Perspective: fundamental and clinical concepts on stem cell homing and engraftment: a journey to niches and beyond

In vitro generation of epidermal keratinocytes from human CD34-positive hematopoietic stem cells

The epidermis is largely composed of keratinocytes (KCs), and the proliferation and differentiation of KCs from the stratum basale to the stratum corneum is the cellular hierarchy present in the epidermis. In this study, we explore the differentiation abilities of human hematopoietic stem cells (HSCs) into KCs. Cultured HSCs positive for CD34, CD45, and CD133 with prominent telomerase activity were induced with keratinocyte differentiation medium (KDM), which is composed of bovine pituitary extract (BPE), epidermal growth factor (EGF), insulin, hydrocortisone, epinephrine, transferrin, calcium chloride (CaCl2), bone morphogenetic protein 4 (BMP4), and retinoic acid (RA). Differentiation was monitored through the expression of cytokeratin markers K5 (keratin 5), K14 (keratin 14), K10 (keratin 10), K1 (keratin 1), transglutaminase 1 (TGM1), involucrin (IVL), and filaggrin (FLG) on day 0 (D0), day 6 (D6), day 11 (D11), day 18 (D18), day 24 (D24), and day 30 (D30) using immunocytochemistry, fluorescence microscopy, flow cytometry, qPCR, and Western blotting. The results revealed the expression of K5 and K14 genes in D6 cells (early keratinocytes), K10 and K1 genes in D11-D18 cells (mature keratinocytes) with active telomerase enzyme, and FLG, IVL, and TGM1 in D18-D24 cells (terminal keratinocytes), and by D30, the KCs were completely enucleated similar to cornified matrix. This method of differentiation of HSCs to KCs explains the cellular order exists in the normal epidermis and opens the possibility of exploring the use of human HSCs in the epidermal differentiation.

Competitive Repopulation and Allo-Immunologic Pressure Determine Chimerism Kinetics after T Cell-Depleted Allogeneic Stem Cell Transplantation and Donor Lymphocyte Infusion

After allogeneic stem cell transplantation (alloSCT), patient-derived stem cells that survived the pretransplantation conditioning compete with engrafting donor stem cells for bone marrow (BM) repopulation. In addition, donor-derived alloreactive T cells present in the stem cell product may favor establishment of complete donor-derived hematopoiesis by eliminating patient-derived lymphohematopoietic cells. T cell-depleted alloSCT with sequential transfer of potentially alloreactive T cells by donor lymphocyte infusion (DLI) provides a unique opportunity to selectively study how competitive repopulation and allo-immunologic pressure influence lymphohematopoietic recovery. This study aimed to determine the relative contribution of competitive repopulation and donor-derived anti-recipient alloimmunologic pressure on the establishment of lymphohematopoietic chimerism after alloSCT. In this retrospective cohort study of 281 acute leukemia patients treated according to a protocol combining alemtuzumab-based T cell-depleted alloSCT with prophylactic DLI, we investigated engraftment and quantitative donor chimerism in the BM and immune cell subsets. DLI-induced increase of chimerism and development of graft-versus-host disease (GVHD) were analyzed as complementary indicators for donor-derived anti-recipient alloimmunologic pressure. Profound suppression of patient immune cells by conditioning sufficed for sustained engraftment without necessity for myeloablative conditioning or development of clinically significant GVHD. Although 61% of the patients without any DLI or GVHD showed full donor chimerism (FDC) in the BM at 6 months after alloSCT, only 24% showed FDC in the CD4⁺ T cell compartment. In contrast, 75% of the patients who had received DLI and 83% of the patients with clinically significant GVHD had FDC in this compartment. In addition, 72% of the patients with mixed hematopoiesis receiving DLI converted to complete donor-derived hematopoiesis, of whom only 34% developed clinically significant GVHD. Our data show that competitive repopulation can be sufficient to reach complete donor-derived hematopoiesis, but that some alloimmunologic pressure is needed for the establishment of a completely donor-derived T cell compartment, either by the development of GVHD or by administration of DLI. We illustrate that it is possible to separate the graft-versus-leukemia effect from GVHD, as conversion to durable complete donor-derived hematopoiesis following DLI did not require induction of clinically significant GVHD.

Fractionated Infusion of Hematopoietic Progenitor Cells Does Not Improve Neutrophil Recovery or Survival in Allograft Recipients

Allogeneic hematopoietic cell transplantation (HCT) offers a potentially curative therapy in patients with hematologic malignancies; however, nonrelapse mortality (NRM) remains a concern. Strategies to improve neutrophil recovery and immune reconstitution are needed to decrease NRM. Murine models of allogeneic HCT suggest that fractionated hematopoietic progenitor cell (HPC) infusion may improve engraftment through improved access of HPCs to a viable hematopoietic niche. The primary objective of the present study was to determine the impact of fractionated infusion versus unfractionated (bulk) infusion of HPCs on the time to achieve neutrophil engraftment. Secondary objectives included the effect of fractionated versus bulk infusion of HPCs on platelet engraftment, immune reconstitution, the incidence of acute graft-versus-host disease (GVHD) grade II-IV, NRM, and overall survival (OS). In this randomized phase 2 study, patients with hematologic malignancies undergoing allogeneic HCT were randomized to receive HPC infusion as a bulk (bulk arm) or in fractions (fractionated arm): 4 × 106 CD34+ cells/kg recipient weight infused on day 0, with the remaining HPCs CD34+ cell-selected then infused in equally distributed aliquots on days 2, 4, and 6 post-HCT. Randomization was stratified by type of transplant, unmodified (i.e. T cell-replete graft) versus CD34+ cell-selected (T cell-depleted graft). Patients whose donor failed to collect at least 7 × 106 CD34+ cells/kg of recipient weight received bulk HPC infusions regardless of randomization, for safety. These patients continued the HCT process on study but were replaced until each arm reached the prespecified accrual target. Per protocol, these patients were not included in this modified intention-to-treat analysis. A total of 116 patients were enrolled. Donors of 42 patients failed to mobilize the minimum CD34+ cell dose (7 × 106 cells/kg recipient weight) and were excluded from the analysis. The 74 evaluable patients included 38 randomized to the bulk arm and 36 randomized to the fractionated arm. All patients engrafted. The median time to an absolute neutrophil count of ≥0.5 × 109/L was 11 days on both arms. The day +180 median CD4+ cell count was 179 cells/µL in the bulk arm and 111 cells/µL in the fractionated arm (P = .779). The cumulative incidence of grade II-IV acute GVHD on post-transplant day +100 was 32% in the bulk arm and 17% in the fractionated arm (P = .131). Two patients in the bulk arm, but none in the fractionated arm, experienced grade III-IV GVHD. The 4-year OS was 60% in the bulk arm and 62% in the fractionated arm (P = .414), whereas the 4-year cumulative incidences of NRM and relapse were similar in the 2 arms. Fractionated infusion of HPCs in allogeneic HCT recipients did not impact neutrophil or CD4+ cell recovery, NRM, relapse, or OS when compared with bulk HPC infusion. We also observed that with current mobilization techniques, it was unlikely that more than 60% of healthy donors would be able to collect >7 × 106 CD34+ cells/kg recipient weight for adult recipients. © 2021 American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc.

