Soluble factor(s) from bone marrow cells can rescue lethally irradiated mice by protecting endogenous hematopoietic stem cells

Elucidating the Morphogenic and Signaling Roles of Defined Growth Factors Controlling Human Endothelial Cell Lumen Formation Versus Sprouting Behavior

Five growth factors [ie, insulin, fibroblast growth factor-2 (FGF-2), stem cell factor, IL-3, and stromal-derived factor 1α] in combination are necessary for human endothelial cells (ECs) to undergo tube morphogenesis, a process requiring both lumen formation and sprouting behavior. This study investigated why these factors are required by subdividing the factors into 4 separate groups: insulin-only, insulin and FGF-2, no FGF-2 (all factors but without FGF-2), and all factors. The study found that the insulin-only condition failed to support EC morphogenesis or survival, the insulin and FGF-2 condition supported primarily EC lumen formation, and the no FGF-2 condition supported EC sprouting behavior. By comparison, the all-factors condition more strongly stimulated both EC lumen formation and sprouting behavior, and signaling analysis revealed prolonged stimulation of multiple promorphogenic signals coupled with inhibition of proregressive signals. Pharmacologic inhibition of Jak kinases more selectively blocked EC sprouting behavior, whereas inhibition of Raf, phosphatidylinositol 3-kinase, and Akt kinases showed selective blockade of lumen formation. Inhibition of Src family kinases and Notch led to increased sprouting coupled to decreased lumen formation, whereas inhibition of Pak, Mek, and mammalian target of rapamycin kinases blocked both sprouting and lumen formation. These findings reveal novel downstream biological and signaling activities of defined factors that are required for the assembly of human EC-lined capillary tube networks.

Allogeneic adipose-derived stem cells mitigate acute radiation syndrome by the rescue of damaged bone marrow cells from apoptosis

Acute radiation syndrome (ARS) is the radiation toxicity that can affect the hematopoietic, gastrointestinal, and nervous systems upon accidental radiation exposure within a short time. Currently, there are no effective and safe approaches to treat mass population exposure to ARS. Our study aimed to evaluate the therapeutic potential of allogeneic adipose‐derived stem cells (ASCs) for total body irradiation (TBI)‐induced ARS and understand the underlying mitigation mechanism. We employed 9.25 Gy TBI dose to C57BL/6 mice and studied the effect of allogeneic ASCs on mice survival and regeneration of the hematopoietic system. Our results indicate that intraperitoneal‐injected ASCs migrated to the bone marrow, rescued hematopoiesis, and improved the survival of irradiated mice. Our transwell coculture results confirmed the migration of ASCs to irradiated bone marrow and rescue hematopoietic activity. Furthermore, contact coculture of ASCs improved the survival and hematopoiesis of irradiated bone marrow in vitro. Irradiation results in DNA damage, upregulation of inflammatory signals, and apoptosis in bone marrow cells, while coculture with ASCs reduces apoptosis via activation of DNA repair and the antioxidation system. Upon exposure to irradiated bone marrow cells, ASCs secrete prosurvival and hematopoietic factors, such as GM‐CSF, MIP1α, MIP1β, LIX, KC, 1P‐10, Rantes, IL‐17, MCSF, TNFα, Eotaxin, and IP‐10, which reduces oxidative stress and rescues damaged bone marrow cells from apoptosis. Our findings suggest that allogeneic ASCs therapy is effective in mitigating TBI‐induced ARS in mice and may be beneficial for clinical adaptation to treat TBI‐induced toxicities. Further studies will help to advocate the scale‐up and adaptation of allogeneic ASCs as the radiation countermeasure.

Bone marrow stromal cell therapy improves survival after radiation injury but does not restore endogenous hematopoiesis

