Haematopoietic stem cell survival and transplantation efficacy is limited by the BH3-only proteins Bim and Bmf

Non-viral DNA delivery and TALEN editing correct the sickle cell mutation in hematopoietic stem cells

Sickle cell disease is a devastating blood disorder that originates from a single point mutation in the HBB gene coding for hemoglobin. Here, we develop a GMP-compatible TALEN-mediated gene editing process enabling efficient HBB correction via a DNA repair template while minimizing risks associated with HBB inactivation. Comparing viral versus non-viral DNA repair template delivery in hematopoietic stem and progenitor cells in vitro, both strategies achieve comparable HBB correction and result in over 50% expression of normal adult hemoglobin in red blood cells without inducing β-thalassemic phenotype. In an immunodeficient female mouse model, transplanted cells edited with the non-viral strategy exhibit higher engraftment and gene correction levels compared to those edited with the viral strategy. Transcriptomic analysis reveals that non-viral DNA repair template delivery mitigates P53-mediated toxicity and preserves high levels of long-term hematopoietic stem cells. This work paves the way for TALEN-based autologous gene therapy for sickle cell disease.

Lentivirus-Mediated BCL-XL Overexpression Inhibits Stem Cell Apoptosis during Ex Vivo Expansion and Provides Competitive Advantage Following Xenotransplantation

Hematopoietic reconstitution after hematopoietic stem cell transplantation (HSCT) is influenced by the number of transplanted cells. However, under certain conditions donor cell counts are limited and impair clinical outcome. Hematopoietic stem and progenitor cell (HSPC) expansion prior to HSCT is a widely used method to achieve higher donor cell counts and minimize transplantation-related risks such as graft failure or delayed engraftment. Still, expansion in a non-physiological environment can trigger cell death mechanisms and hence counteract the desired effect. We have shown earlier that during HSCT a relevant amount of HSPCs were lost due to apoptosis and that cell death inhibition in donor HSPCs improved engraftment in xenotransplantation experiments. Here, we assessed the effect of combined ex vivo expansion and cell death inhibition on HSPC yield and their reconstitution potential in vivo. During expansion with cytokines and the small molecule inhibitor StemRegenin 1, concomitant lentiviral overexpression of antiapoptotic BCL-XL resulted in an increased yield of transduced HSPCs. Importantly, BCL-XL overexpression enhanced the reconstitution potential of HSPCs in xenotransplantation experiments in vivo. In contrast, treatment with caspase and necroptosis inhibitors had no favorable effects on HSPC yields nor on cell viability. We postulate that overexpression of antiapoptotic BCL-XL, both during ex vivo expansion and transplantation, is a promising approach to improve the outcome of HSCT in situations with limited donor cell numbers. However, such apoptosis inhibition needs to be transient to avoid long-term sequelae like leukemia.

Blastocyst complementation and interspecies chimeras in gene edited pigs

The only curative therapy for many endstage diseases is allograft organ transplantation. Due to the limited supply of donor organs, relatively few patients are recipients of a transplanted organ. Therefore, new strategies are warranted to address this unmet need. Using gene editing technologies, somatic cell nuclear transfer and human induced pluripotent stem cell technologies, interspecies chimeric organs have been pursued with promising results. In this review, we highlight the overall technical strategy, the successful early results and the hurdles that need to be addressed in order for these approaches to produce a successful organ that could be transplanted in patients with endstage diseases.

SNAI2-Mediated Repression of BIM Protects Rhabdomyosarcoma from Ionizing Radiation

Ionizing radiation (IR) and chemotherapy are mainstays of treatment for patients with rhabdomyosarcoma, yet the molecular mechanisms that underlie the success or failure of radiotherapy remain unclear. The transcriptional repressor SNAI2 was previously identified as a key regulator of IR sensitivity in normal and malignant stem cells through its repression of the proapoptotic BH3-only gene PUMA/BBC3. Here, we demonstrate a clear correlation between SNAI2 expression levels and radiosensitivity across multiple rhabdomyosarcoma cell lines. Modulating SNAI2 levels in rhabdomyosarcoma cells through its overexpression or knockdown altered radiosensitivity in vitro and in vivo. SNAI2 expression reliably promoted overall cell growth and inhibited mitochondrial apoptosis following exposure to IR, with either variable or minimal effects on differentiation and senescence, respectively. Importantly, SNAI2 knockdown increased expression of the proapoptotic BH3-only gene BIM, and chromatin immunoprecipitation sequencing experiments established that SNAI2 is a direct repressor of BIM/BCL2L11. Because the p53 pathway is nonfunctional in the rhabdomyosarcoma cells used in this study, we have identified a new, p53-independent SNAI2/BIM signaling axis that could potentially predict clinical responses to IR treatment and be exploited to improve rhabdomyosarcoma therapy. SIGNIFICANCE: SNAI2 is identified as a major regulator of radiation-induced apoptosis in rhabdomyosarcoma through previously unknown mechanisms independent of p53.

