Primitive Human Hematopoietic Progenitor Cells Research Articles

PB1702 EXPRESSION OF ANGIOTENSIN‐CONVERTING ENZYME IN PATIENTS WITH ACUTE LEUKEMIAS

Background:Blast cells are characterized by aberrant expression normal markers of early hematopoietic progenitor cells (HPC). Angiotensin‐converting enzyme (ACE/CD143), a key regulator of renin‐angiotensin system, was shown to be expressed in blood‐forming tissues of the human embryo and primitive human HPC. Recent studies indicated that CD143 is over‐expressed in leukemic myeloid blast cells.Aims:To evaluate the surface expression of CD143 in blast cells in patients with newly diagnosed acute leukemia (AL).Methods:The expression of CD143 was estimated in patients with newly diagnosed untreated acute leukemia, using eight‐color flow cytometry. The blast gate was defined on the basis of CD45dim expression and side‐scatter characteristics. Anti‐CD143 monoclonal antibody (clone BB9, BD) conjugated with phycoerythrin was used for analysis of CD143 expression. The percentage of positive cells (PPC) and the mean fluorescence intensity (MFI) was applied to describe expression. The expression of CD143 was devided into three groups according range of PPC ≤1%, 1 – 20 %, and ≥20 %.Results:A total of untreated 68 patients with newly diagnosed acute leukemia were included in the study group. They were diagnosed with non‐promyelocytic acute myeloid leukemia (non‐APL AML) – 35 (51.5 %), of them 3 (8.5 %) patients were related to favorable genetic group, 9 (25.7 %) – intermediate, 23 (65.8 %) – adverse; promyelocytic leukemia (APL) – 10 (14.7 %); B acute lymphoblastic leukemia/lymphoma (B‐ALL) – 13 (19.1 %), including 2 patients with Ph‐positive B‐ALL; T acute lymphoblastic leukemia/lymphoma (T‐ALL) – 10 (14.7 %), including 2 cases of early T‐precursor ALL (ETP). Median age of patients was 35.5 years, 29 male, 39 female.In total group with CD143 expression with PPC≥20% was observed in 7% (5/68) of all cases AL. CD143 expression in non‐APL AML with a range of PPC 1 – 20 % was detected in 12/35 cases (34.2 %), PPC ≥20 % was observed in 2/35 cases with complex karyotype (5.7 %). In group with APL 2/10 (20 %) patients had 1 – 20%, and 2/10 (20 %) patients had >20 %. Among T‐ALL only 1/10 (10 %) had a CD143 expression. ACE expression in B‐ALL was absent (defined as ≤1 %). Median MFI of ACE expression was higher in patients with APL – 477 (range 159 – 4796) than in non‐APL AML ‐ 175 (range 43 – 3253). CD143 expression with PPC≥1% was significantly higher in patients with AML (non‐APL AML and APL), than in ALL (B‐ and T‐ALL), 40 % (18/45) vs. 4.3 % (1/23) (p = 0.0016).Summary/Conclusion:Thus, CD143 expression with PPC≥20% was observed only in 7% (5/68) of all cases AL, 2 was observed in AML with PML‐RARA, 2 ‐ AML with complex karyotype and 1 in ETP‐ALL. No expression of CD143 was detected in B‐ALL. CD143 expression was significantly higher in myeloid blast cells than lymphoid. No correlation was found between the expression of ACE and age, sex, neuroleukemia, leucocytes count, bone marrow blasts, lactic acid dehydrogenase activity and cytogenetic abnormalities in different subgroups. The significance of CD143 expression in AL is not completely clear and has to be studied.

Aldehyde Dehydrogenase Expression in Primitive Human Hematopoietic Progenitor Cells with Side Population Characteristic and in Samples from Patients with Acute Myeloid Leukemia.

