METHODS FOR USE OF Mpl LIGANDS WITH PRIMITIVE HUMAN STEM CELLS

Abstract: Myeloproliferative leukemia receptor (mpl) ligands, such as thrombopoietin, act on a primitive subpopulation of human stem cells having the characteristics of self-renewal and ability to give rise to all hematopoietic cell lineages. Thrombopoietin supports both megakaryocytic differentiation and primitive progenitor cell expansion of CD34<+> and CD<34> sub-populations (CD34<+>Lin<->, CD34<+>Thy-1<+>Lin<->, and CD34<+>Lin<-> Rh123<10>). Thrombopoietin also stimulates quiescent human stem cells to begin cycling. Thus, mpl ligands are useful for expanding primitive stem cells for restoration of hematopoietic capabilities and for providing modified human stem cells for gene therapy applications.

Description: METHODS FOR USE OF Mpl LIGANDS WITH PRIMITIVE HUMAN STEM CELLS Field of the Invention This invention relates to myeloproliferative receptors (mpl), specifically, the use of mpl ligands to expand primitive human stem cell subpopulations with minimal differentiation. Background of the Invention Thrombopoietin (TPO) is a recently isolated ligand of mpl, first identified as the proto-oncogene transduced by the murine myeloproliferative leukemia (MPL) virus. TPO has been shown to independently stimulate megakaryocyte (MK) progenitor division and MK maturation m vi vo and m vi tro . Ir vi vo administration of TPO to thrombocytopenic rodents was found to significantly boost the platelet count as well as increase the number and ploidy of maturing MKs in the bone marrov.. Absence of a c-mpl (TPO receptor) gene in mice was reported to result in thrombocytopema . More recently, it has been demonstrated that MKs can be primed to produce functional platelets in culture after exposure to TPO. Hematopoietic stem cells are rare cells that have been identified in fetal bone marrow, umbilical cord blood, adult bone marrow, and peripheral blood, which are capable of differentiating into each of the myeloerythroid (red blood cells, granulocytes, monocytes), megakaryocyte (platelets) and lymphoid (T-cells, B-cells, and natural killer cells) lineages. In addition, these cells are long-lived, and are capable of producing additional stem cells, a process termed self-renewal. Stem cells initially undergo commitment to lineage restricted progenitor cells, which can be assayed by their ability to form colonies m semisolid media. Progenitor cells are restricted in their ability to undergo multi-lineage differentiation and have lost their ability to self-renew. Progenitor cells eventually differentiate and mature into each of the functional elements of the blood. This maturation process is thought to be modulated by a complex network of regulatory factors including erythropoietin (EPO) , granulocyte colony stimulating factor (G-CSF) , granulocyte-macrophage colony stimulating factor (GM-CSF), thrombopoietin (TPO), steel factor (Stl), Flk-2 ligand and mterleukins (IL) 1-15. Recently, in vi tro assays have been developed to identify human hematopoietic stem cells having self-renewal and multi-lineage differentiative capacity. One assay is the cobblestone area-forming cell (CAFC) assay, based on freshly isolated stromal cells or established stromal cell lines. In the mouse system, late-appearing cobblestone area formation on fresh marrow-derived stroma or on a cloned stromal cell line has been shown to correlate with m vi vo hematopoietic repopulating ability. Correlation of CAFC and long term culture-initiating cell (LTCIC) frequencies using the mouse stromal cell l ne SySl has been demonstrated. In addition, the in vi vo severe combined immunodeficiency (SCID)-hu bone assay has been used to measure long term engraftment of candidate stem cell populations. Both the m vi tro CAFC assay and the SCID-hu bone model permit analysis of B-cell and myeloiα cell generation from candidate pluripotent hematopoietic stem cells (PHSC) . Distinct subpopulations of stem cells may be responsible for different phases of engraftment post transplantation. As early as 1964, differences in the ability of murine spleen colony forming units (CFU-S) to generate secondary CFU-S were defined. Although evidence now indicates that most CFU-S are not involved in repopulating let ally irradiated hosts, heterogeneity in transplantation potential appears to exist even within subpopulations of radioprotective cells. This has been demonstrated with serial bone marrow transplantations. The long-term repopulating ability of the grafts are lost with serial transfers, while a cell population survives which contributes to short-term reconstitution. Further support for the concept that both short-term and long-term reconstituting stem cell populations exist have been derived from studies in which isoenzyme analysis and retroviral gene marking of hematopoietic cells have been utilized to track the fate of stem cells. A mathematical analysis of correlations and variances of donor reconstitution with isoenzyme variants in lethally irradiated mice indicates that a large number of multi-lineage clones are active immediately after reconstitution but rapidly decline, with the ma]orιty being inactive 12 weeks post-transplantation. These observations indicate the existence of a population of cells with multi-lineage short-term engrafting potential in donor murine bone marrow. Similar observations have been made in a large animal transplantation model, where isoenzyme differences have indicated the contribution of multiple clones to short-term engraftment followed by sustained contributions by relatively few stem cell clones. These findings have been confirmed by an eloquent analysis of clonal development after transplantation with retrovirally marked stem cells. Subsets of cells w th differing proliferative potentials may also differ with regards to physical characteristics, and therefore may be isolated and functionally defined. Bone marrow cells responsible for reconstitution following lethal irradiation can be isolated using centrifugation techniques exploiting cell size and density fractionation, or fluorescence-activated cell sorting (FACS) based on uptake of fluorescent vital dyes, lectin binding, or cell surface antigen expression. FACS isolated murine cells that are responsible for engraftment lack lineage markers for B-cells, T-cells, myelomonocytes and erythrocytes and are termed lineage negative (Lin " ) . The Lin ' fraction of murine bone marrow can be further subdivided based on low levels of Thy-1 and expression of the Sca-1 antigen. Murine cells that are Thy-1.l iC Lm " Sca-l * are 1000-fold enriched in radioprotective ability, and contain all of the radioprotective cells found in the bone marrow of syngeneic C57BL/thy-l.1 mice. As few as 100 cells that are Thy-1.l l0 Lin ~ Sca-l * can radioprotect at least 95% of lethally irradiated recipients with long term donor derived reconstitution. At this cell dose, Thy-1.l l0 Lin " Sca-l * cells give rise to donor peripheral blood white blood cells by 10 days post transplant, and to platelets within 14 days of transplant, suggesting that this population contains cells with both short-term and long-term engrafting potential. Thy-1.l lc Lin " Sca-l * cells can be further divided by heterogeneity of cell cycle status and uptake of a fluorescent dye, rhodamine 123. Rhodamme 123 is a dye that identifies active mitochondria, and its efflux from the cell is handled by the multidrug resistance gene product, P-glycoprotem. Those cells that retain small quantities of rhodamine 123 are termed rhodamine dull (or low) and have been shown to possess marrow repopulating ability (MRA) . The cells responsible for reconstituting hematopoiesis m humans receiving a bone marrow transplant reside in a subset cf cells expressing the CD34 antigen (CD34") . This fraction c: cells can be further subdivided based on multiple antigen characteristics including the lack cf lineage specific markers (Lin " ) and expression of the Thy antigen (Thy-1 * ) . In vi vo assays using adult bone marrow and mobilized peripheral blood cells that are CD34 * Thy-l * Lin " have shown that this population contains cells capable of contributing to all hematopoietic lineages. Mpl expression has been detected by polymerase chain reaction (PCR) in human hematopoietic cells throughout the MK lineage, as well as in primitive CD34 * CD38 10 ' " cells. however, little is known about the actions of TPO on primitive cells prior to commitment to the MK lineage. One report has shown that primitive mouse HSC of the phenotype Sca-1 + AA4 * express mpl, and exposure of these cells to TPO in vi tro resulted in MK differentiation. Summary of the Invention It is now surprisingly discovered that human stem cells respond to ligands which bind and activate the myeloproliferative leukemia receptor (mpl) by expansion of primitive non-megakaryocytic blast cells, as well as expansion of megakaryocytic (MK) progenitor cells, commitment to the MK lineage, and MK maturation. Accordingly, mpl ligands are useful for m vi tro and m vi vo stem cell activation and expansion. The subset of primitive human stem cells described and disclosed herein to express mpl and respond to a mpl ligand, thrombopoietin (TPO) , have the ability to reconstitute hematopoiesis in humans receiving a bone marrow transplant. The stem cells useful in this invention are characterized by their ability to undergo substantial self-renewal and to proliferate and differentiate into cells of all the hematopoietic lineages. These cells may be identified by cell surface markers such as the CD34 antigen (CD34 * ) . More preferably, the stem cells useful in the invention are enricheα fcr the ability to undergo substantial self-renewal ana tc proliferate and differentiate into cells of all the hematopoiet-c lineages. A more enriched subpopulation of cel..s may oe identified by the lack of lineage specific markers (Lin ) , and/or expression of the Thy-1 antigen (Thy- 1 " ), e.g., CD34 * Lm " or CD34 * Thy-rLιn " . These cells are highly enriched in pluripotent cells with long-term and short-term repopulating potential. Alternatively, an enriched subpopulation of cells with the desired characteristics may be identified by the cell markers CD34 ' Lm " Rho-° or CD34 * Thy-l"Lιn-Rho 10 . The subpopulations of ctem cells useful in this invention may also be identified by their light scattering characteristics, e.g., low side scatter and low-to-medium forward scatter profiles by FACS analysis, and/or by phenotype, e.g., having a size between mature lymphoid cells and mature granulocytes. The present invention features a method for promoting the survival of long-term repopulating human stem cells by culturing such cells in the presence of a mpl ligand. The mpl ligand of the invention is characterized by the ability to bind mpl such that mpl-mediated biological activity is initiated. In a preferred embodiment, the mpl ligand is TPO, more