Isolating and defining cells to engineer human blood vessels

A great deal of attention has been recently focused on understanding the role that bone marrow-derived putative endothelial progenitor cells (EPC) may play in the process of neoangiogenesis. However, recent data indicate that many of the putative EPC populations are comprised of various haematopoietic cell subsets with proangiogenic activity, but these marrow-derived putative EPC fail to display vasculogenic activity. Rather, this property is reserved for a rare population of circulating viable endothelial cells with colony-forming cell (ECFC) ability. Indeed, human ECFC possess clonal proliferative potential, display endothelial and not haematopoietic cell surface antigens, and display in vivo vasculogenic activity when suspended in an extracellular matrix and implanted into immunodeficient mice. Furthermore, human vessels derived became integrated into the murine circulatory system and eventually were remodelled into arterial and venous vessels. Identification of this population now permits determination of optimal type I collagen matrix microenvironment into which the cells should be embedded and delivered to accelerate and even pattern number and size of blood vessels formed, in vivo. Indeed, altering physical properties of ECFC-collagen matrix implants changed numerous parameters of human blood vessel formation, in host mice. These recent discoveries may permit a strategy for patterning vascular beds for eventual tissue and organ regeneration.

Collagen oligomers modulate physical and biological properties of three-dimensional self-assembled matrices

Elucidation of mechanisms underlying collagen fibril assembly and matrix-induced guidance of cell fate will contribute to the design and expanded use of this biopolymer for research and clinical applications. Here, we define how Type I collagen oligomers affect in-vitro polymerization kinetics as well as fibril microstructure and mechanical properties of formed matrices. Monomers and oligomers were fractionated from acid-solubilized pig skin collagen and used to generate formulations varying in monomer/oligomer content or average polymer molecular weight (AMW). Polymerization half-times decreased with increasing collagen AMW and closely paralleled lag times, indicating that oligomers effectively served as nucleation sites. Furthermore, increasing AMW yielded matrices with increased interfibril branching and had no correlative effect on fibril density or diameter. These microstructure changes increased the stiffness of matrices as evidenced by increases in both shear storage and compressive moduli. Finally, the biological relevance of modulating collagen AMW was evidenced by the ability of cultured endothelial colony forming cells to sense associated changes in matrix physical properties and alter vacuole and capillary-like network formation. This work documents the importance of oligomers as another physiologically-relevant design parameter for development and standardization of polymerizable collagen formulations to be used for cell culture, regenerative medicine, and engineered tissue applications.

Collagen matrix physical properties modulate endothelial colony forming cell-derived vessels in vivo

Developing tissue engineering approaches to generate functional vascular networks is important for improving treatments of peripheral and cardiovascular disease. Endothelial colony forming cells (ECFCs) are an endothelial progenitor cell (EPC) population defined by high proliferative potential and an ability to vascularize collagen-based matrices in vivo. Little is known regarding how physical properties of the local cell microenvironment guide vessel formation following EPC transplantation. In vitro evidence suggests that collagen matrix stiffness may modulate EPC vessel formation. The present study determined the ability of 3D collagen matrix physical properties, varied by changing collagen concentration, to influence ECFC vasculogenesis in vivo. Human umbilical cord blood ECFCs were cultured within matrices for 18 h in vitro and then fixed for in vitro analysis or implanted subcutaneously into the flank of immunodeficient mice for 14 days. We report that increasing collagen concentration significantly decreased ECFC derived vessels per area (density), but significantly increased vessel sizes (total cross sectional area). These results demonstrate that the physical properties of collagen matrices influence ECFC vasculogenesis in vivo and that by modulating these properties, one can guide vascularization.

Retroviral vector integration in post-transplant hematopoiesis in mice conditioned with either submyeloablative or ablative irradiation

X-linked chronic granulomatous disease (X-CGD) is an inherited immunodeficiency with absent phagocyte NADPH oxidase activity caused by defects in the gene encoding gp91^phox. Here we evaluated strategies for less intensive conditioning for gene therapy of genetic blood disorders without selective advantage for gene correction, such as might be used in a human X-CGD protocol. We compared submyeloablative with ablative irradiation as conditioning in murine X-CGD, examining engraftment, oxidase activity and vector integration in mice transplanted with marrow transduced with a gamma-retroviral vector for gp91^phox expression. The frequency of oxidase-positive neutrophils in the donor population was unexpectedly higher in many 300 cGy-conditioned mice compared to lethally irradiated recipients, as was the fraction of vector-marked donor secondary CFU-S12. Vector integration sites in marrow, spleen, and secondary CFU-S12 DNA from primary recipients were enriched for cancer-associated genes, including Evi1, and integrations in or near cancer-associated genes were more frequent in marrow and secondary CFU-S12 from 300 cGy-conditioned mice compared to fully ablated mice. These findings support the concept that vector integration can confer a selection bias, and suggest that intensity of the conditioning regimen may further influence the effects of vector integration on clonal selection in post-transplant engraftment and hematopoiesis.

Cleaved high molecular weight kininogen inhibits tube formation of endothelial progenitor cells via suppression of matrix metalloproteinase 2

BACKGROUND AND OBJECTIVE

Endothelial progenitor cells (EPCs) contribute to postnatal neovascularization, thus promoting wide interest in their therapeutic potential in vascular injury and prevention of their dysfunction in cardiovascular diseases. Cleaved high molecular weight kininogen (HKa), an activation product of the plasma kallikrein-kinin system (KKS), inhibits the functions of differentiated endothelial cells including in vitro and in vivo angiogenesis. In this study, our results provided the first evidence that HKa is able to target EPCs and inhibits their tube forming capacity.

METHODS AND RESULTS

We determined the effect of HKa on EPCs using a three-dimensional vasculogenesis assay. Upon stimulation with vascular endothelial growth factor (VEGF) alone, EPCs formed vacuoles and tubes, and differentiated into capillary-like networks. As detected by gelatinolytic activity assay, VEGF stimulated secretion and activation of matrix metallopeptidase 2 (MMP-2), but not MMP-9, in the conditioned medium of 3D culture of EPCs. Specific inhibition or gene ablation of MMP-2, but not MMP-9, blocked the vacuole and tube formation by EPCs. Thus, MMP-2 is selectively required for EPC vasculogenesis. In a concentration-dependent manner, HKa significantly inhibited tube formation by EPCs and the conversion of pro-MMP-2 to MMP-2. Moreover, HKa completely blocked the association between pro-MMP-2 and alphavbeta3 integrin, and its inhibition of MMP-2 activation was dependent on the presence of alphavbeta3 integrin. In a purified system, HKa did not directly inhibit MMP-2 activity.

CONCLUSIONS

HKa inhibits tube forming capacity of EPCs by suppression of MMP-2 activation, which may constitute a novel link between activation of the KKS and EPC dysfunction.

Cleaved high molecular weight kininogen inhibits tube formation of endothelial progenitor cells via suppression of matrix metalloproteinase 2

BACKGROUND AND OBJECTIVE

Endothelial progenitor cells (EPCs) contribute to postnatal neovascularization, thus promoting wide interest in their therapeutic potential in vascular injury and prevention of their dysfunction in cardiovascular diseases. Cleaved high molecular weight kininogen (HKa), an activation product of the plasma kallikrein-kinin system (KKS), inhibits the functions of differentiated endothelial cells including in vitro and in vivo angiogenesis. In this study, our results provided the first evidence that HKa is able to target EPCs and inhibits their tube forming capacity.

METHODS AND RESULTS

We determined the effect of HKa on EPCs using a three-dimensional vasculogenesis assay. Upon stimulation with vascular endothelial growth factor (VEGF) alone, EPCs formed vacuoles and tubes, and differentiated into capillary-like networks. As detected by gelatinolytic activity assay, VEGF stimulated secretion and activation of matrix metallopeptidase 2 (MMP-2), but not MMP-9, in the conditioned medium of 3D culture of EPCs. Specific inhibition or gene ablation of MMP-2, but not MMP-9, blocked the vacuole and tube formation by EPCs. Thus, MMP-2 is selectively required for EPC vasculogenesis. In a concentration-dependent manner, HKa significantly inhibited tube formation by EPCs and the conversion of pro-MMP-2 to MMP-2. Moreover, HKa completely blocked the association between pro-MMP-2 and alphavbeta3 integrin, and its inhibition of MMP-2 activation was dependent on the presence of alphavbeta3 integrin. In a purified system, HKa did not directly inhibit MMP-2 activity.

