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1.  Ischemic Stroke Activates Hematopoietic Bone Marrow Stem Cells 
Circulation research  2014;116(3):407-417.
The mechanisms leading to an expanded neutrophil and monocyte supply after stroke are incompletely understood.
To test the hypothesis that transient middle cerebral artery occlusion (tMCAO) in mice leads to activation of hematopoietic bone marrow stem cells.
Methods and Results
Serial in vivo bioluminescence reporter gene imaging in mice with tMCAO revealed that bone marrow cell cycling peaked 4 days after stroke (p<0.05 versus pre tMCAO). FACS and cell cycle analysis showed activation of the entire hematopoietic tree, including myeloid progenitors. The cycling fraction of the most upstream hematopoietic stem cells increased from 3.34%±0.19 to 7.32±0.52 after tMCAO (p<0.05). In vivo microscopy corroborated proliferation of adoptively transferred hematopoietic progenitors in the bone marrow of mice with stroke. The hematopoietic system’s myeloid bias was reflected by increased expression of myeloid transcription factors, including PU.1 (p<0.05), and by a decline in lymphocyte precursors. In mice after tMCAO, tyrosine hydroxylase levels in sympathetic fibers and bone marrow noradrenaline levels rose (p<0.05, respectively), associated with a decrease of hematopoietic niche factors that promote stem cell quiescence. In mice with genetic deficiency of the β3 adrenergic receptor, hematopoietic stem cells did not enter the cell cycle in increased numbers after tMCAO (naive control, 3.23±0.22; tMCAO, 3.74±0.33, p=0.51).
Ischemic stroke activates hematopoietic stem cells via increased sympathetic tone, leading to a myeloid bias of hematopoiesis and higher bone marrow output of inflammatory Ly6Chigh monocytes and neutrophils.
PMCID: PMC4312511  PMID: 25362208
Bone marrow; stroke; hematopoietic stem cells; monocyte
2.  Macrophages retain hematopoietic stem cells in the spleen via VCAM-1 
Dutta et al. show that targeting VACM-1 expression in splenic macrophages impairs extramedullary hematopoiesis, thus reducing inflammation in mouse ischemic heart and atherosclerotic plaques.
Splenic myelopoiesis provides a steady flow of leukocytes to inflamed tissues, and leukocytosis correlates with cardiovascular mortality. Yet regulation of hematopoietic stem cell (HSC) activity in the spleen is incompletely understood. Here, we show that red pulp vascular cell adhesion molecule 1 (VCAM-1)+ macrophages are essential to extramedullary myelopoiesis because these macrophages use the adhesion molecule VCAM-1 to retain HSCs in the spleen. Nanoparticle-enabled in vivo RNAi silencing of the receptor for macrophage colony stimulation factor (M-CSFR) blocked splenic macrophage maturation, reduced splenic VCAM-1 expression and compromised splenic HSC retention. Both, depleting macrophages in CD169 iDTR mice or silencing VCAM-1 in macrophages released HSCs from the spleen. When we silenced either VCAM-1 or M-CSFR in mice with myocardial infarction or in ApoE−/− mice with atherosclerosis, nanoparticle-enabled in vivo RNAi mitigated blood leukocytosis, limited inflammation in the ischemic heart, and reduced myeloid cell numbers in atherosclerotic plaques.
PMCID: PMC4387283  PMID: 25800955
3.  Differential Contribution of Monocytes to Heart Macrophages in Steady-State and after Myocardial Infarction 
Circulation research  2014;115(2):284-295.
Macrophages populate the steady-state myocardium. Previously, all macrophages were thought to arise from monocytes; however, it emerged that in several organs tissue-resident macrophages may self-maintain through local proliferation.
To study the contribution of monocytes to cardiac resident macrophages in steady-state, after macrophage depletion in CD11bDTR/+ mice and in myocardial infarction.