preferably, human TPO, and still more preferably, recombinant human TPO. Stem cells cultured in the presence of a mpl ligand (e.g., TPO) retain the ability to undergo substantial self-renewal and to proliferate and differentiate into cells of all the hematopoietic lineages, i.e., are pluripotent hematopoietic stem cells. The invention features the use of a mpl ligand on a primitive subpopulation of human stem cells to expand the population of stem cells with minimal concomitant induction of differentiation. The expanded cell population is characterized by the ability to generate cells capable of substantial self-renewal and proliferation and differentiation into cells of all of the hematopoietic lineages. The invention features a therapeutic method for restoring hematopoietic capability to a human subject. Stem cells are purified from cells removed from a subject, expanded by m vi tro exposure to a mpl ligand, and returned to the subject, resulting in restoration of hematopoietic capability to the subject. In addition to expansion in the presence of the mpl ligand thrombopoietin, m vi tro expansion of stem cells can be conducted in the presence of additional cytokines, including for example ιnterleukιn-3 (IL-3) , leukemia inhibitory factor (LIF) , and steel factor (Stl) (also known as c- it ligand (KL) or stem cell factor (SCF)), interleukin 6 (IL-6) , fetal liver tyrosine kinase 2/3 (Flk2/Flt3) , macrophage inhibitory protein-lα (MlP-lα) , granulocyte-macrophage colony stimulating factor (GM-CSF) , granulocyte colony stimulating factor (G-CSF), IL-1, and IL- ll. The invention features a method for activating quiescent stem cells to divide without differentiating by exposing such cells to a mpl ligand. This aspect of the invention permits improved transduction of hematopoietic cells by a retroviral vector for use in gene therapy. Accordingly, in a related aspect, quiescent stem cells are activated in the presence of a mpl ligand, and cultured with a retroviral vector containing a gene of interest. The actively dividing cells integrate the gene of interest, and express the foreign gene product. Such transformed stem cells are useful for gene therapy applications. In the gene therapy aspect of the invention, hematopoietic cells are removed from a subject, transduced in vi tro in the presence of a mpl ligand and a gene delivery agent, e.g., a retroviral vector, and the modified cells returneα to the subject. The modified stem cells and their progeny will express the desired gene product m vi vo, thus providing sustained therapeutic benefit. In addition to the advantages of stem cell expansion w th minimal differentiation and stem cell activation, other aαvartages ana features of the present invention will become apparent to those skilled in the art upon reading this disclosure. Brief Description of the Figures Fig. IA is a graph showing the effect of TPO on the expansion of the CD41b * cell population of cultured CD34 * Lιn " and CD34 * Thy-l * Lιn " cells. Fig. IB is a graph showing the effect of TPO on total cells numbers of cultured CD34 * Lι^ " and CD34 * Thy-rLιn " cells. Detailed Description Before the present invention and methods for using same are described, it is to be understood that this invention is not limited to the particular cell lines, mpl ligand, or methodology described. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention will be limited only by the appended claims. It must be noted that as used in this specification and the appended claims, the singular forms "a", "an" and "the" include plural references unless the context clearly dictates otherwise. Thus, for example, reference to "a stem cell" includes a plurality of cells, including mixtures thereof, and reference to "a mpl ligand" include compounds able to bind mpl with sufficient specificity to elicit mpl-mediated biological activity. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methodology and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now describee. All publications mentioned herein are incorporated herein by reference for the purpose of disclosing ar.c αes::;c:ng the particular materials and methodologies fcr which the reference was cited in connection with. Definitions By the term "mpl ligand" is meant a compound capable of binding to mpl such that one or more mpl-mediated biological actions are initiated. As herein disclosed, mpl-mediated biological activity includes (1) promotion of the survival of stem cells in culture, such that the cell maintains the capability of self-renewal and the ability to give rise to all hematopoietic cell lineages, (2) expansion of stem cell populations, such that the expanded cell population maintains the capability of self-renewal and the ability to give rise to all hematopoietic cell lineages, and (3) activation of a quiescent stem cell, such that the stem cell is activated to divide and the resulting cells maintain the capability of self-renewal and the ability to give rise to all hematopoietic cell lineages. The mpl ligand in the invention initiates at least one mpl-mediated activity, and preferably two or more mpl-mediated activities. Preferably, mpl ligand is thrombopoietin, and more preferably, human thrombopoietin, and still more preferably, recombinant human thrombopoietin. The term "mpl ligand" also includes antibodies to the mpl receptor capable of binding to mpl such that one or more of the above-described mpl-mediated biological actions are initiated. Such antibodies may consist essentially of pooled monoclonal antibodies with different epitopic specificities, or be distinct monoclonal antibodies. The term "mpl ligand" further includes mimetic molecules, e.g., small molecules able to bind mpl such that one or more of the above-described mpl-mediated biological actions are initiated. The methods and assays disclosed herein, combined with methods known to the art, are utilized to construct libraries cf mimetic molecules, and to screen the libraries such that a TPO mimetic is identified having the requisite c clogical activity. By the term "ste cell" is meant hematopoietic cells which are capable of self-regeneration when provided to a human subject m vi vo, and may become lineage restricted progenitors, which further differentiate and expand into specific lineages. As used herein, "stem cells" refers to hematopoietic cells and not stem cells of other cell types. Further, unless indicated otherwise, "stem cells" refers to human hematopoietic stem cells. The term "stem cell" or "pluripotent" stem cell are used interchangeably to mean a stem cell having (1) the ability to give rise to progeny in all defined hematopoietic lineages, and (2) stem cells capable of fully reconstituting a seriously immunocompromised host in all blood cell types and their progeny, including the pluripotent hematopoietic stem cell, by self-renewal. A stem cell or pluripotent stem cell may be identified by expression of the cell surface marker CD34 * . Stem cells constitute only a small percentage of the total number of hematopoietic cells. Hematopoietic cells are identifiable by the presence of a variety of cell surface "markers." Such markers may be either specific to a particular lineage or progenitor cell or be present on more than one cell type. CD34 is a marker found on stem cells and a significant number of more differentiated progenitor cells. U.S. Patent No. 4,714,680 describes a population of cells expressing the CD34 marker. As used herein, "stem cells" or "pluripotent stem cells" refers to a population of hematopoietic cells having all of the long-term engrafting potential m vi vo . Animal models for long-term engrafting potential of candidate human hematopoietic stem cell populations include the SCID-hu bone model and the in u tero sheep model. In vi tro assays for stem ceils induce the long-term culture-initiating cell (LTCIC) assay, oaseo on a limiting dilution analysis of the number of clonogemc cells produced in a stromal co-culture after 5-8 weeks, ana the cobblestone area forming cell (CAFC) assay, shown to correlate with the LTCIC assay and with long-term engrafting potential. For use in the present invention, a highly enriched stem cell population is preferred. An example of an enriched stem cell population is a population of cells selected by expression of the CD34 marker. In LTCIC assays, a population enriched in CD34 * cells will typically have an LTCIC frequency in the range of 1/50 to 1/500, more usually in the lange of 1/50 to 1/200. Preferably, the stem cell population will be more highly enriched for stem cells than that provided by a population selected on the basis of CD34 * expression alone. By use of various techniques described more fully below, a highly enriched stem cell population may be obtained. A highly enriched stem cell population will typically have an LTCIC frequency in the range of 1/5 to 1/100, more usually in the range of 1/10 to 1/50. Preferably, it will have an LTCIC frequency of at least 1/50. Exemplary of a highly enriched stem cell population is a population having the CD34 * Lιn " or CD34 * Thy-l * Lm " phenotype as described in U.S. Patent No. 5,061,620, incorporated herein by reference to disclose and describe such cells. A population of this phenotype will typically have an average LTCIC frequency of approximately 1/20. LTCIC frequencies are known to correlate with CAFC frequencies. Most preferably, the stem cell population will be enriched for the ability to efflux the mitochondrial dye rhodamιnel23 (Rhl23) . The ability to efflux Rhl23 is a characteristic associated with the most primitive pluripotent human stem cell. Thus, in the most preferred embodiment, the stem ceil population is characterized by having the CD34 * Lm " Rhol23 or CD34 * Thy-l * Lιn " Rhol23 10 phenotype. It will be appreciated by those of skill in the art that the enrichment provided in any stem cell population will be dependent both on the selection criteria used, as well as the purity achieved b the given selection techniques. Further, the LTCIC or CAFC frequencies obtained will vary depending on the assay conditions such as the stromal cells and cytokines used, although the enrichment in stem cell activity over that present in whole bone marrow will be comparable. Stem cells may be isolated from any known human source of stem cells, including bone marrow, both adult and fetal, mobilized peripheral blood (MPB) and umbilical cord blood. Initially, bone marrow cells may be obtained from a source of bone marrow, including ilium (e.g. from the hip bone via the iliac crest), tibia, femora, spine, or other bone cavities. Other sources of stem cells include embryonic yolk sac, fetal liver, and fetal spleen. For isolation of bone marrow, an appropriate solution may be used to flush the bone, including saline solution, conveniently supplemented with fetal calf serum (FCS) or other naturally occurring factors, in conjunction with