Resident Bone Marrow Macrophages Research Articles

Leukaemia Exposure Alters the Transcriptional Profile and Function of Macrophages in the Bone Marrow Niche

These authors contributed equally to this work: Martha M. Zarou & Amy Dawson Macrophages are fundamental cells of the innate immune system. Bone marrow (BM) resident macrophages are involved in antigen presentation, phagocytosis, efferocytosis (clearing of apoptotic cells) and play a critical role in regulating haematopoietic stem cell (HSC) function. In chronic myeloid leukaemia (CML), the disease-initiating leukaemic stem cells (LSCs) modulate the BM microenvironment to support leukaemia maintenance and progression. However, whether the CML BM niche alters the function of macrophages remains unknown. Using single-cell RNA sequencing (scRNA-seq) of human BM mononuclear cells, we demonstrate heterogeneity of tissue resident macrophages, isolated from CML patients at diagnosis, which cluster separately from BM macrophages deriving from heathy individuals. As most BM macrophages at diagnosis express BCR::ABL1 and are sensitive to TKI treatment, we further investigated if Philadelphia chromosome negative (Ph -) macrophages play a major role in CML development. We applied a chimeric CML mouse model (SCLtTA/BCR::ABL1) and reveal that temporal depletion of macrophages prior to leukaemia induction (using CSF1R antagonist antibody), increases leukaemia burden and decreases survival of chimeric mice following BCR::ABL1 induction (p<0.05). Comparison of CML exposed Ph - BM macrophages to control counterparts by scRNA-seq demonstrates that the CML niche drives two unique subpopulations of immature and anti-inflammatory macrophages. Functionally, we discover that Ph - BM macrophages exposed to a CML microenvironment display reduced phagocytic and efferocytic capacity, which correlates with reduction in mitochondrial respiration (p<0.05). Mechanistically, we show that diminished efferocytosis is mediated by reduced expression of the plasma membrane fatty acid translocase CD36. Additionally, treatment of primary mouse and human BM derived macrophages (BMDM) with the irreversible CD36 inhibitor sulfosuccinimidyl oleate (SSO), or genetic deletion of CD36 in THP-1 derived macrophages, results in reduced clearance of apoptotic cells in vitro. Furthermore, CD36 deficient macrophages display decreased mitochondrial respiration, similarly to CML exposed Ph - BMDM. Through untargeted liquid chromatography-mass spectrometry approach we identified the immunomodulatory protein lactotransferrin (LTF) as differentially secreted protein by leukaemic mouse stem/progenitor cells, when compared with secretome of normal counterparts (p<0.05). Treatment with LTF replicates the effect of CML exposure on BMDM, including decreased phagocytic function of mouse BMDM and THP-1 derived macrophages (p<0.05). Furthermore, mouse/human BMDM display reduced clearance of CML apoptotic cells following LTF exposure. Notably, LTF treatment reduces CD36 transcript and protein levels on BMDM and/or THP-1 derived macrophages, which correlates with reduced oxygen consumption rate and a 40% decrease in expression of the monocytic marker CD11b during macrophage differentiation, suggesting that secreted LTF is a major contributor to the immature phenotype of CML exposed macrophages. Overall, we present novel findings that dysregulated CML microenvironment modifies BM macrophage function, including their phagocytic and efferocytic capacity. This is at least partially contributed by secretion of LTF from CML stem/progenitor cells and modulation of CD36 expression on altered BM macrophages.

Integrated single cell analysis shows chronic alcohol drinking disrupts monocyte differentiation in the bone marrow.

Chronic heavy alcohol drinking (CHD) rewires monocytes and macrophages toward heightened inflammatory states with compromised antimicrobial defenses that persist after 1-month abstinence. To determine whether these changes are mediated through alterations in the bone marrow niche, we profiled monocytes and hematopoietic stem cell progenitors (HSCPs) from CHD rhesus macaques using a combination of functional assays and single cell genomics. CHD resulted in transcriptional profiles consistent with increased activation and inflammation within bone marrow resident monocytes and macrophages. Furthermore, CHD resulted in transcriptional signatures associated with increased oxidative and cellular stress in HSCP. Differentiation of HSCP invitro revealed skewing toward monocytes expressing "neutrophil-like" markers with greater inflammatory responses to bacterial agonists. Further analyses of HSCPs showed broad epigenetic changes that were in line with exacerbated inflammatory responses within monocytes and their progenitors. In summary, CHD alters HSCPs in the bone marrow leading to the production of monocytes poised to generate dysregulated hyper-inflammatory responses.

Lead exposure suppresses the Wnt3a/β-catenin signaling to increase the quiescence of hematopoietic stem cells via reducing the expression of CD70 on bone marrow-resident macrophages.

Lead (Pb) is a heavy metal highly toxic to human health in the environment. The aim of this study was to investigate the mechanism of Pb impact on the quiescence of hematopoietic stem cells (HSC). WT C57BL/6 (B6) mice treated with 1250 ppm Pb via drinking water for 8 wk had increased the quiescence of HSC in the bone marrow (BM), which was caused by the suppressed activation of the Wnt3a/β-catenin signaling. Mechanically, a synergistic action of Pb and IFNγ on BM-resident macrophages (BM-Mφ) reduced their surface expression of CD70, which thereby dampened the Wnt3a/β-catenin signaling to suppress the proliferation of HSC in mice. In addition, a joint action of Pb and IFNγ also suppressed the expression of CD70 on human Mφ to impair the Wnt3a/β-catenin signaling and reduce the proliferation of human HSC purified from umbilical cord blood (UCB) of healthy donors. Moreover, correlation analyses showed that the blood Pb concentration was or tended to be positively associated with the quiescence of HSC, and was or tended to be negatively associated with the activation of the Wnt3a/β-catenin signaling in HSC in human subjects occupationally exposed to Pb. Collectively, these data indicate that an occupationally relevant level of Pb exposure suppresses the Wnt3a/β-catenin signaling to increase the quiescence of HSC via reducing the expression of CD70 on BM-Mφ in both mice and humans.

