5-Azacytidine depletes HSCs and synergizes with an anti-CD117 antibody to augment donor engraftment in immunocompetent mice.

Depletion of hematopoietic stem cells (HSCs) is used therapeutically in many malignant and nonmalignant blood disorders in the setting of a hematopoietic cell transplantation (HCT) to eradicate diseased HSCs, thus allowing donor HSCs to engraft. Current treatments to eliminate HSCs rely on modalities that cause DNA strand breakage (ie, alkylators, radiation) resulting in multiple short-term and long-term toxicities and sometimes even death. These risks have severely limited the use of HCT to patients with few to no comorbidities and excluded many others with diseases that could be cured with an HCT. 5-Azacytidine (AZA) is a widely used hypomethylating agent that is thought to preferentially target leukemic cells in myeloid malignancies. Here, we reveal a previously unknown effect of AZA on HSCs. We show that AZA induces early HSC proliferation in vivo and exerts a direct cytotoxic effect on proliferating HSCs in vitro. When used to pretreat recipient mice for transplantation, AZA permitted low-level donor HSC engraftment. Moreover, by combining AZA with a monoclonal antibody (mAb) targeting CD117 (c-Kit) (a molecule expressed on HSCs), more robust HSC depletion and substantially higher levels of multilineage donor cell engraftment were achieved in immunocompetent mice. The enhanced effectiveness of this combined regimen correlated with increased apoptotic cell death in hematopoietic stem and progenitor cells. Together, these findings highlight a previously unknown therapeutic mechanism for AZA which may broaden its use in clinical practice. Moreover, the synergy we show between AZA and anti-CD117 mAb is a novel strategy to eradicate abnormal HSCs that can be rapidly tested in the clinical setting.

RUNX3 levels in human hematopoietic progenitors are regulated by aging and dictate erythroid-myeloid balance.

Healthy bone marrow progenitors yield a co-ordinated balance of hematopoietic lineages. This balance shifts with aging toward enhanced granulopoiesis with diminished erythropoiesis and lymphopoiesis, changes which likely contribute to the development of bone marrow disorders in the elderly. In this study, RUNX3 was identified as a hematopoietic stem and progenitor cell factor whose levels decline with aging in humans and mice. This decline is exaggerated in hematopoietic stem and progenitor cells from subjects diagnosed with unexplained anemia of the elderly. Hematopoietic stem cells from elderly unexplained anemia patients had diminished erythroid but unaffected granulocytic colony forming potential. Knockdown studies revealed human hematopoietic stem and progenitor cells to be strongly influenced by RUNX3 levels, with modest deficiencies abrogating erythroid differentiation at multiple steps while retaining capacity for granulopoiesis. Transcriptome profiling indicated control by RUNX3 of key erythroid transcription factors, including KLF1 and GATA1 These findings thus implicate RUNX3 as a participant in hematopoietic stem and progenitor cell aging, and a key determinant of erythroid-myeloid lineage balance.

Blockade of TIM-1 on the donor graft ameliorates graft-versus-host disease following hematopoietic cell transplantation.