an acceptable buffer at low concentration, generally from about 5-25 mM. Convenient buffers include HEPES, phosphate buffers and lactate buffers. Otherwise bone marrow may be aspirated from the bone in accordance with conventional techniques well known to those skilled in the art. Methods for mobilizing stem cells into the peripheral blood are known in the art and generally involve treatment with chemotherapeutic drugs, e.g, cytoxan, cyclophosphamide, VP-16, and cytokines such as GM-CSF, G-CSF, or IL-3, or combinations thereof. Typically, apheresis for total white cells begins when the total white cell count reaches 500-2000 cells/μl and the platelet count reaches 50,000/μl. Daily leukapheris samples may be monitored for the presence of CD34 " and/or Thy-1 " cells to determine the peak of stem cell mobilization and, hence, the optimal time for harvesting peripheral blood stem cells. Various techniques may be employed to separate the cells b- initially removing cells of dedicated lineage ("Imeage-committed" cells) . Monoclonal antibodies are particularly useful for identifying markers associated with particular cell lineages and/or stages of di ferentiation. The antioodies may oe attached to a solid support to allow for cruαe separation. The separation techniques employed should maximize the viability of the fraction to be collected. The use of separation techniques include those based on differences in physical (density gradient centrifugation and counter-flow centrifugal elutriation) , cell surface (lectin and antibody affinity) , and vital staining properties (mitochondria-binding dye rhodam 1 ne 123 and DNA-binding dye Hoechst 33342) . Procedures for separation may include magnetic separation, using antibody-coated magnetic beads, affinity chromatography, cytotoxic agents joined to a monoclonal antibody or used in conjunction with a monoclonal antibody, including complement and cytotoxins, and "panning" with antibody attached to a solid matrix or any other convenient technique. Techniques providing accurate separation include flow cytometry which can have varying degrees of sophistication, e.g., a plurality of color channels, low angle and obtuse light scattering detecting channels, impedance channels, etc. A large proportion of the differentiated cells may be removed by initially using a relatively crude separation, where major cell population lineages of the hematopoietic system, such as lymphocytic and myelomonocytic, are removed, as well as lymphocytic populations, such as megakaryocytic, mast cells, eosinophils and basophils. Usually, at least about 70 to 90 percent of the hematopoietic cells will be removed. Concomitantly or subsequent to a gross separation providing for positive selection, e.g. using the CD34 marker, a negative selection may be carried out, where antibodies to meage-specific markers present on dedicated cells are employed. For the most part, these markers include CD2 " , CD3 " , CD7 , CDS " , CDIO " , CD14 " , CD15 " , CD16 " , CD19 " , CD20 " , CD33 " , CD38 , CD71 " , HLA-DP " , and glycophorin A; preferably including at least C 2 " , CD14 " , CD15 " , CD16 " , CD19 " and glycophorin A; and normally including at least CD14 " and CD15 " . As used herein, Lir. " refers to a cell population lacking at least one lineage specific marker, e.g., a population from which one or more cell lineages has been removed by removal of cells expressing a lineage specific marker selected from the list above. The hematopoietic cell composition substantially depleted of dedicated cells may be further separated using selection for Thy-1 * and/or Rhol23 l0 , whereby a highly enriched stem cell population is achieved. The purified stem cells have low side scatter and low to medium forward scatter profiles by FACS analysis. Cytospin preparations show the enriched stem cells to have a size between mature lymphoid cells and mature granulocytes. Cells may be selected based on light-scatter properties as well as their expression of various cell surface antigens. Preferably, cells are initially separated by a coarse separation, followed by a fine separation, with positive selection of a marker associated with stem cells and negative selection for markers associated with lineage committed cells. Compositions highly enriched in stem cells may be achieved in this manner. The desired stem cells are exemplified by a population with the CD34 * Thy-l * Lιn " phenotype, and are characterized by being able to be maintained in culture for extended periods of time, being capable of selection and transfer to secondary and higher order cultures, and being capable of differentiating into the various lymphocytic and myelomonocytic lineages, particularly B- and T-lymphocytes, monocytes, macrophages, neutrophils, erythrocytes and the like. The stem cells may be grown in culture in an appropriate nutrient medium, including conditioned medium, a co-culture w th an appropriate stromal cell line, or a medium comprising a combination of growth factors sufficient to maintain the growth of hematopoietic cells. For conditioned media or co-cultures, various stromal cell lines may be used. Since human stromal cell lines are not required, other stromal cell lines may be employed, including rodentiae, particularly murine. Suitable murine stromal cell lines include AC3 and AC6, which are described in Whitlock et al. (1987) Cell 48:1009-1021. Preferably, the stromal cell line used is a passage of AC6, AC6.21 (otherwise referred to as SySl). The compositions comprising stem cells can be tested for the ability to produce myeloid cells and lymphoid cells in appropriate stromal cell co-cultures. The stromal cells may come from various sources, including human, porcine or murine, by selection for the ability to maintain stem cells, and the like. Preferably, the stromal cells are AC3 or AC6, most preferably AC6.21, and the ability to produce B lymphocytes and myeloid cells is determined in cultures supplied with LIF and IL-6. The stem cells also give rise to B-cells, T-cells and myelomonocytic cells in the in vi vo assays described below. To demonstrate differentiation to T-cells, fetal thymus is isolated and cultured from 4-7 days at about 25°C, so as to substantially deplete the lymphoid population. The cells to be tested for T-cell activity are then microinjected into the thymus tissue, where the HLA of the population which is injected is mismatched with the HLA of the thymus cells. The thymus tissue may then be transplanted into a scid/scid mouse as described in US Patent No. 5,147,784, herein specifically incorporated by reference, particularly transplanting under the kidney capsule. After 6 weeks, the thymus tissue is harvested and analyzed for donor-derived CD4 * and/or CD8 * T- cells by flow cytometry. Further demonstration of the long-term repopulating ability of the stem cell populations useful in the invention is shown by the detection of continued myeloid and B-lymphoid cell production in the SCID-hu bone model. Human fetal bone is isolated and a longitudinally sliced portion of this bone transferred into the mammary fat pad of a scid/scid animal. The bone cavity is diminished in endogenous cells by whole body irradiation of the mouse host prior to injection of the test donor population. The HLA of the population which is injected is mismatched with the HLA of the recipient bone cells. Stem cells from human hematopoietic sources sustain B lymphopoiesis and myelopoiesis in the SCID-hu bone model. Finally, measurement of burst forming unit-erythroid (BFU-E) units, for example in methylcellulose culture, demonstrates that the cells are capable of developing into the erythroid lineage . Effect of Thrombopoietin on Stem Cells The human mpl transcript in highly purified bone marrow and mobilized peripheral blood CD34 * Thy-l * Lιn " stem cells was demonstrated by RT-PCR (Example 3) . However, it was not known if the receptor is expressed on the cell surface or if a receptor ligand would have any effect on the cells. The present study was initiated to determine whether TPO has any effect on primitive human hematopoietic stem cells ("HSC"), and if so, to define its range of activities. Initially, human TPO was cloned and sequenced based on the published sequence as described in Example 1. Single CD34 * Thy-l * Lin " cells from adult bone marrow (ABM) were cultured individually in Terasaki wells on the murine stromal cell monolayer, SySl, with TPO alone or in combination with cytokines IL-3, KL, IL-6, or LIF (Example 4) . The growth of each cell was tracked by image acquisition, as described in Example 4. The maximum proliferation achieved over a 4 week period by each CD34'Thy-l * Lιn " cell plated under various cytokine conditions from a representative ABM tissue was compared. TPO alone supported a plating efficiency of 74% and an average cell production of 454 cells per cell. Plating efficiency is defined as the number of cells that divided one or more times/number of cells plated. When IL-3 was added, cell prodjctior was enhanced approximately 4-fold. This level of proliferat-cr was similar to that achieved in the presence of combined IL-3, IL-6, LIF and KL. The results show that TPO acts directly on human CD34 * Thy-l * Lm " cells to induce cycling of primitive noncommitted human hematopoietic progenitors (e.g., expressing CD34 but not lineage antigens), and that co-stimulation with IL-3 increases total cell production. Approximately 50% of single CD34 * Thy-l + Lιn " cells grown in the presence of TPO displayed MK differentiation, as measured by size and CD41b antigen expression (Example 4) . CD41b antigen expression is characteristic of cells committed to the MK lineage. These results demonstrate that TPO supports stem cell differentiation into MK lineage. To quantitatively demonstrate that TPO was stimulating single stem cells to produce multiple MK progenitors prior to maturation, the number of CFU-MK present in populations of day 0 ABM CD34 * Thy-l * Lin " cells were compared to the number obtained from wells of proliferating blast cells that grew from a single CD34 * Thy-l * Lin " cell in the presence of TPO. In one representative experiment, day 0 CD34 + Thy-l * Lin " cells produced 0.0026 CFU-MK per cell, compared to blast cell populations expanded from a single CD34 * Thy-l * Lin " cell, which produced an average of 8.3 CFU-MK per original cell (range 1-18) in the presence of TPO and IL-3. This represents an amplification in MK progenitor production during culture of 3,200-fold, and indicates that TPO can act on single human stem cells and permit commitment to the MK lineage, MK progenitor expansion, and MK maturation in culture. In addition to promoting MK proliferation and differentiation, it has now been found that TPO acts on primitive HSC to promote expansion without differentiation into the MK lineage. This finding is surprising in light of the report that only the MK lineage is