Activated T cell death in vivo mediated by proapoptotic bcl-2 family member bim

GILZ, a new target for the transcription factor FoxO3, protects T lymphocytes from interleukin-2 withdrawal-induced apoptosis

Interleukin-2 (IL-2) withdrawal is a physiologic process inducing cell death in activated T lymphocytes. Glucocorticoid-induced leucine zipper (GILZ) has recently been identified as a protein modulating T-cell receptor activation by repressing various signaling pathways. We report here that IL-2 deprivation leads to expression of GILZ in T lymphocytes. We then characterized the human gilz promoter and showed that FoxO3 (Forkhead box class O3) binding to the Forkhead responsive elements identified in the promoter is necessary for induction of gilz expression upon IL-2 withdrawal. To assess the functional consequences of this induction, we used 2 strategies, GILZ overexpression and GILZ silencing in murine IL-2-dependent CTLL-2 cells. GILZ overexpression protects CTLL-2 cells from IL-2 withdrawal-induced apoptosis, whereas cell death is accelerated in cells unable to express GILZ. Concomitantly, the expression of Bim is inhibited in GILZ-overexpressing cells and enhanced when GILZ expression is impaired. Furthermore, GILZ inhibits FoxO3 transcriptional activity that leads to inhibition of Bim expression but also to down-regulation of GILZ itself. Therefore, GILZ is a transiently expressed protein induced upon IL-2 withdrawal that protects T cells from the onset of apoptosis.

Differential tumor necrosis factor receptor 2-mediated editing of virus-specific CD8+ effector T cells

Much of the CD8(+) T cell response in H2(b) mice with influenza pneumonia is directed at the nucleoprotein(366-374) (NP(366)) and acid polymerase(224-233) (PA(224)) peptides presented by the H2D(b) MHC class I glycoprotein. These D(b)NP(366)- and D(b)PA(224)-specific T cell populations are readily analyzed by staining with tetrameric complexes of MHC(+) peptide (tetramers) or by cytokine production subsequent to in vitro stimulation with the cognate peptides. The D(b)PA(224)-specific CD8(+) effector T cells make more tumor necrosis factor (TNF) alpha than the comparable CD8(+)D(b)NP(366)(+) set, a difference reflected in the greater sensitivity of the CD8(+)D(b)PA(224)(+) population to TNF receptor (TNFR) 2-mediated apoptosis under conditions of in vitro culture. Freshly isolated CD8(+)D(b)NP(366)(+) and CD8(+)D(b)PA(224)(+) T cells from influenza-infected TNFR2(-/-) mice produce higher levels of IFN-gamma and TNF-alpha after in vitro stimulation with peptide, although the avidity of the T cell receptor-epitope interaction does not change. Increased numbers of both CD8(+)D(b)PA(224)(+) and CD8(+)D(b)NP(366)(+) T cells were recovered from the lungs (but not the spleens) of secondarily challenged TNFR2(-/-) mice, a pattern that correlates with the profiles of TNFR expression in the TNFR2(+/+) controls. Thus, it seems that TNFR2-mediated editing of influenza-specific CD8(+) T cells functions to limit the numbers of effectors that have localized to the site of pathology in the lung but does not modify the size of the less activated responder T cell populations in the spleen. Therefore, the massive difference in magnitude for the secondary, although not the primary, response to these D(b)NP(366) and D(b)PA(224) epitopes cannot be considered to reflect differential TNFR2-mediated T cell editing.

Regulation of expression of Bcl-2 protein family member Bim by T cell receptor triggering

Bim, a proapoptotic BH3-only member of the Bcl-2 protein family, is required for central and peripheral deletion of T lymphocytes. Mechanisms regulating Bim activity in T cells remain poorly understood. We show that expression of Bim is up-regulated in human T cells after polyclonal or specific T cell receptor triggering. Induction of Bim was affected by the agonistic potency of MHC:peptide ligands. Peptides that failed to induce Bim expression, failed to induce apoptosis in specific T cells, whereas partially agonistic ligands, which trigger death receptor-independent activation-induced cell death (AICD), induced Bim, but were inefficient in up-regulating Bcl-X(L). Activation of protein kinase C and calcineurin appeared to be necessary and sufficient for Bim up-regulation after T cell receptor ligation. Immunosuppressive drugs known to prevent T cell deletion in vivo, such as cyclosporin A or FK506, blocked Bim up-regulation and rescued T cells from death receptor-independent AICD, whereas rapamycin, which allows the development of stable immunological tolerance, did not exhibit these activities. These results define a new mode of Bim regulation, strongly implicate Bim as a mediator of AICD, and suggest that Bim up-regulation can be targeted to influence the outcome of specific immune responses.

B Cell Receptor-Mediated Apoptosis of Human Lymphocytes Is Associated with a New Regulatory Pathway of Bim Isoform Expression

Studies in Bim-deficient mice have shown that the proapoptotic molecule Bim plays a key role in the control of B cell homeostasis and activation. However, the role of Bim in human B lymphocyte apoptosis is unknown. We show in this study that, depending on the degree of cross-linking, B cell receptors can mediate both Bim-dependent and apparent Bim-independent apoptotic pathways. Cross-linked anti-mu Ab-mediated activation induces an original pathway governing the expression of the various Bim isoforms. This new pathway involves the following three sequential steps: 1) extracellular signal-regulated kinase-dependent phosphorylation of the BimEL isoform, which is produced in large amounts in healthy B cells; 2) proteasome-mediated degradation of phosphorylated BimEL; and 3) increased expression of the shorter apoptotic isoforms BimL and BimS.

