A human bone marrow mesodermal-derived cell population with hemogenic potential

The presence, within the human bone marrow, of cells with both endothelial and hemogenic potential has been controversial. Herein, we identify, within the human fetal bone marrow, prior to establishment of hematopoiesis, a unique APLNR+, Stro-1+ cell population, co-expressing markers of early mesodermal precursors and/or hemogenic endothelium. In adult marrow, cells expressing similar markers are also found, but at very low frequency. These adult-derived cells can be extensively culture expanded in vitro without loss of potential, they preserve a biased hemogenic transcriptional profile, and, upon in vitro induction with OCT4, assume a hematopoietic phenotype. In vivo, these cells, upon transplantation into a fetal microenvironment, contribute to the vasculature, and generate hematopoietic cells that provide multilineage repopulation upon serial transplantation. The identification of this human somatic cell population provides novel insights into human ontogenetic hematovascular potential, which could lead to a better understanding of, and new target therapies for, malignant and nonmalignant hematologic disorders.

Correction: A human bone marrow mesodermal-derived cell population with hemogenic potential

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Boosting Hematopoietic Engraftment after in Utero Transplantation through Vascular Niche Manipulation

In utero hematopoietic stem/progenitor cell transplantation (IUHSCT) has only been fully successful in the treatment of congenital immunodeficiency diseases. Using sheep as a large animal model of IUHSCT, we demonstrate that administration of CD146⁺CXCL12⁺VEGFR2⁺ or CD146⁺CXCL12⁺VEGFR2⁻ cells prior to, or in combination with, hematopoietic stem/progenitor cells (HSC), results in robust CXCL12 production within the fetal marrow environment, and significantly increases the levels of hematopoietic engraftment. While in the fetal recipient, donor-derived HSC were found to reside within the trabecular bone, the increased expression of VEGFR2 in the microvasculature of CD146⁺CXCL12⁺VEGFR2⁺ transplanted animals enhanced levels of donor-derived hematopoietic cells in circulation. These studies provide important insights into IUHSCT biology, and demonstrate the feasibility of enhancing HSC engraftment to levels that would likely be therapeutic in many candidate diseases for IUHSCT.

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After IUHSCT, HSC engraft in the trabecular bone of the metaphysis

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CD146^++(+/−) cells engraft in diaphysis and make hematopoiesis-supporting cytokines

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Donor cell-derived CXCL12 and VEGFR2 significantly increase HSC engraftment

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IUHSCT of CD146⁺CXCL12⁺VEGFR2⁺ cells prior to HSC could be curative in several diseases

Influence of a dual-injection regimen, plerixafor and CXCR4 on in utero hematopoietic stem cell transplantation and engraftment with use of the sheep model

BACKGROUND AIMS

Inadequate engraftment of hematopoietic stem cells (HSCs) after in utero HSC transplantation (IUHSCT) remains a major obstacle for the prenatal correction of numerous hereditary disorders. HSCs express CXCR4 receptors that allow homing and engraftment in response to stromal-derived factor 1 (SDF-1) ligand present in the bone marrow stromal niche. Plerixafor, a mobilization drug, works through the interruption of the CXCR4-SDF-1 axis.

METHODS

We used the fetal sheep large-animal model to test our hypotheses that (i) by administering plerixafor in utero before performing IUHSCT to release fetal HSCs and thus vacating recipient HSC niches, (ii) by using human mesenchymal stromal/stem cells (MSCs) to immunomodulate and humanize the fetal BM niches and (iii) by increasing the CXCR4(+) fraction of CD34(+) HSCs, we could improve engraftment. Human cord blood-derived CD34(+) cells and human bone marrow-derived MSCs were used for these studies.

RESULTS

When MSCs were transplanted 1 week before CD34(+) cells with plerixafor treatment, we observed 2.80% donor hematopoietic engraftment. Combination of this regimen with additional CD34(+) cells at the time of MSC infusion increased engraftment levels to 8.77%. Next, increasing the fraction of CXCR4(+) cells in the CD34(+) population albeit transplanting at a late gestation age was not beneficial. Our results show engraftment of both lymphoid and myeloid lineages.

CONCLUSIONS

Prior MSC and HSC cotransplantation followed by manipulation of the CXCR4-SDF-1 axis in IUHSCT provides an innovative conceptual approach for conferring competitive advantage to donor HSCs. Our novel approach could provide a clinically relevant approach for enhancing engraftment early in the fetus.

Temporal definition of haematopoietic stem cell niches in a large animal model of in utero stem cell transplantation

The fetal sheep model has served as a biologically relevant and translational model to study in utero haematopoietic stem cell transplantation (IUHSCT), yet little is known about the ontogeny of the bone marrow (BM) niches in this model. Because the BMmicroenvironment plays a critical role in the outcome of haematopoietic engraftment, we have established the correlation between the fetal-sheep and fetal-human BM niche ontogeny, so that studies addressing the role of niche development at the time of IUHSCT could be accurately performed. Immunofluorescence confocal microscopic analysis of sheep fetal bone from gestational days (gd) 25-68 showed that the BM microenvironment commences development with formation of the vascular niche between 25 and 36 gd in sheep; correlating with the events at 10-11 gestational weeks (gw) in humans. Subsequently, between 45 and 51 gd in sheep (c. 14 gw in humans), the osteoblastic/endosteal niche started developing, the presence of CD34(+)  CD45(+) cells were promptly detected, and their number increased with gestational age. IUHSCT, performed in sheep at 45 and 65 gd, showed significant haematopoietic engraftment only at the later time point, indicating that a fully functional BM microenvironment improved engraftment. These studies show that sheep niche ontogeny closely parallels human, validating this model for investigating niche influence/manipulation in IUHSCT engraftment.

In utero transplantation: Disparate ramifications

In utero stem cell transplantation, which promises treatment for a host of genetic disorders early in gestation before disease effect stems from Ray Owen’s seminal observation that self-tolerance, is acquired during gestation. To date, in utero transplantation (IUT) has proved useful in characterizing the hematopoietic stem cell. Recent observations support its use as an in vivo method to further understanding of self-tolerance. Preclinical development continues for its application as a treatment for childhood hematolymphoid diseases. In addition, IUT may offer therapeutic options in the treatment of diabetes among other diseases. Thus IUT serves as a technique or system important in both a basic and applied format. This review summarizes these findings.

Mesenchymal stem cells contribute to endogenous FVIII:c production

Besides the liver, it has been difficult to identify which organ(s) and/or cellular component(s) contribute significantly to the production of human FVIII:c (FVIII). Thus far, only endothelial cells have been shown to constitute a robust extrahepatic source of FVIII, possibly explaining both the diverse presence of FVIII mRNA in the body, and the observed increase in FVIII levels during liver failure. Here, we investigate whether human mesenchymal stem cells (MSC), ubiquitously present in different organs, could also contribute to FVIII production. MSC isolated from human lung, liver, brain, and bone marrow expressed FVIII message as determined by quantitative-RT-PCR. Using an antibody specific for FVIII, confocal microscopy, and umbilical cord-derived endothelial cells (HUVEC) as a negative control, we demonstrated that, in MSC, FVIII protein was not stored in granules; rather, it localized to the perinuclear region. Furthermore, functional FVIII was detected in MSC supernatants and cell lysates by aPTT and chromogenic assays. These results demonstrate that MSC can contribute at low levels to the functional FVIII pool, and advance the understanding of the physiology of FVIII production and secretion.

