Hematopoietic reconstitution of syngeneic mice with a peripheral blood-derived, monoclonal CD34-, Sca-1+, Thy-1(low), c-kit+ stem cell line

Autologous Stem Cell Therapy for Chronic Lower Extremity Wounds: A Meta-Analysis of Randomized Controlled Trials

Lower extremity chronic wounds (LECWs) commonly occur in patients with diabetes mellitus (DM) and peripheral arterial disease (PAD). Autologous stem cell therapy (ASCT) has emerged as a promising alternative treatment for those who suffered from LECWs. The purpose of this study was to assess the effects of ASCT on LECWs. Two authors searched three core databases, and independently identified evidence according to predefined criteria. They also individually assessed the quality of the included randomized controlled trials (RCTs), and extracted data on complete healing rate, amputation rate, and outcomes regarding peripheral circulation. The extracted data were pooled using a random-effects model due to clinical heterogeneity among the included RCTs. A subgroup analysis was further performed according to etiology, source of stem cells, follow-up time, and cell markers. A total of 28 RCTs (n = 1096) were eligible for this study. The pooled results showed that patients receiving ASCT had significantly higher complete healing rates (risk ratio (RR) = 1.67, 95% confidence interval (CI) 1.28–2.19) as compared with those without ASCT. In the CD34+ subgroup, ASCT significantly led to a higher complete healing rate (RR = 2.70, 95% CI 1.50–4.86), but there was no significant difference in the CD34− subgroup. ASCT through intramuscular injection can significantly improve wound healing in patients with LECWs caused by either DM or critical limb ischemia. Lastly, CD34+ is an important cell marker for potential wound healing. However, more extensive scale and well-designed studies are necessary to explore the details of ASCT and chronic wound healing.

Bone Tissue Engineering: Past-Present-Future

Bone is one of the few tissues to display a true potential for regeneration. Fracture healing is an obvious example where regeneration occurs through tightly regulated sequences of molecular and cellular events which recapitulate tissue formation seen during embryogenesis. Still in some instances, bone regeneration does not occur properly (i.e. critical size lesions) and an appropriate therapeutic intervention is necessary. Successful replacement of bone by tissue engineering will likely depend on the recapitulation of this flow of events. In fact, bone regeneration requires cross-talk between microenvironmental factors and cells; for example, resident mesenchymal progenitors are recruited and properly guided by soluble and insoluble signaling molecules. Tissue engineering attempts to reproduce and to mimic this natural milieu by delivering cells capable of differentiating into osteoblasts, inducing growth factors and biomaterials to support cellular attachment, proliferation, migration, and matrix deposition. In the last two decades, a significant effort has been made by the scientific community in the development of methods and protocols to repair and regenerate tissues such as bone, cartilage, tendons, and ligaments. In this same period, great advancements have been achieved in the biology of stem cells and on the mechanisms governing "stemness". Unfortunately, after two decades, effective clinical translation does not exist, besides a few limited examples. Many years have passed since cell-based regenerative therapies were first described as "promising approaches", but this definition still engulfs the present literature. Failure to envisage translational cell therapy applications in routine medical practice evidences the existence of unresolved scientific and technical struggles, some of which still puzzle researchers in the field and are presented in this chapter.

Bone marrow-derived mesenchymal stem cells migrate to healthy and damaged salivary glands following stem cell infusion

Xerostomia is a severe side effect of radiation therapy in head and neck cancer patients. To date, no satisfactory treatment option has been established. Because mesenchymal stem cells (MSCs) have been identified as a potential treatment modality, we aimed to evaluate stem cell distribution following intravenous and intraglandular injections using a surgical model of salivary gland damage and to analyse the effects of MSC injections on the recruitment of immune cells. The submandibular gland ducts of rats were surgically ligated. Syngeneic adult MSCs were isolated, immortalised by simian virus 40 (SV40) large T antigen and characterized by flow cytometry. MSCs were injected intravenously and intraglandularly. After 1, 3 and 7 days, the organs of interest were analysed for stem cell recruitment. Inflammation was analysed by immunohistochemical staining. We were able to demonstrate that, after intravenous injection, MSCs were recruited to normal and damaged submandibular glands on days 1, 3 and 7. Unexpectedly, stem cells were recruited to ligated and non-ligated glands in a comparable manner. After intraglandular injection of MSCs into ligated glands, the presence of MSCs, leucocytes and macrophages was enhanced, compared to intravenous injection of stem cells. Our data suggest that injected MSCs were retained within the inflamed glands, could become activated and subsequently recruited leucocytes to the sites of tissue damage.

Is CD34 truly a negative marker for mesenchymal stromal cells?

The prevailing school of thought is that mesenchymal stromal cells (MSC) do not express CD34, and this sets MSC apart from hematopoietic stem cells (HSC), which do express CD34. However, the evidence for MSC being CD34(-) is largely based on cultured MSC, not tissue-resident MSC, and the existence of CD34(-) HSC is in fact well documented. Furthermore, the Stro-1 antibody, which has been used extensively for the identification/isolation of MSC, was generated by using CD34(+) bone marrow cells as immunogen. Thus, neither MSC being CD34(-) nor HSC being CD34(+) is entirely correct. In particular, two studies that analyzed CD34 expression in uncultured human bone marrow nucleated cells found that MSC (BMSC) existed in the CD34(+) fraction. Several studies have also found that freshly isolated adipose-derived MSC (ADSC) express CD34. In addition, all of these ADSC studies and several other MSC studies have observed a disappearance of CD34 expression when the cells are propagated in culture. Thus the available evidence points to CD34 being expressed in tissue-resident MSC, and its negative finding being a consequence of cell culturing.

Chimerism of bone marrow mesenchymal stem/stromal cells in allogeneic hematopoietic cell transplantation: is it clinically relevant?

Multipotent mesenchymal stem/stromal cells (MSCs) have been extensively used as a transplantable cell source for regenerative medicine and immunomodulatory therapy. Specifically in allogeneic hematopoietic stem cell transplantation (HSCT), co-transplantation or post-transplant infusion of MSCs derived from bone marrow (BM) of non-self donors has been implicated in accelerating hematopoietic recovery, ameliorating graft-vs.-host disease, and promoting tissue regeneration. However, irrespective of the use of MSC co-administration, post-transplant chimerism of BM-derived MSCs after allogeneic HSCT has been reported to remain of host origin, suggesting that the infused donor MSCs are immunologically rejected or not capable of long-term engraftment in the host microenvironment. Also, hematopoietic cell allografts currently used for HSCT do not seem to contain sufficient amount of MSCs or their precursors to reconstitute host BM microenvironment. Since the toxic conditioning employed in allo-HSCT may impair the function of host MSCs to maintain hematopoietic/regenerative stem cell niches and to provide a local immunomodulatory milieu, we propose that new directions for enhancing immunohematopoietic reconstitution and tissue repair after allogeneic HSCT include the development of strategies to support functional replenishment of residual host MSCs or to support more efficient engraftment of infused donor MSCs. Future areas of research should include in vivo tracking of infused MSCs and detection of their microchimeric presence in extra-marrow sites as well as in BM.