Caspase cleavage of BimEL triggers a positive feedback amplification of apoptotic signaling

Members of the Bcl-2 protein family that share only the Bcl-2 homology 3 (BH3) domain are known mostly as sentinels for apoptotic stimuli and initiators of apoptosis. One BH3-only protein, Bim, is the major physiological antagonist of the prosurvival proteins in B and T lymphocytes. It is required for hematopoietic homeostasis and to preclude autoimmunity. Here, we show that the Bim(EL) isoform, which was predominant in T cells, existed in both phosphorylated and unphosphorylated forms. Whereas the unphosphorylated Bim(EL) was sequestered to microtubules by means of a direct interaction with tubulin, the phosphorylated protein was released from microtubules. The freed Bim(EL) was subjected to caspase cleavage at an early stage of apoptosis induced by stimuli that activate either the mitochondria- or death receptor-dependent apoptosis pathway. The N-terminally cleaved Bim(EL) became hyperactive in inducing apoptosis because of its more efficient targeting of Bcl-2. Thus, unlike many other BH3-only proteins, Bim(EL) can be activated downstream of the caspase cascade, leading to a positive feedback amplification of apoptotic signals.

B-cell homeostasis: digital survival or analog growth?

Maintenance of B-lymphocyte homeostasis requires balanced cell production, death, and proliferation. To coordinate these processes, B cells are dependent on cell extrinsic signals. In lymphocyte development, precursor cells are dependent on Fms-like tyrosine kinase ligand 3 (Flt3L), and pre-B cells are dependent on the cytokine interleukin-7. Transitional B cells require B-lymphocyte stimulator (BLyS) for survival. Mature B cells require B-cell receptor (BCR) signals and also remain sensitive to their microenvironment. An emerging model suggests that extrinsic signals do not regulate B-cell survival through a digital mechanism where cells are simply instructed to survive or die. Instead, availability and competition for extrinsic signals regulates cellular physiology and metabolism in an analog fashion that then influences cell commitment to apoptosis or proliferation. Decreases in cellular metabolism may sensitize cells to activation and action of the pro-apoptotic Bcl-2 family members, Bak and Bax, and promote apoptosis. In contrast, increases in metabolism may predispose cells to proliferate. Analog control of cell physiology can, thus, be integrated with other inputs by individual cells to produce a fate decision for survival, proliferation, or apoptosis and prevent diseases of cell death, such as immunodeficiency, and cell activation and proliferation, such as autoimmunity or cancer.

Ligand-independent redistribution of Fas (CD95) into lipid rafts mediates clonotypic T cell death

Clonotypic elimination of activated T cells through Fas-Fas ligand (CD95-CD95L) interactions is one mechanism of peripheral self-tolerance. T cell receptor (TCR) stimuli trigger FasL synthesis but also sensitize activated T cells to Fas-mediated apoptosis through an unknown mechanism. Here we show that TCR restimulation of activated human CD4(+) T cells resulted in Fas translocation into lipid raft microdomains before binding FasL, rendering these cells sensitive to apoptosis after stimulation with bivalent antibody or FasL. Disruption of lipid rafts reduced sensitivity to Fas-mediated apoptosis after TCR restimulation. Thus, the redistribution of Fas and other tumor necrosis factor family receptors into and out of lipid rafts may dynamically regulate the efficiency and outcomes of signaling by these receptors.

IL-7 promotes the transition of CD4 effectors to persistent memory cells

After transfer to adoptive hosts, in vitro–generated CD4 effectors can become long-lived memory cells, but the factors regulating this transition are unknown. We find that low doses of interleukin (IL) 7 enhance survival of effectors in vitro without driving their division. When in vitro–generated effectors are transferred to normal intact adoptive hosts, they survive and rapidly become small resting cells with a memory phenotype. CD4 effectors generated from wild-type versus IL-7 receptor^−/− mice were transferred to adoptive hosts, including intact mice and those deficient in IL-7. In each case, the response to IL-7 was critical for good recovery of donor cells after 5–7 d. Recovery was also IL-7–dependent in Class II hosts where division was minimal. Blocking antibodies to IL-7 dramatically decreased short-term recovery of transferred effectors in vivo without affecting their division. These data indicate that IL-7 plays a critical role in promoting memory CD4 T cell generation by providing survival signals, which allow effectors to successfully become resting memory cells.

Reactive oxygen species regulate quiescent T-cell apoptosis via the BH3-only proapoptotic protein BIM

The survival of quiescent T cells in the peripheral immune system is dependent on signals transmitted from the extracellular environment. The requirement for survival factors is also manifested in vitro, providing a robust system to examine molecular mechanisms underlying T-cell death. We show that peripheral T cells cultured in the absence of survival factors accumulate reactive oxygen species (ROS), upregulate BIM (Bcl-2-interacting mediator of death) and inducible nitric oxide synthase (iNOS) expression, culminating in Fas-independent neglect-induced death (NID). We have examined ROS, iNOS and cytokine modulation of T-cell NID. Antioxidants inhibit BIM induction, caspase activation and apoptosis but do not promote cell cycle entry. iNOS-deficient T cells are protected from apoptosis, implicating iNOS in the regulation of NID via suppression of Bcl-x(L) expression and consequent inhibition of BIM activity. Finally, we show that the prosurvival cytokine IL-7 elevates Bcl-x(L) expression and transcriptionally regulates iNOS but not BIM expression in T cells.

c-FLICE inhibitory protein expression inhibits T-cell activation

Cellular FLICE-inhibitory protein (c-FLIP) inhibits death receptor-mediated apoptosis by specific interaction with FADD and procaspase-8, and may thus interfere with activation events mediated by FADD and caspase-8. Recent studies, however, suggest that c-FLIP also transmits activation signals. The role of c-FLIP on T-cell activation was examined here using several transgenic mice with variable c-FLIP expression. In all c-FLIP-transgenic mice, Fas-mediated apoptosis and in vitro activation-induced T-cell death were suppressed, and T-cell proliferation and IL-2 production were inhibited. c-FLIP transgene also promoted in vivo thymocyte death. Higher c-FLIP transgene expression was correlated with a more profound suppression of T-cell activation and a prominent disturbance in mature thymocyte development. There was no evidence of increased activation and proliferation in all c-FLIP-transgenic T cells examined. Instead, suppression of T-cell activation in c-FLIP-transgenic T cells could be a combinatory effect of FADD/caspase-8-dependent signals and c-FLIP-specific activities.

