolerance imposed by regulatory T cells and immunosuppression mediated by interleukin-10 and transforming growth factor-beta. T- and B-cell antigen receptor ligation induces an increase in intracellular calcium levels as well as activating additional signaling pathways that are further potentiated by costimulatory receptors. In this review, we argue that cell-intrinsic programs of peripheral anergy and tolerance are imposed by sustained calcium signaling in lymphocytes. We address in particular the role of the calcium-dependent transcription factor nuclear factor for activation of T cells, which is activated by antigen receptor stimulation and, depending on the presence or absence of input from its transcriptional partner, activator protein-1, dictates two distinct transcriptional programs: activation or tolerance.

Interleukin 2 gene transcription is regulated by Ikaros-induced changes in histone acetylation in anergic T cells

In T cells anergy may be evoked by an unbalanced stimulation of the T-cell receptor in the absence of costimulation. Anergic T cells are unresponsive to new antigen receptor engagement and do not produce interleukin 2. We present evidence that anergizing stimuli induce changes in histone acetylation, which mediates transcriptional repression of interleukin 2 expression. In response to calcium signaling, anergic T cells up-regulate the expression of Ikaros, a zinc finger transcription factor essential for lymphoid lineage determination. Ikaros binds to the interleukin 2 promoter where it induces histone deacetylation. Confirming the role of Ikaros in the induction of T-cell anergy, cells with reduced Ikaros activity show defective inactivation in response to an anergizing stimulus. We propose a model in which tolerizing stimuli induce epigenetic changes on the interleukin 2 locus that are responsible for the stable inhibition of the expression of this cytokine in anergic T cells.

Discrete T cell populations with specificity for a neo-self-antigen bear distinct imprints of tolerance

Mice expressing the Torpedo acetylcholine receptor alpha-chain as a neo-self-Ag exhibit a reduced frequency of T cells responding to the immunodominant epitope Talpha146-162 indicating a degree of tolerance. We characterized tolerance induction in these animals by analyzing the residual Talpha146-162-responsive T cell population and comparing it to that of nontransgenic littermates. Using CD4(high) sorting, we isolated the vast majority of Ag-reactive T cells from both strains of mice. Quantitative studies of the CD4(high) populations in transgenic mice following immunization with Talpha146-162 revealed a diminished expansion of cells expressing the canonical TCRBV6 but not other TCRBV gene segments when compared with nontransgenic littermates. In addition, CD4(high) cells from transgenic mice were functionally hyporesponsive to Talpha146-162 in terms of proliferation and cytokine secretion regardless of TCRBV gene segment use. TCR sequence analysis of transgenic Vbeta6(+)CD4(high) cells revealed a reduced frequency of cells expressing a conserved motif within the TCRbeta CDR3. Thus, the canonical Talpha146-162 responsive, Vbeta6(+) population demonstrates both quantitative and qualitative deficits that correlate with an altered TCR repertoire whereas the non-Vbeta6 population in transgenic mice exhibits only a reduction in peptide responsiveness, a qualitative defect. These data demonstrate that discrete autoreactive T cell populations with identical peptide/MHC specificity in Torpedo acetylcholine receptor-alpha-transgenic animals bear distinct tolerance imprints.

Negative signaling contributes to T-cell anergy in trauma patients

OBJECTIVE

Maintenance of postinjury T-lymphocyte immune paralysis or anergy could result from failure to activate costimulatory receptors during T-cell receptor activation and/or from chronic stimulation of a competing set of elevated corepressor receptors. Our objective was to assess whether elevated posttrauma T-lymphocyte surface expression of corepressor receptors was associated with immunodepressed lymphocyte responses and corresponded to increased inhibitory and decreased activating signal transduction molecules.

DESIGN

Prospective observational study.

SETTING

University trauma intensive care unit and research laboratory.

PATIENTS

Sixty-one severe thermal and mechanical trauma patients.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Isolated trauma patients' and controls' peripheral blood T cells were assayed for negative and positive costimulation receptor expression. These receptor expression levels were compared (flow cytometry) between the two groups and correlated to T-cell levels of inhibitory and activating signal transduction molecules and proliferation capacity. Patients' proliferation hyporesponsive (anergic) T cells had increased expression of novel inhibitory receptors (corepressors) PD-1 (p < .05) and CD47 (p < .05) vs. patients' T-cell proliferation competent or controls' T cells. Patients' T-cell CD152 (CTLA-4) expression was also elevated vs. controls. Only patients' anergic T cells had simultaneously increased levels of the inhibitory signal transduction proteins, c-Cbl, a ubiquitin-ligase (p < .01) and SHP-1, a phosphatase (p < .01), concomitant to depressed phosphorylation of the activating signal kinases Erk, Zap70, and CD3Euro. T-cell receptor complex phosphorylation and activation of the interleukin-2 pivotal transcriptional complex protein CREB were also simultaneously depressed as c-Cbl and SHP-1 were elevated.