Peripheral blood stem cells: phenotypic diversity and potential clinical applications

A small proportion of cells in peripheral blood are actually pluripotent stem cells. These peripheral blood stem cells (PBSCs) are thought to be heterogeneous and could be exploited for a variety of clinical applications. The exact number of distinct populations is unknown. It is likely that individual PBSC populations detected by different experimental strategies are similar or overlapping but have been assigned different names. In this mini review, we divide PBSCs into seven groups: hematopoietic stem cells (HSCs), CD34- stem cells, CD14+ stem cells, mesenchymal stem cells (MSCs), very small embryonic-like (VSEL) stem cells, endothelial progenitor cells (EPCs), and other pluripotent stem cells. We review the major characteristics of these stem/progenitor cell populations and their potential applications in ophthalmology.

Selective targeting of genetically engineered mesenchymal stem cells to tumor stroma microenvironments using tissue-specific suicide gene expression suppresses growth of hepatocellular carcinoma

BACKGROUND

The use of engineered mesenchymal stem cells (MSCs) as therapeutic vehicles for the treatment of experimental pancreatic and breast cancer has been previously demonstrated. The potential application of MSCs for the treatment of hepatocellular carcinoma (HCC) has been controversial. The general approach uses engineered MSCs to target different aspects of tumor biology, including angiogenesis or the fibroblast-like stromal compartment, through the use of tissue-specific expression of therapeutic transgenes. The aim of the present study was (1) to evaluate the effect of exogenously added MSCs on the growth of HCC and (2) the establishment of an MSC-based suicide gene therapy for experimental HCC.

METHODS

Mesenchymal stem cells were isolated from bone marrow of C57/Bl6 p53(-/-) mice. The cells were injected into mice with HCC xenografts and the effect on tumor proliferation and angiogenesis was evaluated. The cells were then stably transfected with red fluorescent protein (RFP) or Herpes simplex virus thymidine kinase (HSV-Tk) gene under control of the Tie2 promoter/enhancer or the CCL5 promoter. Mesenchymal stem cells were injected intravenously into mice with orthotopically growing xenografts of HCC and treated with ganciclovir (GCV).

RESULTS

Ex vivo examination of hepatic tumors revealed tumor-specific recruitment, enhanced tumor growth, and increased microvessel density after nontherapeutic MSC injections. After their homing to the hepatic xenografts, engineered MSCs demonstrated activation of the Tie2 or CCL5 promoter as shown by RFP expression. Application of CCL5/HSV-TK transfected MSCs in combination with GCV significantly reduced tumor growth by 56.4% as compared with the control group and by 71.6% as compared with nontherapeutic MSC injections. CCL5/HSV-TK(+) transfected MSCs proved more potent in tumor inhibition as compared with Tie2/HSV-TK(+) MSCs.

CONCLUSION

Exogenously added MSCs are recruited to growing HCC xenografts with concomitant activation of the CCL5 or Tie2 promoters within the MSCs. Stem cell-mediated introduction of suicide genes into the tumor followed by prodrug administration was effective for treatment of experimental HCC and thus may help fill the existing gap in bridging therapies for patients suffering from advanced HCCs.

Radiation rescue: mesenchymal stromal cells protect from lethal irradiation

BACKGROUND

Successful treatment of acute radiation syndromes relies on immediate supportive care. In patients with limited hematopoietic recovery potential, hematopoietic stem cell (HSC) transplantation is the only curative treatment option. Because of time consuming donor search and uncertain outcome we propose MSC treatment as an alternative treatment for severely radiation-affected individuals.

METHODS AND FINDINGS

Mouse mesenchymal stromal cells (mMSCs) were expanded from bone marrow, retrovirally labeled with eGFP (bulk cultures) and cloned. Bulk and five selected clonal mMSCs populations were characterized in vitro for their multilineage differentiation potential and phenotype showing no contamination with hematopoietic cells. Lethally irradiated recipients were i.v. transplanted with bulk or clonal mMSCs. We found a long-term survival of recipients with fast hematopoietic recovery after the transplantation of MSCs exclusively without support by HSCs. Quantitative PCR based chimerism analysis detected eGFP-positive donor cells in peripheral blood immediately after injection and in lungs within 24 hours. However, no donor cells in any investigated tissue remained long-term. Despite the rapidly disappearing donor cells, microarray and quantitative RT-PCR gene expression analysis in the bone marrow of MSC-transplanted animals displayed enhanced regenerative features characterized by (i) decreased proinflammatory, ECM formation and adhesion properties and (ii) boosted anti-inflammation, detoxification, cell cycle and anti-oxidative stress control as compared to HSC-transplanted animals.

CONCLUSIONS

Our data revealed that systemically administered MSCs provoke a protective mechanism counteracting the inflammatory events and also supporting detoxification and stress management after radiation exposure. Further our results suggest that MSCs, their release of trophic factors and their HSC-niche modulating activity rescue endogenous hematopoiesis thereby serving as fast and effective first-line treatment to combat radiation-induced hematopoietic failure.

Bone marrow mesenchymal stromal cells remain of recipient origin after allogeneic SCT and do not harbor the JAK2V617F mutation in patients with myelofibrosis

The close association of the myeloproliferative neoplasms with the activating non-receptor tyrosine kinase JAK2V617F mutation is well established. To further clarify the pathomechanisms of this mutation in patients with myelofibrosis, we performed screening with quantitative real-time PCR for the respective mutation in in vitro expanded bone marrow (BM) mesenchymal stromal cells (MSCs) and compared the results with BM/peripheral blood (PB). Eight patients with primary/secondary myelofibrosis were investigated before (n = 4) or after allogeneic stem cell transplantation (n = 4). All patients had systemic evidence of the JAK2V617F mutation in BM/PB (mutation ratios 0.2-23.5) at the time of investigation in contrast to negative results in the MSCs (n = 7) or a very low (0.004) mutation ratio (n = 1) which was probably due to hematopoietic contamination. The four patients post-transplant had systemic donor chimerism between 96.5 and 100% in BM/PB, while MSCs showed no evidence of donor-specific alleles. In conclusion, in myelofibrosis, the JAK2V617F mutation is restricted to hematopoietic cells, and cannot explain the stromal alterations being observed in this disorder. Further, the MSCs remain of recipient origin after allogeneic SCT, which might contribute to the increased risk of graft dysfunction or failure in myelofibrosis patients after allogeneic transplantation.