CONCLUSIONS

HKa inhibits tube forming capacity of EPCs by suppression of MMP-2 activation, which may constitute a novel link between activation of the KKS and EPC dysfunction.

Defining human endothelial progenitor cells

There is no specific marker to identify an endothelial progenitor cell (EPC) and this deficiency is restricting the ability of an entire field of research in defining these cells. We will review current methods to define EPC in the human system and suggest approaches to define better the cell populations involved in neoangiogenesis. PubMed was used to identify articles via the search term 'endothelial progenitor cell' and those articles focused on defining the term were evaluated. The only human cells expressing the characteristics of an EPC, as originally proposed, are endothelial colony forming cells. A variety of hematopoietic cells including stem and progenitors, participate in initiating and modulating neoangiogenesis. Future studies must focus on defining the specific hematopoietic subsets that are involved in activating, recruiting, and remodeling the vascular networks formed by the endothelial colony forming cells.

Working hypothesis to redefine endothelial progenitor cells

Since 1997, postnatal vasculogenesis has been purported to be an important mechanism for neoangiogenesis via bone marrow (BM)-derived circulating endothelial progenitor cells (EPCs). Based on this paradigm, EPCs have been extensively studied as biomarkers to assess severity of cardiovascular disease and as a cell-based therapy for several human cardiovascular disorders. In the majority of studies to date, EPCs were identified and enumerated by two primary methodologies; EPCs were obtained and quantified following in vitro cell culture, or EPCs were identified and enumerated by flow cytometry. Both methods have proven controversial. This review will attempt to outline the definition of EPCs from some of the most widely cited published reports in an effort to provide a framework for understanding subsequent studies in this rapidly evolving field. We will focus this review on studies that used cell culture techniques to define EPCs.

Improved transduction of primary murine hepatocytes by recombinant adeno-associated virus 2 vectors in vivo

Adeno-associated virus 2 (AAV) vectors are currently in use in Phase I/II clinical trials for gene therapy of cystic fibrosis and hemophilia B. Although 100% of murine hepatocytes can be targeted by AAV vectors, the transgene expression is limited to approximately 5% of hepatocytes. Since the viral genome is a single-stranded DNA, and single strands of both polarities are encapsidated with equal frequency, it has been suggested that failure to undergo DNA strand-annealing accounts for the lack of efficient transgene expression. We and others, on the other hand, have proposed that failure to undergo viral second-strand DNA synthesis attributes to the observed low efficiency of transgene expression. We have previously documented that a cellular protein, designated FKBP52, when present in phosphorylated forms, inhibits the viral second-strand DNA synthesis, and consequently, limits transgene expression in nonhepatic cells, whereas unphosphorylated forms of FKBP52 have no effect. To further evaluate whether phosphorylated FKBP52 is also involved in regulating AAV-mediated transgene expression in murine hepatocytes, we generated transgenic mice overexpressing the cellular T-cell protein tyrosine phosphatase (TC-PTP) protein, known to catalyze dephosphorylation of FKBP52, as well as mice deficient in FKBP52. We demonstrate here that dephosphorylation of FKBP52 in TC-PTP transgenic (TC-PTP-TG) mice, and removal of FKBP52 in FKBP52-knockout (FKBP52-KO) mice results in efficient transduction of murine hepatocytes following tail-vein injection of recombinant AAV vectors. We also document efficient viral second-strand DNA synthesis in hepatocytes from both TC-PTP-TG and FKBP52-KO mice. Thus, our data strongly support the contention that the viral second-strand DNA synthesis, rather than DNA strand-annealing, is the rate-limiting step in the efficient transduction of hepatocytes, which should have implications in the optimal use of recombinant AAV vectors in human gene therapy.

Embryonic hematopoiesis in mice and humans

AIM

The aim of this report is to review briefly the ontogeny of hematopoiesis in mice and humans and to discuss recent evidence for an intraembryonic source of hematopoietic stem cells.

METHODS

The hematopoiesis overview summarizes information present in the PubMed online database and from experiments conducted in our laboratories.

RESULTS

The major sites of hematopoiesis change throughout development in mice and humans. Recent evidence suggests that hematopoietic cells may emerge from mesoderm precursors within the embryo as well as in the yolk sac.

CONCLUSION

The ontogeny of hematopoiesis is similar in mice and humans. The murine system is a useful model to study the earliest events involved in forming hematopoietic stem cells.

Homing, cell cycle kinetics and fate of transplanted hematopoietic stem cells

Homing of transplanted hematopoietic stem cells to recipient bone marrow is a critical step in engraftment and initiation of marrow reconstitution. At present, only partial understanding of the cellular and molecular mechanisms governing homing exists. Likewise, only an incomplete list of adhesion molecules implicated in directing the trafficking of stem cells to the marrow microenvironment is available. Opposing hypotheses that attribute homing to an orderly and orchestrated cascade of events or to random migration of circulating cells find ample experimental support. Also unsettled is the fate of marrow-homed cells shortly after transplantation and the rapidity at which they begin to proliferate in their new marrow microenvironment. The limited number of studies in this field and disparities in their experimental design intensifies the confusion surrounding these critical aspects of stem cell biology. However, this area of research is moving forward rapidly and results capable of clarifying many of these issues are forthcoming.

Yolk-sac hematopoiesis: the first blood cells of mouse and man

OBJECTIVE

To review the process of blood-cell formation in the murine and human yolk sac.

DATA SOURCES

Most articles were selected from the PubMed database.

DATA SYNTHESIS

The yolk sac is the first site of blood-cell production during murine and human ontogeny. Primitive erythroid cells originate in the yolk sac and complete their maturation, including enucleation, in the bloodstream. Though species differences exist, the pattern of hematopoietic progenitor cell emergence in the yolk sac is similar in mouse and man. In both species, there is a stage of development where both primitive red blood cells and definitive erythroid progenitors are produced in the yolk sac. An "embryonic" hematopoietic stem cell that engrafts in myeloablated newborn but not adult mice can be detected in the murine yolk sac and embryo. Stem-cell activity in the human yolk sac has not been reported.

CONCLUSIONS

The yolk sac is the sole site of embryonic erythropoiesis. However, definitive erythroid, myeloid, and multipotential progenitors also originate in the yolk sac. The relationship between these progenitors and the "embryonic" hematopoietic stem cell has not been elucidated. Yolk sac-derived progenitor cells may seed the developing liver via the circulation and serve as the immediate source of the mature blood cells that are required to meet the metabolic needs of the rapidly growing fetus.

Adeno-associated virus 2-mediated transduction and erythroid lineage-restricted long-term expression of the human beta-globin gene in hematopoietic cells from homozygous beta-thalassemic mice

Adeno-associated virus 2 (AAV), a nonpathogenic human parvovirus, has gained attention as a potentially useful vector for human gene therapy. Here, we report successful AAV-mediated stable transduction and high-efficiency, long-term, erythroid lineage-restricted expression of a human beta-globin gene in primary murine hematopoietic stem cells in vivo. Bone marrow-derived primitive Sca-1(+), lin(-) hematopoietic stem cells from homozygous beta-thalassemic mice were transduced ex vivo with a recombinant AAV vector containing a normal human beta-globin gene followed by transplantation into low-dose-irradiated B6.c-kitW(41/41) anemic recipient mice. Six months posttransplantation, tail-vein blood samples were analyzed by PCR amplification to document the presence of the transduced human beta-globin gene sequences in the peripheral blood cells. Semiquantitative PCR analyses revealed that the transduced human beta-globin gene sequences were present at approximately 1 copy per cell. The efficiency of the human beta-globin gene expression was determined to be up to 35% compared with the murine endogenous beta-globin gene by semiquantitative RT-PCR analyses. Peripheral blood samples from several positive recipient mice obtained 10 months posttransplantation were fractionated to obtain enriched populations of granulocytes, lymphocytes, and erythroid cells. PCR analyses revealed the presence of the human beta-globin gene sequences in granulocytes and lymphocytes, indicating multilineage reconstitution. However, only the erythroid population was positive following RT-PCR analyses, suggesting lineage-restricted expression of the transduced human beta-globin gene. Southern blot analyses of total genomic DNA samples isolated from bone marrow cells from transplanted mice also documented proviral integration. These results provide further support for the potential use of recombinant AAV vectors in gene therapy of beta-thalassemia and sickle-cell disease.

