Bclx regulates the survival of double-positive thymocytes

ATP citrate lyase is an essential player in the metabolic rewiring induced by PTEN loss during T-ALL development

ABSTRACT

Alterations inactivating the tumor suppressor gene PTEN drive the development of solid and hematologic cancers, such as T-cell acute lymphoblastic leukemia (T-ALL), in which phosphatase and tensin homolog (PTEN) loss defines poor-prognosis patients. We investigated the metabolic rewiring induced by PTEN loss in T-ALL, aiming to identify novel metabolic vulnerabilities. We showed that the enzyme adenosine triphosphate (ATP) citrate lyase (ACLY) is strictly required for the transformation of thymic immature progenitors and the growth of human T-ALL, which remain dependent on ACLY activity even upon transformation. Although Pten-mutant mice all died within 17 weeks, the concomitant Acly deletion prevented disease initiation in 70% of the animals. In these animals, ACLY promoted B-cell lymphoma (BCL-2) epigenetic upregulation and prevented the apoptosis of premalignant double-positive thymocytes. Transcriptomic and metabolic analysis of primary T-ALL cells next translated our findings to the human pathology, showing that PTEN-altered T-ALL cells activate ACLY and are sensitive to its genetic targeting. ACLY activation thus represents a metabolic vulnerability with therapeutic potential for high-risk patients with T-ALL.

Hypoxia-adenosinergic regulation of B cell responses

Hypoxic microenvironments induce widespread metabolic changes that have been shown to be critical in regulating innate and adaptive immune responses. Hypoxia-induced changes include the generation of extracellular adenosine followed by subsequent signaling through adenosine receptors on immune cells. This evolutionarily conserved "hypoxia-adenosinergic" pathway of hypoxia → extracellular adenosine → adenosine receptor signaling has been shown to be critical in limiting and redirecting T cell responses including in tumor microenvironments and the gut mucosa. However, the question of whether hypoxic microenvironments are involved in the development of B cell responses has remained unexplored until recently. The discovery that germinal centers (GC), the anatomic site in which B cells undergo secondary diversification and affinity maturation, develop a hypoxic microenvironment has sparked new interest in how this evolutionarily conserved pathway affects antibody responses. In this review we will summarize what is known about hypoxia-adenosinergic microenvironments in lymphocyte development and ongoing immune responses. Specific focus will be placed on new developments regarding the role of the hypoxia-adenosinergic pathway in regulating GC development and humoral immunity.

The partitioning of TCR repertoires by thymic selection

αβ T cells are critical components of the adaptive immune system; they maintain tissue and immune homeostasis during health, provide sterilizing immunity after pathogen infection, and are capable of eliminating transformed tumor cells. Fundamental to these distinct functions is the ligand specificity of the unique antigen receptor expressed on each mature T cell (TCR), which endows lymphocytes with the ability to behave in a cell-autonomous, disease context-specific manner. Clone-specific behavioral properties are initially established during T cell development when thymocytes use TCR recognition of major histocompatibility complex (MHC) and MHC-like ligands to instruct survival versus death and to differentiate into a plethora of inflammatory and regulatory T cell lineages. Here, we review the ligand specificity of the preselection thymocyte repertoire and argue that developmental stage-specific alterations in TCR signaling control cross-reactivity and foreign versus self-specificity of T cell sublineages.

Don't fear the reaper: The role of regulated cell death in tumorigenesis and BH3-mimetics for cancer therapy

From its earliest characterization, it has been recognized that there is a role for regulated (programmed) cell death in cancer. As our understanding of the different types of programmed cell death processes and their molecular control has advanced, so have the technologies that allow us to manipulate these processes to, for example, fight against cancer. In this review, we describe the roles of the different forms of regulated cell death in the development of cancer as well as their potential therapeutic exploitation. In that vein, we explore the development and use of BH3-mimetics, a unique class of drugs that can directly activate the apoptotic cell death machinery to treat cancer. Finally, we address key challenges that face the field to improve the use of these therapeutics and the efforts that are being undertaken to do so.

The thymus road to a T cell: migration, selection, and atrophy

The thymus plays a pivotal role in generating a highly-diverse repertoire of T lymphocytes while preventing autoimmunity. Thymus seeding progenitors (TSPs) are a heterogeneous group of multipotent progenitors that migrate to the thymus via CCR7 and CCR9 receptors. While NOTCH guides thymus progenitors toward T cell fate, the absence or disruption of NOTCH signaling renders the thymus microenvironment permissive to other cell fates. Following T cell commitment, developing T cells undergo multiple selection checkpoints by engaging with the extracellular matrix, and interacting with thymic epithelial cells (TECs) and other immune subsets across the different compartments of the thymus. The different selection checkpoints assess the T cell receptor (TCR) performance, with failure resulting in either repurposing (agonist selection), or cell death. Additionally, environmental cues such as inflammation and endocrine signaling induce acute thymus atrophy, contributing to the demise of most developing T cells during thymic selection. We discuss the occurrence of acute thymus atrophy in response to systemic inflammation. The thymus demonstrates high plasticity, shaping inflammation by abrogating T cell development and undergoing profound structural changes, and facilitating regeneration and restoration of T cell development once inflammation is resolved. Despite the challenges, thymic selection ensures a highly diverse T cell repertoire capable of discerning between self and non-self antigens, ultimately egressing to secondary lymphoid organs where they complete their maturation and exert their functions.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Resistance to the BCL-XL degrader DT2216 in T-cell acute lymphoblastic leukemia is rare and correlates with decreased BCL-XL proteolysis

PURPOSE

The BCL-2 family of anti-apoptotic proteins, BCL-2, BCL-XL and MCL-1, can mediate survival of some types of cancer. DT2216 is a PROteolysis-TArgeting Chimera (PROTAC) that degrades BCL-XL specifically and is in phase 1 trials. We sought to define the frequency and mechanism of resistance to DT2216 in T-cell acute lymphoblastic leukemia (T-ALL) cell lines.

METHODS

We measured cell survival and protein levels of BCL-XL, BCL-2, MCL-1 and the pro-apoptotic BIM in 13 distinct T-ALL cell lines after exposure to varying concentrations of DT2216.

RESULTS

We identified concentrations of DT2216 which were cytotoxic to each T-ALL cell line. These concentrations have no correlation with the initial protein levels of BCL-XL, BCL-2, MCL-1 or BIM in each cell line. However, there was a correlation between survival to DT2216 and the efficiency of degradation of BCL-XL by DT2216. Only one cell line, SUP-T1, had significant resistance to DT2216, defined as an IC50 above what is achievable in murine tumors in vivo.

