Characterization of spontaneous bone marrow recovery after sublethal total body irradiation: importance of the osteoblastic/adipocytic balance

Transient expansion and myofibroblast conversion of adipogenic lineage precursors mediate bone marrow repair after radiation

Radiation causes a collapse of bone marrow cells and elimination of microvasculature. To understand how bone marrow recovers after radiation, we focused on mesenchymal lineage cells that provide a supportive microenvironment for hematopoiesis and angiogenesis in bone. We recently discovered a nonproliferative subpopulation of marrow adipogenic lineage precursors (MALPs) that express adipogenic markers with no lipid accumulation. Single-cell transcriptomic analysis revealed that MALPs acquire proliferation and myofibroblast features shortly after radiation. Using an adipocyte-specific Adipoq-Cre, we validated that MALPs rapidly and transiently expanded at day 3 after radiation, coinciding with marrow vessel dilation and diminished marrow cellularity. Concurrently, MALPs lost most of their cell processes, became more elongated, and highly expressed myofibroblast-related genes. Radiation activated mTOR signaling in MALPs that is essential for their myofibroblast conversion and subsequent bone marrow recovery at day 14. Ablation of MALPs blocked the recovery of bone marrow vasculature and cellularity, including hematopoietic stem and progenitors. Moreover, VEGFa deficiency in MALPs delayed bone marrow recovery after radiation. Taken together, our research demonstrates a critical role of MALPs in mediating bone marrow repair after radiation injury and sheds light on a cellular target for treating marrow suppression after radiotherapy.

Getting blood out of a stone: Identification and management of patients with poor hematopoietic cell mobilization

Hematopoietic cell transplantation (HCT) has become a primary treatment for many cancers. Nowadays, the primary source of hematopoietic cells is by leukapheresis collection of these cells from peripheral blood, after a forced egress of hematopoietic cells from marrow into blood circulation, a process known as "mobilization". In this process, mobilizing agents disrupt binding interactions between hematopoietic cells and marrow microenvironment to facilitate collection. As the first essential step of HCT, poor mobilization, i.e. failure to obtain a desired or required number of hematopoietic cell, is one of the major factors affecting engraftment or even precluding transplantation. This review summarizes the available mobilization regimens using granulocyte-colony stimulating factor (G-CSF) and plerixafor, as well as the current understanding of the factors that are associated with poor mobilization. Strategies to mobilize patients or healthy donors who failed previous mobilization are discussed. Multiple novel agents are under investigation and some of them have shown the potential to enhance the mobilization response to G-CSF and/or plerixafor. Further investigation of the risk factors including genetic factors will offer an opportunity to better understand the molecular mechanism of mobilization and help develop new therapeutic strategies for successful mobilizations.

HIV-1 Tat protein inhibits the hematopoietic support function of human bone marrow mesenchymal stem cells

Most HIV-1-infected patients experience hematopoiesis suppression complications. Bone marrow mesenchymal stem cells (BMSCs) are involved in regulation of hematopoietic homeostasis, so we investigated the role of Tat, a protein released by infected cells in bone marrow and impacted differentiation potential of mesenchymal stem cells, in the BMSC hematopoietic support function. BMSCs were treated with HIV-1 Tat protein (BMSC_Tat-p), transfected with HIV-1 Tat mRNA (BMSC_Tat-m) or treated with solvent (PBS) (BMSC_con) for 20 days. Then, the hematopoietic support function of BMSC_Tat-p, BMSC_Tat-m and BMSC_con was analyzed via ex vivo expansion of hematopoietic stem cells (HSCs) grown on the BMSCs and via in vivo cotransplantation of HSCs and BMSCs. In addition, the hematopoiesis-supporting gene expression patterns of BMSC_Tat-p, BMSC_Tat-m and BMSC_con were compared. The results showed that BMSC_Tat-p and BMSC_Tat-m displayed reduced expansion, a decline in the number of colony forming units (CFUs) and a decreased proportion of the primitive subpopulation of hematopoietic stem cells under coculture conditions compared with BMSC_con. The ability of BMSC_Tat-p to support hematopoietic recovery was also impaired, which was further confirmed by the patterns in gene expression analysis. In conclusion, Tat treatment reduced the function of BMSCs in hematopoietic support, likely by downregulating the expression of a series of hematopoietic cytokines.

