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

The role of inflammation in the generation and maintenance of memory T cells

Following infection or vaccination, antigen-specific T cells undergo enormous expansion in numbers and differentiate into effector cells that control infection and modulate other aspects of innate and adaptive immunity. The effector T-cell expansion phase is followed by an abrupt period of contraction, during which 90-95% of antigen-specific T cells are eliminated. The surviving pool of T cells subsequently differentiates into long-lived memory populations that can persist for the life of the host and mediate enhanced protective immunity following pathogen re-infection. The generation and maintenance of memory T-cell populations are influenced by a multitude of factors, including inflammatory cytokines that can act on T cells at various points during their differentiation. Herein, we discuss our current understanding of how inflammation shapes not only the quantity and quality of memory T cells, but also the rate at which functional memory T-cell populations develop.

Age-associated increase in lifespan of naive CD4 T cells contributes to T-cell homeostasis but facilitates development of functional defects

With age, T-cell generation from the thymus is much reduced, yet a substantial naïve T-cell pool is maintained even in aged animals, suggesting that naïve T cells either persist longer or turn over faster to maintain T-cell homeostasis. We found that with age, naïve CD4 T cells became progressively longer-lived. Their longer lifespan did not depend on recognition of self-peptide/class II. Newly generated naïve T cells derived from aged stem cells had a shorter lifespan, like that of young naïve T cells. Conversely, naïve CD4 T cells derived from middle-aged thymectomized mice were longer-lived in vivo, and their development of functional defects was accelerated. These observations suggest that naïve T cells develop their longer lifespan during their sojourn in the periphery. Increased longevity of naïve CD4 T cells correlated well with reduced expression of proapoptotic molecule Bim. We suggest that the intrinsic increase in longevity helps maintain naïve T-cell homeostasis but facilitates the development of functional defects in mice.

Generation of NO by bystander human CD8 T cells augments allogeneic responses by inhibiting cytokine deprivation-induced cell death

Nitric oxide (NO), generated by inducible NO synthase (iNOS) in bystander human CD8 T cells, augments the accumulation of allogeneically activated human CD8 T cells in vitro and in vivo. Here, we report that iNOS-derived NO does not affect T-cell proliferation but rather inhibits cell death of activated human CD8 T cells after activation by allogeneic endothelial cells in culture. Exogenous NO did not affect activation-induced cell death of human CD8 T cells but specifically reduced death of activated T cells due to cytokine deprivation. NO-mediated inhibition of T-cell death did not involve cGMP signaling, and NO did not affect the expression of Bcl-2-related proteins known to regulate cytokine deprivation-induced cell death. However, NO inhibited the activity of caspases activated as a consequence of cytokine deprivation in activated T cells. This protective effect correlated with S-nitrosylation of caspases and was phenocopied by z-VAD.fmk and z-LEHD.fmk, pharmacological inhibitors of caspases. In summary, our findings indicate that NO augments the accumulation of activated human T cells principally by inhibiting cytokine deprivation-induced cell death through S-nitrosylation of caspases.

Bid cleavage, cytochrome c release and caspase activation in canine coronavirus-induced apoptosis

A previous study demonstrated that infection of a canine fibrosarcoma cell line (A-72 cells) by canine coronavirus (CCoV) resulted in apoptosis (Ruggieri et al., 2007). In this study, we investigated the cell death processes during infection and the underlying mechanisms. We found that CCoV-II triggers apoptosis in A-72 cells by activating initiator (caspase-8 and -9) and executioner (caspase-3 and -6) caspases. The proteolytic cleavage of poly(ADP-ribose) polymerases (PARPs) confirmed the activation of executioner caspases. Furthermore, CCoV-II infection resulted in truncated bid (tbid) translocation from the cytosolic to the mitochondrial fraction, the cytochrome c release from mitochondria, and alterations in the pro- and anti-apoptotic proteins of bcl-2 family. Our data indicated that, in this experimental model, both intrinsic and extrinsic pathways are involved. In addition, we demonstrated that the inhibition of apoptosis by caspase inhibitors did not affect CCoV replication, suggesting that apoptosis does not play a role in facilitating viral release.

Cell cycle progression following naive T cell activation is independent of Jak3/common gamma-chain cytokine signals

T cell proliferation following activation is an essential aspect of the adaptive immune response. Multiple factors, such as TCR signaling, costimulation, and signals from cytokines, each contribute to determine the magnitude of T cell expansion. In this report, we examine in detail the role of Jak3/common gamma-chain-dependent cytokines in promoting cell cycle progression and proliferation of naive T cells. Using naive CD4+ T cells from Jak3-deficient mice and wild-type CD4+ T cells treated with a small molecule inhibitor of Jak3, we find that these cytokine signals are not required for proliferation; instead, they are important for the survival of activated T cells. In addition, we show that the percentage of cells entering the cell cycle and the percentage of cells in each round of cell division are comparable between Jak3-deficent and wild-type T cells. Furthermore, cell cycle progression and the regulated expression of key cell cycle proteins are independent of Jak3/common gamma-chain cytokine signals. These findings hold true over a wide range of TCR signal strengths. However, when CD28 costimulatory signals, but not TCR signals, are limiting, Jak3-dependent cytokine signals become necessary for the proliferation of naive T cells. Because CD28 signaling has been found to be dispensable for autoreactive T cell responses, these data suggest the potential for interfering with autoimmune T cell responses by inhibition of Jak3 signaling.

CD95, BIM and T cell homeostasis

The relative importance of the intrinsic and extrinsic apoptotic pathways in the control of haematopoietic cell homeostasis has been a matter of debate for many years. Cell death is omnipresent in this cellular compartment and ensures the removal of cells that are not properly equipped to assume their function as well as those that have assumed function but are no longer required. In this Review we focus on the roles of CD95 (also known as FAS) and BCL-2-interacting mediator of cell death (BIM), two major regulators of apoptosis in T cell homeostasis, with a particular emphasis on their cooperation in the shutdown of T cell responses.