CONCLUSION

Resistance to DT2216 is rare in a wide variety of T-ALL cells but when it occurs is correlated with decreased BCL-XL degradation. Resistance to DT2216 in T-ALL is not predicted by initial BCL-XL or BIM protein levels, or BCL-2 or MCL-1 levels before or after treatment. These data imply that a phase 2 clinical trial of DT2216 in T-ALL should be widely available and not limited to a subset of patients.

The path to venetoclax resistance is paved with mutations, metabolism, and more

Venetoclax is a B cell lymphoma 2 (BCL-2)-selective antagonist used to treat chronic lymphocytic leukemia (CLL) and acute myelogenous leukemia (AML). Although this has been a promising therapeutic option for these patients, many of these patients develop resistance and relapsed disease. Here, we summarize the emerging mechanisms of resistance to venetoclax treatment, discuss the promising combination strategies, and highlight the combinations that are currently in clinical trials. Efforts to understand mechanisms of resistance are critical to advance the development of new targeted therapeutic strategies and further our understanding of the biological functions of BCL-2 in tumor cells.

Tcf-1 promotes genomic instability and T cell transformation in response to aberrant β-catenin activation

Understanding the mechanisms promoting chromosomal translocations of the rearranging receptor loci in leukemia and lymphoma remains incomplete. Here we show that leukemias induced by aberrant activation of β-catenin in thymocytes, which bear recurrent Tcra/Myc-Pvt1 translocations, depend on Tcf-1. The DNA double strand breaks (DSBs) in the Tcra site of the translocation are Rag-generated, whereas the Myc-Pvt1 DSBs are not. Aberrantly activated β-catenin redirects Tcf-1 binding to novel DNA sites to alter chromatin accessibility and down-regulate genome-stability pathways. Impaired homologous recombination (HR) DNA repair and replication checkpoints lead to retention of DSBs that promote translocations and transformation of double-positive (DP) thymocytes. The resulting lymphomas, which resemble human T cell acute lymphoblastic leukemia (T-ALL), are sensitive to PARP inhibitors (PARPis). Our findings indicate that aberrant β-catenin signaling contributes to translocations in thymocytes by guiding Tcf-1 to promote the generation and retention of replication-induced DSBs allowing their coexistence with Rag-generated DSBs. Thus, PARPis could offer therapeutic options in hematologic malignancies with active Wnt/β-catenin signaling.

Wnt/β-Catenin Protects Lymphocytes from HIV-Mediated Apoptosis via Induction of Bcl-xL

HIV infection mediates the apoptosis of lymphocytes, the mechanism of which is multifaceted. Here, we evaluated the role of Wnt/β-catenin signaling in HIV-associated T cell apoptosis, as Wnt/β-catenin regulates the transcriptional activity of genes impacting apoptosis. We specifically investigated the role of the Wnt/β-catenin pathway in the HIV-associated apoptosis of CD4+ T cells and CD4dimCD8bright T cells, a population that is infected by HIV. We found that the induction of β-catenin, via a 6-bromoindirubin-3-oxime (BIO), significantly rescued HIV-infected CD4+ and CD4dimCD8bright T cells from apoptosis by >40−50%. Further, a small-molecule inhibitor of the Wnt/β-catenin pathway (PNU-74654) reversed BIO-mediated protection from HIV-associated apoptosis. BIO also induced Bcl-xL, an anti-apoptotic protein, and a target gene of Wnt/β-catenin, in CD4+ and CD4dimCD8bright T cells by approximately 3-fold. Inhibiting Bcl-xL by WEHI-539 abrogated β-catenin-mediated apoptotic protection in infected CD4+ and CD4dimCD8bright T cells. Collectively, these findings demonstrate that engaging Wnt/β-catenin signaling in HIV-infected T cells protects them from HIV-associated apoptosis by inducing Bcl-xL.

What can we learn from mice lacking pro-survival BCL-2 proteins to advance BH3 mimetic drugs for cancer therapy?

In many human cancers the control of apoptosis is dysregulated, for instance as a result of the overexpression of pro-survival BCL-2 proteins. This promotes tumorigenesis by protecting nascent neoplastic cells from stress and renders malignant cells resistant to anti-cancer agents. Therefore, several BH3 mimetic drugs targeting distinct pro-survival proteins have been developed. The BCL-2 inhibitor Venetoclax/ABT-199, has been approved for treatment of certain blood cancers and tens of thousands of patients have already been treated effectively with this drug. To advance the clinical development of MCL-1 and BCL-XL inhibitors, a more detailed understanding of their distinct and overlapping roles in the survival of malignant as well as non-transformed cells in healthy tissues is required. Here, we discuss similarities and differences in pro-survival BCL-2 protein structure, subcellular localisation and binding affinities to the pro-apoptotic BCL-2 family members. We summarise the findings from gene-targeting studies in mice to discuss the specific roles of distinct pro-survival BCL-2 family members during embryogenesis and the survival of non-transformed cells in healthy tissues in adults. Finally, we elaborate how these findings align with or differ from the observations from the clinical development and use of BH3 mimetic drugs targeting different pro-survival BCL-2 proteins.

The manipulation of apoptosis for cancer therapy using BH3-mimetic drugs

Apoptosis is a form of programmed cell death that is regulated by the balance between prosurvival and proapoptotic BCL-2 protein family members. Evasion of apoptosis is a hallmark of cancer that arises when this balance is tipped in favour of survival. One form of anticancer therapeutic, termed 'BH3-mimetic drugs', has been developed to directly activate the apoptosis machinery in malignant cells. These drugs bind to and inhibit specific prosurvival BCL-2 family proteins, thereby mimicking their interaction with the BH3 domains of proapoptotic BCL-2 family proteins. The BCL-2-specific inhibitor venetoclax is approved by the US Food and Drug Administration and many regulatory authorities worldwide for the treatment of chronic lymphocytic leukaemia and acute myeloid leukaemia. BH3-mimetic drugs targeting other BCL-2 prosurvival proteins have been tested in preclinical models of cancer, and drugs targeting MCL-1 or BCL-X_L have advanced into phase I clinical trials for certain cancers. As with all therapeutics, efficacy and tolerability need to be carefully balanced to achieve a therapeutic window whereby there is significant anticancer activity with an acceptable safety profile. In this Review, we outline the current state of BH3-mimetic drugs targeting various prosurvival BCL-2 family proteins and discuss emerging data regarding primary and acquired resistance to these agents and approaches that may overcome this. We highlight issues that need to be addressed to further advance the clinical application of BH3-mimetic drugs, both alone and in combination with additional anticancer agents (for example, standard chemotherapeutic drugs or inhibitors of oncogenic kinases), for improved responses in patients with cancer.