EphB2 isolates a human marrow stromal cell subpopulation with enhanced ability to contribute to the resident intestinal cellular pool

To identify human bone marrow stromal cell (BMSC) subsets with enhanced ability to engraft/contribute to the resident intestinal cellular pool, we transplanted clonally derived BMSCs into fetal sheep. Analysis at 75 d post-transplantation showed 2 of the 6 clones engrafting the intestine at 4- to 5-fold higher levels (5.03±0.089 and 5.04±0.15%, respectively) than the other clones (P<0.01), correlating with the percentage of donor-derived Musashi-1(+) (12.01-14.17 vs. 1.2-3.8%; P<0.01) or leucine-rich repeat-containing G-protein coupled receptor 5 (Lgr5)(+) cells within the intestinal stem cell (ISC) region. Phenotypic and transcriptome analysis determined that the clones with enhanced intestinal contribution expressed high levels of Ephrin type B receptor 2 (EphB2). Intestinal explants demonstrated proliferation of the engrafted cells and ability to generate crypt-like structures in vitro still expressing EphB2. Additional transplants based on BMSC EphB2 expression demonstrated that, at 7 d post-transplant, the EphB2(high) BMSCs engrafted in the ISC region at levels of 2.1 ± 0.2%, while control EphB2(low) BMSCs engrafted at 0.3 ± 0.1% (P<0.01). Therefore we identified a marker for isolating and culturing an expandable subpopulation of BMSCs with enhanced intestinal homing and contribution to the ISC region.

Distinct contribution of human cord blood-derived endothelial colony forming cells to liver and gut in a fetal sheep model

Although the vasculogenic potential of circulating and cord blood (CB)-derived endothelial colony-forming cells (ECFC) has been demonstrated in vitro and in vivo, little is known about the inherent biologic ability of these cells to home to different organs and contribute to tissue-specific cell populations. Here we used a fetal sheep model of in utero transplantation to investigate and compare the intrinsic ability of human CB-derived ECFC to migrate to the liver and to the intestine, and to define ECFC's intrinsic ability to integrate and contribute to the cytoarchitecture of these same organs. ECFCs were transplanted by an intraperitoneal or intrahepatic route (IH) into fetal sheep at concentrations ranging from 1.1-2.6 × 10(6) cells/fetus. Recipients were evaluated at 85 days posttransplant for donor (human) cells using flow cytometry and confocal microscopy. We found that, regardless of the route of injection, and despite the IH delivery of ECFC, the overall liver engraftment was low, but a significant percentage of cells were located in the perivascular regions and retained the expression of hallmark endothelial makers. By contrast, ECFC migrated preferentially to the intestinal crypt region and contributed significantly to the myofibroblast population. Furthermore, ECFC expressing CD133 and CD117 lodged in areas where endogenous cells expressed those same phenotypes.

CONCLUSION

ECFC inherently constitute a potential source of cells for the treatment of intestinal diseases, but strategies to increase the numbers of ECFC persisting within the hepatic parenchyma are needed in order to enhance ECFC therapeutic potential for this organ.

Modulation of human mesenchymal stem cell immunogenicity through forced expression of human cytomegalovirus us proteins

Background

Mesenchymal stem cells (MSC) are promising candidates for cell therapy, as they migrate to areas of injury, differentiate into a broad range of specialized cells, and have immunomodulatory properties. However, MSC are not invisible to the recipient's immune system, and upon in vivo administration, allogeneic MSC are able to trigger immune responses, resulting in rejection of the transplanted cells, precluding their full therapeutic potential. Human cytomegalovirus (HCMV) has developed several strategies to evade cytotoxic T lymphocyte (CTL) and Natural Killer (NK) cell recognition. Our goal is to exploit HCMV immunological evasion strategies to reduce MSC immunogenicity.

Methodology/Principal Findings

We genetically engineered human MSC to express HCMV proteins known to downregulate HLA-I expression, and investigated whether modified MSC were protected from CTL and NK attack. Flow cytometric analysis showed that amongst the US proteins tested, US6 and US11 efficiently reduced MSC HLA-I expression, and mixed lymphocyte reaction demonstrated a corresponding decrease in human and sheep mononuclear cell proliferation. NK killing assays showed that the decrease in HLA-I expression did not result in increased NK cytotoxicity, and that at certain NK∶MSC ratios, US11 conferred protection from NK cytotoxic effects. Transplantation of MSC-US6 or MSC-US11 into pre-immune fetal sheep resulted in increased liver engraftment when compared to control MSC, as demonstrated by qPCR and immunofluorescence analyses.

Conclusions and Significance

These data demonstrate that engineering MSC to express US6 and US11 can be used as a means of decreasing recognition of MSC by the immune system, allowing higher levels of engraftment in an allogeneic transplantation setting. Since one of the major factors responsible for the failure of allogeneic-donor MSC to engraft is the mismatch of HLA-I molecules between the donor and the recipient, MSC-US6 and MSC-US11 could constitute an off-the-shelf product to overcome donor-recipient HLA-I mismatch.

Phenotypic correction of hemophilia A in sheep by postnatal intraperitoneal transplantation of FVIII-expressing MSC

We recently re-established a line of sheep that accurately mimics the clinical symptoms and genetics of severe hemophilia A (HA). Here, we tested a novel, nonablative transplantation therapy in two pediatric HA animals. Paternal mesenchymal stem cells (MSC) were transduced with a porcine FVIII-encoding lentivector and transplanted via the intraperitoneal route without preconditioning. At the time of transplantation, these animals had received multiple human FVIII treatments for various spontaneous bleeds and had developed debilitating hemarthroses, which produced severe defects in posture and gait. Transplantation of transduced MSC resolved all existent hemarthroses, and spontaneous bleeds ceased. Damaged joints recovered fully; the animals regained normal posture and gait and resumed normal activity. Despite achieving factor-independence, a sharp rise in pre-existent Bethesda titers occurred following transplantation, decreasing the effectiveness and duration of therapy. Postmortem examination revealed widespread engraftment, with MSC present within the lung, liver, intestine, and thymus, but particularly within joints affected at the time of transplantation, suggesting MSC homed to sites of ongoing injury/inflammation to release FVIII, explaining the dramatic improvement in hemarthrotic joints. In summary, this novel, nonablative MSC transplantation was straightforward, safe, and converted life-threatening, debilitating HA to a moderate phenotype in a large animal model.

Bone marrow stem cells and liver regeneration

Development of new approaches to treat patients with hepatic diseases that can eliminate the need for liver transplantation is imperative. Use of cell therapy as a means of repopulating the liver has several advantages over whole-organ transplantation because it would be less invasive, less immunogenic, and would allow the use, in some instances, of autologous-derived cells. Stem/progenitor cells that would be ideal for liver repopulation would need to have characteristics such as availability and ease of isolation, the ability to be expanded in vitro, ensuring adequate numbers of cells, susceptibility to modification by viral vector transduction/genetic recombination, to correct any underlying genetic defects, and the ability of restoring liver function following transplantation. Bone marrow-derived stem cells, such as hematopoietic, mesenchymal and endothelial progenitor cells possess some or most of these characteristics, making them ideal candidates for liver regenerative therapies. Here, we will summarize the ability of each of these stem cell populations to give rise to functional hepatic elements that could mediate repair in patients with liver damage/disease.

Clinical and molecular characterization of a re-established line of sheep exhibiting hemophilia A

BACKGROUND

Large animal models that accurately mimic human hemophilia A (HA) are in great demand for developing and testing novel therapies to treat HA.

