Persisting multilineage transgene expression in the clonal progeny of a hematopoietic stem cell

Many applications of hematopoietic gene therapy require selection for clones with active transgene expression. However, it was unclear whether the clonal progeny of a retrovirally transduced hematopoietic stem cell would be capable of maintaining transgene expression through serial repopulation and multilineage differentiation. Such investigations require simultaneous analyses of clonality, multilineage activity and transgene copy numbers. Using a mouse model, the present study demonstrates that a single hematopoietic stem cell expressing a marker gene from one or two insertions of a simple retroviral vector actively maintains multilineage transgene expression in the vast majority (80-99%) of bone marrow and peripheral blood cells. Gene expression persisted through serial transplantations for at least 97 weeks post gene transfer and was observed in the lymphoid (B, T and NK cells), myeloid (CD11b(+), Gr-1(+)), erythroid (Ter119(+), mature red blood cells) and megakaryocytic (as indicated by platelets) progeny. Therefore, a single immunoselection for hematopoietic stem cells expressing the transgene in vivo was sufficient to establish a completely chimeric hematopoiesis. These observations imply that the retroviral vectors used in this study contain cis-elements that mediate expression through massive clonal expansion and multilineage differentiation, provided the insertion occurred in genetic loci permissive for expression in hematopoietic stem cells.

Functional identification of secondary mutations inducing autonomous growth in synergy with a truncated interleukin-3 receptor Implications for multi-step oncogenesis

OBJECTIVE

A truncated common beta chain (Deltabeta(C)) of the interleukin-3 (IL-3) receptor complex was previously identified as a key factor in inducing autonomous growth of IL-3-independent mutants. Expression of Deltabeta(C) in IL-3-dependent hematopoietic cells does not result in immediate factor-independent growth, but increases the frequency of obtaining autonomous mutants by three to four orders of magnitude. This study was designed to delineate the mechanisms by which Deltabeta(C) increases the frequency to autonomous growth.

DESIGN AND METHODS

Retroviral vectors were used to express Deltabeta(C) into IL-3-dependent myeloid cells, which were then tested for factor-independent growth. To determine if secondary genetic events were required for conversion to autonomous growth, elements of the Cre-loxP recombinant system were used to excise Deltabeta(C) in factor-independent clones.

RESULTS

Excision of Deltabeta(C) in factor-independent clones revealed two types of phenotypes: reversion to factor-dependent growth (1/8) or continued IL-3-dependent growth (7/8). Analysis of cells that remained factor independent revealed constitutive activation of STAT5, not observed in factor-dependent revertants. Analysis of revertant cells demonstrated the presence of interacting secondary mutations that synergize with Deltabeta(C)-induced proliferation. A cysteine residue within the truncated extracellular domain of Deltabeta(C) was also found to be required for its oncogenic potential, supporting a model of dimerization for receptor activation.

CONCLUSIONS

The high incidence of obtaining factor-independent mutants from cells expressing Deltabeta(C) results from the selection of mutations that either complement Deltabeta(C) expression to promote proliferation or that singly or in synergy with other secondary mutations negate the requirement of Deltabeta(C) expression for proliferation.

Retroviral vector-mediated expression of HoxB4 in hematopoietic cells using a novel coexpression strategy

Retroviral vector-mediated expression of the homeoboxgene, HoxB4, in hematopoietic cells has been reported to mediate a benign expansion of gene-modified hematopoietic stem and precursor cells in vivo. In the present study, we used a novel coexpression strategy for coordinated expression of HoxB4 along with a cytoplasmic protein from a retroviral vector. The novel coexpression strategy, based on cotranslational protein separation mediated by the 2A sequence of foot-and-mouth disease virus (FMDV), allows an indirect quantification of HoxB4 expression levels when inserting a reporter such as the enhanced green fluorescent protein (GFP) in the retroviral vector. Presence of the 2A sequence did not interfere with the correct subcellular localization of HoxB4 (nuclear) and GFP (cytoplasmic), nor with the titer of bicistronic vectors, and mediated functional long-term coexpression (at least 1 year) of GFP and HoxB4 after transplantation of transduced mouse bone marrow cells in mice.

Improved post-transcriptional processing of an MDR1 retrovirus elevates expression of multidrug resistance in primary human hematopoietic cells

We describe the functional analysis of a novel retroviral vector, SF91m3, which was designed for improved expression of the in vivo selectable marker, multidrug resistance 1 gene (MDR1), in hematopoietic cells. SF91m3 combines several promising features. The vector backbone lacks viral coding sequences and AUG-start codons 5' of the MDR1 cDNA. A point mutation of a cryptic splice acceptor of the MDR1 cDNA increases the probability of transferring an intact provirus. The titer of a PG13 packaging cell clone containing a single proviral integration is high (>2 x 10(6) particles/ml from frozen stocks of serum-free vector harvests). Human hematopoietic cells transduced with SF91m3 reliably express MDR1 before and after passage through NOD/SCID mice, as shown by quantitative PCR and efflux assays with rhodamine 123 or Hoechst 33342. Finally, SF91m3 mediates resistance to escalated doses of cytotoxic agents, as shown by survival and differentiation of transduced colony-forming cells in the presence of colchicine at 48 ng/ml (>10 x IC(50)). Thus, SF91m3 may represent an interesting candidate for future trials addressing the safety and utility of MDR1 gene transfer; moreover, this study demonstrates that sequence alterations improving post-transcriptional processing of retroviral vectors have a substantial impact for gene expression in hematopoietic cells.

Recombinant expression of lymphocytic choriomeningitis virus strain WE glycoproteins: a single amino acid makes the difference

Cytoplasmic vector systems are generally used for expression of lymphocytic choriomeningitis virus (LCMV) proteins. However, we achieved high levels of cell surface glycoproteins using a standard nuclear expression plasmid. Expression was independent of other LCMV proteins but was blocked by a missense mutation within the original LCMV(WE) glycoprotein cDNA.

cis-Active elements of Friend spleen focus-forming virus: from disease induction to disease prevention

The polycythemic strain of the Friend spleen focus-forming virus (SFFVp) is a replication-defective, acutely transforming retrovirus inducing a bistage erythroleukemia in susceptible mice. The first stage of the disease is an acute polyclonal erythroblastosis induced by the proliferation-promoting effect of gp55. gp55 is expressed from a spliced subgenomic message of SFFVp and stimulates the cellular receptor for erythropoietin. Using a selectable SFFVp that otherwise mimics the specificity of the disease, we demonstrate that the kinetics of the polyclonal expansion depends on the transcriptional strength of the retroviral cis-active elements. By exchanging gp55 for apathogenic genes, we show that SFFVp enhancer and splice signals can be successfully utilized for the development of retroviral vectors mediating very efficient transgene expression in hematopoietic cells. Apathogenic selectable SFFVp-based vectors carrying distinct enhancer alterations are a valuable tool to analyze transcriptional control of leukemia viruses in the absence of oncogenic proteins. Moreover they might have therapeutic potential.