The only available option to treat radiation-induced hematopoietic syndrome is allogeneic hematopoietic cell transplantation, a therapy unavailable to many patients undergoing treatment for malignancy, which would also be infeasible in a radiological disaster. Stromal cells serve as critical components of the hematopoietic stem cell niche and are thought to protect hematopoietic cells under stress. Prior studies that have transplanted mesenchymal stromal cells (MSCs) without co-administration of a hematopoietic graft have shown underwhelming rescue of endogenous hematopoiesis and have delivered the cells within 24 h of radiation exposure. Herein, we examine the efficacy of a human bone marrow-derived MSC therapy delivered at 3 h or 30 h in ameliorating radiation-induced hematopoietic syndrome and show that pancytopenia persists despite MSC therapy. Animals exposed to radiation had poorer survival and experienced loss of leukocytes, platelets, and red blood cells. Importantly, mice that received a therapeutic dose of MSCs were significantly less likely to die but experienced equivalent collapse of the hematopoietic system. The cause of the improved survival was unclear, as complete blood counts, splenic and marrow cellularity, numbers and function of hematopoietic stem and progenitor cells, and frequency of niche cells were not significantly improved by MSC therapy. Moreover, human MSCs were not detected in the bone marrow. MSC therapy reduced crypt dropout in the small intestine and promoted elevated expression of growth factors with established roles in gut development and regeneration, including PDGF-A, IGFBP-3, IGFBP-2, and IGF-1. We conclude that MSC therapy improves survival not through overt hematopoietic rescue but by positive impact on other radiosensitive tissues, such as the intestinal mucosa. Collectively, these data reveal that MSCs could be an effective countermeasure in cancer patients and victims of nuclear accidents but that MSCs alone do not significantly accelerate or contribute to recovery of the blood system.

Effects of CpG oligodeoxynucleotide 1826 on acute radiation-induced lung injury in mice

Background

The radiation-induced lung injury is a common complication from radiotherapy in lung cancer. CpG ODN is TLR9 activator with potential immune modulatory effects and sensitization of radiotherapy in lung cancer. This study aimed to examine the effect of CpG ODN on acute radiation-induced lung injury in mice.

Methods and results

The mouse model of radiation-induced lung injury was established by a single dose of 20 Gy X-rays exposure to the left lung. The results showed that the pneumonia score was lower in RT+CpG group than in RT group on 15th and 30th days. Compared with RT group, CpG ODN reduced the serum concentrations of MDA (P < 0.05) and increased the serum concentrations of SOD, GSH (P < 0.05). The serum concentration of TNF-α in RT+CpG group was lower on 15th and 30th days post-irradiation (P < 0.05).

Conclusion

The study demonstrated that CpG ODN has preventive effects of acute radiation-induced lung injury in mice. Lung inflammatory reaction and oxidative stress are promoted in the initiation of radiation-induced pneumonia. CpG ODN may reduce the injury of reactive oxygen species and adjust the serum TNF-α concentration in the mice after irradiation, which reduces the generation of the inflammatory cytokines.

Mouse xenograft modeling of human adult acute lymphoblastic leukemia provides mechanistic insights into adult LIC biology

The distinct nature of acute lymphoblastic leukemia (ALL) in adults, evidenced by inferior treatment outcome and different genetic landscape, mandates specific studies of disease-initiating mechanisms. In this study, we used NOD/LtSz-scid IL2Rγ null(c) (NSG) mouse xenotransplantation approaches to elucidate leukemia-initiating cell (LIC) biology in primary adult precursor B (pre-B) ALL to optimize disease modeling. In contrast with xenografting studies of pediatric ALL, we found that modification of the NSG host environment using preconditioning total body irradiation (TBI) was indispensable for efficient engraftment of adult non-t(4;11) pre-B ALL, whereas t(4;11) pre-B ALL was successfully reconstituted without this adaptation. Furthermore, TBI-based xenotransplantation of non-t(4;11) pre-B ALL enabled detection of a high frequency of LICs (<1:6900) and permitted frank leukemic engraftment from a remission sample containing drug-resistant minimal residual disease. Investigation of TBI-sensitive stromal-derived factor-1/chemokine receptor type 4 signaling revealed greater functional dependence of non-t(4;11) pre-B ALL on this niche-based interaction, providing a possible basis for the differential engraftment behavior. Thus, our studies establish the optimal conditions for experimental modeling of human adult pre-B ALL and demonstrate the critical protumorogenic role of microenvironment-derived SDF-1 in regulating adult pre-B LIC activity that may present a therapeutic opportunity.

Modelling hematological parameters after total body irradiation

PURPOSE

The time- and dose-dependent reconstitution of hematopoiesis after radiation exposure is strongly related to the stem cell population and can be used to predict hematological parameters. These parameters allow further insight into the hematopoietic system and might lead to the development of novel stem cell transplantation models.

