A signaling pathway coupled to T cell receptor ligation by MMTV superantigen leading to transient activation and programmed cell death

Ebola Virus Binding to Tim-1 on T Lymphocytes Induces a Cytokine Storm

Ebola virus (EBOV) disease (EVD) results from an exacerbated immunological response that is highlighted by a burst in the production of inflammatory mediators known as a "cytokine storm." Previous reports have suggested that nonspecific activation of T lymphocytes may play a central role in this phenomenon. T-cell immunoglobulin and mucin domain-containing protein 1 (Tim-1) has recently been shown to interact with virion-associated phosphatidylserine to promote infection. Here, we demonstrate the central role of Tim-1 in EBOV pathogenesis, as Tim-1^-/- mice exhibited increased survival rates and reduced disease severity; surprisingly, only a limited decrease in viremia was detected. Tim-1^-/- mice exhibited a modified inflammatory response as evidenced by changes in serum cytokines and activation of T helper subsets. A series of in vitro assays based on the Tim-1 expression profile on T cells demonstrated that despite the apparent absence of detectable viral replication in T lymphocytes, EBOV directly binds to isolated T lymphocytes in a phosphatidylserine-Tim-1-dependent manner. Exposure to EBOV resulted in the rapid development of a CD4^Hi CD3^Low population, non-antigen-specific activation, and cytokine production. Transcriptome and Western blot analysis of EBOV-stimulated CD4⁺ T cells confirmed the induction of the Tim-1 signaling pathway. Furthermore, comparative analysis of transcriptome data and cytokine/chemokine analysis of supernatants highlight the similarities associated with EBOV-stimulated T cells and the onset of a cytokine storm. Flow cytometry revealed virtually exclusive binding and activation of central memory CD4⁺ T cells. These findings provide evidence for the role of Tim-1 in the induction of a cytokine storm phenomenon and the pathogenesis of EVD.IMPORTANCE Ebola virus infection is characterized by a massive release of inflammatory mediators, which has come to be known as a cytokine storm. The severity of the cytokine storm is consistently linked with fatal disease outcome. Previous findings have demonstrated that specific T-cell subsets are key contributors to the onset of a cytokine storm. In this study, we investigated the role of Tim-1, a T-cell-receptor-independent trigger of T-cell activation. We first demonstrated that Tim-1-knockout (KO) mice survive lethal Ebola virus challenge. We then used a series of in vitro assays to demonstrate that Ebola virus directly binds primary T cells in a Tim-1-phosphatidylserine-dependent manner. We noted that binding induces a cytokine storm-like phenomenon and that blocking Tim-1-phosphatidylserine interactions reduces viral binding, T-cell activation, and cytokine production. These findings highlight a previously unknown role of Tim-1 in the development of a cytokine storm and "immune paralysis."

Ebola Virus Binding to Tim-1 on T Lymphocytes Induces a Cytokine Storm

Ebola virus (EBOV) disease (EVD) results from an exacerbated immunological response that is highlighted by a burst in the production of inflammatory mediators known as a "cytokine storm." Previous reports have suggested that nonspecific activation of T lymphocytes may play a central role in this phenomenon. T-cell immunoglobulin and mucin domain-containing protein 1 (Tim-1) has recently been shown to interact with virion-associated phosphatidylserine to promote infection. Here, we demonstrate the central role of Tim-1 in EBOV pathogenesis, as Tim-1^-/- mice exhibited increased survival rates and reduced disease severity; surprisingly, only a limited decrease in viremia was detected. Tim-1^-/- mice exhibited a modified inflammatory response as evidenced by changes in serum cytokines and activation of T helper subsets. A series of in vitro assays based on the Tim-1 expression profile on T cells demonstrated that despite the apparent absence of detectable viral replication in T lymphocytes, EBOV directly binds to isolated T lymphocytes in a phosphatidylserine-Tim-1-dependent manner. Exposure to EBOV resulted in the rapid development of a CD4^Hi CD3^Low population, non-antigen-specific activation, and cytokine production. Transcriptome and Western blot analysis of EBOV-stimulated CD4⁺ T cells confirmed the induction of the Tim-1 signaling pathway. Furthermore, comparative analysis of transcriptome data and cytokine/chemokine analysis of supernatants highlight the similarities associated with EBOV-stimulated T cells and the onset of a cytokine storm. Flow cytometry revealed virtually exclusive binding and activation of central memory CD4⁺ T cells. These findings provide evidence for the role of Tim-1 in the induction of a cytokine storm phenomenon and the pathogenesis of EVD.IMPORTANCE Ebola virus infection is characterized by a massive release of inflammatory mediators, which has come to be known as a cytokine storm. The severity of the cytokine storm is consistently linked with fatal disease outcome. Previous findings have demonstrated that specific T-cell subsets are key contributors to the onset of a cytokine storm. In this study, we investigated the role of Tim-1, a T-cell-receptor-independent trigger of T-cell activation. We first demonstrated that Tim-1-knockout (KO) mice survive lethal Ebola virus challenge. We then used a series of in vitro assays to demonstrate that Ebola virus directly binds primary T cells in a Tim-1-phosphatidylserine-dependent manner. We noted that binding induces a cytokine storm-like phenomenon and that blocking Tim-1-phosphatidylserine interactions reduces viral binding, T-cell activation, and cytokine production. These findings highlight a previously unknown role of Tim-1 in the development of a cytokine storm and "immune paralysis."

