Bim and Bcl-2 mutually affect the expression of the other in T cells

Integrated signaling and transcriptome analysis reveals Src family kinase individualities and novel pathways controlled by their constitutive activity

The Src family kinases (SFKs) Lck and Lyn are crucial for lymphocyte development and function. Albeit tissue-restricted expression patterns the two kinases share common functions; the most pronounced one being the phosphorylation of ITAM motifs in the cytoplasmic tails of antigenic receptors. Lck is predominantly expressed in T lymphocytes; however, it can be ectopically found in B-1 cell subsets and numerous pathologies including acute and chronic B-cell leukemias. The exact impact of Lck on the B-cell signaling apparatus remains enigmatic and is followed by the long-lasting question of mechanisms granting selectivity among SFK members. In this work we sought to investigate the mechanistic basis of ectopic Lck function in B-cells and compare it to events elicited by the predominant B-cell SFK, Lyn. Our results reveal substrate promiscuity displayed by the two SFKs, which however, is buffered by their differential susceptibility toward regulatory mechanisms, revealing a so far unappreciated aspect of SFK member-specific fine-tuning. Furthermore, we show that Lck- and Lyn-generated signals suffice to induce transcriptome alterations, reminiscent of B-cell activation, in the absence of receptor/co-receptor engagement. Finally, our analyses revealed a yet unrecognized role of SFKs in tipping the balance of cellular stress responses, by promoting the onset of ER-phagy, an as yet completely uncharacterized process in B lymphocytes.

Valosin-containing protein (VCP/p97) inhibition reduces viral clearance and induces toxicity associated with muscular damage

Valosin-containing protein (VCP)/p97 has emerged as a central regulator of the ubiquitin-proteasome system by connecting ubiquitylation and degradation. The development of CB-5083, an ATPase D2-domain-selective and orally bioavailable inhibitor of VCP/p97, allows targeting of the ubiquitin-proteasome system in human diseases. In this study, we evaluated the effect of CB-5083 on the immune response in mice by using the lymphocytic choriomeningitis virus (LCMV) as an infection model. We demonstrate that LCMV infection increased the susceptibility to CB-5083 treatment in a CD8-independent manner. Administration of CB-5083 to mice reduced the cytotoxic T cell response and impaired viral clearance. Compared to uninfected cells, CB-5083 treatment enhanced the unfolded protein response in LCMV-infected cells. Administration of CB-5083 during the expansion of CD8⁺ T cells led to strong toxicity in mice within hours, which resulted in enhanced IL-6 levels in the serum and accumulation of poly-ubiquitinated proteins. Furthermore, we linked the observed toxicity to the specific formation of aggregates in the skeletal muscle tissue and the upregulation of both lactate dehydrogenase and creatine kinase in the serum.

Overcoming the acquired resistance to gefitinib in lung cancer brain metastasis in vitro and in vivo

In our previous work, PC-9-Br, a PC-9 brain seeking line established via a preclinical animal model of lung cancer brain metastasis (LCBM), exhibited not only resistance to epidermal growth factor receptor-tyrosine kinase inhibitor (EGFR-TKI) gefitinib in vitro, but also chemotherapy regimens of cisplatin plus etoposide in vivo. Using this cell line, we investigated novel potential targeted therapeutics for treating LCBM in vitro and in vivo to combat drug resistance. Significant increases in mRNA and protein expression levels of Bcl-2 were found in PC-9-Br compared with parental PC-9 (PC-9-P), but no significant changes of Bcl-XL were observed. A remarkable synergistic effect between EGFR-TKI gefitinib and Bcl-2 inhibitors ABT-263 (0.17 ± 0.010 µM at 48 h and 0.02 ± 0.004 µM at 72 h), or ABT-199 (0.22 ± 0.008 µM at 48 h and 0.02 ± 0.001 µM at 72 h) to overcome acquired resistance to gefitinib (> 0.5 µM at 48 h and 0.10 ± 0.007 µM at 72 h) in PC-9-Br was observed in MTT assays. AZD9291 was also shown to overcome acquired resistance to gefitinib in PC-9-Br in MTT assays (0.23 ± 0.031 µM at 48 h and 0.03 ± 0.008 µM at 72 h). Western blot showed significantly decreased phospho-Erk1/2 and increased cleaved-caspase-3 expressions were potential synergistic mechanisms for gefitinib + ABT263/ABT199 in PC-9-Br. Significantly decreased protein expressions of phospho-EGFR, phospho-Akt, p21, and survivin were specific synergistic mechanism for gefitinib + ABT199 in PC-9-Br. In vivo studies demonstrated afatinib (30 mg/kg) and AZD9291 (25 mg/kg) could significantly reduce the LCBM in vivo and increase survival percentages of treated mice compared with mice treated with vehicle and gefitinib (6.25 mg/kg). In conclusion, our study demonstrated gefitinib + ABT263/ABT199, afatinib, and AZD9291 have clinical potential to treat LCBM.

Loss-of-function of Fbxo10, encoding a post-translational regulator of BCL2 in lymphomas, has no discernible effect on BCL2 or B lymphocyte accumulation in mice