Dysplastic Erythropoiesis in Stag2 Loss Exhibits Defective Nuclear Condensation and Hemophagocytosis

Chromosomes fold in a hierarchy of increasingly complex structures from nucleosomes to chromosomal territories. Dynamic chromosomal architecture establishes regulatory cues that allow for local reorganization during cellular differentiation. The ring-shaped cohesin complex, composed of SMC1, SMC3, RAD21, and either STAG1 or STAG2, plays a critical role in forming chromatin loops, permitting genomic partitioning, enhancer-promoter interactions, and topological control of gene expression. While both STAG1 and STAG2 play a role in regulating gene expression and genome organization, we and others have defined distinct roles, owing to their differential cellular localization and range of interactions. Given that STAG2 is recurrently mutated in various cancers, including acute myeloid leukemia and myelodysplastic syndrome, understanding the role of STAG2 in gene regulation, hematopoiesis, and tumor suppression could provide novel therapeutic and clinical insights. Prior work by our group has demonstrated that Stag2 deficiency results in altered hematopoietic stem cell (HSC) self-renewal, impaired HSC differentiation, and lineage skewing. In particular, Stag2 deficiency resulted in marked features of myelodysplasia. Here, we demonstrate that STAG2 loss results in erythroid atypia and hemophagocytosis. We observe decreased terminal erythroid maturation via scRNA-seq of Stag2-/- CFU-E (Lineage-cKit+Sca1−CD34−Fcγ−CD105+). To further investigate the mechanistic basis of this impaired maturation, bone marrow from Stag2-/-, Stag2+/-, and Stag2+/+ mice was stained with H&E, where marked nuclear-cytoplasmic dyssynchrony of erythroblasts was noted in a Stag2 dose dependent manner. Additionally, erythrocytes and erythrocyte debris was observed in bone marrow resident macrophages, suggesting the presence of hemophagocytosis (Fig. 1A). Immunohistochemistry (IHC) for Ter119, an erythroid specific protein, showed markedly reduced surface expression of Ter119, which was confirmed via flow cytometry (p < 0.001). In addition to the overall decrease in Ter119+ cells, the remaining IHC staining was concentrated into distinct foci with Stag2 loss. Immunohistochemistry for Iba1 showed an increased number of Iba1+ macrophages (Mφ) in Stag2-/- mice. Colocalization studies by confocal microscopy further validated hemophagocytosis (Fig.1B). To determine if Mφ overactivation was driving hemophagocytosis, Iba1+ Mφ from Stag2+/+ and Stag2-/- mice were stimulated with IL-4, IL-6, IFN-γ, and LPS and no differences observed as a function of Stag2 genotype. As no anemia present in Stag2-/- mice, we assayed serum erythropoietin and identified a significant increase in erythropoietin in Stag2-/- mice (p < 0.05), suggesting compensatory erythropoietic stimulation. Flow cytometric analysis of Stag2+/+ and Stag2-/- bone marrow stained for CD44 showed significant alterations to terminal erythroid differentiation, with an increase in Stage II erythroblasts (p = 0.01) and decreases in Stage IV/V erythroblasts (p = 0.029 and 0.031, respectively). Subsequent ATAC sequencing of sorted Stag2+/+ and Stag2-/- erythroid progenitors showed an overall more relaxed chromatin state and hyperaccessibility with Stag2 loss, suggesting defective nuclear condensation from the earliest stages of erythropoiesis. Preliminary data from STAG2 shRNA knockdown in human CD34+ cells in vitro suggests that STAG2 may impair chromatin condensation and enucleation during terminal erythroid differentiation. Whether the hyperaccessibility observed with STAG2 loss persists through terminal erythroid differentiation is being investigated to determine the mechanism by which STAG2 loss influences defective nuclear condensation and nuclear extrusion during erythroid differentiation. These data suggest that Stag2 loss negatively affects chromatin condensation, resulting in a compensatory erythropoietin secretion and increased hemophagocytosis. These data extend the role of Stag2 from lineage priming to the latest stages of erythropoiesis and define the critical physiologic role of Stag2 in erythropoiesis. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