Intracellular free radical production by peripheral blood T lymphocytes from patients with systemic sclerosis: role of NADPH oxidase and ERK1/2

Introduction

Abnormal oxidative stress has been described in systemic sclerosis (SSc) and previous works from our laboratory demonstrated an increased generation of reactive oxygen species (ROS) by SSc fibroblasts and monocytes. This study investigated the ability of SSc T lymphocytes to produce ROS, the molecular pathway involved, and the biological effects of ROS on SSc phenotype.

Methods

Peripheral blood T lymphocytes were isolated from serum of healthy controls or SSc patients by negative selection with magnetic beads and activated either with PMA or with magnetic beads coated with anti-CD3 and anti-CD28 antibodies. Intracellular ROS generation was measured using a DCFH-DA assay in a plate reader fluorimeter or by FACS analysis. CD69 expression and cytokine production were analyzed by FACS analysis. Protein expression was studied using immunoblotting techniques and mRNA levels were quantified by real-time PCR. Cell proliferation was carried out using a BrdU incorporation assay.

Results

Peripheral blood T lymphocytes from SSc patients showed an increased ROS production compared to T cells from healthy subjects. Since NADPH oxidase complex is involved in oxidative stress in SSc and we found high levels of gp91phox in SSc T cells, SSc T cells were incubated with chemical inhibititors or specific siRNAs against gp91phox. Inhibition of NADPH oxidase partially reverted CD69 activation and proliferation rate increase, and significantly influenced cytokine production and ERK1/2 activation.

Conclusions

SSc T lymphocityes are characterized by high levels of ROS, generated by NADPH oxidase via ERK1/2 phosphorylation, that are essential for cell activation, proliferation, and cytokine production. These data confirm lymphocytes as key cellular players in the pathogenesis of systemic sclerosis and suggest a crucial link between ROS and T cell activation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0591-8) contains supplementary material, which is available to authorized users.

Energy metabolism during anchorage-independence. Induction by osteopontin-c

The detachment of epithelial cells, but not cancer cells, causes anoikis due to reduced energy production. Invasive tumor cells generate three splice variants of the metastasis gene osteopontin, the shortest of which (osteopontin-c) supports anchorage-independence. Osteopontin-c signaling upregulates three interdependent pathways of the energy metabolism. Glutathione, glutamine and glutamate support the hexose monophosphate shunt and glycolysis and can feed into the tricarboxylic acid cycle, leading to mitochondrial ATP production. Activation of the glycerol phosphate shuttle also supports the mitochondrial respiratory chain. Drawing substrates from glutamine and glycolysis, the elevated creatine may be synthesized from serine via glycine and supports the energy metabolism by increasing the formation of ATP. Metabolic probing with N-acetyl-L-cysteine, L-glutamate, or glycerol identified differential regulation of the pathway components, with mitochondrial activity being redox dependent and the creatine pathway depending on glutamine. The multiple skewed components in the cellular metabolism synergize in a flow toward two mechanisms of ATP generation, via creatine and the respiratory chain. It is consistent with a stimulation of the energy metabolism that supports anti-anoikis. Our findings imply a coalescence in cancer cells between osteopontin-a, which increases the cellular glucose levels, and osteopontin-c, which utilizes this glucose to generate energy.

Modulation of T Cell Metabolism and Function through Calcium Signaling

As a vital second messenger in the activation of lymphocytes, the divalent cation Ca(2+) plays numerous roles in adaptive immune responses. Importantly, Ca(2+) signaling is essential for T cell activation, tolerance of self-antigens, and homeostasis. Supporting the essential role of Ca(2+) signaling in T cell biology, the Ca(2+) regulated protein phosphatase calcineurin is a key target of pharmacologic inhibition for preventing allograft rejection and for autoimmune therapy. Recent studies have highlighted the unique role of Stim1 and Orai1/2 proteins in the regulation of store-operated/calcium release activated calcium (CRAC) channels in the context of T cells. While Ca(2+) is known to modulate T cell activation via effects on calcineurin and its target, nuclear factor of activated T cells (NFAT), this second messenger also regulates other pathways, including protein kinase C, calmodulin kinases, and cytoskeletal proteins. Ca(2+) also modulates the unique metabolic changes that occur during in distinct T cell stages and subsets. Herein, we discuss the means by which Ca(2+) mobilization modulates cellular metabolism following T cell receptor ligation. Further, we highlight the crosstalk between mitochondrial metabolism, reactive oxygen species (ROS) generation, and CRAC channel activity. As a target of mitochondrial ROS and Ca(2+) regulation, we describe the involvement of the serine/threonine kinase DRAK2 in the context of these processes. Given the important roles for Ca(2+) dependent signaling and cellular metabolism in adaptive immune responses, the crosstalk between these pathways is likely to be important for the regulation of T cell activation, tolerance, and homeostasis.