MATERIALS AND METHODS

CD4-/- C57Bl/6 mice, transgenic for human CD4 and HLA-DR3, were irradiated in a single (3, 6, 8 and 12 Gy) and fractionated (6 × 1 Gy, 6 × 1.5 Gy, 6 × 2 Gy; twice daily) dose regimen. Blood was analyzed weekly for red blood cells (RBC), hemoglobin concentration (Hb), hematocrit (HCT) and white blood cells (WBC). Organ and tissue damage after irradiation were examined by histopathology.

RESULTS

The recovery curves for RBC, Hb, HCT and WBC showed the same velocity (< 1 week) for all radiation doses (3-12 Gy) starting at different, dose-dependent times. The only dose-dependent parameter was defined by the beginning of the recovery process (dose-dependent shift) and higher doses were related to a later recovery of the hematopoietic system. The RBC, Hb and HCT recovery was followed by a saturation curve reaching a final concentration independent of the radiation dose. Histological analysis of the bone marrow in the single dose cohort showed a dose-dependent reduction of the cellularity in the bone marrow cavities. The fractioned radiation dose cohort resulted in a regeneration of all bone marrow cavities.

CONCLUSION

Specific functions were developed to describe the reconstitution of hematological parameters after total body irradiation.

Frequency of acute myeloid leukaemia-associated mouse chromosome 2 deletions in X-ray exposed immature haematopoietic progenitors and stem cells

Exposure to ionising radiation can lead to an increased risk of cancer, particularly leukaemia. In radiation-induced acute myeloid leukaemia (rAML), a partial hemizygous deletion of mouse chromosome 2 is a common feature in several susceptible strains. The deletion is an early event detectable 24h after exposure in bone marrow cells using cytogenetic techniques. Expanding clones of bone marrow cells with chromosome 2 deletions can be detected less than a year after exposure to ionising radiation in around half of the irradiated mice. Ultimately, 15-25% of exposed animals develop AML. It is generally assumed that leukaemia originates in an early progenitor cell or haematopoietic stem cell, but it is unknown whether the original chromosome damage occurs at a similar frequency in committed progenitors and stem cells. In this study, we monitored the frequency of chromosome 2 deletions in immature bone marrow cells (Lin(-)) and haematopoietic stem cells/multipotent progenitor cells (LSK) by several techniques, fluorescent in situ hybridisation (FISH) and through use of a reporter gene model, flow cytometry and colony forming units in spleen (CFU-S) following ex vivo or in vivo exposure. We showed that partial chromosome 2 deletions are present in the LSK subpopulation, but cannot be detected in Lin(-) cells and CFU-S12 cells. Furthermore, we transplanted irradiated Lin(-) or LSK cells into host animals to determine whether specific irradiated cell populations acquire an increased proliferative advantage compared to unirradiated cells. Interestingly, the irradiated LSK subpopulation containing cells carrying chromosome 2 deletions does not appear to repopulate as well as the unirradiated population, suggesting that the chromosomal deletion does not provide an advantage for growth and in vivo repopulation, at least at early stages following occurrence.

High-dose ionizing radiation-induced hematotoxicity and metastasis in mice model

Radiotherapy induces untargeted effects on normal tissues such as bone marrow. So alteration of microenvironment by ionizing irradiation is supposed to influence dynamic host-cancer ecosystem affecting cancer behavior including metastasis. Herein, the incidence of lung metastasis after high-dose irradiation has been investigated using mice model having real-time condition of leucopenia. C57BL/6 mice were pre-exposed to a X-irradiation dose of 6 Gy on previous days 2, 5, 7, 10. Complete hematological parameters including lymphocyte subpopulation in blood and lung tissues were analyzed. Additionally, a group of mice including a non-irradiated group were inoculated with B16F10 cells (3 × 10(5)/200 μl) via tail vein at the same day, and lung metastasized colonies were compared among groups at day 14 of post-inoculation. We observed that (i) total leucocytes and platelet were gradually depleted by day 10; (ii) lung tissue showed gradual infiltration of leucocytes including neutrophils and lymphocytes; (iii) pulmonary colonies were maximum and minimum on day 5 and 10 respectively; (iv) lymphocyte subpopulation analysis showed most number of natural killer (NK) cells in lung tissues on day 10; (v) gene expression of platelet/endothelial cell adhesion molecule (PECAM) in lung tissues peaked on day 5. To sum-up the study, severity of leucopenia did not influence the incidence of metastasis but blood platelets and microenvironment alteration of targeting tissue may be responsible factors for lung metastasis in our experimental model.