Control of Bcl-2 expression by reactive oxygen species

Reactive oxygen species (ROS) mediate apoptosis in many different cell types. We have previously shown that the antioxidant Mn(III) tetrakis(5,10,15,20-benzoic acid)porphyrin (MnTBAP) decreased intracellular ROS and prevented the apoptosis of activated T cells in vitro. To determine the mechanism(s) by which MnTBAP afforded such protection, we used Affymetrix (Santa Clara, CA) gene arrays to compare gene expression in T cells activated with staphylococcal enterotoxin B in vivo then cultured with or without MnTBAP. This analysis showed that the antioxidant increased the expression of Bcl-2, an antiapoptotic molecule whose levels are normally decreased by T cell activation. Culture with MnTBAP revealed a tight inverse correlation between the levels of Bcl-2 and ROS within T cells. In vivo, production of ROS in activated T cells occurred before Bcl-2 down-regulation. Furthermore, MnTBAP's ability to prevent death required the expression of Bcl-2 in most T cells. Finally, neither ROS production nor the effects on Bcl-2 expression required Bim, the Bcl-2 antagonist that mediates the death of activated T cells in vivo. Taken together, our results suggest that ROS sensitize T cells to apoptosis by decreasing expression of Bcl-2.

Shutdown of an acute T cell immune response to viral infection is mediated by the proapoptotic Bcl-2 homology 3-only protein Bim

We used mutant Fas-deficient (lpr) or Bim-deficient mice to investigate the role of the death receptor and Bcl-2-regulated apoptotic pathways in terminating a physiological T cell response to herpes simplex virus infection. In WT and lpr mice CD8+ antigen-specific T cells were deleted after viral clearance. In contrast, the immune response was not terminated in Bim-deficient mice despite viral clearance, and CD8+ antigen-specific T cells accumulated in the spleen. Thus, Bim is dispensable for viral clearance but is necessary for the death of activated T cells when immune responses are terminated. These findings have implications for the therapeutic manipulation of immune responses to infections and immunization.

The Bcl-2 family: roles in cell survival and oncogenesis

Apoptosis, the cell-suicide programme executed by caspases, is critical for maintaining tissue homeostasis, and impaired apoptosis is now recognized to be a key step in tumorigenesis. Whether a cell should live or die is largely determined by the Bcl-2 family of anti- and proapoptotic regulators. These proteins respond to cues from various forms of intracellular stress, such as DNA damage or cytokine deprivation, and interact with opposing family members to determine whether or not the caspase proteolytic cascade should be unleashed. This review summarizes current views of how these proteins sense stress, interact with their relatives, perturb organelles such as the mitochondrion and endoplasmic reticulum and govern pathways to caspase activation. It briefly explores how family members influence cell-cycle entry and outlines the evidence for their involvement in tumour development, both as oncoproteins and tumour suppressors. Finally, it discusses the promise of novel anticancer therapeutics that target these vital regulators.

VavP-Bcl2 transgenic mice develop follicular lymphoma preceded by germinal center hyperplasia

In human follicular lymphoma the t(14; 18) chromosome translocation activates the antiapoptotic oncogene Bcl2 by linking it to the immunoglobulin heavy chain (IGH) locus. Transgenic mice expressing Bcl2 controlled by an Igh enhancer (E mu) do not develop follicular lymphoma, although they do have an increased incidence of other B-lymphoid neoplasms. We have now analyzed tumorigenesis in mice bearing a Bcl2 transgene controlled by Vav gene regulatory sequences (VavP), which confer expression in multiple hematopoietic lineages. Unlike E mu-Bcl2 mice, many VavP-Bcl2 mice older than 10 months developed follicular lymphoma. Young VavP-Bcl2 mice had an overabundance of enlarged germinal centers and greatly elevated numbers of cycling B cells that had undergone IgH class switching and V-gene hypermutation. The peripheral T-cell compartment was larger in the VavP-Bcl2 mice than in E mu-Bcl2 strains and, notably, CD4 T cells were 5-fold increased over normal. The germinal center hyperplasia required CD4 T cells, because it could be abolished by anti-CD4 antibody in vivo. VavP-Bcl2 mice also had a propensity to develop kidney disease of the autoimmune type. We suggest that the increased survival capacity of B and T cells fosters prolonged germinal center reactions, and that autoreactivity and hypermutation conspire to generate follicular lymphoma.

T-helper cell tolerance to ubiquitous nuclear antigens

Systemic autoimmune diseases are characterized by the development of antinuclear autoantibodies. In order to understand the immunologic events leading to the development of such antibodies, knowledge of mechanisms of immune tolerance to nuclear antigens is required. By utilizing adoptive T-cell transfer strategies with transgenic mouse models expressing nuclear neo-self antigens, T-cell tolerance to the lupus-related nuclear antigens human La and nRNP A has been demonstrated. These findings also indicate the existence in normal animals of autoreactive B cells continuously presenting nuclear antigen, suggesting that nuclear antigens are not sequestered from the immune system. Investigations of CD4+ T-cell tolerance to non-nuclear antigens have revealed a number of mechanisms that protect the host from autoreactivity, including autoreactive T-cell deletion, regulatory T-cell development and anergy induction. Recent studies using T-cell receptor and neo-self nuclear antigen transgenic mice are revealing the importance of such mechanisms in maintaining tolerance to nuclear antigens. Mechanisms of tolerogenic antigen presentation, identification of tolerogenic antigen source(s) and the pathways leading to loss of tolerance to nuclear antigens in systemic autoimmune disease states are currently being sought.