Acute graft-versus-host disease (GVHD) is a leading cause of mortality after allogeneic hematopoietic cell transplantation (HCT) mediated by dysregulated T-cell immune reconstitution. Given the role of the T-cell immunoglobulin and mucin 1 (TIM-1) surface protein in many immune processes, including organ transplantation tolerance, we asked if TIM-1 might drive post-transplant inflammation and acute GVHD. TIM-1 binds to phosphatidylserine (PtdSer), and agonism of TIM1 on immune cells is proinflammatory. HCT conditioning results in a significant supply of PtdSer from apoptosis and cellular debris. Using murine models, treatment with an antagonistic anti-TIM-1 monoclonal antibody (mAb) protects against acute GVHD while maintaining graft-versus-tumor effects. In contrast, the addition of exogenous free PtdSer worsened GVHD in a TIM-1-dependent manner. Importantly, TIM-1 blockade did not alter the expansion of donor T cells in vitro or in vivo. Instead, TIM-1 blockade reduces proinflammatory cytokines and promotes anti-inflammatory factors like carbonic anhydrase 1 and serum amyloid A1 in the gut tissue. This is mediated by TIM-1 on donor cells, as HCT of wild-type (WT) bone marrow (BM) and conventional T (Tcon) cells into TIM-1-/- knockout (KO) recipient mice showed little survival advantage compared with WT recipients, whereas WT recipients of TIM-1-/- KO Tcon cells or TIM1-/- KO BM had improved survival, in part due to the expression of TIM-1 on donor invariant natural killer T cells, which drives inflammation. Finally, in a humanized mouse xenograft GVHD model, treatment with anti-human TIM-1 antagonist mAb reduced GVHD disease burden and mortality. This supports TIM-1 as important for GVHD pathogenesis and as a target for the prevention of GVHD.

Selective hematopoietic stem cell ablation using CD117-antibody-drug-conjugates enables safe and effective transplantation with immunity preservation.

Hematopoietic stem cell transplantation (HSCT) is a curative therapy for blood and immune diseases with potential for many settings beyond current standard-of-care. Broad HSCT application is currently precluded largely due to morbidity and mortality associated with genotoxic irradiation or chemotherapy conditioning. Here we show that a single dose of a CD117-antibody-drug-conjugate (CD117-ADC) to saporin leads to > 99% depletion of host HSCs, enabling rapid and efficient donor hematopoietic cell engraftment. Importantly, CD117-ADC selectively targets hematopoietic stem cells yet does not cause clinically significant side-effects. Blood counts and immune cell function are preserved following CD117-ADC treatment, with effective responses by recipients to both viral and fungal challenges. These results suggest that CD117-ADC-mediated HSCT pre-treatment could serve as a non-myeloablative conditioning strategy for the treatment of a wide range of non-malignant and malignant diseases, and might be especially suited to gene therapy and gene editing settings in which preservation of immunity is desired.

Anti-CD117 antibody depletes normal and myelodysplastic syndrome human hematopoietic stem cells in xenografted mice.

The myelodysplastic syndromes (MDS) represent a group of clonal disorders that result in ineffective hematopoiesis and are associated with an increased risk of transformation into acute leukemia. MDS arises from hematopoietic stem cells (HSCs); therefore, successful elimination of MDS HSCs is an important part of any curative therapy. However, current treatment options, including allogeneic hematopoietic cell transplantation (HCT), often fail to ablate disease-initiating MDS HSCs, and thus have low curative potential and high relapse rates. Here, we demonstrate that human HSCs can be targeted and eliminated by monoclonal antibodies (mAbs) that bind cell-surface CD117 (c-Kit). We show that an anti-human CD117 mAb, SR-1, inhibits normal cord blood and bone marrow HSCs in vitro. Furthermore, SR-1 and clinical-grade humanized anti-human CD117 mAb, AMG 191, deplete normal and MDS HSCs in vivo in xenograft mouse models. Anti-CD117 mAbs also facilitate the engraftment of normal donor human HSCs in MDS xenograft mouse models, restoring normal human hematopoiesis and eradicating aggressive pathologic MDS cells. This study is the first to demonstrate that anti-human CD117 mAbs have potential as novel therapeutics to eradicate MDS HSCs and augment the curative effect of allogeneic HCT for this disease. Moreover, we establish the foundation for use of these antibody agents not only in the treatment of MDS but also for the multitude of other HSC-driven blood and immune disorders for which transplant can be disease-altering.

Age-associated changes in human hematopoietic stem cells.