Critical roles of Bim in T cell activation and T cell-mediated autoimmune inflammation in mice

Bim, the B cell lymphoma 2-interacting (Bcl2-interacting) mediator, maintains immunological tolerance by deleting autoreactive lymphocytes through apoptosis. We report here that Bim is also, paradoxically, required for the activation of autoreactive T cells. Deletion of Bim in hematopoietic cells rendered mice resistant to autoimmune encephalomyelitis and diabetes, and Bim-deficient T cells had diminished cytokine production. Upon T cell receptor activation, Bim-deficient T cells exhibited severe defects in both calcium release and dephosphorylation of nuclear factor of activated T cells (NFAT) but maintained normal levels of activation of NF-kappaB and MAPKs. The defective calcium signaling in Bim-deficient T cells was associated with a significant increase in the formation of an inhibitory complex containing Bcl2 and the inositol triphosphate receptor (IP3R). Thus, in addition to mediating the death of autoreactive T cells, Bim also controlled T cell activation through the IP3R/calcium/NFAT pathway. These results indicate that a single protein is used to control both the activation and apoptosis of autoreactive T cells and may explain why Bim-deficient mice do not reject their own organs despite lacking thymic negative selection.

Opposing effects of TGF-beta and IL-15 cytokines control the number of short-lived effector CD8+ T cells

An effective immune response against infectious agents involves massive expansion of CD8(+) T cells. Once the infection is cleared, the majority of these effector cells die through unknown mechanisms. How is expansion controlled to maximize pathogen clearance and minimize immunopathology? We found, after Listeria infection, plasma transforming growth factor beta (TGF-beta) titers increased concomitant with the expansion of effector CD8(+) T cells. Blocking TGF-beta signaling did not affect effector function of CD8(+) T cells. However, TGF-beta controlled effector cell number by lowering Bcl-2 amounts and selectively promoting the apoptosis of short-lived effector cells. TGF-beta-mediated apoptosis of this effector subpopulation occurred during clonal expansion and contraction, whereas interleukin-15 (IL-15) promoted their survival only during contraction. We demonstrate that the number of effector CD8(+) T cells is tightly controlled by multiple extrinsic signals throughout effector differentiation; this plasticity should be exploited during vaccine design and immunotherapy against tumors and autoimmune diseases.

Apoptotic signal transduction and T cell tolerance

The healthy immune system makes use of a variety of surveillance mechanisms at different stages of lymphoid development to prevent the occurrence and expansion of potentially harmful autoreactive T cell clones. Disruption of these mechanisms may lead to inappropriate activation of T cells and the development of autoimmune and lymphoproliferative diseases [such as multiple sclerosis, rheumatoid arthritis, lupus erythematosus, diabetes and autoimmune lymphoproliferative syndrome (ALPS)]. Clonal deletion of T cells with high affinities for self-peptide-MHC via programmed cell death (apoptosis) is an essential mechanism leading to self-tolerance. Referred to as negative selection, central tolerance in the thymus serves as the first checkpoint for the developing T cell repertoire and involves the apoptotic elimination of potentially autoreactive T cells clones bearing high affinity T cell receptors (TCR) that recognize autoantigens presented by thymic epithelial cells. Autoreactive T cells that escape negative selection are held in check in the periphery by either functional inactivation ("anergy") or extrathymic clonal deletion, both of which are dependent on the strength and frequency of the TCR signal and the costimulatory context, or by regulatory T cells. This review provides an overview of the different molecular executioners of cell death programs that are vital to intrathymic or extrathymic clonal deletion of T cells. Further, the potential involvement of various apoptotic signaling paradigms are discussed with respect to the genesis and pathophysiology of autoimmune disease.

MEK/ERK-mediated phosphorylation of Bim is required to ensure survival of T and B lymphocytes during mitogenic stimulation

Survival and death of lymphocytes are regulated by the balance between pro- and antiapoptotic members of the Bcl-2 family; this is coordinated with the control of cell cycling and differentiation. Bim, a proapoptotic BH3-only member of the Bcl-2 family, can be regulated by MEK/ERK-mediated phosphorylation, which affects its binding to pro-survival Bcl-2 family members and its turnover. We investigated Bim modifications in mouse B and T lymphoid cells after exposure to apoptotic stimuli and during mitogenic activation. Treatment with ionomycin or cytokine withdrawal caused an elevation in Bim(EL), the most abundant Bim isoform. In contrast, in mitogenically stimulated T and B cells, Bim(EL) was rapidly phosphorylated, and its levels declined. Pharmacological inhibitors of MEK/ERK signaling prevented both of these changes in Bim, reduced proliferation, and triggered apoptosis of mitogen-stimulated T and B cells. Loss of Bim prevented this cell killing but did not restore cell cycling. These results show that during mitogenic stimulation of T and B lymphocytes MEK/ERK signaling is critical for two distinct processes, cell survival, mediated (at least in part) through phosphorylation and consequent inhibition of Bim, and cell cycling, which proceeds independently of Bim inactivation.

Aberrant tissue localization of fungus-specific CD4+ T cells in IL-10-deficient mice

Aspergillus fumigatus, a common environmental fungus, can cause lethal invasive infections in immunocompromised hosts. In immunocompetent individuals, however, inhaled A. fumigatus spores prime CD4(+) T cells and activate immune responses that prevent invasive infection. Calibration of inflammatory responses to levels that prevent fungal invasion without inducing collateral tissue damage is essential for host survival, but the underlying regulatory mechanisms remain undefined. Although IL-10 is a validated regulatory cytokine that suppresses immune responses, and IL-10 deficiency or blockade generally enhances immune responses, we find that A. fumigatus-specific T cell frequencies are markedly reduced in airways of IL-10-deficient mice. T cell priming, proliferation, and survival were unaffected by IL-10 deficiency and did not account for decreased frequencies of A. fumigatus-specific T cells in the airways of IL-10-deficient mice. Instead, IL-10 deficiency results in redistribution of A. fumigatus-specific T cells from infected lungs to the gut, a process that is reversed by antibiotic-mediated depletion of intestinal microbes. Our studies demonstrate that disregulated immune responses in the gut can result in dramatic redistribution of pathogen-specific T cells within the host.

Lymphocyte apoptosis in murine Pneumocystis pneumonia

BACKGROUND

Apoptosis of lymphocytes is important in the termination of an immune response to infection but has also been shown to have detrimental effects in animal models of systemic infection and sepsis. We sought to characterize lymphocyte apoptosis in an animal model of pneumonia due to Pneumocystis murina, an infection localized to the lungs.

METHODS

Control mice and mice depleted of CD4+ lymphocytes were inoculated with Pneumocystis. Apoptosis of lung and spleen lymphocytes was assayed by flow cytometry and PCR assay of apoptotic proteins.

