Developmental-stage-specific expression and regulation of an amphotropic retroviral receptor in hematopoietic cells

Development of HIV-1 vectors pseudotyped with envelope proteins of other retroviruses

In the past three decades, human immunodeficiency virus type 1 (HIV-1)-derived vectors were evolved and became indispensable to transduce therapeutic genes into a range of different target cell types to facilitate a variety of gene therapeutic strategies. To achieve this, i) the biosafety profile of the vectors was incrementally enhanced and ii) the CD4-restricted tropism mediated by the envelope proteins (Env) of the parental virus needed to be directed towards recruitment of other receptors expressed on the desired target cells. Here, a closer look is first taken at the development of vector components and the mechanisms of Env incorporation into particles. While envelope proteins originating from a broad range of very diverse virus species were successfully utilized, members of the Retroviridae family most frequently provided Env or further engineered variants thereof to form transduction-competent HIV-1 pseudotype vector particles. The development of these vectors is reviewed and anticipated to further contribute to the future progression of somatic gene therapy.

Loss of PiT-1 results in abnormal endocytosis in the yolk sac visceral endoderm

PiT-1 protein is a transmembrane sodium-dependent phosphate (Pi) transporter. PiT-1 knock out (KO) embryos die from largely unknown causes by embryonic day (E) 12.5. We tested the hypothesis that PiT-1 is required for endocytosis in the embryonic yolk sac (YS) visceral endoderm (VE). Here we present data supporting that PiT-1 KO results in a YS remodeling defect and decreased endocytosis in the YS VE. The remodeling defect is not due to an upstream cardiomyocyte requirement for PiT-1, as SM22αCre-specific KO of PiT-1 in the developing heart and the YS mesodermal layer (ME) does not recapitulate the PiT-1 global KO phenotype. Furthermore, we find that high levels of PiT-1 protein localize to the YS VE apical membrane. Together these data support that PiT-1 is likely required in YS VE. During normal development maternal immunoglobulin (IgG) is endocytosed into YS VE and accumulates in the apical side of the VE in a specialized lysosome termed the apical vacuole (AV). We have identified a reduction in PiT-1 KO VE cell height and a striking loss of IgG accumulation in the PiT-1 KO VE. The endocytosis genes Tfeb, Lamtor2 and Snx2 are increased at the RNA level. Lysotracker Red staining reveals a loss of distinct AVs, and yolk sacs incubated ex vivo with phRODO Green Dextran for Endocytosis demonstrate a functional loss of endocytosis. As yolk sac endocytosis is controlled in part by microautophagy, but expression of LC3 had not been examined, we investigated LC3 expression during yolk sac development and found stage-specific LC3 RNA expression that is predominantly from the YS VE layer at E9.5. Normalized LC3-II protein levels are decreased in the PiT-1 KO YS, supporting a requirement for PiT-1 in autophagy in the YS. Therefore, we propose the novel idea that PiT-1 is central to the regulation of endocytosis and autophagy in the YS VE.

Sodium-dependent phosphate cotransporters and phosphate-induced calcification of vascular smooth muscle cells: redundant roles for PiT-1 and PiT-2

OBJECTIVE

Elevated serum phosphate has emerged as a major risk factor for vascular calcification. The sodium-dependent phosphate cotransporter, PiT-1, was previously shown to be required for phosphate-induced osteogenic differentiation and calcification of cultured human vascular smooth muscle cells (VSMCs), but its importance in vascular calcification in vivo and the potential role of its homologue, PiT-2, have not been determined. We investigated the in vivo requirement for PiT-1 in vascular calcification using a mouse model of chronic kidney disease and the potential compensatory role of PiT-2 using in vitro knockdown and overexpression strategies.