Regulation of the anti-apoptotic BCL2 protein determines cell survival and is frequently abnormal in B cell lymphomas. An evolutionarily conserved post-translational mechanism for over-expression of BCL2 in human B cell lymphomas and the BCL2 paralogue CED-9 in Caenorhabditis elegans results from loss-of-function mutations in human FBXO10 and its C.elegans paralogue DRE-1, a BCL2/CED-9-binding subunit of the SKP-CULLIN-FBOX (SCF) ubiquitin ligase. Here, we tested the role of FBXO10 in BCL2 regulation by producing mice with two different CRISPR/Cas9-engineered Fbxo10 mutations: an Asp54Lys (E54K) missense mutation in the FBOX domain and a Cys55SerfsTer55 frameshift (fs) truncating mutation. Mice homozygous for either mutant allele were born at the expected Mendelian frequency and appeared normal in body weight and appearance as adults. Spleen B cells from homozygous mutant mice did not have increased BCL2 protein, nor were the numbers of mature B cells or germinal centre B cells increased as would be expected if BCL2 was increased. Other lymphocyte subsets that are also regulated by BCL2 levels also displayed no difference in frequency in homozygous Fbxo10 mutant mice. These results support one of two conclusions: either FBXO10 does not regulate BCL2 in mice, or it does so redundantly with other ubiquitin ligase complexes. Possible candidates for the latter include FBXO11 or ARTS-XIAP. The difference between the role of FBXO10 in regulating BCL2 protein levels in C. elegans and in human DLBCL, relative to single-gene deficient mouse leukocytes, should be further investigated.

miR24-3p activity after delivery into pancreatic carcinoma cell lines exerts profound tumor-inhibitory effects through distinct pathways of apoptosis and autophagy induction

Pancreatic cancer is among the most detrimental tumors, with novel treatment options urgently needed. The pathological downregulation of a miRNA in tumors can lead to the overexpression of oncogenes, thus suggesting miRNA replacement as novel strategy in cancer therapy. While the role of miR24 in cancer, including pancreatic carcinoma, has been described as ambiguous, it may hold great promise and deserves further studies. Here, we comprehensively analyze the effects of miR24-3p replacement in a set of pancreatic carcinoma cell lines. Transfection of miR24-3p mimics leads to profound cell inhibition in various 2D and 3D cell assays, based on the induction of apoptosis, autophagy and ROS. Comprehensive analyses of miR24-3p effects on the molecular level reveal the transcriptional regulation of several important oncogenes and oncogenic pathways. Based on these findings, miRNA replacement therapy was preclinically explored by treating tumor xenograft-bearing mice with miR24-3p mimics formulated in polymeric nanoparticles. The obtained tumor inhibition was associated with the induction of apoptosis and necrosis. Taken together, we identify miR24-3p as powerful tumor-inhibitory miRNA for replacement therapy, and describe a complex network of oncogenic pathways affected by miR24.

Fine-tuning of antigen-specific immune responses by regulatory T cells activated via antigen recognition-independent and humoral factor-dependent mechanisms

CD4⁺ CD25⁺ Foxp3⁺ regulatory T cells (Tregs) are highly sensitive to IL-2, one of the many cytokines produced during immune responses, for their development, survival and functions. Although the effects of IL-2 administration on Tregs in vivo are well characterized, the effects on Tregs elicited by IL-2 produced during an immune response have not been elucidated. Hence, in this study, Treg behaviour during IL-2-producing immune responses was explored using in vivo and in vitro murine systems. The use of murine mixed lymphocyte culture (MLC) revealed that a large proportion of Tregs increased in size, accompanied by both cell death and proliferation status. Further, these large Tregs, which were found to not recognize specific antigens, were observed in MLCs as being functionally activated by various cytokines, including IL-2, produced by antigen-specific T cells. This 'bystander Treg activation' was also observed in mice with graft-versus-host reactions (GvHRs). Alternatively, effector cells from Treg-depleted MLCs exhibited lower antigen-specific responses or higher cross-reactivity as compared to control MLCs with Tregs. Taken together, these results suggest that Tregs are activated by cytokines, mainly IL-2, released from T cells that are activated by a specific antigen. Subsequently, these activated bystander Tregs contribute to the fine-tuning of highly antigen-specific immune responses.

Dynein light chain binding determines complex formation and posttranslational stability of the Bcl-2 family members Bmf and Bim

The BH3-only class of Bcl-2 family proteins triggers mitochondrial apoptosis. Several mechanisms are used to restrain the pro-apoptotic activity of these proteins. Dynein light chain (DYNLL) 1 and 2 has been proposed to negatively regulate the activity of Bim and Bmf, respectively, and the Bim-DYNLL1 interaction leads to the formation of large protein complexes on mitochondria. Here we found that Bim and Bmf interact with both isoforms of DYNLL (DYNLL1 and DYNLL2). DYNLL1/2 not only induced homo-dimerization of Bim and Bmf but also led to the formation of ternary complexes (Bim-DYNLL-Bmf), both in cell-free and in cellular systems. DYNLL-induced oligomerization stabilized Bmf in cultured cells and inhibited its degradation by the ubiquitin-independent 20S proteasome in a cell-free system. Surprisingly, overexpression of wild-type Bmf but not of a DYNLL-binding-deficient mutant induced degradation of endogenous Bim in different cell lines, but both variants sensitized to apoptosis. Mutant Bmf incapable of interacting with anti-apoptotic Bcl-2 proteins and of inducing apoptosis still caused Bim degradation. These results suggest that Bmf overexpression-induced Bim degradation is not due to the displacement of Bim from anti-apoptotic Bcl-2 proteins but a direct consequence of the modulation of Bim-DYNLL association. A peptide derived from the DYNLL-binding domain of Bim also led to the degradation of Bim as well as of its preferred binding partner Mcl-1. Thus DYNLL regulates the mitochondrial pathway of apoptosis by determining the stability of Bmf, Bim, and Mcl-1 proteins.

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.