Expression of interferon regulatory factors (IRF-1 and IRF-2) during radiation-induced damage and regeneration of bone marrow by transplantation in mouse

Interferon regulatory factors (IRF-1 and IRF-2) are transcription factors of IRF-family that regulate expression of genes for cytokines, chemokines and growth factors in mammalian cells. IRF-1 and IRF-2 play crucial roles in the differentiation of bone marrow cells for immune response. Bone marrow (BM) is the soft lymphoid organ that contains many types of stem cells and produces different types of cells of the blood and immune system. Genetic alterations and damage of the bone marrow cells can lead to different types of blood and immune system-related diseases including anemia and cancer. We have studied the expression of IRF-1 and IRF-2 during radiation-induced damage and regeneration of bone marrow cells after transplantation of freshly isolated bone marrow cells in the mouse. Cell cycle analysis, colony forming unit-fibroblast (CFU-F) assay and bone marrow histology showed that after radiation-induced damage, the bone marrow transplantation resulted in regeneration of the bone marrow up to 24-35% recovery. Real-time quantitative reverse transcription-polymerase chain reaction (Q-RT-PCR) for the mRNA expression showed that IRF-1 and IRF-2 were expressed at higher levels in the bone marrow cells of the irradiated (4.34× fold for IRF-1, and 3.87× fold for IRF-2) compared to control and transplanted (1.13× fold for IRF-1, and 1.12× fold IRF-2) mice and immuno-fluorescence analysis for the protein expression showed that IRF-1 and IRF-2 were expressed at higher levels in the bone marrow cells of the irradiated (2.12× fold for IRF-1 and 1.71× fold for IRF-2) compared to control and transplanted (1.73× fold for IRF-1 and 1.21× fold for IRF-2) mice. Thus, IRF-1 and IRF-2 are sensitive and responsive to radiation-induced damage in the bone marrow cells and may also be involved in the bone marrow regeneration process.

MRI Study on the Changes of Bone Marrow Microvascular Permeability and Fat Content after Total-Body X-Ray Irradiation

In this study, we investigated microvascular perfusion status, changes to fat content and fatty acid composition in the bone marrow of rat femurs after total-body irradiation by quantitative permeability parameters of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and ex vivo high-resolution magic angle spinning (HRMAS) ¹H nuclear magnetic resonance spectroscopy (NMRS). Thirty-six Sprague-Dawley rats were randomly assigned to either an irradiated or nonirradiated control group. Permeability imaging using DCE-MRI and HRMAS ¹H NMRS was performed before irradiation, as well as at days 4 and 7 postirradiation. The volume transfer constant (K^trans) values increased to 2.219 ± 0.418/min ( P < 0.01) at day 4 and to 2.760 ± 0.217/min at day 7 ( P < 0.01) postirradiation. The plasma fraction (v_p) values gradually decreased. The proportion of (n-6) polyunsaturated fatty acids (PUFA) gradually reached a peak at day 7, the proportion of (n-3) PUFA gradually decreased and the proportion of saturated fatty acids gradually increased. After irradiation, K^trans at different times showed significant negative correlation with (n-3) PUFA ( r = -0.6393, P < 0.01) and significant positive correlation with (n-6) PUFA ( r = 0.6841, P < 0.05). These findings indicate that bone marrow microcirculation perfusion and vascular permeability correlated with fat content at an early time point after irradiation. A pathophysiological mechanism may exist based on fat-vascular permeability in the case of injury to bone marrow microcirculation.