Methods and Results
Using in vivo fate mapping and flow cytometry, we estimated that during steady-state the heart macrophage population turns over in about one month. To explore the source of cardiac resident macrophages, we joined the circulation of mice using parabiosis. After 6 weeks, we observed blood monocyte chimerism of 35.3±3.4% while heart macrophages showed a much lower chimerism of 2.7±0.5% (p<0.01). Macrophages self renewed locally through proliferation: 2.1±0.3% incorporated BrdU 2 hours after a single injection and 13.7±1.4% heart macrophages stained positive for the cell cycle marker Ki67. The cells likely participate in defense against infection, as we found them to ingest fluorescently labeled bacteria. In ischemic myocardium, we observed that tissue resident macrophages died locally while some also migrated to hematopoietic organs. If the steady-state was perturbed by coronary ligation or diphtheria toxin-induced macrophage depletion in CD11bDTR/+ mice, blood monocytes replenished heart macrophages. However, in the chronic phase after myocardial infarction, macrophages residing in the infarct were again independent from the blood monocyte pool, returning to the steady-state situation.
In this study we show differential contribution of monocytes to heart macrophages during steady-state, after macrophage depletion or in the acute and chronic phase after myocardial infarction. We found that macrophages participate in the immunosurveillance of myocardial tissue. These data correspond with previous studies on tissue-resident macrophages and raise important questions on the fate and function of macrophages during the development of heart failure.
PMCID: PMC4082439  PMID: 24786973
Macrophage; monocyte; heart; myocardial infarction; myocardial
4.  In vivo silencing of the transcription factor IRF5 reprograms the macrophage phenotype and improves infarct healing 
The aim of the study was to test wether silencing of the transcription factor Interferon Regulatory Factor 5 (IRF5) in cardiac macrophages improves infarct healing and attenuates post-MI remodeling.
In healing wounds, M1➛M2 macrophage phenotype transition supports resolution of inflammation and tissue repair. Persistence of inflammatory M1 macrophages may derail healing and compromise organ functions. The transcription factor IRF5 promotes genes associated with M1 macrophages.
Here we used nanoparticle-delivered siRNA to silence the transcription factor IRF5 in macrophages residing in myocardial infarcts (MI) and in surgically induced skin wounds in mice.
Infarct macrophages expressed high levels of IRF5 during the early inflammatory wound healing stages (day 4 after coronary ligation) whereas expression of the transcription factor decreased during the resolution of inflammation (day 8). Following in vitro screening, we identified an siRNA sequence that, when delivered by nanoparticles to wound macrophages, efficiently suppressed expression of IRF5 in vivo. Reduction of IRF5 expression, a factor that regulates macrophage polarization, reduced inflammatory M1 macrophage markers, supported resolution of inflammation, accelerated cutaneous and infarct healing and attenuated development of post-MI heart failure after coronary ligation as measured by protease targeted FMT-CT imaging and cardiac MRI (p<0.05 respectively).
This work identifies a new therapeutic avenue to augment resolution of inflammation in healing infarcts by macrophage phenotype manipulation. This therapeutic concept may be used to attenuate post-MI remodeling and heart failure.
PMCID: PMC3992176  PMID: 24361318
5.  Chronic variable stress activates hematopoietic stem cells 
Nature medicine  2014;20(7):754-758.