Multipotent mesenchymal stem cells acquire a lymphendothelial phenotype and enhance lymphatic regeneration in vivo

BACKGROUND

The importance and therapeutic value of stem cells in lymphangiogenesis are poorly understood. We evaluated the potential of human and murine mesenchymal stem cells (MSCs) to acquire a lymphatic phenotype in vitro and to enhance lymphatic regeneration in vivo.

METHODS AND RESULTS

We assessed the lymphendothelial differentiation of human and murine MSCs after induction with supernatant derived from human dermal microvascular endothelial cells, isolated lymphatic endothelial cells, and purified vascular endothelial growth factor (VEGF)-C in vitro. We used human or murine progenitor MSC lines and then characterized the lymphatic phenotype by morphology, migratory capacity, and the expression of lymphatic markers such as Prox-1, podoplanin, Lyve-1, VEGF receptor-2, and VEGF receptor-3. Using a murine lymphatic edema model, we assessed the potential of these cells to form a functional lymphatic vasculature in vivo after injection of syngeneic MSCs. Incubation with supernatant from lymphatic endothelial cells induced an endothelium-like morphology and the expression of lymphendothelial markers in both human and murine MSCs in vitro. MSCs showed migratory activity along a VEGF-C gradient, which was enhanced by VEGF-C conditioning. In vivo, the local application of MSCs resulted in a significant decrease in edema formation (-20.1%; P<0.01 versus untreated tails) after 3 weekly cell injections and restored the drainage of intradermally injected methylene blue after 7 weekly injections.

CONCLUSIONS

MSCs were capable of expressing a lymphatic phenotype when exposed to lymph-inductive media and purified VEGF-C. Migratory activity toward VEGF-C in vitro suggests homing capability in vivo. Restoration of lymphatic drainage after injection of MSCs in a lymphedema model indicates that MSCs play a role in lymphatic regeneration. The potential clinical application of MSC in wound healing and reduction of lymphatic edema warrants further research.

CD34 Modulates the Trafficking Behavior of Hematopoietic CellsIn Vivo

The CD34 surface antigen has been recognized as a marker of hematopoietic stem cells (HSCs) and is widely used for HSC selection as well as for quality control in HSC transplantation. CD34 has been implicated in cytoadhesion signaling, and its expression has been suggested to reflect the activation state of hematopoietic progenitor cells. However, the function of CD34 remains essentially unknown. Here we analyzed the effects of ectopic CD34 expression in vivo in a bone marrow transplantation model. We transduced murine bone marrow stem cells with retroviral vectors encoding either murine full-length or the alternative splice product truncated CD34. Transduced cells were transplanted into syngeneic, marrow ablated hosts. For comparison, "control" animals received either enhanced green fluorescent protein (eGFP)-transduced or mock-transduced cells. Six months post-transplantation, transduced differentiated blood cells ectopically expressing murine CD34 showed decreased migration from peripheral blood to both bone marrow and thymus, an effect that was more pronounced with full-length CD34 than with the truncated variant. In contrast, no influence of transgene expression on trafficking of differentiated blood cells was seen in the eGFP control group. Our data indicate that CD34 expression in mature blood cells has a suppressive effect on cellular trafficking to hematopoietic stroma organs, thereby supporting a modulating role of the CD34 molecule in cytoadhesion.

Nonmyeloablative stem cell therapy enhances microcirculation and tissue regeneration in murine inflammatory bowel disease

BACKGROUND AND AIMS

Reduced microcirculation has been implicated in the pathogenesis of inflammatory bowel disease (IBD). Stem cells or endothelial progenitor cells are thought to contribute to tissue regeneration through neoangiogenesis or vasculogenesis in ischemia- or inflammatory-related diseases. We therefore hypothesized that adult stem cells facilitate epithelial repair in IBD.

METHODS

Moderate-severe colitis in mice was induced by dextran sulfate sodium (DSS) and 2.0 x 10(6) immortalized CD34(-) stem cells infused twice via the tail vein during an observation period of 35 days in a nonmyeloablative setting.

RESULTS

Here, we demonstrate that adult stem cells home to the damaged digestive tract in the large intestine and facilitate mucosal repair in moderate-severe colitis. Nonmyeloablative stem cell therapy resulted in increased survival in severe colitis (P < .0001). Moreover, clinical activity and histologic evaluation of the colitis severity score were reduced significantly in moderate (P = .0003 or P = .03) and severe (P < .0001 or P < .03) colitis after 35 days, in addition to the DSS-induced shortening of colon length (P = .002 and P < .0002). Genetically marked stem cells were detected predominantly in the submucosa of the damaged colon epithelium. Epithelial repair in experimental IBD was mediated either by induction of improved vasculogenesis or by the differentiation of the transplanted stem cells into endothelial cells, as demonstrated by the promotion of Tie2 activity in the infused cells at the site of the damaged mucosa.

CONCLUSIONS

Our findings indicate that systemically administered adult stem cells respond to an adequate tissue lesion in murine IBD by enhancing microcirculation, resulting in accelerated tissue repair.

Bone engineering: combining smart biomaterials and the application of stem cells

Remarkable preclinical progress and achievements in the isolation and characterization of stem cells from different sources have been made, together with the ability to control proliferation and differentiation in vitro. Researchers and physicians pin their hope on stem cells to develop novel approaches in regenerative medicine. Stem cell therapy is no longer a scientific discipline in the laboratory or in animal models only, as it is already performed routinely in the human therapeutical and clinical treatment for hematological malignancies and clinical trials for bone repair. It has further been recognized that angiogenesis plays a pivotal role for the successful transplantation of in vitro engineered tissues like bone structures, for the repair of large bone defects to facilitate engraftment. We intended to generate transplantable composite grafts, which consist of in vitro generated bone tissue from multipotent adult progenitor cells, maintaining an angiogenic potential to ease and optimize a successful engraftment of long-term sustainable and viable bone tissue. The other component was generated from several synthetically produced three-dimensional tissues like fibrin, thrombin, collagen, hydroxyapatite, and beta-tricalciumphosphate in different compositions and concentrations, all mimicking natural body structures of bone tissue as biocompatible grafts. We established an in vitro mouse model, using bone marrow and peripheral blood-derived murine progenitor cells as well as industrially manufactured materials generating composite grafts. Based on our preliminary results, we propose to pursue research on composite grafts for an extended graft survival and successful engraftment of transplanted tissue.

