Transplantation of vascular endothelial cells mediates the hematopoietic recovery and survival of lethally irradiated mice

Thrombopoietin improves the functions of bone marrow endothelial progenitor cells via METTL16/Akt signalling of haematological patients with chemotherapy-induced thrombocytopenia

Bone marrow endothelial progenitor cells (BM EPCs) are crucial in supporting haematopoietic regeneration, while the BM EPCs of haematological patients with chemotherapy-induced thrombocytopenia (CIT) are unavoidably damaged. Therefore, the present study aimed to examine the effect of thrombopoietin (TPO) on the recovery of BM EPCs of CIT patients and to identify the underlying mechanisms. The cell functions were determined by 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil)-acetylated low-density lipoprotein (Dil-Ac-LDL) uptake and fluorescein isothiocyanate (FITC)-labeled Ulex europaeus agglutinin-I (FITC-UEA-I) binding assay, as well as proliferation, migration and tube formation experiments. Endothelial cells were transfected with METTL16 lentivirus, followed by methylated RNA immunoprecipitation sequencing. Zebrafish with vascular defect was used as the in vivo model. TPO significantly improved the quantity and functions of BM EPCs from CIT patients in vitro and restored the subintestinal vein area of zebrafish with vascular defect in vivo. Mechanically, TPO enhanced the BM EPC functions through Akt signal mediated by METTL16, which was downregulated in BM EPCs of CIT patients and involved in the regulation of endothelial functions. The present study demonstrates that TPO improves the recovery of BM EPCs from CIT patients with haematological malignancies via METTL16/Akt signalling, which provides new insights into the role of TPO in treating CIT in addition to direct megakaryopoiesis.

Endothelial Cell Derived Extracellular Vesicles and Hematopoiesis

Extracellular vesicles (EVs) have been recognized as a novel way of cell-to-cell communication in the last several decades. It is believed that EVs exert their functions on nearby or distant cells through transfer of the cargo that they carry. In this review, we focus on EVs produced by endothelial cells, with emphasis on their role in hematopoiesis. We first describe how endothelial cells interact with hematopoietic stem/progenitor cells during development and in disease conditions. We then discuss EVs, ranging from their subtypes to isolation methods and analysis of EVs. With the above background information, we next review the literature related to endothelial cell derived EVs (ECEVs), including physiological functions and their clinical uses. In the last sections, we summarize the current results about the effect of ECEVs on hematopoiesis under physiological and stress conditions.

Repair of dysfunctional bone marrow endothelial cells alleviates aplastic anemia

Aplastic anemia (AA) is a life-threatening disease characterized by bone marrow (BM) failure and pancytopenia. As an important component of the BM microenvironment, endothelial cells (ECs) play a crucial role in supporting hematopoiesis and regulating immunity. However, whether impaired BM ECs are involved in the occurrence of AA and whether repairing BM ECs could improve hematopoiesis and immune status in AA remain unknown. In this study, a classical AA mouse model and VE-cadherin blocking antibody that could antagonize the function of ECs were used to validate the role of BM ECs in the occurrence of AA. N-acetyl-L-cysteine (NAC, a reactive oxygen species scavenger) or exogenous EC infusion was administered to AA mice. Furthermore, the frequency and functions of BM ECs from AA patients and healthy donors were evaluated. BM ECs from AA patients were treated with NAC in vitro, and then the functions of BM ECs were evaluated. We found that BM ECs were significantly decreased and damaged in AA mice. Hematopoietic failure and immune imbalance became more severe when the function of BM ECs was antagonized, whereas NAC or EC infusion improved hematopoietic and immunological status by repairing BM ECs in AA mice. Consistently, BM ECs in AA patients were decreased and dysfunctional. Furthermore, dysfunctional BM ECs in AA patients led to their impaired ability to support hematopoiesis and dysregulate T cell differentiation toward proinflammatory phenotypes, which could be repaired by NAC in vitro. The reactive oxygen species pathway was activated, and hematopoiesis- and immune-related signaling pathways were enriched in BM ECs of AA patients. In conclusion, our data indicate that dysfunctional BM ECs with impaired hematopoiesis-supporting and immunomodulatory abilities are involved in the occurrence of AA, suggesting that repairing dysfunctional BM ECs may be a potential therapeutic approach for AA patients.

Cord Blood-Derived Endothelial Progenitor Cells Promote In Vivo Regeneration of Human Hematopoietic Bone Marrow

PURPOSE

Victims of acute radiation exposure are susceptible to hematopoietic toxicity due to bone marrow damage and loss of mature blood elements. Here, we evaluated cord blood-derived endothelial progenitor cells (CB-EPCs) as a potential cellular therapy for mitigation of hematologic acute radiation syndrome. CB-EPCs express endothelial cell markers and maintain their growth characteristics beyond 10+ passages without diminishing their doubling capacity. Further, CB-EPCs can be cryopreserved in vapor-phase liquid nitrogen and easily recovered for propagation, making them an attractive nonimmunogenic cellular therapy for off-the-shelf use. Importantly, we show CB-EPCs have the capacity to potently expand adult human bone marrow hematopoietic progenitor cells both in vitro and in vivo.

METHODS AND MATERIALS

To demonstrate the role of CB-EPCs in promoting in vivo human immune reconstitution after irradiation, we employed a novel humanized mouse model established by transplant of CD34+ bone marrow cells from 9 unique adult organ donors into immunocompromised NSG-SGM3 mice. The response of the humanized immune system to ionizing irradiation was then tested by exposure to 1 Gy followed by subcutaneous treatment of CB-EPCs, Food and Drug Administration-approved growth factor pegfilgrastim (0.3 mg/kg), or saline.

RESULTS

At day 7, total human bone marrow was decreased by 80% in irradiated controls. However, treatment with either growth factor pegfilgrastim or CB-EPCs increased recovery of total human bone marrow by 2.5-fold compared with saline. Notably, CB-EPCs also increased recovery of both human CD34+ progenitors by 5-fold and colony-forming capacity by 3-fold versus saline. Additionally, CB-EPCs promoted recovery of endogenous bone marrow endothelial cells as observed by both increased vessel area and length compared with saline.

CONCLUSIONS

These findings indicate the feasibility of using humanized mice engrafted with adult bone marrow for radiation research and the development of CB-EPCs as an off-the-shelf cellular therapy for mitigation of hematologic acute radiation syndrome.

Organ-Specific Endothelial Dysfunction Following Total Body Irradiation Exposure

As the single cell lining of the heart and all blood vessels, the vascular endothelium serves a critical role in maintaining homeostasis via control of vascular tone, immune cell recruitment, and macromolecular transit. For victims of acute high-dose radiation exposure, damage to the vascular endothelium may exacerbate the pathogenesis of acute and delayed multi-organ radiation toxicities. While commonalities exist between radiation-induced endothelial dysfunction in radiosensitive organs, the vascular endothelium is known to be highly heterogeneous as it is required to serve tissue and organ specific roles. In keeping with its organ and tissue specific functionality, the molecular and cellular response of the endothelium to radiation injury varies by organ. Therefore, in the development of medical countermeasures for multi-organ injury, it is necessary to consider organ and tissue-specific endothelial responses to both injury and candidate mitigators. The purpose of this review is to summarize the pathogenesis of endothelial dysfunction following total or near total body irradiation exposure at the level of individual radiosensitive organs.