Glycolipid Antigen Drives Rapid Expansion and Sustained Cytokine Production by NK T Cells

NKT cells are enigmatic lymphocytes that respond to glycolipid Ags presented by CD1d. Although they are key immunoregulatory cells, with a critical role in immunity to cancer, infection, and autoimmune diseases, little is known about how they respond to antigenic challenge. Current theories suggest that NKT cells die within hours of stimulation, implying that their direct impact on the immune system derives from the initial cytokine burst released before their death. Here we show that NKT cell disappearance results from TCR down-regulation rather than apoptosis, and that they expand to many times their normal number in peripheral tissues within 2-3 days of stimulation, before contracting to normal numbers over subsequent days. This expansion is associated with ongoing cytokine production, biased toward a Th1 (IFN-gamma(+) IL-4(-)) phenotype, in contrast to their initial Th0 (IFN-gamma(+)IL-4(+)) phenotype. This study provides critical new insight into how NKT cells can have such a major impact on immune responses, lasting many days beyond the initial stimulation of these cells.

Loss of the pro-apoptotic BH3-only Bcl-2 family member Bim inhibits BCR stimulation-induced apoptosis and deletion of autoreactive B cells

During development, the stochastic process assembling the genes encoding antigen receptors invariably generates B and T lymphocytes that can recognize self-antigens. Several mechanisms have evolved to prevent the activation of these cells and the concomitant development of autoimmune disease. One such mechanism is the induction of apoptosis in developing or mature B cells by engagement of the B cell antigen receptor (BCR) in the absence of T cell help. Here we report that B lymphocytes lacking the pro-apoptotic Bcl-2 family member Bim are refractory to apoptosis induced by BCR ligation in vitro. The loss of Bim also inhibited deletion of autoreactive B cells in vivo in two transgenic systems of B cell tolerance. Bim loss prevented deletion of autoreactive B cells induced by soluble self-antigen and promoted accumulation of self-reactive B cells developing in the presence of membrane-bound self-antigen, although their numbers were considerably lower compared with antigen-free mice. Mechanistically, we determined that BCR ligation promoted interaction of Bim with Bcl-2, inhibiting its survival function. These findings demonstrate that Bim is a critical player in BCR-mediated apoptosis and in B lymphocyte deletion.

Impaired T cell death and lupus-like autoimmunity in T cell-specific adapter protein-deficient mice

T cell-specific adaptor protein (TSAd) is a T lineage-restricted signaling adaptor molecule that is thought to participate in the assembly of intracellular signaling complexes in T cells. Previous studies of TSAd-deficient mice have revealed a role for TSAd in the induction of T cell interleukin 2 secretion and proliferation. We now show that TSAd-deficient mice are susceptible to lupus-like autoimmune disease. On the nonautoimmune-prone C57BL/6 genetic background, TSAd deficiency results in hypergammaglobulinemia that affects all immunoglobulin (Ig)G subclasses. Older C57BL/6 TSAd-deficient mice (1 yr of age) accumulate large numbers of activated T and B cells in spleen, produce autoantibodies against a variety of self-targets including single stranded (ss) and double stranded (ds) DNA, and, in addition, develop glomerulonephritis. We further show that immunization of younger C57BL/6 TSAd-deficient mice (at age 2 mo) with pristane, a recognized nonspecific inflammatory trigger of lupus, results in more severe glomerulonephritis compared with C57BL/6 controls and the production of high titer ss and ds DNA antibodies of the IgG subclass that are not normally produced by C57BL/6 mice in this model. The development of autoimmunity in TSAd-deficient mice is associated with defective T cell death in vivo. These findings illustrate the role of TSAd as a critical regulator of T cell death whose absence promotes systemic autoimmunity.

The Structure of a Bcl-xL/Bim Fragment Complex

After antigen-driven expansion, the majority of T cells involved in an immune response die rapidly by apoptosis dependent on the Bcl-2 related proteins, Bim and Bax or Bak. The details of how these proteins are activated and interact are still unclear. The crystal structure of mouse Bcl-x(L) bound to a long helical fragment of Bim indicates that the structure of Bim is very different from proteins with a Bcl-2-like fold and may leave the BH3 region of Bim constitutively exposed. Based on the structural homology between Bcl-x(L) and Bax, we predicted that binding of Bim to Bax would require displacement of the Bax penultimate alpha helix. Consistent with this prediction, truncation of this short helix was required for Bim/Bax interaction and led to spontaneous activation of Bax. Our results suggest a way in which both Bim and Bax/Bak might be required for activated T cell apoptosis.

A1/Bfl-1 expression is restricted to TCR engagement in T lymphocytes

We analyzed regulation of the prosurvival Bcl-2 homologue A1, following T-cell receptor (TCR) or cytokine receptor engagement. Activation of CD4(+) or CD8(+) T cells by antigenic peptides induced an early but transient IL-2-independent expression of A1 and Bcl-xl mRNA and proteins, whereas expression of Bcl-2 was delayed and required IL-2. Cytokines such as IL-2, IL-4, IL-7 or IL-15 prevented apoptosis of activated T cells that effect being associated with the maintenance of Bcl-2, but not of A1 expression. However, restimulation of activated or posteffector T cells with antigenic peptide strongly upregulated A1 mRNA and maintained A1 protein expression. IL-4, IL-7 or IL-15 also prevented cell death of naive T cells. In those cells, cytokines upregulated Bcl-2, but not A1 expression. Therefore, in naive, activated and posteffector T cells, expression of A1 is dependent on TCR but not on cytokine receptor engagement, indicating that A1 is differently regulated from Bcl-xl and Bcl-2.