Glucocorticoids in T cell development, differentiation and function

Glucocorticoids (GCs) are small lipid hormones produced by the adrenals that maintain organismal homeostasis. Circadian and stress-induced changes in systemic GC levels regulate metabolism, cardiovascular and neural function, reproduction and immune activity. Our understanding of GC effects on immunity comes largely from administration of exogenous GCs to treat immune or inflammatory disorders. However, it is increasingly clear that endogenous GCs both promote and suppress T cell immunity. Examples include selecting an appropriate repertoire of T cell receptor (TCR) self-affinities in the thymus, regulating T cell trafficking between anatomical compartments, suppressing type 1 T helper (T_H1) cell responses while permitting T_H2 cell and, especially, IL-17-producing T helper cell responses, and promoting memory T cell differentiation and maintenance. Furthermore, in addition to functioning at a distance, extra-adrenal (local) production allows GCs to act as paracrine signals, specifically targeting activated T cells in various contexts in the thymus, mucosa and tumours. These pleiotropic effects on different T cell populations during development and immune responses provide a nuanced understanding of how GCs shape immunity.

Noncanonical Cell Fate Regulation by Bcl-2 Proteins

Bcl-2 proteins are widely known as key controllers of mitochondrial outer membrane permeabilization, arguably the most important step of intrinsic apoptosis. Accumulating evidence indicate that most, if not all, members of the Bcl-2 protein family also mediate a number of apoptosis-unrelated functions. Intriguingly, many of these functions ultimately impinge on cell fate decisions via apoptosis-dependent or -independent mechanisms, delineating a complex network through which Bcl-2 family members regulate cell survival and death. Here, we critically discuss the mechanisms through which Bcl-2 proteins influence cell fate as they regulate autophagy, cellular senescence, inflammation, bioenergetic metabolism, Ca²⁺ fluxes, and redox homeostasis.

Bcl-xL overexpression decreases GILZ levels and inhibits glucocorticoid-induced activation of caspase-8 and caspase-3 in mouse thymocytes

Glucocorticoids promote thymocyte apoptosis and modulate transcription of numerous regulators of thymic apoptosis. Among these, glucocorticoid-induced leucine zipper (GILZ) is strongly upregulated in the thymus. We have previously demonstrated that GILZ decreases Bcl-xL expression, activates caspase-8 and caspase-3, and augments apoptosis in mice thymocytes. To better understand the causal links between glucocorticoids, GILZ, Bcl-xL, caspase-8, and caspase-3, we analyzed the thymocytes of Bcl-xL-overexpressing transgenic mice with or without glucocorticoid stimulation in vitro. Overexpression of Bcl-xL inhibited the glucocorticoid-induced up-regulation of GILZ in murine thymocytes as well as the glucocorticoid-dependent activation of caspase-8 and caspase-3. By contrast, no appreciable change in caspase-9 activation was observed upon Bcl-xL overexpression. Thus, these experiments highlighted a novel thymocyte apoptotic pathway in which Bcl-xL overexpression inhibited the glucocorticoid-induced activation of caspase-8 and caspase-3, but not caspase-9, as well as the accumulation of GILZ protein. These findings, together with our previous results showing that caspase-8 protects GILZ from proteasomal degradation, suggest the presence of a glucocorticoid-induced apoptosis self-amplification loop in which GILZ decreases Bcl-xL expression with a subsequent activation of caspase-8 and caspase-3; caspase-8 activation then enhances the stability and accumulation of GILZ and ensures the unimpeded and irreversible progression of apoptosis. By contrast, inappropriate increases in Bcl-xL levels could have catastrophic effects on thymic apoptosis as it would shut down caspase-8/3 activation, diminish the expression of GILZ, and impair the fine control necessary for thymic generation of a healthy immune repertoire.

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Bcl-xL is the main anti-apoptotic molecule of the BCl-2 family expressed in the thymus.

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Bcl-xL and GILZ are linked in a loop to influence each other’s expression.

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Overexpression of Bcl-xL inhibits GC-induced expression of Gilz, activation of caspase-8 and blocks thymocyte apoptosis.

The Role of Inhibition of Apoptosis in Acute Leukemias and Myelodysplastic Syndrome

Avoidance of apoptosis is a key mechanism that malignancies, including acute leukemias and MDS, utilize in order to proliferate and resist chemotherapy. Recently, venetoclax, an inhibitor of the anti-apoptotic protein BCL-2, has been approved for the treatment of upfront AML in an unfit, elderly population. This paper reviews the pre-clinical and clinical data for apoptosis inhibitors currently in development for the treatment of AML, ALL, and MDS.

Cutting Edge: HDAC3 Protects Double-Positive Thymocytes from P2X7 Receptor-Induced Cell Death

Intricate life-versus-death decisions are programmed during T cell development, and the regulatory mechanisms that coordinate their activation and repression are still under investigation. In this study, HDAC3-deficient double-positive (DP) thymocytes exhibit a severe decrease in numbers. The thymic cortex is rich in ATP, which is released by macrophages that clear apoptotic DP thymocytes that fail to undergo positive selection. We demonstrate that HDAC3 is required to repress expression of the purinergic receptor P2X7 to prevent DP cell death. HDAC3-deficient DP thymocytes upregulate the P2X7 receptor, increasing sensitivity to ATP-induced cell death. P2rx7/HDAC3-double knockout mice show a partial rescue in DP cell number. HDAC3 directly binds to the P2rx7 enhancer, which is hyperacetylated in the absence of HDAC3. In addition, RORγt binds to the P2rx7 enhancer and promotes P2X7 receptor expression in the absence of HDAC3. Therefore, HDAC3 is a critical regulator of DP thymocyte survival and is required to suppress P2X7 receptor expression.

