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Critical role for BIM in T cell receptor restimulation-induced death
Abstract and Figures
Upon repeated or chronic antigen stimulation, activated T cells undergo a T cell receptor (TCR)-triggered propriocidal cell death important for governing the intensity of immune responses. This is thought to be chiefly mediated by an extrinsic signal through the Fas-FasL pathway. However, we observed that TCR restimulation still potently induced apoptosis when this interaction was blocked, or genetically impaired in T cells derived from autoimmune lymphoproliferative syndrome (ALPS) patients, prompting us to examine Fas-independent, intrinsic signals.
Upon TCR restimulation, we specifically noted a marked increase in the expression of BIM, a pro-apoptotic Bcl-2 family protein known to mediate lymphocyte apoptosis induced by cytokine withdrawal. In fact, T cells from an ALPS type IV patient in which BIM expression is suppressed were more resistant to restimulation-induced death. Strikingly, knockdown of BIM expression rescued normal T cells from TCR-induced death to as great an extent as Fas disruption.
Our data implicates BIM as a critical mediator of apoptosis induced by restimulation as well as growth cytokine withdrawal. These findings suggest an important role for BIM in eliminating activated T cells even when IL-2 is abundant, working in conjunction with Fas to eliminate chronically stimulated T cells and maintain immune homeostasis.
This article was reviewed by Dr. Wendy Davidson (nominated by Dr. David Scott), Dr. Mark Williams (nominated by Dr. Neil Greenspan), and Dr. Laurence C. Eisenlohr.
Figures - available via license: Creative Commons Attribution 2.0 Generic
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Supplementary resources (8)
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... Patients carrying deleterious variants in FADD and caspase 8 exhibit ALPS-like lymphoproliferation and broader, more severe phenotypes consistent with immunodeficiency, given the pleiotropic roles of both proteins in other signaling pathways (Chun et al., 2002;Bolze et al., 2010;Savic et al., 2015;Kohn et al., 2020). ALPS-like phenotypes linked to diminished RICD sensitivity can even extend to pathogenic variants in genes outside the FAS/ CD95 signaling pathway, including KRAS/NRAS (RAS-associated leukoproliferative disease) and STAT5 (Table 1) (Snow et al., 2008;Calvo et al., 2015;Majri et al., 2018). ...
... In CD8 + T cells, FAS plays a minor role in RICD susceptibility-TCR-induced apoptosis is largely intact in CD8 + effector T cells derived from lpr/gld mice or ALPS patients (Zimmermann et al., 1996;Snow et al., 2008). In fact, in vivo Ag-driven T cell expansion and contraction is relatively normal in conditional knockout mice where FAS is selectively eliminated in T cells, despite the persistence of chronically stimulated DNTs and autoantibody production (Stranges et al., 2007). ...
... Intriguingly, several lines of evidence suggest BIM is also involved in RICD, particularly in CD8 + T cells (Grayson et al., 2006). First, we and others found that BIM is efficiently upregulated in human effector T cells after TCR restimulation, even in the presence of high IL-2 (Sandalova et al., 2004;Snow et al., 2008). Second, siRNA-mediated silencing of BIM upregulation partially rescued human effector T cells from RICD, with more pronounced resistance observed in CD8 + T cells (Snow et al., 2008). ...
For effective adaptive immunity, T lymphocytes must rapidly expand and contract in an antigen-specific manner to effectively control invading pathogens and preserve immunological memory, without sustaining excessive collateral damage to host tissues. Starting from initial antigen encounter, carefully calibrated programmed cell death pathways are critical for maintaining homeostasis over distinct phases of the T cell response. Restimulation-induced cell death (RICD), a self-regulatory apoptosis pathway triggered by re-engagement of the T cell receptor (TCR), is particularly important for constraining effector T cell expansion to preclude overt immunopathology; indeed, genetic disorders affecting key molecules involved in RICD execution can manifest in excessive lymphoproliferation, malignancy, and autoimmunity. Herein we review our current knowledge of how RICD sensitivity is ultimately regulated over the course of an immune response, including recent revelations on molecules that tune RICD by enforcing resistance or promoting susceptibility in expanding versus mature effector T cells, respectively. Detailed dissection of the molecular and temporal control of RICD also illuminates novel therapeutic strategies for correcting abnormal T cell responses noted in various immune disorders by ultimately tuning RICD sensitivity.