Recombinant human parvovirus B19 vectors: erythrocyte P antigen is necessary but not sufficient for successful transduction of human hematopoietic cells

The blood group P antigen, known to be abundantly expressed on erythroid cells, has been reported to be the cellular receptor for parvovirus B19. We have described the development of recombinant parvovirus B19 vectors with which high-efficiency, erythroid lineage-restricted transduction can be achieved (S. Ponnazhagan, K. A. Weigel, S. P. Raikwar, P. Mukherjee, M. C. Yoder, and A. Srivastava, J. Virol. 72:5224-5230, 1998). However, since a low-level transduction of nonerythroid cells could also be detected and since P antigen is expressed in nonerythroid cells, we reevaluated the role of P antigen in the viral binding and entry into cells. Cell surface expression analyses revealed that approximately 75% of primary human bone marrow mononuclear erythroid cells and approximately 31% of cells in the nonerythroid population were positive for P antigen. Two human erythroleukemia cell lines, HEL and K562, and a human promyelocytic leukemia cell line, HL-60, were also examined for P antigen expression and binding and entry of the vector. HEL and K562 cells showed intermediate levels, whereas HL-60 cells demonstrated high levels of expression of P antigen. However, the efficiency of vector binding to these cells did not correlate with P antigen expression. Moreover, despite P antigen positivity and efficient viral binding, HEL, K562, and HL-60 cells could not be transduced with the vector. Low levels of P antigen expression could also be detected in two primary cell types, human umbilical vein endothelial cells (HUVEC) and normal human lung fibroblasts (NHLF). In addition, vector binding occurred in both cell types and was inhibited by globoside, indicating the involvement of P antigen in virus binding to these cells. These primary cells could be efficiently transduced with the recombinant vector. These data suggest that (i) P antigen is expressed on a variety of cell types and is involved in binding of parvovirus B19 to human cells, (ii) the level of P antigen expression does not correlate with the efficiency of viral binding, (iii) P antigen is necessary but not sufficient for parvovirus B19 entry into cells, and (iv) parvovirus B19 vectors can be used to transduce HUVEC and NHLF. These studies further suggest the existence of a putative cellular coreceptor for efficient entry of parvovirus B19 into human cells.

Spatial and temporal emergence of high proliferative potential hematopoietic precursors during murine embryogenesis

During mouse embryogenesis, two waves of hematopoietic progenitors originate in the yolk sac. The first wave consists of primitive erythroid progenitors that arise at embryonic day 7.0 (E7.0), whereas the second wave consists of definitive erythroid progenitors that arise at E8.25. To determine whether these unilineage hematopoietic progenitors arise from multipotential precursors, we investigated the kinetics of high proliferative potential colony-forming cells (HPP-CFC), multipotent precursors that give rise to macroscopic colonies when cultured in vitro. No HPP-CFC were found at presomite stages (E6.5-E7.5). Rather, HPP-CFC were detected first at early somite stages (E8.25), exclusively in the yolk sac. HPP-CFC were found subsequently in the bloodstream at higher levels than the remainder of the embryo proper. However, the yolk sac remains the predominant site of HPP-CFC expansion (>100-fold) until the liver begins to serve as the major hematopoietic organ at E11.5. On secondary replating, embryonic HPP-CFC give rise to definitive erythroid and macrophage (but not primitive erythroid) progenitors. Our findings support the hypothesis that definitive but not primitive hematopoietic progenitors originate from yolk sac-derived HPP-CFC during late gastrulation.

Homing and engraftment potential of Sca-1(+)lin(-) cells fractionated on the basis of adhesion molecule expression and position in cell cycle

Engraftment potential of hematopoietic stem cells (HSCs) is likely to be dependent on several factors including expression of certain adhesion molecules (AMs) and degree of mitotic quiescence. The authors investigated the functional properties and engraftment potential of Sca-1(+)lin(-) cells subfractionated on the basis of expression, or lack thereof, of CD11a, CD43, CD49d, CD49e, or CD62L and correlated that expression with cell cycle status and proliferative potential of engrafting fractions. Donor-derived chimerism in mice receiving CD49e(+) or CD43(+) Sca-1(+)lin(-) cells was greater than that in mice receiving cells lacking these 2 markers, while Sca-1(+)lin(-) cells positive for CD11a and CD62L and bright for CD49d expression mediated minimal engraftment. AM phenotypes enriched for engraftment potential contained the majority of high proliferative potential-colony forming cells, low proliferative potential-colony forming cells, and cells providing rapid in vitro expansion. Cell cycle analysis of AM subpopulations revealed that, regardless of their bone marrow repopulating potential, Sca-1(+)lin(-) AM(-) cells contained a higher percentage of cells in G(0)/G(1) than their AM(+) counterparts. Interestingly, engrafting phenotypes, regardless of the status of their AM expression, were quicker to exit G(0)/G(1) following in vitro cytokine stimulation than their opposing phenotypes. When engrafting phenotypes of Sca-1(+)lin(-) AM(+) or AM(-) cells were further fractionated by Hoechst 33342 into G(0)/G(1) or S/G(2)+M, cells providing long-term engraftment were predominantly contained within the quiescent fraction. These results define a theoretical phenotype of a Sca-1(+)lin(-) engrafting cell as one that is mitotically quiescent, CD43(+), CD49e(+), CD11a(-), CD49d(dim), and CD62L(-). Furthermore, these data suggest that kinetics of in vitro proliferation may be a good predictor of engraftment potential of candidate populations of HSCs. (Blood. 2000;96:1380-1387)

Adeno-associated virus 2-mediated transduction and erythroid lineage-restricted expression from parvovirus B19p6 promoter in primary human hematopoietic progenitor cells

Human parvovirus B19 gene expression from the viral p6 promoter (B19p6) is restricted to primary human hematopoietic cells undergoing erythroid differentiation. We have demonstrated that expression from this promoter does not occur in established human erythroid cell lines in the context of a recombinant parvovirus genome (Ponnazhagan et al. J Virol 69:8096-8101, 1995). However, abundant expression from this promoter can be readily detected in primary human bone marrow cells (Wang et al. Proc Natl Acad Sci USA 92:12416-12420, 1995; Ponnazhagan et al. J Gen Virol 77:1111-1122, 1996). In the present studies, we investigated the pattern of expression from the B19p6 promoter in primary human bone marrow-derived CD34+ HPC undergoing differentiation into myeloid and erythroid lineages. CD34+ cells were transduced with recombinant adeno-associated virus 2 (AAV) vectors containing the beta-galactosidase (lacZ) gene under the control of the following promoters/enhancers: the cytomegalovirus promoter (vCMVp-lacZ), B19p6 promoter (vB19p6-lacZ), B19p6 promoter with an upstream erythroid cell-specific enhancer element (HS-2) from the locus control region (LCR) from the human beta-globin gene cluster (vHS2-B19p6-lacZ), and the human beta-globin gene promoter with the HS-2 enhancer (vHS2-beta p-lacZ). Transgene expression was evaluated either 48 h after infection or following erythroid differentiation in vitro for 3 weeks. Whereas high-level expression from the CMV promoter 48 h after infection diminished with time, low-level expression from the B19p6 and the beta-globin promoters increased significantly following erythroid differentiation. Furthermore, in HPC assays, there was no significant difference in the level of expression from the CMV promoter in myeloid or erythroid cell-derived colonies. Expression from the B19p6 and the beta-globin promoters, on the other hand, was restricted to erythroid cell colonies. These data further corroborate that the B19p6 promoter is erythroid cell-specific and suggest that the recombinant AAV-B19 hybrid vectors may prove useful in gene therapy of human hemoglobinopathies in general and sickle cell anemia and beta-thalassemia in particular.

Murine yolk sac and bone marrow hematopoietic cells with high proliferative potential display different capacities for producing colony-forming cells ex vivo

Increasing evidence suggests that hematopoietic stem and progenitor cells from embryonic and fetal tissues demonstrate proliferative capacities greater than cells isolated from adult hematopoietic tissues. Few studies have explored the organization of the high proliferative potential hematopoietic progenitor hierarchy in the murine yolk sac. We have demonstrated the appearance of high proliferative potential colony-forming cells (HPP-CFC) in the yolk sac as early as embryonic day 8 (E8). Yolk sac HPP-CFC colony size and differentiated cellular composition were similar to adult marrow HPP-CFC. The frequency of yolk sac HPP-CFC at E11 was greater than HPP-CFC frequency in the adult marrow. Replating of primary yolk sac HPP-CFC resulted in significantly greater HPP-CFC and multipotent progenitors than replated adult marrow primary HPP-CFC. Similar results were obtained when AA4.1-expressing yolk sac and adult marrow cells that bind wheat germ agglutinin (WGA) were isolated via flow cytometry. These results support growing evidence that fetal, and perhaps embryonic, hematopoietic tissues may be excellent alternative sources of highly proliferative hematopoietic cells as targets for somatic gene therapy.