BH3-Mimetic Drugs: Blazing the Trail for New Cancer Medicines

Defects in apoptotic cell death can promote cancer and impair responses of malignant cells to anti-cancer therapy. Pro-survival BCL-2 proteins prevent apoptosis by keeping the cell death effectors, BAX and BAK, in check. The BH3-only proteins initiate apoptosis by neutralizing the pro-survival BCL-2 proteins. Structural analysis and medicinal chemistry led to the development of small-molecule drugs that mimic the function of the BH3-only proteins to kill cancer cells. The BCL-2 inhibitor venetoclax has been approved for treatment of refractory chronic lymphocytic leukemia and this drug and inhibitors of pro-survival MCL-1 and BCL-XL are being tested in diverse malignancies.

T Cells and Regulated Cell Death: Kill or Be Killed

Cell death plays two major complementary roles in T cell biology: mediating the removal of cells that are targeted by T cells and the removal of T cells themselves. T cells serve as major actors in the adaptive immune response and function by selectively killing cells which are infected or dysfunctional. This feature is highly involved during homeostatic maintenance, and is relied upon and modulated in the context of cancer immunotherapy. The vital recognition and elimination of both autoreactive T cells and cells which are unable to recognize threats is a highly selective and regulated process. Moreover, detection of potential threats will result in the activation and expansion of T cells, which on resolution of the immune response will need to be eliminated. The culling of these T cells can be executed via a multitude of cell death pathways which are used in context-specific manners. Failure of these processes may result in an accumulation of misdirected or dysfunctional T cells, leading to complications such as autoimmunity or cancer. This review will focus on the role of cell death regulation in the maintenance of T cell homeostasis, as well as T cell-mediated elimination of infected or dysfunctional cells, and will summarize and discuss the current knowledge of the cellular mechanisms which are implicated in these processes.

RORγt limits the amount of the cytokine receptor γc through the prosurvival factor Bcl-xL in developing thymocytes

The cytokine receptor subunit γc provides critical signals for T cell survival and differentiation. We investigated the molecular mechanism that controls the cell surface abundance of γc during T cell development in the thymus. We found that the amount of γc was low on CD4⁺CD8⁺ double-positive (DP) thymocytes before their positive selection to become mature T cells. The transcription factor RORγt was abundant in immature DP thymocytes, and its loss resulted in an increase in the abundance of surface γc, specifically on preselection DP cells. Rather than directly repressing expression of the gene encoding γc, RORγt acted through the antiapoptotic protein Bcl-x_L to reduce the abundance of surface γc, which resulted in decreased cytokine signaling and was associated with inhibition of cell metabolism and mitochondrial biogenesis. Accordingly, overexpression of Bcl-x_L in RORγt-deficient thymocytes restored the amount of surface γc to that present on normal preselection DP cells. Together, these data highlight a previously unappreciated role for RORγt and Bcl-x_L in limiting γc abundance at the cell surface and reveal a signaling circuit in which survival factors control cytokine signaling by limiting the abundance and surface distribution of a receptor subunit shared by several cytokines.

Differential effects of Vav-promoter-driven overexpression of BCLX and BFL1 on lymphocyte survival and B cell lymphomagenesis

Overexpression of BCLX and BFL1/A1 has been reported in various human malignancies and is associated with poor prognosis and drug resistance, identifying these prosurvival BCL2 family members as putative drug targets. We have generated transgenic mice that express human BFL1 or human BCLX protein throughout the haematopoietic system under the control of the Vav gene promoter. Haematopoiesis is normal in both the Vav-BFL1 and Vav-BCLX transgenic (TG) mice and susceptibility to spontaneous haematopoietic malignancies is not increased. Lymphoid cells from Vav-BCLX TG mice exhibit increased resistance to apoptosis in vitro while most blood cell types form Vav-BFL1 TG mice were poorly protected. Both transgenes significantly accelerated lymphomagenesis in Eμ-MYC TG mice and, surprisingly, the Vav-BFL1 transgene was the more potent. Unexpectedly, expression of transgenic BFL1 RNA and protein is significantly elevated in B lymphoid cells of Vav-BFL1/Eμ-MYC double-transgenic compared to Vav-BFL1 mice, even during the preleukaemic phase, providing a rationale for the potent synergy. In contrast, Vav-BCLX expression was not notably different. These mouse models of BFL1 and BCLX overexpression in lymphomas should be useful tools for the testing the efficacy of novel human BFL1- and BCLX-specific inhibitors.

BET-inhibition by JQ1 promotes proliferation and self-renewal capacity of hematopoietic stem cells

Although inhibitors of bromodomain and extra terminal domain (BET) proteins show promising clinical activity in different hematologic malignancies, a systematic analysis of the consequences of pharmacological BET inhibition on healthy hematopoietic (stem) cells is urgently needed. We found that JQ1 treatment decreases the numbers of pre-, immature and mature B cells while numbers of early pro-B cells remain constant. In addition, JQ1 treatment increases apoptosis in T cells, all together leading to reduced cellularity in thymus, bone marrow and spleen. Furthermore, JQ1 induces proliferation of long-term hematopoietic stem cells, thereby increasing stem cell numbers. Due to increased numbers, JQ1-treated hematopoietic stem cells engrafted better after stem cell transplantation and repopulated the hematopoietic system significantly faster after sublethal myeloablation. As quantity and functionality of hematopoietic stem cells determine the duration of life-threatening myelosuppression, BET inhibition might benefit patients in myelosuppressive conditions.

Life and Death of Activated T Cells: How Are They Different from Naïve T Cells?

T cells are pivotal in immunity and immunopathology. After activation, T cells undergo a clonal expansion and differentiation followed by a contraction phase, once the pathogen has been cleared. Cell survival and cell death are critical for controlling the numbers of naïve T cells, effector, and memory T cells. While naïve T cell survival has been studied for a long time, more effort has gone into understanding the survival and death of activated T cells. Despite this effort, there is still much to be learnt about T cell survival, as T cells transition from naïve to effector to memory. One key advance is the development of inhibitors that may allow the temporal study of survival mechanisms operating in these distinct cell states. Naïve T cells were highly reliant on BCL-2 and sensitive to BCL-2 inhibition. Activated T cells are remarkably different in their regulation of apoptosis by pro- and antiapoptotic members of the BCL-2 family, rendering them differentially sensitive to antagonists blocking the function of one or more members of this family. Recent progress in understanding other programmed cell death mechanisms, especially necroptosis, suggests a unique role for alternative pathways in regulating death of activated T cells. Furthermore, we highlight a mechanism of epigenetic regulation of cell survival unique to activated T cells. Together, we present an update of our current understanding of the survival requirement of activated T cells.