The Bcl-2 family pro-apoptotic molecule, BNIP3 regulates activation-induced cell death of effector cytotoxic T lymphocytes

BNIP3 is a recently described pro-apoptotic member of the Bcl-2 family and in BNIP3 cDNA-transfected cell lines, cell death occurs via a caspase-independent pathway with opening of the mitochondrial permeability transition (PT) pore and rapid loss of mitochondrial transmembrane potential (Delta psi m). However, its expression or function in physiologic cell types is not known. Our results using the T-cell receptor transgenic mice P14, specific for lymphocyte choreomeningitis virus (LCMV) glycoprotein, show that in contrast to the other Bcl-2 family pro-apoptotic molecules, BNIP3 is transcriptionally highly up-regulated in effector cytotoxic T lymphocytes (CTL). Because CTL have a propensity to undergo activation-induced cell death (AICD) upon restimulation, we tested for other features associated with BNIP3-induced cell death. AICD of CTL was caspase-independent as determined by measuring caspase activation during target cell killing as well as by lack of inhibition with caspase inhibitors. Moreover, similar to BNIP3-induced cell death, CTL apoptosis was associated with increased production of reactive oxygen species and decreased Delta psi m. Finally, retroviral transduction of BNIP3 antisense RNA diminished AICD in effector CTL. These results suggest that BNIP3 may play an important role in T-cell homeostasis by regulating effector CTL numbers.

Control of apoptosis in the immune system: Bcl-2, BH3-only proteins and more

Apoptotic cell death plays a critical role in the development and functioning of the immune system. During differentiation, apoptosis weeds out lymphocytes lacking useful antigen receptors and those expressing dangerous ones. Lymphocyte death is also involved in limiting the magnitude and duration of immune responses to infection. In this review, we describe the role of the Bcl-2 protein family, and to a lesser extent that of death receptors (members of the tumor necrosis factor receptor family with a death domain), in the control of lymphoid and myeloid cell survival. We also consider the pathogenic consequences of failure of apoptosis in the immune system.

The requirements for Fas-associated death domain signaling in mature T cell activation and survival

Fas-associated death domain (FADD) is a death domain containing cytoplasmic adapter molecule required for the induction of apoptosis by death receptors. Paradoxically, FADD also plays a crucial role in the development and proliferation of T cells. Using T cells from mice expressing a dominant negative form of FADD (FADDdd), activation with anti-TCR Ab and costimulation or exogenous cytokines is profoundly diminished. This is also seen in wild-type primary T cells transduced with the same transgene, demonstrating that FADD signaling is required in normally differentiated T cells. The defective proliferation does not appear to be related to the early events associated with TCR stimulation. Rather, with a block in FADD signaling, stimulated T cells exhibit a high rate of cell death corresponding to the initiation of cell division. Although CD4 T cells exhibit a moderate deficiency, this effect is most profound in CD8 T cells. In vivo, the extent of this defective accumulation is most apparent; lymphocytic choriomenigitis virus-infected FADDdd-expressing mice completely fail to mount an Ag-specific response. These results show that, in a highly regulated fashion, FADD, and most likely caspases, can transduce either a signal for survival or one that leads directly to apoptosis and that the balance between these opposing outcomes is crucial to adaptive immunity.

The role of CD95 in the regulation of peripheral T-cell apoptosis

Apoptosis of activated peripheral T cells during the termination phase of an immune response is critical to maintain T-cell homeostasis. Activated T cells can be removed by two mechanisms: activation-induced cell death (AICD) and death by neglect. AICD is triggered by death receptors, whereas death by neglect is induced by cytokine withdrawal. CD95 (APO-1/Fas) belongs to the subfamily of death receptors and plays a major role in AICD. In this review, we focus on the molecular mechanisms of AICD, in particular those involving the CD95 system. Moreover, we discuss the relative contribution of AICD and death by neglect to terminate a T-cell immune response. In order to become fully activated, T cells require a second signal provided by antigen-presenting cells. We discuss how these costimulatory signals counteract pro-apoptotic signals and, finally, which signals might protect T cells from death to generate a pool of memory T cells.

Migration and T-lymphocyte effector function

The development of immunity depends upon the capacity of responding T cells to become mobilized from lymphoid tissues where they are primed to sites of antigen exposure, wherever they occur in the body. Activation-induced alterations in the ability of T cells to migrate signify a fundamental change in biological function. Considerable attention is now focused on identifying mechanisms that regulate the migration and persistence of T cells that disseminate to non-lymphoid compartments as effector cells, and those that are retained in the lymphoid compartment. There are many unanswered questions about the developmental relationships and roles in protective immunity of antigen-experienced T cells that partition in different tissues.

Activation-induced cell death in T cells

A properly functioning immune system is dependent on programmed cell death at virtually every stage of lymphocyte development and activity. This review addresses the phenomenon of activation-induced cell death (AICD) in T lymphocytes, in which activation through the T-cell receptor results in apoptosis. AICD can occur in a cell-autonomous manner and is influenced by the nature of the initial T-cell activation events. It plays essential roles in both central and peripheral deletion events involved in tolerance and homeostasis, although it is likely that different forms of AICD proceed via different mechanisms. For example, while AICD in peripheral T cells is often caused by the induction of expression of the death ligand, Fas ligand (CD95 ligand, FasL), it does not appear to be involved in AICD in thymocytes. This and other mechanisms of AICD are discussed. One emerging model that may complement other forms of AICD involves the inducible expression of FasL by nonlymphoid tissues in response to activated T lymphocytes. Induction of nonlymphoid FasL in this manner may serve as a sensing mechanism for immune cell infiltration, which contributes to peripheral deletion.