Indirect presentation in the thymus limits naive and regulatory T-cell differentiation by promoting deletion of self-reactive thymocytes

Acquisition of T-cell central tolerance involves distinct pathways of self-antigen presentation to thymocytes. One pathway termed indirect presentation requires a self-antigen transfer step from thymic epithelial cells (TECs) to bone marrow-derived cells before the self-antigen is presented to thymocytes. The role of indirect presentation in central tolerance is context-dependent, potentially due to variation in self-antigen expression, processing and presentation in the thymus. Here, we report experiments in mice in which TECs expressed a membrane-bound transgenic self-antigen, hen egg lysozyme (HEL), from either the insulin (insHEL) or thyroglobulin (thyroHEL) promoter. Intrathymic HEL expression was less abundant and more confined to the medulla in insHEL mice compared with thyroHEL mice. When indirect presentation was impaired by generating mice lacking MHC class II expression in bone marrow-derived antigen-presenting cells, insHEL-mediated thymocyte deletion was abolished, whereas thyroHEL-mediated deletion occurred at a later stage of thymocyte development and Foxp3⁺ regulatory T-cell differentiation increased. Indirect presentation increased the strength of T-cell receptor signalling that both self-antigens induced in thymocytes, as assessed by Helios expression. Hence, indirect presentation limits the differentiation of naive and regulatory T cells by promoting deletion of self-reactive thymocytes.

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).

Combined venetoclax and alvocidib in acute myeloid leukemia

More effective treatment options for elderly acute myeloid leukemia (AML) patients are needed as only 25-50% of patients respond to standard-of-care therapies, response duration is typically short, and disease progression is inevitable even with some novel therapies and ongoing clinical trials. Anti-apoptotic BCL-2 family inhibitors, such as venetoclax, are promising therapies for AML. Nonetheless, resistance is emerging. We demonstrate that venetoclax combined with cyclin-dependent kinase (CDK) inhibitor alvocidib is potently synergistic in venetoclax-sensitive and -resistant AML models in vitro, ex vivo and in vivo. Alvocidib decreased MCL-1, and/or increased pro-apoptotic proteins such as BIM or NOXA, often synergistically with venetoclax. Over-expression of BCL-XL diminished synergy, while knock-down of BIM almost entirely abrogated synergy, demonstrating that the synergistic interaction between alvocidib and venetoclax is primarily dependent on intrinsic apoptosis. CDK9 inhibition predominantly mediated venetoclax sensitization, while CDK4/6 inhibition with palbociclib did not potentiate venetoclax activity. Combined, venetoclax and alvocidib modulate the balance of BCL-2 family proteins through complementary, yet variable mechanisms favoring apoptosis, highlighting this combination as a promising therapy for AML or high-risk MDS with the capacity to overcome intrinsic apoptosis mechanisms of resistance. These results support clinical testing of combined venetoclax and alvocidib for the treatment of AML and advanced MDS.

Temporal Expression of Bim Limits the Development of Agonist-Selected Thymocytes and Skews Their TCRβ Repertoire

CD8αα TCRαβ⁺ intestinal intraepithelial lymphocytes play a critical role in promoting intestinal homeostasis, although mechanisms controlling their development and peripheral homeostasis remain unclear. In this study, we examined the spatiotemporal role of Bim in the thymic selection of CD8αα precursors and the fate of these cells in the periphery. We found that T cell-specific expression of Bim during early/cortical, but not late/medullary, thymic development controls the agonist selection of CD8αα precursors and limits their private TCRβ repertoire. During this process, agonist-selected double-positive cells lose CD4/8 coreceptor expression and masquerade as double-negative (DN) TCRαβ^hi thymocytes. Although these DN thymocytes fail to re-express coreceptors after OP9-DL1 culture, they eventually mature and accumulate in the spleen where TCR and IL-15/STAT5 signaling promotes their conversion to CD8αα cells and their expression of gut-homing receptors. Adoptive transfer of splenic DN cells gives rise to CD8αα cells in the gut, establishing their precursor relationship in vivo. Interestingly, Bim does not restrict the IL-15-driven maturation of CD8αα cells that is critical for intestinal homeostasis. Thus, we found a temporal and tissue-specific role for Bim in limiting thymic agonist selection of CD8αα precursors and their TCRβ repertoire, but not in the maintenance of CD8αα intraepithelial lymphocytes in the intestine.

Non-Hematopoietic and Hematopoietic SIRPα Signaling Differently Regulates Murine B Cell Maturation in Bone Marrow and Spleen

B lymphocyte development occurs in the bone marrow, while final differentiation and maturation can occur in both the bone marrow and the spleen. Here we provide evidence that signal regulatory protein α (SIRPα), an Ig-superfamily ITIM-receptor expressed by myeloid but not by lymphoid cells, is involved in regulating B cell maturation. Lack of SIRPα signaling in adult SIRPα-mutant mice resulted in a reduced maturation of B cells in the bone marrow, evident by reduced numbers of semi-mature IgD⁺IgM^hi follicular type-II (F-II) and mature IgD⁺IgM^lo follicular type-I (F-I) B cells, as well as reduced blood B cell numbers. In addition, lack of SIRPα signaling also impaired follicular B cell maturation in the spleen. Maturing BM or splenic B cells of SIRPα-mutant mice were found to express higher levels of the pro-apoptotic protein BIM and apoptosis was increased among these B cells. Bone marrow reconstitution experiments revealed that the B cell maturation defect in bone marrow and blood was due to lack of SIRPα signaling in non-hematopoietic cells, while hematopoietic SIRPα signaling was important for follicular B cell maturation in the spleen. Adding on to our previous findings of a stromal cell defect in SIRPα-mutant mice was the finding that gene expression of receptor activator of nuclear factor-ĸB ligand (RANKL) was significantly lower in cultured bone marrow stromal cells of SIRPα mutant mice. These data suggest a novel and opposite contribution of SIRPα signaling within non-hematopoietic and hematopoietic cells, respectively, to maintain B cell maturation and to prevent apoptosis in the bone marrow and spleen of adult mice.

miR-148a is upregulated by Twist1 and T-bet and promotes Th1-cell survival by regulating the proapoptotic gene Bim

Repeatedly activated T helper 1 (Th1) cells present during chronic inflammation can efficiently adapt to the inflammatory milieu, for example, by expressing the transcription factor Twist1, which limits the immunopathology caused by Th1 cells. Here, we show that in repeatedly activated murine Th1 cells, Twist1 and T-bet induce expression of microRNA-148a (miR-148a). miR-148a regulates expression of the proapoptotic gene Bim, resulting in a decreased Bim/Bcl2 ratio. Inhibition of miR-148a by antagomirs in repeatedly activated Th1 cells increases the expression of Bim, leading to enhanced apoptosis. Knockdown of Bim expression by siRNA in miR-148a antagomir-treated cells restores viability of the Th1 cells, demonstrating that miR-148a controls survival by regulating Bim expression. Thus, Twist1 and T-bet not only control the differentiation and function of Th1 cells, but also their persistence in chronic inflammation.

