A new member of the tumor necrosis factor/nerve growth factor receptor family inhibits T cell receptor-induced apoptosis

Dexamethasone-Induced Thymocyte Apoptosis: Apoptotic Signal Involves the Sequential Activation of Phosphoinositide-Specific Phospholipase C, Acidic Sphingomyelinase, and Caspases

Glucocorticoid hormones (GCH) have been implicated as regulators of T-lymphocyte growth and differentiation. In particular, it has been reported that GCH can induce thymocyte apoptosis. However, the molecular mechanisms responsible for this GCH-induced death have not been clarified. In this work, the biochemical events associated with apoptosis induced by Dexamethasone (Dex), a synthetic GCH, in normal mouse thymocytes, have been analyzed. Results indicate that Dex-induced thymocyte apoptosis is attributable to an early ceramide generation caused by the activation of an acidic sphingomyelinase (aSMase). Caspase activity plays a crucial role in Dex-induced apoptosis and is downstream the aSMase activation in that inhibition of the early ceramide generation inhibits caspase activation and thymocyte death. Moreover, Dex treatment rapidly induces diacylglycerol (DAG) generation, through a protein kinase C (PKC) and G-protein–dependent phosphatidylinositol-specific phospholipase C (PI-PLC), an event which precedes and is required for aSMase activation. Indeed, PI-PLC inhibition by U73122 totally prevents Dex-induced aSMase activity, ceramide generation, and consequently, caspase activation and apoptosis. All these effects require Dex interaction with GCH receptor (GR), are countered by the GR antagonist RU486, and precede the GCH/GR-activated transcription and protein synthesis. These observations indicate that GCH activates thymocyte death through a complex signaling pathway that requires the sequential activation of different biochemical events.

Dexamethasone-Induced Thymocyte Apoptosis: Apoptotic Signal Involves the Sequential Activation of Phosphoinositide-Specific Phospholipase C, Acidic Sphingomyelinase, and Caspases

Glucocorticoid hormones (GCH) have been implicated as regulators of T-lymphocyte growth and differentiation. In particular, it has been reported that GCH can induce thymocyte apoptosis. However, the molecular mechanisms responsible for this GCH-induced death have not been clarified. In this work, the biochemical events associated with apoptosis induced by Dexamethasone (Dex), a synthetic GCH, in normal mouse thymocytes, have been analyzed. Results indicate that Dex-induced thymocyte apoptosis is attributable to an early ceramide generation caused by the activation of an acidic sphingomyelinase (aSMase). Caspase activity plays a crucial role in Dex-induced apoptosis and is downstream the aSMase activation in that inhibition of the early ceramide generation inhibits caspase activation and thymocyte death. Moreover, Dex treatment rapidly induces diacylglycerol (DAG) generation, through a protein kinase C (PKC) and G-protein–dependent phosphatidylinositol-specific phospholipase C (PI-PLC), an event which precedes and is required for aSMase activation. Indeed, PI-PLC inhibition by U73122 totally prevents Dex-induced aSMase activity, ceramide generation, and consequently, caspase activation and apoptosis. All these effects require Dex interaction with GCH receptor (GR), are countered by the GR antagonist RU486, and precede the GCH/GR-activated transcription and protein synthesis. These observations indicate that GCH activates thymocyte death through a complex signaling pathway that requires the sequential activation of different biochemical events.

Identification of a novel activation-inducible protein of the tumor necrosis factor receptor superfamily and its ligand

Among members of the tumor necrosis factor receptor (TNFR) superfamily, 4-1BB, CD27, and glucocorticoid-induced tumor necrosis factor receptor family-related gene (GITR) share a striking homology in the cytoplasmic domain. Here we report the identification of a new member, activation-inducible TNFR family member (AITR), which belongs to this subfamily, and its ligand. The receptor is expressed in lymph node and peripheral blood leukocytes, and its expression is up-regulated in human peripheral mononuclear cells mainly after stimulation with anti-CD3/CD28 monoclonal antibodies or phorbol 12-myristate 13-acetate/ionomycin. AITR associates with TRAF1 (TNF receptor-associated factor 1), TRAF2, and TRAF3, and induces nuclear factor (NF)-kappaB activation via TRAF2. The ligand for AITR (AITRL) was found to be an undescribed member of the TNF family, which is expressed in endothelial cells. Thus, AITR and AITRL seem to be important for interactions between activated T lymphocytes and endothelial cells.