Investigating the Abscopal Effects of Radioablation on Shielded Bone Marrow in Rodent Models Using Multimodality Imaging

The abscopal effect is the response to radiation at sites that are distant from the irradiated site of an organism, and it is thought to play a role in bone marrow (BM) recovery by initiating responses in the unirradiated bone marrow. Understanding the mechanism of this effect has applications in treating BM failure (BMF) and BM transplantation (BMT), and improving survival of nuclear disaster victims. Here, we investigated the use of multimodality imaging as a translational tool to longitudinally assess bone marrow recovery. We used positron emission tomography/computed tomography (PET/CT), magnetic resonance imaging (MRI) and optical imaging to quantify bone marrow activity, vascular response and marrow repopulation in fully and partially irradiated rodent models. We further measured the effects of radiation on serum cytokine levels, hematopoietic cell counts and histology. PET/CT imaging revealed a radiation-induced increase in proliferation in the shielded bone marrow (SBM) compared to exposed bone marrow (EBM) and sham controls. T₂-weighted MRI showed radiation-induced hemorrhaging in the EBM and unirradiated SBM. In the EBM and SBM groups, we found alterations in serum cytokine and hormone levels and in hematopoietic cell population proportions, and histological evidence of osteoblast activation at the bone marrow interface. Importantly, we generated a BMT mouse model using fluorescent-labeled bone marrow donor cells and performed fluorescent imaging to reveal the migration of bone marrow cells from shielded to radioablated sites. Our study validates the use of multimodality imaging to monitor bone marrow recovery and provides evidence for the abscopal response in promoting bone marrow recovery after irradiation.

A T Cell View of the Bone Marrow

The majority of T cells present in the bone marrow (BM) represent an activated/memory phenotype and most of these, if not all, are circulating T cells. Their lodging in the BM keeps them activated, turning the BM microenvironment into a “memory reservoir.” This article will focus on how T cell activation in the BM results in both direct and indirect effects on the hematopoiesis. The hematopoietic stem cell niche will be presented, with its main components and organization, along with the role played by T lymphocytes in basal and pathologic conditions and their effect on the bone remodeling process. Also discussed herein will be how “normal” bone mass peak is achieved only in the presence of an intact adaptive immune system, with T and B cells playing critical roles in this process. Our main hypothesis is that the partnership between T cells and cells of the BM microenvironment orchestrates numerous processes regulating immunity, hematopoiesis, and bone remodeling.

Direct bone marrow HSC transplantation enhances local engraftment at the expense of systemic engraftment in NSG mice

Direct bone marrow (BM) injection has been proposed as a strategy to bypass homing inefficiencies associated with intravenous (IV) hematopoietic stem cell (HSC) transplantation. Despite physical delivery into the BM cavity, many donor cells are rapidly redistributed by vascular perfusion, perhaps compromising efficacy. Anchoring donor cells to 3-dimensional (3D) multicellular spheroids, formed from mesenchymal stem/stromal cells (MSC) might improve direct BM transplantation. To test this hypothesis, relevant combinations of human umbilical cord blood-derived CD34(+) cells and BM-derived MSC were transplanted into NOD/SCID gamma (NSG) mice using either IV or intrafemoral (IF) routes. IF transplantation resulted in higher human CD45(+) and CD34(+) cell engraftment within injected femurs relative to distal femurs regardless of cell combination, but did not improve overall CD45(+) engraftment at 8 weeks. Analysis within individual mice revealed that despite engraftment reaching near saturation within the injected femur, engraftment at distal hematopoietic sites including peripheral blood, spleen and non-injected femur, could be poor. Our data suggest that the retention of human HSC within the BM following direct BM injection enhances local chimerism at the expense of systemic chimerism in this xenogeneic model.

Marrow damage and hematopoietic recovery following allogeneic bone marrow transplantation for acute leukemias: Effect of radiation dose and conditioning regimen

BACKGROUND AND PURPOSE

Total body irradiation (TBI) is a common component of hematopoietic cell transplantation (HCT) conditioning regimens. Preclinical studies suggest prolonged bone marrow (BM) injury after TBI could contribute to impaired engraftment and poor hematopoietic function.