OBJECTIVES

To re-establish a line of sheep exhibiting a spontaneous bleeding disorder closely mimicking severe human HA, fully characterize their clinical presentation, and define the molecular basis for disease.

PATIENTS/METHODS

Sequential reproductive manipulations were performed with cryopreserved semen from a deceased affected ram. The resultant animals were examined for hematologic parameters, clinical symptoms, and responsiveness to human FVIII (hFVIII). The full coding region of sheep FVIII mRNA was sequenced to identify the genetic lesion.

RESULTS AND CONCLUSIONS

The combined reproductive technologies yielded 36 carriers and 8 affected animals. The latter had almost non-existent levels of FVIII:C and extremely prolonged aPTT, with otherwise normal hematologic parameters. These animals exhibited bleeding from the umbilical cord, prolonged tail and nail cuticle bleeding time, and multiple episodes of severe spontaneous bleeding, including hemarthroses, muscle hematomas and hematuria, all of which responded to hFVIII. Inhibitors of hFVIII were detected in four treated animals, further establishing the preclinical value of this model. Sequencing identified a premature stop codon and frame-shift in exon 14, providing a molecular explanation for HA. Given the decades of experience using sheep to study both normal physiology and a wide array of diseases and the high homology between human and sheep FVIII, this new model will enable a better understanding of HA and facilitate the development and testing of novel treatments that can directly translate to HA patients.

Differences amid bone marrow and cord blood hematopoietic stem/progenitor cell division kinetics

Human hematopoietic stem/progenitor cells (HSC) isolated based upon specific patterns of CD34 and CD38 expression, despite phenotypically identical, were found to be functionally heterogeneous, raising the possibility that reversible expression of these antigens may occur during cellular activation and/or proliferation. In these studies, we combined PKH67 tracking with CD34/CD38 immunostaining to compare cell division kinetics between human bone marrow (BM) and cord blood (CB)-derived HSC expanded in a serum-free/stromal-based system for 14 days (d), and correlated CD34 and CD38 expression with the cell divisional history. CB cells began dividing 24 h earlier than BM cells, and significantly higher numbers underwent mitosis during the time in culture. By d10, over 55% of the CB-cells reached the ninth generation, whereas BM-cells were mostly distributed between the fifth and seventh generation. By d14, all CB cells had undergone multiple cell divisions, while 0.7-3.8% of BM CD34(+) cells remained quiescent. Furthermore, the percentage of BM cells expressing CD34 decreased from 60.8 +/- 6.3% to 30.6 +/- 6.7% prior to initiating division, suggesting that downmodulation of this antigen occurred before commencement of proliferation. Moreover, with BM, all primitive CD34(+)CD38(-) cells present at the end of culture arose from proliferating CD34(+)CD38(+) cells that downregulated CD38 expression, while in CB, a CD34(+)CD38(-) population was maintained throughout culture. These studies show that BM and CB cells differ significantly in cell division kinetics and expression of CD34 and CD38, and that the inherent modulation of these antigens during ex vivo expansion may lead to erroneous quantification of the stem cell content of the expanded graft.

Immune ontogeny and engraftment receptivity in the sheep fetus

OBJECTIVE

The biologic explanation for fetal receptivity to donor engraftment and subsequent long-term tolerance following transplantation early in gestation is not known. We investigated the role fetal immune ontogeny might play in fetal transplantation tolerance in sheep.

METHODS

Engraftment of allogeneic and xenogeneic HSC was determined 60 days following transplantation at different time points in sheep fetal gestation. Parallel analysis of surface differentiation antigen expression on cells from lymphoid organs of timed gestational age fetal sheep was determined by flow cytometry using available reagents.

RESULTS

An engraftment window was identified after day 52 gestation lasting until day 71 (term gestation: 145 days). This period was associated with the expression of the leukocyte common antigen CD45 on all cells in the thymus. Double-positive and single-positive CD4 and CD8 cells began appearing in the thymus just prior (day 45 gestation) to the beginning of the engraftment window, while single-positive CD4 or CD8 cells do not begin appearing in peripheral organs until late in the engraftment period, suggesting deletional mechanisms may be operative. In concert, surface IgM-positive cells express CD45 in the thymus at day 45, with a comparable delay in the appearance of IgM/CD45 cells in the periphery until late in the engraftment window.

CONCLUSIONS

These findings support a central role for the thymus in multilineage immune cell maturation during the period of fetal transplantation receptivity. Further, they suggest that fetal engraftment receptivity is due to gestational age-dependent deletional tolerance.

Docosahexaenoic acid induces dose dependent cell death in an early undifferentiated subtype of acute myeloid leukemia cell line

Acute myeloid leukemia (AML) is the most frequently diagnosed adulthood leukemia, yet current therapies offer a cure rate of less than 30%. This may be due in part to the fact that the leukemia-initiating cells in AML reside within the rare and highly primitive CD34(+)CD38(-) hematopoietic stem/progenitor cell (HSC) population that are often resistant to chemotherapy. Docosahexanoic acid (DHA), a major component of fish oil, has previously been shown to inhibit the induction and progression of breast, prostate and colon cancer, and increase the therapeutic effects of numerous chemotherapeutics, often by enhancing apoptosis. In the present studies, we investigated DHA's effect on the primitive and undifferentiated AML cell line KG1a, to explore the potential of this fatty acid to serve as adjuvant therapy for AML. Treatment of KG1a cells with DHA for 96 hours did not lead to maturation or cell cycle modification when compared to an untreated KG1a control (n = 4). However, DHA treatment of KG1a cells resulted in a progressive loss of viability, DNA fragmentation, and an increase in Annexin V expression, demonstrating DHA-induced apoptosis (n = 4). Moreover, expression of the pro-apoptotic protein Bax was increased, with resultant skewing in the Bax/bcl-2 ratio, providing a mechanistic explanation for the observed DHA-induced increase in apoptosis. Since we also show that DHA does not have a detrimental effect on normal hematopoiesis our results suggest that DHA could potentially serve as an well-tolerated adjuvant in the treatment of AML patients.

Development and characterization of a novel CD34 monoclonal antibody that identifies sheep hematopoietic stem/progenitor cells

OBJECTIVE

We and many others have long used sheep as a predictive model system in which to explore stem cell transplantation. Unfortunately, while numerous markers are available to identify and isolate human hematopoietic stem cells (HSC), no reagents exist that allow HSC/progenitors from sheep to be identified or purified, greatly impeding the application of this well-established large animal model to the study of autologous or allogeneic HSC transplantation. The current studies were undertaken to create a monoclonal antibody to sheep CD34 that would enable isolation and study of sheep HSC/progenitors.

MATERIALS AND METHODS

A partial cDNA to the extracellular domain of the sheep CD34 antigen was polymerase chain reaction cloned, characterized, and used to genetically immunize mice and create hybridomas.

RESULTS

The resultant monoclonal antibody to sheep CD34 allows flow cytometric detection of sheep HSC/progenitors present within bone marrow, cord blood, and mobilized peripheral blood. Moreover, this antibody can be used to enrich for HSC/progenitors with enhanced in vitro colony-forming potential, and also identifies endothelial cells in situ within paraffin-embedded tissue sections, similarly to antibodies to human CD34.

CONCLUSIONS

The availability of this monoclonal antibody recognizing the stem cell antigen CD34 in sheep will greatly facilitate the study of autologous and allogeneic HSC transplantation using this clinically relevant large animal model.