Reactive oxygen intermediate-induced pathomechanisms contribute to immunosenescence, chronic inflammation and autoimmunity

Deregulation of reactive oxygen intermediates (ROI) resulting in either too high or too low concentrations are commonly recognized to be at least in part responsible for many changes associated with aging. This article reviews ROI-dependent mechanisms critically contributing to the decline of immune function during physiologic - or premature - aging. While ROI serve important effector functions in cellular metabolism, signalling and host defence, their fine-tuned generation declines over time, and ROI-mediated damage to several cellular components and/or signalling deviations become increasingly prevalent. Although distinct ROI-associated pathomechanisms contribute to immunosenescence of the innate and adaptive immune system, mutual amplification of dysfunctions may often result in hyporesponsiveness and immunodeficiency, or in chronic inflammation with hyperresponsiveness/deregulation, or both. In this context, we point out how imbalanced ROI contribute ambiguously to driving immunosenescence, chronic inflammation and autoimmunity. Although ROI may offer a distinct potential for therapeutic targeting along with the charming opportunity to rescue from deleterious processes of aging and chronic inflammatory diseases, such modifications, owing to the complexity of metabolic interactions, may carry a marked risk of unforeseen side effects.

Molecular mechanisms of metastasis

Metastasis formation is an essential aspect of cancer, for which the molecular underpinning has long been subject to debate. Although the organ preference for dissemination is governed by tumor-host interactions on the epigenetic level there is a genetic basis to the ability of cancer cells to disseminate. Metastasis genes encode homing receptors, their ligands, and extracellular matrix-degrading proteinases, which jointly cause invasion and anchorage-independence. They are developmentally non-essential stress response genes that physiologically mediate the homing of immune system cells. Metastatic potential is conferred to cancer cells by aberrant expression or splicing of these genes. Oncogenes act upstream of metastasis genes. In cancer cells, oncogenic signaling activates distinct genetic programs leading to cell cycle progression and invasiveness, respectively. The expression of metastasis genes is regulated by multi-subunit transcription factor complexes. The identification of genes that direct cancer metastasis implicates them as candidate drug targets.

An osteopontin splice variant induces anchorage independence in human breast cancer cells

In malignant tumors, metastasis genes are typically deregulated by aberrant expression or splicing. Osteopontin is expressed at high levels by various cancers and contributes importantly to their invasive potential. In contrast, osteopontin derived from host cells induces cellular immunity and could bolster antitumor protection by cytotoxic T lymphocytes. Here we show that breast cancer cells express multiple splice variants of osteopontin. According to RT-PCR analysis of human breast tissue specimens, the splice variant osteopontin-c is a highly specific marker for transformed cells, which is not expressed in their surrounding normal tissue. The full-length form of osteopontin aggregates in the presence of physiologic amounts of calcium and, in this state, leads to enhanced cell adhesion. Ostensibly, this effect is inhibitory for tumor cell dissemination. The shortest splice variant, osteopontin-c, does not aggregate in the presence of calcium and enhances clone formation in soft agar. According to microarray analysis, osteopontin-c induces the expression of oxidoreductases, consistent with protection from anoikis during anchorage-independent growth. These studies define a third functional domain of osteopontin, beside the C-terminal CD44-binding site and the central integrin-binding site. They also provide evidence for a bifunctional character of osteopontin, with the soluble form supporting invasiveness and the aggregated form promoting adhesion.

Phospholipase A2 inhibitors in development

To date, three isoforms of phospholipase A2 (PLA2) have been identified. Of these, the two Ca2+-dependent isoforms, secretory (sPLA2) and cytosolic phospholipase A2 (cPLA2), are targets for new anti-inflammatory drugs. The catalytic mechanisms and functions of the third isoform, Ca2+-independent cytosolic phospholipase A2 (iPLA2), are unknown at present. sPLA2 and cPLA2 are both implicated in the release of arachidonic acid and prophlogistic lipid mediators. However, recent findings provide evidence that cPLA2 is the dominant isoform in various kinds of inflammation, such as T-cell-mediated experimental arthritis. A triple function of PLA2-derived lipid mediators has been suggested: causing immediate inflammatory signs, involvement in secondary processes, e.g., superoxide free radical (O2) generation, apoptosis, or tumour necrosis factor-alpha (TNF-alpha)-cytotoxicity, and controlling the expression and activation of pivotal proteins implicated in inflammation and cell development, e.g., cytokines, adhesion proteins, proteinases, NF-kappaB, fos/jun/AP-1, c-Myc, or p21ras. In the past, research predominantly focused on the development of sPLA2 inhibitors; however, present techniques enable discrimination of cPLA2, sPLA2, and iPLA2, and specific inhibitors of each of the three isoforms are likely to appear soon. Over the last decade, between 40 and 50 sPLA2 inhibitors have been described; and the list is growing. However, of these, few have the potential for clinical success, and those that do are predominantly active site-directed inhibitors, e.g., BMS-181162, LY311727, ARL-67974, FPL67047, SB-203347, Ro-23-9358, YM-26734, and IS-741. At present, there are no likely clinical candidates emerging from the ranks of cPLA2 and iPLA2 inhibitors in development. Indications for which PLA2 inhibitors are being pursued include, sepsis, acute pancreatitis, inflammatory skin and bowel diseases, asthma, and rheumatoid arthritis. The three main obstacles to the successful development of PLA2 inhibitors include, insufficient oral bioavailability, low affinity for the enzyme corresponding to low in vivo efficacy and insufficient selectivity.