Aging has a broad impact on the function of the human hematopoietic system. This review will focus primarily on the effect of aging on the human hematopoietic stem cell (HSC) population. With age, even though human HSCs increase in number, they have decreased self-renewal capacity and reconstitution potential upon transplantation. As a population, human HSCs become more myeloid-biased in their differentiation potential. This is likely due to the human HSC population becoming more clonal with age, selecting for myeloid-biased HSC clones. The HSC clones that come to predominate with age may also contain disease-causing genetic and epigenetic changes that confer an increased risk of developing into an age-associated clonal hematopoietic disease, such as myelodysplastic syndrome, myeloproliferative disorders, or leukemia. The selection of these aged human HSC clones may be in part due to changes in the aging bone marrow microenvironment. While there have been significant advances in the understanding of the effect of aging on mouse hematopoiesis and mouse HSCs, we have comparatively less detailed analyses of the effect of aging on human HSCs. Continued evaluation of human HSCs in the context of aging will be important to determine how applicable the findings in mice and other model organisms are to the human clinical setting.

Myeloid Cell Origins, Differentiation, and Clinical Implications.

The hematopoietic stem cell (HSC) is a multipotent stem cell that resides in the bone marrow and has the ability to form all of the cells of the blood and immune system. Since its first purification in 1988, additional studies have refined the phenotype and functionality of HSCs and characterized all of their downstream progeny. The hematopoietic lineage is divided into two main branches: the myeloid and lymphoid arms. The myeloid arm is characterized by the common myeloid progenitor and all of its resulting cell types. The stages of hematopoiesis have been defined in both mice and humans. During embryological development, the earliest hematopoiesis takes place in yolk sac blood islands and then migrates to the fetal liver and hematopoietic organs. Some adult myeloid populations develop directly from yolk sac progenitors without apparent bone marrow intermediates, such as tissue-resident macrophages. Hematopoiesis also changes over time, with a bias of the dominating HSCs toward myeloid development as animals age. Defects in myelopoiesis contribute to many hematologic disorders, and some of these can be overcome with therapies that target the aberrant stage of development. Furthermore, insights into myeloid development have informed us of mechanisms of programmed cell removal. The CD47/SIRPα axis, a myeloid-specific immune checkpoint, limits macrophage removal of HSCs but can be exploited by hematologic and solid malignancies. Therapeutics targeting CD47 represent a new strategy for treating cancer. Overall, an understanding of hematopoiesis and myeloid cell development has implications for regenerative medicine, hematopoietic cell transplantation, malignancy, and many other diseases.

Sleep disruption impairs haematopoietic stem cell transplantation in mice.

Many of the factors affecting the success of haematopoietic cell transplantation are still unknown. Here we show in mice that donor sleep deprivation reduces the ability of its haematopoietic stem cells (HSCs) to engraft and reconstitute the blood and bone marrow of an irradiated recipient by more than 50%. We demonstrate that sleep deprivation downregulates the expression of microRNA (miR)-19b, a negative regulator of the suppressor of cytokine signalling (SOCS) genes, which inhibit HSC migration and homing. Accordingly, HSCs from sleep-deprived mice have higher levels of SOCS genes expression, lower migration capacity in vitro and reduced homing to the bone marrow in vivo. Recovery of sleep after sleep deprivation restored the reconstitution potential of the HSCs. Taken together, this study provides insights into cellular and molecular mechanisms underlying the effects of sleep deprivation on HSCs, emphasizing the potentially critical role of donor sleep in the success of bone marrow transplantation.

BLT-humanized C57BL/6 Rag2-/-γc-/-CD47-/- mice are resistant to GVHD and develop B- and T-cell immunity to HIV infection.