Generation of tissue-specific cells from MSC does not require fusion or donor-to-host mitochondrial/membrane transfer

Human mesenchymal stem cells (MSC) hold great promise for cellular replacement therapies. Despite their contributing to phenotypically distinct cells in multiple tissues, controversy remains regarding whether the phenotype switch results from a true differentiation process. Here, we studied the events occurring during the first 120 h after human MSC transplantation into a large animal model. We demonstrate that MSC, shortly after engrafting different tissues, undergo proliferation and rapidly initiate the differentiative process, changing their phenotype into tissue-specific cells. Thus, the final level of tissue-specific cell contribution is not determined solely by the initial level of engraftment of the MSC within that organ, but rather by the proliferative capability of the ensuing tissue-specific cells into which the MSC rapidly differentiate. Furthermore, we show that true differentiation, and not cell fusion or transfer of mitochondria or membrane-derived vesicles between transplanted and resident cells, is the primary mechanism contributing to the change of phenotype of MSC upon transplantation.

Adult mesenchymal stem cells: a pluripotent population with multiple applications

Mesenchymal stem cells (MSCs) have been isolated not only from bone marrow, but also from many other tissues such as adipose tissue, skeletal muscle, liver, brain and pancreas. Because MSC were found to have the ability to differentiate into cells of multiple organs and systems such as bone, fat, cartilage, muscle, neurons, hepatocytes and insulin-producing cells, MSCs have generated a great deal of interest for their potential use in regenerative medicine and tissue engineering. Furthermore, given the ease of their isolation and their extensive expansion rate and differentiation potential, mesenchymal stem cells are among the first stem cell types that have a great potential to be introduced in the clinic. Finally, mesenchymal stem cells seem to be not only hypoimmunogenic and thus be suitable for allogeneic transplantation, but they are also able to produce immunosuppression upon transplantation. In this review we summarize the latest research in the use of mesenchymal stem cells in transplantation for generalized diseases, local implantation for local tissue defects, and as a vehicle for genes in gene therapy protocols.

Efficient generation of human hepatocytes by the intrahepatic delivery of clonal human mesenchymal stem cells in fetal sheep

Alternative methods to whole liver transplantation require a suitable cell that can be expanded to obtain sufficient numbers required for successful transplantation while maintaining the ability to differentiate into hepatocytes. Mesenchymal stem cells (MSCs) possess several advantageous characteristics for cell-based therapy and have been shown to be able to differentiate into hepatocytes. Thus, we investigated whether the intrahepatic delivery of human MSCs is a safe and effective method for generating human hepatocytes and whether the route of administration influences the levels of donor-derived hepatocytes and their pattern of distribution throughout the parenchyma of the recipient's liver. Human clonally derived MSCs were transplanted by an intraperitoneal (n = 6) or intrahepatic (n = 6) route into preimmune fetal sheep. The animals were analyzed 56-70 days after transplantation by immunohistochemistry, enzyme-linked immunosorbent assay, and flow cytometry. The intrahepatic injection of human MSCs was safe and resulted in more efficient generation of hepatocytes (12.5% +/- 3.5% versus 2.6% +/- 0.4%). The animals that received an intrahepatic injection exhibited a widespread distribution of hepatocytes throughout the liver parenchyma, whereas an intraperitoneal injection resulted in a preferential periportal distribution of human hepatocytes that produced higher amounts of albumin. Furthermore, hepatocytes were generated from MSCs without the need to first migrate/lodge to the bone marrow and give rise to hematopoietic cells.

CONCLUSION

Our studies provide evidence that MSCs are a valuable source of cells for liver repair and regeneration and that, by the alteration of the site of injection, the generation of hepatocytes occurs in different hepatic zones, suggesting that a combined transplantation approach may be necessary to successfully repopulate the liver with these cells.

The human-sheep chimeras as a model for human stem cell mobilization and evaluation of hematopoietic grafts' potential

OBJECTIVE

To investigate whether the sheep xenograft model of human hematopoiesis can be used to mimic mobilization of human hematopoietic stem cells in vivo.

MATERIAL AND METHODS

Sheep transplanted with 3.6 x 10(6) CD34+ from human adult bone marrow were mobilized 1.5 years posttransplantation with human granulocyte colony-stimulating factor for 5 days. At day 3 and 4 of mobilization, human cells were harvested from peripheral blood (PB) and bone marrow (BM) and were injected into secondary sheep recipients (n = 6) and these animals were analyzed for the presence of human cells in their BM and PB, starting at 3.5 months posttransplantation.

RESULTS

Maximum mobilization of human cells in PB occurred at day 3, with a 21-fold increase in total numbers of human cells, and a recovery of 5.5 x 10(4)/mL CD34+. In the BM, maximal numbers of human cells were achieved at day 4, with a 6.3-fold increase and a recovery of 1.5 x 10(4)/mL CD34+ cells. PB and BM mobilized human cells were then transplanted into new sheep recipients, and analysis at 3.5 months posttransplantation demonstrated that levels of human cell engraftment in BM of the group transplanted with mobilized PB were significantly lower than those transplanted with BM cells (0.6% +/- 0.1% vs 8.0% +/- 1.8%). Furthermore, in sheep transplanted with mobilized PB, the levels of human cells in circulation remained 2.5-fold higher than the levels of human cells found in their BM.

CONCLUSION

Mobilization of human cells in the sheep model parallels human PB and BM hematopoietic stem cells (HSC) mobilization in healthy human donors in their ability to engraft, differentiate, and repopulate secondary hosts. Thus, this model can become a useful tool to study mobilization regimens, mechanisms, and quality of products obtained.

A Stro-1(+) human universal stromal feeder layer to expand/maintain human bone marrow hematopoietic stem/progenitor cells in a serum-free culture system

OBJECTIVE

To compare the ability of allogeneic versus autologous purified human Stro-1(+) mesenchymal stem cell (MSC) populations from different human donors to support the ex vivo expansion and maintenance of human hematopoietic stem/progenitor cells (HSCs). Furthermore, we compared the results obtained with MSC as a feeder layer to traditional allogeneic stromal layers grown in long-term bone marrow culture media (LT-ST).

METHODS

Adult human bone marrow CD34(+)-enriched cells were cultured in serum-free medium for 2 to 3 weeks over the respective MSC-irradiated feeder layers or over traditional allogeneic LT- ST stromal layers in the presence of stem cell factor, basic fibroblast growth factor, leukemia inhibitory factor, and Flt-3 and analyzed every 2 to 4 days for expansion, phenotype, and clonogenic ability.

RESULTS

There was a progressive expansion of total numbers of cells in all the experimental groups; however, allogeneic MSCs were more efficient at expanding CD34(+)CD38(-) cells and showed a higher clonogenic potential than both allogeneic LT-ST and autologous MSCs. The differentiative potential of cells cultured on both MSC and LT-ST was primarily shifted toward myeloid lineage; however, only MSCs were able to maintain/expand a CD7(+) population with lymphocytic potential. Importantly, transplantation into preimmune fetal sheep demonstrated that the HSCs cultured over MSCs retained their engraftment capability.

CONCLUSION

These results indicate that purified Stro-1(+) MSCs may be used as a universal and reproducible stromal feeder layer to efficiently expand and maintain human bone marrow HSCs ex vivo.