Continuous T cell receptor signals maintain a functional regulatory T cell pool

Regulatory T (Treg) cells maintain immune homeostasis and prevent inflammatory and autoimmune responses. During development, thymocytes bearing a moderately self-reactive T cell receptor (TCR) can be selected to become Treg cells. Several observations suggest that also in the periphery mature Treg cells continuously receive self-reactive TCR signals. However, the importance of this inherent autoreactivity for Treg cell biology remains poorly defined. To address this open question, we genetically ablated the TCR of mature Treg cells in vivo. These experiments revealed that TCR-induced Treg lineage-defining Foxp3 expression and gene hypomethylation were uncoupled from TCR input in mature Treg cells. However, Treg cell homeostasis, cell-type-specific gene expression and suppressive function critically depend on continuous triggering of their TCR.

Bcl-2-enhanced efficacy of microtubule-targeting chemotherapy through Bim overexpression: implications for cancer treatment

Bcl-2 is commonly overexpressed in tumors, where it is often associated with unfavorable outcome. However, it has also been linked to a favorable sensitivity to microtubule-targeting agents (MTAs). We show that Bcl-2-overexpressing lung and breast cancer cells were more sensitive to both paclitaxel and vinorelbine. Bcl-2 over-expression also significantly potentiated in vivo efficacy of paclitaxel, in terms of tumor volume decrease and survival benefits, in models of nude mice bearing lung cancer xenografts. To further investigate this favorable effect of Bcl-2, a genomic approach was taken. It revealed that Bcl-2 overexpression induced up-regulation of the proapoptotic protein Bim in lung cancer cells and that, conversely, Bcl-2 silencing decreased Bim expression level. A gene regulation study implicated the transcription factor Forkhead box-containing protein, class O3a in Bim up-regulation. Lastly, we show that Bim was responsible for MTA-triggered lung cancer cell death through a dynamin-related protein 1-mediated mitochondrial fragmentation. The Bcl-2-governed Bim induction evidence offers for the first time an explanation for the favorable higher sensitivity to treatment shown by Bcl-2-overexpressing cells. We suggest that Bim could be a powerful predictive factor for tumor response to MTA chemotherapy. Our data also give new insight into some failures in the efficacy of therapies targeted against Bcl-2.

Examination of a second node of translational control in the unfolded protein response

The unfolded protein response (UPR) is a largely cytoprotective signaling cascade that acts to re-establish homeostasis of the endoplasmic reticulum (ER) under conditions of stress by inducing an early and transient block in general protein synthesis and by increasing the folding and degradative capacity of the cell through an extensive transcriptional program. It is well established that the mechanism for the early translational attenuation during ER stress occurs through phosphorylation of eukaryotic initiation factor 2 α (eIF2α) by activated PERK. Our data demonstrate that when eIF2α is dephosphorylated translation is not fully restored to pre-stressed levels. We found that this correlates with reduced mTOR activity and as a result decreases phosphorylation of 4E-BP1, which negatively regulates assembly of the eIF4F complex and cap-dependent translation. The decrease in mTOR activity and 4E-BP1 phosphorylation is associated with activation of AMP kinase, a negative regulator of mTOR, and in the case of some stress conditions, downregulation of signaling through key components of the PI3K pathway. Furthermore, we show that there is a subset of mRNAs that does not recover from UPR-induced translational repression, including those whose translation is particularly sensitive to loss of eIF4F, such as cyclin D1, Bcl-2 and MMP-9. Together these data implicate reduced mTOR activity and 4E-BP1 hypophosphorylation as a second, more restricted mechanism of translational control occurring somewhat later in the UPR.

IL-15 Fosters Age-Driven Regulatory T Cell Accrual in the Face of Declining IL-2 Levels

We and others have shown that regulatory T cells (T_reg) accumulate dramatically with age in both humans and mice. Such T_reg accrual contributes to age-related immunosenescence as they reduce the response to tumors and parasite infection. While we reported earlier that aged T_reg have decreased expression of the pro-apoptotic molecule Bim and germline deletion of Bim promoted earlier accumulation of T_reg, it remains unclear whether the effects of Bim are: (i) T_reg intrinsic and (ii) dominant to other BH3-only pro-apoptotic molecules. Further, the mechanism(s) controlling Bim expression in aged Treg remain unclear. Here we show that T_reg-specific loss of Bim is sufficient to drive T_reg accrual with age and that additional loss of the downstream apoptotic effectors Bax and Bak did not exacerbate T_reg accumulation. Further, our results demonstrate that a subpopulation of T_reg expands with age and is characterized by lower expression of CD25 (IL-2Rα) and Bim. Mechanistically, we found that IL-2 levels decline with age and likely explain the emergence of CD25^loBim^lo T_reg because T_reg in IL-2^−/− mice are almost entirely comprised of CD25^loBim^lo cells, and IL-2 neutralization increases CD25^loBim^lo T_reg in both young and middle-aged mice. Interestingly, the T_reg population in aged mice had increased expression of CD122 (IL-2/IL-15Rβ) and neutralization or genetic loss of IL-15 led to less T_reg accrual with age. Further, the decreased T_reg accrual in middle-aged IL-15^−/− mice was restored by the additional loss of Bim (IL-15^−/−Bim^−/−). Together, our data show that aging favors the accrual of CD25^lo T_reg whose homeostasis is supported by IL-15 as IL-2 levels become limiting. These data have implications for manipulating T_reg to improve immune responses in the elderly.