MATERIALS AND METHODS

We studied the longitudinal changes in the marrow environment in patients receiving allogeneic HCT with myeloablative (MA, n=42) and reduced intensity (RIC, n=56) doses of TBI from 2003-2013, including BM cellularity, histologic features of injury and repair, hematologic and immunologic recovery.

RESULTS

Following MA conditioning, a 30% decrease in the marrow cellularity persisted at 1 year post-transplant (p=0.03). RIC HCT marrow cellularity transiently decreased but returned to baseline by 6 months even though the RIC group received mostly umbilical cord blood (UCB) grafts (82%, vs. 17% in the MA cohort, p<0.01). There was no evidence of persistent marrow vascular damage or inflammation. Recipients of more intensive conditioning did not show more persistent cytopenias with the exception of a tendency for minimal thrombocytopenia. Immune recovery was similar between MA and RIC.

CONCLUSIONS

These findings suggest that TBI associated with MA conditioning leads to prolonged reductions in marrow cellularity, but does not show additional histological evidence of long-term injury, which is further supported by similar peripheral counts and immunologic recovery.

Long-term loss of osteoclasts and unopposed cortical mineral apposition following limited field irradiation

Late-onset fragility fractures are a common complication following radiotherapy for metastatic disease and soft tissue sarcomas. Using a murine hindlimb focal irradiation model (RTx), we quantified time-dependent changes in osteoclasts and mineral apposition rate (MAR). Mice received either a single, unilateral 5 Gy exposure or four fractionated doses (4 × 5 Gy). Osteoclast numbers and MAR were evaluated histologically at 1, 2, 4, 8, 12, and 26 weeks post-RTx. Radiation induced an early, transient increase in osteoclasts followed by long-term depletion. Increased osteoclast numbers correlated temporally with trabecular resorption; the resorbed trabeculae were not later restored. Radiotherapy did not attenuate MAR at any time point. A transient, early increase in MAR was noted in both RTx groups, however, the 4 × 5 Gy group exhibited an unexpected spike in MAR eight weeks. Persistent depletion of osteoclasts permitted anabolic activity to continue unopposed, resulting in cortical thickening. These biological responses likely contribute to post-radiotherapy bone fragility via microdamage accumulation and matrix embrittlement in the absence of osteoclastic remodeling, and trabecular resorption-induced decrease in bone strength. The temporal distribution of osteoclast numbers suggests that anti-resorptive therapies may be of clinical benefit only if started prior to radiotherapy and continued through the following period of increased osteoclastic remodeling.

Hematopoietic recovery of acute radiation syndrome by human superoxide dismutase-expressing umbilical cord mesenchymal stromal cells

BACKGROUND AIMS

Acute radiation syndrome (ARS) leads to pancytopenia and multi-organ failure. Transplantation of hematopoietic stem cells provides a curative option for radiation-induced aplasia, but this therapy is limited by donor availability.

METHODS

We examined an alternative therapeutic approach to ARS with the use of human extracellular superoxide dismutase (ECSOD)-modified umbilical cord mesenchymal stromal cells (UCMSCs). This treatment combines the unique regenerative role of UCMSCs with the anti-oxidative activity of ECSOD.

RESULTS

We demonstrated that systemically administered ECSOD-UCMSCs are able to protect mice from sub-lethal doses of radiation and improve survival by promoting multilineage hematopoietic recovery. The therapeutic effect of this treatment is related to the decrease in radiation-induced O(2)(-) and apoptosis.

CONCLUSIONS

Our data highlight the clinical potential of this two-pronged approach to the treatment of ARS, thereby serving as a rapid and effective first-line strategy to combat the hematopoietic failure resulting from a radiation accident, nuclear terrorism and other radiologic emergencies.