Loss of FancC function results in decreased hematopoietic stem cell repopulating ability

Fanconi anemia (FA) is a complex genetic disorder characterized by progressive bone marrow (BM) aplasia, chromosomal instability, and acquisition of malignancies, particularly myeloid leukemia. We used a murine model containing a disruption of the murine homologue of FANCC (FancC) to evaluate short- and long-term multilineage repopulating ability of FancC -/- cells in vivo. Competitive repopulation assays were conducted where "test" FancC -/- or FancC +/+ BM cells (expressing CD45.2) were cotransplanted with congenic competitor cells (expressing CD45.1) into irradiated mice. In two independent experiments, we determined that FancC -/- BM cells have a profound decrease in short-term, as well as long-term, multilineage repopulating ability. To determine quantitatively the relative production of progeny cells by each test cell population, we calculated test cell contribution to chimerism as compared with 1 x 10(5) competitor cells. We determined that FancC -/- cells have a 7-fold to 12-fold decrease in repopulating ability compared with FancC +/+ cells. These data indicate that loss of FancC function results in reduced in vivo repopulating ability of pluripotential hematopoietic stem cells, which may play a role in the development of the BM failure in FA patients. This model system provides a powerful tool for evaluation of experimental therapeutics on hematopoietic stem cell function.

Adeno-associated virus type 2-mediated gene transfer: role of epidermal growth factor receptor protein tyrosine kinase in transgene expression

Adeno-associated virus type 2 (AAV), a single-stranded, DNA-containing, nonpathogenic human parvovirus, has gained attention as a potentially useful vector for human gene therapy. However, the transduction efficiency of AAV vectors varies greatly in different cells and tissues in vitro and in vivo. We have recently documented that a cellular tyrosine phosphoprotein, designated the single-stranded D-sequence-binding protein (ssD-BP), plays an important role in AAV-mediated transgene expression (K. Y. Qing et al., Proc. Natl. Acad. Sci. USA 94:10879-10884, 1997) and that a strong correlation exists between the phosphorylation state of the ssD-BP and AAV transduction efficiency in vitro as well as in vivo (K. Y. Qing et al., J. Virol. 72:1593-1599, 1998). In this report, we document that treatment of cells with specific inhibitors of the epidermal growth factor receptor protein tyrosine kinase (EGF-R PTK) activity, such as tyrphostin, leads to significant augmentation of AAV transduction efficiency, and phosphorylation of the ssD-BP is mediated by the EGF-R PTK. Treatment of cells with EGF results in phosphorylation of the ssD-BP, whereas treatment with tyrphostin causes dephosphorylation of the ssD-BP and consequently leads to increased expression of the transgene. Furthermore, AAV transduction efficiency inversely correlates with expression of the EGF-R in different cell types, and stable transfection of the EGF-R cDNA causes phosphorylation of the ssD-BP, leading to significant inhibition in AAV-mediated transgene expression which can be overcome by the tyrphostin treatment. These data suggest that the PTK activity of the EGF-R is a crucial determinant in the life cycle of AAV and that further studies on the interaction between the EGF-R and the ssD-BP may yield new insights not only into its role in the host cell but also in the successful use of AAV vectors in human gene therapy.

c-Kit and CD38 are expressed by long-term reconstituting hematopoietic cells present in the murine yolk sac

Murine fetal liver (FL) and adult bone marrow (BM) hematopoietic stem cells (HSCs) are characterized by cell surface expression of CD38 and c-kit. Because murine yolk sac (YS) HSC activity precedes the initiation of FL hematopoiesis, we investigated whether YS-derived HSCs also expressed c-kit and CD38. c-Kit+ CD38+ lineage- cells derived from day 9 YS as well as adult BM were found to be enriched in high proliferative potential colony-forming cells. c-Kit+ CD38+ lineage- YS or adult BM cells were capable of long-term reconstitution (>6 months) of busulfan-conditioned newborn or lethally irradiated adult mice, respectively. In contrast, c-kit+ CD38- lineage- populations from both tissues were enriched in lineage-committed progenitors and had no long-term HSC activity. We concluded that c-kit and CD38 are cell surface markers of HSCs expressed throughout murine ontogeny.

Recombinant human parvovirus B19 vectors: erythroid cell-specific delivery and expression of transduced genes

A novel packaging strategy combining the salient features of two human parvoviruses, namely the pathogenic parvovirus B19 and the nonpathogenic adeno-associated virus type 2 (AAV), was developed to achieve erythroid cell-specific delivery as well as expression of the transduced gene. The development of such a chimeric vector system was accomplished by packaging heterologous DNA sequences cloned within the inverted terminal repeats of AAV and subsequently packaging the DNA inside the capsid structure of B19 virus. Recombinant B19 virus particles were assembled, as evidenced by electron microscopy as well as DNA slot blot analyses. The hybrid vector failed to transduce nonerythroid human cells, such as 293 cells, as expected. However, MB-02 cells, a human megakaryocytic leukemia cell line which can be infected by B19 virus following erythroid differentiation with erythropoietin (N. C. Munshi, S. Z. Zhou, M. J. Woody, D. A. Morgan, and A. Srivastava, J. Virol. 67:562-566, 1993) but lacks the putative receptor for AAV (S. Ponnazhagan, X.-S. Wang, M. J. Woody, F. Luo, L. Y. Kang, M. L. Nallari, N. C. Munshi, S. Z. Zhou, and A. Srivastava, J. Gen. Virol. 77:1111-1122, 1996), were readily transduced by this vector. The hybrid vector was also found to specifically target the erythroid population in primary human bone marrow cells as well as more immature hematopoietic progenitor cells following erythroid differentiation, as evidenced by selective expression of the transduced gene in these target cells. Preincubation with anticapsid antibodies against B19 virus, but not anticapsid antibodies against AAV, inhibited transduction of primary human erythroid cells. The efficiency of transduction of primary human erythroid cells by the recombinant B19 virus vector was significantly higher than that by the recombinant AAV vector. Further development of the AAV-B19 virus hybrid vector system should prove beneficial in gene therapy protocols aimed at the correction of inherited and acquired human diseases affecting cells of erythroid lineage.

Identification of human and mouse hematopoietic stem cell populations expressing high levels of mRNA encoding retrovirus receptors

One obstacle to retrovirus-mediated gene therapy for human hematopoietic disorders is the low efficiency of gene transfer into pluripotent hematopoietic stem cells (HSC). We have previously shown a direct correlation between retrovirus receptor mRNA levels in mouse HSC and the efficiency with which they are transduced. In the present study, we assayed retrovirus receptor mRNA levels in a variety of mouse and human HSC populations to identify HSC which may be more competent for retrovirus transduction. The highest levels of amphotropic retrovirus receptor (amphoR) mRNA were found in cryopreserved human cord blood HSC. The level of amphoR mRNA in Lin- CD34(+) CD38(-) cells isolated from frozen cord blood was 12-fold higher than the level in fresh cord blood Lin- CD34(+) CD38(-) cells. In mice, the level of amphoR mRNA in HSC from the bone marrow (BM) of mice treated with stem cell factor and granulocyte-colony stimulating factor was 2.8- to 7.8-fold higher than in HSC from the BM of untreated mice. These findings suggest that HSC from frozen cord blood and cytokine-mobilized BM may be superior targets for amphotropic retrovirus transduction compared with HSC from untreated adult BM.