CONCLUSIONS

Up-regulated corepressor receptor expression is novelly shown to characterize trauma patients' anergic T cells and correlate with predominance of inhibitory overactivating signal transduction molecules during T-cell stimulation. This could contribute to postinjury immunosuppression.

CD4+CD25+ T cell-dependent inhibition of autoimmunity in transgenic mice overexpressing human Bcl-2 in T lymphocytes

Regulation of lymphocyte survival is essential for the maintenance of lymphoid homeostasis preventing the development of autoimmune diseases. Recently, we described a systemic lupus erythematosus associated with an IgA nephropathy in autoimmune-prone (NZW x C57BL/6)F(1) overexpressing human Bcl-2 (hBcl-2) in B cells (transgenic (Tg) 1). In the present study, we analyze in detail a second line of hBcl-2 Tg mice overexpressing the transgene in all B cells and in a fraction of CD4(+) and CD8(+) T cells (Tg2). We demonstrate here that the overexpression of hBcl-2 in T cells observed in Tg2 mice is associated with a resistance to the development of lupus disease and collagen type II-induced arthritis in both (NZW x C57BL/6)F(1) and (DBA/1 x C57BL/6)F(1) Tg2 mice, respectively. The disease-protective effect observed in autoimmune-prone Tg2 mice is accompanied by an increase of peripheral CD4(+)CD25(+) hBcl-2(+) regulatory T cells (T(regs)), expressing glucocorticoid-induced TNFR, CTLA-4, and FoxP3. Furthermore, the in vivo depletion of CD4(+)CD25(+) T(regs) in (DBA/1 x C57BL/6)F(1) Tg2 mice promotes the development of a severe collagen type II-induced arthritis. Taken together, our results indicate that the overexpression of hBcl-2 in CD4(+) T cells alters the homeostatic mechanisms controlling the number of CD4(+)CD25(+) T(regs) resulting in the inhibition of autoimmune diseases.

A novel negative regulatory function of the phosphoprotein associated with glycosphingolipid-enriched microdomains: blocking Ras activation

In primary human T cells, anergy induction results in enhanced p59Fyn activity. Because Fyn is the kinase primarily responsible for the phosphorylation of PAG (the phosphoprotein associated with glycosphingolipid-enriched microdomains), which negatively regulates Src-kinase activity by recruiting Csk (the C-terminal Src kinase) to the membrane, we investigated whether anergy induction also affects PAG. Analysis of anergic T cells revealed that PAG is hyperphosphorylated at the Csk binding site, leading to enhanced Csk recruitment and inhibitory tyrosine phosphorylation within Fyn. This together with enhanced phosphorylation of a tyrosine within the SH2 domain of Fyn leads to the formation of a hyperactive conformation, thus explaining the enhanced Fyn kinase activity. In addition, we have also identified the formation of a multiprotein complex containing PAG, Fyn, Sam68, and RasGAP in stimulated T cells. We demonstrate that PAG-Fyn overexpression is sufficient to suppress Ras activation in Jurkat T cells and show that this activity is independent of Csk binding. Thus, in addition to negatively regulating Src family kinases by recruiting Csk, PAG also negatively regulates Ras by recruiting RasGAP to the membrane. Finally, by knocking down PAG, we demonstrate both enhanced Src kinase activity and Ras activation, thereby establishing PAG as an important negative regulator of T-cell activation.

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.

Deficiency of Bim in dendritic cells contributes to overactivation of lymphocytes and autoimmunity

Apoptosis in dendritic cells (DCs) can potentially regulate DC homeostasis and immune responses. We have previously observed that inhibition of the Fas signaling pathway in DCs results in spontaneous T-cell activation and the development of systemic autoimmunity in transgenic mice. However, the role for different apoptosis pathways in DCs in regulating DC homeostasis and immune tolerance remains to be determined. Bim, a BH3-only protein of the Bcl-2 family, was expressed at low levels in DCs and was significantly up-regulated by signaling from CD40 or toll-like receptors (TLRs). Because Bim(-/-) mice develop spontaneous systemic autoimmunity, we investigated whether Bim(-/-) DCs contributed to lymphoproliferation and autoimmunity in these mice. Bim(-/-) DCs showed decreased spontaneous cell death, and induced more robust T-cell activation in vitro and in vivo. Moreover, Bim(-/-) DCs induced autoantibody production after adoptive transfer. Our data suggest that Bim is important for regulating spontaneous cell death in DCs, and Bim-deficient DCs may contribute to the development of autoimmune diseases in Bim(-/-) mice.