Lack of functional Pit-1 and Pit-2 expression on hematopoietic stem cell lines

Hematopoietic stem cells (HSC) are an important target for retroviral gene transfer. However, transduction efficiency in these HSC is extremely low compared to fibroblasts or more mature hematopoietic cells. This infection block was analyzed in the HSC line FDC-Pmix. The infection frequency with the amphotropic murine leukemia virus (MLV-A) is more than 100-fold lower in FDC-Pmix cells as compared to fibroblasts. Pseudotyping with the env of the 10A1 strain (MLV-10A1), which uses both the amphotropic receptor (Pit-2) and the receptor for gibbon ape leukemia virus (Pit-1), did not improve the infection efficiency. Vectors pseudotyped with VSV G protein were found to overcome the infection block in FDC-Pmix, confirming that the block is at the level of virus binding and possibly penetration. Accordingly, we could not detect virus binding of MLV-A or MLV-10A1 to FDC-Pmix cell lines. Northern blot analysis was performed to detect whether the defect is at the level of transcription. Surprisingly, similar levels of Pit-2 receptor transcripts were detected in all cell types. The overexpression of rat Pit-2 DNA in CHO but not in FDC-Pmix cells improved amphotropic infection frequency after introducing rat Pit-2 DNA into the cells. Taken together these results show that the inefficient infection of FDC-Pmix is due to a lack of functional receptors. Either the receptor protein is incorrectly processed in these cells or a cofactor is missing in FDC-Pmix cells that is necessary for efficient binding and/or penetration.

Transcriptional activation of the granulocyte-macrophage colony-stimulating factor receptor gene in cell mutants

Retroviral insertional mutagenesis has proven to be a powerful in vivo approach for identifying genetic mutations involved in tumorigenesis or developmental abnormalities. Applying this approach to an in vitro system, where experimental design can be readily manipulated, would greatly increase its efficacy. In this study, we sought to determine whether retroviral insertional mutagenesis could be used to isolate cell mutants, in which the transcriptional activation of a receptor gene has occurred. Cells of the myeloid progenitor cell line FDC-P1(M), which do not express the alpha receptor subunit (GMRalpha) for granulocyte-macrophage colony-stimulating factor (GM-CSF), were infected and selected for growth in GM-CSF. Over 100 mutants were isolated at a frequency up to ninefold higher than that of uninfected controls. Expression of GMRalpha in these mutants was confirmed by blocking proliferation with GM-CSF antibodies, detection of high-affinity receptors, and Northern blot analysis. Significantly, in 7/18 mutants analyzed, gross DNA rearrangements had occurred in the GMRalpha locus. These rearrangements were demonstrated to be due to intergenic rearrangements, juxtaposing an active enhancer/promoter upstream of the GMRalpha gene. In one mutant it could be demonstrated that the wild-type allele was also expressed, providing evidence that secondary mutations had occurred. The implications of these results for retroviral insertional mutagenesis are discussed.

CD34 splice variant: an attractive marker for selection of gene-modified cells

This study presents a promising selection system for gene-modified cells other than human hematopoietic progenitor and endothelial cells based on transgenic expression of human CD34. Three retrovirally transduced variants of CD34 were compared, differing in the length of their cytoplasmic domains. These were the full-length transmembrane protein (flCD34), a truncated form (tCD34) that is found as a naturally occurring splice variant and has a partial deletion of the cytoplasmic domain for signal transduction, and an engineered variant which is completely deprived of its cytoplasmic tail (dCD34). All three variants allowed selection of gene-modified cells using commercially available immunoaffinity technology. However, examination by flow cytometry as well as by Southern, Northern, and Western blot revealed that dCD34, as opposed to tCD34, is not stably anchored in the membrane and thus is expressed at low levels on the surface of transduced cells. Therefore, tCD34 was chosen as the more promising candidate for a clinically applicable cell surface marker. We show that gene-modified human primary T lymphocytes expressing tCD34 can be enriched to high purity (>95%) using clinically approved immunoaffinity columns. In addition, we demonstrate the utility of tCD34 for surface marking of murine hematopoietic cells in vivo, including primary T lymphocytes detected 9 weeks after bone marrow transplantation.

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).

Effect of estradiol on insulin secreting INS-1 cells overexpressing estrogen receptors

BACKGROUND

Estrogen has been shown to have profound effects on insulin and glucose metabolism in vivo. Indeed, estrogens were recently shown to modulate ion channel and secretory activities in endocrine cells.

DESIGN AND METHODS

To investigate whether estrogenic influences are caused by direct effects on pancreatic beta-cells, we equipped INS-1 insulinoma cells with estrogen receptors and monitored insulin content and Ca(2+) fluxes as well as basal and stimulated insulin secretion upon different stimuli including glucose, the Ca(2+) ionophore ionomycin, the Ca(2+) channel agonist BayK8644, the protein kinase C activator TPA, and the adenylate cyclase activator forskolin.

RESULTS AND CONCLUSION

Our data reveal that estradiol has no significant direct effect on proliferation rate, insulin content, basal and stimulated insulin output as well as Ca(2+) fluxes of insulin secreting cells in vitro, indicating that in vivo responses to estrogen on insulin and glucose metabolism result from indirect betacytotropic effects.

Retroviral vector-mediated gene expression in hematopoietic cells

Gene transfer vectors based on simple retroviruses and more complex lentiviruses are currently the most reliable tools for stable establishment of transgenes in hematopoietic cells. While important hurdles in basic gene transfer technologies have been overcome in recent years, there is still some uncertainty in the choice of the cis-regulatory elements of the vector. These elements dictate the overall level, clonal variability, response to differentiation and persistence of transgene expression in vivo and thus have a significant influence on the outcome of therapeutic applications of somatic gene transfer. The rationale underlying the further improvement of such cis-elements is reviewed here.