APPROACH AND RESULTS

Mice with targeted deletion of PiT-1 in VSMCs were generated (PiT-1(Δsm)). PiT-1 mRNA levels were undetectable, whereas PiT-2 mRNA levels were increased 2-fold in the vascular aortic media of PiT-1(Δsm) compared with PiT-1(flox/flox) control. When arterial medial calcification was induced in PiT-1(Δsm) and PiT-1(flox/flox) by chronic kidney disease followed by dietary phosphate loading, the degree of aortic calcification was not different between genotypes, suggesting compensation by PiT-2. Consistent with this possibility, VSMCs isolated from PiT-1(Δsm) mice had no PiT-1 mRNA expression, increased PiT-2 mRNA levels, and no difference in sodium-dependent phosphate uptake or phosphate-induced matrix calcification compared with PiT-1(flox/flox) VSMCs. Knockdown of PiT-2 decreased phosphate uptake and phosphate-induced calcification of PiT-1(Δsm) VSMCs. Furthermore, overexpression of PiT-2 restored these parameters in human PiT-1-deficient VSMCs.

CONCLUSIONS

PiT-2 can mediate phosphate uptake and calcification of VSMCs in the absence of PiT-1. Mechanistically, PiT-1 and PiT-2 seem to serve redundant roles in phosphate-induced calcification of VSMCs.

Pit2 assemblies at the cell surface are modulated by extracellular inorganic phosphate concentration

Pit2 is a type III sodium-dependent phosphate transporter and the cell surface receptor for amphotropic murine leukemia virus. Indirect arguments have previously suggested that retrovirus receptor assembly play a role in triggering membrane fusion. Using CHO cells expressing physiological amounts of functional versions of human Pit2 fused to various tagging epitopes, we provide evidence that Pit2 forms assemblies at the cell surface. Living cells were exposed to cross-linking reagents and protein extracts were treated with trifluoroacetic acid (TFA), a chemical that destroys all protein interactions but covalent links. Assemblies were also detected in the absence of cross-linking and TFA treatment, indicating that they are partially resistant to detergent denaturation. The formation of homo-oligomers was documented by the coimmunoprecipitation of differently tagged molecules. The amounts of Pit2 assemblies detected in the presence or in the absence of cross-linking reagents varied with extracellular inorganic phosphate concentration ([P(i)]). Variation of signal intensity was in the range of twofold, occurred in the absence of de novo protein synthesis and took place at the cell surface. These results indicate that Pit2 assemblies exhibit variable conformations at the surface of living cells. Susceptibility to virus infection and phosphate uptake also vary with extracellular [P(i)]. A model is proposed in which cell surface Pit2 assemblies switch from a compacted to an expanded configuration in response to changes of extracellular [P(i)], and possible relationships with the variation of biological activities are discussed.

Alterations in the sensing and transport of phosphate and calcium by differentiating chondrocytes

During endochondral bone formation and fracture healing, cells committed to chondrogenesis undergo a temporally restricted program of differentiation that is characterized by sequential changes in their phenotype and gene expression. This results in the manufacture, remodeling, and mineralization of a cartilage template on which bone is laid down. Articular chondrocytes undergo a similar but restricted differentiation program that does not proceed to mineralization, except in pathologic conditions such as osteoarthritis. The pathogenesis of disorders of cartilage development and metabolism, including osteochondrodysplasia, fracture non-union, and osteoarthritis remain poorly defined. We used the CFK2 model to examine the potential roles of phosphate and calcium ions in the regulatory pathways that mediate chondrogenesis and cartilage maturation. Differentiation was monitored over a 4-week period using a combination of morphological, biochemical, and molecular markers that have been characterized in vivo and in vitro. CFK2 cells expressed the type III sodium-dependent phosphate transporters Glvr-1 and Ram-1, as well as a calcium-sensing mechanism. Regulated expression and activity of Glvr-1 by extracellular phosphate and parathyroid hormone-related protein was restricted to an early stage of CFK2 differentiation, as evidenced by expression of type II collagen, proteoglycan, and Ihh. On the other hand, regulated expression and activity of a calcium-sensing receptor by extracellular calcium was most evident after 2 weeks of differentiation, concomitant with an increase in type X collagen expression, alkaline phosphatase activity and parathyroid hormone/parathyroid hormone-related protein receptor expression. On the basis of these temporally restricted changes in the sensing and transport of phosphate and calcium, we predict that extracellular phosphate plays a role in the commitment of chondrogenic cells to differentiation, whereas extracellular calcium plays a role at a later stage in their differentiation program.