Dendritic cell immunotherapy for the treatment of neoplastic disease

It has long been promised that dendritic cell immunotherapy would revolutionize the treatment of neoplastic disease. Now, more than 10 years since the publication of the first clinical data, a firmer understanding of immunology and dendritic cell biology is beginning to produce interesting clinical results. This article reviews the clinical trials that established many of the concepts with which today's investigators are achieving improved results, discusses issues in dendritic cell immunotherapy that are currently unresolved, and offers a perspective on the strategies that the authors believe will be important for the design of future vaccine trials, including the use of Toll-like receptor agonists as maturation agents, the accessory use of the plasmacytoid dendritic cell subset, and the maximization of T-cell help.

The role of immune tolerance in asthma pathogenesis

Immune-mediated tolerance encompasses a wide spectrum of mechanisms that can prevent unnecessary and potentially harmful inflammatory responses. An increasing number of scientific publications provide proof for the concept that an impairment of immune-tolerance mechanisms might be causally related to the development of unwanted Th2-driven, allergen-induced airway diseases. In this review, we discuss immune tolerance and the evidence supporting its role in asthma pathogenesis.

Foxp3 controls autoreactive T cell activation through transcriptional regulation of early growth response genes and E3 ubiquitin ligase genes, independently of thymic selection

To elucidate the mechanisms of autoreactive T cell activation and expansion, we used endogenous viral superantigens (VSAg)-reactive T cells as a model of self-antigens in two strains of Foxp3-mutant mice. These two strains, together with wild-type mice, provided us with an advantage to simultaneously study the positively and negatively selected as well as rescued autoreactive T cells. We show here that while both VSAg-reactive and non-VSAg-reactive T cells are equally activated in Foxp3-mutant mice, only the VSAg-reactive T cells are preferentially expanded independently of their selected states in the thymus. The T cell activation appears to be controlled by Foxp3 through transcriptional regulation of early growth response (Egr) genes Egr-2 and Egr-3, and E3 ubiquitin (Ub) ligase genes Cblb, Itch and GRAIL, subsequently affecting degradation of two key signaling proteins, PLCgamma1 and PKC-theta. Physiologically, the positively, but not negatively selected VSAg-reactive T cells are spontaneously activated without significant expansion. The results suggest that autoreactive T cell activation is controlled by Foxp3 through transcriptional regulation of early growth response genes and E3 ubiquitin ligase genes, independently of thymic selection.

Cell-specific expression and pathway analyses reveal alterations in trauma-related human T cell and monocyte pathways

Monitoring genome-wide, cell-specific responses to human disease, although challenging, holds great promise for the future of medicine. Patients with injuries severe enough to develop multiple organ dysfunction syndrome have multiple immune derangements, including T cell apoptosis and anergy combined with depressed monocyte antigen presentation. Genome-wide expression analysis of highly enriched circulating leukocyte subpopulations, combined with cell-specific pathway analyses, offers an opportunity to discover leukocyte regulatory networks in critically injured patients. Severe injury induced significant changes in T cell (5,693 genes), monocyte (2,801 genes), and total leukocyte (3,437 genes) transcriptomes, with only 911 of these genes common to all three cell populations (12%). T cell-specific pathway analyses identified increased gene expression of several inhibitory receptors (PD-1, CD152, NRP-1, and Lag3) and concomitant decreases in stimulatory receptors (CD28, CD4, and IL-2Ralpha). Functional analysis of T cells and monocytes confirmed reduced T cell proliferation and increased cell surface expression of negative signaling receptors paired with decreased monocyte costimulation ligands. Thus, genome-wide expression from highly enriched cell populations combined with knowledge-based pathway analyses leads to the identification of regulatory networks differentially expressed in injured patients. Importantly, application of cell separation, genome-wide expression, and cell-specific pathway analyses can be used to discover pathway alterations in human disease.

Involvement of NFAT1 in B cell self-tolerance

B cells from anti-lysozyme Ig/soluble lysozyme double-transgenic mice are chronically exposed to self-Ag in the periphery, resulting in an anergic phenotype. Chronic exposure to self-Ag leads to nuclear translocation of NFAT1 and NFAT2, suggesting that they are involved in anergy. To directly test a role for NFAT1 in B cell anergy, NFAT1-deficient mice were crossed with anti-lysozyme Ig transgenic mice. As expected, B cell anergy was evident in the presence of self-Ag based on reduced serum anti-lysozyme levels, percentage and number of mature B cells, and reduced B cell responsiveness. By contrast, B cell anergy was relieved in NFAT1(-/-) mice expressing soluble self-Ag. Bone marrow development was equivalent in NFAT1-sufficient and -deficient mice, suggesting that loss of anergy in the latter is due to selection later in development. Taken together, these studies provide direct evidence that the transcription factor NFAT1 is involved in B cell anergy.