Synergistic activation of the CMV promoter by NF-kappaB P50 and PKG

Several DNA binding NF-kappaB subunits are substrates for cGMP-dependent kinase (PKG) and their transactivation from cognate sites is induced by phosphorylation. This includes p50, which does not have a transcriptional activation domain and therefore needs to bind to other proteins to mediate gene expression. Here, we describe the synergistic transactivation by p50 and PKG from the CMV promoter. This is caused not only by phosphorylation of p50, leading to increased DNA binding, but also by PKG-dependent activation of CRE sites in the promoter. One of the CRE sites is located directly adjacent to a NF-kappaB site and is essential for p50-mediated induction of transcription. According to the binding of CREB to p50 in pull-down assays and according to the inhibition of p50-dependent transactivation by dominant-negative CREB, this reflects the formation of a transcription factor complex containing CREB and p50. The nuclear translocation of NF-kappaB is insufficient to distinguish among the multitude of promoters that harbor cognate recognition sites. The phosphorylation of multiple transcription factors by an upstream kinase, such as PKG, can lead to the formation of transcription factor complexes and differential transactivation from a subset of NF-kappaB sites. These interactions may be relevant for the activation of viral gene expression.

T cell receptor stimulation, reactive oxygen species, and cell signaling

In the immune system, much of the focus on reactive oxygen species (ROS) has been regarding their role in antimicrobial defense as part of the innate immune system. In addition to this role, it is now becoming clear that ROS are used by cells of the adaptive immune system as regulators of signal transduction by cell surface receptors. The activation of T lymphocytes through their specific antigen receptor [T cell receptor (TCR)] is vital in regulating the immune response. Much experimental evidence has suggested that activation of T cells is redox dependent and recent studies have shown that engagement of the TCR induces rapid production of ROS. This review examines the evidence for TCR-stimulated generation of ROS and discusses the role(s) of receptor-stimulated ROS production in T cell signal transduction and gene expression.

Intracellular Thiols Contribute to Th2 Function via a Positive Role in IL-4 Production

A number of lung diseases, including many interstitial lung diseases and HIV infection, are associated with decreases in intracellular thiols. Altered Th1/Th2 T cell balance has also been associated with disease progression in many of the same diseases. IFN-gamma and IL-4 are critical effector cytokines of Th1 and Th2 cells, respectively. To determine the effect of thiols on the production of IFN-gamma and IL-4 by splenocytes, cells were incubated in the presence and the absence of N-acetylcysteine (NAC) and stimulated with alphaCD3 or alphaCD3 and IL-12. Augmenting intracellular soluble thiol pools ( approximately 2-fold) with 15 mM NAC blocked induction of IFN-gamma and increased production of IL-4 without causing significant changes in intracellular glutathione levels. The effect of NAC on IL-4 production was not linked to an increase in STAT6 phosphorylation, as STAT6 levels were decreased, nor did the increase in IL-4 occur with purified CD4 cells. We found that NAC increased splenocyte IL-4 production via an effect on APCs. We also found that NAC increased two IL-4 relevant transcription factors (AP-1) and NFATc. These studies suggest that increasing intracellular reduced thiol pools decreases IL-12 signaling and IFN-gamma production, while increasing IL-4 production. The sum of these effects may contribute to alterations in the balance between Th1 and Th2 responses in lung diseases associated alterations in intracellular thiol pools.

Generation and function of reactive oxygen species in dendritic cells during antigen presentation

Although reactive oxygen species (ROS) have long been considered to play pathogenic roles in various disorders, this classic view is now being challenged by the recent discovery of their physiological roles in cellular signaling. To determine the immunological consequence of pharmacological disruption of endogenous redox regulation, we used a selenium-containing antioxidant compound ebselen known to modulate both thioredoxin and glutaredoxin pathways. Ebselen at 5-20 micro M inhibited Con A-induced proliferation and cytokine production by the HDK-1 T cell line as well as the LPS-triggered cytokine production by XS52 dendritic cell (DC) line. Working with the in vitro-reconstituted Ag presentation system composed of bone marrow-derived DC, CD4(+) T cells purified from DO11.10 TCR-transgenic mice and OVA peptide (serving as Ag), we observed that 1) both T cells and DC elevate intracellular oxidation states upon Ag-specific interaction; 2) ebselen significantly inhibits ROS production in both populations; and 3) ebselen at 5-20 micro M inhibits DC-induced proliferation and cytokine production by T cells as well as T cell-induced cytokine production by DC. Thus, Ag-specific, bidirectional DC-T cell communication can be blocked by interfering with the redox regulation pathways. Allergic contact hypersensitivity responses in BALB/c mice to oxazolone, but not irritant contact hypersensitivity responses to croton oil, were suppressed significantly by postchallenge treatment with oral administrations of ebselen (100 mg/kg per day). These results provide both conceptual and technical frameworks for studying ROS-dependent regulation of DC-T cell communication during Ag presentation and for testing the potential utility of antioxidants for the treatment of immunological disease.