The corepressor NCOR1 regulates the survival of single-positive thymocytes

Nuclear receptor corepressor 1 (NCOR1) is a transcriptional regulator bridging repressive chromatin modifying enzymes with transcription factors. NCOR1 regulates many biological processes, however its role in T cells is not known. Here we show that Cd4-Cre-mediated deletion of NCOR1 (NCOR1 cKOCd4) resulted in a reduction of peripheral T cell numbers due to a decrease in single-positive (SP) thymocytes. In contrast, double-positive (DP) thymocyte numbers were not affected in the absence of NCOR1. The reduction in SP cells was due to diminished survival of NCOR1-null postselection TCRβhiCD69+ and mature TCRβhiCD69- thymocytes. NCOR1-null thymocytes expressed elevated levels of the pro-apoptotic factor BIM and showed a higher fraction of cleaved caspase 3-positive cells upon TCR stimulation ex vivo. However, staphylococcal enterotoxin B (SEB)-mediated deletion of Vβ8+ CD4SP thymocytes was normal, suggesting that negative selection is not altered in the absence of NCOR1. Finally, transgenic expression of the pro-survival protein BCL2 restored the population of CD69+ thymocytes in NCOR1 cKOCd4 mice to a similar percentage as observed in WT mice. Together, these data identify NCOR1 as a crucial regulator of the survival of SP thymocytes and revealed that NCOR1 is essential for the proper generation of the peripheral T cell pool.

Anti-apoptotic BCL-2 family members in development

Almost 30 years ago it was first appreciated that anti-apoptotic B-cell lymphoma-2 (BCL-2) prevents the induction of apoptosis not only in malignant cells, but also in normal cellular lineages. This critical observation has rapidly evolved from merely identifying new BCL-2 family members to understanding how their biochemical interactions trigger the cell death process, and, more recently, to pharmacological inhibition of anti-apoptotic BCL-2 function in disease. Indeed, the proper regulation of apoptosis is important in many aspects of life including development, homeostasis, and disease biology. To better understand these processes, scientists have used many tools to assess the contribution of individual anti-apoptotic BCL-2 family members. This review will focus on the prominent roles for BCL-2 and other pro-survival family members in promoting the development of mammals during early embryogenesis, neurogenesis, and hematopoiesis.

The life and death of immune cell types: the role of BCL-2 anti-apoptotic molecules

Targeting survival mechanisms of immune cells may provide an avenue for immune intervention to dampen unwanted responses (e.g. autoimmunity, immunopathology and transplant rejection) or enhance beneficial ones (e.g. immune deficiency, microbial defence and cancer immunotherapy). The selective survival mechanisms of the various immune cell types also avails the possibility of specific tailoring of such interventions. Here, we review the role of the BCL-2 anti-apoptotic family members (BCL-2, BCL-XL, BCL-W, MCL-1 and A1) on cell death/survival of the major immune cell types, for example, T, NK, B, dendritic cell (DC) lineages. There is both selectivity and redundancy among this family. Selectivity comes partly from the expression levels in each of the cell types. For example, plasmacytoid DC express abundant BCL-2 and are susceptible to BCL-2 antagonism or deficiency, whereas conventional DC express abundant A1 and are susceptible to A1 deficiency. There is, however, also functional redundancy; for example, overexpression of MCL-1 can override BCL-2 antagonism in plasmacytoid DC. Moreover, susceptibility to another anti-apoptotic family member can be unmasked, when one or other member is removed. These dual principles of selectivity and redundancy should guide the use of antagonists for manipulating immune cells.

Dying to protect: cell death and the control of T-cell homeostasis

T cells play a critical role in immune responses as they specifically recognize peptide/MHC complexes with their T-cell receptors and initiate adaptive immune responses. While T cells are critical for performing appropriate effector functions and maintaining immune memory, they also can cause autoimmunity or neoplasia if misdirected or dysregulated. Thus, T cells must be tightly regulated from their development onward. Maintenance of appropriate T-cell homeostasis is essential to promote protective immunity and limit autoimmunity and neoplasia. This review will focus on the role of cell death in maintenance of T-cell homeostasis and outline novel therapeutic strategies tailored to manipulate cell death to limit T-cell survival (eg, autoimmunity and transplantation) or enhance T-cell survival (eg, vaccination and immune deficiency).

Coronins and their role in immunological phenomena

Coronins are a large family of proteins occurring in many eukaryotes. In mammals, seven coronin genes have been identified, evidencing that coronins 1 to 6 present classic coronin structure, while coronin 7 is a tandem coronin particle, without a spiral domain, although the best characterised coronin, in terms of both structure and function, is the mammalian coronin 1. It has been proven that they are related to regulation of actin dynamics, e.g. as a result of interaction with the complex of proteins Arp2/3. These proteins also modulate the activity of immune system cells, including lymphocyte T and B cells, neutrophils and macrophages. They are involved in bacterial infections with Mycobacterium tuberculosis, M. leprae and Helicobacter pylori and participate in the response to viral infections, e.g. infections of lymphocytic choriomeningitis virus (LCMV) and vesicular stomatitis Indiana virus (VSV). Also their involvement in autoimmune diseases such as lupus erythematosus has been recorded.

Kinase-independent function of RIP1, critical for mature T-cell survival and proliferation

The death receptor, Fas, triggers apoptotic death and is essential for maintaining homeostasis in the peripheral lymphoid organs. RIP1 was originally cloned when searching for Fas-binding proteins and was later shown to associate also with the signaling complex of TNFR1. Although Fas exclusively induces apoptosis, TNFR1 primarily activates the pro-survival/pro-inflammatory NF-κB pathway. Mutations in Fas lead to lymphoproliferative (lpr) diseases, and deletion of TNFR1 results in defective innate immune responses. However, the function of RIP1 in the adult lymphoid system has not been well understood, primarily owing to perinatal lethality in mice lacking the entire RIP1 protein in germ cells. This current study investigated the requirement for RIP1 in the T lineage using viable RIP1 mutant mice containing a conditional and kinase-dead RIP1 allele. Disabling the kinase activity of RIP1 had no obvious impact on the T-cell compartment. However, T-cell-specific deletion of RIP1 led to a severe T-lymphopenic condition, owing to a dramatically reduced mature T-cell pool in the periphery. Interestingly, the immature T-cell compartment in the thymus appeared intact. Further analysis showed that mature RIP1^−/− T cells were severely defective in antigen receptor-induced proliferative responses. Moreover, the RIP1^−/− T cells displayed greatly increased death and contained elevated caspase activities, an indication of apoptosis. In total, these results revealed a novel, kinase-independent function of RIP1, which is essential for not only promoting TCR-induced proliferative responses but also in blocking apoptosis in mature T cells.