The glycolytic enzyme PFKFB3 determines bone marrow endothelial progenitor cell damage after chemotherapy and irradiation

Bone marrow (BM) endothelial progenitor cell (EPC) damage of unknown mechanism delays the repair of endothelial cells (EC) and recovery of hematopoiesis after chemo-radiotherapy. We found increased levels of the glycolytic enzyme PFKFB3 in the damaged BM EPC of patients with poor graft function, a clinical model of EPC damage-associated poor hematopoiesis after allogeneic hematopoietic stem cell transplantation. Moreover, in vitro the glycolysis inhibitor 3-(3-pyridinyl)- 1-(4-pyridinyl)-2-propen-1-one (3PO) alleviated the damaged BM EPC from patients with poor graft function. Consistently, PFKFB3 overexpression triggered BM EPC damage after 5-fluorouracil treatment and impaired hematopoiesis-supporting ability in vitro. Mechanistically, PFKFB3 facilitated pro-apoptotic transcription factor FOXO3A and expression of its downstream genes, including p21, p27, and FAS, after 5-fluorouracil treatment in vitro. Moreover, PFKFB3 induced activation of NF-κB and expression of its downstream adhesion molecule E-selectin, while it reduced hematopoietic factor SDF-1 expression, which could be rescued by FOXO3A silencing. High expression of PFKFB3 was found in damaged BM EC of murine models of chemo-radiotherapy-induced myelosuppression. Furthermore, a murine model of BM EC-specific PFKFB3 overexpression demonstrated that PFKFB3 aggravated BM EC damage, and impaired the recovery of hematopoiesis after chemotherapy in vivo, effects which could be mitigated by 3PO, indicating a critical role of PFKFB3 in regulating BM EC damage. Clinically, PFKFB3-induced FOXO3A expression and NF-κB activation were confirmed to contribute to the damaged BM EPC of patients with acute leukemia after chemotherapy. 3PO repaired the damaged BM EPC by reducing FOXO3A expression and phospho-NF-κB p65 in patients after chemotherapy. In summary, our results reveal a critical role of PFKFB3 in triggering BM EPC damage and indicate that endothelial-PFKFB3 may be a potential therapeutic target for myelosuppressive injury.

Prophylactic NAC promoted hematopoietic reconstitution by improving endothelial cells after haploidentical HSCT: a phase 3, open-label randomized trial

BACKGROUND

Poor graft function (PGF) or prolonged isolated thrombocytopenia (PT), which are characterized by pancytopenia or thrombocytopenia, have become serious complications after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Our previous single-arm trial suggests that N-acetyl-L-cysteine (NAC) prophylaxis reduced PGF or PT after allo-HSCT. Therefore, an open-label, randomized, phase 3 trial was performed to investigate the efficacy and tolerability of NAC prophylaxis to reduce PGF or PT after allo-HSCT.

METHODS

A phase 3, open-label randomized trial was performed. Based on the percentage of CD34+VEGFR2 (CD309)+ endothelial cells (ECs) in bone marrow (BM) detected by flow cytometry at 14 days before conditioning, patients aged 15 to 60 years with acute leukemia undergoing haploidentical HSCT were categorized as low-risk (EC ≥ 0.1%) or high-risk (EC < 0.1%); patients at high risk were randomly assigned (2:1) to receive NAC prophylaxis or nonprophylaxis. The primary endpoint was PGF and PT incidence at +60 days post-HSCT.

RESULTS

Between April 18, 2019, and June 24, 2021, 120 patients with BM EC <0.1% were randomly assigned for NAC (group A, N = 80) or nonprophylaxis (group B, N = 40), and 105 patients with EC≥0.1% (group C) were also analyzed. The +60 days incidence of PGF and PT was 7.5% (95% CI, 1.7 to 13.3%) and 22.5% (95% CI, 9.1 to 35.9%) in group A and group B (hazard ratio, 0.317; 95% CI, 0.113 to 0.890; P = 0.021) and 11.4% (95% CI, 5.2 to 17.6%) in group C (hazard ratio, 0.643; 95% CI, 0.242 to 1.715; P = 0.373). Consistently, NAC prophylaxis gradually improved BM ECs and CD34+ cells in group A, whereas reduced their reactive oxygen species (ROS) levels post-HSCT. Within 60 days post-HSCT, the most common grade 3 to 5 adverse events for the NAC and control groups were infections (19/80 [24%] vs. 10/40 [25%]) and gastrointestinal adverse events (16/80 [20%] vs. 7/40 [18%]). There were no treatment-related deaths.

CONCLUSIONS

N-Acetyl-L-cysteine prophylaxis can prevent the occurrence of poor hematopoietic function and is well tolerated in haploidentical HSCT. It may offer a potential pathogenesis-oriented therapeutic approach for patients with poor hematopoietic function.

TRIAL REGISTRATION

This trial was registered at ClinicalTrials.gov as #NCT03967665.

Vascular Regulation of Hematopoietic Stem Cell Homeostasis, Regeneration, and Aging

Purpose of Review

Hematopoietic stem cells (HSCs) sit at the top of the hierarchy that meets the daily burden of blood production. HSC maintenance relies on extrinsic cues from the bone marrow (BM) microenvironment to balance stem cell self-renewal and cell fate decisions. In this brief review, we will highlight the studies and model systems that define the centralized role of BM vascular endothelium in modulating HSC activity in health and stress.

Recent Findings

The BM microenvironment is composed of a diverse array of intimately associated vascular and perivascular cell types. Recent dynamic imaging studies, coupled with single-cell RNA sequencing (scRNA-seq) and functional readouts, have advanced our understanding of the HSC-supportive cell types and their cooperative mechanisms that govern stem cell fate during homeostasis, regeneration, and aging. These findings have established complex and discrete vascular microenvironments within the BM that express overlapping and unique paracrine signals that modulate HSC fate.

Summary

Understanding the spatial and reciprocal HSC-niche interactions and the molecular mechanisms that govern HSC activity in the BM vascular microenvironment will be integral in developing therapies aimed at ameliorating hematological disease and supporting healthy hematopoietic output.

Neuropilin 1 regulates bone marrow vascular regeneration and hematopoietic reconstitution

Ionizing radiation and chemotherapy deplete hematopoietic stem cells and damage the vascular niche wherein hematopoietic stem cells reside. Hematopoietic stem cell regeneration requires signaling from an intact bone marrow (BM) vascular niche, but the mechanisms that control BM vascular niche regeneration are poorly understood. We report that BM vascular endothelial cells secrete semaphorin 3 A (SEMA3A) in response to myeloablation and SEMA3A induces p53 - mediated apoptosis in BM endothelial cells via signaling through its receptor, Neuropilin 1 (NRP1), and activation of cyclin dependent kinase 5. Endothelial cell - specific deletion of Nrp1 or Sema3a or administration of anti-NRP1 antibody suppresses BM endothelial cell apoptosis, accelerates BM vascular regeneration and concordantly drives hematopoietic reconstitution in irradiated mice. In response to NRP1 inhibition, BM endothelial cells increase expression and secretion of the Wnt signal amplifying protein, R spondin 2. Systemic administration of anti - R spondin 2 blocks HSC regeneration and hematopoietic reconstitution which otherwise occurrs in response to NRP1 inhibition. SEMA3A - NRP1 signaling promotes BM vascular regression following myelosuppression and therapeutic blockade of SEMA3A - NRP1 signaling in BM endothelial cells accelerates vascular and hematopoietic regeneration in vivo.

Atorvastatin improves bone marrow endothelial progenitor cell function from patients with immune-related hemocytopenia

Background

Immune-related hemocytopenia (IRH) is a type of autoimmune disease that targets bone marrow hematopoietic cells. This study investigated the influence of atorvastatin on the functional aspects of bone marrow endothelial progenitor cells (BM EPCs) in IRH patients.

Methods

BM EPCs were isolated from 15 patients with IRH and 20 normal controls. The isolated BM EPCs were characterized by flow cytometry. Cell counting kit-8, flow cytometry, and Transwell migration assays were used to determine the proliferation, apoptosis, and migration of BM EPCs, respectively. Protein levels were determined by western blot assay.