The mechanism for the ameliorative effect of CpG-oligodeoxynucleotides on bone marrow hemopoiesis radiation injury

Bone marrow is a major site of radiation injury. The extreme sensitivity of bone marrow cells to genotoxic stress largely determines the adverse side effects of radiation. CpG-oligodeoxynucleotide (ODN) is known to be radioprotective in extramedullary hemopoiesis, but its effect on bone marrow hemopoiesis remains unknown. In this study, we investigated whether CpG-ODN ameliorated hemopoiesis radiation injury when administered after total-body irradiation (TBI). Mice were treated with 50 μg of CpG-ODN via intraperitoneal injection (i.p) 30 min., 24 and 48 hr after TBI. Our results show that CpG-ODN was able to mediate the activation of nuclear factor κB (NF-κB) via degradation of inhibitor NF-κB (IκB-α), and some oxidative stress parameters (malondialdehyde, glutathione and superoxide dismutase) showed significant differences between the radiation control group and the radiation and administration of CpG-ODN group. White blood cell count, bone marrow cell count and bone marrow histological examination indicated that CpG-ODN minimized bone marrow damage induced by radiation. Exogenous colony-forming unit-spleen count indicated that CpG-ODN reduced primitive hemopoietic stem cell damage and reconstituted the hemopoietic system after TBI. The survival of mice was also enhanced after various levels of TBI. The calculated dose reduction factor was 1.2. Thus, we conclude that CpG-ODN may contribute to the amelioration of hemopoiesis radiation injury.

Alkylating chemotherapeutic agents cyclophosphamide and melphalan cause functional injury to human bone marrow-derived mesenchymal stem cells

The adverse effects of melphalan and cyclophosphamide on hematopoietic stem cells are well-known; however, the effects on the mesenchymal stem cells (MSCs) residing in the bone marrow are less well characterised. Examining the effects of chemotherapeutic agents on patient MSCs in vivo is difficult due to variability in patients and differences in the drug combinations used, both of which could have implications on MSC function. As drugs are not commonly used as single agents during high-dose chemotherapy (HDC) regimens, there is a lack of data comparing the short- or long-term effects these drugs have on patients post treatment. To help address these problems, the effects of the alkylating chemotherapeutic agents cyclophosphamide and melphalan on human bone marrow MSCs were evaluated in vitro. Within this study, the exposure of MSCs to the chemotherapeutic agents cyclophosphamide or melphalan had strong negative effects on MSC expansion and CD44 expression. In addition, changes were seen in the ability of MSCs to support hematopoietic cell migration and repopulation. These observations therefore highlight potential disadvantages in the use of autologous MSCs in chemotherapeutically pre-treated patients for future therapeutic strategies. Furthermore, this study suggests that if the damage caused by chemotherapeutic agents to marrow MSCs is substantial, it would be logical to use cultured allogeneic MSCs therapeutically to assist or repair the marrow microenvironment after HDC.

Migratory strategies of normal and malignant stem cells

The regulated migration of stem cells is critical for organogenesis during development and for tissue -homeostasis and repair during adulthood. Human bone marrow (BM) represents an accessible reservoir containing regenerative cell types from hematopoietic, endothelial, and mesenchymal-stromal lineages that together coordinate hematopoiesis and promote the repair of damaged vasculature and tissues throughout the body. Thus, a detailed understanding of lineage-specific stem cell mobilization, homing, and subsequent engraftment in areas of injury or disease is of critical importance to the rational development of novel cell-mediated regenerative therapies. Stem cell trafficking via the circulation from site of origin to peripheral tissues requires fundamental molecular pathways governing (1) niche-specific deadhesion of progenitor cells; (2) chemoattraction to guide progenitor cell homing; and (3) interstitial navigation and adhesion/retention of recruited progenitor cells. This overview chapter summarizes the diversity of migratory strategies employed by hematopoietic, endothelial, and mesenchymal-stromal progenitor cells during repair and regeneration after tissue damage. Further elucidation of stem cell homing and migration pathways will allow greater application of stem cells for targeted cell therapy and/or drug delivery for tissue repair. Strikingly similar migratory mechanisms appear to govern the in vivo migration of recently characterized cancer stem cells (CSC) in leukemias and solid tumors, indicating that conserved principles of stem cell migration and niche specificity will provide new information to target CSC in anticancer therapy.