RESULTS

In control mice, apoptosis of lung lymphocytes was maximal just after the infection was cleared from lung tissue and then declined. However, in CD4-depleted mice, apoptosis was also upregulated in recruited lymphocytes in spite of progressive infection. In splenic lymphocytes, apoptosis was observed early at 1 week after inoculation and then declined. Apoptosis of lung lymphocytes in control mice was associated with a decrease in mRNA for Bcl-2 and an increase in mRNA for Bim. In CD4-depleted mice, lavaged CD8+ cells did change intracellular Bcl-2 but showed increased mRNA for Bim.

CONCLUSION

Apoptosis of both pulmonary and extrapulmonary lymphocytes is part of the normal host response to Pneumocystis but is also triggered in CD4-deficient animals with progressive infection. In normal mice apoptosis of pulmonary lymphocytes may serve to terminate the immune response in lung tissue. Apoptosis of lung lymphocytes takes place via both the intrinsic and extrinsic apoptotic pathways and is associated with changes in both pro- and anti-apoptotic proteins.

Regulation of Fas-mediated immune homeostasis by an activation-induced protein, Cyclon

Activation-induced cell death (AICD) plays an essential role in the contraction of activated T cells after eradication of pathogen. Fas (APO-1/CD95) is one of the key cell surface proteins that mediate AICD in CD4(+) and CD8(+) T cells. Despite its prime importance in cell death, regulation of Fas expression in T cells is poorly understood. Here we show that Cyclon, a newly identified cytokine-inducible protein, is induced in T cells on T-cell receptor ligation and important for immune homeostasis. Transgenic expression of Cyclon ameliorated autoimmune phenotype in mice lacking subunits of IL-2R. Transgenic expression of Cyclon markedly enhanced AICD through increased expression of Fas whose expression is essential for Cyclon action. Finally, we demonstrated that activated but not resting CD4(+) T cells with targeted deletion of a Cyclon allele show reduced AICD and expression of Fas, indicating a critical role of Cyclon in Fas expression in activated T cells. We think that our data provide insight into expression regulation of Fas in T cells.

Diversity in CD8(+) T cell differentiation

CD8(+) T cells are key effector cells of the adaptive immune system, however their activity must be tightly regulated to allow pathogen clearance whilst preventing immunopathology and autoimmunity. In this review, we summarise the diversity of responses that CD8(+) T cells make to antigenic stimulation with a focus on how CD8(+) T cell responses are regulated to achieve different immune outcomes. In particular, we discuss phenotypic diversity during tolerance induction as well as signals that drive effector and memory cell differentiation in response to infection.

The Bcl-2 family in autoimmune and degenerative disorders

Members of the Bcl-2 family are essential regulators of programmed cell death and thus play a major role in the development and function of many tissues. The balance between pro-survival and pro-apoptotic members of the family decides whether a cell will live or die. This mechanism allows organisms to get rid of cells that are no longer needed or have become dangerous. Deregulation of apoptosis is a major contributing factor in the development of many diseases. A deeper understanding of how the Bcl-2 family proteins orchestrate death in normal and pathologic conditions is thus relevant not only for disease etiology, but also to try to prevent these various disorders. Experiments with transgenic and gene-ablated mice have helped elucidate the function of the different members of the Bcl-2 family and their physiological roles. The present review highlights the role of Bcl-2 family members in autoimmune and degenerative disorders, with a particular focus on the mouse models that have been used to study their function.

Surviving the crash: transitioning from effector to memory CD8+ T cell

One outcome of infection is the formation of long-lived immunological memory, which provides durable protection from symptomatic re-infection. In response to infection or vaccination, T cells undergo dramatic proliferation and differentiate into effector T cells that mediate removal of the pathogen. Following pathogen clearance, the majority of effector cells die, restoring lymphocyte homeostasis. However, a small number of antigen-specific cells survive and seed the memory T cell population. Here, we focus on recent advances in identifying the key proteins and transcription factors that allow a portion of effector CD8(+) T cells to persist after contraction of the immune response, forming a memory cell population programmed for long-term self-renewal and survival. We also examine new findings addressing the role of environmental cues such as cytokines and co-stimulatory molecules in CD8(+) memory T cell formation and how the cell-extrinsic cues influence the molecular players of intracellular pathways important for memory formation.

The Epstein-Barr virus Bcl-2 homolog, BHRF1, blocks apoptosis by binding to a limited amount of Bim

Current knowledge suggests that the balance between life and death within a cell can be controlled by the stable engagement of Bcl-2-related proapoptotic proteins such as Bak, Bax, and Bim by survival proteins such as Bcl-2. BHRF1 is a prosurvival molecule from Epstein-Barr virus that has a high degree of homology to Bcl-2. To understand how BHRF1 blocks apoptosis, BHRF1 and mutants of BHRF1 were expressed in primary cells and an IL-2-dependent T cell line. BHRF1 bound the Executioner Bak and, when cells were cultured without cytokines, BHRF1 associated with Bim. A point mutation that lost the ability to bind Bak retained its ability to bind Bim and to protect cells. This result demonstrated that it was the capacity of BHRF1 to bind Bim, not Bak, that provided protection. Interestingly, the amount of Bim bound by BHRF1 was minimal when compared with the amount of Bim induced by apoptosis. Thus, BHRF1 does not act by simply absorbing the excess Bim produced while cells prepare for death. Rather, BHRF1 may act either by binding preferentially the most lethal form of Bim or by acting catalytically on Bim to block apoptosis.

The many roles of FAS receptor signaling in the immune system

FAS belongs to the subgroup of the tumor necrosis factor receptor (TNF-R) family that contains an intracellular "death domain" and triggers apoptosis. Its physiological ligand FASL is a member of the TNF cytokine family. Studies with mutant mice and cells from human patients have shown that FAS plays critical roles in the immune system, including the killing of pathogen-infected cells and the death of obsolete and potentially dangerous lymphocytes. Fas thereby functions as a guardian against autoimmunity and tumor development. FAS triggers apoptosis through FADD-mediated recruitment and activation of caspase-8. In certain cells such as hepatocytes, albeit not lymphocytes, FAS-induced apoptosis requires amplification through proteolytic activation of the proapoptotic BCL-2 family member BID. Curiously, several components of the FAS signaling machinery have been implicated in nonapoptotic processes, including cellular activation, differentiation, and proliferation. This review describes current understanding of Fas-induced apoptosis signaling and proposes experimental strategies for future advances.