Results

The BM EPCs isolated from IRH patients showed reduced proliferation, increased apoptosis, and attenuated migratory ability compared to those from normal controls. Western blot analysis showed that the protein level of p-p38 was significantly increased, while that of Phosphorylated protein kinase B (p-AKT) was significantly decreased in the BM EPCs from IRH patients, compared to BM EPCs from healthy subjects. Cell proliferation and migration were significantly enhanced by atorvastatin, recombinant human thrombopoietin, and SB20358 compared to the untreated BM EPCs from IRH patients. Atorvastatin, Recombinant human thrombopoietin (TPO), and SB20358 treatment significantly suppressed the protein levels of p-p38 protein, but increased those of p-AKT in BM EPCS from IRH patients.

Conclusions

In summary, atorvastatin increases the number and function of BM EPCs in IRH patients by regulating the p38 and AKT signaling pathways.

Extracellular Vesicles for the Treatment of Radiation Injuries

Normal tissue injury from accidental or therapeutic exposure to high-dose radiation can cause severe acute and delayed toxicities, which result in mortality and chronic morbidity. Exposure to single high-dose radiation leads to a multi-organ failure, known as acute radiation syndrome, which is caused by radiation-induced oxidative stress and DNA damage to tissue stem cells. The radiation exposure results in acute cell loss, cell cycle arrest, senescence, and early damage to bone marrow and intestine with high mortality from sepsis. There is an urgent need for developing medical countermeasures against radiation injury for normal tissue toxicity. In this review, we discuss the potential of applying secretory extracellular vesicles derived from mesenchymal stromal/stem cells, endothelial cells, and macrophages for promoting repair and regeneration of organs after radiation injury.

Hematopoietic Stem Cell Niche During Homeostasis, Malignancy, and Bone Marrow Transplantation

Self-renewal and multidirectional differentiation of hematopoietic stem cells (HSCs) are strictly regulated by numerous cellular components and cytokines in the bone marrow (BM) microenvironment. Several cell types that regulate HSC niche have been identified, including both non-hematopoietic cells and HSC-derived cells. Specific changes in the niche composition can result in hematological malignancies. Furthermore, processes such as homing, proliferation, and differentiation of HSCs are strongly controlled by the BM niche and have been reported to be related to the success of hematopoietic stem cell transplantation (HSCT). Single-cell sequencing and in vivo imaging are powerful techniques to study BM microenvironment in hematological malignancies and after HSCT. In this review, we discuss how different components of the BM niche, particularly non-hematopoietic and hematopoietic cells, regulate normal hematopoiesis, and changes in the BM niche in leukemia and after HSCT. We believe that this comprehensive review will provide clues for further research on improving HSCT efficiency and exploring potential therapeutic targets for leukemia.

A Potential Role for Excess Tissue Iron in Development of Cardiovascular Delayed Effects of Acute Radiation Exposure

Murine hematopoietic-acute radiation syndrome (H-ARS) survivors of total body radiation (TBI) have a significant loss of heart vessel endothelial cells, along with increased tissue iron, as early as 4 mo post-TBI. The goal of the current study was to determine the possible role for excess tissue iron in the loss of coronary artery endothelial cells. Experiments used the H-ARS mouse model with gamma radiation exposure of 853 cGy (LD50/30) and time points from 1 to 12 wk post-TBI. Serum iron was elevated at 1 wk post-TBI, peaked at 2 wk post-TBI, and returned to non-irradiated control values by 4 wk post-TBI. A similar trend was seen for transferrin saturation, and both results correlated inversely with red blood cell number. Perls' Prussian Blue staining, used to detect iron deposition in heart tissue sections, showed myocardial iron was present as early as 2 wk following irradiation. Pretreatment of mice with the iron chelator deferiprone decreased tissue iron but not serum iron at 2 wk. Coronary artery endothelial cell density was significantly decreased as early as 2 wk vs. non-irradiated controls (P<0.05), and the reduced density persisted to 12 wk after irradiation. Deferiprone treatment of irradiated mice prevented the decrease in endothelial cell density at 2 and 4 wk post-TBI compared to irradiated, non-treated mice (P<0.03). Taken together, the results suggest excess tissue iron contributes to endothelial cell loss early following TBI and may be a significant event impacting the development of delayed effects of acute radiation exposure.

[Thrombopoietin promotes megakaryopoiesis via protecting bone marrow endothelial function in patients undergoing chemotherapy for hematological malignancies]

OBJECTIVE

To explore whether thrombopoietin (TPO) can rescue megakaryopoiesis by protecting bone marrowderived endothelial progenitor cells (BM-EPCs) in patients receiving chemotherapy for hematological malignancies.

METHODS

Bone marrow samples were collected from 23 patients with hematological malignancies 30 days after chemotherapy and from 10 healthy volunteers. BM-EPCs isolated from the samples were identified by staining for CD34, CD309 and CD133, and their proliferation in response to treatment with TPO was assessed using CCK8 assay. DiL-Ac-LDL uptake and FITC-UEA-I binding assay were performed to evaluate the amount of BM-EPCs from the subjects. Tube-formation and migration experiments were used for functional assessment of the BM-EPCs. The BM-EPCs with or without TPO treatment were co-cultured with human megakaryocytes, and the proliferation of the megakaryocytes was detected with flow cytometry.

RESULTS

Flow cytometry indicated that the TPO-treated cells had high expressions of CD34, CD133, and CD309. CCK8 assay demonstrated that TPO treatment enhanced the proliferation of the BM-EPCs, and the optimal concentration of TPO was 100 μg/L. Double immunofluorescence assay indicated that the number of BM-EPC was significantly higher in TPO-treated group than in the control group. The TPO-treated BM-EPCs exhibited stronger tube-formation and migration abilities (P < 0.05) and more significantly enhanced the proliferation of co-cultured human megakaryocytes than the control cells (P < 0.05).

CONCLUSIONS

TPO can directly stimulate megakaryopoiesis and reduce hemorrhage via protecting the function of BM-EPCs in patients following chemotherapy for hematological malignancies.

Dental Pulp Stem Cell-Derived Extracellular Vesicles Mitigate Haematopoietic Damage after Radiation

Radiation therapy can cause haematopoietic damage, and mesenchymal stem cells (MSCs) derived extracellular vesicles (EVs) have been shown to reverse this damage. Our previous research showed that dental pulp stem cells (DPSCs) have a strong proliferation capacity and can produce abundant amounts of EVs to meet the requirements for use in vitro and in vivo. DPSCs derived EVs (DPSCs-EVs) are evaluated for their effect on reducing haematopoietic damage. Haematopoietic stem cell (HSC) numbers and function were assessed by flow cytometry, peripheral blood cell counts, histology and bone marrow transplantation. Epidermal growth factor (EGF) was used as a reference for evaluating the efficiency of EVs. miRNA microarray was employed to find out the changes of miRNA expression after cells being irradiated in vivo and the role they may play in mitigation the radiation caused injury. We observed the effect of DPSCs-EVs on promoting proliferation and inhibiting apoptosis of human umbilical vein endothelial cells (HUVECs) and FDC-P1 cells in vitro. We found that DPSCs-EVs and EGF could comparably inhibit the decrease in WBC, CFU count and KSL cells in vivo. We also verified that EVs could accelerate the recovery of long-term HSCs. In summary, DPSCs-EVs showed an apoptosis resistant effect on HUVECs and FDC-P1 cells after radiation injury in vitro. EVs from DPSCs were comparable to EGF in their ability to regulate haematopoietic regeneration after radiation injury in vivo. Radiation could alter the expression of some miRNAs in bone marrow cells, and EVs could correct these changes to some extent.