Physiological restraint of Bak by Bcl-xL is essential for cell survival

Lee et al. describe a mutation in mouse and human Bak that specifically disrupts its interaction with the prosurvival protein Bcl-x_L. In vivo, loss of Bcl-x_L binding to Bak led to significant defects in T-cell and blood platelet survival.

Due to the myriad interactions between prosurvival and proapoptotic members of the Bcl-2 family of proteins, establishing the mechanisms that regulate the intrinsic apoptotic pathway has proven challenging. Mechanistic insights have primarily been gleaned from in vitro studies because genetic approaches in mammals that produce unambiguous data are difficult to design. Here we describe a mutation in mouse and human Bak that specifically disrupts its interaction with the prosurvival protein Bcl-x_L. Substitution of Glu75 in mBak (hBAK Q77) for leucine does not affect the three-dimensional structure of Bak or killing activity but reduces its affinity for Bcl-x_L via loss of a single hydrogen bond. Using this mutant, we investigated the requirement for physical restraint of Bak by Bcl-x_L in apoptotic regulation. In vitro, Bak^Q75L cells were significantly more sensitive to various apoptotic stimuli. In vivo, loss of Bcl-x_L binding to Bak led to significant defects in T-cell and blood platelet survival. Thus, we provide the first definitive in vivo evidence that prosurvival proteins maintain cellular viability by interacting with and inhibiting Bak.

Killing Two Cells with One Stone: Pharmacologic BCL-2 Family Targeting for Cancer Cell Death and Immune Modulation

A crucial component of regulating organismal homeostasis is maintaining proper cell number and eliminating damaged or potentially malignant cells. Apoptosis, or programed cell death, is the mechanism responsible for this equilibrium. The intrinsic apoptotic pathway is also especially important in the development and maintenance of the immune system. Apoptosis is essential for proper positive and negative selection during B- and T-cell development and for efficient contraction of expanded lymphocytes following an immune response. Tight regulation of the apoptotic pathway is critical, as excessive cell death can lead to immunodeficiency while apoptotic resistance can lead to aberrant lymphoproliferation and autoimmune disease. Dysregulation of cell death is implicated in a wide range of hematological malignancies, and targeting various components of the apoptotic machinery in these cases is an attractive chemotherapeutic strategy. A wide array of compounds has been developed with the purpose of reactivating the intrinsic apoptotic pathway. These compounds, termed BH3 mimetics are garnering considerable attention as they gain greater clinical oncologic significance. As their use expands, it will be imperative to understand the effects these compounds have on immune homeostasis. Uncovering their potential immunomodulatory activity may allow for administration of BH3 mimetics for direct tumor cell killing as well as novel therapies for a wide range of immune-based directives. This review will summarize the major proteins involved in the intrinsic apoptotic pathway and define their roles in normal immune development and disease. Clinical and preclinical BH3 mimetics are described within the context of what is currently known about their ability to affect immune function. Prospects for future antitumor immune amplification and immune modulation are then proposed.

An Enhancer of the IL-7 Receptor α-Chain Locus Controls IL-7 Receptor Expression and Maintenance of Peripheral T Cells

The IL-7R plays critical roles in lymphocyte development and homeostasis. Although IL-7R expression is strictly regulated during lymphocyte differentiation and the immune response, little is known regarding its in vivo regulation. To address this issue, we established a mouse line with targeted deletion of the conserved non-coding sequence 1 (CNS1) element found 3.6 kb upstream of the IL-7Rα promoter. We report that IL-7Rα is expressed normally on T and B cells in thymus and bone marrow of CNS1(-/-) mice except for in regulatory T cells. In contrast, these mice show reduced IL-7Rα expression in conventional CD4 and CD8 T cells as well as regulatory T, NKT, and γδ T cells in the periphery. CD4 T cells of CNS1(-/-) mice showed IL-7Rα upregulation in the absence of growth factors and IL-7Rα downregulation by IL-7 or TCR stimulation, although the expression levels were lower than those in control mice. Naive CD4 and CD8 T cells of CNS1(-/-) mice show attenuated survival by culture with IL-7 and reduced homeostatic proliferation after transfer into lymphopenic hosts. CNS1(-/-) mice exhibit impaired maintenance of Ag-stimulated T cells. Furthermore, IL-7Rα upregulation by glucocorticoids and TNF-α was abrogated in CNS1(-/-) mice. This work demonstrates that the CNS1 element controls IL-7Rα expression and maintenance of peripheral T cells, suggesting differential regulation of IL-7Rα expression between central and peripheral lymphoid organs.

Harnessing the apoptotic programs in cancer stem-like cells

Elimination of malignant cells is an unmet challenge for most human cancer types even with therapies targeting specific driver mutations. Therefore, a multi-pronged strategy to alter cancer cell biology on multiple levels is increasingly recognized as essential for cancer cure. One such aspect of cancer cell biology is the relative apoptosis resistance of tumor-initiating cells. Here, we provide an overview of the mechanisms affecting the apoptotic process in tumor cells emphasizing the differences in the tumor-initiating or stem-like cells of cancer. Further, we summarize efforts to exploit these differences to design therapies targeting that important cancer cell population.

Transcription factors and target genes of pre-TCR signaling

Almost 30 years ago pioneering work by the laboratories of Harald von Boehmer and Susumo Tonegawa provided the first indications that developing thymocytes could assemble a functional TCRβ chain-containing receptor complex, the pre-TCR, before TCRα expression. The discovery and study of the pre-TCR complex revealed paradigms of signaling pathways in control of cell survival and proliferation, and culminated in the recognition of the multifunctional nature of this receptor. As a receptor integrated in a dynamic developmental process, the pre-TCR must be viewed not only in the light of the biological outcomes it promotes, but also in context with those molecular processes that drive its expression in thymocytes. This review article focuses on transcription factors and target genes activated by the pre-TCR to drive its different outcomes.