Human CD4+ memory T cells are preferential targets for bystander activation and apoptosis

There is much evidence that T cells may be activated via mechanisms that act independently of direct TCR ligation. Despite this, the question of whether such forms of bystander T cell activation occur during immune responses is hotly debated. To address some outstanding questions, we set up an in vitro system within which to analyze bystander T cell activation in human T cells, in the absence of the possibility for TCR cross-reactivity. In addition, we have investigated the genetic, phenotypic, and functional characteristics of bystander-activated T cells. In this study, we show that bystander T cell activation is, indeed, observed during a specific immune response, and that it occurs preferentially among CD4(+) memory T cells. Furthermore, bystander-activated T cells display a distinct gene expression profile. The mechanism for bystander T cell activation involves soluble factors, and the outcome is an elevated level of apoptosis. This may provide an explanation for the attrition of T cell memory pools of heterologous specificity during immune responses to pathogens such as viruses.

Mechanisms regulating the susceptibility of hematopoietic malignancies to glucocorticoid-induced apoptosis

Glucocorticoids (GCs) are commonly used in the treatment of hematopoietic malignancies owing to their ability to induce apoptosis of these cancerous cells. Whereas some types of lymphoma and leukemia respond well to this drug, others are resistant. Also, GC-resistance gradually develops upon repeated treatments ultimately leading to refractory relapsed disease. Understanding the mechanisms regulating GC-induced apoptosis is therefore uttermost important for designing novel treatment strategies that overcome GC-resistance. This review discusses updated data describing the complex regulation of the cell's susceptibility to apoptosis triggered by GCs. We address both the genomic and nongenomic effects involved in promoting the apoptotic signals as well as the resistance mechanisms opposing these signals. Eventually we address potential strategies of clinical relevance that sensitize GC-resistant lymphoma and leukemia cells to this drug. The major target is the nongenomic signal transduction machinery where the interplay between protein kinases determines the cell fate. Shifting the balance of the kinome towards a state where Glycogen synthase kinase 3alpha (GSK3alpha) is kept active, favors an apoptotic response. Accumulating data show that it is possible to therapeutically modulate GC-resistance in patients, thereby improving the response to GC therapy.

Many checkpoints on the road to cell death: regulation of Fas-FasL interactions and Fas signaling in peripheral immune responses

Interactions between the TNF-family receptor Fas (CD95) and Fas Ligand (FasL, CD178) can efficiently induce apoptosis and are critical for the maintenance of immunological self-tolerance. FasL is kept under strict control by transcriptional and posttranslational regulation. Surface FasL can be cleaved by metalloproteases, resulting in shed extracellular domains, and FasL can also traffic to secretory lysosomes. Each form of FasL has distinct biological functions. Fas is more ubiquitously expressed, but its apoptosis-inducing function is regulated by a number of mechanisms including submembrane localization, efficiency of receptor signaling complex assembly and activation, and bcl-2 family members in some circumstances. When apoptosis is not induced, Fas-FasL interactions can also trigger a number of activating and proinflammatory signals. Harnessing the apoptosis-inducing potential of Fas for therapy of cancer and autoimmune disease has been actively pursued, and despite a number of unexpected side-effects that result from manipulating Fas-FasL interactions, this remains a worthy goal.

Autophagy and lymphocyte homeostasis

Lymphocyte homeostasis is tightly regulated in vivo by various factors including cytokines, antigens, and costimulatory signals. Central to this regulation is the intricate balance between survival and apoptosis determined by pro- and antiapoptotic factors, including Bcl-2/Bcl-xL of the Bcl-2 family in the intrinsic death pathway and Fas/FADD of the TNF death receptor superfamily in the extrinsic death pathway. Recent studies have identified a critical role for autophagy, a well-conserved catabolic process in eukaryotic cells, in T and B lymphocyte homeostasis. Autophagy is essential for mature T lymphocyte survival and proliferation. In addition, autophagy can promote T cell death in defined physiologic or pathologic conditions. Autophagy also contributes to the survival of subsets of B lymphocytes, including developing pre-B cells as well as B1 B cells in vivo. Thus, autophagy represents a novel pathway regulating both developing and mature lymphocytes. Future studies are required to investigate the role of autophagy in regulating T and B cell homeostasis during immune responses to pathogens, as well as to define the mechanisms by which autophagy regulates lymphocyte death and survival.

TCR down-regulation controls virus-specific CD8+ T cell responses

The CD3gamma di-leucine-based motif plays a central role in TCR down-regulation. However, little is understood about the role of the CD3gamma di-leucine-based motif in physiological T cell responses. In this study, we show that the expansion in numbers of virus-specific CD8(+) T cells is impaired in mice with a mutated CD3gamma di-leucine-based motif. The CD3gamma mutation did not impair early TCR signaling, nor did it compromise recruitment or proliferation of virus-specific T cells, but it increased the apoptosis rate of the activated T cells by increasing down-regulation of the antiapoptotic molecule Bcl-2. This resulted in a 2-fold reduction in the clonal expansion of virus-specific CD8(+) T cells during the acute phase of vesicular stomatitis virus and lymphocytic choriomeningitis virus infections. These results identify an important role of CD3gamma-mediated TCR down-regulation in virus-specific CD8(+) T cell responses.

IFN-gamma expression in CD8+ T cells regulated by IL-6 signal is involved in superantigen-mediated CD4+ T cell death

Infection with pathogens containing superantigens (Sags) can result in massive excessive CD4+ T cell activation and death in such conditions as toxic shock, food poisoning and autoimmune diseases. We here showed how enhancement of IL-6 signaling suppresses Sag-mediated activated CD4+ T cell death. Sag-induced CD4+ T cell death increased in IL-6 knockout (KO) mice, whereas it decreased in mice characterized by enhanced IL-6-gp130-STAT3 signaling. The serum concentration of IFN-gamma was inversely correlated with the magnitude of IL-6 signaling, and IFN-gamma deficiency inhibited Sag-induced activated CD4+ T cell death, suggesting that IL-6 suppresses CD4+ T cell death via IFN-gamma expression. Interestingly, depletion of activated CD8+ T cells inhibited Sag-mediated increases in IFN-gamma expression in IL-6 KO mice as well as the augmented CD4+ T cell death. The results demonstrate that IL-6-gp130-STAT3 signaling in activated CD8+ T cells contributes to Sag-induced CD4+ T cell death via IFN-gamma expression, highlighting this signaling axis in CD8+ T cells as a potential therapeutic target for Sag-related syndromes.