Pleiotropic Roles of CXCR4 in Wound Repair and Regeneration

Wound healing is a multi-step process that includes multiple cellular events such as cell proliferation, cell adhesion, and chemotactic response as well as cell apoptosis. Accumulating studies have documented the significance of stromal cell-derived factor-1 (SDF-1)/C-X-C chemokine receptor 4 (CXCR4) signaling in wound repair and regeneration. However, the molecular mechanism of regeneration is not clear. This review describes various types of tissue regeneration that CXCR4 participates in and how the efficiency of regeneration is increased by CXCR4 overexpression. It emphasizes the pleiotropic effects of CXCR4 in regeneration. By delving into the specific molecular mechanisms of CXCR4, we hope to provide a theoretical basis for tissue engineering and future regenerative medicine.

Hematopoietic stem and progenitor cells use podosomes to transcellularly cross the bone marrow endothelium

Bone marrow endothelium plays an important role in the homing of hematopoietic stem and progenitor cells upon transplantation, but surprisingly little is known on how the bone marrow endothelial cells regulate local permeability and hematopoietic stem and progenitor cells transmigration. We show that temporal loss of vascular endothelial-cadherin function promotes vascular permeability in BM, even upon low-dose irradiation. Loss of vascular endothelial-cadherin function also enhances homing of transplanted hematopoietic stem and progenitor cells to the bone marrow of irradiated mice although engraftment is not increased. Intriguingly, stabilizing junctional vascular endothelial-cadherin in vivo reduced bone marrow permeability, but did not prevent hematopoietic stem and progenitor cells migration into the bone marrow, suggesting that hematopoietic stem and progenitor cells use the transcellular migration route to enter the bone marrow. Indeed, using an in vitro migration assay, we show that human hematopoietic stem and progenitor cells predominantly cross bone marrow endothelium in a transcellular manner in homeostasis by inducing podosome-like structures. Taken together, vascular endothelial-cadherin is crucial for BM vascular homeostasis but dispensable for the homing of hematopoietic stem and progenitor cells. These findings are important in the development of potential therapeutic targets to improve hematopoietic stem and progenitor cell homing strategies.

All hematopoietic stem cells engraft in submyeloablatively irradiated mice

Significant controversy exists regarding the impact of hematopoietic stroma damage by irradiation on the efficiency of engraftment of intravenously transplanted stem cells. It was previously demonstrated that in normal syngenic mice, all intravenously transplanted donor stem cells, present in the bone marrow, compete equally with those of the host. In this study, we comprehensively compared the blood cell production derived from transplanted donor stem cells with that from the host stem cells surviving various doses of submyeloablative irradiation. We compared the partial chimerism resulting from transplantation with theoretical estimates that assumed transplantation efficiencies ranging from 100% to 20%. The highest level of consensus between the experimental and the theoretical results was 100% for homing and engraftment (ie, the utilization of all transplanted stem cells). These results point to a very potent mechanism through which intravenously administered hematopoietic stem cells are captured from circulation, engraft in the hematopoietic tissue, and contribute to blood cell production in irradiated recipients. The damage done to hematopoietic stroma and to the trabecular bone by submyeloablative doses of ionizing radiation does not negatively affect the homing and engraftment mechanisms of intravenously transplanted hematopoietic progenitor and stem cells.

Immune manipulation for Graves' disease: re-exploring an unfulfilled promise with modern translational research

Although Graves' disease is the commonest autoimmune thyroid disorder, current therapeutics typically center on the eradication of the antigenic stimulus (i.e. thyroid gland) rather than radically tackling the underlying autoimmune processes. Consequently, it is not a surprising fact that Graves' disease remains essentially a chronic drug-dependent ailment afflicting untold numbers worldwide for decades despite progress in deciphering its autoimmune nature. Addressing the latter is key to a future cure as underscored by appropriate, albeit crude, proof-of-concept scenarios of clinical remissions achieved with hematopoietic stem cell transplantation, immune down-regulation during pregnancy, use of corticosteroids or immunosuppressives, and cytokine biologics in animal models. Ongoing basic and translational research to further elucidate and refine our understanding of the pathogenesis of Graves' disease holds the promise of unraveling novel immune manipulative techniques that will bring the world a step closer to the elusive cure of the underlying autoimmunity amidst skepticisms on the value of the science from the present lack of paralleled advances at the bedside. We review the updated literature and describe the forms of immune manipulation hitherto explored that will offer a route to a future cure, from thionamides, hematopoietic stem cell transplantation to the latest immunomodulatory agents.

Transition pattern and mechanism of B-lymphocyte precursors in regenerated mouse bone marrow after subtotal body irradiation

Little is known about the effects of ionizing radiation on the transition and the related signal transduction of progenitor B cells in the bone marrow. Thus, using an NIH Swiss mouse model, we explored the impact of ionizing radiation on the early stage of B-cell development via an examination of the transition of CLP to pro-B to pre-B cells within bone marrow as a function of radiation doses and times. Our results showed that while the total number of bone marrow lymphoid cells at different stages were greatly reduced by subtotal body irradiation (sub-TBI), the surviving cells continued to transition from common lymphoid progenitors to pro-B and then to pre-B in a reproducible temporal pattern. The rearrangement of the immunoglobulin heavy chain increased significantly 1-2 weeks after irradiation, but no change occurred after 3-4 weeks. The rearrangement of the immunoglobulin light chain decreased significantly 1-2 weeks after sub-TBI but increased dramatically after 3-4 weeks. In addition, several key transcription factors and signaling pathways were involved in B-precursor transitions after sub-TBI. The data indicate that week 2 after irradiation is a critical time for the transition from pro-B cells to pre-B cells, reflecting that the functional processes for different B-cell stages are well preserved even after high-dose irradiation.