Inhibition of S-adenosyl-L-homocysteine hydrolase alleviates alloimmune response by down-regulating CD4+ T-cell activation in a mouse heart transplantation model

Background

Transmethylation reactions play an important role on lymphocyte activation and function. S-adenosyl-L-homocysteine hydrolase (SAHH) inhibitors prevent the feedback of transmethylation reactions by S-adenosyl-L-homocysteine (SAH) accumulation, a competitive antagonist of S-adenosylmethionine (SAM)-dependent methyltransferases. However, the role of SAH in solid organ transplantation is currently unclear.

Methods

A murine model of cardiac transplantation (BALB/C to C57B/6) was established to assess allograft survival, histology, and T cell infiltration. The reversible SAHH inhibitor, DZ2002, and irreversible SAHH inhibitor, adenosine dialdehyde (AdOx), were used to assess their immunosuppressive effects in murine cardiac transplantation, compared with mice with DMSO.

Results

Both SAHH inhibitors prolonged the survival of cardiac allografts and alleviated alloimmune response. Notably, AdOx and DZ2002 both eliminated frequencies of Th1 and Th17 in CD4⁺ T cells in cardiac transplantation, and reduced the frequency of active CD4⁺ T cell (CD44⁺ CD62L⁻). The irreversible SAHH inhibitor facilitated the differentiation of regulatory T cells (Tregs) and increased Bim expression. Furthermore, both SAHH inhibitors alleviated infiltration of CD4⁺ T cells in cardiac allografts.

Conclusions

The SAHH inhibitors (AdOx and DZ2002) alleviates allograft rejection in cardiac transplantation by inhibition of CD4⁺ T alloimmune response. SAHH inhibitors, especially DZ2002, is a promising complementary therapeutic agent in organ transplantation.

Inhibition of the anti-apoptotic protein MCL-1 severely suppresses human hematopoiesis

BH3-mimetics inhibiting anti-apoptotic BCL-2 proteins represent a novel and promising class of antitumor drugs. While the BCL-2 inhibitor venetoclax is already approved by the Food and Drug Administration, BCL-XL and MCL-1 inhibitors are currently in early clinical trials. To predict side effects of therapeutic MCL-1 inhibition on the human hematopoietic system, we used RNA interference and the small molecule inhibitor S63845 on cord blood-derived CD34⁺ cells. Both approaches resulted in almost complete depletion of human hematopoietic stem and progenitor cells. As a consequence, maturation into the different hematopoietic lineages was severely restricted and CD34⁺ cells expressing MCL-1 shRNA showed a very limited engraftment potential upon xenotransplantation. In contrast, mature blood cells survived normally in the absence of MCL-1. Combined inhibition of MCL-1 and BCL-XL resulted in synergistic effects with relevant loss of colony-forming hematopoietic stem and progenitor cells already at inhibitor concentrations of 0.1 mM each, indicating “synthetic lethality” of the two BH3- mimetics in the hematopoietic system.

BCL-XL expression is essential for human erythropoiesis and engraftment of hematopoietic stem cells