... Numerous lines of evidence demonstrate a critical role for Bim activity in regulating T cell development, survival, and function. 135,182 For example, knock-out of Bim in mice results in a severe block of T cell development in the thymus, 183 while in peripheral T cells, the absence of Bim reduces rates of AICD induced in both in vivo infection and in vitro stimulation.184,185 Similarly, in human T cells knockdown of Bim by siRNA also protects cells from AICD.185 ...
... 135,182 For example, knock-out of Bim in mice results in a severe block of T cell development in the thymus, 183 while in peripheral T cells, the absence of Bim reduces rates of AICD induced in both in vivo infection and in vitro stimulation.184,185 Similarly, in human T cells knockdown of Bim by siRNA also protects cells from AICD.185 ...
The latest advances in next-generation sequencing studies and transcriptomic profiling over the past decade have highlighted a surprising frequency of genes regulated by RNA processing mechanisms in the immune system. In particular, two control steps in mRNA maturation, namely alternative splicing and alternative polyadenylation, are now recognized to occur in the vast majority of human genes. Both have the potential to alter the identity of the encoded protein, as well as control protein abundance or even protein localization or association with other factors. In this review, we will provide a summary of the general mechanisms by which alternative splicing (AS) and alternative polyadenylation (APA) occur, their regulation within cells of the immune system, and their impact on immunobiology. In particular, we will focus on how control of apoptosis by AS and APA is used to tune cell fate during an immune response.
... 34 We also noted no significant differences between control and FOXP3 KD cells in terms of the expression of BIM ( Supplementary Fig. 2E, F), a proapoptotic BCL2 family protein that facilitates RICD in late-stage effectors after TCR-induced upregulation. 35 Regardless of FOXP3 status, BIM expression varied considerably between donors before and after TCR restimulation ( Supplementary Fig. 2F). ...
... In addition, FAS ligand (FASL) and BIM expression were largely unaffected by FOXP3 KD in early Tcons and likely do not contribute substantially to residual RICD compared with late-stage effectors. 35,53 Instead, FOXP3 and CD48 KD T cells showed increased p53 accumulation upon restimulation, suggesting that signals resulting from cellular stress (e.g., autophagy vs. apoptosis) govern RICD susceptibility in expanding Tcons. Clearly, RICD sensitivity is carefully calibrated throughout the life of a T cell via different mechanisms, although the RICD/AICD literature has primarily focused on thymocyte selection and the terminal stages of effector T-cell differentiation. ...
The adaptive immune response relies on specific apoptotic programs to maintain homeostasis. Conventional effector T cell (Tcon) expansion is constrained by both forkhead box P3 (FOXP3)⁺-regulatory T cells (Tregs) and restimulation-induced cell death (RICD), a propriocidal apoptosis pathway triggered by repeated stimulation through the T-cell receptor (TCR). Constitutive FOXP3 expression protects Tregs from RICD by suppressing SLAM-associated protein (SAP), a key adaptor protein that amplifies TCR signaling strength. The role of transient FOXP3 induction in activated human CD4 and CD8 Tcons remains unresolved, but its expression is inversely correlated with acquired RICD sensitivity. Here, we describe a novel role for FOXP3 in protecting human Tcons from premature RICD during expansion. Unlike FOXP3-mediated protection from RICD in Tregs, FOXP3 protects Tcons through a distinct mechanism requiring de novo transcription that does not require SAP suppression. Transcriptome profiling and functional analyses of expanding Tcons revealed that FOXP3 enhances expression of the SLAM family receptor CD48, which in turn sustains basal autophagy and suppresses pro-apoptotic p53 signaling. Both CD48 and FOXP3 expression reduced p53 accumulation upon TCR restimulation. Furthermore, silencing FOXP3 expression or blocking CD48 decreased the mitochondrial membrane potential in expanding Tcons with a concomitant reduction in basal autophagy. Our findings suggest that FOXP3 governs a distinct transcriptional program in early-stage effector Tcons that maintains RICD resistance via CD48-dependent protective autophagy and p53 suppression.