Hematopoietic Stem Cell Mobilization and Homing after Transplantation: The Role of MMP-2, MMP-9, and MT1-MMP

Hematopoietic stem/progenitor cells (HSPCs) are used in clinical transplantation to restore hematopoietic function. Here we review the role of the soluble matrix metalloproteinases MMP-2 and MMP-9, and membrane type (MT)1-MMP in modulating processes critical to successful transplantation of HSPC, such as mobilization and homing. Growth factors and cytokines which are employed as mobilizing agents upregulate MMP-2 and MMP-9. Recently we demonstrated that MT1-MMP enhances HSPC migration across reconstituted basement membrane, activates proMMP-2, and contributes to a highly proteolytic bone marrow microenvironment that facilitates egress of HSPC. On the other hand, we reported that molecules secreted during HSPC mobilization and collection, such as hyaluronic acid and thrombin, increase MT1-MMP expression in cord blood HSPC and enhance (prime) their homing-related responses. We suggest that modulation of MMP-2, MMP-9, and MT1-MMP expression has potential for development of new therapies for more efficient mobilization, homing, and engraftment of HSPC, which could lead to improved transplantation outcomes.

The ins and outs of hematopoietic stem cells: studies to improve transplantation outcomes

Deciphering the mechanisms of hematopoietic stem/progenitor cell (HSPC) mobilization and homing is important for the development of strategies to enhance the efficacy of HSPC transplantation and achieve the full potential of HSPC-based cellular therapy. Investigation of these mechanisms has revealed interdependence among the various molecules, pathways and cellular components involved, and underscored the complex nature of these two processes. This review summarizes recent progress in identifying the specific factors implicated in HSPC mobilization and homing, with emphasis on our own work. Particularly, we will discuss our studies on stromal cell-derived factor-1 and its interaction with its receptor CXCR4, proteases (matrix metalloproteinases and carboxypeptidase M), complement proteins (C1q, C3a, C5a, membrane attack complex), sphingosine-1-phosphate, and pharmacologic agents such as the histone deacetylase inhibitor valproic acid and hyaluronic acid.

After the bomb drops: a new look at radiation-induced multiple organ dysfunction syndrome (MODS)

PURPOSE

There is increasing concern that, since the Cold War era, there has been little progress regarding the availability of medical countermeasures in the event of either a radiological or nuclear incident. Fortunately, since much is known about the acute consequences that are likely to be experienced by an exposed population, the probability of survival from the immediate hematological crises after total body irradiation (TBI) has improved in recent years. Therefore focus has begun to shift towards later down-stream effects, seen in such organs as the gastrointestinal tract (GI), skin, and lung. However, the mechanisms underlying therapy-related normal tissue late effects, resulting from localised irradiation, have remained somewhat elusive and even less is known about the development of the delayed syndrome seen in the context of whole body exposures, when it is likely that systemic perturbations may alter tissue microenvironments and homeostasis.

CONCLUSIONS

The sequence of organ failures observed after near-lethal TBI doses are similar in many ways to that of multiple organ dysfunction syndrome (MODS), leading to multiple organ failure (MOF). In this review, we compare the mechanistic pathways that underlie both MODS and delayed normal tissue effects since these may impact on strategies to identify radiation countermeasures.

Targeting of mesenchymal stem cells to ovarian tumors via an artificial receptor

Background

Mesenchymal Progenitor/Stem Cells (MSC) respond to homing cues providing an important mechanism to deliver therapeutics to sites of injury and tumors. This property has been confirmed by many investigators, however, the efficiency of tumor homing needs to be improved for effective therapeutic delivery. We investigated the feasibility of enhancing MSC tumor targeting by expressing an artificial tumor-binding receptor on the MSC surface.

Methods

Human MSC expressing an artificial receptor that binds to erbB2, a tumor cell marker, were obtained by transduction with genetically modified adenoviral vectors encoding an artificial receptor (MSC-AR). MSC-AR properties were tested in vitro in cell binding assays and in vivo using two model systems: transient transgenic mice that express human erbB2 in the lungs and ovarian xenograft tumor model. The levels of luciferase-labeled MSCs in erbB2-expressing targeted sites were evaluated by measuring luciferase activity using luciferase assay and imaging.

Results

The expression of AR enhanced binding of MSC-AR to erbB2-expressing cells in vitro, compared to unmodified MSCs. Furthermore, we have tested the properties of erbB2-targeted MSCs in vivo and demonstrated an increased retention of MSC-AR in lungs expressing erbB2. We have also confirmed increased numbers of erbB2-targeted MSCs in ovarian tumors, compared to unmodified MSC. The kinetic of tumor targeting by ip injected MSC was also investigated.

Conclusion

These data demonstrate that targeting abilities of MSCs can be enhanced via introduction of artificial receptors. The application of this strategy for tumor cell-based delivery could increase a number of cell carriers in tumors and enhance efficacy of cell-based therapy.

Bone marrow-derived stem cells and radiation response

The recovery of tissues and organs from ionizing irradiation is critically dependent on the repopulation of resident stem cells, defined as the subset of cells with capacity for both self-renewal and differentiation. Stem cells of both hematopoietic and epithelial origin reside in defined areas of the cellular microenvironment (recently defined as the stem cell "niche"). Experiments using serial repopulation assays in serial generations of total body irradiated mice receiving transplanted marrow and in continuous bone marrow cultures both identified specific microanatomic sites that comprise the bone marrow stem cell niche. Supportive cells of the hematopoietic microenvironment not only contribute to stem cell repopulation capacity but also to the maintenance of their quiescent or nonproliferative state, which allows the most primitive hematopoietic stem cells to stay in a noncycling state protected from both direct ionizing radiation-induced cell-cycle phase-specific killing and indirect cytokine and free radical mediated killing. Recent evidence has defined both cell contact and humoral mechanisms of protection of hematopoietic stem cells by stromal cells. There is also recent evidence for multilineage differentiation capacity of cells of the hematopoietic microenvironment termed bone marrow stromal cells (mesenchymal stem cells). Both hematopoietic stem cells and mesenchymal stem cell populations have been shown to be involved in the repair of ionizing irradiation damage of distant epithelial as well as other hematopoietic sites through their capacity to migrate through the circulation. The radiobiology of these 2 bone marrow stem cell populations is the subject of intense investigation. This review defines the status of research in the areas of stem cell quiescence, niche contact, and migratory responses to ionizing irradiation.

A chromosome 16 quantitative trait locus regulates allogeneic bone marrow engraftment in nonmyeloablated mice

Identifying genes that regulate bone marrow (BM) engraftment may reveal molecular targets for overcoming engraftment barriers. To achieve this aim, we applied a forward genetic approach in a mouse model of nonmyeloablative BM transplantation. We evaluated engraftment of allogeneic and syngeneic BM in BALB.K and B10.BR recipients. This allowed us to partition engraftment resistance into its intermediate phenotypes, which are firstly the immune-mediated resistance and secondly the nonimmune rejection of donor BM cells. We observed that BALB.K and B10.BR mice differed with regard to each of these resistance mechanisms, thereby providing evidence that both are under genetic control. We then generated a segregating backcross (n = 200) between the BALB.K and B10.BR strains to analyze for genetic linkage to the allogeneic BM engraftment phenotype using a 127-marker genome scan. This analysis identified a novel quantitative trait locus (QTL) on chromosome 16, termed Bmgr5 (logarithm of odds 6.4, at 11.1 cM). The QTL encodes susceptibility alleles, from the BALB.K strain, that are permissive for allogeneic BM engraftment. Further identification of Bmgr5 genes by positional cloning may reveal new and effective approaches for overcoming BM engraftment obstacles.