P799: SPATIAL ANALYSIS IDENTIFIES A SPECTRUM OF IMMUNE DYSREGULATION IN ACQUIRED BONE MARROW FAILURE CONDITIONS

Background: Poor Graft Function (PGF), manifested by multilineage cytopenias with complete donor chimerism post allogeneic stem cell transplantation, and acquired Aplastic Anaemia (AA) are acquired bone marrow failure syndromes. Both are thought to result from T cell activation resulting in excessive interferon-γ (IFNγ) production, which in turn suppresses haematopoiesis. In AA, this can promote clonal stem cell selection and potentially progression to myeloid malignancy. Despite similarities in the clinical presentation and proposed mechanism, no studies to date have compared the immunobiology of the bone marrow (BM) microenvironment in these conditions. Aims: To examine the immune microenvironment of the BM in archival AA and PGF BM trephines and identify common immunopathologies. Methods: Archival BM trephines were sourced from patients with PGF (n=20), AA at Diagnosis (AA_DX) (n=15), AA at Progression (AA_PROG) to MDS/AML (n=15) and normal controls (NC) (n=20). 6 x 300µm regions identified by dual CD3/CD45 immunofluorescent staining were analysed per BM trephine using NanoString GeoMX™ digital spatial profiling for the expression of 57 proteins with an immunology focused panel. Data was analysed using a Limma-Voom bioinformatics pipeline. Results: Significant dysregulation was identified across multiple proteins. Overall, 22 proteins had significantly different expression in AA_DX samples (13 up, 9 down) compared to NC with AA_PROG and PGF having 8 (4 up, 4 down) and 14 (7 up, 7 down) respectively. As would be expected given the hypocellular BM, expression of CD45 was significantly reduced compared to NC across AA_DX (adj P = 5.413e-10), AA_PROG (adj P = 8.299e-8) and PGF (adj P = 8.634e-9) groups. When AA_DX patients were stratified into those who progressed (n=6) vs those who did not (n=9), or when AA_DX vs AA_PROG was compared in patients with matched samples (n=6) there were no significantly differentially expressed proteins identified. This suggests that while AA provides an immune microenvironment that is permissive to MDS/AML pathogenesis, it may not directly impact on clonal evolution. Total monocytes were unchanged by CD11c expression. However, AA and PGF samples exhibited upregulation of CD163 compared to NC (AA_DX adj P = 4.5e-6; AA_PROG adj P = 1.2e-5; PGF adj P = 1.58e-8) suggesting an increase in monocyte/macrophage lineage cells. Expression of CD66b was also downregulated in AA_DX (adj P = 8.459e-24), AA_PROG (adj P = 1.193e-10) and PGF (adj P = 2.244e-14) suggesting a reduction in non-classical monocytes, neutrophils or granulocytic MDSCs. Importantly, VISTA was downregulated across AA_DX (adj P = 6.84e-27), AA_PROG (adj P = 1.1e-5) and PGF (adj P = 1.58e-8) and STING was downregulated in AA_DX compared to NC (adj P = 6.139e-13) with a trend towards reduced expression in PGF. Decreased expression of these key immunoregulatory proteins may have wide ranging effects on BM resident macrophages, dendritic cells, MDSCs and T cells leading to reduced immunoregulation and increased T cell activation/IFNγ production, ultimately resulting in stem cell depletion. Data on cell specific expression is currently being obtained to refine this model. Summary/Conclusion: Spatial analysis demonstrated that patients with AA and PGF exhibit similar patterns of protein expression likely resulting in a BM immune microenvironment of decreased immunoregulation. AA_DX samples exhibited a greater degree of dysregulation of multiple markers compared to PGF suggesting that these diseases represent a spectrum of immune dysregulation.

Deleterious effect of bone marrow-resident macrophages on hematopoietic stem cells in response to total body irradiation

AbstractBone marrow (BM) resident macrophages interact with a population of long-term hematopoietic stem cells (LT-HSCs) but their role on LT-HSC properties after stress is not well defined. Here, we show that a 2 Gy-total body irradiation (TBI)-mediated death of LT-HSCs is associated with increased percentages of LT-HSCs with reactive oxygen species (ROS) and of BM resident macrophages producing nitric oxide (NO), resulting in an increased percentage of LT-HSCs with endogenous cytotoxic peroxynitrites. Pharmacological or genetic depletion of BM resident macrophages impairs the radio-induced increases in the percentage of both ROS+ LT-HSCs and peroxynitrite+ LT-HSCs and results in a complete recovery of a functional pool of LT-HSCs. Finally, we show that after a 2 Gy-TBI, a specific decrease of NO production by BM resident macrophages improves the LT-HSC recovery, whereas an exogenous NO delivery decreases the LT-HSC compartment. Altogether, these results show that BM resident macrophages are involved in the response of LT-HSCs to a 2 Gy-TBI and suggest that regulation of NO production can be used to modulate some deleterious effects of a TBI on LT-HSCs.

Piezo1-mediated mechanosensation in bone marrow macrophages promotes vascular niche regeneration after irradiation injury.

Background: Irradiation disrupts the vascular niche where hematopoietic stem cells (HSCs) reside, causing delayed hematopoietic reconstruction. The subsequent recovery of sinusoidal vessels is key to vascular niche regeneration and a prerequisite for hematopoietic reconstruction. We hypothesize that resident bone marrow macrophages (BM-Mφs) are responsible for repairing the HSC niche upon irradiation injury.Methods: We examined the survival and activation of BM-Mφs in C57BL/6 mice upon total body irradiation. After BM-Mφ depletion via injected clodronate-containing liposomes and irradiation injury, hematopoietic reconstruction and sinusoidal vascular regeneration were assessed with immunofluorescence and flow cytometry. Then enzyme-linked immunosorbent assay (ELISA) and flow cytometry were performed to analyze the contribution of VEGF-A released by BM-Mφs to the vascular restructuring of the HSC niche. VEGF-A-mediated signal transduction was assessed with transcriptome sequencing, flow cytometry, and pharmacology (agonists and antagonists) to determine the molecular mechanisms of Piezo1-mediated responses to structural changes in the HSC niche.Results: The depletion of BM-Mφs aggravated the post-irradiation injury, delaying the recovery of sinusoidal endothelial cells and HSCs. A fraction of the BM-Mφ population persisted after irradiation, with residual BM-Mφ exhibiting an activated M2-like phenotype. The expression of VEGF-A, which is essential for sinusoidal regeneration, was upregulated in BM-Mφs post-irradiation, especially CD206+ BM-Mφs. The expression of mechanosensory ion channel Piezo1, a response to mechanical environmental changes induced by bone marrow ablation, was upregulated in BM-Mφs, especially CD206+ BM-Mφs. Piezo1 upregulation was mediated by the effects of irradiation, the activation of Piezo1 itself, and the M2-like polarization induced by the phagocytosis of apoptotic cells. Piezo1 activation was associated with increased expression of VEGF-A and increased accumulation of NFATC1, NFATC2, and HIF-1α. The Piezo1-mediated upregulation in VEGF-A was suppressed by inhibiting the calcineurin/NFAT/HIF-1α signaling pathway.Conclusion: These findings reveal that BM-Mφs play a critical role in promoting vascular niche regeneration by sensing and responding to structural changes after irradiation injury, offering a potential target for therapeutic efforts to enhance hematopoietic reconstruction.