Adeno-associated virus type 2-mediated gene transfer: correlation of tyrosine phosphorylation of the cellular single-stranded D sequence-binding protein with transgene expression in human cells in vitro and murine tissues in vivo

Although the adeno-associated virus type 2 (AAV)-based vector system has gained attention as a potentially useful alternative to the more commonly used retroviral and adenoviral vectors for human gene therapy, the single-stranded nature of the viral genome, and consequently the rate-limiting second-strand viral DNA synthesis, significantly affect its transduction efficiency. We have identified a cellular tyrosine phosphoprotein, designated the single-stranded D sequence-binding protein (ssD-BP), which interacts specifically with the D sequence at the 3' end of the AAV genome and may prevent viral second-strand DNA synthesis in HeLa cells (K. Y. Qing et al., Proc. Natl. Acad. Sci. USA 94:10879-10884, 1997). In the present studies, we examined whether the phosphorylation state of the ssD-BP correlates with the ability of AAV to transduce various established and primary cells in vitro and murine tissues in vivo. The efficiencies of transduction of established human cells by a recombinant AAV vector containing the beta-galactosidase reporter gene were 293 > KB > HeLa, which did not correlate with the levels of AAV infectivity. However, the amounts of dephosphorylated ssD-BP which interacted with the minus-strand D probe were also as follows: 293 > KB > HeLa. Predominantly the phosphorylated form of the ssD-BP was detected in cells of the K562 line, a human erythroleukemia cell line, and in CD34+ primary human hematopoietic progenitor cells; consequently, the efficiencies of AAV-mediated transgene expression were significantly lower in these cells. Murine Sca-1+ lin- primary hematopoietic stem/progenitor cells contained predominantly the dephosphorylated form of the ssD-BP, and these cells could be efficiently transduced by AAV vectors. Dephosphorylation of the ssD-BP also correlated with expression of the adenovirus E4orf6 protein, known to induce AAV gene expression. A deletion mutation in the E4orf6 gene resulted in a failure to catalyze dephosphorylation of the ssD-BP. Extracts prepared from mouse brain, heart, liver, lung, and skeletal-muscle tissues, all of which are known to be highly permissive for AAV-mediated transgene expression, contained predominantly the dephosphorylated form of the ssD-BP. Thus, the efficiency of transduction by AAV vectors correlates well with the extent of the dephosphorylation state of the ssD-BP in vitro as well as in vivo. These data suggest that further studies on the cellular gene that encodes the ssD-BP may promote the successful use of AAV vectors in human gene therapy.

Adeno-associated virus type 2-mediated transduction in primary human bone marrow-derived CD34+ hematopoietic progenitor cells: donor variation and correlation of transgene expression with cellular differentiation

Although the adeno-associated virus type 2 (AAV) is known to possess a broad host range that transcends the species barrier, we suggested in an earlier study that AAV infection of human cells is receptor mediated (S. Ponnazhagan et al., J. Gen. Virol. 77:1111-1122, 1996). In the present studies, we investigated the ability of AAV to infect primary human hematopoietic progenitor cells capable of multilineage differentiation. Bone marrow-derived CD34+ cells from 12 hematologically normal volunteer donors were infected with a recombinant AAV containing the beta-galactosidase gene under the control of the cytomegalovirus immediate-early promoter (vCMVp-lacZ). Whereas 15 to 80% of the cells from approximately 50% of the donors showed various levels of lacZ gene expression, the expression was undetectable in cells from the remaining donors. However, if cells from both sets of donors were stimulated with various combinations of cytokines to induce differentiation into myeloid and lymphoid lineages following AAV infection, then the level of expression of the transduced gene increased up to 20-fold over a period of 14 days. The results of virus-binding assays suggested that the observed difference between the two groups was due to the differential susceptibility of CD34+ cells to AAV infection rather than to differences in transcription and translation of the transduced gene. To corroborate these results, CD34+ cells from the two donor groups, KB (human nasopharyngeal carcinoma) cells, and M07e (human megakaryocytic leukemia) cells were infected with vCMVp-lacZ. KB cells served as a positive control for AAV infection, and M07e cells served as a negative control. Whereas abundant hybridization to the single-stranded viral DNA on Southern blots was detected in KB and CD34+ cells that were positive for lacZ gene expression, little activity was detected in M07e and CD34+ cells that did not show expression of the lacZ gene. These results suggest that the levels of expression of the putative cellular receptor for AAV vary widely in CD34+ cells from different donors. These studies have implications for the potential use of AAV vectors in human gene therapy involving primary human primitive hematopoietic stem and progenitor cells.

A1 adenosine receptors potently regulate heart rate in mammalian embryos

A1 adenosine receptors (A1ARs) have been recently shown to be expressed in rodent embryonic hearts at very early stages of development. To determine the functional significance of fetal cardiac A1AR expression during embryogenesis, murine fetal heart preparations were studied between postconceptual days 9 and 12. Dose-response curves generated using a variety of adenosine agonists revealed that A1AR activation potently regulated fetal heart rates. The A1AR agonist, N6-cyclopentyladenosine, inhibited heart rates in a dose-dependent manner (half-maximal effective concentration = 3.6 x 10(-8) M) and stopped fetal cardiac contractions in 63% of preparations. In contrast, A2a and A2b receptor activation did not alter heart rates, and activation of A3 receptors produced modest declines in heart rates. Endogenous adenosine also acted tonically to suppress fetal heart rates, as demonstrated by the A1AR antagonist 1,3-dipropyl-8-cyclopentylxanthine, increasing heart rates, whereas the adenosine reuptake blocker dipyridamole lowered fetal heart rates. Pertussis toxin treatment blocked A1AR action, showing that A1AR action was G protein mediated. Using drugs that alter cAMP levels and ion channel action, we were able to show that A1AR action involves events mediated by cAMP, ATP-dependent K, L-type calcium, sodium, and chloride channels, and the pacemaker current. These data show that adenosine and A1ARs potently regulate mammalian heart rates via multiple effector systems at very early stages of prenatal development.

Characterization of definitive lymphohematopoietic stem cells in the day 9 murine yolk sac

The site of origin of lymphohematopoietic stem cells (HSC) that initiate definitive blood cell production in the murine fetal liver is controversial. Contrary to reports that the preliver yolk sac does not contain definitive HSC, we observed that CD34+ day 9 yolk sac cells repopulated multiple blood cell lineages in newborn hosts for at least 1 year. Furthermore, 100 CD34+c-Kit+ day 9 yolk sac or para-aortic splanchnopleura (P-Sp) cells, known to give rise to embryonic HSC, similarly repopulated hematopoiesis in recipient hosts. Surprisingly, 37-fold more CD34+c-Kit+ cells reside in the day 9 yolk sac than in the P-Sp. In sum, definitive HSC are coexistent, but not equal in number, in the murine yolk sac and P-Sp prior to fetal liver colonization.

A recombinant human fibronectin fragment facilitates retroviral mediated gene transfer into human hematopoietic progenitor cells

Gene transfer into hematopoietic progenitor cells is increased when these cells are adherent to the carboxy-terminal chymotryptic fragment of human plasma fibronectin (FN30/35) containing the heparin binding domain (HBD) and the alternatively spliced type three connecting segment (IIICS) region. We report herein the production of a recombinant human fibronectin fragment comprised of the HBD and IIICS regions expressed by Sf9 insect cells following recombinant baculovirus infection. The resulting isolated peptide, HuBacFN, facilitated gene transfer into human hematopoietic cell lines and primary human hematopoietic progenitors to a level achieved with the purified human plasma FN30/35 peptide.

Adeno-associated virus type 2-mediated transfer of ecotropic retrovirus receptor cDNA allows ecotropic retroviral transduction of established and primary human cells

The cellular receptors that mediate binding and internalization of retroviruses have recently been identified. The concentration and accessibility of these receptors are critical determinants in accomplishing successful gene transfer with retrovirus-based vectors. Murine retroviruses containing ecotropic glycoproteins do not infect human cells since human cells do not express the receptor that binds the ecotropic glycoproteins. To enable human cells to become permissive for ecotropic retrovirus-mediated gene transfer, we have developed a recombinant adeno-associated virus type 2 (AAV) vector containing ecotropic retroviral receptor (ecoR) cDNA under the control of the Rous sarcoma virus (RSV) long terminal repeat (LTR) promoter (vRSVp-ecoR). Established human cell lines, such as HeLa and KB, known to be nonpermissive for murine ecotropic retroviruses, became permissive for infection by a retroviral vector containing a bacterial gene for resistance to neomycin (RV-Neo(r)), with a transduction efficiency of up to 47%, following transduction with vRSVp-ecoR, as determined by the development of colonies that were resistant to the drug G418, a neomycin analog. No G418-resistant colonies were present in cultures infected with either vRSVp-ecoR or RV-Neo(r) alone. Southern and Northern blot analyses revealed stable integration and long-term expression, respectively, of the transduced murine ecoR gene in clonal isolates of HeLa and KB cells. Similarly, ecotropic retrovirus-mediated Neo(r) transduction of primary human CD34+ hematopoietic progenitor cells from normal bone marrow was also documented, but only following infection with vRSVp-ecoR. The retroviral transduction efficiency was approximately 7% without prestimulation and approximately 14% with prestimulation of CD34+ cells with cytokines, as determined by hematopoietic clonogenic assays. No G418-resistant progenitor cell colonies were present in cultures infected with either vRSVp-ecoR or RV-Neo(r) alone. These results suggest that sequential transduction of primary human cells with two different viral vectors may overcome limitations encountered with a single vector. Thus, the combined use of AAV- and retrovirus-based vectors may have important clinical implications for ex vivo and in vivo human gene therapy.