defective in mpl- deficient transgenic mice (Guerney et al. (1994) Science 265:1445) . Approximately 50% of the single CD34 * Thy-l * Lin " cells from ABM that grew in the presence of TPO did not display MK differentiation. A small proportion (10% of cells that grew) developed directly into large granular cells with macrophage characteristics. The remaining 40% of cells that grew formed tightly apposed clusters of blast cells (cobblestone areas) which did not differentiate to large refractile MK or large granular macrophage cells over a four week period. The addition of IL-3 to TPO increased the frequency of cells exhibiting a non-megakaryocytic blast cell expansion pattern from 40% to approximately 80% of single cells that grew, and increased the proliferation of single cells approximately 4-fold such that some wells reached the maximum capacity of the Terasaki well (approximately 10,000 cells) . To determine whether other cytokines alone, or in combination with TPO could support proliferation of primitive CD34 * cells, cytokines with known activities on early hematopoietic cells were tested. Although CD34 * cells were detected in cell populations expanded using IL-6+LIF+IL-3+KL, the highest percentages of CD34' cells were always found in wells which contained TPO (Table 1) . A typical TPO-expanded blast population of approximately 1,000 cells, of which 2% are CD34 positive, represents an approximate 20-fold expansion of CD34 * (Table 1) . A stromal support layer (SySl) was used in the long-term culture assays herein described because previous observations showed that single or small numbers of HSC cultured under stroma-free conditions displayed more rapid differentiation and lower total cell production than those cultured on a stromal layer. SySl culture supematants alone cannot substitute for the stromal cells for support of hematopoietic progenitors. In addition, single HSC plated on SySl without exogenous human cytokine addition show an extremely low growth frequency. The above findings made with single HSC grown on a stromal layer were substantiated by showing that in bulk cultures grown in the absence of stroma, purified recombinant TPO alone can activate qu escert HSC. PKH26 s a fluorescent dye which laoels cell memoranes and reduces in intensity with each cell α v.s or.. The effect of TPO on PKH26 fluorescence relative tc C 34 expression was analyzed in stromal-free bulk cultures. For these experiments, an enriched quiescent (CD34'Lιn " Rhl23 * ) stem cell population was tested. PKH26 dye marking of cell membranes was used to obtain quantitation of cell divisions in conjunction with expression of the CD34 antigen to determine the effects of various cytokine combinations on stem cell proliferation and differentiation. TPO alone was able to stimulate these quiescent HSC to begin cycling w hin 3 days, and still retain high levels of CD34 expression after 6 days of culture. TPO alone caused greater activation than KL alone at 3 and 6 days. When TPO and KL were combined, they acted synergistically to increase cell cycling by day 6. IL- 3 plus TPO produced much more cell proliferation than TPO alone, but caused more differentiation than TPO alone or TPO+KL (e.g., loss of CD34 expression). Therapeutic Use of Thrombopoietin The present invention is based on the discovery that mpl ligand, thrombopoietin, has unique and unexpected effects on human stem cells. As established by the evidence provided herein, exposure of human long-term repopulating stem cells to TPO results in cell expansion with minimal differentiation. TPO also causes expansion of human stem cells with subsequent differentiation to the MK lineage, as well as non-MK cells. Further, TPO is shown to activate quiescent human stem cells into cycle more quickly than other known cytokines. Additionally, TPO is herein shown to promote the survival of human stem cells in m vi tro culture. The suoset of primitive human hematopoietic cells shown herein to respond to TPO are long-term repopulating or pluripotent stem cells, characterized by the ability to give rise to cel_s which retain the capability of self-renewal, and to proliferate and differentiate into cells of all hematopoietic lineages. Use cf TPO for Expansion of Long-Term Repopulating Cell Populations. The ability of TPO to induce extensive proliferation of a primitive human stem cell subpopulation without loss of the pluripotent capacity has important clinical implications for restoration of hematopoietic capability in subjects in which hematopoietic capability is lost or threatened. Accordingly, the invention features the use of TPO for m vi tro expansion of a human long-term repopulating cell population. This is particularly useful for re-establishing hematopoietic capability in patients in which native hematopoietic capability has been partially, substantially, or completely compromised. Stem cells from any tissue are removed from a human subject, expanded m vi tro by exposure to TPO, and the expanded cells are returned to the patient. If necessary, the process may be repeated to ensure substantial repopulation of the stem cells. The expanded stem cell population returned to the subject retain pluripotent characteristics, e.g., self-renewal and ability to generate cells of all hematopoietic lineages. By combination of various cytokines the expanded cell population will include progenitor cells and more mature cells of the various hematopoietic lineages (e.g., megakaryocytes, neutrophils) in addition to stem cells to provide a cell population that will provide both short-term and long-term repopulation potential. Stems cells are preferably isolated from bone marrow or mobilized peripheral blood, and more preferentially from bone marrow. Expansion procedures may be conducted either with or without stromal cells. Stromal cells may be freshly isolated from bone marrow or from cloned stromal cell lines. Such lines may be human, murine, or porcine; preferably, the cell line is AC6.21, as described in the Examples below. For clinical applications, it is preferred to culture the stem cells in the absence of stromal cells. During expansion, TPO may be present only during the initial course of the stem cell growth and expansion, usually at least 24 hours, more usually at least about 48 hours to 4 days, or more preferably is maintained during the course of the expansion. During cell expansion, TPO is present in a concentration range of 10 - 200 ng/ml; more preferably, TPO is present in a concentration range of about 10 - 100 ng/ml. In addition, TPO may be combined with the use of other growth factor and cytokines, such as IL-3. During stem cell expansion, TPO is present in a concentration range of between about 1 ng/ml to about 200 ng/ml, preferably in the range of between about 50 r.-/ml and 100 ng/ml, most preferably about 50 ng/ml. When cell expansion is conducted in the presence of TPO and IL-3, IL-3 is present in a concentration range of between about 1 ng/ml to about 100 ng/ml, preferably in the range of between about 5 ng/ml and 25 ng/ml, most preferably about 10 ng/ml. Additional regulatory factors may be present, for example, Stl (50 - 100 ng/ml), LIF (50 ng/ml), IL-6 (2-100 ng/ml), MlP-lα (2-100 ng/ml), Flk2/Flt3 (2-100 ng/ml), G-CSF (2-20 ng/ml), GM-CSF (2-20 ng/ml), IL-1 (2-100 ng/ml), and IL-ll (2-100 ng/ml) . Various in vi tro and m vi vo tests known to the art may be employed to ensure that the pluripotent capability of the stem cells has been maintained. Use of Thrombopoietin in Gene Therapy. TPO has been shown herein to stimulate a quiescent human long-term repopulating stem cell population to begin actively dividing without differentiation, e.g., culturing long-term repopulating stem cells in the presence of TPO results in the generation of cells which retain the capability of self- renewal and ability to give rise to cells of all hematopoietic lineages. This discovery is particularly important for transduction of human stem cells with exogenous genes because retroviral vectors require target cells to be cycling for stable integration of the retroviral DNA. The ability to modify a human pluripotent cell population should provide long-term repopulation of an individual with the modified cells and their progeny, which will express the desired gene product. By contrast, gene transfer into more mature hematopoietic cells, such as T cells, at best, provides only transient therapeutic benefit. Thus, the use of TPO for transduction of stem cells satisfies the current world-wide effort to find effective methods of genetically modifying stem cells. While retroviral vectors may be used to genetically modify a population of human long-term repopulating stem cells, other methods may be used, such as liposome-mediated gene transfer or adeno-associated viral vectors. Retroviral vectors have been the primary vehicle due to the generally high rate of gene transfer obtained in experiments with cell lines, and the ability to obtain stable integration of the genetic material, which ensures that the progeny of the modified cell will contain the transferred genetic material. Hematopoietic stem cells are removed from a human patient, and a population of long-term repopulating stem cells isolated. These cells may be optionally expanded prior to or after modification by transduction with a vector carrying the desired gene. The modified cells are then restored to the human patient for expression of the foreign gene. The patient may be treated to partially, substantially, or completely ablate the native hematopoietic capability prior to restoration of the modified stem cells. Preferably, after completion of the treatment of the host, the modified stem cells may then be restored to the host to provide for expression of the foreign gene. The methods of stem cell removal, host ablation and stem cell repopulation are known in the art. If necessary, the process may be repeated to ensure substantial repopulation of the modified stem cells. During optional expansion, TPO alone or combined with other growth factors may be present only during the initial course of the stem cell growth and expansion, usually at least 24 hours, more usually at least about 48 hours to 4 days, or may be maintained during the course of the expansion. Example 8 belovv describes a protocol for stem cell transduction in the presence of TPO. For use in clinical settings, it is preferable to transduce the stem cells without prior or subsequent expansion. Transouction may be accomplished by the direct co-culture of stem cells with producer cells, e.g. by the method of Bregni et al. (1992) Blood 80:1418-1422. For clinical applications, however, transduction by culturing the stem cells with viral supernatant alone or with purified viral preparations, in the absence of stromal cells, is preferred. Transductions may be performed by culturing the stem cells with the virus for from about four hours to six