T cell receptor-stimulated generation of hydrogen peroxide inhibits MEK-ERK activation and lck serine phosphorylation

Previous studies indicated that antigen receptor (TcR) stimulation of mature T cells induced rapid generation of reactive oxygen species (ROS). The goal of the current study was to examine the role(s) of ROS in TcR signal transduction, with a focus upon the redox-sensitive MAPK family. TcR cross-linking of primary human T blasts and Jurkat human T cells rapidly activated the ERK, JNK, p38 and Akt kinases within minutes, and was temporally associated with TcR-stimulated production of hydrogen peroxide (H(2)O(2)). TcR-induced activation of ERK was selectively augmented and sustained in the presence of pharmacologic antioxidants that can quench or inhibit H(2)O(2) production (NAC, MnTBAP and Ebselen, but not DPI), while activation of JNK and Akt were largely unaffected. This was paralleled by concurrent changes in MEK1/2 phosphorylation, suggesting that ROS acted upstream of MEK-ERK activation. Molecular targeting of H(2)O(2) by overexpression of peroxiredoxin II, a thioredoxin dependent peroxidase, also increased and sustained ERK and MEK activation upon TcR cross-linking. Enhancement of ERK phosphorylation by antioxidants correlated with increased and sustained serine phosphorylation of the src-family kinase lck, a known ERK substrate. Thus, the data suggest that TcR-stimulated production of hydrogen peroxide negatively feeds back to dampen antigen-stimulated ERK activation and this redox-dependent regulation may serve to modulate key steps in TcR signaling.

Zinc-binding sites in the N terminus of Mycoplasma arthritidis-derived mitogen permit the dimer formation required for high affinity binding to HLA-DR and for T cell activation

Zinc-dependent superantigens can be divided into two subfamilies based on how they use zinc ions for interactions with major histocompatibility complex (MHC) class II molecules. Members of the first subfamily use zinc ions for interactions with histidine 81 on the beta-chain of MHC class II molecules, whereas members of the second subfamily use zinc ions for dimer formation. The zinc-binding motif is located in the C terminus of the molecule in both subfamilies. While our recent studies with Mycoplasma arthritidis-derived mitogen (MAM) have provided the first direct evidence demonstrating the binding to MHC class II molecules in a zinc-dependent manner, it still not known how zinc coordinates the interaction. Data presented here show that the zinc ion is mainly required to induce MAM/MAM dimer formation. Residues in the N terminus of MAM are involved in dimer formation and MHC class II binding, while histidine 14 and aspartic acid 31 of the MAM sequence are the major residues mediating MAM/MAM dimerization. Zinc-induced dimer formation is necessary for MAM binding, MHC class II-induced cell-cell adhesion, and efficient T cell activation. Together these results depict the unique mode of interaction of MAM in comparison with other superantigens.

Phosphorylation of NF-kappaB proteins by cyclic GMP-dependent kinase. A noncanonical pathway to NF-kappaB activation

The transcription factor NF-kappaB is activated in cellular stress responses. This requires rapid regulation of its function, which is accomplished, in part, by various modes of phosphorylation. Even though diverse DNA binding subunits of NF-kappaB proteins may transactivate from distinct recognition sequences, the differential regulation of transcription from the large number of NF-kappaB responsive sites in various gene promoters and enhancers has been incompletely understood. The cyclic GMP-dependent kinase (PKG) is an important mediator of signal transduction that may induce gene expression through cAMP response element binding protein (CREB) and through other, yet undefined, mechanisms. We have previously characterized a signal transduction pathway that leads to activation-induced cell death in T-lymphocytes and involves the activation of PKG. Here we demonstrate that the NF-kappaB proteins p65, p49 (also called p52), and p50 are specific substrates for this kinase. PKG dose-dependently increases the transactivating activity of p65 from the NF-kappaB consensus sequence. It also mediates dose-dependently an increase in transcriptional activity by p49 or p50 from a unique CCAAT/enhance binding protein (C/EBP)-associated NF-kappaB site, but not from the consensus site. Phosphorylation of p65, p50, or p49 does not alter their subcellular distribution. Because the release of cytosolic p65/p50 heterodimers into the nucleus is by itself insufficient to differentiate all the numerous NF-kappaB promoter sequences, phosphorylation of the DNA-binding subunits reveals a form of differential regulation of NF-kappaB activity and it implies a novel pathway for PKG-induced gene transcription. These observations may bear on mechanisms of programmed cell death in T-lymphocytes. They may also be relevant to ongoing efforts to induce cancer cell apoptosis through activation of PKG.