In vivo repopulating hematopoietic stem cells are present in the murine yolk sac at day 9.0 postcoitus

The murine yolk sac, being the first site of embryonic blood cell production, has long been theorized to contain the migrating hematopoietic stem cells (HSC) that seed the liver and initiate hematopoiesis on day 10.0 postcoitus (pc). However, it remains controversial whether yolk sac cells isolated before day 11.0 pc possess any long-term repopulating HSC activity upon transplantation into adult recipient mice. We hypothesized that failure to demonstrate engraftment of day <11.0 yolk sac cells in adult hosts may result from an inability of yolk sac cells to home to the active adult hematopoietic sites (spleen and bone marrow). In the present studies, we transplanted yolk sac cells into conditioned newborn mice in whom the liver, as well as the spleen and bone marrow, concomitantly function as a site of blood cell formation. We report that yolk sac cells isolated from day 9.0 pc embryos provide long-term multilineage reconstitution for at least 11 months in primary conditioned newborn mice and for at least 6 months in secondary recipients. Donor yolk sac HSC progeny repopulated mature peripheral blood, thymus, spleen, and bone marrow lymphoid, myeloid, and erythroid compartments. Thus, day 9.0 pc yolk sac HSC can contribute to definitive multilineage hematopoiesis in transplanted recipients. Determination of HSC activity in the day 9.0 pc murine yolk sac suggests that yolk sac HSC are available to seed the liver on day 10.0 pc when definitive hematopoiesis is initiated.

Adeno-associated virus 2-mediated gene transfer in vivo: organ-tropism and expression of transduced sequences in mice

Adeno-associated virus 2 (AAV), a non-pathogenic human parvovirus, is gaining attention as a vector for its potential use in human gene therapy. However, few studies have examined the safety and the efficacy of this vector system in vivo. We report here that recombinant AAV vectors, when directly injected intravenously in mice, accumulated predominantly in liver cells, suggesting that AAV may possess in vivo organ-tropism for liver. The transduced lacZ reporter gene was expressed in hepatocytes in the liver and, at the level examined, did not appear to induce any detectable cytotoxic T lymphocyte response against beta Gal. AAV-mediated transduction of murine hematopoietic progenitor cells ex vivo followed by transplantation into lethally irradiated syngeneic mice also revealed high-efficiency gene transfer into progeny cells without any observable cytotoxicity or deleterious effect. The transduced reporter gene sequences were also expressed in mice in vivo. The AAV-based vectors may thus prove useful as a potentially safe alternative to the more commonly used retrovirus- and adenovirus-based vector systems.

Adeno-associated virus type 2-mediated transduction of murine hematopoietic cells with long-term repopulating ability and sustained expression of a human globin gene in vivo

Adeno-associated virus type 2 (AAV), a nonpathogenic human parvovirus, is gaining attention as a vector for potential use in human gene therapy. We and others have described AAV-mediated beta-globin gene transfer and expression in established human and murine erythroleukemia cell lines in vitro. However, successful AAV-mediated globin gene transduction of hematopoietic stem cells and long-term expression in vivo in progeny cells have not been documented. We report here that infection of murine hematopoietic bone marrow cells ex vivo with a recombinant AAV vector containing the genomic copy of a normal human globin gene followed by transplantation of these cells into lethally irradiated congenic mice resulted in efficient gene transfer into hematopoietic cells with long-term repopulating ability as detected by the presence of the human globin gene sequences in bone marrow and spleen in primary recipient mice for at least 6 months. Long-term expression of the human globin gene was also detected in bone marrow, but not in spleen, in primary recipient mice. Furthermore, in secondary-transplant experiments, we were also able to document the presence as well as expression of the transduced human globin gene in mouse bone marrow for up to 3 months. These results provide further support for potential use of the AAV-based vector system in gene therapy of human hemoglobinopathies in general and sickle-cell anemia and beta-thalassemia in particular.

Engraftment of embryonic hematopoietic cells in conditioned newborn recipients

Yolk sac hematopoiesis is characterized by restricted hematopoietic cell differentiation. Although multipotent hematopoietic progenitor cells have been identified in the early yolk sac, long-term multilineage repopulating (LTMR) hematopoietic stem cell (HSC) activity has not been demonstrable before day 11 postcoitus (PC) using standard transplantation assays. In the present study, day-10 PC yolk sac hematopoietic cells were infused into myeloablated congenic newborn pups and donor cell engraftment and multilineage reconstitution of peripheral blood cells for at least 11 months in primary recipients was observed. In contrast, transplantation of day-10 PC yolk sac cells into congenic adult recipients did not result in engraftment despite pretransplant conditioning of the recipients or use of recipients that were genetically deficient in stem cells. Although fresh yolk sac cells were incapable of reconstitution when injected into adult recipient mice, yolk sac donor-derived cells residing in the bone marrow of primary newborn transplant recipients were capable of efficient reconstitution of conditioned secondary recipient adult mice. Primary newborn and secondary adult recipient animals engrafted with yolk sac cells were observed to have normal peripheral blood white blood cell counts. Lymphocyte subsets in peripheral blood, thymus, and spleen were also similar to control animals. The distribution and frequency of lineage-restricted progenitors derived from bone marrow of secondary transplant recipients were normal. These results indicate that day-10 PC yolk sac HSCs are capable of engrafting and reconstituting the hematopoietic system of conditioned newborn but not adult recipient animals. Furthermore, the ability of the yolk sac HSCs to differentiate into all hematopoietic lineages in these recipients strongly suggests that the local cellular microenvironment plays a prominent role in regulating yolk sac HSC differentiation.

Controlled prospective randomized comparison of high-frequency jet ventilation and conventional ventilation in neonates with respiratory failure and persistent pulmonary hypertension

OBJECTIVE

The objective of this study was to evaluate the efficacy and safety of high-frequency jet ventilation in near-term and term neonates with persistent pulmonary hypertension.

STUDY DESIGN

Subjects for this prospective, randomized, controlled comparison study were recruited from neonates treated in a level-three neonatal intensive care unit that accepts referrals for extracorporeal membrane oxygenation.

RESULTS

In patients treated with high-frequency jet ventilation (n = 11) acute improvement in oxygenation (p = 0.008), ventilation (p < 0.001), and oxygen indices (p < or = 0.01) was demonstrated while stable peak and mean airway pressures were maintained. Control group patients receiving high-frequency positive pressure ventilation with a conventional ventilator required increasingly higher peak inspiratory pressures (p = 0.005) to maintain oxygenation, ventilation, and oxygen indices. There were no significant differences in survival without use of extracorporeal membrane oxygenation, nor were there differences in duration of oxygen therapy, ventilation, and hospitalization; need for extracorporeal membrane oxygenation; or incidence of chronic lung disease.

CONCLUSIONS

High-frequency jet ventilation acutely improves oxygenation and ventilation without significantly increasing morbidity. Therefore high-frequency jet ventilation may be a useful adjunct for stabilization of the conditions of neonates with severe persistent pulmonary hypertension. Conclusions about the efficacy of high-frequency jet ventilation in improving survival without the need for extracorporeal membrane oxygenation await multicentered, collaborative investigations with large cohorts of patients.