... Among the benefits of using bioengineering approaches, other therapy applications include boosting T cell growth and differentiation into central memory-like and effector memory subsets for therapy applications. Gray et al. [131] and Snow et al. [132] discussed the potential applications of 3D bioprinting and T cell engineering in the field of neurosurgery, including creating immune chips to elucidate links between immune response and disease. Recently, 3D bioprinting has shown great potential in advancing immunotherapy, particularly in the context of natural killer (NK) cells, a type of immune cell that possess the ability to recognize and eliminate target without prior sensitization. ...
Parkinson’s disease (PD) and Alzheimer’s disease (AD) are neurodegenerative disorders caused by the interaction of genetic, environmental, and familial factors. These diseases have distinct pathologies and symptoms that are linked to specific cell populations in the brain. Notably, the immune system has been implicated in both diseases, with a particular focus on the dysfunction of microglia, the brain’s resident immune cells, contributing to neuronal loss and exacerbating symptoms. Researchers use models of the neuroimmune system to gain a deeper understanding of the physiological and biological aspects of these neurodegenerative diseases and how they progress. Several in vitro and in vivo models, including 2D cultures and animal models, have been utilized. Recently, advancements have been made in optimizing these existing models and developing 3D models and organ-on-a-chip systems, holding tremendous promise in accurately mimicking the intricate intracellular environment. As a result, these models represent a crucial breakthrough in the transformation of current treatments for PD and AD by offering potential for conducting long-term disease-based modeling for therapeutic testing, reducing reliance on animal models, and significantly improving cell viability compared to conventional 2D models. The application of 3D and organ-on-a-chip models in neurodegenerative disease research marks a prosperous step forward, providing a more realistic representation of the complex interactions within the neuroimmune system. Ultimately, these refined models of the neuroimmune system aim to aid in the quest to combat and mitigate the impact of debilitating neuroimmune diseases on patients and their families.
... Bcl-2-like protein 11 (BCL2L11), a proapoptotic BH3-only protein within the Bcl-2 family, is highly expressed in the immune system, including in bone marrow and lymph nodes. High expression of BCL2L11 promotes lymphocyte apoptosis, whereas lack of BCL2L11 can increase the survival of autoreactive B cells and T cells [137][138][139]. This indicates that BCL2L11 plays a critical role in regulating lymphocyte homeostasis. ...
Kawasaki disease (KD) is an acute systemic vasculitis that occurs predominantly in children under 5 years of age. Despite much study, the etiology of KD remains unknown. However, epidemiological and immunological data support the hygiene hypothesis as a possible etiology. It is thought that more sterile or clean modern living environments due to increased use of sanitizing agents, antibiotics, and formula feeding result in a lack of immunological challenges, leading to defective or dysregulated B cell development, accompanied by low IgG and high IgE levels. A lack of B cell immunity may increase sensitivity to unknown environmental triggers that are nonpathogenic in healthy individuals. Genetic studies of KD show that all of the KD susceptibility genes identified by genome-wide association studies are involved in B cell development and function, particularly in early B cell development (from the pro-B to pre-B cell stage). The fact that intravenous immunoglobulin is an effective therapy for KD supports this hypothesis. In this review, I discuss clinical, epidemiological, immunological, and genetic studies showing that the etiopathogenesis of KD in infants and toddlers can be explained by the hygiene hypothesis, and particularly by defects or dysregulation during early B cell development.
... This form of cell death cannot be rescued by overexpression of anti-apoptotic BCL-2 family members suggesting that it is independent of cytochrome c release . However, this interpretation may be too simplistic as the pro-apoptotic BCL-2 protein BIM is upregulated upon TCR re-stimulation and deletion of this protein results in reduced sensitivity to AICD suggesting that there is an overlap between the two pathways (Snow et al., 2008). Increases in the expression of death receptors and their ligands is not the only reason activated T cells have a higher propensity to undergo AICD. ...
