A missense mutation in the MLKL brace region promotes lethal neonatal inflammation and hematopoietic dysfunction.

MLKL is the essential effector of necroptosis, a form of programmed lytic cell death. We have isolated a mouse strain with a single missense mutation, MlklD139V, that alters the two-helix 'brace' that connects the killer four-helix bundle and regulatory pseudokinase domains. This confers constitutive, RIPK3 independent killing activity to MLKL. Homozygous mutant mice develop lethal postnatal inflammation of the salivary glands and mediastinum. The normal embryonic development of MlklD139V homozygotes until birth, and the absence of any overt phenotype in heterozygotes provides important in vivo precedent for the capacity of cells to clear activated MLKL. These observations offer an important insight into the potential disease-modulating roles of three common human MLKL polymorphisms that encode amino acid substitutions within or adjacent to the brace region. Compound heterozygosity of these variants is found at up to 12-fold the expected frequency in patients that suffer from a pediatric autoinflammatory disease, chronic recurrent multifocal osteomyelitis (CRMO).

Interconversion between Tumorigenic and Differentiated States in Acute Myeloid Leukemia.

Tumors are composed of phenotypically heterogeneous cancer cells that often resemble various differentiation states of their lineage of origin. Within this hierarchy, it is thought that an immature subpopulation of tumor-propagating cancer stem cells (CSCs) differentiates into non-tumorigenic progeny, providing a rationale for therapeutic strategies that specifically eradicate CSCs or induce their differentiation. The clinical success of these approaches depends on CSC differentiation being unidirectional rather than reversible, yet this question remains unresolved even in prototypically hierarchical malignancies, such as acute myeloid leukemia (AML). Here, we show in murine and human models of AML that, upon perturbation of endogenous expression of the lineage-determining transcription factor PU.1 or withdrawal of established differentiation therapies, some mature leukemia cells can de-differentiate and reacquire clonogenic and leukemogenic properties. Our results reveal plasticity of CSC maturation in AML, highlighting the need to therapeutically eradicate cancer cells across a range of differentiation states.

Intrinsic apoptosis circumvents the functional decline of circulating platelets but does not cause the storage lesion.

The circulating life span of blood platelets is regulated by the prosurvival protein BCL-XL It restrains the activity of BAK and BAX, the essential prodeath mediators of intrinsic apoptosis. Disabling the platelet intrinsic apoptotic pathway in mice by deleting BAK and BAX results in a doubling of platelet life span and concomitant thrombocytosis. Apoptotic platelets expose phosphatidylserine (PS) via a mechanism that is distinct from that driven by classical agonists. Whether there is any role for apoptotic PS in platelet function in vivo, however, is unclear. Apoptosis has also been associated with the platelet storage lesion (PSL), the constellation of biochemical deteriorations that occur during blood bank storage. In this study, we investigated the role of BAK/BAX-mediated apoptosis in hemostasis and thrombosis and in the development of the PSL. We show that although intrinsic apoptosis is rapidly induced during storage at 37°C, it is not detected when platelets are kept at the standard storage temperature of 22°C. Remarkably, loss of BAK and BAX did not prevent the development of the PSL at either temperature. BAK/BAX-deficient mice exhibited increased bleeding times and unstable thrombus formation. This phenotype was not caused by impaired PS exposure, but was associated with a defect in granule release from aged platelets. Strikingly, rejuvenation of BAK/BAX-deficient platelets in vivo completely rescued the observed hemostatic defects. Thus, apoptotic culling of old platelets from the bloodstream is essential to maintain a functional, hemostatically reactive platelet population. Inhibiting intrinsic apoptosis in blood banked platelets is unlikely to yield significant benefit.

Hhex Regulates Hematopoietic Stem Cell Self-Renewal and Stress Hematopoiesis via Repression of Cdkn2a.