A novel dual function retrovirus expressing multidrug resistance 1 and O6-alkylguanine-DNA-alkyltransferase for engineering resistance of haemopoietic progenitor cells to multiple chemotherapeutic agents

Following transduction with a retrovirus (SF1MIH) expressing both the multidrug resistance 1 (MDR1) and O6-alkylguanine-DNA-alkyltransferase (ATase) proteins, human erythroleukaemic progenitor (K562) cells were isolated which were resistant to killing by the MDR1 substrate, colchicine. In colony-forming survival assays, K562-SF1MIH cells exhibited resistance to colchicine and doxorubicin, as well as to the O6-alkylating agents N-Methyl-N-nitrosourea (MNU) and temozolomide. Furthermore, the resistance to doxorubicin was abolished by preincubation with the MDR1 inhibitor verapamil while resistance to MNU was ablated by the specific ATase inactivator, O6-benzylguanine (O6-beG) confirming that resistance to doxorubicin and MNU was conferred by MDR1 and ATase, respectively. When K562-SF1MIH were exposed to combinations of colchicine and MNU or doxorubicin and temozolomide, simultaneous resistance to these agents was observed. Thus, transduction of K562 with SF1MIH conferred dual resistance to these cells. These data offer the prospect of designing vectors that will confer resistance to entire regimens of chemotherapy rather than just to individual components of such drug cocktails, thereby substantially increasing the efficacy of therapy. Furthermore, the use of such dual expression constructs is likely to be highly informative for the design of effective in vivo selection protocols, an issue likely to make a major impact in a clinical context in gene therapy in the near future.

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.

Retroviral vectors pseudotyped with lymphocytic choriomeningitis virus

Pseudotyping can improve retroviral vector stability and transduction efficiency. Here, we describe a novel pseudotype of murine leukemia virus packaged with lymphocytic choriomeningitis virus (LCMV). This pseudotype was stable during ultracentrifugation and infected several cell lines from different species. Moreover, LCMV glycoproteins were not cell toxic.

Bicistronic retroviral vectors for combining myeloprotection with cell-surface marking

We have developed a retroviral vector coexpressing the multidrug-resistance 1 (MDR1) cDNA for inducing cancer drug resistance and the truncated version of the low-affinity nerve growth factor receptor (DeltaLNGFR) for cell-surface marking of transduced cells. The vector is based on the FMEV backbone which mediates high levels of gene expression in hematopoietic cells. To achieve optimal expression levels of both cDNAs, untranslated regions from MDR1 and DeltaLNGFR were removed and three different connections were tested: retroviral splice signals, an internal ribosomal entry site (IRES) from encephalomyocarditis virus, and an internal promoter from the chicken beta-actin gene. As determined by two-color flow cytometry, the best correlation of the expression of both cDNAs was obtained using the vector SF1mSdelta which utilized retroviral splice signals for co-expression. Simultaneous expression of both cDNAs at the single cell level was also shown by confocal laser microscopy. Lymphoid and hematopoietic progenitor cells, including primary human CD34+ cells, transduced with SF1mSdelta acquired dominant multidrug resistance. Transduced primary CD34+ cells could be enriched in vitro based on expression of DeltaLNGFR, avoiding exposure to cytostatic agents. Thus, monitoring the selection of chemotherapy-resistant cells and analyzing their biological properties may be alleviated, both in vitro and in vivo.

Heparan sulfate proteoglycan expression is induced during early erythroid differentiation of multipotent hematopoietic stem cells

Heparan sulfate (HS) proteoglycans of bone marrow (BM) stromal cells and their extracellular matrix are important components of the microenvironment of hematopoietic tissues and are involved in the interaction of hematopoietic stem and stromal cells. Although previous studies have emphasized the role of HS proteoglycan synthesis by BM stromal cells, we have recently shown that the human hematopoietic progenitor cell line TF-1 also expressed an HS proteoglycan. Immunochemical, reverse transcriptase-polymerase chain reaction (RT-PCR), and Northern blot analysis of this HS proteoglycan showed that it was not related to the syndecan family of HS proteoglycans or to glypican. To answer the question of whether the expression of HS proteoglycans is associated with the differentiation state of hematopoietic progenitor cells, we have analyzed the proteoglycan synthesis of several murine and human hematopoietic progenitor cell lines. Proteoglycans were isolated from metabolically labeled cells and purified by several chromatographic steps. Isolation and characterization of proteoglycans from the cell lines HEL and ELM-D, which like TF-1 cells have an immature erythroid phenotype, showed that these cells synthesize the same HS proteoglycan, previously detected in TF-1 cells, as a major proteoglycan. In contrast, cell lines of the myeloid lineage, like the myeloblastic/promyelocytic cell lines B1 and B2, do not express HS proteoglycans. Taken together, our data strongly suggest that expression of this HS proteoglycan in hematopoietic progenitor cell lines is associated with the erythroid lineage. To prove this association we have analyzed the proteoglycan expression in the nonleukemic multipotent stem cell line FDCP-Mix-A4 after induction of erythroid or granulocytic differentiation. Our data show that HS proteoglycan expression is induced during early erythroid differentiation of multipotent hematopoietic stem cells. In contrast, during granulocytic differentiation, no expression of HS proteoglycans was observed.

Design of 5' untranslated sequences in retroviral vectors developed for medical use

Utilizing genetic modules of simple retroviruses, we have developed a novel generation of gene transfer vectors with improved therapeutic potential. In the 5' untranslated "leader" sequences, all AUG codons which may aberrantly initiate translation and all viral coding sequences were removed. Thus, the probability of expressing unwanted peptides and the potential for homologous recombination with retroviral genes were largely reduced, and the cloning capacity was increased. The transgene was inserted to replace the viral gag sequences, and a new minimal splice acceptor was introduced, resulting in increased expression with all genes tested (those coding for human multidrug resistance 1 and enhanced green fluorescent protein, as well as the lacZ gene). These vectors may represent attractive tools for human gene therapy, because they increase the efficiency of transgene expression and may also increase safety in medical applications.

Downregulation of c-kit (stem cell factor receptor) in transformed hematopoietic precursor cells by stroma cells

We show a dramatic downregulation of the stem cell factor (SCF) receptor in different hematopoietic cell lines by murine stroma. Growth of the human erythroid/macrophage progenitor cell line TF-1 is dependent on granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3). However, TF-1 cells clone and proliferate equally well on stroma. Independent stroma-dependent TF-1 clones (TF-1S) were generated on MS-5 stroma. Growth of TF-1S and TF-1 cells on stroma still requires interaction between c-kit (SCF receptor) and its ligand SCF, because antibodies against c-kit inhibit growth to less than 2%. Surprisingly, c-kit receptor expression (RNA and protein) was downregulated by 2 to 3 orders of magnitude in TF-1S and TF-1 cells grown on stroma. This stroma-dependent regulation of the kit receptor in TF-1 was also observed on exposure to kit ligand-negative stroma, thus indicating the need for heterologous receptor ligand interaction. Removal of stroma induced upregulation by 2 to 4 orders of magnitude. Downregulation and upregulation of c-kit expression could also be shown for the megakaryocytic progenitor cell line M-07e and was comparable to that of TF-1, indicating that stroma-dependent regulation of c-kit is a general mechanism. Downregulation may be an economic way to compensate for the increased sensitivity of the c-kit/ligand interaction on stroma. The stroma-dependent c-kit regulation most likely occurs at the transcriptional level, because mechanisms, such as splicing, attenuation, differential promoter usage, or mRNA stability, could be excluded.