CD4+ T cell activation is critical for the initiation of the adaptive immune response. In particular, through the provision of help to B cells, CD4+ T cells are essential for the generation of high-affinity, class-switched antibodies specific for epitopes on invading pathogens. CD4+ T cells also augment the activation of CD8+ cytotoxic T lymphocytes and modulate the effector function of innate immune cells. These features of the immune response are essential for the clearance of many pathogens and are conditional on the ability of a small population of antigen-specific CD4+ T cells to rapidly expand in response to antigenic challenge. In this study we show that this expansion is strongly dependent on the activity of the WNK1 kinase, and that in the absence of WNK1, CD4+ T cells are unable to support a class-switched antibody response. WNK1 has been extensively studied in the distal nephron of the kidney, where it regulates ion transport, and consequently blood pressure, via the STK39 and OXSR1 kinases and the SLC12A-family of ion co-transporters. Here we show that this osmoregulatory function of WNK1 is required for TCR signalling in CD4+ T cells and the subsequent entry of these cells into G1 phase of the cell cycle. Furthermore, having entered the cell cycle, WNK1-deficient T cells show a reduced rate of DNA replication and activate the ATR-mediated cell cycle checkpoint, resulting in a G2/M blockade. CD4+ T cells carrying mutations in both Oxsr1 and Stk39 phenocopy WNK1-deficient T cells, although the defects in TCR-induced proliferation are less severe. Taken together, these data suggest that WNK1 regulates cell cycle progression via the OXSR1 and STK39 signalling pathways, as well as via another, non-canonical pathway. Importantly, the defective TCR signalling and G1 entry exhibited by WNK1-deficient CD4+ T cells can be rescued by activating the cells in hypotonic medium. These novel findings reveal fundamental roles for WNK1 activity and transmembrane water movement in antigen receptor signalling and cell cycle dynamics.
... In late-stage effector T cells, TIM-3 knockdown decreased the magnitude of proximal TCR signaling as assessed by total phospho-tyrosine signal ( Fig. 2A). Downstream, TIM-3 knockdown also decreased TCR restimulation-induced expression of pro-apoptotic proteins connected with RICD execution, BIM and FAS ligand (FAS-L) 32,33 (Fig. 2B-F). Specifically, we noted a reduction in the cytoplasmic N-terminal fragment of FASL produced upon cell surface exposure and cleavage by matrix metalloproteinases, indicative of enhanced FASL surface expression. ...
Immune homeostasis depends upon effective clearance of pathogens while simultaneously preventing autoimmunity and immunopathology in the host. Restimulation-induced cell death (RICD) is one such mechanism where by activated T cells receive subsequent antigenic stimulation, reach a critical signal threshold through the T cell receptor (TCR), and commit to apoptosis. Many details of this process remain unclear, including the role of co-stimulatory and co-inhibitory proteins that influence the TCR signaling cascade. Here we characterize the role of T cell immunoglobulin and mucin domain containing 3 (TIM-3) in RICD regulation. TIM-3 protected newly activated CD8 ⁺ effector T cells from premature RICD during clonal expansion. Surprisingly, however, we found that TIM-3 potentiated RICD in late-stage effector T cells. The presence of TIM-3 increased proximal TCR signaling and proapoptotic protein expression in late-stage effector T cells, with no consistent signaling effects noted in newly activated cells with or without TIM-3. To better explain these differences in TIM-3 function as T cells aged, we characterized the temporal pattern of TIM-3 expression in effector T cells. We found that TIM-3 was expressed on the surface of newly activated effector T cells, but remained largely intracellular in late-stage effector cells. Consistent with this, TIM-3 required a ligand to prevent early RICD, whereas ligand manipulation had no effects at later stages. Of the known TIM-3 ligands, carcinoembryonic antigen‐related cell adhesion molecule (CEACAM1) showed the greatest difference in surface expression over time and also protected newly activated cells from premature RICD, with no measurable effects in late-stage effectors. Indeed, CEACAM1 enabled TIM-3 surface expression on T cells, implying a co-dependency for these proteins in protecting expanding T cells from premature RICD. Our findings suggest that co-signaling proteins like TIM-3 and CEACAM1 can alter RICD sensitivity at different stages of the effector T cell response, with important implications for checkpoint blockade therapy.