Golli protein negatively regulates store depletion-induced calcium influx in T cells

Calcium influx is crucial for T cell activation and differentiation. The detailed regulation of this process remains unclear. We report here that golli protein, an alternatively spliced product of the myelin basic protein gene, plays a critical role in regulating calcium influx in T cells. Golli-deficient T cells were hyperproliferative and showed enhanced calcium entry upon T cell receptor stimulation. We further found that golli regulates calcium influx in T cells through the inhibition of the store depletion-induced calcium influx. Mutation of the myristoylation site on golli disrupted its association with the plasma membrane and reversed its inhibitory action on Ca2+ influx, indicating that membrane association of golli was essential for its inhibitory action. These results indicate that golli functions in a unique way to regulate T cell activation through a mechanism involving the modulation of the calcium homeostasis.

The quantal theory of immunity

Exactly how the immune system discriminates between all environmental antigens to which it reacts vs. all self-antigens to which it does not, is a principal unanswered question in immunology. As set forth in this review, because of the advances in our understanding of the immune system that have occurred in the last 50 years, for the first time it is possible to formulate a new theory, termed the "Quantal Theory of Immunity", which reduces the problem from the immune system as a whole, to the individual cells comprising the system, and finally to a molecular explanation as to how the system behaves as it does.

Transgenic expression of a CD83-immunoglobulin fusion protein impairs the development of immune-competent CD4-positive T cells

The murine transmembrane glycoprotein CD83 is an important regulator for both thymic T cell maturation and peripheral T cell response. CD83 deficiency leads to a block in the thymic maturation of CD4-positive T cells, and interference with peripheral CD83/CD83 ligand interaction by addition of soluble CD83 suppresses immune responses in vivo and in vitro. Here we report the generation of a mouse transgenic for a fusion protein consisting of the extracellular domain of murine CD83 fused to the constant part of human IgG1 heavy chain. Thymic selection of CD4-positive T cells was unchanged in CD83Ig transgenic and in CD83Ig/OT-2 double-transgenic mice. However, thymic and peripheral CD4-positive T cells derived from CD83Ig/OT-2 transgenic mice displayed a reduced cytokine response to antigenic stimulation in vitro, whereas CD83Ig/OT-1-derived CD8-positive T cells showed normal cytokine secretion. The T cell defect was relevant in vivo, since a sub-lethal infection with Trypanosoma cruzi led to an increased parasitemia and reduced survival rate of CD83Ig transgenic mice compared to wild-type C57BL/6 mice. In contrast, in vivo application of recombinant CD83Ig did not result in an increase in parasitemia. Taken together our data suggest that thymic selection in the presence of CD83Ig leads to an intrinsic T cell defect of CD4-positive T cells resembling the phenotype described for CD4-positive T cells derived from CD83-deficient mouse strains.

Small Interfering RNA-Mediated Knockdown of Notch Ligands in Primary CD4+ T Cells and Dendritic Cells Enhances Cytokine Production

The key interaction in the adaptive immune system's response to pathogenic challenge occurs at the interface between APCs and T cells. Families of costimulatory and coinhibitory molecules function in association with the cytokine microenvironment to orchestrate appropriate T cell activation programs. Recent data have demonstrated that the Notch receptor and its ligands also function at the APC:T interface. In this study, we describe synthetic small interfering RNA (siRNA) sequences targeting the human Notch ligands Delta1, Jagged1 and Jagged2. Transfection of these siRNAs into human primary CD4(+) T cells and monocyte-derived dendritic cells leads to knockdown of endogenous Notch ligand message. Knockdown of any one of these three Notch ligands in dendritic cells enhanced IFN-gamma production from allogeneic CD4(+) T cells in MLR. In contrast, Delta1 knockdown in CD4(+) T cells selectively enhanced production of IFN-gamma, IL-2, and IL-5 in response to polyclonal stimulation, while Jagged1 or Jagged2 knockdown had no effect. Strikingly, blockade of Notch cleavage with a gamma secretase inhibitor failed to affect cytokine production in this system, implying that Delta1 can influence cytokine production via a Notch cleavage-independent mechanism. These data show for the first time that the Notch pathway can be targeted by siRNA, and that its antagonism may be a unique therapeutic opportunity for immune enhancement.