The anti-apoptotic BCL-2 proteins (BCL-2, BCL-XL, MCL-1, A1, BCL-W) counteract apoptotic signals emerging during development and under stress conditions, and are thus essential for the survival of every cell. While the “BCL-2 addiction” of different cell types is well described in mouse models, there is only limited information available on the role of different anti-apoptotic BCL-2 proteins in a given human cell type. Here we characterize the role of BCL-XL for survival and function of human hematopoietic cells, with the aim to predict hematological side effects of novel BCL-XL-inhibiting BH3-mimetics and to identify hematological malignancies potentially responsive to such inhibitors. Earlier clinical studies have shown that the combined BCL-2/BCL-XL/BCL-W inhibitor, Navitoclax (ABT-263) induces severe thrombocytopenia caused by direct platelet demise and counteracted by increased megakaryopoiesis. In contrast, murine studies have reported important contribution of BCL-XL to survival of late erythroid cells and megakaryocytes. Using lentiviral knockdown, we show that the roles of BCL-XL for human hematopoietic cells are much more pronounced than expected from murine data and clinical trials. Efficient genetic or chemical BCL-XL inhibition resulted in significant loss of human erythroid cells beginning from very early stages of erythropoiesis, and in a reduction of megakaryocytes. Most importantly, BCL-XL deficient human hematopoietic stem cells and multipotent progenitors were reduced in numbers, and they showed a severely impaired capacity to engraft in mice during xenotransplantation. BCL-XL deficiency was fully compensated by BCL-2 overexpression, however, loss of its antagonist BIM did not result in any rescue of human erythroid or stem and progenitor cells. We thus conclude that novel and specific BCL-XL inhibitors might be efficient to treat malignancies of erythroid or megakaryocytic origin, such as polycythemia vera, acute erythroid leukemia, essential thrombocytosis or acute megakaryocytic leukemia. At the same time, it can be expected that they will have more severe hematological side effects than Navitoclax.

Context-specific regulation of cell survival by a miRNA-controlled BIM rheostat

Knockout of the ubiquitously expressed miRNA-17∼92 cluster in mice produces a lethal developmental lung defect, skeletal abnormalities, and blocked B lymphopoiesis. A shared target of miR-17∼92 miRNAs is the pro-apoptotic protein BIM, central to life-death decisions in mammalian cells. To clarify the contribution of miR-17∼92:Bim interactions to the complex miR-17∼92 knockout phenotype, we used a system of conditional mutagenesis of the nine Bim 3' UTR miR-17∼92 seed matches. Blocking miR-17∼92:Bim interactions early in development phenocopied the lethal lung phenotype of miR-17∼92 ablation and generated a skeletal kinky tail. In the hematopoietic system, instead of causing the predicted B cell developmental block, it produced a selective inability of B cells to resist cellular stress; and prevented B and T cell hyperplasia caused by Bim haploinsufficiency. Thus, the interaction of miR-17∼92 with a single target is essential for life, and BIM regulation by miRNAs serves as a rheostat controlling cell survival in specific physiological contexts.

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.

In vitro and in vivo evaluation of possible pro-survival activities of PGE2, EGF, TPO and FLT3L on human hematopoiesis

Myelosuppression is a major and frequently dose-limiting side effect of anticancer therapy and is responsible for most treatment-related morbidity and mortality. In addition, repeated cycles of DNA damage and cell death of hematopoietic stem and progenitor cells, followed by compensatory proliferation and selection pressure, lead to genomic instability and pave the way for therapy-related myelodysplastic syndromes and secondary acute myeloid leukemia. Protection of hematopoietic stem and progenitor cells from chemo- and radiotherapy in patients with solid tumors would reduce both immediate complications and long-term sequelae. Epidermal growth factor (EGF) and prostaglandin E2 (PGE2) were reported to prevent chemo- or radiotherapy-induced myelosuppression in mice. We tested both molecules for potentially protective effects on human CD34⁺ cells in vitro and established a xenograft mouse model to analyze stress resistance and regeneration of human hematopoiesis in vivo. EGF was neither able to protect human stem and progenitor cells in vitro nor to promote hematopoietic regeneration following sublethal irradiation in vivo. PGE2 significantly reduced in vitro apoptotic susceptibility of human CD34⁺ cells to taxol and etoposide. This could, however, be ascribed to reduced proliferation rather than to a change in apoptosis signaling and BCL-2 protein regulation. Accordingly, 16,16-dimethyl-PGE2 (dmPGE2) did not accelerate regeneration of the human hematopoietic system in vivo. Repeated treatment of sublethally irradiated xenograft mice with known antiapoptotic substances, such as human FLT3L and thrombopoietin (TPO), which suppress transcription of the proapoptotic BCL-2 proteins BIM and BMF, also only marginally promoted human hematopoietic regeneration in vivo.