Identification of a new member of the tumor necrosis factor family and its receptor, a human ortholog of mouse GITR

The tumor necrosis factor (TNF) and TNF receptor (TNFR) gene superfamilies regulate diverse biological functions, including cell proliferation, differentiation, and survival [1] [2] [3]. We have identified a new TNF-related ligand, designated human GITR ligand (hGITRL), and its human receptor (hGITR), an ortholog of the recently discovered murine glucocorticoid-induced TNFR-related (mGITR) protein [4]. The hGITRL gene mapped to chromosome 1q23, near the gene for the TNF homolog Fas/CD95 ligand [5]. The hGITR gene mapped to chromosome 1p36, near a cluster of five genes encoding TNFR homologs [1] [6]. We found hGITRL mRNA in several peripheral tissues, and detected hGITRL protein on cultured vascular endothelial cells. The levels of hGITR mRNA in tissues were generally low; in peripheral blood T cells, however, antigen-receptor stimulation led to a substantial induction of hGITR transcripts. Cotransfection of hGITRL and hGITR in embryonic kidney 293 cells activated the anti-apoptotic transcription factor NF-kappaB, via a pathway that appeared to involve TNFR-associated factor 2 (TRAF2) [7] and NF-kappaB-inducing kinase (NIK) [8]. Cotransfection of hGITRL and hGITR in Jurkat T leukemia cells inhibited antigen-receptor-induced cell death. Thus, hGITRL and hGITR may modulate T lymphocyte survival in peripheral tissues.

Interleukin-6 (IL-6) Prevents Activation-Induced Cell Death: IL-2–Independent Inhibition of Fas/fasL Expression and Cell Death

Triggering of the TCR/CD3 complex with specific antigen or anti-CD3 monoclonal antibody initiates activation-induced cell death (AICD) in mature T cells, an effect also mediated by the Fas/FasL system. We have previously shown that CD2 stimulation rescues T cells from TCR/CD3-induced apoptosis by decreasing the expression of Fas and FasL. In the present study, we examined whether the endogenous production of IL-2 plays a role in the effects mediated by CD2 triggering. The results indicated that transcription of Fas/FasL is controlled by interleukin-2 (IL-2) production and that CD2 triggering rescues a T-cell hybridoma from AICD via decreased production of IL-2. To ascertain whether modulation of IL-2 may be a general mechanism of AICD control, we examined other stimuli, capable of modulating the expression of the Fas/FasL system and the ensuing AICD, for ability to affect production of IL-2. We found that IL-6 reduced the level of TCR/CD3-induced apoptosis and the expression of Fas/FasL, yet failed to inhibit IL-2 production. Because IL-2 is involved in both apoptosis and activation events, these results indicate that, in contrast to CD2, which inhibits apoptosis and T cell activation, IL-6 inhibits apoptosis but not IL-2–induced activation. These observations may provide the basis for differential control of T-cell activation and apoptosis.

Interleukin-6 (IL-6) Prevents Activation-Induced Cell Death: IL-2–Independent Inhibition of Fas/fasL Expression and Cell Death

Triggering of the TCR/CD3 complex with specific antigen or anti-CD3 monoclonal antibody initiates activation-induced cell death (AICD) in mature T cells, an effect also mediated by the Fas/FasL system. We have previously shown that CD2 stimulation rescues T cells from TCR/CD3-induced apoptosis by decreasing the expression of Fas and FasL. In the present study, we examined whether the endogenous production of IL-2 plays a role in the effects mediated by CD2 triggering. The results indicated that transcription of Fas/FasL is controlled by interleukin-2 (IL-2) production and that CD2 triggering rescues a T-cell hybridoma from AICD via decreased production of IL-2. To ascertain whether modulation of IL-2 may be a general mechanism of AICD control, we examined other stimuli, capable of modulating the expression of the Fas/FasL system and the ensuing AICD, for ability to affect production of IL-2. We found that IL-6 reduced the level of TCR/CD3-induced apoptosis and the expression of Fas/FasL, yet failed to inhibit IL-2 production. Because IL-2 is involved in both apoptosis and activation events, these results indicate that, in contrast to CD2, which inhibits apoptosis and T cell activation, IL-6 inhibits apoptosis but not IL-2–induced activation. These observations may provide the basis for differential control of T-cell activation and apoptosis.