Gap Junctions in the Bone Marrow Lympho-Hematopoietic Stem Cell Niche, Leukemia Progression, and Chemoresistance

Abstract: The crosstalk between hematopoietic stem cells (HSC) and bone marrow (BM) microenvironment is critical for homeostasis and hematopoietic regeneration in response to blood formation emergencies after injury, and has been associated with leukemia transformation and progression. Intercellular signals by the BM stromal cells in the form of cell-bound or secreted factors, or by physical interaction, regulate HSC localization, maintenance, and differentiation within increasingly defined BM HSC niches. Gap junctions (GJ) are comprised of arrays of membrane embedded channels formed by connexin proteins, and control crucial signaling functions, including the transfer of ions, small metabolites, and organelles to adjacent cells which affect intracellular mechanisms of signaling and autophagy. This review will discuss the role of GJ in both normal and leukemic hematopoiesis, and highlight some of the most novel approaches that may improve the efficacy of cytotoxic drugs. Connexin GJ channels exert both cell-intrinsic and cell-extrinsic effects on HSC and BM stromal cells, involved in regenerative hematopoiesis after myelosuppression, and represent an alternative system of cell communication through a combination of electrical and metabolic coupling as well as organelle transfer in the HSC niche. GJ intercellular communication (GJIC) in the HSC niche improves cellular bioenergetics, and rejuvenates damaged recipient cells. Unfortunately, they can also support leukemia proliferation and survival by creating leukemic niches that provide GJIC dependent energy sources and facilitate chemoresistance and relapse. The emergence of new strategies to disrupt self-reinforcing malignant niches and intercellular organelle exchange in leukemic niches, while at the same time conserving normal hematopoietic GJIC function, could synergize the effect of chemotherapy drugs in eradicating minimal residual disease. An improved understanding of the molecular basis of connexin regulation in normal and leukemic hematopoiesis is warranted for the re-establishment of normal hematopoiesis after chemotherapy.

Role of angiocrine signals in bone development, homeostasis and disease

Skeletal vasculature plays a central role in the maintenance of microenvironments for osteogenesis and haematopoiesis. In addition to supplying oxygen and nutrients, vasculature provides a number of inductive factors termed as angiocrine signals. Blood vessels drive recruitment of osteoblast precursors and bone formation during development. Angiogenesis is indispensable for bone repair and regeneration. Dysregulation of the angiocrine crosstalk is a hallmark of ageing and pathobiological conditions in the skeletal system. The skeletal vascular bed is complex, heterogeneous and characterized by distinct capillary subtypes (type H and type L), which exhibit differential expression of angiocrine factors. Furthermore, distinct blood vessel subtypes with differential angiocrine profiles differentially regulate osteogenesis and haematopoiesis, and drive disease states in the skeletal system. This review provides an overview of the role of angiocrine signals in bone during homeostasis and disease.

Dickkopf-1 Treatment Stimulates Hematopoietic Regenerative Function in Infused Endothelial Progenitor Cells

Acute high-dose radiation injury damages the bone marrow hematopoietic stem and progenitor cell compartment. This damage compromises the functional ability of the bone marrow to produce mature blood cells and results in an increased risk of death due to hematopoietic complications. Past work has shown that the bone marrow endothelium provides critical cues, which promote hematopoietic stem cell regeneration after injury. Additionally, transfusion of endothelial cells after radiation injury has been shown to promote recovery of both the bone marrow vasculature and hematopoietic systems. In this work, we examined the regenerative capacity of intravenous infusion of umbilical cord-blood derived endothelial progenitor cells (EPCs) since this is a cell source which is easy to obtain, expand and cryopreserve. We show that pre-treatment with the Wnt-antagonist Dickkopf1 (Dkk1) augments EPC regenerative function in an allogeneic mouse transplant model. Here, hematopoietic recovery was assessed in Balb/c mice after 5 Gy total-body irradiation and transplantation with C57/BL6-derived EPCs either with or without Dkk1 pre-treatment. The Dkk1-treated EPC group had significantly faster recovery of peripheral white blood cells, total bone marrow cellularity, bone marrow progenitors and BM endothelial cells compared to EPC treatment alone or saline controls. Importantly, after an LD_50/30 dose of 8 Gy in the Balb/c mouse, Dkk1-treated EPCs were able to rescue 100% of irradiated mice versus 80% in the EPC control group and only 33% in the saline-treated group. To understand how Dkk1 induces regenerative function in the EPCs, we screened for pro-regenerative factors secreted by the EPC in response to Dkk1. Dkk1-treated EPCs were observed to secrete high levels of the anti-fibrotic protein follistatin as well as several proteins known to promote regeneration including EGF, VEGF and G-CSF. This work demonstrates the potential for Dkk1-treated EPCs as a rescue therapeutic for victims of acute radiation injury.

Endothelial Cell-Derived Extracellular Vesicles Mitigate Radiation-Induced Hematopoietic Injury

PURPOSE

Extracellular vesicles (EVs) are shed vesicles that bear a combination of nucleic acids and proteins. EVs are becoming recognized as a mode of cell-to-cell communication. Because hematopoietic stem cells reside in proximity to endothelial cells (ECs), we investigated whether EC-derived EVs could regulate hematopoietic stem cell regeneration after ionizing radiation.

METHODS AND MATERIALS

We generated EVs derived from primary murine marrow ECs. We sought to determine the response of irradiated hematopoietic stem and progenitor cells to syngeneic or allogeneic EVs in culture assays. Starting 24 hours after either sublethal or lethal irradiation, mice were treated with EVs or saline or cultured primary marrow endothelial cells to determine the hematopoietic response in vivo.

RESULTS

We demonstrate that EVs bear nuclear material and express EC-specific markers. Treatment with EVs promoted cell expansion and increased the number of colony-forming units compared to irradiated, hematopoietic cell cultures treated with cytokines alone. After total body irradiation, EV-treated mice displayed preserved marrow cellularity, marrow vessel integrity, and prolonged overall survival compared with controls treated with saline. Treatment of irradiated hematopoietic stem/progenitor cells (HSPCs) with EVs from different genetic strains showed results similar to treatment of HSPCs from syngeneic EVs. Mechanistically, treatment of irradiated HSPCs with EVs resulted in decreased levels of annexin V+ apoptotic cell death, which is mediated in part by tissue inhibitor of metalloproteinase-1.

CONCLUSIONS

Our findings show that syngeneic or allogeneic EVs could serve as cell-derived therapy to deliver physiologic doses of nucleic acids and growth factors to hematopoietic cells to accelerate hematopoietic regeneration.

Mouse serum protects against total body irradiation-induced hematopoietic system injury by improving the systemic environment after radiation

Reactive oxygen species (ROS) play a critical role in total body irradiation (TBI)-induced hematopoietic system injury. However, the mechanisms involved in ROS production in hematopoietic stem cells (HSCs) post TBI need to be further explored. In this study, we demonstrated that hematopoietic system injury in mice radiated with TBI was effectively alleviated when the blood circulation environment was changed via the injection of serum from non-radiated mice. Serum injection increased the survival of radiated mice and ameliorated TBI-induced hematopoietic system injury through attenuating myeloid skew, increasing HSC frequency, and promoting the reconstitution of radiated HSCs. Serum injection also decreased ROS levels in HSCs and regulated oxidative stress-related proteins. A serum proteome sequence array showed that proteins related to tissue injury and oxidative stress were regulated, and a serum-derived exosome microRNA sequence assay showed that the PI3K-Akt and Hippo signaling pathways were affected in radiated mice injected with serum from non-radiated mice. Furthermore, a significant increase in cell viability and a decrease in ROS were observed in radiated lineage^-c-kit⁺ cells treated with serum-derived exosomes. Similarly, an improvement in the impaired differentiation of HSCs was observed in radiated mice injected with serum-derived exosomes. Taken together, our observations suggest that serum from non-radiated mice alleviates HSC injury in radiated mice by improving the systemic environment after radiation, and exosomes contribute to this radioprotective effect as important serum active component.

Epithelial-mesenchymal transition in haematopoietic stem cell development and homeostasis

Epithelial-mesenchymal transition (EMT) is a morphogenetic process of cells that adopt an epithelial organization in their developmental ontogeny or homeostatic maintenance. Abnormalities in EMT regulation result in many malignant tumours in the human body. Tumours associated with the haematopoietic system, however, are traditionally not considered to involve EMT and haematopoietic stem cells (HSCs) are generally not associated with epithelial characteristics. In this review, we discuss the ontogeny and homeostasis of adult HSCs in the context of EMT intermediate states. We provide evidence that cell polarity regulation is critical for both HSC formation from embryonic dorsal aorta and HSC self-renewal and differentiation in adult bone marrow. HSC polarity is controlled by the same set of surface and transcriptional regulators as those described in canonical EMT processes. With an emphasis on partial EMT, we propose that the concept of EMT can be similarly applied in the study of HSC generation, maintenance and pathogenesis.