Bim and Bmf in tissue homeostasis and malignant disease

Among all BH3-only proteins known to date, most information is available on the biological role and function of Bim (Bcl-2 interacting mediator of cell death)/BOD (Bcl-2 related ovarian death agonist), whereas little is still known about its closest relative, Bcl-2 modifying factor (Bmf). Although Bim has been implicated in the regulation of cell death induction in multiple cell types and tissues in response to a large number of stimuli, including growth factor or cytokine deprivation, calcium flux, ligation of antigen receptors on T and B cells, glucocorticoid or loss of adhesion, Bmf seems to play a more restricted role by supporting Bim in some of these cell death processes. This review aims to highlight similarities between Bim and Bmf function in apoptosis signaling and their role in normal development and disease.

Enhanced T cell apoptosis within Drak2-deficient mice promotes resistance to autoimmunity

Clonal expansion of T cells is vital to adaptive immunity, yet this process must be tightly controlled to prevent autoimmune disease. The serine/threonine kinase death-associated protein kinase-related apoptosis-inducing kinase 2 (DRAK2) is a negative regulator of TCR signaling and sets the threshold for the activation of naive and memory T cells and selected thymocytes. Despite enhanced T cell activation, Drak2(-/-) mice are resistant to experimental autoimmune encephalomyelitis, an autoimmune demyelinating disease that resembles multiple sclerosis. However, the basis for this autoimmune resistance is currently unknown. In this study, we show that, in the absence of DRAK2 signaling, T cells require greater tonic signaling for maintenance during clonal expansion. Following stimulation, Drak2(-/-) T cells were more sensitive to an intrinsic form of apoptosis that was prevented by CD28 ligation, homeostatic cytokines, or enforced Bcl-x(L) expression. T cell-specific Bcl-x(L) expression also restored the susceptibility of Drak2(-/-) mice to experimental autoimmune encephalomyelitis and enhanced thymic positive selection. These findings demonstrate that DRAK2 is selectively important for T cell survival and highlight the potential that DRAK2 blockade may lead to permanent autoimmune T cell destruction via intrinsic apoptosis pathways.

BH3-only proteins in rheumatoid arthritis: potential targets for therapeutic intervention

Rheumatoid arthritis (RA) is a debilitating disease, resulting in the destruction of bone and cartilage, and in the permanent disfigurement of joints. Although the precise cause of RA is currently unresolved, it has become clear that the damaging effects are a result of the toxic milieu caused by an influx of inflammatory cells and the resulting heightened proinflammatory state within the joint. As the amount of literature suggesting that this preponderance of cells is a result of decreased local apoptosis in the joint continues to increase, in this review, we describe how Bcl-2 family pro-apoptotic BH3-only proteins, particularly Bim and Bid, could act to protect against the development of the disease. We also suggest a role for BH3-mimetic drugs as potential therapeutics in the treatment of RA.

The Erk2 MAPK regulates CD8 T cell proliferation and survival

The magnitude of T cell responses is determined by proliferation and survival decisions made by the responding cells. We now demonstrate that the Erk MAPK pathway plays a critical role in these cell fate decisions within CD8 T cells. While Erk1 is dispensable for all aspects of CD8 T cell activation, Erk2 is required for the proliferation of CD8 T cells activated in the absence of costimulation. Surprisingly, Erk2 is not required for proliferation following the addition of a costimulatory signal in vitro, or upon viral infection in vivo, but regulates the size of the responding population by enhancing cell survival. An important component of this Erk2-derived signal is the transcriptional regulation of Bcl-2 family members Bcl-x(L) and Bim, and impaired Erk2-deficient CD8 T cell survival can be rescued by genetic ablation of Bim. These studies ascribe multifaceted functions specific to Erk2 in CD8 T cell activation, proliferation, and survival.

Involvement of BH3-only proteins in hematologic malignancies

The interaction between anti-apoptotic and pro-apoptotic members of the Bcl-2 family proteins determines life or death for cancer cells. In this context, BH3-only proteins (such as Bim), members of the pro-apoptotic Bcl-2 family proteins, act as key initiators of apoptosis by activating Bax and Bak through liberating them from anti-apoptotic Bcl-2 members. This then leads to the disruption of mitochondrial outer membrane, and eventually promotes proteolytic cascades for cellular dismantling. We here review the growing evidence of how BH3-only proteins are involved in tumorigenesis and in apoptosis induced by anti-cancer agents in hematologic malignancies. A deeper understanding of the roles of BH3-only proteins in cell death regulation may yield crucial insights for the further development of more effective and rational cell killing strategies. Recent developments in the direct therapeutic manipulation of Bcl-2 proteins using BH3-mimicking agents, such as ABT-737 or GX15-070, for hematologic malignancies are also summarized.

Fas expression on antigen-specific T cells has costimulatory, helper, and down-regulatory functions in vivo for cytotoxic T cell responses but not for T cell-dependent B cell responses

Fas-mediated apoptosis is an important contributor to contraction of Ag-driven T cell responses acting only on activated Ag-specific T cells. The effects of targeted Fas deletion on selected cell populations are well described however little is known regarding the consequences of Fas deletion on only activated Ag-specific T cells. We addressed this question using the parent-into-F(1) (P-->F(1)) model of acute or chronic (lupus-like) graft-vs-host disease (GVHD) as a model of either a CTL-mediated or T-dependent B cell-mediated response, respectively. By transferring Fas-deficient lpr donor T cells into Fas-intact F(1) hosts, the in vivo role of Ag-specific T cell Fas can be determined. Our results demonstrate a novel dichotomy of Ag-specific T cell Fas function in that: 1) Fas expression on Ag-activated T cells has costimulatory, helper, and down-regulatory roles in vivo and 2) these roles were observed only in a CTL response (acute GVHD) and not in a T-dependent B cell response (chronic GVHD). Specifically, CD4 T cell Fas expression is important for optimal CD4 initial expansion and absolutely required for help for CD8 effector CTL. Donor CD8 T cell Fas expression played an important but not exclusive role in apoptosis and down-regulation. By contrast, CD4 Fas expression played no detectable role in modulating chronic GVHD induction or disease expression. These results demonstrate a novel role for Ag-specific T cell Fas expression in in vivo CTL responses and support a review of the paradigm by which Fas deficiency accelerates lupus in MRL/lpr lupus-prone mice.