The use of C57BL/6 Rag2(-/-)γc(-/-) mice as recipients for xenotransplantation with human immune systems (humanization) has been problematic because C57BL/6 SIRPα does not recognize human CD47, and such recognition is required to suppress macrophage-mediated phagocytosis of transplanted human hematopoietic stem cells (HSCs). We show that genetic inactivation of CD47 on the C57BL/6 Rag2(-/-)γc(-/-) background negates the requirement for CD47-signal recognition protein α (SIRPα) signaling and induces tolerance to transplanted human HSCs. These triple-knockout, bone marrow, liver, thymus (TKO-BLT) humanized mice develop organized lymphoid tissues including mesenteric lymph nodes, splenic follicles and gut-associated lymphoid tissue that demonstrate high levels of multilineage hematopoiesis. Importantly, these mice have an intact complement system and showed no signs of graft-versus-host disease (GVHD) out to 29 weeks after transplantation. Sustained, high-level HIV-1 infection was observed via either intrarectal or intraperitoneal inoculation. TKO-BLT mice exhibited hallmarks of human HIV infection including CD4(+) T-cell depletion, immune activation, and development of HIV-specific B- and T-cell responses. The lack of GVHD makes the TKO-BLT mouse a significantly improved model for long-term studies of pathogenesis, immune responses, therapeutics, and vaccines to human pathogens.

Hematopoietic stem cell and progenitor cell mechanisms in myelodysplastic syndromes.

Myelodysplastic syndromes (MDS) are a group of disorders characterized by variable cytopenias and ineffective hematopoiesis. Hematopoietic stem cells (HSCs) and myeloid progenitors in MDS have not been extensively characterized. We transplanted purified human HSCs from MDS samples into immunodeficient mice and show that HSCs are the disease-initiating cells in MDS. We identify a recurrent loss of granulocyte-macrophage progenitors (GMPs) in the bone marrow of low risk MDS patients that can distinguish low risk MDS from clinical mimics, thus providing a simple diagnostic tool. The loss of GMPs is likely due to increased apoptosis and increased phagocytosis, the latter due to the up-regulation of cell surface calreticulin, a prophagocytic marker. Blocking calreticulin on low risk MDS myeloid progenitors rescues them from phagocytosis in vitro. However, in the high-risk refractory anemia with excess blasts (RAEB) stages of MDS, the GMP population is increased in frequency compared with normal, and myeloid progenitors evade phagocytosis due to up-regulation of CD47, an antiphagocytic marker. Blocking CD47 leads to the selective phagocytosis of this population. We propose that MDS HSCs compete with normal HSCs in the patients by increasing their frequency at the expense of normal hematopoiesis, that the loss of MDS myeloid progenitors by programmed cell death and programmed cell removal are, in part, responsible for the cytopenias, and that up-regulation of the "don't eat me" signal CD47 on MDS myeloid progenitors is an important transition step leading from low risk MDS to high risk MDS and, possibly, to acute myeloid leukemia.

Anemia in the elderly.

PURPOSE OF REVIEW: There have been several large-scale epidemiologic studies, including the National Health and Nutrition Examination Survey III (NHANES III), which have described the prevalence and impact of anemia in the elderly. The information derived has been critically important. However, given the large number of patients surveyed, these reports necessarily relied substantially on the laboratory-based screening evaluations. There are now two recent reports describing the cause of anemia in elderly outpatients, and although the numbers are smaller than the large scale surveys, they constitute comprehensive hematologic evaluations with therapeutic interventions and clinical follow-up. The purpose of this review is to compare these different analyses. RECENT FINDINGS: There are distinct differences and similarities in the two types of studies, which are derived from patients seen in hematology clinics. Despite comprehensive hematologic evaluation, the puzzling entity of unexplained anemia of the elderly is confirmed and found to account for 30-46% of patients. NHANES III classified iron-deficiency anemia with other nutritional anemias, a classification that might be correct in the developing third world, but in North America and Western Europe, iron deficiency is more often caused by blood loss and the cause must be sought and dealt with. The myelodysplastic syndromes are an important cause of anemia in the elderly, with a prevalence of at least 4%. SUMMARY: Large-scale screening studies of anemia in the elderly are of great importance, and when complemented by comprehensive hematologic evaluations, provide a more accurate picture of the clinical situation.