Poor graft function after allogeneic hematopoietic stem cell transplantation-an old complication with new insights☆

Poor graft function (PGF), characterized by pancytopenia, is a life-threatening complication following allogeneic hematopoietic stem cell transplantation (allo-HSCT). PGF has become a growing obstacle that contributes to high morbidity and mortality after allo-HSCT, especially with the increasing use of haploidentical allo-HSCT, and clinical management 81870139, is challenging. Emerging evidence demonstrates that the bone marrow (BM) microenvironment plays a crucial role in maintaining and regulating hematopoiesis. Recent prospective case-control studies demonstrated that impaired BM microenvironments are involved in the pathogenesis of PGF. Moreover, in vitro treatment with N-acetyl-L-cysteine, a reactive oxygen species scavenger, could enhance the defective hematopoietic stem cells by repairing the dysfunctional BM microenvironment of PGF patients. Consequently, a better understanding of the pathogenesis of PGF may guide effective therapy and eventually improve the prognosis of allo-HSCT. Here, based on new insights into the BM microenvironment in PGF patients, we provide an overview of the pathogenesis and promising treatment strategies for PGF patients.

JAK2V617F-bearing vascular niche enhances malignant hematopoietic regeneration following radiation injury

Myeloproliferative neoplasms are clonal stem cell disorders characterized by hematopoietic stem/progenitor cell expansion. The acquired kinase mutation JAK2V617F plays a central role in these disorders. Abnormalities of the marrow microenvironment are beginning to be recognized as an important factor in the development of myeloproliferative neoplasms. Endothelial cells are an essential component of the hematopoietic vascular niche. Endothelial cells carrying the JAK2V617F mutation can be detected in patients with myeloproliferative neoplasms, suggesting that the mutant vascular niche is involved in the pathogenesis of these disorders. Here, using a transgenic mouse expressing JAK2V617F specifically in all hematopoietic cells (including hematopoietic stem/progenitor cells) and endothelial cells, we show that the JAK2V617F-mutant hematopoietic stem/progenitor cells are relatively protected by the JAK2V617F-bearing vascular niche from an otherwise lethal dose of irradiation during conditioning for stem cell transplantation. Gene expression analysis revealed that chemokine (C-X-C motif) ligand 12, epidermal growth factor, and pleiotrophin are up-regulated in irradiated JAK2V617F-bearing endothelial cells compared to wild-type cells. Our findings suggest that the mutant vascular niche may contribute to the high incidence of disease relapse in patients with myeloproliferative neoplasms following allogeneic stem cell transplantation, the only curative treatment for these disorders.

Epidermal Growth Factor and Granulocyte Colony Stimulating Factor Signaling Are Synergistic for Hematopoietic Regeneration

Hematopoietic regeneration following chemotherapy may be distinct from regeneration following radiation. While we have shown that epidermal growth factor (EGF) accelerates regeneration following radiation, its role following chemotherapy is currently unknown. We sought to identify EGF as a hematopoietic growth factor for chemotherapy-induced myelosuppression. Following 5-fluorouracil (5-FU), EGF accelerated hematopoietic stem cell regeneration and prolonged survival compared with saline-treated mice. To mitigate chemotherapy-induced injury to endothelial cells in vivo, we deleted Bax in VEcadherin⁺ cells (VEcadherinCre;Bax^FL/FL mice). Following 5-FU, VEcadherinCre;Bax^FL/FL mice displayed preserved hematopoietic stem/progenitor content compared with littermate controls. 5-FU and EGF treatment resulted in increased cellular proliferation, decreased apoptosis, and increased DNA double-strand break repair by non-homologous end-joining recombination compared with saline-treated control mice. When granulocyte colony stimulating factor (G-CSF) is given with EGF, this combination was synergistic for regeneration compared with either G-CSF or EGF alone. EGF increased G-CSF receptor (G-CSFR) expression following 5-FU. Conversely, G-CSF treatment increased both EGF receptor (EGFR) and phosphorylation of EGFR in hematopoietic stem/progenitor cells. In humans, the expression of EGFR is increased in patients with colorectal cancer treated with 5-FU compared with cancer patients not on 5-FU. Similarly, EGFR signaling is responsive to G-CSF in humans in vivo with both increased EGFR and phospho-EGFR in healthy human donors following G-CSF treatment compared with donors who did not receive G-CSF. These data identify EGF as a hematopoietic growth factor following myelosuppressive chemotherapy and that dual therapy with EGF and G-CSF may be an effective method to accelerate hematopoietic regeneration. Stem Cells 2018;36:252-264.

Endothelial transplantation rejuvenates aged hematopoietic stem cell function

Age-related changes in the hematopoietic compartment are primarily attributed to cell-intrinsic alterations in hematopoietic stem cells (HSCs); however, the contribution of the aged microenvironment has not been adequately evaluated. Understanding the role of the bone marrow (BM) microenvironment in supporting HSC function may prove to be beneficial in treating age-related functional hematopoietic decline. Here, we determined that aging of endothelial cells (ECs), a critical component of the BM microenvironment, was sufficient to drive hematopoietic aging phenotypes in young HSCs. We used an ex vivo hematopoietic stem and progenitor cell/EC (HSPC/EC) coculture system as well as in vivo EC infusions following myelosuppressive injury in mice to demonstrate that aged ECs impair the repopulating activity of young HSCs and impart a myeloid bias. Conversely, young ECs restored the repopulating capacity of aged HSCs but were unable to reverse the intrinsic myeloid bias. Infusion of young, HSC-supportive BM ECs enhanced hematopoietic recovery following myelosuppressive injury and restored endogenous HSC function in aged mice. Coinfusion of young ECs augmented aged HSC engraftment and enhanced overall survival in lethally irradiated mice by mitigating damage to the BM vascular microenvironment. These data lay the groundwork for the exploration of EC therapies that can serve as adjuvant modalities to enhance HSC engraftment and accelerate hematopoietic recovery in the elderly population following myelosuppressive regimens.

Calcium/calmodulin-dependent kinase kinase 2 regulates hematopoietic stem and progenitor cell regeneration

Hematopoietic stem and progenitor cells (HSPCs) are predominantly quiescent in adults, but proliferate in response to bone marrow (BM) injury. Here, we show that deletion of Ca²⁺/calmodulin (CaM)-dependent protein kinase kinase 2 (CaMKK2) promotes HSPC regeneration and hematopoietic recovery following radiation injury. Using Camkk2-enhanced green fluorescent protein (EGFP) reporter mice, we found that Camkk2 expression is developmentally regulated in HSPC. Deletion of Camkk2 in HSPC results in a significant downregulation of genes affiliated with the quiescent signature. Accordingly, HSPC from Camkk2 null mice have a high proliferative capability when stimulated in vitro in the presence of BM-derived endothelial cells. In addition, Camkk2 null mice are more resistant to radiation injury and show accelerated hematopoietic recovery, enhanced HSPC regeneration and ultimately a prolonged survival following sublethal or lethal total body irradiation. Mechanistically, we propose that CaMKK2 regulates the HSPC response to hematopoietic damage by coupling radiation signaling to activation of the anti-proliferative AMP-activated protein kinase. Finally, we demonstrated that systemic administration of the small molecule CaMKK2 inhibitor, STO-609, to irradiated mice enhanced HSPC recovery and improved survival. These findings identify CaMKK2 as an important regulator of HSPC regeneration and demonstrate CaMKK2 inhibition is a novel approach to promoting hematopoietic recovery after BM injury.