FADD and caspase-8 control the outcome of autophagic signaling in proliferating T cells

Fas-associated death domain protein (FADD) and caspase-8 (casp8) are vital intermediaries in apoptotic signaling induced by tumor necrosis factor family ligands. Paradoxically, lymphocytes lacking FADD or casp8 fail to undergo normal clonal expansion following antigen receptor cross-linking and succumb to caspase-independent cell death upon activation. Here we show that T cells lacking FADD or casp8 activity are subject to hyperactive autophagic signaling and subvert a cellular survival mechanism into a potent death process. T cell autophagy, enhanced by mitogenic signaling, recruits casp8 through interaction with FADD:Atg5-Atg12 complexes. Inhibition of autophagic signaling with 3-methyladenine, dominant-negative Vps34, or Atg7 shRNA rescued T cells expressing a dominant-negative FADD protein. The necroptosis inhibitor Nec-1, which blocks receptor interacting protein kinase 1 (RIP kinase 1), also completely rescued T cells lacking FADD or casp8 activity. Thus, while autophagy is necessary for rapid T cell proliferation, our findings suggest that FADD and casp8 form a feedback loop to limit autophagy and prevent this salvage pathway from inducing RIPK1-dependent necroptotic cell death. Thus, linkage of FADD and casp8 to autophagic signaling intermediates is essential for rapid T cell clonal expansion and may normally serve to promote caspase-dependent apoptosis under hyperautophagic conditions, thereby averting necrosis and inflammation in vivo.

Fas-mediated apoptosis regulates the composition of peripheral alphabeta T cell repertoire by constitutively purging out double negative T cells

Background

The Fas pathway is a major regulator of T cell homeostasis, however, the T cell population that is controlled by the Fas pathway in vivo is poorly defined. Although CD4 and CD8 single positive (SP) T cells are the two major T cell subsets in the periphery of wild type mice, the repertoire of mice bearing loss-of-function mutation in either Fas (lpr mice) or Fas ligand (gld mice) is predominated by CD4⁻CD8⁻ double negative αβ T cells that also express B220 and generally referred to as B220⁺DN T cells. Despite extensive analysis, the basis of B220⁺DN T cell lymphoproliferation remains poorly understood. In this study we re-examined the issue of why T cell lymphoproliferation caused by gld mutation is predominated by B220⁺DN T cells.

Methodology and Principal Findings

We combined the following approaches to study this question: Gene transcript profiling, BrdU labeling, and apoptosis assays. Our results show that B220⁺DN T cells are proliferating and dying at exceptionally high rates than SP T cells in the steady state. The high proliferation rate is restricted to B220+DN T cells found in the gut epithelium whereas the high apoptosis rate occurred both in the gut epithelium and periphery. However, only in the periphery, apoptosis of B220⁺DN T cell is Fas-dependent. When the Fas pathway is genetically impaired, apoptosis of peripheral B220⁺DN T cells was reduced to a baseline level similar to that of SP T cells. Under these conditions of normalized apoptosis, B220⁺DN T cells progressively accumulate in the periphery, eventually resulting in B220⁺DN T cell lymphoproliferation.

Conclusions/Significance

The Fas pathway plays a critical role in regulating the tissue distribution of DN T cells through targeting and elimination of DN T cells from the periphery in the steady state. The results provide new insight into pathogenesis of DN T cell lymphoproliferation.

The adjuvant effect in infection and autoimmunity

Infections are widely believed to serve as a trigger for initiating autoimmune disease in humans. An infectious agent may activate lymphocytes in an antigen-specific manner and can also provide the nonantigen-specific second signal necessary to induce a pathogenic adaptive immune response. Collectively, the secondary signaling necessary for induction of an autoimmune disease has been referred to as the adjuvant effect. Examples of an adjuvant effect have been described in the induction of experimental thyroiditis where the administration with thyroglobulin of muramyl dipeptide of mycobacteria or lipopolysaccaride of Gram-negative bacilli provide the necessary adjuvant effect. Other commonly used adjuvants fail to induce disease, although they may elicit autoantibody formation. Myocarditis can be induced in susceptible mice by infection with cardiotrophic coxsackievirus B3 and even induced in resistant mice if an additional adjuvant effect is provided through proinflammatory cytokines like interleukin-1 beta and tumor necrosis factor-alpha. The adjuvant effect is usually exerted early after infection during the innate immune response, operating at least in part through toll-like receptors and mast cells to direct the subsequent pathogenic adaptive immune response.

Putting endotoxin to work for us: monophosphoryl lipid A as a safe and effective vaccine adjuvant

The development of non-infectious subunit vaccines greatly increases the safety of prophylactic immunization, but also reinforces the need for a new generation of immunostimulatory adjuvants. Because adverse effects are a paramount concern in prophylactic immunization, few new adjuvants have received approval for use anywhere in the developed world. The vaccine adjuvant monophosphoryl lipid A is a detoxified form of the endotoxin lipopolysaccharide, and is among the first of a new generation of Toll-like receptor agonists likely to be used as vaccine adjuvants on a mass scale in human populations. Much remains to be learned about this compound's mechanism of action, but recent developments have made clear that it is unlikely to be simply a weak version of lipopolysaccharide. Instead, monophosphoryl lipid A's structure seems to have fortuitously retained several functions needed for stimulation of adaptive immune responses, while shedding those associated with pro-inflammatory side effects.

ERK-dependent Bim modulation downstream of the 4-1BB-TRAF1 signaling axis is a critical mediator of CD8 T cell survival in vivo

During an acute immune response, CD8 T cells undergo rapid expansion followed by a contraction phase during which the majority of activated T cells die, leaving a few survivors to persist as memory cells. The regulation of T cell survival is critical at each stage of this response. 4-1BB, a TNFR family member, has been implicated in prolonging the survival of activated and memory CD8 T cells; however, the precise mechanisms by which 4-1BB sustains T cell survival are incompletely understood. Upon aggregation on T cells, 4-1BB associates with two TNFR-associated factors (TRAF), TRAF1 and TRAF2. TRAF2 is essential for downstream signaling from 4-1BB; however, the role of TRAF1 in 4-1BB signaling has not been elucidated and there have been conflicting data as to whether TRAF1 provides a positive or a negative signal in T cells. In this study, we report that TRAF1 plays a critical role in survival signaling downstream of 4-1BB during CD8 T cell expansion in response to viral infection in vivo. Further analysis reveals that TRAF1-deficient cells are impaired in their ability to up-regulate the prosurvival Bcl-2 family member Bcl-x(L) and show increased levels of the proapoptotic Bcl-2 family member Bim following 4-1BB signaling. TRAF1-deficient CD8 T cells fail to activate ERK in response to 4-1BB ligation and inhibition of ERK signaling downstream of 4-1BB in wild-type cells leads to increased Bim levels. Thus, TRAF1 has a prosurvival effect in CD8 T cells via the 4-1BB-mediated up-regulation of Bcl-x(L) and ERK-dependent Bim down-modulation.