days. Preferably, transduction is carried out for three days, with the media replaced daily with media containing fresh retrovirus. Alternatively, the stem cells may be cultured in the presence of virus for several hours, e.g., four hours, daily for three to four days, with fresh media replacing the virus-containing media each day. Polycations, such as protamine sulfate, polybrene and the like, will generally be included to promote binding. Protamine sulfate and polybrene are typically used in the range of 4 μg/ml. Other cytokines may also be added, including, e.g., IL- 3, IL-6, LIF, steel factor (Stl) GM-CSF, G-CSF, MlP-lα, and Flk2/Flt3, preferably including Stl. The factors employed may be naturally occurring or synthetic, e.g., prepared recombinantly, and preferably human. The amount of the factors will generally be in the range of about 1 ng/ml to 200 ng/ml. Generally, for TPO, the concentration will be in the range of about 1 ng/ml to 200 ng/ml, more usually 5 ng/ml to 100 ng/ml, and optimally about 10 ng/ml to 50 ng/ml; for Stl, the concentration range will be in the range of 10 ng/ml to 200 ng/ml, and more usually 50 ng/ml to 100 ng/ml; for LIF, the concentration will be in the range of about 1 ng/ml to 100 ng/ml, more usually 10 ng/ml to 80 ng/ml, and optimally aoout 50 ng/ml; for IL-3, the concentration will be in the range of about 5 ng/ml to 100 ng/ml, more usually 5 ng/ml to 5C ng/ml; for IL-6, the concentration will be in the range of about 5 ng/ml to 50 ng/ml, more usually 5 ng/ml to 20 ng/m, and for GM-CSF, the concentration will generally be 5 ng/ml to 50 ng/ml, more usually 5 ng/ml to 20 ng/ml. To ensure that the stem cells have been successfully modified, PCR may be used to amplify vector specific sequences in the transduced stem cells or their progeny. In addition, the cells may be grown under various conditions to ensure that they are capable of maturation to all of the hematopoietic lineages while maintaining the capability, as appropriate, of the introduced DNA. Various in v ' fro and in vivo tests described above may be employed to ensure that the pluripotent capability of the stem cells has been maintained. Gene Therapy Applications. Gene transfer into stem cells is useful to treat a variety of neoplastic, infectious or genetic diseases. For example, one may introduce genes that confer resistance to chemotherapeutic agents, thereby protecting the progeny hematopoietic cells, allowing higher doses of chemotherapy and thereby improving the therapeutic benefit of treatment. For instance, the mdrl gene may be introduced into stem cells to provide increased resistance to a wide variety of drugs including taxol, which are exported by the mdrl gene product, in combination with the administration of chemotherapeutics such as taxol, e.g. for oreast cancer treatment. Similarly, genes that provide increased resistance to alkylating agents, such as melphalan, may be introduced into stem cells in conjunction with high dose chemotherapy. For viral infections that primarily affect hematolymphoid cells, stem cells are modified to endow the progeny with resistance to the infectious agent. In the case of human immnunodeficiency virus (HIV) , for example, specific antisense or ribozyme sequences may be introduced that interfere w th viral infection or replication in the target cells. Alternatively, the introduced gene products may serve as "oecoys" oy binding essential viral proteins, thereby interfering with the normal viral life cycle and inhibiting repl catior . Alternatively, stem cells may be modified to produce a product to correct a genetic deficiency, or where the host has acquired a genetic deficiency through a subsequent disease. Genes that may correct a genetic deficiency include adenosine oeaminase for the treatment of ADA severe combined immunodeficiency; glucocerebrosidase for the treatment of Gaucher's disease; β-globin for the treatment of sickle cell anemia; and Factor VIII or Factor IX for the treatment of hemophilia; tumor antigen-specific T-cell receptors for immunotherapy of cancer, and cytokines for treatment of cancer and cytokine-related disorders. Expression of the transferred gene can be controlled in a variety of ways depending on the purpose of gene transfer and the desired effect. Thus, the introduced gene may be put under the control of a promoter that will cause the gene to be expressed constitutively, only under specific physiologic conditions, or in particular cell types. Examples of promoters that may be used to cause expression of the introduced sequence in specific cell types include Granzyme A and Granzyme B for expression in T-cells and NK cells, the CD34 promoter for expression in stem and progenitor cells, the CD8 promoter for expression in cytotoxic T-cells, and the CDllb promoter for expression in myeloid cells. Inducible promoters may be used for gene expression under certain physiologic conditions. The therapeutic benefit may be further increased by targeting the gene product to the appropriate cellular location, for example, the nucleus, by attaching the appropriate localizing sequences. In addition, by appropriate use of inducible promoters, expression of various protein products can be achieved in response to particular stimuli such as chemicals, chemo-attractants, particular ligands, and the like. Example 1. Cloning and Expression of Throrr-oopoietin TPC cDNA was cloned using the polymerase chain reaction from human fetal liver cDNA (Clontech, Palo Alto, CA) , using oiigonucleotides based on the published sequence, de Sauvage et al. Nature (1994) 369: 533, and expressed in COS cells. TPO-containing COS supematants were shown to be active in stimulating the proliferation of mpl-expressing BaF3 cells (activity was 3 x 105 U/ml, where 50 U/ml = 50% maximum activity) , which were constructed by stable transfection of BaF3 cells with a human mpl expression construct as previously described (de Sauvage et al. (1994) supra) . Mock transfected COS supernatant (COS control) was harvested from COS cells transfected with an empty expression vector, and had no effect on the proliferation of BaF3 cells. Exa ple 2. Cell Selection Enrichment of progenitor cells from human adult bone marrow and mobilized peripheral blood. Human adult bone marrow (ABM) CD34+ cells were enriched as follows. Briefly, fresh 20 ml bone marrow aspirates from normal volunteer donors were obtained from Stanford University Medical Center (Palo Alto, CA) or Scripps Research Institute (La Jolla, CA) . Mononuclear cells (density <1.077) were isolated from a Ficoll (Pharmacia, Milwaukee, WI) density gradient in cell separation medium [RPMI 1640 (JRH Biosciences, Lenexa, KS) , 10% fetal bovine serum (FBS) (Gemini Bioproducts, Calabasas, CA) , 50 U/ml penicillin and 50 μg/ml streptomycin (JRH Biosciences) ] . CD34 positive selection was performed by the method of Sutherland et. al (1992) Exp. Hematol. 20:590) . The glycoprotease from Pasteurella haemolyti ca used for the bead release step in this procedure has been shown not to effect subsequent ex vi vo expansion of progenitors (Marsh et al. (1992) Leukemia 6:929) . Stem cells were mobilized into the peripheral blood (MPB) cf multiple myeloma patients and collected by leukapheresis as described by Murray et al. Blood (1995) 85: 368-378. MPB CD34 ' cells were enriched from leukapheresis products by using a procedure involving elutriation, phenylmethylester lysis of granulocytes, and ammonium chloride lysis of red blood cells and CD34 positive selection as described by Sutherland et al. (1992) supra. Antibodies. TuK3 (antι-CD34) directly conjugated to sulpharhodamme (SR) and GM201 (anti-human Thy-1) directly conjugated to phycoerythrin (PE) were used to purify primitive progenitors by flow cytometry. FLOPC 21 mouse IgG3 (Sigma, St. Louis, MO) and purified mouse IgGl (Becton Dickinson, Mountain View, CA) were conjugated to SR and PE, respectively, and used as controls for CD34 and Thy-1 staining. Fluorescein isothiocyanate (FITC) -conjugated lineage antibodies were used to exclude lineage positive cells from selection [Leu-5b (antι-CD2) , Leu-M3 (antι-CD14), Leu-Ml (antι-CD15), Leu-lla (antι-CD16) , Leu-12 (antι-CD19) ] , FITC-conjugated mouse IgGl and IgG2a, PE and FITC-conjugated HPCA2 (antι-CD34) and mouse IgGl were purchased from Becton Dickinson. FITC-conjugated antibody D2.10 (anti-glycophorin) was purchased from AMAC (Westbrook, ME) . FITC-conjugated mouse IgM was purchased from Sigma (St. Louis MO) . Human gamma globulin (Gamimune) , was purchased from Miles Inc. (Elkhart, IN) . Fluorescent labeling and flow cytometry. CD34 * enriched hematopoietic cells were adjusted to a concentration of 2 x 10 e /ml in staining medium [RPMI 1640 without phenol red, 2% FBS, and 10 mM HEPES (Sigma, St. Louis, MO)] and labeled using the following procedure performed on ice. Heat inactivated human gamma globulin (Gamimune) was added to the cell suspension at a concentration of 1 mg/ml to block Fc receptor binding sites. Cells were incubated with antι-CD34-SR (10 μg/ml), antι-Thy-1-PE (10 μg/ml), and a mixtμre of FITC-conjugated antibodies directed against a panel of lineage markers. The mixture included antι-CD2, antι-CD14, antι-CD15, antι-CD16, antι-CD19, and anti-glycophorin A. Irrelevant mouse IgC-2-SF (IC μg/ml) and IgGl-PE (10 μg/ml) antibodies were used as controls for CD34 and Thy-1 staining. FITC-conjugated mouse IgGl, IgG2a, and IgM were used as controls for the lineage panel. Incubation time for staining was 20 minutes followed by a three ml wash with staining medium. Following antibody staining, cells were resuspended at a concentration of 10 δ cells/ml in staining medium containing 1 μg/ml propidium iodide (Pl) (Molecular Probes Inc., Eugene OR) . Fluorescently stained cells were analyzed and sorted on a FACStar Plus cell sorter (Becton Dickinson) . Dead cells (Pl positive) and lineage positive cells (stained with an intensity above the isotype matched control antibody staining) were excluded from the sort gate. The CD34 * Thy-l * and, in some cases, CD34 * Thy-l " , fractions were sorted from the live, lineage-negative cells (CD34 τ Thy-l * Lin ~ or CD34 * Thy- l " Lin " ) . CD34 * selected ABM cells were loaded with rhodamine 123 dye (Rhl23) (Molecular Probes, Eugene, OR) , and allowed to efflux at 37°C for 20 min, according to the method of Spangrude et al. (1988) Science 241:58-62, herein specifically incorporated by reference. The cells were then chilled to 4°C and reacted with anti-CD34-SR and the same lineage marker antibodies as described above except conjugated to PE. Cells with a CD34 * Lin " Rhl23- c phenotype were purified by flow cytometry. Example 3. PCR Detection of MPL in Bone Marrow and Mobilized Peripheral Blood Samples. Cell lysates were prepared from total human bone marrow cells and bone marrow or mobilized peripheral blood cells of the phenotypes CD34 * Thy-l * Lin " (Thy * ) and CD34 * Thy-l " Lin " (Thy " ), isolated as described in Example 2. RNA was prepared using RNA STAT60 (Tel-Test, Inc.) according to the manufacturer's instructions. cDNA was made from each of the RNA samples using BRL Superscript RT following the manufacturer's instructions. One-fifth of the cDNA was then used for the first degenerate PCR reaction. The degenerate PCR primers designed to amplify sequences related to the Epo receptor and mpl receptor of the class I hemopoietin receptor family are as follows: Sense primer HEpMl-5 (SEQ ID NO:l): 5 '-GCTATTGCGGCCGCGAATTCGGARGAYYTIINITGYTTYTGG-3' Antisense primer HEpM2-3 (SEQ ID NO:2) : 5'-GCTATTCTCGAGATCGATSWCCAITCRCTCCAIIINCC-3' Antisense primer HEpM3-3 (SEQ ID NO:3): 5'-GCTATTCTCGAGATCGATSWCCAINIRCTCCAIIINCC-3' (R = A,G; Y = C,T, I = inosine; S = G,C; W = A,T) . Restriction sites were added at the 5' end of each primer. One set of the reaction was performed with HEpMl-5 and HEpM2- 3, and another reaction with was performed with HEpMl-5 and HEpM3-3. In each case, a smear was obtained. The two PCR products (2 μl of each) were mixed for the second PCR reaction. The second PCR was carried out using specific primers to mpl. The following two sets of primers were used: Set A: sense primer mpl-5' (317-336) (SEQ ID N0:4) : 5'-CGCTGCACCTCTGGGTGAAG-3' antisense primer mpl-3' (568-588) (SEQ ID N0:5) : 5 ' -AGCAGGGCAGCAGGTTTCTGT-3' Set B: sense primer mpl-5' (317-336) (SEQ ID N0:4) and antisense primer mpl-3-HI (754-77 ) (SEQ ID N0:6) : 5 '-GCTGCGCAGCTGCAGCCAGTA-3' Results. In total bone marrow samples, mpl was detected with mpl-5' /mpl-3' primers alone. In sorted bone marrow samples, mpl was detected in both Thy * and Thy " samples using both sets of primers, mpl was detected using both sets of primers in peripheral blood samples (Thy * and Thy " ) . Example 4. Effect of TPO on Single Cell Cultures of Stem Cells. Terasaki plates used for single cell deposition were preseeded with a murine stromal cell line (AC6.21, also referred to as SySl) described previously (Baum et al. (1992) Proc. Natl. Acad. Sci. USA 89:2804; DiGiusto et al. (1994) Blood 84:421; Murray et al. (1995) supra) , at approximately 50 cells per well in 20 μl of stromal cell culture medium [RPMI 1640, 5% FBS, 10 "5 M 2-mercaptoethanol (2-ME) (Sigma), 4 mM glutamine (JRH Biosciences) , 50 U/ml penicillin, and 50 μg/ml streptomycin] . Stromal cells were allowed to form a confluent monolayer by culturing for 4-7 days. At confluence monolayers were irradiated (1,000 rads from a cesium source) to prevent overgrowth. Immediately prior to seeding of hematopoietic cells, 10 μl of medium was removed and replaced with 10 μl stem cell culture medium (SCCM) [IMDM (JRH Biosciences), 5% pooled human plasma (HP), 7.5 X 10 " M α-TG (Aldrich Chemical Co. Milwaukee, WI), 10 "5 M 2-ME, 100 mM Na pyruvate (JRH Biosciences) , 4 mM glutamine, 50 U/ml penicillin, 50 μg/ml streptomycin] . The culture medium was chosen for maintenance of MK integrity which included human plasma (HP) and α-TG. SCCM was supplemented with combinations of cytokines as indicated. Cytokines included recombinant human (rh)IL-3 (10 ng/ml) (Genzyme, Cambridge, MA), rhIL-β (10 ng/ml) (Sandoz, Basel, Switzerland), rhKL (25 ng/ml) and rhLIF (10 ng/ml) (Tago/Biosource, Camarillo, CA) and 10% supernatant collected from COS cells transfected with a TFO expression vector or 50 ng/ml rhTPO (R & D Systems, Minneapolis, MN) . Supernatant collected from COS cells transfected with an empty expression vector was used as a control for TPO-containmg COS supernatant. Single CD34 ' Thy-l * Lιn " adult bone marrow (ABM) cells were cultured in the presence of 10% control COS supernatant (controls) , 10% COS supernatant from COS cells transfected with TPO, 10% TPO COS supernatant + 10 ng/ml IL-3, 10% TPO COS supernatant + 25 ng/ml KL, or in the presence of 10 ng/ml IL-3, IL-6, and LIF, and 25 ng/ml KL. Imaging and cell number quantitation. Images of Tcrasaki well single cell cultures were digitized and stored twice weekly. Magnification was adjusted such that the entire well was visible in each image. An inverted Nikon Diaphot microscope and Hamamatsu C2400 Newvicon CCD camera were used for image acquisition. Image-1 image analysis software and Compaq 66-MHz 486 computer (digitizing hardware) were used (Universal Imaging, Philadelphia, PA) . A Panasonic LF-7010 rewritable optical disk drive was used for image storage. Cell numbers were determined from the image series by direct counting (<100 cells) ; or by estimating the proportion of the well filled (>100 cells) . Average cell number in confluent wells was determined by hemacytometer counting. Phenotypic analysis of subconfluent and confluent culture wells. Analysis of CD34 expression on cultured cells by flow cytometry was carried out as follows. Briefly, single cells which grew to fill an entire Terasaki well with blast cells were individually analyzed for the percentage of primitive progenitors (CD34 * cells) . The contents of subconfluent Terasaki wells (30-90% filled with blast cells) were pooled for analysis. Hematopoietic cells were removed from the wells by gentle pipetting, resuspended in 100 μl staining medium, ano stained with antι-CD34 (HPCA2)-PE. A pool maoe from a small aliquot from each of the wells was used for control staining with IgGl-PE. A well was ccrsidereα to contain a CD34 " population if greater than 1% cf the cells showed a level of fluorescence above that of the isotype cortroi. MK Progenitor Fibrin Clot Assay. The serum-depleted fibrin clot assay for colony-forming unit-megakaryocyte (CFU-MK) has been described previously (Bruno et al. (1989) Blood 73:671; Briddell et al. (1989) Blood 74:145) . This assay compared the number of CFU-MK present in populations of day 0 ABM CD34 * Thy-l * Lιn " cells to the number obtained from wells of proliferating blast cells that grew from a single CD34 * Thy-l " Lιn " cell in the presence of TPO. Freshly sorted cells were plated at 10 1 cells/ml, or blast populations harvested from Terasaki wells after 3 weeks of culture were plated in the fibrin clot assay in the presence of IL-3 at 10 ng/ml (Sandoz, Basel, Switzerland), GM-CSF at 2 ng/ml, and KL at 100 ng/ml (R&D Systems, Minneapolis, MN) . Fibrin clots were fixed and analyzed for CD41b * colonies at 12 days. Results. The effects of TPO on single cell cultures grown in the presence or absence of other cytokines, were compared, as well as the plating efficiency (number of single cells that divided one or more times/number of cells plated) and the average cell proliferation achieved by all single cells plated. Control COS supernatant alone did not stimulate proliferation of CD34 * Thy-l * Lιn * cells. TPO- containing COS supernatant supported a plating efficiency of 74% and an average cell production of 454 cells per cell. When 25 ng/ml KL was used in combination with TPO COS supernatant, no augmentation of growth over TPO alone was observed. TPO plus 10 ng/ml of IL-3 enhanced cell production over TPO alone approximately 4-fold (1,681 cells/input cell). This level of proliferation was similar to that achieved with a combination of four cytokines (IL-3, IL-6, LIF, and KL) . Results were similar when purified rhTPO was used. TPO Expansion of Blast Cells. To determine whether TPO alone, or m combination with other cytokines could support proliferation of primitive CD34 * cells, single CD34VThy-l * Lιn " cells were cultured ir the presence of 5% TPO COS supernatant, TPO^IL-3, TPO+KL, IL-3+IL-6+KL+LIF, and IL-3+IL- 6+K1+LIF+TP0. Cells were harvested from culture wells at 4-6 weeks, and either analyzed individually or pooled with other wells grown under like conditions. Cell populations were fluorescently stained with antι-CD34 antibody and analyzed by flow cytometry as described above. These populations contained from 0.5-4.4% CD34 " cells, average 2.8% for TPO alone compared to 1% for TPO+IL-3, 1.2% for TPO+KL, 0.5% for IL-3+IL-6+LIF+KL, and 2.1% for TPO+LIF+KL+IL-3+IL-6. Thus, in these cultures TPO increased the expansion of CD34 * cells, compared to other cytokines tested, without differentiation into the MK lineage. Blast cell outgrowths did not express CD41b by immunocytochemical staining. TPO Effect on Growth Morphology of CD34:Thy-rLιn: Cells. The effect of TPO on MK differentiation was determined by size, phenotype, and expression of CD41b antigen expression by the methods described above. Approximately 50% of single CD34 * Thy-l * Lιn ' cells from ABM which grew in the presence of TPO displayed MK differentiation, as determined by a growth pattern of initial expansion and dispersal of 10-200 refractile blast cells over the stromal cell layer, followed by differentiation into large 25-50 nm refractile cells. This process occurred over a 2-3 week period from initial plating. Wells containing cells with this growth morphology were harvested and analyzed for CD41b expression by immunocytochemical staining. Essentially all hematopoietic cells in these populations stained positively for CD41b. This included the large cells as well as those that were still quite small in size. To quantitatively demonstrate that TPO was stimulating single stem cells to produce multiple MK progenitors prior to maturation, the number of CFU-MK present in populations of day 0 ABM CD34 * Thy-l * Lm " cells were compared to the number obtained from wells of proliferating blast cells that grew from a single CD34 * Thy-l"Lιn " cell in the presence of TPO. Freshly sorted cells, or blast populations harvested from Terasaki wells after three weeks of culture were placed in the fibrin clot assay. Colonies containing MKs were enumerated after immunofluorescent staining of the fibrin clot cultures with antι-CD41b antibody. In one representative experiment, 5,000 day 0 CD34 * Thy-l * Lιn " cells produced 13 MK colonies or 0.0026 colonies per cell. Seven individual blast cell populations, each arising from one cell, produced an average of 8.3 CFR-MK per cell (range 1-18) in the presence of TPO and IL-3. This represents a 3, 200- fold amplification of MK progenitor production during culture. These findings indicate that TPO can act on single human stem cells and permit commitment to the MK lineage, MK progenitor expansion, and MK maturation in culture. Example 5. Effects of TPO on Cobblestone Area Forming Cells (CAFC) . Limiting dilution (LD) cultures were established by plating CD34 * Thy-l * Lιn " cells in Whitlock/Witte medium (50% IMDM (JRH Biosciences), 50% RPMI 1640, 10% FCS, 4 x 10 " ' M 2- mercaptoethanol, 10 mM HEPES, 100 U/ml penicillin, 100 mg/ml streptomycin, and 4 mM glutamine) at limiting dilution (from 100-0.8 cells/well) on preformed SySl stromal monolayers, as described previously (Murray et al. (1995) supra) . Four cytokine conditions were compared: (1) 5% control mock-transfected COS supernatant; (2) 5% TPO-containing COS supernatant; (3) 50 ng/ml LIF + 10 ng/ml IL-6; and (4) 50 ng/ml LIF i- 10 ng/ml