Blood on the tracks: hematopoietic stem cell-endothelial cell interactions in homing and engraftment

Cells of the hematopoietic system undergo rapid turnover. Each day, humans require the production of about one hundred billion new blood cells for proper function. Hematopoietic stem cells (HSCs) are rare cells that reside in specialized niches and are required throughout life to produce specific progenitor cells that will replenish all blood lineages. There is, however, an incomplete understanding of the molecular and physical properties that regulate HSC migration, homing, engraftment, and maintenance in the niche. Endothelial cells (ECs) are intimately associated with HSCs throughout the life of the stem cell, from the specialized endothelial cells that give rise to HSCs, to the perivascular niche endothelial cells that regulate HSC homeostasis. Recent studies have dissected the unique molecular and physical properties of the endothelial cells in the HSC vascular niche and their role in HSC biology, which may be manipulated to enhance hematopoietic stem cell transplantation therapies.

Regulation of the hematopoietic stem cell lifecycle by the endothelial niche

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) predominantly reside either in direct contact or in close proximity to the vascular endothelium throughout their lifespan. From the moment of HSC embryonic specification from hemogenic endothelium, endothelial cells (ECs) act as a critical cellular-hub that regulates a vast repertoire of biological processes crucial for HSC maintenance throughout its lifespan. In this review, we will discuss recent findings in endothelial niche-mediated regulation of HSC function during development, aging and regenerative conditions.

RECENT FINDINGS

Studies employing genetic vascular models have unequivocally confirmed that ECs provide the essential instructive cues for HSC emergence during embryonic development as well as adult HSC maintenance during homeostasis and regeneration. Aging of ECs may impair their ability to maintain HSC function contributing to the development of aging-associated hematopoietic deficiencies. These findings have opened up new avenues to explore the therapeutic application of ECs. ECs can be adapted to serve as an instructive platform to expand bona fide HSCs and also utilized as a cellular therapy to promote regeneration of the hematopoietic system following myelosuppressive and myeloablative injuries.

SUMMARY

ECs provide a fertile niche for maintenance of functional HSCs throughout their lifecycle. An improved understanding of the EC-HSC cross-talk will pave the way for development of EC-directed strategies for improving HSC function during aging.

Cellular Therapies for Treatment of Radiation Injury: Report from a NIH/NIAID and IRSN Workshop

In recent years, there has been increasing concern over the possibility of a radiological or nuclear incident occurring somewhere in the world. Intelligence agencies frequently report that terrorist groups and rogue nations are seeking to obtain radiological or nuclear weapons of mass destruction. In addition, there exists the real possibility that safety of nuclear power reactors could be compromised by natural (such as the tsunami and subsequent Fukushima accident in Japan in March, 2011) or accidental (Three Mile Island, 1979 and Chernobyl, 1986) events. Although progress has been made by governments around the world to prepare for these events, including the stockpiling of radiation countermeasures, there are still challenges concerning care of patients injured during a radiation incident. Because the deleterious and pathological effects of radiation are so broad, it is desirable to identify medical countermeasures that can have a beneficial impact on several tissues and organ systems. Cellular therapies have the potential to impact recovery and tissue/organ regeneration for both early and late complications of radiation exposure. These therapies, which could include stem or blood progenitor cells, mesenchymal stromal cells (MSCs) or cells derived from other tissues (e.g., endothelium or placenta), have shown great promise in treating other nonradiation injuries to and diseases of the bone marrow, skin, gastrointestinal tract, brain, lung and heart. To explore the potential use of these therapies in the treatment of victims after acute radiation exposure, the National Institute of Allergy and Infectious Diseases co-sponsored an international workshop in July, 2015 in Paris, France with the Institut de Radioprotection et de Sûreté Nucléaire. The workshop included discussions of data available from testing in preclinical models of radiation injury to different organs, logistics associated with the practical use of cellular therapies for a mass casualty incident, as well as international regulatory requirements for authorizing such drug products to be legally and readily used in such incidents. This report reviews the data presented, as well as key discussion points from the meeting.

Extrinsic regulation of hematopoietic stem cells in development, homeostasis and diseases

Lifelong generation of blood and immune cells depends on hematopoietic stem cells (HSCs). Their function is precisely regulated by complex molecular networks that integrate and respond to ever changing physiological demands of the body. Over the past several years, significant advances have been made in understanding the extrinsic regulation of HSCs during development and in homeostasis. Propelled by technical advances in the field, the cellular and molecular components of the microenvironment that support HSCs in vivo are emerging. In addition, the interaction of HSCs with their niches is appreciated as a critical contributor to the pathogenesis of a number of hematologic disorders. Here, we review these advances in detail and highlight the extrinsic regulation of HSCs in the context of development, homeostasis, and diseases. WIREs Dev Biol 2017, 6:e279. doi: 10.1002/wdev.279 For further resources related to this article, please visit the WIREs website.

Atorvastatin enhances endothelial cell function in posttransplant poor graft function

Dysfunctional BM EPCs were found in subjects with PGF postallotransplant. BM EPCs from subjects with PGF were enhanced by atorvastatin through downregulation of the p38 MAPK pathway.

Endothelial-specific inhibition of NF-κB enhances functional haematopoiesis

Haematopoietic stem cells (HSCs) reside in distinct niches within the bone marrow (BM) microenvironment, comprised of endothelial cells (ECs) and tightly associated perivascular constituents that regulate haematopoiesis through the expression of paracrine factors. Here we report that the canonical NF-κB pathway in the BM vascular niche is a critical signalling axis that regulates HSC function at steady state and following myelosuppressive insult, in which inhibition of EC NF-κB promotes improved HSC function and pan-haematopoietic recovery. Mice expressing an endothelial-specific dominant negative IκBα cassette under the Tie2 promoter display a marked increase in HSC activity and self-renewal, while promoting the accelerated recovery of haematopoiesis following myelosuppression, in part through protection of the BM microenvironment following radiation and chemotherapeutic-induced insult. Moreover, transplantation of NF-κB-inhibited BM ECs enhanced haematopoietic recovery and protected mice from pancytopenia-induced death. These findings pave the way for development of niche-specific cellular approaches for the treatment of haematological disorders requiring myelosuppressive regimens.

The complex microenvironmental signalling pathways that govern haematopoietic stem cell (HSC) activity remain poorly defined. Here, the authors identify endothelial NF-κB signalling as regulating regenerative HSC function, accelerating haematopoietic recovery following myelosuppressive injury in mice.

Endothelial Cells Promote Expansion of Long-Term Engrafting Marrow Hematopoietic Stem and Progenitor Cells in Primates

Successful expansion of bone marrow (BM) hematopoietic stem and progenitor cells (HSPCs) would benefit many HSPC transplantation and gene therapy/editing applications. However, current expansion technologies have been limited by a loss of multipotency and self-renewal properties ex vivo. We hypothesized that an ex vivo vascular niche would provide prohematopoietic signals to expand HSPCs while maintaining multipotency and self-renewal. To test this hypothesis, BM autologous CD34⁺ cells were expanded in endothelial cell (EC) coculture and transplanted in nonhuman primates. CD34⁺ C38^- HSPCs cocultured with ECs expanded up to 17-fold, with a significant increase in hematopoietic colony-forming activity compared with cells cultured with cytokines alone (colony-forming unit-granulocyte-erythroid-macrophage-monocyte; p < .005). BM CD34⁺ cells that were transduced with green fluorescent protein lentivirus vector and expanded on ECs engrafted long term with multilineage polyclonal reconstitution. Gene marking was observed in granulocytes, lymphocytes, platelets, and erythrocytes. Whole transcriptome analysis indicated that EC coculture altered the expression profile of 75 genes in the BM CD34⁺ cells without impeding the long-term engraftment potential. These findings show that an ex vivo vascular niche is an effective platform for expansion of adult BM HSPCs. Stem Cells Translational Medicine 2017;6:864-876.