An Unbalanced Monocyte Macrophage Polarization in the Bone Marrow Microenvironment of Patients with Poor Graft Function after Allogeneic Hematopoietic Stem Cell Transplantation

BackgroundPoor graft function (PGF) remains a life-threatening complication following allogeneic hematopoietic stem cell transplantation (allo-HSCT), and the underlying mechanisms have not yet been elucidated. Considerable evidence from murine studies has demonstrated that effective hematopoiesis depends on the specific bone marrow (BM) microenvironment, where hematopoietic stem cells (HSCs) reside. In this regard, we previously reported that PGF patients had impaired BM endosteal cells and endothelial progenitor cells, as well as dysregulated T cell responses in the BM microenvironment, which may be involved in the occurrence of PGF (BBMT 2013; BMT2016; Oncotarget 2016; BBMT 2016; Blood 2016; J Transl Med2017). Macrophage is one of the important components of BM immune microenvironment. Meanwhile, Murine studies suggest that macrophages regulate the retention of HSCs by regulating osteoblastic cell activity and nestin-expressing cells. Increasing evidence shows that BM resident macrophages are indispensable for HSCs function and BM erythroid output. However, little is known about the quantity and function of BM macrophages and whether the unbalanced BM macrophages directly interact with HSCs in PGF patients post allo-HSCT.AimsTo compare the number and function of BM macrophages between patients with PGF and GGF after allo-HSCT. Moreover, to investigate whether the unbalanced BM macrophages directly interact with HSCs in PGF patients.MethodsThis prospective nested case-control study enrolled 30 patients with PGF, 60 matched patients with good graft function (GGF), defined as persistent successful engraftment after allotransplant, and 30 healthy donors (HD). Standard monocyte subsets, defined by cluster of differentiation CD14 and CD16, were quantified by flow cytometry. In addition, based on the phenotype of polarized macrophages in vitro, classically activated inflammatory macrophage (M1:CD68+CCR2+) and alternatively activated anti-inflammatory macrophage (M2:CX3CR1+CD163+/CD206+) from BM samples were analyzed by flow cytometry. BM CD14+ monocytes were isolated from BM mononuclear cell (BMMNCs) and were cultured with 100 ng/ml recombinant human M-CSF for 10 days to obtain macrophages. The functions of BM derived macrophages were evaluated by migration assay and phagocytosis assay. CD34+ cells from healthy donors were co-cultured with BM derived macrophages from PGF patients, and the levels of reactive oxygen species (ROS) and apoptosis in CD34+ cells were analyzed by flow cytometry after 5 days co-culture.ResultsAlterations in standard monocyte subsets (classical, intermediate and non-classical) were found when comparing subjects among PGF, GGF and normal controls. In particular, elevated intermediate and non-classical monocyte subsets were observed in PGF patients when compared with those in GGF patients. Moreover, PGF patients displayed an imbalanced M1/M2 ratio compared with the GGF and normal controls, attributable to a reduction in M2 and an increase in M1. Dysfunctional BM derived macrophages, which were characterized by impaired proliferation, migration, phagocytosis and higher level of apoptosis, were revealed in patients with PGF. BM CD34+ cells from healthy donors were co-cultured respectively with BM derived macrophages from patients with PGF and normal controls. Apoptosis and intracellular levels of ROS were markedly increased in CD34+ bone marrow cells which were co-cultured with PGF patients' BM derived macrophages compared with normal controls.Summary/ConclusionIn summary, the current study demonstrated the abnormal number and impaired function of BM macrophages and unbalanced BM macrophages directly interact with HSCs in PGF patients. We speculate that reduction of anti-inflammatory M2 monocytes while increase of pro-inflammatory M1 monocytes in PGF patient BM have a negative impact on HSCs in BM microenvironment. Thus, our results indicate that unbalanced BM M1/M2 and dysfunctional BM macrophages may play an important role in the pathogenesis of PGF following allo-HSCT. Therefore, it would be of value to investigate the appropriate approach to balance the M1/M2 and enhance function of BM macrophages, which may be a promising therapeutic approach for PGF patients. DisclosuresNo relevant conflicts of interest to declare.

Lead in Synergism With IFNγ Acts on Bone Marrow-Resident Macrophages to Increase the Quiescence of Hematopoietic Stem Cells.

Lead (Pb) is a highly toxic heavy metal that broadly exists in our living environment. Although Pb has been shown to influence the development of immune cells, to date, the impact of Pb on hematopoietic stem cells (HSCs) in the bone marrow (BM) remains unknown. As people are ubiquitously exposed to Pb and HSC are essential for human health, understanding the impact of Pb on HSC is significant for public health. In this study, we found that wild-type B6 mice treated with 1250 ppm Pb, but not 125 ppm Pb via drinking water for 8 weeks had increased quiescence of HSC in the BM. Functional analyses demonstrated that wild-type mice treated with 1250 ppm Pb had increased potential for HSC to repopulate the immune system and engraft to the niche in the BM under a competitive chimeric microenvironment of lethally irradiated recipients. Moreover, we found that Pb-increased quiescence of HSC critically relied on a synergetic action of Pb and interferon γ (IFNγ) on BM-resident macrophages (BM-MΦ), but not a direct action of Pb on HSC. Specifically, in steady state, BM-MΦ promoted HSC proliferation; and upon Pb treatment, IFNγ was induced in the BM, and thereafter Pb in synergism with IFNγ acted on BM-MΦ to cause BM-MΦ to become suppressive for HSC proliferation, thus leading to increased quiescence of HSC. Our study suggests that Pb increased the quiescence of HSC via a synergetic action of Pb and IFNγ on BM-MΦ, which was previously unrecognized toxicity of Pb.