The time course of engraftment of human mesenchymal stem cells in fetal heart demonstrates that Purkinje fiber aggregates derive from a single cell and not multi-cell homing

OBJECTIVE

To study the early time course of engraftment of human mesenchymal stem cells in fetal sheep heart and determine the relative roles of proliferation and homing in formation of aggregates of human Purkinje fiber cells.

METHODS

The human sheep xenograft model was utilized for these studies. Prior to injection in the preimmune fetus, human cells were labeled with fluorescent dyes to be able to track human cells at early times of engraftment.

RESULTS

Human stem cells were detected in fetal hearts between 29 and 39 hours after intraperitoneal injection. Engraftment was primarily in the Purkinje fiber system. By 45 hours engrafted human cells had a cardiac phenotype. When two groups of human mesenchymal stem cells, each labeled with a different fluorescent dye, were combined prior to injection, aggregates of human Purkinje fiber cells contained cells labeled with either one dye or the other, no aggregate contained cells labeled with both dyes.

CONCLUSIONS

Human mesenchymal stem cells introduced into fetal sheep rapidly enter the myocardium. The swift differentiation into a cardiac phenotype indicates that the cardiac milieu has a strong influence on the fate of engrafting human mesenchymal stem cells. The absence of any aggregates of human Purkinje fiber cells containing both fluorescent dyes demonstrates that each aggregate of human Purkinje fiber cells is derived from a single mesenchymal stem cell and not from homing of multiple cells to a hotspot.

Male germ-line cells are at risk following direct-injection retroviral-mediated gene transfer in utero

The recent observation of vector sequences in the semen of men undergoing clinical gene therapy for hemophilia has highlighted the need to evaluate the risk of inadvertent germ-line transduction in a clinically relevant animal model. In the present study, we used three different approaches to investigate whether the germ line is at risk of inadvertent alteration following in utero retroviral gene transfer in the clinically relevant, random-bred sheep model. First, we conducted breeding studies. All organs from the 10 resultant offspring were devoid of proviral DNA, suggesting that the germ line had not been altered. As a second approach, we performed PCR on gradient-enriched, forensically purified sperm cells from in utero-transduced rams. The purified sperm cells from 6 of 19 of these rams were PCR positive for provirus, providing compelling evidence that the germ line had been transduced. As a third approach, we performed immunohistochemistry on sections of the testis from in utero-transduced sheep. Numerous somatic cells and very low levels of germ cells within the male reproductive tissues were transduced. In conclusion, our analysis on over 3 x 10(9) sperm cells suggests that the direct-injection approach employed in these studies may result in the inadvertent transduction of very low numbers of male germ cells.

Human embryonic stem cell-derived hematopoietic cells are capable of engrafting primary as well as secondary fetal sheep recipients

The human/sheep xenograft model has proven valuable in assessing the in vivo hematopoietic activity of stem cells from a variety of fetal and postnatal human sources. CD34+/lineage- or CD34+/CD38- cells isolated from human embryonic stem cells (hESCs) differentiated on S17 feeder layer were transplanted by intraperitoneal injections into fetal sheep. Chimerism in primary transplants was established with polymerase chain reaction (PCR) and flow cytometry of bone marrow and peripheral blood samples. Whole bone marrow cells harvested from a primary recipient were transplanted into a secondary recipient. Chimerism was established as described before. This animal was stimulated with human GM-CSF, and an increase in human hematopoietic activity was noted by flow cytometry. Bone marrow aspirations cultured in methylcellulose generated colonies identified by PCR to be of human origin. We therefore conclude that hESCs are capable of generating hematopoietic cells that engraft primary recipients. These cells also fulfill the criteria for long-term engrafting hematopoietic stem cells as demonstrated by engraftment and differentiation in the secondary recipient.

In vivo haematopoietic potential of human neural stem cells

The fetal sheep model was used to compare the in vivo haematopoietic potential of human neural stem cells (NSC) versus bone marrow (BM)-derived haematopoietic stem cells (HSC). To this end, sheep were transplanted with either 8 x 10(5) NSC (n = 11) or HSC, CD34(+)Lin(-) (n = 5), and subsequently analysed for haematopoietic chimaerism. While HSC-transplanted sheep displayed robust donor-derived haematopoiesis starting at less than 2 months post-transplant, NSC recipients exhibited haematopoietic engraftment at much later time points. Nevertheless, chimaerism persisted in both groups throughout the course of this study. Transplantation of secondary recipients with human CD45(+)/HLA-DR(+) cells from the BM of NSC primary recipients at 14 and 16 months post-transplant demonstrated that long-term engrafting HSC were present in these animals. At 6 months post-transplant, both NSC- and HSC-transplanted sheep were mobilised with granulocyte colony-stimulating factor. In contrast to HSC-transplanted animals, levels of human blood cells in peripheral blood of NSC-transplanted sheep remained low throughout mobilisation. Our results show that, although human NSC were able to give rise to multilineage haematopoiesis in our model, the levels, timing of blood cell production and the ability to respond to cytokine mobilisation were different, suggesting that human NSCs latent haematopoietic potential is inherently different from that of true HSC.

A human stromal-based serum-free culture system supports the ex vivo expansion/maintenance of bone marrow and cord blood hematopoietic stem/progenitor cells

OBJECTIVE

We investigated the role of human stromal layers (hu-ST) on the ex vivo expansion/maintenance of human hematopoietic stem/progenitor cells (HSC) from adult bone marrow (BM) and umbilical cord blood (CB).

MATERIALS AND METHODS

BM and CB CD34(+)-enriched cells were cultured in serum-free medium supplemented with SCF, bFGF, LIF, and Flt-3, in the presence or absence of stroma, and analyzed for proliferation, phenotype, and clonogenic potential.

RESULTS

Significant expansion of BM and CB CD34(+) and CD34(+)CD38(-) cells were achieved in the presence of hu-ST. The differentiative potential of both BM and CB CD34(+)-enriched cells cocultured with hu-ST was primarily shifted toward the myeloid lineage, while maintaining/expanding a CD7(+) population. Clonogenic analysis of the expanded cells showed increases in progenitors of the myeloid lineage, including colony-forming unit-granulocyte, macrophage (CFU-GM) and colony-forming unit-granulocyte, erythroid, macrophage, megakaryocyte (CFU-Mix) for both BM (stroma and stroma-free conditions) and CB cells in the presence of stroma.

CONCLUSIONS

These results indicate that adult hu-ST in the presence of appropriate cytokines can be used to efficiently expand/maintain myeloid and lymphoid cell populations from human BM and CB HSC.

The ethics of gene therapy and abortion: public opinion

On the grounds that the public should be consulted in decisions concerning the legitimate scope of germ-line genetic therapy (GLGT), survey data on the ethics of GLGT were collected from a large (n = 1,403) representative national sample of Australians in 2002. The data show that opinion is quite divided over GLGT in the case of a 'death sentence' genetic defect: 36% would forbid it, 23% have mixed feelings and 41% would allow it. For less serious conditions there is more opposition to GLGT. Thus, 48% would forbid GLGT to remedy a minor physical defect and 52% would oppose GLGT to counteract a propensity to violence, but fully 73% would disallow GLGT for cosmetic reasons. The data also show that opposition to abortion is lower than opposition to GLGT in the case of a 'death sentence' genetic defect, but at about the same level as, or greater than, opposition to GLGT for less serious issues. The questions show good measurement properties, including low missing data rates, so they are likely to provide an accurate picture of the public's views on the ethics of GLGT. It is suggested that a system for monitoring public opinion on these issues be developed.