IL-6 + 5% TPO supernatant. In one experiment, purified TPO (R&D Systems) was used at 10 ng/ml. Cultures were fed at weekly intervals and scored at week 4 for cobblestone area formation. Wells containing cobblestone areas were analyzed for CD34 expression. TPO Effect on Size and Frequency of Cobblestone Areas. LD cultures grown with medium alone or with 5% control COS supernatant showed almost no growth, whereas cultures grown with 5 TPO-containing COS supernatant showed increased cell growth, including growth of cobblestone areas. LD cultures grown in the presence of LIF and IL-6, showed cobblestone area formation as observed previously (Murray et al. (1995) supra) . However, when TPO was added to LIF and IL-6, the cobblestone areas appeared earlier, at higher frequency (Table 2), and grew to a much larger size by 4 weeks. When these wells were analyzed for CD34 expression, 69% contained CD34+ cells compared to 38% of wells grown in LIF and IL-6 alone. Similar results were obtained with purified rhTPO. In agreement with the findings obtained with single cell culture, TPO increased the expansion of CD34+ cells from primitive progenitors cultured at limiting dilution (Table 3) . Example 6. Expansion Effects of TPO CD34 * , CD34 * Lin " or CD34 + Thy-1 + Lin " ABM cells were cultured on SySl stromal cells in SCCM with the indicated cytokines and analyzed at days 7 and 11. Expansion Effect of TPO on ABM CD341 selected cells. The purity of glycoprotease-selected CD34 * cells ranged from 60-93%. TPO alone induced a 1.3-fold expansion of CD34 * by day 11, at which time a mean of 52.8% MKs were observed (Table 4) . Wright-Giemsa staining showed both small and large MKs, the latter with multilobulated nuclei, indicative of polyploidization. Immunocytochemistry confirmed that both small and large cells were positive for CD41b expression, showing commitment to the MK lineage. Although IL-3 is known as an MK stimulatory factor, only 3% MKs were detectable at day 11 when CD34 * cells were cultured m IL-3 alone. A combination of TPO, IL-3, and KL resulted in a 4.7-fold lower percentage of MKs at day 11, while the number of total cells increased 5.5-fold, producing a similar or slightly higher number of MKs at day 11. Results are shown in Table 5. Expansion Effect of TPO on ABM CD34 * . Lιnl selected cells. CD34 " Lm " cells (depleted of CD2 " , CD15 * , and CD19 * cells) purified Oy FACS sorting had purities from 87% to 96%. In the presence of TPO alone, total cell expansion was greater than that observed with CD34' cells (Table 6) . By day 11, there was a 10-fold greater total cell expansion in cells cultured with TPO + IL-3 compared to TPO alone, but the proportion of MKs was approximately 3.5-fold lower (Table 5) . TPO + IL-3 generated a 2-fold higher number of total MKs by day 11, but the MKs were a minority of the population, the majority of cells appearing myeloid (Table 5) . The addition of GM-CSF to TPO resulted in a 2.8-fold greater cell expansion by day 11, but the percentage of MKs remained slightly higher than with TPO + IL-3 (33.7% vs. 20.6%) (Table 5) . A higher absolute number of MKs at day 11 could be produced by addition of either IL-3 or GM-CSF to TPO, but at the expense of the purity of the MK population. Effect of TPO on ABM CD34:Thy-lILιn:. Using the original sort gates, purity of the Thy-1 * cell subpopulations ranged from 77-90% (mean 84%) and of the Thy-1 " cell subpopulations from 65-93% (mean 86%) . There was significant cell expansion from the Thy-1 * population and a high percentage of MKs were consistently observed by day 11 (61.9-77.6%), indicating that CD34 * Thy-l * Lιn " stem cells can mature into MKs under the stimulation of TPO alone. The differences in MK potential between the Thy-1 * and Thy-1 " cell subpopulations were not significant (Figs. IA and +B) . Table 6 summarizes the mean total cell expansion and MK production of each cell subpopulation at days 7 and 11 under these conditions. Example 7. TPO Stimulates Activation of Quiescent Stem Cells into Cycle. To determine if TPO could activate quiescent stem cells into cycle, the effect of TPO on primitive hematopoietic cells which could efflux Rhl23, (CD34 " Lm " Rhl23- ) was tested. This cell population was labeled with the me orane dye PKH26, so that cell division could be observed by loss cf PKH fluorescence. Labeling was performed per r,sr.._fact rer's instructions (Zynaxis Cell Sciences Inc., Malvern, PA). The CD34 " Lιn " Rhl22- cells labeleα with PKH26 aye were cultured for up to 6 days as follows: approximately 10' cells were seeoeα in SCCM without stroma at 100 μl/well in round bottom 96 well plates with either no added cytokines (negative control), or in the presence of TPO, KL, or IL-3 alone, TPO and KL, or TPO and IL-3, and then analyzed at day 3 and day 6. Cultures were harvested, stained with antι-CD34-FITC antibody, and analyzed by flow cytometry for CD34 expression versus PKH26 dye retention on days 3 and 6 of culture. To obtain a day 0 measurement, an aliquot of cells was taken following staining and incubated overnight at 37°C to allow complete efflux of the Rhl23 dye. Cells were then stained with antι-CD34-FlTC and analyzed by flow cytometry. Cycling of cells was indicated by loss of PKH fluorescence, while differentiation was indicated by loss of CD34 expression. The cells were then subjected to FACS analysis. In serum-containing medium, in the absence of added cytokines, the cells remained in the upper right quadrant (there was no loss of PKH26 fluorescence or CD34 expression) showing that the cells remained quiescent. By day 3 of culture with TPO, about 8% of Rhol23- r cells had divided. By day 6, 32% of the cells were cycling, and 2/3 of these expressed CD34. At day 3 the effect of TPO and KL was similar, but by day 6 higher percentages of CD34+ cells were retained in the presence of TPO. When rhTPO and KL were combined, activation of cells into cycle by day 3 was more than additive, and occurred without loss of CD34 expression. There was apparent synergy between KL and TPO by day 6 with 69% of the cells entering cycle, however, 62% of cycling cells had differentiated to CD34- cells. IL-3 alone stimulated the greatest degree of cycling by day 6, however, the vast majority of these cells had lost CD34 expression, indicating that IL-3 pushed differentiation at the expense of self-renewal. K th the addition of TPO to IL-3, more dividing cells retained CD34 expression at days 3 and 6 than seen w th IL-2 alone, although fewer dividing cells were CD34' as compared to cultures w th both TPO and KL. Similar results were also obtained in two previous experiments using 10% TPO-containing COS supernatant. Example 8. Effect of TPO on Transduction of Stem Cells. CD34 * Thy-l * Lιn " stem cells are selected from mobilized peripheral blood or ABM as described above. Transduction experiments are conducted by methods known to the art in combination with the present disclosure. In addition to TPO (10-100 ng/ml purified recombinant TPO) the transduction medium may contain cytokines such as KL, IL-3, IL-6, etc. The LN retroviral vector contains the bacterial neo gene as a selectable marker under the control of the Herpes Simplex Virus thymidine kinase (HSV-tk) promoter, with the therapeutic gene of interest inserted under the control of the Moloney murine leukemia virus long terminal repeat (MMLV- LTR) (Miller & Rosman (1989) Biotechniques 7:980) . The retroviral vector DNA is electroporated (Chu et al. (1987) Nucleic Acids Res. 15:1311) into the amphotropic PA317 packaging cells (Miller & Buttimore (1986) Mol. Cell. Biol. 6:2895) and the cells are selected in 600 mg/ml G418 (Gibco¬ BRL) . Virus from resistant PA317 cells are used to inoculate GP-E86 cells (Markowitz et al. (1988) J. Virol. 62:1120) and G418-resιstant cell populations are again established. Virus from GP-E86 cells are used to inoculate PA317 cells at high multiplicity of infection (>10) and individual G418-resιstant PA317 cell clones are isolated. A single clone producing high titer viral supematants (>1 x IO 6 G418-resιstant CFU/ml as determined on NIH 3t3 cells) is selected. The producer cells test negative for replication competent retrovirus (RCR) in S+L- assays on PG-4 cells (Haapala et al. (1985) J. Virol. 53:827) . Vector-producing cells are grown in DMEM with high glucose and 10% FCS. Supernatant is harvested 24 hr after a media change on confluent monolayers and stored at Viral supernatant is diluted 1:1 in 2x transduction medium (Whitlock/Witte medium plus cytokines and 4 ug/ml protamine sulfate) prewarmed to 37oc, and sorted cells are suspended in the diluted viral supernatant at about 5 x 10' cells/ml. The same procedure is followed in the absence of TPO (control) . After 4 hrs, the medium is removed from the cells, and the cells resuspended in fresh medium (without virus) with or without TPO and/or additional cytokines, and the cells repipetted into the original well. The cultures are incubated for another 20 hrs. The process is repeated for three consecutive days with the cells incubated in the presence of virus for 4 hr each day. After 72 hrs, a viable cell count is performed. Transduction frequency is improved when the cells are transduced as described above but sample are centrifuged at 2800 x g at 21°C during the 4 hr incubation period. To determine transduction frequency, 2.5-5 x 10 3 cells from each transduction are added to 5 ml of methylcellulose (Stem Cell Technologies) containing the following cytokines: KL, 10 ng/ml; GM-CSF, 25 ng/ml; G-CSF, 25 ng/ml; IL-3, 10 ng/ml; and rhEPO, 2 units/ml. 1.1 ml of the cell/cytokine methylcellulose mixture is plated onto four 3 cm gridded plates using a 5 ml syringe and 16.5 gauge needle, and the plates are placed in a 37oC incubator for 2 weeks. After 14 days, single methylcellulose colonies are picked and suspended in 50 μl Lysing Buffer (75 mM KCI, 10 mM Tris-HCl, pH 9.25, 1.5 mM MgCl : , 0.5% Tween-20, 0.5% NP40, 1 mg/ml proteinase K) and PCR used to amplify vector-specific sequences in the transduced cells. PCR products are visualized on ethidium bromide agarose gels. Long term stromal cultures are used to evaluate gene markings of more primitive progenitor cells. Briefly, 10,000 cells from each transduction are cultured for 4 weeks on a pre-established monolayer of mouse stromal cells (AC6.21) in media supplemented with 20 ng/ml LIF and 20 ng/ml IL-6. Plates are fed weekly by demi-depletion and growth positive wells are analyzed by PCR for the presence of the transgene. Results. Cells transduced in the presence of TPO will exhibit improved cell viability relative to cell transduced in the absence of TPO, hence resulting in higher numbers of transduced stem cells. Transduction frequencies are improved in the presence of IL-3 and KL, in addition to TPO, which promote cell viability, cell cycling and binding of amphotropic retrovirus to human hematopoietic stem