Noncontact laser microdissection and catapulting for pure sample capture

The understanding of the molecular mechanisms of cellular function, growth, and proliferation is based on the accurate identification, isolation, and finally characterization of a specific single cell or a population of cells and its subsets of biomolecules. For the simultaneous analysis of thousands of molecular parameters within one single experiment as realized by DNA, RNA, and protein microarray technologies, a defined number of homogeneous cells derived from a distinct morphological origin are required. Sample preparation is therefore a very crucial step preceding the functional characterization of specific cell populations. Laser microdissection and laser pressure catapulting (LMPC) enables pure and homogeneous sample preparation resulting in an increased specificity of molecular analyses. With LMPC, the force of focused laser light is utilized to excise selected cells or large tissue areas from object slides down to individual single cells and subcellular components like organelles or chromosomes. After microdissection, the sample is directly catapulted into an appropriate collection vial. As this process works entirely without mechanical contact, it enables pure sample retrieval from morphologically defined origin without cross-contamination. LMPC has been successfully applied to isolate and catapult cells from, for example, histological tissue sections, from forensic evidence material, and also from tough plant matter, supporting biomedical research, forensic science, and plant physiology studies. Even delicate living cells like stem cells have been captured for recultivation without affecting their viability or stem cell character, an important feature influencing stem cell research, regenerative medicine, and drug development. The combination of LMPC with microinjection to inject drugs or genetic material into individual cells and to capture them for molecular analyses bears great potential for efficient patient-tailored medication.

Concise review: multipotent mesenchymal stromal cells in blood

Peripheral blood-derived multipotent mesenchymal stromal cells circulate in low number. They share, most although not all, of the surface markers with bone marrow-derived multipotent mesenchymal stromal cells, possess diverse and complicated gene expression characteristics, and are capable of differentiating along and even beyond mesenchymal lineages. Although their origin and physio-pathological function are still unclear, their presence in the adult peripheral blood might relate to some interesting but controversial subjects in the field of adult stem cell biology, such as systemic migration of bone marrow-derived multipotent mesenchymal stromal cells and the existence of common hematopoietic-mesenchymal precursors. In this review, current studies/knowledge about peripheral blood-derived multipotent mesenchymal stromal cells is summarized, and the above-mentioned topics are discussed.

Multipotent mesenchymal stem cells reduce interstitial fibrosis but do not delay progression of chronic kidney disease in collagen4A3-deficient mice

Multipotent mesenchymal stem or stromal cells (MSC) have shown to improve outcome of acute renal injury models, but whether MSC can delay renal failure in chronic kidney disease is not known. We injected primary MSC or saline into mice that lack the alpha3-chain of type IV collagen (COL4A3), a model of chronic kidney disease with close similarities to human Alport disease. Weekly injections of MSC from week 6 to 10 of life prevented the loss of peritubular capillaries and reduced markers of renal fibrosis, that is, interstitial volume, numbers of smooth muscle actin-positive interstitial cells, and interstitial collagen deposits as compared to saline-injected COL4A3-deficient mice. However, renal function, that is, blood urea nitrogen, creatinine levels, proteinuria as well as survival of COL4A3-deficient mice were not affected by MSC injections. Although MSC were found to localize to kidneys of COL4A3-deficient mice after injection, differentiation into renal cells was not detected. However, MSC expressed growth factors, that is, vascular endothelial growth factor (VEGF) and bone morphogenetic protein-7 under basal culture conditions. In fact, VEGF mRNA levels were increased in kidneys of MSC-injected COL4A3-deficient mice and MSC supernatants enhance endothelial cell proliferation in vitro. Thus, weekly injections with MSC prevent loss of peritubular capillaries possibly owing to local production of growth factors rather than by differentiation into renal cells. The maintenance of interstitial vasculature is associated with less interstitial fibrosis but, is insufficient to delay renal failure and survival of COL4A3-deficient mice.

Expression and functional significance of osteocalcin splicing in disease progression of hematological malignancies

The aim of this study was to investigate the expression of osteocalcin (OCN) splicing variants in hematological malignancies. We analysed bone marrow obtained from two patients with chronic myeloid leukemia (CML), seven patients with other myeloproliferative diseases (MPD) and four patients with acute myeloid leukemia (AML). RT-PCR analyses were performed in order to assess and quantify spliced (OCNs) and unspliced (OCNu) mRNA, the associated transcription factors (AML1 and AML3) as well as c-KIT which is a marker for activated stem cells. Our data indicate that OCNs mRNA and OCN protein is expressed in c-KIT positive neoplastic stem cells in hematological malignancies.

Human adult CD34- progenitor cells functionally express the chemokine receptors CCR1, CCR4, CCR7, CXCR5, and CCR10 but not CXCR4

The homing and tissue-specific recruitment of bone marrow-derived progenitor cells is a major issue in stem cell research and therapy. Chemokine biology plays a central role in the homing and trafficking of leukocytes. Here we show functional expression of the chemokine receptors CCR1, CCR4, CCR7, CCR10, and CXCR5 on primary isolates of CD34- mesenchymal progenitor cells as well as immortalized mesenchymal stem cell (MSC) lines. Although mRNA expression of CXCR4 was detected in both primary cells and immortalized clones, the receptor was not expressed on the cell surface. On the basis of this expression profile, the MSC could potentially home to secondary lymphatic organs (CCR7, CXCR5), skin (CCR4, CCR10), small intestine (CCR10), and salivary glands (CCR10). To study tissue-specific homing, murine CD34- MSC lines showing concordant chemokine receptor expression were either transiently labeled with CMFDA, or were stably transfected with green fluorescent protein (GFP) expression plasmids. The MSC were then injected into syngeneic healthy mice, and the distribution of the cells determined. The injected cells efficiently homed to spleen, thymus, and lymph nodes. In addition, cells were found in the mucosa of the small intestine, skin, and salivary gland. No significant recruitment to bone marrow, liver, or kidney was seen. Chemokine biology may play an important role in the homeostasis and potentially tissue recruitment of early adult progenitor cells.