Gestational age of recipient determines pattern and level of transgene expression following in utero retroviral gene transfer

The direct vector injection approach was used in the fetal sheep model of in utero gene therapy to determine the effects of the recipient gestational age on the efficacy and pattern of liver, lung, and brain transduction and transgene expression. The livers contained foci of transgene-expressing hepatocytes and demonstrated an inverse correlation between recipient age and hepatocyte transduction/transgene expression, with higher levels of gene transfer/expression early in gestation and lower levels late in gestation. Conversely, the percentage of transgene-expressing cells within the lungs of these same animals increased with gestational age, with the majority of transduction occurring in epithelium and fibroblasts. In contrast to the lung and liver, transgene-expressing cells within the brain were extremely limited at all gestational ages tested. Our results demonstrate that numerous nonhematopoietic cells within the liver and lung are transduced following direct injection of murine retroviral vectors into fetal sheep and suggest that the developmental stage of each organ at the time of injection may determine its susceptibility to in utero gene transfer and subsequent levels of transgene expression. Our results suggest that with further vector optimization this approach may be useful for treating diseases that involve the lung and liver early in development.

The effect of hypoxia and stem cell source on haemoglobin switching

This study investigated whether relative changes that accompany the naturally occurring shifts in haematopoietic sites during human development play a role in haemoglobin (Hb) switching or whether Hb switching is innately programmed into cells. CD34(+)/Lineage(-) haematopoietic stem/progenitor cells (HSCs) were isolated from human fetal liver (F-LVR), cord blood (CB), and adult bone marrow (ABM), and the Hb was characterized by flow cytometry on cultures that generated enucleated red cells. All feeder layers (stroma from F-LVR, ABM, and human fetal aorta) enhanced cell proliferation and erythropoiesis but did not affect Hb type. HSCs from CB and F-LVR generated the same Hb profile under normoxia and hypoxia. HSCs from ABM had single-positive HbA and double-positive HbA and HbF cells at normoxia and almost entirely double-positive cells at hypoxia. Further characterization of these ABM cultures was determined by following mRNA expression for the transcription factors erythroid Kruppel-like factor (EKLF) and fetal Kruppel-like factor (FKLF) as a function of time in cultures under hypoxia and normoxia. The erythroid-specific isoform of 5-amino-levulinate synthase (ALAS2) was also expressed under hypoxic conditions. We conclude that Hb switching is affected by the environment but not all HSCs are preprogrammed to respond.

Improved transduction of human sheep repopulating cells by retrovirus vectors pseudotyped with feline leukemia virus type C or RD114 envelopes

Gene therapy for hematopoietic diseases has been hampered by the low frequency of transduction of human hematopoietic stem cells (HSCs) with retroviral vectors pseudotyped with amphotropic envelopes. We hypothesized that transduction could be increased by the use of retroviral vectors pseudotyped with envelopes that recognize more abundant cellular receptors. The levels of mRNA encoding the receptors of the feline retroviruses, RD114 and feline leukemia virus type C (FeLV-C), were significantly higher than the level of gibbon ape leukemia virus (GaLV) receptor mRNA in cells enriched for human HSCs (Lin- CD34+ CD38-). We cotransduced human peripheral blood CD34+ cells with equivalent numbers of FeLV-C and GALV or RD114 and GALV-pseudotyped retroviruses for injection into fetal sheep. Analysis of DNA from peripheral blood and bone marrow from recipient sheep demonstrated that FeLV-C- or RD114-pseudotyped vectors were present at significantly higher levels than GALV-pseudotyped vectors. Analysis of individual myeloid colonies demonstrated that retrovirus vectors with FeLV-C and RD114 pseudotypes were present at 1.5 to 1.6 copies per cell and were preferentially integrated near known genes We conclude that the more efficient transduction of human HSCs with either FeLV-C- or RD114-pseudotyped retroviral particles may improve gene transfer in human clinical trials.

A large animal noninjury model for study of human stem cell plasticity

In this paper, we describe an experimental model that allows evaluation of the full potential of stem cells under normal physiological conditions and in the absence of genetic or injury-induced dysfunction, thus serving as a valuable tool for the study of the mechanism(s) underlying stem cell differentiation. The fetal sheep model of human stem cell transplantation permits the robust formation of donor-derived tissue-specific cells in the absence of selective pressure given its unique characteristics: the preimmune status of the fetus allows donor stem cell engraftment without significant rejection and the existence of the naturally occurring migratory patterns in the fetus facilitates the widespread efficient distribution of donor stem cells throughout the body. The cells are then influenced by the stimulatory environment of the specific tissue or organ to undergo proliferation and directed differentiation. The versatility of this noninjury fetal model of human stem cell plasticity was demonstrated by revealing the differentiative potential of different populations of both human hematopoietic stem cells and human mesenchymal stem cells (MSC).

The sheep model of in utero gene therapy

Once its full clinical potential has been realized, hematopoietic stem cell based gene therapy (GT) promises to cure a wide array of both inborn and acquired diseases. For many genetic disorders, early onset and irreparable tissue and organ damage necessitate innovative methods that allow therapeutic intervention early in development, if a full cure is to be realized. Performing GT in utero would allow early correction prior to disease onset and is thus one of the few therapeutic modalities that could promise the birth of a healthy infant. Several features of the developing fetus may circumvent obstacles that have thus far been observed in GT trials. For example, the immune naïveté of the early gestational fetus may evade immune reactions to the vector and transgene product. Furthermore, fetal exposure to foreign antigens can result in sustained tolerance, suggesting that induction of tolerance to the vector/transgene product could allow postnatal treatment to be performed successfully. In addition to these immunologic advantages, the fetal hematopoietic system promises to be more amenable to retrovirus-mediated gene transfer than either the neonate or adult as a result of both proliferation and expansion of the stem/progenitor cell pool that take place during fetal development. To investigate whether these characteristics of the developing fetus could be used to advantage to efficiently transduce hematopoietic stem cells, we developed an approach to in utero GT, in which retroviral vectors were directly injected into the peritoneal cavity of preimmune fetal sheep. This approach resulted in the transfer and long-term (>5 years) expression of exogenous genes within the hematopoietic system of primary and secondary recipients, albeit at relatively low levels that would not likely be therapeutic in most diseases. These studies also demonstrated that the direct injection of retroviral vectors into preimmune fetal sheep results not only in the successful transduction of long-term engrafting hematopoietic stem cells, but also in the widespread distribution of vector to all other tissues examined, including the reproductive organs. In an effort to increase the hematopoietic cell transduction to clinically relevant levels, we repeated our initial studies with 1,000-fold higher titer vectors. This led to only a modest (two- to fourfold) increase in the transduction levels, suggesting that factors other than absolute vector dosage were responsible for the low levels of gene transfer. For this reason, we have more recently begun evaluating the effect of recipient gestational age on the efficiency of gene transfer to both hematopoietic and nonhematopoietic tissues. Thus far, we have observed an inverse relation between the gestational age at the time of vector administration and the level of transduction and expression of the transgene within the hematopoietic system, such that fetuses injected earlier in gestation have higher levels of hematopoietic cell transduction. These elevated levels have persisted for at least 1 year after injection, suggesting that the enhancement is at the level of primitive stem/progenitor cells. When analyzing the liver sections from animals that had received the vector at different gestational ages, we also observed an inverse correlation between recipient age and efficiency of gene transfer to the hepatocytes, such that a high efficiency of gene transfer occurred at early ages, while very little occurred at later stages of gestation. In contrast to the findings in the hematopoietic system and in the liver, analysis of the lungs of these same animals revealed that the efficiency of transduction of nonhematopoietic lung tissue increased with increasing gestational age. These results demonstrate that both hematopoietic cells and nonhematopoietic cells within liver and lung are transduced following direct injection of murine retroviral vector supernatants into the peritoneal cavity of preimmune fetal sheep and suggest that the developmental stage of each organ at the time of injection may determine its etermine its susceptibility to in utero gene transfer.