Radioprotection of bone marrow hematopoiesis by CpG-oligodeoxynucleotides administered to mice after total-body irradiation

CpG-oligodeoxynucleotide (ODN), a synthetic analog of bacteria DNA, has attracted attention because it activates cells of an adaptive immune system and the innate immune system. In this study, we investigated whether CpG-ODN has radioprotective effects, when administered after total-body irradiation (TBI). Mice were treated with 50 µg CpG-ODN via intraperitoneal injection (i.p) within 30 min, 24 h and 48 h after TBI. Our results showed that the survival rate was enhanced at various levels of TBI. The calculated dose reduction factor (DRF) was 1.2. Bone marrow cell count and bone marrow histological examination indicated that CpG-ODN minimized the bone marrow damage induced by TBI. The data of the white blood cell (WBC) count, exogenous (CFU-S) and endogenous (endoCFU-S) colony forming unit-spleen count demonstrated that CpG-ODN reduced primitive hematopoietic stem cells damage and reconstituted hematopoiesis after TBI. Thus, we suggested that CpG-ODN had the potential to contribute to the improvement of the survival rate and limitation of myelosuppression induced by TBI.

Sensitivity of salivary glands to radiation: from animal models to therapies

Radiation therapy for head and neck cancer causes significant secondary side-effects in normal salivary glands, resulting in diminished quality of life for these individuals. Salivary glands are exquisitely sensitive to radiation and display acute and chronic responses to radiotherapy. This review will discuss clinical implications of radiosensitivity in normal salivary glands, compare animal models used to investigate radiation-induced salivary gland damage, address therapeutic advances, and project future directions in the field.

Hematopoietic stem cell transplant in mice by intra-femoral injection

In traditional bone marrow transplantation, the majority of peripherally introduced stem cells are trapped in peripheral organs, such as the lung and liver. The frequency of cells homed in bone marrow by such method is extremely low. This circumstance adds difficulty to the research of hematopoietic stem cell (HSC), a rare population to begin with. By introducing HSC directly into bone marrow cavity, the peripheral loss of HSC can be minimized. Thus, intra-femoral injection of HSC is a useful method for HSC study.

IL-12 facilitates both the recovery of endogenous hematopoiesis and the engraftment of stem cells after ionizing radiation

OBJECTIVE

Severe myelosuppression is a common side effect of radiotherapy or chemotherapy. Methods have been developed to protect patients by stimulating white blood cell or red blood cell recovery/production using growth factors such as G-CSF or EPO. However, there is no available means to stimulate the full-lineage blood cell recovery from severe myelosuppression. In this study, we used lethally or sublethally irradiated animal models to evaluate the hematopoiesis stimulating effect of IL-12.

MATERIALS AND METHODS

IL-12-treated lethally or sublethally irradiated animals were examined for the survival/lifespan, the function assays (bone marrow transplantation, CFU-S(12), CFC) of bone marrow cell subsets, and apoptosis assay.

RESULTS

Using a low dose of IL-12 (10 times lower than previously reported dose), 91.4% of lethally irradiated animals survived long term without adverse effects on the gastrointestinal (GI) system. The reconstituted hematopoietic system was derived from long-term reconstituting hematopoietic stem cells (LTR HSC), which reconstituted hematopoiesis both endogenously after lethal radiation and in secondary recipients by bone marrow transplantation (BMT). IL-12 significantly attenuated the decline of blood cell counts in sublethally irradiated animals. The IL-12-stimulated hematopoiesis recovery resulted in a full-lineage blood cell production, including white and red blood cells, and platelets. There was no detectable expression of IL-12 receptor on LTR HSC. In IL-12-treated animals, the number of Sca-1(+) cells was significantly higher than in animals without IL-12 treatment.

CONCLUSION

In this study, we showed a low dose of IL-12 has hematopoietic-protecting effects, which can attenuate severe myelosuppresion caused by lethal or sublethal irradiation. This study, together with previous studies showing IL-12 is also an anti-tumor and anti-angiogenic agent, suggest IL-12 may have clinical significance in cancer treatment and BMT.

Sensing radiosensitivity of human epidermal stem cells

PURPOSE

Radiosensitivity of stem cells is a matter of debate. For mouse somatic stem cells, both radiosensitive and radioresistant stem cells have been described. By contrast, the response of human stem cells to radiation has been poorly studied. As epidermis is a radiosensitive tissue, we evaluated in the present work the radiosensitivity of cell populations enriched for epithelial stem cells of human epidermis.