Human bone marrow hematopoietic stem cells are increased in frequency and myeloid-biased with age.

In the human hematopoietic system, aging is associated with decreased bone marrow cellularity, decreased adaptive immune system function, and increased incidence of anemia and other hematological disorders and malignancies. Recent studies in mice suggest that changes within the hematopoietic stem cell (HSC) population during aging contribute significantly to the manifestation of these age-associated hematopoietic pathologies. Though the mouse HSC population has been shown to change both quantitatively and functionally with age, changes in the human HSC and progenitor cell populations during aging have been incompletely characterized. To elucidate the properties of an aged human hematopoietic system that may predispose to age-associated hematopoietic dysfunction, we evaluated immunophenotypic HSC and other hematopoietic progenitor populations from healthy, hematologically normal young and elderly human bone marrow samples. We found that aged immunophenotypic human HSC increase in frequency, are less quiescent, and exhibit myeloid-biased differentiation potential compared with young HSC. Gene expression profiling revealed that aged immunophenotypic human HSC transcriptionally up-regulate genes associated with cell cycle, myeloid lineage specification, and myeloid malignancies. These age-associated alterations in the frequency, developmental potential, and gene expression profile of human HSC are similar to those changes observed in mouse HSC, suggesting that hematopoietic aging is an evolutionarily conserved process.

Reduced ribosomal protein gene dosage and p53 activation in low-risk myelodysplastic syndrome.

Reduced gene dosage of ribosomal protein subunits has been implicated in 5q- myelodysplastic syndrome and Diamond Blackfan anemia, but the cellular and pathophysiologic defects associated with these conditions are enigmatic. Using conditional inactivation of the ribosomal protein S6 gene in laboratory mice, we found that reduced ribosomal protein gene dosage recapitulates cardinal features of the 5q- syndrome, including macrocytic anemia, erythroid hypoplasia, and megakaryocytic dysplasia with thrombocytosis, and that p53 plays a critical role in manifestation of these phenotypes. The blood cell abnormalities are accompanied by a reduction in the number of HSCs, a specific defect in late erythrocyte development, and suggest a disease-specific ontogenetic pathway for megakaryocyte development. Further studies of highly purified HSCs from healthy patients and from those with myelodysplastic syndrome link reduced expression of ribosomal protein genes to decreased RBC maturation and suggest an underlying and common pathophysiologic pathway for additional subtypes of myelodysplastic syndrome.

CD47 is upregulated on circulating hematopoietic stem cells and leukemia cells to avoid phagocytosis.

Macrophages clear pathogens and damaged or aged cells from the blood stream via phagocytosis. Cell-surface CD47 interacts with its receptor on macrophages, SIRPalpha, to inhibit phagocytosis of normal, healthy cells. We find that mobilizing cytokines and inflammatory stimuli cause CD47 to be transiently upregulated on mouse hematopoietic stem cells (HSCs) and progenitors just prior to and during their migratory phase, and that the level of CD47 on these cells determines the probability that they are engulfed in vivo. CD47 is also constitutively upregulated on mouse and human myeloid leukemias, and overexpression of CD47 on a myeloid leukemia line increases its pathogenicity by allowing it to evade phagocytosis. We conclude that CD47 upregulation is an important mechanism that provides protection to normal HSCs during inflammation-mediated mobilization, and that leukemic progenitors co-opt this ability in order to evade macrophage killing.

CD47 is an adverse prognostic factor and therapeutic antibody target on human acute myeloid leukemia stem cells.