Human cytomegalovirus persists in myeloid progenitors and is passed to the myeloid progeny in a latent form

CD34+ progenitor cells can harbour latent human cytomegalovirus (HCMV); however, the mechanisms of HCMV latency remain unclear. We have investigated the effects of the haematopoietic lineage restriction on the establishment and spread of the latent HCMV to progeny cells. In vitro-infected and latently-infected haematopoietic progenitor cells derived from HCMV seropositive donors were studied. The presence of HCMV DNA in bone marrow progenitor (BMP) cells was determined by single colony polymerase chain reaction and fluorescent in situ hybridization (FISH). The presence of CMV DNA was found to be restricted to myeloid progenitors and the percentage of HCMV-infected cells was lower in naturally-infected cells than in in vitro-infected cells. Erythroid differentiation resulted in an abortive infection with persistence of the viral nucleic acids in red cell precursors. In BMP cells from HCMV seronegative donors, HCMV DNA was localized in the nucleus. Bone marrow progenitors in the presence of granulocyte-macrophage colony stimulating factor (GMCSF) maintained HCMV DNA for extended periods of time. No viral production could be detected throughout the culture but the comparison of the numbers of latently-infected cells prior to and after the culture suggests that proliferation of haematopoietic progenitor cells may lead to the expansion of latently-infected cells.

Formation of human hepatocytes by human hematopoietic stem cells in sheep

We took advantage of the proliferative and permissive environment of the developing preimmune fetus to develop a noninjury large animal model in sheep, in which the transplantation of defined populations of human hematopoietic stem cells resulted in the establishment of human hematopoiesis and led to the formation of significant numbers of long-lasting, functional human liver cells, with some animals exhibiting levels as high as 20% of donor (human) hepatocytes 11 months after transplantation. A direct correlation was found between hepatocyte activity and phenotype of transplanted cells, cell dose administered, source of cells used on a cell-per-cell basis (bone marrow, cord blood, mobilized peripheral blood), and time after transplantation. Human hepatocytes generated in this model retained functional properties of normal hepatocytes, constituted hepatic functional units with the presence of human endothelial and biliary duct cells, and secreted human albumin that was detected in circulation. Transplanting populations of hematopoietic stem cells can efficiently generate significant numbers of functional hepatic cells in this noninjury large animal model and thus could be a means of ameliorating or curing genetic diseases in which a deficiency of liver cells or their products threatens the life of the fetus or newborn.

The fetal sheep: a unique model system for assessing the full differentiative potential of human stem cells

The naturally occurring stem cell migratory patterns, the availability of expanding homing and engraftment sites, and the presence of tissue/organ-specific signals in the developing mammalian fetus provide the ideal setting for stem cells to exhibit their full biological potential. These characteristics combined with the relative immunological naivete of the early gestational age fetus that permits the engraftment and long-term persistence of allogeneic and xenogeneic donor stem cells make it possible to use the developing fetus to assess the in vivo potential of a variety of stem cells. We have taken advantage of these permissive characteristics of the fetus to develop a large animal model of human hematopoiesis in sheep that permits not only the long-term engraftment of human hematopoietic stem cell/progenitor cells and their differentiation into the full range of lymphohematopoietic elements, but also the relatively robust expression of their potential to contribute to the formation of non-hematopoietic tissues.

Human mesenchymal stem cells form Purkinje fibers in fetal sheep heart

BACKGROUND

We have investigated the usefulness of a model of cardiac development in a large mammal, sheep, for studies of engraftment of human stem cells in the heart.

METHODS AND RESULTS

Adult and fetal human mesenchymal stem cells were injected intraperitoneally into sheep fetuses in utero. Hearts at late fetal development were analyzed for engraftment of human cells. The majority of the engrafted cells of human origin formed segments of Purkinje fibers containing exclusively human cells. There were no differences in engraftment of human mesenchymal stem cells from adult bone marrow, fetal brain, and fetal liver. On average, 43.2% of the total Purkinje fibers in random areas (n=11) of both ventricles were of human origin. In contrast, approximately 0.01% of cardiomyocytes were of human origin.

CONCLUSIONS

Human mesenchymal stem cells preferentially engraft at high levels in the ventricular conduction system during fetal development in sheep. These findings raise the possibility that stem cells contribute to normal development of the fetal heart.

Human haematopoietic stem cells that mediate long-term in vivo engraftment are not susceptible to infection by human cytomegalovirus

A human/sheep xenograft model was used to evaluate whether long-term engrafting haematopoietic stem cells (HSC) are susceptible to human cytomegalovirus (HCMV) infection. CD34+ Lin- HSC were isolated by fluorescence-activated cell sorting (FACS) from the bone marrow (BM) of HCMV-positive and HCMV-negative normal donors. Cells from the latter group were infected in vitro with HCMV. HCMV DNA was detected in both cell populations by nested-polymerase chain reaction (PCR) and fluorescence in situ hybridization. Cells were transplanted into separate groups of fetal sheep at concentrations of 1.3-5.0 x 105 cells per fetus. Multilineage human haematopoietic cell engraftment, including CD34+ cells, was detected in the BM and peripheral blood of recipients up to 16 months post-transplant as assessed by FACS analysis and PCR for HLA-DQalpha. Levels of engraftment varied (1.2-24.3%) but no sheep exhibited HCMV-positive cells. To ensure that our inability to detect HCMV-positive cells was not due to immune-elimination of HCMV-infected cells, 3.8-10 x 105 HCMV-positive uncharacterized BM stromal cells were transplanted into fetal sheep. At 5 weeks post-transplant several organs were HLA-DQalpha- and HCMV-positive, confirming that HCMV was detectable. These results provide evidence that the long-term engrafting HSC is not a primary target of HCMV and suggest that HCMV infection of human haematopoietic cells is exercised at the level of committed progenitors.

Plasticity of Human Stem Cells in the Fetal Sheep Model of Human Stem Cell Transplantation

Experimental models that allow the evaluation of the full potential of stem cells under normal physiological conditions and in the absence of genetic or injury-induced dysfunction would serve as valuable tools for the study of the mechanisms underlying stem cell differentiation. Ideally, such a model would also permit the robust formation of donor-derived tissue-specific cells. Because studies have shown that the differentiation of stem cells into cells of a different germinal layer is highly inefficient in the absence of selective pressure, it is very unlikely that a healthy adult animal can fulfill these requirements. In this review, we describe the advantages of the permissive aspects of the developing preimmune fetus in the early gestational age that led us to develop the sheep as a large-animal model of human stem cell plasticity.