Isolation of primitive human bone marrow hematopoietic progenitor cells using Hoechst 33342 and Rhodamine 123

In addition to possessing multilineage differentiation and self-renewal capabilities, pluripotent hematopoietic stem cells are believed to be mitotically quiescent and metabolically inactive. Fractions of human bone marrow (BM) CD34+ cells can be further enriched for primitive hematopoietic progenitor cells (HPC) by using a number of cell-surface markers. All of these fractions, however, contain cells that are still heterogeneous as far as their metabolic and mitotic activities are concerned. We therefore used Hoechst 33342 (Hst) to identify quiescent cells and Rhodamine 123 (Rh123) to identify metabolically inactive cells. CD34+HstdimRh123dim (CD34+d/d) and CD34+HstbrightRh123bright (CD34+b/b) cells were isolated by flow cytometry to examine the hematopoietic functions of mitotically and metabolically homogeneous progenitors. Cell-cycle status, progenitor cell content, maintenance of in vitro hematopoiesis, and long-term hematopoietic culture-initiating cell (LTHC-IC) content of CD34+d/d and CD34+b/b cells were compared with CD34+HLA-DR- cells, a well-defined phenotype of primitive HPC. Whereas 99.2 +/- 0.5% of freshly isolated CD34+d/d cells were in G0/G1 phase of the cell cycle, only 74.4 +/- 11.5% of CD34+b/b and 75.6 +/- 1.1% of CD34+HLA-DR- cells were in G0/G1. The number of multipotential progenitors (colony-forming units-granulocyte/erythroid/ macrophage/megakaryocyte [CFU-GEMM]) detected in CD34+d/d cells was twice that observed in CD34+HLA-DR- cells and eight times that in CD34+b/b cells. In stromal cell-free long-term cultures maintained for 10 weeks, production of assayable progenitors in cultures initiated with CD34+d/d cells exceeded that detected in CD34+HLA-DR- cultures by more than three-fold. Only in CD34+d/d cultures were high proliferative potential colony-forming cell (HPP-CFC)-derived colonies detected over a period of 6 weeks. Limiting dilution analysis revealed that the frequency of LTHC-IC was highest among CD34+d/d cells (7.2 +/- 3.3%), followed by a frequency of 4.5 +/- 4.8% for CD34+HLA-DR- cells and 2.2 +/- 3.5% for CD34+b/b cells. The primitive nature of HPC identified by CD34, Hst, and Rh123 was confirmed by the ability of as few as 200 murine marrow cells isolated by this technique to radioprotect and fully reconstitute lethally irradiated recipients. These results indicate that Hst and Rh123 staining can be used in combination with CD34 immunofluorescence to isolate a quiescent subpopulation of human primitive hematopoietic progenitor cells. Cells isolated by this technique appear to have functional properties associated with stem cells, suggesting that they may be ideal candidates for studies requiring primitive HPC, such as ex vivo expansion and somatic gene therapy.

Differential expression of insulin-like growth factor binding proteins in murine hematopoietic stromal cell lines

The expression of the insulin-like growth factor (IGF) system in the hematopoietic microenvironment during ontogeny was studied utilizing immortalized murine cell lines established from several sites of hematopoiesis. Conditioned media was obtained from tissue cultures of murine yolk sac endoderm and mesoderm, fetal liver and adult bone marrow stromal cell lines. IGF-I and -II were quantified by radioimmunoassay and RT-PCR. The presence of insulin-like growth factor binding proteins (IGFBPs) in conditioned media was determined by Western ligand blot and Western immunoblot. IGF-I and IGF-II were present in conditioned media from every stromal cell line and differential expression of the IGFBPs was found among the hematopoietic sites. Stimulation of the cell lines with interleukin-1 alpha altered the IGFBPs in yolk sac endoderm and bone marrow conditioned media. We report that IGF-I and -II are expressed in stromal cell lines obtained from different ontogenic sites of hematopoiesis and IGFBPs are differentially expressed by these sites. The expression of IGFBPs, but not IGFs, is in part regulated by interleukin-1 alpha.

A novel method of myeloablation to enhance engraftment of adult bone marrow cells in newborn mice

This report describes the successful development of a transplantation model in which we engrafted adult murine bone marrow hematopoietic cells in congenic day-old murine pups. Newborn animals conditioned with high doses (> or = 7.0 Gy) of total-body irradiation (TBI) demonstrated severe growth retardation and untoward effects on the developing central nervous system. Newborn pups conditioned with a sublethal dose (5.0 Gy) of TBI, however, achieved normal growth and development up to 6 months posttransplantation, and donor type hematopoietic cell reconstitution exceeded 50% in these animals. A second conditioning regimen, comprising one or two doses of busulfan (15 mg/kg) administered intrapartum, resulted in significant myelosuppression in untransplanted newborn pups. This second regimen was associated, however, with engraftment and full multilineage reconstitution of the conditioned newborn recipient animals with adult bone marrow cells, and the engrafted pups grew and developed without apparent defect. A third conditioning regimen combining a single dose of busulfan with sublethal TBI also permitted engraftment and full multilineage reconstitution of the recipient animals. Thus, successful transplantation of hematopoietic cells into newborn recipient mice may be achieved without significant morbidity if a specific conditioning regimen is employed.

Ex vivo expansion of murine hematopoietic progenitor cells generates classes of expanded cells possessing different levels of bone marrow repopulating potential

The objective of ex vivo expansion of primitive hematopoietic progenitor cells (HPC) is to increase the number of progeny cells possessing hematopoietic potential similar to the original HPC. In the context of bone marrow (BM) transplantation in mice, this implies that expanding a number of HPC sufficient for long-term rescue of one lethally irradiated animal should generate enough cells to rescue more than one lethally irradiated recipient. In the present study, Sca-1+Lin- cells from male C57Bl/6 mice were expanded in vitro with stem cell factor (SCF), interleukin-1alpha (IL-1alpha), IL-3, and IL-6 and used to transplant lethally irradiated syngeneic female recipients. Expanded cells were tracked in vitro with the fluorescent membrane dye PKH2, which becomes evenly distributed among dividing daughter cells, and fractionated on day 7 into Sca-1+ cells which did not divide (Sca-1+PKH2bright), those which had divided 1 to 2 times (Sca-1+PKH2moderate), or those which had divided four or more times (Sca-1+PKH2dim). Grafts of expanded cells consisted of either the same number of fresh cells proven to rescue lethally irradiated animals [3X10(3) cells; referred to as one repopulating dose (1 RD)] or the expansion equivalent (EE) of these cells. One EE of cells represented 3X10(3) multiplied by the fold increase in the number of cultured cells on day 7. All animals transplanted with 3X10(3) freshly isolated Sca-1+Lin- cells survived long-term. Only 53% of animals receiving 1 EE of all cultured day-7 cells survived. One RD from all three PKH2 fractions (bright, moderate, and dim) of day-7 cultured Sca-1+ cells failed to rescue more than 30% of lethally irradiated recipients. Comparable survival rates were obtained when 1 EE of Sca-1+PKH2dim or only 4 RD of Sca-1+PKH2bright cells were used as grafts, suggesting that a larger frequency of long-term repopulating cells may have been retained within the fraction of Sca-1+ cells undergoing minimal or no proliferation in culture. Engraftment of male ex vivo expanded cells in recipients was confirmed by polymerase chain reaction (PCR) analysis with Y chromosome-specific primers. When analyzed for their cell cycle status, Sca-1+PKH2bright cells were mostly quiescent, whereas a higher percentage of Sca-1+PKH2dim cells were in active phases of cell cycle. These data suggest that ex vivo expansion does not augment the number of BM repopulating HPC and that ex vivo expansion generates classes of progenitor cells with different BM repopulating potentials depending on their proliferative history. These studies also suggest that the cell cycle status of graft cells may affect the ability of these cells to engraft in myeloablated hosts.

Parvovirus B19 promoter at map unit 6 confers autonomous replication competence and erythroid specificity to adeno-associated virus 2 in primary human hematopoietic progenitor cells

The pathogenic human parvovirus B19 is an autonomously replicating virus with a remarkable tropism for human erythroid progenitor cells. Although the target cell specificity for B19 infection has been suggested to be mediated by the erythrocyte P-antigen receptor (globoside), a number of nonerythroid cells that express this receptor are nonpermissive for B19 replication. To directly test the role of expression from the B19 promoter at map unit 6 (B19p6) in the erythroid cell specificity of B19, we constructed a recombinant adeno-associated virus 2 (AAV), in which the authentic AAV promoter at map unit 5 (AAVp5) was replaced by the B19p6 promoter. Although the wild-type (wt) AAV requires a helper virus for its optimal replication, we hypothesized that inserting the B19p6 promoter in a recombinant AAV would permit autonomous viral replication, but only in erythroid progenitor cells. In this report, we provide evidence that the B19p6 promoter is necessary and sufficient to impart autonomous replication competence and erythroid specificity to AAV in primary human hematopoietic progenitor cells. Thus, expression from the B19p6 promoter plays an important role in post-P-antigen receptor erythroid-cell specificity of parvovirus B19. The AAV-B19 hybrid vector system may also prove to be useful in potential gene therapy of human hemoglobinopathies.