... The cell phenotype (Tem) of the suspension group was not significantly changed at day 10 compared with the mock group, but the percentage of Tef was lower, indicating that more CD45RO + CD45RA + T-cells appeared (figures 4(B) and (C)). In human body, Tem are present in the blood and have the ability to traffic through peripheral organs [22]. After encountering the pathogen, circulating effector memory CD4 + T-cells (Tem) are recruited into the inflamed tissue to enhance the immune response by secret cytokines [23]. ...
T-cell immunotherapy holds promise for the treatment of cancer, infection, and autoimmune diseases. Nevertheless, T-cell therapy is limited by low cell expansion efficiency ex vivo and functional deficits. Here we describe two 3D bioprinting systems made by different biomaterials that mimic the in vivo formation of natural lymph vessels and lymph nodes which modulate T-cell with distinct fates and functions. We observe that coaxial alginate fibers promote T-cell expansion, less exhausted and enable CD4+ T-cell differentiation into central memory-like phenotype (Tcm), CD8+ T-cells differentiation into effector memory subsets (Tem), while alginate-gelatin scaffolds bring T-cells into a relatively resting state. Both of the two bioprinting methods are strikingly different from a standard suspension system. The former bioprinting method yields a new system for T-cell therapy and the latter method can be useful for making an immune-chip to elucidate links between immune response and disease.
... Last, our study has provided some insight into the mechanism by which ncNF-B signaling by the 4-1BB domain of the CAR promotes T cell survival. The pro-apoptotic protein Bim plays a central role in T cell survival, is increased in abundance with T cell activation, and is repressed by ERK signaling (87)(88)(89)(90)(91)(92). ERK-mediated Bim suppression was previously identified as a key mechanism by which endogenous murine 4-1BB enhances T cell survival (53); however, ncNF-B signaling is implicated with ERK signaling in Bim regulation within multiple myeloma, suggesting that these two pathways are not mutually exclusive (35). ...
Clinical response to chimeric antigen receptor (CAR) T cell therapy is correlated with CAR T cell persistence, especially for CAR T cells that target CD19 ⁺ hematologic malignancies. 4-1BB–costimulated CAR (BBζ) T cells exhibit longer persistence after adoptive transfer than do CD28-costimulated CAR (28ζ) T cells. 4-1BB signaling improves T cell persistence even in the context of 28ζ CAR activation, which indicates distinct prosurvival signals mediated by the 4-1BB cytoplasmic domain. To specifically study signal transduction by CARs, we developed a cell-free, ligand-based activation and ex vivo culture system for CD19-specific CAR T cells. We observed greater ex vivo survival and subsequent expansion of BBζ CAR T cells when compared to 28ζ CAR T cells. We showed that only BBζ CARs activated noncanonical nuclear factor κB (ncNF-κB) signaling in T cells basally and that the anti-CD19 BBζ CAR further enhanced ncNF-κB signaling after ligand engagement. Reducing ncNF-κB signaling reduced the expansion and survival of anti-CD19 BBζ T cells and was associated with a substantial increase in the abundance of the most pro-apoptotic isoforms of Bim. Although our findings do not exclude the importance of other signaling differences between BBζ and 28ζ CARs, they demonstrate the necessary and nonredundant role of ncNF-κB signaling in promoting the survival of BBζ CAR T cells, which likely underlies the engraftment persistence observed with this CAR design.
When the body mounts an immune response against a foreign pathogen, the adaptive arm of the immune system relies upon clonal expansion of antigen-specific T cell populations to exercise acquired effector and cytotoxic functions to clear it. However, T cell expansion must be modulated to effectively combat the perceived threat without inducing excessive collateral damage to host tissues. Restimulation-induced cell death (RICD) is an apoptotic program triggered in activated T cells when an abundance of antigen and IL-2 are present, imposing a negative feedback mechanism that constrains the growing T cell population. This autoregulatory process can be detected via increases in caspase activation, Annexin V binding, and loss of mitochondrial membrane potential. However, simple changes in T cell viability through flow cytometric analysis can reliably measure RICD sensitivity in response to T-cell receptor (TCR) restimulation. This protocol describes the in vitro polyclonal activation, expansion, and restimulation of human primary T cells isolated from donor peripheral blood mononuclear cells (PBMC). This simple procedure allows for accurate quantification of RICD via flow cytometry. We also describe strategies for interrogating the role of specific proteins and pathways that may alter RICD sensitivity. This straightforward protocol provides a quick and dependable tool to track RICD sensitivity in culture over time while probing critical factors that control the magnitude and longevity of an adaptive immune response. Graphic abstract: In-vitro simulation of restimulation-induced cell death in activated human T cells.