The hematopoietically expressed homeobox transcription factor (Hhex) is important for the maturation of definitive hematopoietic progenitors and B-cells during development. We have recently shown that in adult hematopoiesis, Hhex is dispensable for maintenance of hematopoietic stem cells (HSCs) and myeloid lineages but essential for the commitment of common lymphoid progenitors (CLPs) to lymphoid lineages. Here, we show that during serial bone marrow transplantation, Hhex-deleted HSCs are progressively lost, revealing an intrinsic defect in HSC self-renewal. Moreover, Hhex-deleted mice show markedly impaired hematopoietic recovery following myeloablation, due to a failure of progenitor expansion. In vitro, Hhex-null blast colonies were incapable of replating, implying a specific requirement for Hhex in immature progenitors. Transcriptome analysis of Hhex-null Lin- Sca+ Kit+ cells showed that Hhex deletion leads to derepression of polycomb repressive complex 2 (PRC2) and PRC1 target genes, including the Cdkn2a locus encoding the tumor suppressors p16Ink 4a and p19Arf . Indeed, loss of Cdkn2a restored the capacity of Hhex-null blast colonies to generate myeloid progenitors in vitro, as well as hematopoietic reconstitution following myeloablation in vivo. Thus, HSCs require Hhex to promote PRC2-mediated Cdkn2a repression to enable continued self-renewal and response to hematopoietic stress. Stem Cells 2017;35:1948-1957.

MOZ and BMI1 act synergistically to maintain hematopoietic stem cells.

Chromatin plays a central role in maintaining hematopoietic stem cells and during their stepwise differentiation. Although a large number of histone modifications and chromatin-modifying enzymes have been identified, how these act in concert to produce specific phenotypic outcomes remains to be established. MOZ (KAT6A) is a lysine acetyltransferase and enhances transcription at target gene loci. In contrast, the Polycomb group protein BMI1 (PCGF4) is part of the transcriptionally repressive PRC1 complex. Despite their opposing effects on transcription, MOZ and BMI1 regulate biological systems in a similar manner. MOZ and BMI1 are required for the development of transplantable HSCs, for restraining cellular senescence, for the proper patterning of the anterior-posterior axis during development and for the specification and maintenance of the B-cell lineage. Thus, we set out to explore the relationship between MOZ and BMI1. We recently established that MOZ and BMI1 have opposing effects on the initiation of Hox gene expression during embryonic development and that defects in body segment identity specification observed in single Moz and Bmi1 mutants were rescued in compound mutants. We report here the relationship between MOZ and BMI1 in hematopoiesis. Using Moz+/-;Bmi1+/- compound mutant mice, we found that MOZ and BMI1, but not the BMI1-related protein MEL18 (PCGF2), play synergistic roles in maintaining adult HSCs. Although BMI1 restrains premature senescence, we established that MOZ acts to maintain the quiescent state of HSCs. Our work revealed that MOZ and BMI1 regulate HSCs in a synergistic manner by acting on distinct processes required to maintain HSCs.

Growth and Differentiation Factors.

This review is restricted to neutrophilic granulocytes (granulocytes), monocytes (macrophages), and eosinophils, with only passing reference to cells that are also usually included in the "myeloid" category-megakaryocytes, mast cells, and erythroid cells. Although some dendritic cells are of myeloid origin, they are discussed elsewhere. The validity of the information to be described depends on two assumptions: (a) that in vitro data are applicable to events in vivo and (b) that mouse data reflect events in man. Both assumptions are likely to be broadly correct.

Rapid Inflammation in Mice Lacking Both SOCS1 and SOCS3 in Hematopoietic Cells.

The Suppressors of Cytokine Signalling (SOCS) proteins are negative regulators of cytokine signalling required to prevent excess cellular responses. SOCS1 and SOCS3 are essential to prevent inflammatory disease, SOCS1 by attenuating responses to IFNγ and gamma-common (γc) cytokines, and SOCS3 via regulation of G-CSF and IL-6 signalling. SOCS1 and SOCS3 show significant sequence homology and are the only SOCS proteins to possess a KIR domain. The possibility of overlapping or redundant functions was investigated in inflammatory disease via generation of mice lacking both SOCS1 and SOCS3 in hematopoietic cells. Loss of SOCS3 significantly accelerated the pathology and inflammatory disease characteristic of SOCS1 deficiency. We propose a model in which SOCS1 and SOCS3 operate independently to control specific cytokine responses and together modulate the proliferation and activation of lymphoid and myeloid cells to prevent rapid inflammatory disease.

MOZ (KAT6A) is essential for the maintenance of classically defined adult hematopoietic stem cells.