Murine embryonic yolk sac cells promote in vitro proliferation of bone marrow high proliferative potential colony-forming cells

To examine the influence of the hematopoietic microenvironment on hematopoietic cell proliferation and differentiation during the yolk sac phase of hematopoiesis, we have recently established cell lines from embryonic yolk sac visceral endoderm (YSE) and mesoderm (YSM). In the present experiments, we compared in vitro growth of adult murine bone marrow high proliferative potential colony-forming cells (HPP-CFC) in coculture with YSE- and YSM-derived or adult bone marrow stromal cell lines. Whereas both yolk sac-derived and adult stromal cell lines supported the proliferation of HPP-CFC during coculture, YSE- and YSM-derived cells stimulated a significant increase in total HPP-CFC compared with adult bone marrow stromal cell lines. Conditioned media from both YSE- and YSM-derived cell lines also stimulated the growth of HPP-CFC in vitro, but only in combination with exogenous recombinant hematopoietic growth factors. Although multiple hematopoietic growth factor mRNAs were detected in the yolk sac-derived cells by polymerase chain reaction, only macrophage colony-stimulating factor (M-CSF) activity was detected in conditioned media using an enzyme-linked immunosorbent assay. A neutralizing polyclonal antibody against M-CSF did not diminish the YSE- or YSM-derived cell line conditioned media promotion of HPP-CFC colony formation. These results suggest that murine yolk sac-derived cell lines produce a novel soluble factor(s) that recruits primitive bone marrow hematopoietic cells to grow in vitro in response to a combination of hematopoietic growth factors.

Matrix molecule interactions with hematopoietic stem cells

We have reviewed some aspects of the production, distribution, and organization of fibronectin in the bone marrow ECM and discussed HSC-fibronectin interactions. Many questions remain. Which isoforms of fibronectin are produced in the bone marrow during ontogeny, normal hematopoiesis, and pathophysiologic challenges to the marrow microenvironment? Do HSCs interact with ED-A- and ED-B-containing fibronectin isoforms the same way they interact with plasma fibronectin? Are different fibronectin isoforms selectively expressed in different regions of the bone marrow, in keeping with the spatial organization of HSC and hematopoietic progenitor cell compartments? Addressing these questions may give us a better understanding of the role of fibronectin in HSC localization, proliferation, and differentiation. Considerable information regarding the synthesis and secretion of other ECM molecules by adherent stromal cells has been derived from in vitro analysis of LTBMCs [3]. As with fibronectin, much remains to be learned of the precise distribution and composition of bone marrow ECM molecules during in vivo development; how in vivo bone marrow ECM secretion, turnover, and remodeling are regulated during normal and pathologic hematopoietic states; and the nature and importance of in vivo cellular interactions that occur between PHSC and individual constituents of the bone marrow ECM.

Myeloproliferative sarcoma virus directed expression of beta-galactosidase following retroviral transduction of murine hematopoietic cells

The introduction of genetic sequences into hematopoietic stem cells (HSC) has allowed study of HSC proliferation in vivo by proviral-sequence molecular analysis in the DNA of progeny. Analysis of HSC proliferation could be enhanced by development of a retroviral vector that encodes a reporter gene that allows sensitive detection of transduced cells. We developed a recombinant retrovirus vector encoding the reporter gene lacZ under the transcriptional control of the myeloproliferative sarcoma virus long-terminal repeat (LTR). Bone marrow cells from C3H mice were co-cultured on retrovirus producer cell lines and cultured for growth of colony-forming unit granulocyte/macrophage (CFU-GM) and high proliferative potential colony-forming cells (HPP-CFC) in semisolid media or were transplanted into irradiated recipients. In other experiments, recombinant retrovirus was injected in vivo into the liver of developing fetal rat pups, and circulating hematopoietic cells of the postnatal rats were analyzed for evidence of proviral integration and expression of beta-galactosidase. Expression of lacZ was detected in both CFU-GM and HPP-CFC that were cultured immediately following in vitro infection of mouse bone marrow. Beta-galactosidase activity from the retrovirus was also detected in both marrow cells isolated from reconstituted mice 22 weeks following transplantation as well as in blood cells of postnatal rats transduced in utero with the recombinant retrovirus. This strategy may be especially useful for characterizing proliferation of transduced populations of hematopoietic cells and in the development of protocols for somatic gene therapy.

Murine yolk sac endoderm- and mesoderm-derived cell lines support in vitro growth and differentiation of hematopoietic cells

The mechanisms involved in the induction of yolk sac mesoderm into blood islands and the role of visceral endoderm and mesoderm cells in regulating the restricted differentiation and proliferation of hematopoietic cells in the yolk sac remain largely unexplored. To better define the role of murine yolk sac microenvironment cells in supporting hematopoiesis, we established cell lines from day-9.5 gestation murine yolk sac visceral endoderm and mesoderm layers using a recombinant retrovirus vector containing Simian virus 40 large T-antigen cDNA. Obtained immortalized cell lines expressed morphologic and biosynthetic features characteristic of endoderm and mesoderm cells from freshly isolated yolk sacs. Similar to the differentiation of blood island hematopoietic cells in situ, differentiation of hematopoietic progenitor cells in vitro into neutrophils was restricted and macrophage production increased when bone marrow (BM) progenitor cells were cultured in direct contact with immortalized yolk sac cell lines as compared with culture on adult BM stromal cell lines. Yolk sac-derived cell lines also significantly stimulated the proliferation of hematopoietic progenitor cells compared with the adult BM stromal cell lines. Thus, yolk sac endoderm- and mesoderm-derived cells, expressing many features of normal yolk sac cells, alter the growth and differentiation of hematopoietic progenitor cells. These cells will prove useful in examining the cellular interactions between yolk sac endoderm and mesoderm involved in early hematopoietic stem cell proliferation and differentiation.

High proliferative potential colony-forming cell heterogeneity identified using counterflow centrifugal elutriation

Murine high proliferative potential colony-forming cells (HPP-CFC) are known to be heterogenous with respect to proliferative capacity and in vitro responsiveness to hematopoietic growth factors. We have separated HPP-CFC into several subpopulations using counterflow centrifugal elutriation. Although HPP-CFC were identified in all of the elutriated fractions of both C3H/HeJ and C57BI/6J bone marrow cells, the distribution of HPP-CFC as well as of colony-forming units-granulocyte-macrophage (CFU-GM) in each fraction differed between these two strains of inbred mice. Six subsets of HPP-CFC were resolved that differed in growth factor responsiveness. A low-density HPP-CFC subpopulation was isolated that was distinct from day-12 spleen colony-forming units (CFU-S12), CFU-GM, and bone marrow stromal cells. This unique subpopulation of HPP-CFC is rate (3% to 9% of total HPP-CFC), appears to be lymphocyte-like in morphology, and behaves the most primitive of the HPP-CFC subsets by requiring multiple hematopoietic growth factors for optimal in vitro cloning. Further characterization of this subpopulation of HPP-CFC will determine the position of these cells in the HPP-CFC heirarchy.

Association of maternal lithium exposure and premature delivery

Lithium is widely used and the treatment of choice for patients with manic-depressive illness. For pregnant patients with manic-depressive illness, however, the use of lithium during the first trimester of pregnancy may present an increased risk for fetal maldevelopment. We have recently cared for several large-for-gestational-age, prematurely born infants whose mothers were treated with lithium throughout pregnancy. To determine whether maternal lithium use during pregnancy may predispose to the onset of premature labor and fetal macrosomia, we reviewed records from the International Register of Lithium Babies and from a cohort of manic-depressive pregnant women. More than one third (36%) of infants reported to the International Register were born prematurely, and 37% of the premature infants were large for gestational age; 15% of the term infants were born large for gestational age. In the cohort group, manic-depressive mothers who received lithium during pregnancy had a 2.5-fold higher incidence of premature births than manic-depressive pregnant patients who did not receive lithium treatment. The incidence of large-for-gestational-age births in lithium-treated women in the cohort was not different from that of the general population or from manic-depressive women not treated with lithium. In summary, an association between maternal lithium therapy and premature delivery is reported. We recommend that women receiving lithium therapy during pregnancy be closely monitored for the onset of premature labor.