Lessons from gain- and loss-of-function models of pro-survival Bcl2 family proteins: implications for targeted therapy

Cell survival depends on the maintenance of mitochondrial integrity controlled by a well-balanced interplay between anti- and pro-apoptotic B cell lymphoma 2 (Bcl2) family members. Given their frequent deregulation in human pathologies, including autoimmunity and cancer, significant research efforts have increased our molecular understanding of how Bcl2 proteins control cell death. This has fostered the development of small non-peptidic compounds, so-called BH3-mimetics, that show excellent prospects of passing clinical trials and entering daily use for targeted therapy. Possible limitations in clinical application may, to a certain degree, be predicted from loss-of-function phenotypes gathered from studies using gene-modified mice that we attempt to summarize and discuss in this context.

Altered expression of immune-related genes in children with Down syndrome

Individuals with Down syndrome (DS) have a high incidence of immunological alterations with increased susceptibility to bacterial and viral infections and high frequency of different types of hematologic malignancies and autoimmune disorders. In the current study, we profiled the expression pattern of 92 immune-related genes in peripheral blood mononuclear cells (PBMCs) of two different groups, children with DS and control children, to identify differentially expressed genes that might be of pathogenetic importance for the development and phenotype of the immunological alterations observed in individuals with DS. PBMCs samples were obtained from six DS individuals with karyotypically confirmed full trisomy 21 and six healthy control individuals (ages 2–6 years). Gene expression was profiled in duplicate according to the manufacturer's instructions provided by commercially available TaqMan Human Immune Array representing 92 immune function genes and four reference genes on a 96-plex gene card. A set of 17 differentially expressed genes, not located on chromosome 21 (HSA21), involved in immune and inflammatory pathways was identified including 13 genes (BCL2, CCL3, CCR7, CD19, CD28, CD40, CD40LG, CD80, EDN1, IKBKB, IL6, NOS2 and SKI) significantly down-regulated and four genes (BCL2L1, CCR2, CCR5 and IL10) significantly up-regulated in children with DS. These findings highlight a list of candidate genes for further investigation into the molecular mechanism underlying DS pathology and reinforce the secondary effects of the presence of a third copy of HSA21.

The catalytic activity of the mitogen-activated protein kinase extracellular signal-regulated kinase 3 is required to sustain CD4+ CD8+ thymocyte survival

Extracellular signal-regulated kinase 3 (ERK3) is an atypical member of the mitogen-activated protein kinase (MAPK) family whose function is largely unknown. Given the central role of MAPKs in T cell development, we hypothesized that ERK3 may regulate thymocyte development. Here we have shown that ERK3 deficiency leads to a 50% reduction in CD4(+) CD8(+) (DP) thymocyte number. Analysis of hematopoietic chimeras revealed that the reduction in DP thymocytes is intrinsic to hematopoietic cells. We found that early thymic progenitors seed the Erk3(-/-) thymus and can properly differentiate and proliferate to generate DP thymocytes. However, ERK3 deficiency results in a decrease in the DP thymocyte half-life, associated with a higher level of apoptosis. As a consequence, ERK3-deficient DP thymocytes are impaired in their ability to make successful secondary T cell receptor alpha (TCRα) gene rearrangement. Introduction of an already rearranged TCR transgene restores thymic cell number. We further show that knock-in of a catalytically inactive allele of Erk3 fails to rescue the loss of DP thymocytes. Our results uncover a unique role for ERK3, dependent on its kinase activity, during T cell development and show that this atypical MAPK is essential to sustain DP survival during RAG-mediated rearrangements.

Maturation stage of T-cell acute lymphoblastic leukemia determines BCL-2 versus BCL-XL dependence and sensitivity to ABT-199

Acute lymphoblastic leukemia (ALL) is a hematopoietic malignancy derived from immature B-lymphoid and T-lymphoid cells (T-ALL). In T-ALL, there is an early T-cell progenitor (ETP) subgroup that has a very high risk for relapse. In this study, we used mitochondrial BH3 profiling to determine antiapoptotic protein dependencies in T-ALL. We found that T-ALL cell lines and primary patient samples are dependent upon BCL-XL, except when the cancer bears an ETP phenotype, in which case it is BCL-2 dependent. These distinctions directly relate to differential sensitivity to the BH3 mimetics ABT-263 and ABT-199, both in vitro and in vivo. We thus describe for the first time a change of antiapoptotic protein dependence that is related to the differentiation stage of the leukemic clone. Our findings demonstrate that BCL-2 is a clinically relevant target for therapeutic intervention with ABT-199 in ETP-ALL.

SIGNIFICANCE

ETP T-ALL is a treatment-resistant subtype of T-ALL for which novel targeted therapies are urgently needed. We have discovered, through BH3 profiling, that ETP-ALL is BCL-2 dependent and is very sensitive to in vitro and in vivo treatment with ABT-199, a drug well tolerated in clinical trials.

Generation of induced pluripotent stem cell-derived mice by reprogramming of a mature NKT cell

The successful generation of iPSC-derived mouse strains to study NKT cells

NKT cells are characterized by their expression of an NKT-cell-specific invariant antigen-receptor α chain encoded by Vα14Jα18 gene segments. These NKT cells bridge the innate and acquired immune systems to mediate effective and augmented responses; however, the limited number of NKT cells in vivo hampers their analysis. Here, two lines of induced pluripotent stem cell-derived mice (NKT-iPSC-derived mice) were generated by reprogramming of mature NKT cells, where one harbors both rearranged Vα14Jα18 and Vβ7 genes and the other carries rearranged Vα14Jα18 on both alleles but germline Vβ loci. The analysis of NKT-iPSC-derived mice showed a significant increase in NKT cell numbers with relatively normal frequencies of functional subsets, but significantly enhanced in some cases, and acquired functional NKT cell maturation in peripheral lymphoid organs. NKT-iPSC-derived mice also showed normal development of other immune cells except for the absence of γδT cells and disturbed development of conventional CD4 αβT cells. These results suggest that the NKT-iPSC-derived mice are a better model for NKT cell development and function study rather than transgenic mouse models reported previously and also that the presence of a pre-rearranged Vα14Jα18 in the natural chromosomal context favors the developmental fate of NKT cells.