cells. Table 2. Effect of TPO on CAFC frequency Table 3. Limiting dilution culture: analysis of CD34 * cells Table 4. Effect of TPO and IL-3 with or without KL on ABM CD34 + selected cells % CD41 b + cells total cell no. x 10 4 Addition day 4 day 7 day 11 day 4 day 7 day 11 0 4.4 ± 4.6 ± ND 7.8 ± 6.0 ± ND 0.8 2.2 1.1 1.6 IL-3 6.0 ± 7.2 ± 3.0 ± 12.5 ± 13.1 ± 15.6 ± 1.4 1.8 0.2 0.5 1.9 3.2 TPO 20.8 ± 42.4 ± 52.8 ± 1 1.2 ± 1 1 .5 ± 13.2 ± 0.0 7.0 2.8 0.2 0.4 0.8 TPO + 20.1 ± 25.0 ± 32.0 ± 18.8 1 1.0 32.9 IL-3 5.3 3.6 22.2 IL-3, 6.7 ± 1 1.0 ± 1 1.3 ± 34.1 ± 34.8 ± 72.1 ± TPO + 2.6 1 .0 3.7 17.6 7.9 4.3 KL Table 5. Effect of TPO with or without IL-3 or GM-CSF on CD34 * Un cells % CD41b + cells total cell no. x 10 4 AB Addition day 4 day 7 day day day day 11 M 11 4 7 1 TPO 19.8 70.6 70.1 34.0 28.5 34.2 TPO + IL-3 14.8 14.0 15.2 67.0 95.3 276.4 TPO + GM- 16.0 30.7 34.0 25.0 52.3 104.6 CSF 2 TPO 39.7 43.7 61.4 20.6 28.0 74.2 TPO + IL-3 15.2 17.1 21.0 90.0 132.0 204.6 TPO + GM- 29.3 31 .9 33.3 27.4 88.8 195.4 CSF 3 TPO 27.5 53.5 75.6 15.5 40.3 19.7 TPO + IL-3 22.0 31.2 25.6 23.5 131.6 355.8 TPO + IL-3 + 16.5 23.7 16.3 68.6 164.6 1 17.7 GM-CSF Table 6. Comparison of ABM CD34 + Cell Subpopulations Population Additions Total cell no. No. CD41b + % CD41b + x 10 4 cells cells Day 7 CD34 + TPO 11.5 4.9 42.4 TPO + IL-3 11.0 2.8 25.0 CD34 + Lιn TPO 32.3 18.1 55.9 TPO + IL-3 120.0 25.0 20.8 CD34 + Thy + Lιn TPO 18.4 9.2 49.6 TPO + IL-3 14.5 7.1 48.8 Day 11 CD34 * TPO 13.2 7.0 52.8 TPO + IL-3 32.9 10.5 32.0 CD34 * Lιn TPO 42.7 29.5 69.0 TPO + IL-3 279.0 57.5 20.6 CD34'Thy * Lιn TPO 10.5 7.1 67.5 TPO + IL-3 10.5 7.7 73.7 SEQUENCE LISTING (1) GENERAL INFORMATION: (i) APPLICANT: Lesley J. Murray Judy C. Young (ii) TITLE OF INVEN ION: Methods for use of mpl ligands with primitive human stem cells. (iii) NUMBER OF SEQUENCES: 6 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Fish & Richardson (B) STREET: 2200 Sand Hill Road, Suite 100 (C) CITY: Menlo Park (D) STATE: California (E) COUNTRY: USA (F) ZIP: 94025 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: AscIII (vi ) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: (B) FILING DATE: (C) CLASSIFICATION: (viu) ATTORNEY/AGENT INFORMATION: (A) NAME: Valeta Gregg (B) REGISTRATION NUMBER: 35,127 (C) REFERENCE/DOCKET NUMBER: 07532/003001 (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: (415) 322-5070 (B) TELEFAX: (415) 854-0875 (2) INFORMATION FOR SEQ ID NO: 1 : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 42 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 1 : GCTATTGCGG CCGCGAATTC GGARGAYYTI INITGYTTYT GG 42 (2) INFORMATION FOR SEQ ID NO: 2 : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 38 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 2 : GCTATTCTCG AGATCGATSW CCAITCRCTC CAIIINCC 38 (2) INFORMATION FOR SEQ ID NO: 3 : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 38 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 3 : GCTATTCTCG AGATCGATSW CCAINIRCTC CAIIINCC 38 (2) INFORMATION FOR SEQ ID NO: 4 : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4 : CGCTGCACCT CTGGGTGAAG 20 (2) INFORMATION FOR SEQ ID NO: 5 : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 21 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 5 : AGCAGGGCAG CAGGTTTCTG T 21 (2) INFORMATION FOR SEQ ID NO: 6 : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 21 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: cDNA (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 6 : GCTGCGCAGC TGCAGCCAGT A 21

Claims: What is claimed is: 1. A method for promoting the survival of a stem cell in culture, comprising culturing said cell in the presence of a myeloproliferative receptor (mpl) ligand, wherein said ligand binds mpl and mpl-mediated biological activity is initiated. 2. The method of claim 1, wherein said mpl ligand is thrombopoietin. 3. The method of claim 2, wherein said thrombopoietin is human thrombopoietin. 4. The method of claim 3 , wherein said thrombopoietin is recombinant human thrombopoietin. 5. The method of claim 1, wherein said cell cultured in the presence of said mpl ligand is characterized by the capability of self-renewal and ability to give rise to all hematopoietic cell lineages. 6. The method of claim 1, wherein said cell is a human stem cell . 7. The method of claim 6, wherein said cell is CD34 * . 8. The method of claim 6 wherein said cell oε CD34 * Thy-r . 9. The method of claim 6, wherein said cell is Cr34~Lin ~ . 10. The method of claim 6, wherein said cell is CD34 * Thy * Lin " . 11. The method of claim 6, wherein said cell is CD34 * +Lin " Rho 10 or CD34 * Thy * Lin " Rho lc . 12. The method of claim 4, wherein said recombinant human thrombopoietin is present in a concentration of about 1 ng/ml to about 100 ng/ml. 13. A method of expanding a population of stem cells, comprising exposing a stem cell to a mpl ligand, wherein said cell proliferates to form an expanded population of stem cells . 14. The method of claim 13, wherein said mpl ligand is thrombopoietin. 15. The method of claim 14, wherein said thrombopoietin is human thrombopoietin. 16. The method of claim 15, wherein said thrombopoietin is recombinant human thrombopoietin. 17. The method of claim 13, wherein said expanded cell population is characterized by the ability to undergo substantial self-renewal and ability to give rise to all hematopoietic cell lineages. 18. The method of claim 13 wherein said cells are human stem cells . 19. The method of claim 18, wherein said cell is CD34 * . 20. The method of claim 19, wherein said cell is CD34 * Lin " . 21. The method of claim 19 wherein said cell is CD34 * Thy-1 * . 22. The method of claim 19, wherein said cell is CD34 * Thy~Lin ~ . 23. The method of claim 19, wherein said cell is CD34 * Lin ' Rho 10 or CD34 * Thy * Lin " Rho lc . 24. The method of claim 16, wherein εaid recombinant human thrombopoietin is present in a concentration of about 1 ng/ml to about 100 ng/ml. 25. A therapeutic method for restoring hematopoietic capability to a human subject, said method comprising the stepε of : (a) removing εtem cells from a human subject; (b) expanding said cells in the presence of a mpl ligand to form an expanded population of stem cells from a human subject; and (c) returning said expanded cells to said subject, wherein hematopoietic capability is restored to said patient. 26. The method of claim 25, wherein said expanded population of stem cells are characterized by the capability of self-renewal and ability to give rise to all hematopoietic cell lineages. 27. The method of claim 25, wherein said mpl ligand is thrombopoietin. 28. The method of claim 25, wherein said thrombopoietin iε human thrombopoietin. 29. The method of claim 28, wherein said thrombopoietin is recombinant human thrombopoietin. 30. The method of claim 29, wherein said recombinant human thrombopoietin is present in a concentration of about 1 ng/ml to about 100 ng/ml. 31. The method of claim 25, wherein said cells are expanded in the presence of one or more additional cytokines. 32. The method of claim 31, wherein said cytokines are selected from the group consiεting of interleukin 3 (IL-3) , interleukin 6 (IL-6) , leukemia inhibitory factor (LIF) , c-kit ligand (KL) , granulocyte-macrophage colony stimulating factor (GM-CSF) , granulocyte colony stimulating factor (G-CSF) , and steel factor (Stl) . 33. The method of claim 32, wherein said cytokine or one of said cytokines is IL-3. 34. A method for activating a quiescent stem cell to divide, comprising exposing said quiescent cell to a mpl ligand, wherein said cell iε activated to divide. 35. The method of claim 34, wherein said mpl ligand is thrombopoietin . 36. The method of claim 35, wherein said thrombopoietin is human thrombopoietin. 37. The method of claim 36, wherein said thrombopoietin is recombinant human thrombopoietin. 38. The method of claim 34, wherein εaid eel 1 iε a human εtem cell. 39. The method of claim 38, wherein said cell is CD34 * . 40. The method of claim 38, wherein said cell is CD34 * Lin " . 41. The method of claim 38, wherein said cell is CD34 * Thy-1 * . 42. The method of claim 38, wherein said cell iε CD34~Thy~Lin " . 43. The method of claim 38, wherein εaid cell is CD34 * Lin ' Rho 10 or CD34 * Thy * Lin " Rho lc . 44. The method of claim 34, wherein cells formed from said activated cell are characterized by the capability of self-renewal and ability to give rise to all hematopoietic cell lineages . 45. The method of claim 37, wherein said recombinant human thrombopoietin is present in the concentration range of about I ng/ml to about 100 ng/ml. 46. A method for modifying a stem cell, comprising the steps of : (a) inserting a foreign gene into a viral vector; (b) culturing a quiescent εtem cell in the presence of a mpl ligand, wherein said cell is activated to divide; and (c) exposing said activated cell to said viral vector, wherein said foreign gene is integrated into the DNA of said stem cell . 47. The method of claim 46, wherein said mpl ligand is thrombopoietin. 48. The method of claim 47, wherein said thrombopoietin is human thrombopoietin. 49. The method of claim 48, wherein said thrombopoietin is recombinant human thrombopoietin. 50. A method for providing gene therapy to a subject, comprising (a) inserting a foreign gene into a viral vector; (b) culturing a quiescent stem cell in the presence of a mpl ligand, wherein said cell is activated to divide; and (c) exposing said activated cell to εaid viral vector, wherein εaid foreign gene is integrated into the DNA of said stem cell; and (d) providing εaid stem cell thus modified to a subject in need thereof. 51. The method of claim 46, wherein said foreign gene encodes a protein selected from the group consisting of the mdrl gene product, adenosine deaminase, glucocerebrosidase, β-globin, Factor VIII, Factor IX, mdr related protein, T- cell receptors, cytokines, and a protein interfering with replication of HIV. 52. The method of claim 46, wherein said foreign gene is an antisense or ribozyme sequence. 53. Use of an mpl ligand in a method of culturing stem cells . 54. The use according to claim 53 when the mpl ligand is thrombopoietin. 55. The use according to claim 53 or 54 when the stem cells are CD34+ cells. 56. The use of an mpl ligand in the production of a cellular composition for use in gene therapy or hematopoietic reconstitution. 57. A cell culture medium comprising an mpl ligand and one or more cytokines . 58. The cell culture medium according to claim 57 comprising 10-200 mg/ml thrombopoietin and 1-100 ng/ml IL-3.

Inventors: MURRAY LESLEY J, YOUNG JUDY C, MURRAY, LESLEY, J., YOUNG, JUDY, C.
Applicants: SANDOZ AG, SANDOZ LTD, SYSTEMIX INC, SANDOZ LTD., SANDOZ-ERFINDUNGEN VERWALTUNGSGESELLSCHAFT MBH, SANDOZ-PATENT-GMBH, SYSTEMIX, INC.
Application year: 1996
Main classification: C12N5
IPC codes: A61K31, A61K35, A61K38, A61K48, A61P7, C07K14, C12N5, C12N15, C12Q1