Stable colony-stimulating factor 1 fusion protein treatment increases hematopoietic stem cell pool and enhances their mobilisation in mice

BackgroundPrior chemotherapy and/or underlying morbidity commonly leads to poor mobilisation of hematopoietic stem cells (HSC) for transplantation in cancer patients. Increasing the number of available HSC prior to mobilisation is a potential strategy to overcome this deficiency. Resident bone marrow (BM) macrophages are essential for maintenance of niches that support HSC and enable engraftment in transplant recipients. Here we examined potential of donor treatment with modified recombinant colony-stimulating factor 1 (CSF1) to influence the HSC niche and expand the HSC pool for autologous transplantation.MethodsWe administered an acute treatment regimen of CSF1 Fc fusion protein (CSF1-Fc, daily injection for 4 consecutive days) to naive C57Bl/6 mice. Treatment impacts on macrophage and HSC number, HSC function and overall hematopoiesis were assessed at both the predicted peak drug action and during post-treatment recovery. A serial treatment strategy using CSF1-Fc followed by granulocyte colony-stimulating factor (G-CSF) was used to interrogate HSC mobilisation impacts. Outcomes were assessed by in situ imaging and ex vivo standard and imaging flow cytometry with functional validation by colony formation and competitive transplantation assay.ResultsCSF1-Fc treatment caused a transient expansion of monocyte-macrophage cells within BM and spleen at the expense of BM B lymphopoiesis and hematopoietic stem and progenitor cell (HSPC) homeostasis. During the recovery phase after cessation of CSF1-Fc treatment, normalisation of hematopoiesis was accompanied by an increase in the total available HSPC pool. Multiple approaches confirmed that CD48−CD150+ HSC do not express the CSF1 receptor, ruling out direct action of CSF1-Fc on these cells. In the spleen, increased HSC was associated with expression of the BM HSC niche macrophage marker CD169 in red pulp macrophages, suggesting elevated spleen engraftment with CD48−CD150+ HSC was secondary to CSF1-Fc macrophage impacts. Competitive transplant assays demonstrated that pre-treatment of donors with CSF1-Fc increased the number and reconstitution potential of HSPC in blood following a HSC mobilising regimen of G-CSF treatment.ConclusionThese results indicate that CSF1-Fc conditioning could represent a therapeutic strategy to overcome poor HSC mobilisation and subsequently improve HSC transplantation outcomes.

CD4+ T-cell killing of multiple myeloma cells is mediated by resident bone marrow macrophages

AbstractCD4+ T cells may induce potent antitumor immune responses through interaction with antigen-presenting cells within the tumor microenvironment. Using a murine model of multiple myeloma, we demonstrated that adoptive transfer of idiotype-specific CD4+ T cells may elicit curative responses against established multifocal myeloma in bone marrow. This finding indicates that the myeloma bone marrow niche contains antigen-presenting cells that may be rendered tumoricidal. Given the complexity of the bone marrow microenvironment, the mechanistic basis of such immunotherapeutic responses is not known. Through a functional characterization of antitumor CD4+ T-cell responses within the bone marrow microenvironment, we found that killing of myeloma cells is orchestrated by a population of bone marrow–resident CD11b+F4/80+MHC-IIHigh macrophages that have taken up and present secreted myeloma protein. The present results demonstrate the potential of resident macrophages as powerful mediators of tumor killing within the bone marrow and provide a basis for novel therapeutic strategies against multiple myeloma and other malignancies that affect the bone marrow.

CD163 expression defines specific, IRF8-dependent, immune-modulatory macrophages in the bone marrow

Scavenger receptor CD163 is exclusively expressed on monocytes/macrophages and is widely used as a marker for alternatively activated macrophages. However, the role of CD163 is not yet clear. We sought to examine the function of CD163 in steady-state as well as in sterile and infectious inflammation. Expression of CD163 was analyzed under normal and inflammatory conditions in mice. Functional relevance of CD163 was investigated in models of inflammation in wild-type and CD163-/- mice. We describe a subpopulation of bone marrow-resident macrophages (BMRMs) characterized by a high expression of CD163 and functionally distinct from classical bone marrow-derived macrophages. Development of CD163+ BMRMs is strictly dependent on IFN regulatory factor-8. CD163+ BMRMs show a specific transcriptome and cytokine secretion pattern demonstrating a specific immunomodulatory profile of these cells. Accordingly, CD163-/- mice show a stronger inflammation in allergic contact dermatitis, indicating a regulatory role of CD163. However, CD163-/- mice are highly susceptible to S aureus infections, demonstrating the relevance of CD163 for antimicrobial defense as well. Our data indicate that anti-inflammatory and immunosuppressive mechanisms are not necessarily associated with a decreased antimicrobial activity. In contrast, our data define a novel macrophage population that controls overwhelming inflammation on one hand but is also necessary for an effective control of infections on the other hand.