Phosphorylation of the pro-apoptotic protein Bim in lymphocytes is associated with protection from apoptosis

Bim is a pro-apoptotic member of the Bcl-2 protein family. Bim has three isoforms, EL, L, and S, of which the EL form is the least cytotoxic. We show here that Bim is serine phosphorylated in lymphocytes, predominantly on the EL form. Withdrawal of IL-2 from IL-2-dependent T lymphocytes or culture of thymocytes leads to reduced Bim phosphorylation and apoptosis induction. This decrease in Bim phosphorylation occurs when most cells in culture are still viable, indicating that reduction of Bim phosphorylation may be an early event in apoptosis signaling of lymphocytes.

The control of apoptosis in lymphocyte selection

The stochastic nature of rearrangement and diversification of the gene segments encoding immunoglobulins (Igs) and T cell receptors (TCRs) inevitably gives rise to immature B and T lymphocytes that lack antigen receptors or express useless or dangerous (self-antigen-specific) ones. Signaling through antigen receptors promotes survival, proliferative expansion and further differentiation of useful cells and deletion of the useless and dangerous ones. During immune responses, pathogen-specific B and T lymphocytes, as well as cells of the innate immune system, undergo extensive proliferation and develop effector functions, such as antibody secretion, cytotoxicity or cytokine production. To prevent tissue damage by these effector molecules, activated lymphocytes are removed when an infection has been overcome. Together with other mechanisms, including developmental arrest and induction of unresponsiveness (anergy), programmed cell death (apoptosis) of autoreactive lymphocytes safeguards immunological tolerance to self and assists in the development of an effective immune system. We have been investigating the molecular mechanisms that control programmed cell death. This review describes some of our experiments using transgenic and knockout mice, which overexpress or lack apoptosis regulators, that led to discoveries on how life and death decisions are made during development and functioning of the immune system.

Caspases and T lymphocytes: a flip of the coin?

In this review, we consider the role caspases play in cell death downstream of death receptors and cell intrinsic death mechanisms. In particular, we focus on these mechanisms in antigen-induced cell death, a mechanism which regulates the number of surviving T cells at the end of an immune response. The relative role of the apoptosome as an amplifier rather than an initiator of apoptosis is considered. Several factors that regulate the susceptibility to activation-induced cell death are considered. These factors emanate from the stimulation of the T-cell receptors and include multiple pathways. Recent work has shown that death receptor signaling can play an interesting role in cell proliferation in both humans and animals. These recent findings are discussed in the light of models of death receptor signaling.

Apoptosis and transplantation tolerance

Self-tolerance is maintained by several mechanisms including deletion (via apoptosis) and regulation. Acquired tolerance to allogeneic tissues and organs exploits similar strategies. One key difference between alloantigens and peptide antigens is the enormous number of T cells that are alloreactive. Accumulating evidence suggests that in the face of this large mass of potentially graft-destructive T cells, tolerance requires an initial wave of deletion. This creates a more level playing field in which a smaller number of regulatory T cells can then act to maintain an established tolerant state. Deletion of alloreactive T cells by apoptosis actively promotes immunoregulation as well, by interfering with proinflammatory maturation of antigen presenting cells. This article reviews the immune response to alloantigens, the development and use of both necrotic and apoptotic means of cell death during the evolution of the immune response, and the likely role and mechanisms by which apoptosis promotes, and may even be required for, transplantation tolerance.

Mitochondria in HIV-1-induced apoptosis

It is now well admitted that HIV infection leading to AIDS is associated with an abnormal susceptibility of T cells to undergo apoptosis. Recent progress in research into programmed cell death has resulted in the identification of the principal pathways involved in this process. Thus the "extrinsic" as well as the "intrinsic" pathways converge to the mitochondria considered as the main sensor of programmed cell death. This review summarizes our knowledge of the influence of mitochondrial control on T cell death during HIV and SIV infections.

Caspase-independent cell death in T lymphocytes

T lymphocyte death is essential for proper function of the immune system. During the decline of an immune response, most of the activated T cells die. Cell death is also responsible for eliminating autoreactive lymphocytes. Although recent studies have focused on caspase-dependent apoptotic signals, much evidence now shows that caspase- independent, necrotic cell death pathways are as important. An understanding of the molecular control of these alternative pathways is beginning to emerge. Damage of organelles including mitochondria, endoplasmic reticulum or lysozymes, leading to an increase in calcium and reactive oxygen species and the release of effector proteins, is frequently involved in caspase-independent cell death.

Negative selection--clearing out the bad apples from the T-cell repertoire

Dead cells are a prominent feature of the thymic landscape as only 5% of developing thymocytes are exported as mature T cells. The remaining thymocytes die by one of two mechanisms; most thymocytes die because they are not positively selected and do not receive a survival signal, whereas a minority of thymocytes undergo T-cell receptor (TCR)-mediated apoptosis, a process known as negative selection. Negative selection is extremely important for establishing a functional immune system, as it provides an efficient mechanism for ridding the T-cell repertoire of self-reactive and potentially autoimmune lymphocytes. This review discusses several cellular and molecular aspects of negative selection.

Apoptosis in the development and maintenance of the immune system

Programmed cell death is essential for the development and maintenance of cellular homeostasis of the immune system. The Bcl-2 family of proteins comprises both pro-apoptotic and anti-apoptotic members. A subset of pro-apoptotic members, called 'BH3-only' proteins, share sequence homology only in the minimal death domain, designated the Bcl-2 homology 3 (BH3) domain. BH3-only proteins operate as upstream sentinels, selectively sensing both intrinsic and extrinsic death stimuli. They communicate this information to the pro-apoptotic 'multidomain' members Bax or Bak--a process that is antagonized by anti-apoptotic members of the Bcl-2 family. The functional balance of pro-apoptotic versus anti-apoptotic influences, which operates at organelles, determines whether a lymphocyte will live or die. BH3-only molecules, often working in concert, compete for downstream multidomain pro- and anti-apoptotic BCL-2 members to control serial stages of lymphocyte development and homeostasis.