Enhancement of retrovirus-mediated gene transfer to rat liver in vivo by infusion of hepatocyte growth factor and triiodothyronine

BACKGROUND/AIMS

Gene transfer using recombinant Moloney murine leukemia viruses (rMoMuLV) requires mitosis of the target cell. Previously, we and others have used partial hepatectomy for induction of hepatocellular proliferation for gene transfer to the liver in vivo by exsanguineous perfusion with rMo-MuLV. We hypothesized that induction of hepatocellular proliferation by combined administration of two hepatocellular mitogens, hepatocyte growth factor (HGF) and triiodothyronine (T3), should permit rMo-MuLV-mediated gene transfer into liver without invasive approaches.

METHODS

HGF (1 mg/kg) was perfused continuously into the portal vein of Wistar male rats and T3 (2 mg/kg) was injected subcutaneously. Twenty-four hours after injecting HGF and T3, the state of proliferation of hepatocytes was estimated from the incorporation of 5'-bromo-2'-deoxy-uridine (BrdU). The amphotropic retroviral receptor (Ram-1) expression of liver was evaluated at different time points after injecting HGF and T3 by means of Northern blotting using Ram-1 cDNA probe. In order to evaluate the role of hormone treatment on gene transfer, the liver was perfused exsanguineously with rMoMuLV 24 h after injection with hormones.

RESULTS

Rats treated with a combination of HGF and T3 expressed BrdU and beta-galactosidase in 8.3% and 0.7% of hepatocytes, respectively. On the other hand, there was near absence of gene transfer in untreated rats perfused with rMoMuLV Twenty-four hours after the initial manipulation, abundant expression of Ram-1 mRNA was observed in rat hepatocytes treated with HGF plus T3.

CONCLUSIONS

Stimulation of hepatocellular mitosis and upregulation of Ram-1 expression by HGF and T3 augment retrovirus-mediated gene transfer into hepatocytes.

Lentiviral vectors pseudotyped with envelope glycoproteins derived from gibbon ape leukemia virus and murine leukemia virus 10A1

Lentiviral vectors pseudotyped with the envelope glycoproteins (Env) of amphotropic murine leukemia virus (MLV) and the G protein of vesicular stomatitis virus (VSV-G) have been successfully used in recent preclinical gene therapy studies. We report here the generation of infectious HIV-1-derived vector particles pseudotyped with the Env of the molecular clone 10A1 of MLV and with chimeric envelope glycoprotein variants derived from gibbon ape leukemia virus (GaLV) and MLV. Formation of infectious HIV-1 (GaLV) pseudotype vectors was only possible with the substitution of the cytoplasmic tail of GaLV Env with that of MLV. The lentiviral vectors exhibited a host cell range identical with that of MLV(GaLV) and MLV(10A1) vectors, which are known to enter cells either via the GaLV-receptor Glvr-1 (Pit-1) or via the amphotropic receptor Ram-1 (Pit-2) in addition to Glvr-1, respectively. Thus, HIV-1(GaLV) and HIV-1(10A1) pseudotype vectors may be useful for efficient gene transfer into a variety of human tissues like primary human hematopoietic cells.