Thymocyte selection: not by TCR alone

During development of T cells in the thymus, T-cell receptor (TCR)-mediated recognition of self-MHC/self-peptide complexes on thymic stroma dictates the developmental fate of immature CD4+CD8+ (double positive) thymocytes. Intriguingly, TCR-generated intracellular signals can elicit two entirely different cellular responses in such thymocytes: apoptosis or further differentiation. The critical issue in understanding end-stage T-cell development is how TCR occupancy can be perceived in such markedly different ways by the TCR. Here, we review the cytoplasmic and nuclear events that result from TCR signaling during thymocyte selection. Studies aimed at distinguishing molecular components involved in positive selection (resulting in signals for further differentiation) and negative selection (resulting in apoptosis) will help solve this fascinating feature of T-lymphocyte biology. We also discuss how non-TCR-derived signaling might serve to fine tune the TCR-driven selection events in thymocytes. Central to this aspect of the conceptual framework needed to explain thymocyte selection is the observation that thymic antigen-presenting cells appear to be specialized in the induction of either positive or negative selection. Finally, we suggest a hypothesis that integrates the facts currently available on developing thymocytes, and which may serve to refine our exploration of unresolved issues in thymocyte selection. This hypothesis expands our focus to include signals from receptors other than TCRs as modulating and amplifying factors in thymocyte signaling.

Death-inducing functions of ligands of the tumor necrosis factor family: a Sanhedrin verdict

Members of the tumor necrosis factor ligand family can kill cells in a rather straightforward manner. They induce their receptors to recruit and activate caspases, enzymes that are critically involved in the death process, and this activation is further amplified by intracellular mitochondria-associated mechanisms. The potentially hazardous expression of the ligands occurs widely in the body; it is antigen-restricted only in the lymphocytes. Yet, in addition to control modes affecting ligand expression, there are numerous inhibitory mechanisms that act within target cells, to make doubly sure of avoiding an undue 'death verdict', while allowing the cells to exhibit other, noncytocidal effects of the ligands.

Application of differential cDNA screening techniques to the identification of unique gene expression in tumours and lymphocytes

Development, differentiation and cell death in all organisms are mediated by tightly regulated programs of differential gene expression. Furthermore, changes in gene expression profiles are responsible for tumour formation and tumour progression, as well as for many other human diseases. Thus, the definition of distinct patterns of gene expression is not only essential for understanding complex biological processes but also leads to the identification of novel targets for therapy of various diseases.

TWEAK, a new secreted ligand in the tumor necrosis factor family that weakly induces apoptosis

The members of the tumor necrosis factor (TNF) family play pivotal roles in the regulation of the immune system. Here we describe a new ligand in this family, designated TWEAK. The mouse and human versions of this protein are unusually conserved with 93% amino acid identity in the receptor binding domain. The protein was efficiently secreted from cells indicating that, like TNF, TWEAK may have the long range effects of a secreted cytokine. TWEAK transcripts were abundant and found in many tissues, suggesting that TWEAK and TRAIL belong to a new group of widely expressed ligands. Like many members of the TNF family, TWEAK was able to induce interleukin-8 synthesis in a number of cell lines. The human adenocarcinoma cell line, HT29, underwent apoptosis in the presence of both TWEAK and interferon-gamma. Thus, TWEAK resembles many other TNF ligands in the capacity to induce cell death; however, the fact that TWEAK-sensitive cells are relatively rare suggests that TWEAK along with lymphotoxins alpha/beta and possibly CD30L trigger death via a weaker, nondeath domain-dependent mechanism.

A new dexamethasone-induced gene of the leucine zipper family protects T lymphocytes from TCR/CD3-activated cell death

By comparing mRNA species expressed in dexamethasone (DEX)-treated and untreated murine thymocytes, we have identified a gene, glucocorticoid-induced leucine zipper (GILZ), encoding a new member of the leucine zipper family. GILZ was found expressed in normal lymphocytes from thymus, spleen, and lymph nodes, whereas low or no expression was detected in other nonlymphoid tissues, including brain, kidney, and liver. In thymocytes and peripheral T cells, GILZ gene expression is induced by DEX. Furthermore, GILZ expression selectively protects T cells from apoptosis induced by treatment with anti-CD3 monoclonal antibody but not by treatment with other apoptotic stimuli. This antiapoptotic effect correlates with inhibition of Fas and Fas ligand expression. Thus, GILZ is a candidate transcription factor involved in the regulation of apoptosis of T cells.