A number of murine T-cell hybridomas undergo apoptosis within a few hours of activation by specific antigens, mitogens, antibodies against the T-cell antigen receptor, or a combination of phorbol ester and calcium ionophore. This phenomenon has been extensively studied as a model for clonal deletion in the immune system, in which potentially autoreactive T cells eliminate themselves by apoptosis after activation, either in the thymus or in the periphery. Here we show that the Fas/CD95 receptor, which can transduce a potent apoptotic signal when ligand, is rapidly expressed following activation of T-cell hybridomas, as is its functional, membrane-bound ligand. Interference with the ensuing Fas/Fas-ligand interaction inhibits activation-induced apoptosis. Because T-cell receptor ligation can induce apoptosis in a single T hybridoma cell, we suggest that the Fas/Fas-ligand interaction can induce cell death in a cell-autonomous manner.
A significant proportion of previously activated human T cells undergo apoptosis when triggered through the CD3/T cell receptor complex, a process termed activation-induced cell death (AICD). Ligation of Fas on activated T cells by either Fas antibodies or recombinant human Fas-ligand (Fas-L) also results in cytolysis. We demonstrate that these two pathways of apoptosis are causally related. Stimulation of previously activated T cells resulted in the expression of Fas-L mRNA and lysis of Fas-positive target cells. Fas-L antagonists inhibited AICD of T cell clones and staphylococcus enterotoxin B (SEB)-specific T cell lines. The data indicate AICD in previously stimulated T cells is mediated by Fas/Fas-L interactions.
Receptor crosslinking of T-cell hybridomas induces cell activation followed by apoptosis. This activation-induced cell death requires de novo synthesis of RNA and proteins, but the actual gene products that provide the death signal have not been identified. We show here that receptor crosslinking induces Fas ligand and upregulates Fas, and that the ensuing engagement of Fas by Fas ligand activates the cell-death programme. Cell death, but not activation, can be selectively prevented by a soluble Fas-immunoglobulin fusion protein. Thus, Fas and Fas ligand are the death-gene products, and their interaction accounts for the molecular mechanism of activation-induced T-cell death.
A significant proportion of previously activated human T cells undergo apoptosis when triggered through the CD3/T cell receptor complex, a process termed activation-induced cell death (AICD). Ligation of Fas on activated T cells by either Fas antibodies or recombinant human Fas-ligand (Fas-L) also results in cytolysis. We demonstrate that these two pathways of apoptosis are causally related. Stimulation of previously activated T cells resulted in the expression of Fas-L mRNA and lysis of Fas-positive target cells. Fas-L antagonists inhibited AICD of T cell clones and staphylococcus enterotoxin B (SEB)-specific T cell lines. The data indicate AICD in previously stimulated T cells is mediated by Fas/Fas-L interactions.
THE APO-l/(Fas/CD95) cell surface receptor is a member of the nerve growth factor (NGF)/tumour necrosis factor (TNF) receptor superfamily and mediates apoptosis1 -4. Peripheral activated T cells (ATC) from lymphoproliferation (Ipr/lpr) mutant mice that express a reduced number of APO-1 receptors have a defect in T-cell receptor (TCR)-induced apoptosis5,6. This suggests that TCR-induced apoptosis involves APO-1. We tested this hypothesis in various human T cells: (1) malignant Jurkat cells, (2) an allo-reactive T-cell clone (S13), and (3) peripheral ATC. TCR triggering through immobilized anti-CD3 antibodies or Staphylococcus enterotoxin B (SEB) superantigen induced expression of the APO-1 ligand and apoptosis in these cells. Anti-CD3-induced apoptosis of Jurkat cells was demonstrated even in single-cell cultures. In all cases apoptosis was substantially inhibited by blocking anti-APO-1 antibody fragments and soluble APO-1 receptor decoys. The APO-1 ligand was found in the supernatant of activated Jurkat cells as a soluble cytokine. We propose that TCR-induced apoptosis in ATC can occur through an APO-1 ligand-mediated autocrine suicide. These results provide a mechanism for suppression of the immune response and for peripheral tolerance by T-cell deletion.