Intrinsic role of FoxO3a in the development of CD8+ T cell memory

CD8(+) T cells undergo rapid expansion during infection with intracellular pathogens, which is followed by swift and massive culling of primed CD8(+) T cells. The mechanisms that govern the massive contraction and maintenance of primed CD8(+) T cells are not clear. We show in this study that the transcription factor, FoxO3a, does not influence Ag presentation and the consequent expansion of CD8(+) T cell response during Listeria monocytogenes infection, but plays a key role in the maintenance of memory CD8(+) T cells. The effector function of primed CD8(+) T cells as revealed by cytokine secretion and CD107a degranulation was not influenced by inactivation of FoxO3a. Interestingly, FoxO3a-deficient CD8(+) T cells displayed reduced expression of proapoptotic molecules BIM and PUMA during the various phases of response, and underwent reduced apoptosis in comparison with wild-type cells. A higher number of memory precursor effector cells and memory subsets was detectable in FoxO3a-deficient mice compared with wild-type mice. Furthermore, FoxO3a-deficient memory CD8(+) T cells upon transfer into normal or RAG1-deficient mice displayed enhanced survival. These results suggest that FoxO3a acts in a cell-intrinsic manner to regulate the survival of primed CD8(+) T cells.

Protecting and rescuing the effectors: roles of differentiation and survival in the control of memory T cell development

Vaccines, arguably the single most important intervention in improving human health, have exploited the phenomenon of immunological memory. The elicitation of memory T cells is often an essential part of successful long-lived protective immunity. Our understanding of T cell memory has been greatly aided by the development of TCR Tg mice and MHC tetrameric staining reagents that have allowed the precise tracking of antigen-specific T cell responses. Indeed, following acute infection or immunization, naïve T cells undergo a massive expansion culminating in the generation of a robust effector T cell population. This peak effector response is relatively short-lived and, while most effector T cells die by apoptosis, some remain and develop into memory cells. Although the molecular mechanisms underlying this cell fate decision remain incompletely defined, substantial progress has been made, particularly with regards to CD8(+) T cells. For example, the effector CD8(+) T cells generated during a response are heterogeneous, consisting of cells with more or less potential to develop into full-fledged memory cells. Development of CD8(+) T cell memory is regulated by the transcriptional programs that control the differentiation and survival of effector T cells. While the type of antigenic stimulation and level of inflammation control effector CD8(+) T cell differentiation, availability of cytokines and their ability to control expression and function of Bcl-2 family members governs their survival. These distinct differentiation and survival programs may allow for finer therapeutic intervention to control both the quality and quantity of CD8(+) T cell memory. Effector to memory transition of CD4(+) T cells is less well characterized than CD8(+) T cells, emerging details will be discussed. This review will focus on the recent progress made in our understanding of the mechanisms underlying the development of T cell memory with an emphasis on factors controlling survival of effector T cells.

Helios marks strongly autoreactive CD4+ T cells in two major waves of thymic deletion distinguished by induction of PD-1 or NF-κB

Expression of the transcription factor Helios identifies thymocyte divergence during positive and negative selection.

Acquisition of self-tolerance in the thymus requires T cells to discriminate strong versus weak T cell receptor binding by self-peptide–MHC complexes. We find this discrimination is reported by expression of the transcription factor Helios, which is induced during negative selection but decreases during positive selection. Helios and the proapoptotic protein Bim were coinduced in 55% of nascent CCR7⁻ CD4⁺ CD69⁺ thymocytes. These were short-lived cells that up-regulated PD-1 and down-regulated CD4 and CD8 during Bim-dependent apoptosis. Helios and Bim were also coinduced at the subsequent CCR7⁺ CD4⁺ CD69⁺ CD8⁻ stage, and this second wave of Bim-dependent negative selection involved 20% of nascent cells. Unlike CCR7⁻ counterparts, Helios⁺ CCR7⁺ CD4⁺ cells mount a concurrent Card11- and c-Rel–dependent activation response that opposes Bim-mediated apoptosis. This “hollow” activation response consists of many NF-κB target genes but lacks key growth mediators like IL-2 and Myc, and the thymocytes were not induced to proliferate. These findings identify Helios as the first marker known to diverge during positive and negative selection of thymocytes and reveal the extent, stage, and molecular nature of two distinct waves of clonal deletion in the normal thymus.

The BH3-only proteins Bim and Puma cooperate to impose deletional tolerance of organ-specific antigens

Although the proapoptotic BH3-only protein, Bim, is required for deletion of autoreactive thymocytes, Bim-deficient mice do not succumb to extensive organ-specific autoimmune disease. To determine whether other BH3-only proteins safeguard tolerance in the absence of Bim, we screened mice lacking Bim as well as other BH3-only proteins. Most strains showed no additional defects; however, mice deficient for both Puma and Bim spontaneously developed autoimmunity in multiple organs, and their T cells could transfer organ-specific autoimmunity. Puma- and Bim-double-deficient mice had a striking accumulation of mature, single-positive thymocytes, suggesting an additional defect in thymic deletion was the basis for disease. Transgenic mouse models of thymocyte deletion by peripheral neoantigens confirmed that the loss of Bim and Puma allowed increased numbers of autoreactive thymocytes to escape deletion. Our data show that Puma cooperates with Bim to impose a thymic-deletion checkpoint to peripheral self-antigens and cement the notion that defects in apoptosis alone are sufficient to cause autoimmune disease.