Rapid and efficient cloning of proviral flanking fragments by kanamycin resistance gene complementation

We have developed a technique for the rapid cloning of unknown flanking regions of transgenic DNA. We complemented a truncated kanamycin resistance gene of a bacterial plasmid with a neomycin resistance gene fragment from a gene transfer vector. Optimized transformation conditions allowed us to directly select for kanamycin-resistant bacteria. We cloned numerous proviral flanking fragments from growth factor-independent cell mutants that were obtained after infection with a replication incompetent retroviral vector and identified integrations into the cyclin D2 and several unknown genomic sequences. We anticipate that our method could be adapted to various vector systems that are used to tag and identify genes and to map genomes.

FMEV vectors: both retroviral long terminal repeat and leader are important for high expression in transduced hematopoietic cells

FMEV retroviral vectors combine the long terminal repeat of Friend mink cell focus-forming viruses with the 5' untranslated leader region of the murine embryonic stem cells virus. These modules were connected to achieve high transgene expression in hematopoietic progenitor and stem cells. Here, we report the cloning of safety-improved and versatile FMEV vectors allowing module-wise exchange of crucial elements for comparative studies. By transfer and expression of four different marker genes (neomycin phosphotransferase, lacZ, enhanced green fluorescent protein and truncated low affinity nerve growth factor receptor), we formally demonstrate that both the long terminal repeat and the leader contribute to the high expression of FMEV in transduced hematopoietic cells. Most prominent are the data recorded in the absence of selection in myelo-erythroid progenitor cells. Here, FMEV vectors mediate up to two orders of magnitude increased transgene expression levels when compared with vectors based on the Moloney murine leukemia virus.

Carboxyl-truncated STAT5beta is generated by a nucleus-associated serine protease in early hematopoietic progenitors

Hematopoiesis is tightly controlled by a family of cytokines that signal through a related set of receptors. The pleiotropic and overlapping response of a cell to different cytokines is reflected in the number and complex pattern of activated signal transducers. Of special interest is STAT5, which is stimulated by a large and diverse set of cytokines. In addition to the two highly homologous proteins, STAT5A and STAT5B, encoded by duplicated genes, expression and activation of a dominant-negative, carboxyl-truncated form has also been described in early hematopoietic progenitors. We show here that a protease expressed in early hematopoietic cells cleaves the alpha forms of STAT5A/5B (p96/p94) to generate carboxyl-truncated beta forms (p80/p77). Inhibition studies assigned this protease to the serine class of endopeptidases. Cell fractionation experiments showed that the protease is associated with the nucleus in a constitutively activated form and does not require an activated STAT5 substrate. The ability of a protease to modulate the specificity of an activated transcription factor is unprecedented and underlines the importance of proteases in regulation of cell functions.

Both stroma and stem cell factor maintain long-term growth of ELM erythroleukemia cells, but only stroma prevents erythroid differentiation in response to erythropoietin and interleukin-3

Defining how the stromal requirements of hematopoietic progenitors change during leukemia progression is an important topic that is not well understood at present. The murine ELM erythroleukemia is an interesting model because the erythroid progenitors retain dependence on bone marrow-derived stromal cells for long-term growth in vitro, and they also undergo erythroid differentiation in the presence of erythropoietin (EPO) and interleukin-3 (IL-3). In this report, we have shown using neutralizing antibodies that stem cell factor (SCF), insulin-like growth factor (IGF)-1, and integrin signaling pathways are all involved. We then determined whether ELM cells can be maintained long-term without stroma in various combinations of growth factors produced by stroma cells or growth factors for which ELM cells have receptors. This showed that ELM cells could be maintained with high efficiency in SCF alone; furthermore, the cells remained absolutely SCF-dependent and did not become more tumorigenic than cells maintained on stroma. In contrast, ELM cells underwent clonal extinction when serially cloned in IGF1; any cells that survived long-term growth in IGF-1 were found to be IGF1-independent. One important difference between maintaining ELM cells on stroma and growth in SCF is that stroma reversibly inhibits their differentiation in response to EPO and IL-3, whereas SCF does not.

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.

Host cis-mediated extinction of a retrovirus permissive for expression in embryonal stem cells during differentiation

The use of retroviral vectors for gene transfer into animals has been severely hampered by the lack of provirus transcription in the early embryo and embryonic stem (ES) cells. This primary block in provirus expression is maintained in differentiated cells by a cis-acting mechanism that is not well characterized. Retroviral vectors based on the murine embryonal stem cell virus (MESV), which overcome the transcriptional block in ES cells, were constructed to investigate this secondary mechanism. These vectors transferred G418 resistance to ES cells with the same efficiency as to fibroblasts, but overall transcript levels were greatly reduced. A mosaic but stable expression pattern was observed when single cells from G418-resistant clones were replated in G418 or assayed for expression of LacZ or interleukin-3. The expression levels in independent clones were variable and correlated inversely with methylation. However, a second, more pronounced, block to transcription was found upon differentiation induction. Differentiation of the infected ES cells to cells permissive for retroviral expression resulted in repression and complete extinction of provirus expression. Extinction was not accompanied by increased levels of methylation. Provirus expression is thus regulated by two independent cis-acting mechanisms: (i) partial repression in the undifferentiated state, accompanied by increased methylation but compatible with long-term, low expression of retroviral genes, and (ii) total repression and extinction during early stages of differentiation, apparently independent of changes in methylation. These results indicate a time window early during the transition from an undifferentiated to a differentiated stage in which provirus expression is silenced. The mechanisms are presently unknown, but elucidation of these events will have an important impact on vector development for targeting stem cells and for gene therapy.