Concise Review: Paracrine Functions of Vascular Niche Cells in Regulating Hematopoietic Stem Cell Fate

The functions of endothelial cells (ECs) in regulating oxygen delivery, nutrient exchange, coagulation, and transit of inflammatory cells throughout the body are well‐‐established. ECs have also been shown to regulate the maintenance and regeneration of organ‐specific stem cells in mammals. In the hematopoietic system, hematopoietic stem cells (HSCs) are dependent on signals from the bone marrow (BM) vascular niche for their maintenance and regeneration after myelosuppressive injury. Recent studies have demonstrated the essential functions of BM ECs and perivascular stromal cells in regulating these processes. In the present study, we summarize the current understanding of the role of BM ECs and perivascular cells in regulating HSC maintenance and regeneration and highlight the contribution of newly discovered EC‐derived paracrine factors that regulate HSC fate. Stem Cells Translational Medicine2017;6:482–489

A Molecular Profile of the Endothelial Cell Response to Ionizing Radiation

Ionizing radiation exposure can cause acute radiation sickness (ARS) by damaging the hematopoietic compartment. Radiation damages quiescent hematopoietic stem cells (HSCs) and proliferating hematopoietic cells, resulting in neutropenia, thrombocytopenia and increased risk for long-term hematopoietic dysfunction and myelodysplasia. While some aspects of the hematopoietic response to radiation injury are intrinsic to hematopoietic cells, the recovery of the HSC pool and overall hematopoiesis is also dependent on signals from bone marrow endothelial cells (BM ECs) within the HSC vascular niche. The precise mechanisms through which BM ECs regulate HSC regeneration remain unclear. Characterization of the altered EC gene expression that occurs in response to radiation could provide a roadmap to the discovery of EC-derived mechanisms that regulate hematopoietic regeneration. Here, we show that 5 Gy total-body irradiation substantially alters the expression of numerous genes in BM ECs within 24 h and this molecular response largely resolves by day 14 postirradiation. Several unique and nonannotated genes, which encode secreted proteins were upregulated and downregulated in ECs in response to radiation. These results highlight the complexity of the molecular response of BM ECs to ionizing radiation and identify several candidate mechanisms that should be prioritized for functional analysis in models of hematopoietic injury and regeneration.

Understanding the Pathophysiology and Challenges of Development of Medical Countermeasures for Radiation-Induced Vascular/Endothelial Cell Injuries: Report of a NIAID Workshop, August 20, 2015

After the events of September 11, 2001, a decade of research on the development of medical countermeasures (MCMs) to treat victims of a radiological incident has yielded two FDA-approved agents to mitigate acute radiation syndrome. These licensed agents specifically target the mitigation of radiation-induced neutropenia and infection potential, while the ramifications of the exposure event in a public health emergency incident could include the entire body, causing additional acute and/or delayed organ/tissue injuries. Anecdotal data as well as recent findings from both radiation accident survivors and animal experiments implicate radiation-induced injury or dysfunction of the vascular endothelium leading to tissue and organ injuries. There are significant gaps in our understanding of the disease processes and progression, as well as the optimum approaches to develop medical countermeasures to mitigate radiation vascular injury. To address this issue, the Radiation and Nuclear Countermeasures Program of the National Institute of Allergy and Infectious Diseases (NIAID) organized a one-day workshop to examine the current state of the science in radiation-induced vascular injuries and organ dysfunction, the natural history of the pathophysiology and the product development maturity of potential medical countermeasures to treat these injuries. Meeting presentations were followed by a NIAID-led open discussion among academic investigators, industry researchers and government agency representatives. This article provides a summary of these presentations and subsequent discussion from the workshop.

Spleen hypoplasia leads to abnormal stress hematopoiesis in mice with loss of Pbx homeoproteins in splenic mesenchyme

The spleen plays critical roles in immunity and also provides a permissive microenvironment for hematopoiesis. Previous studies have reported that the TALE-class homeodomain transcription factor Pbx1 is essential in hematopoietic stem and progenitor cells (HSPCs) for stem cell maintenance and progenitor expansion. However, the role of Pbx1 in the hematopoietic niche has not been investigated. Here we explored the effects that genetic perturbation of the splenic mesenchymal niche has on hematopoiesis upon loss of members of the Pbx family of homeoproteins. Splenic mesenchyme-specific inactivation of Pbx1 (SKO) on a Pbx2- or Pbx3-deficient genetic background (DKO) resulted in abnormal development of the spleen, which is dysmorphic and severely hypoplastic. This phenotype, in turn, affected the number of HSPCs in the fetal and adult spleen at steady state, as well as markedly impairing the kinetics of hematopoietic regeneration in adult mice after sub-lethal and lethal myelosuppressive irradiation. Spleens of mice with compound Pyx deficiency 8 days following sublethal irradiation displayed significant downregulation of multiple cytokine-encoding genes, including KitL/SCF, Cxcl12/SDF-1, IL-3, IL-4, GM-CSF/Csf2 IL-10, and Igf-1, compared with controls. KitL/SCF and Cxcl12/SDF-1 were recently shown to play key roles in the splenic niche in response to various haematopoietic stresses such as myeloablation, blood loss, or pregnancy. Our results demonstrate that, in addition to their intrinsic roles in HSPCs, non-cell autonomous functions of Pbx factors within the splenic niche contribute to the regulation of hematopoiesis, at least in part via the control of KitL/SCF and Cxcl12/SDF-1. Furthermore, our study establishes that abnormal spleen development and hypoplasia have deleterious effects on the efficiency of hematopoietic recovery after bone marrow injury.

Mesenchymal Stem and Progenitor Cells in Normal and Dysplastic Hematopoiesis-Masters of Survival and Clonality?

Myelodysplastic syndromes (MDS) are malignant hematopoietic stem cell disorders that have the capacity to progress to acute myeloid leukemia (AML). Accumulating evidence suggests that the altered bone marrow (BM) microenvironment in general, and in particular the components of the stem cell niche, including mesenchymal stem cells (MSCs) and their progeny, play a pivotal role in the evolution and propagation of MDS. We here present an overview of the role of MSCs in the pathogenesis of MDS, with emphasis on cellular interactions in the BM microenvironment and related stem cell niche concepts. MSCs have potent immunomodulatory capacities and communicate with diverse immune cells, but also interact with various other cellular components of the microenvironment as well as with normal and leukemic stem and progenitor cells. Moreover, compared to normal MSCs, MSCs in MDS and AML often exhibit altered gene expression profiles, an aberrant phenotype, and abnormal functional properties. These alterations supposedly contribute to the “reprogramming” of the stem cell niche into a disease-permissive microenvironment where an altered immune system, abnormal stem cell niche interactions, and an impaired growth control lead to disease progression. The current article also reviews molecular targets that play a role in such cellular interactions and possibilities to interfere with abnormal stem cell niche interactions by using specific targeted drugs.

Angiocrine functions of organ-specific endothelial cells

Endothelial cells that line capillaries are not just passive conduits for delivering blood. Tissue-specific endothelium establishes specialized vascular niches that deploy sets of growth factors, known as angiocrine factors. These cues participate actively in the induction, specification, patterning and guidance of organ regeneration, as well as in the maintainance of homeostasis and metabolism. When upregulated following injury, they orchestrate self-renewal and differentiation of tissue-specific resident stem and progenitor cells into functional organs. Uncovering the mechanisms by which organotypic endothelium distributes physiological levels of angiocrine factors both spatially and temporally will lay the foundation for clinical trials that promote organ repair without scarring.