Hematopoietic stem cells (HSCs) are conventionally thought to be at the apex of a hierarchy that produces all mature cells of the blood. The quintessential property of these cells is their ability to reconstitute the entire hematopoietic system of hemoablated recipients. This characteristic has enabled HSCs to be used to replenish the hematopoietic system of patients after chemotherapy or radiotherapy. Here, we use deletion of the monocytic leukemia zinc finger gene (Moz/Kat6a/Myst3) to examine the effects of removing HSCs. Loss of MOZ in adult mice leads to the rapid loss of HSCs as defined by transplantation. This is accompanied by a reduction of the LSK-CD48-CD150+ and LSK-CD34-Flt3- populations in the bone marrow and a reduction in quiescent cells in G0 Surprisingly, the loss of classically defined HSCs did not affect mouse viability, and there was no recovery of the LSK-CD48-CD150+ and LSK-CD34-Flt3- populations 15 to 18 months after Moz deletion. Clonal analysis of myeloid progenitors, which produce short-lived granulocytes, demonstrate that these are derived from cells that had undergone recombination at the Moz locus up to 2 years earlier, suggesting that early progenitors have acquired extended self-renewal. Our results establish that there are essential differences in HSC requirement for steady-state blood cell production compared with the artificial situation of reconstitution after transplantation into a hemoablated host. A better understanding of steady-state hematopoiesis may facilitate the development of novel therapies engaging hematopoietic cell populations with previously unrecognized traits, as well as characterizing potential vulnerability to oncogenic transformation.

Acute myeloid leukemia requires Hhex to enable PRC2-mediated epigenetic repression of Cdkn2a.

Unlike clustered HOX genes, the role of nonclustered homeobox gene family members in hematopoiesis and leukemogenesis has not been extensively studied. Here we found that the hematopoietically expressed homeobox gene Hhex is overexpressed in acute myeloid leukemia (AML) and is essential for the initiation and propagation of MLL-ENL-induced AML but dispensable for normal myelopoiesis, indicating a specific requirement for Hhex for leukemic growth. Loss of Hhex leads to expression of the Cdkn2a-encoded tumor suppressors p16(INK4a) and p19(ARF), which are required for growth arrest and myeloid differentiation following Hhex deletion. Mechanistically, we show that Hhex binds to the Cdkn2a locus and directly interacts with the Polycomb-repressive complex 2 (PRC2) to enable H3K27me3-mediated epigenetic repression. Thus, Hhex is a potential therapeutic target that is specifically required for AML stem cells to repress tumor suppressor pathways and enable continued self-renewal.

IL-18 Production from the NLRP1 Inflammasome Prevents Obesity and Metabolic Syndrome.

Interleukin-18 (IL-18) is activated by Caspase-1 in inflammasome complexes and has anti-obesity effects; however, it is not known which inflammasome regulates this process. We found that mice lacking the NLRP1 inflammasome phenocopy mice lacking IL-18, with spontaneous obesity due to intrinsic lipid accumulation. This is exacerbated when the mice are fed a high-fat diet (HFD) or a high-protein diet, but not when mice are fed a HFD with low energy density (high fiber). Furthermore, mice with an activating mutation in NLRP1, and hence increased IL-18, have decreased adiposity and are resistant to diet-induced metabolic dysfunction. Feeding these mice a HFD further increased plasma IL-18 concentrations and strikingly resulted in loss of adipose tissue mass and fatal cachexia, which could be prevented by genetic deletion of IL-18. Thus, NLRP1 is an innate immune sensor that functions in the context of metabolic stress to produce IL-18, preventing obesity and metabolic syndrome.

Polycomb repressive complex 2 component Suz12 is required for hematopoietic stem cell function and lymphopoiesis.