Bone marrow regeneration promoted by biophysically sorted osteoprogenitors from mesenchymal stromal cells

Human tissue repair deficiencies can be supplemented through strategies to isolate, expand in vitro, and reimplant regenerative cells that supplant damaged cells or stimulate endogenous repair mechanisms. Bone marrow-derived mesenchymal stromal cells (MSCs), a subset of which is described as mesenchymal stem cells, are leading candidates for cell-mediated bone repair and wound healing, with hundreds of ongoing clinical trials worldwide. An outstanding key challenge for successful clinical translation of MSCs is the capacity to produce large quantities of cells in vitro with uniform and relevant therapeutic properties. By leveraging biophysical traits of MSC subpopulations and label-free microfluidic cell sorting, we hypothesized and experimentally verified that MSCs of large diameter within expanded MSC cultures were osteoprogenitors that exhibited significantly greater efficacy over other MSC subpopulations in bone marrow repair. Systemic administration of osteoprogenitor MSCs significantly improved survival rates (>80%) as compared with other MSC subpopulations (0%) for preclinical murine bone marrow injury models. Osteoprogenitor MSCs also exerted potent therapeutic effects as "cell factories" that secreted high levels of regenerative factors such as interleukin-6 (IL-6), interleukin-8 (IL-8), vascular endothelial growth factor A, bone morphogenetic protein 2, epidermal growth factor, fibroblast growth factor 1, and angiopoietin-1; this resulted in increased cell proliferation, vessel formation, and reduced apoptosis in bone marrow. This MSC subpopulation mediated rescue of damaged marrow tissue via restoration of the hematopoiesis-supporting stroma, as well as subsequent hematopoiesis. Together, the capabilities described herein for label-freeisolation of regenerative osteoprogenitor MSCs can markedly improve the efficacy of MSC-based therapies.

A dual-radioisotope hybrid whole-body micro-positron emission tomography/computed tomography system reveals functional heterogeneity and early local and systemic changes following targeted radiation to the murine caudal skeleton

The purpose of this study was to develop a longitudinal non-invasive functional imaging method using a dual-radioisotope hybrid micro-positron emission tomography/computed tomography (PET/CT) scanner in order to assess both the skeletal metabolic heterogeneity and the effect of localized radiation that models therapeutic cancer treatment on marrow and bone metabolism. Skeletally mature BALB/c female mice were given clinically relevant local radiation (16 Gy) to the hind limbs on day 0. Micro-PET/CT acquisition was performed serially for the same mice on days -5 and +2 with FDG and days -4 and +3 with NaF. Serum levels of pro-inflammatory cytokines were measured. Significant differences (p < 0.0001) in marrow metabolism (measured by FDG) and bone metabolism (measured by NaF) were observed among bones before radiation, which demonstrates functional heterogeneity in the marrow and mineralized bone throughout the skeleton. Radiation significantly (p < 0.0001) decreased FDG uptake but increased NaF uptake (p = 0.0314) in both irradiated and non-irradiated bones at early time points. An increase in IL-6 was observed with a significant abscopal (distant) effect on marrow and bone metabolic function. Radiation significantly decreased circulating IGF-1 (p < 0.01). Non-invasive longitudinal imaging with dual-radioisotope micro-PET/CT is feasible to investigate simultaneous changes in marrow and bone metabolic function at local and distant skeletal sites in response to focused radiation injury. Distinct local and remote changes may be affected by several cytokines activated early after local radiation exposure. This approach has the potential for longer-term studies to clarify the effects of radiation on marrow and bone.

The changing balance between osteoblastogenesis and adipogenesis in aging and its impact on hematopoiesis

Osteoblasts (OBs) and adipocytes (APs) share a common mesenchymal ancestor. It is now clear that mesenchymal stem cell (MSC) maturation along the OB lineage comes at the expense of adipogenesis and vice versa. During aging, this balance increasingly favors the formation of APs. Hematopoiesis also slowly declines during the aging process. The role of OB lineage cells in hematopoiesis has been studied, but less is known about how APs regulate hematopoiesis. A few studies have demonstrated a negative relationship between APs and hematopoiesis; however, there is also evidence that brown adipose tissue (BAT) may promote hematopoiesis. This review will examine the current knowledge of how adipogenesis and osteogenesis change with aging and the implications of this changing environment on hematopoeisis.