TBI with lung dose reduction does not improve hematopoietic cell homing to BM during allogeneic transplantation

Purpose

To determine the effects of total body irradiation (TBI) dose, fractionation, and lung shielding on hematopoietic stem cell homing to the bone marrow.

Material and Methods

Bone marrow (BM) cells were extracted from tibiae and femurs of B6-GFP mice and were transplanted into B6 mice. Recipient mice had either: 1) no radiation, 2) single dose TBI at 13.6 Gray (Gy), 3) single dose TBI at 13.6 Gy with reduced lung exposure to 0.4 Gy by shielding, 4) split dose TBI at 12 Gy to twice/day over four days, or 5) split dose TBI at 12 Gy to twice/day over four days with reduced lung exposure to 0.36 Gy by shielding. The last radiation exposure preceded tail vein injection by 4–6 hours. Mice were sacrificed after 18 hours.

Results

Homing of GFP positive, lineage negative cells was not significantly improved in any irradiated group compared to control. Homing of GFP positive, lineage negative, Kit positive cells was significantly worse in all irradiated groups.

Conclusion

TBI does not improve the homing of lineage negative donor BM cells to the recipient marrow. Homing of lineage negative, Kit positive donor BM cells was significantly worse following TBI, with or without lung dose reduction.

CD34+ cell responsiveness to stromal cell-derived factor-1alpha underlies rate of engraftment after peripheral blood stem cell transplantation

BACKGROUND

Stromal cell-derived factor (SDF)-1, a chemokine produced in the bone marrow (BM), is essential for the homing of hematopoietic stem/progenitor cells (HSPCs) to the BM after transplantation. This study examines whether there is a correlation between the in vitro chemotaxis of CD34+ HSPC toward an SDF-1 gradient and in vivo hematopoietic engraftment.

STUDY DESIGN AND METHODS

Thirty-five patients underwent granulocyte-colony-stimulating factor HSPC collection and autologous transplant with a median dose of 7.7 (range, 3.9-41.5) x 10(6) CD34+ cells per kg body weight. The chemotactic index (CI) of CD34+ cells isolated from leukapheresis products collected from these patients was calculated as the ratio of the percentages of cells migrating toward an SDF-1 gradient to cells migrating to media alone. Expression of the SDF-1 receptor CXCR4 on CD34+ cells was measured by flow cytometry.

RESULTS

Spontaneous cell migration (range, 3.1 +/- 0.6 to 26.5 +/- 7.7%) and SDF-1-directed chemotaxis (11.1 +/- 0.7 to 54.9 +/- 8.3%) of CD34+ cells did not correlate with time to neutrophil engraftment, which occurred at a median of 10 days (range, 8-16 days). Nonparametric tests showed a negative correlation (r = -0.434) between CI and CD34+ cell dose such that neutrophil recovery occurred within the same period in patients transplanted with a lower dose of CD34+ cells but having a high CI as in those transplanted with a higher dose of CD34+ cells but having a low CI. Moreover, CI correlated (r = 0.8) with surface CXCR4 expression on CD34+ cells.

CONCLUSION

In patients transplanted with a relatively lower CD34+ cell dose who achieved fast engraftment, a higher responsiveness to SDF-1 and high CI could have compensated for the lower cell dose. However, to apply the CI as a prognostic factor of the rate of engraftment requires validation in a larger number of patients.

CXCR4 induction in hematopoietic progenitor cells from Fanca(-/-), -c(-/-), and -d2(-/-) mice

OBJECTIVE

Bone marrow failure is a near-universal occurrence in patients with Fanconi anemia (FA) and is thought to result from exhaustion of the hematopoietic stem cell (HSC) pool. Retrovirus-mediated expression of the deficient protein corrects this phenotype and makes FA a candidate disease for HSC-directed gene therapy. However, inherent repopulation deficits and stem cell attrition during conventional transduction culture prevent therapeutic chimerism.

MATERIALS AND METHODS

We previously reported rapid transduction protocols to limit stem cell losses after ex vivo culture. Here we describe a complementary strategy intended to improve repopulation through upregulation of chemokine receptor (CXCR) 4, a principal factor in hematopoietic homing.

RESULTS

Using murine models with transgenic disruption of Fanca, -c, and -d2, we found that c-kit(+) and sca-1(+) progenitor cells express levels of CXCR4 comparable with those of wild-type littermates. Lineage-depleted progenitor populations rapidly upregulated CXCR4 transcript and protein in response to cytokine stimulation or hypoxia, regardless of genotype. Hypoxia conditioning of lineage-depleted Fancc(-/-) progenitors also reduced oxidative stress, improved in vitro migration and led to improved chimerism in myeloablated recipients after transplantation.

CONCLUSION

These studies provide evidence that CXCR4 regulation in progenitor cells from transgenic mice representing multiple FA genotypes is intact and that modulation of homing offers a potential strategy to offset the FA HSC repopulation deficiency.

The Stromal Activity of Mesenchymal Stromal Cells

SUMMARY: The mechanism that regulates self-renewal and differentiation of hematopoietic stem cells (HSC) is a central question in stem cell biology that might ultimately lead to reliable protocols for in vitro expansion of HSC. Cellular fate is governed by cell-cell interaction with the microenvironment in the bone marrow, the stem cell niche. Mesenchymal stromal cells (MSC) are precursors of the cellular components, and they secrete extracellular matrix proteins of the bone marrow stroma. Therefore, MSC feeder layer might provide a suitable in vitro model system for the stem cell niche. In vitro assays demonstrate that MSC maintain the stem cell function of HSC and that MSC from bone marrow have a higher hematopoiesis supportive activity than MSC from adipose tissue. Co-cultivation with MSC might pave the way for expansion of long-term repopulating HSC, and various clinical trials indicate that co-transplantation of HSC and MSC might enhance engraftment. Thus, MSC are promising tools to elucidate the underlying mechanism of the cellular microenvironment. The large variety of preparative protocols for isolation and cultivation of MSC affects their stromal activity. Standardized isolation methods and molecular characterization of MSC are of utmost importance for reproducible isolation of hematopoiesis supportive stromal cells and for their potential clinical application.