Critical role for BIM in T cell receptor restimulation-induced death

BACKGROUND

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.

RESULTS

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.

CONCLUSION

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.

REVIEWERS

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.

Cutting edge: contributions of apoptosis and anergy to systemic T cell tolerance

Multiple pathways can induce and maintain peripheral T cell tolerance. The goal of this study was to define the contributions of apoptosis and anergy to the maintenance of self-tolerance to a systemic Ag. Upon transfer into mice expressing OVA systemically, OVA-specific DO11 CD4+ T cells are activated transiently, cease responding, and die. Bim is the essential apoptosis-inducing trigger and apoptosis proceeds despite increased expression of Bcl-2 and Bcl-x. However, preventing apoptosis by eliminating Bim does not restore proliferation or cytokine production by DO11 cells. While Foxp3 is transiently induced, anergy is not associated with the stable development of regulatory T cells. Thus, apoptosis is dispensable for tolerance to a systemic self-Ag and cell-intrinsic anergy is sufficient to tolerize T cells.

The anti-apoptotic Bcl-2 family member Mcl-1 promotes T lymphocyte survival at multiple stages

T lymphocyte development and function are tightly regulated by the intrinsic death pathway through members of the Bcl-2 family. Genetic studies have demonstrated that the Bcl-2 family member Mcl-1 is an important anti-apoptotic protein in the development of multiple cell types including T lymphocytes. However, the expression pattern and anti-apoptotic roles of Mcl-1 in T lymphocytes at different developmental stages remain to be fully determined. In this study, we examined the expression pattern of Mcl-1 in different populations of T cells at the single-cell level and found that Mcl-1 protein is constitutively expressed in all T cell populations and up-regulated upon TCR stimulation. We then investigated the role of Mcl-1 in the survival of these different populations by conditionally deleting Mcl-1 at various T cell stages. Our results show that Mcl-1 is required for the survival of double-negative and single-positive thymocytes as well as naive and activated T cells. Furthermore, we demonstrate that Mcl-1 functions together with Bcl-xL to promote double-positive thymocyte survival. Thus, Mcl-1 is a critical anti-apoptotic factor for the survival of T cells at multiple stages in vivo.

Locally produced C5a binds to T cell-expressed C5aR to enhance effector T-cell expansion by limiting antigen-induced apoptosis

Our recent studies have shown that immune cell-produced complement provides costimulatory and survival signals to naive CD4(+) T cells. Whether these signals are similarly required during effector cell expansion and what molecular pathways link locally produced complement to T-cell survival were not clarified. To address this, we stimulated monoclonal and polyclonal T cells in vitro and in vivo with antigen-presenting cells (APCs) deficient in the complement regulatory protein, decay accelerating factor (DAF), and/or the complement component C3. We found that T-cell expansion induced by DAF-deficient APCs was augmented with diminished T-cell apoptosis, whereas T-cell expansion induced by C3(-/-) APCs was reduced because of enhanced T-cell apoptosis. These effects were traced to locally produced C5a, which through binding to T cell-expressed C5aR, enhanced expression of Bcl-2 and prevented Fas up-regulation. The results show that C5aR signal transduction in T cells is important to allow optimal T-cell expansion, as well as to maintain naive cell viability, and does so by suppressing programmed cell death.

Stabilized beta-catenin potentiates Fas-mediated T cell apoptosis

In response to Ag stimulation, Ag-specific T cells proliferate and accumulate in the peripheral lymphoid tissues. To avoid excessive T cell accumulation, the immune system has developed mechanisms to delete clonally expanded T cells. Fas/FasL-mediated apoptosis plays a critical role in the deletion of activated peripheral T cells, which is clearly demonstrated by superantigen (staphylococcal enterotoxin B)-induced deletion of Vbeta8(+) T cells. Using transgenic mice expressing a stabilized beta-catenin (beta-cat(Tg)), we show here that beta-catenin was able to enhance apoptosis of activated T cells by up-regulating Fas. In response to staphylococcal enterotoxin B stimulation, beta-cat(Tg) mice exhibited accelerated deletion of CD4(+)Vbeta8(+) T cells compared with wild type mice. Surface Fas levels were significantly higher on activated T cells obtained from beta-cat(Tg) mice than that from wild type mice. Additionally, T cells from beta-cat(Tg) mice were more sensitive to apoptosis induced by crosslinking Fas, activation-induced cell death, and to apoptosis induced by cytokine withdrawal. Lastly, beta-catenin bound to and stimulated the Fas promoter. Therefore, our data demonstrated that the beta-catenin pathway was able to promote the apoptosis of activated T cells in part via up-regulation of Fas.

Multisite phosphorylation regulates Bim stability and apoptotic activity

The proapoptotic BH3-only protein Bim is established to be an important mediator of signaling pathways that induce cell death. Multisite phosphorylation of Bim by several members of the MAP kinase group is implicated as a regulatory mechanism that controls the apoptotic activity of Bim. To test the role of Bim phosphorylation in vivo, we constructed mice with a series of mutant alleles that express phosphorylation-defective Bim proteins. We show that mutation of the phosphorylation site Thr-112 causes decreased binding of Bim to the antiapoptotic protein Bcl2 and can increase cell survival. In contrast, mutation of the phosphorylation sites Ser-55, Ser-65, and Ser-73 can cause increased apoptosis because of reduced proteasomal degradation of Bim. Together, these data indicate that phosphorylation can regulate Bim by multiple mechanisms and that the phosphorylation of Bim on different sites can contribute to the sensitivity of cellular apoptotic responses.