Adult Scl+/+Murine Hemangioblasts Persist in Allogeneic Mutant Blastocysts but Fail to Rescue the Scl−/−Phenotype

Isolated and expanded scl (+) adult murine progenitors show a strong endothelial and hematopoietic differentiation potential and have been considered to be the adult equivalent of the hemangioblast. These unique cells may provide effective therapeutic approaches to tissue damage resulting from hypoxemia or chronic ischemia. Here, we study the fate of adult scl (+/+) during development and their ability to reverse genetic defects in scl expression. scl (+/+) adult stem cells (clone RM26) did not persist during embryonic development after injection into blastocysts of allogeneic wild-type mice on day E 3.5. However, GFP(+)-marked scl (+/+) cells were detected in all possible genotypes from allogeneic scl (+/+) intercrosses (scl (+/+), scl (+/-), scl (-/-) on day E 9.5 after the cloned cells were injected into scl-mutant blastocysts on day E 3.5. Nevertheless, there was no indication of phenotypic rescue of the mutant blastocysts despite the continued presence of scl (+/+) RM26 cells in the allogeneic embryonic environment. The results show that differentiated stem cells providing a defective gene may exert effects during development when there is a reparative demand, but they are not capable of reversing the effects of a mutant phenotype during embryonic development. These effects should be considered when evaluating the efficacy of stem cells for therapeutic reversal of inborn errors of development.

Milieu-Adopted in vitro and in vivo Differentiation of Mesenchymal Tissues Derived from Different Adult Human CD34-Negative Progenitor Cell Clones

Adult mesenchymal stem cells with multilineage differentiation potentially exist in the bone marrow, but have also been isolated from the peripheral blood. The differentiation of stem cells after leaving their niches depends predominately on the local milieu and its new microenvironment, and is facilitated by soluble factors but also by the close cell-cell interaction in a three-dimensional tissue or organ system. We have isolated CD34-negative, mesenchymal stem cell lines from human bone marrow and peripheral blood and generated monoclonal cell populations after immortalization with the SV40 large T-antigen. The cultivation of those adult stem cell clones in an especially designed in vitro environment, including self-constructed glass capillaries with defined growth conditions, leads to the spontaneous establishment of pleomorphic three-dimensional cell aggregates (spheroids) from the monoclonal cell population, which consist of cells with an osteoblast phenotype and areas of mineralization along with well-vascularized tissue areas. Modifications of the culture conditions favored areas of bone-like calcifications. After the transplantation of the at least partly mineralized human spheroids into different murine soft tissue sites but also a dorsal skinfold chamber, no further bone formation could be observed, but angiogenesis and neovessel formation prevailed instead, enabling the transplanted cells and cell aggregates to survive. This study provides evidence that even monoclonal adult human CD34-negative stem cells from the bone marrow as well as peripheral blood can potentially differentiate into different mesenchymal tissues depending on the local milieu and responding to the needs within the microenvironment.

Transcriptional regulation of the SCL locus: identification of an enhancer that targets the primitive erythroid lineage in vivo

The stem cell leukemia (SCL) gene, also known as TAL-1, encodes a basic helix-loop-helix protein that is essential for the formation of all hematopoietic lineages, including primitive erythropoiesis. Appropriate transcriptional regulation is essential for the biological functions of SCL, and we have previously identified five distinct enhancers which target different subdomains of the normal SCL expression pattern. However, it is not known whether these SCL enhancers also regulate neighboring genes within the SCL locus, and the erythroid expression of SCL remains unexplained. Here, we have quantitated transcripts from SCL and neighboring genes in multiple hematopoietic cell types. Our results show striking coexpression of SCL and its immediate downstream neighbor, MAP17, suggesting that they share regulatory elements. A systematic survey of histone H3 and H4 acetylation throughout the SCL locus in different hematopoietic cell types identified several peaks of histone acetylation between SIL and MAP17, all of which corresponded to previously characterized SCL enhancers or to the MAP17 promoter. Downstream of MAP17 (and 40 kb downstream of SCL exon 1a), an additional peak of acetylation was identified in hematopoietic cells and was found to correlate with expression of SCL but not other neighboring genes. This +40 region is conserved in human-dog-mouse-rat sequence comparisons, functions as an erythroid cell-restricted enhancer in vitro, and directs beta-galactosidase expression to primitive, but not definitive, erythroblasts in transgenic mice. The SCL +40 enhancer provides a powerful tool for studying the molecular and cellular biology of the primitive erythroid lineage.

Adult stem cell lines in regenerative medicine and reconstructive surgery

In recent years, there has been a tremendous increase in the understanding of stem cell biology. The potential clinical applications lead to an extended interest in the use of stem cells in many medical disciplines. Multipotent adult stem cells seem to be almost comparable to embryonic stem cells with respect to their ability to differentiate into various tissues in vitro and in vivo, a function that has been termed "stem cell plasticity". In vivo experiments in rodents have shown that adult stem cells participate in tissue- and organ regeneration in almost all lesions. Although stem cell populations isolated from the bone marrow are usually a heterogeneous mix of different subpopulations, cloned adult stem cell lines from any source also show a broad spectrum of differentiation potential, e.g., osteogenesis, myogenesis, neurogenesis, or angiogenesis in wound healing. Angiogenesis in particular is a subject in tissue regeneration with tremendous implication in reconstructive surgery. This comprehensive plasticity makes it possible to use stem cell lines for biomedical research, tissue engineering, regenerative surgery, and organ repair. Adult stem cell lines are molecularly well defined with respect to transcription factors, active signal transduction pathways, and expression of receptors/ligand pairs. We performed experiments with adult stem cell lines, which are not subject to stem cell heterogeneity. Results obtained with stem cell lines can reliably be ascribed to the stem cell population under scrutiny. Adult stem cell lines can be obtained with the necessary quality and quantity also to study many effects of human stem cells in vitro and in vivo. In this paper, we summarize some of the tremendous therapeutic implications of adult stem cell lines in surgery and surgical research.

Improved Arteriogenesis with Simultaneous Skeletal Muscle Repair in Ischemic Tissue by SCL+ Multipotent Adult Progenitor Cell Clones from Peripheral Blood

BACKGROUND

The CD34(-) murine stem cell line RM26 cloned from peripheral blood mononuclear cells has been shown to generate hematopoietic progeny in lethally irradiated animals. The peripheral blood-derived cell clones expresses a variety of mesodermal and erythroid/myeloid transcription factors suggesting a multipotent differentiation potential like the bone marrow-derived 'multipotent adult progenitor cells' (MAP-C).