The PI-3 kinase/Akt pathway and T cell activation: pleiotropic pathways downstream of PIP3

Ligation of the T cell receptor for antigen (TCR) and/or costimulatory receptor CD28 results in rapid activation of phosphoinositide-3 kinase (PI-3 kinase). It remains unclear, however, precisely how this activation occurs and also how the newly generated phospholipid products trigger the various events associated with T cell activation. Here we discuss the current understanding of how PI-3 kinase is activated by the TCR and CD28 and what roles its products play in T cell activation. We also review recent advances in understanding the function of Akt in particular, especially its role in CD28 costimulation. Several functional targets of Akt are discussed in this regard: inducible transcription, cell survival, glucose metabolism, and the cellular translational machinery. These pathways have been associated with TCR/CD28 costimulation, and they have also been implicated as targets of Akt.

The role of the Bcl-2 protein family in cancer

Seminal studies on the proto-oncogene bcl-2 have first demonstrated that mutations that inhibit programmed cell death (apoptosis) can promote lymphomagenesis and influence the sensitivity of tumour cells to chemotherapy or radiotherapy. It is now widely believed that neoplastic transformation of many, perhaps even all, cell types requires mutational changes that interfere with the cell death programme. In this review, we describe current knowledge of the molecular control of cell death and discuss the role of pro- and anti-apoptotic members of the Bcl-2 protein family in tumourigenesis and anti-cancer therapy.

A1 is a growth-permissive antiapoptotic factor mediating postactivation survival in T cells

The regulation of cell death in activated naive T cells is not well understood. We examined the expression of A1, an antiapoptotic member of the Bcl-2 family, following activation of naive mouse splenocytes. A1 gene expression was strongly but transiently induced during the first day of activation, with a peak at 2 to 6 hours, whereas Bcl-2 mRNA was simultaneously transiently down-regulated. Transgenic (Tg) overexpression of A1-a in T cells via the lck distal promoter resulted in decreased apoptosis following activation either with concanavalin A or with antibodies to CD3 and CD28 and led to a doubling of T-cell yield by 5 days. Tg A1-a also partially protected thymocytes from several proapoptotic stimuli but did not protect T-cell blasts from cell death induced by reactivation via the T-cell receptor. Tg Bcl-2 and Tg A1-a showed a similar ability to reduce apoptosis in both resting and activated T cells. However, in activated splenocyte cultures, the increase in 5-day T-cell yield observed with Tg Bcl-2 was only half that produced by Tg A1-a. This difference could be attributed at least in part to the fact that A1, unlike Bcl-2, did not inhibit S-phase entry of activated cells. The A1 protein may represent an adaptation of the Bcl-2 gene family to the need for survival regulation in the context of a proliferative stimulus.

CD28 signal enhances apoptosis of CD8 T cells after strong TCR ligation

High avidity ligation of the TCR induces negative selection in the thymus and can also induce apoptosis of peripheral T cells. Costimulation through CD28 enhances T cell activation and facilitates negative selection in the thymus, but the role of CD28 in peripheral T cell deletional tolerance has not been investigated. We used 2C CD28 wild-type and 2C CD28-deficient strains to assess the effects of CD28 and TCR avidity on peripheral T cell expansion and apoptosis. We compared the activation, division, expansion, and apoptosis of CD28(+/+) and CD28(-/-) 2C cells in response to self-Ag (K(b)), alloantigens with intermediate (K(bm3)), high (L(d)), or very high (L(d) + QL9 peptide) avidity. With intermediate avidity alloantigen, the CD28 signal enhanced T cell activation and expansion. However, when T cells encountered high avidity alloantigen, the CD28 signal reduced T cell expansion and increased apoptosis. These results indicate that the CD28 signal can down-regulate peripheral T cell responses by increasing apoptosis when TCR ligation exceeds a critical threshold.

Thymic selection revisited: how essential is it?

Intrathymic T cell development represents one of the best studied paradigms of mammalian development. Lymphoid committed precursors enter the thymus and the Notch1 receptor plays an essential role in committing them to the T cell lineages. The pre-T cell receptor (TCR), as an autonomous cell signaling receptor, commits cells to the alphabeta lineage while its rival, the gammadeltaTCR, is involved in generating the gammadelta lineage of T cells. Positive and negative selection of immature alphabetaTCR-expressing cells are essential mechanisms for generating mature T cells, committing them to the CD4 and CD8 lineages and avoiding autoimmunity. Additional lineages of alphabetaT cells, such as the natural killer T cell lineage and the CD25+ regulatory T cell lineage, are formed when the alphabetaTCR encounters specific ligands in suitable microenvironments. Thus, positive selection and receptor-instructed lineage commitment represent a hallmark of the thymus. Ectopically expressed organ-specific antigens contribute to thymic self-nonself discrimination, which represents an essential feature for the evolutionary fitness of mammalian species.