METHODS AND MATERIALS

The total keratinocyte population was enzymatically isolated from normal human skin. We used flow cytometry and antibodies against cell surface markers to isolate basal cell populations from human foreskin. Cell survival was measured after a dose of 2Gy with the XTT assay at 72h after exposure and with a clonogenic assay at 2 weeks. Transcriptome analysis using oligonucleotide microarrays was performed to assess the genomic cell responses to radiation.

RESULTS

Cell sorting based on two membrane proteins, alpha6 integrin and the transferrin receptor CD71, allowed isolation of keratinocyte populations enriched for the two types of cells found in the basal layer of epidermis: stem cells and progenitors. Both the XTT assay and the clonogenic assay showed that the stem cells were radioresistant whereas the progenitors were radiosensitive. We made the hypothesis that upstream DNA damage signalling might be different in the stem cells and used microarray technology to test this hypothesis. The stem cells exhibited a much more reduced gene response to a dose of 2Gy than the progenitors, as we found that 6% of the spotted genes were regulated in the stem cells and 20% in the progenitors. Using Ingenuity Pathway Analysis software, we found that radiation exposure induced very specific pathways in the stem cells. The most striking responses were the repression of a network of genes involved in apoptosis and the induction of a network of cytokines and growth factors.

CONCLUSION

These results show for the first time that keratinocyte populations enriched for stem cells from human epidermis are radioresistant. Based on both repressed and induced genes, we found that the major response of the irradiated stem cell population was the regulation of genes functionally related to cell death, cell survival and apoptosis.

Hematopoietic stem cells promote the expansion and function of adoptively transferred antitumor CD8 T cells

Depleting host immune elements with nonmyeloablative regimens prior to the adoptive transfer of tumor-specific CD8(+) T cells significantly enhances tumor treatment. In the current study, superior antitumor efficacy was achieved by further increasing the intensity of lymphodepletion to a level that required HSC transplantation. Surprisingly, the HSC transplant and not the increased lymphodepletion caused a robust expansion of adoptively transferred tumor-specific CD8(+) T cells. The HSC-driven cell expansion of effector, but not of naive, CD8(+) T cells was independent of in vivo restimulation by MHC class I-expressing APCs. Simultaneously, HSCs also facilitated the reconstitution of the host lymphoid compartment, including inhibitory elements, not merely via the production of progeny cells but by enhancing the expansion of cells that had survived lymphodepletion. Profound lymphodepletion, by myeloablation or by genetic means, focused the nonspecific HSC boost preferentially toward the transferred tumor-specific T cells, leading to successful tumor treatment. These findings indicate that CD8(+) T cell-mediated tumor responses can be efficiently driven by HSCs in the myeloablative setting and have substantial implications for the design of new antitumor immunotherapies.

Radiation and the microenvironment - tumorigenesis and therapy

Radiation rapidly and persistently alters the soluble and insoluble components of the tissue microenvironment. This affects the cell phenotype, tissue composition and the physical interactions and signalling between cells. These alterations in the microenvironment can contribute to carcinogenesis and alter the tissue response to anticancer therapy. Examples of these responses and their implications are discussed with a view to therapeutic intervention.

How do stem cells find their way home?

Migration of hematopoietic stem cells through the blood, across the endothelial vasculature to different organs and to their bone marrow (BM) niches, requires active navigation, a process termed homing. Homing is a rapid process and is the first and essential step in clinical stem cell transplantation. Similarly, homing is required for seeding of the fetal BM by hematopoietic progenitors during development. Homing has physiological roles in adult BM homeostasis, which are amplified during stress-induced recruitment of leukocytes from the BM reservoir and during stem cell mobilization, as part of host defense and repair. Homing is thought to be a coordinated, multistep process, which involves signaling by stromal-derived factor 1 (SDF-1) and stem cell factor (SCF), activation of lymphocyte function-associated antigen 1 (LFA-1), very late antigen 4/5 (VLA-4/5) and CD44, cytoskeleton rearrangement, membrane type 1 (MT1)-matrix metalloproteinase (MMP) activation and secretion of MMP2/9. Rolling and firm adhesion of progenitors to endothelial cells in small marrow sinusoids under blood flow is followed by trans-endothelial migration across the physical endothelium/extracellular matrix (ECM) barrier. Stem cells finalize their homing uniquely, by selective access and anchorage to their specialized niches in the extravascular space of the endosteum region and in periarterial sites. This review is focused on mechanisms and key regulators of human stem cell homing to the BM in experimental animal models and clinical transplantation protocols.