METHODS

SCL(+) CD34(-) RM26 cells were transfused intravenously into mice suffering from chronic hind-limb ischemia, evaluating the effect of stem cells on collateral artery growth and simultaneous skeletal muscle repair.

RESULTS

RM26 cells are capable of differentiating in vitro into endothelial cells when cultured on the appropriate collagen matrix. Activation of the SCL stem cell enhancer (SCL(+)) is mediated through the binding to two Ets and one GATA site and cells start to express milieu- and growth condition-dependent levels of the endothelial markers CD31 (PECAM) and Flt-1 (VEGF-R1). Intravenously infused RM26 cells significantly improved the collateral blood flow (arteriogenesis) and neo-angiogenesis formation in a murine hind-limb ischemia transplant model. Although transplanted RM26 cells did not integrate into the growing collateral arteries, cells were found adjacent to local arteriogenesis, but instead integrated into the ischemic skeletal muscle exclusively in the affected limb for simultaneous tissue repair.

CONCLUSION

These data suggest that molecularly primed hem-/mesangioblast-type adult progenitor cells can circulate in the peripheral blood improving perfusion of tissues with chronic ischemia and extending beyond the vascular compartment.

Cell therapy: filling the gap between basic science and clinical trials October 15-17, 2001, Rome, Italy

Summarized here, and in forthcoming issues of, are the concepts that emerged at a recent international workshop on cell therapy organized by The Istituto Superiore di Sanità in Rome in collaboration with Istituto Dermatopatico dell'Immacolata, Rome; Istituto Nazionale Ricerca Cancro-Centro Biotecnologie Avanzate, Genova; and University G. D'Annunzio, Chieti. The meeting intent was to provide an overview of the most recent developments in cell therapy, the future perspectives for these clinical trials, and the regulatory issues they involve, as well as a progress report on the clinical protocols that have been approved up to now in Italy. The meeting included six scientific sessions (Immunotherapy, Epithelium, Osteoregeneration, Hematopoiesis, Future Perspectives, and Overview of the National and International Regulations) and involved lectures from Italian and foreign scientists.

Identification of a putative pathway for the muscle homing of stem cells in a muscular dystrophy model

Attempts to repair muscle damage in Duchenne muscular dystrophy (DMD) by transplanting skeletal myoblasts directly into muscles are faced with the problem of the limited migration of these cells in the muscles. The delivery of myogenic stem cells to the sites of muscle lesions via the systemic circulation is a potential alternative approach to treat this disease. Muscle-derived stem cells (MDSCs) were obtained by a MACS(R) multisort method. Clones of MDSCs, which were Sca-1+/CD34-/L-selectin+, were found to adhere firmly to the endothelium of mdx dystrophic muscles after i.v. or i.m. injections. The subpopulation of Sca-1+/CD34- MDSCs expressing L-selectin was called homing MDSCs (HMDSCs). Treatment of HMDSCs with antibodies against L-selectin prevented adhesion to the muscle endothelium. Importantly, we found that vascular endothelium from striate muscle of young mdx mice expresses mucosal addressin cell adhesion molecule-1 (MAdCAM-1), a ligand for L-selectin. Our results showed for the first time that the expression of the adhesion molecule L-selectin is important for muscle homing of MDSCs. This discovery will aid in the improvement of a potential therapy for muscular dystrophy based on the systemic delivery of MDSCs.

Highly efficient retroviral gene transfer into immortalized CD34(-) cells and organ distribution after transplantation into NOD/SCID mice

BACKGROUND

CD34(-) stem cells are apparently the earliest progenitors of hematopoiesis and mesenchymal tissues. The majority of those progeny rests in the BM as fibroblast-like cells, but can also circulate the peripheral blood. Nevertheless, CD34(-), fibroblast-like cells can be isolated from BM aspirates and PBMC, mediated by their ability to adhere to the plastic surface of tissue culture flasks. In standard colony assays, CD34(-), fibroblast-like cells produce a significant number of colony-forming-units (CFUs), mainly CFU-F (fibroblast).

METHODS

Despite advanced cell-culture techniques and the application of various growth factors, the life span of those multipotent stem cells is limited. Therefore, we immortalized and cloned fibroblast-like, CD34(-) stem cells and used retroviral constructs containing the green-fluorescence protein (GFP) to determine the gene-transfer efficiency and their use for gene marking prior to transplantation into NOD/SCID mice.

RESULTS

We could demonstrate a highly efficient retroviral gene transfer into those immortalized CD34(-), fibroblast-like hematopoietic cells (up to 95% transduced cells), maintaining their ability to produce CFUs, as well as a distinct organ distribution after transplantation into the recipient animals, functioning as SCID-repopulating cells (SRC). Transplanted cells could be detected in the BM, as well as other parenchymal organs, such as the lung, liver, skin, small intestine and brain.

DISCUSSION

CD34(-), fibroblast-like progenitor cells can give rise to hematopoietic progeny, but also home to mesenchymal organ sites in recipient animals. There is increasing evidence that pluripotent CD34(-) stem cells can be isolated from various sources and still maintain their capabilities to generate progeny of different tissues. This could be a promising approach to using peripheral-blood derived stem cells for cellreplacement therapy and tissue engineering.

GATA transcription in a small rhodamine 123(low)CD34(+) subpopulation of a peripheral blood-derived CD34(-)CD105(+) mesenchymal cell line

OBJECTIVE

Based on previous animal experiments that suggest the plasticity of peripheral blood-derived, CD34(-) stem cell lines, the aim of this study was to isolate CD34(-) stem cell lines from human peripheral blood cells and obtain evidence of their multipotency and plasticity.

MATERIALS AND METHODS

Adherent growing cells were isolated from peripheral blood mononuclear cells from a healthy volunteer donor and different cell clones were established after SV40 large-T-antigen-mediated immortalization. The immunophenotype of the cell lines was investigated by flow cytometry. One particular cell clone, V54/2, was stained with rhodamine 123, and the Rh123(low) and Rh123(high) subpopulations were sorted for a reverse transcriptase polymerase chain reaction gene expression survey and distinct differences in morphology and biologic behavior.

RESULTS

The peripheral blood-derived and fibroblast-like cell line V54/2 expressed high levels of CD10 and CD105 and showed only a very low level expression of CD34 (<1.0%) and CD117 (c-kit). Among the entire CD34(-)CD105(+) cell population that transcribed factors such as Myb, Tie-1, and VEGF, there was a small Rh123(low)CD34(+) subpopulation that transcribed significant levels of several members of the GATA family of transcription factors. The morphology of the Rh123(low)CD34(+) (also expressing the P-glycoprotein) was different compared to the Rh123(high)CD34(-) population. Mesenchymal differentiation into glial fibrillary acidic protein (GFAP)(+) glial cells could be shown from the entire CD34(-)CD105(+) cell population.