Effect of changes in expression of the amphotropic retroviral receptor PiT-2 on transduction efficiency and viral titer: implications for gene therapy

The aim of this study was to quantify the impact of amphotropic retroviral receptor (PiT-2) levels on susceptibility to transduction and to determine whether the low level of PiT-2 found on CD34+ hematopoietic cells is within the range likely to compromise gene transfer. Receptor-deficient Chinese hamster ovary (CHO) cells were transfected with a PiT-2 construct that could be induced by the removal of tetracycline. The level of PiT-2 expression measured by virus binding in uninduced and in fully and partially induced transfectants correlated with the efficiency of transduction by an amphotropic retroviral reporter vector. Fully induced CHO-PiT-2 cells gave apparent viral titers similar to NIH 3T3 fibroblasts while addition of tetracycline reduced titers by up to 140-fold. Binding of the same vector preparation to purified CD34+ peripheral blood stem cells (PBSCs) was always less than to uninduced CHO-PiT-2 transfectants even after preincubation in 10-ng/ml concentrations of IL-3, IL-6, and stem cell factor, which increased retroviral binding by an average of 35%. The level of expression of the amphotropic retroviral receptor PiT-2 thus significantly limits transduction efficiency within the range observed in target cells of importance in human gene therapy.

Improved retroviral transduction of hematopoietic progenitors by combining methods to enhance virus-cell interaction

One of the factors required for successful retroviral transduction is contact between viral particles and target cells. We hypothesized that combining agents that improve virus-target cell interaction via different mechanisms will increase transduction efficiency. We examined the transduction efficiency of leukemic K562 cells, primary normal and chronic myelogenous leukemia CD34+ cells with the amphotropic retroviral vector, G1Na, packaged in PA317 by enumerating G418-resistant colonies in semisolid media. We evaluated the ability of the recombinant fibronectin fragment, CH296, cationic lipids, or a transwell flow-through system, alone or in combination to improve retroviral transduction. Transduction of K562 cells improved 1.5 to two-fold with lipids or CH296, while their combination improved transduction 2.5-fold. Transduction of K562 cells in the transwell flow-through system improved transduction three-fold. Transduction of normal (NL) CD34+ CFC improved 10-fold with lipids and 20-fold with CH296. Lipid and CH296 had synergistic effects. The transwell flow-through system improved transduction of normal CD34+ CFC 30-fold. Finally, similar to what was seen for K562 cells, transduction of CML CFC improved two- to three-fold with either CH296 or lipids, whereas the combination had synergistic effects. We conclude that any physical means that enhances contact between viral particles and target cells improves transduction. Two such methods that have different action mechanisms have additive or synergistic effects on transduction.

DeltahGHR, a novel biosafe cell surface-labeling molecule for analysis and selection of genetically transduced human cells

We describe a new selectable marker for retroviral transduction and selection of human and murine cells. The molecule expressed on the cell surface of the transduced population is a truncated version of human growth hormone receptor (deltahGHR), capable of ligand (hGH) binding, but devoid of the domains involved in signal triggering. We demonstrate that the engineered molecule is stably expressed in the target cells as an inert protein unable to trigger proliferation or to rescue the cells from apoptosis after ligand binding. This new marker will probably have a wide application spectrum, since hGHR in the human adult is highly expressed only in liver cells, and lower levels have been reported in certain lymphocyte cell populations. The deltahGHR label has high biosafety potential, as it belongs to a well-characterized hormonal system that is nonessential in adults, and there is extensive clinical experience with hGH administration in humans. This record allows us to hypothesize the lack of relevant clinical consequences resulting from massive transgene expression caused by successful replacement of a large tissue with genetically transduced cells. We take advantage of the differential binding properties of several monoclonal antibodies (MAbs) in describing a cell rescue method in which the antibody used to select deltahGHR-transduced cells is eluted by competition with hGH or, alternatively biotinylated hGH is used to capture tagged cells. In the latter system, the final purified population would be recovered free of attached antibodies in hGH (a substance approved for human use)-containing medium, providing additional biosafety relative to currently existing methods that rely on the use of murine MAb to rescue genetically labeled cells.