The hematopoietic stem cell and its niche: a comparative view

Stem cells have been identified as a source of virtually all highly differentiated cells that are replenished during the lifetime of an animal. The critical balance between stem and differentiated cell populations is crucial for the long-term maintenance of functional tissue types. Stem cells maintain this balance by choosing one of several alternate fates: self-renewal, commitment to differentiate, and senescence or cell death. These characteristics comprise the core criteria by which these cells are usually defined. The self-renewal property is important, as it allows for extended production of the corresponding differentiated cells throughout the life span of the animal. A microenvironment that is supportive of stem cells is commonly referred to as a stem cell niche. In this review, we first present some general concepts regarding stem cells and their niches, comparing stem cells of many different kinds from diverse organisms, and in the second part, we compare specific aspects of hematopoiesis and the niches that support hematopoiesis in Drosophila, zebrafish and mouse.

Expression of Fas and Fas-ligand in donor hematopoietic stem and progenitor cells is dissociated from the sensitivity to apoptosis

OBJECTIVE

The interaction between the Fas receptor and its cognate ligand (FasL) has been implicated in the mutual suppression of donor and host hematopoietic cells after transplantation. Following the observation of deficient early engraftment of Fas and FasL-defective donor cells and recipients, we determined the role of the Fas-FasL interaction.

METHODS

Donor cells were recovered after syngeneic (CD45.1-->CD45.2) transplants from various organs and assessed for expression of Fas/FasL in reference to lineage markers, carboxyfluorescein succinimidyl ester dilution, Sca-1 and c-kit expression. Naïve and bone marrow-homed cells were challenged for apoptosis ex vivo.

RESULTS

The Fas receptor and ligand were markedly upregulated to 40% to 60% (p < 0.001 vs 5-10% in naïve cells) within 2 days after syngeneic transplantation, while residual host cells displayed modest and delayed upregulation of these molecules ( approximately 10%). All lin(-)Sca(+)c-kit(+) cells were Fas(+)FasL(+), including 95% of Sca-1(+) and 30% of c-kit(+) cells. Fas and FasL expression varied in donor cells that homed to bone marrow, spleen, liver and lung, and was induced by interaction with the stroma, irradiation, cell cycling, and differentiation. Bone marrow-homed donor cells challenged with supralethal doses of FasL were insensitive to apoptosis (3.2% +/- 1% vs 38% +/- 5% in naïve bone marrow cells), and engraftment was not affected by pretransplantation exposure of donor cells to an apoptotic challenge with FasL.

CONCLUSION

There was no evidence of Fas-mediated suppression of donor and host cell activity after transplantation. Resistance to Fas-mediated apoptosis evolves as a functional characteristic of hematopoietic reconstituting stem and progenitor cells, providing them competitive engraftment advantage over committed progenitors.

Junctional adhesion molecule-A, JAM-A, is a novel cell-surface marker for long-term repopulating hematopoietic stem cells

Junctional adhesion molecule-A (JAM-A/JAM-1/F11R) is a cell adhesion molecule expressed in epithelial and endothelial cells, and also hematopoietic cells, such as leukocytes, platelets, and erythrocytes. Here, we show that JAM-A is expressed at a high level in the enriched hematopoietic stem cell (HSC) fraction; that is, CD34(+)c-Kit(+) cells in embryonic day 11.5 (E11.5) aorta-gonod-mesonephros (AGM) and E11.5 fetal liver (FL), as well as c-Kit(+)Sca-1(+)Lineage(-) (KSL) cells in E14.5 FL, E18.5FL, and adult bone marrow (BM). Although the percentage of JAM-A(+) cells in those tissues decreases during development, the expression in the HSC fraction is maintained throughout life. Colony-forming assays reveal that multilineage colony-forming activity in JAM-A(+) cells is higher than that in JAM-A(-) cells in the enriched HSC fraction in all of those tissues. Transplantation assays show that long-term reconstituting HSC (LTR-HSC) activity is exclusively in the JAM-A(+) population and is highly enriched in the JAM-A(+) cells sorted directly from whole BM cells by anti-JAM-A antibody alone. Together, these results indicate that JAM-A is expressed on hematopoietic precursors in various hematopoietic tissues and is an excellent marker to isolate LTR-HSCs.

KIT receptor-positive cells in the bovine corpus luteum are primarily theca-derived small luteal cells

The tyrosine kinase KIT receptor, the protooncogene CD117, plays a key role in growth and maturation of oocytes and follicles. Relevant data are sparse for the corpus luteum (CL). We first confirmed the presence ofKITmRNA and KIT protein in bovine CL homogenates. We then localized KIT-positive (KIT+) cells in CL sections by immunohistochemistry. At the CL stage of early development, the former theca transforming into capsule/septa showed a strong band-like KIT+ immunoresponse. In addition, CD45+ leukocytes in septa included subpopulations of CD45+/KIT+ and CD14+/KIT+ leukocytes as validated by double immunofluorescence localization. At the early secretory stage, KIT+ cells appeared within the septa/capsule region and in the periphery of the CL parenchyma, there forming a complex network. This was separate from the capillary bed as determined by double staining for CD117 and FVIII-related endothelial cell antigen (FVIIIr). The KIT+ network coincided with cells positive for cytochrome P450 17α-hydroxylase, a thecal cell-specific enzyme. The late secretory stage was defined by an advanced manifestation of the KIT+ network in the CL periphery. At the stage of regression, the KIT+ network was absent. The CL of pregnancy expressed high levels ofKITmRNA and KIT protein uniformly throughout pregnancy. The KIT+ immunolocalization revealed small fibroblast-like cells, luteal cells with granules, and clusters of large luteal cells with staining of the cell membrane. We conclude that a majority of KIT+ cells in the bovine CL are primarily theca-derived small luteal cells, and that a minority represent KIT+ leukocytes, in some cases KIT+ monocytes.

Expression and function of cannabinoid receptors CB1 and CB2 and their cognate cannabinoid ligands in murine embryonic stem cells

BACKGROUND

Characterization of intrinsic and extrinsic factors regulating the self-renewal/division and differentiation of stem cells is crucial in determining embryonic stem (ES) cell fate. ES cells differentiate into multiple hematopoietic lineages during embryoid body (EB) formation in vitro, which provides an experimental platform to define the molecular mechanisms controlling germ layer fate determination and tissue formation.

METHODS AND FINDINGS

The cannabinoid receptor type 1 (CB1) and cannabinoid receptor type 2 (CB2) are members of the G-protein coupled receptor (GPCR) family, that are activated by endogenous ligands, the endocannabinoids. CB1 receptor expression is abundant in brain while CB2 receptors are mostly expressed in hematopoietic cells. However, the expression and the precise roles of CB1 and CB2 and their cognate ligands in ES cells are not known. We observed significant induction of CB1 and CB2 cannabinoid receptors during the hematopoietic differentiation of murine ES (mES)-derived embryoid bodies. Furthermore, mES cells as well as ES-derived embryoid bodies at days 7 and 14, expressed endocannabinoids, the ligands for both CB1 and CB2. The CB1 and CB2 antagonists (AM251 and AM630, respectively) induced mES cell death, strongly suggesting that endocannabinoids are involved in the survival of mES cells. Treatment of mES cells with the exogenous cannabinoid ligand Delta(9)-THC resulted in the increased hematopoietic differentiation of mES cells, while addition of AM251 or AM630 blocked embryoid body formation derived from the mES cells. In addition, cannabinoid agonists induced the chemotaxis of ES-derived embryoid bodies, which was specifically inhibited by the CB1 and CB2 antagonists.