Serum microRNAs are early indicators of survival after radiation-induced hematopoietic injury

Accidental radiation exposure is a threat to human health that necessitates effective clinical planning and diagnosis. Minimally invasive biomarkers that can predict long-term radiation injury are urgently needed for optimal management after a radiation accident. We have identified serum microRNA (miRNA) signatures that indicate long-term impact of total body irradiation (TBI) in mice when measured within 24 hours of exposure. Impact of TBI on the hematopoietic system was systematically assessed to determine a correlation of residual hematopoietic stem cells (HSCs) with increasing doses of radiation. Serum miRNA signatures distinguished untreated mice from animals exposed to radiation and correlated with the impact of radiation on HSCs. Mice exposed to sublethal (6.5 Gy) and lethal (8 Gy) doses of radiation were indistinguishable for 3 to 4 weeks after exposure. A serum miRNA signature detectable 24 hours after radiation exposure consistently segregated these two cohorts. Furthermore, using either a radioprotective agent before, or radiation mitigation after, lethal radiation, we determined that the serum miRNA signature correlated with the impact of radiation on animal health rather than the radiation dose. Last, using humanized mice that had been engrafted with human CD34(+) HSCs, we determined that the serum miRNA signature indicated radiation-induced injury to the human bone marrow cells. Our data suggest that serum miRNAs can serve as functional dosimeters of radiation, representing a potential breakthrough in early assessment of radiation-induced hematopoietic damage and timely use of medical countermeasures to mitigate the long-term impact of radiation.

Vascular Platform to Define Hematopoietic Stem Cell Factors and Enhance Regenerative Hematopoiesis

Hematopoietic stem cells (HSCs) inhabit distinct microenvironments within the adult bone marrow (BM), which govern the delicate balance between HSC quiescence, self-renewal, and differentiation. Previous reports have proposed that HSCs localize to the vascular niche, comprised of endothelium and tightly associated perivascular cells. Herein, we examine the capacity of BM endothelial cells (BMECs) to support ex vivo and in vivo hematopoiesis. We demonstrate that AKT1-activated BMECs (BMEC-Akt1) have a unique transcription factor/cytokine profile that supports functional HSCs in lieu of complex serum and cytokine supplementation. Additionally, transplantation of BMEC-Akt1 cells enhanced regenerative hematopoiesis following myeloablative irradiation. These data demonstrate that BMEC-Akt1 cultures can be used as a platform for the discovery of pro-HSC factors and justify the utility of BMECs as a cellular therapy. This technical advance may lead to the development of therapies designed to decrease pancytopenias associated with myeloablative regimens used to treat a wide array of disease states.

Extramedullary hematopoiesis (EMH) in laboratory animals: offering an insight into stem cell research

Extramedullary hematopoiesis (EMH) is a pathological process secondary to underlying bone marrow (BM) insufficiency in adults. It is characterized by the emergence of multipotent hematopoietic progenitors scattered around the affected tissue, most likely in the spleen, liver, and lymph node, etc. EMH in patients frequently receives less medical attention and is neglected unless a compressive or obstructive hematopoietic mass appears to endanger the patient's life. However, on a biological basis, EMH reflects the alteration of relationships among hematopoietic stem and progenitor cells (HSPCs) and their original and new microenvironments. The ability of hematopoietic stem cells (HSCs) to mobilize from the bone marrow and to accommodate and function in extramedullary tissues is rather complicated and far from our current understanding. Fortunately, many reports from the studies of drugs and genetics using animals have incidentally found EMH to be involved. Thereby, the molecular basis of EMH could further be elucidated from those animals after cross-comparison. A deeper understanding of the extramedullary hematopoietic niche could help expand stem cells in vitro and establish a better treatment in patients for stem cell transplantation.

Pleiotrophin mediates hematopoietic regeneration via activation of RAS

Hematopoietic stem cells (HSCs) are highly susceptible to ionizing radiation-mediated death via induction of ROS, DNA double-strand breaks, and apoptotic pathways. The development of therapeutics capable of mitigating ionizing radiation-induced hematopoietic toxicity could benefit both victims of acute radiation sickness and patients undergoing hematopoietic cell transplantation. Unfortunately, therapies capable of accelerating hematopoietic reconstitution following lethal radiation exposure have remained elusive. Here, we found that systemic administration of pleiotrophin (PTN), a protein that is secreted by BM-derived endothelial cells, substantially increased the survival of mice following radiation exposure and after myeloablative BM transplantation. In both models, PTN increased survival by accelerating the recovery of BM hematopoietic stem and progenitor cells in vivo. PTN treatment promoted HSC regeneration via activation of the RAS pathway in mice that expressed protein tyrosine phosphatase receptor-zeta (PTPRZ), whereas PTN treatment did not induce RAS signaling in PTPRZ-deficient mice, suggesting that PTN-mediated activation of RAS was dependent upon signaling through PTPRZ. PTN strongly inhibited HSC cycling following irradiation, whereas RAS inhibition abrogated PTN-mediated induction of HSC quiescence, blocked PTN-mediated recovery of hematopoietic stem and progenitor cells, and abolished PTN-mediated survival of irradiated mice. These studies demonstrate the therapeutic potential of PTN to improve survival after myeloablation and suggest that PTN-mediated hematopoietic regeneration occurs in a RAS-dependent manner.

Dysfunction of bone marrow vascular niche in acute graft-versus-host disease after MHC-haploidentical bone marrow transplantation

Acute graft-versus-host disease (aGvHD) is the most common complication of allogeneic hematopoietic stem cell transplantation (HSCT), which is often accompanied by impaired hematopoietic reconstitution. Sinusoidal endothelial cells (SECs) constitute bone marrow (BM) vascular niche that plays an important role in supporting self-renewal capacity and maintaining the stability of HSC pool. Here we provide evidences that vascular niche is a target of aGvHD in a major histocompatibility complex (MHC)–haploidentical matched murine HSCT model. The results demonstrated that hematopoietic cells derived from GvHD mice had the capacity to reconstitute hematopoiesis in healthy recipient mice. However, hematopoietic cells from healthy donor mice failed to reconstitute hematopoiesis in GvHD recipient mice, indicating that the BM niche was impaired by aGvHD in this model. We further demonstrated that SECs were markedly reduced in the BM of aGvHD mice. High level of Fas and caspase-3 expression and high rate of apoptosis were identified in SECs, indicating that SECs were destroyed by aGvHD in this murine HSCT model. Furthermore, high Fas ligand expression on engrafted donor CD4⁺, but not CD8⁺ T cells, and high level MHC-II but not MHC-I expression on SECs, suggested that SECs apoptosis was mediated by CD4⁺ donor T cells through the Fas/FasL pathway.

Circulating mouse Flk1+/c-Kit+/CD45- cells function as endothelial progenitors cells (EPCs) and stimulate the growth of human tumor xenografts

Background

Endothelial progenitor cells (EPCs) have been demonstrated to have stem-cell like as well as mature endothelial functions. However, controversy remains as to their origins, immunophenotypic markings, and contribution to the tumor vascular network and tumor survival.

Methods

Flow cytometric analysis and sorting was used to isolate Flk-1+/c-Kit+/CD45- cells. Matrigel and methycellulose assays, flow cytometry, and gene array analyses were performed to characterize several murine EPC cell populations. Human tumor xenografts were used to evaluate the impact of EPCs on tumor growth and vascular development.

Results

Flk-1+/c-Kit+/CD45- cells were present at low levels in most murine organs with the highest levels in adipose, aorta/vena cava, and lung tissues. Flk-1+/c-Kit+/CD45- cells demonstrated stem cell qualities through colony forming assays and mature endothelial function by expression of CD31, uptake of acLDL, and vascular structure formation in matrigel. High passage EPCs grown in vitro became more differentiated and lost stem-cell markers. EPCs were found to have hemangioblastic properties as demonstrated by the ability to rescue mice given whole body radiation. Systemic injection of EPCs increased the growth of human xenograft tumors and vessel density.

Conclusions

Flk-1+/C-Kit+/CD45- cells function as endothelial progenitor cells. EPCs are resident in most murine tissue types and localize to human tumor xenografts. Furthermore, the EPC population demonstrates stem-cell and mature endothelial functions and promoted the growth of tumors through enhanced vascular network formation. Given the involvement of EPCs in tumor development, this unique host-derived population may be an additional target to consider for anti-neoplastic therapy.