Elevated Mcl-1 inhibits thymocyte apoptosis and alters thymic selection

T cells developing in the thymus undergo rigorous positive and negative selection to ensure that those exported to peripheral lymphoid organs bear T-cell receptors (TCRs) capable of reacting with foreign antigens but tolerant of self. At each checkpoint, whether a thymocyte survives or dies is determined by antiapoptotic and proapoptotic Bcl-2 family members. We used Mcl-1 transgenic (tg) mice to investigate the impact of elevated expression of antiapoptotic Mcl-1 on thymocyte apoptosis and selection, making a side-by-side comparison with thymocytes from BCL-2tg mice. Mcl-1 was as effective as Bcl-2 at protecting thymocytes against spontaneous cell death, diverse cytotoxic insults and TCR-CD3 stimulation-driven apoptosis. In three different TCR tg models, Mcl-1 markedly enhanced positive selection of thymocytes, as did Bcl-2. In H-Y TCR tg mice, elevated Mcl-1 and Bcl-2 were equally effective at inhibiting deletion of autoreactive thymocytes. However, in the OT-1tg model where deletion is mediated by a peripheral antigen whose expression is regulated by Aire, Mcl-1 was less effective than Bcl-2. Thus, the capacity of Mcl-1 overexpression to inhibit apoptosis triggered by TCR stimulation apparently depends on the thymocyte subset subject to deletion, presumably due to differences in the profiles of proapoptotic Bcl-2 family members mediating the deletion.

Bcl-2, Bcl-x(L), and Bcl-w are not equivalent targets of ABT-737 and navitoclax (ABT-263) in lymphoid and leukemic cells

The BH3-mimetic ABT-737 and an orally bioavailable compound of the same class, navitoclax (ABT-263), have shown promising antitumor efficacy in preclinical and early clinical studies. Although both drugs avidly bind Bcl-2, Bcl-x(L), and Bcl-w in vitro, we find that Bcl-2 is the critical target in vivo, suggesting that patients with tumors overexpressing Bcl-2 will probably benefit. In human non-Hodgkin lymphomas, high expression of Bcl-2 but not Bcl-x(L) predicted sensitivity to ABT-263. Moreover, we show that increasing Bcl-2 sensitized normal and transformed lymphoid cells to ABT-737 by elevating proapoptotic Bim. In striking contrast, increasing Bcl-x(L) or Bcl-w conferred robust resistance to ABT-737, despite also increasing Bim. Cell-based protein redistribution assays unexpectedly revealed that ABT-737 disrupts Bcl-2/Bim complexes more readily than Bcl-x(L)/Bim or Bcl-w/Bim complexes. These results have profound implications for how BH3-mimetics induce apoptosis and how the use of these compounds can be optimized for treating lymphoid malignancies.

Alternative splicing of Bim and Erk-mediated Bim(EL) phosphorylation are dispensable for hematopoietic homeostasis in vivo

The pro-apoptotic BH3-only protein Bim has a major role in hematopoietic homeostasis, particularly in the lymphocyte compartment, where it strongly affects immune function. The three major Bim isoforms (Bim(EL), Bim(L) and Bim(S)) are generated by alternative splicing. Bim(EL), the most abundant isoform, contains a unique sequence that has been reported to be the target of phosphorylation by several MAP kinases. In particular, Erk1/2 has been shown to interact with Bim(EL) through the DEF2 domain of Bim(EL) and specifically phosphorylate this isoform, thereby targeting it for ubiquitination and proteasomal degradation. To examine the physiological importance of this mechanism of regulation and of the alternative splicing of Bim, we have generated several Bim knock-in mouse strains and analyzed their hematopoietic system. Although mutation in the DEF2 domain reduces Bim(EL) degradation in some circumstances, this mutation did not significantly increase Bim's pro-apoptotic activity in vivo nor impact on the homeostasis of the hematopoietic system. We also show that Bim(EL) and Bim(L) are interchangeable, and that Bim(S) is dispensable for the function of Bim. Hence, we conclude that physiological regulation of Bim relies on mechanisms independent of its alternative splicing or the Erk-dependent phosphorylation of Bim(EL).

Proapoptotic protein Bim is differentially required during thymic clonal deletion to ubiquitous versus tissue-restricted antigens

Positive and negative selection of thymocytes in the thymus are critical for the development of a mature and self-tolerant T-cell repertoire. The proapoptotic Bcl-2 family member Bim is important for negative selection by inducing apoptosis in thymocytes receiving a strong signal through their antigen receptor. However, in the case of ubiquitous self-antigens (UbA), Bim is not required for the clonal deletion of self-reactive thymocytes, suggesting the existence of nonapoptotic clonal deletion mechanisms. Unlike UbA, clonal deletion to tissue-restricted antigens (TRAs) requires positive selection and CCR7-mediated migration to the medulla. This led us to hypothesize that Bim is required for the latter. To study the role of Bim in clonal deletion to TRA, we constructed bone marrow (BM) chimeras using OT-I Bim-deficient or -sufficient donor bone marrow and recipients that express membrane bound chicken ovalbumin under control of the rat insulin promoter (Rip-mOVA). We found that clonal deletion to TRA was completely abrogated in the absence of Bim and large numbers of mature OT-I CD8 T cells survived in the periphery. Despite the large numbers of autoreactive T cells, the chimeras did not develop diabetes and OT-I Bim-deficient T cells from these chimeras were functionally impaired. Collectively, these data provide unique evidence of a differential, thymocyte-intrinsic, molecular requirement downstream of the T-cell receptor (TCR) for clonal deletion to UbA versus TRA and highlight the profound ability of other tolerance mechanisms to control T-cell autoreactivity in the absence of thymic clonal deletion.