CONCLUSIONS

This work has not been addressed previously and yields new information on the function of cannabinoid receptors, CB1 and CB2, as components of a novel pathway regulating murine ES cell differentiation. This study provides insights into cannabinoid system involvement in ES cell survival and hematopoietic differentiation.

Antitumor activity of human CD34+ cells expressing membrane-bound tumor necrosis factor-related apoptosis-inducing ligand

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) selectively induces apoptosis in a variety of transformed cells while sparing normal cells. To enhance the therapeutic index of soluble (s)TRAIL, we used CD34+ cells transduced with a replication-deficient adenovirus encoding the human TRAIL gene (CD34-TRAIL+) for the systemic delivery of membrane-bound (m)TRAIL to lymphoid tumors. CD34-TRAIL+ cells were evaluated for their activity in vitro and in vivo in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice xenografted with sTRAIL-sensitive and -resistant tumors. In vitro, coculturing CD34-TRAIL+ cells with sTRAIL-sensitive or -resistant lymphoma cell lines induced significant levels of caspase-dependent tumor cell death. In vivo, CD34-TRAIL+ cells significantly increased the survival of NOD/SCID mice bearing sTRAIL-sensitive or -resistant lymphoid tumors at an early or advanced stage of disease. No obvious toxicity was observed on administration of CD34-TRAIL+ cells. Histological analysis revealed high-level expression of the agonistic receptor TRAIL-R2 by tumor endothelial cells, and efficient tumor homing of transduced cells. Injection of CD34-TRAIL+ cells resulted in extensive damage of tumor vasculature followed by hemorrhagic necrosis exhibiting a perivascular distribution. These results show that CD34-TRAIL+ cells might be an efficient vehicle for mTRAIL delivery to tumors, where they exert a potent antitumor effect possibly mediated by both direct tumor cell killing and indirect vascular-disrupting mechanisms.

Stem cell fate analysis revisited: interpretation of individual clone dynamics in the light of a new paradigm of stem cell organization

Many experimental findings on heterogeneity, flexibility, and plasticity of tissue stem cells are currently challenging stem cell concepts that assume a cell intrinsically predefined, unidirectional differentiation program. In contrast to these classical concepts, nonhierarchical self-organizing systems provide an elegant and comprehensive alternative to explain the experimental data. Here we present the application of such a self-organizing concept to quantitatively describe the hematopoietic stem cell system. Focusing on the analysis of individual-stem-cell fates and clonal dynamics, we particularly discuss implications of the theoretical results on the interpretation of experimental findings. We demonstrate that it is possible to understand hematopoietic stem cell organization without assumptions on unidirectional developmental hierarchies, preprogrammed asymmetric division events or other assumptions implying the existence of a predetermined stem cell entity. The proposed perspective, therefore, changes the general paradigm of thinking about stem cells.

Fas ligand enhances hematopoietic cell engraftment through abrogation of alloimmune responses and nonimmunogenic interactions

Early after transplantation, donor lineage-negative bone marrow cells (lin(-) BMC) constitutively upregulated their expression of Fas ligand (FasL), suggesting an involvement of the Fas/FasL axis in engraftment. Following the observation of impaired engraftment in the presence of a dysfunctional Fas/FasL axis in FasL-defective (gld) donors or Fas-defective (lpr) recipients, we expressed a noncleavable FasL chimeric protein on the surface of donor lin(-) BMC. Despite a short life span of the protein in vivo, expression of FasL on the surface of all the donor lin(-) BMC improved the efficiency of engraftment twofold. The FasL-coated donor cells efficiently blunted the host alloimmune responses in primary recipients and retained their hematopoietic reconstituting potential in secondary transplants. Surprisingly, FasL protein improved the efficiency of engraftment in syngeneic transplants. The deficient engraftment in lpr recipients was not reversed in chimeric mice with Fas(-) stroma and Fas(+) BMC, demonstrating that the host marrow stroma was also a target of donor cell FasL. Hematopoietic stem and progenitor cells are insensitive to Fas-mediated apoptosis and thus can exploit the constitutive expression of FasL to exert potent veto activities in the early stages of engraftment. Manipulation of the donor cells using ectopic FasL protein accentuated the immunogenic and nonimmunogenic interactions between the donor cells and the host, alleviating the requirement for a megadose of transplanted cells to achieve a potent veto effect. Disclosure of potential conflicts of interest is found at the end of this article.

Annexin II expressed by osteoblasts and endothelial cells regulates stem cell adhesion, homing, and engraftment following transplantation

Differentiation of hematopoietic stem cells (HSCs) after birth is largely restricted to the bone marrow cavity, where HSCs are associated closely with osteoblasts (OBs). How OBs localize HSCs to the endosteal niche remains unclear. To explore adhesive interactions between HSCs and OBs, a cell blot analysis was used that revealed 2 major bands that corresponded to monomers and multimers of annexin II (Anxa2). Immunohistochemistry revealed that OBs and marrow endothelial cells express Anxa2 at high levels. Function-blocking studies confirmed that Anxa2 mediates HSC adhesion mainly via the N-terminal portion of the Anxa2 peptide. Adhesion of HSCs to OBs derived from Anxa2-deficient animals (Anxa2(-/-)) was significantly impaired compared with OBs obtained from wild-type animals (Anxa2(+/+)). Moreover, fewer HSCs were found in the marrow of Anxa2(-/-) versus Anxa2(+/+) animals. Short-term lodging, engraftment, and survival of irradiated mice with whole marrow cells were substantially inhibited by N-terminal peptide fragments of Anxa2 or anti-Anxa2 antibodies. Similar findings were noted in long-term competitive repopulation studies. Collectively, these findings reveal that Anxa2 regulates HSC homing and binding to the bone marrow microenvironment and suggest that Anxa2 is crucial for determining the bone marrow niche of HSCs.

Stem cell homing

PURPOSE OF REVIEW

Transplantation of hematopoietic stem cells is dependent upon the successful homing, engraftment and repopulation of stem cells in the bone marrow. Stem cell homing through the circulation to the bone marrow is the critical first step in this process. This review discusses the latest progress in defining the molecular processes underlying stem cell homing and the specialized niches where stem cells reside.