Exposure to psychosocial stress is a risk factor for many diseases, including atherosclerosis1,2. While incompletely understood, interaction between the psyche and the immune system provides one potential mechanism linking stress and disease inception and progression. Known crosstalk between the brain and immune system includes the hypothalamic–pituitary–adrenal axis, which centrally drives glucocorticoid production in the adrenal cortex, and the sympathetic–adrenal–medullary axis, which controls stress–induced catecholamine release in support of the fight–or–flight reflex3,4. It remains unknown however if chronic stress changes hematopoietic stem cell activity. Here we show that stress increases proliferation of these most primitive progenitors, giving rise to higher levels of disease–promoting inflammatory leukocytes. We found that chronic stress induced monocytosis and neutrophilia in humans. While investigating the source of leukocytosis in mice, we discovered that stress activates upstream hematopoietic stem cells. Sympathetic nerve fibers release surplus noradrenaline, which uses the β3 adrenergic receptor to signal bone marrow niche cells to decrease CXCL12 levels. Consequently, elevated hematopoietic stem cell proliferation increases output of neutrophils and inflammatory monocytes. When atherosclerosis–prone ApoE−/− mice encounter chronic stress, accelerated hematopoiesis promotes plaque features associated with vulnerable lesions that cause myocardial infarction and stroke in humans.
PMCID: PMC4087061  PMID: 24952646
6.  The innate immune system after ischemic injury — lessons to be learned from the heart and brain 
JAMA neurology  2014;71(2):233-236.
Innate immune cells are critically involved in ischemic complications of atherosclerosis. While new insight emerged on the origin and role of leukocytes in steady state, the knowledge about myeloid cell's sources, functions and fate after stroke is limited. In our review, we highlight open questions in this important area while examining potential parallels in the immune response after stroke and myocardial infarction. We stress the need to better understand systemic interactions between ischemic tissue, immunity and hematopoiesis, as turn over of leukocytes in inflammatory sites can be rapid, and cell production and supply may serve as future therapeutic targets to modulate inflammation in the vessel wall, the brain and heart.
PMCID: PMC3946050  PMID: 24296962
7.  Monocyte-directed RNAi targeting CCR2 improves infarct healing in atherosclerosis-prone mice 
Circulation  2013;127(20):2038-2046.
Exaggerated and prolonged inflammation after myocardial infarction (MI) accelerates left ventricular remodeling. Inflammatory pathways may present a therapeutic target to prevent post-MI heart failure. However, the appropriate magnitude and timing of interventions are largely unknown, in part because noninvasive monitoring tools are lacking. We here employed nanoparticle-facilitated silencing of CCR2, the chemokine receptor that governs inflammatory Ly-6Chigh monocyte subset traffic, to reduce infarct inflammation in apoE−/− mice after MI. We used dual target PET/MRI of transglutaminase factor XIII (FXIII) and myeloperoxidase (MPO) activity to monitor how monocyte subset-targeted RNAi altered infarct inflammation and healing.
Methods and Results
Flow cytometry, gene expression analysis and histology revealed reduced monocyte numbers and enhanced resolution of inflammation in infarcted hearts of apoE−/− mice that were treated with nanoparticle-encapsulated siRNA. To follow extracellular matrix crosslinking non-invasively, we developed a fluorine-18 labeled PET agent (18F-FXIII). Recruitment of MPO-rich inflammatory leukocytes was imaged using a molecular MRI sensor of MPO activity (MPO-Gd). PET/MRI detected anti-inflammatory effects of intravenous nanoparticle-facilitated siRNA therapy (75% decrease of MPO-Gd signal, p<0.05) while 18F-FXIII PET reflected unimpeded matrix crosslinking in the infarct. Silencing of CCR2 during the first week after MI improved ejection fraction on day 21 after MI from 29 to 35% (p<0.05).
CCR2 targeted RNAi reduced recruitment of Ly-6Chigh monocytes, attenuated infarct inflammation and curbed post-MI left ventricular remodeling.
PMCID: PMC3661714  PMID: 23616627
myocardial infarction; remodeling; monocytes; RNAi; PET/MRI
8.  Myocardial infarction accelerates atherosclerosis 
Nature  2012;487(7407):325-329.