Bcl-2 allows effector and memory CD8+ T cells to tolerate higher expression of Bim

As acute infections resolve, most effector CD8(+) T cells die, whereas some persist and become memory T cells. Recent work showed that subsets of effector CD8(+) T cells, identified by reciprocal expression of killer cell lectin-like receptor G1 (KLRG1) and CD127, have different lifespans. Similar to previous reports, we found that effector CD8(+) T cells reported to have a longer lifespan (i.e., KLRG1(low)CD127(high)) have increased levels of Bcl-2 compared with their shorter-lived KLRG1(high)CD127(low) counterparts. Surprisingly, we found that these effector KLRG1(low)CD127(high) CD8(+) T cells also had increased levels of Bim compared with KLRG1(high)CD127(low) cells. Similar effects were observed in memory cells, in which CD8(+) central memory T cells expressed higher levels of Bim and Bcl-2 than did CD8(+) effector memory T cells. Using both pharmacologic and genetic approaches, we found that survival of both subsets of effector and memory CD8(+) T cells required Bcl-2 to combat the proapoptotic activity of Bim. Interestingly, inhibition or absence of Bcl-2 led to significantly decreased expression of Bim in surviving effector and memory T cells. In addition, manipulation of Bcl-2 levels by IL-7 or IL-15 also affected expression of Bim in effector CD8(+) T cells. Finally, we found that Bim levels were significantly increased in effector CD8(+) T cells lacking Bax and Bak. Together, these data indicate that cells having the highest levels of Bim are selected against during contraction of the response and that Bcl-2 determines the level of Bim that effector and memory T cells can tolerate.

BH3-only protein Bik is involved in both apoptosis induction and sensitivity to oxidative stress in multiple myeloma

Background:

Although gene expression profile of multiple myeloma (MM) patients shows a wide range of Bik/Nbk expression, varying from absent to high, its regulation and function in myeloma cells is poorly understood. Thus, we addressed these questions in MM.

Methods:

Human myeloma cell lines (HMCLs) and primary purified myeloma cells were studied for Bcl-2 family protein expression by western blot and further correlation analysis was performed. Correlative study between Bik and thyrotroph embryonic factor (TEF) transcription factor expression was analysed by PCR. Stress oxidative response was analysed by flow cytometry.

Results:

A strong expression of Bik protein was found only in one out of three of HMCL and correlated to Bcl-2 expression (P=0.0006). We demonstrated that Bik could be regulated at the protein level by Bcl-2 and at the transcriptional level by TEF. Bik overexpression sensitises myeloma cells to oxidative stress whereas Bik silencing increases resistance to H₂O₂ oxidative stress. Furthermore, Bik ectopic expression disrupts Bim/Bcl-2 and Bim/Bcl-xL endogenous complexes triggering Bim release that could induce Bax and Bak activation.

Conclusions:

Ours results suggest that Bik has a role in both, apoptosis induction and sensitivity to oxidative stress in myeloma cells. Small BH3 mimetic molecules should be considered for further apoptosis-based therapy in myeloma cells expressing endogenous Bik/Bcl-2 complexes.

Contracting the 'mus cells'--does down-sizing suit us for diving into the memory pool?

Maintenance of T-cell homeostasis is critical for normal functioning of the immune system. After thymocyte selection, T cells enter the peripheral lymphoid organs, where they are maintained as naive cells. Transient disruption of homeostasis occurs when naive T cells undergo antigen-driven expansion and acquire effector functions. Effector T cells then either undergo apoptosis (i.e. contraction at the population level) or survive to become memory cells. This apoptotic process is crucial: it resets T-cell homeostasis, promotes protective immunity, and limits autoimmunity. Although initial studies using in vitro models supported a role for death receptor signaling, more recent in vivo studies have implicated Bcl-2 family members as being critical for the culling of T-cell responses. While several Bcl-2 family members likely contribute to T-cell contraction, the pro-apoptotic molecule Bim and its anti-apoptotic antagonist Bcl-2 are essential regulators of the process. This review discusses the progress made in our understanding of the mechanisms underlying contraction of T-cell responses and how some cells avoid this cell death and become memory T cells.

Bim dictates naive CD4 T cell lifespan and the development of age-associated functional defects

With age, peripheral naive CD4 T cells become both longer lived and functionally impaired and they express reduced levels of Bim, a proapoptotic Bcl family member. In this study, we show that reduced Bim expression by naive CD4 T cells intrinsically mediates their longer lifespan in the periphery. Moreover, using mixed bone marrow chimeras reconstituted with Bim(+/+) and Bim(+/-) bone marrow cells, Bim(+/-) naive CD4 T cells exhibit accelerated development of age-associated dysfunctions, including reduced proliferation and IL-2 production and defective helper function for B cells, without any increase in their turnover. However, newly generated Bim(+/-) naive CD4 T cells in middle-aged mice are not defective, indicating an additional requirement for their persistence in the periphery. These age-associated immune defects develop independently of the "aged" host environment and without extensive division, distinguishing them from classic "senescence." We suggest that the reduction of Bim levels with age in naive CD4 T cell is the initiating step that leads to increased cellular lifespan and development of age-associated functional defects.

Bcl-2-regulated cell death signalling in the prevention of autoimmunity. Cell Death Dis. 2010;1:e48. [PMID: 21364654 DOI: 10.1038/cddis.2010.27]

Cell death mediated through the intrinsic, Bcl-2-regulated mitochondrial apoptosis signalling pathway is critical for lymphocyte development and the establishment of central and maintenance of peripheral tolerance. Defects in Bcl-2-regulated cell death signalling have been reported to cause or correlate with autoimmunity in mice and men. This review focuses on the role of Bcl-2 family proteins implicated in the development of autoimmune disorders and their potential as targets for therapeutic intervention.

Bim-mediated apoptosis is not necessary for thymic negative selection to ubiquitous self-antigens

T cell education in the thymus is critical for establishing a functional, yet self-tolerant, T cell repertoire. Negative selection is a key process in enforcing self-tolerance. There are many questions that surround the mechanism of negative selection, but it is currently held that apoptosis initiated by Bim and/or Nur77 is critical for negative selection. Recent studies, however, have questioned the necessity of Bim in maintaining both central and peripheral T cell tolerance. To reconcile these apparently contradictory findings, we examined the role of Bim in negative selection in the well-characterized, physiological HY(cd4) mouse model. We found that while Bim expression was required for CD4(+)CD8(+) double-positive thymocyte apoptosis, it was not required for negative selection. Furthermore, Bim deficiency did not alter the frequency or affinity of male reactive cells that escape negative selection in an oligoclonal repertoire. Collectively, these studies indicate that negative selection occurs efficiently in the absence of apoptosis and suggest that the current paradigm of negative selection requiring apoptosis be revisited.