Activation-induced cell death in human T cells is a suicidal process regulated by cell density but superantigen induces T cell fratricide

Repeated ligation of the TCR results in apoptosis (activation-induced cell death; AICD). Superantigens such as Staphylococcal enterotoxin B (SEB) are particularly efficient at inducing AICD in T cells. We investigated whether apoptosis in human T cell subsets was due to fratricide (killing of neighboring cells) or suicide (cell autonomous death). AICD of Th1, Th2, Tc1, and Tc2 effector cells was dramatically enhanced at low cell densities and could be observed in single cell microcultures. AICD was unaffected by adhesion molecules or neighboring cells undergoing AICD, confirming the predominance of a suicidal mechanism. However, SEB was able to induce fratricidal apoptosis of type 1, but not type 2 cells. Fratricide was also observed when unstimulated T cells were exposed to activated Tc1 effector cells. Thus, AICD is tightly regulated to allow clonal T cell expansion and memory cell generation, but superantigens may subvert this process by allowing T cell fratricide.

Discrete generation of superoxide and hydrogen peroxide by T cell receptor stimulation: selective regulation of mitogen-activated protein kinase activation and fas ligand expression

Receptor-stimulated generation of reactive oxygen species (ROS) has been shown to regulate signal transduction, and previous studies have suggested that T cell receptor (TCR) signals may involve or be sensitive to ROS. In this study, we have shown for the first time that TCR cross-linking induced rapid (within 15 min) generation of both hydrogen peroxide and superoxide anion, as defined with oxidation-sensitive dyes, selective pharmacologic antioxidants, and overexpression of specific antioxidant enzymes. Furthermore, the data suggest the novel observation that superoxide anion and hydrogen peroxide are produced separately by distinct TCR-stimulated pathways. Unexpectedly, TCR-stimulated activation of the Fas ligand (FasL) promoter and subsequent cell death was dependent upon superoxide anion, but independent of hydrogen peroxide, while nuclear factor of activated T cells (NFAT) activation or interleukin 2 transcription was independent of all ROS. Anti-CD3 induced phosphorylation of extracellular signal-regulated kinase (ERK)1/2 required hydrogen peroxide generation but was unaffected by superoxide anion. Thus, antigen receptor signaling induces generation of discrete species of oxidants that selectively regulate two distinct redox sensitive pathways, a proapoptotic (FasL) and a proliferative pathway (ERK).

Costimulation by extracellular matrix proteins determines the response to TCR ligation

Although ligation of the T-cell antigen receptor (TCR) is central to the responsiveness and antigen specificity of T-cells, it is insufficient to elicit a response. To determine whether the need for costimulation reflects inadequate strength of signal transduction through the TCR or an absolute block of signaling in the absence of a coligand, we studied T-cell activation under serum-free conditions eliminating costimulation by various extracellular matrix proteins which otherwise have an omnipresent and frequently overlooked effect. Engagement of the TCR leads to induction of Fas, but not to measurable IL-2 secretion or apoptosis. Those activation parameters are induced by costimulation through integrin alphaVbeta3. Furthermore, T-cell survival or elimination is determined by the type of ligand binding to this coreceptor with vitronectin, fibronectin, and fibrinogen efficiently inducing apoptosis and IL-2 production while osteopontin and entactin mediate IL-2 secretion comparably without causing programmed cell death. Consistent with the cytokine properties of these ligands, differential costimulation depends on their presentation in soluble rather than immobilized form. The determination of elimination versus survival of activated T-cells by coligation of beta3-integrins may have bearing on the fundamental postthymic mechanisms that shape the T-cell repertoire.

Superantigen-induced CD4 T cell tolerance mediated by myeloid cells and IFN-gamma

We have previously shown that systemic staphylococcal enterotoxin A (SEA) injections cause CD4 T cells in TCR-transgenic mice to become tolerant to subsequent ex vivo restimulation. An active IFN-gamma-dependent mechanism of suppression was responsible for the apparent unresponsiveness of the CD4 T cells. In this study, we analyze the response of CD4 T cells isolated throughout the first 10 days of the in vivo response to injected SEA. We show that CD4 T cells isolated at the peak of the in vivo response undergo very little activation-induced cell death after sterile FACS sorting or restimulation in the presence of neutralizing Abs to IFN-gamma. We also show that the IFN-gamma-dependent tolerance develops soon after SEA injection in the spleens of both normal and TCR-transgenic mice. This suppression is dependent upon myeloid cells from the SEA-treated mice and is optimal when inducible NO synthase activity and reactive oxygen intermediates are both present. The data indicate that IFN-gamma, myeloid cells, and a combination of NO and reactive oxygen intermediates all contribute to a common pathway of T cell death that targets activated or responding CD4 T cells. Sorted Gr-1(+) cells from SEA-treated mice also directly suppress the response of naive CD4 T cells in mixed cultures, indicating that this tolerance mechanism may play a role in down-regulating other vigorous immune responses.

Final common pathways in neurodegenerative diseases: regulatory role of the glutathione cycle

Attempts to unify diverse mechanisms of neurotoxicity have led to the concept of final common pathways which characterize frequently occurring cellular responses to disruption of homeostasis. The clinical presentation and common patho-biochemistry of reactive oxygen intermediates of Guam's disease have suggested that such pathways may be operative in three major neurodegenerative disorders: Alzheimer's dementia, amyotrophic lateral sclerosis and Parkinson's disease. A candidate-signaling pathway in this regard is characterized by the cascade arachidonic acid/HPETE/*OH/cGMP followed by activation of cGMP-dependent kinase and phosphorylation of NF-kB proteins and possibly CREB. This sequence may lead to apoptosis as well as long-term potentiation and memory and constitutes a biochemical correlate to excitotoxicity. The predominant control of *OH release from HPETE, a checkpoint in this pathway, is exerted by the glutathione cycle, a central biochemical process that is also intimately associated with the synthesis of the neurotransmitters glutamate and GABA and is connected to energy metabolism. Modifications in the activity of the glutathione cycle may provide treatment options.