High activity of the detoxifying enzyme aldehyde dehydrogenase (ALDH) has been attributed to primitive hematopoietic stem cells in the murine and human system. ALDH activity can be measured by a functional assay employing BODIPY-aminoacetaldehyde (BAAA) as a fluorescent intracellular substrate. High ALDH activity within hematopoietic stem cells is associated with CD34 and AC133 surface expression.Given the co-expression of these markers in acute myeloid leukemia (AML), we sought to investigate whether ALDH-activity can be detected in early blood stem cell disorders and in primitive hematopoietic progenitor cells with the side population phenotype. Employing a commercial Aldefluor™ -kit thirteen peripheral blood and bone marrow samples from patients with AML were stained for ALDH activity. Four samples exhibited a clearly distinctive positive staining pattern when compared to the inhibitor control, i.e. 16–91% of AML blast cells were considered as ALDH positive. The staining pattern was similar to marrow samples from healthy donors or peripheral blood sample controls from G-CSF-mobilised healthy stem cell donors.In addition we investigated ALDH activity in primitive bone marrow side population (SP) cells from healthy individuals. Although the ALDH substrate BAAA is actively extruded out of cells and especially SP stem cells by an ABC-transporter-activity, we established a co-staining method for Hoechst 33342 and BAAA by inclusion of specific ABC-transporter inhibitors.Both SP cells and ALDH-positive cells appeared as a clear distinctive population with some overlap between both populations. Quantitative RT-PCR of different sorted stem cell samples confirmed that SP cells had higher ABCG2-expression than ALDH positive cells and ABCG2-expression was higher in ALDH positive cells compared to the total cell fraction or granulocytes.We conclude that ALDH is expressed in some but not all AML blast cells. This is in accordance with the cytogenetic heterogeneity of this disease and may have therapeutic implications because of the detoxifying activity of the ALDH enzyme. In addition ALDH activity can also be found on a minor population of very primitive marrow progenitor cells. Whether high ALDH activity is a feature of the leukemic stem cell requires further investigation.

Uptake of Protamine Sulphate Complexed Fluorescent Nano-Particles Is Defined by Cell Cycle Status in Primary Human CD34+ Cells: Use of a Multi-Color p27 kip1 Based Flow Cytometric Assay.

The use of iron based nano-particles for multi-modal imaging is gaining interest, since it allows high resolution non-invasive in vivo imaging of human hematopoietic homing and engraftment events in xenograft models. The uptake of ferridex nano-particles complexed to cationic protamine sulphate is believed to be non-specific through mechanisms like endocytosis, but this has not been well defined for hematopoietic stem cells (HSC). In defining ex vivo cultivation strategies for manipulation of human HSC, a key factor is the responsiveness of the most primitive cells to the in vitro conditions, with the aim of maintaining viability without inducing terminal differentiation. Here, we present a novel flow cytometry assay which assesses the earliest molecular responses to a defined clinically applicable ex vivo protocol, aimed at facilitating labeling of human stem/progenitor cells using protamine sulphate complexed nano-particles for subsequent in vivo imaging. We used intracellular staining for the cell cycle inhibitor p27kip1, which is present in the highest levels in non-cycling cells, as the primary flow cytometric marker in combination with CD34, CD133 and Alexa 488, 647 and 750 conjugated ferridex nano-particles and the membrane dye PKH26. An assay was developed to simultaneously assess the molecular events occurring in individual human cord blood Lin− or CD34+ cells while they were cultured for up to 72 hours in X-Vivo 15 serum free medium supplemented with Flt3, SCF and TPO on Retronectin (RN) coated plates with or without nano-particles. Co-expression of p27kip1, CD34 or CD133 in the cultured cells slowly decreases from 86.1% CD34+p27kip1 (T=0) to 76.7%+/−12.2% (T=72) and from 89.6% CD133+p27kip1+ (T=0) to 54.1%+/−10.4% (T=72). We suggest that this slow decrease represents cells dividing and potentially differentiating over the time course of the ex vivo cultivation period. Assessing uptake of fluorescent conjugated nano-particles over a 72 hr period showed that the uptake of particles in CD34+ and CD133+ cells declined significantly after the first 24 hrs., from 32.5+/−3.7% nano-positive CD34+ cells to 19.2+/−2.9% at 48 hours ex vivo with a more significant decline to only 8.3+/−3.7% nano positive CD34+ cells in the culture after 72 hours ex vivo. The same decline in uptake over time was observed in cultured human CB cells that were positive for CD133. PKH26 co-staining demonstrated that the majority of cells that undergo cell division within the first 24 hours of ex vivo culture are the most likely to uptake the nano-particles. In summary, using a multi color p27kip1 based flow-cytometry assay, we found that human Lin−, CD133+, and CD34+ cells uptake Fe-Pro in a fashion which is not entirely cell cycle independent as previously suggested. These data indicate that cell cycle or metabolic status may influence the ability of human hematopoietic stem and progenitor subsets to uptake the protamine sulphate-complexed nano-particles. These findings emphasize the need to carefully develop ex vivo conditions for nano-particle labeling of primary human stem cells in order to perform accurate in vivo imaging of the most primitive human hematopoietic stem and progenitor cells.