Dominant selection of hematopoietic progenitor cells with retroviral MDR1 co-expression vectors

When transferring the human multidrug resistance 1 (MDR1) cDNA, FMEV retroviral vectors mediate high-dose multidrug resistance and, thus, background-free selection in primary human hematopoietic progenitor cells. Here, we analyzed strategies for co-expression of a second gene from an FMEV:MDR1 vector. When linking the cDNAs with the internal ribosomal entry site (IRES) of poliovirus or retroviral splice signals, almost all multidrug-resistant hematopoietic colonies simultaneously coexpressed the 3' positioned second gene, neomycin-phosphotransferase (neoR). The IRES strategy allowed functional co-transfer of a 4.2-kb lacZ-neoR fusion gene, resulting in a total proviral genome size of 11 kb, corresponding to the packaging limit of retroviral vectors. Preselection based on multidrug resistance elevated the expression of the second gene in IRES constructs, but not in splice vectors. Moreover, three intriguing observations were made. First, up to 30% of cells preselected for functional transfer of the 3' positioned cDNA (neoR) showed infunctional MDR1; this occurred irrespective of the linking principle and was associated with instability of the MDR1 transcription unit. Second, the levels of multidrug resistance achieved with the co-expression vectors were moderately lower (15-30% reduced) than those mediated by the monocistronic counterpart. Third, transduction with FMEV:MDR1 co-expression vectors still resulted in high-dose cancer drug resistance and background-free selection of hematopoietic progenitor cells (including primary human CD34+ colony-forming units). Thus, for the first time, we describe MDR1 co-expression vectors that maintain their desired function in early and primary human hematopoietic cells. However, careful interpretation of the data reveals that further vector improvements are required to obtain clinically useful MDR1 co-expression vectors.

Proteoglycan synthesis in haematopoietic cells: isolation and characterization of heparan sulphate proteoglycans expressed by the bone-marrow stromal cell line MS-5

Proteoglycans of bone-marrow stromal cells and their extracellular matrix are important components of the haematopoietic microenvironment. Recently, several studies have indicated that they are involved in the interaction of haematopoietic stem and stromal cells. However, a detailed characterization of the heparan sulphate proteoglycans synthesized by bone-marrow stromal cells is still lacking. Here we report on the isolation and characterization of proteoglycans from the haematopoietic stromal cell line MS-5, that efficiently supports the growth and differentiation of human and murine haematopoietic progenitor cells. Biochemical characterization of purified proteoglycans revealed that the haematopoietic stromal cell line MS-5 synthesizes, in addition to chondroitin sulphate proteoglycans, several different heparan sulphate proteoglycans. Immunochemical analysis, using specific antibodies against the different members of the syndecan family, glypican, betaglycan and perlecan, showed that MS-5 cells synthesize all these different heparan sulphate proteoglycans. These data were further supported by reverse-transcriptase PCR and confirmed by sequence and Northern blot analysis. The relative abundance of the different heparan sulphate proteoglycans was estimated on the protein and mRNA levels.

Selective immunoaffinity-based enrichment of CD34+ cells transduced with retroviral vectors containing an intracytoplasmatically truncated version of the human low-affinity nerve growth factor receptor (deltaLNGFR) gene

Human hematopoietic stem cells remain one of the most promising target cells for gene therapeutic approaches to treat malignant and nonmalignant diseases. To rapidly characterize transduced cells and to isolate these from residual nontransduced, but biologically equivalent, cells, we have used a Moloney murine leukemia virus (Mo-MuLV)-based retroviral vector containing the intracytoplasmatically truncated human low-affinity nerve growth factor receptor (deltaLNGFR) cDNA as a marker gene. Supernatant transduction of CD34+ cells (mean purity 97%) in fibronectin-coated tissue culture flasks resulted in 5.5-45% (mean 26%) transduced cells expressing deltaLNGFR (LNGFR+ cells). After transduction, more than 65% of the transduced cells remained CD34+. Compared with control (mock- and nontransduced) CD34+ cells, transduction did not decrease the cloning efficiency of CD34+ cells. Immunomagnetic selection of the transduced cells with a monoclonal anti-LNGFR antibody resulted in >90% LNGFR+ cells. Further phenotypic characterization of these highly enriched LNGFR+ cells indicated that the majority co-expressed the CD34 and CD38 antigens. These results show that transduced cells expressing an ectopic cell-surface protein can be rapidly and conveniently quantitated and characterized by fluorescence-activated cell sorting (FACS) analysis and fast and efficiently enriched by immunoadhesion using magnetic beads. The use of cell-surface reporters should facilitate optimization of methods of gene transfer into more primitive hematopoietic progenitors.

The role of soluble growth factors in inducing transient growth and clonal extinction of stroma cell dependent erythroblastic leukemia cells

A coculture system of a murine erythroblastic leukemia cell line (ELM-D) with its supportive stromal cell line (MS-5) was established. Long-term growth of ELM-D cells is strictly stroma cell dependent. Interaction between stem cell factor (SCF) and its receptor, c-kit, was demonstrated to be important for stroma cell-dependent growth by anti c-kit neutralizing monoclonal antibody (mAb) inhibition experiments. Significantly, soluble growth factors such as granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) or SCF of MS-5 stromal cells (MS-5 CM) could replace the requirement of stroma cells for a considerable period. However, ELM-D cells maintained in these growth factors underwent clonal extinction after 3-6 weeks unless contact with stroma was re-established. Furthermore, IL-3 or GM-CSF acted in a dominant manner in inducing cell death in the presence of stroma cells. Cells showing clonal extinction undergo programmed cell death and do not differentiate. These altered growth properties of ELM-D cells exposed to soluble growth factors or to stroma cells appear to be analogous to those described for T or B cells primed by antigen presenting cells and then grown in growth factors.

The potent enhancer activity of the polycythemic strain of spleen focus-forming virus in hematopoietic cells is governed by a binding site for Sp1 in the upstream control region and by a unique enhancer core motif, creating an exclusive target for PEBP/CBF

The polycythemic strain of the spleen focus-forming virus (SFFVp) contains the most potent murine retroviral enhancer configuration known so far for gene expression in myeloerythroid hematopoietic cells. In the present study, we mapped two crucial elements responsible for the high activity of the SFFVp enhancer to an altered upstream control region (UCR) containing a GC-rich motif (5'-GGGCGGG-3') and to a unique enhancer core (5'-TGCGGTC-3'). Acquisition of these motifs accounts for half of the activity of the complete retroviral enhancer in hematopoietic cells, irrespective of the developmental stage or lineage. Furthermore, the UCR motif contains the major determinant for the enhancer activity of SFFVp in embryonic stem (ES) cells. Using electrophoretic mobility shift assays, we show that the UCR of SFFVp, but not of Friend murine leukemia virus, is targeted by the ubiquitous transcriptional activator, Sp1. The core motif of SFFVp creates a specific and high-affinity target for polyomavirus enhancer binding protein/core binding factor (PEBP/CBF) and excludes access of CAAT/enhancer binding protein. Cotransfection experiments with ES cells imply that PEBP/CBF cooperates with the neighboring element, LVb (the only conserved Ets consensus in the SFFVp enhancer), and that the Sp1 motif in the UCR stimulates transactivation through the Ets-PEBP interaction. Putative secondary structures of the retroviral enhancers are proposed based on these data.