The Role of Interferon, Inflammation and Infection in Aplastic Anemia

Bone marrow failure (BMF) syndromes are a group of acquired and inherited diseases characterized by failed production of blood and immune cells, and profound bone marrow aplasia. Acquired BMF, including severe aplastic anemia (SAA), has been linked to radiation, chemical exposure, and infection, and the precise mechanisms of disease likely differ on a case-by-case basis. Inflammation and immune-mediated destruction of the marrow is a key driver of SAA, thus disease management centers on immunosuppressive therapy (IST) and, especially in young patients, bone marrow transplantation (BMT). However, not all patients are good transplant candidates and IST responsiveness varies, therefore, more specific treatments are necessary. HSC loss is a key feature of SAA, though it has been unclear if inflammation depletes HSCs directly or does so through the microenvironment. Interferons (IFNs) are proteins produced in response to microbial infections that play key roles in limiting pathogen spread and eradicating infections. While transient inflammatory responses and IFN production are often necessary for appropriate host defense, elevated levels of IFN-gamma (IFNγ) have long been associated with BMF syndromes, and type I IFNs (alpha and beta) are well documented to cause bone marrow aplasia during viral infection 1. In models of infection and inflammation IFNs can activate HSCs to proliferate and differentiate, but can also impair proliferation, and in both cases result in restricting HSC self-renewal 2-5. The precise mechanisms whereby IFNs mediate SAA and whether IFNs act directly on HSCs have remained elusive. In a mouse model of SAA driven by T cell-derived IFNγ we found that IFNγ-dependent HSC loss was associated with reduced marrow stromal cells, but a marked preservation of bone marrow-resident macrophages (MΦs) 6. Our data were consistent with findings from SAA patient marrow that also demonstrates MΦ persistence, despite significant reductions in nearly all other stromal and hematopoietic cell types 7. Moreover, we found that IFNγ was necessary for the maintained pool of MΦs. Depleting MΦs using Clodronate-loaded liposomes, or abrogating IFNγ signaling specifically in MΦs was shown to rescue HSCs and reduce SAA-associated mortality. Targeting MΦs during SAA improved thrombocytopenia, increased BM megakaryocytes, preserved platelet-primed HSCs, and increased the platelet-repopulating capacity of transplanted HSCs 6. During SAA IFNγ and MΦs were required for elevated levels of particular chemokines in the bone marrow, including CCL3, CCL4, and CCL5, which were further identified as downstream targets for mitigating SAA pathogenesis. The identification of MΦ function as a key determinant of IFNγ-dependent HSC loss in SAA furthers our understanding of disease pathogenesis and illustrates an important role for the microenvironment in SAA. These studies may reveal potential therapeutic strategies to complement current treatments for SAA that avoid generalized immunosuppression and potentially improve long-term outcomes. Smith JN, Kanwar VS, MacNamara KC. Hematopoietic Stem Cell Regulation by Type I and II Interferons in the Pathogenesis of Acquired Aplastic Anemia. Frontiers in immunology. 2016;7:330. Baldridge MT, King KY, Boles NC, Weksberg DC, Goodell MA. Quiescent haematopoietic stem cells are activated by IFN-gamma in response to chronic infection. Nature. 2010;465(7299):793-797. de Bruin AM, Demirel O, Hooibrink B, Brandts CH, Nolte MA. Interferon-gamma impairs proliferation of hematopoietic stem cells in mice. Blood. 2013;121(18):3578-3585. MacNamara KC, Jones M, Martin O, Winslow GM. Transient activation of hematopoietic stem and progenitor cells by IFNgamma during acute bacterial infection. PLoS One. 2011;6(12):e28669. Matatall KA, Jeong M, Chen S, et al. Chronic Infection Depletes Hematopoietic Stem Cells through Stress-Induced Terminal Differentiation. Cell reports. 2016;17(10):2584-2595. McCabe A, Smith JNP, Costello A, Maloney J, Katikaneni D, MacNamara KC. Hematopoietic stem cell loss and hematopoietic failure in severe aplastic anemia is driven by macrophages and aberrant podoplanin expression. Haematologica. 2018;103(9):1451-1461. Park M, Park CJ, Jang S, et al. Reduced expression of osteonectin and increased natural killer cells may contribute to the pathophysiology of aplastic anemia. Appl Immunohistochem Mol Morphol. 2015;23(2):139-145. Disclosures No relevant conflicts of interest to declare.

Clodronate-Liposome Mediated Macrophage Depletion Abrogates Multiple Myeloma Tumor Establishment In Vivo

Multiple myeloma is a fatal plasma cell malignancy that is reliant on the bone marrow microenvironment. The bone marrow is comprised of numerous cells of mesenchymal and hemopoietic origin. Of these, macrophages have been implicated to play a role in myeloma disease progression, angiogenesis, and drug resistance; however, the role of macrophages in myeloma disease establishment remains unknown. In this study, the antimyeloma efficacy of clodronate-liposome treatment, which globally and transiently depletes macrophages, was evaluated in the well-established C57BL/KaLwRijHsd murine model of myeloma. Our studies show, for the first time, that clodronate-liposome pretreatment abrogates myeloma tumor development in vivo. Clodronate-liposome administration resulted in depletion of CD169+ bone marrow–resident macrophages. Flow cytometric analysis revealed that clodronate-liposome pretreatment impaired myeloma plasma cell homing and retention within the bone marrow 24 hours postmyeloma plasma cell inoculation. This was attributed in part to decreased levels of macrophage-derived insulin-like growth factor 1. Moreover, a single dose of clodronate-liposome led to a significant reduction in myeloma tumor burden in KaLwRij mice with established disease. Collectively, these findings support a role for CD169-expressing bone marrow–resident macrophages in myeloma disease establishment and progression and demonstrate the potential of targeting macrophages as a therapy for myeloma patients.

On Demand Recruitment of Macrophages Is Required for Erythroid Niche Formation during Stress Erythropoiesis in the Bone Marrow