The mirn23a and mirn23b microrna clusters are necessary for proper hematopoietic progenitor cell production and differentiation

Mice deficient for microRNA (miRNA) cluster mirn23a exhibit increased B lymphopoiesis at the expense of myelopoiesis, whereas hematopoietic stem and progenitor cell (HSPC) populations are unchanged. Mammals possess a paralogous mirn23b gene that can give rise to three mature miRNAs (miR-23b, miR-24-1, and miR-27b) that have identical seed/mRNA-targeting sequences to their mirn23a counterparts. To assess whether compound deletion of mirn23a and mirn23b exacerbates the hematopoietic phenotype observed in mirn23a^-/- mice, we generated a compound mirn23a^-/-mirn23b^fl/fl:Mx1-Cre conditional knockout mouse and assayed hematopoietic development after excision of mirn23b. Loss of both genes in adult bone marrow further skewed HSPC differentiation toward B cells at the expense of myeloid cells, demonstrating a dosage-dependent effect on regulating cell differentiation. Strikingly, double-knockout (DKO) mice had decreased bone marrow cellularity with significantly decreased hematopoietic stem cell and HSPC populations, a phenotype not observed in mice deficient for mirn23a alone. Competitive transplantation assays showed decreased contribution of mirn23a^-/-mirn23b^-/- HSPCs to hematopoietic lineages at 6 and 12 weeks after transplantation. Defects in the proliferation of mirn23a^-/-b^-/- HSPCs was not observed; however, DKO cells were more apoptotic compared with both wild-type and mirn23a^-/- cells. Together, our data show that complete loss of mirn23a/mirn23b miRNAs results in decreased blood production and affects lineage output in a concentration-dependent manner.

Transient apoptosis inhibition in donor stem cells improves hematopoietic stem cell transplantation

During hematopoietic stem cell transplantation, a substantial number of donor cells are lost because of apoptotic cell death. Transplantation-associated apoptosis is mediated mainly by the proapoptotic BCL-2 family proteins BIM and BMF, and their proapoptotic function is conserved between mouse and human stem and progenitor cells. Permanent inhibition of apoptosis in donor cells caused by the loss of these BH3-only proteins improves transplantation outcome, but recipients might be exposed to increased risk of lymphomagenesis or autoimmunity. Here, we address whether transient inhibition of apoptosis can serve as a safe but efficient alternative to improve the outcome of stem cell transplantation. We show that transient apoptosis inhibition by short-term overexpression of prosurvival BCL-XL, known to block BIM and BMF, is not only sufficient to increase the viability of hematopoietic stem and progenitor cells during engraftment but also improves transplantation outcome without signs of adverse pathologies. Hence, this strategy represents a promising and novel therapeutic approach, particularly under conditions of limited donor stem cell availability.

Reduced Erg Dosage Impairs Survival of Hematopoietic Stem and Progenitor Cells

ERG, an ETS family transcription factor frequently overexpressed in human leukemia, has been implicated as a key regulator of hematopoietic stem cells. However, how ERG controls normal hematopoiesis, particularly at the stem and progenitor cell level, and how it contributes to leukemogenesis remain incompletely understood. Using homologous recombination, we generated an Erg knockdown allele (Erg^kd ) in which Erg expression can be conditionally restored by Cre recombinase. Erg^kd/kd animals die at E10.5-E11.5 due to defects in endothelial and hematopoietic cells, but can be completely rescued by Tie2-Cre-mediated restoration of Erg in these cells. In Erg^kd/+ mice, ∼40% reduction in Erg dosage perturbs both fetal liver and bone marrow hematopoiesis by reducing the numbers of Lin^- Sca-1⁺ c-Kit⁺ (LSK) hematopoietic stem and progenitor cells (HSPCs) and megakaryocytic progenitors. By genetic mosaic analysis, we find that Erg-restored HSPCs outcompete Erg^kd/+ HSPCs for contribution to adult hematopoiesis in vivo. This defect is in part due to increased apoptosis of HSPCs with reduced Erg dosage, a phenotype that becomes more drastic during 5-FU-induced stress hematopoiesis. Expression analysis reveals that reduced Erg expression leads to changes in expression of a subset of ERG target genes involved in regulating survival of HSPCs, including increased expression of a pro-apoptotic regulator Bcl2l11 (Bim) and reduced expression of Jun. Collectively, our data demonstrate that ERG controls survival of HSPCs, a property that may be used by leukemic cells. Stem Cells 2017;35:1773-1785.