Retroviral vector-mediated gene expression in human CD34+CD38- cells expanded in vitro: cis elements of FMEV are superior to those of Mo-MuLV

A novel murine stromal cell line, HESS-M28, was established, which supports the expansion of human CD34+CD38- cells more than 300-fold in vitro in the presence of human IL-3 and SCF. These cells were used in an attempt to evaluate cis-acting elements of retroviral vectors in human primitive hematopoietic cells. Cord blood cells were cultured on top of the mixed cell layers of the stromal cell line, HESS-M28, and retroviral vector-producing cells. The FMEV-type vector SF/Lyt contained the spleen focus-forming virus U3 and the MESV primer-binding site (PBS), while MO3/Lyt contained the U3 region and PBS from Mo-MuLV. After transduction by the FMEV-type and Mo-MuLV-based vectors, expression of the marker gene murine CD8 (mCD8) was examined in CD34-, CD34+, and CD34+CD38- cells. In CD34+ and CD34+CD38- cells, expression of mCD8 was higher with the FMEV-type vector, SF/Lyt, compared with the cells transduced by the Mo-MuLV-based vector MO3/Lyt, although the expression was comparable in CD34- cells. Expression of marker genes was also confirmed in long-term culture-initiating cells (LTC-ICs) and SCID-repopulating cells (SRCs).

Prospects for fetal gene therapy

Advances in prenatal diagnosis and gene transfer technology have allowed consideration of prenatal gene therapy. A compelling argument can be made for this strategy in treating genetic diseases that are fatal in the prenatal or perinatal period. In other diseases, the fetal environment may offer unique biological advantages that favor a prenatal gene therapy strategy over treatment after birth. Although issues of safety and efficacy must be resolved before clinical application, the development of fetal gene therapy may become a new molecular therapeutic arm in the field of prenatal intervention.

Up-regulation of amphotrophic retroviral receptor expression in human peripheral blood CD34+ cells

Retroviral-mediated gene transfer into hematopoietic stem cells provides the only means of stable transduction of these cells and their progeny for use with a variety of potentially therapeutic genes. Expression of the Moloney amphotropic retroviral receptor-pit-2 or GLVR-2-is critical to the recognition and entry of Moloney leukemia virus-derived viruses into human target cells such as CD34+ hematopoietic cells. GLVR-2 functions as a sodium-dependent phosphate transporter as well as a receptor. We have previously shown that the expression of the murine homologue of the amphotropic receptor Ram 1, also a phosphate transporter, is developmentally regulated in murine hematopoietic fetal liver cells. We also demonstrated that culture of murine fetal liver cells in phosphate-free (PO(4)-free) medium increases levels of receptor mRNA and makes murine fetal liver cells susceptible to Moloney amphotropic viral gene transfer. We now examine the effect of culture conditions on the expression of GLVR-2 in human CD34+ cells. In this report, we demonstrate that there is a 2-3 fold increase in GLVR-2 mRNA levels in CD34+ cells after 3 days in culture with interleukin 3, interleukin 6, and stem-cell factor. In addition, the use of PO(4)-free medium increases expression of GLVR-2 an additional 2-fold in these cells during this time. These results indicate that GLVR-2 expression can be up-regulated on these cells, and may permit improved retroviral gene transfer efficiencies.

Efficient gene transfer by hybrid retroviral vectors to murine spermatogenic cells