CONCLUSIONS

The findings provide evidence that it is possible to isolate CD34(-)CD105(+) mesenchymal stem cell lines from human peripheral blood cells that contain a small subpopulation of CD34(+) and GATA-transcribing cells. Those cells are potential hematopoietic progenitors and can be recruited from the CD34(-) stem cell pool. The plasticity of stem cells seems to require essential molecular tools, such as a panel of transcription factors, to respond to the environmental demand within a biologic system.

The co-expression of CD117 (c-kit) and osteocalcin in activated bone marrow stem cells in different diseases

It is still difficult to identify a potential stem cell in the bone marrow which can give rise to haematopoiesis and mesenchymal cells. In the past, the stem cells for both tissues were considered to be from different stem cell pools, but it has been shown recently in vitro and in vivo that there is an unexpected plasticity at least among early haemopoietic progenitors which can give rise also to mesenchymal tissue. In an attempt to identify stem cells in the bone marrow, which are common precursors to both lineages, we observed that fibroblast-like periosteal cells changed their morphology towards an osteoblastic differentiation with a more cuboidal or triangular morphology especially close to metastatic infiltrates. As a marker for haemopoietic progenitors, we used an antibody against the tyrosine kinase receptor c-kit (CD117) and an anti-osteocalcin antibody to stain mesenchymal cells with a osteoblastic potential. Normal bone marrow specimens only showed a discrete expression pattern of CD117 and osteocalcin, but periosteal stem cells, which strongly co-express the applied haemopoietic and mesenchymal markers, were found particularly in the bone marrow of patients with infiltrates of malignant lymphoma or metastasis from prostate or breast cancer. The evaluation of bone marrow specimens from patients with an aplastic syndrome or myelodysplastic syndrome showed a more heterogenous expression pattern. Our results show that a stem cell candidate common to haematopoiesis and mesenchymal progeny can be detected in bone marrow specimens after activation, as demonstrated by the co-expression of CD117 and osteocalcin, which also seems to be associated with haematological diseases or metastatic infiltration in the bone marrow.

Intraarterial injection of muscle-derived CD34(+)Sca-1(+) stem cells restores dystrophin in mdx mice

Duchenne muscular dystrophy is a lethal recessive disease characterized by widespread muscle damage throughout the body. This increases the difficulty of cell or gene therapy based on direct injections into muscles. One way to circumvent this obstacle would be to use circulating cells capable of homing to the sites of lesions. Here, we showed that stem cell antigen 1 (Sca-1), CD34 double-positive cells purified from the muscle tissues of newborn mice are multipotent in vitro and can undergo both myogenic and multimyeloid differentiation. These muscle-derived stem cells were isolated from newborn mice expressing the LacZ gene under the control of the muscle-specific desmin or troponin I promoter and injected into arterial circulation of the hindlimb of mdx mice. The ability of these cells to interact and firmly adhere to endothelium in mdx muscles microcirculation was demonstrated by intravital microscopy after an intraarterial injection. Donor Sca-1, CD34 muscle-derived stem cells were able to migrate from the circulation into host muscle tissues. Histochemical analysis showed colocalization of LacZ and dystrophin expression in all muscles of the injected hindlimb in all of five out of five 8-wk-old treated mdx mice. Their participation in the formation of muscle fibers was significantly increased by muscle damage done 48 h after their intraarterial injection, as indicated by the presence of 12% beta-galactosidase-positive fibers in muscle cross sections. Normal dystrophin transcripts detected enzymes in the muscles of the hind limb injected intraarterially by the mdx reverse transcription polymerase chain reaction method, which differentiates between normal and mdx message. Our results showed that the muscle-derived stem cells first attach to the capillaries of the muscles and then participate in regeneration after muscle damage.

Hematopoietic stem cells can be CD34+ or CD34-

Until recently, it was thought that the most primitive HSC have a fixed phenotype within a hierarchical differentiation system, and that changes in engraftment and renewal potential occur in a stepwise fashion linked with differentiation. In this review, we summarize the data from several different species and different animal models of hematopoietic stem cell function. Taking into account all of the published data, it becomes clear that the hematopoietic stem cell compartment contains more than one phenotypically identifiable population capable of self-renewal and long term pluripotent engraftment. It is clear that some stem cells express CD34, and others do not. The exact phenotypic progression between these cells needs to be further defined, because different in vivo and ex vivo manipulations may shift the stem cells from one phenotype to another, and this can complicate interpretation of experimental transplant data.

Perspectives on the morphology and biology of CD34-negative stem cells

The CD34 antigen is the classical indicator molecule of pluripotent hematopoietic stem cells. But there is more and more evidence that progenitors of a yet uncommitted stem cell population do not express this surrogate marker. The bone marrow and other sites of hematopoiesis consist also of fibroblast-like stromal cells, quiescent hematopoietic stem cells, and mesenchymal stem cells. Depending on their stage of differentiation, CD34- stem cells cannot only generate hematopoietic progenitors, but also more specified mesenchymal precursors, such as osteoblasts, chondrocytes, myocytes, adipocyts, and others. The stromal cell compartment produces not only matrix proteins, such as collagens, fibronectin and others, but also the essential growth factors, which initiate and support the differentiation of primary quiescent, but eventually activated CD34- stem cells into CD34+ hematopoietic progenitors. In vivo studies have shown that long-term hematopoietic and mesenchymal reconstitution can be achieved with CD34- stem cell lines, isolated from various sources, although the frequency of CD34- stem cells seams to be quite low among the progenitor population. Some authors deny the reconstitution ability of CD34- cells. The majority of CD34- stem cells are quiescent fibroblast-like cells, which can be identified in the bone marrow biopsy as "bone lining cells". Some of those bone lining cells show protein synthesis and contain secretory vesicles. Recent studies have demonstrated that there is a surprising plasticity of the earliest stem cell population, consisting of cells with stromal cell function as well as hematopoietic and mesenchymal progenitors. The new insights into the biology of totipotent stem cells give us novel perspectives for cell- and gene therapy of various malignant and nonmalignant diseases and the possibility to replace defective organ functions with autologous CD34- stem cells.