RECENT FINDINGS

Over the past decade, remarkable advances have been made in characterizing the complex sequence of events involved in stem cell homing to the bone marrow. Specifically, the molecular basis of stem cell adhesion and rolling along bone marrow sinusoidal endothelial cells has been defined, and mechanisms underlying endothelial transmigration and enlodgement in bone marrow niches have now been identified. The processes governing hematopoietic stem cell homing to the bone marrow also regulate hematopoietic stem cell migration to extramedullary tissues and the metastasis of cancer stem cells. Improved understanding of these processes has catalyzed the development of therapies to facilitate stem cell mobilization for clinical purposes.

SUMMARY

Several components of the essential process of stem cell homing have now been characterized. Cell adhesion molecules and their ligands, extracellular matrix components, chemokines, and specialized bone marrow niches all participate in the precise regulation of this process.

Adult neural stem cells, neurogenic niches, and cellular therapy

Niches are specialized microenvironments that regulate stem cells activity. In the nervous system, during development, niches control neural stem cells (NSCs) maturation and the formation of the neuronal network. In the adult, neurogenesis occurs in discrete areas of the brain, the subventricular zone and the hippocampus, where neurogenic niches have been identified and characterized. These niches, an angiogenic and an astroglial niche, control NSCs self-renewal and differentiation. Although the molecular and cellular mechanisms underlying the interactions between NSCs and their environment remain to be elucidated, neurogenic niches share similar developmentally conserved pathways with other niches. It is hypothesized that neurogenic niches underlie the properties and functions of NSCs in the adult central nervous system. Hence, neurogenic niches may not only hold the key to our understanding of neurogenesis in the adult brain, but also of the developmental potential of adult NSCs, and their potential for cellular therapy.

Continuous in vivo infusion of interferon-gamma (IFN-gamma) enhances engraftment of syngeneic wild-type cells in Fanca-/- and Fancg-/- mice

Fanconi anemia (FA) is a heterogeneous genetic disorder characterized by bone marrow (BM) failure and cancer susceptibility. Identification of the cDNAs of FA complementation types allows the potential of using gene transfer technology to introduce functional cDNAs as transgenes into autologous stem cells and provide a cure for the BM failure in FA patients. However, strategies to enhance the mobilization, transduction, and engraftment of exogenous stem cells are required to optimize efficacy prior to widespread clinical use. Hypersensitivity of Fancc-/- cells to interferon-gamma (IFN-gamma), a nongenotoxic immune-regulatory cytokine, enhances engraftment of syngeneic wild-type (WT) cells in Fancc-/- mice. However, whether this phenotype is of broad relevance in other FA complementation groups is unresolved. Here we show that primitive and mature myeloid progenitors in Fanca-/- and Fancg-/- mice are hypersensitive to IFN-gamma and that in vivo infusion of IFN-gamma at clinically relevant concentrations was sufficient to allow consistent long-term engraftment of isogenic WT repopulating stem cells. Given that FANCA, FANCC, and FANCG complementation groups account for more than 90% of all FA patients, these data provide evidence that IFN-gamma conditioning may be a useful nongenotoxic strategy for myelopreparation in FA patients.

The tale of early hematopoietic cell seeding in the bone marrow niche

Since introduction of the notion of a "niche" that hosts engraftment and activity of hematopoietic cells, there is a massive effort to discover its structure and decipher its function. Our understanding of the niche is continuously changing with reinterpretation of traditional concepts and apprehension of new insights into the biology of hematopoietic cell homing, seeding, and engraftment. Here we discuss some of the early events in hematopoietic stem cell seeding and engraftment and propose a perspective based on visualization of labeled bone marrow cells in real time in vivo. Primary seeding of hematopoietic cells in the bone marrow niches evolves as a complex and dynamic process; however, it follows discrete topological and chronological patterns. Initial seeding occurs on the endosteal surface of the marrow, which includes heterogeneous niches for primary seeding. Several days after transplantation the endosteal niches become more restrictive, hosting primarily mitotically quiescent cells, and gradual centripetal migration is accompanied by engagement in proliferation and differentiation. The hematopoietic niches evolve as heterogeneous three-dimensional microenvironments that are continuously changing over time.

Cardiomyogenic potential of mesenchymal progenitors derived from human circulating CD14+ monocytes

Previously, we reported a unique CD14+CD45+CD34+type I collagen+ cell fraction derived from human circulating CD14+ monocytes, named monocyte-derived mesenchymal progenitors (MOMPs). These primitive cells differentiate along mesenchymal lineages, including bone, cartilage, fat, and skeletal muscle. Here, we demonstrate that CD14+ monocytes generate MOMPs that differentiate into cardiomyocytes. MOMPs labeled with a fluorescent marker and co-cultivated with rat cardiomyocytes for 4 weeks expressed the cardiomyocyte-specific transcription factors Nkx2.5, GATA-4, eHAND, and MEF2 and the hematopoietic/monocytic markers CD45 and CD14 within 10 days and retained their proliferative capacity for up to 16 days. A subpopulation of MOMPs subsequently expressed the cardiomyocyte-specific markers micro-sarcomeric actinin, troponin I, and atrial natriuretic peptide on day 21. Furthermore, fluorescence-labeled, spontaneously beating cells that formed gap junctions with adjacent rat cardiomyocytes appeared in these cultures and these cells exhibited electrophysiological properties typical of ventricular myocytes. The co-cultivation of human MOMPs with rat GFP-tagged cardiomyocytes resulted in the generation of human cardiomyocytes lacking green fluorescent protein (GFP) staining, suggesting that our observations could not solely be explained by cell fusion. Our results demonstrate for the first time that human circulating CD14+ monocytes include progenitors capable of proliferating and differentiating along the cardiomyogenic lineage via their differentiation into MOMPs.

CXCR4 and CCR5 mediate homing of primitive bone marrow-derived hematopoietic cells to the postnatal thymus

Factors governing the entry of cells into the postnatal thymus are poorly understood. We aimed to define molecular mechanisms mediating the homing of bone marrow cells to the thymus using a sublethally irradiated in vivo murine model. Entry of unfractionated and lineage-depleted bone marrow cells to the thymus, but not bone marrow, was a Galphai-mediated phenomenon. Lineage-depleted cells that had homed to the thymus expressed abundant CXCR4 and CCR5 mRNA, alone of 17 chemokine receptors evaluated by QPCR. Thymic-homed cells were distinct from cells that had homed to bone marrow in expression of CXCR4 and CCR5 by mRNA quantification and cell-surface expression of protein. Abrogation of CXCR4 and CCR5 function by genetic, antibody, or pharmacologic means impaired homing of lineage-depleted cells to the thymus, although not in a synergistic manner, implying interdependency of these receptors in the homing process. Competitive repopulation experiments demonstrated that inhibiting CXCR4-mediated homing adversely affected the double-negative cell pool at 2 weeks, suggesting that cells with prothymocytic activity may in part home via CXCR4. Overall, our data demonstrate differential homing mechanisms governing entry of unfractionated and lineage-depleted cells to irradiated bone marrow or thymus, with thymic homing of immature cells being pertussis-sensitive and mediated by the chemokine receptors CXCR4 and CCR5.