Retrovirus molecular conjugates: a versatile and efficient gene transfer vector system for primitive human hematopoietic progenitor cells

In principle, transient nongenetic modification of a noninfectious gene transfer virus enabling a one time infection and transduction of human cells could eliminate the risk of formation of replication competent virus. Formation of a molecular conjugate vector by conjugation of noninfective ecotropic murine Moloney leukemia virus to polylysine (eMMLV-PL) enabled high-efficiency transduction of human HPC using in vitro and in vivo assays. Xenotransplanted NOD-SCID mice durably expressed the transgene in human leukocytes and human progenitor cells with eMMLV-PL achieving three-fold increased transduction efficiency when directly compared to optimized amphotropic MMLV (aMMLV) transduction. Both aMMLV and eMMLV assembled conjugate vectors showed similar transduction efficiency indicating predominant polylysine-mediated uptake. Integration of retroviral sequences was determined from individual human HPC recovered from eMMLV-PL-xenotransplanted animals. This simple and versatile concept of conjugate gene transfer vectors has the potential to enhance transduction efficiency as well as to improve certain safety aspects of human gene therapy. Moreover, because it permits effective cellular internalization of particles, this concept of molecular conjugates can be used as research tool to investigate the interactions of otherwise noninfectious viruses or modified viral particles at the genomic level.

Interaction of Stem Cells and Their Niche: Behavior and Gene Expression Profiles of CD34+/CD38− Cells upon Co-Cultivation with AFT024.

Cell-cell contact between stem cells and cellular determinants of the microenvironment plays an essential role in the regulation of self-renewal and differentiation. The stromal cell line derived from murine fetal liver (AFT024) has been shown to support maintenance of primitive human hematopoietic progenitor cells (HPC) in vitro. We have studied the interaction between HPC (defined as CD34+/CD38− umbilical cord blood cells) and AFT024 and the impact of co-cultivation on the behavior and gene expression of HPC. By time lapse microscopy the mobility and behavior of CD34+/CD38− cells were monitored. Approximately 30% of the CD34+/CD38− cells adhered to the cellular niche through an uropod. CD44 and CD34 were co-localized at the site of contact. Gene expression profiles of CD34+/CD38− cells were then compared upon co-cultivation either with or without AFT024. After cultivation for 16h, 20h, 48h or 72h the HPC were separated form the feeder layer cells by a second FAC-Sort. Differential gene expression was analyzed using our Human Genome cDNA Microarray of over 51,145 ESTs. Among the genes with the highest up-regulation in contact with AFT024 were several genes involved in cell adhesion, proliferation and DNA-modification including tubulin genes, ezrin, complement component 1 q subcomponent 1 (C1QR1), proto-oncogene proteins c-fos and v-fos, proliferating cell nuclear antigen (PCNA), HLA-DR, gamma-glutamyl hydrolase (GGH), minichromosome maintenance deficient 6 (MCM6), uracil-DNA glycolase (UNG) and DNA-methyltransferase 1 (DNMT1). In contrast, genes that were down-regulated after contact with AFT024 included collagenase type iv (MMP2), elastin (ELN) and hemoglobin genes. Differential expression of six genes was confirmed by RT-PCR. Other authors have reported on the differential gene expression profiles of CD34+ cells derived from the bone marrow versus those from G-CSF mobilized blood. As CD34+ cells from the bone marrow might represent cells exposed to the natural HPC niche we have then compared our findings with these experiments. In these comparisons we identified several overlapping genes that are involved in regulation of cell cycle and DNA repair including PCNA, DNMT1, MCM6, MCM2, CDC28 protein kinase regulatory subunit 1B (CKS1B), Topoisomerase II (TOP2a), DNA Ligase 1 (LIG1) and DNA mismatch repair protein MLH1. All these genes were up-regulated among CD34+/CD38− cells upon co-culture with AFT024, as well as among CD34+ cells derived from the bone marrow versus those from peripheral blood. Our studies support the hypothesis that intimate contact and adhesive interaction of HPC with their niche profoundly influenced their proliferative potential and their differentiation program.