Both leukaemic and normal peripheral B lymphoid cells are highly sensitive to the selective pharmacological inhibition of prosurvival Bcl-2 with ABT-199

Overexpression of the prosurvival protein Bcl-2 marks many B-lymphoid malignancies and contributes to resistance to many commonly used chemotherapeutic agents. The first effective BH3 mimetic inhibitors of Bcl-2, ABT-737 and navitoclax, also target Bcl-xL, causing dose-limiting thrombocytopenia. This prompted the development of the Bcl-2-selective antagonist, ABT-199. Here we show that in lymphoid cells, ABT-199 specifically causes Bax/Bak-mediated apoptosis that is triggered principally by the initiator BH3-only protein Bim. As expected, malignant cells isolated from patients with chronic lymphocytic leukaemia are highly sensitive to ABT-199. However, we found that normal, untransformed mature B cells are also highly sensitive to ABT-199, both in vitro and in vivo. By contrast, the B-cell precursors are largely spared, as are cells of myeloid origin. These results pinpoint the probable impact of the pharmacological inhibition of Bcl-2 by ABT-199 on the normal mature haemopoietic cell lineages in patients, and have implications for monitoring during ABT-199 therapy as well as for the clinical utility of this very promising targeted agent.

Signal transducer and activator of transcription 3 (Stat3) contributes to T-cell homeostasis by regulating pro-survival Bcl-2 family genes

The naive T-cell pool in peripheral lymphoid tissues is fairly stable in terms of number, diversity and functional capabilities in spite of the absence of prominent stimuli. This stability is attributed to continuous tuning of the composition of the T-cell pool by various homeostatic signals. Despite extensive research into the link between signal transducer and activator of transcription 3 (Stat3) and T-cell survival, little is known about how Stat3 regulates homeostasis by maintaining the required naive T-cell population in peripheral lymphoid organs. We assessed whether the elimination of Stat3 in T cells limits T-cell survival. We demonstrated that the proportion and number of single-positive thymocytes as well as T cells in the spleen and lymph nodes were significantly decreased in the Stat3-deficient group as a result of the enhanced susceptibility of Stat3-deleted T lymphocytes to apoptosis. Importantly, expression of the anti-apoptotic Bcl-2 and Bcl-xL was markedly decreased in Stat3-deleted single-positive thymocytes and T lymphocytes, suggesting that Stat3 helps to maintain the T-cell pool in the resting condition by promoting the expression of Bcl-2 family genes. These findings suggest the importance of Stat3 in the integration of homeostatic cues for the maintenance and functional tuning of the T-cell pool.

Impact of bone marrow stromal cells on Bcl-2 family members in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most common form of adult leukemia in the Western world. High levels of Bcl-2 family anti-apoptotic proteins are responsible for apoptosis resistance. Besides anti-apoptotic proteins, the microenvironment provides substantial survival signals to CLL leukemic cells. However, in-depth knowledge on the role of individual Bcl-2 family members in the context of the microenvironment is still limited. We performed a comprehensive analysis of transcripts and proteins of 18 Bcl-2 family members using an "apoptosis array microfluidic card" in primary cells before and after stromal co-cultures. Our data showed that five of six anti-apoptotic members (excluding Bcl-b), two of three pro-apoptotic members (excluding Bok) and six of nine BH3-only members were present at detectable mRNA levels in CLL cells. Importantly, stromal-mediated extended survival of CLL cells was strongly associated with elevated global transcription. Upon co-culturing with stromal cells, there was an early response of an increase in anti- (2/5) and pro-apoptotic protein (3/8) transcripts on day 1, while an increase in anti-apoptotic proteins was observed on day 3, with no significant change in pro-apoptotic proteins. Our study revealed a differential pattern of expression of both transcripts and proteins following stromal co-cultures, proposing a significance of Bcl-2 family members in the stromal microenvironment.

Antiapoptotic Mcl-1 is critical for the survival and niche-filling capacity of Foxp3⁺ regulatory T cells

Foxp3⁺ regulatory T (Treg) cells are a crucial immunosuppressive population of CD4⁺ T cells, yet the homeostatic processes and survival programs that maintain the Treg cell pool are poorly understood. Here we report that peripheral Treg cells markedly alter their proliferative and apoptotic rates to rapidly restore numerical deficit through an interleukin 2-dependent and costimulation-dependent process. By contrast, excess Treg cells are removed by attrition, dependent on the Bim-initiated Bak- and Bax-dependent intrinsic apoptotic pathway. The antiapoptotic proteins Bcl-xL and Bcl-2 were dispensable for survival of Treg cells, whereas Mcl-1 was critical for survival of Treg cells, and the loss of this antiapoptotic protein caused fatal autoimmunity. Together, these data define the active processes by which Treg cells maintain homeostasis via critical survival pathways.

On guard: coronin proteins in innate and adaptive immunity

Recent work has implicated members of the evolutionarily conserved family of coronin proteins - in particular coronin 1 - in immune homeostasis. Coronins are involved in processes as diverse as pathogen survival in phagocytes and homeostatic T cell signalling. Notably, in both mice and humans, coronin mutations are associated with immune deficiencies and resistance to autoimmunity. In this article, we review what is currently known about these conserved molecules and discuss a potential common mechanism that underlies their diverse activities, which seem to involve cytoskeletal interactions as well as calcium-calcineurin signalling.

Getting away with murder: how does the BCL-2 family of proteins kill with immunity?

The adult human body produces approximately one million white blood cells every second. However, only a small fraction of the cells will survive because the majority is eliminated through a genetically controlled form of cell death known as apoptosis. This review places into perspective recent studies pertaining to the BCL-2 family of proteins as critical regulators of the development and function of the immune system, with particular attention on B cell and T cell biology. Here we discuss how elegant murine model systems have revealed the major contributions of the BCL-2 family in establishing an effective immune system. Moreover, we highlight some key regulatory pathways that influence the expression, function, and stability of individual BCL-2 family members, and discuss their role in immunity. From lethal mechanisms to more gentle ones, the final portion of the review discusses the nonapoptotic functions of the BCL-2 family and how they pertain to the control of immunity.

Delving deeper: MCL-1's contributions to normal and cancer biology

BCL-2 molecules are regulators of programmed cell death and defects in this pathway contribute to human diseases. One family member, MCL-1, is unique because its expression is tightly regulated and it is essential for promoting the survival of myriad cellular lineages. Additionally, MCL-1 promotes the maintenance of normal mitochondrial morphology and energy production. Dissection of these functions revealed recently that they depend on separate mitochondrial sublocalizations. MCL-1's antiapoptotic activity is restricted to the outer mitochondrial membrane (OMM), whereas its function in mitochondrial physiology requires localization to the matrix. These findings provide an attractive model for how MCL-1's diverse functions may contribute to normal cell homeostasis and function. MCL-1 is highly amplified in human cancer, suggesting that these functions may contribute to malignant cell growth and evasion of apoptosis.