Antigen receptor stimulation of mature alpha beta T lymphocytes can lead either to proliferation or death. Programmed cell death, termed apoptosis, leads to the clonal deletion of both thymocytes and mature T cells that establishes tolerance. How a mature T cell selects between proliferation and death is not understood. Here I show that interleukin-2 (IL-2) is a critical determinant of the choice between these two fates. Both CD4+ and CD8+ T cells previously exposed to IL-2 undergo apoptosis after antigen-receptor stimulation. Antibody blockade of IL-2 but not IL-4 reverses the marked reduction of lymph node V beta 8+ T cells caused in mice by the bacterial superantigen Staphylococcus aureus enterotoxin B. IL-2 may thus participate in a feedback regulatory mechanism by predisposing mature T lymphocytes to apoptosis.
T-cell receptor-induced apoptosis regulates immune responses and can result from interactions between Fas (Apo1/CD95) and Fas ligand (FasL). Mutations in the genes for Fas and FasL cause disorders resembling human autoimmune diseases in lpr and gld mice, respectively. However, peripheral T-cell deletion takes place in lpr mice, and autoimmune syndromes occur in mouse strains without Fas or FasL defects. Here we show that tumour necrosis factor (TNF) can mediate mature T-cell receptor-induced apoptosis through the p75 TNF receptor. Blockage of both TNF and FasL is required to abrogate T-cell death and TNF mediates the death of most CD8+ T cells, whereas FasL mediates the death of most CD4+ T cells. Our results suggest that autoregulatory apoptosis of the mature T cells can occur by two distinct molecular mechanisms.
The lpr gene encodes a defective form of the fas gene that mediates apoptosis, and its expression results in autoantibodies and massive lymphadenopathy. bcl-2, another gene locus that affects programmed cell death, acts to inhibit apoptosis. Since multiple mechanisms controlling programmed cell death may contribute to systemic autoimmunity, the effect of the bcl-2 transgene on the lpr model was examined by crossing bcl-2 transgenic and C57BL/6-lpr mice. Compared with bcl-2-/lpr mice, bcl-2+/lpr showed dramatic increases in lymphadenopathy and T cell accumulation, but not in autoantibodies or B cell numbers. Short term transfer studies demonstrated that double negative T cells normally have a limited lifespan, and their survival is enhanced by the bcl-2 transgene. Thus, defects in separate apoptosis mechanisms may combine to produce enhanced pathologic effects.
We found that mature nontransformed CD4+ and CD8+ T lymphocytes could be made susceptible to T cell receptor(TcR)-mediated apoptosis by pretreatment with interleukin-4 (IL-4) or interleukin-2 (IL-2). The degree of susceptibility to death could be correlated with the level of cell cycling as measured by thymidine incorporation, cell doubling times, or the number of cells incorporating bromodeoxyuridine during S phase. However, using pharmacologic cell cycle blocking agents, we found that progression through the cell cycle was not required for cell death. Rather, we found that cells must be in a certain phase of the cell cycle to be susceptible to TcR-mediated death. Cells blocked in G1 phase were resistant to T cell receptor-induced apoptosis, whereas cells blocked in S phase were susceptible. These observations suggest that an important feature of growth lymphokines is their ability to drive T cells into portions of the cell cycle where they are sensitive to antigen receptor-induced apoptosis. Furthermore, these results provide additional evidence that the T cell growth lymphokines IL-2 and IL-4 may participate in the down-regulation of T cell responses by apoptosis-a pathway we have termed "propriocidal regulation".