Using murine spermatogenic cell lines GC-1 spg and GC-2 spd(ts) as target cells, an attempt was made to design a retroviral vector that would transduce genes efficiently. Promoter activities of various retroviral long terminal repeats (LTRs) were examined by using chloramphenicol acetyltransferase (CAT) as a reporter. The U3 region of spleen focus-forming virus (SFFVp) showed higher enhancer activity than that of Moloney murine leukemia virus (Mo-MuLV) in both cell lines. The U3 region of myeloproliferative sarcoma virus (MPSV) showed higher activity only in GC-1 spg cells. Expression was suppressed by the repressor element of the primer-binding site (PBS) of the Moloney-related virus. The efficiency of transduction of the multidrug-resistance gene (mdr-1) by an Mo-MuLV-based vector was compared with hybrid vectors consisting of the murine embryonic stem cell virus (MESV) PBS and the LTR of either SFFVp or MPSV. Rhodamine efflux assays and colchicine-resistant colony-forming assays demonstrated higher gene expression by the hybrid vectors. Amphotropic and ecotropic receptors were found to be expressed and functional in both cell lines. Thus, these hybrid vectors represent a powerful tool by which to transfer genes into spermatogenic cells.

In vivo transduction of mouse epidermis with recombinant retroviral vectors: implications for cutaneous gene therapy

Gene-based therapies may provide a way to treat inherited skin disorders but current approaches suffer serious limitations. The surgical procedures required to transplant ex vivo modified keratinocytes are likely to result in scarring and contracture, thereby limiting the area that can be treated. In addition, none of the methods currently available for in vivo gene transfer to epidermis leads to long-term transgene expression. The goal of this study was to develop a means for in vivo gene transfer to epidermis that would result in long-term transgene expression. We report here the first successful in vivo gene transfer that results in sustained transgene expression in epidermis. Hyperplastic mouse skin was transduced by direct injection of VSV-G pseudotyped retroviral vectors encoding the LacZ reporter gene. In mice tolerant to beta-galactosidase (beta-gal), transgene expression was noted in hair follicles and interfollicular epidermis for the duration of the experiment (16 weeks after transduction). Based on the kinetics of epidermal turnover in mouse skin, expression for this length of time strongly suggests stem cell transduction. In immunocompetent mice intolerant to beta-gal, transgene expression was lost by 3 weeks after transduction, concurrent with the onset of host immune responses to the transgene product.

High-efficiency transduction and long-term gene expression with a murine stem cell retroviral vector encoding the green fluorescent protein in human marrow stromal cells

Bone marrow stromal cells (MSCs) are unique mesenchymal cells that have been utilized as vehicles for the delivery of therapeutic proteins in gene therapy protocols. However, there are several unresolved issues regarding their potential therapeutic applications. These include low transduction efficiency, attenuation of transgene expression, and the technical problems associated with drug-based selection markers. To address these issues, we have developed a transduction protocol that yields high-level gene transfer into human MSCs, employing a murine stem cell virus-based bicistronic vector containing the green fluorescent protein (GFP) gene as a selectable marker. Transduction of MSCs plated at low density for 6 hr per day for 3 days with high-titer viral supernatant resulted in a gene transfer efficiency of 80+/-6% (n = 10) as measured by GFP fluorescence. Neither centrifugation nor phosphate depletion increased transduction efficiency. Assessment of amphotropic receptor (Pit-2) expression by RT-PCR demonstrated that all MSCs expressing the receptor were successfully transduced. Cell cycle distribution profiles measured by propidium iodide staining showed no correlation with the susceptibility of MSCs to transduction by the retroviral vector. Human MSCs sequentially transduced with an adenoviral vector encoding the ecotropic receptor and ecotropic retroviral vector encoding GFP demonstrated that all MSCs are susceptible to retroviral transduction. We further showed that both genes of bicistronic vector are expressed for at least 6 months in vitro and that transgene expression did not affect the growth or osteogenic differentiation potential of MSCs. Future studies will be directed toward the development of gene therapy protocols employing this strategy.