Impact of modeled microgravity on migration, differentiation, and cell cycle control of primitive human hematopoietic progenitor cells

Objective Migration, proliferation, and differentiation of bone marrow (BM) hematopoietic stem cells (HSC) are important factors in maintaining hematopoietic homeostasis. Homeostatic control of erythrocytes and lymphocytes is perturbed in humans exposed to microgravity (μ-g), resulting in space flight–induced anemia and immunosuppression. We sought to determine whether any of these anomalies can be explained by μ-g-induced changes in migration, proliferation, and differentiation of human BM CD34 + cells, and whether such changes can begin to explain any of the shifts in hematopoietic homeostasis observed in astronauts. Materials and methods BM CD34 + cells were cultured in modeled μ-g (mμ-g) using NASA's rotating wall vessels (RWV), or in control cultures at earth gravity for 2 to 18 days. Cells were harvested at different times and CD34 + cells assessed for migration potential, cell-cycle kinetics and regulatory proteins, and maturation status. Results Culture of BM CD34 + cells in RWV for 2 to 3 days resulted in a significant reduction of stromal cell–derived factor 1 (SDF-1α)-directed migration, which correlated with decreased expression of F-actin. Modeled μ-g induced alterations in cell-cycle kinetics that were characterized by prolonged S phase and reduced cyclin A expression. Differentiation of primitive CD34 + cells cultured for 14 to 18 days in RWV favored myeloid cell development at the expense of erythroid development, which was significantly reduced compared to controls. Conclusions These results illustrate that mμ-g significantly inhibits the migration potential, cell-cycle progression, and differentiation patterns of primitive BM CD34 + cells, which may contribute to some of the hematologic abnormalities observed in humans during space flight.

Stem cell defect in aplastic anemia: reduced long term culture-initiating cells (LTC-IC) in CD34+ cells isolated from aplastic anemia patient bone marrow.

Aplastic anemia is associated with quantitative and functional abnormalities in the hematopoietic stem cell compartment. Currently, one of the most primitive human hematopoietic progenitor cells that can be functionally assayed in vitro is the long-term culture-initiating cell (LTC-IC). This assay identifies primitive cells that are capable of producing colonies after five weeks of long-term culture using a limiting dilution method. Previous investigators have demonstrated a significant reduction in the frequency of LTC-IC in bone marrow mononuclear cells (BMMC) isolated from aplastic anemia patients when compared to normal donors. However, immunosuppression of hematopoiesis is a prevalent feature of aplastic anemia. Therefore, we assayed the frequency of LTC-IC in cells expressing the CD34 antigen isolated from bone marrow, a population highly enriched for LTC-IC, of both aplastic anemia patients that had received immunosuppressive therapies (IST, n=13) and normal donors (n=28), thereby minimizing continued immunosuppression by T-cells and regulation by other autologous CD34- cells. We describe a significant reduction of LTC-IC frequency in purified CD34+ populations from aplastic patients (P>0.0001), thus demonstrating a functional difference between this phenotypically defined cell population from patients and normal donors.

Enhanced transgene expression in primitive hematopoietic progenitor cells and embryonic stem cells efficiently transduced by optimized retroviral hybrid vectors.

Oncoretroviral vectors have been successfully used in gene therapy trials, yet low transduction rates and loss of transgene expression are still major obstacles for their application. To overcome these problems we modified the widely used Moloney murine leukemia virus-derived retroviral vector pMX by replacing the 3'LTR with the spleen focus-forming virus LTR and inserting the woodchuck hepatitis B virus post-translational regulatory element. To compare requirements crucial for efficient transgene expression, we generated the hybrid retroviral vectors pMOWS and pOWS that harbor the complete murine embryonic stem cell virus (MESV)-leader sequence or a shortened MESV-leader not comprising primer binding site (PBS) and splice donor (SD). Applying these retroviral vectors significantly augmented transgene expression in hematopoietic cell lines and progenitor cells. For transduction of murine embryonic stem (ES) cells the retroviral vector pMOWS that harbors the MESV-PBS and -SD was superior resulting in 65% green fluorescent protein (GFP) expressing ES cells. Surprisingly, in murine and human primitive hematopoietic progenitor cells (HPC), the highest efficiency of up to 66% GFP expressing cells was achieved with pOWS, a retroviral vector that retains the negative regulatory element coinciding with the MoMuLV-PBS. In summary our hybrid retroviral vectors facilitate significantly improved transgene expression in multipotent cells and thus possess great potential for reconstituting genes in primary cells of disease models, as well as for gene therapy.