Role of the stem cell factor (SCF) receptor and the alternative forms of its ligand (SCF) in the induction of long-term growth by stroma cells

Self renewal and differentiation of hematopoietic stem cells is regulated by the hematopoietic microenvironment. The Stem Cell Factor (SCF) has been shown to be one of the essential factors that governs stem cell maintenance. In this abstract we describe a functional analysis of the membrane bound and soluble SCF within the context of stroma cocultures with hematopoietic cells. We report that transmembrane SCF is essential for long term growth, whereas soluble SCF supports only short term proliferation of stroma dependent hematopoietic cells. We also show that the SCF/c-kit interaction can be substituted by an unknown mechanism. To determine the molecular mechanism involved in maintenance of hematopoietic cells on stroma we analyzed the c-kit expression during coculture. We demonstrate that c-kit expression was downregulated during coculture. Downregulation was induced by the coculture itself and not by external factors.

[Will transfer of cytostatic drug resistance genes increase hematopoiesis resistance in the treatment of malignant tumors?]

The aim of aggressive antitumor chemotherapy is to kill the tumor with the largest possible dose of a cytotoxic drug. The maximum dose tolerated by the patient is limited by the toxicity to normal tissue, hematopoiesis being frequently the most sensitive system. Transfer of drug resistance genes to hematopoietic cells could protect them against chemotherapy-related toxicity and thus could be a way of gene therapy in cancer. Methylating and chloroethylating derivatives of nitrosourea are effective anticancer drugs, however, acute hematopoietic toxicity and late risk of leukemia are serious side effects. The major lesion responsible for toxic and mutagenic effects of alkylnitrosoureas is O6-alkylation of guanine in DNA. This lesion is specifically repaired by O6-alkylguanine-DNA-alkyltransferase and hematopoietic cells can be protected against toxic and mutagenic effect of nitrosoureas by alkyltransferase gene transfer. Endogenous alkyltransferase in tumor tissue could be inactivated by administration of O6-benzylguanine, while hematopoietic cells could still be chemoprotected by inhibitor-resistant alkyltransferase gene transfer. This approach could increase the therapeutic efficacy of nitrosoureas in gene therapy augmented cancer treatment.

High-dose multidrug resistance in primary human hematopoietic progenitor cells transduced with optimized retroviral vectors

Retroviral transfer of the multidrug-resistance 1 (mdr1) cDNA into primary human hematopoietic progenitor cells (HPC) of cancer patients undergoing high-dose chemotherapy has been proposed to protect the bone marrow from the dose-limiting cytotoxicity of cytostatic agents. Preclinical studies performed with vectors derived from the Moloney murine leukemia virus (MoMuLV) or the related Harvey murine sarcoma virus have established that chemoprotection of HPC is feasible. The efficacy of vector-mediated multidrug-resistance under high doses of cytostatic agents, however, remained unclear. We report here that this goal can only be achieved with improved vector design. Novel vectors termed SF-MDR and MP-MDR, which are based on the spleen focus-forming virus or the myeloproliferative sarcoma virus for the enhancer and the murine embryonic stem cell virus for the leader, significantly elevate survival of transduced primary human HPC under moderate doses of colchicine and paclitaxel in vitro when compared with a conventional MoMuLV-based vector. Importantly, SF-MDR and also MP-MDR confer an absolute advantage at high doses of paclitaxel in vitro corresponding to peak plasma levels achieved in patients during chemotherapy. This observation has important consequences for a variety of ongoing and planned gene therapy trials.

Improved retroviral vectors for hematopoietic stem cell protection and in vivo selection

Therapeutic gene transfer into hematopoietic cells is critically dependent on the evolution of methods that allow ex vivo expansion, high-frequency transduction, and selection of gene-modified long-term repopulating cells. Progress in this area needs elaboration of defined culture and transduction conditions for long-term repopulating cells and improvement of gene transfer systems. We have optimized retroviral vector constructions based on murine leukemia viruses (MuLV) to overcome the transcriptional repression encountered with the use of conventional Moloney MuLV (MoMuLV) vectors in early hematopoietic progenitor cells (HPC). Novel retroviral vectors, termed FMEV (for Friend-MCF/MESV hybrid vectors), were cloned that mediate greatly improved gene expression in the myeloerythroid compartment. Transfer of the selectable marker multidrug resistance 1 (mdr1), FMEV, in contrast to conventional MoMuLV-related vectors currently in use for clinical protocols, mediated background-free selectability of transduced human HPC in the presence of myeloablative doses of the cytostatic agent paclitaxel in vitro. Furthermore, FMEV also greatly improved chemo-protection of hematopoietic progenitor cells in a murine model system in vivo. Finally, when a second gene was transferred along with mdr1 in an FMEV-backbone, close to 100% coexpression was observed in multidrug-resistant colonies. These observations have significant consequences for a number of ongoing and planned gene therapy trials, for example, stem cell protection to reduce the myelotoxic side effects of anticancer chemotherapy, correction of inherited disorders involving hematopoietic cells, and antagonism of HIV infection.

Proteoglycan synthesis in human and murine haematopoietic progenitor cell lines: isolation and characterization of a heparan sulphate proteoglycan as a major proteoglycan from the human haematopoietic cell line TF-1

Proteoglycans of bone-marrow stromal cells and their extracellular matrix are important components of the microenvironment of haematopoietic tissues. Proteoglycans might also be involved in the interaction of haematopoietic stem and stromal cells. Recently, several studies have been reported on the proteoglycan synthesis of stromal cells, but little is known about the proteoglycan synthesis of haematopoietic stem or progenitor cells. Here we report on the isolation and characterization of proteoglycans from two haematopoietic progenitor cell lines, the murine FDCP-Mix A4 and the human TF-1 cell line. Proteoglycans were isolated from metabolically labelled cells and purified by several chromatographic steps, including anion-exchange and size-exclusion chromatography. Biochemical characterization was performed by electrophoresis or gel-filtration chromatography before and after digestion with glycosaminoglycan-specific enzymes or HNO2 treatment. Whereas FDCP-Mix A4 cells synthesize a homogeneous chondroitin 4-sulphate proteoglycan, isolation and characterization of proteoglycans from the human cell line TF-1 revealed, that TF-1 cells synthesize, in addition to a chondroitin sulphate proteoglycan, a heparan sulphate proteoglycan as major proteoglycan. For this heparan sulphate proteoglycan a core protein size of approx. 59 kDa was determined. Immunochemical analysis of this heparan sulphate proteoglycan revealed that it is not related to the syndecan family nor to glypican.