During progression of atherosclerosis, myeloid cells destabilize lipid-rich plaque in the arterial wall and cause its rupture, thus triggering myocardial infarction and stroke. Survivors of acute coronary syndromes have a high risk of recurrent events for unknown reasons. Here we show that the systemic response to ischemic injury aggravates chronic atherosclerosis. After myocardial infarction or stroke, apoE−/− mice developed larger atherosclerotic lesions with a more advanced morphology. This disease acceleration persisted over many weeks and was associated with markedly increased monocyte recruitment. When seeking the source of surplus monocytes in plaque, we found that myocardial infarction liberated hematopoietic stem and progenitor cells from bone marrow niches via sympathetic nervous system signaling. The progenitors then seeded the spleen yielding a sustained boost in monocyte production. These observations provide new mechanistic insight into atherogenesis and provide a novel therapeutic opportunity to mitigate disease progression.
PMCID: PMC3401326  PMID: 22763456
10.  Therapeutic siRNA silencing in inflammatory monocytes 
Nature biotechnology  2011;29(11):1005-1010.
Inflammatory monocytes -- but not the non-inflammatory subset -- depend on the chemokine receptor CCR2 for distribution to injured tissue and stimulate disease progression. Precise therapeutic targeting of this inflammatory monocyte subset could spare innate immunity's essential functions for maintenance of homeostasis and thus limit unwanted effects. Here we developed siRNA nanoparticles targeting CCR2 expression in inflammatory monocytes. We identified an optimized lipid nanoparticle and silencing siRNA sequence that when administered systemically, had rapid blood clearance, accumulated in spleen and bone marrow and showed high cellular localization of fluorescently tagged siRNA inside monocytes. Efficient degradation of CCR2 mRNA in monocytes prevented their accumulation in sites of inflammation. Specifically, the treatment attenuated their number in atherosclerotic plaques, reduced infarct size following coronary artery occlusion, prolonged normoglycemia in diabetic mice after pancreatic islet transplantation and resulted in reduced tumor volumes and lower numbers of tumor-associated macrophages. Taken together, siRNA nanoparticle-mediated CCR2 gene silencing in leukocytes selectively modulates functions of innate immune cell subtypes and may allow for the development of specific anti-inflammatory therapy.
PMCID: PMC3212614  PMID: 21983520
12.  IL-6-Dependent PGE2 Secretion by Mesenchymal Stem Cells Inhibits Local Inflammation in Experimental Arthritis 
PLoS ONE  2010;5(12):e14247.
Based on their capacity to suppress immune responses, multipotent mesenchymal stromal cells (MSC) are intensively studied for various clinical applications. Although it has been shown in vitro that the immunomodulatory effect of MSCs mainly occurs through the secretion of soluble mediators, the mechanism is still not completely understood. The aim of the present study was to better understand the mechanisms underlying the suppressive effect of MSCs in vivo, using cells isolated from mice deficient in the production of inducible nitric oxide synthase (iNOS) or interleukin (IL)-6 in the murine model of collagen-induced arthritis.
Principal Findings
In the present study, we show that primary murine MSCs from various strains of mice or isolated from mice deficient for iNOS or IL-6 exhibit different immunosuppressive potential. The immunomodulatory function of MSCs was mainly attributed to IL-6-dependent secretion of prostaglandin E2 (PGE2) with a minor role for NO. To address the role of these molecules in vivo, we used the collagen-induced arthritis as an experimental model of immune-mediated disorder. MSCs effectively inhibited collagen-induced inflammation during a narrow therapeutic window. In contrast to wild type MSCs, IL-6-deficient MSCs and to a lesser extent iNOS-deficient MSCs were not able to reduce the clinical signs of arthritis. Finally, we show that, independently of NO or IL-6 secretion or Treg cell induction, MSCs modulate the host response by inducing a switch to a Th2 immune response.
Our data indicate that MSCs mediate their immunosuppressive effect via two modes of action: locally, they reduce inflammation through the secretion of anti-proliferative mediators, such as NO and mainly PGE2, and systemically they switch the host response from a Th1/Th17 towards a Th2 immune profile.
PMCID: PMC2998425  PMID: 21151872

Results 1-12 (12)