Constitutive expression of the homeobox transcription factor HOXB4 amplifies human primitive hematopoietic progenitor cells in vitro and in vivo

Constitutive expression of the homeobox transcription factor HOXB4 amplifies human primitive hematopoietic progenitor cells in vitro and in vivo

Gp130-stimulating designer cytokine Hyper-interleukin-6 synergizes with murine stroma for long-term survival of primitive human hematopoietic progenitor cells.

Experimental strategies for ex vivo expansion of human hematopoietic stem/progenitor cells involve the use of exogenous cytokines as well as direct interaction with stromal elements. We examined the use of the interleukin-6/soluble interleukin-6 receptor (IL-6/sIL-6R) fusion protein Hyper-IL-6 (H-IL-6), which interacts directly with gp130, in conjunction with stromal support for expansion of human progenitors. Peripheral blood CD34+ cells were cultured on the murine stromal cell line FBMD1 or in suspension for up to 28 days with different cytokines. Cells were evaluated at various time points for phenotype, proliferative and clonogenic capacity, and long-term hematopoietic activity. The combination of Flt3 ligand and H-IL-6 was markedly more effective than Flt3 ligand and IL-6/sIL-6R for expansion of CD34+ cells in suspension culture and on FBMD1. Addition of kit ligand but not thrombopoietin to Flt3 ligand and H-IL-6 significantly augmented proliferation and enhanced colony formation three-fold. However, long-term cobblestone area-forming cell assays indicated that although multipotent progenitors were maintained up to 21 days on FBMD1 in the presence of Flt3 ligand alone and were amplified three-fold by addition of H-IL-6, CD34+ cells cultured in the absence of stromal support rapidly lost their cobblestone area-forming cell potential. Immunophenotyping revealed that stromal support prevented up-regulation of IL-6R on CD34+ cells, which was induced within 3 days in stroma-free cultures and was enhanced in the presence of kit ligand. Delayed addition of H-IL-6 to the cultures resulted in reduced proliferation and colony-forming unit potential. H-IL-6 synergizes with stromal elements to effectively enhance proliferation and maintenance of primitive hematopoietic progenitors under prolonged ex vivo culture conditions.

Adenoviral vector design for high-level transgene expression in primitive human hematopoietic progenitors.

Adenoviral vector-mediated transient gene expression can provide new possibilities for ex vivo manipulation of quiescent hematopoietic stem cells (HSC). In order to define a suitable expression cassette for high levels of transgene expression in HSCs, we have studied the level of transgene expression in human CD34+CD38- cells using adenoviral vectors with various gene expression cassettes encoding the enhanced green fluorescence protein (EGFP) gene. CD34+ hematopoietic cells were cultured in serum-free medium with megakaryocyte growth and development factor (MGDF) alone for supporting the survival of primitive progenitors or with MGDF, c-kit ligand (KL) and flt3 ligand (FL) for inducing proliferation of primitive progenitors. With all the vectors tested, higher percentages of EGFP expressing cells were found in CD34+CD38- cells than those in CD34+CD38high cells from all donors tested. The phosphoglycerate kinase (PGK)-1 promoter was found to allow higher levels of EGFP expression than the human cytomegalovirus (HCMV) promoter in CD34+CD38- cells. Replacing the SV40 polyadenylation signal with the human beta-globin gene IVS2 and polyadenylation signal in the expression cassette (Ad5xPGK-EGFP-beta-globin) enhanced the level of EGFP expression markedly further. These results provide a guideline for the development of adenoviral vectors for gene expression in human primitive hematopoietic progenitor cells. Gene Therapy (2000) 7, 2132-2138.