Amphotropic retrovirus transduction of hematopoietic stem cells

Mice treated with cytokines for 5 days have large numbers of hematopoietic stem cells (HSCs) in their peripheral blood and bone marrow at 1 and 14 days after the last injection. We fractionated the HSCs from the bone marrow of these mice using elutriation at flow rates of 25, 30 and 35 ml/min. The subpopulations of HSCs from cytokine-treated mice show a 3- to 8-fold higher level of mRNA encoding the amphotropic retrovirus receptor (amphoR) compared with the corresponding HSC subpopulation from untreated mouse bone marrow. In an earlier study with mouse HSCs we showed a direct correlation between high levels of amphoR mRNA and efficient retrovirus transduction. We have now utilized our gene transfer protocol to assay amphotropic retrovirus transduction efficiency using HSCs from the bone marrow of mice treated with granulocyte-colony stimulating factor/stem cell factor (G-CSF/SCF). To extend these findings to a more clinically relevant protocol we analyzed the amphoR mRNA levels in HSCs from human cord blood and adult bone marrow. The amphoR mRNA level in HSCs from human bone marrow and fresh cord blood was detectable at an extremely low level compared with the HSC population in cryopreserved cord blood samples. The 12- to 22-fold increase in amphoR mRNA in HSCs from cryopreserved cord blood renders these HSCs likely candidates for high efficiency, gene transfer.

Isolation of stem cell-specific cDNAs from hematopoietic stem cell populations

We have begun to isolate gene sequences that are specifically expressed in hematopoietic stem cells (HSCs). There are at least three fundamental requirements for the isolation of HSC-specific transcripts. First, highly enriched populations of HSCs, and an HSC-depleted cell population for comparison must be isolated. Secondly, the gene isolation procedures must be adapted to accommodate the small amounts of RNA obtained from purified HSCs. Finally, a defined screening strategy must be developed to focus on sequences to be examined in more detail. In this report, we describe the characterization of populations of HSCs that are highly enriched (Lin- c-kitHI) or depleted (Lin- c-kitNEG) of HSCs. We compared two methods for gene isolation, differential display polymerase chain reaction (DD-PCR) and subtractive hybridization (SH), and found that the latter was more powerful and efficient in our hands. Lastly we describe the strategy that we have developed to screen clones for further study.

Influence of cell polarity on retrovirus-mediated gene transfer to differentiated human airway epithelia

Gene transfer with recombinant murine leukemia viruses (MuLV) provides the potential to permanently correct inherited lung diseases, such as cystic fibrosis (CF). Several problems prevent the application of MuLV-based recombinant retroviruses to lung gene therapy: (i) the lack of cell proliferation in mature pulmonary epithelia, (ii) inefficient gene transfer with a vector applied to the apical surface, and (iii) low titers of many retroviral preparations. We found that keratinocyte growth factor (KGF) stimulated proliferation of differentiated human tracheal and bronchial epithelia. Approximately 50% of epithelia divided in response to KGF as assessed by bromodeoxyuridine histochemistry. In airway epithelia stimulated to divide with KGF, high-titer ampho- and xenotropic enveloped vectors preferentially infected cells from the basal side. However, treatment with hypotonic shock or EGTA transiently increased transepithelial permeability, enhancing gene transfer with the vector applied to the mucosal surfaces of KGF-stimulated epithelia. Up to 35% of cells expressed the transgene after gene transfer. By using this approach, cells throughout the epithelial sheet, including basal cells, were targeted. Moreover, the Cl- transport defect in differentiated CF airway epithelia was corrected. These findings suggest that barriers to apical infection with MuLV can be overcome.

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.

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.

Entry of amphotropic and 10A1 pseudotyped murine retroviruses is restricted in hematopoietic stem cell lines

Although transduction with amphotropic murine leukemia virus (MLV) vectors has been optimized successfully for hematopoietic differentiated progenitors, gene transfer to early hematopoietic cells (stem cells) is still highly restricted. A similar restriction to gene transfer was observed in the mouse stem cell line FDC-Pmix compared with transfer in the more mature myeloid precursor cell line FDC-P1 and the human erythroleukemia cell line K562. Gene transfer was not improved when the vector was pseudotyped with gp70SU of the 10A1 strain of MLV, which uses the receptor of the gibbon ape leukemia virus (Pit1), in addition to the amphotropic receptor (Pit2). Although 10A1 and amphotropic gp70SU bound to FDC-P1, K562, and fibroblasts, no binding to FDC-Pmix cells was detected. This indicates that FDC-Pmix cells lack functional Pit2 and Pit1 receptors. Pseudotyping with the vesicular stomatitis virus G protein improved transduction efficiency in FDC-Pmix stem cells by 2 orders of magnitude, to fibroblast levels, confirming a block to retroviral infection at the receptor level.