Polycomb repressive complex 2 (PRC2) is a chromatin modifier that regulates stem cells in embryonic and adult tissues. Loss-of-function studies of PRC2 components have been complicated by early embryonic dependence on PRC2 activity and the partial functional redundancy of enhancer of zeste homolog 1 (Ezh1) and enhancer of zeste homolog 2 (Ezh2), which encode the enzymatic component of PRC2. Here, we investigated the role of PRC2 in hematopoiesis by conditional deletion of suppressor of zeste 12 protein homolog (Suz12), a core component of PRC2. Complete loss of Suz12 resulted in failure of hematopoiesis, both in the embryo and the adult, with a loss of maintenance of hematopoietic stem cells (HSCs). In contrast, partial loss of PRC2 enhanced HSC self-renewal. Although Suz12 was required for lymphoid development, deletion in individual blood cell lineages revealed that it was dispensable for the development of granulocytic, monocytic, and megakaryocytic cells. Collectively, these data reveal the multifaceted role of PRC2 in hematopoiesis, with divergent dose-dependent effects in HSC and distinct roles in maturing blood cells. Because PRC2 is a potential target for cancer therapy, the significant consequences of modest changes in PRC2 activity, as well as the cell and developmental stage-specific effects, will need to be carefully considered in any therapeutic context.

Aberrant actin depolymerization triggers the pyrin inflammasome and autoinflammatory disease that is dependent on IL-18, not IL-1ß.

Gain-of-function mutations that activate the innate immune system can cause systemic autoinflammatory diseases associated with increased IL-1ß production. This cytokine is activated identically to IL-18 by an intracellular protein complex known as the inflammasome; however, IL-18 has not yet been specifically implicated in the pathogenesis of hereditary autoinflammatory disorders. We have now identified an autoinflammatory disease in mice driven by IL-18, but not IL-1ß, resulting from an inactivating mutation of the actin-depolymerizing cofactor Wdr1. This perturbation of actin polymerization leads to systemic autoinflammation that is reduced when IL-18 is deleted but not when IL-1 signaling is removed. Remarkably, inflammasome activation in mature macrophages is unaltered, but IL-18 production from monocytes is greatly exaggerated, and depletion of monocytes in vivo prevents the disease. Small-molecule inhibition of actin polymerization can remove potential danger signals from the system and prevents monocyte IL-18 production. Finally, we show that the inflammasome sensor of actin dynamics in this system requires caspase-1, apoptosis-associated speck-like protein containing a caspase recruitment domain, and the innate immune receptor pyrin. Previously, perturbation of actin polymerization by pathogens was shown to activate the pyrin inflammasome, so our data now extend this guard hypothesis to host-regulated actin-dependent processes and autoinflammatory disease.

Early lineage priming by trisomy of Erg leads to myeloproliferation in a Down syndrome model.

Down syndrome (DS), with trisomy of chromosome 21 (HSA21), is the commonest human aneuploidy. Pre-leukemic myeloproliferative changes in DS foetal livers precede the acquisition of GATA1 mutations, transient myeloproliferative disorder (DS-TMD) and acute megakaryocytic leukemia (DS-AMKL). Trisomy of the Erg gene is required for myeloproliferation in the Ts(1716)65Dn DS mouse model. We demonstrate here that genetic changes specifically attributable to trisomy of Erg lead to lineage priming of primitive and early multipotential progenitor cells in Ts(1716)65Dn mice, excess megakaryocyte-erythroid progenitors, and malignant myeloproliferation. Gene expression changes dependent on trisomy of Erg in Ts(1716)65Dn multilineage progenitor cells were correlated with those associated with trisomy of HSA21 in human DS hematopoietic stem and primitive progenitor cells. These data suggest a role for ERG as a regulator of hematopoietic lineage potential, and that trisomy of ERG in the context of DS foetal liver hemopoiesis drives the pre-leukemic changes that predispose to subsequent DS-TMD and DS-AMKL.

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-ß. In vivo, this precipitates an elevation in IFN-ß 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.

Lymphoid tissue and plasmacytoid dendritic cells and macrophages do not share a common macrophage-dendritic cell-restricted progenitor.

The relationship between dendritic cells (DCs) and macrophages is often debated. Here we ask whether steady-state, lymphoid-tissue-resident conventional DCs (cDCs), plasmacytoid DCs (pDCs), and macrophages share a common macrophage-DC-restricted precursor (MDP). Using new clonal culture assays combined with adoptive transfer, we found that MDP fractions isolated by previous strategies are dominated by precursors of macrophages and monocytes, include some multipotent precursors of other hematopoietic lineages, but contain few precursors of resident cDCs and pDCs and no detectable common precursors restricted to these DC types and macrophages. Overall we find no evidence for a common restricted MDP leading to both macrophages and FL-dependent, resident cDCs and pDCs.