Lentiviral gene transfer into primary and secondary NOD/SCID repopulating cells

AbstractThe ability of lentiviral vectors to transfer genes into human hematopoietic stem cells was studied, using a human immunodeficiency virus 1 (HIV-1)–derived vector expressing the green fluorescence protein (GFP) downstream of the phosphoglycerate kinase (PGK) promoter and pseudotyped with the G protein of vesicular stomatitis virus (VSV). High-efficiency transduction of human cord blood CD34+cells was achieved after overnight incubation with vector particles. Sixteen to 28 percent of individual colony-forming units granulocyte-macrophage (CFU-GM) colonies derived from cord blood CD34+ cells were positive by polymerase chain reaction (PCR) for the GFP gene. The transduction efficiency of SCID-repopulating cells (SRC) within the cord blood CD34+population was assessed by serial transplantation into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. When 400 000 cord blood CD34+ cells were transplanted into primary recipients, all primary and secondary recipients contained and expressed the transgene. Over 50% of CFU-GM colonies derived from the bone marrow of these primary and secondary recipients contained the vector on average as determined by PCR. Transplantation of transduced cells in limiting dilution generated GFP+ lymphoid and myeloid progeny cells that may have arisen from a single SRC. Inverse PCR analysis was used to amplify vector-chromosomal junctional fragments in colonies derived from SRC and confirmed that the vector was integrated. These results show that lentiviral vectors can efficiently transduce very primitive human hematopoietic progenitor and stem cells.

Lentiviral gene transfer into primary and secondary NOD/SCID repopulating cells

The ability of lentiviral vectors to transfer genes into human hematopoietic stem cells was studied, using a human immunodeficiency virus 1 (HIV-1)-derived vector expressing the green fluorescence protein (GFP) downstream of the phosphoglycerate kinase (PGK) promoter and pseudotyped with the G protein of vesicular stomatitis virus (VSV). High-efficiency transduction of human cord blood CD34(+) cells was achieved after overnight incubation with vector particles. Sixteen to 28 percent of individual colony-forming units granulocyte-macrophage (CFU-GM) colonies derived from cord blood CD34(+) cells were positive by polymerase chain reaction (PCR) for the GFP gene. The transduction efficiency of SCID-repopulating cells (SRC) within the cord blood CD34(+) population was assessed by serial transplantation into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. When 400,000 cord blood CD34(+) cells were transplanted into primary recipients, all primary and secondary recipients contained and expressed the transgene. Over 50% of CFU-GM colonies derived from the bone marrow of these primary and secondary recipients contained the vector on average as determined by PCR. Transplantation of transduced cells in limiting dilution generated GFP(+) lymphoid and myeloid progeny cells that may have arisen from a single SRC. Inverse PCR analysis was used to amplify vector-chromosomal junctional fragments in colonies derived from SRC and confirmed that the vector was integrated. These results show that lentiviral vectors can efficiently transduce very primitive human hematopoietic progenitor and stem cells. (Blood. 2000;96:3725-3733)

Proliferative history and hematopoietic function of ex vivo expanded human CD34+ cells

We read with interest the paper of Glimm and Eaves,[1][1] in which the effect of in vitro division on the function of primitive human hematopoietic progenitor cells (HPC) was examined. The interest stemmed from the fact that this paper investigated an issue that we previously examined in a series of

Adenoviral vector–mediated gene transfer to primitive human hematopoietic progenitor cells: assessment of transduction and toxicity in long-term culture

AbstractAdenoviral gene transfer to primitive hematopoietic progenitor cells (HPCs) would be useful in gene therapy applications where transient, high-level transgene expression is required. In the present investigations, we have used an adenoviral vector expressing the green fluorescent protein (AdGFP) to quantify transduction of primitive HPCs and assess adenoviral-associated toxicity in long-term culture. Here we show that a cytokine cocktail protects mass populations of CD34+ cells and primary colony forming unit–cultures (CFU-Cs) from toxicity, enabling transduction of up to 79% of CD34+ cells. Transduction of CFU-Cs and more primitive HPCs was quantified following fluorescence activated cell sorting for green flourescence protein expression. Our results demonstrate transduction of 45% of primary CFU-Cs, 33% of week-5 cobblestone area forming cells (CAFCs), and 18% of week-5 CFU-Cs. However, AdGFP infection inhibited proliferation of more primitive cells. Although there was no apparent quantitative change in week-5 CAFCs, the clonogenic capacity of week-5 AdGFP-infected cells was reduced by 40% (P < .01) when compared with mock-infected cells. Adenoviral toxicity specifically affected the transduced subset of primitive HPCs. Transduction of primitive cells is therefore probably underestimated by week-5 CFU-Cs and more accurately indicated by week-5 CAFCs. These studies provide the first functional and quantitative evidence of adenoviral transduction of primitive HPCs. However, further investigations will be necessary to overcome adenoviral toxicity toward primitive HPCs before adenoviral vectors can be considered a safe option for gene therapy.