A novel hematopoietic multilineage clone, Myl-D-7, is stromal cell-dependent and supported by an alternative mechanism(s) independent of stem cell factor/c-kit interaction

A strictly stroma-dependent hematopoietic clone, Myl-D-7, with lympho-myeloid potential has been isolated. A subset of cells expresses myeloid-macrophage (Mac-1 and Gr-1), erythroid (TER119), and lymphoid (Thy-1 and B220) lineage markers. Spontaneous differentiation to the myeloid-macrophage, erythroid, or lymphoid pathway can be seen by morphologic criteria, detection of beta major globin synthesis, or expression of the early lymphoid specific transcription factor, Ikaros. By sorting lineage marker (Mac-1, Gr-1, B220, and TER119)-negative (LIN-) cells, we showed that the LIN- population actively self-renews on top of MS-5 stromal cells, and differentiates to LIN+ cells. Removal of stroma induces apoptosis and none of the growth factors tested can prevent apoptosis. Granulocyte-macrophage colony-stimulating factor accelerates the differentiation towards the myeloid-macrophage lineage. Using this clone, we show that (1) contact with stroma induces expression of bcl-2, (2) stromal cells derived from SI/SI homozygous fetuses can support long-term growth, and (3) conditioned media of specific stromal cells contains an activity that supports proliferation and self-renewal of the clone. Myl-D-7 can thus be used as an indicator cell for unknown factors that may provide stromal cell support.

Long-term protection of hematopoiesis against the cytotoxic effects of multiple doses of nitrosourea by retrovirus-mediated expression of human O6-alkylguanine-DNA-alkyltransferase

A human O6-alkylguanine-DNA-alkyltransferase (ATase) cDNA-containing retrovirus was used to infect murine long-term primary bone marrow cultures. High levels of ATase expression were obtained, and colony-forming cells of the granulocyte-macrophage lineage from the cultures transduced with the human ATase retrovirus were three times more resistant to the alkylating agent, N-methyl-N-nitrosourea (MNU), than control cultures. Furthermore, expression of the human ATase protected long-term hematopoiesis, measured as the output of progenitor cells to the nonadherent fraction of the culture, against the cytotoxic effects of repeated exposures to MNU. These results clearly show that a human ATase cDNA-containing retrovirus can be used to infect long-term primary bone marrow cultures and that this attenuates their sensitivity to nitrosoureas.

Transcriptional regulation of the c-fms proto-oncogene mediated by granulocyte/macrophage colony-stimulating factor (GM-CSF) in murine cell lines

Differentiation of blood cells is paralleled by a timely ordered expression of cytokine receptor genes. We show here that the expression of the c-fms gene which encodes the lineage-specific receptor for macrophage colony-stimulating factor (M-CSF or CSF-1) is directly linked to ligand-mediated activation of the receptor for the granulocyte/macrophage colony-stimulating factor (GM-CSF). In interleukin-3 (IL-3) dependent multipotent progenitor cells, FDC-Pmix GMV#2 cells, GM-CSF treatment results in the rapid formation of full-length c-fms transcripts. Surprisingly, this upregulation of c-fms transcripts is also observed in mouse NIH3T3 fibroblasts stably transfected with genes coding for the alpha- and beta-subunits of the GM-CSF receptor. These results indicate a direct control by the GM-CSF receptor that takes place regardless of cell differentiation. Furthermore, a 2.1 kb genomic fragment containing the c-fms proximal promoter directs GM-CSF-inducible expression of a reporter gene, suggesting a regulation of c-fms gene expression on the transcriptional level.

Activity of Friend mink cell focus-forming retrovirus during myelo-erythroid hematopoiesis

Friend mink cell focus-forming (FMCF) viruses are recombinants between the Friend murine leukemia virus (F-MuLV) and endogenous polytropic retroviruses involved in a number of retrovirus-induced malignancies of the myelo-erythroid compartment. To analyze the contribution of the viral cis regulatory elements to the host range determinants within the hematopoietic system, we performed a series of marker gene experiments using both transient transfection and retroviral-mediated stable transduction of indicator cell lines representing distinct developmental stages. According to our data, the U3 region in the long terminal repeat (LTR) of FMCF viruses possesses an enhancer assembly that allows efficient transcription in both early and late myelo-erythroid stem and progenitor cells. Retroviral gene expression, however, is subjected to stage-dependent transcriptional controls during blood cell maturation. We obtained evidence that a repressor element overlapping with the primer binding site in the viral leader region compromises U3-mediated gene expression in a stage-dependent manner, with the strongest restriction observed in the most primitive cells analyzed, FDCP-mix. In addition, our data indicate a second hurdle for retroviral gene expression in early hematopoietic cells that is independent of the primer binding site and most likely related to inefficient utilization of U3-located enhancers. These data shed light on the mechanisms of host range restriction within the hematopoietic system and define a basis for the design of retroviral vectors aimed to overcome transcriptional inefficiency in early hematopoietic cells. Thus, we developed novel retroviral vectors combining FMCF-type U3 regions with a permissive leader from the murine embryonic stem cell virus. These vectors are highly efficient for gene transfer and expression in both early and late myelo-erythroid cells, indicating that they will be of great use for a variety of experimental and therapeutic applications.

Novel retroviral vectors for efficient expression of the multidrug resistance (mdr-1) gene in early hematopoietic cells

We present data that retroviral gene expression in early hematopoietic cells is subjected to transcriptional controls similar to those previously described for embryonic stem cells. Transient transfection experiments revealed that both the viral enhancer region in the U3 region of the long terminal repeat as well as a repressor element coincident with the primer binding site of Moloney leukemia viruses are limiting for expression in hematopoietic cells in a differentiation-dependent manner. Within the group of Moloney leukemia virus-related viruses, only the myeloproliferative sarcoma virus showed high enhancer activity in myeloid (including erythroid) cells. In contrast, enhancer regions related to the Friend mink cell focus-forming viruses mediate much higher gene expression levels in both multipotent and lineage-committed myeloid cells. In addition, transcriptional repression related to sequences in the primer binding site of Moloney leukemia virus-derived vectors is also found in early hematopoietic cells and can be overcome by using the corresponding sequences of the murine embryonic stem cell virus. On the basis of these results, two types of novel retroviral hybrid vectors were developed; they combine the U3 regions of either the Friend mink cell focus-forming virus family or the myeloproliferative sarcoma virus with the primer binding site of the murine embryonic stem cell virus. When used to express the human multiple drug resistance gene, these vectors substantially improve protection to cytostatic drugs in transduced hematopoietic cell lines FDC-Pmix, TF-1, and K-562 in comparison with Moloney leukemia virus-derived vectors presently used for the stem cell protection approach in somatic gene therapy.