Modulation of the Bcl-2 family blocks sepsis-induced depletion of dendritic cells and macrophages

This study examined the fate of dendritic cells (DCs) and macrophages (M Phi) in vivo in a murine model of sepsis. Wild-type, knockout, and transgenic mice were used to examine the role of Bcl-2 family members on the regulation of splenic DCs and M Phi survival. Bim knockout (Bim) mice and mice overexpressing Bcl-2 in selected hematopoietic cells were used: (a) overexpression of Bcl-2 in all hematopoietic cells using a vav promoter (Vav-Bcl-2) and (b) overexpression of Bcl-2 in all Major histocompatibility complex (MHC) class I cells (H-2K-Bcl-2). Mice underwent sham surgery or cecal ligation and puncture, and absolute numbers of splenic DCs and M Phi were determined. Importantly, two distinct M Phi populations, that is, well-differentiated "mature" M Phi population and a less differentiated "immature," "monocyte-like" (IM Phi) population were identified that demonstrated differential susceptibility to apoptosis. In wild-type mice, sepsis induced a 64% +/- 7% and a 77% +/- 3% decrease in absolute cell numbers of splenic DCs and IM Phi, respectively (n = 7, P < 0.05). Mature M Phi were not depleted in sepsis. No significant cell depletion was evident in Vav-Bcl-2, H-2K-Bcl-2, or Bim mice. We conclude that sepsis induces a major depletion of developing M Phi as well as DCs, and this depletion may be an important mechanism of immune suppression in sepsis.

Enhancement of dendritic cell-based vaccine potency by anti-apoptotic siRNAs targeting key pro-apoptotic proteins in cytotoxic CD8(+) T cell-mediated cell death

Dendritic cells (DCs) have become an important measure for the treatment of malignancies. Current DC preparations, however, generate short-lived DCs because they are subject to cell death from various apoptotic pressures. Antigen-specific CD8(+) cytotoxic T lymphocytes (CTLs) is one of the main obstacles to limit the DC-mediated immune priming since CTLs can recognize the target antigen expressing DCs as target cells and kill the DCs. CTLs secret perforin and serine protease granzymes during CTL killing. Perforin and serine protease granzymes induce the release of a number of mitochondrial pro-apoptotic factors, which are controlled by members of the BCL-2 family, such as BAK, BAX and BIM. FasL linking to Fas on DCs triggers the activation of caspase-8, which eventually leads to mitochondria-mediated apoptosis via truncation of BID. In this study, we tried to enhance the DC priming capacity by prolonging DC survival using anti-apoptotic siRNA targeting these key pro-apoptotic molecules in CTL killing. Human papillomavirus (HPV)-16 E7 antigen presenting DCs that were transfected with these anti-apoptotic siRNAs showed increased resistance to T cell-mediated death, leading to enhanced E7-specific CD8(+) T cell activation in vitro and in vivo. Among them, siRNA targeting BIM (siBIM) generated strongest E7-specific E7-specific CD8(+) T cell immunity. More importantly, vaccination with E7 presenting DCs transfected with siBIM was capable of generating a marked therapeutic effect in vaccinated mice. Our data indicate that ex vivo manipulation of DCs with siBIM may represent a plausible strategy for enhancing dendritic cell-based vaccine potency.

Genetic control of programmed cell death during animal development

The elimination of unwanted cells by programmed cell death is a common feature of animal development. Genetic studies in the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse have not only revealed the molecular machineries that cause the programmed demise of specific cells, but have also allowed us to get a glimpse of the types of pathways that regulate these machineries during development. Rather than serving as a broad overview of programmed cell death during development, this review focuses on recent advances in our understanding of the regulation of specific programmed cell death events during nematode, fly, and mouse development. Recent studies have revealed that many of the regulatory pathways involved play additional important roles in development, which confirms that the programmed cell death fate is an integral aspect of animal development.

A novel method for long term bone marrow culture and genetic modification of murine neutrophils via retroviral transduction

Neutrophils are a critical component of the innate immune response to invading microbial pathogens. However, an excessive and/or prolonged neutrophil response can result in tissue injury that is thought to underlie the pathogenesis of various inflammatory diseases. The development of novel therapeutic strategies for inflammatory diseases depends on an improved understanding of regulation of neutrophil function. However, investigations into neutrophil function have been constrained in part by the difficulty of genetically modifying neutrophils using current techniques. To overcome this, we have developed a novel method for the genetic modification of murine bone marrow derived progenitor cells using retroviral transduction followed by long term bone marrow culture to generate mature neutrophils. These neutrophils are functionally mature as determined by morphology, surface marker (Gr1, CD11b, CD62L and CXCR2) expression, and functional attributes including the ability to generate superoxide, exocytose granule contents, chemotax, and phagocytose and kill bacteria. Further, the in vitro matured neutrophils are capable of migrating to an inflammatory site in vivo. We utilized this system to express the Bcl-2 transgene in mature neutrophils using the retroviral vectors pMIG and pMIT. Bcl-2 overexpression conferred a substantial delay in spontaneous apoptosis of neutrophils as assessed by annexin V and 7-amino-actinomycin D (7AAD) staining. Moreover, Bcl-2 overexpression did not alter granulopoiesis, as assessed by morphology and surface marker expression. This system enables the genetic manipulation of progenitor cells that can be differentiated in vitro to mature neutrophils that are functional in vitro and in vivo.