Proliferation and functional characterization of human CD34+ bone marrow cells cultured in simulated microgravity

The ability of the rotating wall vessel (RWV) bioreactor, developed by NASA for benchtop simulation of microgravity (μ-G), to support expansion of primitive human hematopoietic progenitor cells (HPC) was assessed during short-term cultures. Human bone marrow (BM) enriched for CD34+ cells (mean purity = 85%, n = 3) was seeded at 1.1 to 2.0 × 10e5 cells/mL in 10 mL in RWV, and incubated in the presence of SCF, IL-3, IL-6, GM-CSF, Flt-3L, and MGDF in either simulated μ-G or earth gravity (1-G). Proliferation of these cells after 5 days of culture averaged 11± 10-fold in μ-G, compared to 4.1 ± 3-fold in 1-G control cultures. Expansion of CD34+ cells was also greater in μ-G cultures compared to 1-G (2.6 ± 2.6 vs. 1.0 ± 0.8, respectively), as well as expansion of more primitive CD34+ CD38−/lo cells (1.9 ± 1.4 vs. 0.8 ± 0.6, respectively). To further examine the hematopoietic potential of CD34+ cells propagated in μ-G, long-term repopulating potential was assessed in NOD/SCID recipients. Conditioned NOD/SCID mice were transplanted with either fresh day 0 CD34-enriched BM cells (570K to 990K/mouse), or progeny of equivalent cell numbers expanded in μ-G (2.5 to 11.4 × 10e6/mouse) or 1-G (1.5 to 3.9 × 10e6/mouse). Fresh CD34+ cells provided 39 ± 14% chimerism at 8 weeks of post-transplantation. Despite possessing in μ-G were deficient in NOD/SCID-repopulating potential compared to 1-G controls (2.1 ± 1.4, n = 4 vs. 14.8 ± 13, n = 4, respectively). To examine whether diminished homing may contribute to the reduced engraftment of μ-G cells, adhesion molecule expression was analyzed. While no differences were found between μ-G and 1-G CD34+ cells in their adhesion molecule repertoire, these cells expressed considerably different levels of CD11a, CD43, CD44, CD49d, CD49e, CD62L, and CXCR4 in comparison to day 0 CD34+ cells, possibly contributing to their reduced engraftment potential. These results in simulated μ-G shed further light on stem cell biology, and may begin to explain some of the hematologic changes occurring in humans exposed to true microgravity.

Regulation of self-renewal divisions of primitive human hematopoietic progenitor cells

Whether self-renewal divisions of primitive hematopoietic progenitor cells is subject to extrinsic control mechanisms remains unclear. Using single cell assays, we examined whether the proliferative potential and self-renewal divisions of daughter cells of individual long-term culture-initiating cells (LTC-IC) can be modulated by exogenous cytokine stimulation. Single G0CD34+CD38−/lo cells were deposited in individual wells of microtiter plates containing complete medium and the three cytokine combination SCF, Flt-3 ligand and MGDF (SFM). All wells were observed daily and after the first division of every primary cell (when two cells were evident in the well between days 1 and 7), one of the two daughter cells was transferred to a new plate prepared with a cytokine combination containing SFM plus IL3, IL6, GM-CSF and erythropoietin (SFM36GE). The destination of each daughter cell was recorded so that clonal pairs could be tracked. If cell growth in one well of a pair was not detected, the pair was discarded. Cells of each clone were harvested on day 7 and examined individually for their HPC content in a 6 week LTC-IC assay. Both daughter cells of 1259 clones split at the 2-cell stage supported continued cell proliferation. However, only 9% of clones in SFM contained ≥ 8 cells on day 7 compared to 45% of their counterparts maintained with SFM36GE. Furthermore, while only 4.5% of clones remaining in SFM were scored at LTC-IC, 19.2% of the SFM36GE clones were determined to be LTC-IC, suggesting that this 7 cytokine combination recruited dormant LTC-IC into cell cycle. Approximately 1/3 of SFM LTC-IC clones were also scored as LTC-IC in SFM36GE suggesting that one of every three divisions of primitive HPC is a self-renewal division. However, of these pairs only 24% divided up to three times by day 7 in SFM compared to 65% in SFM36GE indicating that proliferation of progeny cells following self-renewal divisions may be extrinsically modulated. These results suggest that since the percentage of daughter cell-derived clones scored as LTC-IC and the proliferative potential of every LTC-IC were different under different cytokine conditions, cell cycle activation and fate may be extrinsically controlled.