Acute myeloid leukemia (AML) is organized as a cellular hierarchy initiated and maintained by a subset of self-renewing leukemia stem cells (LSC). We hypothesized that increased CD47 expression on human AML LSC contributes to pathogenesis by inhibiting their phagocytosis through the interaction of CD47 with an inhibitory receptor on phagocytes. We found that CD47 was more highly expressed on AML LSC than their normal counterparts, and that increased CD47 expression predicted worse overall survival in three independent cohorts of adult AML patients. Furthermore, blocking monoclonal antibodies directed against CD47 preferentially enabled phagocytosis of AML LSC and inhibited their engraftment in vivo. Finally, treatment of human AML LSC-engrafted mice with anti-CD47 antibody depleted AML and targeted AML LSC. In summary, increased CD47 expression is an independent, poor prognostic factor that can be targeted on human AML stem cells with blocking monoclonal antibodies capable of enabling phagocytosis of LSC.

Identification of a major susceptibility locus for lethal graft-versus-host disease in MHC-matched mice.

Graft-vs-host disease (GVHD) is the major cause of morbidity and mortality after allogeneic hemopoietic cell transplantation. From a genetic perspective, GVHD is a complex phenotypic trait. Although it is understood that susceptibility results from interacting polymorphisms of genes encoding histocompatibility Ags and immune regulatory molecules, a detailed and integrative understanding of the genetic background underlying GVHD remains lacking. To gain insight regarding these issues, we performed a forward genetic study. A MHC-matched mouse model was used in which irradiated recipient BALB.K and B10.BR mice demonstrate differential susceptibility to lethal GHVD when transplanted using AKR/J donors. Assessment of GVHD in (B10.BR x BALB.K)F(1) mice revealed that susceptibility is a dominant trait and conferred by deleterious alleles from the BALB.K strain. To identify the alleles responsible for GVHD susceptibility, a genome-scanning approach was taken using (B10.BR x BALB.K)F(1) x B10.BR backcross mice as recipients. A major susceptibility locus, termed the Gvh1 locus, was identified on chromosome 16 using linkage analysis (logarithm of the odds, 9.1). A second locus was found on chromosome 13, named Gvh2, which had additive but protective effects. Further identification of Gvh genes by positional cloning may yield new insight into genetic control mechanisms regulating GVHD and potentially reveal novel approaches for effective GVHD therapy.

Genomic instability and aging-like phenotype in the absence of mammalian SIRT6.

The Sir2 histone deacetylase functions as a chromatin silencer to regulate recombination, genomic stability, and aging in budding yeast. Seven mammalian Sir2 homologs have been identified (SIRT1-SIRT7), and it has been speculated that some may have similar functions to Sir2. Here, we demonstrate that SIRT6 is a nuclear, chromatin-associated protein that promotes resistance to DNA damage and suppresses genomic instability in mouse cells, in association with a role in base excision repair (BER). SIRT6-deficient mice are small and at 2-3 weeks of age develop abnormalities that include profound lymphopenia, loss of subcutaneous fat, lordokyphosis, and severe metabolic defects, eventually dying at about 4 weeks. We conclude that one function of SIRT6 is to promote normal DNA repair, and that SIRT6 loss leads to abnormalities in mice that overlap with aging-associated degenerative processes.

Mammalian SIRT1 limits replicative life span in response to chronic genotoxic stress.

The Saccharomyces cerevisiae chromatin silencing factor Sir2 suppresses genomic instability and extends replicative life span. In contrast, we find that mouse embryonic fibroblasts (MEFs) deficient for SIRT1, a mammalian Sir2 homolog, have dramatically increased resistance to replicative senescence. Extended replicative life span of SIRT1-deficient MEFs correlates with enhanced proliferative capacity under conditions of chronic, sublethal oxidative stress. In this context, SIRT1-deficient cells fail to normally upregulate either the p19(ARF) senescence regulator or its downstream target p53. However, upon acute DNA damage or oncogene expression, SIRT1-deficient cells show normal p19(ARF) induction and cell cycle arrest. Together, our findings demonstrate an unexpected SIRT1 function in promoting replicative senescence in response to chronic cellular stress and implicate p19(ARF) as a downstream effector in this pathway.