Expression of mouse mammary tumor virus superantigen mRNA in the thymus correlates with kinetics of self-reactive T-cell loss

Mouse mammary tumor virus (MMTV) encodes a superantigen (Sag) that is expressed at the surface of antigen-presenting cells in conjunction with major histocompatibility complex (MHC) type II molecules. The Sag-MHC complex is recognized by entire subsets of T cells, leading to cytokine release and amplification of infected B and T cells that carry milk-borne MMTV to the mammary gland. Expression of Sag proteins from endogenous MMTV proviruses carried in the mouse germ line usually results in the deletion of self-reactive T cells during negative selection in the thymus and the elimination of T cells required for infection by specific milk-borne MMTVs. However, other endogenous MMTVs are unable to eliminate Sag-reactive T cells in newborn mice and cause partial loss of reactive T cells in adults. To investigate the kinetics of Sag-reactive T-cell deletion, backcross mice that contain single or multiple MMTVs were screened by a novel PCR assay designed to distinguish among highly related MMTV strains. Mice that contained Mtv-17 alone showed slow kinetics of reactive T-cell loss that involved the CD4(+), but not the CD8(+), subset. Deletion of CD4(+) or CD8(+) T cells reactive with Mtv-17 Sag was not detected in thymocytes. Slow kinetics of peripheral T-cell deletion by Mtv-17 Sag also was accompanied by failure to detect Mtv-17 sag-specific mRNA in the thymus, despite detectable expression in other tissues, such as spleen. Together, these data suggest that Mtv-17 Sag causes peripheral, rather than intrathymic, deletion of T cells. Interestingly, the Mtv-8 provirus caused partial deletion of CD4(+)Vbeta12(+) cells in the thymus, but other T-cell subsets appeared to be deleted only in the periphery. Our data have important implications for the level of antigen expression required for elimination of self-reactive T cells. Moreover, these experiments suggest that mice expressing endogenous MMTVs that lead to slow kinetics of T-cell deletion will be susceptible to infection by milk-borne MMTVs with the same Sag specificity.

Chronic deletion, escape from deletion and activation of mouse mammary tumor virus superantigen-reactive T cells in C57BL/10 mice

Though C57BL/10 mice express the mouse mammary tumor virus superantigens (sag) encoded by Mtv-8 and Mtv-9, it has been thought that these sag do not bind to the MHC class II molecule H2-Ab and consequently do not affect the T cell repertoire. However, we show that cells bearing TCR Vbeta chains specific for Mtv-8 and -9 sag are chronically deleted in C57BL/10 mice. Thymocytes and peripheral T cells escaping deletion by Mtv sag display a small reduction in the level of cell surface CD4. T cells escaping thymic deletion respond variably to endogenous Mtv sag with some, but not all, reactive populations appearing overrepresented in the activated/memory subset. The data suggest that in normal mice fine modulation of coreceptor expression levels may be a common way by which thymocytes escape elimination, that systems utilizing potentially Mtv sag-reactive TCR on a C57BL background may be inappropriate for the measurement of the affinity of TCR/MHC/peptide interactions required in thymic selection, and that detection of the activity of human sag may be aided by analysis of CD4 levels and activation markers on T cells in conjunction with studies of the frequency of cells bearing specific TCRVbeta chains.

Activation of T cells by superantigen: cytokine production but not apoptosis depends on MEK-1 activity

Engagement of the TCR may result in proliferation and cytokine release or programmed cell death. These two outcomes may be the consequence of distinct T cell receptor-coupled signal transduction pathways or may reflect quantitative differences in signaling strength via a single pathway. Here we show that genetic inhibition of MAP kinase kinase (MEK) by a dominant negative mutant or through chemical inhibition by PD98059 inhibits IL-2 secretion but not programmed cell death after TCR ligation by superantigen. This supports the hypothesis that T cell cytokine release and apoptosis result from signaling through distinct pathways and implies that the molecular signaling mechanisms regulating apoptosis of mature T cells and negative selection of thymocytes may be similar.

A novel Bcl-x isoform connected to the T cell receptor regulates apoptosis in T cells

We define a novel Bcl-x isoform, Bcl-x gamma, that is generated by alternative splicing and characterized by a unique 47 amino acid C-terminus. Bcl-x gamma is expressed primarily in thymocytes, where it may depend on an interaction between the TCR and host MHC products, and in mature T cells, where its expression is associated with ligation of the T cell receptor. Overexpression of Bcl-x gamma in T cells inhibits activation-induced apoptosis; inhibition of Bcl-x gamma, after stable expression of Bcl-x gamma antisense cDNA, enhances activation-induced apoptosis. In contrast to other Bcl-x isoforms, cells that fail to express Bcl-x gamma after CD3 ligation undergo programmed cell death, while activated T cells that express Bcl-x gamma are spared. Identification of Bcl-x gamma helps provide a molecular explanation of T cell activation and death after antigen engagement.