Bim-mediated deletion of antigen-specific CD8 T cells in patients unable to control HBV infection

HBV-specific CD8(+) T cells are critical for a successful immune response to HBV infection. They are markedly diminished in number in patients who fail to control the virus, but the mechanisms resulting in their depletion remain ill defined. Here, we dissected the defective HBV-specific CD8(+) T cell response associated with chronic HBV infection by gene expression profiling. We found that HBV-specific CD8(+) T cells from patients with different clinical outcomes could be distinguished by their patterns of gene expression. Microarray analysis revealed that overlapping clusters of functionally related apoptotic genes were upregulated in HBV-specific CD8(+) T cells from patients with chronic compared with resolved infection. Further analysis confirmed that levels of the proapoptotic protein Bcl2-interacting mediator (Bim) were upregulated in HBV-specific CD8(+) T cells from patients with chronic HBV infection. Blocking Bim-mediated apoptosis enhanced recovery of HBV-specific CD8(+) T cells both in culture and directly ex vivo. Consistent with evidence that Bim mediates apoptosis of CD8(+) T cells expressing low levels of CD127 (IL-7R), the few surviving HBV-specific CD8(+) T cells were CD127(hi )and had elevated levels of the antiapoptotic protein Mcl1, suggesting they were amenable to IL-7-mediated rescue from apoptosis. We therefore postulate that Bim-mediated attrition of HBV-specific CD8(+) T cells contributes to the inability of these cell populations to persist and control viral replication.

Effects of IL-7 on memory CD8 T cell homeostasis are influenced by the timing of therapy in mice

IL-7 is integral to the generation and maintenance of CD8(+) T cell memory, and insufficient IL-7 is believed to limit survival and the persistence of memory CD8(+) T cells. Here, we show that during the mouse T cell response to lymphocytic choriomeningitis virus, IL-7 enhanced the number of memory CD8(+) T cells when its administration was restricted to the contraction phase of the response. Likewise, IL-7 administration during the contraction phase of the mouse T cell response to vaccinia virus or a DNA vaccine potentiated antigen-specific CD8(+) memory T cell proliferation and function. Qualitatively, CD8(+) T cells from IL-7-treated mice exhibited superior recall responses and improved viral control. IL-7 treatment during the memory phase stimulated a marked increase in the number of memory CD8(+) T cells, but the effects were transient. IL-7 therapy during contraction of the secondary CD8(+) T cell response also expanded the pool of memory CD8(+) T cells. Collectively, our studies show differential effects of IL-7 on memory CD8(+) T cell homeostasis and underscore the importance of the timing of IL-7 therapy to effectively improve CD8(+) T cell memory and protective immunity. These findings may have implications in the clinical use of IL-7 as an immunotherapeutic agent to bolster vaccine-induced CD8(+) T cell memory.

The apoptotic pathway contributing to the deletion of naive CD8 T cells during the induction of peripheral tolerance to a cross-presented self-antigen

The maintenance of T cell tolerance in the periphery proceeds through several mechanisms, including anergy, immuno-regulation, and deletion via apoptosis. We examined the mechanism underlying the induction of CD8 T cell peripheral tolerance to a self-Ag expressed on pancreatic islet beta-cells. Following adoptive transfer, Ag-specific clone 4 T cells underwent deletion independently of extrinsic death receptors, including Fas, TNFR1, or TNFR2. Additional experiments revealed that the induction of clone 4 T cell apoptosis during peripheral tolerance occurred via an intrinsic death pathway that could be inhibited by overexpression of Bcl-2 or targeted deletion of the proapoptotic molecule, Bim, thereby resulting in accumulation of activated clone 4 T cells. Over-expression of Bcl-2 in clone 4 T cells promoted the development of effector function and insulitis whereas Bim-/- clone 4 cells were not autoaggressive. Examination of the upstream molecular mechanisms contributing to clone 4 T cell apoptosis revealed that it proceeded in a p53, E2F1, and E2F2-independent manner. Taken together, these data reveal that initiation of clone 4 T cell apoptosis during the induction of peripheral tolerance to a cross-presented self-Ag occurs through a Bcl-2-sensitive and at least partially Bim-dependent mechanism.

Apoptosis regulators Bim and Fas function concurrently to control autoimmunity and CD8+ T cell contraction

Throughout most of adult life, lymphocyte number remains constant because of a balance of proliferation and apoptosis. Mutation of Bim, a proapoptotic protein in the intrinsic death pathway, or Fas, a tumor necrosis factor receptor (TNFR) superfamily member of the extrinsic pathway, results in late-onset autoimmunity and increased antigen-specific CD8(+) T cell responses during viral infection. However, virus-specific immune responses eventually return to amounts comparable to those for nonmutant mice. Here, we show that loss of both Bim and Fas function resulted in a synergistic disruption of lymphoid homeostasis, rapid-onset autoimmunity, and organ-specific blocks on contraction of antiviral immune responses. When lymphocytic choriomeningitis virus (LCMV)-specific immune responses were quantitated, double-mutant mice had 100-fold more antigen-specific memory CD8(+) T cells in their lymph nodes than wild-type mice. Our results demonstrate that multiple death pathways function concurrently to prevent autoimmunity and downsize T cell responses.

Loss of the BH3-only protein Bmf impairs B cell homeostasis and accelerates gamma irradiation-induced thymic lymphoma development

Members of the Bcl-2 protein family play crucial roles in the maintenance of tissue homeostasis by regulating apoptosis in response to developmental cues or exogenous stress. Proapoptotic BH3-only members of the Bcl-2 family are essential for initiation of cell death, and they function by activating the proapoptotic Bcl-2 family members Bax and/or Bak, either directly or indirectly through binding to prosurvival Bcl-2 family members. Bax and Bak then elicit the downstream events in apoptosis signaling. Mammals have at least eight BH3-only proteins and they are activated in a stimulus-specific, as well as a cell type-specific, manner. We have generated mice lacking the BH3-only protein Bcl-2-modifying factor (Bmf) to investigate its role in cell death signaling. Our studies reveal that Bmf is dispensable for embryonic development and certain forms of stress-induced apoptosis, including loss of cell attachment (anoikis) or UV irradiation. Remarkably, loss of Bmf protected lymphocytes against apoptosis induced by glucocorticoids or histone deacetylase inhibition. Moreover, bmf(-/-) mice develop a B cell-restricted lymphadenopathy caused by the abnormal resistance of these cells to a range of apoptotic stimuli. Finally, Bmf-deficiency accelerated the development of gamma irradiation-induced thymic lymphomas. Our results demonstrate that Bmf plays a critical role in apoptosis signaling and can function as a tumor suppressor.