Predictive factors for long-term engraftment of autologous blood stem cells

Data from 170 consecutive patients aged 19-66 years (median age 46 years) who underwent unmanipulated autologous blood stem cell transplant (ASCT) were analyzed to determine if total CD34+ cells/kg infused, CD34+ subsets (CD34+41+, CD34+90+, CD34+33-, CD34+38-, CD34+38-DR-), peripheral blood CD34+ cell (PBCD34+) count on first apheresis day, or various clinical factors were associated with low blood counts 6 months post ASCT. Thirty-four patients were excluded from analysis either because of death (n = 17) or re-induction chemotherapy prior to 6 months post ASCT (n = 13), or because of lack of follow-up data (n = 4). Of the remaining 136 patients, 46% had low WBC ( < 4 x 10(9)/l), 41% low platelets (<150 x 10(9)/l), and 34% low hemoglobin ( < 120 g/l) at a median of 6 months following ASCT. By Spearman's rank correlation, both the total CD34+ cell dose/kg and the PBCD34+ count correlated with 6 month blood counts better than any subset of CD34+ cells or any clinical factor. The PBCD34+ count was overall a stronger predictor of 6 month blood counts than was the total CD34+ cells/kg infused. Both factors retained their significance in multivariate analysis, controlling for clinical factors. In conclusion, subsets of CD34+ cells and clinical factors are inferior to the total CD34+ cell dose/kg and PBCD34+ count in predicting 6 month blood counts following ASCT.

Analysis of the microenvironment necessary for engraftment: role of the vascular smooth muscle-like stromal cells

This is a review of recent data concerning the phenotype of human and murine stroma, as grown in long-term cultures. Using data on cytoskeletal and extracellular matrix protein expression, a sequential model of differentiation from mesenchymal (stem) cells to vascular-smooth muscle-like stromal cells is proposed. This model would apply, at least in the mouse, to stromal cells generated from different sites of hematopoiesis (bone marrow, fetal liver, spleen, and yolk sac). The in vivo counterparts of vascular-smooth muscle-like stromal cells in the different sites of definitive hematopoiesis are discussed.

Comparison of the hematopoietic activity of flt-3 ligand and granulocyte-macrophage colony-stimulating factor acting alone or in combination

The hematopoietic sequelae of intramuscular administration of flt-3 ligand (FL) and granulocyte-macrophage colony-stimulating factor (GM-CSF) alone, or in combination, were compared in BALB/c mice. Changes in hematopoiesis were measured in the marrow, spleen and blood using an in vitro colony-forming unit (CFU) assay and flow cytometrically (expression of CD34 and stem cell antigen (Sca)-1). FL administration was associated with a significant increase in the absolute number of CFU and CD34+ cells in the marrow and CFU, CD34+, Sca-1+, and CD34+ Sca-1+ cells in the spleen and blood. These data demonstrate that FL expands and mobilizes a range of hematopoietic progenitors. By comparison, GM-CSF administration was associated with a significant increase in the number of CFU in the spleen and a significant reduction in marrow CD34+, Sca-1+, and CD34+Sca-1+ cells. These data suggest that GM-CSF-driven expansion of CFU may be at the expense of more primitive cells. The pattern of progenitor cell expansion associated with FL + GM-CSF administration was similar to that of FL alone with the following exceptions. The numbers of spleen and blood CFU were significantly greater and the number of marrow CD34+Sca-1+ cells were significantly less, than with FL alone. These data suggest that co-administration of these cytokines may combine the expansion of the more primitive cell populations (associated with FL) with the expansion of the more mature CFU population (associated with GM-CSF) to yield a greater overall CFU expansion and elevation of CFU in the blood. However, increasing the expansion and mobilization of the relatively mature, rather than the more primitive, hematopoietic progenitors, may be of limited value as a mobilization strategy, if the goal is the expansion and isolation of increased numbers of "high-quality," primitive cells for transplantation.

Evidence of peripheral blood-derived, plastic-adherent CD34(-/low) hematopoietic stem cell clones with mesenchymal stem cell characteristics

The hematopoietic system of vertebrates can be completely reconstituted with hematopoietic stem cells derived from the bone marrow, fetal liver, or cord blood, or even from peripheral-blood-derived cells. A cellular marker to identify those cells is the proteoglycan CD34, although we have shown that the earliest identifiable hematopoietic stem cell is a CD34(-) fibroblast-like cell which can differentiate into CD34(+) hematopoietic precursors. Peripheral blood mononuclear cells were isolated from the heparinized blood of a dog and incubated in tissue culture in the presence of interleukin 6. After 10-14 days, an adherent layer of fibroblast-like cells had developed and cells were immortalized using the SV-40 large T antigen. Cells were cloned and subcloned by measures of limiting dilution, and various fibroblast-like clones were established. These fibroblast-like cells either do not express the CD34 antigen or express CD34 on a low level, although transcribing CD34. The CD34(-/low) cells express osteocalcin as a mesenchymal cell marker. The fibroblast-like cells eventually differentiate spontaneously in vitro into CD34(+) precursors and show colony formation. Prior to autologous stem cell transplantation, one clone of choice (IIIG7) was transfected with a retroviral construct containing the green-fluorescence protein (GFP). The recipient dog was totally irradiated with 300 cGy and received a stem cell transplant with GFP-containing, immortalized, fibroblast-like monoclonal autologous stem cells (0.5 x 10(8)/kg dog). No additional growth factors were applied. The peripheral blood counts recovered after 23 days (WBC >500; platelets >10,000). A peripheral blood smear showed some dim but definite, although timely, limited expression of the GFP protein in nucleated peripheral blood cells just five weeks after transplantation. A bone marrow biopsy showed GFP-positive cells in the marrow cavity predominantly as "bone-lining cells."

Isolation of primary and immortalized CD34-hematopoietic and mesenchymal stem cells from various sources

Based on historical radiation experiments in rodents, the hematopoietic stem cell was defined by its biological properties and later by the expression of certain surface antigens (e.g., CD34), as well as the absence of lineage-specific markers (e.g., DR). Quite recently it was shown that hematopoietic reconstitution can also be achieved by CD34- stem cells, which can be isolated from the bone marrow, peripheral blood and cord blood cells. CD34-stem cells are considered to be predominately part of the quiescent stem cell pool of hematopoietic and mesenchymal stem cells. Due to novel techniques, CD34-stem cells can be expanded on the level of a true stem cell but also directed towards their differentiation into specified tissues or organ systems. This requires the establishment of primary fibroblast-like CD34- stem cells in vitro and their possible reversible and transient immortalization with optimized vector systems.