MHC class II-dependent peptide antigen versus superantigen presentation to T cells

T lymphocytes expressing the CD4 coreceptor can be activated by two classes of major histocompatibility complex (MHC) class II-bound ligands. The elaboration of a conventional T-cell mediated immune response involves recognition of an antigenic peptide bound to the MHC class II molecules by a T-cell receptor (TCR) specific to that particular antigen. Conversely, superantigens (SAgs) also bind to MHC class II molecules and activate T cells, leading to a completely different functional outcome; indeed, SAg-responsive T cells die through apoptosis following stimulation. Superantigens are proteins that are secreted by various bacteria. They interact with the TCR using molecular determinants that are distinct from the residues involved in the recognition of nominal antigenic peptides. Despite the similarities between the recognition of the two classes of ligands by the TCR, considerable structural difference is observed. Here, we discuss the current knowledge on the presentation of SAgs to T cells and compare the different aspects of the SAg response with the recognition of antigenic peptide/MHC complexes.

Differences Between Graft-Versus-Leukemia and Graft-Versus-Host Reactivity. I. Interaction of Donor Immune T Cells With Tumor and/or Host Cells

Graft-versus-leukemia (GVL) and Graft-versus-host (GVH) reactions were compared after systemic transfer of allogeneic antitumor immune T lymphocytes from B10.D2 (H-2d; MIsb) into DBA/2 (H-2d; MIsa) mice. Before immune cell transfer, recipient DBA/2 mice were sublethally irradiated with 5 Gy to prevent host-versus-graft reactivity. Recipients were either bearing syngeneic metastatic ESb lymphomas (GVL system) or were normal, non–tumor-bearing mice (GVH system). We previously reported that this adoptive immunotherapy protocol (ADI) had pronounced GVL activity and led to immune rejection of even advanced metastasized cancer. In this study, monoclonal antibodies were used for immunohistochemical analysis of native frozen tissue sections from either spleen or liver to distinguish donor from host cells, to differentiate between CD4 and CD8 T lymphocytes, and to stain sialoadhesin-positive macrophages at different time points after cell transfer. The kinetics of donor cell infiltration in spleen and liver differed in that the lymphoid organ was infiltrated earlier (days 1 to 5 after transfer) than the nonlymphoid organ (days 5 to 20). After reaching a peak, donor cell infiltration decreased gradually and was not detectable in the spleen after day 20 and in the liver after day 30. The organ-infiltrating donor immune cells were mostly T lymphocytes and stained positive for CD4 or CD8 T-cell markers. A remarkable GVL-associated observation was made with regard to a subset of macrophages bearing the adhesion molecule sialoadhesin (SER+ macrophages). In the livers of tumor-bearing mice, their numbers increased between days 1 and 12 after ADI by a factor greater than 30. Double-staining for donor cell marker and SER showed that the sialoadhesin-expressing macrophages were of host origin. The SER+ host macrophages from GVL livers were isolated by enzyme perfusion and rosetting 12 days after ADI, when they reached peak values of about 60 cells per liver lobule, and were tested, without further antigen addition, for their capacity to stimulate an antitumor CD8 T-cell response. The results of this immunologic analysis suggest that these cells in the liver function as scavengers of the destroyed metastases and as antigen-processing and -presenting cells for antitumor immune T cells.

Positive and negative selection invoke distinct signaling pathways

During T cell development, interaction of the T cell receptor (TCR) with cognate ligands in the thymus may result in either maturation (positive selection) or death (negative selection). The intracellular pathways that control these opposed outcomes are not well characterized. We have generated mice expressing dominant-negative Ras (dnRas) and Mek-1 (dMek) transgenes simultaneously, either in otherwise normal animals, or in animals expressing a transgenic TCR, thereby permitting a comprehensive analysis of peptide-specific selection. In this system, thymocyte maturation beyond the CD4+8+ stage is blocked almost completely, whereas negative selection, assessed using an in vitro deletion protocol, is quantitatively intact. This suggests that activation of the mitogen-activated protein kinase (MAPK) cascade is necessary for positive selection, but irrelevant for negative selection. Generation of gamma/delta and of CD4-8- alpha/beta T cells proceeds normally despite blockade of the MAPK cascade. Hence, only cells that mature via conventional, TCR-mediated repertoire selection require activation of the MAPK pathway to complete their maturation.

Receptor-ligand interaction between CD44 and osteopontin (Eta-1)

The CD44 family of surface receptors regulates adhesion, movement, and activation of normal and neoplastic cells. The cytokine osteopontin (Eta-1), which regulates similar cellular functions, was found to be a protein ligand of CD44. Osteopontin induces cellular chemotaxis but not homotypic aggregation, whereas the inverse is true for the interaction between CD44 and a carbohydrate ligand, hyaluronate. The different responses of cells after CD44 ligation by either osteopontin or hyaluronate may account for the independent effects of CD44 on cell migration and growth. This mechanism may also be exploited by tumor cells to promote metastasis formation.