DNA Damage-Induced HSPC Malfunction Depends on ROS Accumulation Downstream of IFN-1 Signaling and Bid Mobilization

Mouse mutants with an impaired DNA damage response frequently exhibit a set of remarkably similar defects in the HSPC compartment that are of largely unknown molecular basis. Using Mixed-Lineage-Leukemia-5 (Mll5)-deficient mice as prototypical examples, we have identified a mechanistic pathway linking DNA damage and HSPC malfunction. We show that Mll5 deficiency results in accumulation of DNA damage and reactive oxygen species (ROS) in HSPCs. Reduction of ROS efficiently reverses hematopoietic defects, establishing ROS as a major cause of impaired HSPC function. The Ink4a/Arf locus also contributes to HSPC phenotypes, at least in part via promotion of ROS. Strikingly, toxic ROS levels in Mll5^-/- mice are critically dependent on type 1 interferon (IFN-1) signaling, which triggers mitochondrial accumulation of full-length Bid. Genetic inactivation of Bid diminishes ROS levels and reverses HSPC defects in Mll5^-/- mice. Overall, therefore, our findings highlight an unexpected IFN-1 > Bid > ROS pathway underlying DNA damage-associated HSPC malfunction.

Long-term serial xenotransplantation of juvenile myelomonocytic leukemia recapitulates human disease in Rag2-/-γc-/- mice

Juvenile myelomonocytic leukemia is a clonal malignant disease affecting young children. Current cure rates, even with allogeneic hematopoietic stem cell transplantation, are no better than 50%-60%. Pre-clinical research on juvenile myelomonocytic leukemia is urgently needed for the identification of novel therapies but is hampered by the unavailability of culture systems. Here we report a xenotransplantation model that allows long-term in vivo propagation of primary juvenile myelomonocytic leukemia cells. Persistent engraftment of leukemic cells was achieved by intrahepatic injection of 1×10(6) cells into newborn Rag2(-/-)γc(-/-) mice or intravenous injection of 5×10(6) cells into 5-week old mice. Key characteristics of juvenile myelomonocytic leukemia were reproduced, including cachexia and clonal expansion of myelomonocytic progenitor cells that infiltrated bone marrow, spleen, liver and, notably, lung. Xenografted leukemia cells led to reduced survival of recipient mice. The stem cell character of juvenile myelomonocytic leukemia was confirmed by successful serial transplantation that resulted in leukemia cell propagation for more than one year. Independence of exogenous cytokines, low donor cell number and slowly progressing leukemia are advantages of the model, which will serve as an important tool to research the pathophysiology of juvenile myelomonocytic leukemia and test novel pharmaceutical strategies such as DNA methyltransferase inhibition.

Antagonism between MCL-1 and PUMA governs stem/progenitor cell survival during hematopoietic recovery from stress

Understanding the critical factors that govern recovery of the hematopoietic system from stress, such as during anticancer therapy and bone marrow transplantation, is of clinical significance. We investigated the importance of the prosurvival proteins myeloid cell leukemia-1 (MCL-1) and B-cell lymphoma-extra large (BCL-XL) in stem/progenitor cell survival and fitness during hematopoietic recovery from stress. Loss of a single Mcl-1 allele, which reduced MCL-1 protein levels, severely compromised hematopoietic recovery from myeloablative challenge and following bone marrow transplantation, whereas BCL-XL was dispensable in both contexts. We identified inhibition of proapoptotic p53 upregulated modulator of apoptosis (PUMA) as the key role of MCL-1 in both settings, with Mcl-1(+/-);Puma(-/-) mice completely protected from the deleterious effects of loss of 1 Mcl-1 allele. These results reveal the molecular mechanisms that govern cell survival during hematopoietic recovery from stress.

Apoptotic caspases suppress mtDNA-induced STING-mediated type I IFN production

Activated caspases are a hallmark of apoptosis induced by the intrinsic pathway, but they are dispensable for cell death and the apoptotic clearance of cells in vivo. This has led to the suggestion that caspases are activated not just to kill but to prevent dying cells from triggering a host immune response. Here, we show that the caspase cascade suppresses type I interferon (IFN) production by cells undergoing Bak/Bax-mediated apoptosis. Bak and Bax trigger the release of mitochondrial DNA. This is recognized by the cGAS/STING-dependent DNA sensing pathway, which initiates IFN production. Activated caspases attenuate this response. Pharmacological caspase inhibition or genetic deletion of caspase-9, Apaf-1, or caspase-3/7 causes dying cells to secrete IFN-β. In vivo, this precipitates an elevation in IFN-β levels and consequent hematopoietic stem cell dysfunction, which is corrected by loss of Bak and Bax. Thus, the apoptotic caspase cascade functions to render mitochondrial apoptosis immunologically silent.