A reporter mouse reveals lineage-specific and heterogeneous expression of IRF8 during lymphoid and myeloid cell differentiation.

The IFN regulatory factor family member 8 (IRF8) regulates differentiation of lymphoid and myeloid lineage cells by promoting or suppressing lineage-specific genes. How IRF8 promotes hematopoietic progenitors to commit to one lineage while preventing the development of alternative lineages is not known. In this study, we report an IRF8-EGFP fusion protein reporter mouse that revealed previously unrecognized patterns of IRF8 expression. Differentiation of hematopoietic stem cells into oligopotent progenitors is associated with progressive increases in IRF8-EGFP expression. However, significant induction of IRF8-EGFP is found in granulocyte-myeloid progenitors and the common lymphoid progenitors but not the megakaryocytic-erythroid progenitors. Surprisingly, IRF8-EGFP identifies three subsets of the seemingly homogeneous granulocyte-myeloid progenitors with an intermediate level of expression of EGFP defining bipotent progenitors that differentiation into either EGFP(hi) monocytic progenitors or EGFP(lo) granulocytic progenitors. Also surprisingly, IRF8-EGFP revealed a highly heterogeneous pre-pro-B population with a fluorescence intensity ranging from background to 4 orders above background. Interestingly, IRF8-EGFP readily distinguishes true B cell committed (EGFP(int)) from those that are noncommitted. Moreover, dendritic cell progenitors expressed extremely high levels of IRF8-EGFP. Taken together, the IRF8-EGFP reporter revealed previously unrecognized subsets with distinct developmental potentials in phenotypically well-defined oligopotent progenitors, providing new insights into the dynamic heterogeneity of developing hematopoietic progenitors.

Suppressor of cytokine signaling 4 (SOCS4) protects against severe cytokine storm and enhances viral clearance during influenza infection.

Suppressor of cytokine signaling (SOCS) proteins are key regulators of innate and adaptive immunity. There is no described biological role for SOCS4, despite broad expression in the hematopoietic system. We demonstrate that mice lacking functional SOCS4 protein rapidly succumb to infection with a pathogenic H1N1 influenza virus (PR8) and are hypersusceptible to infection with the less virulent H3N2 (X31) strain. In SOCS4-deficient animals, this led to substantially greater weight loss, dysregulated pro-inflammatory cytokine and chemokine production in the lungs and delayed viral clearance. This was associated with impaired trafficking of influenza-specific CD8 T cells to the site of infection and linked to defects in T cell receptor activation. These results demonstrate that SOCS4 is a critical regulator of anti-viral immunity.

RIPK1 regulates RIPK3-MLKL-driven systemic inflammation and emergency hematopoiesis.

Upon ligand binding, RIPK1 is recruited to tumor necrosis factor receptor superfamily (TNFRSF) and Toll-like receptor (TLR) complexes promoting prosurvival and inflammatory signaling. RIPK1 also directly regulates caspase-8-mediated apoptosis or, if caspase-8 activity is blocked, RIPK3-MLKL-dependent necroptosis. We show that C57BL/6 Ripk1(-/-) mice die at birth of systemic inflammation that was not transferable by the hematopoietic compartment. However, Ripk1(-/-) progenitors failed to engraft lethally irradiated hosts properly. Blocking TNF reversed this defect in emergency hematopoiesis but, surprisingly, Tnfr1 deficiency did not prevent inflammation in Ripk1(-/-) neonates. Deletion of Ripk3 or Mlkl, but not Casp8, prevented extracellular release of the necroptotic DAMP, IL-33, and reduced Myd88-dependent inflammation. Reduced inflammation in the Ripk1(-/-)Ripk3(-/-), Ripk1(-/-)Mlkl(-/-), and Ripk1(-/-)Myd88(-/-) mice prevented neonatal lethality, but only Ripk1(-/-)Ripk3(-/-)Casp8(-/-) mice survived past weaning. These results reveal a key function for RIPK1 in inhibiting necroptosis and, thereby, a role in limiting, not only promoting, inflammation.