Phosphate depletion enhances the stability of the amphotropic murine leukemia virus receptor mRNA

Through its specific receptor, the amphotropic murine leukemia virus (MLV) infects cells from many mammals, including humans. We have previously demonstrated that levels of human amphotropic MLV receptor (pit2) mRNA varied considerably in different human cell lines. Removal of phosphate from the culture medium led to increases in the amount of pit2 mRNA and the quantity of a 71-kDa protein specifically recognized by antibodies against Pit2. To determine if the increases in pit2 mRNA and protein levels were due to a transcriptional effect, the pit2 promoter region was cloned. This region was characterized and found to contain a functional TATA-less promoter that under our experimental conditions does not respond to phosphate depletion. Instead, pit2 mRNA was found to be more stable in response to Pi depletion. These results suggest that the increase in pit2 mRNA levels observed in response to Pi depletion occurs at a posttranscriptional level and is due to enhanced mRNA stability.

Amphotropic or gibbon ape leukemia virus retrovirus binding and transduction correlates with the level of receptor mRNA in human hematopoietic cell lines

The low level of amphotropic retrovirus mediated gene transfer into human hematopoietic stem cells (HSC) has been an impediment to gene therapy for hematopoietic diseases (1). We have previously shown that mouse and human HSC have low levels of the mRNA encoding PiT-2, the amphotropic retrovirus receptor. We hypothesized that the low level of PiT-2 mRNA was responsible for the low frequency of transduction of HSC by amphotropic retroviral vectors (2). In this study we compared the level of PiT-2 and PiT-1, the Gibbon Ape Leukemia Virus receptor (GaLV), in 5 human tissue culture cell lines. PiT-2 and PiT-1 mRNA levels were highest in K562 cells and lowest in HL60 cells. In hematopoietic cell lines, the level of PiT-2 or PiT-1 mRNA correlated directly with retrovirus binding and transduction with the appropriate (amphotropic or GaLV) retrovirus vector. The level of expression of PiT-2 and PiT-1 mRNA could be increased by treatment of HL60 cells with either PMA or Interleukin-1alpha. The increase in the level of PiT-2 and PiT-1 mRNA correlated with increased transduction with both amphotropic and GaLV retroviral vectors. We conclude that the improved transduction was a direct effect of the increased levels of receptor mRNA and unrelated to changes in the cell cycle status.

Gene transfer into haemopoietic cells

The therapeutic potential achievable by efficient transfer and expression of genes into haemopoietic stem cells (HSC) is enormous. In addition to inherited disorders such as haemoglobinopathies and lysosomal storage disorders, this technology can be applied to acquired disorders such as myelosuppression induced by anticancer chemotherapy or infection with human immunodeficiency virus (HIV). To date retroviral vectors are the most attractive modality for gene transfer into HSC. Unfortunately, the expectations of gene therapy are more advanced than the methodology needed to fulfil the goals. In this chapter, the current concepts and limitations in the genetic manipulation of haemopoietic cells are presented. Overcoming these limitations requires not only improvement in isolation and expansion of HSC that contribute to long-term repopulation, but also development of better retroviral transfer systems. Current restrictions occur at various levels in the viral transfer process, including efficient cell entry, regulated expression levels, and sustained expression. The analysis of retroviral mutants has proven to be a successful approach to developing effective retroviral vectors for HSC gene therapy.