Simultaneous bone marrow and intestine transplantation promotes marrow-derived hematopoietic stem cell engraftment and chimerism

Organ allografts have been shown to provide a syngeneic microenvironment for organ-based donor hematopoietic stem cells to maintain long-lasting chimerism after transplantation. We hypothesized that organ allografts would also support engraftment and hematopoiesis of adjunctively infused donor marrow stem cells, syngeneic to organ grafts, in nonmyeloablated recipients. In BN-to-LEW and GFP-to-ACI rat combinations, donor bone marrow (BM) infusion together with small intestine transplantation (SITx) under short-course tacrolimus immunosuppression resulted in persistent macrochimerism (more than 5%) for 150 days. In contrast, after BM infusion or SITx alone, chimerism was temporary and disappeared by day 100. Y-chromosome polymerase chain reaction (PCR) in sex-mismatched male BM plus female intestine or female BM plus male intestine transplantation into female recipients suggested that persistent macrochimerism was derived from infused BM. BM infusion together with lymphoid-depleted intestine grafts also supported macrochimerism development; however, third-party intestine grafts did not. After GFP-positive BM plus wild-type (WT) SITx into ACI, large numbers of GFP-positive leukocytes were found in WT intestine grafts. Isolated cells from WT intestine grafts developed GFP-positive CFU-Cs and propagated multilineage GFP-positive leukocytes when adoptively transferred into lethally irradiated WT recipients. These findings suggest that intestine allograft supports simultaneously infused donor (syngeneic to organ grafts) marrow stem cell engraftment, differentiation, and persistence of chimerism.

Multiple sources of non-embryonic multipotent stem cells: processed lipoaspirates and dermis as promising alternatives to bone-marrow-derived cell therapies

A body of evidence points to the existence of stem cell stores in adult tissues, in addition to the well-known hematopoietic stem cells from bone marrow. Many reports describe the ability of these multipotent cells (developmentally non-compromised with their organs of origin) to give rise to many different cell types in response to specific stimuli. This apparent plasticity provides new perspectives in tissue engineering and suggests the usefulness of these cells in future protocols of autologous transplantation, gene therapy, and tissue reconstitution in a number of pathological processes. Lipoaspirates and dermis represent accessible sources for obtaining such cells, with minimal discomfort to the donor, and might be promising candidates for cell therapy procedures once their features are experimentally accessed. The intention of the present work has been to gather reports on the phenotypic characteristics, profile, and plastic potential of these stem cells.

Antigen-presenting property of mesenchymal stem cells occurs during a narrow window at low levels of interferon-gamma

Mesenchymal stem cells (MSCs) are mostly found around the vasculature system of the adult bone marrow (BM). They function as immune suppressors, express MHC-II, are phagocytic, and support T-cell cytotoxicity. We hypothesize that these contradictory properties of MSCs are important for BM homeostasis and occur partly through antigen presentation (antigen-presenting cells [APCs]) within a narrow window. Indeed, we have verified APC functions of MSCs to recall antigens, Candida albicans and Tetanus toxoid. The target cells have been identified to be CD4(+) T cells. APC assays with IFNgamma-knockdown MSCs and with anti-IFNgamma receptor confirmed that MHC-II expression requires autocrine stimulation by IFNgamma. During APC functions, as IFNgamma levels become elevated, there was a concomitant decrease in MHC-II on MSCs. This observation was correlated with flow cytometry studies showing a gradual decrease in MHC-II expression as IFNgamma levels were increased. The reduced levels of MHC-II correlated with losses in their allogeneic potential, as indicated in mixed lymphocyte reaction. In summary, endogenous and low levels of IFNgamma are required for MHC-II expression on MSCs, and for APC functions. APC functions occur during a narrow window before IFNgamma levels are increased. The study has implications for BM protection against infection and exacerbated inflammatory responses.

Mice in the world of stem cell biology

The ability of embryos to diversify and of some adult tissues to regenerate throughout life is directly attributable to stem cells. These cells have the capacity to self-renew-that is, to divide and to create additional stem cells-and to differentiate along a specific lineage. The differentiation of pluripotent embryonic stem cells along specific cell lineages has been used to understand the molecular mechanisms involved in tissue development. The often endless capacity of embryonic stem cells to generate differentiated cell types positions the field of stem cells at the nexus between developmental biologists, who are fascinated by the properties of these cells, and clinicians, who are excited about the prospects of bringing stem cells from bench to bedside to treat degenerative disorders and injuries for which there are currently no cures. Here we highlight the importance of mice in stem cell biology and in bringing the world one step closer to seeing these cells brought to fruition in modern medicine.

An SDF-1 Trap for Myeloid Cells Stimulates Angiogenesis

In this issue of Cell, Grunewald et al. (2005) examine the role of hematopoietic cells in the formation of new blood vessels. They show that organ-specific expression of vascular endothelial growth factor (VEGF) is sufficient to mobilize and recruit hematopoietic cells from the bone marrow to the blood, but retention of the proangiogenic subpopulation of hematopoietic cells in peripheral organs requires an additional factor, stromal-derived factor 1 (SDF-1).

Hematopoietic Progenitor Cells and Cellular Microenvironment: Behavioral and Molecular Changes upon Interaction

Cell-cell contact between stem cells and cellular determinants of the microenvironment plays an essential role in controlling cell division. Using human hematopoietic progenitor cells (CD34+/CD38-) and a stroma cell line (AFT024) as a model, we have studied the initial behavioral and molecular sequel of this interaction. Time-lapse microscopy showed that CD34+/CD38- cells actively migrated toward and sought contact with stroma cells and 30% of them adhered firmly to AFT024 stroma through the uropod. CD44 and CD34 are colocalized at the site of contact. Gene expression profiles of CD34+/CD38- cells upon cultivation with or without stroma for 16, 20, 48, or 72 hours were analyzed using our human genome cDNA microarray. Chk1, egr1, and cxcl2 were among the first genes upregulated within 16 hours. Genes with the highest upregulation throughout the time course included tubulin genes, ezrin, c1qr1, fos, pcna, mcm6, ung, and dnmt1, genes that play an essential role in reorganization of the cytoskeleton system, stabilization of DNA, and methylation patterns. Our results demonstrate directed migration of CD34+/CD38- cells toward AFT024 and adhesion through the uropod and that upon interaction with supportive stroma, reorganization of the cytoskeleton system, regulation of cell division, and maintenance of genetic stability represent the most essential steps.

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.