Every second more than 2 million new erythrocytes are released from the bone marrow of human adults, highlighting the tremendous turn-over of these cells. In parallel to hematopoietic stem cell niches, the last stages of erythropoiesis take place in specialized bone marrow niches, termed ‘erythroid niches’. Concretely, the erythroid niche is composed of a ‘central macrophage’ which is surrounded by erythroid progenitor cells. Regardless of steady-state or stress erythropoiesis, iron availability is, beside erythropoietin, a key factor determining erythroid output and red blood cell quality, as reflected by hemoglobin content of these cells. It is well established that systemic iron availability for erythropoiesis, in the form of iron saturated transferrin (Tf), is mainly maintained via a recycling process of senescent red blood cells, which takes place in macrophages of the reticuloendothelial system. Yet, it is still a matter of debate if also central macrophages are involved in iron supply for red blood cell development in a more direct way due to their close proximity to developing red blood cells. Using a myeloid-specific knockout mouse strain, lacking the solely known iron exporter ferroportin (Fpn; Fpnfl/flLysMCre+/+ mice) and specific reporter mice (ROSA26tdTomatofl/fl Cx3cr1CreERT2 mice), we examined the connection between iron metabolism, erythropoiesis and central macrophages.Analysis of Fpnfl/flLysMCre+/+ animals at steady state revealed microcytic anemia, higher tissue iron loading, reduced hepatic hepcidin expression and distorted erythroid precursor population distribution in the bone marrow with no significant chances in Tf saturation (Tf-Sat). The latter is giving a first hint, that local bone marrow Fpn expression on macrophages may be important for iron supply for erythropoiesis. Strikingly, further work up via flow cytometry demonstrated that disturbances seen in bone marrow erythropoiesis were accompanied by nearby loss of resident bone marrow macrophages (defined as CD11blo, F4/80pos, MerTKpos). In parallel, a CD11bhi, F4/80pos, MerTKpos population came into existence, suggesting that these cells may compensate for the loss of ‘canonical’ central macrophages.Attempting to explain these intriguing results, we sought to investigate differentiation pathways and turnover of bone marrow central macrophages. First, we used the ROSA26tdTomatofl/fl Cx3cr1CreERT2 monocyte-specific reporter mice and techniques of transient monocyte labelling in utero and in adult phlebotomized animals to determine the origin of central macrophages. We could show that those cells undergo constant replenishment by circulating monocytes. Notably, the rate of this process got markedly increased upon recovery from blood loss and concomitant expansion of the central macrophage population. Second, by administration of a CCR2/CCR5 inhibitor (cenicriviroc), diminishing monocyte egress from the bone marrow and tissue infiltration, we could demonstrate decreased reticulocyte count during stress erythropoiesis, thus strengthening the direct impact of macrophages to support effective erythroid output.Next, effects of stress-induced erythropoiesis were investigated in Fpnfl/flLysMCre+/+ compared to Fpnfl/flLysMCre-/- mice. Amelioration of anemia after phlebotomy was extended, microcytosis was more pronounced and reticulocyte egress was diminished but prolonged. Of interest, Fpnfl/flLysMCre+/+ mice on a diet containing an 8-times higher iron content during phlebotomy, thus transiently increasing Tf-Sat, recovered from anemia wildtype-like. These results indicate that stress erythropoiesis with a high iron demand depends, under normal iron availability, in part on central macrophages and their nursing function to overcome the increased demand of iron.Ongoing experiments aim to identify how recruited bone marrow macrophages, i.e. central macrophages, contribute to erythropoiesis during stress - if central macrophages directly supply developing erythroid cells with iron in a Tf-free fashion or, if they are suppliers of additional growth factors that work synergistically with the Tf-bound iron to drive hemoglobin production. In summary our data clearly show that macrophages need to be recruited to the bone marrow for effective erythroid output during stress erythropoiesis. DisclosuresWeiss:Kymab Ltd.: Consultancy. Theurl:Kymab Ltd.: Consultancy, Research Funding.

Self-repopulating recipient bone marrow resident macrophages promote long-term hematopoietic stem cell engraftment

AbstractDistinct subsets of resident tissue macrophages are important in hematopoietic stem cell niche homeostasis and erythropoiesis. We used a myeloid reporter gene (Csf1r-eGFP) to dissect the persistence of bone marrow and splenic macrophage subsets following lethal irradiation and autologous hematopoietic stem cell transplantation in a mouse model. Multiple recipient bone marrow and splenic macrophage subsets survived after autologous hematopoietic stem cell transplantation with organ-specific persistence kinetics. Short-term persistence (5 weeks) of recipient resident macrophages in spleen paralleled the duration of extramedullary hematopoiesis. In bone marrow, radiation-resistant recipient CD169+ resident macrophages and erythroid-island macrophages self-repopulated long-term after transplantation via autonomous cell division. Posttransplant peak expansion of recipient CD169+ resident macrophage number in bone marrow aligned with the persistent engraftment of phenotypic long-term reconstituting hematopoietic stem cells within bone marrow. Selective depletion of recipient CD169+ macrophages significantly compromised the engraftment of phenotypic long-term reconstituting hematopoietic stem cells and consequently impaired hematopoietic reconstitution. Recipient bone marrow resident macrophages are essential for optimal hematopoietic stem cell transplantation outcomes and could be an important consideration in the development of pretransplant conditioning therapies and/or chemoresistance approaches.

Mercury impact on hematopoietic stem cells is regulated by IFNγ-dependent bone marrow-resident macrophages in mice

Although immunotoxic effects of mercury (Hg) have been extensively investigated, the influence of Hg on hematopoietic stem cells (HSC) remains elusive. The aim of this study was to investigate the effects of Hg on HSC. B10.S (H-2s) and DBA/2 mice (H-2d) were treated with Hg chloride (25, 50 or 100 μM HgCl2) or methyl Hg (1.25, 3.75 or 6.25 μM MeHg) via drinking water for 4 weeks, and thereafter, HSC in the bone marrow (BM) were evaluated. The number of HSC in B10.S mice was increased after treatment with 50 μM HgCl2 and decreased after treatment with 100 μM HgCl2; the number of HSC in DBA/2 mice was reduced after treatment with 50 μM HgCl2 and unaffected after treatment with 25 μM HgCl2. These effects from the HgCl2 treatments were associated with alterations of HSC proliferation, IFNγ expression and BM-resident macrophages. In vivo neutralization of IFNγ diminished the HgCl2-driven HSC proliferation, and in vivo replenishment of recombinant IFNγ eliminated the HgCl2 suppression of HSC proliferation and allowed HgCl2 enhancement of proliferation, suggesting a pivotal role of IFNγ in HSC proliferation regulated by HgCl2. In vivo depletion of macrophages and an in vitro co-culture assay indicated that BM-resident macrophages promoted HSC proliferation during HgCl2 exposure. Furthermore, the induction of BM-resident macrophages was critically dependent on IFNγ. In contrast, MeHg did not influence HSC in B10.S or DBA/2 mice. Collectively, HgCl2, but not MeHg, affects HSC through regulating IFNγ-dependent BM-resident macrophages in mice. These findings reveal a previously unknown toxicity of Hg.

Expanding the Role of Thyroid-Stimulating Hormone in Skeletal Physiology.