Excision of specific DNA-sequences from integrated retroviral vectors via site-specific recombination

Vectors for gene transfer and gene therapy were developed which combine the advantages of the integrase and recombinase systems. This was achieved by inserting two loxP sites for specific DNA excision into an MESV based retroviral vector. We show that this 'retroviral lox system' allows the infection of cells and the expression of transferred genes. In addition, we constructed an efficient retrovirus-based expression system for a modified Cre recombinase. Functional tests for DNA excision from integrated retroviral lox vectors were performed by the use of a negative selectable marker gene (thymidine kinase). Cre expression in cells infected with retroviral lox vectors and subsequent BrdU selection for cells in which site-specific recombination has occurred results in large numbers of independent cell clones. These results were confirmed by detailed molecular analysis. In addition we developed retroviral suicide vectors in which the enhancer/promoter elements of both LTRs were replaced by lox sequences. We show that lox-sequences located in the LTRs of retroviral vectors are stable during retroviral replication. Potential applications of this system would be the establishment of revertants of retrovirus-infected cells by controlled excision of nearly the complete proviral DNA.

The fusion of TEL and ABL in human acute lymphoblastic leukaemia is a rare event

We have recently identified a common ALL patient which harboured a chromosomal fusion between the TEL gene on chromosome 12 and the ABL gene on chromosome 9. We designed an RT-PCR assay to screen 186 adult ALL and 30 childhood ALL patients for this novel translocation. We were unable to identify any additional cases with a TEL/ABL fusion product.

Sequential mutations in the interleukin-3 (IL3)/granulocyte-macrophage colony-stimulating factor/IL5 receptor beta-subunit genes are necessary for the complete conversion to growth autonomy mediated by a truncated beta C subunit

An amino-terminally truncated beta C receptor (beta C-R) subunit of the interleukin-3 (IL3)/granulocyte-macrophage colony-stimulating factor/IL5 receptor complex mediates factor-independent and tumorigenic growth in two spontaneous mutants of a promyelocytic cell line. The constitutive activation of the JAK2 protein kinase in these mutants confirms that signaling occurs through the truncated receptor protein. Noteworthily, in addition to a 10-kb deletion in the beta C-R subunit gene encoding the truncated receptor, several secondary and independent mutations that result in the deletion or functional inactivation of the allelic beta C-R subunit and the closely related beta IL3-R subunit genes were observed in both mutants, suggesting that such mutations are necessary for the full oncogenic penetrance of the truncated beta C-R subunit. Reversion of these mutations by the expression of the wild-type beta C-R in the two mutants resulted in a fivefold decrease in cloning efficiency of the mutants in the absence of IL3, confirming a functional interaction between the wild-type and truncated proteins. Furthermore, expression of the truncated beta C-R subunit in factor-dependent myeloid cells did not immediately render the cells autonomous but increased the spontaneous frequency to factor-independent growth by 4 orders of magnitude. Implications for both leukemogenic progression and receptor-subunit interaction and signaling are discussed.

Selectable retrovirus vectors encoding Friend virus gp55 or erythropoietin induce polycythemia with different phenotypic expression and disease progression

The Friend spleen focus-forming virus induces a massive expansion of erythroid progenitor cells resulting in polycythemia and splenomegaly. The pathogenic agent is the membrane glycoprotein gp55, encoded by the env gene. Recent evidence indicates that gp55 binds to and activates the erythropoietin (Epo) receptor. It is not clear, however, whether gp55 completely mimics the natural receptor ligand (Epo). To directly compare both effectors, we constructed selectable retroviral vectors which carry either the env or the Epo gene. The selection marker allowed for clonal analysis of infected cells. After infection of DBA/2J mice, the spleen weight, hematological indices, and Epo titer of peripheral blood were monitored. Although both viruses induced an acute erythrocytosis, there were significant differences in disease phenotype and progression. The Epo virus caused an enhanced increase of hematocrit and erythrocytes, whereas with the env virus the pool of late progenitors (CFU-erythroid) was dramatically expanded, resulting in a more severe splenomegaly. The distribution of cytologically recognizable erythroid precursors was shifted towards immature cell types by the env vector compared with Epo. These data suggest that Epo and gp55 differentially affect proliferation and differentiation. Gp55 appears to promote proliferation over differentiation, whereas Epo preferentially drives differentiation.

Coordinate expression of the lineage-specific growth factor colony-stimulating factor (CSF)-1 and its receptor selectively promotes macrophage maturation during differentiation of multipotential progenitor cells

The multipotent hematopoietic precursor line A4GMV#2, derived by infection of FDCP-mix cells with a retroviral vector expressing the granulocyte-macrophage colony-stimulating factor (CSF) gene, proliferates continuously in interleukin 3 and presents the unique advantage of synchronous granulocyte and macrophage differentiation upon interleukin 3 withdrawal. Using this system, we showed previously that the mRNAs for lineage-specific receptors (granulocyte-CSF receptors, CSF-1 receptors, and Erythropoietin receptors) and ligands (granulocyte-CSF and CSF-1) are up-regulated during myeloid maturation. Here we address the specific question of the regulation of the expression of CSF-1 and its receptor and of their relevance to macrophage differentiation. Both genes were transcribed with equal efficiency in undifferentiated and differentiating cells. CSF-1 mRNA was detected in undifferentiated cells and increased slightly in the early phases of differentiation. CSF-1 receptor mRNA, absent in undifferentiated cells, accumulated early in differentiation (24 h) and remained constant thereafter. The production of both proteins, detected later during the differentiation of A4GMV#2 cells and of bone marrow-derived myeloid precursors, was therefore controlled at the posttranscriptional level. CSF-1 was produced by cells of the macrophage lineage and accumulated in mature phagocytes. A neutralizing anti-CSF-1 serum selectively impaired macrophage differentiation of A4GMV#2 cells and, most significantly, of primary myeloid precursors. These data indicate that CSF-1 and its receptor interact productively during differentiation and that the resulting autocrine stimulation selectively promotes macrophage maturation.