Purification of interstitial cells of Cajal by fluorescence-activated cell sorting

Interstitial cells of Cajal (ICC) in the gastrointestinal tract generate and propagate slow waves and mediate neuromuscular neurotransmission. Although damages to ICC have been described in several gastrointestinal motor disorders, analysis of their gene expression in health and disease has been problematic because of the difficulties in isolating these cells. Our goal was to develop techniques for large-scale purification of ICC. Murine ICC were identified in live gastrointestinal muscles with fluorescent Kit antibodies. Because this technique also labels resident macrophages nonspecifically, we attempted to separate ICC from these cells by fluorescence-activated cell sorting with or without immunomagnetic presorting. Efficacy and specificity of ICC purification were tested by quantitative RT-PCR of cell-specific markers. Fluorescence-based separation of small intestinal ICC from unlabeled cells and macrophages tagged with F4/80 antibodies yielded 30,000-40,000 cells and approximately 60-fold enrichment of c-kit mRNA. However, the macrophage marker CD68 was also enriched approximately 6-fold. Magnetic presorting of ICC did not significantly improve selectivity. After labeling contaminating cells with additional paramagnetic (anti-CD11b, -CD11c) and fluorescent antibodies (anti-CD11b) and depleting them by magnetic presorting, we harvested approximately 2,000-4,000 cells from single gastric corpus-antrum muscles and detected an approximately 30-fold increase in c-kit mRNA, no enrichment of mast cells, and an approximately 4-fold reduction of CD68 expression. Adding labeled anti-CD45 antibody to our cocktail further increased c-kit enrichment and eliminated mast cells and macrophages. Smooth muscle cells and myenteric neurons were also depleted. We conclude that immunofluorescence-based sorting can yield ICC in sufficiently high numbers and purity to permit detailed molecular analyses.

Modelling of ex vivo expansion/maintenance of hematopoietic stem cells

In this study, we described the modelling of the expansion/maintenance of human hematopoietic stem/progenitor cells from adult human bone marrow. CD 34(+)-enriched cell populations from bone marrow were cultured in the presence and absence of human stroma in serum-free media containing bFGF, SCF, LIF and Flt-3 ligand for several days. The cells in the culture were analysed for expansion and phenotype by flow cytometry. Although significant expansion of bone marrow cultures occurred in the presence and absence of human stroma, the results of expansion were effectively better in the presence of a stromal layer. In both situations the phenotypic analysis demonstrated a great expansion of CD 34(+)38(-) cells. The differentiative potential of bone marrow CD 34(+) cells co-cultured with human stroma was primarily shifted towards the myeloid lineage with the presence of CD 15 and CD 33.

Reversible expression of CD34 by adult human bone marrow long-term engrafting hematopoietic stem cells

OBJECTIVE

We previously reported that CD34(-) population of bone marrow (BM) cells from adult humans contains cells capable of engraftment and multilineage differentiation. We also reported on the reversibility of CD34 expression by murine hematopoietic stem cells. Based on long-term observations in primary, secondary, and tertiary sheep recipients, we now present definitive evidence for the long-term engrafting capability of human BM CD34(-) cells, and the reversibility of CD34 expression by human BM hematopoietic stem cells (HSC) in vivo.

MATERIALS AND METHODS

We used serial transplantations into primary, secondary, and tertiary preimmune fetal sheep recipients to evaluate and compare the long-term engraftment and differentiation of adult human bone marrow-derived CD34(-) and CD34(+) cells in vivo.

RESULTS

In primary hosts CD34(-) or CD34(+) cells produced multilineage human cell activity that persisted for 31 months. To confirm the long-term engrafting characteristics of CD34(-) cells and determine whether CD34 expression on human HSC is reversible, we transplanted human CD34(-) and CD34(+) cells obtained from primary hosts into secondary sheep recipients. Multilineage engraftment occurred in all secondary hosts, and in tertiary hosts transplanted with CD34(-) or CD34(+) cells obtained from BM of secondary recipients.

CONCLUSION

These results demonstrate that human BM CD34(-) cells are capable of long-term multilineage engraftment in vivo. The finding that both CD34(-) and CD34(+) cells from primary/secondary groups engraft secondary/tertiary hosts indicates that CD34 expression on human HSC is reversible, a process that does not impair HSC function in vivo.

Differentiative potential of human metanephric mesenchymal cells

OBJECTIVE

To evaluate the ability of mesenchymal cells derived from nonhematopoietic organs to form blood and other tissues in vitro and in vivo.

MATERIALS AND METHODS

Because of its mesodermic derivation, human fetal kidney was used as a source of mesenchymal cells. Two populations of kidney cells were studied at a nonclonal level: a crude preparation, and an adherent fraction that was derived from the first by propagation in vitro (MNMC). Both populations were transplanted into sheep fetuses and analyzed at intervals for the presence of human cells in different organs by flow cytometry, PCR, immunohistochemistry, and in situ hybridization. Secondary transplantation studies were performed using human hematopoietic cells obtained from the bone marrow (BM) of primary recipients.

RESULTS

MNMC were Thy-1(+), CD51(+), CD44(+), CD45(-), and vimentin(+), a phenotype consistent with that of metanephric mesenchyme. The crude population displayed the same phenotype but was contaminated with 0.4% CD34(+)CD45(+) cells. Cells with hepatocyte-like morphology and phenotype were obtained from the MNMC after culture in specific inducing media. After transplantation, both populations of cells produced multilineage hematopoietic engraftment and gave rise to CD34(+) cells. Successful hematopoietic engraftment in secondary recipients demonstrated the generation of long-term engrafting hematopoietic stem cells from MNMC. PCR analysis confirmed human hematopoietic engraftment and revealed that human cells were also present within other organs. Liver sections of transplanted animals contained human albumin-producing hepatocyte-like cells.

CONCLUSION

A human metanephric mesenchymal cell population simultaneously gave rise to human blood and liver-like cells, suggesting that mesenchymal cells may represent a broad population of putative stem cells in multiple adult organs.

Human natural killer cell development in a xenogeneic culture system

In vivo and in vitro xenogeneic models have shown the ability of a non-human environment in supporting human haemopoiesis. In the present study, we evaluated the effect of fetal sheep thymic stroma in the in vitro development of natural killer (NK) cells from human haemopoietic progenitors. CD34+HLA-DR+ (CD34+ DR+)Lin- and CD34+DR-Lin- bone marrow (BM) progenitors were cultured for 3 weeks with or without interleukin 2 (IL-2), in fetal sheep thymic stroma contact and transwell cultures. Both progenitors gave rise to NK cells, defined as CD45+CD56+ cells, in the presence or absence of IL-2; however, the percentage of NK cells originated in cultures with IL-2 was significantly higher. Direct contact with stroma seemed to be required for the most immature progenitors, CD34+DR-Lin-, to differentiate along the NK cell lineage. Functional assays revealed that only cells grown in the presence of IL-2 were cytolytic against K562 targets and, curiously, NK cells derived from CD34+DR-Lin- progenitors were more cytotoxic that NK cells derived from CD34+DR+Lin- progenitors. These studies suggest that the ability of fetal sheep thymic stroma in promoting the generation of human NK cells from haemopoietic progenitors may have relevance in terms of NK cell ontogeny and induction of tolerance in transplantation.

Strong public support for treatment and research using fetal tissue, particularly among those accepting the scientific world-view

Data from four large representative national sample surveys in Australia between 1993 and 2000 show strong public support for treatment using fetal tissue. Support remains high even if the treatment has only a remote chance of success, or if the goal is research rather than treatment. Regression analysis reveals striking new evidence on the links between science and moral beliefs. Even adjusting for religious denomination and numerous demographic characteristics, acceptance of evolution and of aspects of astronomy suggesting that life on earth is not unique greatly increases support for using fetal tissue. So does trust in scientists.