Adenoviral vector–mediated gene transfer to primitive human hematopoietic progenitor cells: assessment of transduction and toxicity in long-term culture

Adenoviral gene transfer to primitive hematopoietic progenitor cells (HPCs) would be useful in gene therapy applications where transient, high-level transgene expression is required. In the present investigations, we have used an adenoviral vector expressing the green fluorescent protein (AdGFP) to quantify transduction of primitive HPCs and assess adenoviral-associated toxicity in long-term culture. Here we show that a cytokine cocktail protects mass populations of CD34+ cells and primary colony forming unit–cultures (CFU-Cs) from toxicity, enabling transduction of up to 79% of CD34+ cells. Transduction of CFU-Cs and more primitive HPCs was quantified following fluorescence activated cell sorting for green flourescence protein expression. Our results demonstrate transduction of 45% of primary CFU-Cs, 33% of week-5 cobblestone area forming cells (CAFCs), and 18% of week-5 CFU-Cs. However, AdGFP infection inhibited proliferation of more primitive cells. Although there was no apparent quantitative change in week-5 CAFCs, the clonogenic capacity of week-5 AdGFP-infected cells was reduced by 40% (P &lt; .01) when compared with mock-infected cells. Adenoviral toxicity specifically affected the transduced subset of primitive HPCs. Transduction of primitive cells is therefore probably underestimated by week-5 CFU-Cs and more accurately indicated by week-5 CAFCs. These studies provide the first functional and quantitative evidence of adenoviral transduction of primitive HPCs. However, further investigations will be necessary to overcome adenoviral toxicity toward primitive HPCs before adenoviral vectors can be considered a safe option for gene therapy.

Efficient adenoviral vector transduction of human hematopoietic SCID-repopulating and long-term culture-initiating cells.

This article presents our studies on the adenoviral transduction efficiency, level of transgene expression, cell cycle status, and multilineage reconstitution ability of human CD34+ hematopoietic cells transduced under proliferating and survival growth conditions. Bone marrow and umbilical cord blood CD34+ cells were cultured in serum-free medium under survival conditions with thrombopoietin (Tpo) alone, or under proliferating conditions with Tpo, c-Kit ligand (KL), and Flt3 ligand (FL). Adenoviral vectors carrying the enhanced green fluorescent protein (EGFP) gene under the control of the PGK-1 promoter were used to transduce CD34+ cells. Approximately 10% of CD34+ cells were EGFP+ under both culture conditions. In contrast, up to 50% of CD34+CD38- cells were EGFP+, whereas a maximum of 8% of CD34+CD38(high) cells were EGFP+ (p < 0.001). Both colony-forming unit cells (CFU-C) and 5-week long-term culture-initiating cells (LTC-ICs) were efficiently transduced. Under survival conditions, a substantial fraction of transduced CD34+ cells remained quiescent. The nondividing CD34+EGFP+ cells contained LTC-ICs capable of reconstituting longterm culture for as long as 10 weeks. CD34+EGFP+ cells also retained the ability to engraft and multilineage-reconstitute NOD/SCID mice. These observations demonstrate that primitive human hematopoietic progenitor cells can be efficiently transduced by adenoviral vectors.

AAV and RT-PCR: true or false?

Recombinant AAV vectors are capable of long-term gene expression with low toxicity in vivo (1). Therapeutic and sustained protein expression and correction of animal models of disease are now being reported, and human clinical trials are in progress. Several laboratories have evaluated AAV-mediated transduction by a variety of methods, including the technique of RT-PCR (2–13). However, there is a potential flaw with RT-PCR analysis in this context: Specifically, a false-positive signal can be obtained from contaminating AAV single-stranded DNA (ssDNA).