The dogma that thyroid-stimulating hormone (TSH) solely regulates the production of thyroid hormone from the thyroid gland has hampered research on its wider physiological roles. The action of pituitary TSH on the skeleton has now been well described; in particular, its action on osteoblasts and osteoclasts. It has also been recently discovered that the bone marrow microenvironment acts as an endocrine circuit with bone marrow-resident macrophages capable of producing a novel TSH-β subunit variant (TSH-βv), which may modulate skeletal physiology. Interestingly, the production of this TSH-βv is positively regulated by T3 accentuating such modulation in the presence of thyroid overactivity. Furthermore, a number of small molecule ligands acting as TSH agonists, which allosterically modulate the TSH receptor have been identified and may have similar modulatory influences on bone cells suggesting therapeutic potential. This review summarizes our current understanding of the role of TSH, TSH-β, TSH-βv, and small molecule agonists in bone physiology.

Cultured Human Macrophages As a Model for Central Macrophages in Erythroblastic Islands

Erythropoiesis occurs in erythroblastic islands, specific structures in the bone marrow comprising a central macrophage surrounded by erythroid precursors at different stages of terminal differentiation. The central macrophage of the erythroblastic island supports proliferation and differentiation of erythroblasts, as well as phagocytosis of the extruded erythroblast nuclei, the pyrenocytes. Its identity, however, has been poorly characterized. We previously showed that macrophages also enhance in vitro erythropoiesis because they support hematopoietic stem cell (HSC) survival [Heideveld et al. 2015]. Thus, bone marrow macrophages affect all stages of erythropoiesis. The aim of our study is to characterize the relevant human bone marrow macrophages and unravel the mechanism by which they support erythropoiesis with the ultimate goal (i) to optimize erythroblast culture systems that produce erythrocytes for transfusion purposes, and (ii) to target macrophages in vivo to improve erythropoiesis in anemic patients.Macrophages are a heterogeneous population, that can be divided into pro-inflammatory M1 and anti-inflammatory M2 macrophages. Macrophages that we showed to support stem cell survival, and subsequently enhance the yield of erythroid cell cultures, were characterized as a subclass: M2c-like macrophages. These macrophages were derived from CD14+ cells isolated from human peripheral blood mononuclear cells that were cultured in serum-free media supplemented with stem cell factor, erythropoietin and dexamethasone. Within three days these macrophages expressed CD163high, CD169, mannose receptor (MR), CXCR4 and HLA-DR and harbored characteristics of bone marrow resident macrophages. This differentiation process was dependent on glucocorticoid receptor activation. Mass spectrometry of monocytes cultured in presence and absence of dexamethasone showed that expression of CD163 and MR was strictly Dex-dependent, underscoring the role of glucocorticoids in the phenotype of M2c macrophages. Protein ontology analysis revealed dexamethasone-mediated enrichment of lysosome, endocytosis and endothelial development (e.g. STAB1, IL13RA1, CD81, SLC1A3 and FKBP5). We wondered whether these macrophages with increased endosomal and lysosomal capacity not only support stem cell survival and enable erythroid commitment, but also support erythroblastic islands. In mice, it has been shown that clearance of the pyrenocytes by central macrophages occurs presumably via TAM-receptors on the macrophages. Indeed, mRNA expression of cultured M2c-like macrophages showed increased levels of TAM family members MerTK and AXL. Functionally, these macrophages have the capacity to phagocytose zymosan and to bind nuclei. Furthermore, co-culture of the M2c-like macrophages with erythroblasts yielded GPA+(erythroid marker)/CD14+ cell aggregates that suggested the formation of erythroblastic islands. Interestingly, M2c-like macrophages expressing CD163high, MR and CD169 were also observed in human bone marrow aspirates and human fetal livers resembling macrophages induced in in vitro cultures in presence of dexamethasone.Currently, we investigate the mechanism by which glucocorticoids induce monocytes to differentiate into macrophages that may be used to model erythroblastic island-mediated erythropoiesis. Knowledge on the function of such a erythroblastic island is lacking by the absence of an in vitro model. Furthermore, targeting this mechanism in vivo may enhance the recovery of erythropoiesis following bone marrow transplantation.CD14+ cells from peripheral blood positively regulate hematopoietic stem and progenitor cell survival resulting in increased erythroid yield. (2015) Heideveld E, Masiello F, Marra M, Esteghamat F, Yağcı N, von Lindern M, Migliaccio AR, van den Akker E. Haematologica. 100(11):1396-1406 DisclosuresNo relevant conflicts of interest to declare.

In Vivo Imaging of Antileukemic Drug Asparaginase Reveals a Rapid Macrophage-Mediated Clearance from the Bone Marrow.

In vivo localization of 111In-labeled Escherichia coli asparaginase was performed in C57BL/6 mice by both small-animal SPECT/CT and ex vivo biodistribution studies. Mice were treated with liposomal clodronate to investigate the effect of macrophage depletion on tracer localization and drug clearance in vivo. Moreover, macrophage cell line models RAW264.7 and THP-1, as well as knockout mice, were used to identify the cellular and molecular components controlling asparaginase pharmacokinetics. In vivo imaging and biodistribution studies showed a rapid accumulation of asparaginase in macrophage-rich tissues such as the liver, spleen, and in particular bone marrow. Clodronate-mediated depletion of phagocytic cells markedly prolonged the serum half-life of asparaginase in vivo and decreased drug uptake in these macrophage-rich organs. Immunohistochemistry and in vitro binding assays confirmed the involvement of macrophagelike cells in the uptake of asparaginase. We identified the activity of the lysosomal protease cathepsin B in macrophages as a rate-limiting factor in degrading asparaginase both in vitro and in vivo. We showed that asparaginase is rapidly cleared from the serum by liver-, spleen-, and bone marrow-resident phagocytic cells. As a consequence of this efficient uptake and protease-mediated degradation, particularly bone marrow-resident macrophages may provide a protective niche to leukemic cells.