CD28/B7 regulation of anti-CD3-mediated immunosuppression in vivo

FcR-binding "classical" anti-CD3 mAb is a potent immunosuppressive drug that alters CD4(+) and CD8(+) T cell function in vivo via anergy induction and programmed cell death (PCD). Anti-CD3-mediated PCD was Fas independent but was mediated by the mitochondria-initiated apoptosis that was abrogated in Bcl-x(L)-transgenic T cells. The PCD was more pronounced in CD28-deficient mice consistent with defective Bcl-x(L) up-regulation. Residual T cells isolated from anti-CD3-treated wild-type, CD28(-/-), and Bcl-x(L)-transgenic mice were hyporesponsive. The hyporesponsiveness was more pronounced in CD28(-/-) and wild-type mice treated with anti-B7-2, suggesting that CD28 interaction with B7-2 regulates T cell responsiveness in anti-CD3-treated animals. Finally, anti-CD3 treatment led to indefinite cardiac allograft survival in wild-type but not Bcl-x(L) animals. Together these results implicate CD28/B7 signaling in the regulation of both anti-CD3-induced T cell depletion and hyporesponsiveness in vivo, but T cell depletion, not hyporesponsiveness, appears to be critical for anti-CD3 mAb-mediated long-term immune regulation.

HIV-1 Tat targets microtubules to induce apoptosis, a process promoted by the pro-apoptotic Bcl-2 relative Bim

Depletion of CD4(+) T cells is the hallmark of HIV infection and AIDS progression. In addition to the direct killing of the viral-infected cells, HIV infection also leads to increased apoptosis of predominantly uninfected bystander cells. This is mediated in part through the HIV-1 Tat protein, which is secreted by the infected cells and taken up by uninfected cells. Using an affinity-purification approach, a specific and direct interaction of Tat with tubulin and polymerized microtubules has been detected. This interaction does not affect the secretion and uptake of Tat, but is critical for Tat to induce apoptosis. Tat binds tubulin/microtubules through a four-amino-acid subdomain of its conserved core region, leading to the alteration of microtubule dynamics and activation of a mitochondria-dependent apoptotic pathway. Bim, a pro-apoptotic Bcl-2 relative and a transducer of death signals initiated by perturbation of microtubule dynamics, facilitates the Tat-induced apoptosis. Our findings reveal a strategy by which Tat induces apoptosis by targeting the microtubule network. Thus HIV-1 Tat joins a growing list of pathogen-derived proteins that target the cytoskeleton of host cells.

Nonlymphoid Fas ligand in peptide-induced peripheral lymphocyte deletion

Peripheral lymphocyte deletion is required for reduction of lymphocyte numbers after expansion in response to antigen. Peripheral deletion is mediated in part by the activation of apoptosis by engagement of the death receptor, Fas (CD95), by its ligand, Fas ligand (FasL; CD95L), among other mechanisms. Here we used T cell receptor (TCR) transgenic animals to examine the role of inducible expression of nonlymphoid FasL in response to peptide antigen. Antigenic challenge of TCR transgenic mice resulted in increased expression of FasL in a number of nonlymphoid tissues including the epithelium of the small intestine. Similar results were obtained in an adoptive transfer system in which TCR transgenic T cells were transferred into recipient animals. The functional relevance of nonlymphoid FasL in peripheral deletion is supported by the observation that FasL-deficient gld animals showed a significantly reduced rate of clearance of transferred antigen-specific lymphocytes, although the lymphocytes themselves were wild type for FasL. These observations were supported further by studies in a transgenic mouse model where lacZ was expressed under the control of the proximal promoter of the FasL gene. Using these transgenic mice, we observed induced activity of the FasL promoter in intestinal epithelial cells throughout the crypts and villi, where we also observed infiltration of activated T cells. These data demonstrate that nonlymphoid FasL is expressed in response to peripheral T cell activation and participates in the regulation of T cells that infiltrate peripheral tissues.

Making and breaking tolerance

The lymphocyte's decision between tolerance and immunity/autoimmunity is regulated at many levels. Two important parameters in this decision are the maturation state of the antigen presenting cells (APCs) and the amount of self antigen that is detected by the immune system. Maturation of APCs occurs as a consequence of signals received by the innate immune system and may lead to the breakdown of tolerance. Particularly relevant to this process are the Toll-like receptors and mechanisms of cross presentation of self antigens. In addition, genetic alterations in a variety of cell surface receptors, signalling components and regulators of apoptosis/survival can break tolerance and lead to autoimmunity in vivo.

Interferon gamma is required for activation-induced death of T lymphocytes

The effector cytokine interferon gamma (IFN-gamma) may play a role in T cell homeostasis. We have examined the requirement for IFN-gamma in one mechanism that regulates T cell expansion and survival, activation-induced cell death (AICD). CD4(+) T cells lacking IFN-gamma or the Stat1 transcription factor are resistant to AICD. IFN-gamma is required for the production of caspases, and retrovirus-mediated expression of caspase-8 restores the sensitivity of Stat1-deficient T cells to AICD. In vitro, IFN-gamma limits the expansion of T cells that are stimulated through their antigen receptors. Thus, IFN-gamma may function to control the expansion and persistence of T cells by promoting caspase-8-dependent apoptosis.

Role of cell cycle regulator p19ARF in regulating T cell responses

Although it is well established that the processes of cellular proliferation and apoptosis are linked, the role of cell cycle regulators in T cell responses in vivo is not well understood. In recent years, tumor suppressor molecule p19(ARF) has emerged as a key cell cycle regulator important in cellular apoptosis against strong mitogenic stimuli. In this study, we compared the antigen-specific T cell responses between wild type (+/+) and p19(ARF)-deficient (p19-/-) mice following an acute infection with lymphocytic choriomeningitis virus (LCMV). p19-/- mice mounted a potent CD8 T cell response and the magnitude of expansion of LCMV-specific CD8 T cells was comparable to that of +/+ mice. Further, the clonal downsizing of the expanded virus-specific CD8 T cells and establishment of long-term T cell memory were minimally affected by p19(ARF) deficiency. Therefore, p19(ARF) function is not essential to regulate T cell responses following an acute viral infection.

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.