miR-133a enhances the protective capacity of cardiac progenitors cells after myocardial infarction

miR-133a and miR-1 are known as muscle-specific microRNAs that are involved in cardiac development and pathophysiology. We have shown that both miR-1 and miR-133a are early and progressively upregulated during in vitro cardiac differentiation of adult cardiac progenitor cells (CPCs), but only miR-133a expression was enhanced under in vitro oxidative stress. miR-1 was demonstrated to favor differentiation of CPCs, whereas miR-133a overexpression protected CPCs against cell death, targeting, among others, the proapoptotic genes Bim and Bmf. miR-133a-CPCs clearly improved cardiac function in a rat myocardial infarction model by reducing fibrosis and hypertrophy and increasing vascularization and cardiomyocyte proliferation. The beneficial effects of miR-133a-CPCs seem to correlate with the upregulated expression of several relevant paracrine factors and the plausible cooperative secretion of miR-133a via exosomal transport. Finally, an in vitro heart muscle model confirmed the antiapoptotic effects of miR-133a-CPCs, favoring the structuration and contractile functionality of the artificial tissue.

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miR-1 and miR-133a have a role in adult cardiac progenitor cells (CPCs)

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miR-133a-CPCs protect cardiac function

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miR-133a-CPCs increase vascularization and protect against hypertrophy

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miR-133a is enriched in CPCs-derived exosomes

Improved HSC reconstitution and protection from inflammatory stress and chemotherapy in mice lacking granzyme B

Granzyme B is expressed by hematopoietic stem cells (HSCs) and stromal cells in response to bacterial products or chemotherapy agents and limits HSC reconstitution potential.

The serine protease granzyme B (GzmB) is stored in the granules of cytotoxic T and NK cells and facilitates immune-mediated destruction of virus-infected cells. In this study, we use genetic tools to report novel roles for GzmB as an important regulator of hematopoietic stem cell (HSC) function in response to stress. HSCs lacking the GzmB gene show improved bone marrow (BM) reconstitution associated with increased HSC proliferation and mitochondrial activity. In addition, recipients deficient in GzmB support superior engraftment of wild-type HSCs compared with hosts with normal BM niches. Stimulation of mice with lipopolysaccharide strongly induced GzmB protein expression in HSCs, which was mediated by the TLR4–TRIF–p65 NF-κB pathway. This is associated with increased cell death and GzmB secretion into the BM environment, suggesting an extracellular role of GzmB in modulating HSC niches. Moreover, treatment with the chemotherapeutic agent 5-fluorouracil (5-FU) also induces GzmB production in HSCs. In this situation GzmB is not secreted, but instead causes cell-autonomous apoptosis. Accordingly, GzmB-deficient mice are more resistant to serial 5-FU treatments. Collectively, these results identify GzmB as a negative regulator of HSC function that is induced by stress and chemotherapy in both HSCs and their niches. Blockade of GzmB production may help to improve hematopoiesis in various situations of BM stress.

Deregulated cell death and lymphocyte homeostasis cause premature lethality in mice lacking the BH3-only proteins Bim and Bmf

BH3 domain-only proteins (BH3-only) proteins are members of the Bcl-2 family that play crucial roles in embryogenesis and the maintenance of tissue homeostasis by triggering apoptotic cell death. The BH3-only protein Bim is critical for developmental apoptosis of lymphocytes, securing establishment of tolerance and for the termination of immune responses. Bim is believed to act in concert with other BH3-only proteins or members of the tumor necrosis factor receptor family in getting rid of unwanted cells. Bmf, a related BH3-only protein, was shown to play a role in B-cell homeostasis and to mediate cell death in response to certain apoptotic triggers, including glucocorticoid, histone deacetylase inhibitors, and overexpression of the c-Myc proto-oncogene. Here we show that Bim and Bmf have overlapping functions during mouse development and coregulate lymphocyte homeostasis and apoptosis in a nonredundant manner. Double deficiency of Bim and Bmf caused more B lymphadenopathy than loss of either BH3-only protein alone, and this was associated with autoimmune glomerulonephritis and a range of malignancies in aged mice. Thus, our results demonstrate that Bim and Bmf act in concert to prevent autoimmunity and malignant disease, strengthening the rational for the development of BH3-only protein mimicking therapeutics for the treatment of such disorders.